Semiconductor interfaces, where the spatial gradient in built-in potentials governs the electronic and optoelectronic processes, are an essential building block for modern semiconductor devices. Recent advance of atomically thin van der Waals (vdW) materials and their heterostructures provide a new opportunity to realize atomically sharp interfaces in the ultimate quantum limit. In this presentation, we will discuss construction and characterization of an atomically thin vertical vdW interfaces. Schottky junction, p-n heterojunction and superconductor-metal junctions based on the vdW assembly of transition metal dichalcogenides and graphen have been realized. Unlike conventional semiconductor heterostructures, charge transport and photovoltaic response of the devices are found to critically depend on the interlayer recombination process between majority carriers mediated by tunneling across the interface. We demonstrate the enhanced electronic optoelectronic performances in the vdW heterostructures, tuned by applied gate voltages, suggesting that these a few atom thick interfaces may provide a fundamental platform to realize efficient, fast and tunable bipolar electronics, photovoltaics, and optoelectronics.

Semiconductor heterostructures provide a powerful platform to engineer the dynamics of excitons for fundamental and applied interests. However, the functionality of conventional semiconductor heterostructures is often limited by inefficient charge transfer due to the interfacial imperfection caused by lattice mismatch. Here we demonstrate that MoS₂/WS₂ heterostructures consisting of monolayer MoS₂ and WS₂ stacked in the vertical direction can enable equally efficient interlayer exciton relaxation regardless the epitaxy and orientation of the stacking. This is manifested by a decrease of photoluminescence intensity by two orders of magnitude in both epitaxial and non-epitaxial MoS₂/WS₂ heterostructures. Both heterostructures are also found showing improved absorption beyond the simple super-imposition of the absorptions of monolayer MoS₂ and WS₂. Our result indicates significant implications of 2D heterostructures for the development of photonic devices that request efficient exciton separation and strong light absorption, such as solar cells, photodetectors, modulators, and photocatalysts. It also suggests that a simply stacking of dissimilar 2D materials with random orientations is a viable strategy to fabricate complex 2D heterostructures, which would show similar optical functionality as the counterpart with perfect epitaxy.

Semiconducting two-dimensional (2D) materials have become a focus of research in recent years. One of the unique properties of these materials is the sensitivity of their electronic structure and vibrational spectrum to inter-layer coupling, despite the weak van der Waals interaction between neighboring layers. It is expected that controlling inter-layer coupling in homo- and hetero-structures of 2D semiconductors would lead to crossover between 2D and 3D, as well as a wide range of interesting physical effects. We demonstrated that the inter-layer coupling and the resultant physical properties can be modulated thermally and mechanically, and are sensitive to in-plane crystal structure. We employ a diamond anvil cell to apply high hydrostatic pressures onto 2D structures, and probe the resultant optical reflection, absorption and emission and Raman spectrum.

Van der Waals heterojunctions have shown interesting electronic and optoelectronic properties. Pristine interfaces with sharp band edges, free of dangling bonds contribute to improved transport properties compared to bulk semiconductors. In particular, MoS₂/WSe₂ heterojunction diodes have shown good rectification behavior, with ideality factor ~1, emphasizing the existence of trap-free interfaces.^[1]-[3] The absence of depletion region in atomically thin p-n junctions allows the observation of interlayer tunneling recombination of majority carriers across the van der Waals gap, which affects electronic and optoelectronic properties of these p-n junctions. However, clear signature of tunneling has not been observed so far in these van der Waals heterostructures.^[1],[4]
In this work, we fabricate gated p-n didoes with stacked transition metal dichalcogenides. The band alignment of the hetero-stacks can be tuned from a type III (broken gap) configuration to type II configuration by the application of gate voltage. Negative differential resistance in forward bias is observed for the diode under an extreme gate bias. As the gate bias is lowered, the diode moves into backward diode operation, with appreciable tunneling current in the reverse bias, and the forward bias current is lower than the reverse bias current. With further reduction of the gate voltage, the diode is observed to be rectifying, with an ideality factor ~1 at room temperature, and a switching ratio of > 10⁴. The ability to tune the diode from the regime where Esaki behavior (akin to the condition where the n and p sides are degenerately doped) is observed to the regime where the diode is rectifying, merely by the application of gate bias and without using any doping scheme, is novel and has not been observed in three dimensional semiconductors.
REFERENCES:
[1] H. Fang et al., PNAS, 2014
[2] T. Roy et al., ACS Nano, 2014
[3] S. Xiao et al., IEEE DRC, 2014
[4] C. Lee et al., Nature Nano, 2014

Two-dimensional (2D) layered materials such as graphene or transition metal dichalcogenides (TMDs) have attracted considerable interest due to their unique layer-number dependent electronic and optical properties. Vertical integration of layered materials can allow for flexible integration of highly disparate materials, and enable totally new possibilities for the design of future electronic and photonic devices. Here I will discuss a variety of heterostructures made of various 2D materials for novel design of electronic and optoelectronic devices. We first will discuss a general strategy for the vertical integration of various layered materials to obtain both p- and n-channel transistors for complementary logic functions. We demonstrate a complementary inverter with larger than unit voltage gain by vertically stacking the layered materials of graphene, Bi₂Sr₂Co₂O₈ (p-channel), graphene, MoS₂ (n-channel), and metal thin film in sequence. The ability to simultaneously achieve a high on-off ratio, high current density, and logic integration in the vertical heterostructures can open up a new dimension for future electronics to enable three-dimensional integration. Next we show that similar vertical heterostructures (e.g. graphene-MoS₂-graphene) can be explored for highly efficient photocurrent generation. The partial “electrostatic transparency” of graphene allows integration of both a top- and a bottom-gate above and under the heterostructures, to allow using an external electrical field to modulate the band slope in MoS₂ and manipulate the photocarrier separation and transport processes, and thus modulating the amplitude and polarity of the photocurrent in the gated vertical heterostructures. It can open up exciting opportunities for the design of a new generation of photodetection and photovoltaic devices. Lastly, we further show that vertically stacked TMD heterojunctions (MoS₂/WSe₂) and synthetic lateral junctions (MoS₂/WSe₂) can also be created for efficient photon harvesting, photoelectrical conversion and light emitting devices.

Transition metal dichalcogenides (TMDs), e.g. MoS₂, MoSe₂, WS₂, and WSe₂, have risen as a new generation of materials with remarkable properties. While much research has been directed to explore the novel electronic properties of 2D van der Waals heterostructures, it is also important to study their vibrational properties that may affect device performance through electron-phonon interactions. In particular, it is intriguing to explore the possibility of generating new phonon modes through hybridization of different 2D crystals. In this research we achieved the first observation of interlayer phonon modes in atomically thin van der Waals heterostructures. We measured the low-frequency Raman response of MoS₂/WSe₂ and MoSe₂/MoS₂ heterobilayers. We discovered a distinctive Raman mode (30 - 35 cm^-1) that cannot be found in any individual monolayers. By comparing with Raman spectra of bilayer (2L) MoS₂, 2L MoSe₂ and 2L WSe₂, we identified the new Raman mode as the LBM arising from the perpendicular vibration between the two rigid TMD layers. The heterogeneous LBM Raman intensity correlates strongly with the suppression of photoluminescence (PL) that arises from interlayer charge transfer. The LBM only emerges in bilayer regions with atomically close layer-layer proximity and clean interface. In addition, the LBM frequency exhibits noticeable dependence on the relative orientation between the two TMD layers, which implies a change of interlayer separation and interlayer coupling strength with the layer stacking. Our results reveal that LBM generally exists in van der Waals heterostructures and can serve as an effective probe to the interface environment and interlayer interactions in these materials.

MoS₂ and transition metal dichalcogenides have opened numerous research directions and potential applications for this diverse family of nanomaterials. The combination of these 2D materials in heterostructures can result in a huge number of potentially interesting new materials. Most of the attention in this field is focused on heterostructures composed of different 2D materials. In my talk, I will present some of our recent efforts in this direction, oriented towards realizing combinations of 2D and 3D materials into van der Waals heterostructures. I will report on high-performance photodetectors based on 2D/3D heterostructures that can operate with internal gain and high sensitivity. Our devices also show very low noise, due to the unique architecture of the 2D/3D heterojunction. Next, I will give an update on our efforts to realize high-performance electrical circuits based on TMD materials.

Recent years have seen rapid progress in the growth of two-dimensional (2D) transition-metal dichalcogenide MX₂ (M=Mo, W; X=S, Se) nanosheets composed of one to a few monolayers using chemical vapor deposition (CVD) with large-scale polycrystalline MX₂ nanosheets demonstrated, for example, polycrystalline nanosheets MoS₂, WS₂, MoSe₂, MoS_2(1-x)Se_2x, and Mo_1-xW_xS₂ in size of 2-inch-wafer-scale [Nanoscale4, 6637 (2012)], sub-centimeter [ACS Nano7, 5235 (2013)], 1x1-cm² [Nano Letters14, 2419 (2014)], sub-centimeter [Advanced Materials26, 2648 (2014)], and 2-inch-wafer-scale [Nanoscale6, 624 (2014)] respectively have been reported; however, the size of single-crystallineMX₂ nanosheets is still limited under a few hundreds of microns. To the best of our knowledge, the reported largest size of single-crystalline MX₂ nanosheets is 123µm, 135µm, ~370µm, ~50µm, 80µm, and ~50nm for MoS₂, MoSe₂, WS₂, WSe₂, MoS_2(1-x)Se_2x, and Mo_1-xW_xS₂, respectively. [refs: Nature Materials 12, 554 (2013); ACS Nano8, 5125 (2014); Nanoscale 6, 12096 (2014); ACS Nano 8, 923 (2014); JACS136, 3756 (2014); and Nanoscale6, 624 (2014).] For high-performance device applications, large-area single-crystalline MX₂ nanosheets without grain boundaries are indispensable. Hence, one of the important tasks for 2D MX₂ research is to reliably achieve large-scale single-crystalline MX₂ monolayers using a unique synthesis approach.
Here we report a vapor-trapping enhanced CVD growth approach, which has been successfully employed for nanowire and graphene growth in the previous, for large-area growth of high-quality single-crystalline MX₂ nanosheets with one to a few monolayers. The samples grown by vapor-trapping CVD show significantly larger size and better uniformity than those grown with same growth parameters but without vapor-trapping. The vapor-trapping enhancement effect is likely due to the increased local sulfur vapor pressure based on our study so far. The size of the single-crystalline MoS₂ monolayers grown by vapor-trapping CVD as of now is comparable to the existing reported largest size. More importantly, the size of single-crystalline MX₂ has shown a trend of rapid increase during the past several months ever since vapor-trapping was employed in the CVD growth. We believe the breakthrough of single-crystalline MX₂ nanosheet size is likely coming soon and vapor-trapping CVD approach paves a reliable way for large-scale single-crystalline MX₂ nanosheet growth.
The number of layers, crystallinity, and uniformity of the as-grown single-crystalline MX₂ was characterized using Raman and photoluminescence spectra and mapping. The mobility was characterized by both Hall effect directly and field-effect transistor transport measurements. The Hall bar devices were fabricated by lithography and dry-etching of the as-grown single-crystalline MX₂ nanosheets and. In the field-effect transistor transport measurements, both top gate and bottom gate geometry were measured.

Chemical vapour deposition (CVD) growth of monolayer MoS₂ crystal domains and films has been recently apprehended, which has facilitated extensive studies regarding their unique direct-band-gap photoluminescence and electrical transport properties. The CVD synthesis of WS₂ monolayer crystals is slightly more challenging than MoS₂ because of the higher melting points of the W-based precursors, such as WO₃, compared to the Mo-based precursor counterparts, and the resulting higher growth temperatures needed. In the past decade, the CVD approach has proved to be extremely useful for production of large-area single crystalline graphene domains which can diminish grain boundary defects for transport devices. Although the creation of a single crystalline structure of monolayer WS₂ has been recently realized, the controlled CVD growth of large-area monolayers with homogenous crystallinity still remains an experimental challenge and requires continual investigation.
The approach for most reports of CVD growth of WS₂ relies on the reaction of sulphur (S) vapour with tungsten (W)-precursor at elevated temperatures, with both precursor placed in a single furnace. Not having individual control over each precursor's temperature limits the parameter space of the chemical reaction. To overcome this limitation, we utilize a CVD approach that separates the two precursors into different furnaces, one containing the elemental S and the other containing both the WO₃ and the growth substrates (SiO₂/Si). Both the S vapour and W-precursor vapour are carried downstream to the growth substrate by pure Ar gas. As a result, we successfully showed that controlling the introduction time and the amount of sulphur (S) vapour relative to the WO₃ precursor during the CVD growth of WS₂ is critical to achieving large crystal domains on the surface of silicon wafers with a 300 nm oxide layer. Accurate control of the S introduction time enabled the formation of extra-large triangular WS₂ monolayer domains with edges up to 370 mu;m which are visible to the naked eye. The WS₂ domains exhibit room-temperature photoluminescence with a peak value around 635 nm and a full-width at half-maximum (FWHM) of 12 nm. Selected area electron diffraction from different regions of the triangular WS₂ domains shows that they are of single crystalline structure.
One major challenge for growing continuous sheets of monolayer WS₂ using the above-mentioned approach was that the S vapour would quench the bulk WO₃ precursor and turn it into WS₂ bulk powder. Our improved CVD system with a double-walled quartz tube permitted the isolated introduction of S vapour and the W-precursor vapour into the growth chamber. Therefore, we fabricated monolayer WS₂ continuous films with application of the controlled sulphur introduction timing.

The electronic and optical properties of semiconducting layers of transition metal dichalcogenides (TMDs) depend strongly on the layer thickness. Here we employ time-resolved photoluminescence spectroscopy to investigate the exciton dynamics in monolayer, bilayer, and trilayer WS₂ two-dimensional (2D) crystals. We observed remarkably different exciton dynamics in WS₂ monolayers and few-layers: (i) at low exciton density limit, we obtained an exciton lifetime of 856 ± 3 ps, 435 ± 1 ps, and 305 ± 2 ps for WS₂ monolayer, bilayer, and trilayer respectively, which can be explained by the different radiative and non-radiative rates between monolayers and few-layers; (ii) the decay dynamics of monolayer WS₂ showed a strong excitation density dependent behavior, which can be well described using exciton-exciton annihilation model. The exciton-exciton annihilation rate for monolayer, bilayer, and trilayer of WS₂ was determined to be 0.41 ± 0.02, (6.00 ± 1.09) × 10^-3 and (1.88 ± 0.47) × 10^-3 cm²/s respectively. Notably, exciton-exciton annihilation rate is almost 100 folds larger in monolayer than in bilayer and trilayer. We explain the slower in exciton-exciton annihilation rate in bilayer and trilayer by phonon-assistance exciton-exciton annihilation of indirect exciton.

The inefficient dissipation of heat is a very crucial problem that limits the reliability and performance of all electronic devices, especially those in military applications. Since devices have become progressively smaller, more powerful, and more complex, they dissipate much larger amounts of heat. Currently, thermal greases, epoxy-based composites, and solders are the most commonly used types of thermal interface materials (TIMs) used for enabling the efficient dissipation of heat. Herein, we describe next-generation TIMs with thermal resistivity lower than existing solder TIMs and with stiffness comparable with existing epoxy-based TIMs. The approach has involved the incorporation of soft-ligand functionalized boron nitride nanosheets (BNNS) in a metal matrix to fabricate nanocomposite TIMs.
The filler, BNNS, was prepared by the mechanically assisted cleavage of h-BN flakes through ultrasonication in water. It was found that the size and thickness of the nanosheets can be adjusted via varying the sonication intensity and time. Then, the prepared BNNS was functionalized with thiosemicarbazide through Lewis acid-base interactions to form functionalized BNNS (f-BNNS), existence of which was confirmed using IR spectroscopy. Then, f-BNNS was dispersed in a copper matrix using a novel electrocodeposition approach where copper and f-BNNS are deposited on a cathode in the presence of an electric field and potential difference to form nanocomposite TIMs.
The thermal properties of the nanocomposite TIMs were investigated using the laser flash analysis and differential scanning calorimetry (DSC). The specific heat capacity of nanocomposite ranged from 0.32 J/g.K to 0.36 J/g.K for a filler loading ranging from 15 wt% to 30 wt%. The thermal conductivies were found to be in between 95 W/m.K and 110 W/m.K.
Upon determining thermal properties of the developed TIMs, it was focused on obtaining their mechanical properties. To this end, nanoindentation technique was employed. The average E_r values were ranged from 18.6 GPa to 41.9 GPa for a filler loading ranging from 30 wt% to 15 wt%, respectively. As desired, these values are much smaller than that of the electroplated pure copper thin films (99-125 GPa). There was roughly a five-fold decrease in the reduced modulus of copper matrix when copper matrix was loaded with ~30% functionalized BNNSs. The average hardness value corresponding to this sample was 177 MPa, which is about five-to-fifteen times lower than the measured hardness values of pure copper (1.1-2.8 GPa).
In summary, we have shown that electrocodeposited nanocomposites with thermal conductivies greater than 100 W/K.m and elastic modulus values less than 25 GPa can be produced. For these materials, the total resistance across interface was estimated to be 1.8-3.8'10^-3 cm².K/W for a thickness of 20-50 mu;m. Considering two extreme cases of epoxy-based and pure copper shim TIMs, our results are significant in advancing the current state of art for TIMs.

Of the known layered transition metal dichalcogenides, MoS₂ has been the most widely studied for its promising semiconducting properties and potential applications in nanoelectronics and optoelectronics. While fabrication techniques such as chemical vapor deposition and exfoliation have been extensively investigated, the controlled synthesis of highly crystalline MoS₂ atomic layers is still a challenge, and studies on solid-state formation of MoS₂ through physical vapor deposition techniques are severely limited. Herein, we report the successful fabrication of highly crystalline few-layer MoS₂ on silver substrates by pulse laser deposition (PLD) at the relatively low temperature of 5000C. The growth proceeds by conventional epitaxy and is facilitated by the in-situ formation of a nearly lattice-matching Ag₂S phase on the silver surface. The number of layers of the resulting film is easily controlled by selection of appropriate laser energy and deposition time, while the crystalline quality was found to be improved by higher laser energies and slower cooling profiles.
The growth process of MoS₂ on metal substrates was also thoroughly investigated. In the second part of this work, MoS₂ was deposited by PLD on four different metals, Ag, Al, Ni and Cu. Highly crystalline few-layer MoS₂ was successfully grown on Ag, but is absent in Al, Ni and Cu under specific growth conditions. This discrepancy was attributed to either excessively strong or insufficient adlayer-substrate interactions. In the case of Al, the effects of the strong interface interactions can be offset by increasing the amount of source atoms supplied, thereby producing semi-crystalline few-layer MoS₂. The results show that despite PLD being a physical vapor deposition technique, both physical and chemical processes play an important role in MoS₂ growth on metal substrates.

Crystalline quality is the most influential parameter for two-dimensional (2D) materials, making exfoliated flakes the most popular forms for scientific studies. Non-exfoliated forms are technologically more relevant, but they suffer from poorly ordered atoms for large area (>100 mm) 2D materials, and are characterized by high grain or domain boundary densities. The ability to increase or tune the domain size in vapor deposited 2D materials has been elusive because these materials are typically resistant to high temperature treatments to re-structure for any appreciable improvements in lateral coherence over large areas. In this study, we exploit the lower activation energy for surface diffusion in comparison to grain boundary or bulk diffusion by fabricating freestanding 2D molybdenum disulfide (MoS₂) and amorphous boron nitride (BN) specimens to expose both surfaces. We performed in situ heating in a transmission electron microscope (TEM) to observe the domain restructuring in real time as a function of temperature. The freestanding specimens showed up to 100x increase in crystalline domain size at temperatures around 600 °C, much lower than the 850-1000 °C range cited in the literature. Remarkably, the amorphous BN transformed in to polycrystalline hexagonal BN (h-BN), which is otherwise unattainable at this temperature. The evidence of lower temperature domain engineering hints that high performance devices can be fabricated via scalable growth techniques for 2D materials and their heterostructures with post-treatment methods to obtain highly ordered layers for atomic (h-BN) and molecular (MoS₂) structures.

The study of two-dimensional (2D) materials is one of the central topics in condensed matter research [1-3]. To further expand this field, exploration and preparation of new classes of materials in 2D form is strongly desired. Complex oxides are well known to exhibit exotic behaviors due to their electron correlation, such as colossal magnetoresistance (CMR) [4] and high-Tc superconductivity [5] in bulk, as well as new electronic ground states at heterointerfaces [6]. However, many of these materials are not cleavable and studies of freestanding oxide films have been limited due to the technical difficulty in obtaining high quality samples applicable to a wide range of oxides [7, 8].
Here, we demonstrate a general method to fabricate atomic-scale freestanding complex oxide films by using selective buffer layer etching. This method involves epitaxial growth of thin films on an epitaxial buffer layer, which is stabilized on an oxide substrate, followed by etching of the buffer layer to detach the thin film from the substrate. We fabricated freestanding SrTiO₃/La_0.7Sr_0.3MnO₃ (STO/LSMO) superlattices, a representative oxide heterostructure showing CMR properties, as well as STO and LSMO single-layer freestanding films. The lateral sizes of those obtained freestanding films were typically on the millimeter scale. The surface morphology and the structural coherency were maintained in the freestanding structures most notably the atomic-scale precision in the superlattices. Interestingly, the saturation magnetizations of the STO/LSMO superlattices were enhanced compared to the epitaxial ones, the origin of which will be discussed together with the details of the fabrication process. We believe that the successful fabrication and characterization of freestanding complex oxide thin films brings many opportunities both in the fields of fundamental scientific research and device applications.
References:
[1] A. K. Geim and K. S. Novoselov, Nature Materials 6, 183 (2007).
[2] M. Xu et al., Chem. Rev. 113, 3766 (2013).
[3] A. K. Geim and I. V. Grigorieva, Nature 499, 419 (2013).
[4] S. Jin et al., Science 264, 413 (1994).
[5] J. G. Bednorz and K. A. Muller, Z. Phys. B 64, 189 (1986).
[6] A. Ohtomo and H. Y. Hwang, Nature 427, 423 (2004).
[7] W. H. Chen et al., Appl. Phys. Lett. 91, 121114 (2007).
[8] Q. Gan et al., Appl. Phys. Lett. 72, 978 (1998).

Atomic or molecular preferential adsorption on a surface template provides a facile and feasible means of fabricating ordered low-dimensional nanostructures with tailored functionality for novel applications. In this study, we demonstrate that functionality of C doped BN nanomesh can be tailored by an external electric field which modifies the strength of the adsorbate binding to the nanomesh. Specifically, selective binding of H, O, H₂ and O₂ at various sites of the C doped nanomesh - within the pore, on the wire, and at an intermediate site - is investigated using density functional theory. The calculated results find that atomic species are bound, but the molecular species are not bound to the nanomesh. We have shown that it is possible to modify the adsorbate binding energy with the application of an external field, such that the molecular H₂ can be bound at the pore region of the nanomesh. Interestingly, the work function of the nanomesh has a close correlation with the adsorbate binding energy with the BN nanomesh.

The inherent problem of a zero-band gap in graphene generates motivation to search for the next-generation electronic materials in the post-Si era. This spurs the recent surge of interest in 2D transition metal dichalcogenides materials such as MoS₂. In this study, we consider a triangular MoS₂ quantum dot (QD), investigating the effects of passivation, additional layer, and the h-BN substrate on its geometry, energetics, and electronic properties. The results of density functional theory calculations show that the monolayer QD is metallic in nature, mainly due to the coordinatively unsaturated Mo atoms at the edges. This is reaffirmed by the passivation of the S edge atoms which does not significantly modify the metallic character of the QD. Analysis of the chemical topology of the QD shows that Mo-S bonds are predominantly covalent at the under-coordinated atoms despite the presence of metallic states. A bilayer QD is more stable than its monolayer counterpart, mainly due to stabilization of the dangling bonds of the edge atoms. The additional layer reduces the degree of the metallic character considerably as demonstrated by the I-V characteristics. We find the binding strength of a monolayer QD to the h-BN substrate to be weak. The substrate-induced modifications in the electronic structure of the quantum dot are therefore not discernible. The metallic character of the QD deposited on the insulating substrate can therefore be exploited to extend the functionality of MoS₂-based nanostructures in catalysis and electronic applications at the nanoscale level.

Engineering of atomic-scale defect densities in silicon has significantly increased its utility as a semiconducting material. It is anticipated that similar control of electrical properties can be attained in 2D transition meal dichalcogenides (TMDs), however common synthesis techniques permit limited means for adjustment of defect densities during synthesis. Selection of uniform crystalline domain sizes from 5-1000 nm within continuous, vapor-phase grown TMD films with thickness on the order of 1 nm has recently been achieved on diverse substrates including SiO₂ and graphene. The structure of these materials was confirmed independently with Raman spectroscopy, atomic force microscopy, and transmission electron microscopy. In vacuo X-ray electron spectroscopy was also used to evaluate the composition and nature of chemical bonding of TMD film constituents with domain size. In both experiment and atomistic simulation, electrical resistance spans several orders of magnitude over the examined range of domain sizes. Measurements of pristine TMD surfaces with a range of domain sizes and prepared in ultra high vacuum conditions were compared to ex situ measurements to evaluate effects associated with environmental exposure. Measurements of TMD composition with ambient air exposure time were correlated to device properties and performance. The effect of domain size on the potential to “regenerate” degraded devices after air exposure was evaluated. Experimental measurements of composition, structure, and electrical properties of TMD-based transistor devices are correlated to atomistic simulation results to better understand the relationship between domain size, interactions with the ambient environment, and charge transport.

Interest in two-dimensional molybdenum disulfide (2D MoS₂) has been rapidly growing for applications of optoelectronic and electronic devices. The monolayer MoS₂ exhibits interesting electronic structure of indirect-to-direct band-gap transition and extremely higher mobility as compared to bulk. A transistor with extremely high carrier mobility over 200 cm²V^-1s^-1 using monolayer MoS₂ as a channel was reported. The high performance 2D MoS₂ layers are typically obtained by mechanical exfoliation. The exfoliation method, however, cannot be applied for large-scale production. Forming large-area MoS₂ layers is still a very challenging issue. In this work, we demonstrated a simple method to form very thin and large-area MoS₂ layers by sulfurizing a thin Mo in a vacuum. The Mo film was firstly deposited on soda-lime glass with varying the thickness from 2 Å to 10 Å using a sputtering method. The Mo films were successively sulfurized by reactive sulfur atoms provided by a thermal cracker. The Raman spectroscopic and photoluminescence (PL) measurements were used to observe the number of layers and the band-gap of the fabricated MoS₂. The MoS₂ layers formed from the 5-Å- and 10-Å-thick Mo films showed the lower values of peak (E¹_2g)-to-peak (A_1g) distance than that of bulk, while there were no peaks in 2-Å-thick sample. The result obtained from the 5-Å-thick Mo indicates that nearly 2-layered MoS₂ was successfully fabricated. Moreover, the intensity, FWHM, and position of the Raman peaks, obtained at several different positions in centimeter-scale substrates, demonstrated that the MoS₂ film was fabricated very uniformly. The positions of PL peaks were nearly constant of 725 nm (= 1.71 eV) in the samples with different Mo thicknesses. The present work suggests a promising technique to deposit a few layered MoS₂ using scalable methods including sputter deposition and thermal cracker without toxic gases. The electrical and optical properties of devices will be reported using these MoS₂ layer as an active layer.

Semiconducting transition metal dichalcogenides (TMDs) are attracting increasing interest in the field of electronics and optoelectronics owing to their layered structure and the indirect-to-direct band gap transition when approaching the single-layer limit. 2H-MoTe₂ is a semiconducting TMD with a bulk band gap of around 1.0 eV. This compound shows very high mobility at room temperature and strong absorption throughout the solar spectrum. Previous studies demonstrated the possibility to achieve gate-induced ambipolar transport at the surface [1]. 2H-MoTe₂ is thus an attractive candidate for novel optoelectronic devices such as light-emitting diodes, photo detectors and solar cell technology. Controlling the atomic nature and density of defects and dopants is crucial for the development of the aforementioned applications and devices. We shall present a detailed STM/STS investigation and corresponding DFT modeling of native dopants and atomic scale defects and their influence on the local electron density of states.
[1] I. Gutiérrez Lezama et al. 2D Materials 1, 021002 (2014)

The emerging field of two-dimensional transmetalldichalcogenides shows a huge potential for a large variety of new applications. In some cases however, morphological modifications to the atomically flat crystal are needed to tune its properties accordingly. For example, nanopores in single layer MoS₂ are expected to be applicable for DNA sequencing [1]. On the other hand defects and edges in MoS₂ can enhance the catalytic activity of MoS₂ for the hydrogen evolution reaction by acting as active sites [2]. Therefore tools to specifically taylor the morphology of SL MoS₂ are of uttermost importance.
In this contribution it will be demonstrated that swift heavy ion irradiation (typical energy in the 100 MeV regime) can be used as a tool for such defect engineering in two-dimensional materials [3]. Using atomic force microscopy it is shown that nanopores in the range of a few nanometer as well as slit pores with a width of under 10 nm and an aspect ratio of up to 500:1 can be introduced in thin MoS₂ layer. The transition from nanopores to slit pores and the aspect ratio of the slit pores can be controlled by the incidence angle of the impinging ion. The size and width of these pores can be adjusted as well by the energy of the swift heavy ion. Furthermore, two-temperature model calculations are performed in order to reveal the physical mechanism which leads to the formation of these pores.
[1] Farimani et al. ACS Nano 8:7914 (2014)
[2] Jaramillo et al. Science 317:100 (2007)
[3] Ochedowski et al. Appl .Phys. Lett. 102:153103 (2013)

Molybdenum disulfide (MoS₂), one of transition metal dichalcogenides (TMDCs), has been widely researched owing to its outstanding optical properties as well as excellent electrical characteristics. Multilayer MoS₂ can be more advantageous than single layer MoS₂ for optoelectronic devices in terms of higher density-of-states and wider spectral response from UV to near-IR due to its smaller bandgap (1.2 eV). However, the responsivity of multilayer MoS₂ phototransistors (~100 mA/W) has remained much lower than that of single layer MoS₂ photodevices (880 A/W) due to its indirect bandgap. In this presentation, we report on multilayer MoS₂ phototransistors with a local bottom-gate structure, which can significantly amplify the optical properties of the device without any pre- or post- treatment on the active channel region. As-fabricated multilayer MoS₂ TFTs showed reasonable room temperature mobilities (>30 cm²V^-1s^-1), robust current saturation, and negligible shifts in the threshold voltages during illumination. In our local bottom-gate structure, gate length is shorter than the active channel length, leading to a non-overlapped (gate underlap) region between gate and channel. Simulations based on optical absorption, transmission probability, and transistor current equations indicate that the gate underlap region plays a key role for the giant amplification of photoresponsivity in the local bottom-gate multilayer MoS₂ phototransistors.

Recently, two-dimensional (2D) transition-metal dichalcogenide (TMD) materials have gained significant attention due to their potential applications in atomic-film devices. Graphene, one of the most popular of these 2D materials, has been widely studied but has limited utility as a semiconductor because it lacks an energy band gap. Unlike graphene, TMD materials such as MoS₂, MoSe₂, and WSe₂, possess a band gap and semiconducting properties. Among those TMD materials, Molybdenum disulphide (MoS₂) which has a direct band gap of 1.8 eV as a single layer have been gain a lot of interest. So, the ability to tailor the electrical properties of MoS₂ FETs would provide numerous advantages in nanoelectronics in terms of conventional semiconducting engineering. When high energy beams of electrons, ions, or protons are irradiated on the semiconductor nanodevices such as field effect transistor (FET) devices made with semiconducting nanowires, the electrical characteristics of the devices will be affected by the interface or bulk trap charges generated by the high energy particles [1]. A few experimental studies have investigated the irradiation effect of MoS₂ materials with high energy particles. However, a comprehensive study of high energy particle beams on MoS₂ atomic thin films-based FET devices have not yet been investigated.
We investigated the irradiation effects on MoS₂ FETs by proton beams with a high energy of 10 MeV. We measured and compared the electrical characteristics of the MoS₂ FET devices before and after the proton irradiation systematically with beam fluence conditions of 10¹², 10¹³, and 10¹⁴ cm^-2, which corresponds to 20, 200, and 2000 sec for beam irradiation time, respectively. For a low proton beam fluence condition of 10¹² cm^-2, the electrical properties of the MoS₂ FETs almost remained the same regardless of the proton irradiation. In contrast, for high enough proton beam fluence conditions of 10¹³ or 10¹⁴ cm^-2, the current level and conductance of the devices significantly decreased after the proton irradiation compared with those prior to the proton irradiation. The electrical changes are originated from the proton-irradiation-induced traps such as positive oxide-charge traps in the SiO₂ layer and the trap states at the interface between the MoS₂ channel and the SiO₂ layer. Our study will enhance the understanding of the influence by the high energetic particles on MoS₂-based nanoelectronic devices [2]. If time allowed, I will present the result of similar experiments of proton-beam irradiation on organic pentacene FETs. The conductance of the pentacene FETs increased after the proton beam irradiation with low fluence condition, however, that decreased after the proton beam irradiation with high fluence condition. These electrical changes are due to the negative trap charges at the pentacene/SiO₂ interface.
References
[1] W.-K. Hong et al., ACS Nano. 4, 811 (2010).
[2] T.-Y. Kim et al., ACS Nano. 8, 2774 (2014).

Two-dimensional materials, such as graphene, transition metal dichalcogenides (TMDCs) are promising materials for next generation flexible electronics. Especially, molybdenum disulfide (MoS₂) has been attracted considerable interests due to its fascinating electronic and mechanical properties. Here, we show the first comprehensive investigation of flexible process using polyimide substrate and organic gate dielectric layer (SU-8 2000.5, Microchem, and report on a robust electrical-stability of flexible multi-layer MoS₂ thin film transistors (TFTs) under mechanical stresses. Current-voltage characteristic curves of flexible MoS₂ TFTs showed intrinsic mobility of 186.8 cm²/Vs, an I_on/I_off ratio more than 10⁶. In order to enhance mechanical flexibility of the device, transparent and flexible protective films were laminated on both sides of the MoS₂ TFTs array, which placed in stress-free zone, i.e. neutral mechanical plane. Reliability of TFT performance under mechanical stresses will be also discussed with respect to various bending radiuses. The overall electrical and mechanical results suggest that high-mobility MoS₂ transistor on flexible polyimide substrate can offer a compelling application for a potentially flexible electronics or human-centric soft electronics.

2D nanomaterials presenting interesting electro-optical properties have emerged as promising components for electronic and optical applications. Following the first reporting on the production of graphene [1], liquid phase exfoliation has revealed as a convenient technique to produce not only graphene but a broad range of different self-standing 2D structures from layered compounds, including transition metal chalcogenides, metal halides, clays or metal oxides [2,3,4].
However, only a few layered materials are available in nature. Therefore different synthesis approaches, including coprecipitation, thermal decomposition or direct reaction of the elements were utilized to produce the bulk layered materials up to date [5].
In the present work different suitable synthesis routes for the production of nanomaterials based on wet-chemical synthesis are presented. Results show the successful synthesis of different micron- and nanometre- sized semiconducting materials, including nanoparticles, nanotubes and platelets easily exfoliable in liquids.
[1] Novoselov et al. Science 306 (2004) 666.
[2] Nicolosi et al. Science 340 (2013) 21.
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Aerogels are porous solids used in a wide range of applications including sorbents, filtration, insulation, hydrogen storage, catalysis, batteries, and supercapacitors. Their unique properties are related to their high internal surface, low-density, and small pore/particle size. Two-dimensional (2D) nanomaterials, such as boron nitride and dichalcogenides, also exhibit a range of distinct optical, electronic, and mechanical properties, but are typically limited to thin films and coatings. Assembling 2D nanomaterials into monolithic aerogels expands their application space to include technologies and manufacturing processes that require a macroscopic 3D form factor. Furthermore, placing the novel intrinsic properties of 2D materials in a low-density, high surface area architecture has the potential to unlock exciting new properties and features only displayed in the aerogel system. Here, we present synthesis schemes for aerogels made from several different 2D materials, (e.g. boron nitride, graphene, dichalcogenides, etc.). Hybrid aerogel synthesis, combining two or more layered materials, will also be presented. Characterization of aerogels will include textural, transport, mechanical properties, and more.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

The metal phosphorous trichalcogenide semiconductor materials of formula MPX₃ (M = metal, X = S or Se) are Van der Waals stacked monolayers that show great potential for future electronic and optoelectronic applications [1]. The monolayers, which can be isolated [2], experience the effects of quantum confinement in one dimension and are predicted to possess remarkable qualities, such as low formation energies and a bandgap range of 1.77 to 3.94 eV [3]. Some of the bandgaps are direct, and the range spans the visible and near-UV portions of the electromagnetic spectrum, including the critical and challenging “green gap”. The monolayers have the potential to be used in tandem with other two-dimensional materials, such as graphene, boron nitride, and other transition metal chalcogenides, to create ultra-thin, flexible optoelectronic heterostructure devices for use as light-emitters, water-splitters [3] and photovoltaics. We pursue MnPS₃ as a first test material, motivated by its useful bulk band gap of ~2.7 eV [4][5] and similar lattice constant to other trichalcogenides of interest [5]. Mechanical exfoliation has been used to isolate monolayers, but the technique is not scalable and yields crystals on the order of 40 nm across, too small for conventional optoelectronic applications [3]. We have previously established that a carrier gas-assisted tube furnace physical vapor deposition technique can grow MoS₂ crystals on the order of several hundred microns in size. We now present on this technique&’s use for growing large, single-domain MnPS₃ crystals on Si coated Si wafer from bulk MnPS₃ (bulk MnPS₃ is first synthesized via self-flux sublimation of 1:1:3 Mn P and S). Preliminary results have yielded hexagonal crystals, reflecting the structure of the internal honeycomb-shaped crystal lattice of MnPS₃. The crystals have a width of ~10 µm and height ~200 nm. The crystals initiate from seed nucleation sites on the substrate surface and expand laterally; nearby growths merge and terminate, demonstrating the potential for continuous substrate coverage. Future trials will optimize growth parameters to consistently produce large-scale (>100 µm) monolayers of MnPS₃ and other trichalcogenides, which can then be integrated into heterostructures with other 2D materials to prototype novel optoelectronic devices.
[1] Taylor, J. Steger, and A. Wold, J. Solid State Chem. 7, 461 (1973)
[2] Frindt, R., Yang, D., Westreich, P. J. Mater. Res., Vol. 20, No. 5, (2005)
[3] Liu, J., Li, X., Wang, D. , Lau, W., Peng, P., and Liu, L. The Journal of Chemical Physics 140, 054707 (2014)
[4] Piryatinskaya et al., Fiz. Nizk. Temp. 38, 1097-1101 (2012)
[5] Le Flem, G., Brec, R., Ouvard G., Louisy, G., Segransan, P., J. Phys. Chem. Solids Vol. 43, No. 5, pp. 455-461, (1982)

Our MoS₂ biosensor electrically detects prostate specific antigen (PSA) in a highly sensitive and label-free manner. Unlike previous MoS₂-FET-based biosensors, the device configuration of our MoS₂ biosensors does not require a dielectric layer such as HfO₂ due to the hydrophobicity of MoS₂. Such an oxide-free operation can provide Ohigher device sensitivity as well as a simpler device structure. Moreover, the use of an off-current as an indicator of sensitivity allows additional improvement in device sensitivity in our MoS₂ FET biosensors. In order to explore the potential for the quantitative and selective detection of PSA antigen target with MoS₂ biosensor, anti-PSA antibody was immobilized on the sensor surface in a label-free immunoassay format. After anti-PSA (a concentration of 100 mu;g/mL) are immobilized to the overall MoS₂ device surface, the off-state current increases significantly. Since anti-PSA is positively charged at the measurement condition (PBS, pH = 7.2), the binding of anti-PSA(receptor) to the MoS₂ surface increases the channel electron concentration during at the off-state. Then, the off-state current for a MoS₂ transistor decreased as a function of target PSA concentration introduced in the solution, indicating that the amount of the adsorbed PSA on the antibody immobilized MoS₂ surface is proportional to the PSA concentration. The variation of “off-current” is due to the specific binding of negatively charged PSA with the antibodies. It allows us to monitor highly sensitive detection of PSA markers from 1 pg/mL to 10 ng/mL, and to compute quantitative bioassay from the binding of a charged biological species. The minimum detectable concentration of PSA is 1 pg/mL, which is below the clinical cut-off level of 4 ng/mL. In addition to the experimental demonstration, we provide a systematic theoretical analysis of the sensor platform - including the charge state of protein at the specific pH level, self-consistent channel transport in the presence of interface defects. Taken together, the experimental demonstration and the theoretical framework provide a comprehensive description of the performance potential of MoS₂-based biosensor technology.

Transition metal dichalcogenides (TMDs) with layered structures are recently being considered as promising candidates for next generation flexible electronic and optoelectronic devices because of their superior electrical, optical, and mechanical properties. Their thickness scalability down to a monolayer and Van der Waals expitaxial structure without surface dangling bonds (consequently, native oxides) makes TMD-based thin film transistors (TFTs) immune from short channel effect (SCE) and provides very high field effect mobility (~200 cm²/V-sec that is comparable to universal mobility of Si), respectively. In addition, an excellent photo-detector with a wide spectral range from ultraviolet (UV) to near infrared (IR) is achievable since their energy bandgap varies between 1.2 eV (bulk) and 1.8 eV (monolayer), depending on their layer thickness. However, one of the critical issues that hinder the successful integration of such 2D electronic and optoelectronic devices is the lack of reliable and controllable doping method.
Although Fang et al. controlled the doping level on TMDs by adjusting the exposure time to potassium, the range of doping was limited within the heavily doped (degenerate) regime, where TMDs work as a near- metallic layer. Since adjusting the electrical and optical properties of TMDs is possible within a non-degenerate doping regime, wide-range doping capability including non-degenerate and degenerate regime is a critical point in design and fabrication for TMD-based electronic and optoelectronic devices. However, it is very challenging to achieve a non-degenerate doping on 2D semiconductors and secure its wide-range doping controllability because a method such as ion implantation cannot be applied to the doping process. In this work, we first demonstrate a wide-range controllable n-doping method of TMDs on phosphorus silicate glass (PSG) insulating layer, which (1) activates the doping phenomenon by thermal/optical process and (2) adjusts the doping level by controlling the thermal/optical process conditions or weight percentage of P atoms during in-situ doped PSG growth step. In particular, this controllable n-doping process is also demonstrated on exfoliated thin (try-layer) and bulk TMD films. In addition, the proposed doping method is investigated in detail through Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), electrical measurements (I_D-V_G and I_D-V_D), and atomic force microscopy (AFM).

Two-dimensional (2D) materials often show a range of intriguing electronic, catalytic and optical properties that differ greatly from conventional nanoparticles^1,2. However, 2D materials generally tend to form stacks in order to reduce the overall surface energy. Such densely packed structures become detrimental when access to high surface area is required, such as in heterogeneous catalysis and fast sensing applications. In this presentation, we demonstrate a chemical strategy to generate Pd three-dimensional (3D) structures from its flexible 2D nanosheets³. By tuning the solvent polarity with carboxylic acid of different carbon chain length, the final morphology of these 2D stacked nanosheets can crumple to form 3D open structures. Our data indicate when these Pd 3D materials were integrated into homemade hydrogen sensing devices. Moreover, when integrated into a sensing device, response time of the crumpled 2D nanosheets can be further accelerated due to the easy accessibility of hydrogen gas to the Pd sheet surfaces when comparing with flat sheets. In principle, the hydrogen diffusion can occur from both sides of the crumpled 2D nanosheets, leading to a response time increase by a factor of four. Overall, a response time was found to be an order of magnitude faster than their 2D-constrained counterparts. A mass transfer model of atomic hydrogen diffusion within the Pd lattice was developed. By applying Fick&’s second law of diffusion under appropriate boundary conditions, I will show that hydrogen sensing is a diffusion limited process and the response time is proportional to the square of sheet thickness.
References
1. Geim, A. K. Science324, 1530-1534 (2009).
2. Butler, S. Z. et al.ACS Nano7, 2898-2926 (2013).
3. Pan, Y.-T., Yin, X., Kwok, K. S. & Yang, H. Nano Lett.14, 5953-5959 (2014).

Two-dimensional (2D) semiconductor molybdenum disulphide (MoS₂) has received considerable attention following the discovery and research in monolayer graphene that initiated intensive research of ultrathin materials. Different from graphene, MoS₂ has a natural and tunable bandgap which is more useful in electronic and optoelectronic device applications [1].
Recent studies of field effect transistors (FETs) based on MoS₂ single crystal prepared by chemical vapor deposition (CVD), has been demonstrated and their performances were comparable to exfoliated MoS₂ devices [2]. However, producing high quality single crystal MoS₂ over a large area remains a challenge, which limits its practical implementation. In this work, we produced MoS₂ thin films a few nm thick on 300 nm SiO₂ on Si substrates using CVD and fabricated high performance FETs. As-grown MoS₂ thin films were continuous and uniform over the entire substrate surface with an area of a few centimeters. Back gated FETs on the MoS₂ thin films were fabricated to evaluate the electrical performance. Without any dielectric engineering, our long channel device (W/L=40/14µm) exhibited a high on/off ratio of 5×10⁶ and maximum on-current up to 1.5µA at room temperature. Most significantly, the drain current of the MoS₂ thin films FETs experienced robust saturation over a wide range of drain voltage under all gate biases, while current saturation is seldom observed in FETs composed of 2D semiconducting materials. This is an important characteristic for transistors working toward applications in advanced display technologies, such as thin film transistors in OLED displays operating in the saturation region [3]. Our work indicated that the as-grown MoS₂ thin films could be an alternative candidate to single crystal MoS₂, for application in future electronic industries.
1. B. Radisavljevic, et al, Nature Nano 6 147 (2011)
2. X Wang, et al, JACS 135 14 (2013)
3. S Kim, et al , Nature Comms 3 1 (2012)

Here we report the processing aspects of 2D layered materials. Firstly, we show that few-layered MoS₂ flakes can be thinned down to monolayer level by thermal annealing via a sublimation mechanism [1]. The thermal-annealed flakes are generally composed of a mixture between N and N-1 layers, consistent with a layer-by-layer thinning process. Secondly, we developed an etching-free patterning method with SU-8 resist to pattern MoSe₂ atomically thin films from chemical vapor deposition (CVD) synthesis [2]. Such SU-8 patterning can be used to define the conduction channel of MoSe₂ film for back-gated field effect transistors (FET) [2,3]. The SU-8 patterning is useful for device insulation and for suppression of leakage current, thus allowing in-depth characterizations on the device. The electrical transport on CVD-MoSe₂ film can be explained with Mott variable range hopping, suggesting a carrier localization due to disorder within the film. Thirdly, we discuss consequences of catalyst usage in CVD synthesis of MoSe₂ nanoflakes. Better repeatability and higher nanoflakes yield can be obtained with the use of PTAS catalyst as compared to that from uncatalyzed synthesis. However, catalyst particles might be attached on the nanoflakes and remained as impurity on nanoflake-substrate interface. Additionally, FETs from catalyzed MoSe₂ flakes showed poorer electrical performance in terms of mobility and on-off ratio. We hypothesized that those catalyst particles act as scattering center, introduce defect states, and increase trap density. Meanwhile, unseeded FET from CVD-MoSe₂ nanoflakes exhibited n-type behavior with mobility of up to 63 cm²V^-1s^-1 and 10⁶ on-off ratio at room temperature, with inverter (NOT gate) gain of ~3.
References: [1] X. Lu, M. I. B. Utama, Q. H. Xiong, et al., Nanoscale 2013, 5, 8904. [2] M. I. B. Utama, X. Lu, Q. H. Xiong, et al., Nanoscale 2014, 6, 12376. [3] X. Lu, M. I. B. Utama, Q. H. Xiong, et al., Nano Lett. 2014, 14, 2419.

Correlation between band gap and morphology was found on CVD-grown MoS₂ using STM/STS measurements. Existence of local strain on MoS₂ was confirmed by FFT of an atomic resolution image which shows elongated peaks toward the center, which means variation of lattice parameter. To further clarify the correlation between band gap and morphology, line spectroscopy was carried out across the wrinkle geometry which is simpler than two-dimensional corrugation. The line spectroscopy shows the correlation again without ambiguity. Band gap size was plotted as a function of radius of curvature of morphology. The band gap size is increasing and eventually saturated as increasing the radius of curvature of morphology. In addition, bright and dark defects exist and dark defects are dense, relatively. Defects show defect-induced states near the conduction band edge (valence band edge) with Fermi level shifts of n-type (p-type).

The mechanical properties of perfect and defective graphene have been well characterized in recent years both experimentally and theoretically. However, besides graphene other covalent monolayers will gain enormous practical importance in the near future due to complementary properties that become relevant for various technological applications. Knowledge of their mechanical behavior is not only needed to comply with basic physical requirements but also to control mechanical nano-size and edge effects as well as quantum phenomena. An analytical two-dimensional (2D) Morse-type microscopic fracture model is derived that allows the estimation of the bond breaking force and line or edge energy of 2D solids from calculated or experimentally determined 2D Young&’s moduli and 2D intrinsic fracture strengths. The nanoscopic length scale needed for the evaluation of the breaking force can be calculated from the interaction potential of the corresponding covalent bonds in the monolayer. Besides the conductor graphene, the direct band gap semiconductor molybdenum disulfide (MoS₂) and the isolator hexagonal boron nitride (h-BN) are of particular interest in electronic applications. Using the thicknesses of these layers in the sub-nanometer range the line energy can be transformed into a formal surface energy and compared with surface energies of well known solids to judge their relevance. The 2D microscopic model gives access to any of the following three mechanical quantities, the 2D elastic modulus, 2D intrinsic strength, and breaking force or line energy, when the other two are known from experiment or theory. The versatile analytical expression is a valuable tool to determine intrinsic material properties of 2D solids. In addition, their internal consistency can be controlled, when measurements are lacking or hard to perform, as in the case of difficult-to-prepare monolayers and complex layered systems such as biomaterials or composite materials. Especially the breaking forces, derived line or edge energies, and surface energies are difficult to measure for monolayers and therefore their exploration is still at an early stage. For this reason, deviations by a factor of two are not surprising for these particular quantities. While the evaluated breaking forces and line energies vary in a relatively narrow range for graphene, MoS₂, and h-BN, a large change of the deduced 3D properties is observed between the two one-atom layers graphene and h-BN and the multi-atom layer of MoS₂. This seems to be partly due to the large differences in the layer thicknesses of nearly a factor of two between these one- and multi-atom layers. The generally accepted convention to take the equilibrium layer thickness of the layered bulk material for estimating 3D properties could be one reason pointing to a necessary redefinition of the effective thickness. In any case, these thickness-derived mechanical properties should be used only for comparison with values of 3D solids.

For reliable functioning of final electronic and optoelectronic applications, it is required to understand bending behavior of silicene, a graphene-like 2D nanomaterial composed of silicon (Si). In this research, we calculate bending modulus of silicene using atomistic simulations with several different interatomic potential models including Murty-Atwater [1-2], Stillinger-Weber [3], MEAM [4], EDIP [5] and Tersoff [6]. Simulations are conducted under bending periodic boundary condition [7]. Since all these interatomic potential models were constructed to fit the bulk Si properties, the 2D elastic properties predicted by these potentials do not show a consistent manner throughout the potentials. For example, bending modulus is predicted as 0.542 eV in SW model. This value is approximately a seventieth of that calculated by Reax FF model [8]. Ab initio calculations are also conducted for verifying the bending modulus obtained from atomistic simulations.
Acknowledgement
This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education (2013R1A1A2063917).
References
[1] M.V. Ramana Murty and Harry A. Atwater, Empirical interatomic potential for Si-H interactions, Phys. Rev. B, Vol. 51, No. 8, pp. 4889-4893, 1995
[2] M. Kim and B. Lee, Science than art of Si many-body potentials: Reproducibility and transferability, EPL, 102, 33002, 2013
[3] Frank H. Stillinger and Thomas A. Weber, Computer simulation of local order in condensed phases of silicon, Phys. Rev. B, Vol. 31, No. 8, pp. 5262-5271, 1985
[4] M. I. Baskes, Modified embedded-atom potentials for cubic materials and impurities, Phys. Rev. B, Vol. 46, No. 5, pp. 2727-2742, 1992
[5] Martin Z. Bazant et. al., Interatomic potential for silicon defects and disordered phases, Phys. Rev. B, Vol. 58, No. 5, pp. 2539-2550, 1998
[6] J. Tersoff, New empirical approach for the structure and energy of covalent systems, Phys. Rev. B, Vol. 37, No. 12, pp. 6991-6999, 1988
[7]Wei Cai et al., Torsion and bending periodic boundary conditions for modeling the intrinsic strength of nanowires, JMPS, 56, pp. 3242-3258, 2014
[8] Ruth E. Roman and Steven W. Cranford, Mechanical properties of silicene, Comp. Mater. Sci., 82, pp. 50-55, 2014

Two-dimensional crystals with a wealth of exotic dimensional-dependent properties are promising candidates for
next-generation ultrathin and flexible optoelectronic devices. For the first time, we demonstrate that few-layered InSe
photodetectors, fabricated on both a rigid SiO2/Si substrate and a flexible polyethylene terephthalate (PET) film, are capable of
conducting broadband photodetection from the visible to near-infrared region (450minus;785 nm) with high photoresponsivities of
up to 12.3 AW^minus;1 at 450 nm (on SiO₂/Si) and 3.9 AW^minus;1 at 633 nm (on PET). These photoresponsivities are superior to those of
other recently reported two-dimensional (2D) crystal-based (graphene, MoS2, GaS, and GaSe) photodetectors. The InSe devices
fabricated on rigid SiO₂/Si substrates possess a response time of sim;50 ms and exhibit long-term stability in photoswitching. These
InSe devices can also operate on a flexible substrate with or without bending and reveal comparable performance to those devices
on SiO₂/Si. With these excellent optoelectronic merits, we envision that the nanoscale InSe layers will not only find applications
in flexible optoelectronics but also act as an active component to configure versatile 2D heterostructure devices.

We have hybridized layered double hydroxide (LDH) with Angelica gigas Nakai root extract through reconstruction method. LDHs having well-ordered hydrotalcite-like crystal structure and average size ~ 250 nm were prepared by hydrothermal method. The root of Angelica gigas Nakai, which have been utilized in the treatment of inflammation and female disorders as herbal medicine, were treated with methanol to obtain extract. For the hybridization, LDHs were calcined at 400 ^oC for 8 hours to obtain layered double oxide (LDO), which was further dispersed into extract solution with various extract/LDO weight ratio, 0.11, 0.21 and 0.43. The extract contents in hybrid increase upon increasing extract/LDO ratio, showing maximum contents of ~ 12% at extract/LDO ratio of 0.43. The surface charge of LDH dropped from +44 mV to +20 mV upon extract incorporation, which was attributed to the adsorption of negatively charged extract moiety on LDH surface. The scanning electron microscopic results exhibited that the hybrid had house-of-card structures, resulting form random stacking of LDH nanolayers during hybridization process. We quantified the amount of active ingredients in Angelica gigas Nakai such as flavonoid, polyphenol, decursin and decursinol angelate by UV-vis spectroscopy. It was revealed that the active ingredients were effectively concentrated in the hybrids by 1.2-3.0 times.

Molybdenum disulphide (MoS₂), a layered quasi-two dimensional (2D) chalcogenide material, is a subject of intense research because of its electronic, optical, mechanical and physicochemical properties[1]. Since the monolayer MoS₂ is a direct-gap semiconductor, it is widely used in the field of light-emitting area[2]. However, its photoluminescence (PL) efficiency is very low due to excessive doping in monolayer MoS₂ and rich non-radiative centers[3].
Here, we report two methods to enhance photoluminescence (PL) of monolayer MoS₂. Firstly, we find that by simple ambient annealing treatment in air in the range of 200^oC to 400 ^oC , the PL emission of the samples can be greatly enhanced by a factor up to two orders of magnitude. This enhancement can be attributed to two factors first, the formation of Mo-O bonds during ambient exposure introduces an effective p-doping in the MoS₂ layer; second, localized electrons formed around Mo-O bonds related defective sites where the electrons can be effectively localized with higher binding energy resulting in efficient radiative excitons recombination. The PL intensity almost keeps constant over a large range of temperature (300K~800K) in the in the vacuum environment due to the large binding energy of the excitons. Time resolved PL decay measurement showed that longer lifetime of the treated sample consistent with the higher quantum efficiency in PL. Secondly, We synthesized gold nanoparticles and dispersed them on the surface of the MoS₂ samples by means of a spin-coating. We find a great enhancement in the PL intensity of the monolayer sample. Our work shows that gold nanoparticles may impose an obvious p-doping effect to the monolayer samples to enhance the PL.
Our findings shed light on the understanding of the luminescent properties of MoS₂.
[1] Qing HuaWang, Kourosh Kalantar-Zadeh, Andras Kis, et al. Nat Nano 7(11), 699(2012).
[2] R. S. Sundaram, M. Engel, A. Lombardo, R. Krupke, et al. Nano Letters 13(4), 1416(2013).
[3] Xiaoxu Wei, Zhihao Yu, Junzhuan Wang et al. AIP Advances 4(12),123004(2014).

Transition metal dichalcogenide (TMD) monolayers, as atomically thin direct-gap semiconductors, have attracted substantial attention due to their remarkable physical properties and their potential applications in optoelectronics. However, this atomically thin nature limits the capacity for light absorption, which restricts the practical applicability of TMDs in optoelectronics. To enhance the light-matter interactions, hybridization methods of TMDs with plasmonic nanostructures have been used. With this technique, the local-field enhancement effect of localized surface plasmons improves the absorption and emission performances of TMDs. To allow for strong near-field interactions in the hybrids, however, metallic materials should be located near TMD layers. The contact between the metal and the monolayer TMD inevitably results in strong perturbations in emission behaviors of exciton complexes such as band gap pinning, metallic doping, peak shift, and emission quenching. Therefore, a systematic investigation of the charge interaction mechanism in metal-TMD hybrids would lead to promising improvements in the optoelectronic performance of TMD devices.
The conventional method of simply approaching metal nanoparticles to TMDs for plasmonic hybridization does not sufficiently exploit the unique benefit of atomic-scale thickness. Moreover, this approach requires the precise tuning between the plasmon resonance of the metallic structure and the emission wavelength. The benefit in optical performances via the plasmonic enhancement is limited by the narrow-band optical wavelength which needs to be improved, especially for light-harvesting devices such as photodetectors and solar cells, which require wideband spectra in optical absorption.
In this article, we investigated the exciton quenching phenomena for three types of hetero-stacked layers. In a single sample, monolayer MoS₂, monolayer MoS₂ / Au (25 nm) / Cr (5 nm), and monolayer MoS₂ / hBN (5 nm) / Au (25 nm) / Cr (5 nm) on a glass substrate are prepared by the dry transfer method for mechanically exfoliated flakes. Raman spectroscopy measurements revealed a slight p-type doping effect for the MoS₂ samples in contact with Au and hBN/Au layers. Photoluminescence experiments revealed that the dominant emission quenching of negatively charged excitons attributed to the transfer of photo-excited electron from the emitter to the Au layer. Furthermore, we propose a metal-dielectric-TMD hybrid structure that provides a strong plasmonic confinement effect for broadband light sources. This was implemented by inserting monolayer MoS₂ into the narrow dielectric gap between metal films and TiO₂ nanowires, where the unique properties of the atomically thin TMD layer are efficiently exploited. Analysis by finite-difference time-domain numerical simulations showed that plasmonic near-fields are strongly confined near the monolayer MoS₂ emitter for varying wavelengths of excitation lasers.

Monolayer transition metal dichalcogenides have attracted lots of attention lately due to their potentials in nano-electronics, nano-optoelectronics and the novel spintronic and valleytronic devices. We have carried out the growth experiments of monolayer and submonolayer MoSe₂ thin films using molecular-beam epitaxy (MBE) on highly oriented pyrolytic graphite (HOPG) - a prototypical van der Waals (vdW) epitaxial system. We follow the morphological evolutions at the initial stage deposition of MoSe₂ by scanning tunneling microscopy (STM), revealing the characteristics and growth properties of MoSe₂ on HOPG, including the epitaxial growth mode and formation of line defects, etc. Particularly, we find a method to manipulate the line defect formation by tuning the MBE conditions and adopting the post-growth annealing procedure. Electronic properties of the MBE-grown MoSe₂ films and the line defects are also investigated by scanning tunneling spectroscopy (STS) revealing some interesting properties. This project is supported by a collaborative research fund (No. HKU9-CRF-13G) from the Research Grant Council (RGC) of Hong Kong Special Administrative Region (HKSAR) of China.

Piezoelectricity offers precise and robust conversion between electricity and mechanical force, which originates from the broken inversion symmetry of atomic structure. Reducing the size of bulk materials has been suggested to enhance piezoelectricity where quantum confinement modifies electronic states. Yet when materials approach single molecular layer, the large surface energy can cause piezoelectric structures to be thermodynamically unstable. Recently transition metal dichalcogenides (TMDC) have been shown to retain atomic structures down to single layer without lattice reconstruction even in ambient conditions, and theoretical calculations predicted piezoelectricity can exist in such a two-dimensional (2D) crystal due to their broken inversion symmetry. Here we report experimental evidence of piezoelectricity in the free-standing single layer of molybdenum disulfide (MoS₂) crystal, with measured piezoelectric coefficient e₁₁ = 2.9×10^-10 C/m. The free-standing measurement of the intrinsic piezoelectricity is free from the substrate effects, such as doping and parasitic charge. We observed oscillation of piezoelectric response in MoS₂ in odd and even number of layers due to breaking and recovery of inversion symmetry, respectively, in sharp contrast to bulk piezoelectric materials. Through the angular dependence of electro-mechanical coupling, we uniquely determined the 2D crystal orientation. The piezoelectricity discovered in single molecular membrane promises new applications in low-power logic switch and ultrasensitive biological sensors scaled down to single atomic unit cell - the ultimate material limit.

We report the UV light response and NO₂ gas sensor based on few-layer WS₂ nanosheets synthesized by a simple hydrothermal method. For the UV sensor, response time was observed to be 15 s whereas the recovery time was 56 s. Few layer WS₂ nanosheets based sensor device were tested for various concentration of NO₂ . The response time was observed to be ~ 60 s whereas recovery time was in few tens of min. Further, response and recovery time can be shortened by UV illumination or by removing the absorbed gas by heating the device at higher temperature. Our results open up the new avenues for gas sensor based on two-dimension inorganic layered materials.

Spectral analysis of the field emission current fluctuations of few layer WS₂ nanosheets has been carried out at the ultra high vacuum ~1 × 10^minus;8 mbar. The field emission current stability investigated at preset value of 2 µA were characterized by ‘step&’ like fluctuation. The spectral analysis were also performed on a Fast Fourier Transform analyzer which revealed that the observed noise is of 1/f^α type, with the value of α as ~ 1.05. The estimated value of α implies that the current fluctuations are mainly due the various processes occurring at atomic scale like adsorption, migration, and/or desorption of the residual gas species on the WS₂ nanosheet emitter surface.

Recently, there is a great interest in transition metal dichalcogenides such as MoS₂ because of their interesting electronic and optical properties. However, high performance MoS₂ thin-film transistors require the formation of low-resistivity metal-MoS₂ junctions since non-ideal electric contacts formed on MoS₂ can fundamentally hamper any attempts to improve transistor performance. The understanding of metal-MoS₂ junctions is further complicated as electronic properties of naturally-occurring MoS₂ can be variable. In this presentation, we report the variability of electrical properties of Ti contacts in back-gated multilayer MoS₂ thin-film transistors based on mechanically exfoliated flakes. By measuring current-voltage characteristics from room temperature to 240°C, we demonstrate the formation of both ohmic and Schottky contacts at the Ti-MoS₂ junctions of MoS₂ transistors fabricated using identical electrode materials under the same conditions. While MoS₂ transistors with ohmic contacts exhibit a typical signature of band transport, those with Schottky contacts indicate thermally activated transport behavior for the given temperature range. These results provide the experimental evidence of the variability of Ti metal contacts on MoS₂, highlighting the importance of understanding the variability of electronic properties of naturally-occurring MoS₂ for further investigation.

Compound semiconductor two-dimensional (2D) nanomaterials such as MoS₂, MoSe₂ and WSe₂ have been researched intensively by reason of its excellent mobility, flexibility, and optical properties. Meanwhile, silicon (Si) 2D nanomaterials, so called as silicine, has not been researched yet due to the difficulty in the 2D growth of cubic structured silicon. Silicene is compatible with existing CMOS technique and should has attractive properties such as band gap opening, direct band gap transition, and high carrier mobility. Here we report freestanding-, a few layer silicene grown by chemical vapor deposition (CVD) process. The ratio of gases, silicon tetrachloride for silicon source and argon gas for carrier gas, is controlled to make a very low supersaturation state. Hydrogen gas is used to passivate silicon (111) surface. Under the conditions, a few layer silicene having 1 to 20 nm thickness and several hundreds of area with (111) surface could be grown on the Si sunstrate. Surface cleaning and treatment of the substrate by Buffered Oxide Etch (BOE) leaded epitaxial growth and made possible to control the width and growth direction. It showed strong thickness-dependent photoluminescence in visible range including red, green, and blue (RGB) emissions with the associated band-gap energies ranging from 1.6 to 3.2 eV; these emission energies were greater than those from Si quantum dots (SiQDs) of the similar sizes. Out results suggest that a few layer silicene would be helpful for silicon based integrated, flexible electronic device and its epitaxial and freestanding growth would be applied to three-dimensional devices.

Molybdenum disulfide (MoS2) monolayer is a very interesting material for various applications because of its two-dimensional crystal structure and the existence of direct band gap. However, the synthesis of MoS2 monolayer over a large area remains a great challenge. Here we show the uniform growth of high-quality, centimeter-scale, continuous MoS2 monolayer by chemical vapor depostion (CVD). Characterization by scanning electron microscopy (SEM), Raman spectroscopy, atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), photoluminescence (PL) shows the continuous growth of centimeter-scale MoS2 monolayer on sapphire substrates. Transmission electron microscopy (TEM) provides further information on the crystallinity and crystal orientation of the MoS2 monolayer. The synthesis approach reported in this study demonstrates the feasibility of device-quality monolayer MoS2 films with excellent uniformity and quality.

Transition metal dichalcogenides (TMDs) are layered compounds like graphite that can be reduced from three-dimensions to two-dimensional (2D) form, up to the monolayer limit, due to their strong in-plane bonding and weak interlayer van der Waals coupling. In contrast with their indirect gap character in the bulk, single layer TMDs are direct gap semiconductors with great promise for optoelectronic and photonic devices. Towards the development of such devices, the investigation of 2D materials response under intense photoexcitation by ultrashort pulses, as well as of their ultrafast optical properties, is undoubtedly important for various optoelectronic and photonic applications, including phototransistors, light emitters, and heterojunction solar cells.
In this work, the effect of intense femtosecond laser excitation on the structure of bulk and monolayer of MoS₂ and MoSe₂ under conditions ranging from lattice heating to material damage is systematically investigated. The evolution of the Raman active out of plane and in plane vibrational modes was recorded as a function of irradiation intensity and total exposure time. Experiments reveal large differences in the ultrafast laser excitation response of monolayer compared to the bulk, as far as the lattice distortion as well as the lattice morphology at the onset of optical damage. The role of two-photon versus one photon absorption effects is investigated and discussed.

BN mono-layers are sp²-hybrided structural sisters of graphenes. Electrically insulating BN nanosheets possess similar thermal conductivity while superior thermal and chemical stabilities comparing with electrically conductive graphenes. However, the current production of BN nanosheets is rather limited, opposite to massive graphene production. Here I talk about two high-yield syntheses developed recently for BN nanosheets. We synthesized for the first time gram-level high-quality nanosheets by "chemical-blowing" method and biomass carbothermal reaction.
"Chemical-blowing" relies on making large bubbles with atomically-thin boron-nitrogen-hydrogen-polymer walls, resembling blowing up balloons. After annealing thin-walled polymer bubbles at high temperature, crystalline BN bubbles are achieved. High-yield mono- and few- layered BN nanosheets with large lateral dimensions are then obtained by following separation.^[1] Biomass carbothermal is a variant of a carbothermal reduction reaction coupled with pre-vapor-transport and post-nitridation processes using cheap biomass, boron oxide and nitrogen gas. BN nanosheets are spatially converted on the sites where vegetation precursors existed, and they entirely construct and copy the appearances of vegetations. The throughput of ca. 20 g single-crystalline, morphologically pure BN nanosheets per a single production run is documented.^[2]
BN nanosheets have wide applications with complementary functionalities to graphenes, e.g. dielectric gates, ultraviolet illuminants and advanced composites. BN-based composites are presented here essentially based on our large-quantity production. BN nanosheets are featured as perfect fillers to make thermoconductive and electrically insulating composites, e.g. PMMA/BN and epoxy/BN composites with 14-17 fold increase in thermal conductivity. They are envisaged for the heating-release insulting packaging of down-sizing faster cooler electronics. Mechanically reinforced nanosheet-containing composites are also discussed here. We additionally extend the blowing method to make 3D strutted-graphene useful for realizing a highest-power-density supercapacitor.^[3]
References:
[1] X.B. Wang, C.Y Zhi, L. Li, H.B. Zeng, C. Li, M. Mitome, D. Golberg, Y. Bando, Adv. Mater., 23, 4072 (2011).
[2] X.B. Wang, Q.H. Weng, X. Wang, X. Li, J. Zhang, F. Liu, X.F. Jiang, H.X. Guo, N.S. Xu, D. Golberg, Y. Bando, ACS Nano, 8, 9081-9088 (2014).
[3] X.B. Wang, Y.J. Zhang, C.Y. Zhi, X. Wang, D.M. Tang, Y.B. Xu, Q.H. Weng, X.F. Jiang, M. Mitome, D. Golberg, Y. Bando, Nat. Commun.,4, 2905 (2013).

Large-scale integration of 2D transition metal dichalcogenides (TMDCs) with controlled layer orientation and numbers is critically important to realize their technological potential in scaled-up applications. Moreover, the rational integration of dissimilar 2D TDMCs into hetero-structures is anticipated to offer exotic materials properties unobtainable from their single components, suggesting tremendous scientific opportunities. However, conventional integration approaches based on mechanical exfoliation and stamping are not controllable, scalable, thus, impractical. In this work, we present a large-scale, one-step chemical synthesis of 2D TMDCs with controlled 2D layer orientation and hetero-structures. We grew various 2D TMDCs such as MoS₂, WS₂, MoSe₂, and WSe₂ by using Mo and W metal seed precursors instead of conventionally used metal oxide precursors. We observed that the thickness of the metal seed precursor critically dictates the growth direction of 2D layers. With thicker metal seeds of > ~2nm, 2D layers grew vertically standing upright, while they grew horizontally lying on growth substrates when thinner seeds were used. An underlying mechanism is discussed in the context of a completion between the surface energy of newly exposed 2D edge sites and the strain energy built up at interfacing 2D layers. Based on the large-scale patterning and morphology-controllability of metal seeds, we also grew 2D TMDC hetero-structures of MoS₂/WS₂ and MoSe₂/WSe₂ with vertically-aligned 2D layers on an area of > 1cm² [1]. The 2D hetero-structures exhibit anisotropic carrier transport properties reflecting the distinct carrier types and transport natures of dissimilar 2D TMDCs. The presented study paves a way for designing and developing a new class of 2D TMDCs in controlled and scalable manners.
[1] Yeonwoong Jung, Jie Shen, Yong Sun, Judy J. Cha. ACS Nano, 2014, 8 (9), pp 9550-9557

Hexagonal boron nitride (h-BN) is a promising 2D dielectric material that is well suited as an insulating substrate and gate dielectric for 2D electronics. The pairing of h-BN with graphene to make 2D electronics devices has attracted a lot of attention in the literature. To date, all of the devices based on graphene/h-BN structures have utilized h-BN films that are either mechanically exfoliated from a bulk crystal, or grown using chemical vapor deposition (CVD) on transition metal substrates and then arduously transferred to substrates such as Si. These lab scale device demonstrations are not manufacturable processes. In parallel to electronic device demonstrations, work is ongoing to increase the size and quality of graphene/h-BN heterostructures grown on micro-fabrication compatible substrates, with the goal of creating viable production pathways for future graphene-inspired 2D electronics and photonics.
In our previous report on growing h-BN by metalorganic chemical vapor deposition (MOCVD), we showed that atomically smooth few-layer thick h-BN can be achieved under a self-terminating growth mode. Since growth behavior is sensitive to substrate surface properties, modification to the surface of a sapphire substrate is expected to play an important role in the growth of BN films. This extends to unintentional nitridation that may occur. Earlier nitridation studies in MOCVD have observed the formation of an amorphous aluminum oxynitride (AlN_xO_1-x) layer, while other studies presented evidence for the formation of relaxed crystalline AlN on the sapphire surface. The surface morphology of sapphire after nitridation was dependent on nitridation conditions, such as nitridation time, temperature, etc.
In this work, we study the effect of nitridation on the growth of ultra-thin h-BN films on sapphire substrates within a MOCVD system. Two nitridation processes were used (1) the same as growth conditions and (2) low temperature and low ammonia-to-hydrogen ratio to control the modification to the surface of the sapphire substrate. The sapphire surface after nitridation was independently characterized to establish a base line for BN thickness measurements on sapphire. These results were further correlated with film thickness estimation by X-ray reflectance (XRR), with simulation and curve fitting using three-layer and two-layer models for h-BN films deposited with and without nitridation. Our results show that nitridation significantly affects many aspects of the deposited films, including morphology and properties of few-layer thick h-BN as characterized by AFM, XRR, and Raman spectroscopy.

Black phosphorus, a single elemental semiconductor with layered structure, has recently been successfully exfoliated to atomic layers by mechanical cleavage technique, and has drawn great attention mainly due to its high carrier mobility, high on/off ratio (>10⁵), tunable direct band gap, and anisotropic electrical properties. Although mechanical exfoliation produces pristine flakes of very high quality, it is limited to research laboratories and is not scalable for mass production. In this work, we show that atomic layers of black phosphorous (known as phosphorene) can be produced in large quantities by liquid phase exfoliation, providing for progress toward practical applications. Our characterizations show that uniform dispersions of atomically thin nanosheets can be obtained in aprotic and polar organic solvents such as Dimethyl formamide (DMF) and Dimethyl sulfoxide (DMSO) and that the flakes remain well protected against degradation in the solution form. Electron microscopy confirms that produced flakes are exfoliated down to monolayers, having perfect crystalline structure and very low impurity content. Field effect transistors made of few layer thick individual flakes show p-type semiconducting behavior with high on/off ratios (>10³). Moreover, thin films of randomly stacked flakes show stable electronic properties in ambient conditions, and hold promise for use in flexible integrated electronic systems.

In spite of the recent heightened interest in molybdenum disulfide (MoS₂) as a two-dimensional material with substantial bandgaps and reasonably high carrier mobility, a method for the layer-controlled and large-scale synthesis of high quality MoS₂ films has not previously been established. Here, we demonstrate that layer-controlled and large-area CVD MoS₂ films can be achieved by treating the surfaces of their bottom SiO₂ substrates with oxygen plasma process. Most of the excellent previous results for MoS₂ films have been obtained with a top-down approach such as mechanical exfoliation or liquid exfoliation. Recently, several bottom-up approaches, such as the sulfurization of deposited molybdenum, molybdenum oxide sulfurization, vapor-solid growth with MoS₂ single powder as the source, and the decomposition of deep coated thiomolybdates, have been reported. With these approaches, much improved scalability has been demonstrated: MoS₂ monolayers have been fabricated with sizes up to wafer scale, mobilities in the range 0.02-1.2 cm²/Vs, and on/off ratios in the range 10⁴-10⁷. However, the development of a layer-controlled large-scale technique for the synthesis of high-quality MoS₂ films for two-dimensional nanodevice applications remains a significant challenge. In this work, the layer-controlled CVD growth of large-area MoS₂ films is achieved by performing the plasma surface treatment of the SiO₂ substrate and a molybdenum oxide sulfurization.
Raman mapping, UV-Vis, and PL mapping are performed to show that mono, bi, and trilayer MoS₂ films grown on the plasma treated substrates fully cover the centimeter scale substrates with a uniform thickness. Our high-resolution TEM images also present the single crystalline nature of the monolayer MoS₂ film and the formation of the layer-controlled bi- and tri-layer MoS₂ films. Back-gated transistors fabricated on these MoS₂ films are found to exhibit high current on/off ratio of ~ 10⁶ and high mobility values of 3.6 cm²V^-1s^-1 (monolayer), 8.2 cm²V^-1s^-1 (bilayer), and 15.6 cm²V^-1s^-1 (trilayer).
Our results show that the layer-controlled preparation of large-area and high quality MoS₂ films, which are vital for a variety of devices and fundamental physics applications, can be achieved by modifying the surface chemistry of their substrates, which are expected to have significant impact on further studies of the MoS₂ growth mechanism as well as on the scaled layer-controlled production of high quality MoS₂ films for a wide range of applications.

In this talk, we first report the controlled vapor phase synthesis of transition metal dichalcogenide (TMD) atomic layers and their hetero structures. The atomic structure and morphology of the grains and their boundaries are examined and first-principles calculations are applied to investigate their energy landscape. The impacts of grain boundaries on physical properties of TMD atomic layers are discussed. More interestingly, if precise two-dimensional domains of metallic, semiconducting and insulating atomic layers can be seamlessly stitched together, in-plane heterostructures with interesting electronic applications could pote
ntially be created. Here, we show that planar graphene/h-BN/h-BNC heterostructures can be formed either by growinggraphene in lithographically-patterned h-BN atomic layers or by a direct chemical conversion process. We also report a one-step growth strategy for the creation of high-quality vertically stacked as well as in-plane interconnected heterostructures of WS₂/MoS₂ via control of the growth temperature. Finally, we demonstrate how self-assembled monolayers with a variety of end termination chemistries can be utilized to tailor the physical properties of single-crystalline MoS₂ atomic-layers. Our data suggests that combined interface-related effects of charge transfer, built-in molecular polarities, varied densities of defects, and remote interfacial phonons strongly modify the electrical and optical properties of MoS₂, illustrating an engineering approach for local and universal property modulations in two-dimensional atomic-layers.

Two dimensional (2D) materials with a monolayer of atoms represent an ultimate control of material dimension in the vertical direction. Molybdenum sulfide (MoS2) monolayers, with a direct bandgap of 1.8 eV, offer an unprecedented prospect of miniaturizing semiconductor science and technology down to a truly atomic scale. Recent studies have indeed demonstrated the promise of 2D MoS2 in fields including field effect transistors, low power switches, optoelectronics, and spintronics. However, device development with 2D MoS2 has been delayed by the lack of capabilities to produce large-area, uniform, and high-quality MoS2 monolayers. Here we present a self-limiting approach that can grow high quality monolayer and few-layer MoS2 films over an area of centimeters with unprecedented uniformity and controllability. This approach is compatible with the standard fabrication process in semiconductor industry. It paves the way for the development of practical devices with 2D MoS2 and opens up new avenues for fundamental research.
We will also introduce a new technique we have developed for the perfect transfer of the synthesized 2D MoS₂ films. Generally, it is problematic to translate the well-established transfer processes for graphene to MoS₂ due to different growth mechanisms and surface properties. We have developed a surface energy-assisted process that can perfectly transfer centimeter-scale monolayer and fewlayer MoS₂ films from original growth substrates onto arbitrary substrates with no observable wrinkles, cracks, and polymer residues. The unique strategies used in this process include leveraging on the penetration of water between hydrophobic MoS₂ films and hydrophilic growth substrates to lift off the films and dry transferring the film after the lift off. This is in stark contrast with the previous transfer process for synthesized MoS₂ films, which explores the etching of the growth substrate by hot base solutions to lift off the films. Our transfer process can effectively eliminate the mechanical force caused by bubble generations, the attacks from chemical etchants, and the capillary force induced when transferring the film outside solutions as in the previous transfer process, which consist of the major causes for the previous unsatisfactory transfer. Our transfer process also benefits from using polystyrene (PS), instead of PMMA that was widely used previously, as the carrier polymer. PS can form more intimate interaction with MoS₂ films than PMMA and is an important for maintaining the integrity of the film during the transfer process. This surface energy-assisted approach can be generally applied to the transfer of other 2D materials, like WS₂.

The direct-gap property of the semiconducting transition metal dichalcogenide (TMD) monolayers are attractive for optoelectronics and energy harvesting. Recent success in vapor phase growth of micron-sized TMD monolayer[1-2] has stimulated the research in CVD growth as well as its emerging applications. Here we would like to briefly discuss the synthesis and characterizations of sub-mm sized MoS₂, WSe₂ and WS₂ monolayers obtained by vapor phase reaction between metal oxides and S or Se powders. These layer materials can be transferred to desired substrates to form either TMD/TMD or TMD/graphene stacking structures. By using micro-beam X-ray photoelectron spectroscopy, the band offsets may be determined.[3] Several emerging applications based on TMDs and their heterostructures such photodetectors, solar cells, ultra-sensitive detection and future electronics will be discussed.[4-7]
References
1.Huang, J. K. et al. Large-Area Synthesis of Highly Crystalline WSe2 Mono layers and Device Applications. Acs Nano 8, 923 (2014).
2. Lee, Y. H. et al. Synthesis of Large-Area MoS2 Atomic Layers with Chemical Vapor Deposition. Advanced Materials 24, 2320 (2012).
3. M.-H. Chiu et al. Determination of band alignment in transition metal dichalcogenides heterojunctions.arXiv:1406.5137
4. M.-L Tsai et. alMonolayer MoS2 Heterojunction Solar Cells.ACS Nano 8, 8317 (2014).
5. P. T. K. Loan et al. Graphene/MoS2 Heterostructures for Ultrasensitive Detection of DNA Hybridisation.Adv. Mater. DOI: 10.1002/adma.201401084
6. W. Zhang et al. Ultrahigh-Gain Photodetectors Based on Atomically Thin Graphene-MoS2 HeterostructuresScientific Reports 4 (3826), 1-8 (2014)
7. M.-C. Chen et al. Hybrid Si/2D Electronic Double Channels Fabricated Using Solid Few-Layer MoS2 Stacking for Vth Matching and CMOS-compatible 3DFETs. IEDM (2014).

Atomic-scale molybdenum disulfide (MoS2) is a promising semiconductor for flexible and stretchable electronics devices such as displays and wearable sensors. Stretchable devices are typically generated through exfoliation or through lift-off methods, but these approaches are not commercially viable. Unfortunately, commercial scale growth of high quality MoS2 films requires high temperatures, which is not compatible with stretchable polymeric materials. Here, we present a simple and scalable approach to creating MoS2 films on stretchable polymeric materials. Specifically, magnetron sputtering from a MoS2 target is used to create a film at low temperature on PDMS. These films act as a precursor, which is subsequently laser annealed to form high quality MoS2. This combination of sputtering and laser annealing is commercially scalable and lends itself well to patterning. The presentation will include Raman, Scanning Probe Microscopy and x-ray photoelectron spectroscopy to confirm our assertions and illustrate annealing mechanisms. Electrical properties of simple devices built on flexible substrates will be correlated to annealing processesAs our presentation will demonstrate, this simple approach is a significant step towards commercial-scale stretchable 2D nanoelectronic devices.

This talk will discuss the synthesis of large-area, high-quality monolayers of nitrogen-, boron- and silicon-doped graphene sheets on Cu foils using ambient-pressure chemical vapor deposition (AP-CVD). Scanning tunneling microscopy (STM) and spectroscopy (STS) reveal that the defects in the doped graphene samples arrange in different geometrical configurations exhibiting different electronic and magnetic properties. Interestingly, these doped layers could be used as efficient molecular sensors and electronic devices. In addition, the synthesis of hybrid carbon materials consisting of sandwich layers of graphene layers and carbon nanotubes by a self-assembly route will be discussed. These films are energetically stable and could well find important applications as field emission sources, catalytic supports, gas adsorption materials and super capacitors.
Beyond graphene, the synthesis of other 2-Dimensional materials will be described. In particular, we will discuss the synthesis of WS₂ and MoS₂ triangular monolayers, as well as large area films using a high temperature sulfurization of WO_x clusters deposited on insulating substrates. We will show that depending on the substrate and the sizes of the oxide clusters, various morphologies of layered dichalcogenides could be obtained. In addition, photocurrent measurements on these materials will be presented. Our results indicate that the electrical response strongly depends on the laser photon energy. The excellent response observed to detect different photon wavelengths in MoS₂, WS₂ and WSe₂ materials, suggest these materials could be used in the fabrication of novel ultrafast photo sensors. We will also show that these techniques are able to produce in-plane heterojunctions with sharp interfaces of MoS₂ and WS₂. The material exhibits novel excitonic effects that will be discussed.
From the theoretical stand point, we have found using first principles calculations, that by alternating individual layers of different metal chalcogenides (e.g. MoS₂, WS₂, WSe₂ and MoSe₂) with particular stackings, it is possible to generate direct band gap bi-layers ranging from 0.79 eV to 1.157 eV. Interestingly, in this direct band gap, electrons and holes are physically separated and localized in different layers. We demonstrate that it is possible to alternate different chalcogenide layers with graphene and hBN with unprecedented optical and physico-chemical properties.
This research is supported by the Army Research Office through MURI Grant #W911NF-11-1-0362.
1. Lv, R., et al. Scientific Reports2, 586 (2012). DOI: 10.1038/srep00586.
2. H. R. Gutiérrez, et al. Nano Lett. 13, 3447-3454 (2013). DOI: 10.1021/nl3026357.
3. A. Berkdemir, et al. Scientific Reports3, 1755 (2013). DOI:10.1038/srep01755.
4. A. L. Elías, et al. ACS Nano, 7, 5235-5242 (2013).
5. N. Perea-Loacute;pez, et al. Advanced Functional Materials23, 5510 (2013).
6. H. Terrones, F. Lopez-Urias, M. Terrones Scientific Reports3, 1549 (2013). DOI: 10.1038

Heterogeneous engineering of two-dimensional layered materials, including metallic graphene and semiconducting transition metal dichalcogenides, presents an exciting opportunity to produce highly tunable electronic and optoelectronic systems. The direct growth of such heterostructures is desirable to engineer pristine layers and their combination as van der Waals heterostructures. We reported the direct and epitaxial growth of crystalline tungsten diselenide (WSe₂) and molybdenum disulfide (MoS₂) monolayer on epitaxial graphene (EG) grown on SiC and the applications on these two single-junctions, WSe₂/EG, and MoS₂/EG, respectively. With an atomically sharp interface, the vertical transport measurement across the WSe₂/EG junction provides evidence that the interlayer gap between the layers adds a barrier to carrier transport, while the photosensing measurements carried out on the MoS₂/EG photosensors shows a 20 x increased photon responsibility comparing with bare 1L MoS₂ photosensors. In the end, the possibility to directly grow MoS₂/WSe₂/EG and WSe₂/MoS₂/EG, the double-junction having strong interlayer coupling due the type-II band alignment is presented.

Two-dimensional (2D) atomic crystals, best exemplified by graphene, have emerged as a new class of material that may impact future science and technology. Drawing from our experiences in graphene study, I will discuss new 2D materials beyond graphene, including few-layer black phosphorus and 1T-TaS₂ thin film - two new materials with vastly different properties. We explore their electronic properties while the doping and dimensionality of the 2D systems are modulated.

Transition metal dichalcogenides (TMDCs) are a promising class of two-dimensional (2D) materials for use in 2D electronics. This is due to an inherent band-gap that can be direct or indirect, depending on the number of layers, unlike graphene which has no bandgap in its pristine condition. For a wide range of applications, TMDC heterostructures or more broadly van der Waals (vdW) heterostructures are expected to show interesting properties due to interfacial effects. Furthermore, vdW heterostructures avoid any problems arising from lattice mismatch that affect other types of heterostructures. Current studies on heterojunctions between TMDCs have primarily focused on mechanical (or chemical) exfoliation or co-vapor growth methods. These methods are limited by the size and non-uniform distribution of the heterostructures on the substrate. Any application of TMDC heterostructures needs a viable method to produce large-area, patternable heterostructures.
Recently we have demonstrated a large-area synthesis of TMDC thin films via vapor transport. In this presentation, I will discuss a one-step synthesis procedure we have developed for stacked heterostructures of multi-layer MoS₂ and WS₂. This heterostructure forms a p-n junction, so potential applications include the formation of thin, flexible 2D transistors. This method has the key advantage over current methods that it can be patterned, is large-area, and is suitable for scaled synthesis of devices. We have characterized our synthesized heterostructures with Raman Spectroscopy and cross-sectional transmission electron microscopy. Our device shows transport measurement consistent with what we would expect for a p-n junction device. This work lays the ground work for further studies on patterned synthesis of TMDC heterostructures.

Monolayer transition metal dichalcogenide crystals like MoS₂ and MoSe₂ are direct band-gap semiconductors with distinct, but energetically degenerate valleys at the K and K&’ points of the Brillouin zone. Recent research has shown that it is possible to access selectively this valley degree of freedom using circularly polarized radiation, where resonant left-handed light excites one valley and resonant right-handed light excites the other. The correctness of this theoretically predicted valley selectivity has been demonstrated by analysis of the polarization state of photoluminescence excited by circularly polarized radiation.
In this paper, we extend these studies to examine the influence of a perpendicular magnetic field on the valley characteristics of monolayer MoSe₂ crystals. The presence of a magnetic field is predicted to break the time-reversal symmetry of the two valleys and to lift their energy degeneracy. This lifting of the degeneracy can be described as the result of the presence of valley magnetic moments, which are opposite to one another in the K and K&’ valleys and cause the bands to shift in opposite directions under application of a magnetic field.
We have verified these predictions and also determined the magnitude of the field-induced shifts in the transition energies of the two valleys by magneto-optic spectroscopy. Specifically, we measured the emission energy for circularly polarized light of the two handednesses as a function of the strength of the applied magnetic field. Signatures of the magnetic field were observed in the emission of both neutral and charged exciton states. The results demonstrated the controlled lifting of the valley degeneracy and the creation of a valley polarization in the presence of charge carriers in the crystal. We discuss induced changes in both the transition energies and strength of the photoluminescence features and relate these observations to the predictions of modification of the electronic structure of the material under a magnetic field. This research was carried out in collaboration with the team of Zhiqiang Li and Dmitry Smirnov at the National High Magnetic Field Laboratory, Tallahassee, FL.

A fundamental understanding of the intrinsic optoelectronic properties of atomically thin transition metal dichalcogenides (TMDs) is crucial step for integration into high performance semiconductor devices. Here, we investigate the transport properties of chemical vapor deposition (CVD) grown monolayer molybdenum disulfide (MoS₂) under photoexcitation using correlated scanning photocurrent microscopy and photoluminescence imaging. We examined the effect of local phase transformation underneath the metal electrodes on the generation of photocurrent across the channel length with diffraction-limited spatial resolution. While maximum photocurrent generation occurs at the Schottky contacts of semiconducting (2H phase) MoS₂, after the metallic transformation (1T phase), the photocurrent peak is observed towards the center of the device channel, suggesting a strong reduction of native Schottky barriers. Analysis using the bias and position dependence of the photocurrent indicated that the Schottky barrier heights were few meV for 1T contacted and ~200 meV for 2H contacted devices. The obtained results pave a pathway for the fundamental understanding of intrinsic optoelectronic properties of atomically thin TMDs where Ohmic contacts are needed for high efficiency devices.

Two-dimensional transition metal dichalcogenides (2D TMDs) — whose generalized formula is MX₂, where M is a transition metal of groups 4-7 and X is a chalcogen — exhibit versatile chemistry and consist of a family of over 40 compounds that range from complex metals to semiconductors to insulator. Complex metal TMDs assume the 1T phase where the transition metal atom coordination is octahedral. The 2H phase is stable in semiconducting TMDs where the coordination of metal atoms is trigonal prismatic. We have been studying the 1T phase in semiconducting TMDs. In particular, we have focused on mechanisms involved in inducing the meta-stable 1T phase, kinetics of phase transformation and fundamental structural and electronic properties. We have implemented phase-engineered materials as hydrogen evolution reaction (HER) catalysts and as low resistance contact electrodes in electronic devices. We have also exploited the metallic nature of the 1T phase to functionalize a variety of TMDs. The attachment of functional groups leads to dramatic changes in the opto-electronic properties of the material.

Crystal structure plays an important role in the physical properties of a material and its interaction with external stimuli. Layered materials, such as graphite, hexagonal boron nitride (h-BN) and many transition metal dichalcogenides (TMDCs) are centrosymmetric in their bulk three-dimensional (3D) form, but they may exhibit different symmetry when thinned down to a single atomic layer. In graphene, inversion symmetry is preserved since both atoms in the unit cell are identical, while monolayer h-BN and TMDCs become non-centrosymmetric due to the absence of an inversion center. Due to the broken of inversion symmetry, single-atomic-layer h-BN, MoS₂, MoSe₂ and WTe₂ have also been theoretically predicted to exhibit piezoelectricity. Here we report the first experimental study of the piezoelectric properties of two-dimensional (2D) MoS_2. We find that cyclic stretching and releasing of thin MoS₂ flakes with an odd number of atomic layers produces oscillating piezoelectric voltage and current outputs, while no output is observed for flakes with an even number of layers. The output increases with decreasing thickness and reverses sign when the strain direction is rotated by 90 degrees. The coupling between piezoelectricity and semiconducting properties in 2D nanomaterials may enable applications in powering nanodevices, adaptive bio-probes and tunable/stretchable electronics/optoelectronics.

Two-dimensional materials are capable of handling high rates of charge in ion batteries due to absence of diffusion in a 3-D lattice structure. However, graphene, which is the most well-studied 2-D material, is known to have no Li capacity. Here, we investigate the possibility of using graphene with defects and other two dimensional materials such as transition metal dichalcogenides and MXens as anode materials for Li as well as Na, Ca, Mg and Al ion batteries. Compared to materials currently used in high-rate Li and Na ion battery anodes, 2D materials shows promise in increasing overall battery performance. In the second part of the talk I will talk about application of 2D materials as catalysts for hydrogen evolution. Efficient evolution of hydrogen through electrocatalysis at low overpotentials holds tremendous promise for clean energy. Hydrogen evolution can be easily achieved by electrolysis at large potentials that can be lowered with expensive platinum-based catalysts. Replacement of Pt with inexpensive, earth-abundant electrocatalysts would be significantly beneficial for clean and efficient hydrogen evolution. Here we report monolayered nanosheets of chemically exfoliated WS₂ and MoS₂ as efficient catalysts for hydrogen evolution with very low overpotentials. Analyses indicate that the enhanced electrocatalytic activity of WS₂ is associated with the high concentration of the strained metallic 1T (octahedral) phase in the as-exfoliated nanosheets. Our results suggest that chemically exfoliated WS₂ nanosheets are interesting catalysts for hydrogen evolution.

Atomically thin two dimensional materials like Graphene and monolayers of transition metal dichalcogenides have received significant research attention in the recent past. In particular, monolayers of molybdenum disulfide, owing to a dramatic increase in bandgap from its bulk counterpart, have been investigated for FET applications [1]. It has been demonstrated in these studies that the carrier mobility in MoS₂ monolayers can be engineered [2] by using a high-κ dielectric as the gating material. The large bandgap (1.8 eV) and the tunable mobility of MoS₂ monolayer could be leveraged to obtain favorable thermoelectric power factors (S²σ) where S is Seebeck coefficient and σ is electrical conductivity. Semi-classical calculation [3] of thermoelectric power factor in suspended MoS₂ monolayer showed that the theoretical peak thermoelectric power factor at room temperature is 2.8 x 10⁴ µW/m K² at an electron concentration of 10^-12 cm^-2. Here, we show experimental results of the thermoelectric power factor measurements on exfoliated monolayer MoS₂. Exfoliated MoS₂, unlike the CVD grown layer, does not suffer from loss of mobility resulting from structural defects and grain boundaries. This results in more tunability of mobility in the monolayers. We use HfO₂ in our study as a high-κ dielectric material to screen the coulomb scattering and increase the mobility in the monolayers.
References:
1. B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, A. Kis, Single-layer MoS2 transistors, Nature Nanotechnology, Vol.6, pp.147-150, 2011.
2. B. Radisavljevic, A. Kis, Mobility engineering and a metal-insulator transition in monolayer MoS2, Nature Materials, Vol.12, pp.815-820, 2013.
3. H. Babaei, J. M. Khodadadi, S. Sinha, Large Theoretical Thermoelectric Power Factor of Suspended Single-Layer MoS2, Applied Physics Letters, In Review.

Tunability of electronic properties of semiconductors is vital for obtaining large efficiencies in thermoelectrics. The figure of merit for thermoelectric efficiency is defined as ZT =S²σT/k where S is the Seebeck coefficient and σ and k are the electrical and thermal conductivities of the material respectively. The electronic performance is determined by the powerfactor, S²σ which can be shown to be directly related to the mobility and effective mass of the semiconductor. With large mobilities (65 cm²/V-s) and large conduction band effective mass (0.67m₀) few-layer MoS₂ samples provide a promising avenue for thermoelectrics. Our experiments demonstrate that the gate-tuned powerfactor in bilayer MoS₂ can be as large as 8.5 mW/m-K² which is twice as large as commercially viable Bismuth Telluride. We further discuss the origin of the large Seebeck Coefficient in single-layer MoS₂. Our results open up new possibilities of using 2D materials for thermoelectrics.

We present a computational study using Density Functional Theory methods to determine the structural stability and electronic properties in single wall Molybdenum di-Sulfide nanotubes (NT-MoS₂) in zig-zag and arm chair chirality configurations; NT-MoS₂ were subjected to external hydrostatic pressure ranging from 1-15 GPa along y-axis. The simulations was done using the Cambridge Serial Total Energy (CASTEP) package using revised Perdew-Burke-Ernzerhof functional, general gradient approximation, cutoff energy of 290 eV in the reciprocal space with 1x1x4 key point set in the Brillouin zone, a self-consistent field (SCF) convergence threshold of 1 x 10^-6 eV/atom. Nanotubes, present deformation as expected and several electronic phenomena were determined after performing density of states and band structure calculations for deformed geometrical nano-structures.

The solvent-assisted exfoliation of 2D transition metal dichalchogenides (TMDs) is a promising route to achieve inexpensive, high-performance semiconductor devices. However, low concentrations and the restacking/aggregation of the TMD layers remain challenges to achieve controlled thin-film formation. Here we present advances in the exfoliation and solution processing of semiconducting MoS₂ and WSe₂ that are subsequently leveraged to prepare large-area thin-film devices. Specifically, we show how an efficient sonopolymer-assisted exfoliation (5% in yield) affords stable dispersions of single and few-layer TMD flakes in the 2H phase. Alkyl-trichlorosilane surfactants are used to further stabilize the 2D TMD flakes in highly concentrated dispersion (up to 85 mg ml^-1) and induce the expanded restacking of layers in film formation processes.[1] By varying the alkyl-trichlorosilane chain length and solution-based deposition method, we report insights into the effects of layer restacking, flake orientation, and film homogeneity on the measured mobility (in the 0.1 cm² V^-1 s^-1 range) in multi-flake films using space-charge limited current devices (transport perpendicular to layer alignment) and field-effect transistors (transport parallel to layer alignment). Finally we show that the controlled and homogeneous thin-film formation enables large-area TMD devices for solar energy conversion. In particular, efforts to optimize MoS₂:WSe₂ bulk heterojunction photovoltaic devices and WSe₂ photocathodes for solar water reduction are presented. Initial photocurrents of over 1 mA cm^-2 under AM 1.5G illumination are reported and routes to improvement are discussed.
[1] Yu, X.; Prévot, M. S.; Sivula, K. Chem. Mater. 2014, DOI: 10.1021/cm502378g.

Transition metal dichalcogenides (TMDCs) are layered materials (MX₂: M = Mo, W; X = S, Se) have attracted great attention as a chemical vapor sensing materials with high sensitivity, owing to their very high surface-to-volume ratios and semiconducting properties. Even many TMDCs have been considered as a promising gas sensing materials, most of research have been focused on gas sensing properties of molybdenum disulfide (MoS₂). The MoS₂ nanosheets for gas sensing have been synthesized by mechanical exfoliation or chemical vapor deposition (CVD). Recently, mechanically exfoliated MoS₂ field-effect transistor (FET) gas sensors shown highly-sensitive NO₂ sensing properties under positive gate bias (15V) due to the electrons accumulate at the MoS₂. Although the sensitivity was high, MoS₂ was unable to detect the NO₂ below 200 ppm under zero bias. Hence, to develop the low-concentration detectable TMDCs-based gas sensor, studies for gas sensing properties of other TMDCs and investigations of high performance gas sensor are important.
Here, we fabricated two-terminal chemiresistive gas sensor using large area tungsten disulfide (WS₂) synthesized using atomic layer deposition (ALD). The number of WS₂ layer was controlled by controlling the number of ALD cycles. At low concentration of NO₂ and acetone, the immediate response which is attributed to the change of conductivity of WS₂ were exhibited. The WS₂-based gas sensors show clear detection of NO₂ and acetone down to 25 ppm and 0.5 ppm, respectively. We demonstrated that the WS₂ exhibit p-type characteristics to react oxidizing gas (NO₂) and reduction gas (acetone). Also, we demonstrated the gas sensing properties of silver nanowires (Ag NWs) -functionalized WS₂.

Materials that are used in conventional (opto)electronic devices are characterized by covalent bonding in three dimensions (3D). In recent years it has become clear that 2D materials with functional properties complementary to graphene are highly appealing for the realization of new devices with improved functionalities. In order to benefit from the maturity of conventional Si technology, an integration of 2D materials with the 3D material Si is desirable. This requires a detailed understanding of the mechanisms that determine the interface structure between 2D and 3D materials.
We shed some light on this question by investigating Sb₂Te₃ layers grown on top of Si(111) using MBE¹. Sb₂Te₃ is an exemplary 2D material since it is formed by quintuple layers (QL) which are bonded to each other by van der Waals bonds, it possesses robust topological insulating surface states² and thanks to its 2D nature it serves as a building block of interfacial phase-change memory³ and thermoelectric superlattices⁴. The present study led to the discovery of new epitaxial paradigms for the growth of 2D layered materials onto 3D bonded substrates. In fact the presence of covalent bonds at the interface can result in the formation of surface reconstruction induced coincidence lattices between 2D materials and 3D substrates.
We will also present some insight on the growth of 2D materials on 2D bonded substrates by employing Sb₂Te₃ grown on several passivated Si surfaces as well as on epitaxial monolayer and bilayer graphene as a model system to study van der Waals epitaxy.
¹ J.E. Boschker, J. Momand, V. Bragaglia, R. Wang, K. Perumal, A. Giussani, B.J. Kooi, H. Riechert, and R. Calarco, Nano Lett. 14, 3534 (2014).
² Y. Takagaki, A. Giussani, J. Tominaga, U. Jahn, and R. Calarco, J. Phys. Condens. Matter 25, 345801 (2013).
³ J. Tominaga, A. V. Kolobov, P. Fons, T. Nakano, and S. Murakami, Adv. Mater. Interfaces 1, 1300027 (2014).
⁴ R. Venkatasubramanian, T. Colpitts, E. Watko, M. Lamvik, and N. El-Masry, J. Cryst. Growth 170, 817 (1997).

Recently, modulation of band gap in two dimensional (2D) transition metal dichalcogenides (TMDCs) has been actively studied for versatility of 2D TMDCs in optoelectronic and photovoltaic devices such as photodiodes, phototransistors and solar cell. It is well known that band gap of 2D TMDCs can be tuned by control the layer numbers. Moreover, alloying is another method to modulate the band gap of 2D TMDCs based on their good thermodynamical stability at room temperature. However, the reported synthesis methods of 2D TMDCs alloy such as mechanical exfoliation and chemical vapor deposition could not systematically control the composition and layer numbers, although they are essential needs to modulate band gap in 2D TMDCs. Thus, an improved process to satisfy these requirements in synthesis of 2D TMDCs alloy nanosheets is required.
Here, we investigated the synthesis of 2D Mo_1-xW_xS₂ alloy through sulfurization of super-cycle atomic layer deposition (ALD) Mo_1-xW_xO₃ thin film. We systematically controlled the composition and layer numbers (from mono- to tri-layers) of 2D Mo_1-xW_xS₂ alloy by controlling the cycle ratio between ALD MoO₃ and WO₃. The band gap of 2D Mo_1-xW_xS₂ alloy is modulated by control the composition and layer numbers. Also, mixing of Mo and W atoms with sharing metal site in the monolayer Mo_1-xW_xS₂ alloy is observed. Furthermore, we demonstrate that our process is feasible to synthesize a composition graded Mo_1-xW_xS₂ multilayer, which has Mo rich concentration in the upper side and W rich concentration in the bottom side. We examined light absorption property of a composition graded Mo_1-xW_xS₂ multilayer.

Molybdenum disulfide (MoS₂), one of the transition-metal dichalcogenides, is a 2-dimensional semiconducting material that has a layered structure. It is known that MoS₂ shows different physical properties depending on the number of layers. Single-layer MoS₂ has a direct band gap (Eg=1.8 eV), while bulk MoS₂ has an indirect band gap (Eg=1.2 eV). In addition, single-layer MoS₂ transistors have shown excellent properties. For example, it is reported that the field-effect mobility in single-layer MoS₂ transistors was as high as 217 cm²V^-1s^-1. Owing to these optical and electronic properties, single-layer MoS₂ is expected to use for various devices, such as transistors and flexible displays. We fabricated 2-dimensional MoS₂ films by sputtering deposition. In general, the sputtering deposition has a superior mass productivity and the film thickness can be controlled accurately only by the deposition duration. However, details of sputtering deposited MoS₂ film in the ultra-thin region have not been revealed sufficiently yet. In this study, we investigated the physical properties of sputtering deposited MoS₂ film in the sub-10-nm region by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and transmission electron microscopy (TEM). As the results of Raman spectroscopy investigations, we observed two Raman modes, E¹_2g and A_1g, in the 2-dimensional MoS₂ films. As the thickness of the MoS₂ film decreased, the peak frequency difference between E¹_2g and A_1g modes increased. From the XPS investigations, we confirm S reduction from the 2-dimensional MoS₂ films. We considered that the sulfur vacancies in the MoS₂ film affected the Raman peak position.

The discovery of graphene has intrigued worldwide research on two dimensional (2D) materials. The zero band gap of monolayer graphene severely limits its application in microelectronics, light emitting diodes and other optical/electronic devices. The band gap of bilayer and trilayer graphene opened by asymmetric electric field between the top and bottom layer is still limited below 250 meV. On the other hand, opening the band gap of graphene through cutting it down a few nanometers in width is also not very effective. Under this context, scientists switch their attentions to new 2D materials such as MoS₂, WSe₂, et al. which have a suitable band gap for logic circuits and optoelectronic devices. Here, we have proposed and experimentally demonstrated a novel method of band gap engineering of graphene through alloying silicon with graphene by synthesizing 2D silicon dicarbide (SiC₂) and ultrathin silicon carbide (SiC). Thermal stable 2D SiC and SiC₂ can be formed by free carbon and silicon atoms using chemical vapor deposition, which was firmly supported by ab initio Born-Oppenheimer molecular dynamics simulations. High resolution transmission electron microcopy shows the single crystal nature of 2D SiC₂ and it can be stable over 3 months in air atmosphere. Additionally, the thickness of 2D SiC measured by atomic force microscope is commonly below 10 nm. The exciton binding energy of 2D ultrathin SiC can reach as huge as 0.33 eV while the band gap is around 3.82 eV. The first production of 2D SiC₂ and SiC make the band gap engineering in the graphene lattice plane possible and open a new research direction for 2D materials. [1]. S. S. Lin. The Journal of Physical Chemistry C, 116, 3951-3955, (2012). [2]. S. S. Lin, S. J. Zhang, et al. Unpublished. [3]. S. J. Zhang, et al. Unpublished.

Two-dimensional layered TMDs (Two-dimensional transition metal dichalcogenides) has recently attracted much attention due to their transition from indirect to direct bandgap, showing excellent physical properties with outstanding potential for applications in optoelectronics, energy harvesting, Li-ion batteries and supercapacitors. However, the synthesis approach by CVD requires the use of high temperature, restricting the utilization of substrates that can sustain such temperatures. Furthermore, the films have to be transferred to an additional substrate due to the damage caused by the original substrate used for synthesis. Therefore, further improvement of the TMDs synthesis method is an important step. Here, we used a laser approach to simultaneously grow and pattern WS₂ and MoS₂ on insulating substrates without necessity of additional transfer process. Raman and X-ray photoelectron Spectroscopies supported by TEM observations confirm the quality and crystallinity of the synthesized films. Moreover, this method can be expanded to the synthesis of other TMDs TMD materials. Laser assisted synthesis of TMD materials provides a new approach that is fast, cheap and patternable forward the development of TMD materials for industry application.

Emerging two-dimensional atomically thin materials (2D-ATMs) such as grapehene, monolayer MoS₂, and WS₂ have attracted great interest due to their exotic properties. Though significant studies during the last decade have revealed the unexpected characteristics of 2D-ATMs, controlling physical properties of 2D-ATMs has still been far away from ‘on-demand&’ control in wide range. Heterostructuring can be a suitable method to overcome the current limitation of 2D-ATMs and to explore unexpected physical phenomena in 2D-ATMs.
Here, we present a heterostructure composed of conventional semiconductor (Germanium) and 2D-ATM (monolayer MoS₂).
Doped and undoped Ge thin films were grown on monolayer MoS₂ via low-pressure chemical vapor deposition. The structural characterizations with cross-sectional transmission electron microscopy confirmed that the grown Ge thin film is single crystalline. The heterojunction of p-type Ge and n-type MoS₂ shows clear rectifying current-voltage characteristic curves. The source-drain current level was also tunable with gate voltage and optical excitation.

Monolayer transition metal dichalcogenides have attracted a lot of interest due to their diverse properties, including direct energy band gaps that make them intriguing candidates for future optoelectronic devices. With a high on/off ratio that is difficult to reach with graphene and reasonable values of mobility, transition metal dichalcogenides are ideal candidates for bio-sensing purposes and many other applications.
Various approaches have been demonstrated for growth on insulating substrates of molybdenum disulphide (MoS₂), but to date growth of isolated crystalline flakes has been only at random locations. We have developed a method to obtain MoS₂ flakes in precisely defined locations. By patterning molybdenum source material that acted both as material feedstock and growth seed at predetermined areas across a wafer, we were able to grow isolated flakes of MoS₂ at these locations with micrometre-scale resolution. These MoS₂ flakes are predominantly of monolayer thickness and high material quality, as confirmed by atomic force microscopy, transmission electron microscopy, Raman and photoluminescence spectroscopy. Since the monolayer flakes are isolated and in predetermined locations, fabrication of transistor structures requires only a single lithographic step. Thus we are able to obtain multiple arrays of MoS₂ transistors, that are highly crystalline and monolayer, making this method ideal for large scale production. Device measurements showed a carrier mobility and on/off ratio that exceeded 10 cm²V^-1s^-1 and 10⁶, respectively.
This growth technique provides a path for in-depth physical analysis of monolayer MoS₂ as well as fabrication of MoS₂-based integrated circuits. We have also begun to explore applications for this high quality MoS₂, including integration into diodes and biosensors via combinations with other nano- and bio-materials.

Chemical vapor deposition has been a very viable approach for growing large area of transition metal dichalcogenides (TMDs). There have been numerous reports on the CVD growth of MoS₂ and - more recently - other TMDs, such as WS₂, MoSe₂ and WSe₂. Though each of these TMDs has uniquely special properties, it has been theorized in literature that the combination of TMDs could give rise to a hybrid material which will have the combined advantages of both materials. In this regards, we were recently successful in growing a material that is a hybrid of two different TMDs: MoS₂ and WS₂. In fact, both single-layered and multilayered flakes were obtained, a processed achieved by optimizing the amounts of growth precursors which in this case was sulfur and the oxides of both molybdenum and tungsten.
The Raman spectra of this material showed the signature peaks of both MoS₂ and WS₂ with comparable peak intensities, which correspond to equivalent amounts of both the materials. The photoluminescence (PL) peak of this hybrid material showed a blue shift with respect to MoS₂ and red shift with respect to WS₂. Additionally, the intensity of this peak was significantly higher than the PL peak of MoS₂. Both of these results lend credence to the hypothesis that the flakes are made of a hybrid material, rather than of pure MoS₂ or WS₂ flakes layered on top of one another.
Due to the hybrid nature of this material, it is suitable for applications which need the combined properties of two different TMDs. For example, it has been shown in literature that WS₂ is a very good catalyst for hydrogen evolution reaction and MoS₂ is electrically more conductive than WS₂; this material could be used in an application which requires both high catalytic activity and good electrical conductivity.
Having improved the growth methods for preparing these CVD single-layer flakes, we hope to not only further characterize these Mo-W hybrid flakes, but also extend it to alternate transition metals. The results of material synthesis and composition of these hybrids, as well as the optical and electronic properties, will also be explored and discussed.

Recently, a new family of nanolaminated two-dimensional (2D) carbides and nitrides, called MXenes, has been synthesised [1]. These 2D materials are derived from nanolaminated ternary compounds (M_n+1AX_n, where M is a transition metal, A an A-group element, X is carbon or nitrogen, and n = 1-3), and are composed of a single M_n+1X_n octahedra layer, functionalized by hydroxyl and fluorine groups. In addition to their promising capabilities for Li-battery applications [2], MXenes exhibit outstanding surface properties, because of their 2D nature and subsequent high aspect ratio, which perfectly match the growing field of plasmonics, i.e., the manipulation of light at subwavelength scales through its coupling with surface plasmons (SPs). This field of research has rarely been explored in MXenes, and understanding their intrinsic electronic structure and dielectric properties (i.e. at the single grain level) is required to that end. Combining high-resolution electron energy-loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) with density functional theory calculations, we investigate the electronic structure and plasmonic response at the nm scale in MXenes [3].
The dielectric properties of 2D Ti₃C₂ stacked sheets are studied in the 0.2-30 eV energy range. Bulk and surface plasmon modes are analysed with particular emphasis on their dependence on the number of MXene sheets. Intense SPs are evidenced at low energies in the mid-infrared, a spectral range of particular interest for chemical sensing. Several factors influencing the remarkable intensity and possible tunability of these SPs will be discussed: (i) The screening dynamics of the free electrons in MXenes are similar to those in common plasmonic materials such as silver or gold. (ii) The surface modes are the dominant screening process since the probability of bulk excitations is inherently reduced from the weakly interacting 2D M_n+1X_n sheets. We interpret this weak interaction between MXene layers as the primary origin to explain their outstanding ability for facile intercalation by a host of organic molecules and cations. (iii) Due to the interplay between electronic structure and boundary effects, the SP energies can in principle be tunable within the mid-infrared (from 0.2 to 0.7 eV) by controlling the sheets&’ functionalization and/or thickness. The overall tunability and enhancement of SPs in MXenes, attributed to their unique nanostructure, makes this new class of 2D materials very promising for SP-based applications.
[1] M. Naguib et al., Adv. Mater. 23, 4248 (2011)
[2] O. Mashtalir et al., Nat. Commun. 4, 1716 (2013)
[3] V. Mauchamp et al., Phys. Rev. B 89 235428 (2014)

Transition metal dichalcogenides (TMDs) with layered structures similar to that of graphite have shown to be promising candidates for electronic applications. Their weak interlayer attraction allows exfoliation into two-dimensional flakes with a few layers thickness. Unlike graphene, these two-dimensional materials exhibit versatile and tunable properties depending on the chemistry of the transition metal element and the chalcogen. Many reports have explored the application of TMDs on electronic devices, such as field-effect transistors, Li-ion batteries, and electrocatalysts. One of the most studied TMD semiconductor materials is MoS₂. MoS₂ films formed from MoS₂ flakes via liquid exfoliation allow large area applications. Here, we form MoS₂ films employing different solution process approaches: liquid exfoliation from MoS₂ powder using N-Methyl-2-pyrrolidone (NMP) as dispersing solvent and MoS₂ colloidal nanocrystals synthesized from thermolysis of organometallic precursor in presence of sulfur powder. For MoS₂ films formed from different synthesis/processing conditions, we study the band structures by measuring work function, ionization potential, and bandgap. The phase and composition of these films are probed using Raman spectroscopy, and are correlated to structural characterization using transmission electron microscopy (TEM), electron diffraction, and X-ray diffraction (XRD). We also examine the stoichiometry of MoS₂ films by X-ray photoelectron spectroscopy (XPS), and film morphology by atomic force microscopy (AFM). In addition, we explore the application of these solution-deposited MoS₂ films with high work function as hole transport layer (HTL) in bulk heterojunction organic photovoltaic devices. We will also examine the extension of the exfoliation and thermolysis approaches for the formation of thin films of other TMDs that have different electronic properties, e.g. low work function or wide bandgap.

Family of transition metal dichalcogenide (TMDC) materials have received recently rising interest since they posses unique physical properties and have variety of potential applications. Optical absorption has been extensively used to probe electronic transitions in single and few layer TMDC materials. Typically absorption measurements done through measuring differential reflectance, however it has been shown that differential reflectance of the thin film on a substrate is affected by absorption and refractive index of the material, making not possible exact extracting of the absorption from the differential reflectance spectra without knowing the refractive index.
Refractive index as well as it's spectral dependence are important physical quantities of the material by itself. For a few layer MoS₂, it was estimated previously through measuring visibility contrast of the sheet on Si/SiO₂ wafer at different wavelengths, however no dependence from number of layers was reported, as well as spectral information was limited by 9 band pass filters used in the work.
Here we show that by complementing differential reflectance measurement with differential transmittance it is possible to eliminate dependence of refractive index on absorption spectra, furthermore, we show the way to accurately extract both absorption and refractive index from the measured spectra. To probe spectral dependence of reflectance and transmittance supercontinuum laser source with acousto-optical tunable filter was used allowing us to study the spectral dependence of absorption and refractive index in the range of 400 - 1100 nm.
We present our study conducted on three members of TMDC family: MoS₂, MoSe₂ and WSe₂, single- to four layer sheets were analyzed. We show extracted spectral dependences of refractive index and extinction coefficient, as well as variations of those parameters with number of layers. Developed technique can be applied toward investigating the optical properties of other two-dimensional materials.

Copper chalcogenide nanoparticles are compound semiconductors remarkable for exhibiting highly tunable localized surface plasmon resonances (LSPRs) in the near-infrared range. In the presented work, we utilize modified liquid-crystalline (LC) molecular templates as a means of realizing copper sulfide nanostructures with highly confined 2D morphologies reminiscent of graphene and naturally layered transition metal chalcogenides.
Mesogenic Cu-alkanethiolates are lamellar coordination polymers with atomically thick Cu-S networks which serve as single-source precursors for Cu_2-XS nanocrystals upon thermal treatment. Due to thermal disordering of the alkyl groups, heating also results in structural rearrangement to a LC phase. The LC phase can be utilized as an elegant platform for designing Cu_2-XS nanoparticles with extreme aspect ratios better realized through a molecular template than through colloidal growth. Thermal characteristics of these mesogens are tailored by altering the alkyl chain length and by installing functional groups to suppress the LC phase transition and allow thermolysis to occur within the lamellar phase, resulting in 2D sheet-like copper sulfide nanoparticles. Additionally, incorporated halide ions interact strongly with the the LC mesophase effecting profound morphological changes by confining the growth of copper sulfide crystals to 2D planes and giving rise to nanosheets with thicknesses ~2 nm.

To search new functionalities in 2-dimentional (2D) nanomaterials for the next generation of electronic and opto-electronic devices, we report synthesis graphene (G) on boron nitride nanosheets (G/BNNSs). BNNSs are emerging 2D material having similar crystal structure and identical lattice parameter to that of graphene; therefore often known as white graphene.
Synthesis process was carried out in two different sequences: graphene directly deposited on BNNSs and graphene mechanically transferred to the surface of BNNSs. Hot filament chemical vapor deposition (HFCVD) technique used for the synthesis of graphene while BNNSs were prepared by short-pulse laser-plasma deposition technique. HFCVD technique was preferably applied to avoid sputtering of atomic layer BNNSs. The crystalline quality of the G/BNNSs heterostructures was evaluated by Raman spectroscopy mapping in order to analyze the phonon-phonon interactions between the layered structures. The physical properties of the films were carefully studied by: sheet resistance, thermal conductivity, energy band-gap, X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy. Electronic- and opto-electronic properties of G/BNNSs hetrostructures were thoroughly investigated in each sequences of the deposition process.

The capacitor is one of the important passive components for various electronic applications, and requirements of high performance and miniaturized capacitors are on the steady rise due to downsizing of electronic devices. The dielectric permittivity of Ba_1-xSr_xTiO₃, which is successfully commercialized as dielectric materials, is drastically reduced with decreasing their thickness. This decline is particularly known for damage caused by the effect of thermal strain during post-annealing process, so this process will need to be modified to more closely match the latest requirement.
Recant advances in two-dimensional (2D) materials are promising for advanced electronic application because they produce special properties such high conductivity, great magnetic and optical properties. Especially, 2D perovskite dielectric thin films have been emerging as a dielectric thin film material because of not only their high-k material undisturbed for thickness but also applicability of thin film fabrication process without post-annealing and vacuum process.
Therefore, here we synthesis dielectric nanosheets with formula of Sr_2(1-x)Bi_2xNb₃O₁₀(SBNO, x = 0 to 0.3), and SBNO nanosheets thin film, tens of nanometer thickness, were fabricated through Langmuir-Blodgett deposition method at room temperature. The SBNO nanosheets were exfoliated from layered perovskites KSr_2(1-x)Bi_2xNb₃O₁₀ through 2-step solution based cation exchange process, and we measured dielectric and electrical properties of thin film. We also discuss dielectric properties of bulk precursors as well. Structural properties were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), and high-resolution transmission electron microscopy (HR-TEM). The dielectric and electrical properties were measured from an impedance analyzer and current-voltage measurement. The experimental results show that SBNO film could be applied to a miniaturized capacitor because of its great dielectric properties.

We report a novel highly-flexible and ultrathin thermoelectric fabrics based on layered topological insulator (TI) Bi₂Se₃ Nanoplates/Polyvinylidene Fluoride #8203;(PVDF) Composite, which show a high room temperature Seebeck coefficient, electrical conductivity, and figure of merit ZT -80 µV/K, 5000 S/m, 0.02, respectively. This results demonstrate that Bi₂Se₃ Nanoplates/PVDF composite exhibit favorable thermoelectric characteristics, which opens a new avenue to fabricate highly-flexible and lightweight sustainable energy sources that could be compatible with portable/wearable electronic devices. The low thermal conductivity of the composites (~0.42 W/(mK)) suggests the nonconducting host polymer matrix PVDF serves to bind the conducting topological insulator (TI) Bi₂Se₃ while still maintaining an adequate power factor and figure of merit. The flexible thermoelectric fabrics based on layered topological insulator Bi₂Se₃ Nanoplates/PVDF composite that with comparable thermoelectrical efficiency is only a typical example that showing the promising of the present method for further applications of 2D topological insulator like Bi₂Se₃, Bi₂Te₃ and Sb₂Te₃. At their current performance, if enough thermal energy is available, the composites could be used to provide sufficient thermoelectric power for low powered personal and portable electronics.

Bismuth Telluride (Bi₂Te₃) is a promising material for extensive applications including thermoelectric power generator and quantum computation due to the excellent thermoelectric properties and the unique topologically protected surface states. However, the small bandgap nature (E_g = 160 meV) in the bulk material restricted its optoelectronic characteristics, particularly on the emission properties. Here, we show that the bandgap of Bi₂Te₃ can be tuned to the visible and even ultraviolet region by quantum confinement in all three directions. Emissive Bi₂Te₃ quantum dots (QDs) with sizes lower than 5 nm and optical energy bandgap of 3.5 eV has been prepared using a reflux assisted cutting of Bi₂Te₃ nanoplates. As a result, the prepared Bi₂Te₃ QDs show excellent photoluminescence that cover the ultraviolet and visible region and demonstrate highly efficient quantum yield of ~28% under 356 nm excitation. First principle calculation confirms the effective quantum confinement and reveals the size dependent band gap opening mechanism. This highly emissive wide-band-gap Bi₂Te₃ QDs offer scope for applying this unique material in optoelectronics devices like light emitting diodes and solar cells.

When light is incident on 2D TMDCs, limited absorption and weak light-matter interaction due to the thin body and high refractive index have hindered their further applications in optoelectroncis and photonics. Concurrently, incident light also engages in multiple reflections within underlying substrates, producing interferences that lead to enhancement or attenuation of the incoming and outgoing strength of light. Here we report a simple method to engineer the light outcoupling in semiconducting TMDCs by modulating their dielectric surroundings. We reveal that by modulating the thicknesses of underlying substrates and capping layers, the interference caused by substrate can significantly enhance the light absorption and emission of WSe₂, tuning out a ~11 times increase in Raman signal and a ~30 times increase in PL intensity of WSe₂. Based on the interference model, we also propose a strategy to control the photonic and optoelectronic properties of thin-layer WSe₂. This works demonstrate the utilization of outcoupling engineering in 2D materials and offers a new route towards the realization of novel optoelectronic devices, such as flexible LEDs and transparent solar cells.

With high carrier mobility, enhanced light absorption, and improved external quantum efficiency in vertical heterostructures, few-layer transition metal dichalcogenides are becoming preferable candidates for optoelectronic applications. Here, we investigate the photocurrent response at few-layer MoS₂-metal electrode junctions with polarized laser excitation. When incident photon energy is above the direct bandgap in few-layer MoS₂, the maximum photocurrent response at the junctions is observed in the light polarization direction parallel to the metal electrode edge. In contrast, with low energy photons, the photocurrent response is maximized when incident light is polarized in the perpendicular direction. Moreover, when the MoS₂ transistor is approaching “off” state, the photocurrent anisotropy ratio significantly increases with low energy photons, which may be attributed to the polarization-dependence of laser-induced carrier density. The anisotropic photocurrent response in MoS₂ shed light on the understanding of photocurrent generation as well as other fundamental physics in two-dimensional materials.

Many research groups have exhibited extensive research activities in two-dimensional (2D) materials, such as graphene, molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂), due to their unique material properties applicable to flexible electronic device. Graphene has high carrier mobility ge; 1000 cm²/Vs and high transmittance due to the thickness with 0.35 nm. Also, it has high flexibility because of very tightly bonded carbon atoms. However, graphene has a zero bandgap in pristine form without functionalization or structural modification like a ribbon shape, resulting in poor transistor performance. In order to solve these problems, transition metal dichalcogenide (TMDC), such as 2D MoS₂ and WS₂, has been researched as a channel for transistors due to its suitable bandgap. However these materials similar to graphene, mechanical exfoliation are the main method to form electric devices. And these methods are not compatible with current integrated circuit manufacturing processes at all. Furthermore, conventional chemical vapor deposition methods are difficult to apply flexible substrate due to its high process temperatures. From the device point of view, future flexible and wearable electronics need higher performance, lower processing temperature, less power consumption and flexibility.
Tin disulfide (SnS₂) with 2D structure can be a candidate to compete with current 2D materials. The 2D SnS₂ comprising earth-abundant constituents has S-Sn-S tri-atomic planar molecular arrangements with weak van der Waals bonding among molecules. The properties of exfoliated 2D SnS₂ with bandgap of 2.1-2.4 eV can lead to high performance transistors with large I_on/I_off and high mobility. However, this is not compatible with current device fabrication techniques.
In this study, we deposited single and few layers of 2D SnS₂ using Tetrakis(dimethylamino)tin and hydrogen sulfide at 150°C with thermal atomic layer deposition (ALD) method. ALD is atomic scale deposition technique using sequential processing and self-limiting, and it allows atomic scale deposition at low temperatures. Further, it has advantage of excellent uniformity over large area and precise #8491; level control of film thickness. Therefore, we conclude that ALD process is the most appropriate deposition method for 2D materials over large area. The thickness dependence of SnS₂ properties were analyzed by RAMAN, AFM, XPS. And the transistors using few layers of SnS₂ were fabricated and their electrical properties were investigated. More results will be presented in Meeting.

Recently, two-dimensional (2-D) materials have been extensively studied due to their unusual physical phenomena and their potential application in many electronic and optoelectronic devices. Among the various 2-D layered materials, tin sulfides (SnS₂, SnS) system has very interesting point, because each of those has two different types of 2-D layered structures with different electrical conduction types. Although many investigations have been performed in order to synthesize nanostructures of both SnS₂ and SnS, the synthesized crystals were too small to apply device fabrication, but also single crystals directly grown on substrate have not been reported, to the best of our knowledge. Here, we present a controlled synthesis of the polymorphous 2-D layered structures of SnS and SnS₂ directly on substrate via control of the growth atmosphere. The growth behavior is supported by the thermodynamic study. The transmission electron spectroscopy (TEM) results show that a few-layered SnS₂ and SnS crystal have hexagonal and orthorhombic symmetry, and the optical band gap measured by spectral photoresponsivity is 2.69 eV and 1.26 eV for SnS₂ and SnS, respectively. Moreover, our layered crystals shows n-type semiconductor characteristics for SnS₂, and p-type semiconductor characteristics for SnS, respectively. In addition to this, we demonstrate p-n heterojunction device based on these polymorphous 2-D system. The fabricated SnS₂-SnS heterojunction devices show the gate tunable rectifying characteristics, and show obvious photovoltaic effect across the p-n junction with EQE of 0.13 % at the wavelength of 405 nm. Furthermore, we also demonstrate CMOS inverter characteristics based on a SnS₂ n-FET and a SnS p-FET.

The rise of research in graphene spawned the recent growing interest in other two-dimension (2D) materials with low aspect ratio. Metal-thiolate nanosheet is an emerging 2D layered organometallic material with applications in nanolithography, molecular electronics and biophysics. Silver alkanethiolate (AgSCn) lamellar crystals with various number of layers can be systematically grown by a new layer-control synthesis method, with single layer nanosheet as the thinnest species (~2-5 nm) obtained. The single layer structure is confirmed by atomic force microscopy (AFM) and X-ray reflectivity (XRR). Nanocalorimetry (NanoDSC) is applied to measure the melting characteristics of 1-layer AgSC15. Its attribute of high sensitivity enables the characterization of single layer species. The fast heating (~50,000 K/s) and cooling (~10⁴ K/s) rates employed allow an in situ study of lamella layer evolution. By controlling the maximum temperature (T_max) achieved during heating/cooling cycles, the samples can be either melted or annealed. If T_max is larger than sample melting point (T_m), the first NanoDSC pulse shows the melting behavior of the as-synthesized nanosheet. The following rapid cooling (quenching) causes crystallinity loss. If T_max is smaller than T_m, electrical annealing takes place and partially recovers the quenched layered structure, but the melting enthalpy never reaches that of the first pulse. The successful measurement of the thermodynamic properties of single layer nanosheet highlights the capability of studying extremely thin materials via NanoDSC.
References:
1. Z. Ye, L. P. de la Rama, L. H. Allen, et al., Nanocalorimetry study of the evolution of melting characteristics of single layer silver alkanethiolate lamella: Fast heating/cooling and electrical annealing, Thermochimica Acta, In press (2014).
2. L. P. de la Rama, Z. Ye, L. H. Allen, et al., Size Effect and Odd-Even Alternation in the Melting of Single and Stacked AgSCn Layers, JACS, 135, 14286 (2013).
3. L. Hu, L. P. de la Rama, L. H. Allen, et al., Synthesis and Characterization of Single-Layer Silver Decanethiolate Lamella, JACS, 133 (12), 4367 (2011).

Materials of two-dimensional (2D) form have attracted much attention in recent research. A variety of single layer materials based on weak van der Waals interactions have been extracted from their bulk counterparts [1]. In addition to expanding 2D derivatives from layered materials, further development of novel 2D materials (and not limited to a layered structure) will expand the realm of 2D materials. Especially the transition metal oxides in the 2D limit can be a new platform for exploring new functionalities based on the degrees of freedom in charge, spin, and orbital [2]. Advances in oxide thin film growth allow us to create artificial heterostructures with atomically sharp interfaces [3], which can be applied to the fabrication of freestanding oxide layers. An epitaxial sacrificial layer and the oxide film of a few unit cells&’ thickness have been grown on traditional perovskite substrates (SrTiO₃) by pulsed laser deposition (PLD), and the sacrificial layer is selectively etched to create freestanding oxide layers which can be transferred to arbitrary substrates.
The large area, single crystalline oxide layer provides an intriguing system to study the well-known theoretical prediction suggesting the absence of crystalline phases in 2D lattices [4]. Here we report a structural study on freestanding layers of SrTiO₃, a perovskite material stable in ambient conditions. In a series of transmission electron microscopy experiments, we observe a structural transition from a single crystalline film to an amorphous layer, as the thickness of SrTiO₃ approaches the extreme 2D limit. This transition occurs despite the fact that the original unreleased epitaxial film is crystalline in all cases. These experimental studies will help to understand the fundamentals of long-range order from 2D to 3D, as well as developing materials design principles for 2D materials.
[1] K. S. Novoselov et al. Proc. Natl Acad. Sci. USA 102, 10451 (2005).
[2] Y. Tokura and N. Nagaosa, Science 288, 389 (2000).
[3] H. Y. Hwang et al., Nat. Mater. 11, 103 (2012).
[4] N. D. Mermin, Phys. Rev. 176, 250 (1968).

The preparation and characterization of two-dimensional (2D) and sheet-like materials have recently generated much excitement in the materials research community due to the extraordinary mechanical, electronic, and thermal properties these materials display, and the potential for revolutionizing device architectures. The layering of such quasi-2D building blocks of different materials in controlled sequences comprises a natural “next frontier,” as it provides further sophistication in materials design. Expanding on our recently established ability to prepare intergrowths of layered chalcogenides using the modulated elemental reactants approach, we demonstrate a preparation method for preparing more than 500 new compounds in the Sn-Mo-Se system by controlling the size and sequence of stacked SnSe and MoSe₂ layers by locally nucleating the individual component structures from designed thin film precursors. X-ray reflectivity, laboratory and synchrotron XRD, STEM and HRTEM imaging, and electron microprobe analysis provide conclusive evidence of the formation of layered intergrowths with well-defined structure and composition. Synchrotron X-ray diffraction, Raman spectroscopy, and first-principles calculations reveal a size-induced structural transition occurs in the SnSe component as the layer thickness is increased.

Recent theoretical and experimental studies have suggested that mono- and few-layer two-dimensional layered materials such as graphenes and molybdenum disulphide (MoS2) as novel materials with exceptional properties for applications including nanoelectronics, energy storage, fuel cells, and electrochemical sensing [1-4]. However, current production methods usually involve chemical reactions, ion intercalation and surfactants [5, 6], which introduce defects in the crystal structure of these materials. Development of surfactant free liquid phase exfoliation of these materials from bulk layered materials are still very limited and of low yield [7, 8]. Here we demonstrate the facile and scalable production of few-layer two-dimensional layered materials graphene, MoS2, tungsten disulfide (WS2) and boron nitride (BN) by cosolvent exfoliation. Systematic UV-visible spectroscopy was performed to investigate the exfoliation concentration for different cosolvent concentration and hence optimal cosolvent concentration for the effective exfoliation. We found that the cosolvency significantly influence the exfoliation yield of graphene and MoS2, which may be attributed to the large separation of exfoliated layers by the aggregates formed by strong hetero-association among solvent molecules through hydrogen bonding. This founding is further confirmed by the Fourier transform infrared (FTIR) spectroscopy characterization of mixed solvents. Detailed high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), micro Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) characterizations revealed that very low defects in the as-prepared samples were generated during the exfoliation. It is also noteworthy from a practical point of view that the developed cosolvent exfoliation method is amenable to industrial-scale production. [1] H. Hwang, et al., Nano Letters, 11, 4826, 2011. [2] Y. Liang, et al., Advanced Materials, 23, 640, 2011. [3] J. C. Tokash and B. E. Logan, Int. J. Hydrogen Energy, 36, 9439, 2011. [4] R. M. Westervelt, Science, 320, 324, 2008. [5] R. J. Smith, et al., Adv. Mater. 23, 3944, 2011. [6] S. Stankovich, et al., Carbon, 45, 1558, 2007. [7] J. N. Coleman, et al., Science, 331, 568, 2011. [8] U. Halim, et al., Nat Commun, 4, 2013.

Hexagonal boron nitride (h-BN) is primarily grown on various transition metals. Cu was chosen due to its low BN solubility which results in surface-mediated growth mechanisms. However, due to its unique binary atomic configuration, single crystal domains usually exist in the form of triangular-shaped due to the asymmetric N- and B-terminating edge energies, as differed from graphene. Recently, we observed for the first time hexagonal-shaped domains, which involved the presence of alternating N- and B-terminating edges. These hexagons were only grown on highly electropolished Cu. Although the mechanisms to achieve hexagonal-shaped domains remained elusive, we further investigate the influence of Cu surface roughness and orientation towards the growth of h-BN single crystals and its effect on the evolution from triangular-shaped into a hexagonal-shaped domain.

The synthesis of atomically thin molybdenum disulfides (MoS₂) with layer controllability and wafer-scale uniformity is an essential for their application in electronic and optical devices. In this regards, several studies have been reported to prepare atomically thin MoS₂ nanosheets, including exfoliation, sulfurization of Mo and MoO₃ thin films and chemical vapor deposition (CVD) using MoO₃ and S powder. However, these methods have insufficient uniformity in large area and lack in layer controllability. Therefore, an improved synthesis process for atomically thin MoS₂ nanosheets with exact number of layers controllability and large area uniformity is required.
In this work, we describe a process for the synthesis of MoS₂ nanosheets using atomic layer deposition (ALD). ALD MoS₂ nanosheets show wafer scale area (1.5x9 cm²) uniformity (up to 95%) and layer controllability from mono- to tri-layer. The X-ray photoemission spectroscopy, Raman, photoluminescence and transmission electron microscopy measurements exhibit that the ALD MoS₂ nanosheets have good stoichiometry, clear Raman shift and bandgap dependence as a function of the layer numbers, and honeycomb-like structure. The electrical properties of the monolayer ALD MoS₂ nanosheet are measured using a field-effect transistor (FET) with a bottom SiO₂ gate insulator. The electron mobility and on/off current ratio of monolayer ALD MoS₂ are comparable to that of CVD grown MoS₂.

Since the discovery of graphene, two-dimensional layered materials, especially the transition metal dichalcogenide (TMD), have drawn lots of attention for the study of new sciences and technological applications. Unlike the semi-metal graphene, molybdenum disulfide (MoS₂) is one of the TMD family that has a semiconductor behavior with a band gap between 1~2 eV depending on its thickness. Single-layer MoS₂ exhibits a high on/off current ratio exceeding 10⁸ and a room-temperature carrier mobility of around 10² cm²/V-s which is comparable to silicon thin film. Although MoS₂ has shown its fascinating intrinsic properties for electronics, the interface between the contact metal and itself may severely limit the device performance. However, the surface/interface properties of MoS₂ are still vague so far. S. McDonnell et al. addressed that the intrinsic defects in MoS₂ dominate the metal/MoS₂ interface resistance and provide a small Schottky barrier independent of metal contact work function. C. Gong et al. proposed that the partial Fermi level pinning occurred at the metal/MoS₂ interface due to the metal work function modification and the production of gap states. In our study, we reported for the first time the changes in surface states in few-layered polycrystalline MoS₂ thin film with various thicknesses grown on the SiO₂/Si substrate. Based on the XPS analyses, the Mo-3d and S-2p characteristic peaks all shift toward low binding energy as thickness increases, but the carbon, silicon (substrate), and oxygen (substrate) do not show such tendency. This phenomenon could be attributed to the inhomogeneous surface band bending resulting from the step-edge effects on MoS₂ surface as seen in the AFM images. We believe our finding could help clarify the surface/interface properties of MoS₂ and facilitate the development of MoS₂-based electronics.

Two-dimensional, layered van-der Waals materials such as MoS₂ are of fundamental as well as practical interest for use in future device applications in the areas of electronics, spintronics, optoelectronics, solar energy conversion, and sensing [1]. The properties of such atomistic thin nanomembranes are strongly influenced by their interactions with their environment and substrate. In particular, the modification of supporting substrates by interfacial chemistry is a critical aspect for many applications. In order to improve the performance of MoS₂-based nanodevices, modulation of interlayer screening effects can be achieved by tuning the electrostatic potential between the substrate and MoS₂ flakes [2]. We use organophosphonate chemistry as an alternative to siloxane chemistry for the controlled modification of silicon-based support substrates [3]. In particular, two aromatic phosphonic acids, benzylphosphonic acid and 9,10-diphenyl-2,6-diphosphonoanthracene were used to fabricate homogeneous self-assembled monolayers (SAMs) on a silicon oxide support. Electron irradiation was then used to crosslink the aromatic SAMs on the SiO₂ support surface, creating nanopatterns with minimal layer thickness. We find that the interaction of single layered MoS₂ with these substrates results in a significant redshift of the Raman-active out-of-plane vibration mode of the two-dimensional crystal. This finding points towards a reduction of the substrate induced n-type doping of MoS₂ membranes on aromatic SAMs and clearly indicates that organophosphonate interfacial chemistry can play an important role in controlling the performance of 2D materials.
[1] Q.H. Wang et al. Nature Nanotech. 7, 699 (2012).
[2] Y. Li, C-Y Xu, PA Hu, L. Zhen, ACS Nano 7, 7795 (2013).
[3] a) M. Auernhammer, S. J. Schoell, M. Sachsenhauser, K.-C. Liao, J. Schwartz, I. D. Sharp, A. Cattani-Scholz, Appl. Phy. Letters 100, 101601 (2012); b) R. Caterino, R. Csiki, M. Wiesinger, M. Sachsenhauser, G. Speranza, S. D. Janssens, K. Haenen, M. Stutzmann, J. A. Garrido, A. Cattani-Scholz, ACS Appl. Mater. Interfaces 6 (16), 13909 (2014).

Two-dimensional (2D) layered semiconductors have attracted much attention in recent years for highly-scaled transistors and flexible electronics [1, 2]. However, only zero band gaps like graphene or relatively wide gaps like MoS₂, WSe₂, WS₂ (1.4-1.8 eV) [2, 3] have been available among 2D materials initially studied. Intermediate band gaps, preferably in the range of Ge and Si (0.6-1.1 eV), would be better suited for devices with high current density and low power operation valued in modern electronics.
Here, we report on the use of chemical vapor transport (CVT) techniques to grow high-quality, millimetric bulk crystals of the Group IV selenides (HfSe₂, ZrSe₂) and Group V tellurides (MoTe₂, WTe₂), spanning projected monolayer band gaps between 0.4 to 1.2 eV. Mechanical exfoliation is then used to isolate mono- to few-layer flakes, characterized by Raman and Energy Dispersive X-ray (EDX) spectroscopy. For HfSe₂ and ZrSe₂, we monitor thin-film oxidation as a function of time and processing parameters via Auger Electron Spectroscopy (AES) and Transmission Electron Microscopy (TEM). We identify strategies for air-stabilization of few-layer flakes, as well as prospects for high-performance layered semiconductors with native high-dielectric-constant oxides (high-κ HfO_x, ZrO_x), long pursued for CMOS integration.
For MoTe₂ and WTe₂, we demonstrate facile bulk crystal growth from commercially-available molecular precursors. Sub-850 0C growth of MoTe₂ gives the semiconducting α form (-2H; MoS₂ structure), from which few-layer transistors are fabricated on SiO₂(90 nm)/Si substrates using a number of contact metals and processing conditions. Drive current densities exceeding tens of µA/µm are achieved, including both ambipolar and unipolar current response for complementary logic. Conversely, WTe₂ is found only in a metallic -1T&’ distortion of the CdI₂ structure, with a complex Raman response, supporting intrinsic current densities up to MA/cm². Prospects for phase-engineering between metallic and semiconducting forms of the strain-engineered tellurides are discussed, and the experimental results are further elucidated in conjunction with electronic structure calculations based on density functional theory (DFT).
[1] K. Majumdar, C. Hobbs, and P.D. Kirsch, IEEE EDL, 35, 402 (2014)
[2] B. Radisavljevic et al., Nature Nanotechnology6, 147 (2012).
[3] H. Fang et al., Nano Lett.12, 3788 (2011).

Raman spectroscopy has already been shown to be a powerful tool for quantitatively probing the level of disorder in graphene¹, however, a systematic investigation of defective single-layer molybdenum disulphide (1L-MoS₂) has not been performed. MoS₂ is another two-dimensional (2-D) material that has exciting properties, and its semiconducting nature may lead to its inclusion in application areas where graphene will not be suitable. To this end, high-throughput quantification of defect density for this material is of commercial interest as the deterministic introduction of defects can be exploited to engineer the properties of 1L-MoS₂. For example, vacancy defects can enhance the photoluminescence², and the presence of edge-sites can improve the reactivity of MoS₂ for electrocatalysis applications³.
Here, we report the evolution of Raman spectrum for increasingly disordered 1L-MoS₂ (Fig.1), where ion-bombardment was used to controllably introduce vacancy defects. An increase in ion dose results in the observation of specific Raman signatures, which were correlated to the level of disorder. The material were chemically and structurally characterised with corroborating techniques and supported by density functional theory (DFT) calculations of the phonon dispersion and phonon density of states.
1. M. M. Lucchese, F. Stavale, E. H. Martins Ferreira, C. Vilani, M. V. O. Moutinho, R. B. Capaz, C. A. Achete and A. Jorio, Carbon48, 1592-1597 (2010).
2. S. Tongay, J. Suh, C. Ataca, W. Fan, A. Luce, J. S. Kang, J. Liu, C. Ko, R. Raghunathanan, J. Zhou, F. Ogletree, J. Li, J. C. Grossman, and J. Wu, Scientific Reports 3, 2657 (2013).
3. Jakob Kibsgaard, Zhebo Chen, Benjamin N Reinecke, and Thomas F Jaramillo, Nature materials11, 963 (2012).

Sitting under the same column in the periodic table, carbon and silicon have many things in common, e.g. a diamond-cubic structure due to sp3 hybrid. They, however, begin to differ with the size and/or dimension reduced [1, 2]. For example, carbon shows sp2 hybrid making a flat plane called graphene the two-dimensional ground state, compared to a buckled one called silicene for silicon. Rich physics of silicene are yet to be explored, and the mechanics is one of the critical properties for protection against a mechanical failure when integrated into electromechanical and optoelectronic systems.
Here we report the mechanical properties of silicene under tension and compression from first principles calculations, and discuss the relationship between the electronic structures and mechanical properties. In particular, the local environment is compared across dimensionality and connected to the chemical bonding of one-dimensional to three-dimensional structures in silicon. We have found that the so-called bond-order effects are not as significant as believed in the past.
In addition, we propose a multiscale-modeling process, in which the mechanics predicted from first principles is projected to empirical potentials that are to be used in large-scale atomistic calculations. Tersoff potential is examined as a prototype, and a one-to-one relationship between material properties and potential parameters is exploited. We have found that transferability can be improved with a careful choice of the fitting database such that the prediction outside the training set is effectively an interpolation than an extrapolation from the training set. Finally, the fundamental limitations inherent from the selection of a potential function are introduced and the future direction is discussed.
Acknowledgment
This research was supported by Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (2010-0025566).
References
[1] R. T. Senger et al., Phys. Rev. B 72, 075419 (2005).
[2] S. Cahangirov et al., Phys. Rev. Lett. 102, 236804 (2009).

Exfoliated boron nitride nanosheets (BNNSs) have received an increasing amount of attention in recent years on account of a range of attractive intrinsic materials properties. These include high mechanical strength, high chemical and thermal stability, gas diffusion barrier and radiation shielding properties, as well being electrically insulating. It is clear however, that in order to efficiently utilize BNNSs in a range of technological applications that chemical functionalization strategies will be required to compatibilize and integrate BNNSs within solvent systems, molecular and condensed phase materials. Therefore there exists a growing demand to explore controlled functionalization strategies for BNNSs which creates hybrid functionality or facilitate dispersion and integration.
In this work, we report the covalent functionalization of exfoliated boron nitride nanosheets (BNNSs) with reactive dye molecules and quantum dots. Initial functionalization of the BNNSs is achieved by introducing hydroxyl groups to B atoms in the sp² hybridized BN lattice. Hydroxyl groups were derivatized with both conventional color and fluorescent dye molecules, while extended covalent coupling facilitated the attachment of terminal functionality to enable self-assembly of quantum dot species. The introduction of dye species to the BNNSs imparts color to the white material in the bulk phase, which is speculated to offer application as a filler material in composite and printing applications where color coded nanomaterials may be utilized. This approach is further extended by demonstrating the covalent functionalization of BNNSs with fluorescent dye molecules. By these means, fluorescent imaging for BNNSs is made possible which is investigated using high resolution confocal imaging in order to examine the spatial nature of the functionalization on the BNNSs. Building on molecular coupling approaches, coupling of condensed phase quantum dots to BNNSs was achieved by directing self-assembly of the species in solution via functional molecules coupled to the BNNS lattice. Controlled decoration of the nanosheets was demonstrated by manipulation concentration. The Implications of both molecular and quantum dot functionalization of BNNSs is considered in the context of developing sensor platforms and diagnostic materials.

Black phosphorus (BP) is a thermodynamically stable allotrope of phosphorus. Similar to graphite and transition metal dichalcogenides, it presents a lamellar structure, with bonds presenting a high Van der Waals character along one crystal axis. This structure then allows one to obtain two-dimensional crystals by mechanical exfoliation. Indeed, recently this technique has been used to produce few-layer black phosphorus samples, which have been named phosphorene [1-4]. Since its crystal structure is highly anisotropic, presenting valleys that run along one axis in the basal plane, it is convenient to study the optical properties as a function of polarization of the incident and scattered light. This work presents Raman spectroscopy measurements in mechanically exfoliated black phosphorus crystals on a silicon substrate. Both the incident and analyzed polarizations are varied relative to the crystal&’s orientation. Results are analyzed using group theory and the theory of Raman scattering in crystals [5]; through these, we confirm the crystal&’s symmetry and identify its orientation. More importantly, we observe and
explain an anomalous behavior of the Ag2 Raman mode with polarization. In addition to the polarization dependence measurements, Raman images were also made and changes in the Raman spectra were observed near defects. We attribute this effect to translational symmetry breaking in the sample.
[1] XIA, F., WANG, H., and JIA, Y. Nat. Commun. 5, 4458 (2014).
[2] BUSCEMA, M., GROENENDIJK, D. J., BLANTER, S. I., STEELE, G. A., ZANT, H. S. VADER, and CASTELLANOS, A., Nano Lett. 14, (6) 3347-3352 (2014).
[3] KOENIG, S.P., DONOGANOV, R. A. SCHMIDT, H., CASTRO NETO, A. H., and ÖZYILMAZ, B., Appl. Phys. Lett. 104, 103106 (2014).
[4] ZHANG, S., YANG, J., XU, R., WANG, F., LI, W., GHUFRAN, M., ZHANG, YONG-WEI, YU, Z., ZHANG, G., QIN, Q., AND LU, Y., ACS Nano, 8 (9), 9590-9596 (2014)
[5] LOUDON, R. Adv. Phys. 13, (52) 423-482 (1964).

When two-dimensional materials with different lattice constants stack on each other, a heterostructure is formed with a periodic supercell structure called moiré pattern. The friction between the two materials can be very low and this brings a phenomenon of superlubrication into the system ^[1], which has a vast application prospect in the field of micro-nano technology. Using contact mode of an atomic force microscope (Veeco Multimode V), we conducted a series of experiments on a monolayer graphene/h-BN heterostructure sample. The samples are obtained from Prof. Xiaoming Xie and Haomin Wang's group in Shanghai, which consists of graphene monolayer grown on h-BN with the method of chemical vapor deposition (CVD) as described in Ref. [2].
The friction was measured between the probe tips and the graphene/h-BN heterostructure. Hexagonal moiré pattern is clearly visible for the graphene islands with a periodicity of roughly 14 nm, which exactly matches the theoretical calculation of 13.78 nm when the lattice vectors of graphene is parallel to the lattice vectors of h-BN. Atomic resolution stick-slip behavior with graphene lattice periodicity is also detected. The atomic scale friction between the probe tips and graphene/h-BN is larger than that between probe tips and the h-BN substrate. The lateral force also show a long-period stick-slip like variation with a periodicity comparable to the size of moiré pattern showing that the force increases slowly and then decreases suddenly along the scanning direction. The images of trace and retrace scanning are not perfectly coincided but with a certain amount of offset. We attribute this phenomenon to the periodic deformation of graphene during the tip scanning process in contact mode. It has been observed that for the moiré pattern with the periodicity of 14 nm, the narrow regions along the edges of the hexagon shows high Young&’s modulus, and on the contrary, Young&’s modulus is relatively low in the large areas in the centre of the moiré pattern ^[3]. This inhomogeneity in the mechanical properties in heterostructure may account for the intermittent deformation during sliding.
[1] Wang L F, Ma T B, Hu Y Z, et al. Superlubricity of two-dimensional fluorographene/MoS2 heterostructure: a first-principles study [J]. Nanotechnology, 2014, 25(38): 385701.
[2] Tang S, Wang H, Zhang Y, et al. Precisely aligned graphene grown on hexagonal boron nitride by catalyst free chemical vapor deposition[J]. Scientific reports, 2013, 3.
[3] Woods C R, Britnell L, Eckmann A, et al. Commensurate-incommensurate transition in graphene on hexagonal boron nitride [J]. Nature Physics, 2014, 10(6): 451-456.

Hexagonal boron nitride (BN) nanosheets, structurally analogous to graphene nanosheets but electrically insulating, are the perfect sidekicks of graphenes. They are preferable to standard SiO₂ substrates for graphene-based electronics in exhibiting exciting physics of graphene and are able to enhance the carrier mobility or to tune the bandgap of graphenes. Meanwhile, BN nanosheets are ideal fillers for insulating thermoconductive polymeric composite due to the high thermal conductivity of the 2D crystal. However, dozens of gram of high quality BN nanosheet production is still a big challenge, limiting the further studies and applications of BN nanosheets. Herein we developed a biomass-directed carbothermal reduction procedure for high-throughput production of high-crystal-quality BN nanosheets. Single-crystalline BN nanosheets with regular morphologies, were spatially converted on the original biomass sites. A high yield of 20 g can be achieved in a single experimental run. The BN nanosheets filled epoxy composites shows a 14-fold increase in thermal conductivity compared to blank epoxy, which are very promising heat-releasing and electrically-insulating packaging materials.
References:
X.B. Wang, Q.H. Weng, X. Wang, X. Li, J. Zhang, F. Liu, X.F. Jiang, H.X. Guo, N.S. Xu, D. Golberg, Y. Bando, ACS Nano, 8, 9081-9088 (2014).

Although MoS₂ is the most widely studied transition metal dichalcogenide (TMD) with highly acclaimed electrical properties, performance inadequacies such as large variations in threshold voltage (V_th) and significant source/drain contact resistance for MoS₂ transistors represent the present-day reality.
In this talk, we will first discuss to develop the methods to bi-directionally tune the V_th of MoS₂ transistors, which is achieved by performing minor stoichiometry change in the MoS₂ surface. We propose to perform either sulfur or hydrogen surface treatments on MoS₂ such that the amount of sulfur vacancies in the basal plane reduces or increases, respectively, which in turn results in right and left shifting of the V_th of MoS₂ transistors, respectively. It is worth noting that our transistors fabricated on sulfur-treated MoS₂ flakes show a very positive V_th with 2-fold improvement in electron mobility compared to that of pristine MoS₂ flakes. Remarkably, the V_th of such transistors can be left-shifted to a smaller value by our proposed hydrogen treatment without any degradation in terms of electron mobility, on/off current ratio and subthreshold swing. PL and XPS studies are presented to elucidate the physical changes in the MoS₂ surface that give rise to the V_th shifting. First principles calculations were performed to quantitatively explain the phenomenon observed.
In addition, we will also discuss approach to achieve low resistance metal contacts to MoS₂ with the use of Ni-etched-graphene as source/drain electrodes for MoS₂ transistors, instead of pure metals. The proposed fabrication scheme of Ni-etched-graphene electrodes optimizes both metal-graphene and graphene-MoS₂ interfaces and hence we have measured ultralow contact resistance (262 #8486;.µm) to MoS₂, which aligns with the ITRS requirement at the 22 nm device technology node. In particular, optimization of the metal-graphene interface is achieved through realization of “end-contacted” metal-graphene contacts (ACS Nano 2014, 8 (1), 994-1001). Our MoS₂ transistors with Ni-etched-graphene-MoS₂ contacts exhibit performance enhancement in terms of mobility (3-fold), on/off current ratio and subthreshold swing over standard metal-MoS₂ contacts. First-principles calculations indicate that the much reduced contact resistance arises from the lower work function in the Ni-graphene electrodes, together with the much weaker, van der Waals interaction between the Ni-graphene electrodes and MoS₂ (compared to strong covalent bonding for Ni directly contacted with MoS₂).
Remarkably, all of the above-mentioned approaches are both facile and CMOS compatible, and hence represent viable processes in the development of device technology.

Graphitic carbon nitride (g-C₃N₄) has received great attention as a fascinating semiconductor material with a unique 2D-structure. Owing to an appropriate band gap of 2.7 eV, g-C₃N₄ shows excellent performance in photocatalytic reactions. Moreover, with high thermal and chemical stability, g-C₃N₄ exhibits a promising prospect on inexpensive metal-free visible-light photocatalysis. Therefore, it is important to obtain high quality g-C₃N₄ and study its optical properties. The main synthesis method of g-C₃N₄ is direct annealing of reagent such as cyanamide, dicyandiamide, or melamine in air. However, this one-step method has very low yield because polymerization temperature is higher than the sublimation point of reagent and the reagent may escape before reaction occurs. Here we demonstrate the high-yield synthesis of g-C₃N₄ via the encapsulated high temperature synthesis. By sealing the reagent in a vacuum ampoule, the average yield could be improved to as high as 68%. In order to understand the synthesis mechanism and produce g-C₃N₄ with high quality and purity, we obtained the samples under a variety of temperatures (from 450°C to 650°C). In non-optimal annealing of melamine, intermediate and overheated products may also coexist with g-C₃N₄. With comprehensive characterizations by thermal gravity analysis (TGA), X-Ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), we can distinguish the components of the product and find the optimal condition for g-C₃N₄ preparation. Finally, optical spectroscopy investigations including photoluminescence, infrared and Raman spectroscopy shed light on the growth mechanism and their optical properties for excellent photocatalysis.

Artificial deoxyribonucleic acid (DNA) nanostructures are currently being used in various nano-scale research fields including spintronics, nanoelectronics, biosensors, and nanophotonics. Due to the incredible flexibility of DNA base sequence design, diverse nanomaterials like proteins, nanoparticles (NPs), nanowires (NWs), and metal ions have been successfully attached to DNA nanostructures by simply modifying molecules in the DNA. However, most research related to DNA-based nanostructures is stalled in the growth/synthesis stage and cannot yet be incorporated into the fabrication of electronic and optoelectronic devices.
Meanwhile, transition metal dichalcogenide (TMD) materials with two-dimensional (2D) layered structures are considered promising materials for next-generation wearable, flexible, stretchable, and transparent electronics due to their superior electrical, optical, and mechanical properties. Since controllable doping methods like ion implantation can cause fatal crystal damage to 2D TMD materials, resent research has focused on developing a safe doping method that avoids crystal damage and allow for the successful integration of TMD-based 2D electronic and optoelectronic devices. Although theses doping method protected the crystallinity of TMD materials, they all are in a high-level doping concentration regime (degenerate), treating TMD materials as a near-metallic layer. In this light, a non-degenerate light doping technique (where TMD materials serve as semiconductors) that does not cause crystal damage is crucial for the design and fabrication of TMD-based 2D electronic and optoelectronic devices.
Here, we report a non-degenerate doping phenomenon for TMD materials (MoS₂ and WSe₂, which represent n- and p-channel materials, respectively) using DNA and slightly modified DNA by metal ions (Zn²⁺, Ni²⁺, Co²⁺, and Cu²⁺), named as M-DNA. This study is an example of interdisciplinary convergence research between DNA nanotechnology and TMD-based 2D device technology. The phosphate backbone (PO₄^-) in DNA attracts and holds hole carriers in the TMD region, n-doping the TMD films. Conversely, M-DNA nanostructures, which are functionalized by intercalating metal ions, have positive dipole moments and consequently reduce the electron carrier density of TMD materials, resulting in p-doping phenomenon. The observed doping levels are in the non-degenerate regime, allowing for the proper design of performance parameters of TMD-based electronic and optoelectronic devices (V_TH, on-/off-currents, field-effect mobility, photo-responsivity, and detectivity). In addition, by controlling the metal ions used, the p-doping level of TMD materials, which also influences their performance parameters, can be controlled. This interdisciplinary convergence research will allow for the successful integration of future layered semiconductor devices requiring extremely small and very complicated structures.

Two dimensional (2D) materials such as graphene, hexagonal boron nitride (h-BN), and Transition Metal Dichalcogenides (TMDC) (eg. MoS2, MoSe2, WS2, WSe2), have attracted great interest during the past decade, ever since the demonstration of mechanical exfoliation of these materials from their three dimensional bulk counterparts. Among the 2D materials, the semiconducting TMDCs have an intrinsic bandgap between 1.5 to 2.5 eV, which makes them ideal candidates for electronic devices. While monolayer TMDCs have direct band gap, their multilayer (ML) counterparts are indirect semiconductors, resulting in suppressed photoluminescence (PL). Monolayers, while direct band gap, have small optical densities, which limits their potential use in practical optical devices. To circumvent this issue, recent attempts at engineering an indirect to direct bandgap crossover in the ML-TMDCs have focused on doping, defect engineering, and intercalation. The partial intercalation method, in particular, is tremendously advantageous for device applications. It reduces the inter-layer overlap of electronic states in ML-TMDCs by modulating the vdW-gaps between the layers, which can lead to an indirect-to-direct transition of the bandgap that is desirable for optoelectronic applications
In this study, we study the modulation of the vdW gap and bandstructure in few-layer MoS2 and WS2 via partial intercalation of oxygen into the vdW gaps of these materials. Changes in the film thickness and PL spectra due to partial intercalation are analyzed using ab-initio simulations. Calculations employing hybrid functionals combined with semi-empirical corrections to account for vdW interactions are used to quantitatively compare to experimental results. It was found that, as the layer thickness increases from 2 layers to 4 layers, the indirect to direct transition vdW gap distance increases by 45%. A symmetric increase of the vdW gap distance by 30% with respect to the equilibrium distance between each monolayer, is sufficient to observe the indirect to direct crossover in the bandgap. This observation is consistent with our recent experimental observation.
This research was supported by DOE Award No. DE-FG02-07ER46376, National Science Foundation (NSF) Grant Nos. 1124733 and 1128304 and FAME, one of six centers of STARnet, a Semiconductor Research Corporation program sponsored by MARCO and DARPA. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by NSE, Grant No. OCI-1053575, and Information Technology at Purdue University, West Lafayette, IN, USA.

The synthesis of hexagonal boron nitride (h-BN) has recently been the subject of an intense research effort. This has in large part been driven by the suitability of h-BN for integration into heterostructures with other 2-dimensional materials, such as graphene.¹ We report the synthesis of h-BN on polycrystalline Ni foils by molecular beam epitaxy (MBE) from elemental B and N. The precise control over growth conditions, such as pressure, substrate temperature, and precursor flux offered by MBE make it a valuable tool for fundamental studies of h-BN growth. In this study, samples were synthesized at substrate temperatures ranging from 520°C to 750°C. The presence of a well-ordered, crystalline h-BN film on the Ni foil substrate was first confirmed using Raman spectroscopy, which revealed a sharp, symmetric peak at 1360 cm^-1. This peak arises from the E_2g-symmetric, doubly degenerate, in-plane optical phonons of h-BN.² The surface morphology resulting from this growth procedure was evaluated using atomic force microscopy (AFM), which also allowed the continuity of the atomically thin h-BN films to be confirmed. In addition to the topographic features of the underlying Ni foil (such as terraces and step edges), a cellular array of linear features was easily discernible which we identified as wrinkles in the h-BN film. The ubiquity of the wrinkle structure in numerous AFM scans, together with the uninterrupted observation of the h-BN Raman signal, offer strong evidence of a continuous h-BN film. Using shorter duration growths we were able to gain insight into the nucleation and growth behavior of h-BN prior to the formation of a closed film. According to scanning electron microscopy (SEM) images, the morphology of sub-monolayer h-BN islands evolved from star-shaped to much larger compact triangles with increasing substrate temperature. SEM micrographs also clearly showed points of increased contrast at the approximate geometric center of the islands, suggesting the h-BN nucleated heterogeneously.
1. Dean, C. R. et al. Boron nitride substrates for high-quality graphene electronics. Nat. Nanotechnol.5, 722-6 (2010).
2. Reich, S. et al. Resonant Raman scattering in cubic and hexagonal boron nitride. Phys. Rev. B71, 205201 (2005).

Hybrid heterojunctions of conjugated polymers and inorganic nanomaterials offer numerous advantages for good performances of solar cells (SC) [1]. Despite their promising properties one main factor limiting the performances of polymer/QDs SC is the poor charge transport between the QDs in blends due to long organic ligands [2]. Finding an efficient ligand exchange process of QDs incorporated in blends with polymers is still a challenge. Alternative non toxic inorganic nanomaterials which do not require ligands during and after their synthesis are tungsten disulfide (WS₂) nanotubes (NTs) [3,4].
In this work for the first time we have explored the possibility to use WS₂ both as only acceptor material blended with a polymer and in systems of polymer and QDs to improve the performance of this system [5]. We present spectroscopic investigatigation of binary blends, of poly(3-hexylthiophene) (P3HT) and WS₂ NTs (P3HT/WS₂), and ternary blends of P3HT and cadmium selenide (CdSe) QDs and WS₂ NTs (P3HT/CdSe/WS₂). We report and discuss fluorescence quenching effects in ternary system P3HT/CdSe/WS2 due to NTs addition to P3HT and QDs blends. Very interestingly static and time-resolved fluorescence support an efficient energy transfer from the QDs to the WS₂ NTs in ternary blend, responsible of fluorescence quenching. The evidence of energetic interaction between WS₂ NTs and QDs opens new fields of application to WS₂ NTs. Moreover, our results bring new insights into the physics of hybrid blends and promising improvements of the hybrid SC performances.
References
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M. C. Beard, A. G. Midgett, M. C. Hanna, J. M. Luther, B. K. Hughes and A. J. Nozik, Nano Lett. 2010, 10, 3019.
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R. Tenne, L. Margulis, M. Genut and G. Hodes, Nature, 1992, 360, 444.
A Bruno, C Borriello, SA Haque, C Minarini, T Di Luccio PCCP 2014, 16, 17998

Single-layers of two-dimensional Mo- and W-dichalcogenide compounds differ from graphene in an important respect: they can potentially exist in more than one crystal structure. Some of these monolayers exhibit hints of a poorly understood structural metal-to-semiconductor transition with the possibility of long metastable lifetimes. If controllable, such a transition could bring an exciting new application space to monolayer materials. We have discovered that mechanical deformations provide a route to switching thermodynamic stability between a semiconducting and a metallic crystal structure in some of these monolayer materials. Our DFT-based calculations reveal that single-layer MoTe₂ exhibits a phase boundary at a few percent tensile strain, potentially accessible with flexible substrate approaches. The potential application space for this work ranges from information and energy storage to electronic and optical electronic devices.

Single layer transition metal dichalcogenides are 2D semiconducting systems with unique electronic band structure. Two-valley energy bands along with strong spin-orbital coupling lead to valley dependent career spin polarization, which is the basis for recently proposed valleytronic applications. These systems also exhibit unusually strong many body affects, such as strong exciton and trion binding, due to reduced dielectric screening of Coulomb interactions. There is not much known about the impact of strong many particle correlations on spin and valley polarization dynamics in these systems. Here we report direct measurements of ultrafast valley specific relaxation dynamics in single layer MoS₂ and WS₂. We found that excitonic many body interactions significantly contribute to the relaxation process. Biexciton formation reveals hole valley/spin relaxation time. Our results suggest that initial fast intervalley electron scattering and electron spin relaxation leads to loss of valley polarization for holes through an electron-hole exchange mechanism.

The recent emergence of two-dimensional transition metal dichalcogenides (TMDs) provides a new laboratory for exploring the internal quantum degrees of freedom of electrons for new electronics [1]. These include the real electron spin and the valley pseudospin that labels the degenerate band extrema in momentum space. The generation and control of spin and valley pseudospin currents are at the heart of spin and valley based electronics. We will discuss two mechanisms for generating spin and valley currents of electrons in 2D transition metal dichalcogenides: (I) the valley and spin Hall current arising from the Berry curvatures [2,3]; and (II) the nonlinear valley and spin currents arising from Fermi pocket anisotropy [4]. The two effects have distinct scaling with the field and different dependence of the current direction on the field direction and crystalline axis. We discuss the possibility to observe and distinguish the two effects as distinct patterns of polarized electroluminescence at pn junction in monolayer TMDs. We show that the nonlinear current response allow two unprecedented possibilities to generate pure spin and valley flows without net charge current, either by an AC bias or by an inhomogeneous temperature distribution. This points to a new route towards electrical and thermal generations of spin and valley currents for spintronic and valleytronic applications. We will also discuss the valley Hall effect of charged excitons in monolayer TMDs. The exchange interaction between the electron and hole constituents of the exciton gives rise to an effective coupling of the excitonic valley pseudospin to its center of mass motion, which results in a valley Hall effect [5]. The valley Hall effect of charge excitons can be detected from the light emission with contrasted circular polarization on the opposite edges, which leave behind valley and spin polarized electrons.
The work is supported by the Croucher Foundation under Croucher Innovation Award, and the Research Grant Council of Hong Kong (HKU705513P, HKU17305914P, HKU9/CRF/13G).
Ref:
[1] X. Xu, W. Yao, D. Xiao and T. F. Heinz, Spins and pseudospins in layered transition metal dichalcogenides, Nature Physics 10, 343 (2014).
[2] D. Xiao, W. Yao and Q. Niu, Valley Contrasting Physics in Graphene: Magnetic Moment and Topological Transport, Phys. Rev. Lett. 99, 236809 (2007).
[3] D. Xiao, G. Liu, W. Feng, X. Xu and W. Yao, Coupled spin and valley physics in monolayers of MoS2 and other group VI dichalcogenides, Phys. Rev. Lett. 108, 196802 (2012).
[4] H. Yu, Y. Wu, G. Liu, X. Xu and W. Yao, Nonlinear valley and spin currents from Fermi pocket anisotropy in 2D crystals, Phys. Rev. Lett. 113, 156603 (2014).
[5] H. Yu, G. Liu, P. Gong, X. Xu and W. Yao, Dirac cones and Dirac saddle points of bright excitons in monolayer transition metal dichalcogenides, Nature Communications 5, 3876 (2014).

Phosphorus is one of the most abundant elements preserved in earth, constructing with a fraction of ~0.1% of the earth crust. In general, phosphorus has several allotropes. The two most commonly seen allotropes, white and red phosphorus, are widely used in explosives and safety matches. In addition, black phosphorus, though rarely mentioned, is a layered semiconductor and has great potentials in optical and electronic applications. Remarkably, this layered material can be reduced to one single atomic layer in the vertical direction owing to the van der Waals structure, known as phosphorene, where the physical properties can be tremendously different from its bulk counterpart. In this talk, we trace back to the 100 years research history on black phosphorus from the synthesis to material properties, and extend the topic from black phosphorus to phosphorene. The physical and transport properties are highlighted, aiming at further applications in electronic and optoelectronics devices.

The recent interest in thin semiconducting layered transition metal dichalcogenides (TMDCs) has in part been boosted by their fascinating optical properties. Single-layer MoS₂, WS₂, WSe₂ and MoSe₂ have very similar optoelectronic characteristics: a direct bandgap in the monolayer regime at the K valley, strong excitonic effects and coupled valley and spin degrees of freedom due to their peculiar crystal symmetry. In these materials, one can optically excite carriers exclusively in the K or the K&’ valley by near-resonant, circularly polarized illumination. Here, we perform time-resolved Kerr rotation (TRKR) measurements on mechanically exfoliated single-layer MoS₂ and WS₂ in order to probe the valley dynamics for different excitonic features at low temperatures. In this two-beam setup, a circularly polarized pump beam excites a valley polarization in the sample. The change in the linear polarization of the probe beam having the same energy as the pump beam is used to extract the valley polarization degree in the singlelayer TMDC for different time delays between the two beams.
The samples are first characterized by photoluminescence measurements in order to assign the different many-particle-states present in the flakes. For this purpose, we also transfer TMDC monolayers onto substrates with prestructured Ti:Au contacts via an all-dry deterministic transfer process in order to apply a gate voltage. Excitons, charged excitons and surface-impurity bound excitons can be identified. The TRKR technique reveals valley lifetimes of about 20 to 60 ps at a temperature of 4 K in singlelayer MoS₂ for resonant excitation. At higher temperatures, we observe a dramatic decrease of the valley lifetimes, indicating increased intervalley scattering and spin-flip processes in addition to a decreased carrier lifetime. The MoS₂ valley lifetime at low temperatures can be significantly enhanced by mild annealing of the samples (250° C in vacuum). This suggests a dominant role of surface impurities during valley depolarization. For charged excitons in single-layer WS₂ we measure similar valley lifetimes as in the MoS₂ flakes. However, the two TMDCs have very different spin-splitting in the valence band. This indicates, that intravalley spin-flip transitions of holes are negligible for the valley depolarization.

Group VI transition metal dichalcogenides monolayers MX2 (MoS2, MoSe2, WS2,and WSe2) feature a spin splitting with opposite sign in the two degenerate but inequivalent valleys located at corners of 1st Brillouin zone. This spin-valley coupling, particularly pronounced in tungsten dichalcogenides, along with nonzero but contrasting Berry curvatures at K valleys can benefit potential spintronics and valleytronics with the important consequences of spin-valley interplay and the suppression of spin and valley relaxations. In this talk we address the spin-valley coupling in 2D WS2 with optical spectroscopy and photocurrent spectroscopy. This giant spin-valley coupling, together with the valley dependent Berry curvature, may lead to rich possibilities for manipulating spin and valley degrees of freedom in these 2D semiconductors.

Controlling the flow of electric charges enables nanoelectronics and modernizes the information technology. The spin degree of freedom of electrons embraces emerging spintronics, important to solid-state computing. The atomic membrane of transition metal dichalcogenides (TMDs) possesses a nonequivalent carrier distribution in crystal momentum space. The protection from its broken inversion symmetry makes the valley index of charge carriers a new degree of freedom for information processing. A variety of valleytronic devices such as valley filters, valves, and thermoelectric valley current have been proposed. Optical control and detection of valley polarization has been reported in monolayer TMDs due to the valley optical selection rules. However, electrical generation and control of valley-polarized carriers that is the key to the utilization of the valley degree of freedom in nanoelectronics, yet remains a formidable challenge. Here we experimentally demonstrate valley-polarized light emission by electrical spin-polarized carriers injection into monolayer WS₂ using the ferromagnetic semiconductor, (Ga, Mn)As, as a spin aligner. The valley polarization is electrically generated due to the unique spin-valley locking, i.e., valley polarization is achieved through spin polarization of the charge carriers. Our valley polarization heterojunction becomes a circularly polarized light source due to the direct band gap and valley degree of freedom of monolayer WS₂. The unique correlation between spin and valley indices of electric carriers opens the new dimension in utilizing both spin and valley for next-generation electronics and computing.

In many crystals the Bloch bands have degenerate but inequivalent energy extrema in the momentum space, known as valleys. The valley index constitutes a well-defined discrete degree of freedom for low-energy carriers that may be used to encode information. This has led to the concept of valleytronics, a new type of electronics based on manipulating the valley index of carriers. In this talk, I will describe a general scheme based on inversion symmetry breaking to control the valley index, which is based on the Berry phase effect of massive Dirac electrons. A number of valley-dependent phenomena, such as valley Hall effect, valley-dependent optical selection rule, spin-valley coupling, optical generation of pure valley current, as well as Berry-phase induced energy splitting of valley excitons will be discussed.

Two dimensional atomic sheets, such as graphene and transition metal dichalcogenides (TMDs), have been widely studied as electronic materials for flexible nanoelectronics applications due to the high flexibility enabled by their natural 2D layered crystal structure. However, with the growing need for both high speed and low power consumption in realistic applications, TMDs with relatively low mobility and graphene with zero band gap are facing critical challenges to satisfy practical requirements. Recently, few-layer phosphorene, a new candidate in the portfolio of 2D crystals, has demonstrated high room temperature mobility and high on/off ratio, which is very attractive for advanced flexible nanoelectronics.
In this work, we present the first phosphorene flexible field effect transistors (BP-FETs), fundamental circuits and an audio receiver. For BP-FETs based on exfoliated phosphorene flakes with thickness between 5nm to 15nm, clear ambipolar characteristics and negligible hysteresis were achieved, attributed to a dielectric capping layer, which significantly enhanced long-term air stability. Outstanding device performance were achieved at room temperature; hole mobility and current on/off ratio are 300 cm²/Vs and 10⁵, respectively. With significantly enhanced ambipolar characteristics, electron mobility of 100 cm²/Vs was observed. In this work, high performance electronic circuit blocks, including digital inverters, frequency doublers, inverting and non-inverting amplifiers were realized for the first time on plastics. Furthermore, we demonstrate a phosphorene flexible radio receiver which effectively demodulates amplitude modulated audio signals. In conclusion, our results indicate that few layer black phosphorus is the most competitive 2D material for future high speed and low power flexible electronics beyond the low mobility of TMDs and zero bandgap of graphene.

In this talk I will describe liquid exfoliation of layered crystals to give 2D materials. This is based on the exfoliation of graphite to give graphene. The simplest way to do this is to sonicate graphite in certain, stabilising solvents. When the solvent surface energy matches that of graphene the energetic cost of exfoliation is minimised and some of the graphite is converted to graphene. Graphite can also be exfoliated to give graphene by sonication in surfactants or polymer solutions. The resulting graphene is free of oxides and basal-plane defects and consists of nanosheets with lateral size of 200-2000 nm and thickness from 1-10 layers. This material can be used in arrange of applications, such as reinforced composites or strain sensors.
This process can be extended to a host of other layered crystals including BN, MoS2, MoO3 and GaS. The exfoliation of MoS2 will be used as an example. In liquid exfoliation, it is important to know both the lateral size and thickness of the material being exfoliated. In the case of MoS2, we will show that this information is contained within the optical absorption spectrum. I will describe simple metrics which can be extracted from the MoS2 absorption spectrum and used to give the flake length and thickness directly. Access to such metrics allows the exfoliation process to be optimised to maximise monolayer content. As a result we can obtain monolayer-rich MoS2 dispersions which are photoluminescent. These methods can be applied to a range of 2D materials. To illustrate this, I will describe the production of MoTe2 dispersions which are monolayer-rich and photoluminescent.
For any of the applications described above to be successful, a method of producing very large quantities of exfoliated nanosheets will be required. We have developed such a method based on high shear mixing. We have demonstrated the scalability of this method and envisaged the exfoliation mechanism. I will describe the production of batches of liquid exfoliated graphene with volumes of up to (but not limited to) 300 L. In addition, I will describe the large scale production of MoS2 nanosheets by shear mixing in a kitchen blender. The size measurement metrics described above allow the measurement of nanosheet size and thickness as a function of processing parameters. Interestingly, we find one of the processing parameters can be used to control nanosheet dimensions during the exfoliation process.

Using a combination of complementary in-situ x-ray photoelectron spectroscopy (XPS) and x-ray diffraction (XRD) we study the fundamental mechanisms underlying the chemical vapor deposition (CVD) of hexagonal boron nitride (h-BN) on polycrystalline Cu. The nucleation and growth of h-BN layers is found to be isothermally, i.e. at constant elevated temperature, on the Cu surface during exposure to borazine. A Cu lattice expansion during borazine exposure and B precipitation from Cu upon cooling highlight that B is incorporated into the Cu bulk, i.e. that growth is not just surface-mediated. On this basis we suggest that B is taken up in the Cu catalyst while N is not (by relative amounts), indicating element-specific feeding mechanisms including the bulk of the catalyst. We further show that oxygen intercalation readily occurs under as-grown h-BN during ambient air exposure, as common in further processing, and that this negatively affects the stability of h-BN on the catalyst. For extended air exposure Cu oxidation is observed and upon re-heating in vacuum an oxygen-mediated disintegration of the h-BN film via volatile boron oxides occurs. Important thereby is that this disintegration is catalyst mediated i.e. occurs at the catalyst/h-BN interface and depends on the level of oxygen fed to this interface. In turn however, deliberate feeding of oxygen during hexagonal boron nitride deposition can positively affect control over film morphology. We discuss the implications of these observations in the context of corrosion protection and relate to challenges in process integration and hetero-structure CVD.
References
Kidambi et al. Chem. Mat. (2014). Just Accepted.
Kidambi et al. Nano Letters 13 (10), 4769-4778 (2013).
Kidambi et al. J. Phys.Chem. C. 116, 42, 22492-22501 (2012).
Kidambi et al. PSS RRL. 5, 9, 341-343 (2011).

Boron nitride has attracted a great deal of attention as a 2D insulator for substrate and gate dielectric applications in 2D electronics. Structurally, similar to graphene, h-BN is a layered material possessing strong in-plane bonding with a hexagonal lattice, and only weak van de Waals forces holding together neighboring layers. However, unlike graphene h-BN is partially ionic making it a wide band gap (5.9 eV) insulator. Additionally, h-BN is more resistant to oxidation than graphite and other 2D materials making it an important dielectric capping layer for 2D electronics. Fabrication of atomically smooth BN substrates suitable for 2D electronics is very challenging due to the difficulty of growing mono-few layer BN on non-metallic substrates. Work on the growth of BN on microelectronic substrates, like sapphire and Si, will be presented. The MOCVD growth of BN directly on sapphire with a self-terminating growth mode will be discussed. Growth on arbitrary substrates using a catalyzing Cu thin film will also be presented.

Silicene, a 2D silicon analogue of graphene, possesses a buckled honeycomb lattice with mixed sp²-sp³ hybridization. Owing to its predicted Dirac band structure and its buckled nature, silicene has the potential to be a widely tunable 2D material for future innovative nanoelectronics, where external fields and surface interactions can be exploited to influence fundamental properties. Despite recent progress on epitaxial synthesis of silicene, experimental fabrication of silicene devices have been an obstacle so far due to its air-stability issue. Here, we demonstrate the first silicene field-effect transistors, corroborating theoretical expectations on ambipolar Dirac charge transport with measured room temperature mobilities in agreement with the range of theoretical predictions. Analysis based on measurements suggest a bandgap opening within an order of magnitude of thermal of energy.
These original experimental results are enabled by a growth-transfer-fabrication process based on a sandwich transfer method of silicene with double sided encapsulation. Key innovations include: i) epitaxial silicene synthesis on deposited Ag(111) thin films instead of expensive single crystal Ag substrates, ii) delamination transfer, and iii) native Ag film as contact electrodes. Our work paves the path towards room-temperature silicene devices and its low temperature synthesis and affinity with bulk silicon suggest a more direct path for silicene integration with ubiquitous semiconductor technology compared to other 2D materials.

Two-dimensional materials are one of the most active areas of nanomaterials research. Here we report the structural stability, electronic and vibrational properties of different monolayer configurations of the group IV elemental materials silicene and germanene. The structure of the stable configurations is calculated and in the case of planar and a low (<1 Å) atomic buckling configuration, linear band dispersion giving rise to massless Dirac Fermions with a Fermi velocity about two-thirds that of graphene is found. The stability of the layer is shown to be directly attributed to the phonons present with the instability being driven by the out-of-plane ZA and ZO phonon modes. Despite the lower phonon frequencies, associated with the heavier atomic masses, high carrier mobilities of these materials are predicted as the electron-optical phonon coupling matrix elements are found to be about a factor of 25 times smaller than in graphene. As a result the high field electrical transport in these materials is not thought to be adversely affected by phonon scattering.
[1] Nathanael J Roome and J David Carey, ACS Appl. Mater. Interfaces 6, 7743 (2014).

Black phosphorus (BP) has become the second element after carbon to be separated into sheets each a single atom thick [1]. The ultrathin material, dubbed phosphorene, could prove superior to its carbon counterpart for use in next-generation electronics. Just as in graphene, phosphorene atoms are arranged hexagonally, but in phosphorene the anisotropic structure with pucker is amazing [2]. With its special structure, phosphorene is becoming an exciting material with outstanding physical and chemical properties.
Previous attempts adopted the sticky tape approach developed to isolate graphene sheets, to phosphorene. Unfortunately, this method is hardly scalable and unable to provide single-layers of phosphorene. The methodology of low yield is strangling the possibility of its other applications, which are required high mass.
Here, we introduced a highly scalable, facile method to produce mono- and few-layered phosphorene based on the sonication of BP in a N-Methyl-2-pyrrolidone (NMP) solution, which give a possibility for using it in high-mass required applications. We have also explored the use of BP as an anode material for Li- and Na-ion batteries [3] and found the two-step reaction mechanism of intercalation and conversion. For the second step of conversion leading to losing pucker-layer structure, the anisotropic expansion of BP along the y- and z-axis direction upon sodiation and results in its poor electrochemical performance. In order to overcome these issues, we have designed a new, sandwiched phosphorene-graphene structure. This newly developed material has established outstanding electrochemical properties with a high discharge capacity of 2440 mA h g^-1 (close to its theoretical capacity) at a C/50 rate and an 83 % capacity retention after 100 cycles.
[1] E. S. Reich, Nature506, 19 (2014).
[2] L. Li, et al,Nature Nanotechnol.9, 372-377 (2014).
[3] J. Sun, et al,Nano Lett.14, 4573-80 (2014).

MXenes, a new family of two-dimensional (2D) materials, combine hydrophilic oxygen and OH-terminated surfaces with metallic conductivity [1]. Delamination of MXene produces single-layer nanosheets with thickness of about a nanometer and lateral size of the order of micrometers. The MXenes have demonstrated their potential as promising electrode materials for supercapacitors, with volumetric capacitance exceeding commonly used carbon materials. [2]
In order to further improve the rate performance of MXene-based supercapacitors, herein, flexible MXene/carbon composite papers composed of alternating layers of titanium carbide-based MXene and carbon nanoparticles, including 0D onion-like carbons, 1D carbon nanotubes, and 2D graphene, were fabricated. The resulting flexible composite “papers” have high electrical conductivity and a structure accessible to electrolyte ions. When employed as electrodes for supercapacitors, a capacitance of ~440 F/cm³ was achieved in aqueous electrolyte. Besides, the composites showed ~75% increase in rate performance compared to pure MXene films and exhibited no capacity degradation after 10,000 cycles. These conductive, flexible and sufficiently strong papers may also find use as electrochemical actuators, electrode materials for Li-ion batteries, fuel cells, and other energy devices.
[1] M. Naguib, V.N. Mochalin, M.W. Barsoum, Y. Gogotsi, MXenes: A New Family of Two-Dimensional Materials, Advanced Materials, 26, 992-1005 (2014)
[2] M. R. Lukatskaya, O. Mashtalir, C. E. Ren, Y. Dall&’Agnese, P. Rozier, P.-L. Taberna, M. Naguib, P. Simon, M. W. Barsoum, Y. Gogotsi, Cation Intercalation and High Volumetric Capacitance of Two-dimensional Titanium Carbide, Science, 341, 1502-1505 (2013)

We have synthesized atomically thin MoSe₂ specimen with chemical vapor deposition (CVD). Films and isolated nanoflakes were obtained separately by different parameters. The films can be produced in mono- and few-layered thickness in large area (~1 cm²). From Raman spectroscopy, breathing mode A_1g and shear mode E_2g¹ show pronounced shifts from monolayer to bilayer. Furthermore, the low-frequency interlayer shear mode (E_2g²) is identified in bilayer, while it disappears in monolayer due to the nature of interlayer vibration. From photoluminescence spectroscopy, we identify a strong emission from A exciton at room temperature, and B exciton with a splitting energy of 200 meV from A exciton is also observed. On MoSe₂ nanoflakes, we have studied how polytypism in layer stacking affects the properties of atomically thin samples. Scanning transmission electron microscope (STEM) confirmed that there are both 3R and quasi-3R stacking. For the first time, all the interlayer shear modes, including those that are Raman-inactive in samples with 2H stacking, are observed in our CVD-grown 2-7 layered MoSe₂ flakes.

Phosphorene is the 2-dimensional (2-D) structural analogue of the layered black phosphorus (BP) allotrope and is a semiconductor with a high hole carrier mobility. It has the potential to revolutionise electronics based on two-dimensional materials because it is a direct band gap semiconductor, that can be tuned in energy according to the thickness of the flakes (e.g. bulk black phosphorous has a band gap of ~ 0.3 eV and monolayer phosphorene ~ 1.0 eV).
Liquid exfoliation is an important route towards 2-D materials, with the resulting suspension of 2-D flakes forming a suitable precursor for inks. To date graphene, h-BN and 2-D analogues of inorganic layered compounds, such as MoS₂ have been produced by this route. Inkjet printing provides an economical way to achieve additive patterning and direct writing of organic inks combined with low cost and scalability for large area deposition of material. The effective use of inkjet printing as a way to deploy 2D materials has already been demonstrated for graphene and MoS₂ nanosheets.
We have recently demonstrated that liquid exfoliation of BP in N-methyl-2-pyrrolidone (NMP) is a simple and facile route toward the production of few-layer phosphorene of predominantly 3 -5 layers or 1-2 layers height, depending on the exfoliation time used [1]. We have used liquid exfoliation of phosphorene to produce inks suitable for deployment onto substrates by inkjet printing. Phosphorene nanosheets have been dispersed in relatively volatile solvents using surfactant-assisted ultrasonic exfoliation to produce stable colloids. We demonstrate how the size of the phosphorene nanosheets can be controlled by judicious choice of surfactant, solvent and exfoliation conditions to ensure that inks are homogeneous. Semiconducting layers of phosphorene have been printed onto flexible substrates, and we will demonstrate how phosphorene inks have been paired to silver nanoparticle electrodes to form prototype photodetector devices. Such semiconducting devices will be important in a future electronics industry based on 2-D materials.
[1] Brent, J. R.; Savjani, N.; Lewis, E. A.; Haigh, S. J.; Lewis, D. J.; O&’Brien, P. ‘Production of Few-Layer Phosphorene by Liquid Exfoliation of Black Phosphorus Chem. Commun. 2014, DOI:10.1039/C4CC05752J.

Bismuth telluride (Bi₂Te₃), bismuth selenide (Bi₂Se₃) and antimony telluride (Sb₂Te₃) are conventional thermoelectric materials and also known as topological insulators. When a two-dimensional (2D) nanosheet structure is applied to these materials, the surface conduction originated from the topological surface states becomes dominant and their physical properties can be greatly enhanced owing to the fact that topological surface states are protected against surface defects or non-magnetic impurities. In this work, elastic properties of suspended Bi₂Te₃ 2D nanosheets with different thicknesses were investigated by applying mechanical deformations. The chemical vapor transport (CVT) method was used to synthesis Bi₂Te₃ 2D nanosheets on an atomically smooth mica substrate, and the thickness of as-grown 2D nanosheets is only 2-10 nm owing to the van der Waal epitaxial growth. Using poly(dimethylsiloxane) (PDMS) as stamps and poly(methyl methacrylate) (PMMA) film as sacrificial layers, 2D nanosheets were dry-transferred onto a pre-fabricated substrate such that 2D nanosheets are suspended over 1.5-micrometer wide holes. Atomic force microscopy (AFM) tips in contact mode or force/distance mode were then used to deform the suspended 2D nanosheets. It was found that the 2D elastic constant is 240-590 N/m for 2D nanosheets with thicknesses of 6-10 nm, and the derived Young&’s modulus is ~60 GPa. Our data is consistent with the result of the density functional theory simulation (61.6 GPa). However, the Young&’s modulus is smaller than other non-graphene 2D nanosheets, such as monolayer molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) (~270 GPa), and graphene oxide (200 GPa). 2D Bi₂Te₃ nanosheets have shown good elastic properties and a potential for flexible energy devices. Understanding Bi₂Te₃ 2D nanosheet mechanical properties will help to improve their thermoelectric properties under mechanical strains towards practical energy conversion applications.

Two dimensional (2D) materials have gained significant attention with features of ultrathin thickness, transparency, superior transport properties and promising potential for next generation electronic devices. Transition metal dichalcogenides (TMDCs), which possess a wide spectrum of electronic, optical, chemical and thermal properties, has triggered tremendous interests in recent years. Several semiconducting monolayer TMDCs, complementary to graphene, have intrinsic direct bandgaps ~1-2 eV, suggesting TMDCs great candidates for new field-effect transistors applications. However, the limitation of small size TMDCs obtained in conventional processes has hindered further development towards FET-based devices. As a result, fabrication of large-scale single crystal monolayer TMDCs has been one the focal keys in pursue of advanced applications.
In this work, we present an innovative growth technique to fabricate large-scale single crystal TMDCs. Using ultra-thin epitaxial transition metal oxide film as a reactant pre-layer, the high quality epitaxial thin layer serves as an elegant template for well-controlled sulfidation, selenization and tellurization process. To demonstrate such a process, MoO₃ ultra-thin epitaxial layer was grown on atomic flat c-plane sapphire using laser-MBE system, in which the thickness can be precisely controlled by monitoring the layer-by-layer growth mode. As a second step, well-controlled replancing process has further transformed metal oxide film into single crystall TMDCs. Chalcogens replaceing reaction was taken in a quartz tube, in which the hydrogen ions react with oxygen atoms in the oxide pre-layers. Through well-controlled selenization process, large area single crystal monolayer MoSe₂ can be obtained. Several advantages can be derived by using such a process. First, we can grow epitaxial films in wafer scale, suggesting manufacture of large-scale single crystal TMDCs is possible. Second, this process can be used to discover new TMDC systems, which are rarely seen or very hard to be fabricated by conventional processes. Last but not least, different TMDCs heterostructures can be manufactured by growing different transition metal oxide pre-layers as multi-layers or superlattices. Designed heterostructures can be used in light-emitting diodes, diode laser, solar cell and high-speed transistors. Our result sheds light on not only fundamental studies, but also the promising applications of novel TMDC systems

Air-stable doping of transition metal dichalcogenides is of fundamental importance to enable a wide range of optoelectronic and electronic devices while exploring their basic material properties. Here we demonstrate the use of benzyl viologen (BV), which has one of the highest reduction potentials of all electrondonor organic compounds, as a surface charge transfer donor for MoS₂ flakes. The n-doped samples exhibit excellent stability in both ambient air and vacuum. Notably, we obtained a high electron sheet density of sim;10¹³ cm^minus;2, which corresponds to the degenerate doping limit for MoS₂. The BV dopant molecules can be reversibly removed by immersion in toluene, providing the ability to control the carrier sheet density as well as selective removal of surface dopants on demand. By BV doping of MoS₂ at the metal junctions, the contact resistances are shown to be reduced by a factor of >3. As a proof of concept, top-gated field-effect transistors were fabricated with BV-doped n⁺ source/drain contacts self-aligned with respect to the top gate. The device architecture, resembling that of the conventional Si transistors, exhibited excellent switching characteristics with a subthreshold swing of sim;77 mV/decade.
Ref: D. Kiriya et al. J. Am. Chem. Soc. 2014, 136, 7853.

The quantitative correlation of the catalytic activity with the microscopic structure of heterogeneous catalysts is a major challenge for the fi eld of catalysis science. It requests synergistic capabilities to tailor the structure with atomic scale precision and to control the catalytic reaction to proceed through well-defi ned pathways. Here we leverage on the controlled growth of MoS2 atomically thin fi lms to demonstrate that the catalytic activity of MoS2 for the hydrogen evolution reaction decreases by a factor of ~ 4.47 for the addition of every one more layer. Similar layer dependence is also found in edge-riched MoS2 pyramid platelets. This layer-dependent electrocatalysis can be correlated to the hopping of electrons in the vertical direction of MoS2 layers over an interlayer potential barrier. Our experimental results suggest the potential barrier to be 0.119 V, consistent with theoretical calculations. Diff erent from the conventional wisdom, which states that the number of edge sites is important, our results suggest that increasing the hopping effi ciency of electrons in the vertical direction is a key for the development of high-effi ciency two-dimensional material catalysts.

We present scanning photocurrent measurements on CVD-grown monolayer films of molybdenum disulfide, molybdenum diselenide and the alloys of these materials. Our experiments reveal a pronounced effect of the current on excitation in the gap region between contacts, as opposed to directly at the electrodes. Measurements at different irradiation intensity, irradiation position and bias shed light on the charge transfer processes in this material system. Thermal effects are ruled out by complementary measurements of thermal transport using infrared imaging.

Two-dimensional (2D) semiconductors such as MoS₂ have recently garnered attention as materials for thin-film transistors as well as flexible and transparent electronics. In order to scale such materials for practical applications, they must be grown by chemical vapor deposition (CVD). However, existing CVD growth studies typically employ pre-growth substrate treatments and/or do not comment on the variability of devices across the chip [1-5]. In order to achieve 2D films suitable for integration with other circuits or flexible/wearable electronics, a method for growing uniform, high-quality MoS₂ must be developed, and a fabrication and integration process that reduces device variability must be engineered.
In this study, we present the first statistical analysis of large-area MoS₂ thin films and devices grown by CVD from solid precursors. By properly selecting growth pressure and source-substrate geometries, we balance vapor-phase mean free path and surface diffusion length such that an approximately uniform ~2 cm² film of MoS₂ with ~100 nm grain sizes is produced. In addition to electrical characterization of several hundreds of devices, we utilize Raman spectroscopy and atomic force microscopy (AFM) to evaluate both the film thickness and the quality of the MoS₂ transistors.
Raman spectroscopy at locations ~0.5 cm apart on a single chip show peak separation between the E_2g and A_1g modes ranging between 22 and 24 cm^-1, as well as a full width at half maximum (FWHM) average near 6 cm^-1 for both peaks. Although such results may naïvely suggest the presence of low-quality 2 to 4 layer films, AFM reveals the MoS₂ to be a polycrystalline film comprised of individual triangular domains of varying thickness, which may explain the widened Raman peaks. Despite consistent results from both Raman spectroscopy and AFM scans across a single chip, we find that the field effect mobility of these devices can vary by several orders of magnitude. We uncover that the electrical variability is influenced by residual photoresist, contact resistance, and grain boundary scattering, and we propose techniques for reducing this variability across the entire substrate. This work represents the first large-scale study of the effects that processing can have on MoS₂ thin film devices, including the device variability that exists across a single chip.
[1] Y.-H. Lee, et al., Adv. Mater.24, 2320 (2012).
[2] Y.-C. Lin, et al., Nanoscale4, 6637 (2012).
[3] S. Najmaei, et al., Nature Mater.12, 754 (2013).
[4] M. R. Laskar, et al., Appl. Phys. Lett.102, 252108 (2013).
[5] V. Senthilkumar, et al., Nano Research7, DOI: 10.1007/s12274-014-0535-7 (2014).

We demonstrate field-effect transistors using heterogeneously stacked two-dimensional materials for all of the components, including the semiconductor, insulator and metal layers. Specifically, MoS₂ is used as the active channel material, hexagonal-BN as the top-gate dielectric, and graphene as the source/drain and the top-gate contacts. CVD grown bilayer or monolayer graphene (Gr) on a Cu foil was wet-transferred onto a Si/SiO₂ substrate. Ni/Au (30 nm/30 nm) bond pads were then formed by electron-beam (e-beam) lithography, evaporation and resist lift-off. Gr was subsequently patterned using e-beam lithography and oxygen plasma etching to define the S/D electrodes of the FET. MoS₂, h-BN and Gr multilayers as the active channel, gate dielectric and the gate electrode, respectively were sequentially stacked on the Gr S/D contacts using a pick and transfer process. The device, thus fabricated, was annealed in forming gas (5% H₂, 95% N₂) for 3 h at 200 ⁰C in order to improve the interface properties. MoS₂/WSe₂ heterojunction diodes with Gr contacts were also fabricated using a similar transfer approach, except that MoS₂ and WSe₂ layers were sequentially transferred such that each flake is in contact with only one pre-patterned Gr electrode, with a MoS₂/WSe₂ overlap region in the middle of the device.
The all-2D transistor exhibits n-type behavior with an ON/OFF current ratio of >10⁶, and an electron mobility of ~33 cm²/V-s. Uniquely, the mobility does not degrade at high gate voltages, presenting an important advantage over conventional Si transistors where enhanced surface roughness scattering severely reduces carrier mobilities at high gate-fields. The WSe₂-MoS₂ diode with graphene contacts exhibits excellent rectification behavior and a low reverse bias current, suggesting high quality interfaces between the stacked layers. In this work, all interfaces are based on van der Waals bonding, presenting a unique device architecture where crystalline, layered materials with atomically uniform thicknesses are stacked on demand, without the lattice parameter constraints. The results demonstrate the promise of using an all-layered material system for future electronic applications.^[1]
REFERENCE:
[1] T. Roy et al., ACS Nano, 2014.

Black phosphorus (BP) is a thermodynamically stable allotrope of phosphorus [1], a layered nanomaterial with high carrier mobilities (~1000 cm²V^-1s^-1) [2] and a band gap (~0.3 eV) [3]. BP electronic [4] and optoelectronic [5] applications have already been realized using mechanically exfoliated material. Nonetheless, little is known about the stability of exfoliated BP flakes in normal ambient conditions, an important consideration for the viability of BP applications. We find that in normal ambient environments, unprotected, exfoliated BP flakes chemically degrade. Ambient BP degradation proceeds by an irreversible conversion to oxidized phosphorus, as evidenced by atomic force microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and electrostatic force microscopy. We exfoliate BP on both hydrophilic SiO₂ and hydrophobic octadecyltrichlorosilane (OTS) on SiO₂, helping us to assess the role of water in BP ambient degradation. BP degrades two fold faster on hydrophobic OTS versus hydrophilic SiO₂, implicating O₂ saturated H₂O as a major source of the BP ambient degradation. For unprotected BP field-effect transistors (FETs), degradation causes large threshold voltage increases after 6 hours in ambient, followed by a ~10³ decrease in mobility and FET current on/off ratio after 48 hours. However, atomic layer deposited overlayers of AlO_x halt the ambient degradation, protecting the BP morphologically, structurally, and chemically. Importantly, AlO_x encapsulated BP FETs maintain high mobilities of ~100 cm²V^-1s^-1 and on/off ratios of ~10³ when exposed to ambient for over three weeks. Our method for protecting BP against ambient degradation should complement ongoing fundamental BP work and hasten BP electronic and optoelectronic applications.
[1] J. C. Jamieson, Science 139, 1291 (1963); [2] A. Morita, Appl. Phys. A 39, 227 (1986); [3] Y. Akahama et al., J. Phys. Soc. Jpn. 52, 2148 (1983); L. Li et al., Nat. Nanotechnol. 9, 372 (2014); M. Engel et al., Nano Lett. ASAP, DOI: 10.1021/nl502928y (2014).

Hexagonal Boron Nitride (h-BN) has recently emerged as an interesting 2D material for implementation in novel nanoelectronic device applications. The hexagonal crystal lattice makes it an ideal template on which to grow other 2D hexagonal materials¹. Because of its wide band gap, h-BN can be used as a dielectric interlayer for heterostructures coupling layers of graphene or transition metal dichalcogenides (TMD) enabling new all-2D heterostructures. This opens the door for the fabrication of novel nanoelectronics such as vertical broken-gap tunnel FETs and the BiSFET with lower power consumption and higher switching speed compared to conventional CMOS technology². In this work, h-BN is grown in by plasma-assisted molecular beam epitaxy (MBE) using a high temperature effusion cell for boron evaporation and a nitrogen RF plasma source. Ultra-high purity (99.9999%) N₂ gas is used for the RF Plasma source to supply active N₂ species for h-BN growth. A temperature dependent study of the growth mechanism including nucleation and 2D growth will be presented. In-situ RHEED characterization is coupled with ex-situ XPS, LEED, XRD, Raman Spectroscopy, STM, and TEM to investigate the chemical bonding, crystal structure and quality, point defects, and experimentally determined band alignments. The impact of substrate lattice mismatch on the strain of the grown h-BN films will be discussed.
1 G. Zhang, et al., Nature Materials 12, 792-797 (2013)
2 A.H. MacDonald, et al., EDL 30 (2), 158 - 160 (2009)

Layered, two-dimensional (2D), transition metal dichalcogenides (TMDs) are very interesting from both a fundamental and a technological point of view. Spin-orbit interactions in TMDs is strong compared to the original 2D material; graphene. Also, and they have a band gap and it can change from direct to the indirect depending on the number of layers. Quantitative modeling of the magnetic and optical properties of 2D materials has not been thoroughly studied as a function of layer number. Here, using density functional theory (DFT) calculations, we predict the electronic, magnetic and optical properties of this subset of TMDs (MX2, M = Mo, Nb, V, Ta, W; X = S, Se, Te) in both the trigonal, prismatic 2H- semiconducting- and octahedral, 1T metallic-phase from 1 to 6 layers. We find that the magnetic and optical properties of these two polymorphs change with layer number. Specifically, our results reveal that the Raman spectra are unique in both frequency and intensity depending on both polymorph and layer number, which can be explain by the chemical strain. We predict that the E_2g phonon mode of Raman spectra will change drastically in the presence of an external magnetic field through a pronounced magneto-phonon resonance (MPR) due to the presence of 4d/5d elements. We will compare these theoretical results with experiments published in the literature and with novel experiments underway in our laboratory.

It is widely predicted that 2D transition metal dichalcogenides (TMDs) might outperform traditional graphene-based materials due to their well-defined semiconductor properties (E_g=1-2 eV), securing their applications in nanoelectronics, sensing and photonics, e.g. as FETs, photodetectors and gas sensors. Among the 2D TMDs materials, WS₂ holds a number of important advantages, as it can be synthetically produced with high and controllable purity and quality. Atomically thin WS₂ possesses high temperature stability and also exhibits exceptionally high modulation range and ambipolar behaviour at room temperature. Recent theoretical studies have also predicted that due to its favourable band structure, this material can outperform other 2D TMDs in FET devices. However, the physical properties of this material, in particular in relation to its thickness and to the substrate employed, are still not sufficiently understood. Here we present recent data using a combination of scanning probe microscopies (SPM) on CVD grown single and few-layer WS₂ on SiO₂(300 nm)/ Si and epitaxial graphene substrates.
Nominally single layer epitaxial graphene was synthesised via Si sublimation from low off-cut semi-insulating (0001) 6H-SiC substrates in a commercial reactor. The chemical vapour deposition of WS₂ was performed using a two-step process whereby the single layer was grown by sulphurisation of WO₃ powder [1].
We employ scanning Kelvin probe, photo-conductive atomic force and scanning tunnelling microscopy techniques, as well as Raman spectroscopy mapping and photoluminescence spectroscopy in order to correlate the structural, electronic and optical properties of WS₂ on SiO₂ and epitaxial graphene at room temperature.
Using the large-scale triangular WS₂ islands on SiO₂, we demonstrate predominantly single layer growth, with additional layer growth proceeding at the edges of the flakes as evidenced by the SPM techniques. The surface potential studies demonstrate a strong dependence on the number of layers, with a single layer showing a higher surface potential as compared to bilayer WS₂ consistent with Raman spectroscopy mapping. This confirms that the 2LA(M) Raman mode is the spectral fingerprint of monolayer WS₂. The observed results are compared to WS₂ grown on epitaxial graphene and the difference attributed to the interactions inherent to this heterostructure. These results provide an insight into the utilisation of TMDs - graphene heterostructures for future device applications.
1. Chong, C. et al., Adv. Opt. Mat. 2, 131 (2014).

The growth of single-layer hexagonal boron nitride (hBN) films over large areas is one of the key requirements for the integration of this material in graphene-based electronic devices. The ability to control the domain size and thickness, as well as the nucleation density, is of great relevance for the production of 2D-based heterostructures. Compared to other synthesis routes, catalytic growth on metal substrates allows careful tailoring of the growth process and more precision over the hBN morphology.
We demonstrate the growth of large (~0.3 mm side length) hBN domains via low pressure CVD on Fe(1000nm)/SiO₂(300nm)/Si substrates using a borazine precursor. We perform a combination of real-time in-situ X-ray diffraction (XRD) and ex-situ secondary ion mass spectrometry (SIMS) measurements to show that Si diffuses from the underlying wafer into the catalyst during the annealing stage and that the presence of this element affects the growth morphology of hBN.
The variation in the domain size, shape and density as well as the formation of interfacial silicates can be attributed to differing Si concentrations in the Fe bulk and surface. We exploit the SiO₂ layer as a Si diffusion barrier that can be tuned supply Si in the optimum amount for growth of the large triangular domains. Diffusion barriers have been used previously by our group to control low-temperature graphene growth from solid carbon sources on Ni substrates [1]. In a recent work we have also illustrated the beneficial effect of a Au-Ni admixture towards CVD graphene domain size and layer control [2]. The Fe(1000nm)/SiO₂(300nm)/Si stack system inherently provides both diffusion and growth control simultaneously. Using complementary selected area electron diffraction (SAED), transmission electron microscopy (TEM) and atomic force microscopy (AFM) we show that the large hBN domains are monolayers and single-crystal. We also demonstrate that an increase in precursor flux leads to a rise in the nucleation density, which can be exploited to achieve full coverage of the Fe catalyst with a uniform and continuous hBN film. We report an example of rational catalyst design and aim to show that optimization of the catalytic process hinges on the understanding of the chemical and structural state of the catalyst during all CVD stages. Our study on the catalyst-precursor and catalyst-substrate interactions provides deeper insights into growth optimization of 2-D layered materials.
[1] Weatherup et al., Nano Lett. (2013)
[2] Weatherup et al., Nano Lett. (2011)

The high surface to weight ratio of 2D materials offers abundant active sites for energy storage in lithium-ion battery, or for energy conversion in electrochemical water splitting. Many key performance characteristics of batteries are determined by the strength of binding between Li and electrode materials. We find a descriptor, called ‘states-filling work&’, for evaluation of the carbon-based electrode performance.^[1] It allows for straightforward assessment of the Li-C binding energy in candidate carbon materials based solely on the pure substrate electronic structure, without the need for explicit evaluation of Li adsorption. It further suggests specific guidelines for designing more effective C-based anodes, and leads to the discovery of a new material, BC₃, as promising anode.^[2] The ‘states-filling work&’ approach also enables quick assessment of the H binding energy on metal dichalcogenides MX₂, based on which we find that some MX₂ are able to catalyze electrochemical H production on their surfaces,^[3] and therefore are superior to other catalysts such as MoS₂ or WS₂ which are only active at the edges. Furthermore, the structure and especially variability of defects in 2D materials bring about additional rich functionality.^[4-5]
[1] Y. Liu, Y. M. Wang, B. I. Yakobson, and B. C. Wood, Phys. Rev. Lett. 113, 028304 (2014).
[2] Y. Liu, V. I. Artyukhov, M. Liu, A. R. Harutyunyan, and B. I. Yakobson, J. Phys. Chem. Lett. 4, 1737 (2013).
[3] Y. Liu et al., submitted (2014).
[4] X. Zou and B. I. Yakobson, Accounts of Chemical Research, submitted (2014).
[5] Y. Liu, F. Xu, Z. Zhang, E.S. Penev, and B.I. Yakobson, Nano Lett., DOI: 10.1021/nl5021393 (2014).

The deep gap states created by defects in semiconductors typically deteriorate the performance of (opto)electronic devices. This has limited the applications of two-dimensional (2D) metal dichalcogenides (MX₂) and underscored the need for a new 2D semiconductor without defect-induced deep gap states. In this work, we demonstrate that a 2D mono-elemental semiconductor is a promising candidate.^[1] This is exemplified by first-principles study of 2D phosphorus (P), a recently fabricated high-mobility semiconductor. Most of the defects, including intrinsic point defects and grain boundaries, are electronically inactive, thanks to the homoelemental bonding, which is not preferred in heteroelemental system such as MX₂. Unlike MX₂, the edges of which create deep gap states and cannot be eliminated by passivation, the edge states of 2D P can be removed from the band gap by hydrogen termination. We further find that both the type and the concentration of charge carriers in 2D P can be tuned by doping with foreign atoms. Our work sheds light on the role of defects in the electronic structure of materials.
[1]Y. Liu et al, Nano Lett., 2014, DOI: 10.1021/nl5021393

Raman Spectroscopy constitutes a powerful tool for the characterization of 2D and layered materials, such as graphene and transition metal dichalcogenides (TMDs). It stands as a non invasive technique to acquire fundamental information about these materials, regarding crystalline order and number of layers, which is well known for the case of graphene. In this work, we will describe recent advances in the interpretation of Raman spectra for some semiconducting TMDs, such as WS₂, WSe₂ and MoSe₂. Resonant Raman Spectroscopy (RRS) and second-order Resonance Raman studies will be described. We will present a RRS study of samples of WSe₂ with one, two, and three layers (1L, 2L, and 3L), as well as bulk 2H-WSe₂, using up to 20 different laser lines covering the visible range. For the case of single-layer WS₂, the 514.5 nm laser excitation generates a second-order Raman resonance involving the longitudinal acoustic mode, LA(M). These observations establish an unambiguous and nondestructive Raman fingerprint for identifying single- and few-layered WS₂ films, which will also be discussed. Finally, important features exhibited by few layered TMD systems will be also addressed, which allow the unequivocal identification of few-layered TMD from their bulk and monolayer counterparts.

The transition metal dichalcogenides belong to a class of materials, referred to as van der Waals materials, which are characterized by the strong in-plane bonds and weak van der Waals coupling between the sub-layers. This weak coupling allow for exfoliation of thin films from their corresponding bulk crystals. A few of the transition metal dichalcogenides reveal a transition to the charge density wave (CDW) collective state. The 1T polytype of TaSe₂ has a CDW transition temperature of 473 K. We used graphene-like mechanical exfoliation of TaSe₂ crystals to investigate the thickness dependence of the transition temperature. The transition to CDW phase was monitored with the micro-Raman spectroscopy. It was established that unlike in other systems [1], the Peierls temperature of 1T-TaSe₂ decreases from 473 K to 413 K with the reduction in the film thickness from 150 nm to 35 nm. The experimentally observed trend was explained using ab initio calculations of the total energy of bulk and monolayer 1T and 2H polytypes of TaSe₂ in their normal and commensurate charge-density-wave phases. Obtained results are important for proposed applications of charge-density-wave materials and other van der Waals materials in electronics and sensors.
This work was supported in part by the NSF and SRC Nanoelectronic Research Initiative (NRI) project NSF-1124733 and by the NSF Emerging Frontiers in Research and Innovation (EFRI-2014) program Two-Dimensional Atomic-layer Research and Engineering (2-DARE). This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by the National Science Foundation grant number OCI-1053575.
[1] P. Goli, J. Khan, D. Wickramaratne, R.K. Lake and A.A. Balandin, "Charge density waves in exfoliated films of van der Waals materials: Evolution of Raman spectrum in TiSe₂," Nano Letters, 12, 5941 (2012).

MoS₂ belongs to the layered transition metal dichalcogenides group. Because of its relatively large energy band gap, reasonable room temperature on-off ratios of the MoS₂ field-effect transistors (FETs) and low standby power dissipation, MoS₂ is a possible material for electronic applications. The level of 1/f noise and relative contributions of the metal contacts and MoS₂ channel are important metrics for possible communication and sensor applications of this material [1]. In this presentation, we report results of our investigation of the 1/f noise in thin-films MoS₂ transistors at room temperature and low temperature. The low temperature behavior of 1/f noise provides valuable information for analysis of the noise mechanisms. The number of layers and thickness of MoS₂ flakes are determined by Raman and AFM. The room temperature (RT) noise spectral density of as fabricated and week old devices were ~4×10^-8 and ~2×10^-7 1/Hz, respectively. As the temperature decreased from RT to 77 K the I-V characteristics revealed the Schottky barrier features, which were attributed to changes in the metal - MoS₂ contacts. The noise spectral density increased slightly, revealing a trend different from than that in graphene devices. The obtained results are important for understanding of the material properties and potential electronic applications of MoS₂.
This work was supported in part by the NSF and SRC Nanoelectronic Research Initiative (NRI) project NSF-1124733 and by the NSF Emerging Frontiers in Research and Innovation (EFRI-2014) program Two-Dimensional Atomic-layer Research and Engineering (2-DARE).
[1] J. Renteria, R. Samnakay, S. L. Rumyantsev, C. Jiang, P. Goli, M. S. Shur, and A. A. Balandin, Low-frequency 1/f noise in MoS2 transistors: Relative contributions of the channel and contacts, Applied Physics Letters 104, 153104 (2014).

We present a series of computer-assisted High Resolution Transmission Electron (HRTEM) simulations to determine Moiré patters by induced twisting effects between slabs at rotational angles of 3°, 5°, 8°, and 16°, for Molybdenum di-Sulfide, Graphene, Tungsten di-Sulfide, and Tungsten Selenide. It was possible to observe formation of honeycomb-like structures, creating what we called a new O-lattice because of different lattice parameters for each case, our analysis indicate that the formation of new superlattice is mean to happen due to overlapping of atoms from top slab to bottom slab when rotated with respect to (001)-basal plane.

Advanced separation membranes can provide drinkable water via an energy-efficient and facile way in reverse or forward osmosis systems. Recently, two dimensional (2D) graphene oxide (GO) membranes have shown good selectivity to inorganic salt ions against organic molecules, for which selectivity still needs to be enhanced. Herein we present a new class of 2D metallic carbides MXenes as a new promising membrane material selective to metal ions. MXene membranes display a range of physical properties that make them ideal for separation membranes including flexibility, high mechanical strength, hydrophilic surfaces, and high conductivity. Through the preparation method of vacuum-assisted filtration, MXene membranes can be made with thicknesses ranging from hundreds of nanometers to several micrometers. Like GO, the MXene membranes have layered structure with nanochannels permeable for ions, molecules, gases and water. Here we report micrometer-thick MXene membranes&’ selectivity towards different cations (Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Ni²⁺and Al³⁺) with counter Cl^- anions. Permeation rates were shown to rely on ions&’ hydrated radius with a critical point around 4.0 Å and on charges of cations, which demonstrates better selectivity than GO membranes with similar thickness. To improve the mechanical stability of MXene membranes, MXene /polymer composite membranes were prepared which showed equal or even better selectivity. Analysis about how the interlayer spacing parameter of MXene layers and negative charges on MXene surfaces affect the permeation of ions is provided.

We present a wet chemical approach to synthesize in-plane heterostructures of transition metal dichalcogenides (TMDs) and reduced graphene oxide (rGO). The method uses spin coating to lay the precursors on a Si/SiO₂ substrate, after annealing, the resulting film has a laminate structure formed by rGO and TMD. Scanning electron microscope, energy dispersive X-ray spectroscopy were used to study the morphology and composition of the samples. Raman spectroscopy and transmission electron microscopy were used to assess the presence of TMD monolayers in the films. The versatility of this method was demonstrated by the synthesis of hybrid films of rGO with WS₂, MoS₂ and W_XMo_1-XS₂. In addition it was also possible to control the ratio of W and Mo in rGO/W_XMo_1-XS₂ samples.

We demonstrated a site selective tunability of the optical properties by measurement of the photoluminescence (PL) of laser-doped monolayer MoS₂. The PL intensity of MoS₂ was increased by the laser-assisted p-type doping with a PH₃ dopant gas. This PL modulation caused by switching between excition PL(X) and trion PL(X-) depending on carrier density in the MoS₂. The laser-assisted selective doping method enabled extraction of carriers in monolayer MoS₂. The laser can elevate a surface temperature of MoS₂, which can create the sulfur vacancy of the surface to incorporate the dopants, just below the dissociation temperature of the MoS₂ (1100°C). But the dissociation temperature of PH₃ dopant gas is much lower (375°C) than MoS₂ so the dopant gas was fully dissociate under laser irradiation spot of the MoS₂. This experiment was developed following a sequential process step. First, mechanically exfoliated monolayer MoS₂ flakes were place on 300 nm thick SiO2/Si(100) substrates. Afterward, the sample was exposed dopant gas and laser annealed in the reaction chamber. For examine the enhancement of optical property by laser-assisted selective doping, a Micro-PL and Raman spectroscopy measurement was used.

Graphitic carbon nitride (g-CN) has attracted a wealth of interest for varied applications due to its mechanical, electrical, and catalytic properties. The wide variety of properties observed in the polymorphs of carbon nitride stem from an abundance of possible microstructures and stoichiometries, which result in turn from the differing polycondensation chemistries and conditions used in its syntheses from organic precursors. Although several computational studies have explored the effects of incorporation of heteroatoms (phosphorus, boron, etc.) into carbon nitrides, synthetically such materials remain largely uninvestigated. Carbon nitrides with well-defined structure and heteroatom incorporation are sought to synthetically elucidate the effects of composition and structure on properties.
We have synthesized hydrogen-free, nitrile-bearing monomers, which we have shown to undergo Cequiv;N-trimerization at relatively low temperatures, both neat and when dissolved in molten salt flux. Phosphorus tricyanide [P(CN)₃], for example, polymerizes both from the crystalline solid and as a film deposited during vacuum sublimation at ge;220 °C. The resulting amorphous, heteroatom carbon nitrides (HetCN) exhibit C₃N₃P stoichiometry with no change in composition from the monomer. Films and bulk amorphous solids exhibit different microstructures, evident in the spectroscopy thereof. Utilization of monomers that undergo no compositional change on polymerization allows us to decouple the effects of stoichiometry and microstructure on HetCN properties.
Polymerization of bulk HetCN from a molten salt flux provides a means of controlling monomer concentration and achieving homogeneous polymerization temperatures throughout the specimen, thereby reproducibly tailoring HetCN morphology. We present the isolation of various heteroatom carbon nitrides from unreactive, low melting, eutectic molten salt fluxes.

Single layer MoS₂ is a functional direct band gap semiconductor with optoelectronic properties that are particularly susceptible to internal and external perturbations. For example, two thirds of its constituent atoms interface directly to an external environment, while intrinsic structural disruptions such as grain boundaries extend through the extent of the material. Thus, it is nearly impossible to fully understand the optoelectronic properties of MoS₂ and other transition metal dichalcogenides without properly addressing the omnipresent disorder within these systems. Indeed, large discrepancies in the photoluminescence efficiency and emission spectrum between single layers on different support substrates confirm that single layer MoS₂ is very sensitive to its surrounding environment. Furthermore, striking microscopic spatial heterogeneity is observed in the photoluminescence of a single layer of MoS₂, and yet little is known about the origins of this optoelectronic disorder. While the natural inclination is only to isolate and minimize the disorder, the presence of the disorder also suggests that MoS₂ could be developed into a high-performance sensing platform by harnessing the amplified sensitivity of its optoelectronic properties.
An advanced hyperspectral optical microscopy technique, photoluminescence excitation (PLE) spectroscopy is performed at discrete spatial points and used to spatially map the optical absorption properties and excited state manifold of single-layers of MoS₂. In contrast to conventional absorption spectroscopy that is sensitive to all possible optical transitions at a given energy, PLE addresses only the higher energy optical transitions that efficiently thermalize to the ground state exciton. This increased selectivity unveils transitions and thermalization processes that are otherwise masked, providing valuable insight into the energetic landscape of the excited states in MoS₂. Mapping the PLE spectrum over the spatial extent of individual of MoS₂ flakes reveals that the energies of the absorption resonances exhibit a pronounced intra-flake spatial inhomogeneity that is also mirrored in the energy of the photoluminescence. Although the optical absorption and radiative recombination energies can be substantially disordered, their energetic spacing is remarkably more homogeneous, providing an important clue for disentangling the underlying origins of the optoelectronic heterogeneity and estimating the extent and magnitude of the corresponding effects.

Transition metal dichalcogenides (TMDs) are a promising class of 2D of nanoelectronic materials for flexible electronics, optoelectronics and applications requiring high mobility. Many atomically thin TMDs have significant band-gaps and don&’t suffer from short channel effects, thus making them a promising class of materials for field-effect transistors (FETs). Devising a simple, widely applicable material approach to Ohmic contacts for 2D TMDs is critical to realizing the potential of this material class in enabling high performance nanoelectronic devices. Using an ultra-thin metal-oxide material as an intermediate contact material, we were able to create TMD/metal contacts with true Ohmic behavior. The presentation will include a direct comparison of the contact resistance between TMDs and several metals with and without an intermediate metal oxide layer. Photo-current barrier height studies and IV characteristics confirm the Schottky barriers of several hundred meV without intermediate layers and Ohmic behavior from contacts that contain an intermediate metal-oxide layer. XPS depth profiling was used to investigate the compositional nature of interfaces of the TMD-metal oxide-metal contact. Overall, we believe this simple and ubiquitous approach to low resistance contacts is significant step towards commercial nanoelectronic devices using TMDs.

The deposition of ultrathin films from liquid-phase is still rare; we have demonstrated that ruthenium dioxide (RuO₂) can be conformally deposited in a disordered, nanoscopic form on a wealth of planar and curved substrates using a scalable, atom-efficient, low-temperature, liquid-phase, self-limiting synthetic protocol [1,2]. The resulting supported nanoskins of RuO₂ are electrically conductive (500-1000 S cm^-1) and broadband transparent over an uncommonly large range for an oxide, from UV to microwave [2]. The frequency dependence of the complex conductivity, obtained by broadband terahertz time-domain spectroscopy shows strong localization effects in RuO₂ nanoskins, whereas sputtered ultrathin RuO₂ films exhibit a more conventional Drude-type (i.e., metallic) response. Temperature-dependent magneto-transport measurements (1.75K2 and an overall conductivity-temperature formalism that fits two-dimensional transport rather than the three-dimensional dependence expected of the RuO₂ rutile structure habit (and obtained for the sputtered, crystalline thin films of RuO₂). Freestanding 10-nm-thick RuO₂ films (released from soluble substrates) exhibit mechanical flexibility with a shear modulus an order of magnitude lower than bulk RuO₂ and more consistent with a 2D material. The freestanding nanosheets allow direct analysis by transmission electron micrography and selected area electron diffraction data confirming the predominant role of disorder in this form of RuO₂. Our characterization of this metallic oxide in its disordered, nanoscopic form using optical, structural, thermal, microscopic, mechanical, and electrochemical measurements has led us to categorize it as a member of a rare n-mu; quadrant of electronic materials: one that exhibits a high concentration of electronic carriers (n) of low mobility (mu;). These atypical physical characteristics of disordered RuO₂ may correlate with its remarkable energy-storage and electrocatalytic properties and guide us to the design of newly functional materials once expressed in disordered, nanoscopically connected forms.
[1] C.N. Chervin, A.M. Lubers, K.A. Pettigrew, J.W. Long, M.A. Westgate, J.J. Fontanella, D.R. Rolison, Nano Lett. 9 (2009) 2316-2321.
[2] J.W. Long, J.C. Owrutsky, C.N. Chervin, D.R. Rolison, J.S. Melinger, U.S. Patent Application 20110091723.

The fabrication of 3-D mesoporous materials using building blocks of oxide nanosheets is demonstrated using both planar sheets and a combination of scrolled and planar sheets. Chemistries of interest include MnO2 and complex niobates and titanates, providing planar and stepped nanosheet geometries. Processing includes exfoliation of micron-size powders to form nanosheet suspensions, followed by control of surface charge to coagulate the sheets in 3-D edge-to-face geometry with surface areas of ~200m2/g. Dispersion of the nanosheets is very sensitive to pH and can be adjusted from full dispersion to full agglomeration. The processing parameters can be modified to produce either highly disordered 3-D structures with spherical pores or crumpled layered structures which have 1-D and 2-D pores. Electrochemical testing included CV curves and galvanostatic charge-discharge curves, using Na, Li, and proton electrolytes. The manganite and niobate nanosheet electrodes show metal ion redox reactions occur and that the electrodes are stable to at least 100 cycles.

MXenes, emerging 2-dimensional materials comprised of inorganic ternary metal carbides (and/or nitrides), have seen interest grow sharply since their introduction in 2011. However, their production from MAX phases (of the formula M_n+1AX_n, where M is a transition metal, A is an A-group element, X is C and/or N, and n = 1 to 3) required etching in concentrated hydrofluoric acid (HF). We have replaced concentrated HF with a simple acid-salt mixture, driving the formation of small amounts of HF in-situ to etch the MAX phase. This etching has been observed for a wide variety of acids and fluoride salts so far.
In the case of Ti₃C₂ (the most studied MXene to date) etched with lithium fluoride and hydrochloric acid, not only did the new etchant improve the safety of the process, but the resulting MXene showed reversible intercalation of water at the unit-cell level to discreet lattice spacings, as is common in clays (this behavior has not been observed for Ti₃C₂ produced by HF, and its mechanism is currently under investigation). The intercalation and swelling can be exploited to drastically reduce the time and materials needed to make aqueous dispersions of single MXene flakes and further opens new processing capabilities. The hydrated MXene ‘clay&’ can be shaped, rolled into films, or diluted and painted with utmost ease. Upon drying, the solid is highly conductive and has properties suited for energy storage - rolled Ti₃C₂ films (containing no additives) have shown capacitances on the order of 900 F/cm³.
This methodology has further been applied to produce MXene films from Ti₂C, which was previously inaccessible due to its high susceptibility to decomposition during the delamination process. It may also allow for the delamination of other MXenes; we have indeed observed etching of other MAX compositions (Nb₂AlC, V₂AlC). As a final note, we have produced MXenes with other acids and fluoride salts, demonstrating the flexibility and potential tunability of the etchant system.

Controlling the stoichiometry, number of the layers, crystallite size, growth location, and areal uniformity is challenging in conventional vapor phase synthesis. Here we introduce a new confined vapor transfer growth (VTG) method that preserves stoichiometry, provides uniform growth conditions over large areas, addresses boundary layer nonuniformity, and digitally provides a limited quantity of precursors to grow 2D materials with a controllable number of layers. The technique is demonstrated for the growth of GaSe and MoSe₂ 2D crystals on SiO₂/Si substrates over large areas, or in pre-defined patterns that are digitally transferred from one substrate to another. A uniform and precise amount of stoichiometric nanoparticles are first synthesized and deposited onto a source substrate by pulsed laser deposition (PLD) at room temperature. This source substrate is then covered with a receiver substrate to form a confined VTG system. By simply heating the source substrate in an inert background gas, a natural temperature gradient is formed that evaporates the confined nanoparticles to grow large, crystalline 2D nanosheets on the cooler receiver substrate. This PLD-VTG synthesis and processing method appears promising for growth of a wide variety of binary and ternary 2D metal chalcogenides as it provides a versatile and digital approach for stoichiometric transfer of a variety of precursor materials onto various source substrates with subsequent synthesis on a number of receiving substrates through confined growth.
Synthesis science including in situ PLD diagnostics, TEM analysis, SEM and AFM studies, bulk crystal growth, and transfer technique development was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (BES), Materials Sciences and Engineering Division and performed in part as a user project at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. Characterization science at CNMS including optical characterization and lithography techniques was supported by the Scientific User Facilities Division, BES.
Biography
Masoud Mahjouri-Samani is currently a postdoctoral research associate in Center for Nanophase Materials Sciences at Oak Ridge National Laboratory. Masoud received his Ph.D. from the University of Nebraska-Lincoln, department of Electrical Engineering. Currently, his research interest is focused on understanding and controlling the synthesis of 0-, 1-, and 2-dimensional nanomaterials utilizing laser-based techniques for use in energy related applications.

Two-dimensional conductors have received great attention due to their unique properties; for example, graphene, the first 2D material extensively studied, is an intrinsically zero-gap semiconductor that has charge carriers with zero effective mass. Beyond graphene there have been many other 2D materials that have recently been discovered, such as MoS₂ and BN. All these 2D conductors share two common features: first, they have intrinsic hexagonal patterning, and second quantum confinement plays a significant role in determining their properties in nanosheet form.
The metal diborides have an intrinsic hexagonal patterning in the layers of boron; however, they are known to be strongly bonded and have excellent electrical and thermal conductivity due to the transition metal atoms that are bonded to the boron layer. The metal atoms contribute d-states near the Fermi level, which prohibit the opening of a band gap, even if a nanostructure could be created. Here we demonstrate that metal diborides without d-states, for example MgB₂, can be transformed from a semimetal to a semiconductor by thinning the crystal to a 2D structure. In the case of MgB₂ the quantum confinement effect is sufficient to open a band gap as large as 0.5 eV, which makes the compound a viable semiconducting 2D material.
In our work, first-principles, density functional theory methods are used along with many-body Green&’s function methods to understand the electronic structure of bulk and nanostructured MgB₂. The difference between MgB₂ and transition metal diborides is explored by comparing MgB₂ to the archetypical metal diboride ZrB₂. The mechanical properties are also examined and it is found that MgB₂ cleaves at stresses 40% that of ZrB₂, which suggests that processing MgB₂ into nanosheets is plausible.

In this work, with the aim of exploring thermal transport properties of Single-Layer MoS₂ (SL-MoS₂), the phonon relaxation times are calculated. We utilize the spectral energy density (SED) [1] method to calculate anharmonic phonon relaxation times. Within the SED approach, the required velocities of atoms in the simulation box at each time step are calculated from ab initio molecular dynamics (AIMD) simulations. VASP simulation package [2] is used for performing the calculations. The conventional cell contains 48 atoms and has the dimensions where is the lattice constant of the MoS₂ crystal. To prevent interaction of the monolayer with the neighboring layers resulting from periodic boundary condition in DFT calculation, the dimension of the conventional cell in the out-of-plane direction is considered to be . The longer side of the simulation box is along the Γ-M direction in reciprocal space. This choice of simulation cell allows us to calculate the relaxation times for phonons with smaller wave vectors in the Γ-M direction. For acoustic phonons, the calculated phonon relaxation times are in the order of 100 ps for very long wavelength phonons and decrease strongly to an average of ~10 ps moving toward short wavelength phonons. For the optical modes, the relaxation times are generally small (in the order of 5 ps). Therefore, we expect a high contribution from long wavelength acoustic modes into thermal conductance.
[1] J. A. Thomas, J. E. Turney, R. M. Iutzi, C. H. Amon, and A. J. McGaughey, Physical Review B 81, 1-4 (2010).
[2] G. Kresse and J. Hafner, Phys. Rev. B 47, 558 (1993).

Surface acoustic waves have been used to probe surface states in bulk semiconductors and 2D electron gases in semiconductor heterostructures^1-3. A commonly used SAW is the Rayleigh wave where the displacement vector u(t) lies in the plane formed by the normal to the surface and a propagation wave vector parallel to the surface. The wave is elliptically polarized in this plane and decays exponentially into the bulk away from the surface over a span of one wavelength. Since most of the energy in the wave is confined within a fraction of the wavelength, it is highly sensitive to any perturbation on the surface. A typical way to generate SAW involves interdigitated transducers on piezoelectric substrates. For non-piezoelectric substrates, these waves can be generated by patterning metallic nanostructures over the substrate and using intense laser beam^{4, 5}. However, these SAWs dissipate their energy to bulk modes due to the mass loading from the substrate and typical value of Quality factor is around 500. Here, we show an improvement in the quality factor of SAWs by implementing a Bragg Filter which reflects the dissipated energy back to the surface. Bragg filters consists of multiple layers of alternating materials with contrasting acoustic impedances. Numerical Simulations predict that the Q Factor can be increased by two orders of magnitude (~26000). These high quality SAWs enable us to accurately study the interactions between 2-D materials such as molybdenum disulphide, Graphene and different substrates.
References:
1. Ayub, F. M.; Das, P. J. Appl. Phys. 1980, 1, 433-436.
2. Das, P.; Motamedi, M.; Gilboa, H.; Webster, R. Journal of Vacuum Science and Technology 1976, 4, 948-953.
3. Wixforth, A.; Scriba, J.; Wassermeier, M.; Kotthaus, J.; Weimann, G.; Schlapp, W. Physical Review B 1989, 11, 7874.
4. Bonello, B.; Ajinou, A.; Richard, V.; Djemia, P.; Cherif, S. J. Acoust. Soc. Am. 2001, 4, 1943-1949.
5. Sadhu, J.; Lee, J.; Sinha, S. Appl. Phys. Lett. 2010, 13, 133106-133106-3.

Since layered transition-metal dichalcogenides(TMD) have demonstrated novel electronic and optoelectronic property, many research have been involved for synthesis and integration into future electronic device. Unlike graphene, TMD materials have opened band gap, and these band structure can be altered as thickness. For successful integration of TMD into device, proper doping should be involved in TMD with high stability in ambient condition and fabrication process. Here, this study demonstrates realization of surface N doping on MoS₂ with polar TiOPc by employing UHV scanning tunneling microscopy (STM). Multilayer MoS₂ flakes were cleaved in ambient condition and transferred into UHV chamber immediately, then TiOPc monolayers were deposited on MoS₂ surfaces by organic molecular beam epitaxy. After deposition, TiOPc forms a monolayer with only few defects, and the crystal structure of monolayer has diamond lattice in a 1.4 x 1.4 nm grid. This crystal structure indicates that each TiOPc in the monolayer is directed outward to vacuum. STS shows the band gap of the monolayer is 1.8 eV, while bulk clean MoS₂ has 1.3eV band gap. Moreover, Fermi level of deposited TiOPc monolayer on MoS₂ is shifted to valence band, consistent with P type shift, while TiOPc monolayer on HOPG has Fermi level in middle of band gap. On the other hands, MoS₂ surface, which was deposited TiOPc layer with less than monolayer coverage, has shifted Fermi level into conduction band, consistent with N type doping, while clean MoS₂ surface show middle Fermi level. It can be hypothesis that this surface doping of MoS₂ with polar TiOPc results from difference of work function between TiOPc layer and MoS₂. Deposited TiOPc layer has very high thermal stability on MoS₂. Normally, deposited TiOPc layer on HOPG starts to be removed at 523K, indicating very weak interaction between TiOPc and HOPG. However, deposited TiOPc layer on MoS₂ does not removed, unless annealing above of 673K. This high thermal stability indicates there are relatively strong interaction between TiOPc and MoS₂ surface. DFT calculation shows 0.056e^- charge transfer from TiOPc to MoS₂, consistent with N doping of MoS₂. Consequently, integration TiOPc layer into MoS₂ results in surface doping with high stability, thus stable doping condition in ambient condition can be expected.

Layered transition metal dichalcogenide (TMDC) with hexagonal lattice structure has six valleys at corners of the Brillouin zone. The nontrivial Berry curvature distribution renders the adjacent valleys with distinguishable valley angular momentum, which enables itself as an ideal 2D valleytronic platform^1,2. Recent studies reported strong excitonic effect in monolayer WS₂ and each excitonic state is identified with a well-defined orbital angular momentum³_, however the anticipated selection rules involve nonlinear optical processes are not clear. Here we show valley angular momentum (VAM) together with exciton angular momentum (EAM) impose different valley-exciton locked selection rules for second harmonic generation (SHG) and two photon luminescence (TPL) in monolayer WS₂. Moreover, the two-photon induced valley populations yield net circular polarized photoluminescence after a sub-ps interexciton relaxation. The work demonstrates a new approach to control valley population at different excitonic states for next generation of optical circuits and quantum information computing.
[1] Mak, K.F., He, K., Shan, J. & Heinz, T.F. Control of valley polarization in monolayer MoS₂ by optical helicity. Nature Nanotechnology7, 494-498 (2012).
[2] Ross, J.S. et al. Electrically tunable excitonic light emitting diodes based on monolayer WSe₂ p-n junctions. Nature Nanotechnology9, 268-272 (2014).
[3] Ye, Z. et al. Probing excitonic dark states in single-layer tungsten disulfide. Nature 513, 214-218 (2014).

In this work, the operation of n- and p-type field-effect transistors (FETs) on the same WSe₂ flake is realized and a complementary logic inverter is demonstrated. The p-FET is fabricated by contacting WSe₂ with a high work function metal Pt which facilities hole injection at the source contact. The n-FET is realized by utilizing selective surface charge transfer doping with potassium to form degenerately doped n+ contacts for electron injection.¹ An ON/OFF current ratio of > 10⁴ is achieved for both n- and p-FETs with similar ON current densities. A DC voltage gain of > 12 is measured for the complementary WSe₂ inverter. This works presents an important advance towards realization of complementary logic devices based on layered chalcogenide semiconductors for electronic applications.
The fabrication process of WSe₂ CMOS devices is as follows. WSe₂ flakes were deposited on Si/SiO₂ (thickness, 260 nm) substrates using micromechanical exfoliation technique. Three electron-beam (e-beam) lithography steps were performed in order to define the p- and n- contacts as well as the top-gate stack. Specifically, a Pt/Au/Pd (10/30/20 nm) metal stack was deposited by e-beam evaporation for the p-side source-drain (S/D) contacts followed by lift-off in acetone. Au (60 nm) contacts were then formed for the n-side S/D electrodes by e-beam evaporation and lift-off in acetone. The gate stacks are formed by e-beam lithography, atomic layer deposition (ALD) of ZrO₂ (thickness, ~20 nm) at 120 #8304;C, evaporation of Ni/Au electrodes (30/30 nm), and lift-off of the entire gate stack in acetone. Subsequently, potassium doping is performed in vacuum at a pressure of 4x10^-5 mbar by evaporating potassium from a commercially available dispenser filament (SAES Getters) by applying a 5 A DC current through it.
[1] Fang, H.; Tosun, M.; Seol, G.; Chang, T. C.; Takei, K.; Guo, J.; Javey, A. Degenerate n-Doping of Few-Layer Transition Metal Dichalcogenides by Potassium. Nano Lett. 2013, 13, 1991-1995.

In this work, we report the type-I heterojunction formation of as-exfoliated MoS₂ flakes by thickness modulation. MoS₂ crystals, are transferred onto Si/SiO₂ chips with 260nm thick SiO₂ using micromechanical exfoliation technique. The flakes of interest, i.e. mono-bulk junctions are located using an optical microscope. These mono-bulk heterojunction flakes are formed during the exfoliation naturally. The monolayer is found as underlying the whole bulk flake and attached to the bulk flake as one single flake. No transfer procedure was carried out. Kelvin probe force microscopy (KPFM) is used to map the surface potential at the monolayer-bulk heterojunction to determine the type-I heterobarrier height at the monolayer-bulk junction.¹
Scanning photocurrent microscopy (SPCM) is done to investigate the spatial photocurrent response along the device including the source and the drain contacts as well as the mono-bulk junction.² The peak photocurrent observed by SPCM at the heterojunction is attributed to the type-I heterojunction that is formed due to the difference in the band gaps and Fermi levels of the monolayer and the bulk MoS₂. A heterobarrier height of around 120meV is determined by KPFM that blocks the photocurrent generation with local emitted light along the device except at the monolayer-bulk MoS₂ heterojunction.
[1] Nonnenmacher, M.; O&’Boyle, M. P.; Wickramasinghe, H. K. Kelvin probe force microscopy. Applied Physics Letters 1991, 58, 2921-2923.
[2] Wu, C.-C.; Jariwala, D.; Sangwan, V. K.; Marks, T. J.; Hersam, M. C.; Lauhon, L. J. Elucidating the Photoresponse of Ultrathin MoS2 Field-Effect Transistors by Scanning Photocurrent Microscopy. The Journal of Physical Chemistry Letters 2013, 4, 2508-2513

We have successfully incorporated either non-radioactive Co(II) and radioactive ⁵⁷Co(II) into MgAl-layered double hydroxide (LDH) nanomaterials, which have been known as potential cellular drug delivery carriers. Pristine MgAl-LDHs with uniform size and morphology was obtained through conventional coprecipitation followed by hydrothermal treatment at 150 ^oC. According to X-ray diffraction patterns and electron microscopy, pristine LDH was determined to have typical hydrotalcite-like structure, uniform 250 nm size and hexagonal plate-like morphology. In order to substitute non-radioactive Co(II) into LDH lattice, suspension of MgAl-LDHs were added to Co(II) solution and hydrothermally treated at 150 ^oC. According to the inductively coupled plasma-atomic emission spectroscopy, the amount of incorporated Co(II) in LDH lattice showed time-dependent increase. The incorporation saturated at 12 hours showing Mg(II)/Co(II)/Al(III) in the LDH lattice of 1/1/1. We found that the crystallinity, particle size and morphology of LDH did not change significantly after Co(II) substitution without evolution of impurity phase. From energy dispersive spectroscopy, solid UV-vis and X-ray absorption spectroscopic results, substituted Co(II) were homogeneously distributed through LDH lattice and stabilized in the octahedral site where Mg(II) originally occupied. We applied the same hydrothermal reaction to incorporate radioactive ⁵⁷Co(II) into LDH lattice. It was revealed that radioactive Co(II) could be successfully incorporated into LDHs, and thus prepared ⁵⁷Co(II)-MgAl-LDH showed efficient cellular uptake.

Hexagonal boron nitride is a two dimensional material with exciting mechanical and structural characteristics. Due to its unique properties, it has been pursued for applications in electronics, thermal management, aerospace technologies, composites, and biomedicine. However, the defect structures of h-BN at various temperatures are still not well understood. We have used aberration corrected transmission electron microscopy at various temperatures to study the temperature dependence of nanoscale defects.

Molybdenum disulfide, atomically layered transition metal dichalcogenide, has emerged as one of the promising candidates for next generation thin-film optoelectronic devices due to its strong optical absorption and high electronic conductivity. There have been several reports demonstrating the excellent photo-response in MoS₂, but most of these reports have been limited to MoS₂ obtained using mechanical exfoliation from the bulk crystal. Furthermore, the observation of the excitonic states in MoS₂ has required isolating MoS₂ from the substrate by placing it on trench. Morover, there have been no reports of using photocurrent spectroscopy to measure the excitonic states in CVD grown MoS₂. In this work we demonstrate that high crystalline quality single-layer MoS₂ flakes can be obtained by using MoO₂ as a precursor instead of the commonly used MoO₃. Raman and Photoluminescence spectra verify the high quality of the grown samples. Photocurrent spectroscopy performed in an FET geometry on as grown CVD samples shows clear excitonic peaks at room temperature. The magnitude of the photocurrent is strongly bias dependent and increases with increasing the source-drain voltage. We measure an excitons binding energy of 655meV and a spin-orbit coupling of 165meV. Finally, we also performed temperature and gate dependence of the exciton binding energy measured from 300 K to 4.2 K. These results demonstrate that intrinsic optoelectronic properties can be realized by controlling the crystalline quality and defect density during CVD synthesis of TMDs.

Isolated monolayer (ML) crystals of hexagonal transition-metal dichalcogenides (h-TMDCs) can be manually stacked to form atomic two-dimensional (2D) heterostructures, and the strong inter-ML coupling produces unusual electronic and photonic responses. Despite of recent explosive investigations, large-area growth of the ML heterostructures is not available, and secondly the precise control over the crystallographic rotations of each ML in such 2D superstructures is lacking, often ensuing a momentum mismatch during inter-ML excitations. Here, we show the heteroepitaxial 2D stacking and stitching of MoS₂ and WS₂ MLs, by manipulation of 2D nucleation kinetics during a sequential vapor-phase growth. It enables to create the hexagon-on-hexagon unit cell stacking without incommensurate interlayer rotations, i.e. free of 2D Moiré interference.

Ionic gate-type field effect transistor is quite powerful for creating new functionalities of 2D systems, because of its high carrier density in the order of 10¹⁴ cm^-2. This device have enabled us ambipolar operation, superconductivity, and electrically switchable light source in transition metal dichalcogenide systems [1-4]. Thanks to the atomically flat surface available, varieties of 2D materials can be target materials to fabricate ionic gating device or electric double layer transistor (EDLT). In the presentation, discussion is given on fabrication of EDLT devices on 2D materials and search for new superconductors [5], followed by the investigation of nature of superconductivity [6]. In addition to the EDLT based functional devices, we are searching for new superconductors with ionic gating technique, based on not only electrostatic charge accumulation but also intercalation of cations. As for the electric field induced superconductivity with ionic liquids, we confirmed that the superconductivity is highly two-dimensional with a thickness of 1~2 nm, which is much smaller than the in-plane coherence length. These values are close to the Thomas-Fermi screening lengths, providing additional evidence that the carriers are accumulated by the electrostatic mechanism. Another important aspect of electric field effect is that the spatial inversion symmetry is inherently broken in the present device, and might produce a serious impact of paring symmetry, particularly in systems with strong spin-orbit interactions. In fact, the in-plane H_c2 was found considerably enhanced by a factor of more than three in comparison to the value of the Pauli limit. This strongly indicates considerable mixture of triplet nature in the paring in electric field induced superconductivity. [1] Y. J. Zhang et al., Nano Lett. 12, 1136 (2012). [2] J. T. Ye et al., Nat. Mater. 9, 125 (2010). [3] J. T. Ye et al., Science 338, 1193 (2012). [4] Y. J. Zhang et al., Science 344, 725 (2014). [5] W. Shi et al., submitted. [6] Y. Saito et al, submitted.

We systematically measured the optical constants of MoS2 film of different layer numbers range from monolayer to 10 layers using ellipsometor. We find out the optical constants of the atomic thin monolayer MoS2 is strikingly large almost comparable to bulk MoS2. And the imaginary part of dielectric constants is getting smaller as layer number increases until reach to 5 layers, then the dielectric constants begin to increase with larger layer numbers. We provided evidences both from experiments and theoretical calculations that the abnormal large optical constants of monolayer MoS2 and the layer dependent behavior both come from broadband excitonic-dominate effect in monolayer and a few layer MoS2 films. This is not like any conventional semiconductor material that excitonic only show very narrow band feature upon the continumm background in the optical spectrum, this work may pave a way for realizing electric control broadband optical properties semiconductor devices.

Two dimensional transition metal dichalcogenides are a recent addition to the 2D electronic materials family. They have shown outstanding electrical and optical properties for new optoelectronic device concepts. In this talk, we will first discuss the unique valley excitonic physics, including optical generation of valley polarization and valley coherence with magnetic manipulation. We will then talk about optoelectronic devices based on monolayer WSe₂, including p-n junctions as light emitting diodes and hybrid monolayer semiconductor/photonic crystal cavity devices for ultralow threshold nanolasers. We will conclude the talk with a discussion of MoSe₂-WSe₂ heterostructures, including demonstration of interlayer excitons in vertical heterostructures and the creation of atomic seamless lateral heterostructures with a uniform hexagonal crystal lattice structure.

Light emission in two-dimensional (2D) transition metal dichalcogenides (TMDs) changes significantly with number of layers and stacking sequence. While the electronic structure and optical absorption are well understood in 2D-TMDs, much less is known about exciton dynamics and radiative recombination. In this talk, we show first principles calculations of intrinsic exciton radiative lifetimes at low temperature (4 K) and room temperature (300 K) in TMD monolayers with chemical formula MX2 (M=Mo,W and X=S,Se), in bilayer and bulk MoS2, and in two MX2 hetero-bilayers. Our results elucidate the time scale and microscopic origin of light emission in TMDs, which have been the subjects of recent intense investigation.
We find radiative lifetimes of a few ps at low temperature and a few ns at room temperature in the monolayers, and slower radiative recombination in bulk and bilayer than in monolayer MoS2. The MoS2/WS2 and MoSe2/WSe2 hetero-bilayers exhibit very long-lived (~30 ns at room temperature) inter-layer excitons constituted by electrons localized on the Mo-based and holes on the W-based monolayer; this finding agrees with very recent ultrafast spectroscopy experiments, and helps resolve a controversy on the topic. In closing, we discuss how the radiative lifetime tunability, together with the ability shown here to predict radiative lifetimes from computations, can be employed to manipulate excitons in TMDs and their heterostructures for application in optoelectronics and solar energy conversion.

Recent efforts have focused on the novel electronic, optical, mechanical, and structural properties exhibited by monolayer transition metal dichalcogenides. In the time domain, much is known about the carrier and exciton dynamics exhibited by these materials, but little is known about their associated ultrafast structural response. Here we describe recent nonlinear optical studies using the second order nonlinear susceptibility as a window into the photo-induced structural response of these materials on sub-picosecond time-scales. Under excitation conditions corresponding to of order 1 exciton / unit cell, large amplitude increases in the second harmonic on few picosecond time-scales are observed within single domain MoS2 monolayers, decaying on time-scales of order 100 picoseconds with no photo-induced change in lattice symmetry observed, as determined by the polarization dependence of the induced second harmonic light. Complemented by spectroscopic optical and terahertz probes in both the above-band-gap and below-gap regimes, and by first principles modeling of the second harmonic response, we show that these effects can be understood in terms of the extreme electronic temperatures that are induced, without modification of the unit cell structure. We will describe also first measurements using time-resolved x-ray scattering approaches to directly investigate the structural response of 2D transition metal dichalcogenides under intense photo-excitation conditions on picosecond time-scales.

Beyond graphene, layered metal chalcogenide two-dimensional (2D) crystals have recently attracted tremendous interest, such as the exotic valleytronic transition metal dichalcogenides (TMD) and the topological insulating bismuth chalcogenides. Dimensionality not only gives rise to novel optical and electronic properties in these metal chalcogenide 2D crystals, but also brings new excitement to the phonon properties. 2D crystals possess lower symmetry compared to that of their bulk counterparts and thus completely different phonon modes and behaviors. Here, using both Raman spectroscopy and first principles calculations, we uncover the ultra-low frequency (5~55 cm^-1) interlayer breathing and shear modes in few-trilayer MX₂ (M=Mo, W; X=S, Se), prototypical layered TMDs, as well as in few-quintuple layer Bi₂X₃ (X=S, Se). Most of the observed interlayer vibrational modes are absent in the bulk limit, due to a higher crystal symmetry. Remarkably, all the shear modes exhibit frequency softening with decreasing crystal thickness, while all the breathing modes show frequency hardening. The frequencies of these modes can be perfectly described using a simple linear chain model with only nearest-neighbour interactions. Our results shed light on a general understanding of the Raman/IR activities of the phonon modes in layered metal chalcogenides and their evolution behaviors from 3D to 2D. The ultralow frequency interlayer vibrational modes are expected to be universal in all layered 2D crystals.

We present measurement of the acoustic phonon dispersion relations of 2D chalcogenide materials, throughout the mesoscopic wavelength range, using confocal Brillouin microscopy. Brillouin spectroscopy is a laser light scattering technique, similar to Raman, that measures scattering from acoustic phonons rather than optical phonons. We determine the longitudinal and transverse sound velocities of layered 2D materials and demonstrate cross-over from bulk-like to confined phonon behavior as a 2D layered material approaches atomic thicknesses. While many investigations focus on electronic properties, equally relevant are the acoustic phonons, which can profoundly impact electronic transport in device application. The information from these studies comes at a crucial moment and provides fundamental knowledge of 2D chalcogenide material phonon behavior.

We perform first-principles calculation of the electron-phonon interactions in phosphorene, a monolayer of black phosphorus, to assess its potential as a thermoelectric material. Electron-phonon matrix elements are extracted from density functional perturbation theory, interpolated to dense meshes using maximally localized Wannier functions, and used in Boltzmann transport equation to calculate electronic transport properties. Simulation results reveal that phosphorene possesses nearly perfect electronic properties for thermoelectric applications: e.g. step-like density of states, anisotropic effective masses and high carrier mobility, which lead to an extraordinary thermoelectric power factor (~1700 mu;W/cm-K² at room temperature, more than 30 times higher than state-of-the-art commercial thermoelectrics). However, the overall thermoelectric performance of phosphorene is largely compromised by its high electronic thermal conductivity. Combined with the calculated lattice thermal conductivity, we predict an optimal zT of ~0.7 in n-type and ~0.8 in p-type up to 800K for a phonon-limited impurity-free phosphorene film.

We explore the role of substrate in the localization and scattering properties of molybdenum disulfide and introduce a new design concept for control of electrical and optical properties of this material. The growing interest in the properties of transition metal dichalcogenides has been followed by understanding the intrinsic properties of these materials. It is known that defects of a variety of types have a major role in modifying the inherent characteristics of MoS₂. The goal has always been to heal or avoid such defects in order to understand and exploit the materials in conventional application that charge carrier scattering and localizations are not desirable. Here we argue that tuning of extrinsic substrate defect states can be utilized as an approach for control of MoS₂ electronic transport and optical properties. We utilize a robust approach of interface engineering based on self assembly of molecular species on conventional oxides to tune and control key MoS₂ electronic and optical properties. Our temperature dependent electronic transport and photoluminescence unveils the nature and energetics of the introduced trap states. We conclude that exploring the substrate defect properties may be a viable approach for material property modification in transition metal dichalcogenide atomic layers.
Najmaei, S.; Zou, X.; Er, D.; Li, J.; Jin, Z.; Gao, W.; Zhang, Q.; Park, S.; Ge, L.; Lei, S.; Kono, J.; Shenoy, V. B.; Yakobson, B. I.; George, A.; Ajayan, P. M.; Lou, J. Tailoring the Physical Properties of Molybdenum Disulfide Monolayers by Control of Interfacial Chemistry. Nano Letters2014, 14, 1354-1361.

Transition metal dichalcogenides (TMDCs), such as MoS2 and WSe2, have recently gained tremendous interest for electronic and optoelectronic applications. MoS2 and WSe2 monolayers are direct bandgap and show bright photoluminescence (PL), whereas multilayers exhibit much weaker PL due to their indirect optical bandgap. This indirect nature of multilayer TMDCs presents an obstacle for a number of device applications involving light harvesting or detection where thicker films with direct optical bandgap are desired.
Strain can be used to modulate the band structure and engineer the properties of a material. Specifically, the lattice constant and van der Waals gap for TMDCs change by strain. This leads to a direct change in the electronic band structure and hence the energies of the conduction band (CB) minima and valence band (VB) maxima for the material. If the energy difference of the indirect and direct bandgaps is small, then it may be possible to achieve a crossover from one to the other using strain. For example, Ge shows an indirect to direct bandgap transition when strained due to the small difference of its two energy bandgaps. The effect of strain on TMDCs has been previously studied for MoS2 as a model TMDC material. However, for MoS2 multilayers, the direct and indirect bandgap differ by a large value (i.e., sim;300 meV for bilayer MoS2) and hence no transition is seen on the application of up to 2.2% uniaxial tensile strain. In contrast to MoS2, WSe2 multilayers have a much smaller difference between the direct and indirect bandgaps, on the order of 40 meV for bilayer WSe2. Thus, a crossover from indirect to direct bandgap should be possible in multilayer WSe2 for practically achievable strain values, similar to the case of Ge.
Here, we experimentally demonstrate a drastic enhancement in PL intensity for multilayer WSe2 (2minus;4 layers) under uniaxial tensile strain of up to 2% (using the two-point bending method). Specifically, the PL intensity of bilayer WSe2 is amplified by sim;35×, making it comparable to that of an unstrained WSe2 monolayer measured under similar experimental conditions. This drastic PL enhancement is attributed to the indirect to direct bandgap transition (Σc-Kv to Kc-Kv) for strained bilayer WSe2, as confirmed by density functional theory (DFT) calculations. Our results present an important advance toward controlling the band structure and optoelectronic properties of few-layer WSe2 via strain engineering, with important implications for practical device applications such as light-emitting diodes and lasers.
Moving forward, band structure and bandgap engineering with strain can be used as a method to change the mobility of carriers in WSe2. In addition to emission properties, future absorption studies under strain would provide great insight into the potential use of WSe2 in applications like photovoltaics and photodetectors.
Ref: S. B. Desai, et. al., Nano Lett. 2014, 14, 4592minus;4597

Building practical devices from 2D semiconducting crystals and their heterostructures is an intensively pursued research area. Currently, chemical vapor deposition (CVD) of monolayer semiconductors is the only practical way to synthesis these materials at industrial scale. However, elastic properties of CVD-grown 2D semiconductors and their heterostructures have not been measured, although their less-defective, exfoliated counterparts have. In this work we experimentally and theoretically characterized the elastic modulus of CVD-grown MoS₂, WS₂ as well as their heterostructures with each other and with graphene. The 2D moduli of heterostructures are slightly lower than the sum of 2D modulus of each layer, but comparable to the corresponding bilayer homo structures, implying similar interactions between hetero monolayers compared to between homo monolayers. The interlayer coupling of different bilayer homo or hetero structures is also qualitatively compared by introducing a sliding coefficient. Our results provide calibrated values of elastic modulus of these structures for various applications, especially in flexible electronic and mechanical devices.

Two-dimensional analogues of traditional semiconducting devices based on novel hybrid van der Waals materials are rapidly being demonstrated and improved. However, device performance has been highly variable due to sample-dependent differences in electronic structure arising from imprecise fabrication and nanoscale defects. Measuring the spatial variations in electronic structure as they relate to device performance has been difficult.
Here we demonstrate the capability to nondestructively image variations in the electronic structure of 2D semiconductors with nanometer spatial resolution using scanning microwave microscopy (SMM). As model systems we use few-layer crystals of the transition metal dichalcogenides (TMD&’s) MoS2 as well as n-doped and p-doped WSe2. We modify the sample charge carrier concentration using an applied tip bias to reversibly optimize and control sample contrast. The optimized contrast allows us to identify spatial variations in sample conductivity occurring over length scales of several microns as well as correlated with localized surface defects. We further perform local bias-dependent spectroscopy, which we combine with finite element modeling to determine the dominant charge carrier type and extract the local dopant concentration. While demonstrated for the case of TMD&’s, this technique can in principle provide nanometer-resolved characterization of electronic structure of any 2D semiconducting material or device.

The recent emergence of two dimensional (2D) materials has enabled the realization of atomically thin heterostructure devices with vertically stacks of disparate 2D materials. The monolayer thick structure of these materials allows electrostatic doping modulation of the overlying layers in a vertically stacked heterostructure.¹ While a majority of work is focused on stacking varying combinations of 2D materials only, an all 2D structure is not necessary to achieve gate-tunable devices. Here, we demonstrate a gate-tunable p-n heterojunction diode using one dimensional semiconducting single-walled carbon nanotubes (s-SWCNTs) and 2D single-layer molybdenum disulfide (SL-MoS₂) as p-type and n-type semiconductors, respectively.² The vertical stacking of these two direct band gap semiconductors forms a heterojunction with electrical characteristics that can be tuned with an applied gate bias over a wide range of charge transport behavior ranging from insulating to rectifying with forward-to-reverse bias current ratios exceeding 10⁴. The transfer characteristics of this p-n heterojunction has a unique 'anti-ambipolar' characteristic with two off-states at either extremes of gate voltage range with a current maxima in between them.² The continuous transition from a positive to negative transconductance in an anti-ambipolar characteristic enables operation of analog communication circuits with a reduced number of circuit components compared to unipolar transistors. This anti-ambipolar characteristic can be widely generalized to heterojunctions of other materials such as s-SWCNTs and n-type amorphous indium gallium zinc oxide (a-IGZO), ultimately leading to all solution processed heterojunctions on a wafer scale.³
References:
1. Jariwala, D.; Sangwan, V. K.; Lauhon, L. J.; Marks, T. J.; Hersam, M. C. ACS Nano 2014, 8, 1102-1120.
2. Jariwala, D.; Sangwan, V. K.; Wu, C.-C.; Prabhumirashi, P. L.; Geier, M. L.; Marks, T. J.; Lauhon, L. J.; Hersam, M. C. Proc. Natl. Acad. Sci. U.S.A. 2013, 110, 18076-18080.
3. Jariwala, D.; Sangwan, V. K.; Seo, J. T.; Xu, W.; Smith, J.; Kim, C. H.; Lauhon, L. J.; Marks, T.; Hersam, M. C. 2014, (submitted).

The functionality of novel nanostructures depends strongly on local physical and electronic properties, increasing the importance of optically probing matter with true nanoscale spatial resolution. In previous work [1], we proposed a novel photonic-plasmonic hybrid Scanning Near-field Optical Microscopy (SNOM) probe called the “campanile” tip. These campanile tips couple the photonic to the plasmonic mode, then adiabatically compress the plasmon mode, over a broad bandwidth. Using these probes, we mapped carrier radiative recombination within individual CVD grown Molybdenum disulfide (MoS₂) flake, observing optoelectronic heterogeneity with deep subwavelength length scale. It had been previously shown that grain boundary and point defects will significantly change the photoluminescence (PL) emission property of these materials. Here, by combining near-field campanile hyperspectral imaging, scanning electron microscopy, scanning electron microscopy and transmission electron microscopy, we have mapped local PL emission properties with nanometer resolution. We show for the first time on such a small length scale how local morphology of MoS₂ changes it&’s the local PL emission properties.
[1] W. Bao et al., Science. 338, 1317 (2012).

2D materials have attracted a great deal of attention for their unique electrical and magnetic properties, but may also play important roles in energy storage applications. Lithium-ion batteries and supercapacitors are widely used to power mobile devices, but the energy and power densities of the electrode materials still need improvement. Many conventional battery materials have layered structures, and hence can be readily exfoliated into 2D nanosheet materials. The high surface area and short ionic diffusion distances in the 2D nanosheets may improve the charging/discharging rates and result in more lithium insertion or surface adsorption. Furthermore, hybrid electrode materials comprised of layers of different cathode materials may be possible by reassembling different nanosheets. These sandwich structures could potentially result in unique synergistic effects and novel redox behavior due to the interactions from different sheets. Finally, we can obtain better understanding of the structure of complex layered cathode materials through exfoliation and high resolution ex-situ microscopy studies.
Here we present our synthesis of 2D nanosheets of two common lithium-ion battery materials, LiCoO₂ (LCO) and LiNi_1/3Mn_1/3Co_1/3O₂ (NMC). Nanosheets were obtained by exfoliation of LCO and NMC particles using traditional osmotic swelling with tetraethylammonium (TEA). TEM, SEM and AFM analysis showed that the particles were successfully exfoliated nanosheets with around 2 nm thickness. XRD and electron diffraction pattern analysis showed that these materials had hexagonal structures with good crystallization. A reassembly process was developed and applied to obtain LCO, NMC, and LCO/NMC hybrid particles. Electrochemical evaluation of the particles as electrodes for lithium-ion batteries and supercapacitors were performed. Our work is a firm step forward on improving understanding of osmotic swelling processes for the synthesis of nanosheets from complex metal oxides as well as the design and fabrication of high performance hybrid electrodes for energy storage applications.

Recently, molybdenum disulfide (MoS₂) has attracted great attention due to its intriguing electrical properties. Despite the merits of MoS₂, large variations in the transport properties of MoS₂ field effect transistor (FET) devices due to extrinsic effects, such as absorption of oxygen and/or water from the environment, may result in limitations for exploring the intrinsic properties and overall stability. Here, we have investigated the gate bias stress effects of exfoliated multi-layered MoS₂ FETs with a back-gated configuration. We observed that when a positive (negative) gate bias stress was applied to the device, the current decreased (increased) and the threshold shifted in the positive (negative) gate bias direction. The electrical instability of the MoS₂ FETs was significantly influenced by the electrical stress type, relative sweep rate, and stress time in an ambient environment. These phenomena can be explained by the charge trapping due to the adsorption or desorption of oxygen and/or water on the MoS₂ surface with a positive or negative gate bias, respectively, under an ambient environment [1]. We also investigated photoresponse characteristics of MoS₂ FET under various measurement conditions. We varied oxygen pressure of the measurement environment and applied different gate bias stress when we measured the photoresponse characteristics of the device [2]. More recently, we are investigating the effect of alkylthiol molecules deposition on the MoS₂ FET. When we deposit thiol molecule on MoS₂, the thiol molecules are chemically absorbed at the sulfur vacancy site of the MoS₂ channel and influence the electrical properties of MoS₂ FET. Our studies will be helpful in understanding the electrical properties of the MoS₂-based electronic devices and will also give insight into the design of desirable MoS₂ devices for electronics applications .
References:
[1] K. Cho, W. Park, J. Park, H. Jeong, J. Jang, T.-Y. Kim, W.-K. Hong, S. Hong, and T. Lee, ACS Nano, 7, 7751 (2013).
[2] K. Cho, T-Y. Kim, W. Park, J. Park, D. Kim, J. Jang, H. Jeong, S. Hong, T. Lee, Nanotechnology, 25, 155201 (2014).

In this communication, we report the successful transfer of CVD (Chemical Vapor Deposition) grown MoS₂ to Au antenna fabricated using electron beam (e-beam) lithography, and we investigate the photoluminescence properties of this hybrid plasmonic-excitonic system. The ultimately thin 2D MoS₂ layer has the great advantage of introducing a well controlled local absorber (and emitter) in the plasmonic near-field of the Au antenna. The work is focused on the plasmonic mediated pumping of the MoS₂ photoluminescence emission. Off- and in-resonance excitation of the surface plasmons showed drastically different behaviors of the photoluminescence emission from the MoS₂. For plasmonically mediated pumping, we found a significant enhancement (~65%) of the photoluminescence intensity, a clear evidence that the optical properties of MoS₂ monolayer are strongly influenced by the nano-antenna surface plasmons. In addition, a systematic photoluminescence broadening and red-shift in nano-antenna locations is observed which is interpreted in terms of plasmonic enhanced optical absorption and subsequent heating of the MoS₂ monolayers. Using a temperature calibration procedure based on photoluminescence spectral characteristics, we were able to estimate the local temperature changes. We found that the plasmonically induced MoS₂ temperature is 4 times larger than the MoS₂ reference temperature. Based on Green Dyadic theory simulations of the plasmonic properties of the Au antenna, combined with heat dissipation calculations, we discuss the contribution of the Au antenna heating to the measured temperature increase. We found that the results can be interpreted in terms of efficient light absorption by the plasmonic antenna and its conversion into electron-hole pair excitations of the 2D MoS₂ layer thus leading to enhanced excitonic photoluminescence and local heating. This study shines light on the plasmonic-excitonic interaction in the hybrid MoS₂/Au semiconductor/metal nano-structures and provides a unique approach for engineering new light-to-current conversion opto-devices such as high sensitivty photodetectors, bio-sensors and plasmonic controlled field-effect transistors.

The vertically heterostructured MoS₂ bilayers display a wide range of lattice registry relative to that in bulk MoS₂, through a single or combined in-plane displacement, out-of-plane displacement, and in-plane rotation. Here using density functional theory and numerical structural analysis, we examine both the atomic and electronic structures of the arbitrarily stacked MoS₂ bilayers. Our analysis shows that the interlayer electronic coupling between two MoS₂ layers yields an indirect band gap, whose magnitude varies with the lattice registry. In particular, the electronic coupling was found to be mainly attributed to the interlayer S-S interaction, through the anti-bonding p_z orbitals located on the inner S atoms. The variation of the coupling strength and the indirect band gap with respect to the lattice registry can be further attributed to the change of the mean interlayer sulfur-sulfur distance due to the displacement of lattice. Importantly, our analysis further shows that the twisted bilayers, except for those displaying the high symmetry stacking sequence, all have a nearly constant mean interlayer S-S distance, regardless of the twist angle or whether a commensurate or incommensurate superstructure is formed. The magnitude of the indirect band gap in the bilayers consequently exhibit a weak angular dependence, until the twist angle recovers the high symmetry stacking sequence when the gap change significantly. Our analysis provides the thorough theoretical explanation to the recently measured photoluminescence spectroscopy in twisted MoS₂ bilayers, and can form the basis for understanding the coupling in vertically heterostructured bilayers composed of other transition-metal dichalcogenides monolayers.

Tuning the electrical and optical properties of two-dimensional (2D) crystals can be realized by controlling the layer numbers and interlayer rotations and by alloying other elements. In this work, we synthesized monolayer and few-layer triangular molybdenum diselenide (MoSe2) and molybdenum tungsten diselenide alloys (Mo1-xWxSe2) through a chemical vapor deposition (CVD) method. The layer numbers of the crystals were controlled by tuning the reaction chamber pressure during the synthesis. Bilayer crystals show exclusively 0 ° or 60 ° interlayer orientations. Two distinct stacking configurations, i.e. 3R and 2H, corresponding to 0 ° and 60 ° interlayer orientations, respectively, were clearly observed at the atomic scale by using scanning transmission electron microscopy, and exhibit different optical properties including the photoluminescence, Raman scattering, and second-harmonic generation. Interestingly, the p-type or n-type characteristics of these 2D semiconducting crystals are determined by the layer numbers (from monolayer to few layers). Moreover, the alloying of tungsten also greatly influences the optical (photoluminescence emission intensity and energy) and electrical (p, n characteristics) of the 2D MoSe2 crystals. This work provides not only a good example for tuning the electrical and optical properties through structure and composition engineering but also guidance in studying the relationship between interlayer orientation, stacking configuration, and electrical and optical properties for 2D semiconducting crystals.
This research was performed at the Center for Nanophase Materials Sciences, a DOE Office of Science user facility. Synthesis science sponsored by the Materials Science and Engineering Division, Office of Science, Basic Energy Sciences, U.S. Department of Energy. Device fabrication sponsored by the Laboratory Directed Research and Development (LDRD) program at Oak Ridge National Laboratory.

With a rapidly increasing interest in the development of ultrathin MoS₂-based devices, measurement methods allowing for multifunctional characterisation of physical properties, easy identification of MoS₂ layer number and interaction of the flakes with a substrate are in high demand. As electronic and optical properties of MoS₂ are strongly thickness and layer-substrate interaction dependent, it is essential to precisely ascribe the measured parameters to individual layers. Here, we have used Raman spectroscopy, scanning Kelvin probe microscopy (SKPM) and atomic and ultrasonic force microscopy (AFM/UFM) for the mapping of mechanically exfoliated MoS₂ flakes with domains of the different thickness with the aim to precisely correlate their optical, nanomechanical and electrostatic properties on the nanoscale as well as to explore the effect of interaction of MoS₂ flakes with a substrate.
Using subsurface sensitive UFM mapping we have identified the change in the nanomechanical properties of the MoS₂ flake indicating effect of the substrate for a range of flake thicknesses (1, 5 and 8 layers) and correlated these regions with SKPM derived surface potential and Raman mapping. We observe an increase in the surface potential contrast for suspended regions of all thicknesses relative to the supported areas, with the monolayer region demonstrating a ~100 mV (~67%) increase, which is believed to be due to suppressed charge transfer for the suspended monolayer compared to the supported one. Furthermore, a corresponding increase in Raman intensity for the E¹_2g and A_1g modes is observed for the monolayer region but not for thicker regions of the flake. This thickness-dependent enhancement arises due to a different crystal orientation on the suspended area compared to the supported region of the monolayer flake. Additionally, we demonstrate a noticeable red shift for both E¹_2g and A_1g modes when MoS₂ is deposited on Si in comparison with the Au substrate. This observation is consistent with a more strained state of the MoS₂ flake on the Si substrate.
These results provide a detailed understanding of the layer properties, which are essential for potential optoelectronic applications by decoupling the optical and electrostatic properties of MoS₂ from substrate-induced effects.

2D materials ‘beyond graphene&’, such as semiconducting transition metal dichalcogenides, have emerged as a broad new family of materials for electronics applications and novel physics. However, electrical transport in these materials has to date been largely dominated by disorder, particularly at low temperatures. It has been unclear whether this disorder arises from extrinsic effects such as surface impurities, or defects and impurities in the materials themselves. We have developed techniques encapsulate 2D materials inside insulating hexagonal boron nitride, and study multi-terminal magneto-transport. In molybdenum disulfide, we observe phonon-limited transport at moderate temperatures, and low-temperature mobility two orders of magnitude larger than previously reported. We are also able to observe quantum oscillations for the first time. I will describe these results and continuing progress on related materials.

Field effect transistors based on single layer or few layer MoS₂ have been demonstrated to have impressive characteristics, such as high ON/OFF current ratio, reasonable electron mobility and a subthreshold swing approaching the theoretical limit at room temperature. However, the metal-MoS₂ contact resistance is still much higher than the metal-Si contact and metal-graphene contact. Here, we study the resistance distribution at the metal-MoS₂ contact through a multi-electrode measurement with specially designed thin electrodes. We find that the sheet resistance of the 2D MoS₂ increases obviously after contacting the metal. The parameters related to the contact resistance, such as transfer length, sheet resistance of the MoS₂ and contact resistivity between the 2D materials and the metal electrode, are all changed by the gate voltage. Furthermore, we study the ways to reduce the contact resistance and interesting results are obtained.

Two dimensional (2D) materials represent the next frontier in advanced materials for electronic applications. Their extreme thinness (3 or less atoms thick) gives them great mechanical flexibility, optical transparency and an unsurpassed surface-to-volume ratio. At the same time, this family of materials has tremendously diverse and unique properties. For example, graphene is a semimetal with extremely high electron and hole mobilities, hexagonal boron nitride forms an almost ideal insulator, while MoS₂ and other dichalcogenides push the limits on large area semiconductors.
This talk will review some of the recent progress in the use of these 2D materials in electronic and optoelectronic systems. Specifically, we will discuss progress along three different research directions. First, we will focus on large-area flexible electronics. The unique electronic and mechanical properties of 2D materials, in combination with the ability to grow them over very large areas by chemical vapor deposition, make these materials ideal for ubiquitous electronics. Two important building blocks for these applications will be reviewed: RF energy harvesters and flexible displays.
The second research direction is the use of 2D materials to enhance the performance of Silicon-based microsystems. Here, a graphene layer is used as a detector of mid-infrared radiation for night vision systems. The integration of this material at the back-end-of-the-line of standard Silicon fabrication allows the demonstration of mid-infrared imagers with record performance.
Finally, the third research direction that will be highlighted in this talk is the use of 2D materials to embed electronics into 3D-printed objects. By developing electronic inks with a high content of 2D material flakes in them, it is possible print full electronic systems seamlessly by using a 3D printer.
In summary, 2D materials offer amazing new properties that are quickly changing the form factor of electronics. Although there are still many challenges ahead, very significant progress has been achieved in the last few years thanks to the joint effort of material scientists, electrical engineers, chemists and physicists. This interdisciplinary team work is indispensable to take full advantage of these materials.
Acknowledgements
The work presented in this talk has been partially supported by the Army-MIT Institute for Soldier Nanotechnology, NASA, and the ONR PECASE award, monitored by Dr. Paul Maki.

Gate dielectric is one of the critical components for field effect (photo-) transistors. Conventional gate dielectrics used for bulk semiconductors have rough interfaces with two-dimensional (2D) semiconductors,^[1] thus containing many charged defects which decrease carrier mobility or photo-responsivity. It has been shown that an abrupt interface could improve the device performance. Layered boron nitride (BN) has been used as gate dielectrics for graphene, and indeed the mobility is significantly higher than those using conventional dielectrics.^[2] Inspired by these experiments, we use first-principle methods to explore the optimal gate dielectrics for 2D semiconductors (MX₂, where M = Mo, W, and X = S, Se, Te). We focus on experimentally available dielectrics which have layered forms and therefore could form abrupt interfaces with 2D semiconductors. The performance is evaluated based on calculations of their band offsets, dielectric constants, and interface defect properties. Promising dielectric materials are identified and their device integration is discussed. Our work offers insight into the fundamental properties of two-dimensional materials, and could potentially advance the nanoelectronics towards practical applications.
[1] S. McDonnell et al., ACS Nano 7, 10354 (2013).
[2] C. R. Dean et al., Nat. Nanotechnol. 5, 722 (2010).

Beyond graphene, 2D materials have shown fascinating electrical, mechanical, optical, and spintronic properties. Particularly, transition-metal dichalcogenides (TMDs) have recently become a focus of major research efforts. These materials exhibit non-zero energy gaps, consistent with the diatomic nature, making them potential alternatives to conventional 3D semiconductors in the highly scaled applications. A detailed investigation of charge transport, especially the intrinsic properties, is crucial for assessing the technological significance of the material as well as for fundamental physical understanding. By contrast, most of the experimental studies in the literature have been carried out under a broad range of extrinsic conditions such as different TMD layer thicknesses, impurity concentrations/sample qualities, and substrates.
In this work, we theoretically investigate the intrinsic electrical transport properties of both electrons and holes in TMDs from first principles. The density functional theory (DFT) formalism is applied to determine the electronic band structure as well as the phonon spectra for all phonon branches. Moreover, the key component is the ab initio calculation of the electron-phonon scattering rates by employing density-functional perturbation theory within the DFT formalism. This technique has the advantage of dealing with arbitrary electronic state and phononic state on an equal footing. Each phonon is treated as a perturbation of the self-consistent potential created by all electrons and ions, thus enabling direct evaluation of the carrier-phonon scattering matrix in the linear response without empirical fitting.
The results obtained thus far for monolayer TMDs illustrate that the acoustic phonons provide the dominant contribution to the carrier-lattice interactions. In the choice of transition-metal elements, TMDs with Mo (MoS₂, MoSe₂) have generally higher scattering rates than those with W (WS₂, WSe₂). Between two chalcogenides (S vs. Se), it is the selenide that scatters the carriers more strongly via the interaction with the phonons. The calculation also illustrates that the holes may suffer comparatively less interaction with phonons than the electrons, while the overall trends are very similar for both types of the carriers. The estimated saturation velocities range in the low-to-mid 10⁶ cm/s, whereas the intrinsic mobility may reach as high as 700 cm²/Vs and 540 cm²/Vs for electrons and holes, respectively. One particularly interesting point is that all four of the studied materials (with a possible exception of MoSe₂) are predicted with competitive hole transport properties compared to bulk silicon, offering attractive alternatives to conventional semiconductors in p-type applications. These and other relevant details of intrinsic carrier transport in TMD based structures will be discussed.

Atomically thin MoS₂ sheets hold tremendous promise for novel spintronics and valleytronics applications. One of the major challenges is that electron transport in these materials is often dominated by band edge disorder and mid-gap localized states. Here, we investigate the intrinsic magneto-transport properties of mono-, bilayer MoS₂ sheets in the high carrier densities regime where conduction occurs via extended states. We achieve high doping by dual gating the layers with ion gel and SiO₂ back gate. We found that carrier mobility saturates with carrier density and with decreasing temperature, indicating that short-range scattering is responsible in limiting carrier mobility all samples. Our magneto-transport studies further reveal a gradual transition from strong to weak localization and to weak anti-localization regime with increasing carrier density beyond 10¹³ cm^-2 at low temperatures. The phase coherence length of both monolayer and bilayer devices was found to be around 10 nm at 2 K and tunable by gate voltages.

Two-dimensional materials (including graphene, MoS₂, etc.) represent a promising class of materials for electronic and photonic devices, benefiting from their unique properties such as extremely high mobility and ultrathin body. In this talk I will present our works on electronic devices based on 2D atomic and molecular semiconducting crystals.
In the first part, we provide direct evidence that sulfur vacancies exist in exfoliated MoS₂, introducing localized midgap donor states. At low carrier density, the charge transport in MoS₂ is by electron hopping through these localized states, leading to much lower mobility than theoretical expectations and insulating behavior. We develop a facile low-temperature thiol chemistry to repair the sulfur vacancies and improve the interface, resulting in significant reduction of the charged impurities and traps in MoS₂. High mobility greater than 80cm² V^-1 s^-1 is achieved in backgated monolayer MoS₂ field-effect transistors. We further develop a theoretical model to quantitatively extract the key microscopic quantities that control the transistor performances, including the density of charged impurities, short-range defects and traps.
In the second part, we demonstrate that high-quality few-layer dioctylbenzothienobenzothiophene molecular crystals can be grown on graphene or boron nitride substrate via van der Waals epitaxy, with precisely controlled thickness down to monolayer, large-area single crystal, low process temperature and patterning capability. As a result of the pristine crystal and interface quality, monolayer dioctylbenzothienobenzothiophene molecular crystal field-effect transistors on boron nitride show record-high carrier mobility up to 10cm²V^-1s^-1. Our work unveils an exciting new class of two-dimensional molecular materials for electronic and optoelectronic applications.
References
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The presence of a direct band gap and in an ultrathin form factor has caused a considerable interest in two-dimensional semiconductors from the transition metal dichalcogenides (TMD) family with molybdenum disulphide (MoS₂) being the most studied representative of this family of materials. While diverse electronic elements¹, integrated circuits² and optoelectronic devices³ have been demonstrated using ultrathin MoS₂ and related materials, very little is known about their performance at high frequencies where commercial devices are expected to function. We fabricated top-gated MoS₂ transistors operating in the gigahertz range of frequencies. The presence of a band gap also gives rise to current saturation,⁴ allowing voltage gain higher than 1.
The MoS₂ FETs are fabricated from exfoliated MoS₂⁵. We have fabricated RF transistors based on MoS₂ layers with different thickness. Electrical contacts were patterned using electron-beam lithography and by depositing Au electrodes. Atomic layer deposition (ALD) was used to deposit HfO₂ as a gate dielectric. All our devices presented transconductance typical of n-type materials with on-state current reaching ~300 µA/µm for V_ds = 2 V and gate voltage V_tg = 10 V in the case of monolayer MoS₂. The current gain of the MoS₂ FETs decreases with increasing frequency and shows the typical 1/f dependence for different thicknesses of 2D MoS₂ crystals. We realized MoS₂ FETs showing current saturation. We found the behavior of the cut-off frequency as a function of the number of layers of MoS₂ FETs. The cut-off frequency rises with increasing number of layers in the ambient atmosphere.
In conclusion, we studied top-gated MoS₂ transistors with a 240 nm gate length. Our MoS₂ RF-FETs show an intrinsic transconductance higher than 50 µS/µm and a drain-source current saturation with a voltage gain higher than 1. All these features allow the operation of MoS₂ transistors in the GHz range of frequencies. Our devices show cut-off frequencies in the GHz range and are able not only to amplify current in this frequency range but also power and voltage, with the maximum operating frequency f_max = 8.2 GHz.
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