The studies of atomic defects and boundaries are of general interest in the fundamental researches and becoming more and more crucial for technological applications of any nanoscale materials> the atomic scale studies can be expanded to the other low-dimensional materials than graphene. Here I demonstrate some examples for atomic-scale imaging and spectroscopy of various low-dimensional materials with interrupted periodicities. Point defects and edge structures of graphene have been intensively studied with atomic precision in the last decade [1-4]. Active 4|8 defects are most recently found to be responsible for plastic deformation of hexagonal boron-nitride (h-BN) layers [5]. Vacancies and edges with radical bonds are also successfully assigned in h-BN [6, 7]. Doping and boundary behaviors of single-layered dichalcogenides (MX₂) are intensively studied because they indeed govern the phase transition behaviors between 2H and 1T phases [8, 9]. Possible nano-device assembly made of metallic and semiconducting MoS₂ single layers will be also proposed.
[1] A. Hashimoto et al., Nature, 430 (2004) pp.870-873
[2] K. Suenaga et al., Nature Nanotech., 2 (2007) pp.358-360
[3] Z. Liu, K. Suenaga, P. Harris and S. Iijima, Phys. Rev. Lett., 102 (2009) 015501
[4] K. Suenaga and M. Koshino, Nature 468, 1088-1090 (2010).
[5] O. Cretu, YC. Lin and K. Suenaga, Nanolett., 14 (2014) pp.1064-1068
[6] C. Jin et al., Phys. Rev. Lett., 102, 195505 (2009)
[7] K. Suenaga, H. Kobayashi, and M. Koshino, Phys. Rev. Lett., 108 075501 (2012).
[8] YC. Lin et al., Adv. Mater. 2014
[9] YC. Lin et al., Nature Nanotech. 2014
[10] The work is partially supported by JST Research Acceleration Programme.

Carbon-based materials of reduced dimensionality have shown to exhibit some extraordinary structural, thermal and electronic properties. One example of this is graphene. Due to its unique properties, graphene is considered as the basis for a new nanoelectronics. However, in its pristine form graphene is a zero bandgap semiconductor, which limits its use in digital transistor applications. In part due to it, there is a renewed interest in other possible carbon-based 2D materials, as for example, the graphyne structures. Proposed by Baughman and co-workers[1], graphyne is a generic name for a carbon allotrope family of 2D structures, where acetylenic groups connect benzenoid rings. Graphyne nanotubes are also possible[2]. These structures share some of graphene unique properties, with the advantage of some of them are non-zero electronic bandgap systems[3]. Similarly to graphene, its hydrogenation can be used to smartly tune its electronic band gap. In this work we have investigated, through fully atomistic reactive molecular dynamics simulations, the dynamics and structural changes of the hydrogenation of graphyne membranes. The extensive MD study was carried out using reactive forcefields(ReaxFF), as implemented in the Large-scale atomic/Molecular Massively Parallel Simulator(LAMMPS) code. This approach has been successfully applied to the study of hydrogenated graphdiyne[4]. The hydrogenation of α, β, and γ graphyne forms[1] were carried out considering an isolated single-layer graphyne sheet immersed into an atmosphere of atomic hydrogen atoms.
Our results showed that the existence of different sites (single, double and triple bonds), makes the process of incorporating hydrogen atoms into graphyne membranes much more complex than the graphene ones. Our results also show that hydrogenation reactions are strongly site dependent and that the sp-hybridized carbon atoms are the preferential sites to chemical attacks. In our cases, the effectiveness of the hydrogenation (estimated from the number of hydrogen atoms covalently bonded to carbon atoms) follows the α, β, γ-graphyne structure ordering. Another important result is that, in contrast to was reported to the case of graphene hydrogenation[5], we did not observe the formation of correlated domains (islands of hydrogenated carbons). This can be a consequence of the porous graphyne structure, which allows larger out-of-plane deformations (in comparison to graphene) and, consequently, an increase in the curvature with an increased local chemical reactivity. Consistently, in the case of graphene fluorination (fluorine is more reactive than hydrogen atoms) the formation of these domains are also suppressed[6].
[1] R Baughman et al. JCP v87,6687(1987)
[2] VR Coluci et al. PRB v68,035430(2003)
[3] Q Peng et al. PCCP. v14,13385(2012)
[4] PAS. Autreto et al. Carbon-in press.DOI: 10.1016/j.carbon.2014.05.088
[5] MZS. Flores et al. Nanotech. v20,465704(2009)
[6] R Paupitz et al. Nanotech. v24,035706(2013)

In the present work, we investigate the relative stability and electronic properties of carbon nitride (C_xN_y) graphene-like structures using a combination of a new bond-counting method and density-functional-theory (DFT) first-principles calculations. The bond-counting method, parametrized by ab initio results and with quantitative predictive power, includes the possible existence of vacancies and larger holes in the planar structures. The method, which is an extension of a bond-energy model previously proposed and applied to B-C-N layers [1-3], permits us to obtain analytical and numerical results for the energetics and the morphology of graphene-like C_xN_y For instance, at high N concentrations, the bond-counting method allows us to search among millions of possible structures, and we find several ones with ab initio formation energies per N atom comparable to, or even smaller than, that of the isolated graphitic N impurity. Those structures are characterized by a variety of nanoporous graphene morphologies. The low-energy C-N structures also present a variety of band gaps, from zero to 1.6 eV, which can be tuned by stoichiometry and porosity. At specific stoichiometries, some structures also present ferro- and antiferromagnetic ground states.
[1] M. S. C.Mazzoni, R. W. Nunes, S. Azevedo, and H. Chacham, Phys. Rev. B 73, 73108 (2006).
[2] J. R. Martins and H. Chacham, ACS Nano. 5, 385 (2011)
[3] J. R. Martins and H. Chacham, Phys. Rev. B 86, 75421 (2012).

Electronic structure and transport properties of N-doped graphene with a single sublattice preference [1] are investigated using both first-principles techniques and a real-space Kubo-Greenwood approach [2]. Such a breaking of the sublattice symmetry leads to the appearance of a true band gap in graphene electronic spectrum even for a random distribution of the N dopants. In addition, a natural spatial separation of both types of charge carriers at the band edge is observed, leading to a highly asymmetric electronic transport. For such N-doped graphene systems, the carrier at the conduction band edge present outstanding transport properties including long mean free paths, high mobilities and conductivities. Such a transport behavior can be explained by a non- diffusive regime (quasi-ballistic transport behavior at the conduction band edge), and originates from a low scattering rate. The presence of a true band gap along with the persistence of carriers traveling in an unperturbed sublattice suggest the use of such N-doped graphene in G-FET applications, where a high ION/IOFF ratio is expected. The present ab initio simulations should encourage more investigation and specific transport measurements on N-doped graphene samples where such an unbalanced sublattice doping is observed.
As strings of monoatomic thickness, chains of sp-hybridized carbon atoms constitute the logical one-dimensional phase of carbon. These 1D systems have been proposed theoretically for a long time until they were observed in electron microscopy studies. However, electrical measurements on these monoatomic chains have not been feasible. Now, by using a measuring system with an STM tip in a TEM specimen stage, carbon chains are not only produced but their electrical properties are also measured. Ab initio simulations (confirmed by MBPT calculations) reveal that strain has a decisive influence on the bandgap of the chain, thus determining its conductivity [3].
[1] Nitrogen-doped graphene : beyond single substitution and enhanced molecular sensing
R. Lv, Q. Li, A.R. Botello-Mendez, et al. Nature - Scientific Reports 2, 586 (2012).
[2] Electronic and transport properties of unbalanced sublattice N-doping in graphene
A. Lherbier, A.R. Botello-Méndez, and J.-C. Charlier, Nano Lett. 13, 1446-1450 (2013).
[3] Electrical transport measured in atomic carbon chains
O. Cretu, A. R. Botello-Mendez, I. Janowska, C. Pham-Huu, J.-C. Charlier, and F. Banhart, Nano Lett. 13, 3487-3493 (2013).

Two-dimensional (2D) layered materials host many interesting physical and chemical phenomena such as topological insulator and intercalation. Their nanostructures represent novel candidates to host those phenomena. Here we present our study on chemistry and physics of 2D layered nanomaterials. First, we have synthesized a range of morphologies including nanoplates, nanoribbons and their heterostructures. Second, we have developed a new method of zero-valent intercalation which allows unprecedented high levels of various metal intercalants inserted into the van der Waals gaps. The resulted optical properties and electrical conductance change drastically. Third, we have fabricated single nanostructure electrical transport devices. Using topological insulator nanostructures, we observed interesting physical phenomena including Aharonov-Bohm oscillations from topological surface electrons, ambipolar transport with effective control of the Fermi level into the bulk bandgap and across the Dirac Point, and localization effects emerging from the surface electrons in confined dimensions. Lastly, we also demonstrate the interesting tunable catalytic property.

The enormous impact of graphene on both fundamental science and potential device applications has rejuvenated interest in other layered materials as well, where individual atomic or molecular layers are weakly coupled through Van der Waals forces. A recent development in this field involves multi-component two-dimensional hybrids obtain via vertical stacking of different layered materials. Hybrid heterostructures of atomically thin membranes with clean interfaces promise devices that combine advantages of ultimate miniaturization and multiple functionality. In this work we demonstrate that metal dichalcogenides, such as Molybdenum disulphide (MoS₂), can be a natural partner to graphene for novel optoelectronic application because of the visible range bandgap, and gate tunable electrical transport in MoS₂. The responsivity of Graphene/MoS₂ hybrids can be as high as 10¹⁰ Ampere/Watt [1], which is nearly thousand times larger than other light-sensitive graphene hybrids. In addition, these devices display persistent photoconductivity that can be exploited to realize programmable optoelectronic switches. I shall outline this emerging concept in material engineering for optoelectronics from Van der Waals heterostructures with examples of different composition and stacking sequences.
References
[1] Kallol Roy, Medini Padmanabhan, Srijit Goswami, T. Phanindra Sai, Gopalakrishnan Ramalingam, Srinivasan Raghavan, and Arindam Ghosh, Nature Nanotechnology 8, 826 (2013).

The quest for novel two-dimensional (2D) materials has led to the discovery of hybrid heterostructures where graphene and other atomic layer films such as monolayer hexagonal boron nitride (hBN) form phase-separated domains or in-plane heterostructures [1-3]. Here, we report on a combined experimental and theoretical study of in-plane heterostructures of graphene-hBN. By implementing the concept of epitaxy to 2D space, we demonstrated a single-atomic layer, in-plane heterostructure: monolayer hBN grows from fresh edges of monolayer graphene with lattice coherence, forming a 1D boundary. More importantly, the crystallography of the hBN is solely determined by that of the graphene, forgoing configurations favored by the supporting Cu substrate [1]. Scanning tunneling microscopy and spectroscopy measurements reveal an abrupt 1D zigzag oriented boundary, which provides a rare opportunity to examine the spatial and energetic distributions of the 1D boundary states in real space. The polar-on-nonpolar 1D boundary between grpahene and hBN is expected to possess peculiar electronic states associated with edge states of graphene and the polarity of hBN. The revealed boundary states are about 0.6 eV below or above the Fermi energy depending on the termination of the hBN at the boundary, and are extended along but localized at the boundary. These results suggest that unconventional physical effects similar to those observed at 2D interfaces can also exist in lower dimensions, opening a route for tuning of electronic properties at interfaces in 2D heterostructures. This research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.
[1] L. Liu, J. Park, D.A. Siegel, K.F. McCarty, K.W. Clark, W. Deng, L. Basile, J.C. Idrobo, A.-P. Li, and G. Gu, Science343, 163 (2014).
[2] Y. Gao, Y. Zhang, P. Chen, Y. Li, M. Liu, T. Gao, D. Ma, Y. Chen, Z. Cheng, X. Qiu, W. Duan, and Z. Liu, Nano Lett. 13, 3439 (2013).
[3] P. Sutter, R. Cortes, J. Lahiri, E. Sutter, Nano Lett. 12, 4869 (2012).

Recent progress in preparing well controlled 2D van der Waals heterojunctions has opened up a new frontier in materials physics. In this paper we address the intriguing energy gaps that are sometimes observed when a graphene sheet is placed on a hexagonal boron nitride substrate, demonstrating that they are produced by an interesting interplay between structural and electronic properties, including electronic many-body exchange interactions. Our theory is able to explain the observed gap behavior by accounting first for the structural relaxation of graphene&’s carbon atoms when placed on a boron nitride substrate and then for the influence of the substrate on low-energy π-electrons located at relaxed carbon atom sites. The methods we employ can be applied to many other van der Waals heterojunctions.

Graphene and 2D layered semiconductors have shown immense potential in the next-generation solar energy harvesting technology. Due to its relatively simple structure and fabrication process (as compared with that of p-n junction), Schottky barrier is used to implement solar cells with metal/semiconductor junction structure. In this work, we investigate chemical-vapor-deposition assembled large-area 2D layered semiconductor tungsten disulfide (WS₂) as a photoactive material for Schottky barrier solar cells, employing graphene as the metal contact. The effects of using monolayer, bilayer, and multilayer graphene as Schottky contact on 2D layered semiconductor WS₂ are explored. The variation of graphene configuration on WS₂ would modify the Schottky barrier behavior significantly, yielding solar cells with much different photoelectric performance. The Schottky barrier height, electric field strength, and Fermi level pinning at graphene/WS₂ interface vary with respect to the junction structure.

Bulk silicon has dominated the development of microelectronics over the past 50 years. However, the recent interest in two-dimensional (2D) materials (e.g., graphene, MoS₂, phosphorene, etc.) naturally raises questions regarding the growth and properties of Si at the 2D limit. Utilizing atomic-scale, ultra-high vacuum (UHV) scanning tunneling microscopy (STM), we have investigated the 2D limits of Si growth on Ag(111). In agreement with previous reports of sp²-bonded silicene phases, ^1,2 we observe the temperature-dependent evolution of ordered 2D phases, which we attribute to apparent Ag-Si surface alloys. At sufficiently high Si coverage, we observe the precipitation of crystalline, sp³-bonded Si(111) domains. These domains are capped with a radic;3 honeycomb phase that is indistinguishable from the radic;3 honeycomb-chained-trimer reconstruction induced by Ag on bulk Si(111).^3,4,5 Furthermore, Raman spectroscopy on these regions shows modes attributed to dimensionally confined, bulk-like sp³ Si. Structural and chemical analysis of the coverage-dependent morphology suggests that silicon intermixing with the Ag(111) substrate is followed by the precipitation of bulk-like, (111)-oriented Si nanosheets. These conclusions are supported by ex-situshy; atomic force microscopy, cross-sectional transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. Even at the 2D limit, scanning tunneling spectroscopy shows that the sp³-bonded silicon nanosheets exhibit semiconducting electronic characteristics.
[1] Vogt, P., et al. Silicene: Compelling Experimental Evidence for Graphene-like Two-Dimensional Silicon. Phys. Rev. Lett., 108(15), 155501 (2012).
[2] Feng, B., et al. Evidence of silicene in honeycomb structures of silicon on Ag(111). Nano Lett., 12(7), 3507-11 (2012)
[3] Le Lay, G. Physics and electronics of the noble-metal/elemental-semiconductor interface formation: A status report. Surf. Sci., 132(1-3), 169-204 (1983).
[4] Aizawa, H., Tsukada, M., Sato, N., & Hasegawa, S. Asymmetric structure of the Si (111)- radic; 3×radic; 3-Ag surface. Surf. Sci., 429 (0-5) (1999).
[5] Ding, Y., Chan, C., & Ho, K. Structure of the (radic; 3×radic; 3) R30° Ag/Si (111) surface from first-principles calculations. Phys. Rev. Lett., 67(11), 1454-1458 (1991).

Two-dimensional (2D) molybdenum oxide is a core member in the family of 2D materials and has received intensive interests recently. A grinding-assisted liquid phase exfoliation method is adopted for the formation of 2D molybdenum oxide flake suspensions, with a flake thickness in the order of sim;1.4 nm. This thickness is equal to the largest unit cell parameter of fully-stoichiometric molybdenum trioxide (MoO₃). The implemented method has the advantage of simplicity and results in a high yield of 2D flakes that are highly crystalline across the planes, which can be incorporated into 2D material-enabled electronic and optical devices. Here, we demonstrate 2D molybdenum oxide flakes for both resistive and optical gas sensing applications in which hydrogen (H₂) is selected as a model gas. While 2D MoO₃ flakes are utilized for the resistive gas sensing platform, their substoichiometric forms (MoO_3-x), induced by solar light illumination, are used as the sensitive material for optical gas sensing applications as they have demonstrated strong plasmon resonances in visible light range. The novel 2D gas sensing platforms are tested towards different concentrations of H₂ gas at various operating temperatures to comprehensively assess their sensing performances. The presented 2D system offers great opportunities for future sensing and optical applications.

An advantage of 2D materials over their 3D counterparts is the ability to apply large amounts of strain to control electrical, optical and thermal properties - essentially ‘strain engineering&’. However, to control precisely the location, magnitude and direction of a strain field it is critical to understand any motion or sliding between a material and its anchor points. Here, we study the friction between three atomically thin materials and a SiO₂ substrate and determine the friction and comment on the design of strain engineering by controlling nanoscale friction.
We measure the high resolution spatial distribution of the Raman spectra in a 2D material membrane sealing a pressurized, cylindrical, micro-chamber etched into the SiO₂ substrate. From the strain, membrane deflection and material properties, we determine the friction and fundamental Grüneisen parameters and shear deformation potentials. We have chosen molybdenum disulfide (MoS₂), hexagonal Boron Nitride (hBN), and Phosphorene as our materials of choice because of their prominence in 2D material research making them likely candidates for strain engineering applications and because of their interesting electrical and optical properties whose strain dependence we can simultaneously explore during our friction measurements. The friction between these three materials and SiO₂ is compared with previous measurements of friction between Graphene and SiO₂ to provide insight into the mechanism of friction at the nanoscale. We extract the Grüneisen parameters and shear deformation potentials for various phonon modes in all three materials, and in the case of MoS₂ and Phosphorene we also measure the strain dependence of the band gap.
These results are essential for friction and strain engineering applications of MoS₂, hBN and Phosphorene because friction is critical in designing precise strain distributions. Knowledge of the Grüneisen parameters and shear deformation potentials enables Raman spectroscopy to be used for accurate strain measurements of devices. This work establishes our technique as the leading method for measuring friction at the nanoscale.

Vanadium pentoxide (V₂O₅)&’s crystal structure contains double chains bonded together with bridge oxygens within its overall layered structure. The 1D aspect manifests itself in the lowest split-off conduction band, which has dispersion primarily along the chain direction. Doping this narrow band, achieved for example in bulk intercalated NaV₂O_5, where it leads to a spin-Peierls transition,[1] may be of additional interest in monolayer form, where it could be achieved by gating, controlled removal of vanadyl oxygen or nanoribbon formation. Here we revisit its electronic band structure using quasiparticle self-consistent (QS) GW calculations in both bulk and monolayer form. We find that QSGW strongly overestimates the gap, which is a signature of the strongly correlated nature of the electronic stucture and related to the underestimate of the screening of the d-electrons in the random phase approximation (RPA). As for other 2D materials, we find that the QSGW gap of monolayer material varies inversely proportional with the size of the vacuum region included in the periodic boundary condition simulation cells. This is related to the similar dependence of the dielectric function. The reduced 2D screening also plays a significant role in the phonon frequencies. We find strong blue shifts of the highest vibrational modes (vanadyl-oxygen vanadium stretch modes) in monolayer compared to bulk [2] and have analyzed their origin in terms of the long-range and short-range contributions to the force constants. Both the screening and Born effective charges are significantly different in bulk and monolayer and affect the long-range contribution to the force constants.
[1] M. Isobe and Y. Ueda, J. Phys. Soc. Jpn. 65, 1178 (1996)
[2] C. Bhandari and W. R. L. Lambrecht, Phys. Rev. B 89, 045109 (2014)

Two-dimensional (2D) materials are extremely interesting as building blocks of next-generation nanoelectronic-devices because of their promising properties. The most widely studied two-dimensional material to date is graphene due to its high carrier mobility (mu;) over 100 000 cm²/Vmiddot;s, but it has considerable limitations in regards to real device applications. As one of alternative 2D materials beyond graphene, molybdenum disulfide (MoS₂) chalcogenide nanoflake recently appeared circumventing the drawbacks of graphene. MoS₂ nanoflake thus has been investigated for field-effect transistors (FETs), photo-detectors, and even logic circuits for NAND, NOR, small signal amplifier, and ring oscillators. However, practically good circuits using more than two MoS₂ based-FETs generally need electron beam (E-beam) lithography for fabrication, since the nanoflakes of 2D MoS₂ are not large enough to pattern by photolithography. Therefore, 2D MoS₂-based circuits are not that easy to be realized by conventional photolithography on one substrate. Hence, more favorable and simple way to use MoS₂-based device may be necessary.
Here, we report a bottom-up fabrication approach which uses quite a conventional method, to integrate different nanoscale building blocks for practical device applications. MoS₂ nanoflake-FET and ZnO nanowire Schottky Diode were coupled in series on the same substrate, to form an electric and photo-electric hybrid inverter. In fact, similar types of devices were very recently reported for complementary inverter and gate tunable p-n diode adopting MoS₂ and Carbon Nano Tube (CNT), but the incorporation of ZnO nanowire device to MoS₂ FET has not been reported yet. Such hybrid nanodevices would be novel because of its multi-functionalities; the properties of MoS₂ nanoflake are different from those of ZnO nanowire but both would be compensating each other. In the present study, we used such different but compensating properties, to realize an electrical /and photo-detecting inverter. For a logic inverter, we used the MoS₂ FET for a high-speed switch while the ZnO Schottky diode was used as a resistor for low-power consumption or high voltage inverter gain. For a visible photo-detecting inverter, MoS₂ FET plays as a photo-sensitive switch due to the visible light-matching band gap of MoS₂ while ZnO nanowire Schottky diode does as photo-blind resistor. For hybrid device processes, we developed a direct imprint (or nanotransfer printing) process and also utilized conventional photolithography, to place MoS₂ flake near ZnO nanowire. As a result, our inverter has been nicely fabricated, demonstrating a high voltage gain of ~50, low power consumption of 50 nW, and also exhibiting good dynamic photo-responses in visible range.

Ultrathin crystals based on dichalcogenides like MoS₂ form a fascinating class of materials that could evoke new components for optoelectronics, spintronics or valleytronics. While bulk MoS₂ represents an indirect semiconductor, monolayer MoS₂ flakes reveal a direct band gap. Few layer structures like bilayers are thus expected to behave like a link between two-dimensional and bulk semiconductors: They have a more complex band structure with both indirect and direct optical transitions, due to two conduction minima (CBM) at the K and the Λ point. This gives an exciting degree of emission control via the carrier population of the specific CBM. Recent studies have demonstrated that injection of carriers into contacted monolayer flakes can dope MoS₂ monolayers efficiently and that this doping can by monitored by photoluminescence (PL) spectroscopy: Depending on the bias, the generation of charged excitons (trions) has been observed. However, these findings are limited to monolayers up to now. Few-layer structures like bilayers have undergone less research and thus, the distribution of carriers between the different CBM after optical and/or electrical injection is still under discussion.
In this contribution, we demonstrate the electrical control of the carrier population in MoS₂ mono- and bilayer crystals via gate voltage. The different recombination channels are monitored by micro PL spectroscopy. MoS₂ flakes were exfoliated mechanically and deposited on Si/SiO₂ substrates. By contacting the flakes with Ti-Au contact lines, we gain an additional tuning knob to manipulate the carrier distribution. The Si substrate serves as back gate contact.
Micro PL measurements at T=5 K reveal a common feature between mono- and bilayer emission: In both cases, the direct transition emission is split into a higher energy line A⁰ and a lower energy line A^- with Lorentzian line profile and an energy separation of ΔE = 25 meV. We attribute A⁰ to an exciton and A^- to a negatively charged trion emission. These characteristic emission lines serve as a sensor for an agglomeration (depletion) of electrons for positive (negative) gate voltage Ug. The PL emission of both mono- and bilayer structures shows a clear change from excitonic to trionic emission, if Ug is increased. Hereby, important differences between mono- and bilayers are found. In MoS₂ monolayers, trions can be observed up to room temperature, whereas they are completely quenched above 200 K for the bilayers. We attribute this to an escape of excess electrons from the K point to the second CBM in the bilayer band structure, which is more probable than in the monolayer due to similar energy of both CBM in the bilayer device. Moreover, in bilayers a distinct change of the indirect transition with respect to the direct one with gate voltage is observed - another consequence of a bias driven redistribution of charge carriers between the CBM.

Ambient and Ultra High Vacuum Scanning Probe Microscopies (SPM) were used to study ferroelectric properties in cleaved 2-D flakes of copper indium thiophosphate, CuIn_IIIP₂S₆ (CITP). CITP is an unusual example of a layered, anti-collinear, uncompensated, two-sublattice ferroelectric system. Presently, it is the only materials known to display “2-D” ferroelectric semiconductor behavior as a van-der-Waals crystal. The material exhibits a #64257;rst-order phase transition of order-disorder type from the paraelectric to the ferrielectric phase at T_c = 315 K. Our observations indicate the presence of stable ferroelectric polarization as evidenced by domain structures, rewritable polarization, and hysteresis loops. We have found that flakes above 100 nm in thickness have bulk-like polarization and domain structures, whereas below 50 nm thick polarization disappears. Furthermore, ionic mobility has been recorded in this material by macroscopic measurements and by formation of surface damage above tip bias of 4 V, likely due to copper reduction, in SPM. We ascribe this behavior to instability of polarization due to depolarization field, and internal screening by mobile Cu ions, supported by their ionic mobility.
Acknowledgement:
Research for (AB, PM, QH, AB, SVK) was supported by the US Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. Research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, US Department of Energy.

Boron nitride nanostructures are generally accepted to be highly inert against oxidation or chemical treatments. Only very reactive species were found to be successful in covalently modifying these nanostructures. Here we present our recent studies on using a seemingly benign chemical approach to effectively functionalize and modify the morphology of boron nitride nanotubes. This was achieved by sonicating the nanotubes in aqueous ammonia solution. The boron nitride nanotubes were found to be sharpened, shortened, and even unzipped, exhibiting similarities to that observed in the oxidation of carbon nanotubes. We envision that this study may pave the way for convenient processing of boron nitride nanotubes and in general other boron nitride nanostructures, which were previously thought to be highly inert. This technique provides a pathway towards controlling their dispersion, purity, size distribution, and electronic properties.

Intense efforts in the field of two-dimensional (2D) crystalline material have been focused on studying isolated monolayers and on developing devices for a variety of applications based on them. However, 2D crystals with dissimilar crystal structure and composition can also bind when positioned on top of each other because of the van der Waals and Coulomb interactions. It is, thus, conceivable to build hetero-structures[1] of layered materials by stacking two or more 2D crystals forming long-range layer sequence along the out-of-plane direction. The unique in situ low-temperature synthesis technique[2] based on atomically-precise deposition of layer precursors that each self-assemble in the desired 2D crystal, enables the synthesis of misfit [(MX)_1+δ]_m(TX₂)_n layered stacks, where TX₂ is a 2D chalcogenides (where T is a transition metal such as Ti, Nb, Ta, W, or Mo, and X = Se or Te) and MX are layers of 3D chalcogenides (such as PbSe, SnSe, BiSe, etc.) δ is the ratio between the unit areas per metal atom in each layer and quantifies the degree of the structural misfit of the two layers.
This synthesis technique makes it possible to explore a large family of hetero-structures that provide an unprecedented configurational richness that maps into a comparably rich spectrum of functionalities that could cover band gaps, effective masses, etc. However, these hetero-structures pose a challenge for the state-of-the-art ab initio methods because the periodic 3D supercells that are used in these methods to model materials can describe only in an approximate fashion the incommensurate geometry of the different layers in these stacks.
Here, we selected (PbSe)₁(MoSe₂)₁[3] for an ab initio study of in which the stack was modeled using a few supercell geometries and the gradient corrected approximation extended with a van der Waals exchange and correlation energy functional. In this system, the misfit ratio δ is zero and the PbSe layer has been observed to keep a square lattice. These supercell models approximate the square symmetry observed in the PbSe layer while introducing only a small deviation of the MoSe2 layer from hexagonal symmetry. Using these models, we studied the dependence of the interlayer binding energy and out-of-plane lattice parameter of the stack on the reciprocal orientation and overlap of the PbSe and MoSe₂ layers. We found an appreciable modulation of the binding energy as a function of the lateral shift between layers for fixed reciprocal orientation. An analysis of the electronic structure shows that the valence band maximum in [(PbSe)_1.00]₁(MoSe₂)₁ originates from the PbSe layer while the conduction band minimum from the MoSe₂ layer.
References:
[1] A. K. Geim et al., Nature 499, 420(2013)
[2] D. C. Johnson, Curr. Opin. Solid. State. Mat. Sci. 3, 159(1998)
[3]C. L. Heideman et al., J. Am. Chem. Soc. 135, 11055(2013)

Molybdenum disulfide (MoS2) is a semiconductor which has the potential to revolutionize the future
design of electronic devices, specifically 2D transistors. Recently mechanical exfoliatiation
has been used to obtain singleshy;layer MoS2 which has an observed band gap. Here we
present a tightshy;binding model for singleshy;layer and multishy;layer MoS2 in order to give
physical meaning to energy bands calculated using abshy;initio methods. We consider
symmetry operations to reduce the number of interaction parameters and thus reproduce
the essential physics with as few parameters as possible. We further generalize our calculations
to other dichalcogenides.

We present first principles theory calculations on the lattice dynamics of single layer MoS₂, revealing the strain, chirality and defect efffects on the atomic vibrations of the materials. A phonon band gap exists near 250cm^minus;1 with the perfect lattice of MoS₂. This band gap keeps existing with the uniaxial tension along the zigzag direction till reaching failure. However, the band gap is vanishing with the uniaxial tension test along the armchair direction before reaching to the failure. With two sulfur missing defects, This band gap is no longer existing, even at no strain state. This phnomena inspire us that the chirlity effect and defect effect might have great impact on the thermal conductivity of the material. The failure mode is also investigated near the ultimate strength of both the perfect MoS₂ lattice and the defect lattice. Different phonon softening behavior has been identified with the two lattice structure (perfect structure and vacancy structure with two sulfur missing) under the uniaxial tension in armchair direction and zigzag direction, respectively. The phonon stability ceriteria is also adopted to validate the formation of possible defect structure of MoS₂.

We study electronic properties of hexagonal boron nitride (h-BN) monolayer and bilayer sheets from the first principles. First, we discuss the electronic band structure of the pristine h-BN monolayer in the framework of the density functional theory and the many-body perturbation theory (GW approximation). Next, we study the impurity-induced electronic states in h-BN monolayer and bilayer sheets for the C doping into both B and N sites [1]. It is confirmed that the C doping can generate both doner and acceptor states. We also address the strain effect on the electronic structure of C doped h-BN sheets [2]. Interestingly, it is found that both compressive and tensile strain on the C-doped h-BN monolayer can reduce the ionization energy of the donor state. In the case of h-BN bilayer sheets, we examine the energetics of several different stacking sequences in detail, and study the compression effects along the interlayer direction in the case of C-doped bilayers. Finally, we address the important role of the nearly free-electron state in the C-doped h-BN sheets as well as in the pristine h-BN sheets.
1. Y. Fujimoto, T. Koretsune, and S. Saito, JPS Conf. Proc. 1, 012066 (2014).
2. Y. Fujimoto, T. Koretsune, and S. Saito, J. Ceramic Soc. Japan, 122, 346 (2014).
This work was partly supported by the JSPS Grant No.26390062, MEXT Grant No.25107005, and the MEXT Elements Strategy Initiative to Form Core Research Center through Tokodai Institute for Element Strategy.

With the explosion of materials that form 2-D structures in the past few years, there have been a much more diverse ecosystem of combinations of characteristics to explore. Yet with the majority of materials investigated, the properties are fixed according to the composition of the material. Ideally, one wishes to have a tunable system similar to the semiconductor alloy systems, such as Al_xGa_1-xAs. There have been some theoretical studies of transition metal dichalogenides, none have been reported experimentally as of this writing. The tertianary alloy of BCN has been synthesized, however it was found that the boron had the tendency to cause phase segregation of the material into domains of graphene and boron nitride. Here we will report on the synthesis of non-phase seperated carbon-nitrogen 2D alloys ranging from graphene (E_g=0 eV) to carbon-nitride, or melon, (E_g=3.1 eV). We will report on synthesis methods and a summary of relevant electronic and material properties of selected alloys.

Heterostructures made of graphene and other 2D van der Waals materials are among the major research interests in recent days. Graphene has been proved to have high quality electronic properties [1-3], and when combined with other materials with complementary properties to form hybrids it shows new and/or improved functionalities compared to only-graphene devices [4]. In our work we have shown that high quality tunable optoelectronic response can be obtained when atomically thin MoS₂, a 2D van der Waals solid having bandgap in optical regime [5], is added to graphene to form planar heterostructures. Photoresponsivity of such heterostructures is as high as ~10¹⁰ A W^-1, and ~10⁸ A W^-1 when measured at low temperature (130 K) and room temperature (300 K) respectively [6,7]. Photoresponse in such hybrid devices can be tuned over ten decades by using a back gate voltage. We also show that a re-writable optoelectronic memory operation can be accomplished from these devices using appropriate sequence of light and gate pulses.
References:
1. Novoselov, K. S. et al. A roadmap for graphene. Nature490, 192-200 (2012).
2. Dean, C. R. et al. Boron nitride substrates for high-quality graphene electronics. Nat. Nanotechnol.5, 722-6 (2010).
3. Bolotin, K. I. et al. Ultrahigh electron mobility in suspended graphene. Solid State Commun.146, 351-355 (2008).
4. Konstantatos, G. et al. Hybrid graphene-quantum dot phototransistors with ultrahigh gain. Nat. Nanotechnol.7, 363-8 (2012).
5. Mak, K. F., Lee, C., Hone, J., Shan, J. & Heinz, T. F. Atomically Thin MoS₂: A New Direct-Gap Semiconductor. Phys. Rev. Lett.105, 136805 (2010).
6. Roy, K. et al. Graphene - MoS₂ hybrid structures for multifunctional photoresponsive memory devices. Nature Nanotechnology 8, 826-830 (2013)
7. Roy, K. et al. Optically active heterostructures of graphene and ultrathin MoS₂. Solid State Commun.1, 1-8 (2013) (http://dx.doi.org/10.1016/j.ssc.2013.09.021).

Semiconducting transition metal dichalcogenides (s-TMDs) have been attracted due to the various applications including the flexible transparent semiconductor device, vertical transistor, photodetector, and solar cell. For the specific applications, engineering the structure of s-TMDs such as nanotube or two dimensional layered structure is of importance. Here, we report the new synthesis way for tungsten diselenide (WSe₂) nanotube by using chemical vapor deposition. WSe₂ nanotube was synthesized from the tip of WO₃ nanowire and 2H-WSe₂ layers was grown on the outside of WO₃ nanowire, forming the core-shell structure of core (WO₃)-shell (WSe₂) nanowire simultaneously. On the other hand, WSe₂ film was synthesized through selenization of tungsten film under hydrogen atmosphere. The control of the amount of tungsten oxide is a crucial to determine the structure of WSe₂ as a film or nanotube. Our finding will contribute to engineer the other s-TMD structure and promote the applications of WSe₂ nanotube.

Large electrical resistance associated with the metal-molybdenum disulfide (MoS₂) interface has been plaguing the performance of MoS₂ field-effect transistors. In recent years, much effort has been devoted to address this critical issue along directions ranging from metal work function engineering to molecular doping, but the reported contact enhancement to date is still far from satisfactory. Here, we report a technique to fabricate low-contact-resistance MoS₂ transistors without degradation of the intrinsic performance of the MoS₂ channel. The technique involves the insertion of a piece of graphene that had been pre-treated with a nickel-mediated etching process as an interlayer at the metal/MoS₂ contacts to enhance the carrier injection from metal into MoS₂. The nickel-mediated etching process generates a significant amount of nano-sized pits in the graphene surface enclosed by pure zigzag edges. [1] More importantly, the zigzag graphene edges are expected to bond to the subsequent nickel metallization in an end-contact geometry thus resulting in optimized nickel-graphene contacts. [2] Our MoS₂ transistors with nickel-etched-graphene electrodes exhibit not only 2 orders of magnitude contact enhancement, but also very low contact resistance that is approaching that required for the silicon-based technology at the 22 nm node. In short, the findings provide an insight into how the contact resistance at the metal-MoS₂ interface can be engineered with the use of graphene as an interlayer, and can possibly bring the use of MoS₂ as a mainstream electronic material to the forefront.
References:
[1] W.S. Leong, H. Gong and J.T.L. Thong: Low-Contact-Resistance Graphene Devices with Nickel-Etched-Graphene Contacts. ACS Nano 8, 994 (2014).
[2] W.S. Leong, C.T. Nai and J.T.L. Thong: What Does Annealing Do to Metal-Graphene Contacts? Nano Letters (2014). (DOI: 10.1021/nl500999r)

Since the discovery of graphene, there has been a fast-growing interest in fabricating ultrathin 2-dimensional (2D) films and nano flakes out of materials with layered structures. Among these materials, molybdenum disulfide (MoS₂) is a semiconductor that shows significant promise for use in electronics, optoelectronics, and catalytic applications. In addition to the monolayer films, stacked 2D structures open up new opportunities for device structures in these applications. This study focuses on the use of atomistic simulations based on density functional theory (DFT) and molecular dynamics (MD) to analyze the effects of strain on the electronic properties as well as the mechanical stability of stacked 2D structures of MoS₂. The electronic band structures of these structures are investigated with emphasis on the effect of van der Waals interactions between each layer. These constructs are further studied under three deformation conditions and correlated using experiments. The effect of strain on their band gap energy and carrier effective masses will be discussed. In addition, the mechanical stability of the stacked 2D structures is investigated using MD simulations as a function of strain and will be presented in this poster.

We have successfully synthesized semiconducting transition metal dichalcogenide (sTMD) bilayer heterostructures by means of a "self cleaning" process in which coupling between stacked monolayers occurs after annealing under vacuum conditions. Monolayered islands of sTMD were independently grown by chemical vapor depostion (CVD). MS₂ (M= W, Mo) monocrystalline islands were subsequently lifted up from their native substrate and transferred onto MSe₂ islands and the stacked monolayers were successfully coupled after annealing under vacuum. Raman spectroscopy and photoluminescence (PL) studies were performed before and after the annealing step, and the differences arising from coupling will be discussed. It is envisaged that these van der Waals heterostructures will have applications in opto-electronics due to their electronic band gap characteristics, as well as in valleytronics due to their lack of inversion symmetry.

Due to the high carrier mobility, broad adsorption spectrum, graphene is attractive in the fabrication of ultrathin optoelectronic devices. The combination of graphene with emerging monolayer transition metal dichalcagenides, such as molybdenum disulfide (MoS₂), tungsten disulfide (WS₂), can further extend its potential in the optoelectronics with higher efficiency and photoresponsivity. The 1nm thick monolayer graphene and MoS₂ based solar cell has been modulated to exhibits power conversion efficiencies (PCE) of up to ~1%. However, the PCE is still limited to the low light absorbance of two monolayer graphene and MoS₂.
According to the band structure of MoS₂, WS₂ and graphene, a novel structure of monolayer graphene/MoS₂/WS₂ heterostructure is proposed to more efficiently absorb photon and separate electron-hole pairs. Generally, the photoexcited electrons in WS₂ is injected to MoS₂, and then transfer to graphene, while the holes transfer from graphene to MoS₂, and then to WS₂, which further inhibit the electron-hole recombination. We experimentally fabricate the heterostructure as solar cell, as well as photodetector. We have made lots of progress on CVD growing large grain size MoS₂ and WS₂, with strong photoluminescence. The device is fabricated by transferring graphene, MoS₂ and WS₂ one by one. We utilize Raman, PL spectroscopy and transmission electron microscopy to study the heterostructure, semiconductor parameter analyzer to measure the electrical curves in the dark and with illumination. The band alignment of the graphene/MoS₂/WS₂ heterostructure will also be studied by the density functional theory (DFT).

The unique anisotropic electrical properties of two-dimensional (2D) materials are highly promising for nanoelectronic applications. Molybdenum disulfide (MoS₂) has been the subject of recent attention as a 2D semiconductor for nanoelectronic devices due to the direct bandgap of 1.8 eV. To ensure optimum performance of MoS2 as a channel material in field effect transistors it is imperative to design the MoS2/metal interface such that partial Fermi level pinning is eliminated to ensure Ohmic contact. Here we present, Conductance Atomic Force Microscopy measurements and Kelvin Probe Force Microscopy (KPFM) of MoS2 devices to better understand the nature of the partial Fermi level pinning and to elucidate questions related to the design of metal-MoS2 interconnects. Raman and 4-point probes measurements are used in conjunction with the KPFM and C-AFM data to correlate the macroscopic properties with the localized nanoscale measurements.

In this work boron nitride (BN) platelets are used to fabricate ultra-high molecular weight polyethylene (UHMWPE)/BN composite fibers under shear. Fibers were subsequently drawn near the polymer melting point. Wide angle X-ray diffraction patterns (WAXD) and Raman spectroscopy of composite fibers show that exfoliation of the BN platelets to single layers occur during drawing processes. Scanning Electron Microscopy (SEM) images illustrate that crystallization of UHMWPE occurs eptaxially on the surface of the BN particles. The polymer-BN interaction was found to contribute exfoliation of BN particles upon drawing. SEM, WAXD, and small angle X-ray scattering studies show that drawing the fibers transitioned the UHMWPE chains from a folded conformation to the extended-chain structure. Computational analysis was used to examine the work associated with changes in the polyethylene chains in the crystals from folded to extended conformations. It was found that the combination of PE-BN interaction forces as well as work associated with chain unfolding during drawing contributed to BN exfoliation in the fibers. These morphological changes resulted increases for both the tensile stress and Young modulus for the composite fibers. This research shows fundamental analysis of the mechanism for exfoliation of layered fillers by polymer materials.

Two dimensional (2D) crystalline transition-metal dichalcogenides have attracted enormous attention due to their exceptional properties. Monolayer molybdenum disulfide (MoS₂), a direct-bandgap semiconductor, is an emergent material for atomically thin electronics and optoelectronics. Synthesis of MoS₂ through chemical vapor deposition (CVD) has been extensively studied. Recently, large-area monolayer MoS₂ has been obtained by sulfurization of molybdenum trioxide on Si/SiO₂ substrate. However, most of these growth methods were non-epitaxial, and individual single-crystal MoS₂ islands are randomly oriented, causing defects and grain boundaries. To achieve large-area single crystal MoS₂, epitaxy growth is preferred. Graphene has been reported as a growth template for van de Waals epitaxial growth of MoS₂. Moreover, the direct synthesis of MoS₂/graphene 2D heterojunction is favorable for the development of new electronic and optical devices. We will present epitaxial growth of MoS₂ on graphene towards large-area single crystal synthesis, and discuss the growth mechanism. The electrical and optical properties of the as-grown MoS₂ and MoS₂/graphene hybrid structure will be studied.

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.

Hexagonal boron nitride (h-BN) has a number of mechanical properties analogous to those of graphite, but exhibits unique electrical properties such as a wide band gap, low dielectric constant, piezoelectricity and good stability at high temperature. It is thus a promising material for many applications. The potential use of mono-layer or few-layer h-BN as a dielectric building block for graphene-inspired nano-electronics and photonics has motivated tremendous research effort toward synthesis of h-BN with controlled atomic layer thickness on non-metal catalyzed substrates, such as sapphire.
In this report, we described a process for achieving atomically smooth few-layer h-BN on sapphire substrates by MOCVD, using Triethylboron (TEB) and ammonia as precursors. In a certain temperature and pressure regime, the effect of V/III ratio on the growth behavior was studied. Various growth modes were observed, from random 3D nucleation to self-terminating growth, by changing the V/III ratio at growth pressures ranging from 20 torr to 300 torr. Infrared reflectance and Raman spectroscopy were used to identify the h-BN phase of these films. Atomic force microscopy measurements confirmed that the surfaces were smooth and continuous even over atomic steps on the surface of the substrate. Using X-ray reflectance measurements, the thicknesses of films grown under a certain range of conditions were determined to consistently terminate at few atomic layer thickness. Termination of growth at a specific thickness was independent of growth time and process parameters, such as temperature, pressure and TEB flux. This self-terminating growth mode enables a way to robustly synthesize h-BN with highly uniform and reliable thickness on non-metal catalyzed substrates.

We report on grafting polymer brushes on large area single layer hexagonal boron nitride nanosheets (hBNNs) via self-initiated photografting and photopolymerization (SIPGP). SIPGP is a catalyst and initiator-free and offers simplicity since no complicated surface-initiated procedure is needed. We have shown that by SIPGP, the preparation of polymer brushes with different monomers can be realized. Not only homogenous layers, but also patterned polymer brushes and patterned block copolymer brushes can be easily fabricated. Moreover, freestanding polymer carpets based on hBN nanosheets can be realized. This approach may offer flexibility for exploiting the potential application of polymeric hBN nanosheet composites, such as dielectric materials, thermal-isolating composites, or even for graphene electronics.

The synthesis of a novel, disordered, sheet-like boron-carbon-sulfur nanostructure is reported. The chemical vapor deposition process used to create this material is a modification of a method successfully used to grow a variety of pure boron nanostructures [1]. The process was originally a thermal vapor deposition reaction, performed in an inert atmosphere, and this was altered through the addition of methane and hydrogen sulfide gas. The product of this new reaction was analyzed using scanning and transmission electron microscopy, optical microscopy, and Raman and Fourier transform infrared (FTIR) spectroscopy. The product consists of flat, very thin (<10 nm), large area amorphous (based on electron diffraction patterns) sheets with numerous folds. Electron energy dispersive spectroscopy and electron energy loss spectroscopy indicated that the sheets are composed primarily of boron and carbon with sulfur, magnesium, and nickel impurities. FTIR and Raman spectroscopy showed that the material possesses the following bonds: C-C, S-S, B-C, C-C, and C-S. Optical microscopy revealed that these materials are essentially transparent, but further work would be needed to determine if the materials are optically insulating or semiconducting. Alterations to the reaction which produced these sheets ended up resulting in multiwall carbon nanotubes. Understanding the unique growth mechanism in play could provide broader insights into the growth of amorphous nanostructures.
References:
R. B. Patel: Synthesis and Characterization of Novel Boron-Based Nanostructures and Composites. 2013, PhD Dissertation New Jersey Institute of Technology.

The heterostructures of two-dimensional (2D) materials offer the possibility to create high performance electronic and optoelectronic devices by combining the different electrical, mechanical, and optical properties. Constructing a high quality heterostructure is an important step toward more detail study. Here, we report a method to synthesize a nanometer size overlapping graphene-MoS₂ heterojunction directly based on the seeding promoter assisted chemical vapor deposition of monolayer MoS₂. Utilizing the avoidance of graphene by the seeding promoter molecule perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS), the MoS₂ monolayer was only grown on the area external to graphene. Transmission electron microscopy (TEM) characterization shows that a nanometer size overlapping graphene-MoS₂ heterojunction was formed along the edge of graphene. Furthermore, a silicon-compatible process was developed to make the graphene-MoS₂ heterojunctions into arbitrary patterns. The heterojunction thus formed was found to be electrically conducting from transport measurements. The photocurrent was measured on a back-gated MoS₂-graphene heterojunction device. This lateral heterostructure presented here shows remarkable advantages over the stacked heterostructure, insofar as our structure allows separate control of the two components of the heterostructur. Our approach has high potential for constructing future on-chip integrated electronics and optoelectronics.

A centimeter long (15mm×10mm×0.5mm) free standing films comprising of graphitic carbon nitride (g-C₃N₄) nanolayers have been synthesized by the pyrolysis of dicyandiamide under low pressure condition of argon ambience ( #820; 3 torr ) at 600 ⁰C in quartz tube. The structural/ microstructural characterization have been done by X-ray Diffractometry (XRD),Scanning electron microscopy(SEM) X-ray Photoelectron spectroscopy(XPS) and Fourier Transform Infrared Spectroscopy(FTIR). These investigations reveals that the films consist of nanolayers of g-C₃N₄. The photoluminescence (PL) measurements have been done by Fluorescence spectrometer (PL, Perkin Elmer, Model No. LS-55, xenon flash lamp) and time-resolved spectroscopy (TRPL, Edinburgh, Model No. FLSP-920, xenon flash lamp). The Films show intense broad emission peak at 459 nm with various excitation wavelengths.The most intense emmission occurs with excitation of 372nm.

Transition metal dichalcogenides (TMDs) have received increased attention in recent years due to their layer dependent properties and potential applications in electronics and optoelectronics. Few layer transition metal disulfides and diselenides can be obtained either by top down or bottom up approaches. Among many synthesis methods, chemical vapor deposition (CVD) has demonstrated to be potentially effective for large scale synthesis of wafer-scale uniformity. A typical CVD process is based on mild sulfurization of transition metal oxides. In this work, we develop an alternative and novel strategy to synthesize MoS₂ monolayers without using oxide precursors. The as grown samples are characterized by atomic force microscopy, Raman, and photoluminescence spectroscopies. We also demonstrate the method proposed can be extended to synthesize unprecedented Mo_xW_1-xS₂ heterostructures. The optical properties of these structures will also be discussed.

Two-dimensional (2D) crystals, i.e. graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenides (TMDs), are fascinating materials for various device applications as they show a wide range of electronic, optical, physical and mechanical properties. Among different 2D materials, h-BN has gained substantial interest as a dielectric material to graphene devices owing to its atomically smooth surface, which is expected to be free of dangling bonds and charge traps. h-BN has a honeycomb structure similar to that of graphene, containing strong sp² covalent bonds. However, while graphene shows a semi-metallic behavior, h-BN is an insulator. Single and few layer h-BN films show interesting properties, including high mechanical strength and thermal conductivity, and excellent chemical inertness. All these fascinating applications can be facilitated via a scalable synthesis process such as chemical vapor deposition (CVD) method. While there have been recently several reports on the synthesis of single/few layer h-BN films, a controllable and reproducible process for growth of h-BN crystals with tunable thickness and size has been elusive. In this study, we report a controlled synthesis of high-quality 2D h-BN crystals via a low-pressure chemical vapor deposition (LPCVD) technique. The synthesized h-BN films are thoroughly characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM) and Raman spectroscopy.

The rise of graphene can be a defining point for various research and development of stable two-dimensional layered materials (2DLM) from layered systems. This breakthrough has stimulated the exploration of other 2D systems such as hexagonal boron nitride (hBN) and transition-metal dichalcogenides (TMDs) in the form of MX₂, where M equals to IVB-VIB transitional metals (IVB: Ti and Zr; V-B: Nb and Ta; VI-B: Mo and W) and X means dichalcogenides, such as S, Se, or Te. TMDs systems include a wide selection of electronic properties from insulating to semiconducting, and arouse broad research interests on their applications. Tungsten ditellruide (WTe₂) was predicted as a semiconducting TMD having 1.1eV narrow bandgap with 2H crystal structure, which holds the key for next generation tunneling effect transistors (FETs) using combination of TMD materials. However, due to natural WTe₂ single-crystal minerals are not available on the market, there is a lack of detailed information to verify and explore the basic physical and electrical properties for layered WTe₂ materials.
Here we presented the most recent and detailed study on layered WTe₂ materials. WTe₂ layered materials are obtained by physical exfoliating WTe₂ single crystals synthesized via halogen vapor transport method. We has found that WTe₂ actually naturally grows in Td structure, instead of typical 2H or 1T structure for most of TMD materials. With density function theory (DFT) calculation, we are able to correlate the vibration modes of Td-WTe₂ structure with the experimental Raman spectrum, and also determined that WTe₂ is a semimetal TMD material with bandgap ~0.103eV that is different from previous 2H-WTe₂ calculation results. Our electrical measurements has provided electrical properties about WTe₂ layered materials and suggested WTe₂ is a semimetal at room temperature. X-ray photoelectron spectroscopy (XPS) results has further showed that WTe₂ layered flakes has lower surface stability in exposure of oxygen, which surface Te may be converted to more stable Te-O bonding. Based on these information, we are able to explore a new direction for TMD materials that helps understanding the physics behind material properties and developing further application for 2D TMDs systems.

The bandgap of tungsten disulfide (WS₂), like other transition metal dichalcogenides(TMDs), decreases with the number of layer. While bulk WSshy;₂ has indirect bandgap of ~ 1.4 eV, monolayer WS₂ has direct bandgap of ~ 1.9 eV. The fact suggests that the Schottky-barrier height between WS₂ and a metal could vary with the thickness of WS₂. For studying the Schottky barrier with various layer of WS₂, we started with identifying their number of layers.
Optical contrast of WS₂ with various thickness have been measured depending on the thickness of SiO₂ on Si by direclty transfering the same-WS₂ flakes to the various substrates. Then the contrast is compared to the calculation based on Fresnel&’s law, the optical contrast map. While the maximized contrast can be obtained on 90-nm-SiO₂/Si wafer with 650-nm light, negative optical contrast has been measured on 180-nm SiO₂/Si substrate. Raman spectroscopy, and photoluminescence (PL) were measured and atomic force microscopy (AFM) confirmed the thickness of WS₂.
After fabricating field-effect transistors (FETs), the Schottky barrier height have been extracted depenging on the thickness. The height decreases with the thickness by 0.3 eV. Device characteristics also investigated with the FETs depending on the barrier height. The result suggests that the possibility of unpinned junction, and therefore, barrier eigneering between 2D semiconductor and metal.

Recent progress in exfoliation techniques has set the foundations for the manufacturing of essentially any given layered bulk material in the monolayer limit. The "standard" production and manipulation of exfoliated two-dimensional monolayers has opened up the possibility to shuffle the different layers in composite materials with new emerging properties. Among 2D materials, some transition metal chalcogenides, in particular MoS₂, are highly promising for novel nanoelectronic and optoelectronic applications. Freestanding 2D materials spontaneously ripple inducing (local) changes into the material&’s properties. Consequently, porous supports such as 2D covalent organic frameworks (COFs) open up the possibility to create MoS₂-COF composite nanodrums.
We studied the dynamics and electronic properties of MoS₂ monolayers supported by different 2D COFs using tight-binding density functional theory (DFTB) and classical molecular dynamics simulations. The COF pore size was tuned with the appropriate choice of connectors and linkers, that also increase the "freestanding" portion of MoS₂ above the pore (connectors: boroxine, hexahydroxybenzene or hexahydroxytriphenylene; linkers: phenyl, biphenyl or triphenyl). Principal component analysis and power spectra were used to identify the system&’s characteristic MoS₂ rippling modes and frequency at the COF pores as well as to guide the sampling of relevant structures. The MoS₂/COF lattice mismatch was minimized by simulating large supercells (up to 40,000 atoms). This required truly state-of-the-art quantum mechanical calculations in order to evaluate the properties of the electronic system.

A recent trend in the research of few- and single-layered materials such as graphene and graphene analogues is the application of large values of uniaxial or biaxial stress [1], and the observation of the resulting modifications in their electronic and structural properties. A class of those investigated two-dimensional materials are transition-metal dichalcogenides such as the semiconducting molybdenum disulfide (MoS₂) and molybdenum diselenide (MoSe₂). In the present work we have investigated theoretically, in the framework of first-principles calculations, the effect of the application of large uniaxial stress on the structure of MoS₂, MoSe₂ and MoSSe alloys. Our results show that the mechanical response of MoS₂ to the applied stress is nearly linear and isotropic for strain values up to 3%. For large strain, the response is nonlinear and anisotropic, with maximum two-dimensional stress values, at point fracture, of 10 N/m and 15 N/m, for applied stress along the zigzag and armchair directions, respectively. This is consistent with recent experimental and theoretical results. However,
for stress applied along the zigzag direction, we predict an anomalous behavior near the point fracture. This behavior is characterized by the reorientation of the MoS₂ structure along the applied stress from zigzag to armchair, with an apparent 30 degree crystal structure rotation. After reorientation, a large plastic deformation results after the stress is removed. This behavior is also observed in MoSe₂ and in MoSSe alloys. This phenomenon is observed in stress-constrained geometry optimizations and in ab initio molecular dynamics simulations.
[1] H. H. Pérez Garza, E. W. Kievit, G. F. Schneider, and U. Staufer, Nano Lett. (2014), DOI: 10.1021/nl5016848.

Studies of low dimensional structures become one of the most intensely investigated areas of the material research in recent years [1]. With a seminal example of graphene, layered transition metal chalcogenides draw attention of researchers. Molybdenum disulfide (MoS₂) a member of the family. Raman scattering spectroscopy appears to be a technique of choice to study its properties. The non-resonant Raman spectrum of MoS₂ is dominated by two basic vibrational modes: E¹_2g and A¹_g. The sensitivity of those peaks to the number of layers has been employed in a standard method of a thin MoS₂ [1] layer characterization.
More complex is the optical spectrum due to the resonant Raman scattering in which second-order Raman scattering processes, enhanced by the electron-phonon interaction are more effective than the modes related to the first-order Raman scattering. While the resonant Raman scattering in bulk MoS₂ has been widely investigated there are only a few reports on the effect of the MoS₂ layer thickness on the resonant Raman scattering [2]. In this communication we report our study on the effect. We analyze experimental spectra obtained on 1ML, 2ML, 3ML and bulk MoS₂ at room temperature and at liquid helium temperature.
In agreement with previous studies [2] we observe substantial changes of the spectrum with decreasing flake thickness. In particular higher order processes become less effective with decreasing number of layers. Moreover similar characteristic change in the lineshape of Raman peaks at ~460 cm^-1 and ~640 cm^-1 can be seen. The bimodal structure of those peaks becomes evident on flakes of a monolayer thickness. In our opinion the behavior can be understood in terms of recently proposed attribution of the Raman peaks to multiphonon replica involving transverse acoustic phonons in MoS₂ [3]. According the attribution the high-energy components of the ~460 cm^-1 and ~640 cm^-1 peaks are due to combined E_1g (M) +XA(M) and E_1g (M) +2 XA(M) processes respectively with XA - the transverse acoustic phonon from M point of the Brilloiun zone. Their intensity decreasing with the decreasing number of layers suggests that the processes involving transverse phonons become less efficient in thin flakes. This is in contrary to combined processes involving longitudinal acustic phonons, with Raman peaks well resovled also in a 1 ML sample.
In our communication we also report on low-temperature thickness-dependent measurements of resonant Raman scattering in MoS₂, which further support our model.
[1] C. Lee, H. Yan, L. E. Brus, T. F. Heinz, J. Hone, and S. Ryu, ACS Nano 4, 2695 (2010).
[2] A. B. Chakraborty, et al, J. Raman Spectr. 44, 92 (2013); H. Li, et al, Adv. Funct. Mater. 22, 1385 (2012).
[3] K.Go#322;asa et al Appl. Phys. Lett. 104, 092106 (2014)

In this talk differences in the Raman spectra in several different few layered transition metal dichalcogenides are discussed comparatively not only in terms of distinguishing one transition metal dichalcogenide from another, but also in providing information about the bonding between each constituent of a given few layered compound, its stacking arrangement, and its symmetry-dependent physical properties.

Since the discovery of the unique properties of graphene, 2D layered materials have exhibited great potential for fundamental research and technical applications in spintronics, electronics, photonics, optoelectronics. However, pristine graphene is facing the big challenge in use for the fabrication of logical circuits because it has no bandgap. Therefore, more and more emerging non-graphene 2D layered semiconductors have gained big interest of researchers. Our group has been actively engaged in the study on synthesis, properties and applications of Transition metal dichalcogenides and metal dichalcogenides,^[1~5] such as 2D layered GaTe and WSe₂ materials. (1) Through both electrical transport measurements at variable temperatures and first-principles calculations, we find Ga ion vacancy is the critical factor that causes high off-state current, low on/off ratio and large hysteresis of GaTe FET at room temperature. By suppressing thermally activated Ga vacancy defects at liquid nitrogen temperature, a GaTe nanosheet FET with on/off ratio ~10⁵, off-state current ~10^-12 A and negligible gate hysteresis is successfully demonstrated. (2) For the first time we demonstrate a straightforward catalyst-free vapor-solid (VS) growth method to synthesize ultrathin, even monolayer, WSe₂ sheets with high yield, regular shapes and high quality optical properties on sapphire substrates. By detailed layer-number dependent photoluminescence measurements, we found the transition of indirect-to-direct gap when the thickness decreases to monolayer. The results prove the high optical and crystal quality of our WSe₂ nanosheets via the VS growth approach, promising its further use in new exciting valley-based electronics, optoelectronics and photonics.
Reference:
[1] Z. X. Wang, K. Xu, Y. C. Li, X. Y. Zhan, M. Safdar, Q. S. Wang, F. M. Wang and J. He*; Role of Ga vacancy on a multilayer GaTe phototransistor, ACS Nano, 2014, 8, 4859-4865.
[2] K. Xu, Z. X. Wang, X. L. Du, M. Safdar, C. Jiang and J. He*; Atomic-layer triangular WSe₂ sheets: synthesis and layer-dependent photoluminescence property, Nanotechnology, 2013, 24, 465705.
[3] Z. X. Wang, M. Safdar, C. Jiang and J. He*; High-performance UV-Visible-NIR broad spectral photodetectorsbased on one-dimensional In2Te3nanostructures, Nano Lett.2012, 12, 4715-4721.
[4] M. Safdar, Q. S. Wang, M. Mirza, Z. X. Wang, K. Xu, and J. He*; Topological Surface Transport Properties of Single-Crystalline SnTe nanowire, Nano Lett.2013, 13, 5344-5349.
[5] Q. S. Wang , M. Safdar , Z. X. Wang , and J. He*; Low-Dimensional Te-Based Nanostructures, Adv. Mater.2013, 25, 3915-3921.

The discovery of graphene has influenced scientists to explore other 2D materials such as transition metal dichalcogenides, boron nitride, etc. MXenes are a newly discovered 2D family of early transition metal carbides produced by selectively etching the A element from the MAX phase, such as Ti₃AlC₂. The end result is 2D layers of M_n+1C_nT_x where M is an early transition metal and T stands for O, OH and/or F terminations. Herein we report on the optical and electrical properties, of Ti₃C₂T_x, Ti₂CT_x, and Nb₂C produced by etching of their respective epitaxial thin films grown on sapphire single crystals. The thin films produced are quite transparent and conductive. For example, the Ti₃C₂T_x thin films transmit 90% in the visible light range. Down to 100 K, the resistivity is metal-like; below 100 K, the resistivity increases slightly with decreasing temperature, due to a weak localization phenomenon characteristic of 2D defective materials. At the lowest temperatures, the magnetoresistance is negative further confirming the weak localization phenomenon of 2D materials. This work is a first step in using these newly discovered 2D for electronic and photonic sensing applications, as well as, transparent conductive electrodes.

Controlled assembly of graphene/two-dimensional (2D) semiconductor van der Waals (VDW) heterostructures is currently being explored to combine their individual electronic and optoelectronic properties for the creation of novel devices with enhanced functionality. Although most VDW heterostructures are currently made by stacking exfoliated 2D crystals, the precise control over interlayer stacking and crystal size necessary for practical applications requires the development of direct VDW epitaxial growth techniques. In this work, we synthesized 2D GaSe crystals directly on graphene through van der Waals epitaxy. CVD graphene synthesized on Cu foils was transferred onto SiO₂/Si substrates and GaSe was deposited from vapor phase evaporation. The 2D GaSe preferentially nucleated on wrinkles of the graphene film and grew into irregularly-shaped islands, in stark contrast with triangular-shaped flakes grown directly on SiO₂/Si. Aberration-corrected annular dark-field scanning transmission electron microscopy and electron diffraction were used to characterize the interlayer stacking of the heterostructures at the atomic scale for comparison with theory and modeling, while Raman and photoluminescence spectroscopy were used to,characterize the optical and electronic properties. The GaSe crystal lattice generally orients slightly rotated (within 10°) to that of the graphene. Such incommensurate crystal orientations can be expected from the large lattice mismatch between GaSe and graphene. A dramatic quenching of photoluminescence was observed in the 2D GaSe on graphene. [DG1] Device measurements indicate that 2D GaSe-graphene VDW heterostructures are highly photoresponsive and may be promising materials for photodetectors.
Synthesis science sponsored by the Materials Science and Engineering Division, Office of 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. A portion of this research was performed at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. Computing resources provided by the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

There is tremendous interest in building artificially stacked van der Waals solids from 2D atomic layer building blocks of varied compositions. We have successfully grown several atomic layer structures using scalable vapor phase methods and the approach, control and possibility of growing hybrid structures will be discussed here. Specifically, compositions of boron, nitrogen and carbon as well as binary and ternary transition metal dichalcogenides will be considered as building blocks to create hybrid atomic layers. Synthesis and atomic scale characerization and evaluation of different physical properties of these 2D units will be described. Most importantly the ability to build in-plane interfaces between 2D domains of different compositions as well the ability to stack some of these layers will be discussed. The challenges in creating hybrid atomic layer structures with control over the stacking order or the domain distribution within individual planes will be discussed.

Transition metal dichalcogenides (TMD) exhibit interesting electronic and opto-electronic properties. Moreover, single crystals can be easily exfoliated in atomic-scale layers making them very promising for novel electronic devices. These TMX₂ compounds (TM = transition metal, X = S, Se, Te) crystallize in layered structure that consists of vertically stacked layers, held together by weak Van der Waals-like interactions. Some exhibit charge density waves (CDW): TaSe₂, TiSe₂, VSe₂ and NbSe₂. In our work, we focused on the diselenides series TMSe₂ (TM = Ti, Zr, Hf, V, Nb, Ta) and carefully investigated their crystal growth conditions, structural and transport properties.
Single crystals of these TMD are generally obtained by the chemical vapor transport (CVT) method because of their high melting point, the strong volatility of components and the poorly known phase diagram. The experiments show that iodine is an excellent vapor transport vector for the growth of large and high quality crystals, except in the case of TaSe₂. We found that the growth of TaSe₂, as well as other heavy TMD, requires the use of a chloride transport agent, TaCl₅. The actual stoichiometry strongly influences the electronic properties of these TMD, so that it is essential to establish the best conditions for growing high quality single crystals. Structural characterizations of these crystals underline that the 1:2 ratio is hard to achieve in presence of light metals like Ti or V, which need to be reacted at low temperature and for long time.
Transport measurements emphasize their semimetallic behavior and confirm the expected CDW transition. The CDW state can be suppressed by intercalation, like in Cu intercalated TiSe₂, and superconductivity occurs. Structural defects and chemical impurities strongly affect the electronic properties and must be tracked down. Scanning tunneling microscopy (STM) coupled to ab initio calculations has allowed identifying the nature of defects in the TiSe₂ structure [1].
[1] B. Hildebrand et al., Phys. Rev. Lett. 112 (2014) 197001

Large-area monolayers of hexagonal boron nitride (hBN) are highly sought for graphene-based electronics due to the range of attractive electronic, chemical and mechanical properties they display. One of the main challenges to their integration in novel device architectures is the scalable growth of high quality films, which requires control over the domain size and thickness. The use of catalytic growth techniques has become the preferred route towards tailored synthesis.
Here we demonstrate the growth of triangular hBN domains during CVD on Fe substrates using a borazine vapor feedstock. For optimized synthesis conditions we show the growth of extremely large (~300 µm edge length) domains and through complementary characterization techniques including selected area electron diffraction (SAED), transmission electron microscopy (TEM) and atomic force microscopy (AFM) we reveal that these large domains are monolayer and single crystalline hBN. We also show that by increasing the precursor flux full coverage of the Fe catalyst with a homogeneous and continuous hBN layer can be achieved. These insulating hBN films are of interest for nanoelectronics due to their design compatibility with other 2D layered materials such as graphene (semi-metal) and WS₂ (semiconductor).
We relate the improvements in growth to our previous results on graphene synthesis, where we have demonstrated controlled dosing of a solid precursor,[1] and catalyst alloying as a means of controlling nucleation and consequently domain size.[2] For the catalyst stack system presented here we apply a combination of these techniques to achieve high-quality hBN monolayers. The understanding developed is relevant for the integration of hBN in device applications, and provides broader insights regarding growth control of two-dimensional materials.
[1] Weatherup et al., Nano Lett. (2013)
[2] Weatherup et al., Nano Lett. (2011)

Semiconducting transition metal dichalcogenides (TMX₂) are attracting the interest of a growing research community because of their possible application in optoelectronic devices. In this family, the electronic properties can be tuned either by choosing the chemical composition or by size quantization effects, upon reduction of one dimension to the atomic scale. The most interesting semiconducting TMX₂ (X=S,Se,Te) are those containing the heaviest transition metal elements, Mo and W, that proved to be the most difficult to be grown in pure crystalline form. In the Chemical Vapor Transport growth (CVT) of heavy TMX₂ (TM=Ta, Mo, W and X=S,Se,Te), elemental halogen transport agents (either Br₂ or I₂) yield either non single phase or very small crystals. With the aim of producing large and high quality crystals of these compounds, and to control the growth of each single polymorph, we have studied the possibility of using chlorides of the transition metals as a source for CVT growth. The study was successful for MoX₂ and TaX₂ by using MoCl₅ and TaCl₅ as a transport agent, respectively. However, WCl₆ appeared so far not to be an effective agent and the best results in growing WX₂ were obtained by using TeCl₄, I₂ and Br₂. We report and discuss the different processing parameters leading to the best quality of crystalline samples. Among semiconducting TMX₂, those containing Te have been less investigated so far, and rather unexplored at the nano-scale. The chloride-driven CVT method proved to be the most suitable for the growth of large MoTe₂ and TaTe₂ crystals. Exfoliated MoTe₂ crystals have been successfully used in ionic liquid-gated transistors and exhibit ambipolar transport at the surface.

Recently, monolayers of layered transition metal dichalcogenides (TMDc), such as MX₂ (M=Mo, W and X=S, Se), have been reported to exhibit excellent optoelectronic performances. Monolayers in this class of materials offered a burgeoning field in fundamental physics, energy harvesting, electronics and optoelectronics. However, growth mechanisms and transfer of CVD-TMD monolayers remain challenge issues.[1~3] Here, we demonstrate the growth of high-quality TMD monolayers using chemical vapor deposition (CVD) with the seeding of aromatic molecules. The growth of monolayer TMD single crystals is achieved on various surfaces and a possible growth mechanism of the seed-activated growth would be presented.
We would like to demonstrate some techniques in transferring the TMD monolayers to diverse surfaces, Some characterization techniques and applications of vdw heterostructures were presented, which may which may stimulate the progress on diverse hybrid structures with TMDc monolayers.
Ref.
[1] Yi-Hsien Lee, et al., Adv. Mater., 24 (17), p.2320-2325 (2012)
[2] Yi-Hsien Lee, et al. Nano Lett., 13 (4), 1852-1857 (2013)
[3] Xi-Ling, Yi-Hsien Lee*, et al., Nano Lett., 14 (2), p.464-472 (2014)
[4] Lili Yu, Yi-Hsien, et al, Nano Lett, 14 (6), p.3055-3063 (2014)
[5] Xin-Quan Zhang et al, (in preparation)

The practical realization of electronic, sensor, solar conversion and other devices made of two-dimensional (2D) materials with semiconductor and dielectric properties critically depends on the availability of suitable synthesis routes, which can allow reproducible, substrate agnostic, scalable, and cost effective processing technologies. This presentation provides results of the research in physical vapor deposition (PVD) methods to grow few monolayer thick 2D materials including transition metal dichalcogenides with semiconducting behavior (such as MoS₂ and others) and boron nitride with dielectric behavior. Magnetron sputtering and pulsed laser deposition were optimized to produce 2D materials from room temperature to about 750 C on a variety of substrate materials, such as amorphous silicon oxide and glass, sapphire, highly oriented pyrolyticgraphite, flexible polymers, and metals. Photoluminescence has confirmed a1.8-1.9 eV direct band gap in MoS₂ monolayers with a high charge mobility from device measurements. The dielectric nature of the produced BN monolayers was confirmed with conductive AFM measurements. Both 2D MoS₂ and BN layers were produced with no holes, macro-defects, wrinkles, etc. at areas of larger than square inch. These semiconducting and dielectric materials were combined in heterostructures and analyzed with TEM, AFM, XPS, Raman and electrical probe methods. The results demonstrate the benefits of the PVD approaches to overcome scalability challenges with traditional chemical vapor deposition or mechanical exfoliation methods.
[1] C. Muratore, J.J. Hu, B. Wang, M.A. Haque, J.E. Bultman, M. L. Jesperson, P.J. Shamberger, M. E. McConney, R.D. Smith, A.A. Voevodin, “Continuous ultra-thin MoS2 films grown by low-temperature physical vapor deposition”, Applied Physics Letters (2014).
[2] N. R. Glavin, M. L. Jespersen, M. H. Check, J. Hu, A. M. Hilton, T. S. Fisher, A. A. Voevodin, “Synthesis of few-layer, large area hexagonal-boron nitride by pulsed laser deposition”, Thin Solid Films (2014).

Atomistically thin 2D semiconductors are being extensively investigated for post silicon nanoelectronics owing to their excellent electrostatic integrity that allows aggressive channel length scaling ^{(1, 2)}. Phosphorene, a single layer of black phosphorus, arranged in a puckered honeycomb like structure, is the newest member of this 2D family ⁽³⁾. The field effect mobility in few layer phosphorene has been demonstrated to be ~1000cm²/V-s at room temperature, which makes this material extremely appealing as an alternative to silicon.
In this article, we employ a novel technique to extract the bandgap from the experimental transfer characteristics of phosphorene field effect transistors. We found that the bandgap changes monotonically from ~0.3eV to ~1.0eV when the flake thickness is scaled down from bulk to a single layer. Using this technique we were also able to extract the Schottky barrier heights for the electron injection into the conduction band and hole injection into the valence band. We found that along with the bandgap, these Schottky barrier heights also change dynamically with the layer number impacting the ON current, the OFF current and the ON/OFF current ratio as well as the asymmetry of the ambipolar conduction.
A non-monotonic trend was observed in the ON state current as a function of the flake thickness, which we could explain using a resistive network model that includes Thomas-Fermi charge screening and inter-layer coupling. In general, it was found that the use of a finite number of phosphorene layers is beneficial for the ON state performance since the impact of the substrate on the device performance can be eliminated. The highest field effect hole mobility was extracted to be ~315cm²/Vs for a 6nm (~10 layers) thick flake. The monotonic increase of the OFF state current and decrease in the current ON/OFF ratio as a function of the increasing layer number, however, was explained using the fact that the bandgap decreases monotonically.
Acknowledgement: The work by W.Z. and A.H. was supported by the U.S. Department of Energy, Office of Science, Materials Science and Engineering Division. The work of Saptarshi Das is supported by the DOE Office of High Energy Physics under DoE contract number DE-AC02-06CH11357. Use of the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02- 06CH11357.
References
Das, S. et al. Where does the current flow in two diemnsional layered system. Nano letters13 (7), 3396-3402 (2013).
Das, S. et al. Evaluating the scalability of multilayer MoS₂ FETs Device Research ConferenceDRC, (2013).
Li, L.; Yu, Y.; Ye, G. J.; Ge, Q.; Ou, X.; Wu, H.; Feng, D.; Chen, X. H.; Zhang, Y. Black Phosphorous Field Effect Transistor. Nature Nanotechnology, (2014).

The attractive potential of hexagonal boron nitride (hBN) as a wide-band gap material was realized after obtaining its high quality single crystals. Recent progress in the synthesis of high purity hBN crystals was achieved by using Ba-BN as a solvent material at high pressure(HP)crystal growth[1]. Taking advantage of the highly luminous properties of hBN (i.e.Eg=6.4eV), an operation of far ultraviolet-plane-emission device was demonstrated [2,3]. It is also emphasized that hBN exhibits superior properties as a substrate of graphene devices [4].
In order to realize these newly developed potential of hBN crystals, more precise insight for its quality control is important. Although the major impurities affects the optical properties of hBN are oxygen and carbon, the details of their contribution are still not well known. For the application of graphene&’s substrates, some unknown point defects may affect its device qualities.
On the other hands, it is well known that high quality diamond single crystals with less impurities of type IIa (i.e. Nitrogen and Boron impurity levels are less than 1ppm) can be obtained under HP. Furthermore, carbon isotope ratio of 12C99.995 #65374; 13C98% in grown diamond crystals can be controlled by using proper carbon source materials[5]. Similar strategy is also effective for the synthesis of high quality graphite single crystals as a source of graphene devices.
In this paper, some recent trials for impurity control in solvent growth process for hBN and graphite single crystals are reported.
REFERENCES
[1] T.Taniguchi, K.Watanabe, J.Cryst.Growth 303,525 (2007).
[2] K.Watanabe, T.Taniguchi and H.Kanda, Nature Materials, 3,404 (2004).
[3]K.Watanabe,T.Taniguchi,A.Niiyama,K.Miya, M.Taniguchi, Nature Photonics 3, 591(2009).
[4] C.R.Dean, A.F.Young, K.Watanabe, T.Taniguchi, P.Kim, et.al.,Nat.Nanotech,
5, 722(2010).
[5] T. Teraji, T. Taniguchi, S. Koizumi, K. Watanabe, M. Liao, Y. Koide, and J. Isoya,
Jpn. J. Appl. Phys. 51, 090194 (2012).

The unique properties of van der Waals heterostructures have motivated their rapid development, but the emergence of new phenomena depends critically on strong electronic coupling between adjacent 2-D sheets. Here we show that black phosphorus, a 3-D layered crystal, exhibits remarkably strong interlayer coupling, resulting in optoelectronic properties with tunability that is unmatched among 2-D materials. We employ liquid exfoliation to efficiently produce and isolate sheets of varying thickness from monolayers to multilayers. With large quantities of high quality 2-D black phosphorus now available, we systematically probe the thickness-dependent transition from the strongly coupled bulk material to quantum-confined 2-D phosphorene, a monolayer of black phosphorus. We find that the band gap increased from 0.35 (3-D) to 2.4 eV (2-D), a range larger than previously anticipated. This unprecedented variability hints at a new class of strongly-coupled van der Waals heterostructures with properties that can be reversibly switched and tuned via assembly and disassembly.

The present study explored the large scale and thickness-modulated growth of atomically thin MoS₂ layers on Si/SiO₂ substrates using the two-step synthesis process of deposition-CVD method. Our process exhibited wafer-scale fabrication and successful thickness modulation of MoS₂ layers ranging from monolayer (0.72 nm) to multilayer (12.69 nm) with high uniformity. Electrical measurements on MoS₂ field effect transistors (FETs) revealed the field effect mobility of ~20 cm²V^-1s^-1 and the current on/off (I_on/off) ratio of ~1.0x10⁶, which is much higher than those of previously reported MoS₂-FETs and commercially available amorphous silicon (a-Si) thin film transistor. Our results show that the two-step CVD is a viable method to synthesize large area, high quality MoS₂ atomic layers that can be adapted in conventional Si-based micro fabrication technology and future flexible electronics, high temperature and optoelectronics.

Two-dimensional layered materials have generated much interest as the mechanical, optical, and electronic properties of the bulk materials differ from the properties of the exfoliated 2D materials. Examples of Van der Waals solids that have been investigated include graphite, hexagonal boron nitride, and transition metal chalcogenides, but there are still a large number of unexplored materials. Here we report chemistry of the TX (T = Si, Ge; X = P, As) family of compounds. They are composed of TX layers which are terminated with pnicogen atoms. The weak lone pair interactions of the terminal pnicogen atoms allow the material to be readily exfoliated through sonication. TX compounds exhibit rich structural chemistry. Layers can be either further separated by the incorporation of the cations in the interlayer space or connected by the replacement of the terminal pnicogen atoms with heavy main group elements, such as Sn or Bi. Structural chemistry, optical, and electrochemical properties of the TX solids and exfoliated materials will be discussed.

Layered two-dimensional (2D) materials have been of long-standing, special interest, due to their anisotropic structure and the emanating distinctive physical properties. Graphene and its inorganic analogues extend the material basis of electronics to an unprecedented versatility, promising improvements of established device concepts as well as the realisation of entirely new device architectures
Pure 2D materials provide a rich chemistry, to fully utilise their potential as well as for the purpose of uncovering new fundamental phenomena, their alloying and doping in order to alter their electronic and magnetic properties is of paramount importance and impact. Besides the challenge in synthesis there is the demand for novel imaging and sensing strategies that can reveal atomic details without altering the as-synthesized structure. Theoretical modelling as well as new sample preparation strategies rely on the feedback from such strategies.
Atomic-scale transmission electron microscopy pushed towards low electron energy by virtue of chromatic aberration correction recently opened a new horizon for direct imaging of atomic details of layered structures. Here it is used to analyse the structure, layer defects and impurities in atomic sheets of graphene, molybdenum disulfide or transition-metal-dichalcogenide based closed cage structures. The structure of novel misfit-layer compound nanotubes will be presented. Line defects and point-defects in molybdenum-disulfide and their electronic properties will be shown. Spectroscopic methods complement the structure analysis by detection of single impurity or dopant atoms, by local charge transfer sensitivity of chemical signals as well as local optical excitation experiments selectively performed on edge or surface states of individual nanostructures.

Electron-beam-induced structural changes can be applied to modify material properties in a controllable manner [1], and to explore structural dynamics that could be difficult to observe under typical thermodynamic conditions [2, 3]. In particular, the electron beam of a scanning transmission electron microscope has been used to modify ribbons of transition-metal dichalcogenide (TMDC) monolayers into highly stable metallic nanowires with MX stoichiometry, which could serve as interconnects in future flexible nanocircuits fabricated entirely from the same monolayer [1]. The same electron beam simultaneously provides atomic-scale images of the dynamic processes that occur, offering time-resolved direct tracking of atomic motions during the structural changes.
In this work, we use a focused electron beam to generate and excite defects in semiconducting MoSe₂ monolayer, and monitor their dynamics through sequential atomic-scale Z-contrast imaging. We find that Se vacancies are first randomly created and then preferentially agglomerate into line defects under the energy transferred from the electron beam. Successive evolution of these line defects induces nucleation of distinct triangular inversion domains within the MoSe₂ layer and generates conductive 60° grain boundaries within the semiconducting matrix. Density functional theory shows that the nucleation of inversion domain lowers the system energy due to the release of vacancy-induced lattice shrinkage. Migration of the grain boundaries can be further activated by deformation of the peripheral lattice, giving rise to the growth of the inversion domain. The grain nucleation and growth process observed under energy transferred from the electron beam can provide new insights into the structure stability of TMDC monolayers under severe (e.g. high temperature) working conditions.
[1] J. Lin, et al, Flexible metallic nanowires with self-adaptive contacts to semiconducting transition-metal dichalcogenide monolayers, Nature Nanotechnology, 9, 436 (2014)
[2] J. Lee, et al, Direct visualization of reversible dynamics in a Si6 cluster embedded in a graphene pore, Nature Communication, 4, 1650 (2013)
[3] Y. Lin, et al, Atomic mechanism of the semiconducting-to-metallic phase transition in single-layered MoS₂, Nature Nanotechnology, 9, 391 (2014)

We report a robust method for engineering the optoelectronic properties of few layer MoS₂ using low energy oxygen plasma treatment. Gas phase treatment of MoS₂ with oxygen radicals generated in an upstream N₂-O₂ plasma is shown to enhance the photoluminescence (PL) of few layer, mechanically exfoliated MoS₂ flakes by up to 20 times, without reducing the layer thickness of the material. A blue shift in the photoluminescence spectra and narrowing of linewidth is consistent with a transition of MoS₂ from indirect to direct band gap material. Atomic force microscopy and Raman spectra reveal the flake thickness actually increases as a result of the plasma treatment. The plasma treatment enables partial intercalation of the interlayer spaces in few layer MoS₂, thereby decoupling the electronic states in the individual layers, causing an indirect to direct band gap transition. While increase of the interlayer separation confines the carriers in two dimensions, the possible creation of defects in MoS₂ gives rise to localized excitonic states with longer lifetimes. Shifts in the plasmon peak in the electron energy loss spectra (EELS) indicate that the oxygen plasma process is accompanied by an increase in the density of valence electrons in the MoS₂, due to the introduction of electron donor atomic species interspersed between the individual MoS₂ layers. With optimized plasma treatment parameters, we observed enhanced PL signals for 32 out of 35 few layer MoS₂ flakes tested, indicating this method is robust and scalable. Monolayer MoS₂, while direct band gap, has a small optical density, which limits it potential use in practical devices. The results presented here provide a method to harness the direct gap nature of monolayer MoS_2, while overcoming it's low absorbance.

We study and report on the exciton dynamics in monolayer transition metal dichalcogenides (TMD) using a transient mapping technique. Our technique provides a direct approach for space and time-resolved visualization of exciton diffusion in these novel material sets. In monolayer MoS₂ grown using a chemical vapor deposition (CVD) technique, we obtain diffusivity of 5.8e-2 cm²/s, an order of magnitude larger than that of a monolayer exfoliated from a natural crystal. This observation along with the lower photoluminescence from the exfoliated films suggests lower density of trapped states in the CVD grown monolayers. Similarly, exciton dynamics in exfoliated WSe₂ films from natural and synthetic crystals, as well as MoS₂ and WSe₂ heterostructures are exploited and will be reported. Through providing a direct approach to study excitons, our transient mapping technique reveals fundamental information on properties of TMD which contribute to the fast-growing fields of related electronic and photonic devices.

Recently, yttria (Y₂O₃) has demonstrated favorable wetting behavior on quasi-freestanding graphene [1-3], and could therefore be a promising dielectric material for the creation of graphene devices. In this study, we carefully investigated the interface between Y₂O₃, a high-κ dielectric, and the transition metal dichalcogenide, MoS₂. The growth of the yttria film was monitored in-situ by monochromatic X-ray photoelectron spectroscopy and scanning tunneling microscopy (XPS and STM). Depositing Yttrium results in a chemical interaction between the adlayer and the substrate as indicated by the comparison of Mo 3d core levels before and after deposition. However, no indication of chemical interaction was detected for the Y₂O₃ growth, formed by evaporating Yttrium in an O₂ environment. This is due to the initial formation of Y-O bonds, before the weak Y-MoS₂ interaction occurs. The XPS and STM results indicate the formation of a dielectric film on MoS₂(0001) surface. Further results of the growth and interfacial properties will be presented.
This work is supported in part by the Southwest Academy for Nanoelectronics (SWAN) Center sponsored by the SRC Nanoelectronics Research Initiative and the National Institute of Standards and Technology.
[1] R. Addou, A. Dahal, and M. Batzill, Nature Nanotechnology 8, 41-45 (2013).
[2] A. Dahal, R. Addou, H. Coy-Diaz, J. Lallo, and M. Batzill, APL Mat. 1, 042107 (2013).
[3] A. Dahal, H. Coy-Diaz, R. Addou, J. Lallo, E. Sutter, and M. Batzill, J. Appl. Phys. 113, 194305 (2013).

Hexagonal boron nitride (h BN) is of great interest as a 2 dimensional (2D) insulator for use as an atomically flat substrate, gate dielectric and tunneling barrier. At this point the most promising and widely used approach for growth of mono to few layer h BN is metal catalyzed chemical vapor deposition (CVD). Bulk Cu foil has been the most popular metal substrate for growth of h-BN and graphene, as such there are well developed processes for substrate preparation and growth. As alternative thin Cu films deposited on an insulating substrate have some advantages over foil, including more uniform thermal contact with substrate heater, better mechanical stability, transfer free processing, and selective area growth. However, Cu films deposited on SiO₂ present their own unique problems like Cu SiO₂ stability and small Cu grain size. Here we present results on the growth on few-layer h-BN by metal organic chemical vapor deposition (MOCVD) on Cu thin films on SiO₂/Si. We explore the effects of substrate preparation and annealing conditions on the Cu morphology in order to understand impact on the h-BN. To minimize the effects of Cu SiO₂ interdiffusion we investigate the use of buffer layers. H BN films were studied both on Cu films and after transfer to SiO₂/Si films using Raman, SEM, AFM and TEM to determine the impact of Cu film microstructure on the nucleation, thickness and morphology of few layer h-BN films.

Recently, the strong role of intrinsic defects in natural MoS₂ on the device performance has been reported [1-3]. We used room temperature scanning tunneling microscopy and spectroscopy (STM and STS), monochromatic x-ray photoelectron spectroscopy (XPS), inductively coupled plasma mass spectrometry (ICPMS), and high-resolution Rutherford backscattering spectrometry (HR-RBS) to study the pristine surface (0001) of MoS₂ immediately after mechanical exfoliation. The variability in the conductivity and the stoichiometry detected across the same sample was correlated to two different types of defects: metallic and structural (see figure). The imaged imperfections are correlated to similar defects previously observed after artificial bombardment [4] or intentionally induced atoms (Re, Li, Na) [5]. In this work, XPS detects only two impurities: Oxygen and Carbon. We will further compare the intrinsic defects detected on surfaces as well as impurity concentrations from different commercial vendors. We conclude that the exfoliated mineral MoS₂ exhibits a high degree of variability and considered to be far from an ideal surface.
This work is supported in part by the Southwest Academy for Nanoelectronics (SWAN) Center sponsored by the SRC Nanoelectronics Research Initiative and the National Institute of Standards and Technology.
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The challenge of obtaining new materials presenting optimum functional properties for electronic and optical applications has located the two-dimensional nanomaterials as the focus of a significant and fruitful research activity. Thus, following the first production of graphene [1], a broad range of layered materials, including transition metal chalcogenides, metal halides, clays or metal oxides were successfully exfoliated in liquid environments [2,3].
Recently, some layered semiconducting materials presenting interesting electro-optical properties were pointed as potential sources to obtain new atomically thin two-dimensional sheets [4]. However, the sources to obtain these precursors are usually scarce and expensive. In the present work we explore synthesis routes for different semiconductor metal chalcogenides and propose alternative routes to produce micron- and nanometre- sized particles, as a previous step for exfoliation and application.
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In recent years, two-dimensional (2D) nanomaterials have attracted increasing interest because of its unique structure and remarkable properties. Besides graphene, other types of 2D nanomaterials such as transition metal dichalcogenides (TMD&’s) have been widely studied and applied in low power electronics and energy storage applications. As a member of TMD family MoS₂ with intrinsic band gap, is considered to be candidate materials for post silicon electronic devices. However it still remains a great challenge to grow uniform, crystalline MoS₂ over large areas with predetermined number of layers. Here, we report controlled growth of MoS₂ on variety of substrates by chemical vapor deposition (CVD) using Mo(CO)₆ and H₂S as precursors.MoS₂ n-layered, n=1, 2...>6, is obtained by quantitative control of the vapor phase supersaturation. The as-deposited layers are p-type, due to Mo deficiency, with hole mobilities up to 7.4 cm²/V-s, the best reported yet for CVD MoS₂ with room temperature current on/off ratios of 10⁶. All reactant sources in our method are outside the growth chamber enabling for successful deposition of graphene or other TMD&’s, opens up possibility for growth of heterostructures for integration of novel electronic and photonic devices.

Hexagonal boron nitride (h-BN) has recently been in the spotlight due to its numerous applications including its being an ideal substrate for two dimensional electronics, a tunneling material for vertical tunneling devices, and a growth template for heterostructures. However, to obtain a large area of h-BN film while maintaining uniform thickness is still challenging and has not been realized. Here, we report the systematical study of h-BN growth on Pt foil by using low pressure chemical vapor deposition with a borazine source. The monolayer h-BN film was obtained over the whole Pt foil (2 x 5 cm²) under < 100 mTorr, where the size is limited only by the Pt foil size. A borazine source was catalytically decomposed on the Pt surface, leading to the self-limiting growth of the monolayer without the associating precipitation, which is very similar to the growth of graphene on Cu. The orientation of the h-BN domains was largely confined by the Pt domain, which is confirmed by polarizing optical microscopy (POM) assisted by the nematic liquid crystal (LC) film. The total pressure and orientation of the Pt lattice plane are crucial parameters for thickness control. At high pressure (~0.5 Torr), thick film was grown on Pt (111) and, in contrast, thin film was grown on Pt (001). Our advances in monolayer h-BN growth will play an important role to further develop a high quality h-BN film that can be used for vertical tunneling, optoelectronic devices and growth templates for a variety of heterostructures.

A big obstacle for developing colloidal quantum dots (QDs) based optoelectronic devices is the surface ligands. They are necessary for preventing the QDs from aggregation, but significantly impede charge transport in QD films since they are typically organic insulators. To improve the charge transport in QD films, short organic ligands, inorganic ligands and atomic ligands have been developed to replace the original long organic ligands. Although they improve the mobility of the charge carriers in the QD films, the existing inter-QD spacing and the boundary of QDs still hinder the charge carrier transport.
Making two-dimensional (2-D) nanosheets (NSs) can effectively reduce these hindrances, yet retain the tunable quantum confinement in one dimension. It has been demonstrated recently that the charge carrier mobility of a single layer of PbS NS is about ten times higher than that from PbS QD films. We report a method of controlled synthesis of colloidal PbS NSs with tunable thickness from 1.2 nm to 4.6 nm. The thickness-dependent photoluminescence spectra from lead-salt nanosheets are observed for the first time. The photoluminescence peaks and the corresponding optical absorption edges overlap and are tunable from 1470 nm to 2175 nm by changing the thickness of the NSs. The one-dimensional confinement energy of these quasi-two-dimensional nanosheets is found to be proportional to 1/L instead of 1/L² (L is the thickness of the nanosheet), which is consistent with results calculated using density functional theory as well as tight-binding theory.

Transition metal dichalcogenides (TMDC) are two dimensional layered inorganic materials analogous to graphene. Much work has been done on MoS₂ recently due to its potential applications to semiconductor and photonic devices. Most of the work on MoS₂ has been done on mechanically exfoliated flakes or on films grown by chemical vapor deposition (CVD). Epitaxial films produced by these methods have been limited in dimension to the micrometer scale. To overcome this problem, MoS₂ films are grown by pulsed laser deposition (PLD). Epitaxial films of homogeneous thickness with areas on the order of 1cm² have been achieved on a variety of substrates. Structural characterization by X-ray diffraction (XRD) shows high quality films which is confirmed by selected area electron diffraction (SAED) and high resolution scanning tunneling electron microscopy (STEM). The topography was determined by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Thickness variation of the films has been studied by Raman spectroscopy. Finally, electronic properties of field effect transistors will be discussed.

Atomically thin two dimensional MoS₂ has been considered as a potential van-der-Waal solid for for low power electronic and opto-electronic applications. For most of the electronic applications it is important to understand the stability of material in condition such as humidity. In view of this the fundamental question is that how the surface energy and the wettability are altered at the nanoscale-in particular with crystal size and orientation need to be addressed. Therefore we synthesized large area MoS₂ thin films on insulating substrates (SiO₂/Si and Al₂O₃) via sulphurization of the Molybdenum coated substrates in reducing atmosphere of H₂+Ar at 1 atmosphere. The thickness of the films was kept at about 7 nm by optimizing the molybdenum coating and surface morphology was tuned by varying the growth temperatures namely at 550 0C, 750 0C and 900 0C. The surface microstructure was examined by using high resolution transmission electron microscopy which reveals a peculiar change in microstructure with increasing the growth temperature. The obtained difference in microstructure with different growth temperatures was further corroborated by the contact angle measurements with water. Wetting results showed that the films grown at 550 0C and 900 0C have hydrophilic and hydrophobic nature respectively. The specific surface energy for few layers of MoS₂ grown at 900 0C is estimated to be about 46.5 mJ/m².

Sonication-assisted liquid phase exfoliation technique is an efficient and low cost approach for large scale fabrication of two dimensional (2D) transition metal disulfides, while maintaining the integrity of their structural and electronic properties. It is known that the organic solvent utilized during the exfoliation is important to the productivity of 2D materials. In this work, we aims to investigate the characteristics of exfoliated 2D molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) using several types of organic solvents. It is found that their morphologies, dimensions, thicknesses, optical absorption and photoluminescence properties all have strong dependence on the choice of organic solvents. In addition, the structural contaminants induced by organic solvents are comprehensively characterized. This work will provide indications on the appropriate solvent selection for large scale fabrication of 2D semiconductors and accelerate their implementations in the development of practical electronic and optical devices.

New synthesis techniques to rapidly explore new 2D layered materials beyond graphene are of significant current interest, as are interesting optoelectronic applications of these materials. Here we demonstrate that pulsed laser deposition (PLD), a well-known and versatile synthesis method principally used for epitaxial oxide thin film growth, can be used to explore the synthesis of functional metal chalcogenide (GaSe) nanosheet networks. Our PLD approach offers a new synthesis solution to address the challenges of conventional vapor phase (e.g. CVD) growth methods, by taking advantage of the spatial confinement of the ablation plume in relatively high background gas pressures to preserve the stoichiometric transfer of material from a bulk target to the substrate while providing sufficient kinetic energy for surface diffusion. We use in situ ICCD imaging to characterize the plume transport used to grow substrate-scale networks of interconnected, crystalline, GaSe nanosheets that exhibit high photoresponsivity, and we show that tuning the digital delivery of the precursor flux to the substrate by PLD can easily control the thickness of the nanosheets or change layer-by-layer in-plane growth to out-of-plane. GaSe crystal structure was characterized by high resolution transmission electron microscopy (HRTEM) and electron diffraction indicating single crystalline nanosheets. The electrical and optical properties of these interesting networks of interconnected GaSe nanosheets are comparable to exfoliated and superior to CVD-grown few-layer GaSe flakes. Field effect transistors showed p-type semiconducting characteristics with mobilities reaching as high as 0.1 cm²V^-1s^-1, with photoresponsivity and external quantum efficiency ranging up to 1.4 AW^-1 and 600%, respectively.
Synthesis science supported by the Materials Science and Energy Division, Office of Basic Energy Sciences, U.S. Department of Energy. Optoelectronic measurements conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

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 optimal cosolvent mass fraction 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 finding 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.

In the recent years, new interest is emerging towards the engineering of atomic layers of carbon, driving the increase of the research effort towards the production of new materials and synthesis methods.
Flames are well-established reactors for synthesis of a wide variety of carbon compounds, from diamond to fullerenes, nanotubes or graphene, by simply changing the operating conditions like temperature, pressure, residence time, stoichiometry and fuel. However there are still unexplored potential.
In this work we demonstrate the possibility to produce atomically thin carbon nanostructures having a disk-like shape when deposited on a substrate. Such compound has the potentiality to be exploitable as is, to become a building block to create functional structures or to be manipulated and engineered into interesting composite layered structures.
The size distribution of the produced compounds has been analyzed on line by means of a differential mobility analyzer (DMA) and the topology by Atomic Force Microscopy. The sphere equivalent mobility diameter in a gas flow is about 2 nm, but when deposited on a mica substrate for AFM analysis the carbon compounds assume the shape of an atomically thin disk with in plane diameter of about 20 nm.
Raman spectroscopy, XPS and UPS have been used to characterize their structure. Results show that Carbon nano-disks contain small aromatic islands with in plane length, La, of about 1.3 nm. These islands are arranged in a network and are connected by non-aromatic bonding to form bi-dimensional structures that assume an atomically thin disk-like shape when deposited on a substrate. Raman spectra evidence a significant amount of disorder which is in a large part due to the quantum confinement in the aromatic islands and the small size of the disks. However, other kind of disorder are also present, including chain-like bridges or dangling bonds or lattice distortion possibly caused by pentarings or sp³ inclusions.
The presence of sp³ hybridized carbon and oxygen inclusions has been investigated by XPS. The nanodisks contain very small percentage of sp³, the O/C ratio is about 6% which includes possible surface contamination from ambient air. The valence band has been analyzed by UPS. The contribution of C 2p p density of state near 3 eV appears as a shoulder in the spectrum, consistently to the picture of small graphenic islands connected by aliphatic bonds.

Molybdenum disulphide (MoS₂), a recently discovered two-dimensional (2D) transition metal dichalcogenide, has attracted substantial interest due to its semiconducting properties, and potential in electronics, optics and energy storage applications. However, electronic transport and optical properties of these 2D materials are affected by the presence of structural disorder such as formation of domain structures and corrugation and interaction with the substrate. Here we report our study on monolayer and multilayer/multidomain MoS₂ flakes synthesized by chemical vapour deposition (CVD) method, a promising technique and widely used method to produce large area MoS₂, for a variety of technological applications. Lattice orientation and domain boundaries in these MoS₂ flakes are identified and investigated in detail using low energy electron microscopy and diffraction (LEEM/LEED). In addition, monolayer and multilayer MoS₂ flakes are differentiated by distinct reflectivity of low-energy electrons in LEEM and in Raman spectroscopy. These investigations show that as-grown single layer MoS₂ samples on SiO₂ substrate as well as samples transferred onto doped Si substrates are substantially corrugated due to the presence of the underlying substrate. Furthermore, we found that in both as-grown and transferred MoS₂ the amount of corrugation decreases with increasing number of MoS₂ layers. These investigations throw significant light in the understanding of multi domain structural existence and substrate mediated structural changes in atomically thin layers of CVD grown MoS₂.

A large-scale production method is crucial for the application of transition metal dichalcogenides (TMDs) in electronics, optoelectronics and valleytronics. Although several methods have been developed for growing thin layers of MoS₂ and other TMDs, these processes produce materials that are limited in their uniformity and/or electrical performance. Here, we report wafer-scale (up to 4-inch) growth of high quality, uniform monolayer MoS₂ using a new metal-organic chemical vapor deposition (MOCVD) technique, in which the flow rate, partial pressure, and thermal decomposition of each gas-phase precursor are carefully regulated over the entire growth region. This enables accurate control of key growth processes, including the nucleation, growth rate, and inter-grain connection. Our resulting MoS₂ is polycrystalline, yet mechanically and electrically continuous with well-connected grain boundaries while the grain size is controllable over three orders of magnitude (ranging from ~100nm to 100mm). In addition, it displays uniform photoluminescence and Raman signatures across the entire sample. Finally, our electrical measurements show high electrical mobility over 10cm²/Vs in ambient conditions.

α-Molybdenum trioxide has attracted considerable attraction in the last few years due to its unique opto-electronic properties that make it a suitable choice for large range of application. In this work, we report fabrication and characterization of hierarchical α-MoO3 nano-lamellar structure, produced by means of Pulsed laser deposition, using a MoO3 target in presence of Oxygen as the background gas. Quasi 1-D tree-like hierarchical nanostructure was obtained by self-assembly from the gas phase for as-deposited and annealed films up to 350 ^oC that is the thermodynamic stability limit of the fully crystalline β phase. And by further increase of temperature, nucleation and growth of lamellar structure, attributed to the well-known α phase was observed. Utilization of the background gas during deposition makes it possible to tune the morphology by varying kinetic energy of deposited particles. Dense structures deposited at 1 Pa up to highly porous film obtained at 90 Pa pressure of O2 were investigated.
Control over the morphological factors such as thickness and porosity of the nano-lamellas was investigated and 2-D lamellar structure was realized for structures annealed at 400 ^oC where a mix α/β phase is present prior to complete crystal growth in single α phase where thick lamellas can be observed. To obtain hierarchical lamellar structure, the residual β phase was removed and a re-deposition and re- annealing process was performed.
Morphological factors of the film i.e. film porosity and density of lamellas per unit area were investigated by Scanning electron microscopy. Crystallinity and relative ratio of the α and β phases were studied by Raman spectroscopy, X-ray diffraction. Kelvin probe force microscopy and UV/Vis spectrophotometer were utilized to understand the surface and optical properties of the structure.
Such structure has the potential to grow several µm nano-lamellas perpendicular to the substrate. As an example of application, to realize photochromic devices, the films were grown directly on FTO glass and followed annealing process. Ability of Li ions intercalation of structures is studied by voltage-composition measurements.

Molybdenum disulphide is a promising 2D nano-material with potential applications in several areas of research due to its semiconducting nature, strong photo-luminescence, catalytic properties etc. In order to realise these properties in industrial applications (solar cell and energy and water splitting devices), dispersed and exfoliated MoS2 needs to be produced on a large scale. Most of the methods described so far are limited by low yields or low production rates. In particular, sonication in liquids is limited in volume (few 100s of ml) due to a rapid fall-off in energy density with distance from the source. Here we report a simple, scalable and an easy method to exfoliate bulk MoS2 into 2D MoS2 nanosheets using rotating blade mixers. A feasibility study was conducted by varying processing parameters such as time, volume, rotations per minute, initial concentration of bulk MoS2 and surfactant concentration. We achieved concentrations of 0.4 mg/ml in few litres of liquid which is advantageous for producing exfoliated MoS2 on a gram scale in a laboratory. The produced 2D MoS2 nanosheet dispersions were characterised by extinction spectroscopy, electron microscopy, Raman spectroscopy, Atomic force microscopy and photo luminescence in order to confirm the dimensionality of the flakes, defects, structure and indications for monolayers. Typical mean dimensions of the flakes are in the range of 0.3 microns and less, mean thicknesses in the order of ~3-10 layers. It is interesting to note that the size of the flake is not affected by the volume of the liquid which is the most critical part when the method is implemented on a large scale. Also, we discovered that both mean lateral size and thickness of the exfoliated MoS2 varies with the initial concentration of the surfactant. Other types of layered materials such as graphene, BN and WS2 can also be exfoliated using this method resulting in dispersions of high concentrations with outstanding production rates.

Emerging two-dimensional (2D) materials such as transition metal dichalcogenides offer unique and hitherto unavailable opportunities to tailor the mechanical, thermal, electronic, and optical properties of polymer nanocomposites. In this study [1], we exfoliated bulk molybdenum disulfide (MoS₂) into nanoplatelets, which were then dispersed in epoxy polymers at loading fractions of up to 1% by weight. We characterized the tensile and fracture properties of the composite and show that MoS₂ nanoplatelets are highly effective at enhancing the mechanical properties of the epoxy at very low nanofiller loading fractions (below 0.2% by weight). Our results show the potential of 2D sheets of transition metal dichalcogenides as reinforcing additives in polymeric composites. Unlike graphene, transition metal dichalcogenides such as MoS₂ are high band gap semiconductors and do not impart significant electrical conductivity to the epoxy matrix. For many applications, it is essential to enhance mechanical properties while also maintaining the electrical insulation properties and the high dielectric constant of the polymer material. In such applications, conductive carbon based fillers such as graphene cannot be utilized. This study demonstrates the application of 2D transition metal dichalcogenides as potential nanocomposite additives.
[1] O. Eksik et al., ACS Nano, 2014, 8 (5), pp 5282-5289

The driving force behind the solar hydrogen generation is the green environment with enormous resources of clean fuels. Semiconducting materials emerge as the prominent media that assist this water splitting into oxygen and hydrogen with the help of sunlight. A systematic computational investigation has been undertaken to predict the enhanced water dissociation activity of recently synthesized transition metal dichalcogenides^[1,2] MX2 (where M= Mo, W, Ti and X=S, Se, Te) from band edge alignment concept. The real catalytic mechanism of water splitting has also been determined for specific photocatalysts after screened through the high throughput study. The rapid advancement of exfoliation^[3] and synthetic techniques immensely motivates us to theoretically explore these exotic single layered materials. The results have certainly provided a deeper understanding of the fundamental aspects of structural, electronic, chemical and catalytic properties of these novel materials.
1. M. Chhowalla, Nature Chemistry, 5, 263, 2013.
2. D. Jariwala et al, ACS Nano, 8, 1102, 2014.
3. Q.H. Wang et al, Nature Nanotechnolgy, 7, 699, 2012.

In recent years, the palate of two-dimensional (2D) materials has been expanded far beyond graphene to include hexagonal boron nitride, the transition metal dichalcogenides and the bismuth chalcogenides among others. However, there are still few methods currently available for the reliable synthesis of high quality mono- and few-layer two-dimensional materials. These techniques all have drawbacks including low yield, low quality, or high cost. Here, we introduce a novel top-down synthesis approach for two-dimensional materials, demonstrated on Bi₂Se₃ and Sb₂Te₃, the 2D forms of which have been explored heavily in the physics and materials science communities due to the presence of topological surface states. Our method uses catalyst-free vapor-transport to first deposit laterally large (> 10 µm) and relatively thick (~1µm) plates of the 2D material of interest. These plates are then annealed under vacuum in a controlled environment to remove material until the two-dimensional limit is reached (1-2 monolayers). Resultant sheets are large (>10µm) and of high quality, as confirmed through many techniques, such as atomic force microscopy and electronic transport measurements. In contrast to other high yield synthesis methods, for example chemical exfoliation, the quality of the material is retained after thinning. The dynamics of this process are investigated using in situ transmission electron microscopy. We observe that the process is self-limiting due to folding at the flake edge, which stabilizes the edge by reducing the number of dangling bonds, thus making the entire flake more robust against sublimation. The kinetic and thermodynamic analysis of this process will presented, including evaporation activation energies, as well as possible mechanisms. In addition to dimensional reduction, it is expected that the evaporative thinning step reduces defect concentration, increasing the quality of the resultant material&’s properties. Examples of the extension of this method to other chalcogenide systems will also be presented. We believe this technique promises to greatly reduce the difficulty involved in isolating large, high quality 2D materials with high yield.

Due to their extraordinary electronic and mechanical properties, there has been tremendous growth in the study of graphene-like 2D nanomaterials. Among these materials, hexagonal boron nitride (hBN), a material isoelectronic to graphite, has wide uses ranging from cosmetics to high temperature lubricants. The advantages of boron nitride nano sheets (BNNS) compared to other conventional fillers include high surface area, high thermal conductivity and electrical resistivity, superb anti-oxidation ability, and low coefficient of friction. Here we present a versatile approach for the preparation of BN/polymer nano composites.
Pristine BNNS was exfoliated by an interfacial trapping technique. This was achieved by modest sonication of hBN in a water/heptane mixture to form a continuous film at the interface between two immiscible liquids. Nanocomposites were successfully prepared in a two-step method. BNNS emulsions were first prepared in an organic/aqueous system. By using a monomer as the organic phase, composites were prepared by polymerizing the organic phase. After the polymerization, the water was removed, resulting in a lightweight, very strong, flame resistant material. The interfacial trapping technique also has been shown to lead to the formation of nanometer thin films of BN by their climbing of hydrophilic surfaces. These thin film were immobilized with PMMA to create nanometer thin transparent coatings that were shown to impart high dielectric properties to the polymer.

Fabrication of devices from materials that are conformal, stretchable, and flexible are critical for next generation nanoelectronic systems. Currently, implementation of two-dimensional materials on flexible substrates is hindered by the lack of large-area coverage from exfoliation techniques and the high required processing temperatures for nucleation from traditional chemical vapor deposition (CVD) methodologies. Growth of two-dimensional, large area, stoichiometric amorphous-boron nitride (a-BN) thin films by low temperature Pulsed Laser Deposition (PLD) can be performed on virtually any substrate with strict control of thickness. In this study, a-BN films of thicknesses from 1 nm to 15 nm were grown on traditional electronic substrates (e.g., sapphire and silicon dioxide), as well as flexible and strainable substrates (e.g., kapton and PDMS). Film thicknesses were measured by cross-sectional TEM and XPS, and the films were structurally characterized by Raman spectroscopy. By patterning the layered structures, through-thickness and in-plane electrical properties were determined including resistivity and dielectric strength. In addition, the electronic and thermal performance of the a-BN films under the influence of strain and flexing are discussed.

The ability to tailor the properties of a material is essential to optimize device functionality. Recently, attention has been focused to tailor the properties of two dimensional molybdenum disulfide (MoS₂), not only by controlling the number of layers but also by means of external controls. In this talk, I will present evidence that the electrical and optical propeties of monolayer and few layers MoS2 can be tuned by controlled exposure of the samples to oxygen plasma. We find that the mobility, on-current and resistance of single layer and multilayered MoS₂ FET vary exponentially by up to four orders of magnitude with respect to the plasma exposure time. PL study show a decrease of PL intensity leading a complete quenching. Raman studies conducted before and after plasma treatment show a significant decrease of intensity of MoS₂ peaks with the creation of new oxidation induced peak, while X-ray photoelectron spectroscopy (XPS) study show peaks associated with MoO₃ after plasma exposure. We suggest that during exposure to oxygen plasma, the energetic oxygen molecules interact with MoS₂ and create MoO₃ rich defect regions, which are insulating. MoO₃ defect regions acts as a tunnel barrier for the injected conduction electrons, giving rise to the exponential increase in resistivity as a function of plasma exposure time. Band structure calculation show that the PL quenching upon plasma exposure is due to the creation of MoO3 defect region which casues a direct to indirect badgap transition in monolayer MoS2. We discuss possible applications of our results in fabricating MoS2 based nanodevices.

Electrochemical charge storage mechanism of chemically exfoliated and restacked nanosheets consisting mostly of metallic 1T phase MoS₂ has been investigated in aqueous electrolytes. Due to the high conductivity of the 1T phase MoS₂ nanosheets, high surface area of the restacked structure and hydrophilic surface behavior, high volumetric capacitance of 650F/cm³ are observed even at high scan rates. Electrodes consisted of 1mg/cm². Our results demonstrate that cations initially intercalate the two dimensional MoS₂ flakes and enhance the accessible surface area for double layer charge storage. Electrochemical measurements were conducted in 0.5M H₂SO₄, Li₂SO₄, Na₂SO₄ and K₂SO₄ electrolytes. Promising cycling stability (98% after 1000 cycles in 0.5M H₂SO₄) of 1T phase MoS₂ electrodes was also observed. Our study provides insight into electrochemical behavior of the layered transition metal chalcogenides in aqueous electrolytes.

Chemical functionalization of low dimensional materials such as nanotubes, nanowires and graphene leads to profound changes in their properties and is essential for solubilizing them in common solvents. Covalent attachment of functional groups is generally achieved at defect sites where electron transfer can be facilitated. Covalent functionalization of two-dimensional (2D) materials such as molybdenum disulfide (MoS₂) where the basal plane is chemically inert is therefore especially challenging and has yet to be reported. Here we describe a simple general method for covalent functionalization of 2D transition metal dichalcogenide (TMD) nanosheets (MoS₂, WS₂, and MoSe₂) in which the functionalization reaction is initiated by electron transfer between the electron-rich metallic 1T phase and an organohalide reactant without the introduction of defects. We show using nuclear magnetic resonance (NMR) that the functional groups are covalently attached on the chalcogen atoms and the degree of functionalization is 20-30%. The attachment of functional groups leads to dramatic changes in the opto-electronic properties of the material. For example, we show that it renders the metallic 1T phase semiconducting, exhibiting robust and tunable photoluminescence (PL) and gate modulation in field effect transistors (FETs). Our general and facile scheme for chemical modification of TMDs should open up new applications and pathways for tailoring their properties.

Ever since Staudinger introduced his definition of polymers in the early 1920s, this class of materials has gained great interest both in academia and industry.^[1] So far, many techniques have been developed to control polymerizations with regard to structure and properties for all kinds of applications of the resulting products. The ongoing progress in technology and the continuous demands of specialized materials for modern life leads to a strong request for innovations thereof in the future. However, the concepts which have been established for common single-stranded (one-dimensional) polymers (1DPs) such as polystyrene cannot yet be applied for congeneric structures with higher dimensionality like e.g. graphene (2D) or diamond (3D).^[2] In this respect, mimicking such nature-inspired structures using simple organic chemistry under mild reaction conditions is one of the key challenges which have to be addressed.
Recently, the synthesis of two-dimensional polymers by single-crystal transformation^[3] as well as by an interfacial-approach^[4] has been achieved. The latter is based on a controlled distribution of an amphiphilic monomer on a Langmuir-Blodgett trough followed by photo-polymerization.
Here, the monomers, containing polar head-groups and photo-reactive diazaanthracene (DAA) tails were spread onto a water subphase and pre-organized by compression in order to assemble the DAA-units for dimerization trough a [4+4]-cycloaddition. This process leads to a free-standing film with covalent net-points and internal periodicity as investigated by in-situ fluorescence spectroscopy, SEM- and tapping mode AFM-analysis.
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3. a) M. J. Kory, M. Wörle, T. Weber, P. Payamyar, S. W. van de Poll, J. Dshemuchadse, N. Trapp, A. D. Schlüter, Nat. Chem.2014, accepted; b) M. J. Kory, M. Bergeler, M. Reiher, A. D. Schlüter, Chem. Eur. J. 2014, 20, 6934-6938.
4. a) P. Payamyar, K. Kaja, C. Ruiz-Vargas, A. Stemmer, D. J. Murray, C. J. Johnson, B. T. King, F. Schiffmann, J. VandeVondele, A. Renn, S. Götzinger, P. Ceroni, A. Schütz, L.-T. Lee, Z. Zheng, J. Sakamoto, A. D. Schlüter, Adv. Mater.2014, 26, 2052-2058; b) Y. Chen, M. Li, P. Payamyar, Z. Zheng, J. Sakamoto, A. D. Schlu#776;ter, ACS Macro Lett.2014, 3, 153-158.

Molybdenum disulfide (MoS₂), a layered transition metal dichalcogenide, has been recently reported that in the presence of a high-k environment, the mobility of monolayer MoS₂ can reach 200 cm²/(V s). This coupled with the presence of a significant band gap (~1.8 eV) renders layered MoS₂ as an attractive candidate for electronics applications, unlike graphene where the absence of band gap inhibits its use despite large reported mobilities (200000 cm²/(V s)). Although the chemical synthesis of layered MoS₂ for electronics applications is still in its infancy, promising results have been demonstrated with mobilities up to 4.7 cm²/(V s), which is comparable to that of the mechanically exfoliated MoS₂ (not in a high-k environment). In this work, we study the Raman spectroscopy and AFM morphology on the chemically synthesized MoS₂ based on the synthesis method introduced by Li et al. (L.-J. Li et al., Nano Lett.12, 1538, 2012). According to the Raman spatial mapping, the sapphire substrates are covered by MoS₂ continuously throughout the wafers. Judging from the peak wavenumber differences from the two characteristic Raman peaks (the E¹_2g mode ~ 382 cm^-1 and the A_1g mode ~ 405 cm^-1), most of the areas are uniformly covered by 2 to 3 layers of MoS₂. On the other hand, the intensities of the Raman modes vary across the wafers, which may result from the variation of the MoS₂ crystallinity. From AFM topologies of transferred MoS₂ (onto SiO₂/Si substrates), we found that there are areas continuously covered by MoS₂ and there are porous areas with less than 100% MoS₂ coverage. Combined with the Raman study, this implies spatial variation to the MoS₂ crystal quality. More detailed micro-characterization (e.g., electron microscopy) needs to be taken in order to verify the conclusion here.

We demonstrate growth of low-dimensional topological insulator nanostructures on designated substrate. By using CVD-based process with chosen precursor, we demonstrate the growth of ultra-thin Sb₂Te₃ nanostructures directly on Si/SiO₂ substrate. Domain size varies from hundreds of nanometers to tens of microns with different thickness. An optimized growth process is developed to yield monolayer Sb₂Te₃. Extensive material characterization, such as XPS, SEM, AFM and TEM, are used to characterize Sb₂Te₃ domains. SEM imaging verifies large amount of hexagonal and triangle shaped thin structures grown with domain size from 200 nm to 400 nm. AFM measurement reveals monolayer growth with ~1 nm thickness. TEM shows crystalline structure with six-fold SAED patterns, indicating the high quality domain growth.

Chemical Vapor Deposition (CVD) growth processes for two-dimensional nanostructures are being actively developed, including graphene, transition metal dichalcogenide, and topological insulators. Hexagonal boron nitride (h-BN) is investigated as a nearly-perfect insulator for graphene electronics. Here we demonstrate direct growth of single-crystalline h-BN on copper surface through atmospheric CVD approach. By tuning critical growth conditions, domain thickness varies from monolayer to multilayer. In particular we investigate the role of hydrogen in h-BN growth. With higher hydrogen flow rate, the number of h-BN layers increases. Similar to the case in graphene growth, hydrogen serves as an etching agent to decrease the size and density of h-BN domains, impeding the formation of monolayer h-BN. In addition, hydrogen terminates h-BN edges, introducing the growth of additional layer. Extensive material characterization, including XPS, SEM, AFM and TEM, are conducted to analyze the single-crystalline h-BN domains.

MoS₂ thin films have received considerable attention because of unique potential, such as controllable band-gap and high carrier mobility. Although significant efforts have been devoted to prepare MoS₂ thin film, techniques to grow flat MoS₂ thin films in large scale have been rare until now. A main purpose of this study is to explore a growth process of MoS₂ films in large area. For this purpose, a three-step procedure was investigated. Here, MoS₂ thin films were prepared through MoO₃ deposition, thermal annealing for oxidation and crystallization and sulfurization by CVD. The Morphology and crystallinity were examined by XRD, AFM and Raman spectroscopy.
MoO₃ powder was thermally evaporated in vacuum to form 15nm-thick MoO_3-x as an initial film on a SiO₂/Si substrate. Subsequently, the film was thermally annealed at 400 °C in O₂ atmosphere for oxidation and crystallization. The prepared MoO₃ film was then placed in a glass tube together with sulfur powder. Reaction between MoO₃ film and sulfur powder was induced instantaneously at 500 °C for 15 min, where the atmosphere in the tube was kept at 11kPa of N₂ gas. Eventually, a 17 nm-thick MoS₂ film, which corresponds to 22 monolayers, was grown on the substrate.
Structural characterization revealed that the oxidation process converted an amorphous MoO_3-x into a crystalline MoO₃ with layered structure. After the sulfurization, the diffraction peaks from MoS₂ were clearly observed. The Raman spectrum shows E¹_2g and A_1g peaks to confirm successful growth of MoS₂ films. XRD patterns measured from MoS₂ films fabricated with and without oxidation process indicate that the oxidation process yielded an intensive diffraction peak from (002) MoS₂ lattice plane, whereas only marginal peaks were observed from the film without oxidation process. Moreover, the oxidation process contributed to the formation of uniform MoS₂ films over 12mm x 8mm size substrate. From these results, it can be concluded that the three-step CVD technique proposed here is promising for growing MoS₂ film in large area.
In this study, thick MoS₂ films were employed to elucidate the detailed growth process. Next plan is thus to decrease the thickness and refine the growth conditions for producing single layer MoS₂.

Graphitic carbon nitride (g-C₃N₄), which is obtained by the pyrolysis of melamine, dicyanodiamide or cyanamide, is considered to be a layered material made of polymeric tris-s-triazine chains connected to each other by hydrogen bonds [1]. g-C₃N₄ has unique features such as a narrow band gap, incorporation of many nitrogen atoms in the framework, insolubility in all solvents and metal-free composition. Therefore, g-C₃N₄ is a potential material for use as electrodes, photocatalysts, catalyst supports, etc. However, g-C₃N₄ has too low surface area of ca. 8 m²/g. Accordingly, highly porous carbon nitrides are desired, because the high surface area of g-C₃N₄ is responsible for such fascinating functions. In our study, we developed template-free and post-synthetic preparations for highly porous carbon nitride material [2]. We discovered that only concentrated sulfuric acid (concd H₂SO₄) was able to peel carbon nitride layers and completely disperse the g-C₃N₄, leading to the production of a novel, highly porous carbon nitride material (labeled as nanoporous carbon nitride, nanoC₃N₄). nanoC₃N₄ was synthesized as follows: g-C₃N₄ was stirred in concd H₂SO₄ to ensure complete dispersion. The concd-H₂SO₄-dispersed g-C₃N₄ was repeatedly washed with H₂O, and the resulting H₂O-swelled material was stirred in a 1 M NaOH aqueous solution and then washed again with H₂O. The H₂O-swelled carbon nitride was separated by centrifugation and then immersed and stirred twice in EtOH to remove the H₂O from the carbon nitride material. nanoC₃N₄ was obtained after evaporating the EtOH at 120 °C. The Brunauer-Emmett-Teller surface area of nanoC₃N₄ was ca. 180 m²/g, more than 20 times as large as that of g-C₃N₄. The Dollimore-Heal pore size of nanoC₃N₄ was widely distributed in the micro- and meso-porosity ranges. Based on the spectroscopic analysis such as XRD and ¹³C CP/MAS NMR, the structure of nanoC₃N₄ was estimated to be disordered polymeric tris-s-triazine chains. The following two processes are very important: 1) In treatment with concd H₂SO₄, the polymeric tris-s-triazine chains are peeled away. 2) Soaking in EtOH removes the H₂O molecules in the H₂O-swelled carbon nitride and prevents the resulting EtOH-swelled material from forming densely aggregated structures by hydrogen bonds between polymeric tris-s-triazine chains and H₂O molecules. nanoC₃N₄ has a stronger basicity than g-C₃N₄; the pH value of an aqueous suspension of nanoC₃N₄ (10 g/L) was 10.0-10.2, whereas that of g-C₃N₄ or NaOH/g-C₃N₄ which had been treated with an NaOH solution, followed by washing with H₂O and soaking in EtOH in the same way as that for nanoC₃N₄, was 5.9-7.3. The number of base sites on nanoC₃N₄ was ca. 0.36 mmol/g by titration. [1] B. V. Lotsh, M. Döblingre, J. Sehnert, L. Seyfarth, J. Senker, O. Oeckler, W. Schnick, Chem. Eur. J.2007,13, 4969. [2] T. Iwamoto, Y. Masui, J.-C. Wang, M. Onaka Chem. Lett.2013,42, 247.

Monolayer transition metal dichalcogenides (TMDC) have recently been highlighted due to their unique properties and potential applications. However, the synthesis of large-area TMDC is still challenge. There are many problems in tems of crystallinity, coverage and contamination from promoter which is used for monolayer TMDC. Here, we report the synthesis of the large-area monolayer WS₂ and WSe₂ on the catalytic metal substrate by using chemical vapor deposition (CVD). Both monolayer WS₂ and WS₂ were obtained as large as few centimeter scale, only limited to the size of metal substrate and CVD chamber. The crystallinity of both WS₂ and WSe₂ was comparable to the exfoliated ones comfirmed by Raman spectroscopy, Photoluminescene and TEM. The precursors were catalytically decomposed on the metal substrate, leading to the self-limiting growth of the monolayer. Our method will be contributed to synthesize of high-quality and large-Scale TMDC including MoS₂ and MoSe₂.

The intriguing physical and chemical properties of layered metal chalcogenides with the general formula of MX₂ (M is a metal and X is a chalcogen) make these materials very promising for electronic, photonic, spintronic, and energy applications. In this work, we have developed simple intercalation and exfoliation chemistry of the group 6 MX₂ (M = Mo, W; X = S, Se) materials to effectively convert the as-grown multi-layer MX₂ nanoflakes with semiconducting 2H phase to nanosheets of metallic 1T phase using both chemical exfoliation with n-butyl lithium and electrochemical lithiation. We found that the 1T-MX₂ nanosheets are significantly more catalytically active towards the hydrogen evolution reaction (HER) and polysulfide reduction reaction. Furthermore, when the metallic 1T-MX₂ materials are integrated with semiconductor light absorbers, the formed heterostructures are highly efficient photocathode systems for photoelectrochemical hydrogen generation. Specifically, heterostructures of 1T-MoS₂ and p-Si have been fabricated via a chemical vapor deposition method followed by an n-butyl lithium treatment. Photocurrents up to 17.6 mA/cm² at 0 V vs. reversible hydrogen electrode were achieved together with an excellent onset of photocurrent and stability. Electrochemical impedance spectroscopy (EIS) and surface photoresponse (SPR) measurements were also conducted to further explain the high performance. The outstanding electrochemical and photoelectrochemical properties of these chemically exfoliated MX₂ nanosheets make them promising alternatives to noble metals for hydrogen evolution reaction and will likely stimulate further explorations of analogous metallic 1T polymorphs of layered metal chalcogenides for energy conversion and other applications.

Two-dimensional (2D) transition metal dichalcogenides and oxides (TMD&Os) are attracting tremendous interest due to their fascinating electronic and optical properties. The presence of an intrinsic bandgap and the possibility to engineer their electronic energy states through a range of techniques is an important feature that makes these materials highly desirable. Molybdenum disulphide (MoS₂) and molybdenum trioxide (MoO₃) are the most popular 2D transition metal dichalcogenide and oxide, respectively.
So far, doping and intercalation techniques have been widely employed for tuning the electronic band structures of TMD&O monolayers, however a precise control over such manipulations is yet to be achieved. The application of localised strain is an attractive approach for tuning the band gaps and electronic transport properties of TMD&O monolayers due to the metal-semiconductor transitions that are known to occur due to strain effects.
In this work, we analyse the effect of strain on the electronic properties of MoS₂ and MoO₃. We perform mechanical nanoindentation on the monolayers by applying highly localised strain. In situ current-voltage (I-V) characteristics are acquired at varying strain levels. Additionally, the mechanical properties of the 2D MoS₂ and MoO₃ are also obtained to assess the elastic nature of the monolayers.
It is shown that electronic transitions in TMD&Os can be engineered using highly localised strains. These localised strains on such 2D layers induce carrier transport alterations, thereby changing their electrical conduction behaviours. Such strain effects offer a potential tool for precisely manipulating the electronic transport properties of 2D TMD&Os, and understanding the interactions of the atomic electronic states in such layered materials.

Abstract
Peeling of layered materials such as graphite, hexagonal boron nitride, molybdenum disulphide, and tungsten disulfide to prepare two-dimensional (2D) nanosheets is currently a hot research topic ^[1-6]. These 2D nanosheets possess unique properties such as high temperature stability, good mechanical strength, and anti-oxidation ability. In this study, 2D graphene, hexagonal boron nitride, molybdenum disulphide, and tungsten disulfide nanosheets are successfully prepared at low temperature using a facile chemical exfoliation method. The proposed exfoliation method is simple, mild, inexpensive, and practical. Further, scanning electron microscopy, atomic force microscopy, high-resolution transmission electron microscopy, X-ray powder diffraction, and diffuse reflectance UV-vis spectroscopy are used to characterize the fabricated 2D nanosheets. All the as-prepared nanosheets exhibit good dispersibility and high solubility in ethanol solution. The proposed exfoliation method has certain universality and can be used for stripping of other layered materials.
Reference:
[1] K. Varoon, X. Y. Zhang, B. Elyassi, D. D.Brewer, M. Gettel, S Kumar et al ., Science, 2011, 334,72.
[2] V. Nicolosi, M. Chhowalla, M. G. Kanatzidis, M. Strano, J. N. Coleman, Science, 2013, 340, 1226419.
[3] Y. Zhang, Y. Zhang, Q. Ji, J. Ju, H. Yuan, J. Shi, T. Gao, D. Ma, M, Liu, Y. Chen, X. Song, H. Hwang, Y. Cui, Z. Liu, ACS Nano, 2013, 7, 8963.
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The synthesis of graphene nanoribbons (GNRs) by unzipping carbon nanotubes (CNTs) has received extensive attention because of controllable width, edge structures and number of layers. This approach makes GNRs very attractive building blocks for the next generation of electronic devices because their shape can be controlled using defined CNTs. Recently several methods have been reported to produce GNRs by unzipping CNTs such as oxidation cutting, Ar plasma etching and metal catalytic cutting, intercalation and exfoliation. GNRs that have been produced using these methods contain functional groups on their edges, which decreased the conductivity and the quality of the nanoribbons. However, the producing clean, high quality, edge structure and conductivity of GNRs still remains some of the fundamental challenges in this area. Hence, it is necessary for developing a clean and efficient method to synthesize GNRs. Here we shall discuss hypervelocity impact of nanotubes against solid targets. As a consequence of this their unzipping along the nanotube axis. Fully atomistic reactive molecular dynamics simulations are used to gain further insights of the pathways and deformation and fracture mechanisms of nanotubes.

Molybdenum ditelluride (MoTe₂) has been interested due to the several applications including solar cell, thermoelectric device, and field-effect transistor. However, to obtain the large-are MoTe₂ film is still challenge. Here, we report the new method to synthesize the large-area MoTe₂ thin film by using chemical vapor deposition. The thin film of molybdenum with a thickness of ~ 100 nm was prepared by using e-beam deposition, and then tellurization under Te atmosphere was carried out at elevated temperature. From the measurement of X-ray diffraction and transmission electron microscopy, it was confirmed that layered MoTe₂ perpendicular to the c-axis was grown with maintaining high quality under the appropriate growth conditions. The amount of Te source and the thickness of molybdenum film were found to be important parameters to determine the crystal structure. The Mo₆Te₈ film was synthesized when the tellurium source is not enough. In addition, randomly oriented MoTe₂ was grown on the thick molybdenum film (>200 nm). This might be attributed to the different kinetics for recrystallization and tellurization of Mo. Our result will be contributed to synthesize the large-area other TMDs such as MoS₂, MoSe₂, WS₂, and WSe₂.

Two-dimensional nanomaterials hold tremendous promise in a breadth of fields. A key challenge that remains is the construction of multi-layer heterostructures from monolayer materials. Progress, however, is hampered by the difficulty of current methods to create these complex heterostructures. Using a combination of chemical and solid-state synthetic techniques, we demonstrate a unique methodology towards the synthesis of complex 2D heterostructures. We intercalate metals into chalcogenide materials, such as MoS₂ and Bi₂Se₃. With subsequent vapor phase reaction, preliminary results show heterostructures of 2D materials such as Cu₃Se₂-Bi₂Se₃. This solid-state synthesis opens the door to the ability to fine-tune the electrical and chemical properties of each layer in a complex heterostructure.

Regioselective chemical reactivity and structural transformations of two-dimensional (2D) layered transition-metal chalcogenide nanocrystals are studied. When TiS₂ nanodiscs is exposed to chemical stimulus, such as Cu ion, chemical reaction selectively occurs at the peripheral edges. This edge reaction is followed by ion diffusion, leading to the formation of a heteroepitaxial TiS_2(disc)-Cu₂S_(shell) intermediate. Such 2D regioselective chemical reactions also take place when various chemical reactants are used. Chemical principles indicate that a general approach exists for building various high-performance heteroepitaxial disc-shell structures. For example, TiS_2(disc)-TiO_2(shell), synthesized by the chemical principle, constitutes a high performance type II heterojunction with a wide range solar energy coverage and high electron transfer property. These chemical reactions can serve as the new design concept which opens up opportunities for a range of novel catalytic applications of the materials.

Since the discovery of single layer graphene (SLG) was reported, there have been extensive research activities in 2D materials with unique material properties applicable to future IC and energy conversion devices. In particular, graphene and 2D-MoS₂ are the most actively researched to exploit their unique characteristics such as high transmittance, high carrier mobility, flexibility, and large specific surface area. However, graphene has a zero bandgap in pristine form without functionalization or structural modification like ribbon or mesh shapes, resulting in poor transistor performance. Besides the bandgap issue, implementation of graphene to flexible devices is difficult due to other limitations such as high processing temperature for gas phase growth, high contact resistance, and difficulty of reliable wafer-scale doping techniques. As an alternative to graphene, 2D MoS₂ has been investigated as a channel material for transistors due to its direct bandgap of 1.8-1.9 eV. Similar to graphene, implementation of 2D MoS₂ to flexible devices is very challenging due to its high processing temperature and high contact resistance. Furthermore, mechanical exfoliation and chemical deposition methods to form 2D MoS₂ produce very small film coverage and are not compatible with current IC manufacturing processes at all. From the device perspective, future flexible and wearable electronics need higher performance, lower processing temperature, less power consumption and uniform film coverage. It is believed that 2D materials can be utilized for these devices, but none of them are still needed to be improved. Therefore, alternative 2D materials compatible with current device processes are in great demand.
We think that tin (Sn)-based semiconducting 2D materials (e.g. SnS₂) can be one of good candidates to compete with current 2D materials such as grapheme, MoS₂ and etc. This Sn based 2D materials can be easily grown at low temperatures with atomic layer deposition which may solve the aforementioned issues in 2D materials. The unique properties of single layer SnS₂ with bandgap of 2.2-2.8 eV can lead to transistors with large I_on/I_off and high mobility. Also, low temperature characteristics of 2D SnS₂ could be an ideal material for flexible devices, where other 2D materials cannot be implemented.
In this study, we deposited single and few layers (2-5 layer) of SnS₂ using Tetrakis(dimethylamino)tin (TDMASn) as a Sn⁴⁺ source and hydrogen sulfide (H₂S) as a sulfur reactant gas below 150°C with atomic layer deposition (ALD). 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.

The unprecedented electronic, optical, and structural properties of black phosphorus have spurred its development as an important two-dimensional (2-D) materials platform in nanoelectronics. Efforts to exfoliate thin sheets of black phosphorus, however, have been hindered due to the instability of the material under ambient conditions. Although this instability in air was first reported in 1979 (J. Phys. Chem. Solids 40, 967-971, 1979), there has not yet been any significant experimental study of the stability of black phosphorus in bulk and near the two-dimensional limit. In order to effectively utilize black phosphorus as a 2-D material, we must understand the principles governing its surface oxidation and develop techniques and procedures for avoiding such degradation. Here we present a fundamental study of the stability of the black phosphorus surface using x-ray photoelectron spectroscopy (XPS). We show that the black phosphorus surface is more susceptible to oxidation in a pure oxygen environment than in water vapor, yielding surface oxidations of 20% and 3%, respectively, after 1 hour in light. These results motivated our investigation of light as a catalyst for oxidation and have provided key insight into the surface reactivity of black phosphorus. This insight enables the development of novel procedures for safe handling and future chemical modification of black phosphorus for effective use as an emerging 2-D material.

Many layered transition metal dichalcogenides (TMDCs), such as MoS₂, MoSe₂, WS₂ and WSe₂, feature a native direct-energy gap in the visible and near-infrared frequency range when scaled down to a monolayer, making them appealing materials for light-emitters and biosensors. Compared to monolayer MoS₂, monolayer WS₂ and WSe₂ exhibit much stronger room-temperature photoluminescence (PL) and spin-orbit coupling but have received less attention. Here, we compare equilibrium and nonequilibrium synthesis approaches for the growth of monolayer WS₂ and WSe₂, and mixed-stoichiometry WS_2(1-x)Se_2x monolayers to explore continuous tuning of the band gap. Traditional low-pressure chemical vapor deposition and atmospheric pressure chemical vapor deposition (involving WO₃ and S/Se evaporation) are compared with pulsed laser vaporization deposition of reactants from solid targets. Their structure, composition, electrical and optical properties were investigated via comprehensive techniques, including atomic-resolution scanning transmission electron microscopy and electron energy loss spectroscopy, absorption spectroscopy and mapping by micro-photoluminescence, Raman spectroscopy. Additionally, field-effect transistors based on WS₂ monolayers were fabricated to investigate the transport properties. High room-temperature photoluminescence observed in WS₂ and WSe₂ monolayers, together with the high mobility extracted from the FET devices, indicates that our WS₂ and WSe₂ monolayers are of high-crystalline quality. Tungsten chalcogenide monolayers with tunable bandgap promise potential applications in flexible optoelectronics, such as photodetectors and solar cells.
Research sponsored by the U.S. Dept. of Energy, Basic Energy Sciences, Materials Science and Engineering Div. (synthesis science) and Scientific User Facilities Div. (characterization science).

Recent research has shown few layer Molybdenum disulfide (MoS₂) to be a promising candidate for improving current semiconducting materials in electronic and photonic devices due to its high carrier mobility and layer dependent band gap transition. One of the current holdbacks is the inability to fabricate low dimensional large area MoS₂. In an attempt to resolve this problem, we have used pulsed laser deposition (PLD) to fabricate large area MoS₂ thin films on a variety of substrates such as SiO₂, sapphire, and silicon. All films were characterized using various techniques such as Raman spectroscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), atomic force microscopy (AFM), electrical measurement, and X-ray photoelectron spectroscopy (XPS). Raman and XRD data confirm the vibrational modes and structural properties of MoS₂ in the films, while AFM provides the morphology of the films. By observing the E¹_2g and A_1g peaks of MoS₂ in Raman spectra the number of layers were determined. Our samples ranged from a few layers to bulk in thickness, proving PLD is flexible enough to take advantage of MoS₂ layer dependent optical band gap. The pattern of XRD data corresponds to the 2H phase of MoS₂ and the peak at 2theta; = 32.9⁰ indicates the preferential growth of the films along the (100) plane. Electrical characterization such as temperature vs resistance measurement confirms the semiconducting nature of thin films. FET devices have been manufactured using a bottom gate design. Once film growth parameters have been optimized, we expect these FETs to perform admirably. Under certain growth parameters and background environment entangled nano-rod and nano-petal structures measuring approximately 100 nm in size have been observed by SEM. The increased surface area of these structures can be beneficial for sensor applications. AFM study has shown our films have good coverage and a low rms surface roughness. By SEM electron diffraction X-ray spectroscopy (EDS) sulphur deficiency have been observed in the films grown by the commercially purchased MoS₂ target. However, we have found better stoichiometry in the films grown from the PLD targets which were made by adding a definite amount of extra sulphur with the commercially purchased MoS₂ micro powder. As observed from XRD and Raman spectroscopy, these films show better crystallinity with less defects. Detailed growth and structural properties will be discussed in the presentation. This work is partially funded by Graduate College, Missouri State University.

Strong in-plane bonding (covalent) and weak van der Waals interplanar interactions characterize a large number of layered solids, as epitomized by graphite. The advent of graphene (Gr), individual atomic 2D layers isolated from mineral graphite using micro-mechanically exfoliation has enabled the ability to pick, place, and stack of arbitrary compositions, which would otherwise be impossible to synthesize using other known techniques. Moreover, this discovery implicated an access to a vast range of materials of all other dimensionalities beyond graphene towards other 2D van der Waals solids and possibly artificially stacking atomic layers forming hybrids / superlattices. Raman spectroscopy, an inelastic light scattering technique, has emerged inarguably and historically as one of the potential analytical tools and an integral part for lattice dynamical structural characterization of solids at the nanoscale, revealing not only the collective atomic/molecular motions but also localized vibrations/modes. Specifically, it is used as a fast and reliable method to determine the number of graphene layers as well as other 2D van der Waals solids. Here we present Raman spectroscopy properties in first-, second-, and higher-order phonon and Raman mapping of graphene layers (mono-, bi-, tri-, few and thicker forming graphite), transition metal dichalcogenides (TMDs) namely, molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) and wide bandgap hexagonal boron nitride (h-BN) monolayers revealing collective and localized molecular vibrations. Higher-order combination modes involving 3 phonons and 4 phonons are observed in mono-, bi- and tri-layer graphene samples prepared by mechanical exfoliation. The Rama data is analyzed in terms of band position and intensity ratio as a function of number (n) of graphene layers. In addition, all higher order modes are observed to upshift in frequency almost linearly with n, betraying the underlying interlayer van der Waals interactions. We also complemented our structural results with the transmission electron microscopy combined with selected-area electron diffraction characterization for all of the samples mentioned above. The results are presented in view of applications of graphene by itself and in-combination with above mentioned 2D solids forming heterostructures or superlattices for nanoelectronic and optoelectronic devices and help better understanding of physical and electronic properties of graphene.
The work is supported by the author's start-up (SG) and NSF-KY EPSCoR (EPS-0814194 and 3048108525-l4-046) grants.

The realization of viable electronics technology based on new semiconductors requires low resistance contacts for achieving practical drive currents. Ultrathin molybdenum disulphide (MoS₂) has emerged as an interesting new semiconductor because of its finite band gap and absence of dangling bonds. Typical contact resistance values between metal and ultra-thin MoS₂ range from 0.7 - 10 k#8486;-µm, leading to Schottky limited transport. Mitigation of large contact resistance is an active area of research in two-dimensional and ultra-thin MoS₂ field effect transistors (FETs) for obtaining large on-state currents and optimizing device performance. In this study, we engineer the metallic 1T phase of MoS₂ as the source and drain electrodes in FETs. We demonstrate that the 1T phase can be locally induced and patterned on to semiconducting 2H phase MoS₂ nanosheets, which decreases the contact resistance to record values (200 - 300 #8486;-µm at zero gate bias compared to ~ 0.7 - 1 k#8486;-µm). FETs with metallic 1T phase electrodes fabricated and tested in air without any annealing exhibit mobility values of ~ 50 cm²/V-s, subthreshold slope values of < 100 mV/decade, on/off ratios of > 10⁷, drive currents approaching ~ 100 µA/µm, and excellent current saturation. We also demonstrate that deposition of different metals on the metallic 1T phase has no influence on the FET performance, suggesting that it is the 1T/2H interface that controls the injection of carriers into the channel. Our results provide a new strategy based on phase engineering for achieving low resistance contacts and reproducible performance of FETs based on ultrathin MoS₂.

This presentation is aimed at demonstrating the progress with the synthesis of new inorganic nanotubes (INT) and fullerne-like (IF) nanoparticles (NP) from 2-D layered compounds. Two important categories of new IF/INT nanostructures will be discussed in particular: 1. Synthesis of Doped IF/INT of WS₂ (MoS₂) by rhenium and niobium; 2. Synthesis of IF and in particular INT of misfit compounds, like PbS-TaS₂, GdS-CrS₂. The synthesis of 1-D nanostructures from this vast group of layered materials is particularly promising.
Re-doped IF-MoS₂ NP exhibit superior solid lubrication behavior in different environment and can find numerous applications in e.g. medical technology, which will be briefly demonstrated. Major progress has been achieved in elucidating the structure of INT and IF using advanced microscopy techniques, like aberration corrected TEM and electron tomography. If time permits, new mechanical measurements of individual WS₂ nanotubes will be demonstrated.

Nanoscrolls consist of sheets rolled up into a papyrus-like form. Their open ends produce great radial flexibility, which can be exploited for a large variety of applications, from actuators to hydrogen storage [1-4]. They have been successfully synthesized from different materials, including carbon [2,3] and boron nitride [5,6]. In this work we have investigated, through fully atomistic reactive molecular dynamics simulations, the dynamics of scroll formation for a series of graphene-like carbon nitride (CN) two-dimensional systems: g-CN, triazine-based (g-C3N4), and heptazine-based (g-C3N4).
Carbon nitride (CN) structures have been attracting great attention since their prediction as super hard materials. Recently, graphene-like carbon nitride (g-CN) structures have been synthesized [7] with distinct stoichiometry and morphologies. By combining these unique CN characteristics with the structural properties inherent to nanoscrolls new nanostructures with very attractive mechanical and electronic properties could be formed.
Our results [8] show that stable nanoscrolls can be formed for all of CN structures we have investigated. Our results also show that carbon nanotubes can be effectively used to induce the start of the scrolling processes. As the CN sheets have been already synthesized, these new scrolled structures are perfectly feasible with our present-day technology. Possible synthetic routes to produce these nanostructures are also addressed.
[1] D. Tomanek, Physica B 2002, 323, 86.
[2] L. M. Viculis, J. J. Mack, R. B. Kaner, Science 2003, 299, 1361.
[3] S. F. Braga, V. R. Coluci, S. B. Legoas, R. Giro, D. S. Galvao, R. H. Baughman, Nano Lett. 2004, 4, 881.
[4] S. F. Braga, V. R. Coluci, R. H. Baughman, D. S. Galvao, Chem. Phys. Lett. 2007, 441, 78.
[5] E. Perim, D. S. Galvao, Nanotechnology 2009, 20, 335702.
[6] X. Li, X. Hao, M. Zhao, Y. Wu, J. Yang, Y. Tian, G. Qian, Adv. Mater. 2013, 25, 2200.
[7] J. Li, C. Cao, H. Zhu, Nanotechnology 2007, 18, 115605.
[8] E. Perim and D. S. Galvao, Chem. Phys. Chem. - in press. DOI: 10.1002/cphc.201402059

Due to the large difference in the character of the intralayer (covalent) and interlayer (van der Waals) bonding of the layered transition metal dichalcogenides (TMDC), their optical, electronic and transport properties are anisotropic. One would also expect a large anisotropy in the dielectric response of the TMDC nanostructure to the external electric field, which is important in their applications in nonlinear optics and optoelectronics. In this presentation, we will describe an unusual electrooptic response of the colloidal transition metal dichalcogenide nanodiscs, where the strong optical anisotropy is induced only by the time-varying electric field, E(t), not by the DC electric field. Under the DC electric field, the colloidal TMDCs nanodiscs examined in this study (TiS₂ and WSe₂) exhibited no measurable absorption anisotropy, suggesting that the anisotropy of the induced dipole is not sufficiently strong to align the nanodiscs to a preferred direction. However, the onset of a strong anisotropic absorption was observed both at the rising and falling edges of the square wave electric field, which subsequently decayed on the time scale of the rotational relaxation of the nanodiscs during the period of the constant electric field regardless of its amplitude. This has been attributed to the ‘transient&’ rotational torque generated during the period of nonzero dE(t)/dt that creates a net rotational polarization, while the rotational relaxation of the nanodiscs occurs during the period of constant E(t). In addition, the direction of the rotational torque was determined by the sign of dE(t)/dt not the sign of E(t). The observed electrooptic response of the colloidal TMDC nanodiscs to the time-varying electric field appears as if it they sense the time-derivative of the electric field, while being blind to the magnitude of electric field. This is in stark contrast to the most of the anisotropic colloidal nanoparticles, where their response to the electric field is dictated by the amplitude of the electric field. The unique response of the colloidal TMDC nanodiscs to the time slope of the electric field observed here demonstrates a possible new way to control their electrooptic properties by shaping dE(t)/dt rather than the amplitude of E(t).

Semiconducting two-dimensional (2D) materials such as the class of transition metal dichalcogenides (TMDCs) have attracted considerable interest in the recent years due to their extraordinary electronic and optical properties. One prominent candidate is molybdenum disulfide (MoS₂), which possesses a direct bandgap of 1.8eV in its single layer form, ideal for a wide range of technological applications. Recently single-layer MoS₂ phototransistors were demonstrated, where the bandgap allows high absorption coefficient and ultra-low dark-current operation.[1,2] However, the large bandgap also limits the spectral absorption to the visible and the atomically thin profile of single-layer MoS₂ provides with extremely short optical absorption length leading to weak light-matter interaction. One facile way to overcome this common roadblock for 2D-materials has been successfully proven on graphene, where the nanosheet material was combined with colloidal lead sulfide (PbS) quantum dots (QDs).[3] The synergism of excellent transport properties in graphene with strong absorption in the QDs lead to ultrahigh gain with responsivity of 10⁷ A/W, yet it suffers from the high dark conductivity of the graphene channel in view of its semi-metallic character.
Following this sensitizing approach, we will demonstrate that the 2D-material MoS₂ offers an ideal platform with high mobility channels and a bandgap for low dark-current operation. We will introduce a hybrid TMDC-QD phototransistor consisting of colloidal p-type PbS quantum dots and few layer n-type MoS₂ (ge; 2 layers).[4] The underlying MoS2 channel was prepared using micromechanical cleavage technique and the hybrid was manufactured by simple spin-coating of the solution-processed QDs. The hybrid benefits from strong and costum-tailored light absorption in the quantum dots throughout the VIS/NIR, efficient charge carrier separation at the p-n-interface and fast carrier transport through the MoS₂ channel. We will show responsivity of up to 10⁶ A/W for the hybrid, which is an enormous improvement to its MoS₂ single counterpart. By investigating the backgate dependent sensitivity of the hybrid device, we will point out its favorable operation in the depletion mode of the transistor channel and show improved light-to-dark current ratio for stronger capacitive coupling to the backgate.
[1] O. Lopez-Sanchez, D. Lembke, M. Kayci, A. Radenovic, A. Kis, Nat. Nanotechnol.2013, 8, 497.
[2] Z. Yin, H. Li, H. Li, L. Jiang, Y. Shi, Y. Sun, G. Lu, Q. Zhang, X. Chen, H. Zhang, ACS Nano2012, 6, 74.
[3] G. Konstantatos, M. Badioli, L. Gaudreau, J. Osmond, M. Bernechea, F. P. Garcia de Arquer, F. Gatti, F. H. L. Koppens, Nat.Nanotechnol.2012, 7, 363.
[4] D. Kufer, I. Nikitskiy, T. Lasanta, G. Navickaite, F. Koppens, G. Konstantatos, (submitted).

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 for hydrogen evolution catalysis and as contact electrode in electronic devices. I will describe a simple general method for covalent functionalization of TMDs in which the functionalization reaction is initiated by electron transfer between the electron-rich metallic 1T phase and an organohalide reactant without the introduction of defects. NMR shows that the functional groups are covalently attached on the chalcogen atoms and the degree of functionalization is 20-30%. The attachment of functional groups leads to dramatic changes in the opto-electronic properties of the material. For example, we show that it renders the metallic 1T phase semiconducting, exhibiting robust and tunable photoluminescence (PL) and gate modulation in field effect transistors (FETs).

Although solution synthetic methods have been successfully utilized for the formation of multi-layered transition metal chalcogenides (TMCs), their synthetic applicability for single-layer formation has been rare and challenging despite emerging unique characteristics of single sheet TMCs. In this study, we first present a diffusion controlled approach for single sheet ZrS₂ where continuous diffusion flux of a reactive chalcogen species, H₂S, is found to be crucial to exclusive a- and b-axial growth for the single layer production. With reduction of incoming H₂S flux, multi-layer is formed through c-axial growth. The H₂S flux can be effectively modulated by choosing proper chalcogen presursors such as dodecanethiol and CS₂. Dodecanethiol supplies sustained incoming H₂S flux which leads to single-layer ZrS₂ nanosheets while CS₂ affords multi-layered ZrS₂ nanodiscs due to the H₂S flux depletion through discontinued H₂S supply. This chalcogen precursor dependent diffusion controlled methodology is successfully extended to single sheet TiS₂.

Intercalation into layered 2D materials can have profound effects on the physical and chemical properties of the material. I will present novel chemical methods to intercalate large amounts of multiple, zero-valent atoms into 2D layered materials. This method is general and can be used for a variety of chalcogenide and oxide 2D layered materials. I show that this technique establishes a new method bandgap engineering in 2D materials, for example achieving new chromic behaviors, as a method of accessing new phase change behaviors, and towards engineering new 2D complex material structures. This work opens new ground for accessing novel opto-electronic properties, structural properties, and technological applications.

Two-dimensional (2D) crystalline semiconductor nanosheets have attracted significant attention in recent years due to their promising potential for applications in electronics and optics. In addition to the unique electronic and optical properties discovered for 2D crystal monolayers, the rich variety of optoelectronic properties arising from interlayer stackings and rotations in 2D crystal bilayers and heterostructures are the subject of many current studies. However, the synthesis of high quality 2D bilayer crystals and heterostructures still remains a challenge and what determines the interlayer stacking and rotations during growth is unclear. Here, we report the synthesis, atomistic characterization, and theoretical analysis of large pyramid-like 2D bilayer GaSe crystals and GaSe/graphene heterostructures with varied interlayer stackings and rotations. The large (tens of microns) triangular bilayer GaSe 2D crystals were synthesized by chemical vapor deposition (CVD). Aberration-corrected annular dark-field scanning transmission electron microscopy and electron diffraction were used to atomistically characterize the interlayer arrangements. Micro-Raman and photoluminescence spectroscopy were correlated to understand the effects of edge structure, stacking and rotation variations on the optical and electronic properties of as-synthesized 2D bilayer crystals and heterostructures. First-principle calculations were used to understand the atomistic origins of the energetics between GaSe layers or GaSe/graphene layers that determine the observed interlayer stackings and rotations in bi- and multi-layered heterostructures that result in corresponding optical properties. The energetic implications guiding the nucleation and growth processes will be discussed.
A portion of this research was performed at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. Synthesis science sponsored by the Materials Science and Engineering Division, Office of Basic Energy Sciences, U.S. Department of Energy.

Based on first-principles calculation we predict two new thermodynamically stable singel and multi-layered-phases of silicon which exhibit strong directionality in the electronic and structural properties. As compared to silicon crystal, they have wider indirect band gaps but also increased absorption in the visible range making them more interesting for photovoltaic applications. Moreover, the intrinsic two-dimensional confinement and strong electron-phonon coupling make them a candidate material for thermoelectricity and superconductivity. These stable phases consist of intriguing stacking of dumbbell (DB) patterned silicene layers having trigonal structure with radic;3×radic;3 periodicity of silicene. We propose a new mechanism for explaining the spontaneous and consequential formation of radic;3×radic;3 structures from 3×3 structures on Ag substrate. We show that the radic;3×radic;3 reconstruction is mainly determined by the interaction between Si atoms and have weak influence from Ag substrate. The proposed mechanism opens the path to understanding of multilayer silicon and silicites.
We extended those studies to Ge and Sn. For the case of Ge, we showed that single and multi--layer germanium grow on a gold (111). Its growth bears strong similarity with the formation of silicene layers on silver (111) templates. One of the phases shows a clear, nearly flat, honeycomb structure. Thanks to thorough synchrotron radiation core-level spectroscopy measurements and advanced Density Functional Theory calculations we identify it to a radic;3xradic;3R(30°) germanene layer in coincidence with a radic;7xradic;7R(19.1°) Au(111) supercell.
For the case of Sn, we predict from the first-principles calculations that the stanene with dumbbell units (DB) is a fully stable two-dimensional (2D) topological insulator with inverted bands around the Γ point and its band-gap can be tuned by compressive strain. The quantum anomalous Hall effect, Chern half metallicity and topological superconductivity are possible to be observed in DB stanene
boron nitride sheet and reconstructed (2×2) InSb(111).
Acknowledgements: This work was supported by the European Commission within the FP7 CRONOS project (ID 280879), the European Research Council Advanced Grant DYNamo (ERC-2010-AdG-267374), Spanish Grant (FIS2010-21282-C02-01), Grupos Consolidados UPV/EHU del Gobierno Vasco (IT578-13)
Work done in collaboration, joint Publications:
Germanene: a novel two-dimensional Germanium allotrope akin to Graphene and Silicene, M.E. Dávila, L. Xian, S. Cahangirov, A. Rubio, G. Le Lay (2014)
Silicite: The new layered allotrope of silicon, S. Cahangirov, V. Ongun Özccedil;elik, A. Rubio, S. Ciraci (2014)
The atomic structure of the radic;3×radic;3 phase of silicene on Ag(111), S. Cahangirov, V. O. Özccedil;elik, L. Xian, Jose Avila, S. Cho, M.C. Asensio, S. Ciraci, A.l Rubio (2014)
Stable two dimensional dumbbell stanene: a quantum spin Hall insulator, P. Tang, P. Chen, W. Cao, H. Huang, S. Cahangirov, L. Xian, Y. Xu, SC. Zhang, W. Duan, A. Rubio (2014)

Hexagonal boron nitride (BN) is a wide bandgap insulating material. However, the electronic and magnetic properties of BN nanoribbons (BNNRs) can differ very much from those of their parent material, and they can present a behavior that goes from insulating up to metallic. Theoretical results suggest that BNNR bandgap tuning can be obtained by combining edge effects and chemical functionalization [1]. A more detailed understanding of the structural and electronic BNNR properties has been limited by the experimental difficulties in producing large quantities of high-quality nanoribbons [1].
Two synthetic methods exist to produce BNNRs, and both are based on the unzipping of boron nitride nanotubes (BNNTs). Current methods achieve unzipping by using either plasma etching [2] or intercalation of potassium atoms into the layers of multi-walled BNNTs [3]. However, the first method is difficult to scale up because it requires many steps, while the second one has low yields [1].
Recently [4], a new method to unzip carbon nanotubes (CNTs), by shooting them at high velocities (6.9 km/s) against solid targets, was demonstrated [4]. This one-step method avoids chemical contamination during the unzipping process and could be scaled-up to produce high-quality nanoribbons in large quantities. In this work we have investigated, through fully atomistic reactive molecular dynamics simulations,whether this same approach could be effective for the case of boron nitride nanotubes (BNNTs).
The simulations were carried out using the reactive force field (ReaxFF), as implemented in the Large-scale Atomic/Molecular Parallel Simulator (LAMMPS), using the same protocols described in [4] for the case of CNTs. Our results show that velocities of just ~3 km/s are enough to effectively produce fully unzipped BNNTs and the unzipping efficiency is highly dependent on the impact angle with the targets.
Although these results are similar to the ones observed for the unzipping of carbon nanotubes [4], there are major differences, as different fracturing mechanisms are at play during the unzipping of multi-walled BNNTs. BNNTs are frequently more fractured rather than unzipped and, in general, only the outer BN walls unzip upon impact, with the inner layers remaining almost intact. Our results suggest that, as in the case of CNTs, unzipping resulting from high velocity impacts of BNNTs with targets can be effectively used to obtain high-quality BN nanoribbons.
[1] Y. Linand J. W. Connell, Nanoscale 4, 6908 (2012).
[2] H. Zeng, C. Zhi, Z. Zhang, X. Wei, X. Wang, W. Guo, Y. Bando, and D. Golberg, Nano Letters 10, 5049 (2010).
[3] K. J. Erickson, A. L. Gibb, A. Sinitskii, M. Rousseas, N. Alem, J. M. Tour, and A. K. Zettl, Nano Letters 11, 3221 (2011).
[4] S. Ozden, P. A. S. Autreto, C. S. Tiwary, S. Khatiwada, L. Machado, D. S. Galvao, R. Vajtai, E. V. Barrera, and P. M. Ajayan, Nano Letters, Article ASAP DOI: 10.1021/nl501753n

The discovery of graphene and other two-dimensional (2D) materials together with recent advances in exfoliation techniques have set the foundations for the manufacturing of single layered sheets from any layered 3D material. The family of 2D materials encompasses a wide selection of compositions including almost all the elements of the periodic table. This derives into a rich variety of electronic properties including metals, semimetals, insulators and semiconductors with direct and indirect band gaps ranging from ultraviolet to infrared throughout the visible range. Thus, they have the potential to play a fundamental role in the future of nanoelectronics, optoelectronics and the assembly of novel ultrathin and flexible devices. We categorize the 2D materials according to their structure, composition and electronic properties. In this work we distinguish atomically thin materials (graphene, silicene, germanene, and their saturated forms; hexagonal boron nitride; silicon carbide), rare earth, semimetals, transition metal chalcogenides and halides, and finally synthetic organic 2D materials, exemplified by 2D covalent organic frameworks. Our exhaustive data collection presented here demonstrates that 2D materials exhibit properties that are unknown from the bulk, like massless Dirac electrons, and a large diversity of electronic properties, including band gaps and electron mobilities. The key points of modern computational approaches applied to 2D materials are presented with special emphasis to cover their range of application, peculiarities and pitfalls.

In this work we report the Raman and PL characteristics of suspended semiconducting transition metal dichalcogenide (sTMDs) films with applied strain. Most of the Raman and photoluminescence (PL) studies of sTMD have been performed on the substrate where the corresponding material was grown or transferred after exfoliation (i.e. SiO₂, Al₂O₃, graphene, etc.). However, the properties of sTMD thin layers are strongly influenced by the underlying substrate. Our suspended sTMD samples were fabricated by transferring monolayers of WS₂ or MoS₂ onto Si₃N₄ substrates that were previously perforated by a focused ion beam (Ga⁺). Strain was applied to the suspended sTMD layers by means of differential pressure. Raman spectra exhibited higher intensity peaks for both of the corresponding main vibrational modes (E&’ and A&’₁) in the suspended region when compared to the regions of the sTMD that were directly onto the Si₃N₄ substrate. Interestingly, the most intense Raman signals were consistently collected at the edges of the holes. On the other hand, the PL spectra showed an increase in intensity in the entire suspended region and no effect of the edges of the holes was noticed. Finally, applied strain on the sTMD layers resulted in shifts of the peak positions in both Raman and PL spectra. The recorded shifts have been analyzed and compared to theoretical simulations.

Layered transition metal dischalcogenides (TMD) and graphene are expected to lead to 2D nanoelectronic devices. Integration of such layers in device structures is best done by CVD processing in a single reactor, which would be difficult if the TMD precursor is inside the CVD reactor at high temperature. To develop a CVD process for TMD in which precursors can be outside the reactor, equilibrium thermodynamic modeling of CVD is carried out with Mo(CO)₆ and H₂S as precursors for MoS₂, by minimizing the total Gibbs free energy of the system. The resulting phase stability diagram indicates the formation of various solid phases for different growth temperature (T), total reactor pressure (P), and Mo(CO)₆:H₂S ratios, and predicts the stability window for the formation of pure MoS₂ in the Mo-C-O-H-S system. When argon is the carrier gas and the molar flow rate ratio Mo(CO)₆:H₂S:Ar=1:10:100, modeling predicts that MoS₂ and MoS₃ are both deposited (at lower temperatures), together with carbon, whose stability domain and proportion are diminished when T is raised, and enhanced when P is raised. Formation of contamination-free MoS₂ with argon as carrier gas is predicted to occur at high T and low P. Modeling shows that the (T, P) process window for pure MoS₂ can be widened considerably when (Ar+H₂) is the carrier gas, with the window widening progressively as the proportion of H₂ in the carrier gas is increased. As the proportion of H₂ in (Ar+H₂) increases, the co-deposition of carbon with MoS₂ occurs over a steadily narrowing range of T and P, forming a camel-like “hump" in the phase stability diagram. With pure H₂ as carrier gas with Mo(CO)₆:H₂S:H₂=1:10:100, formation of carbon-free MoS₂ is predicted at all T for all P>10 torr. Each of the various predictions of thermodynamic modeling has been verified by Raman spectroscopic analysis of deposits made using a homemade CVD reactor, on Si, sapphire, and fused quartz substrates. Formation of monolayered and few-layer MoS₂ requires low supersaturation, which can be achieved by increasing P and/or increasing the partial pressure of H₂. This is consistent with the prediction of modeling that, under such conditions, carbon-free MoS₂ is deposited. Addition of CO to the system to control supersaturation is also investigated. By so reducing supersaturation, n-layered MoS₂ has been predictively obtained. Thermodynamic modeling is also successful in explaining experimental artifacts: it predicts that the addition of a small percentage of O₂ to Ar as carrier gas leads to solid sulfur being formed at lower T, which accounts for the sulfur-dominated films deposited when a minute leak is present in the CVD system. It also explains the partial reduction of the deposited MoS₂ while being cooled in H₂ ambient. A combination of thermodynamic modeling and kinetic control similar to that used in this study can therefore be employed confidently to develop CVD processes for TMDs of interest predictively and efficiently.

A key issue in Resonant Raman scattering studies is the role played by intermediate exciton states. The effect of temperature (at ambient pressure) and pressure (at ambient temperature) on tuning the A and B exciton energies into resonance has been previously demonstrated [1]. In this presentation we further explore the first and second order Raman resonant behavior of bulk 2H-MoS_2, in the T-P range of 6 to 300 K and up to 6 GPa. Consistent with calculations, that take into account the combined effects of T and P on the excitons energies, the maximum in the resonance shifts of the A_1g phonon towards lower pressures with decreasing temperatures [2].
The fundamental and systematic understanding of the multiphonon processes in semiconductors is of great importance. The challenge is to fully characterize the Raman spectra, while providing a guideline of the nature of the scattering landscape. Recently [3], we reassigned the spectrum while systematically detecting in the spectra all the possible second order overtones, combinations and difference bands and the majority of third order bands of four of the M edge phonons. We use a comprehensive symmetry analysis and polarization measurements to support our reinterpretation of the spectra.
The extensively studied “2LA(M)” band at ~ 460 cm^-1 is also reassigned, while analyzing its resonant nature. The band, previously ascribed to 2LA(M), is attributed to Van Hove singularity between K an M. The approach shown applies for a single layer system as well.
[1] T. Livneh and E. Sterer, Phys. Rev. B81, 195209 (2010).
[2] T. Livneh, J.S. Reparaz, and A.R. Goñi, In preparation.
[3] T. Livneh and J.E. Spanier, Submitted

MoS₂, atomically thin layered 2D material, is a promising candidate for next generation electronic devices due to its high on-off current ratio and good electrical performance.Various synthesis methods have been recently used to produce large-area, scalable and high quality MoS₂.Conventionally, growth of large area of MoS₂ thin films is obtained by the sulfurization of Molybdenum trioxide (MoO₃). However, there are still challenges to obtain controllable number of layers of MoS₂ and growth on diverse substrates since the precursor, MoO₃, has low melting and evaporation temperatures.Here we present highly crystalline MoS₂ with mono-, bi-, tri layers on diverse substrates obtained by direct sulfurization of MoO₂ powder as a source. We show growth of single- vs. few-layered samples when synthesis is performed at specific temperatures, without any substrate and precursor pretreatment. We present a number of previously-unreported structural idiosyncrasies of single and few-grained crystals whose formation is directly related to the growth kinetics of these crystals. Further, synthesis on a wide range of technologically relevant substrates has also been accomplished. We present detailed electronic and optical properties of CVD grown monolayer, bilayer and trilayer MoS₂ samples.

In contrast to graphene, which possesses a flat structure in its pristine form, the structure of most atomically thin films is naturally buckled so that their mirror symmetry can be broken by an external electric field. A freestanding silicene sheet (one atom thick crystal of Si), for example, can harbor a rich phase diagram as a function of external electric and magnetic fields and a silicene nanoribbon could thus be used to manipulate spin-polarized currents via gating without the need to switch magnetic fields. Moreover, a rich tapestry of morphologies and topological characteristics is generated when we consider layer-by-layer growth of thin films on various substrates. First principles computations on atomically thin films of many elements and their alloys and ultra-thin films of most 3D topological insulators yield numerous stable structures capable of supporting the 2D quantum spin Hall phase with band gaps large enough in many cases for room temperature applications. Thin films of transition metal dichalcogenides offer yet other exciting opportunities. For example, the band gap in MoSe₂ films changes from being indirect to direct in the single layer limit. These and other aspects of our recent work on atomically thin 2D materials will be discussed [1-6].
1. Yi Zhang, Tay-Rong Chang, Bo Zhou, Yong-Tao Cui, Hao Yan, Zhongkai Liu, Felix Schmitt, James Lee, Rob Moore, Yulin Chen, Hsin Lin, Horng-Tay Jeng, Sung-Kwan Mo, Zahid Hussain, Arun Bansil and Zhi-Xun Shen, Nature Nanotechnology 9, 111 (2014).
2. Gaurav Gupta, Hsin Lin, Arun Bansil, Mansoor Bin Abdul Jalil, Cheng-Yi Huang, Wei-Feng Tsai and Gengchiau Liang, Applied Physics Letters 104, 032410 (2014)
3. Feng-Chuan Chuang, Liang-Zi Yao, Zhi-Quan Huang, Yu-Tzu Liu, Chia-Hsiu Hsu, Tanmoy Das, Hsin Lin and Arun Bansil, Nano Letters 14, 2505 (2014).
4. Timo Saari, Cheng-Yi Huang, Jouko Nieminen, Wei-Feng Tsai, Hsin Lin, and Arun Bansil: Applied Physics Letters 104, 173104 (2014).
5. F.-C. Chuang, C.-H. Hsu, C.-Y. Chen, Z.-Q. Huang, V. Ozolins, H. Lin, and A. Bansil, Applied Physics Letters 102, 022424 (2013).
6. W.-F. Tsai, C.-Y. Huang, T.-R. Chang, H. Lin, H.-T. Jeng, and A. Bansil, Nature Communications 4, 1500 (2013).

Intercalation-based exfoliation has been actively studied for the formation of two-dimensional (2D) single-layer transition metal chalcogenides (TMCs); however, current techniques always require external energies such as sonication and H₂ generation. In this study, we demonstrate tandem molecular intercalation (TMI) strategy for the exfoliation of TMCs under mild condition in the absence of additional energy input. TMI strategy utilizes short and long intercalates molecules where short initiator molecules are first intercalated into the interlayer to widen the pristine interlayer gap, and then, the long primary molecules spontaneously enter the widened gap. Through the process, the interlayer distance becomes large enough to overcome van der Waals (vdW) interaction between the layers, which smoothly provides single-layer nanosheets. TMI strategy is generally applicable from Group IV TMCs (e.g., TiS₂, ZrS₂) to Group VI TMCs (e.g., WSe₂) where the use of proper intercalate is found to be critical for the successful exfoliation of 2D TMCs. Mild Lewis base such as alkyl amine is an efficient intercalate for Group IV TMCs, whereas stronger Lewis base such as alkoxides is required for the intercalation in case of Group VI TMCs.

Bi₂Te₃ has been identified as one of the most promising systems with which to realize a three-dimensional (3D) topological insulator (TI) [1]. However, the bulk, stoichiometric Bi₂Te₃ single crystals often exhibit p-type metallic electrical conduction due to the Bi_Te-type antisite defects, which overshadows the contribution of surface states. We have established that [2], upon group III (indium and/or thallium) doping, the Fermi level of Bi₂Te₃ can be raised from the valence band into the band gap, and eventually shifted into the conduction band. Such doping progressively changes the electrical conduction of Bi_2-xA_xTe₃ (A = In, Tl, and x = 0 minus; 0.30) single crystals from p-type to n-type. This is observed via measurements of both the Hall effect and the Seebeck coefficient in the temperature range of 2 minus; 300 K. At low doping levels, the temperature dependent in-plane electrical resistivity maintains its metallic character as the hole concentration decreases. Heavier doping content, x = 0.20 (0.10) for In (Tl), drives the electrical resistivity into a prominent non-metallic regime displaying the weak anti-localization (WAL) type of magnetoresistance at the lowest temperatures for Bi_1.80In_0.20Te₃. At the highest concentration, the samples revert back into the metallic state with electron dominated conduction. Thermal conductivity measurements of Bi_2-xA_xTe₃ single crystals reveal a generally stronger point defect scattering of phonons with the increasing doping content.
Our recent ab inito study has revealed that [3], through systematic examination of symmetrically nonequivalent configurations using first-principles calculations [4], the ground state of Cu₂Se is constructed by repeating sextuple layers of Se-Cu-Cu-Cu-Cu-Se. The layered nature is in accord with electron and X-ray diffraction studies at and below room temperature and also is consistent with transport properties. Magnetoresistance measurements at liquid helium temperatures exhibit cusp-shaped field dependence at low fields and evolve into quasi-linear field dependence at intermediate and high fields. These results reveal the existence of WAL effect, which has been analyzed using a modified Hikami, Larkin, and Nagaoka model including a quantum interference term and a classical quadratic contribution. Fitting parameters suggest a quantum coherence length L of 175 nm at 1.8 K. With increasing temperature, the classical parabolic behavior becomes more dominant and L decreases as a power law of T ^-0.83.
References:
[1] H. Zhang et al., Nat Phys5, 438 (2009); Y. Chen et al., Science325, 178 (2009); D. Hsieh et al., Nature460, 1101 (2009); Y. Xia et al., Nat Phys5, 398 (2009); Y. Chen et al., Science329, 659 (2010).
[2] H. Chi et al., Phys. Rev. B88, 045202 (2013).
[3] H. Chi et al., Phys. Rev. B89, 195209 (2014).
[4] H. Chi et al., Phys. Rev. B86, 195209 (2012).
*Electronic mail: [email protected]. This work is supported by CSTEC, a DoE EFRC under Award # DE-SC0000957.

We use density functional theory to investigate the atomic structure and electronic properties of zigzag (n,0) and armchair (n,n)-like nanotubes composed of vanadium, niobium and tantalum pentoxide. The electronic band structure dependence on the nanotube diameter, type (V, Nb, or Ta), and structure (armchair or zigzag) reveals that the V₂O₅ nanotubes are all semiconductors, while Nb₂O₅ and Ta₂O₅ are insulators. For all cases, the top of the electronic valence band is dominated by the oxygen p orbitals (3 different types of oxygens: 1, 2 and 3 coordinated) while the bottom of the conduction band is associated with the transition metal d orbitals. Additionally, the bandgap decreases with decreasing nanotube diameter, with the zigzag nanotubes exhibiting a particularly strong dependence. Given the recent success in electrochemical anodization for the synthesis of group IV and V transition metal oxide nanotubes and their potential applicability for batteries, supercapacitors, and photovoltaics, the results of this study provide important new insight into their behavior and intrinsic properties.

Transition metal dichalcogenides have been shown to exhibit extraordinary mechanical, chemical and electronic properties, leading to applications in catalysis, lubrication and nanoelectronics. They are also known to be able to strongly alter their properties due to the intriguing electronic interplay of the transition metal as well as on the delicate electronic structure, strongly influenced by the electronic system that is formed in the transition metal layer. The electronic properties of transition metal dichalcogenides change by the number of layers, by strain, by exposing them to external fields, by the type of defects.
In my presentation I address the situations where transition metal dichalcogenides are particular sensitive, and will provide the rationale for it, as understanding those phenomena are crucial for the rational design of devices based on these species. I will address external stimuli such es electric fields and strain, and external factors such as defects, grain boundaries, particle size, rippling and interlayer effects, and which consequence they have for electric conductivity, band gap and spin-orbit splitting. A large part of the content that will be covered is published in the contributions below.
An Atlas of Two-Dimensional Materials. P. Miro, M. Audiffred, T. Heine, Chem. Soc. Rev. (2014), DOI: 10.1039/c4cs00102h.
Electron Transport in MoWSeS Monolayers in Presence of an External Electric Field. N. Zibouche, P. Philipsen, T. Heine, A. Kuc, Phys. Chem. Chem. Phys. 16 (2014) 11251-11255.
Two Dimensional Materials Beyond MoS2: Noble Transition Metal Dichalcogenides. P. Miro, M. Ghorbani-Asl, T. Heine, Angew. Chem. Intl. Ed. Engl. 53 (2014) 3015-3018.
Defect-induced conductivity anisotropy in MoS2 monolayers. M. Ghorbani-Asl, A. N. Enyashin, A. Kuc, G. Seifert, T. Heine, Phys. Rev. B 88 (2013) 245440.
Electromechanics in MoS2 and WS2: nanotubes vs. monolayers. M. Ghorbani-Asl, N. Zibouche, M. Wahiduzzaman, A. F. Oliveira, A. Kuc, T. Heine, Scientific Reports 3 (2013) 2961.
Spontaneous Ripple Formation in MoS2 Monolayers: Electronic Structure and Transport Effects. P. Miro, M. Ghorbani-Asl, T. Heine, Adv. Materials 25 (2013) 5473-5475.
Electron Transport in MoWSeS Monolayers in Presence of an External Electric Field. N. Zibouche, P. Philipsen, T. Heine, A. Kuc, Phys. Chem. Chem. Phys. 16 (2014) 11251-11255.

We present theoretical results for the structural and electronic properties of MoS₂ nanoribbons (quasi-1D) in comparison to the single-layer (2D) and bulk (3D) material. Our Density Functional Theory (DFT) calculations for the energy of the (10) edge show that MoS₂ ribbons and flakes are stable nanostructures. We discuss the metallic states localized at the edges of the otherwise semiconducting material and their signature in the electronic density of states. Using linear response theory, several optoelectronic properties are obtained. We find that the dielectric constant increases with increasing ribbon width, reaching values which, at large width, approach the dielectric constant found for the monolayer. We interpret our results with a simple model by separately considering the contributions of the metallic edges and the semiconducting interior of the ribbons. The value of the dielectric constant of the single-layer is calculated to be approximately half of the value for the bulk MoS₂. DFT results related to strain effects on the electronic properties of MoS₂ and other transition metal dichalcogenides (TMD) are also presented. The indirect to direct band-gap transition from 3D to 2D is reversed for most types of strained monolayer structures. We also discuss similarities and differences between different TMD. Our DFT results are interpreted with simple models and are shown to be consistent with available experimental data.
Davelou et al, Solid State Commun., 192 (2014) 42

Monolayer transition-metal dichalcogenides (TMDCs) display valley-selective circular dichroism due to the presence of time-reversal symmetry and the absence of inversion symmetry, making them promising candidates for valleytronics. In contrast, in bilayer TMDCs both symmetries are present and these desirable valley-selective properties are lost. Here, by using density-functional tight-binding electronic structure simulations and revised periodic boundary conditions, we show that bending of bilayer MoS₂ sheets breaks band degeneracies and localizes states on separate layers due to bending-induced strain gradients across the sheets. We propose a strategy for employing bending deformations in bilayer TMDCs as a simple yet effective means of dynamically and reversibly tuning their band gaps while simultaneously tuning valley-selective physics.

Recently, the development of new functionality in heterostructures of different two-dimensional (2D) materials like graphene (G), hexagonal Boron Nitride (h-BN) and the transition metal dichalcohenides (TMDC) has gained much interest [1]. While most efforts have focused on few-layer heterostructures, artificial van der Waals solids that are formed by self-assembling 2D monolayers into three-dimensional structures [2] offers an alternate and novel route. Such solids allow for a truly atomic level amalgamation of two different materials into a 3D heterostructure. Further, by simply changing the ratio of the 2D parent materials in the self-assembly process, one can potentially tune the opto-electronic response from one 2D parent to the other. Most importantly, one hopes that interactions between the layers of the different 2D materials will exhibit functionality that is not seen in either parent material.
Here, we use optical pump terahertz probe spectroscopy to measure the ultrafast opto-electronic response of a 3D van der Waals solid made by self-assembling h-BN and G in varying ratios. We demonstrate that the opto-electronic response of the heterostructure tunes from the completely insulating h-BN phase to the conductive G phase as we vary the ratios of the 2D parents in the hybrid solid. Moreover, for solids with comparable ratios of h-BN and G, where one expects significant interface interaction between the hBN and G monolayers, we observe dissipation-less, purely capactive opto-electronic behavior, distinct from the insulating and conductive parents. The results can be explained by assuming charge trapping and dipole formation at the hBN:G interfaces, with obvious applications in charge separation and extraction for solar cells and photodetectors.
[1] A. K. Geim & I. V. Grigorieva, Nature 499, 499 (2013).
[2] G. Gao, W. Gao, E. Cannuccia, J. Taha-Tijerina, et al., Nano Lett. 12, 3518 (2012).

Here we examine and explore the properties of artificial stacks of molybdenum disulfide and tungsten disulfide. Understanding the interactions between hetero-structures of two dimensional materials provides new avenues for control and creation of new characteristics in these two dimensional materials. After careful preparation of these stacks we examine their photoluminescence properties, unveiling an anomalous quenching in tungsten disulfide signal. We examine the interactions between these two materials through first principles calculations and demonstrate that tungsten disulfide will have a direct to indirect band gap transition in this configuration while molybdenum disulfide maintains its monolayer properties. This explains the observed photoluminescence characteristics and predicts a band alignment resulting in charge transfer between the two species. We further examine the properties of these artificial stacks using electrical and photo-current measurements and show their great potential in future optoelectronic applications.

Emerging non-Graphene 2D materials such as hexagonal boron nitride (h-BN) have recently attracted a lot of research interest.[1] While, chemical vapour deposition (CVD) of h-BN on transition metal catalysts has emerged as a preferred route for synthesis, the mechanism underlying the growth of h-BN remain unclear.
Using a combination of complementary in-situ high-pressure time and depth resolved x-ray photoelectron spectroscopy (XPS) and in-situ x-ray diffraction (XRD) at realistic CVD conditions of pressure (~0.001 - 1 mbar) and extreme temperatures (700-1000^oC) we study the fundamental mechanisms underlying the CVD of h-BN on polycrystalline Cu during exposure to precursors (both gaseous and liquid precursors). These measurements allow for a clear understanding of the B and N incorporation in the nanostructure as it happens by identifying the catalyst state and the chemical nature of the species on the surface of the catalyst at any point of time during CVD. These coupled with ex-situ experiments [3,4] allow for development of a comprehensive growth mechanism to rationally engineer h-BN CVD processes.
We observe h-BN layers nucleate and grow isothermally, i.e. at constant elevated temperature, on the Cu surface facets 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. We discuss the implications of these observations in the context of challenges for process integration and hetero-structure growth.
References:
1. Kidambi et al. NanoLetters (submitted).
2. Kidambi et al. Nano Letters 13 (10), 4769-4778 (2013).
3. Kidambi et al. J. Phys.Chem. C. 116, 42, 22492-22501 (2012).
4. Kidambi et al. PSS RRL. 5, 9, 341-343 (2011).

The isolation of graphene constituted a new paradigm in next generation electronic technologies, and even though graphene is considered transformational, it is only the “tip of the iceberg.” Transition metal dichalcogenides (TMDs) and their heterostructures could have an even greater impact on next generation technologies. Molybdenum disulfide (MoS₂) is currently a leading TMD for scientific exploration, but there are a variety of other suitable, less explored, TMDs and heterostructures that exhibit very attractive bandgaps, charge carrier effective masses, and mobilities for electronic applications. Transition-metal dichalcogenides (TMDs) in the form of MeX₂ (where Me = a transition metal such as Mo, W, Ti, Nb, etc. and X = S, Se, or Te) also exhibit extreme flexibility, possession of tunable band gaps, modest electron mobilities, and wide variety of band-offsets. Synthesizing and heterogeneously combining these atomic layered TMDs to form van der Waals (vdW) solids, where each layer may be different from the previous, is a powerful way to develop novel nanoscale materials. Furthermore, having the ability to tune the physics and chemistry with atomic-level precision is the foundation for “properties-on-demand”, which can have an enormous impact on current and future technologies. This talk will elaborate on recent breakthroughs for direct growth of two-dimensional atomic heterostructures (MoS₂, WSe₂, and hBN) on a graphene template, and provide evidence that graphene can be an ideal substrate for building vdW solids.

Graphene and two-dimensional (2D) transition metal dichalcogenides (TMDs) have been studied with intense attention in view of their promising applications to future nanoelectronics. Among them, graphene exhibits a high carrier mobility, but it has critical limitations for circuit application which is intrinsic difficulty caused by its little bandgap. Recently, Molybdenum disulfide (MoS₂) which is a kind of TMDs appeared as an alternative 2D nanosheet material that may overcome the limitations of graphene. Because of the large bandgap, MoS₂-based field-effect transistors (FETs) exhibited great electrical properties such as large on/off ratio, small subthreshold slope and high electron mobility. According to such properties, MoS₂-based FETs are very promising for future nanoelectronic switching devices, however, until now, there are only a few reports about switching circuits whose basis may be complementary inverters. It is probably because its fabrication is quite complex in process.
In this work, we report vertically stacked hybrid complementary inverter using top-gate n-channel MoS₂ FET and bottom-gate p-channel organic thin-film transistor (TFT) on glass substrate, to overcome the parasitic capacitance problem. MoS₂ FET mobility ranged from 5 to 25 cm²/V s. For the p-channel organic TFT, we used indolocarbazole-type small molecule that is 8,16-dihydrobenzo[a]benzo[6,7]indolo[2,3-h]carbazole (heptazole: C₂₆H₁₆N₂) which has almost the same HOMO level as that of pentacene. CYTOP/Al₂O₃ bilayer was used as dielectric layer for MoS₂ FET and organic channel, to control the threshold voltage toward an enhancement device. Using common dielectric and gate electrode, one vertically stacked hybrid CMOS inverter device could be fabricated. Our one cell hybrid complementary inverter shows a high voltage gain (dV_out/dV_in) of more than 20 and very good voltage switching performance at high input voltage frequencies. More details will be discussed in the meeting.

Van der Waals materials are a category of materials with layered structure and van der Waals force as the binding force between the layers. This group of materials have received high interest recently for their novel properties and high potential in various device applications. While graphene is one of the most prominent member of this group, other materials like MoS₂, ZnSe, hexagonal Boron Nitride (h-BN) etc. are also being eagerly investigated. H-BN is of imminent interest since it can serve as a chemical and electrical barrier material for both graphene and metal dichalcogenides. Such heterostructure is supposed to further improve device performance. While CVD is now the dominant method to fabricate large area graphene and h-BN, direct deposition of van der Waals heterostructure remains difficult. Tube furnace CVD method has limited potential in performing doping of material as well as in in-situ heterostructure growth. Molecular Beam Epitaxy (MBE) has a natural advantage in high-quality heterostructure growth thanks to its UHV environment and instant introduction and control of multiple sources.
In this presentation, we report our work of high-quality MBE graphene/h-BN heterostructures. Our group had previously reported MBE based epitaxial growth of graphene. Now further efforts are being made on developing in-situ growth of graphene/h-BN heterostructures. Plasma assisted MBE equipped with separate carbon, boron and nitrogen sources was used to grow these heterostructures. Growths were performed on metal based substrates. Various characterizations including SEM, TEM, XPS, Raman spectroscopy, and other optical and electrical measurements were performed and high-quality graphene/h-BN heterostructures are demonstrated. This research paves a way for wafer scale synthesis of high-quality van der Waals heterostructures for various device applications.

Achieving Ohmic contacts to layered transition metal dichalcogenides (MoS₂, WS₂, WSe₂ and MoSe₂) has been a challenge to researchers owing to the Schottky barrier between metal and semiconductor. This has resulted in low on-currents, mobilities and poor sub-threshold slopes in the devices made with these materials. Here we report a chemical approach to reversibly transform semiconducting phase (2H) and metallic phase (1T), which is universally applicable to all semiconducting Transition metal dichalcogenides. Taking advantage of the metallic phase, we fabricated hybrid transistors which have 1T phase of the material at the contacts and 2H phase of the material as the channel. The 1T phase significantly reduces the Schottky barrier between the metal and the semiconductor thereby mitigating the high contact resistance issues and allowing for probing the true intrinsic optoelectronic properties of transition metal dichalcogenides. Material synthesis, compositional, optical and electrical characterization results will be discussed in detail. This strategy has implications for several applications in optoelectronics, catalysis, supercapacitors and batteries.

Single- and few-layers of atomically thin Transition Metal Dichalcogenides (TMDCs) such as Molybdenum Disulphide (MoS₂), Tungsten Disulphide (WS₂), etc. possess fascinating and lucrative physicochemical properties. For example, the presence of inherent band gap in these materials is a huge advantage over Graphene, a zero band gap material. Though innovative synthesis approaches such as confining graphene into nano-ribbons (GNRs), production of bilayer graphene, doping graphene etc. are used for opening band gap in them, these synthesis routes are often complicated. In that context, the ability to produce few layered TMDCs through simple liquid phase exfoliation process, with inherent optical gaps opens up a huge possibility in utilizing these materials for several nano and optoelectronics applications. Thus in order to assess the viability of these materials for utilization in next generation photo detectors, ultrafast light modulators, optical switches etc. a clear understanding of photo conduction mechanism in these materials is necessary. In this presentation, several aspects of photoconductive properties of thin films of liquid phase exfoliated MoS₂, WS₂ etc. will be discussed. Evidence of ultrafast intrinsic photo response and presence of sub-gap states in these films will be presented. An understanding of the nature of sub-gap states as well as the role of trap states in the photo conduction process of these films will be discussed in light of well established theoretical models.
This work is supported by the U.S. Army Research Office through a MURI grant # W911NF-11-1-0362.

With advent of graphene other two dimensional van der Waals crystals have gained popularity. With graphene being gapless, a gapped material with high mobility is sought after. Black Phosphorus (BP) appears to be a strong candidate, with a high carrier mobility and a bulk gap of 0.3eV. In this work, we present the preparation and detailed characterization of few-layer black phosphorus in a goal to obtain single-layere BP, i.e. phosphorene. In particular we will report on the layer-thickness dependence of electronic, optical, and Raman spectroscopic behavior of this novel system. Applications in the IR region will also be discussed. A systematic development of these systems can lead to significant advances in atomically thin nanoelectronics and optoelectronics.

The functionality of novel nanostructures depends strongly on local physical, morophological, and electronic properties, increasing the importance of optically probing matter with true nanoscale spatial resolution. Within many 2D material systems, the critical dimensions of a number of optoelectronic-related properties - such as exciton diffusion lengths and average defect spacings - are known to be on the nm length scale. However, despite significant excitement surrounding their novel optoelectronic properties, to date all optical investigations have occurred at diffraction-limited (and larger) length scales. Recently, we demonstrated the use of a novel photonic-plasmonic hybrid Scanning Near-field Optical Microscopy (SNOM) probe, called the “campanile” tip [1], which overcomes the majority of near-field optics limitations. Using these probes, we have now mapped carrier radiative recombination within individual CVD grown Molybdenum disulfide (MoS₂) flakes, observing optoelectronic heterogeneity with deeply subwavelength resolution. It has previously been shown that grain boundaries and point defects significantly change the photoluminescence (PL) emission properties of these materials. Here, by combining near-field campanile hyperspectral imaging, scanning electron microscopy and high resolution Atomic and Dielectric Force Microscopy, we have mapped both local PL emission properties and local morphology and surface potential with nanoscale resolution, showing for the first time on such a small length scale how local potential and morphology of MoS₂ changes the local PL emission properties.
[1] W. Bao et al., Science. 338, 1317 (2012).

Transition metal dichalcogenides (TMDs) which show direct bandgap in the monolayer limit, have received much attention lately as an ideal system to investigate light-matter interaction in two-dimensional (2D) atomic crystals. In this context most of the efforts have focused on enhancing the photoluminescence (PL) properties of the monolayer TMDs using cavities within the weak coupling regime. Here we demonstrate for the first time the formation of strongly coupled light-matter quasiparticles (polaritons) in monolayer MoS₂ embedded in an all-dielectric microcavity at room temperature. A Rabi splitting of 46 meV and highly directional emission is observed from the MoS₂ microcavity. The MoS₂ layers are grown via chemical vapor deposition and transferred via aqueous solution onto the microcavity. The MoS₂ layer is sandwiched between two dielectric distributed Bragg reflectors comprising of alternating layers of SiO2 and SiNx. The monolayer nature of the layer was established by studying the PL intensity. The PL spectrum of the transferred MoS₂ shows only one dominant peak at 1.86 eV resulting from the direct bandgap transition of exciton A (ex_A) while the bare cavity (regions with no MoS₂) has its resonance at 1.875 eV with line width of 17 meV. Following the discussion on fabrication of the microcavity, we will show results of angle resolved reflectivity and PL showing the formation of exciton-polariton states in the MoS₂ microcavity and the associated anticrossing of the two polariton branches with Rabi splitting of 46 meV. The angle resolved PL spectra shows distinct polariton emission patterns due to the 2D nature of the excitons in MoS₂. Finally, we will discuss results of low temperature polarization resolved experiments on these microcavity polaritons for exploiting the valley polarization aspect of the excitons in MoS₂.

Two-dimensional (2D) semiconductors such as molybdenum disulphide (MoS₂) hold great promise in advanced optoelectronic application devices and exhibit the exotic quantum behavior such as coupled spin-valley physics. However, low electron mobility of MoS₂ and large Schottky barrier height between MoS₂ and metal electrode, which have hampered efforts to observe its intrinsic quantum transport behaviors. Potential sources of disorder and scattering include extrinsic sources such as charged impurities and remote optical phonons from oxide dielectrics. To reduce extrinsic scattering and approach the intrinsic limit, we developed a van der Waals (vdW) heterostructure device platform where MoS₂ layers are fully encapsulated within hexagonal boron nitride (hBN), and electrically contacted in a multi-terminal geometry using gate-tunable graphene electrodes. Multi-terminal magneto-transport measurements show dramatic improvements in performance, including a ultrahigh Hall mobility at low temperature. This novel device platform therefore opens up a new way toward measurements of intrinsic properties and the study of quantum transport phenomena in 2D semiconducting materials.

This paper reports low substrate temperature, digital control, fast (~1 minute) synthesis of 2D BNNSs and their temperature-related 2D electrical properties. Raman scattering spectroscopy, X-ray diffraction (XRD), scanning electron microscope (SEM), Transmission electron microscopy (TEM) and electrometer were used to characterize the BNNSs.
SEM and TEM measurements clearly indicate that each sample/membrane consists of a large amount of ultra-thin BNNSs with distribution over entire surfaces of substrates (3x3 cm²). The average size of continuous BNNS is up to 4um², and the average thickness is 3nm. High transparency related to high quality of crystalline structures of BNNS has been identified. Electrical characterization reveals the effects of temperature on the electrical resistivity/conductivity of transparent BNNSs highly depend on the directions of observations in the 2D case but vanished from the 3D bulk materials or thick films.
Based on the synthesized BNNSs, two types of deep UV detectors have been fabricated, and sensitivity, response and recovery times, as well as repeatability have been characterized. Substrate effect and quantum tunnelling effect on the detectors are also discussed.

The demonstration of tunable plasmon resonances in two-dimensional (2D) materials is important for the development of future sensors and optical systems based on such nanostructures. The 2D molybdenum oxide flakes are obtained using a grinding-assisted liquid exfoliation method and exposed to simulated solar to acquire its substoichiometric quasi-metallic form. Here, we present that in suspended 2D molybdenum oxide flakes that generated plasmon resonances can be tuned by controlling the doping levels, lateral dimensions of the flakes in the visible light range as well as the exposure to a model protein.

Achieving low contact resistance is key for the realization of electronic devices based on transition metal dichalcogenides (TMDs), which represent the ultimate limit in 2D structures. Due to their weak interaction with the surroundings forming good electrical contacts to these systems remains challenging. Here a model for charge transfer in these systems (e.g. MoS₂, MoSe₂ and WS₂) accounting for the metal and TMD thickness (number of layers) dependence is presented. Realistic parameters for the model of the TMD/electrode interface are obtained using first principles calculations within the density functional theory. The band alignment and charge distribution in these systems for different metal electrodes (e.g. Pd, Ti, Au) are discussed as well as the potential implications to TMD based nanoelectronics. In particular the Fermi level pinning can result in limited charge transfer to layers near the electrode, resulting in large and gate-dependent contact resistances thereby reducing attainable on/off ratios. Results are also compared to those of graphene and hybrid (graphene/TMD) structures.

The unique properties of graphene, such as high electron mobility, large specific surface area and high transparency, sparked enormous research interest in exploration of its electrochemical properties and potential uses in sensing, energy conversion/storage and corrosion protection. Furthermore, other 2D layered materials, particularly transition metal dichalcogenides (TMDC) exhibit promising properties such as catalytic behaviour and a tuneable band-gap. Understanding how the electron transfer kinetics between these 2D crystals and a redox active molecule compare to bulk materials is crucial to the future use of graphene/TMDCs as high-performing electrode materials in fuel cells, solar cells or supercapacitors.
Herein, the electrochemical response of multi-layered graphene and MoS₂, prepared by mechanical exfoliation of the natural crystals, is presented. Experiments were carried on a micrometre scale areas on the 2D material surface, supported on insulating substrates including SiO₂ and organic polymers.^[1] The electron transfer kinetics at the basal plane, edge plane and freshly cleaved 2D layers are compared using voltammetric techniques.
The number of layers and local surface conditions, are shown to have a significant effect on the electrochemical activity, and most importantly, the electron transfer kinetics decrease by several orders of magnitude after the surfaces are exposed to ambient environment, which has significant impact in electrocatalytic applications of both graphene/graphite and MoS₂.^{[2, 3]}.
[1] A.T. Valota, P.S. Toth, Y.J. Kim, B.H. Hong, I.A. Kinloch, K.S. Novoselov, E.W. Hill, R.A.W. Dryfe, Electrochimica Acta2013,110, 9-15.
[2] P.S. Toth, A. Valota, M. Velicky, I. Kinloch, K. Novoselov, E.W. Hill, R.A.W. Dryfe, Chemical Science2014, 5, 582-589.
[3] A.T. Valota, I.A. Kinloch, K.S. Novoselov, C. Casiraghi, A. Eckmann, E.W. Hill, R.A.W. Dryfe, ACS Nano2011, 5, 8809-8815.

Using piezoresponse force microscopy (PFM) to analyze single-layer MoS₂ on periodically poled lithium niobate (PPLN), we find that the MoS₂ layers significantly enhances the surface polarization for the ferroelectric domains that are polarized “up” with respect to the surface, while slightly quenching the surface polarization in the domains that are polarized “down”. This result is indicative of preferential charge doping of the MoS₂ single-layers on the ferroelectric substrate, which is quite unexpected as p-doping of MoS2 is regarded as a challenge. The MoS₂ single-layers where grown via chemical vapor deposition (CVD) on the ferroelectric periodically poled lithium niobate (PPLN) substrates. Furthermore, we find evidence for preferential MoS₂ growth on the ferroelectric domains that are polarized “up” with respect to the surface suggesting that the details of the surface may influence both charge doping and growth of MoS₂. There are profound implications: charge doping of MoS₂ in a ferroelectric gated transistor geometry may allow large ratio for on/off currents similar to what has been reported in graphene [1].
[1] X. Hong, J. Hoffman, A. Posadas, K. Zou, C.H. Ahn, J. Zhu, Appl. Phys. Lett.97 (2010) 033114

We explored fundamental optoelectronic properties of chemical vapor deposition (CVD) grown molybdenum disulfide (MoS₂) using correlated photoluminescence (PL) imaging and scanning photocurrent microscopy (SPCM). By taking advantage of few hundred nm spatial resolutions of the correlated PL and SPCM, we investigated the effect of MoS₂ - metal contact Schottky barrier heights on the optoelectronic properties of monolayer MoS₂. Two opposite ends of semiconducting phase (2H) MoS₂ sheets were converted to its metallic phase (1T) prior to electrodes deposition to reduce the Schottky barrier hence contact resistance, and their optoelectronic properties were compared with devices without 1T phase. We found that the high photocurrent region emerged towards the center of the channel as opposed to the edges as observed in previous reports, suggesting elimination of the Schottky barrier. This elimination of the high contact resistance through phase transformation allows for systematic studies of diffusion length of photo-excited charged carriers and their lifetime by varying the charged carrier density via gate voltages of transistor devices. The obtained results pave a pathway for the fundamental understanding of intrinsic optoelectronic properties of transition metal dichalcogenides (TMDs) for design of TMD-based optoelectronic devices.

Abstract: Transition-metal dichalcogenides (TMDs), such as MoS₂, MoSe₂, WS₂, and WSe₂, have recently attracted considerable interest as a new class of atomically thin 2D layered materials (2DLMs), due to their atomically thin geometry, unique electronic and optical properties and potential applications in integrated nanosystems ^[1]. In view of expanding the application fields, a major opportunity for 2DLMs and their applications lies in the tunability of bandgaps. Here, using a simple one-step physical evaporation process, we realized the simultaneous synthesis of complete composition ternary MoS_2xSe_2(1-x) alloy nanosheets, with the spectral peak position shifting from 668 nm (for pure MoS₂) to 795 nm (for pure MoSe₂) ^[2]. More important, the composition or bandgap engineering has been successfully achieved with single nanosheet structures, including composition graded alloy nanosheets (i.e. continuous composition/bandgap tunability from the center to the edge of such a single sheet) ^[3], and interfacially sharp nanosheet heterostructures. In addition, we will also show that the composition of a pre-grown nanosheet, like MoS_2xSe_2(1-x) , can be controllably exchanged by other elements, like W and Se, thus developing an universal method to achieve bandgap engineered atomically thin 2D layered materials for novel electronic and photoelectric device applications.
References:
[1] W. J. Yu, Z. Li, H. Zhou, Y. Chen, Y. Wang, Y. Huang, X. Duan, Nature Mater., 2013, 12, 246.
[2] H. Li, X. Duan, X. Wu, X. Zhuang, H. Zhou, Q. Zhang, X. Zhu, W. Hu, X. Fan, X. Wang, J. Xu, A. Pan*, X. Duan, J. Am. Chem. Soc., 2014, 136, 3756.
[3] H. Li, X. Wu, X. Zhuang, A. L. Pan*, X. Duan, 2014, submitted.

Quasi-two-dimensional (Q2D) molybdenum disulfide (MoS₂) is a photoluminescence (PL) material with unique properties. The recent demonstration of its PL, controlled by the intercalation of positive ions, can lead to many opportunities for employing this Q2D material in ion-related biological applications. Here, we present two representative models of biological systems, which are operated on the modulation of the PL in the nanoflakes by the intercalation/deintercalation of H⁺ and K⁺ ions in the presence of extrinsic and intrinsic forces, respectively. The first model showed that the ion-driven PL modulation in Q2D MoS₂/glucose oxidase system can efficiently be used for sensing different concentrations of glucose at small externally applied voltages. In the second model, the PL modulation of Q2D nanoflakes was used for revealing the viability of yeast cells, which operated on the intrinsic cell membrane potentials. The work presented in this paper provides a strong base for the future exploration and incorporation of Q2D materials in wider ion-related biological and medical fields, such as investigating metabolic activities, cell regulations, drug developments as well as studying nerve transmissions and blood constituents&’ activities.

MoS₂ and other transition metal dichalcogenides (TMDs) are semiconducting two-dimensional (2D) materials which could find their applications in optoelectronics, spintronics and emerging valleytronics. However, uniform, large-scale MoS₂ films with high electrical performance that are required for these applications have not been demonstrated. Here, we present the massively parallel fabrication of MoS₂ field effect transistors (FETs) on a 4-inch wafer (up to 10,000 devices) for the first time. We use a single layer MoS₂ film directly grown on a Si/SiO_x wafer using a new metal-organic chemical vapor deposition (MOCVD) growth technique developed by our group. Despite the polycrystalline structure of the MoS₂ film, the characterization of our FET devices shows uniform electrical characteristics with electron mobilities of up to 25 cm²/Vs. We also demonstrate the vertical integration of multiple layers of MoS₂ devices with insulating SiO₂ between adjacent device layers. Our work paves the way for integrating MoS₂ and other TMD materials into large-area atomically thin circuitry.

We have studied various optical reposes of monolayer MoS₂ by changing the incident light energy and power, including the photoelectric, photothermal, and photothermoelectric effects. After characterizing the temperature coefficient of resistance (TCR) and Seebeck coefficient of MoS₂ layer, measurement conditions are carefully tuned to distinguish each effect and analyses are made on each phenomenon.

SiO₂(300 nm)/Si substrate has been extensively used for back-gated 2-D layered material devices such as graphene and Molybdenum disulphide (MoS₂) based transistors, because they can be easily visualized using optical microscope on a 300 nm SiO₂ layer on Silicon, due to optical interference. But this substrate can limit the device performance due to low gate coupling, resulting from low dielectric constant and high thickness of SiO₂ gate dielectric. This in turn requires high gating (switching) voltages.
High-K dielectric materials such as Hafnium dioxide (HfO₂) have been used as a gate dielectric in silicon CMOS technology and they will also play vital role in future Nano-electronic devices such as Graphene/MoS₂ based devices, since high-K media is expected to screen the charged impurities located in the vicinity of channel material, which results in the enhancement of carrier mobility.
In this work we report the optimization process for good quality electron beam evaporated HfO₂ films for back gated MoS₂ transistors on silicon substrate. We report on the fabrication, characterization and performance enhancement of back gated MoS₂ transistors on HfO₂/Si substrates, extending our earlier work on back gated Graphene transistor on HfO₂/Si.
Systematic investigations have been performed to understand the influence of oxygen flow rate on the optical, compositional and electrical properties of the HfO₂ films deposited by electron beam evaporation. We found out that the films deposited at 100^oC substrate temperature with 3 SCCM O₂ flow rate during deposition show very good optical, chemical, compositional and electrical properties. Dielectric constant, K= 19 at 100 kHz, and the leakage current density, J= 1.2×10^-6 A/cm² at an oxide electric field of 1MV/cm have been achieved.
MoS₂ flakes of different number of layers, including monolayer MoS₂ have been identified on optimized HfO₂ (32 nm) / Si substrate using optical microscope and subsequently confirmed with Raman spectroscopy and Photo Luminescence (PL) spectroscopy. Our device shows (HfO₂ back gated MoS₂ transistor) gating effect at much lower voltages (Vg~ 2 V) compared to SiO₂ back gated devices (Vg~ 50 V) with high I_ON to I_OFF ratio (I_ON/I_OFF ~10⁶).

Since the discovery of graphene, 2D materials have received renewed attention. We recently discovered that a honeycomb lattice and other 2D lattices could be made by oriented attachment of semiconductor quantum dots (1; 2). On the basis of atomistic calculations, we predicted that zinc-blende semiconductors with honeycomb nanogeometry have valence- and conduction bands with Dirac character, and topological edge states (3). Hence these materials combine the virtues of a semiconductor with those of graphene. In my lecture, I will discuss the fabrication of zinc blende honeycomb semiconductors based on Cd-chalcogenides, and present the first results on their opto-electronic properties.
References
1. Long-range orientation and atomic attachment of nanocrystals in 2D honeycomb superlattices. Boneschanscher, M.P., et al., et al. xx, 2014, Science, Vol. xx, pp. xxx-xxx.
2. Low-Dimensional Semiconductor Superlattices Formed by Geometric Control over Nanocrystal Attachment. Evers, W.H., et al., et al. 6, 2013, Nano Letters, Vol. 13, p. 2317minus;2323.
3. Dirac Cones, Topological Edge States, and Nontrivial Flat Bands in Two-Dimensional Semiconductors with a Honeycomb Nanogeometry. Kalesaki, E., et al., et al. 2014, Physical review X, Vol. 4, p. 011010.

The fundamental properties of a material depend on their atomic structure, nature of bonding and elemental/chemical composition. Confinement of electrons in 2 dimensional planar structures leads to realization of several intriguing properties that are not seen in the bulk 3-dimendional counterparts. In this work, we explore the properties of single and few layer MX (M:Transition metal, X: chalcogen atom) both theoretically and experimentally. Using state of art density functional theory (DFT) we carried out a stability analysis through phonon and electronic, magnetic and elastic structure calculations where M=Cu and X=S, Se, Te. The stacking of transition metal chalcogenide (TMC) monolayers is of the type MX-M2X2 instead of the usual X-M-X stacking found in TMDs. The differences in geometric structure result in many different stable monolayer forms with different electronic and magnetic properties. Depending on the number of layers, MX structures can be found in 2, 3, 4 and 6 MX layer stable configurations. These dimensionality effects predicted by DFT such as energy band structures and Raman active modes are confirmed by experiments. Various different monolayers of MX possess a number of properties that make them highly promising materials for future nanoscale applications.

CVD offers promise for large area growth of monolayer MoS₂. The electrical performance of such flakes, however, has been below that of mechanically exfoliated MoS₂. The reported room temperature mobility for CVD MoS₂ has ranged from 0.1 to 10 cm²/V-s whereas values of > 50 cm²/V-s have been reported for mechanically exfoliated MoS₂. Recently, Schmidt et al (Nanolett. 2014) reported mobility of 45 cm²/V-s for CVD MoS₂ in the highly conducting regime where the high contact resistance is almost independent of gate bias. This high contact resistance results from the Schottky barrier between the metal (typically gold) and semiconductor (MoS₂) and can reduced by annealing in inert conditions for sufficient amount of time. Instead of annealing, we decreased the contact resistance between the electrodes and the channel by patterning metallic 1T phase on MoS₂ flake and deposited gold on them as metal contacts while keeping the channel region of MoS₂ at its naturally occurring 2H phase. The lowering of the Schottky barrier height was confirmed through detailed analysis of low temperature measurements. We observed enhanced field transport, linear current-voltage characteristics which showed excellent saturation and field effect mobility values that are comparable to single layer mechanically exfoliated MoS₂. In the talk, I will be showing SEM/TEM images which show the sharp interface of the 1T-2H MoS₂ phases, fluorescence images showing the PL quenching in 1T MoS₂, electrical and optoelectronic measurements that show efficient carrier conduction in the case of 1T contacted MoS₂ devices.

Topological insulators, as a new platform of coherent spin-polarized electronics, have recently attracted keen interest in designing next generation spintronic devices and exploiting exciting new physics such as topological exciton condensation and superconducting proximity effect. In this study, we report the observation of the weak anti-localization (WAL) effect and the thickness-dependent surface state in Bi_0.5Sb_1.5Te₃ thin films prepared by MBE technique. The positive sign of the Seebeck coefficient and negative sign of the Hall coefficient indicate the Feimi level of Bi_0.5Sb_1.5Te₃ thin films being within the bulk bandgap, while slightly adjusting composition to the Sb₂Te₃ as Bi_0.5-δSb_1.5+δTe₃ introducs p-type conduction of the films. Meanwhile, the resistivity of the 3 nm film shows an insulating behavior indicating the insulating background of film right above the substrate. The sharp cusp appeared in the low field magneto-resistance (MR) of Bi_0.5Sb_1.5Te₃ signals the WAL effect even when film thickness t reached 120 nm. The measured WAL effect well fits the modified Hikami-Larkin-Nagaoka (HLN) theory in wide field range (-5T ~ 5T). We find WAL coefficient α = 1 for film with t = 6 nm, which gradually increases to 1.5 at t = 30 nm and then quickly increases to 7.0 when t = 120 nm. This implies, the transport phenomenon, when t = 6 nm, is very likely attributed to double 2D channels (both the top and bottom surface states) within an insulating bulk state, while with the increase of t, the bulk state starts to take part in which is showed by increased value of α. Further, the attenuated phase coherence length with thickness, from 110 nm at t = 6 nm to 65 nm at t = 120 nm, verifies the WAL effect is not related to the bulk contribution but originates mainly from the surface channels. The low field MR with magnetic field H#8741;a-b plane and H#8741;I is also examined and exhibits a parabolic feature, illustrating the Lorentz type deflection of carriers as evident in semiconductors. Experimental observation of both electron and hole fermions around Bi_0.5Sb_1.5Te₃ and the decoupling of bottom and top surface states in the thin limit would be instructive to explore the above-mentioned exciting new physics.

The excellent physical and semiconducting properties of transition metal dichalcogenide (TMDC) monolayers make them promising materials for many applications. A famous example is MoS₂, which has gained significant attention as a channel material for next-generation transistors. While n-type MoS₂ field-effect transistors (n-FETs) can be fabricated with relative ease, fabrication of p-FETs remains a challenge¹ as the Fermi-level of elemental metals used as contacts are pinned close to the conduction band, leading to large p-type Schottky barrier heights (SBH). Here, we propose the utilization of graphene oxide (GO) as an efficient hole injection layer for MoS₂-based electronic and optoelectronic devices. The utilization of GO in next-generation TMDC devices has many advantages, thanks to its flexibility and easier fabrication protocols². Moreover, GO and MoS₂ are both 2D materials, so they could lead to the creation of hybrid all-2D electronics on flexible substrates.
Using ab-initio simulations, we show that GO forms a p-type contact with monolayer MoS₂, and that the p-type SBH can be made smaller by increasing the oxygen concentration and the fraction of epoxy functional groups in GO. In addition to obtaining low SBH values, our calculations also highlight the possibility to tune the SBH (up to nearly 1 eV) at the MoS₂/GO interface by simply controlling the oxygen concentration and the relative fraction of functional groups in GO. We demonstrate that this effect is due to the high work function of GO and the relatively weak Fermi-level pinning at the MoS₂/GO interfaces, compared to the traditional MoS₂/metal systems³. Finally, we extend our argument to other functionalized graphene materials⁴ and highlight their potential in TMDC-based devices.
Overall, we highlight the remarkable potential held by GO and functionalized graphene structures in general for next-generation MoS₂-based electronic and optoelectronic devices.
REFERENCES:
[1] Chuang, S., Battaglia, C., Azcatl, A., McDonnell, S., Kang, J. S., Yin, X., et al. (2014). MoS 2P-type Transistors and Diodes Enabled by High Work Function MoO xContacts. Nano Letters, 140227162128008. doi:10.1021/nl4043505
[2] Loh, K. P., Bao, Q., Eda, G., & Chhowalla, M. (2010). Graphene oxide as a chemically tunable platform for optical applications. Nature Chemistry, 2(12), 1015-1024. doi:10.1038/nchem.907
[3] Gong, C., Colombo, L., Wallace, R. M., & Cho, K. (2014). The Unusual Mechanism of Partial Fermi Level Pinning at Metal-MoS2 Interfaces - Nano Letters (ACS Publications). Nano Letters.
[4] Karlický, F., Kumara Ramanatha Datta, K., Otyepka, M., & Zbo#345;il, R. (2013). Halogenated Graphenes: Rapidly Growing Family of Graphene Derivatives. ACS Nano, 7(8), 6434-6464. doi:10.1021/nn4024027

The engineering of contacts to two dimensional transition metal dichalcogenides (2DTMD) presents unique challenges for device preparation. Since mobility and underlying device performance are highly dependent on carrier injection from the contact materials, understanding and careful design are paramount. This study will investigate the interaction of various metals with 2DTMDs to elucidate the transport behavior at the interface (Ohmic vs. Schottky-barrier type). Furthermore, this investigation aims to answer questions about the role of surface defects at the interface. To accomplish this task metals was deposited using various techniques (i.e. magnetron sputtering and e-beam evaporation) on 2DTMD samples with various degrees of surface defects. The use of multiple deposition sources allows us to control the energy of the atoms nucleating at the surface, and thus gives us a mechanism to engineer the metal to semiconductor interface. This represents a critical technology for the incorporation of TMD into next generation electronics. Moreover, the subsequent films have been evaluated using X-ray photoelectron spectroscopy, Raman spectroscopy, atomic force microscopy, transmission electron microscopy, and electrical characterization.

Germanane (GeH), a two-dimensional material comprised of thin layers of sp³- bonded hydrogen-terminated Germanium, has recently been synthesized. It has been predicted to have high electron mobility, and a band gap suitable for use in electronics. Just like silicon nanoelectric devices, the performance and reliability of novel electronic devices made from GeH can degrade with increasing local device temperature that sensitively depend on its thermal properties, which are largely unknown. Here, we report a measurement of the basal-plane thermal conductivity of exfoliated GeH with the use of suspended micro-devices with integrated heaters and thermometers. The obtained values for several samples are in the range of 0.6 - 1.0 Wm^-1K^-1 at room temperature, and increase with increasing temperature between 100-300 K. Both crystalline and amorphous samples were characterized, and the experimental values were compared to a minimum thermal conductivity model that was derived in this work for two-dimensional systems. Comparison between the experiment results and the theoretical model suggests that defects within and between layers can lead to the thermal conductivity approaching the amorphous limit.

Semiconducting transition metal dichalcogenides such as MoS₂ consist of discrete 2D layers bound together by van der Waals forces. The band structure varies with multilayer thickness, transitioning from a direct bandgap in single layer MoS₂ to an indirect bandgap in two-layer and multilayer thick materials. The variation of band structure with material thickness suggests the intriguing possibility of a fundamentally new type of semiconductor heterojunction in which discontinuities in the energy bands arise from abrupt discontinuities in physical thickness. Here we report the first properties of this new type of semiconductor junction and its energy conversion characteristics. Devices are formed by depositing contacts on opposite sides of monolayer/multilayer junctions, producing field effect transistors (FETs) with a discrete variation in channel thickness. As a consequence of the monolayer/multilayer junction, the devices exhibit nonlinear, rectifying behavior in accumulation that is not observed for devices of uniform thickness. Furthermore, this rectification is tunable with gate voltage, which has recently been demonstrated to be a useful property of such ultrathin junctions. Moreover, we report photovoltaic characteristics and a gate tunable photovoltage of monolayer/multilayer heterojunction devices when under illumination.
To explore the origin of these surprising photovoltaic and rectification characteristics, detailed electrical and opto-electrical studies were carried out on devices both with and without discontinuities in channel thickness. In addition to conventional current versus voltage (I-V) measurements, scanning photocurrent microscopy (SPCM) was used to profile energy bands in situ in operating devices. In SPCM, a focused laser beam is raster scanned across the device while simultaneously measuring the photocurrent, spatially mapping photocurrent under conditions of varying drain-source voltage, gate voltage, and illumination wavelength. The present experimental studies, conducted in air, were complemented with finite element modeling using a commercial device simulator (Sentaurus TCAD). The measured photocurrent generation under applied bias is explained using a model incorporating a type II band alignment between monolayer and multilayer MoS₂, and it is concluded that that hot carriers contribute to energy conversion at zero applied bias. These results demonstrate and elucidate a new type of semiconductor heterojunction where energy band discontinuities arise only from discontinuities in physical thickness due to quantum confinement.

Photo detection and photodetectors made by two-dimensional (2D) nano-materials and nanostructures are with significant importance and emerging research interests in the recent years. However, the efforts of fabricating photodetectors based on 2D materials are greatly hindered by the issues of repeatability and scalability in material synthesizing. Despite mechanical exfoliation could provide high quality layered materials, the devices are lack of consistency due to the poor control over the number of layers and the device geometries. Here, we report a novel silicon-integrated approach of fabricating large-scale 2D photodetectors based on monolayer MoS₂. Chemical vapor deposition (CVD) has been used to synthesize both high density single-crystal triangular MoS₂ and large-scale mono-layered MoS₂ film with decent crystallization, which enables a mass production of 2D nano-devices. Compared to commonly-used single-crystal triangle domains, the device made by MoS₂ film exhibited significantly stronger sensitivity in photo-detections. A thorough study on the influences of light intensity (1-1000 W/m²), light wavelength (514 to 785 nm), drain-source voltage (up to 20 V) and device area on photoresponse has been performed. The response time (tau;) of the photodetector is 2 ms at the exitation laser wavelength of 543 nm. These results could provide in-depth understanding of the working mechanism and pave the way to the fabrication of large scale photodetectors based on 2D materials.

When monolayer graphene was exfoliated from graphite by Novoselov et al. in 2004, the scientific community surprized at the disproof of the established theories and experiments. Researchers have now great expectations of applicability of graphene to nanoelectronics and nanomechanics because of its existence in two-dimension and of its superior electronic and mechanical properties; the two-dimensionality matches well the conventional processes of thin film technology. Since pristine graphene is a zero-bandgap semiconductor, methods of opening its bandgap have been investigated. On the other hand, the success of exfoliation of graphene stimulated researchers to seek other graphene-like two-dimensional materials such as transition metal dichalcogendies, among which molybdenum disulfide (MoS₂) has attracted much attention because it is a direct bandgap semiconductor. Mechanical and solution-based exfoliation methods are used to fabricate monolayer MoS₂; a solution-based exfoliation method for fabricating large-area two-dimensional flakes of MoS₂ has recently been reported by Pachuri et al. (2013), and its device applications are expected to become a real thing. However, the structures and electronic properties of monolayer MoS₂ are not yet fully understood. For example, it was reported a molayer MoS₂ made by the mechanical exfoliation is trigonal prismatic (Qin et al., 1991) while a monolayer MoS₂ made by the solvent-based exfoliation is octahedral (Gordon et al., 2002). Furthermore, a distorted octahedral structure is also identified by electron crystallography (Heising et al., 1999). In this paper, we investigate the structural stability and electronic bandgaps of various configurations of MoS₂ flakes using the first-principles calculations with the help of the VASP (Vienna Ab initio Simulation Package); this package is based on the density functional theory and uses the projector augmented (PAW) method. The exchange-correlation functionals constructed through the Perdew-Burke-Ernzerhof parameterization under the generalized gradient approximation (GGA) are adopted. The energies of the structures are minimized by means of the conjugate gradient energy minimization technique. The Monkhorst-Pack method is adopted for k point-sampling in the reciprocal lattice space. The free energies of various MoS₂ flakes are compared, and their stabilities are discussed. The bandgap of two-dimensional monolayer MoS₂ in infinite extent has been reported to be about 1.9 eV. How much the bandgaps of MoS2 flakes vary from this reference value is investigated.

Hexagonal boron nitride (hBN), as the inorganic analog of graphite, has a morphology consisting of stacks of hexagonal two-dimensional sheets. While graphite and boron nitride have similarities, hBN differs in its high dielectric constant, high thermal stability and high transparency. The lack of an effective method for exfoliation and functionalization, however, limits the potential applications of hBN. The state of hBN is much like that of graphite before methods of exfoliation and suspension of GO in water were developed. Here we describe the formation of the hBN equivalent of GO: water dispersible, functionalized, and exfoliated boron nitride (BNO) sheets by a thermal method suitable for the large-scale production.
When heated in air, a mass gain was observed in hBN, which we attributed to the incorporation of oxygen into the hBN lattice. Following the heat treatment, stirring the material in deionized water resulted in hydroxylated and exfoliated boron nitride (BNO). These sheets formed a stable suspension without the need for sonication or surfactants. Yields of water suspended sheets were ~65%, with the balance precipitating from the aqueous suspension as larger, less hydroxylated material that remained stacked.
Characterization of BNO using atomic force microscopy (AFM), thermo-gravimetric analysis (TGA), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD) and FTIR confirmed the presence of exfoliation and chemical functionalization. Field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and dynamic light scattering (DLS) were used to study the morphology and further revealed the mechanism of formation of single sheets. The chemical reactivity of the hydroxyl groups was demonstrated by their reaction with phenylisocyanate to form BN/organic hybrid materials. Polymer modified BNO was prepared via atom transfer radical polymerization (ATRP) grown poly(methyl methacrylate) (PMMA) from initiators attached via the hydroxyl groups. The obtained composites possessed an enhanced Young&’s modulus and elongation as well as good dispersity of the PMMA-fBN within polymeric matrix.
The availability of large quantities of exfoliated, functionalized BNO is expected to allow for the incorporation of BNO as a nanofiller as well as providing material for mechanical reinforcement. Other potential applications include more fully utilize the thermal conductivity, chemical inertness, and electrical insulating properties of this promising material.

First-principles computations were carried out to predict the electronic and magnetic properties of cadmium selenide (CdSe) nanoribbons (CdSeNRs) with zigzag and armchair edges. It is found that armchair nanoribbons are nonmagnetic and semiconducting, independent of their width and edge passivation status. The band gap of both bare and hydrogen-terminated armchair nanoribbons decreases monotonically with the increasing width. Bare zigzag nanoribbons show magnetic and metallic behavior, and the net magnetic moment will increase with the increasing width. By analyzing the spin density distribution, we found that the magnetization mainly localized on the Se atoms at the edge of the bare nanoribbons. Once the edges of zigzag nanoribbons are passivated, the zigzag nanoribbons will transfer to nonmagnetic metal.

Chalcogenides have been studied as strong material of family in thermoelectric materials. They are also attracting attention as candidate materials for achieving new class of physics ideas, topological insulators. Among them, bismuth compound is the leading group of material due to the heavy mass of the element.
Chalcogenides of either magnetic or nonmagnetic elements of d electrons, such as copper, nickel or iridium are interesting material in terms of electronic structure or the thermoelectric application. If the magnetism plays important role in the electronic structure, the electron interaction is expected to be strong in the material. This aspect is usually neglected in both of aforementioned topological insulator or thermoelectric studies. We focus on the binary copper chalcogenides and synthesized single crystals of copper compounds. Measurement of electric resistivity and magneto-resistance revealed that the unique band structure is achieved in the material. Band calculation shows that the defect of the copper can tune the electronic structure of the material. We will report the possibility of Dirac semimetals and the unique electronic structure of the material from several measurements.

The recent discovery of the enhanced optical activity of direct-bandgap semiconducting group-IV transition metal dichalcogenide (TMD) monolayers, including the disulfides of molybdenum and tungsten, has instigated many efforts to grow high-quality single-crystal films for various optoelectronic applications. As a facile measure of the quality of such syntheses, room-temperature Raman and photoluminescence (PL) spectroscopic methods have been extensively utilized. However, there exists a discrepancy in the conclusions drawn from such methodologies. Enhanced photoluminescence intensity is to be expected from the indirect-to-direct bandgap conversion with decreasing number of van der Waals-bonded layers. As a result, high PL intensity is often cited as an indicator of high sample quality. However, a number of reports indicate inhomogeneous PL intensity in vapor-deposited TMD films and enhanced PL in atomically defective mechanically-exfoliated samples.
We investigate these conflicting ideas and systematically study the evolution of the Raman and PL spectra of increasingly defective group-IV TMD films. We observe the appearance of a low-energy shoulder at room temperature on the main PL peak from exfoliated and vapor-deposited tungsten disulfide (WS₂) monolayers centered around 1.98 eV with increasing argon ion bombardment. We believe such treatments induce sulfur vacancies, on the basis of x-ray photoelectron spectra from extensively bombarded samples. We utilize a combination of Raman, PL and scanning Auger electron spectroscopy as well as transmission electron microscopy to elucidate the influence of atomic-scale defects on the Raman and PL spectra of group-IV TMD films.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have been actively studied because of their intrinsic semiconducting properties, interesting luminescence responses, mechanical flexibility, and optical transparency. In particular, proper modification of their properties is required to use it in various applications, and many studies have shown the tuning of the properties with doping elements. However, the decoration with extra dopants on the 2D nano-flakes may have drawbacks such as the change of work function, low stability and lattice damage. As a new approach to solve the issues, we introduce a novel doping method for molybdenum disulfide (MoS2) by using patterning based on silicon-containing block copolymer (BCP). In the study, we used self-assembled poly(styrene-b-dimethylsiloxane) BCP, where the poly(dimethylsiloxane) block is converted into silica nanostructures with an excellent etch resistance when exposed to oxygen plasma. Therefore, the silica nanostructures can effectively block the underlying MoS2 to protect it from ion bombardment, while partially exposed area is etched by plasma. As a result, the nano-patterned MoS2 has the step structures with high-density edge sites consisting of sulfur vacancies. We used field-effect transistors (FETs) to characterize the controllable electrical properties through local defect generation. The photoluminescence (PL) and FET analysis data suggest that generated edge sites act as n-type dopant and the carrier concentration can be finely tuned by etching conditions. In addition, X-ray photoelectron spectroscopy (XPS) results show that the edge sites are easily oxidized due to sulfur vacancy formation. As an example of device application, a gas sensor based on the step-structured MoS2 is fabricated and shows an extremely low detection limit to NOx gases. This new doping strategy for 2D materials can extend their applications such as FET, photo detector and sensors.

Transition metal dichalcogenides (TMDCs) have attracted much attention due to their unique properties as the mechanical, optical, and electronic properties of single-layer materials differ from the properties of the bulk materials. For example, significant attention has been directed to MoS₂, since its single-layer presents a large intrinsic bandgap (1.8 eV) and large in-plane mobility (> 200 cm²/Vs). For the further improvement of material design and electron device fabrication of TMDCs, process and characterization on a nanometer scale have become increasingly important. Since the unique properties of TMDCs depend on the number of layers of TMDC, thinning of TMDC is one of important processes during device fabrication. In this work, we present the investigation results of morphology and electrical characterization of thinning-process-treated MoS₂ flakes.
As thinning-processes of MoS₂, we studied gas phase etching processes and a liquid exfoliation process. The etching of MoS₂ flakes by O₂ plasma (ashing) or Ar CCP-RIE was evaluated. MoS₂ flakes were prepared by micromechanical exfoliation of bulk crystals on highly doped silicon substrate covered with thick SiO₂ layers. Liquid exfoliated MoS₂ flakes were prepared by Li-intercalation method using n-buthyllithium in hexane, followed by deposition of the exfoliated MoS₂ flakes onto 300 nm-thick SiO₂/Si(001) substrates. Optical microscope (OM), Atomic force microscope (AFM), Raman spectroscopy were used for characterization of thinning-process-treated MoS₂ morphologies. For electrical characterization, we used electrical AFM, ‘Nanotester&’, which is an electrical testing equipment with metal probes in a sample chamber of scanning electron microscope [1], and back-gated FETs with the thinning-process-treated MoS₂ flakes as channel layers.
OM and AFM analysis showed achievement of layer-by-layer etching by O₂ plasma or Ar CCP-RIE, whereas crystalline of etched MoS₂ deteriorated with progress of the etching. The back-gated FET with a MoS₂ flake etched by O₂ plasma and Ni/Au contacts shows a 10⁶ on/off current ratio, n-type conduction, room-temperature mobility of ~16 cm² V^-1 s^-1. The detailed results including AFM surface morphologies and properties of liquid exfoliated MoS₂ flakes will be discussed at the presentation.
Acknowledgments
This study was supported by NIMS Nanofabrication Platform in "Nanotechnology Platform Project"
Reference
[1] S. Shimbori et al., Abstract of STM&’05, Wed-Pos-69 (2005).

MoO₃ is an attractive 2D material widely used in electrochromic and photochromic devices owing to its unique ability to reversibly change color between transparent and blue. Despite its significant application potential, MoO₃ performance is largely limited by its low absorption coefficient in the colored state. We demonstrated a novel methodology to enhance the absorption of MoO₃ through intercalation-driven bandgap engineering. The color of MoO₃ is reversibly altered using intercalation of zero-valent metal coupled with a disorder-order phase transition. First, we introduced a layer of zero-valent metal atoms (Sn, Co, Cu) into the Van der Waals gap of 2D α-MoO₃ ribbon. The metal inserts intermediate electronic states between the valence band and the conduction band of MoO₃, altering the color from transparent to dark blue. Then with heating, we introduced a disorder-order phase transition to revert the color back to transparent. The chromism induced by intercalation of zero valent metals and disorder-order phase transition overcomes a major limitation of MoO₃ application and offers much promise for future device technologies.

As one of the promising two dimensional (2D) semiconductors, molybdenum disulfide (MoS₂) nanosheet has recently been attracting much attention from researchers in respects of its relatively high mobility and distinct energy band gap of 1.2~1.8 eV, which is different from that of graphene, the gapless 2D semiconductor. The magnitude of MoS₂ energy band gap is dependent on the nanosheet thickness; single layer sheet appears to have ~1.8 eV while bulk-type thick layers show 1.2 eV as their band gap. Many applications using MoS₂ nanosheet has thus been reported, and those were mostly about top- or bottom-gated field-effect transistors (FETs). However, P-N diode or Schottky diode devices using MoS₂ have hardly been reported yet except few while only a couple of P-N diode reports are mostly found with WSe₂ nanosheets, although such diode components would be also as important as FETs in circuit. Schottky diode applications are particularly rare in any nanosheets. It is probably because researchers have been mainly interested in the ohmic-Schottky contact issue of nanosheet FET source-drain, rather than in Schottky effect-driven device engineering. Hence, reports on the dynamic rectifications of diode type devices are even less to find yet and any diode circuit integrations for voltage output have not been shown, either.
In the present study, we have fabricated a Palladium (Pd)-driven MoS₂ Schottky diode and its related circuit which has quite a simple form but turns out to be very useful toward multifunctional applications based on the Schottky barrier effects at the Pd/ a few layer MoS₂ interface. Our Schottky diode circuits demonstrate desirable static and dynamic behavior as an electrical rectifier, a visible light sensor, and a hydrogen gas sensor with voltage output. Among all the applications our hydrogen gas sensor circuits with 2, 4, and 7 layer (L) MoS₂ appeared as probably the most attractive, to exploit the Schottky barrier between Pd and MoS₂. In particular, Pd/2L-MoS₂ interface turns out to be the most efficient for fast switching dynamics in hydrogen gas sensing due to its largest barrier height among other interfaces with thicker MoS₂.

The influence of a perpendicular external electric field on low-dimensional transition-metal chalcogenide materials TX₂ (T= Mo, W; X= S, Se) has been investigated by means of density functional theory, including full relativistic effects. Our results indicate a remarkable stability of the electronic properties of monolayers with respect to gate voltages, whereas in case of bilayers, a semiconductor-metal transition occurs at practical electric fields. In centro-symmetric bilayers, the inversion symmetry is broken due to the applied electric field, leading to spin-orbit splittings in the band structures, so-called Stark effect. The induced spin-orbit splitting values in the bilayers rapidly increase with the electric field strengths and saturate at similar values to those of the respective monolayers.

The transition-metal chalcogenides (TMCs) produced from different methods may include topological defects which can significantly alter the electronic and transport properties of these materials. In this work, we study the effect of several defects, e.g. point vacancies, grain boundaries and line defects, on the electron transport through MoS₂ ML. Our simulations are performed by using the density-functional based tight-binding (DFTB) method in conjunction with the Green&’s function (GF) approach. The results show that point defects cause localized mid-gap states in MoS₂ ML. As a result, the conductance of imperfect ML is lowered due to the backscattering effects from the defective sites. Although the electron transport is almost isotropic in pristine MoS₂, strong anisotropy is observed in the presence of defects. Our results could help to better understand the impact of topological structure on the electronic performance of the TMC-based nanoelectronic devices.
References:
[1] M. Ghorbani-Asl, A. N. Enyashin, A. Kuc, G. Seifert and T. Heine, Phys. Rev. B 88, 245440 (2013).

We observe highly efficient non-radiative energy transfer from cadmium selenide (CdSe) quantum dots to monolayer and few-layer molybdenum disulfide (MoS₂). The quenching of the donor quantum dot photoluminescence increases as the MoS₂ flake thickness decreases, with the highest efficiency (>95%) observed for monolayer MoS₂. This counterintuitive result arises from reduced dielectric screening in thin layer semiconductors having unusually large permittivity and a strong in-plane transition dipole moment, as found in MoS₂. Excitonic energy transfer between a 0D emitter and a 2D absorber is fundamentally interesting and enables a wide range of applications including broadband optical down-conversion, optical detection, photovoltaic sensitization, and color shifting in light-emitting devices.

Two-dimensional phosphorus, a new type of elemental 2-D material, has recently attracted considerable attention due to its widely tunable optical properties, direct band gap and high electronic mobility. Because of the material&’s relatively low absorption cross section, however, single monolayers are insufficient to design many classes of optoelectronic devices. Here we describe several methods to assemble optically thick films that retain the optoelectronic properties of discrete sheets and identify design principles that will guide future applications. We compare the structure and electronic properties of thick films produced by vacuum filtration, convective assembly, dip coating, and spray deposition. Through the study of these films we have found that grain boundary resistance dominates charge transport behavior, motivating an approach to improve electronic coupling between adjacent sheets. Employing a model recently developed by our lab to estimate grain boundary resistance (Nat. Mater., 12, 842-849, 2013), we engineer grain boundary resistance via modification of carrier concentrations and surface state trap densities. Towards building thick film devices, we also identify ohmic and Schottky contacts that enable exciting new classes of semiconductor devices. These results offer a way to realize efficient optoelectronic devices based on 2-D phosphorus in the near future.

The emergence of two dimensional materials has enabled rapid advances in nanocomposites, field effect transistors, and water purification. Despite these advances, the integration of these materials into photovoltaics, LEDs, and near IR photodetectors has remained a challenge because current materials lack both the desired bandgap and charge mobility for an ideal device. Here we report the optoelectronic properties of a new atomically thin material, phosphorene. In our approach, we combine UV-vis-near IR transmission spectroscopy with transmission electron microscopy (TEM) on colloidal suspensions of phosphorene. The TEM technique determines the number of layers and lateral size of phosphorus samples while transmission spectroscopy evaluates the optical absorption and bandgap of the solution. By correlating transmission spectroscopy and TEM analyses, we determined the optical bandgap of colloidal suspensions of thin phosphorus species to be 2.4 eV and direct in character. Although these solutions initially contain species of varying number of layers, further isolation via centrifugation yields suspensions of primarily monolayer and bilayer phosphorus. Most importantly, we observe that the bandgap of thin phosphorus species is dependent on the number of layers. Indeed, with this approach we can selectively isolate phosphorene suspensions of a particular thickness and onset of optical absoprtion, ranging from 2.0 eV for a suspension of semi-bulk black phosphorus (five to six layers) to 3.0 eV for a suspension of monolayer and bilayer phosphorene.

Since the discovery of graphene, layered materials have attracted great attention due to their unique electronic, optical, structural and mechanical properties. Moreover, intercalated layered compounds are thought to be good candidates for Li-ion battery electrode, thermoelectric materials, and superconductors. Recently, molybdenum disulfide (MoS₂) is of interest as a next-generation device channel material. It has been known that the weak interaction between the MoS₂ layers allows alkali metal ions to intercalate without breaking the layered structure [1]. Aiming to create hybrid inorganic/organic system by intercalation of chemical reagents such as metal ions, solvent molecules, and functional organic molecules, we have been working on intercalation into MoS₂ using liquid phase process. In this work, we discuss the mechanism of Li-intercalation reaction of MoS₂.
To afford Li-intercalated compound, MoS₂ crystals were soaked in a hexane solution of n-buthyllithium [2]. In order to investigate the structure of the intermediate lithiated phase, the intermediate was exfoliated by hydrolysis. After purification by dialysis, the dispersion was sonicated for 5 min in a sonic bath. Thus, we obtained highly dispersed MoS₂ nanosheets in water. To determine the thickness of the exfoliated MoS₂ nanosheets, we prepared MoS₂ thin films onto 300 nm thick SiO₂/Si(001) substrates. AFM observation shows that the exfoliated MoS₂ nanosheets mainly consist of two or three layers. This process can control the number of layers MoS₂ nanosheets by reaction conditions. This results suggest that the Li intercalation reaction to MoS₂ is a multi-step process the same as metal halide intercalation reaction to graphite [3]. We suppose the layer material technology opens up a new way to develop various devices.
[1] P. Joensen, R. F. Frindt, S. Roy Morrison, Mat. Res. Bull., 21, 457 (1986). [2] Y. Taguchi, R. Kimura, R. Azumi, H. Tachibana, N. Koshizaki, M. Shimomura, N. Momozawa, H. Sakai, M. Abe, M. Matsumoto, Langmuir, 14, 6550 (1998). [3] E. Stumpp, Proc. Of the Yamada conf. 105B, 9 (1981).

Quasi-two-dimensional (Q2D) transition metal dichalcogenide semiconductors offer unique electronic and optical properties. It is known that the electronic structure of Q2D molybdenum disulfide (MoS₂), which is the most popular member of the family, depends on the number of layers. Its electronic structure alters dramatically at near atomically thin morphologies, producing strong photoluminescence (PL). Developing processes for controlling the Q2D MoS₂ PL is essential to efficiently harness many of its optical capabilities. So far, it has been shown that this PL can be electrically or mechanically gated. Here, we introduce an electrochemical approach to actively control the PL of liquid-phase-exfoliated Q2D MoS₂ nanoflakes by manipulating the amount of intercalated ions including Li⁺, Na⁺, and K⁺ into and out of the Q2D crystal structure, given that Q2D MoS₂ has an extraordinary capacity to accommodate large ionic charges. Simultaneously, this ion-driven electrochemical approach reversibly transforms the semiconducting 2H MoS₂ phase into its metallic intercalated phase. This leads to the generation of sufficient free electron densities that produce plasmon resonance in the commercial wavelengths of visible and near UV regions, depending on the degrees of ion doping. The exhibition of tunable PL and plasmon resonances in Q2D materials offers the opportunity to develop novel biosensors and future nanophotonics.

Atomically thin two-dimensional (2D) semiconductors such as molybdenum disulphide (MoS₂) hold great promise in electrical, optical, and mechanical devices, and display novel physical phenomena such as the valley Hall effect. However, MoS₂ and all 2D materials are by nature extremely sensitive to environmental influences. For example, the charge carrier mobility can be strongly degraded by external sources of scattering such as surface optical (SO) phonons and charged impurities, and MoS₂ field-effect transistor (FET) performance is seen to substantially degrade over time. Here we develop a van der Waals (vdW) heterostructure device platform in which MoS₂ layers are fully encapsulated by hexagonal boron nitride (hBN), and multi-terminal contacts are formed using graphene. Two-terminal FET measurements reveal high quality electrical contacts and reduced hysteresis. The hBN provides excellent protection from environmental factors, resulting in highly stable device performance, even at elevated temperatures. Most strikingly, extrinsic scattering is dramatically reduced: multi-terminal magneto-transport measurements reveal a Hall mobility that reaches 34,000 cm²/Vs at 1.5 K, an improvement over previous reports by over two orders of magnitude. This novel device platform opens up a new way toward measurements of intrinsic properties of environmentally-sensitive 2D materials.

Two-dimensional (2D) materials, such as molybdenum disulfide (MoS₂) are interesting as building blocks of nanoelectronic devices due to excellent electrical and optical properties for many applications. Bulk MoS₂ is known to have an indirect bandgap of ~1.2 eV, although the few angstrom-thin single-layered MoS₂ has been reported to exhibit a direct bandgap of 1.8 eV. Thus, MoS₂ nanosheet has been investigated for field-effect transistors (FETs), photo-detectors, and even integrated logic circuits for NAND, NOR, inverter, and photo transistors.
In the present work, we demonstrate a multifunctional inverter type nanodevice based on a 2-D semiconductor MoS₂. The inverter device was comprised of patterned back-gate and top-gate field-effect transistors (FETs) on one substrate, which work respectively as driver and load for integrated electronic circuit with multifunctional applications of high gain logic circuit, photodetection, and humidity sensing. We used a nano transfer printing method using polydimethylsiloxane for the fabrication of patterned back-gate MoS₂ nanoflake FETs, so that they might be placed near the top gated MoS₂ FET which was initially fabricated. Electrical logic inverter at room-temperature shows a voltage gain as high as 20 at supply voltage of 5V. We characterized the inverter behavior under different V_in modulation frequencies included 1 Hz,10 Hz, 100 Hz, and 1 KHz, where the voltage (V_out) dynamics for a high frequency of 1 KHz reveals a rising time around 150 mu;s.
Moreover, When the patterned back-gate FET controls the circuit as driver to sensitively detect visible lights using its open channel, the device effectively operates as a photo-inverter detecting visible photons, because MoS₂ FETs appear very photosensitive while top-gate FET with metal gate is blind to visible lights. Using this configuration we could extract voltage signal as output which is more practical but rare to see for MoS₂ based photo-detectors. In addition, the patterned back-gate FET due to its open channel can adsorb and desorb the moisture on its surface, therefore the threshold voltage and hysteresis change. These properties make them suitable for humidity sensing, so that we could extract voltage signal as output. More details will be presented in the meeting.

Carbon nanomaterials such as zero-dimensional (0D) fullerenes, one-dimensional (1D) single-walled carbon nanotubes (SWCNTs), quasi-1D multi-walled (MWCNTs) and carbon nanofibers (CNFs), and three-dimensional (3D) graphite are derivatives of graphene&’s 2D honeycomb crystal lattice structure. The sp2 bonding of carbon atoms in graphitic carbon nanomaterials generally affords them their exceptional materials properties, such as a high-charge-carrier mobility, high-thermal conductivity and excellent mechanical strength and flexibility, which make them attractive for a range of applications, such as in electronics, photonics, sensors, and flexible electronics, to name a few. The investigation of graphene as a model 2D system has impacted a diverse array of fields spanning physics, chemistry, materials science, and engineering. While great strides have been made recently for applications that have stemmed from graphene&’s unique properties, the absence of a band-gap in graphene poses concerns for its attractiveness to enable high ON/OFF ratio transistors for digital electronics applications. Although a band-gap in graphene is induced through quantum confinement by creating graphene nanoribbons, the band-gaps nonetheless are small (up to few hundred meV) and it is challenging to maintain pristine edge chirality due to defects that are induced during nanofabrication of the ribbons. Recently, layered 2D crystals of other materials similar to graphene have been realized, which include the transition metal di-chalcogenides, that display properties ranging from semiconducting, semimetallic, metallic, and superconducting to insulating. The device applications of such systems show promising characteristics; for example transistors derived from 2D monolayers of MoS2 show ON/OFF ratios many orders of magnitude larger than the best graphene transistors at room temperature, with comparable mobilities. In this talk, an overview of new initiatives on beyond graphene materials and devices will be presented, which includes a collaborative effort between the National Science Foundation&’s Emerging Frontiers in Research and Innovation (EFRI) Program, specifically the 2-Dimensional Atomic-layer Research and Engineering (2-DARE) initiative, and the Air Force Office of Scientific Research&’s Basic Research Initiative (BRI) on 2D Materials and Devices beyond Graphene. With a total budget of approximately $45M anticipated between the NSF EFRI 2-DARE program and the AFOSR BRI program, this should help kick start important research activities in this exciting area over the next five years.

Single-layer two-dimensional (2D) materials hold great promise to foster important discoveries in the new class of low-dimensional physics and for future electronic and optoelectronic applications. Among 2D materials, graphene has been widely investigated; however, its low absorption of solar photons greatly limits its use in many applications. As a result of their intrinsic bandgap, layered transition metal dichalcogenides (TMDs) have been emerging as new 2D semiconducting candidates for device engineering. Molybdenum disulfide (MoS₂) has an interesting transition from direct bandgap (1.8eV) in monolayer MoS₂ to indirect bandgap (1.2eV) in bulk MoS₂. In order to tailor the properties of MoS₂, controlling the number of layers as well as applying external controls such as chemical doping, strain engineering, or plasma treatment have been considered in recent studies. However, external control will become the only option for tailoring the properties in single-layer MoS₂ device configurations.
In this study, we performed the systematic study of the effect of oxygen plasma exposure on photoluminescence (PL) of single-layer molybdenum disulfide (MoS₂). The properties can be tuned significantly by applying time-dependent exposure to oxygen plasma. With the variation of the plasma exposure time, the intensity of PL changes from a high to complete quenching accompanied by clear changes in Raman spectra measured with gradual reduction of MoS₂ peaks and appearance of an oxidization-induced peak of Mo-O bonds formation. Interestingly, we found a larger disturbance of Mo-S bonds in monolayer MoS₂ compared to the effect of external control on multi-layer MoS₂ from other studies. Using band structure calculations, we showed that the creation of MoO₃ disordered domains upon exposure to oxygen plasma leads to a plasma-induced direct-to-indirect bandgap transition in defected single-layer MoS_2, resulting in PL quenching and clear changes in Raman signal. The proposed approach suggests new opportunities to tailor 2D TMDs properties related future electronic and optoelectronic applications.

Over the last decade, since demonstration of the exceptional physical, chemical and electrical properties of graphene, there has been a lot of interest in two-dimensional materials. Of these new materials significant effort has been focused on transition metal dichalcogenides (TMDs) due to their various possible applications. Initial work on TMDs, similar to that of graphene, has depended on exfoliated samples. In this work we present controlled large-area synthesis of highly crystalline few to monolayers of various TMDs (MoS2, WS2, WSe2) using both solid and gas precursors. Characterization of the TMDs are done using a combination of conventional techniques such as Raman and Photoluminescence spectroscopy, Atomic force microscopy, scanning and transmission electron microscopy. New characterization tools with the capability of localized dielectric mapping (Microwave impedance microscopy) and elemental identification of individual layers and their interfaces (using Time of Flight SIMS) are demonstrated as extremely useful for studying these 2d materials. Electrical device characterization and paths of optimization are also presented.

Although MoS₂ has drawn significant research attention since the discovery that a single layer is a 2D direct band gap semiconductor, numerous questions regarding the optoelectronic properties of this intriguing system remain largely unanswered. Large discrepancies in the photoluminescence efficiency and emission spectrum are found in single layers depending upon the underlying support substrate, indicating that single layer MoS₂ is sensitive to subtle electronic properties of its surrounding environment. Furthermore, striking microscopic spatial heterogeneity is observed in the photoluminescence properties of a single layer of MoS₂, and yet little is known about the origins of this optoelectronic disorder and its broader implications on the utility of single-layer MoS₂ as an operating 2D semiconducting material. Finally, unambiguous quantification of the exciton binding energy and electronic band gap of the single-layer system remains elusive.
Using a novel hyperspectral optical microscopy technique, momentum-selective absorption spectroscopy is performed at discrete spatial points and used to construct maps of the optical absorption properties of individual single-layers of MoS₂. In contrast to traditional, non-selective absorption spectroscopy, which is sensitive to all possible optical transitions at a given energy, the momentum-selective absorption isolates and probes only the higher energy optical transitions that efficiently thermalize to the ground state exciton. This increased selectivity unveils transitions that are masked in less discriminant traditional absorption measurements, providing important information for determining the nature and position of the higher-energy exciton states and possibly the energetic position of the band-edge. Mapping the absorption spectrum over the full extent of individual MoS₂ flakes reveals that the energies of the absorption resonances exhibit a pronounced intra-flake spatial inhomogeneity that is identically mirrored in the corresponding heterogeneity of the photoluminescence energy. These correlated spatial maps of the absorption and emission resonances demonstrate that the optical absorption and radiative recombination within single-layer MoS₂ are equally disordered, providing an important clue for determining the underlying origins of the optoelectronic heterogeneity. Finally, correlated multimodal imaging with, for example, electron and atomic force microscopy of the same single-layers of MoS₂ enables a more precise determination of the physical origins of this optoelectronic disorder.

Two-dimensional (2D) atomic layers of molybdenum disulfide (MoS2) have attracted much recent attention due to their unique electronic properties. In addition to charge and spin, electrons in MoS2 monolayers possess a new valley degree of freedom (DOF) that has finite Berry curvatures. As a result, not only optical control of the valley DOF is allowed, but each valley is also predicted to exhibit a Hall effect in the absence of a magnetic field whose sign depends on the valley index. In this talk, we will discuss our recent observation of this new valley Hall effect (VHE) in monolayer MoS2 transistors. Experimentally, this is manifested as a finite anomalous Hall effect when circularly polarized light is used to preferentially excite electrons into a specific valley. We will describe the dependence of the anomalous Hall conductivity on photon helicity, photon energy, doping levels and crystal symmetry, and will compare these observations with theoretical predictions. Finally, possibilities of using the valley DOF as an information carrier in next-generation electronics and optoelectronics will also be discussed.

Photoluminescence (PL) properties of single-layer MoS₂ are shown to have strong correlations with the surrounding dielectric environments. Single-layer MoS₂ synthesized by chemical vapor deposition (CVD) was immersed into a series of non-ionic solvents with different dielectric constants, and blue shifts of up to 40 meV of the exciton or trion PL peaks were observed as a function of the dielectric constant of the environmental solvents. The trion/exciton intensity ratio was also tuned by at least one order of magnitude accordingly. These results can be explained by a scaling relationship from the dielectric screening effect of the Coulomb potential, and a revised mass action model. Our findings are helpful to better understand the tightly bound exciton properties in strongly quantum-confined systems, and provide a simple approach to the selective and separate generation of excitons or trions with potential applications in excitonic interconnects and valleytronics.

Heterojunctions between well studied materials opens up novel avenues of study for fundamental science and technological applications. Graphene, whose growth methods and properties have been heavily studied and characterized, makes a convenient framework on which to develop heterojunction devices. Bi₂Se₃ is a layered material made up of quintuple layers, which are 5 alternating layers of bismuth and selenium. By introducing 3D confinement onto the system, a band gap open up in Bi₂Se₃ nanostructures shown by photoluminescence in this work. Additionally, in this work, we will present the synthesis, characterzation, and optical and electronic properties of Bi₂Se₃-graphene heterojunctions grown via chemical vapor deposition.

Black phosphorus (BP) is a re-emerging elemental semiconductor. In its bulk form it has a small energy band gap of 0.3eV. Moreover, black phosphorus has a layered atomic structure which enables the preparation of thin layers analogous to graphene through its mechanical exfoliation. Its small energy band gap makes it a potentially promising candidate for broad-band photodetection. Here we report on the photoresponse of few- layer BP in a field-effect transistor configuration. We will present results on the responsitivity and time response of our BP-photodectectors as a function of the light wavelength and the applied electric fields. Based on our findings we will comment on the implications and future directions of black phosphorus photodetectors.

Intense efforts in the last few years have focused on the novel electronic, optical, and structural properties exhibited by molybdenum disulfide and related two-dimensional transition metal dichalcogenides. Very little is known about the dynamical electronic and structural processes that underlie these novel effects or the associated differences in the dynamics exhibited by few layer compared to bulk materials. In particular, the extreme stresses these materials are capable of withstanding point towards novel opportunities for dynamic control of these functional responses using light and other time-dependent stimuli. Although prior time-resolved optical pump-probe studies have investigated carrier relaxation and scattering processes at low photo-excitation conditions, the structural and electronic dynamics of two-dimensional systems have not been investigated in a regime associated with large amplitude strains, temperature jumps, or electronic excitation conditions. Here we report first measurements on the dynamical response of single-layer transition metal dichalcogenide MoS₂ to intense above-bandgap photoexcitation using the nonlinear-optical second-order susceptibility as a direct probe of the electronic and structural dynamics. Excitation conditions corresponding to of order one exciton per unit cell generate large amplitude increases in the second harmonic from monolayer films occurring on few picosecond timescales. These recover on 100 picosecond timescales and are reversible at megahertz repetition rates with no photo-induced change in lattice symmetry observed despite the extreme excitation conditions. These studies open up new possibilities for all-optical and reversible dynamic tuning of the electronic properties of monolayer transition metal dichalcogenides.

We investigate tightly bound excitons in monolayer molybdenum disulfide (MoS₂) and other monolayer transition metal dichalcogenites (TMDC) via low-temperature photocurrent spectroscopy of annealed suspended samples. We focus on the three most prominent excitonic features of MoS₂ spectrum: direct band-edge A- and B- excitons (at ~1.9 eV, ~2.1 eV respectively), and above-bandgap C-exciton (at ~2.9 eV). Photocurrent spectroscopy technique allows us to directly estimate binding energy of band-edge A-exciton and show that it exceeds 500meV. Using combination of experimental techniques, first-principle calculations and simple mathematical models we investigate properties of C-exciton. This state is associated with a van Hove singularity of MoS₂ and can be described as a quasiparticle with a Mexican hat-like dispersion. We use source-drain bias dependence of the photocurrent spectrum to investigate the difference between photoconversion mechanisms for A/B- and C-excitons, and study effects of environment, gating and illumination intensity on excitonic states in MoS₂ and other TMDCs.

Utilizing the inherent optical band gap present in 2 dimensional (2D) Transition Metal Dichancogenides (TMDCs) can possibly lead to devices with multifunctional applications. In particular high performance TMDC based FETs are thought of as ideal candidate for low power opto-electronics. For example, it was recently demonstrated that FETs based on few-layered WSe₂ can display room temperature hole-mobilities approaching those of hole-doped Silicon. Here we report on the photo response of few-layers of thin WSe₂ flakes exfoliated from crystals grown using chemical vapor transport technique. Field dependant electronic transport measurement (two-probe) on devices fabricated using these materials show an ambipolar behavior with typical field effect electron and hole mobility of 38 cm²V^-1S^-1 and 93 cm²V^-1S^-1 respectively at 275K. Temperature dependent (25 K < T < 275K) photoconductivity measurements investigated using a continuous laser of #120582;= 658nm (E=1.88eV), over a broad range of illuminating laser power, P (0.1 mu;W < P < 2mu;W) indicate a fractional power dependence of steady state photocurrent (I_ph) on P (I ~ P^γ, with 0.5 < γ < 1). The devices show fast photo switching with responsivities ~ 500 mA/W at 275 K. We further show that the responsivity and hence the quantum efficiency of these devices can be substantially increased upon application of gate voltage. These findings indicate the possibility of developing WSe₂ based FETs for highly efficient and tunable photo detectors and can potentially be used for several other opto-electronic applications.
This work is supported by the U.S. Army Research Office MURI grant W911NF-11-1-0362. The NHMFL is supported by NSF through NSF-DMR-0084173 and the State of Florida.

Research on van der Waals (vdW) heterostructures formed by stacking up various 2D crystals and held together by vdW interactions is an emerging field because the numerous possible interfaces may lead to new physics not necessarily associated with the constituent 2D materials and, therefore, to novel applications. Orientational alignment in vdW heterostructures is required for certain novel phenomena to arise, and can only be obtained by epitaxial growth. Superficially, one would expect crystallography and symmetry similarities between the epilayer and substrate to be important, if not determining, factors to orientational alignment. Also, in epitaxy, strong epilayer-substrate interactions are considered to enhance registry, which naturally leads to orientational alignment between the epilayer and the substrate.
Here, using selected-area low energy electron diffraction analysis, we show strict orientational alignment of chemical vapor deposition-grown monolayer hexagonal boron nitride (h-BN) crystallites with Cu(100) surface lattices of the Cu foil substrate. Despite the crystallographic similarity between graphene and h-BN, the graphene/Cu(100) system is known to assume a wide range of rotations, in sharp contrast. Our density functional theory calculations uncovered the origin of this surprising difference: the crystallite orientation is determined at the early nucleation stage by interactions between the cluster edge and the substrate. Unlike the weaker B- and N-Cu interactions, strong C-Cu interactions rearrange surface Cu atoms, resulting in the aligned geometries not being distinctive minima in total energy. The present discovery runs counter the long-held conventional wisdom that strong epilayer-substrate interactions enhance orientational alignment in epitaxy, and sheds light on the determination of orientational relation in van der Waals epitaxy of two-dimensional materials.

Two-dimensional (2D) layered materials have attracted great interest due to their different types of interlayer and intralayer bonding, which results in unusual thermal and electrical transport properties. Previous results on tungsten diselenide (WSe₂) suggest MoS₂, with a similar structure, may exhibit ultralow thermal conductivity. We present the large area growth of MoS₂ films by a scalable catalyst-free chemical vapor deposition (CVD) method. Films grown under optimal conditions were found to have layered structures oriented parallel to the substrate. The anisotropic thermal conductivities of such MoS₂ films are charaterized using time-domain thermoreflectance (TDTR) technique. More interestingly, one of the unique properties of 2D layered materials is the ability to intercalate guest ions to the van der Waal gaps. In this work, we demonstrate the intercalation of Li⁺ ions and investigate the resulting thermal conductivity changes in MoS₂ thin films.

A low absorption rate of the atomically thin layer is one of the most significant challenges to apply the 2D materials in photodetectors. Although the internal quantum efficiency of these layered materials can be high, the external quantum efficiency is attenuated by the limited absorption rate. So an internal or external gain is critical to amplify the photoresponse to a detectable level. Here three amplification mechanisms are discussed and compared: trapping states due to gating, charge carrier multiplication by ionization collision and local field enhancement by plasmonon resonance. Their effects to response time, signal to noise ratio, and spectral response are explained in details. Also the possibility to combine these internal and external gains is explored, and a proper combination can result in significant improvement on the device performances. An amplification of 100 times and a fast response time less than 80 µs is achieved.

Similar to how carbon can be sculpted into low-dimensional allotropes such as fullerenes, nanotubes, graphene, and graphene nanoribbons, our major research premise is that the connectivity of atoms for any crystalline solid can be constrained along specific axes to produce single atom thick (<1 nm) dimensionally-reduced “allotropes” with transformative physical phenomena. Here, we will describe our recent efforts on the synthesis, stability, properties and applications of germanium and tin graphane analogues. We have synthesized for the first time, mm-scale crystals of hydrogen and organic-terminated single atom thick hexagonal germanium/tin graphane analogues from the topochemical deintercalation of different Zintl phases. We will describe our recent studies on manipulating the optical and electronic properties of these systems through covalent surface modification, doping, and alloying the main group element. These materials represent a new class of covalently terminated graphane analogue building blocks and have great potential for a wide range of optoelectronic and spintronic applications.

We study the energetics and electronic properties of silicene and its composite atomic-layer materials in the framework of the density functional theory. We first discuss the stability of isolated silicene layer with holes/electrons doped to the system. In the case of hole-doped silicene, the sheet is found to become unstable with negative-frequency phonon mode. On the other hand, the electron-doped silicene is found to be stable. We then study the interlayer interaction energy between silicene and graphene and that between silicene and the hexagonal boron-nitride (h-BN) sheet, and also the effect of the interlayer interaction on the electronic properties of silicene composite materials. In the case of the compoite bilayer of silicene and graphene, due to the buckled geometry of the silicene layer, the interlayer interaction breaks the Si sublattice symmetry of silicene and therefore induce the finite gap for the silicene electronic states at the Fermi level. Finally, we discuss the edge reconstruction of silicene for both "zigzag" and "armchair" edges. It is found that the edge reconstruction is much larger in silicene than in graphene, especially for the armchair edge.

Using first principle calculations, we propose functionalized germanene (GeX,
X=H,F,Cl,Br,I,OH,CH3) as emerging nanomaterials. While germanene has no band gap, complete functionalization with H induces band gap of around 1.80 eV. 50% H
functionalization shows a dangling band at the Fermi level. GeI is a 2D Topological
Insulators (TI). GeH,GeF,GeCl and GeBr can be transformed into TI by applying
strain.The methyl-functionalized two-dimensional germanium monolayer sheets have been synthesized with a facile, one-step metathesis approach from CaGe2 crystals. We find that tensile strain can induce topological phase transition with band inversion at Gamma point. The band gap opened by spin-orbit coupling in this quantum spin Hall insulator can be as large as 0.1 eV ample for practical applications at room temperature.

Ever since the discovery of graphene, a natural two-dimensional polymer (2DP), it has become evident that macromolecular sheets consisting of covalently bonded repeating units bear many interesting prospects for nanotechnology. Up until recently, a bottom-up preparation of such structures based on a carbon scaffold has not been achieved. Here we report that noncentrosymmetric, enantiomorphic single crystals of a simple-to-make monomer^[1] can be photochemically converted into chiral 2DP van der Waals crystals on the multigram scale, and by thermal treatment also cleanly reversed back to monomer^[2]. The crystal structure of the two-dimensional polymer was elucidated by single crystal XRD which provides unequivocal structural proof of this synthetic macromolecule. The highly robust monomer crystals can be grown to sizes larger than 1 mm and the 2DP crystals can be exfoliated to nanometer-thin sheets. Both monomer and polymer crystals show pronounced second-harmonic generation which makes them interesting materials for applications in the field of nonlinear optics.
[1] M. J. Kory, M. Bergeler, M. Reiher, A. D. Schlüter Chem. Eur. J. 2014, 20, 6934 - 6938.
[2] M. J. Kory, M. Wörle, T. Weber, P. Payamyar, S. W. van de Poll, J. Dshemuchadse, N. Trapp, A. D. Schlu#776;ter, Nat. Chem.2014 accepted.

Recently, two-dimensional (2D) layered materials beyond graphene are being extensively studied. Many advantages of these materials originate from the 2D geometry. For example, the inherently low series resistance can substantially minimize the internal energy loss. Mostly importantly, the excitonic effects due to the decreased dielectric screening in 2D materials contributes significantly to the enhanced optical absorptions, which has also been experimentally observed in MoS₂ and WS₂.These merits motivate the explorations of other more “conventional” semiconductors in the 2D form for solar cell applications. One material of interest is CuInSe₂, with CuInSe₂-based solar cells among the most efficient thin-film technologies.
Here, we report, for the first time, the synthesis of single-crystal CuInSe₂ nanosheets with the thickness on the nanometer scale by a solid-state chemical reaction between Cu and single-crystal exfoliated In₂Se₃ nanosheets. Carrier dynamics in single nanosheets was studied via the measurements of the transient optical reflectivity using a femto-second optical pump-probe technique. Hot carrier cooling was suggested to dominate the carrier dynamics within a few ps following the optical excitation. The ambipolar hot carrier diffusion coefficient was obtained by spatially resolved pump-probe measurements on single nanosheets. The dependence of the hot carrier diffusion coefficient on the nanosheet thickness provides insight into the limiting mechanisms of hot carrier transport, and can be used to gauge the possibility of efficient hot carrier collection in nanostructured CuInSe₂ solar cells.