Symposium QN03—2D Materials---Tunable Physical Properties, Heterostructures and Device Applications

Neuromorphic computing systems using electronic synapses and neurons have been extensively studied in recent years due to its ability to perform analog computing for artificial intelligence. It is also expected to overcome the high energy and various limitations of today’s computing systems. Most of all analog switching device based on the two-terminal memristor, such as conductive bridging random access memory (CBRAM) and oxide based resistive random access memory (RRAM). The memristors are operated by movement of anion vacancies or forming of metal filament, which exhibit relatively good retention and endurance for neuromorphic circuits. However, since the anion vacancies or metal ions must move within the oxide by an electric field, these devices necessarily require high operating voltages of several V or more even few-nm switching oxide. The high operating voltage causes problems such as reduced overall device stability and increased device operation energy.
On the other hand, TMDC (transition metal dichacogenide), a kind of two dimensional (2D) material, has been recently developed with great interest in growth method, process technology, and so on. Especially, the TMDC growth method using the MOCVD technique provides a poly-crystalline layered film from mono-layer to few-layers with layer controllability. In addition, the transfer method using the polymer support has been intensively studied and reached to a level at which almost no defect occurs in the process.
In this presentation, we presented a memristor device using these TMDC materials and show the electrical characteristics of memristor applications. The TMDC materials effectively blocks metal ion transport due to the high bonding energy and the close packing structure of atoms within the single layer. However, the metal ions can be transported through defect sites such as grain boundaries and vacancies, thus enabling the operation of the TMDC memristor. In particular, the bi-layer TMDC, in which the defect site is effectively controlled through transfer process, has the advantage of significantly improving the device-to-device uniformity and endurance. This is because the transfer process with rotation converts most of the defect sites to point defects. In contrast, the mono-layer TMDC has high possibility of line defect such as grain boundary, so that the uniformity and endurance of memristor are not secured.
Finally, we fabricated the memristors of metal / 2L-MoS2 / metal structure using MOCVD technique and rotated transfer method. These devices show bipolar memristor behavior with a lower set/reset voltage of +0.5 V/-0.47 V than the conventional oxide based memristor. The resistance changes of the device from tens of to hundreds of ohms-um^2 is found to occur mainly in 2D materials through external voltage measurement. It also shows the possibility of analog memory whose resistance varies linearly with bias applied from 0.3 V. Also, we confirmed STDP (spike-timing-dependent plasticity) operation in a single device, and tested various applications using a cross-bar structure. This work is an important foundation for developing devices physics to understand the memristive characteristics of 2D material-based memristor and providing neuromorphic computing systems based on such memristors.

Emulation of brain-like signal processing with thin-film devices could lay the foundation for building artificially intelligent learning circuitry in future. Encompassing higher functionalities into single artificial neural elements will allow the development of robust neuromorphic circuitry emulating biological adaptation mechanisms with drastically lesser neural elements, mitigating strict process challenges and high circuit density requirements necessary to match the computational complexity of the human brain. Here, 2D transition metal di-chalcogenide (TMDC) (MoS₂) neuristors are designed to mimic intracellular ion endocytosis-exocytosis dynamics / neurotransmitter-release in chemical synapses using three approaches: (i) electronic-mode: a defect modulation approach where the traps at the semiconductor-dielectric interface are perturbed, (ii) ionotronic-mode: where electronic responses are modulated via ionic gating and (iii) photoactive-mode: harnessing persistent photoconductivity or trap-assisted slow recombination mechanisms. Exploiting a novel multi-gated architecture incorporating electrical and optical biases, this incarnation not only addresses different charge-trapping probabilities to finely modulate the synaptic weights, but also amalgamates neuromodulation schemes to achieve “plasticity of plasticity-metaplasticity” via dynamic control of Hebbian spike-time dependent plasticity and homeostatic regulation. Co-existence of such multiple forms of synaptic plasticity increases the efficacy of memory storage and processing capacity of artificial neuristors, enabling design of highly efficient novel neural architectures.

The charge density wave (CDW) phase is a quantum state consisting of a periodic modulation of the electronic charge density accompanied by a periodic distortion of the atomic lattice in quasi-1D or quasi-2D metallic crystals. Several layered transition metal dichalcogenides (TMDs) exhibit unusually high transition temperatures to different CDW symmetry-reducing phases [1-2]. For example, crystals of 1T-TaS₂ have the CDW transition between the nearly-commensurate (NC-CDW) and the incommensurate (IC-CDW) phases at temperature of 350 K. In this talk, I review our recent experimental results on controlling the CDW phase transitions with an applied electric field and the number of the atomic planes in the quasi-2D films [2]. We have demonstrated a room-temperature voltage controlled oscillator (VCO), which operates on the basis of the NC-to-IC CDW transition in 1T-TaS₂ channels, triggered by the applied voltage. We found that the “all-metallic” 1T-TaS₂ CDW devices reveal exceptional hardness against X-ray and other radiations [3]. We explained this property of the CDW devices by the high carrier concentration in all their resistive states, and the quasi-2D channel geometry. We have also shown that CDW devices can be used for transistor-less information processing [4]. In the second part of the talk, I will highlight the extension of the quasi-2D van der Waals materials research to the systems of lower dimensionality – quasi-1D van der Waals materials [5].

This work was supported, in part, by the National Science Foundation (NSF) through the Emerging Frontiers of Research Initiative (EFRI) 2-DARE project: Novel Switching Phenomena in Atomic Heterostructures for Multifunctional Applications (NSF EFRI-1433395), by the Semiconductor Research Corporation (SRC) contract 2018-NM-2796: One-Dimensional van-der-Waals Metals: Ultimately Downscaled Interconnects with Exceptional Current-Carrying Capacity and Reliability, and by the University of California – National Laboratory Collaborative Research and Training Program LFR-17-477237.

[1] R. Samnakay, D. Wickramaratne, T. R. Pope, R. K. Lake, T. T. Salguero, and A. A. Balandin, Zone-folded phonons and the commensurate-incommensurate charge-density-wave transition in 1T-TaSe₂ thin films, Nano Letters, 15, 2965 (2015).
[2] G. Liu, B. Debnath, T. R. Pope, T. T. Salguero, R. K. Lake, and A. A. Balandin, A charge-density-wave oscillator based on an integrated tantalum disulfide–boron nitride–graphene device operating at room temperature, Nature Nanotechnology, 11, 845 (2016).
[3] G. Liu, E. X. Zhang, C. Liang, M. Bloodgood, T. Salguero, D. Fleetwood, A. A. Balandin, Total-ionizing-dose effects on threshold switching in 1T-TaS₂ charge density wave devices,” IEEE Electron Device Letters, 38, 1724 (2017).
[4] A. G. Khitun, A. K. Geremew, and A. A. Balandin, Transistor-less logic circuits implemented with 2-D charge density wave devices, IEEE Electron Device Letters, 39, 1449 (2018).

Two-dimensional layered semiconductors have recently emerged as attractive building blocks for next-generation low-power non-volatile memories. However, challenges remain in the controllable sub-micron fabrication of bipolar resistively switching circuit components from these novel materials. Here we report on the scalable experimental realisation of lateral on-dielectric memtransistors from monolayer molybdenum disulfide (MoS₂) utilising a focused helium ion beam.

Site-specific localised irradiation with the focused probe of a helium ion microscope (HIM) allows for the creation of charged defects in the MoS₂ lattice, confining the damaged region to < 5 nm. The reversible drift of these locally-seeded defects in the applied electric field changes the resistance of the semiconducting channel, opening up versatile memristive functionality due to additional tuning by the field effect. The device can reliably retain its resistance ratios and SET voltages for hundreds of switching cycles at sweep frequencies of up to 2.9 V/s. High-resolution Raman and PL spectroscopy mapping has been employed to reveal the role of mobile defective sites in the switching mechanism of the device by proxy monitoring of adsorbate-enhanced luminescence.

Cryogenic charge transport studies down to 1.5 Kelvin have revealed the impact of defect drift on device operation; parametrised by large differences between the low and high resistance states in: threshold voltages, electron mobility scaling, carrier localisation lengths, Schottky barrier heights, amplitude of 1/f noise and current density saturation. In addition, an early onset metal-insulator transition occurs in the low resistance state of the memtransistor due to the high degree of reversible doping brought about by the drift of mobile n-type donors. Moreover, we demonstrate long term potentiation and depression with sharp habituation that promises rich applications in future neuromorphic architectures, as well as the ability to tune device conductance by cross-terminal stressing with perpendicular contacts.

This work advances the down-scaling progress of memristive devices without sacrificing key performance parameters such as power consumption (< 20 nW) or applicability to synaptic emulation. In addition, we push forward the understanding of resistive switching in atomically thin devices and demonstrate the utility of field-controllable dopant drift in facilitating exotic phenomena in two dimensions.

This talk will present the latest research progress on 2D memory materials and devices, otherwise known as atomristors. In particular the talk will highlight our work on fundamental research on switching mechanisms, performance limits, and the new application of zero-power non-volatile radio-freqeuncy (RF) switches. The fundamental understanding is enabled by in-situ scanneling tunneling microscopy studies, materials characterization, and first principles modeling. The monolayer memory devices offer fast switching below 15ns, relatively high retention and the prospects of high endurance with further studies. From an application perspective, the non-volatile RF switches offer superior scalability compared phase-change memory RF switches.

Lateral heterogeneities in atomically-thin two-dimensional (2D) materials provide an innovative platform to construct novel optoelectronic and electronic devices. Specifically, heterojunctions and grain boundaries (GBs) have demonstrated large on/off ratios and memristive responses. Despite the unique opportunities these systems offer, lateral heterogeneities also have the potential to induce local temperature rises and thus require careful examination to ensure device performance and reliability. In this study, we probe the temperature rise distribution across these heterogeneities in monolayer transition metal dichalcogenide (TMD) devices using nanoscale characterization techniques, including scanning thermal microscopy and high-resolution S/TEM. Our results directly demonstrate that lateral heterojunctions between MoS₂ and WS₂ do not appreciably impact the temperature rise distribution, but that GBs in MoS₂ clearly localize heat and give rise to non-uniform current densities within the device. S/TEM reveals that the atomic sublattices are stitched nearly flawlessly together across heterojunctions but can be quite defective across GBs. These results suggest that the interfacial atomic structure plays a crucial role in enabling efficient and uniform charge transport without inducing localized heating. Through ongoing work correlating electronic and thermal phenomena with the local atomic and defect structure at 2D interfaces, we aim for a more holistic understanding of charge transport across lateral heterogeneities in 2D TMD systems.

Monolayers of 2D materials have ultrahigh mechanical strength, which enables application of very large elastic strains (e.g., ~11% for MoS₂ and 25% for graphene). The ability of sustaining such large elastic strain offers unprecedented opportunities to engineer the physicochemical properties by applying mechanical strain. In this talk, I will present two examples on the effect of strain on the properties of MoS₂ layer. The first example shows that elastic tensile strain reduces the bandgap of MoS2. When a gradient strain field is applied to MoS2 monolayer, the created bandgap gradient acts as an efficient funnel of photogenerated excitons that leads to enhanced photoluminescence. The second example shows that when elastic strain is applied to the sulfur vacancy on the basal planes of monolayer 2H-MoS2, the strain modifies the local electronic structure and catalytic activity. The proper combinations of S-vacancy and strain allows us to achieve higher intrinsic activity for hydrogen evolution reaction than the edge site of MoS2.

Liquid exfoliation has become an important production technique to give access to large quantities of two-dimensional nanosheets in colloidal dispersion. Importantly, this is a highly versatile technique that can be applied to numerous layered materials. In this talk, recent advancements in the liquid-exfoliation and optical characterization of a range of 2D-materials will be summarized. Unifying principles among various classes of materials in both centrifugation and optical properties were identified recently. Materials under study include various commercially available TMDs, GaS, InSe, h-BN, hydroxides, and more exotic candidates such as RuCl₃, PdSe₂, LaSeTe₂, NiPS₃ or even novel synthetic organic sheet stacks (2D polymers and COFs).
All materials can be exfoliated and size-selected in a similar way yielding nanosheet dispersions with well-defined changes in their lateral dimensions and thickness. This allowed us to develop a model to understand the exfoliation in high energy processes in greater depth. In brief, the "exfoliation efficiency" (defined by the combination of layer number and lateral size that can be produced) is dependent on both interlayer and intralayer interactions. In addition, similar to the effects previously observed for MoS₂ and WS₂, the optical extinction and absorbance spectra show systematic changes across all material classes. This is of great practical importance as it allows for a rapid size determination via optical spectroscopy which can be used for further optimization of the sample preparation. For example, centrifugation can be designed in such a way to enrich dispersions in monolayers without significantly changing their lateral dimensions.
In addition, we show that liquid-exfoliated nanosheets and the fundamental understanding of their optical properties is an ideal foundation to track potential degradation due to reaction with water and oxygen by time and temperature dependent optical measurements of the dispersions. This in turn allows us to elaborate ways to chemically passivate defect sites and quantify the efficiency of various passivation pathways.

Two-dimensional group IV (Ge, Sn) monochalcogenides – compound analogues of phosphorene – are of fundamental interest due to their anisotropic crystal structure and predicted characteristics such as large exciton binding energies, tunable band offsets and charge separation in heterostructures, selective valley polarization, and multiferroic order in semiconductors with bandgaps in the visible and near-infrared range. To access these predicted properties and pave the way for applications of this family of 2D semiconductors, robust approaches toward single-layer group IV chalcogenides are required. However, realizing monolayers of this class of materials has proven challenging, both by top-down (exfoliation) and bottom-up (growth) approaches.
Here, we discuss results from in-situ low-energy electron microscopy that shed light on the fundamental growth mechanisms and identify the factors that complicate the synthesis of monolayers [1]. We have used this insight to determine avenues for growing ultrathin crystals with thickness down to a single layer, as well as few-layer heterostructures, e.g., between mono- and dichalcogenide phases involving the same metal species. The functional properties of these materials are explored through charge transport in devices, and in particular via measurements of light-matter interactions with nanometer resolution using cathodoluminescence spectroscopy in scanning transmission electron microscopy (STEM-CL) [2,3]. Using a tightly focused electron beam as an excitation source, STEM-CL provides spectroscopy and mapping of light emission far below the diffraction limit to address band-edge luminescence, excitonic effects, charge separation in heterostructures, and confined photonic modes in monochalcogenide mesostructures.
[1] P. Sutter and E. Sutter, ACS Applied Nano Materials 1, 3026 (2018).
[2] E. Sutter and P. Sutter, ACS Applied Nano Materials 1, 1042 (2018).
[3] P. Sutter, C. Argyropoulos, and E. Sutter, Nano Letters, 18, 4576 (2018).

Magnetism of graphene has attracted much attention because of its combined charge and spin manipulation useful for futuristic spintronic devices. In addition, the magnetism of carbon-based materials is particularly important for lightweight actuator/sensor devices and electromagnetic wave shielding in flexible electronics. The previous studies on graphene’s magnetism were mainly focused on paramagnetism and ferromagnetism that are related to electron spins, but the diamagnetism induced by the orbital motion of two-dimensional electron gas has been studied only by theoretical approaches due to the lack of measurable amount of samples. Recent advances in large-scale synthesis of graphene enabled a facile preparation of massive graphene samples by multiple stacking and transfer. Here we report a giant orbital diamagnetism in ultra-thin, lightweight, flexible, and transparent graphene films grown by chemical vapor deposition (CVD), where the diamagnetic susceptibility in perpendicular direction is measured to be ~100 times greater than the strongest diamagnetic materials such as bismuth. As a result, the graphene film also exhibits two orders stronger electromagnetic interference (EMI) attenuation characteristics normalized to the film thickness than conventional EMI shielding or absorbing materials over wide frequency ranges. The EM wave induces an oscillating magnetic moment generated by the orbital motion of moving electrons, which efficiently absorbs the EM energy and dissipate it as a thermal energy. In this case, the mobility of electron is more important than the conductivity, because the EMinduced diamagnetic moment is directly proportional to the speed of electron in an orbital motion. To control the charge carrier mobility of graphene we functionalized substrates with self-assembled monolayers (SAM). As the result, we find that the graphene showing the Dirac voltage close to zero can be more efficiently heated by EM waves. In addition, the temperature gradient also depends on the number of graphene. We expect that the efficient and fast heating of graphene films by EM waves can be utilized for smart heating windows and defogging windshields.

Many of the intriguing properties of single-crystal 2D devices rely on the relative rotational alignment between layers. For instance, in the graphene/hBN system, substantial band structure modulation can occur at specific interlayer alignments [1], but a misalignment may be beneficial if innate graphene properties are to be examined, as in the case with near-commensurate bilayer graphene showing superconductivity [2]. To allow for scalable graphene/hBN heterostructure formation, this work investigates hBN growth on single-crystal epitaxial graphene (EG) on macrostepped SiC(0001) substrates. The presented results suggest that the EG/SiC(0001) macrosteps influence hBN epitaxy such that a metastable, 30° in-plane hBN/EG rotational alignment is more favorable in certain growth regimes than the fully commensurate hBN/EG alignment, despite their similar crystal structures.

Plasma-enhanced chemical beam epitaxy (PE-CBE), an ultra-high vacuum (UHV) compatible process, was utilized to provide a clean environment for examination of the hBN structural, electrical, and chemical properties via in-situ and in-vacuo characterization methods. To determine the effect of substrate macrostep morphology, SiC (0001) substrates with a 4°-offcut toward <11-20> and nominally on-axis substrates were tested. The alignment of the hBN/EG/SiC(0001) heterostructure was studied by relating in-situ electron diffraction to nuclei edge directions found using ex-situ atomic force microscopy (AFM). Preferential alignment of the hBN nuclei edges to the SiC macrosteps was found in growths with a lower precursor flux (~1 nm/hr. growth rate), while far lower preference was found for higher-flux depositions (16 nm/hr. growth rate). In addition, cross-sectional TEM confirmed the registry and rotational alignment of the hBN to the EG/SiC substrate for both growth conditions, while plan-view TEM showed a single crystal alignment. Energy dispersive X-ray spectroscopy (EDS) during scanning TEM showed the graphene layers remained after the growth process and an atomically sharp hBN/EG interface was present. No hBN/EG intermixing between the layers was detected in either EDS or X-ray photoelectron spectroscopy (XPS). High-resolution topographic measurements and electrical measurements confirming the dielectric integrity of the hBN layer were performed using in-vacuo scanning tunneling microscopy (STM) and conducting AFM. The macrostep-directed epitaxy of hBN on EG highlighted in this work shows how different levels of commensurability can be achieved solely by tuning growth parameters during van der Waals epitaxy, thus reducing the reliance on manual rotation during film transfer and increasing the viability of scalable, single-crystal heterostructure growth.

[1] M. Yankowitz, et al., Nat. Phys. 8, 382 (2012).
[2] Y. Cao, et al., Nature, 43, 556 (2018)

Two-dimensional (2D) layered 3-rhombohedral (3R) phase transition metal dichalcogenides (TMDs) have received significantly increased research interest in nonlinear optical applications due to their unique crystal structures and the broken inversion symmetry. However, controlled growth of 2D 3R phase TMDs still remains a great challenge. In this work, we report a universal growth method for large area TMDs atomic layers with controllable crystal phases via a developed temperature selective physical vapor deposition route. Large area triangular 3R phase TMDs (e.g. WS₂ and WSe₂) layers are synthesized at a lower deposition temperature. Steady state and time-resolved photoluminescence (PL) spectroscopy and Raman spectroscopy have been used to study the unique properties of 3R phase layers due to different layer stacking and interlayer coupling. More importantly, with broken inversion symmetry, 3R phase layers show a quadratically increased second harmonic generation (SHG) intensity with respect to layer numbers. Furthermore, by polarization-resolved SHG, we observe a uniform polarization preference in bilayer and trilayer 3R phase WS₂, which could be a benefit for practical applications. Our results not only contribute to the controlled growth of 2D TMDs layers with different phases but also pave the way to the promising nonlinear optical devices.

Two-dimensional materials such as graphene and transition metal dichalcogenides (TMDCs) show intriguing properties compared to their bulk counterparts. Most of these materials crystallize in hexagonal systems with covalent bonding in two dimensions and weak van der Waals' bonding in the out-of-plane direction. Recently, two-dimensional transition metal oxides have attracted enormous attention due to their unique differences compared to the other chalcogenides in terms of structure, properties, and chemistry which are important for potential applications. Most syntheses so far have been achieved via exfoliation and sol-gel methods which result in good yield but lack size and quality control. In this work, we explore the synthesis of ultra-thin molybdenum dioxide (MoO₂) nanosheets using chemical vapor deposition. Unlike Mo-based chalcogenides, MoO₂ crystallizes in a monoclinic structure with anisotropy. We achieve single-crystal flakes with lateral size of tens of microns and thickness of ~5 nm as confirmed via atomic force microscopy. The crystal structure is verified with high resolution transmission electron microscopy and grazing incidence X-ray diffraction whereas Raman, FTIR, and X-ray photoelectron spectroscopies reveal the intricate changes in the bond vibration and binding energy shifts. The flakes show strong optical absorption at ~550 nm and high metallic conductivity as verified by transport measurements. The structural anisotropy of the as-synthesized MoO₂ flakes was verified with polarized Raman spectra for vibration modes at 205 cm^-1 and 745 cm^-1. Experiments for hydrogen evolution reaction showed good catalytic properties of the as-synthesized MoO₂ nanoflakes at -0.4V. The mechanical properties of the flakes were investigated with nanoindentation and nanoscratch experiments. The work shows a facile way of synthesizing metallic nanosheets with potential opticelectronic, catalytic, and mechanical applications.

2-Dimensional covalent organic frameworks (2D-COFs) are crystalline porous materials comprising aligned columns of π-stacked building blocks. Regarding potential applications of COFs in organic electronics and optoelectronics, access to oligothiophene-based COFs would be of great interest. However, the realization of such materials has remained a challenge, in particular concerning the laterally conjugated imine-linked COFs. We have developed a new building block design, implementing an asymmetric modification on an otherwise symmetric backbone that allows us to obtain a series of highly crystalline quaterthiophene (4T)-derived COFs with tunable electronic properties.¹ Studying the optical response of these materials, we have observed for the first time the formation of a charge transfer state between the COF subunits across the imine bond.

In this work, we have developed the first quaterthiophene-based 2D covalent organic frameworks comprising ordered π-stacked columns of 4T and pyrene moieties. Applying an asymmetric functionalization strategy of the otherwise C2-symmetric 4T backbone allowed us to incorporate alkyl chains for optimized solubility while still retaining the ability of the building blocks to stack in close-packed face-on thiophene columns. We also demonstrate that this approach provides a facile route for modifying the electronic properties of the 4T backbone via incorporation of electron-deficient subunits, thus forming donor-acceptor type chromophores. The absorption and emission spectra confirm that the 4T-based building blocks are electronically integrated into the framework. Spectral features below the energy of the π-π* transition and the analysis of the corresponding emission decay time traces reveal the fast and efficient formation of a charge transfer state between the imine-linked pyrene and quaterthiophene subunits. We believe that our new asymmetric building block design provides a general strategy for the synthesis of well-ordered COFs from various extended building blocks. This will greatly expand the range of applicable molecules for realizing frameworks with tailor-made optoelectronic properties.

[1] Keller, N.; Bessinger, D.; Reuter, S.; Calik, M.; Ascherl, L.; Hanusch, F. C.; Auras, F.; Bein, T., Oligothiophene-Bridged Conjugated Covalent Organic Frameworks. J. Am. Chem. Soc. 2017, 139, 8194-8199.

PdSe₂ is a high-mobility 2D material that has an unusual pentagonal structure, leading to buckled and puckered layers. Its strong interlayer coupling results in highly interesting and asymmetric optoelectronic properties and makes it difficult to exfoliate. However, to date it has not been directly synthesized on substrates. In this work, we report the bottom-up, vapor transport growth of few-layer PdSe₂ crystals that are rectangular and tens of microns in size. The high quality of the PdSe₂ crystals were confirmed by low-frequency Raman spectroscopy, atomic force microscopy (AFM), and high resolution annular dark field scanning transmission electron microscopy (ADF-STEM). Second-harmonic generation (SHG) mapping analysis of the PdSe₂ flakes was used to characterize their strong optical anisotropy. Field-effect transistors made from the few-layer PdSe₂ crystals revealed tunable ambipolar charge carrier conduction with a high electron mobility of ~78 cm²V^-1s^-1, which is comparable to that of exfoliated PdSe₂, indicating the promise of this anisotropic 2D material for electronics.

Synthesis science was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (BES), Materials Sciences and Engineering Division and characterizations were performed at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.

The continuous miniaturization of electronic devices has propelled modern technologies to higher performance and efficiency, but future progress requires a shift from traditional semiconductors towards new multifunctional systems and integration technologies. The development of two-dimensional (2D) van der Waals (vdW) crystals has led to the discovery of new phenomena and the realization of several functional devices. Among these vdW crystals, the metal chalcogenide InSe compound represents a promising semiconductor [1-6]. Our recent demonstration of fast broad-band photodiodes [1], field effect transistors (FETs) with electron mobility higher than in Si-FETs [3], and quantum devices [4, 6] has revealed the great potential of this material. From the growth and fundamental studies to the demonstration of prototype devices, this talk will describe how these systems can provide a platform for scientific investigations and new routes to 2D electronics and optoelectronics.

References
[1] G.W. Mudd et al., Advanced Materials 25, 5714 (2013); ibidem 27, 3760 (2015).
[2] G.W. Mudd et al., Scientific Reports 6, 39619 (2016).
[3] D.A. Bandurin et al., Nature Nanotechnology 12, 223, (2017).
[4] Z.R. Kudrynskyi et al., Physical Review Letters 119, 157701 (2017).
[5] N. Balakrishnan et al., 2D Materials 4, 25043 (2017); ibidem 2D Materials 5, 035026 (2018).
[6] J. Hamer et al., Nano Letters 18, 3950 (2018).

One current problem with the fabrication of devices using two-dimensional (2D) materials as the semiconducting channel is the formation of a high-quality semiconductor-dielectric interface. Seeding for atomic layer deposition (ALD) films is notoriously difficult on van der Waals (vdW) layers due to the absence of dangling bonds. One method to improve the seeding on vdW materials is to use plasma-enhanced atomic layer deposition (PEALD). However, PEALD often damages the top-most layers which prevents the use of PEALD on monolayer materials.
Here we investigate the use of few-layer hexagonal boron nitride (h-BN) as an interfacial buffer layer transferred on top of monolayer molybdenum disulfide (MoS₂) before deposition of the top dielectric. Top-gated monolayer MoS₂ transistors with various dielectric stacks are fabricated and the h-BN layers’ impact is characterized via optical and electrical measurements.

Two-dimensional (2D) metal dichalcogenides (MX2) are the most common type of 2D semiconductors and have shown great potential for a wide range of chemical and physical applications. However, they are limited by a low electron/hole mobility, which has been recognized as one of the major challenges impeding their further developments, and urges efforts to understand the mobility-limiting factors and discovery of higher-mobility alternatives. Here using density functional perturbation theory and Wannier interpolation of the electron–phonon matrix to study a wide range of MX2, we find that the intrinsic carrier mobility, in contrast to common belief, neither correlates with the effective mass nor can be assessed by the widely used deformation potential theory; instead it is limited by the longitudinal optical (LO) phonon scattering for most MX2, while for MoS2 and WS2, the mobility is limited by the longitudinal acoustic (LA) phonon scattering. Furthermore, we find that the LO scattering strength is strongly correlated with the magnitude of the Born effective charge, suggesting that the carrier transport is greatly affected by the electric polarization change induced by the atomic vibration. This finding enables us to use the Born effective charge to rapidly screen the 2D MX2 database for high-mobility semiconductor candidates. Our work reveals the underlying factors governing the intrinsic carrier mobility of 2D MX2, and offers a practical descriptor for discovering high-mobility candidates.

Ref: L. Cheng, Y. Liu, JACS, 2018, DOI: 10.1021/jacs.8b07871

We have performed an extensive high-throughput screening of known inorganic materials, in order to identify those that could be exfoliated into novel two-dimensional monolayers and multilayers [1]. The screening protocol first identifies bulk materials that appear layered according to a simple and robust chemical definition of bonding, determining then for all of these the binding energies of the respective monolayers, and their electronic state (metallic vs insulating), magnetic configuration (ferro-,ferri- or antiferro-magnetic), and phonon dispersions (to evaluate mechanically stability). Such protocol identifies a portfolio of close to 2,000 inorganic materials that appear either easily or potentially exfoliable, to be investigated further for promising properties. First focus has been on the determination of the effective masses and mobilities (from the full solution of the Boltzmann transport equation) for electronic applications; of topological invariants; of superconductivity and charge-density waves; and of photocatalytic parameters for water splitting. Thanks to the use of the AiiDA (http://aiida.net) materials' informatics platform, all the high-throughput calculations can be performed and streamlined in fully searchable and reproducible ways, they are stored in a database with their full provenance tree of all parent and children calculations, and can be shared with the community at large in the form of raw or curated data via the Materials Cloud (http://www.materialscloud.org) dissemination portal.

[1] Nicolas Mounet, Marco Gibertini, Philippe Schwaller, Davide Campi, Andrius Merkys, Antimo Marrazzo, Thibault Sohier, Ivano Eligio Castelli, Andrea Cepellotti, Giovanni Pizzi and Nicola Marzari, Two-dimensional materials from high-throughput computational exfoliation of experimentally known compounds, Nature Nanotechnology 13, 246–252 (2018).

Controlling the circular polarization of light is a key aspect for the development of functional nanophotonics for emerging applications. Monolayer transition metal dichalcogenides (TMDCs) are excellent candidates for spintronic, valleytronic and optoelectronic devices because the two inequivalent valleys in the Brillouin zone can give rise to valley-polarized photoluminescence (PL) under excitation with circular polarized light (CPL). However, the atomic monolayer thickness is a significant challenge for WS₂ photoluminescence emission due to its weak light-matter interaction. We investigated the feasibility of enhancing the light-matter interaction in monolayer WS₂ by means of the exciton-plasmon interaction with spin-orbit coupling of light. This objective was achieved by designing and fabricating plasmonic spiral rings with subwavelength dimensions on a hybrid substrate for WS₂, and by synthesizing high-quality WS₂. Specifically, we developed controlled growth of heterogeneous domains in CVD-grown monolayer WS₂ single crystal on SiO₂/Si substrate using tungsten oxide and sulfur precursors at T=850 degree C. Spatially resolved PL, Raman, X-ray photoelectron spectroscopy and Kevin probe force microscopy images revealed the formation of homojunctions in these single crystals which imply a direct correlation between the chemical stoichiometry and optoelectronic heterostructure. The WS₂ were integrated with a plasmonic spiral ring metasurface with subwavelength sized elements, designed to enhance the light-matter interaction. Our plasmonic chiral metasurface consisted of a gold back reflector, a 20-nm-thick SiO₂ dielectric layer followed by a monolayer heterogeneous WS₂ layer on which we fabricated a gold spiral array. By optically pumping the plasmonic chiral metasurface/WS₂ heterostructure with CPL and measuring the resulting spatially resolved CP emission (P_circ) at room temperature and low temperature (80K), we found the optical chirality of WS₂ was enhanced by more than 10 times relative to WS₂ layers. Additionally, by proper designs of the dimensions of the chiral metasurface structure, a linearly-polarized incident light can be converted to circularly-polarized light. These results suggest a new pathway of manipulating the valley-polarized PL emission in 2D materials via plasmonic chiral metasurfaces, which may be further applied to the development of valley-polariton optoelectronic devices.

2D semiconductors such as transition metal dichalcogenides (TMDC) recently attracted significant attention of the research community due to large number of fascinating optoelectronic and photocatalytic properties of these materials originating from their 2D nature. Defect engineering in these materials and defect-bound excitons are of particular importance due to their potential use as qubits for quantum computing.
Lateral heterostructures of different TMDCs is one of possible ways of creating controlled defects.
Tip enhanced optical spectroscopy (TEOS) which encompasses tip enhanced Raman scattering (TERS) and tip enhanced photoluminescence (TEPL) is a fast developing method that allows spectroscopic characterization of 2D materials with nanoscale spatial resolution and cross-correlation of this optical information with various properties revealed by scanning probe microscopy.
Here we report observation of strongly enhanced PL from WS₂ at the grain boundary in CVD-grown monolayer of WS₂–MoS₂ lateral heterostructures transferred onto gold. Scanning Kelvin probe microscopy revealed strongly profiled saw-tooth-like junction between the MoS₂ and WS₂ sub-crystals with typical size of the features being from few hundreds to over 1000 nm. TEPL measurements showed that PL peak corresponding to WS₂ gets strongly enhanced within approximately 100nm next to the heterojunction. TERS spectra suggested alloying within few tens of nanometers away from formal boundary between MoS₂ and WS₂. We argue that observed strongly localized nanoscale enhancement of the WS₂ PL is associated with defects located at heterojunction and subsequently excitons bound to these defects. We’ll discuss prospective use of such structures as single photon emitters with prospective application as qubits for quantum computing.

Atomically thin two-dimensional layered materials receive great interest because of their unique physical properties. Particularly, monolayers of semiconducting transition metal dichalcogenides, such as MoS₂, excel due to their strong light-matter interaction that is dominated by exciton phenomena [1-3]. Key to the integration of monolayer 2D materials into atomistic circuitries is the possibility to tune and engineer their properties on demand and on-chip e.g. by defects, dielectric environment and local doping [4-8]. We will introduce a methodology based on helium ion microscopy (HIM) to controllably realize single optically active emission centers in MoS₂, which show clear indications of quantum dot-like behavior. Our results demonstrate the potential of nanoscopically focused ion beams to engineer the optical properties of semiconducting 2D materials at the nanometer scale [5,9].

We thank J. Klein, A. Kuc, F. Sigger, F. Merbeler, J. Wierzbowski, M. Altzschner, F. Kreupl, K. Müller, M. Kaniber, M. Florian, M. Lorke, M. Knap, R. Schmidt, J.J. Finley, and U. Wurstbauer for a fruitful collaboration and the DFG via NIM, eConversion, and project HO3324/9-1 for financial support.

References
[1] U. Wurstbauer, et al. J. Phys. D: Appl. Phys., 2017, 50, 173001.
[2] S. Funke, et al., J. Phys.: Condens. Matter, 2016, 28, 385301.
[3] B. Miller, et al., Nano Lett., 2017, 17(9), 5229.
[4] S. Diefenbach, et al., J. Phys. Chem. C, 2018, 122 (17), 9663.
[5] J. Klein, et al., 2D Materials, 2018, 5, 011007.
[6] J. Wierzbowski, et al., Nature Scientific Reports, 2017, 7, 12383.
[7] M. Florian, et al., Nano Lett., 2018, 18, 2725.
[8] E. Parzinger, et al., Nature 2D materials, 2017, 1, 40.
[9] J. Klein, et al. 2019.

Two-dimensional (2D) materials are promising due to unique optical and electrical properties. New physics observed only in 2D materials allow for development of new-concept devices by using their valleys, tunneling effect, engineered band offset, and strong light-matter interaction. Recently, van der Waals heterostructures (vdWH) have been achieved by putting these 2D materials onto another, in the similar way to build Lego blocks. Assembled 2D blocks provide a big playground for engineers and physicists to investigate unprecedented properties of 2D materials and fabricate multi-functional optoelectronic devices. In this talk, I introduce our recent achievement in optoelectronic devices based on van der Waals heterostructures. By engineering band alignment of the stacked 2D materials and utilizing work function tunability of graphene, light emitting devices with exceptional multi-functions were fabricated. The electrically tunable light emitting transistors and tunnel devices can emit a strong light with modulation of charge (or trion) density. So, wavelength and intensity of emitted light can be controlled by electric field. Further, light emission from interlayer exciton generated in the stacked 2D semiconductors was demonstrated in device operation for the first time. Our work shows a great promise of van der Waals heterostructures for optoelectronic applications. I will also introduce our recent progress in high integration of graphene devices (or graphene leads). We developed a novel patterning technique by using XeF₂ gas and fluorographene via contacts for graphene interconnects embedded in a van der Waals heterostructure. Our via contacts can be useful for 3D integration of 2D devices for future 2D electronics.

Graphene is an ideal material for optoelectronic applications. Its photonic properties give several advantages and complementarities over Si photonics. I will show that graphene-based integrated photonics could enable ultrahigh spatial bandwidth density, low power consumption for next generation datacom and telecom applications. Heterostructures based on layers of atomic crystals have a number of properties often unique and very different from those of their individual constituents and of their three dimensional counterparts. I will show how these can be exploited in novel light emitting devices, such as single photon emitters, and tuneable light emitting diodes

Van der Waals heterostructures (vdWhs) are an emerging class of semiconductor heterostructures formed by stacking discrete monolayers of 2D materials such as the transition metal dichalcogenides (TMDs). In these structures, it is possible to create interlayer excitons (ILEs), spatially indirect, bound electron-hole pairs with the electron localized in one TMD layer and the hole in an adjacent layer. MoSe₂/WSe₂ forms a bilayer vdWh with a type II band alignment, such that the top of the valence band is formed predominantly by W states and the bottom of the conduction band by Mo states. Photoluminescence (PL) from the ILE state is expected to occur in the range of 1.35 - 1.4 eV, well separated from the room-temperature emission energies of the isolated MoSe₂ (1.55 eV) and WSe₂ (1.65 eV) monolayers. We fabricate MoSe₂/WSe₂ heterostructures from individual monolayer components synthesized by chemical vapor deposition and transferred and flattened using state-of-the-art preparation techniques [1] to insure intimate interlayer contact. We indeed observe a strong ILE feature at room temperature comparable to the emission arising from the constituent monolayers [2]. We are able to clearly resolve two distinct emission peaks in this ILE feature at low temperature, separated by 24 meV. These peaks have nearly equal intensity, indicating they are of common character, and have opposite circular polarizations when excited with circularly polarized light. Ab initio calculations successfully account for these observations – they show that both emission features originate from excitonic transitions that are indirect in momentum space and derive from the spin-orbit splitting of the conduction band, in contrast with previous work that attributes one feature to a direct transition. In addition, the electron is strongly hybridized between both the MoSe₂ and WSe₂ layers, with significant weight in both layers, contrary to the commonly assumed model. Thus, the transitions are not purely interlayer in character. We find that including interlayer hybridization is essential to theoretically determine the ILE character. The hybridized electron eigenstates are superpositions of both spin states, which tilts the electron spins, so that both ILEs are optically bright with opposite polarizations. This work represents a significant advance in our understanding of the properties of TMD heterostructures, with implications for future electronic and valleytronic applications.

[1] M.R. Rosenberger et al, ACS Appl. Materials & Inerfaces 10, 10379-10387 (2018). DOI: 10.1021/acsami.8b01224
[2] A.T. Hanbicki et al, ACS Nano 12, 4719-4726 (2018). DOI: 10.1021/acsnano.8b01369

Two dimensional transitional metal dichalcogenides (TMDs) have been widely investigated in recent years for their electronic, optical and catalytic properties as well as specific mechanical properties. In addition, the characteristics of TMDs would be determined by the layer numbers or layer stacking configurations. Thus, it is essential to design a method for developing new 2D-architecture by tailoring the properties of the material. In this work, we provided a sub 10nm localized thinning technique of atomic layers WS₂ via in-situ scanning/transmission electron microscopy (STEM/TEM). The entire process was visualized at atomic scale and conducted at high temperature. In the beginning, we shortly presented a methodical CVD system to synthesize the crystalline atomic-layers WS₂, followed by transferring the as-grown samples onto specialized TEM chips. Subsequently, evidenced by the TEM results, we successfully shaped the WS₂ layer by partially peeled off each WS₂ monolayer. Additionally, during the thinning process, the exclusive 2D-behaviors such as nanolayers migration, coalescence and etching were observed. Finally, we utilized atomic resolution STEM images to identify layer variation and structural configuration, which helped us deduce the exact atomic stacking sequence. Further EDS/EELS analysis provide elemental information to elucidate the sulfur depletion mechanism. This sculpting technique allows for sub-10nm thinning features while preserving the crystallinity, and also paves the way for the rational design of WS₂-based quantum and optoelectronic devices.

Semiconducting 2D materials, such as the Transition Metal Dichalcogenides (TMDs), have been widely explored for use in the fabrication of devices such as; field-effect transistors (FETs), sensors, and light emission devices, due to their attractive and size tuneable electrical properties. In particular, FETs manufactured around single mechanically exfoliated flakes with mobilities greater than 700 cm².V^-1.s^-1 have been widely reported. TMDs can also be handled as liquid phase dispersions, enabling the formulation of inks for use in high throughput and low cost printed electronic devices. Liquid dispersions of TMDs therefore represent an attractive alternative to current organic semiconductor technology, as mobility and on/off ratio enhancements may be possible.
However, current literature demonstrates that FETs manufactured using solution processed TMDs suffer from low mobilities and on/off current ratios. The reason for the observed shortcomings in such devices is unknown, but possible reasons can be broadly narrowed down to contamination/oxidation of the flakes, surface defects, or poor flake-flake contacts within a film of solution processed TMD material.
Here, we will present results which elucidate possible mechanisms for the poor electronic performance of solution processed TMD thin film devices. First, we have developed new methods of thin film assembly from solution processed few layer 2D material dispersions including; liquid/liquid interfacial assembly, and Langmuir-Blodgett assembly via a Pickering emulsion intermediate phase. This has resulted in average film thicknesses of less than 5 nm. Subsequently, photo and field-effect transistors were assembled around the resultant thin films. Considerable improvements to the electrical properties were achieved by the application of dielectric polymer passivation layers and careful separation of monolayer enriched material from the liquid dispersions. Alternate exfoliation routes such as chemical and electrochemical exfoliation and the effects on resulting thin film transistors have also been explored.
Revealed herein are examples of FET devices manufactured around thin films of TMDs assembled utilising various simple, low cost, and highly scalable assembly methods. This work provides insights into the manufacturability of solution processed 2D FETs by elucidating and resolving some of the failure modes associated with solution processing methods. Finally, a robust and simple all-solution processed manufacture technique for high performance thin film devices made from dispersions of 2D TMD material is demonstrated.

Atomically-thin two dimensional (2D) layered materials have exceptional mechanical flexibility. Therefore, 2D nano-sheets can be strained, stretched and folded into various origami structures and superlattices. 2D material origami structures possess unprecedented physical properties emerged from the folded edge, the interface, and the rotational angle of crystal lattices induced by the highly controllable folding direction.

In this talk, we will introduce a deterministic folding technique for creating controllable 2D material origami structures. This technique allows us to accurately fold a 2D atomic sheet at any position and direction pre-defined by standard electron beam lithography. 2D origami structures of layer-by-layer stacked heterostructure of 2D materials and large-scale CVD monolayer array will be demonstrated as examples of the scalability and controllability of this technique. We also looked deep into the molecular-level dynamics to understand the fundamental interactions underlying the folding process. Through simulation, we found the free energy of graphene increased after folding while the entropy of graphene decreased.

In addition, we will demonstrate various applications of our origami technique. 2D superlattice with controlled rotational angle can be prepared using the folding approach, which can be a powerful platform for exploring a rich array of physical phenomenon. For example, we found a pronounced Raman enhancement in a twisted graphene sample prepared by folding, suggesting the existence of Van Hove singularities. In addition, we were able to reconfigure the functionalities of 2D material electronics through folding. I will introduce a novel flash memory device built with this approach.

In summary, we developed a technique to deterministically fold 2D materials. Not only can this technique serve as a platform for studying nano-mechanics and exploring emerging physical phenomenon, but also it could be promising for realizing foldable and adaptive electronics, nano-actuating, and bio-nano interfaces.


Acknowledgment: This work is supported by the AFOSR FATE MURI program (Grant no. FA9550-15-1-0514).

Traditional silicon bipolar junction transistor (BJT) is widely used in current amplification and high power circuits. With its compatibility to planar process, BJTs with different parameters such as doping level and base width are dedicatedly designed and fabricated to achieve large desired gains. For traditional silicon BJT, gain is determined intrinsically by these parameters and nearly impossible to tune in active region. Here we report a tunable graphene-Si-graphene BJT device with graphene functioning as both emitter and collector, and a thin silicon channel in between acting as the base. An additional layer of Cr/Au is placed over the emitter and collector regions with a thin layer of top dielectrics to function as the top gate. During device operation, electrons in emitter overcome the graphene/Si Schottky barrier to diffuse into the base under a positive voltage, and are subsequently collected by collector under a large reverse bias. Uniquely, with a top gate voltage, the doping and thus Fermi level of emitter graphene could be further tuned, leading to a tunable gain of graphene BJT. Further details of device operation mechanism and device characteristics will be discussed during the conference. The tunability demonstrated here for graphene-Si-graphene BJT can also be readily applied to other type of two dimensional material–Si hybrid devices for better control and performance.

Two dimensional (2D) van der Waals bonded materials exhibit a range of electronic properties spanning from semi-metals through to wide bandgap semiconductors. Potential applications include electronics, sensors and display technologies [1]. In relation to practical device applications, research is focussed on large area growth [2 - 6], stable approaches to doping and contacting [7,8]. We report on the structural properties and carrier mobility in thin films (2-10nm) of MoS₂ grown in a commercial 300mm atomic layer deposition reactor. The MoS₂ films are grown on sapphire and SiO₂/Si substrates at temperatures in the range of 350^oC to 550^oC by a chemical vapour deposition process using Mo(CO)₆ and H₂S precursors.
Analysis of the films with Raman, X-ray photoelectron spectroscopy and electron backscattered diffraction all confirm the films as MoS₂. The carrier concentration, carrier type and carrier mobility are studied with Hall measurements. Excellent ohmic behaviour is achieved on MoS₂ (10nm, 550^oC with no post grown annealing) deposited on sapphire and a-Al₂O₃/sapphire substrates. Room temperature Hall analysis of the MoS₂ films indicates that the non-intentionally doped MoS₂ films are n-type with very low carrier concentrations of ~10¹⁴cm^-3. Comparison of the free carrier concentrations in the grown films (~10¹⁴cm^-3 ) to Hall analysis of natural and synthetic MoS₂ crystals (1x10¹⁶ to 1x10¹⁷ cm^-3) [9], is consistent with the presence of electrically active defect states in the energy gap of the MoS₂ for the polycrystalline CVD grown films [10]. MoS₂ films grown at 350^oC and subsequently annealed at 550^oC in a H₂S/H₂ ambient indicate a significant increase in the electron concentration (~8x10¹⁶ cm^-3).
The electron mobility is in the range of 3 to 17 cm²/V.s for films grown at 550^oC (with comparable values for MoS₂ films grown at 350^oC and subsequently annealed at 550^oC in a H₂S/H₂). Electron mobility values up to ~ 17 cm²/Vs for a MoS₂ grain size in the 5nm to 20nm range in this work, compared to values in the range of 25 to 30 cm²/Vs reported by K. Kang et al., [3] for monolayer MoS₂ with a grain size around 1µm. This motivates the need to investigate scattering mechanisms and the influence of grain size on electron mobility in CVD grown MoS₂, and preliminary results will be shown on the temperature dependence of electron concentration and electron mobility in the CVD grown MoS₂ films.
References
[1] Geim,A.K.& Grigorieva,I.V. Van der Waals Heterostructures. Nature 499,419–425 (2013).
[2] Lin,Y.-C.etal. Wafer-scale MoS₂ thin layers prepared by MoO₃ sulfurization. Nanoscale 4, 6637–6641(2012).
[3] Kang et al., Nature, 2015, 520, 656–660
[4] S. M. Eichfeld, L. Hossain, Y. C. Lin, A. F. Piasecki, B. Kupp, A. G. Birdwell, R. A. Burke, N. Lu, X. Peng, J. Li, A. Azcatl, S. McDonnell, R. M. Wallace, M. J. Kim, T. S. Mayer, J. M. Redwing, and J. A. Robinson, ACS Nano, 9, 2080 (2015).
[5] . M. O’Brien, N. McEvoy, N. Hallam, H. Kim, N. C. Berner, D. Hanlon, K. Lee, J. N. Coleman, and G. S. Duesberg, Sci. Rep., 4, 7374 (2014).
[6] L. A. Walsh, R. Addou, R. M. Wallace, and C. L. Hinkle, in Molecular Beam Epitaxy: From Research to Mass Production, Second ed., M. Henini, Editor, p. 515, Elsevier, New York (2018).
[7] C. Zhou, Y. Zhao, S. Raju, Y. Wang, Z. Lin, M. Chan, Y. Chai, Adv. Funct. Mater. 2016, 26, 4223
[8] Gioele Mirabelli, Michael Schmidt, Brendan Sheehan, Karim Cherkaoui, Scott Monaghan, Ian Povey, Melissa McCarthy, Alan P Bell, Roger Nagle, Felice Crupi, Paul K Hurley, Ray Duffy, “Back-gated Nb-doped MoS2 junctionless field-effect-transistors” AIP Advances, 6, 2 , 025323 (2016)
[9] Monaghan, S. Gity, F., Duffy, R., Mirabelli, G., McCarthy, M., Cherkaoui, K., Povey, I.M., Nagle, R.E., Hurley, P.K., Lindemuth, J.R., Napolitani, E. “Hall-effect mobility for a selection of natural and synthetic 2D semiconductor crystals” Proceedings of ULIS 2017, 27, 7962592 (2017)
[10] Arend M. van der Zande et al., Nature Materials, 12, 554 (2013)

Monolayer transition metal dichalcogenides (TMDs) have demonstrated great potential in next-generation electronics due to their unique optical and electronic properties. Considerable efforts have been made to achieve large-scale synthesis of high-quality TMDs monolayers. However, it remains challenging to produce uniform high-quality monolayer TMDs over large scale. Vapor phase deposition methods have been known to produce high quality TMDs suitable for large-scale device fabrication, but the nucleation and growth of 2D crystals is significantly affected by the gas flow and the geometric effect of chamber in these systems. Direct sulfurization method holds great promise in achieving large-scale synthesis, but the obtained materials suffer from small grain size and multilayer regions.
In this work, low-cost glass substrate is used to address the issues in direct sulfurization method. The synthesized monolayer MoS₂ single crystals possess the largest average crystal size (~100 μm) ever reported for MoS₂ grown by direct sulfurization method, with large-area uniformity which is only limited by the size of the substrate. We confirm that the ability of glass to incorporate pre-deposited precursors into its molten state is the key to the facile production of high-quality monolayer MoS₂ crystals. A combination of low-cost, uniformity, scalability, simplicity and high quality is achieved in our method which showed great promise for the development of wafer scale electronic devices based on 2D materials. Our work also provides a new look at the role substrates could play in the synthesis and the possibilities of using other glass or liquid substrates for the growth of high-quality 2D materials.

Molybdenum trioxide (α-MoO₃) is a 2D layered oxide that has shown suitability in electrochromic and photochromic devices owing to the tunability of its optical properties. Through reversible intercalation of zero-valent metals, the color of α-MoO₃ can be altered from transparent white to dark blue. In this study we show how intercalation also changes the elasto-optic properties of α-MoO₃ via intercalation of Sn, Cu, Mo, and Co. Brillouin spectroscopy is a non-invasive, non-contact, laser-based technique that can be used to measure the elastic properties of a material. We determine the full elastic stiffness tensor of α-MoO₃ and show how intercalated metals alter the elastic properties.

Two-dimensional layered materials (2DLM), Bi2Se3, MoSe2, Si2Te3, and GeS were intercalated with atomic manganese and investigated to 7 GPa using a diamond anvil cell and Raman spectroscopy. There have only been a few studies of intercalation compounds at high pressure. Of those, most have focused on the effects of staging. The behavior of a specific intercalant in different hosts remains unexplored. These studies demonstrate that intercalation of manganese into 2DLMs can result in the appearance of new Raman peaks with pressure, and a change in initial Raman frequency coincident with no change in the frequency shift with pressure.

In recent years, phosphorene, a two-dimensional (2D) form of elemental black phosphorus, has attracted a great deal of attention because of its unique properties and numerous potential applications. This material exhibits a highly tunable bandgap, high carrier mobility, and strong in-plane anisotropy. Its potential applications extend onto various fields, including electronic, optoelectronic and spintronic devices, sensors and actuators, as well as, different energy conversion and storage technologies. Here, we report our recent study on the fabrication, characterization, and high-pressure properties of phosphorene samples. We used a diamond anvil cell and conducted a series of in-situ Raman spectroscopy measurements at pressures up to ~10 GPa. Simultaneous theoretical calculations, performed using the density functional theory (DFT) framework, as implemented in the Vienna Ab-initio Simulation Package (VASP), were also conducted. We used a short transport growth method for obtaining bulk black phosphorus, followed by the liquid mechanical exfoliation to produce flakes of phosphorene of different layer number thickness. These samples were thoroughly characterized using an array of structural and optical methods. For the in-situ high-pressure Raman experiments, two types of flakes, one with a few layers and the other with many layers, were selected. In both cases, systematic shifting of the primary Raman modes, i.e. A_1g, B_2g, and A_2g, with increasing pressure was observed. Such shifts were also found in our theoretical calculations. The detailed behavior and the magnitude of Raman shifts directly reflected the anisotropic nature of phosphorene, as well as the difference in the number of layers. Our study provides a better understanding of the relationship between the strain and the vibrational structure in phosphorene. Together with similar studies, it can enable rational strain engineering towards additional functionalities and device applications of phosphorene and other 2D materials.

To create complex, scalable device structures from 2D materials, it is necessary for patterned growth techniques to be developed. While several methods have been developed for the seeding or exclusion of 2D nucleation on conventional substrates (eg. SiO₂, mica, sapphire), to date there have been few reports of patterned growth of one atomically thin material on top of another. Here, we demonstrate a low temperature (< 260 ^oC) and generalizable deposition technique for the selective-area growth of few-layer Bi₂Te₃ on chemical vapor deposition (CVD)-grown WS₂ single crystalline monolayers. These heterostructures are observed to grow with an epitaxial registry despite considerable lattice mismatch, and display both electronic and optical properties distinct from their constituent layers. These results are explained by Density Functional Theory calculations, which confirm that Bi₂Te₃/WS₂ possesses an electronic structure unique from the layers from which it is comprised. The nature of the interlayer electronic coupling in this novel system is explored further through computation of its band structure upon variation of both the interlayer distance and orientation.

Two-dimensional layered transition-metal dichalcogenides (TMDs) containing a Group VI metal (MoS₂, MoSe₂, WS₂, WSe₂) have been explored recently for use in a variety of devices and applications, including transistors, ultra-thin flexible electronics, sensors, drug delivery, and bio-imaging. A variety of methods have been developed for covalent and non-covalent functionalization of the relatively inert basal plane of TMDs to improve their application-specific suitability. We are investigating the use of reductant-activated functionalization to increase the coverage of functional groups on covalently modified surfaces of 1T′ MoS₂ relative to existing methods, where one-electron reductants are used as sacrificial electron donors that can transfer negative charge to the MoS₂. The reduced MoS₂ can then react with electrophiles, similar to the transfer of negative charge from freshly intercalated and exfoliated MoS₂.
Our results for reductant-activated addition of methyl and propyl groups to 1T′ MoS₂ show that we are able to increase the coverage of these groups by 50-100% and facilitate functionalization with weak electrophiles using strong one-electron reductants. We also observe that the rate of reaction increases with increased reduction potential, resulting in higher coverages under the same conditions. Coverage of functional groups was quantified using XPS and TGA, and the covalent C-S bond and carbon species observed through FTIR and solid-state ¹³C-CPMAS NMR. By enabling greater coverage and facilitating reactions with weak electrophiles, reductant-activated functionalization is a promising method to broaden the set of obtainable functionalized TMD materials.

Multilayer graphene has been considered as a promising material for various optoelectonic devices because of its tunable electronic structure under an vertical electric field (E-field). Introduction of desired E-field inside the multilayer graphene is essential to fully exploit its intriguing electronic properties. However, applying desired E-field inside multilayer graphene has been remained challenge because external E-field is mostly screened by the outer graphene layer. Here, we report a one-step chemical vapor deposition for the synthesis of Bernal-like stacked graphene that S atoms are gradientlly-doped in c-axis. The thickness and built-in E-field of synthesized graphene can be simultaneously controlled in wide range at the growth stage. We demonstrated tunability of electronic structure of synthesized graphene by dual-gate field-effect transistor and infrared absorption meausrement. Taking advantage of built-in E-field, we fabricated barristor-type graphene photodetectors that has both high phororesponsivity and on/off current ratio.

Graphene oxide (GO) serves as a precursor to various technologies, which include batteries, biosensors, solar cells, and supercapacitors. Metal hybrids of GOx enhance the sensitivity and electrical conductivity for sensing and catalytic applications. In this project we use silver and palladium nanoparticles independently to modify the surface of GOx. Silver nanoparticles exhibit optical properties that play a role in bacterial resistance, demonstrating its role in various biomedical sensors. Palladium nanoparticles are highly sensitive and functional in room temperature; making it an ideal metal for catalytic applications. Here we report the synthesis of functional graphene oxide from graphite flakes, followed by both successful carboxylation and reduction. In this experiment, carboxylation of GOx was followed by insertion of silver nanoparticles through an oleylamine ligand. Reduction of GOx resulted in a highly conductive, monolayer graphene quantum dot, which will be coated with palladium nanoparticles and utilized in catalytic applications, primarily Suzuki cross-coupling reactions. The Pd-GOx will serve as a cheaper and stable alternative as compared to traditional palladium-based catalysts used in these reactions. Initial results of our experiment will be presented.
We acknowledge funding from NSF MRI 1626326.

Infrared imaging has applications in multiple fields, including point-of-care diagnostics in medical care and gas sensing in air quality monitoring. While solutions for these applications exist, commercially available technologies are expensive due to reliance on costly materials. Here we report an inexpensive and scalable device for infrared sensing, implemented using chemical vapor deposition (CVD) grown two-dimensional materials. A MoS₂ field-effect transistor (FET) is combined with a graphene photoactive layer, allowing transport of charges through a tunable Schottky barrier. The MoS₂ FET has a lower dark current and higher on-off current ratio than graphene alone, while the graphene component permits detection of light beyond the natural absorption spectra of MoS₂. The resulting heterostructure device is capable of broadband detection. Detailed mechanism, device sensitivity, and viability for photodetector arrays will be discussed in the presentation.

Silicene, a buckled honeycomb structure of Si atoms, has received increased interest in recent years because of its exotic properties and compatibility with existing semiconductor technology. Theoretical studies have predicted silicene to be thermodynamically stable, but its synthesis remains a challenge due to the high reactivity of sp² hybridized Si structures. So far, silicene has been synthesized on a metal substrate under ultra-high vaccum. The major drawback of this method is the high reactivity of silicene surface and its strong coupling with the metal substrate. A promising alternative is the synthesis of functionalized silicene that can preserve the honeycomb structure of silicene and reduce its surface reactivity.
Functionalized silicene can be synthesized using a top-down method starting with siloxene. Siloxene is a layered, honeycomb silicon structure with hydride and hydroxide functionalization. Previous research has focused on the hydrosilylation reaction of siloxene as an exfoliation method. In our work, we show that stable dispersions of siloxene nanosheets can be synthesized via liquid-assisted exfoliation without hydrosilylation and conduct a detailed study of their structure and stability.
Electron Energy Loss Spectroscopy (EELS) and Electron Diffraction show that the structure of siloxene nanosheets is sensitive to air exposure. The nanosheets synthesized in ambient environment show amorphous silica-like characteristic while in the inert synthesis they retain the hexagonal crystalline structure. Our in-depth EELS study shows that crystalline nanosheets have a signature distinct from pure Si and SiO₂. We have also identified a method for the large-scale synthesis of these nanosheets which enables size-selection. This study, conducted in ambient environment, shows a blue shift in the band-gap and luminescence with decrease in size of the nanosheets likely due to a combined effect of oxidation and size. This method can be potentially extended to the synthesis in inert environment to yield size-selected dispersions of crystalline siloxene nanosheets.
In this work, we provide the first EELS study of siloxene nanosheets to understand its structure and stability in different environments. We have also identified a unique solution-based synthesis method and the conditions under which crystalline siloxene nanosheets can be obtained. This provides a feasible synthesis for silicon-based nanosheets and opens up the possibility of integrating them with the existing silicon technology.

By employing 2-step chemical vapor deposition (CVD) method, we realize heterostructures of molybdenum disulfide (MoS₂) with carbon nanomaterials such as graphene and carbon nanotubes (CNTs). On the first CVD step, either highly aligned CNTs (HA-CNTs) or graphene is first synthesized. On the second CVD step, mono to few-layer MoS₂ is directly grown on previously grown carbon nanomaterials forming heterostructures. In the case of HA-CNTs, MoS₂ grows preferentially along CNTs, forming mixed-dimensional heterostructures, which become visible as line-like features under an optical microscope. Post-growth characterizations such as atomic force microscopy, Raman and PL measurements confirm the formation of heterostructures with mostly few-layer crystalline MoS₂ flakes along HA-CNTs with low defect densities. On the other hand, vertical heterostructures of graphene/MoS₂ are prepared by the CVD deposition of triangular MoS₂ flakes on the graphene surface. Efficient charge transfers from MoS₂ to carbon nanomaterials are found on both types of heterostructures. By utilizing efficient light absorption and charge transfer in MoS₂ with high mobility channels (CNT or graphene), we demonstrate photodetectors with high responsivities using both types of heterostructures. Strategies to improve the temporal responses of photodetectors will also be discussed.

Graphene has been used in supercapacitors as an electrode material in recent times due to its high surface area. The reduced graphene oxide (rGO) has a high surface area, which is a key parameter for high capacitance. In addition, there is another parameter i.e. equivalent series resistance (ESR), which influences the performance of supercapacitors. ESR should be controlled, particularly at high scan rates, for the high capacitance of the electrical double layer capacitors (EDLC). The resistance at the interface between the current collector and active electrode material is the major component of ESR. In this report, chemical vapor deposition (CVD) method grown graphene has been introduced between the rGO and Ni-foil to minimize this resistance. The rGO having the surface area, ~367 m²g^-1, has been used as an active electrode material and Ni-foil as the current collector. The CVD-graphene, grown directly on Ni-foil at various cooling rates i.e. 20, 40 and 100 ⁰C/min. during the growth process, has been used between rGO and Ni-foil to fabricate electrodes for optimizing the supercapacitor properties. The electrodes with CVD-graphene grown at faster cooling rates exhibited better electrochemical characteristics even at high scan rates. The specific areal capacitance of electrodes with CVD-graphene grown at 40 ⁰C/min. cooling rate is 58.3 μFcm^-2 (213.8 Fg^-1) and 39.1 μFcm^-2 (143.3 Fg^-1) at scan rates 0.05 and 10 Vs^-1, respectively. The electrodes with CVD-graphene grown at 100 ⁰C/min., exhibit better stability, and low voltage drops at various current densities while maintaining quasi-rectangular shapes in C-V characteristics. The introduction of CVD-graphene between the rGO and Ni-foil reduced the ESR significantly from 0.8 Ω to ~0.38 Ω at high frequencies.

Changes in the concentration of volatile organic compounds (VOCs) in a person’s breath can be used to detect cancer, liver failure, and other medical issues. Graphene and its derivatives have been studied for applications in VOC chemiresistors because of their enhanced limit of detection over other materials. Most of these chemiresistors are fabricated with graphene nanoplatelets and its functionalized derivatives, which have lower carrier mobilities than a monolayer of graphene grown by chemical vapor deposition. This difference in mobility suggests that the integration of a monolayer of graphene into a functional sensor could be more sensitive than one using graphene nanoplatelets, allowing for more precise detection of VOCs. As the intrinsic response of graphene to most analytes is low, modification of the basal plane is necessary. However, current functionalization techniques can be irreproducible between different growths of graphene, due to the differences in grain size and defect density. New methods to increase the response of a monolayer of graphene, such as integration with a polymer or low-volatility organic molecules, are needed to avoid the difficulties of specific, identical functionalization. In addition, as detection with a monolayer of graphene is based on the adsorption of analytes on the exposed portions of the basal plane, integrating a monolayer of graphene into a chemiresistor limits the available surface area for typical sensor response. Thus, good chemiresistors involving monolayer graphene must use methods beyond surface adsorption to detect a sensor response. This work demonstrates that a highly sensitive chemiresistor can be made from the integration of a monolayer graphene-polymer layer with a patterned substrate, relying on changes in lattice strain due to the perturbation of a polymer overlayer to yield a significant response where signal was previously low or undetectable. Graphene-polymer layers on the patterned substrate showed responses over 1% for polar analytes, including acetone, ethyl acetate, and tetrahydrofuran, with sensitivities for each analyte between 0.4 and 0.6. In comparison, sensors fabricated from graphene alone or a graphene-polymer layer on a flat, unpatterned substrate showed little to no response. Further tests on substrates with patterns of varying height and pitch have shown that systematic changes to how much the graphene is suspended off the substrate surface controls the degree of response from the chemiresistors. In addition, altering the polymer overlayer on the graphene also allows for control over the sensor response to specific analytes. When the patterned substrates were tested with a poly(ethylene co-vinyl acetate) overlayer, an enhanced response to polar analytes was detected, as expected. These measurements are also repeatable as the full magnitude of sensor response can be recovered after multiple rounds of testing. This sensor design shows promise for improving the response of graphene-polymer chemiresistors for use in biomedical applications.

Before the discovery and creation of molecularly thin, stable, semiconducting transition metal dichalcogenide (TMD) sheets, FETs with multiple independent gates were realized mostly by use of ultra-thin SOI channels. Due to the van der Waals attraction between a single layer sheet of TMDs (e.g. MoS₂) and atomically smooth, defect free, insulating hexagonal boron nitride (h-BN), single layer MoS₂ FETs with two gates (above and below the channel), to independently control the charge concentration in the channel, can be fabricated using standard dry transfer techniques developed for the construction of van der Waals bonded FETs made from TMDs and h-BN [1-2]. The functionality of these devices as circuit elements is of great interest.

In this work, we focus on fabricating single layer MoS₂ FETs with two independent gates for use as front-end single transistor mixers to up-convert weak, low frequency input signals to a higher frequency of around 5 kHz. Chopper stabilization is a circuit technique that uses frequency modulation to up-convert a low frequency input signal, such as a brainwave, to a higher frequency (> 5kHz) where there is a reduction in 1/f noise before further amplification (i.e. using an op-amp). We show that a single layer, double gate MoS₂ FET can be used as a chopper amplifier. The input signal of around 100Hz is applied to one of the two gates of the MoS₂ FET. A modulation frequency of a few kHz is applied to the second gate. Heterodyning is observed albeit with a low conversion gain due to linearity created by large contact resistance. We improve the non-linearity (I ~ V_gate², saturation characteristics) by increasing the channel length of our FETs so that the contact resistance does not dominate the total resistance. We note that a differential pair of double gate MoS₂ FETs requires less overall FETs (two) as compared to the traditional chopper consisting of four silicon FETs. The pristine interfaces between MoS₂ and h-BN could result in less noise caused by interface states as compared to traditional semiconductors. We also note that due to the van der Waals bonding of MoS₂ and h-BN, these two-dimensional materials can be fabricated on a variety of substrates, making them of interest for front-end input signal frequency modulation. A biomedical signal may be acquired by use of an electrode gel sticking to a part of the body. Along with using two traditional metal gates to modulate the charge of the MoS₂ channel, we will show results of an single transistor MoS₂ chopper amplifier in which one of the gates is a traditional metal gate while the other is replaced with an ion-gel for charge modulation via ionic-gating. By applying a low frequency input signal to the ion-gel gate while applying a few kHz signal to the traditional metal gate to show up-conversion of the input signal, we demonstrate the double gate MoS₂ FET’s broader applications as a chopper amplifier to up-convert biomedical signals. Additionally, we present a new double gate MoS₂ FET device structure for reducing contact resistance. From the necessity for electrostatic gating to reduce source/drain contact resistance in intrinsically low doped TMD FETs, we fabricate a double gated FET in which MoS₂ is both on top and bottom of the source and drain metal, thereby resulting in electrostatic doping of the contact regions from both the top and bottom gate rather than only one gate as in the traditional double gate MoS₂ FET structure. This new structure is made possible due to the van der Waals attraction between TMD and h-BN layers; such a structure could not be fabricated easily using conventional semiconductor materials.

[1] Movva, HCP, et al. "High-Mobility Holes in Dual-Gated WSe₂ Field-Effect Transistors." Acs Nano, vol. 9, no. 10, 2015, pp. 10402-10410.
[2] Larentis, S., et al. "Reconfigurable Complementary Monolayer MoTe₂ Field-Effect Transistors for Integrated Circuits." Acs Nano, vol. 11, no. 5, 2017, pp. 4832-4839.

We present the results of a density functional theory high-throughput screening approach for identifying potential solar cell materials. In order to evaluate a materials efficiency, we calculate spectroscopy limited maximum efficiency (SLME) using frequency dependent dielectric function data obtained using meta-GGA TBmBJ formalism. Additionally, we use effective carrier mass information to further screen candidate materials. Finally, we present the results of a specific example screening case for finding potential 2D-bulk materials with high-SLME. Out of 30000 materials in the JARVIS-DFT database, we have 15000 TBmBJ frequency dependent dielectric function data. The dielectric functions were used to calculate SLME. We further investigate the elements that contribute towards high efficiency solar cell materials. We find elements such as Pd, Cu, In, Se, S are some of them. Furthermore, we screened 58 2D-bulk materials with high-SLME, low effective mass, and energy above convex hull <0.1 eV/atom.

The quest towards miniaturization and smart futuristic technologies in electronic, optoelectronic, healthcare and environmental technologies has fuelled research in material science and engineering. The memristive/ resistive switching devices are a promising candidate for upcoming innovations in electronics with high packing density. Recently, it is being envisioned that an optical control over memristors may open new paradigms in optoelectronic devices for photonic integrated circuits, optical fibre telecommunications, neuromorphic computing and artificial intelligence. Graphene and ZnO nanostructures as optically active semiconductors have already been studied in designing memristor/ resistive switches as well as photodetectors, individually. This encouraged us to further design a photomemristor using the two. We have prepared ZnO /rGO heterostructure using wet chemistry and spin coating the materials on ITO coated glass substrate. In heterostructure device, ZnO acts as a photoconducting layer, whereas, rGO film serves as a light absorbing material and an efficient (top) transport layer that facilitates the movement of charge carriers across the interface. The device showed a resistive switching behaviour with I_LHS/I_HRS ratio of ~ 10¹ and photoswitching with I_ON/I_OFF ratio of ~ 10¹ under 370 nm illumination. The results indicate that ZnO/rGO heterostructures may form promising photomemristors.

Molybdenum disulfide (MoS₂), a semiconductor in the family of transition metal dichalcogenide (TMD) materials, has recently drawn great research interest in a wide range of applications such as electronic transistors, light sensing, batteries, and hydrogen production. However, the limited control over the geometry, location, yield, and size distribution of the products using current synthesis methods has severely limited their large-scale applicability. Herein, we identify the ability to use metal (e.g., Au) nanoparticles to seed the growth of MoS₂ layered structures (including lateral monolayer and vertical wings) and thereby provide a means to achieve programmable and controllable synthesis. In this study, pre-patterned Au seeds are used as heterogeneous nucleation sites to induce the formation of desired geometries of MoS₂ layers via chemical vapor deposition. Our experimental and theoretical results shed light on the growth mechanism driving the formation of MoS₂ monolayers at these sites, revealing that the “seeding effect” originates from the favorable formation energy of MoS₂ on the Au surface. A field-effect transistor with a pre-designed MoS₂ monolayer channel exhibits electronic performance that compares nicely with previously reported MoS₂ monolayer devices. The vertical wing-like MoS₂ layers, with an abundance of edge-terminated active sites, are found to exhibit dramatically improved electrocatalytic activity for the hydrogen evolution reaction. We believe this study contributes fundamental insights into controlled synthesis of layered TMDs, making integration of these materials into emerging electronic and energy systems more attainable.

We present a paradigm for encoding strain into two dimensional materials (2DM) to create and deterministically place single photon emitters (SPEs) in arbitrary locations with nanometer-scale precision. Our material platform consists of a 2DM placed on top of a deformable polymer film. Upon application of sufficient mechanical stress using an atomic force microscope (AFM) tip, the polymer layer plastically deforms. While the AFM tip is in contact with the sample, the AFM tip forces the 2DM to deform with the polymer layer, resulting in tensile strain buildup in the 2DM. When the AFM tip is removed, the adhesive interaction between the polymer and the 2DM prevents the 2DM from relaxing back to its original, strain-free geometry, resulting in a permanent and highly localized strain field in the 2DM. We demonstrate control of indent size by modifying the applied load. Also, we show excellent repeatability of both indent shape and size. We show that SPEs are created and localized at nanoindents in a WSe₂/PMMA structure, as confirmed by antibunching measurements. The SPEs exhibit single photon emission up to 60 K, which is the highest temperature reported for SPEs in WSe₂. This quantum calligraphy allows deterministic placement and real time design of arbitrary patterns of SPEs for facile coupling with photonic waveguides, cavities, and plasmonic structures. In addition to enabling versatile placement of SPEs, these results present a general methodology for imparting strain into 2DM with nanometer-scale precision, providing an invaluable tool for further investigations and future applications of strain engineering of 2DM and 2DM devices.

Reference:
Rosenberger et al., “Quantum Calligraphy: Writing Single-Photon Emitters in a Two-Dimensional Materials Platform,” ACS Nano, 2019, https://pubs.acs.org/doi/10.1021/acsnano.8b08730.

Recently, two-dimensional monolayer chromium triiodide (CrI₃) with intrinsic magnetism was experimentally discovered, which shows promising applications in many technologies ranging from sensing to data storage where thermal transport plays a critical role. However, so far, the effect of spin-lattice coupling on the thermal transport properties has not been explored yet. In this talk, I will present the giant effect of spin-lattice coupling on the lattice thermal conductivity (κ_L) of monolayer CrI₃. The lattice thermal conductivity is more than two orders of magnitude enhanced by considering the spin-lattice coupling. The effect is found to be especially stronger for the acoustic phonon modes, which dominates thermal transport with spin-lattice coupling. The mechanism lies in the weakened phonon anharmonicity by spin-lattice coupling. The bond angle and atomic position are changed due to the spin-lattice coupling, making the structure much stiffer and more symmetric, which lead to the weaker phonon anharmonicity, and thus the enhanced thermal conductivity. This study uncovers the effect of spin-lattice coupling on the thermal transport, which would deepen our understanding on thermal transport and shed light on future research of thermal transport in magnetic materials.

Forming van der Waals multilayer structures with two-dimensional materials is a promising new method for material discovery. The weak van der Waals interlayer interaction without atomic correspondence relaxes lattice matching requirement and allows formation of high-quality interfaces with virtually any combination of two-dimensional materials. However, the weak nature of the van der Waals interaction also makes it challenging to harness emergent properties of such multilayer materials. Previous studies have indicated that in transition metal dichalcogenide bilayer heterostructures, the interlayer charge and energy transfer is highly efficient. Therefore, it is important to understand interlayer coupling in these materials and its role on charge and energy transfer. Here we show that in a MoSe2/WSe2/WS2 trilayer, the interlayer coupling is strong enough to form layercoupled states in the conduction band with the electron wave function extends to all three layers. Density functional theory calculations reveal that the layer-coupled states in Q valley are about 0.1 eV below the individual monolayer states in K valley, which is consistent with photoluminescence measurements. Transient absorption measurements show that these layer-coupled states provide a channel for ultrafast interlayer charge transfer between the top WS2 and the bottom MoSe2 layers. In this process, electrons from the K valley of the individual monolayers are scattered to the layer-coupled states in Q valley. Such a partial charge transfer allows formation of partial-indirect excitons with the holes in one monolayer while electrons shared by three layers. The formation of layer-coupled states is promising for harnessing emergent properties of transition metal dichalcogenide multilayer heterostructures. Our findings also provide new ingredient to understand charge and energy transfer in transition metal dichalcogenide heterobilayers, as the layer-coupled states can play important roles in the efficient transfer observed in these systems.

Using semiconducting van der Waals materials in the ultrathin limit as a foundation for engineering new function in electronic materials, we demonstrate the tunable properties provided by direct growth and/or assembly of 0-D materials (quantum dots and organic molecules) on 2D materials (transition metal dichalcogenides and metal monochalcogenides). We address the challenge of controlling nucleation and growth on a non-reactive (vdW) substrate by developing a range of substrate pre-treatments prior to initiating van der Waals epitaxy via metal organic vapor phase deposition. Furthermore, the alternate pulsing of precursors enables the realization of self-limited growth using physisorbed species. We use this approach to grow p-type SnS on n-type MoS₂ in both 2-D/2-D and 0-D/2-D heterojunctions. To understand the role of shape, size, structure, and composition on the physical behaviors and device characteristics of mixed-dimensional heterojunctions, we employ Kelvin probe force microscopy, conductive atomic force microscopy, and scanning photocurrent microscopy to analyze the induced photovoltage and photocurrent generated by these mixed-dimensional van der Waals heterojunctions. Further chemical tunability of the photoresponse is explored by the direct deposition and assembly of organic donor or acceptor molecules on 2-D n-type or p-type materials, respectively, with subsequent characterization by scanning probes. While the energies of molecular orbitals and band edges can in general be used to predict the response, we find that local electrostatic effects at these unusual interfaces also play a role. Finally, in operando studies of the effects of adsorbing dopant molecules and metal oxide precursors will be described to provide new insights into mechanisms that enable tunable doping.

Precise and scalable control of individual quantum states remains a significant barrier in the way of using two-dimensional (2D) materials as a device platform for quantum information. Recent works have demonstrated strain-actuated spatially varying electron/hole potential surfaces in stacked heterobilayers^1–3, where different monolayer transition metal dichalcogenides (TMDCs) are vertically assembled to form strained Moiré superlattices. Further isolating these periodic quantum wells can be achieved by deposition of small islands of a minority TMDC on top of a majority TMDC substrate; experimentally, these islands have been observed to form nanoscale triangles and hexagons. In this work, we utilize a continuum k.p model informed by existing first-principles results to search for massive Dirac fermion states in the quantum wells that naturally emerge in these heterostructures, validating these results with a tight-binding approach. We then study geometric and finite-size effects on the stability of these 2D-confined quantum disk states over an areal range of 3 to 200nm². Using the MoS₂/WS₂ heterostructure as our model system, we find that both minimum and maximum disk areas exist to support a coherent, discrete ground state in the minority TMDC, and that geometrically confined shapes (triangles) can support ground states at larger side lengths (20nm) than rounder shapes (hexagons, 10nm). As the dot area is increased, the energy of the ground state decreases until it becomes indistinguishable from the bottom of the well, at which point confinement is lost and the eigenstate space becomes continuous. The energy-size scaling relationship is found to depend on both the magnitude of the band offset and smoothness of the potential surface, which we probe by applying the Moiré superlattice boundary conditions experimentally observed in MoS₂/WSe₂ heterobilayers⁴. In this case, the degree of quantum confinement is subjected to a competition between increased potential smoothness (decreasing confinement) and increased separation of neighboring dots (increased confinement) as the Moiré cell grows in size. We summarize these scaling results in a phase diagram format that can be used to design TMDC quantum disk configurations with maximally discrete, isolated ground states for confinement of low-energy excited fermions.

References:
1. Yu, H., Liu, G.-B., Tang, J., Xu, X. & Yao, W. Moiré excitons: From programmable quantum emitter arrays to spin-orbit–coupled artificial lattices. Sci. Adv. 3, e1701696 (2017).
2. Wu, F., Lovorn, T. & Macdonald, A. H. Topological Exciton Bands in Moiré Heterojunctions. Phys. Rev. Lett. 118, (2017).
3. Johari, P. & Shenoy, V. B. Tuning the Electronic Properties of Semiconducting Transition Metal Dichalcogenides by Applying Mechanical Strains. ACS Nano 6, 5449–5456 (2012).
4. Zhang, C. et al. Interlayer couplings, Moiré patterns, and 2D electronic superlattices in MoS ₂ /WSe ₂ hetero-bilayers. Sci. Adv. 3, e1601459 (2017).

Solid state photovoltaic devices made of monolayer two-dimensional (2D) transition metal dichalcogenide (TMDs) monolayers such as WSe₂, MoS₂, and MoS₂/WSe₂have achieved 1-3% power conversion efficiency. However, photo-generated charge carriers in the middle of the device typically traverse tens of micrometers toward end-on electrical contacts at the device edge. The long transport distances parallel to the TMD layers give rise to transport-limited device physics that limit overall device performance. Here we demonstrate ultrathin liquid junction photovoltaic systems that use an iodide-based hole-selective electrolyte contact that facilitates a charge transport pathway perpendicular to the 3-atom thick TMD materials. These proof-of-concept indium doped tin oxide (ITO)|monolayer TMD|I^–/I₂|Pt cells exhibit peak internal quantum efficiencies (IQE) of 7.1%, 34.5%, and 8.2% and open circuit voltages (V_OC) of 0.46 V, 0.26 V, and 0.28 V for WSe₂, MoS₂, and MoS₂/WSe₂ heterojunctions. The photocurrent efficiency values are competitive with solid state devices. Surprisingly, we observe a non-linear scaling relation between IQE and layer thickness for homojunction and heterojunction TMDs that can be attributed to either thickness-dependent charge carrier dynamics or a buffer layer screening effect at the ITO/MX₂interface. We discuss opportunities to improve the fill factor and V_OCby interface engineering and tuning the energy level offset between the TMD composition and redox electrolyte potential.

The performance of thermoelectric materials is limited by the anti-correlation between the Seebeck coefficient and electrical/thermal conductivities. Local magnetic impurities arising from atomic vacancies in two-dimensional (2D) nanosheets are predicted to have a profound effect on charge transport due and therefore manipulate the thermoelectric properties on demand. However, 2D material systems are extremely sensitive to external fluctuations and turbostratic interfaces that conceal intrinsic properties from these impurities, therefore, at an experimental level; it is highly challenging to probe the correlation of magnetic impurities to thermoelectric parameters. In this work, we demonstrate that by placing Molybdenum Disulfide on a hexagonal Boron Nitride (h-BN) substrate, a colossal spin splitting of the conduction sub-band up to ~50.05.0 meV is observed at the sulfur vacancies, which is evident of the existence of local magnetic states. In a device environment, interacting strongly with the local magnetic defects through the Kondo effect, most of the electrons anomalously scattered back to the hot side, leading to a large anomalous positive Seebeck coefficient in highly conducting n-type MoS₂. Furthermore, modulated by an external electric field, a record power factor of 50mW/mK² was achieved. Our device is planar and compact and could become a viable option for high-performance thermoelectric devices.

There have been extensive studies on monolayer graphene where unique properties result from the symmetry of the honeycomb lattice. Few layer graphene systems are of interest as they offer interesting opportunities to study effect of electronic interactions [1] due to flatter bands while monolayer graphene was largely understood in terms of independent electron picture. In addition, few layer graphene offer an opportunity to break simple symmetries and study their consequence [2].
Berry’s phase can be probed using magneto-transport measurements. These measurements can throw up interesting questions in multiband system of few layer graphene. I will briefly discuss our efforts to measure and understand Berry’s phase in a complex multiband system.

1. Biswajit Datta, et al. Nature Communications 8, 14518 (2017).
2. Biswajit Datta, et al. Physical Review Letters 121, 056801 (2018).

Two-dimensional materials hosting optically addressable electronic states offer a unique physical platform for quantum engineering due to intrinsic spatial confinement and the ability to create multifunctional layered materials. Defect-related quantum emitters exhibiting room-temperature photoluminescence (PL) have been identified in the wide-bandgap van der Waals material hexagonal boron nitride (h-BN), although many questions remain regarding the defects’ chemical and electronic structure [1], and evidence for spin-related effects has so-far been elusive. In this talk I will present new measurements of room-temperature, spin-dependent quantum emission in h-BN [2]. Select defects exhibit striking, anisotropic PL modulations in response to an applied magnetic field, with corresponding changes in the photon emission statistics consistent with an electronic model featuring a spin-dependent inter-system crossing between triplet and singlet states. The discovery of optically addressable spins in h-BN ushers in a new platform for spin-based quantum technologies with the potential for atom-scale engineering and versatile functionality.
[1] A. L. Exarhos, D. A. Hopper, R. R. Grote, A. Alkauskas, and L. C. Bassett, ACS Nano 11, 3328 (2017)
[2] A. L. Exarhos, D. A. Hopper, R. N. Patel, M. W. Doherty, and L. C. Bassett, arXiv:1804.09061 [quant-ph]
This work was supported by the Army Research Office (W911NF-15-1-0589).

The recent discovery of 2D magnets provides new ways to study 2D magnetism by harnessing the unique features of atomically-thin materials, such as electrical control for magnetoelectronics and van der Waals (vdW) engineering for novel interface phenomena. In this talk, I will describe our recent magneto-optical spectroscopy experiments on vdW magnets. I will discuss the layered antiferromagnetic properties of CrI3 with electrical control, giant tunneling magnetoresistance through spin filtering effect in vdW magnetic tunnel junctions, magnetic dynamics in zigzag antiferromagnets, and progress on exfoliable 2D magnets with stable magnetic order near room temperature.

The ability to tune aqueous ion conduction across nanoporous membranes is a key ingredient in a wide variety of applications in nanofluidics and is a critical function of protein ion channels, which control salinity in living cells for a myriad of biological processes. Unsurprisingly, the inspiration for achieving controlled ion flow in artificial ion channels historically came from biophysics. Here, we instead draw the initial inspiration from the chemistry of complex formation between crown ether molecules and dissolved metal ions to demonstrate strain-gated permeation across locally suspended sub-nanoporous graphene and hexagonal molybdenum disulfide (MoS₂) monolayers. Using extensive all-atom molecular dynamics simulations informed by density functional theory calculations, we investigate electrostatically driven aqueous ion permeation across precisely engineered sub-nanometer pores in two-dimensional (2D) membranes under the influence of tensile strain.

In graphene, highly strain-sensitive solid-state ion channels are achieved via membrane-embedded crown ether pores, which exhibit trapping of certain aqueous cation species in the pores due to ion-crown complexation at room temperature. The resulting highly mechanosensitive permeation of potassium ions is shown to arise from a strain-controlled competition between ion-crown and ion-solvent interactions, enabled by the atomic thinness of graphene [1]. Additionally, ion trapping in crown-porous graphene is demonstrated to enable strain-controlled sieving of salt mixtures. In MoS₂ monolayers, mechanosensitive sub-nanopores selective to either anions or cations can be achieved by precise multivacancy engineering, in principle not requiring additional chemical functionalization of the pore lining, because MoS₂ contains two atom species with significantly different electronegativities and thus considerable partial electrostatic charges. The high strain sensitivity in ion permeation across sub-nanoporous MoS₂ membranes is shown to arise from the reduction of ion-pore repulsive interactions upon pore dilation caused by lattice strain. For both graphene and MoS₂-based membranes, current modulation at the order-of-magnitude level per a few percent of lattice strain is demonstrated. Pore structures similar to those considered here have been reported experimentally and thus our results potentially lay ground for a new class of solid-state membranes with ion permeation properties sensitively controlled by externally applied tensile lattice strain.

References
1. Fang, A., et al., Highly mechanosensitive ion channels from graphene-embedded crown ethers. Nature Materials, 2018. (in press).

Nanoscale bubbles (nanobubbles) are a common feature of two-dimensional materials. Nanobubbles form when surface contaminates, such as water vapor, become trapped between the top atomic layer and the substrate and aggregate, forming a pressure pocket of trapped gas. Though often undesired for electronic applications, they have gained recent scientific interest as an experimental tool to engineer novel material properties of the top atomic layer. For instance, graphene nanobubbles have been shown to host large pseudo-magnetic fields. In the case of the semiconducting transition metal dichalcogenides (TMDs), the strain from nanobubbles can have dramatic effects on the exciton species, through changing of the bandgap, but also modification of the exciton-phonon coupling, and inducement of a piezo-electric field. These effects are conventionally studied by optical methods such as Raman scattering and photoluminescence spectroscopy. However, the nano- nature of nanobubbles (~<100 nm) limits the information attainable by conventional far-field optics where the finest resolution is given by the diffraction limit (~250 nm).
To investigate the effects of the nanobubble strain on the opto-electronic properties of TMDs we have performed hyperspectral tip-enhanced photoluminescence (TEPL) and tip-enhanced Raman scattering (TERS) of nanobubbles of tungsten TMDs, WSe₂ and WS₂. Combining elastic plate theory with atomic force microscopy we calculate the corresponding strain and correlate the value with the photoluminescence and Raman signals. TEPL and TERS imaging of nanobubbles shows dramatic heterogeneity over the nanobubbles. For both WSe₂ and WS₂ on Au substrates, TEPL and TERS signals show large enhancement with increasing strain from the nanobubble. Additionally, longer wavelength bound exciton luminescence is observed in both WSe₂ and WS₂ nanobubbles at room temperature. This extreme effect of nanobubble strain on both the Raman scattering and exciton photoluminescence show the promise of using nanobubble strain to engineer the properties of excitons in monolayer TMDs for novel opto-electronic applications.

There is wide interest in using strain-engineering to modify the physical properties of 2D materials, for both basic science and applications. Deformations of graphene, for example, can lead to the opening of band gaps, as well as the generation of pseudo-magnetic fields and novel quantum Hall states. We demonstrate how controllable, device-compatible strain patterns in graphene can be engineered by depositing graphene on corrugated substrates. We discuss several techniques for creating corrugated substrates, focusing on periodic spherical curvature patterns in the form of closely packed nanospheres. We show how the smaller nanospheres induce larger tensile strain in graphene, and explain the microscopic mechanism of this. We also present experimental results demonstrating how a nearly periodic array of underlying nanospheres creates a strain superlattice in graphene, exhibiting mini-band conductance dips and magnetic field effects that depend on the magnitude of induced strain.

Two dimensional materials offer a unique set of advantages as sensors for space applications. For instance, two dimensional materials such as graphene and molybdenum disulfide have high surface-to-volume ratio and low Johnson noise, resulting in highly sensitive gas sensors. In addition, great mechanical, thermal, and electrical stability, as well as radiation hardness due to minute cross-sectional area, make them useful in space applications. Furthermore, lightweight, low power consumption and small size make the two dimensional materials suitable for targeted missions with limited resources. Two dimensional material-based environmental sensors can be used for the in situ detection of neutrals, trace gases, volatile organics and environmental parameters in a number of disciplines, including Planetary Science, Earth Science and Heliophysics. For example, these nanosensors can detect gases that can fingerprint various biological and abiotic processes on outer planetary bodies. They can be used to simultaneously detect methane, ammonia and other gases difficult to detect using mass spectrometry due to mass interference issues, and thus can be used in tandem with mass spectrometers. In addition, these miniaturized sensors can be used for quick screening of samples in sample return missions without adding significantly to the mass and power of the payload. Furthermore, Human exploration missions can benefit from these highly sensitive sensors to detect leaks of toxic gases that are commonly onboard such as hydrazine, ensuring human safety and mission success. In this talk, we will discuss our work on developing a multifunctional sensor platform with a suite of environmental sensors fabricated with a variety of nanomaterials.

Atomically thin crystals, i.e. two-dimensional (2D) materials, are known to be much more stretchable than their bulk counterparts. It is therefore possible to use mechanical strain to dramatically tune their electronic, optical, and magnetic properties. Recently, nano-bubbles and nano-tents formed by two-dimensional (2D) materials have seen a surge of interest because they are able to induce in-plane strain via out-of-plane deformation. Existing MD (molecular dynamics) simulations for those nano-structures are not sufficient in revealing the governing factors or guiding the future design. However, analytical modeling of these nano-structures is a nontrivial task attributing to the atomic thinness of 2D materials, which features intertwined in-plane and out-of-plane deformation as well as unusual 2D materials-to-substrate interaction. By examining experimentally measured profiles of a variety of nano-bubbles and nano-tents, we found all of them to collapse into a simple power law. Using the membrane limit of the Föppl–von Kármán equations, we can analytically unveil what sets the in-plane strains in terms of the shape characteristics. As a validation, our analysis can predict the Raman shift and electromagnetic responses of graphene bubbles and tents, which turn out in good agreement with our experiments. Besides pointing to an analytical solution for determining the strain distributions out of measurable profile, our analysis suggests a number of guidelines for the strain engineering of 2D materials. Crucially, we show that, both the strain level and the strain distribution can be tuned by the 2D-material-substrate interface adhesion and friction properties. By measuring the profiles of the nano-bubbles and nano-tents, interface 2D materials-to-substrate interface properties such as adhesion and shear stress can also be extracted.

Strain engineering is widely used in material science to tune the (opto-)electronic properties of materials and enhance the performance of devices. Two-dimensional atomic crystals are a versatile playground to study the influence of strain, as they can sustain very large deformations without breaking. Various optical techniques have been employed to probe strain in two-dimensional materials, including Raman and photoluminescence spectroscopy. Here we demonstrate that optical second harmonic generation constitutes an even more powerful technique, as it allows extraction of the full strain tensor with a spatial resolution below the optical diffraction limit. Our method is based on the strain-induced modification of the nonlinear susceptibility tensor due to a photoelastic effect. Using a three-point bending technique, we determine the photoelastic tensor elements of four different transition metal dichalcogenide monolayers: MoS₂, MoSe₂, WS₂ and WSe₂. Once identified, these parameters allow us to spatially image the two-dimensional strain field in an inhomogeneously strained sample.

Two-dimensional (2D) materials provide a wide range of unique properties that enable light generation, modulation, and detection for photonic applications, and could offer an alternative solution for heterogeneous integration by the advent of sufficiently strong van der Waals (vdW) force combined with mature and low-cost CMOS fabrication technology [1]. Hence, it is important to understand the interaction of 2D materials with Silicon photonic platform since their real vs. imaginary part of the optical indices decay away from the exciton resonance have different impacts on the telecom-operating photonic structures [2]. In order to achieve, a detailed understanding of the loss and index change on Si photonics, as per example, the precise and cross-contamination free transfer of 2D materials is essential. In reality, the state-of-the-art of TMDs transfer techniques are not benign with taped-out chip technology due to the inability to place a single 2D material flake on the pre-fabricated photonic chip without incurring significant cross-contamination (e.g. transfer of undesired flakes). We recently provided a solution for this challenge developing a 2D material printer enabling cross-contamination-free transfers without impacting the underlying photonic waveguide structures [3,4]. Finally, we discuss our latest results on passive coupling tunability and index determination using a semi-empirical approach with heterogeneously integrating TMDs layer on Si micro-ring resonator for broadband on-chip modulators and photodetectors with high-speed and high responsivity [5]. Our results indicate that 2D materials could be an essential building block for a fully integrated photonic circuit.
References
[1] A. Nourbakhsh et al. Heterogeneous Integration of 2D Materials and Devices on a Si Platform, 2018.
[2] R. Maiti et al. Microring Resonators Coupling Tunability by Heterogeneous 2D Material Integration, arXiv:1807.03945, 2018.
[3] R. Hemnani et al. Towards a 2D Printer: A Deterministic Cross Contamination-free Transfer Method for Atomically Layered Materials. 2D Materials, 2018.
[4] https://sorger.seas.gwu.edu/2d-material-printer.html.
[5] R. Maiti et al. Loss and Coupling Tuning via Heterogeneous Integration of MoS2 Layers in Silicon Photonics, arXiv:1810.09823, 2018.

Graphene, owing to its excellent electron transport together with a high mechanical strength, has opened up a new pathway in electronics since its discovery. Device applications of graphene, owing to its zero band gap, however, have been limited, and significant efforts have been made to introduce a band gap by including defects in pristine graphene. Patterning of graphene is also employed and graphene nanoribbons (GNR), quantum dots (GQD), nanomesh (GNM), etc. have been used for band gap opening and transform graphene from semi-metal to semi-conductor for potential applications in visible to infrared plasmonics. A sizeable band gap can be engineered based on quantum confinement induced by patterned graphene, making the patterning of graphene an important research area.
Lithographic techniques along with plasma etching are most commonly used for graphene nanopatterning. However, referring to the violent nature of plasma etching, graphene patterning based on lithographic methods lead to disordered graphene edges subsequently resulting in low carrier mobility. In addition, such lithographic methods require complex mask preparation and graphene is contaminated due to photoresist processing. Focused Ion Beam (FIB) milling is an alternative, offering direct fabrication without any mask preparation and post processing, providing opportunities for graphene patterning. FIB milling, however, induces localization effects and lateral damage in graphene, which adds to nanopatterning inaccuracies. The key challenges are precision, size tunability, high periodicity avoiding the lateral damage in nanopatterning.
In this work, a facile and systematic approach for graphene nanopatterning with ion beam is presented, and size tunable graphene nanomesh on a suspended single layer graphene is shown. The beam overlap function and step size in a gallium FIB are used for nanomesh fabrication with tunable size (30-100 nm) and period (50-100 nm). The fidelity of FIB milling for graphene patterning is demonstrated and revealed by SEM, TEM and AFM imaging. The effect of fabrication parameters is evaluated and graphene cutting conditions are optimized, in addition to patterning. This is important from a technological perspective and graphene device applications. Fabricated nanomesh is size tunable, offering potential graphene plasmonics applications. The versatile nature of proposed approach avoiding lateral damage for graphene nanopatterning allows flexibility, and we also demonstrate the fabrication of size tunable graphene nanoribbons. Gold-graphene nanomesh, based on the developed graphene nanopatterning approach, is further presented and illustrated for graphene plasmonics and optical modulation of visible light. The plasmonic response, calculated with finite-difference time-domain (FDTD) electromagnetic simulations, offered through composite graphene-gold nanomesh architecture is in the visible range, and is tunable through the geometrical parameters, providing an opportunity for FIB nanopatterning for graphene plasmonics. Gold-graphene nanomesh offers field enhancements through patterned nanomesh, which arises from localized surface plasmons at the nanostructures significantly, improve the performance of graphene plasmonic devices. In summary, a nanopatterning approach for freestanding size tunable single layer graphene nanomesh is developed and graphene plasmonics is further illustrated through the development of composite graphene-gold nanomesh for optical modulation in the visible spectrum. The demonstrated approach shows endeavour for graphene patterning and future applications of graphene for plasmonics, gas sensing, water desalination etc. with just one processing step.

In recent days, the healthcare 3.0 has been emerging as one of the most important concepts regarding human life. The primary goal of the healthcare 3.0 is to secure the longevity of human beings through close connection with technology. With the development of various healthcare devices, all kinds of signals from human body can be monitored in real time and inter-connection of the devices, or Internet of Things (IoT) can provide valuable healthcare information. Among various signals from human body, gas composition in exhaled breath can provide clues for diagnosis of various diseases. For examples, patients with asthma have nitrogen dioxide (NO₂) and those with renal disease have ammonia (NH₃) in their breath. Therefore, the gas sensors for a precise detection of the exhaled breath are highly demanded.
The most challenging point for the examination of exhaled breath is the highly humid condition of the breath higher than 90% relative humidity (RH90). The water molecules are well-known for their interference in adsorption and desorption of gas molecules on gas sensing materials. In order to overcome the interference of water molecules, high operating temperature has been adopted but that limited their practical application due to high power consumption. For the real application of the gas sensors for the breath analysis, those with a low power consumption, promising gas sensing performance under highly humid condition, portable size and simple operation should be achieved.
In that aspect, we focused on chemi-resistive type gas sensors with simple structures. The heterojunctions between tin dioxide (SnO₂) and tin sulfide (SnS) in vertically aligned 1-dimensional (1D) nanostructures were utilized for the gas sensing materials. Our SnS/SnO₂ heterojunctions prepared by a direct sulfurization of SnO₂ nanorods (NRs) exhibited superior gas response and recovery to 1 ppm NO₂ under RH90 with extremely low theoretical detection limit. Furthermore, noble metal decoration on SnS/SnO₂ NRs exhibited diverse selectivity toward various gases. The gas sensors developed in this study will provide a new perspective of heterojunction nanostructures for the examination of exhaled breath.

We demonstrate a facile method of modifying graphene oxide (GO) to enhance performance of the perovskite solar cells. Prior to doping GO into hole transport layer, chemical and morphological characteristics of the GO were tailored by irradiating it with ₆₀Co γ-ray. γ-ray irradiation on the aqueous GO induced oxidation on the GO particles, by producing reactive hydroxyl radical from water that oxidizes the pristine GO sheets. Furthermore, the treatment significantly altered the size of the GO particles and facilitated GO particles’ even distribution in the hole transport layer, (PEDOT:PSS) which resulted in enhanced hole mobility. In addition, tuning the surface chemistry of GO subsequently contributed to improved crystallinity of the perovskite layer. The combined effect of enhanced hole mobility and the perovskite’s crystallinity lead to the improved electrical parameters and power conversion efficiency of the devices, with enhanced charge extraction and generation efficiencies. [1]

(1) J. S. Cho, W. Jang, S. C. Mun, M. Yi, J. H. Park, and D. H. Wang, Carbon, 2018, 139, 564

Owing to their two-dimensional (2-D) character, transition metal di-chalcogenides of the type MX₂ (where M=W or Mo and X = S, Se or Te) possess intriguing physical properties [1]. These physical parameters strongly depend upon the number of layers. For instance, in bulk form they have an indirect band gap whereas a monolayer of TMDCs possesses a direct band gap. In WS₂ monolayer, the direct band gap is ≈ 2.1 eV indicating its use light sensitive devices. Further, due to their 2-D character these materials also possess high surface-to-volume ratio, an essential property, important for materials used for sensor devices [2]. The optical and electronics properties of these layered materials have been extensively investigated, but due to their higher cost of fabrication, gas sensing behaviors of 2-D WS₂ has not yet completely explored [3]. In view of these aspects, it is important to investigate the sensing performance of layered nanostructures of WS₂.

To overcome these limitations, here, we have fabricated a highly sensitive and selective room temperature ammonia sensor via low cost probe sonication. The sensor has detection limit as low as 2 ppm, which is below the prescribed occupation exposure limit of NH₃ gas. The response of sensor increases monotonically from 120% to 650% for ammonia concentration 2 ppm to 20 ppm, respectively. The room temperature Raman spectra from these nanostructures display dominant Raman active modes arising from in-plane motion (E¹_2g mode) and out-of-plane motion (A_1g mode) of the W and S atoms. These modes display a blue shift when sample is exposed to NH₃, indicating interaction of NH₃ molecules with WS₂. We will show that WS₂ nanostructures work effectively as ammonia sensor at room temperature. Thus, we conclude that WS₂ nanostructures based sensor is highly selective to ammonia as compared to ethanol, acetone and other gases at room temperature. Present work justifies the potential of 2-D nanostructures as high-performance gas sensors.

References:

1. Q. H. Wang, K. Zadeh , A. Kis, J. N. Coleman, M. S. Strano, Electronics and optoelectronics of two-dimensional transition-metal dichalcogenides. Nat. Nanotechnol 7 (2012) 699–712.
2. F. K. Perkins, A. L. Friedman, E. Cobas, P. M. Campbell, G. G. Jernigan, and B. T. Jonker, Chemical Vapor Sensing with Monolayer MoS₂. Nano Lett. (2013), 13, 668−673.
3. X. Li, X. Li, Z. Li, J. Wang, J. Zhang WS2 nanoflakes based selective ammonia sensors at room temperature Sensors and Actuators B 240 (2017) 273–277

We report on the electrical properties of natural quasi 1D p-type GeSe nanoflakes, which were mechanically exfoliated from GeSe single crystals by the polydimethylsiloxane (PDMS) stamp method, using a back-gate field effect transistor (FET) measured in a vacuum probe station at room temperature. In this study, we used two contact metals, including Au and Cr metals, as the Ohmic contacts to the GeSe nanoflake FETs, resulting in an Ohmic behavior with the Au contacts, with a total resistance of 5.5×10⁶ Ω. We also found that the 40-nm-thick GeSe nanoflake FET exhibits clear p-type semiconductor behavior with a field-effect mobility of ~1.0×10^-3 cm²/(Vs) and a current on/off ratio of ~10⁴ at room temperature.

The photoluminescence (PL) emerging from monolayer transition metal dichalcogenides (TMD) with their direct bang gap nature [1] and high binding energies of excitons [2] is prospective for multitude of nanoscale optoelectronic applications. However, the impeded amount of light absorption imposed by the monolayer thickness impairs the realization of viable TMD based optoelectronic devices. Using a van der Waals (vdW) heterostructure of monolayer MoS₂ stacked on top of multilayer SnSe₂, we demonstrate PL enhancement of MoS₂ by more than an order of magnitude over the entire overlap area. Counteracting the PL quenching from ultrafast charge transfer across the highly staggered conduction bands of MoS₂ and SnSe₂, PL of MoS₂ enhances by 14x with 633 nm excitation and 5x with 532 nm excitation at room temperature. Similar enhancement is observed with monolayer WS₂ stacked on multilayer SnSe₂. The enhancement factor is found to be a strong function of the thickness of SnSe₂ flake. The PL enhancement retains up to 5x even with the insertion of hBN as thick as 10 nm across MoS₂ and SnSe₂.

This kind of robust enhancement is facilitated through sensitization of PL of TMD by strongly resonant dipole-dipole coupled energy transfer [3] to monolayer TMD from SnSe₂ which acts as a scavenger of light unabsorbed by the monolayer. Multilayer SnSe₂ has a strong optical absorption with a direct bandgap of about 2 eV [4] that closely matches with A_1s exciton state energy in MoS₂ or WS_2. These kind of layered heterostructures are lucrative for exploiting the energy transfer mechanism because of efficient dipole-dipole coupling at the closest possibly spaced in-plane dipoles that match in momentum and relaxed energy conservation enabled by energy transfer from free electron-hole pairs to highly luminescent A_1s exciton states. At low temperatures (from 200 K to 4 K), we observe a quenching of the PL instead of enhancement, due to suppression of SnSe₂ absorption upon reducing temperature. Quenching is also observed when the donor material is replaced from SnSe₂ to a weakly absorbing material like TaS₂ [5]. The mechanism is further confirmed by polarization resolved PL data and the effect of different parameters on the factor of PL enhancement is analyzed upon constructing a microscopic energy transfer model. Such large area, uniform, highly efficient photoluminescence enhancement through non-radiative energy transfer is promising for the design of optoelectronic devices with layered materials.

References:
1. Splendiani, Andrea et al. Nano Letters 10.4 (2010): 1271–1275.
2. Chernikov, Alexey et al. Physical Review Letters 113.7 (2014).
3. Lyo, S.K. Physical Review B 62.20 (2000): 13641.
4. Evans, B.L. and R.A. Hazelwood. Journal of Physics D: Applied Physics 2.11 (1969): 1507.
5. Barker Jr, A.S et al. Physical Review B 12.6 (1975): 2049.

Two-dimensional materials such as MoS2 have been show to exhibit large Seebeck coefficient and are promising candidates for thermoelectric energy conversion [1]. The ab-initio calculations predict a high intrinsic thermoelectric power factor if the substrate effects are excluded [2]. But the overall power factor is typically limited by its low electrical conductivity. This is possibly due to localized states arising from the impurities and the adsorbates introduced from the substrate [2]. In this work, we measure Seebeck coefficient and electrical conductance of CVD grown MoS2 flakes on Si/SiO2 substrates [3]. We observe hopping transport from the localized states introduced by the substrate at low temperatures and low electrostatic doping. To reduce the observed defect states, we use 2D hexagonal boron nitride (hBN) as a substrate because it provides less charge inhomogeneity and atomically flat surface. The CVD grown MoS2 monolayer is placed on top of hBN using a dry transfer printing process and a PDMS stamp [4]. We observe an enhanced thermoelectric power factor of from MoS2 on hBN substrate over Si/SiO2. However, we still find the localized states to be dominating the extended states from Seebeck measurements at low temperatures and doping. We attribute these defect states observed in hBN/MoS2 to arise from the dry transfer printing process that could introduce strain heterogeneity. We systematically characterize the defect states through electrical measurements on similarly transfer printed monolayer MoS2 on Si/SiO2 substrates. This work advances the understanding of substrate effects on transfer printed 2D materials for improved thermoelectric performance.
1. Wu, Jing, et al. Nano letters 14.5 (2014): 2730-2734.
2. Babaei, Hasan, J. M. Khodadadi, and Sanjiv Sinha. Applied Physics Letters 105.19 (2014): 193901.
3. Lee, Gwan-Hyoung, et al. ACS nano 7.9 (2013): 7931-7936.
4. Lee, Chul-Ho, et al. Nature nanotechnology 9.9 (2014): 676-681.

Graphene is a 2D allotrope of carbon with fascinating chemical and physical properties. Its unique electronic structure and surface sensitivity qualify this single layer of sp²-carbon atoms as a promising material for sensor applications in the form of field-effect devices. As descendants of the ion selective field-effect transistor (FET), graphene FETs have been developed in the past years as part of the future of sensing devices. During measurements, the Dirac point of graphene is tracked. Stimuli on the surface of graphene influence the position of this point of minimal conductivity. Examples for this effect range from graphene’s pH-sensitivity [1], over charge transfer doping effects e.g. by gas molecules [2], to the detection of biomolecules based on changes in the electrostatic charge distribution on the surface of graphene [3]. Using covalent chemical functionalization the physical properties and selectivity of graphene can be tuned to realize selective sensor surfaces.[4] In this work, we focus on non-covalent interactions between redox active molecules and graphene.

Graphene has been applied as an electrode material as well. Being used as a working electrode in electrochemical studies, it shows high electron transfer rates for canonical redox couples such as [Fe(CN)₆]^3-/4-.[5] The Marcus-Gerischer theory can be applied to describe the electron transfer between a redox active species and an electrode. Therein, the position of the occupied or unoccupied states of the redox active species relative to the electronic structure of the electrode material plays a key role. Due to its unique electronic structure, graphene is particularly interesting in this context being neither metal nor semi-conductor, which separates it from other 2D materials beyond graphene. The Marcus-Gerischer theory has been successfully used to explain charge transfer doping of graphene by the O₂/H₂O redox couple.[6]

In this study, we have investigated the interfacial interactions of redox active molecules with graphene, based on measurements using electrochemically gated graphene FETs and cyclic voltammetry. [Fe(CN)₆]^3-/4- was used as the main probe. The influence of different factors was tested including the pH of the electrolyte solution, the initial oxidation state of the probe as well as the position of the standard potential of the redox couple with respect to the Dirac point. We showed that, while the electrochemical activity is independent of the initial oxidation state of the redox-couple, only the oxidized form [Fe(CN)₆]^3- exhibits charge transfer doping onto graphene. In this presentation, we explore systematically the influence of several factors on the observed doping effect.

[1] L. Zuccaro (et al.), Sci. Rep. 2015, 5, 11794.
[2] F. Schedin (et al.), Nat. Mater. 2007, 6, 652–655.
[3] L. Zuccaro (et al.), ACS Nano 2015, 9, 11166–11176.
[4] T. J. Neubert (et al.), Appl. Surf. Sci. 2017, 421, 755-760.
[5] M. Velicky (et al.), ACS Nano, 2014, 8, 10089–10100.
[6] P. L. Levesque (et al.), Nano Lett. 2011, 11, 132–137.

Ultrathin two-dimensional (2D) group VI transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS₂) have attracted much attention in opto-electronics due to their extraordinary properties. As bulk materials, they feature a layered crystal structure consisting of stacked monolayers weakly bound by Van der Waals interactions. The combination of these weak out-of-plane forces with strong in-plane chemical bonds enable ultrathin mono-and few-layered TMDs to be produced using top-down exfoliation methods, vacuum deposition approaches and, more recently, solution-based wet-chemistry methods. Vacuum deposition approaches have arisen as key methods to produce large area monolayer films, yet their high cost and substrate dependence, still remain critical concerns. In this respect could colloidal synthesis offer an inexpensive and scalable alternative as the production and processing of materials in solution opens ample possibilities for large-area device fabrication with a high degree of tunability. [1]

Over the past years, much work has been devoted to understanding photophysical processes occuring in MoS₂ flakes made by exfoliation or CVD. Using a series of spectroscopic techniques it was shown that their physics is being largely governed by enhanced many-body Coulomb interactions which result from quantum confinement and reduced screening. [2] These effects have shown to give rise to pronounced exciton binding energy and band gap renormalization effects. Such studies have not only contributed to a broader fundamental knowledge but were also pivotal for advancing the use of MoS₂ in a broad range of applications. To date, few, if any, studies have addressed the photophysical properties of MoS₂ produced by colloidal synthesis. An overview study is therefore relevant and timely as these materials are produced through an entirely different chemistry - low temperature solution phase versus high temperature gas phase (cf. CVD) and bottom-up versus top-down (cf. exfoliation). In addition would the knowledge obtained in this study undoubtedly pave the way for a more rational fine-tuning of emerging synthetic protocols.

In this contribution, we address the relaxation dynamics of few layer colloidal MoS₂ sheets after photoexcitation by means of femtosecond transient absorption (TA) spectroscopy at visible and near infrared wavelengths. We show that the transient absorbance after photoexcitation results from the joint effect of (a) a reduced oscillator strength of the so-called A and B exciton and (b) a redshift of the entire absorbance spectrum; features we assign to state filling and band-gap renormalization, respectively. Both components exhibit a close to first order decay, yet their rate constants differ markedly. All signatures of state filling disappear from the transient absorbance at a rate of 4 ps^-1. In line with the work by Cunningham et al. [3], we attribute the loss of state filling to the trapping of charge carriers - electrons in this case - in midgap states. The bandgap renormalization decays at a rate of 0.1 ps^-1, which reflects the rate of the eventual non-radiative charge-carrier recombination. Although being made by an entirely different chemistry, colloidal MoS₂ nanosheets have shown a great resemblance to the state-of-art CVD grown flakes. These results are promising for the future development of colloidal TMDs and their rational fine-tuning according through state-of-the art post-treatment techniques which would undoubtely pave the way for their incorporation in device applications.

[1] Son, D., et al., Advanced Materials, 2016, 28, 9326-9332.
[2] Pogna, E., et al., ACS nano, 2016, 10.1: 1182-1188.
[3] Cunningham, P.D., et al., JPCC, 2016, 120, 5819-5826.

The bridge between conventional topological insulators as a playground for novel physics and as a platform for novel electronic devices is control. For example, a perpendicular magnetic field applied to select areas of a thin film topological insulator can eliminate the two-dimensional surface states, creating re-writable dissipation-free edge states around those areas. This method is available in topological crystalline insulators (TCIs) such as SnTe but with different symmetry-breaking stimuli: strain or electric fields. However, the ability to grow TCIs thin and uniform enough to permit spatial control has remained elusive. This work presents a method of achieving uniform, single-domain ultra-thin SnTe films on SrTiO₃ substrates down to 5nm. X-ray diffraction and atomic force microscope confirm that an optimized molecular beam epitaxy approach yields films sufficiently thin and uniform to create a continuous two-dimensional surface state. A quantum transport effect, consistent with a 2D topological state, is observed in magnetotransport varying with film thickness. To identify the next steps towards TCI state control, details of the topological states as a function of film thickness are extracted by fitting the quantum transport to theory.

In this work we present a facile vapor phase assisted growth approach to synthesize MoS₂ monolayers on Si/SiO₂ substrates, by using H₂S for sulfurization of MoO₃ precursor films. We describe the tuning of the most important process parameters in order to obtain the desired monolayers. These are the reactor temperature and a specific time delay for the reaction onset, which is realized by a delayed H₂S injection in the reaction chamber. Finely tuning these parameters allows guiding the process towards a more favorable chemical vapor-phase reaction pathway.
The influence of the processing parameters on the morphology, homogeneity, and optical properties of the obtained material are investigated. XPS analysis was performed to corroborate the proposed vapor-phase reaction mechanisms. Raman and photoluminescence (PL) spectroscopy were performed on the micrometer to millimeter scale to confirm the large scale formation of MoS₂ monolayer material. It was found that optimum material quality is reached in the temperature range of 950 to 1000 °C. The obtained samples were compared with exfoliated MoS₂ monolayers, showing very similar PL yield for both sample types, demonstrating a good optical quality of the as-grown monolayers. The influence of the processing parameters on the layer morphology and homogeneity in the nanometer and micrometer regime was further investigated by high resolution probing techniques such as TEM and AFM, indicating possibilities to control the resulting heterogeneity of the layers on the nanometer scale.

Graphene-on-silicon (G/Si) solar cell has attracted numerous interest due to its potential to be an alternative to the Si wafer-based photovoltaic technologies. However, such heterojunction photovoltaic cells suffer from long-term instability in power conversion efficiency. This is mainly due to the interfacial charge carrier recombination owing to the low G/Si Schottky barrier height. Therefore, it is critical to overcome this challenge via interfacial engineering principles such as introduction of a non-reacting add-layer. Here, we have introduced two-dimensional (2D) hexagonal boron nitride (h-BN) crystal as a tunneling inter-layer in graphene/Si heterojunctions, which also passivates the dangling bonds on the surfaces of Si. Thin films of h-BN are directly synthesized on lightly-doped Si surface via a bottom-up chemical-surface-adsorption strategy followed by the transfer of chemical vapor deposited graphene. The 2D-on-2D-on-3D nanoarchitecture of graphene/h-BN/silicon forms a metal-insulator-semiconductor (MIS)-type junction, where h-BN acts as an electron-blocking or hole-transporting layer and it avoids interfacial charge carrier recombination. A 4-fold increase in open-circuit voltage (VOC) is found for graphene/h-BN/silicon heterojunction cell (0.52 V) in contrast to the graphene/silicon cell (0.13 V), which may be due to the decrease in the interfacial charge carrier recombination. Furthermore, relevant temperature and stability study has been done on this cell. Device performance decreases ~4% with every 10K temperature increase and has a temporal stability for more than 6 months. This result paves a new way for the higher performance and stable photovoltaics.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) such as molybdenum disulfide (MoS₂) are very promising candidates for future optoelectronics due to their enhanced light-matter interactions and stability at atomic thicknesses. We present here a new technique for high-throughput synthesis of 2D MoS₂ using rapid thermal processing (RTP) of a solution-based precursor, which increases MoS₂ synthesis speed by an order of magnitude relative to competing techniques. We also report progress towards advanced opto-electronic devices that utilize carrier selective contact schemes and post-fabrication treatments to further improve device performance.

For the RTP-based growth method, precursor solution is made by dissolving ammonium tetrathiomolybdate ((NH₄)₂MoS₄) powder into ethylene glycol within an ultrasonicator¹, followed by spin-coating onto plasma-treated substrates. Samples are then loaded in the RTP tool, which can achieve temperature ramps of 120 °C/s, allowing for super-fast heating and cooling of samples for rapid processing. A two-step annealing process in H₂/N₂ environment at atmospheric pressure is used to first turn MoS₄ into MoS₂ at 350 °C, followed by a crystallization step at 1000 °C. The RTP tool can reach these temperatures within seconds, and the entire heating/cooling process takes 15 minutes. Several concentrations of the MoS₄ solution are examined and MoS₂ as thin as 3 molecular layers is grown with great thickness control. X-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and transmission spectroscopy is performed to study the phase, crystal structure, composition, stoichiometry and other characteristic qualities of the synthesized 2D MoS₂. The usual 2H-phase MoS₂ in-plane (100) and (110) peaks are found at 2θ = 33.1° and 2θ = 58.9°, respectively, in XRD. Raman spectroscopy shows the MoS₂ 2H E¹_2g and A_1g peaks at 381 cm^-1 and 407 cm^-1, respectively. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) are also performed to measure film thickness and surface roughness and shows very good uniformity and homogeneity of the 2D films.

Bottom-gate lateral devices have been fabricated with Ti/Au contacts and mobility of 0.022 cm²/V-s and on/off ratio of 10² is measured without any post-processing of the devices. Photodetectors with external quantum efficiency up to 0.02% are also fabricated and measured. We are currently developing carrier-selective Schottky-type photovoltaic devices that utilize different hole and electron-collecting top and bottom contacts. Finally, we have reported in the past that post-fabrication treatment using 1,2–dichloroethane (DCE) can boost device photocurrent significantly, which we will apply to these devices to further improve their optoelectronic performance.

¹Lim, Yi Rang, et al. "Wafer-Scale, Homogeneous MoS₂ Layers on Plastic Substrates for Flexible Visible-Light Photodetectors." Advanced Materials 28.25 (2016): 5025-5030.

The growing research interest in black phosphorus (BP) as a remarkable material with applications in electronics and optoelectronics has made the study and development of device fabrication and integration processes an essential research area. Specifically, the weak material integrity and oxidation chemistry of BP makes the development of robust passivation techniques vital to preventing degradation in BP. The rapid interactions of BP with O₂ and H₂O in air leads to formation of oxides and quick degradation of its electrical and optical properties. An approach, compatible with electronic and optoelectronic device fabrication, that can protect and preserve BP properties is the dielectric capping of the 2D crystal. In this work we will explore and develop scalable (< 10 nm) processes for deposition of high-k dielectrics as capping layers on BP using plasma-enhanced atomic layer (PEALD). We demonstrate how the high energy chemical processes and minor defect formations at the interfaces in the PEALD process can enhance the nucleation of the dielectrics on the inert surfaces of the BP to enable high quality HfO₂ deposition down to 5 nm. We will examine the integrity and the time evolution of the material and interface stack properties of the BP-dielectric heterostructures using electrical characterization, Raman spectroscopy, and XPS. We demonstrate a BP field-effect transistor with a ~5 nm PEALD HfO₂ passivation layer that exhibits high performance and maintains their properties for long periods of time. These results show another option for passivating BP crystals with ultrathin dielectrics using PEALD.

Valley pseudo-spin and broken inversion symmetry leads to valley-specific optical selection rules in monolayer transition metal dichalcogenides (TMDC). Specifically, the degenerate K and K’ valleys can be selectively populated with right- and left-hand circularly polarized light. This control of the band edge states has fueled interest in the potential of TMDCs for Valleytronic applications, where the momentum state is manipulated to perform computational and/or logical operations.
Recently, it was discovered that intense circularly polarized light fields can be used to break the degeneracy between the K and K’ valley, allowing the energy levels of each valley to be independently tuned via a valley-selective form of the well-known optical Stark effect^1,2 and lesser-known Bloch-Siegert³ effects. For high-intensity sub-resonant illumination with right-hand circularly polarized light, the K-valley A-exciton absorption band will blue-shift to higher energies. In the past, this has been described by the dressed-atom picture, which predicts that the effect reverses to a red-shift for above-resonant illumination. Additionally, application of Floquet theory to describe those observations has led to the prediction of topologically protected chiral edge states for above-resonant light fields.
Here, we report that above resonant excitation does not lead to the predicted red-shift in monolayer WS₂, but instead continues to produce a blue-shift of the A-exciton K-valley absorption band. We use ultrafast transient absorption spectroscopy to show that the degree of Stark-shift follows the field strength amplitude of the applied optical light pulse in time, confirming that it remains an optical Stark effect. We also report the power and wavelength dependence of this effect, where we show that the Stark-shift is resonantly enhanced, reaching several 10s of meV for moderate optical fields.
For near-resonant pumping of the K-valley, no Stark shift is observed for the opposite (i.e. K’-) valley, confirming the valley-selective nature of the optical Stark effect in TMDCs. We do not see a Stark shift associated with intervalley biexcitons.⁴ Instead, we do observe the appearance of a short-lived photo-induced absorption resonance for cross-circular polarizations associated with the coherent creation of intervalley biexcitons with binding energy near 50 meV. There is evidence of multiple biexciton peaks associated with the biexciton fine structure within this photoinduced absorption.⁵ These observations persist at room temperature.
Our results demonstrate the breakdown of the two-level dressed-atom picture for TMDCs. For low-dimensional semiconductors, we suggest that exciton-exciton repulsion⁶ needs to accounted for to predict the near-resonance behavior of the optical Stark effect. Our results may have implications in the search for topologically protected chiral states in TMDCs.


References
1. Kim, et al., Science 346,1205 (2014)
2. Sie, et al., Nat. Mater. 14290 (2015)
3. Sie, et al., Science 355, 1066 (2017)
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Thin film materials that possess features of being electrically conductive, flexible, semi-transparent, and able to be fabricated at low cost are crucial for realizing future highly-efficient and low-cost electronics, including solar cells, touchscreens, displays, and printed electronics. So far, there have been several thin film materials with such potentials, such as transparent conductive oxides, including fluorinated tin oxide (FTO) and indium tin oxide (ITO), conductive polymers, nanostructured metals, and carbon-based materials, including carbon nanotubes, fullerene, and other carbon allotropes. One of the most notable carbon-based materials discovered in recent years is graphene due to its excellent electronic properties. However, synthesizing large-scale pristine graphenes with good quality and at low cost is still a great challenge. One feasible solution to this problem has been extensively developed in recent years as a high yielding, robust chemical synthesis known as Hummers’ method [1], where graphite is exfoliated by oxidizing agent into graphene oxide (GO). Subsequently, GO is treated in reducing condition to remove oxygen containing functional groups gained during the oxidation, thus, reverting it back to graphene-like state, known as reduced graphene oxide (rGO).
So far, there have been several reducing agents widely used in the reduction process of rGO. However, many of them are considered environmentally toxic and unsafe to work with, such as hydrazine, sodium borohydride and hydroiodic acid. Therefore, green and environmentally friendly reducing agents would be more practical and sustainable for scaled-up production of rGO. Recently, extracts from fruit peels have been demonstrated to be an effective reducing agent for indigo dyes [2]. Thus, in this work, we investigate the utilization of extracts from several fruit peels, such as banana peel and apple peel, as the reducing agent for the reduction process of rGO. The utilization of fruit peel extracts is desirable due to their environmental friendliness, low cost, high availability and simple production and synthesis routine. In addition, this method also increases the value of household wastes. We also compare the electrical property of the rGOs reduced by the fruit peel extracts with those reduced by other commercially available green reducing agents, such as amino acids, and organic acids.
GO is synthesized by improved Hummers’ method [3], which utilizes more environmentally friendly phosphoric acid instead of sodium nitrate in conventional Hummers’ method, where toxic nitrogen oxide species are generated during the synthesis process. The fruit peel extracts are obtained by adding fruit peels to DI water with the amount of 1 g of either banana peel or apple peel per 5 mL of DI water. The mixture is boiled for 30 minutes, and the solid residual is removed to obtain the fruit peel extracts. The fruit peel extract is then mixed with the synthesized GO, which is dispersed in DI water with the concentration of 0.5 mg/mL. The mixture is then filtered after being left to react for 48 hours to obtain the rGO. Finally, we evaluate the electrical property of the rGO reduced by the fruit peel extracts by comparing their conductivity measured by using four-point probe technique with those reduced by amino acids (alanine and cysteine), and organic acids (caffeic acid and l-ascorbic acid).

References
[1] W. S. Hummers et. al., J. Am. Chem. Soc. 80, 1339 (1958)
[2] D. Hossain et. al., J. Polym. Environ. 25, 1219 (2017)
[3] C. M. Daniela et. al., ACS Nano 4, 4806 (2010)

Converse flexoelectricity induces mechanical stress under an inhomogeneous electric field in all dielectrics. Although this electromechanical behavior has great potential as an actuation mechanism, flexoelectric actuators have not been widely studied, which is mainly attributed to the fact that the effect of flexoelectricity is considerably small in general bulk dielectrics. Two-dimensional (2D) materials can be a practical candidate as an active material due to the scaling effect of flexoelectricity. Here, we report a molybdenum disulfide (MoS₂) thin-film actuator driven by the converse flexoelectric effect under electric field gradient. To fabricate an actuator device, single-layer chemical vapor deposition (CVD)-grown MoS₂ was transferred on the substrate with an insulator on the bottom electrode, followed by comb-shaped top electrode deposition for the electric field gradient through fringe fields. To characterize the actuators, we measured out-of-plane displacements at the first harmonic resonance frequency of the input AC signal using a laser vibrometer. When we characterized the actuation performance, the measured displacement relative to the active layer thickness of our MoS₂ actuator showed more than an order of magnitude higher performance compared to other existing flexoelectric actuators. In addition, in a cyclic test, the MoS₂ device exhibited consistent performance after 10⁹ cycles of actuation, which suggests stable operation in air conditions. The converse flexoelectricity combined with distinct properties of 2D materials will open up unique opportunities in the field of nanoelectromechanical systems (NEMS).

Electrical disorder in CVD (chemical vapor deposition) graphene limits its performance in many applications [1, 2]. Recent works on graphene sensing applications have shown that disorder induces uncertainties in the reading of Dirac voltage, which undermines their sensitivity [3]. However, the sources of electrical disorder have not been thoroughly understood yet in literature. This work presents a comprehensive study of the sources of disorder in CVD graphene. Utilizing Raman spectroscopy mapping and atomic force spectroscopy (AFM), we correlated the carrier concentration distribution [4] of transferred CVD graphene to its topographic image. The results show that due to the embedded ripple structure, which originates from the thermal expansion coefficient mismatch against the growth substrate, graphene forms uneven separation to SiO₂ substrate according to the ripple array. Consequently, the hole doping received from SiO₂ [5] alternates in a periodical manner. Further, we bisected the mapping data of graphene according to its ripple structure into valley (topographically lower) and peak (elevated by ripples) groups, and show that valley region is more disordered. Such additional disorder is from the enhanced charge interaction with the impurities in the substrate [6]. The results suggest that the disorder in graphene depends on the morphology of the ripple array, which is eventually determined by the degree of mismatch between graphene and the growth substrate [7]. Finally, we performed AFM and Raman spectroscopy mapping on the same sample after rapid thermal annealing (RTA). The results indicate that annealing heals the height unevenness from ripple, and therefore alleviates the non-uniformity in doping distribution across the ripples. However, as thermal annealing relocates graphene closer to the substrate [8, 9], graphene suffers more disorder from SiO₂. This study suggests that ripple is essentially not a disorder source, whereas it contributes to disorder when there is substrate doping. The authors acknowledge funding from Boston Scientific Corporation. Portions of this work were also carried out in the University of Minnesota Characterization Facility, which received capital equipment funding from the University of Minnesota MRSEC under NSF Award DMR-1420013.
[1] Olson, E et al., ACS Appl. Mater. & Interfaces, vol. 7, no. 46, pp.25804-25812, 2015. [2] Zhang, Y et al., ACS Appl. Mater. & Interfaces, vol. 9, no. 44, pp.38863-38869, 2017. [3] M. Ebrish et al., ACS Appl. Mater. & Interfaces, vol. 6, no. 13, pp. 10296-10303, 2014. [4] J. Lee et al., Nat. Commun., vol. 3, pp. 1024, 2012. [5] S. Berciaud et al., Nano Lett., vol. 9, no. 1, pp. 346-352, 2009. [6] F. Forster et al., Phys. Rev. B, vol. 88, pp. 085419, 2013. [7] D. W. Kim et al., J. Mater. Chem. C, vol. 1, pp. 7819-7824, 2013. [8] Z. Cheng et al., Nano Lett., vol. 11, no. 2, pp. 767–771, 2011. [9] S. Ryu et al., Nano Lett., vol. 10, no. 12, pp. 4944-4951, 2010.

Stacking orientation of atomically-thin 2D layers may serve as an interesting way to tune their optical and electronic properties for optoelectronic applications. However, it is a challenge to engineer it in a desirable way. Here, we demonstrate a synthetic strategy to enable the growth of bilayer crystals exhibiting a wide range of stacking configurations. Detailed second harmonic generation (SHG), low-frequency Raman spectroscopy, and atomic-resolution scanning transmission electron microscopy (STEM) studies of bilayer WSe₂ ribbons are performed to reveal that the defects on the grain boundaries facilitate the growth of bilayers with a wider range of stacking angles. First-principles calculations proposed possible mechanisms of the orientationally arrangement of bilayers. We also show that the low-frequency Raman serves as a distinct signature for fast differentiation of new stacking configurations. Our finding shows a promising way to direct synthesis of bilayer crystals with weak interlayer coupling and specific mutual monolayer orientations, which is important for their future optoelectronic applications.

The properties of 2D materials motivate the exploration of new atomically thin materials. One class of 2D materials, demonstrated experimentally, is 2D electrides, which consist of a layer of atoms and layers of electrons on the surface. The layers of electrons are delocalized into layers of 2D electron gas. As a result, 2D electrides, such as 2D Ca₂N, have exciting properties like high electrical mobility, high carrier concentrations, and ultra-low work functions. To synthesize 2D Ca₂N, we exfoliated the layered bulk crystal Ca₂N in appropriate organic solvents to shear few-layer sheets. Careful study using X-ray diffraction and transmission electron microscopy coupled with selected area diffraction showed that the sonicated Ca₂N yielded crystalline 2D sheets. Measurements by ultraviolet photoelectron spectroscopy and UV-Vis-NIR spectroscopy confirmed that the 2D sheets retain the metallic electronic structure of the bulk and could serve as excellent transparent conducting materials. To build on this result conceptually, we performed computations using density functional theory (DFT) that suggest that the 2D electrides, when brought into contact with other 2D materials, donate massive amounts of electron density (up to 0.5 e-/formula unit of acceptor) and form quasi-covalent bonds between materials. This unique behavior constitutes a conceptually new type of heterostructure with properties distinct from either monolayer. Here we report calculations of electride/graphene, electride/hBN, electride/black phosphorus, and electride/ZrS₂ heterostructures. Extensive electron transfer from the 2D electride to the acceptor creates a strong electrostatic attraction, which results in an interlayer distance (~2.0 A) much shorter than traditional van der Waals distances. In the case of ZrS₂, the high degree of electron transfer into ZrS₂ raises the Fermi level of ZrS₂, which populates its conduction band and converts the wide bandgap semiconductor into a metal. By calculating different orientations of the heterostructures, we show that the electron transfer and the interlayer distance are inherently coupled. We provide calculations of the anisotropic work function and give analysis of the electronic structure as one example of the exciting properties of this new type of heterostructure.

The most common form of data communication in present day is via electrons shuttling through metal wires. However, this presents severe losses leading to incoherent communication for long-reach applications. Light propagating through a well-defined path, such as in a waveguide, offers the most promising future in data communication. Optical on-chip and inter-chip connections provide high-bandwidth communication in short-reach applications as well as very little losses for long-reach applications. We are to explore a broad range of 2D metal dichalcogendies and their alloys such as MoS₂, MoSe₂, WS₂ and WSe₂, for applications as advanced opto-electronic materials. The combination of tunable bandgap, interface-determined structure in the absence of a bulk and Fermi-level control by electrical contacts renders them a materials system with great potential for application as emitters and sensors, e.g. for on-chip and inter-chip optical communication. The high stability and facile preparation of MoS₂ and related materials suggests great potential in this system, rendering them an ideal target for a research project that seeks novel materials for mastering energy and information on the nanoscale to create new technologies. We use the Molecular Foundry nanofabrication facility to fabricate on-chip waveguides out of a silicon nitride on silicon dioxide substrate including edge-couplers initially and grating-couplers that are tailored to the bandgap of common TMD materials between 1.5 and 1.9 eV. These grating couplers act as seed locations via etching and depositing specific metals, initially molybdenum, via e-beam evaporation due to its directional nature. Preliminary data utilizing waveguides with couplers at unsuitable wavelength (but within the abilities of the lithographic processes available to us) show structurally promising result, but lack functionality. Preliminary seeding growth results show monolayer MoS2 single islands on side walls. More results will be acquired using the optical wave-guide testbed system built in a nitrogen enclosed glove-box environment.

Sodium ion batteries are a promising more environmentally sustainable alternative to Li-ion batteries. However, the larger sized sodium ion cannot be hosted by the graphene anode material used in Li-ion batteries, and so significant research is devoted to the development of disordered carbon materials with a more open structure to serve as an alternative. One promising candidate, hard carbon, is composed of nanodomain of stacked rippled graphene.
Na intercalation into the galleries of this material is found experimentally to be energetically favorable; however, its d-spacing, while larger than graphite, is smaller than the theoretically predicted d-spacing for favorable intercalation. In this work, we use density functional theory, and classical molecular dynamics to examine the role that rippling of the sheets plays, how it altering the local d-spacing, and changing the energy of the Na interaction. The increase of surface curvature and the rippling wavelength decreases will lead to larger d-spacing within a range. Experimentalists have been able to successfully tune the curvature of graphene rippling. Our work will be able to help find the best-performed curvature and design the anode material for sodium-ion or other larger ion batteries.

The piezoelectric properties of various thin films have been widely studied for potential applications in mechanical-to-electrical energy conversion and harvesting. Among these, two dimensional (2D) nanocrystal sheets of MoS₂ thin films offer numerous advantages such as excellent scalability (which is ideal for flexible electronics), high surface area, relatively high mobility, maximum electrostatic control due to atomic thickness dimensions, optical transparency, chemical sensing capability, and mechanical flexibility. The recent experimental study on the piezoelectric properties of MoS₂ has shown that the cyclic stretching and releasing of MoS₂ exhibits promising piezoelectric characteristics for the odd number of layers. However, the state-of-the-art relies on the mechanical exfoliation technique and its relatively small size (e.g., flakes) limits the practical adoption of MoS₂ for applications requiring large-area device or system integration. In this work, we applied an unique CVD (chemical vapor deposition) technique to grow a large surface area, easy-transfer MoS₂ (2cm × 2cm) film on a SiO₂ substrate, which was thoroughly characterized by using a variety of structural and electrical analysis methodologies, including Raman spectroscopy and piezoelectric testing on the flexible substrate. This work will provide a new insight on the practical application domain where the large-area, piezoelectric MoS₂ can be best utilized for energy harvesting purposes.

In the last four decades, integrated circuit manufacturers have successfully continued Moore’s Law by effective scaling silicon-based transistors. However, as scaling technology advances, passive power density increases and becomes a major power consumption problem [1]. Furthermore, the demand for information processing has accelerated rapidly due to advancements in cloud computing, social networking, mobile internet, and data analytics. However, the energy used to manipulate a single bit of information is currently ~10⁵ times greater than the theoretical limit which creates an opportunity to significantly reduce energy consumption in information processing devices [2]. Additionally, there has been remarkable progress on the application of two-dimensional layered materials, such as graphene and transition-metal dichalcogenides (TMDs) for electronic circuitry. Monolayers of TMDs have very distinct electrical properties from their bulk counterparts, primarily due to the confinement of charge carriers in two dimensions. A useful property of some TMDs is that their bandgaps (>1eV) are highly sensitive to strain. For example, bi-layered MoS₂ has a direct bandgap of ~1.9 eV and has a strain sensitivity of -.1167 eV/%strain which enables a large range of electrical conductivity from semiconducting to conducting [3]. In addition, MoS₂ has shown exceptional strength with a fracture point at 11% tensile strain [4].
To address power consumption in electronics, a switch was recently proposed for low-voltage applications, which uses the bandgap deformation potential exhibited by TMDs. In this device, a MEMS actuator strains a thin TMD membrane causing it to theoretically increase its conductivity by six orders of magnitude when subjected to 6% strain [5]. In this paper, we demonstrate the first experimental results of the device showing an increase of conductivity of up to 3,000 times using bi-layered MoS₂. A poly-Si_0.35Ge_0.65-based MEMS actuator is fabricated and used to strain the MoS₂. Additionally, optical measurements showed evidence of strain induced on the MoS₂ flakes. The amount of strain induced in the MoS₂ flakes was determined to be as high as 3% using the model fitted to the experiment data.



[1] B. Meyerson, Semico Impact Conference, Taiwan, 2004.

[2] "Center for Energy Efficient Electronics," [Online]. Available: https://www.e3s-center.org/research/. [Accessed 01 August 2017].

[3] S. Manzeli, A. Allain, A. Ghadimi and A. Kis, "Piezoresistivity and Strain-induced Band Gap Tuning in Atomically Thin MoS2," Nano letters, vol. 15, no. 8, pp. 5330-5335, Jul 2015.

[4] S. Bertolazzi, J. Brivio and A. Kis, "Stretching and Breaking of Ultrathin MoS2," ACS nano, vol. 5, no. 12, pp. 9703-9709, Nov 2011.

[5] S. Almeida, "Monolayer strain by NEMS for low power application," in Energy Efficient Electronic Systems (E3S), 2015 Fourth Berkeley Symposium, Berkeley, 2015.

Non-equilibrium synthesis and processing enables a wide variety of materials systems with tuned properties. Here we report how a prototype setup allows laser illumination to be coupled into a (scanning) transmission electron microscope (TEM) for real-time observations of non-equilibrium synthesis. The pulsed laser deposition of atomically-thin 2D materials can be accomplished at elevated temperatures from amorphous nanoparticle 'building blocks', and is suspected to occur by the general process known as crystallization by particle attachment (CPA). To investigate how amorphous nanoparticles of transition metal dichalcogenides (TMDs) crystallize into atomically-thin 2D layers, the process is broken into two steps. First, nanoparticles are deposited at room temperature onto TEM grids by pulsed laser deposition, and then are rapidly heated by a laser built within a TEM to observe how they crystallize, fuse, and recrystallize in situ. Characterization of these “building blocks” by high-resolution electron microscopy and spectroscopy, as well as first principles modelling, is used to infer valuable information on the time scales and kinetics of the growth processes of these nanomaterials. Especially, the mobility of the nanoparticulate species on the TEM grids is highly dependent on the grid material. In addition to commercial ultrathin SiO₂ grids, graphene and other 2D crystalline layers grown by chemical vapor deposition are utilized as platforms for synthesis, which yielded different microstructures and different kinetics. Electron energy loss spectroscopy (EELS) is utilized to understand the electronic properties and composition of synthetic 2D crystals and their heterostructures in the TEM. The observation with EELS at cryogenic temperatures with a mono-chromated TEM allows also to map with high spatial resolution the distribution of excitons. The energy of the excitons is very sensitive to the local structure and can be directly compared to photoluminescence and Raman spectra. In addition, post-growth atomic-resolution ADF-Z-STEM measurements permit the characterization of crystal phases, defects and grain boundaries at the atomic scale. These results are compared with ex situ laser annealing experiments and optical spectroscopic measurements. These nanoscale measurements to understand the growth mechanisms of crystalline 2D films from amorphous precursors should enable more rapid advancement of laser processing at the macroscale.

Synthesis science was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (BES), Materials Sciences and Engineering Division. Characterization and computational science at CNMS were supported by the Scientific User Facilities Division, BES.

Highly-sensitive surfaced-enhanced Raman scattering (SERS) platforms were prepared by depositing graphene oxide (GO) layers on ZnO thin films via suction filtration followed by proper heat treatments. FTIR spectra show that oxygen-containing function groups (FGs), were significantly increased after specific heat treatments. SERS measurements show that the FGs benefit enhancement factors (EFs) of SERS effects. In the work, the effects of GO thickness, heating temperature/time, and atmosphere on the FGs and SERS were investigated. Ultraviolet photoelectron spectroscopy (UPS) was employed to characterize electronic structures of the GO/ZnO composite with different heat treatments. The GO/ZnO composite annealing in Ar atmosphere at 100°C for 3h demonstrates a high EF value of exceeding 70000 for Rhodamine 6G detection. Raman mapping shows uniform intensity distribution of enhanced signals from the detection molecules. The SERS platform shows a high retention of more than 75% after aging in dark for six months. The metal-free, high-sensitive, and stable GO/ZnO composites are promising for fabricating low-cost and high-performance SERS platforms for chemical and bio- sensing.

Molybdenum disulfide (MoS₂) is a kind of transition metal dichalcogenide (TMD) materials which has attracted great attention due to its various interesting properties[1][2]. MoS₂ is very promising for electronic and optoelectronic devices because of indirect band gap (~1.2 eV) for few layer and direct band gap (~1.8 eV) for monolayer MoS₂ [3]. In MoS₂ based Schottky devices, Schottky barrier height depends on the thickness of MoS₂ because of its tunable electronic properties[4][5]. MoS₂ have been deposited by different methods such as exfoliation, CVD, hydrothermal synthesis and other techniques. Here, we have used DC sputtering technique to fabricate metal-semiconductor junction of MoS₂ with platinum (Pt) metal contacts.
In this work, MoS₂ thin film (~10 nm) was deposited on p-Silicon (111) using DC sputtering technique at optimized parameters. Schottky metallization of Pt metal (diameter~1mm) was also done using DC sputtering. Temperature dependent current-voltage (I-V-T) characteristics of Pt/MoS₂ Schottky diodes have been investigated in the temperature range 80-300 K in the steps of 20K. Forward I-V characteristics Pt/MoS₂ junction are analyzed to calculate different Schottky parameters such as Schottky barrier height, ideality factor, series resistance, Richardson constant and standard deviation. Schottky barrier height increases and ideality factor decreases on increasing the temperature from 80-300K. The I-V-T measurements suggest the presence of barrier inhomogeneities at the metal-semiconductor (Pt/MoS₂) junction. Schottky barrier inhomogeneities occur if the interface is not flat or rough, then the Schottky barrier height is not remain constant and vary locally. Current transport through the junction is a temperature activated process. The electrons at low temperature pass over the lower barriers and therefore the transport will be dominated by current flowing through the patches of lower Schottky barriers and results in larger ideality factor. As the temperature increases, more and more electrons overcome the barriers and this causes the ideality factor to be close to unity and apparent barrier height increases. Further, the modified Richardson equation and the Gaussian distribution of Schottky barrier height in addition to the thermionic emission model were used to analyze the I-V-T data of the Schottky interface. The average barrier height, Richardson constant and standard deviation were calculated from the modified Richardson equation and the Gaussian model.

References
[1] Q. H. Wang, K. Kalantar-Zadeh, A. Kis, J. N. Coleman, and M. S. Strano, “Electronics and optoelectronics of two-dimensional transition metal dichalcogenides,” Nature Nanotechnology, vol. 7, no. 11. pp. 699–712, 2012.
[2] Neetika, A. Sanger, V. K. Malik, and R. Chandra, “One step sputtered grown MoS2 nanoworms binder free electrodes for high performance supercapacitor application,” Int. J. Hydrogen Energy, vol. 43, no. 24, pp. 11141–11149, Jun. 2018.
[3] A. Kuc, T. Heine, and A. Kis, “Electronic properties of transition-metal dichalcogenides,” MRS Bull., vol. 40, no. 7, pp. 577–584, 2015.
[4] N. Kaushik et al., “Schottky barrier heights for Au and Pd contacts to MoS2,” Appl. Phys. Lett., vol. 105, no. 11, 2014.
[5] H. Dong et al., “Schottky Barrier Height of Pd/MoS2 Contact by Large Area Photoemission Spectroscopy,” ACS Appl. Mater. Interfaces, vol. 9, no. 44, pp. 38977–38983, 2017.

Vanadium diselenide (VSe₂ ) usually exists at the 1T crystal-phase in bulk form. While the DFT calculation predicts that both 1T and 2H phase are possible stable structures at 2D limit, though 2H phase VSe₂ is not reported as yet. In this work, we report the phase transition from 1T phase to 2H phase by annealing multilayer VSe2 at about 650K accompanying evidence of metal-insulator transition and Raman scattering.

Two-dimensional transition metal dichalcogenides (TMDs) have attracted researchers for their broad application in semiconductor industry. As their controllable band gap, electronic characteristic and unique optical characteristic are suitable to fabricate electric device such as field-effect transistor. Here we show a synthesis method to control the quality of the bulk MoSe₂ using chemical vapor deposition. Argon and hydrogen are the most popular carrier gas and reducing gas, respectively. The ratio of Ar and H₂ determines the quality of the MoSe₂. Furthemore, the crystallinity increased with increasing gas flow rate. To investigate the quality and uniformity of the bulk MoSe₂, Raman spectroscopy, scanning electron microscope, energy dispersive analysis of X-ray, and X-ray photoelectron spectroscopy were performed. MoSe₂ thin film transistor was then fabricated with typical bottom gate sturucture on a SiO₂/Si substrate with Al₂O₃ passivation. The room-temperature carrier mobility for the device was about 21 cm²/Vs. This study demonstrates the novel approach in growth MoSe₂ with high crystallinity and its application to electronic devices.

Semiconductor two dimensional (2D) materials have shown great potential in optoelectronic devices because of their high carrier mobility and very thin thickness. Recently, many groups have exploited the heterojunction structures based on semiconducting 2D materials for photovoltaic applications as ultrathin p-n didoes. For high-performance photovoltaic devices, the electron-hole balance is an important parameter because it has effect on the formation of the built-in field across the device. In this study, we exhibit multi-layer black phosphorus (BP)/WS₂ p-n heterojunction device and demonstrate the photovoltaic effect of the device by electrostatically controlling carrier concentration into BP. The gate-controlled heterojunction device exhibits a high current rectification of 10³, external quantum efficiency of 4.4% under the 405-nm laser illumination, and solar efficiency of 4.6% under the AM 1.5 solar spectrum. Our works suggest that making a balance of charge carriers in the 2D heterojunction p-n diode is highly prioritized for the high performance 2D heterojunction photovoltaic device.

Atomically thin graphene exhibits fascinating mechanical properties, although its hardness and transverse stiffness are inferior to those of diamond. So far, there has been no practical demonstration of the transformation of multilayer graphene into diamond-like ultrahard structures. Here, we show that during nanoindentation at room temperature, 1-layer graphene on SiC(0001) (1 layer graphene + buffer layer) exhibits a transverse stiffness and hardness comparable to diamond, showing resistance to perforation with a diamond indenter and a reversible drop in electrical conductivity upon indentation. Density functional theory calculations suggest that, upon compression, the 1-layer graphene film transforms into a diamond-like film, producing both elastic deformations and sp² to sp³ chemical changes. Experiments and calculations show that this reversible phase change is not observed for a single buffer layer on SiC or graphene films thicker than two layers. Indeed, calculations show that whereas in 1-layer graphene layer-stacking configuration controls the conformation of the diamond-like film, in a multilayer film it hinders the phase transformation. Our findings identify supported 1-layer graphene as an interesting candidate for pressure-activated adaptive ultrahard and ultrathin coatings and for force-controlled dissipation switches. Our study opens up new ways to investigate graphite-to-diamond phase transitions at room temperature in low-dimensional systems. Finally, this work suggests a new route to produce and pattern single-layer diamond in graphene through stabilization of the diamond-like phase, with applications ranging from nanoelectronics to spintronics.

As elements of electronics are downsized to the nanoscale, the microstructure is no longer stable under current stress in practical applications. For example, conventional interconnect metals (i.e., poly-crystalline Cu and W) suffer from size effects related to both surface and grain boundary scatterings, thus impeding current injection into the materials contributed by the exponentially increased resistivity.^[1] Accordingly, high current-carrying van der Waals (vdW) metals such as quasi-one-dimensional (quasi-1D) transition metal (TM) chalcogenides (i.e., TiS₃, TaSe₃, and WTe₂)^[2-5] have drawn new attention as alternatives for downscaled interconnects owing to their dangling-bond-free surface and quasi-1D atomic intermetallic bonds, which allow highly mobile carriers to reduce their diffusive motion. However, a lack of reliable bottom-up approaches to obtain high-quality 1D single crystals on a desired substrate hinders systematical electrical characterizations and further applications of the materials. Moreover, a high-bias analysis of TM chalcogenides depending on size under different ambient environments remains to be studied, which will allow us to understand their electrical breakdown mechanism and energy transfer through different interface dimensions.

In this study, we report an investigation of the low- and high-field electrical properties of single-crystalline WTe₂ nanobelts, mainly focusing on their electrical failure and current-carrying capacity. To synthesize stoichiometric, quasi-1D WTe₂ crystals, a eutectic alloy (e.g., Cu_xTe_y or Au_xTe_y)^[5] was introduced to provide a liquid-like Te-rich ambient condition. Because of this approach, a systemic investigation of the electrical properties of the crystals became possible as a function of channel dimensions by avoiding degraded edges by harsh etching^[6] and random crystalline orientations,^[4] which are unavoidable for the exfoliated flakes. The resultant WTe₂ nanobelts could sustain significantly high breakdown current densities (J_B) of up to ~100 MA/cm² and electrical input power per channel length of ~16.4 W/cm, which are higher than those of conventional interconnect metals and any other TM chalcogenides to date.^[2-4] The energy dissipation and carrier scattering models exploited in this study revealed that the high current-carrying capacity of the quasi-1D products is because of a rather scattering-free transport behavior as well as suppressed electromigration in the encapsulated devices until the channels’ breakdown, as compared to the air-exposed ones. We believe that our strategy sufficiently promises the potential for applications in future downscaled nanoelectronics owing to the unique fundamental electrical characteristics of the quasi-1D layered WTe₂ single crystals compared to other vdW materials.

[1] Shacham-Diamand, Y., Osaka, T., Datta, M. & Ohba, T. Advanced nanoscale ULSI interconnects: fundamentals and applications (Springer, 2009).
[2] Stolyarov, M. A. et al. Breakdown current density in h-BN-capped quasi-1D TaSe₃ metallic nanowires: prospects of interconnect applications. Nanoscale 8, 15774-15782 (2016).
[3] Molina-Mendoza, A. J. et al. High current density electrical breakdown of TiS₃ nanoribbon-based field-effect transistors. Adv. Funct. Mater. 27, 1605647 (2017).
[4] Mleczko, M. J. et al. High current density and low thermal conductivity of atomically thin semimetallic WTe₂. ACS Nano 10, 7507-7514 (2016).
[5] Kwak, J. et al., Single-crystalline nanobelts composed of transition metal ditellurides. Adv. Mater. 30, 1707260 (2018).
[6] Zhang, F., Lee, C.-H., Robinson, J. A. & Appenzeller, J. Exploration of channel width scaling and edge states in transition metal dichalcogenides. Nano Res. 11, 1768-1774 (2017).

Two-dimensional layered transition metal dichalcogenides (TMDs) like WS₂ are seen as promising materials for back-end-of-line applications in nanoelectronics including low-power devices.¹ The synthesis of crystalline TMD multi-layers using atomic layer deposition (ALD) has attracted a lot of interest lately as ALD offers monolayer thickness control, scalability and low temperature growth (T ≤450°C). However, these multi-layered ALD grown films have been reported in literature to exhibit a high density of out-of-plane 3D structures in addition to the desired 2D horizontal layers.^2,3 The presence of such 3D structures can hinder charge transport and consequently raise film resistivity, which hampers device application. In this work, we demonstrate a novel approach to suppress the growth of such 3D structures during plasma-enhanced ALD (PEALD) of WS₂ and effectively lower the film resistivity (by one order of magnitude) while maintaining the inherent merits of ALD.

Conventionally, PEALD is a two-step (AB) cyclic deposition technique, based on sequential use of self-limiting steps: (A) Precursor dosing and (B) plasma activated co-reactant exposure. We report here, on a novel three-step (ABC) WS₂ process involving an additional Ar and/or H₂ plasma step (C) at a low temperature of 450°C. The energetic ions and/or the highly reactive species in the plasma (step C) can assist in the removal/etching of the undesired 3D structures. In line with this hypothesis, transmission electron microscopy (TEM) studies of multi-layered WS₂ films grown using the ABC process showed a significant reduction in the areal density of 3D structures relative to the reference AB process. Interestingly, in-depth analysis of the TEM images revealed that the reduction in areal density of 3D structures goes hand-in-hand with grain size enlargement. Based on these findings, we postulate a growth model explaining the evolution and subsequent removal/etching of the 3D structures in multi-layered WS₂ films, which can be applicable to other crystalline semiconducting TMDs grown by ALD. Furthermore, we report an optimization of the plasma gas composition that minimizes density of undesired 3D structures. The density reduction of the 3D structures was accompanied by an order of magnitude drop in film resistivity (10x) relative to the AB process, as-determined from four-point probe measurements.

To summarize, we report a novel method to suppress the growth of undesired 3D structures in multi-layered, PEALD grown WS₂ films at a low deposition temperature of 450°C. The low density of 3D structures can allow efficient charge carrier transport in the films and the low film resistivity ensured by these novel processes showcase a strong potential for implementation in electronic devices.

¹ T. Schram et al, 47^th European Solid-State Device Research Conference, Leuven, Belgium, Sept 11−14, 212-215 (2017).
² A. Sharma et al, Nanoscale 10, 8615–8627 (2018).
³ T.A. Ho et al, Chem. Mater. 29, 7604-7614 (2017).

Controlled heterojunction design was combined with diameter-tailored electrospinning to prepare mesostructured photocatalysts, leading to strongly enhanced photocatalytic water splitting rates.
The (111)-layered perovskite Ba₅Ta₄O₁₅ [1] in heterojunction with Ba₃Ta₅O₁₅ [2] was for the first time prepared as nanofibers by means of electrospinning, including variation of the fiber diameter.[3] The result is a tailored mesostructured heterojunction leading to hydrogen evolution rates from water/methanol mixtures of up to 4.4 mmol h⁻¹ without co-catalyst, and remarkable water splitting activity after Rh−Cr₂O₃ decoration with hydrogen production rates up to one mmol h⁻¹. Our studies also confirm the photocurrent doubling effect of sacrificial alcohols exhibiting α-H atoms with this heterojunction, in contrast to tert-butanol.

[1] N. C. Hildebrandt, J. Soldat, R. Marschall, Small 2015, 11, 2051−2057.
[2] R. Marschall, J. Soldat, M. Wark, Photochem. Photobiol. Sci. 2013, 12, 671−677.
[3] A. Bloesser, R. Marschall, ACS Appl. Energy Mater. 2018, 1, 2520−2525.

Making adjustable two-dimensional (2D) materials is an emerging route to reach a superior control of functional properties and access new device performances or applications. With this aim in mind, here I will give consideration to two distinct cases. On one hand is the case of the epitaxial Xenes, an emerging class of 2D monoelemental lattice beyond graphene; on the other hand is the anisotropy design in MoS₂ nanosheets. By close analogy with graphene, epitaxial Xenes are comprised of monoelemental atoms arranged in a honeycomb lattice but unlike graphene, Xenes are epitaxially grown on substrates and exhibit a varying degree of buckling and/or puckering in the lattice structure [1]. Examples in this respect are silicene, germanene, stanene, borophene, epitaxial phosphorene, and recently synthesized antimonene and tellurene. Buckling in Xenes can be taken as a leverage to tune the electronic and quantum properties making it possible for Xenes to appear as semiconductors, semimetals, metals topological and trivial insulators. Not only the wealth of electronic states in the Xenes makes them suitable as nanotechnology platform, but also topological transitions among these electronic states are predicted to take place as a function of an external solicitation (e.g. vertical electric field, applied stress) thus paving the way to the full exploitation of topology in devices at the 2D level. I will focus on silicene as representative story [2]. Then I will show the route and challenges for Xenes to be integrated in nanoelectronic devices by briefly describing a universal approach to Xene processing and eventually the concept of a topological field effect transistor.
A different case of morphology design at the 2D level is the chemical vapour deposition of MoS₂ nanosheets on patterned substrates. The highly conformal character of the MoS₂ growth allows for the achievement of an anisotropically modulated MoS₂ nanosheet where the phonon and electronic properties are observed to be strongly morphology dependent. The so-induced morphological anisotropy is reflected in the anisotropy of the physical characteristics, such as the phonon spectrum, intrinsic charge fluctuations, and the exciton dynamics. Implications on the band-gap and exciton engineering will be discussed, and the potential for applications envisioned [3].

[1] A. Molle, J. Goldberger, M. Houssa, Y. Xu, S.-C. Zhang, and D. Akinwande, Nature Mater. 2017, 16, 163.
[2] A. Molle, C. Grazianetti, L. Tao, D. Tanneja, Md. H. Alan, and D. Akinwande, Chem. Soc. Rev. 2018, 47, 6370.
[3] C. Martella, C. Mennucci, E. Cinquanta, A. Lamperti, E. Cappelluti, F. Buatier de Mongeot, and A. Molle., Adv. Mater. 2018, 30, 1705615.

The development of vapor sensors with tunable sensitivity and selectivity based on rational molecular design is challenging, however at the same time of great interest due to the manifold applications in our everyday life, e.g. in air quality monitoring or food control. Here, we introduce a design concept for photonic vapor sensors based on stimuli-responsive, swellable 2D nanosheet materials. Antimony and tantalum phosphate (H3Sb3P2O14 and HMP2O8 (M = Sb, Ta) nanosheet-based Fabry-Pérot interference sensors as well as 1D Photonic Crystals were endowed with chemical sensitivity to various volatile organic compounds (VOCs) by ion exchange or by modifying the nanosheets with primary and tertiary alkylamines [1-3]. Intercalative amine modification was optically tracked in real time [3,4] and shown to be reversible after exposure to competitive amine vapors. For rational sensor design, amine intercalation was used to fine tune the sensors’ sensitivity towards various volatile analytes based on tailored host—guest interactions. Spatial control of amine intercalation was used to demonstrate the fabrication of single-chip colorimetric sensor arrays that can be read out by the naked eye. Finally, we highlight the exceptional sensitivity of these sensing platforms to humidity, which allows for optical tracking of the motion of a finger across the sensor surface in real time, due to the humidity sheath surrounding the finger [5,6]. Such high sensitivity, paired with subsecond response times, bodes well for the development of touchless positioning interfaces for touchless displays based on humidity responsive 2D nanosheets.

[1] P. Ganter, K. Szendrei, B.V. Lotsch, Adv. Mater. 2016, 28, 7436–7442.
[2] P. Ganter, L. Schoop, B.V. Lotsch, Adv. Mater. 2017, 29, 1604884.
[3] P. Ganter, L.M. Schoop, M. Däntl, B.V. Lotsch, Chem. Mater. 2018, 30, 2557–2565.
[4] K. Szendrei-Temesi, A. Jiménez-Solano, B.V. Lotsch, Adv. Mater. 2018, DOI: 10.1002/adma.201803730.
[5] K. Szendrei-Temesi, O. Sanchez-Sobrado, S. Betzler, K.M. Durner, T. Holzmann, B.V. Lotsch, Adv. Funct. Mater. 2018, 28, 1705740.
[6] K. Szendrei, P. Ganter, O. Sánchez-Sobrado, R. Eger, A. Kuhn, B.V. Lotsch, Adv. Mater. 2015, 27, 6341–6348.

The non-equilibrium nature of laser-based synthesis and processing provides a promising alternative for the synthesis of van der Waals alloys and heterostructures of 2D transition metal dichalcogenides (TMD). Using chalcogen plasma plumes generated by pulsed excimer laser ablation of solid S, Se, or Te targets in vacuum and low-pressure background gases, the chalcogen composition of existing 2D crystal monolayers or multilayers can be digitally modulated to form new alloys or vertical heterostructures in order to controllably alter and explore their optoelectronic properties. In this talk, starting with existing monolayer and bilayer WS₂ or WSe₂ grown on various substrates (and suspended) we demonstrate the versatility of this approach to adjust the composition and property-tune mixed WSe_xWS_2-x monolayers, and partially convert bilayers to form vertical heterojunctions. In both cases, depending on the global strain inherent in the initial TMD crystal (resulting from the initial growth at elevated temperatures), the plasma-induced exchange of S-Se atoms in the TMD layer creates an additional biaxial strain that can subsequently induce unfavorable cracks or wrinkles in the 2D alloys. However, by choosing a suitable thermal expansion coefficient for the substrate for the initial growth of the 2D crystals, we show that the additional biaxial strain-induced deformation induced by chalcogen replacement can be mediated to allow the 2D crystals to avoid wrinkles and cracks, while retaining their lattice structure. Using this pulsed laser deposition conversion approach, we demonstrate that a homogenous bilayer TMD (e.g., a WS₂ bilayer) grown by chemical vapor deposition (CVD) can be converted into vertical heterojunctions, such as bilayer WSe₂/WS₂. The conversion of different bilayer stacking configurations is explored. Photoluminescence and Raman spectroscopy, and scanning transmission electron microscopy confirm that the original bottom layer is nearly intact, even when the top layer is fully converted into another chalcogen composition. Second harmonic generation measurements reveal that the degree of conversion of the top layer depends on the crystallographic stacking between the top and bottom layers. For example, while the 2H-stacking configuration of bilayer WS₂ is found to be the most stable under Se plume irradiation, other stacking configurations are found to have lower thresholds for chalcogen replacement, enabling full conversion of the top WS₂ layer to WSe₂ at 350 ^oC to form a vertical heterostructure. This approach provides a simple and efficient way to fabricate both 2D alloys and vertical heterostructures.

Two-dimensional (2D) materials have the potential to enable flexible electronic devices with exceptional properties for future wearable sensors. One major hurdle still to overcome is the limited growth and processing schemes to manipulate these unique materials on flexible substrates. In this talk, we will discuss using lasers and other light sources to locally transform 2D MoS₂, WSe₂ and heterostructures on polymeric substrates for controlled crystal structure, chemistry and functionalization at both a local (<10 ) and global (>1 cm²) scale. To understand the transformation process, kinetics studies using in-situ spectroscopy enabled insight into fundamental crystal formation and conversion of amorphous to crystalline MoS₂. This information enabled direct laser writing of 2D materials on flexible substrates for controlled crystal structure for sensing applications. Additionally, circuits were constructed out of the laser transformation of 2D materials to form various crystal structures and oxide phases with altering electronic properties. Finally, large area, wafer-scale crystallization of stretchable 2D photodetectors with the use of a broadband pulsed lamp source demonstrate the feasibility of large scale transformation as a means to create unique device constructs. The versatile technique of laser manipulation of 2D materials directly on polymer substrates can potentially enable truly revolutionary flexible electronics based on the unique van der Waals structures.

Two-dimensional (2D) layered semiconductors, notably transitional metal dichalcogenides (TMDs), have recently attracted considerable interest, due to their atomically thin geometry, unique electronic and optical properties and potential applications in integrated nanosystems. Band gap and interface engineering within these nanostructures is fundamentally important to design and create semiconductor heterostructures with tailored electronic band structures for diverse functions and applications^{[1, 2]}. Although significant effort and progress have been made to realize composition modulation among different chalcogenide nanostructures, precisely spatial band gap modulation within single 2D semiconductor nanostructures has always been a considerable challenge. In this talk, I will report our recent progress on the band gap and interface engineering of 2D atomically thin layered materials. I will show how to realize composition and band gap continuously modulated 2D semiconductors through alloying of two semiconductor compound with different band gaps^[3-5], and how to achieve sharp interfacial composition profiles of 2D semiconductor heterostructures through the real-time composition control^{[6, 7]}. More importantly, the growth control of 2D atomic layered heterostructures: lateral epitaxy vs. vertical stack^[8], and some interesting optical and optoelectronic properties and device applications of these novel 2D nanostructures will also be exhibited. In addition, I will reveal the strain-tuning atomic substitution mechanism of 2D semiconductor materials, which is driven by the strain between the substituted and host lattices^{[9, 10]}.

References:
[1] X. Duan, C. Wang, A. Pan*, et al. Chem. Soc. Rev., 2015, 44, 8859.
[2] H. Li, X. Wang, A. Pan*, et al. Chem. Soc. Rev., 2018, 47, 7504.
[3] H. Li, X.Duan, A. Pan*, et al. J. Am. Chem. Soc., 2014, 136, 3756.
[4] X. Duan, C. Wang, A. Pan*, et al. Nano Lett., 2016, 16, 264.
[5] H. Li, Q. Zhang, A. Pan*, et al. J. Am. Chem. Soc., 2015, 137, 5284.
[6] X. Duan, C. Wang, A. Pan*, et al. Nat. Nanotechnol. 2014, 9, 1024.
[7] B. Zheng, C. Ma, A. Pan*, et al. J. Am. Chem. Soc., 2018, 140, 11193.
[8] T. Yang, B. Zheng, A. Pan*, et al. Nat. Commun., 2017, 8, 1906.
[9] H. Li, X. Wu, A. Pan*, et al. ACS Nano, 2017, 11, 961.
[10] H. Li, H. Liu, A. Pan*, et al. ACS Nano, 2018, 12, 4853.

There has been tremendous interest in recent years to study two-dimensional (2D) atomic layers which form building blocks of many bulk layered materials. This was started by the spectacular discovery of graphene. This talk will focus on the materials science of the emerging field of 2D atomic layers of various compositions. Several aspects that include synthesis, characterization of defects, doping and manipulation will be explored with the objective of achieving functional structures with atomic layer building blocks. The concept of nanoscale engineering and the goal of creating new artificially stacked van der Waals solids and 3D constructs will be discussed through a number of examples including graphene and other 2D layer compositions. The talk will explore the emerging landscape of 2D materials systems that include hybrid compositions and multi-component 2D alloys. Some of the anticipated applications of these materials will also be discussed.

This talk will present recent highlights from our research on two-dimensional (2D) materials and devices including graphene, boron nitride (h-BN), and transition metal dichalcogenides (TMDs). The results span from fundamental measurements and simulations, to devices, to system-oriented applications which take advantage of unusual 2D material properties. On the fundamental side, we have measured record velocity saturation in graphene [1,2], as well as the thermal properties of graphene nanoribbons [3]. These are important for electronic applications, which can exhibit substantial self-heating during operation [4]. Taking advantage of low cross-plane thermal conductance, we found unexpected applications of graphene as ultra-thin electrode to reduce power consumption in phase-change memory [5]. We have also demonstrated wafer-scale graphene systems for analog dot product computation [6].
We have grown monolayer 2D semiconductors by chemical vapor deposition over cm² scales, including MoS₂ with low device variability [7], WSe₂, MoSe₂ – and multilayer TMDs MoTe₂ and WTe₂ [8]. Importantly, ZrSe₂ and HfSe₂ have native high-K dielectrics ZrO₂ and HfO₂, which are of key technological relevance [9]. Improving the electrical contact resistance [10], we demonstrated 10 nm transistors using monolayer MoS₂, with the highest current reported to date (>400 µA/µm), approaching ballistic limits [11]. We also directly measured the saturation velocity and saturation current in monolayer MoS₂, finding it is thermally-limited (i.e. by device self-heating and phonon scattering) to about one-third that of silicon and about one-tenth that of graphene [12]. Using Raman thermometry, we uncovered low thermal boundary conductance (~15 MW/m²/K) between MoS₂ and SiO₂, which could limit heat dissipation in 2D electronics [13]. We are presently exploring unconventional applications including thermal transistors [14], which could enable nanoscale control of heat in “thermal circuits” analogous with electrical circuits. These studies reveal fundamental limits and new applications of 2D materials, taking advantage of their unique properties.

References: [1] V. Dorgan, M.-H. Bae, E. Pop, Appl. Phys. Lett. 97, 082112 (2010). [2] M. Yamoah, et al., ACS Nano 11, 9914 (2017). [3] M.-H. Bae et al., Nature Comm. 4, 1734 (2013). [4] S. Islam, et al., IEEE Electron Device Lett. 34, 166 (2013). [5] A. Behnam et al., Appl. Phys. Letters. 107, 123508 (2015). [6] N. Wang et al., IEEE VLSI Tech. Symp., Jun 2016, Honolulu HI. [7] K. Smithe et al., ACS Nano 11, 8456 (2017). [8] M. Mleczko et al., ACS Nano 10, 7507 (2016). [9] M. Mleczko, E. Pop, et al., Science Adv. 3, e1700481 (2017). [10] C. English et al., Nano Lett. 16, 3824 (2016). [11] C. English et al., IEEE Intl. Electron Devices Meeting (IEDM), Dec 2016. [12] K. Smithe et al., Nano Lett. 18, 4516 (2018). [13] E. Yalon, E. Pop, et al., Nano Lett. 17, 3429 (2017). [14] A. Sood, E. Pop et al. Nature Comm. 9, 4510 (2018).

Two-dimensional materials exhibit unique and dramatically tunable optical and electronic properties. In this seminar, we will describe the fundamental exploration of some of these extraordinary properties, and how these materials are leveraged to create improved or even fundamentally novel devices that point towards impactful applications. A common theme throughout will be the value of highly interdisciplinary and collaborative discovery.

Highly voltage-tunable optical properties will be described in the context of graphene/gold heterostructure metasurfaces. Smooth modulation is observed in the reflected phase of infrared light of over 200 degrees at an infrared wavelength of 8.7 microns [1]. This fast optoelectronic tuning mechanism could enable photonic phased arrays for beamsteering and other high speed reconfigurable elements.

Atomically thin materials also can exhibit novel functionality which is without direct analog in bulk media. For example, an active material developed by oxidation of 2D boron nitride demonstrates formation of atomic scale conductive filaments under electronic bias, exhibiting memristive switching with femtojoule energy consumption [2].

Many emerging device concepts exploit the strong anisotropy of 2D materials and their properties. Multilayers comprised of 2D semiconducting MoS₂ exhibit strong anisotropy between lateral within-layer conduction and interlayer vertical transport, as well as inhomogeneous vertical carrier distribution. From systematic study of charge transport in these structures, a new mechanism for negative transconductance (NTC) device implementation is observed [3].

Sherrott, M. et al. Nano Lett. 2017, 17 (5) 3027-3034
Zhao, H. et al. Adv. Mater. 2017, 29 (47) 1703232
Liu, Y. et al. Nano Lett. 2017, 17 (9) 5495-5501

The advent of graphene in the recent years has led to widespread investigations on two-dimensional (2D) semiconductors of layered metal chalcogenides (LMCs). This is mostly directed towards their ultrathin and flexible electronic applications as many 2D LMCs interact strongly with light, exhibiting high-power-density optoelectronic devices. This is especially true for those 2D LMCs which comprise of post-transition metals (like In₂Se₃ here), when compared to their transition metal counterparts. Although vacuum deposition and micromechanical cleavage are the most popular techniques for producing high quality 2D LMCs, they suffer from limitations like costly high temperature instrumentation and polydisperse sampling (respectively). Colloidal chemistry, in this regard, offers an alternative low temperature synthetic route with unique morphological control. In this presentation, we will describe the colloidal synthesis of single-layer β-In₂Se₃ nanosheets with lateral sizes tunable from ∼300 to ∼900 nm. Such synthesis of a single layer of this material, which is five-atom thick (Se-In-Se-In-Se), was accomplished by using short aminonitriles (dicyandiamide or cyanamide) as shape controlling agents in a two-stage hot injection process. Discerning the phase of our synthesized nanosheets was particularly challenging since the In₂Se₃ system exhibits intricate polymorphism, especially in the few layer range. A combined analysis comprising of HRTEM and XRD with ED was employed for this. The intensity ratio between two diffraction peaks from two-dimensional slabs of the various phases were compared, which were determined by diffraction simulations in the first place. This was further corroborated by comparing and fitting the experimental X-ray diffraction pattern with Debye formula simulated patterns and also with side-view HRTEM imaging and simulation. The as-synthesized β-In₂Se₃ nanosheets were found to be indirect band gap semiconductors (Eg = 1.55 eV). Single nanosheets were then contacted by Ti/Al or Ti/Au electrodes in order to fabricate photodetectors which showed high photoresponsivities (of the order of 10³ A/W at few pW laser power) range at low laser power and fast response times (order of few milliseconds). These values of photoresponsivity and response times are comparable to the best photodetectors based on single- and few-layer 2D semiconductors that were reported so far, and in particular to those of multilayer β- In₂Se₃, which implies a higher photoresponsivity per layer in the monolayer regime.

Optical metasurface refers to a kind of nanostructured material with sub-wavelength thickness and on-demand optical properties. By rationally engineering the metasurface structures, one could achieve new capabilities for the manipulation of light, e.g., ultra-thin flat lenses, waveplates and holographic plates. In this talk, I will present the unique capability of metasurfaces to address the fundamental limitations in traditional optical materials and realize ultra-compact optoelectronic devices with various functionalities, which are highly desirable for practical applications.
I will discuss about recent progress on mid-infrared modulation based on metasurface integrated with active tuning element. I will first discuss novel design concepts and device implementation of modulators with large modulation depth and broad bandwidth, followed by a summary of our recent progress on system implementation of spatial light modulation and wavefront control.
I will also present how to exploit the great flexibility of metasurfaces to manipulate light and enhance light matter interaction in nanometer-size structures to enhance the performance and enable unique functionalities of photodetectors. I will discuss about various schemes we have implemented to enhance infrared photodetection in graphene-metasurface hybrid structures. Moreover, I will also briefly discuss about a full-stokes polarimetric imager sensor with chip-integrated metasurfaces.

Strain engineering has been a promising approach to control exciton dynamics and photoluminescence properties of two-dimensional (2D) semiconductors. In particular, strain concentrated on submicron area of 2D WSe₂ or WS₂ is known to create quantum emitters by exciton funneling and exciton confinement through bandgap gradient. While the previous approaches have introduced 2D layers suspended on top of nano- or micro-patterns to achieve spatially deterministic strain, predictabilty and modulability of strain have been limited due to challenges in interface control (e.g., uniformity) and dynamic configurability (e.g., rigid substrate). Here, we present predictable and reconfigurable strain gradient in atomically-thin WSe₂ via three-dimensional (3D) elastomeric wrinkle architectures. This approach allows 2D WSe₂ to be deformed conformally along the wrinkled substrate comprised of a stiff skin layer (SiO₂) on top of a soft polydimethylsiloxane (PDMS) substrate. As a result, the strain exerted on WSe₂ was periodically changed to tensile and compressive strains at peaks and valleys of the wrinkles, respectively. By tuning wrinkling parameters (e.g., prestrain, skin layer thickness) and encapsulation methods, we were able to achieve photoluminescence (PL) emission shift of as much as 123 meV, corresponding to approximately 2.3% strain. Furthermore, thanks to the use of elastomeric substrate, the applied strain can be tuned reconfigurably by post stretching/releasing processes, with PL shift dynamically modulated. Time-resolved PL decay measurements revealed heterogeneous exciton recombination where tensile strain leads to longer exciton lifetime compared to unstrained or compressive strained regions. Our results demonstrate a 3D architecture based active platform not only for exciton engineering of 2D semiconductors, but also for other strain-responsive 2D devices which can exploit the merits of spatially and dynamically controllable strain engineering.

Two-dimensional layered materials have recently garnered burgeoning amount of interest due to their unique electronic, optical, thermal, mechanical properties emerging at the mono-to-few layer thicknesses. Over the past decade, we have witnessed significant research activity on the area of two-dimensional materials. Most of research has focused on conventional 2D materials such as graphene and 2D TMDCs, having isotropic electronic and optical properties due to their crystal symmetry. Recently, layered materials such as black phosphorus and hexagonal boron nitride investigated for anisotropic crystal structure. In this talk, I will present anisotropic plasmonic and photonic properties of black phosphorus at the visible and infrared frequencies. Monolayer black phosphorus exhibit plasmonic characteristics at far-infrared wavelengths. Using full-field electromagnetic simulations, we demonstrate that monolayer black phosphorus nanostructures support localized surface plasmons at different wavelengths for different polarization owing to its puckered crystal structure. Moreover, we investigate polarization dependent absorption properties of few-layer black phosphorus films at visible frequencies. Symmetric and asymmetric metal nanoantennas are used to control absorption intensity in few-layer black phosphorus films. I will also discuss our recent theoretical investigations on mono-layer borophene plasmons at optical frequencies. In the second part of my talk, I will discuss hexagonal boron nitride thin films, which exhibit out-of-the-plane anisotropic phononic response at mid-IR wavelengths. We designed and experimentally demonstrated an anisotropic phononic absorber around 8 and 12 micron wavelengths. Anisotropic plasmonic, photonic and phononic materials provide additional freedom in controlling polarization dependent properties and could find applications in wide range of fields where polarization control plays important role.

Recent findings on ReX₂ compounds show that their optical properties differ markedly from that of other members of the transition metal dichalcogenides (TMDs) family. Due to their distorted lattice structure and resulting low symmetry, unique properties arise in ReX₂; even an electronic decoupling of the bulk layers similar to the monolayer case has been proposed [1]. However, the degree of interlayer interaction is currently debated. Analysis of the response of the electronic band structure to pressure can allow further insights to clarify the nature of the interaction between adjacent layers.
We present an ab-initio investigation at the density functional level and experimental photoreflectance spectroscopy results of the direct optical transitions of ReS₂ and ReSe₂ at high hydrostatic pressures. We analyse the consequences of increasing pressure on the band structure and draw important conclusions about the nature of the band gap and the interlayer interactions using orbital theory. Our results suggest that there is an appreciable interlayer interaction which confirms recent direct measurements of the band dispersion [2,3]. Another intriguing result is that the tunability of the direct band gap of ReX₂ with pressure exhibits the opposite trend than for conventional TMDs like MoS₂ and WS₂: while the pressure coefficient of the direct band gap energy in conventional TMDs like MoS₂ and WS₂ is positive, it is negative in ReX₂.

[1] S. Tongay et al. (2014), Nature Communications, 5: 3252, DOI: 0.1038/ncomms4252
[2] L. S. Hart et al. (2017), Scientific reports, 7: 5145, DOI: 10.1038/s41598-017-05361-6
[3] D. Biswas et al. (2017), Physical Review B 96, 085205, DOI: 10.1103/PhysRevB.96.085205

Atomically-thin two-dimensional (2D) materials provide a unique and promising platform for optoelectronic applications due to their optical, electronic, and mechanical properties. Monolayer molybdenum disulfide (MoS₂) is one of the most widely studied semiconducting 2D materials for photosensors with its direct bandgap. Despite of advantageous features of 2D materials, the low optical absorption of monolayer MoS₂, resulting from its single-atom-thickness, limits higher photoresponsivity. In this presentation, we report a mechanically self-assembled, crumpled MoS₂ photosensor with graphene electrodes. We use graphene as electrodes in our all-2D photosensor because of its mechanical robustness, electrical conductivity and barrier-free contact to MoS₂. Our photosensor exhibits (i) enhanced photosensitivity from crumpled MoS₂ structure, (ii) high stretchability with mechanical durability, and (iii) anisotropic photocurrent generation from anisotropic topography via exciton funneling effect. We demonstrate enhanced internal photoresponsivity of our crumpled MoS₂ photosensor, by comparing with flat MoS₂ photosensor. Our photosensor was further stretched up to 150% without introducing major structural damage, and mechanical durability of the sensor was verified with 1000 cycles of stretching-releasing test. Our approach to mechanical self-assembly of atomically-thin materials offers a simple but effective way to enhance photosensitivity and suggests a new way to engineer exciton funneling in 2D material system.

Two-dimensional (2D) elemental materials have emerged as a new class of nanosheets whose properties are highly reliant on layer thickness. These single element materials possess a wide range of exciting tunable electronic and optical properties. Materials such as black phosphorus were initially hampered due to their ambient instability.
We investigated the fundamental origins of material degradation and developed a surface-engineering approach to render few-layer black phosphorus stable for several months in ambient conditions. The findings revealed that the key cause of degradation is photo-oxidation which is specifically accelerated when the material is exposed to UV light. [1,2] Building on this knowledge, we used an ionic liquid on the surface that sequesters reactive oxygen species (ROS) that are responsible for surface deterioration.[3] We further created a dry-etch procedure to control the layer-thickness (hence bandgap) and engineer controlled defects in the material.[4] Subsequently, the defect engineered layers were exploited to generate unique photoresponse in BP which was deployed to mimic neural synaptic interfaces using optical stimuli.
In summary, we have conducted a full suite of studies ranging from understanding the fundamentals of degradation to developing protection techniques without interfering with the desirable properties of 2D BP. Subsequently, we report a simple defect engineering strategy and exploit them for various functionalities including the creation of an artificial optical synapse.

[1] S. Walia, Y. Sabri, T. Ahmed, M.R. Field, R. Ramanathan, A. Arash, S.K. Bhargava, S. Sriram, M. Bhaskaran, V.Bansal and S. Balendhran “Defining the role of humidity in the ambient degradation of few-layer black phosphorus,” 2D Materials 4 015025 (2016)
[2] T. Ahmed, S. Balendhran, Md. N. Karim, E. Mayes, M. Field, R. Ramanathan, M. Singh, V. Bansal, S. Sriram, M. Bhaskaran and S. Walia<span style="font-size:10.8333px"> </span>“Degradation of black phosphorus is contingent on UV–blue light exposure,” Nature Partner Journal: 2D Materials and Applications 1 8 (2017)
[3] S. Walia,<span style="font-size:10.8333px"> </span>S. Balendhran, T. Ahmed, M. Singh, C. El-Badawi, M.D. Brennan, P. Weerathunge, Md. N. Karim, F. Rahman, A. Rassell, J. Duckworth, R. Ramanathan, G.E. Collis, C.J. Lobo, M. Toth, J.C. Kotsakidis, B. Weber, M. Fuhrer, J.M. Dominguez-Vera, M.J.S. Spencer, I. Aharonovich, S. Sriram, M. Bhaskaran and V. Bansal “Ambient protection of few-layer black phosphorus via sequestration of reactive oxygen species,” Advanced Materials 29 201700152 (2017)
[4] S. Kuriakose, T. Ahmed, S. Balendhran, G.E. Collis, V. Bansal, I. Aharonovich, S. Sriram, M. Bhaskaran, S. Walia “Effects of plasma-treatment on the electrical and optoelectronic properties of layered black phosphorus,” Applied Materials Today 12 244 (2018)

While graphene is well known to have exceptional electrical properties, those properties can be greatly affected by its substrate. Electron-phonon scattering has been shown to contribute to the optical absorption and interaction with polar substrate phonon modes can limit electron transport. Instead of polar substrates, we have investigated graphene and graphene nanoribbons on BaF₂. The BaF₂ is grown epitaxially on either Si, resulting in (111) BaF₂, or GaAs, resulting in (100) BaF₂, which is lattice matched. As a result, unlike SiO₂, we have an atomically crystalline surface upon which CVD grown graphene sheets are placed and then formed into nanoribbons by ion lithography. BaF₂ has a wide Restrahlen band around 50um, making it a good candidate for coupling to surface phonon plasmons in the far IR. To study the substrate effects, we use EFM AFM which provides a relationship between the electrostatic potential and the voltage applied to the tip. By mapping the phase change vs voltage, we can obtain the work function. EFS measures the force gradient versus the force providing a better resolution of the electrical properties. Since BaF₂ grows (111) on Si by forming triangles in three directions, we can use STM to determine edge effects which can be polar. In building, heterosturctured materials it is important to understand the valence band offset and the heterostructure interface. We use XPS spectra of the valance band to obtain the valance band offset formed from a BaF₂/graphene structure. Finally, we explain the large red shift in Raman peaks we observed as due to the lower frequency phonons from the much larger Ba atom.

The bottom-up wet chemical synthesis of graphene nanoribbons (GNRs) opens interesting opportunities for tailoring the GNR structure with atomic precision [1]. Atomically precise porous GNRs are a new chemically synthesized variation for which the fabrication procedure yielding multiple pores in a single ribbon and the electronic details of the ribbon have not been reported. In this work, porous GNRs are dry contact transferred in ultrahigh vacuum to clean silicon and III-V semiconducting substrates and examined using UHV scanning tunneling microscopy (STM) and spectroscopy (STS). STM imaging confirms the expected porous structure and indicates a unique electronic feature at the graphene nanopores, and STS measurements indicate a 2.0 eV bandgap. These results are compared to first-principles DFT simulations in which an increased local density of states at the pores is predicted. A GW correction predicts a 3.24 eV bandgap. Illumination of pore effects in GNRs contributes to an increased understanding of the tunability of GNR electronic structure. Porous GNRs have potential applications in molecular filtration, detection and DNA sequencing.

References

[1] Vo, T. H., Shekhirev, M, Kunkel, D. A., Morton, M. D., Berglund, E., Kong, L., Wilson, P. M., Dowben, P. A., Enders, A., and Sinitskii, A., Nat. Commun. 2014, 5, 3189.

We report the first observation of the non-magnetic Barkhausen effect in van der Waals layered crystals, specifically, between the
T_d and 1T' phases in type-II Weyl semimetal MoTe₂. Thinning down the MoTe₂ crystal from bulk material to about 25nm results in a drastic strengthening of the hysteresis in the phase transition, with the difference in critical temperature increasing from ~40K to more than 300K. The Barkhausen effect appears for thin samples and the temperature range of the Barkhausen zone grows approximately linearly with reducing sample thickness, pointing to a surface origin of the phase pinning defects. The distribution of the Barkhausen jumps shows a power law behavior, with its critical exponent α = 1.27, in good agreement with existing scaling theory. Temperature-dependent Raman spectroscopy on MoTe₂ crystals of various thicknesses shows results consistent with our transport measurements.
Reference: C. Cao et al., 2D Materials 5, 044003 (2018)

In recent years, graphene has attracted significant amount of research efforts on multiple fronts in the photonics field, mainly thanks to its attractive material properties such as high carrier mobility at room temperature and large tunability of carrier concentration via electrostatic gating. Development of reliable and high-throughput processes of graphene transfer also make integrating graphene with a wide variety of other materials, structures and devices relatively straightforward. Moreover, in the mid-infrared to terahertz (THz) spectral range, graphene can support highly confined surface plasmon modes with relatively long lifetime, and therefore drastically enhance the light-matter interactions in various processes which can be exploited by a wide range of applications. There has been a plethora of proof-of-concept demonstrations and further developments of graphene-based photonic devices for different applications and spectral regions, and research in these fields is growing rapidly. In this talk, I will present some of our recent work on employing graphene as a key functional material in various photonic devices operating in the THz spectral region, including high-speed modulators, beam steerers, photodetectors and molecular sensors. These devices may facilitate the ongoing development of THz technologies and applications such as THz wireless communications, imaging and sensing.

Low dimensional materials constitute an exciting and unusually tunable platform for investigation of integer and fractional quantum Hall states. Here I will present our results on transport measurements of high quality graphene and phosphorene devices. In rhomohedral-stacked tetralayer graphene, we have observed a large intrinsic gap at half filling, up to 80 meV, that arises from electronic interactions[1]. In Bernal-stacked-tetralayer, we have observed multiple Lifshitz transitions as charge density, magnetic field and displacement field are tuned[2]. In few-layer black phosphorus devices with mobility up to 50,000 cm²/Vs, integer QH states are resolved at a few Tesla, and fractional states at filling factor -4/3 and -0.56±0.1 are observed[3]. These results underscore the strong electron correlation in atomically thin systems.

1. K. Myhro, S. Che, Y. Shi, Y. Lee, K. Thilahar, K. Bleich, Dmitry Smirnov, C. N. Lau, “Large tunable intrinsic gap in rhombohedral-stacked tetralayer graphene at half filling”, 2D Materials, in press (2018).
2. Y. Shi, S. Che, K. Zhou, S. Ge, Z. Pi, T. Espiritu, T. Taniguchi, K. Watanabe, R. Lake, and C.N. Lau, “Tunable Lifshitz Transitions and Multiband Transport in Tetralayer Graphene”, Phys. Rev. Lett., 120, 096802 (2018).
3. J. Yang, S. Tran, J. Wu, S. Che, P. Stepanov, T. Taniguchi, K. Watanabe, H. Baek, D. Smirnov, R. Chen, C.N. Lau, “Integer and Fractional Quantum Hall effect in Ultra-high Quality Few-layer Black Phosphorus Transistors”, Nano Letters, 18, 229 (2018).

The properties of 2D materials can be modified by structural defects, so controlling the nature and concentration of such defects provides a means of tailoring material properties for specific applications [1]. This study examines the effect of ion-irradiation on damage production in graphene, with particular emphasis on the effect of ion fluence and nuclear stopping power (S_n) on the concentration and structure of defects; the significance of collective effects associated with molecular ion irradiation; and the role of the substrate in damage production.

Experiments are conducted on supported (G/SiO₂) and suspended (Gs) graphene films, with the former produced by exfoliation of highly oriented pyrolytic graphite (HOPG) and the latter by chemical vapor deposition (CVD). The films are irradiated with 30 keV H^-, Li^-, C^-, Si^- and Ge^- ions to fluences (Ø) in the range 10¹⁰–10¹⁶ ions/cm² and analysed using Raman Spectroscopy (RS). The density and structure of defects are determined from Raman analysis using the phenomenological models of Lucchese et al. [2] and Eckmann et. al. [3], respectively. Substrate effects are assessed by comparing damage in G/SiO₂ and Gs irradiated under identical conditions and by comparing damage in G/SiO₂ irradiated with 30keV C at incident angles of 30°, 45° and 60° relative to the surface normal. G samples are also irradiated with 60 keV C₂^- ions to assess collective effects resulting from multi-ion interactions.

It is shown that defect production scales with the nuclear stopping power of the ions, and that the structure of the defects evolves with increasing ion-fluence from an initial mix of Stone-Wales defects and divacancies, to defects with an increasing fraction of higher-order topological defects. A comparison of the defects created in suspended graphene films by 30 keV C^- ions and 60 keV C₂^- ions further showed that the damage created by dimer ions closely matched that produced by monomer ions with twice the fluence, suggesting that there no significant collective effects associated with the molecular ion irradiation.

The predictions of reference [4] that ion-irradiation damage in supported graphene films is dominated by recoiling ions and sputtered atoms from the substrate is also tested by comparing defect accumulation in supported and suspended films irradiated under identical with 30 keV C^- ions. No significant difference in defect density is observed for the two samples, clearly showing that defect production is dominated by direct collisions with C atoms in the graphene layer, a conclusion that is further supported by data from variable incident angle studies.

References
[1] P. Miro, M. Audiffred, T. Heine, Chem Soc Rev, 43 (2014) 6537.
[2] M.M. Lucchese et al., Carbon, 48 (2010) 1592.
[3] A. Eckmann, et al., Nano Lett, 12 (2012) 3925.
[4] W.S. Li, et al., Sci Reports, 5 (2015) 6.

Due to their unparalleled mechanical strength, and stability down to a monolayer, atomically-thin membranes made from two dimensional (2D) materials like graphene and MoS₂ represent the ultimate limit in size of mechanical atomic membranes. Yet, many of the most interesting properties of 2D materials and new functionality arise from the interfaces between layers and in engineering multilayer heterostructures. The last few years have led to a revolution in nanoelectronics from 2D heterostructures where layer-by-layer stacking of monolayer 2D materials with disparate electronic properties leads to functional nanometer scale electronic devices. An open question is how to integrate the outstanding mechanical properties of 2D materials with the enhanced functionality of heterostructures to enable new technologies like 3D foldable circuitry, nanoscale origami, or resonant NEMS utilizing 2D heterostructures as active electronic and mechanical components. In this presentation, we will examine the mechanics of deformation of 2D atomic membranes, the emergence of interlayer slip in van der Waals interfaces, and their impact on nanoelectromechanical systems.
We measure the morphology of multilayer graphene transferred onto steps in a hBN substrate. We find the graphene conforms with nanoscale curvatures down to 1 nm, which is allowed by the introduction of slip dislocations between the graphene layers. From the curvature, we find that the bending modulus of the slipped graphene is linearly dependent on the number of layers rather than the cubic dependence that would be predicted from continuum mechanics. Next, we examine the evolution of buckle and fold instabilities in uniaxially compressed graphene and MoS₂ by measuring the topography under increased compression. We observe a material dependent transition from delamination and new fold generation to slip and growth of existing folds. Through a simple energy conservation model, we predict this transition and use it to extract the material dependent interfacial friction. Finally, we engineer electromechanical resonators from atomically thin bimorphs of graphene/MoS2 as well as twisted bilayer graphene. We observe new features in the tuning behavior of the resonators, including kinks and discrete jumps. These features are not predicted by the tensioned membrane model used to describe the behavior of monolayer 2D resonators, and we attribute them to atomic scale stick-slip behavior in the resonators.
Taken together, these experiments show that interfacial slip strongly impacts the mechanics of 2D materials and heterostructures and leads to membranes which are orders of magnitude more deformable than conventional 3D materials. As a result, slip is a critical phenomena which must be considered when engineering mechanically active devices from 2D materials and heterostructures.

In this talk, I will discuss our recent success in confining optical fields to atomic scale allowing to visualise vibrational and electronic dynamics of single-molecule [1,2]. Optical confinement is achieved via plasmonic coupling between gold nanostructure placed over a metallic film (NPoM geometry), generating strongly red-shifted resonances with intense local field amplification on the nanoscale [3,4]. This allows to directly sense individual atoms and molecules and watch their dynamics in real time at room temperature, also allowing to couple to excitations in monolayer semiconductors [5,6]. In this talk, I will also discuss our latest work on the sub-nm precision assembly of single-molecule using DNA origami technique, which allows to directly map optical near-field with deterministic single-molecule emission in strong-coupled systems [7,8]. I will also show how these can produce sensing systems which become realistic for widespread application.

Reference:
[1] Nature 535, 127 (2016); Single-molecule strong coupling at room temperature in plasmonic nanocavities.
[2] Science 354, 726 (2016); Single-molecule optomechanics in pico-cavities.
[3] ACS Photonics, 4, 469 (2017); How Ultranarrow Gap Symmetries Control Plasmonic Nanocavity Modes: From Cubes to Spheres in the Nanoparticle-on-Mirror.
[4] ACS Nano 11, 850 (2017); Revealing Nanostructures through Plasmon Polarimetry.
[5] Nature Comm, 8, 994 (2017); Plasmonic Tunnel Junctions for Single-Molecule Redox Chemistry.
[6] Nature Comm, 8, 1296 (2017); Strong-coupling of WSe2 in ultra-compact plasmonic nanocavities at room temperature.
[7] ACS Photonics, 5, 186 (2018); Suppressed Quenching and Strong-Coupling of Purcell-Enhanced Single-Molecule Emission in Plasmonic Nanocavities.
[8] Nano Letters, 18, 405 (2018); Mapping Nanoscale Hotspots with Single-Molecule Emitters Assembled into Plasmonic Nanocavities Using DNA Origami.

Designing materials with structural hierarchy is of fundamental interest in materials science since it uncovers unique physical properties and functions. The field of two-dimensional (2D) materials is highly active and rapidly expanding, but hierarchical material design in 2D materials has been challenging. Here, we report a novel 2D semiconductor with a hierarchical structure composed of covalently linked Re₆Se₈ clusters. The material, a 2D structural analogue of the Chevrel phase, is prepared via mechanical exfoliation of the van der Waals solid Re₆Se₈Cl₂. Using scanning tunneling spectroscopy, photoluminescence and ultraviolet photoelectron spectroscopy, and first-principles calculations, we determine the electronic band gap (1.58 eV), optical band gap (indirect, 1.48 eV), and exciton binding energy (100 meV) of the material. The latter is consistent with the partially 2D nature of the exciton. Re₆Se₈Cl₂ is the first member of a new family of 2D semiconductors whose structure is built from superatomic building blocks instead of simply atoms; such structures will expand the conceptual design space for 2D materials research.
To demonstrate its unique properties, we bring our attention to hierarchical phonons of the material. Phonon couplings to electrons, excitons and other phonons play key roles in material properties, such as charge transport, thermal transport, light emission, energy transfer, and superconductivity. In conventional atomic solids, such as Si or GaAs, there are only extended phonons as determined by bonding and crystalline symmetry. Phonons are uniquely important in nanomaterials due to quantum confinement and spatial localization in zero, one, and two dimensions (0D, 1D, 2D). In quantum dots, the 0D confinement and local symmetry/interfaces can lead to strong electron-phonon coupling. In 1D carbon nanotubes and 2D graphene or transition metal dichalcogenides, the reduced dimensions also enhance phonon coupling to electrons or excitons. In these materials, their phonon modes are intimately related to their dimensionalities, as demonstrated from radial breathing modes, interlayer sliding modes, and interlayer breathing modes. Here we expand the phonon space by adding localized 0D phonon modes to delocalized phonon manifolds in a hierarchical semiconductor. With coherent phonon spectroscopy, we observe three distinct phonons showing strong electron-phonon coupling and correlated with three different steps of dimensional hierarchy. Additionally, these phonons, including the localized 0D breathing of each superatom, the delocalized twisting of superatoms in each 2D layer, and interlayer breathing modes, can be concurrently generated by above-gap impulsive excitation. The presence of local phonon modes in an extended crystal opens the door to controlling material properties from hierarchical phonon engineering.

In the present report, chemical functionalization is demonstrated to enhance the electrical performance of two-dimensional (2D) layered transition metal dichalcogenides (TMDCs) field-effect transistors (FETs) using a one step-dipping process utilizing ammonium sulfide [(NH₄)₂S] solution. The adsorption process at the surface of WSe₂ via chemical treatment is elucidated at the molecular level using scanning tunneling microscopy (STM) and spectroscopy (STS). Molecularly resolved STM and STS reveal that the molecular adsorption on monolayer (ML) WSe₂ surface induces a band structure transition with the reduction of electronic band gap from 2.1 eV to 1.1eV and the Fermi level shift towards the valence band edge (VBE), consistent with an increase in the density of positive charge carriers. The electronic transformation of WSe₂ with this one-step chemical treatment is confirmed by density functional theory (DFT) calculations which reveal that molecular ‘SH’ adsorption on the WSe₂ surface induces a Fermi level shift towards its VBE along with a reduction in its band gap. As a result of the chemical treatment, the p-branch ON-current (I_ON) of few-layer WSe₂ FETs is enhanced about two orders of magnitude with a ~ 6 X increase in the hole field-effect mobility and a subthreshold swing decrease of about 40 %. Consequently, this one-step chemical functionalization technique can serve as a model method to control the electronic band structure and enhance the performance of devices based on layered TMDCs.

The heterojunction band offset, a critical design parameter for optoelectronic devices, generally cannot be changed after fabrication of the interface. Local strain engineering can manipulate the band structure of a semiconductor, enabling the ability to actively tailor the band offset in a van der Waals heterostructure. Here, we demonstrate continuous variation of the band alignment between the constituent monolayers in monolayer transition metal dichalcogenide (TMDC) heterostructures (i.e. MoS₂-WSe₂ and MoSe₂-WSe₂) by applying periodic mechanical strain. We interpret the strain-induced photoluminescence peak position shift of the interlayer exciton as modulation of the heterojunction band alignment. We spatially resolve the energy shift of the interlayer exciton in a heterostructure subject to alternating tensile and compressive strains with periods of ~1 mm. Comparing the MoS₂-WSe₂ and MoSe₂-WSe₂heterostructures, we observe that peak shifts of ~50 nm are possible through application of in-plane strain. Raman and absorption spectroscopy further confirmed the effects of local strain and the resultant topographical deformation. Our results suggest that strain control of band alignment affects the interlayer exciton which can have a broad impact on van der Waals optoelectronic devices, including improved photovoltaic device performance and enhanced excitonic light emission.

Origami is perhaps best known as an art form that transforms flat sheets of paper into intricate three-dimensional (3D) structures. Inspired by this technique, recent works have demonstrated the folding of two-dimensional (2D) materials into micron-scale structures actuated by thermal and chemical perturbations. “Capillary origami,” in which the surface tension of a liquid droplet on a flat sheet causes the sheet to fold, is a particularly attractive origami technique because it can be designed for liquid-liquid interfaces, enabling many biological and chemical applications. Here, we extend capillary origami techniques to 2D materials, using droplets in liquid systems to fold micron-scale polyhedra from sheets of monolayer molybdenum disulfide (MoS₂), a 2D semiconductor. The geometries of the final folded origami are preprogrammed by lithographic design of different starting shapes of MoS₂ and the introduction of rigid metal panels to define hinges. These origami structures are further made self-folding by manipulating the miscibility of the droplets on the sheets, growing and shrinking the droplets to dynamically control folding. This work demonstrates a new avenue for actuating 2D materials into pre-designed 3D structures.

First-principles calculations predict that epitaxial growth of Au on 2D transition metal dichalcogenides (TMDCs) should induce selective strain in the top layer of a multilayer stack^1,2. The ability to manipulate strain in this way would have implications for the nanomanufacturing of these materials for opto-electronic devices³, such as the ability to modulate the band gap within a single monolayer.

4D scanning transmission electron microscopy (4D STEM) is used to investigate the strain states present in the Au-MoS₂ system. Where Au is evaporated onto MoS₂, we find consistent compressive biaxial strains up to a stunning 9% in MoS₂, as well as previously unreported epitaxial relationships, imposing anisotropic strains, between the two species. Further, it appears that the strain is confined to the monolayer in contact with the Au, while other layers in the structure remain unstrained. These findings open up new avenues of inquiry in the investigation of TMDC-metal interactions.

We will present the results of extensive characterization of the various morphologies and strain states found at the Au-MoS₂ interface via 4D STEM. This technique involves recording a selected area electron diffraction (SAED) pattern every few nanometers across a two-dimensional sample area, and the resulting data is a 2-D array of 2-D diffraction patterns, comprising a 4-D dataset. Extremely large, information-rich datasets are produced, which requires a large-scale approach to data analysis. In order to meet this need for high-throughput analysis, we develop a novel machine learning approach that identifies, indexes, and characterizes the 10,000s of SAED patterns in these 4D STEM datasets utilizing a neural network (NN). The NN is trained on simulated diffraction patterns to recognize different crystal species and orientations (e.g., MoS₂ [001], Au [111], Au [211]). The strain states of the identified crystals are then characterized using conventional analytical scripts.

These findings provide empirical motivation for the Au-assisted exfoliation of TMDCs for anticipated applications in nanoelectronics, photonics, and sensing. Moreover, this evidence of ultrahigh strains bears implications for band gap engineering in monolayer TMDCs. This multidisciplinary work directly integrates modern computer science deep learning techniques with microscopy and materials science, bridging a knowledge gap in the field and opening the door to rapid characterization of nanoscale structures.

1. Y. Zhou, D. Kiriya, E. E. Haller, J. W. Ager, A. Javey, and D. C. Chrzan: Compliant substrate epitaxy: Au on MoS₂. Phys. Rev. B 93(5), 1 (2016).
2. H. Sun, E. W. Sirott, J. Mastandrea, H. M. Gramling, Y. Zhou, M. Poschmann, H. K. Taylor, J. W. Ager, and D. C. Chrzan: Theory of thin-film-mediated exfoliation of van der Waals bonded layered materials. Phys. Rev. Mater. 2(9), 094004 (2018).
3. S. B. Desai, S. R. Madhvapathy, M. Amani, D. Kiriya, M. Hettick, M. Tosun, Y. Zhou, M. Dubey, J. W. Ager, D. Chrzan, and A. Javey: Gold-Mediated Exfoliation of Ultralarge Optoelectronically-Perfect Monolayers. Adv. Mater. 28(21), 4053 (2016).

Recent progress on automatic driving technology further requires infrared detectors with faster response and room temperature operation. Although fast response is achieved for quantum type HgCdTe and quantum-dot infrared photodetector (QDIP), the thermal noise inherently prevents the infrared detection by narrow gap semiconductors at room temperature [1]. Here, bilayer graphene (BLG) phototransistor with the electrically tunable band gap is theoretically suggested to have the IR detectivity even at room temperature because the gate action due to the photoexcited carriers amplify the photocurrent and this amplification can overcome the thermal noise [2]. In order to experimentally prove the current amplification by phototransistor action, the understanding of the basic mechanism behind the photoresponse without phototransistor action is critical. In this study, we investigated the photoresponse to the visible light in h-BN encapsulated dual gate BLG-FETs on SiO₂/Si and quartz substrates.
For the visible light detection, a Ni top gate electrode with ~20 nm thickness and ~5 μm width was used. In the transport measurement at 10 K for the BLG heterostructure FET on the SiO₂/Si substrate, a pronounced photoresponse was observed around the charge neutrality point and the photoresponse at the charge neutrality point increased with increasing the band gap. Moreover, it was found that photoresponse diminished above 100 K. On the other hand, for the device on the quartz substrate, the photoresponse was quite limited even at 10 K. These comparison suggests that the photoresponse in the present device is dominated by bolometric effect, not by photoconduction. In previously reported BLG bolometer with SiO₂ top gate [3], the responsivity at the displacement field of -0.65 V/nm disappeared at ~40K, while the responsivity of the present device at the same displacement field was retained up to ~80K. This improvement is because the reduction of charged impurities in the heterostructure with h-BN resulted in stronger temperature dependence of channel resistance at the charge neutrality point.
As a summary, the dominant photoresponse without phototransistor action in h-BN encapsulated dual gate BLG-FETs is bolometric effect and the bolometric response is much improved than that in the BLG encapsulated with oxides. The phototransistor action will become prominent when the top gate length is reduced to the carrier diffusion length in BLG (~250 nm).

[1] P. Martyniuk, A. Rogalski, Prog. Quantum electronics 32 (2008) 89.
[2] Y. Ryzhii & M. Ryzhii Phys. Rev. B 79 (2009) 245311.
[3] J. Yan et al. Nat. Nanotechnol. 7 (2012) 472.

Graphene absorbs light of extremely wide wavelength range owing to its gapless bandstructure. This makes graphene a promising material for ultra-broadband photodetection [1]. In addition, due to high mobility, the graphene photodetectors can exhibit high speed operation [2]. However, there are two important roadblocks that prohibit graphene from being a highly sensitive photodetector. First, graphene being extremely thin material, can absorb only ~2.3% of the incident photons in a monolayer. Second, the lifetime of the photo-generated carriers is very short (~ ps). Thus, it is important to separate them in a time scale less than this to obtain a sizable photocurrent. Most of the graphene detectors, for example, metal-graphene junction [2], graphene p-n junction [3] and biased bilayer graphene [4], thus exhibit low responsivity. In this regard graphene detectors based on photogating effect are attractive which exhibit large responsivity [5-7]. However, these structures often rely on light absorption by a different material, like MoS₂ [5] and quantum dot [6], thereby the wavelength of operation is limited to the light absorption threshold of the respective absorbing material. These high-gain photogating devices also exhibit slow transient response with reminiscent photocurrent, and a gate pulse is often required to flush trapped carriers and restore initial state [5, 7] – necessitating additional circuit.

We propose and experimentally demonstrate a new class of photodetector using graphene/WS₂/MoS₂ van der Waals heterojunction in a floating gate structure. Graphene is used both as light absorbing medium as well as conducting channel in plane. A fraction of the photoelectrons generated in graphene is transferred vertically in an ultra-fast time scale to floating monolayer MoS₂ through the WS₂ spacer barrier layer. The electrons, after being transferred to MoS₂, thermalize in short time scale to the conduction band edge of MoS₂. While they can return to graphene by tunneling through WS₂ bandgap, a steady state population of electrons is created in a short time scale in MoS₂. The transferred photo-electrons in MoS₂ in turn electrostatically gate the graphene channel by strong capacitive coupling through the ultra-thin WS₂ spacer layer. When light is turned off, the captured electrons in MoS₂ swiftly tunnel through WS₂ to graphene due to the built-in field, automatically shutting off the gating action. This suppresses reminiscent photocurrent and no extra gate pulse is required to flush the confined electrons.

The stack is systematically characterized photoluminescence quenching measurement as a function of temperature. The photodetector exhibits a responsivity in excess of 10⁶ A/W in the NIR regime with a specific detectivity of 1.6x10¹¹Jones, which is higher than that of any existing graphene-absorption-based detector by several orders of magnitude. The photodetector also maintains high speed of operation (sub-millisecond transient response). This marks an important step towards building graphene based high speed, ultra-wideband photodetector with high sensitivity across bands.

References:
[1] F. H. L Koppens et al., Nat. Nanotechnol., vol. 9, pp. 780, 2014.
[2] T. Mueller et al., Nat. Photonics., vol. 4, pp. 297, 2010.
[3] M. Freitag et al., Nano Lett., vol. 13, pp. 1644, 2013.
[4] D. Spirito et al., Appl.Phys.Lett vol. 104, pp. 061111, 2014
[5] K. Roy et al., Nat. Nanotechnol., vol. 8, pp. 826, 2013.
[6] G. Konstantatos et al., Nat. Nanotechnol., vol. 7, pp. 363, 2012.
[7] C. H. Liu et al., Nat. Nanotechnol., vol. 9, pp. 273, 2014.