Session P15: 2D Materials: Preparation and Characterization

Electronic Transport Properties of New 2-D Materials GeH and NaSn$_{2}$As$_{2}$

2-D materials potentially have superior thermoelectric properties compared to traditional 3-D materials due to their layered structure. Here we present electrical and thermoelectric transport properties of 2 types of 2-D materials, GeH and NaSn$_{2}$As$_{2}$. GeH is a graphane analog which is prepared using chemical exfoliation of CaGe$_{2}$ crystals. Intrinsic GeH is proven to be a highly resistive material at room temperature. Resistance and Seebeck coefficient of Ga doped GeH are measured in a cryostat with a gating voltage varying from -100V to 100V. NaSn$_{2}$As$_{2}$ is another 2-D system, with Na atom embedded between nearly-2D Sn-As layers. Unlike GeH, NaSn$_{2}$As$_{2}$ is a metal based of Hall measurements, with p-type behavior, and with van der Pauw resistances on the order of ~5m$\Omega$/square. Thermoelectric transport properties of NaSn$_{2}$As$_{2}$ will be reported.   

Microwave Assisted 2D Materials Exfoliation

Two-dimensional materials have emerged as extremely important materials with applications ranging from energy and environmental science to electronics and biology. Here we report our discovery of a universal, ultrafast, green, solvo-thermal technology for producing excellent-quality, few-layered nanosheets in liquid phase from well-known 2D materials such as such hexagonal boron nitride (h-BN), graphite, and MoS$_{\mathrm{2}}$. We start by mixing the uniform bulk-layered material with a common organic solvent that matches its surface energy to reduce the van der Waals attractive interactions between the layers; next, the solutions are heated in a commercial microwave oven to overcome the energy barrier between bulk and few-layers states. We discovered the minutes-long rapid exfoliation process is highly temperature dependent, which requires precise thermal management to obtain high-quality inks. We hypothesize a possible mechanism of this proposed solvo-thermal process; our theory confirms the basis of this novel technique for exfoliation of high-quality, layered 2D materials by using an as yet unknown role of the solvent.   

A \textit{first principles} investigation of point defects in monolayer, few-layer, and bulk WS$_{2}$

We present the results of a systematic study of physics of point defects in 2D WS$_{2}$ materials conducted by means of density functional theory. First, we investigate the physics of point defects in monolayer (ML) WS$_{2}$. Second, we examine the impact of point defects on the physical properties of multi-layer defective WS$_{2}$ as a function of slab thickness. The studied point defects are: monovacancies, interstitials and anti-sites, and the considered physical properties include local geometry, defect formation energy, electronic structure and magnetism. Van der Waals interaction, spin-polarization and spin-orbit coupling effects are also incorporated in the calculations to ensure accurate results. In a ML WS$_{2}$, we predict that I$_{S}$ is the most favorable defect inside WS$_{2}$ having a low formation energy of 1.21 eV. W$_{S}$ and W$_{S2}$ anti-sites result in a total magnetic moment of 2 $\mu_{B}$. By studying ML, few-layer (up to 4 layers), and bulk WS$_{2}$ slabs we find that, all point defects cause only localized perturbation, thus have little influence on the thickness-dependent evolution of the physical properties. The depth-dependence of the defect formation energy is also found: V$_{S}$ prefers to stay on the surface, while V$_{W}$ prefers the slab center.   

Towards the Intrinsic Limit in As-Exfoliated MoS2

In recent years, two-dimensional transition metal dichalcogenide (TMDC) semiconductors have been intensively studied as exciting non-zero band gap analogs to graphene. For example, molybdenum disulfide (MoS2), a TMDC, is a van der Waals material which can be thinned down to single atomic layers less than a nanometer thick via micro-mechanical cleavage. In this regime, quantum confinement effects give rise to properties not seen in the bulk crystal. The attractive properties of ultrathin MoS2 have inspired myriad applications, including spin- and valley-tronics, and LED and photo-detecting devices. As the performance of these devices is optimized, a method of modulating these properties is strongly desired. Through exfoliating MoS2 on various substrates in an inert glovebox environment, we have produced as-exfoliated MoS2 doped at the intrinsic level. We study the changes in the MoS2 via Raman and photoluminescence spectra and see shifts in excitonic behavior. The ability to create intrinsic MoS2 without the need for chemical doping or gating has exciting implications for optical studies of the material in addition to device applications such as photovoltaic, photocatalytic, and LED devices.   

Preparation and Electronic Characterization of Substrate-Scale MoS$_{2}$ Single-Layer Films

Using a novel high vacuum chemical vapor deposition process we synthesize substrate scale (2x2 cm) homogeneous monolayer MoS$_{2}$ films. Our process involves exposure of a hot Mo filament to organic chalcogen precursors that volatilize MoS$_{x}$ species which then precipitate on a thermally-controlled substrate. The resultant films are photoluminescent at 1.87 eV as expected for monolayer material; their Raman modes are indistinguishable from exfoliated material. Metal contact formation to these films was investigated under UHV conditions utilizing X-Ray Photoelectron Spectroscopy. These measurements permit us to follow the formation of a Schottky Barrier with increasing metal film thickness on the Angstrom scale. We utilize core level spectroscopy to indicate the evolution of the MoS$_{2}$ valence band under metal deposition. Our measurements provide direct indication on the charge transfer direction at metal contacts and the ensuing band-bending in two-terminal devices.~   

Synthesis and electronic structure of single-layer TaS$_{2}$

Bulk TaS$_{2}$ is an intriguing material that exhibits charge density wave phases, Mott physics, and superconductivity; however, little work has been done on single-layer (SL) TaS$_{2}$. Progress in this area demands a method for controllably fabricating high-quality, uniform samples with low defect densities. We have succeeded in epitaxially growing SL TaS$_{2}$, using the Au(111) substrate. The monolayer exhibits a well-defined orientation with respect to the substrate, a strong preference toward forming triangular islands, and a moire superstructure. Furthermore, long deposition times lead to smooth layer-by-layer growth of TaS$_{2}$. In this talk, I will present band structure measurements acquired by angle-resolved photoemission spectroscopy (ARPES) on TaS$_{2}$/Au samples fabricated in situ at the SGM3 end station of the ASTRID2 synchrotron facility in Denmark. Scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) elucidate the material's structural properties and interaction with the substrate.   

Observation of Interlayer Phonons in Transition Metal Dichalcogenide Heterostructures

Interlayer phonon modes in transition metal dichalcogenide (TMD) heterostructures are observed for the first time. We measured the low-frequency Raman response of MoS2/WSe2 and MoSe2/MoS2 heterobilayers. We discovered a distinct Raman mode (30 - 35 cm-1) that cannot be found in any individual monolayers. By comparing with Raman spectra of Bernal bilayer (2L) MoS2, 2L MoSe2 and 2L WSe2, we identified the new Raman mode as the layer breathing vibration arising from the vertical displacement of the two TMD layers. The layer breathing mode (LBM) only emerges in bilayer regions with atomically close layer-layer proximity and clean interface. In addition, the LBM frequency exhibits noticeable dependence on the rotational angle between the two TMD layers, which implies a change of interlayer separation and interlayer coupling strength with the layer stacking.   

Distinct Reconstruction Patterns and Spin-Resolved Electronic States along the Zigzag Edges of Transition Metal Dichalcogenides

Two-dimensional transition metal dichalcogenides are a new class of materials exhibiting various intriguing physical, chemical, and mechanical properties. Integration of such materials for potential device applications will necessarily encounter creation of different boundaries. Using first-principles approaches, here we investigate the structural, electronic, and magnetic properties along two inequivalent M- or X-terminated zigzag edges of MX$_{\mathrm{2}}$ (M$=$Mo, W; X$=$S, Se). Along the M-terminated edges, we discover a previously unrecognized but energetically strongly preferred (2x1) reconstruction pattern, which is universal for all the MX$_{\mathrm{2}}$, characterized by place exchanges of the outmost X and M edge atoms. In contrast, the X-terminated edges undergo a more moderate (2x1) or (3x1) reconstruction for MoX$_{\mathrm{2}}$ or WX$_{\mathrm{2}}$, respectively. We further use the prototypical examples of zigzag MoX$_{\mathrm{2}}$ nanoribbons to demonstrate that the M- and X-terminated edges possess distinctly different electronic and magnetic properties, which can be exploited for a broad range of spintronic and catalytic applications   

Low-frequency Raman modes as fingerprints of layer stacking configurations of transition metal dichalcogenides

The tunable optoelectronic properties of stacked two-dimensional (2D) crystal monolayers are determined by their stacking orientation, order, and atomic registry. Atomic-resolution Z-contrast scanning transmission electron microscopy (AR-Z-STEM) can be used to determine the exact atomic registration between different layers in few-layer 2D stacks; however, fast and relatively inexpensive optical characterization techniques are essential for rapid development of the field. Using two- and three-layer MoSe2 and WSe2 crystals synthesized by chemical vapor deposition, we show that the generally unexplored low-frequency (LF) Raman modes (<50 cm-1) that originate from interlayer vibrations can serve as fingerprints to characterize not only the number of layers, but also their stacking configurations [Puretzky and Liang et al, ACS Nano 2015, 9, 6333]. First-principles Raman calculations and group theory analysis corroborate the experimental assignments determined by AR-Z-STEM and show that the calculated LF mode fingerprints are related to the 2D crystal symmetries. Our combined experimental/theoretical work demonstrates the LF Raman modes potentially more effective than HF Raman modes to probe the layer stacking and interlayer interaction for 2D materials.   

Excitons and exciton-phonon interactions in 2D MoS2, WS2 and WSe2 studied by resonance Raman spectroscopy

The 2D materials exhibit a very strong exciton binding energy, and the exciton-phonon coupling plays an important role in their optical properties. Resonance Raman spectroscopy (RRS) is a very useful tool to provide information about excitons and their couplings with phonons. We will present in this work a RRS study of different samples of 2D transition metal dichalcogenides (MoS2, WS2 and WSe2) with one, two and three layers (1L, 2L, 3L) and bulk samples, using more than 30 different laser excitation lines covering the visible range. We have observed that all Raman features are enhanced by resonances with excitonic transitions. From the laser energy dependence of the Raman excitation profile (REP) we obtained the energies of the excitonic states and their dependence with the number of atomic layers.. In the case of MoS2, we observed that the electron-phonon coupling is symmetry dependent, and our results provide experimental evidence of the C exciton recently predicted theoretically. The RRS results WSe2 show that the Raman modes are enhanced by the excited excitonic states and we will present the dependence of the excited states energies on the number of layers.   

\textit{Giant blue shifted photoluminescence peak from the edges of CVD grown monolayer MoS}$_{2}$

To probe the electronic and optical properties of direct band-gap monolayer transition metal dichalcogenides, such as band structure and excitons, micro-photoluminescence spectroscopy has become an attractive and standard tool. Here, we present our experimental work on spatial scanning of photoluminescence of monolayer MoS$_{\mathrm{2}}$ grown by chemical vapor deposition (CVD) using an ultrasmall blue laser (wavelength 405 nm) beam spot with beam diameter as small as $\sim 200$ nm. We have observed a giant blue shift, as large as $\sim 40$ nm or $\sim 100$ meV, of the $A$-excitonic peak in the photoluminescence spectra from the edges when compared to luminescence from the inside. This giant blue shift may result from the following: (i) compressive strain at the edges; (ii) the enhanced dielectric screening caused by the increased electron density at the metallic Mo-edges; and (iii) chemical impurities.   

\textbf{High Resolution X-ray investigation of few-layer Molybdenum Disulfide (MoS2) Based Heterostructures}

Due to its favorable band gap, few-layer MoS2 can play an important role in optoelectronics and magneto-optics applications. Device applications necessitate a heterostructure combination of MoS2 with other compatible materials. Here we report the growth and characterization of structural properties of few-layer MoS2 based prototypes on Si substrates deposited by means of magnetron sputtering. A number of heteorstructure combinations such as MoS2/BN, MoS2/SiO2 shall be analyzed using high resolution X-ray reflectivity, scattering and diffraction methods. Our preliminary work already indicates that MoS2 deposited on BN is quite favorable for optoelectronic applications [1]. But substantial work remains in order to obtain abrupt interfaces and atomic-level control. High resolution X-ray analysis can provide the essential understanding into the various structural aspects (crystal structure, interface roughness, density thickness) which could be valuable for developing a diversity of optoelectronic applications using MoS$_{\mathrm{2\thinspace }}$or other \textit{transition metal dichalcogenides.} Ref 1: Wasala, Samassekou, et al. (under review).   

Anisotropic Dielectric Breakdown of Hexagonal Boron Nitride Film

Hexagonal boron nitride (h-BN) is considered as ideal substrate for 2D material devises. However, the reliability of insulating properties of h-BN itself has not been clarified yet. In this study, the anisotropic dielectric breakdown of h-BN is studied. We have found that the dielectric breakdown in $c$ axis direction using a conductive atomic force microscope proceeded in the layer-by-layer manner. The obtained dielectric field strength was \textasciitilde 12 MV/cm, which is comparable to the conventional SiO$_{2}$. On the other hand, to estimate the dielectric field strength in a direction perpendicular to $c$ axis, voltage is applied to a relatively thick h-BN (10-60 nm) through Cr/Au electrodes fabricated on the h-BN. We realized that the absorbed water on h-BN significantly affect the \textit{IV} characters and the breakdown voltage. After the adsorbed water was removed by the heating in vacuum, the dielectric field strength was determined to be \textasciitilde 3 MV/cm, which is the same order as that in $c$ axis direction. This value could be increased when we consider the effect of electric field concentration around the metal electrode. Although the large difference in dielectric filed strength for two directions was initially expected due to the highly-anisotropic layered structure with the van der Waals bonding, it was not the case because the sp2 bonding should be broken for dielectric breakdown regardless of its direction.   

The effective bending rigidity of a fluctuating ribbon

We study the vibration of a two-dimensional ribbon using molecular dynamics. We find the effective bending rigidity tends to a constant which can be orders of magnitude larger than the bare bending rigidity in the limit that the bare bending rigidity goes to zero, consistent with theoretical expectations. Experimental realizations include graphene, molybdenum disulfide and some doped membranes.