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Register a new userNew insights in the lattice dynamics of monolayers, bilayers, and trilayers of WSe2 and unambiguous determination of few-layer-flakes thickness
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Among the most common few-layers transition metal dichalcogenides (TMDs), WSe2 is the most challenging material from the lattice dynamics point of view. Indeed, for a long time the main two phonon modes (A1g and E12g) have been wrongly assigned. In the last few years, these two modes have been properly interpreted, and their quasi-degeneracy in the monolayer has been used for its identification. In this work, we show that this approach has a limited validity and we propose an alternative, more general approach, based on multi-phonon bands. Moreover, we show and interpret all the peaks (about 40) appearing in the Raman spectra of monolayers, bilayers, and trilayers of WSe2 by combining experimental wavelength- and polarization-dependent Raman studies with density-functional theory calculations providing the phonon dispersions, the polarization-resolved first-order Raman spectra, and the one- and two-phonon density of states. This complete study not only offers a method to distinguish between monolayers, bilayers, and trilayers with no need of optical images and atomic force microscopy, but it also sheds light on the interpretation of single and multi-phonon bands appearing in the inelastic light scattering experiments of layered WSe2; some of these bands were never observed before, and some were observed and uncertainly assigned. We promote the full understanding of the lattice dynamics of this material that is crucial for the realization of optoelectronics devices and of novel phononic metamaterials, such as TMDs superlattices.
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The emergence of transition metal dichalcogenides (TMDs) as 2D electronic materials has stimulated proposals of novel electronic and photonic devices based on TMD heterostructures. Here we report the determination of band offsets in TMD heterostructures by using microbeam X-ray photoelectron spectroscopy ({mu}-XPS) and scanning tunneling microscopy/spectroscopy (STM/S). We determine a type-II alignment between $textrm{MoS}_2$ and $textrm{WSe}_2$ with a valence band offset (VBO) value of 0.83 eV and a conduction band offset (CBO) of 0.76 eV. First-principles calculations show that in this heterostructure with dissimilar chalcogen atoms, the electronic structures of $textrm{WSe}_2$ and $textrm{MoS}_2$ are well retained in their respective layers due to a weak interlayer coupling. Moreover, a VBO of 0.94 eV is obtained from density functional theory (DFT), consistent with the experimental determination.
The optical properties of two-dimensional transition metal dichalcogenide monolayers such as MoS$_2$ or WSe$_2$ are dominated by excitons, Coulomb bound electron-hole pairs. Screening effects due the presence of hexagonal-BN surrounding layers have been investigated by solving the Bethe Salpeter Equation on top of GW wave functions in density functional theory calculations. We have calculated the dependence of both the quasi-particle gap and the binding energy of the neutral exciton ground state E$_b$ as a function of the hBN layer thickness. This study demonstrates that the effects of screening at this level of theory are more short-ranged that it is widely believed. The encapsulation of a WSe$_2$ monolayer by three sheets of hBN (around 1 nm) already yields a 20 % decrease of E$_b$ whereas the maximal reduction is 27% for thick hBN. We have performed similar calculations in the case of a WSe$_2$ monolayer deposited on stacked hBN layers. These results are compared to the recently proposed Quantum Electrostatic Heterostructure approach.
Two-dimensional (2D) layered tungsten diselenides (WSe2) material has recently drawn a lot of attention due to its unique optoelectronic properties and ambipolar transport behavior. However, direct chemical vapor deposition (CVD) synthesis of 2D WSe2 is not as straightforward as other 2D materials due to the low reactivity between reactants in WSe2 synthesis. In addition, the growth mechanism of WSe2 in such CVD process remains unclear. Here we report the observation of a screw-dislocation-driven (SDD) spiral growth of 2D WSe2 flakes and pyramid-like structures using a sulfur-assisted CVD method. Few-layer and pyramid-like WSe2 flakes instead of monolayer were synthesized by introducing a small amount of sulfur as a reducer to help the selenization of WO3, which is the precursor of tungsten. Clear observations of steps, helical fringes, and herring-bone contours under atomic force microscope characterization reveal the existence of screw dislocations in the as-grown WSe2. The generation and propagation mechanisms of screw dislocations during the growth of WSe2 were discussed. Back-gated field-effect transistors were made on these 2D WSe2 materials, which show on/off current ratios of 106 and mobility up to 44 cm2/Vs.
When a crystal becomes thinner and thinner to the atomic level, peculiar phenomena discretely depending on its layer-numbers (n) start to appear. The symmetry and wave functions strongly reflect the layer-numbers and stacking order, which brings us a potential of realizing new properties and functions that are unexpected in either bulk or simple monolayer. Multilayer WTe2 is one such example exhibiting unique ferroelectricity and non-linear transport properties related to the antiphase stacking and Berry-curvature dipole. Here we investigate the electronic band dispersions of multilayer WTe2 (2-5 layers), by performing laser-based micro-focused angle-resolved photoelectron spectroscopy on exfoliated-flakes that are strictly sorted by n and encapsulated by graphene. We clearly observed the insulator-semimetal transition occurring between 2- and 3-layers, as well as the 30-70 meV spin-splitting of valence bands manifesting in even n as a signature of stronger structural asymmetry. Our result fully demonstrates the possibility of the large energy-scale band and spin manipulation through the finite n stacking procedure.
The electronic and thermoelectric properties of one to four monolayers of MoS$_{2}$, MoSe$_{2}$, WS$_{2}$, and WSe$_{2}$ are calculated. For few layer thicknesses,the near degeneracies of the conduction band $K$ and $Sigma$ valleys and the valence band $Gamma$ and $K$ valleys enhance the n-type and p-type thermoelectric performance. The interlayer hybridization and energy level splitting determine how the number of modes within $k_BT$ of a valley minimum changes with layer thickness. In all cases, the maximum ZT coincides with the greatest near-degeneracy within $k_BT$ of the band edge that results in the sharpest turn-on of the density of modes. The thickness at which this maximum occurs is, in general, not a monolayer. The transition from few layers to bulk is discussed. Effective masses, energy gaps, power-factors, and ZT values are tabulated for all materials and layer thicknesses.
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