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Register a new userSpin splitting and strain in epitaxial monolayer WSe$_2$ on graphene
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We present the electronic and structural properties of monolayer WSe$_{2}$ grown by pulsed-laser deposition on monolayer graphene (MLG) on SiC. The spin splitting in the WSe$_{2}$ valence band at $overline{mathrm{K}}$ was $Delta_mathrm{SO}=0.469pm0.008$ eV by angle-resolved photoemission spectroscopy (ARPES). Synchrotron-based grazing-incidence in-plane X-ray diffraction (XRD) revealed the in-plane lattice constant of monolayer WSe$_{2}$ to be $a_mathrm{WSe_2}=3.2757pm0.0008 mathrm{r{A}}$. This indicates a lattice compression of -0.19 % from bulk WSe$_{2}$. By using experimentally determined graphene lattice constant ($a_mathrm{MLG}=2.4575pm0.0007 mathrm{r{A}}$), we found that a 3$times$3 unit cell of the slightly compressed WSe$_{2}$ is perfectly commensurate with a 4$times$4 graphene lattice with a mismatch below 0.03 %, which could explain why the monolayer WSe$_{2}$ is compressed on MLG. From XRD and first-principles calculations, however, we conclude that the observed size of strain is negligibly small to account for a discrepancy in $Delta_mathrm{SO}$ found between exfoliated and epitaxial monolayers in earlier ARPES. In addition, angle-resolved, ultraviolet and X-ray photoelectron spectroscopy shed light on the band alignment between WSe$_{2}$ and MLG/SiC and indicate electron transfer from graphene to the WSe$_{2}$ monolayer. As further revealed by atomic force microscopy, the WSe$_{2}$ island size depends on the number of carbon layers on top of the SiC substrate. This suggests that the epitaxy of WSe$_{2}$ favors the weak van der Waals interactions with graphene while it is perturbed by the influence of the SiC substrate and its carbon buffer layer.
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We report on the superlubric sliding of monolayer tungsten disulfide (WS2) on epitaxial graphene (EG) on silicon carbide (SiC). WS2 single-crystalline flakes with lateral size of hundreds of nanometers are obtained via chemical vapor deposition (CVD) on EG and microscopic and diffraction analyses indicate that the WS2/EG stack is predominantly aligned with zero azimuthal rotation. Our experimental findings show that the WS2 flakes are prone to slide over graphene surfaces at room temperature when perturbed by a scanning probe microscopy (SPM) tip. Atomistic force field based molecular dynamics simulations indicate that through local physical deformation of the WS2 flake, the scanning tip releases enough energy to the flake to overcome the motion activation barrier and to trigger an ultra-low friction roto-translational displacement, that is superlubric. Experimental observations indicate that after the sliding, the WS2 flakes rest with a rotation of npi/3 with respect to graphene. Atomically resolved investigations show that the interface is atomically sharp and that the WS2 lattice is strain-free. These results help to shed light on nanotribological phenomena in van der Waals (vdW) heterostacks and suggest that the applicative potential of the WS2/graphene heterostructure can be extended by novel mechanical prospects.
We report the observation and gate manipulation of intrinsic dark trions in monolayer WSe$_2$. By using ultraclean WSe$_2$ devices encapsulated by boron nitride, we directly resolve the weak photoluminescence of dark trions. The dark trions can be tuned continuously between negative and positive charged trions with electrostatic gating. We also reveal their spin triplet configuration and distinct valley optical emission by their characteristic Zeeman splitting under magnetic field. The dark trions exhibit large binding energy (14-16 meV). Their lifetime (~1.3 ns) is two orders of magnitude longer than the bright trion lifetime (~10 ps) and can be tuned between 0.4 to 1.3 ns by electrostatic gating. Such robust, optically detectable, and gate tunable dark trions provide a new path to realize electrically controllable trion transport in two-dimensional materials.
Strong Coulomb correlations together with multi-valley electronic bands in the presence of spin-orbit interaction and possible new optoelectronic applications are at the heart of studies of the rich physics of excitons in semiconductor structures made of monolayers of transition metal dichalcogenides (TMD). In intrinsic TMD monolayers the basic, intravalley excitons are formed by a hole from the top of the valence band and an electron either from the lower or upper spin-orbit-split conduction band subbands: one of these excitons is optically active, the second one is dark, although possibly observed under special conditions. Here we demonstrate the s-series of Rydberg dark exciton states in monolayer WSe$_2$, which appears in addition to a conventional bright exciton series in photoluminescence spectra measured in high in-plane magnetic fields. The comparison of energy ladders of bright and dark Rydberg excitons is shown to be a method to experimentally evaluate one of the missing band parameters in TMD monolayers: the amplitude of the spin-orbit splitting of the conduction band.
Monolayer WSe$_2$ hosts a series of exciton Rydberg states denoted by the principal quantum number n = 1, 2, 3, etc. While most research focuses on their absorption properties, their optical emission is also important but much less studied. Here we measure the photoluminescence from the 1s - 5s exciton Rydberg states in ultraclean monolayer WSe$_2$ encapsulated by boron nitride under magnetic fields from -31 T to 31 T. The exciton Rydberg states exhibit similar Zeeman shifts but distinct diamagnetic shifts from each other. From their luminescence spectra, Zeeman and diamagnetic shifts, we deduce the binding energies, g-factors and radii of the 1s - 4s exciton states. Our results are consistent with theoretical predictions and results from prior magneto-reflection experiments.
Many of the fundamental optical and electronic properties of atomically thin transition metal dichalcogenides are dominated by strong Coulomb interactions between electrons and holes, forming tightly bound atom-like excitons. Here, we directly trace the ultrafast formation of excitons by monitoring the absolute densities of bound and unbound electron-hole pairs in monolayers of WSe$_2$ following femtosecond non-resonant optical excitation. To this end, phase-locked mid-infrared probe pulses and field-sensitive electro-optic sampling are used to map out the full complex-valued optical conductivity of the non-equilibrium system and to discern the hallmark low-energy responses of bound and unbound pairs. While free charge carriers strongly influence the infrared response immediately after above-bandgap injection, up to 60% of the electron-hole pairs are bound as excitons already on a sub-picosecond timescale, evidencing extremely fast and efficient exciton formation. During the subsequent recombination phase, we still find a large density of free carriers in addition to excitons, indicating a non-equilibrium state of the photoexcited electron-hole system.
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