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Register a new userMeasurement of high exciton binding energy in the monolayer transition-metal dichalcogenides WS2 and WSe2
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Monolayer transition-metal dichalcogenides are direct gap semiconductors with great promise for optoelectronic devices. Although spatial correlation of electrons and holes plays a key role, there is little experimental information on such fundamental properties as exciton binding energies and band gaps. We report here an experimental determination of exciton excited states and binding energies for monolayer WS2 and WSe2. We observe peaks in the optical reflectivity/absorption spectra corresponding to the ground- and excited-state excitons (1s and 2s states). From these features, we determine lower bounds free of any model assumptions for the exciton binding energies as E2sA - E1sA of 0.83 eV and 0.79 eV for WS2 and WSe2, respectively, and for the corresponding band gaps Eg >= E2sA of 2.90 and 2.53 eV at 4K. Because the binding energies are large, the true band gap is substantially higher than the dominant spectral feature commonly observed with photoluminescence. This information is critical for emerging applications, and provides new insight into these novel monolayer semiconductors.
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The optical properties of monolayer transition metal dichalcogenides (TMDC) feature prominent excitonic natures. Here we report an experimental approach toward measuring the exciton binding energy of monolayer WS2 with linear differential transmission spectroscopy and two-photon photoluminescence excitation spectroscopy (TP-PLE). TP-PLE measurements show the exciton binding energy of 0.71eV around K valley in the Brillouin zone. The trion binding energy of 34meV, two-photon absorption cross section 4X10^{4}cm^{2}W^{-2}S^{-1} at 780nm and exciton-exciton annihilation rate around 0.5cm^{2}/s are experimentally obtained.
The exceptionally strong Coulomb interaction in semiconducting transition-metal dichalcogenides (TMDs) gives rise to a rich exciton landscape consisting of bright and dark exciton states. At elevated densities, excitons can interact through exciton-exciton annihilation (EEA), an Auger-like recombination process limiting the efficiency of optoelectronic applications. Although EEA is a well-known and particularly important process in atomically thin semiconductors determining exciton lifetimes and affecting transport at elevated densities, its microscopic origin has remained elusive. In this joint theory-experiment study combining microscopic and material-specific theory with time- and temperature-resolved photoluminescence measurements, we demonstrate the key role of dark intervalley states that are found to dominate the EEA rate in monolayer WSe$_2$. We reveal an intriguing, characteristic temperature dependence of Auger scattering in this class of materials with an excellent agreement between theory and experiment. Our study provides microscopic insights into the efficiency of technologically relevant Auger scattering channels within the remarkable exciton landscape of atomically thin semiconductors.
Monolayer transition metal dichalcogenides $MX_2$ ($M$ = Mo,W and $X$ = Te, Se, S) in 1T structure were predicted to be quantum spin Hall insulators based on first-principles calculations, which were quickly confirmed by multiple experimental groups. For a better understanding of their properties, in particular their responses to external fields, we construct a realistic four-band tight-binding (TB) model by combining the symmetry analysis and first-principles calculations. Our TB model respects all the symmetries and can accurately reproduce the band structure in a large energy window from -0.3 eV to 0.8 eV. With the inclusion of spin-orbital coupling (SOC), our TB model can characterize the nontrivial topology and the corresponding edge states. Our TB model can also capture the anisotropic strain effects on the band structure and the strain-induced metal-insulator transition. Moreover, we found that although $MX_2$ share the same crystal structures and have the same crystal symmetries, while the orbital composition of states around the Fermi level are qualitatively different and their lower-energy properties cannot fully described by a single k $cdot$ p model. Thus, we construct two different types of k $cdot$ p model for $M$S$_2$,$M$Se$_2$ and $M$Te$_2$, respectively. Benefiting from the high accuracy and simplicity, our TB and k $cdot$ p models can serve as a solid and concrete starting point for future studies of transport, superconductivity, strong correlation effects and twistronics in 1T-transition metal dichalcogenides.
Monolayer transition-metal dichalcogenides (ML-TMDs) offer exciting opportunities to test the manifestations of many-body interactions through changes in the charge density. Tuning the charge density by a gate voltage leads to profound changes in the optical spectra of excitons in ML-TMDs. We review the band-gap renormalization and dynamical screening as a function of charge density, and then incorporate these effects through various approximations that model long-wavelength charge excitations in the Bethe-Salpeter Equation (BSE). We then show that coupling between excitons and shortwave charge excitations is essential to resolve several experimental puzzles. Unlike ubiquitous and well-studied plasmons, driven by collective oscillations of the background charge density in the long-wavelength limit, we discuss the emergence of shortwave plasmons that originate from the short-range Coulomb interaction through which electrons transition between the $mathbf{K}$ and $-mathbf{K}$ valleys. We study the coupling between the shortwave plasmons and the neutral exciton through the self-energy of the latter. We then elucidate how this coupling as well as the spin ordering in the conduction band give rise to an experimentally observed optical sideband in electron-doped W-based MLs, conspicuously absent in electron-doped Mo-based MLs or any hole-doped ML-TMDs. While the focus of this review is on the optical manifestations of many-body effects in ML-TMDs, a systematic description of the dynamical screening and its various approximations allow one to revisit other phenomena, such as nonequilibrium transport or superconducting pairing, where the use of the BSE or the emergence of shortwave plasmons can play an important role.
The valley degree of freedom is a sought-after quantum number in monolayer transition-metal dichalcogenides. Similar to optical spin orientation in semiconductors, the helicity of absorbed photons can be relayed to the valley (pseudospin) quantum number of photoexcited electrons and holes. Also similar to the quantum-mechanical spin, the valley quantum number is not a conserved quantity. Valley depolarization of excitons in monolayer transition-metal dichalcogenides due to long-range electron-hole exchange typically takes a few ps at low temperatures. Exceptions to this behavior are monolayers MoSe$_2$ and MoTe$_2$ wherein the depolarization is much faster. We elucidate the enigmatic anomaly of these materials, finding that it originates from Rashba-induced coupling of the dark and bright exciton branches next to their degeneracy point. When photoexcited excitons scatter during their energy relaxation between states next to the degeneracy region, they reach the light cone after losing the initial helicity. The valley depolarization is not as fast in monolayers WSe$_2$, WS$_2$ and likely MoS$_2$ wherein the Rashba-induced coupling is negligible.
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