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Register a new userIntra-Domain Periodic Defects in Monolayer MoS$_2$
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We present an ultra-high vacuum scanning tunneling microscopy (STM) study of structural defects in molybdenum disulfide thin films grown on silicon substrates by chemical vapor deposition. A distinctive type of grain boundary periodically arranged inside an isolated triangular domain, along with other inter-domain grain boundaries of various types, is observed. These periodic defects, about 50 nm apart and a few nanometers in width, remain hidden in optical or low-resolution microscopy studies. We report a complex growth mechanism that produces 2D nucleation and spiral growth features that can explain the topography in our films.
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In this work, we have systematically studied the role of point defects in the recombination time of monolayer MoS$_2$ using time-dependent ab initio non-adiabatic molecular dynamics simulations. Various types of point defects, such as S vacancy, S interstitial, Mo vacancy and Mo interstitial have been considered. We show that defects strongly accelerate the electron-hole recombination, especially interstitial S atoms do that by 3 orders of magnitude higher compared to pristine MoS$_2$. Mo defects (both vacancy and interstitial) introduce a multitude of de-excitation pathways via various defect levels in the energy gap. The results of this study provide some fundamental understanding of photoinduced de-excitation dynamics in presence of defects in highly technologically relevant 2D MoS$_2$.
Bound quasiparticles, negatively charged trions and neutral excitons, are associated with the direct optical transitions at the K-points of the Brillouin zone for monolayer MoS$_2$. The change in the carrier concentration, surrounding dielectric constant and defect concentration can modulate the photoluminescence and Raman spectra. Here we show that exposing the monolayer MoS$_2$ in air to a modest laser intensity for a brief period of time enhances simultaneously the photoluminescence (PL) intensity associated with both trions and excitons, together with $sim$ 3 to 5 times increase of the Raman intensity of first and second order modes. The simultaneous increase of PL from trions and excitons cannot be understood based only on known-scenario of depletion of electron concentration in MoS$_2$ by adsorption of O$_2$ and H$_2$O molecules. This is explained by laser induced healing of defect states resulting in reduction of non-radiative Auger processes. This laser healing is corroborated by an observed increase of intensity of both the first order and second order 2LA(M) Raman modes by a factor of $sim$ 3 to 5. The A$_{1g}$ mode hardens by $sim$ 1.4 cm$^{-1}$ whereas the E$^1_{2g}$ mode softens by $sim$ 1 cm$^{-1}$. The second order 2LA(M) Raman mode at $sim$ 440 cm$^{-1}$ shows an increase in wavenumber by $sim$ 8 cm$^{-1}$ with laser exposure. These changes are a combined effect of change in electron concentrations and oxygen-induced lattice displacements.
We discuss here the effect of band nesting and topology on the spectrum of excitons in a single layer of MoS$_2$, a prototype transition metal dichalcogenide material. We solve for the single particle states using the ab initio based tight-binding model containing metal $d$ and sulfur $p$ orbitals. The metal orbitals contribution evolving from $K$ to $Gamma$ points results in conduction-valence band nesting and a set of second minima at $Q$ points in the conduction band. There are three $Q$ minima for each $K$ valley. We accurately solve the Bethe-Salpeter equation including both $K$ and $Q$ points and obtain ground and excited exciton states. We determine the effects of the electron-hole single particle energies including band nesting, direct and exchange screened Coulomb electron-hole interactions and resulting topological magnetic moments on the exciton spectrum. The ability to control different contributions combined with accurate calculations of the ground and excited exciton states allows for the determination of the importance of different contributions and a comparison with effective mass and $kcdot p$ massive Dirac fermion models.
Accurately described excitonic properties of transition metal dichalcogenide heterobilayers (HBLs) are crucial to comprehend the optical response and the charge carrier dynamics of them. Excitons in multilayer systems posses inter or intralayer character whose spectral positions depend on their binding energy and the band alignment of the constituent single-layers. In this study, we report the electronic structure and the absorption spectra of MoS$_2$/WS$_2$ and MoSe$_2$/WSe$_2$ HBLs from first-principles calculations. We explore the spectral positions, binding energies and the origins of inter and intralayer excitons and compare our results with experimental observations. The absorption spectra of the systems are obtained by solving the Bethe-Salpeter equation on top of a G$_0$W$_0$ calculation which corrects the independent particle eigenvalues obtained from density functional theory calculations. Our calculations reveal that the lowest energy exciton in both HBLs possesses interlayer character which is decisive regarding their possible device applications. Due to the spatially separated nature of the charge carriers, the binding energy of inter-layer excitons might be expected to be considerably smaller than that of intra-layer ones. However, according to our calculations the binding energy of lowest energy interlayer excitons is only $sim$ 20% lower due to the weaker screening of the Coulomb interaction between layers of the HBLs. Therefore, it can be deduced that the spectral positions of the interlayer excitons with respect to intralayer ones are mostly determined by the band offset of the constituent single-layers. By comparing oscillator strengths and thermal occupation factors, we show that in luminescence at low temperature, the interlayer exciton peak becomes dominant, while in absorption it is almost invisible.
We show that pristine MoS$_2$ single layer (SL) exhibits two bandgaps $E_{gparallel}=1.9$ eV and $E_{gperp}=3.2$ eV for the optical in-plane and out-of-plane susceptibilities $chi_parallel$ and $chi_perp$, respectively. In particular, we show that odd states bound to vacancy defects (VDs) lead to resonances in $chi_perp$ inside $E_{gperp}$ in MoS$_2$ SL with VDs. We use density functional theory, the tight-binding model, and the Dirac equation to study MoS$_2$ SL with three types of VDs: (i) Mo-vacancy, (ii) S$_2$-vacancy, and (iii) 3$times$MoS$_2$ quantum antidot. The resulting optical spectra identify and characterize the VDs.
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