Laser Power Dependent Optical Properties of Mono- and Few-Layer MoS2 (original) (raw)
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Laser Power Dependent Optical Properties of Mono- and Few-Layer MoS2
Journal of Nanoscience and Nanotechnology, 2015
We report on the exponential decay of the red-shift of the photoluminescence A-exciton peak in monolayer molybdenum disulfide (MoS 2 ) with the excitation laser power. The linear relationship found for the thermal variation of the same peak suggests that the laser power effect goes beyond the exciton dynamics associated to temperature variations. Laser exitation power effect on the broadening and red-shifting of the A 1g and E 1 2g phonon peaks observed by Raman spectroscopy reflect the damping of vibration due local thermal heating induced by the laser. Our results point out the laser excitation power dependence on the photoluminescence properties of monolayer MoS 2 .
Optoelectronic response and excitonic properties of monolayer MoS2
Journal of Applied Physics, 2016
Ab initio, electronic energy bands of MoS2 single layer are reported within the local density functional approximation. The inclusion of spin orbit coupling reveals the presence of two excitons A and B. We also discuss the change of physical properties of MoS2 from multilayer and bulk counterparts. The nature of the band gap changes from indirect to direct when the thickness is reduced to a single monolayer. The imaginary and real dielectric functions are investigated. Refractive index and birefringence are also reported. The results suggest that MoS2 is suitable for potential applications in optoelectronic and photovoltaic devices. The ab initio study is essential to propose the crucial parameters for the analytical model used for A-B exciton properties of the monolayer MoS2. From a theoretical point of view, we consider how the exciton behavior evolves under environmental dielectrics.
Lecture Notes in Nanoscale Science and Technology, 2013
We review vibrational and electronic properties of single and a few layer MoS 2 relevant to understand their resonant and non-resonant Raman scattering results. In particular, the optical modes and low frequency shear and layer breathing modes show significant dependence on the number of MoS 2 layers. Further, the electron doping of the MoS 2 single layer achieved using top-gating in a field effect transistor renormalizes the two optical modes A 1g and E 1 2g differently due to symmetry-dependent electron-phonon coupling. The issues related to carrier mobility, the Schottky barrier at the MoS 2-metal contact pads and the modifications of the dielectric environment are addressed. The direct optical transitions for single layer-MoS 2 involve two excitons at K-point in the Brillouin zone and their stability with temperature and pressure has been reviewed. Finally, the Fermi-level dependence of spectral shift for a quasiparticle, called trion, has been discussed.
Large Excitonic Effects in the Optical Properties of Monolayer MoS2
The band structure and absorption spectrum of monolayer MoS2 is calculated using the G0W0 approximation and the Bethe-Salpeter equation (BSE), respectively. We find that the so-called A and B peaks in the absorption spectrum arise from strongly bound excitons (0.7-0.8 eV) localized in distinct regions of the Brillouin zone and not from a split valence band as commonly assumed. Furthermore, we find the minimum band gap to be of the indirect type. This seems to conflict with recent experimental results showing stong luminescence in this material. However, our results indicate that the luminescence is a consequence of the large binding energy of the lowest exciton which stabilizes it against thermal relaxation. PACS numbers: 71.20.Nr, 78.20.Bh, 78.60.Lc Nanostructured forms of the semi-conductor MoS 2 have recieved much attention due to their potential as catalysts for desulferization of crude oil and more recently for (photo)-electrochemical hydrogen evolution . Bulk MoS 2 is composed of two-dimensional sheets held together by weak van der Waals forces and individual sheets can be isolated by exfoliation techniques similar to those used to produce graphene . Single layers of MoS 2 therefore comprise highly interesting twodimensional systems with a finite band gap and have recently been proposed for nano-electronics applications .
Broadband optical properties of monolayer and bulk MoS2
npj 2D Materials and Applications
Layered semiconductors such as transition metal dichalcogenides (TMDs) offer endless possibilities for designing modern photonic and optoelectronic components. However, their optical engineering is still a challenging task owing to multiple obstacles, including the absence of a rapid, contactless, and the reliable method to obtain their dielectric function as well as to evaluate in situ the changes in optical constants and exciton binding energies. Here, we present an advanced approach based on ellipsometry measurements for retrieval of dielectric functions and the excitonic properties of both monolayer and bulk TMDs. Using this method, we conduct a detailed study of monolayer MoS2 and its bulk crystal in the broad spectral range (290–3300 nm). In the near- and mid-infrared ranges, both configurations appear to have no optical absorption and possess an extremely high dielectric permittivity making them favorable for lossless subwavelength photonics. In addition, the proposed approac...
Investigation of Optical Properties of layered MoS 2
2012
Few atomic layers of molybdenum disulphide (MoS 2) have been prepared using chemical exfoliation techniques. Optical properties of few atomic layers of MoS 2 were investigated experimentally using Raman and Photoluminescence spectroscopy. Ab-initio density functional theory has been used to understand the electronic structure and lattice dynamics of monolayered and bilayered MoS2.
Photoluminescence Kinetics of Dark and Bright Excitons in Atomically Thin MoS 2
physica status solidi (RRL) – Rapid Research Letters, 2021
The fine structure of the exciton spectrum, containing optically allowed (bright) and forbidden (dark) exciton states, determines the radiation efficiency in nanostructures. We study time-resolved micro-photoluminescence in MoS2 monolayers and bilayers, both unstrained and compressively strained, in a wide temperature range (10-300 K) to distinguish between exciton states optically allowed and forbidden, both in spin and momentum, as well as to estimate their characteristic decay times and contributions to the total radiation intensity. The decay times were found to either increase or decrease with increasing temperature, indicating the lowest bright or lowest dark state, respectively. Our results unambiguously show that, in an unstrained monolayer, the spin-allowed state is the lowest for a series of A excitons (1.9 eV) with the dark state being <2 meV higher, and that the splitting energy can increase several times at compression. In contrast, in the indirect exciton series in bilayers (1.5 eV), the spin-forbidden state is the lowest, being about 3 meV below the bright one. The strong effect of strain on the exciton spectrum can explain the large scatter among the published data and must be taken into account to realize the desired optical properties of 2D MoS2.