Temperature dependence of the double‐resonance Raman bands in monolayer MoS 2 (original) (raw)
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Laser Power Dependent Optical Properties of Mono- and Few-Layer MoS2
We report on the exponential decay of the red-shift of the photoluminescence A-exciton peak in monolayer molybdenum disulfide (MoS2) 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 A1g and E12g 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 MoS2.
This work describes a resonance Raman study performed in domes of monolayer MoS2 using 23 different laser excitation energies covering the visible and near-infrared (NIR) ranges. The multiple-excitation results allowed us to investigate the exciton-phonon interactions of different phonons (A1’, E’ and LA) with different electronic and excitonic optical transitions in biaxially strained monolayer MoS2. The analysis of the intensities of the two first-order peaks, A1’ and E’, and the double-resonance 2LA Raman band as a function of the laser excitation furnished the values of the energies of the indirect gap and the excitonic transitions in the strained MoS2 domes. It was noticed that the out-of-plane A1’ phonon mode is significantly enhanced only by the indirect gap I and the C exciton, whereas the in-plane E’ mode is only enhanced by the C exciton of MoS2 dome, revealing thus a weak interaction of these phonons with the A and B excitons in the strained MoS2 domes. On the other hand,...
Theoretical and Experimental Study of Phonon Spectra of Bulk and Nano-Sized MoS2 Layer Crystals
Nanoscale Research Letters
Theoretical analysis of Raman scattering spectra (RS) for single-crystal MoS 2 sample and atomically thin MoS 2 sample consisting from one to few layers was performed in order to explain the change of MoS 2 vibrations at transition from a monoatomic layer to a bulk crystal. Experiments have shown that changes of frequencies of the most intensive bands arising from the in-plane, E 1 2g , and out-of-plane, A 1g , vibrations, as a function of number n of layers looks differently. Thus, the frequency of ω(A 1g) is increasing with growth of n, whereas the frequency of ω E 1 2g is decreasing. Such a change of the ω E 1 2g frequency was explained as the effect of "strong increase of the dielectric tensor when going from single layer to the bulk" sample. In the present work, we show that the reason of different dependences of frequencies can be related to both the van der Waals (vdW) interlayer interaction and the anharmonic interaction of noted fundamental vibrations with the corresponding combination tones (CT) of layer that manifests itself due to Fermi resonance in the layer. Overjumping of these phonon pairs (s, s ') owing to interlayer interaction,Ṽ p s;s 0 ;q , to other layers at growth of number n, results in the change of frequencies for each interacting pair of A 1g or E 1 2g symmetry. The alteration of pair frequencies depends on the ratio of constantsṼ p s;s 0 ;q describing the interaction of studied states s and s '.
Anharmonicity in Raman-active phonon modes in atomically thin MoS2
Physical Review B
Phonon-phonon anharmonic effects have a strong influence on the phonon spectrum; most prominent manifestation of these effects are the softening (shift in frequency) and broadening (change in FWHM) of the phonon modes at finite temperature. Using Raman spectroscopy, we studied the temperature dependence of the FWHM and Raman shift of E 1 2g and A 1g modes for single-layer and natural bilayer MoS 2 over a broad range of temperatures (8 < T < 300 K). Both the Raman shift and FWHM of these modes show linear temperature dependence for T > 100 K, whereas they become independent of temperature for T < 100 K. Using first-principles calculations, we show that three-phonon anharmonic effects intrinsic to the material can account for the observed temperature dependence of the linewidth of both the modes. It also plays an important role in determining the temperature dependence of the frequency of the Raman modes. The observed evolution of the linewidth of the A 1g mode suggests that electron-phonon processes are additionally involved. From the analysis of the temperature-dependent Raman spectra of MoS 2 on two different substrates-SiO 2 and hexagonal boron nitride-we disentangle the contributions of external stress and internal impurities to these phonon-related processes. We find that the renormalization of the phonon mode frequencies on different substrates is governed by strain and intrinsic doping. Our work establishes the role of intrinsic phonon anharmonic effects in deciding the Raman shift in MoS 2 irrespective of substrate and layer number.
Probing combinations of acoustic phonons in MoS2 by intervalley double-resonance Raman scattering
Physical Review B
In this work, we present measurements of the temperature dependence of the resonance Raman spectra of MoS 2 in two-dimensional and bulk forms, performed to identify the processes related to different combinations of two acoustic phonons involved in the intervalley scattering. The resonance Raman spectra of samples of different thicknesses (single layer, bilayer, trilayer, and bulk) were measured near the resonance with the A excitonic transition and at different temperatures. Measurements of the Raman spectra of bulk MoS 2 were performed using several laser energies across the resonances with the A and B excitonic transitions. Based on the electronic and vibrational structures of the samples with different thicknesses and the evolution of the bands as a function of the laser excitation energy and temperature, we propose correct assignments to the Raman bands appearing at approximately 380, 395, and 405 cm −1. According to our measurements and data analysis, the peaks at 380, 395, and 405 cm −1 correspond to the combinations of 2TA around the K point, LA and TA phonons around the M point, and LA and out-of-plane acoustic (ZA) phonons around the M point, respectively. This work sheds light on the double-resonance processes of MoS 2 and how it is related to the electronic structure of this material. The results presented here establish the assignment and the scattering mechanism for some two-phonon and double-resonance Raman bands whose origins were still a matter of debate in the literature. It can also be an important basis to explain the double-resonance processes in other transition-metal dichalcogenides since we present the fundamental electron scattering mechanism near the resonance with the A and B excitonic transitions, and how it is affected by thermal effects.
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.
The Effect of Substrate on Vibrational Properties of Single-Layer MoS_2
Acta Physica Polonica A, 2016
We report on the Raman scattering from single-layer molybdenum disulfide (MoS2) deposited on various substrates: Si/SiO2, hexagonal boron nitride (h-BN), sapphire, as well as suspended. Room temperature Raman scattering spectra are investigated under both resonant (632.8 nm) and non-resonant (514.5 nm) excitations. A rather weak influence of the substrate on the Raman scattering signal is observed. The most pronounced, although still small, is the effect of h-BN, which manifests itself in the change of energy positions of the E and A 1 Raman modes of single-layer MoS2. We interpret this modification as originating from van der Waals interaction between the MoS2 and h-BN layers.