Temperature dependence of the double‐resonance Raman bands in monolayer MoS 2 (original) (raw)
Solid State Communications, 2018
We report on resonant Raman scattering studies of monolayer MoS 2 as a function of the excitation laser energy (1.959-2.033 eV) and temperature (T=7-295 K). In complementary reflectivity contrast experiments we determined the temperature evolution of the A exciton and trion resonances. We focus our studies on the dispersive, second order 'b' mode related to the resonant two phonon Raman process of successive emissions of the acoustic LA and TA phonons at K points. We found that when excitation laser energy is tuned across the A exciton level this mode shifts almost linearly to lower frequency with the rate equal -83 and -71 cm - 1 /eV at T=7 and 295 K, respectively, which is about two times higher rate than those reported in the previous studies of monolayer MoS 2 but very close the relevant rate recorded for bulk MoS 2 . We interpret this effect as related to the difference of concentration of two dimensional electron gas. We also determined, using excitation with the He-Ne laser the temperature shifts of the Raman peaks of dispersive 'b' and dispersionless E' and A 1 ' modes. We found that absolute value of the temperature coefficient of 'b' mode, equals 3.5×10 -2 cm -1 /K, is much higher than those of E' and A 1 ' modes, equal 0.4×10 -2 and 0.8×10 -2 cm -1 /K, respectively.
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.
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 .
Temperature Dependent Thermal Boundary Conductance of Monolayer MoS_2 by Raman Thermometry
2017
The electrical and thermal behavior of nanoscale devices based on two-dimensional (2D) materials is often limited by their contacts and interfaces. Here we report the temperature-dependent thermal boundary conductance (TBC) of monolayer MoS_2 with AlN and SiO_2, using Raman thermometry with laser-induced heating. The temperature-dependent optical absorption of the 2D material is crucial in such experiments, which we characterize here for the first time above room temperature. We obtain TBC 15 MWm^-^2K^-^1 near room temperature, increasing as T^0^.^2 up to 300 C. The similar TBC of MoS_2 with the two substrates indicates that MoS_2 is the "softer" material with weaker phonon irradiance, and the relatively low TBC signifies that such interfaces present a key bottleneck in energy dissipation from 2D devices. Our approach is needed to correctly perform Raman thermometry of 2D materials, and our findings are key for understanding energy coupling at the nanoscale.
Temperature Dependent Phononic Response of Few Layered MoS2 Nanosheets
Volume 4,Issue 5,2018, 2018
From the temperature dependent phononic studies of few layered liquid phase exfoliated MoS2 nanosheets we find that the E12g (in-plane) and A1g (out-of-plane) Raman modes follow red shift with increase in temperature and exhibits non-linear temperature dependence in the entire temperature range (80 to 600 K). The first-order temperature coefficients for E12g and A1g modes are found to be -0.0133 cm-1K-1 and -0.0092 cm-1K-1, respectively. The physical origin of the non-linear temperature dependence is analyzed using an analytical model that includes contribution of the thermal expansion and an-harmonic effects to the lattice potential. Our analysis suggests that the non-linear temperature dependence of E12g and A1g modes mainly originates from the an-harmonic contributions from three-phonon and four-phonon scattering.
Nature communications, 2017
Double-resonance Raman scattering is a sensitive probe to study the electron-phonon scattering pathways in crystals. For semiconducting two-dimensional transition-metal dichalcogenides, the double-resonance Raman process involves different valleys and phonons in the Brillouin zone, and it has not yet been fully understood. Here we present a multiple energy excitation Raman study in conjunction with density functional theory calculations that unveil the double-resonance Raman scattering process in monolayer and bulk MoS2. Results show that the frequency of some Raman features shifts when changing the excitation energy, and first-principle simulations confirm that such bands arise from distinct acoustic phonons, connecting different valley states. The double-resonance Raman process is affected by the indirect-to-direct bandgap transition, and a comparison of results in monolayer and bulk allows the assignment of each Raman feature near the M or K points of the Brillouin zone. Our work...
Nanotechnology, 2021
We present comprehensive temperature dependent Raman measurements for chemical vapor deposition grown horizontally aligned layered MoS2 in a temperature range of 4–330 K under a resonance condition. Our analysis of temperature dependent phonon frequency shift and linewidth suggests a finite role of three and four phonon anharmonic effect. We observe Davydov splitting of the out-of-plane (A 1g ) and in-plane ( E 2 g 1 ) modes for both three layer (3L) and few layer (FL) systems. The number of Davydov splitting components are found more in FL compared to 3L MoS2, which suggests that it increases with an increasing number of layers. Further, Davydov splitting is analyzed as a function of temperature. Temperature evaluation of the Raman spectra shows that the Davydov splitting, especially for A 1g mode, is very strong and well resolved at low temperature. We observe that A 1g mode shows the splitting at low temperature, while E 2 g 1 mode is split even at room temperature, which suggest...
Spin-Orbit Splitting in Single-Layer MoS_{2} Revealed by Triply Resonant Raman Scattering
Physical Review Letters, 2013
Although new spintronic devices based on the giant spin-orbit splitting of single-layer MoS 2 have been proposed, such splitting has not been studied effectively in experiments. This Letter reports the valence band spin-orbit splitting in single-layer MoS 2 for the first time, probed by the triply resonant Raman scattering process. We found that upon 325 nm laser irradiation, the second order overtone and combination Raman modes of single-layer MoS 2 are dramatically enhanced. Such resonant Raman enhancement arises from the electron-two-phonon triple resonance via the deformation potential and Fro ¨hlich interaction. As a sensitive and precise probe for the spin-orbit splitting, the triply resonant Raman scattering will provide a new and independent route to study the spin characteristics of MoS 2 .
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 .