Abstract
We report the temperature-dependent evolution of Raman spectra of monolayer WS2 directly CVD-grown on a gold foil and then transferred onto quartz substrates over a wide temperature range from 84 to 543 K. The nonlinear temperature dependence of Raman shifts for both and A1g modes has been observed. The first-order temperature coefficients of Raman shifts are obtained to be −0.0093 (cm−1/K) and −0.0122 (cm−1/K) for and A1g peaks, respectively. A physical model, including thermal expansion and three- and four-phonon anharmonic effects, is used quantitatively to analyze the observed nonlinear temperature dependence. Thermal expansion coefficient (TEC) of monolayer WS2 is extracted from the experimental data for the first time. It is found that thermal expansion coefficient of out-plane mode is larger than one of in-plane mode, and TECs of and A1g modes are temperature-dependent weakly and strongly, respectively. It is also found that the nonlinear temperature dependence of Raman shift of mode mainly originates from the anharmonic effect of three-phonon process, whereas one of A1g mode is mainly contributed by thermal expansion effect in high temperature region, revealing that thermal expansion effect cannot be ignored.
Highlights
Since the discovery of graphene, atomically thin two-dimensional (2D) layered materials have drawn intense attention due to their unique physical, chemical, and mechanical properties1,2. 2D transition metal dichalcogenides (TMDCs) such as MX2 (M =Mo, W and X =Se, S) have been successfully synthesized and prove to be one of the most stable atomically thin 2D materials[1,3]
Monolayer WS2 sample studied here was grown on gold foil substrates by chemical vapor deposition (CVD), and transferred onto a quartz substrate for Raman measurement and a SiO2/Si substrate for good optical contrast (See Fig. 1(a))
The details of the synthesis and crystal-quality characterization of monolayer WS2 can be found in ref
Summary
Since the discovery of graphene, atomically thin two-dimensional (2D) layered materials have drawn intense attention due to their unique physical, chemical, and mechanical properties1,2. 2D transition metal dichalcogenides (TMDCs) such as MX2 (M =Mo, W and X =Se, S) have been successfully synthesized and prove to be one of the most stable atomically thin 2D materials[1,3]. The temperature dependence of Raman shift of monolayer and few layer MoS2 has been extensively studied over a heating process[13,14], a cooling process[15], and a wide temperature range from 77 K to 623 K16. Thripuranthaka et al reported the first experimental investigation on temperature-dependent Raman shifts of mechanically exfoliated monolayer WS2 transferred onto a Si substrate over a wide temperature range from 77 K to 623 K19, and observed an obvious nonlinear temperature dependence of Raman shifts of E12g and A1g modes. Gaur et al studied the temperature dependence of Raman shifts of CVD-grown monolayer WS2 on Al2O3 substrates over a wide range from 83 to 573 K22, and observed a weakly nonlinear temperature dependence of Raman shift of A1g mode. We investigate temperature dependence on Raman shift of monolayer WS227, directly
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