Temperature-dependent phonon dynamics and anharmonicity of suspended and supported few-layer gallium sulfide

Abstract

Phonons play a fundamental role in the electronic and thermal transport of 2D materials which is crucial for device applications. In this work, we investigate the temperature-dependence of A$^1_{1 \mathrm{g}}$ and A$^2_{1 \mathrm{g}}$ Raman modes of suspended and supported mechanically exfoliated few-layer gallium sulfide (GaS), accessing their relevant thermodynamic Grüneisen parameters and anharmonicity. The Raman frequencies of these two phonons soften with increasing temperature with different $\theta = \partial\omega/\partial T$ temperature coefficients. The first-order temperature coefficients θ of A$^2_{1 \mathrm{g}}$ mode is ∼ −0.016 cm−1/K, independent of the number of layers and the support. In contrast, the θ of A$^1_{1 \mathrm{g}}$ mode is smaller for two-layer GaS and constant for thicker samples (∼ −0.006 2 cm−1 K−1). Furthermore, for two-layer GaS, the θ value is ∼ −0.004 4 cm−1 K−1 for the supported sample, while it is even smaller for the suspended one (∼ −0.002 9 cm−1 K−1). The higher θ value for supported and thicker samples was attributed to the increase in phonon anharmonicity induced by the substrate surface roughness and Umklapp phonon scattering. Our results shed new light on the influence of the substrate and number of layers on the thermal properties of few-layer GaS, which are fundamental for developing atomically-thin GaS electronic devices.