Raman spectroscopic study of the layer-dependent Davydov splitting and thermal conductivity of chemically vapor deposited two-dimensional MoSe2

Abstract

Atomically thin MoSe2 is of interest from the perspective of estimating the layer-dependent material properties necessary for the translation of two-dimensional materials into devices. This work presents Raman spectroscopic protocols to determine a multitude of material parameters of two-dimensional MoSe2 films, including the layer thickness as well as the layer-dependent thermal conductivity, interlayer interactions, and anharmonicity. The Davydov splitting (factor-group splitting) observed in an out-of-plane A1g Raman mode, being layer-dependent in both the number and the peak positions, provides a method for estimating the number of layers. Furthermore, this work demonstrates the determination of the thermal conductivity ( K) from the temperature-dependent Davydov split Raman modes of the multi-layers. The measurement of K by conventional methods is otherwise challenging for the micrometer sizes of the two-dimensional materials. The value of K thus determined increases significantly from 9 W m−1 K−1 for a four-layer thick MoSe2 film to 52 W m−1 K−1 for a monolayer. The diminishing effect of anharmonicity observed in the monolayer as compared to multi-layer MoSe2 supports the layer-dependent trend in the thermal conductivity. Overall, the findings are relevant for the applications of 2D MoSe2 in low power electronic, optoelectronic, and thermoelectric devices.

Interlayer Vibronic Interactions and Phonon Anharmonic Decay in Few-Layer WS2 Nanoflakes

“…However, no splitting has been observed for the monolayer and bulk WS 2 . A similar explanation was given by Singh et al, 35 for the * , and A 1g peaks by 4, 4.5, and 5.5 cm −1 respectively, has been observed between 200 and 600 K. This shift emerges from lattice vibrations induced by temperature, affecting both metal-chalcogen and chalcogenchalcogen vibrations, along with the interlayer interactions between them. The shift in the peak position of Raman modes with temperature has been fitted with a polynomial, as shown in Figure 3.…”

“…However, no splitting has been observed for the monolayer and bulk WS 2 . A similar explanation was given by Singh et al, 35 for the splitting of the A 1g mode of MoSe 2 at an excitation wavelength of 488 nm; the TD behavior of the Davydov components of the A 1g mode has been reported. Figure 2b represents the intensity vs Raman shift at various temperatures.…”

“…The splitting in the out-of-plane A 1g mode is due to weak interlayer interactions present in the few-layered TMDs. Furthermore, this splitting depends on the number of layers present in the sample as well as on the laser excitation wavelength. − The Davydov splitting is reported to be a characteristic feature of few-layered 2D materials. The splitting of A 1g mode for WS 2 and other 2D-TMDs has also been reported earlier. − For instance, a study by Staiger et al reported the splitting of the A 1g mode of WS 2 .…”

“…Furthermore, this splitting depends on the number of layers present in the sample as well as on the laser excitation wavelength. − The Davydov splitting is reported to be a characteristic feature of few-layered 2D materials. The splitting of A 1g mode for WS 2 and other 2D-TMDs has also been reported earlier. − For instance, a study by Staiger et al reported the splitting of the A 1g mode of WS 2 . It was observed that the resonance of the excitation wavelength of 633 nm with the A exciton results in the splitting of the A 1g phonon mode.…”