Temperature-dependent Raman study and determination of anisotropy ratio and in-plane thermal conductivity of low-temperature CVD-grown PdSe2 using unpolarized laser excitation

Abstract

PdSe₂ is a promising two-dimensional noble metal dichalcogenide with inherent in-plane anisotropy due to its pentagonal structure, and it finds applications in electronic, photonic, and thermoelectric devices. Herein, we present the low-temperature (250–290 °C) chemical vapour deposition (CVD) growth of an air-stable atomically thin PdSe₂ layer and study its Raman temperature coefficients, anisotropy ratio and thermal conductivity as a function of the layer thickness. Optical microscopy (OM), atomic force microscopy (AFM), and micro-Raman analyses lead to confirmation of the as-grown ultrathin sheet-like bilayer and ribbon-like few-layer PdSe₂ on a mica substrate. We found the reciprocal lattice basis vectors |*| = 0.17 nm and |*| = 0.16 nm in single-crystalline PdSe₂, acquired via electron diffraction analysis. A low-temperature Raman study has been conducted to understand the phonon dynamics of the as-grown PdSe₂ in the temperature range of 83–295 K. This work opens up a new avenue for calculation of the anisotropic ratio without using a polarized laser, which is significant for designing new functional materials. Analysis of the temperature-dependent Raman data revealed the anisotropic ratios to be 1.42 and 1.28 for bilayer and few-layer PdSe₂, respectively. Our results affirm that the anisotropic ratio decreases with the increasing layer number. Furthermore, we perform a comparative analysis of the temperature coefficients for transition metal di-chalcogenides (TMDs) and noble transition metal di-chalcogenides (NTMDs) that have been reported so far. For bilayer to few-layer (∼5-layer) CVD-synthesized PdSe₂ the temperature coefficients change from −0.006 cm⁻¹ K⁻¹ to −0.024 cm⁻¹ K⁻¹, respectively, which shows a wider variation with thickness compared with conventional TMDs. Via analysis of the mini-planes of PdSe₂, we propose the precise vibrational planes responsible for different Raman modes. Broadening of the Raman spectral linewidth with increasing temperature is explained based on the phonon decay process. Finally, the in-plane thermal conductivities of CVD-grown few-layer and bilayer PdSe₂ are estimated to be ∼10.1 W m⁻¹ K⁻¹ and ∼36.8 W m⁻¹ K⁻¹, respectively, using a non-contact Raman measurement technique. Our results are significant for the low-temperature CVD growth of bilayer and few-layer PdSe₂ on an arbitrary substrate, including mica, and for gaining insight into electron–phonon and phonon–phonon interactions in noble transition metal chalcogenide 2D materials.