Vibrational Properties of a Naturally Occurring Semiconducting van der Waals heterostructure

TL;DR

This study investigates the vibrational properties and thermal transport in Franckeite, a naturally occurring van der Waals heterostructure, using Raman spectroscopy and first-principles simulations to reveal layer-specific vibrational modes and temperature effects.

Contribution

It provides a detailed characterization of Franckeite's vibrational modes and thermal properties, combining experimental Raman data with theoretical first-principles calculations.

Findings

Identification of low-frequency intralayer Raman modes.

Layer-specific temperature coefficients for vibrational modes.

Correlation of vibrational patterns with thermal transport properties.

Abstract

We present vibrational properties of Franckeite, which is a naturally occurring van der Waals heterostructure consisting of two different semiconducting layers. Franckeite is a complex layered crystal composed of alternating SnS$_2$ like pseudohexagonal and PbS-like pseudotetragonal layers stacked on top of each other, providing a unique platform to study vibrational properties and thermal transport across layers with mass density and phonon mismatches. By using micro-Raman spectroscopy and first-principles Raman simulations, we found that the PbS-like pseudotetragonal structure is mostly composed of Pb$_3$SbS$_4$. We also discovered several low-frequency Raman modes that originate from the intralayer vibrations of the pseudotetragonal layer. Using density functional theory, we determined all vibrational patterns of Franckeite, whose signatures are observed in the Raman spectrum. By studying temperature dependent Raman spectroscopy (300 K - 500 K), we have found different temperature coefficients for both pseudotetragonal and pseudohexagonal layers. We believe that our study will help understand the vibration modes of its complex heterostructure and the thermal properties at the nanoscale.