Thickness-Dependent Polaron Crossover in Tellurene

TL;DR

This study investigates how the nature of polarons in tellurene changes from large to small as the material's thickness decreases below 10 nm, revealing a thickness-dependent polaron crossover driven by reduced dielectric screening.

Contribution

It provides the first detailed analysis of thickness-dependent polaron formation in tellurene, linking phonon behavior and electronic transport to polaron size transition in low-dimensional materials.

Findings

Abrupt change in phonon frequency and linewidth below 10 nm thickness.

Rapid decrease in field effect mobility for tellurene thinner than 10 nm.

Theoretical model reproduces phonon hardening and broadening in few-layer tellurene.

Abstract

Polarons, quasiparticles arising from electron-phonon coupling, are crucial in understanding material properties such as high-temperature superconductivity and colossal magnetoresistance. However, scarce studies have been performed to investigate the formation of polarons in low-dimensional materials with phonon polarity and electronic structure transitions. In this work, we studied polarons of tellurene that are composed of chiral chains of tellurium atoms. The frequency and linewidth of the A1 phonon, which becomes increasingly polar for thinner tellurene, exhibit an abrupt change when the thickness of tellurene is below 10 nm. Meanwhile, the field effect mobility of tellurene drops rapidly as the thickness is smaller than 10 nm. These phonon and transport signatures, combined with the calculated phonon polarity and band structure, suggest a crossover from large polarons for bulk tellurium to small polarons for few-layer tellurene. Effective field theory considers the phonon renormalization in the strong coupling (small polaron) regime, and semi-quantitatively reproduces the observed phonon hardening and broadening effects in few-layer tellurene. This polaron crossover stems from the quasi-1D nature of tellurene where modulation of the interchain distance reduces the dielectric screening and promotes electron-phonon coupling. Our work provides valuable insights into the influence of polarons on phononic, electronic, and structural properties in low-dimensional materials.