Giant Thermomechanical Bandgap Engineering in Quasi-two-dimensional Tellurium

TL;DR

This paper demonstrates a large, non-volatile thermomechanical bandgap tuning in quasi-2D tellurium nanoflakes through controlled strain during synthesis, enabling significant optical property modulation for optoelectronic applications.

Contribution

It introduces a novel thermomechanical strain engineering method that achieves unprecedented bandgap modulation in quasi-2D tellurium, surpassing previous volatile strain techniques.

Findings

Achieved 2.3 eV bandgap modulation with ~600 meV/% rate.

Demonstrated robust blue photoemission with 79.9% IQE.

Strain inhibits exciton-exciton annihilation, enhancing radiative recombination.

Abstract

Mechanical straining-induced bandgap modulation in two-dimensional (2D) materials has been confined to volatile and narrow modulation due to substrate slippage and poor strain transfer. We report the thermomechanical modulation of the inherent bandgap in quasi-2D tellurium nanoflakes (TeNFs) via non-volatile strain induction during hot-press synthesis. We leveraged the coefficient of thermal expansion (CTE) mismatch between TeNFs and growth substrates by maintaining a high-pressure enforced non-slip condition during thermal relaxation (623 to 300K) to achieve the optimal biaxial compressive strain of -4.6 percent in TeNFs/sapphire. This resulted in an enormous bandgap modulation of 2.3 eV, at a rate of up to ~600 meV/%, which is two-fold larger than reported modulation rate. Strained TeNFs display robust band-to-band radiative excitonic blue photoemission with an intrinsic quantum efficiency (IQE) of c.a. 79.9%, making it promising for energy efficient blue LEDs and nanolasers. Computational studies reveal that biaxial compressive strain inhibits exciton-exciton annihilation by evading van-Hove singularities, hence promoting radiative-recombination. Bandgap modulation by such nonvolatile straining is scalable to other 2D semiconductors for on-demand nano(opto)-electronics.