Inverse Funnel Effect of Excitons in Strained Black Phosphorus

TL;DR

This paper investigates how strain influences exciton behavior in black phosphorus, revealing an inverse and highly anisotropic funnel effect that differs from other 2D materials, with potential applications in optoelectronics.

Contribution

It demonstrates the inverse and anisotropic funnel effect of excitons in strained black phosphorus, contrasting with MoS₂, and explores its implications for optoelectronic device design.

Findings

Inverse funnel effect causes excitons to move away from high strain regions.

Funnel distances are larger along the armchair direction.

Potential for improved solar cell designs using this effect.

Abstract

We study the effects of strain on the properties and dynamics of Wannier excitons in monolayer (phosphorene) and few-layer black phosphorus (BP), a promising two-dimensional material for optoelectronic applications due to its high mobility, mechanical strength and strain-tuneable direct band gap. We compare the results to the case of molybdenum disulphide (MoS$_2$) monolayers. We find that the so-called funnel effect, i.e. the possibility of controlling exciton motion by means of inhomogeneous strains, is much stronger in few-layer BP than in MoS$_2$ monolayers and, crucially, is of opposite sign. Instead of excitons accumulating isotropically around regions of high tensile strain like in MoS$_2$, excitons in BP are pushed away from said regions. This \emph{inverse} funnel effect is moreover highly anisotropic, with much larger funnel distances along the armchair crystallographic direction, leading to a directional focusing of exciton flow. A strong inverse funnel effect could enable simpler designs of funnel solar cells, and offer new possibilities for the manipulation and harvesting of light.