Exciton

Chemistry

Metallurgy

Materials science

Black phosphorus

Optoelectronics

Chemical physics

Inverse

Geometry

Phosphorus

Physics

Funnel

Organic chemistry

Mathematics

Condensed matter physics

Physical Review X

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.

Inverse Funnel Effect of Excitons in Strained Black Phosphorus