Inverse Funnel Effect of Excitons in Strained Black Phosphorus

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.