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-tunable direct band gap. We compare the results to the case of molybdenum disulphide (MoS) 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 monolayers and, crucially, is of opposite sign. Instead of excitons accumulating isotropically around regions of high tensile strain like in MoS, excitons in BP are pushed away from said regions. This 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.We acknowledge financial support from MINECO (Spain) through the Ramón y Cajal Programs No. RYC-2013-14645 and No. RYC-2011-09345, and Grants No. FIS2011-23713, No. FIS2013-47328-C2-1-P, No. FIS2014-58445-JIN, and No. FIS2014-57432, and The María de Maeztu Programme for Units of Excellence in R&D (MDM-2014-0377), as well as from the Comunidad Autónoma de Madrid (CAM) MAD2D-CM Program (S2013/MIT-3007), the European Commission under the Graphene Flagship, Contract No. CNECTICT-604391, and the European Research Council, through Grant No. 290846.Peer reviewe