摘要
Nonradiative processes govern efficiencies of semiconductor nanocrystal(NC)-based devices.A central process is hot exciton cooling,or the nonradiative relaxation of a highly excited electron/hole pair to form a band-edge exciton.Due to quantum confinement effects,the timescale and mechanism of cooling are not well understood.A mismatch between electronic energy gaps and phonon frequencies has led to the hypothesis of a phonon bottleneck and extremely slow cooling,while enhanced electron-hole interactions have suggested ultrafast cooling.Experimental measurements of the cooling timescale range six orders of magnitude.Here,we develop an atomistic approach to describe phonon-mediated exciton dynamics and simulate cooling in NCs of experimentally relevant sizes.We find that cooling occurs on~30 fs timescales in CdSe NCs,in agreement with the most recent measurements,and that the phonon bottleneck is circumvented through a cascade of multiphonon-mediated relaxation events.Furthermore,we identify NC handles for tuning the cooling timescale.
基金
E.R.acknowledges support from the U.S.Department of Energy,Office of Science,Office of Advanced Scientific Computing Research,Scientific Discovery through Advanced Computing(SciDAC)program under Award No.DE-SC0022088
Methods used in this work were provided by the Center for Computational Study of Excited State Phenomena in Energy Materials(C2SEPEM),which is funded by the U.S.Department of Energy,Office of Science,Basic Energy Sciences,Materials Sciences and Engineering Division,via Contract No.DE-AC02-05CH11231
as part of the Computational Materials Sciences Program.D.J.acknowledges the support of the Computational Science Graduate Fellowship from the U.S.Department of Energy under Grant No.DE-SC0019323.
