Electron Migration Limits the Rate of Photobrightening in Thin Films of CdSe Quantum Dots in a Dry N2 (g) Atmosphere

Abstract

This paper describes a study of the mechanism of photoinduced photoluminescence enhancement, termed “photobrightening” (PB), of ∼150-nm-thick films of CdSe quantum dots (QDs) in a dry N2 (g) atmosphere. Steady-state photoluminescence (PL) and ultrafast transient absorption measurements of films photoexcited continuously and shot-wise revealed that: (i) PL enhancement occurred in all of the close-packed films during periods of continuous photoexcitation and continued after the excitation source was turned off; (ii) the time-dependence of PB (both during excitation and in the dark) was initially linear and became exponential as the PB reached saturation; (iii) the rate of PB and the maximum PB achieved by the film depended on the degree of surface passivation of the QDs; (iv) the PL peak shifted to lower energy and broadened during PB; and (v) rates of nonradiative trapping of excitonic electrons decreased during PB. These data were utilized to construct a model for PB based on migration of photoexcited ele...