title: Size and composition dependent multiple exciton generation efficiency in PbS, PbSe, and PbS(x)Se(1-x) alloyed quantum dots.
abstract: Using ultrafast transient absorption and time-resolved photoluminescence spectroscopies, we studied multiple exciton generation (MEG) in quantum dots (QDs) consisting of either PbSe, PbS, or a PbSxSe1-x alloy for various QD diameters with corresponding bandgaps (Eg) ranging from 0.6 to 1 eV. For each QD sample, we determine the MEG efficiency, ηMEG, defined in terms of the electron-hole pair creation energy (εeh) such that ηMEG = Eg/εeh. In previous reports, we found that ηMEG is about two times greater in PbSe QDs compared to bulk PbSe, however, little could be said about the QD-size dependence of MEG. In this study, we find for both PbS and PbSxSe1-x alloyed QDs that ηMEG decreases lineally with increasing QD diameter within the strong confinement regime. When the QD radius is normalized by a material-dependent characteristic radius, defined as the radius at which the electron-hole Coulomb and confinement energies are equivalent, PbSe, PbS, and PbSxSe1-x exhibit similar MEG behaviors. Our results suggest that MEG increases with quantum confinement, and we discuss the interplay between a size-dependent MEG rate versus hot exciton cooling.
authors.affiliation: National Renewable Energy Laboratory , Golden, Colorado 80401, United States.
authors.forename: Aaron G
authors.initials: AG
authors.lastname: Midgett
authors.forename: Joseph M
authors.initials: JM
authors.lastname: Luther
authors.forename: John T
authors.initials: JT
authors.lastname: Stewart
authors.forename: Danielle K
authors.initials: DK
authors.lastname: Smith
authors.forename: Lazaro A
authors.initials: LA
authors.lastname: Padilha
authors.forename: Victor I
authors.initials: VI
authors.lastname: Klimov
authors.forename: Arthur J
authors.initials: AJ
authors.lastname: Nozik
authors.forename: Matthew C
authors.initials: MC
authors.lastname: Beard
mesh_terms: D000497:Alloys; D003198:Computer Simulation; D004735:Energy Transfer; D007854:Lead / Q000737:chemistry*; D008163:Luminescent Measurements / Q000379:methods*; D008422:Materials Testing; D008956:Models, Chemical; D010316:Particle Size; D045663:Quantum Dots; D018036:Selenium Compounds / Q000737:chemistry*; D013440:Sulfides / Q000737:chemistry*; D020349:Surface Plasmon Resonance / Q000379:methods*
chemical_list: D000497:Alloys; D018036:Selenium Compounds; D013440:Sulfides; C088065:lead selenide; C018391:lead sulfide; D007854:Lead