title: Wave function engineering for ultrafast charge separation and slow charge recombination in type II core/shell quantum dots.
abstract: The size dependence of optical and electronic properties of semiconductor quantum dots (QDs) have been extensively studied in various applications ranging from solar energy conversion to biological imaging. Core/shell QDs allow further tuning of these properties by controlling the spatial distributions of the conduction-band electron and valence-band hole wave functions through the choice of the core/shell materials and their size/thickness. It is possible to engineer type II core/shell QDs, such as CdTe/CdSe, in which the lowest energy conduction-band electron is largely localized in the shell while the lowest energy valence-band hole is localized in the core. This spatial distribution enables ultrafast electron transfer to the surface-adsorbed electron acceptors due to enhanced electron density on the shell materials, while simultaneously retarding the charge recombination process because the shell acts as a tunneling barrier for the core localized hole. Using ultrafast transient absorption spectroscopy, we show that in CdTe/CdSe-anthraquinone (AQ) complexes, after the initial ultrafast (~770 fs) intra-QD electron transfer from the CdTe core to the CdSe shell, the shell-localized electron is transferred to the adsorbed AQ with a half-life of 2.7 ps. The subsequent charge recombination from the reduced acceptor, AQ(-), to the hole in the CdTe core has a half-life of 92 ns. Compared to CdSe-AQ complexes, the type II band alignment in CdTe/CdSe QDs maintains similar ultrafast charge separation while retarding the charge recombination by 100-fold. This unique ultrafast charge separation and slow recombination property, coupled with longer single and multiple exciton lifetimes in type II QDs, suggests that they are ideal light-harvesting materials for solar energy conversion.
authors.affiliation: Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA.
authors.forename: Haiming
authors.initials: H
authors.lastname: Zhu
authors.forename: Nianhui
authors.initials: N
authors.lastname: Song
authors.forename: Tianquan
authors.initials: T
authors.lastname: Lian
mesh_terms: D000880:Anthraquinones / Q000737:chemistry*; D002104:Cadmium / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D053758:Nanoparticles / Q000737:chemistry*; D045663:Quantum Dots; D012643:Selenium / Q000737:chemistry*; D013691:Tellurium / Q000737:chemistry*
chemical_list: D000880:Anthraquinones; D002104:Cadmium; D012643:Selenium; D013691:Tellurium