Optical transitions in charged CdSe quantum dots

Abstract

Using a many-body approach based on single-particle pseudopotential wave functions, we calculate the dependence of the optical transitions in CdSe nanocrystals on the presence of ‘‘spectator’’ electrons or holes. We find that~i! as a result of the different localization of the electron and hole wave functions, the absorption lines shift by as much as 22 meV/unit charge when electrons or holes are loaded into the quantum dot. ~ii! The lowest emission line is significantly red shifted with respect to the lowest allowed absorption line. ~iii! Trapping of a ‘‘spectator’’ hole in a surface state is predicted to lead to dramatic changes in the absorption spectrum, including the appearance of new transitions. Semiconductor quantum dots can be charged by deliberate injection of carriers ~via electrical contacts, 1 or via a scanning-tunneling-microscopy tip, 2 ! by photoionization processes removing one or more carriers from the quantum dot, 3 or by capture of external charges. 4 The effects of charging on the optical properties of self-assembled InAs/GaAs quantum dots have been recently measured both in absorption 5 and emission. 6 It was found that when electrons are progressively loaded into the quantum dots, the absorption and emission energies are redshifted relative to the neutral dots. Furthermore, low-energy lines disappear from the absorption spectrum, 5 while new high-energy lines appear in the photoluminescence spectrum. 6 In colloidal quantum dots, charging of surface states is believed to be at the origin of a variety of unusual phenomena, including the occurrence of a permanent dipole moment even in zincblende dots, 7 intermittency ~blinking! of photoluminescence, 3 spectral diffusion and Stark shift, 8 upconversion of photoluminescence, 9 and possibly even the occurrence of long spin lifetimes. 10 However, there are still no reports on the absorption or emission spectra of charged colloidal dots. The effects of charging on the interband optical transitions can be examined using a screened Hartree-Fock model, where the initial and final states are expressed as Slater determinants. The energy DEh,e(Nes ) required to optically excite an electron from the valence-band state h to the conduction-band state e in the presence of N es ‘‘spectator electrons’’ ( e s ) is: