Characteristic Energies and Shifts in Optical Spectra of Colloidal IV−VI Semiconductor Nanocrystals

We investigate structural, electronic, and optical properties of colloidal IV-VI semiconductor quantum dots (QDs) using an ab initio pseudopotential method and a repeated supercell approximation. In particular, rhombo-cuboctahedral quantum dots consisting of PbSe, PbTe, and SnTe with a pseudohydrogen passivation shell are investigated for different QD sizes. The obtained dependence of the confinement energy on the QD size questions the use of three-dimensional spherical potential well models for small QD structures. The predicted band gaps are almost in agreement with measured values. The calculated Franck-Condon shifts vary significantly with the QD size. Only for PbSe they may explain the Stokes shift between optical absorption and emission. For the tellurides, spectral properties such as the oscillator strength are more important.