The Peculiar Electronic Structure of PbSe Quantum Dots

PbSe is a pseudo-II-VI material distinguished from ordinary II-VI's (e.g., CdSe, ZnSe) by having both its valence band maximum (VBM) and its conduction band minimum (CBM) located at the fourfold-degenerate L-point in the Brillouin zone. It turns out that this feature dramatically affects the properties of the nanosystem. We have calculated the electronic and optical properties of PbSe quantum dots using an atomistic pseudopotential method, finding that the electronic structure is different from that of ordinary II-VI's and, at the same time, is more subtle than what k.p or tight-binding calculations have suggested previously for PbSe. We find the following in PbSe dots: (i) The intraband (valence-to-valence and conduction-to-conduction) as well as interband (valence-to-conduction) excitations involve the massively split L-manifold states. (ii) In contrast to previous suggestions that the spacings between valence band levels will equal those between conduction band levels (because the corresponding effective-masses me approximately mh are similar), we find a densely spaced hole manifold and much sparser electron manifold. This finding reflects the existence of a few valence band maxima in bulk PbSe within approximately 500 meV. This result reverses previous expectations of slow hole cooling in PbSe dots. (iii) The calculated optical absorption spectrum reproduces the measured absorption peak that had previously been attributed to the forbidden 1Sh --> 1Pe or 1Ph --> 1Se transitions on the basis of k.p calculations. However, we find that this transition corresponds to an allowed 1Ph --> 1Pe excitation arising mainly from bulk states near the L valleys on the Gamma-L lines of the Brillouin zone. We discuss this reinterpretation of numerous experimental results.