Electronic and optical properties of InAs nanocrystals

An atomistic pseudopotential method is used to investigate the electronic and opti- cal properties of spherical InAs nanocrystals. Our calculated interband (valence-to- conduction) absorption spectra reproduce the features observed experimentally both qualitatively and quantitatively. The results relative to intraband (valence-to-valence and conduction-to-conduction) absorption successfully reproduce the recently measured photoinduced absorption spectra, which had so far been addressed only qualitatively. They exclude the hypothesis of a thermal activation process between dot-interior delocalised hole states to explain the temperature dependence observed experimentally. Furthermore, based on the agreement of our data with the experimental valence inter- sublevel transitions and the almost complete overlap of the latter with STM measure- ments, we question the simplistic attribution of the observed STM peaks obtained for negative bias.
Motivated by the excellent agreement of our calculated results with the STM, PLE and PIA spectra, we therefore extend our knowledge to a detailed theoretical investigation of the electronic structure and optical properties of InAs nanocrystals at the transition from spheres to rods. We predict that despite the qualitative similarity of both intra- and inter-band optical spectra, for NCs with R > 15 ̊A even slight elongations should result in shifts of the order of hundreds of meV in the spacings between STM peaks measured
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in the positive bias regime, in the position of the intra-band absorption peaks associated with transitions in the conduction band and in the separation between the first and the fifth peak in PLE experiments. Our results suggest that, based on the spectroscopic data, it should be possible to discriminate between spherical and elongated NCs with aspect ratios of length over diameter as small as 1.2. Indeed our results suggest that many nominally spherical experimental samples contained a large fraction of slightly elongated structures.
Additionally, the atomistic pseudopotential approach is also applied to a study of the electronic and optical properties of InAs quantum rods as a function of increasing length- to-diameter ratio. We show that, as the aspect ratio increases, energy levels cross in both conduction and valence bands, reflecting their different dependence on confinement along a specific direction. Unlike in CdSe and InP quantum rods, however, the position of the crossover between highest occupied molecular orbitals with different symmetries is found to be size-dependent and the value of the aspect ratio at the crossing to increase with the rod diameter. We find that the level crossings at the top of the valence band are crucial to explain the evolution with elongation of all optical properties in these systems. Their transformation from 0- to quasi-1-dimensional structures is characterised by a common monotonic behaviour of band gap, Stokes shift, degree of linear polarisation and radiative lifetime, closely linked to the variation with aspect ratio of the electronic structure of the nanocrystal valence band edge. This characteristic feature was not observed in elongated CdSe structures, whose optical properties exhibited instead a distinctive non-monotonic evolution with length, with a turning point associated with a crossover at the top of the valence band, similar to that found here between states with σ and π symmetries.