The effect of small elongations on the electronic and optical signatures in InAs nanocrystal quantum dots

Abstract

Abstract.We present a detailed theoretical investigation of the electronic structure and optical properties of InAs nanocrystals at the transition from spheres to rods. Using a semiempirical pseudopotential approach, we predict that, despite the qualitative similarity of both intra-and inter-band optical spectra, for NCs with R > 15Å even slight elongations should result in shifts of the order of hundreds of meV in the spacings between STM peaks measured in the positive bias regime, in the position of the intraband 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 show 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.

Direct and Inverse Auger Processes in InAs Nanocrystals: Can the Decay Signature of a Trion Be Mistaken for Carrier Multiplication?

“…It has been recently suggested, based on a detailed theoretical analysis of the observed optical spectra, − that slightly elongated NCs (quantum rods, QRs) with aspect ratios between the two main diameters of the order of about 1.2 may be present in the experimental (core-only) samples synthesized according to the procedure, detailed by Guzelian et al ., followed by Schaller et al . for the synthesis of their core-only NCs.…”

Monotonic Evolution of the Optical Properties in the Transition from Three- to Quasi-Two-Dimensional Quantum Confinement in InAs Nanorods

“…This effect is illustrated in Figure , where we compare the absorption spectra calculated with different line broadenings. The proximity of the two strongest lowest-energy absorption transitions (whose separation decreases with increasing rod length due to the increase in the VB density of states) causes the absorption maximum to blue-shift for broadenings larger than their separation: in panels (b) and (c), the two well separated absorption peaks that can be resolved for small broadenings (10 meV, thin red lines), yielding shifts between absorption and emission peaks of the order of a few meV, are shown to merge into a broad blue-shifted single featureless peak (thick red lines) for Γ = 80 meV. This leads to apparent Stokes shifts that are much larger than the separation between the lowermost excitonic states responsible for absorption and emission, which we call the real Stokes shift (henceforth referred to simply as Stokes shift, black line in Figure ).…”

“…In Table , we compare our results for spherical (ρ = 1.0) NCs with D = 4.0 nm with the available experimental data for spherical dots (unfortunately, no lifetime data have been published so far for InAs rods). Results for slightly elongated NCs (with ρ = 1.2) were also included, as our previous calculations suggested that many “nominally” spherical experimental samples contained a large fraction of slightly elongated structures. Given that the experimental data are all relative to core/(multi)shell structures, it is perhaps not surprising that the lifetimes calculated using large values for the matrix dielectric constant (4.5 ≤ ε out ≤ 6.0) give the best agreement with experiment.…”

“…Rods with different aspect ratios L / D are generated starting from zinc-blende InAs spherical dots (lattice constant a = 6.0584 Å) with the desired diameter (see ref for details), and inserting a cylindrical segment along the [001] direction. In order to mimic the experimental situation of surfactant passivation, the dangling bonds at the surface of the NCs are passivated by pseudohydrogen atoms …”

“…The unique properties of CdSe quantum rods, such as a non-monotonic change of the global Stokes shift, , the polarization ratio, , and the radiative lifetime with increasing aspect ratio, are just beginning to show the important role of shape in these systems. Detailed theoretical studies of InAs NRs are, however, still not available despite extensive experimental investigations conducted recently. , After the success of our pseudopotential study on the optical properties of spherical and slightly elongated InAs nanocrystals, we will therefore extend here our investigation to the transformation of the electronic and optical structure from three- to quasi-two-dimensional quantum confinement.…”