We use external magnetic fields to identify the band edge emitting statc in CdSe quantum dms. The field dependence of emission decays and LO phonon spectra show the importance of exciton spin dynamics in the recombination mechanism. To interpret our results we calculate the band edge exciton structure, including the effects of the electron-hole exchange interaction and a nonspherical shape. The exchange term, negligible in the bulk. is strongly enhanced by quantum confinement and allows the observation of an optically passive "dark" excitonic state.

Observation of the dark exciton in CdSe quantum dots

Exciton fine structures in cubic III-V semiconductor GaAs, InAs and GaN quantum dots are investigated systematically and the exciton spin relaxation in GaN quantum dots is calculated by first setting up the effective exciton Hamiltonian. The electron-hole exchange interaction Hamiltonian, which consists of the long- and short-range parts, is derived within the effective-mass approximation by taking into account the conduction, heavy- and light-hole bands, and especially the split-off band. The scheme applied in this paper allows the description of excitons in both the strong- and weak-confinement regimes. The importance of treating the direct electron-hole Coulomb interaction unperturbatively is demonstrated. We show in our calculation that the light-hole and split-off bands are negligible when considering the exciton fine structure, even for GaN quantum dots, and the short-range exchange interaction is irrelevant when considering the optically active doublet splitting. We point out that the long-range exchange interaction, which is neglected in many previous works, contributes to the energy splitting between the bright and dark states, together with the short-range exchange interaction. Strong dependence of the optically active doublet splitting on the anisotropy of dot shape is reported. Large doublet splittings up to 600 $\ensuremath{\mu}$eV, and even up to several meV for small dot size with large anisotropy, are shown in GaN quantum dots. The spin relaxation between the lowest two optically active exciton states in GaN quantum dots is calculated, showing a strong dependence on the dot anisotropy. Long exciton spin relaxation time is reported in GaN quantum dots. These findings are in good agreement with the experimental results.

Theory of excitons in cubic III-V semiconductor GaAs, InAs and GaN quantum dots: Fine structure and spin relaxation

Doped and codoped silicon nanocrystals: The role of surfaces and interfaces

Using time-resolved photoluminescence spectroscopy over a wide range of temperatures, we were able to probe both radiative and nonradiative relaxation processes in luminescent porous silicon. By comparing the photoluminescence decay times from freshly prepared and oxidized porous silicon, we show that radiative processes should be linked with quantum confinement in small Si nanocrystallites and are not affected by oxidation. In contrast, nonradiative relaxation processes are associated with the state of oxidation where slower relaxation times characterize hydrogen-terminated porous silicon. These results are in a good agreement with the extended vibron model for small Si nanocrystallites.

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Radiative and nonradiative relaxation phenomena in hydrogen- and oxygen-terminated porous silicon

Ultrafast exciton dynamics in one-dimensional (1D) silicon nanowires (SiNWs) have been investigated using femtosecond transient absorption techniques. A strong transient bleach feature was observed from 500 to 770 nm following excitation at 470 nm. The bleach recovery was dominated by an extremely fast feature that can be fit to a triple exponential with time constants of 0.3, 5.4, and ∼75 ps, which are independent of probe wavelength. The amplitude and lifetime of the fast component were excitation intensity-dependent, with the amplitude increasing more than linearly and the lifetime decreasing with increasing excitation intensity. The fast decay is attributed to exciton–exciton annihilation upon trap state saturation. The threshold for observing this nonlinear process is sensitive to the porosity and surface properties of the sample. To help gain insight into the relaxation pathways, a four-state kinetic model was developed to explain the main features of the experimental dynamics data. The model sugges...

Ultrafast Exciton Dynamics in Silicon Nanowires.

The excitonic fine structure, including splitting due to direct and exchange interactions, has experimentally been resolved from silicon nanocrystals and from silicon nanorods. We have found the hierarchy of levels for silicon nanorods to be different from that of silicon nanocrystals with the slower semidark state located above the faster semibright state. The results are analyzed in terms of spin and orbital selection rules indicating that the dimensionality of the exciton determines the relative contribution of the direct Coulomb and the exchange interactions in these nanostructures.

Fine structure and selection rules for excitonic transitions in silicon nanostructures

Time-resolved photoluminescence spectroscopy has been utilized to reveal excitonic transitions in silicon nanowires and silicon nanocrystals. While previous works on porous silicon and silicon nanocrystals have shown a two-level splitting, e.g., singlet-triplet states, our measurements reveal the fine structure of the excitons including three semi-bright states and a ground dark excitonic state. Surprisingly, for silicon nanowires we have found the slowest semi-dark exciton to fall above the faster semi-bright excitonic state as opposed to silicon nanocrystals. The results are analyzed in terms of spin and orbital selection rules showing that the interchange in the level's hierarchy corresponds to a swap between spin-forbidden and orbitally-allowed states. We assign this surprising phenomenon to the relationship between the exchange interaction and the direct Coulomb interaction, which is affected by the dimensionality of the exciton. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Y. Shalibo

Papers

Fine structure and selection rules for excitonic transitions in silicon nanostructures

Excitonic transitions in silicon nanostructures probed by time‐resolved photoluminescence spectroscopy