We have measured the low-temperature (4.2 K) exciton lifetimes of zinc-blende CdTe nanocrystal quantum dots (NQDs), 2.6-3.8 nm in diameter, in magnetic fields up to 30 T. The exciton photoluminescence decay time decreases with both dot size and magnetic field. We explain the decrease in decay time in magnetic fields by the mixing of bright and dark exciton states due to a small shape asymmetry in the zinc-blende CdTe NQDs. We show that this behavior resembles that of wurtzite CdSe NQDs, and we demonstrate that an asymmetry of NQDs caused by either shape or crystal structure leads to similar exciton decay dynamics.

We investigate the electronic properties of GaAs1−xBix by photoluminescence at variable temperature (T=10–430K) and high magnetic field (B=0–30T). In GaAs0.981Bi0.019, localized state contribution to PL is dominant up to 150K. At T=180K the diamagnetic shift of the free-exciton states reveals a sizable increase in the carrier effective mass with respect to GaAs. Such an increase cannot be accounted for by an enhanced localized character of the valence band states, solely. Instead, it suggests that also the Bloch states of the conduction band are heavily affected by the presence of bismuth atoms.

We determine the low-temperature optical properties of dark-exciton states in CdSe/CdS nanocrystal quantum dots ͑NQDs͒. By using resonant laser excitation we distinguish zero-phonon from phonon-assisted photoluminescence. The NQDs show a decreasing zero-phonon intensity with decreasing temperature, resulting in a redshift of the nonresonant photoluminescence. This redshift is undone by application of a magnetic field. Our results show that dark-exciton luminescence originates from the intricate competition of phonon-assisted and zero-phonon transitions, the latter of which are enhanced by dark-bright-exciton mixing due to unpassivated surface states or a magnetic field.

We report the compositional dependence of the exciton reduced mass, ^exc, of GaAs1-xBix in a very large Bi concentration range (x =0-10.6%). Photoluminescence under high magnetic fields (B up to 30 T) shows that fiexc increases rapidly until x~ 1.5% and then oscillates around ~0.08 m0, m0 being the electron mass in vacuum, up to about x =6%. Surprisingly, for x > 8% the exciton reduced mass decreases below the GaAs value, in agreement with the expectations of a k • p model. Such a behavior reveals the existence of different concentration intervals, where continuum states of the valence and conduction band hybridize with Bi-related levels at different extents, thus conferring to the band edges a localized or bandlike character for x < 6% and x > 8%, respectively.

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We show that the thermal annealing of thiol-capped PbS colloidal quantum dots provides a means of narrowing the nanoparticle size distribution, increasing the size of the quantum dots and facilitating their coalescence preferentially along the 100 crystallographic axes. We exploit these phenomena to tune the photoluminescence emission of an ensemble of dots and to narrow the optical linewidth to values that compare with those reported at room temperature for single PbS quantum dots. We probe the influence of annealing on the electronic properties of the quantum dots by temperature dependent studies of the photoluminescence and magneto-photoluminescence.

A magneto-optical study of the energy and spin structure of charged excitons in a 20-nm-thick CdTe/Cd 0.65 Mg 0.35 Te quantum well is performed in strong magnetic fields up to 51 T. The type of resident carriers (holes or electrons) in the quantum well is controlled optically by above-barrier illumination, permitting a direct comparison of positively (T + ) versus negatively (T − ) charged excitons. The binding energies of the singlet states of these complexes behave qualitatively differently with increasing magnetic field B; namely, the binding energy decreases for T + and increases for T − with B. The triplet state of T + is identified in strong fields with a binding energy smaller than that of the T − triplet state.

We measure the energy levels and charging spectra of holes in self-assembled InAs quantum dots using capacitance-voltage and polarized photoluminescence spectroscopy in high m agnetic fields. The pronounced circular polarization of the optical emission, together with the optical selection rules for orbital and spin quantum numbers, allows us to separate the individual electron and hole levels. The magnetic field dependence of the single-particle hole energy levels can be understood by considering a spin-orbit coupled valence band and agrees well with the observed behavior of the charging peaks in the capacitance-voltage spectra.