Individual single-wall carbon nanotubes (SWNTs) and double-wall carbon nanotubes (DWNTs) were suspended in water for optical studies using sodium-cholate and other surfactants. We used time-resolved photoluminescence (PL) spectroscopy to study the influence of tube chirality and diameter as well as of the environment on nonradiative decay in small diameter tubes. The studies provide evidence for PL from small diameter core tubes in DWNTs and for a correlation of nonradiative decay with tube diameter and exciton red shift as induced by interaction with the environment.

We have studied the electronic structure and charge-carrier dynamics of individual single-wall carbon nanotubes (SWNTs) and nanotube ropes using optical and electron-spectroscopic techniques. The electronic structure of semiconducting SWNTs in the band-gap region is analyzed using near-infrared absorption spectroscopy. A semi-empirical expression for E S 11 transition energies, based on tight-binding calculations is found to give striking agreement with experimental data. Time-resolved PL from dispersed SWNT-micelles shows a decay with a time constant of about 15 ps. Using time-resolved photoemission we also find that the electron-phonon (e-ph) coupling in metallic tubes is characterized by a very small e-ph mass-enhancement of 0.0004. Ultrafast electronelectron scattering of photo-excited carriers in nanotube ropes is finally found to lead to internal thermalization of the electronic system within about 200 fs.

A study of the bandgap character of compressively strained GeSn0.060-0.091/Ge(001) quantum wells grown by molecular beam epitaxy is reported. The built-in strain in GeSn wells leads to an increased separation between L and Γ conduction band minima. The prevalent indirect interband transitions in GeSn were probed by photoluminescence spectroscopy. As a result we could simulate the L-valley bowing parameter in GeSn alloys, bL = 0.80 ± 0.06 eV at 10 K. From this we conclude that even compressively strained GeSn/Ge(001) alloys could become direct band gap semiconductors at the Sn-fraction higher than 17.0 at. %.

Si nanocrystals designed for memory applications were prepared in a layered arrangement by using a SiOx∕SiO2 multilayer structure with a variation of the stoichiometry parameter x from 0.9 to 1.63. The stoichiometry of the SiOx layers is controlled by adjusting the oxygen pressure during the growth which influences the resulting area density of the Si nanocrystals after high temperature annealing from around (2.8–0.93)×1012∕cm2. The tuning of the Si nanocrystal area density in the layers is demonstrated by transmission electron microscopy as well as by comparison of capacitance-voltage and photoluminescence measurements. The influence of the nanocrystal density on the charge behavior is demonstrated and discussed. Our method realizes a simple way to control the area density by maintaining equally sized nanocrystals, that gives unique possibilities to study the influence of the nanocrystal density on the electrical properties.

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We discuss the formation of a Si/Ge-superlattice (SL) generated by molecular beam epitaxy. Specific growth parameter were chosen to optimize the periodic structure of vertically stacked Ge islands. Optimized SLs show a strong photoluminescence at a wavelength in the region of 1.55 μm up to room temperature. The luminescence is explained by a recombination of electrons in a miniband and holes localized in the Ge islands. The morphology and the crystal structure of the SL, which are influenced by the growth parameters, were analyzed by transmission electron microscopy techniques. It is demonstrated that doping of the SL structure by antimony improves both structural and optical properties.

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Transient photoluminescence from a series of asymmetric InAs quantum-dot bilayers with a GaAs barrier layer thickness varying from 30 to 60 monolayers between the quantum-dot planes is investigated. The interdot carrier transfer process is analyzed. In the framework of a three-level system, interdot carrier transfer times between 200 and 2500 ps are derived and compared with similar data from the literature. Within the semiclassical Wentzel-Kramers-Brillouin approximation, the observed "transfer time-barrier thickness-relation" supports nonresonant tunneling as the microscopic carrier transfer mechanism.