Energy conversion in optically active semiconductor nanostructures probed by optical spectroscopy
Tilmar Kümmel, Werkstoffe der Elektrotechnik and CENIDE, Universität Duisburg-Essen, Duisburg, GermanyWolf Quitsch, Werkstoffe der Elektrotechnik and CENIDE, Universität Duisburg-Essen, Duisburg, GermanyOliver Pfingsten, Werkstoffe der Elektrotechnik and CENIDE, Universität Duisburg-Essen, Duisburg, GermanyDaniel Sager, Werkstoffe der Elektrotechnik and CENIDE, Universität Duisburg-Essen, Duisburg, GermanyGerd Bacher, Werkstoffe der Elektrotechnik and CENIDE, Universität Duisburg-Essen, Duisburg, Germany
The enhanced surface-to-volume ratio and the size dependent band structure make semiconductor nanomaterials highly attractive for applications in optoelectronics, photovoltaics and photo catalysis. In order to get insight into the underlying mechanisms of energy conversion (i.e. electrical energy into light and vice versa, light into chemical energy), time-resolved optical spectroscopy with high spatial resolution (< 500 nm) has been demonstrated to be a powerful experimental tool.
III-V semiconductor nanowires based on GaAs are highly attractive for concentrator solar cells, i.e. converting light into electrical energy, because of the direct bandgap and the enhanced light collection due to the specific nanowire architecture. We investigated GaAs nanowires with an axial heterojunction by spatially and time-resolved photoluminescence spectroscopy and photocurrent measurements. The impact of doping on surface band bending  is shown and a reduction of carrier losses by a core-shell approach is demonstrated. Core-shell architectures in addition represent a key element for efficient and fast switchable nanowire LEDs, i.e. converting electrical energy into light. Planar InGaN/GaN LEDs grown on the c-plane suffer from long carrier lifetimes and thus limited modulation capability due to strong piezoelectric fields. These fields are absent in m-plane quantum wells grown in a core-shell nanowire geometry. We could show that the carrier lifetime is thus drastically reduced, giving way to a GHz electrical operation of the device . In order to convert light into chemical energy, a further reduction in size towards ultrasmall nanocrystals is even more promising. We investigate oxide nanoclusters like VxOy bound to metal oxide particles. A wavelength selective catalytic decomposition of methyl orange dye is found in a dispersion of these particles. Degradation of the dye under illumination with UV light demonstrates catalytic activity, while the dye remains stable under visible light illumination.
 D. Sager et al., J. Appl. Phys. 113, 174303 (2013)
 R. Koester et al., Nano Letters 15, 2318 (2015)