M. Kreye

Origin of the near-band-edge photoluminescence emission in aqueous chemically grown ZnO nanorods

The optical properties of ZnO nanorods realized by an advanced low-temperature aqueous chemical growth on both silicon and plastic substrates are presented. Systematic photoluminescence investigations in the temperature range of 4–293K reveal strong and well-resolved near-band-edge emission even for rods on plastic substrate, and a weak deep-level emission. At intermediate temperatures phonon replicas of excitonic lines are observable. The optimum molar concentration range of the solution for obtaining nanorods of good optical quality is shown to lie between 0.025M and 0.075M. The large linewidth of the near-band-edge emission (∼10meV), its temperature dependence, and the absence of sharp excitonic transitions indicate that this emission is a result of transitions from a band of donor states.

Controlled low-temperature fabrication of ZnO nanopillars with a wet-chemical approach

Aqueous chemical growth (ACG) is an efficient way to generate wafer-scale and densely packed arrays of ZnO nanopillars on various substrate materials. ACG is a low-temperature growth approach that is only weakly influenced by the substrate and even allows growth on flexible polymer substrates or on conducting materials. The advanced fabrication of wafer-scale and highly vertically aligned arrays of ZnO nanopillars on various substrate materials is demonstrated. Moreover, it is possible to control the morphology in diameter and length by changing the growth conditions. Photoluminescence characterization clearly shows a comparatively strong band-edge luminescence, even at room temperature, that is accompanied by a rather weak visible luminescence in the yellow/orange spectral range.

An optical in-situ study of the re-oxidation kinetics of mixed valent Yb3Al5O12

The optical spectra of blue Yb-garnet single crystals have been studied in the range of 200 to 1200 nm. The spectra exhibit absorption bands due to Yb2+ and Yb3+ ions including 4f–4f transitions on Yb3+ as well as 4f–5d transitions on Yb2+ ions, and a band due to Yb2+–Yb3+ intervalence charge-transfer (IVCT) responsible for the blue colour of the crystal. At high temperatures, the blue colour is found to bleach in an oxidising atmosphere. The oxidation reaction was studied by means of optical spectroscopy at in-situ conditions in the temperature range between 861 and 1065°C. Basically, the oxidation kinetics were found to follow a parabolic rate law. The kinetics have been analysed in the framework of a refined model of internal oxidation accounting for both the transport of the oxidising defects and the diffusion of the reduced component Yb2+.

Chemical diffusion with non-constant Dδ and the appearance of a parabolic rate law: Model study on SrTiO3

The chemical diffusion of oxygen in Fe-doped SrTiO3 single crystals driven by large changes in the ambient oxygen partial pressure pO2 is studied. The stoichiometry dependence of the chemical diffusion coefficient Dδ is derived from the concept of conservative ensembles for two independent trapping reactions. Finite difference simulations show that the stoichiometry dependence of Dδ can lead to the appearance of a parabolic rate law (which typically applies to two- or multiple-phase systems) even in single-phase systems, provided a strong increase of Dδ occurs in the outer sample region.

Optical Characterisation of Low‐Temperature Grown ZnO Nanorods

Photoluminescence studies have been carried out on ZnO nanorods from an aqueous chemical growth. The results reveal well‐resolved near‐band‐edge emission with a broad linewidth of about 10 meV. Nanorods of different lengths and diameters on Si and even on plastic substrates showed similar optical properties indicating a good control of the growth process without much influence on the optical properties. Time‐resolved photoluminescence measurements show a very fast decay time of about 70 ps for the as‐grown samples but longer decay time of about 175 ps for the annealed samples.