To gain an understanding of the fundamental physical properties of novel (nano)materials and to provide feedback for our synthesis as well as device fabrication projects we employ a number of experimental techniques. In essence, we investigate the interaction of our materials with light. This includes emission and absorption of photons both by large ensembles of nanocrystals as well as by single crystals. We want to understand on which timescales excitations as well as energy and charge transfer processes occur and how they are affected by the materials' composition and form. For more in-depth information on specific methods follow the links below.
Micro-photoluminescence (or µ-PL) is a technique in which PL studies are conducted with a spatial resolution of typically around or below 1 µm. This enables either a spatial mapping in larger crystals and films or imaging single objects such as nanocrystals. For this, we prepare samples so that the individual NCs are separated by several micrometers and insert the NC samples into one of our two µ-PL setups. Optical excitation is accomplished with a wavelength-tunable white light laser, and detection depends on the NC properties we wish to study.
Optical transitions, which typically occur at fixed energies can be spectrally broadened through several phenomena. Inhomogeneous broadening, due to non-identical nanocrystals and homogeneous broadening, mainly due to carrier-phonon interactions can strongly wash out and blur individual features. By going to low temperatures, the homogeneous broadening is strongly suppressed and features become clearer. For this, we employ a closed-cycle cryostat, which lets us set the temperature of the sample between 4K and 350K.
To integrate our nanomaterials into working devices, we must be able to inject charges or energy into these. Understanding how charge and/or energy carriers diffuse within thin films of these nanocrystals can help to optimize the nanomaterials for enhanced transport capabilities. To investigate charge carrier diffusion in our systems we employ a microscope that lets us spatially and temporally resolve photoluminescence in bulk and NC-films after optical excitation. From this in turn, we can extract important properties such as diffusion coefficients.