Energy Transfer in Nanostructured Materials

Nanostuctured materials allow us to tailor material properties by controlling composition and structure on nanometer length scales. Integrating these nanostructured materials into devices while preserving their unique and tunable features, however, is very challenging. One approach is to simply create a film or layer composed of many individual nanostructures and hope that the ensemble preserves the properties of the individual nanostructures. One of the fundamental obstacles to this approach is that nanoparticles are never identical. Consequently, the interactions and energy transfer in an ensemble of nanostructures can be very different than either isolated nanostructures or bulk crystals. Characterizing these interactions and energy transfer mechanisms is challenging because the energy transfer occurs on extremely short time scales. Similar questions about energy transfer and relaxation dynamics in nanostructures are important to the development of new devices that are centered around single nanostructures. We characterize energy transfer in nanostructured materials using ultrafast optical techniques, including time-resolved optical spectroscopy and transient absorption spectroscopy.

One of the simplest nanostructures is a defect in a semiconductor crystal. These defects typically act as traps that facilitate nonradiative relaxation of excited electrons or holes, limiting the lifetime of carriers. We can characterize materials by measuring the photoluminescence (PL) lifetime of a material, which tells us about the minority carrier lifetime. We make these measurements with time-resolved photoluminescence (TRPL) spectroscopy. Unfortunately, the situation is a bit complicated: the PL lifetime we measure depends very strongly on the experimental conditions, particularly the power of the exciting laser. This dependence is related to saturation of the traps under high excitation fluence conditions, which turns off a nonradiative recombination pathway. As shown in the figure, we have developed an approach that can fit an analytic solution to the full Shockley-Read-Hall equation for trap state relaxation to experimental data that measures PL lifetime as a function of excitation laser fluence. The only free parameter in the fit is the trap state density. This approach provides a rapid nondestructive way to obtain an unambiguous measure of the material quality -the trap state density.

Another application of TRPL is to study energy transfer in films of colloidal quantum dots. Working with our collaborators, we have shown that the assembly of layers of QDs with a controlled layer-by-layer change in QD size allows us to control the flow of energy through the ensemble. Of particular importance is the demonstration that this controlled ordering allows the recycling of excitions bound in surface traps, as depicted in the figure. This is important because surface traps are very difficult to eliminate in colloidal QDs, particularly when good electrical transport properties are desired, and these trap states provide locations for nonradiative recombination that limits device efficiency. Efforts are underway to design and understand more complex “heterostructures” of colloidal quantum dots that could be used to tailor energy transfer.

Although TRPL is a powerful technique, it is limited to a time resolution of about 50 ps by the instrument response function of the avalanche photodiodes. Phonon relaxation processes in nanostructures, however, often happen on much timescales of 1-10 ps. To measure these dynamics, we are developing a transient absorption spectroscopy system that has sub-ps time resolution derived from the delay between optical pump and probe pulses. Using this approach, we will investigate the dynamics of energy transfer in a variety of new nanostructures.