Lea Nienhaus

From imaging excitons at the nanoscale to emerging device applications

Understanding light-induced processes in materials is critical for tailoring their optical and electronic properties to applications in chemical conversion, light harvesting, or energy transfer. Nanomaterials are prime candidates to study light-matter interactions on both the single-particle and the ensemble level. However, they are prone to defects which can be detrimental to their function in optoelectronic devices. Furthermore, many optoelectronic and photocatalytic systems are based on hybrid interfaces combining both inorganic and organic materials. The exact energy transfer mechanism at these hybrid interfaces is often obscure, particularly when both the macroscopic donor and acceptor materials consist of many separately interacting moieties. Here, I describe the detailed photophysics of a PbS nanocrystal-based light-harvesting device, and further demonstrate a technique for single-particle visualization of absorption in various nanomaterials.

I present exchange-mediated spin-triplet exciton transfer from semiconducting PbS nanocrystals to the triplet state of the organic molecule rubrene. Diffusion-mediated triplet-triplet annihilation in rubrene generates higher-energy emissive spin-singlet states, and shows promise in sub-bandgap sensitization of silicon. We combine transient photoluminescence spectroscopy with a kinetic model to unravel the underlying photophysics of the relevant energy transfer processes occurring in the upconverting device.

To further investigate light-harvesting at the nanoscale, I employ single molecule absorption detected by scanning tunneling microscopy. This technique is based on a change in the local density of states upon absorption, and thus visualizes the localized excitation. Taking advantage of Stark shifts caused by the electric field in the STM, different energy levels can be shifted into resonance with the excitation wavelength.