Imaging excited states of nanomaterials*

Nanomaterials are promising for applications in photocatalysis, photosensitization, photodetection, photovoltaics and optoelectronics. In these applications, nanomaterials are first photoexcited, then the generated energy or charge is either used to break chemical bonds or transferred to nearby electrodes, particles or molecules of interest. Understanding photoexcited nanomaterials at their characteristic length scale is critical for performance optimization, however, is challenging given their small size and fast relaxation. We develop and use single-molecule adsorption scanning tunneling microscopy (SMA-STM), a powerful technique capable of imaging photoexcited nanomaterials with sub-nanometer spatial resolution, to investigate photoexcited quantum dots (QDs), carbon nanotubes (CNTs) and their interactions. Images of individual photoexcited QDs (absorption images) vary significantly from dot-to-dot. For a single QD, different excited states are probed by changing the applied electric field. Using the STM tip to nudge and roll the QDs on the surface, different images of the excited state at different angles are obtained. Energy transfer in arrays of QDs, QD-CNT interactions are also imaged and manipulated at individual nanoparticle level. Finally, I will discuss my recent work on probing molecular-scale catalytic interactions of oxygen with an oxygen reduction molecular catalyst using another optical STM technique, STM tip-enhanced Raman spectroscopy (STM-TERS).