Abstract: Optical interactions and photophysical processes hinge on structure and the local environment in nanoscale systems, it is critically important to develop experimental approaches which can characterize these optical properties and correlate them with atomic-scale morphology and electronic structure. Over the past three decades, ultra-high vacuum (UHV) scanning tunneling microscopy/spectroscopy (STM/STS) and associated surface preparation techniques have demonstrated atomic-scale control over nanoscale structures. In parallel, single-particle laser spectroscopy has elucidated photophysics, quantum coherence, and optical properties with ultrahigh spectral resolution at the single quantum absorber and emitter level.

In this talk, I will focus on our efforts to extend these studies to the atomic scale on surfaces by combining UHV STM and AFM and single particle laser spectroscopy. I will discuss our work exploring the structural, electronic, and transport properties of donor-acceptor molecular heterojunctions (HJs) self-assembled from C60 and pentacene as a potential platform for exploring photophysics at the molecular scale. We have resolved a surprising structure and charge transfer in in-plane molecular HJs, and demonstrated extremely strong (and spatially dependent) current rectification in transport for a stacked molecular HJ at the monolayer level. I will discuss recent STM/STS measurements on defects in bilayer WSe2, which may be related to the single-photon emitting defects observed in laser spectroscopy experiments, revealing the local electronic structure and demonstrate the ability to control the charge state of these defects. Time permitting, I will discuss UHV STM measurements on Cu2O (111) and (110) surfaces and our efforts to understand their physical and electronic structure in light of their photocatalytic activity. I will also discuss our future plans enabled by the recent DOE NSRC QIS AQuISS award.