Lawrence Berkeley National Laboratory

An in situ methodology was developed and implemented to characterize electronic band properties at the apex of high-aspect-ratio nanoscale solid-state electrochemical devices. Model devices with materials relevant to various energy applications were composed of nano-thin metal oxide films coated onto metallic scanning tunneling microscopy (STM) probe tips. Electronic properties of these films at the device apex were measured by inverting the tunneling spectroscopy paradigm: vacuum tunneling spectroscopy between functionalized STM probe tips with ambiguous electronic properties and metallic substrates – a method we here introduce as inverted scanning tunneling spectroscopy (I-STS). Additionally, concentration of metal species within the apex of these metal-oxide devices can be controlled through nanoscale solid-state electrochemistry within the same experimental platform, allowing subsequent in situ I-STS measurements that directly reveal how electronic properties change with composition. We first demonstrated and validated the I-STS approach by quantifying the bandgap of nano-thin aluminum oxide (ca. 7.1 eV). Next, bandgaps (or lack thereof) of nano-thin stoichiometric lithium cobalt oxide (LCO, ca. 1.6 eV) and deeply sub-stoichiometric LCO (0 eV) were observed, directly measuring LCO band evolution along its delithiation-induced semiconductor-to-metal transition. I-STS reveals bandgaps at the nanoscale apex that are substantially narrower than bulk values, demonstrating pronounced modulation of electronic structure in electrochemically active oxide nanostructure termini. Ultimately, I-STS uniquely enables electrical characterization of small, high-aspect-ratio model devices at their apex – a spatial regime inaccessible with existing techniques – and provides a pathway to explore ion-coupled electronic phenomena in advanced energy storage and low-power neuromorphic computing devices using standard STM instrumentation.