(Invited) Estimating Band Edge Energies of Monolayer-Tethered Semiconductor Nanocrystals: Comparison of Photoemission and Spectroelectrochemical Approaches and the Implications for Photocatalysis
Authors: Armstrong Neal , Saavedra S. Scott , Pyun Dong-Chul , Sun Weijun , Shallcross R. Clayton , Carothers Kyle , Olikagu Chisom Sylvia , Pattadar Dhruba 
Published: July 2021

This presentation will focus on recent studies to characterize the energetics of semiconductor nanocrystals (NCs - e.g. CdSe, CdS and hetero-structured CdSe@CdS nanorods and tetrapods) using both spectroelectrochemistry and X-ray and UV-photoemission spectroscopies (XPS/UPS), for nanocrystals tethered as monolayers to ITO electrodes (spectroelectrochemistry) or Au (XPS/UPS). Understanding the parameters which control conduction and valence band edge energies in semiconductor nanocrystals is challenging and directly impacts on our ability to design new photocatalysts and photoelectrochemical platforms to produce fuels from sunlight. We have recently shown that UPS and XPS can be used to determine valence band maximum (VBM) energies and vacuum-level corrected ionization potentials (IP), and using the bandgap energies, conduction band energies (CBM) and electron affinities (EA), for monolayers of CdSe NCs tethered to Au, where processing occurred in ambient conditions or in completely inert (O2, H2O free) environments. These studies demonstrated the extreme sensitivity of band edge energies for these nanocrystals to even small changes in surface composition, and if NCs are processed in inert environments, the IP/EA values track the diameter and bandgap energy of the NC quite closely to the trends predicted by the effective mass approximation (EMA). Ambient processed NCs show evidence for loss of the chalcogenide from the surface, with large differences in local vacuum level, and poor tracking of the EMA as NC diameter is varied. More recently we have returned to the characterization of monolayer-tethered, ligand capped CdSe NCs on ITO electrodes, using attenutated-total internal reflectance (ATR) spectroelectrochemical methods, which provide an optical approach to monitoring electron injection into these low surface coverage NCs, allowing us to estimate CBM for isolated NCs in contact with electrolyte solutions. In our most recent studies we find that these estimates of CBM versus NC diameter functionally track the EMA approximation, but with significant shifts in electrochemical potentials (on the vacuum scale) relative to energies estimated via photoemission. In the presence of polar solutions and high concentrations of supporting electrolyte the CBM is shifted by up to 800 meV farther from vacuum than for what is observed via photoemission, emphasizing the critical role played by solvent, surrounding ions, and even the electrode surface in screening the extra charge in the isolated NCs acquired as a result of electrochemical "reduction." For NCs which are precisely synthesized, where processing conditions are optimized, and where size dispersity is minimized, however, it appears that it will eventually be possible to quantify these effects in ways that will be useful in the design of photocatalysts, where VBM/CBM energies control the rates of electron transfer to electron donors/acceptors as part of fuel forming processes.