Waveguide-Based Spectroelectrochemical Characterization of Band Edge Energies in Submonolayers of CdSe Quantum Dots Tethered to Indium-Tin Oxide Electrodes

We present here high sensitivity attenuated total reflectance (ATR) spectroelectrochemical studies of electron injection (reduction) into surface-tethered, submonolayer to monolayer coverages of CdSe quantum dots (QDs) linked to indium-tin oxide (ITO) electrodes using a strong X-type bifunctional phosphonic acid (PA) surface linker, octanediphosphonic acid (ODiPA). Estimates of conduction band energies (ECB) were obtained from the onset of absorbance bleaching as a function of QD diameter (3.2-6.4 nm) and as a function of the supporting electrolyte (LiClO4) concentration. For CdSe QDs created from combinations of moderately strong stearic acid, hexadecylamine, trioctylphosphine oxide, and trioctylphosphine ligands, surface-tethering was accompanied by decreases in QD diameter and loss of up to 25% volume for the largest QDs. For QDs prepared with PA ligands, followed by aggressive (3×) pyridine exchange to produce QDs with weak capping ligands, no size reduction was observed as a result of adsorption to the ODiPA/ITO surface. For both types of tethered CdSe QDs, significant stabilization of the reduction product of the surface-tethered QD was observed with ca. 700 meV lowering of ECB relative to estimates of ECB obtained from our recent in vacuuo UV-photoemission studies of bare CdSe QDs tethered to Au surfaces. A sizeable fraction of that stabilization is proposed to arise from the tethering of these asymmetric QDs to a complex, high dielectric constant interface region. At least 200 meV of the stabilization arises from concentration-dependent charge screening by the solution counter ion (Li+), with no evidence for the incorporation of Li+ as a result of the electron injection process. The overall stabilization in the reduced form of these tethered QDs is larger than seen for previous spectroelectrochemical studies of QD reduction, in solution, tethered at higher coverages to transparent electrodes, or as electrophoretically deposited multilayer QD thin films. This waveguide ATR spectroelectrochemical approach to estimating energetics for QDs tethered to semiconductor or oxide substrates at low surface coverages is likely to be relevant for a wide array of energy conversion and energy storage processes using nanomaterials and may be especially useful for studying the effects of surface coverage, type of surface linker, contacting solvent/electrolytes, and adsorbed molecular reactants.