Spectroelectrochemical Characterization of Energetics in Type I vs Quasi-Type II Heterojunctions in CdSe@CdS Nanorod Films

Abstract

We present here the spectroelectrochemical characterization of band edge energetics in Type I versus quasi-Type II core/shell CdSe@CdS nanorod (NR) ensembles, tethered via biphosphonic acids to indium−tin oxide (ITO) electrodes. These investigations targeted either 2.2 or 3.5 nm diameter CdSe seeds embedded in CdS rods with diameters of 5.7 and 7.3 nm and rod lengths of 33 and 37 nm, respectively (CdSe 2.2 @CdS 33 and CdSe 3.5 @CdS 37 ). CdS shell thicknesses around the CdSe seeds in these NRs were estimated to be ca. 1.7−1.9 nm. Following UVozone activation of the NR films, the bleaching of the lowest energy excitonic absorbance features of both the CdSe seeds and the CdS rods was independently monitored as a function of applied (negative) potential. The midpoint potential for bleaching of these excitonic features, associated with electron injection into either the rod or the seed, corrected to a common vacuum scale, provided for estimates of the conduction band edge (E CB ) for both the CdSe seed and the CdS rod, with good energy resolution. E CB values in CdSe 2.2 @CdS 35 NRs were found to be the same (ca. −3.99 eV) for both the CdS rod and the CdSe seed, suggesting the formation of a "quasi-Type II" CdS/CdSe heterojunction for CdSe seeds of this diameter, with no detectable energetic offset in E CB . E CB values in the larger seed CdSe 3.5 @CdS 35 NRs were resolved (ca. −4.03 eV for the CdS rods and ca. −3.95 eV for the CdSe seeds), consistent with the formation of a Type I CdS/CdSe heterojunction. These spectroelectrochemical experiments suggest that electrons can be directly injected into both the buried CdSe seed and the CdS rod for Type I heterostructures. Such spectroelectrochemical characterizations provide a direct pathway for estimation of conduction band energies in a variety of complex heterostructured semiconductor nanomaterials.

Optimizing Electron Delocalization in CdSe-CdS Heteronanostructures: Insights from Single-Particle Photoluminescence Studies

“…Two commonly adopted techniques to comprehend the delocalization of conduction band electrons in HNSs are time-resolved transient absorption (TA) spectroscopy and single photon counting. By adopting these spectroscopic techniques, exciton delocalization efficiency in type I and quasi-type II systems has been demonstrated by various groups. − In addition, theoretical methods, and spectroelectrochemical and scanning tunneling spectroscopic techniques have been adopted to understand exciton delocalization in HNSs. − Since the direct exciton recombination is highly dependent on the electron–hole wave function overlap, the extent of delocalization can be extracted from their photoluminescence (PL) lifetime. However, HNSs often exhibit multiple charge carrier recombination pathways, such as band-edge- and trap-mediated recombinations.…”