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

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@CdS₃₃ and CdSe_3.5@CdS₃₇). CdS shell thicknesses around the CdSe seeds in these NRs were estimated to be ca. 1.7-1.9 nm. Following UV-ozone 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@CdS₃₅ 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@CdS₃₅ 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.