Element-Selective Analysis of Carrier Behavior in Plasmonic Metal–Zinc Oxide (M = Al or Ga) Nanocrystals

The ability to correlate changes in free carrier densities with changes in atomic structure in plasmonic semiconducting nanocrystals (NCs) can improve our understanding of these materials and lend insight into their rational design. Zinc oxide (ZnO) is of particular interest in this class of materials because of its uniquely wide band gap, which can be manipulated through the addition of dopants (e.g., Al3+ and Ga3+). Herein, we demonstrate the utility of solid-state NMR (SSNMR) in tandem with other spectroscopic techniques to probe changes in atomic structure and elucidate the relationship between these changes and free carrier densities in a series of ZnO NCs doped with varying concentrations of Al3+ and Ga3+ (i.e., AZO and GZO, respectively). 67Zn SSNMR is employed to draw comparisons between the bulk ZnO phase and the ZnO NC phases. 67Zn SSNMR is also employed to study how differences in dopant size and concentration affect dopant deactivation and, hence, free carrier densities and the resulting frequency of the plasmon. 27Al and 71Ga SSNMR are employed to probe the effects of dopant incorporation on the zinc oxide lattice and gain insight into the preferred coordination environments of each dopant. In the AZO NCs, four-, five-, and six-coordinate Al3+ environments are observed, whereas in the GZO NCs, only four- and six-coordinate Ga3+ environments are identified. In both, higher doping levels result in increased dopant populations in six-coordinate environments. These samples also exhibit the largest distributions of 67Zn Knight shifts, which is consistent with the highest carrier densities determined from fitting mid-IR localized surface plasmon resonance (LSPR) extinction spectra, inferring that the nature and populations of the dopants play important roles in their determination. This raises the tantalizing possibility of using such information to rationally design new types of NCs with tunable carrier densities and LSPR frequencies.