SECANS investigated transition metal nitride semiconductor thin films for solar-to-chemical energy conversion, with a focus on how composition, symmetry, and defect chemistry define optoelectronic and interfacial behavior. By combining nonequilibrium synthesis with advanced structural, electrical, and spectroscopic characterization, the project established systematic approaches for correlating microscopic chemical and structural modifications with macroscopic functional properties. In addition to providing important insights into several known transition metal nitride materials, this work also identified promising new compounds, including both crystalline and amorphous phases.
A recurring theme throughout the project was the role of impurity incorporation and local coordination environments in stabilizing crystal structures and tuning carrier concentrations. Initial investigations of nitrogen-rich tantalum nitride demonstrated that oxygen can stabilize metastable structures and strongly modify free carrier densities, providing wide tunability between semiconducting and degenerate regimes. Subsequent work extended these insights to additional materials and revealed that oxygen influences not only electronic properties but also operational stability under photoelectrochemical conditions. Moreover, controlled impurity doping was shown to enhance photocarrier lifetimes, reduce carrier localization processes, and improve photoelectrochemical performance.
Building on these foundations, later phases of the project expanded the materials space to include ternary and quaternary nitrides and oxynitrides. Notable outcomes included the identification of new ternary compounds such as TaZrN3, where cation ordering and site symmetry were shown to play key roles in defining the electronic structure and carrier transport characteristics. In parallel, SECANS researchers demonstrated that amorphous nitrides can retain high carrier mobility and photoactivity despite extreme structural disorder. This finding has the potential to open a new class of compounds that can be synthesized under comparatively mild conditions, thereby providing a route to sustainable thin-film devices for energy applications and beyond.
Interface engineering and methodological development formed a complementary aspect of the project. Atomic layer deposition processes were developed to create phase-tunable catalytic and protective overlayers, enabling control of semiconductor/electrolyte interfaces. In addition, advanced operando and time-resolved experimental techniques were developed and utilized to directly observe photocarrier dynamics, surface adsorbates, and light- and bias-induced transformations. Together, these approaches provided key mechanistic insights into photoelectrochemical energy-conversion mechanisms.
Overall, the project expanded the experimentally accessible range of transition-metal nitride semiconductors and clarified how composition, symmetry, and defect engineering can be used to control optoelectronic and interfacial properties. While these studies were primarily oriented toward solar-to-chemical energy conversion, the resulting materials advances also open new application spaces in electronic and photonic thin film technologies where defect tolerance and tunable carrier concentrations are critical. The outcomes of SECANS have been disseminated through 22 peer-reviewed publications and numerous international presentations, as well as public outreach events. Exploitation occurs through follow-on academic research projects, continued methodological development, and the transfer of trained researchers into both academic and high-technology industrial environments.
