Periodic Reporting for period 4 - SECANS (Solar-to-Chemical Energy Conversion with Advanced Nitride Semiconductors)
Periodo di rendicontazione: 2025-01-01 al 2025-12-31
As photovoltaic technologies gain prominence, an outstanding challenge remains the development of systems capable of robustly storing solar energy with high density. In this regard, the capture of sunlight and its direct conversion to chemical fuels in artificial photosystems represents a promising route for sustainably meeting future energy demands. However, a central difficulty is the lack of materials systems that can efficiently direct light-generated excitations toward desired chemical products while maintaining stability under harsh reaction conditions. Recent advances in the use of thin protective films have relaxed the requirement for intrinsically stable semiconductors and opened new perspectives for the rational design of materials and interfaces. Within this context, the SECANS project set out to establish the scientific basis for solar-to-chemical energy conversion devices enabled by the targeted exploration and optimization of an underexplored class of materials: transition-metal nitride semiconductors. The electronic structures of these compounds offer a potentially favorable balance between high charge-carrier mobility and tolerance to structural defects. At the same time, the expansive range of possible nitrides has historically remained largely unexplored due to the chemical stability of molecular nitrogen and the limited availability of suitable synthesis and characterization methods. SECANS addressed these challenges through an interdisciplinary approach combining non-equilibrium semiconductor deposition, interface engineering, and advanced operando and time-resolved spectroscopies. The project aimed to develop and understand novel nitride semiconductor materials optimized for solar-to-chemical energy conversion, to elucidate the roles of defects and disorder in photochemical reaction cycles, and to generate new insights into the mechanisms governing photochemical stability at semiconductor interfaces. These objectives were realized through the discovery and development of nitride and oxynitride materials, as well as the application of advanced experimental methods for probing the elementary steps of light-to-chemical energy conversion. Through this work, SECANS provided important new insights into how composition, symmetry, and defects can be tailored to control optoelectronic properties and photoelectrochemical characteristics. In this way, SECANS expanded the accessible range of transition metal nitride semiconductors, laying a foundation for future advances in not only solar-driven chemical energy conversion but also related semiconductor technologies.
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.
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.
SECANS advanced the state of the art in semiconductor materials for solar-to-chemical energy conversion by expanding the experimentally accessible range of nitride and oxynitride semiconductors and by elucidating how composition, cation ordering, local site symmetry, and controlled oxygen incorporation govern their electronic and photoelectrochemical behavior. The project provided the first detailed electronic-structure analyses of previously unexplored nitride systems, demonstrated that amorphous phases can retain high carrier mobilities despite structural disorder, and resolved long-standing ambiguities regarding the role of oxygen in defining both defect equilibria and operational stability. In parallel, the establishment of operando and time-resolved spectroscopic methods enabled observation of interfacial and structural dynamics under illumination, linking microscopic chemical processes with macroscopic functional characteristics. These results provide key insights that can guide rational defect and interface engineering in nitride thin films for not only energy conversion but also related optical and electronic applications.