Insights into Na2O Photocathodes: First-Principles Investigation of Electronic, Vibrational, Optical, Transport, and Surface Properties

With potential air stability and competitive photoemissive response, alkali-metal oxides (X₂O, X = Na, K, Rb) are emerging photocathode candidates, although key properties influencing their photoemission, electronic, optical, thermal, transport, and surface remain largely unexplored. This study offers an in-depth assessment of Na₂O’s properties that govern its photoemission. The material exhibits a wide bandgap of 3.52 eV and, in the visible region, a significantly lower optical absorption coefficient than alkali-metal-antimonide photocathodes. The low room-temperature lattice thermal conductivity of 8.25 Wm^–1 K^–1 is attributed to strong phonon–phonon scattering in Na₂O. Additionally, Na₂O exhibits room-temperature electron and hole mobility of 35.43 cm²/(V s) and 0.35 cm²/(V s), respectively, limited primarily by strong polar optical phonon (POP) scattering. The most stable Na-terminated (111) surface shows an ionization potential of 3.70 eV, intermediate with the work functions of conventional metallic and semiconducting photocathodes.