Achieving a Large Net “Negative Electron Affinity” on Diamond (100) via Molecular Oxygen and Lithium Functionalization

Toward the realization of thermally and ambient-stable diamond surfaces with negative electron affinity (NEA), advances in surface engineering are critical for high-performance electron-emission devices, including thermionic and field emitters, and next-generation energy converters. Here, we develop and systematically investigate a novel "molecular oxygen" oxidation method for (100)-oriented single-crystal diamond, comparing it with the benchmark UV-ozone treatment. Using the state-of-the-art surface analysis techniques, we quantify surface oxygen coverage and characterize the electronic structure following lithium deposition. The molecular oxygen treatment achieves ∼90% surface coverage and produces an NEA of -1.68 eV, outperforming UV-ozone oxidation (-1.31 eV). Although air stability is slightly limited, the NEA is fully recoverable upon reactivation (-1.56 eV). This study demonstrates that the new oxygen termination provides a practical, high-performance route to optimized NEA diamond surfaces, offering a scalable platform for next-generation electronic and energy applications.