Mechanistic Origins of Atomic-Scale Control in Plasma-Thermal Atomic-Layer Etching of Nickel

Abstract We present a mechanistic density functional theory (DFT) investigation of atomic-layer etching of nickel previously modified by surface nitride formation. Etching is performed via gas-phase formic acid, and we focus on the influence of nitrogen coverage on reaction energetics and mechanisms. Using two models for the etching process, removing a partial or full surface Ni atomic layer, we examine formic acid adsorption and reaction by dehydrogenation and hydrogen transfer to nitrogen. We show that the mechanism for Ni etching initiates with the formation of metastable nickel-formate surface complexes, partially decoordinated from other Ni on the surface, and their desorption with accompanying ammonia release. Product stability calculations reveal that polynuclear nickel-formate-ammonia complexes are thermodynamically favored over mononuclear species, with meta-generalized gradient approximation (GGA) functionals (SCAN, R2SCAN, R2SCAN-rVV10) providing consistently lower (more favorable) reaction energies than Perdew–Burke–Ernzerhof (PBE) and better in line with experimental observations. Mechanistic pathways on the stepped Ni(843) surface demonstrate a strong dependence on nitrogen coverage: low coverage preserves surface crystallinity but limits the thermodynamic driving force for product desorption, whereas high coverage populates terrace and subsurface sites, weakens Ni–Ni bonds, and facilitates surface rearrangements, forming nickel-formate surface complexes that render early desorption steps accessible. These findings underscore the critical role of nitrogen content in enabling controlled and efficient etching and provide atomistic insight into surface complex formation, hydrogen stabilization, and nickel removal. This work establishes a computational framework to optimize atomic-layer etching of nickel nitride surfaces.