Wideband gap diamond-based materials are attracting huge interest for energy harvesting and quantum applications. We have used density functional theory code (DFT) to explore tin (Sn) as a potential negative electron affinity (NEA) imparting termination on the oxygen terminated diamond (OTD) surface. Large adsorption energies (~−6 eV) were obtained for half monolayer of Sn atoms on oxygen (O) terminated diamond surface with an NEA of up to −1.37 eV. We are also demonstrating experimentally, the formation of SnO nano cluster layer on the surface of diamond which resulted in NEA and reduction in the work function (WF) by 1.8 eV. The SnO termination, clearly distinguishable from more stable and widely known SnO2, was found to be stable in ambient conditions, a crucial point for device application. A comparison of lithium (Li) and Li/Sn oxide terminations of single crystal diamond (1 0 0) is presented in terms of the stability of the surface layer and the electronic properties thus induced, using the photoemission spectroscopic techniques. The intercalation of Li into SnO planes results in the increased stability of LiO on the surface of diamond and WF reduction by 2.3 eV which paves way for a more efficient termination on the diamond surface.