A hybrid density functional theory investigation of silicon and germanium at the (100) diamond surface: routes to stable negative electron affinity for electron-emission applications

• Adsorption energies and electron affinities for Si and Ge on diamond were calculated using DFT. • All configurations except one possess a negative electron affinity (NEA). • Adsorbate-to-diamond charge transfer quantified using both Mulliken and Bader analyses. • A planar dipole model is used to correlate the magnitude of the surface dipole with the observed NEA. Termination of the diamond (100) surface with silicon and germanium is modelled using hybrid density functional theory (DFT) for coverages up to 1 monolayer (ML). Adsorption energies and electron affinities were calculated using the B3LYP exchange–correlation functional. Calculations predict that all configurations except one possess a negative electron affinity (NEA). In general, Si adsorption results in slightly higher exothermic adsorption energies, but Ge adsorption produces surfaces with larger NEA values. Additionally, we quantify the charge transfer from the adsorbates to the diamond surface using both Mulliken and Bader’s “atoms-in-molecules” charge partitioning (QTAIM) analyses. We suggest that the suppressed NEA in the case of Si may be the result of the more negative atomic basin dipole moments for Si.