Spatial effect of C-H dipoles on the electron affinity of diamond (100)-2x1 adsorbed with organic molecules

Cycloaddition of allyl organics on the dimer rows of a clean C(100)-2x1 diamond surface can be used for the controlled functionalization of such a surface. Sticking probability measurements confirm that appreciable uptake of acetylene and butadiene occur on the clean diamond surface at room temperature. The change in electron affinity of the surface as a function of the coverage of the organic molecules is investigated with periodic DFT calculations. The presence of C-H dipoles on these adsorbates modify the surface charge density and gives rise to an induced dipolar layer that modifies the electrostatic potential outside the surface. There is a significant reduction of up to 2.5 eV in electron affinity following the chemisorption of ethylene. Therefore, the adsorbed molecules play the same role as surface hydrogen in inducing the NEA condition on the clean diamond. The change in electron affinity does not scale linearly with the coverage of the organic molecules, because the spatial profile of the C-H dipoles as well as the orientation of the molecules on the surface have a predominant effect on the surface charge density.