The modification of the diamond surface through adsorbants offers the opportunity to adjust the electronic and electron emission properties of the surface. In the study presented here, we deposited between 0.1 and 100 monolayers of carbon from an electron beam evaporation source on polycrystalline diamond films. Photoelectron pectroscopy in the ultraviolet and X-ray regime was employed to characterize the surface. Observations on a (100) polycrystalline diamond film show, that the surface is first depleted of hydrogen and subsequent growth of an amorphous carbon film (a-C) occurs on the reconstructed surface. The deposition of these ultrathin carbon films allows the controlled introduction of sp~2 carbon and p- pi states onto the diamond surface. The field emission current increases considerably with the amount of sp~2-carbon accumulated at the diamond surface. The current-voltage characteristics only partially follow the Fowler-Nordheim equation, and the results obtained for different films are described and possible emission mechanism discussed.
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