Effect of work function and surface microstructure on field emission of tetrahedral amorphous carbon

The work function of tetrahedral amorphous carbon (ta-C) has been measured by Kelvin probe to lie in the range 4-5 eV, irrespective of its sp(3) conte nt or nitrogen addition. This implies that the surface barrier to emission is dominant and that emission changes caused by sp(3) bonding or nitrogen a ddition are not directly due to changes in work function. Hydrogen, oxygen, and argon plasma treatments are all found to increase the emission of a-C, but hydrogen and argon treatments are found to reduce the work function wh ile oxygen treatment increases it. Detailed studies of the surface with var ying plasma treatment conditions suggest that the changes in emission arise mainly from changes in the surface microstructure, such as the formation o f sp(2) regions within the sp(3) bulk. The need for local field enhancement mechanisms to account for emission over the sizeable barrier is emphasized , which may arise from local chemical nonhomogeneity, or formation of nanom eter-size sp(2) clusters embedded in an sp(3) matrix. (C) 2000 American Ins titute of Physics. [S0021-8979(00)01622-4].