Novel ultrananocrystalline diamond (UNCD) films under investigation at Argonne National Laboratory exhibit a phase-pure diamond microstructure with 2-5 nm equiaxed grains, atomically abrupt grain boundaries and extremely smooth surfaces (RMS roughness ~30-40 nm) due to a unique growth process based on the insertion of C2 dimers (the dominant growth species in the microwave CH2/Ar or C60/Ar plasmas used for film growth) in the growing film lattice\. TEM studies have shown that the grain boundaries of UNCD films are atomically abrupt and devoid of non-diamond secondary phases, although UV Raman spectroscopy shows the presence of a few percent of sp2 bonded carbon atoms, which according to theoretical calculations are to be found at the grain boundaries of this material.
Cu-Li alloy coatings developed in our group provide a chemically and thermally stable, self-replenishing, segregated lithium monolayer on the surface of the host Cu layer, resulting in a substantial reduction of the work function of the alloy, compared to that of a thick Li layer or a pure Cu surface. Both UNCD and Cu-Li alloy films exhibit unique electron field emission properties controlled by different mechanisms.
UNCD films exhibit relatively high/stable electron emission currents when exposed to electric fields. Quantum photoyield measurements of the UNCD films revealed that these films have an enhanced density of states within the bulk diamond band gap that is correlated with a reduction in the threshold field for electron emission. In addition, scanning tunneling microscopy studies indicate that the emission sites from UNCD films are related to minima or inflection points in the surface topography, and not to surface asperities. These data in conjunction with tight binding pseudopotential calculations indicate that grain boundaries play a critical role in the electron emission properties of UNCD films, such that these boundaries: (a) provide a conducting path from the substrate to the diamond-vacuum interface, (b) produce a geometric enhancement in the local electric field via internal structures, rather than surface topography, and (c) produce an enhancement in the local density of states within the bulk diamond band gap. Cu-Li alloy films show a thirteen-fold reduction in the threshold voltage for electron emission, compared with uncoated Si surfaces. The segregated Li monolayer on the surface of the alloy leads to charge transfer between the metal conduction band and the Li adsorbate atom, giving rise to an effective work function even lower than that of the elemental alkali metal that results in relatively high/stable field-induced electron emission.
The work performed by our group to understand the mechanism responsible for field-induced electron emission from UNCD and Cu-Li alloy films is providing valuable insights into the composition and/or microstructure-property relationships related to this phenomenon in both materials. The fundamental knowledge acquired in this work is providing the basis for applied research directed at developing new multifunctional devices based on the field emission properties of the materials described in this FWP. Both UNCD and Cu-Li alloy films may play critical roles in the development of a whole new generation of devices such as field emission flat panel displays, high frequency travel wave tubes, field emission-based electron microscopes, x-ray tubes, and particle accelerators among many other.
J.A. Carlisle, D.M. Gruen, M.Q. Ding (visitor), O. Auciello (collaborator), R.A. Nemanich (North Carolina State University), V.P. Dravid (Northwestern University)