Diamond nanowires and the insulator-metal transition in ultrananocrystalline diamond films

Abstract

Further progress in the development of the remarkable electrochemical, electron field emission, high-temperature diode, and optical properties of $n$-type ultrananocrystalline diamond films requires a better understanding of electron transport in this material. Of particular interest is the origin of the transition to the metallic regime observed when about 10% by volume of nitrogen has been added to the synthesis gas. Here, we present data showing that the transition to the metallic state is due to the formation of partially oriented diamond nanowires surrounded by an $s{p}^{2}$-bonded carbon sheath. These have been characterized by scanning electron microscopy, transmission electron microscopy techniques (high-resolution mode, selected area electron diffraction, and electron-energy-loss spectroscopy), Raman spectroscopy, and small-angle neutron scattering. The nanowires are $80--100\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in length and consist of $\ensuremath{\sim}5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ wide and $6--10\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ long segments of diamond crystallites exhibiting atomically sharp interfaces. Each nanowire is enveloped in a sheath of $s{p}^{2}$-bonded carbon that provides the conductive path for electrons. Raman spectroscopy on the films coupled with a consideration of plasma chemical and physical processes reveals that the sheath is likely composed of a nanocarbon material resembling in some respects a polymer-like mixture of polyacetylene and polynitrile. The complex interactions governing the simultaneous growth of the diamond core and the $s{p}^{2}$ sheath responsible for electrical conductivity are discussed as are attempts at a better theoretical understanding of the transport mechanism.