Highlights

Diamond Nanowires and the Mechanism of Electrical Conductivity in Ultrananocrystalline Diamond (UNCD) Films

Scientific Achievement

Ultrananocrystalline diamond (UNCD) films consist of randomly oriented 3-5nm crystallites surrounded by atomically abrupt 0.2nm wide, largely sp² bonded grain boundaries.  Such films, typically synthesized from hydrogen-poor, 1% CH₄-99% Ar microwave plasmas, are highly electrically insulating but are rendered highly conducting (up to several hundred S/cm) by substituting up to 20% of N₂ for Ar in the synthesis gas.  In recent work, the origin of the n-type metal-insulator transition in UNCD films has been shown to be strongly correlated to the formation of partially oriented diamond nanowires when nitrogen is added to the synthesis gas.  The nanowires, which have been characterized by SEM, HRTEM, EELS and Raman studies, are 80-100nm in length and consist of 5nm wide 6-10nm long segments of diamond crystallites exhibiting atomically sharp interfaces separated by twin boundaries.  Each nanowire is enveloped in a 1-2nm thick sheath of sp² bonded carbon that provides the conductive path for electrons.

Significance

These films are the only currently available source of n-type diamond material conducting at ambient temperatures.  These studies provide a better understanding of the mechanism underlying the insulator-metal transition of these films. In fact, we concluded that this transition is strongly correlated with the formation of these diamond NWs.  These NWs are enveloped by an amorphous carbon layer that seems to provide the conductive path for electrons.  Thus these works provide further insight into the interesting properties of these films.

This work has been presented in invited and contributed talks at the presented in invited and contributed talks at the NanoSMat-07 International Conference on Surfaces, Coatings and Nanostructured Materials, Algarve, Portugal, July 9-11, 2007, and at the MRS Fall Meeting, Boston, MA, Dec. 1-5, 2008.  This work has been published in Phys. Rev. B 75, 195431 (2007).  It has been submitted to Applied Physics Letters.

Performers

P. Bruno, D. M. Gruen, (Argonne-MSD); R. Arenal (Laboratoire d'Etude des Microstructures, ONERA-CNRS)