Physics
Scientific paper
Mar 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004aps..mars15001r&link_type=abstract
American Physical Society, March Meeting 2004, March 22-26, 2004, Palais des Congres de Montreal, Montreal, Quebec, Canada, MEET
Physics
Scientific paper
In the recent years, diamonds with diameter distributions peaked around 3 nanometers have been evidenced in an intriguing variety of environments : on meteorites, comets and in detonation residues, but also in diamond films produced under specific (low) hydrogen partial pressures in CVD experiments and even in petroleum (diamondoids). If diamond particles get more stable than their graphitic counterparts in the nanometer range, the competition between those two structures and the effect of size reduction make the properties of nanoscale diamond unique. We address the properties of nanoscale diamond using first-principles molecular dynamics simulation. Structural models are obtained for sizes up to 3 nanometers and their optical properties, computed using Time-Dependent Density Functional Theory, are compared to X-ray absorption and emission spectra. We conclude that, contrary to silicon and germanium, there is no more a quantum confinement effect on the optical gap above sizes of 2-3 nanometers [1]. Also for these sizes, specific surface reconstructions that have signatures in the aborption spectra occur : the particle's diamond core gets surrounded by fullerene-like caps. We studied surface reconstructions as a function of hydrogen chemical potential. These model simulations indicate that for sizes of about 2 nanometers, surface hydrogen tends to be released, favoring fullerene-like surface reconstructions. This provides an explanation for the similar size distributions of nanodiamonds produced under extremely different temperature/pressure conditions [2]. We also show that below some hydrogen potential value threshold, the CVD process of diamond film would lead to an agglomeration of nanodiamonds (called UltraNanocrystallineDiamond) rather than the growth of bulk diamond, as observed experimentally. Finally we discuss the possibility of N-doping nanodiamonds and show that the nitrogen incorporation is very much dependent on the particle size and affected by the surface reconstructions. This work has been done in collaboration with Dr. Giulia Galli under the auspices of the U.S. DOE at the University of California/LLNL under contract No W-7405-Eng-48 and was supported by the FNRS. [1] Physical Review Letters 90 (2003) 037401-1 [2] Nature Materials 2 (2003) 792
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