Synthesis of Nanocrystalline Hexagonal Diamond Films in Organic Solvents by Femtosecond Laser Irradiation
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Synthesis of Nanocrystalline Hexagonal Diamond Films in Organic Solvents by Femtosecond Laser Irradiation Anming Hu1, and Walt W. Duley2 1 Physics, University of Waterloo, Waterloo, ON, N2L 3G1, Canada 2 Physics, University of Waterloo, 200 Univ. Ave. West, Waterloo, ON, N2L 3G1, Canada ABSTRACT Nanocrystalline diamond films have been synthesized on various substrates by 800 nm fs laser irradiation of a variety of organic solvents including acetone, pentane, hexane, heptane, octane, dodecane, and hexadecane in the presence of a transition metal catalyst. 632 nm Raman spectra display a strong vibration mode at 1308 cm-1 characteristic of hexagonal diamond. X-ray photoelectron spectra confirm that the film is mainly sp3-bonded carbon containing a low concentration of sp2-bonded inclusions. Film microstructure shows that these films are assembled from nanoparticles having an average size of < 100 nm. Analysis of the liquid after irradiation using HPLC techniques indicates that polyyne molecules are also synthesized during irradiation. It is possible that these species are formed as the products of ion chemistry following Coulomb explosion. This process may enable a new method for the creation of nanocrystalline hexagonal diamond layers for micro-electronic and other applications. INTRODUCTION Nanocrystalline diamond films possess very high thermal conductivity, enhanced transparency and good chemical stability. They also exhibit a low threshold for electron emission and mechanical properties (smooth surface, high hardness and a low coefficient of friction) similar to that of crystalline diamond. These materials are then useful as substrates and in applications requiring hard coatings and high IR transmission. It is well known that diamond films can be fabricated by microwave plasma- or hot filament assisted-chemical vapor deposition often involving high temperatures (600-1200oC) in the presence of reactive gases [1-4]. These harsh deposition environments are not readily compatible with metal and III-V semiconductor devices. Pulsed laser ablation using ns excimer laser radiation is an alternative approach in the synthesis of diamond-like films at room temperature, however, these films consist of a sp3hybridized bonded tetrahedral carbon matrix with embedded graphitic clusters, with a sp3 concentrations ranging from 60 to 80% [1, 2, 5]. It is important to find alternative methods of growing diamond films having high sp3 content at room temperature. Hexagonal diamond (lonsdaleite) is the hexagonal phase of the sp3-bonded diamond structure. It is energetically unfavored and is therefore observed much less frequently than the normal cubic phase. It has been found in the crystalline boundary of CVD diamond films [6], where it accommodates large anisotropic stresses. Hexagonal diamond can be synthesized by static compression of crystalline graphite at ≈ 13 GPa and temperatures > 1000oC [7, 8]. Recently, Misra et al. [9] successfully deposited hexagonal diamond films on strained gallium nitride-coated quartz subst
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