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Thin diamond foils are needed in many particle accelerator experiments regarding nuclear and atomic physics, as well as in some interdisciplinary research. Particularly, nanodiamond texture is attractive for this purpose as it possesses a unique combination of diamond properties such as high thermal conductivity, mechanical strength and high radiation hardness; therefore, it is a potential material for energetic ion beam stripper foils. At the ORNL Spallation Neutron Source (SNS), the installed set of foils must be able to survive a nominal five-month operation period, without the need for unscheduled costly shutdowns and repairs. Thus, a single nanodiamond foil about the size of a postage stamp is critical to the entire operation of SNS and similar sources in U.S. laboratories and around the world. We are investigating nanocrystalline, polycrystalline and their admixture films fabricated using a hot filament chemical vapor deposition (HFCVD) system for H- stripping to support the SNS at Oak Ridge National Laboratory. Here we discuss optimization of process variables such as substrate temperature, process gas ratio of H2/Ar/CH4, substrate to filament distance, filament temperature, carburization conditions, and filament geometry to achieve high purity diamond foils on patterned silicon substrates with manageable intrinsic and thermal stresses so that they can be released as free standing foils without curling. An in situ laser reflectance interferometry tool (LRI) is used for monitoring the growth characteristics of the diamond thin film materials. The optimization process has yielded free standing foils with no pinholes. The sp3/sp2 bonds are controlled to optimize electrical resistivity to reduce the possibility of surface charging of the foils. The integrated LRI and HFCVD process provides real time information on the growth of films and can quickly illustrate growth features and control over film thickness. The results are discussed in the light of development of nanodiamond foils that will be able to withstand a few MW proton beam and hopefully will be able to be used after possible future upgrades to the SNS to greater than a 3MW beam.
Carbon is a favorable alternative as counter electrode material for dye sensitized solar cells (DSSC) as compared to Pt. Various carbon materials such as carbon nanotubes (CNT), activated carbon (AC) and carbon nanofibers have been investigated as counter electrodes for DSSC applications, based on their high electrochemical activity, high specific surface area, chemical inertness and high electrical conductivity. Among various phases of carbon, diamond is the most robust and chemical inert material that can be used for electrode application. It has band gap of 5.5 eV, high thermal conductivity. its electrical resistivity can be tuned by doping such as boron. In this work, we investigate boron doped diamond thin film electrode for DSSCs. The conductive diamond thin electrode films were grown using Blue Wave hot wire chemical vapor deposition (HWCVD) system. The electrical resistance in diamond thin films was tuned by controlling grow temperature, filament power, dopant concentration and sp3/sp2 ratio in the film, it thickness, and initial seeding process. Scanning electron microscopy, Raman spectroscopy and electrical resistivity measurement were used to characterize morphology, diamond quality and electrode conductivity, respectively. Diamond film electrodes with optimized surface morphology and electrical characteristics were used for DSSC fabrication. We used nanocrystalline TiO2 paste (P25 Degussa) with average particle size of 25nm as an active layer, the electrolyte comprised of a LiI/I2 electrolyte in acetonitrile (CH3CN), a Ru based metal complex dye [cis-diisothiocyanato-bis(2,2’-bipyridyl-4,4’-dicarboxylato) ruthenium(II) bis(tetrabutylammonium)] OR N719 was used as sensitizer. The photovoltaic performance was determined using J-V characteristics under standard illumination conditions and was compared to a reference DSSC with Pt counter electrode. Results are discussed in the context of diamond electrical and durability and chemical stability of diamond films against most commonly used family of iodine based electrolytes.
A compact and efficient hot filament chemical vapor deposition system has been designed for growing electronic-grade diamond and related materials. We report here the effect of substrate rotation on quality and uniformity of HFCVD diamond films on 2” wafers, using two to three filaments with power ranging from 500 to 600 Watt. Diamond films have been characterized using x-ray diffraction, Raman Spectroscopy, scanning electron microscopy and atomic force microscopy. Our results indicate that substrate rotation not only yields uniform films across the wafer, but crystallites grow larger than without sample rotation. Well-faceted microcrystals are observed for wafers rotated at 10 rpm. We also find that the Raman spectrum taken from various locations indicate no compositional variation in the diamond film and no significant Raman shift associated with intrinsic stresses. Results are discussed in the context of growth uniformity of diamond film to improve deposition efficiency for wafer-based electronic applications.
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