High Temperature Oxidation And Wetting Properties Of CVD Diamond Films

As a functional material with many excellent properties, diamond has beenwidely applied in many fields. The fabrication and properties of diamond micro-ornano-structures have attracted great attention. By using two approaches of “bottomup” and “top down”, kinds of diamond structures have been synthesized. The opticaland electrical properties of the structures were also studied. Recently, the wettabilityproperty of diamond and the related application were widely investigated. It wasfound that the wettability property was strongly affected by crystal orientation,surface modification and surface micro-or nano-structures. Therefore, it is a verynew issue to find a way to fabricate a new structure of diamond which had acontrollable wettability property.In this thesis, free-standing CVD diamond films were fabricated by MPCVDmethod. After high temperature thermal treated in air, diamond films with specialsurface morphology were obtained. The wettability properties of the diamond filmswere studied. The main content and innovation of this thesis are listed as following:1. Using a MPCVD system, free-standing (100) oriented diamond films werefabricated by introducing little nitrogen. The films were consisted of (100) diamondmicro-columns and grain boundaries with nanodiamond and nondiamond phase.After a thermal treatment in air under a high temperature of600-800oC, the grainboundaries were oxidized to CO and CO2. Because of the stability of diamond (100)planes, the growth side of the films contains (100)-oriented micrometer-sizedcolumns, while its nucleation side is formed of nano-sized tips. It is believed that thedifferent growth processes for the different side and the instability of grain boundaries under thermal treatment lead to the special structure. After a long time (40min)treatment in air, the film was fully broken, only a large amount of separated singlecrystal diamond columns and spindle-shaped needles remained, which furtherdemonstrates the column growth mechanism of (100)-oriented diamond film.2. The wettability properties of the thermally treated diamond films wereexamined. It is suggested that the growth surface (contact angle (CA),87o) andnucleation surface (70o) of the as-treated diamond films are hydrophilicity,respectively, which is enhanced by oxygen thermal treatment (CA,7880o). Whilethe nucleation surface present superhydrophilicity (CA,018o). Because of aftertreated in hydrogen plasma, the wettability of nucleation side converted fromsuperhydrophilicity to high hydrophobicity (CA,97109o), while the hydrophilicityof the growth side does not change significantly (CA,8287o). It is because that thesurface is of hydrogen termination after the plasma treatment. Since the largedifference of surface morphology between growth side and nucleation side, thechange of the wettability properties is different. The findings open a possibility forrealizing freestanding diamond films having not only novel surface structures but alsomultifunction applications, especially propose on the selected growth side ornucleation side in one product. The observations are useful for extending the industryapplications of diamond in numerous fields.3. Without the addition of N2, free standing diamond films without (100) orientedsurface were deposited. The films were mainly consisting of (111) planes with theother planes without visible nanodiamond or nondiamond phase. After treated with800oC, from XRD and SEM, it is observed that the (100) grains were remainedbecause of its thermal stability, while grains with other faces showing a porousstructure. The as-deposited diamond films surface show a hydrophilic characteristic,the CAs of growth surface and nucleation surface are contact angle52oand65o. Afterthe thermal treatment (800oC,10min), they became superhydrophilic (CAs:~0oand27o). This result is different from that of the (100) oriented films, indicating that thegrain orientation, surface modification and micro-nano structure dominant thewettability property of the film.In this thesis, we designed and fabricated free standing diamond films with different surface structures and wettability properties on the growth side andnucleation side and discussed the effects of the thermal treatment and hydrogenplasma etching on the variation of the film. The observations are useful for furtherunderstanding the growth mechanism, thermal oxidization, and wettability propertyof CVD diamonds to extend their industry applications in numerous fields.