Structural Analysis of Planar sp3 and sp2 Films: Diamond-Like Carbon and Graphene Overlayers
KAUST DepartmentPhysical Sciences and Engineering (PSE) Division
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AbstractThe special electronic configuration of carbon enables the existence of wide ranging allotropes taking all possible dimensionalities. The allotropes of carbon are characterized by the type of hybridized bonding forming its structure, ranging from pure sp2 as in graphene, carbon nanotubes and fullerenes, to pure sp3 as in diamond. Amorphous and diamond-like carbon consists of a mixture of both hybridizations. This variation in hybridization in carbon materials enables a wide spectrum of properties, ranging from high bulk mechanical hardness, tribological properties and chemical inertness made possible by moving towards pure sp3 bonding to the extraordinary electrical conductivity, optical properties and in-plane mechanical strength resulting from pure sp2 bonding. Two allotropes at the extremes of this spectrum, diamond like carbon (DLC) and graphene, are investigated in this thesis; the former is investigated as a protective coating in hard drive applications, while the latter is investigated in the context of chemically derived graphene as material for transparent conducting electrode applications. DLC thin films are a main component in computer hard drives, acting as a protective coating against corrosion and mechanical wear of the magnetic layer and read-write head. The thickness of DLC films greatly affects the storage density in such devices, as larger separation between the read/write head and the magnetic layer decreases the storage density. A targeted DLC thickness of 2 nm would increase the storage density towards 1 Tbits/inch2. However, difficulty achieving continuous films at such thicknesses by commonly used sputtering methods challenges the industry to investigate alternative methods. Filtered cathodic vacuum arc (FCVA) has been proposed as an efficient technique to provide continuous, smooth and ultra-thin DLC films. We investigate the influence of deposition angle, deposition time, and substrate biasing to define the optimum process window to obtain smooth and sp3-rich DLC films on model Si substrates. Graphene has attracted worldwide attention since its recent discovery in 2004, due to its extraordinary properties. One of the most promising applications of graphene is its use as a transparent conducting electrode in photovoltaic and display applications. Unfortunately, large scale deposition of graphene is still a challenge and a limiting factor. Solution processing of graphene oxide, a type of chemically derived graphene that forms well dispersed single sheet solutions, is an easy method of depositing large scale graphene films. Subsequent reduction allows recovering part of the properties of graphene. However, residual defects and oxide groups remain in the sheet even after optimal reduction processes, making it difficult to achieve the extraordinary electrical and transport properties of graphene. We investigate the structural, chemical, and morphological implications of plasma oxidation of CVD graphene to better understand the nature of graphene oxide. One of the common threads of this study is the use of Raman spectroscopy (RS) X-ray photoelectron spectroscopy (XPS), and Atomic force microscopy (AFM) to understand the structural and compositional changes of both FCVA-deposited DLC films and plasma-oxidized graphene. In both cases, the analysis allows to obtain detailed insight into structural evolution of sp3 and sp2-rich films during plasma processing. Optimum FCVA deposited DLC films were obtained at a deposition angle of 70° for 30 sec deposition time on a biased substrate (-100 V), in terms of both surface roughness and sp3 content. RMS roughness of deposited films were around 0.15 nm with a ~50% sp3 content. Structural and chemical changes of oxygen plasma treated CVD graphene were shown to vary over three distinct stages depending on the time of plasma exposure. Initial exposure up to 3 seconds was mainly accompanied by structural changes without net uptake of oxygen, introducing more defects and smaller crystallite size into the graphene sheet. Extended exposure resulted in a nano-crystalline structure with a crystallite size of around 4 nm with a significant uptake of oxygen. Significant degree of disorder and loss of materials were eventually observed at exposure times larger than 10 s. A three stage model of graphene oxidation was suggested summing up the observed variations.