Modifiye kimyasal buhar birikimi ile sentezlenen azot doplu grafen üzerinde araştırmalar

Graphene, fundamental building block of carbon based materials, has gained more attention from both scientific and technological communities not only due to its unique properties such as quantum hall effect at room temperature, tunable band gap, high elasticity, and an ambipolar electric field effect but also due to its electrochemical functionalities. Within a plane of graphene, which consists of honeycomb lattice of sp2 hybridized carbon atoms, there are three in plane σ bonds and one π orbital for each of the atoms. This bonding nature difference provide spectacular properties anisotropy for graphene. Starting from the early years of last decade that science community's focus was brought to bear on graphene, various synthesis techniques have been developed. Among those, mechanical exfoliation and epitaxial growth have gained more attention. However, low crystallinity of the former one and lack of scalability of the latter, are the main drawbacks of these methods. Chemical Vapor Deposition (CVD) of various carbon precursors on different substrates opened a new era in reaching to a desired low cost, high quality and large scale graphene. Beside feasibility of pristine graphene to be used in different applications, it is possible to modify its properties with different chemical methods and to introduce new features to it. One of the main strategies in graphene modification is chemical doping. Addition of a foreign element into its bonding network can turn graphene into electrochemically active material. Nitrogen doped graphene is a new type of metal free electro-catalyst which offers great potential in terms of activity, durability and selectivity. Several methods have been proposed to synthesize N doped graphene. These methods, which generally are classified to direct synthesis and post treatment, include different types of nitrogen precursors such as NH3, N2, N2H4, etc. as well as various synthesis techniques like CVD, segregation, plasma treatment, etc. The main features which determine the electrochemical activity of N doped graphene are nitrogen concentration and its configuration in graphene lattice. It is well-known that nitrogen has three possible configuration (pyridinic, pyrrolic and graphitic) in graphene with different electrochemical properties. In this study, multi-layer graphene films in centimeter scale were synthesized by CVD method on multi-crystalline Cu foils. Methane was used as the precursor of carbon. The impacts of different parameters such as total pressure, growth time, growth temperature, methane flow rate and heating rate were investigated in a great detail by the application of Raman spectroscopy. Graphene has some obvious fingerprints in its Raman spectrum that make it possible to determine both the quality of the graphene film (from defect density point of view) and also the number of layers. The results showed that copper substrate temperature and hydrogen exposure time are two important control parameters in order to have a high crystalline graphene. Moreover, the total pressure of the system was found to be critical. The presence of hydrogen at elevated temperatures (900 °C and higher) makes the Cu surface smooth, eliminate the contaminations and enlarge grain size. CVD grown graphene was doped with nitrogen atoms by using N2 plasma. The results obtained from Raman spectroscopy and microscopy, X-ray photoelectron spectroscopy (XPS) and also atomic force microscopy (AFM) were cross-examined to investigate homogeneity of nitrogen doping distribution as well as its atomic configuration. It was proved that more than 35% nitrogen was doped into the graphene structure including all three types of C-N bonding. Further investigations indicated that pyridinic and pyrrolic sites are the most favorable configurations and nitrogen atoms tend to occupy these sites at low nitrogen coverage. Heat treatment of these samples did not result in a configurational change. However, in N doped graphene samples with high concentration of nitrogen, where all three types of nitrogen were present, heating up to mild temperatures caused graphitic nitrogen to get converted to pyridinic and pyrrolic nitrogen, which are presumably more electrochemically active. The sp2 honeycomb structure of graphene was not damaged during heat treatment.