Investigation On Growth Of Graphene Nanostructures On Transition Metal Surfaces

Graphene and graphene-based nanostructures possess extraordinary electronic properties, they are also potential candidates whithin thermal, magnatic applications.Precision during synthesis shown crucial importance for the practical use of graphene-based materials,compared to the tradition way like unzipping carbon nanotubes (CNTs), we use aromatic molecules as precursors for bottom-up fabrication of graphene. Scanning tunneling microscopy (STM) was used for direct observation of growth process of graphene, and density functioinal theory (DFT) based calculation was used to further explore the influence of graphene-substrate and intermolecular interaction on growth of graphene-based nanostructures.First we studied he bottom-up preparation of graphene nanostructures on the Ru(10 10) substrate using DBBA as precursor. STM experiment demonstrates that practically all the N=7 GNRs aligned along the [1210] azimuth of the Ru(1010) substrate. Upon annealing the sample at higher temperature, lateral attachments of GNRs occurred, and ribbon-like graphene nanoflakes with varies of orientations and armchair and zigzag boundaries were observed. The dominating Moire patterns are formed by graphene overlayer on Ru (1010) with rotation angles of 0° and 30° respect to the [1210] direction of the substrate. DFT calculation demonstrated the adsorption energy versus rotation angle showed a maximum at rotation angle of 30°, and a minumun at rotation angle of about 21°. The projected DOS of graphene and Ru showed significant overlaps between pDOS of graphene and the Ru substrate for graphene overlayer with 30° orientation. Formation of GNRs aligned along the [1210] direction of the Ru (1010) substrate and the domination of graphene overlayers with rotation angles of 0° and 30° suggest that the interaction between GNR/graphene and the substrate control the orientation of graphene overlayer and the growth of graphene on Ru (1010).Then,we carried out investigations on the self-assembly of epitaxial graphene on the Ru(0001) substrate using 1,3,5-triphenylbenzene as precursor. DFT calculations showed an increase in adsorption energy from monomer to dimer adsorption, which is well consistent with the accumulation and close-packing behavior of the precursors observed in STM measurements. Further NEB theory-based calculations showed that the intermolecular vdW interaction has crucial influence on the graphitization process of 1,3,5-triphenylbenzene, and the increment in the dehydrogenation barrier from 1.27 for monomer adsorption to 1.62eV for dimer adsorption has a well agreement with the observed drastic reduction in the graphitization temperature at lower precursor coverage,we also investigated the graphene nanoclusters grown on the Ru(0001) substrate by controlling the coverage of 1,3,5-triphenylbenzene precursor and annealing temperature. Graphene nanoclusters (GNCs) with different geometries can be easily identified during STM measurement, further DFT-based calculations were employed to explore the plausible geometries of two GNCs, and 7C6 was found to be the more reasonable one compares to the 21-3C and agreed much better with the morphology during STM observation, DFT simulation was also carried out for the two meta-stable GNCs,which help us gain preliminary understanding of 3C6, 5C6,7C6,9C6 GNCs, and initial growth and nucleation process of graphene.