Graphene, one-atom-thick planar sheet of carbon atoms densely packed in a honeycomb crystal lattice, has grabbed appreciable attention due to its exceptional electronic, mechanical and optical properties. Chemical functionalization schemes are needed to integrate graphene with the different materials required for potential applications. Molecular self-assembly behavior on graphene is a key method to investigate the mechanism of interaction between molecules and graphene and the promising applications related to molecular devices. In this thesis, we report the molecular self-assembly behavior of phenyl-C61-butyric acid methyl ester (PCBM), C60, perylenetetracarboxylic dianhydride (PTCDA) and Gd3N@C80 on flat and rippled graphene 2D material by the experimental methods of scanning tunneling microscope (STM) and atomic force microscope (AFM) and by the theoretical method of density functional theory (DFT). We found that molecules form ordered structures on flat graphene, while they form disordered structure on rippled graphene. For example, PCBM forms bilayer and monolayer structures, C60 and Gd3N@C80 form hexagonal close packed (hcp) structure on flat graphene and PTCDA forms herringbone structure on flat graphene surface. Although C60 and Gd3N@C80 both form hcp structure, C60 forms a highly ordered hcp structure over large areas with little defects and Gd3N@C80 forms hcp structure only over small areas with many defects. These differences of structure that forms on flat graphene is mainly due to the molecule-molecule interactions and the shape of the molecules. We find that the spherical C60 molecules form a quasi-hexagonal close packed (hcp) structure, while the planar PTCDA molecules form a disordered herringbone structure. From DFT calculations, we found that molecules are more effected by the morphology of rippled graphene than the molecule-molecule interaction, while the molecule-molecule interaction plays a main role during the formation process on flat graphene. The results of this study clearly illustrate significant differences in C60 and PTCDA molecular packing on rippled graphene surfaces. / Doctor of Philosophy / As the first physical isolated two-dimensional (2D) material, graphene has attracted exceptional scientific attention. Due to its impressive properties including high carrier density, flexibility and transparency, graphene has numerous potential applications, such as solar cell, sensors and electronics. 2D molecular self-assembly is an area that focuses on organization and interaction between self-assembly behaviors of molecules on surface. Graphene is an excellent substrate for the study of molecular self-assembly behavior, and study of molecular study is very important for graphene due to potential applications of molecules on graphene. In this thesis, we present investigations of the molecular self-assembly of PCBM, C60, PTCDA and Gd3N@C80 on graphene substrate.
First, we report the two types of bilayer PCBM configuration on HOPG with a step height of 1.68 nm and 1.23 nm, as well as two types of monolayer PCBM configuration with a step height of 0.7 nm and 0.88 nm, respectively. On graphene, PCBM forms one type of PCBM bilayer with a step height of 1.37 nm and one type of PCBM monolayer with a step height of 0.87 nm. By building and analyzing the models of PCBM bilayers and monolayers, we believe the main differences between two configurations of PCBM bilayer and monolayer is the tilt angle between PCBM and HOPG, which makes type I configuration the higher molecule density and binding energy.
Secondly, we report the investigation of self-assembly behaviors of C60 and PTCDA on flat graphene and rippled graphene by experimental scanning tunneling microscope (STM) and theoretical density functional theory (DFT). On flat graphene, C60 forms hexagon close pack (hcp) structure, while PTCDA forms herringbone structure. On rippled graphene, C60 forms quasi-hcp structure while PTCDA forms disordered herringbone structure. By DFT calculation, we study the effect of graphene curvature on spherical C60 and planar PTCDA.
Finally, we report a STM study of a monolayer of Gd3N@C80 on graphene substrate. Gd3N@C80 forms hcp structure in a small domain with a step height of 0.88 nm and lattice constant of 1.15 nm. According to our DFT calculation, for the optimal organization of Gd3N@C80 and graphene, the gap between Gd3N@C80 and graphene is 3.3 Å and the binding energy is 0.95 eV. Besides, the distance between Gd3N@C80 and Gd3N@C80 is 3.5 Å and the binding energy is 0.32 eV.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/97366 |
| Date | 18 March 2020 |
| Creators | Li, Yanlong |
| Contributors | Physics, Heflin, James R., Robinson, Hans D., Cheng, Shengfeng, Tao, Chenggang |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/x-zip-compressed |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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