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The Investigation Of The Growth And Properties Of Organic Semiconductors And Graphene On Metal Surfaces

Posted on:2016-11-29Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y L CaiFull Text:PDF
GTID:1108330470460874Subject:Condensed matter physics
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The orgainic semiconductor (OSC) devices are attracting more and more attention due to their low costs, variety of materials and structures available, sutability for large area fabrication and their flexibility. Majority of the OSC devices are based on the film/metal interface structure, the morphology of the film grown on the metal substrate and the interfacial electronic structure influence the deveice performance. For example, improving the uniformitry of the grown film will increase the carrier mobility, while the energy levels alignment between OSC and metal will influence the interfacial carrier injection. Therefore, the investigations on the organic film/metal inetfaces are instructive for improving the OSC device performance.This dissertation focuses on two types of materials:the first one is metal phthalocyanine (MPc). Due to its conjugated molecular plane, chemical stability and the strong sunlight absorption ability, MPc has drawn much attention in the reseach area of organic field effect transistor (OFET) and organic phtovotaic (OPV) devices. The second one is graphene. The fascinating properties of graphene, such as half integer quntum Hall effect, extremely high carrier mobility, bipolar band structure and extremely high strength, make graphene a promising material in lots of application areas. For example, taking advantage of the long ballistic carrier mean free path at room temperature, it is promising to fabricate high performce OFET from graphene via the chemical doping or the adsorption of specific atoms or molecules on graphene; taking graphene as the acceptor, mixing it with OSC donors, it is possible to synthesize high performance organic photovoltaic cells (OPVC); graphene could be used to fabricate transparent and high strength electrode in touch screen, OPVC, organic light-emitting diode (OLED); or due to its chemical stability, the moire structure of graphene on metal substrate could be utilized as a template for the growth of nano structures.In this dissertation, The electronic property of colbalt phthalocyanine (CoPc)/Au(111) interface was firstly investigated by angle-resolved ultra-violet photoelectron spectroscopy (ARUPS). Combining the ARUPS data with density functional theory (DFT) simulation, the correspondences between the emission peaks and the molecular levels were established. The interfacial charge transfer and the variation of the spin polarization of the adsorbed CoPc and the underneath gold atom indicated a strong interaction between the adsorbate and the substrate. Several conductance resonance peaks at specific molecular energy levels for CoPc molecule, and an adsorbate-substrate interaction brought modification to the quantum conductance spectrum for CoPc/Au(111) system were found by the first principle transport calculations.Then, the transition behavior of iron phthalocyanine (FePc) on Ag (110) was investigated by scanning tunneling microcopy (STM) and DFT simulation. After FePc molecules deposited on Ag (110) surface, two adsorption structures were observed. The pathway between these different configurations was located by Nudged Elastic Band (NEB) theory, and a transition barrier of 0.236eV was calculated.Also combining STM and DFT, a bottom-up fabrication of graphene via molecular self-assembly of p-Terphenyl on Ru(0001) was investigated. Upon the annealing of sample to 450℃, the intermediate stage in which the adsorbed p-Terphenyl molecules and graphitized flakes converted from the molecules coexist is observed, implying the onset of dehydrogenation of p-Terphenyl. At the annealing temperature of 480℃, the graphitized flakes start to convert into graphene. An adsoption energy of 5.99eV is calculated for an individual p-Terphenyl molecule on Ru(0001), denoting a strong interaction between the adsorbate and substrate. The intermolecular interaction brings extra adsorption energy of 0.28eV for each molecule in the di-molecule adsorption system. During the conversion process from adsorbed molecule into graphene, the intermolecular interaction increases the dehydrogenation barrier from 1.52eV to 1.64eV.At the end, the adsorption properties of cobalt phthalocyanine (CoPc) on monolayer graphene/Ru(0001) [MG/Ru(0001)] was investigated by STM and DFT. At monolayer coverage, CoPc forms an ordered Kagome lattice, and a slight deformation for one lobe of CoPc and charge transfer of 0.084e-from CoPc to Ru(0001) substrate take place. The existence of the defect (vacancy) in graphene on Ru(0001) increases the coupling between the Ru substrate and the epitaxial graphene. Such an increase of the coupling brings about an overall CoPc molecular energy level shift toward the low binding energy, which subsequently results in a central topographical contrast between the CoPc molecules on the intact and defective MG/Ru(0001).
Keywords/Search Tags:Organic Semiconductor, Graphene, Phthalocyanine, Ru(0001), Scanning Tunneling Microscopy, Ultraviolet Photoelectron Spectroscopy, Density Functional Theory
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