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Investigations On The Electronic Properties And Adsorption Structures Of Organic Semiconductors On Ordered Surfaces

Posted on:2016-06-27Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y S TaoFull Text:PDF
GTID:1108330470460877Subject:Condensed matter physics
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In the past decades, because of the compatibilities of high-throughput, low-cost solution processable and the capabilities of precisely functionalization for desired performance by different synthesis methods, organic semiconductors have shown great potentials in the applications of organic electronic devices, such as organic light-emitting diodes (OLEDs), organic photovoltaic cells, gas sensors, heterojunctions and ultrafast optical switches. Both the properties of organic materials and organic/electrode interface play important roles in determining devices performance. More importantly, various organic/metal interfaces are involved in such devices, and hence the interfacial properties are of key importance. It has been demonstrated that the molecular orientation of crystalline organic thin films have great impact on device performance of organic electronic devices. Therefore, it is critical to have a thoroughly understanding about the electronic properties and the adsorption geometries at the interface between organic materials and substrate. In this thesis, the properties at the interface between the active material and electrode were studied base on the ultraviolet photoemission spectroscopy (UPS) together with density functional theory (DFT) calculations.First, the adsorption of tetracene on the Ag(110) surface has been studied using UPS and first-principles DFT calculations. The five emission features originating from the organic molecules locate at 3.4,5.7,6.3,7.2, and 8.8 eV below the Fermi level, respectively. There is no apparent binding energy shift of these features with the increasing coverage of tetracene, indicating a relatively weak interaction between tetracene and the Ag(110) substrate. Moreover, our DFT calculation shows that tetracene molecules prefer to adsorb between two Ag atomic rows of the first layer with its molecular axis along the [001] direction of the substrate, and the center of its second benzene ring locates at top site of Ag atoms of the second layer. With this optimized adsorption configuration, the π-d interactions between tetracene and Ag(110) are maximized. Comparing with the densities of states (DOS) before and after tetracene adsorption, no apparent intensity change can be observed for the DOS from p-orbital of tetracene, suggesting that the coupling between the substrate d-band and the filled p-orbitals of tetracene are relatively weak-Then the electronic and structural properties at the copper phthalocyanine (CuPc)/graphene interface have been studied using UPS and first-principles DFT calculations. The five emission features α, β, γ,δ and ε originating from the CuPc molecules locate at 1.48,3.66,4.98,6.90, and 9.04 eV, respectively. These features shift in binding energy with the increasing CuPc coverage. The LUMO orbital of CuPc is originated from Cu 3 d orbital and the HOMO orbital is originated from C 2p orbital. Further theoretical calculation indicates that the adsorption of CuPc on the top site is the most favorable configuration, and the separation between the adsorbate and graphene is about 3.47 A. According to the density of states before and after CuPc adsorption, the LUMO of CuPc is slightly occupied, while the Dirac point of graphene slightly shift towards higher energy, suggesting that a small amount of electron transfer from graphene to CuPc upon contact.
Keywords/Search Tags:Tetracene/Ag(110), CuPC/graphene, Photoemission spectroscopy, DFT calculations, Adsorption and electronic structures
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