Electronic and thin film stacking structure of Organic Semiconductors

Presented here is a study of the electronic properties and molecular stacking structure of four novel X-shaped anthracene based organic semiconductors utilizing near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and density functional theory (DFT) calculations. These materials have been found to exhibit high charge carrier mobility when used in organic thin film transistors without an annealing step. Angle resolved NEXAFS show local molecular order through polarization dependence in C 1s → π* transitions, and that the plane of the anthracene core is oriented nearly normal to the plane of the substrate. DFT calculations were used examine electronic structure and the effects of molecular geometry, showing that the highest occupied molecular orbital (HOMO) conjugation extends to the thiophene end groups. The attachment of the thiophene end group is determined to modify intermolecular interaction, resulting in either a cofacial or herringbone structure. With the understanding of how these materials form an ordered crystal structure, future fabrication of new materials may be directed towards a preference for crystallization without annealing. A study with applications for organic photovoltaic devices was also undertaken to examine the thin film stacking structure of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). NEXAFS measurements show that the side chain lifts the energy degeneracy of the C60 molecular orbitals around the chain attachment. This breaks the spatial π -orbital symmetry of the lowest unoccupied molecular orbital (LUMO) of the C60 backbone which is observed through polarization dependence of π* transitions. The intensity dependence is further analyzed to determine the bulk crystal structure of PCBM

Band engineering of graphene using metal mediated oxidation

In the study of materials for electronic devices, there is a continuous search for new materials with useful properties. In the early 2000’s, the 2D semi-metal carbon material graphene was isolated and characterized experimentally, and found to have a variety of desirable electronic properties. Since that time research on graphene and graphene related materials has progressed at an ever growing rate as researchers seek to understand, manipulate, and enhance graphene for use in electronic devices. One arm of this research seeks to manipulate the band structure of graphene such that it behaves like a semiconductor in devices. This thesis reports a study of four graphene systems investigated to attempt to manipulate the electronic structure in graphene; Graphene/Cu, Co/Graphene/Cu, Graphene/Co/SiO2, Co/Graphene/SiO2. The properties of these systems were investigated using various X-ray spectroscopy and surface science techniques. The analysis showed that the band structure of Graphene/SiO2 may be manipulated by depositing cobalt on the graphene surface. At a low concentration, the cobalt is completely oxidized into primarily CoO, and the graphene is not heavily damaged. Oxide groups form on the graphene surface but are found to be proportional to the cobalt thickness below 1 nm. Using X-ray spectroscopy a band gap of up to 0.30 ± 0.10 eV is observed in graphene 2p states when a low concentration of cobalt forms islands on the graphene surface. The mechanism of band gap opening was interpreted using electronic structure calculations to have a contribution from both graphene oxide formation as well as the presence of CoO. These results have implications for graphene electronics and spintronics where magnetic metals can be used to induce a band gap in graphene that is stable at room temperature and under atmospheric exposure

Graphene Thin Films and Graphene Decorated with Metal Nanoparticles

The electronic, thermal, and optical properties of graphene-based materials depend strongly on the fabrication method used and can be further manipulated through the use of metal nanoparticles deposited on the graphene surface. Metals that strongly interact with graphene such as Co and Ni can form strong chemical bonds which may significantly alter the band structure of graphene near the Dirac point. Weakly interacting metals such as Au and Cu can be used to induce shifts in the graphene Fermi energy, resulting in doping without significant alteration to the graphene band structure. The deposition and nucleation conditions such as deposition rate, annealing temperature and time, and annealing atmosphere can be used to control the size and distribution of metal nanoparticles. Under ideal conditions, self-assembled arrays of nanoparticles can be obtained on graphene-based films for use in new types of nano-devices such as evanescent waveguides

Atomic and electronic structure of a copper/graphene interface as prepared and 1.5 years after

We report the results of X-ray spectroscopy and Raman measurements of as-prepared graphene on a high quality copper surface and the same materials after 1.5 years under different conditions (ambient and low humidity). The obtained results were compared with density functional theory calculations of the formation energies and electronic structures of various structural defects in graphene/Cu interfaces. For evaluation of the stability of the carbon cover, we propose a two-step model. The first step is oxidation of the graphene, and the second is perforation of graphene with the removal of carbon atoms as part of the carbon dioxide molecule. Results of the modeling and experimental measurements provide evidence that graphene grown on high-quality copper substrate becomes robust and stable in time (1.5 years). However, the stability of this interface depends on the quality of the graphene and the number of native defects in the graphene and substrate. The effect of the presence of a metallic substrate with defects on the stability and electronic structure of graphene is also discussed.Comment: 18 pages, 6 figures, accepted to Appl. Surf. Sc

Protecting, copper, by, graphene, coating:, XPS, and, DFT, studies

This work is partially supported by Russian Foundation for Basic Research (grant No. 14-02-00006)

Selective Area Band Engineering of Graphene using Cobalt-Mediated Oxidation

This study reports a scalable and economical method to open a band gap in single layer graphene by deposition of cobalt metal on its surface using physical vapor deposition in high vacuum. At low cobalt thickness, clusters form at impurity sites on the graphene without etching or damaging the graphene. When exposed to oxygen at room temperature, oxygen functional groups form in proportion to the cobalt thickness that modify the graphene band structure. Cobalt/Graphene resulting from this treatment can support a band gap of 0.30 eV, while remaining largely undamaged to preserve its structural and electrical properties. A mechanism of cobalt-mediated band opening is proposed as a two-step process starting with charge transfer from metal to graphene, followed by formation of oxides where cobalt has been deposited. Contributions from the formation of both CoO and oxygen functional groups on graphene affect the electronic structure to open a band gap. This study demonstrates that cobalt-mediated oxidation is a viable method to introduce a band gap into graphene at room temperature that could be applicable in electronics applications

Polish literature in the slavonic library of „Wahazar" Publishers

„Wahazar” Publishers from 1992 issues in Russian translation „Collection of the Polish Literature” and from 2003 - bilingual „Polish-Russian Poetic Library”. These series (as well as „Collection of the Serbian Literature” and „Serbian-Russian Poetic Library”) form integral part of the Slavonic library, which’s aim is to make united information-field and to resist primitivization of the whole-Slavonic cultural space. Research, artistic and enlightening activity of the Publishers is the element of constructive alternative in the epoch of aggressive chaotization of the world, serves for communication upon new-arisen prohibitions. Productive existence of this publishing utopia shows, on a level with large number of other facts, that in spite of onset of dehumanizing globalization, in spite of guided devaluation of culture texts - nowadays increases the tendency to form open community of Slavonic (and wider - East-European) cultures, which is not subjected to ethnic patterns, ideomyths, negative inertia. New times could become the stage of really friendly contacts between the peoples - without pauses, conceals and distortions, the stage of continually coming news about things which are important and lasting, not artificially played up, marginal and ephemeral, about those true things which can help to consolidation - not frittering away