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

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

A Review of Three-Dimensional Scanning Near-Field Optical Microscopy (3D-SNOM) and Its Applications in Nanoscale Light Management

In this article, we present an overview of aperture and apertureless type scanning near-field optical microscopy (SNOM) techniques that have been developed, with a focus on three-dimensional (3D) SNOM methods. 3D SNOM has been undertaken to image the local distribution (within ~100 nm of the surface) of the electromagnetic radiation scattered by random and deterministic arrays of metal nanostructures or photonic crystal waveguides. Individual metal nanoparticles and metal nanoparticle arrays exhibit unique effects under light illumination, including plasmon resonance and waveguiding properties, which can be directly investigated using 3D-SNOM. In the second part of this article, we will review a few applications in which 3D-SNOM has proven to be useful for designing and understanding specific nano-optoelectronic structures. Examples include the analysis of the nano-optical response phonetic crystal waveguides, aperture antennae and metal nanoparticle arrays, as well as the design of plasmonic solar cells incorporating random arrays of copper nanoparticles as an optical absorption enhancement layer, and the use of 3D-SNOM to probe multiple components of the electric and magnetic near-fields without requiring specially designed probe tips. A common denominator of these examples is the added value provided by 3D-SNOM in predicting the properties-performance relationship of nanostructured systems

Self-Ordering Properties of Functionalized Acenes for Annealing-Free Organic Thin Film Transistors

Presented here is a study of the molecular self-ordering properties of four bis­(phenylethynyl) anthracene based organic semiconductors related to their electronic structure employing X-ray spectroscopy techniques and density functional theory (DFT) calculations. The local molecular order through polarization dependence of C 1<i>s</i> → π* transitions revealed ordered π-stacking nearly perpendicular to the substrate due to van der Waals interactions between alkyl groups. DFT calculations were used to deconvolute the measured electronic structure and examine effects of small changes in molecular geometry in relation to measured charge carrier mobility in top contact field effect transistors. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are found to be conjugated from the anthracene core across the bridging ethynyl groups to the thiophene and phenyl end groups. The inclusion of ethynyl bridges connecting the thiophenes has a twofold effect of both reducing the rotational freedom of this functional group and increasing HOMO/LUMO conjugation across the molecules. These features help create a more rigid upright structure for HB-ant-THT with better molecular orbital conjugation and subsequent higher mobility. With this understanding of how different functional groups interact with an acene core, future synthesis of new materials may be directed toward annealing-free organic semiconducting materials

Impact of sputtered ZnO interfacial layer on the S-curve in conjugated polymer/fullerene based-inverted organic solar cells

The impact of crystalline structure changes of sputtered ZnO interfacial layer on performances of inverted organic solar cells (OSCs) has been investigated. We find that the structural modification of the ZnO cathode interfacial layer, obtained by thermal annealing, plays a crucial role in the origin and solving of the S-curve in conjugated polymer/fullerene photovoltaics. Our results show that the crystallization (i.e. crystallites size) of poly(3hexylthiophene) (P3HT) evolves as a function of that of ZnO according to the annealing temperature. This evolution can directly impact the interfacial orientation and organization of the chains of P3HT at the ZnO buried interface. Such an ordered profile favors the vertical phase segregation and raises the carrier mobility, which explains the disappearance of the S-shape observed in current density-voltage device characteristics for annealing temperatures above 200 degrees C. These results adequately address recent research and provide an important insight into the interfacial layers of inverted OSCs. (C) 2014 Elsevier B.V. All rights reserved

Efficient energy transfer from ZnO to Nd3+ ions in Nd-doped ZnO films deposited by magnetron reactive sputtering

In this paper, a detailed study of the luminescent properties of Nd3+ ions in sputtered ZnO thin films is reported for the first time. Experimental evidence is provided showing that Nd is inserted and optically active in the ZnO matrix. Despite the small amount (<2%) of rare earth in these thin ZnO films, intense luminescence signals have been collected, indicating efficient infrared emission of Nd3+ in ZnO. Direct excitation of Nd3+ ions in the ZnO matrix was possible, suggesting that most of the Nd atoms are in the 3+ form at all deposition temperatures. Moreover, intense Nd3+ emission has been recorded also when the host was excited, indicating that an efficient energy transfer occurs from ZnO to Nd ions. Both the transfer efficiency and the Nd3+ concentration seem to depend on the deposition temperature. In particular, indirect excitation of the sample deposited at 400 degrees C generates a richer emission pattern compared to lower temperatures. The careful analysis of the luminescence data indicated that the new pattern comes from Nd sites that cannot be efficiently directly excited, but that are characterized by intense emission under indirect excitation of the host. The possible transfer mechanisms leading to this behavior will be outlined

Photon management properties of rare-earth (Nd,Yb,Sm)-doped CeO2 films prepared by pulsed laser deposition

CeO2 is a promising material for applications in optoelectronics and photovoltaics due to its large band gap and values of the refractive index and lattice parameters, which are suitable for silicon-based devices. In this study, we show that trivalent Sm, Nd and Yb ions can be successfully inserted and optically activated in CeO2 films grown at a relatively low deposition temperature (400 [degree]C), which is compatible with inorganic photovoltaics. CeO2 thin films can therefore be efficiently functionalized with photon-management properties by doping with trivalent rare earth (RE) ions. Structural and optical analyses provide details of the electronic level structure of the films and of their energy transfer mechanisms. In particular, we give evidence of the existence of an absorption band centered at 350 nm from which energy transfer to rare earth ions occurs. The transfer mechanisms can be completely explained only by considering the spontaneous migration of Ce3+ ions in CeO2 at a short distance from the RE3+ ions. The strong absorption cross section of the f-d transitions in Ce3+ ions efficiently intercepts the UV photons of the solar spectrum and therefore strongly increases the potential of these layers as downshifters and downconverters