bluuebunny/crossref_metadata_2025 huggingface dataset:
Photoluminescence Imaging of III-V Substrates and Epitaxial Heterostructures
Authors: Jantz W. , Baeumler M. , Wang Z.M. , Windscheif J.
Published: March 2011

The characterization of III-V compound semiconductor substrates and epitaxial layers with photoluminescence imaging is reviewed. The luminescence patterns of semi-insulating GaAs are dominantly determined by the concentration and distribution of nonradiative recombination centers, as shown by comparison with spectroscopic temperature and lifetime topography of photoexcited carriers. Wafers fabricated with various growth and annealing procedures are evaluated. Presently available informations on nonradiative centers in GaAs are summarized and discussed. The correlation of luminescence, absorption and resistivity topograms of InP substrates shows various interrelated influences of the Fe acceptor distribution. High resolution luminescence images of growth induced, strain induced and substrate induced defects in epitaxial heterostructures are obtained. The generation of relaxation dislocations in pseudomorphic layers is influenced by growth parameters, layer structures, layer doping and also by substrate properties. Nonradiative recombination center patterns replicate the arrangement of threading dislocations in the substrate.
Dominant Midgap Levels in the Compensation Mechanism in GaAs
Authors: Shiraxi H. , Tokuda Y. , Tohyama E. , Sassa K. , Toyama N.
Published: March 2011

Properties of midgap levels in n-type GaAs crystals were studied by using trap density spectroscopy (TDS), capacitance-voltage and near infrared (NIR) absorption measurements. The TDS analysis for various n-type samples showed that the concentrations of the midgap level were from 2.0 × 1016 cm−3 to 3.4 × 1016 cm−3. However, those of EL2 in the same crystals were found to be from 1.1 × 1016 cm−3 to 1.3 × 1016 cm−3 by NIR absorption measurements at room temperature. On the other hand, in undoped semi-insulating GaAs crystals with carbon concentrations ranging from 6 × 1014 cm−3 to 2.4 × 1016 cm−3, the densities of the ionized EL2 determined by NIR absorption were only about 30 % of those of carbon acceptor determined by the localized vibrational mode absorption at room temperature. These differences suggest the presence of another midgap donor which does not give NIR absorption. The concentrations of this trap were estimated to be 0.8 to 1.6 times those of EL2 by decomposition of the TDS spectra into their components.
Band Edge Optical Absorption of Molecular Beam Epitaxial GaSb Grown on Semi-Insulating GaAs Substrate
Authors: Shah M. , Manasreh M.O. , Kaspi R. , Yen M. Y. , Philips B. A. , Skowronski M. , Shinar J.
Published: March 2011

The optical absorption of the band edge of GaSb layers grown on semi-insulating GaAs substrates by the molecular beam epitaxy (MBE) technique is studied as a function of temperature. A free exciton absorption peak at 0.807 eV was observed at 10 K. The free exciton line is observed in either thick samples (5μm thick) or samples with ∼0.1 μm thick AlSb buffer layers. The latter samples suggest that the AlSb buffer layer is very effective in preventing some of the dislocations from propagating into the MBE GaSb layers. The fitting of the band gap of the GaSb layers as a function of temperature gives a Debye temperature different than that of the bulk GaSb calculated from the elastic constants.
A Tem Study of Defect Structure in GaAs Film on Si Substrate
Authors: Nahm Sahn , Lee Hee-Tae , Kim Sang-Gi , Cho Kyoung-Ik
Published: March 2011

For the GaAs buffer layer deposited on Si substrate at 80°C and annealed at 300°C for 10 min, the size of most GaAs islands was observed as ∼ 10 nm but large islands (∼ 40 nm) were also seen. According to the calculation of spacing of moire fringes, large GaAs islands are considered to be rotated about 4 ° with respect to the Si substrate normal. However, for the main GaAs film overgrown on the GaAs buffer layer at 580 °C, moire fringes with the spacing of 5 nm (GaAs film without rotation) completely covered the surface of Si substrate. Misfit dislocations and stacking faults were already formed at the growth stage of buffer layer. Stacking faults and misfit dislocations consisting of Lomer and 60 ° dislocations were observed in GaAs films grown at 580 °C. However, after rapid thermal annealing at 900 °C for 10 sec, only Lomer dislocations with 1/2[110] and 1/2[-110] Burgers vectors were observed.
The Growth of InAsSb/InGaAs Strained-Layer Superlattices by Metal-Organic Chemical Vapor Deposition
Authors: Biefeld R. M. , Baucom K. C. , Kurtz S. R. , Follstaedt D. M.
Published: March 2011

We have grown InAsl-xSbx/Inl-yGayAs strained-layer superlattice (SLS) semiconductors lattice matched to InAs using a variety of conditions by metal-organic chemical vapor deposition. The V/III ratio was varied from 2.5 to 10 at a temperature of 475 °C, at pressures of 200 to 660 torr and growth rates of 3 - 5 A/s and layer thicknesses ranging from 55 to 152 Å. The composition of the InAsSb ternary can be predicted from the input gas molar flow rates using a thermodynamic model. At lower temperatures, the thermodynamic model must be modified to take account of the incomplete decomposition of arsine and trimethylantimony. Diodes have been prepared using Zn as the p-type dopant and undoped SLS as the n-type material. The diode was found to emit at 3.56 μm. These layers have been characterized by optical microscopy, SIMS, x-ray diffraction, and transmission electron diffraction. The optical properties of these SLS's were determined by infrared photoluminescence and absorption measurements.
Electron Beam Induced Degradation of 2Deg in AlGaAs/GaAs Heterostructure
Authors: Wada T. , Kanayama T. , Ichimura S. , Sugiyama Y. , Komuro M.
Published: March 2011

The effects of low-energy electron irradiation on the two-dimensional electron gases (2DEG's) in AlGaAs/GaAs heterostructures have been investigated. Not only the electron mobility of the 2DEG's but also the two-dimensional (2D) carriers are found to be reduced by the electron irradiation with the incident energies between 3.5 k and 8 keV and the electron dose of 1×1016 and 1×1017 /cm2. The degraded mobility and the removed carriers by the low-energy electron irradiation are shown to recover by isochronal annealing to some extent, but not completely below 400°C. It is also found that considerable amount of scatterers which are created by an electron irradiation at room temperature are also created by an irradiation at 90 K. Comparing the experimental results with the Monte Carlo simulation, we speculate that the mobility degradation and the 2D carrier compensation are partly caused by the formation of complex defects in the GaAs buffer layer which are due to the excitations of core electrons of As, and that the mobility is further degraded by the formation of short-range scatterers in the heterointerface.
D− Impurities in a Quasi-Two-Dimensional System: Statistics and Screening
Authors: Li W. J. , Wang J. L. , Cheng J.-P. , Holmes S. , Wang Y. J. , Mccombe B. D. , Schaff W.
Published: March 2011

Far infrared spectroscopic studies of electron-density- and magnetic-field-dependenceof manyelectron effects on silicon donor impurities confined in GaAs quantum wells are presented. At low excess electron densities, transitions from D− singlet and triplet states are observed. Temperature- and polarization-dependence measurements show that the relative absorption strengths of various spectroscopic features (D0, D−, and CR) are in qualitative agreement with a statistical calculation in thermal equilibrium in high magnetic fields. At large excess electron densities, the “D−” transition energy shifts to higher energy when electron density is increased, and the magnetic-field dependence of the transition energy exhibits discontinuities in slope at integer filling factors. The spectroscopic features and their relation to excess free carriers and the role of screening and correlation are presented and discussed.
Density of States of Hydrogenic Impurities in GaAs/GaAlAs Quantum Wires
Authors: LeÃo Salviano A. , HipÓlito O. , Ferreira Da Silva A.
Published: March 2011

We investigate the occurrence of impurity density of states of hydrogenic impurities placed on the axis of a cylinder quantum-well wire of GaAs/GaAlAs structure. The effects of disorder are taken into account in the calculation. It is shown that for a specific radius of the wire, the peak energy of the density of states as a function of the impurity concentration increases very fast enlarging the bandwidth. The impurity bands are considered for the observed binding energy and radius as well as for the impurity concentration of experimental interest.
Single-Source Precursors to Niobium Nitride and Tantalum Nitride Films
Authors: Winter Charles H. , Jayaratne Kumudini C. , Proscia James W.
Published: March 2011

Herein we report our efforts to prepare new precursors to niobium nitride (NbN) and tantalum nitride (TAN, Ta3N5) thin films. Treatment of MC15 (M = Nb, Ta) with tertbutylamine in benzene solvent affords complexes of the formula [MCI2 (NtBu)(NHtBu)(NH2tBu)]2. The niobium complex [NbC12(NtBu)(NHtBu)- (NH2tBu)]2 affords NbN films on glass substrates between 500-600 °C, while the tantalum analog [TaC12(NtBu)(NHtBu)(NH2tBu)]2 gives films of Ta3N5 in this temperature range. Treatment of MCI5 (M = Nb, Ta) with 1,1-dimethylhydrazine in dichloromethane solvent affords complexes of the formula [MC12(NNMe2)(NHNMe2)(NH2NMe2)]n. These hydrazido complexes afford cubic MN films upon glass substrates at ≥400°C. Analyses of the films are presented.
Manufacturing of Covalent Ceramics by Exothermic Reactions
Authors: Deevi Seetharama C.
Published: March 2011

Manufacturing of high temperature materials is an energy expensive process, and conventional techniques require long periods of homogenization to ensure uniform chemical composition in large batches of material. Several covalent ceramics, and high temperature materials can be manufactured by utilizing the exothermicity of the reactants. Some of the salient features of combustion synthesis, approaches to densify the combustion synthesized product, and other processes that utilize exothermic reactions for the synthesis of materials have been discussed.
Ceramic Coatings for PBR Components
Authors: Barletta Robert. E. , Ludewig H. , Powell J. R. , Vanier P. E.
Published: March 2011

Particle bed reactors (PBRs) have the potential for providing compact power sources for both space power and propulsion applications. The reactors operate in the temperature range of 2000 to 3000 K and, for propulsion systems, utilize hydrogen as a coolant. Given these overall operating conditions, the need exists to develop and verify the performance of various reactor components, particularly those which must operate at the more extreme conditions of temperature and hydrogen flow rate. These components must undergo multiple thermal cycles between cryogenic and exit temperatures. In addition, some of the components are subject to a thermal gradient of thousands of degrees Kelvin over a few centimeters.

A number of approaches directed towards meeting these needs have been developed. These include both monolithic ceramics (e.g. boron nitride) as well as coated composites consisting of refractory carbides (ZrC, NbC and TaC) on carbon-based substrates. Full and partial scale components have been fabricated and tested under prototypic conditions. The results of this developmental effort will be described.
New Covalent Ceramics: MgSiN2
Authors: Groen Wilhelm A. , Kraan Marcel J. , de With Gijsbertus , Viegers Mathias P.A
Published: March 2011

The ternary nitride MgSiN2 crystallizes in a diamond-like structure with a band gap of 4.8 eV. These characteristics make the compound promising for a variety of applications. Recently we succeeded in preparing fully dense ceramics by sintering at 1550°C. Phase composition, chemical properties and the mechanical properties of the as-prepared ceramics are described. The ceramics are resistant to oxidation in air at least up to 920°C. The thermal conductivity at room temperature has been found to be 20 W/m·K. A considerable improvement of the thermal conductivity is expected when improved processing conditions, well within reach, are realized. A reasonable strength of 270 MPa and a fairly good fracture toughness of about 4.3 MPa·m½ are obtained. A hardness of about 15 GPa and a Young's modulus of 235 GPa have been measured. These new ceramics show that it is still possible to find improved materials as compared to the usual oxide and non-oxide ceramics.
Carbon Nitride Thin Films Grown by Pulsed Laser Deposition
Authors: Treece Randolph E. , Horwitz James S. , Chrisey Douglas B.
Published: March 2011

Thin films of diamond and diamond-like carbon (DLC) are technologically important materials that serve as hard, scratch resistant and chemically inert coatings for tools and optics. Recent calculations suggest that β-C3N4 should be harder than diamond. We have deposited carbon nitride (CNx) thin films by pulsed laser deposition. The films were grown from a graphite target in a nitrogen background. The nitrogen source was either (a) a N2 gas atmosphere, or (b) a N2+/N+ ion beam generated by a Kaufman ion gun. A wide range of deposition parameters were investigated, such as deposition pressure (0.3-900 mTorr N2), substrate temperature (50 and 600°C), and laser fluence (1-4 J/cm2) and laser repetition rate (1-10 Hz). The films have been characterized by Rutherford Backscattering Spectroscopy, thin-film X-ray diffraction, scanning electron microscopy, and micro-Raman spectroscopy. In general, the films were nitrogen deficient with a maximum nitrogen to carbon ratio (N/C) of 0.45 and a shift in the G band Raman peak consistent with amorphous CNx (a-CNx).
Chemical Vapor Infiltration of Non-Oxide Ceramic Matrix Composites
Authors: Besmann Theodore M. , Stinton David P. , Lowden Richard A.
Published: March 2011

Continuous fiber ceramic composites are enabling new, high temperature structural applications. Chemical vapor infiltration methods for producing these composites are being investigated, with the complexity of filament weaves and deposition chemistry merged with standard heat and mass transport relationships. Silicon carbide-based materials are, by far, the most mature, and are already being used in aerospace applications. This paper addresses the state-of-the art of the technology and outlines current issues.
Textures in AiN-SiC Composite Ceramics
Authors: Sandlin Michael S. , Bowman Keith J.
Published: March 2011

Historically, texture analysis has been useful in explaining processing-induced property anisotropy in metals and deformation history in geological materials. Texture analysis, however, has not been fully utilized in explaining similar phenomena in ceramic systems. The scarcity of texture analysis in ceramic systems could conceivably stem from the classical conception that textures rarely develop in ceramic materials because of their lack of plasticity. While it is true that slip-based textures occur in only a limited number of ceramic systems, textures occur in a multitude of ceramic systems in which dislocation plasticity is not observed. The most prominent non-slip-based texture development mechanism in ceramic systems is grain rotation during processing. Other possible mechanisms for texture development in ceramics are seeded solid solution formation and oriented grain growth. These non-slip-based texture development mechanisms and other phenomena related to the measurement of textures in covalent ceramics are discussed in the context of the A1N-SiC system.
Reconstructions at Si- and C-Terminated Surfaces of 2H-SiC: an ab Initio Molecular Dynamics Study
Authors: Hwang Yong Gyoo , Chen Bin , Boguslawski Piotr , Bernholc J.
Published: March 2011

The reconstruction patterns of Si- and C-terminated surfaces of wurtzite SiC are studied by ab initio molecular dynamics. The (1×1) relaxed, (1×2) buckled, and (2×1) л-bonded chain geometries are considered. In the (l×1) geometry, inward relaxation of the C-terminated surface is much stronger than that of the Si-terminated surface. The C-terminated surface is stable with respect to (2×1) buckled reconstruction, but the Si-terminated surface is not. At both surfaces, the (2×1) л-bonded chain geometry is energetically the most favorable. The Si-C л-bonded chains are buckled and slightly dimerized. In both cases, the outermost atoms are carbons, due to the bond length constraints of the л-bonded chain reconstruction.
Interface Roughness Effects on Adhesion of Ta2N Films.
Authors: Moody N. R. , Venkataraman S. , Nelson J. , Worobey W. , Gerberich W. W.
Published: March 2011

In this study, we employed continuous microscratch testing to determine the effects of interface roughness on adhesion and fracture toughness of thin Ta2N films. These films were sputterdeposited on single crystal sapphire and polycrystal alumina substrates to a thickness of 0.5 μm. Comparison of the results showed that the interfacial fracture energy increased from 0.4 J/m2 for films on the single crystals to 1.5 J/m2 for films on the polycrystals with a corresponding increase in fracture toughness values. These results are consistent with crack deflection and interface roughness models and are useful for understanding adhesion and toughness on the submicron scale. The results also show that the continuous microscratch technique is a viable approach to determining adhesion and toughness of bi-material systems.
Properties and Thermal Behaviour of Si3N4 Thin Films
Authors: Knotek O. , LÖffler F. , Wolkers L.
Published: March 2011

The applications of thin film technologies are increasing. A variety of different ceramic and hard coating materials are deposited. Silicon nitride is one of the materials which seems to be quite interesting for the use as coating material. An use as a wear and corrosion protective coating seems to be conceivable. Silicon nitride is known to have a good thermalshock resistivity, thermal and chemical stability and good mechanical and wear resistant properties. If these properties are also given for deposited metastable PVD Si-N-coatings has to be evaluated yet.

This paper describes the influence of different coating process parameters of the MSIP process like DC and RF mode, reactive and non reactive sputter processes and target orientation on the properties of silicon nitride films. It is shown, that coatings deposited with a 100 oriented single crystal target have the highest deposition rate and the best mechanical properties. The thermal stability of these thin films is insufficient although it can be improved by a carbon incorporation.
Group 12 Thiolates: Syntheses, Characterization and Decomposition Pathways
Authors: KrÄuter Gertrud , Goedken Virgil L. , NeumÜller Bernhard , Rees W. S.
Published: March 2011

Zinc- and cadmium bis(alkylthiolate) compounds have been prepared and converted into the appropriate binary metal sulfides by thermal treatment. Several mercury chlorothiolates have been synthesized and characterized by single crystal X-ray diffraction. Their decomposition pathways are discussed. The prepared binary metal sulfides have been studied by XRPD and - in selected cases - by particle size determination. The volatile co-products have been isolated and characterized by GC/MS.
Why Covalent Ceramics are Hard
Authors: Gilman John J.
Published: March 2011

Pure metals with nearly the same elastic properties as hard, covalent ceramics are often very soft, even at low temperatures. Why the difference? “Directed bonds” is the common answer. Or, “the Peierls-Nabarro stress is large”. However, the former is only qualitative, while the latter does not distinguish between covalent and metallic bonding, and is quantitatively incorrect. The rules of quantum chemistry, as developed to account for chemical reaction rates, indicate why dislocation mobilities are low in crystals bonded through hybrid sp and spd orbitals. Resistance to the motion of kinks on dislocation lines arises from the way in which the initial electronic states of these kinks are correlated with the final states (after a unit of motion has occurred). The initial bonding states correlate with the final antibonding states, and vice versa. Thus, the correlation lines cross, creating substantial barriers to the motion. The barrier magnitudes can be calculated from the energy gaps (e.g., for Sic, the HOMO-LUMO gap); together with the quadratic dependencies of bond energy on bond-bending. The calculated results agree with observations. A second prototype, TiC is also discussed.
The Electronic Structure of Carbon Nitride
Authors: Valladares Ariel A. , Sansores L. Enrique
Published: March 2011

Recently there has been very much interest in understanding the structure of amorphous carbon nitride thin films, it has been shown that the introduction of nitrogen in amorphous carbon leads to better properties. In this paper we use four different cluster to study the changes in the local density of states at the carbon and nitrogen sites introduced by different environments. The method used is a SCF Hartree-Fock using pseudo potentials. We have considered carbons with sp3 and nitrogens with sp2 and sp3 hybridization. The results show that when N occupies a tetrahedral position (sp3) it introduces a localized state in the gap, while in the sp2 configuration, it introduces states near the edges of the bands.
Processing of Cunse2 Films for Photovoltaics
Authors: Basol BÜlent M. , Kapur Vijay K.
Published: March 2011

CuInSe2and related I-HI-VI2 compound semiconductors containing Ga and S are important photovoltaic materials with electrical and optical properties that can be tuned for optimum device performance. Polycrystalline thin films of ternary and multinary compounds from the Cu(In,Ga)(Se,S)2family have already yielded laboratory size solar cells with over 15 % efficiency. Large area photovoltaic modules with over 10% efficiency have also been demonstrated using these materials. In this contribution we present a brief review of the CuInSe2 material characteristics which are important to solar cell performance and their correlation with the processing variables. We concentrate on films obtained by the selenization technique. Selenization of Cu-In precursors is an approach that has yielded good quality CuInSe2films which have been used for the fabrication of high efficiency solar cells.
Room-Temperature Synthesis of CuInQ2 (Q = S or Se) in Non-Aqueous Solution Using an Organoindium Reagent
Authors: Hepp Aloysius F. , Andras Maria T. , Landry Christopher C. , Barron Andrew R.
Published: March 2011

We have discovered a novel two-phase synthesis of CuInSe2 at 25° from Cu2Se and (C5H5)3In in 4-methylpyridine (4-MePy). An analogous reaction to produce CulnS2 must be run at 140°C in refluxing 4-MePy in the presence of 2-mercaptopyridine. Microscopy of CuInSe2 produced at 25°C shows it to be platelet-shaped crystallites with an approximate particle size of 10 microns, less than 2% C and H, with a small amount of unidentified crystalline impurity. Our results demonstrate that it is possible to produce from solution a material that is ordinarily synthesized in bulk or films at much higher temperatures or using extraneous reagents and/or electrons.
Luminescence Behavior of Self-Assembled Multilayer Heterostructures of Poly (Phenylenevinylene)
Authors: Ferreira Marysilvia , Rubner M. F. , Hsieh B. R.
Published: March 2011

Multilayer thin film heterostrutuctures comprised of alternating layers of a poly (phenylene vinylene) (PPV) precursor and various polyanions were fabricated using a new self-assembly technique. Thermal treatment of these films produced multilayers of the fully conjugated form of PPV. Alternating Multilayers fabricated with inert polyanions such as sulfonated polystyrene exhibited a very strong photoluminescence whereas alternating multilayers fabricated with electron acceptor polyanions such as sulfonated C?O and poly (thiophene acetic acid) displayed a highly quenched luminescence. This latter behavior suggests that self-assembled Multilayers can be used to examine photoinduced charge transfer interactions between PPV and various electron acceptors.
Urea Crystals Grown by Physical Vapour Transport
Authors: Paorici C. , Zanotti L. , Zha M. , Franzosi P.
Published: March 2011

A new facility for growing single crystals of urea by physical vapour transport in effusive cells is reported. The best crystals, as characterised by X-ray diffraction topography, are largely dislocation-free and do not contain microprocipitates.
Molecular Composites Based on Rigid Rod Polymers for Electrooptical Applications
Authors: Wegner Gerhard , Neher D. , Heldmann C. , Servay Th.K. , Winkelhahn H.-J. , Schulze M. , Kang C.-S.
Published: March 2011

Novel rigid rod polymers substituted with NLO-active chromophores have been developed towards application in electrooptical signal Modulation. The Materials are based on rigid polyesters and polyesteramids, in which the chromophores are either covalently linked to the backbone by short flexible spacers or directly incorporated into the main chain. In the bulk these systems form macroscopically ordered structures with layers of rigid rod backbones separated by the side chain segments. The properties and stability of the induced polar order can thus be adjusted by morphological parameters like the layer distance or the orientation of the main chains relative to the substrate. The relaxation of the NLO activity with time is described by a Multi-exponential decay and shows enhanced time-temperature stability even above 100°C. The temperature dependence of the relaxation times exhibits unusual features that distinguishes these systems from conventional NLO side chain polymers.
Oligomers Containing Mixed Thiophene and Pyrrole Units
Authors: Cava Michael P. , Parakka James P. , Lakshmikantham M. V.
Published: March 2011

The synthesis and properties of polythiophenes, polypyrroles and their oligomers has evolved into a major area of conducting polymer research. Mixed systems containing both thiophene and pyrrole units have received much less attention. In this paper, the chemistry of thiophene-pyrrole oligomers containing up to seven heterocyclic units is reviewed.
Optical Absorption, Luminescence, And Redox Switching Properties Of Polyphenylene Derivatives
Authors: Reynolds John R. , Child Andrew D. , Ruiz Jose P. , Musfeldt Janice L. , Sankaran Bala , Larmat Fernando , Balanda Peter , Tanner David B.
Published: March 2011

A series of polyphenylene derivative polymers with a variety of heterocyclic units along the main chain, and various pendant groups, have been synthesized and their optoelectrochemical properties examined. Polyphenylenes containing electronically isolated emitter centers have optical absorption and luminescence characteristics which are a function of the length of the conjugated unit. A water soluble electroactive rigid-rod poly (p-phenylene), specifically Poly[2,5-bis (propoxy-3-sulfonate)-1,4-phenylene-alt,4-phenylene], has been prepared. Solution cast films of this polymer exhibit optical evidence for bipolarons via both p-type and n-type electrochemical doping. Further poly (p-phenylene) (PPP) derivatives are being pursued, including PPP's bearing pendant donor molecules for charge transfer complex formation. A series of polymers containing alternating p-phenylene units with bithienylene and bifuranylene units, specifically poly[1,4-bis (2-heterocycle)-2,5-disubstituted-1,4-phenylenes], have been synthesized with both alkyl and alkoxy pendant groups. Steric interactions disrupt the ability of the redox doped polymers to attain planarity, strongly affecting the polymer's oxidation potentials and DC conductivities. The substituent length has little effect on the electronic properties of the polymer, but greatly affects the polymer's solubility and transport properties during redox switching.
Polyaniline Surface Morphology During The Doping Process Using Atomic Force Microscopy
Authors: Buckley Leonard J.
Published: March 2011

Chemically synthesized polyaniline films were studied during the doping and dedoping process by imaging the polymer surface using in-situ Atomic Force Microscopy (AFM). The polymer, which was initially in the base non-conducting form was doped using aqueous acidic solutions of both tosylic (pH=0.2) and HCl (pH of 0.2 and 1.0). Dedoping was accomplished by exposing the same doped polymer surface to NH4OH (pH=12) base solution. AFM images showed that it was necessary to cycle the polymer surface 3 times between acid and base before a reproducible surface morphology was established between the doped and dedoped states. For the case of doping with tosylic acid AFM images showed that the polyaniline surface was immediately roughened. In addition there appeared to be an increase in the size of surface channels and cracks. When doping with HC1 (pH=1.0), no change in surface morphology was observed; however, noticeable surface roughening occurred over 10 Minutes for the case of the lower pH=0.2 solution. The complex permittivity and DC conductivity was measured to estimate the level of protonation in the AFM samples.
Influence of Structural Parameters of Conducting Polymers on their Microwave Properties
Authors: Hourquebie P.
Published: March 2011

The complex dielectric constant ε* (ε* = ε′-iε″) of conducting polymers (polyaniline and poly (3-alkyl thiophene)) is studied over a frequency range spanning from 130 MHz to 20 GHz. The effects of structural parameters (counter-anion size, Molar Mass, length of the alkyl chain on the substituted Monomer) on electrical properties (σdc and ε* values) have been investigated.
Mps3 Layered Intercalates as Insulating Magnets: Towards Nanocomposites Associating Ferrimagnetism and Non-Linear Optical Properties
Authors: Clement Rene , Lacroix Pascal G.
Published: March 2011

The layered MIIPS3 compounds posess a unique cation exchange intercalation chemistry, which allows the synthesis of a wide range of air stable M1-xPS3G2x intercalates (G stands here for a Monocation). The positive charge of the guest cations is counterbalanced by intralayer metallic vacancies.

Intercalation exerts a dramatic influence on the magnetic properties of these compounds. Whereas pristine MPS3 compounds order antiferromagnetically (M = Mn, Fe‥), Many intercalates become magnets below an ordering temperature that can be as high as 92 K. The role of intercalation is discussed in terms of the creation of intralayer metallic vacancies, which destroy the spin balance that prevails in pristine MPS3 compounds.

The MPS3 host lattice has the capability of causing spontaneous poling of the dimethylamino-N-Methyl stilbazolium chromophore (DAMS), leading to materials that exhibit strong second harmonic generation. (SHG). Inserting DAMS into MnPS3 therefore leads to a composite that possess both spontaneous Magnetization (below Tc = 40 K) and a strong efficiency for SHG.
Fermi Surface Study Of An Organic Superconductor θ- (Bedt-Ttf)2I3
Authors: Tamura M. , Kuroda H. , Uji S. , Aoki H. , Tokumoto M. , Swanson A. G. , Brooks J. S. , Agosta C. C. , Hannahs S. T.
Published: March 2011

The temperature and field dependence of the de Haas-van Alphen oscillations in θ- (BEDT-TTF)2l3 is analyzed. The cyclotron masses are estimated to be 2.0 Me and 3.6 me for slow and fast oscillations, respectively. The indication of magnetic breakdown effect is discussed in terms of the geometry of the Fermi surface. The results are compared with those of the infrared reflectance spectra.
Optoelectronic and Structural Properties of Vacuum-Deposited Crystalline Organic Thin Films
Authors: Forrest S. R. , Burrows P. E. , Haskal E. I. , Zhang Y.
Published: March 2011

Recently, it has been discovered that crystalline organic thin films can be deposited in nearly single crystalline form on a variety of substrates such as glass, polymers, etc. Since then, this discovery has led to the growth of crystalline organic quantum wells, waveguides, coupler/switches, and organic/ inorganic heterojunction devices such as field effect transistors and avalanche photodiodes. Organic light emitting diodes (LEDs) which luminesce in the red, green and blue have also been demonstrated. In this paper, we will report on several recent advances in the growth of organic thin films deposited by organic molecular beam deposition. We report on modeling of organic monolayer growth based on the atom-atom potential Method. The Model provides insight into the factors which control “quasi-epitaxial growth” i.e. the ordered growth of one layer of an organic film which is incommensurate with the substrate lattice. We also observe large optical nonlinearities which are a feature of both single and Multi-layer crystalline organic films. The growth of organic, nonlinear optically active crystalline organic compounds are also discussed.
Conjugated Polymer Fluorescence: Interplay Of Correlations And Alternation
Authors: Soos Z. G. , Galvão D. S. , Etemad S. , Keplero R. G.
Published: March 2011

Conjugated polymers have been intensively studied in connection with high conductivity on doping, large nonlinear optical responses in their semiconducting state, and recently[1] as novel light-emitting diodes (LEDs). Many different polymers are realized in Fig. 1 on varying the substituents R, including the best characterized examples[2,3] that have centrosymmetric conjugated backbones in their idealized extended geometry. The PS and PPV families fluoresce strongly, the PT family Moderately, and the PDA's or PA hardly at all. We have related [4,5] polymer fluorescence to the nature of the lowest singlet excited state, Si, which in C2h symmetry is either a dipole-allowed Bu or a two-photon allowed Ag state. Since the ground state is Ag, S1 is either 1B or 2A and fluorescence requires E (1B) < E(2A). The idea is simply that radiationless decay wins out for the low excitation energies and many vibrational degrees of freedom of conjugated polymers unless Si has a strong transition dipole to the ground state.
Molecular Design of Nonlinear Optical Polymers for Photonic Device Applications
Authors: Kaino Toshikuni , Ooba Naoki , Tomaru Satoru , Kurihara Takashi , Yamamoto Takakazu
Published: March 2011

Molecular design of processable nonlinear optical (NLO) polymers is discussed for applications to devices such as ultrafast optical switches. Channel waveguides must be fabricated in order to develop polymeric NLO switching devices that can be driven with lower laser power. For these purposes, we propose molecular structures that reduce attenuation loss and allow the development of highly processable NLO polymers. Third-order nonlinear optical properties, χ(3), of the novel NLO polymers are around 10−10 esu in the 1.5 μm wavelength region. Even in off-resonant regions, the χ(3) values are around 5 × 10−11 esu. Single Mode channel waveguides of the polymers are fabricated using a standard photo-process. Attenuation loss through a film and also for a single mode waveguide at 1.32 μm wavelength is Measured. Nonlinear optical properties of novel, processable, heteroaromatic polymers with large χ(3) are also discussed. Several types of heteroaromatic polymers are developed with a structure in which π-electron excessive heteroaromatic compounds and π-electron deficient heteroaromatic compounds are combined to induce strong nonlinearity. The charge transfer between heteroaromatic rings on the π-conjugated sequences is thought to influence the properties of certain excited states that enhance χ(3) These polymers have excellent potential for use in fabrication of optical switching devices.
Second-Order Nonlinear Optical Properties Of Thiophene Containing Chromophores With Extended Conjugation
Authors: Gilmour Sandra , Jen Alex K-Y. , Marder Seth R. , Jennifer-Neissink A. , Perry Joseph W. , Skindhøj Jørgen , Cai Young Ming
Published: March 2011

The synthesis and first hyperpolarizabilities (β) of various donor-acceptor substituted thiophene containing compounds with extended conjugation are reported. Results indicate that replacing phenyl rings with less aromatic thiophene moieties enhances the second-order hyperpolarizability. Incorporating the acceptor group, N,N' diethylthiobarbituric acid, that can gain aromaticity upon charge-separation has also led to an increase in the nonlinearity. Some of the molecules have been incorporated into poIy (Methyl Methacrylate) and the electro-optic coefficients of these host-guest polymers were Measured.
Polymer Physics Of High Temperature Polymers For Second Order Nonlinear Optics
Authors: Stacey Fu C. Y. , Prdddy Duane B. , Lyle Greg D. , Mcgrath James E. , Lackritz Hilary S.
Published: March 2011

A new class of high temperature polymeric materials capable of second order nonlinear optical effects has been developed and characterized. These polymers, Bisphenol A-poly (arylene ether) phosphine oxides (Bis A-PEPO), F6 Bis A-PEPO, phenolphthalein•PEPO (PP-PEPO), and phenolphthaIein-anilide•PEPO (PP-PEPO), doped with nonlinear optical chromophores are being examined using second harmonic generation. Corona poling is used to orient the chromophores into the noncentrosymmetric structure required to obtain the second harmonic signal. These high glass transition temperature (>200°C) polymers have strong hydrogen bonding sites that can interact with the chromophores. This work describes the basic polymer physics including local Mobility, chromophore/polymer interactions, and polymer steric effects that control the thermal and temporal stability of chromophore orientation in these guest-host polymeric systems.
Novel Organic Sol-Gel Materials For Second-Order Nonlinear Optics
Authors: Jeng R. J. , Hsiue G. H. , Chen J. I. , Marturunkakul S. , Li L. , Jiang X. L. , Masse C. , Kumar J. , Tripathy S. K.
Published: March 2011

A new class of organic sol-gel second-order nonlinear optical (NLO) Materials based on hexa (Methoxymethyl) Melamine has been investigated. NLO active chromophores, 4- (4′-nitrophenylazo) aniline and 4-amino-4′-nitrobiphenyl, were incorporated into the Melamine Matrices. Samples exhibit second-order optical nonlinearity after poling and curing at 220 °C for 30 Min. The synthesis, preparation, and characterization of these materials are reported.
Laser-Induced Pretransitional Ordering And Nonlinear Optical Properties Of Rigid-Rod Polymer Suspensions
Authors: Vugmeister Boris E. , Malcuit Michelle S. , Kralik John C. , Stevens Colleen
Published: March 2011

We investigate the pretransitional behavior in laser-induced alignment of rigid rod-like polytetraflouroethylene (PTFE) suspensions. Using a laser-induced birefringence experiment, we measure both the orientational order parameter and the orientational relaxation time. We find that both increase as the volume fraction of colloidal particles approaches the critical value for the isotropic-nematic phase transition. Experimental results are compared with theory which takes into account the possibility of a first-order phase transition induced by a laser electric field.
Negative Third Order Optical Susceptibilities of Squaraines
Authors: Shi R. F. , Zhou Q. L. , Garito A. F.
Published: March 2011

Quantum Many-electron calculations based on Multiple-excited configuration interaction have been implemented on squaraine Molecules. It is found that certain squaraines possess negative third order optical susceptibilities (γ) even in the zero frequency limit. Our calculations agree well with the experimental results obtained by Dirk and co-workers[1] for anilinium squarylium (BSQ) in terms of the sign and Magnitude. High-lying two-photon states in squaraines are found to be always critically important. The sign of γ is found to depend on the electron donating ability of the two side groups in squaraines.
Demonstration of Dispersive Optical Bistability With a Third Order Nonlinear Optically Active Polymer
Authors: Druy Mark A. , He Guang S. , Casstevens Martin K. , Burzynski Ryszard
Published: March 2011

Dispersive optical bistability in a Fabry-Perot (F-P) cavity with a third order nonlinear optical polymer was demonstrated. This effort involved the development of several nonlinear optical polyimides and an evaluation of their linear optical and nonlinear optical properties. These Materials were processed into waveguiding films having good homogeniety and optical properties. Several of these films were shown to have desirable third order nonlinearities, while exhibiting good mechanical and environmental stability. Characterization of these materials involved the measurement of their absorption profiles, film thicknesses, optical waveguide propagation constants and nonlinear optical susceptibilities. Based on the measured results, a specific polyimide was chosen and used to demonstrate an all-optical instrinsic bistable device operating at a wavelength of 1.06 μ. Using ∼ 100 ps input pulse widths at 1.06 μ, dispersive optical bistability originating from the induced refractive index change, δn, due to an electronic nonlinearity of the polyimide was observed.
Novel Polydiacetylenes as Materials for Second and Third Order Nonlinear Optics.
Authors: Kim W. H. , Masse C. E. , Bihari B. , Kumar J. , Tripathy S. K.
Published: March 2011

The objective of this investigation was to develop stable nonlinear optical (NLO) Materials which possess both second and third order NLO properties. These Materials were envisioned as having a polydiacetylene (PDA) backbone with a second order active NLO chromophoric substituent consisting of a donor group, a π-conjugated bridge, and an acceptor group. The choice of a PDA backbone in this investigation was twofold. In terms of third order NLO properties, the extensive π-conjugation of the PDA backbone leads to large ultrafast third order effects. In terms of second order NLO properties, the high thermal stability and rigidity of the PDA Matrix in a perfect polymeric single crystal is expected to prevent the randomization of the NLO chromophores in the noncentrosymmetric environment. This paper focuses on the synthesis and NLO properties of the unsymmetrical diacetylene Monomers, ((9-Butoxy carbonyl) Methyl urethanyl) -l- (4-urethanyl-4'-nitrobiphenyl) -nona-2,4-diyne, and ((9-Butoxy carbonyl) Methyl urethanyl) -l- (4-urethanyl-4'-nitroazobenzene) -nona-2,4-diyne. The high entropy flexible urethanyl side group was chosen to satisfy the monomer packing requirements for polymerization and enhance the solubility of the PDAs.
Temperature Dependence of the Exciton-Exciton Annihilation Rate Constant in Poly (Di-N-Hexylsilane)
Authors: Kepler R. G. , Soos Z. G.
Published: March 2011

Fluorescence depolarization studies of polysilane chains in solution have shown that energy transfer along the polymer chains occurs for only a very short time relative to the excited state lifetime and only over short distances before the excited states become trapped in long, low-energy segments. However, in solid films we have shown in previous work that excitons are highly Mobile throughout their 600 ps lifetime at room temperature, presumably because energy transfer among neighboring, parallel chain segments becomes possible. In this paper we report that the exciton-exciton annihilation rate constant decreases by only a factor of five between room temperature and 12 K, showing that the excitons do not become trapped even at low temperatures.
Observation of Excitonic n-Strings In A Quasi-one-Deviensional Charge-Transfer Crystal
Authors: Peyghambarian N. , Meissner K. , Fluegel B. , Mazumdar S. , Guo F. , Kuwata-Gonokami M. , Sato Y. , Ema K. , Shimano R. , Tokihiro T. , Ezaki H. , Hanamura E.
Published: March 2011

Collective excitations, such as plasmons, rotons, electron-hole liquid, and excitonic Molecules, have been extensively studied in condensed Matter.1 Wannier excitons in inorganic semiconductors are bound by the exchange interactions between two electrons of the exciton, and the bound state of More than two excitons is not possible. We demonstrate here a new type of collective excitations,2 bound states of Multiple charge-transfer (CT) excitons. Coulomb interactions in one dimension are shown to bind a train of several (More than two) CT excitons. Experimental evidence for these new type of elementary excitations is reported in a quasi-one-dimensional CT crystal of anthracene PMDA. High density excitation by femtosecond light pulses generates Multi-exciton chains, which we refer to as excitonic n-strings with n = 1, 2, 3, etc., along the stack axis of the crystal. Both the n = 2 excitonic string (biexciton) and the n = 3 string (tri-exciton) are observed. This report provides evidence for an n > 2 exciton chain in this system.2 The stability of the n-string exciton is supported by our theoretical calculations based on the extended Hubbard Hamiltonian in one dimension.
Measurement of Nonlinear Index n2(λ,τ) By Using White Laser Pulses
Authors: Zhao Jianming , Qian Qiuming , Wang Zhijiang
Published: March 2011

A Modified Z-scan Method was proposed to measure nonlinear index n2 (1, t) by using white laser pulses
Novel Oxidative Nucleophilic Chemistry Enroute to The Synthesis of C60 Fullerols
Authors: Chiang Long Y. , Wang Lee Y. , Swirczewski John W.
Published: March 2011

We have demonstrated a novel cyclosulfation chemistry for the functionalization of C60 Molecules and a new route to the synthesis of fullerol. The cyclosulfation reaction of C60 was performed in neat fuming sulfuric acid at 55–60 °C under N2. Hydrolysis of these derivatives in the presence of water at 85–90 °C or in aqueous NaOH solution at ambient temperature gave the corresponding polyhydroxylated fullerenes (fullerols) in high yield. Various spectroscopie methods were used to resolve the chemical structure of fullerol. An average of 10 to 12 hydroxyl addends was found in fullerols that can be correlated to the structure of polycyclosulfated fullerene precursors containing 5 to 6 cyclosulfate units. The cyclosulfation chemistry is, presumably, initiated by an one-electron oxidation reaction on C60 Molecules, followed by the attack of nucleophilic anionie sulfate species onto the resulting cationic C60 radical intermediates. Further electron oxidation and intramolecular cyclization of the resulting hydrogen sulfated C60 yielded the desired polycyclosulfated C60 products.
Diacetylene Containing Polyesters and Segmented Polyurethanes with Body Temperature Sensitive Thermochromic Transitions
Authors: Zemach Ken D. , Rubner M. F.
Published: March 2011

Several diacetylene containing polyesters with thermochromic transitions at or around room temperature have been synthesized. Some of these polymers have been shown to be highly sensitive to body temperature, giving rise to either reversible or irreversible thermochromic changes, depending upon both the molecular structure and the processing conditions. By changing the structure and processing conditions, the thermochromic transition can be moved over a wide range of temperatures. Segmented polyurethanes using these diacetylene functionalized polyesters as soft segments and containing diacetylene groups in the hard segments have also been produced. Through a selective polymerization process, it is possible to selectively cross-polymerize the hard segment diacetylene groups or cross-polymerize both the hard and soft segments thereby making it possible to examine the effects of hard and soft segment crosslinking on mechanical properties.
Dielectric and Absorbate Effects on the Optical Properties of Phosphazenes
Authors: Ferris KIM F. , Samuels W. D. , Morita Y. , Exarhos G. J.
Published: March 2011

The optical response of polyphosphazenes can be directly related to the π (out-of-plane) and π′ (in-plane) bonding interactions intrinsic to the electronic structure of these Materials. Altering this structure either by hydrogen bonding or absórbate effects, affects both the linear and nonlinear optical susceptibilities. In this paper, we have performed electronic structure calculations on the cyclic Molecules, P3N3 (NHCH3)6, P3N3(SCH3)6, P3N3 (OCH3)6 and P4N4 (NHCH3)8 as model systems for the polymer. Charge distribution arguments are discussed to explain the influence of a polarizing electric field on the π bonding systems, and are used to suggest methods to enhance their nonlinearities.