bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of Single and Double Stranded Dna Binding to Polypyrrole.
Authors: Pande Rajiv , Lim Jeong - Ok , Marx Kenneth A. , Tripathy Sukant K. , Kaplan David L.
Published: March 2011

The polycation conducting polymer, oxidized polypyrrole (PPy), possesses the ability to form complexes with DNA. Our previously proposed diffusion limited binding model for double helical DNA was also found to be applicable to single stranded DNA in this study. Single stranded DNA was found to bind PPy at a nearly identical level to that of double helical DNA. An investigation of electropolymerized PPy film morphology using SEM revealed two distinctly differing surface morphologies for the Platinum (Pt) electrode face (smooth) and polymeric growth face (rough). The DNA uptake levels were found to be consistently different on either surface, being higher on the rough surface. DNA penetrated into the disk interior with increasing time periods of exposure while a similar phenomenon but to a lesser extent was observed for single stranded DNA.
The Enzymatic Mediated Polymerization of Phenol and Aniline Derivatives on a Langmuir Trough
Authors: Bruno Ferdinando F. , Akkara Joseph A. , Samuelson Lynne A. , Kaplan David L. , Marx Kenneth A. , Tripathy Sukant K.
Published: March 2011

Enzymatic polymerization of a number of monomers in an ordered lattice on a Langmuir trough (L-T) was investigated. The assembly and polymerization of two mixed monomeric systems consisting of 4-tetradecyloxyphenol (C14PP) with phenol, and 4-hexadecylaniline (C16PA) with aniline, in various ratios were carried out. Polymerization was obtained with C14PP and phenol (in the ratio 1:10), and C16PA and aniline (in the ratio 1:2) on buffered MilliQ water (pH 7.5) using the enzyme horseradish peroxidase (HRP) at 20°C. Polymerized monolayers were then transferred to appropriate substrates for UV-Vis, third order non linear optical (NLO) properties, and thickness measurements. Thermogravimetric analysis (TGA) was performed on the final polymers. Results suggest that the lattice controlled polymerization results in highly ordered conjugated polymers with improved functional, electronic, NLO and processability properties, which form a basis for intelligent materials design and applications.
Formation of Silk Monolayers
Authors: Muller Wayne S. , Samuelson Lynne A. , Fossey Stephen A. , Kaplan David L.
Published: March 2011

Cast silk membranes exhibit useful properties. However, there is limited control over the molecular architecture in these structures. The Langmuir-Blodgett technique can enhance the control of the membrane structure and allow improved control over membrane properties. We have formed natural silk monolayers using the Langmuir technique. Silk fibroin, regenerated from Bombvx mori cocoons, formed stable monolayers evident from pressure/area isotherms. Multilayers of the silk fibroin monolayers were deposited on a number of substrates and characterized. Transmission Electron Microscopy (TEM) and ellipsometry data provide basic information about the physical characteristics of the monolayer. Preliminary analysis of electron diffraction data of the monolayer indicate polycrystalline structure, consistent with the known structure of silk. Infrared spectrometric analysis of the monolayer using Attenuated Total Reflectance (ATR) gave wavenumbers for Amide I, II, III and V bands which compare with the silk II conformation reported for cast silk membranes.
The Physical Properties of a Hyaluronic Acid Based Bioresorbable Membrane for the Prevention of postsurgical Adhesions
Authors: Greenawalt K. , Masi L. , Muir C. , Burns J.
Published: March 2011

We have evaluated the physical properties and animal efficacy of a hyaluronic acid (HA) based bioresorbable membrane for the prevention of post-surgical adhesions. Test methods were developed to measure the dry and wet tensile properties and ia Xilm tissue adhesiveness of the membranes. The thin membranes were found to have sufficient strength and flexibility in the dry state for surgical handling. When hydrated in buffered saline, the membranes became weaker and more elastic. The membranes exhibited a high degree of tissue adhesiveness and significantly reduced adhesion formation in a rat cecal abrasion model.
Preliminary Characterization of Resilin Isolated from the Cockroach,Periplaneta Americana
Authors: Lombardi Elizabeth Craig , Kaplan David L.
Published: March 2011

We would like to mimic the mechanical properties of animal systems for the development of novel materials. Insect cuticle serves as one source of inspiration for the design of these materials. Cuticle is composed of chitin embedded in a protein matrix which may also contain plasticizers, fillers, crosslinkers, and minerals. The specific properties of the cuticle depend on the type, amount and interactions between each component. We are renewing the investigation of the elastic cuticle, resilin. Resilin, a protein-based elastomer first described in the early 1960s, has properties which have been reported to be most like those of ideal rubbers. We have examined resilin isolated from the prealar arms of the cockroach,Periplaneta americana. The results of amino acid analysis are in good agreement with earlier data reported for resilin. A series of tryptic fragments have been isolated and sequenced. These peptides have been used for the design of oligonucleotide probes for the identification of the gene(s) from a teneral cockroach cDNA library. A biopolymer, based on one tryptic fragment, has been designed and synthesized. We are continuing to treat resilin with residue specific proteases in order to map the resilin protein.
The Adhesive Glycoprotein of the Orb Web ofArgiope Aurantia(Araneae, Araneidae)
Authors: Tillinghast Edward K. , Townley Mark A. , Wight Thomas N. , Uhlenbruck Gerhard , Janssen Eveline
Published: March 2011

A phosphorylated, glycoprotein preparation has been obtained from orb webs of the araneid spiderArgiope aurantia. This preparation probably contains proteins from more than one gland type, but resolution of these proteins has not yet been achieved. Nevertheless, a major component appears to be the adhesive glycoprotein(s) from the adhesive spiral. A product of the aggregate glands, this glycoprotein(s) occurs as discrete nodules along the core fibers of the adhesive spiral, within the viscid, aqueous droplets.

The glycoprotein preparation has a high apparent molecular weight (> 200 kDa) and is polydisperse. The only monosaccharide constituent identified by gas-liquid chromatography or in lectin studies isN-acetylgalactosamine and this is at least primarilyO-linked to threonine. By electron microscopy, linear, unbranched and apparently flexible filaments are observed. Phosphorylated serine and threonine residues are present in the preparation and glycine, proline and threonine together account for about 57 mole % of the preparation's amino acid content. Thus, in some, but not all, respects, this glycoprotein preparation is reminiscent of a secretory mucin.
Luminescent Properties of Polymer Electrolytes Modified by a Rare-Earth Salt
Authors: Carlos L.D. , AssunÇĀO M. , Abrantes T.M. , Alcacer L.
Published: March 2011

The excitation and fluorescence spectra of PEOx EuBr3 (x=8, 16, 20, 24, 32) and PPO32 EuBr3 were recorded and analyzed in terms of the barycenters of each level of the ground septet of Eu3+. The site symmetry of the Europium ion is tentatively assigned to the C1,2, C5 or C2v cases. The maximum fluorescence intensity for each composition, together with the measured values for the lifetimes, suggest the possibility of these two polymeric electrolytes being used as fluorescent materials.
Concerted Ion and Electron Transfer Across Electronically Conductive Polymer Membranes
Authors: Martin Charles R. , Lawson Del R. , Liang Wenbin
Published: March 2011

We describe in this paper an experiment involving an electronically conductive polymer (ECP) which, to our knowledge, has not been described previously. A free-standing ECP (polypyrrole)-based membrane separates a solution of an electron donor from a solution of an electron acceptor. Because the ECP is both electronically and anionically conductive, the membrane can transport electrons from the donor solution to the acceptor solution, and anions in the opposite direction, such that a sustainable electron-transfer reaction is driven across the ECP membrane. We demonstrate such transmembrane electron/ion-transfer processes using both an inorganic and a biochemical electron donor/acceptor system. The biochemical case is of particular interest because we show that the reduced form of the enzyme glucose oxidase can give its electrons directly to the synthetic metal. Direct electron transfer is usually not possible at inorganic metals.
Cell Attachment and Protein Adsorption to Polypyrrole thin Films
Authors: Wong Joyce Y. , Langer Robert S. , Ingber Donald E.
Published: March 2011

Thin films of polypyrrole were synthesized using both chemical oxidative and electrochemical methods. The resulting oxidized films were characterized by UV/VIS spectroscopy, contact angle and conductivity measurements. In vitro studies suggest that extracellular matrix molecules, such as fibronectin, adsorb efficiently onto polypyrrole thin films and that 3T3 Balb/c mouse fibroblasts attach and spread normally on polypyrrole.
Voltammetric Studies of Poly(carbon disulfide)
Authors: Geng L. , Lee H.S. , Xu J. , McBreen J. , Prasad Sathya , Skotheim T.A.
Published: March 2011

Poly(carbon disulfide) was studied by cyclic voltammetry using glassy carbon and platinum macro- and microdisk electrodes. The electron transfer kinetic is significantly faster at glassy carbon electrodes than at Pt electrodes. It is chemically reversible with moderate electron transfer rates. Voltammetric results of poly(carbon disulfide) are in good agreement with our battery testing data. The k0 value measured at a Pt microdisk electrode is 7×10−3 cm/sec. Electrochemical data suggest that PCS can be a potential cathode material for low current density lithium batteries.
Decomposition of Universal AC Ionic Conductivity Response into Two Ranges with Constant Exponents
Authors: Lim B.S. , Nowick A.S.
Published: March 2011

Many disordered materials both crystalline and glasses show universal dynamic response [1], which is characterized by appropriate power-laws. The ac-conductivity as a function of frequency, is given by: σ(ω)=σ(0)+ Aωs where σ(0) is the dc-conductivity and the exponent s (≤1) generally decreases with increasing temperature. In previous work on ionically conducting crystals and glasses, we have observed two ranges in which s = constant, independent of temperature: at high temperatures, s=so (a value between 0.5 and 0.6) and, at low temperatures, s = 1. Further for the s = so region, the parameter A is activated with activation energy related to that of σ(0), while for the s = 1 region, A varies only slowly with temperature. For the cases of borate and silicate glasses as well as for crystalline CaTiO3 doped with 30% A13+, careful analysis was carried out over the region in which the effective s falls from 1.0 to so. The results show that this region can be described by the relation: σ(ω)= σ(0) + Aωs0 + A'ωl.0 with parameters A and A’ consistent with those obtained from the high and low- temperature regions, respectively. Thus, we conclude that two separate types of universal behavior are superimposed throughout the entire temperature region; each is described by a constant value of s, and is due to an independent mechanism of non-Debye relaxational behavior.
Molecular Dynamics Simulation of Aerogel Silica on Parallel Computers
Authors: Nakano Aiichiro , Kalia Rajiv K. , Vashishta Priya
Published: March 2011

Molecular dynamics (MD) simulations of porous silica, in the density range 2.2 - 0.1 g/cm3, are carried out on a 41,472 particle system using two- and three-body interatomic potentials. Calculated results for fractal dimension and small-angle neutron scattering data are in good agreement with neutron scattering experiments. Results for structural correlations reveal crossovers from the short- to intermediate range (< 8 Å) and fractal to large-scale regime (10 ∼ 100 Å).

The MD program simulations are carried out on distributed-memory MIMD computers using a domain-decomposition algorithm. The algorithm employs the linked-celllist method and separable three-body force calculation. The force calculation is accelerated by the multiple-time-step method. The parallel algorithm is highly efficient (parallel efficiency = 0.974), as it involves only 3 % communication overhead.
Is the Rigid-Band Model Applicable in Lithium Intercalation Compounds?
Authors: Julien C. , Balkanski M.
Published: March 2011

Numerous layered compounds are interesting materials in which lithium intercalation occurs primarily without destruction of the host lattice. In many cases a rigid-band model is a useful first approximation for describing the changes in electronic properties of the host material with intercalation. This paper presents some recent experimental results obtained on transition-metal chalcogenide compounds and on transition-metal oxides as well. We shall observe, nevertheless, that the rigid-band model is not applicable to all of the intercalated materials. The applicability of the rigid-band model may be used as a test for the most desirable properties of a good intercalation material. This needs to be more extensively documented for their possible applications as insertion electrodes in solid state batteries.
Nafion® Membranes: Molecular Diffusion, Proton Conductivity and Proton Conduction Mechanism
Authors: Kreuer Klaus-Dieter , Dippel Thomas , Meyer Wolfgang , Maier Joachim
Published: March 2011

H-PFG-NMR diffusion and proton conductivity data for NAFION® 117 is presented as a function of temperature and water content. From this, H2O diffusion on a molecular scale is concluded to be similar to the one in bulk water. But “geometrical” restrictions of diffusion lead to a decreasing pre-exponential factor, i.e. decreasing macroscopic diffusion coefficient with decreasing water content. It is shown that proton conductivity is correlated with water diffusion for low degrees of hydration (vehicle mechanism). With increasing hydration there is an increasing contribution from “structure diffusion” culminating in an amplification factor of A = 2.5 for fully hydrated protonic NAFION® 117 at room temperature. The bonding of water in NAFION® as well as the local environment (hydration) of the acidic proton are concluded to be similar as in acidic aqueous solutions.
Proton Conducting Solid Electrolytes for High Temperature Humidity Sensing
Authors: Greenblatt Martha , Feng Shouhua
Published: March 2011

The properties of various proton conducting solid electrolytes were investigated by electrochemical galvanic cell and ac impedance type measurements for applications as high temperature (T>200°C) humidity sensors. The basic characteristics of proton conductors required for humidity sensing are reviewed. Results of ionic conductivity and EMF measurements and their implications for the mechanism of ion transport are discussed for two prototype ceramic humidity sensors: β-Ca(PO3)2 and a NASICON-type ceramic composite, HZr2P3O12/ZrP2O7.
X-Ray Crystal Diffraction Study of Zr,Na-β-Alumina
Authors: Pearson Wayne H. , Lomax Joseph F.
Published: March 2011

The reaction of crystalline lithium-stabilized Na-β”-Alumina with zirconium(IV) chloride at 400°C for 303h has caused sodium ions to exchange with zirconium ions. An X-ray crystal diffraction study of both the parent lithium stabilized Na-β”-alumina (1) and the partially zirconium exchanged product (2) have been made at 147 K. Crystal data for 1: rhombohedral, R 3 m, a = 5.6037 (3), c = 33.6210 (86) Å, V = 914.31 (24) Å3. Crystal data for 2: rhombohedral, R 3 m, a = 5.5994 (5), c = 33.7711 (51) Å, V = 916.96 (16) Å3. Zirconiums in 2 reside, predominately, in the Beevers-Ross type site.
Fast-Ion Conduction and Disorder in Cation and Anion Arrays in Y2(Zry Ti1−y)2O7 Pyrochlores Induced by Zr Substitution: a Neutron Rietveld Analysis
Authors: Heremans C. , Wuensch B.J. , Stalick J.K. , Prince E.
Published: March 2011

Structural refinements have been carried out on the Y2(ZryTi1−y)2O7 system for increasing amounts of Zr substitution. The structure changes from an ordered pyrochlore structure (superstructure of the fluorite structure) to an anion-deficient fluorite structure. Separate processes govern the disordering of the cation and anion arrays. In particular, the anions begin to disorder at lower values of y than the cations. For intermediate compositions, a minor amount of fluorite phase coexists with the primary partially-disordered pyrochlore phase.
XPS Spectra and Optical Reflectivity of Yttrium Stabilised Zirconia
Authors: Camagni P. , Depero L.E. , Parmigiani F. , Samoggia G. , Sangaletri L. , Scagliotri M.
Published: March 2011

X-ray photoelectron spectroscopy measurements and synchrotron radiation reflectivity and measurements in the photon energy range between 5eV and 30 eV are reported for yttrium stabilised zirconia single crystals. From the present data some behaviour of the valence band and conduction band electronic structure are elucidated. Two features present in the valence band XPS spectra, not explained on the basis of one-clectron theory, are tentatively assigned to simultaneous two electron excitation.
Potassium Beams from Solid State Electrochemical Cells
Authors: Bruce P.G. , Roy S. , Stewart H. , Prior K. A.
Published: March 2011

A solid state electrochemical cell, K1-xFe11O17/potassium beta-alumina/metal gauze, is presented which is capable of producing a beam of potassium vapour. The generation of potassium vapour has been demonstrated and the cell has been used to dope ZnSe during growth of the chalcogenide by molecular beam epitaxy, thus forming p-type ZnSe. The electrochemical source has the advantage that the potassium flux is controlled by the current passing through the cell and eliminates the need to handle potassium metal. Cyclic voltammetric measurements at 350'C suggest that potassium metal is first generated electrochemically by a mechanism of nucleation and growth, then evaporates from the cell.
AFM Imaging of the Crystalline-To-Amorphous Transition On the Surface of Ion-Implanted Mica
Authors: Eby Ray K. , Henderson Grant S. , Wicks Fred J. , Arnold George W.
Published: March 2011

Scanning microscopy is the only technique available for directly imaging the short-range structural features of the crystalline-to-amorphous (C-A) transition. Cleaved sheets of muscovite were implanted with 600 keV argon ions in order to induce radiation damage; thus producing the C-A transition. AFM images of Ar-implanted muscovite show the surface structural features of the C-A transition. Low-resolution images show a progressive increase in the surface roughness of muscovite with increasing ion dose, as seen by the development of hummocky microtopography, whose individual hummocks are 25 nm across. High-resolution AFM images of unimplanted muscovite reveal a highly crystalline structure, as shown by the hexagonal arrangement of the (SiO4)-tetrahedral layers. By scraping through the uppermost surface layer of the implanted samples, using a cantilever forces of 50 - 100 nN, the C-A transition is recognized by a decrease in the long-range order of the tetrahedral layers. Accordingly, crystal defects and highly disordered regions increase in frequency and size with Ar-dose. At the highest Ardose, disordered regions dominate the structure, lending complications to AFM image interpretation. Surface hardness decreases with higher radiation doses, as evidenced by the greater ease of scraping through atomic layers in the more damaged samples.
AFM Imagings of Ferritin Molecules Bound to LB Films of Poly-1-Benzyl-L-Histidine: Imaging the ordered arrays of water-soluble protein ferritin with the atomic force microscope
Authors: Ohnishi Satomi , Hara Masahiko , Furuno Taiji , Knoll Wolfgang , Sasabe Hiroyuki
Published: March 2011

A water-soluble protein, ferritin, on a silicon surface has been imaged in pure water at room temperature with the atomic force microscope (AFM). The samples were prepared by binding ferritin molecules electrostatically to a charged polypeptide layer of poly-l-benzyl-L-histidine (PBLH). The hexagonal arrangement of ferritin molecules was imaged with high reproducibility, since the force between tip and the sample surface could be kept sufficiently lower than 10-10 N. The applied force can be stabilized and weakened mainly due to a “self-screening effect” of the surface charges of the ferritin-PBLH layer. We demonstrate that the electrostatic-binding sample preparation is one of the suitable methods for soft biological specimens to achieve the nondestructive. low-force AFM imagings.
Surface Structure of Oxide Catalyst Microcrystals: High Resolution Electron Microscopy Study
Authors: Kryukova G. N. , Chuvilin A. L. , Sadykov V. A.
Published: March 2011

The surface structure of “pure” oxides as well as the same materials after promotion by alkali metals, models of the oxide surface based on results of image simulation procedure have been proposed. The surface structure reconstruction of the catalysts upon electron beam irradiation was also observed. For the specimens under investigation surface steps and kinks were shown to be essential for strong interaction between oxides and supported Pt particles preventing their migration and suppressing the sintering process.
In-Plane Asymmetries on the Ge(111)-c(2×8) Surface Mapped with the Scanning Tunneling Microscope
Authors: Molinàs-Mata P. , Zegenhagen J. , Böhringer M. , Takeuchi N. , Selloni A.
Published: March 2011

We report on new experimental studies of the Ge(111)-c(2×8) reconstruction performed with low-energy electron diffraction. (LEED) and scanning tunneling microscopy (STM). Weak quarter-order reflections are present in the c(2 × 8) LEED pattern in agreement with previous observations and results of ab initio calculations. In order to gain insight into the predicted splitting of dangling bond states, we compare constant current topographs (CCT's) performed at high-tunneling currents (40.nA) with first-principles calculations of the local density of states (LDOS) 1Å above the surface adatoms and obtain good qualitative agreement. We finally discuss to what extent the STM CCT's at high tunneling currents (small sample-tip distances (STD)) are sensitive to surface states outside the Г point.
Effect of the Angular Momentum and Magnetic Quantum Numbers on Auger and Photoelectron Scattering
Authors: Ramaker D. E. , Yang H. , Idzerda Y. U.
Published: March 2011

Angle-resolved electron intensity distributions for Auger and photo- electrons are calculated using a curved-wave electron scattering formalism and compared with experiment. The distributions are found to depend significantly on both the ℓ and mℓ quantum numbers of the scattered electron. We consider the simple metals Ir and Al. Although the intensity patterns depend strongly on the ℓ quantum number at relatively low energy, even at high energies (around 1500 eV), a significant ℓ effect remains, which we find arises from nearest neighbor single scattering and from the multiple scattering or defocussing effect. The relative contribution of each ℓ and mℓ partial wave is determined not only by the quantum mechanicaf matrix elements, but also by the energy, polarization, and direction of the excitation beam.
Atomic Force Microscopic Imaging of Biologically Important Materials
Authors: Siperko Lorraine M. , Landis William J.
Published: March 2011

Atomic force microscopy (AFM) has been used to image the surface atomic structure of hydroxyapatite [Ca10(PO4)6 (OH)2], HA, and brushite [CaHPO4 · 2H2O], DCPD. Compared to HA, the surface of DCPD was found to be much less complex. Identification has been made of one crystal plane of DCPD that has atoms commensurate with those of one of the observed crystal planes of HA.
Characterization of Internal Interfaces by Atom Probe Field Ion Microscopy
Authors: Miller M. K. , Jayaram Raman
Published: March 2011

The near atomic spatial resolution of the atom probe field ion microscope permits the elemental characterization of internal interfaces, grain boundaries and surfaces to be performed in a wide variety of materials. Information such as the orientation relationship between grains, topology of the interface, and the coherency of small precipitates with the surrounding matrix may be obtained from field ion microscopy. Details of the solute segregation may be obtained at the plane of the interface and as a function of distance from the interface for all elements simultaneously from atom probe compositional analysis. The capabilities and limitations of the atom probe technique in the characterization of internal interfaces is illustrated with examples of grain boundaries and interphase interfaces in a wide range of materials including intermetallics, model alloys, and commercial steels.
The Origins of High Spatial Resolution Secondary Electron Microscopy
Authors: Scheinfein M. R. , Drucker J. S. , Liu J. , Weiss J. K. , Hembree G. G. , Cowley J. M.
Published: March 2011

The secondary electron generation process is studied in an ultra-high vacuum scanning transmission electron microscope using electron coincidence spectroscopy. Production pathways for secondary electrons are determined by analyzing coincidences between secondary electrons and individual excitation events. The ultimate spatial resolution available in scanning electron microscopy is limited by the delocalization of the secondary electron generation process. This delocalization is studied using momentum resolved coincidence electron spectroscopy. The fraction of secondary electrons resulting from localized excitations can explain the high spatial resolution observed in secondary electron microscopy images.
New a-Si:H Photo-Detectors for Long-Term Charge Storage
Authors: Lee H. , Cho G. , Drewery J.S. , Hong W.S. , Jing T. , Kaplan S.N. , Mireshghi A. , Perez-Mendez V. , Wildermuth D.
Published: March 2011

Using the high light absorption properties of amorphous silicon, we developed a new device configuration that can detect photons and store the induced charges for relatively long time. This device, coupled to a scintillator such as CsI(Tl) in an array form, could be used as a scintillation camera, or for long-term photo-detection such as radionuclide labeled chromatography. The detector has a simple sandwich structure consisting of a scintillator followed by a top metal layer, p-i-n layers of hydrogenated amorphous silicon (a-Si:H), a second metal layer, a thin insulating layer and a bottom metal layer. The electron-hole pairs generated in the i-layer by the interaction with the incident light will be separated by the imposed electric field and be stored in the central metal-insulator interface. Readout will be done by switching the external bias to ground after the storage time, which depends on the needs for the specific application. Prototype devices were fabricated and tested. The performances of the devices were analyzed in connection with the storage time and the background signal produced by the thermally generated charges.
An Examination of H Effusion in a-Si:H Using Infrared Spectroscopy
Authors: Mahan A.H. , Johnson E.J. , Webb J.D.
Published: March 2011

We present the results of H diffusion studies on glow discharge (GD) and hot wire (HW) deposited a-Si:H films studied by infrared (IR) spectroscopy. In this technique, a-Si:H films deposited on crystalline silicon are annealed isochronally for fixed times, and after each anneal the decrease in the amount of SiH infrared absorption is carefully measured. We then incorporate these data into the rate equation for loss of SiH bonds due to annealing, enabling the determination of an activation energy to break these bonds and place the H in a site that cannot be measured by IR. The activation energies for device quality GD films are similar to those found in the a-Si:H diffusion literature, suggesting that diffusion parameters can be measured by this method. On the other hand, a similar analysis of data for HW films containing similar H contents is not clear cut. GD and HW samples with lower H content, deposited at higher Ts, are also examined. We discuss these results in the context of film microstructure.
Laser Dehydrogenation of PECVD Amorphous Silicon
Authors: Mei Ping , Boyce J.B. , Hack M. , Lujan R.A. , Johnson R.I. , Anderson G.B. , Fork D.K. , Ready S.E. , Smith D.L.
Published: March 2011

A low temperature process for laser dehydrogenation and crystallization of hydrogenated amorphous silicon has been studied. The key feature of this process is the removal of hydrogen from the amorphous silicon thin films while crystallizing the films at the same time. Studies of transient phenomena, hydrogen loss, and crystallinity, using transient reflectivity analyses, transmission electron microscopy and quadrupole mass spectrometry, find that hydrogen out-diffusion depends strongly on film structure and the melt duration controlled by the laser energy density. Utilizing this process, for which the maximum temperature is 350 °C, both high quality polycrystalline and amorphous silicon TFT's have been fabricated on the same Corning 7059 glass substrate.
Evidence for Different Kinds of Dangling Bond Defects in a-Si:H
Authors: Tran Minh , Fritzsche H. , Stradins P.
Published: March 2011

Comparing the photoconductivity σp of undoped samples with different native defect concentrations we find that light-induced metastable defects decrease the electron lifetime more strongly than the native defects. We discuss the differences between native and metastable dangling bond defects. We find that the constant photocurrent method underestimates the defect concentration in undoped a-Si:H.
Signature of the Weak Bond-Dangling Bond Conversion Process in a-Si:H as Seen by Total Photoelectron Yield Spectroscopy
Authors: Graf W. , Leihkamm K. , Ristein J. , Ley L.
Published: March 2011

Photoelectron Yield Spectroscopy is one of the most direct methods to study the density of gap states of amorphous semiconductors. We have made use of it to investigate the transient changes of the density of gap states of a-Si:H upon illumination with above-band-gap light. An excitation density of 50 mW/cm2 of 532 nm light from a frequency doubled Nd-YAG laser was modulated with frequencies between 0.1 Hz and 10 KHz and the transient response of the photoelectron yield signal was monitored by gated electron counting or a lock-in amplifier. From this we deduce a reversible increase of the occupied density of defect states under illumination with a maximum Δg ∼ 1017 cm3 eV-1 at about 0.3 eV below the Fermi energy and a decrease below EF− 0.75 eV, i. e. in the region of deep tail states. The effect exhibits a wide spectrum of time constants involved. Based on additional transient surface photo voltage measurements as well as numerical simulations we argue that the observed redistribution of states is not due to a light induced change in surface band bending or due to a mere excitation of electrons from occupied to empty states. Instead, we interpret the observed effect as a conversion of weak to dangling bonds, i.e. a precursor of the Staebler-Wronski Effect, lacking however the final step of hydrogen stabilisation necessary for the creation of metastable defects.
Photoluminescence and Light-Induced ESR in a-Si:H Studied with Subgap Excitation
Authors: Saleh Rosari , Ulber Isabell , Fuhs Walther
Published: March 2011

We have studied the influence of the photon energy of the exciting light on the distribution of the carriers in the gap of a-Si:H by light-induced electron spin resonance (LESR) and photoluminescence (PL). In the LESR spectra the number of spins in the broad line (h) is larger than in the narrow line (e or e+db) by a factor of 2-3. For excitation in the bulk of the sample this asymmetry does not depend on the light intensity and photon energy, which suggests that it is not possible to relate the asymmetry to a specific defect structure. LESR and PL show that both bandtails can be populated by absorption of IR light (1060 nm) by optical transitions from occupied and into empty defect states. The recombination kinetics suggest that these absorption processes occur as a two-step process at the same defect
Structural Equilibration in Pure and Hydrogenated Amorphous Silicon
Authors: Müller Gerhard , KrÖtz Gerhard
Published: March 2011

Chemically pure (a-Si) and hydrogenated amorphous silicon (a-Si:H) are metasta- ble materials which are thermodynamically unstable with respect to crystalline silicon (c-Si). In both materials, however, partial thermal equilibria can be established between certain structural, configurational and electronic degrees of freedom. The present paper discusses experiments on both amorphous (a-) materials showing that two kinds of structural change can take place within random Si networks: structural relaxation and configurational equilibration. The first process can be observed in both materials indicating that it is supported by intrinsic degrees of freedom of the random Si networks. During these changes partial thermal equilibria between distorted and broken bonds are established via irreversible and relatively long-range relaxation processes. The second kind of change can only be observed in a-Si:H, indicating that it is H-related. The H-related degrees of freedom support reversible valence alternation reactions in which the local bonding configuration of the dopant and defect sites is changed and in which their charge states are altered. These latter interactions establish a strong coupling between the electronic system and the configurational degrees of freedom of the random Si networks. Formally, these latter changes bear strong similarity to the electrochemical processes that take place in liquid electrolytes.
Ellipsometry Studies of (μc-Si:H/ZnO) and (μc-Si:H/a-Si:H) Interfaces in Magnetron Sputtering System
Authors: Yang Y.H. , Katiyar M. , Abelson J.R. , Maley N.
Published: March 2011

We have studied the microstructure of μc-Si:H/ZnO and μc-Si:H/a-Si:H interfaces in reactive magnetron sputtering (RMS) using in situ spectroscopic ellipsometry (SE). For (μc- Si:H deposited on ZnO, the real time ellipsometry trajectory shows that there is 20 volume % void apparent inside the first 100Å film. Then the film becomes densified, the initial void layer propagates as a surface layer, and its thickness decreases to 53Å when the bulk film reaches 200Å. Both unhydrogenated and hydrogenated silicon films deposited on ZnO show similar nucleation and coalescence behavior. This indicates that the atomic hydrogen rich plasma under μc-Si growth conditions doesn't reduce the ZnO substrate. For the μc-Si:H/a-Si:H study, we also observe the delayed crystallite nucleation after a ∼ 100Å high H content interface layer forms. Part of this layer results from H implanted into the a-Si:H substrate (∼ 45Å deep). The thickness of this interface layer decreases as the crystallite nucleation begins. This is the first direct experimental evidence showing sub-surface μc-Si:H formation.
Hydrogen and Lithium Diffusion in Amorphous Silicon
Authors: Beyer Wolfhard , Zastrow Uwe
Published: March 2011

Diffusion processes of hydrogen and lithium in amorphous silicon are compared. While similarities in the diffusion energies and in a doping dependence exist, the charge state of the diffusing particles and the type of bonding of diffusing particles in traps differ. In case of Li diffusion, oxygen contamination plays an important role.
Annealing of Ion-Implanted Hydrogenated Amorphous Silicon: Stable and Removable Damage
Authors: Berntsen A.J.M. , Stolk P.A. , VAN DER WEG W.F. , Saris F.W.
Published: March 2011

Hydrogenated amorphous silicon (a-Si:H) films were irradiated with 1-MeV Si+ ions. The accumulation and annealing of ion damage was investigated by Raman scattering, optical reflection and transmission, and conductivity measurements. For damage levels up to 0.003 displacements per atom, electrical defects are created with no measurable effect on the structural properties. These defects can be completely annealed out at 180°C. Further irradiation results in an increase in the average bond-angle variation in the films. This structural disorder causes a decrease of the optical band gap with 0.46 eV. The structural changes caused by high-dose implantation can not be reversed by annealing at 180° C, which results in the formation of anneal-stable electrical defects.
Inhomogeneity in the Network Order of Device Quality a-Si:H
Authors: Morell G. , Katiyar R.S. , Weisz S.Z. , Gomez M. , Balberg I.
Published: March 2011

In this paper we show that the degree of order of the Si network in a-Si:H is increasing with two length scales from the surface into the bulk. The major manifestation of the disorder is the variation in the Si-Si bond-stretching rather than the variation in the width of the dihedral angle distribution. The results are interpreted in terms of the decrease of the hydrogen concentration from the free surface into the bulk.
Impurity Content and Defect Density in 42 a-Si:H Films
Authors: Nakata Masami , Wagner S. , Magee C.W. , Peterson T.M. , Park H.R
Published: March 2011

In an earlier study of 37 a-Si:H films we identified positive correlations between the saturated light-induced defect density Nsat, the hydrogen content CH measured by IR absorption, and the optical gap. Impurities also have been implicated in the production of metastable defects. Yet no conclusive evidence has been produced that non-dopant impurities cause defects. This situation led us to analyze our samples for impurity content. Our present 42 samples were made in six laboratories by ten combinations of deposition technique. The concentrations of H, B, C, N, O, F, Na, Cr, and Ge were determined by secondary ion mass spectroscopy. Na, Cr, or Ge were not detected. The content of C, N and O varies by a factor ∼1,000 and depends on deposition system and source gas. We report a comprehensive evaluation of our data with emphasis on their correlation with optoelectronic properties. We find the annealed state defect density Nann, and the saturated light-induced defect density Nsat surprisingly independent of impurity content.
Numerical Simulation and Experimental Investigation of the Time-Of-Flight Technique Applied to a-Si:H/a-SiGe:H-Heterojunctions
Authors: Brüggemann Rudi , Bernhard Norbert , Main Charles , Bauer Gottfried H.
Published: March 2011

We report on a comparative study, employing the TOF-technique for the characterization of the properties of a-Si:H/a-SiGe:H-heterojunctions. Both the simulated and experimental electron current transients exhibit a pronounced difference depending on the direction of movement of the excess carriers (from the a-Si:H into the a-SiGe:H or vice versa). For the movement from the low bandgap a-SiGe:H into the a-Si:H we find a char¬acteristic increase in the transient current at room temperature, which, as the simulation can reveal, is attributed to the higher drift mobility in the a-Si:H. A drop in the current is observed when the direction for the transit is from a-Si:H to a-SiGe:H. The post-transit behaviour is dominated by the large amount of trapped carriers which remain on the SiGe side of the sample. The situation for a high barrier between a-Si:H and a-SiC:H and the influence of various parameters on the shape of the current transient are discussed.
Electron Time-of-Flight Measurements in a-Si1−XCX:H
Authors: Wang Qi , Schiff Eric A. , Li Yuan-Min
Published: March 2011

We have measured the temperature-dependent electron drift mobility in a series of hydrogenated amorphous silicon-carbon alloys using time-of-flight. The specimens were prepared at Solarex using the gas mixture procedures which have recently yielded improvement in the solar conversion efficiency of wide bandgap solar cells. As the bandgap increased due to carbon alloying the electron drift mobility decreased by as much as a factor 30 at some temperatures. The cells with 1.75 eV, 1.81 eV, and 1.87 eV bandgaps had thermally activated drift mobilities over the temperature range 120 K ‐ 200 K; this is associated with simple multiple-trapping behavior. Specimens with bandgaps near 1.90 eV did not have simply activated drift mobilities; we have not accounted for this behavior, but it suggests that the bandtail broadening description used to account for the effects of germanium alloying on the electron drift mobility may not be simply applicable to carbon alloying.
Revealing Mysteries of Hall Experiments on a-Si:H and a-SiC:H
Authors: Nebel C. E. , Street R.A
Published: March 2011

Hall experiments performed on intrinsic, phosphorus and boron doped a-Si:H and a-SiC:H in the temperature regime 200 K ≤ T ≤400 K are introduced and discussed. The data confirm the double sign anomaly meaning the Hall coefficient is p-type on intrinsic and phosphorus doped a-Si:H and a-SiC:H, and n-type on boron doped a-Si:H. The Hall mobilities, μH, are significantly smaller than the drift mobilities, decreasing with increasing doping and/or carbon content. μH of holes is about half that for electron which indicate that μH scales approximately with 1/Eg where Eg is the tail slope. The presented interpretation of the Hall coefficient, that is introduced to be a function of Hall- and drift-mobility, enables the accurate determination of the carrier density in intrinsic and doped amorphous silicon and alloys from Hall data.
Recombination of Excess Carriers at High Excitation Density in Amorphous Silicon
Authors: Kunst M. , Haffer C. , Swiatkowski C.
Published: March 2011

Excess electron-hole recombination at high excitation densities in a-Si:H is investigated by comparison of the transient photoconductivity during and just after excitation in the nanosecond time range to numerical models. A simple one parameter recombination model fits the experimental data satisfactorily. An extension of the model where deep trapping is taken into account explains also the transient photoconductivity in Boron doped a-Si:H.
Monte-Carlo Simulation of Energy Relaxation of Interacting Carriers in a-Si:H Under Arbitrary Electric Fields
Authors: Cleve B. , Hess R. , Baranovskii S.D. , Thomas P.
Published: March 2011

A new Monte-Carlo simulation algorithm has been applied to verify the concept of the effective temperature recently suggested as a description of energy relaxation of carriers in band tails of amorphous semiconductors under the presence of an applied electric field. The algorithm allows the simulation with arbitrary applied field and finite temperature. The results of the simulation agree favourably with the theoretical prediction.
Ambipolar Diffusion Coefficients in a-SiC:H Alloys in Steady- State and Transient Grating Measurements
Authors: Weinert H. , Petrauskas M. , Kolenda J. , Galeckas A. , Wang F. , Schwarz R.
Published: March 2011

Recently we have determined surprisingly large values of the ambipolar diffusion coefficient D of 3 − 9 cm2/s in amorphous silicon-based alloys and a-Si:H/a-SiC:H multilayer structures from transient grating decays in the psec time domain. The steady-state photocarrier grating method, however, resulted in much lower D values (∼10−4 cm2/s) in the same samples. Since high carrier densities of typically 1019 cm−3 are reached in the psec domain, the Einstein relation may no longer be valid. The large diffusivity of non-equilibrium carriers decreases, however, rapidly in time due to energy relaxation and carrier recombination until a stable trap occupation under steady-state condition is reached.
Study on Structural, Electrical and Optical Properties of Microcrystalline Si:H and SiC:H Films
Authors: Demichelis F. , Crovini G. , Pirri C.F. , Tresso E. , Rubino A. , Nobile G. , Terzini E. , Conte G.
Published: March 2011

We report results on a study on μc-Si:H and (μc-SiC:H films deposited by PECVD. The crystallinity fraction and the crystal sizes have been evaluated by X-ray diffractometry, Raman spectroscopy and Transmission Electron Microscopy(TEM). Infrared vibrational spectra of both μc-Si:H and |μc-SiC:H samples have been studied to obtain information on their structure. A comparison between the structure of the amorphous and diphasic amorphous-microcrystalline samples has been performed. Optical properties were obtained by transmission-reflectance and PDS measurements. Electronic transport mechanisms through the conductivity measurements in a wide range of temperatures (50-500 K) have been defined. Structural and electron density models have been discussed and used to interpret the experimental results.
Excimer Laser Amorphous Silicon Film Crystallization: A Study of Time Resolved Reflectivity Measurements
Authors: Summonte C. , Bianconi M. , Govoni D.
Published: March 2011

Time Resolved Reflectivity during XeCl pulsed laser irradiation of amorphous silicon films deposited on glass was measured. Simulation of the process by a Heat Flow Calculation in which explosive crystallization was not forced to occur, predicts the coexistence of partial bulk nucleation and a traveling molten layer. Optical simulation of Time Resolved Reflectivity was used to critically examine the Heat Flow Calculation results, substantially confirming the existence of a mixture of thermodynamical phases.
Internal Electric Field in Light-Degraded p-i-n a-Si:H Solar Cells
Authors: Smole Franc , Furlan JoŽe , TopiČ Marko
Published: March 2011

Based on results of a-Si:H solar cell computer simulation solving numerically the Poisson, transport and continuity equations, using a rather complex density of states distribution being in agreement with experimental observations, shows that light-soaking degradation can be attributed to both, light induced effects in heterojunction region and to changes in the i-layer, which together tend to reduce the built-in electric field inside of the i-region. Measurements of spectral sensitivity using small intensity monochromatic illumination in combination with an accurate computer analysis provides the possibility of ascertaining which of the degrading effects is the dominating one in a particular a-Si:H solar cell structure.
Trapping Dynamics And Charged Defects: Light Induced Studies in a-Si:H and a-Si1-xGe:H
Authors: Hautala J. , Cohen J.D.
Published: March 2011

Bandtail defect carrier trapping dynamics in high quality photo-CVD grown a-Si1-xGex:H (x=0.3 and 0.62) and PECVD deposited a-Si:H films were studied using photo-modulated electron spin resonance (PMESR). Comparison of the dark spin densities with junction capacitance (drive-level profiling) methods indicate that a majority of the defects are neutral in the a-Si:H and 30% Ge samples, but the majority of the defects in the 62% Ge sample are charged. Lineshape deconvolution of the PMESR signals in the in-phase and quadrature phase as a function of photo-modulation frequency, excitation energy and excitation intensity for both intrinsic (state A) and light soaked (state B) states revealed three significant insights: in state A all samples behave similarly, state B behavior of the a-Si:H is dramatically different from the state B alloyed samples, and strong evidence for 'safe hole traps' (SHT) is observed in the state B a-Si:H material.
Structural and Optical Properties of the Thermally Stable Amorphous Si1−xBx Alloy.
Authors: Carlsson Johan R. A. , Li X.H. , Gong S.F. , Hentzell H.T.G.
Published: March 2011

Thin amorphous Si1−xBx films were co-evaporated onto pre-oxidized (100) Si wafers and quartz substrates, by using a high vacuum system with 2 electron guns. Films were deposited in a composition range from x=0 to x=0.5. In order to study how the structural and optical properties depended on concentration and annealing temperature, heat treatments were carried out at temperatures from 400 up to 1000°C. The films were characterized by means of transmission electron microscopy (TEM), Auger electron spectroscopy (AES), and light absorption spectrophotometry. It is shown that the amorphous Si1−xBx alloy is stable up to 1000°C at certain compositions and that the optical band gap of the alloy increases gradually with increasing annealing temperature up to 700 - 900°C, and then increases rapidly when annealed at a higher temperature by about 0.5 eV. These changes can be associated with microstructural alterations. The relationship between the microstructure and the band gap of the films is discussed.