bluuebunny/crossref_metadata_2025 huggingface dataset:
Atomistic Studies of Crack Branching at Bimaterial Interfaces: Preliminary Results
Authors: Krishnan Sriram , Buehler Markus J
Published: April 2011

In this paper, we summarize recent progress in applying atomistic studies of cracking along interfaces of dissimilar materials under quasi-static crack growth conditions. We consider two linear-elastic material strips in which atoms interact with harmonic potentials, with a different spring constant in each layer leading to a soft and a stiff strip. The two strips are bound together with a tunable potential, which allows to independently control the interface and bulk fracture surface energy. An initial crack serves as initiation point for the failure. This provides a model system to investigate how elastic properties and interface strength interplay and determine the crack growth direction, leading to either interfacial cracking or branching into the film material. We observe a clear transition to interface failure when the interface fracture energy is less than 80% of the bulk fracture energy. We further find that branching in the film material is controlled by the elastic properties of the film material, suggesting interfacial cracking for extremely soft films and branching for stiffer films. Analysis of the virial stress field around the crack suggests that the circumferential hoop stress controls the branching behavior.
Osteoblast Response to Amino Acid-modified Surfaces
Authors: Todd Simon , Rawsterne Rachel , Ulijn Rein , Gough Julie
Published: April 2011

The behavior of cells at the material surface is of critical importance in the design of biomaterials. To understand and control cell responses to surfaces, different surface chemistries are commonly compared. In this study we exploit the chemical diversity of the 20 naturally occurring amino acids to make 20 surfaces with different chemical properties. The biological response of osteoblasts to these surfaces was examined. The osteoblast response to the surfaces was found to be dependant on the wettability of the surface with polar amino acid surfaces (Cys<Ser<Thr<Gln<Asn) promoting osteoblast spreading more than hydrophobic aromatic amino acid surfaces (Phe <Tyr <Trp), aliphatic amino acid surfaces (Leu<Ile<Val<Ala<Met<Gly), and Pro. The surface charge also affected the cellular response with positively charged amino acid surfaces (His<Arg<Lys) producing a higher percentage of spreading cells compared to the negatively charged surfaces (Asp<Glu). Amino acid surfaces provide a range of well defined chemical functionalities that are useful in studying the interactions of cells with material surfaces.
Culture of Mammalian Cells on Single Crystal SiC Substrates
Authors: Coletti Camilla , Jaroszeski Mark J. , Hoff Andrew M. , Saddow Stephen E.
Published: April 2011

Crystalline silicon carbide (SiC) has the potential to become an important biomaterial and a versatile interface between the electronic and biological world. In this work, single crystal SiC biocompatibility is investigated by culturing mammalian cells directly on SiC substrates. The cell morphology and the quality of the cell adhesion have been studied using fluorescence microscopy, while MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assays have been performed to quantify cell viability and number. Standard culture-wells and silicon (Si) substrates were used as controls in the final assessment of crystalline SiC biocompatibility.
DNA Hydrogels
Authors: Um Soong Ho , Park Nokyoung , Luo Dan
Published: April 2011

A hydrogel has been developed into a promising scaffold for cell encapsulation or drug delivery useful in various biorelated research areas. DNA, with its outstanding specificity and flexibility and intrinsic biocompatibility, has been utilized as a new building block in the construction of this new three dimensional material-a DNA based hydrogel. Here, we report the synthesis, properties and applications of a DNA based hydrogel system. The DNA hydrogels are entirely constructed from branched DNA building blocks such as X-shaped DNA (X-DNA), Y-shaped DNA (Y-DNA), and T-shaped DNA (T-DNA) through an enzyme-catalyzed reaction. These DNA hydrogels can be easily patterned into different sizes with millimeter or micrometer scale and different shapes including cylindrical, rectangular, cross, star, and even the “CORNELL” logo. By adjusting the initial concentrations and original shapes of the different building blocks, the external and internal morphology and chemical/physical properties of the DNA hydrogel were easily tuned: The X-DNA based hydrogel (X-DNA gel) showed both the most swelling degree and mechanical strength as well as the higher resistance to chemical and biological degradation. Inspired by these characteristics of the DNA hydrogels, we have developed them into a drug delivery system, an artificial extracellular matrix, and even a cell-free protein producing template.
Photo-oxidation of Ge Nanocrystals: Kinetic Measurements by In Situ Raman Spectroscopy
Authors: Sharp I. D. , Xu Q. , Yuan C. W. , Beeman J. W. , Ager J. W. , Chrzan D. C. , Haller E. E.
Published: April 2011

Ge nanocrystals are formed in silica by ion beam synthesis and are subsequently exposed by selective HF etching of the silica. Under ambient conditions, the exposed nanocrystals are stable after formation of a protective native oxide shell of no more than a few monolayers. However, under visible laser illumination at room temperature and in the presence of O2, the nanocrystals rapidly oxidize. The oxidation rate was monitored by measuring the Raman spectra of the Ge nanocrystals in-situ. The intensity ratio of the anti-Stokes to the Stokes line indicated that no significant laser-induced heating of illuminated nanocrystals occurs. Therefore, the oxidation reaction rate enhancement is due to a photo-chemical process. The oxidation rate varies nearly linearly with the logarithm of the laser intensity, and at constant laser intensity the rate increases with increasing photon energy. These kinetic measurements, along with the power dependencies, are described quantitatively by an electron active oxidation mechanism involving tunneling of optically excited electrons through the forming oxide skin and subsequent transport of oxygen ions to the Ge nanocrystal surface.
Terahertz Detection Related to Plasma Excitations in Nanometer Gate Length Field Effect Transistor.
Authors: Knap Wojciech , El Fatimy A. , Tauk R. , Tombet S. Boubanga , Teppe F.
Published: April 2011

The channel of nanometre field effect transistor can act as a resonant cavity for plasma waves. The frequency of these plasma waves is in the Terahertz range and can be tuned by the gate length and the gate bias. During the last few years Terahertz detection and emission related to plasma wave instabilities in nanometre size field effect transistors was demonstrated experimentally. In this work we review the recent results on sub-THz and THz detection by 50-300nm gate length III-V HEMTs and Si MOSFETs. We present experimental results on the resonant and nonresonant (overdamped) plasma wave detection and discuss possible applications of nanometre field effect transistors as new detectors of THz radiations.
Nano-sized Crystals of Silicon Embedded in Silica Glass: Large Scale Models and Aspects of the Electronic Structure
Authors: Kroll Peter , Schulte Hendrik J.
Published: April 2011

We construct quasi-spherical Si nanocrystallites consisting of 17, 29, 47, 71, and 99 atoms with diameters from 0.8 to 1.6 nm embedded in an SiO2 network. All atoms have saturated bonds: Si is four-fold connected and O is two-fold connected. The models comprise 400-600 atoms and have lattice parameters of about 2 nm. The networks are subjected to a bond switching algorithm yielding models of nanocrystalline Si embedded in amorphous silica.

Subsequently, we employ density functional methods. As a result of the DFT-optimization we find that the Si nanocrystals are free of defects at the interface to the host matrix. Si-Si distances within the Si nanocrystallites are strained, the strain itself tailors off to the suboxide interface. The excess energy of the optimized models with respect to crystalline silicon and vitreous silica scales linearly with the surface of the Si nanoparticles. The interfacial energy of the nc-Si/SiO2 interface is calculated to 1.5 J/m2. We observe an increase of the band gap with decreasing cluster size due to the quantum-confinement effect. The highest occupied states of the valence band are located at Si-Si bonds close to the interface; the corresponding charge density forms a shell-like structure around the central core of the nanocrystal. The lowest unoccupied states are centered within the nanocrystal.
Gas Cluster Ge Infusion for Si(1−x)Ge(x)Strained-Layer Applications
Authors: Tetreault Thomas G. , Shao Yan , Huang Mengbing , Hautala John
Published: April 2011

Materials processing with a gas cluster ion beam (GCIB) is an emerging technology that has been shown to produce novel material properties in the very near-surface (<40nm) regime. We have termed the process of doping with GCIB, infusion. Its principal characteristic enables room temperature shallow processing with high concentration gradients and no sharp interfaces. Results are presented from an investigation of GCIB GeH4 infusion into Si(100) substrates for the purpose of producing a strained Si(1-x)Ge(x) layer relative to the Si substrate. The as-infused samples exhibit surface Ge concentrations up to 100%, with a steep concentration gradient over ∼40 nm. Specifically, a broad series of post-processing tube-furnace anneals was investigated over a temperature range of 400 °C to 1200 °C and the samples were analyzed by channeling/RBS to look at quality of recrystallization and for evidence of strain in the lattice. Some anneal schedules included a VLTA (very low temperature anneal) first step followed by subsequent HTA (high temperature anneal). Evidence is shown for improved results with no Ge loss using two-step anneals. Channeling analysis was performed along the <100> and <110> directions relative to the Si(100) substrate. Measurements of the min for both the Ge and Si were made to assess crystal quality and to determine the degree of epitaxial alignment with the Si lattice. High-resolution axial scans along <110> were performed on selected samples to look for strain relative to the Si lattice. SIMS analysis of higher temperature anneals showed significant diffusion of Ge. Some samples were also examined by cross-sectional TEM. The best infusion samples show crystal quality comparable to a commercially available epi-grown Si75Ge25 control sample.
Biological Properties of Nanocrystalline Silicon Particles for Biomedical Applications
Authors: Sato Keisuke , Yanagisawa Satoshi , Funakubo Akio , Fukui Yasuhiro , Hirakuri Kenji , Higami Tetsuya
Published: April 2011

We have studied the biological properties of nanocrystalline silicon (nc-Si) particles after injection at various places in a mouse. The nc-Si particles with a size of 2.5 nm and a concentration of 1.3 mg/ml were dispersed in a normal saline solution (NSS). The NSS dispersible nc-Si particles were safely injected into the mouse. When the nc-Si particles in the NSS were directly injected into the subcutaneous vein and the coronary artery of the heart by syringe, the condition of bloodstream at each place was confirmed by the red luminescence (peak wavelength at 720 nm) from the nc-Si particles under the ultraviolet (UV) light-irradiation. Moreover, the nc-Si particles in the NSS, which were injected into the vein in the sole, smoothly flowed to the small intestine, and the smooth fluidity of nc-Si particles was also observed for the condition of the peristalsis of the small intestine. The nc-Si particles in the small intestine emitted red light during peristalsis under the UV light-irradiation. The red luminescence at each place was very bright and could be clearly seen with the naked eye. These phenomenons were achieved by the utilization of the harmless material, the formation of nc-Si particles with the single-order-size and the realization of the stable surface modification to the nc-Si particles.
Effect of Surface Passivation on Stability of Luminescence from Nanocrystalline Silicon Particles in Pure Water
Authors: Hiruoka Masaki , Sato Keisuke , Hirakuri Kenji
Published: April 2011

We have investigated the stability of luminescence in pure water from a nanocrystalline silicon (nc-Si) particles passivated with various chemical elements such as a hydrogen, carbon and oxygen atoms. Each sample emitted red light with a peak wavelength in a range from 690 to 800 nm. When the hydrogen- and/or carbon-passivated samples were immersed in pure water, the intensity of red luminescence was decreased by aging after a short period of time. At the same time, the peak wavelength was also shifted toward shorter wavelength. These were caused by the generation of defects (Pb-centers) and the reduction of particle size due to the desorption of hydrogen and/or carbon atoms and the replacement from the Si-H and/or Si-C bonds to the Si-O bond, respectively, at the surface of nc-Si particles. On the other hand, the oxygen-passivated sample showed stable luminescence in addition to the slight blue-shift of peak wavelength under the immersion in pure water for 400 hours. The good stability was attributed to the formation of stable surface condition. These results are a strong indication that the stability of the luminescence in pure water can be remarkably improved by the oxygen-passivation to the surface of nc-Si particles.
Time Resolved Photoconduction Studies of Uniformly Doped and p-n Junction Si Nanowires
Authors: Tsakalakos Loucas , Michael Darryl , Fronheiser Jody , Balch Joleyn , Wortman Robert , Wilson Paul , White David , Boone Rolf , LeBoeuf Stephen
Published: April 2011

The time resolved and DC photoconduction characteristics of Si nanowire devices are described. Si nanowires with diameters ranging from 20-100 nm were grown using the vapor-liquid-solid (VLS) growth mechanism under standard conditions and devices were fabricated in a back-gate field effect transistor (FET) configuration using simple photolithography. It is shown that under certain biasing conditions, illumination with light from light emitting diodes with wavelengths ranging from 480 nm to 625 nm causes changes in current as high as 4%. On the other hand, illumination by a broadband incandescent source causes a ∼4.1% percent change in current. Photoconductive decay curves show bi- and tri-exponential behavior, indicative of multiple potential recombination mechanisms occurring within the Si nanowire devices. p-n doped Si nanowires show similar behavior. Studies under various drain and gate voltages provides insight into the proposed mechanism. It is argued that the Shottky barrier plays a strong role in the observed photoconduction process in these wires, as do transitions involving surface and deep level trap states.
Surface Reactions of Metal Catalysts in Ethanol-CVD Ambient at Low-pressure Studied by in-situ Photoelectron Spectroscopy
Authors: Maeda Fumihiko , Suzuki Satoru , Kobayashi Yoshihiro , Takagi Daisuke , Homma Yoshikazu
Published: April 2011

We succeeded in growing carbon nanotubes in a photoelectron spectroscopy analysis system using thermal chemical vapor deposition and analyzed the chemical states of the Co catalysts by in-situ x-ray photoelectron spectroscopy before and after the growth. We found that almost all of the Co particles are metallic after the growth in both cases; Co particles are formed from a Co oxide thin film and a metallic Co thin film. This shows that the metallic state is stable for Co under low-pressure ethanol ambient in our growth condition for carbon nanotubes.
Irradiation assisted interaction behavior of Regioreguler Poly(3-hexylthiophene) nanotube composites
Authors: Adhikari Ananta , Bakhru Hassaram , Chipara Mircea , Ryu Chang Y. , Ajayan Pullickel
Published: April 2011

The basic step to optimize the properties of filler in composite is the interfacial interaction between the matrix and the filler. Irradiation, a novel technique, was used in this work to introduce a wide range of defects in Single-walled Carbon nanotubes (SWNTs) as filler prior to composite formation. The thermal stability along with phase transition behavior of an organic conducting polymer Poly(3-hexylthiophene) (P3HT) loaded with pristine and ion implanted SWNTs has been investigated using Thermogravimetry Analysis (TGA), Differential Scanning Calorimetry (DSC), and Electron Spin Resonance (ESR). Interestingly, we observed substantial improvement on the thermal stability and the phase transition behavior of the composite with pristine and irradiated fillers.
Controlled Carbon Nanotube Networks and its Corresponding Channel Effect at High Bias
Authors: Huang Jun , Rao Bangalore Kiran , Vedala Harindra , Kim Do-Hyun , Jeon Minhyon , Park Wanjun , Choi Wonbong
Published: April 2011

Geometrically controlled single-walled carbon nanotube (SWNT) and multi-walled carbon nanotube (MWNT) networks were fabricated by a width confinement technique to characterize their electrical characteristics. The results demonstrated non-linear resistance decay with the number of conducting channels. The current-voltage characteristics at high field were studied until the electrical breakdown took place. Large current (∼2 mA), low resistance (∼5 KΩ) and current densities exceeding ∼108 A/cm2 were demonstrated from multi-channel MWNT networks confined in a 10 μm × 15 μm trench. Additionally, chronological SEM imaging was used to identify the breakdown sequences in the carbon nanotube networks, which revealed a strong tendency for CNT breakdown to occur in the vicinity of CNT-CNT intersections. Our results offer insights for interconnect applications using CNT networks.
A Compact Single-Walled Carbon Nanotube Transistor Integrated with a Silicon MOSFET Using a Single Common Gate
Authors: Lin Hao , Park Yong Wook , Tiwari Sandip
Published: April 2011

Single-walled carbon nanotubes have been integrated on top of an individual CMOS device using lithography, chemical vapor deposition and chemical mechanical planarization. Hybrid integration of silicon and nanotube transistors is demonstrated for the first time by making an inverter with a p-type CNTFET placed on top of an n-type MOSFET. A polysilicon gate is positioned between the silicon device channel and carbon nanotube for dense integration of opposite-polarity devices. Such a configuration offers many advantages for high-density electronics and ultra-sensitive detection.
EDLC Properties Depend on the Diameter of Mass Produced Multi-walled Carbon Nanotubes
Authors: Kim Yong-Jung , Abe Yusuke , Yanagiura Takashi , Kitani Massaki , Kodama Tsuyoshi , Higuchi Keita , Endo Morinobu
Published: April 2011

Mass produced multi-walled carbon nanotubes have examined their feasibility as an electrode for supercapacitors. Activation behavior of MWNTs with different diameter is quite different. In this study, the effect by diameter on KOH activation of MWNTs are confirmed.
Wide Bandgap Semiconductors - Nanowires of p- and n-type Silicon Carbide
Authors: Friedel Bettina , Greulich-Weber Siegmund
Published: April 2011

Monocrystalline nanowires of cubic silicon carbide were synthesized using a combined sol-gel and carbothermal reduction process in which tetraethoxysilane was used as primary sili-con and sucrose as carbon source. The diameters of the as-grown nanowires varied depending on process parameters from several tens to several hundreds nanometers, whereas the length of the wires was located in the millimetre region. By precisely controlling the atomic ratio of Si / C, silicon carbide nano wires were synthesized exclusively and pure without the presence of resid-ual carbon or unwanted silica, thus leads to semi-insulating behaviour. Supported by their consis-tence the silicon carbide micro or nano wires can be processed to textile or felt structures and are therefore usable for many applications such as for fireproof clothing, high temperature or chemi-cal filters and composite materials. Additionally during sol-gel synthesis the silicon carbide mi-cro / nano wires were easily doped to achieve p- or n-conduction, guiding to new applications in the field of wide bandgap semiconductors. The structure of 3C-SiC micro and nano wires was determined using scanning electron microscopy, X-ray diffraction, nuclear magnetic resonance and fourier transform infrared spectroscopy. The electronic properties were studied using elec-tron paramagnetic resonance spectroscopy, Hall effect and current-voltage measurements.
Micromachined Silicon Grids for Direct TEM Characterization of Carbon Nanotubes Grown by CVD
Authors: Choi Yongho , Johnson Jason , Moreau Ryan , Perozziello Eric , Ural Ant
Published: April 2011

Transmission electron microscopy (TEM) is a key technique in the structural characterization of carbon nanotubes. For device applications, carbon nanotubes are typically grown by chemical vapor deposition (CVD) on silicon substrates. However, TEM requires very thin samples, which are electron transparent. Therefore, for TEM analysis, CVD grown nanotubes are typically deposited on commercial TEM grids by post-processing. This procedure has two problems: It can damage the nanotubes, and it does not work reliably if the nanotube density is too low. The ability to do TEM directly on as-grown nanotubes lying on the silicon substrate would solve these two problems. In this work, for this purpose, we have fabricated micromachined TEM grids from silicon substrates. In particular, we have wet-etched large membranes from the back side of silicon wafers with a thin layer of thermal oxide on them. We have then etched a large array of long and narrow open slits on these membranes from the top side using a deep silicon etcher. Subsequently, we have grown nanotubes on these micromachined TEM grids by CVD, and characterized the nanotubes by high resolution TEM (HRTEM), micro-Raman spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM). Since the nanotubes grown on the micromachined substrates are completely suspended over the width of the open slits, these substrates form a natural TEM grid for direct imaging of CVD-grown nanotubes. Furthermore, the signal from the substrate is significantly reduced during micro-Raman spectroscopy, resulting in a better signal-to-noise ratio. In addition, the silicon membranes are strong enough to support AFM and SEM characterization. As a result, these substrates provide a low cost, mass producible, efficient, and reliable platform for direct TEM, Raman, AFM, and SEM analysis of as-grown nanotubes or other nanomaterials on the same substrate, eliminating the need for any post-processing after CVD growth.
Reactive Force Field Studies of Large-Deformation of Hybrid Carbon Nanotube-Metal Nanowires
Authors: Wang Xin , King Ryan , Buehler Markus J.
Published: April 2011

We use the ReaxFF reactive force field to model extreme tensile deformation of a (10,10) armchair carbon nanotube. The ReaxFF force field has been developed based on DFT quantum mechanical calculations without any empirical parameters (Duin et al., 2001). We report an analysis of the stress-strain relationship for the elastic and plastic regime, including a description of the microscopic fracture mechanisms. We find Young's modulus to be around 1 TPa, close to experimental values. Our modeling yields a fracture tensile strain of approximately 30%, with a maximum tensile stress of approximately 300 GPa. Fracture of the CNT originates from formation of 5-7 Stone-Wales-like defects, leading to formation of micro-cracks. We also report preliminary results of straining hybrid CNTs embedding a nickel nanowire.
Characterization of TFT-LCD and OLEDs Devices by Phase Modulated Spectroscopic Ellipsometry for Display Applications
Authors: Teboul Eric , Yan Li , Gaillet Melanie
Published: April 2011

Spectroscopic ellipsometry is one of the most accurate and reliable optical technique to characterize polymers, liquid crystals (LCs) and organic light emitting diodes (OLEDs). Because these devices are formed by complex structure including optical anisotropy, absorbing and graded materials, the correct use of spectroscopic ellipsometer required a combination of the proper choice of hardware and the appropriate ellipsometric model. In this work, we presents ellipsometric results obtained by a commercially available phase modulated spectroscopic ellipsometer (PMSE) on a full TFT-LCD structure characterized from UV to NIR. As expected, strong anisotropy and inhomogeneous optical properties were found respectively on LCs and ITO materials. We also presents measurements of film thickness and optical constants of each layer constituing an OLED structure
Study on Electron Injection Effect of Low Molecular Organic Light-emitting Diodes Utilizing LiF/Al and Li/Al Cathode
Authors: Kin Zenken , Kajii Hirotake , Ohmori Yutaka
Published: April 2011

The electron injection effects of organic light-emitting diodes (OLED) from the cathode interface between LiF or Li and Al on the tris-(8-hydroxyquinolinato) aluminum (Alq3) emissive layer have been investigated. Efficient electron injection was achieved in the case of thin layer of LiF or Li deposited on Alq3 emissive layer at the thickness of 0.5 nm and 2 nm, respectively. The results indicate that the coexistence of LiF and Al layer or the Li and Al layer on the Alq3 emissive layer result in the efficient electron injection. Electron injection effect in the case of LiF/Al and Li/Al electrode configurations and also the effect of air explosion during the electrode formation have been discussed. In the case of the device with Li, the diffused Li in the Al layer acts as the efficient carrier injection process.
New Concepts for the Development of Active Functional Polymers for p and n-Type OFET- Applications
Authors: Janietz Silvia , Assawapirom Udom , Sainova Dessislava
Published: April 2011

Here we present a concept to improve the field effect transistor performance of P3HT in terms of threshold voltage stability as well as the stability in ambient atmosphere by introducing a strong acceptor dopant in the main polymer chain. In our concept the direct introduction of the acceptor dopant in the polymer main chain ensures the strucural stability against diffusion processes. P3HTs with different contents of acceptor molecules which are fixed linked in the main chain of the polymer , have been synthesized using the McCullough Grignard metathesis method. As acceptor unit has been integrated tetrafluorbenzene (TFB). The introduced dopant amount has been varied in order to obtain an optimum between the processability of the polymers and the resultant transistor performance.

Compared to the p-type semionducting polymers the n-type organic materials are markedly less developed. Recently an interesting solution to this task has been proposed in the form of a conjugated ladder-type poly (benzo-bisimidazobenzo-phenanthroline) (BBL) showing either ambipolar or n-type field effect properties dependent upon the sample preparation and processing. However this rigid-chain ladder polymer is not soluble in the common organic solvents resulting in a rather complicated technological transfer. We report the significant improvement of the BBL-processing utilizing aqueous colloidal dispersions and their OFET-application. The resultant devices demonstrate ambipolar electronic transport with charge carrier mobilities in the range of 10−5 cm2/Vs without specific optimization procedures..
Extracting the Device Parameters from Organic Thin Film Transistors
Authors: Park Eung Seok , Kang Pil Soo , Kim Gyu Tae
Published: April 2011

Organic thin film transistors(OTFTs) were simulated by a SPICE model adopted in hydrogenated amorphous TFTs(a-Si:H). The gate voltage-dependent mobilities were assumed to fit the representative current-voltage characteristics. The optimal fitting procedures were suggested to compare the experimental data with the mathematical expressions used in the amorphous TFTs. Each SPICE parameter explains the gate dependent mobilities in OTFTs originating from the distribution of trap sites for the hopping conduction.
Orientation Control of Standing Epitaxial Pentacene Monolayers Using Surface Steps and In-plane Band Dispersion Analysis by Angle Resolved Photoelectron Spectroscopy
Authors: Suzuki Tadamasa , Shimada Toshihiro , Ueno Keiji , Ikeda Susumu , Saiki Koichiro , Hasegawa Tetsuya
Published: April 2011

We prepared quasi single crystalline pentacene monolayer films on Bi-terminated Si(111) by using bunched steps on vicinally-cut surfaces as an orientation template. Band dispersion in the conduction plane of pentacene was clearly observed by angle-resolved photoelectron spectroscopy.
Interfacial Phenomena Affecting Charge Transport In Small Molecule Organic Thin-Film Transistors
Authors: Facchetti Antonio , Kim Choongik , Yoon Myung-han , Marks Tobin J
Published: April 2011

Organic semiconductors exhibiting p-, n-type, or ambipolar majority charge transport are grown on six different bilayer dielectric structures consisting of various spin-coated polymers / HMDS on 300 nm SiO2/p+-Si, and are characterized by AFM, SEM, and WAXRD, followed by field-effect transistor (FET) electrical characterization. It is observed that in case of air-sensitive n-type semiconductors, dielectric surface modifications induce large variations in the corresponding OTFT performance parameters although the film morphologies and microstructures remain similar. In marked contrast, the device performance of air-stable n-type and p-type semiconductors is not significantly affected by the same dielectric surface modifications. This study provides key information on the chemical origin of the charge trapping sites at the FET dielectric-semiconductor interface. In parallel, bottom-contact FETs of n-type oligothiophenes were investigated by a combination of microscopy/electrical measurements and new insights for the poor electron injection efficiency from the Au contacts are presented
Electronic Memory Effects in Zinc Oxide Nanoparticle -Polystyrene Devices with a Calcium Top Electrode
Authors: Verbakel Frank , Meskers Stefan C. J. , Janssen René A. J.
Published: April 2011

Diodes with an active layer of solution processed zinc oxide (ZnO) nanoparticles and polystyrene are studied. Poly(3,4-ethylenedioxythiophene)- polystyrenesulfonate (PEDOT:PSS) on indium doped tin oxide (ITO) is used as the bottom electrode and aluminum or calcium are used as top electrode. Pristine devices show diode behavior in their current-voltage characteristics. The conductivity of the device in reverse bias can be raised three orders of magnitude by applying a positive voltage or by illumination with UV light. In this high conductivity state we observe reversible electronic memory effects. The electronic memory effects are attributed to a reversible electrochemical process at the PEDOT:PSS/ZnO interface. Memory effects in diodes with Al and Ca metal electrode are found to be very similar, consistent with the view that the memory effects arise at the PEDOT:PSS/ZnO interface.
Organic Field-Effect Based Devices for Pressure Detection
Authors: Manunza Ileana , Bonfiglio Annalisa
Published: April 2011

We describe pressure sensors realised starting from completely flexible organic thin film transistors (OTFTs). A flexible and transparent plastic foil (Mylar) is employed both as substrate and gate dielectric; gold source and drain electrodes are patterned on the upper side of the foil while the gate electrode lies on the opposite side; a vacuum-sublimed pentacene film acts as active layer. The pressure dependence of the output current has been investigated by applying to the gate side of the device a mechanical stimulus by means of a pressurized air flow. Experimental results show a reversible current dependence on pressure; further data analysis suggests that current variations are due to pressure-induced variations of mobility and threshold voltage.
The Control of Electronic States Spreading Outside the Conjugated Polymer Surface
Authors: Hao Xiao Tao , Hosokai Takuya , Mitsuo Noritaka , Kera Satoshi , Sakamoto Kazuyuki , Okudaira Koji Kamiya , Ueno Nobuo
Published: April 2011

The surface electronic structures of conjugated regio regular and regio random poly (3- hexylthiophene) (rr-P3HT and rra-P3HT) thin films were studied by near edge X-ray absorption fine structure spectroscopy, ultraviolet photoelectron spectroscopy and Penning ionization electron spectroscopy (PIES). The distribution of the surface electronic states was controlled on rr-P3HT and rra-P3HT thin films with different molecular ordering by varying the coating process and PIES was adopted to observe the electronic states existing outside the surface.
Structural Transformation in PECVD Ultralow-k Material during Porogen Removal by UV Assisted Thermal Curing
Authors: Zenasni Aziz , Jousseaume Vincent , Gourhant Olivier , Favennec Laurent , Maury Patrick , Mage Lucile
Published: April 2011

Next-generation microelectronic interconnects require the use of dielectrics with continuously lower permittivity (k) to overcome limitations induced by crosstalk parasitic signal delay. Using PECVD, Ultralow-k film (ULK, k ≤ 2.5) can be developed by creating pore inclusions within an organosilicate matrix through porogen approach. Both ULK deposition and subsequent curing process has to be adjusted in order to achieve optimized mechanical and electrical properties and maintain stability during integration. For this concern, the attention was recently focused on ultraviolet (UV) radiation to sustain the thermal curing. In the present work, a fundamental understanding of structural transformations occurring during porogen extraction from as-deposited ULK materials when exposed to thermal-assisted UV radiation is proposed. This thermal-assisted UV cure technique is very efficient in porogen removal since in a few minutes the desired porosity is reached. During the first stage of curing, the film shrinks strongly whereas the porosity is created. After porogen removal step, the porous film continues shrinking under UV radiation leading to an increase of SiOSi bonds concentration (film densification). The normalized FTIR SiOSi peak increase during UV curing (related in literature to an improvement of mechanical properties) is mainly due to the film densification, in addition to the SiOSi bridging bond creation. In this case, correlation is found between shrinkage and elastic modulus.
Electrical and Structural Properties of Ruthenium Film Grown by Atomic Layer Deposition Using Liquid-Phase Ru(CO)3(C6H8) Precursor
Authors: Chung Sung-Hoon , Vasilyev Vladislav , Gorokhov Evgeni , Song Yong-Won , Jang Hyuk-Kyoo
Published: April 2011

We investigated effects of thermal annealing on Ru films deposited on the 8 inch Si substrates using a volatile liquid-phase Ru precursor, tricarbonyl-1,3-cyclohexadienyl ruthenium (Ru(CO)3(C6H8)) by an atomic layer deposition (ALD) technique. Structural and electrical properties of the films were characterized by scanning probe microscopy, X-ray diffractometry, sheet resistance. Grazing incidence X-ray diffraction (GIXRD) patterns show typical Ru hexagonal polycrystalline peaks as annealing temperature was increased. At the highest annealing temperature condition, Ta = 700 °C electrical resistivity become 6 times less than in as-deposited films.
Time Evolution of Nanoscale Surface Topography of Tungsten Carbide Coatings on “Hot” Silicon Carbide Electronics Devices
Authors: Muntele Claudin , Walker Daniel , Elsamadicy Abdalla , Ila Daryush
Published: April 2011

We are reporting here on the current status of our investigations on the time evolution of nanoscale surface morphology of thermally evaporated tungsten carbide coatings on silicon carbide substrates. The purpose of the study is to develop a recipe for creating thermally and chemically stable electrical contacts on silicon carbide electronic devices able to work at elevated temperatures (up to 800 °C) in oxidizing environments. We used thermal evaporation and tungsten carbide (WC) powder as a starting material to produce the thin layer deposition on semi-insulating silicon carbide (6H). Our intended applications are for devices working at 800 °C; therefore, our investigations are carried out at 1 hr intervals of time the samples spent at this temperature, in air at atmospheric pressure. We used Rutherford Backscattering Spectrometry (RBS) for measuring the stoichiometry and depth profile, and Atomic Force Microscopy (AFM) to monitor the surface morphology change.
Electroless Deposition of Gold Nanoparticles Over Silicon-based Substrates
Authors: Borteh Hassan , Ferrell Nick , Butler Randall , Olesik Susan , Hansford Derek
Published: April 2011

The use of proteins and peptides to deposit and pattern metallic nanoparticles is becoming increasingly important. This method provides an inexpensive procedure to produced patterned and continuous metallic nanoparticles on variety of substrates such as silicon dioxide, silicon nitride, and polyimide. In this work, we explore the use of proteins and peptides for patterning and depositing gold nanoparticles. We used two different peptides or proteins for this experiment. One is 3XFLAG peptide from Sigma-Aldrich and the other one is bovine serum albumin (BSA). To pattern the peptides on the substrates we used two different methods, photolithography and micro-transfer molding. In photolithography, S1813 photoresist was patterned on the substrates through clean room process. In the micro-transfer molding process, a PDMS mold was made out of photoresist pattern. Polypropylmethacrylate (PPMA) was spin coated on the PDMS mold and stamped on the substrate in a high temperature. The proteins were adsorbed on the surface either physically or covalently. To deposit gold nanoparticles, the substrates with adsorbed proteins were covered with aqueous HAuCl4 solution. The proteins catalyze gold nanoparticles reduction from the solution. To characterize nanoparticles we used SEM and Electron Beam Induced Current (EBIC).
ITO-Channel Ferroelectric-Gate Thin Film Transistor with Large On/off Current Ratio
Authors: Tokumitsu Eisuke , Senoo Masaru , Shin Etsu , Fujimura Tomofumi
Published: April 2011

Indium tin oxide (ITO)-channel ferroelectric-gate thin film transistor (TFT) with large on/off current ratio is demonstrated by using mechanical polishing process to planarize the surface of ferroelectric bottom gate insulator (Bi,La)4Ti3O12 (BLT). It is shown that the mechanical polishing of the sol-gel derived polycrystalline ferroelectric BLT films causes no degradation in electrical properties. ITO channel layer was then deposited on the planarized BLT gate insulator to fabricate ferroelectric-gate TFTs. The off-current of the ITO/BLT TFT fabricated with the polishing process is drastically reduced to around 10−12 A, which is four orders of magnitude lower than that of the TFT fabricated without the polishing process. The obtained on/off current ratio is more than 107. In addition, a subthreshold voltage swing as small as 200 mV/decade was obtained.
Composition Control in the Deposition of Cu(InGa)(SeS)2Thin Films
Authors: Shafarman William , Hanket Gregory , Nishiwaki Shiro
Published: April 2011

The pentenary chalcopyrite compound Cu(InGa)(SeS)2provides several potential advantages over Cu(InGa)Se2as the absorber layer in thin film solar cells, especially with wide bandgap alloys. The effects of S addition to the quaternary alloy are investigated with films deposited by elemental thermal co-evaporation and by the reaction of metallic precursors in hydride gases. With co-evaporated films the addition of S complicates the control of composition through the film. The incorporation of the chalcogen species Se and S depend on the relative Cu in the film and, for films with excess Cu, on the relative group III composition. For the precursor reaction process the addition of S by the inclusion of H2S gas in the reaction enables control of the relative Ga concentration and bandgap of the film. With both processes the incorporation of S during deposition also effects the morphology and grain size. The co-evaporated films have smaller grains with S while the reacted films have larger grains which may be due to the higher TSS the S reaction enables.
Fabrication of (Cu,Ag)InSe2 Thin films by a Combination of Mechanochemical and Screen Printing/Sintering Processes
Authors: Nomura Syuusuke , Matsuo Yoshihiro , Wada Takahiro
Published: April 2011

We successfully prepared (Cu1-XAgx)InSe2 solid solution with 0 ¡Ü x ¡Ü 1.0 by a mechanochemical process without any additional heating. The obtained fine powder was suitable for screen-printing. Particulate precursors were deposited in a thin layer by a screen-printing technique and then the porous precursor layer was sintered into a dense polycrystalline film by atmospheric-pressure firing. The crystal structures of the powder and the film were analyzed by x-ray diffraction and the microstructure of the film was observed in an SEM. For the (Cu1-XAgx)InSe2 films with x ¡Ü 0.2, the (Cu,Ag)InSe2 films had a good microstructure for the solar cell absorbers.
All-CSS processing of CdS/CdTe thin-film solar cells with thin CdS layers
Authors: Davies Alan , Sites J. R. , Enzenroth R. A. , Sampath W. S. , Barth K. L.
Published: April 2011

Cadmium Sulfide/Cadmium Telluride (CdS/CdTe) thin-film solar cells were fabricated by an in-line, close-space-sublimation (CSS) process at Colorado State University. Source temperature control was used to reduce the deposited CdS thickness. Quantum efficiency (QE) showed CdS thicknesses that varied over a range from 250 to 10 nm. Current-Voltage (J-V) measurements showed increased Jsc as CdS was thinned. Thin CdS resulted in reduced voltage (800 mV to 350 mV) and fill factor, which offset gains in current, and caused efficiencies to drop from 12.6% for thick CdS layers to 4.5% for devices with the thinnest CdS. These performance trends are consistent with calculations assuming parallel junctions of CdS/CdTe and SnO2/CdTe. Localized weak-junction formation was characterized by high-resolution laser-beam-induced current (LBIC) mapping. Greater incidence of spatial non-uniformities in photocurrent response accompanied thinning of the CdS layer, with 638-nm spectral response varying spatially by 4.5% for thin CdS devices compared to variations less than 1% for devices with thicker CdS. Non-uniformities of cells with thin CdS are highly sensitive to voltage bias and are likely indicative of parallel p-n and Schottky-type junctions.
Formation of Cu2ZnSnS4 and Cu2ZnSnS4-CuInS2 Thin Films Investigated by In-Situ Energy Dispersive X-Ray Diffraction
Authors: Weber Alfons , Kötschau Immo , Schorr Susan , Schock Hans-Werner
Published: April 2011

Chalcopyrite CuInS2 and the structurally related kesterite Cu2ZnSnS4 are known as photovoltaic absorber materials. In this study different precursor thin films of the quaternary Cu-Zn-Sn-S system (stacking: Mo/CuS/ZnS-SnS) and of the pentenary Cu-In-Zn-Sn-S system (stacking: Mo/CuIn/ZnS-SnS) were annealed in sulfur atmosphere. The predominant crystalline phases were detected by in-situ energy dispersive X-ray diffraction (EDXRD). Additionally the X-ray fluorescence signals of the film components were recorded to detect diffusion effects. For the quaternary system we found ZnS, CuS, Cu2-xS, Sn2S3 and SnS as main binary phases during annealing. The Sn2S3-SnS phase transition had a significant impact on the later formation of ternary/quaternary phases. A high diffusivity of copper can explain the little influence of the precursor stacking on the reaction path and may also be responsible for the poor adhesion of the films. For annealing temperatures above 450°C Cu2ZnSnS4 can be identified clearly by XRD. The incorporation of indium in the system leads to new diffraction peaks which can be explained by the formation of solid solutions in the system CuInS2-Cu2ZnSnS4.
Characterisation of Thin Films CuIn1-xAlxSe2 Prepared by Selenisation of Magnetron Sputtered Metallic Precursors
Authors: Zoppi Guillaume , Forbes Ian , Nasikkar Paresh , Miles Robert W.
Published: April 2011

Thin films of CuIn1-xAlxSe2 have been produced by the selenisation of magnetron sputtered Cu/In/Al precursor layers using elemental selenium and the chemical and physical properties of the layers have been determined for different conditions of synthesis. For optimum conditions of synthesis it was found possible to produce single phase films with the chalcopyrite structure. These films were pinhole free, had good adhesion and were conformal to the substrate. The films had uniform depth profiles as determined using the MiniSIMS. The layers were highly photoactive, indicating that they have the potential to be used to fabricate thin film photovoltaic solar cell devices.
Optical Properties of Strained Polycrystalline CuInS2 Layers
Authors: Eberhardt Jens , Metzner Heinrich , Goldhahn Rüdiger , Hudert Florian , Schulz Kristian , Reislöhner Udo , Hahn Thomas , Cieslak Janko , Witthuhn Wolfgang
Published: April 2011

Using molecular beams, polycrystalline thin CuInS2 (CIS) films of different thicknesses were grown on Si substrates covered with a sputtered Mo-buffer layer. Systematic photoluminescence and photoreflectance measurements were performed to investigate the influence of strain - introduced during growth - on the optical properties. The transition energy of the free A-exciton (FXA) decreases with increasing tensile strain caused by (i) increasing thickness of the Mo buffer layer and (ii) decreasing thickness of the CIS layer. Furthermore, the energetic splittings between FXA, FXB, and FXC increase with increasing tensile strain. When combined with X-ray diffraction data, the oscillator strengths of the excitonic transitions yield information on the strain distribution within the films.
SnS Thin Films in Chemically Deposited Solar Cell Structures
Authors: Avellaneda David , Nair M. T. S. , Nair P. K.
Published: April 2011

We report photovoltaic cell structures on SnO2:F (TCO) coated glass substrates. Thin films of CdS, SnS, and CuS or PbS were deposited sequentially from chemical baths to produce the solar cell structures: SnO2:F-CdS- SnS (A)-CuS-Ag; SnO2:F-CdS- SnS (A)-PbS-Ag; and SnO2:F-CdS- SnS (B)-PbS-Ag. Heating SnS-CuS films results in the formation of Cu2SnS3, and sequential depositions of SnS and PbS to obtain solar cells produce stratified layers as required for solar cells. The photovoltaic characteristics, Voc 340 mV and Jsc 6 mA/cm2 in these structures suggest that absorber thin films based on tin sulfide are worth investigating as a relatively abundant and non-toxic material for solar cells.
Revisiting CdS-PbS Solar Cell Structure
Authors: Garcia Harumi Moreno , Gómez-Daza O. , Campos J. , Nair M. T. S. , Nair P. K.
Published: April 2011

CdS and PbS are well known semiconductor materials. Starting in 1969 and into 1970's CdS-PbS cells were reported with open circuit voltage (Voc) up to 450 mV and short circuit current density (Jsc) < 1 mA/cm2. However, further reports are scarce. These two materials are also the most investigated by chemical deposition technique. In this work we revisit this type of photovoltaic junctions and present the photovoltaic behavior of distinct type of cell structures prepared by chemical deposition: glass/CdS/PbS/Ag, SnO2:F/CdS/PbS/Ag, and SnO2:F/CdS/(Bi2S3 or/and CdSe)/PbS/Ag. Depending on the cell type, Voc of > 500 mV or Jsc of > 3 mA/cm2 could be obtained under illumination of 1-3 kW/m2. This work opens up possibilities for developing simple solar cell structures by sequential chemical deposition of semiconductors.