bluuebunny/crossref_metadata_2025 huggingface dataset:
Reliability and degradation modes of 280 nm deep UV LEDs on sapphire
Authors: Gong Zheng , Chhajed Sameer , Gaevski Mikhail , Sun Wenhong , Advivarahan Vinod , Shatalov Maxim , Khan M. Asif
Published: April 2011

In this paper, we report a study of the degradation of AlGaN-based 280 nm LEDs, which were grown on sapphire substrates using migration-enhanced metalorganic chemical vapor deposition process (MEMOCVD). Electroluminescence (EL), atomic force microscopy (AFM), cathodoluminescence (CL), and scanning electron microscopy (SEM) observations showed that the degradation of deep UV LEDs generally fell into two categories: catastrophic degradation and gradual degradation. The catastrophic degradation was found to be mostly caused by the non-uniformity of surface morphology. The gradual power reduction had two characteristic time constants indicating two possible degradation mechanisms as found from temperature and bias dependent LED power degradation measurements. The faster time constant was bias dependent and virtually constant with temperature whereas the second time constant (slower) varied exponentially with junction temperature. For this temperature dependent part, the activation energies of degradation were determined to be 0.23 eV and 0.27 eV under injected current density of 100 A/cm2 and 200 A/cm2 respectively.
Morphological study of InN films and nanorods grown by H-MOVPE
Authors: Park Hyun Jong , Kang Sang Won , Kryliouk Olga , Anderson Tim
Published: April 2011

Hydride-Metalorganic Vapor Phase Epitaxy (H-MOVPE) was used to grow a series of films on c-Al2O3 substrates. Depending on the growth temperature and HCl/TMIn molar ratio, InN deposited as a continuous film or a collection of micro or nanorods, or no InN growth was observed. A chemical equilibrium analysis of the In-N-H-Cl system predicts both InN growth and etching regimes with the nanorod growth observed near the growth-etching transition. All InN rod structures demonstrated well faceted hexagonal structure with a near random orientation of the rods, while the films were polycrystalline.
Study of GaN Light-Emitting Diodes Obtained by Laser-Assisted Debonding
Authors: Chan C. P. , Yue T. M. , Surya C. , Ng A. M. C. , Djurišić A. B. , Liu C. K. , Li M.
Published: April 2011

We report experimental investigation of laser-assisted debonding of GaN-based light emitting diodes (LEDs) grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrates. The device was photo-electrochemically (PEC) etched to form hexagonal pyramid hillocks on the debonded surface. The luminous intensity of the debonded and roughened LEDs increased by 60% when compared to the LEDs on sapphire. This increase is attributed to the enhancement in photon extraction efficiency.
Role of deep levels in DC current aging of GaN/InGaN Light-Emitting Diodes studied by Capacitance and Photocurrent Spectroscopy
Authors: Castaldini Antonio , Cavallini Anna , Rigutti Lorenzo , Meneghini Matteo , Levada Simone , Meneghesso Gaudenzio , Zanoni Enrico , Härle Volker , Zahner Thomas , Zehnder Ulrich
Published: April 2011

We present a combined Capacitance-Voltage (C-V), Deep Level Transient Spectroscopy (DLTS) and Photocurrent (PC) study of short-term instabilities of InGaN/GaN LEDs submitted to forward current aging tests at room temperature. C-V profiles detect changes consisting in apparent doping and/or charge concentration increase within the quantum wells. This increase is correlated to dramatic modifications in the DLTS spectrum when the reverse bias and filling pulse are properly adjusted in order to probe the quantum well region. The new distribution of the electronic levels detected by DLTS could explain the observed decrease in the light emission efficiency [1,2] of the device, as the deep levels generated during the stress may provide alternative recombination paths for free carriers. The photocurrent spectra do not change in shape during stress, although their amplitude slightly decreases. This is related to a decrease of the device yield, in this photodetector configuration, with increasing aging time. Thus, we can suggest that the introduction of new defect levels in the bulk material lowers the free carrier mobility.
Double-Ion-Implanted GaN MESFETs with Extremely Low Source/Drain Resistance
Authors: Nomoto Kazuki , Ito Nobuyuki , Tajima Taku , Kasai Takeshi , Mishima Tomoyoshi , Inada Taroh , Satoh Masataka , Nakamura Tohru
Published: April 2011

Incorporation of Si ion implantation to GaN metal semiconductor field effect transistor (MESFET) processing has been demonstrated. The channel and source/drain regions formed using Si ion implantation into undoped GaN on sapphire substrate. In comparison with the conventional devices without ion implanted source/drain structures, the ion implanted devices showed excellent device performance. On-state resistance reduces from 210 Ω-mm to 105 Ω-mm. Saturation drain current and maximum transconductance increase from 36 mA/mm to 78 mA/mm and from 3.8 mS/mm to 10 mS/mm, respectively.
Fabrication and device characteristics of bulk GaN-based Schottky diodes
Authors: Zhou Yi , Wang Dake , Ahyi Claude , Tin Chin-Che , Williams John , Park Minseo , Williams N. Mark , Hanser Andrew
Published: April 2011

In this investigation, Schottky diodes with different device sizes (150μm, 420μm and 700μm) were fabricated on the Ga-face of a free-standing n--GaN wafer produced by Kyma Technologies, Inc. Full area back side ohmic contact was prepared on the N-face of the bulk GaN using Ti/Al. Without any edge-termination scheme, a relatively high reverse breakdown voltage of 240V was achieved. The reverse breakdown voltage decreases as the device size increases. The forward turn-on voltage was as low as 2.4V at room temperature for the 150μm diameter Schottky diodes. The best on-state resistance was 7.56 mΩcm2 for diodes with VB=240V, producing a figure-of-merit (VB2/RON) of 7.6 MWcm-2. The Schottky diode also showed an extremely short reverse recovery time (< 20 ns) switching from forward bias to reverse bias.
Anti-diffusion barriers for gold-based metallization to GaN
Authors: Piotrowska Anna , Kaminska Eliana , Guziewicz Marek , Dynowska Elzbieta , Stonert Anna , Turos Andrzej , Figge Stephan , Kröger Roland , Hommel Detlef
Published: April 2011

We propose a new metallization scheme to p-GaN, where thin-film transition metal nitrides have been applied to improve thermal stability of gold-based metallization. In this metallization scheme the Pd/Au bilayer was used to form low-resistivity ohmic contact to p-GaN, while Ta-Si-N, Ti-Si-N, and Ti-W-N anti-diffusion barriers were used to protect contact metallization from interaction with Au overlayer. We present the details of optimization of process parameters of barrier layer fabrication and show that Ta0.34Si0.25Ni0.41 and Ti0.26Si0.0.17Ni0.57 thin films fabricated by reactive magnetron sputtering show excellent barrier properties under high temperature stress.
Planarization of GaN by the Etch-Back Method
Authors: Williams Adrian , Moustakas T.D.
Published: April 2011

The planarization of GaN has been mainly addressed by chemo-mechanical polishing methods in the literature. Such techniques were found to be successful in planarizing the nitrogen polar (000-1) surface of GaN. However, planarization of the gallium polar (0001) surface presents a challenge due to its higher degree of chemical inertness. Moreover, studies on the planarization of the remaining crystal orientations remain a topic with sparse representation in the literature. In this paper, we report our planarization studies of GaN films using an etch-back technique. Photoresist is employed as the sacrificial layer and a chlorine inductively coupled plasma (ICP) used for etching. We demonstrate the planarization of rough C-plane and A-plane GaN films grown by hydride vapor phase epitaxy (HVPE). Specifically, an rms roughness of approximately 2.1nm and 4.1nm was realized after iterative etch-back for a representative C-plane and A-plane sample with initial rms roughness of 134nm and 414nm respectively.
GaN-based Light Emitting Diode with Transparent Nanoparticles-Embedded p-Ohmic Electrodes
Authors: Song June-O , Kang Hun , Nicol David , Ferguson Ian T , Hong Hyun-Gi , Seong Tae-Yeon
Published: April 2011

Optoelectronics related to GaN-based semiconductors (i.e., InGaN, GaN, and AlGaN) are new technologies that have the potential to far exceed the energy efficiencies of incandescent and fluorescent lighting sources. Among the GaN-based optoelectronic devices like light emitting diode (LED) and laser diode (LD), the GaN-based LEDs are of interest for the next generation illumination because of the representative characteristics such as small, highly radiant, reliably long, and fast responding, compared with the existing general lighting systems.

Achievement of high luminous intensity by flip-chip LED (FCLED) with Ag-metallic reflector or using top emitting LED (TELED) with highly transparent ITO contact is required to improve the external quantum efficiency (EQE) and light output of GaN-based LEDs. However, since the work function of Ag and ITO is lower than 5.0 eV, it is difficult to produce low-resistance p-ohmic electrode with Ag-metallic reflector or ITO only.

In this study, in order to develop new ohmic contact materials having low contact resistance and high transmittance, transparent nanoparticles-embedded p-ohmic electrode, Mg-doped indium oxide (MIO) (3 nm)/indium tin oxide (ITO) (400 nm) ohmic contact for high brightness TELEDs for solid-state lighting, was suggested. The MIO/ITO contact become ohmic with specific contact resistances of 2.64 × 10-3 Ωcm2 and give transmittance higher than 94.6 % at a wavelength of 450 nm when annealed at 630 °C for 1 min in air. GaN-based LEDs fabricated with the annealed MIO/ITO p-contact layer give a forward-bias voltage of about 3.38 V at injection current of 20 mA. It is further shown that the output power of the LEDs with the MIO/ITO contact is enhanced by about 1.86 times at 20 mA as compared with that of LEDs with the conventional Ni/Au contact.
MgZnO Nanocrystallites: Photoluminescence and Phonon Properties
Authors: Morrison John L. , Chen Xiang-Bai , Huso Jesse , Hoeck Heather , Mitchell James , Bergman Leah , Zheleva Tsvetanka
Published: April 2011

We report on the ultraviolet photoluminescence (UV-PL), Raman and structural properties of wurtzite MgxZn1-xO nanopowders of average size ∼ 30 nm that were synthesized via the thermal decomposition method. For the studied composition range of, the room temperature UV-PL was found to be tuned by ∼ 0.24 eV towards the UV-spectral range, and the PL emission was established to be due to an excitonic-type recombination mechanism. The first-order LO Raman mode was found to exhibit a blueshift of ∼ 33 cm-1. The LO-mode of the nanopowders is discussed in terms of a mixed A1-E1 symmetry phonon known as a quasi-LO mode. The observed 30 cm-1 blueshift indicates that the E1 is the principle symmetry component in the Raman scattering of the MgxZn1-xO nanopowders.
Characteristics of a phosphorus-doped p-type ZnO film by MBE
Authors: Xiu Faxian , Yang Zheng , Leelaprasanna Mandalapu J. , Liu Jianlin
Published: April 2011

A solid-source GaP effusion cell was used to provide phosphorus dopants to achieve p-type ZnO with molecular-beam epitaxy (MBE). Room temperature (RT) Hall-effect measurements reveal that phosphorus-doped ZnO has a strong p-type conduction with a hole concentration of 6.5×1018 cm-3 and a hole mobility of 9.0 cm2/V s. X-ray diffraction measurements show a preferential growth orientation along <11-20> by θ-2θ scan and a tilt of ZnO (11-20) plane relative to the substrate surface by rocking curve and reciprocal space map. Photoluminescence (PL) spectra at 8.5 K show a dominant acceptor-bound exciton emission at 3.319 eV. The acceptor energy level of the phosphorus dopant is calculated to be 0.18 eV above the valence band from PL spectra, which is consistent with the temperature dependence of PL measurements.
Room temperature Strong coupling in low finesse GaN microcavities
Authors: Sellers Ian , Semond Fabrice , Leroux Mathieu , Massies Jean , Disseix Pierre , Malpuech Guillaume , Henneghien A-L. , Leymarie Joel , Vasson A
Published: April 2011

We present experimental results demonstrating strong-light matter coupling at low and room temperature in bulk GaN microcavities. Angle dependent reflectivity measurements demonstrate strong-coupling with a Rabi-energy of 50meV at room temperature which is well reproduced with transfer matrix simulations. The absence of strong coupling in the photoluminescence is attributed to the low finesse of the microcavity (Q=60) and is confirmed by simulations which indicate a quality factor of 90 is required to observe strong-coupling in the emission.
Surface Recombination and Vacuum/GaN/AlGaN Surface Quantum Wells
Authors: Zhang Xiyao , Wellenius Ian Patrick , Cai Ailing , Muth John , Roberts John , Rajagopal Pradeep , Cook Jim , Piner Eddie , Linthicum Kevin
Published: April 2011

Surface quantum wells of gallium nitride have been grown by Metal Organic Vapor Phase Epitaxy on top of AlGaN/GaN heterostructures. One boundary of the quantum well is vacuum (or air)/GaN interface, the other is GaN/AlGaN interface, and the width of the quantum well is the thickness of gallium nitride cap, and quantum confinement is demonstrate by the energy shift in photoluminescence, and cathodoluminescence as the GaN cap thickness is varied. The efficiency of the quantum well emission is sensitive to the surface environment and resulting surface recombination velocity. In this study the surface is altered by surface preparation treatments and resulting in changes in the luminescence. The changes in the efficiency of quantum well luminescence with surface treatments are attributed to changes in surface recombination velocity and surface electric fields.
A microspectroscopic study of cap damage in annealed RE-doped AlN-capped GaN
Authors: Nogales Emilio , Lorenz K. , Wang K. , Roqan I.S. , Martin R.W. , O'Donnell K.P. , Alves E. , Ruffenach S. , Briot O.
Published: April 2011

Integrated AlN nanocaps are used to protect gallium nitride epilayers during high temperature annealing treatments following high-energy implantation of rare earth (RE) ions. Cracks formed in thicker caps due to the lattice mismatch between AlN and GaN lead to the creation of microscopic surface defects at annealing temperatures higher than around 1200 °C. GaN dissociates locally to produce holes in the caps. Simultaneous cathodoluminescence/wavelength dispersive X-ray microanalysis in a modified electron probe microanalyzer allows study of the compositional and light emission variations near these microscopic defects. The intensity of the 5D0 – 7F2 transition related emission is enhanced and spectral changes can be observed, which indicate changes in the structure and/or composition of a very thin layer that forms the walls of holes in the caps. We also report some preliminary observations on the influence of the annealing atmosphere (nitrogen or ammonia) on cap damage.
Environmental Impact of Crystalline Silicon Photovoltaic Module Production
Authors: Alsema Erik , de Wild Mariska J
Published: April 2011

Together with a number of PV companies an extensive effort has been made to collect Life Cycle Inventory data that represents the current status of production technology for crystalline silicon modules. The new data covers all processes from silicon feedstock production to cell and module manufacturing. All commercial wafer technologies are covered, that is multi- and monocrystalline wafers as well as ribbon technology. The presented data should be representative for the technology status in 2004, although for monocrystalline Si crystallisation further improvement of the data quality is recommended. On the basis of the new data it is shown that PV systems on the basis of c-Si technology are in a good position to compete with other energy technologies. Energy Pay-Back Times of 1.7-2.7 yr are found for South-European locations, while life-cycle CO2 emission is in the 30-46 g/kWh range. Clear perspectives exist for further improvements with roughly 40-50%.
Life Cycle Analysis of Solar Module Recycling Process
Authors: Müller Anja , Wambach Karsten , Alsema Erik
Published: April 2011

Since June 2003 Deutsche Solar AG is operating a recycling plant for modules with crystalline cells. The aim of the process is to recover the silicon wafers so that they can be reprocessed and integrated in modules again. The aims of the Life Cycle Analysis of the mentioned process are (i) the verification if the process is beneficial regarding environmental aspects, (ii) the comparison to other end-of-life scenarios, (iii) the ability to include the end-of-life phase of modules in future LCA of photovoltaic modules. The results show that the recycling process makes good ecological sense, because the environmental burden during the production phase of reusable components is higher than the burden due to the recycling process. Moreover the Energy Pay Back Time of modules with recycled cells was determined.
Novel Energetic Composite Materials
Authors: Nable Jun , Mercado Andrew , Sherman Andrew
Published: April 2011

A screening of novel energetic materials is presented. These materials are selected according to their energy content and heat release during ignition. These material mixtures will be fabricated into composites that can also perform structural functions. A composite fabrication route is developed using fluidized bed chemical vapor deposition; coating a metallic layer onto the particles as a method to control mechanical properties and reaction kinetics. These reactive metal matrix composites can be developed to offer optimum performance with specific strength, modulus and energetic capabilities.
Towards Cyclophosphazene Based Dendrimers For Energetic Binders
Authors: Greenland Barny , Steinke Joachim H. G. , Widdowson David A. , Golding Peter , Trussell Steve
Published: April 2011

Dendrons and dendrimers containing cyclophosphazenes units in each generation have been synthesised by the convergent route for the first time. We also demonstrate the synthesis of dendrons containing precise numbers of benzyl protected alcohol functionalities.
Ignition of Aerosolized Reactive Particles at High Heating Rates
Authors: Mohan Salil , Shoshin Yuriy L , Dreizin Edward L
Published: April 2011

This paper presents an experimental methodology, respective heat transfer model, and initial results describing ignition of rapidly heated, aerosolized powders of different materials. The experimental setup uses a CO2 laser as a heat source. The interaction of the laser beam with particles is particle size-dependent and only a narrow range of particle sizes is heated effectively. Therefore, the heat transfer model needs to be only analyzed for the particles with this specific size, which greatly simplifies the interpretation of experiments. The powder is aerosolized inside a plate capacitor by charging particles contacting the capacitor’s electrodes. A thin, laminar aerosol jet is carried out by an oxidizing gas through a small opening in the top electrode and is fed into a laser beam. The velocities of particles in the jet are about 1 m/s. The laser power is increased until the particles are observed to ignite. The ignition is detected optically. The ignition thresholds for spherical magnesium and aluminum powders were measured. The experimental data for magnesium, for which ignition kinetics is well known, were used to calibrate the detailed heat transfer model. The model was used to evaluate the ignition kinetics for aluminum powder.
Dynamic Impact Characterization of Al+Fe2O3+30% Epoxy Composites Using Time Synchronized High-Speed Camera and VISAR Measurements
Authors: Ferranti Louis , Thadhani Naresh N.
Published: April 2011

Reverse Taylor anvil-on-rod impact experiments were conducted on Al+Fe2O3+30% epoxy composites to measure their viscoelastic and fracture response to dynamic loading. Impact velocities ranged from 80 to 200 m/s. High-speed camera images capturing transient deformation reveal these materials exhibit significant elastic recovery in both the longitudinal and radial directions. Images were time synchronized with free surface velocity measurements, using VISAR, to track elastic/plastic wave interactions attributed to the material’s dynamic loading response. Some specimens underwent brittle fracture once a critical areal strain was exceeded while the axial strain response appeared unaltered.
A Multi-Phasic, Continuum Damage Mechanics Model of Mechanically Induced Increased Permeability in Tissues
Authors: O’Neill Brian , Quinn Timothy P , Frenkel Victor , Li King C P
Published: April 2011

Recently, we have reported enhanced permeability of tissues due to in vivo treatment with pulsed high intensity focused ultrasound (pHIFU). This new therapy has shown promise as a way of increasing the penetration of large drug molecules, both out of the vasculature and through the tissue. To date, no clear physical model of tissue exists that can account for these effects.

A new model is proposed that clearly establishes the link between tissue structure and fluid flow properties on one hand, and the history of applied mechanical forces on the other. The model draws inspiration from two different theoretical fields of materials science, multi-phase theory and continuum damage mechanics. The theory differs from the traditional bi-phasic solid-fluid model of tissues in that the fluid part here is broken into trapped (moving with the solid) and free (moving through the solid) parts. A damage-like variable links the effective elasticity of the tissue to the ratio of the trapped to free fluids. As the damage increases, the tissue becomes, in effect, less stiff and more permeable. Release of elastic energy drives the process. A distribution of energy barriers opposes the process and governs how the fluid is released as damage increases.
Micromechanical and Structural Analysis of Compromised Dental Tissues
Authors: Mahoney Erin , Kilpatrick Nicky , Swain Michael
Published: April 2011

With the continual development of new dental materials and treatment techniques, dental materials research is now more important than ever. Although the traditional method of materials and treatment technique assessment was laboratory based, there is a need for investigations into the clinical outcomes of in-vitro research. The aim of this paper is to discuss the micromechanical and structural analysis of compromised dental tissues that affect young children, using a biomaterial and clinical approach.
Biphasic composite of Tricalcium phosphate reinforced with Hydroxyapatite Whiskers
Authors: Viswanath B , Narayanan Ravishankar
Published: April 2011

Biphasic calcium phosphates have received considerable attention due to their optimum dissolution rate in the human body after implantation. These materials are composed of hydroxyapatite (HA) and resorbable tricalcium phosphate (TCP). In the present investigation, HA whiskers are reinforced into TCP to enhance the mechanical properties of this biphasic composite. Various amounts (30-50 wt%) HA whiskers are reinforced in TCP matrix. Microstructural characterization has been carried out using field-emission scanning electron microscope. Mechanical properties have been investigated by microindentation in a universal testing machine (UTM). As TCP is resorbable, it will dissolve in body fluid and there is a strong possibility for the faceted HA whiskers to interact with functional groups present in the body fluid surroundings.
Functional aspects of the hydration of two natural materials – insect cuticle and pine-cone bract
Authors: Vincent Julian
Published: April 2011

Water is a somewhat neglected parameter in the mechanical properties of biological materials. In some instances it takes over a role of actuation, even in dead structures.
Nanoindentation Measurements of Bone Viscoelasticity as a Function of Hydration State
Authors: Bembey Amanpreet Kaur , Oyen Michelle , Bushby Andrew , Boyde Alan
Published: April 2011

Bone is an anisotropic material, and its mechanical properties are determined by its microstructure as well as its composition. Mechanical properties of bone are a consequence of the proportions of, and the interactions between, mineral, collagen and water. Water plays an important role in maintaining the mechanical integrity of the composite, but the manner in which water interacts within the ultrastructure is unclear. Dentine being an isotropic two-dimensional structure presents a homogenous composite to examine the dehydration effects. Nanoindentation methods for determining the viscoelastic properties have recently been developed and are a subject of great interest. Here, one method based on elastic-viscoelastic correspondence for ‘ramp and hold’ creep testing (Oyen, J. Mater. Res., 2005) has been used to analyze viscoelastic behavior of polymeric and biological materials. The method of ‘ramp and hold’ allows the shear modulus at time zero to be determined from fitting of the displacement during the maximum load hold. Changes in the viscoelastic properties of bone and dentine were examined as the material was systematically dehydrated in a series of water:solvent mixes. Samples of equine dentine were sectioned and cryo-polished. Shear modulus was obtained by nanoindentation using spherical indenters with a maximum load hold of 120s. Samples were tested in different solvent concentrations sequentially, 70% ethanol to 50% ethanol, 70 % ethanol to 100% ethanol, 70% ethanol to 70% methanol to 100% methanol, and 70% ethanol to 100% acetone, after storage in each condition for 24h. By selectively removing and then replacing water from the composite, insights in to the ultrastructure of the tissue can be gained from the corresponding changes in the experimentally determined moduli, as well as an understanding of the complete reversibility of the dehydration process.
Effects of gelatin on mechanical properties of hydroxyapatite-gelatin nano-composites
Authors: Ko Ching-Chang , Oyen Michelle , Fallgatter Alison M , Hu Wei-Shou
Published: April 2011

A biomimetic reactor has been developed to synthesize hydroxyapatite-gelatin (HAP-GEL) nanocomposites that mimic ultra-structures of natural bone. We hypothesize that in the reactor, gelatin concentration controls morphology and packing structures of HAP crystals. To test the hypothesis, three types of mechanical tests were conducted, including nanoindentation, compression, and fracture tests. Nanoindentation tests in conjunction with computer modeling were used to assess effects on gelatin-induced microstructures of HAP. The results showed that increasing gelatin content increased both the plane strain modulus and the fracture toughness. The gelatin appeared to shorten the HAP crystal distance, which consolidated the internal structure of the composite and made the material more rigid. The fracture toughness KIC increased partially due to the effect of fiber bridging between gelatin molecules. The highest fracture toughness (1.12 MPa·m1/2) was equivalent to that of pure hydroxyapatite. The compressive strength of the HAP-GEL (107.7±6.8 MPa) was, however, less sensitive to microstructural changes and was within the range of natural cortical bone (human 170 MPa, pig: 100 MPa). The compression strength was dominated by void inclusions while the nanoindentation response reflected ultra-structural arrangement of the crystals. The gelatin concentration is likely to modify crystal arrangement as demonstrated in TEM experiments but not void distribution at macroscopic levels.
Role of Interfacial Interactions on Mechanical Properties of Biomimetic Composites for Bone Tissue Engineering
Authors: Verma Devendra , Bhowmik Rahul , Mohanty Bedabibhas , Katti Dinesh R , Katti Kalpana S
Published: April 2011

Interfaces play an important role in controlling the mechanical properties of composites. Optimum mechanical strength of scaffolds is of prime importance for bone tissue engineering. In the present work, molecular dynamics simulations and experimental studies have been conducted to study effect of interfacial interactions on mechanical properties of composites for bone replacement. In order to mimic biological processes, hydroxyapatite (HAP) is mineralized in presence of polyacrylic acid (PAAc) (in situ HAP). Further, solid and porous composites of in situ HAP with polycaprolactone (PCL) are made. Mechanical tests of composites of in situ HAP with PAAc have shown improved strain recovery, higher modulus/density ratio and also improved mechanical response in simulated body fluid (SBF). Simulation studies indicate potential for calcium bridging between –COO− of PAAc and surface calcium of HAP. This fact is also supported by infrared spectroscopic studies. PAAc modified surfaces of in situ HAP offer means to control the microstructure and mechanical response of porous composites. Nanoindentation experiments indicate that apatite grown on in situ HAP/PCL composites from SBF has improved elastic modulus and hardness. This work gives insight into the interfacial mechanisms responsible for mechanical response as well as bioactivity in biomaterials.
Influence of Strain Rate on the Mechanical Behaviour of Human Interstitial Bone at the Microstructural Level
Authors: Maximilien Vanleene , Pierre-Emmanuel Mazeran , Marie-Christine Ho Ba Tho
Published: April 2011

Investigation of bone mechanical properties is of importance for bone pathology researches, development of biomaterials and prosthesis. Due to the bone complex multi-scale structure, assessment of bone microstructure is an important step of bone mechanical behaviour understanding. In this study, we performed dynamic nanoindentation tests in order to investigate visco-plastic and visco-elastic properties of bone interstitial lamellae.
A Theoretical Basis for Biomimetics
Authors: Vincent Julian , Bogatyreva Olga , Bogatyrev Nikolay , Pahl Anja-Karina , Bowyer Adrian
Published: April 2011

The Russian “Theory of Inventive Problem Solving” was developed to solve technological problems, but it can equally well be applied to biology and the way organisms have solved similar problems. As such it can also be used to bridge the gap between biology and engineering, and to provide a theoretical basis for Biomimetics.
Modeling receptor-mediated endocytosis via mechanics of cell adhesion
Authors: Shi Wendong , Gao Huajian , Freund Lambert Ben
Published: April 2011

A mathematical model describing how a cell membrane with diffusive mobile receptors wraps around a ligand coated cylindrical or spherical particle has been recently developed to model particle size effects in receptor mediated endocytosis. The model predicted an optimal particle size for the smallest wrapping time, as well as a minimum and a maximum particle size for successful wrapping. The results showed that particles in the size range of tens to hundreds of nanometers can enter cells even in the absence of clathrin or caveolin coats. Here we report some further progresses on modeling a spontaneous membrane curvature induced by clathrin or caveolin coats at the inner membrane leaflet as well as simultaneous entry of many particles into cells. It is found that a spontaneous membrane curvature narrows the particle size range for successful wrapping and there exists an optimal particle density for maximum particle adsorptivity.
Crystalline TiO2 Macrocellular Foams with Highly Nano-Mesoporous Framework
Authors: Carn Florent , Reculusa Stéphane , Deleuze Hervé , Backov Rénal
Published: April 2011

Titanium dioxide macro-cellular monolith-type materials have been obtained with emphases toward controlling porosity at the meso- and macroscopic length scales, leading thus to hierarchically organized porous architectures. First, at the microscopic length scale, either monophasic Anatase, biphasic Anatase-Rutile or monophasic Rutile allotropic forms are generated by varying the applied thermal treatment. At the meso- and/or nanoscopic length scales either lyotropic templates or latex colloids have been used to promote meso- or super-mesoporosity. Particularly, a Pluronic copolymer P-123 combined with cationic surfactant (TTAB) induces vermicular-like mesoporosity associated with a specific surface area around 450 m2·g-1. At the macroscopic length scale a non-static air-liquid foam strategy allows a strong control over the open-cell morphologies.
Engineered Three-dimensional Colloidal Crystals Containing a Planar Defect
Authors: Massé Pascal , Agricole Beatrice , Sellier Elisabeth , Ravaine Serge
Published: April 2011

We have fabricated three-dimensional (3D) colloidal crystals containing a two-dimensional (2D) defect as the middle layer by the Langmuir-Blodgett (LB) technique. Either a homogeneous sheet made of hundreds layers of behenic acid or a single monolayer of silica colloidal particles of various sizes was inserted between two opal films of silica spheres. The presence of the extrinsic defect led to an impurity mode within the photonic stop band, which was observed as a pass band in the near-infrared (NIR) spectra. The position of this defect mode was found to vary periodically with the value of the ratio of the thickness of the defect layer to the diameter of the colloids of the upper and lower opals.
Nucleation and growth of polymorphs of barbital on chemically modified surfaces in microfluidic channels
Authors: MacDonald John C. , Biyikli Kasim , Zugic Branko , Ebersole Garrett , Allor Joshua
Published: April 2011

Crystallization experiments have been carried out in microfluidic devices to screen for polymorphs by crystallization on a range of surfaces. These devices consist of PDMS (polydimethylsiloxane) patterned with microchannels and then bonded to self-assembled monolayers (SAMs) of organic molecules on gold substrates. Barbital was crystallized in microchannels over five different SAMs functionalized with polar and nonpolar organic groups. Growth of polymorphs was examined under thermodynamic conditions from solutions at room temperature and under kinetic conditions by rapid cooling. The results of these experiments and the influence of chemically modified surfaces in microchannels in controlling polymorphism are discussed.
Protein-Directed Self-Assembly of Gold Nanoparticles
Authors: Vertegel Alexey , Shang Wen , Dordick Jonathan , Siegel Richard
Published: April 2011

We have employed protein-protein interactions for controlled assembly of gold nanoparticles. Stoichiometric 1:1 protein:nanoparticle conjugates were prepared for proteins known to strongly interact with each other and these interactions were used to self-assemble nanoparticles. Mixing equivalent amounts of the antigen-nanoparticle and antibody-nanoparticle conjugates resulted in the formation of nanoparticle dimers with a yield of about 60%. Trimers (yield ∼30%) can be obtained by mixing 2:1 antigen-nanoparticle with 1:1 antibody-nanoparticle conjugates in a molar ratio of 1:2. The structures are destroyed at low pH when the antibody-antigen complex dissociates.
Electronic structure of helically coiled carbon nanotubes
Authors: Guzman-Verri Gian Giacomo , Lew Yan Voon Lok C. , Willatzen Morten , Gravesen Jens
Published: April 2011

In the present work we calculate the electronic band structure of single-wall helical carbon nanotubes following an effective-mass approach. We include curvature effects and strain due to bending in the band structure. The curvature energy ΔE, and the change in the electronic energy ΔEs due to strain, depend upon the coil pitch and coil diameter of the tube. We find 0.003 ≤|ΔE|≤ 1.3 eV and 0 ≤ΔEs ≤ 4.0 eV for the single-wall helical carbon nanotubes considered here.
Self-Assembled GdSi2 Nanostructures Grown on Si(001) Studied by TEM and STM
Authors: Ye Gangfeng , Crimp Martin A. , Nogami Jun
Published: April 2011

Self-assembled gadolinium silicide (GdSi2) nanostructures grown on Si(001) were studied by transmission electron microscopy (TEM) and scanning tunneling microscopy (STM). Cross-sectional TEM and plan-view TEM moiré fringe pattern analyses show that the GdSi2 nanostructures can be divided into two classes: elongated nanowires with hexagonal crystal structure and rectangular islands with orthorhombic or tetragonal crystal structure.
Multifunctional nanocomposite transparent UV-blocking films sythesized by sol-gel process for optical, automotive and aeronautic applications
Authors: Ayalasomayajula Phani Ratna , Santucci S.
Published: April 2011

Development of UV blocking thin films with effective cut-off features with steep edges and high transmission in the visible and IR region have been developed. The unique optical, mechanical and chemical properties of silica and ceria nanocomposites with surface functional groups making them most promising candidate for applications in opto-electronic, automotive, and aeronautic industries. On the other hand, highly hydro and oleophobic films are being actively considered in optical, automotive and aeronautic industries to increase adhesion and scratch, abrasion resistance properties. In order to fill the gap, and fulfill the requirements to meet both ends, it could be proved that morphological changes in the nanometer range influences the water contact angles and their hystersis of low energy materials. Nanocomposite films of SiO2 and CeO2 with surface functionalisation with decafluorooctly-triethoxy silane itself forms nano-hemispheres (similar to lotus leaf) at and above 100°C favoring an increase in water contact angle from 122° (25°C) to 145°(400°C). The structural, optical, and hydrophobic properties have been examined by employing X-ray diffraction, UV-visible spectroscopy, contact angle techniques, respectively. The cut-off behavior of the deposited and annealed nanocomposite thin films have been tuned by varying different amounts of CeO2 in SiO2.
Effect of Diameter on Electron Field Emission of Carbon Nanotube Bundles
Authors: McClain Devon , DeRoss Mason , Tavan Noel , Jiao Jun , McCarter Coralee M , Richards Robert F , Mesarovic Sinisa , Richards Cecilia D , Bahr David F
Published: April 2011

Arrays of multi-walled carbon nanotunbe (CNT) bundles were fabricated on silicon [100] substrate with iron-nitrate sol-gel catalyst patterned via standard photolithographic techniques. Nanotube bundles with diameters ranging from 400µm to 15µm were grown in a chemical vapor deposition reactor and electrically characterized using a scanning-anode probe apparatus. Results showed a relatively low number of graphitic layers in individual nanotubes and a definite increase in field emission performance with decreasing bundle diameter. A 400µm wide matt of CNTs yielded a turn-on field of 6.7 V/µm and field enhancement of 602 while 15µm bundles performed significantly better with turn-on fields of 1.6 V/µm and field enhancement factors of 2425. The overall trend strongly suggests that the field emission character of CNT based aggregate structures such as those presented here is proportional to their aspect ratio.
Novel Synthesis of Gold Nanoparticles in Aqueous Media
Authors: Sugunan Abhilash , Dutta Joydeep
Published: April 2011

Aqueous synthesis of glutamate functionalized single crystal gold nanoparticles was achieved by reducing a solution of chloroauric acid with monosodium glutamate. Highly uniform size distributions of the nanoparticles can be obtained by having an excess of monosodium glutamate in the starting solution. Similar to the conventional synthesis of gold nanoparticles by reduction of chloroauric acid with trisodium citrate, the obtained particle sizes were varied in the range of 14 – 25 nm without loosing the uniformity in its size distribution by varying the concentration of the reducing agent.
Self Assembly of Anisotropic Organic Molecules: Diffusion versus Sticking Anisotropy
Authors: Berkebile Stephen , Koller Georg , Hlawacek Gregor , Oehzelt Martin , Resel Roland , Netzer Falko P. , Ramsey Michael G.
Published: April 2011

The molecular/crystal orientation and morphology of active molecular structures is a key determinant for the function of nanoscaled organic devices. In π-conjugated systems, both charge transport and optical properties will strongly depend on the molecular orientation due to the highly anisotropic charge carrier mobility in these organic crystals and the anisotropic absorption and luminescence behavior of the molecules. Although the importance of organic on inorganic interface formation and thin film growth is widely acknowledged, little is known regarding the growth kinetics. A better understanding of the processes driving molecular self-assembly is necessary if the self-assembly process is to be controlled. Moreover, it is interesting as the anisotropy of the molecular building blocks presents a fundamental difference from what is known from inorganic growth. Here we show that either sticking or diffusion anisotropy can control the growth depending on preparation conditions. This is illustrated by an investigation into the growth of sexiphenyl (6P) on the anisotropic TiO2(110)-(1×1) surface for temperatures between 80K and 400K using in-situ UHV photoemission, x-ray absorption spectroscopy, synchrotron x-ray diffraction and ex-situ atomic force microscopy. For 6P adsorption even at 80K we found that the molecules orient parallel to the TiO2 oxygen rows and form small crystallites. At 300K this molecular orientation is retained and large micrometer sized 6P(203) oriented needles running perpendicular to oxygen substrate rows are formed. In contrast, for growth at elevated temperatures the 6P molecular axis is near perpendicular to the surface and large islands elongated parallel to the substrate rows are formed. These differences in crystallite orientation and morphology can be explained by the domination of the growth kinetics by either sticking or diffusion anisotropy depending on growth temperature.
Fabrication and Defect Designs on DNA Linked 2-D Colloidal Photonic Crystals Using a Nd:YAG Pulsed Laser
Authors: Asmatulu Ramazan , Kim Sejong , Bright Robin , Yu Phillip , Papadimitrakopoulos Fotios , Marcus Harris
Published: April 2011

Controlled defects were created on DNA linked 2-D colloidal photonic crystals using a Nd:YAG pulsed laser. The 2-D photonic crystals were self-assembled using 1.8 μm polystyrene (PS) microspheres on functionalized glass substrates. To synthesize the hexagonal close packed crystalline samples, both substrate and particles attached single-strand DNA, sequence A on the substrate and sequence B on the particles. The DNA was hybridized using the DNA linker with complementary single-strand A’B’ that anchored the particles to the substrate during self-assembly. The 532 nm second harmonic wavelength beam of the pulsed Nd:YAG laser (1064 nm) with a pulse width of 10 ns was used for the removal of individual colloidal particles from the self assembled photonic crystals. In the present tests, the diameter of the laser beam was optically reduced from 7 mm to about 1.8 μm. Controlled line defects and geometrical shapes (e.g., hexagonal and triangle) were created in the 2D arrays in an aqueous medium.
Nanostructured Ceramic Film Formation on Self-Assembled Monolayers via a Biomimetic Approach
Authors: Cho Junghyun , Blom Douglas A. , Coffey Dorothy W. , Allard Lawrence F. , Cho Junghyun
Published: April 2011

A biomimetic approach is employed to deposit ceramic films on organic self-assembled monolayers (SAMs) coated substrates. Specifically, zirconia (ZrO2) films are grown in a zirconium sulfate precursor solution at near room temperatures (∼70°C). This process, directed by the nanoscale organic template, mimics the controlled nucleation and growth of the biominerals such as bones and teeth. The resultant zirconia films consist of nanosized particles (5-10 nm) that are precipitated out in a supersaturated precursor solution. Cross-sectional TEM and STEM works were performed to quantitatively analyze the film structure and chemistry, as well as interfacial region of the ceramic-SAM films. A stepwise deposition process was developed to avoid excessive formation of aggregation. Further, the dynamic nanoindentation testing was employed to assess the thickness and film-only intrinsic mechanical properties for direct comparison among the films processed with different processing parameters and microstructures. The films with finer particulate structure displayed higher intrinsic modulus than did those with coarser structure.
A General Route to Nanoparticle Thin Films and Coatings
Authors: Mukherjee Bratindranath , Narayanan Ravishankar
Published: April 2011

Nanoparticles thin films have wide range of applications such as nanoelectronics, magnetic storage devices, SERS substrate fabrication, optical grating and antireflective coating. Present work describes a method to prepare large area nanoparticles thin film of the order of few square centimeters. Thin film deposition has been done successfully on a wide range of conducting as well as non conducting substrates such as carbon-coated copper grid, silicon, m-plane of alumina, glass and (100) plane of NaCl single crystal. SEM, TEM and AFM studies have been done for microstructural characterization of the thin films. A basic mechanism has been proposed towards the understanding of the deposition process.
Pulsed Laser Deposition of Cluster-Assembled Thin Films with Controlled Nanostructure
Authors: Li Bassi Andrea , Casari Carlo Spartaco , Di Fonzo Fabio , Bailini Alessandro , Fusi Matteo , Dellasega David , Russo Valeria , Baserga Andrea , Cattaneo Diego , Bottani Carlo Enrico
Published: April 2011

Thin films synthesized by assembling clusters present interesting chemical and physical properties and a large specific surface, and are appealing for functional applications (e.g. sensing and catalysis). Also, clusters supported on surfaces are interesting both for nanocatalysis applications and for fundamental research. By means of pulsed laser deposition (PLD) in a background atmosphere we can induce cluster aggregation in the ablation plume and control the deposition kinetic energy of the clusters. These phenomena depend on the plume expansion dynamics and their influence on the properties of the deposited films has been investigated as a function of the background gas mass and pressure. The control of these parameters permits variation of the film surface morphology, from a compact structure with a very smooth surface, to a film with a controlled roughness at the nanoscale, to an open, low density meso- and nanostructure characterized by a high fraction of voids and by a large specific area. Thin films of WOx, TiOx, Pd/PdO, and Ag were deposited and characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM) and Raman spectroscopy. Post-deposition annealing permits control of the crystalline degree of the films, which in the case of tungsten and titanium oxide is found to depend on the original nanostructure, while a different degree of oxidation can be induced by controlling the amount of oxygen in the deposition chamber. In-situ scanning tunneling microscopy (STM) was employed to study the first stages of growth of W films on different substrates. This opens the possibility to tailor the material properties through the control of the building nano-units.