bluuebunny/crossref_metadata_2025 huggingface dataset:
Ferromagnetism in lightly Gd doped GaN: The role of defects
Authors: Mishra J. K. , Dhar S. , Khaderabad M. A. , Brandt O.
Published: September 2011

Gd:GaN layers grown with different Gd concentrations by molecular beam epitaxy (MBE) are studied using photoconductivity and photo-thermoelectric power spectroscopy. Our study reveals that the incorporation of Gd produces a large concentration of acceptor-like defects in the GaN lattice. The defect band is found to be located ~450meV above the valence band. Moreover, the concentration of defects is found to increase with the Gd concentration. The effect of annealing on the structural and the magnetic properties of GaN implanted with Gd is also investigated. A clear correlation between the saturation magnetization and the defect density is observed in implanted samples. The colossal magnetic moment per Gd ion and the ferromagnetism observed in this material is explained in terms of the formation of giant defect cluster around each Gd ion.
Structural, Magnetic and Electronic Properties of Dilute MnScN(001) Grown by RF Nitrogen Plasma Molecular Beam Epitaxy
Authors: Constantin Costel , Wang Kangkang , Chinchore Abhijit , Chia Han-Jong , Markert John , Smith Arthur R.
Published: September 2011

The structural, magnetic, and electronic properties of dilute Mn-doped scandium nitride thin films grown by radio frequency N-plasma molecular beam epitaxy are explored. The results indicate a small magnetization extending up to as high as 350K. There is a slight dependence on the manganese concentration, with the lower Mn concentration showing a larger saturation magnetization.
Synthesis of New Flexible Aerogels from Di- and Trifunctional Organosilanes
Authors: Hayase Gen , Kanamori Kazuyoshi , Nakanishi Kazuki , Hanada Teiichi
Published: September 2011

Recent years, although silica aerogels are expected to be the promising material for energy savings, the lack of mechanical strength prevents from commercial applications such as to low-density thermal insulators. To improve mechanical properties, methyltrimethoxysilane (MTMS) and dimethyldimethoxysilane (DMDMS) are used in this study as the co-precursor of aerogels because the network becomes flexible due to the relatively low cross-linking density and to the unreacted methyl groups. Because of the strong hydrophobicity of MTMS/DMDMS-derived condensates, phase separation occurs in aqueous sol and must be suppressed to obtain uniform and monolithic gel. We employed surfactant n-hexadecyltrimethylammonium chloride (CTAC) in starting compositions to control phase separation during a 2-step acid/base sol-gel reaction. By changing the starting composition, various microstructures of pores are obtained. In the uniaxial compression test, the aerogel showed high flexibility and spring-back to the original shape after removing the stress.
Low temperature CVD growth of graphene nano-flakes directly on high K dielectrics
Authors: Rümmeli Mark H. , Bachmatiuk Alicja , Dianat Arezoo , Scott Andrew , Börrnert Felix , Ibrahim Imad , Zhang Shasha , Borowiak-Palen Ewa , Cuniberti Gianaurelio , Büchner Bernd
Published: September 2011

The potential of MgO and ZrO2 as catalytically active substrates for graphene formation via thermal CVD is explored. Experimental observations show the growth of single and multi-layer graphene nano-flakes over MgO and ZrO2 at low temperatures. The graphene nano-flakes are found to anchor at step sites. Ab initio calculations indicate step sites are crucial to adsorb and crack acetylene.
Metal-catalyzed graphitization in Ni-C alloys and amorphous-C/Ni bilayers
Authors: Saenger Katherine L. , Lavoie Christian , Carruthers Roy , Bol Ageeth A. , Mcardle Timothy J. , Chu Jack O. , Tsang James C. , Grill Alfred
Published: September 2011

Metal-catalyzed graphitization from vapor phase sources of carbon is now an established technique for producing few-layer graphene, a candidate material of interest for post-silicon electronics. Here we describe two alternative metal-catalyzed graphene formation processes utilizing solid phase sources of carbon. In the first, carbon is introduced as part of a cosputtered Ni-C alloy; in the second, carbon is introduced as one of the layers in an amorphous carbon (a-C)/Ni bilayer stack. We examine the quality and characteristics of the resulting graphene as a function of starting film thicknesses, Ni-C alloy composition or a-C deposition method (physical or chemical vapor deposition), and annealing conditions. We then discuss some of the competing processes playing a role in graphitic carbon formation and review recent evidence showing that the graphitic carbon in the a-C/Ni system initially forms by a metal-induced crystallization mechanism (analogous to what is seen with Al-induced crystallization of amorphous Si) rather than by the dissolution-upon-heating/precipitation-upon-cooling mechanism seen when graphene is grown by metal-catalyzed chemical vapor deposition methods.
On the Formation of Carbon Nanotube Serpentines: Insights from Multi-Million Atom Molecular Dynamics Simulation
Authors: Machado Leonardo D. , Legoas Sergio B. , Soares Jaqueline S. , Shadmi Nitzan , Jorio Ado , Joselevich Ernesto , Galvao Douglas S.
Published: September 2011

In this work we present preliminary results from molecular dynamics simulations for carbon nanotubes serpentine dynamics formation. These S-like nanostructures consist of a series of parallel and straight nanotube segments connected by alternating U-turn shaped curves. Nanotube serpentines were experimentally synthesized and reported in recent years, but up to now no atomistic simulations have been carried out to address the dynamics of formation of these structures. We have carried out fully atomistic molecular dynamics simulations in the framework of classical mechanics with a standard molecular force field. Multi-million atoms structures formed by stepped substrates with a carbon nanotube (about 1 micron in length) placed on top of them have been considered in our simulations. A force is applied to the upper part of the tube during a short period of time and then turned off and the system set free to evolve in time. Our results showed that these conditions are sufficient to form robust serpentines and validate the general features of the ‘falling spaghetti mechanism’ previously proposed to explain their formation.
Curvature-induced Symmetry Lowering and Anomalous Dispersion of Phonons in Single-Walled Carbon Nanotubes
Authors: Reppert Jason , Podila Ramakrishna , Li Nan , Loebick Codruta Z. , Stuart Steven J. , Pfefferle Lisa D. , Rao Apparao M.
Published: September 2011

Here we report rich and new resonant Raman spectral features for several sub-nanometer diameter single wall carbon nanotubes (sub-nm SWNTs) samples grown using chemical vapor deposition technique operating at different temperatures. We find that the high curvature in sub-nm SWNTs leads to (i) an unusual S-like dispersion of the G‑band frequency due to perturbations caused by the strong electron-phonon coupling, and (ii) an activation of diameter-selective intermediate frequency modes that are as intense as the radial breathing modes (RBMs). Furthermore, an analytical approach which includes the effects of curvature into the overlap integral and the energy gap between the van Hove singularities is discussed. Lastly, we show that the phonon spectra for sub-nm SWNTs obtained from the molecular dynamic simulations which employs a curvature-dependent force field concur with our experimental observations.
Properties Modeling of Low-Dimensional Carbon Nanostructures
Authors: Basteev Andrew , Bazyma Leonid , Ugryumov Mykhaylo , Chernishov Yuriy , Slepicheva Margarita
Published: September 2011

The modeling of single wall carbon nanotubes properties (length, diameter, chirality, defective wall structure) influence on sorption capability at different thermodynamic conditions (T= 80-273 К; P = 2-12 MPa) is presented in this work. The applied simulation procedure is the molecular dynamics as well the new event-driven simulation algorithm has been used. In the frameworks of this event-driven simulation algorithm the modeling of structure formation for carbon nanotubes have been done with different chirality and with wall defects presence. The analysis of obtained results and their comparison with published experimental and theoretical results are performed.
Effect of Sidewall Fluorination on the Mechanical Properties of Catalytically Grown Multi-Wall Carbon Nanotubes
Authors: Ganesan Yogeeswaran , Peng Cheng , Ci Lijie , Khabashesku Valery , Ajayan Pulickel M. , Lou Jun
Published: September 2011

We report on the usage of a simple microfabricateddevice, that works in conjunction with a quantitative nanoindenter inside a scanning electron microscope (SEM), for the in situ quantitative tensile testing of individual sidewall fluorinated multi-wall carbon nanotubes (MWNTs). The stress vs. strain curves and the tensile strength values for five fluorinated specimens have been presented and compared to those of pristine MWNT specimens (data reported earlier). The fluorinated specimens were found to deform and fail in a brittle fashion similar to pristine MWNTs. However, sidewall fluorination was found to have considerably degraded the mechanical properties (tensile strength and load bearing capacity) of the MWNTs.
Dynamics of Graphene Nanodrums
Authors: Brunetto Gustavo , Legoas Sergio B. , Coluci Vitor R. , Lucena Liacir S. , Galvao Douglas S.
Published: September 2011

Recently, it was proposed that graphene sheets deposited on silicon oxide can act as impermeable atomic membranes to standard gases, such as helium, argon, and nitrogen. It is assumed that graphene membrane is clamped over the surface due only to van der Waals forces. The leakage mechanism can be experimentally addressed only indirectly. In this work we have carried out molecular dynamics simulations to study this problem. We have considered nano-containers composed of a chamber of silicon oxide filled with gas and sealed by single and multi-layer graphene membranes. The obtained results are in good qualitative agreement with the experimental data. We observed that the graphene membranes remain attached to the substrate for pressure values up to two times the largest value experimentally investigated. We did not observe any gas leakage through the membrane/substrate interface until the critical limit is reached and then a sudden membrane detachment occurs.
Characterizing the effect of uniaxial strain on the surface roughness of Si nanowire MEMS-based microstructures
Authors: Escobedo-Cousin E. , Olsen S.H. , Pardoen T. , Bhaskar U. , Raskin J.-P.
Published: September 2011

This work addresses the paucity of roughness measurements by reporting on roughness parameters in uniaxial strained Si beams relevant for state of the art MOSFETs, nanowire and MEMS devices, with varying degrees of strain. Roughness is characterized by high resolution AFM and strain is characterized by Raman spectroscopy. Microstructures comprising a silicon nitride actuator are used to induce a wide range of stress levels in Si beams. The microstructures also allow the comparison of surface evolution in the strain direction (along the Si beam) compared with the unstrained direction (across the Si beam). A gradual reduction in rms roughness amplitude and increase in roughness correlation length in the direction of the applied stress are found for increasing values of strain. In contrast, surface roughness in the direction perpendicular to the applied stress remained largely unchanged from the unstrained initial state.
Improving PZT Thin Film Texture Through Pt Metallization and Seed Layers
Authors: Sanchez L.M. , Potrepka D.M. , Fox G.R. , Takeuchi I. , Polcawich R.G.
Published: September 2011

Leveraging past research activities in orientation control of lead zirconate titanate (PZT) thin films [1,2], this work attempts to optimize those research results using the fabrication equipment at the U.S. Army Research Laboratory so as to achieve a high degree of {001}- texture and improved piezoelectric properties. Initial experiments examined the influence of Ti/Pt and TiO2/Pt thins films used as the base-electrode for chemical solution deposition PZT thin film growth. In all cases, the starting silicon substrates used a 500 nm thermally grown silicon dioxide. The Pt films were sputter deposited onto highly textured titanium dioxide films grown by a thermal oxidation process of a sputtered Ti film [3]. The second objective targeted was to achieve highly {001}-textured PZT using a seed layer of PbTiO3 (PT). A comparative study was performed between Ti/Pt and TiO2/Pt bottom electrodes. The results indicate that the use of a highly oriented TiO2 led to highly {111}-textured Pt, which in turn improved both the PT and PZT orientations. Both PZT (52/48) and (45/55) thin films with and without PT seed layers were deposited and examined via x-ray diffraction methods (XRD) as a function of annealing temperature. As expected, the PT seed layer provides significant improvement in the PZT {001}-texture while suppressing the {111}-texture of the PZT. Improvements in the Lotgering factor (f) were observed upon comparison of the original Ti/Pt/PZT process (f=0.66) with samples using the PT seed layer as a template, Ti/Pt/PT/PZT (f=0.87), and with films deposited onto the improved Pt electrodes, TiO2/Pt/PT/PZT (f=0.96).
Mechanical and Material Characterization of Bilayer Microcantilever-based IR detectors
Authors: Lin I-Kuan , Du Ping , Zhang Yanhang , Zhang Xin
Published: September 2011

Infrared radiation (IR) detection and imaging are of great importance to a variety of military and civilian applications. Microcantilever-based IR detectors have recently gained a lot of interest because of their potential to achieve extremely low noise equivalent temperature difference (NETD) while maintaining low cost to make them affordable to more applications. However, the curvature induced by residual strain mismatch within the microcantilever severely decreases the device performance. To meet performance and reliability requirement, it is important to fully understand the deformation of IR detectors. Therefore, the purpose of this study is threefold: (1) to develop an engineering approach to flatten IR detectors, (2) to model and predict the elastic deformation of IR detectors using finite element analysis (FEA), and (3) to study the inelastic deformation during isothermal holding.
Investigation and Study of a Transparent and Translucent Facade System, with a Minimum Thickness, a Free Form Design, and a High Thermal Insulation Value to be used in to Architectural Development
Authors: Alonso Luis A. , Lauret Benito , Alonso Fernando
Published: September 2011

This article examines, from the energy viewpoint, a new lightweight, slim, high energy efficient, light-transmitting envelope system, providing for seamless, free-form designs for use in architectural projects. The research was based on envelope components already existing on the market, especially components implemented with granular silica aerogel insulation, as this is the most effective translucent thermal insulation there is today. The tests run on these materials revealed that there is not one that has all the features required of the new envelope model, although some do have properties that could be exploited to generate this envelope, namely, the vacuum chamber of vacuum insulated panels (VIP), the monolithic aerogel used as insulation in some prototypes, and reinforced polyester barriers. By combining these three design components — the high-performance thermal insulation of the vacuum chamber combined with monolithic silica aerogel insulation, the free-form design potential provided by materials like reinforced polyester and epoxy resins—, we have been able to define and test a new, variable geometry, energy-saving envelope system.
Sub-Micron Feature Patterning of Thermoplastics using Multi-Scale BMG Tooling
Authors: Stratton Dermot J. , Byrne Cormac , Mulcahy James , Browne David J.
Published: September 2011

One potential application for Bulk Metallic Glasses (BMGs) is in dies with micro- and nano-sized features. Three basic characteristic sets inherent to BMGs make them ideal materials for micro/nano-tooling applications: (1) excellent compressive strength, wear and corrosion resistance; (2) amorphous structure which presents no microstructural length scale limitation to cutting and forming operations; (3) the presence of a glass transition temperature above which they can be easily formed. There are many potential applications for multi-scale BMG tooling, including in production of microfluidic and other precision biomedical devices. In the current work, discs were cut from 5 mm diameter cylindrical specimens of Zr44Cu40Al8Ag8 BMG produced via arc melting and casting into water-cooled copper molds. The cylindrical specimens were then thermoplastically formed into thin coin-like disc samples. The thin disc-shaped plates were then ground and polished to create a smooth flat surface. Sub-micron-sized features were patterned into the plates via a focused ion beam. We demonstrated that such feature sizes are not achievable in conventional crystalline metallic tool materials. The patterned BMG tools were then set in a compression press where the platen temperature was precisely controlled and a series of load-controlled embossing trials were carried out in which the features of the BMG tooling were replicated in poly(methyl methacrylate) (PMMA) sheet. An exercise in mapping out the size limitation of such a multi-scale embossing operation is reported.
Digital Pulsed Force Mode AFM and Confocal Raman Microscopy in Drug-Eluting Coatings Research
Authors: Haugstad G. , Wormuth K.
Published: September 2011

Controlled release of amorphous drug from a polymer matrix depends intimately upon the degree of mixing of drug and polymer, the susceptibility of the drug to crystallization, and the ability of the drug to dissolve and diffuse through water-swollen polymer. Characterization methods ideally would follow these processes on the molecular level in situ and in real time. We move closer to this ideal state of characterization through application of two imaging methods: digital pulsed force mode atomic force microscopy (D-PFM AFM) and confocal Raman microscopy (CRM). We examine model spin-coated films ~1 μm thick containing the drug dexamethasone dispersed in poly(n-alkyl methacrylate) homopolymer and blend coatings. We report aqueous-immersion studies of surface and subsurface structural changes due to drug elution over time frames ranging from very fast (a few minutes) to slow (tens of hours).
Circular AFM Mode: A New AFM Mode for Investigating Surface Properties
Authors: Noel Olivier , Mazeran Pierre-Emmanuel , Nasrallah Hussein
Published: September 2011

For the first time, a new AFM mode is presented that simultaneously allows the measuring of adhesion and friction forces at different constant and continuous sliding velocities. Our methodology consists of implementing a circular relative displacement of the contact to reach a constant sliding velocity, with no stop periods. Some of the main advantages of performing a circular displacement is that continuous and high sliding velocities (more than 1 mm/s) can be reached compared to the low sliding velocities (up to 10 μm/s) available when using commercial AFM. Also, a stationary state is reached when doing measurements. Moreover, the circular mode can be coupled with the classical operating mode, for instance, force spectrum. Main applications of this circular mode are related to metrological measurements in physics that require high speed displacements. As an example, we report the evolution of friction and adhesive forces measured in air at different high sliding velocities.
Scanning Probe Microscopy with Diamond Tip in Tribo-nanolithography
Authors: Lysenko Oleg , Grushko Vladimir , Mitskevich Evgeni , Mamalis Athanasios
Published: September 2011

The results obtained by direct nano-patterning demonstrate the potential of the SPM-based techniques that include surface scratching to create 3D nanostructures. Such techniques became known as tribo-nanolithography and have prospects of being successfully implemented in the future nanofabrication industry. An important obstacle to this, however, is the effect of wear at the nanometer scale which is critical to the stability of tribo-nanolithoraphic processes. Such stability is achievable via in-depth theoretical and experimental studies of friction at the nanoscale along with the development of pioneering equipment. Our work presents the results of experimental fabrication of nanostructures formed by nanoscratching with the use of the multifunctional scanning tunneling microscopy previously developed by the authors. The authors attempted scratching the silicon surface by using a boron-doped diamond tip. This operation was undertaken in the same direction sequentially with the tip sliding a side of the groove by one of the tip’s facets and the consequent surface scanning. Although not being applicable to non-conductive surfaces, the proposed technique has significant advantages. One advantage is related to the high stiffness of the tunneling probe as compared to the stiffness of the AFM cantilever. High stiffness and perpendicularity of the tip to the surface during surface processing eliminates bending beam effects on the typical AFM and ensures machining effectiveness. Purposely synthesized boron-doped single-crystal diamonds were used as a tip material. The results of experimental fabrication of nanostructures formed by nanoscratching with the use of the multifunctional scanning probe are demonstrated and discussed.
Liquid-crystals-plasmonics for ultrafast broadband all-optical switching
Authors: Khoo I. C. , Huang J. , Zhou S. , Liou J. , Hong K. L. , Ma Yi
Published: September 2011

We present the results and critical analyses of recent studies of ultrafast optical nonlinearities of liquid crystals in the isotropic and ordered phases for time scales spanning femtoseconds – microseconds. Pure undoped liquid crystals as well as liquid crystals containing plasmonic nano-particles have been investigated. Individual molecular electronic optical nonlinearities are found to be useful for femtoseconds – nanoseconds nonlinear transmission clamping applications. On the other hand, laser induced order parameter and birefringence modification in aligned nematic cells allow very rapid transmission switching of visible as well as near infrared lasers with response times in the sub-microseconds - few nanoseconds regime.
π-π Stacking Molecular Interactions Between Carbon Nanotubes and 5CB Liquid Crystal
Authors: Georgiev Georgi Y. , Gombos Erin A. , McIntyre Michael B. , Cebe Peggy
Published: September 2011

Multiwall Carbon Nanotubes (MWCNT) align by coupling to the liquid crystals’ (LC) nematic director in LC/MWCNT dispersions. This coupling is so strong that the LC molecules act as molecular motors to reorient the MWCNTs when an electric field is applied across oriented electro optic cells. On the other hand, MWCNTs also improve the LC order and modify the crystal phase of LCs. We investigate the physical reasons for those strong effects by studying the molecular interactions between a host LC and MWCNTs. It has been predicted theoretically that the aromatic rings could stack with their π orbitals in 4-Cyano-4’-pentylbiphenyl (5CB) and MWCNT nanocomposites. Experimentally 5CB modifies the MWCNTs Raman breathing modes in the same nanocomposites. In turn, we look for evidence of this interaction between MWCNTs and LCs at the 5CB molecules. Using FTIR spectroscopy we found that the modes corresponding to 5CB aromatic rings vibrations are affected in the presence of MWCNTs which confirms that π-π stacking of 5CB’s biphenyl rigid core to the carbon rings on the MWCNTs’ surface may indeed be major mechanism for MWCNT/LC nematic coupling. It shows also that the Raman breathing mode effects on MWCNTs can be due to this π-π stacking interaction with 5CB. Further investigations of the MWCNTs interactions with 5CB can lead to developing of a complete model of this phenomenon and help applications for electro optic cells, nanoswitches, new crystal forms for optics, communication technology and others.
Atomic structure of metal-free and catalyzed Si nanowires
Authors: Nicotra Giuseppe , Bongiorno Corrado , Convertino Annalisa , Cuscunà Massimo , Martelli Faustino , Spinella Corrado
Published: September 2011

Metal-free and Au-catalyzed silicon nanowires (Si-NWs) grown at low temperatures have been analyzed through transmission electron microscopy (TEM) and scanning electron microscopy (SEM), and their crystalline phase studied. All the observed nanowires are crystalline, grow along two different directions, <110> or <112>, and contain high density of planar defects, such as stacking faults (SFs) and twins. The defect size is comparable to the wire diameter for the metal-free process whilst it is much larger than the wire diameter for the Aucatalyzed Si-NWs. In this latter case parallel SFs may re-arrange and transform in a metastable rhombohedral 9R polytype structure whose formation mechanism is discussed.
Electrical Characterization of Ge Nanocrystals in Oxide Matrix
Authors: Capan Ivana , Buljan Maja , Misic-Radic Tea , Pivac Branko , Radic Nikola , Grenzer Joerg , Holy Vaclav , Levichev S. , Bernstorff Sigrid
Published: September 2011

We report on electrical properties in [(Ge+SiO2)/SiO2]×2 films deposited by magnetron sputtering on a periodically corrugated-rippled substrate and annealed in vacuum and forming gas. The rippled substrate caused a self-ordered growth of Ge quantum dots, while annealing in different environments enabled us to separate charge trapping in quantum dots from the trapping at the dot-matrix and matrix-substrate interfaces. We show that the charge trapping occurs mainly in Ge quantum dots in the films annealed in the forming gas, while Si–SiO2 interface trapping is dominant for the vacuum annealed films.
RHEED Pole Figure Measurements of Biaxial Thin Film Growth Front Evolution
Authors: Wang Gwo-Ching , Liu Yu , Gaire Churamani , Yuan Wen , Lu Toh-Ming
Published: September 2011

The most frequently used characterization technique for biaxial texture formation in thin films is x-ray pole figure analysis. However, x-rays interact weakly with matter and can penetrate a few microns deep into the film. The texture obtained by x-rays is therefore an average texture from the entire thickness of the film. As the texture of a film often changes during growth, information on the basic mechanisms that control the final texture is often lost. In contrast electrons interact strongly with matter and they have very limited penetration and escape depths of a few nm. In this paper we will show how we can use our newly developed reflection high energy electron diffraction (RHEED) surface pole figure technique to probe the surface texture evolution of the growth front from the initial stage (nm thick) to the later stage. The RHEED pole figure technique is a surface-sensitive technique that allows us to obtain information on the dynamic behavior of texture evolution of the growth front during film deposition. We shall explain the principle, measurement, and construction of such RHEED surface pole figures. An example of the biaxial texture evolution of CaF2 due to the atomic shadowing effect during oblique angle deposition is described.
Enhanced CO Oxidation Catalysis of Pt0.1Cu0.9/Fe2O3 Synthesized by Radiolytic Process
Authors: Yamamoto Takao A. , Kitagawa Ryota , Seino Satoshi , Nakagawa Takashi
Published: September 2011

Catalysts in which Pt and Cu are immobilized on support particles of γ-Fe2O3 were synthesized by the radiolytic process and were evaluated for CO oxidation in a gas flow mixture (1% CO, 0.5% O2, 67.2% H2 and N2 balance) by measuring the CO concentration in the outlet gas. The Pt/Cu atomic ratios of the as-synthesized catalysts were determined to be 100:0, 90:10, 78:22, 50:50, 21:79, and 11:89, and the total metal loadings determined by chemical analyses were 10 wt%. Material characterization was performed using X-ray diffraction, X-ray absorption near edge structure, and transmission electron microscopy, and it was indicated that the composite catalysts consist of Pt-Cu bimetallic grains immobilized on the support at higher Pt-loading, while CuO with poor crystallinity is also observed at lower Pt-loading. The catalytic activity decreased as the Pt-loading was decreased to 50 at%, and also with increasing temperature. However, as the Pt-loading was further decreased, the activity contrariwise increased, and increased with increasing temperature up to 100 °C. The sample containing only 11 at% Pt exhibited the highest activity at 100 °C, which is higher than that of the commercial catalyst measured for comparison, and given at a lower temperature than that for the commercial catalyst. This enhanced activity, despite the low Pt-loading, could be attributed to oxygen supply via CuO from the O2-poor atmosphere to PtCu bimetallic grains trapping CO molecules. This new material is promising for use as a catalyst to purify hydrogen gas fed to a polymer electrolyte fuel cell.
Effects of Al3Ti Addition on the Sintering of TiB2
Authors: Yoshida Masashi
Published: September 2011

Titanium diboride (TiB2) has been sintered using spark plasma sintering (SPS). With the addition of Al3Ti, the Vickers hardness, Hv, of TiB2 increases up to as high as 2100 by the sintering at 1273K, while that of the sample sintered without Al3Ti is as low as 20. Such a remarkable improvement is caused by the formation of rigid direct contacts between TiB2 grains in addition to the effect that Al3Ti fills in the space between TiB2 grains and acts as a binder.
Synthesis and Comparison of Boron Nitride Nanostructure Prepared on Silicon and Molybdenum Substrates by Laser Plasma Deposition Technique
Authors: Sajjad M. , Zhang H. X. , Feng P. X.
Published: September 2011

The synthesis of boron nitride nanowires on silicon (Si) and nanorods on molybdenum (Mo) substrates at the same experimental conditions was composed. Fine tip nanowires with diameters around 50 nm were produced on Si substrates, whereas, nanorods with diameter around 100 nm were formed on Mo substrates. The change in length from 5 μm to 100 μm for nanowires and 0.2 μm to 0.8μm for nanorods following variation of substrate temperature were studied systematically.

Scanning Electron Microscopy was used to analyze the surface images of BN nanowires and nanorods. Energy Dispersive X-Ray spectroscopy (EDS) was used to analyze boron and nitrogen concentration in the samples. The crystal structures of BN samples were investigated using Raman spectroscopy and x-ray diffraction. The experimental results showed that the nanorods are hexagonal mixed with cubic, whereas the nanowires are hexagonal.
Sonosensitive nanoparticle formulations for cavitation-mediated ultrasonic enhancement of local drug delivery
Authors: Wagstaffe Sarah J. , Arora Manish , Coussios Constantin-C , Schiffter Heiko A.
Published: September 2011

Inertial cavitation, namely the rapid expansion and subsequent violent collapse of micron-sized cavities under the effect of ultrasound-induced pressure variations, has widely been considered as an undesirable phenomenon forin-vivobiomedical applications. This is mainly because of its highly stochastic nature and difficulties in its reliable initiationin vivousing moderate ultrasound pressure levels. Methods of lowering the pressure required to initiate cavitation, which is known as the cavitation threshold, has been previously addressed with the use of ultrasound contrast agents in form of encapsulated stabilized micron sized bubbles. However, such agents do not readily extravasate into tumours and other target tissues due to their relatively large size. This paper investigates the engineering of core-shell nanoparticles and examines their ability to initiate inertial cavitation in the context of ultrasound-enhanced local drug delivery. The nanoparticulate formulations are size-engineered to target tumour vasculature whilst presenting high surface roughness, facilitating surface air entrapment upon drying. The core-shell nanoparticles have been demonstrated to substantially lower the cavitation threshold in aqueous solution, allowing the initiation of inertial cavitation with moderate ultrasound amplitudes and the low energy levels typically deployed by diagnostic systems. The peak focal pressure where the probability of cavitation is greater than 0.5 was found to decrease by factors of five to ten fold, dependant on particle size, total surface area and surface morphology.
Formation of dense and aligned planar arrangements of Pb nanoparticles at silica/silicon interfaces
Authors: Luce Flavia P. , Kremer Felipe , Sanchez Dario F. , Fabrim Zacarias E. , Reboh Shay , Zawislak Fernando C. , Fichtner Paulo F. P.
Published: September 2011

The ion beam synthesis of Pb nanoparticles (NPs) in silica/silicon films is studied in terms of the combination of a two-step annealing process consisting of a low temperature long time aging treatment followed by a high temperature short time furnace annealing. The samples are analyzed through Rutherford Backscattering Spectrometry and Transmission Electron Microscopy. The aging process leads to the suppression of the classical homogeneous nucleation of metallic Pb NPs in the silica, thus promoting Pb redistribution during the high temperature annealing. This causes the formation of dense bi-dimensional NP arrays located at the silica-silicon interface, presenting small size dispersion.
Tuning Electronic and Structural Properties of Triple Layers of Intercalated Graphene and Hexagonal Boron Nitride: An Ab-initio Study.
Authors: Coutinho Samir S. , Azevedo David L. , Galvão Douglas S.
Published: September 2011

Recently, several experiments and theoretical studies demonstrated the possibility of tuning or modulating band gap values of nanostructures composed of bi-layer graphene, bi-layer hexagonal boron-nitride (BN) and hetero-layer combinations. These triple layers systems present several possibilities of stacking. In this work we report an ab initio (within the formalism of density functional theory (DFT)) study of structural and electronic properties of some of these stacked configurations. We observe that an applied external electric field can alter the electronic and structural properties of these systems. With the same value of the applied electric field the band gap values can be increased or decreased, depending on the layer stacking sequences. Strong geometrical deformations were observed. These results show that the application of an external electric field perpendicular to the stacked layers can effectively be used to modulate their inter-layer distances and/or their band gap values.
Biodegradable Microfluidic Scaffolds with Tunable Degradation Properties from Amino Alcohol-based Poly(ester amide) Elastomers
Authors: Wang Jane , Kniazeva Tatiana , Campbell Carly F. , Langer Robert , Ustin Jeffrey S. , Borenstein Jeffrey T.
Published: September 2011

Biodegradable polymers with high mechanical strength, flexibility and optical transparency, optimal degradation properties and biocompatibility are critical to the success of tissue engineered devices and drug delivery systems. In this work, microfluidic devices have been fabricated from elastomeric scaffolds with tunable degradation properties for applications in tissue engineering and regenerative medicine. Most biodegradable polymers suffer from short half life resulting from rapid and poorly controlled degradation upon implantation, exceedingly high stiffness, and limited compatibility with chemical functionalization. Here we report the first microfluidic devices constructed from a recently developed class of biodegradable elastomeric poly(ester amide)s, poly(1,3-diamino-2-hydroxypropane-co-polyol sebacate)s (APS), showing a much longer and highly tunable in vivo degradation half-life comparing to many other commonly used biodegradable polymers. The device is molded in a similar approach to that reported previously for conventional biodegradable polymers, and the bonded microfluidic channels are shown to be capable of supporting physiologic levels of flow and pressure. The device has been tested for degradation rate and gas permeation properties in order to predict performance in the implantation environment. This device is high resolution and fully biodegradable; the fabrication process is fast, inexpensive, reproducible, and scalable, making it the approach ideal for both rapid prototyping and manufacturing of tissue engineering scaffolds and vasculature and tissue and organ replacements.
Polyimide-Silica Hybrid Aerogels with High Mechanical Strength for Thermal Insulation Applications
Authors: Dong Wenting , Rhine Wendell , White Shannon
Published: September 2011

High performance polyimides have been widely investigated as materials with excellent thermal, mechanical, and electronic properties due to their highly rigid structures. Aspen has developed an approach to prepare polyimide aerogels which have applications as low dielectric constant materials, separation membranes, catalyst supports and insulation materials. In this paper, we will discuss the preparation of polyimide-silica hybrid aerogel materials with good mechanical strengths and low thermal conductivities. The polyimide-silica hybrid aerogels were made by a two-step process and the materials were characterized to determine thermal conductivity and compressive strength. Results show that compressive moduli of the polyimide-silica hybrid aerogels increase dramatically with density (power law relationship). Thermal conductivity of the aerogels is dependent on the aging conditions and density, with the lowest value achieved so far being ~12 mW/m-K at ambient conditions. The relationship between aerogel density and surface area, thermal stability, porosity and morphology of the nanostructure of the polyimide-silica hybrid aerogels are also described in this paper.
Evaluation of the Adsorption Potential of Synthesized Anatase Nanoparticles for Arsenic Removal
Authors: Kocabaş Z. Özlem , Yürüm Yuda
Published: September 2011

Titanium dioxide has been extensively tested in environmental applications, especially in separation technologies. In the present study, anatase nanoparticles were synthesized by using a sol-gel method, and batch adsorption experiments were carried out to analyze arsenic removal capacity of the anatase nanoparticles from water. The maximum arsenic removal percentages were found ~ 84 % for As(III) at pH 8 and ~98% for As(V) at pH 3, respectively, when 5 g/l anatase nanoparticles were used at an initial arsenic concentration of 1 mg/l. The results of the sorption experiments, which take into consideration the effects of equilibrium concentration on adsorption capacity, were analyzed with two popular adsorption models, Langmuir and Freundlich models. From the comparison of R2 values, the adsorption isotherm for As(III) was fitted satisfactorily well to the Langmuir equation (R2 > 0.996) while the adsorption behavior of As(V) on anatase nanoparticles was described better with Freundlich equation (R2 > 0.991). This study proposes the potential adsorbent material for water which is contaminated with arsenic species.
Site Dependence of Electronic Structure of Gd Impurities in GaN
Authors: Cheiwchanchamnangij Tawinan , Punya Atchara , Lambrecht Walter R. L.
Published: September 2011

Electronic structure calculations are reported for Gd in GaN on Ga as well as on N site and for pairs of Gd on neighboring Ga and N sites, using the full-potential linearized muffin-tin orbital method in the local spin density approximation with Hubbard U corrections (LSDA+U). The energy of formation for the N site is found to be much higher than for the Ga site even after relaxations are included. The GdN configuration is found to be at best metastable (in ZB). In WZ and in the pair configurations, the Gd is found to move toward an interstitial site leaving a nitrogen vacancy behind. The electronic structure of these structures and their magnetic moments are discussed.
First-principles Study of Nitrogen Vacancies in GdN
Authors: Punya Atchara , Cheiwchanchamnangij Tawinan , Thiess Alexander , Lambrecht Walter R. L.
Published: September 2011

The electronic structure of nitrogen vacancies in gadolinium nitride are studied using the full-potential linearized muffin-tin orbital method in the local spin density approximation with Hubbard U corrections (LSDA+U). The vacancy is found to have two localized defect levels in the gap, one of each spin. The third electron of each vacancy in the neutral state dopes the conduction band. The single positive state is found to be the ground state for Fermi levels located anywhere within the band gap. The vacancy has a net magnetic moment of 1 μB in the neutral charge state. The presence of the vacancy is found to increase the average exchange interactions between Gd atoms and hence the Curie temperature but only by about a factor 2 compared to GdN without vacancies.
Solid Bridging during Pattern Collapse (Stiction) Studied on Silicon Nanoparticles
Authors: Peter Daniel , Dalmer Michael , Lotnyk Andriy , Kienle Lorenz , Lechner Alfred , Bensch Wolfgang
Published: September 2011

The high surface to volume ratio of nanoparticles allows a detailed experimental study of the surface phenomena associated with solid bridging. Besides bulk analyses, the local view on the structure and composition via HRTEM is particularly essential. 50 nm core shell particles consisting of a silicon (Si) core and a SiO2 shell were used as model system to understand surface phenomena appearing for Si-based nanostructures. Evaporative drying from de-ionized water shows the most significant bridging effect based on SiO2. There is only a localized deposition of oxides between the particles during the drying process and no overall oxidation. For the deposition material, silicates are the most likely candidates.
Preparation of Nanoporous MgAl2O4 by Combined Utilization of Sol-Gel Process and Combustion of Biorenewable Oil
Authors: Hörtz Christian , Ladd Danielle M. , Seo Dong-Kyun
Published: September 2011

Nanoporous MgAl2O4 particulates with high porosities were successfully prepared from sol-gel reactions, solvent exchange with castor oil and subsequent combustion and calcination at 700 °C. The products were crystalline and semitransparent. Changes in the metal precursor concentrations allowed control of pore volumes from 0.7 to 1.1 cm3/g and average pore sizes from 14 to 19 nm. The specific surface areas are about 200 m2/g regardless of the precursor concentrations. After heating at 1000 °C for 10 hours, the products kept about 70% of their original pore volume and about 60% of the original surface area. Heating at 1100 °C caused a drastic reduction of pore volume and surface area to 40 and 36%, respectively, as the average particle size increased to 23 nm.
The Adsorption and Chemistry of Tripropyl phosphate (TPP) into Microporous and Mesoporous NaX Zeolites
Authors: Meng Qingguo , Doetschman David C. , Rizos Apostolos K. , Lee Min-Hong , Schulte Jürgen T. , Spyros Apostolos , Kanyi Charles W.
Published: September 2011

Adsorption and chemistry of tripropylphosphate (TPP) in mesoporous NaX zeolite, which was templated by cationic templated polymer (polydiallyldimethylammonium chloride, PDADMAC) with two different length chains, was investigated. The structural properties of the zeolites were characterized by X-ray diffraction (XRD) and nitrogen adsorption analysis. The chemical activities of different zeolites toward the decomposition of TPP were determined with solid state 31P NMR spectra. After exposure of zeolites to TPP was sufficient and equilibrium was reached, a stoichiometric amount of water was also adsorbed and hydrolysis was observed. The TPP decomposition yields in different NaX zeolites were compared.
Mechanical Performance of Tubular Microtruss Materials Reinforced With Nanocrystalline Sleeves
Authors: Bele Eral , Azhar Mishaal , Hibbard Glenn D.
Published: September 2011

Microtruss cellular materials are assemblies of struts with characteristic features in the μm to mm scale, arranged in a periodic, three-dimensional architecture. Compared to conventional cellular architectures (e.g. stochastic foams and honeycombs), they can possess improved structural efficiency, because externally applied loads are resolved axially along the constituent struts. We have recently fabricated composite microtruss materials by electrodepositing reinforcing nanocrystalline sleeves on tubular polymeric scaffolds. These materials can offer enhanced structural performance by exploiting advantageous properties along three length scales: the inherent strength of the electrodeposited material (grain size reduction to the nm scale), its location away from the bending axis of the struts (cross-sectional efficiency in the μm scale), and the spatial arrangement of the struts (architectural efficiency in the mm scale). This study uses finite element analysis and experimental methods to characterize the mechanical properties of these composite materials.
Transport Model for Microfluidic Device for Cell Culture and Tissue Development
Authors: Inamdar Niraj , Griffith Linda , Borenstein Jeffrey T.
Published: September 2011

In recent years, microfluidic devices have emerged as a platform in which to culture tissue for various applications such as drug discovery, toxicity testing, and fundamental investigations of cell-cell interactions. We examine the transport phenomena associated with gradients of soluble factors and oxygen in a microfluidic device for co-culture. This work focuses on emulating conditions known to be important in sustaining a viable culture of cells. Critical parameters include the flow and the resulting shear stresses, the transport of various soluble factors throughout the flow media, and the mechanical arrangement of the cells in the device. Using analytical models derived from first principles, we investigate interactions between flow conditions and transport in a microfluidic device. A particular device of interest is a bilayer configuration in which critical solutes such as oxygen are delivered through the media into one channel, transported across a nanoporous membrane, and consumed by cells cultured in another. The ability to control the flow conditions in this membrane bilayer device to achieve sufficient oxygenation without shear damage is shown to be superior to the case present in a single channel system. Using the results of these analyses, a set of criteria that characterize the geometric and transport properties of a robust microfluidic device are provided.
Crystalline Alignment and In-plane Texture Improvement of Buffer Layers Deposited on NiW Tapes
Authors: Liu Linfei , Li Yijie , Liu Huaran , Song Xiaokun , Hong Dan , Wang Ying , Xu Da
Published: September 2011

In order to deposit YBCO coated conductor with high critical current densities on rolling assisted biaxially textured Ni-W tapes, this paper has systematically studied the influence of deposition conditions on the orientation, in-plane texture and surface morphology of buffers and superconducting layers. It was found that the crystalline alignment and the in-plane texture of cerium oxide cap-layers were well improved by optimizing deposition parameters. The full width at half maximum of phi-scan x-ray diffraction peaks were reduced from original values of 7-8 degrees to 5-6 degrees. A high critical current density of 4.6×106 A/cm2 has been achieved on optimized buffer layers. This value is comparable with the critical current density of YBCO thin films deposited on single crystalline substrates.