bluuebunny/crossref_metadata_2025 huggingface dataset:
Chemical Force Microscopic Study of UV Excimer Laser Irradiated Polyamide
Authors: Yip Joanne , Chan Kwong , Sin Kwan Moon , Lau Kai Shui
Published: April 2011

Recently, there has been great interest in physico-chemical treatment for modifying polymer surfaces. UV Excimer laser irradiation is of particular interest in morphological and chemical modifications. Depending on the laser energies used, effects of laser treatment fall into two groups: above the ablation threshold the ripple structures of micrometer size form (high-fluence), and below the ablation threshold the sub-micron structures emerge (low-fluence).

Traditional methods of studying chemical properties are Fourier-transform infrared spectroscopy, x-ray photoelectron spectroscopy, and mass spectroscopy. The ability of a relatively new technique involving chemical force microscopy (CFM) can be used to image and discriminate the areas exposing different functional groups on polymers. Gold-coated AFM tip modified with carboxylic acid (-COOH) terminated self-assembled alkanethiol monolayers (SAMs) was used to measure the adhesive forces between the tip and the laser treated samples in a water or hexane medium.

The CFM results showed that high-fluence laser treated polyamide has the highest adhesive force with the modified tip in a water medium when compared with the control and low-fluence ones. The adhesive force is due to electrostatic attraction between the negatively charged tips terminating in COO- groups and the positively charged sample terminating in NH+ groups. This indicates that the high-fluence laser treated sample results in the formation of many amine end-groups on the polymer surface. In comparison, only low-fluence treated surface shows adhesive force with the modified tip in a hexane medium. This implies that the low-fluence laser treated polyamide has substantially more hydrophilic groups than the untreated and high-fluence laser treated samples. The adhesion force measurements by CFM allow one to have better understanding of surface chemical modifications induced by UV excimer laser.
Atomic and Electronic Structures of Au/TiO2 Catalyst – First-Principle Calculations –
Authors: Okazaki Kazuyuki , Morikawa Yoshitada , Tanaka Shingo , Ichikawa Satoshi , Tanaka Koji , Kohyama Masanori
Published: April 2011

The atomic and electronic structures of Au/TiO2(110) systems have been theoretically investigated based on the density functional theory. We have examined Au adsorption on the stoichiometric TiO2(110) surface and on the Ti-rich surface formed by the removal of bridging-oxygen atoms (VOB surface) and the O-rich surface formed by the removal of 6-fold titanium and bridging-oxygen atoms (VTi6OB surface). For the stoichiometric surface, the stable site for the Au adatom is the hollow site of one bridging-oxygen and two in-plane oxygen atoms or the on-top site above 5-fold titanium atom. For the Ti-rich VOB surface, the bridging site of 6-fold titanium atoms along [001] direction is the most stable. In addition, the vacant site of 6-fold titanium atom is the most stable for the O-rich VTi6OB surface. The adhesive energies between the Au adlayer and the TiO2 surface are larger for the non-stoichiometric surfaces than that for the stoichiometric surface. The charge transfer between the Au adatom and the substrate is small for stoichiometric surface. For the Ti-rich VOB surface, the electron transfer occurs from the 6-fold Ti to the Au, while from the Au to the in-plane oxygen for the O-rich VTi6OB surface. It can be said that the TiO2 surface conditions such as defects or non-stoichiometry strongly affect the adsorption energy and electron structure of the Au adsorbed system. This point should be closely related the catalytic property of the Au/TiO2 system.
Microstructure of precipitated Au Nanoclusters in Single Crystal MgO
Authors: Wang C. M. , Thevuthasan S. , Shutthanandan V. , Cavanagh A. , Walton J. , Jiang W. , Weber W. J. , Thomas L. E. , Wang L. M. , Lian Jie
Published: April 2011

Gold nanoclusters dispersed in single crystal MgO were prepared by ion implantation and subsequent annealing at 1000 °C. The morphological feature, size, crystallographic orientation of the Au nanoclusters with respect to the MgO matrix, and the interface structure between the Au nanoclusters and MgO were investigated using transmission electron microscopy. During annealing, the Au clusters nucleate coherently in the MgO lattice, leading to an epitaxial orientation relationship of [010] MgO// [010]Au and (200)MgO//(200)Au that is maintained for all the Au clusters. A critical size for the coherent-semicoherent interface transition is observed to be in the range from ∼5 to 8 nm for Au clusters in MgO. Au clusters larger than this critical size exhibit faceting on the {001} planes. The precipitated Au also exhibits internal dislocations.
Characterization of High-k Dielectric Films with Tunneling AFM
Authors: Wang Xiang-Dong , Kulik Joe , Edwards N. V. , Samavedam S. B. , Lu Shifeng
Published: April 2011

In this paper, we report the leakage current characterization of HfO2 high-k dielectric thin films by using tunneling AFM, which utilizes a conducting AFM probe to detect current passing through the sample and the probe while simultaneously acquiring a topographic image. We have studied tunneling current behavior of HfO2 films by characterizing the hot spots, which are characterized by excessive local leakage current, as well as the overall current distribution. Tunneling AFM results show sensitive dependence of tunneling current with variation of film thickness. The current distribution can be described approximately by a log-normal distribution, which is consistent with the characteristics of the thickness variation. Furthermore, the film structure and thickness were also characterized with TEM and spectroscopic ellipsometry.
Analysis of Stacking Structure of Cup-Stacked Type Carbon Nanofibers
Authors: Oshida Kyoich , Nakazawa Tatsuo , Osawa Kozo , Hayashi Takuya , Endo Morinobu , Yanagisawa Takashi , Dresselhaus Mildred S.
Published: April 2011

The lattice images of the new type (cup-stacked type) carbon nanofibers were observed by high resolution transmission electron microscopy (HRTEM) and the detailed structure was analyzed by image processing. The angle of opposed graphitic basal planes was measured from power spectrum obtained by 2 dimensional first Fourier transform. The structures of the slanted (00·1) lattice planes, which appear at the cross section of the fiber wall, were shown by means of this image analysis technique. Defects were observed in the ordered structure of this material. The (00·1) lattice planes show large fluctuation in alignment of them and the (10·0) lattice planes make several angles with the (00·1) lattice planes. The average repeat distance d002 was estimated on the basis of d100 by HRTEM image and image analysis.
The Use of Surface Enhanced Raman Scattering for the Detection of Dipicolinic Acid on Silver Nanoparticles
Authors: Phillips Terry E. , Sample Jennifer L. , Scholl Peter F. , Miragliotta Joseph
Published: April 2011

We report on the use of surface enhanced Raman scattering (SERS) for the detection of dipicolinic acid (DPA) adsorbed on a silver (Ag) nanoparticle substrate. We have examined the interaction of DPA with Ag nanoparticles in a slightly basic, aqueous solution and determined that the molecule adsorbs as a dipicolinate anion on the metal surface. For micro molar or lower DPA concentrations in the colloid solution, no SERS activity from the adsorbed molecule was observed until nanoparticle aggregation was induced by reducing the pH with the addition of nitric acid. Following aggregation, the SERS response exhibited vibrational bands associated with both the pyridine ring and the carboxylate moieties in the adsorbed dipicolinate species. With proper control of the colloidal solution chemistry, the dipicolinate vibrational features could be observed in the SERS spectra at concentrations as low as 20 nano molar, a limit determined by the presence of solution-based contaminants on the Ag surface. In addition to the controlled DPA analyte studies, SERS was also able to detect the release of this molecule fromBacillus globigiispores, a non-toxic simulant forBacillus anthracis, which demonstrated the potential of this optical spectroscopy for the detection of biological and chemical agents.
Hardness and Abrasion Resistance of Nanocrystalline Nickel Alloys Near the Hall-Petch Breakdown Regime
Authors: Schuh Christopher A. , Nieh T. G.
Published: April 2011

The breakdown of classical Hall-Petch scaling remains an area of scientific interest, and will govern the limiting strength of nanocrystalline alloys for structural applications. In this work we discuss the hardness and scratch resistance of nanocrystalline nickel and nickel-tungsten solid solution alloys, assessed through nanoindentation and nano-scratch techniques. The materials have been prepared by electrodeposition, and are fully dense with grain sizes between 6 and 22 nm. In this range of grain sizes, there is some evidence for a breakdown of Hall-Petch scaling, reflected in both the hardness and abrasion data. The role of solid solution alloying on this breakdown is also discussed.
Silver Nanodisk: Synthesis, Characterization and Self-Assembly
Authors: Chen Sihai , Fan Zhiyong , Carroll David L.
Published: April 2011

A new form of silver nanostructured materials, a silver nanodisk, is generated by a solution-phase approach. In this method, two main steps are applied: the first is the generation of the truncated triangular silver nanoplates, which are obtained by seed-mediated growth of silver particles using cetyltrimethylammonium bromide (CTAB) as the soft templates. The second is the mild aging of the above triangular silver nanoplate solution at 40 °C to get the desired silver nanodisks. Transmission electron microscopy and atomic force microscopy studies show that the nanodisk has a thickness of the order of 20 – 30 nm, and a diameter around 60 nm. X-ray and electron diffraction analysis reveal that the nanodisk is single crystal and with its basal plane as (111) lattice plane. These nanodisks display a strong surface plasmon absorption band at 475 nm; this band can be continuously tuned within 420 nm to 560 nm through adjusting the aging time. The formation of self-assembled monolayer of CTAB on the basal plane is suggested to account for both the anisotropic growth from triangular nanoplates to nanodisks, and the formation of large-scale necklace-like structures.
Simulation of Carbon Nanotube Pull-outWhen Bonded to a Polymer Matrix
Authors: Frankland S. J. V. , Harik V. M.
Published: April 2011

A carbon nanotube pulling through a polyethylene matrix was simulated using molecular dynamics. The interfacial sliding was characterized in terms of a nanoscale friction model, which is parametrized from the molecular dynamics simulation, and involves determining the critical pull-out force on the nanotube and the effective viscosity at the nanotube/polymer interface. Comparison was made of the pull-out behavior of non-bonded and functionalized nanotube composites. Chemical bonds between the polymer and the nanotube increased the critical pullout force, the resistance to interfacial sliding, and the interfacial viscosity.
Nanocomposite Fibers
Authors: Kim Yong K. , Lewis Armand F. , Patra Prabir K. , Warner Steven B. , Mhetre Shamal K. , Shah Mithun A. , Nam Daejin
Published: April 2011

Nanocomposite fibers involve the concept of integrally dispersing nanosized particles of the second phase inorganic material into fiber forming polymers such as nylon or polyester. The material goal involves obtaining bi-phasic fibers with high mechanical stiffness and strength, electrical conductivity and/or enhanced other features such as thermal stability. Thus far, the main difficulties toward achieving nanocomposite fibers are: (1) the inability of obtaining large quantities of nanoparticles in a pure, un-agglomerated state and (2) obtaining a uniform, intimate dispersion of single entity nanoparticles in a fiber polymer matrix. These problems have been approached in the context of studying the properties of experimentally prepared nano-silica reinforced fibers and attempts to prepare carbon nanotube, CNT, containing fiber material. Modulus and tenacity tests on experimentally prepared nanosilica filled PET (polyethyleneterephthalate) fibers showed the silica nanoparticles reduced the modulus and tenacity (tensile strength) of the filled PET fiber materials. DSC and shrinkage studies on nano-silica/PET fiber show that polymer crystallinity is influenced by the presence of the silica nano-particles. Heats of melting are found to increase as the filler loading increases. In further shrinkage studies, the “shrinkage modulus” of these nanosilica/PET fibers was found to increase by the addition of silica nanoparticles. Attempts to prepare nylon reinforced CNTs (multi-wall) from a commercially available CNT/nylon resin “concentrate” failed due to the poor melt dispersion processing of the blended polymer. Only very weak CNT reinforced nylon fibers could be prepared.
Plastic Relaxation Mechanics in Systems with a Twist-Bonded Layer
Authors: Priester Catherine , Grenet Geneviève
Published: April 2011

With a view to investigating how a thin film twist-bonded to a host substrate can have compliant behavior from a plasticity point of view, the onset and spread of edge dislocations throughout a mesa are studied. The discussion focuses on the energy relaxed by such dislocations in a mesa made from two coherently bonded lattice-mismatched layers twist-bonded onto a host substrate and patterned down to the film/host substrate interface. Our theoretical results show that the confinement of threading dislocations into a thin twist-bonded film is energetically favorable allowing the overgrowth of a mismatched layer exempt of any threading dislocation at least as far as mesas are concerned.
Electrodeposition of Ni Catalyst on Tungsten Substrates and Its Effect on the Formation of Carbon Nano- and Micro-coils
Authors: Einarsson Erik , Jiao Jun , Prado Josie , Coia George M. , Petty Jeremy , Love Logan
Published: April 2011

Carbon micro-coils and nano-coils were produced in high yield from nickel catalyst particles electrochemically deposited onto tungsten substrates. Various electrochemical deposition techniques were used to produce the nickel catalyst particles. These particles catalyzed the chemical vapor deposition (CVD) of acetylene at 800°C, resulting in growth of carbon microand nano-coils. Linear-sweep (cyclic) voltammetry produced catalyst particles which resulted in single- filament coils intermixed with thin nanotubes over most of the substrate surface. Passing the same amount of charge by constant current electrolysis produced nickel particles that decomposed carbon but did not grow coils. Catalyst particles deposited by constant current electrolysis grew nanocoils similar to linear-sweep voltammetry but in small, localized, high-yield patches. The coils produced were either spring- like micro-coils of low pitch and large diameter or rope- like nano-coils of higher pitch and smaller diameter. High-resolution transmission electron microscopy (HRTEM) showed typical graphite fringes in the filaments, but did not reveal an internal tubular structure. Catalyst particles were often detected at the ends of the coils.
Nucleation Induced Nanostructures
Authors: ten Bosch A.
Published: April 2011

Numerical simulations in films and aggregates have repeatedly shown the presence of vibrations during a phase transition and/or the appearance of periodic structures. A phase transition could be controlled and novel nanostructures created by astute manipulation of such phenomena. In order to study the occurrence and effect of wave-like phenomena, the dynamics of a first order phase transition is described using kinetic theory. At a first order phase transition, the initial phase is replaced in time by the new phase on propagation of a density front through the sample. The dynamic stability analysis studies the transition to the uniform phase by propagation of the front and provides the conditions for the formation of transient periodic structures by a local increase of density. The results apply also to spherical geometry and a discussion of cluster dynamics follows the planar case.
Nanophase Alumina/Poly(L-Lactic Acid) Composite Scaffolds for Biomedical Applications
Authors: Dulgar Tulloch Aaron J. , Bizios Rena , Siegel Richard W.
Published: April 2011

Three-dimensional composites of nanophase alumina and poly(L-lactic acid) with an interconnected porous network and an overall porosity in excess of 90% are cytocompatible. Osteoblast proliferation on the nanophase ceramic/polymer composites is a function of time of cell culture and of nanoceramic loading in the biomaterial substrates.
Synthesis of Aerogel-Metal Cluster Composites by Gamma Radiolysis
Authors: Bertino Massimo F. , Hund Jared F. , Zhang Guohui , Sotiriou-Leventis Chariklia , Leventis Nicholas , Tokuhiro Akira T. , Farmer John
Published: April 2011

Noble metal clusters (Ag, Au) were formed in a silica aerogel matrix by gamma irradiation of hydrogel precursors loaded with aqueous solutions containing Ag+ or [AuCl4]- ions. Hydrogels exposed to gamma rays assumed the color expected for colloidal suspensions of Ag (respectively Au) clusters. The hydrogels were subsequently washed and supercritically dried, without any evident change in color, indicating that the metal clusters were not removed during drying. Typical gamma ray doses were between 3 and 3.5 kGy, and achieved complete reduction of hydrogels containing metal ion concentrations in the 10-4-10-3 M range. Metal clusters in the aerogel monoliths were characterized with optical absorption, transmission electron microscopy, X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy. These techniques have shown that the clusters have a crystalline fcc structure. Au clusters consist of pure Au, while surface oxidation of Ag clusters was observed with XPS.
Molecular Dynamics Simulations on Nanocomposites Formed by Intermetallic Dispersoids of L12 Type and Aluminum Matrices
Authors: Namkung Min , Paik Sun Mok , Wincheski Buzz
Published: April 2011

Molecular dynamics simulations were performed to characterize the lattice morphology in the region adjacent to the interfaces in nanocomposite systems of a Ni3Al dispersoid embedded in Al matrix (Ni3Al/Al) and an Al3Nb dispersoid embedded in aluminum matrix (Al3Nb/Al). A nearly perfect coherent interface is obtained in the Al3Nb/Al system with the lattice planes of dispersoid and matrix aligned parallel in all directions. The simulation results show the presence of the matrix atom-depleted regions near the dispersoid boundary for most cases. Detailed analysis revealed that certain sites immediately next to the dispersoid are energetically favored for the matrix atoms to occupy. The matrix atoms occupying these sites attract other atoms producing the depleted regions. In certain specific situations of Al3Nb/Al system, however, the wetting of a rotated dispersoid is overwhelmingly complete prompting the need of further study for better understanding. The order parameters of dispersoids calculated for Ni3Al in aluminum is nearly constant while that for Al3Nb in aluminum is rapidly decreasing function of temperature in the range of 300 to 1800K.
Condensation of Carbon Vapour in the Microwave Oven
Authors: Kharissova Oxana V. , Lopez Israel Nieto , Méndez Ubaldo Ortiz , Aguilar Juan A. , Rivera Moisés Hinojosa
Published: April 2011

This work is devoted to microwave heating of graphite for studying the processing of carbon nanotubes (CNTs) by graphite vaporization. We have applied heating by microwaves (MW) (power 800W, frequency 2.45 GHz) in air at 20–90 min. The oven temperature was approximately 1200°C. The condensed material was collected on a fused silica target. After deposition, the morphology of carbon nanotubes was studied by Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), and Transmission Electron Microscopy (TEM). The samples were found to contain nanotubes, nanoparticles and fibers (at 1.30–2.80 micrometers to 6–11 micrometers) which appeared to be highly graphitized. It was observed that multi-walled nanotubes (MWNT's) were produced by this method.
Deformation-induced crystallization and amorphization of Al-based metallic glasses
Authors: Hebert Rainer J. , Perepezko John H.
Published: April 2011

A main requirement for the application of nanostructured materials for structural applications is their thermal stability. Structural materials are often exposed to mechanically-induced stress states. Nanomaterials for structural applications should therefore retain their microstructure not only within a defined temperature range but also under applied load. Cold-rolling experiments with melt-spun Al87Ni10Ce3ribbons containing a dispersion of nanocrystallites in an amorphous matrix demonstrate that during the continued deformation through repeated rolling and folding crystallization as well as amorphization reactions could be induced. The results indicate that in addition to the microstructure control through annealing of precursor materials, deformation processing represents an effective approach to the synthesis of amorphous and nanophase composite materials.
Densification and Sintering of a Microwave-Plasma-Synthesized Iron Nanopowder
Authors: Kecskes L. J. , Woodman R. H. , Klotz B. R.
Published: April 2011

Powder compacts made from microwave-plasma-synthesized iron (Fe) nanopowder (Materials Modification Inc., Fairfax, VA) were sintered under hydrogen (H2). Results showed that without the application of pressure, the powder could not be sintered to full density. In subsequent experiments, the powder was consolidated to final densities near 80% of the theoretical full density, using plasma pressure compaction (P2C). To provide an explanation for the difficulty in attaining full densification, the precursor powder and compacted pellet characteristics were examined by several methods. These included residual porosity by mercury pycnometry, surface area measurement by Brunauer, Emmett, and Teller (BET) analysis, and structural change by x-ray diffraction (XRD) and scanning electron microscopy (SEM). It was observed that the nanosized primary particles were aggregated into micrometer-sized dendritic structures, with the appearance of tumbleweeds. The observed limitations in the densification and associated porous microstructures of the pressureless-sintered and P2C-densified compacts were rationalized in terms of the behavior of the dendritic aggregate structures rather than that of the nanosized primary particles.
Continuum Mechanics-Discrete Defect Modeling and Bubble Raft Simulation of Cracked Specimen Response in Nanoscale Geometries
Authors: Starr Michael J. , Drugan Walter J. , Lopez-Garcia Maria d. C. , Stone Donald S.
Published: April 2011

In a continuation of prior work, a new group of Bragg bubble model experiments have been performed to explore the effects of nanoscale crack size and nanoscale structural geometry on atomically-sharp crack tip dislocation emission behavior. The experiments have been designed to correspond to the theoretical limits that bound the expected crack tip response. Continuum elasticity analyses of these situations have also been carried out, in which the leading-order terms in the Williams expansion (the K and T terms) are determined, and the predictions of these continuum analyses coupled with discrete dislocation theory are compared with the experimental results. The experiments exhibit fascinating changes in crack tip dislocation emission direction with changing crack and structural size, crack location and loading conditions, as well as substantial changes in the magnitude of the resolved shear stress that drives dislocation emission. These changes are predicted well by the continuum elasticity-discrete dislocation model down to extremely small dimensions, on the order of a few atomic spacings. Preliminary experiments were performed with layered and two-atom basis rafts to establish crucial comparisons between theory and experiment that validate the applicability of continuum elasticity theory to make predictions directly related to nanoscale fracture behavior.
Structure and Characterization of Sol-gel and Aerogel Materials and Oxidation Products from the Reaction of (CH3O)4Si and C16H33Si(OCH3)3
Authors: Tillotson Thomas M. , Reynolds John G.
Published: April 2011

Reactions of (CH3O)4Si and C16H33Si(OCH3)3 at various mole ratios were performed in methanol using base and acid catalysis. Sol-gel materials were formed that have low surface areas (∼10 m2/g). Subsequent supercritical drying using CO2 at 40 °C produced materials that are very different than traditional aerogels, with surface areas around the same values as the corresponding sol-gels, as well as no detectable meso-pore features. In some cases, the aerogels even melted upon heating. Spectroscopic characterization, using IR, 29Si and 13C NMR revealed normal Si-O substitution as well as incorporation of the carbon chain into the Si framework.

Heating of the stable forms of the materials in air at different temperatures yielded, depending upon oxidation conditions, several materials with much higher surface areas, typical of aerogels. Pore size distribution measurements revealed meso-pore features with a narrow distribution around 35Å. Spectroscopic characterization revealed the disappearance of the R-Si substitution and the appearance of an oxidized intermediate.

This paper will describe the chemistry and characterization of these unusual sol-gels, aerogels, and oxidation products, as well as potential applications.
Nanomechanical Characterization on Zinc and Tin Oxides Nanobelts
Authors: Zhao Minhua , Mao Scott , Wang Zhong Lin , Xu Fengting , Barnard John A
Published: April 2011

Nanobelts are a group of materials that have a rectangle-like cross section with typical widths of several hundred nanometers, width-to-thickness ratios of 5 to 10, and lengths of hundreds of micron meters. In this paper, nanoindentations were made on individual ZnO, SnO2 nanobelts and (0001) bulk ZnO by using AFM and Hysitron Triboscope indenters. It was shown that the indentation size effect was still obvious for the indentation depth under 50 nm. It is also demonstrated that nanomaching is possible on nanobelt using AFM tip.
Interfacial Effects on the Premature Failure of Polycrystalline Silicon Structural Films
Authors: Muhlstein C.L. , Stach E.A. , Ritchie R.O.
Published: April 2011

Although bulk silicon is not known to be susceptible to cyclic fatigue, micron-scale structures made from mono and polycrystalline silicon films are vulnerable to degradation by fatigue in ambient air environments. Such silicon thin films are used in small-scale structural applications, including microelectromechanical systems (MEMS), and display “metal-like” stress-life (S/N) fatigue behavior in room temperature air environments. Previously, the authors have observed fatigue lives in excess of 1011 cycles at high frequency (∼40 kHz), fully-reversed stress amplitudes as low as half the fracture strength using a surface micromachined, resonant-loaded, fatigue characterization structures. Stress-life fatigue, transmission electron microscopy, infrared microscopy, and numerical models were used to establish that the mechanism of the fatigue failure of thin-film silicon involves the sequential oxidation and environmentally-assisted crack growth solely within the native silica layer, a process that we term “reaction-layer fatigue”. Only thin films are susceptible to such a failure mechanism because the critical crack size for catastrophic failure of the entire silicon structure can be exceeded by a crack solely within the native oxide layer. The importance of the interfacial geometry on the mechanics of the reaction-layer fatigue mechanism is described.
Controlling Casimir Forces in Mems and Nems
Authors: Esquivel-Sirvent R. , Villarreal C.
Published: April 2011

We present a theoretical study of vacuum effects in systems of length scales of micrometers and nanometers. In particular, we calculate the Casimir force between dielectric plates with different compositions. We show that by an appropriate choice and configurations of materials, the Casimir forces develop noticeable changes than could be used to inhibit or modulate the action of vacuum fluctuations in MEMS and NEMS.
Tailoring of Stress Development in MEMS Packaging Systems
Authors: Walwadkar Satyajit S. , Cho Junghyun , Farrell P.W. , Felton Lawrence E.
Published: April 2011

A better understanding of the origin and evolution of the stresses is a crucial step in improving reliability of packaging systems for microelectromechanical systems (MEMS). Given its importance, we examine the stresses developed in hermetically packaged MEMS inertial sensors. For this purpose, an optical surface profilometer is employed to assess the stresses by measuring the curvature of dummy silicon dies (3.5×3.5 mm2) assembled in different types of packages and die attach adhesives. We also explore a temporal evolution of stresses during thermal exposure of the test packages in an effort to emulate actual packaging processes and device operation conditions. The result shows different levels of stresses generated from various adhesives and package types, and also a stress evolution during packaging processes. The mechanical stress data also show a good agreement with MEMS performance data obtained from actual accelerometers. Therefore, the stress data will not only be useful in better understanding performance of MEMS packages, but the testing protocol can also provide a diagnostic tool for very small packaging systems.
Defect-Induced Shifts in the Elastic Constants of Silicon
Authors: Allred Clark L. , Borenstein Jeffrey T. , Weinberg Marc S. , Yuan Xianglong , Bazant Martin Z. , Hobbs Linn W.
Published: April 2011

As MEMS devices become ever more sensitive, even slight shifts in materials properties can be detrimental to device performance. Radiation-induced defects can change both the dimensions and mechanical properties of MEMS materials, which will be of concern to designers of MEMS for applications involving radiation exposure, such as those in a reactor environment or in space. We have performed atomistic simulations of the effect that defects and amorphous regions, such as could be produced by radiation damage, have on the elastic constants of silicon. We have then applied the results of the elastic constant shift calculations to a hypothetical MEMS device, and calculated the difference that would be generated by this effect.
Design and Performance of a Microengine Realized with Arrays of Asymmetrical Electrothermal Polysilicon Surface Micromachined Microactuators
Authors: Kolesar Edward S. , Ruff Matthew D. , Odom William E. , Jayachandran Joseph A. , McAllister Justin B. , Ko Simon Y. , Howard Jeffery T. , Allen Peter B. , Wilken Josh M. , Boydston Noah C. , Bosch Jorge E. , Wilks Richard J.
Published: April 2011

This research focuses on the design and experimental characterization of two types of MEMS asymmetrical electrothermal microactuators. Both microactuator design variants use resistive (Joule) heating to generate thermal expansion and movement. Deflection and force measurements of both microactuators as a function of applied electrical power are presented. Also described is the practical integration of the electrothermal microactuators in a monolithic microengine that is capable of rotating a set of gears.
Manufacturing of 3-D Microstructures Using Novel Upsams Process for Mems Applications
Authors: Sharon André , Bilsing Axel , Lewis Gordon , Zhang Xin
Published: April 2011

This paper introduces a new microfabrication technique, Ultraprecision Manufacturing of Self-Assembled Micro-Systems (UPSAMS), which combines ultraprecision micromachining (milling/turning/drilling/grinding) with sacrificial/structural multi-layer manufacturing processes to produce self-assembled, 3-D microsystems and associated mesoscale interfaces from a variety of materials for MEMS applications. With this process, a new class of microsystems can be developed that is highly three dimensional, precisely machined, and automatically assembled.
Stress Adjustment and Characteristics Improvement in a 1.8GHz Range Film Bulk Acoustic Wave Resonator by Using Multi-layer Structure of ZnO/A12O3/SiO2
Authors: Takeuchi Masaki , Yamada Hajime , Kawamura Hideki , Goto Yoshihiko , Nomura Tadashi , Fujino Hiroyuki , Yoshino Yukio , Makino Takahiro , Arai Seiichi
Published: April 2011

Stress adjustment and improvement of electrical characteristics in film bulk acoustic wave resonator (BAW) have been successfully carried out in 1.8GHz range by using a multi-layer structure of ZnO / Al2O3 / SiO2. The BAW resonator, designed on secondary harmonics at about 1.8GHz, has Al / ZnO / Al / Al2O3 / SiO2 structure. ZnO and SiO2 thin films have a negative and a positive temperature coefficient of sound velocity, respectively. So temperature coefficient of frequency (TCF) of the BAW resonator can be controlled by the thickness ratio of ZnO and SiO2 thin films. Since both ZnO and SiO2 have compressive stress, and Al2O3 has tensile one, the stress of the membrane is reduced by combining these thin films so that the membrane can avoid deformation. The BAW resonator, with thickness of ZnO / Al2O3 / SiO2=1.2 /0.45/1.25 microns, was designed by finite element method (FEM) simulation and fabricated. The value of quality factor (Q factor) and the TCF of the BAW resonator were realized over 1000 and −20 ppm/degree C, respectively. The Q of ZnO / Al2O3 / SiO2 structure was higher than that of ZnO / SiO2 one with keeping small TCF.
Self-aligned Split Gate Electrodes Fabricated on Suspended Carbon Nanotubes
Authors: Lee S.-B. , Robinson L.A.W. , Teo K.B.K. , Chhowalla M. , Amaratunga G.A.J. , Milne W.I. , Hasko D.G. , Ahmed H.
Published: April 2011

We describe the fabrication of self-aligned split gate electrodes on suspended multiwalled carbon nanotube structures. A suspended multiwalled carbon nanotube structure was used as an evaporation mask for the deposition of metal electrodes resulting in the formation of discontinuous wire deposition. The metal deposits on the nanotubes are removed with lift-off due to the poor adhesion of metal to the nanotube surface. Using Al sacrificial layers, it was possible to fabricate self-aligned contact electrodes and control electrodes nanometers from the suspended carbon nanotubes with a single lithography step. It was also shown that the fabrication technique may also be used to form nano-gaped contact electrodes. The technique should prove useful for the fabrication of nano-electromechanical systems.
Whole-Wafer Optical Mapping of Defects in Insulating Silicon Carbide Wafers
Authors: Mier Millard G. , Boeckl John J. , Hill David A. , Bertrand Scott D. , Ramakrishnan Easwar , Roth Matthew D. , Balkas Cengiz , Nelson Matthew P.
Published: April 2011

Plotting defect locations in insulating SiC presents a challenge because the total number of locations on a wafer is so large. We scan the wafer with visible light at an appropriate resolution and sort out transmissions appropriate for the defects we are looking for. Under these conditions, we find that voids and micropipes reduce the pixel transmission to 0.3 to 0.5. Sorting for this transmission reduces the number of pixels of interest to a manageable number, especially with recent progress in growing lower defect SiC. Now a commercial plotting program can easily display defect locations within a circle representing the wafer boundary. We verify the defect locations by scanning electron microscope secondary electron images and scanning optical microscope visible-light images at several resolutions.
Field Enhancement Mechanisms and Electron Field Emission Properties of Ion Beam Synthesized and Modified SiC/Si Heterostructures
Authors: Tsang W. M. , Wong S. P. , Lindner J. K. N.
Published: April 2011

SiC/Si heterostructures were synthesized by high dose carbon implantation into silicon using a metal vapor vacuum arc ion source. Their electron field emission properties were studied and correlated with results from other characterization techniques including atomic force microscopy (AFM), conducting AFM, Fourier transform infrared absorption spectroscopy, x-ray diffraction and photoelectron spectroscopy. It is clearly demonstrated that there are two types of field enhancement mechanisms responsible for the improvement of the electron field emission properties of these ion beam synthesized SiC/Si heterostructures, namely, the surface morphology effect and the local electrical inhomogeneity effect. The dependence of the FE properties on the carbon implant dose and thermal annealing conditions could be understood in terms of these two field enhancement mechanisms. It is also demonstrated that improvement in the FE properties can be achieved by implanting tungsten ions into these SiC/Si heterostructures.
The Effect of Annealing on High-resistivity and Semi-insulating 4H-SiC
Authors: Smith S. R. , Evwaraye A. O. , Mitchel W. C.
Published: April 2011

We have examined specimens of high-resistivity, and semi-insulating, 4H-SiC before and after thermal annealing at 1600 °C, using Optical Admittance Spectroscopy. We have found enhanced ultraviolet response in most specimens. Enhanced activation of previously undetected V impurities has also been observed. Peaks believed to be attributable to complex V-related defects were greatly reduced by annealing. The annealing was in addition to a thermal oxidation at 1150 °C for 4 hours. The purpose of the oxidation was to remove surface-related deep levels known to be present in polished SiC. Transition metal impurities in these bulk specimens were quantified by SIMS. In specimens where Ti was not detected by SIMS, no further activation of Ti centers was detected by Optical Admittance Spectroscopy.
The Effect of Channel Recess and Passivation on 4H-SiC MESFETs
Authors: Cha Ho-Young , Thomas Christopher I. , Koley Goutam , Eastman Lester F. , Spencer Michael G.
Published: April 2011

Channel-recessed 4H-SiC MESFETs were fabricated and demonstrated excellent small signal characteristics. A saturated current of 250 − 270 mA/mm at Vgs = 0 V and a maximum transconductance of 40 − 45 mS/mm were measured for channel-recessed devices with a gate length of 0.45 m. The three-terminal breakdown voltages (Vds) range from 120 V to 150 V. The Ft and Fmax of the 2 × 200 m devices were measured to be 14.5 GHz and 40 GHz, respectively. The channel recess technique results in a lower saturation current but higher breakdown voltage which makes it possible for the devices to operate at high voltages. Si3N4 passivation suppresses the instability in DC characteristics and improves CW power performance by reducing the surface effects. Less dispersion in the drain current during a power sweep was observed after passivation.
System Design Considerations for Optimizing the Benefit by Unipolar SiC Power Devices.
Authors: Rupp Roland , Zverev Ilia
Published: April 2011

Focussing on unipolar SiC power devices a variety of applications are described, where cost reduction can be a achieved on system level even for SiC device costs being several times higher than the costs of the competing Si devices. Based on the specific properties of SiC devices like Schottky diodes and JFETs it is explained with the help of these examples how this is attainable.
X-ray Spectroscopic Studies of the Bulk Electronic Structure of InGaN Alloys
Authors: McGuinness Cormac , Downes James E. , Ryan Philip , Smith Kevin E. , Doppalapudi Dharanipal , Moustakas Theodore D.
Published: April 2011

Synchrotron radiation excited soft x-ray emission and soft x-ray absorption spectroscopies are applied to the study of the electronic structure of InxGa1-xN alloys with (0 ≤ x ≤ 0.29). The elementally resolved partial density of states of the valence and conduction bands may be measured using these spectroscopies. The x-ray absorption spectra indicate that the conduction band broadens considerably with increasing indium incorporation. The band gap evolution as a function of indium content derives primarily from this broadening of the conduction band states. The emission spectra indicate that motion of the valence band makes a smaller contribution to the evolution of the band gap. This gap evolution differs from previous studies on the AlxGa1-xN alloy system, which observed a linear valence band shift through the series (0 < x < 1). Instead in the case of InxGa1-xN the valence band exhibits a significant shift between x = 0 and x = 0.1 with minimal movement thereafter. Furthermore, evidence of In 4d -N 2p and Ga 3d- N 2p hybridisation is reported. Finally, the thermal stability of an In011Ga089N film was investigated. Both emission and absorption spectra were found to have a temperature dependent shift in energy, but the overall definition of the spectra was unaltered even at annealing temperatures well beyond the growth temperature of the film.
An In-Situ TEM-Cathodoluminescence Study of Electron Beam Degradation of Luminescence from GaN and In0.1Ga0.9N Quantum Wells
Authors: Boyall Nicholas M. , Durose Ken , Watson Ian M.
Published: April 2011

The effect of electron beam irradiation on the cathodoluminescence (CL) emission from In0.1Ga0.9N/GaN single quantum wells (QW) has been investigated by in-situ measurement of CL in a transmission electron microscope. Analysis of CL quenching over 600s showed that the QW luminescence decayed more quickly than the barrier emission. Both the In0.1Ga0.9N and GaN CL decay curves could be fitted to a simple recombination based model suggesting the decay was due to the introduction of non-radiative centres.
Room-Temperature Time–Resolved Photoluminescence Studies of UV Emission from GaN/AlN Quantum Wells
Authors: Furis Madalina , Chen Fei , Cartwright Alexander N. , Wu Hong , Schaff William J.
Published: April 2011

Room temperature time-resolved photoluminescence (TRPL) studies of multiple quantum well (MQW) structures of the binaries GaN and AlN grown by molecular beam epitaxy are reported. The eventual application of these structures is for GaN intersubband IR light emitters. However, as an initial study, the structures are evaluated at UV to investigate materials parameters relevant to IR light emission. The nominally 0.9, 1.3 and 1.5 nm GaN quantum wells are clad by 6nm of AlN on top of a thick AlN buffer grown on sapphire. All samples consisted of 20 quantum wells. The observed peak energy of the emission spectrum is in excellent agreement with a model that includes the strong confinement present in these structures and the existence of the large built-in piezoelectric field and spontaneous polarization present inside the wells. Furthermore, consistent with screening of the in-well field as carriers are injected in the well, a clear blue shift of the emission is observed at short times after carrier injection. Subsequently, as the carriers recombine, the peak emission red-shifts and the screening of the field is reduced. Moreover, the observed lifetimes were energy dependent as should be expected from field dependent elongation of lifetimes due to spatial separation of the injected carriers. Specifically, the decay time at high energies can be fitted by a stretched exponential with a beta value of 0.8 which is consistent with carrier spatial separation. The lifetimes obtained from the fitting are of the order of 1ns, longer than the reported recombination lifetimes in similar GaN/AlGaN MQW's. On the low energy side of the PL feature the intensity time decay becomes exponential with lifetimes ranging from 3 to 10ns. The strong UV emission at room temperature makes these structures promising for UV emitters.
A Study of Indium Nitride Films Grown Under Conditions Resulting in Apparent Band-gaps from 0.7 eV to 2.3 eV
Authors: Butcher K. S. A. , Wintrebert-Fouquet M. , Motlan  , Shrestha S. K. , Timmers H. , Prince K. E. , Tansley T. L.
Published: April 2011

The band-gap of indium nitride has long been believed to be about 1.9eV with slight variations due to band-tailing in polycrystalline samples and degenerate doping. Recently, other values as low as 0.7 eV have apparently been observed. We have compared samples spanning this apparent range of band-gap using secondary ion mass spectroscopy (SIMS), X-ray Photoelectron Spectroscopy (XPS) and heavy ion elastic recoil detection analysis (ERDA), in conjunction with spectral optical density measurements. Once structural inhomogeneiteies are taken into account, we show that much of the conflicting data are compatible with direct photoionisation with a threshold energy of about 1.0eV. This feature was first reported in polycrystalline indium nitride over 15 years ago and attributed to a ∣p> like defect state. We ask whether the feature may instead be a direct band-gap.
Optical Properties of Controllable Self-Assembled Lateral Nanostructures on InN, InAlN, and AlN Thin Films
Authors: Danylyuk Yuri , Romanov Dmitri , McCullen Eric , Haddad Daad , Naik Ratna , Auner Gregory
Published: April 2011

Utilizing plasma source molecular beam epitaxy (PSMBE), we have grown epitaxial InxAl1-xN films on (0001) sapphire substrates; the indium concentration, x, varied from 0 to 1. The atomic force microscopy of the films reveals characteristic surface patterns of nanometer scale. The feature size distribution is determined by the film composition and thickness. Both absorption and reflection spectra of the films have additional peaks below the fundamental absorption threshold. These peaks cannot be associated with N vacancies or any other known crystal defects and impurities. We attribute the peaks to electron confinement in the hillocks of the lateral structure by the strong electric field of piezoelectric and spontaneous polarization that is characteristic to nitride semiconductor compounds. The calculated values of the electron energy levels are in good agreement with the spectroscopic data; moreover, the electron confinement model explains the observed temperature dependence of the additional peaks. The hillock size control will allow one to control the optical and transport properties of the films.
Plane-wave pseudopotential study on mechanical and electronic properties for group III-V binary phases
Authors: Wang S. Q. , Ye H. Q.
Published: April 2011

The result of first-principles density functional calculations of the bulk modulus and related structural and electronic properties of the total 25 group III-V binary phases with zinc-blende and wurtzite structures are presented. The behavior of energy band structure variation under high pressures is also studied. It is found that the bulk modulus is more sensitive to the local atom configuration than the lattice structure. The crystallographic geometry plays an important role in the electronic property of these phases.
Transport Studies on Two-subband-populated AlGaN/GaN Heterostructures
Authors: Hang D. R. , Huang C. F. , Chen Y. F. , Shen B.
Published: April 2011

We report an investigation of electronic properties of two-dimensional electron gas (2DEG) confined at AlGaN/GaN heterostructures by magnetotransport measurements. The second-subband population is manifested by the multi-frequency in the Shubnikov-de Haas (SdH) oscillations. The modulated patterns of SdH oscillations which are due to the two-subband occupancy can be drastically enhanced by employing the microwave modulation technique. This unique advantage enables us to provide direct experimental evidence that the 2DEG in the second subband has a higher mobility than that in the first subband in the modulation-doped Al0.22Ga0.78N/GaN heterostructures by means of microwave-modulated magnetotransport measurements. The carrier concentrations and 2DEG Fermi energy for each subband were determined. It was found that the second-subband population ratio increases with spacer thickness up to 5 nm, while the subband separation decreases.