bluuebunny/crossref_metadata_2025 huggingface dataset:
Development of SiC-based Gas Sensors for Aerospace Applications
Authors: Hunter G. W. , Neudeck P. G. , Xu J. , Lukco D. , Trunek A. , Artale M. , Lampard P. , Androjna D. , Makel D. , Ward B. , Liu C. C.
Published: April 2011

Silicon carbide (SiC) based gas sensors have the ability to meet the needs of a range of aerospace applications including leak detection, environmental control, emission monitoring, and fire detection. While each of these applications require that the sensor and associated packaging be tailored for that individual application, they all require sensitive detection. The sensing approach taken to meet these needs is the use of SiC as a semiconductor in a Schottky diode configuration due to the demonstrated high sensitivity of Schottky diode-based sensors. However, Schottky diode structures require good control of the interface between the gas sensitive metal and SiC in order to meet required levels of sensitivity and stability. Two examples of effort to better control the SiC gas sensitive Schottky diode interface will be discussed. First, the use of chrome carbide as a barrier layer between the metal and SiC is discussed. Second, we report the first use of atomically flat SiC to provide an improved SiC semiconductor surface for gas sensor deposition. An example of the demonstration of a SiC gas sensor in an aerospace applications is given. It is concluded that, while significant progress has been made, the development of SiC gas sensor systems is still at a relatively early level of maturity for a number of applications.
Epitaxial Growth of 2 inch 3C-SiC on Si Substrates by Atmospheric Hot Wall CVD
Authors: Zhu Jiliang , Chen Yi , Mukai Yusuke , Shoji Akira , Nishiguchi Taro , Ohshima Satoru , Nishino S.
Published: April 2011

As a high mobility, wide bandgap semiconductor, 3C-SiC has great promise. In this paper, we examined to obtain 3C-SiC epilayer on Si substrates using hot-wall CVD furnace and report the use of hexamethyledisilane (HMDS) and propane as reaction gases to grow uniform thickness on 2 inch (100), (111), (110) and (211) orientation of Si substrates. A horizontal atmospheric pressure CVD reactor was used. A reaction zone was specially designed. To obtain uniform thickness of the epilayer, inside of the suscceptor hole was intentionally tapered along flow direction as follows; inlet of the square hole is 13 mm × 60 mm and outlet of the hole is 7 mm × 60 mm, and laminar channel for changing the gas flow profile was managed. The susceptor was surrounded by graphite foam. Temperature of the suscepotor was measured at inside wall of the susceptor by optical pyrometer. H2 flow rate for etching was 3 slm. An initial carbonization procedure was performed using 0.9 sccm propane at 1250 oC for 2-3 minutes. During the growth of SiC at 1300 °C, the flow rate of HMDS was 0.75-1.2 sccm and the flow rate of propane was 0.1 – 0.5 sccm. The hydrogen carrier gas flow rate was 3-10 slm. Typical growth rate was 4.5 micron /h. Uniform thick 3C-SiC was obtained. The samples were examined using ultra violet light spectrometer and RHEED.
Photoluminescence Characterization of Defects Introduced in 4H-SiC During High Energy Proton Irradiation and Their Annealing Behavior
Authors: Ahoujja M. , Crocket H. C. , Scott M. B. , Yeo Y.K. , Hengehold R. L.
Published: April 2011

We report on the optical properties of defects introduced in epitaxial 4H-SiC by 2 MeV protons using photoluminescence spectroscopy. The near band edge characteristics of nitrogen-doped n-type 4H-SiC are present in the optical spectrum of the as-grown samples. Following a proton irradiation, the material is altered and the luminescence of the shallow centers is attenuated almost entirely with the emergence of deeper shallow traps at energies greater than 300 meV below the conduction band. Subsequent hightemperature thermal annealing of the material results in an increase in the emission spectrum at both the near band edge region (Eg = 3.25 eV) and between 2.65 and 2.95 eV. Recovery of the characteristic nitrogen-related peaks at the near band edge following high-temperature annealing is identified, but is not complete even at 1500 °C. In the deep trap region below 2.95 eV, activation of trap centers with annealing results in a sharp increase in the signal intensity of an irradiation-induced defect trap (2.90 eV) as well as the associated phonon replicas. Based on previous ion-implantation and other radiation studies in 4H-SiC, the emergence of the 2.90 eV defect complex and associated phonon replicas after high temperature anneal is the well known D1 photoluminescence. The observed lines in the D1 spectrum are due to exciton recombination at isoelectronic defect centers.
Saturation and Flow Rate Effects on the Response of a Pd/AIN/SiC Hydrogen Sensor
Authors: Rahman M.H. , Ng K.Y.S. , McCullen E.F. , Naik R. , Newaz G. , Danylyuk Y. , Rimai L. , Baird R.J. , Auner G.W.
Published: April 2011

A Pd/AlN/SiC device was fabricated with a 550 Å thick AlN layer in between the SiC substrate and Pd gate. At fixed flow rate, the response to Hydrogen initially increases roughly linearly with concentration, but eventually becomes saturated. The magnitude of the response as well as the saturating concentration increase with temperature. At 120° C the response saturates around 60 ppm of H2 whereas at 250° C, the onset of saturation occurs at higher ppm. The magnitude of the response also increases with the total flow rate, up to a flow rate of 250 sccm beyond which it remains constant. In the linear region, the magnitude of the response increases by a factor of 4 between 120° and 250° C, at a flow rate of 250 sccm.
The Electrical Behavior of Pd/AIN/Semiconductor Thin Film Hydrogen Sensing Structures
Authors: Rimai L. , Rahman M.H. , McCullen E.F. , Zhang L. , Thakur J.S. , Naik R. , Newaz G. M. , Ng K.Y.S. , Baird R.J. , Auner G.W.
Published: April 2011

The device on Si substrates behaves as an MIS capacitor and the response to hydrogen is given by a shift of the capacitance vs. bias profile along the bias voltage axis, whereas the device on SiC behaves as a rectifying diode and the presence of hydrogen causes a shift of the forward current vs. voltage plot. The relatively large forward current, in both cases, indicates that there is measurable electrical transport across the AlN layer, but at the same temperature the turn on bias is different. Either structure contains two rectifying contacts in series, namely a Schottky contact between Pd and AlN and a heterojunction between AlN and the substrate.
MOS Interface Properties and MOSFET Performance on 4H-SiC{0001} and Non-Basal Faces Processed by N2O Oxidation
Authors: Kimoto T. , Kanzaki Y. , Noborio M. , Kawano H. , Matsunami H.
Published: April 2011

4H-SiC(0001), (000-1), and (11-20) have been directly oxidized by N2O at 1300°C, and the MOS interfaces have been characterized. The interface state density has been significantly reduced by N2O oxidation on any face, compared to conventional wet O2 oxidation at 1150°C. Planar n-channel MOSFETs fabricated on lightly-doped 4H-SiC(0001), (000-1) and (11-20) faces have shown an effective channel mobility of 26, 43, and 78 cm2/Vs, respectively. The mobility decreased with increasing the doping concentration of p-body. SIMS analyses have revealed a clear pile-up of nitrogen atoms near the MOS interface. The thickness of interfacial SiCxOy layer can be decreased by utilizing N2O oxidation. The crystal face dependence of interface structure is discussed.
Copper CMP Formulation for 65 nm Device Planarization
Authors: Stauf Gregory T. , Boggs Karl , Wrschka Peter , Ragaglia Craig , Darsillo Michael , Roeder Jeffrey F. , King Mackenzie , Liu Jun , Baum Thomas
Published: April 2011

To achieve 65 nm technology node requirements, CMP processes must provide improved control of selectivity, topography, wire cross section, and process robustness. Slurries and processes must also be compatible with fragile low k materials by providing low erosion and shear forces. We present data on a unique step 1 bulk Cu removal slurry with high selectivity, removal rates over 8000 Å/min, and extremely low liner removal/erosion in high (90%) density structures. This is achieved through a combination of surface modified abrasives and alternative inhibitors which provide superior performance and reduced electrochemical activity compared to benzotriazole, a commonly used inhibitor. The step 1 slurry was used with a step 2 liner removal slurry that can be chemically tuned to adjust relative selectivities of Cu:Ta:oxide from the nominal ratio of 1:0.9:1.6, allowing its use with a variety of integration schemes. Results of CMP planarization experiments on 200 mm blanket and patterned single damascene test wafers are described, including electrical data which demonstrates low overpolish sensitivity.
A Study on the Self-Stopping CMP Process for the Planarization of the High Step Height(@step height>1.5um) Pattern
Authors: Kim Kwang-Bok , Lee Hyo-Jin , Jang Ki-Hoon , Ko Joung-Duk , Kim Kyung-Hyun , Hwang In-Seac , Ko Yong-Sun , Song Chang-Lyong
Published: April 2011

CMP(Chemical Mechanical Planarization) process is widely used to reduce step height in semiconductor fabrication processes. As a design rule shrinks, a highly planar surface becomes inevitable within wafer scales. In order to get a high degree of a planarization, self-stopping characteristics of a ceria-based slurry should be studied and used in semiconductor process. In this study, threshold polishing pressure for a self-stopping characteristics was obtained by optimizing down pressure, pad conditioning, and mixing ratio of ceria abrasive and additive. A series of experiments were made to optimize the threshold polishing pressure in variable line & space patterns that consist of 0.8um step height and unit oxide film. As a result, self-stopping cmp process is twice batter than conventional silica-based process with respect to planarity and WIWNU. In addition, WIWNU and step height was dramatically decreased to less than 1000Å when applying to real fabrication devices over 2um step height.
CMP Modeling and Characterization for Polysilicon MEMS Structures
Authors: Tang Brian , Boning Duane
Published: April 2011

The current bedrock technology for integrated circuit (IC) planarization, chemical-mechanical polishing is beginning to play an important role in microelectromechnical systems (MEMS). However, MEMS devices operate with bigger feature sizes in comparison to ICs, in order to fulfill mechanical functions. We present an experiment to characterize and model a polysilicon CMP process with the specific goal of examining MEMS-sized test structures. We utilize previously discussed CMP models and examine whether assumptions from IC CMP can be applied to MEMS CMP. An analysis of the data collected points to a polishing dependence on not only pattern density, but also partly on feature size or feature configuration. The existing pattern density and step height CMP models are able to capture the major trends in up and down area polishing. However, certain layout features relevant to MEMS are difficult to predict, motivating the need for further model development and application.
Cerium Oxide Abrasives – Observations and Analysis
Authors: Evans David R.
Published: April 2011

The use of cerium oxide (ceria) as an abrasive for dielectric chemical mechanical polishing has a “checkered” history to say the least. Nevertheless, its use remains attractive for this purpose because of favorable polishing characteristics that are generally not obtainable using conventional fumed or colloidal silica abrasives. To be specific, large differences are commonly observed between removal rates of thin film silicon oxide, silicon nitride, and/or polysilicon. Moreover, such rate selectivity invariably favors removal of oxide films, which of course, is precisely what is desirable for fabrication of modern shallow trench isolation schemes. Even so, CMP using ceria abrasive often exhibits unusual characteristics that cannot be explained adequately by conventional polishing models based on pad/asperity elasticity or pressure distribution over features. Most notably, non-conventional, observed behaviors can be collected under the rubric of “slow start phenomena”. In this work, it is asserted that specific polishing characteristics of ceria slurry are largely due to the detailed surface chemistry of ceria particles and their interaction with silica. In any case, it is further shown that modification of slurry lubrication can alleviate slow-start and shift CMP process characteristics back toward more conventional behavior.
Rare-earth doped microlasers for microphotonic applications
Authors: Yang Lan , Min Bumki , Vahala K. J.
Published: April 2011

Sol gels provide a highly flexible technique for preparation of both planar and non-planar oxide thin films. They also enable the incorporation of various dopants into the films. In this work we describe the application of erbium-doped solgel films to surface functionalize optical microresonators. The resulting microlaser devices are especially interesting because their emission band falls in the important 1.5 μm window used for optical fiber communications. Both microsphere and ultra-high-Q microtoroid resonators-on-a-chip were functionalized into lasers and then characterized [1]. The erbium-doped sol-gel films were applied to the resonator surface and subsequently a CO2 laser was used to induce flow and densification of the sol-gel film on the surface. Optical quality thin films were obtained after the CO2 laser induced anneal. By varying the doping concentration and thickness of the applied sol-gel layers in microsphere resonators, we can vary the laser dynamics so that both continuous-wave and pulsation operation are possible. Single mode performance with high differential quantum efficiency was also obtained using the ultra-high-Q microtoroid resonator. These chip-based microlasers enable integration with other optical or electronic functions [2-3].
In-situ Control of Nitrogen Content and the Effect on PL Properties of SiNx Films Grown by Ion Beam Sputter Deposition
Authors: Kim Kyung Joong , Moon Dae Won , Yang Moon-Seung , Shin Jung H.
Published: April 2011

Strong visible luminescence was observed in silicon nitride (SiNx) thin films grown by ion beam sputter deposition (IBSD) using nitrogen ion as a sputtering source. Nitrogen content (x) of the films was controlled by variation of the sputtering N2 ion flux and analysed by in-situ x-ray photoelectron spectroscopy (XPS). Relative sensitivity factors of Si and N peaks could be calculated by Rutherford backscattering spectroscopy. The photoluminescence (PL) spectra of the post-annealed samples showed visible luminescence at blue-green region. PL energy showed a blue-shift due to quantum confinement with decreased excess Si and intensity showed a maximum value near x = 1.1. These PL properties are well correlated with the formation of Si nanocrystals (nc-Si). We found that there is a great increase of PL energy of SiNx thin films compared with SiOx thin films, which indicate that the surface state of Si nanocrystals plays an important role to increase PL energy and intensity.
Optical and Nanomechanical Characterization of an Omnidirectional Reflector Encompassing 850 nm Wavelength
Authors: Deopura Manish , Fink Yoel , Schuh Christopher A.
Published: April 2011

We demonstrate that multilayers composed of nineteen alternating layers of tin sulfide and silica can function as omnidirectional reflectors. These materials exhibit omnidirectional reflectivity for a range of frequencies in the near infra-red (NIR) encompassing the 850 nm wavelength. A refractive index contrast of 2.7/1.46 is achieved, one of the highest values demonstrated until now in NIR photonic bad gaps. In addition, new nanoindentation procedures have been developed to measure mechanical properties of these fine laminate materials, and demonstrate that tin sulfide-silica multilayers are mechanically stable for practical applications.
Structural Characterization of Molecular Beam Epitaxy Grown GaInNAs and GaInNAsSb Quantum Wells by Transmission Electron Microscopy
Authors: Gugov Tihomir , Wistey Mark , Yuen Homan , Bank Seth , Harris James S.
Published: April 2011

In the past decade, the quaternary GaInNAs alloy has emerged as a very promising material for lasers in the 1.2-1.6 μm range with application in telecommunication fiber-optic networks. While most of the challenges in growing high quality laser material with emission wavelength out to 1.3 μm have been successfully resolved, extending the emission beyond 1.3 μm has proven to be quite difficult. Achieving emission out to 1.5 μm requires higher In (up to 40%) and N (up to 2%) compositions. This makes the growth of this thermodynamically unstable alloy quite difficult with phase segregation occurring even at lower growth temperatures. Recently, adding small amounts of antimony has dramatically improved the quality of the material and high luminescence has been demonstrated at wavelengths beyond 1.5 μm. In this study, high-resolution transmission electron microscopy (HRTEM) was used in a novel way in conjunction with dark-field (DF) TEM to elucidate the role of antimony in improving the material quality. The results show that antimony improves the material uniformity via reduction of the local compositional fluctuations of indium.
Multilevel Nonvolatile Memories Based on Nanowire / Molecular Wire Heterostructures
Authors: Li Chao , Fan Wendy , Lei Bo , Zhang Daihua , Han Jie , Meyyapan M.
Published: April 2011

Nonvolatile nanoscale memories with ultra-long retention times have been demonstrated for both binary and multilevel applications. These devices were based on nanowires functionalized with a self- assembled monolayer of redox active molecular wires, where the bit was represented by the charge stored in the redox molecules and the nanowire conductance was used as the readout. Our devices exhibited reliable operation, on/off ratios ∼ 104 and retention times ∼ one month, one of the longest retention times ever achieved with nanoscale devices. These devices were further tailored for multilevel data storage with appreciable noise margins, representing a new concept for functional devices. Our work clearly demonstrates the potential of combining nanowires and molecular wires for superior performance.
Characterization and Manipulation of Exposed Ge Nanocrystals
Authors: Sharp I.D. , Xu Q. , Liao C.Y. , Yi D.O. , Ager J.W. , Beeman J.W. , Yu K.M. , Zakharov D.N. , Liliental-Weber Z. , Chrzan D. C. , Haller E.E.
Published: April 2011

Isotopically pure 70Ge and 74Ge nanocrystals embedded in SiO2 thin films on Si substrates have been fabricated through ion implantation and thermal annealing. Nanocrystals were subsequently exposed using a hydrofluoric acid etching procedure to selectively remove the oxide matrix while retaining up to 69% of the implanted Ge. Comparison of transmission electron micrographs (TEM) of as-grown crystals to atomic force microscope (AFM) data of exposed crystals reveals that the nanocrystal size distribution is very nearly preserved during etching. Therefore, this process provides a new means to use AFM for rapid and straightforward determination of size distributions of nanocrystals formed in a silica matrix. Once exposed, nanocrystals may be transferred to a variety of substrates, such as conducting metal films and optically transparent insulators for further characterization.
Laser-Grown Silicon Nanoparticles and Photoluminescence Properties
Authors: Herlin-Boime N. , Jursikova K. , Trave E. , Borsella E. , Guillois O. , Fabbri F. , Vicens J. , Reynaud C.
Published: April 2011

Light-emitting silicon nanocrystals (Si nc) have attracted much interest due to their possible application as optoelectronic devices. The interest for Si nanopowders is enhanced by their photoluminescence (PL) emission intensity that can be very strong at room temperature. Due to the intrinsic biocompatibility of Si nanoparticles, this strong optical emission intensity as well as the long decay time (mean life time around hundred microseconds) make these powders potential candidates as tracers for in-vivo applications.

Si nanopowders were obtained in gram quantities by CO2 laser pyrolysis of silane. The particles in the produced powders are in the size range 10-15 nm. These nanoparticles exhibit strong red photoluminescence after heat treatment. The appearance of intense PL emission is clearly related to the surface oxidation of the powders which must be carefully controlled. Several steps have been identified in the oxidation process. This paper presents a detailed study of the evolution of both the photoluminescence intensity and spectral dependence and of the crystalline structure as a function of the heat treatment. We also show that the nanopowders can be dispersed in liquids and incorporated in gel samples while keeping their intense photoluminescence. This result opens a route towards the fabrication of novel devices
Silicon Nanoparticle Synthesis Using Constricted Mode Capacitive Silane Plasma
Authors: Bapat A. , Dong Ying , Perrey C. R. , Carter C. B. , Campbell S.A. , Kortshagen U.
Published: April 2011

Crystalline semiconductor nanoparticles are of interest for a variety of electronic and opto-electronic applications. We report experimental studies of the synthesis and characterization of crystalline silicon nanoparticles using a constricted-mode capacitive RF plasma in continua- tion of results reported earlier from an RF inductively coupled plasma [1]. The constricted-mode discharge is based on a thermal plasma instability yielding a high-density plasma filament, which rotates at a high frequency. Silane is dissociated, leading to particle nucleation and growth. Particles are extracted by passing the particle-laden gas through an orifice to form a beam and col- lected by inertial impaction.

We are able to reproducibly synthesize highly oriented freestanding single-crystal silicon nanoparticles. Monodisperse particle size distributions centered at a 35nm particle diameter with a geometric standard deviation of 1.3 are obtained. Transmission electron microscope (TEM) studies show uniformly shaped cubic particles. Selected-area electron diffraction patterns indi- cate the particles have the diamond-cubic silicon structure. To study the electrical properties of these particles, metal-semiconductor-metal structures were fabricated and analyzed.
TEM 3D Tomography of Noble Metal Nanowires Growth Inside SiO2 Mesoporous Aggregates
Authors: Arbiol J. , Rossinyol E. , Cabot A. , Peirò F. , Cornet A. , Morante J. R.
Published: April 2011

The use of bright-field transmission electron microscopy 3-D tomographic reconstruction allowed us to localize the synthesized nanostructures within the pores of mesoporous nanotemplates. Using this technique, we found that noble metal impregnation resulted in the inclusion of metal nanostructures within the SBA-15 mesoporous silica hexagonal pores (from nanoclusters to nanowires). The method allowed corroborating if the nanostructures were synthesized inside the pores or if they were alternatively on the nanotemplate aggregates exterior surface.
Defect Formation in Surface Modified TiO2 Nanostructures
Authors: Prokes S.M. , Gole James L.
Published: April 2011

The defect formation in surface modified TiO2 has been studied. Anatase TiO2 structures in the 3-20 nm range formed by a wet chemical technique were surface modified and nitridation of the highly reactive TiO2nanocolloid surface was achieved by a quick and simple treatment in alkyl ammonium compounds. The nitriding process was also accompanied by a metal surface coating process using electroless plating techniques, resulting in a thin metal surface layer on the modified TiO2 nanostructures. The crystal structure of the resultant TiO2-xNx nano-colloids remained anatase and the freshly prepared samples exhibited a strong light emission near 560nm (2.21 eV), which red shifted to 660 nm (1.88 eV) and dropped in intensity with aging in the atmosphere. This behavior was also evident in some of the combined nitridized and metal coated TiO2 nano-colloids. Electron Spin Resonance performed on these samples identified a resonance at g = 2.0035, which increased significantly with nitridation. This resonance is attributed to an oxygen hole center created near the surface of the nanocolloid, which correlates well with the observed optical activity.
Synthesis of Co@Au Core-Shell Nanoparticles in Non-Aqueous Solution and their Characterization
Authors: Ban Zhihui , O'Connor C. J.
Published: April 2011

A homogeneous non-aqueous solution reactions method has been developed to prepare gold-coated cobalt (Co@Au) nanoparticles. After the sample was washed with 8% HCl, XRD (X-Ray Diffraction), TEM (transmission electron microscopy), and magnetic measurements SQUID (Superconducting Quantum Interference Device) are utilized to characterize the nanocomposites. XRD shows the pattern of sample, which is responding to gold and cobalt, no cobalt oxide was found. TEM results show that the average size of Co@Au nanoparticles is about 10 nm and we can find core-shell structure of the sample. SQUID results show that the particles are ferromagnetic materials at 300K. So the gold- coated cobalt nanoparticles (Co@Au) can be successfully prepared by the homogeneous nonaqueous approach. This kind of core-shell materials is stable in acid condition, which would give many opportunities for bio- application.
Finite-Size Effects and Surface-Enhanced Raman Scattering in Noble-Metal Nanoparticles
Authors: Pustovit Vitaliy N. , Shahbazyan Tigran V.
Published: April 2011

We study the role of a strong electron confinement on the surface-enhanced Raman scattering from molecules adsorbed on small noble-metal nanoparticles. We describe a novel enhancement mechanism which originates from the different effect that confining potential has on s-band and d-band electrons. We demonstrate that the interplay between finite-size and screening efects in the nanoparticle surface layer leads to an enhancement of the surface plasmon local field acting on a molecule located in a close proximity to the metal surface. Our calculations show that the additional enhancement of the Raman signal is especially strong for small nanometer-sized nanoparticles.
Growth of Co Nanoclusters on SiC Honeycomb Templates
Authors: Chen Wei , Loh Kian Ping , Xu Hai , Wee A.T.S.
Published: April 2011

A honeycomb-like SiC reconstructed surface with regular, periodic porosity in the nano-scale range has been used as an effective template for the formation of monodispersed Co nanoclusters. In-situ scanning tunneling microscopy (STM) was used to study the nucleation process of the Co nanoclusters on this template. The deposition of Co at different substrate temperature was investigated by STM. It is found that the failure in the deposition of Co nanoclusters on the SiC honeycomb template with substrate temperatures higher than room temperature (RT) might be due to the high desorption rate of the adsorbed Co atoms.
Atomistic Simulations of fcc Pt75Ni25 and Pt75Re25 Cubo-octahedral Nanoparticles
Authors: Wang Guofeng , Van Hove M.A. , Ross P.N. , Baskes M.I.
Published: April 2011

We have developed interatomic potentials for Pt-Ni and Pt-Re alloys within the modified embedded atom method (MEAM). Furthermore, we applied these potentials to study the equilibrium structures of Pt75Ni25 and Pt75Re25 nanoparticles at T=600 K using the Monte Carlo method. In this work, the nanoparticles are assumed to have disordered fcc cubo-octahedral shapes (terminated by {111} and {100} facets) and contain from 586 to 4033 atoms (corresponding to a diameter from 2.5 to 5 nm). It was found that, due to surface segregation, (1) the Pt75Ni25 nanoparticles form a surface-sandwich structure: the Pt atoms are enriched in the outermost and third atomic shells, while the Ni atoms are enriched in the second atomic shell; (2) the equilibrium Pt75Re25 nanoparticles adopt a core-shell structure: a Pt-enriched shell surrounding a Pt-deficient core.
Molecular-Dynamics Study of the Structural Dependence of the Young Modulus of Au Nanowires
Authors: Kameoka S. , Shintani K.
Published: April 2011

The deformation of Au nanowires of helical multi-shell (HMS) structures and the fcc structure under a tensile external force is addressed by molecular-dynamics simulation. The modified embedded-atom method (MEAM) potential is employed for calculating the interaction between Au atoms. At first, a model nanowire is equilibrated at a specified temperature. Next, the external force in the axial direction is imposed on the Au atoms at the ends of the nanowire. We conclude that the Young modulus of a Au nanowire depends on its atomic structure.
Room-Temperature Lasing Characteristics of Zinc Sulfide Single Nanoribbons
Authors: Zapien J. A. , Jiang Y. , Meng X.-M. , Chen W. , Lifshitz Y. , Lee S.-T.
Published: April 2011

We present a study of the photoluminescence and lasing characteristics of individual single-crystal zinc sulfide nanoribbons. We show that ZnS presents optical gain at the near band gap emission of ∼338 nm. Optical gain together with the optical cavity defined by the perfect crystallographic structure with a rectangular cross section results in lasing activity at optical pumping power densities of ∼ 50 kW/cm2. Small collection angle measurements show that nanoribbons form excellent optical cavities and gain medium with record (FWHM<0.1nm) lasing modes free of PL background. Large collection angles (as previously employed for nanowire measurements) add a broad, non-resonant PL component and obscure the original high-quality lasing of the nanowires/nanoribbons.
Controlling the Growth Direction of ZnO Nanowires on c-Plane Sapphire
Authors: Nikoobakht Babak , Davydov Albert , Stranick Stephan J.
Published: April 2011

Well oriented vertical ZnO nanowires (NWs) are grown on c-plane sapphire via a vapor- phase transport process using an Au thin film as a catalyst. This new finding is unexpected due to the fact that the lattice mismatch between the zinc oxide and the underlying substrate is 18%. X- ray diffraction (XRD) analysis shows that single-crystal, wurtzite NWs grow in the [0001] direction normal to the basal sapphire plane, which proves that a-plane sapphire is not essential for growth of vertical ZnO NWs, as has been previously stated.[1] We have found that by controlling the thickness of the Au-film and pre-growth annealing of the Au/sapphire substrate NWs can be grown either tilted or vertical. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) studies on Au films with thicknesses ranging from 1 to 10 nm show that in the absence of film annealing, NWs can be grown 32° tilted from the surface normal, whereas pre-annealed Au films result in growth of NWs in the surface normal direction. We attribute the formation of the normal and tilted growth directions to the surface concentration of O and Al ions on sapphire.
Targeting Magnetic Nanoparticles in High Magnetic Fields for Drug Delivery Purposes
Authors: Asmatulu Ramazan , Claus Richard.O. , Riffle Judy S. , Zalich Michael
Published: April 2011

Biodegradable magnetic nanoparticles were synthesized using Poly(L-Lactic Acid) and magnetite nanoparticles (∼14 nm) at different dosages, and then these nanaoparticles (nanocomposites) and pure magnetic particles were targeted in external magnetic fields by changing the test parameters. The magnetic field test results showed that magnetic saturation, fluid speed, magnetic field distance and particle size were extremely effective for a magnetic guidance system that is needed for an effective drug delivery approach. Thus, it is assumed that such nanoparticles can carry drugs (chemotherapy) to be able to cure cancer tumors as well as many other diseases.
Carbon Nanotube-Based Permeable Membranes
Authors: Holt Jason K. , Park Hyung Gyu , Bakajin Olgica , Noy Aleksandr , Huser Thomas , Eaglesham David
Published: April 2011

A membrane of multiwall carbon nanotubes embedded in a silicon nitride matrix was fabricated for use in studying fluid mechanics on the nanometer scale. Characterization by fluorescent tracer diffusion and scanning electron microscopy suggests that the membrane is void-free near the silicon substrate on which it rests, implying that the hollow core of the nanotube is the only conduction path for molecular transport. Assuming Knudsen diffusion through this nanotube membrane, a maximum helium transport rate (for a pressure drop of 1 atm) of 0.25 cc/sec is predicted. Helium flow measurements of a nanoporous silicon nitride membrane, fabricated by sacrificial removal of carbon, give a flow rate greater than 1×10-6 cc/sec. For viscous, laminar flow conditions, water is estimated to flow across the nanotube membrane (under a 1 atm pressure drop) at up to 2.8×10-5 cc/sec (1.7 μL/min).
Integrating Carbon Nanotubes For Atomic Force Microscopy Imaging Applications
Authors: Ye Qi , Cassell Alan M. , Liu Hongbing , Han Jie , Meyyappan Meyya
Published: April 2011

Carbon nanotube (CNT) related nanostructures possess remarkable electrical, mechanical, and thermal properties. To produce these nanostructures for real world applications, a large-scale controlled growth of carbon nanotubes is crucial for the integration and fabrication of nanodevices and nanosensors. We have taken the approach of integrating nanopatterning and nanomaterials synthesis with traditional silicon microfabrication techniques. This integration requires a catalyst or nanomaterial protection scheme. In this paper, we report our recent work on fabricating wafer-scale carbon nanotube AFM cantilever probe tips. We will address the design and fabrication considerations in detail, and present the preliminary scanning probe test results. This work may serve as an example of rational design, fabrication, and integration of nanomaterials for advanced nanodevice and nanosensor applications.
Novel Chemical Approach to Achieve Advanced Soft Lithography by Developing New Stiffer, Photocurable PDMS Stamp Materials
Authors: Choi Kyung M. , Rogers John A.
Published: April 2011

Recent advances in microfabrication technology allow us to develop a number of novel devices with high performance. In microfabrication technology, a new development, ‘soft lithography’, is widely used by making stamps, molding, and microcontact-printing due to the low cost and easy processability. The resolution of soft lithography significantly relies on the performance of stamping materials. However, pattern transfers using commercially available PDMS stamp materials often end up with mechanical failures such as collapse or sag due to their low physical stiffness. Additionally, most of those commercial PDMS materials are thermally curable systems, which results in significant thermal deformations. These limitations have motivated us to start this work, which demonstrates a ‘chemical approach’ to overcome those limits by developing new stiff, photocurable PDMS stamp materials with attached designed functionalities. Molecular modification of PDMS materials results in advanced soft lithography, which produces enhanced physical toughness, lower polymerization shrinkage, and photopatterning capability.
A Multi-layer Technology for Biocompatible Polymer Microsystems with Integrated Fluid and Electrical Functionality
Authors: Moss Eileen D. , Han Arum , Frazier A. Bruno
Published: April 2011

A method to fabricate biocompatible polymer microfluidic systems with integrated electrical and fluid functionality has been established. The process flow utilizes laser ablation, microstenciling, and heat staking as the techniques to realize multi-layered polyimide based microsystems with microchannels, thru and embedded fluid / electrical vias, and metallic electrodes and contact pads. As an application of the fabrication technology, a six layer multi-functional cellular analysis system has been demonstrated. The electrophysiological analysis system contains fluid microchannel / via networks for cell positioning and chemical delivery as well as electrical detectors and electrodes for impedance spectroscopy and patch clamping studies. Multiple layers of 50.8µm thick Kapton® sheets with double-sided polyimide adhesive layers were used as the primary material-of-construction. Microchannels with widths of 400µm as well as thru hole vias with 3.71µm diameters (aspect ratios of over 12:1) were laser ablated through the polyimide sheets using an excimer laser and a CO2laser. Electrical traces and contact pads with features down to 20µm were defined on the flexible polyimide sheets using microstenciling. The patterned layers were bonded using heat staking at a temperature of 350°C, a pressure of 1.65MPa for 60 minutes. This multi-layer technology can be used to create microfluidic devices for many application areas requiring biocompatibility, relatively high temperature operation, or a flexible substrate material.
Tuning the Mechanical Properties of Poly-silicon Film by Surface Modification Using Plasma Treatment
Authors: Su Wang-Shen , Fang Weileun , Tsai Ming-Shih
Published: April 2011

The mechanical properties of thin film are very critical for the performance of MEMS devices. Since Poly-silicon film is of great use in MEMS, this study investigates the surface modification by various plasma treatments to finely tune the chemical and mechanical properties of poly-silicon film. Various plasma treatments, including H2, O2, and NH3, were implemented to modify the original Si-Si film bonding, Young's modulus, and hardness of poly-silicon film. These were significant Si-O, Si-OH/Si-H and Si-NH2/Si-N bonds formed after O2, H2 and NH3 plasma treatment, respectively. According to the H analysis from SIMS depth profile of, the thickness of surface modified layer would be ranged from 50 to 120 nm. In summary, the surface modification with H2 plasma can reduce the elastic modulus of poly-silicon film for about 32.3%; moreover, the following vacuum annealing will further reduce the elastic modulus for about 60.2%. Therefore, surface modification with an adequate plasma treatment would be an effective method to change the chemical and mechanical properties of poly-silicon film.
Fabrication of Ordered Sub-Micron Topographies on Large-Area Poly(Urethane Urea) by Two-Stage Replication Molding
Authors: Milner Keith R. , Balmer Mallory , Donahue Henry J. , Snyder Alan J. , Siedlecki Christopher A.
Published: April 2011

It has been established that material surface topography can have a significant effect on biological cell adhesion, in the absence of changes in surface chemistry. Such investigations were typically performed using surface features with size on the order of microns, comparable to the dimensions of the cells. It has been demonstrated that sub-micron sized topographies that cannot be created via contact lithography also influence cell behavior. The ability to affect cell adhesion is a prime consideration in the development of novel biomaterials. This study reports a two-stage replication molding process for fabricating ordered sub-micron sized features over a large area of biomedical polyether(urethane urea). Such a technique has great applicability in the area of long-term implantable materials as a method for influencing cell-material interactions.
Formation of Three Dimensional Ni Nanostructures for Large Area Catalysts
Authors: Carey J. D. , Henley S. J. , Mendoza E. , Giusca C. E. , Adikaari A. A. D. T. , Silva S. R. P.
Published: April 2011

The formation of Ni nanostructures to act as catalysts in the growth of carbon nanotubes is reported. The changes in the surface morphology of Ni produced by three methods - thermal evaporation and annealing of thin films, pulsed laser ablation and annealing of Ni, and the use of metal containing macromolecules - have been investigated by atomic force microscopy and scanning electron microscopy. In the case of thermal annealing of thin metal films in the temperature range 300-500°C we observe an increase in the mean diameter of the islands formed, accompanied by a reduction in the mean island density with increasing temperature. We attribute this effect to mass transport of weakly bound individual Ni atoms and/or small island clusters across the surface to form larger isolated islands, in a process similar to Ostwald ripening. Using a pulsed KrF excimer laser for ablation of a Ni target we show that nanometre smooth Ni thin films can be produced provided a sufficient number of laser shots is used. The surface morphology of these smooth films can then be altered by laser annealing to form Ni droplets. It is found that the mean diameter of the Ni droplets depends not only on the initial Ni thickness but also the laser fluence. It is also found that the nanostructuring of the film depends on the presence of an oxide under layer, with a higher fluence required on thinner oxides and no nanostructuring observed on bare Si. Finally, we show that Ni nanostructuring can be formed by suitable annealing of a Ni containing aqueous dendrimer solutions.
Debonding Under Fatigue Loading at Polymer/Inorganic Interfaces
Authors: Sharratt Bree M. , Dauskardt Reinhold H.
Published: April 2011

The mechanisms associated with cycle-by-cycle damage accumulation resulting in fatigue crack propagation between a highly constrained polymer layer and an adjacent elastic substrate are explored. Specifically, cyclic fatigue-induced crack growth between a bisphenol F model epoxy system and a passivated silicon substrate under Mode I loading is reported. Preliminary findings regarding the effects of fatigue load ratio on interfacial crack growth rates are presented. While intermediate crack growth rates were significantly accelerated under cyclic loading, the near-threshold crack growth behavior under cyclic and monotonic loading was surprisingly similar.
Testing Multiwall Carbon Nanotubes on Ion Erosion for Advanced Space Propulsion
Authors: Yap Yoke Khin , Menda Jitendra , Vanga Lakshman Kumar , Kayastha Vijaya , Wang Jiesheng , King Lyon B. , Dimovski Svetlana , Gogotsi Yury
Published: April 2011

Are carbon nanotubes more resistant than diamonds against ion erosion? Here, we report an evaluation of multiwall carbon nanotubes (MWNTs) as the protective coating against plasma erosion in advanced space propulsion systems. We have compared polycrystalline diamond films with MWNTs, amorphous carbon (a-C) and boron nitride (BN) films. Two types of MWNTs were investigated including vertically aligned (VA) MWNTs, and those horizontally laid on the substrate surfaces. Only diamond films and VA-MWNTs survived erosion by 250 eV krypton ions of a flight-quality Hall-effect thruster. VA-MWNTs are found to bundle at their tips after ion erosion.
Twin Boundaries and Stacking Faults in Monazite (Monoclinic LaPO4)
Authors: Hay Randall S.
Published: April 2011

Monazite (LaPO4) was indented at room temperature. Deformation twin boundaries and stacking faults were characterized by high resolution transmission electron microscopy. Kinked deformation twins were also characterized and analyzed. Three types of stacking faults associated with climb-dissociated partial dislocations were observed. Two were found on twin boundaries, and a third in the lattice. Formation mechanisms are discussed. The superimposition of stacking faults along twin boundaries during deformation twinning and the glide of climb-dissociated partial dislocations allowed by stacking fault migration are discussed. The possible relationship between the formation mechanisms for these defects and the low- temperature recrystallization and self-annealing of defects in monazite is considered.
The Mechanical Properties in the Vicinity of Grain Boundaries in Ultrafine-Grained and Polycrystalline Materials Studied by Nanoindentations
Authors: Schweitzer E. , Durst K. , Amberger D. , Göken M.
Published: April 2011

The strength of structural materials strongly depends on the structure and properties of grain boundaries. Interfaces usually act as barriers to dislocation motion and therefore strengthen materials with decreasing grain size, quantitatively described by the well-known Hall-Petch relation. However, interfaces in nanocrystalline materials are often covered with impurities or second phases, which may influence the mechanical properties. With nanoindentation testing it is now possible to probe the strength of interfaces like grain boundaries directly on a nanometer scale. Therefore this method was used to investigate the properties in the vicinity of grain boundaries in polycrystalline materials with conventional grain size and in ultrafine-grained metals prepared by equal channel angular pressing (ECAP), where no impurities are introduced during processing. Measurements on an austenitic steel clearly show a decreasing hardness close to the interface opposite to the general expected behavior of strengthening. In this case segregation effects strongly influence the mechanical properties near the boundaries. The nanoindentation investigations on ultrafine-grained Al and Cu show a strong strain rate sensitivity. Inelastic effects are also found between unloading-loading segments during indentations.
Bimodal Microstructures in Nanocrystalline Al and Al-Mg Alloy Powders Prepared by Cryogenic Ball Milling
Authors: Zhou F. , Lee Z. , Lavernia E. J. , Nutt S.R.
Published: April 2011

The microstructures of nanocrystalline Al and Al-7.6 at% Mg alloy powders processed by cryogenic ball milling (i.e., cryomilling) and subsequent thermal annealing were characterized using transmission electron microscopy. The as-milled samples primarily consisted of equiaxed grains with average sizes of ∼ 26 nm. The annealing treatments at elevated temperatures resulted in bimodal grain-size distributions, with sub-micrometer-sized grains embedded in a matrix of nanocrystalline (<50 nm) grains. The bimodal microstructures were formed during the processes of recovery and recrystallization.
Hyperelastic effects in brittle materials failure
Authors: Buehler Markus J. , Abraham Farid F. , Gao Huajian
Published: April 2011

A fact that has been neglected in most theories of brittle fracture is that the elasticity of a solid clearly depends on its state of deformation. Metals will weaken, or soften, and polymers stiffen as the strain approaches the state of materials failure. It is only for infinitesimal deformation that the elastic moduli can be considered constant and the elasticity of the solid linear. We show by large-scale atomistic simulations that hyperelasticity, the elasticity of large strains, can play a governing role in the dynamics of fracture and that linear theory is incapable of capturing all phenomena. We introduce a new concept of a characteristic length scale ξ for the energy flux near the crack tip and demonstrate that the local hyperelastic wave speed governs the crack speed when the hyperelastic zone approaches this energy length scale. The new length scale ξ, heretofore missing in the existing theories of dynamic fracture, helps to form a comprehensive picture of crack dynamics, explaining super-Rayleigh and supersonic fracture. We further address the stability of cracks, and show agreement of the Yoffe criterion with the dynamics of cracks in harmonic systems. We find that softening hyperelastic effects lead to a decrease in critical instability speed, and stiffening hyperelastic effects leads to an increase in critical speed. The main conclusion is that hyperelasticity plays a critical role in forming a complete picture of dynamic fracture.
Extremely Thin Absorber Layers in Solid State Solar Cells
Authors: Bellaidi A. , Ernst K. , Könenkamp R.
Published: April 2011

We report on recent progress in fabricating an inorganic solid state solar cell based on a nano-structured substrate with an extremely thin absorber, the so-called eta-cell. The cell uses HgCdTe as an absorber layer and TiO2 as a substrate. By adjusting the Hg/Cd ratio the conduction band edges in these two materials can be brought into alignment. This tuning provides an ideal tool to optimize the optical and electrical properties of the cell. In qualitative agreement with earlier work we find a band offset of 0.25 eV to give maximum transfer rates across the interface. The optimized cell has a short circuit current of 15 mA/cm2 and an open-circuit voltage of 0.6 V. We discuss the optical and electrical properties of the cell, and outline ideas to further improve the performance.