bluuebunny/crossref_metadata_2025 huggingface dataset:
Nanometer Metrology of Periodic Structures With Ultrafast Optoacoustics
Authors: Grimsley Thomas J , Antonelli George Andrew , Yang Fan , Maris Humphrey J , Nurmikko Arto
Published: August 2010

We present an optoacoustic method to non-destructively measure the average dimensions of a periodic array of simple structures with aspect ratios greater than 10:1, which are inaccessible to AFM techniques. The technique that we describe could be used as the basis of an inline metrology tool for wafer inspection. The samples examined were test structures with high precision lithographically defined lines of silicon dioxide deposited on a silicon substrate. The thickness of the silicon dioxide was around 400 nm, and the gaps between the lines ranged from 100 nm down to smaller than 40 nm. A drop of water was placed on the sample, and an optoacoustic transducer was placed on top; measurements were taken with a water thickness less than 1 micron between the optoacoustic transducer and the sample. The water filled the spaces between the lines due to the hydrophilic nature of the sample surface. Using the picosecond ultrasonics technique, acoustic pulses are generated in a special optoacoustic transducer, transmitted through a coupling fluid (water), scattered off of the sample being examined and then return to the transducer. The returning acoustic signal shows nanometer sensitivity to the height of the lines and the specific details of their profile.
Indium Gallium Arsenide Based Non-Volatile Memory Devices with Site-Specific Self-Assembled Germanium Quantum Dot Gate
Authors: Chan Pik-Yiu , Gogna Mukesh , Suarez Ernesto , Alamoody Fuad , Karmakar Supriya , Miller Barry , Ayers John , Jain Faquir
Published: August 2010

This paper presents the implementation of indium gallium arsenide field-effect transistors (InGaAs FETs) as non-volatile memory using lattice-matched II-VI gate insulator and quantum dots of GeOx-cladded Ge as the floating gate. Studies have been done to show the ability of II-VI materials to act as a tunneling gate material for InGaAs based FETs, and GeOx-cladded Ge quantum dots having the ability to store charges in the floating gate of a memory device. Proposed structure of the InGaAs device is presented.
Co/HfO2 core shell nanocrystal memory
Authors: Zhou Huimei , Dorman James Anthony , Perng Ya-Chuan , Gachot Stephanie , Huang Jian , Mao Yuanbing , Chang Jane , Liu Jianlin
Published: August 2010

Metal/high-k dielectric core shell nanocrystal memory capacitor was demonstrated. This kind of MOS memory shows good performance in charge storage capacity, programming and erasing speed. By using a self-assembled Block Co-Polymer, Co/HfO2 core shell nanocrystals were well arrayed and showed uniform dot size and inter distance between dots. Compared with traditional metal nanocrystal fabrication process with E-Beam Evaporation followed by RTA (Rapid Thermal Annealing), core shell nanocrystal memory prepared by Block Co-Polymer produces a wide memory window of 8.4V at the ±12 V voltage sweep. Co/HfO2 core shell nanocrystals prepared by low-temperature Block Co-polymer process ensure high reliability of the devices.
Effect of Ion-implantation on Forming and Resistive Switching Response of NiO Thin-Films
Authors: Elliman Robert , Saleh Muhammad , Kim Sung , Venkatachalam Dinesh , Kim Taehyun , Belay Kidane
Published: August 2010

The forming voltage and set/reset response of sputter-deposited NiO thin films is studied as a function of implant fluence for samples implanted with Ni and O ions. The forming voltage of the films is shown to decrease with increasing ion fluence and to scale with the damage production rate of the different ions. In contrast, the set/reset response of the films was largely unaffected by the ion-implantation. These results are discussed in terms of the filamentary model of conduction and the thermochemical model of resistive switching.
Nanocrystal Memory Device Utilizing GaN Quantum Dots by RF MBE
Authors: Dimitrakis Panagiotis , Iliopoulos Eleftherios , Normand Pascal
Published: August 2010

The growth of GaN-QDs by radio frequency plasma assisted molecular beam deposition (RF-MBD) on thin SiO2 films for non-volatile memories (NVM) applications is demonstrated. Thermal budget modification during the deposition allows tuning of the size and density of the QDs. Preliminary electrical characterization of GaN-QD MOS devices reveals efficient electron injection at very low voltages from the Si accumulation layer to the QDs. The observed limitation in hole injection relates adequately to the energy band diagram of the structure.
Charge Trapping Memories With Atomic Layer Deposited High-k Dielectrics Capping Layers
Authors: Nikolaou Nikolaos , Dimitrakis Panos , Normand Pascal , Giannakopoulos Konstantinos , Mergia Konstantina , Ioannou-Sougleridis Vassilios , Kukli Kaupo , Niinisto Jaakko , Ritala Mikko , Leskela Markku
Published: August 2010

In this work, we examine the influence of hafnium and zirconium oxides ALD precursor chemistry on the memory properties of SiO2/Si3N4/ZrO2 and SiO2/Si3N4/HfO2 non-volatile gate memory stacks. Approximately 10 nm thick ZrO2 and HfO2 layers were deposited on top of a SiO2/Si3N4 structure, functioning as blocking oxides. Both metal oxides were deposited using either alkylamides or cyclopentadienyls as metal precursors, and ozone as the oxygen source. In the case of the ZrO2 gate stacks a memory window of 6 V was determined, comprised of 4 V write window and 2 V erase window. Although no dramatic differences were evident between the ZrO2 layers, ZrO2 grown from alkylamide provided structures with higher dielectric strength. The memory structures with HfO2 blocking layers indicate that the memory window and the dielectric strength are significantly affected by the precursor. The structures with the HfO2 formed from alkylamide showed a write window of 7 V, while the films grown from cyclopentadienyl possessed window of 5 V. Comparison between the memory windows obtained using ZrO2 and HfO2 as control oxides reveals that the former provides memory structures with higher electron trapping efficiency.
Low Temperature Deposition of Ferromagnetic Ni-Mn-Ga Thin Films From Two Different Targets via rf Magnetron Sputtering
Authors: Lourenço A. C. , Figueiras F. , Das S. , Amaral J. S. , Kakazei G. N. , Karpinsky D. V. , Soares N. , Peres M. , Pereira M. J. , Tavares P. B. , Sobolev N. , Amaral V. , Santos N. M. , Kholkin Andrei L.
Published: August 2010

Low temperature (400°C) deposition of ferromagnetic Ni-Mn-Ga thin films is successfully performed via rf magnetron sputtering technique using co-deposition of two targets Ni50Mn50 and Ni50Ga50 on sapphire (0001) and Si (100) substrates. The films are in part amorphous with significant degree of crystallinity. The obtained crystallographic structure is shown to be substrate-dependent. Films on both substrates are ferromagnetic at room temperature (Curie temperature ∼ 332.5K) with well-defined hysteresis loops, low coercivity (∼ 100 Oe) and a saturation magnetization of ∼ 200 emu/cc. At low temperature (5 K), both films show increased magnetization value with wider hysteresis loops having higher coercivity and remanent magnetization. The process is therefore effective in achieving the appropriate thermodynamic conditions to deposit thin films of the Ni-Mn-Ga austenitic phase (highly magnetic at room temperature) at relatively low substrate temperature without the need for post-deposition annealing or further thermal treatment, which is prerequisite for the device fabrication.
Magnetostatic Interactions of Two-Dimensional Arrays of Magnetic Strips
Authors: Malkinski Leszek Mariusz , Yu Minghui , Scherer II Donald
Published: August 2010

A series of arrays consisting of Permalloy stripes with dimensions of 100 nm × 300 nm × 1500 nm was fabricated using electron beam nanolithography and magnetron sputtering followed by the lift-off process. In order to elucidate the effect of magnetostatic interactions among nanosized stripes on magnetic properties of the arrays, the separation between the stripes in different arrays was varied in the range between 100 nm and 2000 nm. Magnetic hysteresis loops of the arrays were measured using SQUID magnetometer for different orientations of the applied field with respect to the arrays. Magnetic anisotropy of the arrays was determined based on ferromagnetic resonance measurements at 9.8 GHz using EPR spectrometer. The measurements were carried out for different directions of in-plane magnetic bias filed. The angular dependence of the resonance field of the main resonant peak indicated presence of the uniaxial magnetic anisotropy due to elongated shape of the stripes. Comparison between angular curves of resonant fields for different arrays leads to the conclusion that increasing strength of magnetostatic interactions among the stripes leads to a suppression of the uniaxial anisotropy. The stripes separated by 2000 nm behave almost like non-interacting objects, but the effect of interactions becomes particularly significant for separations smaller than 600 nm. The properties of the arrays with the smallest separations resembled those of continuous films. Magnetostatic modes have been observed in the FMR spectra in addition to the main resonant peak. These modes are believed to result from dimensional confinement of lateral spinwaves in the magnetic stripes. No such modes were observed in the reference samples of solid Py films, with the in-plane applied magnetic field.
Nonvolatile Floating Gate Memory Devices Containing AgInSbTe-SiO2 Nanocomposite Thin Film Prepared by Sputtering Method
Authors: Hsieh Tsung-Eong , Chiang Kuo-Chang
Published: August 2010

AgInSbTe (AIST)-SiO2 nanocomposite layer prepared by a one-step sputtering process utilizing target-attachment method was implanted in the nonvolatile floating gate memory (NFGM) devices. Device sample subjected to post annealing at 400°C for 2 min in atmospheric ambient exhibited a significant hysteresis memory window (ΔVFB) shift = 5.91V and charge density = 5.22×12 cm-2 after ±8V voltage sweep. During the retention time test, a ΔVFB shift about 3.50 V and charge loss about 28.4% were observed in the sample after a ±5V voltage stress for 104 sec. Cross-sectional TEM revealed that the nanocomposite layer contains the crystalline AIST nanoparticles with the sizes about 5 to 7 nm embedded in SiO2 matrix. XPS analysis indicated that annealing induces the reduction of antimony oxides to form metallic Sb nanocrystals and suppresses the oxygen defects and charge loss in nanocomposite layer. Analytical results illustrated that the utilization of AIST-SiO2 nanocomposite layer may simplify the preparation of NFGM device with satisfactory electrical properties, implying a promising feasibility of such a nanocomposite layer to NFGM devices.
Crystalline Path Formation in Nanoglasses of PCM
Authors: Nardone Marco , Simon Mark Alexander , Karpov Ilya V. , Karpov Victor G.
Published: August 2010

This work presents an analytical model of crystalline phase formation in nanoglasses of phase change memory. We describe a data loss mechanism when the cell resistance changes significantly at elevated temperatures over long periods of time with no electrical bias applied. Unlike the standard approach, which relates crystalline shunt formation to aggregates of crystalline particles forming the percolation cluster, we look at the rare events of almost rectilinear path formation in very thin structures. They can occur at crystalline volume fractions considerably lower than the critical volume fraction required for percolation. We find the characteristic parameters which can describe statistics of these rare events.
The role of vacancies in the pressure amorphisation phenomenon observed in Ge-Sb-Te phase change alloys
Authors: Krbal Milos , Kolobov Alex , Fons Paul , Tominaga Junji , Haines Julien , Pradel Annie , Ribes Michel , Levelut Claire , Le Parc Rozenn , Hanfland Michael
Published: August 2010

We demonstrate, both experimentally and by computer simulation, that while the metastable face-centered cubic (fcc) phase of Ge-Sb-Te becomes amorphous under hydrostatic compression at about 15 GPa, the stable trigonal phase remains crystalline. We present evidences that the pressure-induced amorphisation phenomenon strongly depends on the concentration of vacancies included in the Ge/Sb sublattice, but is thermally insensitive. Upon higher compression, a body-centered cubic phase is obtained in both cases at around 30 GPa. Upon decompression, the amorphous phase is retained when starting with the fcc phase while the initial structure is recovered when starting with the trigonal phase. We argue that the presence of vacancies and the associated subsequent large atomic displacements lead to nanoscale phase separation and the loss of the initial structure memory in the fcc staring phase of Ge-Sb-Te. We futher compare the amorphous phase obtained via the pressure route with the melt quenched amorphous phase.
Understanding Phase Change Memory Reliability and Scaling by Physical Models of the Amorphous Chalcogenide Phase
Authors: Ielmini Daniele
Published: August 2010

Phase change memory (PCM) devices are based on the electrically-induced change of phase within an active chalcogenide material. PCM features large resistance window, fast threshold/phase switching and high endurance, thus motivating a broad interest as potential Flash replacement and/or nonvolatile storage class memory. Despite the relatively mature progress of research and technology, there is still a wide debate about the ultimate scaling perspective for PCMs. Structural relaxation, crystallization and noise affecting the amorphous chalcogenide phase need to be addressed by accurate physical models for a realistic scaling projection. This work discusses the scaling of PCM devices in terms of the conduction mechanisms and structural stability of the amorphous chalcogenide phase. Resistance window narrowing, current fluctuations, resistance drift and crystallization in the amorphous phase will be explained by a unified model for thermal excitation of the structure by many-phonon phenomena. The downscaling of the reset current, needed to reduce the cell area in memory arrays, and thermal disturb between adjacent cells during reset will be finally addressed to assess the scaling capability of high-density PCM crossbar architectures.
Stress Limited Scaling of Ge2Sb2Te5
Authors: Simpson Robert Edward , Krbal Milos , Fons Paul , Kolobov Alex , Uruga Tomoya , Tanida Hajime , Tominaga Junji
Published: August 2010

The influence of stress on the phase change behaviour of Ge2Sb2Te5 encapsulated by ZnS-SiO2 and TiN is investigated using temperature dependent Extended X-ray Asbsorption Fines Structure and Ellipsometry to determine the crystallisation temperature. The encapsulation material surrounding the Ge2Sb2Te5 has an increasingly dominant effect on the material's ability to change phase and can cause a profound increase in its crystallization temperature. We have experimentally shown that the increased crystallization temperature originates from compressive stress exerted from the encapsulation material. By minimizing the stress we have maintained the bulk crystallization temperature in Ge2Sb2Te5 films just 2 nm thick.
GeTe-filled Carbon Nanotubes for Data Storage Applications
Authors: Giusca Cristina E. , Stolojan Vlad , Sloan Jeremy , Shiozawa Hidetsugu , Silva Ravi
Published: August 2010

By virtue of their unique electronic properties, nanometer-diameter sized single-walled carbon nanotubes represent ideal candidates to function as active parts of nanoelectronic memory storage devices. We show for the first time that GeTe, a phase change material, currently considered to be one of the most promising materials for data-storage applications, can efficiently be encapsulated within single-walled carbon nanontubes of 1.4 nm diameter. Structural investigations on the encapsulated GeTe nanowires have been carried out by high resolution transmission electron microscopy. The electronic interactions between the filling material and the host nanotube have been examined using ultraviolet photoelectron spectroscopy experiments and show that the electronic structure of the encapsulating nanotube and that of the encased filling are not perturbed by the presence of each of the other component.

The newly formed hybrids offer potential to operate as active elements in non-volatile electronic memory storage devices.
Band Offset Control by Interfacial Oxygen Content at GaAs:HfO2 interfaces
Authors: Wang Weichao , Wallace Robert M. , Cho Kyeongjae
Published: August 2010

The impact of interfacial oxygen content on the band offsets of GaAs:HfO2 interfaces was investigated using the density functional theory (DFT) method. Reference potential method was used to determine the band offsets. Moreover, GW correction was utilized to find more accurate value of the valence band edge of HfO2 and hence obtain more accurate band offsets. With gradually decreasing the interfacial O content from 100% to 30% (by changing O chemical potential corresponding to varying the growth condition), the valence band offset increases from 1.06 to 3.34 eV. It is found that this increase of the valence band offsets is inversely proportional to the charge loss of interfacial Ga atoms. Specifically, less charge loss of interfacial Ga induces less charge transfer from GaAs to HfO2 side. Consequently, the less charge loss of interfacial Ga essentially leads to an increase of the valence band offsets.
Optimization of Light Emission from Silicon Nanocrystals Grown by PECVD
Authors: Ishikawa Satoshi , Cheng Szu-Lin , Gong Yiyang , Vuckovic Jelena , Nishi Yoshio
Published: August 2010

Light emission from Si nanocrystals ( SiNCs ) embedded in Si oxide was studied in this work. SiNCs were fabricated by annealing a Si-rich oxide ( SRO ) deposited by a plasma-enhanced chemical vapor deposition ( PECVD ) system. The gas flow ratio between SiH4 and N2O of a precursor gas was changed by varying a N2O gas flow rate and the annealing temperature was varied from 800 to 1100°C. The highest PL intensity was obtained with a N2O flow rate of 125sccm, a SiH4 flow rate of 1400sccm and annealing temperature of 900°C. The PL wavelength was also controlled by N2O gas flow rate and annealing temperature, with blue shifting to the visible wavelengths for increasing N2O flow rate and decreasing annealing temperature. In addition, forming gas ( 4% H2 ) anneal for 1 hour, which is a common method to passivate Si surface, at 500°C to SiNCs was used to further enhance the emission intensity. To approach emission at shorter wavelength, the Si oxide with SiNCs / SiO2 multi layer structure ( MLS ) was also fabricated by similar methods. The SiO2 layer was used as a diffusion barrier to extra Si on vertical direction during the annealing process. Such a barrier can effectively reduce the diameter of SiNCs and shift the emission peak to shorter wavelength. A blue shift from PL was clearly observed as the thickness of Si oxide layer with SiNCs in MLS reduces. Finally, the PIN light emitting diode which consisted of n-type poly-Si / Si oxide with SiNCs / p-type poly-Si structure was also fabricated to study the electroluminescence ( EL ) of SiNCs. The current under the forward bias was about 10 times higher than under the reverse bias. The carrier injection mechanism assumed that Poole-Frenkel type conduction or hopping conduction dominates under a low electric field and Fowler-Nordheim tunneling dominates under a high electric field. EL was obtained with a forward bias voltage of around 6V and EL emission efficiency was proportional to the current density.
Distance Controlled and Electrically Driven Photoluminescence Quench From Quantum Dot-Au Complexes
Authors: Kang Zhitao , Xu Jie , Andreasen Dinal , Wagner Brent Karl
Published: August 2010

Quantum Dots (QDs) bound to gold nanoparticles have shown photoluminescence (PL) quenching dependent on distance between the two particles. The incident light from the QD couples to plasmon excitation of the metal when the frequencies of the light and the surface plasmon resonance (SPR) coincide, leading to a reduction in emitted PL in the system. The quenching effect of gold nanoparticles on QDs was used to study protein-protein interactions with the potential for drug screening applications. CdTe and CdHgTe QDs with emission wavelengths from 500˜900nm were synthesized and gold nanospheres and nanorods with controlled absorption in the visible and near-infrared (NIR) wavelength regions were prepared. The PL quenching of QD-Protein-Protein-Au complexes was studied as a function of Au concentration, QD size and protein type. A quenching efficiency of up to 90% was observed. The QD-Au complexes were also studied for electric potential sensing. The surface of the QDs was negatively charged due to thiol ligands capping. By applying a positive potential on the gold or gold nanoparticle attached substrate, the local electric field between the substrate and the statically charged QDs would pull the QDs closer to the gold surface and quench the QD PL. PL quenching of QD with Au was studied as a function of electric signal and QD type. In this methodology, electric signals were effectively converted to optical signals.
Silicon Quantum Dots Functionalized for siRNA Delivery
Authors: Stefanie Klein , Zolk Oliver , Schrödl Falk , Kryschi Carola
Published: August 2010

Biocompatible, water-soluble, green luminescent silicon quantum dots (SiQDs) were developed as transfection tool for small interfering RNA (siRNA). The research goal was to down-regulate via the RNA interference (RNAi) mechanism the P-glycoprotein expression of the multidrug resistant gene 1 (MDR1) in a human colon carcinoma cell line (Caco-2). The internalization of 2-vinylpyridine terminated SiQDs (2-vipySiQDs) by Caco-2 cells as observed in confocal laser scanning microscopy imaging studies occurs via endocytosis. Experiments employing agarose gel electrophoresis revealed that 2-vipySiQD-siRNA complexes are formed through electrostatic interactions. The release of siRNA in the cytosol with subsequently RNAi induced down-regulation of the P-glycoprotein translation was verified by detecting a reduced ABCB1 mRNA level in transfected Caco-2 cells employing real-time PCR. Additional evidence for successful ABCB1 gene silencing was obtained by measuring a significant decrease of the P-glycoprotein transporter efficiency for the fluorescent substrate Rhodamine 123 (Rh123).
Multivalent Biomimetic Glyconanoparticle Platforms as QCM Amplifiers for Lectin-carbohydrate Interactions
Authors: Mahon Eugene , Aastrup Teodor , Barboiu Mihail
Published: August 2010

The Lectin-Carbohydrate interaction was investigated by combining the Quartz Crystal Microbalance-QCM technique with signal amplifying biomimetic nanoparticle-NP platforms. A library of glyconanoparticles was prepared and the avidity of these NPs for immobilised lectin layers was then evaluated by a QCM setup. Large responses were observed as a result of surface recognition by nanoparticles displaying the appropriate molecular functionality. Large affinity enhancements were also in evidence due to the biomimetic nature of the glyconanoparticle assemblies' carbohydrate presentation demonstrating evidence of the cluster glycoside effect.
Template-Assisted Growth of Tungsten Oxide Nanorods on Substrates and Their Electrochemical Properties
Authors: Park Sun Hwa , Song Jaeyong , Park Hyun Min , Yu Hyunung
Published: August 2010

Tungsten oxide nanorods (TONs) with the diameters of 40 nm and the length of 130 nm have been synthesized on substrates using two step electrochemical anodizing processes. The TONs were vertically well-ordered on the substrates with the average interdistance of 100 nm. The TONs had amorphous structure and was mainly composed of W, Al, and O elements, of which the contents varied gradually along the nanorod length from the top surface to the bottom. The cyclic voltammograms (CVs) and galvanostatic charge-discharge analyses showed that TONs had the typical electrochemical pseudocapacitive features of rectangular CV hysteresis and symmetric charge-discharge behaviors, respectively. When the TONs were heat-treated at 600℃ in vacuum, they showed the maximum specific capacitance of 660 ㎌/cm2, which was higher, by an order of magnitude, than that (68 ㎌/cm2) of the TONs annealed at 300 ℃ in ambient atmosphere.
Block Copolymer Templating for Formation of Quantum Dots and Lattice Mismatched Semiconductor Structures
Authors: Jha Smita , Liu Chi-Chun , Park Joo Hyung , Wiedmann Monika K. , Kuan T S , Babcock Susan E. , Mawst Luke J. , Nealey Paul , Kuech Thomas F.
Published: August 2010

Templated growth for the fabrication of semiconductor nanostructures such as quantum dots and lattice-mismatched structures has been employed in this study. Self assembly of block copolymers (BCP) has been exploited to create a regular array of nanoscale patterns on a substrate to generate the growth template. These patterned templates were used for the selective area growth of pseudomorphic quantum dots, allowing for precise control over the dot size and spatial distribution. Strain relaxation in lattice-mismatched structures grown past the pseudomorphic limit was also studied. Analysis of the grown structures suggests that this approach using block copolymer templating followed by selective growth can be used for defect reduction in lattice-mismatched materials.
Interfacial Energy Level Alignment at Acid Oxidized Carbon Nanotube - Triphenyldiamine Contacts
Authors: Tan Li Wei , Hatton Ross A. , Giusca Cristina E. , Silva Ravi
Published: August 2010

We report an ultraviolet photoelectron spectroscopy study of the energetics at the interface between acid oxidised carbon nanotubes and the archetypical molecular N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'biphenyl-4,4'diamine(TPD). Electrical equilibrium is achieved across both interfaces within the experiment time frame due to the formation of an interfacial dipole layer which abruptly shifts the vacuum level at the interface. To the authors knowledge this is the first reported measurement of the electronic structure of a carbon nanotube / organic semiconductor interface; a system in which the magnitude of the dipole layer formed at the interface upon contact formation is proportional to the difference in work function between the substrate and organic semiconductor overlayer.
Spatial Variations in the Mechanical Properties and Electrical Properties of Carbon Nanotube Turfs
Authors: Qiu Anqi , Lopez Melinda , Bellou Aikaterini , McClain Devon , Jiao Jun , Bahr David
Published: August 2010

The mechanical properties of arrays of nominally vertically aligned carbon nanotubes, often referred to as turfs, have been measured using nanoindentation and the electrical properties have been measured using electrical contact resistance (ECR) nanoindentation. The elastic properties do not vary significantly between the top and the bottom of the same carbon nanotube turf. Within a single turf the lateral spatial variation is less than 10% when volumes of μm's are probed with the indenter, indicating that each turf can be treated mechanically as continuum on this scale. The electrical properties vary significantly within a single turf on the same scale. This suggests that the use of average mechanical properties for a given vertically aligned turf should be suitable for design purposes without the need to account for spatial variation in structure, and variations in mechanical properties on the micrometer scale are not dependent on spatially distinct defects. However, local contact behavior appears to dominate the electrical behavior on this same length scale.
Characterization of an Ultra Thin Dense Hafnium Oxide Compact Layer with Electrochemical Impedance Spectroscopy for Dye-Sensitized Solar Cell Application
Authors: Bills Braden , Shanmugam Mariyappan , Farrokh Baroughi Mahdi , Galipeau David
Published: August 2010

The performance of dye-sensitized solar cells (DSSCs) is limited by the back-reaction of photogenerated electrons from the photoelectrode back into the electrolyte solution. An atomic layer deposited (ALD) hafnium oxide (HfO2) ultra thin, a few nanometers, compact layer was grown on the surface of the transparent conducting oxide (TCO) and its effects on the performance of DSSCs were studied with dark and illuminated current-voltage and electrochemical impedance spectroscopy (EIS) measurements. Further, the theory of electron recombination at the TCO/electrolyte interface was developed and used to explain the improved DSSC performance with an ALD HfO2 compact layer.
Structural and optical characterization of Si quantumdots in a SiC matrix
Authors: Kuenle Matthias , Loeper Philipp , Rothfelder Marcel , Janz Stefan , Eibl Oliver , Nickel Klaus Georg
Published: August 2010

Amorphous hydrogenated Si1-xCx/SiC multilayers consisting of alternating Si1-xCx and stoichiometric SiC layers were prepared using Plasma Enhanced Chemical Vapour Deposition (PECVD). Annealing at temperatures up to 1100°C was done targeting the size controlled crystallization of Si nanocrystals (NCs) in a SiC matrix. The influence of annealing temperature on the nanostructure of the multilayers was studied using Glancing Incidence X-ray Diffraction (GIXRD), Raman spectroscopy and Transmission Electron Microscopy (TEM). GIXRD reveal the crystallization of Si and SiC, when annealing temperatures exceed 900°C. The crystallization of Si and SiC was confirmed by TEM bright-field imaging and electron diffraction. Annealing at 900°C, leads to the formation of Si NCs with a size of 3 nm, whereas the SiC NCs also have a size of 3 nm. However, a large amount of Si is still amorphous as shown by Raman spectroscopy. Annealing at temperatures exceeding 900°C reduces the amorphous phase and a further growth of Si NCs occurs.
Quaternary Ammonium Borohydride Adsorption in Mesoporous Silicate MCM-48
Authors: Wolverton Michael Joseph , Daemen Luc L. , Hartl Monika A.
Published: August 2010

Inorganic borohydrides have a high gravimetric hydrogen density but release H2 only under energetically unfavorable conditions. Surface chemistry may help in lowering thermodynamic barriers, but inclusion of inorganic borohydrides in porous silica materials has proved hitherto difficult or impossible. We show that borohydrides with a large organic cation are readily adsorbed inside mesoporous silicates, particularly after surface treatment. Thermal analysis reveals that the decomposition thermodynamics of tetraalkylammonium borohydrides are substantially affected by inclusion in MCM-48. Inelastic neutron scattering (INS) data show that the compounds adsorb on the silica surface. Evidence of pore loading is supplemented by DSC/TGA, XRD, FTIR, and BET isotherm measurements. Mass spectrometry shows significant hydrogen release at lower temperature from adsorbed borohydrides in comparison with the bulk borohydrides. INS data from partially decomposed samples indicates that the decomposition of the cation and anion is likely simultaneous. These data confirm the formation of Si-H bonds on the silica surface upon decomposition of adsorbed tetramethylammonium borohydride.
NaBX4-MgX2 Composites (X= D,H) Investigated by In situ Neutron Diffraction
Authors: Pottmaier Daphiny , Garroni Sebastiano , Brunelli Michela , Vaughan Gavin B. M. , Castellero Alberto , Menéndez Enric , Baró Maria Dolors , Baricco Marcello
Published: August 2010

Light element complex hydrides (e.g. NaBH4) together with metal hydrides (e.g. MgH2) are considered two primary classes of solid state hydrogen storage materials. In spite of drawbacks such as unfavourable thermodynamics and poor kinetics, enhancements may occur in reactive hydride composites by nanostructuring of reactant phases and formation of more stable product phases (e.g. MgB2) which lower overall reaction enthalpy and allow reversibility. One potential system is based on mixing NaBH4 and MgH2 and subsequent ball milling, which in a 2:1 molar ratio can store considerable amounts of hydrogen by weight (up to 7.8 wt%). A study of the 2NaBX4 + MgX2 → MgB2 + 2NaX + 4X2 (X=D,H) reaction is assessed by means of in-situ neutron diffraction with different combinations of hydrogen and deuterium on the X position. The desorption is established to begin at temperatures as low as 250 °C, starting with decomposition of nanostructured MgX2 due to joint effects of nanostructured MgX2 and its reducing effect at NaBX4. Analyses of background profile, due to the high incoherent neutron scattering of hydrogen, as a function of temperature demonstrate direct correlation of H/D desorption reactions with relative phases amount.
Neutron Scattering to Characterize Cu/Mg(Li) Destabilized Hydrogen Storage Materials
Authors: Braga Maria Helena , Wolverton Michael Joseph , Lobet-Megias Anna , Daemen Luc L.
Published: August 2010

Cu-Li-Mg-(H,D) was studied as an example of destabilizer of the Ti-(H,D) system. A Cu-Li-Mg alloy was prepared resulting in the formation of a system with 60.5 at% of CuLi0.08Mg1.92, 23.9 at% of CuMg2 and 15.6 at% of Cu2Mg. Titanium was added to a fraction of this mixture so that 68.2 at% (47.3 wt%) of the final mixture was Ti. The mixture was ground and kept at 200 °C/473 K for 7h under H2 or 9h under D2 at P = 34 bar. Under those conditions, neutron powder diffraction shows the formation of TiD2, as well as of the deuteride of CuLi0.08Mg1.92. Similarly inelastic neutron scattering shows that at 10 K TiH2 is present in the sample, together with the hydride of CuLi0.08Mg1.92. Interestingly, at 10 K TiH2 is very clearly detected and at 300 K TiH2 is still clearly present as indicated by the neutron vibrational spectrum, but CuLi0.08Mg1.92-H is not detected anymore. These results indicate that Ti(H,D)2 is possibly formed by diffusion of hydrogen from the Cu-Li-Mg-(H,D) alloys. This is an intriguing result since TiH2 is normally synthesized from the metal at T > 400°C/673 K (and most commonly at T ˜ 700 °C/973 K). In the presence of CuLi0.08Mg1.92, TiH2 forms at a temperature that is 300 – 400 K lower than that needed to synthesize it just from the elements.
Nanostructured Metal Hydrides for Hydrogen Storage Studied by In Situ Synchrotron and Neutron Diffraction
Authors: Yartys Volodymyr , Denys Roman , Maehlen Jan Petter , Webb Colin J , Gray Evan MacA , Blach Tomas , Poletaev Andrey A. , Solberg Jan Ketil , Isnard Olivier
Published: August 2010

This work was focused on studies of the metal hydride materials having a potential in building hydrogen storage systems with high gravimetric and volumetric efficiencies of H storage and formed / decomposed with high rates of hydrogen exchange. In situ diffraction studies of the metal-hydrogen systems were explored as a valuable tool in probing both the mechanism of the phase-structural transformations and their kinetics. Two complementary techniques, namely Neutron Powder Diffraction (NPD) and Synchrotron X-ray diffraction (SR XRD) were utilised. High pressure in situ NPD studies were performed at D2 pressures reaching 1000 bar at the D1B diffractometer accommodated at Institute Laue Langevin, Grenoble. The data of the time resolved in situ SR XRD were collected at the Swiss Norwegian Beam Lines, ESRF, Grenoble in the pressure range up to 50 bar H2 at temperatures 20-400°C.

The systems studied by NPD at high pressures included deuterated Al-modified Laves-type C15 ZrFe2-xAlx intermetallics with x = 0.02; 0.04 and 0.20 and the CeNi5-D2 system. D content, hysteresis of H uptake and release, unit cell expansion and stability of the hydrides systematically change with Al content.

Deuteration exhibited a very fast kinetics; it resulted in increase of the unit cells volumes reaching 23.5 % for ZrFe1.98Al0.02D2.9(1) and associated with exclusive occupancy of the Zr2(Fe,Al)2 tetrahedra.

For CeNi5 deuteration yielded a hexahydride CeNi5D6.2 (20°C, 776 bar D2) and was accompanied by a nearly isotropic volume expansion reaching 30.1% (∆a/a=10.0%; ∆c/c=7.5%). Deuterium atoms fill three different interstitial sites including Ce2Ni2, Ce2Ni3 and Ni4. Significant hysteresis was observed on the first absorption-desorption cycle. This hysteresis decreased on the absorption-desorption cycling.

A different approach to the development of H storage systems is based on the hydrides of light elements, first of all the Mg-based ones. These systems were studied by SR XRD. Reactive ball milling in hydrogen (HRBM) allowed synthesis of the nanostructured Mg-based hydrides.

The experimental parameters (PH2, T, energy of milling, ball / sample ratio and balls size), significantly influence rate of hydrogenation. The studies confirmed (a) a completeness of hydrogenation of Mg into MgH2; (b) indicated a partial transformation of the originally formed -MgH2 into a metastable -MgH2 (a ratio / was 3/1); (c) yielded the crystallite size for the main hydrogenation product, -MgH2, as close to 10 nm. Influence of the additives to Mg on the structure and hydrogen absorption/desorption properties and cycle behaviour of the composites was established and will be discussed in the paper.
Insights into Stress Corrosion Cracking Mechanisms from High-Resolution Measurements of Crack-tip Structures and Compositions
Authors: Bruemmer Stephen Michael , Thomas Larry
Published: August 2010

Results are presented employing cross-sectional analytical transmission electron microscopy (ATEM) to examine intergranular stress corrosion cracking (IGSCC) of austenitic stainless alloys in high-temperature water environments. Microstructural, chemical and crystallographic characterization of buried interfaces at near-atomic resolutions is used to investigate corrosion/oxidation reactions, composition changes and deformation events at crack tips. Information obtained by a wide variety of high-resolution imaging and analysis methods indicates the processes occurring during crack advance and provides insights into the mechanisms controlling SCC. Examples of crack tips produced in oxidizing and hydrogenated water are presented for both Fe-base stainless steels (SSs) and Ni-base stainless alloys. Cracks in SSs show similar characteristics in both environments, with oriented oxide films at crack walls and cracks ending in few-nm-wide tips. Many of these same features are seen for alloy 182 in oxidizing water suggesting a common mechanism, generally consistent with a slip oxidation process. A distinct difference is seen at alloy 600 and alloy 182 tips produced in hydrogenated water. Penetrative attack along grain boundaries without evidence for significant plastic deformation is believed to indicate a major role of active-path corrosion/oxidation in the SCC process.
Nanostructures from Hydrogen and Helium Implantation of Aluminum
Authors: Ong Markus D. , Yang Nancy , Depuit Ryan J. , McWatters Bruce R. , Causey Rion A.
Published: August 2010

This study investigates a pathway to nanoporous structures created by hydrogen and helium implantation in aluminum. Previous experiments for fusion applications have indicated that hydrogen and helium ion implantations are capable of producing bicontinuous nanoporous structures in a variety of metals. This study focuses specifically on implantations of hydrogen and helium ions at 25 keV in aluminum. The hydrogen and helium systems result in remarkably different nanostructures of aluminum at the surface. Scanning electron microscopy, focused ion beam, and transmission electron microscopy show that both implantations result in porosity that persists approximately 200 nm deep. However, hydrogen implantations tend to produce larger and more irregular voids that preferentially reside at defects. Implantations of helium at a fluence of 1018 cm-2 produce much smaller porosity on the order of 10 nm that is regular and creates a bicontinuous structure in the porous region. The primary difference driving the formation of the contrasting structures is likely the relatively high mobility of hydrogen and the ability of hydrogen to form alanes that are capable of desorbing and etching Al (111) faces.
Grain Boundary Analysis of HT9 Steel after Accelerated Creep Testing
Authors: Leng Zhe , Field David
Published: August 2010

Ferritic/martensitic steels are attractive materials for use as components in nuclear reactors because of their high strength and good swelling resistance. Grain boundary specific phenomena (such as segregation, voiding, cracking, etc) are prevalent in these materials so grain boundary character is of primary importance. Certain types of boundaries are more susceptible to thermal creep damage whereas others tend to resist damage. If more damage resistant boundaries can be introduced into the structures, this will result in steel that is more resistant to the processes of degradation that prevail in high-temperature environments. We have characterized the grain boundary structure in HT9 steel by electron backscatter diffraction to identify boundaries that are resistant to degradation and those that are more susceptible to damage in extreme environments. It is found that intergranular damage is mitigated by a high fraction of low energy boundaries, and certain kinds of grain boundaries are more favored by intergranular cracks.
Fluorination of Uranium Metal to UF3 and UF4 by Nitrogen Trifluoride: Evidence for Elusive UF2
Authors: McNamara Bruce K , Scheele Randall , Casella Andrew M , Kozelisky Anne E , Neiner Doinita
Published: August 2010

We have recently found that uranium and plutonium metals will react with nitrogen trifluoride (NF3) at temperatures below 120°C. These are the first reported instances of such low temperature fluorination reactions using NF3 and implicate metal catalyzed dissociation of the NF3 bond. We additionally present preliminary evidences for a surface mediated product distribution. Reaction of uranium metal with NF3 promotes products that are apparently determined by the concentration of the fluorinating reagent between temperatures of 60 to 120°C.
Evolution of the Magnetic and Superconducting States in UCoGe With Fe and Ni Substitution
Authors: Hamlin James Jeffrey , Baumbach Ryan Eagle , Huang Kevin , Janoschek Marc , Kanchanavatee Noravee , Zocco Diego A. , Maple M. Brian
Published: August 2010

The very small number of known ferromagnetic superconductors places the study of such compounds at the frontier of superconductivity research. Recently, UCoGe has emerged as a new member of the class of materials exhibiting coexistence of ferromagnetism and superconductivity (Curie temperature TCurie = 3 K; superconducting critical temperature Ts = 0.8 K). This compound has generated much excitement, in part because it has been proposed that the superconductivity derives from spin triplet pairing mediated by ferromagnetic interactions. Therefore, a key question is how changes in the magnetic state of UCoGe affect the superconducting properties. We have carried out a comprehensive study of the UCo1-xFexGe and UCo1-xNixGe series of compounds across the entire range of composition 0 ≤ x ≤ 1. We report the results of x-ray diffraction, electrical resistivity, and magnetization measurements to elucidate the magnetic and superconducting phase diagram of the U[Fe, Co, Ni]Ge system. Substitution of either Ni or Fe into UCoGe initially results in an increase in the Curie temperature. At higher dopant concentrations (x), the ferromagnetic state crosses over to paramagnetism in UCo1-xFexGe and antiferromagnetism in UCo1-xNixGe.
Stability of Cs-Ionsiv in Portland cement blends for radioactive waste disposal
Authors: Hyatt Neil Christian , Jenni Andreas , Stennett Martin Christopher
Published: August 2010

The suitability of Portland cement blends for encapsulation of Cs-Ionsiv in a monolithic wasteform was investigated. No evidence of reaction or dissolution of the Cs-Ionsiv in the cementitious environment was found by scanning electron microscopy and X-ray diffraction. However, a small fraction (≤1.6 wt%) of the Cs inventory was released from the encapsulated Ionsiv during leaching experiments carried out on hydrated samples. Cs release was enhanced by exchange of K and Na present in the cementitious pore water. Cement systems lower in K and Na, such as slag based blends, showed lower Cs release than the fly ash based analogues.
Migration Behavior of Multivalent Radionuclides From Fully Radioactive Waste Glass in Compacted Sodium Bentonite
Authors: Fujiwara Kenso , Iijima Kazuki , Mitsui Seiichiro , Odakura Makoto , Kohara Yukitoshi , Kikuchi Hiroshi
Published: August 2010

In a repository of high-level radioactive waste, radionuclides will leach from the waste glass and migrate into the surrounding bentonite after very long time. These processes occur simultaneously in the bentonite and should be evaluated to confirm the reliability of individual models and data for the performance assessment of high-level radioactive waste repository.

Previous study [1] reported the results of the in-diffusion experiments of Cs in compacted sodium bentonite (Kunigel V1®) in contact with fully radioactive waste glass for 15 to 300 days under aerobic conditions. And Cs migration was successfully interpreted using fundamental one dimensional diffusion model. However, migration of other radionuclides in fully radioactive waste glass were extremely slow because of low solubility, low effective diffusivity and high distribution ratio, especially multivalent elements of actinide and lanthanide.

In this study, the similar in-diffusion experiment reported by Ashida et al. [1] was carried out for about 15 years to evaluate the migration behavior of multivalent actinide and lanthanide elements. The bentonite was compacted into a stainless steel cell with 20 mm in diameter and 18 mm in length to produce dry densities of 0.5 and 1.0 Mg m-3saturated distilled water. The form of fully radioactive waste glass was borosilicate glass by using vitrified in CPF (Chemical Processing Facility). The glass sample was sliced into the disc with 4 mm in thickness and sandwiched by two pieces of the saturated bentonite sample in the diffusion cell under aerobic conditions. After 15 years, bentonite sample was sliced and immersed into the HNO3 to extract the radionuclides from the bentonite. Then profiles of Cs, Eu, Pu, Am and Cm in the bentonite sample were evaluated. The concentration profile of Cs in the bentonite was constant due to its high diffusivity.

The experimental concentrations of Am, Cm and Eu in contact with compacted sodium bentonite were good agreement with the solubilities calculated by thermodynamic data. On the other hand, the profiles of Am and Cm show two parts with different slopes which cannot be fitted by simple one-dimensional diffusion model considering single specie. Leaching and migration behavior of radio nuclides will be discussed based on the one-dimensional diffusion model considering other mechanism of several species.

[1] T. Ashida, et al. Migration behavior of cesium released from fully radioactive waste glass in compacted sodium bentonite. PNC Technical Report, TN8410 98-014(1998).
Microstructural modifications in ion-irradiated nuclear materials: characteristic features of fluorite-structured oxides
Authors: Debelle Aurelien , Garrido Frederico , Thome Lionel
Published: August 2010

The behavior of two nuclear materials, namely cubic zirconia and urania, is investigated under different irradiation conditions in the low and medium ion energy range (tens of keV to a few MeV). In each case, these materials display a multi-step damage build-up, as revealed by both RBS/C and XRD measurements. It is demonstrated that each step exhibits characteristic features such as damage fraction, elastic strain, nature of defects, and thus presents a specific microstructure. The transition from one step to the following involves radiation defect re-organization which arises to lower the energy of the system.