bluuebunny/crossref_metadata_2025 huggingface dataset:
High Removal Rate CMP Process on TSV Thick Cu Overburden
Authors: Caramto Raymond , Qureshi Jamal , Mase Jerry
Published: August 2010

In 3D packaging, Through Silicon Vias (TSVs) with high aspect ratios and depths measuring tens of microns are filled by advanced Cu electroplating processes. Today's 300mm TSV plating platforms are intended to produce bottom-up via fill, no seam voids, and minimal, controlled overburden. To achieve this, the preceding Cu-seed coverage must be continuous at a constant resistance, and the plating chemistries optimized. However, other factors such as hardware configurations and simple depletion of the plating formulations during the extended TSV process can lead to high Cu overburden thickness requiring a high removal rate (HRR) chemical-mechanical polish (CMP) process to remove the thick Cu layer.

Presented here are CMP results of a thick Cu overburden (~6um) resulting from the fill of 5 x 25um TSVs on 300mm wafers. The goal is to uniformly polish the overburden and utilize the tool's endpoint system at Cu clear before the next step of conventional barrier CMP. Slurries were screened for removal rate, uniformity, planarization ability, and defectivity. This study focuses on achieving a removal rate of >2.5um/min through evaluation of several commercial slurries in a multi-step polish application. Cross-sections post-plating show the Cu overburden, barrier and liner layer. Cross-sections post-CMP quantify Cu recess, dielectric loss, and presence/absence of seam defects. The process selected is demonstrated to achieve good planarization results with low occurrence of polish defects, at a rate and selectivity suitable for emerging 3D TSV Cu CMP applications.
An Alternative Non-contact Planarization Technique by Utilizing the Electrokinetic Phenomenon
Authors: Butler David , Leo Cheng Seng , Ng Sum Huan , Danyluk Steven
Published: August 2010

In this paper, the authors introduce and present some findings on an alternative non-contact material removal technique. Material removal is made possible by utilizing the electrokinetic and hydrodynamic effects of suspended particles to manipulate their trajectories to impact onto the surface of the workpiece. The research was previously demonstrated and reported where the removal rate can be precisely controlled by varying the electrical field and the flow rate of the slurry across the surface of the workpiece. New findings are reported on the application of the technique to different materials that will highlight the attractiveness of this alternative approach to producing surfaces with roughness in the order of nanometres.
Characterization and Modeling of Pad Asperity Response in CMP
Authors: Boning Duane , Fan Wei
Published: August 2010

A model is proposed to understand the interactions between CMP pad asperities and the wafer. Pad asperity reduced modulus and height distribution are included in the model. Physical measurements of asperity properties are performed: asperity reduced modulus is measured by nanoindentation, and pad asperity height distribution is scanned by profilometry. The measured results are used in the model to predict the contact area percentage between the pad and wafer in the CMP process.
Copper Migration During Tungsten via Formation
Authors: Gambino Jeff , Cooney Ed , Murphy Will , Luce Cameron , Mongeon Steve , Lai Ning , Zwonik Bob , Anderson Felix , Schutz Laura , Lee Tom C , McDevitt Tom
Published: August 2010

A yield problem is observed with tungsten vias formed on copper interconnects. Copper migration can occur during chemical vapor deposition (CVD) of tungsten, if there are defects in the liner inside the via. Copper can react quickly with SiH4during the early stages of tungsten deposition, when SiH4-reduction of WF6 is used. Under severe conditions, large amounts of copper diffuse out of the underlying metal layer, resulting in copper silicide formation in the via and leaving voids in the copper wire. Copper migration can be minimized by reducing the time that the wafers are exposed to SiH4.
Towards the Understanding of Resistive Contrast Imaging in in-situ Dielectric Breakdown Studies Using a Nanoprober Setup
Authors: Lambrinou Konstantina , Arstila Kai , Hantschel Thomas , Rummel Andreas , Tőkei Zsolt , Pantouvaki Marianna , Croes Kristof , Czarnecki Piotr , Carbonell Laure , Richard Olivier , Kleindiek Stephan , De Wolf Ingrid , Vandervorst Wilfried
Published: August 2010

This work aims at attaining a more complete understanding of the principles governing resistive contrast imaging (RCI) of copper/low-k interconnects used for dielectric breakdown studies in a nanoprober scanning electron microscope (SEM) system. RCI is employed in such in situ dielectric breakdown studies to facilitate the localization of interconnect defect sites related to various stages in the degradation process of the low-k dielectric material. This work shows that RCI is suitable for detecting high-resistance sites, like opens, in copper/low-k interconnects. Moreover, RCI demonstrates potential in locating defects that lie deep in the test structure and are, thus, not detectable by SEM. A model is also proposed to explain the formation of RCI images of specific interconnect test structures with complex layout.
Fabrication of Prototype Magnetic Coupled Spin-torque Devices for Non-volatile Logic Applications
Authors: Leem Larkhoon , Harris James S. , Rettner Charles , Hughes Brian , Jiang Xin , Yang See-hun , Parkin Stuart
Published: August 2010

A Magnetic Coupled Spin-torque Device (MCSTD) is a collective system of three interacting magnetic tunnel junctions (MTJs) that forms a novel magnetic logic gate. The fundamental principle of the MCSTD is the modification of the energy barrier for spin-torque magnetization switching of a central (output) MTJ device arising from changes in the magnetic state of two input MTJ devices. The input MTJs are placed in close proximity of a few tens of nm of the output MTJ such that their magnetic fringing fields are strong enough (> 10 Oersted) to modulate the switching characteristics of the output device. By changing the magnetic states of the two input MTJs four possible net magnetic fields at the center MTJ can be generated. A single MCSTD thereby enables NAND, NOR and XOR operations. In this paper, the fabrication of a prototype MCSTD device is described and preliminary experiment results are reported.
Size-dependent Temperature Instability in NiO–based Resistive Switching Memory
Authors: Ielmini Daniele , Nardi Federico , Cagli Carlo , Lacaita Andrea L.
Published: August 2010

Resistive switching memory (RRAM) is attracting a strong interest as novel nonvolatile memories for high-density storage. Anyway this technology has to overcome two main issues before its use in real applications which are the high current needed for program operations and data retention stability. These two problems are here investigated from experimental and theoretical points of view to clarify the possibilities of NiO RRAMs to become a real competitive alternative to mainstream Flash technology.
CVD Growth and Passivation of W and TiN Nanocrystals for Non-volatile Memory Applications
Authors: Gay Guillaume , Belhachemi Djamel , Colonna Jean-Philippe , Minoret Stéphane , Beaurain Arnaud , Pelissier Bernard , Roure Marie-Christine , Lafond Dominique , Jalaguier Eric , Molas Gabriel , Baron Thierry , De Salvo Barbara
Published: August 2010

In this paper, we present CVD (Chemical Vapor Deposition) growth and passivation of tungsten (W) and titanium nitride (TiN) nanocrystals (NCs) on silicon dioxide and silicon nitride for use as charge trapping layer in floating gate memory devices. NCs are deposited in an 8 inches industrial CVD Centura tool. W and TiN are chosen for being compatible with MOSFET memory fabrication process. For protecting NCs from oxidation, a silicon shell is selectively deposited on them. Moreover, for a better passivation, TiN NCs are encapsulated in silicon nitride (Si3N4) in order to get rid of oxidation issues. After high temperature annealing (1050°C under N2 during 1 minute) XPS measurements point out that NCs are still metallic, which makes them good candidates for being used as charge trapping layer in floating gate memories.
Low Temperature Growth of Silicon Structures for Application in Flash Memory Devices
Authors: Mih Thomas Attia , Paul Shashi , Cross Richard BM
Published: August 2010

An in-depth study of the structural and electrical properties of silicon (Si) films deposited by a novel low temperature technique at temperatures less than 400°C in a 13.56 MHz RF PECVD reactor is reported. The method is based on substrates having to undergo some initial preparatory steps (IPS) before the deposition of Si films in the PECVD chamber. The optical band gap of Si films deposited using this novel technique narrowed to 1.25 eV from 1.78 eV using the traditional a-Si:H deposition recipe. No annealing of any form was performed on the films to attain this band gap. Furthermore, photosensitivities for these films under various deposition conditions were of order 100 compared to 104 for a-Si:H films deposited under like conditions. Using metal-insulator-semiconductor devices, the Si films grown by this novel technique exhibit charge storage and memory behaviour unlike their amorphous counterparts. However, device endurance has been found to be inadequate, probably due to the presence of some contaminants - notably interstitial oxygen - which has been found elsewhere to have adverse effects on the electrical characteristics of Si films. If well harnessed, we suggest Si structures grown by this novel growth technique could be well-suited for flash memory applications, particularly 3-D flash which requires process temperatures to be less than 400 °C.
Fabrication of IGZO and In2O3-channel Ferroelectric-gate Thin Film Transistors
Authors: Oiwa tomohiro , Tokumitsu Eisuke
Published: August 2010

We have fabricated and characterized ferroelectric-gate TFTs using In-Ga-Zn-O (IGZO) or In2O3 as a channel material. The ferroelectric gate insulator used in this work is (Bi,La)4Ti3O12 (BLT). We observed normal n-channel transistor operation for both IGZO and In2O3-channel TFTs. However, a charge injection type hysteresis was observed for IGZO channel TFTs in drain current – gate voltage (ID-VG) characteristics. Post fabrication anneal at 300oC reduced the charge-injection-tyoe hystereesis and the subthreshold swing was also improved from 0.27 to 0.19 V/decade. On the other hand, when the In2O3 was used as a channel, hysteresis due to the ferroelectric gate insulator was clearly observed in ID-VG characteristics. A memory window of 2V, a subthreshold voltage swing of 0.35V/decade, a field-effect mobility of 1.6 cm2/Vs, and a on/off drain current ratio of more than 10^6 were obtained.
Phase Change Memory with Chalcogenide Selector (PCMS): Characteristic Behaviors, Physical Models and Key Material Properties
Authors: Karpov Ilya V. , Kencke David , Kau Derchang , Tang Stephen , Spadini Gianpalo
Published: August 2010

We present a novel scalable and stackable nonvolatile solid state memory. Each cell consists of a storage element, based on phase change memory (PCM) element, and an integrated selector, using an Ovonic threshold switch (OTS). The cell is implemented to enable a true cross-point array. The main device characteristics and behaviors, corresponding physical processes in different operation modes, and key material properties are discussed.
Memory retention of doped SbTe phase change line cells measured isothermally and isochronally
Authors: Oosthoek Jasper L.M. , Kooi Bart J. , Attenborough Karen , Hurkx Fred A.M. , Gravesteijn Dirk J.
Published: August 2010

Doped SbTe phase change (PRAM) line cells produced by e-beam lithography were cycled for at least 100 million times. The memory retention of the PRAM cell was measured both isothermally and isochronally which showed excellent agreement. An activation energy for growth of 1.7 eV was found (after 100 million cycles) for both measurements. Similar isothermal and isochronal measurements were performed on PRAM cells produced by optical lithography which yielded activation energies of 3.0 eV and 3.3 eV, respectively. Our results show that the same phase-change material can show large differences in retention behavior depending on the way the cells are produced.
Charge Transport in Nanoglasses of Phase-Change Memory
Authors: Simon Mark Alexander , Nardone Marco , Kostylev sergey , Karpov Ilya V. , Karpov Victor G.
Published: August 2010

We discuss possible mechanisms for Poole-Frenkel type of non-ohmic conduction in chalcogenide glasses in the range of room temperatures. Overall, we list 8 such mechanisms, only one of which (Schottky emission) can be ruled out as inconsistent with the observations. Seven others can give more or less satisfactory fits of the observed non-linear IV curves. Our analysis calls upon indicative facts that would enable one to discriminate between the various alternative models.
Amorphization of Crystalline Phase Change Material by Ion Implantation
Authors: Raoux Simone , Cohen Guy , Shelby Robert M. , Cheng Huai-Yu , Jordan-Sweet Jean L
Published: August 2010

Germanium ion implantation at an energy of 30 keV was used as a different method to re-amorphize thin films of crystalline phase change material Ge2Sb2Te5 (GST). It was found that rather low doses of 5×1013 cm-2 were sufficient to re-amorphize GST. Amorphization was determined by X-ray diffraction (XRD) as well as reflectivity measurements. Re-crystallization characteristics of ion-implantation-amorphized samples was studied using time-resolved XRD. It showed that samples re-crystallize at an increased crystallization temperature with increasing dose compared to as-deposited material. A static laser tester was applied to measure the crystallization times of material that was (1) as–deposited amorphous; (2) crystallized by annealing and re-amorphized by melt-quenching using a laser pulse; and (3) crystallized by annealing and re-amorphized by ion implantation. It was found that as-deposited amorphous and high-dose ion implanted samples had longer crystallization times while melt-quenched amorphous and low-dose ion implanted samples had shorter crystallization times.
Local Order and Crystallization of Laser Quenched and Ion Implanted Amorphous Ge1-xTex Thin Films
Authors: Carria Egidio , De Bastiani Riccardo , Gibilisco Santo , Mio Antonio Massimiliano , Miritello Maria , Pennisi Agata Roberta , Bongiorno Corrado , Grimaldi MariaGrazia , Rimini Emanuele
Published: August 2010

Amorphous films of Ge1-xTex (x=0.37, 0.51, 0.64) prepared by sputtering, by melt quenching or by ion irradiation were annealed up to 450°C. Different phase stability, i.e. crystallization temperature, was observed varying the amorphous status in stoichiometric and Te-rich alloys while no variation was obtained in the Ge-rich alloy. Laser and ion irradiated stoichiometric alloy exhibits lower stability with respect to the sputtered film while irradiated amorphous Te-rich samples are more stable than the as deposited amorphous sample. An enhancement of edge-sharing GeTe4 tetrahedra Raman signal at the expenses of Ge-rich tetrahedra signal occurs in the irradiated samples with respect to the as-deposited amorphous layers both in the stoichiometric and in the Te-rich alloy. The crystallization temperature decreases in GeTe since the system during irradiation is promoted to a state closer to the crystalline phase while in Te-rich alloy the stability increases with the density of Ge-Te bonds since this local rearrangement delays Te precipitation and the subsequent GeTe crystallization. Irradiation does not affect the stability of Ge rich alloy in which crystallization is limited by the Ge mobility and the induced local rearrangements is probably prevented by the low atomic diffusivity.
Influence of Bottom Contact Material on the Selective Chemical Vapor Deposition of Crystalline GeSbTe Alloys
Authors: Schrott Alejandro , Chen Chieh-Fang , Breitwisch Matthew J. , Joseph Eric A. , Dasaka Ravi K , Cheek Roger W. , Zhu Yu , Lam Chung H
Published: August 2010

Selective Chemical Vapor Deposition of Crystalline Ge-Sb-Te alloys initiating at the bottom metal contact of vias of various sizes has been accomplished. The method is based on selecting Sb and Te precursors which do not decompose on dielectric surfaces in the utilized temperature range.
Investigation of the Thermal Stability of Strained Ge Layers Grown at Low Temperature by Reduced-pressure Chemical Vapour Deposition on Si0.2Ge0.8 Relaxed Buffers
Authors: Dobbie Andy , Myronov Maksym , Liu Xue-Chao , Nguyen Van Huy , Parker Evan , Leadley David
Published: August 2010

High quality strained Ge (s-Ge) epitaxial layers are a promising candidate to achieve high mobility channel MOSFETs suitable for the 22 nm technology node and beyond, due to the intrinsically higher mobility of Ge compared to Si, and the additional performance enhancements from strain [1]. In order to achieve an s-Ge channel more than a few monolayers thick it is necessary to engineer a relaxed Si1-xGex buffer with a high Ge content (x > 0.5). We have recently reported high quality s-Ge layers grown by RP-CVD at low temperature (T ≤ 450 °C), on a fully relaxed Si0.2Ge0.8 buffer [2]. By using a reverse-grading approach, we achieved a high Ge composition in the buffer, with a smooth surface (rms surface roughness of ~2 nm), low threading dislocations density (~ 4 x 106 cm-2) and much thinner (~ 2.1 μm) than can be achieved with conventional linear grading [3].

In this work, the thermal stability of s-Ge epilayers (up to 80 nm thick) grown on relaxed Si0.2Ge0.8 buffers has been investigated by in-situ annealing in H2 ambient at temperatures up to 650 °C. These temperatures are similar to those currently used during fabrication of advanced CMOS devices. All s-Ge layers were grown at 400 °C using GeH4 gaseous precursor. The relaxation of the annealed layers has been studied using high-resolution XRD reciprocal space maps (RSMs), and was found to depend strongly on both annealing temperature and thickness of the Ge epilayer. Strained Ge layers up to 50 nm thick remained fully strained after annealing at 450 °C, whereas after annealing at 550 °C s-Ge layers thicker than 20 nm were on the onset of relaxation; after annealing at 650 °C all s-Ge layers showed significant relaxation with defects clearly visible at the Si0.2Ge0.8/Ge interface. All annealed s-Ge layers exhibited higher surface roughness than s-Ge control samples without annealing (rms ~ 2 nm). Annealing at 450 °C resulted in only a slight increase in surface roughness (rms ~ 3 nm), almost independent of s-Ge thickness. However, annealing at 550 °C and 650 °C resulted in significant surface roughening (with maximum rms values of 5 nm and 35 nm, respectively) due to the formation of Ge islands, which were observed by AFM. At these higher temperatures, the surface roughness of the s-Ge layers was found to be thickness dependent, with a Ge smoothing effect observed for layers greater than 50 nm.

These results are particularly important for the fabrication of s-Ge MOSFETs, for which the surface passivation prior to gate stack formation is critical to the performance of the device. Based on the results presented here, the thermal budget should be kept below 550 °C to avoid relaxation and roughening of the s-Ge epilayer, which could degrade the device performance.
Novel Solid Nanohybrids that Stabilize Oil/Water Emulsions and Catalyze Reactions at the Interface
Authors: Faria Jimmy Alexander , Shen Min , Resasco Daniel E
Published: August 2010

A novel system has been developed to catalyze reactions at the oil/water interface of a biphasic liquid system. Stabilization of emulsions was accomplished through the use of nanohybrids composed of hydrophilic oxide particles and hydrophobic Single-Walled Carbon Nanotubes (SWNT), generated in the CoMoCAT process. These nanohybrids are inherently amphiphilic, and tend to adsorb at the interface of a biphasic water/oil liquid system. When enough energy is added to the system, these particles stabilize emulsions by suppressing the coalescence of the droplets and increasing the viscoelastic or pseudoplastic character of the liquid film between droplets. , Depending on contact angle of the particles at the liquid-liquid interface it was possible to stabilize water-in-oil or oil-in-water emulsions. The resulting emulsions are remarkably stable against coalescence and sedimentation, and can be easily separated by filtration or centrifugation, which make them suitable for applications in interfacial catalytic processes in which the catalyst can be easily recovered after reaction. Catalytic activity was imparted by transition metal clusters supported onto the nanohybrids. These metals selectively catalyze reactions at the Oil/Water interface. The proof-of-concept of the biphasic hydrogenation and condensation catalysis was obtained with three reactions of interest in biorefining. The first example was the hydrodeoxygenation of vanillin (4-hydroxy-3-methoxybenzaldehyde). The second example was the conversion of molecules that were exclusively soluble in the aqueous or the organic phase, like glutaraldehyde (water phase) and octanal (oil phase). In the third example we explored a tandem reaction sequence in which Pd-catalyzed hydrogenation was paired with a preceding Aldol-condensation of 5- methylfurfural and acetone. It was demonstrated that with these nanohybrids it is possible to selectively accomplish hydrodeoxygenation and condensation reactions at the water/oil interface of a biphasic system, followed by migration of the products to the oil phase. This contribution provides a proof-of-concept for a promising catalytic system with many potential applications in the liquid phase, such as bio-oil upgrading, production of specialty chemicals, and pharmaceutical applications in which selective reaction and product separations, based on water solubility can be desirable.
Structural and magnetic properties of pure and cobalt doped Gallium Nitride nanocrystals
Authors: Ganesh Vottikondala , Sundaram Suresh , Baskar Krishnan
Published: August 2010

In the present study pure and doped gallium nitride (GaN) nanocrystals were synthesized using gallium trichloride (GaCl3), ethylene diamine tetra acetic acid (EDTA) and cobalt chloride as raw materials at a temperature of 900 °C in ammonia (NH3) atmosphere. The XRD spectrum for pure and cobalt doped GaN nanocrystals shows the formation of single phase wurtzite structure. No impurity phases were observed in the X-ray diffraction pattern for 5% Co doped sample whereas secondary phases were observed when the doping concentration exceeds 5 %. Shift in X-ray diffraction peaks were observed in Co doped samples towards lower angle side compared to pure GaN, it confirms that the Co atoms introduces in to the GaN lattice. Transmission electron microscopy images were taken for pure and Co doped GaN. Hexagonal morphology was observed in pure GaN samples. The average size of the particle was found to be ˜20 nm for pure and Co doped GaN. The magnetic measurements were carried out for the Co (5% & 10%) doped samples both at 10K and 300K. Clear hysteresis loop in the magnetization curve suggest the presence of ferromagnetic behavior in cobalt doped GaN. Temperature dependent magnetization (M-T) measurements were also carried out for doped samples using Super Conducting Quantum Interface Device (SQUID) from 10K to 300K The results have been discussed and correlated to structural and magnetic properties of the materials.
Polymeric/Inorganic Multifunctional Nanoparticles for Simultaneous Drug Delivery and Visualization
Authors: Fornara Andrea , Recalenda Alberto , Qin Jian , Sugunan Abhilash , Ye Fei , Laurent Sophie , Muller Robert N , Zou Jing , Usama Abo-Ramadan , Toprak Muhammet S , Muhammed Mamoun
Published: August 2010

Nanoparticles consisting of different biocompatible materials are attracting a lot of interest in the biomedical area as useful tools for drug delivery, photo-therapy and contrast enhancement agents in MRI, fluorescence and confocal microscopy. This work mainly focuses on the synthesis of polymeric/inorganic multifunctional nanoparticles (PIMN) based on biocompatible di-block copolymer poly(L,L-lactide-co-ethylene glycol) (PLLA-PEG) via an emulsion-evaporation method. Besides containing a hydrophobic drug (Indomethacin), these polymeric nanoparticles incorporate different visualization agents such as superparamagnetic iron oxide nanoparticles (SPION) and fluorescent Quantum Dots (QDs) that are used as contrast agents for Magnetic Resonance Imaging (MRI) and fluorescence microscopy together. Gold Nanorods are also incorporated in such nanostructures to allow simultaneous visualization and photodynamic therapy. MRI studies are performed with different loading of SPION into PIMN, showing an enhancement in T2 contrast superior to commercial contrast agents. Core-shell QDs absorption and emission spectra are recorded before and after their loading into PIMN. With these polymeric/inorganic multifunctional nanoparticles, both MRI visualization and confocal fluorescence microscopy studies can be performed. Gold nanorods are also synthesized and incorporated into PIMN without changing their longitudinal absorption peak usable for lased excitation and phototherapy. In-vitro cytotoxicity studies have also been performed to confirm the low cytotoxicity of PIMN for further in-vivo studies.
Facile Alignment of Carbon Nanotubes Mediated by Tethered Maghemite Nanoparticles
Authors: Kim Il Tae , Nunnery Grady A , Jacob Karl I , Schwartz Justin , Liu Xiaotao , Tannenbaum Rina
Published: August 2010

We describe a novel, facile method for the alignment of multi-walled carbon nanotubes (MWNTs) in a magnetic field facilitated by the decoration of the MWNTs with monodisperse γ-Fe2O3 magnetic (maghemite) nanoparticles. The tethering of the nanoparticles was achieved by the attachment of γ-Fe2O3 nanoparticles that were synthesized using a modified sol-gel process, onto the carboxylate-activated MWNTs. Sodium dodecylbenzene sulfonate (NaDDBS) was used to prevent the formation of an iron oxide 3D network. Various characterization methods were employed to confirm the formation of homogeneously-distributed and nearly-monodispersed iron oxide nanoparticles, and show that they were indeed tethered to the walls of the MWNTs. The γ-Fe2O3 nanoparticles imparted magnetic characteristics to the MWNTs, which in turn, were oriented parallel to the direction of an externally-applied magnetic field. This facile alignment of MWNT could promote the enhancement of various properties, e.g. mechanical and electrical properties, of the resulting composites. Moreover, this facile alignment at low magnetic fields, made possible by the magnetization of the carbon nanotubes through the tethering of maghemite nanoparticles, may be applied to a variety of other useful nanofillers, such a glass fibers, clay nanoparticles and cellulose nanowhiskers.
Carbon Nanomaterials in Silica Aerogel Matrices
Authors: Hamilton Christopher E. , Chavez Manuel E , Duque Juan G. , Gupta Gautam , Doorn Stephen , Dattelbaum Andrew M. , DeFriend Obrey Kimberly A.
Published: August 2010

Silica aerogels are ultra low-density, high surface area materials that are extremely good thermal insulators and have numerous technical applications. However, their mechanical properties are not ideal, as they are brittle and prone to shattering. Conversely, single-walled carbon nanotubes (SWNTs) and graphene-based materials, such as graphene oxide, have extremely high tensile strength and possess novel electronic properties. By introducing SWNTs or graphene-based materials into aerogel matrices, it is possible to produce composites with the desirable properties of both constituents. We have successfully dispersed SWNTs and graphene-based materials into silica gels. Subsequent supercritical drying results in monolithic low-density composites having improved mechanical properties. These nanocomposite aerogels have great potential for use in a wide range of applications.
Strength and Electrical Conductivity of Carbon Nanotube Yarns
Authors: Schauer Mark W , Lashmore David , Lewis Diana , Lewis Benjamin M. , Towle Erick C.
Published: August 2010

The strength and electrical conductivity of Carbon Nanotube (CNT) yarns is dramatically affected by the handling of the material after the nanotubes are produced. Our nanotube production process involving Chemical Vapor Deposition (CVD) using the floating catalyst method produces a mass of entangled bundles of single-walled nanotubes in a gas suspension. Simply collecting and spinning this material produces a yarn with strength and electrical conductivity far less than the properties of the individual nanotubes due to the poor alignment of the bundles on the microscopic scale. We have developed methods of aligning the CNT material that are analogous to the techniques used in the textile industry for spinning staple yarns, but modified to be appropriate for nano-scale material. The result is a dramatic improvement in strength and electrical conductivity of our CNT yarns.
High Electron Gain from Forests of Multi-Walled Carbon Nanotubes
Authors: Michan Mario , Nojeh Alireza
Published: August 2010

Carbon nanotubes are attractive candidates for electron field-emitters due to their high aspect ratio, mechanical stability, and electrical conductivity. It has previously been shown that an electron beam hitting the tip of a carbon nanotube biased near the threshold of field-emission can stimulate the emission of a large number of electrons from the nanotube tip. Here we report on similar experiments on arrays of free-standing multi-walled carbon nanotubes (nanotube forests) interacting with a scanning electron microscope's primary beam. Electron gains of up to 19,000 were obtained. This can enable applications such as electron detection and multiplication, and vacuum transistors.
Influence of reaction parameters on the photoluminescence properties of free standing functionalized silicon nanocrystals
Authors: Gupta Anoop , Wiggers Hartmut
Published: August 2010

While silicon nanostructures acquire novel optical properties due to miniaturization, the stability of light emission is severely limited because of exciton trapping due to surface oxidation coming along with the formation of defects. Grafting of organic molecules on a hydrogen-terminated silicon surface via hydrosilylation provides a promising route to stabilize their surface against oxidation. In this communication, we report on the effect of surface passivation on the optical properties of freestanding silicon nanocrystals (Si-NCs). The surface functionalization of hydrogen-terminated Si-NCs with organic molecules was achieved via liquid phase hydrosilylation. We demonstrate that surface functionalization does not preserve the original emission of hydrogen-terminated Si-NCs. It is observed that the emission spectrum of green emitting hydrogen-terminated Si-NCs is red shifted after surface functionalization. We find that the direction of shift does not depend on the type of organic ligands and the reaction conditions, however, the amount of shift can be altered. The factors influencing the shift in the emission spectra of functionalized Si-NCs with respect to hydrogen-terminated samples are discussed.
Improved Spectral Response of Quantum Dot Solar Cells Using InAs Multi-stack High Density Quantum Dot Molecules
Authors: Tangmettajittakul Ong-arj , Changmoang P , Thainoi Supachok , Changmoang Pornchai , Kanjanachuchai Songphol , Rattanathammaphan Somchai , Panyakeow Somsak
Published: August 2010

In this paper, we report the spectral response measurement of Schottky solar cells with conventional quantum dots (QDs) and high-density quantum dot molecules (HD-QDMs) as active layers. 3-stack HD-QDMs Schottky structure is compared to 15-stack QD Schottky structure and control sample without QDs. 15-stack conventional InAs QDs give narrow response peak centered at 907 nm while 3-stack InAs HD-QDMs give broad peak between 915 and 985 nm. Both spectral responses are extended beyond the band edge of GaAs, i.e. 870 nm. 42 % more photovoltaic power could be evident from the extended spectral response curve comparing to that of GaAs bulk sample without dots.
The Use of Thermally Decomposable Ligands for Conductive Films of Semiconductor Nanocrystals
Authors: Wills Andrew , Kang Moon Sung , Khare Ankur , Gladfelter Wayne L. L. , Norris David
Published: August 2010

Poor conductivity is a bottleneck hindering the production of nanocrystal-based devices. In most nanocrystal syntheses, ligands with long alkyl chains are used to prepare monodisperse, crystalline particles. When these nanocrystals are incorporated into devices as films, the bulky ligands form an insulating layer that prevents charge transfer between particles. While annealing or post-deposition chemical treatments can be used to strip surface ligands, each of these approaches has disadvantages. Here we demonstrate the use of a novel family of ligands comprised of primary alkyl dithiocarbamates to stabilize PbSe/CdSe core-shell nanocrystals. Primary dithiocarbamates, which can bind to cadmium and lead, are known to decompose to the corresponding sulfides when heated under mild conditions. In our scheme, PbSe/CdSe core-shell nanocrystals are first synthesized with standard ligands. These ligands are then exchanged to short chain dithiocarbamates in solution. When a film is cast and annealed at low temperature, the dithiocarbamates are removed. Electron microscopy reveals that the particles move closer together, and, along with x-ray diffraction, shows that the nanocrystals remain quantum confined. Transport measurements show a 10,000-fold increase in conductivity after annealing.
Excited State Charge Transfer in Dyads of ZnO Nanocrystals and Organic or Transition Metal Dyes
Authors: Saunders Julia E. , Huss Adam S. , Bohnsack Jon , Mann Kent R. , Blank David A. , Gladfelter Wayne L. L.
Published: August 2010

To better understand the specific charge transfer events that occur within a dye-sensitized solar cell (DSSC), we synthesized well-defined ZnO:dye dyads. The ZnO nanocrystals were synthesized following literature procedures from zinc acetate and a hydroxide source in ethanol. The absorption onset of the ZnO nanocrystals was observed using UV-vis measurements, from which estimated nanocrystal diameters were determined. At room temperature, the synthesis yielded nanocrystals ranging in diameter from 2-4 nm. Dispersions of ZnO nanocrystals in ethanol were mixed with solutions containing 5΄΄-phenyl-3΄,4΄-di(nbutyl)-[2,2΄:5΄,2΄΄] terthiophene-5-carboxylic acid. Using FT-IR and fluorescence spectroscopy, it was verified that the dye molecules were adsorbed to the ZnO surface via their carboxylate groups while the number of dye molecules adsorbed to the surface was quantified using a combination of techniques. Adsorption isotherms were employed to probe surface coverage of the dye onto the nanocrystals to yield an adsorption equilibrium constant of 1.5 ± 0.2 x 105 M-1. The ability of ZnO nanocrystals to quench the emission of the dye by an electron transfer mechanism was observed and elucidated using ultra-fast laser spectroscopy where the time-scale for electron injection from the dye to the ZnO was determined to be 5.5 ps.
Capacitance-Voltage Characterization of Solar Cells With CdS in CdTe Matrix
Authors: Jha Rashmi , Liu Xiangxin , Wieland Kristopher , Ordosgoitti Jorhan , Paudel Naba , Sun Kai , Compaan Alvin
Published: August 2010

Solar cells fabricated from a sputtering target with 5% cadmium sulfide (CdS) intermixed cadmium telluride (CdTe) material was studied using Capacitance-Voltage (CV) profiling. The average efficiency of a set of these novel solar cells under one sun illumination was observed to be 7.56%. In contrast with standard sputtered CdS/CdTe cells, the junction capacitance of the mixed compound device was observed to show minimal change in the entire reverse bias range. The element mapping of the film using Bright Field Scanning Transmission Electron Microscopy (BF-STEM) was used to determine the distribution of S, Te, Cd, O, and Cl in CdTe film. From these observations, it is believed that the morphology and composition of the film resulted in a built-in potential which was sufficient to completely deplete the film.
Direct Observations of Relaxation of Si/SiGe/Si on Insulator
Authors: Ma Tongda , Tu Hailing
Published: August 2010

Microstructural evolution is directly observed when the cross-sectional film specimen of Si/SiGe/Si on insulator (Si/SiGe/SOI) is heated from room temperature (R.T., 291 K) up to 1113 K in high voltage transmission electron microscope (HVEM). The misfit dislocation at the lower interface of the SiGe layer begins to extend downwards even at 913 K. The lower interface takes the lead in roughening against the upper interface of the SiGe layer. The roughened interface is ascribed to elastic relaxation. As misfit strain is partially transferred to SOI top Si layer and misfit dislocation is prolonged at the lower interface, the roughened interface turns smooth again. Thereafter, the misfit dislocations are introduced into the upper roughened interface of the SiGe layer to release the increased misfit strain. It is suggested that the microscopic relaxation of the SiGe layer is related to dislocation behavior and strain transfer.
Neutron Imaging Methods for the Investigation of Energy Related Materials (Fuel Cells, Battery, Hydrogen Storage and Nuclear Fuel)
Authors: Lehmann Eberhard Helmar , Oberholzer Pierre , Boillat Pierre
Published: August 2010

This article underlines with examples of studies for energy related materials and processes how important and useful the technique of neutron imaging can be for our future energy supply. With the help of the particularly designed configurations for each such study it becomes possible to derive essential information for the material properties and their change in a non-invasive manner.

The four mentioned examples (PEM fuel cell, Li-battery, hydrogen storage and nuclear fuel inspection) cover a very wide range of applications and demonstrate the high potential of the various used methods in neutron imaging.
XANES Studies on Eu-doped Fluorozirconate Based Glass Ceramics
Authors: Henke Bastian , Keil Patrick , Pablick Christian , Vogel Dirk , Rohwerder Michael , Wiegand Marie-Christin , Johnson Jacqueline A. , Schweizer Stefan
Published: August 2010

The influence of adding InF3 as a reducing agent on the oxidation state of Eu in fluoro-chloro- (FCZ) and fluorobromozirconate (FBZ) glass ceramics was investigated using x-ray ab-sorption near edge (XANES) and photoluminescence (PL) spectroscopy. For both materials, it was found that InF3 decreases the Eu2+-to-Eu3+ ratio significantly. PL spectroscopy proved that an annealing step leads to the formation of Eu-doped BaCl2 and BaBr2 nanocrystals in the FCZ and FBZ glasses, respectively. In the case of FCZ glass ceramics the hexagonal phase of BaCl2 could be detected in indium-free and InF3-doped ceramics, but only for InF3 containing FCZ glass ceramics a phase transition of the nanoparticles from hexagonal to orthorhombic structure is observed. For the FBZ glass ceramics, the hexagonal phase of BaBr2 can be formed with and without indium doping, but only in the indium-free case a phase transition to orthorhombic BaBr2 could be found.
Mechanism of Ultrafast (Dis)charging of Li Ion Batteries by Heterogeneous Doping of LiFePO4
Authors: Adams Stefan , Prasada Rao R. , Choo Haiping
Published: August 2010

Molecular dynamics (MD) simulations with a dedicated force-field and our bond valence (BV) pathway analysis have been employed to reproduce and explain the experimentally observed ultrafast Li+ transport in surface modified LixFePO4-δas a consequence of heterogeneous doping, i.e. the Li+redistribution in the vicinity of the interface between LixFePO4and a pyrophosphate glass surface layer. Over the usual working temperature range of LIBs Li+ion conductivity in the surface modified LixFePO4phase is enhanced by 2-3 orders of magnitude, while the enhancement practically vanishes for T > 700K. Simulations for the bulk phase reproduce the experimental conductivities and the activation energy of 0.57eV (for x ≈ 1). A layer-by-layer analysis of structurally relaxed multilayer systems indicates a continuous variation of Li+mobility with the distance from the interface and the maximum mobility close to the interface, but Li+diffusion rate remains enhanced (compared to bulk values) even at the center of the simulated cathode material crystallites. Our BV migration pathway analysis in the dynamic local structure models shows that the ion mobility is related to the extension of unoccupied accessible pathway regions. The change in the extent of Li redistribution across the interface with the overall Li content constitutes a fast pseudo-capacitive (dis)charging contribution.
Visible-light Photoresponse of Nitrogen-doped TiO2: Excited State Studies Using Time-dependent Density Functional Theory and Equation-of-Motion Coupled Cluster Methods
Authors: Govind Niranjan , Rousseau Roger , Andersen Amity , Kowalski Karol
Published: August 2010

To shed light on the nature of the electronic states at play in N-doped TiO2 nanoparticles, we have performed detailed ground and excited state calculations on pure and N-doped TiO2 rutile using an embedding model. We have validated our model by comparing ground-state embedded results with those obtained from periodic DFT calculations. Our results are consistent with periodic calculations. Using this embedding model we have performed B3LYP based TDDFT calculations of the excited state spectrum. We have also studied the lowest excitations using high-level equation-of-motion coupled cluster (EOMCC) approaches involving all single and inter-band double excitations. We compare and contrast the nature of the excitations in detail for the pure and doped systems using these calculations. Our calculations indicate a lowering of the bandgap and confirm the role of the N3- states on the UV/Vis spectrum of N-doped TiO2 rutile supported by experimental findings.
The influence of palladium impurities on vacancy diffusion in cubic silicon carbide
Authors: Roma Guido
Published: August 2010

The basic properties of palladium impurities in silicon carbide, such as solubility or diffusion mechanisms, are far from being well understood. In a recent paper I presented a systematic study of stability and kinetic properties of Pd in cubic silicon carbide using first principles calculations. In this paper I focus on the effect of the presence of palladium in silicon carbide, even in very low concentrations, on the kinetic properties of carbon vacancies. I apply a odel of Pd diffusion through a vacancy mechanism on the carbon sublattice and extract the correlation factors leading to an enhancement of vacancy migration, due to the coupling of iffusion fluxes between vacancies and palladium impurities.
Chemical Compatibility at High Temperature between the Carbide Fuel UC or (U,Pu)C and SiC Cladding for the Gas cooled Fast Reactor
Authors: Berche Alexandre , Alpettaz Thierry , Chatain Sylvie , Gossé Stephane , Guéneau Christine , Rado Cyril
Published: August 2010

The chemical compatibility at high temperature between the fuel kernel (U,Pu)C and SiC cladding, the reference materials for the GFR reactor, is studied. For that purpose, a thermodynamic database on the U-Pu-C-Si system was developed with the Calphad method to calculate the phase diagrams. Differential thermal analysis experiments were performed to measure phase transition temperatures in Si-U and C-Si-U systems. According to the calculated isopleth section between the hyperstoichiometric uranium carbide UC1.02 and SiC, the materials shall not react below 2056 K, the temperature at which a liquid phase shall form. These calculations are in good agreement with two chemical compatibility tests performed at 1873 K and 2073 K between the materials. Calculations were also performed to study the chemical interaction between the mixed carbide (U,Pu)C1.04 and SiC. The presence of plutonium in the fuel kernel lowers the liquid formation temperature of 167 K.
Structural Evolution of Nuclear Glasses under Forcing Conditions (Irradiation, Alteration)
Authors: Calas Georges , Galoisy Laurence , Cormier Laurent , Delaye Jean Marc , Jollvet Patrick , Peuget Sylvain
Published: August 2010

Assessing the long-term behavior of nuclear glass implies the prediction of their long-term performance, and more precisely of their evolution under irradiation and during interaction with water. After briefly recalling the major characteristics of the local and medium-range structure of borosilicate glasses of nuclear interest, we will present some structural features observed under forcing conditions. Specific structural tools (EXAFS/XANES, Neutron/x-ray diffraction, solid state spectroscopic methods…) are correlated with numerical simulations to determine the local structure of glass and provide selective information on glass surface using total electron yield detection. During alteration in near- or under-saturated conditions, some elements such as Fe change coordination, as other elements such as Zr only suffer structural modifications in under-saturated conditions. These structural modifications may explain the chemical dependence of the initial alteration rate and the transition to the residual regime. They also illustrate the molecular-scale origin of the processes at the origin of the glass-to-gel transformation. Molecular scale processes help in predicting the properties of new generations of nuclear glasses required by future production of nuclear energy. Under irradiation, various structural effects are observed, including coordination change, ion migration or disorder effects. These studies show that glasses with a simplified composition do not show the same behavior as more realistic glasses. Molecular dynamics (MD) simulations provide complementary information on elastic effects. Recent direct evidence for B-coordination change under external irradiation together with structural models derived from MD sheds light on the structural mechanisms at the origin of radiation-induced modifications of glass properties, emphasizing the importance of the thermal regime in the cascade core.
Influence of Operational Conditions on Retardation Parameters Measured by Diffusion Experiment in Compacted Bentonite
Authors: Yasuo Ishii , Seida Yoshimi , Tachi Yukio , Yoshikawa Hideki
Published: August 2010

Influence of operation factors in diffusion test of compacted bentonite (such as agitation of test solution in the reservoir, feed rate of the test solution and mass transfer resistance in the filter) on the diffusion data was examined by reservoir depletion (RD) test method using Cs+. The influence of these factors on the diffusion data was also analyzed based on the mathematical sorption-diffusion model which considered the feed of test solution and mass transfer resistance in the filter as well. The reservoir depletion data showed some remarkable influences of these operational conditions, especially in the system with low ionic strength. Change in mass transfer resistance at filter-compacted bentonite due to the operational conditions was found to be potential factor which disturb the diffusion data. The influence was reduced in the system with high ionic strength of solution.