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The high theoretical lithium storage capacity of Sn makes it an enticing anode material for Li-ion batteries (LIBs); however, its large volumetric expansion during Li-Sn alloying must be addressed. Combining Sn with metals that are electrochemically inactive to lithium leads to intermetallics that can alleviate volumetric expansion issues and still enable high capacity. Here, we present the cycling behavior of a nanostructured MnSn

We present an in-depth study of metal–insulator interfaces within granular metal (GM) films and correlate their interfacial interactions with structural and electrical transport properties. Nominally 100 nm thick GM films of Co and Mo dispersed within yttria-stabilized zirconia (YSZ), with volumetric metal fractions (φ) from 0.2–0.8, were grown by radio frequency co-sputtering from individual metal and YSZ targets. Scanning transmission electron microscopy and DC transport measurements find that the resulting metal islands are well-defined with 1.7–2.6 nm average diameters and percolation thresholds between φ = 0.4–0.5. The room temperature conductivities for the φ = 0.2 samples are several orders of magnitude larger than previously-reported for GMs. X-ray photoemission spectroscopy indicates both oxygen vacancy formation within the YSZ and band-bending at metal–insulator interfaces. The higher-than-predicted conductivity is largely attributed to these interface interactions. In agreement with recent theory, interactions that reduce the change in conductivity across the metal–insulator interface are seen to prevent sharp conductivity drops when the metal concentration decreases below the percolation threshold. These interface interactions help interpret the broad range of conductivities reported throughout the literature and can be used to tune the conductivities of future GMs.

The high theoretical lithium storage capacity of Sn makes it an enticing anode material for Li-ion batteries (LIBs); however, its large volumetric expansion during Li-Sn alloying must be addressed. Combining Sn with metals that are electrochemically inactive to lithium leads to intermetallics that can alleviate volumetric expansion issues and still enable high capacity. Here, we present the cycling behavior of a nanostructured MnSn2intermetallic used in LIBs. Nanostructured MnSn2is synthesized by reducing Sn and Mn salts using a hot injection method. The resulting MnSn2is characterized by x-ray diffraction and transmission electron microscopy and then is investigated as an anode for LIBs. The MnSn2electrode delivers a stable capacity of 514 mAh g-1after 100 cycles at a C/10 current rate with a Coulombic efficiency >99%. Unlike other Sn-intermetallic anodes, an activation overpotential peak near 0.9 V versus Li is present from the second lithiation and in subsequent cycles. We hypothesize that this effect is likely due to electrolyte reactions with segregated Mn from MnSn2. To prevent these undesirable Mn reactions with the electrolyte, a 5 nm TiO2protection layer is applied onto the MnSn2electrode surface via atomic layer deposition. The TiO2-coated MnSn2electrodes do not exhibit the activation overpotential peak. The protection layer also increases the capacity to 612 mAh g-1after 100 cycles at a C/10 current rate with a Coulombic efficiency >99%. This higher capacity is achieved by suppressing the parasitic reaction of Mn with the electrolyte, as is supported by x-ray photoelectron spectroscopy analysis.

Neutron sensing is critical in civilian and military applications. Conventional neutron sensors are limited by size, weight, cost, portability and helium supply. Here the microfabrication of gadolinium (Gd) conversion material–based heterojunction diodes for detecting thermal neutrons using electrical signals produced by internal conversion electrons (ICEs) is described. Films with negligible stress were produced at the tensile-compressive crossover point, enabling Gd coatings of any desired thickness by controlling the radiofrequency sputtering power and using the zero-point near p(Ar) of 50 mTorr at 100 W. Post-deposition Gd oxidation–induced spallation was eliminated by growing a residual stress-free 50 nm neodymium-doped aluminum cap layer atop Gd. The resultant coatings were stable for at least 6 years, demonstrating excellent stability and product shelf-life. Depositing Gd directly on the diode surface eliminated the air gap, leading to a 200-fold increase in electron capture efficiency and facilitating monolithic microfabrication. The conversion electron spectrum was dominated by ICEs with energies of 72, 132 and 174 keV. Results are reported for neutron reflection and moderation by polyethylene for enhanced sensitivity, and γ- and X-ray elimination for improved specificity. The optimal Gd thickness was 10.4 μm for a 300 μm-thick partially depleted diode of 300 mm2 active surface area. Fast detection (within 10 min) at a neutron source-to-diode distance of 11.7 cm was achieved with this configuration. All ICE energies along with γ-ray and Kα,β X-rays were modeled to emphasize correlations between experiment and theory. Semi-conductor thermal neutron detectors offer advantages for field-sensing of radioactive neutron sources.

Here, we study nanoporous-carbon (NPC) grown via pulsed laser deposition (PLD) as an electrically conductive anode host material for Mg2+ intercalation. NPC has high surface area, and an open, accessible pore structure tunable via mass density that can improve diffusion. We fabricate 2032 coin cells using NPC coated stainless-steel disk anodes, metallic Mg cathodes, and a Grignard-based electrolyte. NPC mass density is controlled during growth, ranging from 0.06–1.3 g/cm3. The specific surface area of NPC increases linearly from 1,000 to 1,700 m2/g as mass density decreases from 1.3 to 0.26 g/cm3, however, the surface area falls off dramatically at lower mass densities, implying a lack of mechanical integrity in such nanostructures. These structural characterizations correlate directly with coin cell electrochemical measurements. In particular, cyclic voltammetry (CV) scans for NPC with density ~0.5 g/cm3 and BET surface area ~1500 m2/g infer the possibility of reversible Mg-ion intercalation. Higher density NPC yields capacitive behavior, most likely resulting from the smaller interplanar spacings between graphene sheet fragments and tighter domain boundaries; lower density NPC results in asymmetrical CV scans, consistent with the likely structural degradation resulting from mass transport through soft, low-density carbon materials.

We study nanoporous-carbon (NPC) grown via pulsed laser deposition (PLD) as a sorbent coating on 96.5-MHz surface-acoustic-wave (SAW) devices to detect trihalomethanes (THMs), regulated byproducts from the chemical treatment of drinking water. Using both insertion-loss and isothermal-response measurements from known quantities of chloroform, the highest vapor pressure THM, we optimize the NPC mass-density at 1.05 ± 0.08 g/cm3 by controlling the background argon pressure during PLD. Precise THM quantities in a chlorobenzene solvent are directly injected into a separation column and detected as the phase-angle shift of the SAW device output compared to the drive signal. Using optimized NPC-coated SAWs, we study the chloroform response as a function of operating temperatures ranging from 10–50°C. Finally, we demonstrate individual responses from complex mixtures of all four THMs, with masses ranging from 10–2000 ng, after gas chromatography separation. As a result, estimates for each THM detection limit using a simple peak-height response evaluation are 4.4 ng for chloroform and 1 ng for bromoform; using an integrated-peak area response analysis improves the detection limits to 0.73 ng for chloroform and 0.003 ng bromoform.

Using galvanostatic pulse deposition, we studied the factors influencing the quality of electroformed Bi1–xSbx nanowires with respect to composition, crystallinity, and preferred orientation for high thermoelectric performance. Two nonaqueous baths with different Sb salts were investigated. The Sb salts used played a major role in both crystalline quality and preferred orientations. Nanowire arrays electroformed using an SbI3-based chemistry were polycrystalline with no preferred orientation, whereas arrays electroformed from an SbCl3-based chemistry were strongly crystallographically textured with the desired trigonal orientation for optimal thermoelectric performance. From the SbCl3 bath, the electroformed nanowire arrays were optimized to have nanocompositional uniformity, with a nearly constant composition along the nanowire length. Nanowires harvested from the center of the array had an average composition of Bi0.75Sb0.25. However, the nanowire compositions were slightly enriched in Sb in a small region near the edges of the array, with the composition approaching Bi0.70Sb0.30.

Vertically aligned, untangled planarized arrays of multiwall carbon nanotubes (MWNTs) with Ohmic back contacts were grown in nanopore templates on arbitrary substrates. The templates were prepared by sputter depositing Nd-doped Al films onto W-coated substrates, followed by anodization to form an aluminum oxide nanopore array. The W underlayer helps eliminate the aluminum oxide barrier that typically occurs at the nanopore bottoms by instead forming a thin WO3 layer. The WO3 can be selectively etched to enable electrodeposition of Co catalysts with control over the Co site density. This led to control of the site density of MWNTs grown by thermal chemical vapor deposition, with W also serving as a back electrical contact. Ohmic contact to MWNTs was confirmed, even following ultrasonic cutting of the entire array to a uniform height.

We investigate the structure of $$ \frac{1}{3}\langle 0\bar{1}11\rangle $$ dislocations observed in Bi2Te3 nanowires. This particular type of dislocation is interesting because it has a large Burgers vector (b = 1.048 nm) with a component normal to the basal planes equal to the thickness of one full Bi2Te3 quintuple unit (i.e., c/3). Atomic-resolution high-angle annular dark-field scanning transmission electron microscopy observations show that the dislocations form with a complex dissociated core structure. This structure consists of two partial dislocations that separate a defected region consisting of a seven-plane-thick septuple unit, consistent with a local patch of Bi3Te4, rather than the normal Bi2Te3 quintuple layer structure. As we discuss, details of the core structure can be understood from an analysis of the crystallographic parameters of the observed partial dislocations. This analysis suggests a mechanism to accommodate the loss of tellurium through the heterogeneous nucleation and growth of seven-layer defects at $$ \frac{1}{3}\langle 0\bar{1}11\rangle $$ —type dislocations.

Achieving control of crystalline quality is a key barrier to developing thermoelectric (TE) nanowires. We show that the structural properties of free-standing Bi2(Te.97Se.03)3 nanowire arrays on substrates can be improved by postdeposition annealing. Nanowires were electrochemically deposited into anodized aluminum oxide nanopore templates formed directly on metallized Si(100). The templates were chemically removed prior to annealing in a 3% H2/Ar environment to prevent microcrack formation that results from thermal stresses. Grain sizes grew exponentially with annealing temperature until reaching the full 75-nm diameter of the nanowires at 300 °C; growth was linear above this temperature since grains could grow further only in the axial directions. Crystalline quality, along with the development of the preferred (110) orientation for optimal TE properties, improved with increasing annealing temperature between 200 and 400 °C. However, continued loss of Te composition with annealing led to a mixed phase of Bi2Te3 and Bi4Te3 at 500 °C.

We report on a nanoelectrode array sensor for the detection of gaseous iodine at levels below 20 ppb within short exposure times. The sensor is constructed on a free-standing anodic aluminum oxide wafer with 120 nm diameter, 50 um length nanopores. On each wafer surface, continuous metal films are deposited: on one surface, a gold film is deposited such that it covers the nanopores, and, on the opposite surface, a platinum film is deposited such that the nanopores remain open and their volumes in contact with the environment. The gold film serves as the working electrode and the platinum film serves as the auxiliary and pseudo-reference electrode. The nanopores are filled with a pH 9 buffer aqueous solution to create an array of billions/cm2 electrochemical cells in parallel. Ionic conductivity is maintained for environments with relative humidity levels greater than ~30%. Here we report on sensor performance improvements that allow using this sensor in more arid environments, as well as in the presence of other relevant interferent analytes (e.g. Cl2). The detection mechanism is the electrochemical oxidation of anionic species formed by the dissolution and subsequent hydrolysis of gas phase I2 in pH 9 buffer electrolyte. Gas phase I2 dissolves in the buffer following Henry's Law and then is hydrolyzed to several species, including the anionic species iodide and tri-iodide. The hydrolysis mechanism of I2 in the pH 9 buffer enables it to function as a concentrator for the anionic species. When the sensor is exposed to a gas stream containing iodine, continuous accumulation of the electrochemically detectable anionic species is possible, enabling preconcentration of the incoming flow-stream by several orders of magnitude. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Lithium-ion batteries (LIBs) find applications ranging from portable electronics to electric vehicles, due to their higher energy densities (~ 250 Wh.hr kg-1) than other rechargeable batteries. Since the invention of LIBs, commercialized anodes have mostly consisted of carbon-based materials such as natural graphite, mesophase carbon microbeads (MCMB) massive artificial graphite (MAG), and hard carbons. However, the employment of LIBs in electric vehicles is increasing demand for higher energy density LIBs. Graphite-based anodes are limited to a charge storage capacity of 372 mAh g-1 due to the tight interplanar spacing (3.35 Å) between the graphene sheets. Several alternative anode materials have been proposed to replace graphite. In particular, Si and Sn alloys are extensively studied, but enormous volume change while cycling is hindering their commercialization. Lithium titianate (Li4Ti5O12) anodes are successfully commercialized, but low lithium storage capacity (175 mAh g-1) and high voltage (~1.5 V) prevents their use for high energy density applications. Carbon allotropes with modified structures and crystallinity can achieve higher capacity relative to graphite, so they are potential candidates to enable higher energy density LIBs. Carbon materials vary drastically in terms of interplanar spacing, termination groups on the edges of graphite/graphene fragments, defects, and porosity within the carbon structures. These variations significantly affect the Coulombic efficiency, lithium storage capacity, irreversible capacity loss during the first cycle, and the voltage range for lithiation. We will present an investigation of nanoporous carbon (NPC) prepared via pulsed laser deposition as a model system to understand lithiation mechanisms in carbon anodes. NPC is grown directly on stainless steel current collectors at room temperature, resulting in randomly-oriented stacks consisting of a few layers of aligned nm-sized graphene sheets. NPC mass density can be controlled from 2.0 g/cm3 to below 0.1 g/cm3 by varying the carbon deposition energetics, enabling the tuning of interplanar spacing and porosity within NPC. Low-density NPC exhibits as much as a 55% larger interplanar spacing compared to graphite-like high-density NPC and larger open pore spaces between stacks of sheets. The ability to control interplanar spacings and porosity in otherwise identical carbon samples allows for a systematic study to understand how these characteristics affect lithiation and de-lithiation processes in graphene-like carbon materials. We will discuss how the mass density of this 3D graphene-like NPC nanomaterial affects lithiation mechanisms and will demonstrate that NPC anode storage capacity systematically increases with decreasing mass density. High capacity is enabled by a plethora of grain boundaries, large and controlled interplanar spacings between graphene sheets, and nanopores between the randomly-aligned sheet fragments, all potentially contributing to extra lithium storage sites. The primary mechanisms for lithium storage in nanoporous carbon varies with mass density. For example, the capacity increases with cycling for the lowest mass density materials, potentially indicating that lithium plates within the nanopores as electrolyte penetrates deeper into the structure with each cycle. NPC mass density also correlates with Coulombic efficiency, such that lower density NPC demonstrates lower coulombic efficiency. This may be related to solid electrolyte interphase formation in the lower mass density samples since the electrolyte is more likely to penetrate into the structure and enable higher carbon-electrolyte interface surface areas. However, initial data suggests that lower Coulombic efficiency with lower mass density is likely related to the trapping of lithium ions at termination groups on the edges of graphene sheets. This trapping of lithium can be reversed by increasing the voltage during delithiation. Finally, we will also present data showing that NPC can be sodiated. Sodiation processes mimic those of lithiation, with sodium storage capacity increasing with decreasing NPC mass-density. We thank Lyle Brunke for assistance growing NPC films and Carlos Gutierrez for programmatic guidance. This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories, a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA-0003525. SAND2019-4730 A

AlGaN-channel high electron mobility transistors (HEMTs) were operated as visible- and solar-blind photodetectors by using GaN nanodots as an optically active floating gate. The effect of the floating gate was large enough to switch an HEMT from the off-state in the dark to an on-state under illumination. This opto-electronic response achieved responsivity >108 A/W at room temperature while allowing HEMTs to be electrically biased in the off-state for low dark current and low DC power dissipation. The influence of GaN nanodot distance from the HEMT channel on the dynamic range of the photodetector was investigated, along with the responsivity and temporal response of the floating gate HEMT as a function of optical intensity. The absorption threshold was shown to be controlled by the AlN mole fraction of the HEMT channel layer, thus enabling the same device design to be tuned for either visible- or solar-blind detection.

The room temperature electronic transport properties of 1 μm thick Bi0.4Sb1.6Te3 (BST) films correlate with overall microstructural quality. Films with homogeneous composition are deposited onto fused silica substrates, capped with SiN to prevent both oxidation and Te loss, and postannealed to temperatures ranging from 200 to 450 °C. BST grain sizes and (00l) orientations improve dramatically with annealing to 375 °C, with smaller increases to 450 °C. Tiny few-nanometer-sized voids in the as-deposited film grain boundaries coalesce into larger void sizes up to 300 nm with annealing to 350 °C; the smallest voids continue coalescing with annealing to 450 °C. These voids are decorated with few-nanometer-sized Sb clusters that increase in number with increasing annealing temperatures, reducing the Sb content of the remaining BST film matrix. Resistivity decreases linearly with increasing temperature over the entire range studied, consistent with improving crystalline quality. The Seebeck coefficient also improves with crystalline quality to 350 °C, above which void coalescence and reduced Sb content from the BST matrix correlate with a decrease in the Seebeck coefficient. Nevertheless, a plateau exists for an optimal power factor between 350 and 450 °C, implying thermal stability to higher temperatures than previously reported.The room temperature electronic transport properties of 1 μm thick Bi0.4Sb1.6Te3 (BST) films correlate with overall microstructural quality. Films with homogeneous composition are deposited onto fused silica substrates, capped with SiN to prevent both oxidation and Te loss, and postannealed to temperatures ranging from 200 to 450 °C. BST grain sizes and (00l) orientations improve dramatically with annealing to 375 °C, with smaller increases to 450 °C. Tiny few-nanometer-sized voids in the as-deposited film grain boundaries coalesce into larger void sizes up to 300 nm with annealing to 350 °C; the smallest voids continue coalescing with annealing to 450 °C. These voids are decorated with few-nanometer-sized Sb clusters that increase in number with increasing annealing temperatures, reducing the Sb content of the remaining BST film matrix. Resistivity decreases linearly with increasing temperature over the entire range studied, consistent with improving crystalline quality. The Seebeck coefficient also impr...

We report on a nanoelectrode array sensor for the detection of I2 (iodine gas) at levels as low as near 0,02 ppm within tens of seconds of exposure. The sensor is constructed on a free-standing anodic aluminum oxide wafer with nanopores of 120 nm diameter and 50 um length. On each wafer surface, continuous metal films of tens of nm thickness are deposited: on one surface, a gold film is deposited such that it covers the nanopores, and, on the opposite surface, a platinum film is deposited such that the nanopores remain open and their volumes in contact with the environment. The gold film serves as the working electrode and the platinum film serves as the auxiliary and pseudo-reference electrode. The nanopores are filled with a pH 9 buffer aqueous solution to create an array of electrochemical cells in parallel, and ionic conductivity is maintained for environments with relative humidity levels greater than about 30%. The detection mechanism is the electrochemical oxidation of anionic species of I2 that form from the dissolution and subsequent hydrolysis of gas phase I2 in the pH 9 buffer electrolyte: the gas phase I2 dissolves in the buffer in proportion to Henry's Law constant and then the dissolved I2 is hydrolyzed to several species, including the anionic species iodide and tri-iodide. The detection of I2 is achieved by measuring oxidizing current during the cyclic voltammetry technique bounded between -0,2 and 0,5 V vs the Pt pseudoreference electrode. The hydrolysis mechanism of I2 in the pH 9 buffer enables the buffer to function as a concentrator for the anionic species of I2. When the sensor is exposed to a gas stream containing I2, practically continuous accumulation of the electrochemically detectable anionic species of I2 is possible, with a saturation point where the pH 9 buffer contains > 100 ppm iodide anion. For context, the sensor can detect iodide anions in pH 9 buffer at levels as low as 0,01 ppm. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Nanoporous carbon (NPC) is a purely graphitic material with highly controlled densities ranging from less than 0.1 to 2.0 g/cm3, grown via pulsed-laser deposition. Decreasing the density of NPC increases the interplanar spacing between graphene-sheet fragments. This ability to tune the interplanar spacing makes NPC an ideal model system to study the behavior of carbon electrodes in electrochemical capacitors and batteries. We examine the capacitance of NPC films in alkaline and acidic electrolytes, and measure specific capacitances as high as 242 F/g.

High crystalline quality single- and multiwalled carbon nanotubes (CNTs) grow using thermal chemical vapor deposition (CVD) at temperatures ranging from 530 to 630 °C. Using identical Ni catalyst layers and a constant CO reduction annealing process at 600 °C, the number of walls in the resulting CNTs increases linearly from one to eight over this growth temperature range. The highest temperature used in the growth process appears to control the resulting inner core diameter, which is ∼1 nm for all CNTs grown ≤ 610 °C, near the CO reduction anneal used for all the samples. The corresponding CNT outer diameters also increase linearly from 1 to 5 nm up to 610 °C. Using growth temperatures measurably higher than that of the reduction anneal results in both larger inner and outer diameters, inferring that the Ni catalyst islands grow with increasing temperature. These results suggest that independent control of the number of walls and the inner core diameter in CNTs is possible.

Graphite is the state-of-the-art anode for Li-ion batteries, but graphite has limited lithium storage capacity (372 mAh/g) due to the small inter-planar spacing (3.35 Å) between graphene sheets. Graphene, in the form of reduced graphene oxide (RGO), has recently been demonstrated as an alternative anode. The theoretical capacity of graphene is double (744 mAh/g) that of graphite; Li+ can adsorb on both sides of the graphene sheets because the sheets pack at a lower density than graphite due to disorder and the presence of functional groups formed during synthesis. Typically, RGO anodes are fabricated by preparing a slurry of RGO, conductive carbon, and binder. This process does not allow control of the interplanar spacing between the graphene sheets and leads to a porous, disorganized electrode. RGO electrodes typically exhibit capacity fade and copious solid electrolyte interphase (SEI) formation. We study graphene as an anode material for Li-ion batteries via deposition of nanoporous carbon (NPC) by pulsed laser deposition (PLD). This binderless deposition provides a ‘clean’ three-dimensional graphene material, enabling systematic and controlled lithiation studies. NPC grows at room temperature, is stable up to 600 °C, and assembles as nanometric, randomly-oriented stacks of several aligned graphene sheets. Most importantly, the interplanar spacing can be controlled during deposition such that the mass density can be varied from 2.25 g/cm3 (graphite-like) to 0.1 g/cm3 (55% expanded interplanar spacing relative to graphite). The ability to control interplanar spacing allows systematic study to understand how interplanar spacing affects lithiation of graphene. NPC demonstrates higher specific capacity than the theoretical values for graphene, likely arising from the plethora of grain boundaries and large, controlled interplanar spacing between sheets. We find that lower-density NPC typically leads to higher capacity, perhaps arising from the larger spacing between sheets for lithiation. The Coulombic efficiency (CE) of the first cycle also correlates with mass-density, such that the higher capacity, lower mass-density samples also exhibit lower CE. This indicates SEI forms more readily during the first cycle in lower mass-density samples. The CE rises after the first two cycles to settle in the range of 98 to >99% and is not very dependent on NPC density, indicating that SEI formation is minimal after the first few cycles. In addition to enabling systematic, controlled lithiation studies in a three-dimensional form of graphene, NPC is also useful as a scaffold to support higher capacity materials. We demonstrate this concept using Si with a theoretical capacity (~4200 mAh/g) much larger than carbons. Si suffers from >300% volume expansion upon alloying with Li, causing severe capacity fade and excessive SEI formation as the material pulverizes with electrochemical cycling. Many strategies have improved capacity fade. However, reversible capacity after many cycles remains far lower than theoretical. RGO and Si nanocomposites have been prepared to cushion volume expansion, but pulverization of the composite can still occur because the Si particles are large relative to the spacing between RGO sheets. Our current work focuses on depositing Si atoms and/or small clusters between NPC sheets (Si-NPC composites). We take advantage of the expanded NPC interplanar spacing to provide a scaffold where Si atoms can reside and are free to expand with lithiation without pulverizing the NPC host. Si-NPC composites are prepared by co-depositing Si during the NPC PLD process. We have successfully deposited at least 7 atomic % Si in NPC. The formation of composites between Si and NPC is confirmed by mass-density measurements, which are below the mass-densities of other potential products such as Si, graphite, and silicon carbide (all greater than 2.2 g/cm3). Initial data shows improved capacity with addition of Si. The CE in the first cycle is comparable to NPC without Si and CE rises to >99% in some samples. The high CE values indicate little additional SEI formation with Si addition and suggest that these composites do not pulverize with cycling, which would expose new surface area for SEI to form. We expect that the Si-NPC architectures can be tuned for high capacity, CE, and cycle life. We thank Lyle Brunke for assistance growing NPC films, Graham Yelton, Kyle Fenton, and Kyle Klavetter for discussions, and Carlos Gutierrez for programmatic guidance. This work is supported by the Laboratory Directed Research and Development program at Sandia National Laboratories, a multi-mission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Carbon nanotubes (CNTs) offer great potential for numerous cold-cathode field emission applications. A less studied need is for high-current cathodes. While work to date has focused on the use of tangled webs of single-wall CNTs, much understanding about field emission has occurred from studies using multi-wall CNTs with controlled geometries. However, the crystalline nature of these multi-wall CNTs typically is far inferior to that of single-wall CNTs. We use high-resolution transmission electron microscopy to demonstrate that growth at temperatures ≤ 630 °C via thermal chemical vapor deposition can produce highly crystalline multi-wall CNTs, with structures consisting entirely of concentric graphene cylinders. Conversely, growth at temperatures ≥ 650 °C results in crystalline CNTs embedded in a nanocrystalline graphite, or glassy carbon, sheath. This sheath material is likely a poor electrical conductor, due to phonon scattering, and will have deleterious effects on field emission. Field emission measurements taken from such films are consistent with the best field-emitting multi-wall CNT films in the literature, in terms of total current for a given applied field, but without the benefit of the preferred perpendicular orientation. These results are promising toward the development of reliable high-current field emission cathodes.

Chemical solution deposition (CSD) is used to grow high-quality (100)-oriented films of SrTiO3 (STO) on CSD Ba0.2 Ca0.8TiO3(100) (BCT) templates on textured W-doped Ni(100) (Ni:W) tape substrates. The BCT template films form a thin layer or “skin” that bridges its significant porosity. STO films grown at 1000 °C appear optimized for heteroepitaxial orientation, surface coverage, and film smoothness. Both interfaces in the STO(100)/BCT(100)/Ni:W(100) stack demonstrate excellent atomic registry and compositional abruptness. Doping STO with a few atomic percent of Nb reduces oxygen diffusion into the film by an order of magnitude and provides greater protection to the Ni interfacial surface from oxidation during the growth of additional functional oxides requiring relatively higher p(O2) high-temperature processing, such as superconducting YBa2Cu3O7−δ. CSD growth of BCT and STO also planarizes pre-existing grooves in the Ni:W(100) tapes while maintaining a high degree of orientation by forming facets at the interfaces.

A variety of solution deposition routes have been reported for processing complex perovskite-based materials such as ferroelectric oxides and conductive electrode oxides, due to ease of incorporating multiple elements, control of chemical stoichiometry, and feasibility for large area deposition. Here, we report an extension of these methods toward long length, epitaxial film solution deposition routes to enable biaxially oriented YBa2Cu3O7−δ (YBCO)-coated conductors for superconducting transmission wires. Recent results are presented detailing an all-solution deposition approach to YBCO-coated conductors with critical current densities Jc (77 K)>1 MA/cm2 on rolling-assisted, biaxially textured, (200)-oriented Ni–W alloy tapes. Solution-deposition methods such as this approach and those of other research groups appear to have promise to compete with vapor phase methods for superconductor electrical properties, with potential advantages for large area deposition and low cost/kA·m of wire.

YBa2Cu3O7� d (YBCO) films, deposited on buffered metal substrates, are the primary candidate for second-generation superconducting (SC) wires, with applications including expanded power grid transmission capability, compact motors, and enhanced sensitivity magnetic resonance imaging. Feasibility of manufacturing such superconducting wires is dependent on high processing speed, often a limitation of vapor and solution-based YBCO deposition processes. In this work, YBCO films were fabricated via a new diethanolamine-modified trifluoroacetic film solution deposition method. Modifying the copper chemistry of the YBCO precursor solution with diethanolamine enables a hundredfold decrease in the organic pyrolysis time required for MA/cm 2 current density (Jc) YBCO films, from multiple hours to � 20 s in atmospheric pressure air. High quality, � 0.2 lm thick YBCO films with Jc (77 K) values P 2 MA/cm 2 at 77 K are routinely crystallized from these rapidly pyrolyzed films deposited on LaAlO3. This process has also enabled Jc (77 K) ¼ 1.1 MA/cm 2 YBCO films via 90 m/h dip-coating on Oak Ridge National Laboratory RABiTSe textured metal tape substrates. This new YBCO solution deposition method suggests a route toward inexpensive and commercializable � $10/kA m solution deposited YBCO coated conductor wires. � 2003 Elsevier B.V. All rights reserved.

We study nanoporous carbon (NPC) as an adsorbent coating on surface acoustic wave (SAW) chemical microsensors for a wide range of analyte gases. By use of pulsed-laser deposition in a controlled inert gas ambient, NPC grows at room temperature with negligible residual stress and, hence, can coat most surfaces to any desired thickness. Acetone adsorption isotherms for NPC-coated SAW devices with mass density ranging from 0.18 to 1.08 g/cm3 indicate that the device frequency response relates to NPC density. Data analysis suggests the possibility of detecting acetone below parts-per-billion concentrations. We find NPC to be highly sensitive to a variety of other volatile organic and toxic industrial compounds. Transmission electron microscopy reveals that lower-density NPC has both larger and greater numbers of nanopores than higher-density NPC and that decreasing NPC density also increases the interplanar spacing between graphene sheet fragments within the ultrathin carbon wall structures. These physical differences effectively increase the available surface area for analyte gas adsorption with decreasing NPC density, with only the structural integrity of the internal NPC wall structures a limiting factor in determining the lowest useful density NPC coating.

The superconducting and structural properties of YBa 2 Cu 3 O 7− δ (YBCO) films grown from chemical solution deposited (CSD) metallofluoride-based precursors improve by using high heating rates to the desired growth temperature. This is due to avoiding the nucleation of undesirable a -axis grains at lower temperatures, from 650 to 800 °C in p (O 2 )=0.1%. Minimizing time spent in this range during the temperature ramp of the ex situ growth process depresses a -axis grain growth in favor of the desired c -axis orientation. Using optimized conditions, this results in high-quality YBCO films on LaAlO 3 (1 0 0) with J c (77 K) ∼ 3 MA/cm 2 for films thicknesses ranging from 60 to 140 nm. In particular, there is a dramatic decrease in a -axis grains in coated-conductors grown on CSD Nb-doped SrTiO 3 (1 0 0) buffered Ni(1 0 0) tapes.

We grow multiwalled carbon nanotubes (CNTs) via thermal chemical vapor deposition from a sputtered 4-nm-thick nickel catalyst film on a tungsten-coated silicon substrate. CNTs grow from a mixture of nitrogen and acetylene gases at temperatures ranging from 630 to 790 °C, resulting in CNT outer diameters of 5–350 nm. CNT diameters increase exponentially with temperature. These results define regimes for template growth fabricated in catalytically active anodized aluminum oxide (AAO) with controlled pinhole sizes ranging from 10 to 50 nm. We measure a threshold electron emission field of 3 V/μm and a field enhancement factor β=5230 on randomly oriented 10-nm diameter CNTs.

The effect of high-boiling-point solvent addition on the morphology of low-p(O2) processed, sol-gel YBa2Cu3O7−δ (YBCO) films is discussed. Proper selection and addition of a high-boiling-point solvent prevents film roughening during precursor pyrolysis, while permitting at least a tenfold reduction in pyrolysis time compared to standard film processing in air or O2. Use of such solvents appears to increase film plasticity, avoiding elastic compressive stress related buckling. High-quality YBCO films on 〈100〉 LaAlO3 with a critical current density (Jc) ∼ 3–4 MA/cm2 at 77 K, are routinely crystallized with this new sol-gel process. Diethanolamine-based, sol-gel YBCO films deposited on sol-gel SrTiO3-buffered 〈100〉 Ni have Jc(77 K) > 1 MA/cm2, demonstrating a route toward all-sol-gel superconducting wires.

We study the use of nanoelectrode arrays (NEAs), consisting of billions/cm2 of ~ 75 nm diameter electrodes, for the electrochemical analysis of ionic halogen species in water. Such NEA sensors were previously demonstrated to detect part-per-billion (ppb) levels of Pb-ions in aqueous solutions without supporting electrolytes. Briefly, the individual electrodes in the NEA can be tailored with sufficient separation coupled with pore-depth control so that each experiences non-overlapping hemispherical diffusion zones within which the ionic flux of an analyte is independent of time. This geometry is well-suited for trace-level detection, as fast voltage sweeps result in a large and steady-state current density. With optimized design, the additive response from the plethora of nanoelectrodes leads to an orders-of-magnitude analytical advantage for electrochemical detection compared to conventional (mm) sized disc electrodes using sweep or pulse voltammetric techniques. NEAs are fabricated by anodizing RF-sputter deposited Al films on a substrate-of-choice, resulting in billions/cm2 tailored with controllable pore diameters. Each nanopore is partially filled via electrochemical plating with an appropriate working electrode material, such as Au, Pt or Ni. A thin W film is beneath the nanopore template for both processing assistance and to provide a backside electrical contact in parallel to every nanoelectrode. In the case that the NEA is recessed within the porous alumina, a thin conducting film can be added to the topside to function as a counter/pseudo-reference electrode. In some situations, the counter electrode can also serve as an ion collector, preconcentrating ionic species near the pore openings prior to beginning an electrochemical measurement. Using such NEA sensors, we will demonstrate trace detection levels for both Cl and I ionic species in water, and compare the limits-of-detection with conventional electrodes. In addition, we explore the possibility of using NEA sensors to detect halogens in the gas phase. By manipulating the NEA structure, the monitoring capability may be extended to gas phase detection of Cl2 or I2. Such gas-phase detection is possible by utilizing the natural humidity in air condensing on the nanopore walls above the recessed working electrodes. Indirect sensing of such gases occurs after they dissolve in the pore water to form electrochemically detectable halogen species. Notably, the electrochemical measurement is achieved with simple, condensed humidity as the sole ionically conducting phase, without the need for a supporting electrolyte. This is because the high resistivity of water† is mitigated by the short ionic path length: the net solution resistance is small because of the sub-micron distance separating the NEA electrodes from the counter/pseudo-reference electrode sputtered upon the surface of the opposite (open) end of the pore. †The conductivity of humidity levels we consider in this study range from 10-6 S/m (humidity from DI water) to 10-3-10-4 S/m (conductivity range of natural humidity across the USA as monitored by the National Atmospheric Deposition Program’s National Trend Network). This work is supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

We study the feasibility of nanoporous-carbon (NPC) as an electrically conductive host material for reversible Li-ion energy storage. The efficiency of energy storage devices based on intercalation of Li-ions highly depends on the interstitial spacing of the host material, which can significantly limit the diffusion of ions. NPC offers with controlled mass density and surface area considerable advantages over other carbon allotropes. For example, the internal structure of NPC, which self-assembles during growth and consists of nano-fragments of aligned graphene sheet stacks, exhibits as much as a 55% larger interplanar spacing compared to crystalline graphite. Hence, NPC porosity is well controlled via its mass density as a result of defined process parameters, allowing for the NPC host material to accommodate efficient diffusion, as recently demonstrated for the reversible intercalation of Mg-ions. To demonstrate viability as a host material for Li-ion intercalation, we grow NPC directly on 16 mm diameter stainless-steel disks for study as coin cell anodes. The NPC mass density is chosen to range from 0.8 – 1.7 g/cm3 for a series of coin cells which are electrochemically characterized. By varying the mass densities, a correlation between interplanar spacing and electrochemical response is established, revealing the most favorable NPC structure for rapid Li-ion intercalation. The correlating surface area of the most efficient NPC configurations is obtained through Brunauer–Emmett–Teller measurements as a function of nitrogen adsorption in the NPC framework. Moreover, the various NPC structures are investigated via transmission electron microscopy to verify successful diffusion of Li-ions into the NPC host material. NPC represents a new approach for an efficient host material in Li-ion intercalation based energy storage devices with potentially faster diffusion rates. Using NPC as a host material may also enable implementing other atomic species with larger theoretical Li-storage capacity than graphitic carbons, such as Si. Si suffers from drastic volumetric expansion during cycling which leads to fast degradation as a result of embrittlement of the Si bulk, thus limiting its employment as a battery anode. The high porosity of NPC, however, may allow embedding Si atoms or clusters into an NPC host material. We will report on the ability to embed Si into NPC without collapsing the low mass density structure. The decoration of NPC structures with Si offers obvious advantages to crystalline graphite and Si composite architectures for battery anodes, as both the large Li-storage capacity of Si and rapid ionic intercalation into NPC can be made use of in one combined architecture. This work is supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

The room temperature electronic transport properties of 100 nm-thick thermoelectric Bi0.8Sb0.2 films, sputter-deposited onto quartz substrates and post-annealed in an ex-situ furnace, systematically correlate with the overall microstructural quality, improving with increasing annealing temperature until close to the melting point for the alloy composition. The optimized films have high crystalline quality with ∼99% of the grains oriented with the trigonal axis perpendicular to the substrate surface. Film resistivities and Seebeck coefficients are accurately measured by preventing deleterious surface oxide formation via a SiN capping layer and using Nd-doped Al for contacts. The resulting values are similar to single crystals and significantly better than previous reports from films and polycrystalline bulk alloys.

The inability to rapidly grow thick or multilayer epitaxial films for either electronic or electrical power applications limits the utility of fluoride-based processes for YBa2Cu3O7−δ (YBCO). This problem is due to the use of water vapor in the growth process, necessary for the dissociation of metallofluorides. Flowing wet gas at low temperatures is corrosive to cuprates and responsible for destroying underlying YBCO layers in attempts to grow secondary layers. This is avoided simply by increasing the temperature where vapor is introduced into the growth ambient. Resulting two-layer films of YBCO have properties similar to those of high critical current density single-layer films.

YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// films have been prepared by spin-coating solutions of metal acetates dissolved in trifluoroacetic acid/methanol and trifluoroacetic acid/isopropanol/1,3 propanediol onto LaAlO/sub 3/[100] substrates. By using a rapid, low pO/sub 2/ pyrolysis process, high-quality diol films (0.25 /spl mu/m thick) with J/sub c/ values as high as 14 MA/cm/sup 2/ and 2 MA/cm/sup 2/ at 7 K and 77 K, respectively, were fabricated in one tenth the time, compared to conventional processing schemes. The effect of multicoating, in order to reach YBCO film thicknesses of 1.5 /spl mu/m, on the J/sub c/ was also studied.

In this investigation YBa2Cu3O7-δ (YBCO) films were fabricated via a metal acetate, trifluoroacetic acid based sol-gel route and spin-coat deposited on (100) LaAlO3 with a focus on maximizing Jc, while minimizing processing time. We demonstrate that the use of a low-pO2 atmosphere during the pyrolysis stage can lead to at least a tenfold reduction in pyrolysis time, compared to a 1 atm O2 ambient. High-quality YBCO films on LaAlO3, with Jc values up to 3 MA/cm2 at 77 K, can be routinely crystallized from these rapidly pyrolyzed films.

We report the stability of TlBa{sub 2}CaCu{sub 2}O{sub 7} (Tl-1212) and Tl{sub 2}Ba{sub 2}CaCu{sub 2}O{sub 8} (T1-2212) thin films and by inference, the stability of TlBa{sub 2}Ca{sub 2}Cu{sub 3}O{sub 9} (Tl-1223) and Tl{sub 2}Ba{sub 2}Ca{sub 2}Cu{sub 3}O{sub 10} (Tl-2223) thin films, under a variety of conditions. In general, we observe that the stability behavior of the single Tl-O layer materials (Tl-1212 and Tl-1223)are similar and the double Tl-O layer materials (Tl-2212 and Tl-2223) are similar. All films are stable with repeated thermal cycling to cryogenic temperatures. Films are also stable in acetone and methanol. Moisture degrades film quality rapidly, especially in the form of vapor. Tl-1212 is more sensitive to vapor than Tl-2212. These materials are stable to high temperatures in either N{sub 2}, similar to vacuum for the cuprates, and O{sub 2} ambients. While total degradation of properties (superconducting and structural) occur at the same temperatures for all phases, 600 C in N{sub 2} and 700 C in O{sub 2}, the onset of degradation occurs at somewhat lower temperatures for Tl-1212 than for Tl-2212 films. In all cases, sample degradation is associated with Tl depletion from the films.

Epitaxial thin films of the TI cuprate superconductors are studied with x-ray photoemission spectroscopy.

XPS measurements on epitaxial thin films of the Tl cuprate superconductors Tl2Ba2CaCu2O8, Tl2Ba2Ca2Cu3O10, and Tl0.78Bi0.22Ba0.4Sr1.6Ca2Cu3O9−δ are presented. These data, together with previous measurements in this lab on Tl2Ba2CuO6−δ and TlBa2CaCu2O7−δ, comprise a comprehensive data set for comparison of Tl cuprates in which the number of Tl–O and Cu–O layers, and hence the chemical and electronic properties, vary.

Tl-2212 and Tl-1212 are the two easiest unsubstituted Tl-Ba-Ca-Cu-O superconducting thin films to grow. Both phases exhibit T/sub c//spl ges/95 K, J/sub c/(5 K)>10 MA/cm/sup 2/ and smooth morphologies. We report on the properties of both materials following one hour furnace anneals in dry ambients of pure oxygen or nitrogen at temperatures ranging from 50-700/spl deg/C. The stability of these films to high temperatures in each ambient is important for device processing. Of equal interest is the controlled partial volatilization of Tl-oxide at modest temperatures from nano-site regions within Tl-1212 films. We correlate such modifications with increases in flux pinning strength in Tl-1212 films. Tl-2212 films maintain their superconducting properties to even higher temperatures than Tl-1212 films. Optimum annealing appears to also increase flux pinning, although not to the same degree as for Tl-1212 films.

Epitaxial Tl2Ba2CaCu2O8 (Tl-2212) films ∼5500 Å thick are grown on LaAlO3(100) substrates using a new hybrid two-zone/crucible furnace process enabling precise control of thallination. This method combines the best features of both conventional crucible and two-zone processing for the first time: superb film properties and reduced handling of hazardous Tl-oxide powders. Single-phase, highly caxis oriented Tl-2212 films are grown with smooth morphology, Meissner transition ∼103 K, and critical current density ∼1.1 × 107 A/cm2 at 5 K for twenty consecutive runs without having to change or add to the Tl-oxide source.

We discuss two methods tor the ex-situ furnace growth of pure TIBaCaCuO superconducting thin films on LaAlO3(100) substrates. The traditional approach of “crucible processing” is used to grow high-quality films of several of the TIBaCaCuO phases. “Two-zone processing” promises greater thermodynamic control, yet film properties from optimized films are not as good as those grown from crucibles. The transport kinetics of Tl-oxide vapor from a source to the film surface appears to be a dominant factor. The best results are from single-phase Tl-1212 and Tl-2212 thin films. Tc's are high and sharp, Jc's > 1 × 107 A/cm2 at 5 K, and surface morphologies are relatively smooth.

We review the synthesis methods and properties of single crystal, powder, and thin film TlBaCaCuO high-temperature superconducting (Tl-HTS) materials. With transition temperatures ≥ 100 K for several compounds, Tl-HTS materials present real opportunities for applications above 77 K. Experiments using (1) single crystals: determined precise structural parameters and identified the complex Tl1+–Tl3+ equilibrium model; (2) powders: studied the complex thermodynamic phase diagram; and (3) epitaxial films: studied fundamental properties such as electron pair symmetry and the effect of controlled extrinsic defects on flux pinning strength, as well as providing the large-area surfaces required for device applications.

Furnace anneals of TlBa/sub 2/CaCu/sub 2/O/sub x/ thin films at temperatures above 500/spl deg/C cause partial TlO/sub x/ loss. High resolution transmission electron microscopy images reveal nanometer-scale discontinuities (pinched stacking faults) in the microstructure of annealed films. Significant increases in the vortex pinning potential and critical current density at elevated temperatures in strong magnetic fields are observed and are attributed to the presence of these localized defects.

Microstructural changes in TlBa/sub 2/CaCu/sub 2/O/sub 7-/spl part// (Tl-1212) epitaxial thin films resulting from low oxygen partial pressure furnace anneals at 600/spl deg/C are studied using high resolution transmission electron microscopy (TEM). These post-growth anneals have been shown to significantly raise the superconducting transition temperature from 70 to 90 K, and greatly improve the magnetic flux pinning and the critical current density. Changes occur in both the microstructure and the morphology of the films that correlate with changes in J/sub c/. Plan view TEM and high-resolution cross-sectional TEM analysis of the films before and after anneals demonstrates changes in the lattice fringe image, overall contrast modulation, and nanometer-scale discontinuities.

The importance of near-equilibrium thermodynamics in the growth of TlBaCaCuO materials is well-documented; i.e. the independent control of sample temperature, Tl-oxide partial pressure, and oxygen partial pressure. Here we demonstrate how this is accomplished using a simple version of the crucible process. In addition, we report the relevance of growth kinetics. Superconducting thin films are grown in a two-step process. First, /spl sim/2000 /spl Aring/ thick, amorphous Ba/sub 2/CaCu/sub 2/O/sub x/ and Ba/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ precursors are sputter-deposited onto LaAlO/sub 3/(100) substrates. Films are then thallinated and annealed using the crucible-pellet process. With this method, it is possible to grow essentially single-phase Tl-1212, Tl-1223, Tl-2212, and Tl-2223 thin films. Under similar thermodynamic conditions, we find that different phases dominate the film growth as a function of reaction time. For example, the Tl-x212 phases generally nucleate first, and then, depending on growth conditions, convert to the Tl-1223 or Tl-2223 phase with extended annealing times. We demonstrate that knowledge of both the thermodynamics and the kinetics of growth can yield high-quality films of each of the major Tl-superconducting phases.

The core levels of ${\mathrm{TlBa}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (Tl-1212) epitaxial films have been measured with x-ray photoelectron spectroscopy (XPS). The valence electronic structure has been determined using the full-potential linear muffin-tin-orbital band-structure method and measured with XPS. The calculations show that a van Hove singularity (VHS) lies above the Fermi level (${\mathrm{E}}_{\mathrm{F}}$) for the stoichiometric compound (\ensuremath{\delta}=0), while for 50% oxygen vacancies in the Tl-O layer (\ensuremath{\delta}=0.5) ${\mathrm{E}}_{\mathrm{F}}$ is in close proximity to the VHS. Samples annealed in nitrogen (to reduce the hole overdoping by the removal of oxygen) exhibit higher core-level binding energies and a higher ${\mathrm{T}}_{\mathrm{c}}$, consistent with a shift of ${\mathrm{E}}_{\mathrm{F}}$ closer to the VHS. Comparisons are made to the core levels and valence bands of ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ (Tl-2212) and ${\mathrm{HgBa}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{6+\mathrm{\ensuremath{\delta}}}$ (Hg-1212). The similarity of the Cu ${2\mathrm{p}}_{3\mathrm{/}2}$ spectra for Tl-1212 and Tl-2212 indicates that the number of Tl-O layers has little effect on the Cu-O bonding. However, the Tl-1212 and Hg-1212 Cu ${2\mathrm{p}}_{3\mathrm{/}2}$ signals exhibit differences which suggest that the replacement of ${\mathrm{Tl}}^{3+}$ with ${\mathrm{Hg}}^{2+}$ results in a decrease in the O 2p\ensuremath{\rightarrow}Cu 3d charge-transfer energy and differences in the probabilities of planar vs apical oxygen charge transfer and/or Zhang-Rice singlet-state formation. Differences between the Tl-1212 and the Tl-2212 and Hg-1212 measured valence bands are consistent with the calculated Cu 3d and (Tl,Hg) 6s/5d partial densities of states.

Electron field emission from two amorphous, tetrahedrally bonded diamondlike carbon films, one with (a‐tC:N), and a second without nitrogen doping (a‐tC), prepared by pulsed laser deposition has been investigated using a scanning probe apparatus with micrometer spatial resolution. Electric fields of 100 V/μm (180 V/μm) were required to initiate emission from our a‐tC:N (a‐tC) films; however, once emission was established at a particular location, electrons could be drawn at average fields as low as 10 V/μm (60 V/μm) from the same region. The initiation of emission was concomitant with electrical discharges which were observed by video techniques. These discharges left craters with micrometer dimensions on the surfaces of otherwise smooth films.

We present two complementary methods for controlling the site density of multiwall carbon nanotubes (CNTs) directly as a function of growth conditions from 1011to107CNTs∕cm2. Several potential applications require significant spacing between individual CNTs. The first method shows that the site density varies with the heat of formation of the hydrocarbon gas used during CNT growth by thermal chemical vapor deposition. The second method demonstrates that the site density decreases with increasing residual stress of the metal catalyst∕diffusion barrier layers. These methods are combined for wide-range control of CNT site density.