Acknowledged

• ARL

Abstract
We study macroscopic aspects of crystal surface relaxation in 2+1 dimensions by accounting for near-equilibrium kinetics of transparent steps at the nanoscale. For slowly varying step geometries, we show that step permeability can simply {\it renormalize} a parameter in a known relation between the large-scale surface flux and the step chemical potential. This leads to a nonlinear partial differential equation for the surface height profile.

Acknowledged

• MRSEC

Abstract
Label-free deoxyribonucleic acid (DNA) hybridization detection using carbon nanotube transistor (CNT) arrays is demonstrated. The present scheme is distinguished from other CNT sensing methods as it uses a gate oxide overlayer on top of the carbon nanotubes, which function solely as charge sensors but are not participants in the chemical binding process. Because it involves DNA probe attachment on the gate oxide rather than on the CNT, this approach allows the use of conventional DNA functionalization and bioassay protocols, and is less prone to false positives. The signal sought is a few tens of millivolts in threshold voltage shift due to the increase of surface charges after target hybridization. The hybridization detection is shown to be highly specific and sensitive to a minimum concentration of about 30 nM of 61-mer DNA. Despite differences in the transistor properties due to the spread in the CNT parameters during fabrication, the yields are very high and the sensing characteristics are uniformly consistent in nearly all transistors.

Acknowledged

• NIH

Abstract
In conventional photolithography, diffraction limits the resolution to about one-quarter of the wavelength of the light used. We introduce an approach to photolithography in which multiphoton absorption of pulsed 800-nanometer (nm) light is used to initiate cross-linking in a polymer photoresist and one-photon absorption of continuous-wave 800-nm light is used simultaneously to deactivate the photopolymerization. By employing spatial phase-shaping of the deactivation beam, we demonstrate the fabrication of features with scalable resolution along the beam axis, down to a 40-nm minimum feature size. We anticipate application of this technique for the fabrication of diverse two- and three-dimensional structures with a feature size that is a small fraction of the wavelength of the light employed.

Acknowledged

• NispLab
• MRSEC

Abstract
We study the continuum limit in 2+1 dimensions of nanoscale anisotropic diffusion processes on crystal surfaces relaxing to become flat below roughening. Our main result is a continuum law for the surface flux in terms of a new continuum-scale tensor mobility. The starting point is the Burton, Cabrera and Frank (BCF) theory, which offers a discrete scheme for atomic steps whose motion drives surface evolution. Our derivation is based on the separation of local space variables into fast and slow. The model includes: (i) anisotropic diffusion of adsorbed atoms (adatoms) on terraces separating steps; (ii) diffusion of atoms along step edges; and (iii) attachment-detachment of atoms at step edges. We derive a parabolic fourth-order nonlinear partial differential equation (PDE) for the continuum surface height profile. An ingredient of this PDE is the surface mobility for the adatom flux, which is a nontrivial extension of the tensor mobility for isotropic terrace diffusion derived previously by Margetis and Kohn (2006 Multisc. Model. Simul. 5 729--758). Approximate, separable solutions of the PDE are discussed.

Abstract
Starting from a detailed model for the kinetics of a step edge or island boundary, we derive a Gibbs-Thomson type formula and the associated step stiffness as a function of the step edge orientation angle, $\theta$. Basic ingredients of the model are: (i) the diffusion of point defects (``adatoms'') on terraces and along step edges; (ii) the convection of kinks along step edges; and (iii) constitutive laws that relate adatom fluxes, sources for kinks, and the kink velocity with densities via a mean-field approach. This model has a kinetic (nonequilibrium) steady-state solution that corresponds to epitaxial growth through step flow. The step stiffness, $\tbe(\theta)$, is determined via perturbations of the kinetic steady state for small edge Peclet number, $P$, which is the ratio of the deposition to the diffusive flux along a step edge. In particular, $\tbe$ is found to satisfy $\tbe =O(\theta^{-1})$ for $O(P^{1/3}) <\theta \ll 1$, which is in agreement with independent, equilibrium-based calculations.

Acknowledged

• MRSEC

Abstract
We study the evolution of a many-particle system whose wave function obeys the N-body Schroedinger equation under Bose symmetry. The system Hamiltonian describes pairwise particle interactions in the absence of an external potential. We derive apriori dispersive estimates that express the overall repulsive nature of the particle interactions. These estimates hold for a wide class of two-body interaction potentials which are independent of the particle number, N. We discuss applications of these estimates to the BBGKY hierarchy for reduced density matrices analyzed by Elgart, Erdos, Schlein and Yau.

Acknowledged

• NispLab

Abstract
We employ scanning probe microscopy to reveal atomic structures and nanoscale morphology of graphene-based electronic devices (i.e., a graphene sheet supported by an insulating silicon dioxide substrate) for the first time. Atomic resolution scanning tunneling microscopy images reveal the presence of a strong spatially dependent perturbation, which breaks the hexagonal lattice symmetry of the graphitic lattice. Structural corrugations of the graphene sheet partially conform to the underlying silicon oxide substrate. These effects are obscured or modified on graphene devices processed with normal lithographic methods, as they are covered with a layer of photoresist residue. We enable our experiments by a novel cleaning process to produce atomically clean graphene sheets.

Acknowledged

• MRSEC
• ONR, NSF, IC Postdoc

Acknowledged

• ARL

Abstract
We have studied bulk vortex properties in Bi2Sr2CaCu2O8+d sBSCCOd using transport measurements with a unique Corbino disk contact geometry. This Corbino disk contact geometry allows us to measure true bulk vortex properties free from surface barrier effects. The investigated vortex properties include current-induced vortex dissipation in the vortex liquid and vortex solid phases, vortex matter phase transition svortex lattice melting transitiond, and vortex correlation along the c axis of BSCCO. No discrepancy is found between our measurements and others measurements using the conventional four-probe contact geometry, which renders vortex transport properties susceptible to the Bean-Livingston surface barrier effect. We conclude that the sample bulk vortex matter properties determine the vortex transport behaviors in BSCCO.

Acknowledged

• NSF, DOE

Abstract
Air-stable single-source molecular precursor is applied for controlled size and morphology synthesis of one-dimensional and quasi-one-dimensional CdSe nanostructures. Two different growth approaches are compared to control the growth of nanostructures. When combined with well-defined Au colloidal catalysts, the use of single-source molecular precursor allows diameter control synthesis of monodispersed CdSe nanowires from 10?30 nm via a vapor?liquid?solid mechanism. In addition, a variety of CdSe nanostructures with different morphologies can be achieved and tuned without assistance of metallic catalysts by carefully manipulating dynamic thermal decomposition process of single-source molecular precursor. The new level of synthetic control afforded by our present work opens up new opportunities for using as-synthesized CdSe nanostructures as model systems for fundamental studies as well as building blocks for larger scale functional device assembly. Importantly, we demonstrate that a single CdSe tripod can be natively configured as a nanoscale phototransistor in which photocurrent created between two tripod arms can be efficiently modulated by applying a gate voltage through the third arm.

Acknowledged

• NispLab

Abstract
The magnetic-field-dependent longitudinal and Hall components of the resistivity
xx
H and
xy
H are measured in graphene on silicon dioxide substrates at temperatures 1.6 KT300 K. At charge densities near the minimum conductivity point
xx
H is strongly enhanced and
xy
H is suppressed, indicating nearly equal electron and hole contributions to the current. The data are inconsistent with the standard two-fluid model but consistent with the prediction for inhomogeneously distributed electron and hole regions of equal mobility. At low T and high H,
xx
H saturates to a value h/e2, with Hall conductivity
e2 /h, which may indicate a regime of localized v=2 and v=−2 quantum Hall puddles.

Acknowledged

• MRSEC
• ONR, NSF

Abstract
Since the initial demonstration of the ability to experimentally isolate a single graphene sheet1, a great deal of theoretical work has focused on explaining graphene's unusual carrier-density-dependent conductivity sigma(n), and its minimum value (sigmamin) of nearly twice the quantum unit of conductance (4e2/h) (refs 1, 2, 3, 4, 5, 6). Potential explanations for such behaviour include short-range disorder7, 8, 9, 10, 'ripples' in graphene's atomic structure11, 12 and the presence of charged impurities7, 8, 13, 14, 15, 16, 17, 18. Here, we conduct a systematic study of the last of these mechanisms, by monitoring changes in electronic characteristics of initially clean graphene19 as the density of charged impurities (nimp) is increased by depositing potassium atoms onto its surface in ultrahigh vacuum. At non-zero carrier density, charged-impurity scattering produces the widely observed linear dependence1, 2, 3, 4, 5, 6 of sigma(n). More significantly, we find that sigmamin occurs not at the carrier density that neutralizes nimp, but rather the carrier density at which the average impurity potential is zero15. As nimp increases, sigmamin initially falls to a minimum value near 4e2/h. This indicates that sigmamin in the present experimental samples1, 2, 3, 4, 5, 6 is governed not by the physics of the Dirac point singularity20, 21, but rather by carrier-density inhomogeneities induced by the potential of charged impurities6, 8, 14, 15.

Acknowledged

• MRSEC
• ONR, NSF, IC Postdoc

Acknowledged

• NispLab

Abstract
We report facile in situ biomolecule assembly at readily addressable sites in microfluidic channels after complete fabrication and packaging of the microfluidic device. Aminopolysaccharide chitosan's pH responsive and chemically reactive properties allow electric signal-guided biomolecule assembly onto conductive inorganic surfaces from the aqueous environment, preserving the activity of the biomolecules. A transparent and nonpermanently packaged device allows consistently leak-free sealing, simple in situ and ex situ examination of the assembly procedures, fluidic input/outputs for transport of aqueous solutions, and electrical ports to guide the assembly onto the patterned gold electrode sites within the channel. Both in situ fluorescence and ex situ profilometer results confirm chitosan-mediated in situ biomolecule assembly, demonstrating a simple approach to direct the assembly of biological components into a completely fabricated device. We believe that this strategy holds significant potential as a simple and generic biomolecule assembly approach for future applications in complex biomolecular or biosensing analyses as well as in sophisticated microfluidic networks as anticipated for future lab-on-a-chip devices.

Acknowledged

• MRSEC

Acknowledged

• ARL

Abstract
The morphological relaxation of faceted crystal surfaces is studied via a continuum approach. Our formulation includes: (i) an evolution equation for the surface slope that describes step line tension, $g_1$, and step repulsion energy, $g_3$; and (ii) a condition at the facet edge (a free boundary) that accounts for discrete effects via the collapse times, $t_n$, of top steps. For initial cones and $t_n\approx \tilde t n^4$, we use $\tilde t(g)$ from step simulations and predict self-similar slopes in agreement with simulations for any $g=g_3/g_1>0$. We show that for $g\gg 1$: (i) the theory simplifies to an equilibrium-thermodynamics model; (ii) the slope profiles reduce to a universal curve; and (iii) the facet radius scales as $g^{-3/4}$.

Acknowledged

• NSF-DMR

Abstract
Biological microelectromechanical systems (bioMEMS) provide an attractive approach to understanding and modifying enzymatic pathways by separating and interrogating individual reaction steps at localized sites in a microfluidic network. We have previously shown that electrodeposited chitosan enables immobilization of an enzyme at a specific site while maintaining its catalytic activity. While promising as a methodology to replicate metabolic pathways and search for inhibitors as drug candidates, these investigations also revealed unintended (or parasitic) effects, including products generated by the enzyme either (1) in the homogeneous phase (in the liquid), or (2) nonspecifically bound to microchannel surfaces. Here we report on bioMEMS designs which significantly suppress these parasitic effects. To reduce homogeneous reactions we have developed a new packaging and assembly strategy which eliminates fluid reservoirs that are commonly used for fluidic interconnects with external tubing. To suppress reactions by nonspecifically bound enzyme on microchannel walls we have implemented a cross-flow microfluidic network design so that enzyme flow for assembly and substrate/product for reaction share only the region where the enzyme is immobilized at the intended reaction site. Our results show that the signal-to-background ratio of sequential enzymatic reactions increases from 0.72 to 1.28 by eliminating the packaging reservoirs, and increases to 2.43 by separating the flow direction of enzymatic reaction from that of enzyme assembly step. These techniques can be easily applied to versatile microfluidic devices to minimize parasitic reactions in sequential biochemical reactions.

Acknowledged

• FabLab
• Deutsch Foundation, NSF-EFRI

Acknowledged

• FabLab

Acknowledged

• NispLab

Abstract
Recently, significant interest has emerged in fabricated systems that mimic the behavior of geometrically frustrated materials. We present the full realization of such an artificial spin ice system on a two-dimensional kagome lattice and we demonstrate rigid adherence to the local ice rule by directly counting individual pseudospins. The resulting spin configurations show not only local ice rules and long-range disorder, but also correlations consistent with spin ice Monte Carlo calculations. Our results suggest that dipolar corrections are significant in this system, as in pyrochlore spin ice, and that they open a door to further studies of frustration in general.

Acknowledged

• NispLab

Acknowledged

• FabLab
• ARL, NSF

Abstract
Low-dimensional boundaries between phases and domains in organic thin films are important in charge transport and recombination. Here, fluctuations of interfacial boundaries in an organic thin film, acridine-9-carboxylic acid (ACA) on Ag(111), have been visualized in real time, and measured quantitatively, using Scanning Tunneling Microscopy. The boundaries fluctuate via molecular exchange with exchange time constants of 10-30 ms at room temperature, yielding length mode fluctuations that should yield characteristic f-1/2 signatures for frequencies less than ~100 Hz. Although ACA has highly anisotropic intermolecular interactions, it forms islands that are compact in shape with crystallographically distinct boundaries that have essentially identical thermodynamic and kinetic properties. The physical basis of the modified symmetry is shown to arise from significantly different substrate interactions induced by alternating orientations of successive molecules in the condensed phase. Incorporating this additional set of interactions in a lattice gas model leads to effective multi-component behavior, as in the Blume-Emery-Griffiths (BEG) model, and can straightforwardly reproduce the experimentally observed isotropic behavior. The general multi-component description allows the domain shapes and boundary fluctuations to be tuned from isotropic to highly anisotropic in terms of the balance between intermolecular interactions and moleculesubstrate interactions.

Acknowledged

• NanoCenter as Infrastructure

Abstract
Mechanisms for the formation of crust on planetary bodies remain poorly understood. It is generally accepted that Earth's andesitic continental crust is the product of plate tectonics, whereas the Moon acquired its feldspar-rich crust by way of plagioclase flotation in a magma ocean. Basaltic meteorites provide evidence that, like the terrestrial planets, some asteroids generated crust and underwent large-scale differentiation processes. Until now, however, no evolved felsic asteroidal crust has been sampled or observed. Here we report age and compositional data for the newly discovered, paired and differentiated meteorites Graves Nunatak (GRA) 06128 and GRA 06129. These meteorites are feldspar-rich, with andesite bulk compositions. Their age of 4.52 0.06 Gyr demonstrates formation early in Solar System history. The isotopic and elemental compositions, degree of metamorphic re-equilibration and sulphide-rich nature of the meteorites are most consistent with an origin as partial melts from a volatile-rich, oxidized asteroid. GRA 06128 and 06129 are the result of a newly recognized style of evolved crust formation, bearing witness to incomplete differentiation of their parent asteroid and to previously unrecognized diversity of early-formed materials in the Solar System.

Abstract
Pentacenequinone (PnQ) impurities have been introduced into a pentacene source material in a controlled manner to quantify the relative effects of the impurity content on the grain boundary structure and thin film nucleation. Atomic force microscopy has been employed to directly characterize by weight films grown by using 0.0%–7.5% PnQ in the source material. Analysis of the distribution of capture zone areas of submonolayer islands as a function of impurity content shows that for a large PnQ content, the critical nucleus size for forming a Pn island is smaller than for a low PnQ content. This result indicates a favorable energy for the formation of Pn-PnQ complexes, which, in turn, suggests that the primary effect of PnQ on Pn mobility may arise from the homogeneous distribution of PnQ defects.

Abstract
Charge transport in semiconducting single-walled nanotubes (SWNTs) with Schottky-barrier contacts has been studied at high bias. We observe nearly symmetric ambipolar transport with electron and hole currents significantly exceeding 25
A, the reported current limit in metallic SWNTs due to optical phonon emission. Four simple models for the field-dependent velocity (ballistic, current saturation, velocity saturation, and constant mobility) are studied in the unipolar regime; the high-bias behavior is best explained by a velocity-saturation model with a saturation velocity of 2
107 cm=s.

Acknowledged

• NSF

Abstract
We present real-time, nanoscale temperature mapping using a transmission electron microscope and standard phase transitions in metal islands. Islands are deposited on the reverse side of commercially available silicon nitride membranes, while local thermal gradients are produced by Joule heating in a thin wire on the front side of the membrane. Change in contrast due to the liquid?solid transition in the islands allows the mapping of absolute temperature, as above or below the transition temperature, over the entire field-of-view. Experiments demonstrate nanoscale (<100 nm) resolution and video-rate (>30 thermal-images per second) speed, supported by combined electrical and thermal modeling. This provides a generic and adaptable platform for nanoscale thermal characterization independent of strong probe coupling and optical effects.

Acknowledged

• NispLab

Acknowledged

• NSF

Abstract
The surface of a nanostructure relaxing on a substrate consists of a finite number of interacting steps, and often involves the expansion of facets. Prior theoretical studies of facet evolution have focused on models with an infinite number of steps which neglect edge effects caused by the presence of the substrate. We show that these edge or finite height effects play an important role in the structure's macroscopic evolution under the assumption of diffusion limited kinetics and a homoepitaxial substrate. Specifically, using data from step simulations and a continuum theory, we demonstrate a switch in the time behavior of the facet position when finite height effects become significant. Our analysis and numerical simulations focus on two model systems where steps repel each other through entropic and elastic dipolar interactions. The first model is a vicinal surface consisting of a finite number of straight steps. The second model is an axisymmetric structure consisting of a finite number of circular steps; in this case, we also include curvature effects which cause the steps to collapse under the effect of line tension. In the first case, we show that the facet expansion switches from $O(t^{1/4})$ behavior to $O(t^{1/5})$ (where $t$ is time) and in the second, the behavior switches from $O(t^{1/4})$ to $O(t)$. For the axisymmetric case, we also predict analytically (through a continuum equation) how the individual collapse times are modified by the effects of finite height under the assumption that step interactions are weak compared to the step line tension.

Acknowledged

• MRSEC

Abstract
A simple scalable scheme is reported for fabricating suspended carbon nanotube field effect transistors (CNT-FETs) without exposing pristine as-grown carbon nanotubes to subsequent chemical processing. Versatility and ease of the technique is demonstrated by controlling the density of suspended nanotubes and reproducing devices multiple times on the same electrode set. Suspending the carbon nanotubes results in ambipolar transport behavior with negligible hysteresis. The Hooge's constant of the suspended CNT-FETs (2.6×10-3) is about 20 times lower than for control CNT-FETs on SiO2 (5.6×10-2).

Abstract
We investigate the connections between two field-enhanced phenomena of gold nanoparticles: multiphoton-absorption-induced luminescence (MAIL) and metal-enhanced multiphoton absorption polymerization (MEMAP). We observe a strong correlation between the nanoparticles and aggregates that have high efficiency for each process. The results of our studies indicate that for this system, MEMAP is driven not by field-enhanced two-photon absorption of the photoinitiator but rather by single-photon excitation of the photoinitiator by the MAIL emission.

Acknowledged

• NispLab
• MRSEC

Abstract
We have studied the resistance of single-wall carbon nanotubes measured in a four-point configuration with noninvasive voltage electrodes. The voltage drop is detected using multiwalled carbon nanotubes while the current is injected through nanofabricated Au electrodes. The resistance at room temperature is shown to be linear with the length as expected for a classical resistor. This changes at cryogenic temperature; the four-point resistance then depends on the resistance at the Au-tube interfaces and can even become negative due to quantum-interference effects.

Acknowledged

• NSF

Abstract
Kilauea Iki lava lake formed during the 1959 summit eruption of Kilauea Volcano, then crystallized and differentiated over a period of 35 years. It offers an opportunity to evaluate the fractionation behavior of trace elements in a uniquely well-documented basaltic system. A suite of 14 core samples recovered from 1967 to 1981 has been analyzed for 5 platinum-group elements (PGE: Ir, Os, Ru, Pt, Pd), plus Re. These samples have MgO ranging from 2.4 to 26.9 wt.%, with temperatures prior to quench ranging from 1140 °C to ambient (110 °C). Five eruption samples were also analyzed. Osmium and Ru concentrations vary by nearly four orders of magnitude (0.0006–1.40 ppb for Os and 0.0006–2.01 ppb for Ru) and are positively correlated with MgO content. These elements behaved compatibly during crystallization, mostly likely being concentrated in trace phases (alloy or sulfide) present in olivine phenocrysts or included chromite. Iridium also correlates positively with MgO, although less strongly than Os and Ru. The somewhat poorer correlation for Ir, compared with Os and Ru, may reflect variable loss of Ir as volatile IrF6 in some of the most magnesian samples. Rhenium is negatively correlated with MgO, behaving as an incompatible trace element. Its behavior in the lava lake is complicated by apparent volatile loss of Re, as suggested by a decrease in Re concentration with time of quenching for lake samples vs. eruption samples. Platinum and Pd concentrations are negatively, albeit weakly, correlated with MgO, so these elements were modestly incompatible during crystallization of the major silicate phases. Palladium contents peaked before precipitation of immiscible sulfide liquid, however, and decline sharply in the most differentiated samples. In contrast, Pt appears to have been unaffected by sulfide precipitation. Microprobe data confirm that Pd entered the sulfide liquid before Re, and that Pt is not strongly chalcophile in this system. Occasional high Pt values in both eruption and lava lake samples suggest the presence of unevenly distributed, unidentified Pt-rich trace phases in some Kilauea Iki materials. Estimated mineral (olivine + chromite)/melt D values for Os, Ir, Ru and Pt for equilibrium crystallization for samples from ~ 7 to 27 wt.% MgO are 26, 8.2, 19 and 0.55, respectively. These Os, Ir and Ru estimates are somewhat higher than previous estimates for similar systems. If fractional crystallization is instead assumed, D values are much more similar. Results confirm many prior observations in other mafic systems that olivine (together with included phases) has a major effect on absolute and relative abundances of Re and the PGE. The relatively linear correlations between these elements and MgO potentially permit accurate estimation of the concentrations of these elements in the primary melts of comparable systems, especially in instances where the MgO content of the primary melt is well constrained.

Acknowledged

• NispLab

Abstract
The authors perform nonlocal four-probe spin-valve experiments on graphene contacted by ferromagnetic Permalloy electrodes. They observe sharp switching and often sign reversal of the nonlocal resistance at the coercive field of the electrodes, indicating the presence of a spin current between injector and detector. The nonlocal spin-valve signal changes magnitude and sign with back-gate voltage, and is observed up to T=300 K. The gate voltage variation of the spin-valve signal may result from quantum-coherent transport, as evidenced by Fabry-Pérot-like oscillations of the current.

Acknowledged

• MRSEC
• ONR, NSF

Abstract
A robust and low dead volume world-to-chip interface for thermoplastic microfluidics has been developed. The high pressure fluidic port employs a stainless steel needle inserted into a mating hole aligned to an embedded microchannel, with an interference fit used to increase pressure resistance. Alternately, a self-tapping threaded needle screwed into a mating hole is also demonstrated. In both cases, the flat bottom needle ports seat directly against the microchannel substrate, ensuring low interfacial dead volumes. Low dispersion is observed for dye bands passing the interfaces. The needle ports offer sufficient pull-out forces for applications such as liquid chromatography that require high internal fluid pressures, with the epoxy-free interfaces compatible with internal microchannel pressures above 40 MPa.

Abstract
The 1/ f noise in individual semiconducting carbon nanotubes
s-CNT in a field effect transistor configuration has been measured in ultrahigh vacuum and following exposure to air. The amplitude of the normalized current spectral noise density is independent of source-drain current and inversely proportional to gate voltage, to channel length, and therefore to carrier number, indicating that the noise is due to mobility rather than number fluctuations. Hooge’s constant for s-CNT is found to be
9.3±0.4
10−3 The magnitude of the 1/ f noise is substantially decreased by exposing the devices to air.

Acknowledged

• MRSEC
• NSF, IC Postdoc

Abstract
Models of quantum computing rely on transformations of the states of a quantum memory. We study mathematical aspects of a model proposed by Wu in which the memory state is changed via the scattering of incoming particles. This operation causes the memory content to deviate from a pure state, i.e. induces impurity. For nonrelativistic particles scattered from a two-state memory and sufficiently general interaction potentials in 1+1 dimensions, we express impurity in terms of quaternionic commutators. In this context, pure memory states correspond to null hyperbolic quaternions. In the case with point interactions, the scattering process amounts to appropriate rotations of quaternions in the frequency domain. Our work complements previous analyses by Margetis and Myers (2006 J.\ Phys.\ A 39 11567--11581).

Abstract
The linear dispersion relation in graphene1,2 gives rise to a surprising prediction: the resistivity due to isotropic scatterers, such as white-noise disorder3 or phonons4–8, is independent of carrier density, n. Here we show that electron–acoustic phonon scattering4–6 is indeed independent of n, and contributes only 30 V to graphene’s room-temperature resistivity. At a technologically relevant carrier density of 1 31012 cm22, we infer a mean free path for electron–acoustic phonon scattering of >2 mm and an intrinsic mobility limit of 2 3 105 cm2 V21 s21. If realized, this mobility would exceed that of InSb, the inorganic semiconductor with the highest known mobility (7.7 3 104 cm2 V21 s21; ref. 9) and that of semiconducting carbon nanotubes (1 3 105 cm2 V21 s21; ref. 10). A strongly temperature-dependent resistivity contribution is observed above 200 K (ref. 8); its magnitude, temperature dependence and carrier-density dependence are consistent with extrinsic scattering by surface phonons at the SiO2 substrate11,12 and limit the room-temperature mobility to 4 3 104 cm2 V21 s21, indicating the importance of substrate choice for graphene devices13.

Acknowledged

• MRSEC
• ONR, NSF, IC Postdoc

Abstract
We apply methods of kinetic theory to study the passage from particle evolution schemes to nonlinear partial differential equations (PDEs) in the context of deterministic crystal surface relaxation. Starting with the near-equilibrium motion of $N$ line defects (``steps'') with atomic size $a$, we derive coupled evolution equations (``kinetic hierarchies'') for correlation functions, $F_n^a$, which express correlations of $n$ consecutive steps. We investigate separately evaporation-condensation and surface diffusion dynamics in 1+1 dimensions when each step interacts repulsively with its nearest neighbors. In the limit $a\to 0$ with $Na=O(1)$, where $a$ is appropriately nondimensional, the first equations of the hierarchies reduce to known evolution laws for the surface slope profile. The remaining PDEs take the form of simple continuity equations, which we solve exactly and thereby connect continuous limits of $F_n^a$ with the slope profile. In addition, we construct a particular example of $F_n^a$ asymptotically for small but finite $a$ by regularization of measures. In the limit $a\to 0$, this construction yields singular correlation functions.

Acknowledged

• MRSEC

Acknowledged

• NispLab

Acknowledged

• NSF

Abstract
A two-dimensional microfluidic system is presented for intact protein separations combining isoelectric focusing (IEF) and sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis (PAGE) employing in situ photopolymerized polyacrylamide (PAAm) gels. The PAAm gels are used for multiple functions. In addition to serving as a highly-resolving separation medium for gel electrophoresis, discrete polyacrylamide gel plugs are used to enable the efficient isolation of different on-chip media including anolyte, catholyte, and sample/ampholyte solutions for IEF. The gel plugs are demonstrated as on-chip reagent containers, holding defined quantities of SDS for on-chip SDS–protein complexation, and enabling the use of a discontinuous buffer system for sample band sharpening during SDS-PAGE. The 2-D chip also employs several unique design features including an angled isoelectric focusing channel to minimize sample tailing, and backbiasing channels designed to achieve uniform interdimensional sample transfer. Separation results using E. coli cell lysate are presented using a 10-channel chip with and without the discontinuous buffer system, with resolving power more than doubled in the former case. Further improvements in separation resolution are demonstrated using a 20-channel chip design.

Abstract
Nanotechnology has been increasingly utilized to enhance bone tissue engineering strategies. In particular, nanotechnology has been employed to overcome some of the current limitations associated with bone regeneration methods including insufficient mechanical strength of scaffold materials, ineffective cell growth and osteogenic differentiation at the defect site, as well as unstable and insufficient production of growth factors to stimulate bone cell growth. Among the tremendous technologies of nanoparticles in biological systems, we focus here on the three major nanoparticle research areas that have been developed to overcome these limitations and disadvantages: (a) the generation of nanoparticle-composite scaffolds to provide increased mechanical strength for bone graft, (b) the fabrication of nanofibrous scaffolds to support cell growth and differentiation through morphologically-favored architectures, and (c) the development of novel delivery and targeting systems of genetic material, especially those encoding osteogenic growth factors. These nanoparticle-based bone tissue engineering technologies possess a great potential to ensure the efficacy of clinical bone regeneration.

Acknowledged

• University of Maryland/National Institute of Standards and Technology Center for NanoManufacturing and Metrology

Acknowledged

• DOE

Abstract
Nanostructured devices have the potential to serve as the basis for next-generation energy systems that make use of densely packed interfaces and thin films. One approach to making such devices is to build multilayer structures of large area inside the open volume of a nanostructured template. Here, we report the use of atomic layer deposition to fabricate arrays of metal–insulator–metal nanocapacitors in anodic aluminium oxide nanopores. These highly regular arrays have a capacitance per unit planar area of approx10 microF cm-2 for 1-microm-thick anodic aluminium oxide and approx100 microF cm-2 for 10-microm-thick anodic aluminium oxide, significantly exceeding previously reported values for metal–insulator–metal capacitors in porous templates. It should be possible to scale devices fabricated with this approach to make viable energy storage systems that provide both high energy density and high power density.

Acknowledged

• NispLab
• MRSEC

Acknowledged

• NispLab

Abstract
Nonlinear oscillations of microelectromechanical resonators with curved cross-sections are studied in this effort. The resonators are fabricated as clamped–clamped composite structures, and these structures have both lengthwise and widthwise curvatures induced by residual stresses. Harmonic piezoelectric actuations of these structures were considered in experiments and the spatial responses of these structures were studied. The spatial responses observed for different resonance excitations could not be explained by a previous model of these composite microresonators, where stepwise non-uniform properties along the length and rectangular cross-sections were considered. Here, a curved cross-section model is adopted to refine the previous model and the resulting predictions are found to compare better with the experimental observations. The results show that the cross-section curvature significantly affects the structural stiffness and response, and this is important to consider in system modeling.

Abstract
Quantum computing involves transforming the state of a quantum memory. We view this operation as performed by transmitting nonrelativistic (massive) particles that scatter from the memory. By using a system of (1+1)-dimensional, coupled Schrödinger equations with point interaction and narrow-band incoming pulse wave functions, we show how the outgoing pulse becomes entangled with a two-state memory. This effect necessarily induces decoherence, i.e., deviations of the memory content from a pure state. We describe incoming pulses that minimize this decoherence effect under a constraint on the duration of their interaction with the memory.

Abstract
Thermal transport in carbon nanotubes is explored using different laser powers to heat suspended single-walled carbon nanotubes 5
m in length. The temperature change along the length of a nanotube is determined from the temperature-induced shifts in the G band Raman frequency. The spatial temperature profile reveals the ratio of the contact thermal resistance to the intrinsic thermal resistance of the nanotube. Moreover, the obtained temperature profiles allow differentiation between diffusive and ballistic phonon transport. Diffusive transport is observed in all nanotubes measured and the ratio of thermal contact resistance to intrinsic nanotube thermal resistance is found to range from 0.02 to 17.

Acknowledged

• NispLab
• DOE

Abstract
Multidimensional microfluidic separation systems combining a first dimension microchannel with an array of parallel second dimension microchannels can suffer from non-uniform sample transfer between the dimensions, sample leakage, and injection plug tailing within the second dimension array. These factors can significantly reduce overall two-dimensional separation performance. In this paper, numerical and analytical models reveal an optimized chip design which combines multidimensional backbiasing and an angled channel geometry to ensure leakage-free and uniform interdimensional sample transfer, while also minimizing injected sample plug lengths. The optimized design is validated experimentally using a multidimensional chip containing five second dimension channels.

Abstract
The temperature dependence of 1/ f noise in individual semiconducting carbon nanotube
CNT field-effect transistors is used to estimate the distribution of activation energies of the fluctuators D
E responsible for the noise. D
E shows a rise at low energy with no characteristic energy scale, and a broad peak at 0.4 eV. The peak, responsible for the majority of noise at room temperature, cannot be due to electronic excitations, carrier number fluctuations, or structural fluctuations of the CNT, and likely results from the motion of defects in the dielectric or at the CNT-dielectric interface, or very strongly physisorbed species
binding energy 0.4 eV on the CNT or dielectric surface.

Acknowledged

• MRSEC
• NSF, IC Postdoc

Acknowledged

• NispLab

Abstract
PDMS films of 10 µm thickness can be patterned within 30 min by combining dry etching to achieve substantially vertical sidewalls with wet etching to achieve high etch rates and to protect the underlying substrate from attack. Dry etching alone would have taken 5 h, and wet etching alone would produce severe undercutting. In addition, using either technique alone produces undesirable surface morphologies. The mask used during etching is critical to a successful patterning outcome. E-beam evaporated Al was found to work well, adhering strongly to oxygen-plasma-treated PDMS and holding up well during both dry and wet etching. To prevent wrinkling of the PDMS, a fast deposition rate should be used.

Abstract
We review a variety of micro- and nanoparticle formulations produced with microfluidic methods. A diverse variety of approaches to generate microscale and nanoscale particles has been reported. Here we emphasize the use of microfluidics, specifically microfluidic systems that operate in a continuous flow mode, thereby allowing continuous generation of desired particle formulations. The generation of semiconductor quantum dots, metal colloids, emulsions, and liposomes is considered. To emphasize the potential benefits of the continuous-flow microfluidic methodology for nanoparticle generation, preliminary data on the size distribution of liposomes formed using the microfluidic approach is compared to the traditional bulk alcohol injection method.

Abstract
A single layer of graphite, graphene,[1,2] is a truly 2-dimensional semi-metallic material composed of only one atomic layer of carbon atoms. Graphene’s peculiar band structure suppresses carrier backscattering, leading to extremely high carrier mobility.[2] Narrow graphene ribbons are predicted to have a semiconducting energy gap tunable by width,[3] indicating a path to device fabrication. In addition, because graphene is only one atom in thickness, transport properties are expected to be sensitively influenced by atomic scale defects, adsorbates,[4,5] local electronic environment, and mechanical deformations; consequently, graphene is a promising sensor material. To date, graphene has been obtained by only two methods: mechanical exfoliation of graphite on SiO2/Si[1] or thermal graphitization of a silicon carbide (SiC) surface.[2] In each case, the substrate strongly influences the graphene properties; charge defects in SiO2 are thought to limit the mobility, and strong interaction with SiC introduces a large charge density. Furthermore, the substrate can limit the graphene device possibilities; gating of devices on SiC is difficult, and on SiO2/Si the presence of a conducting backplane (also used as the gate) precludes high-frequency device operation. In this paper, we report the transfer of graphene from one substrate to another to realize flexible, transparent graphene devices with high field effect mobility. This represents the ultimate extension of printing technology to a single atomic layer.

Acknowledged

• MRSEC
• ONR, NSF, IC Postdoc

Abstract
We have combined transmission electron microscopy (TEM) and magnetic measurements to probe the growth and aging of colloidal cobalt (Co) nanocrystals and demonstrated that these two techniques together yield structure and property information in a manner that neither can do alone. During the growth, TEM shows the formation of Co nanocrystals (4.8 nm ± 1.7 nm), while magnetic measurements indicate the presence of paramagnetic Co cluster complexes and weakly interacting Co nanocrystals. At the completion of the synthesis, TEM shows that the average size of the Co nanocrystals has increased, but with a narrower size distribution (10.5 nm ± 1.0 nm). Meanwhile, magnetic measurements demonstrate the strong interactions between the Co nanocrystals. Exchange bias and increased coercivity are observed for the aged Co colloid under field-cooled conditions, indicating the existence of antiferromagnetic/ferromagnetic (AFM/FM) coupling. High-resolution TEM confirms that AFM face-centered cubic cobalt(II) oxide grows on the surface of the FM ?-Co nanocrystals, but this oxide layer is thin and inhomogeneous. These combined results suggest that not only the AFM/FM exchange coupling within individual aged nanocrystal matters but also the strong magnetostatic coupling between the neighboring nanocrystals significantly contributes to the observed exchange bias.

Abstract
We report a biofunctionalization strategy for the assembly of catalytically active enzymes within a completely packaged bioMEMS device, through the programmed generation of electrical signals at spatially and temporally defined sites. The enzyme of a bacterial metabolic pathway, S-adenosylhomocysteine nucleosidase (Pfs), is genetically fused with a pentatyrosine “pro-tag” at its C-terminus. Signal responsive assembly is based on covalent conjugation of Pfs to the aminopolysaccharide, chitosan, upon biochemical activation of the pro-tag, followed by electrodeposition of the enzyme–chitosan conjugate onto readily addressable sites in microfluidic channels. Compared to traditional physical entrapment and surface immobilization approaches in microfluidic environments, our signal-guided electrochemical assembly is unique in that the enzymes are assembled under mild aqueous conditions with spatial and temporal programmability and orientational control. Significantly, the chitosan-mediated enzyme assembly can be reversed, making the bioMEMS reusable for repeated assembly and catalytic activity. Additionally, the assembled enzymes retain catalytic activity over multiple days, demonstrating enhanced enzyme stability. We envision that this assembly strategy can be applied to rebuild metabolic pathways in microfluidic environments for antimicrobial drug discovery.

Acknowledged

• NSF IMI, Deutsch Foundation, USDA

Abstract
We report a versatile approach for covalent surface-assembly of proteins onto selected electrode patterns of pre-fabricated devices. Our approach is based on electro-assembly of the aminopolysaccharide chitosan scaffold as a stable thin film onto patterned conductive surfaces of the device, which is followed by covalent assembly of the target protein onto the scaffold surface upon enzymatic activation of the protein's pro-tag. For our demonstration, the model target protein is green fluorescent protein (GFP) genetically fused with a pentatyrosine pro-tag at its C-terminus, which assembles onto both two-dimensional chips and within fully packaged microfluidic devices in situ and under flow. Our surface-assembly approach enables spatial selectivity and orientational control under mild experimental conditions. We believe that our integrated approach harnessing genetic manipulation, in situ enzymatic activation, and electro-assembly makes it advantageous for a wide variety of bioMEMS and biosensing applications that require facile biofunctionalization of microfabricated devices.

Acknowledged

• FabLab
• NSF-EFRI

Abstract
In the search for a chemical sensing strategy to monitor atomic layer deposition (ALD) processes suitable for real-time application in wafer manufacturing, we have applied downstream mass spectrometry sampling to study process dynamics during ALD cycles for tungsten deposition from WF6 and SiH4. The ALD reactor has UHV cleanliness conditions and incorporated a minireactor chamber to simulate the small reaction volume anticipated for manufacturing tools to achieve adequate throughput. Mass spectrometry revealed essential surface reaction dynamics through real-time signals associated with by-product generation as well as reactant introduction and depletion for each ALD half-cycle. These were then used to optimize process cycle time and to study the effect of process recipe changes on film growth. The reaction by-products were clearly observed as H2 from SiH4 exposure and SiF4 from WF6 exposure. For each of the two half-cycles, rapid increase of by-product leds to steady-state adsorption/reaction conditions, following by by-product decrease and complementary reactant increase as surface saturation was achieved, indicating self-limiting surface reaction. From this direct observation of the surface reactions, exposure times could be minimized without sacrificing ALD growth rate per cycle, as verified experimentally. With gas flow parallel to the wafer surface in the minireactor, deviations from across-wafer uniformity were small when sufficient reactant doses were applied, but uniformity suffered markedly when doses were insufficient for surface saturation. Increasing WF6 concentration accelerated surface saturation as expected. Growth rates per cycle showed a notable temperature dependence, with small but noticeable activation energies
3 kcal/mol consistent with previous reports. The effect of varying gas doses of one reactant while keeping the other constant suggests a complex interdependence between the half-cycles, in which the reactivity in one half-cycle is influenced by the prior dose achieved in the previous half-cycle.

Acknowledged

• NIST, NSF

Abstract
In situ quadrupole mass spectrometry (QMS) has been integrated to an atomic layer deposition (ALD) reactor to achieve real-time chemical diagnostic and wafer-state metrology. The process investigated was tungsten ALD using WF6 and SiH4. The UHV-based substrate-heated ALD reactor incorporated a minireactor chamber to simulate the small reaction volume anticipated for manufacturing tools in order to achieve adequate throughput. Mass spectrometry revealed essential surface reaction dynamics through real-time signals associated with by-product generation as well as reactant introduction and depletion for each ALD half-cycle. The by-product QMS signal was then integrated in real time over each exposure and plotted against process cycle number to directly observe ALD film growth, leading to two valuable metrologies. First, the integrated by-product QMS value changes with cycle number, directly reflecting the nucleation kinetics. Specifically, QMS values increase with cycle number during the nucleation phase and then saturates as the film growth enters its steady-state growth phase. Second, summing the integrated by-product QMS signals over an entire deposition run provides an immediate measure of film thickness. The growth kinetics as measured by QMS is consistent with ex situ film characterization and is strongly dependent on process conditions and reactor chamber status. In the latter case, a clear first wafer effect was apparent when the system was left idle for a few hours, resulting in an apparent QMS signal difference during nucleation phase between the first wafer and nonfirst wafer cases. The dependence of QMS signals on chamber status is attributed to parallel reactions on the chamber wall, where different gas exposure history is encountered. The first wafer effect can be explained in a quantitative manner by considering the chamber wall as an additional wafer inside the ALD reactor. The first wafer effects can be reduced by proper preprocess treatment, and the linear correlation between QMS measurement and film thickness suggests a promising start for QMS-based ALD film thickness metrology. © 2007 American Vacuum Society. DOI: 10.1116/1.2429672

Acknowledged

• NIST, NSF-IMI

Abstract
Ultrathin crystals of the layered transition-metal dichalcogenide MoS2
semiconducting and TaS2
metallic were obtained by mechanical peeling or chemical exfoliation techniques and electrically contacted using electron-beam lithography. The MoS2 devices showed high field-effect mobility in the tens of cm2 /V s and an on/off ratio higher than 105. The TaS2 devices remained metallic despite the fabrication process and showed an enhancement of the superconducting transition temperature and disappearance of the charge density wave phase anomaly at low temperature.

Acknowledged

• MRSEC
• NSF

Acknowledged

• NispLab

Abstract
Most of the world's hydrogen supply is currently obtained by reforming hydrocarbons. 'Reformate' hydrogen contains significant quantities of CO that poison current hydrogen fuel-cell devices. Catalysts are needed to remove CO from hydrogen through selective oxidation. Here, we report first-principles-guided synthesis of a nanoparticle catalyst comprising a Ru core covered with an approximately 1–2-monolayer-thick shell of Pt atoms. The distinct catalytic properties of these well-characterized core–shell nanoparticles were demonstrated for preferential CO oxidation in hydrogen feeds and subsequent hydrogen light-off. For H2 streams containing 1,000 p.p.m. CO, H2 light-off is complete by 30 °C, which is significantly better than for traditional PtRu nano-alloys (85 °C), monometallic mixtures of nanoparticles (93 °C) and pure Pt particles (170 °C). Density functional theory studies suggest that the enhanced catalytic activity for the core–shell nanoparticle originates from a combination of an increased availability of CO-free Pt surface sites on the Ru@Pt nanoparticles and a hydrogen-mediated low-temperature CO oxidation process that is clearly distinct from the traditional bifunctional CO oxidation mechanism.

Acknowledged

• NispLab

Abstract
We report a constant current (CC) based anodization technique to fabricate and control structure of mechanically stable anodic aluminum oxide (AAO) membranes with a long-range ordered hexagonal nanopore pattern. For the first time we show that interpore distance (Dint) of a self-ordered nanopore feature can be continuously tuned over a broad range with CC anodization and is uniquely defined by the conductivity of sulfuric acid as electrolyte. We further demonstrate that this technique can offer new degrees of freedom for engineering planar nanopore structures by fine tailoring the CC based anodization process. Our results not only facilitate further understanding of self-ordering mechanism of alumina membranes but also provide a fast, simple (without requirement of prepatterning or preoxide layer), and flexible methodology for controlling complex nanoporous structures, thus offering promising practical applications in nanotechnology.

Acknowledged

• NispLab

Abstract
In Bose-Einstein condensation (BEC), particles occupy a single-particle quantum state, $\Phi$, macroscopically. At zero temperature, the wavefunction for $\Phi$ is usually described via a nonlinear Schrödinger equation (NSE). Our goal is to study time-dependent nonlocal effects beyond the NSE in trapped atomic gases. We adopt the view that atoms are excited from $\Phi$ in pairs: the scattering from $\Phi$ to other states at positions $\bx$ and $\by$ is described by the pair-excitation function, $K_0(\bx,\by,t)$ (Wu T~T 1961 J Math Phys 2 105--123). This function satisfies a nonlinear, dispersive integrodifferential equation coupled with the NSE. We solve these equations under a slowly varying external potential by assuming that $\Phi$ is stationary. For zero initial excitation ($K_0\equiv 0$ at $t=0$) and sufficiently large $t$, we evaluate $K_0$ asymptotically for any distance $|\bx-\by|$. Implications of these results are discussed, particularly the connection of non-equilibrium properties to the coalescence of critical points in the Fourier space.

Abstract
Metal nanoparticles (NPs) with size comparable to their electron mean free path possess unusual properties and functionalities, serving as model systems to explore quantum and classical coupling interactions as well as building blocks of practical applications. Although advances in strategies for synthesizing metal NPs have enabled control of size, composition and shape, the requirement that defects are simultaneously controlled, to ensure essential perfect nanocrystallinity for physics modelling as well as device optimization, is a potentially more significant issue, but has posed substantial technological challenges. Here we report that crystallinity of monodisperse silver NPs can be well controlled by judicious choice of functional groups of molecular precursors, thus facilitating investigation of their scope for versatile applications. We demonstrate how nanoscale chemical transformation, electron–phonon interactions and nanomechanical properties are modified by nanocrystallinity. Lastly, we find that performance of NP-based molecular sensing devices can be optimized with significant improvement of figure of merit if perfect single-crystalline NPs are applied. Our approach represents a versatile synthetic route for other metal nanomaterials with unprecedented control of their structure, creating a rational pathway for understanding and manipulating nanoscale chemical and physical processes as well as technological applications of metal NPs.

Acknowledged

• NispLab

Abstract
We have fabricated nanotubes using atomic layer deposition (ALD) into nanopore arrays created by anodic aluminum oxidation (AAO), developed and applied a TEM methodology to quantify the ALD conformality in the nanopores (thickness as a function of depth), and compared results to existing models for ALD conformality. ALD HfO2 nanotubes formed in AAO templates were released by dissolution of the Al2O3, transferred to a grid and imaged in TEM. An algorithm was devised to automate the quantification of nanotube wall thickness as a function of position along the central axis of the nanotube, using a cylindrical model for the nanotube. Diffusion limited depletion occurs in the lower portion of the nanotubes and is characterized by a linear slope of decreasing thickness. Experimentally recorded slopes match well with two simple models of ALD within nanopores put forth in the literature. The TEM analysis technique provides a method for rapid analysis of such nanostructures in general and also a means to efficiently quantify ALD profiles in nanostructures for a variety of nanodevice applications.

Acknowledged

• FabLab
• NispLab
• MRSEC
• ITC-irst, MKS Instruments and Inficon Inc

Abstract
Controlled electromigration of gold nanowires of different cross-sectional areas to form nanogap junctions is studied using a feedback method. A linear correlation between the cross-sectional area of the gold nanowires and the power dissipated in the junction during electromigration is observed, indicating that the feedback mechanism operates primarily by controlling the temperature of the junction during electromigration. We also show that the role of the external feedback circuit is to prevent thermal runaway; minimization of series resistance allows controlled electromigration to a significant range of junction resistances with a simple voltage ramp.

Acknowledged

• MRSEC
• DOE

Abstract
We have performed five separate sets of experiments to elucidate the effects of magnetite, ulvöspinel-magnetite solid solution, and pyrrhotite crystallization on the budgets of Au, Cu, and Ag at magmatic conditions. The experiments were done in both hydrous and anhydrous assemblages at temperatures between 800 and 1050 °C, pressures from ambient to 140 MPa, log fO2 from NNO-0.25 to NNO, and log fS2 from –1.5 to –3.0. Nernst-type partition coefficients (±1σ) at 800 °C in a water-saturated assemblage are DAgMt/melt = 2 × 10–4 ± 2 × 10–9, DCuMt/melt = 0.82 ± 0.69, DCuUsp/melt = 26 ± 17, DAuUsp/melt = 50 ± 31, DCuPo/melt = 174 ± 25. Nernst-type partition coefficients (±1σ) at 1050 °C in an anhydrous assemblage are DCuPo/melt ≥ 200, DAgPo/melt = 58 ± 8, DAuPo/melt = 120 ± 50. The calculated values for DAuUsp/melt and DCuUsp/melt indicate that the addition of Ti to magnetite increases significantly the Au- and Cu-scavenging potential of ulvöspinel relative to end-member magnetite. Partition coefficients for Cu and Au between pyrrhotite and melt indicate that a temperature change from 1050 to 800 °C in an anhydrous and hydrous assemblage, respectively, results in no observable change in Cu partitioning. The calculated partition coefficients are used to model the effect of crystal fractionation on the concentrations of Ag, Cu, and Au. Model results suggest that the co-crystallization of magnetite and pyrrhotite sequester no more than 2% Ag, 7% Cu, and 37% Au from the melt over the first 25% solidification. If the melt reaches volatile saturation after 25% crystallization, the presence of end-member magnetite and pyrrhotite do not appear to inhibit the Cu-, Au-, and Ag-ore potential of the magma. Ulvöspinel-magnetite, however, may reduce the Au concentration in the melt by approximately one-third relative to its initial value that decreases the overall Au available to partition into the volatile phase.

Acknowledged

• NispLab

Acknowledged

• NispLab

Abstract
The authors measure the resistance and frequency-dependent gate capacitance of carbon nanotube
CNT thin films in ambient, vacuum, and under low pressure
10−6 Torr analyte environments. They model the CNT film as a RC transmission line and show that changes in the measured capacitance as a function of gate bias and analyte pressure are consistent with changes in the transmission line impedance due to changes in the CNT film resistivity alone; the electrostatic gate capacitance of the CNT film does not depend on gate voltage or chemical analyte adsorption. However, the CNT film resistance is enormously sensitive to low pressure analyte exposure.

Acknowledged

• MRSEC
• ARL, IC Postdoc

Abstract
Electrical power >1 mW is dissipated in semiconducting single-walled carbon nanotube devices in a vacuum. After high-power treatment, devices exhibit lower on currents and intrinsic, ambipolar behavior with near-ideal thermionic emission from Schottky barriers of height one-half the band gap. Upon exposure to air, devices recover p-type behavior, with positive threshold and ohmic contacts. The air-exposed state cannot be explained by a change in contact work function but instead is due to doping of the nanotube.

Acknowledged

• MRSEC
• NSF

Abstract
We unify step bunching (SB) instabilities occuring under various conditions on crystal surfaces below roughening. We show that when attachment-detachment of atoms at step edges is the rate limiting process, the SB of interacting, concentric circular steps is equivalent to the commonly observed SB of interacting straight steps under deposition, desorption, or drift. We derive a continuum Lagrangian partial differential equation, which is used to study the onset of instabilities for circular steps. These findings place on a common ground SB instabilities from numerical simulations for circular steps and experimental observations of straight steps.

Abstract
A continuum theory is used to predict scaling laws for the morphological relaxation of crystal surfaces in two independent space dimensions. Our goal is to unify previously disconnected experimental observations of decaying surface profiles. The continuum description is derived from the motion of interacting atomic steps. For isotropic diffusion of adatoms across each terrace, induced adatom fluxes transverse and parallel to step edges obey different laws, yielding a tensor mobility for the continuum surface flux. The partial differential equation for the height profile expresses an interplay of step energetics and kinetics, and aspect ratio of surface topography that plausibly unifies observations of decaying bidirectional surface corrugations.

Acknowledged

• MRSEC

Abstract
A process for preparing vertical interconnects for flexible electronics using transfer printing is reported. The interconnects are initially prepared on a sacrificial transfer substrate in a four step process that yields a subassembly of upper electrode, interconnect, and dielectric. This subassembly is printed as a unit onto the lower electrodes. The average contact resistance is less than 1 ?/25 ?m2 interconnect cross section. The quality of the resulting conductive paths is established by fabricating and characterizing (to 5 GHz) the inductances and quality factors of a series of square planar spiral inductors.

Acknowledged

• ARL