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Now showing items 1 - 16 of 266

  • Gold Nanoparticle Quantitation by Whole Cell Tomography

    Sanders, Aric W.   Jeerage, Kavita M.   Schwartz, Cindi L.   Curtin, Alexandra E.   Chiaramonti, Ann N.  

    Many proposed biomedical applications for engineered gold nanoparticles require their incorporation by mammalian cells in specific numbers and locations. Here, the number of gold nanoparticles inside of individual mammalian stem cells was characterized using fast focused ion beam–scanning electron microscopy based tomography. Enhanced optical microscopy was used to provide a multiscale map of the in vitro sample, which allows cells of interest to be identified within their local environment. Cells were then serially sectioned using a gallium ion beam and imaged using a scanning electron beam. To confirm the accuracy of single cross sections, nanoparticles in similar cross sections were imaged using transmission electron microscopy and scanning helium ion microscopy. Complete tomographic series were then used to count the nanoparticles inside of each cell and measure their spatial distribution. We investigated the influence of slice thickness on counting single particles and clusters as well as nanoparticle packing within clusters. For 60 nm citrate stabilized particles, the nanoparticle cluster packing volume is 2.15 ± 0.20 times the volume of the bare gold nanoparticles.
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  • Influence of Morphology on Current-Voltage Behavior of GaN Nanowires

    Blanchard, Paul T.   Bertness, Kris A.   Brubaker, Matthew D.   Harvey, Todd E.   Sanders, Aric W.   Sanford, Norman A.  

    We demonstrate the effect that the different morphologies of molecular beam epitaxy-grown GaN nanowires (NWs) can have upon current-voltage (I-V) behavior. Two aspects of NW morphology were investigated. The first aspect was the NW diameter, d(NW). For single-crystal Si-doped GaN NW devices with d(NW) < 120 nm, I-V curves were nonlinear. In contrast, single-crystal Si-doped NWs from the same growth run with d(NW) > 120 nm consistently showed ohmic I-V behavior. This discrepancy is likely the result of the comparatively larger surface depletion in thin NWs, which contributes to 1) an increased contact barrier, and 2) a barrier resulting from an axial band offset between the portion of the NW directly beneath the contact and the portion extending from the contact. The second aspect of NW morphology that we investigated was NW coalescence, which occurs when neighboring NWs fuse together during growth. I-V measurements of undoped coalesced NWs showed that these structures can have a free carrier concentration that is significantly higher than the background carrier concentration that is present in single-crystal (noncoalesced), undoped NWs.
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  • MOSFETs Made From GaN Nanowires With Fully Conformal Cylindrical Gates

    Blanchard, Paul T.   Bertness, Kris A.   Harvey, Todd E.   Sanders, Aric W.   Sanford, Norman A.   George, Steven M.   Seghete, Dragos  

    We report novel metal-oxide-semiconductor field effect transistors (MOSFETs) based on individual gallium nitride (GaN) nanowires with fully conformal cylindrical gates. The W/Al2O3 gates were deposited by atomic layer deposition. Reversebias breakdown voltages exceeded the largest gate voltage tested (-35 V). The nanowire MOSFETs showed complete pinchoff, with threshold voltages between -4 and -12V. Maximum transcon-ductances exceeded 10 mu S, and ON/OFF current ratios higher than 10(8) were measured. Significant gating hysteresis and memory effects were also present, indicative of charge traps. Although further optimization is needed, these results represent a promising step forward in the development of efficient GaN nanowire-based FETs.
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  • Controlled Nucleation of GaN Nanowires Grown with Molecular Beam Epitaxy

    Bertness, Kris A.   Sanders, Aric W.   Rourke, Devin M.   Harvey, Todd E.   Roshko, Alexana   Schlager, John B.   Sanford, Norman A.  

    The location of GaN nanowires is controlled with essentially perfect selectivity using patterned SiN(x) prior to molecular beam epitaxy growth. Nanowire growth is uniform within mask openings and absent on the mask surface for over 95% of the usable area of a 76 mm diameter substrate. The diameters of the resulting nanowires are controlled by the size of the mask openings. Openings of approximately 500 nm or less produce single nanowires with symmetrically faceted tips.
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  • A Simple Metric for Determining Resolution in Optical,Ion,and Electron Microscope Images

    Curtin, Alexandra E.   Skinner, Ryan   Sanders, Aric W.  

    A resolution metric intended for resolution analysis of arbitrary spatially calibrated images is presented. By fitting a simple sigmoidal function to pixel intensities across slices of an image taken perpendicular to light-dark edges, the mean distance over which the light-dark transition occurs can be determined. A fixed multiple of this characteristic distance is then reported as the image resolution. The prefactor is determined by analysis of scanning transmission electron microscope high-angle annular dark field images of Si<110>. This metric has been applied to optical, scanning electron microscope, and helium ion microscope images. This method provides quantitative feedback about image resolution, independent of the tool on which the data were collected. In addition, our analysis provides a nonarbitrary and self-consistent framework that any end user can utilize to evaluate the resolution of multiple microscopes from any vendor using the same metric.
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  • Localization and Number of Au Nanoparticles in Optically Indexed Cells by FIB Tomography

    Sanders, Aric W.   Jeerage, Kavita M.   Curtin, Alexandra E.   Chiaramonti, Ann N.  

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  • MESFETs Made From Individual GaN Nanowires

    Blanchard, Paul T.   Bertness, Kris A.   Harvey, Todd E.   Mansfield, Lorelle M.   Sanders, Aric W.   Sanford, Norman A.  

    In this paper, we demonstrate novel MESFETs based on individual GaN nanowires. The Pt/Au Schottky gates exhibited excellent two-terminal Schottky diode rectification behavior. The average effective Schottky barrier height was 0.87 eV, with an average ideality factor of 1.6. In addition, the Schottky gates efficiently modulated the conduction of the nanowires. The threshold gate voltages required for complete pinch off were as small as -2.6 V, and transconductances exceeded 1.4 mu S. Subthreshold swings approaching 60 mV/decade and ON/OFF current ratios of up to 5 x 10(8) were achieved. These results show that the Schottky gate has the potential to significantly improve the performance of GaN nanowire field-effect devices.
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  • Complex Folding Pathways of Bacteriorhodopsin Revealed by 1-mu s-Resolution Force Spectroscopy

    Siewny, Matthew G. W.   Yu, Hao   Edwards, Devin T.   Sanders, Aric W.   Perkins, Thomas T.  

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  • Using 3D Nanotomography to Visualize Defects in the Fabrication of Superconducting Electronics

    Sanders, Aric W.   Fox, Anna E.   Dresselhaus, Paul D.  

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  • NIST-Traceable Microwave Power Measurement in a Waveguide Calorimeter With Correlated Uncertainties

    Gu, Dazhen   Lu, Xifeng   Jamroz, Benjamin E.   Williams, Dylan E.   Cui, Xiaohai   Sanders, Aric W.  

    To realize microwave power traceability, we apply a newly developed calibration technique for measuring the correction factor of a calorimeter with a vector network analyzer in the WR-15 frequency band. The wave-parameter approach is used to formulate analytic expressions for the correction factor and effective efficiency. This allows us to systematically track the uncertainty with correlation along an unbroken path. Measurement results are consistent with what was previously obtained by use of the conventional method.
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  • NIST-Traceable Microwave Power Measurement in a Waveguide Calorimeter With Correlated Uncertainties

    Gu, Dazhen   Lu, Xifeng   Jamroz, Benjamin F.   Williams, Dylan F.   Cui, Xiaohai   Sanders, Aric W.  

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  • Observation of Plasmon Propagation, Redirection, and Fan-Out in Silver Nanowires

    Sanders, Aric W.   Routenberg, David A.   Wiley, Benjamin J.   Xia, Younan   Dufresne, Eric R.   Reed, Mark A.  

    We report the coupling of free-space photons (vacuum wavelength of 830 nm) to surface plasmon modes of a silver nanowire. The launch of propagating plasmons, and the subsequent emission of photons, is selective and occurs only at ends and other discontinuities of the nanowire. In addition, we observe that the nanowires redirect the plasmons through turns of radii as small as 4 mu m. We exploit the radiating nature of discontinuities to find a plasmon propagation length > 3 +/- 1 mu m. Finally, we observe that interwire plasmon coupling occurs for overlapping wires, demonstrating plasmon fan-out at subwavelength scales.
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  • Custom Modification of AFM Tips for Fast, High Force Resolution Single-Molecule Force Spectroscopy

    Sanders, Aric W.   Faulk, Jaevyn K.   Edwards, Devin T.   Perkins, Thomas T.  

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  • Toward Discrete Axial p-n Junction Nanowire Light-Emitting Diodes Grown by Plasma-Assisted Molecular Beam Epitaxy

    Brubaker, Matt D.   Blanchard, Paul T.   Schlager, John B.   Sanders, Aric W.   Herrero, Andrew M.   Roshko, Alexana   Duff, Shannon M.   Harvey, Todd E.   Bright, Victor M.   Sanford, Norman A.   Bertness, Kris A.  

    In this paper we investigate axial p-n junction GaN nanowires grown by plasma-assisted molecular beam epitaxy (MBE), with particular attention to the effect of Mg doping on the device characteristics of individual nanowire light-emitting diodes (LEDs). We observe that a significant fraction of single-nanowire LEDs produce measurable band-gap electroluminescence when a thin AlGaN electron blocking layer (EBL) is incorporated into the device structure near the junction. Similar devices with no EBL typically yield below-detection-limit electroluminescence, despite diode-like I-V characteristics and optically measured internal quantum efficiencies (IQEs) of similar to 1%. I-V measurements of the p-regions in p-n junction nanowires, as well as nanowires doped with Mg only, indicate low p-type conductivity and asymmetric Schottky-like p-contacts. These observations suggest that imbalanced carrier injection from the junction and p-contact can produce significant nonradiative losses.
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  • Irish vs. Yankees: A Social History of the Boston Schools by James W. Sanders

    Walch, Timothy  

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  • Polarity-Controlled GaN/AlN Nucleation Layers for Selective-Area Growth of GaN Nanowire Arrays on Si(111) Substrates by Molecular Beam Epitaxy

    Brubaker, Matt D.   Duff, Shannon M.   Harvey, Todd E.   Blanchard, Paul T.   Roshko, Alexana   Sanders, Aric W.   Sanford, Norman A.   Bertness, Kris A.  

    We have demonstrated dramatic improvement in the quality of selective-area GaN nanowire growth by controlling the polarity of the underlying nucleation layers. In particular, we find that N-polarity is beneficial for the growth of large ordered nanowire arrays with arbitrary spacing. Herein, we present techniques for obtaining and characterizing polarity-controlled nucleation layers on Si (111) substrates. An initial AlN layer, which is demonstrated to adopt Al-(N-)polarity for N-(Al-)rich growth conditions, is utilized to configure the polarity of subsequently grown GaN layers as determined by piezoresponse force microscopy (PFM), polarity-dependent surface reconstructions, and polarity-sensitive etching. Polarity-dependent surface reconstructions observed in reflection high-energy electron diffraction (RHEED) patterns were found to be particularly useful for in situ verification of the nucleation layer polarity, prior to mask deposition, patterning, and selective-area regrowth of the GaN NW arrays. N-polar templates produced fast-growing nanowires with vertical m-plane side walls and flat c-plane tips, while Ga-polar templates produced slow-growing pyramidal structures bounded by (1 (1) over bar 02) r-planes. The selective-area nanowire growth process window, bounded by nonselective and no-growth conditions, was found to be substantially more relaxed for NW arrays grown on N-polar templates, allowing for long-range selectivity where the NW pitch far exceeds the Ga diffusion length.
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