D. R. Strachan

University of Kentucky, Lexington, Kentucky, United States

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Publications (55)175.24 Total impact

  • Mathias J. Boland · Mohsen Nasseri · D. Patrick Hunley · Armin Ansary · Douglas R. Strachan ·
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    ABSTRACT: Lateral force microscopy (LFM) is used to probe the nanoscale elastic and frictional characteristics of molybdenum disulfide (MoS2). We find that MoS2 edges are effectively flexed over a region of about 10 nm when scanned with sharp single asperity LFM probes, with energies consistent with out-of-plane bending and being slightly stiffer than those of graphene. Additionally, we report the first observation of a striped nanoscale frictional phase on the surface of MoS2. This frictional phase is fixed to the underlying MoS2 with a modulation length of ∼4 nm that is insensitive to scan parameters and has domain sizes that exceed 100 nm. The amplitude of these features is found to be relatively independent of the geometry of the tip asperity and the applied load within the ranges we investigate. Experimental results suggest this periodic friction can be explained by variations in the local strain in the underlying MoS2. These results could have general applicability to understanding the nanomechanical properties of the growing array of laminar materials that are of potential use as atomically-thin coatings to future nanoscale machines.
    RSC Advances 01/2015; 5(112):92165-92173. DOI:10.1039/C5RA20617K · 3.84 Impact Factor
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    ABSTRACT: Nanostructured bi-layer graphene samples formed through catalytic etching are investigated with electrostatic force microscopy. The measurements and supporting computations show a variation in the microscopy signal for different nano-domains that are indicative of changes in capacitive coupling related to their small sizes. Abrupt capacitance variations detected across etch tracks indicates that the nano-domains have strong electrical isolation between them. Comparison of the measurements to a resistor-capacitor model indicates that the resistance between two bi-layer graphene regions separated by an approximately 10 nm wide etch track is greater than about 1 × 10 12 Ω with a corresponding gap resistivity greater than about 3 × 10 14 Ω ⋅ nm . This extremely large gap resistivity suggests that catalytic etch tracks within few-layer graphene samples are sufficient for providing electrical isolation between separate nano-domains that could permit their use in constructing atomically thin nanogap electrodes, interconnects, and nanoribbons.
    Applied Physics Letters 12/2014; 105(24):243109. DOI:10.1063/1.4904709 · 3.30 Impact Factor
  • D. Patrick Hunley · Mathias J. Boland · Douglas R. Strachan ·
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    ABSTRACT: Carbon nanotubes, few-layer graphene, and etch tracks exposing insulating SiO2 regions are integrated into nanoscale systems with precise crystallographic orientations. These integrated systems consist of nanotubes grown across nanogap etch tracks and nanoribbons formed within the few-layer graphene films. This work is relevant to the integration of semiconducting, conducting, and insulating nanomaterials together into precise intricate systems. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    Advanced Materials 12/2014; 27(5). DOI:10.1002/adma.201404060 · 17.49 Impact Factor
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    ABSTRACT: The catalytic activity of nanoparticles to form either crystallographically oriented etch tracks or carbon nanotubes on top of few-layer graphene is tuned through the application of a methane feedstock gas. The catalytic activity for these two processes is found to vary at different rates as a function of the amount of applied feedstock gas. These differences provide a window of growth parameters in which nanotubes can be grown on the surface of few-layer graphene without significant formation of etch tracks. Since this surface growth results in nanotubes which can be crystallographically aligned to the underlying graphene layers, this development could lead to improved electrical interfaces between nanotubes and graphene without the deleterious consequences of catalytic etching. Such improved interfaces could prove to be useful in applications benefiting from low interfacial electrical resistances between one-dimensional and two-dimensional materials, such as in supercapacitor applications.
    Carbon 10/2014; 77:958-963. DOI:10.1016/j.carbon.2014.06.011 · 6.20 Impact Factor
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    ABSTRACT: The effects of encapsulating graphene with poly(methyl methacrylate) (PMMA) polymer are determined through in situ electrical transport measurements. After regenerating graphene devices in dry-nitrogen environments, PMMA is applied to their surfaces. Low-temperature annealing decreases the overall doping level, suggesting that residual solvent plays an important role in the doping. For few-layer graphene devices, we even observe stable n-doping through annealing. Application of solvent onto encapsulated devices demonstrates enhanced hysteric switching between p and n-doped states. The stability and ubiquitous use of PMMA in nanolithography make this polymer a potentially useful localized doping agent for graphene and other two-dimensional materials.
    Applied Physics Letters 12/2013; 103(25):253505-253505-4. DOI:10.1063/1.4851956 · 3.30 Impact Factor
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    ABSTRACT: An analytical closed form diffusive model is developed of Joule heating in a device consisting of a nanowire connected to two contacts on a substrate. This analytical model is compared to finite-element simulations and demonstrates excellent agreement over a wider range of system parameters in comparison to other recent models, with particularly large improvements in cases when the width of the nanowire is less than the thermal healing length of the contacts and when the thermal resistance of the contact is appreciable relative to the thermal resistance of the nanowire. The success of this model is due to more accurately accounting for the heat spreading within the contact region of a device and below the nanowire into a substrate. The heat spreading is achieved by matching the linear heat flow near the nanowire interfaces with a radially symmetric spreading solution through an interpolation function. Additional features of this model are the ability to incorporate contact resistances that may be present at the nanowire-contact interfaces, as well as accommodating materials with a linear temperature-dependent electrical resistivity.
    Journal of Applied Physics 06/2013; 113(23). DOI:10.1063/1.4811444 · 2.18 Impact Factor
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    ABSTRACT: Frictional, adhesive, and elastic characteristics of graphene edges are determined through lateral force microscopy. Measurements reveal a significant local frictional increase at exposed graphene edges, whereas a single overlapping layer of graphene removes this local frictional increase. Comparison of lateral force and atomic force microscopy measurements shows that local forces on the probe are successfully modeled with a vertical adhesion in the vicinity of the atomic-scale graphene steps. Lateral force microscopy performed with carefully maintained probes shows evidence of elastic straining of graphene edges which are consistent with out-of-plane bending of the edges.
    Physical review. B, Condensed matter 03/2013; 87(3):8004-. DOI:10.1103/PhysRevB.87.035417 · 3.66 Impact Factor
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    ABSTRACT: We illustrate a simple method to synthesize highly ordered ZnO axial p-n homojunction-containing nanowires using a low temperature method, and on a variety of substrates. X-ray diffraction, scanning transmission electron microscopy, scanning electron microscopy, and Raman spectroscopy are used to reveal high quality single-crystalline wires with a [001] growth direction. The study of electrical transport through a single nanowire based device and cathodoluminescence via scanning transmission electron microscopy demonstrates that an axial p-n junction exists within each ZnO nanowire. This represents the first low temperature synthesis of axial p-n homojunction-containing ZnO nanowires with uniform and controllable diameters.
    Nanoscale 02/2013; 5(6):2259-63. DOI:10.1039/c3nr31639d · 7.39 Impact Factor
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    ABSTRACT: Lateral force microscopy is used to investigate the local nanoscale frictional variations on single- and multi-layered graphene films. Employing novel calibration methods, quantitative frictional measurements are taken for a range of normal loads. The coverage of specific high-friction regions with a single layer of graphene shows a significant reduction in the frictional characteristics.
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    ABSTRACT: Carbon nanotubes are grown on few-layer graphene films using chemical vapor deposition without a carbon feedstock gas. We find that the nanotubes show a striking alignment to specific crystal orientations of the few-layer graphene films. The nanotubes are oriented predominantly at 60 degree intervals and are offset 30 degrees from crystallographically oriented etch tracks, indicating alignment to the armchair axes of the few-layer graphene films. Nanotubes grown on various thicknesses of few-layer graphene under identical process conditions show that the thinnest films, in the sub-6 atomic layer regime, demonstrate significantly improved crystallographic alignment. Intricate crystallographic patterns are also observed having sharp kinks with bending radii less than the ∼10 nm lateral resolution of the electron and atomic force microscopy used to image them. Some of these kinks occur independently without interactions between nanotubes while others result when two nanotubes intersect. These intersections can trap nanotubes between two parallel nanotubes resulting in crystallographic back and forth zigzag geometries. These interactions suggest a tip-growth mechanism such that the catalyst particles remain within several nanometers of the few-layer graphene surface as they move leaving a nanotube in their wake.
    ACS Nano 08/2011; 5(8):6403-9. DOI:10.1021/nn201573m · 12.88 Impact Factor
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    ABSTRACT: We demonstrate a technique for simultaneously fabricating arrays of electromigrated nanogaps using a single-ramp feedback-controlled voltage clamp. The parallel formation is achieved by controlling the applied bias with a voltage clamp directly adjacent to a nanogap array containing low-impedance shunts. Self-balancing of the electromigration permits the two voltage leads to fix the applied voltage across all the forming nanogaps simultaneously. This single-ramp feedback-controlled voltage clamp method is at least a 100 times faster than previous work utilizing computer feedback control of parallel nanojunctions and also circumvents the deleterious thermal runaway that occurs in the conventional single-ramp technique.
    IEEE Transactions on Nanotechnology 08/2011; 10(4-10):806 - 809. DOI:10.1109/TNANO.2010.2080283 · 1.83 Impact Factor
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    ABSTRACT: Catalytic etching is a promising method for constructing crystallographically defined graphene structures such as nanoribbons. Catalytic etching experiments are performed and shown to contain significant correlation yielding crystallographic graphene nanoribbons. This correlation is investigated as a function of etching conditions and compared to simulations with possible sources discussed. Supported in part by NSF Award No. DMR-0805136, the Kentucky NSF EPSCoR program, the University of Kentucky Center for Advanced Materials, and the University of Kentucky Center for Nanoscale Science and Engineering.
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    ABSTRACT: The electrical breakdown of graphene and few-layer graphene (FLG) structures are investigated. To better understand the dynamics of these nano-scale thermal effects, we investigate graphene and FLG nanowires of various dimensions and find that significant joule heating occurs inducing the structures to evolve. A distinct change in the behavior during electrical stressing indicates that different mechanisms occur at the various stages of evolution. The results are compared to detailed thermal modeling of our structures and could have implications on the development of high current carrying nanoscale graphene devices. Supported in part by NSF Award No. DMR-0805136, the Kentucky NSF EPSCoR program, the University of Kentucky Center for Advanced Materials, and the University of Kentucky Center for Nanoscale Science and Engineering.
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    ABSTRACT: Carbon nanotubes are grown on graphene and few-layer graphene films through chemical vapor deposition. The nanotube growth is found to depend on the thickness of the few-layer graphene films. The thinnest films show significant alignment of the nanotubes with the crystallographic axes of the graphene. This alignment is compared to the orientation of the crystallographic etch tracks, permitting the orientation of the nanotubes to be determined. Related nanotube/graphene structures will also be presented and discussed. Supported in part by NSF Award No. DMR-0805136, the Kentucky NSF EPSCoR program, the University of Kentucky Center for Advanced Materials, and the University of Kentucky Center for Nanoscale Science and Engineering.
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    ABSTRACT: Electrostatic force microscopy (EFM) is a widely used scanning-probe technique for the characterization of electronic properties of nanoscale samples without the use of electrical contacts. Here we review the basic principles of EFM, developing a quantitative framework by which EFM measurements of extended nanostructures can be understood. We support our calculations with experimental data of EFM of carbon nanotubes and conducting or insulating electrospun polyaniline-based nanofibers. Furthermore, we explore routes towards extending EFM as a means of non-invasively probing the local electronic density of states of carbon nanotubes.
    MRS Online Proceeding Library 01/2011; 1025. DOI:10.1557/PROC-1025-B13-03
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    ABSTRACT: A method is reported to pattern monolayer graphene nanoconstriction field-effect transistors (NCFETs) with critical dimensions below 10 nm. NCFET fabrication is enabled by the use of feedback-controlled electromigration (FCE) to form a constriction in a gold etch mask that is first patterned using conventional lithographic techniques. The use of FCE allows the etch mask to be patterned on size scales below the limit of conventional nanolithography. The opening of a confinement-induced energy gap is observed as the NCFET width is reduced, as evidenced by a sharp increase in the NCFET on/off ratio. The on/off ratios obtained with this procedure can be larger than 1000 at room temperature for the narrowest devices; this is the first report of such large room-temperature on/off ratios for patterned graphene FETs.
    Small 12/2010; 6(23):2748-54. DOI:10.1002/smll.201001324 · 8.37 Impact Factor
  • Ye Lu · Brett Goldsmith · Douglas Strachan · A. T. Charlie Johnson ·
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    ABSTRACT: We report an approach to fabricate monolayer graphene nanoconstriction field effect transistors (NCFETs) with critical dimensions below 10 nm, a regime that is not accessible by conventional nanolithography. We start by fabricating a gold nanowire on top of mechanically-exfoliated monolayer graphene. We use Feedback Controlled Electromigration to form a nanoconstriction in the gold wire, which is then used as an etch mask for the graphene during an oxygen plasma patterning step. We observe the opening of a confinement-induced energy gap as the NCFET width is reduced, as evidenced by a sharp increase in the NCEFT on/off ratio, which can be as large as 1100 at room temperature for the thinnest devices. Such devices deliver up to 100microampere current at 50mV bias with an on state resistance of 50kilo ohm, which is at least an order of magnitude lower than graphene nanoribbon FETs with similar on/off ratio. This lower resistance is due to large area contacts in our devices.
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    S L Johnson · A Sundararajan · D P Hunley · D R Strachan ·
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    ABSTRACT: Memristors have recently generated significant interest due to their potential use in nanoscale logic and memory devices. Of the four passive circuit elements, the memristor (a two-terminal hysteretic switch) has so far proved hard to fabricate out of a single material. Here we employ electromigration to create a reversible passive electrical switch, a memristive device, from a single-component metallic nanowire. To achieve resistive switching in a single-component structure we introduce a new class of memristors, devices in which the state variable of resistance is the system's physical geometry. By exploiting electromigration to reversibly alter the geometry, we repeatedly switch the resistance of single-component metallic nanowires between low and high states over many cycles. The reversible electromigration causes the nanowire to be cyclically narrowed to approximately 10 nm in width, resulting in a change in resistance by a factor of two. As a result, this work represents a potential route to the creation of nanoscale circuits from a single metallic element.
    Nanotechnology 03/2010; 21(12):125204. DOI:10.1088/0957-4484/21/12/125204 · 3.82 Impact Factor
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    ABSTRACT: In this talk, we will discuss a new type of memristor - one whose state variable is its physical geometry. Using a single metallic material, we employ electromigration to change the resistance of metallic nanowires. The resistive switching is due to the creation/filling-in of voids in the nanowire as atoms are pushed back and forth by the electrical current. Exploiting electromigration in this manner, we repeatedly switch the resistance of single-component metallic nanowires between low and high states over many cycles. This work thus completes the array of fundamental passive circuit elements, now including the memristor, which can be fabricated from a single metallic material.
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    Sujit S. Datta · Douglas R. Strachan · Johnson · A.T. Charlie ·
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    ABSTRACT: The realization of single-molecule electronic devices, in which a nanometer-scale molecule is connected to macroscopic leads, requires the reproducible production of highly ordered nanoscale gaps in which a molecule of interest is electrostatically coupled to nearby gate electrodes. Understanding how the molecule-gate coupling depends on key parameters is crucial for the development of high-performance devices. Here we directly address this, presenting two- and three-dimensional finite-element electrostatic simulations of the electrode geometries formed using emerging fabrication techniques. We quantify the gate coupling intrinsic to these devices, exploring the roles of parameters believed to be relevant to such devices. These include the thickness and nature of the dielectric used, and the gate screening due to different device geometries. On the singlemolecule ( ~ 1 nm) scale, we find that device geometry plays a greater role in the gate coupling than the dielectric constant or the thickness of the insulator. Compared to the typical uniform nanogap electrode geometry envisioned, we find that nonuniform tapered electrodes yield a significant 3 orders of magnitude improvement in gate coupling. We also find that in the tapered geometry the polarizability of a molecular channel works to enhance the gate coupling.
    Physical review. B, Condensed matter 05/2009; 79(20). DOI:10.1103/PhysRevB.79.205404 · 3.66 Impact Factor

Publication Stats

1k Citations
175.24 Total Impact Points


  • 2009-2014
    • University of Kentucky
      • Department of Physics & Astronomy
      Lexington, Kentucky, United States
  • 2005-2011
    • University of Pennsylvania
      • • Department of Materials Science and Engineering
      • • Department of Physics and Astronomy
      Filadelfia, Pennsylvania, United States
  • 2008
    • William Penn University
      Filadelfia, Pennsylvania, United States
  • 2001-2008
    • University of Maryland, College Park
      • • Department of Physics
      • • Department of Chemical and Biomolecular Engineering
      Maryland, United States