Aidan J. Quinn

Probing intrinsic transport properties of single metal nanowires: Direct-write contact formation using a focused ion beam

The transport characteristics of 70-nm-diameter platinum nanowires (NWs), fabricated using a pore-templated electrodeposition process and individually contacted using a focused ion beam (FIB) method, are reported. This approach yields nanowire devices with low contact resistances (∼400Ω) and linear current–voltage characteristics for current densities up to 65kA∕cm2. The intrinsic nanowire resistivity (33±5μΩcm) indicates significant contributions from surface- and grain-boundary scattering mechanisms. Fits to the temperature dependence of the intrinsic NW resistance confirm that grain-boundary scattering dominates surface scattering (by more than a factor of 2) at all temperatures. Our results demonstrate that FIB presents a rapid and flexible method for the formation of low-resistance ohmic contacts to individual metal nanowires, allowing intrinsic nanowire transport properties to be probed.

Low-temperature electron mobility in Trigate SOI MOSFETs

Evidence of one-dimensional subband formation is found at low temperature in trigate silicon-on-insulator MOSFETs, resulting in oscillations of the I/sub D/(V/sub G/) characteristics. These oscillations correspond to the filling of energy subbands by electrons as the gate voltage is increased. High mobility, reaching 1200 cm/sup 2//Vs, is measured in the subbands at T=4.4 K. Subband mobility decreases as temperature is increased. Conduction in subbands disappears for temperatures higher than 100 K or for drain voltage values that are significantly larger than kT/q.

Temperature effects on trigate SOI MOSFETs

Trigate silicon-on-insulator (SOI) MOSFETs have been measured in the 5-400 K temperature range. The device fin width and height is 45 and 82 nm, respectively, and the p-type doping concentration in the channel is 6/spl times/10/sup 17/ cm/sup -3/. The subthreshold slope varies linearly with temperature as predicted by fully depleted SOI MOS theory. The mobility is phonon limited for temperatures larger than 100 K, while it is limited by surface roughness below that temperature. The corner effect, in which the device corners have a lower threshold voltage than the top and sidewall Si/SiO/sub 2/ interfaces, shows up at temperatures lower than 150 K.

Ion-Transfer Electrochemistry at Arrays of Nanointerfaces between Immiscible Electrolyte Solutions Confined within Silicon Nitride Nanopore Membranes

Ion transfer across interfaces between immiscible liquids provides a means for the nonredox electrochemical detection of ions. Miniaturization of such interfaces brings the benefits of enhanced mass transport. Here, the electrochemical behavior of geometrically regular arrays of nanoscale interfaces between two immiscible electrolyte solutions (nanoITIES arrays) is presented. These were prepared by supporting the two electrolyte phases within silicon nitride membranes containing engineered arrays of nanopores. The nanoITIES arrays were characterized by cyclic voltammetry of the interfacial transfer of tetraethylammonium cation (TEA(+)) between the aqueous phase and the gelled organic phase. Effects of pore radius, pore center-to-center separation, and number of pores in the array were examined. The ion transfer produced apparent steady-state voltammetry on the forward and reverse sweeps at all experimentally accessible scan rates and at all nanopore array designs. However, background-subtraction of the voltammograms revealed the evolution of a peak-shaped response on the reverse sweep with increasing scan rate, indicative of pores filled with the organic phase to a certain extent. The steady-state voltammetric behavior at the nanoITIES arrays on the forward sweep for arrays with significant diffusion zone overlap between adjacent nanoITIES is indicative of the dominance of radial diffusion to interfaces at the edge of the arrays over linear diffusion to interfaces within the arrays. This implies that nanoITIES arrays, which occupy an overall area of micrometer dimensions, behave like a single microITIES of corresponding area to the nanoITIES array.

Single Nanoskived Nanowires for Electrochemical Applications

In this work, we fabricate gold nanowires with well controlled critical dimensions using a recently demonstrated facile approach termed nanoskiving. Nanowires are fabricated with lengths of several hundreds of micrometers and are easily electrically contacted using overlay electrodes. Following fabrication, nanowire device performance is assessed using both electrical and electrochemical characterization techniques. We observe low electrical resistances with typical linear Ohmic responses from fully packaged nanowire devices. Steady-state cyclic voltammograms in ferrocenemonocarboxylic acid demonstrate scan rate independence up to 1000 mV s(-1). Electrochemical responses are excellently described by classical Butler-Volmer kinetics, displaying a fast, heterogeneous electron transfer kinetics, k(0) = 2.27 ± 0.02 cm s(-1), α = 0.4 ± 0.01. Direct reduction of hydrogen peroxide is observed at nanowires across the 110 pM to 1 mM concentration range, without the need for chemical modification, demonstrating the potential of these devices for electrochemical applications.

Polythiophene mesowires: synthesis by template wetting and local electrical characterisation of single wires

We report on the synthesis of semiconducting mesowires from regioregular poly(3-hexylthiophene) via melt injection into a porous alumina template. Following liberation from the template, mesowires with diameters ∼450 nm and average lengths of 10 µm were obtained. For two-terminal electrical contacting of individual mesowires on insulating substrates, the drain electrode was formed by shadow masking and metal evaporation, and a conducting probe atomic force microscope tip was employed as the source electrode. This approach enabled combined topographic imaging and spatially resolved electrical characterisation of individual mesowires, allowing the intrinsic mesowire resistivity (700 ± 300 Ω m) as well as the contact resistance (10 ± 6 GΩ) to be estimated. Fitting the measured data to a thermionic emission–diffusion model yielded a hole mobility ∼2 × 10−5 cm2 V−1 s−1 and a metal–polymer interface barrier height ∼0.1 eV.

Morphology of sputtering damage on Cu(111) studied by scanning tunneling microscopy

The morphology of Cu(111) after bombardment with 1 keV Ar ions and its dependence on temperature have been investigated using STM. Qualitative agreement is found with experiments conducted by other groups on Pt(111), but with more pronounced temperature influences. In addition, interlayer mass transport was observed to have a more important effect on Cu(111). The Cu(111) surface only slowly approaches equilibrium shape at room temperature. A series of images illustrates how the surface changes over a period of several hours following ion bombardment.

Nanoscale dynamics and protein adhesivity of alkylamine self-assembled monolayers on graphene.

Atomic-scale molecular dynamics computer simulations are used to probe the structure, dynamics, and energetics of alkylamine self-assembled monolayer (SAM) films on graphene and to model the formation of molecular bilayers and protein complexes on the films. Routes toward the development and exploitation of functionalized graphene structures are detailed here, and we show that the SAM architecture can be tailored for use in emerging applications (e.g., electrically stimulated nerve fiber growth via the targeted binding of specific cell surface peptide sequences on the functionalized graphene scaffold). The simulations quantify the changes in film physisorption on graphene and the alkyl chain packing efficiency as the film surface is made more polar by changing the terminal groups from methyl (-CH3) to amine (-NH2) to hydroxyl (-OH). The mode of molecule packing dictates the orientation and spacing between terminal groups on the surface of the SAM, which determines the way in which successive layers build up on the surface, whether via the formation of bilayers of the molecule or the immobilization of other (macro)molecules (e.g., proteins) on the SAM. The simulations show the formation of ordered, stable assemblies of monolayers and bilayers of decylamine-based molecules on graphene. These films can serve as protein adsorption platforms, with a hydrophobin protein showing strong and selective adsorption by binding via its hydrophobic patch to methyl-terminated films and binding to amine-terminated films using its more hydrophilic surface regions. Design rules obtained from modeling the atomic-scale structure of the films and interfaces may provide input into experiments for the rational design of assemblies in which the electronic, physicochemical, and mechanical properties of the substrate, film, and protein layer can be tuned to provide the desired functionality.

Asymmetric Pentagonal Metal Meshes for Flexible Transparent Electrodes and Heaters

Metal meshes have emerged as an important class of flexible transparent electrodes. We report on the characteristics of a new class of asymmetric meshes, tiled using a recently discovered family of pentagons. Micron-scale meshes were fabricated on flexible polyethylene terephthalate substrates via optical lithography, metal evaporation (Ti 10 nm, Pt 50 nm), and lift-off. Three different designs were assessed, each with the same tessellation pattern and line width (5 μm), but with different sizes of the fundamental pentagonal unit. Good mechanical stability was observed for both tensile strain and compressive strain. After 1000 bending cycles, devices subjected to tensile strain showed fractional resistance increases in the range of 8-17%, while devices subjected to compressive strain showed fractional resistance increases in the range of 0-7%. The performance of the pentagonal metal mesh devices as visible transparent heaters via Joule heating was also assessed. Rapid response times (∼15 s) at low bias voltage (≤5 V) and good thermal resistance characteristics (213-258 °C cm2/W) were found using measured thermal imaging data. Deicing of an ice-bearing glass coupon on top of the transparent heater was also successfully demonstrated.

Analysis of charge transport in arrays of 28 kDa nanocrystal gold molecules

Arrays of 28 kDa nanocrystal gold molecules behave as weakly-coupled molecular solids comprising discrete nanoscale metallic islands separated by insulating ligand barriers. The key parameters which are found to dominate charge transport are (a) the single-electron nanocrystal charging energy, governed by the core diameter, the dielectric properties of the passivating ligands and classical electrostatic coupling between neighbouring cores; (b) the inter-nanocrystal tunnel barrier resistance that arises from the insulating nature of the ligand bilayers that separate the cores; and (c) the dimensionality of the network of current-carrying paths.