Identifying new synthesis methods to produce single crystal metal nanowires has the potential to expand nanowire based technology integration and increase applications. The catalogue of single crystal metal nanostructures is rather limited compared to the range of high aspect semiconductor nanowires synthesized via well-established vapor liquid solid and atomic layer deposition methods. The surface energy driven growth (SEDG) method opens new possibilities for producing novel metal nanostructures. The method, based on the relative surface energy of the components involved in the synthesis, is presented as a stand-alone method for producing high aspect ratio single crystal metal nanowires. Wire growth is realized following eutectic temperature annealing of individual thin films that make up binary alloy systems. In response to thermal annealing the high surface energy component is observed to crystallize and form wire structures directly from the bilayer material without the introduction of growth precursors. The potential of this growth method has been demonstrated in the several recent examples of nanostructures synthesized using SEDG. Focus here will be on generalizing the method to enable future synthesis via this new method. Controlled positioning of the growth can be achieved through manipulation of dewetting phenomena to control mass flow and facilitated by the placement of defects that nucleate the dewetting phenomenon. The physical principles dictating the growth method are covered in detail to enable fine control over positioning and dewetting during growth. The principles of the method are presented together in detail for the first time with the aim of increasing the accessibility of the method for the wider scientific community.

We describe the fabrication, operation principles, and simulation of a coherent single-atom quantum interference device (QID) structure on Si(100) controlled by the properties of single atoms. The energy and spatial distribution of the wave functions associated with the device are visualized by scanning tunneling spectroscopy and the amplitude and phase of the evanescent wave functions that couple into the quantum well states are directly measured, including the action of an electrostatic gate. Density functional theory simulations were employed to simulate the electronic structure of the device structure, which is in excellent agreement with the measurements. Simulations of device transmission demonstrate that our coherent single-atom QID can have ON-OFF ratios in excess of 103 with potentially minimal power dissipation.

In this work, we demonstrate the position controlled growth of single Cu3Si nanostructures using a Ge–Cu bilayer film that contains a pattern of defects on a Si substrate with a thin oxide layer. The defects act as nucleation centers for growth, while the presence of Ge within the bilayer is critical to ensure effective surface diffusion rates and to eliminate spurious nucleation and growth. The behavior presented is consistent with a surface energy driven growth mechanism. The defect mediated reaction between Cu and the underlying Si substrate insures that the grown nanostructures are in perfect registry with the substrate. The possible applications and alternative implementations of this technology are discussed.

Electrical connectivity in networks of nanoscale junctions must be better understood if nanowire devices are to be scaled up from single wires to functional material systems. We show that the natural connectivity behaviour found in random nanowire networks presents a new paradigm for creating multi-functional, programmable materials. In devices made from networks of Ni/NiO core–shell nanowires at different length scales, we discover the emergence of distinct behavioural regimes when networks are electrically stressed. We show that a small network, with few nanowire–nanowire junctions, acts as a unipolar resistive switch, demonstrating very high ON/OFF current ratios (>105). However, large networks of nanowires distribute an applied bias across a large number of junctions, and thus respond not by switching but instead by evolving connectivity. We demonstrate that these emergent properties lead to fault–tolerant materials whose resistance may be tuned, and which are capable of adaptively reconfiguring under stress. By combining these two behavioural regimes, we demonstrate that the same nanowire network may be programmed to act both as a metallic interconnect, and a resistive switch device with high ON/OFF ratio. These results enable the fabrication of programmable, multi-functional materials from random nanowire networks.

Nanoscale devices that are sensitive to measurement history enable memory applications, and memristors are currently under intense investigation for robustness and functionality. Here we describe the fabrication and performance of a memristor-like device that comprises a single TiO2 nanowire in contact with Au electrodes, demonstrating both high sensitivity to electrical stimuli and high levels of control. Through an electroforming process, a population of charged dopants is created at the interface between the wire and electrode that can be manipulated to demonstrate a range of device and memristor characteristics. In contrast to conventional two-terminal memristors, our device is essentially a diode that exhibits memristance in the forward bias direction. The device is easily reset to the off state by a single voltage pulse and can be incremented to provide a range of controllable conductance states in the forward direction. Electrochemical modification of the Schottky barrier at the electrodes is proposed as an underlying mechanism, and six-level memory operations are demonstrated on a single nanowire.Keywords: engineered vacancies; memristor; multilevel memory; multistate memory; single nanowire; TiO2;

Networks comprised of randomly oriented overlapping nanowires offer the possibility of simple fabrication on a variety of substrates, in contrast with the precise placement required for devices with single or aligned nanowires. Metal nanowires typically have a coating of surfactant or oxide that prevents aggregation, but also prevents electrical connection. Prohibitively high voltages can be required to electrically activate nanowire networks, and even after activation many nanowire junctions remain nonconducting. Nonelectrical activation methods can enhance conductivity but destroy the memristive behavior of the junctions that comprise the network. We show through both simulation and experiment that electrical stimulation, microstructured electrode geometry, and feature scaling can all be used to manipulate the connectivity and thus electrical conductivity of networks of silver nanowires with a nonconducting polymer coating. More generally, these results describe a strategy to integrate nanomaterials into controllable, adaptive macroscale materials.Keywords: activation; conductivity; nanowire; network; sheet resistance; tunable;

In this article, we examine the phenomenon of single-crystal halide salt wire growth at the surface of porous materials. We report the use of a single-step casting technique with a supramolecular self-assembly gel matrix that upon drying leads to the growth of single-crystal halide (e.g., NaCl, KCl, and KI) nanowires with diameters ∼130–200 nm. We demonstrate their formation using electron microscopy and electron-dispersive X-ray spectroscopy, showing that the supramolecular gel stabilizes the growth of these wires by facilitating a diffusion-driven base growth mechanism. Critically, we show that standard non-supramolecular gels are unable to facilitate nanowire growth. We further show that these nanowires can be grown by seeding, forming nanocrystal gardens. This study helps understand the possible prefunctionalization of membranes to stimulate ion-specific filters or salt efflorescence suppressors, while also providing a novel route to nanomaterial growth.Keywords: chemical garden; nanowires; self-assembly; sodium chloride; supramolecular gel

Connectivity in metallic nanowire networks with resistive junctions is manipulated by applying an electric field to create materials with tunable electrical conductivity. In situ electron microscope and electrical measurements visualize the activation and evolution of connectivity within these networks. Modeling nanowire networks, having a distribution of junction breakdown voltages, reveals universal scaling behavior applicable to all network materials. We demonstrate how local connectivity within these networks can be programmed and discuss material and device applications.

Free-standing, single-crystal Cu3Si nanowires were synthesized by annealing a Cu/Ge bilayer film on a SiO2/Si substrate. The grown nanowires exhibit well-defined facets, were characterized using a combination of high-resolution microscopy and local area diffraction, and were shown to be comprised exclusively of the well-known η″ phase. Preliminary electrical measurements indicate that the metallic Cu3Si NWs are excellent conductors with a resistivity of less than 30 μΩ·cm and lower than that found for the corresponding thin film materials.

LiMo3Se3 is a highly anisotropic solid comprised of a regular pattern of quasi-1-D wire-like structures. Solutions of LiMo3Se3 deposited on substrates and TEM grids reveal the presence of two-dimensional network morphologies. High resolution STEM imaging reveals that the junctions within these networks are not formed by discrete overlying LiMo3Se3 fibers or wires. Rather the junctions are continuous in that the wires are seamlessly interwoven from one bundle to the next. We investigated network formation by dynamic light scattering and AFM and demonstrate that the networks are not pre-existent in solution but rather form via self-assembly of nanoscale building blocks that is driven by solvent evaporation.

In this article we map out the thickness dependence of the resistivity of individual graphene strips, from single layer graphene through to the formation of graphitic structures. We report exceptionally low resistivity values for single strips and demonstrate that the resistivity distribution for single strips is anomalously narrow when compared to bi- and trilayer graphene, consistent with the unique electronic properties of single graphene layers. In agreement with theoretical predictions, we show that the transition to bulklike resistivities occurs at seven to eight layers of graphene. Moreover, we demonstrate that the contact resistance between graphene flakes in a graphene network scales with the flake thickness and the implications for transparent conductor applications are discussed.

We introduce a novel wire growth technique that involves simply heating a multilayer film specifically designed to take advantage of the different surface energies of the substrate and film components. In all cases the high surface energy component is extruded as a single crystal nanowire. Moreover we demonstrate that patterning the bilayer film generates localized surface agglomeration waves during the anneal that can be exploited to position the grown wires. Examples of Au and Cu nanowire growth are presented, and the generalization of this method to other systems is discussed.

We consider the reaction of 1,3-cyclohexadiene (1,3-CHD) on Si(100) and show that the observed reactivity and stereoselectivity cannot be explained on the basis of thermodynamics. We postulate the existence of secondary orbital interactions (SOIs) and introduce a simple algorithm that examines all possible secondary interactions between the frontier orbitals of the molecule and the surface. We demonstrate using an orbital symmetry-based algorithm supported by DFT calculations that SOIs favor a particular molecular configuration, consistent with the experimental observations. The potential role of SOIs in controlling surface chemical reactions is discussed.

Conductance imaging atomic force microscopy was used to probe the electrical interface between single-walled carbon nanotubes and metal electrodes. The contact resistance was optimized by applying a local voltage pulse (∼2 s) using a conductive probe with controlled loading force to the region of the metal electrode contacting the nanotube. Using this technique, we show that Pd forms superior contacts, resulting in contact resistance values that are among the lowest ever reported.Keywords: conductance imaging atomic force microscopy (CI-AFM); contact resistance; single-walled carbon nanotubes (SWCNTs)

Transport in single-walled carbon nanotubes (SWCNTs) networks is shown to be dominated by resistance at network junctions which scale with the size of the interconnecting bundles. Acid treatment, known to dope individual tubes, actually produces a dramatic reduction in junction resistances, whereas annealing significantly increases this resistance. Measured junction resistances for pristine, acid-treated and annealed SWCNT bundles correlate with conductivities of the corresponding films, in excellent agreement with a model in which junctions control the overall network performance.

We present a conceptually simple frontier orbital description of an ideal Si(100) surface by extending the standard orbital description for a single Si dimer unit across the surface Brillouin zone. Density functional theory calculations are used to order the predicted frontier wave functions in terms of energy. When applied to the p(2 × 1) and c(4 × 2) reconstructions, this analysis provides a route for the controversial [2 + 2] cycloaddition reaction, which was previously thought to involve a violation of the Woodward−Hoffman rules. The calculated frontier states are shown to be a valuable aid in describing reactivity on Si(100) that is consistent with experiment and provides a rational means to predict allowed reaction products on Si(100).

Singlewalled carbon nanotube/polyvinylalcohol composite nanofibers were electro-spun onto a silicon surface pre-patterned with trenches. These nanofibers were prepared with different loadings of SWCNTs and had radii between 20 and 40 nm. Individual fiber sections were pinned across the trenches and laterally loaded by an AFM tip to yield mechanical response curves. A simple model was exploited to extract the tensile mechanical properties from the lateral force–displacement data. Depending on the fiber composition, the tensile modulus was found to be between 3 and 85 GPa. In addition we have prepared fibers with tensile strength of up to 2.6 GPa. Such optimised fibers break at strains of ∼4% and exhibit toughness of up to 27 MJ/m3.

We report the co-existence and growth of two different types of GdSi2 nanowires (NWs) following Gd deposition on the Si(1 0 0) surface. The first NW type is hexagonal and similar in appearance and orientation to that previously reported in the literature. The structure of the second NW type is unknown, but its orientation, thermodynamic stability and surface appearance are consistent with an orthorhombic structure. Real-time high-temperature STM studies show that these new NWs are thermodynamically preferred and in regions where both wires co-exist, they grow at the expense of their hexagonal counterparts. The implications of these observations for nanoscale interconnects is discussed.

The reaction of 1,3-cyclohexadiene (1,3-CHD) with Si dimers on the Si(100)-2×1 surface was studied via scanning tunnelling microscopy (STM) and density functional theory (DFT). Both intra- and interdimer [2+2] and [4+2] products are observed on the surface under empty-state bias conditions. Comparison of the observed products and their respective distribution on the surface with preliminary DFT calculations suggest a kinetically controlled reaction mechanism.