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.

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 is comprised of 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.Transition metal oxides are an emerging candidate material for the realization of non-volatile, low power memory devices for handheld and portable electronics.1, 2 Non-volatile memories that retain encoded information without power consumption would greatly enhance battery performance for all mobile platform technologies. 3 Recent devices based on transition metal oxides display many characteristics required for the next generation of resistive switching based random access memory (RRAM) technologies. 4-9 TiO2

In spite of the strong interest in brain-like or neuromorphic computation, relatively few devices have emerged whose neuromorphic behavior is embedded in the hardware itself and not reliant on external programming of synaptic weights. We describe here a neuromorphic device based on a TiO2 nanowire that exhibits an associative memory response to the time correlation between voltage and optical stimuli. Memristive characteristics are also observed with current-voltage sweeps showing hysteresis loops and continuum resistance levels. The nanowire device responds to heterogeneous voltage and optical pulse stimuli with spike-like neuromorphic outputs. Moreover, uncorrelated pulses produce a weak response, consistent with the interaction of coincident pulses with adsorbed and bulk oxygen in the surface depletion region, leading to a nonlinear enhancement in conductance. The strength of this learned enhancement depends on the both the time correlation and number of pulse stimuli, consistent with spike timing dependent plasticity. The nanowire devices presented have neural synapse-like properties that could serve as a building block for neuromorphic computation.

As-synthesized single-walled carbon nanotubes (SWCNTs) are a mixture of metallic and semiconducting tubes, and separation is essential to improve the performances of SWCNT-based electric devices. Our chemical sensor monitors the conductivity of an SWCNT network, wherein each tube is wrapped by an insulating metallosupramolecular polymer (MSP). Vapors of strong electrophiles such as diethyl chlorophosphate (DECP), a nerve agent simulant, can trigger the disassembly of MSPs, resulting in conductive SWCNT pathways. Herein, we report that separated SWCNTs have a large impact on the sensitivity and selectivity of chemical sensors. Semiconducting SWCNT (S-SWCNT) sensors are the most sensitive to DECP (up to 10000% increase in conductivity). By contrast, the responses of metallic SWCNT (M-SWCNT) sensors were smaller but less susceptible to interfering signals. For saturated water vapor, increasing and decreasing conductivities were observed for S- and M-SWCNT sensors, respectively. Mixtures of M- and S-SWCNTs revealed reduced responses to saturated water vapor as a result of canceling effects. Our results reveal that S- and M-SWCNTs compensate sensitivity and selectivity, and the combined use of separated SWCNTs, either in arrays or in single sensors, offers advantages in sensing systems.

Access to the full text of the published version may require a subscription. Rights AbstractThe synthesis of Ge nanowires with very high-aspect ratios (greater than 1000) and uniform crystal growth directions is highly desirable, not only for investigating the fundamental properties of nanoscale materials, but also for fabricating integrated functional nanodevices. In this article, we present a unique approach for manipulating the supersaturation, and thus the growth kinetics, of Ge nanowires using Au/Ge bilayer films. Ge nanowires were synthesized on substrates consisting of two parts: a Au film on one half of a Si substrate and a Au/Ge bilayer film on the other half of the 2 substrate. Upon annealing the substrate, Au and Au/Ge binary alloy catalysts were formed on the Au-side and Au/Ge-side of the substrates respectively, under identical conditions. The mean lengths of Ge nanowires produced were found to be significantly longer on the Au/Ge bilayer side of the substrate compared to the Au-coated side, as a result of a reduced incubation time for nucleation on the bi-layer side. The mean length and growth rate on the bilayer side (with a 1 nm Ge film) was found to be 5.5 ± 2.3 µm and 3.7 × 10 -3 µm s -1 respectively, and 2.7 ± 0.8 µm and 1.8 × 10 -3 µm s -1for the Au film. Additionally, the lengths and growth rates of the nanowires further increased as the thickness of the Ge layer in the Au/Ge bilayer was increased. In-situ TEM experiments were performed to probe the kinetics of Ge nanowire growth from the Au/Ge bilayer substrates.Diffraction contrast during in-situ heating of the bilayer films clarified the fact that thinner Ge films,i.e. lower Ge concentration, take longer to alloy with Au than thicker films. Phase separation was also more significant for thicker Ge films upon cooling. The use of binary alloy catalyst particles, instead of the more commonly used elementary metal catalyst, enabled the supersaturation of Ge during nanowire growth to be readily tailored, offering a unique approach to producing very long high aspect ratio nanowires.

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Single crystal iron nanocubes are produced by simply heating a bilayer film. This surface energy driven growth (SEDG) method exploits the difference in surface energies of the components (γ(Fe) ~ 2.2 J m(-2) versus γ(Nd) ~ 0.7 J m(-2)) in the binary alloy Fe-Nd system to produce nanocubes of the higher energy Fe component. The dimensions of the cubes range from tens to hundreds of nanometers in size and can be controlled by changing the initial thickness of iron in the deposited Fe-Nd bilayer prior to annealing at 700 °C. The composition and structure of the nanocubes was confirmed by transmission electron microscopy analysis as single crystal bcc iron in the α-phase. The cubes were found to exist as core-shell structures with the α-phase encased by an intermetallic Fe-Nd phase, characteristic of the SEDG growth mechanism.

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