Quantum transport localization through graphene.

Localization of atomic defect-induced electronic transport through a single graphene layer is calculated using a full-valence electronic structure description as a function of the defect density and taking into account the atomic-scale deformations of the layer. The elementary electronic destructive interferences leading to Anderson localization are analyzed. The low-voltage current intensity decreases with increasing length and defect density, with a calculated localization length ζ = 3.5 nm for a defect density of 5%. The difference from the experimental defect density of 0.5% required for an oxide surface-supported graphene to obtain the same ζ is discussed, pointing out how interactions of the graphene supporting surface and surface chemical modifications also control electronic transport localization.

Self-assembled diacetylene molecular wire polymerization on an insulating hexagonal boron nitride (0001) surface.

The electrical characterization of single-polymer chains on a surface is an important step towards novel molecular device development. The main challenge is the lack of appropriate atomically flat insulating substrates for fabricating single-polymer chains. Here, using atomic force microscopy, we demonstrate that the (0001) surface of an insulating hexagonal boron nitride (h-BN) substrate leads to a flat-lying self-assembled monolayer of diacetylene compounds. The subsequent heating or ultraviolet irradiation can initiate an on-surface polymerization process leading to the formation of long polydiacetylene chains. The frequency of photo-polymerization occurrence on h-BN(0001) is two orders of magnitude higher than that on graphite(0001). This is explained by the enhanced lifetime of the molecular excited state, because relaxation via the h-BN is suppressed due to a large band gap. We also demonstrate that on-surface polymerization on h-BN(0001) is possible even after the lithography process, which opens up the possibility of further electrical investigations.

Self-assembling diacetylene molecules on atomically flat insulators.

Single crystal sapphire and diamond surfaces are used as planar, atomically flat insulating surfaces, for the deposition of the diacetylene compound 10,12-nonacosadiynoic acid. The surface assembly is compared with results on hexagonal boron nitride (h-BN), highly oriented pyrolytic graphite (HOPG) and MoS2 surfaces. A perfectly flat-lying monolayer of 10,12-nonacosadiynoic acid self-assembles on h-BN like on HOPG and MoS2. On sapphire and oxidized diamond surfaces, we observed assemblies of standing-up molecular layers. Surface assembly is driven by surface electrostatic dipoles. Surface polarity is partially controlled using a hydrogenated diamond surface or totally screened by the deposition of a graphene layer on the sapphire surface. This results in a perfectly flat and organized SAM on graphene, which is ready for on-surface polymerization of long and isolated molecular wires under ambient conditions.

Position detection and observation of a conducting filament hidden under a top electrode in a Ta2O5-based atomic switch.

Resistive random access memories (ReRAMs) are promising next-generation memory devices. Observation of the conductive filaments formed in ReRAMs is essential in understanding their operating mechanisms and their expected ultimate performance. Finding the position of the conductive filament is the key process in the preparation of samples for cross-sectional transmission electron microscopy (TEM) imaging. Here, we propose a method for locating the position of conductive filaments hidden under top electrodes. Atomic force microscopy imaging with a conductive tip detects the current flowing through a conductive filament from the bottom electrode, which reaches its maximum at a position that is above the conductive filament. This is achieved by properly biasing a top electrode, a bottom electrode and the conductive tip. This technique was applied to Cu/Ta2O5/Pt atomic switches, revealing the formation of a single Cu filament in a device, although the device had a large area of 5 × 5 µm(2). Change in filament size was clearly observed depending on the compliance current used in the set process. It was also found from the TEM observation that the cross-sectional shape of the formed filament varies considerably, which is attributable to different Cu nuclei growth mechanisms.

Plasmon-mediated photocatalytic activity of wet-chemically prepared ZnO nanowire arrays.

We report on measurements and simulations of the efficient sunlight-driven and visible-active photocatalysts composed of plasmonic metal nanoparticles and ZnO nanowire (NW) arrays fabricated via an all-wet-chemical route. Because of the coupling between the ZnO dielectric response and the excitation of the Ag or Au nanoparticles, efficient electronic excitation can be induced in the vicinity of the metal-ZnO interfaces because optically-excited plasmonic particles can not only concentrate the electromagnetic field at the ZnO/particle interface, but also act as efficient sources of plasmonic hot electrons to be injected into the conduction band of the ZnO catalyst. The catalytic activities of the fabricated ZnO NWs are examined by photodegradation of methylene blue and by photocurrent measurements in a photovoltaic configuration. Numerical electromagnetic simulations were used to understand the behavior of the light on the nanometer-scale to clarify the catalytic enhancement mechanisms in both the ultraviolet (UV) and visible (VIS) regions. In addition, simulation results indicated that a near-surface normal but slightly tilted ZnO NW array geometry would provide an increased optical path length and enhanced multiple scattering and absorption processes arising from the localized surface plasmon resonances of the nanoparticles. The results obtained here clarify the role of the plasmon resonance and provide us with useful knowledge for the development of metal-oxide nano-hybrid materials for solar energy conversion.

Benchtop fabrication of memristive atomic switch networks.

Recent advances in nanoscale science and technology provide possibilities to directly self-assemble and integrate functional circuit elements within the wiring scheme of devices with potentially unique architectures. Electroionic resistive switching circuits comprising highly interconnected fractal electrodes and metal-insulator-metal interfaces, known as atomic switch networks, have been fabricated using simple benchtop techniques including solution-phase electroless deposition. These devices are shown to activate through a bias-induced forming step that produces the frequency dependent, nonlinear hysteretic switching expected for gapless-type atomic switches and memristors. By eliminating the need for complex lithographic methods, such an approach toward device fabrication provides a more accessible platform for the study of ionic resistive switches and memristive systems.

Atomically controlled electrochemical nucleation at superionic solid electrolyte surfaces.

Electrochemical equilibrium and the transfer of mass and charge through interfaces at the atomic scale are of fundamental importance for the microscopic understanding of elementary physicochemical processes. Approaching atomic dimensions, phase instabilities and instrumentation limits restrict the resolution. Here we show an ultimate lateral, mass and charge resolution during electrochemical Ag phase formation at the surface of RbAg(4)I(5) superionic conductor thin films. We found that a small amount of electron donors in the solid electrolyte enables scanning tunnelling microscope measurements and atomically resolved imaging. We demonstrate that Ag critical nucleus formation is rate limiting. The Gibbs energy of this process takes discrete values and the number of atoms of the critical nucleus remains constant over a large range of applied potentials. Our approach is crucial to elucidate the mechanism of atomic switches and highlights the possibility of extending this method to a variety of other electrochemical systems.

Ordered monomolecular layers as a template for the regular arrangement of gold nanoparticles.

Ordered arrays of metal nanoparticles are important for nanoelectronic and nanophotonic applications. Here, we report the formation of self-assembled arrays of gold nanoparticles on molecular layers of diacetylene compounds on a MoS2(0001) substrate. The arrangement of gold nanoparticles is observed using scanning tunneling microscopy. When gold is deposited on a self-assembled monolayer of 10,12-nonacosadiynoic acid or 10,12-octadecadiynoic acid on a MoS2(0001) substrate, the ordered array of diacetylene moieties in the molecular layer serves as a template for the formation of ordered arrays of gold nanoparticles. In contrast, when gold is deposited on a pristine MoS2(0001) surface or on a molecular layer of stearic acid, the gold nanoparticles are randomly distributed on the surface. It is found that the arrangement of gold nanoparticles is largely determined by the deposition rate; faster deposition results in more ordered arrays of gold nanoparticles. Our observations confirm the role of unsaturated π systems in molecules acting as a template for the regular arrangement of gold nanoparticles; this work will open up new possibilities for interfacial nanoarchitectonics.

Double-side-coated nanomechanical membrane-type surface stress sensor (MSS) for one-chip-one-channel setup.

With their capability for real-time and label-free detection of targets ranging from gases to biological molecules, nanomechanical sensors are expected to contribute to various fields, such as medicine, security, and environmental science. For practical applications, one of the major issues of nanomechanical sensors is the difficulty of coating receptor layers on their surfaces to which target molecules adsorb or react. To have measurable deflection, a single-side coating is commonly applied to cantilever-type geometry, and it requires specific methods or protocols, such as inkjet spotting or gold-thiol chemistry. If we can apply a double-side coating to nanomechanical sensors, it allows almost any kind of coating technique including dip coating methods, making nanomechanical sensors more useful with better user experiences. Here we address the feasibility of the double-side coating on nanomechanical sensors demonstrated by a membrane-type surface stress sensor (MSS) and verify its working principle by both finite element analysis (FEA) and experiments. In addition, simple hand-operated dip coating is demonstrated as a proof of concept, achieving practical receptor layers without any complex instrumentation. Because the double-side coating is compatible with batch protocols such as dip coating, double-side-coated MSS represents a new paradigm of one-chip-one-channel (channels on a chip are all coated with the same receptor layers) shifting from the conventional one-chip-multiple-channel (channels on a chip are coated with different receptor layers) paradigm.

Synaptic plasticity and memory functions achieved in a WO3-x-based nanoionics device by using the principle of atomic switch operation.

A compact neuromorphic nanodevice with inherent learning and memory properties emulating those of biological synapses is the key to developing artificial neural networks rivaling their biological counterparts. Experimental results showed that memorization with a wide time scale from volatile to permanent can be achieved in a WO3-x-based nanoionics device and can be precisely and cumulatively controlled by adjusting the device's resistance state and input pulse parameters such as the amplitude, interval, and number. This control is analogous to biological synaptic plasticity including short-term plasticity, long-term potentiation, transition from short-term memory to long-term memory, forgetting processes for short- and long-term memory, learning speed, and learning history. A compact WO3-x-based nanoionics device with a simple stacked layer structure should thus be a promising candidate for use as an inorganic synapse in artificial neural networks due to its striking resemblance to the biological synapse.

A theoretical and experimental study of neuromorphic atomic switch networks for reservoir computing.

Atomic switch networks (ASNs) have been shown to generate network level dynamics that resemble those observed in biological neural networks. To facilitate understanding and control of these behaviors, we developed a numerical model based on the synapse-like properties of individual atomic switches and the random nature of the network wiring. We validated the model against various experimental results highlighting the possibility to functionalize the network plasticity and the differences between an atomic switch in isolation and its behaviors in a network. The effects of changing connectivity density on the nonlinear dynamics were examined as characterized by higher harmonic generation in response to AC inputs. To demonstrate their utility for computation, we subjected the simulated network to training within the framework of reservoir computing and showed initial evidence of the ASN acting as a reservoir which may be optimized for specific tasks by adjusting the input gain. The work presented represents steps in a unified approach to experimentation and theory of complex systems to make ASNs a uniquely scalable platform for neuromorphic computing.

Multilayer silicene nanoribbons.

The synthesis of silicene, graphene-like silicon, has generated very strong interest. Here, we reveal the growth of high aspect ratio, perfectly straight, and aligned silicon nanoribbons, exhibiting pyramidal cross section. They are multistacks of silicene and show in angle-resolved photoemission cone-like dispersion of their π and π* bands, at the X[overline] point of their one-dimensional Brillouin zone, with Fermi velocity of ~1.3 × 10(6) m sec(-1), which is very promising for potential applications.

Enhancing the humidity sensitivity of Ga2O3 /SnO2 core/shell microribbon by applying mechanical strain and its application as a flexible strain sensor.

The humidity sensitivity of a single ß-Ga(2) O(3) /amorphous SnO(2) core/shell microribbon on a flexible substrate is enhanced by the application of tensile strain and increases linearly with the strain. The strain-induced enhancement originates from the increase in the effective surface area where water molecules are adsorbed. This strain dependence of humidity sensitivity can be used to monitor the external strain. The strain sensing of the microribbon device under various amounts of mechanical loading shows excellent reliability and reproducibility with a gauge factor of -41. The flexible device has high potential to detect both humidity and strain at room temperature. These findings and the mechanism involved are expected to pave the way for new flexible strain and multifunctional sensors.

Short-term plasticity and long-term potentiation mimicked in single inorganic synapses.

Memory is believed to occur in the human brain as a result of two types of synaptic plasticity: short-term plasticity (STP) and long-term potentiation (LTP; refs 1-4). In neuromorphic engineering, emulation of known neural behaviour has proven to be difficult to implement in software because of the highly complex interconnected nature of thought processes. Here we report the discovery of a Ag(2)S inorganic synapse, which emulates the synaptic functions of both STP and LTP characteristics through the use of input pulse repetition time. The structure known as an atomic switch, operating at critical voltages, stores information as STP with a spontaneous decay of conductance level in response to intermittent input stimuli, whereas frequent stimulation results in a transition to LTP. The Ag(2)S inorganic synapse has interesting characteristics with analogies to an individual biological synapse, and achieves dynamic memorization in a single device without the need of external preprogramming. A psychological model related to the process of memorizing and forgetting is also demonstrated using the inorganic synapses. Our Ag(2)S element indicates a breakthrough in mimicking synaptic behaviour essential for the further creation of artificial neural systems that emulate characteristics of human memory.

Conductance quantization and synaptic behavior in a Ta2O5-based atomic switch.

Quantized conductance was observed in a cation-migration-based resistive switching memory cell with a simple metal-insulator-metal (MIM) structure using a thin Ta(2)O(5) layer. The observed conductance changes are attributed to the formation and dissolution of a metal filament with an atomic point contact of different integer multiples in the Ta(2)O(5) layer. The results demonstrate that atomic point contacts can be realized in an oxide-based MIM structure that functions as a nanogap-based atomic switch (Terabe et al 2005 Nature 433 47). By applying consecutive voltage pulses at periodic intervals of different times, we also observed an effect analogous to the long-term potentiation of biological synapses, which shows that the oxide-based atomic switch has potential for use as an essential building block of neural computing systems.

Two dimensional array of piezoresistive nanomechanical Membrane-type Surface Stress Sensor (MSS) with improved sensitivity.

We present a new generation of piezoresistive nanomechanical Membrane-type Surface stress Sensor (MSS) chips, which consist of a two dimensional array of MSS on a single chip. The implementation of several optimization techniques in the design and microfabrication improved the piezoresistive sensitivity by 3~4 times compared to the first generation MSS chip, resulting in a sensitivity about ~100 times better than a standard cantilever-type sensor and a few times better than optical read-out methods in terms of experimental signal-to-noise ratio. Since the integrated piezoresistive read-out of the MSS can meet practical requirements, such as compactness and not requiring bulky and expensive peripheral devices, the MSS is a promising transducer for nanomechanical sensing in the rapidly growing application fields in medicine, biology, security, and the environment. Specifically, its system compactness due to the integrated piezoresistive sensing makes the MSS concept attractive for the instruments used in mobile applications. In addition, the MSS can operate in opaque liquids, such as blood, where optical read-out techniques cannot be applied.

Single-molecule detection with enhanced Raman scattering of tungsten oxide nanostructure.

We have found that tungsten oxide nanorods have a very large enhancement effect on Raman scattering. The nanorods with adsorbed 12CO and 13CO at the ratio of 1 : 1 were dispersed on a Si substrate and Raman mapping was performed. The Raman images of 12CO and 13CO were completely different, indicating that a very small number of molecules at the single-molecule level were observed. We also confirmed the characteristic blinking phenomenon when single-molecule detection was performed. The very large enhancement effect of Raman scattering can be attributed to the {001}CS structure of the tungsten oxide nanorods. It was confirmed from the DFT calculation results that the {001}CS structure exhibits two-dimensional electrical conduction properties.

Quantum Conductance in Memristive Devices: Fundamentals, Developments, and Applications.

Quantum effects in novel functional materials and new device concepts represent a potential breakthrough for the development of new information processing technologies based on quantum phenomena. Among the emerging technologies, memristive elements that exhibit resistive switching, which relies on the electrochemical formation/rupture of conductive nanofilaments, exhibit quantum conductance effects at room temperature. Despite the underlying resistive switching mechanism having been exploited for the realization of next-generation memories and neuromorphic computing architectures, the potentialities of quantum effects in memristive devices are still rather unexplored. Here, a comprehensive review on memristive quantum devices, where quantum conductance effects can be observed by coupling ionics with electronics, is presented. Fundamental electrochemical and physicochemical phenomena underlying device functionalities are introduced, together with fundamentals of electronic ballistic conduction transport in nanofilaments. Quantum conductance effects including quantum mode splitting, stability, and random telegraph noise are analyzed, reporting experimental techniques and challenges of nanoscale metrology for the characterization of memristive phenomena. Finally, potential applications and future perspectives are envisioned, discussing how memristive devices with controllable atomic-sized conductive filaments can represent not only suitable platforms for the investigation of quantum phenomena but also promising building blocks for the realization of integrated quantum systems working in air at room temperature.

Chemical wiring and soldering toward all-molecule electronic circuitry.

Key to single-molecule electronics is connecting functional molecules to each other using conductive nanowires. This involves two issues: how to create conductive nanowires at designated positions, and how to ensure chemical bonding between the nanowires and functional molecules. Here, we present a novel method that solves both issues. Relevant functional molecules are placed on a self-assembled monolayer of diacetylene compound. A probe tip of a scanning tunneling microscope is then positioned on the molecular row of the diacetylene compound to which the functional molecule is adsorbed, and a conductive polydiacetylene nanowire is fabricated by initiating chain polymerization by stimulation with the tip. Since the front edge of chain polymerization necessarily has a reactive chemical species, the created polymer nanowire forms chemical bonding with an encountered molecular element. We name this spontaneous reaction "chemical soldering". First-principles theoretical calculations are used to investigate the structures and electronic properties of the connection. We demonstrate that two conductive polymer nanowires are connected to a single phthalocyanine molecule. A resonant tunneling diode formed by this method is discussed.

Macroscopic superconducting current through a silicon surface reconstruction with indium adatoms: Si(111)-(√7 × âˆš3)-In.

Macroscopic and robust supercurrents are observed by direct electron transport measurements on a silicon surface reconstruction with In adatoms [Si(111)-(√7 × âˆš3)-In]. The superconducting transition manifests itself as an emergence of the zero resistance state below 2.8 K. I-V characteristics exhibit sharp and hysteretic switching between superconducting and normal states with well-defined critical and retrapping currents. The two-dimensional (2D) critical current density J(2D,c) is estimated to be as high as 1.8 A/m at 1.8 K. The temperature dependence of J(2D,c) indicates that the surface atomic steps play the role of strongly coupled Josephson junctions.