Nanoionics-based resistive switching memories

Many metal-insulator-metal systems show electrically induced resistive switching effects and have therefore been proposed as the basis for future non-volatile memories. They combine the advantages of Flash and DRAM (dynamic random access memories) while avoiding their drawbacks, and they might be highly scalable. Here we propose a coarse-grained classification into primarily thermal, electrical or ion-migration-induced switching mechanisms. The ion-migration effects are coupled to redox processes which cause the change in resistance. They are subdivided into cation-migration cells, based on the electrochemical growth and dissolution of metallic filaments, and anion-migration cells, typically realized with transition metal oxides as the insulator, in which electronically conducting paths of sub-oxides are formed and removed by local redox processes. From this insight, we take a brief look into molecular switching systems. Finally, we discuss chip architecture and scaling issues.

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.

Quantized conductance atomic switch

A large variety of nanometre-scale devices have been investigated in recent years that could overcome the physical and economic limitations of current semiconductor devices. To be of technological interest, the energy consumption and fabrication cost of these 'nanodevices' need to be low. Here we report a new type of nanodevice, a quantized conductance atomic switch (QCAS), which satisfies these requirements. The QCAS works by controlling the formation and annihilation of an atomic bridge at the crossing point between two electrodes. The wires are spaced approximately 1 nm apart, and one of the two is a solid electrolyte wire from which the atomic bridges are formed. We demonstrate that such a QCAS can switch between 'on' and 'off' states at room temperature and in air at a frequency of 1 MHz and at a small operating voltage (600 mV). Basic logic circuits are also easily fabricated by crossing solid electrolyte wires with metal electrodes.

The effect of hydroxyapatite coating on bone growth into porous titanium alloy implants

In rabbits and goats, test implants with a porous surface of two layers of Tl-6A;-4V beads were examined at intervals for bond strength with bone. Half of the implants were coated with hydroxyapatite by plasma spray. The bonding strength with bone in the coated specimens was about four times greater than that of the uncoated specimens at two weeks, and twice as strong at six weeks. Twelve weeks after implantation, the strengths were similar. The hydroxyapatite coating of the beads provided earlier and stronger fixation.

Atomic switch: atom/ion movement controlled devices for beyond von-neumann computers

An atomic switch is a nanoionic device that controls the diffusion of metal ions/atoms and their reduction/oxidation processes in the switching operation to form/annihilate a conductive path. Since metal atoms can provide a highly conductive channel even if their cluster size is in the nanometer scale, atomic switches may enable downscaling to smaller than the 11 nm technology node, which is a great challenge for semiconductor devices. Atomic switches also possess novel characteristics, such as high on/off ratios, very low power consumption and non-volatility. The unique operating mechanisms of these devices have enabled the development of various types of atomic switch, such as gap-type and gapless-type two-terminal atomic switches and three-terminal atomic switches. Novel functions, such as selective volatile/nonvolatile, synaptic, memristive, and photo-assisted operations have been demonstrated. Such atomic switch characteristics can not only improve the performance of present-day electronic systems, but also enable development of new types of electronic systems, such as beyond von- Neumann computers.

The Way to Nanoarchitectonics and the Way of Nanoarchitectonics

The critical differences between microtechnology and nanotechnology are discussed, and the necessity of a new paradigm, nanoarchitectonics, is proposed for the future development of nanotechnology. An important task in material fabrication is to harmonize various factors and effects, and to combine them into functional nanomaterials and nanosystems. It is the way of architectonics rather than that of an individual technology. Therefore, a novel terminology, nanoarchitectonics (nano + architecto +nics) has been proposed as a new paradigm of materials science and technology on the nanoscale. The statement by Feynman that "there's plenty of room at the bottom" is really true. With nanoarchitectonics in our hands, we can re-open the door to Feynman's huge room.

ZnO-based ultraviolet photodetectors

Ultraviolet (UV) photodetection has drawn a great deal of attention in recent years due to a wide range of civil and military applications. Because of its wide band gap, low cost, strong radiation hardness and high chemical stability, ZnO are regarded as one of the most promising candidates for UV photodetectors. Additionally, doping in ZnO with Mg elements can adjust the bandgap largely and make it feasible to prepare UV photodetectors with different cut-off wavelengths. ZnO-based photoconductors, Schottky photodiodes, metal-semiconductor-metal photodiodes and p-n junction photodetectors have been developed. In this work, it mainly focuses on the ZnO and ZnMgO films photodetectors. We analyze the performance of ZnO-based photodetectors, discussing recent achievements, and comparing the characteristics of the various photodetector structures developed to date.

Formation of Nanometer-Scale Grooves in Silicon with a Scanning Tunneling Microscope

Grooves a few nanometers wide can be formed on a Si(111) surface with a scanning tunneling microscope when the tip is above a critical voltage. This may provide a promising approach to nanodevice fabrication. The dependence of the critical voltage on tunneling current, tip polarity, and tip material was studied with silver, gold, platinum, and tungsten tips. The results are consistent with field emission of positive and negative silicon ions. The variation of critical voltage with current is explained quantitatively by a simple tunneling equation that includes the effect of the contact potential between tip and sample.

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.

Antibacterial activity of silver ions implanted in SiO2 filler on oral streptococci

OBJECTIVE

To evaluate the role of silver ions in composite resin dental materials, an in vivo investigation was conducted into the antibacterial effect of SiO2 filler implanted with silver ions on oral streptococci. METHODS. SiO2 filler samples (0.1g) were implanted with silver ions. The effect of the filler with silver ions (Ag+ filler) was tested on oral streptococci bacteria. These bacterial strains had been isolated predominantly from composite resin surfaces. The organisms tested were anaerobically cultured in 5 mL Trypticase Soy Broth containing 0.5 per cent yeast extract at 37 degrees C for 10-12 h. Each bacterial strain was adjusted to a concentration of 1 x 10(6) cells per mL with reduced transport fluid (RTF). Ag+ filler was immersed in 1 mL of RTF and anaerobically incubated 2, 6 and 12 h to study the antibacterial effect. The survival of bacteria was then estimated by culturing on TSBY agar plates. A plate with approximately 100 discrete colonies was chosen from the serial agar cultures, and the number of colonies was counted at each sampling time.

RESULTS

The Ag+ filler showed significantly more antibacterial activity than the control filler without silver ions.

SIGNIFICANCE

These results indicate that the antibacterial effect found in this study was due to the silver ions released by the Ag+ filler and that it may be useful to add this filler to composite resin dental materials for secondary caries protection.

Monitoring the presence of ionic mercury in environmental water by plasmon-enhanced infrared spectroscopy

We demonstrate the ppt-level single-step selective monitoring of the presence of mercury ions (Hg²⁺) dissolved in environmental water by plasmon-enhanced vibrational spectroscopy. We combined a nanogap-optimized mid-infrared plasmonic structure with mercury-binding DNA aptamers to monitor in-situ the spectral evolution of the vibrational signal of the DNA induced by the mercury binding. Here, we adopted single-stranded thiolated 15-base DNA oligonucleotides that are immobilized on the Au surface and show strong specificity to Hg²⁺. The mercury-associated distinct signal is located apart from the biomolecule-associated broad signals and is selectively characterized. For example, with natural water from Lake Kasumigaura (Ibaraki Prefecture, Japan), direct detection of Hg²⁺ with a concentration as low as 37 ppt (37 × 10⁻¹⁰%) was readily demonstrated, indicating the high potential of this simple method for environmental and chemical sensing of metallic species in aqueous solution.

Generic relevance of counter charges for cation-based nanoscale resistive switching memories

Resistive switching memories (ReRAMs) are the major candidates for replacing the state-of-the-art memory technology in future nanoelectronics. These nonvolatile memory cells are based on nanoionic redox processes and offer prospects for high scalability, ultrafast write and read access, and low power consumption. The interfacial electrochemical reactions of oxidation and reduction of ions necessarily needed for resistive switching result inevitably in nonequilibrium states, which play a fundamental role in the processes involved during device operation. We report on nonequilibrium states in SiO2-based ReRAMs being induced during the resistance transition. It is demonstrated that the formation of metallic cations proceeds in parallel to reduction of moisture, supplied by the ambient. The latter results in the formation of an electromotive force in the range of up to 600 mV. The outcome of the study highlights the hitherto overlooked necessity of a counter charge/reaction to keep the charge electroneutrality in cation-transporting thin films, making it hard to analyze and compare experimental results under different ambient conditions such as water partial pressure. Together with the dependence of the electromotive force on the ambient, these results contribute to the microscopic understanding of the resistive switching phenomena in cation-based ReRAMs.

Shaping and reshaping of salmonid genomes by amplification of tRNA-derived retroposons during evolution

Three families of tRNA-derived repeated retroposons in the genomes of salmonid species have been isolated and characterized. These three families differ in sequence, but all are derived from a tRNA(Lys) or from a tRNA species structurally related to tRNA(Lys). The salmon Sma I family is present in the genomes of two species of the genus Oncorhynchus but not in other species, including five other species of the same genus. The charr Fok I family is present only in four species and subspecies of the genus Salvelinus. The third family, the salmonid Hpa I family, appears to be present in all salmonid species but is not present in species that are not members of the Salmonidae. Thus, the genome of proto-Salmonidae was originally shaped by amplification and dispersion of the salmonid Hpa I family and then reshaped by amplification of the Sma I and Fok I families in the more recently evolved species of salmon and charr, respectively. We speculate that amplification and dispersion of retroposons may have played a role in salmonid speciation.

Forming and switching mechanisms of a cation-migration-based oxide resistive memory

We report detailed current-voltage and current-time measurements to reveal the forming and switching behaviors of Cu/Ta(2)O(5)/Pt nonvolatile resistive memory devices. The devices can be initially SET (from the OFF state to the ON state) when a low positive bias voltage is applied to the Cu electrode. This first SET operation corresponds to the first formation of a metal filament by inhomogeneous nucleation and subsequent growth of Cu on the Pt electrode, based on the migration of Cu ions in the stable Ta(2)O(5) matrix. After the forming, the device exhibits bipolar switching behavior (SET at positive bias and RESET (from the ON state to the OFF state) at negative bias) with increasing the ON resistance from a few hundred Ω to a few kΩ. From the measurements of the temperature stability of the ON states, we concluded that the RESET process consists of the Joule-heating-assisted oxidation of Cu atoms at the thinnest part of the metal filament followed by diffusion and drift of the Cu ions under their own concentration gradient and the applied electric field, disconnecting the metal filament. With ON resistances of the order of a few kΩ, the SET and RESET operations are repeated by the inhomogeneous nucleation and the Joule-heating-assisted dissolution of a small filament on a remaining filament. This switching model is applicable to the operation of cation-migration-based resistive memories using other oxide materials.

Effects of volatile anesthetics on N-methyl-D-aspartate excitotoxicity in primary rat neuronal-glial cultures

BACKGROUND

Volatile anesthetics are known to ameliorate experimental ischemic brain injury. A possible mechanism is inhibition of excitotoxic cascades induced by excessive glutamatergic stimulation. This study examined interactions between volatile anesthetics and excitotoxic stress.

METHODS

Primary cortical neuronal-glial cultures were exposed to N-methyl-D-aspartate (NMDA) or glutamate and isoflurane (0.1-3.3 mM), sevoflurane (0.1-2.9 mM), halothane (0.1-2.9 mM), or 10 microM (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine hydrogen maleate (MK-801). Lactate dehydrogenase release was measured 24 h later. In other cultures, effects of volatile anesthetics on Ca++ uptake and mitochondrial membrane potential were determined in the presence or absence of NMDA (0-200 microM).

RESULTS

Volatile anesthetics reduced excitotoxin induced lactate dehydrogenase release by up to 52% in a dose-dependent manner. At higher concentrations, this protection was reversed. When corrected for olive oil solubility, the three anesthetics offered equivalent protection. MK-801 provided near-complete protection. Ca++ uptake was proportionally reduced with increasing concentrations of anesthetic but did not account for reversal of protection at higher anesthetic concentrations. Given equivalent NMDA-induced Ca++ loads, cells treated with volatile anesthetic had greater lactate dehydrogenase release than those left untreated. At protective concentrations, volatile anesthetics partially inhibited NMDA-induced mitochondrial membrane depolarization. At higher concentrations, volatile anesthetics alone were sufficient to induce mitochondrial depolarization.

CONCLUSIONS

Volatile anesthetics offer similar protection against excitotoxicity, but this protection is substantially less than that provided by selective NMDA receptor antagonism. Peak effects of NMDA receptor antagonism were observed at volatile anesthetic concentrations substantially greater than those used clinically.

Neuromorphic atomic switch networks

Efforts to emulate the formidable information processing capabilities of the brain through neuromorphic engineering have been bolstered by recent progress in the fabrication of nonlinear, nanoscale circuit elements that exhibit synapse-like operational characteristics. However, conventional fabrication techniques are unable to efficiently generate structures with the highly complex interconnectivity found in biological neuronal networks. Here we demonstrate the physical realization of a self-assembled neuromorphic device which implements basic concepts of systems neuroscience through a hardware-based platform comprised of over a billion interconnected atomic-switch inorganic synapses embedded in a complex network of silver nanowires. Observations of network activation and passive harmonic generation demonstrate a collective response to input stimulus in agreement with recent theoretical predictions. Further, emergent behaviors unique to the complex network of atomic switches and akin to brain function are observed, namely spatially distributed memory, recurrent dynamics and the activation of feedforward subnetworks. These devices display the functional characteristics required for implementing unconventional, biologically and neurally inspired computational methodologies in a synthetic experimental system.

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.