Wavelength dependent light tunable resistive switching graphene oxide nonvolatile memory devices

This paper reports on the first optically tunable graphene oxide memristor device. Modulation of resistive switching memory by light opens the route to new optoelectronic devices that can be switched optically and read electronically. Applications include integrated circuits with memory elements switchable by light and optically reconfigurable and tunable synaptic circuits for neuromorphic computing and brain-inspired, artificial intelligence systems. In this report, planar and vertical structured optical resistive switching memristors based on graphene oxide are reported. The device is switchable by either optical or electronic means, or by a combination of both. In addition the devices exhibit a unique wavelength dependence that produces reversible and irreversible properties depending on whether the irradiation is long or short wavelength light, respectively. For long wavelength light, the reversible photoconductance effect permits short-term dynamic modulation of the resistive switching properties of the light, which has application as short-term memory in neuromorphic computing. In contrast, short wavelength light induces both the reversible photoconductance effect and an irreversible change in the memristance due to reduction of the graphene oxide. This has important application in the fabrication of cloned neural networks with factory defined weights, enabling the fast replication of artificial intelligent chips with pre-trained information

Nanoscale junctions for single molecule electronics fabricated using bilayer nanoimprint lithography combined with feedback controlled electromigration

Nanoimprint lithography (NIL) is a fast, simple and high throughput technique that allows fabrication of structures with nanometre precision features at low cost. We present an advanced bilayer nanoimprint lithography approach to fabricate four terminal nanojunction devices for use in single molecule electronic studies. In the first part of this work, we demonstrate a NIL lift-off process using a bilayer resist technique that negates problems associated with metal side-wall tearing during lift-off. In addition to precise nanoscale feature replication, we show that it is possible to imprint micron-sized features while still maintaining a bilayer structure enabling an undercut resist structure to be formed. This is accomplished by choosing suitable imprint parameters as well as residual layer etching depth and development time. We then use a feedback controlled electromigration procedure, to produce room-temperature stable nanogap electrodes with sizes below 2 nm. This approach facilitates the integration of molecules in stable, solid-state molecular electronic devices as demonstrated by incorporating benzenethiol as molecular bridges between the electrodes and characterizing its electronics properties through current-voltage measurements. The observation of molecular transport signatures, showing current suppression in the I-V behaviour at low voltage, which is then lifted at high voltage, signifying on- and off-resonant transport through molecular levels as a function of voltage, is confirmed in repeated I-V sweeps. The large conductance, symmetry of the I-V sweep and small value of the voltage minimum in transition voltage spectroscopy indicates the bridging of the two benzenethiol molecules is by π-stacking

Method to reduce the formation of crystallites in ZnO nanorod thin-films grown via ultra-fast microwave heating

© 2018 This paper discusses the nucleation and growth mechanisms of ZnO nanorod thin-films and larger sized crystallites that form within the solution and on surfaces during an ultra-fast microwave heating growth process. In particular, the work focusses on the elimination of crystallites as this is necessary to improve thin-film uniformity and to prevent electrical short circuits between electrodes in device applications. High microwave power during the early stages of ZnO deposition was found to be a key factor in the formation of unwanted crystallites on substrate surfaces. Once formed, the crystallites, grow at a much faster rate than the nanorods and quickly dominate the thin-film structure. A new two-step microwave heating method was developed that eliminates the onset of crystallite formation, allowing the deposition of large-area nanorod thin-films that are free from crystallites. A dissolution-recrystallization mechanism is proposed to explain why this procedure is successful and we demonstrate the importance of the work in the fabrication of low-cost memristor devices

Printed and flexible organic and inorganic memristor devices for non-volatile memory applications

The electronics market is highly competitive and driven by consumers desire for the latest and most sophisticated devices at the lowest cost. In the last decade there has been increasing interest in printing electronic materials on lightweight and flexible substrates such as plastics and fabrics. This not only lowers fabrication and capital costs but also facilitates many new applications, such as flexible displays and wearable electronics. The printing of computer memory is also desirable since many of these applications require memory to store and process information. In addition, there is now an international effort to develop new types of computer memory that consume ultra-low levels of power. This is not only to lower energy usage worldwide, which is important for reducing CO2 emissions, but it also enables a longer period between the re-charging of devices such as mobile phones, music players and fitness bands. Memory that is non-volatile is an obvious choice since it does not consume power to retain information like conventional SRAM and DRAM. Memristors (or memory resistor) are a new type of memory that are intrinsically non-volatile in nature. Their simple two-terminal architecture, easy method of fabrication and low power consumption means they have received much attention from both the research community and industry. Devices with the lowest fabrication costs are made from organic or hybrid (organic–inorganic) composite materials because of the ability to use low-cost solution processing methods with the advantages of large area deposition under vacuum-free and room temperature ambient conditions. Memristors have excellent device properties, including a large resistance Off/On ratio (up to 5 orders of magnitude), fast switching speeds (less than 15 ns), long endurance (over 1012 cycles), long data storage retention time (∼10 years) and high scalability down to nanoscale dimensions. In this article we review progress in the field of printed and flexible memristor devices and discuss their potential across a wide range of applications

3D-structured mesoporous silica memristors for neuromorphic switching and reservoir computing

Memristors are emerging as promising candidates for practical application in reservoir computing systems that are capable of temporal information processing. Here, we experimentally implement a physical reservoir computing system using resistive memristors based on three-dimensional (3D)-structured mesoporous silica (mSiO2) thin films fabricated by a low cost, fast and vacuum-free sol–gel technique. The in situ learning capability and a classification accuracy of 100% on a standard machine learning dataset are experimentally demonstrated. The volatile (temporal) resistive switching in diffusive memristors arises from the formation and subsequent spontaneous rupture of conductive filaments via diffusion of Ag species within the 3D-structured nanopores of the mSiO2 thin film. Besides volatile switching, the devices also exhibit a bipolar non-volatile resistive switching behavior when the devices are operated at a higher compliance current level. The implementation of mSiO2 thin films opens the route to fabricate a simple and low cost dynamic memristor with a temporal information process functionality, which is essential for neuromorphic computing applications

Reversible optical switching memristors with tunable STDP synaptic plasticity: a route to hierarchical control in artificial intelligent systems

Optical control of memristors opens the route to new applications in optoelectronic switching and neuromorphic computing. Motivated by the need for reversible and latched optical switching we report on the development of a memristor with electronic properties tunable and switchable by wavelength and polarization specific light. The device consists of an optically active azobenzene polymer, poly(disperse red 1 acrylate), overlaying a forest of vertically aligned ZnO nanorods. Illumination induces trans- cis isomerization of the azobenzene molecules, which expands or contracts the polymer layer and alters the resistance of the off/on states, their ratio and retention time. The reversible optical effect enables dynamic control of a memristors learning properties including control of synaptic potentiation and depression, optical switching between short -term and long-term memory and optical modulation of the synaptic efficacy via spike timing dependent plasticity. The work opens the route to the dynamic patterning of memristor networks both spatially and temporally by light, thus allowing the development of new optically reconfigurable neural networks and adaptive electronic circuits

Nanorods Versus Nanoparticles: A Comparison Study of Au/ZnO-PMMA/Au Non-Volatile Memory Devices Showing the Importance of Nanostructure Geometry on Conduction Mechanisms and Switching Properties

Hybrid organic-inorganic devices offer a simple and low cost route to the fabrication of resistive memory devices. However the switching and conduction mechanisms are not well established. This work compares ZnO-based devices made in the same manner but having two different nanostructure geometries, vertically aligned ZnO nanorods and randomly dispersed ZnO nanoparticles, both embedded within a PMMA host material and sandwiched between two gold electrodes in a crossbar device configuration. Both device types do not require a forming step to initiate switching and exhibit bipolar switching at relatively low operating voltages. In the low field regime both device types exhibit Ohmic behavior, however in the high field regime their switching and conduction mechanisms are distinctly different. ZnO nanorod-based devices exhibit smooth I-V curves and smooth switching behavior and a conduction mechanism that changes from Poole-Frenkel to Schottky emission when switching from the ON state to the OFF state. In contrast, ZnO nanoparticle devices exhibit sharp switching properties with SCLC behavior in the OFF state and Ohmic conduction in the ON state. These differences in the conduction and switching properties of devices containing the same materials clearly demonstrates the importance of the nanostructure geometry and device architecture on the switching and conduction properties of memristor devices. For each device type we discuss the results and propose plausible mechanisms to account for their different behavior

Electrodeposition of GeSbTe-based resistive switching memory in crossbar arrays

In this work, we report on the fabrication of resistive random-access memory cells based on electrodeposited GeSbTe material between TiN top and bottom electrodes in a crossbar architecture. The cells exhibit asymmetric bipolar resistive switching characteristics under the same SET and RESET compliance current (CC), showing highly uniform and reproducible switching properties. A multi-state switching behavior can be also achieved by varying the sweeping voltage and CC. Unlike phase-change switching, the switching between the high-resistance state and the low-resistance state in these cells can be attributed to the formation and rupture of conductive Te bridge(s) within the Te-rich GeSbTe matrix upon application of a high electric field. The results point toward the usage of the electrodeposition method to fabricate advanced functional device structures for application in non-volatile memory

Dynamic Electric Field Alignment of Metal-Organic Framework Microrods

Alignment of metal-organic framework (MOF) crystals has previously been performed via careful control of oriented MOF growth on substrates, as well as by dynamic magnetic alignment. We show here that bromobenzene-suspended microrod crystals of the MOF NU-1000 can also be dynamically aligned via electric fields, giving rise to rapid electrooptical responses. This method of dynamic MOF alignment opens up new avenues of MOF control which are important for integration of MOFs into switchable electronic devices as well as in other applications such as reconfigurable sensors or optical systems