Atomic Switch Research Articles

An atomic switch using oxygenated amorphous carbon as solid electrolyte

Atomic switches based on carbon films as electrolyte have received great attention due to the many advantages of carbon, but their low high-resistance state and poor thermal stability of the device have greatly limited their further development. Here, we have constructed an atomic switch in which we use an oxygenated amorphous carbon (a-COx) film as a solid electrolyte through introducing O2 gas during the sputtering of a graphite target. The reversible and steady resistive switching effect with a resistance ratio over two orders of magnitude was achieved. While the sputtered pure C film has a low resistivity, which is not suitable for an atomic switch, the resistance of a-COx increases substantially with the partial pressure of O2. More importantly, we found that the higher the partial pressure of O2, the better the high temperature retention. In combination with the two-step reset process in which the heat and electric fields respectively act and mutable forming voltage with temperature, the achieved high temperature retention for more oxidized carbon films is most likely due to the more reduction of structural defects caused by the greater addition of oxygen during the deposition process and the chemical inertness of a-COx.

Recent Progress on Neuromorphic Synapse Electronics: From Emerging Materials, Devices, to Neural Networks.

To realize intelligent functions in electronic devices like a human brain, it is important to develop the electronic devices that can imitate biological neurons and synapses (synaptic electronics). In this paper, we review the critical learning mechanisms for synaptic plasticity. Different electronic devices were developed to mimic biological synapses, such as atomic switch, phase change memory, ferroelectric memory, and electric-double-layer transistors. More importantly, several groups have realized the artificial neuromorphic network using multi-gate transistor architecture. The leap from synapse to neuron to neural network, thus, has been systematically realized using thin films and nanomaterials. The emerging synaptic electronics can have a broader applications and brighter future in the next-generation intelligent nano-electronics.

Tight focusing properties of a circular partially coherent Gaussian beam.

Tight focusing properties of a circular partially coherent Gaussian (CPCG) beam with linear polarization have been studied based on vectorial Debye theory. Expressions for the intensity distribution and degree of coherence near the focus are derived. Numerical calculations are performed to show the intensity distribution and degree of coherence of the CPCG beam in the focal region. It is interesting to find that after focusing the CPCG beam through a high numerical-aperture objective we can obtain a super-length optical needle (>12λ) with homogeneous intensity along the propagation axis and wavelength beam size (∼λ). Moreover, the numerical calculations of coherence illustrate that, in the range of full width at half-maximum of the optical needle, for any two of the parallel electric field components of the optical needle the coherence is close to 1, but for any two of orthometric electric field components the value of coherence is between 0.4 and 0.9. Such a non-diffracting optical needle may have potential applications in atom optical experiments, such as in atom traps and atom switches.

Effect of constructive rehybridization on transverse conductivity of aligned single-walled carbon nanotube films

Alignment of densely packed single-walled carbon nanotubes (SWNTs) largely preserves the extraordinary electronic properties of individual SWNTs in the alignment direction, while in transverse direction the films are very resistive due to large energy barriers for tunneling between adjacent SWNTs. We demonstrate that chromium atoms inserted between the sidewalls of parallel SWNTs effectively coordinate to the benzene rings of the nanotubes via hexahapto bonds that preserve the nanotube-conjugated electronic structure and serve as a conduit for electron transfer. The atomically interconnected aligned SWNTs exhibit enhanced transverse conductivity, which increases by ∼2100% as a result of the photoactivated organometallic functionalization with Cr. The hexahapto mode of bonding the graphitic surfaces of carbon nanotubes with transition metal atoms offers an attractive route to the reversible chemical engineering of the transport properties of aligned carbon nanotube thin films. We demonstrate that a device fabricated with aligned SWNTs can be reversibly switched between a state of high electrical conductivity (ON) by light and low electrical conductivity (OFF) by applied potential. This study provides a route to the design of novel nanomaterials for applications in electrical atomic switches, optoelectronic and spintronic devices.

Route to achieving giant magnetoelectric coupling in BaTiO3/Sr2CoO3F perovskite heterostructures

Polarization-induced spin switching of atoms in magnetic materials opens the possibilities to design and develop advanced spintronic devices, in particular, storage devices where the magnetic state can be controlled by an electric field. We employ density functional theory calculations to study the magnetic properties of a perovskite strontium cobalt oxyfluoride ${\mathrm{Sr}}_{2}{\mathrm{CoO}}_{3}\mathrm{F}$ (SCOF) in a hybrid perovskite heterostructure, where SCOF is sandwiched between two ferroelectic ${\mathrm{BaTiO}}_{3}$ (BTO) layers. Our calculations show that the spin state of the central Co atom in SCOF can be controlled by altering the polarization direction of the BTO, specifically, to switch from a high-spin state to a low-spin state by changing the relative orientation of the ferroelectric polarization of BTO with respect to SCOF, leading to an unexpected, giant magnetoelectric coupling, ${\ensuremath{\alpha}}_{s}\ensuremath{\approx}21\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\phantom{\rule{0.28em}{0ex}}{\mathrm{G}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}}^{2}/\mathrm{V}$.

Development of a molecular gap-type atomic switch and its stochastic operation

The gap-type atomic switch is a novel neuromorphic device that possesses functions such as analog changes in resistance and short-term/long-term memory-based learning. However, it is difficult to integrate conventional gap-type atomic switches that use a vacuum gap and Ag2+δS, which has restricted their practical use. In this study, we developed a new, easy to fabricate gap-type atomic switch that incorporates a molecular layer as a gap and Ta2O5 as an ionic transfer material. This molecular gap-type atomic switch operates in a manner that is similar to conventional vacuum gap-type atomic switches. We also demonstrate stochastic operations using the aforementioned molecular gap-type atomic switches. These results indicate a higher potential for the practical use of gap-type atomic switches.

Effect of Coulomb blockade on the switching time of Ag–Ag2S–Pt atomic switches

A theoretical model based on the single electron tunneling phenomenon is employed to calculate the time-dependent electrical resistance of an Ag–Ag2S–Pt atomic switch at different applied voltages. While a negative voltage is applied to Pt electrode, Ag atoms precipitate on the surface of Ag2S electrode where they form Ag clusters. The resistance of switch decreases as Ag clusters grow larger between two electrodes. Our model calculations imply the time required to decrease the resistance of switch below the resistance quantum (switching time) is mainly determined by the Coulomb blockade effect of Ag clusters. The switching time is found to decrease exponentially with increasing the applied voltage, which agrees very well with the experimental observations.

CMOS compatible low-power volatile atomic switch for steep-slope FET devices

In this paper, we demonstrate a volatile atomic switch that can be utilized for obtaining steep subthreshold swing (SS) (<5 mV/dec) characteristics in FETs. We propose a CMOS-compatible atomic switch (W/Cu2S/W) that demonstrates volatility and immunity to the voltage–time dilemma. Furthermore, we enhance the device characteristics by examining the composition control, scaling of device size, and film thickness. Then, the atomic switch is integrated with a conventional transistor that has a large SS (>60 mV/dec). The result shows an improvement in the SS, which results from the transition of the atomic switch between the ON and OFF states, which is caused by the formation and rupture of a conductive filament. As a result, excellent switching characteristics are obtained for the FETs, such as low IOFF (∼10−5 μA/μm), high ION/IOFF ratio (∼105), low VDD (∼0.25 V), and steep SS (<5 mV/dec).

Resistive switching characteristics of a modified active electrode and Ti buffer layer in Cu[sbnd]Se-based atomic switch

Atomic switches are well-known promising candidates for future application in non-volatile logic memory devices. The resistive switching characteristics of these devices depend on the formation of a conductive filament (CF) by active metal electrodes. However, the formation of a stable CF is still a challenge owing to filament overgrowth in the solid electrolyte. To achieve controlled CF growth, we have used a modified active electrode with different CuxSe1−x composition ratios (0.01 < x < 0.45) and a titanium (Ti) buffer layer. The optimum composition was determined to be Cu0.11Se0.89/Ti (2.5 nm) for which excellent resistive switching properties were observed, such as a high on/off ratio of 104, low operating voltage, uniform resistance distribution, ten-year data retention at 85 °C, and uniform endurance (2000 cycles). The improvement is can be described from the high controllability of the oxidation-reduction reaction rate of the optimized CuSe modified active electrode by the means of Ti buffer layer and the TiO bond formation at the Ti/Al2O3 interface. In addition, depth profiles of the conductive filament based on Cu0.11Se0.89 with a Ti buffer layer were studied by performing current atomic force microscopy (I-AFM) to evaluate the enhancements in electrical performance and reliability resulting from the insertion of the Ti buffer layer.

Enhancement of resistive switching properties in Al2O3 bilayer-based atomic switches: multilevel resistive switching

Atomic switches are considered to be building blocks for future non-volatile data storage and internet of things. However, obtaining device structures capable of ultrahigh density data storage, high endurance, and long data retention, and more importantly, understanding the switching mechanisms are still a challenge for atomic switches. Here, we achieved improved resistive switching performance in a bilayer structure containing aluminum oxide, with an oxygen-deficient oxide as the top switching layer and stoichiometric oxide as the bottom switching layer, using atomic layer deposition. This bilayer device showed a high on/off ratio (105) with better endurance (∼2000 cycles) and longer data retention (104 s) than single-oxide layers. In addition, depending on the compliance current, the bilayer device could be operated in four different resistance states. Furthermore, the depth profiles of the hourglass-shaped conductive filament of the bilayer device was observed by conductive atomic force microscopy.

Thermal annealing and transient electronic excitations induced interfacial and magnetic effects on Pt/Co/Pt trilayer

Present study investigates the importance of thermal annealing and transient electronic excitations (using 100 MeV oxygen ions) in assisting the interfacial atomic diffusion, alloy composition, and magnetic switching field distributions in Pt/Co/Pt stacked trilayer. X-ray diffraction analysis reveals that thermal annealing results in the formation of the face centered tetragonal L1°CoPt phase. The Rutherford back scattering spectra shows a trilayer structure for as-deposited and as-irradiated films. Interlayer mixing on the thermally annealed films further improves by electronic excitations produced by high energy ion irradiation. Magnetically hard face centered tetragonal CoPt alloy retains its hard phase after ion irradiation and reveals an enhancement in the structural ordering and magnetic stability. Enhancement in the homogeneity of alloy composition and its correlation with the magnetic switching field is evident from this study. A detailed investigation of the contributing parameters shows that the magnetic switching behaviour varies with the type of thermal annealing, transient electronic excitations of ion beams and combination of these processes.

Atomic switch networks as complex adaptive systems

Complexity is an increasingly crucial aspect of societal, environmental and biological phenomena. Using a dense unorganized network of synthetic synapses it is shown that a complex adaptive system can be physically created on a microchip built especially for complex problems. These neuro-inspired atomic switch networks (ASNs) are a dynamic system with inherent and distributed memory, recurrent pathways, and up to a billion interacting elements. We demonstrate key parameters describing self-organized behavior such as non-linearity, power law dynamics, and multistate switching regimes. Device dynamics are then investigated using a feedback loop which provides control over current and voltage power-law behavior. Wide ranging prospective applications include understanding and eventually predicting future events that display complex emergent behavior in the critical regime.

The rate limiting process and its activation energy in the forming process of a Cu/Ta2O5/Pt gapless-type atomic switch

The rate limiting process in first resistive switching, called the “forming process”, is determined by measuring the switching time of an as-fabricated Cu/Ta2O5/Pt atomic switch as a function of the ambient temperature and the Ta2O5 thickness. The temperature dependence is well fitted by the Arrhenius equation, suggesting that a certain activation process dominates the switching phenomenon. The switching time increases linearly as the Ta2O5 thickness increases. The results herein clearly suggest that the rate limiting process is the drift of Cu cations in the Ta2O5 layer. We determine that the activation energy is 0.4 eV.

26pPSB-44 硫化銀原子スイッチの伝導特性およびナノフィラメント生成過程に関する第一原理計算(領域9ポスターセッション,領域9,表面・界面,結晶成長)

26pPSB-44 硫化銀原子スイッチの伝導特性およびナノフィラメント生成過程に関する第一原理計算(領域9ポスターセッション,領域9,表面・界面,結晶成長)

Excellent Resistive Switching Performance of Cu–Se-Based Atomic Switch Using Lanthanide Metal Nanolayer at the Cu–Se/Al2O3 Interface

The next-generation electronic society is dependent on the performance of nonvolatile memory devices, which has been continuously improving. In the last few years, many memory devices have been introduced. However, atomic switches are considered to be a simple and reliable basis for next-generation nonvolatile devices. In general, atomic switch-based resistive switching is controlled by electrochemical metallization. However, excess ion injection from the entire area of the active electrode into the switching layer causes device nonuniformity and degradation of reliability. Here, we propose the fabrication of a high-performance atomic switch based on Cu x-Se1- x by inserting lanthanide (Ln) metal buffer layers such as neodymium (Nd), samarium (Sm), dysprosium (Dy), or lutetium (Lu) between the active metal layer and the electrolyte. Current-atomic force microscopy results confirm that Cu ions penetrate through the Ln-buffer layer and form thin conductive filaments inside the switching layer. Compared with the Pt/Cu x-Se1- x/Al2O3/Pt device, the optimized Pt/Cu x-Se1- x/Ln/Al2O3/Pt devices show improvement in the on/off resistance ratio (102-107), retention (10 years/85 °C), endurance (∼10 000 cycles), and uniform resistance state distribution.

Atomic scale reversible opto-structural switching of few atom luminescent silver clusters confined in LTA zeolites.

Luminescent silver clusters (AgCLs) stabilized inside partially Ag exchanged Na LTA zeolites show a remarkable reversible on-off switching of their green-yellowish luminescence that is easily tuned by a hydration and dehydration cycle, making them very promising materials for sensing applications. We have used a unique combination of photoluminescence (PL), UV-visible-NIR Diffuse Reflectance (DRS), X-ray absorption fine structure (XAFS), Fourier Transform-Infrared (FTIR) and electron spin resonance (ESR) spectroscopies to unravel the atomic-scale structural changes responsible for the reversible optical behavior of the confined AgCLs in LTA zeolites. Water coordinated, diamagnetic, tetrahedral AgCLs [Ag4(H2O)4]2+ with Ag atoms positioned along the axis of the sodalite six-membered rings are at the origin of the broad and intense green-yellowish luminescence in the hydrated sample. Upon dehydration, luminescent [Ag4(H2O)4]2+ clusters are transformed into non-luminescent (dark), diamagnetic, octahedral AgCLs [Ag6(OF)14]2+ with Ag atoms interacting strongly with zeolite framework oxygen (OF) of the sodalite four-membered rings. This highly responsive on-off switching reveals that besides quantum confinement and molecular-size, coordinated water and framework oxygen ligands strongly affect the organization of AgCLs valence electrons and play a crucial role in the opto-structural properties of AgCLs.

Thermally stable resistive switching of a polyvinyl alcohol-based atomic switch

Thermally stable resistive switching is demonstrated in a Ag salt incorporated polyvinyl alcohol-based atomic switch.

Copper Nanowires through Oriented Mesoporous Silica: A Step towards Protected and Parallel Atomic Switches

The formation of copper atomic contacts has been investigated. Copper nanowires were grown by electrochemical deposition, in the scanning electrochemical microscopy (SECM) configuration, from a platinum microelectrode to an indium tin oxide (ITO) substrate. Self-termination leaves copper filaments between the two electrodes with an atomic point contact at the ITO electrode. Histogram analysis shows that the conductance of this contact is close to, or less than, 1 G0. Atomic contacts were also fabricated on ITO electrodes covered with vertically-aligned mesoporous silica films. Scanning Transmission Electron Microscopy images show that copper filaments occupy individual isolated nanopores. Contacts generated on bare ITO break down rapidly in sodium salicylate, whereas those generated in ITO/nanopores are unaffected; the nanopores protect the copper filaments. Finally, atomic switch behaviour was obtained using these ITO and ITO/nanopores electrodes.

A two-dimensional ON/OFF switching device based on anisotropic interactions of atomic quantum dots on Si(100):H

Controlling the properties of quantum dots at the atomic scale, such as dangling bonds, is a general motivation as they allow studying various nanoscale processes including atomic switches, charge storage, or low binding energy state interactions. Adjusting the coupling of individual silicon dangling bonds to form a 2D device having a defined function remains a challenge. Here, we exploit the anisotropic interactions between silicon dangling bonds on n-type doped Si(100):H surface to tune their hybridization. This process arises from interactions between the subsurface silicon network and dangling bonds inducing a combination of Jahn–Teller distortions and local charge ordering. A three-pointed star-shaped device prototype is designed. By changing the charge state of this device, its electronic properties are shown to switch reversibly from an ON to an OFF state via local change of its central gap. Our results provide a playground for the study of quantum information at the nanoscale.

Effects of Cu Ion Doping in HfO2-Based Atomic Switching Devices

Effects of Cu Ion Doping in HfO₂-Based Atomic Switching Devices