A comprehensive review of non-invasive optical and microwave biosensors for glucose monitoring.

Frequent glucose monitoring is essential for effective diabetes management. Currently, glucose monitoring is done using invasive methods such as finger-pricking and subcutaneous sensing. However, these methods can cause discomfort, heighten the risk of infection, and some sensing devices need frequent calibration.

The limits of near field immersion microwave microscopy evaluated by imaging bilayer graphene moiré patterns.

Near field scanning Microwave Impedance Microscopy can resolve structures as small as 1 nm using radiation with wavelengths of 0.1 m. Combining liquid immersion microscopy concepts with exquisite force control exerted on nanoscale water menisci, concentration of electromagnetic fields in nanometer-size regions was achieved.

Localization of lattice dynamics in low-angle twisted bilayer graphene.

Twisted bilayer graphene is created by slightly rotating the two crystal networks in bilayer graphene with respect to each other. For small twist angles, the material undergoes a self-organized lattice reconstruction, leading to the formation of a periodically repeated domain. The resulting superlattice modulates the vibrational and electronic structures within the material, leading to changes in the behaviour of electron-phonon coupling and to the observation of strong correlations and superconductivity.

Quantized conductance coincides with state instability and excess noise in tantalum oxide memristors.

Tantalum oxide memristors can switch continuously from a low-conductance semiconducting to a high-conductance metallic state. At the boundary between these two regimes are quantized conductance states, which indicate the formation of a point contact within the oxide characterized by multistable conductance fluctuations and enlarged electronic noise. Here, we observe diverse conductance-dependent noise spectra, including a transition from 1/f(2) (activated transport) to 1/f (flicker noise) as a function of the frequency f, and a large peak in the noise amplitude at the conductance quantum GQ=2e(2)/h, in contrast to suppressed noise at the conductance quantum observed in other systems.

Characterization of electroforming-free titanium dioxide memristors.

Metal-insulator-metal (MIM) structures based on titanium dioxide have demonstrated reversible and non-volatile resistance-switching behavior and have been identified with the concept of the memristor. Microphysical studies suggest that the development of sub-oxide phases in the material drives the resistance changes. The creation of these phases, however, has a number of negative effects such as requiring an elevated voltage, increasing the device-to-device variability, damaging the electrodes due to oxygen evolution, and ultimately limiting the device lifetime.

A scalable neuristor built with Mott memristors.

The Hodgkin-Huxley model for action potential generation in biological axons is central for understanding the computational capability of the nervous system and emulating its functionality. Owing to the historical success of silicon complementary metal-oxide-semiconductors, spike-based computing is primarily confined to software simulations and specialized analogue metal-oxide-semiconductor field-effect transistor circuits. However, there is interest in constructing physical systems that emulate biological functionality more directly, with the goal of improving efficiency and scale.

Continuous electrical tuning of the chemical composition of TaO(x)-based memristors.

TaO(x)-based memristors have recently demonstrated both subnanosecond resistance switching speeds and very high write/erase switching endurance. Here we show that the physical state variable that enables these properties is the oxygen concentration in a conduction channel, based on the measurement of the thermal coefficient of resistance of different TaO(x) memristor states and a set of reference Ta-O films of known composition. The continuous electrical tunability of the oxygen concentration in the channel, with a resolution of a few percent, was demonstrated by controlling the write currents with a one transistor-one memristor (1T1M) circuit.

Measuring the switching dynamics and energy efficiency of tantalum oxide memristors.

We measured the real-time switching of metal-oxide memristors with sub-nanosecond resolution and recorded the evolution of the current and voltage during both ON (set) and OFF (reset) events. From these we determined the dynamical behavior of the conductivity for different applied bias amplitudes. Quantitative analysis of the energy cost and switching dynamics showed 115 fJ for ON-switching and 13 pJ for OFF-switching when resistance change was limited to 200%.

Sub-nanosecond switching of a tantalum oxide memristor.

We report sub-nanosecond switching of a metal-oxide-metal memristor utilizing a broadband 20 GHz experimental setup developed to observe fast switching dynamics. Set and reset operations were successfully performed in the tantalum oxide memristor using pulses with durations of 105 and 120 ps, respectively. Reproducibility of the sub-nanosecond switching was also confirmed as the device switched over consecutive cycles.

Anatomy of a nanoscale conduction channel reveals the mechanism of a high-performance memristor.

By employing a precise method for locating and directly imaging the active switching region in a resistive random access memory (RRAM) device, a nanoscale conducting channel consisting of an amorphous Ta(O) solid solution surrounded by nearly stoichiometric Ta(2) O(5) is observed. Structural and chemical analysis of the channel combined with temperature-dependent transport measurements indicate a unique resistance switching mechanism.

Observation of two resistance switching modes in TiO2 memristive devices electroformed at low current.

We report the observation of two resistance switching modes in certain 50 nm × 50 nm crossbar TiO(2) memristive devices that have been electroformed with a low-current process. The two switching modes showed opposite switching polarities. The intermediate state was shared by both modes (the ON state of the high-resistance mode or the OFF state of the low-resistance mode) and exhibited a relaxation to a more resistive state, including an initial transient decay.

Impact of geometry on the performance of memristive nanodevices.

We examined the influence of memristor geometry on switching endurance by comparing ribbed and planar TiO(2)-based cross-point devices with 50 nm × 50 nm lateral dimensions. We observed that planar devices exhibited a factor of over four improvement in median endurance value over ribbed structures for otherwise identical structures. Our simulations indicated that the corners in the upper wires of the ribbed devices experienced higher current density and more heating during device forming and switching, and hence a shorter life time.

The switching location of a bipolar memristor: chemical, thermal and structural mapping.

Memristors are memory resistors promising a rapid integration into future memory technologies. However, progress is still critically limited by a lack of understanding of the physical processes occurring at the nanoscale. Here we correlate device electrical characteristics with local atomic structure, chemistry and temperature.

Sketched oxide single-electron transistor.

Devices that confine and process single electrons represent an important scaling limit of electronics. Such devices have been realized in a variety of materials and exhibit remarkable electronic, optical and spintronic properties. Here, we use an atomic force microscope tip to reversibly 'sketch' single-electron transistors by controlling a metal-insulator transition at the interface of two oxides.

Lognormal switching times for titanium dioxide bipolar memristors: origin and resolution.

We measured the switching time statistics for a TiO(2) memristor and found that they followed a lognormal distribution, which is a potentially serious problem for computer memory and data storage applications. We examined the underlying physical phenomena that determine the switching statistics and proposed a simple analytical model for the distribution based on the drift/diffusion equation and previously measured nonlinear drift behavior. We designed a closed-loop switching protocol that dramatically narrows the time distribution, which can significantly improve memory circuit performance and reliability.

A memristor-based nonvolatile latch circuit.

Memristive devices, which exhibit a dynamical conductance state that depends on the excitation history, can be used as nonvolatile memory elements by storing information as different conductance states. We describe the implementation of a nonvolatile synchronous flip-flop circuit that uses a nanoscale memristive device as the nonvolatile memory element. Controlled testing of the circuit demonstrated successful state storage and restoration, with an error rate of 0.

Structural and chemical characterization of TiO2 memristive devices by spatially-resolved NEXAFS.

We used spatially-resolved NEXAFS (near-edge x-ray absorption fine structure) spectroscopy coupled with microscopy to characterize the electronic, structural and chemical properties of bipolar resistive switching devices. Metal/TiO2/metal devices were electroformed with both bias polarities and then physically opened to study the resulting material changes within the device. Soft x-ray absorption techniques allowed isolated study of the different materials present in the device with 100 nm spatial resolution.

Memristor-CMOS hybrid integrated circuits for reconfigurable logic.

Hybrid reconfigurable logic circuits were fabricated by integrating memristor-based crossbars onto a foundry-built CMOS (complementary metal-oxide-semiconductor) platform using nanoimprint lithography, as well as materials and processes that were compatible with the CMOS. Titanium dioxide thin-film memristors served as the configuration bits and switches in a data routing network and were connected to gate-level CMOS components that acted as logic elements, in a manner similar to a field programmable gate array. We analyzed the chips using a purpose-built testing system, and demonstrated the ability to configure individual devices, use them to wire up various logic gates and a flip-flop, and then reconfigure devices.

Microstrip resonators for electron paramagnetic resonance experiments.

In this article we evaluate the performance of an electron paramagnetic resonance (EPR) setup using a microstrip resonator (MR). The design and characterization of the resonator are described and parameters of importance to EPR and spin manipulation are examined, including cavity quality factor, filling factor, and microwave magnetic field in the sample region. Simulated microwave electric and magnetic field distributions in the resonator are also presented and compared with qualitative measurements of the field distribution obtained by a perturbation technique.

Submicron fabrication by local anodic oxidation of germanium thin films.

Here we describe a lithography scheme based on the local anodic oxidation of germanium film by a scanning atomic force microscope in a humidity-controlled atmosphere. The oxidation kinetics of the Ge film were investigated by a tapping mode, in which a pulsed bias voltage was synchronized and applied with the resonance frequency of the cantilever, and by a contact mode, in which a continuous voltage was applied. In the tapping mode we clearly identified two regimes of oxidation as a function of the applied voltage: the trench width increased linearly during the vertical growth and increased exponentially during the lateral growth.

Evolution of thermodynamic potentials in closed and open nanocrystalline systems: Ge-Si:Si(001) islands.

An open (closed) system, in which matter is (not) exchanged through surface diffusion, was realized via growth kinetics. Epitaxially grown Si-Ge:Si (001) islands were annealed in different environments affecting the diffusivity of Si adatoms selectively. The evolution of the driving forces for intermixing while approaching the equilibrium was inferred from Synchrotron x-ray measurements of composition and strain.

Scanning tunneling microscopy of template-stripped Au surfaces and highly ordered self-assembled monolayers.

Template stripping of Au films in ultrahigh vacuum (UHV) produces atomically flat and pristine surfaces that serve as substrates for highly ordered self-assembled monolayer (SAM) formation. Atomic resolution scanning tunneling microscopy of template-stripped (TS) Au stripped in UHV confirms that the stripping process produces a flat, predominantly 111 textured, atomically clean surface. Octanethiol SAMs vapor deposited in situ onto UHV TS Au show a c(4 x 2) superlattice with (square root 3 x square root 3) R30 degrees basic molecular structure having an ordered domain size up to 100 nm wide.