TiO2 and other titanium oxide-based nanomaterials have drawn immense attention from researchers in different scientific domains due to their fascinating multifunctional properties, relative abundance, environmental friendliness, and bio-compatibility. However, the physical and chemical properties of titanium oxide-based nanomaterials are found to be explicitly dependent on the presence of various crystal defects. Oxygen vacancies are the most common among them and have always been the subject of both theoretical and experimental research as they play a crucial role in tuning the inherent properties of titanium oxides. This review highlights different strategies for effectively introducing oxygen vacancies in titanium oxide-based nanomaterials, as well as a discussion on the positions of oxygen vacancies inside the TiO2 band gap based on theoretical calculations. Additionally, a detailed review of different experimental techniques that are extensively used for identifying oxygen vacancies in TiO2 nanostructures is also presented.

The formation and detection techniques of oxygen vacancies in titanium oxide-based nanostructures.

In this review, a comprehensive survey of different oxide-based resistive random-access memories (RRAMs) for neuromorphic computing is provided. We begin with the history of RRAM development, physical mechanism of conduction, fundamental of neuromorphic computing, followed by a review of a variety of RRAM oxide materials (PCMO, HfOx, TaOx, TiOx, NiOx, etc.) with a focus on their application for neuromorphic computing. Our goal is to give a broad review of oxide-based RRAM materials that can be adapted to neuromorphic computing and to help further ongoing research in the field.

Oxide-based RRAM materials for neuromorphic computing

Non-volatile memories will play a decisive role in the next generation of digital technology. Flash memories are currently the key player in the field, yet they fail to meet the commercial demands of scalability and endurance. Resistive memory devices, and in particular memories based on low-cost, solution-processable and chemically tunable organic materials, are promising alternatives explored by the industry. However, to date, they have been lacking the performance and mechanistic understanding required for commercial translation. Here we report a resistive memory device based on a spin-coated active layer of a transition-metal complex, which shows high reproducibility (∼350 devices), fast switching (≤30 ns), excellent endurance (∼1012 cycles), stability (>106 s) and scalability (down to ∼60 nm2). In situ Raman and ultraviolet-visible spectroscopy alongside spectroelectrochemistry and quantum chemical calculations demonstrate that the redox state of the ligands determines the switching states of the device whereas the counterions control the hysteresis. This insight may accelerate the technological deployment of organic resistive memories.

Robust Resistive Memory Devices Using Solution-Processable Metal-Coordinated Azo-aromatics

The red shift and the broadening of the Raman signal from microcrystalline silicon films is described in terms of a relaxation in the q-vector selection rule for the excitation of the Raman active optical phonons. The relationship between width and shift calculated from the known dispersion relation in c-Si is in good agreement with available data. An increase in the decay rate of the optical phonons predicted on the basis of the same model is confirmed experimentally.

The one phonon raman spectrum in microcrystalline silicon

Non-modified (ZnO) and modified (Fe2O3@ZnO and CuO@ZnO) structured films are deposited via aerosol assisted chemical vapor deposition. The surface modification of ZnO with iron or copper oxides is achieved in a second aerosol assisted chemical vapor deposition step and the characterization of morphology, structure, and surface of these new structured films is discussed. X-ray photoelectron spectrometry and X-ray diffraction corroborate the formation of ZnO, Fe2O3, and CuO and the electron microscopy images show the morphological and crystalline characteristics of these structured films. Static water contact angle measurements for these structured films indicate hydrophobic behavior with the modified structures showing higher contact angles compared to the non-modified films. Overall, results show that the modification of ZnO with iron or copper oxides enhances the hydrophobic behavior of the surface, increasing the contact angle of the water drops at the non-modified ZnO structures from 122° to 135° and 145° for Fe2O3@ZnO and CuO@ZnO, respectively. This is attributed to the different surface properties of the films including the morphology and chemical composition.

/pdf/aacvd-synthesis-and-characterization-of-iron-and-copper-4wi7yfie4u.pdf

AACVD Synthesis and Characterization of Iron and Copper Oxides Modified ZnO Structured Films

Different non-isothermal and isothermal techniques are used to find out the phase transformation kinetic parameters (in this case, activation energy [E] of crystallization, Avrami exponent [n] and frequency factor [k
 0]) for the crystallization of a newly developed four component Zr58Cu22Al12Ag8 glassy alloy using differential scanning calorimetry (DSC) data. A comparative study is carried out to understand the effectiveness of different methods of evaluation of crystallization kinetic parameters from the DSC traces. n for the alloy varies in the range of 2.5–4 suggesting that growth varies from one to three dimensional with growth mechanism varying from diffusion controlled 3D growth to an interface controlled 1D growth.

Comparative Studies of Different Methods for Determining Crystallization Kinetics of Metallic Glass

Advancement of the memristor-based artificial synapse (AS) is urgently needed for rapid progress in neuromorphic devices. The precise structural and chemical engineering of metal oxide layers by metal dopants (Ni) is presented as an innovative way to set off a decent performance of the AS. An ON/OFF ratio of 103 as well as data retention and endurance capabilities of 104 s and 103 cycles, respectively, are achieved. With these properties, the symmetric alteration in conductance states, short-term plasticity (STP) and long-term plasticity (LTP) are realized within the same device, and compared with the reported values to establish its excellent cognitive behavioural ability. Our combined experimental and the DFT-based first-principles calculation results reveal that the rational designing of AS using metal-cations (Ni) can promote an ultra-low-power of ∼2.55 fJ per pulse (lower than human brain ∼10 fJ per pulse) for STP, promising for next-generation smart memory devices. Here, Ni endorses strong electronic localization, which in turn familiarizes trap states within the forbidden energy gap and improves short-term memory loss. Further, it modifies the local electrostatic barriers to stimulate modulatory action (as commonly observed in the mammalian brain) for LTP. Overall, this work provides a novel pathway to overcome the technological bottleneck.

Metal-induced progressive alteration of conducting states in memristors for implementing an efficient analog memory: A DFT-supported experimental approach

https://iopscience.iop.org/article/10.1088/1361-6641/aafc8a/pdf

Resistive switching in reactive electrode-based memristor: engineering bulk defects and interface inhomogeneity through bias characteristics

Here, we have presented the results of the detailed theoretical study of thermoelectric properties of two Rashba compounds KSnSb and KSnBi using first principles calculations based on density functional theory and Boltzmann transport theory taking spin–orbit coupling (SOC) into account. As these compounds have layered-type crystal structures, their transport parameters are found to be highly anisotropic. For KSnBi (KSnSb), the calculated lattice thermal conductivity [Formula: see text] along its crystallographic c axis is found to have ultralow value of 0.49 W m−1 K−1 (0.78 W m−1 K−1) even at room temperature, whereas almost twofold larger value of [Formula: see text] is estimated along its crystallographic a axis. However, large values of other transport parameters like electrical conductivity [Formula: see text] and thermopower S desirable for a high power factor [Formula: see text] are found along the a axis of these compounds. For KSnSb, the optimum a axis [Formula: see text] can be reachable for an electron concentration of 3.3 × 1019 cm−3 and at a temperature of 800 K. Comparable value of optimum a axis [Formula: see text] is also noted for KSnBi despite its strong susceptibility to bipolar conduction. Both these non-centrosymmetric compounds exhibit SOC-driven Rashba spin splitting of electronic bands, which affects both thermopower and electrical conductivity of these compounds. However, such Rashba spin splitting induced change in thermopower is almost negated by the concomitant change in electrical conductivity, resulting in no appreciable impact on power factor and hence [Formula: see text] of the studied compounds.

Thermoelectric properties of Rashba compounds KSnX (X = Sb, Bi)

Alteration of transport properties of any material, especially metal oxides, by doping suitable impurities is not straightforward as it may introduce multiple defects like oxygen vacancies (Vo) in the system. It plays a decisive role in controlling the resistive switching (RS) performance of metal oxide-based memory devices. Therefore, a judicious choice of dopants and their atomic concentrations is crucial for achieving an optimum Vo configuration. Here, we show that the rational designing of RS memory devices with cationic dopants (Ta), in particular, Au/Ti1-xTaxO2-δ/Pt devices, is promising for the upcoming non-volatile memory technology. Indeed, a current window of ∼104 is realized at an ultralow voltage as low as 0.25 V with significant retention (∼104 s) and endurance (∼105 cycles) of the device by considering 1.11 at % Ta doping. The obtained device parameters are compared with those in the available literature to establish its excellent performance. Furthermore, using detailed experimental analyses and density functional theory (DFT)-based first-principles calculations, we comprehend that the meticulous presence of Vo configurations and the columnar-like dendritic structures is crucial for achieving ultralow-voltage bipolar RS characteristics. In fact, the dopant-mediated Vo interactions are found to be responsible for the enhancement in local current conduction, as evidenced from the DFT-simulated electron localization function plots, and these, in turn, augment the device performance. Overall, the present study on cationic-dopant-controlled defect engineering could pave a neoteric direction for future energy-efficient oxide memristors.

A. Barman

Papers

Comparative Studies of Different Methods for Determining Crystallization Kinetics of Metallic Glass

Metal-induced progressive alteration of conducting states in memristors for implementing an efficient analog memory: A DFT-supported experimental approach

Resistive switching in reactive electrode-based memristor: engineering bulk defects and interface inhomogeneity through bias characteristics

Thermoelectric properties of Rashba compounds KSnX (X = Sb, Bi)

Aliovalent Ta-Doping-Engineered Oxygen Vacancy Configurations for Ultralow-Voltage Resistive Memory Devices: A DFT-Supported Experimental Study.