RESISTIVE SWITCHING CHARACTERISTICS OF NANOSTRUCTURED AND SOLUTION-PROCESSED COMPLEX OXIDE ASSEMBLIES

Miniaturization of conventional nonvolatile (NVM) memory devices is rapidly approaching the physical limitations of the constituent materials. An emerging random access memory (RAM), nanoscale resistive RAM (RRAM), has the potential to replace conventional nonvolatile memory and could foster novel type of computing due to its fast switching speed, high scalability, and low power consumption. RRAM, or memristors, represent a class of two terminal devices comprising an insulating layer, such as a metal oxide, sandwiched between two terminal electrodes that exhibits two or more distinct resistance states that depend on the history of the applied bias. While the sudden resistance reduction into a conductive state in metal oxide insulators has been known for almost 50 years, the fundamental resistive switching mechanism is a complex phenomenon that is still long-debated, complex process. Further improvements to existing memristor performance require a complete understanding of memristive properties under various operation conditions. Additional technical issues also remain, such as the development of facile, low-cost fabrication methods as an alternative to expensive, ultra-high vacuum (UHV) deposition methods. This collection of work explores resistive switching within metal oxide-based memristive material assemblies by analyzing the fundamental physical insulating material properties. Chapter 3 aims to translate the utility and simplicity of the highly ordered anodic aluminum oxide (AAO) template structure to complex, yet more functional (memristive) materials. Functional oxides possessing ordered, scalable nanoporous arrays and nanocapacitor arrays over a large area is of interest to both the fields of next-generation electronics and energy storing/harvesting devices. Here their switching performance will be evaluated using conductive atomic force microscopy (C-AFM). Chapter 4 demonstrates a convective self-assembly fabrication method that effectively enables the synthesis of a low-cost solution processed memristor comprising binary oxide and perovskite ABO3 nanocrystals of varying diameter. Chapter 5 systematically compares the influence of inter-nanoparticle distance on the threshold switching SET voltage of hafnium oxide (HfO2) memristors. Utilizing shorter phosphonic acid ligands with higher binding affinity on the nanocrystal surface enabled a record-low SET voltage to be achieved. Chapter 6 extends the scope to the fine tuning of solution processed memristors with two types of perovskites nanocrystals. The primary advantage of nanocrystal memristors is the ability to draw from additional degrees of freedom by tuning the constituent nanocrystal material properties. Recent advancement of solution phase techniques enables a high degree of controllability over the nanocrystal size and structure. Thus, this work found in this dissertation aims to understand and decouple the effects of the geometric size and substitutional nanocrystal parameters on resistive switching

OXIDE-BASED MEMRISTIVE DEVICES BY BLOCK COPOLYMER SELF-ASSEMBLY

Oxide-based memristive systems represent today an emerging class of devices with a significant potential in memory, logic, and neuromorphic circuit applications. These devices have a simple capacitor structure and promise superior scalability together with favorable memory performances. This thesis presents a study of resistive switching phenomena in HfOx-based nanoscale memristive devices, with focus on material properties and development of bottom-up approaches for the fabrication of structures with dimension down to the nanoscale. One of the main issues for practical applications regarding device variability is first assessed by doping hafnium oxide films with different concentrations of aluminum atoms. Testing devices are analyzed by physico-chemical and electrical techniques in order to define the effect of oxide doping on the device properties. In the following part of the thesis, the scalability limit is explored in very high density arrays of nanodevices produced exploiting a lithographic approach based on the bottom-up self-assembly of block copolymer templates. This technique allows a tight control over the size and density of the defined features, and the possibilities offered by block copolymer patterning are here discussed. Electrical measurements of the nanodevices are performed through conductive atomic force microscopy. The device variability is examined and related to the inherent oxide non-homogeneity at the nanoscale, while a non-volatile switching of the resistance of the nanodevices is demonstrated. Further, this analysis draws the attention to a crosstalk phenomenon occurring at the nanoscale in a continuous thin film geometry. This result suggests to select different system configurations. A promising technique based on selective reactions with one copolymer block is finally discussed which allows the direct production of oxide patterns from block copolymer templates avoiding a pattern transfer process. In conclusion, the results reported in this thesis highlight the high scalability potential of oxide-based memristive devices, providing a missing piece of information for the understanding and practical development of very high density arrays

Application of Maxwell-Wagner polarisation in monolithic technologies

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Application of Scanning Probe Microscopy for New Physical Measurements and Studies of Surface Chemical Reactions of Materials at the Molecular Level

Scanning probe microscopy (SPM) provides unique capabilities for surface visualization and measurements that reach atomic and molecular dimensions. My research focus is directed toward applying and developing new measurements for analytical and surface chemistry with SPM. Two distinct goals based on studies with atomic force microscopy (AFM) will be described within this dissertation. The primary goal was to develop and apply a new AFM imaging mode for ultrasensitive measurements of the superparamagnetic properties of proteins. Magnetic sample modulation (MSM)-AFM, has capabilities to investigate and map the magnetic response of nanomaterials with unprecedented spatial resolution. The second goal was to apply high resolution AFM to probe the scaling and magnitude of corrosion of copper surfaces as a function of selected chemical parameters. Characterization of the magnetic properties of nanomaterials using a new AFM imaging mode will be described in the first part of the dissertation. Ferritin is a model nanomaterial for SPM studies because of the superparamagnetic iron-oxide (Fe2O3) core and ultra small dimensions of the protein, as described in Chapter 3. Periodic arrays of ferritin architectures were fabricated on surfaces and used as test platforms for measurements with magnetic sample modulation (MSM), for mapping the magnetic domains of ferritin are described in Chapters 4 and 5. The new MSM approach combines contact mode AFM with electromagnetic modulation of samples to measure the vibration and motion of nanomaterials. Proof-of-concept results demonstrate the capabilities for selective mapping of individual ferritin molecules through vibration of the superparamagnetic iron cores. Corrosion by-products from copper plumbing that are released into tap water are known to impact water quality and are detrimental to consumer health. The second part of this dissertation (Chapter 6) presents results for surface changes caused by water chemistry parameters typical of domestic water supplies. In this study, AFM was used to characterize nanoscale changes in surface morphology caused by chemical treatments at the earliest onset of copper corrosion as a function of pH, solution concentration and immersion intervals of copper substrates. Conclusions and future directions for the work of this dissertation will be summarized in Chapter 7

A study of complex magnetic configurations using magnetic force microscopy

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física la Materia Condensada y Nanotecnología. Fecha de lectura:15-06-2018Esta tesis tiene embargado el acceso al texto completo hasta el 15-12-2019The irruption of nanomagnetism in industry has brought remarkable advances in data storage technologies. In addition, further development in this field is expected to revolutionize traditional medicine by addressing diagnosis and disease treatment from a localized approach. Applications require a deep fundamental knowledge on the magnetic behavior of nanostructures. This thesis is framed on the study of non-trivial magnetic configurations and magnetization reversal processes of different nano-objects. The magnetic configuration and magnetization reversal process of cylindrical shaped magnets (nanowires and nanodots) have been studied using Magnetic Force Microscopy. Different mechanisms have been studied to obtain well-defined pinning sites in nanowires with axial magnetization. The magnetization reversal process of nanowires of complex magnetic configuration, due to strong magnetocrystalline anisotropy, has been studied. Finally, results obtained in cylindrical nanodots are presented, where topologically protected spin textures have been unveile

Functionalization of PS-b-P4VP Nanotemplates: towards optoelectronic applications

Self-organization of block copolymers becomes attractive for several branches of the current science and technology, which requires a cheap way of fabrication of well-ordered arrays of various nanoobjects. High ratio between the surface (or the interface) and the volume of the nanoobjects enables development of very efficient devices. The work within this thesis profits from the chemical dissimilarity between blocks of polystyrene-block-poly(4‑vinylpyridine) copolymers, where polystyrene forms “a body” of nanostructures and poly(4‑vinylpyridine) is “a link” for assemblies with low-molar-mass additives. Procedures and phenomena are demonstrated (observed) on few sorts of PS‑b‑P4VP copolymers with respect to their molecular weight and ratio of blocks. Although there are many kinds of nanostructures based on block copolymers, only nanotemplates are involved in the study. Their properties, like an influence of substrate roughness on microphase separation, stability of porous nanotemplates in ionized solutions, or a role of additives in their supramolecular assembly, respectively, are investigated. All of them appears to be important in development of various devices based on the nanotemplates. With respect to optoelectronic applications, electrical current transport and fluorescence are two basic phenomena studied on functionalized nanotemplates, developed in the thesis. DC transport is studied on nanostructures developed via sputtering of chromium into porous nanotemplates. Sputtering process is optimized in dependence of chromium deposition rate, composition and pressure of ambient gas. It is shown that a reactive nature of PS-b-P4VP nanotemplates enables development of resistant organometallic nanotemplates. On the other hand, suppression of the polymer reactivity is achieved by oxidation of a metal during sputtering in a reactive gas, which enables e. g. development of highly ordered TiO2 nanodots. Current-voltage characteristics are measured on “sandwich” devices (like LEDs) with various electrodes and composition. Several recent theoretical models fitting the characteristics are applied together with structural characterization techniques (like AFM or x-ray reflectivity) in order to elucidate relations among surface roughness, distribution of sputtered clusters, and carrier injection and transport. Fluorescence is studied on nanotemplates with organic low-molar-mass dyes, developed either via direct blending with the copolymer or via soaking of porous nanotemplates in dye solutions. Several relations between structure and fluorescence are observed. For instance, excimer emission in pyrene assemblies is supressed after ordering of the nanotemplate. Solvent induced orientation of fluorescein molecules in the nanotemplate results in fluorescence enhancement. Dimerization of Rhodamine 6G is dependent on the way of its impregnation in the nanotemplates (solvent, concentration, speed)

Structural, Electrical and Magnetic Properties of CoFe2O4 and BaTiO3 Layered Nanostructures on Conductive Metal Oxides

Multiferroic materials exhibit simultaneously, magnetic and electric order. In a magnetoelectric composite structure, a coupling is induced via an interfacial elastic interaction between magnetostrictive and piezoelectric materials enabling the control of the magnetisation by applying an electric field and vice versa. However, despite the potential of such coupling, experimental limits of theoretical models were observed. This work sheds some light on these limits by focusing the research on the chemistry of nanocomposite CoFe2O4 and BaTiO3, particularly at the interfaces where the coupling predominates. A comparison of the most common conductive oxides, Nb doped SrTiO3 and SrRuO3, was made for the bottom electrode application. The variation of conductive properties in Nb-SrTiO3 thin films at high temperature has been quantified when artificially strained and 60 nm SrRuO3 film was found to be the best bottom electrode choice for room temperature use. Epitaxial growth of magnetic CoFe2O4 was achieved on various metal oxide substrates despite large lattice mismatches. Crystallographic properties and strain evaluation were investigated and a Stranski-Krastanov growth mechanism, arising from the PLD deposition, was predominant. A notable drop of magnetisation was observed depending on the growth template, particularly on BaTiO3 substrates, the piezoelectric counterpart of the magnetoelectric structures. However, an encouraging magnetoelectric coupling induced by thermal phase transition of BaTiO3 was revealed. For BaTiO3, a control of the growth direction was realised by varying the deposition pressure, and the existence of both 180° and 90° ferroelectric domains was observed for films up to 300 nm in thickness. However, both the ferroelectric and piezoelectric properties were reduced in the thin films due to the clamping effect of the substrate. Finally, highly crystalline multilayers of CoFe2O4 and BaTiO3 were prepared on SrRuO3 buffered SrTiO3 substrates. It was found that the degradation of both magnetic and ferroelectric properties was proportional to the increase in the number of interfaces. A thorough microscopic study revealed interdiffusion and chemical instability occurring between CoFe2O4 and BaTiO3 at the interface. This undesired effect was partially recovered by the insertion of an ultra thin layer of SrTiO3, acting as a barrier layer at every interface. This research shows how interfacial chemistry need to be understood to achieve high magnetoelectric coupling in these types of epitaxial engineered structures

Defect-Rich Size-Selected Nanoclusters and Nanocrystalline Films of Titanium (IV) Oxide and Tantalum (IV) Oxide for Efficient Photocatalyst and Electroforming-Free Memristor Applications

Transition metal oxides, TiO2 and Ta2O5, are two of the most extensively studied wide bandgap semiconductor materials (with high work functions). Due to their suitable band edge positions for hydrogen evolution and exceptional stability against photocorrosion upon optical excitation, their application in heterogeneous photocatalysis has attracted a lot of attention. These oxides are also great components in the field of electronic devices such as field effect transistors, solar cells, and more recently advanced memory devices. Here, we focus on ultrasmall nanoclusters (< 5 nm) and nanocrystalline thin films of defect-rich TiO2 and Ta2O5 and their applications as high-performance photocatalysts in photoelectrochemical water splitting reactions and as resistive switching materials in memory applications. The present work is divided into two main parts. In the first part, ultrasmall nanoclusters (below 10 nm) of defect-rich TiO2 and Ta2O5 are synthesized using a gas phase aggregation technique in a nanocluster generation source based on DC magnetron sputtering. With a careful optimization of the deposition parameters such as aggregation zone length (condensation volume), Ar gas flow rate, deposition temperature and source power, we are able to produce metal/metal oxide nanoclusters with a narrow size distribution. As most of these as-grown nanoclusters are negatively charged, it is possible to conduct size-selection according to their mass-to-charge ratio. Using a quadrupole mass filter (directly coupled to the magnetron source), we achieve precise size-selection of nanoclusters, with the size distribution reduced to below 2% mass resolution. The nearly monosized nanoclusters so produced are deposited onto appropriate substrates to serve as the photoanodes for photoelectrochemical water splitting application. We demonstrate, for the first time, that the precisely size-selected TiO2 nanoclusters can be deposited on H-terminated Si(001) in a soft-landing condition and they can be used as highperformance photocatalysts for solar harvesting, with greater enhancement in the photoconversion efficiency. Three different sizes of TiO2 nanoclusters (4, 6 and 8 nm) are synthesized with appropriate combinations of aggregation length and Ar flow rate. Despite the low amount of material loading (of ~20% of substrate coverage), these supported TiO2 nanoclusters exhibit remarkable photocatalytic activities during photoelectrochemical water splitting reaction under simulated sunlight (50 mW/cm^2). Higher photocurrent densities (up to 0.8 mA/cm^2) and photoconversion efficiencies (up to 1%) with decreasing nanocluster size (at the applied voltage of –0.22 V vs Ag/AgCl) are observed. We attribute this enhancement to the presence of surface defects, providing a large amount of active surface sites, in the amorphous TiO2 nanoclusters as-grown at room temperature. We have further shown that the incorporation of metallic nanoclusters with the semiconductor photocatalysts can enhance the photoconversion efficiency. In this work, we have co-deposited surface oxygen deficient Ta2O5 or TaOx nanoclusters along with Pt nanoclusters of similar nanocluster size (~5 nm), the latter used as a promoter. The electron-hole pairs generated in the water splitting reaction can be effectively separated and stored with the presence of Pt nanoclusters, while the increase in Pt loading as a promoter can enhance the reaction by providing a large number of electrons for H2 evolution. However, loading too much Pt nanoclusters could actually reduce the photoresponse, which is due to blocking of photosensitive TaOx surface by excess Pt nanoclusters. In both cases, the photoconversion efficiency could potentially be enhanced at least 5 times by increasing the amount of nanocluster loading from 20% coverage to a monolayer coverage (e.g., by increasing the amount of deposition time for TiO2 and TaOx nanoclusters). Even higher photoconversion efficiency can be obtained with multiple layers of nanoclusters and by employing nanoclusters with even smaller size and/or with modification by chemical functionalization. These potential improvements could dramatically increase the photoconversion efficiency, making these nanocluster samples to be among the top photoelectrochemical catalysis performers. In the second part of the present work, we employ defect-rich nanocrystalline TiOx and TaOx thin films as active materials for resistive switching for memory application. Based on resistive switching principle, memristive devices (or memristors) provide the unique capability of multistep information storage. The development of memristors has often been hailed as the next evolution in non-volatile memories, low-power remote sensing, and adaptive intelligent prototypes including neuromorphic and biological systems. One major obstacle in achieving high switching performance is the irreversible electroforming step that is required to create oxygen vacancies for resistive switching. Using magnetron sputtering film deposition technique, we have fabricated the heterojunction memristor devices based on nanocrystalline TiOx and TaOx thin films (10-60 nm thick) with a high density of built-in oxygen vacancies, sandwiched between a pair of metallic Pt electrodes (30 nm thick). To avoid the destructive electroforming process and to achieve a high switching performance in the memristor device, we carefully manipulate the chamber pressure and ambient in deposition chamber during deposition to generate the required highly oxygen deficient semiconducting films. The films, as-deposited at room temperature, exhibit a crystallite size of 4-5 nm. In the fabricated Pt/TiOx/Pt memristors, a high electric field gradient can be generated in the TiOx film at a much lower electroforming voltage of +1.5 V, due in part to the nanocrystalline nature, which causes localization of this electric field and consequently enhanced reproducibility and repeatability in the device performance. After the first switching, consecutive 250 switching cycles can be achieved with a low programing voltage of ±1.0 V, along with a high ON/OFF current ratio, and long retention (up to 10^5 s). We further improve this TiOx memristor device by totally removing the electroforming step by fabricating an electroforming-free memristive device based on a heterojunction interface of TiOx and TaOx layers. In the Pt/TiOx/TaOx/Pt architecture structure (with Pt serving as the top and bottom electrodes), a high-κ dielectric TaOx layer is used to facilitate trapping and release of the electronic carriers, while a TiOx layer provides low-bias rectification as an additional oxygen vacancy source. With the incorporation of TaOx layer, the need for the electroforming step can be eliminated. More importantly, the resistance states of the device can be tuned such that switching between the high resistance state and the low resistance state can be achieved even smaller programming voltage of +0.8 V. With the low leakage current properties of TaOx, the high endurance (10^4 repeated cycles) and high retention capabilities (up to 10^8 s) can be enhanced manifold with highly stable ON/OFF current ratio. In both memristor devices, four different junction sizes (5×5, 10×10, 20×20 and 50×50 μm^2) have been evaluated according to their ON/OFF current ratio. We observe that the smaller is the junction size is, the higher is the current ratio. For the Pt/TiOx/TaOx/Pt memristor, we have also analyzed the thickness dependent effect of the switching behavior of devices with four different TaOx layer thicknesses (10, 20, 40 and 60 nm) and a TiOx layer thickness constant at 10 nm. The device with 10 nm thick TaOx (being amorphous in nature) shows unipolar switching with two SETs and two RESETs in one sweep cycle. This is in contrast to the bipolar resistive switching found in devices with the thicker TaOx films with a SET in the positive sweep and a RESET during the negative sweep. We further demonstrate that resistive switching can also occur at very low programming voltage (~50 mV), thus qualifying it as an ultralow power consumption device (~nW). The stable non-volatile bipolar switching characteristics with high ON/OFF current ratio and low power consumption make our devices best suitable for various analog and discrete programmable electric pulses. With the simplicity in the construction, the performance achieved for our memristors represents the best reported to date. This new class of defect-rich metal oxides nanomaterials with an ultrananocrystalline nature shows solid promises for various catalytic and electronic applications and, also, the simple, scalable roomtemperature device fabrication process makes this approach easily migratable further to transparent and/or flexible devices

Defect-Rich Size-Selected Nanoclusters and Nanocrystalline Films of Titanium (IV) Oxide and Tantalum (IV) Oxide for Efficient Photocatalyst and Electroforming-Free Memristor Applications

Transition metal oxides, TiO2 and Ta2O5, are two of the most extensively studied wide bandgap semiconductor materials (with high work functions). Due to their suitable band edge positions for hydrogen evolution and exceptional stability against photocorrosion upon optical excitation, their application in heterogeneous photocatalysis has attracted a lot of attention. These oxides are also great components in the field of electronic devices such as field effect transistors, solar cells, and more recently advanced memory devices. Here, we focus on ultrasmall nanoclusters (< 5 nm) and nanocrystalline thin films of defect-rich TiO2 and Ta2O5 and their applications as high-performance photocatalysts in photoelectrochemical water splitting reactions and as resistive switching materials in memory applications. The present work is divided into two main parts. In the first part, ultrasmall nanoclusters (below 10 nm) of defect-rich TiO2 and Ta2O5 are synthesized using a gas phase aggregation technique in a nanocluster generation source based on DC magnetron sputtering. With a careful optimization of the deposition parameters such as aggregation zone length (condensation volume), Ar gas flow rate, deposition temperature and source power, we are able to produce metal/metal oxide nanoclusters with a narrow size distribution. As most of these as-grown nanoclusters are negatively charged, it is possible to conduct size-selection according to their mass-to-charge ratio. Using a quadrupole mass filter (directly coupled to the magnetron source), we achieve precise size-selection of nanoclusters, with the size distribution reduced to below 2% mass resolution. The nearly monosized nanoclusters so produced are deposited onto appropriate substrates to serve as the photoanodes for photoelectrochemical water splitting application. We demonstrate, for the first time, that the precisely size-selected TiO2 nanoclusters can be deposited on H-terminated Si(001) in a soft-landing condition and they can be used as highperformance photocatalysts for solar harvesting, with greater enhancement in the photoconversion efficiency. Three different sizes of TiO2 nanoclusters (4, 6 and 8 nm) are synthesized with appropriate combinations of aggregation length and Ar flow rate. Despite the low amount of material loading (of ~20% of substrate coverage), these supported TiO2 nanoclusters exhibit remarkable photocatalytic activities during photoelectrochemical water splitting reaction under simulated sunlight (50 mW/cm^2). Higher photocurrent densities (up to 0.8 mA/cm^2) and photoconversion efficiencies (up to 1%) with decreasing nanocluster size (at the applied voltage of –0.22 V vs Ag/AgCl) are observed. We attribute this enhancement to the presence of surface defects, providing a large amount of active surface sites, in the amorphous TiO2 nanoclusters as-grown at room temperature. We have further shown that the incorporation of metallic nanoclusters with the semiconductor photocatalysts can enhance the photoconversion efficiency. In this work, we have co-deposited surface oxygen deficient Ta2O5 or TaOx nanoclusters along with Pt nanoclusters of similar nanocluster size (~5 nm), the latter used as a promoter. The electron-hole pairs generated in the water splitting reaction can be effectively separated and stored with the presence of Pt nanoclusters, while the increase in Pt loading as a promoter can enhance the reaction by providing a large number of electrons for H2 evolution. However, loading too much Pt nanoclusters could actually reduce the photoresponse, which is due to blocking of photosensitive TaOx surface by excess Pt nanoclusters. In both cases, the photoconversion efficiency could potentially be enhanced at least 5 times by increasing the amount of nanocluster loading from 20% coverage to a monolayer coverage (e.g., by increasing the amount of deposition time for TiO2 and TaOx nanoclusters). Even higher photoconversion efficiency can be obtained with multiple layers of nanoclusters and by employing nanoclusters with even smaller size and/or with modification by chemical functionalization. These potential improvements could dramatically increase the photoconversion efficiency, making these nanocluster samples to be among the top photoelectrochemical catalysis performers. In the second part of the present work, we employ defect-rich nanocrystalline TiOx and TaOx thin films as active materials for resistive switching for memory application. Based on resistive switching principle, memristive devices (or memristors) provide the unique capability of multistep information storage. The development of memristors has often been hailed as the next evolution in non-volatile memories, low-power remote sensing, and adaptive intelligent prototypes including neuromorphic and biological systems. One major obstacle in achieving high switching performance is the irreversible electroforming step that is required to create oxygen vacancies for resistive switching. Using magnetron sputtering film deposition technique, we have fabricated the heterojunction memristor devices based on nanocrystalline TiOx and TaOx thin films (10-60 nm thick) with a high density of built-in oxygen vacancies, sandwiched between a pair of metallic Pt electrodes (30 nm thick). To avoid the destructive electroforming process and to achieve a high switching performance in the memristor device, we carefully manipulate the chamber pressure and ambient in deposition chamber during deposition to generate the required highly oxygen deficient semiconducting films. The films, as-deposited at room temperature, exhibit a crystallite size of 4-5 nm. In the fabricated Pt/TiOx/Pt memristors, a high electric field gradient can be generated in the TiOx film at a much lower electroforming voltage of +1.5 V, due in part to the nanocrystalline nature, which causes localization of this electric field and consequently enhanced reproducibility and repeatability in the device performance. After the first switching, consecutive 250 switching cycles can be achieved with a low programing voltage of ±1.0 V, along with a high ON/OFF current ratio, and long retention (up to 10^5 s). We further improve this TiOx memristor device by totally removing the electroforming step by fabricating an electroforming-free memristive device based on a heterojunction interface of TiOx and TaOx layers. In the Pt/TiOx/TaOx/Pt architecture structure (with Pt serving as the top and bottom electrodes), a high-κ dielectric TaOx layer is used to facilitate trapping and release of the electronic carriers, while a TiOx layer provides low-bias rectification as an additional oxygen vacancy source. With the incorporation of TaOx layer, the need for the electroforming step can be eliminated. More importantly, the resistance states of the device can be tuned such that switching between the high resistance state and the low resistance state can be achieved even smaller programming voltage of +0.8 V. With the low leakage current properties of TaOx, the high endurance (10^4 repeated cycles) and high retention capabilities (up to 10^8 s) can be enhanced manifold with highly stable ON/OFF current ratio. In both memristor devices, four different junction sizes (5×5, 10×10, 20×20 and 50×50 μm^2) have been evaluated according to their ON/OFF current ratio. We observe that the smaller is the junction size is, the higher is the current ratio. For the Pt/TiOx/TaOx/Pt memristor, we have also analyzed the thickness dependent effect of the switching behavior of devices with four different TaOx layer thicknesses (10, 20, 40 and 60 nm) and a TiOx layer thickness constant at 10 nm. The device with 10 nm thick TaOx (being amorphous in nature) shows unipolar switching with two SETs and two RESETs in one sweep cycle. This is in contrast to the bipolar resistive switching found in devices with the thicker TaOx films with a SET in the positive sweep and a RESET during the negative sweep. We further demonstrate that resistive switching can also occur at very low programming voltage (~50 mV), thus qualifying it as an ultralow power consumption device (~nW). The stable non-volatile bipolar switching characteristics with high ON/OFF current ratio and low power consumption make our devices best suitable for various analog and discrete programmable electric pulses. With the simplicity in the construction, the performance achieved for our memristors represents the best reported to date. This new class of defect-rich metal oxides nanomaterials with an ultrananocrystalline nature shows solid promises for various catalytic and electronic applications and, also, the simple, scalable roomtemperature device fabrication process makes this approach easily migratable further to transparent and/or flexible devices

Memristive Non-Volatile Memory Based on Graphene Materials

Resistive random access memory (RRAM), which is considered as one of the most promising next-generation non-volatile memory (NVM) devices and a representative of memristor technologies, demonstrated great potential in acting as an artificial synapse in the industry of neuromorphic systems and artificial intelligence (AI), due its advantages such as fast operation speed, low power consumption, and high device density. Graphene and related materials (GRMs), especially graphene oxide (GO), acting as active materials for RRAM devices, are considered as a promising alternative to other materials including metal oxides and perovskite materials. Herein, an overview of GRM-based RRAM devices is provided, with discussion about the properties of GRMs, main operation mechanisms for resistive switching (RS) behavior, figure of merit (FoM) summary, and prospect extension of GRM-based RRAM devices. With excellent physical and chemical advantages like intrinsic Young’s modulus (1.0 TPa), good tensile strength (130 GPa), excellent carrier mobility (2.0 × 105 cm2∙V−1∙s−1), and high thermal (5000 Wm−1∙K−1) and superior electrical conductivity (1.0 × 106 S∙m−1), GRMs can act as electrodes and resistive switching media in RRAM devices. In addition, the GRM-based interface between electrode and dielectric can have an effect on atomic diffusion limitation in dielectric and surface effect suppression. Immense amounts of concrete research indicate that GRMs might play a significant role in promoting the large-scale commercialization possibility of RRAM devices