Graphene–Graphene Oxide Floating Gate Transistor Memory

A novel transparent, flexible, graphene channel floating‐gate transistor memory (FGTM) device is fabricated using a graphene oxide (GO) charge trapping layer on a plastic substrate. The GO layer, which bears ammonium groups (NH₃⁺), is prepared at the interface between the crosslinked PVP (cPVP) tunneling dielectric and the Al₂O₃ blocking dielectric layers. Important design rules are proposed for a high‐performance graphene memory device: i) precise doping of the graphene channel, and ii) chemical functionalization of the GO charge trapping layer. How to control memory characteristics by graphene doping is systematically explained, and the optimal conditions for the best performance of the memory devices are found. Note that precise control over the doping of the graphene channel maximizes the conductance difference at a zero gate voltage, which reduces the device power consumption. The proposed optimization via graphene doping can be applied to any graphene channel transistor‐type memory device. Additionally, the positively charged GO (GO–NH₃⁺) interacts electrostatically with hydroxyl groups of both UV‐treated Al₂O₃ and PVP layers, which enhances the interfacial adhesion, and thus the mechanical stability of the device during bending. The resulting graphene–graphene oxide FGTMs exhibit excellent memory characteristics, including a large memory window (11.7 V), fast switching speed (1 μs), cyclic endurance (200 cycles), stable retention (10⁵ s), and good mechanical stability (1000 cycles).     

Topotactic Phase Transition Driving Memristive Behavior

Redox‐based memristive devices are one of the most attractive candidates for future nonvolatile memory applications and neuromorphic circuits, and their performance is determined by redox processes and the corresponding oxygen‐ion dynamics. In this regard, brownmillerite SrFeO_2.5 has been recently introduced as a novel material platform due to its exceptional oxygen‐ion transport properties for resistive‐switching memory devices. However, the underlying redox processes that give rise to resistive switching remain poorly understood. By using X‐ray absorption spectromicroscopy, it is demonstrated that the reversible redox‐based topotactic phase transition between the insulating brownmillerite phase, SrFeO_2.5, and the conductive perovskite phase, SrFeO₃, gives rise to the resistive‐switching properties of SrFeO_x memristive devices. Furthermore, it is found that the electric‐field‐induced phase transition spreads over a large area in (001) oriented SrFeO_2.5 devices, where oxygen vacancy channels are ordered along the in‐plane direction of the device. In contrast, (111)‐grown SrFeO_2.5 devices with out‐of‐plane oriented oxygen vacancy channels, reaching from the bottom to the top electrode, show a localized phase transition. These findings provide detailed insight into the resistive‐switching mechanism in SrFeO_x‐based memristive devices within the framework of metal–insulator topotactic phase transitions.     

Oxygen Exchange Processes between Oxide Memristive Devices and Water Molecules

Resistive switching based on transition metal oxide memristive devices is suspected to be caused by the electric‐field‐driven motion and internal redistribution of oxygen vacancies. Deriving the detailed mechanistic picture of the switching process is complicated, however, by the frequently observed influence of the surrounding atmosphere. Specifically, the presence or absence of water vapor in the atmosphere has a strong impact on the switching properties, but the redox reactions between water and the active layer have yet to be clarified. To investigate the role of oxygen and water species during resistive switching in greater detail, isotope labeling experiments in a N₂/H₂¹⁸O tracer gas atmosphere combined with time‐of‐flight secondary‐ion mass spectrometry are used. It is explicitly demonstrated that during the RESET operation in resistive switching SrTiO₃‐based memristive devices, oxygen is incorporated directly from water molecules or oxygen molecules into the active layer. In humid atmospheres, the reaction pathway via water molecules predominates. These findings clearly resolve the role of humidity as both oxidizing agent and source of protonic defects during the RESET operation.     

Independent Memcapacitive Switching Triggered by Bromide Ion Migration for Quaternary Information Storage

Memcapacitors are emerging as an attractive candidate for high‐density information storage due to their multilevel and adjustable capacitances and long‐term retention without a power supply. However, knowledge of their memcapacitive mechanism remains unclear and accounts for the limited implementation of memcapacitors for multilevel memory technologies. Here, repeatable and reproducible quaternary memories fabricated from hybrid perovskite (CH₃NH₃SnBr₃) memcapacitors are reported. The device can be modulated to at least four capacitive states ranging from 0 to 169 pF with retention for 10⁴ s. Impressively, an effective device yield approaching 100% for quaternary memory switching is achieved by a batch of devices; each state has a sufficiently narrow distribution that can be distinguished from the others and is superior to most multilevel memories that have a low device yield as well as an overlapping distribution of states. The memcapacitive switching stems from the modulated p–i–n junction capacitance triggered by Br^? migration, as demonstrated by in situ element mapping, X‐ray photoelectron spectra, and frequency‐dependent capacitance measurements; this mechanism is different from the widely reported memristive switching involving filamentary conduction. The results provide a new way to produce high‐density information storage through memcapacitors.     

Atomically Thin Femtojoule Memristive Device

The morphology and dimension of the conductive filament formed in a memristive device are strongly influenced by the thickness of its switching medium layer. Aggressive scaling of this active layer thickness is critical toward reducing the operating current, voltage, and energy consumption in filamentary‐type memristors. Previously, the thickness of this filament layer has been limited to above a few nanometers due to processing constraints, making it challenging to further suppress the on‐state current and the switching voltage. Here, the formation of conductive filaments in a material medium with sub‐nanometer thickness formed through the oxidation of atomically thin two‐dimensional boron nitride is studied. The resulting memristive device exhibits sub‐nanometer filamentary switching with sub‐pA operation current and femtojoule per bit energy consumption. Furthermore, by confining the filament to the atomic scale, current switching characteristics are observed that are distinct from that in thicker medium due to the profoundly different atomic kinetics. The filament morphology in such an aggressively scaled memristive device is also theoretically explored. These ultralow energy devices are promising for realizing femtojoule and sub‐femtojoule electronic computation, which can be attractive for applications in a wide range of electronics systems that desire ultralow power operation.     

Studies on switching mechanisms in Pd-nanodot embedded Nb2O5 memristors using scanning tunneling microscopy

Current imaging tunneling spectrum obtained from scanning tunneling microscopy has been used to probe the formation and/or rupture of conductive filaments responsible for bipolar switching in Pd nano-dots embedded Nb₂O₅ memristors. Filamentary conduction mechanism has been confirmed by scanning tunneling microscopy study using a Pt-Ir tip that enabled performing electroforming and reset operations at the nanoscale. The back and forth transition between the fully oxidized and metallic sub-oxide states of niobium under applied bias, as observed from X-ray photoelectron spectroscopy, is believed to be the source of bipolar switching in Nb₂O₅ memristors. The incorporation of Pd nanodots in Nb₂O₅ matrix plays a critical role by acting as an oxygen ion reservoir and/or by polarizing a large volume of oxygen vacancies. The formation and/or rupture of the conducting filaments through trapping-detrapping phenomena are found to boost the memristive switching performance.     

Mesopore‐Induced Ultrafast Na+‐Storage in T‐Nb2O5/Carbon Nanofiber Films toward Flexible High‐Power Na‐Ion Capacitors

Hybrid Na‐ion capacitors (NICs) are receiving considerable interest because they combine the merits of both batteries and supercapacitors and because of the low‐cost of sodium resources. However, further large‐scale deployment of NICs is impeded by the sluggish diffusion of Na⁺ in the anode. To achieve rapid redox kinetics, herein the controlled fabrication of mesoporous orthorhombic‐Nb₂O₅ (T‐Nb₂O₅)/carbon nanofiber (CNF) networks is demonstrated via in situ SiO₂‐etching. The as‐obtained mesoporous T‐Nb₂O₅ (m‐Nb₂O₅)/CNF membranes are mechanically flexible without using any additives, binders, or current collectors. The in situ formed mesopores can efficiently increase Na⁺‐storage performances of the m‐Nb₂O₅/CNF electrode, such as excellent rate capability (up to 150 C) and outstanding cyclability (94% retention after 10 000 cycles at 100 C). A flexible NIC device based on the m‐Nb₂O₅/CNF anode and the graphene framework (GF)/mesoporous carbon nanofiber (mCNF) cathode, is further constructed, and delivers an ultrahigh power density of 60 kW kg^?1 at 55 Wh kg^?1 (based on the total weight of m‐Nb₂O₅/CNF and GF/mCNF). More importantly, owing to the free‐standing flexible electrode configuration, the m‐Nb₂O₅/CNF//GF/mCNF NIC exhibits high volumetric energy and power densities (11.2 mWh cm^?3, 5.4 W cm^?3) based on the full device, which holds great promise in a wide variety of flexible electronics.     

Electrochemical Tantalum Oxide for Resistive Switching Memories

Redox‐based resistive switching memories (ReRAMs) are strongest candidates for the next‐generation nonvolatile memories fulfilling the criteria for fast, energy efficient, and scalable green IT. These types of devices can also be used for selector elements, alternative logic circuits and computing, and memristive and neuromorphic operations. ReRAMs are composed of metal/solid electrolyte/metal junctions in which the solid electrolyte is typically a metal oxide or multilayer oxides structures. Here, this study offers an effective and cheap electrochemical approach to fabricate Ta/Ta₂O₅‐based devices by anodizing. This method allows to grow high‐quality and dense oxide thin films onto a metallic substrates with precise control over morphology and thickness. Electrochemical‐oxide‐based devices demonstrate superior properties, i.e., endurance of at least 10⁶ pulse cycles and/or 10³I–V sweeps maintaining a good memory window with a low dispersion in R_OFF and R_ON values, nanosecond fast switching, and data retention of at least 10⁴ s. Multilevel programing capability is presented with both I–V sweeps and pulse measurements. Thus, it is shown that anodizing has a great prospective as a method for preparation of dense oxide films for resistive switching memories.     

Gate‐Tunable and Multidirection‐Switchable Memristive Phenomena in a Van Der Waals Ferroelectric

Memristive devices have been extensively demonstrated for applications in nonvolatile memory, computer logic, and biological synapses. Precise control of the conducting paths associated with the resistance switching in memristive devices is critical for optimizing their performances including ON/OFF ratios. Here, gate tunability and multidirectional switching can be implemented in memristors for modulating the conducting paths using hexagonal α‐In₂Se₃, a semiconducting van der Waals ferroelectric material. The planar memristor based on in‐plane (IP) polarization of α‐In₂Se₃ exhibits a pronounced switchable photocurrent, as well as gate tunability of the channel conductance, ferroelectric polarization, and resistance‐switching ratio. The integration of vertical α‐In₂Se₃ memristors based on out‐of‐plane (OOP) polarization is demonstrated with a device density of 7.1 × 10⁹ in.^?2 and a resistance‐switching ratio of well over 10³. A multidirectionally operated α‐In₂Se₃ memristor is also proposed, enabling the control of the OOP (or IP) resistance state directly by an IP (or OOP) programming pulse, which has not been achieved in other reported memristors. The remarkable behavior and diverse functionalities of these ferroelectric α‐In₂Se₃ memristors suggest opportunities for future logic circuits and complex neuromorphic computing.     

Non‐Volatile ReRAM Devices Based on Self‐Assembled Multilayers of Modified Graphene Oxide 2D Nanosheets

2D nanomaterials have been actively utilized in non‐volatile resistive switching random access memory (ReRAM) devices due to their high flexibility, 3D‐stacking capability, simple structure, transparency, easy fabrication, and low cost. Herein, it demonstrates re‐writable, bistable, transparent, and flexible solution‐processed crossbar ReRAM devices utilizing graphene oxide (GO) based multilayers as active dielectric layers. The devices employ single‐ or multi‐component‐based multilayers composed of positively charged GO (N‐GO(+) or NS‐GO(+)) with/without negatively charged GO(‐) using layer‐by‐layer assembly method, sandwiched between Al bottom and Au top electrodes. The device based on the multi‐component active layer Au/[N‐GO(+)/GO(‐)]_n/Al/PES shows higher ON/OFF ratio of ≈10⁵ with switching voltage of ?1.9 V and higher retention stability (≈10⁴ s), whereas the device based on single component (Au/[N‐GO(+)]_n/Al/PES) shows ≈10³ ON/OFF ratio at ±3.5 V switching voltage. The superior ReRAM properties of the multi‐component‐based device are attributed to a higher coating surface roughness. The Au/[N‐GO(+)/GO(–)]_n/Al/PES device prepared from lower GO concentration (0.01%) exhibits higher ON/OFF ratio (≈10⁹) at switching voltage of ±2.0 V. However, better stability is achieved by increasing the concentration from 0.01% to 0.05% of all GO‐based solutions. It is found that the devices containing MnO₂ in the dielectric layer do not improve the ReRAM performance.     

Graphene Oxide Sieving Membrane for Improved Cycle Life in High‐Efficiency Redox‐Mediated Li–O2 batteries

As soluble catalysts, redox‐mediators (RMs) endow mobility to catalysts for unconstrained access to tethered solid discharge products, lowering the energy barrier for Li₂O₂ formation/decomposition; however, this desired mobility is accompanied by the undesirable side effect of RM migration to the Li metal anode. The reaction between RMs and Li metal degrades both the Li metal and the RMs, leading to cell deterioration within a few cycles. To extend the cycle life of redox‐mediated Li–O₂ batteries, herein graphene oxide (GO) membranes are reported as RM‐blocking separators. It is revealed that the size of GO nanochannels is narrow enough to reject 5,10‐dihydro‐5,10‐dimethylphenazine (DMPZ) while selectively allowing the transport of smaller Li⁺ ions. The negative surface charges of GO further repel negative ions via Donnan exclusion, greatly improving the lithium ion transference number. The Li–O₂ cells with GO membranes efficiently harness the redox‐mediation activity of DMPZ for improved performance, achieving energy efficiency of above 80% for more than 25 cycles, and 90% for 78 cycles when the capacity limits were 0.75 and 0.5 mAh cm^‐2, respectively. Considering the facile preparation of GO membranes, RM‐sieving GO membranes can be cost‐effective and processable functional separators in Li–O₂ batteries.     

Fast Supercapacitors Based on Graphene‐Bridged V2O3/VOx Core–Shell Nanostructure Electrodes with a Power Density of 1 MW kg−1

Transition metal oxides (TMOs), with their very large pseudocapacitance effect, hold promise for next generation high‐energy‐density electrochemical supercapacitors (ECs). However, the typical high resistivity of TMOs restricts the reported ECs to work at a low charge–discharge (C–D) rate of 0.1–1 V s⁻¹. Here, a novel vanadium oxides core/shell nanostructure‐based electrode to overcome the resistivity challenge of TMOs for rapid pseudocapacitive EC design is reported. Quasi‐metallic V₂O₃ nanocores are dispersed on graphene sheets for electrical connection of the whole structure, while a naturally formed amorphous VO₂ and V₂O₅ (called as VO_x here) thin shell around V₂O₃ nanocore acts as the active pseudocapacitive material. With such a graphene‐bridged V₂O₃/VO_x core–shell composite as electrode material, ECs with a C–D rate as high as 50 V s⁻¹ is demonstrated. This high rate was attributed to the largely enhanced conductivity of this unique structure and a possibly facile redox mechanism. Such an EC can provide 1000 kW kg⁻¹ power density at an energy density of 10 Wh kg⁻¹. At the critical 45° phase angle, these ECs have a measured frequency of 114 Hz. All these indicate the graphene‐bridged V₂O₃/VO_x core–shell structure is promising for fast EC development.     

In Situ Growth of Graphene Catalyzed by a Phase-Change Material at 400 °C for Wafer-Scale Optoelectronic Device Application

The use of metal foil catalysts in the chemical vapor deposition of graphene films makes graphene transfer an ineluctable part of graphene device fabrication, which greatly limits industrialization. Here, an oxide phase-change material (V₂O₅) is found to have the same catalytic effect on graphene growth as conventional metals. A uniform large-area graphene film can be obtained on a 10 nm V₂O₅ film. Density functional theory is used to quantitatively analyze the catalytic effect of V₂O₅. Due to the high resistance property of V₂O₅ at room temperature, the obtained graphene can be directly used in devices with V₂O₅ as an intercalation layer. A wafer-scale graphene-V₂O₅-Si (GVS) Schottky photodetector array is successfully fabricated. When illuminated by a 792 nm laser, the responsivity of the photodetector can reach 266 mA W⁻¹ at 0 V bias and 420 mA W⁻¹ at 2 V. The transfer-free device fabrication process enables high feasibility for industrialization.     

Two‐Terminal Multibit Optical Memory via van der Waals Heterostructure

2D van der Waals (vdWs) heterostructures exhibit intriguing optoelectronic properties in photodetectors, solar cells, and light‐emitting diodes. In addition, these materials have the potential to be further extended to optical memories with promising broadband applications for image sensing, logic gates, and synaptic devices for neuromorphic computing. In particular, high programming voltage, high off‐power consumption, and circuital complexity in integration are primary concerns in the development of three‐terminal optical memory devices. This study describes a multilevel nonvolatile optical memory device with a two‐terminal floating‐gate field‐effect transistor with a MoS₂/hexagonal boron nitride/graphene heterostructure. The device exhibits an extremely low off‐current of ≈10^?14 A and high optical switching on/off current ratio of over ≈10⁶, allowing 18 distinct current levels corresponding to more than four‐bit information storage. Furthermore, it demonstrates an extended endurance of over ≈10⁴ program–erase cycles and a long retention time exceeding 3.6 × 10⁴ s with a low programming voltage of ?10 V. This device paves the way for miniaturization and high‐density integration of future optical memories with vdWs heterostructures.     

Achieving Insertion‐Like Capacity at Ultrahigh Rate via Tunable Surface Pseudocapacitance

The insertion/deinsertion mechanism enables plenty of charge‐storage sites in the bulk phase to be accessible to intercalated ions, giving rise to at least one more order of magnitude higher energy density than the adsorption/desorption mechanism. However, the sluggish ion diffusion in the bulk phase leads to several orders of magnitude slower charge‐transport kinetics. An ideal energy‐storage device should possess high power density and large energy density simultaneously. Herein, surface‐modified Fe₂O₃ quantum dots anchored on graphene nanosheets are developed and exhibit greatly enhanced pseudocapacitance via fast dual‐ion‐involved redox reactions with both large specific capacity and fast charge/discharge capability. By using an aqueous Na₂SO₃ electrolyte, the oxygen‐vacancy‐tuned Fe₂O₃ surface greatly enhances the absorption of SO₃^2? anions that majorly increase the surface pseudocapacitance. Significantly, the Fe₂O₃‐based electrode delivers a high specific capacity of 749 C g^?1 at 5 mV s^?1 and retains 290 C g^?1 at an ultrahigh scan rate of 3.2 V s^?1. With a novel dual‐electrolyte design, a 2 V Fe₂O₃/Na₂SO₃//MnO₂/Na₂SO₄ asymmetric supercapacitor is constructed, delivering a high energy density of 75 W h kg^?1 at a power density of 3125 W kg^?1.     

Bifunctional Graphene/γ‐Fe2O3 Hybrid Aerogels with Double Nanocrystalline Networks for Enzyme Immobilization

Highly porous hosting materials with conducting (favorable to electron transfer) and magnetic (favorable to product separation) bicontinuous networks should possess great potentials for immobilization of various enzymes in the field of biocatalytic engineering, but the synthesis of such materials is still a great challenge. Herein, bifunctional graphene/γ‐Fe₂O₃ hybrid aerogels with quite low density (30–65 mg cm^?3), large specific surface area (270–414 m² g^?1), high electrical conductivity (0.5–5 × 10^?2 S m^?1), and superior saturation magnetization (23–54 emu g^?1) are fabricated. Single networks of either graphene aerogels or γ‐Fe₂O₃ aerogels are obtained by etching of the hybrid aerogels with acid solution or calcining of the hybrid aerogels in air, indicative of the double networks of the as‐synthesized graphene/γ‐Fe₂O₃ hybrid aerogels for the first time. The resulting bifunctional aerogels are used to immobilize β‐glucuronidase for biocatalytic transformation of glycyrrhizin into glycyrrhetinic acid monoglucuronide or glycyrrhetinic acid, with high biocatalytic activity and definite repeatability.     

Number‐Resolved Single‐Photon Detection with Ultralow Noise van der Waals Hybrid

Van der Waals hybrids of graphene and transition metal dichalcogenides exhibit an extremely large response to optical excitation, yet counting of photons with single‐photon resolution is not achieved. Here, a dual‐gated bilayer graphene (BLG) and molybdenum disulphide (MoS₂) hybrid are demonstrated, where opening a band gap in the BLG allows extremely low channel (receiver) noise and large optical gain (≈10¹⁰) simultaneously. The resulting device is capable of unambiguous determination of the Poissonian emission statistics of an optical source with single‐photon resolution at an operating temperature of 80 K, dark count rate 0.07 Hz, and linear dynamic range of ≈40 dB. Single‐shot number‐resolved single‐photon detection with van der Waals heterostructures may impact multiple technologies, including the linear optical quantum computation.     

3D Graphene Frameworks/Co3O4 Composites Electrode for High‐Performance Supercapacitor and Enzymeless Glucose Detection

3D graphene frameworks/Co₃O₄ composites are produced by the thermal explosion method, in which the generation of Co₃O₄ nanoparticles, reduction of graphene oxide, and creation of 3D frameworks are simultaneously completed. The process prevents the agglomeration of Co₃O₄ particles effectively, resulting in monodispersed Co₃O₄ nanoparticles scattered on the 3D graphene frameworks evenly. The prepared 3D graphene frameworks/Co₃O₄ composites used as electrodes for supercapacitor display a definite improvement on electrochemical performance with high specific capacitance (≈1765 F g^?1 at a current density of 1 A g^?1), good rate performance (≈1266 F g^?1 at a current density of 20 A g^?1), and excellent stability (≈93% maintenance of specific capacitance at a constant current density of 10 A g^?1 after 5000 cycles). In addition, the composites are also employed as nonenzymatic sensors for the electrochemical detection of glucose, which exhibit high sensitivity (122.16 µA mM ^?1 cm^?2) and noteworthy lower detection limit (157 × 10^?9 M , S/N = 3). Therefore, the authors expect that the 3D graphene frameworks/Co₃O₄ composites described here would possess potential applications as the electrode materials in supercapacitors and nonenzymatic detection of glucose.     

Vertical ZnO Nanotube Transistor on a Graphene Film for Flexible Inorganic Electronics

The bottom‐up integration of a 1D–2D hybrid semiconductor nanostructure into a vertical field‐effect transistor (VFET) for use in flexible inorganic electronics is reported. Zinc oxide (ZnO) nanotubes on graphene film is used as an example. The VFET is fabricated by growing position‐ and dimension‐controlled single crystal ZnO nanotubes vertically on a large graphene film. The graphene film, which acts as the substrate, provides a bottom electrical contact to the nanotubes. Due to the high quality of the single crystal ZnO nanotubes and the unique 1D device structure, the fabricated VFET exhibits excellent electrical characteristics. For example, it has a small subthreshold swing of 110 mV dec^?1, a high I_max/I_min ratio of 10⁶, and a transconductance of 170 nS µm^?1. The electrical characteristics of the nanotube VFETs are validated using 3D transport simulations. Furthermore, the nanotube VFETs fabricated on graphene films can be easily transferred onto flexible plastic substrates. The resulting components are reliable, exhibit high performance, and do not degrade significantly during testing.     

Hierarchical 3D Cobalt‐Doped Fe3O4 Nanospheres@NG Hybrid as an Advanced Anode Material for High‐Performance Asymmetric Supercapacitors

Hierarchical nanostructure, high electrical conductivity, extraordinary specific surface area, and unique porous architecture are essential properties in energy storage and conversion studies. A new type of hierarchical 3D cobalt encapsulated Fe₃O₄ nanosphere is successfully developed on N‐graphene sheet (Co?Fe₃O₄ NS@NG) hybrid with unique nanostructure by simple, scalable, and efficient solvothermal technique. When applied as an electrode material for supercapacitors, hierarchical Co?Fe₃O₄ NS@NG hybrid shows an ultrahigh specific capacitance (775 F g^?1 at a current density of 1 A g^?1) with exceptional rate capability (475 F g^?1 at current density of 50 A g^?1), and admirable cycling performance (97.1% capacitance retention after 10 000 cycles). Furthermore, the fabricated Co?Fe₃O₄ NS@NG//CoMnO₃@NG asymmetric supercapacitor (ASC) device exhibits a high energy density of 89.1 Wh kg^?1 at power density of 0.901 kW kg^?1, and outstanding cycling performance (89.3% capacitance retention after 10 000 cycles). Such eminent electrochemical properties of the Co?Fe₃O₄ NS@NG are due to the high electrical conductivity, ultrahigh surface area, and unique porous architecture. This research first proposes hierarchical Co?Fe₃O₄ NS@NG hybrid as an ultrafast charge?discharge anode material for the ASC device, that holds great potential for the development of high‐performance energy storage devices.