Cu2O Photocathodes Modified by Graphene Oxide and ZnO Nanoparticles with Improved Photocatalytic Properties.

The modification of photocathodes based on copper(I) oxide (COPC) by coating with ZnO nanoparticles or graphene oxide (GO) with different compositions and morphologies is considered. To cover the catalyst surface with graphene oxide, a technique was proposed via freezing of a sprayed aqueous suspension of graphene oxide followed by sublimation drying under vacuum conditions. This method improves the uniformity of the GO layer in comparison with the traditional drop-casting method and, as a result, improves the photocatalytic properties of the COPC. The influence of the composition and morphology of graphene oxide on the photocatalytic activity and stability of COPC has been established. ZnO nanoparticles and GO particles in contact with copper(I) oxide increase the photocurrent density by the more efficient separation of light-generated charge carriers, providing higher photocathode stability required in photocatalytic water splitting.

One-Stage Process of Reduction, Fluorination, and Doping with Nitrogen of Graphene Oxide Films.

The possibility of chemical modification of a graphene oxide film deposited on a Si/SiO2 substrate during a one-stage hydrothermal process in the presence of fluorine ions and reducing agents, such as ascorbic acid or hydrazine, is shown. The proposed technique makes it possible to obtain reduced fluorinated graphene nitride oxide (RGOFN) in the form of a thin film with a controlled composition of functional groups by changing the type and concentration of the reducing agent and then transferring the obtained films to any substrate. XPS and IR spectroscopy of the obtained films revealed controlled changes in the structure and composition of graphene oxide associated with the removal of oxygen groups and the incorporation of fluorine ions as well as the reduction of conjugated double bonds and the controlled incorporation of nitrogen into thin RGOFN films. The current-voltage characteristics of the fabricated RGOFN structures showed that their electrical properties are well controlled by doping with nitrogen during the proposed one-stage process.

Resistive Switching in Bigraphene/Diamane Nanostructures Formed on a La3Ga5SiO14 Substrate Using Electron Beam Irradiation.

Memristors, resistive switching memory devices, play a crucial role in the energy-efficient implementation of artificial intelligence. This study investigates resistive switching behavior in a lateral 2D composite structure composed of bilayer graphene and 2D diamond (diamane) nanostructures formed using electron beam irradiation. The resulting bigraphene/diamane structure exhibits nonlinear charge carrier transport behavior and a significant increase in resistance. It is shown that the resistive switching of the nanostructure is well controlled using bias voltage. The impact of an electrical field on the bonding of diamane-stabilizing functional groups is investigated. By subjecting the lateral bigraphene/diamane/bigraphene nanostructure to a sufficiently strong electric field, the migration of hydrogen ions and/or oxygen-related groups located on one or both sides of the nanostructure can occur. This process leads to the disruption of sp3 carbon bonds, restoring the high conductivity of bigraphene.

Graphene/MoS2-xOx/graphene photomemristor with tunable non-volatile responsivities for neuromorphic vision processing.

Conventional artificial intelligence (AI) machine vision technology, based on the von Neumann architecture, uses separate sensing, computing, and storage units to process huge amounts of vision data generated in sensory terminals. The frequent movement of redundant data between sensors, processors and memory, however, results in high-power consumption and latency. A more efficient approach is to offload some of the memory and computational tasks to sensor elements that can perceive and process the optical signal simultaneously. Here, we proposed a non-volatile photomemristor, in which the reconfigurable responsivity can be modulated by the charge and/or photon flux through it and further stored in the device. The non-volatile photomemristor has a simple two-terminal architecture, in which photoexcited carriers and oxygen-related ions are coupled, leading to a displaced and pinched hysteresis in the current-voltage characteristics. For the first time, non-volatile photomemristors implement computationally complete logic with photoresponse-stateful operations, for which the same photomemristor serves as both a logic gate and memory, using photoresponse as a physical state variable instead of light, voltage and memresistance. The polarity reversal of photomemristors shows great potential for in-memory sensing and computing with feature extraction and image recognition for neuromorphic vision.

Formation of Diamane Nanostructures in Bilayer Graphene on Langasite under Irradiation with a Focused Electron Beam.

In the presented paper, we studied bilayer CVD graphene transferred to a langasite substrate and irradiated with a focused electron beam through a layer of polymethyl methacrylate (PMMA). Changes in the Raman spectra and an increase in the electrical resistance of bigraphene after irradiation indicate a local phase transition associated with graphene diamondization. The results are explained in the framework of the theory of a chemically induced phase transition of bilayer graphene to diamane, which can be associated with the release of hydrogen and oxygen atoms from PMMA and langasite due to the "knock-on" effect, respectively, upon irradiation of the structure with an electron beam. Theoretical calculations of the modified structure of bigraphene on langasite and the experimental evaluation of sp3-hybridized carbon fraction indicate the formation of diamane nanoclusters in the bigraphene irradiated regions. This result can be considered as the first realization of local tunable bilayer graphene diamondization.

Geometry-asymmetric photodetectors from metal-semiconductor-metal van der Waals heterostructures.

The functional diversities of two-dimensional (2D) material devices with simple architectures are ultimately limited by immature doping techniques. An alternative strategy is to use geometry-asymmetric metal-semiconductor-metal (GA-MSM) structures, which enable the basic functions of semiconductor junctions such as rectification and photovoltaics. Here, the mixed-dimensional van der Waals heterostructures (MDvdWHs) based on the separation and self-assembly of p-type SnS layered nanosheets (NSs) and n-type SnS2 nanoparticles (NPs) are obtained using an aqueous phase exfoliation (APE) method. Due to the surface charge transfer doping, the carrier transport mechanism of devices based on MDvdWHs turns from thermionic field emission (TFE) to thermionic emission (TE), with the rectification factor (Iforward/Ireverse) changing from 0.7 to 3. To further illustrate the experimental results, the generic current transport models of GA-MSM devices have been established based on the TE and TFE mechanisms in which the TE and TFE mechanisms lead to opposite rectification phenomena in good agreement with the experimental results. The GA-MSM devices show a photovoltaic effect with a high responsivity of 35 A W-1 and detectivity of 3.4 × 1011 cm Hz1/2 W-1. This study not only provides a novel strategy to design photovoltaic devices with MDvdWHs, but more importantly, we have established fundamental models for the rectification behavior of GA-MSM devices.

Molybdenum Disulfide Nanosheet/Quantum Dot Dynamic Memristive Structure Driven by Photoinduced Phase Transition.

MoS2 2D nanosheets (NS) with intercalated 0D quantum dots (QDs) represent promising structures for creating low-dimensional (LD) resistive memory devices. Nonvolatile memristors based 2D materials demonstrate low power consumption and ultrahigh density. Here, the observation of a photoinduced phase transition in the 2D NS/0D QDs MoS2 structure providing dynamic resistive memory is reported. The resistive switching of the MoS2 NS/QD structure is observed in an electric field and can be controlled through local QD excitations. Photoexcitation of the LD structure at different laser power densities leads to a reversible MoS2 2H-1T phase transition and demonstrates the potential of the LD structure for implementing a new dynamic ultrafast photoresistive memory. The dynamic LD photomemristive structure is attractive for real-time pattern recognition and photoconfiguration of artificial neural networks in a wide spectral range of sensitivity provided by QDs.

The effect of atmospheric doping on pressure-dependent Raman scattering in supported graphene.

Atmospheric doping of supported graphene was investigated by Raman scattering under different pressures. Various Raman spectra parameters were found to depend on the pressure and the substrate material. The results are interpreted in terms of atmospheric adsorption leading to a change in graphene charge carrier density and the effect of the substrate on the electronic and phonon properties of graphene. It was found that adsorption of molecules from the atmosphere onto graphene doped with nitrogen (electron doping) compensates for the electron charge. Furthermore, the atmosphere-induced doping drastically decreases the spatial heterogeneity of charge carriers in graphene doped with nitrogen, while the opposite effect was observed for undoped samples. The results of this study should be taken into account for the development of sensors and nanoelectronic devices based on graphene.

Influence of the Quantum Well Structure and Growth Temperature on a Five-Layer InGaMnAs Quantum Well with an InGaAs Buffer Layer.

The influence of quantum well structure and growth temperature on a synthesized multilayer system composed of a five-layer InMnGaAs quantum well with an InGaAs buffer layer grown on semi-insulating (100)-oriented substrates prepared by low temperature molecular beam epitaxy was studied. The magnetization measurements using a superconducting quantum interference device indicated the existence of ferromagnetism with a Curie temperature above room temperature in the five-layer InGaMnAs quantum well structure with an InGaAs buffer layer in a GaAs matrix. X-ray diffraction and secondary ion mass spectroscopy measurements confirmed the second phase formation of ferromagnetic GaMn clusters. The ferromagnetism that exists in the five-layer of the InMnGaAs quantum well with the InGaAs buffer layer results from a superposition of the ferromagnetism of the low temperature region from the substitutional Mn ions into Ga sites or interstitial Mn ions as well as the presence of manganese ions dopant clusters such as GaMn clusters.

Self-assembled MoS2/rGO nanocomposites with tunable UV-IR absorption.

MoS2/reduced graphene oxide (rGO) nanocomposites were synthesized using an ultrasonic pretreatment with a single-stage hydrothermal and reduction process. Self-assembled MoS2 layers in the rGO matrix were obtained. The effect of quantum confinement in the structure, controlled by the degree of reduction of graphene oxide and the size of the 2D MoS2 nanocrystals, made it possible to obtain tunable optical absorption. MoS2/rGO layered nanocomposites exhibit a wide UV-IR absorption in the wavelength range from 280 nm to 973 nm, which is attractive for highly efficient multiband solar cells and advanced photonics.

A patterned single layer graphene resistance temperature sensor.

Micro-fabricated single-layer graphenes (SLGs) on a silicon dioxide (SiO2)/Si substrate, a silicon nitride (SiN) membrane, and a suspended architecture are presented for their use as temperature sensors. These graphene temperature sensors act as resistance temperature detectors, showing a quadratic dependence of resistance on the temperature in a range between 283 K and 303 K. The observed resistance change of the graphene temperature sensors are explained by the temperature dependent electron mobility relationship (~T-4) and electron-phonon scattering. By analyzing the transient response of the SLG temperature sensors on different substrates, it is found that the graphene sensor on the SiN membrane shows the highest sensitivity due to low thermal mass, while the sensor on SiO2/Si reveals the lowest one. Also, the graphene on the SiN membrane reveals not only the fastest response, but also better mechanical stability compared to the suspended graphene sensor. Therefore, the presented results show that the temperature sensors based on SLG with an extremely low thermal mass can be used in various applications requiring high sensitivity and fast operation.

Novel Green Luminescent and Phosphorescent Material: Semiconductive Nanoporous ZnMnO with Photon Confinement.

A novel green luminescent and phosphorescent material of semiconductive nanoporous ZnMnO was synthesized by the thermal nucleation of nanopores in the 20-period Zn0.93Mn0.07O/Zn0.65Mn0.35O multilayer structure. Nanoporous ZnMnO showed an n-type semiconducting property and exhibited an extremely strong green light emission in its luminescence and phosphorescence characteristics. This arises from the formation of the localized energy level (i.e., green emission band) within the energy band gap and the confinement of photons. The results suggest nanoporous ZnMnO to have a great potential for the new type of semiconducting green phosphors and semiconductor light-emitting diodes with lower thresholds, producing an efficient light emission. In-depth analyses on the structural, electrical, and optical properties are thoroughly examined, and the formation mechanism of nanoporous ZnMnO and the origin of the strong green light emission are discussed.

Formation of self-assembled nanoscale graphene/graphene oxide photomemristive heterojunctions using photocatalytic oxidation.

Photocatalytic oxidation of graphene with ZnO nanoparticles was found to create self-assembled graphene oxide/graphene (G/GO) photosensitive heterostructures, which can be used as memristors. Oxygen groups released during photodecomposition of water molecules on the nanoparticles under ultraviolet light, oxidized graphene, locally forming the G/GO heterojunctions with ultra-high density. The G/GO nanostructures have non-linear current-voltage characteristics and switch the resistance in the dark and under white light, providing four resistive states at room temperature. Photocatalytic oxidation of graphene with ZnO nanoparticles is proposed as an effective method for creating two-dimensional memristors with a photoresistive switching for ultra-high capacity non-volatile memory.

Tunable UV-visible absorption of SnS2 layered quantum dots produced by liquid phase exfoliation.

4H-SnS2 layered crystals synthesized by a hydrothermal method were used to obtain via liquid phase exfoliation quantum dots (QDs), consisting of a single layer (SLQDs) or multiple layers (MLQDs). Systematic downshift of the peaks in the Raman spectra of crystals with a decrease in size was observed. The bandgap of layered QDs, estimated by UV-visible absorption spectroscopy and the tunneling current measurements using graphene probes, increases from 2.25 eV to 3.50 eV with decreasing size. 2-4 nm SLQDs, which are transparent in the visible region, show selective absorption and photosensitivity at wavelengths in the ultraviolet region of the spectrum while larger MLQDs (5-90 nm) exhibit a broad band absorption in the visible spectral region and the photoresponse under white light. The results show that the layered quantum dots obtained by liquid phase exfoliation exhibit well-controlled and regulated bandgap absorption in a wide tunable wavelength range. These novel layered quantum dots prepared using an inexpensive method of exfoliation and deposition from solution onto various substrates at room temperature can be used to create highly efficient visible-blind ultraviolet photodetectors and multiple bandgap solar cells.

Multicolor Emission from Poly(p-Phenylene)/Nanoporous ZnMnO Organic-Inorganic Hybrid Light-Emitting Diode.

The voltage-tunable multicolor emission was realized in a poly(p-phenylene)/nanoporous ZnMnO organic-inorganic hybrid light-emitting diode. Red, green, and blue (RGB) colors sequentially appeared with increasing magnitude of the bias voltage (i.e., R → RG → RGB with V↑). At a higher voltage (>2.4 V), eventually, the device emitted the visible light with a mixture of colors including RGB. These unique features may move us a step closer to the application of organic-inorganic hybrid solid-state lighting devices for the full-color display and/or the electrical-to-optical data converter for multivalue electronic signal processes. In-depth analyses on electrical and optical properties are presented, and voltage-controllable multicolor-emission mechanisms are discussed.

MoS2 memristor with photoresistive switching.

A MoS2 nanosphere memristor with lateral gold electrodes was found to show photoresistive switching. The new device can be controlled by the polarization of nanospheres, which causes resistance switching in an electric field in the dark or under white light illumination. The polarization charge allows to change the switching voltage of the photomemristor, providing its multi-level operation. The device, polarized at a voltage 6 V, switches abruptly from a high resistance state (HRSL6) to a low resistance state (LRSL6) with the On/Off resistance ratio of about 10 under white light and smooth in the dark. Analysis of device conductivity in different resistive states indicates that its resistive state could be changed by the modulation of the charge in an electric field in the dark or under light, resulting in the formation/disruption of filaments with high conductivity. A MoS2 photomemristor has great potential as a multifunctional device designed by using cost-effective fabrication techniques.

Luminescent properties of three structures built from 3-cyano-4-dicyanomethylene-5-oxo-4,5-dihydro-1H-pyrrol-2-olate and cadmium.

Yellow-orange tetraaquabis(3-cyano-4-dicyanomethylene-5-oxo-4,5-dihydro-1H-pyrrol-2-olato-kappaN(3))cadmium(II) dihydrate, [Cd(C8HN4O2)2(H2O)4] x 2 H2O, (I), and yellow tetraaquabis(3-cyano-4-dicyanomethylene-5-oxo-4,5-dihydro-1H-pyrrol-2-olato-kappaN(3))cadmium(II) 1,4-dioxane solvate, [Cd(C8HN4O2)2(H2O)4] x C4H8O2, (II), contain centrosymmetric mononuclear Cd2+ coordination complex molecules in different conformations. Dark-red poly[[decaaquabis(mu(2)-3-cyano-4-dicyanomethylene-5-oxo-4,5-dihydro-1H-pyrrol-2-olato-kappa(2)N:N')bis(mu(2)-3-cyano-4-dicyanomethylene-1H-pyrrole-2,5-diolato-kappa(2)N:N')tricadmium] hemihydrate], [Cd3(C8HN4O2)2(C8N4O2)2(H2O)10] x 0.5 H2O, (III), has a polymeric two-dimensional structure, the building block of which includes two cadmium cations (one of them located on an inversion centre), and both singly and doubly charged anions. The cathodoluminescence spectra of the crystals are different and cover the wavelength range from UV to red, with emission peaks at 377 and 620 nm for (III), and at 583 and 580 nm for (I) and (II), respectively.