Engineering of graphene-based composites with hexagonal boron nitride and PEDOT:PSS for sensing applications.

A unique nanomaterial has been developed for sweat analysis, including glucose level monitoring. Simple resusable low-cost sensors from composite materials based on graphene, hexagonal boron nitride, and conductive PEDOT:PSS (poly(3,4-ethylenedioxythiophene)polystyrene sulfonate) polymer have been developed and fabricated via 2D printing on flexible substrates. The sensors were tested as biosensors using different water-based solutions. A strong increase in the current response (several orders of magnitude) was observed for aqua vapors or glucose solution vapors. This property is associated with the sorption capacity of graphene synthesized in a volume of plasma jets and thus having many active centers on the surface. The structure and properties of graphene synthesized in a plasma are different from those of graphene created by other methods. As a result, the current response for a wearable sensor is 3-5 orders of magnitude higher for the reference blood glucose concentration range of 4-14 mM. It has been found that the most promising sensor with the highest response was fabricated based on the graphene:PEDOT:PSS composite. The graphene:h-BN:PEDOT:PSS (h-BN is hexagonal boron nitride) sensors demonstrated a longer response and the highest response after the functionalization of the sensors with a glucose oxidase enzyme. The reusable wearable graphene:PEDOT:PSS glucose sensors on a paper substrate demonstrated a current response of 10-10 to 10-5 A for an operating voltage of 0.5 V and glucose range of 4-10 mM.

Graphene-based multifunctional humidity sensors with an ultrahigh current response.

Prospective composites, based on graphene (G) and hexagonal boron nitride (h-BN) nanoparticles, synthesized using a plasma jet and conducting polymer PEDOT:PSS, were used to create and study a set of sensors in the current study. The composites used were G:PEDOT:PSS (GPP) and G:h-BN:PEDOT:PSS (GBNPP). The PEDOT:PSS content in the composites was 10-3 wt%, and the ratio of G : h-BN was 1 : 1 in GBNPP. The development of these new highly conductive graphene-based composites makes it possible to create an active sensor layer with an ultra-low thickness of several nanometers. The ultra-high sensitivity of the current response, S, was ((2.0-3.3) × 106)% for GPP and GBNPP (2-3 printing layers) for a humidity range of 20-80%. The sensor response in the form of current pulses associated with human breathing has a range of ∼2-3 orders of magnitude. Two different processes are assumed to determine the form of the current pulse: the first is a fast process with a rise time of less than 1-4 seconds; the second is a relatively slow process with a front time of several tens of seconds. When touching with a finger (useful, for instance, for a flexible touchpad), a current response was observed as pulses of ∼2-3 orders of magnitude. We hypothesize that skin sweat is likely to play a critical role in the sensory response. Thus, this work presents an effective approach to creating a highly sensitive humidity sensor based on composite 2D materials. Moreover, the ultra-high sensitivity of the studied sensors is accompanied by their low cost and ease of manufacturing by 2D-printing.

Thin V2O5 films synthesized by plasma-enhanced atomic layer deposition for memristive applications.

In the present study, the V2O5 films synthesized by plasma-enhanced atomic layer deposition on p-Si and fluorinated graphene on Si (or FG/Si) substrates were analyzed for memristive applications. A number of samples were grown with V2O5 films with an average thickness of 1.0-10.0 nm, as determined by ellipsometric measurements. The study of surface morphology by atomic force microscopy showed that an island growth occurs in the initial stages of the film growth. The Raman spectra of the synthesized V2O5 films with an average thickness of more than 2.0 nm on the SiO2/Si substrates exhibit six distinct modes typical of the orthorhombic V2O5 phase. A large hysteresis was found in the C-V characteristics of the V2O5 films with a thickness of 1.0-4.2 nm. In general, the built-in charge in the V2O5 layers with an average thickness of 1.0-4.0 nm is positive and has a value of about ∼(2-8) × 1011 cm-2 at the 1 MHz frequency. Increasing the V2O5 film thickness leads to the accumulation of negative built-in charge up to -(1.7 to 2.3) × 1011 cm-2 at the 1 MHz frequency. The temperature dependence of the conductivity exhibits different electrically active states in V2O5/Si and V2O5/FG/Si structures. Thus, the FG layer can modify these states. V2O5 layers with an average film thickness of 1.0-3.6 nm demonstrate the memristive switching with an ON/OFF ratio of ∼1-4 orders of magnitude. At film thicknesses above 5.0 nm, the memristive switching practically vanishes. V2O5 films with an average thickness of 3.6 nm were found to be particularly stable and promising for memristive switching applications.

Engineering of graphene flakes in the process of synthesis in DC plasma jets.

During the pyrolysis of hydrocarbons in helium plasma jets in a plasma-chemical reactor, graphene flakes of a different structure and resistance were obtained. The presence of hydrogen in these structures was established by physicochemical methods, and its content depends on the pressure in the plasma-chemical reactor and the composition of a plasma-forming system. In addition to hydrogen, a relatively low concentration of oxygen atoms is present in the graphene flakes. Hydrogen is involved in the graphene nucleation, whereas oxygen is absorbed on graphene flakes from the air at low temperatures. It was found that a pressure increase in the reactor (up to 710 Torr) leads to the formation of flakes with a low resistivity (0.12-0.20 kOhm sq-1) and low defect density. In the case of synthesis at a low pressure (350-500 Torr), the resistance of graphene flakes is increased by three orders of magnitude (100-400 kOhm sq-1) with a more complicated defect structure and built-in hydrogen. Moreover, hydrogen is difficult to remove from these flakes, and annealing at relatively high temperatures (up to 300 °C) leads to a weak decrease in the resistance due to flake deformation. Additionally, the functionalization of the graphene flakes synthesized at a low pressure with fluorine atoms is suppressed due to their structural features. In general, the selection of growth parameters (gas pressure in a camera, flow rate and content of impurity atoms) allows one to control the defects in graphene, and its structure and conductivity.

Benefits of Printed Graphene with Variable Resistance for Flexible and Ecological 5G Band Antennas.

The possibility of creating antennas of the 5G standard (5.2-5.9 GHz) with specified electrodynamic characteristics by printing layers of variable thickness using a graphene suspension has been substantiated experimentally and by computer simulation. A graphene suspension for screen printing on photographic paper and other flexible substrates was prepared by means of exfoliation from graphite. The relation between the graphene layer thickness and its sheet resistance was studied with the aim of determining the required thickness of the antenna conductive layer. To create a two-sided dipole, a technology has been developed for the double-sided deposition of graphene layers on photographic paper. The electrodynamic characteristics of graphene and copper antennas of identical design are compared. The antenna design corresponds to the operating frequency of 2.4 GHz. It was found that the use of graphene as a conductive layer made it possible to suppress the fundamental (first) harmonic (2.45 GHz) and to observe radiation at the second harmonic (5.75 GHz). This effect is assumed to observe in the case when the thickness of graphene is lower than that of the skin depth. The result indicates the possibility of changing the antenna electrodynamic characteristics by adjusting the graphene layer thickness.

Graphene: Hexagonal Boron Nitride Composite Films with Low-Resistance for Flexible Electronics.

The structure and electric properties of hexagonal boron nitride (h-BN):graphene composite with additives of the conductive polymer PEDOT:PSS and ethylene glycol were examined. The graphene and h-BN flakes synthesized in plasma with nanometer sizes were used for experiments. It was found that the addition of more than 10-3 mass% of PEDOT:PSS to the graphene suspension or h-BN:graphene composite in combination with ethylene glycol leads to a strong decrease (4-5 orders of magnitude, in our case) in the resistance of the films created from these suspensions. This is caused by an increase in the conductivity of PEDOT:PSS due to the interaction with ethylene glycol and synergetic effect on the composite properties of h-BN:graphene films. The addition of PEDOT:PSS to the h-BN:graphene composite leads to the correction of the bonds between nanoparticles and a weak change in the resistance under the tensile strain caused by the sample bending. A more pronounced flexibility of the composite films with tree components is demonstrated. The self-organization effects for graphene flakes and polar h-BN flakes lead to the formation of micrometer sized plates in drops and uniform-in-size nanoparticles in inks. The ratio of the components in the composite was found for the observed strong hysteresis and a negative differential resistance. Generally, PEDOT:PSS and ethylene glycol composite films are promising for their application as electrodes or active elements for logic and signal processing.

Memristive FG-PVA Structures Fabricated with the Use of High Energy Xe Ion Irradiation.

A new approach based on the irradiation by heavy high energy ions (Xe ions with 26 and 167 MeV) was used for the creation of graphene quantum dots in the fluorinated matrix and the formation of the memristors in double-layer structures consisting of fluorinated graphene (FG) on polyvinyl alcohol (PVA). As a result, memristive switchings with an ON/OFF current relation ~2-4 orders of magnitude were observed in 2D printed crossbar structures with the active layer consisting of dielectric FG films on PVA after ion irradiation. All used ion energies and fluences (3 × 1010 and 3 × 1011 cm-2) led to the appearance of memristive switchings. Pockets with 103 pulses through each sample were passed for testing, and any changes in the ON/OFF current ratio were not observed. Pulse measurements allowed us to determine the time of crossbar structures opening of about 30-40 ns for the opening voltage of 2.5 V. Thus, the graphene quantum dots created in the fluorinated matrix by the high energy ions are a perspective approach for the development of flexible memristors and signal processing.

Resistive switching on individual V2O5 nanoparticles encapsulated in fluorinated graphene films.

Memristors currently attract much attention as basic building blocks for future neuromorphic electronics. Due to their unusual electronic, optical, magnetic, electrochemical, and structural properties, transition metal oxides offer much potential in the development of memristors. Recent trends in the design and fabrication of electronic devices have led to miniaturization of their working elements, with nanometer-sized structures enjoying increasing demand. In the present study, we investigated resistive switching on individual vanadium oxide (V2O5) crystal-hydrate nanoparticles, 2 to 10 nm in size, encapsulated in fluorinated graphene (FG). Measurements using a conductive atomic force microscope (c-AFM) probe showed that the core-shell V2O5/FG nanoparticles make it possible to achieve bipolar resistive switching, reproducible during 104 switching cycles, with the ON/OFF current ratio reaching 103-105. The switching voltage of the structures depends on the thickness of the FG shells of the composite particles and equals ∼2-4 V. It is shown that the encapsulation of V2O5 particles in fluorinated graphene ensures a high stability of the resistive switching effect and, simultaneously, prevents the escape of water from the crystalline vanadium oxide hydrates. A qualitative model is proposed to describe the bipolar resistive switching effect in examined structures. Results reported in the present article will prove useful in creating bipolar nanoswitches.

Graphene Flakes for Electronic Applications: DC Plasma Jet-Assisted Synthesis.

The possibility of graphene synthesis (the bottom-up approach) in plasma and the effective control of the morphology and electrical properties of graphene-based layers were demonstrated. Graphene flakes were grown in a plasma jet generated by a direct current plasma torch with helium and argon as the plasma-forming gases. In the case of argon plasma, the synthesized graphene flakes were relatively thick (2-6 nm) and non-conductive. In helium plasma, for the first time, graphene with a predominance of monolayer flakes and high conductivity was grown in a significant amount using an industrial plasma torch. One-dimensional (1D) flow modeling shows that the helium plasma is a less charged environment providing the formation of thinner graphene flakes with low defect density. These flakes might be used for a water-based suspension of the graphene with PEDOT:PSS (poly(3,4-ethylenedioxythiophene): polystyrene sulfonate) composite to create the structures employing the 2D printing technologies. Good structural quality, low layer resistance, and good mechanical strength combined with the ability to obtain a large amount of the graphene powder, and to control the parameters of the synthesized particles make this material promising for various applications and, above all, for sensors and other devices for flexible electronics and the Internet of things ecosystem.

Fluorinated graphene nanoparticles with 1-3 nm electrically active graphene quantum dots.

A new approach to creating a new and locally nanostructured graphene-based material is reported. We studied the electric and structural properties of partially fluorinated graphene (FG) films obtained from an FG-suspension and nanostructured by high-energy Xe ions. Local shock heating in ion tracks is suggested to be the main force driving the changes. It was found that ion irradiation leads to the formation of locally thermally expanded FG and its cracking into nanoparticles with small (∼1.5-3 nm) graphene quantum dots (GQD), embedded in them. The bandgap of GQD was estimated as 1 -1.5 eV. A further developed approach was applied to correct the functional properties of printed FG-based crossbar memristors. Dielectric FG films with small quantum dots may offer prospects in graphene-based electronics due to their stability and promising properties.

Resistive switching effects in fluorinated graphene films with graphene quantum dots enhanced by polyvinyl alcohol.

Two-layer films of partially fluorinated graphene (PFG) with graphene quantum dots and polyvinyl alcohol (PVA) were prepared by means of 2D printing technology. A stable resistive switching effect with the ON/OFF current ratio amounting from one to 4-5 orders of magnitude is found. The decrease in the PVA thickness leads to a change of the unipolar threshold switchings to the bipolar resistive switchings. The crossbar Ag/PFG/PVA/Ag structures retain their performance up to 6.5% deformation. The switching phenomenon is observed for a period about a year. The traps with characteristic activation energies ∼0.05 eV are suggested to be responsible for resistive switching. The time of charge-carrier emission from the localized states was found to be ∼5 µs. A quality model to describe the resistive switching effect in two-layer films implying the conduction over quantum dots proceeding with the participation of active traps at the PFG/PVA interface is proposed. The structures with the design demonstrated threshold resistive switching have their high potential for development of selector devices integrated to sensor or memristors circuits, for information storage and data processing, for flexible and wearable electronics. The structures with lower PVA thickness and the bipolar threshold switching are perspective for non-volatile memory cells for printed and flexible electronics.