Solution-processed Reduced Graphene Oxide Research Articles

Highly sensitive flexible strain and temperature sensors using solution processed graphene palladium nanocomposite

This work reports highly sensitive, flexible, and fast response strain sensors for load cell application and a temperature sensor with a tunable temperature coefficient of resistance (TCR). The proposed sensors are fabricated with solution-processed reduced graphene oxide (rGO) and Palladium (Pd) nanocomposite. The strain sensor is encapsulated with a Polydimethylsiloxane (PDMS) substrate to ensure flexibility and moisture protection. The strain sensor's performance was studied; the gauge factor was calculated as ~22 to ~198 in the strain range of 0.05–0.625%. The sensor exhibited high durability beyond 1000 cycles, fast response (~39 ms). A binocular type of load cell was designed and simulated using COMSOL Multiphysics software to study the strain profiles under applied load conditions. The designed load cell was tested with a micro universal test machine (UTM) for different loads. Its response was stable, linear, and repeatable with a sensitivity of 23.52 mV/V/Kg and a resolution of 0.0085 Kg/mV at an excitation of 5 V. Additionally, the nanocomposite was investigated for temperature response with different ratios of rGO-Pd and tuned the TCR of the sensor. The temperature sensor displayed good sensitivity and linearity with both negative temperature coefficient (NTC) and positive temperature coefficient (PTC) depending on rGO and Pd nanoparticles (NPs) composition.

Low-cost synthesis of high quality graphene oxide with large electrical and thermal conductivities

A simple and cost-effective method for synthesizing high quality thermally reduced graphene oxide (TrGO) thin films using Shellac, is presented. The synthesis temperature ranges from 550 °C to 900 °C, and is the sole control parameter to obtain a carbon content as high as 97 atomic percentage. For TrGO synthesized at 900 ˚C, its XPS, Raman, and TEM data indicate a large portion of sp2 hybridized carbon atoms and large crystal sizes (~1 µm), which resulted in very high electrical and cross-plane thermal conductivities, 2488 S/cm and 1.3 W/m-K, respectively. Furthermore, this high electrical conductivity of TrGO was observed to remain unaffected under mechanical stress 10 times higher than that completely broke solution-processed reduced graphene oxide (rGO) films, promising its potential for robust, transparent, and cost-effective conductive coating.

Analysis of enhanced hole transport in naphthalene dicarboxyimide (NDI)-based n-type polymer field-effect transistors using solution-processed reduced graphene oxide electrodes

In this study, organic field-effect transistors (OFETs) using a naphthalene dicarboxamide (NDI)-based n-type semiconducting polymer and electrodes comprising either Au or reduced graphene oxide (rGO) were fabricated. Compared with those with Au electrodes, transistors with rGO electrodes exhibited enhanced hole transport characteristics. The analysis of the interaction between the NDI-based polymer and the two electrodes revealed satisfactory hole transport in terms of device performance with the rGO electrode despite the less favorable work function for the injection of holes into the polymer semiconductor, corresponding to the formation of interfacial dipoles of different magnitudes. The electron orbital structure of the rGO electrode induced a smaller shift by the dipole moment at the interface between the electrode and semiconductor compared with that induced by the electron orbital structure of the gold electrode to promote ambipolarity. In both cases, energy barriers for the injection of charge at the interface were determined by ultraviolet photoelectron spectroscopy and Kelvin probe force microscopy analysis. In addition, a complementary logic inverter comprising two identical OFETs based on n-type NDI derivative and rGO electrodes with improved hole transport properties was fabricated.

Solution-processable reduced graphene oxide template layer for molecular orientation control of organic semiconductors

A reduced graphene oxide (rGO) film is first applied to a surface template layer to control the molecular orientation of crystalline organic semiconductors. The ultrathin and ultrasmooth rGO layer was successfully prepared on a substrate without a transfer process by spin-coating a carefully purified GO aqueous dispersion. This rGO layer exhibited a strong templating effect rivaling monolayer graphene, inducing a face-on orientation of copper phthalocyanine molecules leading to significant improvement of vertical carrier mobilities. The highly-conductive and transparent rGO film does not hamper charge transport at the interface and photoabsorption unlike conventional templating materials. This method can be widely used for vertical organic devices that require high carrier mobilities and strong photoabsorption/emission.

Tunable Charge Injection via Solution-Processed Reduced Graphene Oxide Electrode for Vertical Schottky Barrier Transistors

We demonstrate, for the first time, the use of a solution-processed reduced graphene oxide (rGO) layer as a work function tunable electrode in vertical Schottky barrier (SB) transistors. The rGO electrodes were deposited by simple spray-coating onto the substrate. The vertical device structure was formed by sandwiching a N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) organic semiconductor between rGO and Al electrodes. By varying the voltage applied to the gate electrode, the work function of rGO and thus the SB formed at the rGO-PTCDI-C8 interface could be effectively modulated. The resulting vertical SB transistors based on rGO-PTCDI-C8 heterostructures exhibited excellent electrical properties, including a maximum current density of 17.9 mA/cm2 and an on–off current ratio >103, which were comparable with the values obtained for the devices based on a CVD-grown graphene electrode. The charge injection properties of the vertical devices were systematically investigated through temperature-depende...

Solar Light Responsive Photocatalytic Activity of Reduced Graphene Oxide–Zinc Selenide Nanocomposite

Solution processable reduced graphene oxide–zinc selenide (RGO–ZnSe) nanocomposite has been successfully synthesized by an easy one-pot single-step solvothermal reaction. The RGO–ZnSe composite was characterized structurally and morphologically by the study of XRD analysis, SEM and TEM imaging. Reduction in graphene oxide was confirmed by FTIR spectroscopy analysis. Photocatalytic efficiency of RGO–ZnSe composite was investigated toward the degradation of Rhodamine B under solar light irradiation. Our study indicates that the RGO–ZnSe composite is catalytically more active compared to the controlled-ZnSe under the solar light illumination. Here, RGO plays an important role for photoinduced charge separation and subsequently hinders the electron–hole recombination probability that consequently enhances photocatalytic degradation efficiency. We expect that this type of RGO-based optoelectronics materials opens up a new avenue in the field of photocatalytic degradation of different organic water pollutants.

Efficient flexible polymer solar cells based on solution-processed reduced graphene oxide–Assisted silver nanowire transparent electrode

It is reported a kind of solution-processible transparent conductive film consisting of reduced graphene oxide (rGO) nanosheets and silver nanowires (AgNWs). The sandwiched structure of rGO/AgNWs/rGO is readily deposited via layer-by-layer blade-coating on the rigid glass or flexible PET substrate at mild annealing temperature. The rGO nanosheets sandwiching the AgNWs networks not only induce close contact of the AgNWs networks as a clipper to improve the conductivity, but also link the discrete AgNWs as a connector to improve the uniformity of the film conductivity. The transparent rGO/AgNWs/rGO film exhibits sheet resistance as low as 14.29 Ω/□ with transmittance over 90% at 550 nm. Moreover, the rGO/AgNWs/rGO composite film shows good ambient stability due to the rGO coverage and the existing charge interaction between AgNWs and rGO. Polymer solar cells are fabricated on the transparent rGO/AgNWs/rGO-coated glass or flexible PET substrates and they exhibit comparable photovoltaic perfromance to the control devices fabricated on the commercial ITO substrate. More importantly, the PSCs based on the rGO/AgNWs/rGO/PET substrate show more excellent flexibility than that of the device based on the ITO/PET substrate.

Titanum Carbide MXene Flakes as Novel 2D Metallic Solution-Processed Films

Two-dimensional (2D) materials have opened up a field for developing the next generation of optoelectronic devices thanks to their novel properties such as ultrafast charge injection/extraction, strong light-matter interactions (despite being atomically thin) and quantum confinement(1). In addition, recent advancements in liquid phase exfoliation has revealed a new scalable route to investigate these materials(2). A good example is the fabrication of graphene oxide (GO) where the high resistive precursor of graphene that has to be reduced to recover its favorable optoelectronic properties. In most cases, ultra-high temperature (>1000ºC) and toxic chemicals (hydrazine) have to be used. In this talk, we will present a novel solution-processed film containing T3C2Tx monolayers, a 2D crystal from the recently discovered family of MXenes(3). This new MXene 2D family can be synthetized by selectively etching the A element of the MAX phase and formed into stacks of transition metal carbides and carbonitrides. Indeed, MXenes have recently gained attention due their a hydrophilic behavior, high metallic conductivity and excellent mechanical properties(4). Herein, we explore the use of freestanding nanometer-thin films of T3C2Tx for solution-processed transparent conductive electrodes. We demonstrate that our T3C2Tx films possess half of the sheet resistance (437 Ohm/sq) of the best reduced graphene oxide by produced solution-processed methods(5) while preserving an 80% of its transmission. In this talk, we will show that by using a simple spin-coating technique we can control the number of flakes forming the film (Figure 1a) and optimize the optoelectronic properties of our films so that they exhibit a figure of merit twice as large as reduced-GO (Figure 1b). The novelty of our approach is our in deep understanding of the limiting factors in the resistivity of the films such as heat treatments and functional end-groups. In addition, we will illustrate their metallic behavior by on-chip measurements and report for the first time the experimental measurement of their work function, which is crucial for advanced device design. This demonstration of a novel metallic and solution-processed 2D material with high conductivity using T3C2Txflakes with low-temperature fabrication provides a new possible pathway towards scalable manufacturing of ultrathin film devices. 1. F. Bonaccorso et al., Graphene, related two-dimensional crystals, and hybrid systems for energy conversion and storage. Science 347, 1246501 (2015). 2. V. Nicolosi, M. Chhowalla, M. G. Kanatzidis, M. S. Strano, J. N. Coleman, Liquid Exfoliation of Layered Materials. Science 340, 1226419 (2013). 3. M. Naguib, V. N. Mochalin, M. W. Barsoum, Y. Gogotsi, 25th Anniversary Article: MXenes: A New Family of Two-Dimensional Materials. Advanced Materials 26, 992-1005 (2014). 4. M. Naguib, Y. Gogotsi, Synthesis of Two-Dimensional Materials by Selective Extraction. Accounts of Chemical Research 48, 128-135 (2015). 5. H. A. Becerril et al., Evaluation of Solution-Processed Reduced Graphene Oxide Films as Transparent Conductors. ACS Nano 2, 463-470 (2008). Figure 1

Photovoltaic and impedance spectroscopic characteristics of heterojunction of graphene-PEDOT:PSS composite and n-silicon prepared via solution-based process

Recent investigations involving nanoscale energy conversion using two-dimensional nanomaterials such as graphene and transition metal dichalcogenides (TMDs) hold promise to mitigate future energy challenges. The heterojunction devices designed by interfacing 2D layers with 3D bulk semiconductors (2D/3D heterojunctions) including graphene/silicon and TMDs/silicon are widely explored for the photovoltaic characteristics. Developing a thorough understanding of the interfacial chemistry via impedance spectroscopic analysis will leverage the potential of these 2D/3D junctions for large-scale integrations. Here, the non-vacuum solution-processed reduced graphene oxide (rGO), poly(3,4-ethylenedioxythiophene):polystyrene sulphonate (PEDOT:PSS) and their composite (PEDOT:PSS(rGO)) were spin coated on the silicon (n-Si) substrates for fabrication of heterojunctions, with the device construct of rGO/n-Si, PEDOT:PSS/n-Si and PEDOT:PSS(rGO)/n-Si having Ohmic metal contacts in both top and bottom. The significant efficiency improvement of three orders for PEDOT:PSS(rGO)/n-Si device over the rGO/n-Si and PEDOT:PSS/n-Si heterojunction devices can be attributed to (i) increased conducting channels in the active region and (ii) reduced series resistance. Further, the impedance spectroscopy is employed to understand the interfacial chemistry of PEDOT:PSS(rGO)/n-Si solid-state junction, which is explained by a parallel resistance–capacitance circuit model.

Solution-processable reduced graphene oxide films as broadband terahertz wave impedance matching layers

The potential of solution-processable reduced graphene oxide (rGO) films as wave impedance matching layers has been examined in a broad terahertz (THz) spectral bandwidth.

Solution processable reduced graphene oxide decorated ATO electrode for organic solar cells

A novel concept based on the use of solutions containing already qualified crystalline antimony-doped tin oxide SnO2:Sb (ATO) nanoparticles has been developed. ATO nanoparticles are decorated by reduced graphene oxide (rGO) through a hydrothermal synthesis method. The electrical and optical properties of the graphene oxide films are investigated systematically. The sheet resistance (R □) of the ATO–rGO films decreases with the increase in the rGO content in the precursor solution. The R □ can be decreased after the ATO–rGO films annealing in the air for 1 h and can be further decreased by depositing Au on the surface of the films. The optimum property of the ATO–rGO film shows that the R □ is 80 Ω/□ and the transmittance is about 70 %. The ATO–rGO films are used as the anode of the organic solar cells. The anode film impact on the performance of the devices is studied. Finally, the power conversion efficiency (PCE) of the device based on the poly-(3-hexylthiophene): [6, 6]-phenyl C61-butyric acid methyl ester (PCBM) blended is 1.85 %, and the PCE of the device based on the poly-benzo[1,2-b:4,5-b′] dithio-phene thieno[3,4-b] thiophene:PCBM blended is 3.4 %.

Rate and mechanistic investigation of Eu(OTf)₂-mediated reduction of graphene oxide at room temperature.

We describe a fast, efficient, and mild approach to prepare chemically reduced graphene oxide (rGO) at room temperature using divalent europium triflate {Eu(OTf)2}. The characterization of solution-processable reduced graphene oxide has been carried out by various spectroscopic (FT-IR, UV-visible absorption, and Raman), microscopic (TEM and AFM), and powder X-ray diffraction (XRD) techniques. Kinetic study indicates that the bimolecular rate constants for the reduction of graphene oxide are 13.7 ± 0.7 and 5.3 ± 0.1 M(-1) s(-1) in tetrahydrofuran (THF)-water and acetonitrile (ACN)-water mixtures, respectively. The reduction rate constants are two orders of magnitude higher compared to the values obtained in the case of commonly used reducing agents such as the hydrazine derivative, sodium borohydride, and a glucose-ammonia mixture. The present work introduces a feasible reduction process for preparing reduced graphene oxide at ambient conditions, which is important for bulk production of GO. More importantly, the study explores the possibilities of utilizing the unique chemistry of divalent lanthanide complexes for chemical modifications of graphene oxide.

Morphological, optical, and electrical investigations of solution-processed reduced graphene oxide and its application to transparent electrodes in organic solar cells

Morphological, optical, and electrical properties of solution-processed reduced graphene oxide (r-GO) thin-films are investigated by varying on the number of spin-coating cycles and annealing temperature. Spin-coated r-GO thin-films all show full-covered morphologies with highly small rms roughness values ranging from ∼1 to 2.5nm. The sheet resistance of r-GO films can be controlled over two orders of magnitude by controlling both the spin-coating cycles and annealing temperature while conserving the transmittance values of 57–87%. The combination of coating cycles and heat-treatment temperature make the r-GO films as a useful transparent hole-extraction electrode for the application to organic photovoltaic cells.

Solution-processed reduced graphene oxide in light-emitting diodes and photovoltaic devices with the same pair of active materials

In studying the role of GO in LEDs and PVs, a single pair of active materials has been used in both the devices.

Enhancement of electron mobility and photovoltaic performance based on the P3HT:SPRGO organic photovoltaic

The time-of-flight method has been used to study the effect of P3HT:SPRGO composite films on charge mobility. Hole mobility was observed to remain constant at 7–10×10−5cm2/Vs as SPRGO concentration was increased from 0 to 20%, but then electron mobility had enhanced from 1.5 to 35×10−5cm2/Vs as SPRGO concentration was increased from 0 to 20%. The enhancement electron mobility was accompanied by an improved exciton dissociation of P3HT:SPRGO surface. Doping solution processable reduced graphene oxide (SPRGO) into P3HT that improved the organic photovoltaic (OPV) performance and the electron transported ability of P3HT:SPRGO composite films. The lower SPRGO concentration had improved OPV performance. The TOF (time of flight) curve of devices had shown that the electron mobility was enhanced along with increase of the SPRGO concentration. The beat OPV performance had reached 2.28% of power conversion efficiency. At the over high SPRGO concentration, the OPV performance had decreased along with an increase in SPRGO concentration. So the SPRGO is used for enhancing the electron mobility and OPV performance of P3HT:SPRGO.

Low-voltage solution-processed graphene transistors based on chemically and solvothermally reduced graphene oxide

We have developed solution-processed reduced graphene oxide (RGO) transistors with ion gel gate dielectrics. The combination of solution-processed high-capacitance ion gel gate dielectrics and spray-coated RGO films yielded high-performance RGO transistors that operated below 4 V. Two reduction processes were applied to GO: (i) chemical reduction by hydrazine and (ii) solvothermal reduction in N-methyl-2-pyrrolidone. Chemical reduction provided a more efficient route to reduce GO than solvothermal reduction, and the resulting RGO films yielded higher electron and hole mobilities than films based on solvothermal methods. Temperature-dependent transport studies revealed that higher mobilities in RGO films based on chemical reduction result from (i) more effective delocalization of the charge carriers, (ii) more numerous localized states near the Fermi energy, and (iii) a longer optimum hopping distance, compared to those for films based on solvothermal reduction.

Fabrication and Evaluation of Solution-Processed Reduced Graphene Oxide Electrodes for p- and n-Channel Bottom-Contact Organic Thin-Film Transistors

Reduced graphene oxide (RGO) is an electrically conductive carbon-based nanomaterial that has recently attracted attention as a potential electrode for organic electronics. Here we evaluate several solution-based methods for fabricating RGO bottom-contact (BC) electrodes for organic thin-film transistors (OTFTs), demonstrate functional p- and n-channel devices with such electrodes, and compare their electrical performance with analogous devices containing gold electrodes. We show that the morphology of organic semiconductor films deposited on RGO electrodes is similar to that observed in the channel region of the devices and that devices fabricated with RGO electrodes have lower contact resistances compared to those fabricated with gold contacts. Although the conductivity of RGO is poor compared to that of gold, RGO is still an enticing electrode material for organic electronic devices possibly owing to the retention of desirable morphological features, lower contact resistance, lower cost, and solution processability.

Evaluation of Solution-Processed Reduced Graphene Oxide Films as Transparent Conductors

Processable, single-layered graphene oxide (GO) is an intriguing nanomaterial with tremendous potential for electronic applications. We spin-coated GO thin-films on quartz and characterized their sheet resistance and optical transparency using different reduction treatments. A thermal graphitization procedure was most effective, producing films with sheet resistances as low as 10(2) -10(3) Omega/square with 80% transmittance for 550 nm light. Our experiments demonstrate solution-processed GO films have potential as transparent electrodes.