Duality of the interfacial thermal conductance in graphene-based nanocomposites

The thermal conductance of graphene–matrix interfaces plays a key role in controlling the thermal properties of graphene-based nanocomposites. Using atomistic simulations, we found that the interfacial thermal conductance depends strongly on the mode of heat transfer at graphene–matrix interfaces: if heat enters graphene from one side of its basal plane and immediately leaves it through the other side, the corresponding interfacial thermal conductance, G_across, is large; if heat enters graphene from both sides of its basal plane and leaves it at a position far away on its basal plane, the corresponding interfacial thermal conductance, G_non-across, is small. For a single-layer graphene immersed in liquid octane, G_across is ∼150 MW/m²K while G_non-across is ∼5 MW/m²K. G_across decreases with increasing multi-layer graphene thickness (i.e., number of layers in graphene) and approaches an asymptotic value of 100 MW/m²K for 7-layer graphenes. G_non-across increases only marginally as the graphene sheet thickness increases. Such a duality of the interface thermal conductance for different probing methods and its dependence on graphene sheet thickness can be traced ultimately to the unique physical and chemical structure of graphene materials. The ramifications of these results in areas such as the optimal design of graphene-based thermal nanocomposites are discussed.     

Carborane functionalized graphene oxide,a precursor for conductive self-assembled monolayers

A new, advanced graphene oxide based material modified with the extremely stable o-carboranoyl 1,2-C₂B₁₀H₁₁ unit was created. The carboranoyl-functionalized graphene oxide was synthesized from Li[1,2-C₂B₁₀H₁₁] and COCl-functionalized graphene-oxide, which was prepared by converting the graphene oxide –COOH groups to COCl with SOCl₂. The carboranoyl-functionalized graphene oxide was characterized using infrared and X-ray photoelectron spectroscopy. The morphology was investigated via transmission, scanning electron, and atomic force microscopies. The electrochemical properties of o-carboranoyl functionalized graphene oxide was studied using cyclic voltammetry, electrochemical impedance spectroscopy and electric force microscopy. This new graphene-based material could in future be used as a precursor for conductive self-assembled monolayers.     

Electrochemical doping of graphene with toluene

The electrical properties of graphene are known to be modified by chemical species that interact with it. We investigate the effect of doping of graphene-based devices by toluene (C₆H₅CH₃). We show that this effect has a complicated character. Toluene is seen to act as a donor, transferring electrons to the graphene. However, the degree of doping is seen to depend on the magnitude and polarity of an electric field applied between the graphene and a nearby electrode. This can be understood in terms of an electrochemical reaction mediated by the graphene crystal.     

Development of solution-gated graphene transistor model for biosensors

The distinctive properties of graphene, characterized by its high carrier mobility and biocompatibility, have stimulated extreme scientific interest as a promising nanomaterial for future nanoelectronic applications. In particular, graphene-based transistors have been developed rapidly and are considered as an option for DNA sensing applications. Recent findings in the field of DNA biosensors have led to a renewed interest in the identification of genetic risk factors associated with complex human diseases for diagnosis of cancers or hereditary diseases. In this paper, an analytical model of graphene-based solution gated field effect transistors (SGFET) is proposed to constitute an important step towards development of DNA biosensors with high sensitivity and selectivity. Inspired by this fact, a novel strategy for a DNA sensor model with capability of single-nucleotide polymorphism detection is proposed and extensively explained. First of all, graphene-based DNA sensor model is optimized using particle swarm optimization algorithm. Based on the sensing mechanism of DNA sensors, detective parameters (I_ds and V_gmin) are suggested to facilitate the decision making process. Finally, the behaviour of graphene-based SGFET is predicted in the presence of single-nucleotide polymorphism with an accuracy of more than 98% which guarantees the reliability of the optimized model for any application of the graphene-based DNA sensor. It is expected to achieve the rapid, quick and economical detection of DNA hybridization which could speed up the realization of the next generation of the homecare sensor system.     

Analytical modelling of monolayer graphene-based ion-sensitive FET to pH changes

Graphene has attracted great interest because of unique properties such as high sensitivity, high mobility, and biocompatibility. It is also known as a superior candidate for pH sensing. Graphene-based ion-sensitive field-effect transistor (ISFET) is currently getting much attention as a novel material with organic nature and ionic liquid gate that is intrinsically sensitive to pH changes. pH is an important factor in enzyme stabilities which can affect the enzymatic reaction and broaden the number of enzyme applications. More accurate and consistent results of enzymes must be optimized to realize their full potential as catalysts accordingly. In this paper, a monolayer graphene-based ISFET pH sensor is studied by simulating its electrical measurement of buffer solutions for different pH values. Electrical detection model of each pH value is suggested by conductance modelling of monolayer graphene. Hydrogen ion (H⁺) concentration as a function of carrier concentration is proposed, and the control parameter (Ƥ) is defined based on the electro-active ions absorbed by the surface of the graphene with different pH values. Finally, the proposed new analytical model is compared with experimental data and shows good overall agreement.     

Relationship between electrical and thermal conductivity in graphene-based transparent and conducting thin films

We measured and modeled the electrical, optical and thermal properties of transparent and conducting thin films based on graphene and graphitic platelets. Thermal conductivity of our films decreases with increasing electrical conductivity. Our experiments indicate that, for sufficiently large platelets, the influence factor in controlling the thermal conductivity is represented by the junctions between neighboring graphene platelets. The thickness of such junctions is determined by the average number of graphene layers (N) forming each platelet. The fact that both the thermal and electrical properties depend on N allows us to establish a model that leads to a theoretical relationship between the thermal and electrical conductivity in our samples, which is general enough to be applied to a large class of graphene-based thin films.     

Biologically inspired graphene-chlorophyll phototransistors with high gain

We present prominent photoresponse of bio-inspired graphene-based phototransistors sensitized with chlorophyll molecules. The hybrid graphene-chlorophyll phototransistors exhibit a high gain of 10⁶ electrons per photon and a high responsivity of 10⁶ A/W, which can be attributed to the integration of high-mobility graphene and the photosensitive chlorophyll molecules. The charge transfer at interface and the photogating effect in the chlorophyll layer can account for the observed photoresponse of the hybrid devices, which is confirmed by the back-gate-tunable photocurrent as well as the thickness and time dependent studies of the photoresponse. The demonstration of the graphene-chlorophyll phototransistors with high gain envisions a viable method to employ biomaterials for graphene-based optoelectronics.     

Doping graphene films via chemically mediated charge transfer

Transparent conductive films (TCFs) are critical components of a myriad of technologies including flat panel displays, light-emitting diodes, and solar cells. Graphene-based TCFs have attracted a lot of attention because of their high electrical conductivity, transparency, and low cost. Carrier doping of graphene would potentially improve the properties of graphene-based TCFs for practical industrial applications. However, controlling the carrier type and concentration of dopants in graphene films is challenging, especially for the synthesis of p-type films. In this article, a new method for doping graphene using the conjugated organic molecule, tetracyanoquinodimethane (TCNQ), is described. Notably, TCNQ is well known as a powerful electron accepter and is expected to favor electron transfer from graphene into TCNQ molecules, thereby leading to p-type doping of graphene films. Small amounts of TCNQ drastically improved the resistivity without degradation of optical transparency. Our carrier doping method based on charge transfer has a huge potential for graphene-based TCFs.     

Tunable transport characteristics of p-type graphene field-effect transistors by poly(ethylene imine) overlayer

Tunable electrical transport properties of graphene field-effect transistors (GFETs) are achieved by spin-coating a poly(ethylene imine) (PEI) layer on graphene surface. The initially p-doped graphene recovers the ambipolar characteristics with the PEI overlayer. When increasing the PEI concentration in a methanol solvent, a systematic evolution of the transport properties of GFETs is observed. The carrier mobility of graphene is greatly improved by several tens of times and the hole/electron conductivity saturation is shown. The voltage of the neutrality point V_Dirac gets closer to 0 V and the plateau width around the neutrality point ΔV_Dirac becomes much smaller. It is proposed that the long-range Coulomb scattering in graphene is suppressed due to the screening effect of PEI and the performances of GFETs are consequently improved. The hysteretic behaviors of the transfer characteristic curves of GFETs are also influenced by the PEI coating. The gradual reversion of the hysteresis direction is observed when increasing the concentration of PEI, which is probably due to the two competing mechanisms between the charge trapping at the graphene/SiO₂ interface and the capacitive coupling of graphene and the PEI overlayer.     

Transformation of graphene into graphane in the absence of hydrogen

Density functional based tight binding simulations were used to explore the fundamental charging of graphene nanoflakes, either by an electron-beam or electric current, to determine the effective failure limit with respect to induced charge. During this study we find that the failure limit of graphene is sufficiently high as to pose no problem to the operation of graphene-based electronic devices, but that localized regions may be transformed from an sp²-bonded into an sp³-bonded material before the failure limit. These regions are consistent with the carbon framework observed in graphane, but occur in the absence of hydrogen.     

Identifying efficient natural bioreductants for the preparation of graphene and graphene-metal nanoparticle hybrids with enhanced catalytic activity from graphite oxide

The implementation of green approaches towards the preparation of graphene and graphene-based materials with enhanced functionality from graphite oxide has been relatively little explored. Particularly, the use of bioreductants and the testing of their relative efficacies is an incipient area of research. Here, a pool of 20 environmentally friendly, natural antioxidants have been tested for their ability to reduce graphene oxide. These antioxidants were mostly vitamins, amino acids and organic acids. By establishing a protocol to systematically compare and optimize their performance, several new efficient bioreductants of graphene oxide have been identified, namely, pyridoxine and pyridoxamine (vitamin B₆), riboflavin (vitamin B₂), as well as the amino acids arginine, histidine and tryptophan. These biomolecules were used to prepare reduced graphene oxide–silver nanoparticle hybrids that displayed colloidal stability in water in the absence of additional dispersants. Particularly, hybrids prepared with pyridoxamine exhibited a combination of long-term colloidal stability and exceptionally high catalytic activity among silver nanoparticle-based catalysts in the reduction of p-nitrophenol with NaBH₄. Thus, in addition to expanding substantially the number of green reductants available for graphene oxide reduction, the present results underline the idea that proper selection of bioreductant can be relevant to achieve graphene-based materials with improved performance.     

Studies of electrochemical interfaces of thin PT film electrodes by surface conductance

In this paper the dc conductance of thin Pt films has been measured with respect to film thickness and to the surface effects as a function of the electrode potential in the “ideal” double layer, platinum “oxygen” and adsorbed hydrogen regions. The change in the value of dc conductance has been correlated with the electric charge as potential is scanned in these regions. Tentative mechanistic suggestions are offered to explain the data including some results on the deposition of copper on platinum.     

Experimental and theoretical comparative analysis of modified graphene (G-O,Pd-G,Pd-G-O) sensing performance toward SO2 in agricultural greenhouse

Sulfur dioxide (SO₂) is one of the main harmful gas in agricultural greenhouse. Its concentration can effectively reflect the health status of plants in agricultural greenhouse. In this paper, the adsorption characteristics of SO₂ on modified graphene-based were studied in comparison. The adsorption models of epoxy grouped graphene (G-O), palladium doped graphene (Pd-G) and epoxy and palladium co-doped graphene (Pd-G-O) to SO₂ were established based on the first-principles, analyzing in terms of adsorption energy, density of states, orbitals, charge density and desorption time of the adsorption models. The results showed that Pd-G-O exhibited strong chemisorption with adsorption energy of -1.237 eV. Furthermore, three modified graphene-based sensing materials were prepared via the redox method in this paper, and their crystal morphology and chemical elemental composition were investigated by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), respectively. The response performance of the graphene-based sensor to SO₂ was tested by a gas-sensitive testbed. The test results confirmed that the response of the Pd-G-O sample to 50 ppm SO₂ at room temperature was 10.5 (1.567 and 1.235 times than that of G-O, Pd-G). Pd-G-O provides theoretical basis and experimental support for the development of a highly sensitive and fast response sensor for the detection of harmful gases in agricultural greenhouse.     

The role of microwave absorption on formation of graphene from graphite oxide

By means of manipulating the oxygen content in graphite oxides (GO) and/or graphene-based materials, we demonstrate that the microwave absorption capacity of carbon materials is highly dependent on their chemical composition and structure. The increase of oxygen in GO remarkably decreases its microwave absorption capacity due to the size decrease of the π–π conjugated structure in these materials, and vice versa. It was revealed that graphene is an excellent microwave absorbent while GO with poor microwave absorption capacity, the unoxidized graphitic region “impurities” in GO act as the microwave absorbents to initiate the microwave-induced deoxygenation. The addition of a small amount graphene to GO leads to avalanche-like deoxygenation reaction of GO under microwave irradiation (MWI) and graphene formation, which was used for electrode materials in supercapacitors. The interaction between microwaves and graphene or graphene-based materials may be used for the fabrication of a variety of graphene-based nanocomposites with exceptional properties and a wealth of practical applications.     

Long-range and strong ferromagnetic graphene by compensated n–p codoping and π–π stacking

Generating magnetism in graphene and preserving its extraordinary properties are essential to the development of graphene-based spintronics. By using first-principles calculation methods, we propose a new scheme based on compensated n–p codoping and non-covalent π–π stacking with F4-TCNQ, to engineer some hybrid structures of Fe/graphene/F4-TCNQ. Our results firstly show that F4-TCNQ adsorption on graphene can preserve the original electronic properties of graphene by minimizing the distortion of carbon–carbon lattices due to non-covalent π–π interaction between graphene and F4-TCNQ. Moreover, F4-TCNQ adsorption on graphene can lead it to p-type, which is in agreement with experimental finding. Then, n-type Fe adatom on F4-TCNQ/graphene system forms very stable hybrid Fe/graphene/F4-TCNQ system due to compensated n–p codoping interaction. Further calculations reveal that a long-range and strong ferromagnetic coupling of two Fe adatoms is realized by F4-TCNQ exchange interaction in the hybrid Fe/graphene/F4-TCNQ system. This study provides a new method to realize ferromagnetic graphene and preserve its extraordinary properties, which will open a promising way to tailor the electronic properties and magnetic properties of graphene so as to be suited for future application in nanoelectronics and spintronic devices.     

Innovative synthesis of a novel ZnO/ZnBi2O4/graphene ternary heterojunction nanocomposite photocatalyst in the presence of tragacanth mucilage as natural surfactant

Developing interfacial connections is one of the breakthrough strategies to improve the photocatalytic activity of graphene/p-n heterojunction systems. Herein, natural tragacanth mucilage, for the first time, was employed as cost-effective and ecofriendly surfactant to prepare highly efficient ZnO–ZnBi₂O₄/graphene hybrid photocatalyst. The results indicated that the methylene blue (MB) photocatalytic degradation efficiency of ZnO–ZnBi₂O₄/graphene-mucilage heterojunction, containing 10 wt% ZnBi₂O₄ and 1 wt% graphene, was ～1.2, 1.4, 3.1 and 8.3 times higher than that of ZnO–ZnBi₂O₄/graphene, ZnO–ZnBi₂O₄, ZnBi₂O₄ and ZnO samples, respectively. This significant improvement in the photocatalytic performance could be mainly ascribed to the desirable advantages of using natural mucilage as surfactant, including uniform distribution of ZnO–ZnBi₂O₄ nanoparticles on the surface of graphene sheets, increasing of the effective surface area, and improving of the charge carriers separation. Based on the trapping experiments, electron spin resonance and photoelectrochemical Mott-Schottky tests, direct Z-Scheme charge transfer mechanism with hydroxyl radicals as main active species was suggested for photocatalytic degradation of MB on the ZnO–ZnBi₂O₄/graphene-mucilage nanocomposite. This study provides a new insight to fabricate more homogeneous and close contact interfaces in graphene-based hybrid photocatalytic systems for environmental remediation.     

Fabrication of graphene/single wall carbon nanotubes/polyaniline composite gels as binder-free electrode materials

A three-dimensional (3D) graphene-based hydrogels system containing one-dimensional (1D) carbon material-single wall carbon nanotubes (SWCNTs) and pseudocapacitor material-polyaniline (PANI) was prepared by combination of cross-linking, reduced and in situ polymerization. The polyaniline nanoparticles were combined with the reduced graphene sheet by π-π conjugation. The as-perpared composite gels could be directly used as electrode materials without binders. Due to the synergistic effect between SWCNTs, graphene sheet and PANI, the graphene/single wall carbon nanotubes/polyaniline (GH/SWCNTs/PANI) composite gel shows the enhanced electrochemical performances. The resultant GH/SWCNTs/PANI gel electroactive material shows a gravimetric specific capacitance of 145.4 F/g at 0.5 A/g and has improved 45% compared with initial graphene hydrogel (GH) at the same current density. And it keeps high retention of 98.8% of the initial capacity after 10,00 charge/discharge cycles at high current density of 10 A/g. The great cycle stability achieved is fundamentally attributed to the support of graphene sheet and single wall carbon nanotubes, which favors stress distribution and charge transfer during the longtime charge/discharge process. The graphene-based hydrogels could be a potential applicant for high rate charge/discharge applications. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 46948.     

Graphene films decorated with metal sulfide nanoparticles for use as counter electrodes of dye-sensitized solar cells

The composite films of metal sulfide (MS, M = Ni, Co) nanoparticles (NPs)/graphene films were proposed to be novel transparent conductive oxide- and platinum (Pt)-free counter electrodes with high electrocatalytic activity for dye-sensitized solar cells (DSSCs). Such DSSCs show higher photovoltaic conversion efficiencies of 5.25% (NiS/graphene) and 5.04% (CoS/graphene), compared with 5.00% for (Pt/fluorine-doped tin oxide). The excellent DSSC efficiencies are mainly due to the superior electrocatalytic activity of the MS and graphene films, and highly electrical properties of graphene films (9.57 Ω/sq). The excellent charge transfer between MS NPs and graphene films is due to the unique MS NPs and high surface area graphene structure. The graphene films were directly grown on dielectric SiO₂ substrates by chemical vapor deposition. MS NPs were uniformly implanted on the graphene films by dip coating of MS precursors M(C₃H₅OS₂)₂, and further annealed at 400 °C for 30 min under Ar.     

Conductance of Graphene Nanoribbon Junctions and the Tight Binding Model

Planar carbon-based electronic devices, including metal/semiconductor junctions, transistors and interconnects, can now be formed from patterned sheets of graphene. Most simulations of charge transport within graphene-based electronic devices assume an energy band structure based on a nearest-neighbour tight binding analysis. In this paper, the energy band structure and conductance of graphene nanoribbons and metal/semiconductor junctions are obtained using a third nearest-neighbour tight binding analysis in conjunction with an efficient nonequilibrium Green's function formalism. We find significant differences in both the energy band structure and conductance obtained with the two approximations.