Numerical Simulation on Thermal-Electrical Characteristics and Electrode Patterns of GaN LEDs with Graphene/NiO_x Hybrid Electrode

The thermal-electrical characteristic of a GaN light-emitting diode （LED） with the hybrid transparent conductive layers （TCLs） of graphene （Gr） and NiOx is investigated by a finite element method. It is indicated that the LED with the compound TCL of 3-layer Gr and 1 nm NiOx has the best thermal-electrical performance from the view point of the maximum temperature and the current density deviation of multiple quantum wells, and the maximum temperature occurs near the n-electrode rather than p-electrode. Furthermore, to depress the current crowding on the LED, the electrode pattern parameters including p- and n-electrode length, p-electrode buried depth and the distance of n-electrode to active area are optimized. It is found that either increasing p- or n-electrode length and buried depth or decreasing the distance of n-electrode from the active area will decrease the temperature of the LED, while the increase of the n-electrode length has more prominent effect. Typically, when the n-electrode length increases to 0.8 times of the chip size, the temperature of the GaN LED with the inm NiOx/3-1ayer-Gr hybrid TCLs could drop about 7K and the current density uniformity could increase by 23.8%, compared to 0.4 times of the chip size. This new finding will be beneficial for improvement of the thermal- electrical performance of LEDs with various conductive TCLs such as NiOx/Gr or ITO/Gr as current spreading layers.

Electronic Transport Properties of Diblock Co-Oligomer Molecule Devices Sandwiched between Nitrogen Doping Armchair Graphene Nanoribbon Electrodes

We investigate the electronic transport properties of dipyrimidinyl-diphenyl sandwiched between two armchair graphene nanoribbon electrodes using the nonequilibrium Green function formalism combined with a first-principles method based on density functional theory. Among the three models M1–M3, M1 is not doped with a heteroatom. In the left parts of M2 and M3, nitrogen atoms are doped at two edges of the nanoribbon. In the right parts, nitrogen atoms are doped at one center and at the edges of M2 and M3, respectively. Comparisons of M1, M2 and M3 show obvious rectifying characteristics, and the maximum rectification ratios are up to 42.9 in M2. The results show that the rectifying behavior is strongly dependent on the doping position of electrodes. A higher rectification ratio can be found in the dipyrimidinyl-diphenyl molecular device with asymmetric doping of left and right electrodes, which suggests that this system has a broader application in future logic and memory devices.

3D hierarchically porous NiO/Graphene hybrid paper anode for long-life and high rate cycling flexible Li-ion batteries

With the rapid emergence of wearable devices, flexible lithium-ion batteries(LIBs) are much more needed than ever. Free-standing graphene-based composite paper electrodes with various active materials have appealed wide applications in flexible LIBs. However, due to the prone-to-restacking feature of graphene layers, a long cycle life at high current densities is rather difficult to be achieved. Herein, a unique threedimensional(3D) hierarchically porous NiO micro-flowers/graphene paper(fNiO/GP) electrode is successfully fabricated. The resulting fNiO/GP electrode shows superior long-term cycling stability at high rates(e.g., storage capacity of 359 mAh/g after 600 cycles at a high current density of 1 A/g). The facile 3D porous structure combines both the advantages of the graphene that is highly conductive and flexible to ensure rapid electrons/ions transfer and buffer the volume expansion of NiO during charge/discharge,and of the micro-sized NiO flowers that induces hierarchical between-layer pores ranging from nanomicro meters to promote the penetration of the electrolyte and prevent the re-stacking of graphene layers. Such structural design will inspire future manufacture of a wide range of active materials/graphene composite electrodes for high performance flexible LIBs.

Engineering two-dimensional pores in freestanding TiO_2/graphene gel film for high performance lithium ion battery

As the key component of electrochemical energy storage devices, an electrode with superior ions transport pores is the important premise for high electrochemical performance. In this paper, we developed a unique solution process to prepare freestanding TiO_2/graphene hydrogel electrode with tunable density and porous structures. By incorporating room temperature ionic liquids（RTILs）, even upon drying, the non-volatile RTILs that remained in the gel film would preserve the efficient ion transport channels and prevent the electrode from closely stacking, to develop dense yet porous structures. As a result, the dense TiO_2/graphene gel film as an electrode for lithium ion battery displayed a good gravimetric electrochemical performance and more importantly a high volumetric performance.

High-performance self-powered ultraviolet photodetector in SnO_(2)microwire/p-GaN heterojunction using graphene as charge collection medium

Graphene monolayer has been extensively applied as a transparency electrode material in photoelectronic devices due to its high transmittance,high carrier mobility,and ultrafast carrier dynamics.In this study,a high-performance self-powered photodetector,which is made of a SnO_(2)microwire,p-type GaN film,and monolayer graphene transparent electrode,was proposed and fabricated.The detector is sensitive to ultraviolet light signals and illustrates pronounced detection performances,including a peak respon-sivity∼223.7 mA W^(-1),a detectivity∼6.9×10^(12)Jones,fast response speed(rising/decaying times∼18/580μs),and excellent external quantum efficiency∼77%at 360 nm light illumination without exter-nal power supply.Compared with the pristine SnO_(2)/GaN photodetector using ITO electrode,the device performances of responsivity and detectivity are significantly increased over 6×10^(3)%and 3×10^(3)%,respectively.The performance-enhanced characteristics are mainly attributed to the high-quality het-erointerface of n-SnO_(2)/p-GaN,the highly conductive capacity,and the unique transparency of graphene electrodes.Particularly,the built-in potential formed at the SnO_(2)/GaN heterojunction interface could be strengthened by the Schottky potential barrier derived from the graphene electrode and SnO_(2)wire,en-hancing the carrier collection efficiency through graphene as a charge collection medium.This work is of great importance and significance to developing excellent-performance ultraviolet photodetectors for photovoltaic and optoelectronic applications in a self-powered operation manner.

Electrochemical Study and Application on Shikonin at Poly（diallyldimethylammonium chloride） Functionalized Graphene Sheets Modified Glass Carbon Electrode

The electrochemical behaviors of shikonin at a poly（diallyldimethylammonium chloride） functionalized graphene sheets modified glass carbon electrode（PDDA-GS/GCE） have been investigated. Shikonin could exhibit a pair of well-defined redox peaks at the PDDA-GS/GCE located at 0.681 V（Epa） and 0.662 V（Epc）[vs. saturated calo- mel electrode（SCE）] in 0.1 mol/L phosphate buffer solution（pH=2.0） with a peak-to-peak separation of about 20 mV, revealing a fast electron-transfer process. Moreover, the current response was remarkably increased at PDDA- GS/GCE compared with that at the bare GCE. The electrochemical behaviors of shikonin at the modified electrode were investigated. And the results indicate that the reaction involves the transfer of two electrons, accompanied by two protons and the electrochemical process is a diffusional-controlled electrode process. The electrochemical para- meters of shikonin at the modified electrode, the electron-transfer coefficient（a）, the electron-transfer number（n） and the electrode reaction rate constant（ks） were calculated to be as 0.53, 2.18 and 3.6 s^-1, respectively. Under the optimal conditions, the peak current of differential pulse voltammetry（DPV） increased linearly with the shikonin concentra- tion in a range from 9A72×10^-8 mol/L to 3,789×10^-6 mol/L with a detection limit of 3,157× 10^-8 mol/L. The linear regression equation was Ip=O.7366c＋0.7855（R=0.9978; lp： 10-7 A, c： 10-8 mol/L）. In addition, the modified glass carbon electrode also exhibited good stability, selectivity and acceptable reproducibility that could be used for the sensitive, simple and rapid determination of shikonin in real samples. Therefore, the present work offers a new way to broaden the analytical application of graphene in pharmaceutical analysis.

Detection of dopamine at graphene-ZIF-8 nanocomposite modified electrode

A hybrid graphene-ZIF-8（G-ZIF-8） nanocomposite modified electrode was prepared in our work. SEM characterization shows that nanocrystals of zeolitic imidazolate frameworks（ZIF-8） were homogeneously well-intergrown on the surface of graphene and thus the graphene sheets were refrained from restacking, which implies the high accessible surface area. The BET results further testifies that G-ZIF-8composites had a larger surface area than 3D graphene. G-ZIF-8 modified electrode exhibited excellent electroanalytical performance for dopamine. The linear concentration range was from 3.0 mmol/L to1.0 mmol/L with the detection sensitivity of 0.34 m A/mmol/L and the detection limit of 1.0 mmol/L was obtained. The interference study, electrode stability and reproducibility were carried out. In addition, the prepared sensor was applied to the detection of DA in serum sample with recoveries from 96.8% to100.7%. It is believable that the structure characteristic of G-ZIF-8 nanocomposite is favorable for using MOFs to fabricate highly sensitive electrochemical sensor

UV-assisted flash light welding process to fabricate silver nanowire/graphene on a PET substrate for transparent electrodes

Graphene oxide and silver nanowires were bar coated onto polyethylene terephthalate （PET） substrates and then welded using an ultraviolet （UV）-assisted flash light irradiation process to achieve both high electrical conductivity and low haze. The irradiation process connected adjacent silver nanowires by welding, while simultaneously reducing the graphene oxide to graphene. This process was performed using a custom W-assisted flash light welding system at room temperature under ambient conditions and was extremely rapid, with processing time of several milliseconds. The effects of varying the weight fractions of the silver nanowires and graphene oxide and of varying the W-assisted flash light welding conditions （light energy and pulse duration） were investigated. The surface morphologies of the welded silver nanowire/graphene films were analyzed using scanning electron microscopy. Optical characterizations, including transmittance and haze measurements, were also conducted using a spectrophotometer. To test their resistance to oxidation, the welded silver nanowire/graphene films were subjected to high temperature in a furnace （100 ℃）, and their sheet resistances were measured every hour. The flash light welding process was found to yield silver nanowire/graphene films with high oxidation resistance, high conductivity （14.35 Ω·sq-1）, high transmittance （93.46%）, and low haze （0.9%）. This material showed uniform temperature distribution when applied as a resistive heating film.

A nanosilica/exfoliated graphene composite film-modified electrode for sensitive detection of methyl parathion

Graphene nanosheets （GS） were easily prepared through liquid-phase exfoliation of graphite powder in N,N-dimethylformamide （DMF） with the assistance of sodium citrate. Then, GS was coated onto a glassy carbon electrode （GCE） surface by drop to fabricate a GS]GCE nanointerface. Subsequently, by using tetraethylorthosilicate sol as precursor, nanosilica was electrochemically deposited onto the GS]GCE surface to produce a nanocomposite film electrode （nanosilicaJGSJGCE）. Electrochemical behaviors of methyl parathion （MP） on the nanosilica/GS/GCE surface were investigated thoroughly. It was found that the nanosilicaJGS nanocomposites can improve the redox peak currents of MP significantly due to the synergetic effect. The oxidation peak current was linearly related to MP concentration in the range from 0.0005 μmol/L to 5.6 μmol/L. The detection limit was calculated to be 0.07 nmol/L （SJN = 3）. The developed method was used to determine MP in real samples. The recoveries were in the range from 95.4% to 104.2%, demonstrating satisfactory results.

Nanostructured organic electrode materials grown on graphene with covalent-bond interaction for high-rate and ultra-long-life lithium-ion batteries

Nanostructured organic tetralithium salts of 2,5-dihydroxyterephthalic acid （Li4C8H2O6） supported on graphene were prepared via a facile recrystallization method. The optimized composite with 75 wt.% Li4C8H2O6 was evaluated as an anode with redox couples of Li4C8H2O6/Li6C8H2O6 and as a cathode with redox couples of Li4C8H2O6/Li2C8H2O6 for Li-ion batteries, exhibiting a high-rate capability （10 C） and long cycling life （1,000 cycles）. Moreover, in an all-organic symmetric Li-ion battery, this dual-function electrode retained capacities of 191 and 121 mA.h·g-1 after 100 and 500 cycles, respectively. Density functional theory calculations indicated the presence of covalent bonds between Li4CsH206 and graphene, which affected both the morphology and electronic structure of the composite. The special nanostructures, high electronic conductivity of graphene, and covalent-bond interaction between Li4C8H2O6 and graphene contributed to the superior electrochemical properties. Our results indicate that the combination of organic salt molecules with graphene is useful for obtaining high-performance organic batteries.

Fabrication of three-dimensional graphene anode for augmenting performance in microbial fuel cells

In the present work,we have reported the synthesis of 3D graphene structures by simple chemical reduction method for application in microbial fuel cell(MFC)as an anode.The synthesis procedure includes synthesis of graphene oxide by modified Hummers method followed by simultaneous reduction of graphene oxide and the formation of hydrogels.The 3D graphene structures are characterized using X-ray diffraction and Raman spectroscopy techniques.Electrochemical characterization is carried out to study the capacitive properties and impedance studies of electrodes in conductive electrolyte.3D graphene electrodes show remarkably high capacitive current,charge storage and lower charge transfer resistance as estimated through cyclic voltammetry and Nyquist plots.In order to evaluate the performance of 3D graphene electrodes in MFC,power density and polarization curves are recorded in three configurations by varying terminal electron acceptors(TEA)viz.,dissolved oxygen(MFC-DO),potassium ferricyanide and MFC without TEA(MFC-NDO).3D graphene electrodes exhibits maximum power density of 0.49 mW/m^(2) in potassium ferricyanide followed by MFC-DO(0.36 mW/m^(2))and MFC-NDO(0.34 mW/m^(2)).Present work highlights the viability of chemically synthesized 3D graphene electrodes as an efficient anode material in MFC.

Chemical cross-linking and mechanically reinforced carbon network constructed by graphene boosts potassium ion storage

Carbon-based electrodes of potassium-ion batteries are of great research interest ascribed to their low cost and environmentally friendly distinctions.However,traditional carbon materials usually exhibit weak mechanical properties and incomplete crosslinking,resulting in poor stability and electrochemical performance.Herein,we report a new strategy for modifying reduced graphene oxide into a uniform few-layer structure through a sol–gel method combined with acid etching treatment.The obtained chemical cross-linking and mechanically reinforced carbon network constructed by graphene(CNCG)demonstrates excellent electrochemical and mechanical properties.Adopted as a free-standing anode(~7 mg·cm^(−2))for potassium ion battery,the asachieved CNCG delivers a high reversible specific capacity of 317.7 mAh·g^(−1) at a current density of 50 mA·g^(−1) and admirable cycle stability(208.4 mAh·g^(−1) at 50 mA·g^(−1) after 500 cycles).The highly reversible structural stability and fully cross-linked properties during potassiation are revealed by ex-situ characterization.This work provides new ideas for the synthesis of new carbon materials and the development of high-performance electrodes.

Graphene-based flexible and wearable electronics

Graphene with an exceptional combination of electronic, optical and outstanding mechanical features has been proved to lead a completely different kind of 2-D electronics. The most exciting feature of graphene is its ultra-thin thickness, that can be conformally contacted to any kind of rough surface without losing much of its transparency and conductivity. Graphene has been explored demonstrating various prototype flexible electronic applications, however, its potentiality has been proven wherever transparent conductive electrodes（TCEs） are needed in a flexible, stretchable format. Graphene-based TCEs in flexible electronic applications showed greatly superior performance over their conventionally available competitor indium tin oxide（ITO）. Moreover, enormous applications have been emerging, especially in wearable devices that can be potentially used in our daily life as well as in biomedical areas. However, the production of high-quality, defect-free large area graphene is still a challenge and the main hurdle in the commercialization of flexible and wearable products. The objective of the present review paper is to summarize the progress made so far in graphene-based flexible and wearable applications. The current developments including challenges and future perspectives arc also highlighted.

Amorphous Cobalt Boron Alloy@Graphene Oxide Nanocomposites for Pseudocapacitor Applications

Amorphous Co-B alloy nanoparticles grown on graphene sheets were synthesized via a chemical reduc- tion approach and successfully used for an application as a pseudocapacitor. This study aims to improve the capacity and cycling stability of amorphous Co-B alloy nanoparticles grown on conductive graphene sheets. The products were characterized by X-ray powder diffraction, scanning electron microscopy, and transmission electron microscopy. As electrode materials for pseudocapacitors, the amorphous Co-B alloy grown on graphene oxide （Co-B@GO） exhibits a high specific capacitance of 460 F g-1, which is nearly 1.5 times greater than that of bare Co-B nanoparticles at 1 A g-l, The specific capacitance preserved 84% of the initial capacitance even after 1000 cycles at a scan rate of 10 m V-1, suggesting its promising po- tential as pseudocapacitor materials.