Photoinduced Hysteresis of Graphene Field-Effect Transistors Due to Hydrogen-Complexed Defects in Silicon Dioxide.

Photoinduced hysteresis (PIH) of graphene field-effect transistors (G-FETs) has attracted attention because of its potential in developing photoelectronic or nonvolatile memory devices. In this work, we focused on the role of SiO2 dielectric layer on PIH, where G-FETs have only a SiO2 dielectric layer. Adsorbates are effectively removed before the PIH test. The effects of laser wavelength, laser power density, and temperature on the PIH are systematically investigated. The PIH is significantly enhanced by increasing the hydrogen flow in a hydrogen-atmosphere device thermal annealing. This strongly suggests proton-related defects that play a key role. The pure electronic process for PIH is further ruled out by the significant dependence of the doping rate on the temperature. A mechanism of PIH based on proton generation after hole trapping at [O3≡Si-H] is proposed. The proposed mechanism is well-supported by our experimental data: (1) the observed threshold photon energy for PIH is between 2.76 and 2.34 eV, which is close to the energy barrier for [O3≡Si-H], releasing a proton. (2) No obvious carrier mobility degradation after the PIH process suggests that the bulk defects in SiO2 are the major contributors rather than graphene/SiO2 interface defects. (3) The dependence of the doping rate on the temperature and the laser power density matches a theoretical model based on the random hopping of H+. The results in this work are also valuable for the study of degradation of other oxide dielectric materials in various field-effect transistors.

Reduced graphene oxide/CoS2 porous nanoparticle hybrid electrode material for supercapacitor application.

Graphene/transition metal hybrid electrode materials are considered promising electrode materials for supercapacitor applications. However, the stacking of graphene sheets and agglomeration of transition metal parts are still challenging issues to overcome in order to achieve the expected theoretical performances. Herein, a reduced graphene oxide/cobalt disulphide porous nanoparticle hybrid electrode material is fabricated using sulphur as the template precursor. The unique porosity derived from the sulphur template gives favourable open structures for easy diffusion of electrolyte ions and better accessible active sites, and free space for volume changes and results in improved electrochemical performance. In this hybrid material the graphene layers serve as a conductive matrix and physical support for pours cobalt sulphide nanoparticles. On the other hand, the porous cobalt sulphide redox-active material uniformly decorated on rGO can enhance the pseudocapacitive performance of the as synthesized hybrid material. Using the combined advantage of graphene and transition metal sulphide the as synthesized composite electrode material has excellent specific capacitance, excellent rate capability and cycling stability. Thus, our design approach can be considered as a potential candidate to design advanced energy storage devices.

A hierarchical NiCo2S4 honeycomb/NiCo2S4 nanosheet core-shell structure for supercapacitor applications.

Transition metal sulphides are becoming one of the promising materials for energy storage applications. Particularly, an advanced electrode material architecture, which gives favourable electronic and ionic conductivity, is highly in demand. Herein, a hierarchical NiCo2S4 honeycomb/NiCo2S4 nanosheet core-shell structure is reported for supercapacitor applications. The core-shell structure was in situ grown on a nickel foam via two consecutive hydrothermal processes, followed by an electrochemical deposition process. Moreover, we tuned the deposition cycle to get abundant active sites with gaps of suitable sizes between the walls of the honeycomb structure for efficient electrolyte diffusion routes. The 3D honeycomb core structure was used as superhighway for electron transport to the current collector, while the ultrathin shell structure offered a large surface area with short electron and ion diffusion paths, thus leading to the faster kinetics and higher utilization of active materials. Thus, using the synergistic advantages of the core material and the shell material, the as-synthesized optimized electrode material came up with an excellent specific capacitance of 17.56 F cm-2 at a current density of 5 mA cm-2 and the highest cycling stability of 88.2% after 5000 cycles of charge-discharge process. Such advanced electrode architectures are highly promising for the future electrode materials.

A 2D metal-organic framework/reduced graphene oxide heterostructure for supercapacitor application.

Metal organic frameworks (MOFs) with two dimensional (2D) nanosheets have attracted special attention for supercapacitor application due to their exceptional large surface area and high surface-to-volume atom ratios. However, their electrochemical performance is greatly hindered by their poor electrical conductivity. Herein, we report a 2D nanosheet nickel cobalt based MOF (NiCo-MOF)/reduced graphene oxide heterostructure as an electrode material for supercapacitors. The NiCo-MOF 2D nanosheets are in situ grown on rGO surfaces by simple room temperature precipitation. In such hybrid structure the MOF ultrathin nanosheets provide large surface area with abundant channels for fast mass transport of ions while the rGO conductive and physical support provides rapid electron transport. Thus, using the synergistic advantage of rGO and NiCo-MOF nanosheets an excellent specific capacitance of 1553 F g-1 at a current density of 1 A g-1 is obtained. Additionally, the as synthesized hybrid material showed excellent cycling capacity of 83.6% after 5000 cycles of charge-discharge. Interestingly, the assembled asymmetric device showed an excellent energy density of 44 W h kg-1 at a power density of 3168 W kg-1. The electrochemical performance obtained in this report illustrates hybridization of MOF nanosheets with carbon materials is promising for next generation supercapacitors.

Nickel Cobalt Sulfide core/shell structure on 3D Graphene for supercapacitor application.

Three-dimensional (3D) core/shell structure of nickel cobalt sulfide is nano-engineered by using series of hydrothermal steps on a CVD grown graphene for supercapacitor application. This core/shell is composited of NiCo2S4 nanotube (NCS) as core and CoxNi(3-x)S2 (CNS) nanosheets as a shell. The as-synthesized composite exhibits excellent electrochemical properties by using the advantage of NCS nanontube core as superhighway for electron and ion transport, and CNS nanosheets shell as high active area pseudocapacitive material. The 3D graphene layer serves as excellent surface area to support 3D NCS/CNS; moreover, it provides excellent electrical conductivity between nickel foam current collector and the 3D NCS/NCS composite. Using these hybrid advantages the as-synthesized graphene/NCS/CNS composite electrode exhibits high areal capacitance of 15.6 F/cm2 at current density of 10 mA/cm2; excellent cycling stability of 93% after 5000 of cycles and excellent rate capability of 74.36% as current increase from 10 to 100 mA/cm2. Moreover, a prototype of asymmetric device fabricated using graphene/NCS/CNS as positive electrode and RGO as negative electrode exhibits high energy density of 23.9 Wh/kg and power density of 2460.6 W/kg at high operating current of 100 mA. Such high performance electrode material may get great application in future energy storage device.