Graphene quantum dots from chemistry to applications

A significant advance toward the design and fabrication of a novel hierarchical supercapacitor electrode consisting of FeCo2S4-tubes with well-defined square cross-section and intersecting nanosheets built porous shells on a 3D porous Ni backbone via controlled sulfidation is reported. This general method allows template-free synthesis of metal sulfides tubular structures with polygonal cross-sections and also fine control over the nanostructure leading to both maximized porosity and saturation sulfidation. New insights into concentration and time dependent sulfidation reaction kinetics are proposed. The FeCo2S4 electrode achieves a specific capacitance reaching 2411 F g-1 at 5 mA cm-2 and good rate capability, which are superior over those for nanotube arrays of other ternary transition metal sulfides. This is attributed to rich redox reactions, the highly porous but robust architecture as well as high electrical conductivity. Especially such porous shells effectively avoid “dead volume”, thus improve the utilization ratio of the electrode material. Asymmetric solid-state device applying the FeCo2S4 as positive electrode and N-doped graphene hydrogel film as negative electrode has a high cell voltage of 1.6 V and thus delivers considerably higher energy density of 76.1 W h kg-1 (at 755 W kg-1) than those reported for similar devices.

General Controlled Sulfidation toward Achieving Novel Nanosheet-Built Porous Square-FeCo2S4-Tube Arrays for High-Performance Asymmetric All-Solid-State Pseudocapacitors

Graphene and related materials (GRMs) are promising candidates for the fabrication of resistive random access memories (RRAMs) Here, this emerging field is analyzed, classified, and evaluated, and the performance of a number of RRAM prototypes using GRMs is summarized Graphene oxide, amorphous carbon films, transition metal dichalcogenides, hexagonal boron nitride and black phosphorous can be used as resistive switching media, in which the switching can be governed either by the migration of intrinsic species or penetration of metallic ions from adjacent layers Graphene can be used as an electrode to provide flexibility and transparency, as well as an interface layer between the electrode and dielectric to block atomic diffusion, reduce power consumption, suppress surface effects, limit the number of conductive filaments in the dielectric, and improve device integration GRM-based RRAMs fit some non-volatile memory technological requirements, such as low operating voltages ( 10 years, endurance >109 cycles and power consumption ≈10 pJ per transition still remain a challenge More technology-oriented studies including reliability and variability analyses may lead to the development of GRMs-based RRAMs with realistic possibilities of commercialization

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Graphene and Related Materials for Resistive Random Access Memories

Metal–organic frameworks (MOF) used directly in supercapacitors have attracted much attention for their special porous structure and potential high performance. Here, the Ni-MOF is fabricated by one-step facile hydrothermal method with a modification of mixed solution with DMF and water instead of pure DMF. After characterization, the Ni-MOF exhibits loosely stacked layer-cuboid structure with abundant mesoporous, which is beneficial for the charge transfer and ion transport for supercapacitors. In the three-electrode system, this Ni-MOF serving as working electrode shows remarkable specific capacitance of 804 Fg−1 at 1 Ag−1, excellent rate capacitance of 534 Fg−1 at 10 Ag−1, and with 302 Fg−1 retention after 5000 cycles, when measured in 2 M KOH electrolyte solution. To make a further research into the practical utility of the Ni-MOF, the Ni-MOF//AC asymmetrical supercapacitor device is assembled with the Ni-MOF and active carbon acted as positive and negative electrode materials, respectively. This device exhibits high specific energy of 31.5 Wh kg−1, at specific power of 800 W kg−1. All these results demonstrate that this Ni-MOF is a kind of promising electrode material for high-performance supercapacitors.

Facile synthesis of cuboid Ni-MOF for high-performance supercapacitors

Hierarchical flower-like C/NiO composite hollow microspheres and its excellent supercapacitor performance

NiO nanoarrays of a few atoms thickness on 3D nickel network for enhanced pseudocapacitive electrode applications

High performance of organic tandem solar cell is largely dependent on transparent and conductive intermediate layer (IML). The current work reports the design and fabrication of an IML using a simple solution process. The efficiency of a homo-tandem device with poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester as an active layer and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/poly(ethylenimine) as an IML was initially found to be 3.40%. Further enhancement of the cell efficiency was achieved using silver nanoparticles (Ag-NPs) of different sizes and graphene quantum dot embedded IML. A maximum efficiency of 4.03% was achieved using 7 nm Ag-NPs that contribute to a better recombination process. Also, the performance of the tandem cell was solely based on the electrical improvements indicated by the current - voltage measurements, external quantum efficiency and impedance analysis. The use of Ag-NPs in the IML has been shown to lengthen the life time of electron-hole pairs in the device. This study thus paves way to develop such efficient IMLs for more efficient tandem solar cells.

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Enhancement of recombination process using silver and graphene quantum dot embedded intermediate layer for efficient organic tandem cells.

In this study, we investigated the resistive switching behavior of pristine graphene oxide and thermally reduced GO. Impedance spectroscopy and current–voltage analysis were used to verify the possible physical mechanism of the switching operation. Our observations demonstrated that, the switching operation originates from the oxidation/reduction at the top interface of Al electrode and oxygen migration inside the active layer. Reversible redox reaction Al +1  + xO −2  ↔ AlO x is ground for the conduction electrons.

Impedance spectroscopy analysis of the switching mechanism of reduced graphene oxide resistive switching memory

This work covered the application of thermally reduced graphene oxide (rGO) as a hole-transport layer (HTL) to enhance stability of normal P3HT:PCBM-based bulk-heterojunction solar device. This device showed approximately the same performance of device using PEDOT:PSS as HTL (3.14%). After 3 days, exposure of an unsealed device in 1 sun illumination, rGO device resulted in 67% degradation of performance, whereas samples fabricated with PEDOT:PSS or without HTL degraded to more than 88% in efficiency. By simulation into an equivalent circuit, impedance analysis demonstrated that this degradation was strongly depended on the buffer materials. The use of the thermally rGO as buffer material could reduce the change of resistance in both HTL and bulk layer as well as avoid the accumulation of space charges in the active film.

Nhu Thuy Ho

Papers

NiO nanoarrays of a few atoms thickness on 3D nickel network for enhanced pseudocapacitive electrode applications

Enhancement of recombination process using silver and graphene quantum dot embedded intermediate layer for efficient organic tandem cells.

Impedance spectroscopy analysis of the switching mechanism of reduced graphene oxide resistive switching memory

Reliability improvement of bulk‐heterojunction organic solar cell by using reduced graphene oxide as hole‐transport layer

Solution-Processed Transparent Intermediate Layer for Organic Tandem Solar Cell Using Nitrogen-Doped Graphene Quantum Dots