Heterostructures based on layering of two-dimensional (2D) materials such as graphene and hexagonal boron nitride represent a new class of electronic devices. Realizing this potential, however, depends critically on the ability to make high-quality electrical contact. Here, we report a contact geometry in which we metalize only the 1D edge of a 2D graphene layer. In addition to outperforming conventional surface contacts, the edge-contact geometry allows a complete separation of the layer assembly and contact metallization processes. In graphene heterostructures, this enables high electronic performance, including low-temperature ballistic transport over distances longer than 15 micrometers, and room-temperature mobility comparable to the theoretical phonon-scattering limit. The edge-contact geometry provides new design possibilities for multilayered structures of complimentary 2D materials.

One-dimensional electrical contact to a two-dimensional material.

The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Ultracapacitors: Why, How, and Where is the Technology

Graphene, a two-dimensional, single-layer sheet of sp(2) hybridized carbon atoms, has attracted tremendous attention and research interest, owing to its exceptional physical properties, such as high electronic conductivity, good thermal stability, and excellent mechanical strength. Other forms of graphene-related materials, including graphene oxide, reduced graphene oxide, and exfoliated graphite, have been reliably produced in large scale. The promising properties together with the ease of processibility and functionalization make graphene-based materials ideal candidates for incorporation into a variety of functional materials. Importantly, graphene and its derivatives have been explored in a wide range of applications, such as electronic and photonic devices, clean energy, and sensors. In this review, after a general introduction to graphene and its derivatives, the synthesis, characterization, properties, and applications of graphene-based materials are discussed.

Graphene-Based Materials: Synthesis, Characterization, Properties, and Applications

Colloidal nanoparticles are being intensely pursued in current nanoscience research. Nanochemists are often frustrated by the well-known fact that no two nanoparticles are the same, which precludes the deep understanding of many fundamental properties of colloidal nanoparticles in which the total structures (core plus surface) must be known. Therefore, controlling nanoparticles with atomic precision and solving their total structures have long been major dreams for nanochemists. Recently, these goals are partially fulfilled in the case of gold nanoparticles, at least in the ultrasmall size regime (1–3 nm in diameter, often called nanoclusters). This review summarizes the major progress in the field, including the principles that permit atomically precise synthesis, new types of atomic structures, and unique physical and chemical properties of atomically precise nanoparticles, as well as exciting opportunities for nanochemists to understand very fundamental science of colloidal nanoparticles (such as the s...

Atomically Precise Colloidal Metal Nanoclusters and Nanoparticles: Fundamentals and Opportunities

Graphene and other 2D atomic crystals are of considerable interest in catalysis because of their unique structural and electronic properties. Over the past decade, the materials have been used in a variety of reactions, including the oxygen reduction reaction, water splitting and CO2 activation, and have been shown to exhibit a range of catalytic mechanisms. Here, we review recent advances in the use of graphene and other 2D materials in catalytic applications, focusing in particular on the catalytic activity of heterogeneous systems such as van der Waals heterostructures (stacks of several 2D crystals). We discuss the advantages of these materials for catalysis and the different routes available to tune their electronic states and active sites. We also explore the future opportunities of these catalytic materials and the challenges they face in terms of both fundamental understanding and the development of industrial applications.

Catalysis with two-dimensional materials and their heterostructures

Flake-like NiO/WO 3 p-n heterojunction photocathode for photoelectrochemical water splitting

Solution-Processed Lead-Free Perovskite Nanocrystal Scintillators for High-Resolution X-Ray CT Imaging

Self-supported core-shell heterostructure MnO2/NiCo-LDH composite for flexible high-performance supercapacitor

We report the synthesis, characterization and gas sensing performance of reduced graphene oxide (RGO) and RGO/ZnO bilayer thin films prepared by facile sol-airbrush technology at room temperature. We found that RGO/ZnO bilayer films exhibit enhanced surface roughness and more spacing compared to RGO. Sensing performance measurements showed that RGO/ZnO film has 30% enhancement of Δ R/R 0 to chloroform vapor compared to pure RGO film due to the improvement of film structure. A slight enhancement of Δ R/R 0 was also observed on exposure to other vapors (water, ethanol, acetone and formaldehyde), with response and recovery time less than 10 s. To explore the stability of RGO film, we studied the effect of various flow rates of background gas on the electric resistance variation. It was found that electric resistance fell at a larger flow rate, rose at a smaller rate, and Δ R/R 0 became smaller after annealing. These results can be understood by the air pressure-induced deformation of RGO film. We further proposed a new method to calculate the sensing response, which was verified to be more reliable for sensing performance analysis with negligible hysteresis. We expect these findings to shed new light on future development of high-performance gas sensors based on bilayer films.

Miao Zhou

Papers

Flake-like NiO/WO 3 p-n heterojunction photocathode for photoelectrochemical water splitting

Solution-Processed Lead-Free Perovskite Nanocrystal Scintillators for High-Resolution X-Ray CT Imaging

Self-supported core-shell heterostructure MnO2/NiCo-LDH composite for flexible high-performance supercapacitor

Study on gas sensing of reduced graphene oxide/ZnO thin film at room temperature

Enhanced photovoltaic performance of ZnO nanorod-based dye-sensitized solar cells by using Ga doped ZnO seed layer