Guolin Hao

Three-dimensional holey-graphene/niobia composite architectures for ultrahigh-rate energy storage

As with donuts, the holes matter Improving the density of stored charge and increasing the speed at which it can move through a material are usually opposing objectives. Sun et al. developed a Nb2O5/holey graphene framework composite with tailored porosity. The three-dimensional, hierarchically porous holey graphene acted as a conductive scaffold to support Nb2O5. A high mass loading and improved power capability were reached by tailoring the porosity in the holey graphene backbone with higher charge transport in the composite architecture. The interconnected graphene network provided excellent electron transport, and the hierarchical porous structure in the graphene sheets facilitated rapid ion transport and mitigated diffusion limitations. Science, this issue p. 599 A graphene/Nb2O5 composite shows optimized electron and ion transport. Nanostructured materials have shown extraordinary promise for electrochemical energy storage but are usually limited to electrodes with rather low mass loading (~1 milligram per square centimeter) because of the increasing ion diffusion limitations in thicker electrodes. We report the design of a three-dimensional (3D) holey-graphene/niobia (Nb2O5) composite for ultrahigh-rate energy storage at practical levels of mass loading (>10 milligrams per square centimeter). The highly interconnected graphene network in the 3D architecture provides excellent electron transport properties, and its hierarchical porous structure facilitates rapid ion transport. By systematically tailoring the porosity in the holey graphene backbone, charge transport in the composite architecture is optimized to deliver high areal capacity and high-rate capability at high mass loading, which represents a critical step forward toward practical applications.

Large-scale production of ultrathin topological insulator bismuth telluride nanosheets by a hydrothermal intercalation and exfoliation route

A convenient hydrothermal intercalation/exfoliation method for large-scale manufacturing of bismuth telluride (Bi2Te3) nanosheets is reported here. Lithium cations can be intercalated between the layers of Bi2Te3 using the reducing power of ethylene glycol in the common hydrothermal process, and high quality Bi2Te3 nanosheets with thickness down to only 3–4 nm are obtained by removing lithium in the following exfoliating process. Scanning electron microscopy, transmission electron microscopy and Raman spectrum characterizations confirm that the high yield of Bi2Te3 nanosheets with good quality were successfully achieved and the sizes of the immense nanosheets reached 200 nm width and 1 μm length. This hydrothermal intercalation/exfoliation method is general, as it has been extended to other layered materials, such as Bi2Se3 and MoS2. Our results suggest a simple route for the large-scale production of thin and flat Bi2Te3 nanosheets, which may be beneficial to further electronic and spintronics applications.

Synthesis of WS2xSe2–2x Alloy Nanosheets with Composition-Tunable Electronic Properties

Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have recently emerged as a new class of atomically thin semiconductors for diverse electronic, optoelectronic, and valleytronic applications. To explore the full potential of these 2D semiconductors requires a precise control of their band gap and electronic properties, which represents a significant challenge in 2D material systems. Here we demonstrate a systematic control of the electronic properties of 2D-TMDs by creating mixed alloys of the intrinsically p-type WSe2 and intrinsically n-type WS2 with variable alloy compositions. We show that a series of WS2xSe2-2x alloy nanosheets can be synthesized with fully tunable chemical compositions and optical properties. Electrical transport studies using back-gated field effect transistors demonstrate that charge carrier types and threshold voltages of the alloy nanosheet transistors can be systematically tuned by adjusting the alloy composition. A highly p-type behavior is observed in selenium-rich alloy, which gradually shifts to lightly p-type, and then switches to lightly n-type characteristics with the increasing sulfur atomic ratio, and eventually evolves into highly n-doped semiconductors in sulfur-rich alloys. The synthesis of WS2xSe2-2x nanosheets with tunable optical and electronic properties represents a critical step toward rational design of 2D electronics with tailored spectral responses and device characteristics.

Porous Fe2O3 Nanoframeworks Encapsulated within Three-Dimensional Graphene as High-Performance Flexible Anode for Lithium-Ion Battery

Integrating nanoscale porous metal oxides into three-dimensional graphene (3DG) with encapsulated structure is a promising route but remains challenging to develop high-performance electrodes for lithium-ion battery. Herein, we design 3DG/metal organic framework composite by an excessive metal-ion-induced combination and spatially confined Ostwald ripening strategy, which can be transformed into 3DG/Fe2O3 aerogel with porous Fe2O3 nanoframeworks well encapsulated within graphene. The hierarchical structure offers highly interpenetrated porous conductive network and intimate contact between graphene and porous Fe2O3 as well as abundant stress buffer nanospace for effective charge transport and robust structural stability during electrochemical processes. The obtained free-standing 3DG/Fe2O3 aerogel was directly used as highly flexible anode upon mechanical pressing for lithium-ion battery and showed an ultrahigh capacity of 1129 mAh/g at 0.2 A/g after 130 cycles and outstanding cycling stability with a capacity retention of 98% after 1200 cycles at 5 A/g, which is the best results that have been reported so far. This study offers a promising route to greatly enhance the electrochemical properties of metal oxides and provides suggestive insights for developing high-performance electrode materials for electrochemical energy storage.

Formation of ripples in atomically thin MoS2 and local strain engineering of electrostatic properties

Ripple is a common deformation in two-dimensional materials due to localized strain, which is expected to greatly influence the physical properties. The effects of the ripple deformation in the MoS2 layer on their physics, however, are rarely addressed experimentally. We here grow atomically thin MoS2 nanostructures by employing a vapor phase deposition method without any catalyst and observed the ripples in MoS2 nanostructures. The MoS2 ripples exhibit quasi-periodical ripple structures in the MoS2 surface. The heights of the ripples vary from several angstroms to tens of nanometers and the wavelength is in the range of several hundred nanometers. The growth mechanism of rippled MoS2 nanostructures is elucidated. We have also simultaneously investigated the electrostatic properties of MoS2 ripples by using Kelvin probe force microscopy, which shows inhomogeneous surface potential and charge distributions for MoS2 ripple nanostructures with different local strains.

Photoresponse properties of large-area MoS2 atomic layer synthesized by vapor phase deposition

Photoresponse properties of a large area MoS2 atomic layer synthesized by vapor phase deposition method without any catalyst are studied. Scanning electron microscopy, atomic force microscopy, Raman spectrum, and photoluminescence spectrum characterizations confirm that the two-dimensional microstructures of MoS2 atomic layer are of high quality. Photoelectrical results indicate that the as-prepared MoS2 devices have an excellent sensitivity and a good reproducibility as a photodetector, which is proposed to be ascribed to the potential-assisted charge separation mechanism.

Electrostatic properties of few-layer MoS2 films

Two-dimensional MoS2-based materials are considered to be one of the most attractive materials for next-generation nanoelectronics. The electrostatic properties are important in designing and understanding the performance of MoS2-based devices. By using Kelvin probe force microscopy, we show that few-layer MoS2 sheets exhibit uniform surface potential and charge distributions on their surfaces but have relatively lower surface potentials on the edges, folded areas as well as defect grain boundaries.

Ambipolar charge injection and transport of few-layer topological insulator Bi2Te3 and Bi2Se3 nanoplates

We report the electrostatic properties of few-layer Bi2Te3 and Bi2Se3 nanoplates (NPs) grown on 300 nm SiO2/Si substrate. Electrons and holes are locally injected in Bi2Te3 and Bi2Se3 nanoplates by the apex of an atomic force microscope tip. Both carriers are delocalized uniformly over the whole nanoplate. The electrostatic property of topological insulator Bi2Te3 and Bi2Se3 nanoplates after charge injection is characterized by Kelvin probe force microscopy under ambient environment and exhibits an ambipolar surface potential behavior. These results provide insight into the electronic properties of topological insulators at the nanometer scale.

Electrochemically reduced graphene oxide with porous structure as a binder-free electrode for high-rate supercapacitors

A binder-free electrode is prepared by directly depositing electrochemically reduced graphene oxide (ERGO) on the metal current collector. Fourier transform infrared spectroscopy and Raman spectrum have been used to demonstrate the effective reduction of graphene oxide on the electrode, and the porous structure of the ERGO film was further characterized by scanning electron microscopy. The electrochemical properties of ERGO were investigated by cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy (EIS). Electrochemical measurements showed that the binder-free ERGO electrode had high specific capacity, good cycle stability, as well as excellent high-rate capability. The specific capacitance of the constructed electrode was 131.6 F g−1 at a scan rate of 10 mV s−1 and maintained 66.9% of the initial value when the scan rate was increased up to 1000 mV s−1. Owing to its favorable electrochemical performance, this binder-free ERGO electrode with porous structure has great potential in future commercial electrochemical supercapacitors.

Growth and surface potential characterization of Bi2Te3 nanoplates

Topological insulator Bi2Te3 nanoplates with hexagonal, triangular and truncated triangular nanostructures have been fabricated with thickness of ∼10 nm by vacuum vapor phase deposition method. The possible formation mechanism of Bi2Te3 nanoplates with different nanostructures has been proposed. We have examined the surface potentials of Bi2Te3 nanoplates using Kelvin probe force microscopy. The surface potential of Bi2Te3 nanoplates is determined to be about 482 mV on the SiO2/Si substrate, 88 mV and -112 mV on the n-doped and p-doped Si (111) substrates, respectively. The surface potential information provides insight into understanding electronic properties of Bi2Te3 nanoplates, which may open a new door to the exploration of the topological insulators.