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 (NbO) 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.

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.

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/FeO aerogel with porous FeO nanoframeworks well encapsulated within graphene. The hierarchical structure offers highly interpenetrated porous conductive network and intimate contact between graphene and porous FeO as well as abundant stress buffer nanospace for effective charge transport and robust structural stability during electrochemical processes. The obtained free-standing 3DG/FeO 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.

A convenient hydrothermal intercalation/exfoliation method for large-scale manufacturing of bismuth telluride (Bi 2 Te 3 ) nanosheets is reported here. Lithium cations can be intercalated between the layers of Bi 2 Te 3 using the reducing power of ethylene glycol in the common hydrothermal process, and high quality Bi 2 Te 3 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 Bi 2 Te 3 nanosheets with good quality were successfully achieved and the sizes of the immense nanosheets reached 200 nm width and 1 mm length. This hydrothermal intercalation/exfoliation method is general, as it has been extended to other layered materials, such as Bi 2 Se 3 and MoS 2 . Our results suggest a simple route for the large-scale production of thin and flat Bi 2 Te 3 nanosheets, which may be beneficial to further electronic and spintronics applications.

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.

The quantum spin Hall effect and topological insulators Physics Today 63, 33 (2010); https://doi

Atomically thin transition metal dichalcogenides (TMDCs) have drawn much interest for their promising applications in electronic, optoelectronic, valleytronic and sensing fields. Controlled growth of large-scale and high-quality TMDC nanostructures is highly desirable but remains challenging. In the present work, large-scale monolayer, bilayer and few-layer MoSe 2 films have been controllably synthesized by ambient pressure chemical vapor deposition (APCVD). Hydrogen flow rate, growth temperature as well as selenium-metal flux ratio have been systematically investigated, which were demonstrated to play a key role in the synthesis of MoSe 2 nanostructures. We have also reported the successful growth of MoSe 2 nanoribbons with controlled width and length on diverse substrates by APCVD with the assistance of sodium chloride and corresponding growth mechanism was proposed. Our findings highlight the prospects for the controlled growth of novel 1D and 2D TMDC nanostructures for nanoelectronic devices and the development of mixed-dimensional heterostructures.

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