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Lepidocrocite-type titanates that reversibly intercalate sodium ions at low potentials (∼0.6 V vs Na/Na+) are promising anode candidates for sodium-ion batteries. However, large amounts of carbon additives are often used to improve their electrical conductivity and overcome poor cycling performance in the electrode composites. To ameliorate electronic transport issues of lepidocrocite titanate (K0.8Ti1.73Li0.27O4, KTL) in sodium-ion batteries, we have designed and synthesized heterostructures of exfoliated lepidocrocite-type titanium oxide (LTO) nanosheets with alternating carbon layers via a solution-based self-assembly approach. Positively charged dopamine (Dopa) was used as the carbon precursor and intercalated between negatively charged exfoliated titania nanosheets through electrostatic interaction. Dopa-intercalated LTO was then annealed under argon to form conductive carbon layers between titania sheets. The carbon content in the heterostructures was controlled by modifying the self-assembly conditions (i.e., pH, stirring duration, and Dopa-to-LTO ratio). Electrodes were prepared using carbonized heterostructures (LTO-C) without adding more carbon to the composites and tested in sodium half-cell configurations. Higher capacities and improved capacity retention over 250 cycles and lower impedance were observed, as the carbon content of LTO-C heterostructures was increased from 0% (LTO nanosheets) to 30%. These results indicate that the self-assembly approach for 2D heterostructured electrode materials is a promising strategy to overcome electronic transport limitations of layered transition-metal oxides and improve their electrochemical performance for next-generation energy storage applications. [ABSTRACT FROM AUTHOR]

Metal chalcogenide compounds have attracted interest as materials for next-generation semiconductors, catalysts, and device architectures. Hybrid compounds containing both a metal chalcogenide architecture and a supporting organic lattice combine the interesting structural and electronic properties of the material class with a configurable hybrid component that can lead to a wide range of tailorable materials. However, many of the methods available for preparing inorganic coordination polymers in this class require specialized solution-phase chemical preparations that are incompatible with solid-state fabrication techniques. Here, we prepare metal–organic chalcogenolates (MOChas) of copper, indium, lead, and tin from benzeneselenol or benzenethiol directly from the organochalcogen and corresponding metal in a tube furnace at a relatively modest temperature. Scanning electron microscopy and X-ray diffraction confirmed the conversion of the precursors to crystalline MOChas. X-ray photoelectron spectroscopy was used to investigate the chemical bonding for each of the materials and provides insight into the elemental composition of the synthesized compounds. This straightforward approach for preparing crystalline hybrid materials may be generalizable for the preparation of a wide variety of coordination compounds and complexes in form factors useful for subsequent development of device architectures. [ABSTRACT FROM AUTHOR]

In this work, we demonstrate thatcatalyst composition can be usedto direct the crystallographic growth axis of GaN nanowires. By adjustingthe ratio of gold to nickel in a bimetallic catalyst, we achievedselective growth of dense, uniform nanowire arrays along two nonpolardirections. A gold-rich catalyst resulted in single-crystalline nanowiregrowth along the ⟨11̅00⟩ or maxis, whereas a nickel-rich catalyst resulted in nanowire growthalong the ⟨112̅0⟩ or aaxis.The same growth control was demonstrated on two different epitaxialsubstrates. Using proper conditions, many of the nanowires were observedto switch direction midgrowth, resulting in monolithic single-crystalstructures with segments of two distinct orientations. Cathodoluminescencespectra revealed significant differences in the optical propertiesof these nanowire segments, which we attribute to the electronic structuresof their semipolar {112̅2} or {11̅01} sidewalls. [ABSTRACT FROM AUTHOR]

We synthesized Fe3O4nanoparticle/reduced graphene oxide (RGO-Fe3O4) nanocomposites and evaluated their performance as anodes in both half and full coin cells. The nanocomposites were synthesized through a chemical co-precipitation of Fe2橷 Fe3 the presence of graphene oxides within an alkaline solution and a subsequent high-temperature reduction reaction in argon (Ar) environment. The morphology and microstructures of the fabricated RGO-Fe3O4nanocomposites were characterized using various techniques. The results indicated that the Fe3O4nanoparticles had relatively homogeneous dispersions on the RGO sheet surfaces. These as-synthesized RGO-Fe3O4nanocomposites were used as anodes for both half and full lithium-ion cells. Electrochemical measurement results exhibit a high reversible capacity which is about two and a half times higher than that of graphite-based anodes at a 0.05C rate, and an enhanced reversible capacity of about 200 mAh g−1even at a high charge/discharge rate of 10C (9260 mA g−1) in half cells. Most important of all, these fabricated novel nanostructures also show exceptional capacity retention with the assembled RGO-Fe3O4/LiNi1/3Mn1/3Co1/3O2full cell at different C rates. This outstanding electrochemical behavior can be attributed to the unique microstructure, morphology, texture, surface properties of the nanocomposites, and combinative effects from the different chemical composition in the nanocomposites. [ABSTRACT FROM AUTHOR]

As one of the highest mobility semiconductor materials, carbon nanotubes (CNTs) have been extensively studied for use in field effect transistors (FETs). To fabricate surround-gate FETs— which offer the best switching performance—deposition of conformal, weakly-interacting dielectric layers is necessary. This is challenging due to the chemically inert surface of CNTs and a lack of nucleation sites—especially for defect-free CNTs. As a result, a technique that enables integration of uniform high-k dielectrics, while preserving the CNT's exceptional properties is required. In this work, we show a method that enables conformal atomic layer deposition (ALD) of high-k dielectrics on defect-free CNTs. By depositing a thin Ti metal film, followed by oxidation to TiO2 under ambient conditions, a nucleation layer is formed for subsequent ALD deposition of Al2O3. The technique is easy to implement and is VLSI-compatible. We show that the ALD coatings are uniform, continuous and conformal, and Raman spectroscopy reveals that the technique does not induce defects in the CNT. The resulting bilayer TiO2/Al2O3 thin-film shows an improved dielectric constant of 21.7 and an equivalent oxide thickness of 2.7 nm. The electrical properties of back-gated and top-gated devices fabricated using this method are presented. [ABSTRACT FROM AUTHOR]

Review: recent advances in synthetic methods; 35 refs. [ABSTRACT FROM AUTHOR]

Two-dimensional materials with engineered composition and structure will provide designer materials beyond conventional semiconductors. However, the potentials of defect engineering remain largely untapped, because it hinges on a precise understanding of electronic structure and excitonic properties, which are not yet predictable by theory alone. Here, we utilize correlative, nanoscale photoemission spectroscopy to visualize how local introduction of defects modifies electronic and excitonic properties of two-dimensional materials at the nanoscale. As a model system, we study chemical vapor deposition grown monolayer WS 2 , a prototypical, direct gap, two-dimensional semiconductor. By cross-correlating nanoscale angle-resolved photoemission spectroscopy, core level spectroscopy, and photoluminescence, we unravel how local variations in defect density influence electronic structure, lateral band alignment, and excitonic phenomena in synthetic WS 2 monolayers.

Fundamental understanding of structure-property relationships in hierarchically organized nanostructures is crucial for the development of new functionality, yet quantifying structure across multiple length scales is challenging. In this work, we used nondestructive X-ray scattering to quantitatively map the multiscale structure of hierarchically self-organized carbon nanotube (CNT) "forests" across 4 orders of magnitude in length scale, from 2.0 Å to 1.5 μm. Fully resolved structural features include the graphitic honeycomb lattice and interlayer walls (atomic), CNT diameter (nano), as well as the greater CNT ensemble (meso) and large corrugations (micro). Correlating orientational order across hierarchical levels revealed a cascading decrease as we probed finer structural feature sizes with enhanced sensitivity to small-scale disorder. Furthermore, we established qualitative relationships for single-, few-, and multiwall CNT forest characteristics, showing that multiscale orientational order is directly correlated with number density spanning 10 9 -10 12 cm -2 , yet order is inversely proportional to CNT diameter, number of walls, and atomic defects. Lastly, we captured and quantified ultralow-q meridional scattering features and built a phenomenological model of the large-scale CNT forest morphology, which predicted and confirmed that these features arise due to microscale corrugations along the vertical forest direction. Providing detailed structural information at multiple length scales is important for design and synthesis of CNT materials as well as other hierarchically organized nanostructures.

SnS2 nanoparticle-loaded graphene nanocomposites were synthesized via one-step hydrothermal reaction. Their electrochemical performance was evaluated as the anode for rechargeable lithium-ion batteries after thermal treatment in an Ar environment. The electrochemical testing results show a high reversible capacity of more than 800 mA h g(-1) at 0.1 C rate and 200 mA h g(-1) for up to 5 C rate. The cells also exhibit excellent capacity retention for up to 90 cycles even at a high rate of 2 C. This electrochemical behavior can be attributed to the well-defined morphology and nanostructures of these as-synthesized nanocomposites, which is characterized by high-resolution transmission electron microscopy and electron energy-loss spectroscopy.

We synthesized Fe(3)O(4) nanoparticle/reduced graphene oxide (RGO-Fe(3)O(4)) nanocomposites and evaluated their performance as anodes in both half and full coin cells. The nanocomposites were synthesized through a chemical co-precipitation of Fe(2+) and Fe(3+) in the presence of graphene oxides within an alkaline solution and a subsequent high-temperature reduction reaction in argon (Ar) environment. The morphology and microstructures of the fabricated RGO-Fe(3)O(4) nanocomposites were characterized using various techniques. The results indicated that the Fe(3)O(4) nanoparticles had relatively homogeneous dispersions on the RGO sheet surfaces. These as-synthesized RGO-Fe(3)O(4) nanocomposites were used as anodes for both half and full lithium-ion cells. Electrochemical measurement results exhibit a high reversible capacity which is about two and a half times higher than that of graphite-based anodes at a 0.05C rate, and an enhanced reversible capacity of about 200 mAh g(-1) even at a high charge/discharge rate of 10C (9260 mA g(-1)) in half cells. Most important of all, these fabricated novel nanostructures also show exceptional capacity retention with the assembled RGO-Fe(3)O(4)/LiNi(1/3)Mn(1/3)Co(1/3)O(2) full cell at different C rates. This outstanding electrochemical behavior can be attributed to the unique microstructure, morphology, texture, surface properties of the nanocomposites, and combinative effects from the different chemical composition in the nanocomposites.

Dense and aligned TiO2 nanorod arrays are fabricated using oblique-angle deposition on indium tin oxide (ITO) conducting substrates. The TiO2 nanorods are measured to be 800-1100 nm in length and 45-400 nm in width with an anatase crystal phase. Coverage of the ITO is extremely high with 25 x 10(6) mm(-2) of the TiO2 nanorods. The first use of these dense TiO2 nanorod arrays as working electrodes in photoelectrochemical (PEC) cells used for the generation of hydrogen by water splitting is demonstrated. A number of experimental techniques including UV/Vis absorption spectroscopy, X-ray diffraction, high-resolution scanning electron microscopy, energy-dispersive X-ray spectroscopy, and photoelectrochemistry are used to characterize their structural, optical, and electronic properties. Both UV/Vis and incident-photon-to-current-efficiency measurements show their photoresponse in the visible is limited but with a marked increase around approximately 400 nm. Mott-Schottky measurements give a flat-band potential (V(FB)) of +0.20 V, a carrier density of 4.5 x 10(17) cm(-3), and a space-charge layer of 99 nm. Overall water splitting is observed with an applied overpotential at 1.0 V (versus Ag/AgCl) with a photo-to-hydrogen efficiency of 0.1%. The results suggest that these dense and aligned one-dimensional TiO2 nanostructures are promising for hydrogen generation from water splitting based on PEC cells.