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One dimensional boron nitride (BN) nanomaterials with a high aspect ratio are of great interest due to their unique properties and potential applications. However, BN nanomaterials are generally difficult to synthesize. Here, we describe the creation of arrays of vertically aligned pure BN nanofibers and BN coated carbon nanofibers, fabricated on-chip via a straightforward template-assisted chemical conversion reaction. The template, a glassy carbon nanofiber array, is produced by plasma processing of conventional photoresists. The method is highly controllable, patternable, and scalable, and the final arrays can be fabricated over large areas with a controlled fiber length. We characterize the electron field emission properties of the BN-coated carbon nanofiber array and find a large field enhancement factor, low turn-on voltage, and good stability. The outstanding field emission performance results from the small tip size and high aspect ratio of the nanofiber as well as the high chemical stability and high thermal conductivity of the BN coating.

Moiré superlattices enable the generation of new quantum phenomena in two-dimensional heterostructures, in which the interactions between the atomically thin layers qualitatively change the electronic band structure of the superlattice. For example, mini-Dirac points, tunable Mott insulator states and the Hofstadter butterfly pattern can emerge in different types of graphene/boron nitride moiré superlattices, whereas correlated insulating states and superconductivity have been reported in twisted bilayer graphene moiré superlattices1-12. In addition to their pronounced effects on single-particle states, moiré superlattices have recently been predicted to host excited states such as moiré exciton bands13-15. Here we report the observation of moiré superlattice exciton states in tungsten diselenide/tungsten disulfide (WSe2/WS2) heterostructures in which the layers are closely aligned. These moiré exciton states manifest as multiple emergent peaks around the original WSe2 A exciton resonance in the absorption spectra, and they exhibit gate dependences that are distinct from that of the A exciton in WSe2 monolayers and in WSe2/WS2 heterostructures with large twist angles. These phenomena can be described by a theoretical model in which the periodic moiré potential is much stronger than the exciton kinetic energy and generates multiple flat exciton minibands. The moiré exciton bands provide an attractive platform from which to explore and control excited states of matter, such as topological excitons and a correlated exciton Hubbard model, in transition-metal dichalcogenides.

Atomically thin MoS2 grown by chemical vapor deposition (CVD) is a promising candidate for next-generation electronics due to inherent CVD scalability and controllability. However, it is well known that the stacking sequence in few-layer MoS2 can significantly impact electrical and optical properties. Herein we report different intrinsic stacking sequences in CVD-grown few-layer MoS2 obtained by atomic-resolution annular-dark-field imaging in an aberration-corrected scanning transmission electron microscope operated at 50 keV. Trilayer MoS2 displays a new stacking sequence distinct from the commonly observed 2H and 3R phases of MoS2. Density functional theory is used to examine the stability of different stacking sequences, and the findings are consistent with our experimental observations.

Nominally 'diffraction amorphous' materials represent a pervasive challenge in establishing classical structure-property relationships. This stems from the difficulty in defining the structure of nominally amorphous materials and experimentally differentiating the short-range (<10 Å) and medium-range (10 to 30 Å) order as a function of process parameters which are important due to their influence on physical, chemical, or transport properties. Herein, we report on the determination of short- and medium-range order in nominally amorphous zinc and tin co-substituted indium oxide In1.4Zn0.3Sn0.3O3 (a-ZITO) thin films grown by pulsed laser deposition. These thin films are being explored as transparent conductors that exhibit changes in transport properties depending on deposition temperature while remaining 'diffraction amorphous.' Reduced density function (RDF) analysis from electron diffraction accompanied by density functional theory molecular dynamics simulation of a liquid quench suggests ordering at short range. Scanning transmission fluctuation electron microscopy (STFEM) indicates the presence of ordered ZITO regions at medium range. For a-ZITO films grown at different temperatures, RDF analysis showed no significant change in the short range order. However, STFEM revealed notable structural (and symmetry) differences at medium range. The existence of this type of multi-scale local order in amorphous ZITO thin films highlights the importance of a combined approach to unravel the structures of nominally amorphous materials and to understand structure-properties correlations. [ABSTRACT FROM AUTHOR]

Moiré superlattices enable the generation of new quantum phenomena in two-dimensional heterostructures, in which the interactions between the atomically thin layers qualitatively change the electronic band structure of the superlattice. For example, mini-Dirac points, tunable Mott insulator states and the Hofstadter butterfly pattern can emerge in different types of graphene/boron nitride moiré superlattices, whereas correlated insulating states and superconductivity have been reported in twisted bilayer graphene moiré superlattices1-12. In addition to their pronounced effects on single-particle states, moiré superlattices have recently been predicted to host excited states such as moiré exciton bands13-15. Here we report the observation of moiré superlattice exciton states in tungsten diselenide/tungsten disulfide (WSe2/WS2) heterostructures in which the layers are closely aligned. These moiré exciton states manifest as multiple emergent peaks around the original WSe2 A exciton resonance in the absorption spectra, and they exhibit gate dependences that are distinct from that of the A exciton in WSe2 monolayers and in WSe2/WS2 heterostructures with large twist angles. These phenomena can be described by a theoretical model in which the periodic moiré potential is much stronger than the exciton kinetic energy and generates multiple flat exciton minibands. The moiré exciton bands provide an attractive platform from which to explore and control excited states of matter, such as topological excitons and a correlated exciton Hubbard model, in transition-metal dichalcogenides. Moiré superlattice exciton states are observed in WSe2/WS2 heterostructures with closely aligned layers. [ABSTRACT FROM AUTHOR]

Recently, hexagonal boron nitride (h‐BN) nanomaterials, e.g., nanosheets and nanotubes, have been predicted to be effective materials for reversible gas adsorption with high selectivity once charged. However, despite the encouraging theoretical predictions, sensing with h‐BN is difficult to realize experimentally due to its electrically insulating nature stemming from its large band gap. In this research, the controlled synthesis of high surface area hybrid h‐BN/graphene aerogel is reported, using high‐quality graphene as a template, and its application for selective gas sensing. It is discovered for the first time in this system that the difficulty of conversion in template synthesis of h‐BN is positively correlated with the quality of the carbon template, an observation that is verified on both graphene nanosheets and carbon nanotubes. The application of this hybrid material for gas sensing yields ppb level of detection limit and high selectivity for NH3. Through density functional theory calculations, the adsorption energy and charge transfer between NH3 molecules and aerogel are greatly enhanced. Therefore, this innovative approach promises new possibilities for the application of h‐BN in gas sensing, with the potential to play a significant role in gas capture, environmental monitoring, and other related fields. [ABSTRACT FROM AUTHOR]

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010. [ABSTRACT FROM PUBLISHER]

Moiré superlattices enable the generation of new quantum phenomena in two-dimensional heterostructures, in which the interactions between the atomically thin layers qualitatively change the electronic band structure of the superlattice. For example, mini-Dirac points, tunable Mott insulator states and the Hofstadter butterfly pattern can emerge in different types of graphene/boron nitride moiré superlattices, whereas correlated insulating states and superconductivity have been reported in twisted bilayer graphene moiré superlattices 1-12 . In addition to their pronounced effects on single-particle states, moiré superlattices have recently been predicted to host excited states such as moiré exciton bands 13-15 . Here we report the observation of moiré superlattice exciton states in tungsten diselenide/tungsten disulfide (WSe 2 /WS 2 ) heterostructures in which the layers are closely aligned. These moiré exciton states manifest as multiple emergent peaks around the original WSe 2 A exciton resonance in the absorption spectra, and they exhibit gate dependences that are distinct from that of the A exciton in WSe 2 monolayers and in WSe 2 /WS 2 heterostructures with large twist angles. These phenomena can be described by a theoretical model in which the periodic moiré potential is much stronger than the exciton kinetic energy and generates multiple flat exciton minibands. The moiré exciton bands provide an attractive platform from which to explore and control excited states of matter, such as topological excitons and a correlated exciton Hubbard model, in transition-metal dichalcogenides.

Charge transfer at the interface between dissimilar materials is at the heart of electronics and photovoltaics. Here we study the molecular orientation, electronic structure, and local charge transfer at the interface region of C 60 deposited on graphene, with and without supporting substrates such as hexagonal boron nitride. We employ ab initio density functional theory with van der Waals interactions and experimentally characterize interface devices using high-resolution transmission electron microscopy and electronic transport. Charge transfer between C 60 and the graphene is found to be sensitive to the nature of the underlying supporting substrate and to the crystallinity and local orientation of the C 60 . Even at room temperature, C 60 molecules interfaced to graphene are orientationally locked into position. High electron and hole mobilities are preserved in graphene with crystalline C 60 overlayers, which has ramifications for organic high-mobility field-effect devices.