Epitaxial Growth of a Single-Crystal Hybridized Boron Nitride and Graphene layer on a Wide-Band Gap Semiconductor

Vertical and lateral heterogeneous structures of two-dimensional (2D) materials have paved the way for pioneering studies on the physics and applications of 2D materials. A hybridized hexagonal boron nitride (h-BN) and graphene lateral structure, a heterogeneous 2D structure, has been fabricated on single-crystal metals or metal foils by chemical vapor deposition (CVD). However, once fabricated on metals, the h-BN/graphene lateral structures require an additional transfer process for device applications, as reported for CVD graphene grown on metal foils. Here, we demonstrate that a single-crystal h-BN/graphene lateral structure can be epitaxially grown on a wide-gap semiconductor, SiC(0001). First, a single-crystal h-BN layer with the same orientation as bulk SiC was grown on a Si-terminated SiC substrate at 850 oC using borazine molecules. Second, when heated above 1150 oC in vacuum, the h-BN layer was partially removed and, subsequently, replaced with graphene domains. Interestingly, these graphene domains possess the same orientation as the h-BN layer, resulting in a single-crystal h-BN/graphene lateral structure on a whole sample area. For temperatures above 1600 oC, the single-crystal h-BN layer was completely replaced by the single-crystal graphene layer. The crystalline structure, electronic band structure, and atomic structure of the h-BN/graphene lateral structure were studied by using low energy electron diffraction, angle-resolved photoemission spectroscopy, and scanning tunneling microscopy, respectively. The h-BN/graphene lateral structure fabricated on a wide-gap semiconductor substrate can be directly applied to devices without a further transfer process, as reported for epitaxial graphene on a SiC substrate.Comment: 23 pages, 7 figure

Self-catalytic growth of elementary semiconductor nanowires with controlled morphology and crystallographic orientation

While the orientation-dependent properties of semiconductor nanowires have been theoretically predicted, their study has long been overlooked in many fields owing to the limits to controlling the crystallographic growth direction of nanowires (NWs). We present here the orientation-controlled growth of single-crystalline germanium (Ge) NWs using a self-catalytic low-pressure chemical vapor deposition process. By adjusting the growth temperature, the orientation of growth direction in GeNWs was selectively controlled to the ⟨110⟩, ⟨112⟩, or ⟨111⟩ directions on the same substrate. The NWs with different growth directions exhibit distinct morphological features, allowing control of the NW morphology from uniform NWs to nanoribbon structures. Significantly, the VLS-based self-catalytic growth of the ⟨111⟩ oriented GeNW suggests that NW growth is possible for single elementary materials even without an appropriate external catalyst. Furthermore, these findings could provide opportunities to investigate the orientation-dependent properties of semiconductor NWs

Large-Scale Hierarchical Organization of Nanowires for Functional Nanosystems

We review recent studies of solution-based hierarchical organization of nanowire building blocks. Nanowires have been aligned with controlled nanometer to micrometer scale separation using the Langmuir-Blodgett technique, transferred to planar substrates in a layer-by-layer process to form parallel and crossed nanowire structures over centimeter length scales, and then efficiently patterned into repeating arrays of controlled dimensions and pitch using photolithography. The hierarchically-organized nanowires open up key opportunities in several general areas of nanoscale science and technology. First, hierarchically-assembled nanowire arrays have been used as masks to define nanometer scale metal lines and surface features over large areas. Second, hierarchically-assembled nanowire arrays have been used to fabricate fully-scalable centimeter size arrays of field-effect transistors in high yields without requiring alignment of individual nanowires to output electrodes. Diverse applications of this approach for enabling a broad range of functional nanosystems, including macroelectronic and sensing applications, are described

hBN Flake Embedded Al2O3 Thin Film for Flexible Moisture Barrier

Due to the vulnerability of organic optoelectronic devices to moisture and oxygen, thin-film moisture barriers have played a critical role in improving the lifetime of the devices. Here, we propose a hexagonal boron nitride (hBN) embedded Al2O3 thin film as a flexible moisture barrier. After layer-by-layer (LBL) staking of polymer and hBN flake composite layer, Al2O3 was deposited on the nano-laminate template by spatial plasma atomic layer deposition (PEALD). Because the hBN flakes in Al2O3 thin film increase the diffusion path of moisture, the composite layer has a low water vapor transmission ratio (WVTR) value of 1.8 × 10−4 g/m2 day. Furthermore, as embedded hBN flakes restrict crack propagation, the composite film exhibits high mechanical stability in repeated 3 mm bending radius fatigue tests

Tailoring a N-Doped Nanoporous Carbon Host for a Stable Lithium Metal Anode

Li metal is a promising anode candidate due to its high theoretical capacity and low electrochemical potential. However, dendrite formation and the resulting dead Li cause continuous Li consumption, which hinders its practical application. In this study, we realized N-doped nanoporous carbon for a stable Li metal host composed only of lightweight elements C and N through the simple calcination of a nitrogen-containing metal–organic framework (MOF). During the calcination process, we effectively controlled the amount of lithophilic N and the electrical conductivity of the N-doped porous carbons to optimize their performance as Li metal hosts. As a result, the N-doped porous carbon exhibited excellent electrochemical performances, including 95.8% coulombic efficiency and 91% capacity retention after 150 cycles in a full cell with an LFP cathode. The N-doped nanoporous carbon developed in this study can realize a stable Li metal host without adding lithium ion metals and metal oxides, etc., which is expected to provide an efficient approach for reliable Li metal anodes in secondary battery applications