Joung Real Ahn

Wafer-Scale Growth of Single-Crystal Monolayer Graphene on Reusable Hydrogen-Terminated Germanium

Smoothing Graphene Several methods have been reported for the growth of monolayer graphene into areas large enough for integration into silicon electronics. However, the electronic properties of the graphene are often degraded by grain boundaries and wrinkles. Lee et al. (p. 286, published online 3 April) showed that flat, single crystals of monolayer graphene can be grown by chemical-vapor deposition on silicon wafers covered by a germanium layer that aligns the grains. The graphene can be dry-transferred to other substrates, and the germanium layer can be reused for further growth cycles. Wafer-scale single-crystal monolayer graphene can be repeatedly grown on a hydrogen-terminated germanium (110) surface. The uniform growth of single-crystal graphene over wafer-scale areas remains a challenge in the commercial-level manufacturability of various electronic, photonic, mechanical, and other devices based on graphene. Here, we describe wafer-scale growth of wrinkle-free single-crystal monolayer graphene on silicon wafer using a hydrogen-terminated germanium buffer layer. The anisotropic twofold symmetry of the germanium (110) surface allowed unidirectional alignment of multiple seeds, which were merged to uniform single-crystal graphene with predefined orientation. Furthermore, the weak interaction between graphene and underlying hydrogen-terminated germanium surface enabled the facile etch-free dry transfer of graphene and the recycling of the germanium substrate for continual graphene growth.

Unstable Single-Layered Colloidal TiS2 Nanodisks†

Layer-structured materials are an important class of solid materials, and are especially useful as electrode materials. Usually, multilayered materials are formed by the weak interaction between the layers, which serves to stabilize the structure. The intercalation or doping into the interlamellar space of these materials enables materials scientists to tailor their physical properties. It has been suggested that the number of layers is closely related to the physical properties, thus allowing the electrical properties to be tuned by adjusting the number of layers. With the help of the accumulated synthetic knowledge, there has been special interest in monolayered materials. Although the preparation of single-layered materials including graphene is quite challenging, many studies have been conducted in an attempt to find an efficient synthetic route. There are two main ways to prepare single-layered materials; layer separation from multilayered materials or the direct synthesis of monolayered materials. In the former case, the relatively weak interaction between the layers that stabilizes the structure must be overcome. Thus, the physical separation of the layers or, more recently, the chemical functionalization of the layers, has been actively explored. In the latter case, although it is not common, there are rare reports on the direct synthesis of a monolayered structure under specialized conditions. Titanium disulfide (Figure 1) is known to form the layered structural motif (Figure 2b), in which each layer is stacked via relatively weak van der Waals interactions with its

Focused-Laser-Enabled p–n Junctions in Graphene Field-Effect Transistors

With its electrical carrier type as well as carrier densities highly sensitive to light, graphene is potentially an ideal candidate for many optoelectronic applications. Beyond the direct light-graphene interactions, indirect effects arising from induced charge traps underneath the photoactive graphene arising from light-substrate interactions must be better understood and harnessed. Here, we study the local doping effect in graphene using focused-laser irradiation, which governs the trapping and ejecting behavior of the charge trap sites in the gate oxide. The local doping effect in graphene is manifested by large Dirac voltage shifts and/or double Dirac peaks from the electrical measurements and a strong photocurrent response due to the formation of a p-n-p junction in gate-dependent scanning photocurrent microscopy. The technique of focused-laser irradiation on a graphene device suggests a new method to control the charge-carrier type and carrier concentration in graphene in a nonintrusive manner as well as elucidate strong light-substrate interactions in the ultimate performance of graphene devices.

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 °C using borazine molecules. Second, when heated above 1150 °C 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 °C, 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.

Columnar assembly and successive heating of colloidal 2D nanomaterials on graphene as an efficient strategy for new anode materials in lithium ion batteries: the case of In2S3 nanoplates

This study shows that heat-treatment of colloidal inorganic nanoplates with columnar assembly under argon is a good strategy for development of anode materials. The heating of colloidal In2S3 nanoplates under argon resulted in the formation of film-like materials through interconnection of plates in a side by side manner. When the columnarly assembled colloidal In2S3 plates were heated at 400 °C under argon for 2 hours on graphene, more efficient anode materials with smaller diameters were obtained. Interestingly, the heat-treated columnarly assembled In2S3 plates on graphene had a layered structure, which was attributed to the possible existence of carbon materials between plates formed by the heat-treatment of surfactants under argon. The resultant graphene–In2S3 composites showed enhanced discharge capacities, up to 716–837 mA h g−1, as well as excellent stabilities. In addition, the materials showed promising coulombic efficiencies and rate performances. We believe that, based on the strategy in this work, diverse graphene–inorganic nanomaterial composites with a layered structure can be prepared and applied as new anode materials in lithium ion batteries.

Horizontally aligned ZnO nanowire transistors using patterned graphene thin films

Here we report the directed growth of ZnO nanowires on multilayer graphene films (MGFs) without the use of metal seed materials. The ZnO source substance was diffused onto the MGF surface, where nanowires tended to grow in the high surface energy sites. This property was exploited to fabricate top-gate structural nanowire transistors with ZnO nanowires grown in the direction of the exposed sides of 6 × 4 μm patterned MGFs with a SiO2 capping layer. The devices showed an on-current of 160 nA, a threshold voltage of −2.27 V, an on-off current ratio of 3.98 × 105, and a field effect mobility of ∼41.32 cm2/V·s.

Simple, green, and clean removal of a poly(methyl methacrylate) film on chemical vapor deposited graphene

The clean removal of a poly(methyl methacrylate) (PMMA) film on graphene has been an essential part of the process of transferring chemical vapor deposited graphene to a specific substrate, influencing the quality of the transferred graphene. Here we demonstrate that the clean removal of PMMA can be achieved by a single heat-treatment process without the chemical treatment that was adopted in other methods of PMMA removal. The cleanness of the transferred graphene was confirmed by four-point probe measurements, synchrotron radiation x-ray photoemission spectroscopy, optical images, and Raman spectroscopy.

Submicro-polymer particles bearing imidazoline-2-selenone: dual mode adsorbents with color-sensing for halogens and mercury ions

Submicron-sized polymer particles (PSE) containing imidazoline selenones were prepared by co-polymerization of styrene derivative (MSE) bearing an imidazoline selenone moiety with 1,4-divinylbenzene (DVB). The size and chemical composition of PSE were controlled by changing the stoichiometric ratios of MSE to DVB. The physical and chemical properties of PSE were characterized by SEM, EDS and elemental analysis. PSE showed an interesting reactivity towards halogens with vivid color-change from white to red-orange, which is attributed to the reaction of selenium in imidazoline-2-selenone with halogens. Acid treatment of PSE generated the hydrophilic red-orange colored particles (PSEA) which showed very selective adsorption properties towards mercury ions with color change to pale yellow. To figure out the origin of color change, model studies were conducted using 1,3-dimethyl-imidazoline-2-selenone. The dimerization of 1,3-dimethyl-imidazoline-2-selenone through Se–Se bond formation by acid-treatment resulted in color change from colorless to red-orange. The coordination-induced cleavage of the Se–Se bond of the dimerized species by mercury ions resulted in color change from red-orange to pale yellow. These observations indicate that hydrophobic PSE and hydrophilic PSEA are efficient systems for adsorption of halogens and mercury ions with a vivid color-detection.

Atrial local Ca2+ signaling and inositol 1,4,5-trisphosphate receptors.

In atrial myocytes lacking t-tubules, action potential triggers junctional Ca(2+) releases in the cell periphery, which propagates into the cell interior. The present article describes growing evidence on atrial local Ca(2+) signaling and on the functions of inositol 1,4,5-trisphosphate receptors (IP(3)Rs) in atrial myocytes, and show our new findings on the role of IP(3)R subtype in the regulation of spontaneous focal Ca(2+) releases in the compartmentalized areas of atrial myocytes. The Ca(2+) sparks, representing focal Ca(2+) releases from the sarcoplasmic reticulum (SR) through the ryanodine receptor (RyR) clusters, occur most frequently at the peripheral junctions in isolated resting atrial cells. The Ca(2+) sparks that were darker and longer lasting than peripheral and non-junctional (central) sparks, were found at peri-nuclear sites in rat atrial myocytes. Peri-nuclear sparks occurred more frequently than central sparks. Atrial cells express larger amounts of IP(3)Rs compared with ventricular cells and possess significant levels of type 1 IP(3)R (IP(3)R1) and type 2 IP(3)R (IP(3)R2). Over the last decade the roles of atrial IP(3)R on the enhancement of Ca(2+)-induced Ca(2+) release and arrhythmic Ca(2+) releases under hormonal stimulations have been well documented. Using protein knock-down method and confocal Ca(2+) imaging in conjunction with immunocytochemistry in the adult atrial cell line HL-1, we could demonstrate a role of IP(3)R1 in the maintenance of peri-nuclear and non-junctional Ca(2+) sparks via stimulating a posttranslational organization of RyR clusters.

Band engineering of bilayer graphene by metal atoms: First-principles calculations

The continuous change in the electronic band structure of metal-adsorbed bilayer graphene was calculated as a function of metal coverage using first-principles calculations. Instead of modifying the unit cell size as a function of metal coverage, the distance between the metal atoms and bilayer graphene in the same 2×2 unit unit cell was controlled to change the total charges transferred from the metal atoms to bilayer graphene. The validity of the theoretical method was confirmed by reproducing the continuous change in the electronic band structure of K-adsorbed epitaxial bilayer graphene, as shown by Ohta et al. [Science 313, 951 (2006)]. In addition, the changes in the electronic band structures of undoped, n-type, and p-type bilayer graphene were studied schematically as a function of metal coverage using the theoretical method.