Dong-Hwa Oh

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

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.

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.

Influence of graphene-substrate interactions on configurations of organic molecules on graphene: Pentacene/epitaxial graphene/SiC

Pentacene has been used widely in organic devices, and the interface structure between pentacene and a substrate is known to significantly influence device performances. Here we demonstrate that molecular ordering of pentacene on graphene depends on the interaction between graphene and its underlying SiC substrate. The adsorption of pentacene molecules on zero-layer and single-layer graphene, which were grown on a Sifaced 6H-SiC(0001) wafer, was studied using scanning tunneling microscopy (STM). Pentacene molecules form a quasi-amorphous layer on zero-layer graphene which interacts strongly with the underlying SiC substrate. In contrast, they form a uniformly ordered layer on the single-layer graphene having a weak graphene-SiC interaction. Furthermore, we could change the configuration of pentacene molecules on the singlelayer graphene by using STM tips. The results suggest that the molecular ordering of pentacene on graphene and the pentacene/graphene interface structure can be controlled by a graphene-substrate interaction.

Spontaneous assembly of ordered atomic wires with a long interwire distance on a stepped atomic template

Indium atomic wires with a long interwire distance of 5.73 nm were ordered spontaneously at room temperature on a stepped atomic template using a Si(557) surface. The long interwire distance is very interesting because, in general, interwire interactions are needed to order atomic wires in such a way that ordered atomic wires have a short interwire distance of just a few A. The Si(557) surface is composed of four steps, i.e., one (111) step and three (112) steps, with a very similar local structure to each other. However, mobile indium atoms at room temperature were adsorbed specifically onto the second Si(112) step while maintaining the overall structure of the stepped atomic template, as observed by scanning tunneling microscopy, which results in the ordered atomic wires with the long interwire distance. This was supported by first-principles calculations.