Yuta Nakayasu

Exfoliated MoS2 and MoSe2 Nanosheets by a Supercritical Fluid Process for a Hybrid Mg–Li-Ion Battery

The ultrathin two-dimensional nanosheets of layered transition-metal dichalcogenides (TMDs) have attracted great interest as an important class of materials for fundamental research and technological applications. Solution-phase processes are highly desirable to produce a large amount of TMD nanosheets for applications in energy conversion and energy storage such as catalysis, electronics, rechargeable batteries, and capacitors. Here, we report a rapid exfoliation by supercritical fluid processing for the production of MoS2 and MoSe2 nanosheets. Atomic-resolution high-angle annular dark-field imaging reveals high-quality exfoliated MoS2 and MoSe2 nanosheets with hexagonal structures, which retain their 2H stacking sequence. The obtained nanosheets were tested for their electrochemical performance in a hybrid Mg–Li-ion battery as a proof of functionality. The MoS2 and MoSe2 nanosheets exhibited the specific capacities of 81 and 55 mA h g–1, respectively, at a current rate of 20 mA g–1.

Controllable bandgap of Cu2ZnSn(S,Se)4 thin films via simultaneous supercritical fluid chalcogenization

Supercritical ethanol (scEtOH), with its high solubility and reducibility, can be used as a medium to fabricate chalcogenide semiconductors from stable solid chalcogen sources. We fabricated Cu2ZnSn(S,Se)4 films via chalcogenization of Cu–Zn–Sn oxide precursor films using scEtOH to dissolve SeO2 and elemental sulfur (S8). The S/Se molar ratio and the bandgap of Cu2ZnSn(S,Se)4 films were controlled by changing the input ratio of selenium source (SeO2) to sulfur source (S8). Analysis indicated that high-density atomic chalcogens are the dominant reactive species. This process may contribute to the development of non-vacuum fabrication methods for chalcogenide-semiconductor solar cells.

Structure-Based Selective Adsorption of Graphene on a Gel Surface: Toward Improving the Quality of Graphene Nanosheets

Top-down graphene production via exfoliation from graphite produces a mass of graphene with structural variation in terms of the number of layers, sheet size, edge type, and defect density. All of these characteristics affect its electronic structure. To develop useful applications of graphene, structural separation of graphene is necessary. In this study, we investigate the adsorption behavior of different types of graphene fragments using a multicolumn gel chromatography system with a view to developing an efficient method for separating high-quality graphene. The graphene was dispersed in an aqueous sodium dodecyl sulfate (SDS) surfactant solution and flown through allyl-dextran-based gel columns connected in series. In the chromatographic operation, we observed that the small-sized or oxidized graphene fragments tended to bind to the gel and the relatively large-sized graphene with a low oxygen content eluted from the gel column. In this system, the adsorbed SDS molecules on the graphitic surface prevented graphitic materials from binding to the gel and the oxygen functional groups on the graphene oxide or at the abundant edge of small-sized graphene hindered SDS adsorption. We hypothesize that the reduced SDS adsorption density results in the preferential adsorption of small-sized or oxidized graphene fragments on the gel. This type of chromatographic separation is a cost-effective and scalable method for sorting nanomaterials. The structural separation of graphene based on the adsorption priority found in this study will improve the quality of graphene nanosheets on an industrial scale.

Inversion domain boundaries in MoSe2 layers

Structural defects, including point defects, dislocation and planar defects, significantly affect the physical and chemical properties of low-dimensional materials, such as layered compounds. In particular, inversion domain boundary is an intrinsic defect surrounded by a 60° grain boundary, which significantly influences electronic transport properties. We study atomic structures of the inversion domain grain boundaries (IDBs) in layered transition metal dichalcogenides (MoSe2 and MoS2) obtained by an exfoliation method, based on the aberration-corrected scanning transmission electron microscopy observation and density functional theory (DFT) calculation. The atomic-scale observation shows that the grain boundaries consist of two different types of 4-fold ring point shared and 8-fold ring edge shared chains. The results of DFT calculations indicate that the inversion domain grain boundary behaves as a metallic one-dimensional chain embedded in the semiconducting MoSe2 matrix with the occurrence of a new state within the band gap.