Kazuhito SHINTANI (新谷 一人)

Sharp decrease and existence of the minimum of the thermal conductivity (TC) of a graphene/hexagonal boron nitride (hBN) heterobilayer due to interlayer sp3 bonds are demonstarted via nonequilibrium molecular dynamics simulation. Since interlayer sp3 bonds act as phonon scatterers, the TC sharply falls if there are a small number of them. As their number increases, their role as a fastener of the two layers becomes more and more significant, as a result of which the heterobilayer becomes a quasi three-dimensional structure and its TC begins to increase again. This is confirmed by examining the phonon density of states (DOSs) in the heterobilayer with/without interlayer sp3 bonds.

Dynamic encapsulation of pyrenes into a single-walled carbon nanotube (SWCNT) is addressed via molecular dynamics (MD) simulation. It is revealed the final structures of encapsulated pyrenes are classified into two categories according to the magnitude of the SWCNT diameter. For the SWCNT diameters less than 1.470 nm, the azimuthal angles of pyrenes during sliding within the SWCNT are almost zero, and they form a stack, whereas for the SWCNT diameters equal to or greater than 1.470 nm, their azimuthal angles during sliding are nonzero, and they form not a stack but a sequence of trimers. The conjugate-gradient (CG) energy minimization confirms the final stacking structures for the SWCNT diameters less than 1.470 nm obtained via MD are energetically favorable ones. A semi-analytical formula expresses well the tilt angles obtained via MD.

Pyrene is a rhombic-shaped flat polycyclic aromatic hydrocarbon molecule with blue fluorescence property. If pyrenes are encapsulated into a single-walled carbon nanotube (SWCNT), the resulting hybrids might be fluorescent probes for diseased sites in organisms. In this paper, dynamic encapsulation of pyrenes into a SWCNT is simulated using molecular dynamic method. If an edge-opened SWCNT is inserted into an equilibrated pyrene atmosphere, spontaneous encapsulation of pyrenes into the SWCNT starts and continues until the SWCNT is filled with pyrenes. The encapsulated pyrenes are deformed to be of the shape of saddle and inevitably form their dimers with their faces parallel to the SWCNT inner wall. After the SWCNT is filled with such dimers, they start to make angles with the SWCNT axis sequentially from the end of the SWCNT where additional pyrenes enter the SWCNT. The tilt angles of the stacked pyrenes are calculated, and their dependences on the SWCNT diameter are expressed by a semi-analytical formula useful for controlling the morphologies of the stacked molecules.

A graphene/hexagonal boron nitride (hBN) heterobilayer (HBL) is among two-dimensional van der Waals heterostructures. In the present paper, the effects of interlayer bonds in a graphene/hBN HBL on its thermal conductivity (TC) are addressed using molecular dynamics (MD) simulation. The TC is estimated by the method of nonequilibrium MD. It is shown that the TC of a graphene/hBN HBL sharply decreases if only a few interlayer bonds exist, and continues to gradually decrease with increasing the density of the interlayer bonds up to about 25 %. However, it starts to gradually increase if the density of the interlayer bonds exceeds about 25 %. Namely, there exists a minimum TC of the HBL. The reduction of the TC occurs because the interlayer bonds work as phonon scatterers. On the other hand, its increase occurs because the rigidity of the HBL is enhanced via interlayer bonds. Its minimum appears resulting from the balance between the both actions of the interlayer bonds. In order to confirm such an interpretation, the phonon density of states (DOSs) in the HBLs with/without interlayer bonds are examined in details.

The performance index of the Seebeck effect is inversely proportional to the thermal conductivity. Hence, a material with low thermal conductivity can be used as a highly efficient thermoelectric conversion material. In this study, we investigate the influence of the density of interlayer bonds on the thermal conductivity and mechanical properties of a hetero-bilayer structure consisting of graphene and hexagonal boron nitride (hBN). It is found that its thermal conductivity gradually decreases with increasing the density of interlayer bonds. By calculating the local phonon density of states (DOS) and examining the overlapped energy of the phonon DOS, we discuss the mechanism of the decrease of the thermal conductivity.

Gas separation using membrane consumes small energy and is important in the development of clean technology. In recent years, graphene with nanopores has attracted attention as a gas separation membrane. In this study, the usefulness of graphene with nanopores in hydrogen separation from mixed gases is shown, and the mechanism of separation and permeation is revealed. It turned out that there is a big difference in the way gas molecules enter the pores, depending on the shapes of the pores.