Mina Maruyama

Two-Dimensional sp2 Carbon Network of Fused Pentagons: All Carbon Ferromagnetic Sheet

Based on first-principles total-energy calculations, we investigate geometric and electronic structures of two-dimensional stable carbon allotropes comprised of pentagonal rings. We found that the sp2 carbon sheet has a slightly higher total energy than C60 and retains its planar structure up to 1000 K, indicating that the sheet is both energetically and kinetically stable. The electronic structure of the sheet is found to be a metal with a flat dispersion band at the Fermi level, leading to spin polarization on the sheet. The polarized electron spin is ferromagnetically aligned and extends throughout the sheet with a spin moment of 0.62 µB/nm2.

Two-dimensional sp2 carbon networks of fused pentagons

On the basis of first-principles total-energy calculations, we investigated the geometric and electronic structures of two-dimensional (2D) stable carbon allotropes containing acepentalene structures. We found that the sp2 carbon sheet has a total energy that is 0.9 eV/atom higher than that of C60 and retains its planar structure up to 2000 K, indicating that the sheet is a stable 2D carbon allotrope. The electronic structure of the sheet was found to be metallic. The sheet also has a flat dispersion band above the Fermi level throughout the Brillouin zone, indicating the possibility of spin polarization of the sheet by electron doping.

Elemental Semiconductors of Fused Small Fullerenes: Electronic and Geometric Structures of C28 Polymers

We report on first-principles total energy calculations providing geometric and electronic structures for new stable polymerized fullerites consisting of a small fullerene C 28 . We found that C 28...

Geometric and Electronic Structures of Two-Dimensional Networks of Fused C36 Fullerenes

We theoretically designed layered materials with nanometer thickness by assembling C36 fullerenes with D6h symmetry based on first-principles total-energy calculations within the framework of density functional theory. Our calculations show three possible network topologies derived from fused C36 fullerenes depending on the lateral lattice constant, possessing both static and kinetic stabilities. The electronic structures of these materials are semiconducting or metallic depending on their network topologies. We found small dispersion bands near the fundamental gap of the semiconducting systems, resulting from the segmentation of the π network or the strained bonds of four-fold coordinated C atoms. By injecting holes into the valence bands under a normal electric field, C36 sheets exhibit spin polarization with a magnetic moment of approximately 2 μB/nm2.

Geometric and electronic structures of polymerized C32 fullerenes: Electronic structure tuning by fullerene and carbon nanotube filling

We report first-principles total-energy calculations providing the geometric and electronic structures of polymerized small fullerene C32, and C36- and carbon nanotube (CNT)-filled C32 polymers. We found that the pristine C32 polymer is a semiconductor with a direct bandgap of 1.5 eV, while the filled C32 polymers are metals with the carriers distributed on sp2 carbon atoms in the C32 cage. We also found that the C32 polymer possesses a pair of linear dispersion bands in their valence band because of the ? network topology, which is regarded as a graphene-like topology with an internal degree of freedom.

Carrier Transport and Photoresponse in GeSe/MoS2 Heterojunction p–n Diodes

Simple stacking of thin van der Waals 2D materials with different physical properties enables one to create heterojunctions (HJs) with novel functionalities and new potential applications. Here, a 2D material p-n HJ of GeSe/MoS2 is fabricated and its vertical and horizontal carrier transport and photoresponse properties are studied. Substantial rectification with a very high contrast (>104 ) through the potential barrier in the vertical-direction tunneling of HJs is observed. The negative differential transconductance with high peak-to-valley ratio (>105 ) due to the series resistance change of GeSe, MoS2 , and HJs at different gate voltages is observed. Moreover, strong and broad-band photoresponse via the photoconductive effect are also demonstrated. The explored multifunctional properties of the GeSe/MoS2 HJs are expected to be important for understanding the carrier transport and photoresponse of 2D-material HJs for achieving their use in various new applications in the electronics and optoelectronics fields.

Band-Gap Engineering of Graphene Heterostructures by Substitutional Doping with B3N3

We investigated the energetics and electronic structure of B3 N3 -doped graphene employing density functional theory calculations with the generalized gradient approximation. Our calculations reveal that all of the B3 N3 -doped graphene structures are semiconducting, irrespective of the periodicity of the B3 N3 embedded into the graphene network. This is in contrast to graphene nanomeshes, which are either semiconductors or metals depending on the mesh arrangement. In B3 N3 -doped graphene, the effective masses for both electrons and holes are small. The band gap in the B3 N3 -doped graphene networks and the total energy of the B3 N3 -doped graphene are inversely proportional to the B3 N3 spacing. Furthermore, both properties depend on whether or not the graphene region possesses a Clar structure. In particular, the sheets with a Clar structure exhibit a wider band gap and a slightly lower total energy than those without a Clar structure.

Highly Conductive and Transparent Large‐Area Bilayer Graphene Realized by MoCl5 Intercalation

Bilayer graphene (BLG) comprises a 2D nanospace sandwiched by two parallel graphene sheets that can be used to intercalate molecules or ions for attaining novel functionalities. However, intercalation is mostly demonstrated with small, exfoliated graphene flakes. This study demonstrates intercalation of molybdenum chloride (MoCl5 ) into a large-area, uniform BLG sheet, which is grown by chemical vapor deposition (CVD). This study reveals that the degree of MoCl5 intercalation strongly depends on the stacking order of the graphene; twist-stacked graphene shows a much higher degree of intercalation than AB-stacked. Density functional theory calculations suggest that weak interlayer coupling in the twist-stacked graphene contributes to the effective intercalation. By selectively synthesizing twist-rich BLG films through control of the CVD conditions, low sheet resistance (83 Ω ▫-1 ) is realized after MoCl5 intercalation, while maintaining high optical transmittance (≈95%). The low sheet resistance state is relatively stable in air for more than three months. Furthermore, the intercalated BLG film is applied to organic solar cells, realizing a high power conversion efficiency.

Surface-Mediated Aligned Growth of Monolayer MoS2 and In-Plane Heterostructures with Graphene on Sapphire

Aligned growth of transition metal dichalcogenides and related two-dimensional (2D) materials is essential for the synthesis of high-quality 2D films due to effective stitching of merging grains. Here, we demonstrate the controlled growth of highly aligned molybdenum disulfide (MoS2) on c-plane sapphire with two distinct orientations, which are highly controlled by tuning sulfur concentration. We found that the size of the aligned MoS2 grains is smaller and their photoluminescence is weaker as compared with those of the randomly oriented grains, signifying enhanced MoS2-substrate interaction in the aligned grains. This interaction induces strain in the aligned MoS2, which can be recognized from their high susceptibility to air oxidation. The surface-mediated MoS2 growth on sapphire was further developed to the rational synthesis of an in-plane MoS2-graphene heterostructure connected with the predefined orientation. The in-plane epitaxy was observed by low-energy electron microscopy. Transmission electron microscopy and scanning transmission electron microscopy suggest the alignment of a zigzag edge of MoS2 parallel to a zigzag edge of the neighboring graphene. Moreover, better electrical contact to MoS2 was obtained by the monolayer graphene compared with a conventional metal electrode. Our findings deepen the understanding of the chemical vapor deposition growth of 2D materials and also contribute to the tailored synthesis as well as applications of advanced 2D heterostructures.

Radical Spin Interaction of Graphene Flakes Embedded into h-BN Sheet

Based on the first principles total energy calculation within the framework of density functional theory, we investigate the energetics and magnetic properties of graphene flakes with triangular shape embedded into h-BN sheet. Our calculation shows that the spin polarization energy saturates about 100 meV per graphene flake at the separation of 0.8 nm. The spin-spin interaction, J, prefers an antiparallel spin coupling to a parallel one with the energy of 25 meV at the flake-flake distance of 0.5 nm.