Mari Ohfuchi

Anisotropic optical and electronic properties of two-dimensional layered germanium sulfide

Two-dimensional (2D) layered materials, transition-metal dichalcogenides, and black phosphorus have attracted considerable interest from the viewpoints of fundamental physics and device applications. The establishment of new functionalities in anisotropic layered 2D materials is a challenging but rewarding frontier, owing to the remarkable optical properties of these materials and their prospects for new devices. Herein, we report the anisotropic and thickness-dependent optical properties of a 2D layered monochalcogenide of germanium sulfide (GeS). Three Raman-scattering peaks corresponding to the B3g, Ag1, and Ag2 modes with a strong polarization dependence are demonstrated in the GeS flakes, which validates polarized Raman spectroscopy as an effective method for identifying the crystal orientation of anisotropic layered GeS. Photoluminescence (PL) is observed with a peak at ~1.66 eV that originates from the direct optical transition in GeS at room temperature. The polarization-dependent characteristics of the PL, which are revealed for the first time, along with the demonstration of anisotropic absorption, indicate an obvious anisotropic optical transition near the band edge of GeS, which is supported by density functional theory calculations. The significantly thickness-dependent PL is observed and discussed. This anisotropic layered GeS presents opportunities for the discovery of new physical phenomena and will find applications that exploit its anisotropic properties, such as polarization-sensitive photodetectors.

First-principles study of edge-modified armchair graphene nanoribbons

We have used first-principles methods to study the geometries and electronic structures of hydrogen (H), fluorine (F), chlorine (Cl), and hydroxyl (OH) terminated armchair graphene nanoribbons (H-AGNRs, F-AGNRs, Cl-AGNRs, and OH-AGNRs) with ribbon widths N = 7 and 19. The most stable geometries of H-AGNRs have planar configurations, but those of F-, Cl-, and OH-AGNRs have rippled edges. The ripples stem from steric hindrances between neighboring pairs of terminal atoms or groups, and the ripples are strongly localized to the edges. The most stable termination occurs with F atoms owing to strong C-F bonds despite their rippled edge structures. The energy band gaps of F- and Cl-AGNRs are narrower than those of H-AGNRs. This is due to structural deformations rather than chemical effects. For OH-AGNRs, chemical interactions between neighboring OH groups further reduce the band gaps.

Experimental and Theoretical Investigations of Surface-Assisted Graphene Nanoribbon Synthesis Featuring Carbon–Fluorine Bond Cleavage

Edge-fluorinated graphene nanoribbons are predicted to exhibit attractive structural and electronic properties, which, however, still need to be demonstrated experimentally. Hence, to provide further experimental insights, an anthracene trimer comprising a partially fluorinated central unit is explored as a precursor molecule, with scanning tunneling microscopy and X-ray photoelectron spectroscopy analyses, indicating the formation of partially edge-fluorinated polyanthrylenes via on-surface reactions after annealing at 350 °C on Au(111) under ultrahigh-vacuum conditions. Further annealing at 400 °C leads to the cyclodehydrogenation of partially edge-fluorinated polyanthrylenes to form graphene nanoribbons, resulting in carbon-fluorine bond cleavage despite its high dissociation energy. Extensive theoretical calculations reveal a defluorination-based reaction mechanism, showing that a critical intermediate structure, obtained as a result of H atom migration to the terminal carbon of a fluorinated anthracene unit in polyanthrylene, plays a crucial role in significantly lowering the activation energy of carbon-fluorine bond dissociation. These results suggest the importance of transient structures in intermediate states for synthesizing edge-fluorinated graphene nanoribbons.

First-principles electronic transport calculations of graphene nanoribbons on SiO2/Si

We study the electronic transport properties of graphene nanoribbons (GNRs) on SiO2/Si with O-terminated (siloxane) or OH-terminated (silanol) surfaces. The channel length and SiO2 thickness are 9.91 and 0.45 nm, respectively. The GNR shows p-type conduction on both the SiO2/Si surfaces. More holes are injected from OH groups in the silanol SiO2 to GNRs. The on/off current ratio for both GNRs on SiO2/Si is 103–105, which is consistent with recent experiments, and smaller by a factor of 108 than those of freestanding GNRs. The ratio is even smaller on the p side for the silanol SiO2.

Large-Scale Electronic Transport Calculations of Finite-Length Carbon Nanotubes Bridged between Graphene Electrodes with Lithium-Intercalated Contact

We perform density-functional-theory-based large-scale calculations of finite-length carbon nanotubes with caps bridged between graphene electrodes. The electronic transport properties vary with the length of the nanotubes and the contact structure. Despite the use of thin nanotubes expected to show n-type behavior, the Fermi level of the shorter nanotubes is uniformly pinned to the cap state, forming a large conduction gap. Although the longer nanotubes still have a medium conduction gap after forming a Schottky-like contact, the lithium intercalation in the contact area brings about a good ohmic property due to not only doping but also the orbital hybridization.

Stability of two orientations of MoS2 on α-Al2O3(0001)

We have studied the morphology of MoS2 on α-Al2O3(0001) using first-principles calculations. We found that the binding energy between MoS2 and the OH-terminated α-Al2O3(0001) surface is weaker than that for the Al-terminated surface. The band gap reduction of MoS2 is also smaller on the OH-terminated surface than on the Al-terminated surface. The strong chemical interaction between MoS2 and α-Al2O3(0001) seems to result in the larger binding energy and the larger band gap reduction on the Al-terminated surface. Despite the different binding characteristics between the OH- and Al-terminated surfaces, the total energy difference between the two orientations of the MoS2 monolayer related by a 60° rotation is quite small for both surfaces, indicating that the two orientations of MoS2 exist with almost the same amount on the α-Al2O3(0001) regardless of the termination. This result suggests that it is essentially difficult to obtain large-scale MoS2 with a single domain on the α-Al2O3(0001) by chemical vapor deposition.

Graphene-gate transistors for gas sensing and threshold control

Graphene has been employed as gate electrodes of n-channel silicon transistors. When the graphene gate is exposed and gas molecules adsorb on the graphene surface, the work function of graphene changes depending on the gas species and concentrations, thus changing the threshold of the silicon transistor. This novel graphene-gate sensor exhibits sensitivities more than one order of magnitude higher than those of the conventional resistivity-based graphene gas sensors, easily detecting 7 ppb of NO2. The selectivity of several gases also exist. Furthermore, the work function of graphene-gate can be controlled by intentionally depositing proper doping materials on graphene, changing the threshold by up to 620 mV without degrading the subthreshold properties.

Electronic transport properties of graphene channel with metal electrodes or insulating substrates in 10 nm-scale devices

We have studied the electronic transport properties of armchair graphene nanoribbons (AGNRs) bridged between two metal electrodes or supported on insulating substrates in 10 nm-scale devices using the first-principles calculations. The two metal species of Ti and Au are examined as metal electrodes and are compared. The current densities through the AGNR-Ti contact are about 10 times greater than those through the AGNR-Au contact, even though the AGNR width reaches 12 nm. For the insulating substrates, we have investigated the dependence of the channel length on the transport properties using models with two channel lengths of 15.1 and 9.91 nm. Regardless of the channel length, the on/off current ratio is 105 for the AGNRs on an O-terminated surface. This ratio is consistent with the recent experiments and is less by factors of 1016 for the 15.1 nm channel length and 108 for the 9.91 nm channel length compared to the freestanding AGNR.

First-Principles Simulations Applied to Graphene Nanoribbon Transistors

We developed methods to introduce a gate electric field to first-principles electron transport calculations that do not require empirical parameters as a useful tool for investigating novel nanodevices. We can place any number of gates anywhere without increasing the computational cost through the analytical expression of the potential produced by a surface charge as a gate model. Equivalent circuit analysis transforms from the electron density of the surface charge G to the gate voltage VG. Applying these methods to graphene nanoribbon (GNR) transistors, we demonstrated the allocability and multiplicity of gates and showed the dependence on channel length LCh of the transfer characteristics and the quantum capacitance of the channel CQ for specific device structures.

Electronic Structure and Carrier Transport in Thin Graphene Films under a Vertical Electric Field Based on Ab-initio Calculations

We have made ab-initio calculations of graphene films up to four layers without/with a titanium electrode under an electric field, and a semiclassical Monte Carlo study on electron transport in monolayer and bilayer graphene. The ab-initio calculation is a very useful tool for not only material design, but also device design of grapheme films. We can expect graphene channel FETs to have better performance than conventional FETs and it will be possible to use them in millimeter- or sub-millimeter wave devices.