Hideyuki Jippo

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.

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.

Theoretical study on high-frequency graphene-nanoribbon heterojunction backward diode

We propose and analyze a heterojunction backward diode for millimeter- or terahertz-wave detection using edge-modified graphene nanoribbons (GNRs). According to the electron-affinity difference between a hydrogen-terminated GNR and a fluorine-terminated GNR, it is possible to construct a staggered-type lateral heterojunction diode. First-principles calculations reveal that because of band-to-band tunneling, the diode has a nonlinear current of the order of kA/m. The small junction area contributes to the reduction of the intrinsic junction capacitance. Equivalent-circuit analyses show that when the total capacitance is reduced below 100 aF, the diode exhibits a voltage sensitivity of 3.79 × 103 V/W at 300 GHz.

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.