Satoru Konabe

Enhanced Chemical Reactivity of Graphene Induced by Mechanical Strain

Control over chemical reactivity is essential in the field of nanotechnology. Graphene is a two-dimensional atomic sheet of sp(2) hybridized carbon with exceptional properties that can be altered by chemical functionalization. Here, we transferred single-layer graphene onto a flexible substrate and investigated the functionalization using different aryl diazonium molecules while applying mechanical strain. We found that mechanical strain can alter the structure of graphene, and dramatically increase the reaction rate, by a factor of up to 10, as well as increase the final degree of functionalization. Furthermore, we demonstrate that mechanical strain enables functionalization of graphene for both p- and n-type dopants, where unstrained graphene showed negligible reactivity. Theoretical calculations were also performed to support the experimental findings. Our findings offer a simple approach to control the chemical reactivity of graphene through the application of mechanical strain, allowing for a tuning of the properties of graphene.

Crossover from Ballistic to Diffusive Thermal Transport in Carbon Nanotubes

We present a theoretical scheme that seamlessly handles the crossover from fully ballistic to diffusive thermal transport regimes and apply it to carbon nanotubes. At room temperature, micrometer-length nanotubes belong to the intermediate regime in which ballistic and diffusive phonons coexist. According to our scheme, the thermal conductance of these nanotubes exhibit anomalous nonlinear dependence of tube length due to this coexistence. This result is in excellent agreement with molecular-dynamics simulation results showing the nonlinear thermal conductance. Additionally, we clarify the mechanism of crossover in terms of the length-dependent characteristic frequency.

Significant enhancement of the thermoelectric performance of phosphorene through the application of tensile strain

First-principles calculations based on density functional theory showed that the thermoelectric performance of single-atomic-layer phosphorus, referred to as phosphorene, is significantly enhanced by the application of tensile strain along a particular direction. Phosphorene subjected to a 10% strain exhibits a very large power factor exceeding 10 mW/(mK2), which is almost two orders of magnitude greater than the power factors of conventional flexible thermoelectric materials. This extreme enhancement of the thermoelectric performance is explained by the new concept of a strain-induced energy valley.

Robustness and Fragility of a Linear Dispersion Band of Bilayer Graphene under an Electric Field

Based on first-principles total-energy calculations, we investigate the electronic structure of bilayer graphene under a vertical electric field for various stacking arrangements. We find that the ...

Modulation of electrical potential and conductivity in an atomic-layer semiconductor heterojunction

Semiconductor heterojunction interfaces have been an important topic, both in modern solid state physics and in electronics and optoelectronics applications. Recently, the heterojunctions of atomically-thin transition metal dichalcogenides (TMDCs) are expected to realize one-dimensional (1D) electronic systems at their heterointerfaces due to their tunable electronic properties. Herein, we report unique conductivity enhancement and electrical potential modulation of heterojunction interfaces based on TMDC bilayers consisted of MoS2 and WS2. Scanning tunneling microscopy/spectroscopy analyses showed the formation of 1D confining potential (potential barrier) in the valence (conduction) band, as well as bandgap narrowing around the heterointerface. The modulation of electronic properties were also probed as the increase of current in conducting atomic force microscopy. Notably, the observed band bending can be explained by the presence of 1D fixed charges around the heterointerface. The present findings indicate that the atomic layer heterojunctions provide a novel approach to realizing tunable 1D electrical potential for embedded quantum wires and ultrashort barriers of electrical transport.

Enhanced photocurrent in single-walled carbon nanotubes by exciton interactions

We theoretically investigate the photocurrent generation efficiency of single-walled carbon nanotubes by considering the interplay between exciton many-body effects. We calculate the photocurrent by solving rate equations that incorporate the influences of the two competing processes, multiple exciton generation (MEG) and the Auger recombination (AR) processes. We find that MEG substantially enhances photocurrent generation in spite of the competing AR process. Our calculation shows that the generation efficiency is up to 150% higher than that without MEG.

Laser probing of the single-particle energy gap of a Bose gas in an optical lattice in the Mott-insulator phase

We study single-particle excitations of a Bose gas in an optical lattice in the Mott-insulator phase. The characteristic feature of the single-particle spectrum in the Mott-insulator phase is the existence of an energy gap between the particle and hole excitations. We show that the single-particle excitation energies and associated energy gap in the Mott-insulator phase can be directly probed by an output-coupling experiment. We apply the general expression for the output current derived by Luxat and Griffin, which is given in terms of the single-particle Greens functions of a trapped Bose gas, to the Mott-insulator phase using the Bose-Hubbard model. The energy spectrum of the momentum-resolved output current exhibits two characteristic peaks corresponding to the particle and hole excitations, and thus it can be used to detect the transition point from the Mott insulator to superfluid phase where the energy gap disappears.

Instability of a superfluid Bose gas induced by a locked thermal gas in an optical lattice

We use a dissipative Gross–Pitaevskii equation derived from the Bose–Hubbard Hamiltonian to study the effect of the thermal component on the stability of a current-carrying superfluid state of a Bose gas in an optical lattice potential. We explicitly show that the superfluid state becomes unstable at certain quasi-momentum of the condensate due to a thermal component which is locked by an optical lattice potential. It is shown that this instability coincides with the Landau instability derived from the GP equation.

Graphene-diamond hybrid structure as spin-polarized conducting wire with thermally efficient heat sinks

We have theoretically investigated electronic, magnetic, and thermal properties of a graphene-diamond hybrid structure consisting of a graphene nanoribbon with zigzag edges connected to diamond surfaces. From the first-principles calculation, we found that the hybrid structure is stable and that the ferro-magnetically ordered edge state appears around the graphene-diamond. On the other hand, from the non-equilibrium molecular dynamics simulations, we found that the thermal conductance at the interface between the graphene and diamond is 7.01±0.05GWm-2K-1 at the room temperature, which is much larger than that for covalently bonded interface between carbon nanotube and silicon. Thus, we propose that the hybrid structure is a potential candidate for spin-polarized conducting wires with thermally efficient heat sinks.

Auger-Recombination Induced Photocurrents in Single-Walled Carbon Nanotubes

Photocurrents in single-walled carbon nanotubes are theoretically investigated with focus on the excitonic effects and a new method to generate photocurrents due to E11-exciton dissociation through the Auger recombination process is proposed. Those photocurrents whose carriers are dissociated E11-excitons abruptly increase at the threshold laser-intensity, due to the efficient Auger recombination associated with exciton–exciton scattering. Our calculation predicts that by increasing the laser-intensity, the current originating from dissociated E11-excitons significantly increases compared with that from electron–hole pairs of band-to-band excitations.