Kengo Takashima

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.

Conductance fluctuation of edge-disordered graphene nanoribbons: Crossover from diffusive transport to Anderson localization

Conductance fluctuation of edge-disordered graphene nanoribbons (ED-GNRs) is examined using the non-equilibrium Greens function technique combined with the extended Huckel approximation. The mean free path λ and the localization length ξ of the ED-GNRs are determined to classify the quantum transport regimes. In the diffusive regime where the length Lc of the ED-GNRs is much longer than λ and much shorter than ξ, the conductance histogram is given by a Gaussian distribution function with universal conductance fluctuation. In the localization regime where Lc≫ξ, the histogram is no longer the universal Gaussian distribution but a lognormal distribution that characterizes Anderson localization.

Transport phenomena of electrons at the carbon nanotube interface with molecular adsorption

The electric conductance of carbon-nanotube (CNT) films is affected by gas adsorption. Previous studies have shown that the adsorption of gas molecules on the CNT/CNT interface is the key to the changing CNT-film conductance. However, it is still unclear how the gas molecules affect the electric conduction of the CNT/CNT interface or its electron transport properties. We present here a study on the effects of gas-molecule adsorption on the CNT/CNT interface using a fluctuation-induced tunneling (FIT) model of the CNT-film electrical conduction. We demonstrated that the CNT-film conduction follows the FIT model, and the subsequently estimated electrostatic potential between the CNT/CNT interfaces was in good agreement with estimates from density functional theory simulations. Since the FIT model treats the CNT/CNT interface as a parallel-plate capacitor, we propose a modified FIT model that accounts for the change in the dielectric constant at the CNT/CNT interface due to the adsorption of gas molecules. T...

Carrier localization length in edge-disordered graphene nanoribbons with sub-100 nm length

We theoretically and computationally examined carrier localization in semiconducting edge-disordered graphene nanoribbons (ED-GNRs) with sub-100 nm lengths that correspond to the typical gate length for field-effect transistors. We numerically found that the localization length of ED-GNRs is proportional to the square of ribbon width and inversely proportional to the edge-disorder concentration. Furthermore, we obtained an analytical formula of the localization length in terms of the GNR width and the roughness concentration.