Changing from diffusive to superdiffusive transport in carbon nanotube networks via nematic order control

Abstract

Carbon nanotubes (CNTs) exhibit extremely anisotropic electronic and optical properties due to the confinement of quasiparticles in one-dimension. This anisotropy disappears in an ensemble of randomly oriented CNTs, together with some other properties that make them attractive for certain device applications. However, it is unclear if this disorder suppresses other types of behaviors as well. In this work [1], we addressed this question by using a combination of terahertz (THz) emission and photocurrent experiments (Fig. 1a) with out-of-equilibrium numerical simulations to compare the dynamics of quasiparticles under strong electric fields in aligned and random CNT networks. We found that the degree of alignment has a strong influence on the dynamics and thermalization pathways of quasiparticles. In aligned CNTs, the electrons show super-diffusive transport properties and have a high probability of acquiring enough kinetic energy to produce low energy excitons via exciton impact ionization [2]. However, this process is significantly suppressed in random CNTs. Due to the increased impurity-like scatterings in random CNTs that arise from multiple tube distortions in the intersecting tubes, the electrons show diffusive transport properties and do not gain the kinetic energy required for exciton impact ionization. This difference in electron transport regimes (superdiffusive vs diffusive) leads to drastically different bias-dependence of THz emission amplitude and photocurrent in aligned and random CNT networks as shown in Figs. 1b and 1c, respectively.