Hot electrons injection in carbon nanotubes under the influence of quasi-static ac-field

TL;DR

This paper theoretically investigates hot electron injection in carbon nanotubes under a quasi-static ac electric field, revealing conditions for switching from negative to positive differential conductivity, which could enable terahertz radiation generation.

Contribution

It introduces a theoretical model for hot electron injection in CNTs under combined dc and quasi-static ac fields, predicting NDC to PDC transition and potential terahertz applications.

Findings

Initial ohmic conductivity observed

Transition from NDC to PDC at high hot electron injection

Potential for terahertz radiation generation

Abstract

Hot electrons injection in carbon nanotubes (CNTs ) where in addition to applied dc field ($\mathbf{E}$), there exist simultaneously a quasi-static ac electric field (i.e. when the frequency $\omega$ of ac field is much less than the scattering frequency $v$ ($\omega\ll v$ or $\omega\tau\ll 1$, $v =\tau^{-1}$, where $\tau$ is relaxation time) is considered. The investigation is done theoritically by solving semiclassical Boltzmann transport equation with and without the presence of the hot electrons source to derive the current densities. Plots of the normalized current density versus dc field ($\mathbf{E}$) applied along the axis of the CNTs in the presence and absence of hot electrons reveal ohmic conductivity initially and finally negative differential conductivity (NDC) provided $\omega\tau\ll 1$ (i.e. quasi- static case). With strong enough axial injection of the hot electrons , there is a switch from NDC to positive differential conductivity (PDC) about $\mathbf{E} \geq 75 kV/cm$ and $\mathbf{E} \geq 140 kV/cm$ for a zigzag CNT and an armchair CNT respectively. Thus, the most important tough problem for NDC region which is the space charge instabilities can be suppressed due to the switch from the NDC behaviour to the PDC behaviour predicting a potential generation of terahertz radiations whose applications are relevance in current-day technology, industry, and research.