How fast can electrons zip through crystals?  This is an important question, because the faster they go, the more efficient is a resulting device such as a transitor switch.  The major hindrance limiting the speed is scattering, which could be either due to imperfections such as impurities in the crystals, or when electrons collide with the atoms of the crystal, launching a jiggling motion called phonons, which eventually converts to heat.  This behavior is known for several decades.  But the discovery of the atomically thin semiconductor - graphene - forced us to re-think the picture all over again. 

The manner by which an electron interacts with an impurity is through long-range electrical forces, called "Coulomb" interaction.  Inside a thick solid, this interaction is damped by the electron clouds and atomic nuclei: this damping is lumped under the quantity referred to as the 'dielectric constant' of a medium.  However, if the electrons are confined to move in a 2-dimensional crystal such as graphene, the Coulomb interaction is still three-dimensional.  In other words, one can confine electrons to live in two dimensions, but not the electric field!  Since the electric field lines leak out of the 2-dimensional plane, the sourrounding material medium is expected to strongly affect the interactions electrons feel even when they are confined to the plane.  In other words, the surrounding dielectric medium strongly modifies the interaction between electrons and charged objects inside the 2D nanostructure, from which it is spatially separated.  This is an electronic 'remote control'.

Based on this simple idea, we investigated whether there are avenues to use this remarkable feature to speed up electrons in 2D crystals such as graphene, or other new materials such as transition metal dichalcogenides.  The first studies concentrated solely on the interactions between electrons and charged impurities, and a significant boost of electron speed was predicted, and has been ovserved.  But the story ran deeper - if the surrounding dielectric allowed vibrational modes that were also charged, then an electron can remotely set these vibrations in motion, thus suffering scattering and slowing down!  In this project, we explored these fundamental questions by combining theoretical analysis with experimental evaluation.  The study lays the goundwork for dielectric manipulation of the transport properties of semiconductor nanostructures.  It identified situations where significant boosts of electron speeds may be obtained, and these have been verified in several experiments.  In addition, the study pointed out specific properties of the dielectrics such as ionicity and polar nature, that can interfere with the electron speeds.  These have also been verified through experiments.  Each of these findings have significant applications in the semiconductor and electronics industry, where thin nanostructures are at the heart of every transistor switch.  Speeding them up and avoiding unwanted power dissipation is critical to saving us energy, and keeping electronics cool and efficient.

Last Modified: 09/08/2013
Modified by: Debdeep Jena