Efros-Shklovskii variable range hopping in reduced graphene oxide sheets of varying carbon sp2 fraction

TL;DR

This study demonstrates that reduced graphene oxide sheets with varying sp2 fractions exhibit Efros-Shklovskii variable range hopping behavior at low temperatures, with localization length and bandgap decreasing as sp2 content increases.

Contribution

It provides experimental evidence of ES-VRH in RGO sheets and links the sp2 fraction to localization length and bandgap, highlighting the impact of structural disorder.

Findings

ES-VRH observed in all RGO samples at low temperatures

Localization length increases with higher sp2 fraction

Bandgap decreases from 1.43 eV to 0.21 eV as sp2 increases

Abstract

We investigate the low temperature electron transport properties of chemically reduced graphene oxide (RGO) sheets with different carbon sp2 fractions of 55 to 80 %. We show that in the low bias (Ohmic) regime, the temperature (T) dependent resistance (R) of all the devices follow Efros-Shklovskii variable range hopping (ES-VRH) R ~ exp[(T(ES)/T)^1/2] with T(ES) decreasing from 30976 to 4225 K and electron localization length increasing from 0.46 to 3.21 nm with increasing sp2 fraction. From our data, we predict that for the temperature range used in our study, Mott-VRH may not be observed even at 100 % sp2 fraction samples due to residual topological defects and structural disorders. From the localization length, we calculate a bandgap variation of our RGO from 1.43 to 0.21 eV with increasing sp2 fraction from 55 to 80 % which agrees remarkably well with theoretical prediction. We also show that, in the high bias regime, the hopping is field driven and the data follow R ~ exp[(E(0)/E)^1/2] providing further evidence of ES-VRH.