External Bias Dependent Direct To Indirect Bandgap Transition in Graphene Nanoribbon

TL;DR

This paper demonstrates that applying an external bias to armchair graphene nanoribbons can induce a transition from a direct to an indirect bandgap, enabling new device functionalities.

Contribution

It introduces the first demonstration of bias-induced direct to indirect bandgap transition in graphene nanoribbons using self-consistent tight-binding calculations.

Findings

Bias induces direct to indirect bandgap transition.

Carrier effective masses become sharply tunable near transition points.

Bandgap trends verified with Magnus expansion technique.

Abstract

In this work, using self-consistent tight-binding calculations, for the first time, we show that a direct to indirect bandgap transition is possible in an armchair graphene nanoribbon by the application of an external bias along the width of the ribbon, opening up the possibility of new device applications. With the help of Dirac equation, we qualitatively explain this bandgap transition using the asymmetry in the spatial distribution of the perturbation potential produced inside the nanoribbon by the external bias. This is followed by the verification of the bandgap trends with a numerical technique using Magnus expansion of matrix exponentials. Finally, we show that the carrier effective masses possess tunable sharp characters in the vicinity of the bandgap transition points.