Band Structure and Optical Absorption in Multilayer Armchair Graphene Nanoribbons: A Pariser-Parr-Pople Model Study

TL;DR

This study uses the Pariser-Parr-Pople model to analyze how multilayer armchair graphene nanoribbons' electronic and optical properties change with layer number, electric field, and edge alignment, revealing tunable optical absorption features.

Contribution

It provides a detailed theoretical analysis of multilayer AGNRs' electro-optical properties considering layer number, edge alignment, and gate bias effects, which was not comprehensively studied before.

Findings

Energy gap decreases and saturates with increasing layers.

Optical absorption intensity increases with layer number and depends on polarization.

Gate bias causes complex shifts in absorption, differing from bilayer graphene behavior.

Abstract

Using the tight binding and Pariser-Parr-Pople (PPP) model Hamiltonians, we study the electronic structure and optical response of multilayer armchair graphene nanoribbons (AGNRs), both with and without a gate bias. In particular, the influence of the number of layers ($n$), and the strength of the electric field applied perpendicular to layers, for different types of edge alignments, is explored on their electro-optical properties. As a function of increasing $n$, the energy gap initially decreases, eventually saturating for large $n$. The intensity of the linear optical absorption in these systems also increases with increasing $n$, and depends crucially on the polarization direction of the incident light, and the type of the edge alignment. This provides an efficient way of determining the nature of the edge alignment, and $n$, in the experiments. In the presence of a gate bias, the intensity of optical absorption behaves in a nontrivial way. The absorption becomes more intense for the large fields in narrow ribbons exhibiting a red shift of the band gap with the increasing field strength, while in broad ribbons exhibiting a blue shift, the absorption becomes weaker. However, for smaller electric fields, the absorption intensity exhibits more complicated behavior with respect to the field strength. Thus, the effect of the gate bias on optical absorption intensity in multilayer AGNRs is in sharp contrast to the bilayer graphene, which exhibits only enhancement of the absorption intensity with the increasing electric field.