Raman Spectroscopy of Lithographically Patterned Graphene Nanoribbons

TL;DR

This study uses Raman spectroscopy to analyze the properties of lithographically patterned graphene nanoribbons, revealing effects of quantum confinement, edge disorder, and environmental doping on their vibrational spectra.

Contribution

It provides detailed Raman spectral characterization of graphene nanoribbons with varying widths, highlighting edge effects, relaxation lengths, and doping influences not previously quantified.

Findings

D-to-G band ratio increases as width decreases, then decreases below 25 nm.

G band upshift attributed to quantum confinement or edge doping.

2D band remains a reliable layer marker despite broadening.

Abstract

Nanometer-scale graphene objects are attracting much research interest because of newly emerging properties originating from quantum confinement effects. We present Raman spectroscopy studies of graphene nanoribbons (GNRs) which are known to have nonzero electronic bandgap. GNRs of width ranging from 15 nm to 100 nm have been prepared by e-beam lithographic patterning of mechanically exfoliated graphene followed by oxygen plasma etching. Raman spectra of narrow GNRs can be characterized by upshifted G band and prominent disorder-related D band originating from scattering at ribbon edges. The D-to-G band intensity ratio generally increases with decreasing ribbon width. However, its decrease for width < 25 nm, partly attributed to amorphization at the edges, provides a valuable experimental estimate on D mode relaxation length of <5 nm. The upshift in the G band of the narrowest GNRs can be attributed to confinement effect or chemical doping by functional groups on the GNR edges. Notably, GNRs are much more susceptible to photothermal effects resulting in reversible hole doping caused by atmospheric oxygen than bulk graphene sheets. Finally we show that the 2D band is still a reliable marker in determining the number of layers of GNRs despite its significant broadening for very narrow GNRs.