Width-Dependent Band Gap in Armchair Graphene Nanoribbons Reveals Fermi Level Pinning on Au(111)

TL;DR

This study investigates how the band gap of armchair graphene nanoribbons varies with width and reveals Fermi level pinning phenomena at a critical band gap, providing insights for nanoelectronic device interfaces.

Contribution

It demonstrates the width-dependent band gap evolution and Fermi level pinning in aGNRs on Au(111), highlighting a new system for studying interface electronic properties.

Findings

Band gap inversely proportional to nanoribbon width.

Fermi level pinning occurs below a 1.7 eV band gap threshold.

The system allows modification of the adsorbate band gap with stable interface chemistry.

Abstract

We report on the energy level alignment evolution of valence and conduction bands of armchair-oriented graphene nanoribbons (aGNR) as their band gap shrinks with increasing width. We use 4,4-dibromo-para-terphenyl as molecular precursor on Au(111) to form extended poly-para-phenylene nanowires, which can be fused sideways to form atomically precise aGNRs of varying widths. We measure the frontier bands by means of scanning tunneling spectroscopy, corroborating that the nanoribbons band gap is inversely proportional to their width. Interestingly, valence bands are found to show Fermi level pinning as the band gap decreases below a threshold value around 1.7 eV. Such behavior is of critical importance to understand the properties of potential contacts in graphene nanoribbon-based devices. Our measurements further reveal a particularly interesting system for studying Fermi level pinning by modifying an adsorbates band gap while maintaining an almost unchanged interface chemistry defined by substrate and adsorbate.