[Paper Review] Doping of Graphene Nanoribbons via Functional Group Edge Modification

Eduard Carbonell-Sanromà, Jérémy Hieulle+10 more|arXiv (Cornell University)|May 19, 2017

Molecular Junctions and Nanostructures3 references4 citations

TL;DR

This study demonstrates the on-surface synthesis of 7-armchair graphene nanoribbons (7-AGNRs) functionalized with nitrile (CN) groups at their edges via a bottom-up approach on Au(111). Using scanning tunneling microscopy (STM), spectroscopy (STS), and DFT calculations, it is shown that CN groups act as strong n-type dopants, inducing a ~0.3 eV band downshift per group and creating deep impurity levels from nitrogen lone pairs, significantly altering the electronic structure for semiconductor applications.

ABSTRACT

We report on the on-surface synthesis of 7 armchair graphene nanoribbons (7-AGNRs) substituted with nitrile (CN) functional groups. The CN groups are attached to the GNR backbone by modifying the 7-AGNR precursor. While many of these groups survive the on-surface synthesis, the reaction process causes the cleavage of some CN from the ribbon backbone and the on-surface cycloisomerization of few nitriles onto pyridine rings. Scanning Tunneling Spectroscopy and Density Functional Theory reveal that CN groups behave as very efficient n-dopants, significantly downshifting the bands of the ribbon, and introducing deep impurity levels associated to the nitrogen electron lone pairs.

Motivation & Objective

• To develop a precise method for doping graphene nanoribbons (GNRs) using edge-functionalized precursors.
• To investigate the electronic effects of nitrile (CN) groups on the band structure of armchair GNRs.
• To understand how functional group modification via on-surface synthesis alters the electronic properties of GNRs at the atomic level.
• To explore the potential of CN groups as efficient n-dopants in GNR-based electronic devices.

Proposed method

• On-surface synthesis of 7-AGNRs using cyano-substituted dibromo bianthracene precursors (3) via thermal activation on Au(111).
• Sequential annealing at 200 °C (Ullmann coupling) and 350 °C (cyclodehydrogenation) to form the final GNR structure.
• High-resolution scanning tunneling microscopy (STM) and CO-functionalized tip STM for atomic-scale imaging of functional groups and defects.
• Scanning tunneling spectroscopy (STS) to measure local electronic structure and band alignment changes.
• Density functional theory (DFT) calculations using the SIESTA code with van der Waals corrections to model electronic structure and charge redistribution.
• Hirshfeld population analysis to quantify charge transfer and electrostatic potential shifts induced by CN groups.

Experimental results

Research questions

• RQ1 How does the incorporation of nitrile (CN) groups at the edges of 7-AGNRs affect their electronic band structure?
• RQ2 To what extent do CN groups act as n-type dopants in GNRs, and how does this compare to substitutional nitrogen doping?
• RQ3 What are the structural and electronic consequences of CN group cleavage and cycloisomerization into pyridine rings during on-surface synthesis?
• RQ4 How do the electron-withdrawing properties of CN groups influence the electrostatic potential and effective electronegativity of the GNR backbone?

Key findings

• CN groups act as strong n-dopants, inducing a band downshift of approximately 0.3 eV per CN group, as confirmed by STS measurements.
• DFT calculations reveal that the downshift arises from the electron-withdrawing nature of CN groups, which create localized dipoles and increase the ribbon's effective electronegativity.
• The nitrogen lone pairs in CN groups introduce deep impurity levels within the bandgap, contributing to n-type character.
• Some CN groups are cleaved during synthesis, and a portion undergoes on-surface cycloisomerization to form pyridine rings, confirmed by high-resolution STM.
• The bandgap of the GNR is reduced due to the extended π-network interaction with conjugated CN groups, as predicted by DFT.
• The electronic response to CN functionalization is stronger than that observed with substitutional nitrogen edge atoms, indicating superior doping efficiency.