[Paper Review] Quantifying Alignment and Quality of Graphene Nanoribbons: A Polarized Raman Spectroscopy Approach

Rimah Darawish, Jan Overbeck+5 more|arXiv (Cornell University)|Jul 18, 2023

Graphene research and applications57 references4 citations

TL;DR

This study introduces an extended polarized Raman spectroscopy model to quantitatively assess the alignment and quality of 9-atom-wide armchair graphene nanoribbons (9-AGNRs) on various substrates. By fitting the Raman intensity anisotropy to a Gaussian distribution of ribbon orientations and accounting for polarization-independent background signals, the method reveals that low-coverage 9-AGNRs on Au(788) exhibit superior uniaxial alignment due to step-edge growth, which degrades upon transfer, while high-coverage samples maintain alignment and reduced disorder after transfer.

ABSTRACT

Graphene nanoribbons (GNRs) are atomically precise stripes of graphene with tunable electronic properties, making them promising for room-temperature switching applications like field-effect transistors (FETs). However, challenges persist in GNR processing and characterization, particularly regarding GNR alignment during device integration. In this study, we quantitatively assess the alignment and quality of 9-atom-wide armchair graphene nanoribbons (9-AGNRs) on different substrates using polarized Raman spectroscopy. Our approach incorporates an extended model that describes GNR alignment through a Gaussian distribution of angles. We not only extract the angular distribution of GNRs but also analyze polarization-independent intensity contributions to the Raman signal, providing insights into surface disorder on the growth substrate and after substrate transfer. Our findings reveal that low-coverage samples grown on Au(788) exhibit superior uniaxial alignment compared to high-coverage samples, attributed to preferential growth along step edges, as confirmed by scanning tunneling microscopy (STM). Upon substrate transfer, the alignment of low-coverage samples deteriorates, accompanied by increased surface disorder. On the other hand, high-coverage samples maintain alignment and exhibit reduced disorder on the target substrate. Our extended model enables a quantitative description of GNR alignment and quality, facilitating the development of GNR-based nanoelectronic devices.

Motivation & Objective

• To develop a quantitative method for assessing the alignment and quality of atomically precise graphene nanoribbons (GNRs) during device integration.
• To address the challenge of characterizing GNR orientation and surface disorder in nanoelectronic applications.
• To link structural alignment and defect density in GNRs to their electronic performance potential.
• To evaluate the impact of substrate morphology and transfer processes on GNR quality and orientation.

Proposed method

• Employing polarized Raman spectroscopy to measure the angular dependence of the G2 mode intensity in 9-AGNRs.
• Applying an extended model that fits the Raman intensity anisotropy using a Gaussian distribution of ribbon orientation angles.
• Decomposing the total Raman signal into polarization-dependent and polarization-independent components to isolate contributions from alignment and surface disorder.
• Using scanning tunneling microscopy (STM) to validate preferential growth along step edges on Au(788) substrates.
• Comparing low-coverage and high-coverage GNR samples on Au(788) and after substrate transfer to assess alignment and disorder changes.
• Quantifying alignment via the width of the Gaussian angular distribution and disorder via the intensity of the polarization-independent background.

Experimental results

Research questions

• RQ1 How can the alignment of atomically precise 9-AGNRs be quantitatively measured using polarized Raman spectroscopy?
• RQ2 What role does substrate morphology, particularly step edges on Au(788), play in determining GNR alignment during growth?
• RQ3 How does the transfer process affect the alignment and surface disorder of GNRs on different substrates?
• RQ4 To what extent do low-coverage versus high-coverage GNR samples differ in structural stability post-transfer?
• RQ5 Can polarization-independent Raman intensity contributions be reliably attributed to surface disorder on the growth and target substrates?

Key findings

• Low-coverage 9-AGNRs grown on Au(788) exhibit superior uniaxial alignment due to preferential growth along step edges, as confirmed by STM.
• The alignment of low-coverage samples deteriorates significantly after substrate transfer, indicating structural degradation during transfer.
• High-coverage 9-AGNRs maintain their alignment and show reduced surface disorder after transfer to the target substrate.
• The polarization-independent Raman intensity component increases after transfer for low-coverage samples, indicating elevated surface disorder.
• The extended model successfully quantifies alignment through the width of the Gaussian angular distribution and isolates disorder contributions from substrate effects.
• The study establishes a quantitative framework linking Raman spectroscopy data to structural quality and orientation in GNRs for nanoelectronic applications.