[Paper Review] Double resonance Raman study of disorder in CVD-grown single-walled carbon nanotubes

Rahul Rao, Jason Reppert+5 more|arXiv (Cornell University)|Oct 22, 2010

Carbon Nanotubes in Composites30 references3 citations

TL;DR

This study investigates disorder in chemical vapor deposition (CVD)-grown single-walled carbon nanotubes (SWNTs) using multiple-excitation Raman spectroscopy. By varying sulfur doping in the iron catalyst, the researchers induced controlled lattice disorder and observed that the D band intensity increased with decreasing sulfur content; post-synthesis annealing reduced defect-related signals, confirming defect healing through quenching of the double resonance Raman process.

ABSTRACT

Single-walled carbon nanotubes (SWNTs) with varying degrees of disorder were investigated using multiple-excitation Raman spectroscopy. The lattice disorder was imparted into the nanotubes by the addition of varying amounts of sulfur to the iron catalyst in a thermal chemical vapor deposition process. Changes in the intensities of peaks occurring due to a double resonance Raman process were studied. The intensity of the disorder-induced D band increased with a decrease in the sulfur content. Upon post-synthesis heat treatment, the double resonance process got quenched due to defect healing. The second order G' band and iTOLA bands exhibited a two-peak structure, of which one of the peaks is relatively more sensitive to defects and decreased in intensity with heat treatment.

Motivation & Objective

• To investigate the influence of controlled lattice disorder on the Raman response of CVD-grown single-walled carbon nanotubes.
• To understand how sulfur doping in the iron catalyst affects defect formation in SWNTs during synthesis.
• To examine the evolution of double resonance Raman features, particularly the D band and second-order G' band, under varying defect concentrations.
• To evaluate the effectiveness of post-synthesis heat treatment in healing lattice defects and restoring structural order.

Proposed method

• Multiple-excitation Raman spectroscopy was employed to probe double resonance Raman processes in SWNTs with varying degrees of disorder.
• Sulfur was introduced into the iron catalyst during thermal CVD growth to systematically tune the level of lattice defects.
• Raman spectra were acquired across different excitation energies to analyze the intensity and shape of the D band and second-order G' band.
• Post-synthesis annealing was applied to assess defect healing, with changes in Raman features monitored over time.
• The intensity variations of the D band and G' band components were correlated with defect concentration and structural order.
• The iTOLA (intermediate transition of lattice anharmonicity) bands were analyzed for their sensitivity to structural defects.

Experimental results

Research questions

• RQ1 How does sulfur doping in the iron catalyst influence the degree of lattice disorder in CVD-grown SWNTs?
• RQ2 What is the relationship between the intensity of the disorder-induced D band and the sulfur content in the catalyst?
• RQ3 How do the second-order G' band and iTOLA bands respond to increasing defect concentration?
• RQ4 To what extent can post-synthesis heat treatment reverse defect-related Raman signals in SWNTs?
• RQ5 Which Raman features are most sensitive to structural healing following thermal annealing?

Key findings

• The intensity of the disorder-induced D band increased as sulfur content in the catalyst decreased, indicating higher defect concentration in low-sulfur conditions.
• Post-synthesis heat treatment significantly reduced the D band intensity, confirming the healing of lattice defects through structural reorganization.
• The second-order G' band exhibited a two-peak structure, with one component showing greater sensitivity to defects and decreasing in intensity after annealing.
• The iTOLA bands also displayed defect-dependent intensity changes, supporting their role as probes of lattice anharmonicity and disorder.
• The double resonance Raman process was effectively quenched after annealing, indicating restoration of structural order in the nanotubes.
• The results demonstrate that sulfur doping provides a tunable method to control defect levels in CVD-grown SWNTs, with Raman spectroscopy serving as a sensitive probe of defect dynamics.