[Paper Review] Length distribution of single walled carbon nanotubes determined by ac atomic force microscopy

Richard D. Piner, Rodney S. Ruoff|arXiv (Cornell University)|Jun 7, 2002

Carbon Nanotubes in Composites3 citations

TL;DR

This study presents a method using ac atomic force microscopy (AFM) with optimized tuning to measure individual single-walled carbon nanotubes (SWCNTs) dispersed in dimethylformamide (DMF), enabling accurate length distribution analysis. The key result is a length distribution peaking at 300 nm with a mean of 480 nm, achieved through repeated dilution, sonication, and precise AFM parameter control to distinguish single tubes from small bundles.

ABSTRACT

A simple method to disperse individual single walled carbon nanotubes (SWCNT ) on an atomically flat substrate is presented. Proper tuning of ac modes of atomic force microscopes(AFM) is discussed. This is needed to discriminate between individual nanotubes and very small bundles. The distribution of lengths of the nanotubes measured by these methods is reported.

Motivation & Objective

• To develop a reliable method for measuring individual SWCNT lengths despite their tendency to bundle and be easily displaced.
• To overcome challenges in AFM imaging of SWCNTs due to low adhesion and surface smoothness on substrates like mica.
• To establish a reproducible dispersion protocol that yields isolated, stable SWCNTs on atomically flat substrates.
• To optimize ac-AFM tuning parameters to distinguish single tubes from small bundles based on height and contrast.
• To determine the statistical length distribution of SWCNTs in a laser-ablation-derived sample.

Proposed method

• Used repeated dilution (1:1,000,000) and 16-hour sonication in DMF to disperse SWCNTs from a toluene suspension.
• Prepared samples by sandwiching the DMF/tube suspension between two cleaved mica sheets to control drying and uniformity.
• Employed intermittent-contact ac-AFM with unmounted non-contact ultralevers on two Thermomicroscopes AFM systems.
• Optimized AFM tuning by adjusting cantilever chip position to achieve a single resonance peak in amplitude spectrum, using 200 kHz resonant frequency and 500 Hz below resonance.
• Applied feedback gain of 0.5 or higher and scan speeds of 1 Hz per 4 µm image for stable imaging.
• Used reverse-contrast imaging and height measurements to distinguish single tubes (0.8 nm apparent height) from bundles (1.5–1.6 nm), enabling selective length measurement.

Experimental results

Research questions

• RQ1 What dispersion method enables stable, isolated SWCNTs on mica for reliable AFM imaging?
• RQ2 How can ac-AFM be tuned to distinguish individual SWCNTs from small bundles without sample damage?
• RQ3 What is the actual length distribution of SWCNTs in a laser-ablation-derived sample, given their tendency to bundle?
• RQ4 How does environmental humidity affect ac-AFM imaging stability and contrast of SWCNTs?
• RQ5 Can precise AFM tuning and image processing reliably extract individual tube lengths from complex AFM images?

Key findings

• The length distribution of SWCNTs in the sample peaks at 300 nm, with a mean length of 480 nm, based on measurements of 500 individual tubes.
• Proper tuning of ac-AFM, including achieving a single resonance peak via chip repositioning, enabled stable imaging without tube displacement.
• Apparent tube height was measured at 0.8 nm, significantly below the expected 1.2–1.4 nm, likely due to water adsorption on hydrophilic mica in air.
• The method successfully distinguished single tubes from small bundles using contrast and height analysis, with bundles appearing darker and wider in reverse-contrast images.
• The dispersion protocol using DMF dilution and prolonged sonication produced a stable, non-precipitating suspension for over 3 months.
• Imaging stability was maintained between 20–40% relative humidity; below 20%, feedback oscillations and contrast reversal occurred.