[Paper Review] Transport Properties of Carbon Nanotube C$_{60}$ Peapods

C. H. L. Quay, John Cumings+5 more|arXiv (Cornell University)|Jun 12, 2006

Graphene research and applications1 references3 citations

TL;DR

This study investigates the low-temperature transport properties of carbon nanotube C60 peapods, finding that encapsulated C60 molecules do not induce significant electron backscattering near the Fermi level. Despite strong electronic structure modifications at higher energies, peapods exhibit metallic and semiconducting behavior indistinguishable from empty nanotubes, with Coulomb blockade and Kondo effects observed at 250 mK, indicating preserved one-dimensional transport coherence.

ABSTRACT

We measure the conductance of carbon nanotube peapods from room temperature down to 250mK. Our devices show both metallic and semiconducting behavior at room temperature. At the lowest temperatures, we observe single electron effects. Our results suggest that the encapsulated C$_{60}$ molecules do not introduce substantial backscattering for electrons near the Fermi level. This is remarkable given that previous tunneling spectroscopy measurements show that encapsulated C$_{60}$ strongly modifies the electronic structure of a nanotube away from the Fermi level.

Motivation & Objective

• To measure the conductance of C60 peapods from room temperature down to 250 mK to assess low-energy electronic transport.
• To determine whether encapsulated C60 molecules introduce substantial backscattering in carbon nanotubes, which would disrupt one-dimensional transport.
• To perform a statistically rigorous analysis of filling fractions using TEM data to infer the proportion of filled nanotubes in transport devices.
• To compare transport behavior of peapods with that of empty nanotubes and assess the impact of C60 on Coulomb blockade and Kondo effects.
• To reconcile apparent contradictions between tunneling spectroscopy (showing strong high-energy modifications) and transport measurements (showing minimal low-energy effects).

Proposed method

• Fabricated bottom-gated field-effect transistors using C60 peapods synthesized via sublimation and dispersed in chloroform or ortho-dichlorobenzene.
• Employed electron-beam lithography to define Pd source and drain electrodes spaced 250–500 nm apart on a thermal SiO2/Si substrate.
• Used atomic force microscopy (AFM) and transmission electron microscopy (TEM) to identify and locate individual nanotubes and determine filling status.
• Applied Bayes’s Theorem with a uniform prior to estimate the posterior probability distribution of the filling fraction in the device sample, using 92 filled out of 109 imaged nanotubes.
• Calculated the expected number of filled nanotubes in the 7 conductive devices as 5.86 using Bayesian inference to avoid overestimating filling based on sample statistics.
• Measured conductance as a function of gate voltage and temperature (250 mK to 300 K), identifying Coulomb blockade and Kondo effects in low-temperature regimes.

Experimental results

Research questions

• RQ1 Do C60 molecules encapsulated in carbon nanotubes significantly alter electron transport near the Fermi level?
• RQ2 How does the filling fraction of C60 in nanotubes affect the observed conductance and transport behavior in nanotube devices?
• RQ3 To what extent do the transport properties of peapods differ from those of empty nanotubes at low temperatures?
• RQ4 Can the absence of backscattering in peapods be explained by the long-wavelength nature of the perturbation introduced by C60 molecules?
• RQ5 How do the observed transport phenomena (Coulomb blockade, Kondo effect) in peapods compare to those in empty nanotubes, and what does this imply about electron localization and coherence?

Key findings

• The conductance of C60 peapods at room temperature shows metallic or semiconducting behavior indistinguishable from that of empty nanotubes.
• At 250 mK, the devices exhibit Coulomb blockade, indicating the formation of quantum dots with localized charge, consistent with electron transport through a 250–500 nm nanotube segment.
• Both spin-1/2 and spin-1 Kondo effects are observed, indicating strong electron-electron interactions and Kondo screening in the low-temperature regime.
• The statistical analysis using Bayes’s Theorem estimates that 5.86 out of 7 conductive devices are expected to be filled with C60, based on a TEM sample of 92 filled out of 109 nanotubes.
• Despite strong electronic structure modifications at high energies (as seen in tunneling spectroscopy), the absence of backscattering near the Fermi level suggests that C60 perturbations are effectively long-wavelength and do not localize electrons.
• The results imply that C60 peapods preserve the one-dimensional character of electron transport, with minimal scattering, even though the C60 molecules are present in the nanotube core.