Measurement of the absolute absorption cross-section in individual carbon nanotubes

J.-C. Blancon1, D. Christofilos2, A. Ayari1, A. San Miguel1, N. Del Fatti1 and F. Vallée1

1LPMCN & LASIM, Université Lyon 1, Domaine Scientifique de la Doua, 69622 Villeurbanne, France2Physics Division, School of Technology, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece

Carbon nanotubes (CNTs) exhibit unique optical, mechanical and electrical properties depending on their characteristics, i.e., their nature (semiconducting or metallic) and their diameter. There are promising candidates for selective applications, e.g., in optoelectronic and in biology. In this context, the understanding of light-matter interaction processes occurring in these nano-objects is crucial, and more precisely this comprehension should occur at the individual nanotube level. A large number of studies based on resonant Raman spectroscopy and luminescence spectroscopy were performed on individualised CNTs, whereas characterisation of the absorption properties of CNTs is still challenging. Here we show direct measurements of the polarised absorption cross-section of individual CNTs obtained with the spatial modulation spectroscopy technique (SMS) [1]. This method was coupled with other existing techniques: scanning electron microscopy, atomic force microscopy, and Raman spectroscopy.

This study demonstrates the impressive capability of the SMS technique to measure the absolute absorption cross-section (σabs) of CNTs deposited either on transparent or opaque substrates (Fig. 1). For a (18,5) semiconducting CNT, we estimated its resonant absorption cross-section to be σabs = 0.35 nm²/nm (corresponding to about 1.8 x 10-17 cm²/carbon atom), for incident light polarised along the nanotube axis [2]. Polarisation anisotropy of the absorption, measured for few different CVD grown CNTs, always give a contrast factor close to 2.5 : 1 between light polarised parallel and orthogonal to the nanotubes axes (Fig. 2). Absorption spectra in the visible and infra-red ranges were obtained for CNTs deposited on quartz substrates. Thus, we were able to relate the evolution of the absolute absorption cross-section with the CNTs' nature and diameter.

fig1-JMC2012
Fig. 1: Scanning modulation spectroscopy (SMS) image (b) of an individual carbon nanotube deposited on a Si/SiO2 substrate (ΔR/R is the relative reflectivity signal). The same CNT was identified with scanning electron microscopy (a) and atomic force microscopy (c).

fig2-JMC2012
Fig. 2: Light polarisation dependent SMS signal of a (18,5) semiconducting CNT close to its E33 resonance.

References
[1] A. Arbouet, D. Christofilos, N. Del Fatti et al., Phys. Rev. Lett. 2004, 93, 127401.
[2] D. Christofilos, J.-C. Blancon, J. Arvanitidis et al., J. Phys. Chem. Lett. 2012, 3 (9), 1176.