Determination of the Metallic/Semiconducting Ratio in Bulk Single-Wall Carbon Nanotube Samples by Cobalt Porphyrin Probe Electron Paramagnetic Resonance Spectroscopy Sofie Cambre ´, †, * Wim Wenseleers, † Etienne Goovaerts, † and Daniel E. Resasco ‡ † Department of Physics, University of Antwerp, Universiteitsplein 1, 2610 Antwerp, Belgium, and ‡ School of Chemical, Biological and Materials Engineering, University of Oklahoma, 100 East Boyd Street, Norman, Oklahoma 73019, United States S ingle-wall carbon nanotubes (SWCNTs) have attracted tremendous interest because of their unique elec- tronic, optical, thermal, and mechanical properties which are particularly promising for a wide range of applications in (nano)opto-electronics. However, a major obstacle for these applications is the fact that the electronic properties of SWCNTs depend critically on their exact chiral struc- ture (chiral index (n,m)), 1 and all synthesis methods known to date produce a mixture of both metallic (M) and semiconducting (SC) SWCNTs. For a few years now, signifi- cant progress is being made in the prepara- tion of SWCNT samples enriched in either semiconducting or metallic tubes, either at the synthesis level 2-6 or by various post- growth separation methods. 7-17 One of the most promising separation methods is den- sity gradient centrifugation 7 of bile salt sol- ubilized 18 SWCNTs, which not only allows the separation of semiconducting and me- tallic tubes but can also sort them according to chirality and even handedness. 19 How- ever, characterizing the actual content of semiconducting and metallic SWCNTs in a bulk sample remains difficult, because no simple spectroscopic technique giving a re- liable, absolute reading of the M:SC ratio was available until now. Here, we show that the electron paramagnetic resonance (EPR) spectrum of a bulk sample of SWCNTs, to which Co(II)octaethylporphyrin (CoOEP) probe molecules have been added, directly yields such a measurement of the M:SC ratio in the original SWCNT sample, without re- quiring an external calibration, yielding a simple, quantitative spectroscopic tech- nique for the determination of M:SC ratios in bulk SWCNT samples. Several spectroscopic techniques have been proposed in literature for characteriz- ing the M:SC ratio of SWCNT samples. A 2D Raman map, over a sufficiently wide range of laser wavelengths to cover resonances of all SWCNT chiralities in a given sample can in principle be used to determine the com- plete chirality distribution and hence also the M:SC ratio. 20 However, this is extremely tedious and not feasible for arbitrary diam- eter ranges (limited by the available lasers and detection wavelength range). More- over, resonant Raman scattering data are difficult to quantify because these depend *Address correspondence to sofie.cambre@ua.ac.be. Received for review August 31, 2010 and accepted October 12, 2010. Published online October 19, 2010. 10.1021/nn102222w © 2010 American Chemical Society ABSTRACT A simple and quantitative, self-calibrating spectroscopic technique for the determination of the ratio of metallic to semiconducting single-wall carbon nanotubes (SWCNTs) in a bulk sample is presented. The technique is based on the measurement of the electron paramagnetic resonance (EPR) spectrum of the SWCNT sample to which cobalt(II)octaethylporphyrin (CoOEP) probe molecules have been added. This yields signals from both CoOEP molecules on metallic and on semiconducting tubes, which are easily distinguished and accurately characterized in this work. By applying this technique to a variety of SWCNT samples produced by different synthesis methods, it is shown that these signals for metallic and semiconducting tubes are independent of other factors such as tube length, defect density, and diameter, allowing the intensities of both signals for arbitrary samples to be retrieved by a straightforward least-squares regression. The technique is self-calibrating in that the EPR intensity can be directly related to the number of spins (number of CoOEP probe molecules), and as the adsorption of the CoOEP molecules is itself found to be unbiased toward metallic or semiconducting tubes, the measured intensities can be directly related to the mass percentage of metallic and semiconducting tubes in the bulk SWCNT sample. With the use of this method it was found that for some samples the metallic/semiconducting ratios strongly differed from the usual 1:2 ratio. KEYWORDS: electronic type · spin probe EPR · CoMoCat · SWCNTs · electron spin resonance · metallic:semiconducting ratio · EPR spectroscopy ARTICLE www.acsnano.org VOL. 4 ▪ NO. 11 ▪ 6717–6724 ▪ 2010 6717