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Characterization of nanoparticles in general, and carbon nanotubes in particular, remains a technical challenge even though the chemistry of covalent functionalization has been studied for more than a decade. It has been noted by several researchers that the characterization of products represents a constant problem in nanotube chemistry. A systematic tool or suites of tools are needed for adequate characterization of chemically functionalized single-walled carbon nanotubes (SWNTs), and is necessary for declaration of success or failure in functionalization trials.
So far, a wide range of techniques have been applied to characterize functionalized SWNTs: infra red (IR), Raman, and UV/visible spectroscopies, thermogravimetric analysis (TGA), atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), etc. A summary of the attribute of each of the characterization method is given in [link] .
| Method | Sample | Information | Limitations |
| TGA | solid | functionalization ratio | no evidence for covalent functionalization, not specific |
| XPS | solid | elements, functionalization ratio | no evidence of covalent functionalization, not specific, quantification complicated |
| Raman | solid | sp 3 indicated by D mode | not specific, quantification not reliable |
| Infra red (IR) | solid for (ATR-IR) or solution | substituent groups | no direct evidence for covalent functionalization, quantification not possible |
| UV/visible | solution | sidewall functionalization | not specific or quantitative, need highly dispersed sample |
| Solution NMR | solution | substituents | no evidence of covalent functionalization, high solubility of sample |
| Solid state NMR | solid | substituents, sp 3 molecular motions, quantification at high level of funcitionalization | high functionalization needed, long time for signal acquisition, quantification not available for samples with protons on side chains |
| AFM | solid on substrate | topography | only a small portion of sample characterized, no evidence of covalent functionalization, no chemical identity |
| TEM | solid on substrate | image of sample distribution dispersion | only a small portion of sample characterized, no evidence of covalent functionalization, no chemical identity dispersion information complicated |
| STM | solid on substrate | distribution | no chemical identity of functional groups small portion of sample conductive sample only |
Thermogravimetric analysis (TGA) is the mostly widely used method to determine the level of sidewall functionalization. Since most functional groups are labile or decompose upon heating, while the SWNTs are stable up to 1200 °C under Ar atmosphere. The weight loss at 800 °C under Ar is often used to determine functionalization ratio using this indirect method. Unfortunately, quantification can be complicated with presence of multiple functional groups. Also, TGA does not provide direct evidence for covalent functionalization since it cannot differentiate between covalent attachment and physical adsorption.
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