Raman microspectroscopy of soot and related carbonaceous materials: Spectral analysis and structural information

Experimental conditions and mathematical fitting procedures for the collection and analysis of Raman spectra of soot and related carbonaceous materials have been investigated and optimised with a Raman microscope system operated at three different laser excitation wavelengths (514, 633, and 780 nm). Several band combinations for spectral analysis have been tested, and a combination of four Lorentzian-shaped bands (G, D1, D2, D4) at about 1580, 1350, 1620, and 1200 cm⁻¹, respectively, with a Gaussian-shaped band (D3) at ∼1500 cm⁻¹ was best suited for the first-order spectra. The second-order spectra were best fitted with Lorentzian-shaped bands at about 2450, 2700, 2900, and 3100 cm⁻¹. Spectral parameters (band positions, full widths at half maximum, and intensity ratios) are reported for several types of industrial carbon black (Degussa Printex, Cabot Monarch), diesel soot (particulate matter from modern heavy duty vehicle and passenger car engine exhaust, NIST SRM1650), spark-discharge soot (Palas GfG100), and graphite. Several parameters, in particular the width of the D1 band at ∼1350 cm⁻¹, provide structural information and allow to discriminate the sample materials, but the characterisation and distinction of different types of soot is limited by the experimental reproducibility of the spectra and the statistical uncertainties of curve fitting. The results are discussed and compared with X-ray diffraction measurements and earlier Raman spectroscopic studies of comparable materials, where different measurement and fitting procedures had been applied.