Surface Enhanced Raman Spectroscopy (SERS) of ultrafine particles produced in premixed flames

Ultrafine particles produced in combustion are mainly composed by two kind of structures: soot, and organic carbon particles. Soot is currently described as particles larger than 10nm which aggregate forming chain, they are composed by carbon and hydrogen atoms in a ratio of H/C=0.3-0.5. Carbon atoms form small crystallite consisting of three to four aromatic layers with the interplane distance generally larger than that of graphite crystals, which are linked by van der Waals forces or aliphatic domains, randomly oriented in a turbostratic structure. Organic particles and incipient soot particles are smaller than 10 nm. Such particles appear as translucent particles to TEM microscopi with extended regions of amorphous carbon where no crystallite presence has been evidenced. The analysis by IR spectroscopy reveals a molecular-like character for soot precursors whose spectrum is characterized by sharp and narrow bands mainly due to aromatic and aliphatic bond vibrations [1] and recently, Raman spectroscopy of these particles sampled in water showed the presence of the typical G and D bands typical of carbonaceous compounds [2]. The objective of this paper is to study the chemical composition of precursors and incipient soot particles produced in flames across the soot threshold. Raman Spectroscopy is a widely used technique to study carbonaceous compounds from graphite to amorphous carbon, being a complementary technique to infrared spectroscopy to study vibrational transitions and low range order in the structure. While infrared spectroscopy is more sensitive to the aliphatic vibrations, Raman spectra, excited by visible light, is only sensitive to sp2 sites, since they have a much larger cross section than sp3 sites. The most used part of the Raman spectra of soot and amorphous carbon is the region between 1000-1800 cm-1, where two main peaks are observed: the first at about 1850 cm-1 (G peak, for graphite), and the second peak at 1350 cm-1 ( D peak, for disorder). In Raman spectra of carbonaceous material an band at about 2700 cm-1 is also observed, denoted as 2D being the second order of zone-boundary phonons which gives the first order D peak in defected graphite. Its intensity is much lower than D and G bands and it has been rarely used for the study of amorphous carbon. Various methods have been developed to increase the intensity of Raman spectra, like Resonant Raman Spectroscopy, CARS or Surface Enhanced Raman Spectroscopy (SERS). In particular, SERS is a very powerful method able to provide intensity enhancements up to 14 order of magnitude, allowing the detection of a single molecule with the high structural selectivity provided by its Raman spectrum. It rely on the strongly increased Raman signal from molecules close to metallic nanostructures. The enhancement mechanisms are usually divided into a properly electromagnetic field enhancement and chemical effects. The strong enhancement of Raman signal operated by SERS has been exploited in this paper to study 2D Raman bands in soot and soot precursors sampled from premixed ethylene/air flames in addition to first order Raman bands. It also allowed to measure Raman spectra of size selected particles collected by means of a dilution probe-DMA system deposited in very low concentration on SERS substrates.