Surface enhanced Raman spectroscopy (SERS) of particles produced in premixed flame across soot threshold

Surface enhanced Raman scattering (SERS) has been employed for the first time to characterize particles produced in flames across the soot inception.The strong enhancement of Raman signal produced by SERS enabled the measurement of the Raman spectra of size-selected particles collected by a dilution probe-DMA system and deposited by electrophoresis in very low concentrations on SERS substrates. It also enables investigation of second order 2D Raman bands in soot and soot precursors, providing complementary information to the first order Raman bands.Raman spectra, together with the measurement of particle size distribution by DMA, help to characterize particles produced across the soot threshold limit. Three particle modes have been identified: flames with C/O < 0.65 produce only modeI particles with a diameter of d ≈ 2 nm, which present the typical bands of amorphous carbon and have a very low, if any, three-dimensional ordering of graphitic planes. Other aromatic macromolecules smaller than the lower detection limit of the DMA (d < 1.5 nm) are present in the flame, and are collected by thermophoretic sampling. In flames with higher C/O ratio the size distribution becomes bimodal with the formation of modeII particles, d ≈ 4 nm. In non-sooting flames, the SERS spectra of modeI and modeII particles are similar except for features indicating the presence of a higher degree of disorder in modeII, likely due to distorted or five order rings.In soot-forming flames, larger modeIII particles collected with a diameter of 40 nm present a higher ordering and a reduction of graphitic interplane distance, which, nevertheless, still remains larger than 3.50 Ǻ. Two types of particles were observed in smaller modeII particles collected from sooting flame conditions: one with Raman spectra similar to the modeIII particles and the other with spectra similar to particles collected from leaner non-sooting flames.