Imaging Nanocarbon Materials: Soot Particles in Flames are Not Structurally Homogeneous

Abstract

For the first time, nascent soot particles are probed by using helium-ion microscopy (HIM). HIM is a technique that is similar to scanning electron microscopy (SEM) but it can achieve higher contrast and improved surface sensitivity, especially for carbonaceous materials. The HIM microscope yields images with a high contrast, which allows for the unambiguous recognition of smaller nascent soot particles than those observed in previous transmission electron microscopy studies. The results indicate that HIM is ideal for rapid and reliable probing of the morphology of nascent soot, with surface details visible down to approximately 5 nm, and particles as small as 2 nm are detectable. The results also show that nascent soot is structurally and chemically inhomogeneous, and even the smallest particles can have shapes that deviate from a perfect sphere.

Investigation of the size of the incandescent incipient soot particles in premixed sooting and nucleation flames of n-butane using LII, HIM, and 1 nm-SMPS

“…Accurate measurements of particles smaller than 10 nm remain a challenge Zhao et al 2003;Abid et al 2008;D'Anna 2009;Schenk et al 2013). Available experimental techniques for measuring soot particle size and concentration have been discussed by Wang (2011), Desgroux et al (2013), Michelsen (2016), andD'Anna (2009).…”

“…First, these particles may be liquid-like at the very early stage of inception and could tend to splash upon deposition on the TEM grids Abid et al 2008). Second, the TEM images of these nascent particles have poor contrast and the particle structure is prone to be damaged by the electron beam Schenk et al 2013). Due to these limitations, the smallest soot particles measured by the TEM approach are typically about 10 nm Bladh et al 2011).…”

“…Recently, incipient particles with structures as small as 2 nm could be detected by helium-ion microscopy (HIM), a technique which involves much lower beamsample interaction than TEM (Schenk et al 2013). Therefore, HIM seems a reliable alternative to TEM to image the size and morphology of incipient soot particles (Schenk et al 2013).…”

“…Therefore, HIM seems a reliable alternative to TEM to image the size and morphology of incipient soot particles (Schenk et al 2013). Direct comparisons of PSDFs of incipient soot obtained by HIM and SMPS in ethylene-oxygen-argon premixed flames showed that there is generally good agreement in the results of the two techniques at higher distances from the burner exit, where soot particles become more mature and larger (Schenk et al 2013(Schenk et al , 2015. However, large discrepancies are typically observed for newly formed incipient soot particles at low distances from the burner exit.…”

“…However, large discrepancies are typically observed for newly formed incipient soot particles at low distances from the burner exit. The potential causes for the discrepancies were attributed to the sample spread on the HIM substrate and lower sampling efficiency for very small particles of few nanometers (Schenk et al 2013(Schenk et al , 2015. Very recently, Tang et al (2016) have demonstrated that incipient soot particles as small as about 1.5 nm can be measured with a diethylene glycol (DEG) SMPS.…”