Mid-infrared intracavity absorption diagnostics in nanoparticle synthesis flames

• Applicant: Dr. Peter Fjodorow
• Subject Area: Energy Process Engineering
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 528312492

Predictive manufacturing of nanoparticles with tailored properties through gas-phase synthesis requires in-depth understanding of the underlying chemical processes. To this end, quantitative measurements of the concentrations of key intermediate species are indispensable. The most valuable information for supporting a mechanistic understanding is gained from measuring the intermediates that form in the short period between decomposition of the gas-phase precursors and particle growth. However, such species are typically present at low concentrations only and often have unknown spectral characteristics. Conventional absorption-based techniques fail in measuring in such particle laden flows, because the light attenuation is governed through strong broad-band absorption and scattering by particles, preventing the measurement of the target species’ absorption. The innovative solution proposed within this project is the combination of Intracavity Absorption Spectroscopy (ICAS) and novel mid-infrared lasers based on Cr-doped CdSe and Fe-doped ZnSe crystals, together covering the 2.2–5.3 µm spectral range, where relevant species have strong transitions and, therefore, enable measurements with high sensitivity. In the ICAS technique, the absorber is located inside the cavity of a broadband laser: This leads to significant sensitivity enhancement due to record-breaking effective absorption path lengths and makes the measurement independent of broadband losses as they are compensated by the laser gain medium. Generally, absorption techniques can provide quantitative information only if the absorption cross-sections of the species under study are known. However, for the species of interest, spectroscopic data are often unavailable. This proposed project aims at enabling quantitative ICAS measurements of intermediates formed during synthesis of iron-containing nanoparticles in a so far unexplored wavelength range. In particular, two main challenges will be addressed: (1) low concentration of intermediate species, further complicated by broadband losses originating from, e.g., light scattering and absorption by particles, and (2) scarce information concerning the relevant spectral signatures. The approach exploits the recent development of novel broadband and widely tunable mid-infrared lasers and their application as light sources for highly sensitive ICAS measurements. While these mid-infrared laser sources will enable addressing the few known or predicted spectral positions of the target species in the mid-infrared, the utilization of ICAS will facilitate highly sensitive spectroscopic exploration of the corresponding spectral regions and thus pave the way for quantitative measurements of intermediate gas-phase species, such as FeO2, FeCO, Fe(CO)2, Fe(CO)3, Fe2CO, FeOH, Fe(OH)2, Fe(OH)3, and FeOOH, that so far cannot be observed in situ.

• DFG Programme: Research Grants