Fe and Fe-oxide nanoparticles (NPs) have a number of promising potential applications in physical and medical sciences. These include magnetic storage devices, catalysis, sensing, contrast enhancement in magnetic resonance imaging and magnetic hyperthermia [1-3]. Understanding of the metallic Fe NPs formation and the oxidation processes down to atomic scale is paramount for the control of the quality and the optimization of their applications. Although metals oxidation has been studied for decades on bulk and thin film materials [4], there is still a lack of studies on the oxidation of metallic iron at the nanoscale.  A recently modified double aberration corrected JEOL 2200FS (S)TEM [5] has demonstrated the possibility of the analysis of metallic nanoparticles in gas environment at temperature allowing single atom visualisation by HAADF STEM in controlled gas reaction environment [6]. In this study, thin films of Fe were deposited by sputtering ex-situ on C films supported by standard TEM Cu grids. Nanoparticles were produced by annealing in vacuum within the microscope column the pre-sputtered iron thin films (see Figure). Nanoparticle formation and size distribution was monitored in-situ as a function of time and temperature by HAADF STEM imaging. After annealing, nanoparticles were shown to consist of single crystal metallic Fe. The Fe nanoparticles interaction with the Oxygen atmosphere was studied in-situ at 300 °C with an Oxygen partial pressure at the specimen in the range of 2*10^-2 Pa to 2 Pa. The interaction of the nanoparticles with the gas will be discussed in terms of the oxidation mechanism as well as the changes in nanoparticle geometry, composition, size distribution, and crystallinity.

References:

[1] B D Terris and T Thomson, J. Phys. D 38, R199–R222 (2005)

[2] H. Galvis et al. Science 335, 835–838 (2012)

[3] Q A Pankhurst, N T K Thanh, S K Jones, and J Dobson, J. Phys. D 42, 224001 (2009)

[4] N. Cabrera and N. F. Mott, Reports on Progress in Physics, Volume 12, Issue 1, pp. 163-184 (1949)

[5] P L Gai and E D Boyes, Microscopy Research and Technique 72, 153 (2009)

[6] E D Boyes, M R Ward, L Lari, and P L Gai, Annalen der Physik 525, 423 (2013)

Acknowledgement: We thank the EPSRC (UK) for research grants EP/J018058/1 and EP/K03278X/1

Figures:

STEM image showing Fe nanoparticles forming by annealing in vacuum a pre-sputtered Fe thin film.

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