IT-15-IN-1716 Chemically selective spectromicroscopy by soft x-ray scanning transmission x-ray microscopy

Soft X-ray scanning transmission X-ray microscopy (STXM) uses natural near edge X-ray absorption spectral contrast for chemical speciation and quantitative chemical & orientation mapping (geometric & magnetic) in 2d & 3d with <20 nm spatial resolution. Recently STXM capabilities have been expanded to include electron detection for surface studies, X-ray fluorescence for enhanced sensitivity, and ptychography. STXM is ideal for wet samples since soft X-rays readily penetrate a few microns of water. I will outline instrumentation, data analysis, and capabilities of soft X-ray STXM. Examples will include:

Biomagnetism. STXM with circularly polarized light [CLS 10ID1 or ALS 11.0.2] measure magnetism by X-ray magnetic circular polarization (XMCD). We use this to study magnetotactic bacteria [1] which biomineralize intra-cellular chains of ~50 nm magnetite single crystals. In most cases all magnetic moments in a chain point in the same sense. Recently we found cases where there is internal reversal -the magnetic field of one part points opposite to other parts of the chain (Fig. 1). The gap region exhibits an Fe L3 spectrum similar to that of magnetite but without XMCD [2]. These are situations where either magnetite bio-mineralization has failed or the chain is in the act of growing. Our studies provide insights into biomineralization. Use of ptychography to measure XMCD with improved spatial resolution (<10 nm) will be described.

Automotive hydrogen fuel cells. Polymer electrolyte membrane fuel cells (PEM-FC) are being developed for near-future mass production automotive applications. The performance, efficiency and lifetime of PEM-FC depend on composition and nanostructure of electrodes. Optimization is critical for the cathode where the rate limiting oxygen reduction reaction takes place. STXM is a powerful tool to study a wide range of issues in PEM-FC optimization including mapping ionomer in cathodes [3,4]. Most studies to date have been carried out on dry, microtomed samples at ambient temperature (25 C, 0 % RH) which are very different from typical operating conditions of PEM-FC (70 C, 80 % RH). The nanostructure change with temperature and hydration. Instrumentation and methods to examine PEM-FC under more realistic conditions are needed. We have developed an environmental cell for in situ STXM measurements under controlled relative humidity (0-100%) and temperature (-30 - 80 C) (Fig. 2). We study water saturation in cathode and membrane [5] and changes on freezing.

1. K.P. Lam, et al. Chem. Geology 270 (2010) 1101; S. Kalirai et al. ibid 300 (2012) 14.
2. S. Kalirai et al. PLOS One 8 (2013) e53368.
3. V. Berejnov et al PCCP 14 (2012) 4835.
4. V. Berejnov, et al. ECS Trans., 50 (2012) 361.
5. V. Berejnov et al., ECS Trans. 41 (2011) 395.