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Soft X-ray spectromicroscopy using ptychography with randomly phased illumination

Ptychography is rapidly developing into an important imaging tool in X-ray microscopy. The technique involves successively illuminating overlapping regions of a specimen with a coherent probe and recording the resulting diffraction patterns. It is important that the illuminated areas overlap significantly since those common regions provide duplicate information that allows computer algorithms to reconstruct reliably both the sample transmission function and the illuminating probefrom the measured diffraction patterns. A particularly useful feature of these algorithms is that they can recover the amplitude and phase of both these signals.
In the reported experiment the illuminated area on the sample was defined by a 5 µm pinhole located about 1.25 mm upstream of the sample, and the sample was scanned in small rectangular rasters perpendicular to the beam direction, with raster steps ~1 µm to ensure significant overlap between adjacent illuminated areas. As in all coherent diffractive imaging techniques acommon experimental difficulty is the large dynamic range of the diffraction data, where the zero-order component is orders of magnitude more intense than the scattered signal. One way to deal with this problem is to use a diffusing mask to spread the illumination across the detector. In the experiments carried out on the TwinMic beamline, a random array of 40 nm diameter pinholes etched into a thin tungsten film was placed about 1.5 mm upstream of the beam-defining pinhole to act as a diffuser, reducing the dynamic range of the recorded diffraction patterns by about an order of magnitude.

Figure 1:    (a) Plot of the loge(modulus) and phase image signals measured from the CoFe2O4 nanoparticles. The locations of the nanoparticle clusters are highlighted by coloured dots in the inset image (phase image at 711.8 eV). The curves show that five different classes of behaviour as a function of X-ray energy were identified. For comparison, a scaled plot of the TEY signal from similar particles is overlaid on the loge(modulus) plots. The solid and dashed black lines show the signals calculated for a 90-nm thick film of CoFe2O4 derived from the semi-empirical tabulation of atomic scattering factors by Chantler et al., (http://www.nist.gov/pml/data/ffast/). (b) Reconstructed modulus and phase images of a Balb/3T3 mouse fibroblast. The data were collected at X-ray beam energies across the iron L edge, showing the variation in contrast of the CoFe2O4 nanoparticles as a function of energy. Scale bar, 5 µm.


This ptychographic approach was used to image Balb/3T3 mouse fibroblast cells that had been exposed to cobalt ferrite (CoFe2O4) nanoparticles. The ePIE (extended Ptychographic Iterative Engine) algorithm developed at Sheffield University, UK, was used to generate a number of ptychographic reconstructions at a range of X-ray energies around the iron L absorption edge. The modulus (amplitude) and phase contrast from clusters of the nanoparticles varied quite strongly as the incident X-ray energy was changed. The two signals can be related directly to the optical thickness (n-1)t where t is the thickness of the cluster and n=1-δ-iβ the complex refractive index of the material. In practice, five different classes of behavior across the iron L3 edge were identified, as shown by the colour coding of the graph in Figure 1, indicating that the iron present in the nanoparticle clusters was in a number of different chemical states.
The observed variations in modulus contrast across the iron L3 edge are consistent with estimates based on total electron yield (TEY) measurements made by Dr V.S. Coker of Manchester University, UK. However, the use of ptychography to recover the complex sample transmittance has allowed the first direct measurements from cobalt ferrite nanoparticles of the phase variations across the iron L3 edge, with the phase variations showing stronger and clearer features than the modulus data provided.
The ptychographic approach to imaging offers some important advantages for high-resolution X-ray spectromicroscopy.It provides modulus and phase reconstructions with a spatial resolution which is comparable to that achieved when the TwinMic microscope is operated as a conventional STXM, while the use of a relatively large probe on the sample allows a whole fibroblast cell to be covered in a raster of modest size, with no refocusing required when the incident X-ray energy is changed.
In addition, the phase signal provides new information that complements near-edge X-ray absorption fine structure (NEXAFS) measurements, a technique that has become a vital tool for the study and chemical characterization of nanoscale materials. At present, there are few detailed experimental and theoretical data on the fine structure of phase variations near to absorption edges, and we believe that the ability of X-ray ptychography to provide direct, quantitative phase information, in perfect registration with conventional NEXAFS data, will open up a rich new branch of X-ray spectromicroscopy.

This research was conducted by the following research team:

  • Andrew M. Maiden, John M. Rodenburg, University of Sheffield, Sheffield, UK
  • Graeme R. Morrison, University College London, London Centre for Nanotechnology, UK
  • Burkhard Kaulich, Diamond Light Source, Didcot, UK
  • Alessandra Gianoncelli, Elettra-Sincrotrone Trieste S.C.p.A., Trieste, Italy

Contact person:
Alessandra Gianoncelli, email: alessandra.gianoncelli@elettra.eu

Reference

A. M. Maiden, G. R. Morrison, B. Kaulich, A. Gianoncelli, J. M. Rodenburg, Soft X-ray spectromicroscopy using ptychography with randomly phased illumination”, Nature Communications 4, 1669, (2013), DOI: 10.1038/ncomms2640.

Last Updated on Tuesday, 02 July 2013 11:21