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An old tenet of manufacturing holds that if you can't measure it, you can't make it. This has proven a particular concern in the field of nanotechnology, which requires imaging at resolutions well beyond those attainable with conventional optical systems. Meanwhile, analytical chemists and biomedical researchers continue to seek tools that will help them better understand the fundamental building blocks of matter and life. Research groups in a number of countries have reported advances in ultrahigh resolution imaging. Let's take a look.
SEEING BENEATH THE SURFACE
In a dramatic leap forward, an imaging technique called scanning x-ray diffraction microscopy (SXDM) has achieved spatial resolutions on the order of a few tens of nanometers. Developed by collaborators at the Paul Scherrer Institut (PSI) and the École Polytechnique Fédérale de Lausanne (Lausanne, Switzerland), SXDM is a sort of hybrid of diffraction microscopy and scanning transmission x-ray microscopy (STXM). In diffraction microscopy, images are reconstructed from far-field diffraction patterns, requiring complete isolation of the sample under test. The method also suffers from a defocus ambiguity. STXM generates a transmission map of the sample by scanning it with a focused x-ray beam and capturing the transmitted intensity at each point. It's a useful technique but the resolution is limited by the spatial resolution of the beam.
The Swiss collaboration's SXDM system combines the two approaches according to the principles of a method called ptychography. In ptychography, multiple diffraction patterns are captured by scanning a finite illumination area on the sample. The data winds up over-determined because of overlap between adjacent illumination positions, but combined with knowledge of the illumination source, this actually simplifies processing. SXDM follows the same method, but uses a more powerful reconstruction algorithm that extracts from the data both the image of the sample and the profile of the illumination. The data redundancy removes the defocus ambiguity that can plague diffraction microscopy.
In their first demonstration of the method, the cSAXS beamline team of the Swiss Light Source at the PSI scanned the sample under a coherent x-ray probe beam with a 300-nm spot. To ensure the required data redundancy, the step size was smaller than the probe diameter so that adjacent areas were illuminated twice. To capture the transmitted signal, they used a fast single-photon counting detector called Pilatus, developed at the PSI. The 2D hybrid pixel array detector counts each single incoming photon, and yielding negligible readout noise.
The research team imaged a Fresnel zone plate coated with a layer of gold, capturing a raster scan of 201 × 201 diffraction patterns. The modified image reconstruction technique required three hours to generate an image. Their current best spatial resolution for the technique is 20 nm. Data collection typically requires less than an hour; image reconstruction can range from 30 minimum to many hours, although parallel computing can speed this process. In future, the group hopes to achieve resolutions of better than 10 nm and extend the technique to 3-D imaging. The high penetration power of x-rays allows the non-destructive, non-invasive investigation of various samples; applications include electronics and biological imaging.