In a recent paper published in Nature on March 6, 2012 under the daunting title of “Ptychographic electron microscopy using high-angle dark-field scattering for sub-nanometre resolution imaging,” University of Sheffield scientists M.J. Humphry, B. Kraus, A.C. Hurst, A.M. Maiden and principal investigator John M. Rodenburg outlined their achievements in overcoming some of the limitations that have held back the potential of the electron microscope
Researchers at the University of Sheffield have created what sounds impossible – even nonsensical: an experimental electron microscope without lenses that not only works, but is orders of magnitude more powerful than current models. By means of a new form of mathematical analysis, scientists can take the meaningless patterns of dots and circles created by the lens-less microscope and create images that are of high resolution and contrast and, potentially, up to 100 times greater magnification.
The full field-of-view is shown in the inset image (scale bar, 15 nm); the main image is a blow up of the region indicated by the yellow box, showing 0.236 nm atomic plane fringes (scale bar, 5 nm). The modulus and phase of the reconstructions are combined in these images, with phase represented by colour and modulus by brightness, as indicated on the colour wheel scale.
Schematic of the experimental arrangement. Experiments were carried out on an FEI Quanta 600 SEM fitted with a thermally assisted Schottky field emission gun and operating at 30 keV. The probe wavefront was formed using the microscope condenser and objective lenses and was scanned across the specimen using the microscope scanning coils. The specimen was mounted on a compact rig attached to the objective lens pole piece assembly. The door of the microscope was replaced in order to accommodate a flange for a Gatan Orius SC200 CDD camera that was cantilevered into a position below the specimen plane
Diffractive imaging, in which image-forming optics are replaced by an inverse computation using scattered intensity data, could, in principle, realize wavelength-scale resolution in a transmission electron microscope. However, to date all implementations of this approach have suffered from various experimental restrictions. Here we demonstrate a form of diffractive imaging that unshackles the image formation process from the constraints of electron optics, improving resolution over that of the lens used by a factor of five and showing for the first time that it is possible to recover the complex exit wave (in modulus and phase) at atomic resolution, over an unlimited field of view, using low-energy (30 keV) electrons. Our method, called electron ptychography, has no fundamental experimental boundaries: further development of this proof-of-principle could revolutionize sub-atomic scale transmission imaging.
Ultimately, the resolution limit for ptychography will in part be determined by the practicality of preparing very thin specimens to avoid 3D scattering effects or, as discussed above, by our ability to account for these effects during the reconstruction process. It will also be affected by two further issues. The first is specimen damage, which will increase as the radiation per unit area is necessarily increased to realize higher resolution. A possible advantage of ptychography in this respect is that the phase image has high contrast. The second is that inelastic scattering may mask the coherent scattering we rely upon for this technique to work. Exactly how serious this will be is uncertain: it has not affected the results we present here, where no attempt has been made to energy filter the scattered electrons, but clearly further work is required in this area.
It should be emphasized that current results represent a first step in what we believe is a completely new epoch of electron imaging. Many improvements in the experimental setup can be envisaged. The resolution that we achieve here is determined by the angle that the detector subtends at the specimen—a simple, non-fundamental, geometric constraint. Combining an optimal detector configuration with reduced wavelength (by working at normal TEM accelerating voltages: 80–300 keV) could in principle let us achieve much less than 0.05 nm resolution: better than the very best state-of-the-art aberration-corrected machines. Although here we have used a conventional round magnetic lens to form the illumination at the object plane, there are undoubtedly much better ways to configure and optimize a ptychographic microscope. The only requirement on the illumination is that it is reasonably localized (say up to 100 times larger than the final resolution desired) and coherent. There is no need for a high-performance objective lens or any magnification optics. By disposing of so many high-precision components, and moving the imaging process into a computer, we can at last see a route to exploiting the shortness of the electron wavelength for ultimate transmission imaging. No longer does TEM have to be bound by the paradigm of the lens—its Achilles’ heel since its invention in 1933.
Examples of the recorded diffraction patterns. (a) Free-space diffraction pattern, that is, collected when the probe is in free space. The disk is a shadow image cast by the condenser aperture. (b) Diffraction pattern from the sample. The strong bright-field intensity is seen inside the central disk, also known as the Gabor hologram or Ronchigram. (c) The same diffraction pattern as shown in b plotted on a log-intensity scale to show dark-field intensity data: the high-resolution information arises from this data. Scale bar, 1 nm−1. The ring indicates a radius of 0.236 nm−1.
Wide-field ptychographic reconstruction of gold particles and graphitized carbon on a holey carbon support film