Researchers at the
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.
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
Nature Communications - Ptychographic electron microscopy using high-angle dark-field scattering for sub-nanometre resolution imaging - pictures