Previously reported crystalline structures obtained by an iterative phase retrieval reconstruction of their diffraction patterns seem to be free from displaying any irregularities or defects in the lattice, which appears to be unrealistic. We demonstrate here that the structure of a nanocrystal including its atomic defects can unambiguously be recovered from its diffraction pattern alone by applying a direct phase retrieval procedure not relying on prior information of the object shape. Individual point defects in the atomic lattice are clearly apparent. Conventional phase retrieval routines assume isotropic scattering. We show that when dealing with electrons, the quantitatively correct transmission function of the sample cannot be retrieved due to anisotropic, strong forward scattering specific to electrons.We summarize the conditions for this phase retrieval method and show that the diffraction pattern can be extrapolated beyond the original record to even reveal formerly not visible Bragg peaks. Such extrapolated wave field pattern leads to enhanced spatial resolution in the reconstruction.
Main textThe study of nanocrystal structures at atomic resolution is an important topic in nanotechnology, solid state physics and especially in biology, where preparing a large perfect crystal is often a challenge and the synthesis of nanocrystals is preferred 1 . It has recently been demonstrated that the structure of a nanocrystal can directly be obtained by coherent diffraction imaging 2 from an electron or X-ray diffraction pattern by applying phase retrieval algorithms [3][4][5][6][7] . The diffraction pattern of a crystalline structure typically consists of distinct Bragg peaks, whereby each peak is convoluted with the Fourier transform of the crystal shape (shape-transform) [8][9] . In the experiments demonstrated so far 3-7 , a regular crystalline structure at sub-nanometer resolution could be retrieved, but the individual atoms remained unresolved and therefore no atomic defects were revealed. The structure retrieval in the reported experiments require the input of an initial low-resolution image of the sample distribution typically provided by other techniques, as for example by transmission electron microscopy (TEM) imaging 3-4 , scanning electron microscopy (SEM) 5 , high-resolution transmission electron microscopy (HRTEM)