Electron Ptychography At Low Dose

With the development of spherical aberration correctors,scanning transmission electron microscopy(STEM)has achieved sub-angstrom resolution,so it has been widely used in materials science and life science research.However,STEM only uses a part of scattered electrons for imaging.In order to obtain high-resolution images in an effective field of view,STEM needs to use focus probes to collect information at millions of scanning positions,which leads to a high electron dose in this imaging technology.Organic materials,biomaterials,ceramics and zeolites are sensitive to electron beams and are prone to radiation damage.Therefore,STEM imaging technology is difficult to characterize and analyze these samples which are easily damaged by electron beam.Electron ptychography is a new coherent diffraction imaging technology.At present,light element imaging and three-dimensional imaging have been realized using electron ptychography experimentally.Compared with STEM,this technology can greatly reduce the electron dose on the sample.There are two main reasons:first,ptychography can use non-focused probe to scan the sample to collect diffraction patterns,which can greatly reduce the number of scanning positions,and then reduce the electron dose on the sample.Second,ptychography uses the full information of the bright field of the diffraction image to reconstruct.The utilization efficiency of incident electrons is higher.In data acquisition,the development of ultra-fast cameras enables us to collect weak signals at high signal-to-noise ratio(SNR).This also makes it possible to reconstruct the sample which is easily damaged by electron beam at low electron dose.Therefore,this work combines electron ptychography with ultra-fast camera,and chooses single-layer MoS2 which is easy to be damaged by electron beam as the research material.The research is carried out through the combination of experiment and multislice simulation.It is shown that:(1)The electron ptychography under defocus can utilize electrons more effectively,the electron dose efficiency is higher,and it is more robust to noise at low electron dose.(2)The electron ptychography imaging can effectively epitaxy the original diffraction pattern,so that the reconstructed diffraction information exceeds the limit of the convergent aperture(1a truncation point),but the required electron dose can not be less than 2.04×104e-/A2.(3)The lattice fringes of MoS2 can still be obtained at the electron dose of 403 e-/A2.In this experiment,the lowest electron dose is obtained,and the experimental parameters are explored to provide guidance for reconstructing biological samples in the future.Then,the method of reducing the electron dose is further studied in this paper.The electron ptychography is combined with the annular aperture.The generated annular illumination probe is used to scan the sample.Because the annular aperture has not yet been prepared in the experiment,it is mainly studied by multislice simulation.It is shown that:(1)The depth of field increases and the size of probe decreases with the increase of inner diameter of annular aperture.Compared with the point spread function of STEM-HAADF,the depth of field of electron ptychography increases more.(2)When the inner diameter of the annular aperture is greater than 10 mrad,the reconstructed phase increases first and then decreases with the increase of thickness,but there is no phase reversal.(3)When applied to biological samples such as GroEL,the reconstructed phase value increased with the increase of the inner diameter of the aperture.These results show that the combination of electron ptychography and annular aperture can reduce the electron dose without reducing the quality of reconstructed phase.The simulation results provide a theoretical basis for future experiments.In this paper,the study of low electron dose ptychography is very important for characterizing biological samples which are easily damaged by electron beam.With the development of ultrafast cameras and annular aperture,we believe that the electron dose can be further reduced experimentally,so that various irradiation sensitive samples can be characterized.