Multi-beam And Multi-convergence-angle Electron Ptychographic Diffractive Imaging

With the popularization of spherical aberration corrector,high-resolution transmission electron microscopy(TEM)has been widely applied to characterize and analyze materials at the atomic scale.On the one hand,the resolution of TEM needs to be further improved due to the residual aberration of the magnetic lens and the temporal and spatial coherence of the electron beam,etc.On the other hand,conventional TEM and scanning TEM(STEM)could only obtain part of the information of the sample,and electron imaging needs more information about the sample,such as light element information,electric field distribution,defects,etc.Ptychography is an iterative recovery algorithm(Ptychographic Iterative Engine,PIE)based on diffractive imaging,which can recover the missing phase information from the recorded diffraction intensity images,and is widely used for phase recovery in visible light,X-ray and 3D imaging.In recent years,ptychography has been developed rapidly in the field of electron microscopy due to the dramatic increase in computational capability.Combined with high speed electron detector,ptychography has been able to achieve a resolution of 0.39?in monolayer Mo S₂,much higher than the 0.98?achieved by conventional imaging methods under the same conditions.Compared to STEM,ptychography is more sensitive to light elements because ptychography is able to retrieve the phase information,and the dose of electrons needed for same quality images is much lower than that in STEM mode due to high dose efficiency.These advantages made ptychography as a hot-point and frontier research field in electron microscopy,promising for biological,battery and organic materials which are sensitive to electron beams.However,ptychography is still limited by noise,data acquisition time,drifting and other factors,the question of relative long time of data acquisition,and contrast transfer constrained to a small band of spatial frequency,limit the application of ptychography in in-situ characterization and wideband contrast transfer.Based on the current ptychography methods,this work gives solutions to the problems above through theoretical analysis and experiments,the conclusions are as below:(1)Based on single-shot ptychography in X-ray and visible light,this work theoretically proposes a multi-beam electron ptychography method using an array of multiple electron beam sources to obtain diffraction patterns of multiple electron beams in one single illumination.In this work,computational simulations were carried out for the generation of the diffraction patterns and ptychography reconstruction on two simulated samples composed by images and atomic potential fields to verify the feasibility of the multi-beam electron ptychography method.Besides,we explored the noise introduced by the interaction between individual electron beams and the methods to reduce it.The multi-beam electron ptychography method can considerably minimize acquisition time and sample drift,reduce sample irradiation damage,enable parallel data acquisition and therefore,provides a practical solution for in-situ characterization.(2)From the relationship between the contrast transfer and the convergence angle of the ptychography method,this work theoretically designs the multi-convergence-angle electron ptychography method,using electron beams with different convergence angles to illuminate the sample and obtain a mixed diffraction pattern dataset.Firstly,in this work,we use amorphous carbon and Gro EL biomolecules as simulation objects to obtain diffraction pattern at different convergence angles by multislice simulation,and reconstruct datasets with different convergence angles and mixed diffraction pattern dataset using ptychography to verify the relationship between contrast transfer and convergence angle under Poisson noise,and the feasibility of multi-convergence-angle electron ptychography method was verified.Then,this paper experimentally implemented the multi-convergence angle electron stacked diffraction imaging method using Ag-CdS nanoparticles,Au-DNA origami structures and boron nitride nanotubes as experimental samples.From the reconstructed results under different datasets,we demonstrated that multi-convergence-angle electron ptychography method can achieve the mixing of low-frequency information with high-frequency information to obtain high-resolution and high-contrast images with wideband spatial frequency transfer.In the future,as the application of a fast-switch aperture which could change convergence angle in real time during the scanning process,the acquisition of diffraction pattern dataset with multiple convergence angles would be complished during one single scan,ultimately realizing direct high-resolution high-contrast imaging,which would become a new practical high-resolution imaging technology,promising for imaging and characterization of biological,light element and other samples.