5D Multi-channel Hollow Electron Ptychographic Diffractive Imaging

With the application of spherical aberration correctors,high-resolution transmission electron microscopy(TEM)has been extensively used for characterizing and analyzing materials at atomic scale for several decades.However,limited by the residual aberration of magnetic lenses and the coherence of electron beam et al.,it is difficult to further improve the resolution of TEM.As an imaging method based on iterative reconstructed algorithm ePIE,the resolution of ptychographic reconstructions is not limited by the optical system,so ptychography has been successfully implemented to recover the high resolution missing phase information from recorded intensity images or 3D imaging of samples in light and X-ray sources.Recent advances in data processing algorithms and fast direct electron detectors has triggered the interest in electron ptychography.Compared with the conventional STEM images,ptychography can provide higher contrast of light elements,improved resolution and lower electron dose.So that,it can be better applied on electron beam sensitive materials,such as biological,batteries and organic materials.It has been pointed out that ptychography can improve the resolution of monolayer MoS2 to 0.39 A at 80 KeV.Therefore,ptychography has been regarded as one of the important methods to improve the imaging capability of TEM in the future.In this paper,we propose a new 5D multi-channel super-resolution iterative ptychographic reconstructed configuration:hollow ptychography.The monolayer MoS2 located at the focus plane was raster-scanned by a focusing probe.Except for the conventional HAADF image,the hollow diffraction patterns were recorded on the hollow pixelated detector,and the central hollow part would allow electrons to enter EELS spectrometer.Therefore,hollow ptychography will enable the future combination of ptychography with both EELS and HAADF to simultaneously provide high-efficiency phase and Z contrast imaging together with spectroscopic information in a single experiment.In order to prove the feasibility and stability of this method,we compared hollow ptychographic reconstructed phases with ADF image with the same electron dose.It is shown that:(1)The hollow ptychographic reconstructed phases have a resolution of 0.91 A,while the resolution of ADF image is only 1.36A.(2)The signal of effective information in hollow ptychographic results is larger,and the contrast of the lighter 2S atoms is higher.(3)The missing information in the center of diffraction patterns can be retrieved by multiple iterations,which effects the reconstructed phases quality slightly.Similarly,multislice simulation results with equivalent dose confirm the interpretation of the above experimental results,and the frozen phonons and equivalent sourcesize models made multislice simulation closer to the real experiments.Therefore,these results demonstrate that hollow ptychographic reconstructions have higher resolution,lower noise and better image contrast,and the lower signal of difraction patterns has no obvious effect on hollow ptychographic phases.For typical atomic resolution EELS mapping,the actual pixel dwell time and electron beam scanning step size are generally much larger than ADF imaging,so we further explored the effect of scanning step size and accuracy on both the convergence and resolution of hollow ptychographic reconstructions with different hollow inner angles.The results show that:(1)Even in the under-sampling condition,such as a step of0.612 A and 15 mrad hollow inner angle,a resolution of 1.58 A can still be achieved in the reconstructions,while the simultaneously acquired DPC images are completely pixelated.(2)As the thickness increases,the reconstructed phase contrast wraps without structural changes,and the phenomenon of phase wrap can be improved by changing defocus during the reconstructed algorithm.Therefore,the hollow geometry allows it to couple simultaneously with EELS mapping,even under the undersampled and large thickness samples conditions.Generally,this 5D-STEM imaging method based on hollow fast direct electron detectors not only fully integrates various traditional STEM modes,including HAADF,ABF,DPC images,as well as EDS and EELS mappings,but also provides more resolved and quantifiable recovered phases.In the future,with the development of fast hollow detectors that are compatible with an EELS spectrometer,we anticipate that this geometry will enable simultaneous,correlative analysis using both highly sensitive phase information and chemical mapping.