Three Dimensional Tilted Electron Ptychographic Tomography

Transmission electron microscopy is widely used as characterization technology in the fields of both materials and biology science due to its capabilities of achieving twodimensional and three-dimensional imaging in ultra-high spatial resolution.Apart from the conventional TEM and STEM imaging modes,a novel mode called 4D-STEM is now rapidly developed.4D-STEM can provide many different signals in one single experiment,including electron ptychography and differential phase contrast(DPC),etc.Electron ptychography is a high-resolution,low-dose and high contrast imaging technique,allowing both the sample function and the incident electron wave to be reconstructed.In this thesis,two-dimensional and three-dimensional electron ptychography are studied,and main researches are listed below.(1)In this paper,we evaluated the effects of various scanning step-size and detector hollow angle on the imaging qualities among electron ptychography,DPC and STEMADF techniques,using single-layer molybdenum disulfide as a sample.Our results showed that the phase images reconstructed from electron ptychography and integrated DPC have similar resolution in a smaller scanning step(0.2 ?).Whereas,the resolution of electron ptychography becomes better than that of integrated DPC when a large scanning step(0.6 ?)is used.Furthermore,it was proofed by experiments that hollow electron ptychography,which is an improved reconstruction method,is able to reconstruct the image of sample from incomplete diffraction patterns collected by hollow electron detectors without scarifying resolution.(2)We carried out a tomography experiment with a mixed sample of self-assembled DNA origami frame and gold nanoparticles in a cryogenic electron microscope.The reconstruction results showed the positions of the gold nanoparticles in three-dimension.We further analyzed the extracted position coordinates of the gold particles and revealed the difference between true and expected positions.This provided guidance for the future synthesis of relevant structures.However,the contrast of organic samples is too weak in a Cryo-EM.In the above 3D experiment,we also found that unstained weakscattering samples,for example unstained DNA origami,can hardly be three dimensionally analyzed using conventional methods.(3)To overcome the weak contrast of unstained organic samples,we further proposed ptychographic tomography,as a new imaging method for three-dimensional analysis of weak-scattering samples.we carried out electron ptychographic tomography experiments on unstained DNA origami frame with gold nanoparticles.the resolution of a two-dimensional projection image is 2.0 nm in a 300 k V conventional TEM.Based on these images,a three-dimensional result is reconstructed.DNA origami,gold nanoparticles and background can be clearly distinguished.Ptychographic tomography has advantages of high resolution and high contrast,and it is especially important for imaging and three-dimensional reconstruction of weak scattering samples.Therefore,it can be foreseen that electron ptychographic tomography can potentially be used in Cryo-EM for 3D imaging a wide range of weak-scattering samples,such as biological cells,virus and proteins.