Super-resolution Electron Ptychographical Diffractive Imaging

Transmission electron microscope(TEM)is now widely used in material science and biology,especially in the field of nano-materials,because of its high spatial resolution.In the past decade,thanks to the invention of aberration correctors,the resolution of TEM is greatly improved,from about 0.2nm,which is close to atomic resolution,to around 0.05nm,which is beyond atomic resolution.Many materials,for example catalyst,multiferroic materials and 2D materials benefit from this improvement.To further improve resolution,we need to further correct the residual aberrations and improve the TEM stabilities and coherences.Except for using the expensive aberration correctors,an alternate way to improve the resolution is using"lensless" methods.The "lensless" method used in the thesis is called ptychography which sometimes is also called scanning coherent diffractive imaging.Ptychography is now widely used in X-ray community.However,there are very few researches in transmission electron microscope community.An important advantage of this method is not shown,i.e.this method can be used to imaging light elements.For many key materials,for instance,hydrogen storage materials,lithium battery and functional oxide materials,knowing the distributions of the light elements is important to investigate the structure-activity relationship.LaB6 is used for this demonstration.In the experiment,we used a convergent beam so that the diffraction spots are expended to disks and the center transmission beam can thus be collected to prevent from the information loss problem.A diffraction pattern(DP)is record for each scanning position while the electron probe is two dimensional scanned.The probes should "overlap" for every two adjacent positions.That's the origin of the name,where"ptycho" means folding.Because the speed cameras used is slow,we put the sample tens of nanometers away from the focus point so the diameter of the defocused probe is increased and we can collect less diffraction patterns.As the 4D data sets were collected,we used the enhanced ptychographic iterative engine(ePIE)method to finish the reconstruction.The reconstructed result shows that ptychography can be performed to an extended single crystal and we find that the contrast of the low atomic number elements is much better than that in a high angle annual dark field(HAADF)images.We also collected ABF images which can also be used to imaging low atomic number elements.The spectrums of the images show the resolution achieved by ptychography is 83pm,which is better than the simultaneous acquired HAADF and ABF images.The results show that,with the scanning position error correction method,ptychography can detect and correct residual aberrations and this method can ultimately achieve the resolution limited by wavelength.In the simulation,we compared the contrast with different sample thickness.We also considered partial coherence and used the "frozen phonon"method.All these factors should be considered if we want a quantitative reconstruction.We believe that ptychography can be an important technique to imaging light elements since the technique is compatible with current STEM hardware.Over the past years,TEM projects 3D samples to 2D images.We lost much important information alone the optical axis and this may lead to misjudgments.Therefore,3D imaging and analysis are important to understand the relationship between structure and properties of materials.The widely used methods are optical sectioning and tilted tomography while we used a new technique in the thesis.We used two crossed carbon nanotubes and placed them 125nm and 197nm away from the focus point to demonstrate this method.We then two dimensionally scanned the probe and collected 4D data sets.In the final step,we used a method combined with the enhanced ptychographic iterative engine and the inversed multislice method to reconstruct the carbon nanotubes.In the spectrums of the reconstructions,we find(0002)spots which correspond to 0.34nm lateral resolution.By observing the intensity changes of these spots,we achieve an axial resolution of 24-30nm.Further,we draw the transfer function,calculated the theoretical resolutions,which are 0.11 nm and 22nm for lateral and axial resolutions.We then compared and discussed the resolutions differences.3D ptychography method has the advantage that it can achieve 3D reconstruction without rotation.So it collects data fast and may be used to imaging beam sensitive materials.3D ptychography can also be used to in-situ liquid or gas holders,which are hard to be rotated in TEM.At the end of this thesis,we draw conclusions and make a prospect to electron ptychography.Benefited from the development of fast cameras and direct electron detectors,ptychography will be a widely used technique with high sensitivity and high special resolution.Besides,3D ptychography method has the advantage that it can achieve 3D reconstruction without rotation.So this method might be used for special holders,for example,in-situ liquid and gas holders.