Investigations On Several Basic Problems In Modern Transmission Electron Microscopy

Quantitative transmission electron microscopy,aming to determine the atomic structure,the element distribution and three-dimension?3D?structure of nano-scale sample,is promising to be widely used in materials science research.However,this is heavily limitted due to the electronic sensitivity of sample and inaccurate imaging parameters measurement and so on.To solve these problems,this thesis concerns four key points:?1?With decreased accelerating voltage used in TEM for investigating 2D and nano materials,the influence of voltage on traditional image simulation method?multislice method?is not yet clear.?2?It is widely known that beam tilt has great influence on the contrast of high resolution TEM images,whereas this parameters cannot be measured quantitatively to date.?3?3D tomography from tilt series of HAADF STEM images is very popular in determining the morphology of samples.But the influence of imaging parameters,especially detector size,convergence angle etc.,on reconstruction has not been investigated yet.?4?Although through-focus exit wave reconstruction is powerful in image interpretation,the low success rate limits its applications.Reasons are probably the misalignment of images,inaccurate defocus step values and beam tilt.?1?Significant errors occur in conventional multislice simulation?CMS?algorithm when the accelerating voltage decreases to¹00 kV.The propagator corrected multislice simulation?PCMS?algorithm retains high accuracy even when the voltage is as low as²0 kV.But when the voltage decreases to¹0 kV or below,the fully corrected multislice algorithm?FCMS?should be employed for high-accuracy simulation.The performance of all the three algorithms are found to be related with the sample thickness and voltage.When high voltage is used,the simulations of thick sample by the three algorithms all keep the same accuracy.However,when the voltage is low,the accuracy of the three algorithms can only retain the same for thin sample.Since the FCMS algorithm can only be realized using real space method,the consumed time is about 16 times that of the FFT realized PCMS.?2?The phases of some specific diffractogram spots which are formed by only one pair of diffracted beams that have equal amplitudes and inverse directions,are found to be linear with the defocus values.The slope is found to be related with the beam tilt value in imaging process.According to this finding,a quantitative method for measuring such small beam tilts,including its theoretical basis,numerical simulations and experimental verification are presented in this thesis.The advantage of this method is that it is independent of lens aberrations apart from the defocus of the microscope.It is shown that this method can reach a rather good 94%accuracy for small experimental beam tilts of about 0.1°or less.?3?With increasing the inner angle of annular dark field detector in STEM mode,the linearity between colected intensity with sample thickness,namely the projection theorem,can be enhanced,making it more suitable for tilt series 3D tomography.When big inner detector angle is employed,the size of convergence angle has little influence on this linearity relationship.In addition,big convergence angle can diminish the electron channeling effect which usually become significant when the incident beam is along high symetry zone axises.The 3^rd spherical aberration and defocus are found not to influence both the projection theorem and the electron channeling effect.?4?A modified iterative wave function reconstruction?mIWFR?algorithm is proposed in the present thesis.By employing traditional IWFR algorithm as part of the mIWFR algorithm,it is fast and highly efficient in reconstruction.Besides,the newly proposed algorithm aligns the focal series of HRTEM images by correlating the experimental images with calculated image functions,which greatly improves the alignment accuracy.For low-dose through-focus exit wave reconstruction,the present algorithm are found to have better performance than the comercial software and conventional IWFR algorithm.?5?In exit wave reconstruction,the inaccurate defocus step between two successive images can decrease the contrast of reconstructed phase.The beam tilt has significant influence on the reconstructed results,and should be corrected in the reconstruction.When the beam tilt reaches 1 mrad,the elongnation of atom columns remains in the reconstructed wave,while it is not found in the reconstructed wave by mIWFR algorithm which considered the influence of beam tilt.