Convergent Beam Electron Diffraction Measurement Of Structure Factors

Emergent phenomena are pervasive in strong-correlation systems,such as multiferroics,high temperature superconductivity,metal-insulator transitions,et.al,providing us a paradigm for exploring functional properties of materials.These electron strong correlation effects constrain the number of electrons at the given lattice site,inducing a local tangle of the charge,orbital and spin degrees of freedom.Thus,studying the electronic structure of functional materials helps us understand the nature of novelty properties of materials.Among these,the charge density is the most direct expression of electrons.Due to its ultrahigh spatial resolution,transmission electron microscopy(TEM)has become a very important tool for studying the physical and chemical properties of materials.In convergent beam electron diffraction(CBED)mode,electron beam illuminates onto the crystal with quite convergent angle and makes diffraction spots expand into diffraction disks which can provide abundant information.Electrons,which are scattered by the electrostatic potential when the electron beam is incident into a crystal,are sensitive to the valence electron distribution.So,it is suitable for CBED to measure the charge density distribution of crystalline materials.This has important guiding significance for revealing material properties,improving material properties and designing new materials.In this dissertation,we accurately measured the low-order structure factors of crystal materials using CBED.The main contents are summarized as follows:First,the similarities and differences of measuring structure factors using electron diffraction and X-ray diffraction are introduced.Theoretical analysis shows that valence electrons mainly affect low-order structural factors,and electron diffraction is more sensitive to low-order structural factors than X-ray diffraction.So,it is significantly better to measure low-order structural factors using electron diffraction than X-ray diffraction.The multi-beam dynamic electron diffraction theory has been explored in detail.Then,we obtained Si(2-20)systematic row using CBED and got the structure factor of Si(2-20)through refinement combining theoretical calculations.Finally,we analyzed the reasons why the experimental results were not accurate enough.It was probably due to the geometry distortion of the diffraction disks and unsuitable thickness of the sample.