Experimental Atomic Resolution Electron Tomography
This work focus on the efforts made to obtain three-dimensional local information at atomic resolution using electron tomography. To realize this long-standing goal, state-of-the art experimental and computational methods were developed. This thesis describes the experimental portion of the collaborative project that has resulted in the first true atomic resolution reconstruction of a material while making no assumptions of crystallinity.
Chapter 1 will give background information on electron microscopy and tomography. Chapter 2 describes the experimental aspects of atomic scale electron tomography using an uncorrected electron microscope. Chapters 3 and 4 follow from this development, and describe the results obtained from implementing this methodology. By reconstructing the data obtained with these methods with an advanced algorithm, atomic scale features were observed for the first time in three dimensions. As a first demonstration, a gold nanoparticle was reconstructed with 2.4 angstrom resolution, revealing the internal twin boundaries and grain structure of a multiply twinned icosahedral nanoparticle. Next, improvements in data and Bragg filtering revealed the three dimensional atomic structure of edge and screw dislocations, as well as stacking faults, for the first time.
Chapter 5 then discusses the additional experimental considerations needed to employ an aberration corrected electron microscope for the tomographic data. Also in this chapter, a novel needle geometry for tomography is introduced. Chapters 6 and 7 then discuss the results of this methodology applied to both needle and conventional samples. Here, true atomic resolution is finally achieved, with atomic positions being precisely located within the sample, and a point defect identified. With the 3D atomic positions determined, displacement and strain fields can be calculated at high resolution. Work is underway to achieve not only atomic resolution, but also to differentiate between atomic species within the sample.
Electron tomography is a powerful technique for characterization in three dimensions, and this work represents the highest resolution yet achieved, and the first demonstration of true atomic resolution electron tomography. The ability to localize the three-dimensional positions of atoms in a material while making no assumptions of crystallinity, as well as measure point defects and strain, is an important breakthrough and will find broad application in many fields.