Atomic Scale Investigation of Structures and Functionalities in Quantum Matter Heteroepitaxy

Abstract

“…whether, ultimately---in the great future---we can arrange the atoms the way we want; the very atoms, all the way down! What would happen if we could arrange the atoms one by one the way we want them…” Richard Feynman described the exciting blueprint of nanotechnology in his famous lecture entitled “There’s plenty of room at the bottom” in 1959. He devoted a significant part of electron microscopy in his lecture, underlining the possible breakthroughs in many research fields, if we are able to look at the atomic scale. With explosive advances in nanoscience and nanotechnologies, since then, especially after the 1980s, Feynman’s visions have arguably been realized. So far, the arrangement of the atoms can be artificially manipulated with advanced synthesis methods and microscopy techniques, whilst they can be visualized using microscopy techniques with high resolution. The latest generation of electron microscopes is capable of resolutions of 0.39 A. The work in this thesis is dedicated to studying “plenty of room at the bottom” from two perspectives. On the one hand, we fabricated quantum matter heterostructures by arranging the atoms using pulsed laser deposition (PLD) with atomic-layer precision and studied caused physical behaviors. On the other hand, we visualized the actual arrangement of atoms in quantum matter heterostructures, in particular at the heterointerface, using state-of-the-art scanning transmission electron microscopy (STEM) and spectroscopy. The underlying mechanisms of physical behaviors were explained accordingly. Firstly, SrRuO3 (SRO) artificial atoms (AAs) were fabricated by electron beam lithography (EBL) patterning of the epitaxial-grown SRO thin films with atomic layer precision using PLD. The size of AAs is down to 15 nm, which is the smallest size reported so far. Due to the spatial confinement of the electron system, interesting magnetic phenomena occur with decreasing the size of AAs, showing a gradual increase of magnetic Curie temperature and a reorientation of the easy axis towards the in-plane direction. Based on the optimization of TEM sample preparation, we performed atomically resolved STEM observations of the atomic arrangements in different samples. It was found that the chemical distribution keeps unchanged, while the epitaxial strain is gradually relieved as the size of AAs decreases. The strain relaxation leads to the Curie temperature variations, whilst the strain relaxation induces the change of magnetocrystalline anisotropy that results in a rotation of the magnetic easy-axis. The hetero-interface between SrMnO3 (SMO) and SrTiO3 (STO) is then studied atomic column by atomic column using STEM and spectroscopy. We found that the electronic charges accumulate close to the interface layer, reaching the maximum charge density at the first SMO monolayer. A fundamental understanding of the charge accumulation was elaborated with a combination of STEM investigations and numerical calculations. STEM offers atomic-scale information in terms of strain fields in out-of-plane and in-plane directions and two-dimensional distribution of constituent elements. The SMO layer close to the interface is demonstrated to have the largest tensile strain and the strongest intermixing of Mn and Ti than other regions away from the interface. The role of tensile strain, elemental intermixing, charge transfer, and space-charge effects are intensively discussed, proving a comprehensive perspective of the interfacial charge distribution. We also studied the atomic structure and functionality of cracks formed around the heterointerface due to thermal strain relaxations. The atomic arrangement of crack tips was clearly visualized by STEM imaging. Quantitative analyses of STEM images revealed a large strain gradient around the tip of deep crack that is extended into the STO substrate. With the STEM-based lattice displacement analysis, we determined the displacements between the negative center and the positive center unit cell by unit cell and thereby obtained the flexoelectric polarization around crack tips with atomic resolution. The averaged polarization around the tip of the deep crack is about 62 ± 16 µC·cm-2, which is the largest flexoelectric polarization measured so far. The flexoelectric polarization is screened by the electronic charge with a density of 0.7 ± 0.1 e-/uc localized within one unit cell. We suggest that the flexoelectricity plays an essential role in the propagation of cracks, because the flexoelectric energy density was found to be ~3% of that of the elastic energy. Benefited from the unique advantages of STEM in terms of high spatial resolution, this thesis presents three applications about studying the atomic worlds “at the bottom” visioned by Feynman sixty years ago, providing an atomic-scale understanding of structure and properties of quantum matter heterostructures. In the near future, the atomic arrangement in quantum matter heterostructures will be better controllable. Furthermore, the capacity of STEM will undoubtedly be more versatile to explore atomic worlds, such as direct visualization of the electric and magnetic fields on the atomic scale. Many mysterious phenomena in the atomic-scale world are about to be revealed.