Guo-zhen Zhu

Bonding and structure of a reconstructed (001) surface of SrTiO3 from TEM

The determination of the atomic structure and the retrieval of information about reconstruction and bonding of metal oxide surfaces is challenging owing to the highly defective structure and insulating properties of these surfaces. Transmission electron microscopy (TEM) offers extremely high spatial resolution (less than one ångström) and the ability to provide systematic information from both real and reciprocal space. However, very few TEM studies have been carried out on surfaces because the information from the bulk dominates the very weak signals originating from surfaces. Here we report an experimental approach to extract surface information effectively from a thickness series of electron energy-loss spectra containing different weights of surface signals, using a wedge-shaped sample. Using the (001) surface of the technologically important compound strontium titanate, SrTiO3 (refs 4, 5, 6), as a model system for validation, our method shows that surface spectra are sensitive to the atomic reconstruction and indicate bonding and crystal-field changes surrounding the surface Ti cations. Very good agreement can be achieved between the experimental surface spectra and crystal-field multiplet calculations based on the proposed atomic surface structure optimized by density functional calculations. The distorted TiO6−x units indicated by the proposed model can be viewed directly in our high-resolution scanning TEM images. We suggest that this approach be used as a general method to extract valuable spectroscopic information from surface atoms in parallel with high-resolution images in TEM.

Atomic structure and bonding of the interfacial bilayer between Au nanoparticles and epitaxially regrown MgAl2O4 substrates

A unique metal/oxide interfacial bilayer formed between Au nanoparticles and MgAl2O4 substrates following thermal treatment is reported. Associated with the formation of the bilayer was the onset of an abnormal epitaxial growth of the substrate under the nanoparticle. According to the redistribution of atoms and the changes of their electronic structure probed across the interface by a transmission electron microscopy, we suggest two possible atomic models of the interfacial bilayer.

Damage behavior and atomic migration in MgAl2O4 under an 80 keV scanning focused probe in a STEM.

With the dramatic improvement in the spatial resolution of scanning transmission electron microscopes over the past few decades, the tolerance of a specimen to the high-energy electron beam becomes the limiting factor for the quality of images and spectra obtained. Therefore, a deep understanding of the beam irradiation processes is crucial to extend the applications of electron microscopy. In this paper, we report the structural evolution of a selected oxide, MgAl2O4, under an 80 keV focused electron probe so that the beam irradiation process is not dominated by the knock-on mechanism. The formation of peroxyl bonds and the assisted atomic migration were studied using imaging and electron energy-loss spectroscopic techniques.

Atomic-level 2-dimensional chemical mapping and imaging of individual dopants in a phosphor crystal

The ability to visualize and identify individual dopants, as well as measure their local physical and chemical environments in a bulk, provides deep insight for designing new functional materials and predicting their properties. However, a full understanding of dopants inside a solid has been limited by currently available characterization techniques. We demonstrate the first atomic-level 2-dimensional elemental maps of Pr dopants using the electron energy-loss spectroscopy (EELS) technique and we image Al dopants located in a lattice. Based on spectroscopic and imaging evidence we provide plausible local defect configurations of implanted Pr(+) and Al(+) ions within SrTiO3 single crystals. Our results demonstrate the detection of single Pr atoms and the formation of Al-rich nanoscale clusters ranging from 1 to 3 nm in size randomly distributed in the implanted lattice. These results provide insight into the mechanism of red light emission in doped SrTiO3.

Structural investigation of interface and defects in epitaxial Bi3.25La0.75Ti3O12 film on SrRuO3/SrTiO3 (111) and (100)

The structure of La-doped bismuth titanate (BLT), Bi3.25La0.75Ti3O12, is investigated with atomic resolution high-angle annular dark field (HAADF) scanning transmission electron microscopy. The images reveal evidence of the tilting of TiO6 octahedra within the perovskite-like layers of the BLT unit cell. The tendency of La ions to substitute Bi ions and occupy the top part of the (Bi2O2)2+ layer, previously observed from electron energy loss spectroscopy (EELS) mapping experiments, is explained based on the tolerance factors and stress relief mechanism. The atomic resolution HAADF images also reveal the presence of the out-of-phase boundaries (OPBs). The role of OPBs in BLT is discussed in terms of its fatigue resistance as the OPBs provide extra nucleation sites for ferroelectric domains during polarization reversals. Further, we show evidence that the first deposited atomic layer at the interface also governs the subsequent film growth, resulting in the modulation of the “defect-free” and the “defected”...

Two-dimensional X-ray diffraction and transmission electron microscopy study on the effect of magnetron sputtering atmosphere on GaN/SiC interface and gallium nitride thin film crystal structure

The growth mechanisms of high quality GaN thin films on 6H-SiC by sputtering were investigated by X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM). The XRD θ-2θ scans show that high quality (0002) oriented GaN was deposited on 6H-SiC by reactive magnetron sputtering. Pole figures obtained by 2D-XRD clarify that GaN thin films are dominated by (0002) oriented wurtzite GaN and {111} oriented zinc-blende GaN. A thin amorphous silicon oxide layer on SiC surfaces observed by STEM plays a critical role in terms of the orientation information transfer from the substrate to the GaN epilayer. The addition of H2 into Ar and/or N2 during sputtering can reduce the thickness of the amorphous layer. Moreover, adding 5% H2 into Ar can facilitate a phase transformation from amorphous to crystalline in the silicon oxide layer and eliminate the unwanted {33¯02} orientation in the GaN thin film. Fiber texture GaN thin films can be grown by adding 10% H2 into N2 due to the complex reaction between...

Atomic-scale identification of novel planar defect phases in heteroepitaxial YBa2Cu3O7−δ thin films

We have discovered two novel types of planar defects that appear in heteroepitaxial YBa2Cu3O7−δ (YBCO123) thin films, grown by pulsed-laser deposition (PLD) either with or without a La2/3Ca1/3MnO3 (LCMO) overlayer, using the combination of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging and electron energy loss spectroscopy (EELS) mapping for unambiguous identification. These planar lattice defects are based on the intergrowth of either a BaO plane between two CuO chains or multiple Y-O layers between two CuO2 planes, resulting in non-stoichiometric layer sequences that could directly impact the high-Tc superconductivity.

Studying tomorrow's materials today: Insights with quantitative STEM, EELS

The development of aberration correctors for the scanning transmission electron microscope has revolutionized the field of electron microscopy and dramatically improved the analytical “toolkit” of materials scientists. In particular, when combined with electron energy loss spectroscopy (EELS), scanning transmission electron microscopy (STEM) makes it possible to detect compositional and spectroscopic changes at the atomic level that can be used to understand the structure, and ultimately the performance of materials. Here we present some examples of quantitative STEM and EELS as applied to the study of graphene-based materials, complex nanoparticles used in electrocatalysts for fuel cells, group IIInitride nanowires used for light emitting devices, and the defects generated in implanted Si.

High-Resolution STEM and EELS as Tools to Study Structure and Bonding of Oxides

Due to the developments of aberration correctors, bright electron sources, stable microscopes and electron monochromators, electron microscopy has dramatically evolved in the recent years. The current microscopes provide structural, chemical and spectroscopic information with sub-angstrom resolution and with synchrotron-quality spectroscopic performance ranging from the mid-infrared to the hard X-ray regime. Using a combination of scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) with better than 0.1eV energy resolution (down to 10meV), we provide here a review of recent studies where EELS and STEM have allowed us to probe the structure, the local chemistry and the nature of the local electronic structure of a range of complex oxides. These studies show that it is possible to determine the location of particular atomic species used as dopants in a crystal [1], the local coordination and valence of atoms in crystals and at surfaces [2,3], and also the nature of the hybridization and valence in perovskites [4] and superconductors [5,6]. These applications show that EELS and STEM can be used to resolve ambiguities in structure refinements of oxides by deducing the site preference of transition metal atoms and their coordination. We also show that it is possible to extract valence information and localization of electron charge in a range of materials, thus providing essential information on termination at interfaces [7]. With these techniques, we explore defects in materials and the nature of the electronic structure at interfaces.