L. J. Allen

Materials Applications of Aberration-Corrected STEM

The VG Microscopes 100 kV and 300 kV scanning transmission electron microscopes at Oak Ridge National Laboratory were equipped with Nion aberration correctors several years ago. This chapter reviews our experience with these correctors, specifically, the reduction in probe size by more than a factor of two and the associated benefits for materials research, which extend far beyond improved resolution. A smaller, brighter probe brings enhanced image contrast and signal to noise ratio, making it possible to image light atom columns in materials such as oxide perovskites. It vastly increases the sensitivity to single atoms, both for imaging and electron energy loss spectroscopy. In addition, aberration correction greatly improves the collection efficiency for bright field phase contrast imaging, allowing simultaneous, aberration-corrected, Zcontrast and phase contrast imaging. Finally, the larger probe-forming aperture gives a reduced depth of field, giving a depth resolution less than the thickness of a typical specimen. It becomes possible to focus directly on features at different depths in the specimen, and three-dimensional information can be extracted with single atom sensitivity. In conjunction with density functional and elasticity theory, these advances provide a new level of insight into the atomistic origins of materials properties. Several examples are discussedmorexa0» that illustrate the potential for applications including the detection of orbital occupation stripes and interface stacking in complex oxides, the mechanism for improved critical currents in Ca-doped grain boundaries in high-Tc superconductors, the segregation of rare earth dopants in Si3N4 grain boundaries, the quantitative analysis of strain-induced growth phenomena in semiconductor quantum wells, the determination of the three-dimensional distribution of stray Hf atoms in a high dielectric constant device structure, and the origin of the remarkable catalytic activity of Au nanoparticles«xa0less

Absolute-Scale Comparison with Simulation for Quantitative Energy-Dispersive X-Ray Spectroscopy in Atomic-Resolution Scanning Transmission Electron Microscopy

1. School of Physics and Astronomy, Monash University, Melbourne, Australia. 2. School of Applied and Engineering Physics, Cornell University, Ithaca, USA. 3. Monash Centre for Electron Microscopy, Monash University, Melbourne, Australia. 4. Department of Materials Science and Engineering, Monash University, Melbourne, Australia. 5. Department of Materials Science and Engineering, North Carolina State University, Raleigh, USA. 6. School of Physics, University of Melbourne, Melbourne, Australia.

Understanding Imaging and Energy-loss Spectra Due to Phonon Excitation

The “quantum excitation of phonons” model [3] encapsulates the physics necessary to simulate the atomic resolution imaging of crystals based on phonon excitation. Figure 1(a) shows the distribution of thermally scattered electrons with a scanning transmission electron microscopy (STEM) probe placed above various columns of atoms in strontium titanate and Fig. 1(b) shows line scan images across strontium and titanium/oxygen columns in <001> strontium titanate, showing atomic resolution imaging is possible even for electrons scattered through small angles (less than ~20 mrad) [4].

Quantitative Specimen Electric Potential Maps Using Segmented and Pixel Detectors in Scanning Transmission Electron Microscopy

Research into materials for the next generation of computers, batteries and solar cells requires techniques that can characterise both the structural and functional properties of materials, often at atomic resolution. Over the past fifteen years or so, advances in scanning transmission electron microscopy (STEM) have led to a technique capable of atomic resolution imaging of the heavy and light atomic positions in a sample and elemental mapping. Recently, new developments in segmented and pixel detectors for STEM have made it possible to record more detailed information about the interaction of the electron probe with the specimen. This talk will discuss quantitative retrieval of specimen electric fields from segmented and pixel detector data. We explore two different case studies: atomic resolution imaging of monolayer MoS2 and thicker samples of the perovskite SrTiO3, and nanometre scale resolution imaging of the inbuilt electric field of a p-n junction in a GaAs semiconductor.

New Approach to Quantitative ADF STEM

High-angle annular dark-field scanning transmission electron microscopy (HAADF STEM or Z-contrast) has been shown to be remarkably sensitive to atomic number (Z). However, HAADF images are currently formed on an arbitrary intensity scale, thereby limiting the possibility of truly quantitative imaging. Recently, it was reported that a mismatch exists between experimental and simulated image contrast in HAADF STEM [1]. Without an absolute scale, it is impossible to determine the cause of the discrepancy [2]. Additionally, an absolute scale would facilitate composition mapping at atomic resolution. Here we demonstrate that the HAADF detector can measure the incident beam intensity to normalize Z-contrast images onto an absolute intensity scale. We report on a practical approach that ensures that the detector does not saturate and is sufficiently linear over the intensity range of interest. An FEI Titan 80–300 STEM/TEM equipped with a super-twin lens (Cs ∼ 1.2 mm) operating at 300 kV was used for this study.

Nanostructure Functionality through Aberration-Corrected STEM

The 300 kV VG Microscopes’ HB603U STEM at Oak Ridge National Laboratory with a Nion aberration corrector has achieved the first direct image of a crystal at sub-Angstrom resolution, using the incoherent Z-contrast or high-angle annular dark field (HAADF) mode, as shown in Fig. 1a,b. [1] To validate use of the Fourier transform to measure a resolution limit of 0.61A, Fig. 1c compares Fourier transforms of a simulated image and the probe used for the simulation. Excellent agreement is seen for a thin crystal where ideal incoherent imaging applies. Thicker crystals show reduced high frequency transfer, but no spurious sum or difference frequencies [2]. The sub-Angstrom probe allows Z-contrast imaging of oxygen columns next to heavy columns (Fig. 1d). Furthermore, an efficient, simultaneous, aberration corrected, phase contrast image is available using a small axial detector giving improved oxygen visibility, although spurious features are seen between the Sr columns (Fig. 1e). The STEM has become a viable means of acquiring aberration-corrected phase contrast images, with the advantage of simultaneous Z-contrast imaging and EELS.