ICMCTF2007 Session F4: Applications of Analytical Electron Microscopy

The success of aberration correction in the scanning transmission electron microscope (STEM) has pushed the achievable spatial resolution and the sensitivity for imaging and spectroscopy into the sub-angstrom regime, providing a new level of insight into the structure/property relations of complex materials. This level of sensitivity allows us to analyze in great detail the crystal and electronic structures of complex oxides at the atomic scale. Complex oxides with perovskite structure are extremely interesting materials which exhibit an array of exciting physical behaviors including ferroelectricity, colossal magnetoresistance and high Tc superconductivity. In particular, complex oxide thin films are of wide interest to the electronics industry and the emerging field of spintronics. However epitaxial complex oxide ultrathin films and heterostructures can be significantly affected or even dominated by the presence of interfaces and may exhibit intriguing new physical properties quite different from the bulk. In this respect, the combination of STEM and electron energy loss spectroscopy (EELS) is a unique tool that allows simultaneous characterization of the chemistry, structure and also the electronic properties down to the single atom level. This work will present several examples of atomic resolution studies of the relationship between structure and electronic properties of complex oxide thin films and interfaces, with complementary density-functional calculations. Materials examples will include charge transfer across manganite interfaces and hole localization in high Tc superconductors.

Research sponsored by DOE Basic Energy Sciences, Division of Materials Science and Engineering and the Laboratory Directed Research and Development Program of ORNL, managed by UT-Batelle, LLC, for the U.S. Department of Energy under Contract No. DE-AC05-00OR22725.

The colossal magnetoresistance (CMR) effect was found in manganites in THE early 1990s and ever since has constituted a major challenge in our understanding of solid state physics. A large number of reports have shown evidence for the existence of inhomogeneous electronic phases (ferromagnetic metallic, charge ordered insulating, paramagnetic etc) in this system. Most of these studies have been performed by tuning different external conditions, such as the doping, temperature, magnetic field and pressure. Complex phase separation and the ensued phase competition are believed to be the key to understanding the CMR effect. However, the lack of experimental techniques capable of looking at phase separation in bulk materials in real space especially at the nanometer length scale, has obscured our understanding of the peculiar properties in these systems.

Here we report experimental observations of the evolution of the charge ordering (CO) phase and the ferromagnetic (FM) domains in La_1-xCa_xMnO₃ (0.3 < x < 0.5) using electron nano-diffraction and Lorentz imaging. We find that the transition temperature of the observed CMR effect is different than the magnetic domain transition temperature, but coincides with the transition temperature of the nanometer sized CO phase. Our results strongly suggest that the CMR effect can not be interpreted within the picture of percolative sub-micron FM domains as proposed before [1], but it is due to the onset of percolation of the CO phase teself within the still PM host.

[1]. L. Zhang et. al., Science 298, 805 (2002).

With the advent of next generation spectrometers and energy filters, and high-brightness sources for electron microscopy, it is now possible to acquire high information density spectrum images in the same amount of time that it would have taken to acquire an individual spectrum a just few years ago. Such improvements have made these rich data collection and analysis techniques available to nearly all characterization laboratories.

In this talk we will introduce the technique of electron energy-loss spectrum imaging and discuss its advantages and disadvantage as a nanometer scale, analytic technique. We will also discuss advanced data extraction and segmentation techniques. We will close with several case studies taken from: front-end and back-end processing application as well as nano-catalyst applications.

The adhesion mitigating properties of diamond-like carbon (DLC) coatings have generated interest in using them as tool coatings for dry machining of aluminum alloys. The non-hydrogenated DLC coatings, with bonding interlayer, were deposited on M2 steel substrates using an unbalanced magnetron sputtering method. The DLC coatings were subjected to pin-on-disc tests against the aluminum pins (1100 Al and 319 Al) in N ₂ and vacuum (8.78 x 10^-3 Pa).

The cross-sections of DLC coatings with Al pieces adhered on them were cut using a focused ion beam (FIB) micro-cutting (lift-out) technique. The ion channeling contrast images revealed the microstructures of CrN and Cr-based bonding layers. The defects that formed during sliding consisted of interfacial cracks between the substrate and Cr layers that seemed to originate at the carbide phases in the substrate. CrN /M2 interfaces were more resistant to crack growth. The amorphous structure of DLC coatings with occasional graphite platelets has been confirmed by the high resolution (HREM) transmission electron microscope. The adhered Al grains (initial grain size of 30 micrometers) have assumed an equaxied nanocrystalline morphology with an average grain size of only 70 nm.