Papers

ICMCTF2006 Session F2: Applications of Analytical Electron Microscopy

Monday, May 1, 2006 2:30 PM in Room Royal Palm 1-3

Monday Afternoon

Start	Invited?	Item
2:30 PM		F2-4 Recent Progress in Analytical Electron Microscopy on Advanced Materials K. Suenaga (AIST, Japan) Advances of nanotechnology increasingly rely on the progress of electron microscopies. The spectrum imaging approach based on STEM/EELS enable us to obtain chemical map with high sensitivity and resolution, which is crucial to detect dopants or impurities in materials at atomic level (1, 2). Also a classical phase contrast with high time-resolution and high sensitivity allows visualization of point defects during formation and annihilation (3, 4). This presentation highlights our recent achievements in characteraization of advanced carbon nano-materials, involving nanotubes, nano-diamonds, and nano-peapods. Supported by the NEDO NCS project. (1) K. Suenaga, M. Tenc, C. Mory, C. Colliex, H. Kato, T. Okazaki, H. Shinohara, K. Hirahara, S. Bandow and S. Iijima, Science 290 (2000) 2280 (2) A. Hashimoto, H. Yorimitsu, K. Ajima, K. Suenaga, H. Isobe, J. Miyawaki, M. Yudasaka, S. Iijima and E. Nakamura, Proc. Natl. Acac. Sci., 101 (2004) 8527 (3) A. Hashimoto, K. Suenaga, A. Gloter, K. Urita and S. Iijima, Nature 430 (2004) 870 (4) K. Urita, K. Suenaga, T. Sugai, H. Shinohara and S. Iijima, Phys. Rev. Lett., 94 (2005) 155502.
3:10 PM		F2-6 Nanometer-Sized Phase Separation in La_1-xCa_xMnO₃ at 0.33≤x≤0.5 J. Tao, M. Varela, S. Pennycook (Oak Ridge National Laboratory); D. Niebieskikwiat, M.B. Salamon, J.M. Zuo (University of Illinois at Urbana-Champaign); W.D. Luo, S. Pantelides (Vanderbilt University) Colossal magnetoresistance effect (CMR) in manganites attracts considerable attentions since early 1990s. A large amount of theoretical and experimental work has been done in order to explain the CMR effect and other complex physical phenomena exhibited in manganites¹. Decades of research have yet to allow us to completely understand the full range of ordered phases that occur in the system including charge ordering (CO), orbital ordering (OO) and phase separation in variable length scales. Here we report the nanometer-sized phase separation observed in La_1-xCa_xMnO₃. Previous results initiated the picture of the competition between the charge ordered phase and ferromagnetic phase at certain temperatures in La_1-xCa_xMnO₃ at x = 0.33². Similar to what observed before but more interesting structures were revealed using quantitative electron diffraction (ED) in the doping range 0.33≤x≤0.50. Super-reflections in the ED indicate the existence of structurally modified clusters with average size about 3-4 nm. Measured wave vectors and intensities of the super-reflections vary with chemical doping and the temperature, which play important roles in the phase separation. To map the distribution of the structurally modified clusters, convergent beam electron diffraction imaging (CBEDI) was done on the sample at x = 0.45. The sizes and the density of the clusters can be quantified as the first time in real space. The CBEDI obtained at different temperatures clearly show a change of the cluster densities. Z-contrast images and the electron energy loss spectra obtained from this system will be also discussed. ¹M.B. Salamon and M. Jaime, Rev. Mod. Phys. 73, 583(2001)²J.M. Zuo and J. Tao, Phys. Rev. B 63, 060407 (2001).
3:30 PM		F2-7 Structure Characterization and Strain Relaxation Study on Si/Ge Interface H. Chen, J.M. Zuo (University of Illinois at Urbana-Champaign); S. Kim (Seoul National University, Korea) The strained Si/Ge interface is a promising structure for high-speed low-power dissipation CMOS technology because of higher carrier mobilities. In the design and fabrication of strained Si MOSFETs with a sub-micrometer size area, it becomes very important to characterize the structure accurately. The goal of our project is to combine strain-sensitive CBED (Convergent Beam Electron Diffraction) with Z-contrast STEM (Scanning Tranmission Electron Diffraction) to observe the strain-induced change in lattice parameter of Si/Ge interface. In this research project, we prepared Si/Ge interface by CVD growth of Ge epitaxial wetting layer on clean Si (111) surface. We were able to control the thickness of Ge layer by controlling the growth temperature. Lattice parameters of these Ge layers were different depending of their thickness. Ag nanowires of various morphologies were epitaxially grown on these surfaces. Z-contrast STEM was used to measure the thicknesses of Ge layers, and the structure of Ge layers and Ag nanowires was characterized by electron diffraction and high-resolution TEM imaging. Si/Si-Ge interface was studied by CBED and EDX in STEM mode. We have observed the strain and relaxation fields in Si-Ge and interface area. Also, the lattice parameter along with Ge atomic percentage measured from CBED was correlated with the strain and relaxation fields.
3:50 PM		F2-8 Materials Characterization Using Electron Optical Aberration-Corrected Technology P.O.Å. Persson, B. Freitag (FEI Company, Netherlands) With the decreasing dimensions in nano-technology, the transmission electron microscope (TEM) has become an indispensable tool for characterization on the sub nm scale. Traditionally, the fundamental restriction on (S)TEM resolution is the spherical aberration coefficient of the objective lens. With the addition of an image Cs-corrector to a microscope, the point-resolution which is typically about 0.2 nm for an intermediate accelerating voltage TEM is pushed to the information limit which is around 0.10-0.14 nm for a FEG system. For a STEM probe Cs-corrected system the benefit is either more probecurrent for EDX and EELS analysis with the typical STEM spatial resolution of 0.14-0.2 nm or a decreased probe size for sub-Angstrom imaging. This revolutionarily improvement is a significant step forward in nano-characterization. Apart from detection and stability, the energy resolution using electron energy loss spectroscopy (EELS) is determined mainly by the energy spread of the electron source. An electron monochromator, which filters the electrons from the source may increase the energy resolution from a typical value of 0.7 eV to 0.1-0.2 eV which is of particular use for chemical and valence electron analysis with EELS. Transmission electron microscopy with Cs-corrector(s) in combination with a monochromator is rapidly becoming the new trend in TEM instrumentation for advanced nano-characterization but this is also pushing the requirements on overall instrument stability. Application examples in various fields are given, to show the strength of using the new generation of aberration-corrected transmission electron microscopes.
4:10 PM		F2-9 Atomic Characterization of Doped Grain Boundaries M.F. Chisholm (Oak Ridge National Laboratory) The properties of interfaces depend upon the details of their composition down to the level of single atoms. Recent developments have pushed the achievable spatial resolution for imaging and electron energy-loss spectroscopy into the sub-Angstrom regime. The combination of these two state-of-the-art atomic characterization techniques with ab initio theoretical materials simulations are used to investigate the atomic configurations and electronic structure of grain boundaries with and without segregated dopants. Only with this combination of methods are we able to observe the distribution of dopants in the boundary and detect changes in the electronic structure produced by the impurities. Three different systems will be discussed (Cu, Ni₃Al, and Al) in which the properties of the grain boundaries are dramatically influenced by dopant segregation. These three systems will be shown to exhibit three different reactions to the impurity.
4:50 PM		F2-11 HR-TEM Analysis of the Junction Structures in Carbon Nanotubes with Mixed Chirality T. Suzuki (Keio University, Japan); Y. Sato (National Institute of Advanced Industrial Science and Technology, Japan); K. Suenaga (AIST, Japan); Sumio Iijima (National Institute of Advanced Industrial Science and Technology, Japan); H. Wakabayashi (Research Center for Advanced Carbon Materials, Japan) It is well-known that the transport properties of a carbon nanotube(CNT) are essentially determined by its chiral index (n,m). Besides the chiral index, the atomic defects must have also strong influence on the physical properties of CNTs as they generally do for the other materials. Especially a topological defect in CNT involves a serial junction of two constituent nanotubes with different chiral indices (n1,m1) and (n2,m2), and therefore is quite important to be studied towards the realization of functional nano devices based on CNTs. There have been, however, very few researches reported for the defective CNTs. We demonstrate here a HR-TEM study of such a nanotube junction intentionally induced by a simple heat treatment and try to clarify its atomic structures. The CNTs with mixed chirality involving a serial and parallel junction have been prepared by heating the commercially available SWNTs (HIPco) at 1273~2273K in vacuum (10-2 Pa) for 5 hours. Thus obtained complex CNTs were fixed on microscopy grid with holey carbon mesh and examined by a JEOL-2010F HR-TEM operated at 120keV. Analysis of the lattice fringes corresponding to the zig-zag chain of graphen network around the defects associated with image simulation of modeled defect structure will be shown to elucidate the real atomic arrangements near the junction. This work is partially supported by the NEDO NCT project.