Z-contrast STEM imaging of long-range ordered structures in epitaxially grown CoPt nanoparticles

We report on atomic structure imaging of epitaxial L₀CoPt nanoparticles using chemically sensitive high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Highly ordered nanoparticles formed by annealing at 973 K show single-variant structure with perpendicular c-axis orientation, while multivariant ordered domains are frequently observed for specimens annealed at 873 K. It was found that the (001) facets of the multivariant particles are terminated by Co atoms rather than by Pt, presumably due to the intermediate stage of atomic ordering. Coexistence of single-variant particles and multivariant particles in the same specimen film suggests that the interfacial energy between variant domains be small enough to form such structural domains in a nanoparticle as small as 4 nm in diameter. © 2013 Kazuhisa Sato et al.Kazuhisa Sato, Keigo Yanajima and Toyohiko J. Konno, Z-Contrast STEM Imaging of Long-Range Ordered Structures in Epitaxially Grown CoPt Nanoparticles, Journal of Nanomaterials, 2013, 679638

Three-dimensional shapes and distribution of FePd nanoparticles observed by electron tomography using high-angle annular dark-field scanning transmission electron microscopy

We have studied three-dimensional shapes and distribution of FePd nanoparticles, prepared by electron beam deposition and postdeposition annealing, by means of single-axis tilt tomography using atomic number contrasts obtained by high-angle annular dark-field scanning transmission electron microscopy. Particle size, shape, and locations were reconstructed by weighted backprojection (WBP), as well as by simultaneous iterative reconstruction technique (SIRT). We have also estimated the particle size by simple extrapolation of tilt-series original data sets, which proved to be quite powerful. The results of the two algorithms for reconstruction have been compared quantitatively with those obtained by the extrapolation method and those independently reported by electron holography. It was found that the reconstructed intensity map by WBP contains a small amount of dotlike artifacts, which do not exist in the results by SIRT, and that the particle surface obtained by WBP is rougher than that by SIRT. We demonstrate, on the other hand, that WBP yields a better estimation of the particle size in the z direction than SIRT does, most likely due to the presence of a "missing wedge" in the original data set. © 2010 American Institute of Physics.Kazuhisa Sato, Kenta Aoyagi, and Toyohiko J. Konno, "Three-dimensional shapes and distribution of FePd nanoparticles observed by electron tomography using high-angle annular dark-field scanning transmission electron microscopy", Journal of Applied Physics 107, 024304 (2010) https://doi.org/10.1063/1.3280026

Low-temperature synthesis of oriented CoPtCu-MgO and CoFePt-Ag-SiO₂ nanocomposite thin films by rf-magnetron sputtering

Nanocomposite thin films composed of oriented Co₅₀Pt₄₄Cu₆ nanoparticles embedded in a MgO matrix has been synthesized by rf-magnetron co-sputtering onto NaCl(001) substrates kept at 620 K. As the sputtering power increases, (001) orientation is improved and atomic ordering is also promoted. However, superlattice reflections are quite weak and the ordered region is limited in local area of the nanoparticles, indicating low degree of order. Electron diffraction and elemental mapping revealed that additive Cu is alloyed with CoPt. We have also fabricated (Co₂₆Fe₂₀)Pt₄₄Ag₁₀SiO₂ nanocomposite thin films at 675K for comparison. Atomic ordering is promoted by Fe and Ag addition as well as higher substrate temperature; however, it was found that ternary element (Cu, Ag, Fe) addition into CoPt alloy is not so effective to promote atomic ordering. Origin of the slow ordering kinetics is discussed using thermodynamical parameters. © 2014 The Ceramic Society of Japan. All rights reserved.Kazuhisa SATO, Tamotsu KOSAKA, Toyohiko J. KONNO, Low-temperature synthesis of oriented CoPtCu–MgO and CoFePt–Ag–SiO₂ nanocomposite thin films by rf-magnetron sputtering, Journal of the Ceramic Society of Japan, 2014, 122, 1425, p. 317-321

Atomic structure imaging of L1₀ -type FePd nanoparticles by spherical aberration corrected high-resolution transmission electron microscopy

The atomic structure of FePd nanoparticles has been studied by spherical aberration (Cs) corrected high-resolution transmission electron microscopy. The periodic arrangement of atoms arising from chemical order is clearly seen as bright contrast due to the small negative value of corrected Cs. The amount of optimal defocus (Scherzer defocus) is markedly reduced by the small Cs value. The interface between crystalline particles and the amorphous matrix can also be observed, free of imaging artifacts, at a small defocus value. The reconstructed phase image directly shows the projected potential distribution within the specimen and reveals the elemental differences due to chemical order. The clear-cut long-range order is lost when particle size is smaller than about 5 nm, at which locally ordered mixed-phase particles begin to dominate. © 2009 American Institute of Physics.Kazuhisa Sato, Toyohiko J. Konno, and Yoshihiko Hirotsu, "Atomic structure imaging of L1₀-type FePd nanoparticles by spherical aberration corrected high-resolution transmission electron microscopy", Journal of Applied Physics 105, 034308 (2009) https://doi.org/10.1063/1.3074505

Three-dimensional imaging of dislocations in a Ti-35mass%Nb alloy by electron tomography

We have studied three-dimensional (3D) configurations of dislocations in the β phase of a Ti-35mass%Nb alloy by means of single-axis tilt tomography using bright-field scanning transmission electron microscopy (BF-STEM). To visualize dislocations, the hh0 systematic reflections were excited throughout tilt-series acquisition with the maximum tilt angle of 70°. Dislocations in the β grains were clearly reconstructed by the weighted back-projection algorithm. The slip planes of the dislocations were deduced by rotating the reconstructed volumes with the aid of selected area electron diffraction patterns. It was found that BF-STEM images with relatively low contrasts, taken along low-order zone axes, are capable to reproduce and preserve the quality of reconstructed image of dislocations. We also found that tilt angles as low as 40° are practically acceptable to visualize 3D configurations of dislocations, while there exists limitation in resolution due to the existence of a large missing wedge.Sato K, Semboshi S, Konno TJ. Three-Dimensional Imaging of Dislocations in a Ti–35mass%Nb Alloy by Electron Tomography. Materials. 2015; 8(4):1924-1933. https://doi.org/10.3390/ma8041924

Surface-segregation-induced phase separation in epitaxial Au/Co nanoparticles: Formation and stability of core-shell structures

We have studied formation and stability of core-shell structures in epitaxial Au/Co nanoparticles (NPs) by using atomic-resolution scanning transmission electron microscopy. As the particle size reduces, number of NPs having Au-shell increases and their frequency of occurrence reached 65%. Au segregation proceeds during particle growth at 520 K. The core-shell structure formation is particle size-dependent; the critical diameter dividing the Au-shell and the Co-shell structures is about 11 nm, below which the Au-shell is stable. After annealing at 800 K for 3.6 ks, Au-shell NPs were conserved while the Co-shell NPs changed to two-phase structures with a planar interface separating Au and Co. There is a local energy minimum where the Co-shell NP is metastable in the as-deposited state. A simple model based on surface and interfacial energies suggests stability of Au-shell structures. Surface-segregation-induced phase separation in small NPs, due to low surface free energy of Au, will be responsible for the Au-shell formation.Kazuhisa Sato, Yuta Matsushima, and Toyohiko J. Konno, "Surface-segregation-induced phase separation in epitaxial Au/Co nanoparticles: Formation and stability of core-shell structures", AIP Advances 7, 065309 (2017)

Aging effect on microstructure of cold groove-rolled α′-type Ti-12 mass%V-2 mass%Al alloys studied by transmission electron microscopy

Microstructure and phase decomposition of hexagonal α′ martensite in cold groove rolled (CGR) Ti-12 mass% V-2 mass% Al alloys have been studied by transmission electron microscopy and electron diffraction. Acicular structure of the α′ martensite changes into equiaxed α′ grains by CGR with a cold reduction of 75%. After aging at 573K for 500 h, local Moiré fringes disappear and diffraction rings become sharp due to recovery. However, Vickers hardness also increased in spite of the recovery. Partitioning of solute atoms and/or formation of fine precipitates can be possible cause for the observed age-hardening. Although a CGR alloy is characterized by a microstructure with polycrystalline equiaxed grains, following orientation relationship is locally observed between β precipitates and α grains after aging at 673 K: (101)β // (011̄1̄)α, [010]β // (101̄1)α. Electron tomography revealed a heterogeneous nucleation and growth of β precipitates in the deformed α′ matrix. © 2014 The Japan Institute of Metals and Materials