Volume 96, Issue 5

The increasing role of interfaces with decreasing scale is particularly important for small inclusions, grains or precipitates. In this work, we present a brief review of some of our recent TEM investigations of faceted singular interfaces and interface junctions in crystals. Direct observations by high-resolution electron microscopy allow us to relate the size, shape, and local interface structure to the behavior of nanoscale inclusions. The effect of facet junctions on equilibrium shapes of inclusions, on the structure of grain boundary grooves at surfaces and on grain-boundaries in anisotropic bicrystals are illustrated with examples from recent work by this group.

This brief review describes the crystallisation behaviour of selected lanthanide halides (LnX 3 , with Ln = La, Ce, Gd and X = Cl, I), confined to the nanometre-sized inner cavity of single-walled carbon nanotubes. The resulting LnX 3 @SWNT encapsulation composites were characterised using conventional high-resolution transmission electron microscopy techniques, combined with advanced digital image restoration approaches, enabling the complete structural determination of both the encapsulated crystal fragment and the surrounding nanotube. The studies reveal that some encapsulated 1-dimensional lanthanide halide crystal fragments maintain the overall structure of the respective bulk material, while others exhibit unprecedented crystal structure.

We report on the growth of silicon nanowires by means of chemical vapour deposition in ultrahigh vacuum environment using different catalyst materials. Epitaxial growth via the vapour-liquid-solid growth mechanism of silicon nanowires is achieved with gold as catalyst on a (111)-oriented hydrogen-terminated silicon wafer. The resulting wires are mostly (111) oriented. Additionally, iron and dysprosium have been successfully tested as catalyst material for nanowire growth below the eutectic temperature. The nanowires grown with Fe or Dy exhibit a transition from epitaxial growth to polycrystalline growth with increasing height.

Electron energy-loss spectroscopic profiling is a hybrid method between imaging and spectroscopy. It can be applied in a transmission electron microscope with an imaging energy filter operated in spectroscopy mode such that an image is recorded on a two-dimensional detector with an interface parallel to the energy-dispersive direction. This reveals directly, i. e., in one exposure, how a specific energy-loss near-edge structure varies across the interface. The aim of this work is to describe a method called stripeTEM to extract quantitative chemical profiles from spectroscopic imaging, discuss practical implementations, compare the method to more widely used nano-analytical techniques, and to present some recent applications to materials science. Measurements of diffusion and segregation in planar thin films as well as the observation of core-shell structures of nano-particles are reviewed.

We summarize our experimental investigation of the morphology and orientation of Ag nanoclusters on silicon surfaces formed by room-temperature (RT) deposition and subsequent annealing. Using Ag as a model system, we show that nanocluster interfaces and epitaxy depend critically on the cluster size and the substrate lattice. In case of Ag on hydrogen-terminated Si (111), the RT self-assembled clusters grow as mostly single-crystal crystallites with Ag (111)//Si (111), but the in-plane orientation has a dispersion of ~ 9° centering at Si [110] direction. Upon annealing, the Ag clusters drastically rotated to the epitaxial configuration with the in-plane orientation aligned to the Ag [110]//Si [110] direction. The rotation and epitaxy of the Ag nanoclusters are explained based on a coincident site lattice model and interface energy minimization.

The advantage of Z-contrast STEM over high-resolution phase contrast TEM for measuring segregation of (heavy) impurity atoms to grain boundaries is its increased sensitivity to atomic number. However, varying occupancy of segregation sites, insufficient imaging resolution, may make it very difficult to determine the exact position of the segregation site, especially in cases where the resolution does not suffice to resolve atomic columns in the crystal structure of the grains while the boundary is oriented parallel to the electron beam. We will show that, given the proper experimental conditions and due to the absence of lens aberrations in diffraction data, the Fourier transform of a selected-area electron diffraction pattern from the boundary may determine the segregation site with very high accuracy.

Misfit dislocations in epitaxial Pb(Zr 0.52 Ti 0.48 )O 3 (PZT) nanoislands on SrTiO 3 (001) substrates are shown to involve strain fields that distort the PZT lattice in a tube of material with a cross-section of about 4 nm × 8 nm around each misfit dislocation. As a consequence, 10 nm thin nanoislands – around half of their volume being distorted – are not ferroelectric. This finding corresponds to an impact of the interfacial dislocations on the ferroelectric size effect. Moreover, an interface-related, size-dependent imprint effect – i. e., a vertical shift of the ferroelectric hysteresis loop – was observed in polycrystalline PZT and epitaxial BaTiO 3 nanoislands. In result, interfacial engineering is indispensable to obtain nanostructured ferroelectrics with stable polarization and switching properties.

Using in situ high-resolution transmission electron microscopy (HRTEM), we examine the formation and time evolution of the TiO islands on the SrTiO 3 (001) and (011) surfaces at 970 °C. Titanium monoxide (TiO) islands are observed to form mainly on the SrTiO 3 (001) surface but rarely on the (011) surface, which indicates an interface energy anisotropy with interface plane orientations.

The processes of mechanical stress relaxation in lattice-mismatched heteroepitaxial films usually proceed via misfit dislocation formation at the film/substrate interface and are typically accompanied by the generation of a high density of threading dislocations in the bulk of the film. Threading dislocations are deleterious for a wide variety of modern electronic and optoelectronic devices. Relaxation in stressed epitaxial films also gives rise to the characteristic cross-hatch surface morphology of the growing heterostructures. This article presents a short review of recent mesoscopic models describing the evolution of dislocation structures in heteroepitaxial films. Dislocation-assisted crosshatch formation in (001)-oriented fcc film is explained theoretically and discussed on the basis of experimental results. A specific stress relaxation mechanism via threading dislocation inclination in (0001)-oriented wurtzite crystal structure films is considered and analyzed in detail. Finally, a ‘reaction-kinetics’ approach to threading dislocation reduction in growing films is developed.

In the present study, the effect of mechanical stresses on grain growth was studied. The change of the grain-growth kinetics as well as the microstructure evolution in polycrystalline high-purity aluminium under the influence of different mechanical stress fields were investigated. It will be shown that small mechanical stresses can slow down the grain growth kinetics. A model will be introduced which can explain the experimental observations. Especially in the case of small grain sizes, where grain boundaries play a major role for the microstructure evolution and the properties of the material, the possibility to influence the kinetic behaviour of grain boundaries by mechanical stress fields sheds new light on microstructure control and materials design.

Nanocrystalline structures have attracted increasing interest in modern development of hard films for wear-resistant applications. In plasma-assisted vapor-deposited thin films, nanostructures can be generated during growth or post-annealing treatment. In this review, we demonstrate, using the four model-films overstoichiometric TiB 2 , TiN – TiB 2 , Ti(B)N, and Ti 0.33 Al 0.67 N, how interfaces influence the mechanical properties of ceramic thin films. For overstoichiometric TiB 2 films and TiN–TiB 2 , a two-dimensional and three-dimensional nanostructure, respectively, is generated during growth by segregation-driven processes. Growth of less B containing Ti(B)N films and Ti 0.33 Al 0.67 N results in the formation of a supersaturated TiN phase with NaCl structure, where either the non-metallic part or the metallic part is substituted by B or Al, respectively. For these films, the nanostructure forms during post-annealing, due to the decomposition of the supersaturated phases into their stable constituents. As the hardness of a material is determined by resistance to bond distortion and, dislocation formation and motion, which themselves depend on the amount of interfaces and their constitution, there is a direct relation between hardness and nanostructure.

The temporal evolution of γ′(L1 2 )-precipitates in Ni-5.2 Al-14.2 Cr at.% and Ni-9.5 Al-8.6 Cr-2.0 Re at.% alloys are studied utilizing three-dimensional atom-probe microscopy. For the times investigated (0.0833 to 1024 h), these alloys exhibit a high number density (10 22 –10 24 m –3 ) of spheroidal γ′-precipitates, 0.45 –30 nm in radius. For the Ni 5.2 Al-14.2 Cr at.% alloy, nucleation, growth, and coarsening of the γ′-precipitates is observed during the isothermal transformation at 873 K. Precipitate coalescence is present, and its influence on the power-law dependencies of the number density and mean radius during coarsening is discussed and compared to classical models. For Ni-9.5 Al-8.6 Cr-2.0 Re at.%, phase separation occurs during the initial quench leading to a highly interconnected γ′-nanostructure. The temporal evolution of the elemental partitioning behavior at 1073 K between the γ-matrix and γ′-precipitates is studied in detail.

Distribution of TiO 2 dopants and SiO 2 impurities in the bimodal microstructure of Al 2 O 3 with anisotropic and equiaxed grains is systematically analyzed using analytical electron microscopy (AEM). The TiO 2 -doped ceramic materials were hot-pressed at 1500 °C in a reducing environment. Different amounts of Ti solutes in the anisotropic or equiaxed grains were observed after removal of the contamination signal stemming from Ti on the surface. SiO 2 and TiO 2 exhibit a selective segregation behavior. The boundary between the equiaxed grains is segregated mainly by TiO 2 but the boundary at the (0001) basal plane of anisotropic grains is covered with a thin amorphous film made up of mostly SiO 2 . Precipitation of Al 2 TiO 5 occurs at high TiO 2 doping levels. A bimodal microstructure develops in three stages, characterized successively by segregation, solution, and precipitation. The preferential adsorption of SiO 2 to the (0001) basal plane initiates the anisotropic grain growth, starting at low TiO 2 doping level. At higher TiO 2 doping level, bi-level Ti solution occurs, either as a result of equilibration between segregants and solutes, or incorporated as transient Ti solutes in the anisotropic grains due to fast-moving fronts. Further doping starts Al 2 TiO 5 precipitation, which may result in de-wetting of the basal boundary, possibly due to a change of interface energy. The correlation and competition between segregation, solution, and precipitation characterize and dictate the evolution of microstructure, as monitored by the aspect ratio of anisotropic grains.

The development of nanostructured coatings with increased hardness has drawn significant attention since the beginning of the nineties due to their combination of technologically important properties such as high hardness and good oxidation resistance in the case of TiN/Si 3 N 4 and high hardness and low friction of TiC/a-C : H. The large internal surfaces in nanocomposite materials, which exist due to the small crystallite size of a few nanometers, play a dominant role for the macroscopic properties of these materials. The present extended abstract shortly addresses the key role of the interfaces between the nanocrystalline phase and the amorphous phase using two of the best known nanocomposite (nc) thin film materials, nc-TiN/a-Si 3 N 4 and nc-TiC/a-C : H.

The microstructures of liquid-phase sintered SiC with AlN℃Y 2 O 3 additives are systematically investigated by using transmission electron microscopy and analytical electron microscopy. Pure α-SiC as starting powder leads to fine, equiaxied microstructure. Introduction of α-SiC seed crystals into β-SiC powder accelerates the β-to-α-SiC phase transformation through a solution-precipitation process and promotes anisotropic grain growth, which results in a plate-like microstructure. Core/rim structures were found in both cases as a result of AlN dissolution into the re-precipitated part of SiC grains. This changes the liquid composition during sintering and induces crystallization of Y 10 Al 2 Si 3 O 18 N 4 and Y 2 O 3 in the triple-pockets. Amorphous films were observed to wet both grain boundaries and two-phase interfaces. A low ratio of AlN to Y 2 O 3 in the sintering additive accelerates the devitrification of triple-pockets. Additional annealing can further devitrify the triple-pockets as well as the amorphous GB films, leading to a microstructure with potentially higher creep resistance.

In the present paper, calcium deficient hydroxyapatite (Ca-def OHAP) has been examined in a transmission electron microscope. During the observation, the initial Ca-def OHAP was seen to transform first into a transient intermediate phase, then to a final, stable phase. The intermediate phase is most likely monoclinic and is commensurate with the original Ca-def OHAP phase. Computer images calculated for the hydroxyapatite (OHAP) structure show a structural identity with the initial Ca-def OHAP material. No structural match was obtained for the final and intermediate phases. To our knowledge, this is the first time an intermediate phase is observed during the transformation of Ca-def OHAP. Furthermore, the phenomena observed in this paper are specific to Ca-def OHAP as the damage processes. Under electron irradiation on human tooth enamel crystals or on OHAP ceramics are markedly different from the ones observed here. This is probably indicative of specific structural features of Ca-def OHAP.

Experimental tests for zirconia/nickel composites determined lower toughness than expected. In this regard, the combination of ab initio density functional theory calculations and high-resolution transmission electron microscopy observations allowed to describe the nature of the zirconia/ nickel interface and to justify the absence of any toughening mechanism by grain bridging of the metal particles. In the particular case of nanocomposites, enhanced values of hardness were found. Hardness increases have been justified by a novel model which takes into account the Hall – Petch effect and mean particle size according to the percolation theory. It has been found that this model is not exclusive to zirconia/nickel but can be extended to several other superhard composites.