Kui Du

Discrete plasticity in sub-10-nm-sized gold crystals

Although deformation processes in submicron-sized metallic crystals are well documented, the direct observation of deformation mechanisms in crystals with dimensions below the sub-10-nm range is currently lacking. Here, through in situ high-resolution transmission electron microscopy (HRTEM) observations, we show that (1) in sharp contrast to what happens in bulk materials, in which plasticity is mediated by dislocation emission from Frank-Read sources and multiplication, partial dislocations emitted from free surfaces dominate the deformation of gold (Au) nanocrystals; (2) the crystallographic orientation (Schmid factor) is not the only factor in determining the deformation mechanism of nanometre-sized Au; and (3) the Au nanocrystal exhibits a phase transformation from a face-centered cubic to a body-centered tetragonal structure after failure. These findings provide direct experimental evidence for the vast amount of theoretical modelling on the deformation mechanisms of nanomaterials that have appeared in recent years.

A modified sol-gel process for multiferroic nanocomposite films

Multiferroic CoFe2O4–Pb(Zr,Ti)O3 (CFO-PZT) composite films with nanoscale mixture of the two phases were prepared by a modified sol-gel process, in which a mixed precursor solution of both CFO and PZT was used. X-ray diffraction and transmission electron microscopy examinations revealed the coexistence of perovskite PZT and spinel CFO that were mixed in nanoscale with mean grain sizes of 5–10nm. Magnetic properties of the CFO-PZT nanocomposite were examined, which were consistent with their microstructures. The magnetoelectric coupling between CFO and PZT was demonstrated by an external magnetic field induced electric polarization change. This modified sol-gel processing provides an alternative for multiferroic composite films, which is simpler and easier to control compared to the conventional layer-layer sol-gel process for multiferroic composite films.

Effects of oxygen vacancies on the electrochemical performance of tin oxide

Using an aberration-corrected transmission electron microscope, we observed the oxygen vacancies, profiled the concentration in the SnO2−δ nanocrystals on an atomic scale, and estimated the amount of oxygen vacancies to be ca. 3.3 atom%. The SnO2−δ nanocrystals show much improved initial Coulombic efficiency, rate capability and specific capacity compared with stoichiometric SnO2 when used as an anode material for lithium ion batteries.

Interstitial-boron solution strengthened WB3+x

By means of variable-composition evolutionary algorithm coupled with density functional theory and in combination with aberration-corrected high-resolution transmission electron microscopy experiments, we have studied and characterized the composition, structure, and hardness properties of WB3+x (x < 0.5). We provide robust evidence for the occurrence of stoichiometric WB3 and non-stoichiometric WB3+x, both crystallizing in the metastable hP16 (P63/mmc) structure. No signs for the formation of the highly debated WB4 (both hP20 and hP10) phases were found. Our results rationalize the seemingly contradictory high-pressure experimental findings and suggest that the interstitial boron atom is located in the tungsten layer and vertically interconnect with four boron atoms, thus forming a typical three-center boron net with the upper and lower boron layers in a three-dimensional covalent network, which thereby strengthen the hardness.

Transition of dislocation nucleation induced by local stress concentration in nanotwinned copper

Metals with a high density of nanometre-scale twins have demonstrated simultaneous high strength and good ductility, attributed to the interaction between lattice dislocations and twin boundaries. Maximum strength was observed at a critical twin lamella spacing (∼15 nm) by mechanical testing; hence, an explanation of how twin lamella spacing influences dislocation behaviours is desired. Here, we report a transition of dislocation nucleation from steps on the twin boundaries to twin boundary/grain boundary junctions at a critical twin lamella spacing (12–37 nm), observed with in situ transmission electron microscopy. The local stress concentrations vary significantly with twin lamella spacing, thus resulting in a critical twin lamella spacing (∼18 nm) for the transition of dislocation nucleation. This agrees quantitatively with the mechanical test. These results demonstrate that by quantitatively analysing local stress concentrations, a direct relationship can be resolved between the microscopic dislocation activities and macroscopic mechanical properties of nanotwinned metals.

Deformation-induced structural transition in body-centred cubic molybdenum

Molybdenum is a refractory metal that is stable in a body-centred cubic structure at all temperatures before melting. Plastic deformation via structural transitions has never been reported for pure molybdenum, while transformation coupled with plasticity is well known for many alloys and ceramics. Here we demonstrate a structural transformation accompanied by shear deformation from an original <001>-oriented body-centred cubic structure to a <110>-oriented face-centred cubic lattice, captured at crack tips during the straining of molybdenum inside a transmission electron microscope at room temperature. The face-centred cubic domains then revert into <111>-oriented body-centred cubic domains, equivalent to a lattice rotation of 54.7°, and ~15.4% tensile strain is reached. The face-centred cubic structure appears to be a well-defined metastable state, as evidenced by scanning transmission electron microscopy and nanodiffraction, the Nishiyama–Wassermann and Kurdjumov–Sachs relationships between the face-centred cubic and body-centred cubic structures and molecular dynamics simulations. Our findings reveal a deformation mechanism for elemental metals under high-stress deformation conditions.

Correlations between structural and optical properties of GaInNAs quantum wells grown by MBE

Abstract In this work, the structural properties of Ga1−xInxNyAs1−y (GINA) quantum wells (QW) are investigated in terms of interface roughness and chemical composition variations by using conventional and high-resolution transmission electron microscopy (HRTEM). The structural behaviors of these heterostructures are systematically compared with the optical properties investigated by photoluminescence (PL). Our results demonstrate that high PL efficiency of GINA material, irrespective of composition, can be obtained only when the epitaxy is performed under conditions preserving a 2D growth mode. Moreover, composition variations are shown at a local scale using HRTEM and strain mapping.

On the accuracy of lattice-distortion analysis directly from high-resolution transmission electron micrographs.

Lattice‐distortion analysis from high‐resolution transmission electron micrographs offers a convenient and fast tool for direct measurement of strains in materials over a large area. In the present work, we have evaluated the accuracy of the strain measurement when the effects of the realistic experimental variables are explicitly taken into account by the use of image simulation techniques. These variables are focal setting and variation, local thickness and orientation of the sample, as well as misalignments of the sample and the incident beam. The evaluation reveals that consistency of image features and contrast within the micrographs is desired for the analysis to eliminate effects of the variations of local focus value and specimen thickness. After proper orientation of a crystalline specimen, the misorientation of the object will not notably influence the strain measurement even though a local bending may exist within the sample. However, the incident beam of the microscope needs to be aligned carefully as the beam misalignment may introduce a notable artefact around the interface region.

Quantitative assessment of nanoparticle size distributions from HRTEM images Dedicated to Professor Dr. Knut Urban on the occasion of his 65th birthday

Abstract Aiming to determine the size distribution and surface area per unit mass of catalyst nanoparticles for fuel cells, we have developed a novel method of digital image processing that quantitatively determines the size of individual nanoparticles from atomic-resolution transmission electron microscopy images. After introducing the basic concept of this new method, we demonstrate its efficiency using platinum nanoparticles on carbon support as a model system. Special attention is given to the error that can be introduced by estimating the surface area per unit mass from the distribution of particle diameters in the presence of asymmetry of the particle size about the mean.

Formation of nickel nanoparticles in nickel – ceramic anodes during operation of solid-oxide fuel cells

Abstract The microstructure of solid-oxide fuel cell anodes, consisting of nickel particles embedded in either doped zirconia or ceria, was characterized in three different stages: (i) as sintered, (ii) after reduction, and (iii) after fuel cell operation for 500 h in an atmosphere of moist hydrogen. After 500 h of operation, we found that both types of anodes, those based on zirconia and those based on ceria, contained nanoparticles of elemental nickel. We propose that these nanoparticles form from nickel hydroxide, synthesized during fuel cell operation and precipitating elemental nickel where the partial pressure of water is low at the periphery of triple-phase boundaries or on cooling down from the operating temperature to room temperature.