Understanding Micro and Atomic Structures of Secondary Phases in Cu-Doped SnTe.

Highly efficient thermoelectric materials require, including point defects within the host matrix, secondary phases generating positive effects on lowering lattice thermal conductivity (κL ). Amongst effective dopants for a functional thermoelectric material, SnTe, Cu doping realizes the ultra-low κL approaching the SnTe amorphous limit. Such effective κL reduction is first attributed to strong phonon scattering by substitutional Cu atoms at Sn sites and interstitial defects in the host SnTe. However, other crystallographic defects in secondary phases have been unfocused. Here, this work reports micro- to atomic-scale characterization on secondary phases of Cu-doped SnTe using advanced microscopes. It is found that Cu-rich secondary phases begin precipitation ≈1.7 at% Cu (x = 0.034 where Sn1- x Cux Te). The Cu-rich secondary phases encapsulate two distinct solids: Cu2 SnTe3 ( F 4 ¯ 3 m $F\bar{4}3m$ ) has semi-coherent interfaces with SnTe ( F m 3 ¯ m $Fm\bar{3}{\rm{m}}$ ) such that they minimize lattice mismatch to favor the thermoelectric transport; the other resembles a stoichiometric Cu2 Te model, yet is so meta-stable that it demonstrates not only various defects such as dislocation cores and ordered/disordered Cu vacancies, but also dynamic grain-boundary migration with heating and a subsequent phase transition ≈350 °C. The atomic-scale analysis on the Cu-rich secondary phases offers viable strategies for reducing κL through Cu addition to SnTe.

Singular behaviour of atomic ordering in Pt-Co nanocubes starting from core-shell configurations.

The ordered structure of platinum-cobalt (Pt-Co) alloy nanoparticles has been studied actively because the structure influences their magnetic and catalytic properties. On the Pt-Co alloy's surface, Pt atoms preferentially segregate during annealing to reduce the surface energy. Such surface segregation has been shown to promote the formation of an ordered structure near the surface of Pt-Co thin films. Although this phenomenon seems also useful to control the nanoparticle structure, this has not been observed. Here, we have studied the ordered structure in annealed Pt@Co core-shell nanoparticles using a scanning transmission electron microscope. The nanoparticles were chemically synthesized, and their structural changes after annealing at 600 °C, 700 °C, and 800 °C for 3 h were observed. After being annealed at 600 °C and 800 °C, the particles contained the L12-Pt3Co ordered structure. The structure seems reasonable considering an initial Pt : Co ratio of ∼4 : 1. However, we found that the L10-PtCo structure was formed near the nanoparticle surface after annealing at 700 °C. The L10-PtCo structure was thought to be formed from the surface segregation of Pt atoms and insufficient diffusion of Pt and Co atoms to mix them in the particle overall.

Quantitative Characterization of the Thermally Driven Alloying State in Ternary Ir-Pd-Ru Nanoparticles.

Compositional and structural arrangements of constituent elements, especially those at the surface and near-surface layers, are known to greatly influence the catalytic performance of alloyed nanoparticles (NPs). Although much research effort often focuses on the ability to tailor these important aspects in the design stage, their stability under realistic operating conditions remains a major technical challenge. Here, the compositional stability and associated structural evolution of a ternary iridium-palladium-ruthenium (Ir-Pd-Ru) nanoalloy at elevated temperatures have been studied using interrupted in situ scanning transmission electron microscopy and theoretical modeling. The results are based on a combinatory approach of statistical sampling at the sub-nanometer scale for large groups of NPs as well as tracking individual NPs. We find that the solid solution Ir-Pd-Ru NPs (∼5.6 nm) evolved into a Pd-enriched shell supported on an alloyed Ir-Ru-rich core, most notably when the temperature exceeds 500 °C, concurrently with the development of expansive atomic strain in the outer surface and subsurface layers with respect to the core regions. Theoretically, we identify the weak interatomic bonds, low surface energy, and large atomic sizes associated with Pd as the key factors responsible for such observed features.

Subpercent Local Strains Due to the Shapes of Gold Nanorods Revealed by Data-Driven Analysis.

Analysis of subpercent local strain is important for a deeper understanding of nanomaterials, whose properties often depend on the strain. Conventional strain analysis has been performed by measuring interatomic distances from scanning transmission electron microscopy (STEM) images. However, measuring subpercent strain remains a challenge because the peak positions in STEM images do not precisely correspond to the real atomic positions due to disturbing influences, such as random noise and image distortion. Here, we utilized an advanced data-driven analysis method, Gaussian process regression, to predict the true strain distribution by reconstructing the true atomic positions. As a result, a precision of 0.2% was achieved in strain measurement at the atomic scale. The method was applied to gold nanoparticles of different shapes to reveal the shape dependence of the strain distribution. A spherical gold nanoparticle showed a symmetric strain distribution with a contraction of ∼1% near the surface owing to surface relaxation. By contrast, a gold nanorod, which is a cylinder terminated by hemispherical caps on both sides, showed nonuniform strain distributions with lattice expansions of ∼0.5% along the longitudinal axis around the caps except for the contraction at the surface. Our results indicate that the strain distribution depends on the shape of the nanomaterials. The proposed data-driven analysis is a convenient and powerful tool to measure the strain distribution with high precision at the atomic scale.

Highly Stable and Active Solid-Solution-Alloy Three-Way Catalyst by Utilizing Configurational-Entropy Effect.

Since 1970, people have been making every endeavor to reduce toxic emissions from automobiles. After the development of a three-way catalyst (TWC) that concurrently converts three harmful gases, carbon monoxide (CO), hydrocarbons (HCs), and nitrogen oxides (NOx ), Rh became an essential element in automobile technology because only Rh works efficiently for catalytic NOx reduction. However, due to the sharp price spike in 2007, numerous efforts have been made to replace Rh in TWCs. Nevertheless, Rh remains irreplaceable, and now, the price of Rh is increasing significantly again. Here, it is demonstrated that PdRuM ternary solid-solution alloy nanoparticles (NPs) exhibit highly durable and active TWC performance, which will result in a significant reduction in catalyst cost compared to Rh. This work provides insights into the design of highly durable and efficient functional alloy NPs, guiding how to best take advantage of the configurational entropy in addition to the mixing enthalpy.

In-Situ Observation of the Continuous Phase Transition in Determining the High Thermoelectric Performance of Polycrystalline Sn0.98Se.

We report a comprehensive in-situ phase-change study on polycrystalline Sn0.98Se via high-temperature X-ray diffraction and in-situ high-voltage transmission electron microscopy from room temperature to 843 K. The results clearly demonstrate a continuous phase transition from Pnma to Cmcm starting from 573 to 843 K, rather than a sudden transition at 800 K. We also find that the thermal-conductivity rise at high temperature after the phase transition, as commonly seen in pristine SnSe, does not occur in Sn0.98Se, leading to a high thermoelectric figure of merit. Density functional theory calculations reveal the origin to be the suppression of bipolar thermal conduction in the Cmcm phase of Sn0.98Se due to the enlarged bandgap. This work fills the gap of in-situ characterization on polycrystalline Sn0.98Se and provides new insights into the outstanding thermoelectric performance of polycrystalline Sn0.98Se.

Imaging the Polymorphic Transformation in a Single Cu6Sn5 Grain in a Solder Joint.

In-situ observations of the polymorphic transformation in a single targeted Cu6Sn5 grain constrained between Sn-0.7 wt % Cu solder and Cu-Cu3Sn phases and the associated structural evolution during a solid-state thermal cycle were achieved via a high-voltage transmission electron microscope (HV-TEM) technique. Here, we show that the monoclinic η'-Cu6Sn5 superlattice reflections appear in the hexagonal η-Cu6Sn5 diffraction pattern upon cooling to isothermal 140 °C from 210 °C. The in-situ real space imaging shows that the η'-Cu6Sn5 contrast pattern is initiated at the grain boundary. This method demonstrates a new approach for further understanding the polymorphic transformation behavior on a real solder joint.