Direct Atomic-Scale Observation of Intermediate Pathways of Melting and Crystallization in Supported Bi Nanoparticles

Uncovering the evolutional pathways of melting and crystallization atomically is critical to understanding complex microscopic mechanism of first-order phase transformation. We conduct in situ atomic-scale observations of melting and crystallization in supported Bi nanoparticles under heating and cooling within an aberration-corrected TEM. We provide direct evidence of the multiple intermediate state events in melting and crystallization. The melting of the supported nanocrystal involves the formation and migration of domain boundaries and dislocations due to the atomic rearrangement under heating, which occurs through a size-dependent multiple intermediate state. A critical size, which is key to inducing the transition pathway in melting from two to four barriers, is identified for the nanocrystal. In contrast, crystallization of a Bi droplet involves three stages. These findings demonstrate that the phase transformations cannot be viewed as a simple single barrier-crossing event but as a complex multiple intermediate state phenomenon, highlighting the importance of nonlocal behaviors

Real-Time Dynamical Observation of Lattice Induced Nucleation and Growth in Interfacial Solid–Solid Phase Transitions

Uncovering dynamical processes of lattice induced epitaxial growth of nanocrystal on the support is critical to understanding crystallization, solid-phase epitaxial growth, Oswald ripening process, and advanced nanofabrication, all of which are linked to different important applications in the materials field. Here, we conduct direct in situ atomic-scale dynamical observation of segregated Bi layers on SrBi2Ta2O9 support under low dose electron irradiation to explore the nucleation and growth from an initial disordered solid state to a stable faceted crystal by using aberration-corrected transmission electron microscopy. We provide, for the first time, atomic-scale insights into the initial prenucleation stage of lattice induced interfacial nucleation, size-dependent crystalline fluctuation, and stepped-growth stage of the formed nanocrystal on the oxide support at the atomic scale. We identify a critical diameter in forming a stable faceted configuration and find interestingly that the stable nanocrystal presents a size-dependent coalescence mechanism. These results offer an atomic-scale view into the dynamic process at solid/solid interfaces, which has implications for thin film growth and advanced nanofabrication

Surface Science and Colloidal Stability of Double-Perovskite Cs₂AgBiBr₆ Nanocrystals and Their Superlattices

Capping ligand bonding and the thermal and colloidal stability of Cs2AgBiBr6 nanocrystals were studied. Oleylamine and oleic acid bonding to Cs2AgBiBr6 nanocrystals was studied with 1H nuclear magnetic resonance and nuclear Overhauser effect spectroscopy. Both molecules are present in the ionic metathesis synthesis reaction, but only oleylamine remains bound to the nanocrystals after purification. Oleic acid is not a capping ligand; however, the synthesis requires it, and its concentration determines the yield of the reaction while oleylamine primarily affects the uniformity of the sample. Substitution of oleic acid in the reaction with diisooctylphosphinic acid still yielded nanocrystals with similar size, cuboidal shape, uniformity, cubic double-perovskite crystal structure, and optical properties. Nanocrystals were assembled into superlattices and heated in air. Grazing incidence small-angle and wide-angle X-ray scattering showed that the nanocrystals sinter at 250 °C, but the crystal structure and preferred crystal orientation on the substrate does not change. When nanocrystals were dispersed in hexane and exposed to light, they precipitated within 24 h. Scanning electron microscopy showed that the aggregated nanocrystals still retained their initial size and shape and had not coalesced

Real-Time Dynamical Observation of Lattice Induced Nucleation and Growth in Interfacial Solid–Solid Phase Transitions

Uncovering dynamical processes of lattice induced epitaxial growth of nanocrystal on the support is critical to understanding crystallization, solid-phase epitaxial growth, Oswald ripening process, and advanced nanofabrication, all of which are linked to different important applications in the materials field. Here, we conduct direct <i>in situ</i> atomic-scale dynamical observation of segregated Bi layers on SrBi₂Ta₂O₉ support under low dose electron irradiation to explore the nucleation and growth from an initial disordered solid state to a stable faceted crystal by using aberration-corrected transmission electron microscopy. We provide, for the first time, atomic-scale insights into the initial prenucleation stage of lattice induced interfacial nucleation, size-dependent crystalline fluctuation, and stepped-growth stage of the formed nanocrystal on the oxide support at the atomic scale. We identify a critical diameter in forming a stable faceted configuration and find interestingly that the stable nanocrystal presents a size-dependent coalescence mechanism. These results offer an atomic-scale view into the dynamic process at solid/solid interfaces, which has implications for thin film growth and advanced nanofabrication

A Convenient Route for Au@Ti–SiO₂ Nanocatalyst Synthesis and Its Application for Room Temperature CO Oxidation

Small gold nanoparticles of size less than 5 nm encapsulated inside titanium modified silica shell have been reported. Here, a modified sol–gel method, which is a one-step process, produces Au@Ti–SiO2 nanocatalyst with a good control of titanium loading. With a titanium loading of 0.9 and 2.2 wt % in silica, unprecedented low temperature activity (full conversion) is observed for this catalyst for CO oxidation reaction compared to Au@SiO2 catalyst. A combination of optimum sized gold nanoparticles with a large amount of oxygen vacancies created due to Ti incorporation in silica matrix is considered to be the reason for this enhanced catalytic activity. The size of gold nanoparticles is maintained even after high temperature pretreatments, which show the benefit of encapsulation. The effect of the various pretreatments on the catalytic activity has also been reported

Reversible Light-Induced Enhancement of Photoluminescence Lifetime and Intensity in Perovskite-Phase CsPbI₃ Nanocrystals

Light-induced changes in photophysical and electronic properties in metal halide perovskites can affect their performance in photovoltaic devices, light-emitting diodes, and other applications. Here we reveal that light induces a slow, reversible enhancement in photoluminescence (PL) lifetime and intensity in films of perovskite-phase CsPbI3 nanocrystals. When films of CsPbI3 nanocrystals stored in air are photoexcited, their PL lifetime and intensity increase by as much as a factor of 5 over the course of 20–30 min. Several hours later, without additional light excitation, the initial PL lifetime and intensity return. Placing the films under vacuum or nitrogen for several minutes was also found to increase the PL lifetime and intensity. We propose a model of slow, humidity- and light-sensitive surface states in perovskite-phase CsPbI3 nanocrystals

Direct Imaging and Identification of Individual Dopant Atoms in MoS₂ and WS₂ Catalysts by Aberration Corrected Scanning Transmission Electron Microscopy

In the case of the hydrodesulfurization (HDS) processes one of the best catalysts currently available is that based on MoS2 and WS2. A very significant increase in their activity can be achieved by adding Co or Ni as a promoter. In the present report we have used probe aberration corrected STEM (scanning transmission electron microscopy) for the first time to characterize Co doped MoS2/WS2 nanowire catalysts (supported on Al2O3 substrates). The high-resolution imaging reveals clearly the location of Co in the individual catalysts. This has not been possible to date with other experimental techniques because of the insufficient image contrast and/or resolution. On the basis of the HAADF-STEM images, we built two models for the Co−Mo−S and Co−W−S catalysts to illustrate the different morphologies found in the catalysts. With this study it is now possible to better locate, identify, and understand the role of promoters in the design and functioning of catalysts

Controlling Bimetallic Nanostructures by the Microemulsion Method with Subnanometer Resolution Using a Prediction Model

We present a theoretical model to predict the atomic structure of Au/Pt nanoparticles synthesized in microemulsions. Excellent concordance with the experimental results shows that the structure of the nanoparticles can be controlled at subnanometer resolution simply by changing the reactant concentration. The results of this study not only offer a better understanding of the complex mechanisms governing reactions in microemulsions, but open up a simple new way to synthesize bimetallic nanoparticles with ad hoc controlled nanostructures

A Convenient Route for Au@Ti–SiO₂ Nanocatalyst Synthesis and Its Application for Room Temperature CO Oxidation

Small gold nanoparticles of size less than 5 nm encapsulated inside titanium modified silica shell have been reported. Here, a modified sol–gel method, which is a one-step process, produces Au@Ti–SiO2 nanocatalyst with a good control of titanium loading. With a titanium loading of 0.9 and 2.2 wt % in silica, unprecedented low temperature activity (full conversion) is observed for this catalyst for CO oxidation reaction compared to Au@SiO2 catalyst. A combination of optimum sized gold nanoparticles with a large amount of oxygen vacancies created due to Ti incorporation in silica matrix is considered to be the reason for this enhanced catalytic activity. The size of gold nanoparticles is maintained even after high temperature pretreatments, which show the benefit of encapsulation. The effect of the various pretreatments on the catalytic activity has also been reported

Atomic-Scale Interface Modification Improves the Performance of Cu(In_1–<i>x</i>Ga_<i>x</i>)Se₂/Zn(O,S) Heterojunction Solar Cells

Cadmium-free buffer layers deposited by a dry vacuum process are mandatory for low-cost and environmentally friendly Cu­(In1–xGax)­Se2 (CIGS) photovoltaic in-line production. Zn­(O,S) has been identified as an alternative to the chemical bath deposited CdS buffer layer, providing comparable power conversion efficiencies. Recently, a significant efficiency enhancement has been reported for sputtered Zn­(O,S) buffers after an annealing treatment of the complete solar cell stack; the enhancement was attributed to interdiffusion at the CIGS/Zn­(O,S) interface, resulting in wide-gap ZnSO4 islands formation and reduced interface defects. Here, we exclude interdiffusion or island formation at the absorber/buffer interface after annealing up to 200 °C using high-resolution scanning transmission electron microscopy (HR-STEM) and energy-dispersive X-ray spectroscopy (EDX). Interestingly, HR-STEM imaging reveals an epitaxial relationship between a part of the Zn­(O,S) buffer layer grains and the CIGS grains induced by annealing at such a low temperature. This alteration of the CIGS/buffer interface is expected to lead to a lower density of interface defects, and could explain the efficiency enhancement observed upon annealing the solar cell stack, although other causes cannot be excluded