Tuning epitaxial growth on NaYbF4 upconversion nanoparticles by strain management

Behavior of Microstrain in Nd3+-Sensitized Near-Infrared Upconverting Core-Shell Nanocrystals for Defect-Induced Tailoring of Luminescence Intensity

In an attempt to optimize the upconversion luminescence (UCL) output of a Nd3+-sensitized near-infrared (808 nm) upconverting core-shell (CS) nanocrystal through deliberate incorporation of lattice defects, a comprehensive analysis of microstrain both at the CS interface and within the core layer was performed using integral breadth calculation of high-energy synchrotron X-ray (λ = 0.568551 Å) diffraction. An atomic level interpretation of such microstrain was performed using pair distribution function analysis of the high-energy total scattering. The core NC developed compressive microstrain, which gradually transformed into tensile microstrain with the growth of the epitaxial shell. Such a reversal was rationalized in terms of a consistent negative lattice mismatch. Upon introduction of lattice defects into the CS systems upon incorporation of Li+, the corresponding UCL intensity was maximized at some specific Li+ incorporation, where the tensile microstrain of CS, compressive microstrain of the core, and atomic level disorders exhibited their respective extreme values irrespective of the activator ions.

Ag@Pd bimetallic structures for enhanced electrocatalytic CO2 conversion to CO: an interplay between the strain effect and ligand effect

Electrochemical CO₂ reduction reactions provide a promising path to effectively convert CO₂ into valuable chemicals and fuels for industries. Among the many CO₂ conversion catalysts, Pd stands out as a promising catalyst for effective CO₂ to CO conversion. Here, using the misfit strain strategy, Ag@Pd bimetallic nanoparticles with different Pd overlayer contents were prepared as CO₂ reduction catalysts. By varying the Pd overlayer content, all the Ag@Pd bimetallic nanoparticles exhibited superior CO₂ conversion performance over their Pd and Ag nanoparticle counterparts. An optimal Pd-to-Ag ratio of 1.5 : 1 yielded the highest CO faradaic efficiency of 94.3% at -0.65 V vs. RHE with a high CO specific current density of 3.9 mA cm^-2. It was found that the Pd content can substantially affect the interplay between the strain effect and ligand effect, resulting in optimized binding properties of the reaction intermediates on the catalyst surface, thereby enhancing the CO₂ reduction performance.

Unravelling phase and morphology evolution of NaYbF4 upconversion nanoparticles via modulating reaction parameters

The phase and morphology evolutions of NaYbF4 upconversion nanocrystals have been systemically explored through modulating the experiment parameters in a canonical high-temperature co-precipitation method.

First-principles calculations of strain engineering in NaYF4-based nanocrystals with hydroxyl impurities

Lanthanide-based nanocrystals with heterogeneous core-shell structures possess elastic strain due to lattice mismatch and volumetric expansion or shrinkage. Strain relaxation is usually accompanied by lattice defects, especially those point defects and small defect clusters. However, the influence of strain on the formation of lattice defects remains unclear. Using OH^- ions as a representative lattice impurity, first-principles calculations can be used to address the correlation between the thermodynamic stability of OH-based substitutional defects and elastic strain. Moreover, the concentration of OH^- impurities in both strained and relaxed sodium yttrium fluoride lattices can be greatly reduced by increasing the concentration of fluoride-containing precursors. These findings suggest that minimal incorporation of OH^- ions effectively suppresses multiphonon nonradiative relaxation and thus boost the efficiency of upconversion conversion.

Shedding Light on the Role of Misfit Strain in Controlling Core-Shell Nanocrystals

Heteroepitaxial modification of nanomaterials has become a powerful means to create novel functionalities for various applications. One of the most elementary factors in heteroepitaxial nanostructures is the misfit strain arising from mismatched lattices of the constituent parts. Misfit strain not only dictates epitaxy kinetics for diversifying nanocrystal morphologies but also provides rational control over materials properties. In recent years, advances in chemical synthesis along with the rapid development of electron microscopy and X-ray diffraction techniques have enabled a substantial understanding of strain-related processes, which offers theoretical foundation and experimental guidance for researchers to refine heteroepitaxial nanostructures and their properties. Herein, recent investigations on heterogeneous core-shell nanocrystals containing misfit strains are summarized, with a focus on the mechanistic understanding of strain and strain-induced effects such as tuning the epitaxial habit, modulating the optical emission, and enhancing the catalytic activity and magnetic coercivity.