Structure-Function Correlation: Engineering High Quantum Yields in Down-Shifting Nanophosphors.

Lanthanides are routinely incorporated into quantum dots to act as down-shifting and up-converting phosphors in display and lighting applications due to their high photoluminescence quantum yields (PLQY). Recent efforts in the field have demonstrated that trivalent lanthanide, Ln(III), incorporated into ZnAl2O4 spinel nanocrystals can achieve PLQYs of 50% for down-shifting nanophosphors using earth abundant materials. The high PLQY is surprising as the Al(III) site in a spinel is centrosymmetric, which should lead to poor performance for these nanophosphors. However, spinels are prone to formation of an admixture of inverse and normal spinel lattices when the cation size ratio is not optimal. Such behavior can produce local cation disorder that can influence the phosphor performance. Herein, we describe the use of Tb(III) as an optical probe to evaluate the fractional population of the inverse and normal spinel structures within TbxZnAl2-xO4. The experimental data exhibits a Tb(III) concentration dependent change in the fractional population that results in a maximum PLQY of 37% with 3.56% Tb(III) incorporation. A decrease in the degree of inversion (cation disorder) leads to larger amounts of the cubic Fd3m phase resulting in the observed photoluminescence behavior. The correlation of NMR, pXRD, and optical methods provides direct insight into the high PLQY behavior for this class of nanophosphor.

Synthesis of Highly Uniform Nickel Multipods with Tunable Aspect Ratio by Microwave Power Control.

As the importance of anisotropic nanostructures and the role of surfaces continues to rise in applications including catalysis, magneto-optics, and electromagnetic interference shielding, there is a need for efficient and economical synthesis routes for such nanostructures. The article describes the application of cycled microwave power for the rapid synthesis of highly branched pure-phase face-centered cubic crystalline nickel multipod nanostructures with >99% multipod population. By controlling the power delivery to the reaction mixture through cycling, superior control is achieved over the growth kinetics of the metallic nanostructures, allowing formation of multipods consisting of arms with different aspect ratios. The multipod structures are formed under ambient conditions in a simple reaction system composed of nickel acetylacetonate (Ni(acac)2), oleylamine (OAm), and oleic acid (OAc) in a matter of minutes by selective heating at the (111) overgrowth corners on Ni nanoseeds. The selective heating at the corners leads to accelerated autocatalytic growth along the ⟨111⟩ direction through a "lightning rod" effect. The length is proprtional to the length and number of microwave (MW)-on cycles, whereas the core size is controlled by continuous MW power delivery. The roles of heating mode (cycling versus variable power versus convective heating) during synthesis of the materials is explored, allowing a mechanism into how cycled microwave energy may allow fast multipod evolution to be proposed.