Expanding the Concept of van der Waals Heterostructures to Interwoven 3D Structures

Several members of a new family of heterostructures [(LaSe)_1.17]₁V_{<i>n</i>(1+<i>y</i>)+1}Se_2<i>n</i>+2 with <i>n</i> = 1, 2, and 3 were prepared using a diffusion constrained, kinetically controlled synthesis approach. Specular diffraction patterns collected as a function of annealing temperature show the evolution of designed precursors into highly ordered heterostructures. Scanning transmission electron microscopy (STEM) images reveal that the structure of <i>n</i> = 3 consists of rock salt structured LaSe bilayers alternating with vanadium selenide layers of varying thickness, which are structurally closely related to V₃Se₄. Interplanar distances obtained from the STEM images were successfully used as the starting point for Rietveld refinements of the specular diffraction patterns of these crystallographically aligned compounds. Utilizing this unorthodox combined approach to extract detailed structural information unambiguously, we demonstrated that these thin film compounds are the first examples of chalcogenide-based heterostructures, where the bulk structures of both building blocks lack a van der Waals gap, yet a nonepitaxial incommensurate interface forms. Moreover, the refinement results of the <i>n</i> = 2 and 3 heterostructures suggest that the structure of the V–Se layers can be varied ranging from VSe₂ to VSe depending on the film composition. The electrical resistivity of the [(LaSe)_1.17]₁V_{<i>n</i>(1+<i>y</i>)+1}Se_2<i>n</i>+2 heterostructures changes systematically from semiconducting toward metallic behavior with increasing <i>n</i>, showcasing the ability to tune physical properties by precisely controlling the layer sequence in these heterostructures

From Phase Separation to Nanocrystallization in Fluorosilicate Glasses: Structural Design of Highly Luminescent Glass-Ceramics

Tremendous enhancement of optical emission efficiency was achieved in fluorosilicate glasses by growing lanthanide doped fluoride nanocrystals embedded in oxide glass matrix. The formation mechanism of the microstructure was elucidated by combining solid-state NMR, scanning TEM, EDX map, and large-scale molecular dynamics simulations. The results reveal that the growth of fluoride nanocrystals in fluorosilicate glass was originated from fluoride phase separation. Atomic level structures of phase separation of fluoride-rich regions in oxyfluoride glasses matrix were observed from both EDX maps and MD simulations, and it was found that, while silicon exclusively coordinated by oxygen and alkali earth ions and lanthanide mainly coordinated by fluorine, aluminum played the role of linking the two fluoride glass and oxide glass regions by bonding to both oxygen and fluoride ions