Synthesis, Solvent-dependent Self-Assembly and Partial Oxidation of Ultrathin Cerium Fluoride Nanoplatelets

TL;DR

This study presents an optimized synthesis of cerium fluoride nanoplatelets, investigates their partial oxidation to oxyfluoride, and demonstrates how solvent choice influences their self-assembly into different superstructures at interfaces.

Contribution

It reveals the solvent-dependent self-assembly behavior of cerium fluoride nanoplatelets and details their partial oxidation to oxyfluoride during synthesis.

Findings

Solvent choice determines nanoplatelet organization and superstructure formation.

Partial oxidation of CeF3 NPLs results in oxyfluoride composition.

Organized films exhibit either columnar or hexagonal superlattices depending on solvent.

Abstract

Two-dimensional colloidal nanoplatelets (NPLs) with atomically defined thickness exhibit unique physical properties, yet understanding their formation mechanism and assembly remains essential for tuning their collective behavior. We report an optimized synthesis of triangular cerium-based NPLs with narrow size and shape distributions via thermal decomposition of cerium trifluoroacetate. Combining X-ray diffraction, XPS, and high-resolution STEM, we show that the expected CeF3 NPL structure undergoes partial oxidation, yielding an oxyfluoride composition CeOxFy. Beyond their composition, we investigate how these oleic acid-capped NPLs organize in solution and at interfaces. The choice of solvent governs both the solution-phase organization and the resulting superstructures formed upon evaporation at the liquid--air interface. In solvents that promote face-to-face stacking in solution, evaporation produces films organized into columnar assemblies tens of micrometers long, with the NPL planes oriented perpendicular to the interface. In contrast, solvents in which NPLs remain individually dispersed yield extended hexagonally ordered superlattices with edge-to-edge stacking spanning several micrometers, where the NPLs lie parallel to the interface in an edge-to-edge arrangement. These results highlight that solvent-mediated interactions and pre-existing organization in solution are decisive factors in determining the outcome of evaporative self-assembly of colloidal nanocrystals.