Hexagonal BN membranes formation by liquid-phase synthesis: Insights from scanning transmission x-ray microscopy

Our electrochemical approach to fabricating two-dimensional flakes and creating ordered stacks of hexagonal BN (hBN) primarily enhances the synthesis flexibility and expands their applications in the aqueous phase. Using a simple cyclic voltammetry (CV) method, freestanding hBN membranes can be formed on the Au mesh with varying structural conditions and degrees of oxidation. Raman and photoluminescence analyses confirm the high quality and distinct characteristics of the CV hBN membranes, which feature a consistent lattice structure and proper B–N bonding coordination, as in the conventional drop-cast (DC) membranes. At the atomic scale, transmission electron microscopy confirms the presence of a perfect hBN lattice structure for both DC and CV methods. However, the electron diffraction patterns reveal a difference in the number of hexagonal phases: three sets for DC hBN and two sets for CV hBN. Beyond the lattice structure, the nanoscale images from scanning transmission x-ray microscopy highlight the regions of interest and their electronic structures at the B, N, and O K-edge absorptions. Chemical bonds involving oxygen at the N and B sites occur at edges or planar surfaces as a result of the liquid process and electrochemical utilization. The DC hBN shows inequality in lattice structure and mixing stacking due to orbital correlations probed at B and N sites and variation in chemical structure owing to surface oxidation, including tri-coordinated N–O, N=O, and OH bonds. Interestingly, the CV method enhances ordered lattice symmetry and stacking structures, inducing the following N lone pairs and B–O(H) bonds in the chemical structure coordination. The use of CV in insulator hBN membranes provides an alternative approach for growing large-scale membranes, presenting new opportunities for semiconductor devices, catalytic applications, and quantum materials.