Black Phosphorus Nanosheet/Melamine Cyanurate Assemblies as Functional Active Layers for Artificial Synapse Memristors

The aqueous solution for the liquid-phase exfoliation (LPE) of 2D black phosphorus (BP) is rarely used due to the environmental instability of BP caused by oxygen and water molecules. In this study, we demonstrate a melamine-assisted LPE in aqueous conditions to achieve high-quality BP nanosheets with effective exfoliation, where the assembled melamine molecule on BP enhances the peeling force together with the in situ protection against BP oxidation, leading to the well-dispersion of highly crystalline BP in water that enables the eco-friendly and biocompatible process. Moreover, we develop an approach to extract BP together with a supramolecular assembly of melamine-cyanurate (MCA) by adding the cyanuric acid in melamine solution, formatting the hydrogen-bonded supramolecular network together with the incorporated BP nanosheets. Such one-step self-assembly of BP and the supramolecular complex are readily integrated into two-terminal devices, namely memristor and threshold switch (TS) selector, where BP nanosheets bridge the proton transfer of the hydrogen bond network in individual supramolecules, modulating analog resistive switching as synaptic behaviors of the neuromorphic system. TS selector is achieved through stacking pure MCA layer to block proton hopping under the threshold voltage, exhibiting high nonlinearity of 30 mV/dec with on/off resistance ratio up to 10⁴and retention time over 4000 s. This study provides an effective LPE of BP in aqueous solution and explores an extraction route of as-exfoliated BP with a supramolecular complex ready for integration of a memristor, which paves the way for versatile applications of BP on emerging nanoelectronics and optoelectronics.