Fabrication of composite thin-film materials based on black phosphorus (BP) will greatly broaden the applications of BP in various areas. However, it is still a challenge to prepare a BP-based composite film with good stability and controllable structure. In this work, a series of BP-based composite Langmuir−Blodgett (LB) films are prepared by the selfassembly of polyethyleneimine (PEI)-modified BP nanosheets (BPNSs) (BPNS-PEI) and dye molecules. The presence of PEI greatly improves the stability of BPNSs. As for BPNS-PEI and dye molecules, the electrostatic interactions or π−π stacking interactions ensure the formation of stable composite LB films. Due to the protonation and deprotonation of amino groups, the synthesized BPNS-PEI/dye composite films show a sensitive response to acid and alkali gases, which shows wide application prospects as a highly sensitive gas sensor. Furthermore, surface-enhanced Raman scattering (SERS) proves that the prepared LB films exhibit good reproducibility and obvious Raman enhancement effect on rhodamine 6G molecules. In addition, due to the high carrier transfer rate of the obtained composite films, they possess enhanced photocurrent generation performance than pure BPNS-PEI and pure dye films. The current work demonstrates an effective method for preparing the ordered self-assembled BP-based composite LB films with good SERS and photoelectric conversion performance.

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Molecular self–assembly is the automatic formation of functional assemblies of different structural components through weak, reversible, non–covalent interactions on the basis of molecular recognition. Amphiphilic molecules have a natural advantage in self–assembly at the gas/liquid interface. In this work, two amphiphilic molecules with a special molecular structure, indocyanine green (ICG) and a derivative of indocyanine green (CCS), were combined with two dye molecules (tetraphenylporphyrin tetrasulfonic acid hydrate (TPPS) and nickel (II) phthalocyanine–tetrasulfonic acid tetrasodium salt (TsNiPc) for self–assembly through the Langmuir–Blodgett (LB) technique. The nanostructure and assembly behavior in ordered self–assembled films are effectively regulated by inducing dye molecules to form different types of aggregates (H– and J–aggregates). In addition, we prepared composite films containing the same functional components using the conventional drop–casting technique and performed a series of comparative experiments with LB films. The degree of hydrophilicity was found to be related to roughness, with LB composite films being flatter and denser, with the lowest roughness and the best hydrophobicity compared to drop–casting films. Notably, the LB films showed better optoelectronic properties under the same conditions, providing new clues for the application of optoelectronic functional ultrathin film devices.