Monolayer-Precision Colloidal Nanoplatelet Films via Liquid–Air Interface Self-Assembly for Thickness-Engineered Light-Emitting Diodes

Abstract Colloidal semiconductor nanoplatelets (NPLs) offer atomically precise quantum confinement and exceptional optical properties, yet translating these advantages into device-quality thin films with deterministic thickness control remains challenging. Here, liquid–air interface self-assembly is employed to fabricate core–shell CdSe/CdZnS NPL films with exclusively face-down orientation, spanning one to four monolayers. Optical gain characterization verifies that the self-assembly process fully preserves the intrinsic photophysical quality of the NPLs. When integrated as emissive layers into light-emitting diode (LED) architectures, the self-assembled films produce stable, spectrally pure red electroluminescence with Commission Internationale de l’Éclairage chromaticity coordinates of (0.72, 0.28), achieving a maximum external quantum efficiency of 12%, a peak luminance of 18,530 cd/m2, and a current efficiency of 4.8 cd/A. These results demonstrate that liquid–air interface self-assembly transforms the active-region thickness from a stochastic processing outcome into a discretely addressable design parameter, adjustable one monolayer at a time with quantitative predictability. This capability provides a practical platform for systematically investigating the influence of active-layer thickness on carrier recombination, charge balance, and exciton management, and opens pathways toward atomically defined optoelectronic device architectures.