Hierarchical self-constructed molecular layers of small bifunctional molecules with strong molecular dipole moments for improvement of quantum efficiency in inorganic quantum dots

Hierarchically structured self-assembled molecular layers are functionally versatile and structurally tunable, which are nanoscale molecular building blocks with a wide range of applications from biosensing systems to optoelectronic devices. Herein, we present a facile and effective strategy to fabricate hierarchical self-constructed molecular layers comprising small bifunctional molecules with strong molecular dipole moments for improvement of quantum efficiency in inorganic quantum dots (QDs) by using the cooperative interfacial self-assembly of hydrophilic bifunctional molecules on the polar electrode surface. The spontaneous construction of nanostructured molecular layers with different molecular dipole moments was simply achieved through solution-dipping and in situ interfacial self-organization of 3-mercaptopropionic acid or 1-propanethiol molecules on the surface of indium tin oxide (ITO) anode. It is experimentally revealed that these functional molecules formed uniform and ultrathin self-constructed molecular nanolayers on the ITO surface via covalent bonding, where bifunctional 3-mercaptopropionic acid molecules induced a polar molecular surface of thiol groups with strong dipole moments, whereas 1-propanethiol molecules formed a relatively hydrophobic surface of short alkyl chains with weak dipole moments. In addition, the nanoscale ultrathin nature of these molecular layers afforded high visible light transmittance comparable to that of pristine ITO substrate. The electro-optical properties of the QD cells revealed that hierarchical self-constructed molecular nanolayer with strong molecular dipole moments exhibited superior current and luminescent properties with a 162% improvement in external quantum efficiency relative to those of a conventional QD device without molecular layer. Furthermore, the self-assembled molecular nanolayer of 3-mercaptopropionic acid exhibited a higher luminescent efficiency than 1-propanethiol-based nanolayer owing to the increased hole mobility of thiol-based molecular layer with strong dipole moments.