Lithography of Self-assembled Semiconductor Quantum Dots on Templates Fabricated from Mixed Langmuir-Blodgett Films

In this paper, we discuss a useful technique to fabricate patterned single layers consisting of quantum dots on two-dimensional templates fabricated from phase-separated mixed Langmuir-Blodgett (LB) films. The phase-separated structures are governed by two competing interactions-line tension and dipole-dipole interaction-and are tunable by adjusting the intermolecular interactions between the film-forming molecules and the fabrication conditions. The templates can be fabricated from mixed LB films containing silane-coupling agents, which form covalent bonds with Si wafers. The CdS- or CdSe/ZnS-nanoparticles were immobilized on the templates using a chemisorption technique. The samples were analyzed using ultraviolet-visible absorption spectroscopy, emission spectroscopy, atomic force microscopy, scanning electron microscopy, and fluorescence microscopy. The diameters of the CdS-NPs and CdSe/ZnS were estimated to be about 2.6 and 4.5 nm, respectively, using the Brus equation. We successfully obtained a patterned single layer consisting of quantum dots on the templates. The density of the immobilized-CdSe/ZnS-NPs on these templates can be controlled through variations in the concentration of the aqueous dispersion of CdSe/ZnS-NPs. Fluorescence observations indicate that the immobilized-CdSe/ZnS-NPs on the templates serve as light-emitting devices. These results indicate that patterned quantum dots can be formed only through self-assembly processes.