Ligand-Engineered Emissive Quantum Dots for Direct Patterning

Nanocrystal quantum dots (QDs) are a new class of luminophores that stand at the forefront of nearly all light-emitting applications. Specifically, QDs have become mainstream of display technologies, in which the combinations of primary colors realize images in response to electrical or optical excitation. The success of next-generation QD displays demands multicolor QD patterns on desired substrates over a large area with high-precision and high-definition, and most importantly, without compromising on optical or optoelectronic characteristics of QDs.<br/><br/>Previously, transfer printing, inkjet printing and photolithographic patterning have been suggested to realize such QD patterns, but only marginal success has been awarded. Transfer printing can only cover small area patterns, and standard instruments for its use have yet to be developed. Inkjet printing is effective only for low resolution patterns due to the limitation in the feature size of ejected drops, which in return constrains its use for high-definition QD patterns for near eye display applications (<i>i.e</i>, resolution &gt; 3,000 ppi). Photolithography, for which the standard fabrication equipment and procedures are established, promises high-throughput, high-definition QD patterning over a large area. The only problem with conventional photolithography is the use of photoresists and photoinitiators, which deface the optical and electrical properties of QDs.<br/><br/>Here, we devise QD materials that can be processed <i>via</i> photolithographic processes without the presence of photoresists and photoinitiators. Specifically, we endow the essential functions for photocrosslink to the ligands that exist at the perimeter of QDs - the surface of QDs is functionalized with the photocrosslinkable ligands and the dispersing ligands. Photocrosslinkable ligands are photoreactive organic scaffolds that recruit structurally engineered benzophenone moieties whose sensitivity to UV-A improved by two orders of magnitude compared to unsubstituted benzophenone molecules. Consequently, even a short exposure time to UV-A irradiation confer the structural robustness to the QD films. Only a small fraction of photocrosslinkable ligand displacement (less than 10 mol%) is necessary for fully crosslinking, which in return leaves room for extending process compatibility with a range of solution-processing techniques by engineering the rest of the majority dispersing ligands. The success in controlling the structure of both ligands allows direct patterning of QDs <i>via </i>commercialized photolithography (<i>i</i>-line) without altering optical or optoelectronic characteristics of QDs. The advantages of the present approach are well represented by the high-definition QD patterns over a large area achieved by means of standard photolithography equipment (<i>i.e.</i>, pixel resolution for primary colors over 15,000 ppi on a 6-inch wafer).