Patterned Quantumn Dots Layer Via Nanoimprint Lithograph

Quantum dots were demonstrated high quantum yields, extremely narrow emission, spectral tunability and high stability, among other beneficial characteristics and aroused the attention of researchers. At the same time, Quantum dots film especially large area patterned structure with an orderly array in quantum dot film have many important applications in optoelectronic devices, pixels, full-color LED display, organic/inorganic solar cells, and biology.In recent years, there have been commendable attempts to incorporate quantum dot 2D and 3D patterns through traditional lithography strategies such as electron beam lithography(EBL) and photolithography while are expensive and limited by a trade-off between resolution and throughput. So, a simple and convenient method which can realize the fabrication of large area, morphology and structure of controllable pattern of quantum dot film is eager to be found. At present, the patterned methods mainly have: nano-imprinting technology, electron beam etching technology, micro-contact printing, molecular self-assembly, and so on. And each method has two sides. But, the development of nano-imprint lithography(NIL) and soft lithography strategies has enabled high throughput fabrication of QDs features with sub-100 nm resolution via a thermal or UV-curing process. By using this method, patterning colloidal quantum dots for photonic nanostructures such as straight lines, rings and dot patterns are successfully obtained. Nano-imprinting technologies showed many advantages, such as preparation process and preparation cost. In this paper, we combine the template modification with rotary stamping to achieve large area, high yield, orderly structure, morphology density of 2 d and 3 d design quantum dot film. From the results of experiments, we found that the patterned film with template modified has much better morphology. Besides, the patterned quantum dots exhibited enhanced fluorescence property compared with the planar films. Then we carried out the following work:1、Patterned quantum dot layer via thermal printing technology: Firstly, we choose toluene solvent as dispersion solvent of quantum dots, and determined the concentration of the solvent 15 mg/ml. To make sure the quantum dots film with high quality. Secondly, spin-coating the solution onto ITO glass substrates, controlling the conditions of spin-coat to change the thickness of film and improve the density of it. Finally, we press grating structure onto the planar quantum dot film via thermal printing.2、Optimizated the structure of patterned quantum dot film by modifying template: In order to obtain a high quality patterned quantum dot film with large area, ordered array, and uniform density, we treat the template with modifier which contains ammonium fluoride solution with the concertration of 5mol/L and hydrofluoric acid with concertration of 40%. The two solutions mixed with each other with the volume of 6:1. After modification, the surface energy decreased and the force between template and substrate also decreased, so the quantities of quantum dots which were taken away by template are greatly reduced. As a result, the efficiency is improved and the morphology and structure becomes more perfect than before.3、Patterned quantum dot layer via UV-assisted nano-imprint lithography : We make full use of the light sensitivity of NOA61 photoresist, and mix it with quantum dots solution at different proportions. Different amount of quantum dots and photoresist mixed solution was successfully prepared due to the compatibility of QD with the photoresist. Under the precondition of UV-NIL, we try to reduce the amount of photoresist as possible as we can. At the same time, we still ensure the quality of patterned film. Finally, we spin the mixed solution with the proportion of 9:1 onto ITO glass substrate at 4000 r/min, and curing it for 15 minutes under the ultraviolet light with power of eight watts. In this case, two dimension stripes patterns of 700 nm period were directly prepared by one-step imprinting over a large area. More importantly, the morphology and structure of patterned quantum dot film is much better than thermal printing. Complex 3D QD patterning could be realized by expanding the QD direct imprinting process through two-step imprint method by rotating the imprint mold 90°. Most interestingly, the patterned quantum dots exhibited enhanced fluorescence property nearly 3.8 times compared with the planar films.