Quantum dot breakthrough: high-resolution deposition and patterning

       Since scientists proposed the concept of "quantum dot" (QD) in the 1980s, quantum dot, as a zero-dimensional light-emitting semiconductor nanostructure, has attracted the interest of a large number of researchers due to its narrow-band photoluminescence properties due to its quantum confinement effect. Researchers say that quantum dot materials are expected to be used in optoelectronic devices such as solar cells, transistors and light-emitting devices.

　　However, quantum dot technology has not yet achieved widespread commercial application. The biggest bottleneck is the inability to achieve large-scale, high-resolution quantum dot deposition and patterning on substrates.

　　Recently, researchers including Joon-Suh Park from the Korea Institute of Science and Technology (KIST) proposed using traditional photolithography technology combined with electrostatically assisted layer-by-layer (LbL) technology to achieve multi-color, high-resolution, large-scale quantum dot deposition and patterning technology, breaking through the current bottleneck in the practical application of quantum dot technology.

　　The research results were published in the journal Nano Letters on October 11.

　　Quantum dot deposition on a 4-inch quartz wafer recreates the 1967 Andy Warhol artwork of Marilyn Monroe. Image credit: Park et al. ©2016 American Chemical Society

　　Although there are many quantum dot deposition and patterning technologies, the special properties of quantum dots, such as high molecular weight, make evaporation deposition technology difficult to implement. In addition, these methods can only choose a compromise between high resolution and large-scale deposition.

　　New multi-color, high-resolution, large-scale quantum dot patterning technology. (a) Quantum dot patterning technology: photolithography technology and electrostatic assisted layer-by-layer assembly technology. (b) Multi-color quantum dot patterning under 405nm laser excitation. (c) "Marilyn Monroe" on a 4-inch wafer under ultraviolet light excitation. Image source: DOI: 10.1021/acs.nanolett.6b03007

　　Photolithography is a high-resolution, batch-scale patterning technology. However, due to the hydrophobic properties of quantum dots, the organic chemical reagents used in traditional photolithography may destroy and dissolve quantum dots.

　　How can we use traditional photolithography technology without damaging the quantum dots themselves? Park's team modified the quantum dot coating to be hydrophilic, so that the quantum dots will not be dissolved in organic solvents during the photolithography process.

　　In addition, the researchers also used a charged substrate and utilized the electrostatic attraction between the quantum dots and the charged substrate to assist the layer-by-layer assembly (LbL) process of the quantum dots, achieving multi-color, large-scale quantum dot deposition.

　　The researchers combined photolithography technology with layer-by-layer assembly technology, repeatedly performing the photolithography and assembly processes to achieve multi-color, high-resolution, and large-scale uniform quantum dot deposition.

　　"Our new quantum dot patterning technology is compatible with conventional semiconductor manufacturing processes and is expected to solve industry challenges," Park said. "Compared to organic materials, quantum dots are more stable and reliable when exposed to water and oxygen, and their applications will be broader than those currently used by organic materials, such as displays, photodetectors, phototransistors, and solar cells."

　　To verify the practical potential of this new quantum dot deposition technology, the researchers used quantum dots in four colors, red, green, purple and yellow, to graphically reproduce the art painting of Marilyn Monroe created by artist Andy Warhol in 1967 on a 4-inch quartz wafer.

　　The colorful, high-resolution "Marilyn Monroe" proves that the technology can achieve multi-color, high-precision, large-range optoelectronic display effects.

　　In the future, the researchers plan to continue developing new quantum dot patterning technologies to further improve quantum dot optoelectronic display effects.

　　Park said: "Using this method, we can optimize the structure of quantum dot light-emitting diodes (QD-LEDs), reduce the size of QD-LEDs, and obtain higher energy efficiency and higher resolution display effects, and ultimately develop a one-chip, multi-wavelength excitation photodetector."