Methods of Patterned Mist Deposition of Nano-Crystalline Quantum Dot Films
Authors: Sabeeh Ala , Thakur Yash , Chao Ju-Hung , Kshirsagar Aditya , Ruzyllo Jerzy 
Published: February 2020

This study extends an investigation concerned with patterning of ultra-thin films formed by means of mist deposition technique using colloidal solutions of nanocrystalline CdSe nanodots with ZnS shell suspended in organic solvent. The CdSe/ZnS nanocrystalline quantum dots (NQD) show very good promise in display/LED and variety of nano-labeling and counterfeiting applications between others.
Regardless of application, to become a part of the functional device the NQD film must be patterned accordingly. The conventional top-down patterning sequence involving photolithography and etching cannot be used in this case as the process would results in the critical damage to the NQD film. Hence, alternative patterning technologies must be pursued. The goal of this contribution is to review past experiments and to introduce new approach to NQD film patterning in conjunction with mist deposition technology.
In this study NQDs deposited were CdSe/ZnS core/shell NQDs featuring average diameter of 6.5 nm and corresponding red (620 nm) emission wavelength. The method of mist deposition converts colloidal solution, in this case CdSe/ZnS NQDs suspended in toluene, into a fine mist of sub-micrometer sized particles using an atomizer. The mist droplets are carried from the atomizer to the deposition chamber by ultrapure N2 carrier gas and then coalesce on the substrate in the presence of an electric field.
Patterned deposition using mechanical masks of nanocrystalline quantum dot (NQD) films by means of mist deposition on various substrates was demonstrated earlier (e.g. [1]). The process can be used for the variety of purposes including security labeling, barcode printing as well as in photonic detection/conversion device fabrication. In the continuation of this investigation the feasibility of patterned mist deposition of NQD films by surface functionalization applied prior to mist deposition instead of the use of mechanical masks during deposition was explored. With this approach desired pattern can be created through the bottom-up process by properly functionalizing SAM material prior to NQD film formation by mist deposition. Surfaces of substrates used in this experiment, including Si wafers and laboratory glass were functionalized by immersion in trichloro (1H,1H,2H,2H-perfluorooctyl) silane (FOTS) and let dry in an ambient air. UV exposure was carried out in ambient air and at room temperature using apparatus equipped with low-pressure mercury lamp featuring intense 185 and 254 nm wavelengths. Further modifications of the SAM material were accomplished by means of the lamp-based rapid optical surface treatment (ROST) at the temperatures up to 300 oC. In terms of selectivity of deposition the results obtained in this study were strongly affected by the modification of wetting properties of the surface by the presence of electric field accelerating process of mist deposition. This effect, known as electrowetting, was influencing FOTS surface wettability and preventing selectivity of the deposition process. The effect can be partial reversed with the low-temperature thermal treatment in ambient air. The manipulation of the surface wettability of FOTS using UV light, electric field and temperature process is employed to alter selectivity of the NQD film deposition process.
In the extension of this investigation, approach to NQD CdSe/ZnS film patterning based on the principles of a lift-off process is investigated. In this case a negative image of the final NQD pattern is created by means of photolithography in the layer of photoresist and blanket mist deposition of NQD film follows. Subsequently, patterned photoresist is oxygen plasma ashed lifting off NQDs from the resist covered areas and leaving them in the areas not covered with photoresist.
The overall results obtained in this study demonstrate that, unlike other thin film deposition methods using colloidal solutions such as spin-on or micro-spray, mist deposition is compatible with a range of film patterning scenarios. In the full account of this work a complete set of experimental results obtained in the course of this investigation is presented and possible applications of the processes under investigation considered.

References

[1] A. Kshirsagar, et al., ECS J. S.S.S.T, 2(5), R87,
(2013).
[2] J. C. Chao, A. Kshirsagar, and J. Ruzyllo, ECS

Trans., 58(6), 311 (2013).