High-resolution patterning of colloidal quantum dots via non-destructive, light-driven ligand crosslinking

Establishing multi-colour patterning technology for colloidal quantum dots is critical for realising high-resolution displays based on the material. Here, we report a solution-based processing method to form patterns of quantum dots using a light-driven ligand crosslinker, ethane-1,2-diyl bis(4-azido-2,3,5,6-tetrafluorobenzoate). The crosslinker with two azide end groups can interlock the ligands of neighbouring quantum dots upon exposure to UV, yielding chemically robust quantum dot films. Exploiting the light-driven crosslinking process, different colour CdSe-based core-shell quantum dots can be photo-patterned; quantum dot patterns of red, green and blue primary colours with a sub-pixel size of 4 μm × 16 μm, corresponding to a resolution of >1400 pixels per inch, are demonstrated. The process is non-destructive, such that photoluminescence and electroluminescence characteristics of quantum dot films are preserved after crosslinking. We demonstrate that red crosslinked quantum dot light-emitting diodes exhibiting an external quantum efficiency as high as 14.6% can be obtained.

Supplementary Fig. 5 Line edge roughness (LER) analysis of line patterns (3 μm pitch, 4 μm line spacing) a 13-nm sized red-emitting CdSe/CdZnS QDs and b 8-nm sized greenemitting CdSe/CdZnSeS QDs were used for RG line patterns, respectively. The AFM images are converted to grayscale images to enhance the contrast at the edges of line patterns, which are depicted in yellow. LER of the patterns was analyzed using an image analysis tool, ImageJ. The resulting LERs for line patterns comprising red and green-emitting QDs were measured to be 0.14 and 0.15 μm, respectively.  (b) and (d) shows EL spectra of each device. The EL spectrum of a pristine QD-LED is vertically shifted for visual clarity. Fig. 10 Schematic illustration of synthetic route to ethane-1,2-diyl bis(4azido-2,3,5,6-tetrafluorobenzoate). Fig. 13 19 F-NMR spectrum of ethane-1,2-diyl bis(4-azido-2,3,5,6tetrafluorobenzoate). tetrafluorobenzoate).

Supplementary Fig. 15 Effective removal of an uncrosslinked QD layer formed on the
ZnO NP layer a Absorption and b photoluminescence (PL) spectra of a layer of ZnO NPs (black), a layer of QDs coated on the layer of ZnO NPs before (red) and after (blue) rinsing process. All samples were prepared on glass substrates. The samples were partially exposed to the UV source through a photomask and were rinsed with toluene. The blue curves were obtained from the section of the film that was covered with the photomask during the UV exposure, which is expected not to undergo the ligand crosslinking reaction.

Supplementary Discussion
Quantitative description of the photon-to-crosslink conversion process.
The entire incident photon-to-ligand crosslinking efficiency was estimated from the product of i) the number density of photons irradiated to the QD film, ii) the fraction of photons absorbed by LiXer, and iii) the efficiency of LiXer undergoing through the intended alkyl insertion. Each of these factors are obtained as follows. i) The QD films with LiXer were irradiated to a 254 nm light source (0.4 mW cm -2 ) for 5 sec. This corresponds to a dose of 2 mJ cm -2 (2.6 × 10 15 photons cm -2 ).  iii) The efficiency of LiXer undergoing through the intended alkyl insertion reaction could be estimated by considering the quantum efficiency for photo-activation of the fluorinated phenyl azides in LiXer and the subsequent C-H insertion reaction based on the activated nitrene.
It is known that halogenated phenyl azides have a very low probability of triplet or ring expansion reactions. [R2] Thus, we assumed that only the nitrenes in single state are generated.
Based on the change in the areal intensity of azide peak from FT-IR measurement on the film after UV exposure, we estimated that this efficiency to be ~73% (Supplementary Fig. 4). The resulting singlet phenyl-nitrenes can undergo various reactions processes such as (i) ring expansion, (ii) intersystem crossing, and (iii) insertion/addition reaction. Since the surface ligands of QDs we examined are 1-dodecanethiol (DDT), which consists of simple hydrocarbon chain and a thiolate binding group, the possibility of other insertion reaction (e.g., N-H insertion reaction or C=C cycloaddition reaction) is very low. Therefore, we assumed that nearly entire photo-activated nitrenes form chemical bonds with surface ligands of QDs through C-H insertion reaction.
Thus, based on the estimation above that there are ca. 10 molecules of LiXer reside in a single QD-QD linkage, we estimate that 7 molecules participate in the formation of a single QD-QD linkage.

Fabrication of μm-thick QD layers by repeating photo-crosslinking steps
Crosslinked QD films with thicknesses in hundreds of nm can be readily prepared by simply using a QD/LiXer solution of a higher concentration. For example, crosslinking a QD/LiXer film prepared by spin-coating (at 1000 rpm 30 sec) using a 100 mg mL -1 mixture solution yielded 340 nm-thick QD/LiXer films. Blade coating (adjusting the speed of a moving blade (5 cm sec -1 ) and a blade gap (100 μm) with thermal annealing at 70 °C) of the same solution yielded 2 μm-thick QD/LiXer films.
More importantly, the thickness of films can be increased by executing the deposition process multiple times, due to the structural tolerance of the crosslinked QD films against the solvent used to deposit the QD films (e.g, toluene) -the underlying crosslinked QD films would not be dissolved during the processing of the upper QD films. For example, by applying the same spin-coating/crosslinking protocol repeatedly, the thickness of the resulting QD film increased from 340 nm (prepared from the primary deposition) to 645 nm after the secondary deposition step and to 950 nm after the tertiary deposition step. By applying the same barcoating/crosslinking protocol repeatedly, the thickness of the resulting QD film increased from 2 μm (prepared from the primary deposition) to 4 μm after the secondary deposition step.
Successive deposition of a crosslinked QD layer on top of a crosslinked layer resulted in thick QD films yielding a transmission at 460 nm wavelength reaching below 10% ( Supplementary   Fig. 7).

Synthesis of blue-emitting CdZnS/ZnS QDs. CdZnS/ZnS
QDs were prepared via the reported procedure with minor modification. 2 As a typical synthesis, 1mmol of CdO, 10 mmol of Zn(ac)2, and 7 mL of OA were loaded to a flask and the mixture was degassed at 160 o C.
The flask was filled with argon gas before 15 mL of degassed ODE was added. After a clear solution of Cd(OA)2 and Zn(OA)2, the temperature of reaction flask was heated to 310 o C. 1.6 mmol of S dissolved in 2.4 mL of ODE was rapidly injected to the flask and reacted for 8 min.
For further growth of ZnS shell, 8 mmol of S dissolved in 16 mL of ODE was slowly injected and reacted for 2 hours. Synthesized CdZnS/ZnS QDs were purified repeatedly via typical precipitation/redispersion method.

Synthesis of ZnO nanoparticles for device fabrication.
ZnO nanoparticles were synthesised referring the method used by Lim et al. 3 First, 80 mL of 0.1-M zinc acetate dihydrate in methanol was placed in a three-neck round-bottom flask and heated to 60 °C. Then, 40 mL of 0.4-M potassium hydroxide with methanol was injected dropwise into the zinc acetate dihydrate solution with strong agitation. The mixture was kept at 60 °C for 2 h 15 min. After precipitation, the product was redispersed in 1-butanol.