Direct patterning of colloidal quantum dots with adaptable dual-ligand surface

Colloidal quantum dots (QDs) stand at the forefront of a variety of photonic applications given their narrow spectral bandwidth and near-unity luminescence efficiency. However, integrating luminescent QD films into photonic devices without compromising their optical or transport characteristics remains challenging. Here we devise a dual-ligand passivation system comprising photocrosslinkable ligands and dispersing ligands to enable QDs to be universally compatible with solution-based patterning techniques. The successful control over the structure of both ligands allows the direct patterning of dual-ligand QDs on various substrates using commercialized photolithography (i-line) or inkjet printing systems at a resolution up to 15,000 pixels per inch without compromising the optical properties of the QDs or the optoelectronic performance of the device. We demonstrate the capabilities of our approach for QD-LED applications. Our approach offers a versatile way of creating various structures of luminescent QDs in a cost-effective and non-destructive manner, and could be implemented in nearly all commercial photonics applications where QDs are used. A dual-ligand passivation system comprising photocrosslinkable ligands and dispersing ligands enables quantum dots to be universally compatible with solution-based patterning techniques.

C olloidal quantum dots (QDs) are promising materials for use in next-generation light sources due to their wide-ranging bandgap tunability, narrow spectral bandwidths and near-unity luminescence quantum yields (QYs) [1][2][3][4][5] . Together with the capability of cost-effective solution processing, QDs have become the key light-emissive materials for information displays 3,[5][6][7] . A patterned QD downconversion layer on blue light-emitting diodes (LEDs) renders high colour reproduction and ultrahigh image quality in full-colour displays 8,9 . Likewise, a laterally patterned array consisting of red, green and blue (RGB) QD-LEDs, in which QDs convert electrically pumped charge carriers into photons, allows for excellent colour gamut and brightness as well as lightweight, thin and flexible form factors [10][11][12][13][14][15] , which are suited for wearable near-eye displays for virtual reality and augmented reality devices. For these 'mixed-reality' applications, the QD deposition process should enable the patterning of RGB QDs (or RG QDs along with a blank) into few-micrometre subpixels over a large area with high precision and high fidelity 16,17 . At the same time, the process should not disrupt the optical and transport characteristics of QDs and adjacent functional layers.
Current QD-patterning methods include transfer printing 12,13,[18][19][20] , inkjet printing [21][22][23][24][25][26][27][28] and photolithography 11,21,[29][30][31][32][33][34][35] . Transfer printing typically covers small-area patterns, and standard instruments have yet to be developed. Inkjet printing allows for an effective approach in this regard, but works in a certain range of pattering resolution (500 pixels per inch (ppi) at the most with the assistance of a bank) due to the feature size of the ejected drops (highest diameter of 25-30 μm). On the other hand, photolithography has demonstrated a well-established fabrication process to create micro-to submicrometre-scale patterns over a large area on desired substrates. Moreover, from a practical standpoint, it poses great benefit as one can use equipment that is already deployed in display-device manufacturing steps for the patterning process. Thus, to take advantage of the photolithography technique, there is a clear need for a method that enables the non-destructive direct patterning of QDs via photolithography.
Conventional photolithography employs a photopatternable polymer layer, that is, a photoresist, that serves as a masking layer in forming QD patterns underneath 21,[29][30][31] . However, the solution processing for photoresist deposition and development could damage the underlying QD layers 30,31 . Moreover, already-patterned QD layers can readily dissolve as the subsequent QD layer is processed, as the patterning process must be carried out at least three times to get the true tone of RGB images. Without ensuring the structural robustness of QD patterns already in place, conventional photoresist-based photolithography cannot yield high-resolution full-colour QD-LEDs. Direct photolithography, a method in which light exposure directly induces a solubility change in QD layers, is considered as an effective solution to mitigate the aforementioned issues 11,[32][33][34]36 . Several approaches have been proposed, such as the detachment or decomposition of ligands on ultraviolet (UV) irradiation [32][33][34]36 . Yet, corrosive chemicals (for example, acids) are produced as byproducts from these reactions, deteriorating the luminescence efficiency of QDs 32,34 . Alternatively, azide-group-containing Direct patterning of colloidal quantum dots with adaptable dual-ligand surface photoactive crosslinkable additives can form relatively benign direct chemical bonds to the aliphatic ligands of neighbouring QDs 11 . However, the addition of electrically insulating crosslinker molecules can impair the transport properties of QD films and hence lower their electroluminescence performance.
Here we show a simple and versatile approach for the non-destructive direct patterning of QDs within standard microfabrication systems. Specifically, we design and synthesize QDs passivated with dual ligands consisting of photocrosslinkable ligands (PXLs) and dispersing ligands (DLs). PXLs are structurally engineered molecular scaffolds that form covalent bonds with neighbouring ligands in response to UV-A irradiation, enabling high-throughput QD patterning without compromising their optical properties. The freedom to modify DLs confers solvent versatility, allowing the present approach to be compatible with nearly any solution-processing technique including spin coating and inkjet printing. Using dual-ligand QDs, we demonstrate fine QD patterns (up to 15,000 ppi), which can be applied to the light-emissive layers of downconversion and electroluminescent devices, as well as assess the feasibility of the present approach for use in state-of-the-art photonic devices.

Dual-ligand passivation of QDs
We devise QD materials that can be processed via photolithographic processes without the presence of photoresists and photoinitiators. Specifically, the surface of the QDs is functionalized with PXLs and DLs ( Fig. 1). PXLs are linear organic compounds that hold a thiol anchor group (-SH) at one end and a benzophenone derivative at the other end (Fig. 1a,b and Supplementary Figs. 1 and 2). The thiolate end group of PXLs has a greater binding affinity to the QD surface than that of oleate ligands 37,38 . Therefore, PXLs can readily displace the native ligands of as-synthesized QDs at room temperature following the typical ligand exchange protocol (Methods). The benzophenone derivatives on the other end of PXLs are well-known photoresponsive moieties that are extensively used for photoinitiators, photophysical probes or photocrosslinkers [39][40][41][42][43] . On UV irradiation, the carbonyl group of the benzophenone moiety transforms into ketyl radical by abstracting a hydrogen atom from a hydrocarbon group nearby (hydrogen abstraction) and creates a covalent bond between the ligands of neighbouring QDs (Fig. 1c). The chemically crosslinked QD films are no longer dispersible when a solvent is applied. Hence, we can achieve QD patterns by selective UV irradiation on QD films followed by development with good solvents (Fig. 1d,e). We note that only a fraction of PXL displacement, less than 10 mol% of the entire bound ligands, is necessary for fully crosslinking in the QD film ( Supplementary Fig. 3). Thus, we can control the solubility of QDs in different solvents by engineering the rest of the majority ligands (>90 mol%) (Fig. 1b).

Non-destructive photocrosslinking of QD films
Benzophenone has a relatively low absorbance in the UV-A (320-400 nm) region. Thus, there are substantial limitations on its use with industrial-standard light sources for optical lithography (i-line; peak wavelength, 365 nm). To boost the photosensitivity of PXLs to UV-A, we use reverse engineering to design the chemical structure of a benzophenone moiety via density functional theory calculations ( Fig. 2a,b, Supplementary Note 3 and Supplementary Figs. 1 and 2). Specifically, we substitute electron-donating groups (namely, pyrrolidinyl (-N(CH 2 ) 4 ), oxy (-O-) and thio (-S-)) to the para positions of benzophenone to increase the oscillator strength in the UV-A region. The electron-donating groups in place of hydrogen enhance the oscillator strength in the UV-A region of each PXL. The enhancement appears more pronounced with a stronger electron-donating substituent, and the effect is even more evident when two substituents are in place. For example, the extinction coefficient of PXL increases from 60 to 106 and 588 M −1 cm −1 (at 365 nm) by substituting the -O-and -S-groups to the para position of benzophenone in replacement of a proton, respectively, and soars to 2.02 × 10 4 M −1 cm −1 (at 365 nm) with -N(CH 2 ) 4 and -Ssubstituents at both para positions of benzophenone (Fig. 2b). PXLs with different substituents to benzophenone are denoted as O-BP, S-BP and NS-BP.
Because of the greater binding affinity of thiolates to the QD surface, PXLs could readily displace native oleate ligands with a graft yield (the ratio of the grafted versus added PXLs) above 80% ( Supplementary Fig. 4). The extent of grafted PXLs could be altered by varying the added content of PXLs. Ultraviolet-visible, Fourier-transform infrared and 1 H nuclear magnetic resonance (NMR) spectra of dual-ligand passivated QDs show the characteristic peaks of benzophenone moieties even after repeated purifications (Supplementary Figs. [5][6][7][8], indicating that PXLs are strongly bound on the surface of QDs. The small fraction of PXL displacement under the mild reaction condition allows QDs to retain their photophysical characteristics throughout the ligand displacement procedure ( Supplementary Fig. 9). It is noted that graft of less than 10 mol% PXL does not impair the oxidative stability of QD solution or QD films ( Supplementary Fig. 10) . Dual-ligand QDs with 7 mol% PXLs are used for subsequent experiments in the manuscript, unless otherwise noted.
QD deposition followed by exposure to a UV source results in chemically crosslinked QD films. The degree of crosslinking is assessed by monitoring the changes in the absorbance of QD films after soaking the films into the mother solvent used for QD deposition. Figure 2c summarizes the relative variations in the film absorbance for InP (r = 1.2 nm)/ZnSe x S 1-x (h = 2.3 nm) QD films crosslinked under different conditions, which we refer to as the film retention ratio. Due to their enhanced extinction coefficient, QD films bearing NS-BP could be completely crosslinked even under an exposure dose of 35 mJ cm -2 , which corresponds to an exposure time of 1.4 s under commercially available i-line light source (radiation power, 25 mW cm -2 ). Figure 2d contrasts the photochemical response of PXLs on exposure to an i-line light source. Even after dipping into the mother solvent for a few minutes, NS-BP-decorated QD films barely dissolve, whereas other QD films are partially washed off on brief contact with toluene.
The photophysical properties of QDs are preserved throughout the ligand exchange process, photocrosslinking and development ( Fig. 2e and Supplementary Fig. 11). The mild ligand displacement condition prevents the degradation of the photoluminescence (PL) characteristics of PXLs that are grafted onto the surface of QDs. The quick photocrosslinking reaction with the lower-energy photon source is benign enough to retain the photophysical properties of QDs. Specifically, a crosslinked NS-BP-decorated QD film with a film retention ratio greater than 0.95 could be readily obtained without compromising the photoluminescence quantum yield (PL QY), whereas substantial PL QY drops (24-26%) appear during photocrosslinking processes in the cases of S-BP and O-BP (Fig. 2e). Control experiments under an inert atmosphere show that the reactive oxygen species generated under UV-A irradiation, rather than ketyl radicals, are responsible for the PL QY loss of QDs ( Supplementary Fig. 12), implying that the UV irradiation time for photocrosslinking is critical to the optical quality of the QD patterns. By exploiting NS-BP-decorated RGB QDs with minimal exposure to UV-A irradiation, the resulting QD films retain their photophysical properties (PL spectra and PL QY) and film morphologies throughout the photocrosslinking and rinsing steps under ambient conditions (Fig. 2e and Supplementary Fig. 13). We note that InP (r = 1.9 nm)/ZnSe x S 1-x (h = 3.2 nm) QDs, InP (r = 1.2 nm)/ ZnSe x S 1-x (h = 2.3 nm) QDs and Cd x Zn 1-x S (r = 2.7 nm)/ZnS (h = 3.6 nm) QDs are the red (R), green (G) and blue (B) emitters used in this study, respectively (Supplementary Fig. 14

Direct patterning of dual-ligand QDs
We prepared QD photopatterns in three steps: (1) QD deposition onto a substrate, (2) selective UV irradiation (365 nm, 35 mJ cm -2 ) through a patterned photomask and (3) removal of the uncrosslinked QDs using a proper solvent. The present approach using conventional photolithography equipment (for example, stepper or contact aligner) renders well-defined QD patterns of varying shapes and dimensions with high fidelity (Fig. 3a-e). Repeating the processes with red, green and blue QDs renders QD patterns with various colours. For example, the lateral deposition yields RGB QD patterns with a resolution greater than 7,000 ppi ( Fig. 3b and Supplementary Fig. 15), which meets the requirement for near-eye hyper-realistic displays. The vertical stacks of QD patterns enable RGB primary-colour combinations that can express multiple colours, including yellow, cyan and magenta, and ultimately full-colour images after the delicate pixel design (Fig. 3c-e and Supplementary Fig. 16).
The present approach demands PXLs with less than 10 mol% of surface ligands, and thus, the solubility of QDs is still dictated by the remaining ligands. This implies that structural engineering of the remaining DLs into ones containing polar (that is, mono-2-(methacryloyloxy)ethyl succinate (MMES)) or fluorinated (that is, 4-(trifluoromethyl)benzenethiolate (TFMBT)) tails permits dual-ligand QDs to be processed with solvents of different polarities across polar organic solvents (for example, propylene glycol methyl ether acetate (PGMEA) or diethylene glycol monoethyl ether acetate (DGMEA)) and fluorinated solvents (for example, trifluorotoluene (TFT)) ( Fig. 3f and Supplementary Figs. 17 and 18). The present approach renders heteroligand QDs into the desired solvent that even PXL itself can barely dissolve in (that is, hexane, PGMEA, DGMEA and TFT). This clearly contrasts to the previous approaches recruiting crosslinkable polymeric ligands 44 or additives 11 , wherein the solubility of QD dispersions for crosslinking is restricted to the solubility of crosslinkable agents.
Importantly, the versatility in controlling the solvent media offers extensive compatibility of our ligand-engineered QDs with nearly any solution processing methods. For instance, the dual-ligand-passivated QDs can readily integrate with inkjet printing, which is a low-cost, large-area and non-vacuum process capable of depositing micrometre-scale patterns without needless waste of materials 22,25,45,46 . The fluidic characteristics of inks (that is, viscosity, surface tension, density and inertial force) should meet the criteria for inkjet printing 21,47,48 with regard to the minimal volume of ink droplets and uniformity of the deposited films. We accomplish well-defined QD ink droplets by means of ligand engineering and solvent optimization (MMES for the dispersion ligand and PGMEA/DGMEA co-solvent (PGMEA:DGMEA = 40:60 vol%)) that produce clear QD patterns via inkjet printing (Fig. 3g). A short period of UV irradiation onto the printed patterns confers structural robustness to the predeposited QD patterns against exposure to subsequent solution processes, enabling the assembly of multiple functional materials with well-defined interfaces. The process orthogonality eliminates the colour-blurring effect at the crossings of patterns, and each pattern is clearly represented by its own colour ( Fig. 3h and Supplementary Fig. 19).

optoelectronic characteristics of photocrosslinked QD films
The present approach neither changes the transport characteristics of charge carriers (as it does not involve additional photocrosslinkable agents, which are typically electrical insulators) nor does it expand the effective ligand length. We compare electron and hole transport from neighbouring charge transport layers into pristine versus photocrosslinked CdSe (r = 2.5 nm)/Cd x Zn 1-x Se/ZnSe y S 1-y (h = 8.0 nm), CdSe (r = 2.0 nm)/Cd x Zn 1-x Se/ZnSe y S 1-y (h = 7.7 nm) and InP (r = 1.9 nm)/ZnSe x S 1-x (h = 3.2 nm) QD films in the electron-only device and hole-only device, respectively ( Fig. 4a and Supplementary  Fig. 20). Notable differences are not observed in the current densityvoltage characteristics for both devices, indicating that photocrosslinking with PXLs does not impair the transport characteristics of the devices implementing them. The optical properties of QD films and electrical characteristics of devices are well preserved throughout the implantation of PXLs and the photocrosslinking steps, allowing us to fully exploit the performance of optoelectronic devices employing QDs. Figure 4b,c demonstrates the optoelectronic characteristics of LEDs implementing the QD film as the emissive layer. Nearly identical electrical and optoelectronic performances are observed for QD-LEDs with pristine QD films or photocrosslinked QD films even after the development step ( Fig. 4c and Supplementary Fig. 21). As the device performance is determined by electron versus hole injection balance in QDs and the charge-to-photon conversion efficiency, these results confirm that the photochemical reaction of PXLs does not leave electrical or optical defects either at the QDs or in the neighbouring charge transport layers. Finally, we use our approach to create passive-matrix-driven RGB QD-LED arrays by positioning photopatterned RGB QD films The inset shows the semi-log plots of molar extinction spectra for pXLs and the unsubstituted benzophenone between 300 and 450 nm. c,d, Exposure-dose-dependent film retention ratios (c) and fluorescent images of QD films having different pXLs (all the films are exposed to UV radiation with an exposure dose of 630 mJ cm -2 and rinsed with toluene) (d). The error bars in c indicate the standard deviations of the data acquired from five independent runs. Scale bars, 50 μm. e, Normalized pL QYs of QD films (film retention ratio, >0.9) employing different pXLs after the photocrosslinking and rinsing steps under ambient condition. The error bars represent the standard deviations of five independent runs. Exposure-dose-dependent changes in the pL QY of QD films exposed to different wavelengths of UV sources (namely, 365 nm (blue) and 254 nm (purple)) are overlaid for comparison. f, pL spectra of photocrosslinked RGB QD films with NS-Bp. Inp (core radius, r = 1.9 nm)/ZnSe x S 1-x (shell thickness, h = 3.2 nm) QDs, Inp (r = 1.2 nm)/ZnSe x S 1-x (h = 2.3 nm) QDs and Cd x Zn 1-x S (r = 2.7 nm)/ZnS (h = 3.6 nm) QDs are adopted as the red, green and blue emitters, respectively. A fixed amount of pXLs (7 mol%) is grafted to each coloured QD. All the QD films are exposed to UV-A (365 nm; exposure dose, 35 mJ cm -2 ). between common charge transport layers and patterned electrodes ( Fig. 4d-h, Supplementary Video 1 and Supplementary Fig. 22). The use of dual-ligand QDs is not limited to PR-free, direct QD patterning, but can also be used in conventional photopatterning methods. For instance, state-of-the-art displays employ submicrometre-to-micrometre-thick, photoemissive red and green QD patterns directly on blue-emitting organic light-emitting devices (OLEDs), which demands a rather stringent QD-patterning process capable of being conducted at low temperatures (below 100 °C) to prevent OLEDs from thermal damage. NS-BP-decorated QDs blended with transparent polymeric resins can be photocrosslinked by a low-energy UV-light source (i-line) with a higher penetration depth, which are suited for attaining micrometre-thick fine QD patterns for full-colour QD-OLED displays (Supplementary Note 4 and Supplementary Fig. 23).

Conclusions
In summary, we have demonstrated a non-destructive, adaptive approach for the direct pattering of luminescent QDs. We have devised dual-ligand QDs consisting of PXLs and DLs. We have shown that we can perform structural engineering on both ligands to seamlessly integrate them into the industrial-standard microfabrication processes. Structural engineering on a photoreactive benzophenone moiety boosts the photochemical responsivity of PXLs to low-energy UV-light sources, whereas the polarity control of DLs confers solvent versatility. We successfully create well-defined multicoloured QD patterns with commercialized photolithography or inkjet printing systems at no cost to the optical or electrical properties of QDs. In addition, we test these photocrosslinked QDs in state-of-the-art displays. These results demonstrate the effectiveness of the present approach in nearly all photonic applications employing QDs. Device characteristics with pristine QDs (oleic acids only) are shown for comparison. CBp and ZnMgO are used as the hole transport layer (HTL) and electron transport layer (ETL), respectively, for EOD, HOD and QD-LEDs. The inset shows a photograph of the operating QD-LED. d, Schematic showing passive-matrix-driven 10 × 10 RGB QD-LED arrays employing patterned QD films. e,f, Cross-sectional schematic (e) and associated electric circuit of RGB pixels (f). g,h, Electroluminescent images of 10 × 10 RGB QD-LED arrays (g) and QD-LED array of each primary colour (h). Scale bars, 2 mm. All the QD films are prepared by spin casting and photolithography. Supplementary Fig. 24 shows the other device applications.

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