Asymmetric Wettability Interfaces Induced a Large-Area Quantum Dot

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Asymmetric wettability interface induced large-area quantum dots micro-structure toward high-resolution QLEDs. Xiaoxun Li, Binbin Hu, Zu-liang Du, Yuchen Wu, and Lei Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08603 • Publication Date (Web): 15 Jul 2019 Downloaded from pubs.acs.org on July 21, 2019

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ACS Applied Materials & Interfaces

Asymmetric Wettability Interface Induced Largearea Quantum Dots Micro-Structure Toward HighResolution QLEDs Xiaoxun Li† , Binbin Hu†*, Zuliang Du†, Yuchen Wu‡* and Lei Jiang‡

†Key

Laboratory for Special Functional Materials of Ministry of Education, National & Local

Joint Engineering Research Centre for High-efficiency Display and Lighting Technology, School of Materials and Engineering, Collaborative Innovation Centre of Nano Functional Materials and Applications Henan University Kaifeng 475004, P. R. China ‡Key

Laboratory of Bio-inspired Materials and Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences Beijing 100190, P. R. China

KEYWORDS：quantum dots, asymmetric wettability, assembly, micro-structure, QLEDs

ACS Paragon Plus Environment

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ABSTRACT Precisely patterning large-area quantum dot (QD) nanoparticles is an essential technique for enhancing high-resolution and high-performance in the next generation display --- QLEDs. However, conventional solution-based assembly techniques suffer from trade-offs between largescale and spatial precision. As such, large non-defect areas and ordered stacking of QD assembly architectures are difficult to achieve, and both are essential to fabricating a high-performance device. Herein, we demonstrate a facile method for assembling the QD nanoparticles into a micro-structure using an asymmetric wettability template to regulate the dewetting process. The wettability difference of the interface induces the continuous liquid film to recede into individual liquid-bridges, which enabled unidirectional dewetting and regulated the QD solution mass transport. In addition, due to the asymmetric wettability between the substrate and template, large-scale, ultra-fine (1 μm), highly-flat micro-wire QD arrays with precise position and strict alignment are easily assembled and transferred onto the target substrate. The method has been further introduced into the fabrication of high-resolution patterned QLEDs devices, with maximum electroluminescence of 73,490 cd/m2, 4357 cd/m2, and 950 cd/m2, for green, red and blue, respectively. This research provides a novel and facile perspective for manufacturing highresolution and high-performance patterned QLEDs device.


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Introduction Colloidal quantum dots (QDs) have been intensely investigated because of their highly advantageous characteristics, including low-cost solution-processing, narrow emission spectra, tunable emission wavelength, and high luminescent efficiency;1-7 all of which make them the most popular material in quantum dots light-emitting diodes (QLEDs) technology.8-11 Of particular interest is using a patterned QD micro-structure as the light-emitting layer in QLEDs, for the purpose of implementing them into new candidates for the next generation display.12-15 Recently, various approaches have emerged to utilize solution-based fabrication processes in order to deposit patterned QD micro-structures in QLEDs devices, such as inkjet printing,16-21 transfer printing,22-26 and mask printing,27-29 among others.30-32 However, all these techniques suffer from trade-offs between high-cost, complex processing, large-scale, and spatial precision. Thus, developing a facile, feasible, innovative technique to jointly mitigate all the issues associated with assembling a QD micro-structure in QLEDs devices remains a significant challenge. Generally, miniaturized liquid droplets or thin film are located with predetermined patterns for regulating the spatial position, dewetting and evaporation of liquid necessary to assemble a QD micro-structure. However, owing to the solution’s random and uncontrollable dewetting dynamics, the long-range ordered QD nanoparticles are difficult to precisely assemble. The gasliquid-solid three-phase contact line (TCL), which are pinned or unpinned,33-34 (called coffee-ring effect) on the substrate will disorder the assembly blocks in the dewetting process.35-36 Thus, large non-defect areas and ordered stacking of QD assembly architectures are difficult to achieve. Therefore, to obtain a precisely patterned QD micro-structure, a directional force is required to


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induce the QD solution fluidic flow and mass transport. Recently, Jiang et al.,37-42 designed a highly efficient method to manipulate the dewetting dynamics of various solutions, yielding long-range ordered 1D assembly micro-structures. However, the ultra-small size of QD nanoparticles (ca. 10 nm), combined with the fact that they are easily influenced by the coffeering effect35,36 during solution-processed deposition, makes it difficult to induce the fluidic flow and enable mass transport for precise assembling. So, fabricating patterned QD micro-structure arrays with large areas, high density, and strict alignment remains a significant challenge. Herein, we demonstrate a simple and effective method for assembling QD micro-structure arrays by using an asymmetric wettability template to regulate the dewetting process. In this approach, the line-shaped micro-pillar structure on the template plays an important role in generating an asymmetric wettability interface, which manipulates the QD solution unidirectional dewetting. As the solution recedes in the assembly system, the wettability difference of the interface induces the continuous liquid film to recede into individual liquidbridges and become pinned on top of the micro-pillar, which enables unidirectional dewetting and regulates the QD solution mass transport. Due to Laplace pressure,40,43 the directional receding of the QD solution drives the moving nanoparticles from the gaps to the micro-pillar tops. Subsequently, a QD micro-structure with precise position, good homogeneity, and ultrasmooth texture is assembled and transferred onto a select area, accompanied by a controllable TCL receding process of the individual liquid bridge. Two essential elements have the asymmetric wettability in this sandwich system to manipulate the liquid dewetting dynamics, and yield a large non-defect area and long-range order QD micro-structure on the target substrate. One is the asymmetric wettability between the silicon grooves and the micro-pillars, which ensures the formation of individual liquid-bridges. The other is the difference in wettability


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between the flat substrate and micro-structure template, which attains a facile transfer of the assembled micro-structure QD arrays without any residual. Here, we assembled large-scale (300 × 250 μm2), ultra-fine (1 μm), highly-flat micro-wire QD arrays with precise position and strict alignment. Of particular note, the well-assembled QD micro-structure arrays are successfully implemented into the QLEDs device as the high-resolution light-emitting layer. Furthermore, we fabricated the high-resolution and high-performance patterned QLEDs devices, with a maximum electroluminescence of 73,490 cd/m2, 4357 cd/m2, and 950 cd/m2, for green, red and blue, respectively. It is expected that this technique will present a new alternative to enhance highresolution and high-performance of QLEDs display.

Result and Discussion In order to assemble patterned QD micro-structure arrays, a 2 μm wide, 10 μm high, micropillar, structured silicon template was prepared with a 5 μ m gap. An optical image of the top view is shown in Figure S1 (Supporting information). UV micro-replication against a nickel master structure was employed to manufacture the structured template; via deep reactive ion etching (DRIE, details in experimental methods). Next, the substrate was modified with a monolayer of FAS by means of vapor deposition (see experimental methods). Thus, due to interaction between the FAS and the micro-structure, the template became superhydrophobic, and the water contact angle increased from 110° ± 2.1° to 150° ± 2.7° (Figure S2). Therefore, the asymmetric wettability interface was formed and then used for manipulating the unidirectional solution dewetting. Figure 1 schematically depicts the solution dewetting mechanisms and regulating process on the asymmetric wettability interface to yield the long-range ordered assembly. In this sandwich


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system, there are two essential parts of the asymmetric wettability interface that enable QD nanoparticles to assemble on the target substrate. 1) Between the groove and pillar. When the silicon template was modified by FAS, the wettability difference increased between the template’s groove and pillar (Figure S2), which ensures the formation of individual liquidbridges. 2) Between the flat substrate and micro-structure template. As shown in Figure 1a, the oil (octane) contact angle on the template is 45° ± 1° and on the substrate is 2° ± 0.5°, which achieves a facile transfer of the assembled micro-structure QD arrays onto the substrate without any residual. Figure 1c-f is a cartoon schematic that demonstrates in detail this method’s assembly process. At first, a continuous liquid film is immersed into the grooves between the silicon micro-pillars template and the substrate (Figure 1c). With the development of solvent evaporation and solution dewetting, continuous liquid film dewetted to one edge of the template, leaving liquid-bridges on the tops and liquid-tails in the gaps (Figure 1d). After the dewetting of all continuous liquid film, individual liquid-bridges were anchored on top of the micro-pillars (Figure 1e). Consequently, QD nanoparticles were assembled onto the target substrate with regulated positions after the solvents totally evaporated (Figure 1f). Note that the cartoon detailing the assembling process mechanism in the top pinned liquid bridges is shown in Figure S3. To gain further insight into the mechanism of this approach, in situ fluorescent microscopy, with an excitation source under 375 nm, was used to observe the evaporation and dewetting process of the QD solution immersed in the sandwich system (Figure 1g-j). At first, the QD solution was fully filled into the micro-grooves and confined between the bottom micro-pillars and top substrate (Figure 1g). With evaporation of the solvent, the QD solution filled in the silicon gap where unidirectional dewetting was evaporation-driven to the edge of the template at


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the first stage (see Figure S4). Using an optical microscope, we observed the QD solution receding process while confined in the sandwich system. Every 60 sec, the receding process in the silicon groove was imaged. The red line indicates a single groove receding process direction, which is toward the template edge. Yet, the residual QD solution was pinning onto the top of the micro-pillars, and left the individual liquid-bridges anchored on the micro-pillar tops and the crescent liquid-trails in the gaps’ rectangular spacing (Figure 1h). Thus, there is a unidirectional shearing force inside the liquid-trails which forces the movement of QD nanoparticles into the pinning liquid-bridges (Figure 1b). There are two primary reasons these asymmetric dewetting and nanoparticles moving behavior is triggered in this sandwich system: 1) Difference of equivalent capillary size40 and 2) asymmetric wettability interface.37,38 When solution is confined in the sandwich system, due to the equivalent capillary size difference between the template tops and grooves, higher Laplace pressure emerges at the top of the micro-pillars, giving rise to this directional dewetting and nanoparticles movement behavior. Similarly, the asymmetric wettability interface between the micro-grooves and the micro-pillars induces the formation of the individual liquid-bridge. Therefore, a continuous liquid film was divided into individual liquid-bridges anchored on top of the micro-pillars and no QD nanoparticles were left in the gaps, as confirmed by the absence of fluorescence (Figure 1i). After the solvent of each pinned liquid-bridge was totally evaporated, the QDs were assembled into the micro-structure arrays with precise position and strict alignment on the select area (Figure 1j). Finally, due to the asymmetric wettability between substrate and template (Figure 1a), the individual liquid-bridge had an inverted trapezoidal section profile with a different contact angle θ1 and θ2, θ1 > θ2, causing the liquid to easily adhere to the substrate. As such, the patterned QD arrays could be


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easily transferred onto the desired substrate. Therefore, this technique offers a novel and facile approach for assembling QDs into long-range 1D micro-structures. Temperature and solution concentration are the two essential variables, so serial experiments were designed and conducted to identify optimal condition for both. Figure S5 shows that when the temperature was < 140 ℃, all the transferred QD micro-arrays were positioned with precise position and strict alignment. In contrast, when the temperature is > 140 ℃ , which is 15 ℃ higher than the solvent boiling point, the assembled micro-wires became disordered and chaotic. Obviously, a higher temperature accelerates the dewetting speed and shortens the fabricating time. After correlating the relationship between the assembly speed and the ambient temperature, it was determined that the assembly time can be shortened to within 5 minutes, if the temperature is > 100 ℃ (Figure S6). However, a temperature > 100 ℃, will lead to irreversible damage of the QLEDs and other functional layers. With respect to solution concentration, when < 8 mg/mL, the bubble area on the covered substrate emerged because of inadequate solution evaporation (Figure S7). When > 8 mg/mL, the assembled QD micro-arrays were positioned with precise position and strict alignment. Interestingly, using our method, the width of each single microstructure increased from 1 to 3 μm as the concentration increased from 8 mg/mL to 15 mg/mL. As such, high-quality QDs micro-structure could be achieved by adjusting solution concentration and ambient temperature. Using this strategy, we prepared the large-area and long-range ordered QD micro-structure array, with precise position and strict alignment observed in Figure 2. Figure 2a is a TEM image that depicts green, homogeneous, mono-dispersed CdSe/ZnS QD nanoparticles, with a diameter of 11 nm. TEM images for two other QD nanoparticle colors, and their absorption and normalized fluorescence spectrum are also present in Figure S8. Figures 2b and 2c show the


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photoluminescence (PL) image of large-area and long-range ordered green QD assembly arrays under a 375 nm laser as an excitation source and its corresponding optical image. Note that at high-resolution, the patterned QD arrays exhibit good uniformity, with a 1 μ m width, 5 μ m interval, and no residual gaps. The SEM image of the patterned QD micro-wire arrays with an ultra-smooth surface and distinct boundary is illustrated in Figure 2d. PL and SEM images of the large-area (300 × 250 μm2) QD micro-wire arrays are also shown in Figure S9. Furthermore, the zoom-in SEM image (Figure 2e) also depicts the smooth surface and single assembled QD micro-wire straight boundary. The corresponding AFM image and topography diagram (Figure 2f and 2g) further demonstrate the trapezoidal QD micro-structure assembly, of which the height is 150 nm, and the top and bottom width of a typical micro-structure is ~ 1 µm, and 1.5 μ m, respectively. These widths are smaller than that of the 2 µm wide micro-pillars, because of the dewetting and shrinking of the liquid-bridges pinned on top of the micro-pillars. In addition, low surface energy of the FAS modified micro-pillars template is an important property for stably assembling good homogeneity and distinct QD micro-structure boundaries. Without FAS modification, there would be a highly rough surface and disordered boundaries in the QD microstructure (Figure S10). Furthermore, by changing the silicon template’s micro-structure, various shapes of the patterned QD assemble block, such as point, triangle, quadrangle, square, pentagon, and hexagon, were achieved (Figure S11). In this work, an established method of assembling QD micro-arrays has been extended to fabricate a high-resolution patterned QLEDs device (Figure 3). QLEDs are composed of multiple layers, all of which are functional layers that contribute to making it an excellent lighting device43-45 . Essentially, indium tin oxide (ITO), Poly (ethylenedioxythiophene): polystyrene sulphonate (PEDOT:PSS,), a poly (9,9-dioctylfluorene-co-N-(4-(3-methy-lpropyl))


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diphenylamine) (TFB), QDs layer, ZnO layer, all sit top Al electrodes (Figure 3d). An energy level diagram of each layer is depicted in Figure 3e, and theoretically confirms the practicality of this multi-layer device. In this device, TFB is hole transport layer (HTL) and ZnO is electron transport layer (ETL)and the QD film had been assembled between HTL and ETL. The cartoon in Figures 3a-c, schematically demonstrate the process of fabricating integrated patterned QLEDs. The detailed preparation process requires a special substrate (ITO glass) to be successively spin-coated with PEDOT: PSS and TFB layers, then used to cover an asymmetric wettability template to form a sandwich system (Figure 3a). The system was then held at 80 ℃ ambient temperature for 5 minutes. Thus, the patterned QD micro-wire arrays were assembled and transferred onto the target substrate (Figure 3b). As a consequence, we integrated a patterned light-emitting layer into the QLEDs after spin-coating a ZnO layer and evaporating the Al electrode (Figure 3c). Therefore, the QLEDs device with a high-resolution patterned QD layer was simply and effectively generated using the method described. Figure 4 demonstrates the different-color patterned QLEDs devices, for red, green, and blue, respectively; and the performance of these patterned devices were also evaluated. Figures 4a-c, show the Photoluminescence (PL) image of the different color patterned QD micro-arrays, with a 1 μm width and 5 μm interval. Figures 4d-f show the electroluminescence (EL) photographs of the patterned QLEDs operating at 4V. Each of the QLEDs devices exhibits a high-resolution property due to using the patterned QD micro-arrays as the light-emitting layer. For the red, green, and blue, respectively, the maximum luminance of these devices is 4357 cd/m2, 73490 cd/m2, and 950 cd/m2 (Figure 4h); maximum EQE is 0.8%, 1%, and 0.08%; and the normalized electroluminescence spectrum is 634 nm, 530 nm, and 434 nm (Figure S12). Clearly, the QLED devices with a patterned light-emitting layer have much lower luminance and higher current


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density than the QLED device with a membrane layer, because of the smaller luminance area and current leakage (Figure S12). However, these issues can be resolved by using full-color patterned QD micro-arrays with interval red, green, and blue assembled QD nanoparticles to fill up the active layer. This system can then be integrated into full color QLEDs devices for the next generation display. Finally, Figure 4g displays the CIE coordinates of three-color QLEDs at their corresponding EL maximum: the red are in (0.5689, 0.2698), green (0.1698, 0.7402), and blue (0.1635, 0.1437). The wide color extent further indicates that this patterned QLEDs put forward a novel alternative for fabricating high-resolution and high-performance QLEDs display.

Conclusions In summary, we developed and exploited a facile approach to utilizing the asymmetric wettability interface for regulating the liquid dewetting process and effectively applied it in assembling QD nanoparticles into large-area and long-range ordered micro-arrays. In this method, the interface wettability difference induced the continuous liquid film to recede into individual liquid-bridges, which enabled unidirectional dewetting and regulated mass transport of the QD solution. Thus three colors, high-flatness and high-resolution patterned QD microarrays with a 1 μ m width and a 5 μ m interval were assembled on the target substrate. We implemented the patterned QD micro-arrays into the QLEDs lighting device, which resulted in high-resolution, high-performance QLEDs devices with a maximum luminance of 4357 cd/m2, 73490 cd/m2, and 950 cd/m2 for red, green, and blue QLEDs, respectively. Therefore, we demonstrated a novel technique for developing high-resolution patterned QLEDs device.


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Experimental Methods Fabrication of Micropillar-Structured templates: Silicon wafers (10 cm diameter, N doped, oriented, 400 µm thick) were structured using a direct laser writing apparatus (Heidelberg DWL200 Instruments Mikrotechnik, Germany) that transferred the computer predefined design onto the photoresist (Shipley Microposit S1800 Series) coated wafer with ~1 µm precision. After irradiation and development, the wafers were etched using deep reactive ion etching (DRIE, Alcatel 601E) with fluorine-based reagents, from 10 sec - 6 min, depending on the desired height of the structures. Pillar-structured silicon substrates with tunable pillar top areas, pillar gap, and pillar top shapes were fabricated. After resist stripping (Microposit Remover 1165), the substrates were cleaned using ethanol and acetone. The superhydrophobic and highly-adhesive pillar-structured silicon substrates were formed by silanizing the silicon substrates with heptadecafluorodecyltrime-thoxysilane (FAS) in a decompression environment at room temperature for 10 min, and then heating them at 120 ℃ for 3 hr, resulting in reproducibly homogeneous and highly hydrophobic surfaces. Then, the contact angle of water and oil had been measured. (The solvent of our QD solution is octane, and we also use the octane as the oil for contact angle measurement.) Patterning of the quantum dots active layer: To begin, quantum dots, which had previously been deposited on two functional films, were assembled on an indium tin oxide (ITO) substrate and stored in a glove-box. Then a 10 mg/mL QD solution was prepared with the solvent octane. Next, the silica micro-pillar structured templates were laid out on a horizontal desk. Using a


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micro-syringe, 3 μ L of the 10 mg/ml QD solution was dripped on the silicon template; over which the solution spontaneously spread in a semi-ellipse. A sandwich system was then manufactured by covering the silicon template with the ITO substrate containing the two functional layers, and the final specialized structure was kept at 80 ℃, for 30 min. After that, we pierced the ITO substrate with the patterning quantum dots micro-wire arrays and put it into the glove-box for the next procedure. Preparation of the device substrate and finish the devices: The patterned ITO glass substrates were submerged in staining gel mixed with acetone, methanol, and deionized water for 20 min; then subjected to UV-ozone treatment for 10 min. After that, the cleaned substrate was moved into the glove-box, where it was held in an N2 environment. Poly (ethylenedioxythiophene): polystyrenesulfonate (PEDOT: PSS) was spin-coated onto the ITO substrates at 3000 r/min for 30 sec, then annealed at 120 ℃ for 20 min. The PEDOT: PSS film serves as the hole injection layer in the QLEDs luminescent device. Next, 1.5 wt% of 9,9-dioctylfluorene-co-N-(4-(3-methylpropyl)) diphenylamine (TFB) in chlorobenzene was prepared and spin-coated atop the PEDOT: PSS film, in the N2 environment at 2000 r/min for 30 sec. This hole transport layer (HTL), situated atop the hole injection layer, was then baked at 150 ℃ for 30 min; and the device was returned to the glove box. After patterning of the QD luminescent layer via the asymmetric wettability template, ZnO nanoparticles were deposited onto the top layer of the device via spincoating at 2000 r/min for 45 sec, then annealed at 60 ℃ for 20 min. Finally, Al cathodes were deposited sequentially on top of the ZnO layer using the deposition apparatus. The entire sequence will eventually be encapsulated into a quantum dots light-emitting device. Characterization of QDs micro-structure and QLEDs device: Photoluminescence (PL) images and in situ observation image were taken by fluorescence microscope (OLYMPUS, BX53). An


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atomic force microscope (AFM) (FASTSCANBIO, Bruker, Germany) and scanning electron microscope (SEM) (S4800, Hitachi, Japan) were used to image and observe the QD microstructure. The transmission electron microscope (TEM) (JEM2100, JEOL, Japan) imagery was used to assess the QD nanoparticles. The current-voltage-luminance characteristics were measured with a Keithley 236 source-measure unit and a Keithley 2000 multi-meter coupled with a calibrated Si photodiode. The EL spectra of the devices were obtained using a KonicaMinolta CS-1000A spectroradiometer.

Supporting Information Optiacal image; CAs and comparison; In situ optical microscopy observation image; Mechanism cartoon; Photoluminescence (PL) image; Temperature-Time diagram; TEM image and Absorbance image; AFM and corresponding height diagram; PL image and SEM image; Various datas of QLEDs.


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FIGURES

Figure 1. Schematic of assembling patterned QD micro-arrays. (a) The cartoon diagram of a single liquid-bridge with an asymmetric contact angle (CA) between template θ 1 and substrate θ 2.

The CA of the silicon template and target substrate, with an oil CA of 45° ± 1° and 2° ± 0.5°,

respectively. (b) The moving direction of small size QD nanoparticles during the solution dewetting processes in a single capillary trail confined between the template and substrate in the rectangle space. (c-f) Schematic illustration of the asymmetric wettability template technique for assembling patterned QD micro-arrays. A continuous liquid of QD solution is immersing into the groove between silicon template and the substrate is dewetting into individual liquid-bridges owing to the guidance of the template structure, yielding the patterned QD micro-arrays after the solution was total evaporated. (g-j) In situ fluorescent microscopy observation of the liquid dewetting processes and the formation of the divisive liquid-bridges in the sandwich system corresponding to the schematic photograph in (c-f).


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Figure 2. (a) Transmission electron microscopy (TEM) image of green QD nanoparticles. (b) Photoluminescence (PL) image of patterned QD micro-arrays under 375 nm laser as an excitation source. (c) Optical image of patterned QD micro-arrays corresponding to PL image. (d) Scanning electron microscope (SEM) image of QD micro-arrays with precise position and strict alignment. (e) Zoom-in SEM image of a single assembled micro-wire. (f) Atomic force microscopy (AFM) image of a single assemble QD micro-wire. (g) Single QD micro-wire’s smooth surface and straight boundary and its width and height.


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Figure 3. Schematic depicting fabrication of patterned QLEDs device via asymmetric wettability template technique. (a) A functional ITO substrate with PEDOT: PSS and TFB layers covered onto the silicon template to force the immersed and confined solution into the sandwich system. (b) After the total solution has evaporated, the assembled QD micro-arrays are transferred onto the functional ITO substrate surface. (c) The other functional layers are successively fabricated on the ITO to make an integrated patterned QLEDs device. (d) Schematic diagram of ITO/PEDOT: PSS/TFB/patterned QDs/ZnO/Al QLEDs device. e) Energy diagram of our multilayer structured QLEDs device.


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Figure 4. (a-c) Fluorescence image of patterned QD micro-arrays of Red, Green, and Blue under 375 nm laser as an excitation source. (d-f) Electroluminescence (EL) photographs of patterned red, green and blue QLEDs, operating at 4V. (g) The CIE coordinates of three color QLEDs at their corresponding EL maximum. (h) The J-L-V characteristics of patterned red, green and blue QLEDs.


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AUTHOR INFORMATION Corresponding Author Dr. Y. C. Wu：E-mail: [email protected] Dr. B. B. Hu：E-mail: [email protected]

ACKNOWLEDGMENT The authors acknowledge the MOST of China (Grant Nos. 2017YFA0204504), the National Natural Science Foundation (21703268 and 21633014), and Beijing Natural Science Foundation (2182081).


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Asymmetric Wettability Interfaces Induced a Large-Area Quantum Dot

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