Alternative Patterning Process for Realization of Large-Area, Full

Oct 13, 2016 - Alternative Patterning Process for Realization of Large-Area, Full-Color, Active Quantum Dot Display ... To validate that this process ...
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Letter pubs.acs.org/NanoLett

Alternative Patterning Process for Realization of Large-Area, FullColor, Active Quantum Dot Display Joon-Suh Park,*,† Jihoon Kyhm,‡ Hong Hee Kim,§,⊥ Shinyoung Jeong,†,¶ JoonHyun Kang,† Song-ee Lee,†,¶ Kyu-Tae Lee,† Kisun Park,†,∥ Nilesh Barange,† JiYeong Han,† Jin Dong Song,‡ Won Kook Choi,§ and Il Ki Han*,† †

Nanophotonics Research Center, ‡Center for Optoelectronic Materials and Devices, and §Materials and Life Science Research Division, Korea Institute of Science and Technology (KIST), Seoul 02792, Korea ⊥ Department of Materials Science and Engineering, Yonsei University, Seoul 03722, Korea ¶ School of Electrical Engineering, Korea University, Seoul 02841, Korea ∥ Department of Materials Science and Engineering, Korea University, Seoul 02841, Korea S Supporting Information *

ABSTRACT: Although various colloidal quantum dot (QD) coating and patterning techniques have been developed to meet the demands in optoelectronic applications over the past years, each of the previously demonstrated methods has one or more limitations and trade-offs in forming multicolor, high-resolution, or large-area patterns of QDs. In this study, we present an alternative QD patterning technique using conventional photolithography combined with charge-assisted layer-by-layer (LbL) assembly to solve the trade-offs of the traditional patterning processes. From our demonstrations, we show repeatable QD patterning process that allows multicolor QD patterns in both large-area and microscale. Also, we show that the QD patterns are robust against additional photolithography processes and that the thickness of the QD patterns can be controlled at each position. To validate that this process can be applied to actual device applications as an active material, we have fabricated inverted, differently colored, active QD light-emitting device (QD-LED) on a pixelated substrate, which achieved maximum electroluminescence intensity of 23 770 cd/m2, and discussed the results. From our findings, we believe that our process provides a solution to achieving both high-resolution and large-scale QD pattern applicable to not only display, but also to practical photonic device research and development. KEYWORDS: Colloidal quantum dots, patterning, light-emitting device, photolithography

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To deposit QDs onto a substrate, methods such as spincoating,7 phase separation,8 self-assembly,9 dip-coating,10 or layer-by-layer (LbL)11 deposition techniques have been investigated and developed, as the QDs have high molecular weight that current evaporation techniques are unfeasible for QD deposition.12 Such methods, however, deposit monochrome or mixed QDs on to the entire area of the substrate, rendering the separate use of different QDs on single device or

ince the discovery of the colloidal quantum dots (QDs), zero-dimensional nanocrystals, which exhibit size-dependent, narrow band photoluminescence (PL) characteristics originating from the quantum confinement effect,1 have drawn scientists’ and researchers’ high interest in device applications for their high photo/chemical robustness.2 As the QDs are color-tunable throughout the optical spectrum by controlling the size, they have been considered as promising materials for optoelectronic applications such as solar cells,3 transistors,4 or light-emitting devices.5 However, there have been no practical accomplishments that impacted the market because QD deposition and patterning techniques on a selected area of a substrate were the bottleneck in realizing such devices. © 2016 American Chemical Society

Received: July 19, 2016 Revised: October 11, 2016 Published: October 13, 2016 6946

DOI: 10.1021/acs.nanolett.6b03007 Nano Lett. 2016, 16, 6946−6953

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Figure 1. Schematic of patterning different QDs on a substrate using repeated photolithography and LbL assembly. (a) Illustration of the QD patterning process. (b) PL image of patterned QDs on a substrate with 405 nm laser as excitation source. (c) Large-scale QD pattern demonstration with pop art of Marilyn Monroe by Andy Warhol (1967) on 4-in. quartz wafer under UV lamp excitation.

legitimate answer to applying conventional photolithography technique for patterning the QDs.34 In this study, for the first time, we present various multicolor QD patterns with photolithographic lift-off process and chargeassisted LbL assembly using the negatively ionized state of the water-soluble QDs’ functional group.11,24 This method provides an alternative to pattern multicolor QD arrays with controlled thickness for each color and uniform pixel sizes that could achieve large-scale QD array with resolution up to optical limit of the photolithography. In addition, the QD pattern demonstration itself, which we observe through scanning electron microscope (SEM) images, validates the robustness of the QD pattern itself against additional photolithography processes and shows that the thickness of each QD patterns can be controlled by repeating the LbL assembly processes with atomic force microscopy (AFM) results. To illustrate the possibility in realizing a practical device with this method, we fabricate multicolored, active QD-light-emitting device (QDLED) on a single substrate and discuss the device physics of our QD-LED structure with ultraviolet photoelectron spectroscopy (UPS), photoluminescence (PL), and electroluminescence (EL) characteristics. On the basis of the above results, we confirm that the combination of photolithography and LbL processes can be a pragmatic approach in realizing practical display or sensor applications that require patterned array of QDs in large-scale. Figure 1 schematically illustrates the process of patterning QDs on a substrate using photolithography and charge assisted LbL assembly. This process allows repeated fine-patterning of QDs directly onto a substrate with minimal damage to preceding QD patterns. Detailed observation of the patterning process and the stability of the result would be provided in the latter part of this section. In addition, we show that the QD thickness for each pattern can be controlled by repeating LbL deposition of each QD pattern. In this demonstration, we use 1 μm thick SiO2 on Si (100) wafer as a patterning substrate. After conventional photolithography process to form the PR pattern (Figure 1a1,2), the

substrate difficult. Therefore, techniques for patterning QDs are needed for development. Mist deposition with masking technique,13 photolithographic patterning of QD embedded polymers,14 jet printing,15 dip-pen nanolithography,16 3D printing,17 and contact-printing6,18−22 methods have been proposed and researched to meet the demands. Among the various methods, contact-printing methods have been developed and demonstrated to pattern high-resolution, multicolored QDs on single substrate and are considered as one of the promising methods that will help utilize QDs into electronic devices including display applications. This method first requires deposition of monochrome QDs on different substrates, and the QDs are then separately transferred to desired surface using patterned elastomer stamps. Although such a process allows high-resolution patterns of QDs, the total area of which the pattern is formed is restricted by the size and pressure uniformity of the elastomer stamp and the fact that transfer printing requires coating high-density QDs on additional substrates, which impedes the cost-efficiency in manufacturing. An alternative approach to resolve the trade-off issues between scale and resolution of precedent QD coating/ patterning techniques as well as technical difficulties would be conventional photolithography, which is currently being used for fabricating large-area electronic devices with structural resolution up to optical diffraction limit. However, a critical problem exists in adapting such process to repeatable, multicolored QD patterning: most of the chemicals used in the photolithography process are organic solvents that could damage or dissolve precoated QD layers. As most conventional colloidal QDs are functionalized with hydrophobic or nonpolar ligands such as oleic acid, trioctylphospine (TOP), or tri-noctylphospine oxide (TOPO) as a mean to achieve high internal quantum efficiency,23 the QDs were subject to dissolve when they become in contact with the organic solvents, which hindered the repetition of QD patterning process.35−39 Therefore, using hydrophilic QDs that have low solubility, or orthogonality to organic solvents, can be considered as a 6947

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Nano Letters substrate is exposed to oxygen plasma to endow negative charge on the surface of the substrate25 (Figure 1a3). The substrate, whose surface is now hydrophilic by surface activation, is then immersed in poly(diallyldimethylammonium chloride) (PDDA) solution.24 The PDDA, which is positively ionized polyelectrolyte dissolved in water, adheres to the negatively charged substrate surface by electrostatic force. After the immersion process, the substrate is rinsed in deionized (D.I.) water to remove residual PDDA that is not bonded to the substrate, and it is dried with nitrogen gun. The surfaceadhered PDDA layer temporarily provides positively charged surface state.25 After the PDDA treatment, conventional water-soluble II−VI core/shell-type red QDs terminated with carboxylic acid (see Figure S1 in Supporting Information) are diluted in water to approximately 0.01 wt % and pipetted onto the substrate so as to cover the entire patterned area. The water-soluble QDs, whose surfaces are negatively charged by ionized carboxylic acid at the end of their functional group, adhere to the positively charged PDDA on the substrate by electrostatic force. After the QD deposition, the substrate is rinsed in D.I. water to remove excess QDs, which are not strongly bonded to the PDDA layer, and it is dried with nitrogen gun. This results in assembly of QD monolayer on the entire substrate (Figure 1a4), and by repeating the process of immersing in PDDA solution and depositing QDs, one can achieve thicker QD layers with increasing luminescence with respect to the number of LbL cycles (see Supporting Information).24 In the following demonstrations, the LbL assembly process of repeating the PDDA and QD depositions is performed twice for each pattern unless otherwise noted. When the desired thickness of QD pattern is achieved, the PR patterned substrate is then sequentially immersed in acetone and methanol in ultrasonic bath, respectively, to liftoff the PR, carrying along the QD layers coated on the top of the PR (Figure 1a5), which thereby results in first QD pattern on the substrate. The PR patterning, surface activation, QD LbL assembly, and lift-off process are repeated to additionally pattern green and blue colored QDs, respectively (Figure 1a6− 10). Our result in Figure 1, panel b shows the PL image 40 μm × 40 μm square RGB pixels on SiO2 substrate consisting of 10 μm × 40 μm rectangular pattern with 5 μm spacing between each color under 405 nm laser excitation. Also, using the same process, we have fabricated differently colored pop art of “Marilyn Monroe 1967” by Andy Warhol using red, green, violet, and yellow (mixture of red and green) colored QDs on 4-in. quartz wafer to provide evident example of large-scale patterning with this method (Figure 1c). To demonstrate the resolution of our QD patterning technique, we have fabricated 5 μm line pattern of QDs and observed the PR and QD formations step-by-step with SEM. As shown in Figure 2, panel a, the PR patterning process results in tapered PR pattern as the UV light diffracts at the edge of the optical mask. After the PR patterned substrate was exposed to oxygen plasma for surface activation, we found that the plasma minimally etched the PR (Figure 2b), which retained the pattern shape. As the oxygen plasma activates not only the exposed part of substrate, but also the PR itself, the LbL assembly of QDs occurs on both materials (Figure 2c), entirely covering the exposed area. Figure 2, panel d shows the final line pattern of the QDs after PR lift-off process, showing welldefined boundaries. From this result, we were able to conclude

Figure 2. SEM image of QD patterning process on SiO2 substrate. (a) Line patterned positive PR with width and pitch of 5 and 10 μm, respectively. Thickness of the PR is 1.7 μm. (b) PR patterned surface after oxygen plasma treatment. Approximately 200 nm of PR in thickness is found to be etched by oxygen plasma. (c) Close-up image of the patterned substrate after QD LbL deposition. Two cycles of LbL were carried out with red QDs. The inset shows overall image of QD coated substrate and PR pattern. The scale bar in the inset represents 5 μm. (d) Line pattern of QD after PR lift-off process. The dark and bright patches in the line pattern are deposited QD and exposed SiO2 surface, respectively with width of 5 μm. The scale bar in the inset represents 300 nm.

that the resolution of our QD patterning method is only restricted by the optical limitation of photolithography process. To verify that such repeated PR patterning and lift-off process causes minimal damage to the QD layers, we also have repeatedly spin-coated, baked, and stripped PR on the similarly fabricated triple layer QD pattern and confirmed that the height of pattern does not show observable degradation using AFM (Figures 3a,b). Since the water-soluble QDs’ functional group used in this study has low solubility in acetone and methanol, and that those solvents lack the energy to overcome the electrostatic bonding energy between the QDs and the adhered surface, we thus found that this entire process is repeatable without causing significant damage to the preceding QD patterns. In addition to the stability of the pattern itself, we show that thickness control at each site of the pattern is possible by varying the number of repeated LbL assembly processes (Figures 3c,d) and that linear increase in thickness with respect to the number of LbL repetition is recognizable. From our observations so far, we have found that this process is also applicable to different substrates, ranging from III−V substrates (GaAs) and oxide substrates (Y2O3, TiO2, ZnO, or SiO2) to elastomers such as polydimethylsiloxane (PDMS) whose surface can be activated to endow charge on the surface. Moreover, we have shown that the same procedure can be applied to QDs of different core diameters and from different manufacturers, which demonstrate that our method only depends on the surface charge termination of the QDs and that our method can be generalized to other kinds of colloidal QDs with different core materials whose functional group can be exchanged to similar hydrophilic, charged state.25 6948

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Figure 3. Demonstrations of robustness of the QD pattern against repeated photolithography process and on layer thickness control at each position. (a) AFM image of the 5 μm line pattern formed by three cycles of LbL process. (b) AFM line profile of QD line patterns after repeated PR coating, baking, and stripping to demonstrate that the damage to the QD layers by additional photolithography processes are minimal. (c) AFM image and (d) line profile of green QD 10 μm line patterns after individually patterning each line and performing one, two, and three LbL cycles on each site, respectively.

prevent oxidation during EL is performed within nitrogen environment. To confirm that the device fabrication has been performed as desired, cross-sectional transmission electron microscope (TEM) image of resulting device is provided in Figure 4, panel b. The well-defined boundaries between each constituting layer allow effective carrier interactions between each layer without short-circuiting when bias voltage is applied. Figure 4, panel c shows the energy levels of the device constituting materials with respect to the vacuum energy as 0 eV, determined from UPS and PL measurements. From UPS measurements (Figures S2a,b)), we were able to approximate each material’s highest occupied molecular orbital (HOMO), and by measuring the optical band gap of the materials through PL (Figure S2c), determine the lowest unoccupied molecular orbital (LUMO) energies of each material. The ultrathin ZnO layer’s LUMO levels are near that of the QDs that electrons can be transferred efficiently from the ITO to the QD layers. Although the HOMO level of the ZnO is relatively well-below that of the QDs, the existence of the defect states between the HOMO and LUMO levels, as shown in the PL measurements, interferes with the hole-blocking capability of the ZnO layer. Figure 4, panel d shows the EL photograph of our RGCW, active QD-LED with common bias voltage of 6 V, and separately measured I−V-L characteristics and the Commission International de l’Éclairage coordinates (CIE, 1931) of each pixel at their EL maxima are shown in Figure 4, panels e and f, respectively. Maximum EL intensities of red, green, cyan, and warm-white QD-LEDs were 6900 cd/m2 at 6.6 V, 23 770 cd/ m2 at 8.4 V, 4598 cd/m2 at 7 V, and 10 920 cd/m2 at 7.6 V, with their CIE coordinates at (0.68, 0.32), (0.29, 0.68), (0.12,0.49), and (0.51, 0.47), respectively. Each pixel’s EL can be attributed as direct recombination within QD layer as there

Using the demonstrated method, we have fabricated an inverted,26−28 multicolored, active QD-LED on a single substrate through solution-process as a proof of concept to realizing full-color, active QD display. In the present device structure, shown in Figure 4, panel a, ITO is used as cathode, sol−gel processed ZnO layer as electron transport layer (ETL),29,30 poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (poly-TPD) as hole injection layer (HIL) and electron blocking layer (EBL), and MoOx/Ag layer as hole transport layer (HTL) and anode.28 The device fabrication and measurement are performed in ambient clean-room environment except for MoO x /Ag electrode deposition and encapsulation process, which are performed in vacuum and nitrogen glovebox, respectively. The QDs with varying colors are patterned on each of the four ZnO-coated ITO pixel patterns, respectively, using the photolithographic patterning and LbL assembly as demonstrated in the previous section. The ZnO layer, which is a metal-oxide compound, can be successively surface-activated through oxygen plasma treatment so that the same presented LbL assembly can be applied to deposit the QDs. An example of microscale QD patterning on the ITO/ZnO film is provided in the Supporting Information. For each pixel, the LbL assembly process is repeated five times each to ensure formation of QD-based emissive layer,31 and warm-white color pixel is fabricated by sequentially stacking different QDs (2 cyan, 2 green, and 1 red, respectively) with LbL assembly. After QDs are patterned on each pixel, poly-TPD diluted in 1,2dichlorobenzene is spin-coated on to the device. Poly-TPD was chosen for its effective electron blocking capability in the previous reports.28,32 After the process, MoOx/Ag is deposited as cathode using thermal evaporator, and encapsulation to 6949

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Figure 4. Multicolored, active QD-LED demonstration. (a) Inverted QD-LED structure. (b) Cross-sectional TEM image of the fabricated device. (c) Energy diagram of our inverted QD-LED structure. (d) EL image of RGCW QD-LED on single device at common bias voltage of 6 V. (e) Current−voltage−luminance (I−V−L) graph of RGCW QD-LEDs. (f) CIE coordinates of each QD-LED at their corresponding EL maxima.

To provide evidence of our speculation, we have also fabricated stacked QD-LED in the reverse order of warm-white color QD-LED, that the cyan colored QDs are in contact with the HIL, and compared the EL spectrum at EL maxima with that of the warm-white color QD-LED (Figure S3c). The result shows that most of the EL originates from the QDs near the HIL.11 The poly-TPD, serving as both HIL and EBL, can accumulate electron in the QD/poly-TPD interface with its high LUMO level so that the electron−hole recombination is more favorable at QDs near the HIL. As such, characteristics render achieving both EL intensity control, and stable whitelight emission from mixed or stacked QDs through voltage regulation virtually difficult, we can ensure again that pixelating individual color would be more desirable to achieve true white-

is no excess EL spectrum originating from other device materials, and the EL spectrum solely consists of near-band edge emissions of the QDs (Figure S3a). Although red, green, and cyan color QD-LEDs show consistent EL spectrum, the warm-white color pixel exhibits spectrum shift in the color space when applied bias voltage is varied (Figure S3b). In other words, stable white-light emission was not possible throughout the working voltage range when QDs with different bandgap were stacked in the device structure. This result is because each different QD at each layer exhibits different voltage-luminance characteristics as shown in Figure 4, panel e, and the distance of the QDs from the HIL affects the electron−hole recombination probability. 6950

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dehydrate (ACS reagent, ≥98%), ethanolamine (purified by redistillation, ≥99.5%), 2-methoxyethanol (anhydrous, 99.8%), 1,2-dichlorobenzene (anhydrous, 99%), and molybdenum(VI) oxide (MoOx) were purchased from Sigma-Aldrich. AZ5214E and AZ300MIF manufactured by AZ Electronic Materials were used as photoresist and developer, respectively. Poly-TPD (ADS254BE) was purchased from American Dye Source, Inc. CMOS grade acetone and methanol manufactured by J.T. Baker were used throughout this report. D.I. water with resistance of 18.1 MΩ was generated by Barnstead NANOpure Diamond 1190. Photolithography and LbL Assembly. All substrates were cleaned in Branson 3510 40 kHz ultrasonic bath using acetone and methanol for 15 min each and were blown dry with nitrogen gun. AZ5214E and AZ300MIF (AZ Electronic Materials, USA) were used as positive PR and developer, respectively. Spin-coating the PR was carried out at 3000 rpm and was soft-baked on a hot plate. UV exposure to the sample with chrome mask was carried out using MIDAS mask aligner MDA-400 M equipped with Hg arc lamp. PR was developed in AZ300MIF developer and was rinsed in D.I. water after development. PR patterned substrate was then exposed to oxygen plasma to activate the surface. After the surface activation, the substrate was then immersed in PDDA solution and rinsed in D.I. water. Substrate was then blown dry with nitrogen gun, and QD dispersed in water was pipetted to cover entire surface. After the QDs were allowed to adhere to the substrate by electrostatic force, the substrate was rinsed in D.I. water, blown dry with nitrogen gun. The process of immersing in PDDA and QD solution was repeated to obtain thicker QD layers. After QD LbL assembly, the QD-coated substrate was then sequentially immersed in acetone and methanol in the ultrasonic bath to strip the PR from the substrate. AFM measurements were carried out in tapping mode using XE-100 atomic force microscope (Park Systems) with PPP-NCHR noncontact cantilever (Nanosensors). SEM images were obtained using Nova Nano SEM 200 (FEI) with accelerating voltage at 10 kV. Device Fabrication and Characterization. After the patterned indium−tin-oxide (ITO) glass substrate was cleared with acetone and methanol in ultrasonic bath sequentially, the surface of the substrate was then UV−ozone treated to achieve organic residue removal and surface activation. For the ETL, ZnO sol−gel was prepared by dissolving zinc acetate dihydrate in 2-methoxyethanol and adding ethanolamine and then stirring the solution at 80 °C for 2 h and at room temperature for 24 h.30 The solution was filtered using 0.45 μm PFPE filter before being spin-coated on to the patterned ITO substrate. The ZnO spin-coated substrate was thermalized at 200 °C on a hot plate for 2 h. The ZnO spin-coating and thermal annealing were repeated several times. The ultrathin ZnO film isolated the QDs from direct contact with ITO, which could result in exciton quenching11 in the EL device. PR patterning and LbL assembly of QDs were repeatedly performed on each pixel of ITO/ZnO patterns. The substrate was then exposed to vacuum environment overnight to dry. HIL was prepared by dissolving poly-TPD in 1,2-dichlorobenzene with density of 1 wt %. The poly-TPD solution was spin-coated on to the device at 2000 rpm and again dried in vacuum environment. MoOx/Ag (12 nm/200 nm) cathode deposition was performed using thermal evaporator in vacuum followed by glass encapsulation process in nitrogen glovebox. The I−V−L characteristics of the device were measured using a Spectra Scan PR-670 spectroradiometer

light as suggested by previous reports.20,33 However, in terms of true white-light, while previous studies are performed by pixelation through transfer-printing, our method is advantageous in that pixelation is achieved by photolithography, which would be more feasible in large-area device fabrication in low cost. In comparison with the QDs’ high EL emission near the HIL, the QDs near the ZnO thin film show low EL intensity. When the ZnO film is absent in the device structure, however, no EL emission occurs in all QD layers. Therefore, while the ZnO thin-film can serve as an effective electron transport pathway for the QDs by having similar LUMO levels, as shown in Figure 4, panel c, we can also speculate that the ZnO film also functions as a spacer and a buffer that controls the balanced charge injection to the QDs and allows efficient exciton recombination during EL process.27 More details on these analysis are provided in the Supporting Information. In the present study, we have successfully shown novel QD patterning technique using conventional photolithography and provided an alternative to previously studied QD patterning methods. By combining photolithography, which allows largescale, fine patterning, and LbL assembly, which enables uniform deposition of QDs over large area, we believe that the trade-off issues between resolution and size among previously studied methods can be significantly reduced in terms of technical difficulties. As our demonstrated process depends not on the core type of the QDs but on the surface termination, we are able to speculate that this process can be further applied to patterning heavy-metal free QDs as well. In addition to the patterning itself, we have showed thickness control at each site is possible and that the damage to the preexisting QD during repeated photolithography is minimal. Also, the illustrated process in this research is cost-efficient as LbL assembly of QDs is possible with colloidal QDs in low concentration9,24 compared to spin-assisted coating methods in previous studies.8,11,12,18,20,26,28 To provide evident example of this QD patterning process in actual device application, we have successfully demonstrated a first proof of concept, multicolored, active QD-LED on a single substrate. By analyzing the materials and combined device structure with optical and EL characteristics, we were able to assign each of the device layers with their role in the carrier transport and recombination processes during EL emission process, thereby understanding the EL mechanism in our QDLED. We expect that the performance of QD-LED based on this approach has enough room to be improved through further research for which our device structure and fabrication method are not yet fully optimized. As shown by the example of our approach with QD-LED, the presented QD patterning process can be considered expandable to various optoelectronic applications, which require QD patterns such as displays or sensors, in general. We believe that this photolithographic QD patterning method can provide a cost-efficient, practical solution to the bottlenecks in developing high-resolution, large-scale QD-based photonic devices, especially in the area of active QD display applications. Methods. Materials. The II−VI core/shell colloidal QDs for red, green, cyan, and violet QDs were purchased from NanoSquare, and blue colored QDs were purchased from Ocean NanoTech. All QDs were terminated with carboxylic acid, and red, green, cyan, blue, and violet QDs had PL maxima at 620, 540, 500, 450, and 440 nm, respectively. PDDA (average MW < 100 000, 35 wt % in H2O), zinc acetate 6951

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and a Keithley-2601 source-measure unit. Cross-sectional sample of the QD-LED was obtained by using FEI Helios NanoLab 600 focused ion beam (FIB), and the cross-sectional images were taken by Titan 80−300 (FEI) TEM. Optical Characterization. HOMO levels of the materials were determined by ultraviolet photoemission spectroscopy using PHI 5000 VersaProbe (ULVAC PHI) with He discharge lamp (21.2 eV). PL was measured using a third harmonic Ti/ sapphire laser (Mai-tai, Spectra physics), which excited exciton of the sample with 266 nm (4.66 eV) and 7 mW. To suppress oxidation of sample, we used a vacuum chamber (Janis ST-100) and kept the chamber pressure under 10−3 Torr using rotary pump. The PL was collected by the quartz lens pairs and dispersed in a monochromator (SP-2300i, Princeton Instruments) with 1200 g mm −1 grating and 500 nm blaze.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b03007. FT-IR spectroscopy of QD functional group used in this study; optical characterization of the materials in QDLED; EL properties of single color QD-LEDs and stacked QD-LED; optical properties of QD-LED layered structure; optical characterization and study on QD-LED device physics; summary of QD-LED performance; current density−voltage characteristics of QD-LEDs; operational lifetime characteristic of QD-LED; surface morphology of ZnO sol−gel film on ITO substrate; QD line pattern demonstration on quartz/ITO/ZnO substrate; optical characterization of LbL assembled, stacked QDs (PDF)

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Letter

AUTHOR INFORMATION

Corresponding Authors

*E-mail: fi[email protected]. *E-mail: [email protected]. Author Contributions

J.-S.P., J.K., and H.H.K. contributed equally to this work. J.-S.P., J.K., H.H.K., W.K.C., and I.K.H. designed the experiments and analyzed the data. J.-S.P., J.K., and S.J. optimized and performed QD LbL assembly and patterning. J.-S.P., J.K., and S.L. performed AFM measurement and analysis. J.-S.P. and H.H.K. designed the QD-LED structure; J.-S.P., S.J., and H.H.K. fabricated the QD-LED; and K.P. and N.B. optimized ZnO synthesis and coating. J.-S.P., J.K., and J.D.S. performed optical characterizations, and H.H.K. performed EL characterization of the device. K.-T.L. and J.H. performed SEM measurements; S.J., J.H., and H.H.K. performed and analyzed FT-IR spectroscopy; and J-S.P. and H.H.K. analyzed TEM results. J.-S.P., J.K., and H.H.K. wrote the draft. All authors discussed the results and commented on the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (NRF-2013M3C1A3065033). 6952

DOI: 10.1021/acs.nanolett.6b03007 Nano Lett. 2016, 16, 6946−6953

Letter

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DOI: 10.1021/acs.nanolett.6b03007 Nano Lett. 2016, 16, 6946−6953