Photoresist Contact Patterning of Quantum Dot Films - ACS Nano

16 hours ago - Scalable and cost-effective protocols to pattern and integrate colloidal quantum dots (QDs) with high resolution have been challenging ...
0 downloads 0 Views 4MB Size
Download PDF
www.acsnano.org

Photoresist Contact Patterning of Quantum Dot Films Hohyun Keum,†,§ Yiran Jiang,‡,§ Jun Kyu Park,†,§ Joseph C. Flanagan,‡ Moonsub Shim,*,‡ and Seok Kim*,† †

Department of Mechanical Science and Engineering, University of Illinois, Urbana, Illinois 61801, United States Department of Materials Science and Engineering and Frederick Seitz Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801, United States

Downloaded via DUQUESNE UNIV on September 25, 2018 at 01:10:53 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Scalable and cost-effective protocols to pattern and integrate colloidal quantum dots (QDs) with high resolution have been challenging to establish. While their solubility can facilitate certain processes such as spin-casting into thin films, it also makes them incompatible with many conventional patterning techniques including photolithography that require solution processing. In this work, we present “photoresist (PR) contact patterning”, a dry means to pattern QD films over large areas with high resolution while maintaining desired properties. Here, a PR layer on an elastomer substrate is patterned by conventional photolithography and used as a dry contact stamp to selectively peel off QDs in the contact regions, leaving behind a QD film with the negative of the PR pattern. Once patterned, QD films are readily transferred and integrated on foreign substrates by subsequent transfer printing processes. Patterned PR layers can also be transferred from elastomer substrates onto QD films and used as masking layers for subsequent deposition and patterning of additional materials, e.g., patterned metal electrodes or charge transport layers for QD-based devices. The study of the interfacial mechanics and energy of materials associated with PR contact patterning reveals why a lithographically patterned PR is superior for high-resolution QD film patterning. Applicability of PR contact patterning is demonstrated through the fabrication of red, green, and blue (RGB) QD light-emitting diode pixels. PR contact patterning presented in this work not only allows dry patterning of QD films but also enables high-resolution integration of functional multistack structures for future QD-based electronic and optoelectronic devices. KEYWORDS: colloidal quantum dots, patterning, transfer printing, light-emitting diodes within device structures. For example, spin-cast thin films of QDs are in general not compatible with conventional photolithography because QD films can be dissolved away in solvents used with photoresists. Similarly, solvents used for QDs can damage patterned photoresists. One method to address this challenge is to modify the surface of QDs to achieve orthogonality to the solvents used for photolithography. For instance, QDs with ionized hydrophilic surfaces can be assembled layer-by-layer into a film that can survive the photolithography processes.16 More recently, direct optical lithography of QDs has been demonstrated by incorporating photoactive ionic molecules as ligands for the QDs.17 However, many colloidal QD-based optoelectronic devices require organic charge transport layers, which again could be damaged by the conventional photolithography processes.

R

ecent progress in nanoscale materials enables a variety of advanced applications ranging from biomedical diagnostics and solar energy conversion to lighting and displays.1−7 Colloidal quantum dots (QDs) or semiconductor nanocrystals are a class of nanoscale materials that are appealing for the above-mentioned applications especially due to their properties that can be tuned through size, shape, and composition.8−13 For example, in addition to sizedependent photoluminescence emission wavelengths, rodshaped colloidal nanocrystals can provide benefits of directionality/anisotropy.14 Furthermore, the introduction of well-defined heterojunctions into anisotropic nanocrystals enables nanoscale precision in band structure engineering that can lead to opportunities for advanced devices with multiple functionality.15 Additionally, colloidal QDs can allow cost-effective approaches to manufacturing through simple solution processes rather than costly vacuum techniques. While beneficial in many ways, the solubility of QDs can also be a hindrance especially when delicate QD patterns are required © XXXX American Chemical Society

Received: June 12, 2018 Accepted: September 11, 2018

A

DOI: 10.1021/acsnano.8b04462 ACS Nano XXXX, XXX, XXX−XXX

Article

Cite This: ACS Nano XXXX, XXX, XXX−XXX

Article

ACS Nano

Figure 1. Schematic process flow and images for PR contact patterning of a QD film on an ODTS-coated Si substrate. (a) A QD film is spincast on an ODTS-coated Si substrate. (b) The separately prepared PR stamp, i.e., a PR layer patterned on a PDMS substrate, is brought into contact with the QD film. (c) Subsequent peeling of the PR stamp results in a patterned QD film on the Si substrate. (d) Patterned QD film on the Si substrate. (e) Optical microscope image of a PR layer photolithography patterned on a PDMS substrate. (f) Optical microscope image of a patterned QD film on a Si substrate after PR contact patterning. The inset includes a magnified image, which shows clear patterns of individual QD squares. The scale bars represent 50 μm. (g) Optical microscope images of complex star-shape QD patterns with different sizes. The scale bar represents 150 μm. (h) SEM images of a corner of a triangle-shape QD pattern at two different magnifications. The right image shows compactly remaining QDs in the pattern region. The scale bars represent 100 μm (left) and 200 nm (right).

In this work, we present a dry means to pattern QD films over large areas with high resolution by exploring the use of a photolithographically patterned photoresist (PR) layer. We refer to this approach as “PR contact patterning”. A patterned PR layer on an elastomer substrate, i.e., a PR stamp, is first brought into contact with a film of QDs. The subsequent peeling of the PR stamp removes the QDs that are in contact with the PR stamp, thereby forming a pattern on the QD film that is the negative of the photolithographically defined pattern on the PR stamp. This PR contact patterning has several major advantages over other dry approaches to patterning QD films. First, the pattern resolution of PR contact patterning depends on that of a PR stamp which is photolithographically defined and thus inherently high. The pattern design is also generated directly on a PR stamp using photolithography instead of molding processes, which require fabricating different master templates for different pattern designs, impairing versatility and cost-savings. Once different QD films are patterned by PR contact patterning, they can be transfer printed on a common receiving substrate, allowing, for example, vertical stacking or horizontal arrays of different QD films. Furthermore, a PR layer on an elastomer substrate can also be transferred onto a QD film (referred to hereon as “PR contact transfer”), and additional layers such as metal thin films for electrical contacts can be deposited on the entire area. Subsequent peeling of the PR layer allows multiple stacks of heterogeneous materials to be patterned simultaneously. Such abilities of PR contact patterning enable the scalable fabrication of QD-based electronic or optoelectronic devices, in particular, when combined with transfer printing.

Hence, approaches that allow patterning of QD films with resolution as high as photolithography and that permit integration of QD-based devices without damaging device components are highly desirable. Among alternative approaches to pattern and integrate QD films within functional device structures are contact and transfer printing methods. Contact printing is a method where QDs deposited onto a structured polydimethylsiloxane (PDMS) stamp coated with parylene-C are subsequently brought into contact with a receiving substrate, resulting in printing of QDs that are on the protruding regions of the stamp.18 Despite the simplicity and high throughput, this process is limited due to the inking step that requires a solvent compatible with both parylene-C and QDs. This limitation has been resolved by the development of dry transfer printing techniques. Rather than spin-casting directly onto a structured PDMS stamp, QDs spin-cast and dried on a donor substrate are selectively picked up via a stamp, and the retrieved QDs are subsequently printed onto target sites on a receiving substrate.19 Recently, this transfer printing method has been applied to fabricate large-area flexible QD light-emitting diode (QD-LED) displays.20 Further improvements on transfer printing of QDs have been made, including the introduction of a poly(vinyl alcohol) layer to enhance retrieval of QD films21 and an intaglio patterning step to push the patterning resolution limit well beyond current needs for most applications.22 However, these approaches rely on the ratedependent adhesion of PDMS,23,24 often requiring a high preload with a high peeling speed, or expensive intaglio trenches. More recently, a shape memory polymer (SMP) has been utilized in place of PDMS as a stamp material to pattern QD films.25 An SMP stamp can allow reliable patterning of QDs even with a low peeling speed since it exploits the shape memory effect to switch its dry adhesion rather than the peeling rate.26 Nevertheless, the SMP stamp requires thermomechanical loading to enable the shape memory effect, which may decrease the process throughput.

RESULTS AND DISCUSSION Figure 1 represents the process flow to pattern a QD film using PR contact patterning. Colloidal QDs, prepared by established methods,22,27,28 cleaned, and dissolved in octane, are spin-cast onto an octadecyltrichlorosilane (ODTS)-coated Si substrate (Figure 1a). Separately, a PR is spin-cast and photolithoB

DOI: 10.1021/acsnano.8b04462 ACS Nano XXXX, XXX, XXX−XXX

Article

ACS Nano

Figure 2. Photoluminescence (PL) images of patterned QD films on ODTS-coated Si substrates. (a) Red, green, and blue (RGB) QD films patterned in circle, triangle, and star shapes. (b) RGB QD square arrays. (c) Red QD film patterned into “UIUC”. All scale bars are 50 μm. (d) PL spectra of patterned RGB QD films.

defined photolithographically, such as the letters “UIUC” in Figure 2c, can be easily replicated on a QD film. The PL spectra of patterned QD films are shown in Figure 2d, and the narrow full-width at half-maximum (FWHM) of only ∼30 nm is maintained for all cases. The unchanged PL spectral features and brightness of QD films before and after patterning indicate that PR contact patterning does not alter the PL properties of QD films. To understand this simple-yet-effective QD film patterning capability of PR contact patterning, normal stresses at the contact interface between the stamp (PDMS or PR) and the substrate (Si) have been investigated using finite element analysis (FEA). In Figure 3a,b, the top surface of the stamp is pulled with 10 N/m2. Both the top view (Figure 3a) and the side view (Figure 3b) normal stress fringe plots indicate that the stress distribution at the PR−Si contact interface is more uniform than the PDMS−Si contact interface due to the higher Young’s modulus of PR (2 GPa). A more uniform interfacial stress distribution leads to lower local stresses at the contact edges and, thus, a higher pull-off force, which enhances the resolution of patterned QD films on Si substrates. This observation is also consistent with the concept of equal load sharing.29 When a stamp with the same work of adhesion is more rigid than the other, the load is more equally shared along the contact interface when the stamp is pulled from a substrate. This means that local loads at the contact edge and the contact center are less different. In other words, there is a higher tendency for contact edge and contact center regions to fail simultaneously, and thus, a higher pull-off force is expected when the stamp is peeled from the substrate. In order to further understand PR contact patterning, the work of adhesion between different surfaces involved in PR contact patterning is calculated from experimentally measured contact angle data. In Figure 3c, deionized (DI) water and ethylene glycol droplets (5 μL) are placed on QD film, PR, PDMS, ODTS-coated Si, and bare Si surfaces. Their contact angles (θ) are measured as listed in Table S1. DI water and ethylene glycol are selected since both dispersion and polar components of their surface energies are known (γdH2O = 23.9, γpH2O = 48.8, γdEG = 29.2, γpEG = 18.3 mJ/m2),30 which can be used to obtain the analogous components of the surface energy of a solid surface using the following equation.

graphically patterned on a PDMS substrate to form the PR stamp. The PR stamp is then brought into contact with the QD film on the ODTS-coated Si substrate at 65 °C with the patterned PR layer facing the QD film (Figure 1b). Detailed procedures to prepare QD films and PR stamps are given in the Methods section. The elevated temperature and a slight preload enhance the conformal contact between the PR stamp and the QD film. Upon cooling to room temperature and quickly peeling the PR stamp, regions of the QD film that are in contact with the PR stamp are removed, whereas the QDs in the openings of the PR stamp remain on the original Si substrate (Figure 1c). This process leaves behind a highresolution pattern of the QD film, which corresponds to the lithographically defined pattern of the PR stamp (Figure 1d). Examples of QD films patterned on ODTS-coated Si substrates through PR contact patterning are shown in Figure 1e−h. Optical images show a grid-pattern PR layer on a PDMS substrate (Figure 1e) and the corresponding patterned array of QD squares on a Si substrate (Figure 1f). The inset in Figure 1f is a magnified image showing no significant residue in regions where the QDs were in direct contact with the PR stamp and peeled off. The individual QD squares are 12 μm × 12 μm over a 2.5 cm × 2.5 cm area, which demonstrates the feasibility of large-area, high-resolution patterning of QD films through PR contact patterning. Although 12 μm × 12 μm is the smallest pattern size demonstrated in this work, higher precision tools may be adopted to increase the patterning resolution to sub-micrometer level. Figure 1g shows patterning of complex star-shape QD films with various dimensions. As seen in Figure 1h, scanning electron microscopy reveals that densely packed QDs are easily patterned into complex shapes without any discernible damage. To demonstrate the high-quality patterning capability of PR contact patterning for diverse QD materials, red, green, and blue (RGB) QD films have been prepared, patterned, and characterized. Photoluminescence (PL) images of QD patterns shown in Figure 2 are collected with a confocal microscope with a 405 nm laser excitation source using band-pass range ± 10 nm of the PL peak positions, which are 616, 516, and 470 nm for RGB QDs, respectively. Figure 2a shows RGB QD films in circle, triangle, and star shapes, respectively. The square arrays of RGB QD patterns in Figure 2b illustrate the scalability of PR contact patterning. Even complex features C

DOI: 10.1021/acsnano.8b04462 ACS Nano XXXX, XXX, XXX−XXX

Article

ACS Nano

Figure 3. FEA fringe plots of a normal stress distribution near the contact interface between a Si substrate and either PDMS or PR stamp when 10 N/m2 tensile pressure is applied on the top surface of each stamp. Both top view (a) and side view (b) results indicate that a PR stamp involves lower stress concentration at the edge and less stress variation along the contact interface. Note that color scale range for the stress is different for PDMS and PR. The scale bars represent 100 μm. (c) Images of water and ethylene glycol droplets on QD film, PR, PDMS, ODTS-coated Si, and Si surfaces.

γliquid(1 + cos θ ) =

d d 4γsolid γliquid d d γsolid + γliquid

+

p p 4γsolid γliquid p p γsolid + γliquid

printing. Such an ability can significantly facilitate device integration. In particular, the ability to pattern, position, and stack device-relevant layers together through a dry means would be highly appealing. As a simple example, squares of RGB QD films can be transfer printed sequentially on the same site of a glass slide using a PDMS stamp. Optical and PL images of such vertically stacked RGB QD films are shown in Figure S1. In order to develop an approach more relevant to device fabrication, we have modified our PR contact patterning to enable PR contact transfer. Here, the patterned PR layer on the PDMS substrate is directly transferred onto a QD film prepared on an ODTS-coated Si substrate rather than being used only as a stamp to peel off unwanted QDs. The transferred PR layer can then be used as a masking layer for successive material deposition steps. The final peeling of the PR layer using a pressure-sensitive adhesive (PSA), i.e., dry liftoff, then leaves behind a patterned QD film with the same pattern of subsequently deposited materials that are precisely registered to the QD film pattern. To this end, a PSA is gently brought into contact with the deposited layer on the transferred PR layer while taut to ensure that the PSA does not collapse and touch the deposited materials through the PR opening. To demonstrate this procedure, chromium (Cr) and gold (Au) are deposited as additional materials. Figure 4a−d represent the process flow where a patterned PR masking layer initially on a PDMS substrate is transferred onto a QD film on an ODTS-coated Si substrate. In this transfer, the patterned PR layer is in contact with the QD film; then the PDMS substrate is peeled slowly at an elevated temperature (65 °C) (Figure 4a,b). Subsequently, 5 nm thick Cr and 100 nm thick Au are sputter-deposited (Figure 4c). The PR layer is then removed simply by using a commercial PSA to obtain a QD/metal composite layer array (Figure 4d). Figure 4e−h include optical microscope images corresponding to each step of the QD/ metal composite layer array formation. The image of a fully processed composite layer array shows a clear distinction between composite layer regions and the Si surface, indicating successful formation of the QD/metal composite layer array for subsequent transfer printing steps. Although this process based on PR contact transfer is demonstrated with Cr and Au, these metals can be replaced by any material of interest as long as the material is deposited in dry (or with an orthogonal

(1)

where d and p denote the dispersion and polar components of the surface energy, respectively. The calculated surface energies are listed in Table S2. The work of adhesion between two contacting solid surfaces is then calculated using the harmonic mean equation.31 W12 =

4γ1dγ2d γ1d + γ2d

+

4γ1pγ2p γ1p + γ2p

(2)

The subscripts 1 and 2 in eq 2 represent the two contacting solid surfaces. Table S3 lists work of adhesion values at two contacting solid interfaces including the QD and Si interface as a point of reference that helps to explain why an ODTS layer is necessary for PR contact patterning. Without an ODTS layer on a Si substrate, a QD film cannot be well patterned by the PR stamp due to the high work of adhesion between QDs and Si. It is also shown that the work of adhesion between PR and QDs is around 49% larger than that between PDMS and QDs. This larger work of adhesion between PR and QDs along with more uniform interfacial stresses of the PR stamp as shown in the above FEA results validates that PR is a better choice as the stamp material to improve QD film patterning quality. It should also be noted that the work of adhesion between PR and QD is 19% higher than that between PR and PDMS, indicating that it is possible to transfer a PR layer on a PDMS substrate onto a QD film in certain circumstances. For example, the peeling rate of a PR stamp determines whether the PR layer on a PDMS substrate is transferred on a QD layer or not. A high or moderate peeling rate makes the PR layer retrieve QDs. On the other hand, a low peeling rate causes the PR layer to transfer on the QD layer. In addition to these interfacial conditions, the size of the PR layer openings compared with the PR thickness is critical to successful QD patterning. Too large PR layer opening or too small PR thickness may make the PDMS substrate collapse and touch the QD layer through the PR layer openings, leading to irregular QD patterning. Since individually prepared RGB QD films are on low surface energy ODTS-coated Si substrates, patterned RGB QD films can be moved to arbitrary substrates using transfer D

DOI: 10.1021/acsnano.8b04462 ACS Nano XXXX, XXX, XXX−XXX

Article

ACS Nano

Figure 4. Schematic process flow to fabricate a patterned QD and metal composite layer and resulting images. (a, b) A patterned PR layer is transferred onto a QD film. (c) Metal layers are deposited. (d) Patterned QD/metal squares remain after removing the PR layer. Optical microscope images of a grid-patterned PR layer on a PDMS substrate (e), a transferred PR layer onto a QD film (f), successively deposited Cr and Au (g), and a square-patterned QD and metal composite layer after removing the PR layer using a PSA, i.e., dry lift-off (h). All scale bars represent 150 μm.

Figure 5. Schematic process flow to fabricate RGB QD-LEDs and demonstrative images of individual QD-LEDs. (a) A ZnO layer is spin-cast and annealed on top of a QD film on an ODTS-coated Si substrate. (b) A patterned PR layer is transferred on the substrate via PR contact transfer. (c) Al electrodes are formed by depositing and patterning, i.e., dry lift-off, depicted in Figure 4. (d) A patterned QD/ZnO/Al composite layer is picked up by a PDMS or SMP stamp. (e) The composite layers with RGB QDs are assembled on a separately prepared glass/ITO/PEDOT:PSS/TFB substrate via transfer printing. (f) A PI layer is coated on the assembled substrate, and selective openings are made by selective plasma etching. (g) Cr/Au wires are deposited and patterned. (h) Schematic illustration of individually addressable RGB QD-LED pixels. Optical images of activated red (i), green (j), blue (k), red/green (l), green/blue (m), blue/red (n), and red/green/blue (o) QDs. All scale bars represent 150 μm.

layer from the substrates complete the preparation of 100 μm square arrays of QD/ZnO/Al composite layers on donor substrates for the following QD-LED assembly (Figure 5c). The receiving substrate is separately prepared on patterned indium tin oxide (ITO) glass. Sequential spin-casting and annealing of poly(3,4-ethylenedioxythiophene)polystyrenesulfonate (PEDOT:PSS) and poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,40-(N-(4-s-butylphenyl))diphenylamine)] (TFB) as hole injection layer (HIL) and hole transport layer (HTL), respectively, complete the preparation of the receiving substrate for the QD-LED assembly (Figure 5e). Transfer printing of the composite layers is enabled by a PDMS or SMP stamp. Square pixels of each color are picked

solvent) and relatively low temperature conditions such that the PR masking layer is not significantly damaged during the deposition process. To demonstrate device-level applications of PR contact transfer, we have fabricated QD-LED pixels that are defined by individual transfer printing steps. First, RGB QD films are deposited on three different ODTS-coated Si donor substrates. A zinc oxide (ZnO) electron transport layer (ETL) is then deposited on each of these QD films (Figure 5a). Then a patterned PR layer with a 100 μm square-opening array is transferred on top of the ZnO layer by PR contact transfer (Figure 5b). Subsequent deposition of a 100 nm thick aluminum (Al) as the cathode and dry lift-off of the PR E

DOI: 10.1021/acsnano.8b04462 ACS Nano XXXX, XXX, XXX−XXX

Article

ACS Nano

acetate dihydrate (≥98%), 1-butanol (anhydrous, ≥99%), m-xylene (anhydrous, ≥99%), and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ, 97%) were all purchased from Sigma-Aldrich. Synthesis of RGB QDs. Red, green, and blue QDs were synthesized in a standard Schlenk line following established methods with slight modifications.22,27,28 The red CdSe/CdS core/shell QDs were synthesized using a two-pot procedure. In the first reaction flask, 0.06 g of CdO, 0.28 g of ODPA, and 3 g of TOPO were degassed at 150 °C for 1 h before being heated to 350 °C to form Cd-ODPA. After adding 1 mL of TOP, 0.06 g of Se in 0.5 mL of TOP was swiftly injected at 380 °C, and the reaction was quenched after 30 s. The obtained CdSe seed QDs were purified by precipitation with acetone and then redispersed in hexanes. In the second step, 3 mL of ODE, 2 mL of oleyamine, and the solution of CdSe seeds were degassed at 100 °C for 1 h. A 1.2 mL amount of the Cd(OA)2 solution (stock solution prepared in a separate reaction flask by degassing a mixture of 0.128 g of CdO, 0.64 mL of oleic acid, and 5.4 mL of ODE at 100 °C for 30 min before heating to 250 °C) was then injected dropwise into the reaction mixture at 260 °C. After finishing precursor injection, 1.2 equiv (to Cd(OA)2) of 1-octanethiol in ODE was added dropwise to the reaction mixture, followed by a swift injection of 1 mL of oleic acid. The reaction was terminated by cooling with an air jet after 1 h of growth at 310 °C. For purification, excess acetone was added to the reaction mixture. The precipitate after centrifugation was redissolved in hexanes and centrifuged again to remove the insoluble precipitate. The supernatant was then purified twice by precipitating with methanol. The final QD product was dispersed in octane (∼60 mg/ mL) for spin-casting. Transmission electron microscopy (TEM) images of QD core and QD core/shell images of red QD are shown in Figure S3. To synthesize the green CdSe/ZnS alloy QDs, 0.2 mmol of CdO, 4 mmol of Zn acetate, 5 mL of oleic acid, and 15 mL of ODE were degassed at 110 °C for 1 h before being heated to 300 °C. A solution of 0.2 mmol of Se and 3 mmol of S in 2 mL of TOP was then injected into the reaction mixture. The reaction was quenched after 2 min of growth. The blue CdSe/ZnS alloy QDs were synthesized in a similar manner. One mmol of CdO, 9 mmol of Zn acetate, 8 mL of oleic acid, and 15 mL of ODE were degassed at 110 °C for 1 h before being heated to 300 °C. Then a solution of 0.2 mmol of Se and 1.8 mmol of S in 3 mL of ODE was injected. After 10 min of growth, 4 mL of 2 M TBP-S was injected at 30 mL/h using a syringe pump. The reaction was terminated by cooling with an air jet after another 1 h of growth. The green and blue QDs were purified three times by adding chloroform and an excess amount of acetone. The final QD products were dispersed in octane (∼60 mg/mL) for spin-casting. TEM images of QD core and QD core/shell images of green and blue QDs are shown in Figure S4 and Figure S5, respectively. Preparation of RGB QD Films on ODTS-Coated Si Substrates. The preparation of ODTS-coated Si substrates followed an established method20 with slight modifications. Si substrates were cleaned by acetone/2-propanol and dried with N2 flow and then further treated under UV/O3 for 15 min. After UV/O3 exposure, the Si substrates were immediately transferred to ODTS in anhydrous hexane solution (1:600 volume ratio) and left for self-assembly for 1 h. The resulting substrates were sonicated in chloroform to remove excess ODTS molecules and then baked on a hot plate at 120 °C for 20 min. Once the ODTS-coated Si substrate was prepared, solutions of QDs in octane were spin-cast onto the substrate. Preparation of a Patterned PR Layer on a PDMS Substrate. The SPR 220-4.5 photoresist was spin-cast on a 1:10 ratio PDMS substrate at 3000 rpm for 30 s. The PR on the PDMS substrate was soft baked on a hot plate at 65 and 110 °C for 1 and 2 min, respectively. Subsequently, exposure to UV (I-line, ∼210 mJ/cm2) light under a photomask and development with AZ 400 K formed the predesigned patterns on the PR layer. Synthesis and Deposition of ZnO. ZnO nanoparticles were synthesized following a published method2 with modifications. A solution of potassium hydroxide (0.74 g) in methanol (65 mL) was slowly added to zinc acetate dihydrate (2.95 g) in methanol (125 mL) solution, and the reaction mixture was stirred at 60 °C for 2 h. The mixture was cooled to room temperature and allowed to stand and

up from RGB donor substrates using the stamp and printed on the receiver substrate individually to form a group of RGB pixels as shown in Figure 5d,e. After transfer printing, the RGB pixels are encapsulated with a polyimide (PI) layer, which is selectively etched on top of the RGB pixels by reactive ion etching with O2/N2 plasma for making electrical contacts to the metal electrodes (Figure 5f). After depositing and patterning Cr/Au wires (Figure 5g), the fabricated QD-LED device is encapsulated. Detailed procedures for full device fabrication are found in the Methods section. Applying negative voltage to three separate electrodes through three wires with the ITO layer as the common ground enables individually activated RGB pixels. To produce similar brightness, the RGB pixels are driven at 6.1, 9.5, and 8.05 V, respectively. While each RGB pixel is composed of different QDs and gives different electroluminescence characteristics, the applied negative voltage can be modulated to match the brightness of the three RGB pixels (Figure 5h). Figure 5i−k are the optical microscope images of individually activated RGB pixels, respectively. Figure 5l−n are the dual activation of RG, GB, and BR pixels, respectively. Figure 5o is the optical microscope image of all three pixels turned on. Electroluminescence characteristics of these QD-LED pixels are given in Figure S2.

CONCLUSION In summary, PR contact patterning, a method of patterning colloidal QD thin films using a photolithographically patterned PR, has been developed. The process is purely dry and nondetrimental to QDs, while it conveniently enables highresolution patterning over large areas. The patterned QD films can also be transfer printed onto virtually any foreign surface for utilization. In addition to PR contact patterning, PR contact transfer has been developed where a patterned PR can be transferred directly onto a QD film and used as a masking layer for subsequent fabrication steps/processes. The PR contact transfer approach allows additional materials to be deposited prior to removal of the transferred PR, providing a very simple means of vertically stacking multiple layers of various compositions on top of QD films with the same pattern design. As a proof-of-concept, RGB pixels of QD-LEDs have been fabricated on the same substrate using this approach. The patterning capabilities shown here enable a scalable and costeffective approach to heterogeneous integration of QD films with other materials such as charge transport layers and metal electrodes on separately optimized substrates, facilitating manufacturing of advanced QD-based devices. METHODS Chemicals. N-Octadecylphosphonic acid (ODPA) was purchased from PCI Synthesis. TFB was purchased from H.W. Sands Corp. SPR 220-4.5 was purchased from MicroChem. AZ 5214, AZ 400 K, and AZ 917 MIF were purchased from AZ Electronic Materials. PDMS (Sylgard 184) was purchased from Dow-Corning. ITO etchant was purchased from Transene Company. Polyimide precursor (PI-2454) and thinner (T9039) were purchased from HD Microsystems. PEDOT:PSS (CLEVIOS P VP AI 4083) was purchased from Heraeus. Cadmium oxide (CdO), 1-octadecene (ODE, 90%), oleic acid (OA, 90%), trioctylphosphine oxide (TOPO, 99%), trioctylphosphine (TOP, 97%), oleylamine (≥98%), selenium (Se) powder (99.999%), sulfur (S) powder (99.999%), tributylphosphine (TBP), zinc acetate (Zn(OA)2, 99.99%), hexane (anhydrous, 95%), hexanes (≥98.5%), methanol (≥99.8%), acetone (≥99.5%), octane (≥99%) octadecyltrichlorosilane (≥90%), potassium hydroxide (≥85%), zinc F

DOI: 10.1021/acsnano.8b04462 ACS Nano XXXX, XXX, XXX−XXX

Article

ACS Nano precipitate for 24 h. The precipitate was then washed through two cycles of precipitation with methanol and centrifugation before being dissolved in 1-butanol with 30 mg/mL concentration. ZnO nanoparticles obtained after centrifugation were directly weighed and dissolved without an additional drying process. For film deposition, the solution of ZnO nanoparticles was drawn with a syringe and applied onto the device substrate through a 0.2 μm pore size PTFE membrane syringe filter. This solution was spin-cast at 2000 rpm for 30 s, and the substrate was annealed on a hot plate at 120 °C for 30 min. Preparation of ITO Patterned Substrates. ITO substrates were purchased from Sigma-Aldrich (surface resistivity 15−25 Ω/sq). The ITO substrates were patterned by photolithography to obtain a 2 mm pixel width. AZ 5214 photoresist and AZ 917 MIF developer were used for patterning. After developing the photoresist, the substrates were dipped in an ITO etchant bath for 20 min on a 90 °C hot plate, before rinsing with water. The patterned ITO substrates were wiped clean with acetone and a Kimwipe and then rinsed with acetone, 2propanol, DI water, and then 2-propanol again before drying with a nitrogen flow. After this cleaning step, the ITO substrates were exposed to UV/O3 for 15 min for further cleaning and to achieve a more hydrophilic surface. The next step of spin-casting PEDOT:PSS was carried out immediately after the UV/O3 treatment due to the short duration of the UV/O3 effect. Deposition of PEDOT:PSS. A sufficient amount of PEDOT:PSS was drawn with a syringe and applied onto the substrate through a 0.45 μm pore size PVDF membrane syringe filter. The solution was then spin-cast at 4000 rpm for 40 s. The obtained PEDOT:PSS film was annealed on a 125 °C hot plate for 5 min in air to remove moisture before the samples were transferred into a N2-filled glovebox (with O2 and H2O levels