Nano Patterning of Multiple Colored Quantum Dots by

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Direct Micro/Nano Patterning of Multiple Colored Quantum Dots by Large Area and Multilayer Imprinting Yong Suk Oh,† Kyung Heon Lee,† Hyunki Kim,‡ Duk Young Jeon,‡ Seung Hwan Ko,§ Costas P. Grigoropoulos,∥ and Hyung Jin Sung*,† †

Center for Opto-Fluid-Flexible Body Interaction, Department of Mechanical Engineering, ‡Department of Materials Science and Engineering, and §Applied Nano Technology and Science Lab, Department of Mechanical Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Korea ∥ Laser Thermal Lab, Department of Mechanical Engineering, UC Berkeley, Berkeley, California 94720, United States ABSTRACT: A novel direct method of micro/nano quantum dot (QD) patterning via one-step imprinting over a large area at low temperatures and low pressures was demonstrated as an alternative to conventional vacuum deposition and photolithography methods. More complex QD patterning could be demonstrated by expanding the QD direct imprinting process for multiple colored QD and patterning on the multiple layers. Additionally, a self-alignment scheme was developed to pattern multiple layers without the need for laborious alignment steps. Our approach may be useful for QDbased optoelectronic device fabrication and patterning on large flexible substrates due to the low-temperature requirements of this process.

1. INTRODUCTION Nanoimprinting lithography (NIL) offers a cost-effective direct micro/nano patterning alternative to traditional expensive vacuum deposition and photolithography processes. NIL allows the replication of two- and three-dimensional structures with sub-100 nm resolution via a thermal1 or UV-curing process.2 A variety of polymer materials (e.g., thermoplastic polymers,3 UVcurable polymers4) have been used to manufacture micro/nano structures for biological devices,5 photonic devices,6 and organic electronics.7 However, unwanted residual layers after an imprinting process present an important postprocessing challenge. Residual layers are usually eliminated by oxygen reactive ion etching (RIE).8 This technique can damage functional polymer structures and degrade the resolution and fidelity of a pattern.9 Methods that do not require RIE would present good alternatives. Although inorganic materials (e.g., Ag, Au) are preferred for use in functional micro/nano devices due to their excellent optical and electrical properties, polymers are widely used in imprinting process development because they can be easily handled, particularly because they are present as fluids or solutions at low temperatures. Bulk metals are difficult to use in the replication of micro/nano patterns using conventional imprinting processes because metals usually have very high melting temperatures. To alleviate the limitations associated with metal imprinting, Ko et al. recently developed a direct metal nanoimprinting process using a metal nanoparticle ink. This technique exploited the size-dependent melting temperature decrease observed in metal nanoparticles and the © 2012 American Chemical Society

solution-state metal patterning processes available at low temperatures, yielding minimal residual layers.10,11 In addition to metal nanoparticles, quantum dots (QDs) are widely used in biosensors, high-efficiency solar cells, efficient full-color displays, and luminescent devices due to a narrow emission spectrum, high quantum efficiency, and unique sizedependent properties.12,13 To the authors' knowledge, direct imprinting of QDs for optoelectronic and biosensing purposes has not yet been demonstrated. Generally, QD direct patterning has been fabricated via a dry transfer printing14 or solution process like inkjet printing,15 which was limited by poor resolution (∼100 μm) due to a nonuniform surface induced by a coffee-stain problem. Additionally, inkjet printing is a serial patterning process, which suffers from a long processing time and small throughput. Direct QD imprinting can be developed with increased resolution and fidelity of the patterns by controlling solution viscosity, concentration of QD, and designing a polydimethylsiloxane (PDMS) mold. For a more complex device fabrication, more sophisticated imprinting process developments such as multiple component imprinting and multiple layer processing are necessary. However, for a multilayer process of micro/nano imprinting, accurate alignment is a vital challenge.16 To manufacture integrated circuits (ICs), multiple colored electroluminescent devices, and pixilated full-color displays, a multilayer process of micro/ Received: February 11, 2012 Revised: May 8, 2012 Published: May 9, 2012 11728

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nano patterns should be necessarily dealt with efficient alignment. In this paper, we demonstrate a direct micro/nano QD patterning method using a direct solution-based imprinting process. Complex QD patterning could be realized by expanding the QD direct imprinting process for multiple colored QDs and patterning on the multiple layers. A selfalignment scheme was developed to pattern multiple layers without the need for laborious alignment steps. This simple, cost-effective, low-temperature, one-step QD patterning technique does not require conventional expensive vacuum deposition and photolithography processes. This process can be easily scaled up to large substrates, and it combines roll-to-roll processes for the high-throughput production of functional QD patterning. The low-temperature nature of the process permits QD patterning on low-temperature polymer substrates.

Figure 1. (a) Normalized PL spectra of CdSe QDs in an organic solution without precipitation (inset: digital camera image of the photoemission of various-sized CdSe QD in hexane solution excited by a UV lamp). (b) TEM image of CdSe QDs.

2. EXPERIMENTAL DETAILS 2.1. QD Synthesis. CdSe QDs were synthesized using a procedure based on that described in Peng et al.,17 with modifications. CdO (13 mg, Sigma Aldrich), oleic acid (1.2 mL, Sigma Aldrich), and octadecene (10 mL, Sigma Aldrich) were mixed in a 100 mL three-neck flask under an Ar atmosphere. The mixture was heated to 60 °C under vacuum conditions with magnetic stirring for 10 min. As Ar was reintroduced, the reaction was reheated to 270 °C. A stock solution containing selenium (30 mg) in trioctylphosphine (0.8 mL) was swiftly injected into the hot solution. The reaction mixture was allowed to cool to 230 °C for the growth of CdSe nanocrystals. Aliquots were collected at different reaction time intervals. The UV/vis spectra and PL (photoluminescence) spectra of each aliquot were recorded. The nanocrystals were purified by precipitation with ethanol and acetonitrile. The precipitate was collected by centrifugation. The nanocrystals were then dispersed in hexane. 2.2. PDMS Mold Fabrication. The PDMS mold can be easily replicated from a SU-8 master mold. The PDMS mold permits solvent evaporation and provides good conformal contact, thereby minimizing residual layers. A mixture of the PDMS silicon elastomer kit and a curing agent (10:1) was poured onto a SU-8 master in a Petri dish. Stacked glasses held together with two magnets, and a cover glass was used to support the flat PDMS surface. After curing, the PDMS at 65 °C over 5 h, the PDMS replica was carefully released from the SU-8 master. 2.3. Poly-4-vinylphenol (PVP) Layer for Multilayer Imprinting of Multiple Colored QD. To protect the prepatterns, PVP (MW ∼8000 AMU) dissolved in hexanol with a small amount of the cross-linking agent [poly(melamineco-formaldehyde)] was spin-coated (3000 rpm, 1 min) onto the imprinted QD patterns and cross-linked at 150 °C. The cross-linked PVP was transparent and provided a protective layer to maintain separation between the different layers.

subsequent removal of the hexane by heating at 100−150 °C because α-terpineol has a much higher boiling temperature (219 °C) than that of hexane (69 °C). By using α-terpineolbased QD solution, direct QD imprinting process was accomplished with the following steps (Figure 2): (i) the substrate (e.g., Si/SiO2 wafer) was cleaned with acetone and isopropanol and then dried with N2 gas. The QD solution was dispensed onto the Si/SiO2 wafer. (ii) The solution was squeezed into a PDMS mold at low temperatures (80 °C) and low pressures (26 kPa). (iii) The Si/SiO2 wafer was heated at 80 °C for 20 min to allow the QDs to form structures and to evaporate the solvent. (iv) After the substrate was cooled to room temperature, the PDMS mold was carefully removed from the substrate to leave an imprinted QD pattern. The PDMS stamp presented several advantages over a rigid stamp (e.g., silicon or quartz). First, PDMS stamps were easily replicated from an original master mold. Second, evaporating solvent can easily escape through PDMS during the imprinting process. Third, PDMS leaves minimal residual layers on a substrate due to the good conformal contact between the PDMS mold and the substrate. Finally, the flexibility of PDMS facilitates demolding. High-quality imprinted patterns with negligible residual layers were achieved by controlling the QD solution viscosity. Among the organic solvents tested, α-terpineol was found to be suitable for imprinting because the fluid properties could be easily adjusted by varying the temperature. The solvent presented a high viscosity at 25 °C and a low viscosity at 100 °C. The temperature of the imprinting process was optimized by examining the solvent filling in the PDMS mold and by facilitating the pattern replication with minimal residual layers. The optimal conditions had to balance two effects that influenced the solution viscosity: high temperature lowered the viscosity, although this was offset by the viscosity increase due to solvent evaporation. The optimal temperature in the present study was 80 °C. The final quality of the imprinting process was affected by both the fluidic viscosity and the conformal contact property as a mutually complementary relation. The control of the fluidic viscosity by varying temperature was important for completely filling the solvent into the PDMS mold, keeping the structures without collapse and minimizing residual layers. The conformal contact between the PDMS mold and the substrate was successfully achieved by an appropriate external pressure. Very low pressure led to an imprecise structure and residual layers. Also, very high pressure

3. RESULTS AND DISCUSSION CdSe QDs were synthesized and formulated into viscous ink for direct QD imprinting. Figure 1a shows that the synthesized QDs display the size-tunable PL spectra that emit from blue to red. The size of QDs was measured by TEM (Figure 1b). After the QD precipitation, the QDs were collected and dispersed in hexane. The solvent was replaced with α-terpineol from a mixture of the QD hexane solution and α-terpineol through 11729

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Figure 2. Schematic diagram showing the process of direct micro/nano imprinting using colloidal QDs.

made the PDMS mold damaged and deformed as well as unwanted residual layers. Direct micro/nano patterns of multiple colored QDs were achieved on a Si/SiO2 substrate over a large area (up to 2 × 2 cm2). Circular and square-shaped dot arrays fabricated by the direct micro/nano imprinting of QDs were characterized by confocal microscopy (Figure 3). Fluorescent images of square-

Figure 4. (a) Scanning electron microscopy (SEM) images of the QD patterns (diameter, 10 μm). (b) SEM images of the QD patterns (diameter, 18 μm). (c) Fluorescence image of the circle-shaped microdot arrays of QD (diameter, 10 μm), collected by confocal microscopy. (d) Fluorescence image of the donut-shaped microdot arrays of QD (diameter, 18 μm), collected by confocal microscopy.

PDMS mold under the applied pressure strongly affected the QD structures. The top part of the imprinted cylindrical QD structure shown in Figure 4b displayed a nonflat surface, signifying that the imprinted QD structures were more strongly subject to gravity effects (Laplace pressure) than to capillary effects (hydrostatic pressure). Fluorescence images of the circular microdot patterns (10 μm diameter) and donut-shaped microdot patterns (18 μm diameter), collected by confocal microscopy, are shown in Figure 4c,d. Fluorescence images of microdot arrays featuring squares or rectangles (5−30 μm) are shown in Figure 5. The squareshaped microdot arrays shown in Figure 5a−d had widths of 5, 10, 15, and 20 μm, respectively. The fluorescence images shown in Figure 5a−c show completely full QD square-shaped microdot arrays inside the mold patterns. For comparison, the fluorescence image shown in Figure 5d shows partially filled QD square microdot arrays near a mold pattern boundary. The square mcirodot arrays in Figure 5c are slightly distorted into capital letter “X” shapes due to the large deformations of the PDMS mold under high pressures. The empty square-shaped patterns shown in Figure 5d indicate that most QDs collected near the edges of the square, and the center of the square

Figure 3. Fluorescence images of imprinted CdSe QD patterns (confocal microscopy images). (a) Square-shaped dot arrays (d = 12 μm) of red-colored emitting QDs. (b) Square-shaped dot arrays (d = 15 μm) of green-colored emitting QDs. (c) Circular dot arrays (d = 15 μm) of blue-colored emitting QDs. (d) Digital camera image over a large area (1.5 × 1.5 cm2) of the Si/SiO2 wafer under UV irradiation.

shaped dot arrays (d = 12, 15 μm) of red-colored and greencolored emitting QDs and circular dot arrays of blue-colored emitting QDs (d = 15 μm) are shown in Figure 3a−c, respectively. Figure 3d shows digital camera images of the fluorescent micro/nano dot patterns with different pitches on the Si/SiO2 wafer (1.5 × 1.5 cm2) under UV irradiation. SEM images of the QD structural arrays fabricated by direct imprinting are shown in Figure 4a,b. The arrays of cylindrical QD structure imprinted using the PDMS mold of a negative cylindrical structure were 8−20 μm in diameter and a few tens of nanometers to a few micrometers in height. The features of the imprinted QD structures were strongly affected by the amount of QD solution initially present. Incomplete filling into the PDMS mold with the QD solution or deformation of the 11730

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square-shaped microdot arrays near the mold pattern boundary. These results suggest the existence of a characteristic length scale for successful QD imprinting. A feature size smaller than the characteristic length (15 μm) yielded successful QD patterning; however, a feature size greater than the characteristic length yielded nonuniform QD patterning with coffee-stain problems, as shown in Figure 5d,h. In fact, the imprinting of large features or isolated features surrounded by large unstructured areas remains a challenge because complete solution filling into the PDMS mold and minimizing residual layers are difficult. Thus, the fluorescence images of patterns obtained from direct QD imprinting with features larger than the characteristic length did not provide uniform emission properties. The fluorescence images in Figure 6 show the capital letters (“A” and “I”), which were formed from a single 70 μm solid line, as in Figure 6a, or from a collection of many 10 μm circular dots, as shown in Figure 6b. The letters formed from 70 μm solid lines yielded highly nonuniform emission, and most QDs were accumulated at the edges of the letters. The width of the solid lines used to write the letters was larger than the characteristic length for this process (15 μm). To achieve uniform emitting character patterns larger than the characteristic length, the target patterns were decomposed into small circular dots smaller than the characteristic length. Figure 6b shows the successful direct micro/nano imprinting of large uniform letter patterns composed of arrays of 10 μm sized

Figure 5. Fluorescence image of imprinted QD features. (a−d) Square-shaped microdot arrays and (e−h) rectangle-shaped microdot arrays. The insets at the corner of each picture show a magnified view.

patterns were nearly empty due to incomplete QD solution filling into the PDMS mold. Similar features can be observed in the rectangular microdot arrays shown in the fluorescence images of Figure 5e−h, with width/length rations of 7/14, 10/ 20, 15/20, and 20/30 μm, respectively. The fluorescence images shown in Figure 5e−g show completely full QD rectangle-shaped microdot arrays inside the mold patterns. The fluorescence image in Figure 5h shows partially filled QD

Figure 6. Fluorescence images of capital letters (“A” and “I”) formed from (a) a single patterned 70 μm solid line or (b) a collection of many 10 μm small dots. Note that a suffers from the nonuniform QD accumulation at the edges of the pattern (the coffee-stain problem), whereas b shows a more uniform distribution of QDs within the pattern. 11731

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Figure 7. Direct imprinting of multilayered and multicolored QD patterns by self-alignment. (a) Schematic diagram of the multilayered and multicolored QD pattern direct imprinting process using the self-alignment scheme. (i−iii) Direct imprinting of cylindrical red QD patterns on the first layer and an imprinted red QD SEM picture (inset). (iv) Subsequent PVP deposition and cross-linking at 150 °C on the imprinted red QD patterns. (v−vii) Green QD direct imprinting on top of the PVP/cylindrical red QD patterns. (v) Green QD solution dispensing on the PVP/ cylindrical red QD patterns. (vi) Pressing the green QD solution with a flat PDMS block and subsequent green QD accumulation near the PVP/ cylindrical red QD patterns. (vii) Removing the PDMS block after cooling, leaving the donut-shaped green/red QD concentric patterns. Note that the second PDMS mold is flat, whereas the first PDMS mold has the target patterns. (b) Confocal fluorescence images of the multilayered and multicolored QD patterns prepared by direct imprinting and self-alignment. (i) Fluorescence image of the red-colored emitting QD (circle-shaped) patterns. (ii) Fluorescence image of the green-colored emitting QD (donut-shaped) patterns formed around the red-colored emitting QD patterns via self-alignment.

circular dots. The fluorescence images in Figure 6b show much more controlled and uniform emitting patterns than were observed in the uncontrolled case shown in Figure 6a. This technique provides a useful approach to imprinting uniform QD patterns with large feature sizes via direct micro/nano imprinting without (or with negligible) residual layers. More complex and functional QD patterning needs the development of multilayered and multicolored QD patterning methods. Here, we developed a novel method for multiple QD imprinting on the multiple layers. Especially, a new concept for concentric multicolored QD patterning was achieved by developing a self-aligned direct QD imprinting on the multiple layers. The multiple layers with different colored QD were separated by the transparent PVP thin film to protect the prefabricated patterns (Figure 7). First, cylindrical red QD patterns on Si/SiO2 wafer were formed by the regular direct imprinting process [Figure 7a(i−iii)]. Then, PVP in hexanol with a small amount of the cross-linking agent was spin coated and cross-linked (150 °C) on the direct imprinted red cylindrical QD patterns [Figure 7a(iv)]. This PVP layer protects the prepatterns from post imprinting process and also separates successive QD deposits. After the PVP layer deposition, green QD solution was dispensed onto the PVP/ red QD patterns. The imprinting of the second QD (green QD) on the second layer was carried out by the flat and thin

PDMS mold [Figure 7a(v)]. For multiple layer imprinting, a laborious and time-consuming precise alignment process is required. However, a self-alignment process could be developed by exploiting the surface morphology of the first imprinted patterns on the first layer and the subsequent QD solution squeezing effect by the flat PDMS mold. Note that second PDMS mold is flat, while the first PDMS mold has the target patterns on it. A green QD solution was concentrated around the edge of the imprinted cylindrical red QD patterns on the first layer by the conformal PDMS mold contact on the prepatterned surface [Figure 7a(vi)]. After solvent evaporation, the PDMS mold was removed to leave donut-shaped green QD patterns around the circular red QD patterns [Figure 7a(vii)]. Figure 7b shows a fluorescence image of the two colored QD patterns with the donut-shaped green QD patterns around the circular red QD patterns. Note that the red-colored emitting QD patterns are very well located inside the green-colored emitting QD patterns without any alignment process. The imprinted red QD patterns did not overlap with the imprinted green QD due to PVP film. Finally, followed by pattern size, shape, and localization as well as QD size, a variety of fluorescence color patterns on the multilayer can be imprinted. 11732

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(16) Liang, X.; Zhang, W.; Li, M.; Xia, Q.; Wu, W.; Ge, H.; Huang, X.; Chou, S. Y. Nano Lett. 2005, 5, 527−530. (17) Yu, W. W.; Peng, X. Angew. Chem. 2002, 114, 2474−2477.

4. CONCLUSIONS We successfully demonstrated a direct micro/nano QD patterning technique using one-step imprinting over a large area at low temperatures and low pressures. The viscosity of the α-terpineol-based QD solution was easily controlled by varying the temperature during imprinting. A variety of QD microstructures, including microrods, microwires, and microdots, were fabricated with no or negligible residual layers. More complex QD patterning could be realized by expanding the developed QD direct imprinting process to include a variety of colored QDs or patterns with multiple layers. The selfalignment scheme was developed to pattern multiple layers without the need for laborious alignment steps. The residual layers were minimized by the conformal contact between the PDMS mold and the substrate, as well as surface wetting of the QD solution. Our ultralow cost, large-area, multilayer patterning approach has great potential for applications involving multiple colored QDs, such as optoelectronic devices or full-color displays. The low temperature and pressure requirements permit the QD direct imprinting process to be applied to flexible substrates.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Creative Research Initiatives (No. 2012-0000246) program of the National Research Foundation of Korea.

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REFERENCES

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