Langmuir 2006, 22, 2407-2410
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Photopatterned Semiconductor Nanocrystals and Their Electroluminescence from Hybrid Light-Emitting Devices Shinae Jun, Eunjoo Jang,* Jongjin Park, and Jongmin Kim Materials Laboratory, Samsung AdVanced Institute of Technology, Mt.14-1, Nongseo-Ri, Giheung-Eup, Yongin-Si, Gyeonggi-Do, 449-712, Korea ReceiVed June 30, 2005. In Final Form: December 6, 2005 We demonstrate the ability to use a photolithographic method to make patterned nanocrystal film for device applications. Exposing a nanocrystal film to strong UV light allowed the oleic acid ligands on the surface of the nanocrystals to form an insoluble cross-linked network while the unexposed areas were still soluble to toluene solvent. Therefore, the UV light exposure through a shadow mask followed by solvent rinsing produced a small feature size on the order of 2 µm. We also report that the integrated nanocrystal patterns in an organic light-emitting diode show clear electroluminescence from the nanocrystals.
Semiconductor nanocrystals have become a matter of technological interest because of their tunable optoelectronic properties depending on their sizes.1,2 Enthusiastic studies on the synthetic method made wonderful progress toward designing high-quality nanocrystals.3-9 Many reports have disclosed potential applications for light-emitting devices,10-16 photovoltaic cells,17-19 and biolabeling tags.20-22 At this moment, the integration of the nanomaterials in a desired device structure becomes the more critical issue. Until recently, various methods have been tried to fabricate nanocrystal films for potential applications. A self-assembly technique has been applied by using multifunctional agents which make bridges between nanocrystals and the substrate.23,24 An electroluminescent device comprised of one or more layers of nanocrystals was also obtained (1) Alivisatos, A. P. Science 1996, 271, 933. (2) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. ReV. Mater. Sci. 2000, 30, 545. (3) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (4) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989. (5) Kim, S.; Fisher, B.; Eisler, H.; Bawendi, M. J. Am. Chem. Soc. 2003, 125, 11466. (6) Cao, Y. W.; Bannin, U. J. Am. Chem. Soc. 2000, 122, 9692. (7) Talapin, D. V.; Rogach, A. L.; Shevchenko, E. V.; Kornowski, A.; Hasse, M.; Weller, H. J. Am. Chem. Soc. 2002, 124, 5782. (8) Crouch, D. J.; O’Brien, P.; Malik, M. A.; Skabara, P. J.; Wright, S. P. Chem. Commun. 2003, 1454. (9) Afzaal, M.; Malik, M. A.; O’Brien, P. Chem. Commun. 2004, 334. (10) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (11) Dabbousi, B. O.; Bawendi, M. G.; Onitsuka, O.; Rubner, M. F. Appl. Phys. Lett. 1995, 66, 1316. (12) Schlamp, M. C.; Peng, X.; Alivisatos, A. P. J. Appl. Phys. 1997, 82, 5837. (13) Coe, S.; Woo, W. K.; Bawendi, M. G.; Bulovic, V. Nature 2002, 420, 800. (14) Mattoussi, H.; Radzilowski, L. H.; Dabbousi, B. O.; Thomas, E. L.; Bawendi, M. G.; Rubner, M. F. J. Appl. Phys. 1998, 83, 7965. (15) Mattoussi, H.; Radzilowski, L. H.; Dabbousi, B. O.; Fogg, D. E.; Schrock, R. R.; Thomas, E. L.; Rubner, M. F.; Bawendi, M. G. J. Appl. Phys. 1999, 86, 4390. (16) Coe, S.; Woo, W. K.; Steckel, J. S.; Bawendi, M. G.; Bulovic, V. Org. Electron. 2003, 4, 123. (17) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (18) Huynh, W. U.; Peng, X.; Alivisatos, A. P. AdV. Mater. 1999, 11, 923. (19) Milliron, D. J.; Alivisatos, A. P.; Pitois, C.; Edder, C.; Fre´chet, J. M. J. AdV. Mater. 2003, 15, 58. (20) Bruchez, M., Jr.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013. (21) Chan, W. C. W.; Nie, S. Science 1998, 281, 2016. (22) Dubertret, B.; Skourides, P.; Norris, D. J.; Noireaux, V.; Brivanlou, A. H.; Libchaber, A. Science 2002, 298, 1759. (23) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221. (24) Tsuruoka, T.; Akamatsu, K.; Nawafune, H. Langmuir 2004, 20, 11169.
by self-assembly.25,26 Interfacial capillary force was reportedly used for the integration of nanocrystals in preformed polymer patterns.27 The Langmuir-Blodgett technique has been widely used to make a close-packed nanocrystal assembly,28 which was used in a light-emitting diode (LED) device for efficient energy transfer between nanocrystals and quantum wells.29 A layerby-layer adsorption method has been tried to prepare a nanocrystal-polyelectrolyte multilayer in an electroluminescent device.30 Spin coating has been the most common technique. A monolayer nanocrystal film in a hybrid light-emitting device was spin cast by using phase segregation via aromatic-aliphatic repulsion.13,16 Also, a few reports about the patterning of colloidal particles on a substrate were disclosed. Nanocrystals could be arrayed selectively on the organic patterns by the interaction between the nanocrystal surface and specific functional groups, which were produced by exposing a photodegraded protection group to UV light through a mask.31 Another work showed that a photobrightened pattern of nanocrystal film was prepared by simply exposing the film to UV through a mask by photoactivation.32 Still, it is challenging work to develop a proper fabrication technique for the integration of the nanostructures in a working device. In this report, we describe a simple series of processes to fabricate a patterned nanocrystal light-emitting device in detail. The uniform CdSeS nanocrystals were prepared by a published technique.33 CdO powder (Aldrich, 99.99%, 0.4 mmol) was dissolved with oleic acid (Aldrich, 90%, 1.6 mmol) in trioctylamine (Aldrich, 98%, 16 g). The mixture was heated to 320 °C and maintained at that temperature for 10 min with nitrogen blow. The temperature was lowered to 300 °C, and the mixture of elemental Se and S in trioctylphosphine solvent (Strem, 97%, 1 mL) was injected into the medium containing cadmium precursor. The molar ratios of Se to S were adjusted to 1:1, 1:10, (25) Alivisatos, A. P.; Colvin, V. L. U.S. Patent 5, 751, 018. (26) Alivisatos, A. P.; Colvin, V. L. U.S. Patent 5, 537, 000. (27) Cui, Y.; Bjo¨rk, M. T.; Liddle, J. A.; So¨nnichsen, C.; Boussert, B.; Alivisatos, A. P. Nano Lett. 2004, 4, 1093. (28) Kim, F.; Kwan, S.; Akana, J.; Yang, P. J. Am. Chem. Soc. 2001, 123, 4360. (29) Achermann, M.; Petruska, M. A.; Kos, S.; Smith, D. L.; Koleske, D. D.; Klimov, V. I. Nature 2004, 429, 642. (30) Gao, M.; Lesser, C.; Kirstein, S.; Mo¨hwald, H.; Rogach, A. L.; Weller, H. J. Appl. Phys. 2000, 87, 2297. (31) Vossmeyer, T.; Jia, S.; Delonno, E.; Diehl, M. R.; Kim, S. H.; Peng, X.; Alivisatos, A. P.; Heath, J. R. J. Appl. Phys. 1998, 84, 3664. (32) Wang, Y.; Tang, Z.; Correa-Duarte, M. A.; Liz-Marzan, L. M.; Kotov, N. A. J. Am. Chem. Soc. 2003, 125, 2830. (33) Jang, E.; Jun, S.; Pu, Y. Chem. Commun. 2003, 2964.
10.1021/la051756k CCC: $33.50 © 2006 American Chemical Society Published on Web 02/07/2006
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Figure 1. (a) Fluorescence micrograph of the nanocrystal patterns under 365 nm UV illumination (from left, the emission wavelengths of the employed CdSeS nanocrystals were 620, 550, and 480 nm). The number in each photo indicates the real size of the patterned line width. (b) The PL spectra of the original nanocrystal solutions (bright colored red, green, and blue lines) and patterned film (dark colored red, green, and blue lines) excited at 365 nm.
and 1:72 for the nanocrystals emitting light at 620, 530, and 480 nm respectively. After 4 min, the reaction medium was quenched with ethanol and the formed nanocrystals were separated from the solvent by centrifuge. The prepared nanocrystals were redispersed in toluene as 1 wt % solution. At the initial stage of the synthesis, oleic acid surfactant formed a complex with cadmium oxide to make cadmium oleate precursor, which reacted with a mixture of the Se and S precursor. The oleic acid still coordinated on the surface of the CdSeS nanocrystals after the separation, and it suppressed the aggregation of nanocrystals. The finally obtained CdSeS nanocrystals exhibited controllable photoluminescence from red to blue with high quantum efficiency up to 80%. Since oleic acid has a photosensitive double bond in the middle of the carbon chain, it can be photopolymerized by absorbing UV.34,35 Therefore, when the oleic acid coordinated CdSeS nanocrystal film on the substrate was exposed under UV source through a mask, the exposed part of the photosensitive surfactants formed a cross-linked network and became resistibly insoluble to the organic solvent. The following procedures could produce (34) Nichols, P. D.; Shaw, P. M.; Johns, R. B. J. Microbiol. Methods 1985, 3, 311. (35) Yammine, P.; Pavon-Djavid, G.; Helary, G.; Migonney, V. Biomacromolecules 2005, 6, 2630. (36) Hess, B. C.; Okhrimenko, I. G.; Davis, R. C.; Stevens, B. C.; Schulzke, Q. A.; Wright, K. C.; Bass, C. D.; Evans, C. D.; Summers, S. L. Phys. ReV. Lett. 2001, 86, 3132. (37) Nazzal, A. Y.; Wang, X.; Qu, L.; Yu, W.; Wang, Y.; Peng, X.; Xiao, M. J. Phys. Chem. B 2004, 108, 5507.
a well-resolved direct pattern of the nanocrystals. A Si wafer was cleaned in neutral detergent and isopropyl alcohol alternatively several times and treated in an ozone generator for 15 min. PEDOT (Baytron AI 4083, poly(3,4-ethylenedioxythiophene)) was spin coated on the cleaned Si substrate and softly baked at 110 °C for 10 min. Then, 1 wt % nanocrystal solution was spin coated (spin-coating step was 500 rpm/1 s-1500 rpm/ 20 s-500 rpm/5s) on top of the PEDOT film and baked at 50 °C for 2 min and then at 100 °C for 2 min. It was exposed under UV for 5 min through a microscale patterned photomask. A broad range of UV light was used (300-400 nm), and the source power was 33.2 mW/cm2 at 360 nm. After the mask was removed, the unexposed part of the nanocrystal film was rinsed out with toluene (∼0.5 mL/s) for 0.5 min and dried for 1 min at 90 °C. Figure 1a is a photo of the prepared nanocrystal patterns taken with a fluorescence microscope (excited by 365 nm UV light). In each photo, the number below the line pattern indicates the above line width in microscale. The patterns showed good resolution down to two microscale with a clean contrast. The peak wavelengths of the PL spectrums are 620, 550, and 480 nm for the red, green, and blue light-emitting patterns, respectively, and they appeared at the same position of the original CdSeS nanocrystal solutions as shown in Figure 1b. The CdSeS nanocrystals are quite stable against photobleaching. The obtained patterns are retained for more than a year under daylight and maintain the same resolution and light-emitting property. The patterned nanocrystals after strong UV exposure
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Figure 2. IR spectra of the nanocrystal films before and after UV exposure. The peak of alkenyl C-H stretching disappeared after the UV exposure.
even showed enhanced PL efficiency compared to the original film. The exposed part to UV became brighter under a 365 nm UV lamp, and the pattern shape could be distinguished even before rinsing with solvent.32 This can be explained by photoinduced surface transformation. The photopolymerized organic passivation layer of the nanocrystals stabilized the defect sites more efficiently and increased the light-emitting intensity.36,37
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X-ray photospectroscopy revealed that the nanocrystal surfaces coordinate with 30 wt % of oleic acid (Supporting Information, Figure 1). The IR spectra of the original film and UV exposed film were measured to verify the chemical changes of the surfactants in Figure 2. It was noticed that the small peak at 3010 cm-1 from the original film disappeared after UV exposure. The peak at 3010 cm-1 is considered as an alkenyl C-H bond stretching of unsaturated hydrocarbon. There is no indication of a peak at 3010 cm-1 after UV exposure, which implies that the unsaturated hydrocarbon becomes saturated by the photopolymerization of an oleic acid interparticle and/or intraparticle cross-link process to form an insoluble polymeric structure. Since the oleic acid on the nanocrystal surface is able to penetrate into carbon chains on adjacent nanocrystals during film preparation, oleic acids between the particles are close enough to come in contact. Therefore, they can interconnect particles to make an insoluble network after UV exposure. EGA (evolving gas analysis, Supporting Information, Figure 2) data showed that oleic acid was detected around 130 °C in the nanocrystal film before UV exposure and the polymerized structure was decomposed around 300 °C in the UV exposed nanocrystal film. The atomic force microscopy (AFM) images of the nanocrystal films on the PEDOT layer showed that the initially close-packed nanocrystals aggregated themselves to make a less uniform packing structure by the UV exposure (Figure 3a). This could be due to the interpenetration of the surfactant chains to form cross-linked networks. The pattern height in Figure 3b reveals more than two layers of nanocrystals formed when 1 wt % nanocrystals in toluene were spin coated on PEDOT. The numbers of layers are dependent on the nanocrystal concentration of the solution, spin speed, and the natures of the solvent and the substrate. When a chloroform solvent was used to disperse the nanocrystals, the spin-coated film generally consisted of more layers than in the case of toluene solvent since the vapor pressure of the solvent may determine the thickness of the film.
Figure 3. (a) AFM images of the nanocrystal films before and after UV exposure for 300 s (plane view). (b) The AFM images of the nanocrystal pattern (angled view) and the height profile.
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Figure 4. PL spectra of the original nanocrystal solutions (excitation at 365 nm) and the EL spectra of the respective nanocrystal patterns at the current density 65 mA/cm2. Inset photos show that the EL devices emit patterned light. Scheme 1. Preparation Process of the Electroluminescence Device Embedded Pattern of a CdSeS Nanocrystal
We prepared the patterned electroluminescence device using two kinds of CdSeS nanocrystals with different band gaps according to the process as described in Scheme 1. The device structure is similar to a previously known QD-LED device.13,16 We chose PEDOT as a hole transport layer since it was insoluble to organic solvent after soft baking at 100 °C. We prepared a patterned nanocrystal layer on a PEDOT-coated ITO substrate according to the previously introduced procedure in this report. In a LED device, the pattern size was made in 0.5 mm in order to collect I-V-L (current-voltage-brightness) data. On the nanocrystal layer, 3-(4-biphenylyl)-4-phenyl-5-tert-butylphenyl1,2,4-triazole (TAZ, HWSands, sublimed grade) was thermally deposited in 10 nm thickness, and tris-(8-hydroxyquinoline) aluminum (Alq3, HWSands, sublimed grade) was evaporated in 25 nm thickness on top of the TAZ layer. As a cathode, 5 nm of LiF (Aldrich, 99%) and 200 nm of Aluminum (Aldrich, slug for evaporation) were deposited. The unit device finally was encapsulated with cover glass using a thermally curable epoxy (Araldite 2011) in the glovebox for characterization. The PL spectra of the original CdSeS nanocrystal solution appeared at 550 and 618 nm with narrow bandwidths in Figure 4. The EL spectra of the patterned nanocrystals were similar to those of the original solutions with slightly broader bandwidths. The inset photos are the operating patterned light-emitting devices. The device area is 24 mm2 (4 mm × 6 mm, white box), and only the nanocrystal patterned area emits the light at the corresponding
wavelength. The brightness of the device was about 100 Cd/m2 at a current density of 65 mA/cm2. The efficiency of the device (∼0.15 Cd/A) is still at a low level for this stage, but we expect to improve the efficiency by optimizing the fabrication procedures of the EL device. In conclusion, we made a patterned semiconductor nanocrystal light-emitting device. Monodisperse and bright CdSeS nanocrystals were prepared with oleic acid surfactant. When they were spin coated and exposed to UV light, the unsaturated double bond of the oleic acid started to photopolymerize and resulted in a cross-linked insoluble network. The nanocrystal pattern was easily prepared by UV exposure through the photomask and simple development with organic solvent, and this was embedded in the conventional electroluminescent device as a patterned lightemitting layer. The patterned nanocrystal EL device showed a clear emission from the nanocrystal with reasonable efficiency. This novel nanocrystal patterning method would be useful for spatial positioning of a nanocrystal in general electronic and photonic devices. Supporting Information Available: Graphs showing X-ray photospectroscopy and evolving gas analysis of oleic acid. This material is available free of charge via the Internet at http://pubs.acs.org. LA051756K
