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Site-Selective Patterning of Organic Luminescent Molecules via Gas Phase Deposition Juanyuan Hao, Nan Lu,* Qiong Wu, Wei Hu, Xiaodong Chen, Hongyu Zhang, Ying Wu, Yue Wang, and Lifeng Chi* State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin UniVersity, Changchun, 130012, P. R. China, and Physikalisches Institut and Center for Nanotechnology (CeNTech), Westfälische Wilhelms-UniVersität Münster, D-48149 Münster, Germany ReceiVed August 31, 2007. ReVised Manuscript ReceiVed February 1, 2008 In this paper, we present a bottom-up approach to pattern organic luminescent molecules with a feature size down to sub-100 nm over wafer-sized areas. This method is based on the selective gas deposition of organic molecules on self-organized patterned structures, which consist of an organic monolayer with two different phases rather than different materials. The site selectivity is controllable by deposition rate and the pattern features. The reason for the site selectivity may be due to the nucleation and diffusion behaviors of the deposited organic molecules on different monolayer phases.
Introduction Site-selective patterning of organic luminescent molecules with ordered micro- and nanoscopic features is of great importance due to its potential applications in photonics,1,2 optoelectronics,3–6 biochip-based detection,7 biosensor arrays,8 and other related areas. The current strategies for site-selective patterning of organic luminescent materials are usually achieved by combination of top-down and bottom-up concepts, such as nanoimprinting9 and microcontact printing (µCP).10–13 In such approaches, substrate surfaces are chemically modulated with different materials to provide specific energetically and/or chemically favorable binding sites for the molecules that need to be patterned, termed templated self-assembly. The template-directed growth of organic assemblies is usually conducted by wet chemical approaches. Much less attention has been paid to the site-selective patterning of organic luminescent molecules via gas-phase deposition until recently.13–15 * Corresponding uni-muenster.de.
author.
E-mail:
[email protected];
chi@
(1) Gaal, M.; Gadermaier, C.; Plank, H.; Moderegger, E.; Pogantsch, A.; Leising, G.; List, E. J. W. AdV. Mater. 2003, 15, 1165. (2) Kallinger, C.; Hilmer, M.; Haugeneder, A.; Perner, M.; Spirkl, W.; Lemmer, U.; Feldmann, J.; Scherf, U.; Mullen, K.; Gombert, A.; Wittwer, V. AdV. Mater. 1998, 10, 920. (3) Kim, C.; Burrows, P. E.; Forrest, S. R. Science 2000, 288, 831. (4) Koide, Y.; Wang, Q. W.; Cui, J.; Benson, D. D.; Marks, T. J. J. Am. Chem. Soc. 2000, 122, 11266. (5) Lee, T. W.; Zaumseil, J.; Bao, Z. N.; Hsu, J. W. P.; Rogers, J. A. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 429. (6) Veinot, J. G. C.; Yan, H.; Smith, S. M.; Cui, J.; Huang, Q. L.; Marks, T. J. Nano Lett. 2002, 2, 333. (7) Cheek, B. J.; Steel, A. B.; Torres, M. P.; Yu, Y. Y.; Yang, H. J. Anal. Chem. 2001, 73, 5777. (8) Lingerfelt, B. M.; Mattoussi, H.; Goldman, E. R.; Mauro, J. M.; Anderson, G. P. Anal. Chem. 2003, 75, 4043. (9) Hu, W.; Lu, N.; Zhang, H. Y.; Wang, Y.; Kehagias, N.; Reboud, V.; Sotomayor Torres, C. M.; Hao, J. Y.; Li, W.; Fuchs, H.; Chi, L. F. AdV. Mater. 2007, 19, 2119. (10) Choi, H. Y.; Kim, S. H.; Jang, J. AdV. Mater. 2004, 16, 732. (11) Briseno, A. L.; Aizenberg, J.; Han, Y. J.; Penkala, R. A.; Moon, H.; Lovinger, A. J.; Kloc, C.; Bao, Z. N. J. Am. Chem. Soc. 2005, 127, 12164. (12) de la Fuente, J. M.; Andar, A.; Gadegaard, N.; Berry, C. C.; Kingshott, P.; Riehle, M. O. Langmuir 2006, 22, 5528. (13) Briseno, A. L.; Mansfeld, S. C. B.; Ling, M. M.; Liu, S. H.; Tseng, R. J.; Reese, C.; Roberts, M. E.; Yang, Y.; Wudl, F.; Bao, Z. N. Nature 2006, 444, 913. (14) Plain, J.; Pallandre, A.; Nysten, B.; Joans, A. M. Small 2006, 2, 892. (15) Wang, W. C.; Zhong, D. Y.; Zhu, J.; Kalischewski, F.; Dou, R. F.; Wedeking, K.; Wang, Y.; Heuer, A.; Fuchs, H.; Erker, G.; Chi, L. F. Phys. ReV. Lett. 2007, 98, 225504.
Previously, we developed a simple method for patterning nanocrystals into periodic lateral structures based on selective adsorption of nanocrystals on self-organized stripe patterns with submicrometer lateral dimensions, which were obtained by transferring a monolayer of L-R-dipalmitoylphosphatidylcholine (DPPC) onto mica substrates by the Langmuir–Blodgett (LB) technique.16–19 The striped DPPC pattern is composed of liquid expanded (LE) DPPC molecules in the channels and liquid condensed (LC) DPPC molecules in the stripes.20 One of the reasons for the selective adsorption of nanocrystals is the different interfacial energy for the LE DPPC channel (∼31 mJ/m2) and LC DPPC stripe (∼23 mJ/m2).19 Such a structured surface can also serve as a template for self-assembly of inorganic materials from the gas phase.16,21 However, it is still unknown whether it can be used as a template to guide the selective deposition of organic molecules from the gas phase. Herein, we demonstrate that such self-organized templates can be used to pattern organic luminescent molecules via gasphase deposition. The approach is accomplished by two steps: surface patterns composed of organic molecules in two different phases are generated by LB technique; dye molecules deposited from the gas phase nucleate differently on these two regions, which induces site selectivity, as schematically shown in Figure 1.
Experimental Section Materials. L-R-Dipalmitoylphosphatidylcholine (DPPC, purity >99%) and stearic acid (C17-COOH, purity 99.7%) were obtained as a powder from Fluka and used without further purification. Chloroform (HPLC grade) purchased from commercial sources (Guangfu Fine Chemical Research Institute, Tianjin, China) was (16) Gleiche, M.; Chi, L. F.; Fuchs, H. Nature 2000, 403, 173. (17) Lu, N.; Chen, X. D.; Molenda, D.; Naber, A.; Fuchs, H.; Talapin, D. V.; Weller, H.; Muller, J.; Lupton, J. M.; Feldmann, J.; Rogach, A. L.; Chi, L. F. Nano Lett. 2004, 4, 885. (18) Chen, X. D.; Hirtz, M.; Rogach, A. L.; Talapin, D. V.; Fuchs, H.; Chi, L. F. Nano Lett. 2007, 7, 3483. (19) Chen, X. D.; Rogach, A. L.; Talapin, D. V.; Fuchs, H.; Chi, L. F. J. Am. Chem. Soc. 2006, 128, 9592. (20) Chen, X. D.; Lenhert, S.; Hertz, M.; Lu, N.; Fuchs, H.; Chi, L. F. Acc. Chem. Res. 2007, 40, 393. (21) Gleiche, M.; Chi, L. F.; Gedig, E.; Fuchs, H. ChemPhysChem 2001, 2, 187.
10.1021/la7026779 CCC: $40.75 2008 American Chemical Society Published on Web 03/28/2008
5316 Langmuir, Vol. 24, No. 10, 2008
Hao et al. mass accumulation (∆m) on the silver electrode surface according to the following equation
∆F )
2F20 × ∆m
[A ×
1 (µqPq) 2
]
(1)
where ∆F is the change in the resonance frequency [Hz]; F0 is the basic resonant frequency of the silver electrode between the crystals [MHz]; ∆m is the mass accumulation on the silver electrode [g]; A is the silver electrode area [cm2]; µq is the density of the quartz crystal [g · cm-3]; and Pq is shear modulus. Furthermore,
d) Figure 1. Schematic illustration of the procedure for vapor deposition of organic luminescent molecules ANP on patterned substrates. (a) ANP selectively deposited on LE phase of DPPC and formed particles with small amount of ANP; (b) ANP formed particles on both LE and LC phases of DPPC with different sizes by further evaporation. Scheme 1. Molecular Structure of 3(5)-(9-Anthryl) Pyrazole (ANP)
used as solvent for both substances without further purification. Branched poly(ethyleneimine) (PEI) with an average molecular weight of 750 000 (PEI750000, purity 99%) was purchased from Aldrich. 3-(9-anthrye) pyrazole (ANP) (molecular structure is shown in Scheme 1) was synthesized according to refs 22, 23. LB Transfer. The LB films were prepared with a commercial LB trough (NIMA 312D). For the DPPC stripe pattern formation, the temperature of the subphase was controlled by a thermostat (22.0 ( 0.3 °C) and the humidity of the laboratory was 50–70%. First of all, a freshly cleaved mica plate (PLANO, Germany) was inserted into the water subphase (Millipore, resistance 18.2 MΩ · cm). Then, 20 µL of a 1 mg/mL DPPC-chloroform solution was spread on the water surface using a microsyringe. Following the solvent evaporation for 20 min, the monolayer was symmetrically compressed to the target surface pressure with a constant barrier speed of 10 cm2 · min-1. Finally, after waiting for 30 min for the system to equilibrate, the monolayer was transferred onto the mica surface by a vertical dipping method with an upward motion at a constant dipping speed (1–60 mm/min). A similar procedure is used for preparing a monolayer of stearic acid in the LE/LC coexiting region. The temperature of the subphase (PEI750000 solution 0.75 g/L) was controlled by a thermostat (25.0 ( 0.3 °C) and the humidity of the laboratory was 20–40%. The monolayer was transferred onto the mica surface by a vertical dipping method with an upward motion at a constant dipping speed (4 mm · min-1). Fluorescence Patterns. The evaporation of the dye molecules was carried out by a commercial thermal evaporation system in a vacuum (5 × 10-4 Pa) (Shenyang City Keyou Institute of Vacuum Technology, China). The average thickness of evaporated molecular films was read indirectly by a quartz crystal microbalance (QCM). The QCM measurements were performed on 10 MHz QCM devices. The prepared QCM probe was first mounted on the same height as the sample holder. The frequency difference (∆F) is related to the (22) Komai, Y.; Kasai, H.; Hirakoso, H.; Hakuta, Y.; Okada, S.; Oikawa, H.; Adschiri, T.; Inomata, H.; Arai, K.; Nakanishi, H. Mol. Cryst. Liq. Cryst. 1998, 322, 167. (23) Amar, J. G.; Family, F.; Lam, P. M. Phys. ReV. B 1994, 50, 8781.
∆m FA
(2)
where d is the thickness of the evaporated film [nm]; and F is the density of the evaporated material [g · cm-3]. For ANP, 17 Hz in the change of resonance frequency is equal to 1 nm of evaporated dye molecules. The reading error of QCM is only 1 Hz, so the absolute deviation is 0.059 nm. The evaporation rate can be calculated according to the change rate of the resonance frequency and it can be adjusted by alternating heating current. In our system, the sublimation temperature of ANP is only 43 °C and the distance between the QCM and heating boat is 40 cm, so the temperature around QCM is almost invariable when changing heating current from 1.5 to 2.0 A. The error caused by temperature can be neglected. Characterization. Atomic force microscopy (AFM) studies were carried out with a commercial AFM instrument (Digital Instruments, Dimension 3100, Santa Barbara, CA) running in tapping mode. Si cantilevers (Nanosensors) with resonance frequencies of 250–350 kHz were used. Fluorescence microscope images were taken with a commercial fluorescence microscope (Olympus Reflected Fluorescence System BX51, Olympus, Japan). Particle size distribution analysis was carried out by a self-developed program written in the PW-Wave developing environment. The resulting histograms were given by applying a Gaussian fit and gave us the mean values and standard deviation for each measurement.
Results and Discussion An organic molecule ANP (see Scheme 1), which showed interesting luminescent properties upon crystallization states,24 is selected as a probe molecule to study the deposition of organic molecules on self-organized patterns. First, DPPC (periodicity: 1.5 µm; stripe width: 1.3 µm) was transferred onto mica surfaces at a lateral surface pressure of 2 mN/m with a velocity of 20 mm/min to form the striped patterns. The corresponding AFM height image is shown in Supporting Information Figure S1. ANP molecules adsorbed almost exclusively in the channel regions, forming aggregated clusters when 2 nm (average evaporation thickness) of ANP was evaporated onto the DPPC pattern with an evaporation rate (R) of 2.0 nm/min in a vacuum chamber, as observed by AFM shown in Figure 2a. The average particle size is ∼75 nm in radius, which is obtained from AFM data and corrected by taking the widening effect into account. The calculation method is shown in Supporting Information, and all the following particle size is corrected by this method. The corresponding fluorescence micrograph (Figure 2b) confirms that the aggregated molecules retain their light-emitting properties, which has the same fluorescence spectrum as the aggregates with green emission,24 as confirmed by spectroscopic measurements (see Supporting Information Figure S2). The preferential adsorption of ANP molecules onto LE DPPC phase (channel area) may be related to the higher surface energy of LE DPPC phase, resulting in preferential formation of nuclei in the channels. (24) Zhang, H. Y.; Zhang, Z. L.; Ye, K. Q.; Zhang, J. Y.; Wang, Y. AdV. Mater. 2006, 18, 2369.
Site-SelectiVe Patterning of Organic Luminescent Molecules
Figure 2. AFM height images of (a) 2 nm ANP selectively deposited on DPPC-patterned substrates and (c) 10 nm ANP deposited on both LE and LC phases of DPPC. Scan size: 20 × 20 µm; (b) and (d) are fluorescence micrographs of (a) and (c), respectively.
However, for the same geometric feature of DPPC patterns with the same evaporation velocity (2.0 nm/min), when 10 nm of ANP molecules were evaporated onto DPPC patterns, the size of ANP particles increased and some of the ANP molecules aggregated on the LC DPPC phase (stripe area) (Figure 2c). It is worth noting that the average size of the ANP nuclei on the LC phase is smaller than that on LE phase. It is also proven by evaporating ANP on pure LC phase and LE phase of DPPC separately (see Supporting Information Figure S3). The surface densities of the molecular aggregates on pure LE DPPC and pure LC DPPC phases are 0.6 no/µm2 and 12.5 no/µm2 (no: average numbers of the aggregates), and the corresponding average particle sizes are 146 ( 34 nm and 23 ( 5 nm, respectively, at an evaporation rate R of 0.2 nm/min. The fluorescence intensity of ANP on the LE regions is much stronger than that on the LC regions, as shown in Figure 2d. This may be caused by the different amount of dye molecules in both regions. However, we cannot exclude the size effect and selfquenching. It is very interesting to note that the geometric features of the pattern (i.e., the width of the LC stripes D) and the evaporation rate R play important roles in the selectivity of dye molecules on patterned surfaces. For instance, at the evaporation rate of 1.0 nm/min, when 6 nm of ANP was evaporated on the DPPC pattern with line widths (D) of 0.8 µm (Figure 3a), ANP particles only distribute within LE phase channels. However, when the same amount of ANP was deposited on DPPC patterns with D of 1.3 µm (Figure 3b), in addition to the ANP particles within LE phase channels, some ANP aggregates also nucleate on the LC phase stripes. If we further increased D to 4.0 µm, even when only 0.35 nm of ANP was evaporated, ANP particles were found on both stripes and channels (Figure 3c). The above results suggest that the width of the stripes is very important to determine the selectivity of ANP molecules on the pattern surface. The data here also imply that the lateral diffusion length of ANP molecules on different phases is different, which may be the critical reason for the nucleus formation. The evaporation rate affects the selectivity as well. In general, smaller R leads to better selectivity
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(Supporting Information Figure S4 for detailed data). The dependence of selectivity on D and R is quite similar to siteselective growth of organic molecules on prepatterned surfaces by vacuum deposition reported in another system.15 There is a similar phenomenon observed when evaporating ANP molecules on pure LC phase or LE phase of DPPC separately where smaller R leads to low density and larger size of the molecular aggregate (Supporting Information Figure S5). It is reported by other groups that light-emitting properties of molecular fluorescent nanoparticles can be controlled by the particle size, analogous to semiconductive nanoparticles.25–30 Here, we demonstrate the possibility of preparing variable-sized fluorescent nanoparticles by means of surface engineering, although the spectra difference between the different particles was not observed yet (see Supporting Information Figure S5). One possible reason is that the size of ANP particles we prepared is still too big (from 23 nm to 146 nm under different preparation conditions) so that the size confinement effect does not appear. The above results indicate that the nucleation of ANP molecules preferably starts on top of the LE DPPC. To further demonstrate the generality of the phenomena, we also took mica substrates decorated by other LE/LC coexisting monolayers, such as a stearic acid monolayer prepared on a PEI-containing aqueous subphase31–33 or mixed monolayer of lignoceric acid (C23-COOH) and palmitic acid (C15-COOH).34 The pressure–area isotherm and the AFM height image of the first system are shown in the Supporting Information (Figure S6). By evaporating ANP molecules on stearic acid monolayer transferred from the LE/LC coexistence phase, the nucleation of ANP started on top of the LE phase covered region, and on top of the LC phase covered region upon further evaporation. The AFM images and fluorescence microscope images with different evaporation amounts are shown in Figure 4. We also tried other dye molecules and observed similar phenomena (data not shown here). The site-selective deposition is a diffusion-controlled nucleation and growth process. The interesting point is that the molecular diffusion strongly depends on the packing density of the underneath monolayer. As we observed experimentally, the diffusion length (λ) of dye molecules is different on LE (λLE) and LC (λLC) phases (here, we define the diffusion length as half of the average distance between adjacent molecular aggregates): it is shorter on the LC phase than that on the LE phase, i.e., λLC < λLE. Then, why can the molecules still keep moving to LE phase resulting in selective deposition? This can only be understood if the energy barriers for crossing from the LC phase to the LE phase and for crossing from the LE phase to the LC phase are not equal, as schematically depicted in Figure 5. In this case, the rate constant k1 (ANP moving from LC phase to LE phase) is bigger than the rate constant k2 (ANP moving from LE phase to LC phase). So, even with shorter λLC, the ANP molecules will still be able to move onto the LE phase as long as D/2 < (25) Steudel, S.; Janssen, D.; Verlaak, S.; Genoe, J.; Heremans, P. Appl. Phys. Lett. 2004, 85, 5550. (26) Xiao, D. B.; Xi, L.; Yang, W. S.; Fu, H. B.; Shuai, Z. G.; Fang, Y.; Yao, J. N. J. Am. Chem. Soc. 2003, 125, 6740. (27) Fu, H. B.; Loo, B. H.; Xiao, D. B.; Xie, R. M.; Ji, X. H.; Yao, J. N.; Zhang, B. W.; Zhang, L. Q. Angew. Chem., Int. Ed. 2002, 41, 962. (28) Fu, H. B.; Yao, J. N. J. Am. Chem. Soc. 2001, 123, 1434. (29) Kasai, H.; Kamatani, H.; Okada, S.; Oikawa, H.; Matsuda, H.; Nakanishi, H. Jpn. J. Appl. Phys. 1996, 35, L221. (30) Kasai, H.; Yoshikawa, Y.; Seko, T.; Okada, S.; Oikawa, H.; Matsuda, H.; Watanabe, A.; Ito, O.; Toyotama, H.; Nakanishi, H. Mol. Cryst. Liq. Cryst. 1997, 294, 173. (31) Chi, L. F.; Johnston, R. R.; Ringsdorf, H. Langmuir 1992, 8, 1360. (32) Chi, L. F.; Gleiche, M.; Fuchs, H. Langmuir 1998, 14, 875. (33) Chi, L. F.; Anders, M.; Fuchs, H.; Johnston, R. R.; Ringsdorf, H. Science 1993, 259, 213. (34) Gleiche, M.; Chi, L. F.; Fuchs, H. Thin Solid Films 1998, 327–329, 268.
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Figure 3. AFM height images of 6 nm ANP deposited on DPPC patterns with different stripe widths: (a) stripe width, 0.8 µm; (b) stripe widths, 1.3 µm. Scan size: 20 × 20 µm2; (c) AFM height image of 0.35 nm ANP deposited on DPPC pattern with the stripe width of 4.0 µm. Scan size: 40 × 40 µm2. Evaporation rate: 1.0 nm/min.
Figure 4. AFM height images of (a) 6 nm ANP selectively deposited on stearic acid patterned substrates and (b) 18 nm ANP deposited on both LE and LC phase of stearic acid. Scan size: 15 × 15 µm; (c) and (d) are fluorescence micrographs of (a) and (b), respectively.
λLC, since nuclei are preferably formed on the LE phase for energetic reasons. Why λ is shorter on the LC phase than on the LE phase is still an open question. It cannot be understood from the thermodynamic point of view, since the interfacial energy of the LE phase is larger than that of the LC phase, and thus should favor full coverage by other molecules (higher density of nucleation). It should be notd that, on the mica surface, λmica is between λLC and λLE, i.e., λLC < λmica < λLE, as deduced from AFM data (see Supporting Information Figure S3). In the early studies, material-dependent nucleation behaviors and sticking properties of Ag atoms on different LB films were reported;35,36 the comparison was done between monolayer and multilayer, as well as between different materials, but not for one monolayer with difference phases. The explanation given there cannot be simply applied to our systems. Further work has to be carried out to understand the mechanisms. (35) Bubeck, C. AdV. Mater. 1990, 2, 537. (36) Reiter, G.; Bubeck, C.; Stamm, M. Langmuir 1992, 8, 1881.
Figure 5. Kinetic processes of dye molecule transport between LE phase and LC phase of DPPC and the related energy barriers.
Conclusion In summary, we have presented a bottom-up approach to fabricate organic luminescent stripe patterns. The self-assembled template structures consist of two monolayer phases rather than different chemical natures. This approach provides a simple way to control the site-selective deposition from the gas phase. Adjusting the experimental parameters allowed the control of size and position of luminescent organic particles. It is likely to provide a new route to pattern the organic luminescent materials, which have potentially size tunable optical properties. Acknowledgment. We thank Mr. Hirtz Michael and Dr. Nan Li for the particle size distribution program and stripes preparation, respectively. Financial support was given by the National Natural Science Foundation of China (20773052, 20373019, and 50520130316), the Program for New Century Excellent Talents in University, the National Basic Research Program (2007CB808003), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT0422). Supporting Information Available: Additional experimental material as discussed in the text. This material is available free of charge via the Internet at http://pubs.acs.org. LA7026779
