pubs.acs.org/Langmuir © 2010 American Chemical Society
)
Templated Self-Assembly of ZnO Films on Monolayer Patterns with Nanoscale Resolution )
)
)
Luciana Pitta Bauermann,*,†,‡ Peter Gerstel,†,‡ Joachim Bill,†,‡ Stefan Walheim,*,§, Cheng Huang,§, Joerg Pfeifer,§, and Thomas Schimmel ,§
Institute for Materials Science, Universit€ at Stuttgart, 70569 Stuttgart, Germany, ‡Max-Planck-Institute for Metals Research, 70569 Stuttgart, Germany, §Institute of Nanotechnology (INT), Karlsruhe Institute of Technology (KIT) Nothern Campus, 76021 Karlsruhe, Germany, and Institute of Applied Physics and Center for Functional Nanostructures (CFN), Karlsruhe Institute of Technology (KIT) Southern Campus, 76128 Karlsruhe, Germany )
†
Received September 25, 2009. Revised Manuscript Received January 18, 2010 Lithographically defined self-growing ZnO films were prepared by a bioinspired chemical bath deposition technique (CBD). We observed a high selectivity of ZnO deposition: Teflon-like per-fluoro-decyl-trichlorosilane (FDTS) monolayers repelled ZnO primary particles, whereas amino-functionalized areas of the substrate were selectively covered by a highly anisotropic, oriented, and compact ZnO film with a thickness of 50 nm. The size of the primary particles in our methanol-based solution was approximately 2.5 nm. On the amino substrate they formed agglomerates not larger than 30 nm. Monolayer patterns made by polymer blend lithography were templated with an edge resolution of 30 nm. By using a specialized derivative of microcontact printing, we prepared layout-defined silane templates, which reliably determined the growth of a layout-defined, patterned oxide film with submicrometer lateral resolution.
Introduction Thin films of nanosized metal oxides, such as semiconductor ceramics, have a large potential for a broad range of applications1 including the construction of functional devices such as solar cells, photodetectors, light-emitting devices, gas sensors, and surface acoustic wave guides. Additionally, they are suitable for scaling those devices to small sizes. There is considerable interest in simple, cheap, and environmentally friendly approaches that are capable of producing films with new or improved properties. When considering the preparation of thin films for technological application through a chemical route, the research and development processes have to be aware that the industry should emphasize the low generation of waste and the low consumption of raw materials and energy. In this direction, the bioinspired synthesis of materials has developed.1 This approach makes use of functional surfaces to induce film formation. It represents a typical low-temperature, normal pressure, atmosphere process for the deposition of ceramic thin films, thus even temperaturesensitive substrates can be coated. From biomineralization processes it is understood that the nucleation and growth of biominerals (inorganic materials) are carefully controlled by a complex organic matrix of preorganized biopolymers.2,3 These structurally organized organic surfaces can even catalytically or epitaxially induce the growth of specifically oriented inorganic thin films. Self-assembled monolayers (SAMs) are an effective template for controlling the crystal axis orientation and morphologies of inorganic materials. SAMs can also improve heterogeneous nucleation and adjust interaction forces between the substrate and particles generated in solution. The self-assembly of chemically synthesized nanoparticles into large, functional ensembles *Authors to whom correspondence should be addressed. ZnO film synthesis: Luciana Pitta Bauermann (E-mail:
[email protected]). Template structures: Stefan Walheim (E-mail:
[email protected]). (1) Gao, Y.; Koumoto, K. Cryst. Growth Des. 2005, 5, 1983–2017. (2) Mann, S. Nature 1993, 365, 499. (3) Liu, X. Y.; Lim, S. W. J. Am. Chem. Soc. 2003, 125, 888.
3774 DOI: 10.1021/la903636k
(bottom-up) has become popular.4 The ability to control the lateral structure of the SAMs on the nanometer scale provides an attractive method for the fabrication of microelectronic devices with high-resolution patterned surfaces and submicrometer periodicity over large areas.5 Zinc oxide is a semiconductor with a wide band gap at room temperature and functional electrical and optical properties. It has potential applications as displays,6 UV-light-emitting diodes,7 and laser diodes because of its luminescence properties.8,9 Zinc oxide exhibits significant photoconductivity,10 finding relevance also in electronics. It has been investigated as gas sensors,11 electroacoustic transducers,12 varistors,13 and highly transparent conducting windows for solar cells and displays.14 The large field of applications of zinc oxide15 has caused a high demand for new synthesis routes of this material as thin coatings and nanostructures. A new method to obtain nanocrystalline zinc oxide controlled by a supersaturated solution has been achieved by chemical bath deposition (CBD).16,17 This method has been (4) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418–2421. (5) Aizenberg, J.; Black, A. J.; Whitesides, G. M. Nature 1998, 394, 868–871. (6) Nakanishi, Y.; Miyake, A.; Kominami, H.; Aoki, T.; Hatanaka, Y.; Shimaoka, G. Appl. Surf. Sci. 1999, 142, 233–236. (7) Minami, T.; Tanigawa, M.; Yamanishi, M.; Kawamura, T. Jpn. J. Appl. Phys. 1974, 13, 1475–1476. (8) Bagnall, D. M.; Chen, C. F.; Zhu, Z.; Yao, T.; Koyama, S.; Shen, M. Y.; Goto, T. Appl. Phys. Lett. 1997, 70, 2230–2232. (9) Morimoto, K. In Phosphor Handbook; Shionoya, S., Yen, W.M., Eds.; CRC Press: Boca Raton, FL, 1998; pp 561-580. (10) Brown, H. E. Zinc Oxide: Properties and Applications; International Lead Zinc Research Organization: New York, 1976; pp 26-32. (11) Wang J. X.; Sun, X. W.; Yang, Y.; Huang, H.; Lee, Y. C.; Tan, O. K.; Vayssieres, L. Hydrothermally grown oriented ZnO nanorod arrays for gas sensing applications; Nanotechnology 2006, 17 (19), 4995–4998. (12) Foster, N. F.; Rozgonyi, G. A. Appl. Phys. Lett. 1966, 8, 221. (13) Levinson, L. M.; Philipp, H. R. Appl. Phys. Lett. 1974, 24, 75–76. (14) Nakazawa, T.; Ito, K. J. Vac. Soc. Jpn. 1983, 26, 889–894. € ur, U.; Alivov, Ya. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Dogan, S.; (15) Ozg€ Avrutin, V.; Cho, S.-J.; Morkoc-, H. J. Appl. Phys. 2005, 98, 41301. (16) Lipowsky, P.; Hoffmann, R. C.; Welzel, U.; Bill, J.; Aldinger, F. Adv. Funct. Mater. 2007, 17, 2151–2159. (17) Hoffmann, R. C.; Jia, S.; Jeurgens, L. P. H.; Bill, J.; Aldinger, F. Mater. Sci. Eng., C 2006, 26, 41–45.
Published on Web 02/12/2010
Langmuir 2010, 26(6), 3774–3778
Pitta Bauermann et al.
implemented through the principle of bioinspired materials synthesis. ZnO films obtained by this method consist of nanometer-sized zincite crystals with preferential orientations and are smooth, uniform, and stable. The formation of nanocrystalline ZnO under such mild conditions is an important step toward technological applications because the performance of most materials in electronic and optical devices is related to their crystallinity. The crystal size and film thickness are well controlled by the deposition conditions. Their adsorption depends on the characteristics of the surface, most particularly, on the electrostatic charge density. The deposition of ZnO is possible both on strongly positively and negatively charged substrates,18 though deposition does not occur on weakly charged or neutral surfaces.19 Recently, we showed that ZnO layers produced by this technique show photoluminescence18 and show at the same time high mechanical performance.20,21 Patterned ZnO films could be formed after the localized decomposition of a negatively charged SAM by UV irradiation of an area not protected by a photomask.18,22 The lateral resolution was limited by diffraction and the quality of the photomask. Mineralization of polycrystalline ZnO of about 10 μm in length and a height of about 40 nm was obtained by using the same mild conditions.23 Instead of sulfonate-terminated SAMs, immobilized DNA molecules as carriers of surface charge were used to assemble nanocrystalline ZnO selectively. Here DNA acts as a well-defined 1D template. A thin, uniform ZnO coating on DNA was formed because of the electrostatic interaction between the polar ZnO crystallites and negatively charged DNA molecules. The difficulty of aligning the DNA in complex geometries prompted us to look for other templates to achieve large areas with complex nanopatterned polycrystalline ZnO with high lateral resolution. Here we present the regioselective deposition of compact ZnO layers by CBD near room temperature on micro- and nanostructured SAM template structures. To pattern our monolayer templates, we use two different techniques: (1) polymer blend lithography, where self-organized lateral polymer structures are used as stencil masks for SAM deposition and (2) microcontact printing on gold, where the resulting gold structures, after a subsequently performed etching step, are used as lift-off masks for SAM deposition. ZnO structures in the range between 100 nm and several micrometers with a thickness in the 50 nm range are formed. The combination of bioinspired organically mediated synthesis of a metal oxide film with high-resolution templating opens perspectives for the fast, cost-efficient production of devices using ZnO semiconductor nanostructures.
Experimental Details Polymer Blend Lithography. A 3:7 PS/PMMA polymer blend solution (15 mg/mL in methyl ethyl ketone (MEK)) was spin-cast (1500 rpm) onto a silicon wafer at 23 °C and a relative humidity of 45%. The molecular weights of the polymers (PSS/ Mainz/Germany) were 10K (PMMA) and 36K (PS), respectively. Next, the PMMA was selectively dissolved in acetic acid (twice for a duration of 1 min while gently moving the sample). After a short (18) Lipowsky, P.; Hoffmann, R. C.; Welzel, U.; Bill, J.; Aldinger, F. Adv. Funct. Mater. 2007, 17, 2151–2159. (19) Shyue, J.-J.; De Guire, M. R.; Nakanishi, T.; Masuda, Y.; Koumoto, K.; Sukenik, C. N. Langmuir 2004, 20, 8693–8698. (20) Lipowsky, P.; Hirscher, M.; Hoffmann, R. C.; Bill, J.; Aldinger, F Nanotechnology 2007, 18, 165603. (21) Lipowsky, P.; Burghard, Z.; Jeurgens, L. P. H.; Bill, J.; Aldinger, F. Nanotechnology 2007, 18, 345707. (22) Lipowsky, P.; Hedin, N.; Bill, J.; Hoffmann, R. C.; Ahniyaz, A.; Aldinger, F.; Bergst€om, L. J. Phys. Chem. C 2008, 112, 5373–5383. (23) Atanasova, P.; Weitz, T.; Gerstel, P.; Srot, V.; Kopold, P.; van Aken, P. A.; Burghard, M.; Bill, J. Nanotechnology 2009, 20, 365302.
Langmuir 2010, 26(6), 3774–3778
Letter drying process in a nitrogen stream (10 s), the samples were mounted face down inside the lid of a desiccator containing two droplets of FDTS (Aldrich), which then was evacuated to a pressure of 50 mbar. After 3 h of exposure to FDTS vapor, which led to a water contact angle of 110° on a bare silicon substrate, the samples were cleaned with a snow jet in order to remove the PS islands, which covered/sealed the bare oxide substrate. Finally, the deposition of amino-terminated silane amino-propyltriethoxy-silane (APTS) (Aldrich) was done according to the same protocol. Samples thus prepared were stable and functional for 3 months and were cleaned via a CO2 snow-jet treatment prior to ZnO deposition. Preparation of Au Substrates for μCP. Polycrystalline gold substrates with a grain diameter of about 30 nm were prepared by e-beam evaporation in vacuum (
