Published: September 05, 2011 r2011 American Chemical Society 12354 dx.doi.org/10.1021/la202728t | Langmuir 2011, 27, 12354–12360 ARTICLE pubs.acs.org/Langmuir Impact of Temperature on the LB Patterning of DPPC on Mica Michael H. K€ opf,* ,† Heiko Harder, ‡ J€ urgen Reiche, ‡ and Svetlana Santer* ,‡ † University of M€ unster, Institute for Theoretical Physics, Wilhelm-Klemm-Straße 9, D-48149 M€ unster, Germany ‡ University of Potsdam, Institute of Physics and Astronomy, Physics of Nanosystems, Karl-Liebknecht-Straße 24-25, D-14476 Potsdam, Germany ’ INTRODUCTION The application potential of self-organized mesoscopic and nanoscopic patterns strongly motivates an investigation of pattern formation in nature. By following the bottom-up strategy in- herent in biological structures, that is, the formation of complex functional architectures from molecular entities, LangmuirÀ Blodgett (LB) transfer methods have been refined to yield the controlled formation of meso- and nanoscopic surface patterns. 1À14 In the corresponding experiments, a monomolecular layer of an amphiphilic substance such as pulmonary surfactant dipalmitoyl- phosphatidylcholine (DPPC) is prepared on the aqueous surface of an LB trough. The monolayer is then transferred onto a solid substrate by withdrawing it from the trough in a controlled manner. Most of the LB-patterning activities are based on phospholipids 3À11 and fatty acids. 1,2 Gleiche et al. 3 lifted a freshly cleaved mica substrate through a liquid-expanded (LE) DPPC Langmuir film at a constant surface pressure of 3 mN/m and a subphase temperature of 22.5 °C with a dipper speed of 60 mm/min. Their experiment resulted in the transfer of a perio- dic structure consisting of hydrophobic stripes of about 800 nm width and hydrophilic channels of about 200 nm width parallel to the three-phase contact line (subsequently termed horizontal stripes). The formation of these stripes was ascribed to an oscillation at the meniscus that is related to substrate-mediated condensation. 15À19 Subsequent work was focused on the influ- ence of the surface pressure, 9 the dipper speed, 5,6 the composi- tion of binary mixtures, 6 and the substrate treatment 7 on pattern formation and the templated self-assembly of functional com- pounds, 20 polymer brushes, 21 and biological objects 22 on top of patterned surfaces. Varying the surface pressure between 1 and 5 mN/m and the dipper speed between 10 and 60 mm/min revealed a strong influence of both parameters on the structure of the DPPC stripe pattern. In addition to the formation of hori- zontal stripes, the formation of vertical stripes (perpendicular to the three-phase contact line) was observed for higher surface pressure and lower dipper speed. 5,6,9 The stripe patterns observed during the transfer of phos- pholipids are considered to consist of alternating domains of the liquid expanded (LE) and liquid condensed (LC) phases, 5 and the pattern-formation process is thus intrinsically linked to the thermodynamics of the monolayer and, in particular, to the first-order LEÀLC phase transition that is commonly referred to as the main transition. Accordingly, one can expect the transferred patterns to depend on the temperature T of the subphase. Any change in T will change the surface pressureÀ area isotherm and the extent of the LEÀLC coexistence region. In particular, a temperature greater than the critical temperature clearly rules out LEÀLC coexistence such that pattern formation is possible only for lower T. Despite its obvious importance, not much information about the effect of the subphase temperature has been reported in the literature. Gleiche et al. 3 mention that the periodicity of stripes increases at lower temperatures. They report a change in the periodicity by 0.6 μm with a temperature variation of only (0.5 °C and respective changes in the channel widths by 200À300 nm. Furthermore, they report that the periodicity of the stripes increases with increasing humidity. 23 Lenhert et al. 8 heated the meniscus during the transfer locally with a low-power laser beam and thereby induced a change in the structure of the pattern. Received: July 15, 2011 Revised: September 5, 2011 ABSTRACT: The influence of the subphase temperature on the stripe pattern formation during LangmuirÀBlodgett trans- fer (LB patterning) is investigated in a combined experimental and theoretical study. According to our experiments on the LB transfer of dipalmitoylphosphatidylcholine (DPPC) on planar mica substrates, even small temperature changes between 21.5 and 24.5 °C lead to significant changes in the monolayer patterns. For a constant surface pressure and dipper speed, the width of the stripes and the overall spatial period of the patterns increase with increasing subphase temperature. Because the stripe patterns are ascribed to alternating monolayer domains in the liquid-expanded and the liquid-condensed phases, the working regime for the formation of stripes is found to depend strongly on the respective surface pressureÀarea isotherm. These experimental findings are in accordance with the results of a theoretical investigation based on a model that takes hydrodynamics and the monolayer thermodynamics into account.