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C: Surfaces, Interfaces, Porous Materials, and Catalysis
In Situ Wettability Characterization of Chemically Heterogeneous Surfaces Probed by Ionic Liquid Contact Angle in Vacuum: Pentacene on Single Crystal SrTiO (001) 3
Takuya Kinoshita, Shingo Maruyama, and Yuji Matsumoto J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01128 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 2, 2018
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The Journal of Physical Chemistry
In situ Wettability Characterization of Chemically Heterogeneous Surfaces Probed by Ionic Liquid Contact Angle in Vacuum: Pentacene on Single Crystal SrTiO3 (001) Takuya Kinoshita, Shingo Maruyama, and Yuji Matsumoto* Department of Applied Chemistry, School of Engineering, Tohoku University, Sendai 980-8579, Japan
ABSTRACT
Intrinsic surface wettability of pentacene thin films with different island coverages grown on single crystal SrTiO3 (001) was investigated by an in situ contact angle measurement in a high vacuum using ionic liquid as a probe fluid. The contact angle at the quasi-equilibrium state, which was measured at a certain time after its fast decay depending on the liquid viscosity, increased with the deposition amount of pentacene and saturated when the surface was fully covered with pentacene islands. The observed island coverage dependence of the in situ contact angle was well fitted by a three-component Cassie equation composed of surface areas of not
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only the bare substrate and island pentacene but also adsorbed pentacene. The importance of the in situ contact angle measurement was confirmed by the result of air exposure experiment, where the contact angle took almost irrelevant values to the integral coverage of pentacene, suggesting the significant effects of airborne contaminants on the surface wettability.
INTRODUCTION Wettability of a solid surface is a phenomenon of how a liquid spreads on the solid surface, and in general quantitatively represented by its contact angle. The contact angle is usually measured by the sessile drop method.1,2 Due to its simplicity and validity, the contact angle measurement has been used for wide varieties of research fields such as medical engineering,3,4 biology,5,6 solid surface engineering.7–9 However, most of the contact angle measurements are conducted in the atmospheric conditions. In such conditions, the adsorption of airborne contaminants such as hydrocarbons and water on the surface would strongly affect the liquid-solid interactions, resulting in different contact angles from the intrinsic value.10–13 In addition to the contamination problem, the evaporation of a liquid droplet during the contact angle measurement could be another extrinsic factor to change the contact angle.14–17 Therefore, in order to investigate the intrinsic wettability of a substance, it is necessary to eliminate those extrinsic effects as much as possible. For this purpose, the contact angle measurement in vacuum, which is ideally a contamination free environment, is a promising and challenging approach. Ionic liquids (ILs) can be a good candidate as contact angle probe fluids used in vacuum because they have extremely low-volatility, so that the effect of droplet evaporation is negligible even in high vacuum, and have a wide variety of surface tensions. In our previous work, we have evaluated the intrinsic
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wettability of ILs on rutile TiO2 single crystal surfaces prepared in a vacuum by our originally developed in situ high-vacuum contact angle measurement system and revealed that the air exposure strongly affects the interfacial interactions between ILs and the TiO2 surfaces.18 In this study, we extend our previous work to a heterogeneous surface system: pentacene thin films grown on a SrTiO3 (001) single crystal substrate. Since the pentacene thin film grows in a layerby-layer fashion on the SrTiO3 by vacuum deposition,19,20 it would be a good playground to clarify the intrinsic wettability of the multi component surface for its different coverages. Moreover, the importance of in situ characterization would be emphasized for organic materials because it is well known that the physical properties of organic materials are sometimes sensitive to the air exposure.21 From these backgrounds, in situ contact angle measurements with an IL in a vacuum were performed for pentacene thin films with various coverages. As a result, we found that the IL contact angle is strongly affected by the air exposure. The observed coverage dependent in situ contact angle can be well explained by the Cassie equation considering not only the simple two components (bare substrate and island pentacene), but also the existence of absorbed pentacene on part of the substrate surface uncovered with island pentacene, which was discussed by our previous works.19,20,33
EXPERIMENTAL SECTION Ionic liquid The IL used in this study was 1-ethyl-3-methylimidazolium bis(trifluoromethylilsulfonyl)amide ([emim][NTf2]), which was purchased from Tokyo Chemical Industry Co., Ltd. The viscosity of the [emim][NTf2] is 33.9 cP at 298 K,22 which is a relatively low value among ILs commercially
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available and suitable for the present contact angle measurement because the waiting time for the IL to reach the equilibrium contact angle would be short. Note that the surface tension of [emim][NTf2] in vacuum using the pendant drop method was 37.3 mN/m which was not so much different from the reported value measured in air.23 Pretreatment of SrTiO3 substrates Atomically flat TiO2-terminated SrTiO3 (001) substrates with a square size of 15 mm × 15mm (purchased from Shinkosha Co., Ltd.) were used in this study. The substrate was cut into a piece of ~7.5 mm square and then ultrasonically rinsed with acetone and ethanol for 5 minutes, respectively. To photocatalytically remove surface organic contaminants, the substrate was irradiated with ultra violet (UV) light (super high pressure Hg lamp, 3.0 mW/cm2) for 30 minutes in 300 Torr O2 atmosphere in the preparation chamber (Figure 1). After the UV cleaning, the substrate was not exposed to the air until all experimental procedures were finished except the air exposure experiments.
Figure 1. Schematic illustration of the in situ contact angle measurement system.
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Deposition of pentacene The deposition chamber is directly connected to the preparation chamber via the gate valve (Figure 1). The base pressure of the deposition chamber was ~10-8 Torr. Pentacene thin films were deposited on the substrate by a continuous wave infrared (CW-IR) laser deposition technique24 at room temperature. This technique allows us to precisely control the deposition amount of pentacene on a sub-monolayer scale just by on/off laser switching. The thin film deposition rate was monitored by a quartz crystal microbalance (QCM) during the deposition. The deposition rate was controlled to be kept 0.075 Å/s. We deposited pentacene on two substrates simultaneously in one experiment. One was used for in situ contact angle measurement and the other for estimation of the deposition amount and coverage of the pentacene thin film on the substrate by AFM. In situ contact angle measurement In situ contact angle measurements were carried out in our originally developed vacuum chamber with a liquid drop system that was connected to the deposition chamber. The base pressure of the contact angle measurement chamber was 1.2 ML pentacene was deposited, the substrate surface was fully covered with the first layer islands, and the second layer dendritic islands were in turn further developed on the complete first layer. In this way, the successive deposition gave a sort of layer-by-layer growth of pentacene as seen in the AFM images. In Figure 2b, the coverage of each layer is plotted as a function of the integral coverage, i.e. the total amount of the deposited pentacene (ML). The observed growth kinetics of pentacene on the SrTiO3(001) was similar to the result of the real-time growth observation of pentacene islands on SiO2 by low-energy electron microscopy (LEEM).27
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Figure 2. (a) AFM topographic images (5 µm × 5 µm) for pentacene films on a SrTiO3(001) substrate deposited at rates of 0.075Å/sec, respectively, did not affect the contact angle values significantly as seen in Figure 5a. In order to explain the deviation of the experimental data from the two component Cassie equation, we considered a possible effect of adsorbed pentacene molecules on the contact angle. The adsorbed pentacene is a kind of a precursor state of pentacene molecules before forming pentacene islands, existing on the substrate area uncovered with pentacene islands, as illustrated in Figure 5b. Such precursor state, adsorbed pentacene molecules were inferred from our previous studies of the initial growth of pentacene on SrTiO3(001) using in situ low-energy electron diffraction (LEED)/Auger electron spectroscopy (AES).19,20,34 Moreover, similar adsorbed layers prior to forming the islands is reported in many other systems such as pentacene on Si27 and SiO235 and para-
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sexiphenyl36 on mica. Accordingly, we extended the Cassie equation applied for two component systems into a modified Cassie equation to be applied for the present three component system, assuming that the amount of such adsorbed pentacene increases proportional to the total amount of the deposited pentacene and the adsorbing area uncovered with pentacene islands of the time. In this assumption, since the amounts of the 2nd, 3rd-layer pentacene and adsorbed pentacene are negligible below
