The Partially Degraded Hydrophilic Silane Pattern and Its Application

Apr 9, 2009 - We developed a protocol to fabricate hydrophilic patterns over an octadecyltrichlorosilane (OTS) film surface with an atomic force micro...
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The Partially Degraded Hydrophilic Silane Pattern and Its Application in Studying the Structures of Long Chain Alkane Films Yuguang Cai* Department of Chemistry, University of Kentucky, Rose Street, Lexington, Kentucky 40506 Received July 22, 2008 We developed a protocol to fabricate hydrophilic patterns over an octadecyltrichlorosilane (OTS) film surface with an atomic force microscope (AFM). Through a local probe oxidation under a 100% humidity environment, the OTS was converted into a hydrophilic, carboxylic acid-terminated surface (OTSpd). The OTSpd pattern grew with the voltage dwell time applied on the conducting AFM probe. Eighty nanometer to submillimeter sized OTSpd patterns could be fabricated with a single scanning probe. The OTSpd patterns were used to study the spreading of long chain alkanes. Hexatriacontane (C36H74) was dip-coated on an OTSpd pattern. Subsequently, an additional hydrophilic OTSpd region was fabricated surrounding the coated C36H74. The alkane spread over this newly created region when heated above its melting point. After cooling to room temperature, the shape and structures of the solidified alkane patterns were characterized. On the methyl-terminated, low-energy surface, the alkane molecules stood directly on the surface. In contrast, on the hydrophilic, high-energy surface, the alkane formed seaweed-shaped patterns after spreading. On the OTSpd surface, the alkane molecules initially adsorbed on the hydrophilic surface with their alkyl chains parallel to the surface. Additional alkane molecules stood vertically or tilted on top of the parallel layer, forming the seaweed-shaped layer. The seaweed patterns were previously thought to consist of only vertically standing alkane molecules. We found that three additional tilted phases existed in the seaweed-shaped structures.

Introduction Knowledge about adsorption, wetting, and spreading of n-alkanes on solid surfaces has important applications in lubrication, painting, printing, petroleum tertiary recovery, and oil spill cleaning.1,2 Long chain n-alkanes on SiO2 surfaces have been studied by grazing incidence X-ray diffraction (GID), X-ray reflectivity, ellipsometry and atomic force microscopy (AFM).1-10 Novel interfacial phenomena such as surface freezing have been discovered through these studies, which changed our view on the freezing process.11,12 However, the molecular structures of n-alkanes adsorbed on solid surfaces have still not been fully investigated, especially at the submonolayer coverage level. In such case, the liquid alkane drop is known to freely spread over the surface, forming a seaweed-shaped layer upon freezing.2,13 Two models were proposed for the molecular structure of the alkane in the seaweed pattern. The bilayer model suggests that alkane molecules initially adsorb on the SiO2 substrate with their *E-mail: [email protected]. (1) Merkl, C.; Pfohl, T.; Riegler, H. Phys. Rev. Lett. 1997, 79, 4625. (2) Holzwarth, A.; Leporatti, S.; Riegler, H. Europhys. Lett. 2000, 52, 653. (3) Schollmeyer, H.; Ocko, B.; Riegler, H. Langmuir 2002, 18, 4351. (4) Volkmann, U. G.; Pino, M.; Altamirano, L. A.; Taub, H.; Hansen, F. Y. J. Chem. Phys. 2002, 116, 2107. (5) Mo, H.; Taub, H.; Volkmann, U. G.; Pino, M.; Ehrlich, S. N.; Hansen, F. Y.; Lu, E.; Miceli, P. Chem. Phys. Lett. 2003, 277, 99. (6) Schollmeyer, H.; Struth, B.; Riegler, H. Langmuir 2003, 19, 5042. (7) Lazar, P.; Schollmeyer, H.; Riegler, H. Phys. Rev. Lett. 2005, 94, 116101. (8) Trogisch, S.; Simpson, M. J.; Taub, H.; Volkmann, U. G.; Pino, M.; Hansen, F. Y. J. Chem. Phys. 2005, 123, 154703. (9) Bai, M.; Knorr, K.; Simpson, M. J.; Trogisch, S.; Taub, H.; Ehrlich, S. N.; Mo, H.; Volkmann, U. G.; Hansen, F. Y. Europhys. Lett. 2007, 79, 26003. (10) Basu, S.; Satija, S. K. Langmuir 2007, 23, 8331. (11) Wu, X. Z.; Sirota, E. B.; Sinha, S. K.; Ocko, B. M.; Deutsch, M. Phys. Rev. Lett. 1993, 70, 958. (12) Ocko, B. M.; Wu, X. Z.; Sirota, E. B.; Sinha, S. K.; Gang, O.; Deutsch, M. Phys. Rev. E 1997, 55, 3164. :: (13) Knufing, L.; Schollmeyer, H.; Riegler, H.; Mecke, K. Langmuir 2005, 21, 992.

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chains parallel to the surface, and then the additional alkane molecules stand vertically on the parallel layer(s).5,10 In contrast, the one layer model suggests that all alkane molecules stand directly on the surface. They stand perpendicularly on the surface and pack side-by-side.2,6 The microscopic size and the fractal shape of alkane seaweed patterns undermine the thorough investigation of the alkane structure using X-ray or the spectroscopic methods. To solve such controversy about the alkane structure, the precise control of alkane spreading at mesoscale is necessary. Chemical patterns can control the adsorption, shape, and size of liquid at mesoscale.14-16 Chemical patterns can be used to study the liquid alkane spreading and adsorption structures on the surface. The local probe oxidation has been extensively used to fabricate chemical patterns on surfaces. However, in order to use the chemical pattern to tackle alkane spreading problems, the topography and chemical properties of the pattern have to be clearly understood first. The outcome of the local probe oxidation varies with different substrates. The local probe oxidation conducted on a clean silicon surface generates SiO2 on the surface, creating a positive topographic feature under the conducting probe.17,18 The size and height of the SiO2 are functions of the oxidation voltage and oxidation dwell time. These relationships have been well documented in the literature.19-22 On the alkanethiol self-assembled (14) Checco, A.; Cai, Y.; Gang, O.; Ocko, B. M. Ultramicroscopy 2006, 106, 703. (15) Chowdhury, D.; Maoz, R.; Sagiv, J. Nano Lett. 2007, 7, 1770. (16) Darhuber, A. A.; Troian, S. M.; Davis, J. M.; Miller, S. M.; Wagner, S. J. Appl. Phys. 2000, 88, 5119. (17) Day, H. C.; Alleea, D. R. Appl. Phys. Lett. 1993, 62, 2691. (18) Garcia, R.; Tello, M. Nano Lett. 2004, 4, 1115. (19) Calleja, M.; Garcia, R. Appl. Phys. Lett. 2000, 76, 3427. (20) Fontaine, P. A.; Dubois, E.; Stievenard, D. J. Appl. Phys. 1998, 84, 1776. (21) Avouris, P.; Martel, R.; Hertel, T.; Sandstrom, R. Appl. Phys. A: Mater. 1998, 66, S659. (22) Kinser, C. R.; Schmitz, M. J.; Hersam, M. C. Nano Lett. 2005, 5, 91.

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monolayer (SAM) grown on a single crystal gold surface, the local probe oxidation usually leads to the destructive desorption of the alkanethiol, creating a negative topography pattern.23 If the alkanethiol SAM is grown on a gold film coated on a silicon wafer, the outcome of the probe oxidation becomes complicated. Depending on the alkanethiol terminal functionalities and the alkyl chain length, the pattern generated can be either a negative pattern due to the desorption of the alkanethiol molecules, or a positive pattern due to the SiO2 growth after the desorption of alkanethiol molecules.24 The local probe oxidation on a silane SAM has similar reaction pathways. Both a positive topography pattern25,26 and a zero-contrast topography pattern have been reported.27,28 The oxidation of a silane film in a dry environment only yields oxide. The oxidation of shorter silanes also leads to the oxide growth under the tip. Whereas oxidation under an ambient relative humidity is called “constructive lithography”, the conducting AFM probe only oxidizes the terminal methyl group of the octadecyltrichlorosilane (OTS) SAM, converting the methyl group to the carboxylic acid group (OTSox) in an ambient humid environment.27,29-33 The “constructive lithography” generates patterns with almost no apparent topography change on the OTS surface. It is generally agreed that the degree of oxidation causes these differences: incomplete oxidation only converts the terminal group of the silane film, while complete oxidation causes silane desorption from the substrate and the growth of SiO2. The oxidation of OTS is the combination of two simultaneous processes.34,35 One is the degradation of the silane film, which causes the pattern height to decrease; the other is the growth of SiO2 underneath the film, which increases the height. The final pattern height is the summation of these two processes. In the case of constructive lithography, the oxide growth process is negligible under incomplete oxidation conditions. Therefore, only the terminal group is oxidized. The oxidation of a silane film is the function of humidity, voltage, and dwell time. The parameters favoring the oxide formation and the carboxylic acid group formation have been reported by the Schubert group.35 In this paper, we show that silane oxidation does not have just these two outcomes. There is a third possible result: the silane is partially degraded and stays on the surface. Our data show that using the same set-ups as the Sagiv group, but at 100% relative humidity, we can create partially degraded OTS patterns (OTSpd). OTSpd is 10.2 ( 0.7 A˚ lower than the OTS film, which is different from the zero topography contrast OTSox patterns. The OTSpd pattern fabricated in our procedure is also hydrophilic. More interestingly, the OTSpd pattern can grow well beyond the contact area of the AFM probe. By controlling the voltage and dwell time, we can fabricate such partially degraded silane patterns from ∼80 nm to submillimeter size with the same AFM probe. Thus, we extended the capability of the scanning (23) Schoer, J. K.; Zamborini, F. P.; Crooks, R. M. J. Phys. Chem. 1996, 100, 11086. (24) Jang, J. W.; Sanedrin, R. G.; Maspoch, D.; Hwang, S.; Fujigaya, T.; Jeon, Y. M.; Vega, R. A.; Chen, X. D.; Mirkin, C. A. Nano Lett. 2008, 8, 1451. (25) Yang, M. L.; Zheng, Z. K.; Liu, Y. Q.; Zhang, B. L. Nanotechnology 2006, 17, 330. (26) Xie, X. N.; Chung, H. J.; Sow, C. H.; Wee, A. T. S. Chem. Phys. Lett. 2004, 388, 446. (27) Maoz, R.; Cohen, S. R.; Sagiv, J. Adv. Mater. 1999, 11, 55. (28) Sugimura, H. Jpn. J. Appl. Phys. 2004, 43, 4477. (29) Maoz, R.; Cohen, S. R.; Sagiv, J. Adv. Mater. 1999, 11, 55. (30) Maoz, R.; Frydman, E.; Cohen, S. R.; Sagiv, J. Adv. Mater. 2000, 12, 725. (31) Maoz, R.; Frydman, E.; Cohen, S. R.; Sagiv, J. Adv. Mater. 2000, 12, 424. (32) Liu, S. T.; Maoz, R.; Sagiv, J. Nano Lett. 2004, 4, 845. (33) Liu, S. T.; Maoz, R.; Schmid, G.; Sagiv, J. Nano Lett. 2002, 2, 1055. (34) Sugimura, H.; Hanji, T.; Hayashi, K.; Takai, O. Ultramicroscopy 2002, 91, 221. (35) Wouters, D.; Willems, R.; Hoeppener, S.; Flipse, C. F. J.; Schubert, U. S. Adv. Funct. Mater. 2005, 15, 938.

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probe lithography to submillimeter scale, which is beyond the piezo scanner’s range. Because the hydrophilic OTSpd pattern can be fabricated in 80 nm to submillimeter scale with precise control, it is an ideal tool to study alkane spreading. In the second part of this article, we demonstrate the capability of OTSpd to study alkane spreading. We studied alkane spreading over the chemically and topographically homogeneous OTSpd pattern. Since the OTSpd pattern’s depth and chemical identity are known, we can compare the alkane behaviors over the hydrophilic OTSpd pattern and over the hydrophobic OTS surface. We found that the “one-layer model” correctly described the long chain alkane adsorption on hydrophobic surface, whereas the “bi-layer model” was consistent with the adsorption structure on the hydrophilic surface. Through AFM characterization, we also found three new tilted structures of long-chain alkanes over the OTSpd surface.

Experiment Materials. OTS (97%) was purchased from Gelest. Chloroform (HPLC grade) was from Mallinckrdt Baker. Toluene (HPLC grade) was from EMD. n-Hexatriacontane (99%) was from Sigma. 1-Pyrenyldiazomethane (PDAM) was from Invitrogen. All chemicals were used without further purifications. Silicon (100) wafers were from Virginia Semiconductors (Nitrogen doped, 30 ohm cm resistivity), James River Semiconductors (Nitrogen doped, 13 ohm cm resistivity), KC electronics (Nitrogen doped, 5 Ω cm resistivity). Instrumentation. The chemical pattern fabrication and characterization were performed with an Agilent PicoPlus AFM in an environmental chamber and a Veeco Multimode AFM. All images were processed with WSxM.36 After pattern fabrication, the sample was cleaned by a supercritical carbon dioxide snow cleaning system from Applied Surface Technologies. The patterns were characterized in tapping mode with MikroMasch NSC-14 tips. The OTS film thickness and quality were examined with an Angstrom Advanced PhE 101 laser ellipsometer and a Varian Excalibur 3100 Fourier transform infrared spectrometer.37 The resistivities of wafers were measured with a Signatone four-point probe. The fluorescent imaging was conducted using a Nikon ME 600 L microscope. Preparation of OTS Film and Patterning. The preparation of the OTS film has been described in detail elsewhere.32,38 We fabricated the hydrophilic OTSpd pattern on the OTS film using the local probe oxidation lithography invented by the Sagiv group.30,32 However, we modified the experimental conditions. First, the local probe oxidation was conducted at 25 °C in a 100% relative humidity environment instead of ambient relative humidity used by other groups.30,32,35 In addition, a stationary tip (0.1 to 10 s. voltage pulse) was used to fabricate disk shaped patterns, and a slow moving tip was used to fabricate line patterns. Also, the Pt-Ti-coated conducting AFM probe was used during the oxidation process, instead of the highly doped silicon probe. As a result, the pattern we obtained is deeply oxidized and partially destructed. OTSpd is hydrophilic and 10 A˚ lower than the OTS. In contrast, the incompletely oxidized OTSox patterns created in the published conditions are only 0-3 A˚ lower than the OTS.30,32,35 Under our pattern fabrication conditions, a stationary tip generates a disk-shaped pattern. The longer the voltage pulse, the bigger the disk is. Typically, a 1 s 10 V pulse at 100% relative humidity generates a ∼3 μm-diameter deeply oxidized OTSpd disk. (36) Horcas, I.; Fernandez, R.; Gomez-Rodriguez, J. M.; Colchero, J.; GomezHerrero, J.; Baro, A. M. Rev. Sci. Instrum. 2007, 78, 013705. (37) Maoz, R.; Sagiv, J.; Degenhardt, D.; Miihwald, H.; Quint, P. Supramol. Sci. 1995, 2, 9. (38) Cai, Y. G. Langmuir 2008, 24, 337.

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Cai Scheme 1. Experimental Designa

a (a) An OTSpd disk pattern fabricated by the local probe deep oxidation. (b) Dip-coating the n-hexatriacontane on the OTSpd pattern. After briefly dipping the OTSpd pattern in the liquid n-hexatriacontane, the alkane drop adsorbed and solidified on the OTSpd pattern. (c) A new OTSpd area (the blue disk) was fabricated surrounding the coated n-hexatriacontane by local probe deep oxidation again. (d) At room temperature, the coated nhexatriacontane was a solid. There was no spreading. (e) When heated, the alkane drop melted and spread over the OTSpd surface. Upon cooling, the alkane liquid froze. Then the alkane structures could be characterized by AFM.

After the pattern fabrication, we cleaned the sample with a supercritical CO2 snow jet cleaner. Then the pattern was characterized under nitrogen environment with a new tip. Alkane Coating and Spreading. Dip-coating was used to apply the alkane to hydrophilic OTSpd patterns, following Sagiv group’s “Wetting Driven Assembly” approach.15 To deposit the nhexatriacontane on the pattern, the sample was dipped in a small vial containing 5 mL of liquid alkane. The vial temperature was maintained at 120 °C. After 3 min, when the temperature of the sample and the liquid alkane equilibrated, the sample was withdrawn from the hexatriacontane liquid at an estimated speed of 0.1 m/s. To spread the alkane, the sample coated with alkane was heated by a home-built programmable heater for 2 min at 105 °C. The experimental procedure is illustrated in Scheme 1. We first fabricated the OTSpd pattern on the OTS film with AFM probe oxidation lithography (Scheme 1a). Next, we used the OTSpd pattern to direct the assembly of n-hexatriacontane molecules on the pattern through the dip-coating. (Scheme 1b). Subsequently, we used the scanning probe oxidation lithography to create a new OTSpd region enclosing the dip-coated alkane (Scheme 1c). In this step, the tip was moved to a position on the OTS film that was in close proximity with the alkane drop. In a 100% relative humidity environment, we applied 10 V to the sample for 1-100 s. A new deeply oxidized OTSpd disk grew from the current tip position (Scheme 1d). We controlled the disk growth by the voltage dwell time. The new OTSpd disk grew and enclosed the alkane drop. When the disk reached our specified size and all surrounding areas of the alkane drop had been converted to OTSpd, we turned off the voltage. Subsequent AFM tapping mode images showed that the shape and the position of the coated alkane remained intact. In the next step, we made the liquid alkane spread over the newly fabricated OTSpd surface by increasing the temperature to 105 °C, which is higher than the alkane surface freezing temperature (Scheme 1e). After cooling to room temperature, the sample was characterized with AFM. A control test confirmed that our two-step patterning procedure was valid for the spreading experiment. After the spreading experiment, we rinsed the sample with chloroform and completely removed all alkanes on the surface. We characterized the patterned area again. AFM scans showed that the OTSpd disk fabricated the first time (Scheme 1a), and the subsequently fabricated “big” OTSpd disk (Scheme 1e) formed a continuous OTSpd region. No OTS barrier was found between the two OTSpd regions that had been fabricated at different times. The data are summarized in the Supporting Information (Figure S1).

Results and Discussion Part A. Fabricating Chemical Patterns Ranging from 80 nm to Submillimeter Scale with a Single Probe: The Local Probe Oxidation of OTS in a 100% Relative Humidity Environment. Under our patterning conditions, a stationary conducting AFM probe generated a disk-shaped, partially degraded silane pattern (OTSpd) as shown in Figure 1a. 5596

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The OTSpd disk center was the tip contact point. While with the voltage applied to the sample, a slow moving tip generated line patterns (Figure 1c). The line width was much larger than the AFM probe size. The topography images in Figure 1a,c are rendered in the gradient height scale, where dark color means low and bright color means high. The topography image in Figure 1a shows that the disk is lower than the OTS background. In the center, there is a peak, which is higher than the OTS. The corresponding AFM friction image shows that the OTSpd disk has a higher friction than the methyl-terminated hydrophobic OTS background. Since the tip is hydrophilic, the hydrophilichydrophilic interaction would yield a stronger attractive force than the hydrophilic-hydrophobic interaction. Therefore, the high friction value over the OTSpd indicates that the OTSpd disk is hydrophilic. Figure 1a,b also shows that, except the center peak, all regions inside the OTSpd disk are homogeneous, both in topography and in friction. Dipping the pattern in 1% hydrofluoric acid (HF) for 30 s, the disk was intact, except that the center peak became a deep hole. The HF etching test suggests that the center peak is SiO2. The OTSpd disk size increased with the voltage and voltage dwell time in the 100% relative humidity environment. At 100% relative humidity and with 10 V applied to the wafer, a 10 ms long pulse typically generated a 80 nm-diameter disk for a sharp and new tip. When a 60 min-long, 10 V pulse was applied to the sample, the OTSpd disk reached submillimeter size, which is shown in Figure 2. Other scanning probe lithography methods are capable of patterning the surface with the feature size close to the tip size. To create large patterns, the tip needs to raster scan over the specified area. Therefore, the piezo scanner range limits the area that could be patterned. The OTSpd pattern is generated by the electrochemical reaction. Thereby, as long as the voltage is applied, the pattern would grow larger and larger. In this sense, our patterning method demonstrates a unique capability to create surface patterns from nanometer scale to millimeter scale. The Depth of the OTSpd Pattern. The representative topography and friction images of the OTSpd pattern in Figure 1 have clearly demonstrated that OTSpd is different from the oxidized silane patterns from other groups25,26,30,34,35 because the pattern is lower than the OTS and its size is well beyond the tip-surface contact size. However, several factors affect the measurement of the depth of the OTSpd pattern. First, the surface of the oxidized silane is usually charged. In addition, the silane film is known to hold static charge during the voltage writing process.28,39,40 (39) Cohen, H.; Maoz, R.; Sagiv, J. Nano Lett. 2006, 6, 2462. (40) Han, J.; Lee, K. H.; Fujii, S.; Sano, H.; Kim, Y. J.; Murase, K.; Ichii, T.; Sugimura, H. Jpn. J. Appl. Phys. 2007, 46, 5621.

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Figure 1. Representative deeply oxidized OTSpd patterns. (a) Topography image of an OTSpd disk, which was fabricated by a stationary tip. The bright spot in the disk center is the oxide, which can be etched into a deep hole after HF treatment. (b) The corresponding friction image. (c) Line patterns generated by a moving tip (topography). From top right to lower left, the four lines were fabricated using 7 V bias: 0.5, 1, 2, and 4 μm/s tip moving speed, respectively.

Figure 2. An OTSpd disk fabricated by a long voltage pulse in a 100% relative humidity environment. (Optical microscope image) The sample was placed on a cold stage of a microscope. Water condensed on the OTS film as beads, which scattered light and appeared as white. Water condensed on the hydrophilic OTSpd disk (left in the image) and formed a film, which appeared as dark. On the right of the image, the scratch is a mark inscribed on the wafer with a diamond pen.

The surface charge causes topographic artifacts in the AFM scanning. Second, the cross-talks between the topography and friction also distort the topography information.35,41-43 The cross-talks may originate from several sources. The misalignment of the AFM cantilever, the unsymmetrical position of the tip in the cantilever, or the misalignment of the photodetector all cause a fake topographic signal generated from the torsion of the cantilever. Third, damaged AFM tips might also bring unreliable results due to the changes of the tip’s shape, size, and surface property after the oxidation reaction. These factors partially account for the inconsistency in the reported oxidized silane pattern heights. To measure the true depth of the OTSpd pattern, we need to eliminate the above interfering factors. We incubated the sample in 10% hydrochloric acid for 10 min and then cleaned the sample with a supercritical CO2 jet.44 After these procedures, the charge and contaminates were completely removed. We designed three independent methods to measure the depth of the OTSpd pattern. In the first method, we used

(41) (42) (43) (44)

Such, M. W.; Kramer, D. E.; Hersam, M. C. Ultramicroscopy 2004, 99, 189. Hoffmann, A.; Jungk, T.; Soergel, E. Rev. Sci. Instrum. 2007, 78, 016101. Peter, F.; Rudiger, A.; Waser, R. Rev. Sci. Instrum. 2006, 77, 036103. Sherman, R.; Whitlock, W. J. Vac. Sci. Technol. 1990, B8, 563.

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tapping mode AFM to characterize the OTSpd depth, avoiding the topography-friction cross-talk problem in contact mode AFM characterization. In the second method, we imaged the OTSpd pattern in contact mode with different forces applied and scanned the image from different directions. The artifacts in the AFM scan due to surface charge/friction-topography cross-talk are a function of the force set-point and the scanning directions.35 During the scanning, if the pattern height changed with the set-point values or the scanning direction, then we knew that the apparent height in the AFM image was not real. Therefore, we could distinguish the artifact from the real features when we characterized OTSpd in contact mode. In the third method, we designed an indirect method to probe the real height of OTSpd. We incubated the OTSpd sample in a 5 mM OTS toluene solution for overnight. Since the OTSpd pattern has a hydrophilic surface, a second OTS layer grew on top of the OTSpd pattern.27,30 The OTS film has a known height of 26 A˚.45 By measuring the apparent height of the bilayer film (OTSpdOTS) above the OTS background, we can deduce the depth of the OTSpd pattern. In the Supporting Information, we included the detailed procedures and the results for these measurements. All these three methods reached the same conclusion: the OTSpd pattern is 10 A˚ lower than the OTS background. In Figure 3, we show a representative topography image of an array of OTSpd disks. The image was acquired in tapping mode. The topography histogram in Figure 3b shows the height distribution of the OTS background and the OTSpd discs. Measured from the histogram, the apparent heights and standard deviations of OTS and OTSpd are 15.7 ( 0.5 A˚ and 5.0 ( 1.3 A˚, respectively. The depth of OTSpd pattern can be calculated as OTSpdDepth ¼ OTSApparentHeight -OTSpdApparentHeight The standard deviation of the OTSpd depth is σ OTSpdD epth ¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi σ OTSpdA pparentHeight 2 þ σOTSA pparentHeight 2

On the basis of above formulas, the depth of the OTSpd pattern is 10.7 ( 1.4 A˚ for the sample shown in Figure 3a. Similarly, we also measured the depth of the OTSpd pattern of 30 other samples. The average value from these AFM measurements is 10.2 ( 0.7 A˚. The Chemical Identity of the OTSpd Surface. We used PDAM as a molecular probe to test the chemical identity of the OTSpd surface. PDAM can generate fluorescence upon excitation. It specifically reacts with the carboxylic acid group. It has been used to analyze the amount of carboxylic group.46 The reaction of PDAM and the surface bound carboxylic acid group is illustrated below.

(45) Wasserman, S. R.; Whitesides, G. M.; Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Axe, J. D. J. Am. Chem. Soc. 1989, 111, 5852. (46) Yan, B.; Liu, L.; Astor, C. A.; Tang, Q. Anal. Chem. 1999, 71, 4564.

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Figure 3. An array of an OTSpd pattern. (a) The topography image, acquired in tapping mode after the sample was cleaned with the supercritical CO2 snow jet. (b) The histogram of the topography image. From the apparent height distribution profile in the histogram, the OTSpd depth in panel a is measured to be 10.7 ( 1.4 A˚ by AFM.

Figure 4. The PDAM reaction on the OTSpd pattern. (a) Part of an OTSpd disk array. (Topography image, tapping mode 6  6 μm2). The bright spot in the disk center is the SiO2. (b) The exact same area after the sample reacted with PDAM and was wiped in chloroform (topography image acquired in contact mode, 6  6 μm2). Both a and b are rendered in the same 30 A˚ height scale. The PDAM molecules coupled with the carboxylic groups of the OTSpd surface, which led to the increase of the height of the pattern. (Inset in b) The fluorescent image of the pattern after the sample reacted with PDAM and was wiped in chloroform. The PDAM is a fluorescent molecule, which emits fluorescent photons upon excitation. The white box corresponds to the exact area shown in a and b. The fluorescent signal (green) confirms that the species on the OTSpd pattern is PDAM.

We incubated the OTSpd pattern in a 3 mM PDAM methanol solution for 10 h. Then the sample was rinsed and ultrasonicated in tetrahydrofuran (THF). Figure 4a shows a representative topography image of the OTSpd patterns before incubating with the PDAM. Figure 4b is the same area after the PDAM incubation. The disk patterns evolved from lower than OTS to the same height as that of OTS after the PDAM incubation. The inset in Figure 4b is the optical fluorescence image of the pattern. The fluorescence signal confirmed that the species bound on OTSpd was PDAM. We also conducted the “Scotch tape” test and the wiping test. After peeling off a piece of adhesive Scotch tape from the patterned wafer or swabbing the patterned area under chloroform, the PDAM treated pattern remained the same height as that of OTS. In contrast, in the control experiment, the OTSpd patterns incubated with pyrene or hexatriacontane cannot pass the “Scotch tape” test and the wiping test. Pyrene and hexatriacontane were completely removed from the OTSpd discs after these tests. These experimental results show that the PDAM reacted with the carboxylic acid groups on the OTSpd pattern and formed covalent bonds. Since PDAM only reacts with the carboxylic group, our data indicate that OTSpd is carboxylic acid-terminated. In a parallel experiment, we confirmed that the OTSpd surface is not made of SiO2. We fabricated an OTSpd pattern as shown in Figure 5. During the pattern fabrication process, the 5598

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Figure 5. The OTSpd pattern after treated with 1% HF. (a) Topography image, tapping mode. The white arrows show the places of the SiO2 before the HF treatment. The SiO2 always stayed in the center of the OTSpd disks (the joints of the lines) and appeared higher than OTS. After HF treatment, the places that SiO2 used to be became deep holes, indicating that SiO2 was etched by HF, while the depth of the OTSpd pattern remained the same, illustrating that the OTSpd surface does not consist of SiO2 and resisted the HF etching. (b) Height cross-section profile corresponds to the blue line in panel a, showing OTSpd is still 9 A˚ after the HF etching.

biased moving tip briefly halted when the moving direction changed. As a result, the joints of the lines appear as OTSpd discs, where the disk centers were the SiO2 peaks. We reacted the pattern with a 1% HF solution for 5 min and imaged the pattern again after rinsing. Figure 5 shows that the center “peaks” in the disk pattern (pointed out by white arrows in the figure) changed to deep holes after the HF treatment, while other parts of OTSpd remain 9 ( 1 A˚ below OTS. This height change indicates that the center “peak” was made of SiO2, which was not covered by a protective organic layer and was etched away by HF, whereas the fact that the depth of OTSpd disk did not change after the HF etching indicates a “protective” layer existed. The partially degraded OTS film resisted the HF etching. Therefore, the underneath native SiO2 was not removed by HF. Summarizing the experimental data on the OTSpd depth as well as the OTSpd chemical identify, we conclude that OTSpd is a degraded silane layer with -COOH termination. The oxidation of silane consists of two simultaneous processes: the degradation of the silane film, which decreases the pattern height, and the silicon oxide formation, which increases the pattern height. We speculate that these two opposite processes reached equilibrium when the apparent height of OTSpd was 10 A˚ lower than OTS. The 10 A˚-depth roughly corresponds to the dimension of eight CH2 units. However, such similarity should not be explained as that the deep oxidation always truncated exactly eight CH2 units from the OTS backbone. The underneath SiO2 might also grow thicker after oxidation. In addition, the measured 10 A˚-depth of OTSpd does not necessarily mean that all the degraded silane (OTSpd) molecules have the exact height. The depth of the OTSpd pattern is measured using AFM, where the tip contact area is around 10  10 nm2 or larger. Surface features that are smaller than the tip size are subject to the tip convolution. OTS molecules that were truncated further might also coexist with the OTS molecules that were truncated by just eight CH2 units. When “tall” molecules and “short” molecules mixed, AFM can only detect the “tall” molecules because of the tip size. Therefore, the AFM data only suggest that the deep oxidation truncated at least 10 A˚ from the OTS backbone. The mechanism of the deep oxidation needs further investigation. In conclusion, under a 100% relative humidity environment, the local probe oxidation on an OTS film generates the partially degraded, hydrophilic and carboxylic acid-terminated silane pattern (OTSpd) around the tip. The region directly under the Langmuir 2009, 25(10), 5594–5601

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conducting AFM tip is converted to SiO2 as a result of the intense electric field. If the tip is stationary during the electrochemical oxidation, the resulting pattern will have a disk shape, which could be as large as several hundred micrometers in diameter. Part B. The Application of an OTSpd Pattern in Studying Alkane Spreading. The Full Length of the Hexatriacontane Molecules Measured by AFM. The n-hexatriacontane was dip-coated on the OTSpd pattern. The alkane pattern followed the shape of the underneath chemical template. Figure 6 shows a typical n-hexatriacontane pattern dipcoated on an OTSpd disk, corresponding to Scheme 1c. The n-hexatriacontane on OTSpd shows a layer-by-layer structure, which is similar to the previously reported results for the n-dotriacontane.8,9 The solidified alkane drop is composed of many terraces. In Figure 6 we observe a total of 16 layers above the OTS surface. From the second layer in the base to the top layer, all the steps have the same height. The height cross-section (Figure 6b) profiles the height variation from the second layer to the fourteenth layer. The total height of these 13 n-hexatriacontane layers is 659 A˚. Thus, the averaged height of a single layer is 50.7 A˚. The height of n-hexatriacontane from the direct X-ray reflectivity measurement is 52 A˚3, which is similar to our value. On the basis of the height, we assign the observed terraces as perpendicularly packed n-hexatriacontane in all-trans configuration. However, the heights of long n-alkanes obtained from the direct X-ray reflectivity measurement and our experiment are about 10% larger than the theoretical lengths of the fully extended all-trans configuration.1,3 This discrepancy is still not fully understood. The n-Hexatriacontane Structures on the High-Energy Surface and the Low-Energy Surface. By monitoring changes of the OTSpd pattern depth before dip-coating the alkane and after the alkane layer covered the OTSpd, we can figure out the thickness of the adsorbate layer on the OTSpd surface. Figure 7a shows the n-hexatriacontane layer dip-coated on the OTSpd pattern. The amount of liquid coated on the pattern is controlled by the withdrawal speed during the dip-coating.15,16,47 Here we deliberately used a low withdrawal speed. As a result, the OTSpd pattern could not hold an excessive amount of liquid alkane. The alkane overflowed out of the OTSpd pattern. The majority of the alkane layer selectively covered the hydrophilic OTSpd pattern, while part of the alkane layer was on the OTS. The thickness of the overflowed alkane layer is measured to be 50.6 A˚, indicating that the hexatriacontane molecule is perpendicular and stands direct on the methyl-terminated OTS film. Interestingly, this result is consistent with the observed terraced structure shown in Figure 6. In the terraced structure, the alkane molecules also stand up perpendicularly and directly on the underneath alkane layer, which is also methylterminated. Since the alkane film simultaneously covered both the hydrophilic and the hydrophobic surfaces, such “overflowed” structures offered an opportunity to compare the alkane structures on the high-energy OTSpd surface and the low-energy OTS surface. In Figure 7a, the disk that has a dark orange color was the OTSpd pattern before the alkane adsorption, which was 10 A˚ lower than OTS. After the alkane deposition, hexatriacontane covered the disk. In the disk center, the hexatriacontane formed terraced structure. Figure 7b is the phase image corresponding to the topography image (Figure 7a). The OTS surface and the (47) Darhuber, A. A.; Troian, S. M.; Miller, S. M.; Wagner, S. J. Appl. Phys. 2000, 87, 7768.

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Figure 6. A representative n-hexatriacontane drop dip-coated on the OTSpd template. (a) AFM image of a solidified drop. (Tapping mode, topography) The n-hexatriacontane drop has a layer-bylayer structure. (b) The height cross-section profile of the drop (green line in a) shows that 13 layers of n-hexatriacontane have a total height of 659 A˚. Therefore, the height of one layer is 50.7 A˚. (c) The structural model of the n-hexatriacontane drop on the OTSpd. A layer of alkane adsorbs on the OTSpd surface with the chain parallel to the surface. The standing up phase alkane layer is assembled on the parallel layer.

Figure 7. An n-hexatriacontane layer dip-coated on the OTSpd overflowed out of the OTSpd disk. (a) AFM topography image. The dark orange-colored disk was the OTSpd disk pattern before the alkane was dip-coated. (b) Phase image corresponding to the topography image. The phase signals are the same for the alkane molecules inside and outside the OTSpd disk. Therefore, they have the same surface structure. (c) The schematic structure of the alkane films assembled inside and outside of the OTSpd template. (d) Height histogram of the region inside the white box in panel a. The histogram shows that the alkane layer inside the OTSpd pattern is 4.6 ( 0.3 A˚ lower than that of the alkane layer outside the OTSpd disk.

surfaces of alkane layers have the same phase contrast in Figure 7b. Therefore, the surface of OTS, the surface of the hexatriacontane that overflowed on OTS, and the surface of the hexatriacontane layer inside the OTSpd disk are all the same, which are methyl-terminated. On the basis of the phase signal, we conclude that the hexatriacontane molecules inside the disk also stand up perpendicularly, which have the DOI: 10.1021/la9004483

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same configuration as the hexatriacontane molecules on the OTS. The white frame in Figure 7a shows that the first perpendicularly standing alkane layer inside the OTSpd disk is lower than that of the perpendicularly standing alkane layer on OTS. In Figure 7d, we plot the height histogram of the topography image inside the white frame. The histogram shows that the alkane layer inside OTSpd pattern is 4.6 ( 0.3 A˚ lower than that of the perpendicularly standing alkane layer on OTS. Because the alkane molecules inside and outside the OTSpd disk are both perpendicularly aligned and OTSpd is 10.2 ( 0.7 A˚ lower than the OTS, we conclude that there is an additional layer between the perpendicularly standing alkane molecules and the OTSpd surface. The thickness of this layer is 5.6 ( 0.8 A˚. The 5.6 A˚ height implies that the alkane molecules in this additional layer can only adsorb on the OTSpd surface with their backbones parallel to the surface. Therefore, the bilayer model is consistent with our AFM data. We illustrated the speculated hexatriacontane adsorption structures in Figure 7c. In summary, by using the depth of the OTSpd pattern as a topographic reference, we confirmed the existence of the “bilayer” structure proposed by Taub et al. We show that, on high-energy surfaces, such as the OTSpd surface, the long-chain alkanes have a bilayer adsorption structure, whereas, on lowenergy surfaces like the methyl-terminated surface, the alkane molecules stand perpendicularly on the surface directly. New Adsorption Structures of n-Hexatriacontane on the Carboxylic Acid-Terminated OTSpd Surface. The AFM characterization was performed immediately after the spreading experiment (step d to step e in Scheme 1). AFM measurement shows that the OTSpd/OTS edge is reduced from 10 A˚ to 5 A˚ after the spreading. The alkane molecules initially adsorbed on the OTSpd surface with their alkyl chains parallel to the surface until one monolayer-coverage was reached. The additional hexatriacontane molecules spread over the parallel alkane layer, forming seaweed-shaped fractal patterns. Figure 8a shows representative seaweed patterns after the alkane spread over the OTSpd surface. The OTSpd disk we fabricated here is larger than 100 μm in diameter in Figure 8a, which is beyond the field of view of Figure 8a. The background in Figure 8 is the OTSpd surface adsorbed with the parallel alkane layer. Seaweed-shaped alkane layers have also been observed on the SiO2 surfaces.2,13 The alkane molecules in the seaweed pattern are thought to stand vertically on the surface. As a result, all “seaweed” phase layers should have the same height, which equals the alkane length. However, after characterizing 49 such “seaweed” patterns, we found the height of the “seaweed” layer was not always the same. The height distribution of seaweed patterns is plotted in Figure 8b. The heights of seaweed patterns are 50.6 ( 0.3, 44.3 ( 0.3, 38.8 ( 0.4, and 34.1 ( 0.6 A˚. The 50.6 A˚-thick seaweed layer corresponds to the perpendicularly standing alkane. We speculate that the hexatriacontane molecules in the 44.3, 38.8, and 34.1 A˚-thick “seaweed” alkane layers are tilted. This is because the long chain alkane molecules can only tilt in a few quantized angles when packed side-by-side. The alkane chain has a zigzag shape. If the alkane molecules are tilted, the teeth of the zigzags between neighboring molecules have to match to facilitate a snug sideby-side packing. Therefore, an alkane molecule can only shift along its backbone direction in quantized units. The distance of each unit is the C-C bond length projected in the alkyl

chain direction (1.26 A˚). The width of a “tooth” is two units. For the hexatriacontane molecule, if we use 50.6 A˚ as the full molecular length and 4.5 A˚ as the distance between the alkane chains,48-50 a two-unit shifting along the chain direction will yield a 29.25° tilt off the surface normal direction, which corresponds to a layer thickness of 44.1 A˚. Similarly, a threeunit shifting corresponds to a layer thickness of 38.7 A˚, and a four-unit shifting corresponds to a layer thickness of 33.7 A˚. The calculated and the measured layer thickness values are listed in Table 1. The observed values closely match the calculated values from those quantized tilting angles, which confirms our speculation on the alkane structures. Although we observed seaweed patterns with only four types, we speculate that layers made up by alkane molecules of other quantized tilting angles also exist if more samples are examined. In summary, in the first layer, alkane molecules adsorb on OTSpd with the alkyl chain parallel to the surface. This parallel layer evenly covers the whole OTSpd surface. Additional alkane molecules spread over the parallel layer. After cooling down, these alkane molecules solidify into seaweed-shaped patterns. The hexatriacontane molecules in the seaweed patterns either stand perpendicularly, or tilt. Previously, the fittings from the X-ray reflectivity and GID data showed that only the perpendicular phase existed on the SiO2 surface. Our AFM characterization captured the rare structures that were overlooked from the previous X-ray detections. The X-ray characterization only yields statistically averaged structural information of the beam-impacted area. Due to the small fraction of the tilted phase seaweed patterns over the surface, the tilted phases might be neglected by the X-ray studies.

(48) Dai, P.; Wang, S. K.; Taub, H.; Buckley, J. E.; Ehrlich, S. N.; Larese, J. Z.; Binnig, G.; Smith, D. P. E. Phys. Rev. B 1993, 47, 7401.

(49) Morishige, K.; Takami, Y.; Yokota, Y. Phys. Rev. B 1993, 48, 8277. (50) Claypool, C. L.; Faglioni, F.; Goddard, W. A.; Gray, H. B.; Lewis, N. S.; Marcus, R. A. J. Phys. Chem. B 1997, 101, 5978.

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Figure 8. The seaweed-shaped hexatriacontane pattern on the parallel alkane layer. (a) Representative hexatriacontane patterns after spreading over the OTSpd surface (tapping mode, topography image, 45  45 μm2). The OTSpd disk is bigger than the scanned area. Therefore, the OTSpd/OTS border is out of field of view in this figure. (b) The height distribution of seaweed patterns. (c-f) The proposed alkane structural models correspond to different alkane layer thicknesses. The thickness shown in the figure is the calculated value assuming the alkane full length is 50.6 A˚. Table 1. Alkane Tilting Angles and Heights alkane molecule shifting distance (1Unit=1.26 A˚) 0 2 3 4

calculated alkane calculated measured tiltinzg angle off the alkane layer alkane layer surface norm (°) thickness (A˚) thickness (A˚) 0 29.25 40.02 48.24

50.6 44.1 38.7 33.7

50.6 ( 0.3 44.3 ( 0.3 38.8 ( 0.4 34.1 ( 0.6

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Conclusion We developed a new patterning method that can create hydrophilic OTSpd patterns ranging from 80 nm to submillimeter in size on the OTS film surface. The pattern we fabricated is carboxylic acid-terminated and 10.2 ( 0.7 A˚ below OTS. During the local probe oxidation of OTS film, under low humidity, the surface is converted to oxide. Under ambient humidity and fast writing speed, the OTS surface is converted to the OTSox pattern, which is hydrophilic and just 0-3 A˚ lower than OTS, whereas, under 100% relative humidity and slow writing speed, OTS is converted to the partially degraded OTSpd pattern, which is carboxylic acid-terminated and 10 A˚ lower than OTS. We used the OTSpd patterns to study the spreading and adsorption structures of long-chain n-alkanes over a chemically and topographically defined surface. The chemical pattern provided a reference for the comparison of alkane layer heights and surface energies. We confirmed the bilayer alkane adsorption model over the hydrophilic surface. In addition, we found three tilted phases of the adsorbed n-hexatriacontane.

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Acknowledgment. This research is supported by a University of Kentucky faculty start-up grant. We thank Dr. Zach Hilt of the Department of Chemical Engineering, University of Kentucky, for his help in the fluorescence imaging experiment.

Nomenclature Glossary OTSox The incompletely oxidized OTS pattern that has the same height as the OTS. OTSpd The deeply oxidized OTS pattern, which is partially degraded. The OTSpd pattern is lower than OTS. Supporting Information Available: Additional analyses of the depth of OTSpd patterns are included. This material is available free of charge via the Internet at http://pubs. acs.org.

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