Article pubs.acs.org/Langmuir
Atomic Force Microscopy (AFM)-Based Nanografting for the Study of Self-Assembled Monolayer Formation of Organophosphonic Acids on Al2O3 Single-Crystal Surfaces Boray Torun, Berkem Ozkaya,* and Guido Grundmeier Technical and Macromolecular Chemistry, University of Paderborn, Warburger Straße 100, 33098 Paderborn, Germany S Supporting Information *
ABSTRACT: Adsorption, stability, and organization kinetics of organophosphonic acids on single-crystalline alumina surfaces were investigated by means of atomic force microscopy (AFM)-based imaging, nanoshaving, and nanografting. AFM friction and phase imaging have shown that chemical etching and subsequent annealing led to heterogeneities on single-crystalline surfaces with (0001) orientation. Selfassembly and stability of octadecylphosphonic acid (ODPA) were shown to be strictly dependent upon the observed heterogeneities of the surface termination, where it was locally shown that ODPA can loosely or strongly bind on different terminations of the crystal surface. Organization kinetics of ODPA was monitored with nanografting on (0001) surfaces. Supported by measurements of surface wettability and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), it was demonstrated that the lack of organization within the protective adsorbed hexylphosphonic acid (HPA) monolayer on alumina surfaces facilitated the reduced confinement effect during nanografting, such that kinetics information on the organization process of ODPA could be obtained. self-assembly.21,22 Moreover, using nanografting, it has been demonstrated that, because of the spatial confinement effect,24 the grafted patches contain less defects and friction on the patches is always smaller than on the surrounding self-assembled monolayer (SAM) matrix.21 For the mentioned studies, nanografting remains a unique experimental technique, where local studies can be performed and questions can be answered, which are not accessible with bulk spectroscopic methods. Furthermore, the spontaneous adsorption of molecules with high affinity to bind on specific surfaces (for example, thiol adsorption on gold surfaces) makes it difficult to perform AFM-based in situ experiments on monolayer formation. Nanografting is an experimental tool that can be used to compare adsorption of molecules of different chain length and termination. In this study, nanografting and nanoshaving is performed to investigate the adsorption of organophosphonic acids on alumina surfaces with (0001) orientation.
1. INTRODUCTION The surface chemistry of aluminum oxides is of crucial importance in the field of catalysis, corrosion, and adhesion.1−4 While Al oxides are widely used in catalysis,5 Al-oxide-covered aluminum alloys are employed in the construction of lightweight automotive and aerospace parts.6 In most cases, oxide-covered aluminum alloys are either organically coated or adhesively bound.7,8 In this context, the adhesion between the polymer and oxide surface plays a major role for the long-term stability of the industrial parts. Using self-assembled adhesionpromoting1,9 monolayers, the complexity of the surface pretreatment processes could be tremendously reduced. One of the most common coupling agents for oxide surfaces, bifunctional phosphonic acids, was found to form strongly bound monolayers on aluminum covered with an amorphous native oxide film.10 In comparison to amorphous oxide films10−14 single-crystalline surfaces15−19 provide a well-defined experimental and theoretical platform to understand the adsorption mechanism and stability of phosphonic acids. The possibility to prepare atomically flat single-crystalline Al2O3 surfaces allows high-resolution scanning probe measurements. Nanografting and nanoshaving are rather new techniques to study self-assembly processes. Since it was first reported20 in 1997, atomic force microscopy (AFM)-based nanografting has been used as a tool to investigate the adsorption of organic monolayers on noble metals, such as gold.21−23 Recent studies focus on topics such as the effect of termination and chain length on friction coefficients and the comparison of adsorption of bifunctional thiol molecules via nanografting and solution © 2012 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Chemical and Materials. Chemicals were of analytical grade and used without any further purification. Ultrapure water was obtained from a water purification system Ultraclear TWF (SG Wasseraufbereitung, Barsbuettel, Germany). The quality of the purified water was constantly monitored by means of conductivity, which under normal conditions was 0.055 μS/cm. Phosphonic acid solutions were Received: February 16, 2012 Revised: April 3, 2012 Published: April 5, 2012 6919
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Figure 1. Morphological differences and heterogeneities on Al2O3(0001) surfaces. (A and C) Topography images showing typical atomically flat terraces. (B) Friction and (D) phase images reveal different terminations on the same terrace. (E and F) Similarly prepared single crystals showing domains (E, topography image; F, friction image). Measurements were performed under ambient conditions. prepared in ultrapure ethanol (99.99%, Merck KGaA). Octadecylphosphonic acid (ODPA) and hexylphosphonic acid (HPA) were obtained from Alfa Aesar. The epi-polished Al2O3 single crystals with crystallographic orientation of (0001) were purchased from Mateck GmbH (Juelich, Germany), with dimensions of 20 × 20 × 0.5 mm [length, width, and thickness (LWT)]. 2.2. Preparation of Al2O3 Single Crystals. As received epipolished Al2O3 single crystals with crystallographic orientation of (0001) were prepared by means of alkaline and acidic etching. To avoid any inorganic contaminations, all cleaning steps were performed in polytetrafluoroethylene (PTFE) vessels. First, the crystals were treated with a standard RCA-1 procedure (5:1:1 ultrapure water, 30% aqueous NH3, and 30% aqueous H2O2) at 70 °C for 90 s to remove organic contaminants (Caution: RCA-1 is extremely corrosive). The crystals were then rinsed intensively with ultrapure water. Subsequently, crystals were immersed in 85% phosphoric acid for
60 s, followed by intensive rinsing with ultrapure water. This cycle was repeated at least twice for each crystal. The samples were then annealed under an ambient atmosphere at 1450 °C for 36 h. Annealed crystals were checked with AFM for cleanliness, surface roughness, and defects before immersing in the phosphonic acid solutions. Ethanolic solutions with a concentration of 1 mM and immersion times of 12 h were found to be suitable to obtain SAMs. Prepared crystals were thoroughly rinsed with ultrapure ethanol, dried with nitrogen, and immediately used. 2.3. AFM Measurements. AFM topography measurements, nanoshaving, and nanografting were performed with a MFP-3D-SA (Asylum Research, Santa Barbara, CA), equipped with an antivibration table and custom-built antinoise box. Contact or tapping mode imaging, nanoshaving, and nanografting studies were performed using cantilevers with relatively high force constants (NSC-19, MicroMasch, Tallinn, Estonia). A custom-made polyether ether ketone (PEEK) 6920
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Figure 2. XPS measurements of the O 1s region of the Al2O3 (0001) single crystal at (A) 70° and (B) 20° takeoff angle. Spectra were fitted with two components corresponding to oxide and hydroxide species of oxygen. A clear increase of the hydroxyl contribution was found for the spectrum acquired with a lower takeoff angle, resulting in more surface-sensitive measurements. sample holder was attached to the commercial closed fluid cell of the AFM instrument. Electrolytes and precursor solutions were introduced to the cell using a syringe pump. An excessive amount of precursor electrolyte has been flushed through the closed fluid cell to ensure that the targeted molecular concentration (1 mM) is achieved. In a typical nanoshaving/nanografting process, a monolayer adsorbed to a substrate can be manipulated by means of AFM contact mode scanning by applying high loads to the surface. Performed in pure solvents, the force exerted by the AFM tip leads to film removal, known as nanoshaving. When the same procedure was performed in the presence of secondary molecules in the solvent, rapid adsorption of the secondary molecules takes place. This process is commonly referred as nanografting.20 In the case of alumina substrates, although the nanoshaving procedure removes the adsorbed organic layer, it does not damage the crystal. 2.4. X-ray Photoelectron Spectroscopy (XPS) Measurements. XPS was performed in an ESCA+ spectrometer (Omicron Nanotechnology, Taunusstein, Germany) using a monochromatic Al Kα X-ray source and a spherical electron analyzer. The X-ray source was set to a total power output of 150 W, and the analyzer was operated in constant analyzer energy mode at a pass energy of 25 eV. The measurements were performed at a chamber pressure lower than 2 × 10−9 mbar. To modify the surface sensitivity of the measurement, the angle of incidence was adjusted by an automated sample manipulator. A strong charging was observed during XPS measurements. O 1s spectra positions were normalized with respect to the position of C 1s peaks (284.5 eV).
In ambient environments, the adsorbed water on the surface drastically changes the tip surface interactions. Although the detected topographical differences were typically in the order of 100 pm, a clear phase and friction contrast was detected, which is due to the adsorbed water multilayers on the surface. Moreover, on identically prepared single crystals, large elliptical and spherical domains accompanied with smaller islands were observed, as depicted in Figure 1E. These domains also showed a clear difference in friction, which indicates that the termination is different from the rest of the surface. Moreover, ultrahigh vacuum (UHV)−AFM measurements at elevated temperatures (140 °C) indicated that friction contrast was still observable in the absence of physisorbed water (see Figure S3 of the Supporting Information). To date, the origin of the formation of the heterogeneities on the surface is not fully understood. Throughout the text, the surfaces shown in panels A and C of Figure 1 will be referred as type 1 and the surfaces shown in Figure 1E will be referred as type 2. Previous studies on α-Al2O3 (0001) have shown that surfaces prepared under UHV conditions are composed of two different surface terminations, namely, Al and O.26 Moreover, UHV− AFM studies have confirmed the termination with two layers of Al atoms forming a hexagonal structure.27 As opposed to UHV studies, single-crystalline surfaces prepared under ambient conditions differ because of different oxygen and water activities and the adsorption of water. Therefore, the stabilization mechanism of the surface is different from that for UHV conditions. It has been proposed that, under ambient conditions, hydroxylation lowers the surface free energy and hydroxide-covered surfaces are the most stable in water-containing media.28 As explained previously, the surfaces used in this study exhibited domains of different termination. These surfaces are assumed to be terminated by Al and O atoms in the absence of water. As postulated by Alexander et al.,16 alumina surfaces hydroxylate after annealing in the presence of water. Angle-resolved XPS investigations on the Al2O3 (0001) surface prove this assumption, indicating hydroxyl adsorbates (Figure 2). The heterogeneous structure of the α-Al2O3 (0001) surface naturally raises the question of local molecular adsorption considering that the molecular binding strongly depends upon the surface termination. Using nanoshaving and nanografting, the adsorption and stability of ODPA on Al2O3 (0001) surfaces have been analyzed to address this question.
3. RESULTS AND DISCUSSION AFM imaging was performed to characterize the prepared single-crystal surfaces in both ambient conditions and liquid electrolytes. As received, polished single crystals showed a relatively smooth surface, while after the etching and annealing, large, atomically flat terraces were observed. Typically, (0001) surfaces exhibited terraces as large as 1 μm. Although the preparation conditions were not altered, different surface morphologies were encountered on identical single crystals with (0001) orientation prepared according to the same route. Similar height and friction contrasts have been reported in the literature.16,25 Figure 1 shows the AFM topography, friction, and phase images of such heterogeneities on the (0001) alumina surface in ambient conditions. For all cases, the typical stepped terraces with widths in the range of 1 μm have been observed (panels A, C, and E of Figure 1). The heterogeneity because of the termination difference was revealed by the friction and phase contrast within the same terrace, as seen in panels B, D, and F of Figure 1. 6921
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Figure 3. (A) AFM topography of ex situ adsorbed ODPA on Al2O3 (0001). Topography (B) before and (C) after nanoshaving on an area containing two different surface terminations. (D) Same crystal after two AFM scans. ODPA stays stable only on one type of surface termination. Measurements were performed in ethanol.
the ODPA-covered alumina surface, on which homogeneous adsorption was observed. The cross-section reveals two terraces with distinctly higher roughness values (indicated by stars).
On crystal type 1, nanoshaving was performed in pure ethanol, allowing for the analysis of the stability of adsorbed ODPA on the heterogeneous alumina surface. Figure 3A shows 6922
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Figure 4. Nanografting on Al2O3 single-crystal surfaces. (A) Topography and (B) friction images after nanografting in the presence of ethanolic ODPA solution on the HPA-covered (0001) surface.
Figure 5. Selective adsorption of ODPA on HPA-covered type 2 Al2O3 (0001). (A) HPA-covered α-Al2O3 (0001) surface showing characteristic domains. (B) Nanografted ODPA on a 400 × 400 nm heterogeneous area (image is acquired on the square as shown in panel A). (C) Line profiles of nanografted ODPA showing a height difference of 0.8 nm. (D) Line profile across the island-like domains showing an identical height increase (0.8 nm) on the domain but not in the surrounding region. Schematic of the tip and monolayer illustrating possible compressive forces during imaging. Measurements were performed in ethanolic ODPA solution.
high load applied with the AFM cantilever was sufficient to remove ODPA from both surfaces (Figure 3C). Removed molecules immediately dissolve into the solvent, and re-adsorption was not observed. The cross-sectional analysis showed a height difference of ∼2 nm in both directions, marked as 1 and 2, proving that an ODPA SAM was present in both regions.
These terraces are assumed to be covered with a rather disordered and loosely bound layer of ODPA, while the other terraces with a low roughness are covered with a strongly bound SAM of ODPA. Prior to nanoshaving, an area including both types of surfaces was selected, as indicated by the rectangle in Figure 3B. The 6923
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Figure 6. AFM (A) topography and (B) phase image of the HPA-covered Al2O3 (0001) surface in pure ethanol. Topography (C) before and (D) after nanografting of ODPA within a 400 × 400 nm area. (E) Cross-section of the grafted ODPA showing a 0.8 nm height difference compared to the HPA-covered terrace. Measurements were performed in ethanolic ODPA solution.
Previously, it was shown that ODPA SAMs are stable on Al2O3 (0001) in ethanolic solution but not in the presence of water-based electrolytes.17 With an in situ AFM experiment, it was reported that changing the solvent from ethanol to water revealed micellar structures on the surface, indicating the desorption of ODPA from the surface. The instability has been explained by three main reasons: the adsorption free energies of ODPA in competition with water, the adsorption geometry, and finally, the interfacial binding mechanism.17 It has been argued that, in comparison to ODPA, the high adsorption free energy favors the water adsorption for the (0001) surface orientation. Moreover, the mismatch between the distance of the oxygen atoms in the phosphonate group and the Al−Al distance within the (0001) surface was proposed as a reason for preventing the formation of coordination bonds on (0001) surfaces. Within this work, a similar experiment was performed to check the stability of ODPA in ethanol. Without introducing water to the experimental cell, it was observed that mere scanning in contact mode in pure ethanol was sufficient to
remove the monolayer specifically from one type of termination. However, the pressure applied during imaging with the AFM tip was not enough to disturb the strongly bound ODPA. Figure 3D shows a different region of the crystal used for the nanoshaving experiment shown in panels A−C of Figure 3. After two scans within the same region, the intact ODPA monolayer reveals a height difference of 2 nm, as seen in the cross-sectional image. This indicates that ODPA molecules can strongly bind on one type of crystal termination, whereas physisorption is likely to take place on the second crystal termination. 3.1. Nanografting ODPA and Termination-Dependent Adsorption. Nanografting on alumina surfaces was performed using monofunctional phosphonic acid molecules (ODPA). Figure 4 gives an impression of the nanografted phosphonic acid molecules on (0001) single-crystalline alumina surfaces. In the case of adsorption of ODPA on the HPA-covered Al2O3 (0001) surface, a positive height contrast was found. Because in this case, the −CH3 termination on the patch and the rest of 6924
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the surface remains the same, no remarkable friction contrast can be observed (panels A and B of Figure 4). The surface-termination-dependent adsorption was demonstrated by an in situ measurement on crystal type 2. An area on a HPA-covered heterogeneous surface was selected (see Figure 5A). The selected area for nanografting is composed of a smooth surface and includes islands of presumable Al−OH termination. The closed fluid cell was flushed with 1 mM ethanolic ODPA solution, and nanografting was performed in a square region of 400 × 400 nm, as seen in Figure 5B. As predicted, ODPA was successfully grafted on the available sites upon removal of HPA from the flat terrace. On the other hand, a height increase was only observed on the islands. Panels C and D of Figure 5 show a height increase of 0.8 nm, which verifies that grafting occurred only on the islands. The AFM imaging performed right after nanografting has likely removed the physisorbed ODPA molecules around the islands. The observed height difference depends upon the organization of the background monolayer (HPA) and the grafted patch. Moreover, because imaging is also performed with a certain load applied on the monolayer, a possible compression of the molecules should also be taken into account.18 The schematic given in Figure 5 demonstrates such a scenario. 3.2. Organization Kinetics of ODPA on Alumina Surfaces. To trace the organization kinetics, continuous AFM imaging was performed following the nanografting process. As explained previously, to hinder the immediate adsorption of ODPA, the surface was protected with a shorter molecule, HPA. Adsorbed HPA via solution self-assembly exhibited a domain structure on the terraces of the single-crystal surface. Existence of such domains has proven the coverage of the surface, as seen in Figure 6A (indicated by arrows). Moreover, a slight phase contrast was also observed at the edge of the terraces (Figure 6B). The homogeneous domains indicate the full coverage on the surface. Nevertheless, static water contact angle measurements on alumina crystals prepared by solution self-assembly of HPA resulted in a relatively low contact angle of 70°. This indicates a low degree of organization of the short HPA molecules. This value was measured to be 110° on equally prepared ODPA-covered alumina crystals, indicating the SAM formation (see Figure S1 of the Supporting Information). In a typical solution self-assembly process, molecules immediately replace the solvent molecules and lay down on the surface, and subsequently, organization of the molecules starts.10,19 However, even a few minutes after the nanografting process, a considerable large height difference from the background monolayer is observed, which indicates a spontaneous partial organization on the surface. Panels C and D of Figure 6 show the HPA-covered singlecrystal surface before and after nanografting. The image acquired right after grafting shows a height difference of 0.75−0.8 nm. To explain this height difference, a number of parameters have to be carefully considered. First, because of the short chain length of HPA, the level of organization will be considerably lower than that of longer molecules. Second, although tapping mode imaging has been used, the pressure applied with the tip could compress the monolayer and the apparent height difference could be slightly less than expected.18 In contradiction to reported results of quick densification of grafted patches,21,22 nanografted ODPA showed an increase in height difference within the first 45 min after the nanografting process. This indicates a possible ongoing organization process. Within this time period, the packing density of the molecules increases and the van der Waals forces between the alkyl chains
Figure 7. Organization kinetics of nanografted ODPA. Average height difference versus time, showing two distinct regimes of low density and organized phase. (A) Height difference between the HPA-covered Al2O3 (0001) surface and ODPA feature remains constant after 45 min (solid line is drawn to guide the eye). (B and C) Schematic of possible structure of the adsorbed molecules in the organization and densely packed phase.
lead to a dense monolayer. Eventually, this process stabilizes, and height contrast reaches 0.95 nm. Figure 7A depicts the extracted data from AFM measurements showing the kinetics of the organization process. Possible orientations for the molecules right after nanografting and in the final stage are shown in panels B and C of Figure 7. It should be kept in mind that the particularity of nanografting in comparison to solution selfassembly is based on the spatial confinement between the intact matrix SAM and the AFM tip. The assumption of a disordered HPA monolayer would partly diminish this effect; hence, nanografting could be used as a tool to monitor the organization kinetics of ODPA molecules. Ex situ adsorption and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements (see the Supporting Information) on single crystalline surfaces supported this argumentation. The CH2 stretching peaks show a shift to lower wavenumbers on the ODPA-covered surface, which indicates a high level of organization.9 Contact angle and DRIFTS measurements indicate a low degree of organization of adsorbed HPA molecules, which still serve as a background monolayer. The lack of self-assembly in the HPA monolayer leads to a decrease in the confinement effect between the tip and the monolayer and, therefore, makes it possible to observe the organization process in the nanografted patch. Consequently, it is proposed that using shorter adsorbates to cover the surface allows kinetic measurement via nanografting.
4. CONCLUSION Adsorption of organophosphonic acids on single-crystalline alumina surfaces prepared under ambient conditions could be analyzed by means of nanoshaving and nanografting. For the Al2O3 (0001) surface, heterogeneities on the terraces were shown with tapping and contact mode AFM. By means of ex situ adsorption and AFM-based nanoshaving measurements, it 6925
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(7) Jaehne, E.; Oberoi, S.; Adler, H. J. P. Ultra thin layers as new concepts for corrosion inhibition and adhesion promotion. Prog. Org. Coat. 2008, 61 (2−4), 211−223. (8) Phung, L. H.; Kleinert, H.; Fussel, U.; Duc, L. M.; Rammelt, U.; Plieth, W. Influence of self-assembling adhesion promoter on the properties of the epoxy/aluminium interphase. Int. J. Adhes. Adhes. 2005, 25 (3), 239−245. (9) Lim, M. S.; Feng, K.; Chen, X. Q.; Wu, N. Q.; Raman, A.; Nightingale, J.; Gawalt, E. S.; Korakakis, D.; Hornak, L. A.; Timperman, A. T. Adsorption and desorption of stearic acid selfassembled monolayers on aluminum oxide. Langmuir 2007, 23 (5), 2444−2452. (10) Giza, M.; Thissen, P.; Grundmeier, G. Adsorption kinetics of organophosphonic acids on plasma-modified oxide-covered aluminum surfaces. Langmuir 2008, 24 (16), 8688−8694. (11) Dartevelle, C.; McAlpine, E.; Thompson, G. E.; Alexander, M. R. Low pressure plasma treatment for improving the strength and durability of adhesively bonded aluminium joints. Surf. Coat. Technol. 2003, 173 (2−3), 249−258. (12) Van Alsten, J. G. Self-assembled monolayers on engineering metals: Structure, derivatization, and utility. Langmuir 1999, 15 (22), 7605−7614. (13) Ramsier, R. D.; Henriksen, P. N.; Gent, A. N. Adsorption of phosphorus acids on alumina. Surf. Sci. 1988, 203 (1−2), 72−88. (14) Allara, D. L.; Nuzzo, R. G. Spontaneously organized molecular assemblies. 1. Formation, dynamics, and physical properties of nalkanoic acids adsorbed from solution on oxidized aluminum surface. Langmuir 1985, 1 (1), 45−52. (15) Neves, B. R. A.; Salmon, M. E.; Russell, P. E.; Troghton, E. B. Spread coating of OPA on mica: From multilayers to self-assembled monolayers. Langmuir 2001, 17 (26), 8193−8198. (16) Liakos, I. L.; McAlpine, E.; Chen, X.; Newman, R.; Alexander, M. R. Assembly of octadecyl phosphonic acid on the α-Al2O3 (0001) surface of air annealed alumina: Evidence for termination dependent adsorption. Appl. Surf. Sci. 2008, 255 (5), 3276−3282. (17) Thissen, P.; Valtiner, M.; Grundmeier, G. Stability of phosphonic acid self-assembled monolayers on amorphous and single-crystalline aluminum oxide surfaces in aqueous solution. Langmuir 2010, 26 (1), 156−164. (18) Brukman, M. J.; Marco, G. O.; Dunbar, T. D.; Boardman, L. D.; Carpick, R. W. Nanotribological properties of alkanephosphonic acid self-assembled monolayers on aluminum oxide: Effects of fluorination and substrate crystallinity. Langmuir 2006, 22 (9), 3988−3998. (19) Messerschmidt, C.; Schwartz, D. K. Growth mechanisms of octadecylphosphonic acid self-assembled monolayers on sapphire (corundum): Evidence for a quasi-equilibrium triple point. Langmuir 2001, 17 (2), 462−467. (20) Xu, S.; Liu, G. Y. Nanometer-scale fabrication by simultaneous nanoshaving and molecular self-assembly. Langmuir 1997, 13 (2), 127−129. (21) Te Riet, J.; Smit, T.; Gerritsen, J. W.; Cambi, A.; Elemans, J. A. A. W.; Figdor, C. G.; Speller, S. Molecular friction as a tool to identify functionalized alkanethiols. Langmuir 2010, 26 (9), 6357−6366. (22) Yu, J. H.; Ngunjiri, J. N.; Kelley, A. T.; Gano, J. C. Nanografting versus solution self-assembly of α,ω-alkanedithiols on Au(111) investigated by AFM. Langmuir 2008, 24 (20), 11661−11668. (23) Liu, G. Y.; Xu, S.; Qian, Y. L. Nanofabrication of self-assembled monolayers using scanning probe lithography. Acc. Chem. Res. 2000, 33 (7), 457−466. (24) Xu, S.; Laibinis, P. E.; Liu, G. Y. Accelerating the kinetics of thiol self-assembly on goldA spatial confinement effect. J. Am. Chem. Soc. 1998, 120 (36), 9356−9361. (25) Isono, T.; Ikeda, T.; Aoki, R.; Yamazaki, K.; Ogino, T. Structural- and chemical-phase separation on single crystalline sapphire (0001) surfaces. Surf. Sci. 2010, 604 (21−22), 2055−2063. (26) Toofan, J.; Watson, P. R. The termination of the α-Al2O3 (0001) surface: A LEED crystallography determination. Surf. Sci. 1998, 401 (2), 162−172.
was demonstrated that a strong correlation exists between stable binding of phosphonic acids and the surface termination. Nanografting of phosphonic acids with different molecular termination and chain length has been performed for the first time on Al2O3 single crystals. Nanografting is shown to be a suitable experimental method not only to investigate adsorption of organic acids but also to study organization kinetics on oxide surfaces at the liquid−solid interface. As supported by static contact angle and DRIFTS measurements on HPA- and ODPAcovered single-crystalline surfaces, the low organization level of the preadsorbed short-chain organic acid facilitated the kinetic measurements of the SAM because of the insufficiency of spatial confinement between the AFM tip and the protective monolayer. Overall, the presented methodology can be extended to relevant single-crystalline oxide surfaces.
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ASSOCIATED CONTENT
S Supporting Information *
Static contact angle measurements of bare, HPA- and ODPAcovered Al2O3 single crystals (Figure S1), DRIFTS measurements of HPA- and ODPA-covered Al2O3 single crystals (Figure S2), and UHV−AFM measurements of bare Al2O3 crystal at elevated temperature (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the German Research Foundation (DFG) within the Transregional Collaborative Research Center TRR87/1 “Pulsed High Power Plasmas for the Synthesis of Nanostructured Functional Layers” (SFB-TR 87) and within the “Schwerpunktprogramm Partikel im Kontakt” (DFG-SPP 1486).
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