pubs.acs.org/Langmuir © 2009 American Chemical Society
Photopatterning, Etching, and Derivatization of Self-Assembled Monolayers of Phosphonic Acids on the Native Oxide of Titanium Getachew Tizazu,† Ali M. Adawi,‡ Graham J. Leggett,*,† and David G. Lidzey‡ †
Department of Chemistry, University of Sheffield, Brook Hill, Sheffield S3 7HF, U.K., and Department of Physics and Astronomy, University of Sheffield, Hounsfield Road, Sheffield S3 7RH, U.K.
‡
Received April 9, 2009. Revised Manuscript Received May 22, 2009 Electron-hole pair formation at titania surfaces leads to the formation of reactive species that degrade organic materials. Here, we describe the degradation of self-assembled monolayers of alkylphosphonic acids on the native oxide of titanium following exposure to UV light. The rate of degradation was found to decrease as the length of the adsorbate molecule increased. Increasing order in the monolayer, resulting from the enhanced dispersion forces between longer adsorbates, impedes the progress of oxygen-containing molecules to the oxide surface and slows the rate of oxidation. Rates of degradation on titanium oxide are substantially greater than rates of degradation on aluminum oxide because of the photocatalytic effect of the titanium oxide substrate. Micrometer-scale patterns may be fabricated readily using a UV laser in conjunction with a mask, and nanometer-scale patterns may be fabricated using a scanning near-field optical microscope coupled to a UV laser. Photodegraded adsorbates may be replaced by contrasting molecules to yield chemical contrast. Such patterned materials have been utilized to fabricate patterns from polymer nanoparticles. The resist character is switchable; at lower exposures, the monolayer behaves as a positive tone resist, but at higher exposures, it exhibits negative tone behavior. Patterned samples may also be utilized as resists for solution-phase etching of the underlying substrate.
Introduction The modification and control of metal and metal oxide surfaces by the formation of self-assembled monolayers (SAMs) is an active field of research.1,2 SAMs are attractive because of the low cost associated with their preparation, the availability of molecules with a diverse range of functional groups, and their stability in a variety of chemical environments.2 Monolayer preparation is usually carried out by immersion of a clean substrate in a freshly prepared solution.3 Because self-assembled monolayers are monomolecular films, they can be easily tailored to promote or resist the adhesion of specific substances.4-8 These tailored selfassembled monolayers have a variety of applications. They can be *Corresponding author. E-mail:
[email protected].
(1) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (2) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103. (3) Ulman, A. Chem. Rev. 1996, 96, 1533. (4) Pale-Grosdemange, C.; Simon, E. S.; Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 12. (5) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164. (6) Lopez, G. P.; Biebuyck, H. A.; Haerter, R.; Kumar, A.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 10774. (7) Singhvi, R.; Kumar, A.; Lopez, G. P.; Stephanopoulos, G. N.; Wang, D. I. C.; Whitesides, G. M.; Ingber, D. E. Science 1994, 264, 696. (8) Ostuni, E.; Chapman, R. G.; Holmlin, E. R.; Takayama, S.; Whitesides, G. M. Langmuir 2001, 17, 5605. (9) Ducker, R. E.; Leggett, G. J. J. Am. Chem. Soc. 2006, 128, 392. (10) Sun, S.; Montague, M.; Critchley, K.; Chen, M.-S.; Dressick, W. J.; Evans, S. D.; Leggett, G. J. Nano Lett. 2006, 6, 29. (11) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S.; Mirkin, C. A. Science 1999, 283, 661. (12) Amro, N. A.; Xu, S.; Liu, G.-Y. Langmuir 2000, 16, 3006. (13) Demers, L.; Ginger, D. S.; Park, S.-J.; Li, Z.; Chung, S.-W.; Mirkin, C. A. Science 2002, 296, 1836. (14) Lee, K.-B.; Park, S.-J.; Mirkin, C. A.; Smith, J. C.; Mrksich, M. Science 2002, 295, 1702. (15) Liu, G.-Y.; Amro, N. A. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5165. (16) Sun, S.; Chong, K. S. L.; Leggett, G. J. J. Am. Chem. Soc. 2002, 124, 2414. (17) Hyun, J.; Ahn, S. J.; Lee, W. K.; Chilkoti, A.; Zauscher, S. Nano Lett. 2002, 2, 1203. (18) Cheung, C. L.; Camarero, J. A.; Woods, B. W.; Lin, T.; Johnson, J. E.; De Yoreo, J. J. J. Am. Chem. Soc. 2003, 125, 6848. (19) Rosi, N. L.; Mirkin, C. A. Chem. Rev. 2005, 105, 1547.
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used as micro- and nanolithography resists,2,9-19 biosensors,20 to create bioactive surfaces,21 to enhance the efficiency of solar cells,22 for the fabrication of photonic devices,23 as lubricants,24 and to inhibit corrosion.25 There has been a great deal of interest in the formation of patterned structures on SAMs.2,26,27 Although there is extensive literature on the patterning of thiol SAMs on gold and silver,28 there has been much less work on patterning on oxide surfaces, with the one significant exception of monolayers of siloxanes on silicon dioxide surfaces.10,29-35 Phosphonic acids form closepacked, highly ordered monolayers on metal oxide substrates by the reaction of the phosphonate group with the substrate.36-40 (20) Froebort, I.; Skoupa, L.; Pec, P. Food Control 2000, 11, 12. (21) Adden, N.; Gamble, L. J.; Castner, D. G.; Hofmann, A.; Gross, G.; Menzel, H. Langmuir 2006, 22, 8197. (22) Kim, Y.; Sung, Y.; Xia, J.; Lira-Cantu, M.; Masaki, N.; Yanagida, S. J. Photochem. Photobiol. A 2008, 193, 77. (23) Vossmeyer, T.; Jia, S.; DeIonno, E.; Diehl, M. R.; Kim, S. H.; Peng, X.; Alivisatos, A. P.; Heath, J. R. J. Appl. Phys. 1998, 84, 3664. (24) Brukman, M. J.; Marco, G. O.; Dunbar, T. D.; Boardman, L. D.; Carpick, R. W. Langmuir 2006, 22, 3988. (25) Liakos, I. L.; Newman, R. C.; McAlpine, E. O.; Alexander, M. R. Langmuir 2007, 23, 995. (26) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550. (27) Gates, B. D.; Xu, Q.; Stewart, M.; Ryan, D.; Willson, C. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1171. (28) Zharnikov, M.; Grunze, M. J. Vac. Sci. Technol., B 2002, 20, 1793. (29) Maoz, R.; Frydman, E.; Cohen, S. R.; Sagiv, J. Adv. Mater. 2000, 12, 725. (30) Maoz, R.; Frydman, E.; Cohen, S. R.; Sagiv, J. Adv. Mater. 2000, 12, 424. (31) Hoeppener, S.; Maoz, R.; Sagiv, J. Nano Lett. 2003, 3, 761. (32) Jeon, N. L.; Finnie, K.; Branshaw, K.; Nuzzo, R. G. Langmuir 1997, 13, 3382. (33) Finnie, K. R.; Haasch, R.; Nuzzo, R. G. Langmuir 2000, 16, 6968. (34) Pallandre, A.; Glinel, K.; Jonas, A. M.; Nysten, B. Nano Lett. 2004, 4, 365. (35) Gabriel, G. B.; Antoine, P.; Bernard, N.; Alain, M. J. Nanotechnology 2006, 17, 1160. (36) Sun, S.; Leggett, G. J. Nano Lett. 2007, 7, 3753. (37) van Alsten, J. G. Langmuir 1999, 15, 7605. (38) Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, L. Langmuir 1996, 12, 6429. (39) Viornery, C.; Chevolot, Y.; Leonar, D.; Aronsson, B.; Pechy, P.; Mathieu, H. J.; Descouts, P.; Graetzel, M. Langmuir 2002, 18, 2582. (40) Tosatti, S.; Michel, R.; Textor, M.; Spencer, N. D. Langmuir 2002, 18, 3537.
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These monolayers show high stability to mechanical wear,40 to a variety of chemical environments,25 and to a temperature of ca. 90 °C.41,42 Thus, they offer many attractive features for application in a variety of areas. There has previously been some interest in the patterning of alkylphosphonic acid (APA) SAMs on oxide surfaces. Predominantly, effort has focused on monolayers on aluminum oxide. For example, Goetting et al. formed patterns of APA on alumina by microcontact printing,43 whereas Gadegaard et al. have recently reported the use of electron beam lithography to fabricate structures with a resolution of ca. 40 nm.44 In the authors’ laboratory, photochemical methods have been used to pattern APA SAMs on aluminum oxide. It was found that, by exposing monolayers to UV light (λ = 244 nm) through a mask, it was possible to fabricate structures that could be used as resists for solution-phase etching or as templates for the formation of chemical patterns.36 Using a scanning near-field optical microscope coupled to a UV laser (scanning near-field photolithography (SNP)),16,45,46 it was possible to fabricate 100 nm features. In the present work, we describe the use of APA on titanium oxide surfaces for the fabrication of micrometer- and nanometer-scale structures. Titanium oxide was chosen as a substrate because of its potential use in the fabrication of photonic devices.47-50 The photochemical modification of organic films and SAMs on titanium oxide has been reported previously by other workers.51-54 The photocatalytic properties of titania have been reported to yield degradation of organic materials following exposure to UV light,53 making it a potentially valuable tool for the fabrication of patterns,53,54 gradients,55 and other structures. Here, we report a comparative study of the rates of photodegradation of SAMs formed from APA of different lengths and describe the potential for pattern formation by replacing degraded adsorbates with contrasting molecules. The potential for fabrication of nanopatterns by using SNP has been explored, and the functionalization and etching of patterned materials have been investigated.
Experimental Section Titanium (99.9%) was obtained from Goodfellow (Cambridge, U.K.). Methylphosphonic acid, ethylphosphonic acid (EPA), aminobutylphosphonic acid (ABPA), and 2-(Nmorpholino) ethanesulfonic acid (MES) were purchased from Sigma-Aldrich (Poole, U.K.). Butylphosphonic acid (BPA), octylphosphonic acid (OPA), decylphosphonic acid (DPA), octadecylphosphonic acid (ODPA), potassium hydroxide (KOH), and sodium chloride (NaCl) were obtained from Alfa Aesar (Heysham, U.K.). Ethanol (HPLC grade) was obtained from Fisher Scientific Ltd. (Loughborough, U.K.). Aldehyde-modified (41) Neves, R. A.; Salmon, M. E.; Russell, P. E.; Troughton, E. B. Langmuir 2000, 16, 2409. (42) Neves, R. A.; Troughton, E. B.; Russell, P. E. Nanotechnology 2001, 12, 285. (43) Goetting, L. B.; Deng, T.; Whitesides, G. M. Langmuir 1999, 15, 1182. (44) Gadegaard, N.; Chen, X.; Rutten, F. J. M.; Alexander, M. R. Langmuir 2008, 24, 2057. (45) Sun, S.; Leggett, G. J. Nano Lett. 2004, 4, 1381. (46) Leggett, G. J. Chem. Soc. Rev. 2006, 35, 1150. (47) Wang, X.; Fujimaki, M.; Awazu, K. Opt. Express 2005, 13, 1486. (48) Wijnhoven, J. E. G. J.; Vos, W. L. Science 1998, 281, 802. (49) Adawi, A. M.; Chalcraft, A. R. A.; Whittaker, D. M.; Lidzey, D. G. Opt. Express 2007, 15, 14299. (50) Li, B.; Cai, X.; Zhang, Y. Appl. Phys. Lett. 2006, 89, 031103. (51) Haick, H.; Paz, Y. J. Phys. Chem. B 2001, 105, 3045. (52) Haick, H.; Paz, Y. ChemPhysChem. 2003, 4, 617. (53) Avnon, E.; Paz, Y.; Tessler, N. Appl. Phys. Lett. 2009, 94, 013502. (54) Dibbell, R. S.; Soja, G. S.; Hoth, R. M.; Watson, D. F. Langmuir 2007, 23, 3432. (55) Blondiaux, N.; Zurcher, S.; Liley, M.; Spencer, N. D. Langmuir 2007, 23, 3489.
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polystyrene microspheres (4% solid) were purchased from Invitrogen Ltd. (Paisley, U.K.). The glass slides and the glass vials were thoroughly cleaned by soaking them in piranha solution (30% hydrogen peroxide solution and concentrated sulfuric acid in the ratio of 3:7) for 1 h, rinsing them seven times with DI water, and drying them in an oven. Substrates were prepared by evaporating 20-30 nm of titanium or aluminum onto glass slides at a rate of 0.05 nm s-1. Following deposition, the evaporator was allowed to cool before venting to dry nitrogen. The slides were then exposed to the laboratory atmosphere for 20 min, allowing formation of the native oxide film and surface hydroxylation (a necessary precursor to SAM formation).56 Self-assembled monolayers (SAMs) were formed by immersion of the substrates in a 1 mM ethanolic solution of the appropriate phosphonic acid for 48 h. Refunctionalization of UV-modified ODPA monolayers was carried out by immersion of the samples in a 0.1 mM aqueous solution of aminobutylphosphonic acid for 30 min. Aldehyde nanoparticle attachment was carried out by immersing amine-functionalized samples in a 0.1 M 2-(N-morpholino) ethanesulfonic acid (MES) buffer of pH 6.1 that contains 10 μL of the nanoparticle (aldehyde-modified polystyrene nanospheres, diameter 200 nm, Invitrogen) for 30 min. Etching was carried out by immersion of photopatterned samples in solutions of either potassium hydroxide (0.04 g mL-1) in deionized water or piranha solution that had cooled for 20 min. UV irradiation was carried out using two sources. A hand-held UV lamp (R-52G, UV Products, Cambridge, U.K.) with a low intensity (0.5 mW cm-2) was used for a large part of the work described here. Additionally, a Coherent Innova FreD 300C frequency-doubled argon ion laser, emitting at 244 nm with a maximum power of 100 mW, was also used. To expand the illumination area, the UV light from the laser source was passed through diverging and converging lenses before interacting with the sample. The area illuminated following beam expansion was 0.8 cm-2. During patterning, copper electron microscope grids (Agar Scientific, Cambridge, U.K.) were used as masks. Nanopatterning was carried out using a ThermoMicroscopes Aurora II scanning near-field optical microscope (Veeco U.K. Ltd., Cambridge, U.K.). The Aurora utilizes shear-force feedback to control the fiber motions via a tuning fork attached to the fiber. Fused silica probes suitable for UV applications were obtained from Jasco (Great Dunmow, U.K.). Sessile drop static water contact angle measurements were made using a Rame-Hart model 100-00 contact angle goniometer. Deionized water was used. Measurements were made at five different points on the surface of the sample and then averaged. Contact mode and tapping mode AFM images were taken by using Nanoscope III and Nanoscope IV multimode atomic force microscopes (Digital Instruments, Santa Barbara, CA). Triangular silicon nitride probes (Veeco) with force constants of 0.38 N m-1 were used for contact mode measurements. Silicon cantilevers with force constants of 58 N m-1 were used for tapping mode measurements. All images were acquired in ambient conditions. All the samples were thoroughly cleaned by rinsing in appropriate solvents and blown dry by a stream of nitrogen gas prior to imaging to avoid contamination. X-ray photoelectron spectroscopy (XPS) was carried out using a Kratos Axis Ultra DLD X-ray photoelectron spectrometer equipped with a monochromatic Al KR X-ray source in an ultrahigh-vacuum environment. Survey scans were acquired at a pass (56) Pertrays, K.; Thompson, G. E.; Alexander, M. R. Surf. Interface Anal. 2004, 36, 1361.
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energy of 160 eV, and the high-resolution spectra were acquired at a pass energy of 20 eV. During studies of monolayer photodegradation, samples were prepared for XPS analysis using the UV lamp because it had a wide area of illumination and yielded uniform illumination over the XPS sampling area. The data were analyzed using Casa software (Casa, http://www.casaxps. com, U.K.).
Results and Discussion Monolayer Characterization. Sessile drop water contact angles were measured for a range of alkylphosphonic acid (APA) SAMs and are shown as a function of the alkyl chain length, n, in Figure 1a. The contact angle increased with n, from n = 1 to 10; whereafter, it remained approximately constant, in agreement with previously published studies. As the chain length increases, the increasing dispersion force arising from interactions between multiple CH2 groups leads to the adoption of an ordered all-trans configuration that leads to low chain mobility. This produces highly close-packed hydrophobic SAMS for long chains. Thus, adsorbates with short alkyl chains form liquid-like structures, and longer adsorbates form highly ordered crystalline structures. The adoption of this limiting configuration occurs at slightly shorter chain lengths for APA than for SAMs of alkanethiols on Au because of their closer packing. Contact mode AFM images of APA SAMs on titanium oxide indicated, as expected, that the materials possessed smooth, uniform morphologies (Figure 1c). XPS survey scans revealed the presence of only carbon, oxygen, titanium, and phosphorus, again, in agreement with expectations and with the work of other authors, confirming the absence of contamination from the surface. The stability of the SAMs in water was characterized because it was intended to utilize aqueous surface treatments for surface functionalization. Samples were immersed in water, and the contact angle was measured at 40 min intervals (see Figure 1b). For ethyl-, butyl-, and decylphosphonic acid, there was a rapid change in the contact angle with time. The contact angle of an ethylphosphonic acid monolayer fell to nearly 20° after 40 min, but even for decylphosphonic acid, there was a reduction in the contact angle of nearly 30° in the same period. However, the change in contact angle for adsorbates with longer alkyl chains was smaller. ODPA monolayers were particularly stable, with little change being observed after times in excess of 3 h. The SAM stability is clearly influenced by the packing density: the probability of water molecules penetrating between the molecules of the monolayer decreases as the adsorbate chain length increases. Consequently, short chains may not be suitable for application in an aqueous solution. Photooxidation. SAMs of APA on titanium oxide were exposed to UV light (λ=254 nm) from a hand-held lamp with a low intensity (0.5 mW cm-2) and the contact angles measured. Measurements were additionally made for SAMs of decylphosphonic acid on aluminum oxide. The data are shown in Figure 2a. It may be seen that, even for an exposure as short as 10 min (corresponding to 300 mJ cm-2), a significant fall was observed in the contact angles of SAMs of octyl- and decylphosphonic acids on titanium oxide. A rapid change was also seen in the contact angles of ODPA monolayers on titanium oxide, although the rate of change was significantly slower than that for the short-chain monolayers, likely because the longer adsorbates are closer-packed, reducing the rate of diffusion of oxygencontaining species through the film and, hence, providing an increased steric barrier to degradation. Similar influences have been reported elsewhere for alkanethiol monolayers on gold 10748 DOI: 10.1021/la901271c
Figure 1. (a) Variation in the water contact angles of APA SAMs on titanium oxide with the number of carbon atoms, n, in the alkyl chain. (b) The variation in the contact angle with the immersion time in water for SAMs of ethyl-, butyl-, decyl-, and octadecylphosphonic acids on the native oxide of titanium. (c) Contact mode image of an ODPA SAM on titanium oxide: image size, 1 1 μm2; z-scale range, 14.02 nm; rms roughness, 0.64 nm.
and silver,57-59 where the rate-limiting step has been reported to be diffusion of oxygen species through the monolayer to the headgroup-substrate bond.57 After 40 min (1.2 J cm-2), the photodegradation was complete for DPA and OPA and nearly complete for ODPA. (57) Hutt, D. A.; Leggett, G. J. J. Phys. Chem. 1996, 100, 6657. (58) Hutt, D. A.; Cooper, E.; Leggett, G. J. J. Phys. Chem. B 1998, 102, 174–184. (59) Brewer, N. J.; Janusz, S. J.; Critchley, K.; Evans, S. D.; Leggett, G. J. J. Phys. Chem. B 2005, 109, 11247.
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Figure 2. Contact angle as a function of exposure time: (a) for SAMs on titanium oxide and aluminum oxide following illumination by a UV lamp with a power of 0.5 mW cm-2 (λ = 254 nm) and (b) for a DPA SAM on titanium oxide exposed to a UV laser (λ = 244 nm) with a power of 82.5 mW cm-2.
In contrast, the change in the contact angle of the monolayer of DPA on aluminum oxide was negligible even after 40 min. The photooxidation of APA SAMs on aluminum oxide has been reported previously. On the basis of XPS analysis, it was suggested that the photodegradation of phosphonic acid monolayers on aluminum oxide occurs primarily via cleavage of the P-C bond when exposed to UV light.36 The magnitude of the difference in the behavior observed here on titanium oxide films; where even ODPA monolayers degrade significantly faster than the much shorter DPA monolayers do on aluminum oxide; suggests that a different mechanism must operate on titanium oxide. The most likely explanation is that the decomposition of monolayers formed onto titanium oxide is due to the photocatalytic effect of the oxide, which causes a substantial acceleration of the degradation rate. When titania is irradiated with a photon of wavelength less than or equal to 388 nm, electron-hole pairs are produced and migrated to the surface. The holes are powerful oxidants and electrons are good reductants, which degrade any adsorbates rapidly and completely.60,61 This photocatalytic property makes titanium oxide a good substrate on which to photopattern monolayers using weak UV light. To test this further, monolayers formed on the native oxides of titanium and aluminum were exposed to UV light with a wavelength of 325 nm. Monolayers on titanium oxide were oxidized, but monolayers on aluminum oxide were not oxidized. Figure 2b shows the very rapid modification that may be achieved with a more powerful light source, a frequency(60) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (61) Fujishima, A.; Zhang, X. C. R. Chim. 2006, 9, 750.
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doubled argon ion laser. An ODPA monolayer on titanium oxide was exposed to irradiation at 244 nm and at a power of 66 mW or 82.5 mW cm-2. The change in contact angle when the monolayer was exposed to high power was large following just a 2 min exposure (9.9 J cm-2), suggesting that complete removal of the monolayer has occurred. The effects of SAM photodegradation on titanium oxide were analyzed by XPS. Figure 3a,b shows the C 1s and P 2p spectra of ODPA as a function of exposure to light from the UV lamp. As the exposure increased, the C 1s peak area decreased, whereas the count rate from phosphorus increased slightly. This is because the carbon backbones of the ODPA molecules are decomposed following UV exposure. A further consequence of this degradation is that there is a reduced amount of hydrocarbon material to cause attenuation of the phosphorus photoelectrons, leading to the small increase in the strength of the P 2p signal. The C 1s spectrum exhibited asymmetry after photodegradation, as illustrated in Figure 3c. The asymmetry was ascribed to the presence of oxygen-containing functional groups at the surface. It is possible that these were produced by oxidative breakdown of the alkyl chains, although the creation of a bare titanium oxide surface would yield a high surface free energy, which would be expected to lead to rapid adsorption of contamination from the laboratory atmosphere. Very similar spectra have been observed for other “clean”, high-energy surfaces and have been attributed to adventitious contamination; the contaminant molecules typically contain carbon bonded to oxygen.25 The feasibility of adsorbing a replacement monolayer onto a photodegraded surface was investigated. An ODPA monolayer was formed on titanium oxide. The contact angle was measured and found to be 110° (see (i) in Figure 4a). The corresponding C 1s spectrum exhibited a single main component at 285 eV (Figure 4b). After exposure to light from the UV lamp for 80 min (2.4 J cm-2), the contact angle of the monolayer was measured again and found to be 5° ((ii) in Figure 4a), corresponding to a fully oxidized monolayer. The C 1s spectrum was consistent with this. The area of the C 1s peak was much reduced and contained a long tail on the high binding energy side, indicative of the presence of oxidized carbon atoms. This fully oxidized monolayer was immersed again into the original solution. After 18 h, a contact angle of 107° was measured (see (iv) in Figure 4a). The C 1s spectrum was indistinguishable from that of the freshly prepared monolayer. Together, these observations provide clear evidence that there was a complete recovery of the original SAM. From the contact angle and XPS measurements, it is evident that phosphonic acid monolayers removed by photodegradation can be replaced by simply immersing the samples in the original solution. It is clear from Figure 4c that the phosphorus from the headgroup is not removed during the photooxidation process. At (iv) (Figure 4c), the area of the P 2p peak is ca. 2 times that at (i). The subsequent reformation of the APA monolayer thus occurs on an oxide surface covered with a phosphonate monolayer. From the data above, it is clear that the formation of a good-quality monolayer, indistinguishable by contact angle analysis from the virgin film, is not impeded by the presence of the phosphonate groups at the surface. The importance of this property of phosphonic acid monolayers is that it is possible to activate the surface selectively by exposing samples to UV light and immersing them into a solution that contains an adsorbate with a different end group. To further explore this possibility, the feasibility of introducing amine groups by adsorbing aminobutylphosphonic acid (ABPA) to photodegraded materials was examined. Three different DOI: 10.1021/la901271c
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Figure 3. Variation in (a) the C 1s region and (b) the P 2p region with UV exposure for ODPA on the native oxide of titanium. (c) A high-resolution C 1s spectrum acquired for ODPA on titanium oxide after a 45 min exposure (1.4 J cm-2).
Figure 4. (a) Variation in the contact angle during a cycle of formation (i), UV exposure for 80 min (2.4 J cm-2) of a SAM of ODPA on titanium oxide (ii), and subsequent reimmersion in an ODPA solution (iii, iv). (b) High-resolution C 1s spectra and (c) P 2p spectra corresponding to stages in the process in (a).
samples, pure TiO2 film, ABPA/TiO2, and ODPA/TiO2 exposed to UV light and, subsequently, immersed into an aqueous solution of ABPA, were analyzed by XPS. Figure 5 shows the N 1s spectra of the three samples. Figure 5a shows the N 1s spectrum of the ABPA monolayer. Figure 5b shows the N 1s region of the XPS spectrum of the bare titanium oxide surface, indicating that there is no nitrogen present at the surface. Figure 5c shows the corresponding region of the N 1s spectrum of the photodegraded ODPA sample following 30 min immersion in an aqueous solution of ABPA. It is clear that the N 1s peak has a very similar area to that acquired for the ABPA monolayer, suggesting that the ODPA monolayer has been replaced by a monolayer of
ABPA. A 30 min immersion time was used for the ABPA immersion step because of the limited stability of the APA monolayers in water (Figure 1b). However, Figure 5 clearly shows that this was an adequate time period to enable adsorption to occur with high coverage. Micrometer- and Nanometer-Scale Photopatterning. Light from a frequency-doubled argon ion laser (244 nm) was used to photopattern ODPA monolayers on titanium oxide. The samples were exposed to the UV light at a power of 50 mW for 2 min (7.5 J cm-2), using an electron microscope grid as a simple mask. Figure 6a shows a friction force microscopy (FFM) image of the photopatterned monolayer directly following exposure to
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Figure 5. Comparison of high-resolution sans the N 1s region for (a) an ABPA self-assembled monolayer (SAM) on titanium oxide, (b) bare oxide, and (c) an ODPA sample exposed to UV light and, subsequently, immersed in an aqueous ABPA solution.
UV light. The exposed regions (squares) exhibit bright contrast, attributed to the high surface free energy of the oxide surface exposed by photodegradation of the adsorbate molecules, whereas the mask regions (bars) exhibit dark contrast because the hydrophobic alkyl termini of the intact adsorbate molecules confer a low surface free energy. In the FFM experiment, bright contrast (high friction) is associated with a high rate of energy dissipation. Here, differences in adhesion yield differences in the rates of energy dissipation in different areas,62,63 with the polar material at the surface interacting adhesively with the polar surface of the probe, in contrast to the hydrophobic regions, which adhere much less strongly to the probe.64 Figure 6b shows an FFM image of a similar pattern following subsequent immersion in an aqueous solution of aminobutylphosphonic acid. In the exposed regions (squares), the photodegraded adsorbates have been replaced by the more polar aminobutyl molecules, whereas the unexposed parts (bars) retained the original hydrophobic adsorbate molecules. The amine groups yield brighter contrast than that of the ODPA molecules, as expected, because they interact more adhesively with the probe, but the contrast difference is less marked than that in Figure 6a because the surface free energy of the amine-terminated regions (θa ∼ 50°) is significantly lower than that of the clean oxide. To facilitate higher spatial resolution, patterning was also carried out using a scanning near-field optical microscope coupled to the UV laser, an approach we call scanning near-field photolithography (SNP). Figure 6c shows an FFM image of a pattern formed by tracing the near-field probe (a tapered optical fiber attached to a tuning fork, operating in shear-force mode) across an ODPA/TiO2 monolayer. In exposed regions, the adsorbate has been photodegraded, exposing the high surface energy oxide and yielding bright contrast. The patterning was carried out using a laser power of 5 mW (i.e., the power before coupling to the optical fiber, not after; measurement of the power at the tip is not straightforward), and the writing speed was 0.4 μm s-1, similar to the writing speed used in previous studies of nearfield patterning of alkanethiol SAMs. In the present case, a probe (62) Carpick, R. W.; Salmeron, M. Chem. Rev. 1997, 97, 1163. (63) Gnecco, E.; Bennewitz, R.; Gyalog, T.; Meyer, E. J. Phys.: Condens. Matter 2001, 13, R619. (64) Leggett, G. J.; Brewer, N. J.; Chong, K. C. Phys. Chem. Chem. Phys. 2005, 7, 1107.
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Figure 6. (a) 113 113 μm2 FFM image of an ODPA SAM on the native oxide of titanium following exposure through a mask (600 mesh) at 50 mW for 2 min (7.5 J cm-2): z-scale range, 0-517 mV. (b) 150 150 μm2 FFM image of a similar photopatterned sample following backfilling of the exposed regions with ABPA: z-scale range, 0-4.00 V. (c) 20 20 μm2 FFM image of a pattern produced in an ODPA monolayer on titanium oxide using scanning nearfield photolithography: z-scale range, 0-1.00 V.
with a comparatively broad aperture was employed. Thus, the features are some 285 nm wide. However, the potential for nanometer scale patterning of films on titanium oxide using this approach is very clear. Attachment of Aldehyde-Functionalized Nanoparticles to Photopatterned ABPA and L-Lysine. To test for the retention of functionality in masked regions of the sample, monolayers of ABPA were patterned by exposure to light from the UV laser through a mask and then immersed in a solution of an aldehydefunctionalized nanoparticle (polystyrene latex, diamater=100 nm). The nanoparticle was coupled to amine groups at the surface through the formation of an imine bond. Figure 7a shows an AFM topographical image of the resulting structure. The masked regions (bars) exhibit bright contrast because, here, the amine groups are protected and remain at the surface after patterning for coupling to the nanoparticles. In the exposed regions, however, the adsorbates have been degraded, leading to removal of the amine-functionalized molecules and, thus, no nanoparticles are bound to the surface in those regions. Analysis of a cross section through the image yielded a step height of 97 nm, consistent with the formation of a covalently attached monolayer of particles in DOI: 10.1021/la901271c
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Figure 7. (a) AFM topographical image of an ABPA SAM on titanium oxide following exposure to light from a UV laser through a mask and immersion in a solution of aldehyde-functionalized nanoparticle: image size, 100 100 μm2; z-scale range, 0-200 nm. (b) AFM topographical image of an L-lysine film on titanium oxide following UV patterning and coupling with aldehyde-functionalized nanoparticles (diameter = 42 nm): image size, 80 80 μm2; z-scale range, 0-100 nm. (c) AFM topographical image of an ODPA SAM on titanium oxide, following photopatterning, replacement of degraded adsorbates using ABPA, and attachment of aldehyde-functionalized polymer nanoparticles: image size, 100 100 μm2; z-scale range, 0-200 nm. (d) Fluorescence microscopy image of a similar sample to that shown in Figure 7c but where the polymer nanoparticle has been loaded with a fluorescent dye.
the masked regions. Similar results could also be achieved using films of lysine. Figure 7b shows a lysine film following patterning in the same way described for the sample shown in Figure 7a and subsequent derivatization with aldehyde-functionalized polymer nanoparticles (in this case, with a diameter of 42 nm). ODPA monolayers were also photopatterned, and the exposed regions refunctionalized using aldehyde-functionalized nanoparticles, yielding a pattern (see Figure 7c) that was the inverse of that shown in Figure 7a. In this case, a nanoparticle (diameter = 160 nm) that was loaded with a fluorescent dye (fluorescein isothiocyanate (FITC)) was utilized, enabling characterization by fluorescence microscopy (Figure 7d), yielding further evidence for the selectivity of nanoparticle attachment. Switchable Resist Character. The use of high exposures led to the observation of rather different behavior. For ODPA SAMs, treated following exposure by immersion in a solution of ABPA and then derivatized with aldehyde-functionalized nanoparticles, an inversion of contrast was observed, in which the character of the ODPA resist appeared to change from positive to negative tone. Figure 8 shows a series of samples exposed to increasing doses of UV light at this higher irradiation power (100 mW). The transition from positive to negative tone behavior began to be observed through the formation of pronounced edge features around the boundary between the masked and exposed areas (Figure 8a). As exposure increased further, the widths of these edge features increased, and at an exposure of 13.5 J cm-2, the extent of modification around the edges began to appear significantly greater than was observed in the exposed regions (Figure 8b). Eventually, at an exposure of 33.8 J cm-2, the inversion became complete when the masked areas (bars) yielded bright contrast (i.e., indicative of nanoparticle functionalization), 10752 DOI: 10.1021/la901271c
Figure 8. 100 100 μm2 AFM topographical images of patterns formed by exposure of ODPA SAMs on titanium oxide to UV light, immersion in ABPA solutions, and derivatization with aldehyde-functionalized nanoparticles. The exposures are (a) 7.5, (b) 13.5, and (c) 33.8 J cm-2. The z-scale range is 0-200 nm in all images.
presumably following removal of the ODPA and its replacement by ABPA. A very similar kind of behavior was reported previously in studies of photopatterning of APA monolayers on aluminum oxide films. It was suggested that remodeling of the oxide surface may occur at long exposures, rendering it less reactive.36 Hence, at high exposures, the “exposed” regions have become passivated following removal of the SAM. A similar process may occur during UV exposure of SAMs on titianium oxide. The possibility that UV exposure causes thickening of the oxide layer can be discounted because patterned exposure does not yield differential topographical contrast between masked and exposed areas when they are imaged by AFM. Rather, it is more likely that the remodeling takes the form of a transformation of the structure in exposed areas to one that is less reactive (perhaps via some form of annealing). Unfortunately, detailed analysis of O 1s spectra did not provide any insights into the nature of any such transformation. Additional evidence for the switchable nature of APA resists on titanium oxide surfaces was provided by wet etching experiments. When photopatterned SAMs of APA on titanium oxide were exposed to solutions of piranha solution (see Figure 9a), the unexposed regions (bars) remained intact, whereas Langmuir 2009, 25(18), 10746–10753
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nation of APA photochemistry with surface functionalization and/or etching represents a flexible route to patterned, functionalized oxide surfaces.
Figure 9. 100 100 μm2 AFM topographical images showing microstructures produced by etching of photopatterned ODPA monolayers on titanium oxide using (a) piranha solution and (b) potassium hydroxide: z-scale range, 0-100 nm.
the photodegraded regions (squares) were etched. However, when potassium hydroxide was used as the etchant, the opposite was observed: when patterned samples were immersed in a solution of potassium hydroxide, the masked areas were found to be etched and the exposed areas were found to remain intact (Figure 9b). This inverted etching behavior was also observed in the case of ODPA SAMs on aluminum oxide subjected to large UV exposures. However, in the case of monolayers on titanium oxide, the same behavior was observed at all exposures;that is, no inversion in behavior was observed as a function of exposure during the etching experiments. Although a detailed explanation for these observations cannot be provided at present, it is clear that the chemistries of the oxide surfaces are very different in exposed and unexposed areas. It is also evident that careful choice of reagents and processing sequence enables a wide range of behaviors and morphologies to be accessed to order, suggesting that a combi-
Langmuir 2009, 25(18), 10746–10753
Conclusions Self-assembled monolayers of alkylphosphonic acids on titanium oxide substrates may be quickly and conveniently patterned using simple photochemical methods. Exposure to light with a wavelength of ca. 250 nm causes degradation of the adsorbate alkyl chain, probably in a photocatalytic process. The degradation process is much faster than is the case for monolayers of the same adsorbates on aluminum oxide surfaces. The oxide surface is exposed by this process and may readily be refunctionalized with a second, contrasting phosphonic acid, providing a simple, rapid route to a two-component patterned surface when the exposure is carried out through a mask. If a UV laser is coupled to a scanning near-field optical microscope, then patterning may be carried out with much higher resolution. Patterns may readily be functionalized, for example, by aldehyde-functionalized polymer nanoparticles. When photopatterning is followed by chemical etching, there is an inversion of the expected contrast: the phosphonic acid monolayer behaves as a negative tone resist rather than a positive tone resist. Although a full explanation for this remains elusive, the most likely explanation is a remodeling of the structure of the titianium oxide surface following UV exposure, rendering it resistant to etching. Acknowledgment. The authors are grateful to EPSRC (Grant No. EP/D064767/1) for financial support.
DOI: 10.1021/la901271c
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