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Site-Selective Deposition of Gold on Photo-Patterned Self-Assembled Monolayers Yu-Chin Lin,† Bang-Ying Yu,† Wei-Chun Lin,† Ying-Yu Chen,† and Jing-Jong Shyue*,†,‡ Research Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan, Department of Materials Science and Engineering, National Taiwan UniVersity, Taipei 106, Taiwan ReceiVed August 20, 2008. ReVised Manuscript ReceiVed September 9, 2008
Siloxane-anchored self-assembled monolayers on glass were patterned by contact lithography using vacuum UV (193 nm) in air. Regionally separated OTS (octadecyltrichlorosilane) and APTES (3-aminopropyltriethoxysilane) were prepared by sequential deposition. The surface potential of OTS and APTES modified glass was determined with the streaming current between parallel sample plates separated by 100-150 µm. It was found that the isoelectric points (IEP) of OTS and APTES were pH 4.0 and 6.6, respectively. The gold thin film was selectively deposited from an aqueous solution of chloroauric acid (HAuCl4) and L-ascorbic acid. At pH 6.5, OTS is negatively charged and repels the chloroauric ions, so gold deposition was not observed. On the other hand, because the pH was below the IEP of APTES-modified glasses, chloroauric ions were adsorbed by APTES SAMs and reduced in situ by ascorbic acid. After the initial Au was formed, Au was continuously deposited and metalized to form uniform thin films by autocatalysis. A micropattern of Au thin film on a glass substrate was thus successfully fabricated without the use of a seed or any etching process.
Introduction Self-assembled monolayers (SAMs) have many applications due to their ability to control the physicochemical properties of a surface.1 The literature on SAMs focuses largely on assemblies formed by the adsorption of organothiol compounds from solutions or the vapor phase onto planar metal substrates of gold and silver.2-4 A major thrust in the research on SAMs is the continual expansion in both the types of substrates used to support SAMs and the types of surface functional groups.5 With different surface functional groups, different types of SAMs are found to either promote or inhibit the deposition of different ceramic thin films on the basis of their electrostatic interactions.6-9 For example, titania was found to be deposited on SAMs with acidic functional groups and amine groups. On the other hand, vanadia only deposited on amine and alkylammonium SAMs. Consequently, titania-vanadia films can be deposited on all types of SAMs except nonpolar SAMs. * To whom correspondence should be addressed. Tel: 866(2) 2789-8000 #69. E-mail:
[email protected]. † Academia Sinica. ‡ National Taiwan University.
(1) Ulman, A. Chem. ReV. 1996, 96, 1533–1554. (2) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559–3568. (3) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365–385. (4) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5, 723–727. (5) Shyue, J.-J.; De Guire, M. R.; Nakanishi, T.; Masuda, Y.; Koumoto, K.; Sukenik, C. N. Langmuir 2004, 20, 8693–8698. (6) Shyue, J.-J.; De Guire, M. R. In CIMTEC 2002 (10th International Ceramics Congress and 3rd Forum on New Materials), Florence, Italy, July 14-19, 2002; Vincenzini, P., Ed.; Trans Tech Publishers: StafaZurich, Switzerland, 2002; p 469. (7) Shyue, J.-J.; De Guire, M. R. Chem. Mater. 2005, 17, 787–794. (8) Shyue, J.-J.; De Guire, M. R. Chem. Mater. 2005, 17, 5550–5557. (9) Shyue, J.-J.; De Guire, M. R. Trans. MRS J. 2005, 29, 2383–2386.
Recently, micro- and nanopatterned SAMs have attracted increasing interest for electronic, photonic, and biological device fabrication due to demonstrations of selective metal deposition and adsorption in only the desired areas.10 Several notable processes have been achieved in site-selective deposition and micropatterning of functional ceramic thin films. Using patterned SAMs as templates, researchers successfully fabricated ceramic thin films such as TiO2,11,12 SnO2,13 ZrO2,14 Ta2O5,15 and SrTiO3.16 Furthermore, the availability of new types of nanostructures with well-defined shapes and sizes on planar supports (metal structure on silicon wafers or glass slides) and in solution (nanocrystals, template structures) has stimulated the widespread application of SAMs in both stabilizing the structure of these new metallic nanomaterials and in manipulating the interfacial or surface properties of these materials. Physical vapor deposition (PVD) methods like thermalevaporationandelectronbeam(e-beam)evaporation,17,18 sputtering, electrodeposition,19 or electroless deposition20,21 are all common techniques for generating thin films of a wide (10) Dulcey, C. S.; Georger, J. H.; Krauthamer, V.; Stenger, D. A.; Fare, T. L.; Calvert, J. M. Science 1991, 252, 551–554. (11) Masuda, Y.; Sugiyama, T.; Koumoto, K. Chem. Mater. 2002, 12, 2643. (12) Masuda, Y.; Kato, K. Chem. Mater. 2008, 20, 1057–1063. (13) Shirahata, N.; Masuda, Y.; Yonezawa, T.; Koumoto, K. Langmuir 2002, 18, 10379. (14) Gao, Y. F.; Masuda, Y.; Ohta, H.; Koumoto, K. Chem. Mater. 2004, 16, 2615–2622. (15) Masuda, Y.; Wakamatsu, S.; Koumoto, K. J. Eur. Ceram. Soc. 2004, 24, 301. (16) Gao, Y. F.; Masuda, Y.; Yonezawa, T.; Koumoto, K. Chem. Mater. 2002, 14, 5006–5014. (17) Venables, J. A. Introduction to Surface and Thin Film Processes; Cambridge University Press: Cambridge, U.K., 2000. (18) Vossen, J. L. Thin Film Processes; Academic Press: San Diego, CA, 1991. (19) Schlesinger, M. Modern Electroplating; John Wiley & Sons: New York, 2000.
10.1021/cm8022456 CCC: $40.75 2008 American Chemical Society Published on Web 10/10/2008
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Figure 1. Depiction of the processes used to fabricate a micropattern of metallic gold thin film.
range of metals including gold, silver, copper, palladium, platinum, and nickel. Among them, electroless deposition has been widely used for the production of fine metal patterns for numerous applications in template-directed formation of micro- and nanoscale metallic structures.22 This method has significant merits in its lower cost, fast deposition, and its ability to form good membranes at low temperatures. Electroless deposition is the process used to deposit thin films via the chemical reduction of metal salts onto surfaces. One advantage of this method over PVD is that it does not require vacuum processing equipment and only requires mixing chemical solutions that are commercially available. In addition, complicated shaped structures can be coated because deposition from the liquid phase does not rely on a line-of-sight. Furthermore, unlike conventional electrodeposition, electroless deposition does not require a conductive electrode, hence this method can deposit films onto nonconductive materials. Meanwhile, metallic gold films have also attracted much attention due to their myriad of potential applications, especially in developing microcircuits. In addition, gold films that are less that ∼15 nm thick are semitransparent and commonly used as substrates for SAMs in biology.23 Gold is easy to obtain, although expensive, single crystals are available commercially. Gold is an inert metal that does not oxidize at temperatures below its melting point, hence it does not react with atmospheric O2 or most chemicals. Thin films of gold are common substrates used for a number of analytical and spectroscopic techniques such as quartz crystal microbalances (QCM), ellipsometry and SPR spectroscopy, making gold the material of choice for applications that use SAMs as interfaces for studies in biology. Moreover, gold is compatible with cells, which can adhere to and function on gold surfaces without signs of toxicity. In this study, we developed a simple technique for siteselective electroless deposition of gold from a solution onto glass surfaces modified with organic SAMs. We utilized a regionally separated octadecyltrichlorosilane (OTS) SAMand 3-aminopropyltriethoxysilane (APTES) SAM-modified glass slide as a template upon which to fabricate a micropattern of metallic Au thin films. An ∼5 nm thick continuous film was observed in the desired region. (20) Dubrovsky, T. B.; Hou, Z.; Stroeve, P.; Abbott, N. L. Anal. Chem. 1991, 71, 327. (21) Pham, T.; Jackson, J. B.; Halas, N. J.; Lee, T. R. Langmuir 2002, 18, 4915. (22) Hidber, P. C.; Helbig, W.; Kim, E.; Whitesides, G. M. Langmuir 1996, 12, 1375–1380. (23) Kane, R. S.; Takayama, S.; Ostumi, E.; Ingber, D. E.; Whitesides, G. M. Biomaterials 1999, 20, 2363–2376.
Figure 2. Zeta potential of glass, APTES-SAM modified glass, and OTSSAM modified glass.
Experimental Section Figure 1 shows the flow of the experimental procedure. Regional Separated OTS-SAM and APTES-SAM on Glass. One volume percent octadecyltrichlorosilane (OTS, Acros Organics, USA) was dissolved in bicyclohexyl (BCH, Acros Organics, Japan) at room temperature in air. Glass slides were ultrasonically cleaned in ethanol for 5 min before their exposure to vacuum UV in air for 40 min to eliminate organic contaminants. The cleaned glass slides were immersed in a BCH solution of OTS for 6 h for the deposition of OTS and then rinsed with chloroform. The glass slide with the OTS-SAM was irradiated by vacuum UV (193 nm) for 80 min. The center-marked grids (300 mesh, 3.0 mm OD, Ted Pella Inc., USA) were used as a photomask to selectively remove parts of the OTS-SAM. The substrates were subsequently dipped into the acetone solution containing 1 vol % 3-aminopropyltrimethoxysilane (APTES, Acros Organics, Germany) for 6 h at room temperature and then rinsed with acetone. The regionally separated SAMs glass slides were used as templates to promote the site-selective deposition of gold. Site-Selective Deposition of Gold Thin Films. An electroless deposition bath was prepared with hydrogen tetrachloroaurate trihydrate (1 mM, Acros Organics, Germany), and L-ascorbic acid sodium salt (1 mM, Acros Organics, Belgium), which acted as a reducing agent. To compensate for the change in pH, an autotitrator (Mettler Toledo Titration Excellence T70, Switzerland) was used to maintain the pH at 4.5, 5.5, or 6.5 by adding appropriate amounts of NaOH based on the pH change. The substrates with SAMs were immersed in the solution and kept at room temperature for 24 h for the deposition of Au. Characterization. The zeta potential as a function of the pH of OTS and APTES modified glasses was obtained using an electrokinetic analyzer (Anton Paar SurPASS, Germany). Regionally separated SAMs were confirmed by a surface potential microscope (SPoM, Veeco Innova SPM, USA; 75 kHz tapping mode with 100 nm lift height and 2 V excitation) and scanning X-ray photoelectron spectroscopy (XPS/ESCA; PHI 5000 VersaProbe, ULVAC-PHI, Japan; probe size ∼20 µm). The deposited films were examined by a scanning electron microscope (SEM; FEI Nova200 NanoSEM, USA) in low-
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Figure 3. (a) XPS N 1s mapping for OTS-APTES patterned glass. (b) Surface potential mapping of the glass slide with regionally separated OTS and APTES surface. The difference in the work function was 140 mV.
Figure 4. Topographic AFM images of Au thin film deposited on glass substrates from different solution concentrations: (a) 1 and (b) 0.6 mM.
vacuum (0.4-0.5 Torr) mode using the Helix detector, an atomic force microscope (AFM; Veeco Innova SPM, USA), and a transmission electron microscope (TEM, operated at 200 kV; JEOL JEM-2100F, Japan). Cross-sectional TEM specimens were prepared with standard ion-mill technique. Electron spectroscopic images (ESI) were acquired with a postcolumn filter (Tridium, Gatan, USA) using three 50 eV energy windows near the edge.
Results and Discussion Zeta Potential of OTS and APTES Modified Glasses. The zeta potential of OTS and APTES modified glasses was determined using the streaming current across parallel plates separated by 100-150 µm. A dilute electrolyte of 1 mM NaCl was circulated through the measuring cell containing the solid sample, thus creating a pressure difference. A relative movement of the charges in the electrochemical double layer gave rise to the streaming potential. This streaming potential, or alternatively, the streaming current, was detected by electrodes placed at both sides of the sample
and the slope of the current-pressure curve was then converted to the zeta potential of the surface.24,25 Through titration of the electrolyte with acid (HCl) and/or base (NaOH), the potential was measured over a range of pH values. The isoelectric point (IEP), where the zeta potential is zero, gives an important indication of how materials will interact with the surface at a given pH. Above the IEP, the surface is negatively charged and will thus repel negatively charged species like AuCl4-; below the IEP, the surface is positively charged and will attract negatively charged species. It is found that the IEP of OTS and APTES was at pH 4.0 and 6.6, respectively (Figure 2). Therefore, between pH 4.0 and 6.6, negatively charged AuCl4- would be selectively adsorbed on APTES and repelled by OTS. (24) Meter, U.; Ko¨ stler, S.; Ribitsch, V.; Kern, W. Macromol. Chem. Phys. 2005, 206, 210–217. (25) Temmel, S.; Kern, W.; Luxbacher, T. Prog. Colloid Polym. Sci. 2006, 132, 54–61.
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Figure 5. SEM images of the site-selective deposition of Au on UV-patterned glass substrates at (a) pH 4.5, (b) pH 5.5, and (c, d) pH 6.5.
Figure 6. (a). TEM bright-field image, (b) zero-loss peak image, and (c) ESI of Au N4,5-edge of the Au pattern on organic SAMs.
UV-Patterned SAM Substrates. Because of its high surface sensitivity, XPS can be used to study the chemical
composition of the topmost surface. Si, C, N, and O were clearly observed from the APTES-SAM surface while only Si, C, and O were observed from OTS-SAM surface. Using the N 1s mapping, the OTS-APTES patterned glass was imaged (Figure 3a). The SPoM image shows the relative surface potential (work function) of the substrate. It is clear that regionally separated SAMs were fabricated and the difference in surface potential (work function) between OTSSAM and APTES-SAM was 140 mV (Figure 3b). Site-Selective Deposition of Au. The simultaneous mixing of the tetrachloroauric acid and L-ascorbic acid solutions resulted in the formation of stable gold sols as indicated by the appearance of the typical red-purple color in the final dispersion. After the initial Au formation on the substrate, Au was continuously deposited and metallized to form uniform thin films by autocatalysis. The overall reduction reaction responsible for the formation of the dispersed gold is given by 2HAuCl4 + 3C6H8O6 f 2Au0 + 3C6H6O6 + 8HCl. It is clear that the pH will decrease during the reaction. This change in pH will affect the surface potential of the SAM. Therefore, an autotitrator is used to compensate the pH change by the controlled addition of NaOH during the deposition. The Au growth rate in the solution as well as the growth of particle on the deposited films can be controlled by adjusting the concentration of tetrachloroauric acid and L-ascorbic acid solutions. Figure 4 shows the Au film deposited at different concentrations. Low concentration of tetrachloroauric acid and L-ascorbic acid solutions tended
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to cause a slow deposition rate in the formation of Au films made up of homogeneously nucleated particles. Moreover, if the concentration was too low, the deposited film was then composed of small particles that had many grain boundaries. These grain boundaries would reduce the electrical conductivity. On the other hand, high concentration tetrachloroauric acid and L-ascorbic acid solutions tended to promote the rapid growth of Au that was no longer site-selective. By employing regionally separated OTS-SAM and APTESSAM on glass slides, we successfully fabricated micropatterned gold. The surface morphology and site-selective deposition were further evaluated by SEM and AFM. Since the IEP of OTS and APTES was at pH 4.0 and 6.6 respectively, negatively charged species like AuCl4- could be selectively adsorbed on APTES between pH 4.0 and 6.6. The SEM shows the site-selective deposition of Au on UVpatterned substrates at pH 4.5 (Figure 5a), pH 5.5 (Figure 5b), and pH 6.5 (images c and d in Figure 5). At a lower pH (pH4.5 and 5.5), where the hydrophobic OTS SAMs had a weak negative potential, a small amount of chloroauric acid was adsorbed and gold was subsequently deposited. As a result, the pH of the deposition solution was kept at 6.5. OTS is negatively charged (-75 mV) at this pH and repels the chloroauric ions, inhibiting gold deposition. On the other hand, because the pH was still below the IEP of APTES modified glasses, chloroauric ions were adsorbed by the positively charged APTES SAMs and then reduced by ascorbic acid. Cross-section TEM samples of the site-selective deposition of Au on UV-patterned substrates were prepared in order to examine the continuity of Au layer on the substrate (Figure
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6). With the aid of ESI taken with Au N4,5-edge at 334 eV, it is clear that a uniform Au layer (∼5 nm) was present on the surface. On this continuous thin film, nanoparticles were also observed. The HRTEM (inset in Figure 6a) indicated that the thin-layer at the interface was amorphous and the nanoparticles were cubic. The resistivity of prepared films was measured by the resistance across a sample of 1 cm × 1 cm and was about 50 Ω cm. Conclusion By using self-assembled monoalyers with regionally separated functional groups (-NH2 and -CH3 terminal groups) on glass, which function as templates, we achieved site-selective deposition of gold thin film. The substrates were first modified with OTS and partially removed with vacuum UV in air. APTES was then deposited in the region where the OTS was removed. By tailoring the pH of the electroless deposition bath to a value between the IEP of OTS and APTES, we achieved site-selective deposition of gold. After optimization of the deposition conditions (1 mM tetrachloroauric acid, 1 mM ascorbic acid, and maintaining the pH at 6.5), a continuous Au thin film was deposited on the APTES region, whereas no deposition occurred in the OTS region. Acknowledgment. The authors acknowledge the sponsorship by Academia Sinica and the Taiwan National Science Council through Grant 96-2113-M-001-012-MY2. CM8022456
