Langmuir 1999, 15, 3011-3014
Self-Assembled Monolayers on (111) Textured Electroless Gold Zhizhong Hou, Silvia Dante,† Nicholas L. Abbott,‡ and Pieter Stroeve* NSF Center on Polymer Interfaces and Macromolecular Assemblies (CPIMA), Department of Chemical Engineering and Materials Science, University of California, One Shields Avenue, Davis, California 95616 Received November 24, 1998. In Final Form: February 17, 1999
Introduction Thin films of gold deposited on supporting substrates (such as silicon wafers,1-6 glass microscope slides,7-13 and mica14-16) by vacuum evaporation have been extensively used as substrates for self-assembled monolayers (SAMs) formed from alkanethiols. The predominant crystallographic orientation in evaporated gold (EV) has been reported to be Au(111).1,14,17 Densely packed, crystallinelike SAMs with nearly all-trans conformations of alkyl chains can be formed from long chain alkanethiols [CH3(CH2)nSH, n g 9] on the surfaces of EV.1-5,18 The alkyl chains within the SAMs were found to be tilted by ∼30° from the surface normal of the substrate and rotated around the chain axis by an angle (twist angle) of ∼52°.1,2,4,5,18 This note reports a procedure for preparation of gold films using electroless plating that leads to similar orientations of the alkyl chains within the SAMs. We recently reported deposition of gold films onto glass substrates by electroless19 plating and verified formation * To whom correspondence should be addressed. E-mail:
[email protected]. Telephone: (530) 752-8778. Fax: (530) 7521031. † Current address: Hahn-Meitner-Institut Berlin, Glienicker Str. 100, D-14109 Berlin, Germany. ‡ Current address: Department of Chemical Engineering, University of WisconsinsMadison, 1415 Engineering Drive, Madison, WI 53706-1691. (1) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358-68. (2) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-68. (3) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-35. (4) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-69. (5) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152-67. (6) Lee, T. R.; Carey, R. I.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 741-9. (7) Abbott, N. L.; Rolison, D. R.; Whitesides, G. M. Langmuir 1994, 10, 2672-82. (8) Gupta, V. K.; Miller, W. J.; Pike, C. L.; Abbott, N. L. Chem. Mater. 1996, 8, 1366-9. (9) Gupta, V. K.; Abbott, N. L. Langmuir 1996, 12, 2587-93. (10) Gupta, V. K.; Abbott, N. L. Science 1997, 276, 1533-6. (11) Miller, W. J.; Abbott, N. L. Langmuir 1997, 13, 7106-14. (12) Sigal, G. B.; Mrksich, M.; Whitesides, G. M. Langmuir 1997, 13, 2749-55. (13) Kane, V.; Mulvaney, P. Langmuir 1998, 14, 3303-11. (14) Chidsey, C. E. D.; Loiacono, D. N.; Sleator, T.; Nakahara, S. Surf. Sci. 1988, 200, 45-66. (15) Hegner, M.; Wagner, P.; Semenza, G. Surf. Sci. 1993, 291, 3946. (16) Hu, K.; Bard, A. J. Langmuir 1997, 13, 5114-9. (17) Hou, Z.; Abbott, N. L.; Stroeve, P. Langmuir 1998, 14, 3287-97. (18) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437-63. (19) The term “electroless” refers to the fact that there is no external electrical potential applied, in contrast to the case of electrolytic processes.
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of densely packed SAMs from long chain alkanethiols on such gold films.17 The electroless plating of the gold was performed by reduction of a gold salt to elemental gold at the surface of the immersed substrate from a solution containing a reducing agent.17,20 One advantage of electroless plating is that it permits deposition of metals onto complex or internal surfaces. We have developed a new type of support for biological assays by deposition of electroless gold onto silica gel and formation of SAMs from functionalized alkanethiols onto the deposited gold.21 Whereas studies of electroless plating of gold have investigated reaction kinetics and factors such as temperature, formulation, concentration, pH, reducing agent, and substrate,22-27 the crystal structure of electroless gold seems not to have been reported in the literature to our knowledge. In our recent report,17 we have found that electroless gold deposited directly onto glass (EL) has significant (200), (220), and (311) reflections in X-ray diffraction patterns, although (111) occupies the largest portion of crystallographic orientations. The difference in crystal structure between EV and EL results in different orientations of alkyl chains within the SAMs formed on the two types of gold substrates. The tilt angle of the chains on EL is markedly smaller than that on EV.17 In this note, we report that electroless gold deposited on a seed layer of EV (ELEV) is (111) textured, which is similar to that of EV. The orientation of alkyl chains within SAMs formed from hexadecanethiol on such gold films is close to that observed on EV. Because thermal deposition is sometimes not desirable in sequential steps in microfabrication processes and electroless plating is inexpensive compared to thermal deposition, the electroless plating method reported in this note can be used to thicken gold microstructures from thin gold templates while maintaining the crystal structure of gold. Experimental Section We deposited electroless gold onto two types of substrates. The first was high-index glass (LaSFN9, from Hellma, Germany, n ) 1.844). The use of high-index glass other than ordinary glass microscope slides enhances the adhesion between electroless gold and glass. The procedure we used to deposit electroless gold onto high-index glass was based on past reports from other groups20,28 and was described in our past report.17 The plating was carried out at room temperature, at which the deposition rate of gold was ∼50 nm/h. After plating, the sample of electroless gold was treated by thermal annealing (at 250 °C for 3 h) followed by electrochemical cleaning. We found that densely packed monolayers could not be formed from alkanethiols onto electroless gold without such post plating treatments.17 The second type of substrate used for electroless plating of gold was a thin film (5 nm in thickness) of evaporated gold deposited at 0.2 Å/s on a glass microscope slide that was precoated with 1 nm of evaporated Ti.17 The substrate was directly immersed into the gold plating solution17 that contained Na3(20) Menon, V. P.; Martin, C. R. J. Anal. Chem. 1995, 67, 1920-8. (21) Dubrovsky, T. B.; Hou Z.; Stroeve, P.; Abbott, N. L. Anal. Chem. 1999, 71, 327-32. (22) Ali, H. O.; Christie, R. A. Gold Bull. 1984, 17, 118-27. (23) Ganu, G. M.; Mahapatra, S. J. Sci. Ind. Res. 1987, 46, 154-61. (24) Que´au, E.; Stremsdoerfer, G.; Martin, J. R.; Cle´chet, P. Plat. Surf. Finish. 1994, 81 (Jan), 65-9. (25) Gaudiello, J. G. IEEE Trans. Compon., Packag., Manufac. Technol., Part A 1996, 19, 41-4. (26) Hajdu, J. B. Plat. Surf. Finish. 1996, 83 (Sep), 29-33. (27) Srinivasan, R.; Suni, I. I. Surf. Sci. 1998, 408, L698-702. (28) Mallory, G. O., Hajdu, J. B., Eds. Electroless Plating: Fundamentals and Applications; American Electroplaters and Surface Finishers Society: Orlando, FL, 1990; Chapter 17.
10.1021/la981644b CCC: $18.00 © 1999 American Chemical Society Published on Web 03/26/1999
3012 Langmuir, Vol. 15, No. 8, 1999
Notes
Figure 2. Cyclic voltammograms measured in aqueous 0.1 M H2SO4 using (a) bare gold films and (b) the same gold films as in part a but with SAMs of CH3(CH2)15SH formed on the surfaces of gold. The immersed surface area of each gold film is ∼1.5 cm2. The lower level of each scale bar indicates zero current.
Figure 1. X-ray diffraction patterns of films of (a) evaporated gold (EV), (b) a 5 nm seed layer of EV, (c) electroless gold deposited on high-index glass (EL), and (d) electroless gold deposited on 5 nm EV (ELEV). See text for additional details. Au(SO3)2 and formaldehyde at room temperature. The reaction for the deposition of electroless gold is initially catalyzed by the surface of evaporated gold and then “autocatalyzed” by the deposited electroless gold. The sample was treated after plating in the same way as stated above. Self-assembled monolayers were formed by immersion of samples of gold films in ethanolic solutions of hexadecanethiol (∼1 mM) for at least 24 h. The gold films were thoroughly rinsed with ethanol before and after immersion. The gold films and SAMs were characterized by X-ray diffractometry, contact angles, cyclic voltammetry, and grazing angle FTIR as described in our past report.17
Results and Discussion Figure 1 shows X-ray diffraction patterns of gold films. The thickness of each gold film was 100 nm, except for the sample shown in Figure 1b (5 nm in thickness). Samples of EL and ELEV were thermally annealed prior to measurement while samples of EV were measured without annealing. Thermal annealing, as stated earlier, is a part of the post plating treatment that is necessary in order to form densely packed monolayers on the surface of electroless gold but not required when using EV.17 The intensities of the peaks in each pattern are normalized by the intensity of the peak corresponding to Au(111). Note the diffraction pattern in Figure 1b has been amplified by ∼100 times relative to that in Figure 1a. It can be seen from Figure 1a and b that evaporated gold (EV) is highly (111) textured. A peak corresponding to Au(111) can be clearly observed even when the film thickness is as small as 5 nm (Figure 1b). A very small peak corresponding to Au(200) can be identified in addition to the peaks corresponding to Au(111) and (222) when the film thickness is 100 nm (Figure 1a). We can also see that the width of the peak of Au(111) is significantly reduced as the gold film grows from 5 to 100 nm. This phenomenon suggests that the crystallite size in EV increases during the growth of the gold, according to the relation between the peak
width of X-ray diffraction and the crystallite size.29 This result is consistent with past observations using scanning tunneling microscopy (STM).14,30 Electroless gold deposited on high-index glass (EL) shows dispersed crystallographic orientations (Figure 1c). Although the largest peak in the diffraction pattern still corresponds to Au(111), EL has significant (200), (220), and (311) reflections. In contrast, electroless gold deposited on a thin layer of (111) textured EV (ELEV) shows only (111) and (222) reflections (Figure 1d), which is very close to the case for EV. The crystal structure of ELEV 100 nm in thickness is likely determined by that of the thin substrate 5 nm in thickness. We found that the diffraction pattern of ELEV measured prior to thermal annealing [not shown, also with a very small (200) peak] is nearly the same as that shown in Figure 1a. In addition, the very small Au(200) peak in Figure 1a disappeared when EV was thermally annealed, which is similar to the case of the annealed ELEV shown in Figure 1d. The above results suggest that gold grows epitaxially on the surface of evaporated gold during electroless plating. Self-assembled monolayers (SAMs) were then formed from CH3(CH2)15SH on EV, EL, and ELEV. The advancing contact angles of hexadecane (HD) measured on SAMs formed on all three samples were 47°. This value is typical for a densely packed SAM formed from a long chain alkanethiol.3,5,8-11,31 The receding contact angles of HD measured on SAMs formed on ELEV and EL were 36° and 38°, respectively, which are smaller than that (43°) measured using EV. The hysteresis between advancing and receding angles observed on the three gold films likely suggests that the surface roughnesses of ELEV and EL are similar but larger than that of EV.3,17 We also measured cyclic voltammograms in aqueous 0.1 M H2SO4 solutions using the above films of gold before and after the SAMs were formed from CH3(CH2)15SH (Figure 2). The anodic wave starting at +1.1 V corresponds to the oxidation of the gold surfaces, and the cathodic peak at +0.87 V corresponds to the reduction of the surface layer of gold oxide to gold.7,32 We estimated fractional coverages of SAMs by evaluating the decrease in the area of the cathodic (29) Jenkins, R.; Snyder, R. L. Introduction to X-ray Powder Diffractometry; John Wiley & Sons: New York, 1996; p 90. (30) Hwang, J.; Dubson, M. A. J. Appl. Phys. 1992, 72, 1852-7. (31) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87-96.
Notes
Langmuir, Vol. 15, No. 8, 1999 3013 Table 2. Orientation of Alkyl Chains within SAMs Formed from CH3(CH2)15SH on Films of Gold (r, chain tilt; β, Chain Twist) film of gold EVa EL ELEV
R
β
30° 21° 27°
52° 57° 56°
a Values were obtained from typical results in past studies,1,2,4,5,18 which were used as references in calculations.
Figure 3. Grazing-angle FTIR spectra of SAMs formed from CH3(CH2)15SH on gold films. The peaks in the spectra correspond to the following C-H stretching modes: 1, asymmetric methyl stretch (2964 cm-1); 2 and 4, symmetric methyl stretch (2937 and 2877 cm-1); 3, asymmetric methylene stretch (2918 cm-1); and 5, symmetric methylene stretch (2850 cm-1). Table 1. Relative Intensities of Peaks in IR Spectra in Figure 3 relative intensitya peak no.
EV
EL
ELEV
1 2 3 4 5
0.22 0.20 1 0.30 0.45
0.68 0.27 1 0.42 0.31
0.36 0.20 1 0.33 0.33
a The intensities were obtained by fitting each spectrum using a Lorentzian function and then normalized by the intensity of peak 3 in each spectrum.
peaks after the SAMs were formed. The fractional coverages of SAMs on EV, EL, and ELEV were all g 99.9%. Figure 3 shows the C-H stretching region of grazing angle IR spectra of SAMs formed from CH3(CH2)15SH on EV, EL, and ELEV. The positions and shapes of the methylene stretching peaks shown in Figure 3 indicate that the SAMs on all gold samples are densely packed and crystalline-like with a nearly all-trans conformation of alkyl chains. Differences in the relative intensities of the peaks between IR spectra as shown in Table 1 suggest that the orientations of alkyl chains within the SAMs are not identical on the three types of gold films. We used the measured IR spectra shown in Figure 3 and a method that was detailed in our past report17 to calculate the average orientation of alkyl chains. The method is based on a physical model that correlates relative intensities in IR spectra, transition dipole moments of methyl and methylene stretches, and spacial alignment of alkyl chains. The method is also based on the assumptions that the (32) The anodic wave of EV shown in Figure 2a has two peaks that represent the two steps of the electrochemical oxidation of an Au(111) surface.7 The voltammogram of EL in Figure 2a, however, shows only one peak. Although only one peak can be clearly identified in the voltammogram of ELEV in Figure 2a, the anodic wave of ELEV looks different from that of EL. The shapes of the anodic peaks have been used to evaluate the crystallographic orientations at the surface of gold.7 However, the anodic currents may involve a contribution from the oxidation of water and may also depend on the roughness of the samples. These factors make interpretation of the anodic peaks complicated.
surface of the substrate is flat and that all alkyl chains within the SAM are equivalent and present in all-trans conformations. The orientation of alkyl chains is described by the tilt angle (R, from the surface normal) and twist angle (β, around the chain axis) of the chains. The IR spectrum of the SAM formed on EV was used as a reference, and its corresponding orientation of chains (R ) 30°, β ) 52°) was obtained from typical results in past studies.1,2,4,5,18 The average tilt and twist angles of chains calculated for SAMs formed on EL and ELEV are listed in Table 2. One can see that the chain tilt obtained on EL is notably smaller than that obtained on EV. This result was attributed to the fact that EL has significant portions of other crystallographic orientations than (111) while EV is highly (111) oriented.17 The chain tilt obtained on ELEV, however, is close to that obtained on EV. This result reflects that the crystal structures of ELEV and EV are similar (as shown in Figure 1a and d). The slight difference in the chain tilt on ELEV and EV might be caused by the effect of different scales of the surface roughness of those two substrates on the observed IR spectra. The hysteresis of contact angles presented earlier suggests that the surface roughness of ELEV is similar to that of EL but larger than that of EV. Table 2 also shows that the chain twist is not as sensitive to the crystal structure of the substrates as the chain tilt. The chain twists on EL and ELEV are similar although the crystallographic orientations of these two substrates are different. One can also see that the chain twists on EL and ELEV are slightly larger than that obtained on EV. The manner of the difference in chain twists is consistent with that in hysteresis of contact angles and is likely caused by the effect of the roughness of gold films. The results reported in this note demonstrate that judicious choice of substrates for electroless plating of gold can lead to polycrystalline films of gold that present one predominant crystallographic orientation. One can use the structure-property relationships known for SAMs formed on evaporated Au(111) and apply them to SAMs formed on electroless gold. For example, it is now suggested that subtle differences in the conformations of ethylene glycol-terminated SAMs formed on Au and Ag determine the extent of nonspecific adsorption of proteins to the SAMs.33 Conclusions We report preparation of densely packed self-assembled monolayers formed from hexadecanethiol on the surfaces of electroless gold that is deposited on a thin layer of evaporated gold. X-ray diffraction shows that such films of electroless gold are highly (111) textured, which is similar to the crystal structure of evaporated gold. The tilt angles of alkyl chains within the SAMs supported on the electroless gold deposited on evaporated gold are found to be close to that obtained on evaporated gold. In comparison, electroless gold deposited directly on glass (33) Pertsin, A. J.; Grunze, M.; Garbuzova I. A. J. Phys. Chem. B 1998, 102, 4918-26.
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has significant (200), (220), and (311) reflections in addition to the major reflection of (111). The tilt angle of alkyl chains within the SAMs formed on this type of substrate is notably smaller than that obtained on evaporated gold. The expitaxial growth of Au(111) by electroless plating on a seed layer of evaporated gold is useful for microfabrication applications.
Notes
Acknowledgment. This work was partially supported by CPIMA (NSF Grant DMR-9808677). We thank Kay Kanazawa, Vinay Gupta, William Miller, and Lana Jong for their help in X-ray diffraction, FTIR, and cyclic voltammetry. LA981644B
