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Novel Approach to the Formation of Smooth Gold Surfaces C. I. Priest,† K. Jacobs,‡ and J. Ralston*,† Ian Wark Research Institute, ARC Special Research Centre for Particle and Material Interfaces, University of South Australia, Mawson Lakes, South Australia 5095, Australia, and Department of Applied Physics, University of Ulm, D-89069 Ulm, Germany Received August 31, 2001. In Final Form: December 12, 2001
Introduction The self-assembly behavior of silicon- and sulfur-based organic molecules has prompted investigations involving wettability,1-5 micro-6-9 and nanoscale design,10,11 phase separation,1,12,13 biosensors,14 molecular switches,15 and micromachining.16 Such applications may lead to precise machines and sensors. Although these applications indicate the considerable potential of self-assembled monolayers (SAMs), fundamental difficulties exist. The first is the requirement of a suitable substrate. Where alkane thiols are the self-assembled species, an unoxidized metal substrate is required, such as gold. The quality of the gold substrate is dependent on the preparation technique. The latter may be altered to yield a surface suitable for the specific application in mind. Current methods for the preparation of thin gold films vary. Annealing conditions,17,18 evaporation rate,19 and substrate type18 have shown characteristic surface features on vapor-deposited gold. Template-stripping involves * To whom correspondence should be addressed. † University of South Australia. ‡ University of Ulm. (1) Imabayashi, S.-I.; Gon, N.; Sasaki, T.; Hobara, D.; Kakiuchi, T. Langmuir 1998, 14, 2348-2351. (2) Abbott, S.; Ralston, J.; Reynolds, G.; Hayes, R. Langmuir 1999, 15, 8923-8928. (3) Lee, T. R.; Carey, R. I.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 741-749. (4) Engquist, I.; Lestelius, M.; Liedberg, B. Langmuir 1997, 13, 40034012. (5) Abe, K.; Takiguchi, H.; Tamada, K. Langmuir 2000, 16, 23942397. (6) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 20022004. (7) Wilbur, J. L.; Kumar, A.; Kim, E.; Whitesides, G. M. Adv. Mater. 1994, 6, 600-604. (8) Larsen, N. B.; Biebuyck, H.; Delamarche, E.; Michel, B. J. Am. Chem. Soc. 1997, 119, 3017-3026. (9) Yan, L.; Zhao, X.-M.; Whitesides, G. M. J. Am. Chem. Soc. 1998, 120, 6179-6180. (10) Hatzor, A.; Weiss, P. S. Science 2001, 291, 1019-1020. (11) Xu, S.; Miller, S.; Laibinis, P. E.; Liu, G.-Y. Langmuir 1999, 15, 7244-7251. (12) Stranick, S. J.; Atre, S. V.; Parikh, A. N.; Wood, M. C.; Allara, D. L.; Winograd, N.; Weiss, P. S. Nanotechnology 1996, 7, 438-442. (13) Stranick, S. J.; Parikh, A. N.; Tao, Y.-T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636-7646. (14) Cotton, C.; Glidle, A.; Beamson, G.; Cooper, J. M. Langmuir 1998, 14, 5139-5146. (15) Raymo, F. M.; Giordani, S. J. Am. Chem. Soc. 2001, 123, 46514652. (16) Lee, S.-W.; Laibinis, P. E. J. Am. Chem. Soc. 2000, 122, 53955396. (17) Wagner, P.; Hegner, M.; Guntherodt, H.-J.; Semenza, G. Langmuir 1995, 11, 3867-3875. (18) Masens, C.; Schulte, J.; Phillips, M.; Dligatch, S. Microsc. Microanal. 2000, 6, 113-120. (19) Sennett, R. S.; Scott, G. D. J. Opt. Soc. Am. 1950, 40, 203-211.
the removal of the gold.1,17,18,20 Variations within this technique exist. Adhesives and solvent (i.e., THF),17 substrates,18 and annealing17,18 have been considered to enhance both the ease of stripping and the final surface topography. The most common stripping technique uses mica as a substrate due to its atomically flat surface, easy cleavage plane, and thermal stability for annealing. Each of the techniques mentioned produces characteristic surface qualities that are difficult to compare objectively. A surface may be reported as “atomically flat”, for example, but due to annealing or stripping is likely to contain large defect sites of microscopic dimensions. Alternatively, a sample prepared at room temperature may be devoid of such defects, leading to a slightly rougher yet more consistent surface.18,21,22 Where roughness analysis is reported numerically (i.e., root-mean-square (rms) values, the mean roughness (Ra) and peak-to-trough heights from section analysis), the two surfaces cannot be objectively compared. Sample quality must be considered with respect to the desired application. Atomically flat gold has been successfully prepared for scanning tunneling microscopy (STM) measurements of SAM structures23,24 over small surface areas (i.e., less than 1 µm2). Outside of this area, atomically flat gold surfaces contain large defects (>1 µm) and random steps (∼1 nm), making these surfaces unsuitable for large-scale applications (i.e., wettability measurements and micropatterning). In this study, we have employed gypsum as a substrate to improve the large-scale surface quality, minimizing roughness using a template-stripping method without the aid of annealing, solvent, or temperatureassisted removal. The resulting gold contained no trace gypsum and is suitable for applications where consistency of the gold surface is required over quite large areas, ahead of absolute flatness. Experimental Section Thin gold films were prepared by evaporation of gold (99.99%, Aldrich) under vacuum (10-6 Torr) onto freshly cleaved {010} natural gypsum (South Australian Museum) at ambient temperature. The vacuum unit was custom-built by the University of South Australia. The evaporation rate was monitored by a purpose-built quartz crystal microbalance. The total change in frequency for all depositions was 1000 Hz. The thickness corresponding to this frequency change was 50 nm, determined by atomic force microscopy (AFM) analysis (the piezo was calibrated for x, y, and z dimensions by scanning a calibration grid; the error was less than 1%) of the gold step from where the template-stripped gold was removed. Deposition rates of 2 and 10 Hz/s were tested to determine the effect of rate on roughness. For results presented in this report, the deposition rate was 2 Hz/s, or 0.1 nm/s, unless otherwise specified. Adhesion of the thin gold film to a rigid support was achieved using a commercial two-part epoxy glue (EPOTEK 301). Once thoroughly mixed, the glue was placed on an area of defect-free gold. Light pressing of the rigid support spreads the glue evenly, forming a thin film of adhesive containing no visible bubbles. When the adhesive was cured, removing the gypsum was readily (20) Hegner, M.; Wagner, P.; Semenza, G. Surf. Sci. 1993, 291, 3946. (21) Stamou, D.; Gourdon, D.; Liley, M.; Burnham, N. A.; Kulik, A.; Vogel, H.; Duschl, C. Langmuir 1997, 13, 2425-2428. (22) Knarr, R. F.; Quon, R. A.; Vanderlick, T. K. Langmuir 1998, 14, 6414-6418. (23) Takami, T.; Delamarche, E.; Michel, B.; Gerber, C. Langmuir 1995, 11, 3876-3881. (24) Ishida, T.; Mizutani, W.; Tokumoto, H.; Hokari, H.; Azehara, H.; Fujihira, M. Appl. Surf. Sci. 1998, 130-132, 786-791.
10.1021/la011379l CCC: $22.00 © 2002 American Chemical Society Published on Web 02/09/2002
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Figure 1. An AFM cross section of cleaved gypsum showing a step of ∼0.8 nm with atomically flat gypsum faces on either side. Step defects of this magnitude do not significantly affect the overall roughness when translated into gold topography (see Figure 3). achieved with minimum effort. The ease of separation, combined with controlled gold deposition, meant that the process was reproducible for gypsum. In contrast, when mica was tested for comparison purposes, sheets of mica frequently remained attached to the gold surface. A Digital Instruments Nanoscope III was used in TappingMode for AFM imaging of all surfaces. Nanosensors supplied type NCH50 tips. Roughness (rms) values are reported over an area of 1 µm2. Surface analysis to identify trace gypsum (calcium, sulfur, and oxygen) after removal of the gold from the substrate was performed using time-of-flight secondary-ion mass spectrometry (ToF-SIMS). ToF-SIMS revealed that sulfur and oxygen contaminants adsorbed from air were present, as expected for any gold surface; however, no calcium signal was detected, indicating complete removal of the gypsum.
Results and Discussion Gypsum (CaSO4‚2H2O) can be cleaved perfectly along the {010} plane to expose macroscopic regions of clean, atomically flat faces.25 Figure 1 shows an AFM line scan of cleaved gypsum. The image has an ∼0.8 nm step separating two atomically flat regions. Although gypsum can be cleaved to yield atomically flat regions, other factors such as its susceptibility to heat and low pressure must also be considered. Under vacuum and at high temperature, water loss from gypsum is anticipated, as CaSO4‚ 2H2O is the origin of anhydrous CaSO4. According to AFM analysis, the gypsum topography was unchanged at 10-6 Torr over 1 h (the time required for a typical deposition). Where thermally stable substrates are used, it is common to anneal the gold by heating the substrate prior to and throughout the deposition.17,18 The crystal structure of gypsum, however, is sensitive to elevated temperatures; hence, annealing of the gold layer was not performed in this instance. Although annealing is useful in forming atomically flat regions of gold, such surfaces tend to contain an inconsistent distribution of microdomains and microdefects. Annealed surfaces which are atomically flat over very small areas are not sufficient for many applications where surface features need to span tens of microns or where macroscopic analysis is required. In contrast, it has been shown that room-temperature deposition leads to a rougher yet more consistent topography.18,21,22 This increased consistency is critical for applications where even a low concentration of surface defects can distort conclusions significantly. (25) Roberts, W. L.; Rapp, G. R., Jr.; Weber, J. Encyclopedia of Minerals; Van Nostrand Reinhold: New York, 1974; p 253.
Figure 2. A 1 µm2 AFM image/cross section of gold templatestripped from gypsum (scanning parameters: z-scale, 5 nm; scan rate, 1.0 Hz). The arrows mark a peak and adjacent trough. The peak-to-trough height is 0.78 nm.
Since the deposition rate is known to affect the quality of “topside gold”, two deposition rates (0.1 and 0.5 nm/s) were investigated to observe any effect on the templatestripped surfaces. For gold template-stripped from gypsum, typical peak-to-trough and rms values for the 0.1 nm/s deposition were 0.78 and 0.24 nm, respectively. For depositions at 0.5 nm/s, the peak-to-trough and rms values were 1.33 and 0.3 nm, respectively. Clearly, faster depositions lead to rougher stripped gold surfaces, consistent with topside gold observations. To obtain the surfaces presented here, the slower rate (0.1 nm/s) was used to achieve optimal surface quality. Figure 2 shows template-stripped gold removed from gypsum. The image reveals the characteristic particulate texture of the gold, condensed onto the surface as solid beads. The grain size is ∼25 nm; however, the grains are poorly defined. The typical peak-to-valley heights are less than 1 nm, and the roughness (rms) over 1 µm2 is 0.24 nm. A more consistent topography is achieved over larger areas than by alternative methods. The template-stripping of gold from gypsum is an easy task since it can be done manually without the aid of solvents or thermal means. Where atomically flat gold surfaces have been prepared, random steps in the surface exist of equivalent magnitude to the peak-to-valley heights reported here. Although the frequency of the undulations in gold template-stripped from gypsum at room temperature is greater and atomically flat regions are not present, the surface is more consistent and the procedure is greatly simplified.
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Notes
flat gold is not required but where roughness needs to be minimized. Figure 3 shows a 10 µm × 10 µm scan of the sample in Figure 2. Although undulations are present, they are similar in magnitude to the peak-to-trough heights stated earlier. It is the smooth nature that becomes important for measurements such as wettability, where edge effects arising from sharp geometry26 may mask the contribution of surface chemistry. Conclusions
Figure 3. A 10 µm × 10 µm AFM image of template-stripped gold from gypsum showing typical topography but no significant irregularities (scanning parameters: z-scale, 10 nm; scan rate, 1.0 Hz). The horizontal line is likely to be from a step in the gypsum. Step defects are typically of dimensions similar to the undulations of the remaining surface.
Where large sample areas are required, this preparation enables regions greater than 2500 µm2 to be formed. This gypsum technique is ideal where large sample areas are required for either chemical or physical patterning and related applications. The simplicity of the preparation makes this technique attractive for cases where atomically
A simple and consistent technique has been developed to prepare smooth gold (peak-to-trough < 1 nm and rms ) 0.24 nm) over large sample areas (>2500 µm2). Gypsum is used as a suitable substrate. The template-stripping procedure leads to gold with suitable surface qualities for large-scale patterning and macroscopic measurements. Gypsum has distinct advantages: excess adhering gypsum (rarely present) may be removed by water dissolution, and the stripping process is easier and cleaner than that of mica and very reproducible. The resulting gold surfaces may be suitable for the construction of chemical patterns on large sample areas. Acknowledgment. Useful discussions with Dr. Allan Pring of the South Australian Museum, Adelaide, are warmly acknowledged, along with the supply of gypsum. Financial support from the Australian Research Council through the Special Research Centres Scheme is gratefully acknowledged. LA011379L (26) Oliver, J. F.; Huh, C.; Mason, S. G. J. Colloid Interface Sci. 1997, 59, 568.
