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Large Area, Molecularly Smooth (0.2 nm rms) Gold Films for Surface Forces and Other Studies Liraz Chai† and Jacob Klein*,†,‡ Department of Materials and Interfaces, Weizmann Institute of Science, RehoVot 76100, Israel, and Oxford UniVersity, Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford OXI 3QZ, U.K. ReceiVed December 26, 2006. In Final Form: March 20, 2007 Using large-area (cm2) single-crystal mica sheets as the templating substrate, we have created correspondingly large template-stripped (TS) gold films (thickness 82 ( 2 nm) that appear smooth to within 0.2 nm rms roughness over their entire area. These gold films, created without the use of any releasing solvent, are characterized using AFM, X-ray diffraction, multiple beam interferometric fringes of equal chromatic order (FECO), and contact angle measurements. Being molecularly smooth over large areas and (adjustably) semitransparent, these films are especially suitable for use in the surface force balance (SFB), as shown by measurements of the normal force (F) versus distance (D) profiles between such a flat gold surface and a bare mica surface in water. The F(D) profiles are in good agreement with DLVO theory down to molecular contact and indicate that the gold surface is negatively charged under water.
Introduction The creation of large-area gold films, smooth to the angstrom level, is a highly desirable goal in surface science where effects on the order of a nanometer or less are studied. Smooth gold can be used as a support for adsorbed species such as small molecules1or nanotubes,2 and provides the possibility of thiol chemistry that enables the creation of a wide range of functional thin films with correspondingly smooth topologies.3,4 Such smooth gold surfaces may be used with or without adsorbed species with scanning probe microscopes (SPMs) for imaging or force measurements,5,6 as electrodes for the characterization of adsorbed species,7 and also as a conducting support in the study of molecular electronics.8 Because gold deposited by evaporation tends to form grains on the order of 20-60 nm in diameter and ca. 10 nm in height9-11(depending on the evaporation method), limiting the height resolution correspondingly, much effort has been invested to produce flat gold over large areas by other means. Among these are surface annealing and flame annealing,9,12-15 but these * Corresponding author. E-mail:
[email protected]. Tel: +972-8-934-3823. Fax: +972-8-934-41-38. † Weizmann Institute of Science. ‡ Oxford University. (1) Kuhnle, A.; Linderoth, T. R.; Hammer, B.; Besenbacher, F. Nature 2002, 415, 891. (2) Janssen, J. W.; Lemay, S. G.; Kouwenhoven, L. P.; Dekker, C. Phys. ReV. B 2002, 65, 115423. (3) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 11, 321. (4) Ulman, A. An Introduction to Ultrathin Organic Films: From LangmuirBlodgett to Self Assembly; Academic Press: Boston, 1991. (5) Stipe, B. C.; Mamin, H. J.; Stowe, T. D.; Kenny, T. W.; Rugar, D. Phys. ReV. Lett. 2001, 87, 096801. (6) Chaal, L.; Pillier, F.; Saidani, B.; Joiret, S.; Pailleret, A.; Deslouis, C. J. Phys. Chem. B 2006, 110, 21710. (7) Garcia-Araez, N.; Brosseau, C. L.; Rodrigeuz, P.; Lipkowski, J. Langmuir 2006, 22, 10365. (8) Porath, D.; Millo, O. J. Appl. Phys. 1997, 81, 2241. (9) Chidsey, C. E. D.; Loiacono, D. N.; Sleator, T.; Nakahara, S. Surf. Sci. 1988, 200, 45. (10) Rost, M. J.; Quist, D. A.; Frenken, J. W. M. Phys. ReV. Lett. 2003, 91, 026101. (11) Nogues, C.; Wanunu, M. Surf. Sci. 2004, 573, L383. (12) DeRose, J. A.; Thundat, T.; Nagahara, L. A.; Lindsay, S. M. Surf. Sci. 1991, 256, 102. (13) Golan, Y.; Margulis, L.; Rubinstein, I. Surf. Sci. 1992, 264, 312. (14) Dishner, M. H.; Ivey, M. M.; Gorer, S.; Hemminger, J. C.; Feher, F. J. J. Vac. Sci. Technol., A 1998, 16, 3295.
generally result in islands that, although very smooth, have a rather limited area (100 µm2 compared to some 25 µm2 for TS gold), the cold-welding process is limited to specific contact positions and can be carried out in air only. Despite much progress in the field, there is still no procedure for creating macroscopically large (many mm2), molecularly smooth (i.e., rms < 0.5 nm) gold films on solid substrates or for force measurements in the SFB. There are, for example, no reports to our knowledge of forcedistance profiles between such smooth gold and mica or of the confining effect of a smooth metal surface on molecularly thin films. Here we describe a novel method for producing smooth gold surfaces that are suitable for use in the SFB but also for other applications requiring macroscopically large gold surfaces that are molecularly smooth over their entire area. Using our approach, which is based on template stripping,16,17 we can routinely produce almost perfect 〈111〉-oriented gold surfaces with rms roughness of ca. 0.2 nm over areas on the order of 1 cm2, that is, several orders of magnitude larger than previously attained. Moreover, our approach is optimized to eliminate the need for release (27) Smith, C. P.; Maeda, M.; Atanasoska, L.; White, H. S. J. Phys. Chem. 1988, 92, 199. (28) Akbulut, M.; Alig, A. R. G.; Israelachvili, J. N. J. Phys. Chem. B 2006, 110, 22271. (29) Connor, J.; Horn, R. Langmuir 2001, 17, 7194. (30) Horn, R. G.; Bachmann, D. J.; Connor, J. N.; Miklavcic, S. J. J. Phys.: Condens. Matter 1996, 8, 9483. (31) Horn, R. G.; Asadulla, M.; Connor, J. N. Langmuir 2006, 22, 2610. (32) Clasohm, L. Y.; Connor, J. N.; Vinogradova, O. I.; Horn, R. G. Langmuir 2005, 21, 8243. (33) Frechette, J.; Vanderlick, T. K. Langmuir 2001, 17, 7620. (34) Frechette, J.; Vanderlick, T. Langmuir 2005, 21, 985. (35) Frechette, J.; Vanderlick, T. J. Phys. Chem. B 2005, 109, 4007. (36) Levins, J. M.; Vanderlick, T. K. J. Colloid Interface Sci. 1993, 158, 223. (37) Levins, J. M.; Vanderlick, T. K. J. Phys. Chem. 1995, 99, 5067. (38) Levins, J. M.; Vanderlick, T. K. J. Colloid Interface Sci. 1997, 185, 449. (39) Knarr, R. F.; Quon, R. A.; Vanderlick, T. K. Langmuir 1998, 14, 6414.
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solvents, adhesive tape, or adhesion promoters for the glue, which are potential contaminants of the gold surfaces. In the following section, we describe the materials and characterization procedures used and then describe the new approach to creating such large, smooth gold surfaces. We characterize our gold surfaces using several different techniques: atomic force microscope (AFM) to establish the surface roughness, X-ray diffraction to determine the dominant orientation, contact angle measurements to monitor the cleanliness of the gold film with respect to ambient atmospheric contamination, and optical interferometry to examine the interfacial roughness in an SFB configuration. Finally, we demonstrate the feasibility of using our large, smooth gold surfaces as an interacting substrate in the SFB by determining for the first time the normal interactions between smooth (0.2 nm rms) gold and mica surfaces across water down to molecular contact. Materials and Methods We used ruby muscovite mica grade I or V-2 special grade, obtained from S&J Trading Inc (New York) both for the evaporation of gold for template stripping and for the force measurements. The evaporated gold was 99.99% pure from Kurt Lesker, and the evaporation process was carried out in an electron-beam-equipped Edwards 306/2 evaporating system. The mica template (whose preparation is described below) is mounted inside the e-beam evaporating chamber, 30 cm above a quartz lamp (General Electric 82V, 360 W) used for heating the sample. Following pump-down to ca. 5 × 10-6 mbar, the mica template is heated to 100-150 °C for about 2 h before the evaporation process begins. A layer of 82 ( 2 nm of gold (as measured with a precalibrated quartz thickness monitor and verified by AFM) is then evaporated at an average rate of 0.1 Å/s; the pressure inside the chamber is maintained at ca. 5 × 10-5 mbar during the evaporation. When the evaporation is complete, the sample is annealed again (via the lamp) at 150-200 °C for 1.5-2 h and then allowed to cool to room temperature prior to venting the chamber. This postevaporation annealing affects the quality of the X-ray diffraction peaks (see below) but not the rms roughness of the gold surface. AFM imaging of the gold was carried out using a Veeco Nanoscope IV - multi-mode, in tapping mode using Si tips from Nanoprobe Inc. X-ray measurements were performed in a θ-2θ configuration using a DMAX/B Rigaku diffractometer with an IU 200 X-ray generator from Rigaku. Contact angle measurements were made using a Rame´-Hart goniometer with an imaging system. Images of a sessile drop on the gold surface were taken with a CCD camera and analyzed using ImageJ freeware. Fringes of equal chromatic order (FECO) were recorded using a C60 Olympus digital camera with 2816 rows × 1112 columns of pixels. Force versus distance profiles between the template-stripped gold and a back-silvered mica sheet were measured across water that had been prepared by a standard three-stage process: reverse osmosis followed by an ion-exchange column and finally passage through a Barnsted Nanopure Diamond UV/UF system. This treatment resulted in water with a total organic content (TOC) of less than 1 ppb and resistivity of 18.2 MΩ cm-1 (so-called conductivity water). All glassware was cleaned with a 30:70 H2O2/H2SO4 piranha solution and then rinsed with conductivity water and ethanol. Preparation and Characterization of Large-Area Smooth Gold Films. Preparation of Samples for the EVaporation of Gold (Figure 1a).The preparation of the samples includes a few consecutive steps that are described below and schematically illustrated in Figure 1. Prior to the evaporation of gold, we prepare a backing sheet of mica with freshly cleaved single facets of mica placed on it. This procedure has been used traditionally for preparing mica surfaces for the SFB
Large Area, Molecularly Smooth Gold Films
Figure 1. Step-by-step schematic illustration of the sample preparation and the template stripping (TS) procedure. (a) Preparation of the samples: the mica is cleaved into 3-6-µm-thick facets of single-crystalline structure (ai). The facets are cut along their edge (dashed line, ai) with a Pt wire or are torn with tweezers.40 The pieces are then laid on a mica backing sheet onto which they adhere by van der Waals forces (either melt-cut facets, top and right, or torn-off bottom-left facet, partially laid on a step to facilitate peel off,40 aii). (b) Transfer of gold onto a lens: following evaporation and annealing (Materials and Methods), a mica sheet (from aii) is laid, gold side down, on a glue layer on a substrate (cylindrical lens of the SFB in this case), and the mica is stripped off with tweezers as indicated by the solid arrow, leaving a smooth gold film on the substrate as shown in the photograph (c) (lens diameter 1 cm). and has been described at length elsewhere40 and hence will be explained only briefly here: mica is cleaved in a laminar flow hood to expose 3-6-µm-thick (single crystallographic plane) facets that are molecularly smooth on both sides (Figure 1ai). The choice of mica pieces of this thickness, as well as the use of pieces that are molecularly smooth on both sides over their entire area, is crucially important to the success of the template-stripping step. The facets cleaved from the thicker mica sheet are either downstream melt cut or torn off,40 and when placed on a freshly cleaved backing sheet (Figure 1aii), they adhere to it to molecular contact under van der Waals forces. In contrast to the traditional procedure,40 the thin mica sheets are not turned over before they are placed on the backing sheet because it is the top, cleaner side that we are interested in for the evaporation of gold. The mica pieces on the backing sheet (the samples) are kept inside a desiccator (for 2 h at most) until being mounted inside the evaporating chamber, following which we evaporate the gold as explained in the Materials and Methods section. (40) Perkin, S.; Chai, L.; Kampf, N.; Raviv, U.; Briscoe, W.; Dunlop, I.; Titmuss, S.; Seo, M.; Kumacheva, E.; Klein, J. Langmuir 2006, 22, 6142.
Langmuir, Vol. 23, No. 14, 2007 7779 The backing sheet generally bears 7-10 pieces of the gold-coated mica pieces, each of area of a few cm2. It is kept inside a desiccator and may be used at least up to several months after the evaporation without any detectable change. Transfer of Gold to a Planocylindrical Lens for Use in the SFB (Figure 1b). A piece of gold-coated mica (or a part of a larger piece that was scalpel cut as indicated in Figure 1aii by the dashed line) is glued onto a fused-silica planocylindrical lens with the gold facing the glue, either Epon 1004 or Epo-Tek 301-2 (from Shell and Epoxy Technology, Inc, respectively), as in Figure 1b: similar results were obtained with both glues. We make sure that the gold-coated sample is larger than the lens to enable us in due course to grasp an overhanging edge with tweezers for template stripping. After the glue hardens (Epon 1004 melts at 180 °C and hardens at room temperature, Epo-Tek 301-2 is mixed in a ratio 1:0.35 A/B, and cured at 80 °C for 3 h), this edge is raised to strip the mica away and expose the gold film as shown in Figure 1c. As noted above, the choice of mica facets of thickness in the range of d ≈ 3-6 µm is crucial to this step: significantly thinner mica is hard to strip away from the gold, whereas significantly thicker mica has a high bending modulus (varying as d3) and is bent with difficulty by the capillary forces arising from the molten glue to follow the curved geometry of the lens. Rather, it will stick to the top ridge of the lens only. We emphasize that neither a razor blade nor scotch tape nor any solvent is needed to strip the mica smoothly away from the gold, and it is carried out using tweezers only,41 as shown in Figure 1b. We note also that following this step the mica template has a circular, goldfree region in its center (of radius ca. 1 cm equal to that of the lens) and that this circular region is seen, via reflection interferometry of monochromatic light, to retain its integrity and original molecular smoothness on both sides. This is consistent with the observed absence of any mica flakes or ridges corresponding to mica lattice steps on the gold surface following the stripping stage, as indicated both by AFM and scanning electron microscopy at increasing magnification over the entire gold surface. This stripping procedure can be performed on any substrate, curved or flat, and is not limited to use on planocylindrical lenses in the SFB. Indeed, the characterization work described below (other than with the ECO fringes) was performed on gold that had been glued (using either Epon 1004 or EPO-Tek 301-2) onto - mm-thick glass slides. Atomic Force Microscopy (AFM). AFM was used to image the gold surface. Figure 2a,b shows a top view of the mica-stripped gold of a 10 µm × 10 µm and a 1 µm × 1 µm region, respectively, and Figure 2c,d shows the side view of the images in Figure 2a,b. Cross sectional profiles, Figure 2d,e, show the rms roughness over the lines indicated to be 0.195 and 0.05 nm, respectively. The rms roughnesses taken over the entire images are 0.227 nm (Figure 2a) and 0.146 nm (Figure 2b). Similar images and rms values were obtained in a large number of different positions over the entire area of many mm2 of the TS gold film, suggesting that the entire TS film is smooth to that rms value. To the best of our knowledge, this is the lowest value for the rms roughness of a gold surface reported, whereas the continuous macroscopic area over which it appears to apply, on the order of 1 cm2, is some 4 to 5 orders of magnitude larger than that of previously reported TS gold or smooth gold produced by other means.16,17,20-22,39,42 X-ray Diffraction. In addition to the roughness of the surface, the crystallographic structure of the gold film is of considerable interest because it may affect interactions at the gold surface and, in addition, defines the potential of zero charge (PZC)43 at the surface. An X-ray diffraction study of the gold films was carried out to determine their (41) When immersing the nonstripped sample in water, the mica detaches spontaneously. This approach, which avoids exposing the gold surface to air prior to its immersion in water, may be used whenever direct exposure of smooth gold to water is desired. (42) Blackstock, J. J.; Li, Z.; Jung, G.-y. J. Vac. Soc. Technol., A 2004, 22, 602. (43) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley & Sons Inc.: New York, 1980.
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Figure 2. (a, b) AFM images of a gold surface, 10 µm × 10 µm and 1 µm × 1 µm, respectively, obtained using a Veeco Nanoscope IV Multi-mode in noncontact tapping mode (Si tips from Nanoprobe Inc.), taken in air following the stripping of mica as in Figure 1b. The height distributions along 10 µm and 1 µm cross sections (black lines) in a and b are shown in e and f, respectively, giving rms roughnesses of 0.195 nm (the peak-to-trough height over the same line is 1.024 nm) and 0.05 nm (the peak-to-trough height over the same line is 0.5 nm). The rms roughness taken over the entire image (a) is 0.227 nm, and 0.146 nm over image b. Similar images are obtained after immersion in water in an SFB experiment. The images in a and b are shown in 3D in e and f, respectively.
crystallographic orientation. Figure 3 shows the θ-2θ diffraction pattern obtained from the mica-template-stripped gold films together with the expected peak positions for polycrystalline gold (from pattern diffraction file no. 04-0784, International Center for Diffraction Data, Pennsylvania). It shows a well-defined 〈111〉 texture, with a very small peak (inset to Figure 3) corresponding to the 〈311〉 orientation. The intensity of the latter is just above the noise level and is less than 0.1% of the expected intensity of polycrystalline gold. Other peaks corresponding to polycrystalline gold are not visible above the noise level: we conclude that the gold film is composed of almost perfect 〈111〉-oriented domains. The mean size of the domains within the film in the direction perpendicular to the surface
can be calculated from the diffraction pattern using the Scherrer equation:44 Dd )
κλx B cos θ
(1)
Here, Dd is the size of the domains in a direction perpendicular to the surface, λx is the X-ray wavelength (1.54 Å), θ is the angle of incidence, and B is the full-width at half-maximum of the chosen (44) Cullity, B. D. Elements of X-ray Diffraction, 2nd ed.; Addison-Wesley Publishing Company: Reading, MA, 1956.
Large Area, Molecularly Smooth Gold Films
Figure 3. X-ray diffraction pattern of a gold film (on Epo-tek 3012/glass), taken with a DMAX/B Rigaku diffractometer with an IU 200 X-ray generator. The normalized diffracted intensity vs 2θ plot (θ ) angle of incidence) reveals almost perfect 〈111〉 texture over mm2 areas. (The footprint of the X-ray beam is ca. 1.7 mm2 at 2θ ) 38.2.) The lines show the expected peak positions and their relative magnitudes for polycrystalline gold (from pattern diffraction file no. 04-0784, International Center for Diffraction Data, Pennsylvania). On an enlarged scale (inset), a small peak corresponding to a 〈311〉 orientation is seen (arrow) to the left of the 〈222〉 peak, just above the noise level and of a magnitude of less than 0.1% of its expected intensity in polycrystalline gold.
Figure 4. Contact angle of a sessile water drop on the TS gold surface at increasing times following the stripping away of the mica. The increase in the contact angle with time (at ambient conditions) indicates the progressive adsorption of hydrophobic airborne contaminants from the ambient atmosphere. peak on the intensity versus 2θ plot, corrected by instrumental peak broadening as was measured with a Lanthanum HexaBoride (LAB6) standard. κ is a correction for the deviation of the crystalline domain shape from a sphere, and it varies from 0.9 (for a sphere)44 to 1.732 (for a cube).45 Because the shape of the single crystal domains is not defined, we can give a lower and an upper estimate of the domain size. Putting in the appropriate values, we find that D ranges from 58 nm (for κ ) 0.9) to 111 nm (for κ ) 1.732) for the 〈111〉 peak (2θ ) 38.2) and from 52 to 100 nm (for κ ) 0.9-1.732) for the 〈222〉 peak (2θ ) 81.65). Because the thickness of the film is only ca. 82 nm, this calculation suggests that single 〈111〉 domains penetrate through the entire film thickness. Preliminary STM micrographs show the presence of grain boundaries laterally separated by some 20-30 nm at the surface. Contact Angle Measurements. The cleanliness of the TS gold surface is reflected by changes in its surface energy on exposure to air. Clean gold is hydrophilic;46 however, when exposed to ambient air, airborne hydrophobic (probably carbonaceous) contaminants adsorb onto the gold surface and decrease its hydrophilicity. In Figure 4, the static contact angle of a sessile water drop on the gold surface is plotted as a function of the time elapsed from the stripping process. The initial low values (20-25°) of the contact angle show that the (45) Wilson, A. J. C. Proc. Phys. Soc. 1962, 80, 286. (46) Smith, T. J. Colloid Interface Sci. 1980, 75, 51.
Langmuir, Vol. 23, No. 14, 2007 7781 gold surface is reasonably hydrophilic for a few minutes after mica stripping. These results are in good agreement with previous results obtained with both TS gold and platinum42,46 and confirm the cleanliness of the gold at short times following its exposure to air. It is for this reason that, as a rule, whenever we use the gold surface in the SFB experiments we strip the mica off the gold when it is already mounted on its holder and then we immediately place it inside the water-containing chamber where the actual force profiles are measured. The elapsed time between stripping and coverage by water is generally less than 1 min. Multiple Beam Interferometry: Fringes of Equal Chromatic Order (FECO) in the SFB. An important measure of the feasibility of using gold as a substrate in the SFB is the quality of the FECO. The conventional interferometer used in the SFB is composed of a silver/ mica/medium/mica/silver combination, (henceforth intereferometer I) and hence it is a three-layer interferometer (the layers confined between the semireflective silver coatings). In the present study, we have replaced one back-silvered mica surface with a TS gold film to form the two-layer interferometer: gold/medium (air or water in these experiments)/mica/silver (interferometer II). Here, gold is used both as a reflector and as a substrate whose interactions with mica are to be characterized. In Figure 5, we show the FECO formed as a result of constructive interference between the two silver reflectors in interferometer I in air (Figure 5a, top) where the mica surfaces are in adhesive contact. (The surfaces jump spontaneously into contact.) The same is shown in Figure 5b for interferometer II (which is created when one backsilvered mica surface is replaced with gold). Both measurements were performed at similar spectrometer slit widths. Because mica is birefringent, each fringe is a doublet. We calculated the intensity variation and position of the fringes using the multilayer matrix method.47-49 A comparison between the calculated and measured peak positions is given in Tables 1 and 2 and the lower panels in Figure 5a,b compare the full fringe intensities. We note that because interferometer I consists of two mica sheets, whose thickness in contact is double that of the (same facet) single sheet in interferometer II, one expects that for a given wavelength range the fringe spacing in the former will be roughly half that of the latter, as is indeed observed in Figure 5a,b. The fringes in Figure 5 also provide a measure of the smoothness of the gold surface: earlier work by Levins et al.36 predicted (and demonstrated) that, for the (rough) evaporated gold surfaces used in their studies, fringe broadening as well as shifting to longer wavelength would occur relative to values calculated for ideally smooth gold surfaces. However, in our fringes we detect, within the scatter, no shift of the fringes to longer wavelengths. (See Table 1 for the peak positions.) At the same time, the broadening of the fringes (to a width δλ) using interferometer II (relative to interferometer I) may be attributed entirely to the increase in adjacent fringe spacing ∆λ (resulting from a reduction of the thickness of the mica in interferometer II): such fringe broadening is intrinsic to multiple-beam interferometry, for which it may be shown that, for a given slit width, the ratio δλ/∆λ is a function only of the reflectivity of the metal coatings (silver or gold).50 Because ∆λ (interferometer II) is roughly twice ∆λ (interferometer I), one might also expect that δλ (interferometer II) would be roughly twice the value of δλ (interferometer I). An examination of the fringe tips in Figure 5 shows that indeed the extent of peak broadening (interferometer II (47) Clarckson, M. T. J. Phys. D: Appl. Phys. 1989, 22, 475. (48) Born M.; Wolf, E. Principles of Optics; Pergamon Press: London, 1959. (49) The parameters that we used were µm(left) ) 1.5789 + 4.76 × 105/λ2, µm(right) ) 1.5835 + 4.76 × 105/λ2, µs ) [0.05 + i(-1.23 + 0.8734 × 10-3λ)], and µg ) [13.931 - 4.353 × 10-3λ + 3.451 × 10-7λ2 + i(-4.1877 + 1.2089 × 10-3λ)], where µm, µs, and µg are the refractive indices of mica, silver, and gold, respectively, and the coefficients are taken from Johnson, P. B.; Christy, R. W. Phys. ReV. B. 1972, 6, 4370. The thicknesses of the silver and gold films were 500 and 820 Å, respectively. The mica thickness is the average of four values, found by comparing the measured and calculated fringe positions for mica-mica (left and right singlets) and for gold-mica (left and right singlets), and it was found to be Z ) 46215 ( 1 Å. (50) Cook, A. H. Interference of Electromagnetic WaVes; Oxford University Press: London, 1971; pp 102-103.
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Figure 5. Fringes of equal chromatic order (FECO) obtained in the SFB with mica-mica and mica-gold combinations. (a) FECO with an interferometer I configuration: two symmetric back-silvered mica surfaces in air contact. (b) FECO with an interferometer II configuration: one of the back-silvered mica surfaces from a in air contact with a TS gold surface. The transmissivity vs wavelength λ plots in the lower part of the panel are based on the theoretical multilayer matrix calculation for this configuration49 (dashed lines) and are plotted together with the mean intensity at the fringe tips in a and b for comparison (solid lines). (c, d) Height h vs wavelength λ profiles of the FECO in a and b for the regions at the fringe tips outlined with white rectangles. Table 1. Measured and Calculated Values of the Peak Positions in Angstroms (Left, Right Singlets) in Contact (Interferometer I, Figure 5a)a measured ((0.2) multilayer matrix method ((1) a
λP1
λP2
λP3
λP4
5694.3, 5710.2 5694, 5709.8
5804.5, 5819.8 5803.6, 5819.6
5917.1, 5933.2 5917.4, 5933.8
6033.9, 6050.7 6036.2, 6052.8
The scatter is given in parentheses.
Table 2. Measured and Calculated Values of the Peak Positions in Angstroms (Left, Right Singlets) in Contact (Interferometer II, Figure 5b)a measured ((0.2) multilayer matrix method ((1) a
λP1
λP2
5953.3, 5970.8 5953.2, 5970.6
6194.2, 6212.8 6195.4, 6213.6
The scatter is given in parentheses.
relative to I) is in line with this doubling expected and that no significant additional broadening needs to be attributed to any roughness of the gold surface. We also plot the height (h) profiles versus the wavelength (λ) of the flattened fringe peak areas that are marked with rectangles in Figure 5a,b. The height scale is calculated from the calibrated magnification of our optical system. Each data point in the h(λ) plot corresponds to the peak position of a Gaussian fit to the light intensity for the given h value.51 For both interferometers, the spread in the peak position is less than 1 Å, indicating that replacing a half backsilvered mica surface by the reflecting gold surface makes little difference in the accuracy of our wavelength measurements in the force experiments. Overall, we conclude that the quality of FECO in our two-layer interferometer (II) is essentially as good as that in the conventional three-layer interferometer (I), consistent with the AFM profiles that indicate 0.2 nm rms roughness over the entire gold area. Force Profiles between Gold and Mica in Water. To examine the feasibility of using our ultrasmooth TS gold to measure surface forces in the SFB, we measured normal force (F)-separation (D) profiles between our gold surface and a mica surface across (51) At the edges of the fringe, where a Gaussian could not be fitted, we plot the maximum intensity instead.
conductivity water. As noted earlier, the TS gold was generally covered by water within 1 min of being stripped of its protective mica template. Figure 6 shows the normal interaction profile measured between TS gold and mica (full symbols) together with that measured between two mica surfaces of the same thickness (asterisk symbols) (from the same single facet), in both cases across conductivity water. For comparison, we have added the force profile between two mica surfaces from a different single facet (empty triangles).52 In both cases, mica-mica and mica-gold, the profiles are DLVO-like53 and indeed identical within the scatter: they exhibit a long-range osmotic repulsion and a jump-in to contact (due to the mechanical instability of the spring) from a separation of D ) Dj ) 6 ( 2 nm. The inset shows the region close to contact on an expanded scale, with a best fit to the data based on the solution to the PoissonBoltzmann equation at constant surface charge54 (black solid line - this assumes similar surfaces; for the case of dissimilar surfaces, a more detailed calculation is needed55,56). The fit corresponds to a Debye length of ca. 200 nm (corresponding to a 2 × 10-6 M 1:1 ion concentration) and a surface charge of 1.4 mC/m2, corresponding to a surface potential of ca. 140 mV when the surfaces are far apart, similar to surface potentials of gold in water determined in an earlier study.57 This implies that the gold is negatively charged in water, (52) Chai, L.; Klein, J. J. Am. Chem. Soc. 2005, 127, 1104. (53) Derjaguin, B. V.; Churaev, N. V.; Muller, V. M. Surface Forces; Consultants Bureau: New York, 1987. (54) Chan, D. Y. C.; Pashley, R. M.; White, L. R. J. Colloid Interface Sci. 1980, 77, 283. (55) Parsegian, V. A.; Gingell, D. Biophys. J. 1972, 12, 1192. (56) Chan, D. Y. C.; Healy, T. W.; Supasiti, T.; Usui, S. J. Colloid Interface Sci. 2006, 296, 150. (57) Biggs, S.; Mulvaney, P.; Zukoski, C. F.; Grieser, F. J. Am. Chem. Soc. 1994, 116, 9150.
Large Area, Molecularly Smooth Gold Films
Langmuir, Vol. 23, No. 14, 2007 7783 These measurements show unequivocally that the TS gold can readily replace one of the mica sheets by a molecularly smooth metal surface in surface forces experiments. This opens the way to several new avenues of research where the chemistry of gold and its metallic nature are of interest. These include chemical modification of the gold surface by adsorption of a self-assembled monolayer of alkylthiols for SFB measurements in air and especially in liquids. The length of such molecules (ca. 2 nm) is much larger than the 0.2 nm rms smoothness of the TS gold but comparable to the 2-4 nm rms roughness of evaporated gold surfaces used previously to replace one of the mica surfaces.33 Such measurements are of particular interest, for example, in the study of interactions between hydrophobic and hydrophilic surfaces.
Figure 6. Force F vs distance D profiles between a TS gold film and a single bare mica surface in conductivity water (filled symbols) together with force profiles between two symmetric bare mica surfaces in water (asterisks); empty triangles are from an earlier experiment between symmetric mica surfaces.52 D values calculated using matrix layer calculations (for gold-mica) and the standard optical equations (for mica-mica).61 Profiles are in all cases normalized as F/R with respect to the mica radius of curvature R. The inset is an enlarged view of the closest-approach region, indicating the jump-in point (J), where the solid line is a best fit of the constant-surface-charge model (see text) to the data, with κ-1 (Debye length) ) 200 nm and ψ (surface potential at a large separation) ) 140 mV. possibly because of the adsorption of hydroxide ions.58 The jump-in itself (over ca. 0.5 s or less) indicates, as discussed in detail earlier for confinement between two mica surfaces,59,60 that the viscosity of the water remains comparable to its bulk value even when confined between a metal (gold) and mica down to subnanometer films. We note that there is no detectable change in the force profile on a second approach, when the surfaces jump-in to the same contact position. On a third or forth approach, however, there is a change in the profiles, and the separation of the surfaces in contact is larger (as revealed by the FECO shifting to slightly longer wavelength) than on virgin and second contact. We attribute this to possible plastic deformation at the gold surface as was described by Levins et al.37 for gold in contact with mica. However, it is possible to access several different contact points in a single experiment (spanning as much as 5 mm across the gold surface), and each fresh point behaves like a virgin contact in the way described above. Moreover, the contact separation for each fresh point is identical within the scatter in the measurements of (7 Å. In general, the stripped gold surface was immersed in water within less than 1 min, as noted, to minimize hydrophobic contamination from ambient air, but in one of the experiments, measurements were carried out in air following the stripping of the mica template and prior to adding water to establish the air-contact separation. On adding water (after approximately 1 h in the SFB), the surfaces jumped into contact to a separation similar to their air-contact separation (though we cannot rule out the presence of a monolayer or two of water with our resolution). Finally, we note that at the end of an experiment the gold surface emerges wet from the water in the SFB cell. (58) Chen, A.; Lipkowski, J. J. Phys. Chem. B 1999, 103, 682. (59) Raviv, U.; Klein, J. Nature 2001, 413, 51. (60) Raviv, U.; Laurat, P.; Klein, J. J. Chem. Phys. 2002, 116, 5167. (61) Israelachvili, J. N. J. Colloid Interface. Sci. 1973, 44, 259.
Conclusions We describe a novel method, based on template stripping, to produce gold films of many tens of nanometers thickness (or thicker if desired) and area of ca. 1 cm2. We have characterized these films using several complementary techniques, including AFM, X-ray diffraction, contact angle goniometry, and optical interferometry. Our results indicate that the exposed gold surface is smooth to 0.2 nm rms roughness over essentially its entire area and that the film has an almost-perfect 〈111〉 orientation over its entire thickness. This smoothness compares with the 3 Å lattice step on the 〈111〉 gold lattice plane. To our knowledge, these gold surfaces are among the smoothest and are (by several orders of magnitude) the largest that have been reported. Such large, smooth areas provide useful substrates for examining the modification of surface properties by molecules whose dimensions are larger than the surface roughness, especially thiol-anchored, self-assembled layers of surfactants, and the scanning probe microscopy of single molecules of such dimensions (
