7316
J. Phys. Chem. 1993, 97, 7316-7320
Imaging Molecular Defects in Alkanethiol Monolayers with an Atomic Force Microscope Hans-Jiirgen Butt,' Karsten Seifert, and Ernst Bamberg Max- Planck-Inrtitut f u r Biophysik, Kennedyallee 70, 6000 Frankfurt 70, Germany Received: February 3, 1993; In Final Form: April 21, 1993
Octadecanethiol (CHs(CHz)l,SH) monolayers on sputtered gold have been imaged with an atomic force microscope, Octadecanethiol tended to form a hexagonal lattice with a lattice constant of 0.50 nm, despite the fact that the sputtered substrate was not crystalline gold(ll1). This indicates that packing of octadecanethiol on gold is dominated by the hydrocarbon chains. By imaging in electrolyte solution or alcohol we reduced the force applied by the tip to the sample. In this way, different kinds of molecular defects and packing structures of the alkanethiol monolayer could be imaged.
Introduction Many thiols form self-assembled highly ordered monolayers on gold.1.2 By choosing appropriate thiols, monolayers with different properties can be formed in a well-defined and predetermined way.3-7 Thiol monolayers are a model system for studying the correlation between molecular structure and macroscopic properties like wettability.3" In addition, they have been used for electrochemical studies!-" as a substrate for solid supported membranes,l2J3and to investigate the binding of specific adsorbates, e.g. certain ions14J5 or p r ~ t e i n s . ' ~ J ~ Alkanethiol (CHj(CHz),$H) monolayers are one of the best characterized organic monolayer systems on solid supports. By transmission electron diffraction's and low-energy helium diffraction19320it was shown that on gold( 111) alkanethiols form a hexagonal lattice with a lattice constant of 0.50 nm. As 0.50 nm is equal to the second nearest neighbor spacing of gold( 11l), alkanethiols might form a commensurate overlayer over the gold(1 11) surface. Indeed, several experimental results indicate that the gold structure determines the packing of the alkanethiols: On gold( 100) docosanethiol (n = 2 1) forms a base-centered square lattice instead of the hexagonal lattice.18 The axes of the alkanethiol lattice coincide with the axes of the gold( 111) surface. l8 However, it is not clear whether the alkanethiol lattice is commensurate with the gold( 111) surface. On the base of electron diffraction experiments Strong and Whitesides reported that docosanethiol is not commensurate with the underlying gold lattice.19 Chidsey and Loiacono reexamined the electron diffraction patterns and concluded that the alkanethiol monolayer adopts a commensurate overlayer lattice.21 Alkanethiolsongold are tilted by anangleofabout 30' between the alkane backbone and the surface normal.gJ8 The reason for the tilt is supposed to be the following: The gold(ll1) surface forces the thiols to form a lattice with a lattice constant of 0.50 nm. Alkyl chains have a diameter of only 0.42 nm. To avoid empty space between the alkyl chains the alkanethiols are tilted.22v23 Thiol monolayers on gold are a suitable system for scanning tunneling microscope (STM) or atomic force microscope (AFM) studies. Widrig et al. were the first to image thiols on gold( 111) with an STM.24 On ethanethiol ( n = 1) and octadecanethiol monolayers they confirmed the hexagonal packing and the lattice constant of 0.50 nm. In order to understand the imaging mechanism of STMs, Kim et al. imaged thiols with head groups of different physical size.25 The size of the head groups ranged from 0.43 to 1.3 nm. The surface always showed a hexagonal lattice with a lattice constant of 0.50 nm, regardless of the size of the head group. This led to the conclusion that STM images show the electronic distribution of the gold substrate that has been perturbed by the adsorbed thiols rather than actually imaging
the monolayer itself. Alkanethiols of different length have also been imaged with an AFMa26 In all cases the hexagonal arrangement of the molecules with a lattice constant of 0.50 nm was confirmed. In this paper we present AFM images of octadecanethiol monolayers on sputtered gold. The surface of sputtered gold is not crystalline, at least not over areas larger than about 1 nm2. The aim of this study was to reveal the packing of alkanethiols on such a rough gold surface. Octadecanethiols tended to form a hexagonal lattice with a lattice constant of 0.50 nm. AFM images were taken in electrolyte solution or alcohol. In this way the force between tip and sample was reduced27.2* and it was possible to image not only the hexagonal lattice but also defects with molecular resolution. Simultaneously, that is whileimaging, measurements of the electric capacitance indicated a monolayer thickness of 1.9 nm.
Materials and Methods SubstratePreparationand Monolayer Formation. Glass plates of 12-mm diameter were cleaned, dried, and silanized in an atmosphere of dichlorodimethylsilane (Fluka,Buchs Switzerland) for 1 min. Then the plates were kept at 130 O C for 1 h to allow the silane to react with the glass surface. Gold was sputtered under an argon atmosphere of 0.04 mbar at a rate of 0.3 nm/s. The final thickness of the gold layer was 50-100 nm. Silanization was done to make the glass surface hydrophobic and enhance the stability of the gold on the glass; without silanization the gold film sometimes came off the glass when it was put into electrolyte solutions. While sputtering we used masks to obtain well-defined gold-covered areas on the glass. Only the center area with a diameter of 5 mm plus a thin connecting line were covered with gold. For monolayer formation the sputtered gold films were incubated in 1 mM octadecanethiol ( n = 17, Aldrich, Steinheim, Germany) in ethanol for 5 h at 30 'C. Then they were rinsed with ethanol and dried. In a dust-free environment these plates could be stored for several weeks, though we usually used them within the first week after preparation. Atomic Force Microscopy. All images were taken with a Nanoscope I1 (Digital Instruments, Santa Barbara, CA) using sharpened silicon nitride cantilevers (Olympus, Tokio) in constant force mode. To be able to image in ethanol, 2-methyl-2-propano1, or electrolytesolutions, theimaging cell was sealed with an O-ring. The scanner was calibrated laterally with images of mica. Vertically the scanner was calibrated with purple membranes and an inclined plane as described previously.29 To avoid artifacts due to friction between tip and sample it was always verified that back and forth scans were identical, except for a small hysteresis. Only results where a t least two subsequent images showed the same surface details were considered.
0022-365419312097-7316%04.00/0 0 1993 American Chemical Society
Imaging Molecular Defects in Alkanethiol Monolayers
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The Journal of Physical Chemistry, Vol. 97, No. 28, 1993 1311
0c t ad e c ane thiol
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Elgure 1. AFM imageof sputtered gold with an octadecanethiolmonolayer imaged in ethanol. The image was slightly low-pass filtered.
0
0.35 0.40 0.45 0.50 0.55 0.60 Lattice constant / nm Figure 3. Histogram of measured lattice constants of hexagonal lattices. Plot of the frequency at which a certain lattice constant was obsewed in octadecanethiol monolayers on sputtered gold. For comparison, values obtained on mica are also shown. Values for mica were obtained in the following way: Before each series of experiments mica was imaged and the latticeconstant was determined and included in the histogram. Then the calibration factors of the AFM were corrected. All images were corrected for drift before determining lattice constants.
Figure 2. Best image of hexagonally packed octadecanethiol monolayer on sputtered gold. The image was taken in 2-methyl-2-propanol. The best-fit plane was subtracted.
Electric Measurements. To measure the capacitance of the octadecanethiol/gold surfacea current amplifier ( 1O8 or 109V/A) with a low-pass filter (10-100 Hz) was connected to the gold electrode. The connecting line and the contacts were covered with electricallyinsulating nail polish. A voltagecould be applied via an Ag/AgCl or a platinized Pt-electrode counter electrode. Results and Discussion Figure 1 shows a 90 nm wide image of octadecanethiol on sputtered gold. On this scale no significant difference was observed between bare sputtered gold and octadecanethiolcovered gold. Only if the samples were not carefully rinsed with ethanol after incubation with the octadecanethiol were islands often observed with a approximate diameter of 200 nm and a height of about 10 nm. The roughness of sputtered gold and of octadecanethiol/gold was 3-4 nm (for further details see ref 30). On bare sputtered gold no periodic structures were observed. As it is normally possible to resolve the hexagonal lattice of an epitaxial gold surface with an AFM,31we believe that the surface of sputtered gold had no crystalline regions larger than about 1 nm2. Figure 2 shows an image of hexagonallypacked octadecanethiol molecules on sputtered gold. On all samples we found crystalline domains. Typically these domains had diameters around 3 to 7 nm, which agrees with domain sizes reported for alkanethiols on gold( 11 1).18-20J6 Ordered domains were separated by regions without periodic structure. Roughly half of the surface was covered by ordered regions, and about 20% displayed real twodimensional crystals. The lattice constants obtained from many imagesof crystallinedomains were plotted as a histogram (Figure 3). Themean latticeconstantwas0.498 f 0.038 nm. Thenumber
of experiments was not yet sufficient to decide whether the peak at 0.46 nm is significant and represents a different crystalline form. The variation of measured lattice constants was significantly larger than that for lattice constants of 0.5 16 f 0.01 3 nm measured on mica (also shown in Figure 3). Mica also has a hexagonal surface lattice with a lattice constant of 0.5 19 nm. We believe that the large variation of measured lattice constants reflects a variety of slightly different packing structures of octadecanethiols on gold. Sometimes the lattice was not even perfectly hexagonal, e.g. the two lattice constants differed up to 0.05 nm and the angle varied roughly between 1 10' and 130'. Often crystalline regions extended over relatively rough areas (Figure4). In theseareas the substrate wascertainly not a perfect gold( 11 1) surface. This indicates that ordering of the octadecanethiol monolayer is governed by the interaction between the hydrocarbon chains. If the hydrocarbon chains and not the underlying gold substrate determine the packing structure, the question arises why the alkanethiolsare tilted. This tilt, however, is not so surprising. A tilt around 30' is often observed in alkane monolayers (see ref 32 and references therein). Ulman et al. calculated the interaction energy of alkanethiol monolayers for different lattice constants and tilt angles.22 They treated the gold surface as a laterally uniform area. In addition to an energy minimum at a lattice constant of 0.42 nm (tilt angle OO), they found another minimum for a lattice constant of 0.50 nm (tilt angle 30') with about the same energy. Hence, a hexagonal packing of octadecanethiols on gold with a lattice constant of 0.50 nm is not unlikely and the gold(ll1) surface might not be necessary to enforce such a packing. We can only speculate about how it is possible that the crystallineoctanedecanethiol layer binds the noncrystallinegold. One possibility is that each thiol group is not bound to a specific gold atom; the bond is laterally not localized. If the bond is localized (each thiol group forms a stable bond to a specific gold atom) the thiol groups and probably some part of the hydrocarbon chain should be flexible. Often steps were observed in the octadecanethiol monolayer. The height of many steps was measured and plotted in a histogram (Figure 5). Steps occurred preferentially with a height of 0.13, 0.22, and 0.33 nm or larger. One CH2 group spans 0.123 nm along the hydrocarbon chain. As the alkanethiols are supposed
7318 The Journal of Physical Chemistry, Vol. 97, No. 28, 1993
Butt et al.
Figure 6. Single molecular defect in an octadecanethiol monolayer on sputtered gold. The image was taken in ethanol and was slightly lowpass filtered.
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P
n Figure 4. AFM image of hexagonally packed octadecanethiol on gold. The surface plot (a, top) shows that the substrate is relatively rough. In the top view (b, bottom) an average tilt was removed. On the left side a dislocation can be seen. The image was taken in ethanol and was slightly low-pass filtered.
20
15
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5
0
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0.3
/
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Figure 5. Histogram of step heights measured on octadecanethiol monolayers on sputtered gold. Plot of the frequency at which a certain step height was observed. The histogram was fitted with a sum of three Gauss functions. Maxima were at 0.125, 0.222 and 0.331 nm.
to be tilted by 30°, the observable height difference should be 0.123 nm X cos 30' = 0.107 nm per CH2group. Thisagrees with measured step heights. It seems as if in response to the surface roughness the octadecanethiols have slipped one, two, or three CH2 groups. This is another hint that the packing of octadecanethiol monolayers is dominated by the hydrocarbon chains
Figure 7. Single molecules sticking out of an octadecanethiol monolayer on sputtered gold. The image was taken in 2-methyl-2-propanoland was slightly low-pass filtered.
and not by the surface functional group. Otherwise we would have expected to image multiples of the step height of gold, which is 0.25 nm. Measured step heights should, however, not be overinterpreted. Though wecarefullycalibrated the AFM and tried to avoid friction effects, calibration errors at the 0.1-nm scale or errors due to friction effectscannot totally be eliminated. In addition, choosing steps on which the height was measured is a subjective method. Still, the step heights measured are significantly smaller than the values expected for gold. Different types of defects were observed in octadecanethiol monolayers. The hole in the upper left of Figure 6 shows a single molecule which lies at least 0.24 nm deeper in the monolayer. It cannot be decided if it had a shorter alkyl chain, if it slipped two CH2 groups, or if there was no molecule at all. Figure 7 shows single molecules sticking out of the monolayer. Sometimes the hexagonal lattice was curved (Figure 8). Then rows of molecules did not form straight lines anymore. Often rows of molecules or pairs of rows seemed to form stable structures (Figure 9). Defects could usually be scanned many times without changing their structure. This indicates that binding to the gold substrate is relatively tight and that the octadecanethiol molecules do not move laterally. Packing of the octadecanethiol molecules did not depend on the medium in which they were imaged. All types of defects were observed in electrolyte solution, ethanol, and 2-methyl-2propanol. Generally imaging quality in ethanol and 2-methyl2-propanol was better than in electrolytesolution. This is probably partly due to a reduction of van der Waals forces2* The ability to image molecular defects demonstrates the high resolution of AFMs. Giessibland Binnig pointed out that despite
The Journal of Physical Chemistry, Vol. 97, No. 28, 1993 7319
Imaging Molecular Defects in Alkanethiol Monolayers
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Figure 8. Octadecanethiolmonolayerson sputtered gold. The hexagonal lattice is curved and rows of moleculesdo not form straight lines anymore. The image was taken in electrolyte solution (10 m M NaCl, pH 6) and slightly low-pass filtered.
I
Figure 9. Octadecanethiol molecules on sputtered gold tended to form rows or pairs of rows. The image was taken in ethanol. The best-!it plane was subtracted and a weak low-pass filter was applied.
the fact that many groups achieved atomic resolution on samples with a perfect two-dimensional lattice, it was never totally clear whether the resolution was actually atomic.33 In many cases the force between tip and sample was so large that more a one-atom "minitip" must have contributed to the image contrast. Even if the contrast is caused by many contact points between the AFM tip and the sample, it is still possible that the image shows the surface periodicity. Giessibl and Binnig considered the imaging of an atomic step or a point defect as a criterion for true atomic resolution. By imaging a KBr(001) surface at 4.2 K in UHV they were able to reduce the force between tip and sample to less than 1 nN. Under these circumstances they could image monoatomic steps with atomic resolution. Imaging octadecanethiol monolayers on gold with truen molecular resolution (in the sense that molecular defects could reproducibly be imaged) was probably only possible by reducing the force between tip and sample. Alves et al. imaged alkanethiols in air with a force of 10 nN.26 We reduced the force by imaging in electrolyte solution, ethanol, or butanol. Due to the absence of the meniscus force2' and a reduction of the van der Waals force28 we imaged with a force of about 0.3 nN. Interestingly, we also had to use a certain minimal force of about 0.1 nN to get good contrast. For lower forces the image contrast degraded. Low-energy helium diffraction and infrared spectroscopy studies indicated thermal motion of the outermost methyl group at r c " t e m p e r a t ~ r e . ~A . ~minimal ~ . ~ ~ force of 0.1 nN might have been necessary to penetrate the outermost methyl groups. In order to estimate the thickness of octadecanethiol layers imaged with the AFM in electrolyte solution we simultaneously measured their electric capacitance. The "capacitor" consists of
0
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Time / sec Figare 10. Electric current density (top) measured when applying a triangular voltage signal to the counter electrode (bottom). This measurement was done with an Ag/AgCl counter electrode in electrolyte solution (100 mM NaCl, 10 mM MES, pH 6.0). the gold substrate and the electrolytesolutionas the twoelectrodes with the octadecane film as the insulating layer in between. The capacitance is given by C = ee&/d, where e is the dielectric constant of the alkane layer, is the vacuum permittivity, A is the surface area, and d is the thickness of the alkane layer. Triangular voltage signals of amplitude UOwere applied to the counter electrode. To analyze the signals we neglected the resistance of the counter electrode, the resistance of the electrolyte solution, and the capacitance of the counter electrode.9J1 Figure 10 shows the resulting current density. It is approximately the sum of a dominating rectangular signal and a triangular signal. The amplitude of the rectangular signal is the product of the capacitance with the change of the voltage sweep rate. The average capacitance per unit area of the octadecanethiol layer on gold was 1.0 f 0.2 pFlcm2. With c = 2.2,35 the film thickness was 1.9 f 0.4 nm, which agrees with the valueof 1.91 nm expected for octadecane tilted by 3 0 ' . The amplitude of the triangular signal is given by &/R, where R is the resistance per unit area of the octadecanethiol/gold surface. A resistance of about 2 MQ cm2 was measured. Bare gold surfaces, which showed a purely triangular current signal, had a 5 to 10 times lower resistance. However, the results should not be overinterpreted. When estimating the area of our films wedid not consider the roughness of the sputtered gold which might increase the exposed area. Measuring resistances of electrodes is by no means simple. Due to polarization the resistance changed with time and it should be kept in mind that the value reported refers to a measurement done with 0.5 Hz. For a detailed discussion of capacitance and resistance measurements we refer to Porter et a1.9 and Chidsey and Loiacono.2' Still, measuring the electricalcapacitance while imaging allows the correlation of molecular structure with macroscopic electric properties of alkanethiols on gold.
Conclusion Octadecanethiol tends to form a hexagonal lattice even on such a rough surface as sputtered gold. This demonstrates that the two-dimensional packing of octadecanethiols is governed by the interaction between the hydrocarbon chains. The AFM images show that it is possible to obtain "true" molecular resolution on organic monolayers, in the sense that molecular defects can be reproducibly imaged.
Acknowledgment. We would like to thank W. Haase and H. Volk for their help. This work was supported by the Deutsche
1320 The Journal of Physical Chemistry, Vol. 97, No. 28, 1993 Forschungsgemeinschaft SFB 169 and the DFG SPP Neue mikroskopische Techniken far Biologie und Medizin (H.-J.B. and E.B.).
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