Beam Interactions with Materials & Atoms
EISBVIER
Nuclear Instruments and Methods in Physics Research B 131 ( 1997) 245-251
Modification of self-assembled monolayers of alkanethiols on gold by ionizing radiation Mikael Wirde ay*,Ulrik Gelius a, Tim Dunbar b, David L. Allara b a Dept. of Physics, University of Uppsala, Box 530. S-751 21 Uppsala, Sweden b Dept. of Chemistry and Materials Science, Pennsylvania Stare College, University Park, PA 16802, USA
Abstract Self-assembled films of octadecanetbiols on polyctystalhne Au have been studied with monochromated X-ray photoelectron spectroscopy (XPS). The focus has been on the S-Au interface. It is demonstrated that ionizing radiation causes breaking of S-Au bonds and formation of disulfides. The modification of the alkane chains and the S headgroups has been investigated as a function of X-ray exposure. Long time X-ray exposed films have also been characterized by polarized reflection infrared spectroscopy, single-wavelength ellipsometry and water and hexadecane contact angle measurements. All these data show that in addition to the disulfide formation at the gold interface, the originally well ordered hydrophobic film is strongly modified by the ionizing radiation, involving C-H and C-C bond breaking, interchain cross linking and a general disordering of the film. @ 1997 Elsevier Science B.V. Keywords: Self-assembled monolayers; XPS; Surface modification; Ionizing radiation; X-rays
1. Introduction During the last few years, self-assembled monolayers (SAMs) have attracted great attention from researchers in the field of surface science [ l-l 83. So far the most widely used substrate-adsorbate configuration has been that of thiolated compounds adsorbed onto gold [ 5-9,11,12]. Copper and silver have also been shown to be suitable as substrates [ 13-l 81. These films have applications in the field of biosensors, as substrates for polymer growth, and can be used as a basis for further understanding the functionality of lipid bilayers of cell membranes and other organic surfaces. They also provide interesting systems for
* Corresponding author; Fax: +46 18 - 18 36 11; Tel: +46 18 18 36 03; E-mail
[email protected]
studying charge transport in organic materials [ 2,191. n-alkane thiols (CHs (CH2) (“__I)SH, abbreviated C,,) are generally assumed to adsorb as thiolates ( CHs ( CHz) (,,_ 1)S-> on gold surfaces. However, Fenter et al. 1201 have observed a second species of sulfur on the Au surface using grazing incidence X-ray diffraction (GIXD) on Cic, which has been interpreted as being a disulfide. The presence of disulfides has furthermore been confirmed by Zubragel et al. [ 181 by using X-ray photoelectron spectroscopy (XPS) with both monochromated AlKa (1487 eV) and synchrotron (200 eV> excitation on Cz2. XPS is known to be suitable for quantifying the chemical nature and the coverage of organic films, such as the SAlvls, and also for determining the extent of possible surface contaminants. In our study we characterized Cis/Au SAMs using
0168-583X/97/$17.00 @ 1997 Elsevier Science B.V. All tights reserved. PIISO168-583X(97)00140-7
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an XPS instrument with a high-intensity monochromated AlKa source. Like Fenter et al. and Zubragel et al., we observed the presence of two chemically different sulfur species, which we also interpreted as a thiolate and a disulfide. However, we early observed that the relative amount of the disulfide was not constant, but could vary substantially from one measurement to the next. We eventually realized that the amount of disulfide was strongly influenced by the total X-ray dose given to the observed region on the SAM film. In addition, the XPS spectrum from the alkane chain was seen to change with time. In 1987, Nuzzo et al. studied XPS on dimethyl disulfide films [ lo], and in that study it was noted that X-ray irradiation causes disulfide formation on methanethiol monolayers. This motivated us to start a systematic investigation of how the main XPS features of the CrsIAu system depend on the X-ray exposure time.
2. Experimental 2.1. Sample preparation
The samples consisted of glass, whereupon a layer of Au had been sputtered to a thickness of the order of 200 nm, using an adhesion layer of chromium. They were carefully cleaned by immersing them in a 5: 1: 1 H20/NHs/Hz02 solution at 80°C for 2 minutes. The success of this cleaning procedure was confirmed by recording survey spectra with XPS before and after the process. Apart from a small amount of hydrocarbon and water contamination present due to the few minutes of handling the wafer in normal atmosphere, all contamination was reduced to a negligible level. We noted earlier that thiols adsorb spontaneously when gold substrates are exposed to laboratory air for some time. After cleaning, no trace of sulfur was detected. The substrates were immersed in a 1 mM solution of Crs in ethanol (99.5%) at room temperature. The immersion time was varied between one day and one month, without noticeable differences in the quality of the monolayer. After the exposure to Crs, the sample was ultrasonically cleaned in chlorobenzene (99%) for 5 min, rinsed with isopropanol and water, and finally blown dry with Ar prior to mounting on a stainless steel sample holder. All samples - except where otherwise noted - were of dimension 1 x 1 cm.
2.2. The experiments The XPS experiments were performed with a SCIENTA ESCA 300 spectrometer using monochromatic AlKa radiation ( 1487 eV) [ 221. The instrument is provided with a high power rotating anode and the X-rays are monochromated using a wide-angle quartz crystal monochromator. The X-rays irradiate the sample at an incidence angle of about 45” in a narrow, elliptical spot with a major axis of about 5 mm and a minor axis of about 1 mm. All XPS measurements were performed with a pass energy of 150 eV and an analyzer entrance slit width of 1.5 mm for the S2p and C 1s regions, corresponding to an instrument resolution of 0.46 eV. The Au4f lines were also examined for confirming the stability of the energy scale of the instrument. The power of the X-ray electron gun was kept constant at 8 kW for all the experiments (producing a Aux of about 1.7 x lOI2 monochromated AlKa photons s-t at the sample) [ 231. A thin Al X-ray window, placed 30 cm from the sample position, separates the sample analysis chamber from the X-ray monochromator chamber. It has more than 80% AlKa transmission, while it effectively stops all scattered secondary electrons from the X-ray source. Energetic electrons are also created as the X-rays pass through the X-ray window. However, the long sample-X-ray window distance considerably reduces the flux of these electrons at the sample position. The photoelectron take-off angle was 90”. The pressure in the analysis chamber was approximately 23 x lo-” mbar during measurements. The temperature of the sample was 28’C when recording the spectra. Since our results were highly dependent on time, special care was taken in noting when each spectrum was recorded. The XPS experiments were organized in terms of S2plClslAu4f sequences, where each sequence required 11.5 minutes to complete, whereof the S2p region alone took over 9 minutes. The first 168 (software limited) sequences were measured consecutively, to achieve highest possible time resolution at the beginning of the experiment. After 41 h of continuous X-ray irradiation, the measurements were repeated with another 18 sequences. After a total of about 84 h, a final set of 12 sequences were recorded.
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3. Results After acquisition, the XPS spectra were organized so that the first 18 S2p/Cls/Au4f sequences were averaged two-by-two. The rest of the spectra were averaged using six such sequences each. Fig. 1 shows four examples of averaged spectra taken from the S2p region. The first spectrum (O-23 min) shows a normal, single component S2p doublet at 162.0 eV (referred to the Au4fT/2 line at 84.0 eV), with its spin-orbit splitting clearly resolved. This component we denote S 1. After about one hour a new, chemically shifted component (S2) is seen to appear on the high binding energy side. In Fig. 1 its presence is seen from the deviation from the expected 2: 1 ratio of the components in the S2p doublet. The energy shift of this component is +1.3 eV. The relative abundance of the S2 component (the ratio S2lSt01, where S,,, is the total S2p area) as a function
241
of time is obtained from evaluating the corresponding S2p areas in the XPS spectra. To obtain these areas, curve fits were performed using a common set of parameters. A linear background was initially subtracted from the spectra. This background was obtained by fitting a straight line to the first 21 channels on the low binding energy side. The average of all spectra was used to find the slope of the line. The final curve fit on the residual spectra was then performed using a Shirley type background [ 241 and up to three S2p doublets, which were described by pairs of approximate Voigt functions [ 251 with relative intensities of 2: 1 and energy separation 1.19 eV. A weak third component (S3) at 161.3 eV was necessary to introduce on the low binding energy side in order to obtain good fits. This doublet is seen as a small shoulder on the low binding energy side of the S2p peaks. The most probable explanation of this weak component is that it corresponds to sulfur atoms having a larger coordination number to gold atoms than the normal thiolate molecules. This would correspond to thiolates adsorbed on steps, kinks and hollow sites. The time dependence of the S2/S,, ratio is shown with a linear time scale for the first 4 h of continuous X-ray irradiation in Fig. 2a, and with a logarithmic time scale for the full experiment in Fig. 2b. The initial S2 formation is fairly rapid. After about 3 hours of irradiation, half of the Sl type sulfur has been transferred to S2 type sulfur. 5 h later, the ratio reaches a maximum of 64%, after which it decreases slowly towards 50%. Fig. 2c shows the time dependence of Sot, normalized to t = 0. This area is practically constant during the first 4 h. Then it starts to gradually decrease, and after about 8 h, when it has lost about 10% of the initial area, it shows an exponentially decreasing dependence.
4. Discussion The first and most noticeable effect of the irradiation is the occurrence of the S2 component around 163.2 eV in the S2p region (Fig. 1) , which has already been interpreted as being a disulfide [ 181. To confirm that this is a disulfide, an indium sample was scraped and some disulfide, (CH3(CH2) t7S)2, was pressed onto its surface. The resulting film was measured with
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XPS, recording the S2p and the C 1s regions. The C 1s region was then used in an energy scale calibration, since the film was an insulator. Assuming a binding energy of 285.0 eV for the main Cls peak, and shifting the S2p spectrum accordingly, the binding energy of 163.2 eV could be verified for this disulfide sulfur. The influence of the X-ray irradiation on the Cls peak of Cts is shown in Figs. 3 and 4. During irradiation the Cls position shifts 0.5 eV towards low binding energies, and the full width at half maximum (FWHM) increases by 0.25 eV. These trends are signs of a C-C crosslinking process occurring during the irradiation. The Cls intensity gradually decreases, and after 4 days reaches about 2/3 of its initial value. However, this may not be the final value since our experiment ended before a permanent equilibrium was obtained. It is reasonable to assume that the first step in the C-C crosslinking process is a loss of hydrogens from the thiol alkane chains. X-ray photons interact only
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desorption. weakly with the valence electrons of the alkanethiols, making direct valence photoionization a less probable process for causing C-H dissociation. This dissociation is instead dominated by either Auger electron decay, following Cls photoionization, or by valence excitation or ionization induced by inelastic scattering of > 10 eV primary and secondary electrons. The majority of these electrons originate from the gold substrate, as seen from the wide XPS spectrum of the alkanethiol/gold system in Fig. 5. Once two neighbouring C-H bonds have been broken, the two alkane chains can react with each other, forming a crosslink. There are in principle two possible types of reaction processes which can result in formation of C-C crosslinks. The first type of process (denoted type 1) involves only one ionizing particle - photon or electron. After the breaking of one C-H bond, the free hydrogen atom immediately reacts with the neighbouring alkane chain, creating a Hz molecule, which diffuses out of the film and is pumped away. The remaining neighboming alkane chains react with each other, forming a crosslink at this location.
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The second type of process (type 2) involves two independent events. The argument here is that after a C-H bond has been broken, the hydrogen atom is lost to the surroundings. Nothing more happens to the alkane chain until another particle excites or ionizes an adjacent alkane chain at the same height. The type 2 process should therefore not become important until a major fraction of the film has been modified. In both processes the chains are pulled closer together locally in the crosslinked region. When the crosslinking occurs near the thiolate end it is reasonable to expect that also the sulfur atoms will be pulled closer together, and therefore tend to form a disulfide unit, while breaking or weakening the S-Au bond. The shape of the curve in Fig. 2a tells us which of the type 1 or type 2 processes is responsible for the disulfide formation. Except for small deviations due to noise, the initial amount of disulfide increases linearly with time, in accordance with the expected type 1 process, whereas the type 2 process is expected to exhibit a quadratic dependence, with a zero derivative at t = 0. As the number of disulfides increases, the corresponding rate of disulfide formation decreases. This produces the convex curve shape shown in Fig. 2a. The conclusion is therefore that the disulfide formation is explained by the type 1 process. The Cls peak is shifted towards low binding energies during the irradiation, as seen in Fig. 4a. Although there is no sign of stabilization of energy at the end of the experiment, the final measured binding energy closely resembles that of graphite, reported to be at 284.4 eV [ 261. However, the chaotic structure resulting from the irradiation, as seen by the broadening of the Cls peak suggests a film similar to “glassy carbon”, amorphous carbon. It should be noted that the time dependence of the Cls intensity seen in Fig. 4c may not only be affected by the thiols but also by possible hydrocarbon contaminants that adsorb onto the surface, especially after long X-ray irradiation. The films may themselves be the source of these hydrocarbon contaminants, since desorption is likely to occur at all stages of the irradiation. Some desorbed alkane chains are removed from the film completely, while others tend to fall back in a disordered fashion on the film. After ultrasonic cleaning in a solvent, most of these contaminants are probably removed, but since the surface is oleophilic after irradiation, molecules from the lab atmosphere rapidly contaminate the sur-
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5. Summary
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face again. Fig. 6 shows the total S2p/Cls area ratio as a function of time. Let us first assume that the observed initial loss of Cls intensity is only due to desorption of complete alkanethiols from the film. The loss of S2p intensity would then be equal to the loss of the C 1s intensity, implying a constant S2p/Cls ratio. However, Fig. 6 clearly shows an initially increasing S2p/Cls ratio. Fig. 2c shows that the total S2p intensity remains constant during at least the first two hours of irradiation. During this time interval, C-C bond scission must therefore be the process responsible the observed initial Cls intensity loss. The S2p/C 1s ratio continues to increase until after about 5 hours, when the rate of decrease of the S2p area overtakes the rate of decrease of the Cls area. This shows that when the irradiation continues for a longer period of time, sulfur desorption starts to dominate over hydrocarbon desorption. At this point, the amount of disulfides has reached a maximum as seen in Fig. 2b, and the total S2p area begins to decrease noticeably (Fig. 2~). These facts indicate that the sulfur desorption primarily arises from small disulfide units, with no or a few carbon atoms. The desorption of these small disulfide units has become possible by the C-C scission and C-C crosslinking processes discussed above.
X-ray irradiation modifies self-assembled monolayers of alkanethiols on gold through C-H and C-C bond scission, C-C crosslinking and disulfide formation. The time dependence of the XPS data supports the model where a single C-H dissociation close to the sulfur leads to crosslinking and the formation of a disuifide bond. The C-C crosslinking acts as a stabilizing mechanism for retaining a carbon film, yielding a disordered structure reminiscent of “glassy carbon”. Such a stabilizing mechanism is not seen for the sulfur atoms, which desorb as small disulfide units after a long exposure. By extrapolating the data to t = 0, it is concluded that as-prepared SAM samples do not contain disulfides. The recently reported presence of disulfides in self-assembled films of alkanethiols on gold from Xray diffraction and X-ray photoelectron spectroscopy studies may therefore be induced by the radiation dose required to study these films.
Acknowledgements The authors wish to thank Bo Liedberg at the University of LinkGping for providing gold substrates. This project has been supported by the Swedish Natural Science Research Council (NFR), the National Science Foundation (USA) and the Advanced Research Projects Agency (USA).
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[ 191 C.E.D. Chidsey and D.N. Loiacono, Langmuir 6 ( 1990) 682. [20] P Fenter, A. Eberhardt and P Eisenberger, Science 266 (1994) 1216. [21] M. Wirde, T. Dunbar, U. Gelius and D.L. Allara, in manuscript. 1221 U. Gelius, B. Wannberg, P Baltzer, H. Fellner-Feldegg, G. Carlsson, C.-G. Johansson, J. Larsson, P Miinger and G. Vegerfors, J. Electron Spectrosc. Relat. Phenom. 52 ( 1990) 747. [23] The X-ray flux was evaluated from measuring the electron yield from a gold sample using 0.046 as the gold efficiency factor for 1487 eV photons. See M. Krumrey, E. Tegeler, J. Batth, M. Krisch, E Schlfers and R. Wolf, Appl. Opt. 27 (1988) 4336. [24] D.A. Shirley, Phys. Rev. B 5 (1972) 4709. [25] G.K. Wettheim, M.A. Butler, K.W. West and D.N.E. Buchanan, Rev. Sci. Instrum. 45 ( 1974) 1369. 1261 P Bruwhiler, A.J. Maxwell, C. Puglia, A. Nilsson, S. Andersson and N. MSrtensson, Phys. Rev. Lett. 74 (1995) 614.
