Langmuir 1999, 15, 2037-2042
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Attenuation Length of Electrons in Self-Assembled Monolayers of n-Alkanethiols on Gold Christine L. A. Lamont*,† and John Wilkes Department of Chemical and Biological Sciences, University of Huddersfield, Queensgate, Huddersfield HD1 3DH, U.K. Received September 4, 1998. In Final Form: December 9, 1998
The interaction of both photoelectrons and X-rays with self-assembled monolayers of n-alkanethiols on gold has been measured using synchrotron radiation as the photon source in the energy range 140-1100 eV. The attenuation length of photoelectrons (λ) was found to vary from a minimum of ∼5 Å at an electron kinetic energy (E) of 100 eV up to ∼23 Å at a kinetic energy of 1000 eV and can be described by the expression λ ) 0.3E0.64 in the range 300-1000 eV. Exposure of the self-assembled monolayer to X-rays leads to fission of the C-S bond with a cross section of the order of 10-17 cm2 which diplays no apparent dependence on the incident photon energy.
Introduction For application of electron spectroscopies in quantitative analysis the attenuation length (λ) of electrons as a function of their kinetic energy, in different media, must be known accurately. Experimentally, it is often the escape depth of electrons which is measured. The escape depth is defined as the distance normal to the surface at which the probability of an electron escaping without significant energy loss due to inelastic scattering processes drops to e-1 of its original value.1 Thus
I(A) ) I(A0)e-d/λsin(θ)
(1)
where I(A) is the intensity of an Auger or photoelectron peak from an element A in a substrate covered with a film of thickness d, I(A0) is the intensity of the same peak from a clean substrate, and θ is the angle between the plane of the substrate and the detector. If θ ) 90°, the escape depth is equivalent to the attenuation length. Lindau and Spicer2 compiled escape depths for electrons in 20 materials in the kinetic energy range 1-3000 eV. Seah and Dench3 compiled IMFPs (inelastic mean free paths, which are similar to attenuation lengths, but do not contain any contributions from elastic scattering events4) from both experiment and theory for ∼350 materials. Both papers show the well-known “universal curve” which is characterized by long attenuation lengths for low kinetic energies, a minimum at around 100 eV, and an increase again toward higher energy. Although the trends are quite clear, there is considerable disagreement in the absolute values, which may be partly explained if different elements are considered to scatter electrons differently.5 Inelastic scattering of electrons at high energies is due to interactions with core electrons, valence electrons, and plasmons, the energies and extent of which will depend on the chemical nature of the medium. Yet † Tel: ++44 1484 472612. Fax: ++44 1484 472182. E-mail:
[email protected].
(1) Surf. Interface Anal. 1991, 17, 951. (2) Lindau, I.; Spicer, W. E. J. Electron Spectrosc. Relat. Phenom. 1974, 3, 409. (3) Seah, M. P.; Dench, W. A. Surf. Interface Anal. 1979, 1, 2. (4) Jablonski, A. Surf. Interface Anal. 1994, 21, 758. (5) Tanuma, S.; Powell, C. J.; Penn, D. R. Surf. Interface Anal. 1988, 11, 577.
even within measurements on a particular type of material large variations in results occur. For example, attenuation lengths of between ∼10 and ∼100 Å have been reported for organic material at a kinetic energy of 1200 eV.6-14 The discrepancies in these data are most likely due to difficulties in preparing even films of controlled thickness using techniques such as evaporation of C-containing molecules or Langmuir-Blodgett film deposition. Recently, Laibinis et al.8 and Bain and Whitesides9 overcame the latter problem by measuring the attenuation lengths of photoelectrons in the energy range 550-1400 eV from the metal substrates of self-assembled monolayers (SAMs) of thiols adsorbed on Au, Ag, and Cu substrates, using an Al KR source. The advantage of using SAMs in these experiments is that they are very well ordered, uniform adsorbate layers which are usually exactly one monolayer thick. Moreover, the orientation of the molecules on the surface has been determined from infrared measurements,15 allowing the thickness of the monolayer to be accurately calculated. To extend these measurements to a wider range of kinetic energies and to remove any inconsistencies which might arise from the use of different substrates and photoelectrons, λ has been determined by measuring the Au4f7/2, Au4f5/2, and C1s photoelectron peaks for a series of self-assembled monolayers of n-alkanethiols on gold at different photon energies in the range 140-1100 eV, using synchrotron radiation as the photon source. A potential problem with using SAMs in these measurements is that (6) Clark, D. T.; Thomas, H. R. J. Polym. Sci., Chem. Ed. 1977, 15, 2843. (7) Brundle, C. R.; Hopster, H.; Swalen, J. D. J. Chem. Phys. 1979, 70, 5190. (8) Laibinis, P. E.; Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1991, 95, 7017. (9) Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1989, 93, 1670. (10) Cartier, E.; Pfluger, P.; Pireaux, J.-J.; Rei Vilar, M. Appl. Phys. 1987, A44, 43. (11) Hall, S. M.; Andrade, J. D.; Ma, S. M.; King, R. N. J. Electron Spectrosc. Relat. Pnenom. 1979, 17, 181. (12) Hupfer, B.; Schupp, H.; Andrade, J. D.; Ringsdorf, H. J. Electron Spectrosc. Relat. Phenom. 1981, 23, 103. (13) Cadman, P.; Gossedge, G.; Scott, J. D. J. Electron Spectrsoc. Relat. Phenom. 1978, 13, 1. (14) Steinhardt, R. G.; Hudis, J.; Perlman, M. L. Phys. Rev. 1972, 5, 1016. (15) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559.
10.1021/la981168p CCC: $18.00 © 1999 American Chemical Society Published on Web 02/12/1999
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they are prone to radiation damage.16 Therefore, the photon induced decomposition of the thiol films has also been investigated with the aims of identifying the reaction products and of measuring the photodissociation cross section as a function of the photon energy. Experimental Section The substrates used for the SAMs consisted of an approximately 200 nm thick layer of gold on a quartz substrate (Berliner Glass). To prepare a self-assembled monolayer, the substrate was rinsed in dry ethanol, air-dried, flame annealed to a dull red color, cooled slightly, and immediately immersed in a 1 mmol solution of the thiol (Aldrich) in dry ethanol for 24 h. On removal, the SAM was rinsed with dry ethanol and either directly transferred to the ultrahigh vacuum (UHV) chamber, via a load lock system, or stored for up to 48 h prior to insertion. Self-assembled monolayers of n-alkanethiols with chain lengths of 8, 10, 12, 14, 16, and 18 C atoms were prepared in this way. The XPS measurements were performed on a Peterson planegrating monochromator at the Berlin Synchrotron Radiation Source (BESSY). All spectra were recorded with the photon beam incident at 45° to the surface and the electron energy analyzer (CLAM, VG) perpendicular to the surface. Since high signalto-noise rather than high resolution was required in these experiments, the pass energy of the CLAM was set at 100 eV and the resolution of the monochromator degraded to ∼0.4 eV at 200 eV and ∼4.5 eV at 1000 eV. To eliminate anomalous results due to variations in photon flux and long-term stability of the electron energy analyzer, the ratio (I(Au)/I(Au0)) was measured for the Au4f photoelectron peaks rather than an absolute value for I(Au). This was obtained by mounting a clean, flame-annealed gold sample and a SAM side by side, first measuring the Au4f spectrum from the clean sample (I(Au0)) and then, immediately after, from the thiol-covered sample (I(Au)) and normalizing the signal to the beam flux (measured using a gold mesh). The reliability of this technique was tested by measuring the intensity of the Au4f peaks on different regions of the clean sample. Consecutive scans were reproducible to within 3% (and often better than 1%). The consistency of the SAM preparation was checked by performing XPS measurements on two different C18H37SH monolayer samples. Systematic errors were eliminated by recording the XPS spectra for an individual thiol monolayer in a random order of photon energy and also by studying the thiols themselves in a random order. The C1s spectra were recorded using a photon energy of 350 eV. Preliminary experiments demonstrated that SAMs are prone to X-ray damage. To minimize this effect the thiol sample was divided into small squares (1.5 mm × 1.5 mm) slightly larger than the diameter of the photon beam and each scan was measured on a fresh area of the sample. Each spectrum consisted of 50 scans over an energy width of 15 eV giving a total measuring time of 10 min (a compromise between minimizing the effects of radiation damage and obtaining sufficient signal-to-noise). The products of radiation damage were identified by recording XPS spectra of the Au4f, C1s, and S2p regions for a C14H29SH thiol layer before and after a 1 min exposure to zero-order light. (Zeroorder synchrotron radiation, which is nonmonochromatized light, gives a higher degree of photon illumination on the sample than monochromatized light and thus produces a high number of photon induced reactions in a relatively short time.) The reaction cross section was determined by measuring the increase in intensity of the maximum in the Au4f7/2 photoelectron peak from the C14H29SH SAM (for a particular photon energy) over a period of ∼30 min.
Results and Discussion Attenuation Length. The XPS spectra were analyzed by performing a Shirley background subtraction and fitting the peaks with an analytical approximation to a Voigt (16) Nuzzo, R. G.; Zergarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733.
Figure 1. Ratio of the Au4f signal from the thiol-covered surface to that from the clean gold reference (normalized to the photon flux) as a function of photon energy. n refers to the number of C atoms in the thiol chain.
function.17 Although the Doniach-Sˇ unjic line shape should be used to fit metallic XPS peaks a Voigt function should suffice in the present case since the resolution of the photon beam and detector were low. Since the binding energies of the Au4f7/2 and Au4f5/2 photoelectrons are very similar (83.8 and 87.45 eV, respectively18) and since the peaks were unresolved at higher photon energies, the total area under both peaks was used in subsequent analysis. The normalized ratio of the intensity of the Au4f peaks from the SAM (I(Au)) to that of the corresponding peaks from the clean gold sample (I(Au0)) is plotted as a function of the electron kinetic energy in Figure 1. Within experimental error, the ratio I(Au)/I(Au0) decreases with increasing chain length for any kinetic energy and there is reasonable agreement between the results for the two different C18H37SH samples, suggesting that the preparation of the SAMs is reproducible and that the films are homogeneous over the whole surface. The general shape of the curve is the same for each SAM and resembles, to some extent, the “universal curve” in that there is a minimum in the relative intensity of the Au4f photoelectron peaks at a kinetic energy of ∼100 eV. For a particular electron kinetic energy, the attenuation length was determined using a variation of eq 1, i.e.
ln
( )
I(Au) -nd + const ) λ sin θ I(Au0)
(2)
where n is the number of C atoms in the hydrocarbon chain and d is the height equivalent to one C-C bond perpendicular to the surface. For SAMs of thiols on gold the alkyl chains are believed to tilt at an angle of 30° to the surface normal15 giving a value for d of 1.1 Å. From eq 2 a plot of ln(I(Au)/I(Au0)) against n will have a gradient of -d/λ, since θ ) 90°. Sample plots are shown in Figure 2 for electron kinetic energies of 100, 150, 200, and 300 eV. The lines drawn through the points are best fits to the data using least-squares analysis. Values of I(Au)/I(Au0) used in these plots were obtained by either (A) using raw data values or (B) using values obtained from spline (17) McLean, A. B.; Mitchell, C. E. J.; Swanston, D. M. J. Electron Spectrosc. Relat. Phenom. 1994, 65, 125. (18) Handbook of X-ray Photoelectron Spectroscopy; Muilenberg, G. E., Ed.; Perkin-Elmer Corp.: Eden Prairie, MN, 1978.
Attenuation Length of Electrons in Monolayers
Langmuir, Vol. 15, No. 6, 1999 2039
Figure 2. Plots of ln(I(Au)/I(Au0)) versus n used in determining λ for electron kinetic energies of 100, 150, 200, and 300 eV.
Figure 4. Relative intensities of the C1s peaks for different thiols. The line drawn through the data is the best fit to eq 3 giving an attenuation length of 10 ( 2 Å.
atoms the expected intensity as a function of chain length (n) can be estimated from i)n-1
I(Cn) ) I(C1)
Figure 3. Graph showing the attenuation length as a function of photon energy. The closed circles refer to attenuation lengths calculated from the raw experimental data while the open triangles correspond to those calculated from spline function fits to the raw data curves. Table 1. Attenuation Lengths of Photoelectrons in SAMs of n-Alkanethiols on Gold as a Function of Kinetic Energy electron kinetic energy (eV)
attenuation length (Å)
electron kinetic energy (eV)
attenuation length (Å)
50 100 150 200 300 350 400
6.5 ( 1.2 5.4 ( 0.4 7.8 ( 0.7 9.1 ( 1.0 11.7 ( 1.0 12.4 ( 1.0 13.3 ( 1.2
450 500 600 700 800 900 1000
14.8 ( 1.5 15.9 ( 1.7 17.7 ( 2.0 19.9 ( 3.0 21.1 ( 2.3 22.5 ( 2.8 23.4 ( 2.4
function fits to the raw data, both methods giving very similar results (Figure 3). The latter method was thought to be more representative of the complete set of experimental results, and the calculated attenuation lengths are listed in Table 1. The errors, determined from the standard deviation, were generally of the order of ∼10%. Figure 3 contains the essential features of the universal curvesa minimum in λ of ∼5 Å at a kinetic energy of 100 eV and increasing values of λ at both higher and lower kinetic energies. C1s spectra were also recorded for each monolayer and normalized to the Au4f peaks recorded on the clean surface. The relative intensity as a function of the number of C atoms in the alkyl chain is plotted in Figure 4. For C
e-id/λsin(θ) ∑ i)0
(3)
where I(Cn) is the measured intensity of the C1s peak for a self-assembled monolayer of a thiol containing n atoms in its alkyl chain. The line in Figure 4 is the best fit to eq 3 giving a value for λ at 65 eV of 10 ( 2 Å, slightly higher than the values obtained from analysis of the Au4f data in this energy region (Table 1). Since the earlier measurements of λ in carbon-containing materials produced a wide range of values, it is worth considering possible sources of experimental error in the present study. First of all, on examination of the ln(I(Au)/I(Au0)) versus n plots it was noted that the points for the C8H17SH monolayer consistently lay above the lines of best fit while the scatter in the data for the other monolayers was random. This might be because the C8H17SH monolayer adopts a structure different from that of the longer chain thiols.15 Alternatively, the C8H17SH monolayer might not be completely homogeneous and may contain bare patches. The measured gold signal would thus be enhanced by photoelectrons from these bare patches. In either case, the magnitude of the error is quite small and the effect of omitting the data for the C8H17SH monolayer is to increase λ by a maximum of ∼10%. Second, the “clean” gold reference sample was, in fact, slightly contaminated by carbon. Provided, however, that the extent of the carbon contamination remained constant throughout the experiments, then a positive intercept would be expected in Figure 2, but the gradient (and hence λ) would be unchanged. Steps were therefore taken to ensure that the amount of contaminant on the “clean” gold surface did not change significantly throughout the course of the measurements. These included using the same “clean” gold sample as a reference for all of the SAMs and measuring the relative intensity of the C1s/Au4f peaks for the clean surface during acquisition of each data set. The latter was found to remain constant within experimental error. The good agreement between the results for the two C18H37SH samples, despite the fact that one sample was analyzed at the beginning of the set of experiments and the other at the end, provides further confirmation that little change in the reference sample occurred. Another possible source of experimental error is radiation damage of the monolayer while recording the spec-
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trum. This was held to a minimum by collecting the spectrum in 10 min and by measuring on a fresh area of the sample each time. From the studies of the radiationinduced decomposition of the monolayer described below the change in I(Au) due to X-ray damage in 10 min would be about 2-3%. From the above it can be concluded that the only significant source of error in our results is in the data for the C8H17SH layer and so the maximum error is estimated to be ∼20%. We note that our results are in agreement with the calculations of Tanuma et al. for IMFPs in C.5 IMFPs are generally thought to be ∼15-30% higher than attenuation lengths due to elastic scatteringsthe effect being greatest at low kinetic energies and high atomic numbers.5 Therefore the IMFPs and attenuation lengths for C might be expected to be reasonably close. Our results also agree well with the attenuation lengths of electrons in SiO2 measured by Mitchell et al.19sa medium which has a similar calculated IMFP to carbon.5 The attenuation lengths measured by Cartier et al.10 and Hupfer et al.12 and calculated by Ashley20 were higher than those reported here by factors of between 1.5 and 4. Those measured by Clark and Thomas6 and Steinhardt et al.14 were lower by 25-50%. Thus, our results lie in the middle of the range. More significantly, our results are similar to those determined by Laibinis et al. at comparable energies, e.g. 23 ( 2 Å at 554 eV, 24 ( 2 Å at 768 eV, and 28 ( 1 Å at 940 eV 8 but are consistently lower by ∼25%, just outside the quoted error margins. They too obtained good linear fits for ln(I(Au)) versus n plots suggesting that the layers were homogeneous, that the source and detector were stable over time, and that very little radiation damage to the sample occurred during the course of the measurements. One difference in the experimental setup, which might affect the determination of λ, was that their detection angle was 35° with respect to the surface normal while ours was 0°. One consequence of detecting at an angle away from the surface normal is that values of λ tend to be overestimated if the surface is not completely smooth.21 While the method of preparing the gold substrates prior to thiol deposition produces large flat wellordered terraces on the surface, the surfaces are still likely to be a little rough. Laibinis et al. also comment that the acceptance angle of their detector might lead to a systematic overestimation of λ. A combination of the above factors may explain the small difference in the results of the two groups. For wider applicability, it is desirable to be able to determine the attenuation length at any electron kinetic energy. Several mathematical relationships between λ and electron kinetic energy have been proposed, two of which will be considered below. The first is
λ ) kEp
(4)
where k and p are empirically derived constants. Seah and Dench’s3 empirical formula relating attenuation length and kinetic energy for organic compounds
λ(nm) )
49 + 0.11E1/2 2 E
(5)
would predict a value of p of 0.5 in eq 4 at high energies. (19) Mitchell, D. F.; Clark, K. B.; Bardwell, J. A.; Lennard,W. N.; Massoumi, G. R.; Mitchell, I. V. Surf. Interface Anal. 1994, 21, 44. (20) Ashley, J. C. J. Electron Spectrosc. Relat. Phenom. 1982, 28, 177. (21) Fadley, C. S. J. Electron Spectrosc. Relat. Phenom. 1974, 5, 725.
Figure 5. Logarithmic plot of eq 4.
Szajman et al.22 proposed that
λ)
2.12E h E3/4 Ep2
(6)
h is where Ep is the plasmon energy of the medium and E the band gap, thus predicting that p ) 0.75. It is not clear, however, whether such organic monolayers have welldefined plasmons and to what extent such considerations are reasonable. In the earlier study of thiols on gold, Bain and Whitesides found p to be equal to 1.0.9 This was, however, based on only three data points, and on extension of the work to include data from SAMs on Ag and Cu, p was reduced to 0.67.8 The majority of experimentally derived values for p lie between 0.5 and 0.8 eV.3,8,22 From a plot of ln(λ) versus ln(E) (Figure 5) p was determined to be 0.64 ( 0.03, in good agreement with other groups.8,10 k was estimated to be 0.3, in good agreement with Laibinis et al. (k ) 0.31)8 but a factor of 2 lower than Cartier et al.10 The relationship was invalid for kinetic energies below 150 eV, and the deviation from the line of best fit was greater for kinetic energies below 300 eV than above. Tanuma et al.5 also found that the dependence of the IMFP on electron kinetic energy could be described by a modification of the Bethe equation:
λ)
E Ep β ln(γE)
(7)
2
β and γ are empirical constants which Tanuma et al. tried to relate to common properties of a material such as its density and the number of valence electrons.24 Ep, the plasmon energy, was calculated to be 18.6 eV for the thiol self-assembled monolayer.24 Equation 7 is tested in Figure (22) Szajman, J.; Liesegang, J.; Jenkin, J. G.; Leckey, R. C. G. J. Electron Spectrosc. Relat. Phenom. 1982, 23, 97. (23) Wagner, C. D.; Davis, L. E.; Riggs, W. M. Surf. Interface Anal. 1980, 2, 53. (24) In ref 5, the following equations are given for the calculation of β, γ, and Ep.
β ) -0.0252 + 1.05/(Ep2 + Eg2)1/2 + 8.10 × 10-4F γ ) 0.151F-0.49
Ep ) 28.8(FNv/A)1/2
where F is the density of the adsorbate layer (in this case 0.968 g cm-3), Nv is the number of valence electrons (taken as 6 for thiols), A is the atomic weight per unit (14 for a CH2 group), and Eg is the band gap energy for nonconductors (8.8 eV 10). β, γ, and Ep were thus calculated to be 0.027, 0.153, and 18.6 eV, respectively.
Attenuation Length of Electrons in Monolayers
Figure 6. Graph testing the validity of the Bethe equation.
6sagain a linear fit is obtained in the energy range 3001000 eV, suggesting that the equation describes the electron scattering behavior, but the deviation of the points from the line of best fit is greater than for eq 4. The value of β (0.039 ( 0.003 Å-1 eV-1) obtained from least-squares analysis was ∼30% higher than that predicted by the equations in ref 5 while the value of γ (0.022 ( 0.004 eV-1) is very much lower.24 It should be noted that large errors in the calculated values of β and γ were also reported for carbon in ref 5. Thus while the data is reasonably well described by the Bethe equation, the simpler, empirical, eq 4 actually describes it better. A simple mathematical relationship between λ and E for lower kinetic energies was not found. More data and better statistics in the region around 100 eV would be required before such a relationship could be reliably established. Radiation Damage. As mentioned above, self-assembled monolayers of thiols on gold are prone to radiation damage and it is thus important to evaluate the effect this will have on the estimation of attenuation lengths described above. First of all an attempt was made to identify the products of any photon-induced dissociation reactions, and second the cross section for the reaction was measured at different photon energies. The Au4f, C1s, and S2p XPS spectra of a C14H29SH monolayer before and after a 60 s exposure to zero-order light are shown in Figure 7. The considerable increase in the Au4f and S2p peak intensities following UV exposure, together with the decrease in the C1s peak, indicates that photon-induced decomposition of the monolayer occurs quite rapidly. The results are inconsistent with fission of the outer C-C bonds in the thiol chain (as suggested by a low-energy electron-induced decomposition study of alkanethiols on gold by Olsen and Rowntree25) but suggest, instead, that fission occurs in the region of the C-S bond. In particular, the S2p peaks, which are easily detected following UV exposure, are barely discernible beforehand. Removal of just a few C atoms from the thiol chain would not produce such a dramatic effect. As well as being more intense, the S2p spectrum is also much broader after UV exposure, indicating that a new chemical species with a higher binding energy is present. While it was not possible to obtain a good fit to the S2p spectrum (which should be a doublet for each S species due to the S2p3/2/S2p1/2 spin-orbit splitting of 1.2 eV), it can be estimated that the new species has a binding energy (25) Olsen, C. O.; Rowntree, P. A. J. Chem. Phys. 1998, 108, 3750.
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of ∼163 eV. This is very close to the binding energy reported by Zubra¨gel et al. for docosanethiol on gold which they attribute to dimer formation.26 Dimer formation can be excluded from the present study as it would only produce a shift in the binding energy of the S2p peaks and not the significant changes in the intensities of the C1s and Au4f peaks observed. We suggest, therefore, that the peak at ∼163 eV is actually due to atomic sulfur on gold, produced by the fission of the C-S bond and desorption of the hydrocarbon chain fragment. By comparing the relative intensities of the C1s, Au4f, and S2p peaks, as well as identifying a new sulfur species after radiation, we have largely overcome the problems reported by Frydman et al.27 which occur if only the intensity of the C1s peak is used to identify the products. The latter method is incapable of distinguishing changes in the overlayer which do not result in loss of C, e.g. loss of hydrogen or structural transformations. Our results suggest that the formation of such species is not extensive for n-alkanethiols on gold. With identification of the reaction products, quantitative measurements are now possible. Figure 8 (inset) shows a typical plot of I(Au4f) versus time for a photon energy of 480 eV. The signal increases with time, corresponding to removal of the overlayer. The cross section for the reaction, which is defined as the number of reacted photons divided by the product of the number of incoming photons and the number of target molecules, was calculated as follows: I(Au) at time zero was taken as the intensity of the Au4f peaks corresponding to an initial coverage of one monolayer. I(Au0) was calculated using eq 2, assuming the constant term to be negligible. Fission was assumed to occur at the C-S bond. Therefore the measured intensity at time t, I(Aut), can be described by
I(Aut) ) RI(Au)e-nd/λsin(θ) + (1 - R)I(Au0)
(8)
where R is the fraction of the surface covered with thiol at time t. If I(Aut) is known from experiment, then R at time t can be calculated and the cross section (Φ) for the photodissociation reaction can be calculated using
Φ)
(1 - R)A flux × t
(9)
A is the cross-sectional area illuminated by the photon beam and was taken as 0.01 cm2. The relative flux for each measurement was obtained from the drain current on a gold mesh in the photon beam path. To get an absolute flux, the relative flux was multiplied by a conversion factor, based on the assumption that, for this type of monochromator, the flux is typically 1010 photons per second per 100 mA beam current at its maximum throughput. It should be noted that this is a very approximate estimate of the flux, and while the values of the cross sections relative to one another should be accurate, the absolute cross sections could be inaccurate by 1 order of magnitude. The cross section as a function of photon energy is plotted in Figure 8. Within experimental error, the cross section appears to be independent of the photon energy and is of the order of 10-17 cm2. It is worth noting, however, that the measurements were made at widely spaced energies and there may be resonance type features which were not detected. From these measurements it was concluded that (26) Zubra¨gel, Ch.; Deuper, C.; Schneider, F.; Neumann, M.; Grunze, M.; Schertel, A.; Wo¨ll, Ch. Chem. Phys. Lett. 1995, 238, 308. (27) Frydman, E.; Cohen, H.; Maoz, R.; Sagiv, J. Langmuir, 1997, 13, 5089.
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Figure 7. XPS spectra for S2p, C1s, and Au4f in a C14H29SH monolayer, before and after exposure to 60 s radiation with zero-order light. All spectra have been normalized to the beam flux.
Figure 8. Cross section for the photon-induced dissociation of the C-S bond in a C14H29SH monolayer on gold as a function of the incident photon energy. The inset shows a typical measurement of the intensity of the maximum in the Au4f7/2 (normalized to the photon flux) as a function of exposure time.
only 2-3% of the exposed monolayer decomposed in the 10 min required to record an XPS spectrum which would produce negligible error in the calculated attenuation lengths. In contrast, repeated measurements on the same spot would have lead to increasingly large errors, e.g. after five 10 min scans the monolayer would be depleted by up to 15%, giving an increase in I(Au)/I(Au0) of ∼25%.
Conclusions The attenuation of photoelectrons in SAMs of nalkanethiols on gold was measured in the kinetic energy range 50-1000 eV. The results were similar to, yet consistently lower than, those of Laibinis et al. at comparable energies.8 The small differences might be accounted for by considering the different experimental geometry used in collecting the data. Despite this discrepancy, it can be concluded that SAMs are an ideal adsorbate/substrate system for determining attenuation lengths in condensed hydrocarbons, although it should be borne in mind that the density of the hydrocarbons will also play a role. In the electron energy range 300-1000 eV the attenuation length can be calculated using eq 4 with constants k ) 0.3 and p ) 0.64 ( 0.03. At lower energies, more data is required to enable a mathematical relationship between λ and p to be established. Exposure of a SAM to X-rays results in fission of the C-S bond with a reaction cross section of the order of 10-17 cm2 which, within experimental error, is independent of the photon energy. The reaction proceeded using monochromatized light at a rate of 2-3% of a monolayer in 10 min and therefore had minimal effect on the estimation of the attenuation length. Acknowledgment. This work was supported by the EU/TMR program (Grant No. ERBFMGE CT 950031). LA981168P
