J. Phys. Chem. C 2009, 113, 4575–4583
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Energy Level Pinning in Self-Assembled Alkanethiol Monolayers Yan Ge,† Tobias Weidner,‡,§ Heejoon Ahn,†,| James E. Whitten,*,† and Michael Zharnikov*,‡ Department of Chemistry and Center for AdVanced Materials, The UniVersity of MassachusettssLowell, Lowell, Massachusetts 01854, and Angewandte Physikalische Chemie, UniVersita¨t Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany ReceiVed: NoVember 12, 2008; ReVised Manuscript ReceiVed: January 14, 2009
Potassium deposition in ultrahigh vacuum on a self-assembled monolayer (SAM) of 1-dodecanethiol on Au(111) results in penetration of a large fraction of the metal to the gold-SAM interface, as determined by angleresolved X-ray photoelectron spectroscopy (XPS) and high-resolution XPS. This results in a change in the work function (WF) of the surface, due to a change in the WF of the gold substrate beneath the SAM. The measured binding energy of the C1s photoemission peak related to the aliphatic chain also shifts and correlates directly with the change in WF. In contrast, the binding energy of the S2p peak from the thiolate headgroup is independent of the substrate WF. These findings indicate that photoemission from the core levels of the aliphatic chain and headgroup atom in an alkanethiolate SAM should be described in different frameworks, assuming, respectively, either Fermi or vacuum level alignment between the respective parts of the SAM and substrate. The results may be explained by slow dynamics of charge transfer along the aliphatic chain. 1. Introduction Self-assembled monolayers (SAMs) with thiol headgroups have become ubiquitous in science and technology, with applications that include nanotechnology,1 organic electronics,2,3 chemical and biological sensors,4,5 tribology,6 and biofunctional surfaces.7 The design of suitable SAMs for the above applications relies upon extensive characterization of these systems. Because of its chemical sensitivity and relatively small sampling depth (i.e., surface sensitivity), one of the leading analytical techniques used to characterize monolayer-covered surfaces is X-ray photoelectron spectroscopy (XPS). In this technique, the kinetic energies (KEs) of photoelectrons ejected by X-ray irradiation are measured and converted to binding energies (BEs) using the classic Einstein formula (see, for example, ref 8)
KE ) hν - EBF - φsp
(1)
where hν is the X-ray energy, EBF is the BE of the probed state with respect to the Fermi level, and φsp is the work function (WF) of the spectrometer. This formula relies upon the alignment of the Fermi levels of the spectrometer and sample, which is always true for conductive samples. Under this condition, it is only the WF of the spectrometer that affects the BEs of the photoemission peaks; the WF of the sample is expected not to affect the BE. This Fermi level alignment framework is generally believed to be applicable not only to conductive samples but also to organic adsorbates, and SAMs * Corresponding authors. J.E.W.: phone, (978) 934-3666; fax, (978) 9343013; e-mail,
[email protected]. M.Z.: phone, +49 6221 54 4921; fax, +49 6221 54 6199; e-mail,
[email protected] heidelberg.de. † The University of MassachusettssLowell. ‡ Universita¨t Heidelberg. § Present address: National ESCA and Surface Analysis Center for Biomedical Problems (NESAC/BIO), University of Washington, Seattle, WA 98195. | Present address: Department of Fiber and Polymer Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-Gu, Seoul 133-791, Korea.
in particular, on conductive substrates. The insulating nature of the adsorbed molecules can usually be disregarded, since the adsorbate film is thin enough to allow efficient charge transfer from the substrate to neutralize the photoemission hole. In contrast to this generally accepted model, we recently reported data9 showing that the standard Fermi level alignment framework is not fully applicable in the case of SAMs. In that study, potassium was deposited in ultrahigh (UHV) vacuum onto 1-tridecanethiol SAMs assembled on gold surfaces. Potassium deposition provided a convenient means of modifying the work function of the surface, which was measured by ultraviolet photoelectron spectroscopy (UPS). It was demonstrated that the BE of the C1s XPS peak arising from 1-tridecanethiol carbon atoms depends on the work function of the sample, which could only be explained by a model involving alignment of the vacuum levels of the SAM and substrate instead of the Fermi levels. As mentioned above, such a vacuum level pinning of the C1s peak is unexpected in the light of the fact that the molecule is only ca. 20 Å long, and the electronic states of the metal surface are expected to extend at least this far above it. For example, experiments on excited dye molecules have shown that a metal surface quenches their fluorescence and decreases the excitedstate lifetime for distances less than 100 Å.10 While the XPS results were surprising, related observations11,12 have previously been made but not studied or interpreted in detail. A limited number of investigations have been performed of vapor deposition of alkali metals on alkanethiol SAMs. Zhu et al.13,14 used a secondary ion mass spectrometer (SIMS) to compare the reaction of potassium with differently functionalized thiol monolayers. In the case of a methyl-terminated alkanethiol SAM, no chemical reaction was observed. This was in contrast to potassium deposition on SAMs terminated with CO2H and CO2CH3 groups in which CO2K moieties formed at the SAM-vacuum interface.12-14 It is generally believed15 that low reactivity favors penetration of a deposited metal through the SAM and diffusion to the SAM/Au interface, presumably via migration through defects. In the case of alkali metals, diffusion of sodium through methyl-terminated SAMs as a
10.1021/jp809975x CCC: $40.75 2009 American Chemical Society Published on Web 02/20/2009
4576 J. Phys. Chem. C, Vol. 113, No. 11, 2009 function of time indicates diffusion (on the time scale of minutes) through the organic film.16 From these studies, therefore, it can be concluded that potassium and other alkali metals do not react with methyl-terminated alkanethiols assembled on gold surfaces and that diffusion toward the SAM/ Au interface occurs on the time scale of most laboratory experiments. In the present paper, our previous study9 has been extended by performing angle-resolved XPS to determine the exact fate of potassium deposited on nonsubstituted alkanethiolate SAMs, by monitoring the effect of K on the SAM matrix more precisely by measuring the C1s peak shift with high-resolution XPS (HRXPS) and by measuring the S2p peak to determine if carbon atoms in the aliphatic chain and the sulfur headgroup behave differently with respect to photoemission. This latter question could not be addressed previously9 because of the poor signalto-noise ratio for the S2p emission in the XPS experiment. As a test system, we have studied 1-dodecanethiol, which, along with hexadecanethiol and octadecanethiol, is one of the most commonly used alkanethiolate SAMs. It is shown that deposited potassium atoms are not only located at the SAM-substrate interface, as we had assumed in the previous study, but are distributed to some extent throughout the monolayer. It is also demonstrated that while the C1s core level of the spacer atoms is pinned to the vacuum level, the S2p core level remains pinned to the Fermi level because of the proximity of the thiolate headgroup to the metal substrate. 2. Experimental Section 1-Dodecanethiol, C12H25SH (C12), was purchased from Sigma-Aldrich and used without further purification. Gold substrates were prepared by thermal evaporation of 200 nm of gold (99.99% purity) onto polished single crystal Si(111) or Si(100) wafers primed with a 5 nm titanium adhesion layer; the resulting substrates had a dominant (111) orientation.17,18 The dodecanethiol monolayer was formed by immersion of freshly prepared substrates into a 1 mM solution of C12 in absolute ethanol at room temperature for 18-24 h. After immersion, the samples were carefully rinsed with pure ethanol, blown dry with nitrogen or argon, and either used immediately or stored for several days in argon-filled glass containers until the XPS/UPS or synchrotron (HRXPS) experiments were performed. XPS and UPS experiments were carried out in a VG MKII ESCALAB photoelectron spectrometer consisting of three chambers: a load-lock chamber, a preparation chamber, and an analysis chamber. The base pressures in the analysis and preparation chambers were in the 10-10 mbar range. The sample could be rotated such that the angle (referred to as the takeoff angle, TOA) between its plane and the entrance axis of the hemispherical energy analyzer could be varied. Mg KR X-rays with an energy of 1253.6 eV were used for XPS, and He I radiation having an energy of 21.22 eV was used for UPS. The samples were connected to electrical ground for XPS measurements or biased at ca. -6.35 V (accurately measured) for UPS to permit the low kinetic energy region of the spectrum to be recorded. The electron analyzer was operated in fixed analyzer transmission mode, with pass energies of 20 and 2 eV for XPS and UPS, respectively. Potassium deposition was performed using a SAES getter alkali metal source in the preparation chamber of the ESCALAB, with the dose measured by a quartz crystal microbalance (QCM) positioned next to the sample. The deposition was performed in a serial manner (i.e., successive
Ge et al. deposition and characterization cycles were applied to the same sample). The source was thoroughly outgassed prior to experiments; the pressure rose into the 10-9 mbar range during potassium deposition. The substrates were at ambient temperature during alkali metal deposition and sample analyses. A typical experimental sequence consisted of performing UPS and XPS of the C12 monolayer, transferring the sample from the analysis chamber to the preparation chamber, depositing a small amount of potassium, transferring the sample back to the analysis chamber, and repeating the UPS/XPS measurements. Because of possible X-ray-induced degradation,19,20 the number of XPS scans (and X-ray exposure time) was kept to a minimum. Also, it was confirmed by XPS that negligible oxygen contamination occurred during the course of the experiment. Note also that we use units of atoms/cm2 for the potassium doses. A monolayer of potassium deposited on Au(111) at room temperature corresponds approximately to a potassium coverage of 0.45 (with respect to the atomic density in the topmost gold layer),21 equating to 6.2 × 1014 potassium atoms/cm2. Of course, this is not to imply that the potassium necessarily grows in a layer-by-layer fashion, especially in the presence of the SAM. In particular, on the bare Au(111), a partial intermixing of K and Au occurs at potassium coverages exceeding 0.33.21 HRXPS measurements were conducted at the bending magnet beamline D1011 at the MAX II storage ring of the MAXlaboratory synchrotron radiation facility in Lund, Sweden. This beamline is equipped with a Zeiss SX-700 plane-grating monochromator and a two-chamber UHV experimental station with a SCIENTA analyzer. The samples were connected to electrical ground. The Au4f, C1s-K2p, S2p, and O1s regions were recorded in normal emission geometry and at photon energies ranging from 350 to 580 eV. The BE scale was calibrated to the Au 4f7/2 emission of pristine C12/Au at 84.00 eV.22 The energy resolution was better than 100 meV (generally around 70 meV), which is noticeably smaller than the full width at half-maximum (fwhm) of the features observed in the spectra. Similar to the XPS experiments, potassium deposition was performed using a SAES getter alkali metal source. The potassium was deposited in the preparation chamber of the experimental station, with the depositions performed in a serial manner. The source was thoroughly outgassed prior to experiments; the pressure rose into the 10-9 mbar range during potassium deposition. The substrates were kept at ambient temperature during alkali metal deposition and sample analysis. Note that, due to experimental limitations, it was not possible to use a QCM during potassium deposition in the case of the HRXPS experiments. Therefore, the potassium dose was controlled by selection of a specific deposition time. The amount of K was indirectly determined afterward by comparing the potassium-induced shift of the C1s peak to that obtained in the XPS experiments. The HRXPS spectra were fitted by symmetric Voigt functions using a Shirley-type background. To fit the S2p3/2,1/2 doublets, we used two such peaks with the same fwhm, the standard spin-orbit splitting of ≈1.18 eV (verified by fit), and a branching ratio of 2 (S2p3/2/S2p1/2). The fits were performed self-consistently, with identical fitting parameters used for a given spectral region. 3. Results 3.1. UPS and Work Function Measurements. Figure 1a displays He I UPS spectra (with respect to the Fermi level) of C12/Au for various potassium doses, and Figure 1b shows expansion of the region near the Fermi level for the C12/Au
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Figure 2. Work functions of clean gold (open circles) and C12/Au (solid circles) as a function of deposited potassium.
Figure 1. (a) UPS spectra of K/C12/Au acquired in the course of potassium evaporation, with the spectra labeled with their respective potassium doses. (b) Expansion of the region of the UPS spectrum near the Fermi level for C12/Au (i.e., prior to potassium deposition) and following 1.4 × 1015 K atoms/cm2. The spectra are plotted with respect to the Fermi level.
sample prior to and after potassium deposition (1.4 × 1015 atoms/cm2). In the case of the virgin C12/Au sample, UPS features are observed at 3.2, 6.9, and 10.0 eV. The weak peak at 3.2 eV is believed to arise from the thiol sulfur atoms, and the peak at 6.9 eV and shoulder at 10.0 eV are attributed mainly to σ orbitals of the alkyl chains, as similarly observed for 1-tridecanethiol and discussed by Ahn and Whitten.23 As shown in Figure 1b, potassium deposition results in weakening and broadening of these features but does not lead to their disappearance. The work function versus potassium dose for 1-dodecanethiol compared to clean gold is shown in Figure 2. The value for C12/Au sample is 4.3 eV prior to potassium deposition. For clean gold it is 5.2 eV, and a work function decrease of ca. 0.9 eV is consistent with the generally observed behavior for alkanethiol adsorption on gold.23 As potassium is added, the work functions of both clean gold and C12/Au drop in a
progressive fashion. In the case of clean gold, a value of 2.0 eV is reached for doses exceeding 3.0 × 1015 atoms/cm2. For C12/Au, this value is not achieved, at least by 4.1 × 1015 atoms/ cm2. Instead, the final value is ca. 2.6 eV. Furthermore, the work function change is significantly more gradual for deposition on the organic film. Possible reasons for the slower decrease are a lower sticking coefficient of K on C12/Au, as compared to the bare Au substrate,14,16 or that some metal atoms do not completely diffuse to the SAM-substrate interface but get stuck in the aliphatic matrix. The latter possibility will be discussed below in detail, along with the origin of the difference in the saturation values of the work functions for K/Au and K/C12/ Au. The sticking coefficient of K on CH3-terminated surfaces has been estimated as only a few percent compared to reactive substrates.13 Our data in Figure 2, however, suggest that it is significantly higher, since a much slower decrease of the work function with increasing potassium dose would otherwise be observed. However, the sticking coefficient is presumably lower than that of solid surfaces, such as, for example, quartz. The work function results presented for potassium deposition on the C12/Au surface are similar to those observed previously for potassium deposition on a tridecanethiol SAM.9 It should be noted that there is an error in the abscissa of the work function data (the inset) in Figure 2 of ref 9. In that reference, the dose should have been units of 1015 atoms/cm2 instead of 1013 atoms/cm2. It should further be noted that in ref 9 the final work function value was ca. 2.0 eV for K deposition on a tridecanethiol/Au monolayer, with the smaller final work function (compared to the value of 2.6 eV observed in this paper) due to a significantly higher potassium dose (8 × 1015 instead of 4 × 1015 atoms/cm2) being used in ref 9 as compared to the present study. 3.2. XPS Measurements. Figures 3 and 4 show the C1s region (which also contains the K2p doublet in the 293-298 eV BE range) of the potassium-dosed C12/Au samples, for various doses, plotted with respect to the Fermi and vacuum levels, respectively. Each spectrum is the average of 10 scans. In the case of Figure 3, it is seen that the C1s peak exhibits a progressive upward shift with increasing potassium dose. In the given dose range, it shifts from an initial value of 284.9 to a final value of 286.3 eV. The shift in the K2p3/2 peak is
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Figure 3. C1s-K2p XPS spectra of K/C12/Au acquired in the course of potassium evaporation. The binding energy scale is referenced to the Fermi level of the spectrometer. The spectra are labeled with their respective potassium doses.
Figure 5. K2p/C1s intensity ratio for K/C12/Au as a function of takeoff angle for potassium doses of 1.1 × 1015 atoms/cm2 (a), 4.9 × 1015 atoms/cm2 (b), and 7.9 × 1015 atoms/cm2 (b). Experimental data are given by filled circles (for 1.1 and 4.9 × 1015 atoms/cm2) and squares (for 7.9 × 1015 atoms/cm2). Also, theoretical fits (gray solid lines) of the observed dependence as a function of takeoff angle are included; these fits are discussed in detail in section 4. It is assumed that a part of the deposited potassium penetrates to the SAM-gold interface, as indicated on the respective curve, while the remainder is homogeneously distributed in the SAM matrix. In part a, a theoretical curve for complete (100%) penetration of potassium is shown (black solid line).
Figure 4. C1s-K2p XPS spectra of K/C12/Au acquired in the course of potassium evaporation. The binding energy scale is shifted (as compared to Figure 3) in accordance with the work function change with respect to C12/Au (see Figure 2). The spectra are labeled with their respective potassium doses.
significantly less but still visible, with initial and final values of 293.0 and 293.8 eV. The shift of the C1s peak in the course of potassium evaporation correlates with the change in work function. This is evidenced by Figure 4, which was constructed by shifting each spectrum to higher BE by the work function change of the K/C12/Au sample, compared to the pristine one (C12/Au). The C1s peak now remains essentially unchanged in binding energy, illustrating the quantitative correlation between the peak shift and work function change. The change of the work function is caused by potassium evaporation. To gain more information regarding the depth distribution of potassium in the film, angle-resolved XPS has been performed. Figure 5 shows the K2p/C1s intensity ratios
versus TOA for three potassium doses, with the experimental results indicated by data points. The solid curves are theoretical fits to the data; they will be discussed in detail in section 4. As the takeoff angle decreases and the experiment becomes more surface sensitive, the K2p/C1s ratios decrease. This indicates that potassium is neither located completely at the SAM-vacuum interface nor homogeneously distributed in the SAM matrix. This is not surprising in the light of the fact that methyl groups are not expected to be reactive toward the deposited metal atoms; these atoms will diffuse into the SAM matrix and penetrate to the SAM-substrate interface, driven by a tremendous gain of the adhesion energy (metal-metal versus metal-organic). Significantly, the dependence of the K2p/C1s intensity ratio on TOA depends to some extent on the potassium dose. We will discuss this dependence in detail below. 3.3. HRXPS Measurements. C1s and S2p HRXPS spectra of K/C12/Au acquired in the course of potassium evaporation are depicted in Figures 6 and 7, respectively, with the BE scale referenced to the Fermi level. The C1s spectrum of the pristine film exhibits a pronounced peak at a BE of 284.93 eV and a fwhm of 0.85 eV, characteristic of high-quality alkanethiolate SAMs. The S2p spectrum of the pristine film exhibits a single doublet with a BE of 162.0 eV and a fwhm of 0.57 eV characteristic of the thiolate-gold bond in high-quality alkanethiolate SAMs on Au(111).18,24 No features related to atomic sulfur, unbound sulfur, disulfide, or oxidized sulfur are observed. In accordance with the XPS data, the deposition of potassium causes a progressive upward BE shift of the C1s peak; the
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Figure 8. BE position of the C1s peak of C12/Au exposed to potassium as a function of potassium dose. The data were derived from the spectra presented in Figure 6.
Figure 6. C1s HRXPS spectra of K/C12/Au acquired in the course of potassium evaporation (open circles) along with the respective fits (thin solid lines). The spectra are labeled with their respective potassium doses. The binding energy scale is referenced to the Fermi level of the spectrometer. The fwhm of the observed peak is listed next to it.
Figure 9. C1s-K2p HRXPS spectra of C12/Au exposed to potassium (1.4 × 1015 atoms/cm2). The spectra were acquired at photon energies of 350 eV (black curve) and 580 eV (gray curve), respectively. The binding energy scale is referenced to the Fermi level of the spectrometer.
Figure 7. S2p HRXPS spectra of K/C12/Au acquired in the course of potassium evaporation along with the respective fits (thin solid lines). The spectra are labeled with their respective potassium doses. The binding energy scale is referenced to the Fermi level of the spectrometer. The fwhm of the S2p3/2,1/2 peaks are listed next to them. The dashed line is a guide for the eye.
respective dependence on the potassium dose is shown in Figure 8. Simultaneously, a broadening of this peak occurs. In contrast to the C1s spectra, the position of the characteristic S2p doublet does not change in the course of potassium deposition (see Figure 7). The only observable effect is a progressive broadening of the S2p3/2,1/2 components of this doublet, with the extent of the broadening being similar to that in the C1s peak in the respective spectra. Most likely, the broadening of the C1s and S2p features results from progressive disordering of the C12 film upon penetration of potassium into the film and its diffusion
to the substrate-SAM interface. Significantly, this disordering does not affect the anchoring of the SAM constituents to the substrate, as follows from persistence of the single characteristic doublet in the S2p spectra of K/C12/Au, but simply results in inhomogeneity of the exact bonding configurations for the thiolate headgroups at the SAM-gold interface. Penetration of potassium into the film and its diffusion to the gold-SAM interface are evidenced by comparison of the K2p and C1s HRXPS spectra acquired at different photon energies. Spectra collected at photon energies of 350 and 580 eV are depicted in Figure 9. The higher photon energy results in higher photoelectron kinetic energy, which gives a greater escape depth. Since the binding energies of the K2p and C1s core levels are very close to each other, their escape depths are almost the same for a given photon energy: the corresponding values are about 6.4 and 8.7 Å at photon energies of 350 and 580 eV (kinetic energies of 55 and 185 eV), respectively.25 In view of these values, the location of potassium at the SAM-vacuum interface should result in a decrease in the K2p signal relative to the C1s one at the higher photon energy, which is definitely not the case, as shown in Figure 9. Similarly, homogeneous distribution of potassium within the C12 film should lead to independence of the K2p/C1s intensity ratio of the photon energy, which is also not the case. In contrast, the
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relative intensity of the K2p signal increases with respect to the C1s signal with increasing photon energy. This behavior can only be explained with an assumption that a considerable fraction of the deposited potassium penetrates to the SAM-gold interface. This is generally consistent with the angle-resolved XPS results discussed earlier. 4. Discussion 4.1. Location of the Deposited Potassium in K/C12/Au. Both angle-resolved XPS data (Figure 5) and the HRXPS spectra acquired at the different photon energies (Figure 9) indicate that deposited potassium is neither located at the SAM-ambient interface nor homogeneously distributed in the SAM matrix. Furthermore, in view of the different behavior of the work function of K/Au and K/C12/Au (see section 3.1), it is probably not realistic to assume that all potassium atoms penetrate to the SAM-Au interface, as we postulated in our previous work.9 A more realistic model is the assumption that only a portion of the potassium atoms penetrates completely to the SAM-Au interface and that the residual atoms get stuck in the SAM matrix, with a definite distribution or in the form of filaments. This hypothesis is further proven by analysis of the angleresolved XPS data (section 3.2). The intensity of the K2p XPS signal from potassium atoms located at the SAM-Au interface is described by a standard equation (see, for example, ref 8)
IK ) IK0 exp[-dSAM /(λK cos(90° - R))]
(2)
where IK0 is a prefactor representative of the amount of potassium and parameters of the XPS spectrometer, dSAM is the SAM thickness, λK is the mean free path of the K2p photoelectrons at the given kinetic energy, and R is the takeoff angle. The case for the homogeneous distribution of the potassium atoms in the SAM matrix is described by a different equation (see, for example, ref 8)
IK ) IK0{1 - exp[-dSAM /(λK cos(90° - R))]}
(3)
A similar equation describes the intensity of the C1s signal from the SAM matrix
IC ) IC0{1 - exp[-dSAM /(λC cos(90°-R))]}
(4)
where IC0 is a similar prefactor as IK0, and λC is the mean free path of the C1s photoelectrons at the given kinetic energy. Since the binding energies of the C1s and K2p core levels are very similar, the kinetic energies of the respective photoelectrons are essentially identical, so that the same value can be taken for both λK and λC. Assuming a realistic value for λK and λC (24.44 Å),25 we can calculate the dependence of the IK/IC ratio on the takeoff angle and compare it with the experimental data in Figure 5. Assuming that all potassium atoms are located at the SAM-Au interface (100% penetration), eq 2 yields the theoretical curve presented in Figure 5a (the curve is normalized to the 90° TOA value for a potassium coverage of 1.1 × 1015 atoms/ cm2). This curve exhibits a much steeper decrease in the IK/IC ratio with decreasing takeoff angle than indicated by the experimental data, suggesting that the 100% penetration model does not correspond to reality. The next more realistic model mentioned above is penetration of a fraction of the potassium atoms. Assuming that the residual atoms are homogeneously
distributed in the alkyl chain matrix, we can use eqs 2 and 3 weighted with the respective partial coefficients and divide the combination by eq 4. For a potassium dose of 1.1 × 1015 atoms/ cm2, agreement is obtained between theory and experiment for an assumption that 52% of the potassium atoms penetrate to the SAM-Au interface, with the residual 48% getting stuck in the aliphatic matrix (Figure 5a). This result seems to be realistic in view of the much slower dynamics of the work function change of K/C12/Au compared to K/Au (Figure 2). For potassium doses of 4.9 × 1015 and 7.9 × 1015 atoms/cm2, we obtained smaller portions (27% and 24%, respectively) of potassium atoms penetrating to the SAM-substrate interface as compared to 1.1 × 1015 atoms/cm2 (see Figure 5b). Considering these data, we can assume that 52% penetration is only characteristic of the initial stages of potassium deposition. This value drops to 27% and further to 24% at higher doses. Thus, the fraction of potassium atoms penetrating to the SAM-Au interface continuously decreases in the course of the deposition, while the fraction of the atoms caught in the aliphatic matrix increases. Of course, the obtained values of the penetration fraction are rough estimates only, since a homogeneous distribution of the imbedded potassium atoms is only an approximation. However, an inhomogeneous distribution will only slightly affect the penetration values obtained within our model, without changing the general conclusions. These modeling results explain the smaller extent of the work function change for K/C12/Au compared to K/Au (Figure 2) for a particular potassium dose. In the case of K/C12/Au, this change is only induced by a portion of the deposited potassium atoms (i.e., by the atoms penetrating to the SAM-substrate interface). The potassium atoms imbedded in the aliphatic matrix do not induce any change in the work function, since this imbedding does not lead to charge transfer to the substrate. Therefore, at the same deposition dose, the change in the work function for K/C12/Au should be noticeably smaller than that for K/Au, as observed experimentally. Furthermore, the portion of potassium atoms penetrating to the SAM-substrate interface decreases in the course of the deposition, which means that the work function change induced by a particular potassium dose decreases as the dose increases. This results in a faster saturation of the work function versus potassium dose curve as compared to a clean gold substrate, which is exactly what is observed experimentally. An additional factor, especially for high potassium doses, is that the SAM continues to occupy a portion of the atoms on the gold surface. This would cause the measured work function to be a weighted average of K/Au and C12/Au surface sites. We cannot, however, completely exclude the possibility that potassium may insert itself between S and Au atoms, although this is not supported by the S2p XPS spectra. These issues confound a detailed interpretation of the data in Figure 2. Penetration of potassium atoms through the SAM and their imbedding into the aliphatic matrix result in disordering and chemical inhomogeneity of the film, as indicated by the progressive broadening of the C1s peak in the HRXPS spectra in Figure 6. Presumably, these processes result in a sort of “cross-linking” within the matrix, making it less penetratable for the further potassium atoms. This explains the observed decrease of the penetration efficiency with increasing potassium dose. The progressive redistribution of potassium in the aliphatic matrix in the course of its deposition results in an energy shift and a broadening of the K2p3/2,1/2 emissions, as shown by the respective XPS spectra in Figure 3. Note that embedding of potassium into the aliphatic matrix is presumably related to its
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Figure 10. Cartoon illustrating the fate of deposited potassium. While about a half of the deposited atoms penetrate to the SAM/Au interface, the residual atoms remain “stuck” in the organic layer.
high chemical reactivity. An additional, minor contribution to trapping of potassium atoms in the matrix could be provided by irradiation-induced defects (e.g., reactive carbons due to cleavage of C-H bonds), which could form, to a minor extent, even though the X-ray exposure was kept as low as possible (see section 2). Not only the aliphatic matrix but also the SAM-Au interface becomes progressively disordered in the course of the potassium deposition, as follows from the progressive broadening of the S2p3/2,1/2 peaks in the HRXPS spectra in Figure 7. Simultaneously, the BE of the S2p doublet remains unchanged, which means that the bonding character of the SAM constituents (i.e., the pristine thiolate-Au bonds) persists. It can be assumed, therefore, that the potassium atoms that penetrate to the SAM-Au interface are not imbedded between the SAM and Au substrate but located in the same plane as the thiolate headgroups. Since only a limited amount of potassium can be accumulated in this plane, further potassium atoms are presumably located on the top of the lowermost layer, between the bottom parts of the aliphatic chains. A schematic illustration of the K/C12/Au system in view of all the above considerations is shown in Figure 10. 4.2. Fermi and Vacuum Level Pinning in C12/Au. The observed correlation between the progressive BE shift of the C1s peak and the change in the work function of the K/C12/ Au sample (see Figure 4) suggests that vacuum level, but not Fermi level, alignment is a correct description of photoemission from the SAM matrix. The alternative explanations of the C1s core level shift by chemical bonding effects or electron transfer between the carbon atoms and the deposited potassium do not seem reasonable to us in view of the above correlation and existing knowledge related to alkali metals deposited on methylterminated SAMs, as discussed in the Introduction. One cannot, however, simply assume that the vacuum level of a SAM is pinned directly to the vacuum level of the spectrometer, since this will result in a dependence of the C1s peak position on the spectrometer work function (i.e., different positions will be measured with different spectrometers), which is definitely not the case. In our previous tentative model we solved this dilemma by suggesting that the vacuum level of a SAM is pinned to the vacuum level of the conductive substrate, whose Fermi level, in its turn, is pinned to the Fermi level of the substrate.9 Furthermore, we suggested that the vacuum levels of the SAM and substrate are exactly aligned, proposing that the substrate serves as a kind of zero-electron-energy flood gun, which provides charge compensation in the SAM after the photoemission event by tunnelling charge transfer.9 Without such compensation, charging of the SAM should occur in the course of the photoemission experiment, resulting in a stochastical drift of the spectral features. Such a drift is not observed in the case of SAMs, so that charge compensation works, even though this process is quite slow on the time scale of the photoemission process.26 This slowness of charge transfer is presumably the
Figure 11. Energy level diagram for a SAM film on a conductive substrate that is grounded to the spectrometer; the diagram is applicable to the carbon atoms in the aliphatic chain. Vacuum levels (Evac), Fermi levels (Ef), and work functions (φ) of the spectrometer (sp), substrate (s), and SAM (SAM) are shown. ∆φ is the work function difference between the bare and SAM-covered surfaces; hν is the photon energy, and KE is kinetic energy of photoelectrons; EBvac sam and EBvac sp are the BEs of a SAM core level referenced to the vacuum level of the SAM and spectrometer, respectively.
main reason for the nonconventional alignment of the Fermi and vacuum levels in the case of photoemission from SAMs.27 As an alternative to precise vacuum level alignment, one can assume that pinning of the vacuum levels of the SAM and substrate occurs with a bias, corresponding to the work function difference between the bare and SAM-covered surfaces, ∆φ. The respective energy level diagram is shown in Figure 11; all the assignments are explained in the figure caption. Within this diagram, the kinetic energy of the C1s photoelectrons corrected for the work function of the spectrometer (offset) is given by the expression
KEwith offset ) hν - EBvac sam - ∆φ + φs - φsp
(5)
where hV is the energy of the primary X-ray source, EBvac sam is the BE of the C1s core level referenced to the vacuum level of the SAM, and φs is the work function of the substrate. After the standard correction for the work function of the spectrometer (+φsp), this energy does not depend on this parameter (as required) but on the work function of the substrate and the work function change induced by the SAM. The dependence on the work function of the substrate explains all the results regarding the potassium-induced shift of the C1s peak for K/C12/Au (Figures 3 and 6). Potassium deposition results in a decrease of φs, which, according to eq 5, leads to a decrease in the kinetic energy of the C1s photoelectrons. Within the standard Einstein framework (eq 1), which is used for conversion of the kinetic energy axis to the binding energy one by the spectrometer software, such a decrease corresponds to an increase in the C1s BE, which is exactly what was observed in our experiments. In contrast to the C1s emission, the BE position of the S2p doublet persists upon potassium deposition. This means that
4582 J. Phys. Chem. C, Vol. 113, No. 11, 2009 photoemission from the headgroup atoms is described within the standard model, assuming Fermi level pinning. As mentioned in section 1, this situation is characteristic of small adsorbates, which are directly bonded to a metal substrate (however, it can be different for physisorbed adsorbates).28-30 In this case, the electronic systems of the substrate and adsorbate (the headgroup in our case) are coupled, and charge compensation in the photoemission experiment occurs sufficiently fast. This is exactly the difference between the headgroup and the carbon atoms along the molecular chain of the SAM constituents. In contrast to the headgroup, the electronic systems of the SAM matrix and substrate are decoupled in the photoemission experiment because of the lack of unoccupied molecular states for direct charge transfer from the substrate (the Fermi level of the substrate lies within the HOMO-LUMO gap of the SAM). Therefore, charge transfer occurs slowly compared to photoemission26 and only results in charge compensation and not in screening of the photoemission hole. Under these circumstances, screening only occurs by induction of the image charge in the metal substrate, analogous to the case of physisorbed xenon adsorbates.28-30 Because most of the potassium atoms (especially at low doses) are near the SAM-Au interface, they are also in electronic contact with the gold substrate. This is the reason that the K2p peak in Figure 3 does not undergo as significant a change in BE as does the C1s peak. 5. Conclusions We have studied potassium deposition on C12/Au SAMs by UPS, XPS, and HRXPS. It has been shown that a significant portion of potassium atoms penetrates to the SAM-Au interface. However, a large fraction is imbedded in the aliphatic matrix, resulting in its disordering and chemical inhomogeneity. These processes were analyzed within a simplified model assuming a homogeneous distribution of imbedded potassium in the aliphatic matrix. While the real distribution may be different to some extent, the model provides estimates for the relative portions of the penetrating and imbedded potassium atoms. The deposition of potassium is accompanied by a progressive decrease in the work function of the K/C12/Au sample, which is related to the formation of a surface dipole layer at the SAM/ Au interface that, in turn, is mediated by potassium atoms in the vicinity of the interface. The second dominant effect is the upward shift of the BE position of the C1s peak characteristic of the aliphatic chain of the SAM constituents in the course of potassium deposition. This shift correlates exactly with the work function change. In contrast, the BE position of the S2p doublet characteristic of the headgroup of the SAM constituents remained unchanged upon potassium deposition. The results have been rationalized by assuming that different energy level alignment frameworks are applicable to the description of photoemission from the aliphatic chain and headgroup atoms of the SAM constituents. While the standard Fermi level pinning model is suitable for the headgroup, the electronic system of the alkyl matrix is pinned to the substrate and spectrometer in a complex way, involving both Fermi and vacuum level alignment. The different pinning of the electronic systems of the headgroup and matrix is explained by the different proximity of the respective moieties to the metal substrate. While the thiolate headgroup is in electronic contact with the substrate, which ensures rapid charge compensation after photoelectron ejection, the carbon atoms in the methyl and methylene groups are decoupled or, in other words, electrically isolated from the substrate on the time scale of the photoemission process. Charge
Ge et al. compensation in this case occurs on a longer time scale and results only in the lack of charge buildup on the surface. The above findings and considerations suggest that photoemission from SAMs cannot always be described in the standard framework of the chemical shift, even though this description is sufficient in most of cases. There are, however, some special situations when the insulating character of the SAM comes into the foreground, so electrostatic effects should be taken into account. Apart from the potassium deposition case discussed in this work and ref 9, the most prominent examples include the embedded dipole layer in alkanethiolate SAMs27 and the effect of the headgroup dipole in mixed films of unsubstituted and partially semifluorinated alkanethiolates.31 Another “highlight” example of the electrostatic effects, which was for a long time not understood as such,24 is the BE position difference of the C1s emission in alkanethiolate SAMs on Au and Ag.11,18 This difference correlates exactly with the work function difference between Au(111) and Ag(111) and can be perfectly explained by electrostatic effects within, for example, the phenomenological framework of this study (see eq 5). Further experiments on different types of SAMs are clearly needed to fully understand the electrostatic phenomena in these systems. In particular, a tradeoff of electronic coupling versus decoupling by varying the position of the primary C1s hole with respect to the conductive substrate would be interesting. This goal can be, for example, achieved by potassium evaporation experiments on short-chain alkanethiolate SAMs, which we plan in the near future. Acknowledgment. Acknowledgement is made to the Donors of The Petroleum Research Fund, administered by the American Chemical Society, for support of this research at the University of MassachusettssLowell. T.W. and M.Z. thank M. Grunze for the support of this work, S. Watcharinyanon and L. S. O. Johansson (Karlstad University) for the cooperation at MAXLaboratory, and the MAX-Laboratory staff, including A. Preobrajenski, in particular, for the assistance during the HRXPS experiments. This work has been supported by DFG (ZH 63/ 10-1) and the European Community through the IA-SFS project within the Sixth Framework Programme. References and Notes (1) Love, C. J.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103–1169. (2) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99–117. (3) Vuillaume, D.; Lenfant, S. Microelectron. Eng. 2003, 70, 539– 550. (4) Wohltjen, H.; Snow, A. W. Anal. Chem. 1998, 70, 2856–2859. (5) Chaki, N. K. Biosens. Bielectron. 2002, 17, 1–12. (6) Bhushan, B. In Springer Handbook of Nanotechnology, 2nd ed.; Bhushan, B. Ed.; Springer-Verlag: Heidelberg, 2007; pp 1379-1415. (7) Whitesides, G. M.; Jiang, X.; Ostuni, E.; Chapman, R. G.; Grunze, M. Polym. Preprints 2004, 45, 90–91. (8) Ratner, B. D.; Castner, D. G. In Surface AnalysissThe Principle Techniques; Vickerman, J. C., Ed.; Wiley & Sons: Chichester, 1997. (9) Ahn, H.; Zharnikov, M.; Whitten, J. E. Chem. Phys. Lett. 2006, 428, 283–287. (10) Zangwill, A. Physics at Surfaces; Cambridge University Press: Cambridge, 1988; p 334. (11) Tarlov, M. J. Langmuir 1992, 8, 80–89. (12) Czanderna, A. W.; Jung, D. R. Crit. ReV. Sol. State. Mat. Sci. 1994, 19, 1–54. (13) Zhu, Z.; Haynie, B. C.; Winograd, N. Appl. Surf. Sci. 2004, 231/ 232, 318–322. (14) Zhu, Z.; Allara, D. L.; Winograd, N. Appl. Surf. Sci. 2006, 252, 6686–6688. (15) Herdt, G. C.; Jung, D. R.; Czanderna, A. W. Prog. Surf. Sci. 1995, 50, 103–129. (16) Bammel, K.; Ellis, J.; Rubahn, H.-G. Chem. Phys. Lett. 1993, 201, 101–107.
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