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Characterization of benzenethiolate self-assembled monolayer on Cu(1 0 0) by XPS and NEXAFS
Journal of Electron Spectroscopy and Related Phenomena 172 (2009) 64–68
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Journal of Electron Spectroscopy and Related Phenomena journal homepage: www.elsevier.com/locate/elspec
Characterization of benzenethiolate self-assembled monolayer on Cu(1 0 0) by XPS and NEXAFS Marco Beccari a,∗ , Aloke Kanjilal b , Maria Grazia Betti c , Carlo Mariani c , Luca Floreano d , Albano Cossaro d , Valeria Di Castro a a
Dipartimento di Chimica, Università di Roma “La Sapienza”, Piazzale Aldo Moro 2, I-00185 Roma, Italy INFM-CNR Center on nanoStructures and bioSystems at Surfaces (S3), Via G. Campi 231/A, I-41100 Modena, Italy Dipartimento di Fisica, CNISM, CNIS, Università di Roma “La Sapienza”, Piazzale Aldo Moro 2, I-00185 Roma, Italy d Laboratorio TASC-INFM, Basovizza SS14 Km 163.5, I-34012 Trieste, Italy b c
a r t i c l e
i n f o
Article history: Available online 14 March 2009 Keywords: Near-edge X-ray absorption fine structure (NEXAFS) X-ray photoelectron spectroscopy (XPS) Benzenethiolate self-assembled monolayer (SAM) Copper
a b s t r a c t The composition, electronic structure and molecular orientation of a self-assembled monolayer (SAM) of benzenethiolate on the Cu(1 0 0) surface have been investigated by means of X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS). An ordered benzenethiolate SAM with c(2 × 6) superstructure has been obtained by the reaction of diphenyldisulfide with the Cu(1 0 0) surface at room temperature. S 2p and C 1s XP spectra show S–Cu bond formation without significant thiolate decomposition. The strong dichroic signal of C K-edge NEXAFS upon varying polarization angle indicates upstanding of the adsorbed molecules with the benzene ring tilted by 20◦ from the surface normal. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Self-assembled monolayers (SAMs) have attracted a paramount interest in the recent years, for their potential to act as protective coatings, chemical and biological sensors, functionalised interfaces, substrates for crystallization, electronic and optoelectronic devices [1–4]. A detailed knowledge of the molecular configuration of a SAM is of fundamental importance, as it is directly related to the physical and chemical properties at the organic–inorganic interface. Among different functionalised molecules, alkylthiols have been extensively studied on different metals (e.g., Au, Ag, Cu, and Pd) [1–3]. It is now well established that a good quality SAM can be achieved through S–H (or S–S) bond breaking of thiols (or disulfides) on a metal surface [1–3]. More recently the potential application of aromatic thiols in molecular electronic devices [5] has opened up a new avenue of research. Benzenethiol (C6 H5 SH, also known as thiophenol), the simplest aromatic thiol, has been used as a model structure for calculating the interface conductivity between the molecule and a metal electrode [6], and for bridging two metal electrodes [7]. The transport properties at the thiolate–metal interface depend
∗ Corresponding author. Tel.: +39 06 4991 3316; fax: +39 06 4903 24. E-mail addresses: [email protected], [email protected] (M. Beccari). 0368-2048/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2009.03.004
basically on the molecular orientation and the SAM structure. The tendency to form strong metal–S bonds favours a standing-up molecular configuration of the aromatic thiolate, while the interaction between the delocalized orbitals of the aromatic ring and the metal surface states gives rise to a lying-down structure. A number of molecular orientations and configurations for benzenethiol adsorption on different metal surfaces have been found, depending on the molecule–substrate interaction strength, the molecular density, and the actual adsorption conditions, namely whether from the liquid phase or in controlled ultra high vacuum (UHV) environment [8–18]. Recently, we have studied the evolution of the electronic and structural properties of benzenethiolate SAM, adsorbed on Cu(1 0 0) surface as a function of coverage [19,20]. It has been shown that a highly symmetric Cu(1 0 0) surface interacts strongly with thiols, and it can be used as a template for achieving highly ordered SAM by using either a benzenethiol or diphenyldisulfide [(C6 H5 S)2 ] vapours in a controlled UHV condition [20]. Upon increasing coverage, a change in symmetry from the c(4 × 4) to c(2 × 6) has been observed by low-energy electron diffraction (LEED), together with a gradual evolution of the electronic structure, evidencing a structural phase transition. This behaviour was supposed to be associated with a coverage-dependent molecular orientation, namely from an almost flat-lying to a standing-up configuration [19]. In the present work, we focus on the c(2 × 6)-ordered benzenethiolate monolayer on a Cu(1 0 0) substrate, grown at room temperature (RT) in controlled
M. Beccari et al. / Journal of Electron Spectroscopy and Related Phenomena 172 (2009) 64–68
UHV condition, providing direct information of the adlayer composition as well as the orientation of the aromatic ring. A combined X-ray photoelectron spectroscopy (XPS) and C K-edge near-edge X-ray absorption fine structure (NEXAFS) investigation has been employed for our study. Although a recent publication has evidenced a standing-up configuration of the molecule for the c(2 × 6) phase by means of photoelectron diffraction (PhD) and S K-edge NEXAFS measurements, a strong discrepancy has been observed in determining the tilt angle of the S–C bond, which ranges from 10◦ to 24◦ with respect to the surface normal [21]. In this context, the C K-edge NEXAFS, involving the delocalised orbitals of the benzene ring, can provide more reliable information on molecular orientation.
2. Experimental Both XPS and NEXAFS experiments were performed in a UHV apparatus (base pressure better than 1 × 10−10 mbar) at the ALOISA beamline of the ELETTRA synchrotron radiation facility (Trieste) [22]. A high purity (99.999%) Cu(1 0 0) single crystal was mounted on the sample holder with six degrees of freedom and cleaned by repeated sputtering–annealing cycles: Ar+ ion sputtering (1 keV) followed by annealing at 670 K. Surface quality and cleanness were checked by XPS and reflection high-energy electron diffraction (RHEED). The growth of benzenethiolate SAM has been carried out from a pyrex ampoule containing diphenyldisulfide powder (99% purity) through a leak valve, as described in detail elsewhere [20]. The Cu(1 0 0) sample was exposed to diphenyldisulfide molecules at a constant rate of 0.3–0.5 L/min (1 L = 1.33 × 10−6 mbar s) at RT, keeping the base pressure at ∼2 × 10−8 mbar, so that the saturation coverage was reached at about 33 L. The two-domain c(2 × 6) phase has been monitored by RHEED, in perfect agreement with our LEED patterns [19,20]. The saturation phase is considered to be a monolayer. The XPS spectra were measured with photon energy of 400 eV (energy resolution 150 meV), keeping the sample at about 150 K to reduce possible molecular decomposition (i.e., benzene ring desorption) induced by the X-ray irradiation [23]. The binding energy (BE) scale was calibrated using Cu 3p3/2 core level of clean Cu(1 0 0) at 74.9 eV BE. Photoelectrons were taken at normal emission with the hemispherical analyzer in constant pass energy mode (10 eV) [24]. All the XPS spectra were collected using a grazing incidence of ∼4◦ from the surface plane for enhancing the surface sensitivity. The C K-edge NEXAFS spectra (also taken at 150 K) were obtained measuring the partial electron yield (electrons above 200 eV kinetic energy) by means of a channeltron with a −200 V bias on the grid, in order to reject secondary electrons. The linear polarization factor of the X-ray beam is at least 0.95. The photon energy, spanning from 275 eV to 322 eV, was selected by the ALOISA monochromator [22] with a photon energy resolution of 100 meV. A series of NEXAFS spectra was collected as a function of the polar angle between the surface plane and the photon polarization, from 0◦ (electric field polarization in the surface plane) to 90◦ (electric field polarization nearly perpendicular to the surface plane). Thanks to the experimental setup of the ALOISA beamline the required conditions were achieved by rotating the sample around an axis coincident with the X-ray beam, keeping the incidence angle of X-ray fixed at 7◦ from the surface plane. The spot size on the sample is about 1.6 mm × 0.2 mm. The absorption intensities were normalized to a reference signal, i.e. the background few eV below the K-edge. All the spectra were taken using an acquisition time as short as possible, in order to reduce the adlayer damaging due to the secondary electrons. Finally, the quality of the sample was verified by XPS after the NEXAFS experiments, and no evident changes were observed.
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3. Results and discussion 3.1. XPS The S 2p core level photoelectron spectrum of the saturation layer obtained by exposing the Cu(1 0 0) surface to diphenyldisulfide vapour, and the S 2p XP spectrum after annealing the sample at 645 K, are displayed in Fig. 1. The data related to the saturation layer (solid line) show a clear doublet peaked at 162.6 eV and 163.8 eV BE (due to spin orbit splitting), followed by secondary features (at about 161 eV and 167 eV). The main peak at 162.6 eV BE is found at the expected position for the S 2p3/2 signal of the benzenethiolate bonded to copper surface [25–30], confirming the S–S bond breaking induced by the molecule–substrate interaction, followed by the formation of a benzenethiolate layer through S–Cu bonding. The low intensity structure at 161 eV can be assigned to the formation of small quantity (i.e., <5%) of copper sulphide at the surface, due to decomposition of the molecules during the molecule–substrate interaction. This assignment is supported by the evolution of the spectrum after heating the interface to 645 K, where a peak emerges at 161.1 eV, appearing as a dominant structure in the spectrum (dashed line in Fig. 1). The cleavage of the C–S bond has been observed in a previous thermal desorption spectroscopy study, where the maximum desorption of benzene ring was found around 470 K [20]. Our attribution is in agreement with the previously reported values for the similar interfaces, where partial decomposition of the adsorbed molecules was observed [19,29,31,32]. Finally, the low intensity peaks at about 167 eV are consistent with oxidized sulphur species [33–35], which may appear in our case due to the presence of impurities in the disulfide powder. The C 1s photoemission signal (Fig. 2) centred at 284.7 eV is in agreement with the results previously reported in the literature for benzenethiolate layers [8,25,36–39] and it can be ascribed to the aromatic carbons of the benzene ring, confirming the adsorption of the thiolate without decomposition. 3.2. C K-edge NEXAFS The C K-edge NEXAFS spectra, taken at different polar angles (), for the adsorbed benzenethiolate SAM on the Cu(1 0 0) surface are displayed in Fig. 3. There is a remarkable dichroism in the absorption spectra as a function of the polar angle, signifying formation
Fig. 1. XP spectra of the S 2p core levels from benzenethiolate adsorbed at RT on Cu(1 0 0) (solid line) and after annealing at 645 K (dashed line). The data at monolayer coverage were taken at 150 K, the spectrum after annealing was acquired at 345 K. Data taken with photon energy of 400 eV.
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M. Beccari et al. / Journal of Electron Spectroscopy and Related Phenomena 172 (2009) 64–68
Fig. 2. XP spectrum of the C 1s core level from benzenethiolate adsorbed at RT on Cu(1 0 0) surface. Data taken with photon energy of 400 eV, and collected at 150 K.
of an ordered layer with well-oriented thiolate molecules on the copper surface. The absorption spectra, recorded at an intermediate polar angle (45◦ ) along with the corresponding fitting curves, are shown in Fig. 4. Six different peaks, centred at 284.9 eV, 285.5 eV, 286.9 eV, 289.0 eV, 293.4 eV, and 300.5 eV, can be identified. Within a simple single-particle approximation, these peaks can be attributed to the transitions from the C 1s orbitals to the empty molecular states, by comparison with the previously reported results on similar systems. The first resonance at 284.9 eV (i.e., peak ‘1’ in Fig. 4) is associated with the transition from the 1s orbitals of carbons not bonded to sulphur to the 1 * and 2 * (a2 and b1 , respectively) molecular orbitals, diffused on the aromatic ring as pointed out for the aromatic aniline and phenol monolayers on Ag(1 1 0) [40]. The second resonance (peak ‘2’) located at 285.5 eV can be assigned to the transition from C 1s of the carbon atom bonded to sulphur to the 2 *(b1 ) orbital, while the transition to 1 *(a2 ) state is not allowed for the absence of 1 *(a2 ) molecular density on this carbon atom [40]. Furthermore, peak ‘3’ at 286.9 eV is assigned to a transition from the C 1s orbital of the carbon atom bonded to sulphur to the *(S–C) orbital, as for benzenethiol on Mo(1 1 0) [14] and for thiophene on Au(1 1 1) [41]. Peak ‘4’ at 289.0 eV can be ascribed to
Fig. 4. C K-edge NEXAFS spectra (dots) of the benzenethiolate SAM on the Cu(1 0 0) surface, for the 45◦ polar angle. Numbers refer to the different resonances associated with specific transitions, and both the singular and total fitting curves (continuous lines) are displayed (see text).
the C 1s–*3 (b1 ) transition. Two structures have been resolved in case of aniline and phenol SAM deposited on the Ag(1 1 0) surface and they have been assigned to the contribution of unequivalent carbon atoms [40], unresolved in our case. Additionally these two transitions (i.e., peaks ‘3’ and ‘4’) may overlap with the *(C–H) resonances, which falls in this region of the spectrum [40]. Peaks ‘5’ and ‘6’ at 293.4 eV and 300.5 eV, respectively, can be attributed to two different C 1s–*(C–C) transitions. To determine the orientation of the benzenethiolate molecules a quantitative analysis of the intensity of the NEXAFS resonances as a function of the polar angle has been performed. We have used pseudo-Voigt functions for fitting the first three absorption peaks, while an asymmetry parameter has been applied to Gaussian width, taking into account the vibrational effects [42,43], for the wider structures at higher energy. Except the corresponding peak intensities, the other relevant parameters were kept fixed during the fitting process upon varying the polar angle. The K-edge atomic-like jump at 287.7 eV was fitted by a Gaussian-broadened step function with a little linear decay after the jump edge [43]. It lies close to the expected value of the ionization potential, in agreement with the weak interaction of carbon atoms with the surface [42]. The polar angle dependence of the NEXAFS data was obtained by rotating the surface plane, keeping the glancing angle fixed at 7◦ [44,45]. It is possible to represent this particular system with radiation polarization scan, where the coordinate system can be illustrated as the one sketched in Fig. 5. In the dipole approximation, the intensity of the transition from C 1s to a vector-type orbital (* or *) is related to the square modulus of the scalar product between the electric field vector (ε- ) of the incident radiation and the vector orbital (O - ), and can be expressed as [14]: 2 2 I = A|O - · ε- | = A cos ı
Changing the variables from the angle ı to the angles , ˛, and (i.e., expanding the scalar product) gives rise to the following equation: I = A| cos sin cos + sin sin sin sin ˛ + sin cos cos ˛|2
Fig. 3. Set of C K-edge NEXAFS spectra of the benzenethiolate SAM on the Cu(1 0 0) surface, as a function of the polar angle from the surface plane, from 0◦ (radiation polarization in plane) and 90◦ (out of plane). All the spectra were taken at 150 K.
where is the azimuthal orientation, is the angle between the vector orbital and the surface normal, ˛ is the fixed glancing angle between the radiation and the surface plane (i.e., 7◦ ), and is the polar angle. When the polar angle is zero, the radiation polarization vector lies in the plane of the Cu(1 0 0) surface. The azimuthal
M. Beccari et al. / Journal of Electron Spectroscopy and Related Phenomena 172 (2009) 64–68
Fig. 5. Schematic representation of the coordinates used for our analysis. The * orbital involved in the transition is represented by a vector O - , with angle with respect to the surface normal (z axes) and azimuthal angle (with respect to the surface plane, xy). The vector ε- represents the radiation linear polarization. The radiation falls onto the sample surface plane (xy) with a fixed grazing angle ˛. The polar angle is changed on the (xz ) plane. In addition, ı indicates the angle between the vectors O - and ε- .
angle dependence of the intensity can be removed, according to the symmetry of the substrate, and in that case the above expression can be simplified to I=A
1 2
cos2 sin2 +
1 sin2 sin2 sin2 ˛+ sin2 cos2 cos2 ˛ 2
where the scale factor A, and the angle are free parameters in the fitting procedure. From the angle we obtain the tilt angle ˇ of the aromatic ring plane with respect to the surface normal (ˇ = 90◦ − ). The values of the tilt angle ˇ can be estimated from the variation in intensity of the first two transitions to * orbitals (peaks ‘1’ and ‘2’ in Fig. 4) as a function of the radiation polarization, and they are ˇ = 18◦ ( = 72◦ ) and ˇ = 23.5◦ ( = 66.5◦ ), respectively. Considering the experimental error of NEXAFS technique, the different values obtained by analysing peaks ‘1’ and ‘2’ are found to be very close to each other. In any case, a more reliable value can be achieved by summing the areas of the first two peaks, it leads to ˇ = 20 ± 10◦ ( = 70 ± 10◦ ), as shown in Fig. 6, where the experimental intensity dependence and the result of the model fitting are reported. The overall uncertainly of 10◦ takes into account the fit assumptions, the X-ray beam polarization, the finite grazing angle of 7◦ , and the
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misalignment between the surface and the reference manipulator orientation, due to sample mounting. This quantitative determination of the benzenethiolate SAM orientation on Cu(1 0 0) provides an indication for the molecular tilt angle more reliable than the previous results obtained using S K-edge NEXAFS and PhD techniques [21], since it is based on transitions within the aromatic ring and it is not affected by the transitions related to the S–Cu interaction. In any case, the obtained value is in-between the previously reported tilt angles (24◦ and 10◦ ) and the combination of the three independent measurements strongly supports the attribution of a tilt angle of 20◦ to the standing-up molecule. A similar orientation was observed as well for benzenethiolate SAMs on different metals. Tilt angles of 23◦ and 24◦ were reported for the aromatic ring orientation with respect to the surface normal on Mo(1 1 0) [14] and Ag(1 1 1), respectively [10], and the S–C bond was found nearly normal to the surface on Ni(1 0 0) [15], while contrasting results were reported for the SAM orientation on Au(1 1 1), depending on the actual preparation method [8–10,13,18]. The similar orientation of the benzenethiolate SAM on different metals with different surface structures suggests that the SAM arrangement is basically controlled by the strong S–metal bond and the close packing of the adsorbed molecules at monolayer completion. 4. Conclusions Combined XPS and NEXAFS studies enlighten the electronic structure and the molecular orientation of a c(2 × 6) saturated benzenethiolate SAM on a Cu(1 0 0) surface. The S 2p and C 1s XP spectra are consistent with a chemical bonding of the benzenethiolate to the underlying copper substrate through the sulphur atom. A strong dichroism of the NEXAFS signal at the C K-edge is found as a function of the polar angle. The analysis of the experimental data, through a fitting procedure and applying a model calculation, leads to estimate an average tilt angle of 20 ± 10◦ of the molecular plane with respect to the Cu(1 0 0) surface normal. This almost standing-up orientation of the molecules in the ordered monolayer is of interest for possible electronic application and allows the use of the benzenethiolate SAM as a buffer layer for further growth of other molecules maintaining the same orientation. Acknowledgements We are grateful to the Aloisa staff for the invaluable support. Work partially supported by “Facoltà”, “Ateneo”, and “Grandi Attrezzature” grants of the Università di Roma “La Sapienza”. References
Fig. 6. Evolution of the intensity of the observed NEXAFS C 1s–* transitions (sum of the peaks ‘1’ and ‘2’ in Fig. 4), as a function of the polar angle. The best fit is reported as a solid line. We extract from the fit, and we obtain the corresponding tilt angle ˇ of the aromatic ring from the surface normal (ˇ = 90◦ − ).
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Journal of Electron Spectroscopy and Related Phenomena journal homepage: www.elsevier.com/locate/elspec
Characterization of benzenethiolate self-assembled monolayer on Cu(1 0 0) by XPS and NEXAFS Marco Beccari a,∗ , Aloke Kanjilal b , Maria Grazia Betti c , Carlo Mariani c , Luca Floreano d , Albano Cossaro d , Valeria Di Castro a a
Dipartimento di Chimica, Università di Roma “La Sapienza”, Piazzale Aldo Moro 2, I-00185 Roma, Italy INFM-CNR Center on nanoStructures and bioSystems at Surfaces (S3), Via G. Campi 231/A, I-41100 Modena, Italy Dipartimento di Fisica, CNISM, CNIS, Università di Roma “La Sapienza”, Piazzale Aldo Moro 2, I-00185 Roma, Italy d Laboratorio TASC-INFM, Basovizza SS14 Km 163.5, I-34012 Trieste, Italy b c
a r t i c l e
i n f o
Article history: Available online 14 March 2009 Keywords: Near-edge X-ray absorption fine structure (NEXAFS) X-ray photoelectron spectroscopy (XPS) Benzenethiolate self-assembled monolayer (SAM) Copper
a b s t r a c t The composition, electronic structure and molecular orientation of a self-assembled monolayer (SAM) of benzenethiolate on the Cu(1 0 0) surface have been investigated by means of X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS). An ordered benzenethiolate SAM with c(2 × 6) superstructure has been obtained by the reaction of diphenyldisulfide with the Cu(1 0 0) surface at room temperature. S 2p and C 1s XP spectra show S–Cu bond formation without significant thiolate decomposition. The strong dichroic signal of C K-edge NEXAFS upon varying polarization angle indicates upstanding of the adsorbed molecules with the benzene ring tilted by 20◦ from the surface normal. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Self-assembled monolayers (SAMs) have attracted a paramount interest in the recent years, for their potential to act as protective coatings, chemical and biological sensors, functionalised interfaces, substrates for crystallization, electronic and optoelectronic devices [1–4]. A detailed knowledge of the molecular configuration of a SAM is of fundamental importance, as it is directly related to the physical and chemical properties at the organic–inorganic interface. Among different functionalised molecules, alkylthiols have been extensively studied on different metals (e.g., Au, Ag, Cu, and Pd) [1–3]. It is now well established that a good quality SAM can be achieved through S–H (or S–S) bond breaking of thiols (or disulfides) on a metal surface [1–3]. More recently the potential application of aromatic thiols in molecular electronic devices [5] has opened up a new avenue of research. Benzenethiol (C6 H5 SH, also known as thiophenol), the simplest aromatic thiol, has been used as a model structure for calculating the interface conductivity between the molecule and a metal electrode [6], and for bridging two metal electrodes [7]. The transport properties at the thiolate–metal interface depend
∗ Corresponding author. Tel.: +39 06 4991 3316; fax: +39 06 4903 24. E-mail addresses: [email protected], [email protected] (M. Beccari). 0368-2048/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2009.03.004
basically on the molecular orientation and the SAM structure. The tendency to form strong metal–S bonds favours a standing-up molecular configuration of the aromatic thiolate, while the interaction between the delocalized orbitals of the aromatic ring and the metal surface states gives rise to a lying-down structure. A number of molecular orientations and configurations for benzenethiol adsorption on different metal surfaces have been found, depending on the molecule–substrate interaction strength, the molecular density, and the actual adsorption conditions, namely whether from the liquid phase or in controlled ultra high vacuum (UHV) environment [8–18]. Recently, we have studied the evolution of the electronic and structural properties of benzenethiolate SAM, adsorbed on Cu(1 0 0) surface as a function of coverage [19,20]. It has been shown that a highly symmetric Cu(1 0 0) surface interacts strongly with thiols, and it can be used as a template for achieving highly ordered SAM by using either a benzenethiol or diphenyldisulfide [(C6 H5 S)2 ] vapours in a controlled UHV condition [20]. Upon increasing coverage, a change in symmetry from the c(4 × 4) to c(2 × 6) has been observed by low-energy electron diffraction (LEED), together with a gradual evolution of the electronic structure, evidencing a structural phase transition. This behaviour was supposed to be associated with a coverage-dependent molecular orientation, namely from an almost flat-lying to a standing-up configuration [19]. In the present work, we focus on the c(2 × 6)-ordered benzenethiolate monolayer on a Cu(1 0 0) substrate, grown at room temperature (RT) in controlled
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UHV condition, providing direct information of the adlayer composition as well as the orientation of the aromatic ring. A combined X-ray photoelectron spectroscopy (XPS) and C K-edge near-edge X-ray absorption fine structure (NEXAFS) investigation has been employed for our study. Although a recent publication has evidenced a standing-up configuration of the molecule for the c(2 × 6) phase by means of photoelectron diffraction (PhD) and S K-edge NEXAFS measurements, a strong discrepancy has been observed in determining the tilt angle of the S–C bond, which ranges from 10◦ to 24◦ with respect to the surface normal [21]. In this context, the C K-edge NEXAFS, involving the delocalised orbitals of the benzene ring, can provide more reliable information on molecular orientation.
2. Experimental Both XPS and NEXAFS experiments were performed in a UHV apparatus (base pressure better than 1 × 10−10 mbar) at the ALOISA beamline of the ELETTRA synchrotron radiation facility (Trieste) [22]. A high purity (99.999%) Cu(1 0 0) single crystal was mounted on the sample holder with six degrees of freedom and cleaned by repeated sputtering–annealing cycles: Ar+ ion sputtering (1 keV) followed by annealing at 670 K. Surface quality and cleanness were checked by XPS and reflection high-energy electron diffraction (RHEED). The growth of benzenethiolate SAM has been carried out from a pyrex ampoule containing diphenyldisulfide powder (99% purity) through a leak valve, as described in detail elsewhere [20]. The Cu(1 0 0) sample was exposed to diphenyldisulfide molecules at a constant rate of 0.3–0.5 L/min (1 L = 1.33 × 10−6 mbar s) at RT, keeping the base pressure at ∼2 × 10−8 mbar, so that the saturation coverage was reached at about 33 L. The two-domain c(2 × 6) phase has been monitored by RHEED, in perfect agreement with our LEED patterns [19,20]. The saturation phase is considered to be a monolayer. The XPS spectra were measured with photon energy of 400 eV (energy resolution 150 meV), keeping the sample at about 150 K to reduce possible molecular decomposition (i.e., benzene ring desorption) induced by the X-ray irradiation [23]. The binding energy (BE) scale was calibrated using Cu 3p3/2 core level of clean Cu(1 0 0) at 74.9 eV BE. Photoelectrons were taken at normal emission with the hemispherical analyzer in constant pass energy mode (10 eV) [24]. All the XPS spectra were collected using a grazing incidence of ∼4◦ from the surface plane for enhancing the surface sensitivity. The C K-edge NEXAFS spectra (also taken at 150 K) were obtained measuring the partial electron yield (electrons above 200 eV kinetic energy) by means of a channeltron with a −200 V bias on the grid, in order to reject secondary electrons. The linear polarization factor of the X-ray beam is at least 0.95. The photon energy, spanning from 275 eV to 322 eV, was selected by the ALOISA monochromator [22] with a photon energy resolution of 100 meV. A series of NEXAFS spectra was collected as a function of the polar angle between the surface plane and the photon polarization, from 0◦ (electric field polarization in the surface plane) to 90◦ (electric field polarization nearly perpendicular to the surface plane). Thanks to the experimental setup of the ALOISA beamline the required conditions were achieved by rotating the sample around an axis coincident with the X-ray beam, keeping the incidence angle of X-ray fixed at 7◦ from the surface plane. The spot size on the sample is about 1.6 mm × 0.2 mm. The absorption intensities were normalized to a reference signal, i.e. the background few eV below the K-edge. All the spectra were taken using an acquisition time as short as possible, in order to reduce the adlayer damaging due to the secondary electrons. Finally, the quality of the sample was verified by XPS after the NEXAFS experiments, and no evident changes were observed.
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3. Results and discussion 3.1. XPS The S 2p core level photoelectron spectrum of the saturation layer obtained by exposing the Cu(1 0 0) surface to diphenyldisulfide vapour, and the S 2p XP spectrum after annealing the sample at 645 K, are displayed in Fig. 1. The data related to the saturation layer (solid line) show a clear doublet peaked at 162.6 eV and 163.8 eV BE (due to spin orbit splitting), followed by secondary features (at about 161 eV and 167 eV). The main peak at 162.6 eV BE is found at the expected position for the S 2p3/2 signal of the benzenethiolate bonded to copper surface [25–30], confirming the S–S bond breaking induced by the molecule–substrate interaction, followed by the formation of a benzenethiolate layer through S–Cu bonding. The low intensity structure at 161 eV can be assigned to the formation of small quantity (i.e., <5%) of copper sulphide at the surface, due to decomposition of the molecules during the molecule–substrate interaction. This assignment is supported by the evolution of the spectrum after heating the interface to 645 K, where a peak emerges at 161.1 eV, appearing as a dominant structure in the spectrum (dashed line in Fig. 1). The cleavage of the C–S bond has been observed in a previous thermal desorption spectroscopy study, where the maximum desorption of benzene ring was found around 470 K [20]. Our attribution is in agreement with the previously reported values for the similar interfaces, where partial decomposition of the adsorbed molecules was observed [19,29,31,32]. Finally, the low intensity peaks at about 167 eV are consistent with oxidized sulphur species [33–35], which may appear in our case due to the presence of impurities in the disulfide powder. The C 1s photoemission signal (Fig. 2) centred at 284.7 eV is in agreement with the results previously reported in the literature for benzenethiolate layers [8,25,36–39] and it can be ascribed to the aromatic carbons of the benzene ring, confirming the adsorption of the thiolate without decomposition. 3.2. C K-edge NEXAFS The C K-edge NEXAFS spectra, taken at different polar angles (), for the adsorbed benzenethiolate SAM on the Cu(1 0 0) surface are displayed in Fig. 3. There is a remarkable dichroism in the absorption spectra as a function of the polar angle, signifying formation
Fig. 1. XP spectra of the S 2p core levels from benzenethiolate adsorbed at RT on Cu(1 0 0) (solid line) and after annealing at 645 K (dashed line). The data at monolayer coverage were taken at 150 K, the spectrum after annealing was acquired at 345 K. Data taken with photon energy of 400 eV.
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Fig. 2. XP spectrum of the C 1s core level from benzenethiolate adsorbed at RT on Cu(1 0 0) surface. Data taken with photon energy of 400 eV, and collected at 150 K.
of an ordered layer with well-oriented thiolate molecules on the copper surface. The absorption spectra, recorded at an intermediate polar angle (45◦ ) along with the corresponding fitting curves, are shown in Fig. 4. Six different peaks, centred at 284.9 eV, 285.5 eV, 286.9 eV, 289.0 eV, 293.4 eV, and 300.5 eV, can be identified. Within a simple single-particle approximation, these peaks can be attributed to the transitions from the C 1s orbitals to the empty molecular states, by comparison with the previously reported results on similar systems. The first resonance at 284.9 eV (i.e., peak ‘1’ in Fig. 4) is associated with the transition from the 1s orbitals of carbons not bonded to sulphur to the 1 * and 2 * (a2 and b1 , respectively) molecular orbitals, diffused on the aromatic ring as pointed out for the aromatic aniline and phenol monolayers on Ag(1 1 0) [40]. The second resonance (peak ‘2’) located at 285.5 eV can be assigned to the transition from C 1s of the carbon atom bonded to sulphur to the 2 *(b1 ) orbital, while the transition to 1 *(a2 ) state is not allowed for the absence of 1 *(a2 ) molecular density on this carbon atom [40]. Furthermore, peak ‘3’ at 286.9 eV is assigned to a transition from the C 1s orbital of the carbon atom bonded to sulphur to the *(S–C) orbital, as for benzenethiol on Mo(1 1 0) [14] and for thiophene on Au(1 1 1) [41]. Peak ‘4’ at 289.0 eV can be ascribed to
Fig. 4. C K-edge NEXAFS spectra (dots) of the benzenethiolate SAM on the Cu(1 0 0) surface, for the 45◦ polar angle. Numbers refer to the different resonances associated with specific transitions, and both the singular and total fitting curves (continuous lines) are displayed (see text).
the C 1s–*3 (b1 ) transition. Two structures have been resolved in case of aniline and phenol SAM deposited on the Ag(1 1 0) surface and they have been assigned to the contribution of unequivalent carbon atoms [40], unresolved in our case. Additionally these two transitions (i.e., peaks ‘3’ and ‘4’) may overlap with the *(C–H) resonances, which falls in this region of the spectrum [40]. Peaks ‘5’ and ‘6’ at 293.4 eV and 300.5 eV, respectively, can be attributed to two different C 1s–*(C–C) transitions. To determine the orientation of the benzenethiolate molecules a quantitative analysis of the intensity of the NEXAFS resonances as a function of the polar angle has been performed. We have used pseudo-Voigt functions for fitting the first three absorption peaks, while an asymmetry parameter has been applied to Gaussian width, taking into account the vibrational effects [42,43], for the wider structures at higher energy. Except the corresponding peak intensities, the other relevant parameters were kept fixed during the fitting process upon varying the polar angle. The K-edge atomic-like jump at 287.7 eV was fitted by a Gaussian-broadened step function with a little linear decay after the jump edge [43]. It lies close to the expected value of the ionization potential, in agreement with the weak interaction of carbon atoms with the surface [42]. The polar angle dependence of the NEXAFS data was obtained by rotating the surface plane, keeping the glancing angle fixed at 7◦ [44,45]. It is possible to represent this particular system with radiation polarization scan, where the coordinate system can be illustrated as the one sketched in Fig. 5. In the dipole approximation, the intensity of the transition from C 1s to a vector-type orbital (* or *) is related to the square modulus of the scalar product between the electric field vector (ε- ) of the incident radiation and the vector orbital (O - ), and can be expressed as [14]: 2 2 I = A|O - · ε- | = A cos ı
Changing the variables from the angle ı to the angles , ˛, and (i.e., expanding the scalar product) gives rise to the following equation: I = A| cos sin cos + sin sin sin sin ˛ + sin cos cos ˛|2
Fig. 3. Set of C K-edge NEXAFS spectra of the benzenethiolate SAM on the Cu(1 0 0) surface, as a function of the polar angle from the surface plane, from 0◦ (radiation polarization in plane) and 90◦ (out of plane). All the spectra were taken at 150 K.
where is the azimuthal orientation, is the angle between the vector orbital and the surface normal, ˛ is the fixed glancing angle between the radiation and the surface plane (i.e., 7◦ ), and is the polar angle. When the polar angle is zero, the radiation polarization vector lies in the plane of the Cu(1 0 0) surface. The azimuthal
M. Beccari et al. / Journal of Electron Spectroscopy and Related Phenomena 172 (2009) 64–68
Fig. 5. Schematic representation of the coordinates used for our analysis. The * orbital involved in the transition is represented by a vector O - , with angle with respect to the surface normal (z axes) and azimuthal angle (with respect to the surface plane, xy). The vector ε- represents the radiation linear polarization. The radiation falls onto the sample surface plane (xy) with a fixed grazing angle ˛. The polar angle is changed on the (xz ) plane. In addition, ı indicates the angle between the vectors O - and ε- .
angle dependence of the intensity can be removed, according to the symmetry of the substrate, and in that case the above expression can be simplified to I=A
1 2
cos2 sin2 +
1 sin2 sin2 sin2 ˛+ sin2 cos2 cos2 ˛ 2
where the scale factor A, and the angle are free parameters in the fitting procedure. From the angle we obtain the tilt angle ˇ of the aromatic ring plane with respect to the surface normal (ˇ = 90◦ − ). The values of the tilt angle ˇ can be estimated from the variation in intensity of the first two transitions to * orbitals (peaks ‘1’ and ‘2’ in Fig. 4) as a function of the radiation polarization, and they are ˇ = 18◦ ( = 72◦ ) and ˇ = 23.5◦ ( = 66.5◦ ), respectively. Considering the experimental error of NEXAFS technique, the different values obtained by analysing peaks ‘1’ and ‘2’ are found to be very close to each other. In any case, a more reliable value can be achieved by summing the areas of the first two peaks, it leads to ˇ = 20 ± 10◦ ( = 70 ± 10◦ ), as shown in Fig. 6, where the experimental intensity dependence and the result of the model fitting are reported. The overall uncertainly of 10◦ takes into account the fit assumptions, the X-ray beam polarization, the finite grazing angle of 7◦ , and the
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misalignment between the surface and the reference manipulator orientation, due to sample mounting. This quantitative determination of the benzenethiolate SAM orientation on Cu(1 0 0) provides an indication for the molecular tilt angle more reliable than the previous results obtained using S K-edge NEXAFS and PhD techniques [21], since it is based on transitions within the aromatic ring and it is not affected by the transitions related to the S–Cu interaction. In any case, the obtained value is in-between the previously reported tilt angles (24◦ and 10◦ ) and the combination of the three independent measurements strongly supports the attribution of a tilt angle of 20◦ to the standing-up molecule. A similar orientation was observed as well for benzenethiolate SAMs on different metals. Tilt angles of 23◦ and 24◦ were reported for the aromatic ring orientation with respect to the surface normal on Mo(1 1 0) [14] and Ag(1 1 1), respectively [10], and the S–C bond was found nearly normal to the surface on Ni(1 0 0) [15], while contrasting results were reported for the SAM orientation on Au(1 1 1), depending on the actual preparation method [8–10,13,18]. The similar orientation of the benzenethiolate SAM on different metals with different surface structures suggests that the SAM arrangement is basically controlled by the strong S–metal bond and the close packing of the adsorbed molecules at monolayer completion. 4. Conclusions Combined XPS and NEXAFS studies enlighten the electronic structure and the molecular orientation of a c(2 × 6) saturated benzenethiolate SAM on a Cu(1 0 0) surface. The S 2p and C 1s XP spectra are consistent with a chemical bonding of the benzenethiolate to the underlying copper substrate through the sulphur atom. A strong dichroism of the NEXAFS signal at the C K-edge is found as a function of the polar angle. The analysis of the experimental data, through a fitting procedure and applying a model calculation, leads to estimate an average tilt angle of 20 ± 10◦ of the molecular plane with respect to the Cu(1 0 0) surface normal. This almost standing-up orientation of the molecules in the ordered monolayer is of interest for possible electronic application and allows the use of the benzenethiolate SAM as a buffer layer for further growth of other molecules maintaining the same orientation. Acknowledgements We are grateful to the Aloisa staff for the invaluable support. Work partially supported by “Facoltà”, “Ateneo”, and “Grandi Attrezzature” grants of the Università di Roma “La Sapienza”. References
Fig. 6. Evolution of the intensity of the observed NEXAFS C 1s–* transitions (sum of the peaks ‘1’ and ‘2’ in Fig. 4), as a function of the polar angle. The best fit is reported as a solid line. We extract from the fit, and we obtain the corresponding tilt angle ˇ of the aromatic ring from the surface normal (ˇ = 90◦ − ).
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