Depth Distribution of Irradiation-Induced Cross-Linking in Aromatic

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Depth Distribution of Irradiation-Induced Cross-Linking in Aromatic Self-Assembled Monolayers Y. Tai, A. Shaporenko, W. Eck, M. Grunze, and M. Zharnikov* Angewandte Physikalische Chemie, Universita¨ t Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany Received March 16, 2004. In Final Form: June 11, 2004 Pristine and strongly irradiated self-assembled monolayers of [1,1′:4′,1′′-terphenyl]-4,4′′-dimethanethiol (TPDMT) on Au have been characterized by near-edge X-ray absorption fine structure spectroscopy using partial electron yield acquisition mode. The TPDMT films were found to be extremely stable toward electron irradiation, which is explained by cross-linking between the aromatic backbones. In addition, we assume that a large delocalization and a strong relaxation of the initial electronic excitations in the densely packed film contributed to the film stability. The data analysis implies an inhomogeneous distribution of the irradiation-induced dehydrogenation and cross-linking of the terphenyl moieties in the TPDMT film, being most pronounced close to the film-ambient interface. The inhomogeneity was explained by quenching of the electronically excited C-H* states via dipole-dipole interaction with the states’ image at the metal surface, which has a reduced probability with increasing separation from the metal surface. Generally, the results suggest the importance of relaxation processes for the response of self-assembled monolayers to ionizing radiation.

1. Introduction Self-assembled monolayers (SAMs) consisting of chemisorbed chainlike organic molecules attract considerable interest in physics, chemistry, and biology because of their ability to control wetting, adhesion, lubrication, and corrosion on surfaces and interfaces.1,2 So far, most attention was devoted to aliphatic SAMs and, above all, to films of n-alkanethiol (AT) on noble metal substrates.3,4 However, recently, less studied but practically important aromatic SAMs became very popular (see, e.g., refs 5-18) * Corresponding author (e-mail: [email protected]). (1) Ulman, A. An Introduction to Ultrathin Organic Films: LangmuirBlodgett to Self-Assembly; Academic Press: New York, 1991. (2) Ulman, A. Chem. Rev. 1996, 96, 1533. (3) Thin films: self-assembled monolayers of thiols; Ulman A., Ed.; Academic Press: San Diego, CA, 1998. (4) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (5) Sabatani, E.; Cohen-Boulakia, J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9, 2974. (6) Tour, J. M.; Jones, L., II; Pearson, D. L.; Lamba, J. S.; Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. V. J. Am. Chem. Soc. 1995, 117, 9529. (7) Dhirani, A.-A.; Zehner, W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L. J. Am. Chem. Soc. 1996, 118, 3319. (8) Tao, Y.-T.; Wu, C.-C.; Eu, J.-Y.; Lin, W.-L. Langmuir 1997, 13, 4018. (9) Kang, J. F.; Liao, S.; Jordan, R.; Ulman, A. J. Am. Chem. Soc. 1998, 120, 9662. (10) Himmel, H.-J.; Terfort, A.; Wo¨ll, Ch. J. Am. Chem. Soc. 1998, 120, 12069. (11) Ishida, T.; Choi, N.; Mizutani, W.; Tokumoto, H.; Kojima, I.; Azehara, H.; Hokari, H.; Akiba, U.; Fujihira, M. Langmuir 1999, 15, 6799. (12) Leung, T. Y. B.; Schwartz, P.; Scoles, G.; Schreiber, F.; Ulman, A. Surf. Sci. 2000, 458, 34. (13) Frey, S.; Stadler, V.; Heister, K.; Zharnikov, M.; Grunze, M.; Zeysing, B.; Terfort, A. Langmuir 2001, 17, 2408. (14) Ishida, T.; Mizutani, W.; Azehara, H.; Sato, F.; Choi, N.; Akiba, U.; Fujihira, M.; Tokumoto, H. Langmuir 2001, 17, 7459. (15) Kang, J. F.; Ulman, A.; Liao, S.; Jordan, R.; Yang, G.; Liu, G.-y. Langmuir 2001, 17, 95. (16) Fuxen, C.; Azzam, W.; Arnold, R.; Witte, G.; Terfort, A.; Wo¨ll, Ch. Langmuir 2001, 17, 3689. (17) Zharnikov, M.; Ku¨ller, A.; Shaporenko, A.; Schmidt, E.; Eck, W. Langmuir 2003, 19, 4682. (18) Shaporenko, A.; Adlkofer, K.; Johansson, L. S. O.; Tanaka, M.; Zharnikov, M. Langmuir 2003, 19, 4992.

in view of their potential applications in molecular electronics6,19,20 and conventional or chemical lithography.21-25 The lithographic applications of aromatic SAMs are based on their resistance toward ionizing radiation such as X-rays or electrons. Contrary to AT SAMs, which are damaged by such irradiation, aromatic SAMs undergo extended cross-linking, which transforms the film into a quasi two-dimensional polymer and prevents complete loss of the orientational order and an extended damage of the headgroup-substrate interface.22,26 The irradiated aromatic SAMs, thus, become more resistant to wet chemical etching agents than the pristine ones, which allows their use as a negative resist in SAM-based lithography.22,25,27 So far, it was assumed that irradiation-induced crosslinking develops homogeneously over the entire depth of aromatic SAMs. Here, we present evidence that crosslinking occurs more likely close to the SAM-ambient interface. As a test system, we have used SAMs of [1,1′:4′,1′′-terphenyl]-4,4′′-dimethanethiol [HS-CH2(C6H4)3-CH2-SH: TPDMT] on polycrystalline Au(111) substrates.28 Because of the methylene spacer between the thiol group and the aromatic core, these SAMs show high orientational order and tight packing on Au(111),28,29 similar to SAMs of biphenyl-substituted ATs CH3(C6H4)2(19) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705. (20) Adams, D. M.; et al. J. Phys. Chem. B 2003, 107, 7716. (21) Brandow, S. L.; Chen, M.-S.; Aggarwal, R.; Dulcey, C. S.; Calvert, J. M.; Dressick, W. J. Langmuir 1999, 15, 5429. (22) Geyer, W.; Stadler, V.; Eck, W.; Zharnikov, M.; Go¨lzha¨user, A.; Grunze, M. Appl. Phys. Lett. 1999, 75, 2401. (23) Dressick, W. J.; Chen, M.-S.; Brandow, S. L. J. Am. Chem. Soc. 2000, 122, 98. (24) Eck, W.; Stadler, V.; Geyer, W.; Zharnikov, M.; Go¨lzha¨user, A.; Grunze, M. Adv. Mater. 2000, 12, 805. (25) Go¨lzha¨user, A.; Eck, W.; Geyer, W.; Stadler, V.; Weimann, T.; Hinze, P.; Grunze, M. Adv. Mater. 2001, 13, 806. (26) Frey, S.; Rong, H.-T.; Heister, K.; Yang, Y.-J.; Buck, M.; Zharnikov, M. Langmuir 2002, 18, 3142. (27) Zharnikov, M.; Grunze, M. J. Vac. Sci. Technol., B 2002, 20, 1793. (28) Tai, Y.; Shaporenko, A.; Rong, H.-T.; Buck, M.; Eck, W.; Grunze, M.; Zharnikov, M. J. Phys. Chem. B, submitted for publication.

10.1021/la040047o CCC: $27.50 © 2004 American Chemical Society Published on Web 07/21/2004

Irradiation-Induced Cross-Linking in SAMs

(CH2)nSH and 4,4′-terphenyl-substituted ATs C6H5(C6H4)2(CH2)nSH at an odd n, which exhibit a slightly modified (x3 × x3)R30° arrangement on Au(111) with a packing density close to that of AT SAMs.16,30-34 The TPDMT monolayers were characterized by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, which is a powerful technique to study the chemical identity and orientational order in ultrathin organic films.35 In the following section, our experimental procedure will be described. The results are presented in section 3 followed by a discussion and a summary in sections 4 and 5, respectively. 2. Experimental Section The synthesis of the TPDMT compounds is described elsewhere.28 The gold substrates were prepared by thermal evaporation of 100-nm gold (99.99% purity) onto polished single-crystal silicon (100) wafers (Silicon Sense), which had been precoated with a 5-nm titanium adhesion layer. Such evaporated films are standard substrates for thiol-derived SAMs. They are polycrystalline, with a grain size of 20-50 nm as observed by atomic force microscopy. The grains predominantly exhibit a (111) orientation, which is corroborated by the angular distributions of the Au(4f) photoelectrons36 and by the characteristic binding energy shift of the Au(4f) emission from the topmost layers.37 The SAMs were formed by immersion of freshly prepared substrates into a 1 mM TPDMT solution in tetrahydrofuran (stabilized with 0.1% hydroquinone) at room temperature for 24 h. After immersion, the samples were carefully rinsed with chloroform and blown dry with argon. No evidence for impurities or oxidative degradation products was found by X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy. The fabricated films were irradiated by electrons with a kinetic energy of 10 eV, representative for secondary electrons. The obtained results are, therefore, also relevant for the case of X-ray irradiation, because the damage produced in SAMs by X-ray photons can be essentially related to the low-energy secondary electrons arising through the inelastic scattering of the primary electrons created within the photoemission process.27,38-40 The irradiation was performed with fixed doses of 20 000 µC/cm2 and 40 000 µC/cm2. The dose was calculated by multiplication of the exposure time by the current density (≈2.5 µA/cm2), which was calibrated by a Faraday cup. The electron gun was mounted at a distance of ≈15 cm from the sample to ensure uniform illumination. The pristine and irradiated films were characterized by NEXAFS spectroscopy. The measurements were performed at the HE-SGM beamline of the synchrotron storage ring BESSY II in Berlin, Germany. The experiments were carried out at room temperature and under ultrahigh vacuum conditions at a base pressure better than 1.5 × 10-9 Torr. The spectra were acquired at the C K-edge in the partial electron yield mode with a retarding (29) Azzam, W.; Wehner, B. I.; Fisher, R. A.; Terfort, A.; Wo¨ll, Ch. Langmuir 2002, 18, 7766. (30) Zharnikov, M.; Frey, S.; Rong, H.; Yang, Y. J.; Heister, K.; Buck, M.; Grunze, M. Phys. Chem. Chem. Phys. 2000, 2, 3359. (31) Rong, H. T.; Frey, S.; Yang, Y. J.; Zharnikov, M.; Buck, M.; Wu¨hn, M.; Wo¨ll, Ch.; Helmchen, G. Langmuir 2001, 17, 1582. (32) Azzam, W.; Cyganik, P.; Witte, G.; Buck, M.; Wo¨ll, Ch. Langmuir 2003, 19, 8262. (33) Shaporenko, A.; Brunnbauer, M.; Terfort, A.; Grunze, M.; Zharnikov M. J. Phys. Chem. B, in print. (34) Ishida, T.; Mizutani, W.; Akiba, U.; Umemura, K.; Inoue, A.; Choi, N.; Fujihira, M.; Tokumoto, H. J. Phys. Chem. B 1999, 103, 1686. (35) Sto¨hr, J. NEXAFS Spectroscopy; Springer Series in Surface Science 25; Springer-Verlag: Berlin, 1992. (36) Ko¨hn, F. Diploma Thesis, Universita¨t Heidelberg, Heidelberg, Germany, 1998. (37) Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O. J. Phys. Chem. B 2001, 105, 4058. (38) Laibinis, P. E.; Graham, R. L.; Biebuyck, H. A.; Whitesides, G. M. Science 1991, 254, 981. (39) Graham, R. L.; Bain, C. D.; Biebuyck, H. A.; Laibinis, P. E.; Whitesides, G. M. J. Phys. Chem. 1993, 97, 9456. (40) Wirde, M.; Gelius, U.; Dunbar, T.; Allara, D. L. Nucl. Instrum. Methods Phys. Res., Sect. B 1997, 131, 245.

Langmuir, Vol. 20, No. 17, 2004 7167 voltage of -150 V. Linear polarized synchrotron light with a polarization factor of ≈82% was used. The energy resolution was ≈0.40 eV. The incidence angle of the light was varied from 90 (E vector in surface plane) to 20° (E vector near surface normal) in steps of 10-20° to monitor the orientational order in the TPDMT films (see the following).35 To vary the X-ray incidence angle, the sample holder was rotated about the manipulator axis, whereas the PEY detector stayed in a fixed position. The raw NEXAFS spectra were normalized to the incident photon flux by division through a spectrum of a clean, freshly sputtered gold sample. The photon energy (PE) scale was referenced to the pronounced π1* resonance of highly oriented pyrolytic graphite at 285.38 eV.41 In addition to the NEXAFS measurements, the electroninduced modification of the TPDMT films was characterized by XPS. In these experiments, we were mostly interested in monitoring the irradiation stability of the SAM-ambient and SAM-substrate interfaces.

3. Results In the NEXAFS experiment, core level electrons [e.g., C(1s) for a C K-edge spectrum] are excited into nonoccupied molecular orbitals, which are characteristic for a specific bond, a functional group, or a molecule. The PE positions of the respective absorption resonances give then a clear signature of these entities. In addition, information on molecular orientation can be derived from the experimental data, because the cross section of the resonant photoexcitation process depends on the orientation of the electric field vector of the linearly polarized synchrotron light with respect to the molecular orbital of interest (socalled linear dichroism in X-ray absorption). For this purpose, the intensity of the respective resonance I should be monitored as a function of the X-ray incidence angle θ and the resulting dependence evaluated according to theoretical expression (for a vector-type orbital)35

{ [

1 1 I(R, θ) ) A P 1 + (3 cos2 θ - 1)(3 cos2 R - 1) + 3 2 1 (1 - P) sin2 R (1) 2

]

}

where A is a constant, P is a polarization factor of the X-rays, and R is the average tilt angle of the molecular orbital. In some cases, it is useful to avoid the effects of molecular orientation, for which purpose the measurements are performed at the so-called magic angle of the light incidence (≈55°). For this particular orientation, the angle-dependent term in eq 1 vanishes and the measured spectrum is independent of the molecular orientation. C K-edge NEXAFS spectra of the pristine and strongly irradiated (40 000 µC/cm2) TPDMT SAMs acquired at different X-ray incidence angles are presented in Figure 1. The spectra are dominated by the intense π1* resonance of the phenyl rings at ≈285.0 eV, which is accompanied by the second π resonance (π2*) of these entities at ≈288.8 eV, the R*/C-S* resonance at ≈287.8 eV, and several broad σ* resonances at higher photon energies (the assignment has been performed in accordance with refs 13, 16, and 42). The spectra of the pristine TPDMT film in the left panel of Figure 1 exhibit a pronounced linear dichroism, that is, a dependence of the absorption resonance intensity on the incidence angle of X-rays. This suggests high orientational order in these SAMs. The dichroism can be highlighted by so-called difference spectra, that is, the difference between the spectra acquired at a fixed X-ray (41) Batson, P. E. Phys. Rev. B 1993, 48, 2608. (42) Zharnikov, M.; Grunze, M. J. Phys.: Condens. Matter 2001, 13, 11333.

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Figure 1. Carbon K-edge NEXAFS spectra of pristine and irradiated (40 000 µC/cm2) TPDMT SAMs acquired at different X-ray incidence angles.

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resonance intensity at ≈285.0 eV, because the position of the π* resonance of CdC bond matches the position of the π1* resonance of the phenyl ring. However, the possible appearance of the π*(CdC) resonance did not compensate the intensity decrease of the π1* feature, and an overall intensity decrease at 285.0 eV occurs during electron irradiation, as seen in Figure 1. Note that the intensity comparison should be performed for the spectra acquired at the magic X-ray incidence angle, because these spectra are not affected by the orientational effects.35 All other spectra can be affected by these effects because of a partial loss of the orientational order. Note also that the irradiation-induced cross-links are not expected to have any preferred orientation. Even though there is a partial loss of the orientational order in the irradiated TPDMT films, there should be a gradual change (either an increase or a decrease) in the amplitude of the π1* peak with increasing X-ray incidence angle. However, this is not the case, at least for the 90° spectrum, as seen in the right panel of Figure 1. This deviation from the normal behavior is especially pronounced if one considers the respective difference spectra in the right panel of Figure 2; the intensity of the π1* difference peak in the 90-55° spectrum is even lower than that for the 70-55° spectrum. Interestingly, this effect develops gradually during the irradiation. As seen in the middle panel of Figure 2, the π1* difference peak in the 90-55° spectrum of the TPDMT film exposed to a dose of 20 000 µC/Cm2 has an intensity close to that in the respective 70-55° spectrum. 4. Discussion

Figure 2. Differences between the C K-edge NEXAFS spectra of pristine and irradiated (20 000 and 40 000 µC/cm2) TPDMT SAMs acquired at different X-ray incidence angles. The nonzero value of the difference spectra of the irradiated films in the post-edge region (above 300 eV) is presumably related to inhomogeneity effects.

incidence angle and another angle taken as a reference. Such difference spectra for the pristine TPDMT film are presented in the left panel of Figure 2, with the spectrum acquired at the magic angle of X-ray incidence (≈55°), taken as a reference.35 In accordance with the spectra in Figure 1, the difference peak of the π1* resonance is especially pronounced in the difference spectra, gradually varying its amplitude from negative values for the 2055° and 30-55° spectra to positive values for the 70-55° and 90-55° spectra. Such a behavior assumes that the π1* orbital is strongly inclined with respect to the surface normal.35 This means that the terphenyl moieties in the TPDMT films have an upright orientation, because the transition dipole moment of the π1* orbital is perpendicular to the ring plane. The intensity and, especially, linear dichroism of the absorption resonances decreased significantly after prolonged (20 000 and 40 000 µC/cm2) electron irradiation of the TPDMT films, as shown in the right panels of Figures 1 and 2, where the original and difference spectra of the irradiated films are presented. This decrease is associated with a cleavage of C-H bonds followed by cross-linking of the terphenyl backbones and a partial loss of the orientational order.22,27 Among the appearing intermolecular links, only CdC bonds may contribute to the

Because a gradual change in the intensity of an absorption resonance with varying X-ray incidence angle is a common property of NEXAFS spectroscopy (as, e.g., follows from eq 1), the observed abnormal behavior should be related to some specific parameters of our samples or the experimental setup. The analysis of these two factors revealed that the deciding factor is the geometry of the PEY detector, which was placed on the side of the sample holder. Whereas the takeoff angle of the primary Auger and secondary electrons is about 20-40° for X-ray incidence angles of 20, 30, and 55°, it is 50 and 70° for X-ray incidence angles of 70 and 90°, respectively. Considering that an increase in the takeoff angle R is associated with an increasing surface sensitivity according to the 1/(cos R) dependence of the effective sampling depth, one can conclude that the spectra acquired at 90° should contain a higher contribution from the topmost part of the TPDMT SAMs as compared to the spectra acquired at the lower incidence angles. Assuming that the irradiated TPDMT films are inhomogeneous by going from the substrate-headgroup to film-ambient interface, one obtains a consistent explanation for the observed deviation of the 90° spectra from the common behavior. This inhomogeneity, contributing mostly to the 90° spectra, just interferes with the angular dependence of the π1* resonance intensity. Obviously, the intensity of the π1* resonance at the SAM-ambient interface and close to it is smaller than that inside the TPDMT film or close to the headgroup-substrate interface. This means that the irradiation-induced scission of the C-H bonds and the subsequent cross-linking have an inhomogeneous distribution within the TPDMT film; the topmost part of the film is affected to a much higher extent by these processes as compared to the regions close to the headgroupsubstrate interface. A schematic drawing of this distribution is given in Figure 3. Because the excitation of the individual bonds

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Figure 3. Schematic drawing of irradiation-induced inhomogeneous cross-linking in the TPDMT SAMs.

should occur uniformly over the film, such a uniform distribution can only be related to a preferable quenching of the electronically excited C-H* states close to the filmsubstrate interface. In fact, there is a mechanism for such a process, namely, a dipole-dipole coupling of the electronically excited state with its image at the metal surface.43 Such an interaction can result in quenching of the excited state, with the probability for this process decreasing with increasing separation from the metal surface. The quenching is believed to be responsible for the inhomogeneous distribution of irradiation-induced C-H bond scission throughout the alkanethiolate film and, in particular, for a higher extent of the dehydrogenation in the region of the terminal methyl groups as compared to the methylene moieties along the aliphatic backbone.27,43,44 Along with the inhomogeneous distribution of the C-H bond scissions, an extremely high resistance of the TPDMT film to ionizing radiation has to be mentioned. Even after irradiation with a dose of 40 000 µC/cm2, there is still orientational order within the TPDMT film, as exhibited by a pronounced dichroism of the respective NEXAFS spectra in the right panel of Figure 1. This orientational order can be quantitatively evaluated using the average tilt angle of the TPDMT chains as a fingerprint parameter. The angle can be derived by numerical processing of the NEXAFS spectra following eq 1, which should be slightly modified in the case of the aromatic SAMs.17,18 Such a procedure has been applied to the entire sets of the NEXAFS spectra. For the evaluation, the π1* resonance has been selected, as the most intense one in the spectra. To avoid normalization problems, not the absolute intensities but the intensity ratios I(θ)/I(20°) were analyzed, where I(θ) is the intensity of the π1* resonance at an X-ray incidence angle θ. The angular dependencies of the π1* resonance intensity ratio I(θ)/I(20°) for the pristine (squares) and irradiated (circles and triangles) TPDMT SAMs along with the best theoretical fits (solid line) are shown in Figure 4. The values of the derived average tilt angle of the terphenyl moieties in the TPDMT films are given at the respective fit curves. There is a good correlation between the experimental data and theoretical fits with the exception of the θ ) 90° point for the irradiated films (especially for the case of 40 000 µC/cm2), which is understandable in view of the discussed “distortion” of the respective spectra. As to the average tilt angle of the terphenyl moieties, there is an increase in this parameter by only ≈5 and ≈12° for the irradiation with doses of 20 000 and 40 000 µC/cm2, respectively. For comparison, a dose of several thousands µC/cm2 is sufficient for full disordering of AT SAMs27,44 and a dose of 10 000 µC/cm2 results (43) Olsen, C.; Rowntree, P. A. J. Chem. Phys. 1998, 108, 3750. (44) Zharnikov, M.; Geyer, W.; Go¨lzha¨user, A.; Frey, S.; Grunze, M. Phys. Chem. Chem. Phys. 1999, 1, 3163.

Figure 4. Angular dependencies of the π1* resonance intensity ratio I(θ)/I(20°) for pristine (squares) and irradiated (circles for 20 000 µC/cm2 and triangles for 40 000 µC/cm2) TPDMT SAMs along with the best theoretical fits (solid lines). The values of the derived average tilt angle of the terphenyl moieties in the TPDMT films are given at the respective curve fits.

in an increase in the in the average tilt angle of the biphenyl moieties in a SAM of 4,4′-biphenylthiol on Au by ≈10°.22 It can be assumed that the reasons for the extreme stability of the TPDMT films are the higher rigidity of the terphenyl backbones and a high packing density of the terphenyl moieties due to the suitable methylene linker. The cross-linking of the rigid terphenyl backbones prevents a movement of the SAM constituents, which hinders desorption of large fragments or a cleavage of the headgroup-substrate bond.22 A high packing density of the terphenyl moieties leads to a large delocalization and a strong relaxation of the initial electronic excitations, which may result in their effective quenching.26 According to the XPS data, there was only small damage to the SAM-substrate interface (about 5% for 20 000 µC/ cm2) and a small thickness reduction (also about 5% for 20 000 µC/cm2) during the electron irradiation. Most of the terminal thiol groups at the film-ambient interface survived the irradiation as well: there were about 75 and 48% (with respect to the initial amount) of the intact thiol groups after irradiation with doses of 20 000 and 40 000 µC/cm2, respectively. Thus, the SAM-ambient interface of the TPDMT films does not lose its chemical identity even after a strong irradiation, accompanied by an extended cross-linking of the SAM constituents. 5. Conclusion Pristine and strongly irradiated TPDMT films on gold have been characterized by NEXAFS spectroscopy using a PEY acquisition mode. For the irradiated films, a pronounced deviation of the linear dichroism of the absorption resonances from the standard behavior was observed. This deviation was ascribed to an inhomogeneous distribution of irradiation-induced C-H bond scission and the subsequent cross-linking of the terphenyl backbones over the film: its topmost part is affected to a higher extent by these processes as compared to the regions close to the headgroup-substrate interface. As a result of the difference in the effective takeoff angle of the Auger and secondary electrons at different orientations of the sample holder, different probing depths were effectively achieved at the different X-ray incidence angles so that the standard dependence of the absorption resonance intensity was distorted by the inhomogeneous distribution of the respective molecular orbitals.

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The inhomogeneous distribution of the bond scission and cross-linking events was explained by a quenching of the electronically excited C-H* states through dipoledipole coupling with the states’ image at the metal surface. Because the probability for this process is reduced with increasing separation from the metal surface, bond rupture and cross-linking preferentially occur close to the SAMambient interface. The TPDMT films were found to be extremely stable with respect to the ionizing radiation. This behavior was attributed to the high rigidity of the terphenyl backbone and the high orientational order and packing density in the pristine monolayers. The latter parameters are associated with a large delocalization and a strong relaxation of the initial electronic excitations, which may result in their effective quenching. The obtained results underline the importance of the relaxation processes for the response of monomolecular

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films to ionizing radiation. A primary excitation of a functional group does not necessarily cause bond cleavage. There is always a competition between this scenario and quenching of the excitation, which can occur in different ways. Adjusting the molecular architecture of such films, one can effectively tune the extent of the quenching and fabricate systems with desired properties. Acknowledgment. We thank Ch. Wo¨ll (Universita¨t Bochum) for providing us with experimental equipment for the NEXAFS measurements and the BESSY II staff for the assistance during the synchrotron-based experiments. This work has been supported by the Deutsche Forschungsgemeinschaft (JA 883/4-1 and JA 883/4-2), German BMBF (GRE1HD), and the Fonds der Chemischen Industrie. LA040047O