Response of Biphenyl-Substituted Alkanethiol Self ... - ACS Publications

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Langmuir 2002, 18, 3142-3150

Response of Biphenyl-Substituted Alkanethiol Self-Assembled Monolayers to Electron Irradiation: Damage Suppression and Odd-Even Effects S. Frey,† H.-T. Rong,† K. Heister,† Y.-J. Yang,† M. Buck,†,‡ and M. Zharnikov*,† Angewandte Physikalische Chemie, Universita¨ t Heidelberg, Im Neuenheimer Feld 253, D-69120 Heidelberg, Germany, and School of Chemistry, St. Andrews University, North Haugh, St. Andrews, KY16 9ST, United Kingdom Received August 13, 2001. In Final Form: January 3, 2002 The low-energy electron-induced damage in self-assembled monolayers (SAMs) formed from ω-(4′methylbiphenyl-4-yl)alkanethiols CH3(C6H4)2(CH2)nSH (BPn, n ) 0, 1, 4, 5, and 12) on gold substrates was studied. The pristine and heavily (8000 µC/cm2) irradiated films were characterized in detail by X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, infrared reflection absorption spectroscopy, and advancing contact angle measurements. In contrast to SAMs of conventional alkanethiols but similar to pure aromatic thiol-derived systems, only minor damage is observed for the aliphatic-aromatic BPn films. In particular, the orientational order and anchoring to the substrate are retained upon the irradiation. At the same time, C-H bond scissions in the aromatic part occur, leading to a cross-linking between the neighboring biphenyl moieties. Whereas the general behavior of the BPn SAMs with respect to electron irradiation is qualitatively similar, the extent of the irradiation-induced changes depends on the packing of these systems. The densely packed BP1 and BP5 SAMs are much more stable with respect to electron bombardment than the less densely packed BP4 films. The relation between the packing density and the extent of the irradiation-induced changes seems to be a general phenomenon in monomolecular films, which provides a tool to tailor the reaction of these systems toward ionizing radiation for lithographic applications.

1. Introduction The increasing miniaturization of integrated devices and new areas of biology and medicine demand the development of novel methods for the fabrication of microand nanostructures. One of the proposed methods for extending lithography down to nanometer scales applies electron-beam patterning of a new kind of lithographic resistsself-assembled monolayers (SAMs).1,2 These are well-ordered and densely packed two-dimensional ensembles of long-chain molecules, chemisorbed onto the substrate by a suitable headgroup. Due to the extremely small thickness of the SAM resist and the molecular size of its structural elements, patterning on a 0.5-1 nm scale becomes principally possible assuming the respective focusing of the electron beam. Another advantage of using SAMs as resists is that the electron-beam-induced changes can be controlled via the molecular structure. Whereas for aliphatic SAMs electron irradiation causes damage and disorder (a positive resist),3-10 a quasi-polymerization with only partial disordering occurs in aromatic SAMs (a * To whom correspondence should be addressed. Tel.: +49-622154 4921. Fax: +49-6221-54 6199. E-mail: Michael.Zharnikov@ urz.uni-heidelberg.de. † Universita ¨ t Heidelberg. ‡ St Andrews University. (1) Ulman, A. An Introduction to Ultrathin Organic Films: Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. Ulman, A. Chem. Rev. 1996, 96, 1533. (2) Ulman, A., Ed. Thin films: self-assembled monolayers of thiols; Academic Press: San Diego, CA, 1998. (3) Laibinis, P. E.; Graham, R. L.; Biebuyck, H. A.; Whitesides, G. M. Science 1991, 254, 981. (4) Rowntree, P.; Dugal, P.-C.; Hunting, D.; Sanche, L. J. Phys. Chem. 1996, 100, 4546. (5) Ja¨ger, B.; Schu¨rmann, H.; Mu¨ller, H. U.; Himmel, H.-J.; Neumann, M.; Grunze, M.; Wo¨ll, Ch. Z. Phys. Chem. 1997, 202, 263. (6) Olsen, C.; Rowntree, P. A. J. Chem. Phys. 1998, 108, 3750. (7) Mu¨ller, H. U.; Zharnikov, M.; Volkel, B.; Schertel, A.; Harder, P.; Grunze, M. J. Phys. Chem. B 1998, 102, 7949.

negative resist).11-13 In addition, the resistance of the aromatic SAMs toward electron irradiation allows an attachment and subsequent selective, chemical modification of terminal functional groups,14 which enables the fabrication of chemical lithographic patterns and templates.15 The fabricated lithographic patterns can then be used for laterally selective attachment of other molecules or biological objects, such as proteins or cells. The further development of the SAM-based lithography and, in particular, chemical lithography depends on the progress in the technology of electron beam patterning and on the molecular engineering of the SAM resist constituents. Such an engineering includes several aspects. First, different headgroups can be introduced, which makes it possible to extent SAM-based lithography to other substrates than the noble metal substrates used in the case of thiol-derived SAMs (see, e.g., refs 16-20 for the aliphatic SAMs). Second, different functional groups can (8) Zharnikov, M.; Frey, S.; Go¨lzhaüser, A.; Geyer, W.; Grunze, M. Phys. Chem. Chem. Phys. 1999, 1, 3163. (9) Heister, K.; Frey, S.; Go¨lzhaüser, A.; Ulman, A.; Zharnikov, M. J. Phys. Chem. B 1999, 103, 11098. (10) Zharnikov, M.; Frey, S.; Heister, K.; Grunze, M. Langmuir 2000, 16, 2697. (11) Geyer, W.; Stadler, V.; Eck, W.; Zharnikov, M.; Go¨lzhaüser, A.; Grunze, M. Appl. Phys. Lett. 1999, 75, 2401. (12) Go¨lzhaüser, A.; Geyer, W.; Stadler, V.; Eck, W.; Grunze, M.; Edinger, K.; Weimann, Th.; Hinze, P. J. Vac. Sci. Technol., B 2000, 18, 3414. (13) Zeysing, B.; Eickhoff, T.; Drube, W.; Terfort, A. Submitted for publication in Langmuir. (14) Eck, W.; Stadler, V.; Geyer, W.; Zharnikov, M.; Go¨lzhaüser, A.; Grunze, M. Adv. Mater. 2000, 12, 805. (15) Go¨lzhaüser, A.; Eck, W.; Geyer, W.; Stadler, V.; Weimann, T.; Hinze, P.; Grunze, M. Adv. Mater. 2001, 13, 806. (16) Lercel, M. J.; Redinbo, G. F.; Pardo, F. D.; Rooks, M.; Tiberio, R. C.; Simpson, P.; Craighead, H. G.; Sheen, C. W.; Parikh, A. N.; Allara, D. L. J. Vac. Sci. Technol., B 1994, 12, 3663. (17) Baer, D. R.; Engelhard, M. H.; Schulte, D. W.; Guenther, D. E.; Wang, L.-Q.; Rieke, P. C. J. Vac. Sci. Technol., A 1994, 12, 2478.

10.1021/la011288o CCC: $22.00 © 2002 American Chemical Society Published on Web 03/09/2002

Electron Irradiation of Substituted Alkanethiol SAMs

be used for the electron-beam-induced chemical transformations, offering new possibilities for the chemical patterning and molecular attachment (see, e.g., refs 20 and 21). Third, the reaction of a SAM toward electron irradiation can be affected by the variation of the long chain spacer, both through the introduction of electronbeam-sensitive molecular groups9 and through the change of the orientation and the packing density of the SAM constituents,10,22 which can be influenced by the architecture of the molecular chain. A clear example of such an influence was given recently by SAMs of 4,4′-biphenyl-substituted alkanethiols CH3(C6H4)2-(CH2)nSH (BPn, n ) 1-6) on gold and silver substrates.23-25 It was demonstrated that the molecular packing density and orientation of the biphenyl (BP) moieties in these films are noticeably affected by the number of methylene groups in the aliphatic part: A denser molecular packing and a less tilted orientation of the BP moieties occur for an odd number of the CH2 units in the BPn SAMs on Au or for an even number of these entities for BPn on Ag, whereas a reduced density and a larger tilt of the BP moieties are observed for an even number of the CH2 units in the BPn films on Au or for an odd number of these entities for BPn on Ag. The reason for this “odd-even” behavior is a preferential orientation of the aliphatic part imposed by the hybridization of the sulfur headgroup, which is different on Au and Ag (sp3 and sp, respectively). At a given C-C-C bond angle of ∼112° and surface-S-C angles of ∼104° and ∼180°, for the sp3 and sp hybridization, respectively,1,26,27 the spatial orientation of the last CH2-CH2 segment of the alkyl chain and, therefore, the orientation of the adjacent BP moiety change reproducibly by going from odd to even numbers of CH2 units. In particular, the tilt angle of the BP moieties in the BPn SAMs on Au is ∼17° and ∼30° for an odd and even number of CH2 units, respectively,23 which also gives rise to an odd-even variation of the molecular packing density by at least 10%.24 Considering the fundamental differences in the response of aliphatic and aromatic moieties to electron irradiation and the above-mentioned variation of the packing density and molecular orientation of the BP moieties in BPn SAMs, it is of interest to study the electron resist behavior of these SAMs which unify aromatic and aliphatic moieties. In the present work we investigated the electron-induced modification of BPn films with n ) 1, 4, 5, and 12 and SAMs formed from the closely related 4′-methyl-1,1′biphenyl-4-thiol CH3-(C6H4)2-SH (BP0) on gold substrates. The main goal of this study was to explore to what extent the reaction of these systems toward electron beam irradiation is affected by their structure and packing density. Note, that only BP1, BP4, and BP5 films reveal the above-mentioned odd-even structure and packing (18) Seshardi, K.; Froyd, K.; Parikh, A. N.; Allara, D. L.; Lercel, M. J.; Craighead, H. G. J. Phys. Chem. 1996, 100, 15900. (19) Hild, R.; David, C.; Mu¨ller, H. U.; Vo¨lkel, B.; Kayser, D. R.; Grunze, M. Langmuir 1998, 14, 342. (20) Maoz, R.; Cohen, S. R.; Sagiv, J. J. Adv. Mater. 1999, 11, 55. (21) Rieke, P. C.; Baer, D. R.; Fryxell, G. E.; Engelhard, M. H.; Porter, M. S. J. Vac. Sci. Technol., A 1993, 11, 2292. (22) Frey, S.; Heister, K.; Zharnikov, M.; Grunze, M. Phys. Chem. Chem. Phys. 2000, 2, 1979; 2000, 2, 3721 (erratum). (23) Zharnikov, M.; Frey, S.; Rong, H.; Yang, Y.-J.; Heister, K.; Buck, M.; Grunze, M. Phys. Chem. Chem. Phys. 2000, 2, 3359. (24) Rong, H. T.; Frey, S.; Yang, Y.-J.; Zharnikov, M.; Buck, M.; Wu¨hn, M.; Wo¨ll, Ch.; Helmchen, G. Langmuir 2001, 17, 1582. (25) Heister, K.; Rong, H.; Buck, M.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O. J. Phys. Chem. B 2001, 105, 6888. (26) Harris, A. L.; Rothberg, L.; Dubois, L. H.; Levinos, N. J.; Dhar, L. Phys. Rev. Lett. 1990, 64, 2086. (27) Lampert, A. PhD Thesis, Universita¨t Heidelberg, Heidelberg, Germany, 1997.

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density variation and are suited for this purpose. The BP0 and BP12 systems were selected for comparison: BP0 as a pure aromatic SAM28 and BP12 as a mixed aliphaticaromatic film with equal portions of the aromatic and aliphatic parts. The latter system is investigated for the first time, except for a recent electrochemical study by Felgenhauer et al.29 As for BP0, the electron beam modification of pure aromatic SAMs on Au has already been examined,11-14 but it was important to perform a direct comparison with the BPn (n > 1) SAMs within the same experimental procedure. In the following section, the experimental approach and techniques are briefly described. Thereafter, the results are presented in section 3, a detailed analysis of the data is given in section 4, and a summary follows in section 5. 2. Experimental Section Substrates were prepared by evaporation of 200 nm of gold on titanium-primed (5 nm) polished single crystal Si(100) wafers. Such films are commonly used as substrates for thiol-derived SAMs. They predominantly exhibit a (111) orientation, which is, in particular, supported by the observation of the corresponding forward-scattering maxima in the angular distributions of the Au 4f photoelectrons30 and by the characteristic binding energy (BE) shift of the Au 4f surface component.31 The synthesis of the BPn (n > 1) and BP0 substances is described elsewhere.24,32 For this study we have taken some selected members of the BPn series with n ) 1, 4, 5, and 12 because of the time restrictions for in situ characterization, which was performed at a synchrotron. The SAMs were formed by immersion of freshly prepared substrates into a 1 mM BPn or BP0 solution in absolute ethanol at room temperature for 24 h. After immersion, the samples were carefully rinsed with chloroform and pure ethanol and blown dry with argon. No evidence for impurities or oxidative degradation products was found. 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.3,5,8,33,34 The irradiation of all samples was performed with a dose of 8000 µC/cm2. The dose was calculated by multiplication of the exposure time by the current density (∼2.5 µA/cm2). The electron gun was mounted at a distance of ∼15 cm from the sample to ensure uniform illumination. The electron beam damage was monitored in situ by near edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS) in the same ultrahigh vacuum chamber where the electron irradiation occurred. The time for the NEXAFS/XPS characterization was selected as a compromise between the spectra quality and potential damage induced by X-rays during the spectra acquisition. The criterion was to keep the X-ray damage negligible as compared to the electron-induced damage, which was specially proven by independent measurements on irradiation-sensitive alkanethiol (AT) SAMs. Both the irradiation and characterization were performed at room temperature at the base pressure better than 2 × 10-9 (28) The definition “pure aromatic” is related to the pure aromatic spacer of the BP0 molecule. We do not consider the terminal CH3 group as a part of this spacer. (29) Felgenhauer, T.; Yan, C.; Geyer, W.; Rong, H.-T.; Go¨lzhaüser, A.; Buck, M. Appl. Phys. Lett. 2001, 79, 3323. (30) Ko¨hn, F. Diploma Thesis, Universita¨t Heidelberg, Heidelberg, Germany, 1998. (31) Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O. J. Phys. Chem. B 2001, 105, 4058. (32) Rong, H.-T. PhD Thesis, Universita¨t Heidelberg, Heidelberg, Germany, 2001. (33) Graham, R. L.; Bain, C. D.; Biebuyck, H. A.; Laibinis, P. E.; Whitesides, G. M. J. Phys. Chem. 1993, 97, 9456. (34) Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O.; Ulman, A. Langmuir 2001, 17, 8.

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mbar. The experimental chamber was attached to the HE-TGM 2 beamline35 at the German synchrotron radiation facility BESSY I in Berlin. The NEXAFS spectra were acquired at the C 1s absorption edge in the partial yield mode with a retarding voltage of -150 V. The energy resolution was ∼0.65 eV. The incidence angle of the linearly polarized light was varied from 90° (the E-vector in the surface plane) to 20° (the E-vector near the surface normal) in steps of 10°-20° to monitor the orientational order within the BPn films. This approach is based on so-called linear dichroism in X-ray absorption, i.e., the strong dependence of the cross section of the resonant photoexcitation process on the orientation of the electric field vector of the linearly polarized light with respect to the molecular orbital of interest.36 The raw NEXAFS spectra were corrected for the energy dependence of the incident photon flux by division through a spectrum of a clean, freshly sputtered gold sample. For absolute energy calibration, the simultaneously measured photoabsorption signal of a carbon-covered gold grid with a characteristic resonance at ∼285 eV was used. This resonance was separately calibrated to the significant π1* resonance of a graphite sample (HOPG, highly orientented pyrolytic graphite) at 285.38 eV.37 The XPS measurements were performed with a VG CLAM 2 spectrometer using a Mg KR X-ray source (260 W) placed at a distance of several centimeters from the sample. The photoelectron takeoff angle was about 90° and the energy resolution was ∼0.9 eV. The energy scale was referenced to the Au 4f7/2 peak at 84.0 eV.38 For both the pristine and irradiated samples a wide scan spectrum and the C 1s, O 1s, S 2p, and Au 4f narrow scan spectra were recorded. The narrow scan spectra were normalized to the total electron yield to correct for small differences in sample positions and X-ray source intensities39 and fitted by using a Shirley-type background40 and symmetric Voigt functions41 with a Gauss/Lorenz ratio of 4:1. To fit the S 2p3/2,1/2 doublet, we used a pair of such peaks with the same full width at half-maximum (fwhm), the standard38 spin-orbit splitting of ∼1.2 eV (verified by fit), and the branching ratio of 2 (S 2p3/2/S 2p1/2). In addition to the in situ experiments, ex situ characterization of the pristine and electron beam modified BPn films was performed by infrared reflection absorption spectroscopy (IRRAS) and advancing contact angle measurements. IRRAS spectra were acquired with a dry-air-purged Bio-Rad FTIR spectrometer, model FTS 175C, equipped with a liquid nitrogen cooled MCT detector. All spectra were taken with p-polarized light at an incident angle of 80° with respect to the surface normal. The measurements were carried out in the range of 650-4000 cm-1 at a resolution of 2 cm-1 and with at least 1000 scans per spectrum. The spectra are reported in absorbance units A ) -log R/Ro, where R and Ro are the reflectivities of the sample and reference, respectively. As a reference, we used SAMs of perdeuterated hexadecanethiol on Au, whose C-D absorption bands at 2050-2200 cm-1 do not interfere with the spectral regions of interest.42 Advancing contact angles were measured with a Kru¨ss goniometer, model G1. Millipore water and n-hexadecane were used as test liquids. The experiments were performed under ambient conditions with the needle tip in contact with the drop. At least three measurements at different locations on each sample were performed. The averaged values are reported. Deviations from the average were less than (1°. The combination of the applied experimental techniques was chosen because of the diversity and complexity of the possible (35) Bernstorff, S.; Braun, W.; Mast, M.; Peatman, W.; Schro¨der, T. Rev. Sci. Instrum. 1989, 60, 2097. (36) Sto¨hr, J. NEXAFS Spectroscopy; Springer Series in Surface Science 25; Springer-Verlag: Berlin-Heidelberg, 1992. (37) Batson, P. E. Phys. Rev. B 1993, 48, 2608. (38) Moulder, J. F.; Stickle, W. E.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Chastian, J., Ed.; PerkinElmer Corporation: Eden Prairie, MN, 1992. (39) Frey, S.; Heister, K.; Zharnikov, M.; Grunze, M.; Tamada, K.; Colorado, R., Jr.; Graupe, M.; Shmakova, O. E.; Lee, T. R. Isr. J. Chem. 2000, 40, 81. (40) Shirley, D. A. Phys. Rev. B 1972, 5, 4709. (41) Wertheim, G. K.; Butler, M. A.; West, K. W.; Buchanan, D. N. E. Rev. Sci. Instrum. 1974, 45, 1369. (42) Harder, P.; Grunze, M.; Dahint, R.; Whitesides, G. M.; Laibinis, P. E. J. Phys. Chem. 1998, 102, 426.

Frey et al.

Figure 1. The C 1s XPS spectra of pristine (a) and irradiated (b) BPn SAMs. The observed emission structure is fitted by a single Voigt peak; the applied background is shown. The intensities of the C 1s emission for the irradiated films with respect to those for the corresponding pristine films are indicated at the respective spectra in (b). The accuracy of the relative intensities is (3%. damage paths. A single method (e.g., XPS or IRRAS) does not provide sufficient information on different irradiation-induced processes.7,43 Therefore, the data of different techniques should be combined.

3. Results 3.1. XPS Measurements. The C 1s and S 2p XPS spectra of the pristine and irradiated BPn films are shown in Figures 1 and 2, respectively. The relatively poor quality of the S 2p spectra is related to the attenuation of the corresponding signal by the hydrocarbon overlayer and a relatively short acquisition time chosen to reduce possible X-ray-induced damage during the measurement. In the C 1s XPS spectra for the pristine BPn films in Figure 1a, a single C 1s photoemission maximum with a BE of 284.15-284.20 eV for n ) 0, 1, 4, and 5 and of 284.45 eV for n ) 12 is observed (see Table 1 and refs 24, 25, and 31 for details). The fwhm of this peak is 1.20-1.30 eV for n ) 1, 4, 5, and 12 and ∼1.50 eV for n ) 0 (see Table 1). As previously reported,25,31,45 a minor shake-up type component at the higher BE side of the C 1s peak is practically not perceptible at the given energy resolution. The main C 1s emission can be unequivocally attributed to benzene-like carbon atoms, which constitute the major part of the BPn molecules, except for BP12, in which the aliphatic part has a similar weight. In agreement with this molecular composition, the C 1s maximum for BP12 is shifted to a higher BE as compared to the BPn films with a shorter alkyl part (the BE of the C 1s emission of a pure AT SAM on gold substrate is 284.85 eV).10,31 Electron irradiation results in minor changes of the C 1s spectra. Only a very small increase in the BE and fwhm of the C 1s peak occurs (see Table 1). In addition, the (43) Frydman, E.; Cohen, H.; Maoz, R.; Sagiv, J. Langmuir 1997, 13, 5089. (44) Thome, J.; Himmelhaus, M.; Zharnikov, M.; Grunze, M. Langmuir 1998, 14, 7435. (45) Frey, S.; Stadler, V.; Heister, K.; Zharnikov, M.; Grunze, M.; Zeysing, B.; Terfort, A. Langmuir 2001, 17, 2408.


Figure 2. The S 2p XPS spectra of pristine (a) and irradiated (b) BPn SAMs. The observed emission structures are fitted by two doublets of Voigt peaks (see text) related to the pristine thiolate moieties and to the new sulfur species induced by electron irradiation, respectively. The intensities of these emission structures for the irradiated films with respect to the intensity of the thiolate-related S 2p doublet for the corresponding pristine films are indicated at the respective spectra in (b): at the left, for the irradiation-induced doublet; at the right, for the thiolate-related doublet; and above the spectrum, for the entire intensity. The accuracy of the relative intensities is (10%. Table 1. The BEs and fwhm’s of the C 1s Peak for the Pristine and Irradiated BPn Filmsa BE (pristine film), eV BE (irradiated film), eV fwhm (pristine film), eV fwhm (irradiated film), eV % thickness reduction

BP0

BP1

BP4

BP5

BP12

284.20 284.25 1.49 1.53 9.1

284.15 284.18 1.31 1.31 4.5

284.18 284.33 1.28 1.31 15.2

284.20 284.32 1.22 1.24 5.4

284.45 284.75 1.30 1.30 15.0

a The accuracy of the derived values is (0.05. The thickness reduction was calculated assuming an exponential attenuation of the C 1s signal with the mean free path of 27 Å.44

intensity of the C 1s peak decreases, which can be associated with the irradiation-induced desorption of hydrocarbon fragments of the BPn films. For further evaluation, the relative intensities of the C 1s emission for the irradiated films (with respect to those for the pristine films) and the respective thickness reduction were calculated. The results are printed at the corresponding spectra in Figure 1b and are compiled in Table 1, respectively. The reduction of the C 1s intensity in all investigated BPn SAMs is 3-11%; the thickness reduction is 4.5-15%, which is noticeably smaller than that in AT SAMs (27-39%).10 At the same time, the extent of irradiation-induced desorption in the densely packed BP1 and BP5 SAMs is noticeably smaller than that in the less densely packed BP4, equally aromatic/aliphatic BP12 or pure aromatic BP0. In the S 2p XPS spectra for the pristine BPn films in Figure 2a, a single S 2p3/2/S 2p1/2 doublet is observed at approximately the same BE for all systems (see refs 25 and 31 for details). The decomposition of the S 2p3/2/S 2p1/2 doublets by two Voigt peaks with a fixed intensity ratio

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of 2:1 and a fwhm of ∼1.20 eV (see section 2 for details) gives BEs of ∼162.0 eV (S 2p3/2) and ∼163.2 eV (S 2p1/2), characteristic of a thiolate species on Au.25,31 Note, that the position of the S 2p3/2 and S 2p1/2 peaks is nearly the same as for conventional AT SAMs on Au.31 The S 2p spectra of the irradiated BPn films in Figure 2b clearly demonstrate the intensity reduction of the original, thiolate-related doublet and the development of a new doublet at a higher BE. The latter doublet is assumed to be associated with the incorporation of sulfur into the hydrocarbon matrix through its bonding to an irradiation-induced carbon radical in the adjacent aliphatic-aromatic chains.9,13,34 Taking over the BE position and fwhm of the thiolate-related doublet from the S 2p spectra of the pristine films, the evolving structure could be fitted by another doublet with approximately the same peak width as for the thiolate-related doublet and BEs of ∼163.2 eV (S 2p3/2) and ∼164.4 eV (S 2p1/2). Considering that most of the parameters are fixed or common for the “pristine” and “irradiation-induced” S 2p doublets, the results of such a fitting are rather unambiguous despite a rather poor signal-to-noise ratio. The relative intensities of the thiolate-related and irradiation-induced doublets as well as their sum (with respect to the intensity of the thiolate-related S 2p doublet for the corresponding pristine films) are shown at the left, at the right, and above the respective spectra in Figure 2b, respectively. The intensity reduction of the original thiolate-related doublet and the intensity of the irradiation-induced S 2p structure are rather small in all investigated systems, except for BP12. Note for comparison, that at a comparable dose only 2028% of the pristine thiolate species in SAMs of pure aliphatic thiols survive the electron irradiation and the respective extent of irradiation-induced desorption of sulfur-containing fragments is 14-39% of the initial amount of thiolate.10 Besides the reduced damage at the S-Au interface of the BPn films, both the intensity of the irradiation-induced doublet and the intensity reduction of the thiolate-related doublet in the densely packed BP1 and BP5 SAMs are noticeably smaller than those in the less densely packed BP4 or pure aromatic BP0. Note, that the amount of the irradiation-induced sulfur species in the electron beam processed films can be slightly overestimated on the basis of the spectra in Figure 2b, because these species are not necessarily located at the S-Au interface but are distributed over the film (even though in the vicinity of the interface),13,34 which means a weaker attenuation of the respective S 2p signal. 3.2. NEXAFS Measurements. NEXAFS experiments provide both chemical and structural information about the occurrence and average orientation of unoccupied molecular orbitals within organic films of interest. The C 1s NEXAFS spectra of the pristine and irradiated BPn SAMs acquired at a magic X-ray incidence angle of 55° are presented in parts a and b of Figure 3, respectively. The absorption signal at this particular experimental geometry is insensitive to the orientation of molecular orbitals and, therefore, exclusively reflects the electronic structure.36 In addition to the electronic structure, the molecular orbital orientation and the orientational order in the BPn films as well as their evolution in the course of the irradiation are of major interest. A measure of these parameters is the linear dichroism of the NEXAFS spectra, which can be highlighted by the difference of the spectra acquired at X-ray incident angles of 90° and 20°. Such difference spectra for the pristine and irradiated BPn films are depicted in parts a and b of Figure 4, respectively.

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Figure 3. The C 1s NEXAFS spectra of the pristine (a) and irradiated (b) BPn SAMs acquired at a magic X-ray incidence angle of 55°. The characteristic absorption resonances are indicated. The intensities of the π1* resonances for the irradiated films with respect to the corresponding values for the pristine SAMs are indicated at the respective spectra in (b). The accuracy of the relative intensities is (5%.

All spectra of the pristine BPn films in Figure 3a have a similar shape. They exhibit a C 1s absorption edge related to the C 1s f continuum excitations located at ∼287 eV11,45,46 and several pronounced π* and σ* resonances typical for aromatic systems37 and, in particular, aromatic SAMs.11,45,46 The spectra are dominated by the sharp π1* resonance at a photon energy (PE) of ∼285.1 eV. Except for the resonances related to the aromatic part of the BPn molecules, the development of an additional resonance at a PE of ∼287.7 eV near the absorption edge is observed by going from BP0 to BP12. This resonance, usually assigned as the R* resonance47,48 (see refs 36 and 49 for alternative assignments) is characteristic of an alkyl chain and, in the present case, is related to the aliphatic part of the BPn molecules. The other two fingerprint resonances of the alkyl chain, namely, the C-C σ* and C-C′ σ* resonances at PEs of ∼293.4 and ∼301.6 eV, respectively,50-52 overlap with the σ* resonances of the BP moieties and are practically nondistinguishable. Also the R* resonance is overlapped with a low-intensity feature assigned to a Rydberg-type excitation in the aromatic part53,54 or to a C-S σ* resonance.55 (46) Himmel, H.-J.; Terfort, A.; Wo¨ll, Ch. J. Am. Chem. Soc. 1998, 120, 12069. (47) Bagus, P. S.; Weiss, K.; Schertel, A.; Wo¨ll, Ch.; Braun, W.; Hellwig, H.; Jung, C. Chem. Phys. Lett. 1996, 248, 129. (48) Weiss, K.; Bagus, P. S.; Wo¨ll, Ch. J. Chem. Phys. 1999, 111, 6834. (49) Va¨terlein, P.; Fink, R.; Umbach, E.; Wurth, W. J. Phys. Chem. 1998, 108, 3313. (50) Outka, D. A.; Sto¨hr, J.; Rabe, J. P.; Swalen, J. D. J. Chem. Phys. 1988, 88, 4076. (51) Ha¨hner, G.; Kinzler, M.; Wo¨ll, Ch.; Grunze, M.; Scheller, M.; Cederbaum, L. S. Phys. Rev. Lett. 1991, 67, 851; Phys. Rev. Lett. 1992, 69, 694 (erratum). (52) Ha¨hner, G.; Kinzler, M.; Thu¨mmler, C.; Wo¨ll, Ch.; Grunze, M. J. Vac. Sci. Technol. 1992, 10, 2758. (53) Horsley, J. A.; Sto¨hr, J.; Hitchcock, A. P.; Newbury, D. C.; Johnson, A. L.; Sette, F. J. Chem. Phys. 1985, 83, 6099. (54) Weiss, K.; Gebert, S.; Wu¨hn, M.; Wadepohl, H.; Wo¨ll, Ch. J. Vac. Sci. Technol., A 1998, 16, 1017. (55) Sto¨hr, J.; Outka, D. A. Phys. Rev. B 1987, 36, 7891.

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Figure 4. The differences of the C 1s NEXAFS spectra acquired at incident angles of 90° and 20° for the pristine (a) and irradiated (b) BPn films. The π1*, π2*, and R* anisotropy peaks are indicated. The amplitudes of the π1* and R* anisotropy peaks for the irradiated films with respect to the corresponding values for the pristine SAMs are shown at the left and at the right of the curves in (b). For BP0, the relative amplitude of the R* anisotropy peak is not shown, because the BP0 molecule does not contain an alkyl part. The accuracy of the relative amplitudes is (5% and (10% for the π1* and R* anisotropy peaks, respectively.

The molecular orbitals related to the π* and R* resonances are oriented perpendicular to the aromatic rings of the BP moiety and the alkyl chain axis, respectively, whereas the transition dipole moments of the orbitals corresponding to the σ* resonances are parallel to the ring planes and the alkyl chain axis.11,36,45-47,50-52,54 Consequently, a positive sign of the anisotropy peaks for the π* and R* resonances and a negative sign of these peaks for the σ* resonances in the difference spectra in Figure 4b manifest an upright orientation of both aliphatic and aromatic parts of the BPn molecules in the respective SAMs. The well-defined and intense π1* resonance is best suited to analyze the orientation of the aromatic part of the BPn molecules. For BP4 the amplitude of the π1* anisotropy peak is smaller than those for BP1 and BP5. This reveals a larger tilt angle of the BP moieties in the less densely packed BP4 SAMs as compared to those for the BP1 and BP5 films, which have a denser packing.23,24 The respective amplitudes for the BP0 and BP12 SAMs are somewhat intermediate between the values for the BP4 and BP1/BP5 films, which correlates with the relation between the tilt angles of the BP moieties in the pure aromatic biphenylthiol SAMs on Au (23°),45 BP1 and BP5 (17°),23 and BP4 (29°).23 From the similar linear dichroism of the π1* resonance, the average tilt angle of the BP moieties in BP12 seems to be similar to that of BP0. This angle was estimated to be equal to 22.5° based on the assumption of a planar conformation and a herringbone packing of the biphenyl moieties with the same twist angle as in the respective bulk compound (see refs 23, 24, and 45 and references therein). Because there are no crystallographic data for the bulk BPn, we have taken a twist angle of 32° typical for bulk biphenyl compounds.56-58 Note, (56) Trotter, J. Acta Crystallogr. 1961, 14, 1135.


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Table 2. The Average Tilt Angles (deg) of the BP Moieties for the Pristine and Irradiated BPn Filmsa av tilt angle

BP0

BP1

BP4

BP5

BP12

pristine film irradiated film

23 45

18 25

29 43

17 29

22.5 42

a

The accuracy of the derived values is (5°.

that the given values of the tilt angles for the BP moieties in BP0, BP1, BP4, and BP5 are also based on the same value of the twist angle;23,45 a larger twist angle will mean a larger tilt of the BP moieties.24 As for the R* anisotropy peak, its amplitude continuously increases with increasing length of the aliphatic chain. We believe that this increase is predominately related to the change of the R* orbital character associated with its spreading over the longer alkyl chain. Generally, the intensity of the R* resonance is larger for the AT SAMs consisting of longer alkyl chains.10,52 The electron irradiation of the BPn SAMs on Au does not result in noticeable changes of the 55° spectra in Figure 3a except for a reduction of the π1* resonance intensity, which is more pronounced in the case of BP4, as seen in Figure 3b. At the same time, the difference curves in Figure 4b exhibit a pronounced intensity decrease of the π1* and R* anisotropy peaks as compared to the spectra in Figure 4a, which indicates a partial disordering of the BPn SAMs. To monitor this process, the relative amplitudes of the π1* and R* anisotropy peaks for the irradiated films (with respect to the corresponding values in the difference spectra of the pristine SAMs) are shown at the left and at the right of the difference curves in Figure 4b and the respective average tilt angles are given in Table 2. For the densely packed BP1 and BP5 SAMs the reduction of the π1* and R* anisotropy peaks is significantly smaller than those in the less densely packed BP4, the equally aromatic/aliphatic BP12 or the pure aromatic BP0. At the same time, the π1* anisotropy peaks are affected to a much smaller extent by the electron irradiation than the R* anisotropy peaks. In particular, a complete disordering of the aliphatic part can be assumed in BP4 on the basis of the respective spectra in Figure 4b, whereas the BP moieties in this system still keep the (even though reduced) orientational order. Notice also, that the reduction of the π1* and R* anisotropy peaks in BP12 is much larger than those in BP1 and BP5 but very similar to that in BP4. 3.3. IRRAS Characterization. The IRRAS spectra of the pristine and irradiated BPn SAMs are presented in Figures 5 and 6 for two different spectral regions containing characteristic vibrational bands of the aromatic rings and methyl entities. The spectra of the pristine films are depicted in panel a, whereas those for the irradiated SAMs are given in panel b. The IRRAS spectra of the pristine BPn SAMs in Figure 5a exhibit characteristic aromatic semicircle stretch and bending vibration modes, ν(C-C) and δip(C-H) at ∼1500 cm-1 (∼1475 cm-1 for the BP0 film) and ∼1000 cm-1, respectively (see ref 24 and references therein for details). The intensities of the δip(C-H) show a clear odd-even behavior for the BP1, BP4, and BP5 films, whereas this effect is not so pronounced for the ν(C-C) band because of the low intensity of this mode for BP1. This low intensity is a deviation from a common odd-even behavior of the ν(C-C) band in the BPn SAMs (n ) 1-6).24 This is not completely understood at present but we believe that a (57) Kitaigorodskii, I. A. Organic Chemical Crystallography; Consultants Bureau: New York, 1961. (58) Charbonneau, G.-P.; Delugeard, Y. Acta Crystallogr. B 1976, 32, 1420.

Figure 5. The IRRAS spectra of the pristine (a) and irradiated (b) BPn SAMs in the range containing characteristic aromatic semicircle stretch and bending vibration modes, ν(C-C) and δip(C-H) at ∼1500 cm-1 (∼1475 cm-1 for the BP0 film) and ∼1000 cm-1, respectively. The intensities of these modes for the irradiated films with respect to the corresponding values for the pristine SAMs are indicated in (b). The accuracy of the relative intensities is (5% and (7% for the ν(C-C) and δip(C-H) modes, respectively.

Figure 6. The IRRAS spectra of the pristine (a) and irradiated (b) BPn SAMs in the range containing the characteristic vibration band of the methyl entity at ∼1380 cm-1. The intensity of this mode for the irradiated films with respect to the corresponding values for the pristine SAMs is indicated in (b). The accuracy of the relative intensities is (7% except for the values for BP12, BP4, and BP0, which are rather rough estimates because of the low intensity of the considered mode.

significant coupling between the sulfur and the aromatic unit still exists and affects the IR transition dipole moment. The intensity of the νsym(CH3) mode of the methyl entity59 in the IRRAS spectra in Figure 6 also reveals an odd-even behavior for the BP1, BP4, and BP5 SAMs. A

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very interesting observation is that the intensities of the ν(C-C), δip(C-H), and νsym(CH3) bands of BP12 are much closer to those of BP4 than to those of BP1 and BP5. The electron irradiation of the BPn films results in a reduction of the intensity of the ν(C-C), δip(C-H), and νsym(CH3) bands, as follows from comparison of Figures 5a and 6a with Figures 5b and 6b, respectively. For easier comparison, the relative amplitudes of the ν(C-C), δip(C-H), and νsym(CH3) bands for the irradiated films (with respect to the corresponding values in the spectra of the pristine SAMs) are shown at the respective peaks in the IRRAS spectra in Figures 5b and 6b. It is clearly seen, that the intensity reduction of the former band in all BPn films is smaller than those for the two latter bands, which imply that the cleavage of the C-H bonds and a damage of the methyl group are the dominating irradiation-induced processes. Furthermore, the intensity reductions of the ν(C-C), δip(C-H), and νsym(CH3) bands of the densely packed BP1 and BP5 SAMs upon irradiation are noticeably smaller than those in the less densely packed BP4 SAM. The BP12 and BP0 films behave in a similar manner as BP4. Note that the decrease of the IR peak intensities upon irradiation can be due to both damage and orientational disordering of the respective molecular entities. On the basis of the clear evidence of the quasi-polymerization and hydrogen depletion and chemical transformation of the functional groups in pure aromatic SAMs,11,12,14,15 we believe that damage is the dominating contribution. 3.4. Advancing Contact Angle Measurements. The advancing contact angles of water and hexadecane for the pristine (circles) and irradiated (triangles) BPn SAMs are presented in Figures 7a and 7b, respectively. For the pristine BP1, BP4, and BP5 films these values exhibit an odd-even behavior as described previously,24 which seems to be extended to BP12. The odd-even behavior is superimposed on a continuous growth of both contact angles with increasing length of the aliphatic part (see also ref 24). This growth can be presumably associated with an increasing orientational order and a decreasing roughness at the film-vacuum interface, which is related to a generally progressing ordering of the BPn molecules with a longer spacer. The contact angles for BP0 are the smallest ones as compared to the other systems of this study. The electron irradiation of the BPn films results in a decrease of the contact angles (see ref 7 and references therein for comparison with AT SAMs). This decrease is larger for the BP0, BP4, and BP12 films as compared to the BP1 and BP5 SAMs. The largest reduction of the contact angles is observed for the BP12 films. The systematically bigger changes in the contact angle observed for the BPn SAMs with an even n compared to those with an odd n agree with the results from the other techniques and reveal that the electron-induced damage and changes in molecular orientation are more pronounced in the former case. 4. Discussion Before the consideration and analysis of the results presented in Sections 3.1-3.4, let us review the available information on the modification of pure aliphatic and aromatic thiol-derived SAMs by electron irradiation. In AT SAMs both the alkyl chains and the S-Au interface are affected through the electron-induced dissociation of C-H, C-C, C-S, and substrate-thiolate (59) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558.

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bonds.5,8,10 The most noticeable processes are the loss of orientational and conformational order, partial dehydrogenation with CdC double bond formation, desorption of film fragments, reduction of the pristine thiolate species, and the appearance of a new sulfur species, which was recently identified as a dialkyl sulfide distributed over the whole film.3-10,13,60,61 All processes were found to occur at similar rates, except for the formation of CdC double bonds and the desorption of sulfur-containing fragments, which evolve noticeably more slowly.10 The latter process is additionally suppressed in the longer-chain AT SAMs because of the trapping of the sulfur-containing fragments within a thicker alkyl layer.10,34 This phenomenon as well as a quenching of the electronically excited dissociative states through the dipole-dipole interaction with the substrate result in a much higher extent of irradiationinduced desorption processes at the film-vacuum interface as compared to the inner part of the film.6,10 As a general result of all irradiation-induced processes, an almost complete degradation of the well-ordered thioaliphatic film occurs after the irradiation with a large dose, comparable with the dose applied to the BPn SAMs in this study. The film becomes a disordered structure comprising both saturated and unsaturated hydrocarbons. In contrast to the aliphatic SAMs, almost all irradiationinduced processes resulting in the entire damage of these systems were found to be essentially suppressed in the pure aromatic SAMs, as was shown by the examples of 1,1′-biphenyl-4-thiol (very similar to BP0 of this study), 4′-nitro-1,1′-biphenyl-4-thiol, and 1,1′;4′,1′′-terphenyl-4thiol SAMs on gold substrates.11,13,14 Most important of all, the orientational order in the films and the anchoring to the substrate by substrate-sulfur bond are still retained upon irradiation, even though to a somewhat reduced degree. At the same time, C-H bond scissions in the aromatic matrix occur to a significant extent, which subsequently results in irradiation-induced cross-linking between the neighboring aromatic-chain moieties. Also, the bonds within a functional terminal group are cleaved by electrons, which can result in their modification.14 Let us now turn to the results of this study, in which we performed electron beam irradiation of the aromaticaliphatic BPn SAMs and the pure aromatic BP0 film on gold substrates. The aliphatic part of the BPn molecules is adjacent to the substrate-sulfur interface and separates the aromatic part from the sulfur headgroups. The filmvacuum interface is comprised of the methyl groups attached to the biphenyl spacer. On the basis of this layered structure, an intermediate reaction of the BPn films toward electron beam exposure could be expected as compared to the pure aliphatic or pure aromatic SAMs. However, all investigated BPn systems behave essentially like the aromatic SAMs upon electron irradiation, irrespective of n: The orientational order and the anchoring to the substrate are still retained to a large degree, and the extent of irradiation-induced desorption of hydrocarbons is rather small (6-14%) as compared to AT films (30-35%).8,10 Only BP12 exhibits a considerable damage of the initially intact thiolate-gold interface (Figure 2), which is comparable to that in long-chain AT SAMs on Au.8,10 However, the cleavage of thiolatesubstrate bonds in this system is not accompanied by a desorption of complete BPn molecules, because there is no correlation between the amounts of desorbed hydrocarbon fragments and changes at the sulfur-substrate (60) Vo¨lkel, B.; Go¨lzhaüser, A.; Mu¨ller, H. U.; David, C.; Grunze, M. J. Vac. Sci. Technol., B 1997, 15, 2877. (61) Kondoh, H.; Nozoye, H. J. Phys. Chem. B 1998, 102, 2367.


interface, neither for BP12 nor for other investigated systems. The irradiation-induced sulfur species (with or without adjacent hydrocarbon chain) become trapped in the film as follows from the observation of the respective doublet in the S 2p spectra for the irradiated BPn SAMs in Figure 2. Obviously, the aromatic spacer has a profound impact on the stability of the BPn SAMs with respect to electron irradiation. It is the cross-linking between the adjacent BP moieties initiated by the irradiation-induced scission of C-H bonds in the aromatic rings which can be accounted for as the major reason responsible for this stability. The respective processes can be directly inferred in Figure 5, in which the irradiation-induced reduction of the δip(C-H) and ν(C-C) bands reflecting the C-H bond scissions and cross-linking, respectively, is exhibited. Another possible mechanism of a superior stability of the aromatic compared to aliphatic moieties is a delocalization of the primary excitations over the extended π-σ system of the aromatic spacer. Beyond the principal differences between aliphatic and aromatic SAMs, the molecular orientation and packing obviously affect the entire variety of the irradiationinduced processes. The BP1, BP4, and BP5 films, which reveal an odd-even variation of the packing density and molecular orientation of the BP moieties,23-25 exhibit pronounced odd-even effects in their reaction toward electron irradiation. All obtained XPS, NEXAFS, IRRAS, and contact angle data imply that the densely packed BP1 and BP5 SAMs are much more stable with respect to electron bombardment than the less densely packed BP4 films. Even ∼50% of the methyl groups at the filmvacuum interface of the two former systems survive a heavy electron bombardment, whereas this interface becomes almost completely destroyed in BP4. Note, that the high resistance of the terminal methyl groups in BP1 and BP5 toward electron irradiation is very special considering an almost complete damage of the filmvacuum interface in conventional aliphatic thiol-derived SAMs even after a moderate irradiation with a dose of 3000 µC/cm2 (30 eV electrons).7 A complete damage/ transformation of the terminal methyl groups in BP1 and BP5 might occur at a much higher dose than 8000 µC/cm2 used in this study, similar to the pure aromatic 4-nitro1,1′-biphenyl-4-thiol SAMs on gold, in which a dose of ∼35000 µC/cm2 (50 eV electrons) is required for a complete transformation of the terminal nitro functionalities to amino groups.14 A possible explanation of the observed relation between the packing density and molecular orientation of the BPn films and their reaction toward ionizing radiation is a larger delocalization and a faster relaxation of the initial electronic excitation in a better ordered and densely packed monomolecular layer. Recent HRXPS results imply that the BPn SAMs represent highly correlated molecular ensembles.25 A degree of this correlation, intermolecular (van der Waals and electrostatic)62 interaction, and the π-σ conjugation between the adjacent biphenyl moieties should strongly depend on the packing and orientation of the BPn molecules. Comparison of the XPS, NEXAFS, and IRRAS spectra and contact angle data of BP12 with those for BP1, BP4, and BP5 SAMs suggests that the former system reacts to electron irradiation in a similar manner and to a similar extent as BP4. Two different reasons or their combination can be responsible for this behavior. First, the aliphatic chain in BP12 contains an even number of the methylene (62) Lii, J.-H.; Allinger, N. L. J. Am. Chem. Soc. 1989, 111, 8576.

Langmuir, Vol. 18, No. 8, 2002 3149

Figure 7. The advancing contact angles of water (a) and hexadecane (b) for the pristine (circles) and irradiated (triangles) BPn SAMs. The accuracy of the presented values is (1°.

units, which can be associated with a less dense molecular packing, analogous to BP4. In fact, the linear dichroism of the π1* resonance for BP12 is somewhat smaller than that for BP1 and BP5 (Figure 4), which implies a larger tilt angle (∼23°) and suggests a less dense molecular packing of BP12 compared to the two latter systems. Note that there are no systematic data on an odd-even variation of the packing density and molecular orientation for the BPn SAMs with n > 6. Being speculative for the moment, the relation between the electron-induced damage in the BP12 and BP1, BP4, and BP5 films can, however, be considered as an indication that such an odd-even effect might also persist for the BPn SAMs with a longer aliphatic part. Another reason for an enhanced (as compared to BP1 and BP5) sensitivity of BP12 toward electron bombardment can be the longer aliphatic chain. In particular, it manifests itself in the considerable electron-induced damage at the thiolate-gold interface, as implied by the respective S 2p spectra in Figure 2. This damage can be related to the fact that a long alkyl chain can easier accommodate conformational defects associated with the cleavage of either S-Au or S-C bond. The shorter alkyl chains attached to quasi-polymerized (as a result of irradiation) aromatic moieties are much more rigid, which hinders any molecular or atomic shift or movement at and in the vicinity of the thiolate-gold interface. Another singularity of the BP12 (as compared with all other systems of this study) is a larger damage at the filmvacuum interface, which is reflected both by the large reduction of the advancing contact angles (Figure 7) and by a massive decrease in intensity of the methyl-related mode at ∼1380 cm-1 (Figure 6) as a result of the irradiation. The larger damage can be related to a reduced quenching of the electronically excited states at the filmvacuum interface of the comparably thick BP12 film. This quenching is mediated by a dipolar interaction between the excited state and the substrate electrons with the probability for this process decreasing with increasing separation from the metal surface.6,10

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Let us now turn to the fully aromatic BP0 film. Comparison of XPS, NEXAFS, IRRAS, and contact angle data for this system (Figures 1-7) with those for all other films of this study implies that the BP0 SAMs react to electron irradiation in a similar manner and to a similar extent as the less densely packed BPn (n > 0) SAMs. Extending our arguments about the influence of the molecular packing and orientational order in monomolecular films on their reaction toward ionizing radiation (BP4 vs BP1 and BP5), we can conclude that the molecular packing and orientational order in BP0 films are comparable to those in the less densely packed BPn SAMs in agreement with the BP tilt angle relation in all of the above-mentioned SAMs. From the viewpoint of SAM-based lithography, the results of this study show that the reaction of aromatic SAMs toward electron radiation can be substantially influenced by the introduction of an aliphatic spacer between the aromatic part and the thiol headgroup. On the other hand, aromatic rings can be used to stabilize or protect the aliphatic parts and/or the SAM-substrate interface. Considering possible applications of the BPn SAMs as a negative resist for “conventional” lithography or as a template for chemical lithography, it remains to be clarified by further studies for which applications the densely or less densely packed BPn SAMs are better suited. A larger extent of the irradiation-induced effects is observed in the latter systems, whereas the densely packed BPn SAMs are much more stable toward the ionizing radiation, which requires higher doses of irradiation to create their quasi-polymerization or to modify the functional group. The balance between the stability and desirable changes should be addressed in future studies. For this purpose, etching resistance of the pristine and electron beam processed BPn films as well as conversion rates of different functional groups attached to the BPn molecules in the respective SAMs should be monitored in a systematic way. An example of the application of the BPn (n > 0) films as lithographic resists or a template was recently given in ref 29, in which copper deposition on an electron beam patterned BP12 was performed. In agreement with the spectroscopic results of this study, BP12 behaves as a negative resist: The electrochemical copper deposition was only observed on the nonirradiated parts, whereas the irradiated areas exhibited a blocking behavior due to the assumed quasi-polymerization.29 5. Conclusion The low-energy electron-induced modification of the aromatic-aliphatic BPn SAMs (n ) 1, 4, 5, and 12) and pure aromatic BP0 films on gold substrates was studied. The pristine and heavily (8000 µC/cm2) irradiated BPn SAMs were characterized in detail by several complementary experimental techniques such as XPS, NEXAFS spectroscopy, IRRAS, and advancing contact angle measurements. Since the first two techniques were applied in the same vacuum chamber, in which the electron irradiation was performed, in situ monitoring of the irradiationinduced processes was possible. In contrast to aliphatic SAMs, which exhibit a severe degradation upon electron irradiation, damage was found to be largely reduced in BPn SAMs, despite their mixed aliphatic-aromatic character. Most importantly, the orientational order in the BPn films and anchoring to the substrate are retained upon irradiation, even though both are somewhat reduced, especially in BP12 where the anchoring is affected most. At the same time, C-H bond scissions in the aromatic part occur to a noticeable extent, which presumably results in irradiation-induced cross-

Frey et al.

linking between neighboring biphenyl moieties, analogous to pure aromatic SAMs11,13,14 and in agreement with results of ref 29. This cross-linking is indirectly manifested by the suppression of the electron-induced desorption and by a reduction of the aromatic semicircle stretch vibrations at ∼1500 cm-1. Simultaneously with the cross-linking, a partial or complete modification of the terminal methyl groups occurs. Whereas the general behavior of the BPn SAMs with respect to electron irradiation is quite similar, the extent of the irradiation-induced changes clearly depends on the structure of these systems. The BP1, BP4, and BP5 films, which show an odd-even variation of the packing density and molecular orientation of the BP moieties,23-25 exhibit pronounced odd-even effects in their reaction toward electron irradiation. The densely packed BP1 and BP5 SAMs are much more stable with respect to electron bombardment than the less densely packed BP4 films. A possible explanation of this behavior is a larger delocalization and a faster relaxation of the initial electronic excitation in a better ordered and densely packed monomolecular layer. The extent of the irradiation-induced changes in the 50%-50% aromatic-aliphatic BP12 and pure aromatic BP0 is generally similar to that of the BP4 films and is noticeably larger than those in the densely packed BP1 and BP5 SAMs. This behavior correlates with the larger tilt angles of the BP moieties in BP12 and BP0 (which assumes a lower packing density) as compared to those in BP1 and BP5 implying that a relation between the packing density and molecular orientation and the extent of the irradiation-induced changes is a general phenomenon in monomolecular films (first indications of this phenomenon were mentioned in refs 10 and 22). Note, that both the tilt angle of the BP moieties in BP12 and the extent of the irradiation-induced changes in this system indirectly indicate that the structural odd-even variations in BPn SAMs with 0 < n < 7 can be extended to larger n. From the viewpoint of SAM-based lithography, the results of this study show that the reaction of aromatic SAMs toward ionizing radiation can be tailored by the introduction of an aliphatic spacer between the aromatic part and the headgroup. Furthermore, aromatic rings can be used to stabilize or protect some regions within the aliphatic SAMs. The BPn SAMs can be used as a negative resist for “conventional” lithography or as a template for chemical lithography. Depending on the application, either the densely or less densely packed BPn SAMs can be the system of choice. The balance between the radiation stability and desirable changes from the viewpoint of lithographic applications should be addressed in future studies. Other issues of interest are the structural properties of BPn SAMs with n > 6 and the relation between the quality of the BP0 film and its properties, in particular, its resistance toward electron irradiation. Acknowledgment. The authors are very grateful to M. Grunze and G. Helmchen for their support. We also want to thank the BESSY staff, especially M. Mast for technical help, Ch. Wo¨ll (Universita¨t Bochum) for providing us with experimental equipment, and G. Albert for fabrication of the gold substrates. This work has been supported by the German Bundesministerium fu¨r Bildung, Wissenschaft, Forschung und Technologie (BMBF Grants 05 SF8VHA 1 and GRE1HD), by the Deutsche Forschungsgemeinschaft (DFG Grants JA883/3-2 and Bu820/ 11-2), and by the Fonds der Chemischen Industrie. LA011288O

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