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Modification of Alkanethiolate Monolayers by Low Energy Electron Irradiation: Dependence on the Substrate Material and on the Length and Isotopic Composition of the Alkyl Chains M. Zharnikov,* S. Frey, K. Heister, and M. Grunze Angewandte Physikalische Chemie, Universita¨ t Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany Received July 30, 1999. In Final Form: October 21, 1999 The low energy electron induced damage in self-assembled monolayers of dodecanethiolate, octadecanethiolate, and perdeuterated eicosanethiolate on gold and octadecanethiolate on silver has been investigated in situ by X-ray photoelectron spectroscopy and angle resolved near edge X-ray absorption fine structure spectroscopy. All investigated systems exhibit qualitatively similar behavior with respect to low energy electron irradiation. The most noticeable processes are the loss of orientational and conformational order, partial dehydrogenation with CdC double bond formation, desorption of the layer fragments, reduction of the thiolate species, and the appearance of new sulfur species. The cross sections for the rates of the individual irradiation-induced processes have been determined. For the films on gold all these processes are found to evolve with similar rates, except for the formation of CdC double bonds and desorption of sulfur-containing fragments. The extent of the latter process is noticeably smaller in the longer-chain films as compared to their shorter-chain counterparts. The response of the alkyl matrix and the S-Au interface to electron irradiation are not directly correlated. Whereas the irradiation-induced processes in the alkyl matrix are found to be essentially independent of the alkyl chain length and the substrate material, the extent and rate of the thiolate species reduction and new sulfur species formation are mainly determined by the strength and character of the thiolate-substrate bond. No large isotopic effect in the irradiation-induced dehydrogenation process was observed. Deuterated films are found to be only slightly less sensitive to electron irradiation as compared to their hydrogen-containing counterparts.
1. Introduction The ability of definite chainlike molecules to form dense and ordered monolayers on suitable surfaces is widely used in surface science, chemistry, biology, and molecular engineering.1,2 These self-assembled monolayers (SAMs) have been successfully deposited on metals such as gold, silver, copper, platinum, and chromium,3-5 and on oxide and semiconductor surfaces such as SiO2 and GaAs.1,6,7 An interesting area of applied research is SAM modification by light, X-ray, and electron irradiation. This offers a possibility to use SAMs as a resist or chemical template in lithography applications,8-17 but is also important * To whom correspondence should be addressed. Email: o60@ ix.urz.uni-heidelberg.de. (1) Ulman, A. An Introduction to Ultrathin Organic Films: Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (2) Ulman, A. Chem. Rev. 1996, 96, 1533. (3) Stewart, K. R.; Whitesides, G. M.; Godfried, H. P.; Silvera, I. F. Surf. Sci. 1986, 57, 1381. (4) Ulman, A. J. Mater. Ed. 1989, 11, 205. (5) Lee, T. R.; Laibinis, P. E.; Folkers, J. P.; Whitesides, G. M. Pure Appl. Chem. 1991, 63, 821. (6) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. (7) Sheen, C. W.; Shi, J.-X.; Martensson, J.; Parikh, A. N.; Allara, D. L. J. Am. Chem. Soc. 1992, 114, 1514. (8) Tiberio, R. C.; Craighead, H. G.; Lercel, M. J.; Lau, T.; Sheen, C. W.; Allara, D. L. Appl. Phys. Lett. 1993, 62, 476. (9) Lercel, M. J.; Tiberio, R. C.; Chapman, P. F.; Craighead, H. G.; Sheen, C. W.; Parikh, A. N.; Allara, D. L. J. Vac. Sci. Technol. B 1993, 11, 2823. (10) 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. (11) Lercel, M. J.; Redinbo, G. F.; Rooks, M.; Tiberio, R. C.; Craighead, H. G.; Sheen, C. W.; Allara, D. L.; Microelectronic Eng. 1995, 27, 43. (12) Lercel, M. J.; Rooks, M.; Tiberio, R. C.; Craighead, H. G.; Sheen, C. W.; Parikh, A. N.; Allara, D. L. J. Vac. Sci. Technol. B 1995, 13, 1139. (13) Mu¨ller, H. U.; David, C.; Vo¨lkel, B.; Grunze, M. J. Vac. Sci. Technol. B 1995, 13, 2846.
because of radiation induced damage during characterization of SAMs by standard spectroscopic techniques.18-20 The damage produced in SAMs by light and 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.18,20-22 The excitation of the antibonding molecular orbitals and ionization of particular molecular groups mediated by these electrons result in a cleavage of individual molecular bonds with the subsequent partial desorption and chemical modification of the irradiated film.20,23,24 The damage produced in SAMs by low-energy electrons is therefore of fundamental importance for an estimation and understanding of the changes initiated in (14) David, C.; Mu¨ller, H. U.; Vo¨lkel, B. Grunze, M.; Microelectronic Eng. 1996, 30, 57. (15) Lercel, M. J.; Craighead, H. G.; Parikh, A. N.; Seshadri, K.; Allara, D. L. Appl. Phys. Lett. 1996, 68, 1504. Seshadri, K.; Froyd, K.; Parikh, A. N.; Allara, D. L.; Lercel, M. J.; Craighead, H. G. J. Phys. Chem. 1996, 100, 15900. (16) Hild, R.; David, C.; Mu¨ller, H. U.; Vo¨lkel, B.; Kayser, D. R.; Grunze, M. Langmuir 1998, 14, 342. (17) Geyer, W.; Stadler, V.; Eck, W.; Zharnikov, M.; Go¨lzha¨user, A.; Grunze, M. Appl. Phys. Lett. 1999, 75, 2401. (18) Ja¨ger, B.; Schu¨rmann, H.; Mu¨ller, H. U.; Himmel, H.-J.; Neumann, M.; Grunze, M.; Wo¨ll, Ch.; Zeitschrift fu¨ r Phys. Chem. 1997, 202, 263. (19) Wirde, M.; Gelius, U.; Dunbar, T.; Allara, D. L. Nuc. Instrum. Methods Phys. Res. B 1997, 131, 245. (20) Zharnikov, M.; Geyer, W.; Go¨lzha¨user, A.; Frey, S.; Grunze, M.; Phys. Chem. Chem. Phys. 1999, 1, 3163. (21) Laibinis, P. E.; Graham, R. L.; Biebuyck, H. A.; Whitesides, G. M. Science 1991, 254, 981. (22) Graham, R. L.; Bain, C. D.; Biebuyck, H. A.; Laibinis, P. E.; Whitesides, G. M. J. Phys. Chem. 1993, 97, 9456. (23) Rowntree, P.; Dugal, P.-C.; Hunting, D.; Sanche, L. J. Phys. Chem. 1996, 100, 4546. (24) Olsen C.; Rowntree, P. A. J. Chem. Phys. 1998, 108, 3750.
10.1021/la991034r CCC: $19.00 © 2000 American Chemical Society Published on Web 01/22/2000
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SAMs by all types of ionizing radiation. An additional advantage of the related experiments is the exact control over the delivered dose, which can be easily achieved by a calibration of the available electron source. Low-energy electron induced damage has been studied in different SAMs, with special attention to alkanethiolates (AT) on gold substrates.18-20,23-26 Both the alkyl chains and thiolate headgroups providing an anchor to the substrate were found to be simultaneously affected by electron irradiation through the electron-induced dissociation of C-H, C-C, C-S, and Au-thiolate bonds.20 Most noticeable are loss of orientational and conformational order, partial dehydrogenation of saturated hydrocarbons and formation of CdC double bonds, desorption of film fragments, reduction of thiolate species, and the appearance of new sulfur species. Although a general understanding of the damage caused by low energy electrons in AT SAMs is available, there is a lack of information concerning the cross sections of the individual irradiation-induced processes and factors which can influence extents and rates of these processes. This information can be obtained by variation of essential parameters of AT SAMs, such as the chain length, isotopic composition, and the substrate material. We studied the modification of dodecanthiolate [CH3(CH2)11S] (DDT) and perdeuterated eicosanethiolate [CD3(CD2)19S] (PEDT) films on Au substrates and octadecanethiolate [CH3(CH2)17S] (ODT) on Au and Ag substrates by 10 eV electron irradiation. ODT/Au was taken as a reference system, whereas the other SAMs differed from ODT/Au in the length (DDT/Au) and the isotopic composition (PDET/Au) of the alkyl chains and the strength and character of the thiolate-metal bond (ODT/Ag).1 In the following we will give a brief description of the experimental procedure and setup. Thereafter, the results are presented and preliminarily discussed in Section 3. An extended analysis of the data is given in Section 4. Finally, the results are summarized in Section 5. 2. Experimental Section AT monolayers were prepared by immersion of polycrystalline gold (200 nm) and silver (100 nm) films evaporated on titaniumprimed (20 nm) polished single-crystal Si(100) wafers (Silicon Sense) in ethanolic 1 mmol thiol solution.27 DDT and ODT were obtained from Fluka Chemicals, Buchs/Switzerland. PEDT was synthesized from eicosylbromide (Cambridge Isotope Laboratories) and thiourea by the standard procedure.28 After immersion for 24 h the samples were carefully rinsed and cleaned with ethanol and blown dry with pure nitrogen. The fabricated films were irradiated with 10 eV electrons, representative for secondary electrons. The doses were estimated by multiplication of the exposure time with the current density (∼2.5 µA/cm2). The electron gun was mounted in a distance of ∼15 cm from the sample to ensure uniform illumination. The electron beam damage was monitored by near edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS) in the same UHV chamber where the electron irradiation occurred. The time for the NEXAFS/XPS characterization was selected as a compromise between the spectra quality and the damage induced by X-rays during the spectra acquisition. Both the irradiation and characterization were performed at room temperature at the base pressure better than 2 × 10-9 mbar. The experimental chamber (25) Vo¨lkel, B.; Go¨lzha¨user, A.; Mu¨ller, H. U.; David, C.; Grunze, M. J. Vac. Sci. Technol. B 1997, 15, 2877. (26) Mu¨ller, H. U.; Zharnikov, M.; Volkel, B.; Schertel, A.; Harder, P.; Grunze, M. J. Phys. Chem. B 1998, 102, 7949. (27) Strong L.; Whitesides, G. M. Langmuir 1988, 4, 546. (28) Somogyi, A Ä .; Kane, T. E.; Ding, J.-M.; Wysocki, V. H.; J. Am. Chem. Soc. 1993, 115, 5275.
Zharnikov et al. was attached to the HE-TGM 2 beamline29 at the synchrotron radiation facility BESSY-1 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 linear polarized light was varied from 90° (the E-vector in the surface plane) to 20° (the E-vector near the surface normal) to monitor the orientational order within the AT films. This approach is based on the strong dependence of the resonant photoexcitation process on the relative orientation of the light polarization and a molecular orbital of interest.30 The raw NEXAFS spectra were normalized to the incident photon flux by division through a spectrum of a clean, freshly sputtered gold sample. In the case of ODT/Ag the spectrum of a clean silver sample was subtracted from the ODT/Ag spectrum before this normalization. The energy scale was referenced to the π* resonance of graphite (HOPG) at 285.38 eV.20,26,31 The XPS measurement was 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,32 which resulted in a B. E. of 368.1 eV for the Ag 4d5/2 peak in agreement with refs 32 (368.3 eV) and 33 (367.9 eV). For both the pristine and irradiated samples a wide scan spectrum and the C 1s, O 1s, S 2p, and Au 4f (or Ag 3d) narrow scan spectra were measured. No adsorbed oxygen could be observed. The spectra were normalized to the total electron yield to correct for small differences in sample positions and X-ray source intensities34 and then fitted by using a Shirley type background35 and symmetric Voigt functions.36 The film thicknesses were derived through the intensity ratios of the C 1s and Au 4f/Ag 4d emissions37,38 and through the intensity ratios of the Au 4f emission for the AT-covered and clean gold substrate.20,39 The attenuation lengths of the C 1s and Au 4f photoelectrons were taken as 27 and 31 Å, respectively.37,38
3. Results 3.1. NEXAFS Measurements. NEXAFS measurements provide information about the density and average orientation of unoccupied molecular orbitals within the organic monolayer.30 C1s NEXAFS spectra of the pristine (bottom curves) and irradiated DDT/Au, ODT/Au, ODT/ Ag, and PEDT/Au films acquired at an X-ray incident angle of 55° are presented in Figure 1 (a), (b), (c), and (d), respectively. The adsorption signal at this particular magic angle is insensitive to the orientation of the molecular orbitals in the monolayer.30 The corresponding spectra are therefore exclusively representative for electronic structure of the investigated films. In the spectra for the pristine SAMs in Figure 1 a sharp resonance at ∼287.7 eV and two broader resonances at ∼293.4 eV and ∼301.6 eV are observed. The two latter resonances are commonly related to valence, antibonding C-C σ* and C-C′ σ* orbitals while the resonance at 287.7 eV is alternatively (29) Bernstorff, S.; Braun, W.; Mast, M.; Peatman, W.; Schro¨der, T. Rev. Sci. Instrum. 1989, 60, 2097. (30) J. Sto¨hr, NEXAFS Spectroscopy, Springer Series in Surface Science 25, Springer-Verlag: Berlin, 1992. (31) Batson, P. E. Phys. Rev. B 1993, 48, 2608. (32) 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. (33) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy; PerkinElmer Corporation: Eden Prairie, MN, 1979. (34) Frey, S.; Heister, K.; Tamada, K.; Zharnikov, M.; Grunze, M. in preparation. (35) Shirley, D. A. Phys. Rev. B 1972, 5, 4709. (36) Wertheim, G. K.; Butler, M. A.; West, K. W.; Buchanan, D. N. E. Rev. Sci. Instrum. 1974, 45, 1369. (37) Harder, P.; Grunze, M.; Dahint, R.; Whitesides, G. M.; Laibinis, P. E. J. Phys. Chemistry B 1998, 102, 426. (38) Thome, J.; Himmelhaus, M.; Zharnikov, M.; Grunze, M. Langmuir 1998, 14, 7435. (39) Bain C. D.; Whitesides, G. M. J. Phys. Chem. 1989, 93, 1671.
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Figure 1. NEXAFS spectra of pristine (bottom curves) and irradiated DDT/Au (a), ODT/Au (b), ODT/Ag (c), and PEDT/Au (d) acquired at an X-ray incident angle of 55°. The characteristic NEXAFS resonances are indicated by arrows in (a).
attributed to the excitations into pure valence orbitals,30 predominantly Rydberg states,40 and mixed valence/ Rydberg states.41 We will denote this resonance as an R* resonance but take into account a possible admixture of antibonding C-H* orbitals. The molecular orbitals related to the R* resonance are supposed to be oriented perpendicular to the alkyl chains,42-44 whereas the transition dipole moments of the orbitals corresponding to the C-C σ* and C-C′ σ* resonances are believed to be directed along the chain axis.43 Thus, the orientations of these orbitals unequivocally determine the orientation of the alkyl chains in the investigated films. The analysis45 of the respective sets of NEXAFS spectra acquired at different incidence angles of light (not shown here) gives an average alkyl chain tilt angle of ∼33° for the DDT/Au, ODT/Au, and PEDT/Au films and ∼12° for ODT/Ag in good agreement with previous results.44,46-49 A measure of the molecular orbital (40) Bagus, P. S.; Weiss, K.; Schertel, A.; Wo¨ll, Ch.; Braun, W.; Hellwig, H.; Jung, C. Chem. Phys. Lett. 1996, 248, 129. (41) Va¨terlein, P.; Fink, R.; Umbach, E.; Wurth, W. J. Phys. Chem. 1998, 108, 3313. (42) Outka, D. A.; Sto¨hr, J.; Rabe, J. P.; Swalen, J. D.; J. Chem. Phys. 1988, 88, 4076. (43) 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). (44) Ha¨hner, G.; Kinzler, M.; Thu¨mmler, C.; Wo¨ll, Ch.; Grunze, M.; J. Vac. Sci. Technol. 1992, 10, 2758. (45) Kinzler, M.; Schertel, A.; Ha¨hner, G.; Wo¨ll, Ch.; Grunze, M.; Albrecht, H.; Holzhu¨ter, G.; Gerber, Th. J. Chem. Phys. 1994, 100, 7722. (46) Ha¨hner, G.; Wo¨ll, Ch.; Buck, M.; Grunze, M. Langmuir 1993, 9, 1955.
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Figure 2. The differences of the NEXAFS spectra recorded at X-ray incident angles of 90° and 20° for pristine (bottom curves) and irradiated DDT/Au (a), ODT/Au (b), ODT/Ag (c), and PEDT/ Au (d). The positions of the characteristic NEXAFS resonances are indicated by arrows in (a).
orientation and the orientational order in the system is the linear dichroism of the NEXAFS spectra, which can be highlighted by the difference of the spectra acquired at incident angles of 90° and 20°. Such difference spectra for the pristine (bottom curves) and irradiated DDT/Au, ODT/Au, ODT/Ag, and PEDT/Au films are presented in Figure 2 (a), (b), (c), and (d), respectively. In agreement with the found orientation of the alkyl chains, positive and negative anisotropy peaks are observed for the R* and C-C/C-C′ σ* resonances, respectively. The different intensities of these peaks for the pristine films are related to both the different average tilt angles of the alkyl chains on Au and Ag and the different excitation probabilities of the corresponding NEXAFS resonances. The latter difference stems predominantly from the difference of the alkyl chain lengths and can be directly monitored by the NEXAFS spectra acquired at the magic angle of light incidence (the bottom curves in Figure 1). Generally, the excitation probability of the NEXAFS resonances and especially that of the R* resonance increases with alkyl chain length,20,44 which can be explained by a delocalization of the respective molecular orbitals over the entire alkyl chain.30 The electron bombardment results in decreasing intensity of both the NEXAFS resonances (Figure 1) and (47) Fenter, P.; Eisenberger, P.; Liang, K. S. Phys. Rev. Lett. 1993, 70, 2447. Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216. (48) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (49) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G.; J. Am. Chem. Soc. 1991, 113, 7152.
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Table 1. Amplitudes of the Anisotropy Peaks in the Difference Curves for DDT/Au (a), ODT/Au (b), ODT/Ag (c), and PEDT/Au (d) in Figure 2a DDT/Au 0 µC/cm2 1000 µC/cm2 8000 µC/cm2 a
ODT/Au
ODT/Ag
PEDT/Au
R*
C-C σ*
R*
C-C σ*
R*
C-C σ*
R*
C-C σ*
1 0.16 0
1 0.09 0
1 0.29 0
1 0.19 0
1 0.53 0.07
1 0.41 0.04
1 0.32 0.04
1 0.30 0.0
All values are normalized to the respective amplitudes for the pristine films.
Figure 3. Intensities of the π* (circles) and R* (diamonds) resonances extracted from the NEXAFS spectra of DDT/Au (a), ODT/Au (b), ODT/Ag (c), and PEDT/Au (d) in Figure 1. The values are normalized to the absorption edge height.
the respective anisotropy peaks (Figure 2) and in appearance of a new resonance at ∼285.1 eV, characteristic for CdC double bonds (π* resonance). These changes are caused by the orientational and conformational disordering, desorption of hydrogen, and carbon-containing fragments, and the appearance of CdC double bonds in the irradiated films.20,26 The CdC bonds have an isotropic orientation because no anisotropy peaks related to the π* resonance are observed in all investigated films. To compare the extent of the radiation-induced damage in the AT SAMs of this study the intensities of the π* (circles) and R* (diamonds) resonances extracted from the NEXAFS spectra in Figure 1 are presented in Figure 3. The increase of the π* resonance reflects the development of CdC bonds which is correlated to dehydrogenation, while the decrease of the R* resonances is associated with both the dehydrogenation and conformational disorder in the films. To monitor film disordering the amplitudes of the anisotropy peaks in Figure 2 can be used. These values normalized to the respective amplitudes for the pristine films are presented in Table 1. An interesting peculiarity of these normalized amplitudes for the moderately irradiated films (1000 µC/cm2) is their lesser values for the C-C σ* resonance related peaks as compared to those assigned to the R* resonance. Considering that the C-C σ* resonance is exclusively related to the C-C-C backbone, whereas an admixture of antibonding C-H orbitals
Figure 4. The C 1s XP spectra of pristine (bottom curves) and irradiated DDT/Au (a), ODT/Au (b), ODT/Ag (c), and PEDT/Au (d). The observed emission structure is fitted by a single Voigt peak (solid lines).
is an essential factor for an intense R* resonance, the stronger dehydrogenated AT chains seem to be more disordered than the lesser dehydrogenated ones, which can be in principle expected. 3.2. XPS Measurements. XP spectra provide quantitative information on the composition and thickness of the systems investigated. The C 1s and S 2p XP spectra of the neat (bottom curves) and irradiated DDT/Au (a), ODT/Au (b), ODT/Ag (c), and PEDT/Au (d) films are presented in Figures 4 and 5, respectively. The poor quality of the S 2p spectra is related to the low intensity of the respective XPS peaks and the short acquisition time which was necessary to reduce X-ray induced damage during the measurements. In the C 1s XP spectra for the pristine DDT/Au, ODT/ Au, ODT/Ag, and PEDT/Au films a single C 1s emission at ∼284.88 eV, ∼284.95 eV, ∼285.15 eV, and ∼284.92 eV, respectively, is observed. A slightly smaller binding energy (B. E.) for DDT/Au and a higher B. E. for ODT/Ag as compared to ODT/Au correlate well with previous results.50,51 The XPS-derived thicknesses of the pristine films agree well with the theoretical values obtained by
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Figure 6. Positions (circle, solid lines) and fwhm (triangles, dashed lines) of the C 1s XPS peaks for pristine and irradiated DDT/Au (a), ODT/Au (b), ODT/Ag (c), and PEDT/Au (d). The left axes of all subfigures are related to the peak position, whereas the right axes correspond to fwhm. Figure 5. The S 2p XP spectra of pristine (bottom curves) and irradiated DDT/Au (a), ODT/Au (b), ODT/Ag (c), and PEDT/Au (d). The observed emission structures are fitted by two doublets of Voigt peaks (solid lines) with fixed intensity ratio of S 2p3/2/S 2p1/2 ) 2 related to the pristine thiolate moieties and to a new sulfur species induced by electron irradiation, respectively.
multiplication of the respective intact AT chain lengths by the cosines of the relative tilt angles. In the S 2p XP spectra for the pristine AT SAMs a single S 2p3/2/S 2p1/2 doublet related to the Au-thiolate bonds is observed. This doublet can be fitted by two Voigt peaks with a fixed intensity ratio of 0.5 located at ∼162.1 eV and ∼163.3 eV (energy separation of 1.2 eV). The relative intensities of the S 2p3/2/S 2p1/2 doublets for DDT/Au, ODT/ Au, and PEDT/Au correlate with the thickness of the corresponding films. A higher intensity of this doublet for ODT/Ag as compared to the AT/Au films is related to the difference in the total electron yields (TEY) for Au and Ag (all spectra were normalized to the respective TEY, see Section 2). Both C 1s (Figure 4) and S 2p (Figure 5) XP spectra change with progressive irradiation. The position of the C 1s peak shifts toward lower B. E., the width of this peak increases, and its intensity decreases. These changes are related to the irradiation-induced formation of CdC double bonds19,20 and desorption of hydrocarbon fragments resulting in the film thickness reduction.20 The B. E. and widths (fwhm) of the C 1s XPS peaks for both the pristine and irradiated DDT/Au, ODT/Au, ODT/Ag, and PEDT/ Au films are depicted in Figure 6 (a), (b), (c), and (d), respectively. The thicknesses of these films are presented in Figure 7.
The S 2p spectra of the irradiated AT films in Figure 5 clearly demonstrate the development of a new structure which has been previously associated with the formation of disulfide18-20 or an incorporation of sulfur into the alkyl matrix through its bonding to an irradiation-induced carbon radical in the adjacent aliphatic chains.20 The evolving peaks can be fitted by a doublet (the same peaks width, intensity relation, and energy separation as for the thiolate-related doublet) with a B. E. of 163.3 eV (S 2p3/2) close to that of disulfide and alkylsulfide.19,20,52,53 A combination of thiolate and disulfide doublets allows us to fit all spectra in Figure 5. The related intensities (quadrates and triangles, respectively) and the integral S 2p intensity (circles) for the DDT/Au, ODT/Au, ODT/ Ag, and PEDT/Au films are depicted in Figure 8 (a), (b), (c), and (d), respectively. All values are normalized to the S 2p intensities for the pristine films. The desorption of carbon-containing and sulfur-containing fragments occurs in parallel. To compare these desorption channels the ratios of the integral C 1s and S 2p XP peak areas for the DDT/Au, ODT/Au, ODT/Ag, and PEDT/Au films are depicted in Figure 9 (a), (b), (c), and (d), respectively. In agreement with previous results19,20 the C1s/S2p ratio in all investigated systems varies strongly with increasing dose. This suggests that the desorption of complete AT chains is a minor process because an almost constant C1s/S2p ratio could then be expected. The behavior of the C1s/S2p ratio implies that the loss of carbon dominates the loss of sulfur at small doses and a predominate desorption of sulfur-containing units with only a few or without carbon atoms occurs for strongly irradiated samples. It should be noted, however,
(50) Biebuyck, H. A.; Bain, C. D.; Whitesides, G. M. Langmuir 1994, 10, 1825. (51) Himmelhaus, M.; Gauss, I.; Buck, M.; Eisert, F.; Wo¨ll, Ch.; Grunze, M. J. Elec. Spectrosc. Relat. Phenom. 1998, 92, 139.
(52) Rieley, H.; Kendall, G. K.; Zemicael, F. W.; Smith, T. L.; Yang, S. Langmuir 1998, 14, 5147. (53) Heister, K.; Frey, S.; Go¨lzha¨user, A.; Ulman, A.; Zharnikov, M. J. Phys. Chem. 1999, 103, 11098.
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Figure 7. Dose dependences of the DDT/Au (a), ODT/Au (b), ODT/Ag (c), and PEDT/Au (d) films thicknesses. The thicknesses were derived through the intensity ratios of the C 1s and Au 4f/Ag4d emissions (circles, solid lines) and through the intensity ratios of the Au 4f emission for the AT-covered and clean gold substrate (triangles, dashed lines). The thicknesses of the pristine and strongly (8000 µC/cm2) irradiated films are also presented as numbers along with the entire thickness reduction ∆d given with respect to the pristine film thickness d0 and as the absolute value.
Figure 8. Intensities of the S 2p XP emission structures in Figure 5 related to the pristine thiolate moieties (quadrates) and to the irradiation-induced sulfur species (triangles) as well as the integral S 2p intensity (circles) as functions of the irradiation dose. All values are normalized to the S 2p intensities for the pristine films. The data for DDT/Au, ODT/Au, ODT/Ag, and PEDT/Au are presented in (a), (b), (c), and (d), respectively.
that the entire extent of the desorption events decreases significantly with progressive irradiation. 4. Discussion All investigated AT SAMs exhibit qualitatively similar behavior with respect to low energy electron irradiation. The observed development of the NEXAFS and XP spectra agrees rather well with the previous results.18-20,26 Here we address the similarities and differences in the irradiation-induced damage for the investigated systems of different chain lengths, isotopic composition, and substrate metal. To simplify this comparison and provide a qualitative reference for later investigations the cross sections for the observed irradiation-induced processes are determined. Following the formalism of ref 24, these processes are described with a standard saturation function
I ) Isat + (Ipris - Isat) × exp(-σQ/eSirrad)
(1)
where I is the value of a characteristic film parameter in a course of irradiation, Ipris and Isat are the parameter values for the pristine and strongly irradiated AT film (a leveling off behavior), respectively, Q is the cumulative charge delivered to the surface in Coulombs, e is the electron charge, Sirrad is the area irradiated by the electron beam, and the cross section σ (expressed here in cm2) is a measure of a rate at which the saturation behavior is achieved. Equation 1 was used to fit the decaying parameters, such as the intensity of the R* resonance, the film thickness, the total and thiolate-related S 2p intensity, and the increasing (Ipris ) 0) parameters, such
Figure 9. Dose dependences of the XPS C 1s/S 2p total area ratio for DDT/Au (a), ODT/Au (b), ODT/Ag (c), and PEDT/Au (d). The corresponding spectra are presented in Figures 4 and 5.
as the π* resonance intensity and S 2p intensity related to the irradiation-induced sulfur species. The quality of
Low Energy Electron Irradiation of Alkanethiolate Monolayers
Langmuir, Vol. 16, No. 6, 2000 2703
Table 2. Cross Sections of the Electron-Irradiation Induced Processes in Units of cm2 a DDT/Au cleavage of C-H bonds/conformational defects formation of CdC bonds desorption of carbon containing fragments (thickness reduction) desorption of sulfur containing fragments reduction of the thiolate species formation of irradiation-induced sulfur species dissociation of methylene groups (from ref 24) dissociation of methyl groups (from ref 24)
ODT/Ag
PEDT/Au
(1.6 ( 0.2) × 10-16 (0.85 ( 0.1) × 10-16 (1.92 ( 0.2) × 10-16
(1.9 ( 0.2) × 10-16 (0.83 ( 0.1) × 10-16 (1.84 ( 0.2) × 10-16
ODT/Au
(1.4 ( 0.2) × 10-16 (0.77 ( 0.1) × 10-16 (1.84 ( 0.2) × 10-16
(1.9 ( 0.2) × 10-16 (0.7 ( 0.1) × 10-16 (1.92 ( 0.2) × 10-16
(0.8 ( 0.1) × 10-16 (1.8 ( 0.2) × 10-16 (1.7 ( 0.2) × 10-16 1 × 10-16 2.9 × 10-16
(0.27 ( 0.04) × 10-16 (1.8 ( 0.2) × 10-16 (1.7 ( 0.2) × 10-16
(0.15 ( 0.02) × 10-16 (0.65 ( 0.1) × 10-16 (0.65 ( 0.1) × 10-16
(0.15 ( 0.02) × 10-16 (1.44 ( 0.2) × 10-16 (1.44 ( 0.2) × 10-16
5.3 × 10-16 (for HDT/Au)
a
The presented values are a measure of a rate at which a saturation behavior is achieved. The previously24 obtained cross sections for electron stimulated dissociation of methylene and methyl groups in AT SAMs are also presented for comparison.
the fits by the simple exponential saturation function (eq 1) is rather good. The deviations from the saturation behavior, such as the observed continuous quasi-linear growth of the π* resonance and decrease of the entire S 2p intensity, were corrected by multiplication of the eq 1 by corresponding correction functions. The derived cross sections are presented in Table 2. The obtained values agree in order of magnitude with the previously24 found cross sections for electron stimulated dissociation of CH2 and CH3 groups in AT SAMs. In the following we discuss the data of Table 2 in the context of an electron-induced damage comparison for the investigated films. 4.1. Influence of the Alkyl Chain Length. Both DDT/ Au and ODT/Au are characterized by essentially the same structural arrangement and bonding to the substrate, but differ in the alkyl chain length l and a slightly larger number of the conformational defects in the shorter-chain system. The results for DDT/Au and ODT/Au are presented in subfigures (a) and (b) in Figures 1-9. The comparison of the NEXAFS spectra in Figure 1 (a,b) and the NEXAFS resonances intensities in Figure 3 (a,b) implies that DDT/ Au and ODT/Au exhibit a comparable extent of the irradiation induced dehydrogenation and CdC double bond formation. The cross sections of these processes are also very similar (Table 2). At the same time the data presented in Figure 2 (a,b) and Table 1 suggest that the orientational order in the shorter-chain systems is more sensitive to electron irradiation than in their longer-chain counterparts. This difference probably explains the larger increase of the fwhm of the C 1s peak for DDT/Au as compared to ODT/Au. The most interesting finding is essentially the same reductions of the film thickness in DDT/Au and ODT/Au with progressive irradiation (see Figure 7 (a,b) and Table 2). The decrease in film thickness for the strongly irradiated DDT/Au and ODT/Au films are rather different with respect to the initial thickness d0 (39% and 27% of d0 for DDT/Au and ODT/Au, respectively) but have the same absolute value of ∼6 Å. This result along with the respective values for hexadecanethiolate (HDT) on Au (∼7 Å)20 and ODT/Ag (∼6.5 Å) implies an independence of the absolute irradiation-induced thickness reduction on the alkyl chain length. At the same time the character of the irradiation-induced desorption processes depends on this parameter as follows from the comparison of the dose dependences of the integral S 2p intensity in Figure 8 (a,b) and the respective cross sections in Table 2. Whereas the thickness reduction and, subsequently, the extents of carbon-containing fragment desorption from DDT/Au and ODT/Au are essentially identical, the desorption of sulfurcontaining fragments from the former film is significantly stronger than from the latter one. This implies that predominantly the upper part of ODT/Au is affected by irradiation-induced desorption processes, whereas a removal of the complete AT chains is a noticeable contribu-
tion to these processes in DDT/Au. This behavior can be well understood considering an enhanced stability of the long chain AT SAMs as compared to the short chain ones.54-56 The difference in the character of the irradiation-induced desorption from the long and short chain AT SAMs explains the results of recent infrared absorption spectroscopy measurements24 where a larger decrease of the CH3-related absorption peak in longer-chain AT SAMs as compared to their shorter-chain counterparts was found for a given electron dose. Considering that the desorption processes in the former systems affect predominantly their upper part (where methyl groups are placed), whereas these processes in the latter systems are more distributed throughout the whole film, a larger desorption of CH3containing fragment can be expected in longer-chain AT SAMs. It should be noted, however, that even in shorterchain systems the upper part of the film is affected to a larger extent by electrons as compared to its bulk.24 It can be assumed,20,24 that such a uniform distribution of the bond scission events is related to a quenching of the corresponding electronically excited states, with the probability for this process decreasing with increasing separation from the metal surface. Another important process is the trapping of cut off fragments within the alkyl layer. Such trapping can conceivably occur through a bonding of these fragments to the irradiation-induced carbon radicals in the alkyl matrix or through van der Waals interaction of these fragments with the adjacent alkyl chains.57 Such a mechanism can also contribute to the observed difference in the desorption of sulfur-containing fragments from DDT/Au and ODT/Au. In connection with this, an alternative (with respect to the commonly assumed disulfide scenario) assignment of the irradiation-induced sulfur species to sulfur bonded to irradiation-induced carbon radicals in the alkyl matrix should be mentioned.20,53 4.2. Influence of the Headgroup-Substrate Interaction. To evaluate the influence of the interaction between the AT headgroups and the substrate we consider now the results for ODT/Au and ODT/Ag. These films exhibit practically the same spacing between the alkyl chains but differ in structure, lateral density, and bonding to the substrate. The latter parameter is of major importance because both the different lateral density and average tilt angle of alkyl chains in AT/Au and AT/Ag are usually related to the different arrangement of anchoring (54) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo; R. G.; J. Am. Chem. Soc. 1989, 111, 321. (55) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164. (56) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M.; Deutch, J. J. Phys. Chem. 1994, 98, 563. (57) Hutt, D. A.; Cooper, E.; Leggett, G. J. J. Phys. Chem. B 1998, 102, 174.
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thiolate groups, which subsequently results from the differences in the thiolate-Au and thiolate-Ag bonds.2,58 The results for ODT/Au and ODT/Ag are presented in subfigures (b) and (c) in Figures 1-9. A comparison of the NEXAFS spectra in Figure 1 (b,c), the NEXAFS resonances intensities in Figure 3 (b,c), the C 1s XP peak positions in Figure 6 (b,c), and dose dependences of film thickness in Figure 7 (b,c) imply that the alkyl parts of ODT/Au and ODT/Ag exhibit a similar sensitivity toward electron irradiation. Only the electron induced broadening of the C 1s XP peak in the latter system is somewhat smaller than in the former film. However, the rates of some irradiation-induced processes related to the alkyl matrix of ODT/Ag and ODT/Au are noticeably different. In particular, the orientational and conformational order on Ag persists longer in a course of electron irradiation than on Au (see Figure 2 (b,c) and Tables 1 and 2). After an irradiation with a dose of 1000 µC/cm2 the R* and C-C σ* anisotropy maxima for ODT/ Ag reduce, respectively, to 53% and 41% of their initial values, whereas the same maxima for ODT/Au make only 29% and 19% of the respective values for the pristine film. Even after an irradiation with a dose of 8000 µC/cm2 the orientational anisotropy in ODT/Ag is still observable [Figure 2 (c)]. Also, the cross section for the formation of the conformational defects in ODT/Ag is noticeably smaller than the corresponding value in ODT/Au (Table 2). The higher resistance of the orientational and conformational order in ODT/Ag may be related to both the higher lateral package density of this system (18.4 Å2/thiolate) as compared to ODT/Au (21.4 Å2/thiolate), and a different response of the Au-thiolate and Ag-thiolate interfaces to electron irradiation. The data presented in Figure 5 (b,c), Figure 8 (b,c), and Table 2 imply a smaller desorption rate of sulfurcontaining fragments and a significantly smaller rate of formation of the irradiation-induced sulfur species in ODT/ Ag than in ODT/Au. Whereas the formation of these species in ODT/Au occurs rather rapidly in the early stages of irradiation and approaches ∼70% of the overall sulfur at 8000 µC/cm2, the formation of disulfide or alkylsulfide species in ODT/Ag is significantly slower and reaches only ∼27% of the overall sulfur. Considering these differences for the S-metal interface, but a similar irradiationinduced damage of the alkyl matrix of ODT/Ag and ODT/ Au, we conclude that the respective processes in AT SAMs are practically not correlated. A higher stability of the thiolate-substrate bonds for ODT/Ag results merely in a better anchoring of the residual AT chains to the substrate which subsequently leads to a noticeably better retention of the orientational order in ODT/Ag than in ODT/Au. The different rates of irradiation-induced sulfur species formation in ODT/Ag and ODT/Au are presumably related to the different strengths of thiolate-substrate bonding for these two systems.49,59 This was considered60 to be the main reason for different character of thermal desorption of ethanethiol from Ag(110) and Au(110), where sulfurcontaining species were released from Au, whereas on Ag an almost complete retention of sulfur occurred. Moreover, the desorption cross section of sulfur species from HDT on Au with respect to 800 eV He+ ion bombardment was found to be several times larger than that for HDT/Ag.61 (58) Sellers, H.; Ulman, A.; Scnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389. (59) Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2370. (60) Jaffey, D. M.; Madix, R. J. Surf. Sci. 1994, 311, 159. (61) Chenakin, S. P.; Heinz, B.; Morgner, H. Surf. Sci. 1999, 421, 337.
Zharnikov et al.
An increased strength of thiolate-substrate bond for AT/Ag as compared to AT/Au is supposed to result in a weaker S-X bond (X is the alkyl chain) for the latter system.58,60 This means that the irradiation-induced C-S bond cleavage should be more favored for AT/Ag as compared to AT/Au. Considering the similar irradiationinduced damage of the alkyl matrixes of ODT/Ag and ODT/ Au, we can assume that the C-S-bond cleavage channel is of a secondary importance with respect to the entire AT SAM damage. 4.3. Isotopic Variation of the AT Chain Composition. An important irradiation-induced process in AT SAMs is the cleavage of C-H bonds.20,23,24 This process can be affected by deuteration of the alkyl chains. The substitution of the hydrogen atoms in CH2 and CH3 entities by deuterium should result in a change of the respective vibrational frequencies and, subsequently, in a change of the spectra of the electronically excited states responsible for the C-H/C-D bond cleavage in the case of electron stimulated desorption. Semiempirical estimates24 suggest large isotropic variations in the irradiation-induced dehydrogenation process, with deuterated species being noticeably less sensitive to electron-induced reactions than their hydrogen-containing counterparts. As to the pristine ODT and PEDT films, the interchain van der Waals interaction and the strength of C-H/C-D bond should not be noticeably affected by the H/D substitution because these parameters are determined by the electronic structure which is essentially the same for hydrogen and deuterium. In fact, SAMs formed from ODT and PEDT on Au seem to differ only insignificantly [see the bottom curves in Figure 1, 2, 5 (b,d)]. The observed differences, such as slightly higher intensities of the R* resonance and the respective anisotropy maxima for the latter system as compared to the former one can be related to the chain length effects. Despite the expectations24 the low-energy irradiationinduced changes in ODT/Au and PEDT/Au have similar extents and rates [Tables 1 and 2 and Figures 1-9 (b,d)]. A slightly smaller value of the cross section for the formation of the CdC double bonds in PEDT/Au as compared to ODT/Au may be related to a lesser sensitivity of deuterated AT toward electron irradiation as compared to its hydrogen-containing counterparts, but the related difference is rather small. An interesting peculiarity is the somewhat smaller values of the cross sections for the irradiation-induced reduction of the thiolate species and formation of new sulfur species in PEDT/Au as compared to ODT/Au. A slightly different reactivity of the alkyl matrix of these two systems to the electron irradiation is probably insufficient to explain this finding. 5. Summary The low-energy electron induced damage in DDT/Au, ODT/Au, PEDT/Au, and ODT/Ag was studied in situ by XPS and NEXAFS spectroscopy. ODT/Au was chosen as a reference system, to be compared to three other films with different length, and isotopic composition of the alkyl chains and strength of thiolate-substrate bond. All investigated AT SAMs exhibit qualitatively similar behavior with respect to low energy electron irradiation. In agreement with previous results20 both the alkyl chains and the S-Au interface are affected through the electroninduced dissociation of C-H, C-C, C-S, and Au-thiolate bonds. The most noticeable processes are the loss of the orientational and conformational order, partial dehydrogenation with CdC double bond formation, desorption of
Low Energy Electron Irradiation of Alkanethiolate Monolayers
the film fragments, reduction of the pristine thiolate species, and the appearance of new sulfur species. Considering that all these processes evolve rather rapidly in the early stages of irradiation and level off with further treatment, the cross sections for the rates at which the respective saturation behavior is achieved have been determined. For DDT/Au, ODT/Au, and PEDT/Au all irradiation-induced processes are found to evolve with similar rates, except for the formation of CdC double bonds and the desorption of sulfur-containing fragments. The derived values agree well with the previously24 obtained cross section for electron stimulated dissociation of methylene and methyl groups in AT SAMs on gold. The reaction of the alkyl matrix and the S-substrate interface to electron irradiation are found to be not directly correlated. Both the extent and rate of the thiolate species reduction and new sulfur species formation are mainly determined by the strength and character of the thiolatesubstrate bond: The electron irradiation induced damage on the S-metal interface in ODT/Ag is significantly reduced compared to that in DDT/Au, ODT/Au, and PEDT/ Au. A profound irradiation-induced desorption of sulfurcontaining fragments was found only in DDT/Au. In the longer-chain AT SAMs this process is suppressed by a trapping of sulfur-containing fragments within the thicker alkyl layer. Generally, predominantly the upper part of
Langmuir, Vol. 16, No. 6, 2000 2705
AT SAMs is affected by the irradiation-induced desorption process. The dehydrogenation, formation of CdC double bonds, and desorption of the carbon-containing fragments are found to have similar extents and rates for all investigated systems. A higher stability of the orientational and conformational order in ODT/Ag as compared to the films on Au can be related to the larger lateral density and a stronger anchoring to the substrate in the former system. No large isotopic effect in the irradiation-induced dehydrogenated process was observed. Deuterated AT SAMs are found to be only slightly less sensitive to electroninduced reactions than their hydrogen-containing counterparts. Acknowledgment. We would like to thank the BESSY staff, especially M. Mast for technical help, J. Pipper for synthesis of PEDT, G. Albert for the substrate preparation, and Ch. Wo¨ll (Universita¨t Bochum) for providing us with experimental equipment. This work has been supported by the German Bundesministerium fu¨r Bildung, Wissenschaft, Forschung und Technologie through grants No. 05 SF8VHA 1 and 05 SL8VHA 2 and by the Fonds der Chemischen Industrie. LA991034R
