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Langmuir 2001, 17, 8-11
Characterization of X-ray Induced Damage in Alkanethiolate Monolayers by High-Resolution Photoelectron Spectroscopy K. Heister, M. Zharnikov,* and M. Grunze Angewandte Physikalische Chemie, Universita¨ t Heidelberg, Im Neuenheimer Feld 253, D-69120 Heidelberg, Germany
L. S. O. Johansson Department of Physics, Karlstad University, S-65188 Karlstad, Sweden
A. Ulman Department of Chemistry and the NSF MRSEC for Polymers at Engineered Interfaces, Polytechnic University, Brooklyn, New York 11201 Received July 31, 2000. In Final Form: October 23, 2000 Synchrotron-based high-resolution X-ray photoelectron spectroscopy was for the first time applied to investigate the damage in self-assembled monolayers (SAMs) of alkanethiols (AT) on Au caused by soft X-rays. The observed changes in AT SAMs and, in particular, the appearance of a new, irradiation-induced sulfur species are identical to those caused by electron bombardment, implying that most of the damage is produced by the photoelectrons and secondary electrons. The irradiation-induced sulfur species is identified as a dialkyl sulfide distributed within the AT film. Only minutes of monochromatized X-ray irradiation at an undulator beamline destroys the AT adlayer completely.
Self-assembled monolayers (SAMs) are close-packed arrays of amphiphilic molecules, in which the headgroup of an adsorbate covalently bonds to a solid substrate, while the chainlike molecular tail sticks out from the substrate. By a proper molecular design these systems provide a way to tailor surface properties, such as wetting, adhesion, lubrication, and corrosion. Therefore, during the past 2 decades SAMs have moved into the center of interest for scientists in physics, chemistry, and biology.1 The high attraction of these adsorption systems is also driven by the simple preparation procedure, the reproducible film quality, and the long-time stability. An important practical issue in the research of SAMs is their response to irradiation by X-rays, UV light, electrons, or ions.2-11 Irradiation-induced film modification can be used as a lithographic approach to pattern surfaces. SAMs may be applied either as a positive or a negative resist or as a chemical template for a lateral structur* To whom correspondence may be addressed. Tel: +49-622154 4921. Fax: +49-6221-54 6199. E-mail: Michael.Zharnikov@ urz.uni-heidelberg.de. (1) Ulman, A. Chem. Rev. 1996, 96, 1533. (2) Laibinis, P. E.; Graham, R. L.; Biebuyck, H. A.; Whitesides, G. M. Science 1991, 254, 981. (3) Seshadri, K.; Froyd, K.; Parikh, A. N.; Allara, D. L.; Lercel, M. J.; Craighead, H. G. J. Phys. Chem. 1996, 100, 15900. (4) 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. (5) Wirde, M.; Gelius, U.; Dunbar, T.; Allara, D. L. Nucl. Instrum. Methods Phys. Res. B 1997, 131, 245. (6) Mu¨ller, H. U.; Zharnikov, M.; Volkel, B.; Schertel, A.; Harder, P.; Grunze, M. J. Phys. Chem. B 1998, 102, 7949. (7) Zerulla, D.; Chasse, T. Langmuir 1999, 15, 5285. (8) Olsen, C.; Rowntree, P. A. J. Chem. Phys. 1998, 108, 3750. (9) Zharnikov, M.; Frey, S.; Go¨lzha¨user, A.; Geyer, W.; Grunze, M. Phys. Chem. Chem. Phys. 1999, 1, 3163. (10) Heister, K.; Frey, S.; Go¨lzha¨user, A.; Ulman, A.; Zharnikov, M. J. Phys. Chem. B 1999, 103, 11098. (11) Zharnikov, M.; Frey, S.; Heister, K.; Grunze, M. Langmuir 2000, 16, 2697.
ing.12-15 Further, a possible nonintentional irradiationinduced damage has always to be considered using standard spectroscopic techniques. The best investigated SAMs regarding their modification by X-rays and electron irradiation are alkanethiolates (AT) on gold (111) substrates.4-11 It was shown that irradiation causes the loss of the orientational and conformational order, partial dehydrogenation leading to CdC double bond formation, desorption of the film fragments, reduction of the thiolate species, and the appearance of new sulfur species.4,5,9,11 The latter species have been commonly assigned to a disulfide moiety,4,5,7 which is supported by the coincidence of the respective binding energy (BE) of the S 2p emission with that for sulfur in diaminodiphenyl and dialkyl disulfides5,16 and by predominant dialkyl disulfide desorption upon heating of AT SAMs.17-19 However, recent X-ray photoelectron spectroscopy/near edge X-ray absorption fine structure (XPS/NEXAFS) studies proposed an alternative interpretation, namely, an incorporation of sulfur into the alkyl matrix via bonding to irradiation-induced carbon radicals in the adjacent aliphatic chains.9,10 (12) Lercel, M. J.; Craighead, H. G.; Parikh, A. N.; Seshadri, K.; Allara, D. L. Appl. Phys. Lett. 1996, 68, 1504. (13) Hild, R.; David, C.; Mu¨ller, H. U.; Vo¨lkel, B.; Kayser, D. R.; Grunze, M. Langmuir 1998, 14, 342. (14) Geyer, W.; Stadler, V.; Eck, W.; Zharnikov, M.; Go¨lzha¨user, A.; Grunze, M. Appl. Phys. Lett. 1999, 75, 2401. (15) Eck, W.; Stadler, V.; Geyer, W.; Zharnikov, M.; Go¨lzha¨user, A.; Grunze, M. Adv. Mater. 2000, 12, 805. (16) Go¨lzha¨user, A.; Panov, S.; Schertel, A.; Mast, M.; Wo¨ll, Ch.; Grunze, M. Surf. Sci. 1995, 334, 235. (17) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (18) Nishida, N.; Hara, M.; Sasabe H.; Knoll. W. Jpn. J. Appl. Phys. 1996, 35, L799. (19) Kondoh, H.; Kodama, C.; Nozoye, H. J. Phys. Chem. B 1998, 102, 2310.
10.1021/la001101d CCC: $20.00 © 2001 American Chemical Society Published on Web 12/09/2000
Letters
Langmuir, Vol. 17, No. 1, 2001 9 Scheme 1
The goal of the present study is to investigate the X-ray induced damage in AT SAMs on gold and to clarify the nature of the irradiation-induced sulfur species by means of synchrotron-based high-resolution X-ray photoelectron spectroscopy. The use of this technique not only leads to a very high spectral resolution but allows a tuning of the photon energy to vary the surface sensitivity and to maximize the photoionization cross section of the relevant core levels, which is, in particular, crucial to the characterization of the SAM-Au interface.20 We investigated SAMs formed from C12 and C16 ATs [Cn: CH3-(CH2)n-1-SH] on gold substrates. The identification of the chemical state of the irradiation-induced sulfur species was performed using reference samples with either a dialkyl disulfide or sulfide moiety (denoted as disulfide and sulfide below), namely, a SAM of 11(hexylmercapto)undecane-1-thiol [HMUT, CH3(CH2)5S(CH2)11SH], a film of undec-10-ene-1-thiol [UDET, CH2d CH(CH2)9SH], and a bulk sample of dihexadecane disulfide [DHDS, CH3(CH2)15SS(CH2)15CH3]. The gold substrates were prepared by evaporation of 100-300 nm of gold on mica or titanium-primed (20 nm) polished single-crystal Si(100) wafers. Such predominantly (111) textured films21 are commonly used as substrates for AT SAMs. The AT and HMUT SAMs as well as the UDET film were formed by immersion of the gold substrates for 24 h in a 1 mM ethanolic solution of the respective substances. The UDET film was formed by the polymerization of the UDET molecules through a reaction between the CH2dCH and the SH entities according to Scheme 1. For the DHDS reference sample several different preparation methods were used: The substance was pressed on the surface of a scraped indium sample or a special stick pad or dissolved (with a very high concentration) in a solvent and dried on the gold substrate. The high resolution (HR)-XPS measurements were performed at the synchrotron storage ring MAX II in Lund, Sweden, using the D1011 bending magnet beamline and the I311 undulator beamline, which has a significantly higher brilliance and a better energy resolution than the D1011. Both beamlines are equipped with a Zeiss SX-700 plane-grating monochromator and a two-chamber ultrahigh vacuum (UHV) experimental station with a SCIENTA analyzer resulting in an overall energy resolution better than 0.1 eV for the spectra presented here. Some of the measurements on the reference DHDS samples were carried out with a Leybold LHS-12 spectrometer and a nonmonochromated Al KR X-ray source with an energy resolution of ∼1 eV. In the case of HR-XPS, the energy calibration was performed separately for every acquired spectrum to avoid effects related to the instability of the monochromators. The scale was referenced to the pronounced Au 4f7/2 bulk peak at 83.93 eV,20 which was several times independently referenced to the Fermi edge of a clean Pt foil. In the case of DHDS, the energy scale of the S 2p spectra was referenced to the pronounced C 1s XP (20) Heister, K.; Johannson, L. S. O.; Zharnikov, M.; Grunze, M. In preparation. (21) Ko¨hn, F. Diploma Thesis, Universita¨t Heidelberg, Germany, 1998.
Figure 1. The C 1s and S 2p HR-XP spectra of the pristine and strongly irradiated C12 SAMs. The decomposition of the spectra into the components related to the pristine and irradiation-induced species is shown.
peak of the alkyl chain at 284.90 eV (typical for SAMs formed from long-chain AT on Au). The irradiation was performed at standard measurement conditions using monochromatized (150-230 eV) synchrotron radiation. The XP spectra were fitted using a Doniach-Sunjic peak profile with a parabolic background. 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 standard22 spin-orbit splitting of ∼1.2 eV (verified by fit), and the branching ratio of 2 (S 2p3/2/S 2p1/2). The resulting accuracy of the most values given in this Letter is 0.01-0.02 eV. The C 1s and S 2p HR-XP spectra for both the pristine and X-ray irradiated C12/Au are shown in Figure 1. The spectra of the pristine SAM show a single C 1s peak at 284.87 eV and a single S 2p doublet with the S 2p3/2 peak at 161.90 eV. These values are characteristic for undamaged aliphatic chains and thiolate headgroups bonded to the substrate.20 X-ray irradiation results in a desorption of the film fragments (see the increase of the integral Au 4f intensity in Figure 2a) and the same changes of the C 1s and S 2p spectra as at the exposure to low-energy electrons,9,11 which support previous assumptions2,4,9 that not the photons themselves but the subsequent photoelectrons and secondary electrons play the dominant part in the X-ray related damage. In the C 1s spectrum, the irradiation causes a downward shift and broadening of the C 1s peak. This peak can be deconvoluted in two components at BEs of 284.89 and 284.38 eV, which are attributed to CsC and CdC bonds, respectively. Although these components are not resolved in the C 1s spectrum, their existence is indicated by an obvious asymmetry of the joint C 1s peak. Note, that the fwhm of the fitting peaks for the strongly irradiated C12 film is by ∼30% larger than that of the C 1s peak for the pristine film, which is obviously related to the larger chemical and structural inhomogeneity of the former. In the same manner, the fwhm of the thiolate-related S 2p3/2,1/2 peaks continuously increases with progressive X-ray irradiation (Figure 2c) revealing a growing inhomogeneity of the S-Au interface. Such an inhomogeneity and, in particular, irradiation-induced scission of the S-Au bonds results also in an increase of the fwhm of the Au 4f7/2,5/2 peaks as (22) Moulder, J. F.; Stickle, W. E.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corp.: Eden Prairie, MN, 1992.
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Letters
Figure 3. Comparison of the S 2p HR-XP spectra of the pristine and strongly irradiated C12 SAMs with the respective spectra of several samples with a well-defined sulfur identity. The spectrum for DHDS was taken with laboratory XPS equipment with a corresponding poor energy resolution. Figure 2. Evolution of the integral Au 4f intensity (a) and the fwhm’s of the Au 4f7/2,5/2 (b) and the S 2p3/2,1/2 (c) peaks during progressive X-ray irradiation.
demonstrated in Figure 2b. This broadening can be understood based on a very recent HR-XPS study of the SAM-Au interface, where the separation between the Au 4f surface and bulk components for the clean Au surface was found to be significantly larger than that for the ATcovered gold (0.31 vs 0.05 eV).20 So the overall Au 4f fwhm increases with the dissociation of S-Au bonds and can be used as a fingerprint for the surface coverage. The most pronounced change in the S 2p spectrum (Figure 1) is, however, the appearance of an additional S 2p doublet, which is assigned to a new irradiation-induced sulfur species.4,5,9 To reveal the exact nature of this species we compared the S 2p HR-XP spectra of the pristine and strongly irradiated C12/Au with the respective spectra of the HMUT SAM, UDET films, and bulk DHDS having well-defined sulfur moieties (Figure 3). The two former reference samples incorporate sulfide moieties, whereas the latter one contains disulfide entities. Note, that the HMUT and UDET films also contain the thiolate species, but due to the high surface sensitivity and the larger layer thickness as compared to the C12/Au, the corresponding S 2p doublet at ∼162 eV is very weak (HMUT) or even not visible (UDET). At the same time, the sulfide-related S 2p emission for the HMUT SAM is observed at 163.22 eV, while the analogous S 2p doublet for the UDET film is exhibited at 163.3 eV. This slight BE difference is presumable related to the different screening of the S 2p photoemission hole by the substrate electrons because of the different distances of the probed sulfide entities to the Au substrate. The DHDS samples exhibit only one S 2p doublet at ∼163.5 eV (S 2p3/2), which differs from previously reported values of 163.2 eV obtained for thick films of 4,4′diaminodephenylsulfide on Cu(110)16 and dioktadecane disulfide pressed onto the surface of an indium sample.5
We have not reproduced these values for any of the different sample preparation procedures (see above) and different experimental setups used. At the same time, our results are supported by the observation of the somewhat higher BE (by ∼0.4 eV in line with 0.2-0.3 eV in this study) of the S 2p emission for dialkyl disulfide as compared to dialkyl sulfide for the bulk substances containing these moieties.23 The BE of the S 2p emission for the disulfide sample is then with 163.5 eV (S 2p3/2) noticeably higher than that for the irradiation-induced species (163.14 eV), whereas the BEs of the S 2p emissions for the sulfide references are very close to it. This implies that the irradiationinduced sulfur species is most likely a dialkyl sulfide. A slightly smaller BE of the S 2p emission for the irradiated C12/Au as compared to the HMUT and UDET films can be related to the different screening of the photoemission S 2p hole by the substrate electrons associated with a larger separation of the probed sulfide entities in these films from the substrate. In this context the question of the location of the irradiation-induced sulfur species within the irradiated C12 film arises, which is clarified by Figure 4a where the S 2p spectra of an irradiated C12/Au for takeoff angles of 60° (upper plot) and 0° (bottom spectrum) are shown. At a takeoff angle of 60° the intensity ratio of the S 2p doublets related to the thiolate and the irradiation-induced species is significantly lower than that at normal emission. Since the grazing emission geometry is more surface sensitive, the irradiation-induced sulfur species are on the average located closer to the SAM surface than the thiolate moieties at the S-Au interface, which means that the former species are distributed over the alkyl matrix. This finding was supported by a series of the S 2p spectra (not shown) acquired at varying photon energy between 190 and 390 eV. In these spectra a (23) Siegbahn, K.; Nordling, C.; Fahlman, A.; Nordberg, R.; Hamrin, K.; Hedman, J.; Johansson, G.; Bergmark, T.; Karlsson, S.-E.; Lindgren, I.; Lindberg, B. Nova Acta Regiae Soc. Sci. Ups. 1967, IV, 118.
Letters
Figure 4. (a) The S 2p HR-XP spectra of strongly irradiated DDT SAM taken at photoelectron takeoff angles of 0° and 60° (a special care was taken to hit the same place). The intensity ratio of the S 2p doublets related to the thiolate and the irradiation-induced sulfur species is given. (b) A variation of the intensity ratio between the S 2p doublets related to the thiolate and the irradiation-induced sulfur species as a function of photon energy.
variation of the intensity ratio between the S 2p doublets related to the thiolate and the irradiation-induced sulfur species is observed as a function of photon energy, showing a clear minimum at ∼230 eV (Figure 4b). Considering that the electron mean free path reaches its minimum value at the corresponding kinetic energy of ∼65 eV and the surface sensitivity is thereby maximized, this observation is a further verification of the distribution of the irradiation-induced sulfur species over the AT film. This finding supports the dialkyl sulfide assignment for the irradiation-induced sulfur species because the formation of C‚‚‚S-C links after the cleavage of the S-Au bond can occur over the entire film. Note, that the respective trapping of the alkylthiolate fragments in the alkyl matrix was considered earlier as the main factor preventing the desorption of AT molecules after the irradiation-induced S-Au bond cleavage.11 The HR-XP spectra in Figures 1, 3, and 4 were acquired either with the bending magnet beamline or with a very small exit slit of the I311 undulator beamline. For comparison, the successively recorded S 2p spectra of C16/ Au taken with the smallest but one standard exit slit of I311 (23 µm) are shown in Figure 5. These spectra were measured with a time per spectrum of 105 s, a photon flux of ∼2 × 1011 photons/s, and a spot size of 0.05 mm2 (photon fluxes and spot sizes for both beamlines are rough estimates). As seen in Figure 5, a noticeable film degra-
Langmuir, Vol. 17, No. 1, 2001 11
Figure 5. The S 2p HR-XP spectra of C16/Au successively acquired at the I311 undulator beamline with one of the smallest exit slits.
dation starts already during the first two scans and progresses very fast afterward. This example clearly demonstrates that extreme care should be taken if an undulator beamline is used for the SAM characterization. The I311 beamline of this study has a comparable brilliance to other undulator beamlines at third-generation synchrotron light sources over the world. In summary, synchrotron-based HR-XPS was applied to study the X-ray induced damage in AT SAMs. This damage was found to be identical to that induced by a low-energy electron beam, which is a further support of the major role of the photoelectrons and secondary electrons resulting from the photoionization process in the X-ray related AT film modification. The irradiationinduced sulfur species was identified as dialkyl sulfide moiety associated with C‚‚‚S-C links between radicalized AT fragments. These moieties were found to be not exclusively located in the vicinity of the SAM-substrate interface but distributed over the AT film. The results show that extreme care has to be taken with an undulator beamline as the X-ray source to avoid sample damaging during the measurement. The S 2p XP emission can be used as a fingerprint to monitor the film damage. Acknowledgment. We thank the DAAD and BMBF for the financial support, G. Albert for preparing the Au substrates, D. L. Allara and J. Pipper for providing us with the DHDS and UDET substances, D. L. Adams for supplying us with his fitting program FitXPS 1.06, and the MAX-LAB crew for a friendly support during beamtime. A.U. thanks the Alexander von Humboldt Foundation for supporting his stay in Heidelberg. LA001101D
