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Monolayer Damage in XPS Measurements As Evaluated by Independent Methods Eli Frydman,† Hagai Cohen,‡ Rivka Maoz,† and Jacob Sagiv*,† Department of Materials & Interfaces and Chemical Services Unit, The Weizmann Institute of Science, Rehovot 76100, Israel Received November 29, 1996. In Final Form: June 18, 1997X The damage caused to amphiphilic n-alkane monolayers under XPS measurement conditions was assessed in a combined XPS-FTIR study supplemented by additional AFM imaging and contact angle measurements. Nine different self-assembled monolayer/substrate systems were examined, comprising a long chain silane (C18, OTS), a short chain silane (C1, MTS), a functional (COOH-terminated) long chain silane (C18, NTSox), a long chain carboxylic acid (C20, AA), and four different solid substrates (silicon, quartz, glass, and ZnSe). Significant differences were observed in the behavior of the various examined monolayer systems under identical X-ray irradiation conditions. These are interpreted in terms of effects associated with the specific mode of layer-to-surface and intralayer coupling, the size of the monolayer hydrocarbon core, and the presence of radiation-sensitive functional groups in the layer. All these factors and their influence on the degradation path followed by a particular monolayer upon exposure to the X-rays were found to be interrelated, giving rise to a variety of possible damage patterns, including an unexpected overall stabilization effect initiated by the preferential rapid loss of a labile top functional group (NTSox). XPS is shown to be insufficient as a tool for the evaluation of the radiation-induced damage in such ultrathin films, because of its insensitivity to loss of hydrogen and to structural transformations that occur without a net loss of carbon from the surface. Independent methods of surface analysis (mainly FTIR), applied in conjunction with XPS, provide a more comprehensive picture of the induced damage, thus permitting a realistic interpretation of the XPS experimental data as well as the design of improved data acquisition procedures. This could also assist in the tailoring of monolayers with predetermined degradability, for specific purposes. Finally, results of combined AFM-XPS-FTIR-contact angle measurements suggest the possible formation of a “diamond-like” surface film upon extensive X-ray irradiation of an OTS/Si monolayer.
Introduction The recent growing interest in organized molecular films, and the availability of advanced surface analysis techniques, led to numerous studies of solid-supported organic mono- and multilayer systems involving XPS (Xray photoelectron spectroscopy) as a principal tool of investigation.1-30 XPS has been used to provide qualitative as well as detailed quantitative information about the composition and structure of various such films. Depending on the type of information desired, XPS data have been acquired with X-ray exposures ranging from a few minutes to several hours. During such measurements, thin molecular films are susceptible to various chemical and structural transformations caused by the combined action of the X-ray photons, emitted photoelectrons, and ultrahigh vacuum on the organic material. Briggs and Seah31a reported a number of in situ studies of X-ray-induced degradation of polymer surfaces, and Beamson and Briggs31b provided a comprehensive XPS database of various polymers, including a “degradation index” for each polymer along with a discussion of its mode of evaluation. Despite the great general value of this information, its relevance to ultrathin films is rather limited and may even be misleading, the main problems associated with such data being the following: (i) Measurements performed on thick polymeric materials (as those reported in ref 31) tend to underestimate the damage caused to the uppermost polymer surface by the radiation, * To whom correspondence should be addressed: Telephone: 972-08-9342309. Fax: 972-08-9344138. E-mail: bpherut@ weizmann.weizmann.ac.il. † Department of Materials & Interfaces. ‡ Chemical Services Unit. X Abstract published in Advance ACS Abstracts, August 15, 1997. (1) Ulman, A. An Introduction to Ultrathin Organic Films From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991; and references therein.
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as its deterioration frequently reveals deeper, less damaged layers. (ii) The reported degradation indices usually refer to XPS-detected changes in the atomic composition of the examined samples, such as, for example, the fall, upon irradiation, of the Cl/C ratio in some chlorinecontaining polymers.31b Any damage process that does not result in significant changes in the polymer stoichiometry may thus remain unnoticed. This is well demonstrated by Wheeler and Pepper for the case of polytetrafluoroethylene32a and by Buchwalter and Czornyj for poly(methyl methacrylate).32b Obviously, the limited amount of material available for XPS sampling in an organized organic monolayer and the depth-dependent distribution of the different atomic constituents of such films pose much more stringent requirements on their chemical/structural stability during the collection of the XPS data, if the results are to faithfully reflect the real composition and architecture of each examined specimen. Rather surprisingly, the problem of degradation of ultrathin organic films under the XPS measurement conditions has only sporadically been addressed, which encouraged the growing acceptance of XPS as a routine analytical tool for the study of such films, frequently overlooking its possible destructive aspects. Kobayashi, Takaoka, and Ochiai2 used XPS and Fourier transform infrared spectroscopy (FTIR) to study Langmuir-Blodgett (LB) monolayers of eicosanoic acid and its Cd2+, Ca2+, Ba2+ and Pb2+ salts. They reported a decrease in the XPS carbon signal, the decay being faster in the acid case (ca. 40% in 3 h, using a nonmonochromatized Mg KR X-ray source) than in the salt case (ca. 10% in 3 h for the Ca2+ salt). The decrease was ascribed to both irradiation (rupture of molecules) and vacuum effects (acid desorption). No IR data were collected from the X-rayirradiated samples. (2) Kobayashi, K.; Takaoka, K.; Ochiai, S. Thin Solid Films 1988, 159, 267.
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Nakayama et al.3 used angle-resolved XPS (Mg KR nonmonochromatized source operated at a power lower than 240 W) to study the structure of a four-layer arachidic acid LB film. In order to avoid sample damage, short (3 min) acquisition times were used, and each take-off angle measurement was conducted on a different sample. No thickness changes could be detected elipsometrically in one representative sample, before and after the XPS measurement, from which it was concluded that the films were stable under the XPS measurement conditions. Wasserman, Tao, and Whitesides4 carried out an extensive XPS study of various top-functionalized selfassembled (SA) silane monolayers on oxidized silicon using a monochromatized Al KR X-ray source. They reported on the lability of bromine, in a monolayer with brominated terminal vinyl groups, on exposure to the X-rays, but did not provide data regarding the fate of other monolayer functional groups (containing oxygen and fluorine) under the XPS measurement conditions. The possible damaging of such outer exposed functional groups during these measurements is suggested by the reported insensitivity of the angle-resolved XPS spectra of COOH-terminated monolayers to variations in the take-off angle from 90° to 15°. Nuzzo, Dubois, and Allara5 used XPS to study selfassembled monolayers of thiols on gold. They reported extreme sensitivity of their systems to the X-rays (presumably nonmonochromatized), at the extent that although the X-ray flux was kept as low as signal-to-noise constraints allowed, decomposition of the films prevented a quantitative interpretation of the experimental data. Kinloch et al.6 used angle-resolved XPS to study the structure of LB monolayer and bilayer films of 22tricosanoic acid deposited from an aqueous subphase containing Cd2+ ions. Al KR X-rays were used at 280 W power, with total data acquisition times of ca. 5 h. Monitoring of possible sample degradation was done by acquiring the low-angle XPS spectra both at the beginning and at the end of the experiment. Although variations of up to only 5% in the intensities of the different elements were observed, data analysis was problematic and the modeling required introduction of interfacial and surface contamination layers, combined with substrate contributions to all layers. No cadmium-rich layer could be modeled at the expected interlayer sites, the detected low cadmium signal being, apparently, due to cadmium ions present at the outermost surface of the film. Whitesides and co-workers investigated the X-ray damage caused to CF3CO2-7a and CF3CONH-terminated7b monolayers on various substrates. They reported a substantial decrease in the F signal, which was shown to be largely dependent on the electron emission properties of the substrate, being more pronounced in monolayers on substrates with strong electron emission (e.g. Au) than on substrates with weaker such emission (e.g. Si). It was thus suggested that the damage is mainly caused by the emitted photoelectrons rather than the incoming X-ray photons. The observed correlation between the loss of F and O in the CF3CONH system further suggests that the loss of F may arise from cleavage of entire CF3CO units.7b (3) Nakayama, Y.; Takahagi, T.; Soeda, F.; Ishitani, A. J. Colloid Interface Sci. 1989, 131, 153. (4) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074. (5) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (6) Cave, N. G.; Cayless, R. A.; Hazell, L. B.; Kinloch, A. J. Langmuir 1990, 6, 529. (7) (a) Laibinis, P. E.; Graham, R. L.; Biebuyck, H. A.; Whitesides, G. M. Science 1991, 254, 981. (b) Graham, R. L.; Bain, C. D.; Biebuyck, H. A.; Laibinis, P. E.; Whitesides, G. M. J. Phys. Chem. 1993, 97, 9456.
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Heens et al.8 reported a small decrease (ca. 5%) in the XPS carbon signal of arachidic acid and poly(vinyl stearate) LB mono- and multilayers upon several hours of exposure to a monochromatized X-ray beam at 50 W power. The decrease was explained as a result of evaporation of some hydrocarbon species due to rupture of the molecules. Rieke et al.9 studied X-ray and electron beam effects on silane monolayers with a terminal Br or methyl group on silicon. A nonmonochromatic X-ray source was used at 300 W power. The damage was monitored using the XPS signal itself or contact angle measurements. A strong decay of the Br signal was observed, while the carbon signal became broader but did not decrease significantly. Cooling the samples did not change the observed behavior; however, different rates of decay were found in two different spectrometers. This effect was ascribed to the fact that different nonmonochromatic X-ray sources emit different fluxes of low-energy electrons, depending on the type and history of the source, which cause some of the observed damage. Electron beams with an energy of 600 eV were found to cause maximum damage to the monolayer, but the damage manifested itself as a conversion of the hydrocarbon phase into a more electron beam resistant material rather than loss of carbon from the layer. On the basis of a comparison of the electron beam energies causing the onset of damage and maximum damage, and the ionization energy of carbon, the authors suggested a degradation mechanism initiated by carbon ionization and followed by desorption of carbon and/or rearrangements of carbon bonds to form a graphite-like carbonaceous material. Bierbaum et al.10 studied self-assembled monolayers of silanes on oxidized Si surfaces using nonmonochromatic X-ray sources operated between 200 and 300 W of power. XPS survey spectra were recorded prior to and after the XPS experiments in order to check for beam damage to the films. As no changes could be detected, it was concluded that the films are stable under the experimental conditions applied. Using advanced data acquisition and analysis methods, Sun, Castner, and Grainger applied angle-dependent monochromatic XPS, along with ellipsometry, grazingincidence IR spectroscopy, and contact angle measurements, to the characterization of some ultrathin polymeric films self-assembled on gold.11a A similar experimental approach, including also NEXAFS, was used by Rabolt, Castner, Ringsdorf, and co-workers for the structural characterization of a semifluorinated amidethiol monolayer on gold.11b In ref 11a, no account is provided for possible damage of the studied film under the X-ray irradiation, while in ref 11b the degradation of the film is evaluated from the observed fall in the XPS fluorine signal and the accompanying increase in the gold signal only. No IR, ellipsometric or wettability data taken from X-ray-irradiated samples are reported. That the irradiation may have caused non-negligible damage to various functional groups in the film and, possibly, to its overall structure (despite the observed little loss of F) is suggested by some apparently significant quantitative discrepancies between the relative atomic percentages of O, N, and S measured at three different takeoff angles and between these and the corresponding values obtained for the carbon (8) Heens, B.; Gregoire, Ch.; Pireaux, J. J.; Cornelio, P. A.; Gardella, J. A. Appl. Surf. Sci. 1991, 47, 163. (9) Rieke, P. C.; Baer, D. R.; Fryxell, G. E.; Engelhard, M. H.; Porter, M. S. J. Vac. Sci. Technol. 1993, A11 (4), 2292. (10) Bierbaum, K.; Kinzler, M.; Wo¨ll, Ch.; Grunze, M. Langmuir 1995, 11, 512. (11) (a) Sun, F.; Castner, D. G.; Grainger, D. W. Langmuir 1993, 9, 3200. (b) Lenk, T. J.; Hallmark, V. M.; Hoffmann, C. L.; Rabolt, J. F.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1994, 10, 4610.
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atoms bound to O, N, and S (OCN, CN, CS). Consequently, the compositional depth profiles derived from these XPS data, and particularly the distributions of O, N, S, and Au, look rather problematic, which was, however, ascribed to the roughness of the Au substrate, although the same substrate roughness was not reported to have any effect on the ellipsometric and grazing-incidence IR data. Franses and co-workers12 used XPS and FTIR to study the structure and composition of LB films of cadmium stearate. They found that ca. 40% of the IR C-H signals and ca. 60% of the COO- signals were lost after the exposure of a 67-layer film to the X-ray beam (Mg KR nonmonochromatized source at 100 W of power) for 65 min. This was interpreted as indicating decarboxylation, accompanied by a partial loss of hydrocarbon chains and a partial recombination to vacuum-stable species, the nature of which was not specified. XPS measurements of monolayer and two-layer films confirmed the carboxyl loss and, to a lesser extent, alkyl carbon loss. Despite the observed damage, an attempt was made to use the angleresolved XPS data as a basis for a structural analysis of the LB film. Organic monolayers and multilayers of variable thickness were also used by several groups to determine photoelectron mean free paths, the determination being based on measurements of signals from the substrate as well as from the organic overlayer.3,12-21 The wide range of values obtained from these measurements may, among others, reflect different degrees of degradation of the respective films in the course of the XPS measurement, which, in some cases, made impossible the fitting of the experimental data with any electron mean free path value in the reported range.22 We attempted to apply XPS, along with FTIR and other methods of surface analysis, to the characterization of a (12) Marshbanks, T. L.; Jugduth, H. K.; Delgass, W. N.; Franses, E. I. Thin Solid Films 1993, 232, 126. (13) (a) Brundle, C. R.; Hopster, H.; Swalen, J. D. J. Chem. Phys. 1979, 70, 5190. (b) Burns, F. C.; Swalen, J. D. J. Phys. Chem. 1982, 86, 5123. (14) Clarck, D. T.; Thomas, H. R. J. Polym. Sci., Polym. Chem. Ed. 1977, 15, 2843. (15) Hupfer, B.; Schupp, H.; Andrade, J. D.; Ringsdorf, H. J. Electron Spectrosc. 1981, 23, 103. (16) Kajiyama, T.; Morotomi, N.; Hiraoka, S.; Takahara, A. Chem. Lett. 1987, 1737. (17) Akhter, S.; Lee, H.; Hong, H.-G.; Mallouk, T. E.; White, J. M. J. Vac. Sci. Technol. 1989, A7, 1608. (18) Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1989, 93, 1670. (19) Laibinis, P. E.; Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1991, 95, 7017. (20) Kurihara, K.; Kawahara, T.; Sasaki, D. Y.; Ohto, K.; Kunitake, T. Langmuir 1995, 11, 1408. (21) Ohnishi, T.; Ishitani, A.; Ishida, H.; Yamamoto, N.; Tsubomura, H. J. Phys. Chem. 1978, 82, 1989. (22) Byrd, H.; Whipps, S.; Pike, J. K.; Ma, J.; Nagler, S. E.; Talham, D. R. J. Am. Chem. Soc. 1994, 116, 295. (23) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (24) (a) Czanderna, A. W.; King, D. E.; Spaulding, D. J. Vac. Sci. Technol. 1991, A9, 2607. (b) Jung, D. R.; King, D. E.; Czanderna, A. W. J. Vac. Sci. Technol. 1993, A11, 2382. (25) Smith, E. L.; Alves, C. A.; Anderegg, J. W.; Porter, M. D. Langmuir 1992, 8, 2707. (26) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 1330. (27) Tarlov, M. J. Langmuir 1992, 8, 80. (28) Offord, D. A.; Griffin, J. H. Langmuir 1993, 9, 3015. (29) Mino, N.; Ozaki, S.; Ogawa, K.; Hatada, M. Thin Solid Films 1994, 243, 374. (30) Carey, R. I.; Folkers, J. P.; Whitesides, G. M. Langmuir 1994, 10, 2229. (31) (a) Briggs, D.; Seah, M. P. Practical Surface Analysis, 2nd ed.; John Wiley & Sons: Chichester, 1990; p 438 and references therein. (b) Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers; John Wiley & Sons: Chichester, 1992. (32) (a) Wheeler, D. R.; Pepper, S. V. J. Vac. Sci. Technol. 1982, 20 (2), 226. (b) Buchwalter, L. D.; Czornyj, G. J. Vac. Sci. Technol. 1990, A8 (2), 781.
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series of self-assembled molecular films differing in structure, composition, and thickness. It was anticipated that XPS could offer distinct advantages over other spectroscopic methods routinely applied by us in the study of organized monolayers and thin multilayers (FTIR, UVvis), in those situations were direct quantitative information is needed on elements such as metal ions, halogens, sulfur, etc. (usually present in small atomic percentages), which are not readily detected by the latter, as well as an analytical tool capable of providing compositional depth profiles of such layered architectures. In order to take full advantage of the unique quantification capabilities of XPS, prolonged measurements are usually required, during which various molecular components of an ultrathin organic film may undergo substantial chemical and structural transformations. Thus, preliminary XPS measurements performed by us with both nonmonochromatic and monochromatic X-ray sources on some model mono- and bilayer systems with known composition and structure failed to give consistent quantitative results, and IR measurements which followed them indicated substantial degradation of the hydrocarbon core of the films, thus prohibiting their further use. The loss of hydrocarbon material evident from the IR spectra was usually much higher than that derived from the XPS carbon signals themselves. This pointed to chemical and/ or structural transformations not affecting the total carbon content of the damaged film, to which XPS is predominantly sensitive.9 As a result of these observations, we decided to undertake a more systematic investigation of the nature of the X-ray-induced damage in some representative hydrocarbon monolayer systems, using besides XPS, nondestructive methods of surface analysis that are capable of detecting both chemical and structural modifications caused to the examined samples. Here we report some main results of this study, which we expect to guide future work toward a more realistic interpretation of XPS data along with the design of XPS data acquisition and analysis procedures better suited to the specific problems arising in the characterization of ultrathin molecular architectures. We further expect these results to contribute to the future tailoring of monolayer systems with enhanced stability under irradiation, or with predetermined degradability, as required in applications such as lithographic patterning. General Experimental Approach Instrumental Considerations. In view of the inherent limitations of the XPS-derived information, the possible transformations induced in the studied organic monolayers under the XPS measurement conditions (X-ray irradiation + ultrahigh vacuum) were assessed using, in addition to the XPS data themselves, several other analytical methods, that both are nondestructive and offer the desired chemical structural sensitivity. FTIR was selected as a main analytical tool meeting these requirements. The complementarity of the XPS and FTIR techniques allowed us to gain a better understanding of the nature of the observed changes and their mechanisms, by correlating sets of XPS and FTIR results obtained from the same particular film samples before and after their exposure for variable periods of time to the XPS measurement conditions. In addition to FTIR spectroscopy, contact angle measurements were used as a sensitive qualitative indicator of the structural/chemical changes induced in irradiated film areas. Attempts were also made to apply AFM (atomic force microscopy) imaging for the detection of eventual submicroscopic morphological modifications caused by the irradiation. FTIR spectra were taken both in the transmission mode at normal incidence and in the Brewster’s angle geometry,33 which was found particularly useful for the elimination of interference (33) Harrick, N. J. Appl. Spectrosc. 1977, 31, 548.
5092 Langmuir, Vol. 13, No. 19, 1997 fringes in measurements of monolayers on thin double-sidepolished Si wafer substrates.34 These two measurement configurations also respond differently to structural changes in the organic film (vide infra), thus allowing us to distinguish between different film damage processes (such as, for example, loss of material vs molecular disordering without major loss of material from the surface). In order to obtain quantitative results, the IR-sampled film areas had to be confined within the X-rayirradiated spots. This was achieved by the use of aluminum foil masks or by total irradiation of sufficiently small samples, from which IR spectra were taken with a sensitive IR detector (vide infra). Most experiments were performed with X-ray irradiation times (XPS exposure time) of the order of 90 and 180 min, which were selected considering the typical data acquisition times required for quantification of elements different from carbon, usually present in small relative concentrations in ultrathin organic films,6,30 or for angle-resolved measurements6,12,17 (also referred to as “nondestructive” depth profiling). Thus, X-ray exposure times between 1 and 5 h were reported in such XPS monolayer studies.6,12,27,30 Since the irradiated film spots had to be sufficiently large and homogeneous to enable their subsequent examination by FTIR spectroscopy, contact angle measurements, and/or AFM imaging (and considering that the same damage mechanisms should be operative with any type of X-ray source), we used a nonmonochromatized Mg KR X-ray source which gave a spot area of ca. 6 mm × 12 mm on the sample at the relatively large sourcesample distance (ca. 3 cm) employed in this study. This large distance also helped to reduce inhomogeneity and flux variations across the irradiated sample area. The source power was set to 96 W, which is equal to or less than the power setting usually reported in XPS studies of organic monolayers,35 the large sourcesample distance further contributing to a rather low X-ray flux at the sample. With these measurement conditions, we thus expect the observed monolayer degradation to be representative of, while even somewhat underestimating, the actual damage frequently occurring in nonmonochromatized XPS studies of organic monolayers. Since similar degradation effects have also been observed upon irradiation with a monochromatized X-ray beam (see Conclusions section), the general new findings of the present study should bear relevance to monochromatic XPS measurements as well, despite possible differences regarding the extent of the induced damage. Monolayer Systems. To gain insight into some of the various factors influencing the radiation-induced damage process, a comparative study was carried out on a series of representative n-alkane monolayers, involving nine monolayer/substrate systems: OTS (n-octadecyltrichlorosilane, CH3s(CH2)17sSiCl3) on quartz (Q), glass (G), silicon (Si), and ZnSe; AA (arachidic acid, CH3s(CH2)18sCOOH) on glass and ZnSe; and NTSox (18nonadecenyltrichlorosilane, CH2dCHs(CH2)17sSiCl3,29 after the in situ oxidation of its terminal double bond to COOH34,36) on glass and silicon (see Figure 1); and MTS (methyltrichlorosilane, CH3sSiCl3) on silicon. This selection offered the possibility to compare monolayers of the same compound on different substrates (OTS on quartz, glass, silicon, and ZnSe; AA on glass and ZnSe), monolayers with the same kind of hydrocarbon core and on same substrate but with different binding functions and modes of anchoring to the substrate (OTS and AA on glass and on ZnSe), monolayers with the same binding functions and the same hydrocarbon cores but with different outer exposed functional groups (OTS and NTSox), and monolayers with the same binding functions and the same outer exposed groups but with totally different hydrocarbon cores (OTS and MTS on silicon). All monolayers were first tested for stability in the ultrahigh vacuum (UHV) conditions, taking into account also the effect of heating by the X-ray source during the acquisition of XPS data. IR spectra taken from each studied system prior to and after exposure to the UHV for at least 8 h showed no chemical or (34) Maoz, R.; Sagiv, J.; Degenhardt, D.; Mo¨hwald, H.; Quint, P. Supramol. Sci. 1995, 2, 9. (35) Values of 200-300 W for the X-ray source power, operated at source-sample distances smaller than that used in the present study, are common. See for example refs 5, 7, 9, 10, 17, 20, 21, 24, 25, and 29. (36) Maoz, R.; Yam, R.; Bercovic, G.; Sagiv, J. In Thin Films; Ulman, A., Ed.; Academic: San Diego, 1995; Vol. 20, pp 41-68.
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Figure 1. Schematic representation of the long chain monolayer systems selected for the present study, emphasizing the covalent bonding of silanes (OTS, NTSox) and ionic bonding of arachidic acid (AA) to the substrate surface. structural changes. The temperature measured at the sample stub during its direct exposure to the X-ray beam did not exceed 27 °C, while OTS monolayers in UHV were found to be perfectly stable up to ∼350 °C, and a monolayer of AA on glass heated in the spectrometer chamber for 3 h at 80 °C (with the X-ray source off) showed only a small drop of ca. 4% in the peak intensity of its asymmetric CH2 stretching band and 1% in its overall integrated C-H stretch absorbance. Thus, even if heating by the X-ray source would generate local surface temperatures higher than the 27 °C measured at the irradiated stub, damage effects due to heat should be negligible compared to those caused by the X-ray radiation. The observed chemical/structural transformations must therefore be ascribed to the X-ray radiation and photoelectron effects7,9 only, with eventual secondary postirradiation contributions from the UHV in the spectrometer chamber (e.g. evaporation of volatile irradiation products). This is further confirmed by the totally different behavior of OTS and NTSox monolayers under irradiation (vide infra), despite their comparable thermal stability. Evaluation of the Irradiation Induced Damage. Quantitative estimations of the various possible monolayer degradation processes (carbon loss, hydrogen loss, functional group loss, disordering) were made on the basis of changes observed in both the XPS and IR spectra of monolayers exposed to the X-rays for variable periods of time. For organic films of comparable thickness in the range below ca. 30 Å, such as the present investigated monolayers1,34 (i.e. thinner than the characteristic attenuation length of escaping photoelectrons18,19), one may expect the total carbon loss of the film to be roughly proportional to the observed decrease in the corresponding XPS carbon signal area.19 The interpretation of the IR band intensities is somewhat more complex, but also more informative, as the measured signals depend on both the surface concentration and the molecular organization of the film material. For example, in the transmission mode at normal incidence, a transformation from a completely ordered monolayer phase (all paraffinic tails perpendicular to the substrate surface) to a completely disordered one (tails randomly oriented in space), with no material loss, would cause a decrease in the integrated intensities of the C-H stretch bands by a factor of 2/3.37 Such disordering should also cause a blue shift of the peak positions and broadening of the bands.5,34,38 In the Brewster’s angle geometry, disordering of the paraffinic tails will also result in a similar blue shift of the peak positions and broadening of the bands, but the measured band intensities should increase in this case, since disordering of the molecules would allow enhanced interaction of the C-H stretch modes with the out-of-plane (37) Assuming that all paraffinic tails of an ordered monolayer are perpendicular to the substrate surface and that the C-H bonds are parallel to it with random in-plane orientation, half of the C-H bonds should be statistically available for interaction with the electric field of the incoming IR beam in the normal incidence geometry. Upon threedimensional randomization of the paraffinic tails, 1/3 of the C-H bonds will point in the direction of propagation of the IR beam, so that the measured absorbance should decrease by a factor of 2/3 compared to that of a perfectly ordered monolayer. (38) Casal, H.; Cameron, D. G.; Mantsch, H. Can. J. Chem. 1983, 61, 1736.
Monolayer Damage in XPS Measurements component of the incident beam.34,39 Thus, the interpretation of the IR data, taking into account changes both in band intensities and in band widths and peak positions, while comparing normal incidence with Brewster’s angle measurements, gave us the possibility to differentiate between material loss and disordering effects.
Experimental Details Chemicals. OTS (Merck-Schuchardt, “For Synthesis” grade), NTS (10% stock solution in chloroform, kindly supplied by Dr. K. Ogawa, Matsushita Electric Ind. Co. Ltd., Osaka29), MTS (Aldrich, 99%), AA (Sigma, 99%, Sigma Grade), toluene, benzene, ethanol (analytic reagents, Bio-Lab), crown ether (dicyclohexano18-crown-6, C20H36O6, Sigma, 98%, mixed isomers), and potassium permanganate (KMnO4, Fluka, “puriss”) were all used as received. BCH (Bicyclohexyl, Fluka, “puriss”) and HD (nhexdecane, Fluka, “puriss”) were purified by percolation through basic alumina (ICN alumina B, activity grade I, ICN Biomedicals). The solvents used for the RCA cleaning procedure were 37% HCl (Merck), 25% NH4OH (Merck), and 30% H2O2 (Merck, stabilized). The water employed throughout this work was type I analytical grade ultrapure water (resistivity 18.0-18.2 MΩ‚cm) directly withdrawn from a continuously recirculating Barnstead nanopure system equipped with “Macropure” (colloids and bacteria removal), “Ultrapure” (mixed bed ion exchange), and “Organic Free” (active carbon + ion exchange) cartridges and a final 0.2 µm filter. The Nanopure system was fed with water purified via reverse osmosis, two stages of ion exchange, and final passage through 1 and 0.45 µm prefilters. Substrates. Silicon wafers (Virginia Semiconductor, doubleside-polished, 0.015 in. thick 〈111〉, n-type, undoped, resistivity g 50 Ω‚cm) were cut to ca 5 mm × 10 mm pieces before substrate preparation and cleaned according to the RCA procedure;40 the wafer was immersed in a NH4OH:H2O2:H2O (1:1:5) solution at 65 °C for 15 min, rinsed with water for 10 min, then immersed in a HCl:H2O2:H2O (1:1:5) solution at 65 °C for 15 min, rinsed again with water for 10 min, and finally blown dry in a stream of clean nitrogen. Reference IR spectra were collected from each wafer immediately after cleaning, the water rinse/N2 drying being then repeated immediately before monolayer adsorption. Clean wafers emerge completely wet from water. Quartz slides (Westdeutsche Quartzschmelze, UV grade fused silica type Synsil II, polished, 1 mm × 12 × 38 mm) were cleaned by Soxhlet extraction with toluene for 15 min, then immersed in the NH4OH:H2O2:H2O (1:1:5) solution at 65 °C for 5-7 min, rinsed with water for 10 min, and finally blown dry in a stream of dry nitrogen. Clean slides emerge completely wet from water. IR normal incidence transmission measurements of film on quartz slides were performed against a clean reference slide. Glass slides (Gebr. Rettberg, Go¨ttingen, 1 mm × 12 mm × 38 mm) were first sonicated in water for 10 min and then cleaned by Soxhlet extraction with toluene for 15 min, followed by Ar plasma cleaning for 15 min in a Harrick Scientific PDC-3XG plasma cleaner. IR normal incidence transmission measurements of films on glass slides were performed, as were those on quartz, against a clean reference slide. ZnSe prisms (Harrick Scientific, 3 mm × 10 mm × 50 mm ATR prisms) were first cleaned by Soxhlet extraction with toluene for 15 min and then by Ar plasma for ca. 15 min. Reference IR spectra (transmission at normal incidence) were collected from each ZnSe prism immediately after cleaning, followed by immediate monolayer adsorption. The hydrocarbon/carbon content of the cleaned substrates was checked by both XPS and IR measurements. No residual hydrocarbon could be detected in the IR spectra (detection limit below ca. 0.0001 au in the transmission mode at normal incidence, corresponding to less than ca. 2.5% of a complete C18 monolayer). According to XPS, the residual carbon content of the cleaned substrates was as follows: quartz, ca. 9% fractional atomic concentration; glass, ca. 4.5%; Si, ca. 10%; ZnSe, ca. 15%. (39) The out-of-plane component of the electric field provided by the p-polarized beam at the Brewster’s angle of incidence (73.7°, for Si)34 can interact with the out-of-plane C-H vibrations of disordered paraffinic tails, which is not possible in the case of a perfectly ordered monolayer where all C-H vibrations are parallel to the layer plane. (40) Kern, W.; Puotinen, D. A. RCA Rev. 1970, 31, 187.
Langmuir, Vol. 13, No. 19, 1997 5093 Following the adsorption of a complete monolayer (C18-C20), the fractional carbon concentration normally rose to above 45%. The adventitious carbon contamination found on the bare substrates is, apparently, removed upon adsorption of a well defined monolayer,41 so that the carbon content of monolayer-covered surfaces represents the net contribution of the representative monolayer coatings only. Preparation of Self-Assembling Monolayers. OTS. Cleaned Si, quartz, or glass substrates, withdrawn from water and thoroughly blown dry in a clean nitrogen stream, are immersed in a ∼5 mM solution of OTS in BCH for ca. 2 min and then withdrawn and rinsed in clean toluene for ca. 30 s, this adsorption and rinse cycle being then repeated once again.34 The formation of a complete OTS monolayer onto ZnSe required a slightly longer immersion time into the OTS solution (ca. 10 min). OTS-covered substrates emerge completely dry from both the OTS solution and toluene. The quality of each monolayer prepared for this study was checked by contact angle and IR measurements.36,42 Contact angles typical of complete OTS monolayers prepared by the present procedure (regardless of substrate) are 115° for water, 56° for BCH, and 52° for HD (both advancing and receding). The IR peak absorbance of the CH2 asymmetric stretch mode at 2917 cm-1, typical of complete OTS monolayers measured in transmission mode at normal incidence (double-side coating), is 0.0042 for quartz and glass,36 0.0040 for ZnSe, and 0.0030 for Si.43 The corresponding peak absorbance for OTS on Si measured in the Brewster’s angle geometry is 0.0016.34,36 NTS. NTS monolayers were prepared by a procedure identical to that for OTS, using a deposition solution of ca. 1:40 (v/v) NTS (10% stock solution in chloroform) in BCH. Similarly to the case for OTS, NTS-covered substrates emerge completely dry from both the NTS solution and toluene. Typical contact angles on complete NTS monolayers are 103° for water, 52° for BCH, and 46° for HD (both advancing and receding), and the IR absorption due to the CH2 vibrations is practically identical to that of OTS, as both compounds contain the same number (17) of CH2 groups per chain. A longer deposition time (ca. 60 min) was necessary on glass substrates in order to obtain a more stable monolayer under the conditions of the subsequent double-bond oxidation reaction (see NTSox). NTSox. The substrate coated with an NTS monolayer is placed in a ca. 5 mM solution of KMnO4/crown ether (dicyclohexano18-crown-6) in benzene for 48 h (the solution should be refreshed after 24 h), after which it is sonicated in clean benzene for 1 min, then washed in 3.7% HCl for about 3 h (or until the monolayer surface appears visually clean), and finally rinsed with pure water for 10 min. The NTSox monolayer emerges completely wet from water and is blown dry in a stream of clean nitrogen. Typical contact angles on NTSox monolayers are 0° for BCH and HD (advancing and receding) and ∼20° (advancing) and 0° (receding) for water. The IR CH2 absorbance of NTSox is similar to that of NTS, while the CdC stretch absorbance (Si Brewster’s angle, ∼0.000 15 au, 1642 cm-1) is replaced by the COOH absorbance (Si Brewster’s angle, 0.0003 au, 1715 cm-1).34 MTS. MTS monolayers on Si were prepared similarly to OTS and NTS monolayers, using a ∼1 mM solution of MTS in BCH and immersion times of ca. 20-30 s, followed by final sonication (41) We compared the carbon content of a freshly cleaned Si substrate with that of the same substrate on which an MTS (one-carbon-atom silane) monolayer was adsorbed. The fractional carbon concentration of the MTS monolayer (ca. 8%) was consistently lower than that of the uncoated substrate, indicating removal of the adventitious carbon-rich contamination by the silane monolayer, which has a lower carbon content and provides protection against further accumulation of contamination on the coated surface. Using FTIR, it was thus found that the storage of a freshly cleaned Si wafer in the N2-purged sample compartment of the FTIR spectrometer or in glass or plastic containers in the laboratory ambient (from 12 h to 1 weak) results in a gradual accumulation of IR-measurable hydrocarbon contamination (broad peak around 2927 cm-1), which is clearly removed and replaced by MTS (characteristic narrow peaks at 2910 and 2974 cm-1) upon adsorption of a compact monolayer of the latter. (42) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465. (43) These optical density values, determined by us empirically and checked by comparison with analogous LB monolayers,36,42 depend on the refractive index of each of the respective substrates (see, for example: Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J. Phys. Chem. 1990, 94, 62).
5094 Langmuir, Vol. 13, No. 19, 1997 in clean toluene. As with the case for OTS and NTS, MTS-coated wafers emerge dry from both the MTS solution and toluene. Typical contact angles on the MTS monolayers are 81-82° for water, 50-51° for BCH (both advancing and receding), and 41° (advancing) and ca 20° (receding) for HD. The IR peak absorbance (Brewster’s angle geometry, double-side coating) due to the methyl stretch band at ca. 2974 cm-1 was found to be 0.000 13 au, while the symmetric bending mode at ca. 1276 cm-1 is almost tenfold stronger (0.0012 au). Arachidic Acid. AA monolayers were formed on glass or ZnSe by immersing the substrates, immediately after their exposure to the Ar plasma, in a ca. 5 mM solution of AA in BCH, for 1-2 min. The monolayer-covered substrates emerge completely dry from the AA solution, and no further washing is necessary. Typical contact angles for BCH and HD on complete arachidic acid monolayers are similar to those measured on OTS, while water removes AA monolayers on glass and gives lower contact angles on ZnSe.42 The IR peak absorbance of the CH2 asymmetric stretch mode measured in transmission at normal incidence (doubleside coating) is 0.0052 on glass (2916 cm-1, band narrower than that of OTS) and 0.0038 on ZnSe.43 Instrumental Methods and Data Analysis. FTIR. Spectra were recorded with a single-beam Fourier-transform spectrophotometer (Nicolet 730 system) equipped with a “shuttle” accessory which allows automatic translation of the measured sample and reference in and out of the IR beam in a predetermined sequence during data acquisition, thus allowing a quasi-doublebeam measurement.34 A wire grid ZnSe polarizer (Spectra-Tech) was employed in the Brewster’s angle measurements, as described in ref 34. Films on quartz and glass slides were measured in the transmission mode at normal incidence versus a clean slide as reference. Films on ZnSe were also measured in transmission node at normal incidence versus the open beam background, the reference (subtracted mathematically) being an identical measurement of the clean substrate before monolayer adsorption. Silicon wafers were measured using both normal transmission and the Brewster’s angle geometry.33,34 Clean Si wafers were measured as reference (versus the open beam background) before the deposition of the organic film, and the organic film spectra were obtained by subtracting mathematically the reference spectrum from that of the corresponding film-coated wafer obtained by the same procedure. All transmission mode spectra were collected using a DTGS detector, while Brewster’s angle spectra were collected using a liquid nitrogen-cooled MCT detector, allowing the use of a much smaller collection area, as detailed below. The use of a polarized IR beam was found to reduce also spectral fringes when thin Si wafer samples were measured in the normal transmission mode. All reported spectra were obtained at a resolution of 4 cm-1, ca. 400 scans being usually collected for normal transmission measurements and ca. 600 scans for measurements at the Brewster’s angle geometry. In order to obtain quantitative results, the IR-sampled area of the film had to be equal to or smaller than the X-ray-irradiated area (ca. 6 mm × 12 mm). This was achieved using the following measuring procedures: for measurements in the normal transmission mode, the monolayer samples were covered with a clean, thick aluminum foil mask, with a ca. 5 mm diameter hole defining the irradiated area. The entire hole area was X-ray irradiated and IR measured, so that direct assessment of the damage was possible. This configuration, however, gave poor results when used in the Brewster’s angle geometry, for two reasons: the reduction in the measured film area (which was usually ca. 4 cm2, using the DTGS detector) led to a poor signal-to-noise ratio, and reflections between the Si sample and the aluminum mask (due to the shallow angle of incidence (17°) with respect to the sample plane34) resulted in distorted spectra. These difficulties were overcome by using a small area (5 mm × 10 mm) sample and the more sensitive MCT detector, which allowed collection of Brewster’s angle spectra with reasonable S/N ratios. The sample was mounted on the sample holder between two cardboard beam blockers, so that only the part of the IR beam passing through the sample arrives at the detector, resulting in a direct quantitative measurement of the monolayer sample. In this configuration, the entire sample area was X-ray irradiated as well as IR measured.
Frydman et al. Band area calculations were made using a peak integration program available within the manufacturer’s software. The area was numerically calculated for a defined spectral region, with a linear background subtraction. For the C-H stretch vibrations, two regions were defined: a CH3 ν(as) band, usually between 2948 and 2975 cm-1, and an overall C-H stretch band, including the CH2 ν(s), CH2 ν(as), CH3 ν(s), and CH3 ν(as) bands, usually between 2820 and 2975 cm-1. For the samples studied, this band contained at least 95% CH2 contribution and therefore was regarded as a good indicator of the CH2 state in the sample. Optimal integration limits were determined visually for each individual spectrum, taking into account changes in the widths and positions of the peaks caused by the X-ray irradiation. The areas of the functional group bands (e.g. COOH ν, COO- ν) were calculated in the same manner. All investigated samples were double-side-coated with the respective monolayers, while the XPS damage was restricted to the irradiated side of the sample only, as evident from the contact angle measurements performed on both sides of X-ray-irradiated samples. The IR spectra were thus corrected to account for the irradiated side only. This was done by mathematically multiplying the spectrum of the initial unirradiated monolayer by 1/2 (resulting in a corrected “one-side” monolayer spectrum) and subtracting this “halved” spectrum from both the original and the irradiated monolayer spectra, to give the corrected unirradiated and irradiated one-side monolayer spectra, respectively. XPS. Spectra were acquired on a Kratos AXIS HS analytical spectrometer using a relatively broad beam X-ray source with an Mg KR anode (1253.6 eV) operated at 96 W (12 kV, 8 mA) and with a pass energy of 40 eV. The X-ray source was positioned at a distance of ca. 3 cm from the sample. Analysis of the beam flux profile in this configuration, using a clean gold reference sample, showed the beam to be nearly homogeneous across a spot of ca. 6 mm × 12 mm at the sample surface. The pressure in the analysis chamber was ca. 2 × 10-9 Torr during measurements. Since our substrates are poor conductors, a neutralizer electron flood gun was used, usually at 1.8 mA and 0.7 V repulsive bias, in order to compensate surface charging and thus minimize signal shifts. Two control experiments were performed to check the possibility that additional sample damage may be caused by the use of the flood gun. In the first experiment, two identical AA monolayer samples on glass were exposed to the X-ray radiation during a 3 h XPS measurement, with and without the use of a flood gun. The results indicated that the sample exposed to the X-rays without the flood gun was actually more affected (according to the observed decrease in its C-H IR absorbance), possibly because of the electrical fields built across the layer as a result of charge distribution effects. In the second experiment, another AA/G sample was exposed for 3 h to the flood gun operating at 1.9 mA, without any X-ray irradiation. Within the experimental precision of our IR measurements, no damage could be identified in this sample. Thus, it was concluded that the use of a flood gun does not cause additional damage to the monolayer systems of the kind presently investigated, and it might even contribute to a reduction of possible damage caused by surface charging effects. Evaluation of the monolayer damage by XPS was done by monitoring changes in the raw signal area of the studied element (usually carbon). Since changes in the spectra were observed to be faster within the first 10-20 min of irradiation, special care was taken to acquire the first carbon spectrum (and other relevant elements, if necessary) using short sweeps within 1-2 min from the beginning of irradiation (including instrument tuning, which was done at 20% of the measurement flux). Subsequent measurements were longer (ca. 10 min for the C 1s line and 4-7 min for other elements such as Si or O), in order to obtain better signal-to-noise ratios. The stability of the source was checked by following signals of elements in the system that are expected to behave differently from carbon (e.g. an increase in the substrate Si signal concomitant with the decrease in the carbon signal). Signal areas were numerically calculated using an integration program provided by the manufacturer’s software, with linear background subtraction. Slight smoothing (by convolution with a Gaussian of 0.4 eV width) was applied to the spectra before signal area calculation. Curve fittings of the C 1s line (for AA
Monolayer Damage in XPS Measurements
Figure 2. XPS C 1s line (slightly smoothed) of a monolayer of OTS on quartz taken at variable time intervals during 170 min of continuous exposure to the X-ray beam: after (a) 1 min; (b) 67 min; (c) 112 min; (d) 152 min. on ZnSe, AA on glass, and NTSox on Si samples) were done with 50% Gaussian/50% Lorenzian line shapes (60%/40% in Figure 8) and Shirley background subtraction. AFM. Images were acquired at ambient temperature in air, using a Nanoscope 3 (Digital Instruments) scanning probe microscope operated in both the topographic and friction modes. Samples were scanned in the contact mode under the minimum force allowed by the feedback sensitivity (ca. 10 nN), with a commercial Si3N4 tip attached to a cantilever with a spring constant of 0.38 N/m. Separate vertical and torsional motions of the cantilever were detected simultaneously from the same scan. The vertical and torsional motions are proportional to normal forces (topography) and lateral forces (mostly friction on an absolutely flat surface), respectively, acting on the tip in the contact area. The vertical motion was calibrated in absolute height values, while the torsional motion is displayed as noncalibrated voltage data. In the absence of local differences in friction, the “friction” images represent torsions of the tip in the lateral direction caused by surface asperities. Contact Angles. Static contact angles (advancing and receding) were measured under ambient conditions with an NRL contact angle goniometer (model 100, Rame´-Hart) using the sessile drop method.42 The reproducibility of these measurements is ca. (1° angles in the range 30-60° and (2° in the range above 90°.
Results Figure 2 shows the evolution of the XPS C 1s line of a monolayer of OTS on quartz (OTS/Q), taken during 170 min of X-ray irradiation. The overall shape and position of the carbon peak are conserved, while the total band area gradually decreases to a value ca. 8% lower than the initial one, by the end of the experiment. The IR spectra corresponding to the irradiated side of the sample, taken before and after the irradiation (Figure 3), reveal a severe loss of C-H absorption, accounting for more than 50% of the initial intensity, as estimated from the reduced total area of the C-H stretch bands. The decrease in the absorption intensity is accompanied by broadening of the bands and a blue shift of the H-C-H antisymmetric and symmetric stretch modes (from 2917.5 to 2921 cm-1 and from 2849 cm-1 to 2851 cm-1, respectively), which is indicative of a lower degree of structural order in the remaining monolayer material.38 An even more pronounced loss of intensity (about 80%) is clearly observed in the CH3 band at 2958 cm-1. The results of a similar experiment performed on a different system (arachidic acid monolayer on ZnSe (AA/ ZnSe), with an overall irradiation time of 82 min) are presented in Figures 4 (XPS) and 5 (IR). In this case, the decrease in the integrated XPS C 1s signal is significantly larger than that in the OTS/Q case, reaching ca. 20% after 82 min. A careful inspection of the carbon band reveals
Langmuir, Vol. 13, No. 19, 1997 5095
Figure 3. FTIR spectra (transmission at normal incidence, corrected “one-side” spectrum) of the OTS/Q sample of Figure 2 showing the C-H stretch band region before (full line) and after (dashed line) the 170 min exposure to the XPS measurement conditions. The reduction in the overall integrated C-H absorbance is ca. 50%.
Figure 4. XPS C 1s line (slightly smoothed) of a monolayer of arachidic acid on ZnSe taken at variable time intervals during 82 min of continuous exposure to the XPS measurement conditions: after (a) 1 min; (b) 5 min; (c) 12 min; (d) 29 min; (e) 52 min; (f) 73 min (the first measurement was taken with a relatively short acquisition time, resulting in a signal-tonoise ratio lower than that of subsequent measurements).
a similar decrease also in the weaker carboxylic (COO-) carbon around 288-289 eV, which is confirmed by the peak fitting of the C 1s line into carboxylic and hydrocarbon components.2,12 The corresponding IR spectra show again a notable decrease in the C-H stretching bands area (ca. 36%), accompanied by a blue shift of the peak positions (from 2916.5 to 2920 cm-1 and from 2849.5 to 2851 cm-1) and broadening of the bands, which point to enhanced disorder in the remaining monolayer material. A similar intensity decrease is also evident in the CH2 bending band at 1466 cm-1. The area of the COO- band around 1546 cm-1, which, in this system, represents the monolayersubstrate binding function,44 decreases by ca. 28%, while maintaining its shape and position. Finally, a weak CdO band, not found in the initial spectrum, appears in the curve recorded after irradiation, as suggested by the weak, broad feature visible around 1720 cm-1. A somewhat different experiment was performed with a monolayer of arachidic acid on glass (AA/G). In this experiment, the sample was successively exposed to the XPS measurement conditions for different periods of time, (44) As evident from Figure 5, AA binds to ZnSe as the Zn2+ carboxylate salt (see also refs 42 and 45). (45) Gun, J.; Sagiv, J. J. Colloid Interface Sci. 1986, 112, 457.
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Figure 5. FTIR spectra (transmission at normal incidence, corrected “one-side” spectrum) of the AA/ZnSe sample of Figure 4 showing both C-H and COO bands, before (full line) and after (dashed line) the 82 min exposure to the XPS measurement conditions. The reduction in the overall integrated absorbance is ca. 36% for the C-H bands and ca. 28% for the COO- band, the latter representing the damage caused to the anchoring head groups in this monolayer system.
Figure 6. FTIR spectra (transmission at normal incidence, corrected “one-side” spectrum) successively taken from same sample of arachidic acid on glass, showing the gradual decay of the C-H stretch bands following total XPS exposure times of 0, 25, 57, 168, and 337 min (the “intervals” experiment). Note the conservation of the peak positions in the first four spectra.
FTIR spectra being taken after each such exposure (“intervals” experiment). In this manner, it was possible to follow the changes in the sample as a function of the XPS exposure time by both XPS and IR spectroscopy. The successively measured IR spectra in Figure 6 show a gradual decrease in the intensities of the C-H stretch bands. However, in this case no significant changes in the peak positions and band widths were observed during the first 3 h of exposure. Thus, the CH2 ν(as) peak shifts slightly, from 2916.4 to 2917.0 cm-1, while the overall area of the C-H stretch bands decreases by more than 40%. This result (confirmed by a separate measurement on a second AA/G sample) indicates that the molecular order within the remaining monolayer material is largely conserved. It is also apparent in Figure 6 that the CH3 ν(as) peak absorbance near 2958 cm-1 decreases at a rate similar to that of the CH2 bands during the first 3 h of irradiation, while it is barely detectable in the curve taken
Figure 7. Integrated XPS C 1s line and overall IR C-H stretch absorbance as a function of the XPS exposure time of the AA/G monolayer sample of Figure 6, recorded during and after successive exposure intervals, respectively. The data are normalized with reference to the corresponding initial signals (100%) measured at zero (IR) and 2 min (XPS) X-ray exposure times.
after 337 min of X-ray exposure. The combined XPSFTIR results, summarizing the decay of the C 1s XPS signal and of the IR CH absorption as a function of the X-ray exposure time (Figure 7) indicate similar rates of decay during the first 2-3 h, after which the decrease in the IR absorption becomes significantly faster than that of the XPS signal. As mentioned in the Experimental Section, the radiation damage caused to an AA/G monolayer was somewhat larger in the absence of a charge-compensating electron flood gun. This is apparent from both the larger drop in the IR C-H absorption (ca. 53%) and the broadening and blue shift of the C-H stretch bands (ca. 2.3 cm-1 for the asymmetric mode) observed upon the irradiation of an AA/G sample for 190 min under such conditions (compare with data in Table 2). A detailed analysis of the C 1s line in this sample (Figure 8) shows that the loss of oxidized carbon (COOH) exceeds that of total carbon, as its relative
Monolayer Damage in XPS Measurements
Langmuir, Vol. 13, No. 19, 1997 5097
Figure 8. XPS spectra showing the C 1s line of an AA/G monolayer before (left) and after (right) its exposure to the X-ray radiation (for 173 min) without the use of a charge-compensating electron flood gun (except for the relatively short periods of time required for the acquisition of the respective spectra). To facilitate comparison, the two spectra were drawn on identical fully-expanded scales, with their main peak positions fixed at 284.8 eV (curves a and c), as well as on corresponding ×20 expanded scales (curves b and d). Fitting of each curve was done with three components, allowing for variable peak positions and band widths. The relative integrated intensities of the weak component between ca. 287 and 290 eV, ascribed to the carboxyl (COOH) and other CdO carbon species, are 2.74% and 2.04% of the total carbon line area, before (b) and after (d) irradiation, respectively.
intensity drops upon irradiation by more than 25%, from 2.74% to 2.04%46 (compare spectra b and d). In addition, we also see a clear broadening of the carbon line; however, while this effect is of the order of 0.02 eV for the main component ascribed to the nonoxidized carbon species (spectra a and c), the oxidized carbon component broadens by ca. 0.31 eV and its peak shifts from 288.5 to 288.6 eV (46) The intensity of the XPS carbon line arising from the carboxylic carbon is lower than its expected stoichiometric contribution (5%, i.e. one out of the 20 carbon atoms of the AA molecule) because of the COOH groups’ location at the monolayer-substrate interface and the consequent attenuation of the carboxyl carbon signal by the monolayer hydrocarbon core extending away from this interface. Since the expected decrease in thickness of the depleted monolayer following irradiation should cause a relative enhancement of the XPS signals contributed by bottom-located atoms, the actual loss of carboxylic carbon may be even somewhat larger than that directly deduced from the experimental data.
(spectra b and d). The relative intensity of the weak nonoxidized carbon component at ∼285.5 eV, ascribed to C-H stretch vibrations excited during photoionization (see ref 31b, pp 33 and 34), is also seen to decrease upon irradiation (spectra a and c). All this points to changes in chemical composition and structure, in addition to the quantitative depletion of the film. The combined XPS-FTIR experiments performed with the various OTS and AA monolayer systems under normal XPS measurement conditions (using the electron flood gun) are summarized in Tables 1 and 2, and additional data are graphically displayed in Figures 9 and 10. Figure 11 shows IR spectral changes arising from radiationinduced modifications affecting both the hydrocarbon core and the anchoring silane head groups of an OTS monolayer on ZnSe. The experimental results obtained with the top-
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Table 1. Results of the Combined XPS-FTIR Measurements Performed on the Long Chain Silane Monolayersa OTS/Q
OTS/Q
OTS/G
related figures time under XPS conditions (min) IR CH absb area decrease (%)
9 53 24 ( 3
2, 3, 9 170 53 ( 3
9 180 55 ( 5
9 9 94 186 30 ( 10 46 ( 6 (B) 10 ( 4
OTS/Si
OTS/Si
9, 11 88 47 ( 3
IR CH2 ν(as) peak blue shift (cm-1)
0.7
3.7
2.6
4.7
5.0
XPS C 1s signal area decrease (%)
e
8(2
11 ( 2
d (B) 1.4 12 ( 3
13 ( 3
15 ( 4
IR top function abs area decrease (%)
CH3 CH3 CH3 CH3 CH3 70 ( 15 83 ( 10 82 ( 10 d d (B) 15 ( 7
IR bottom function abs area decrease (%) f
f
f
f
f
OTS/ZnSe
CH3 65 ( 15
NTSox/Si
NTSox/G
12 100 8(3 (B) -15 ( 3 (increase39) 2.6 (B) 2.3 4(2 COOH d (B) 70 ( 15
Si-O-Si 20 ( 6 (modified f band shape)c
173 17 ( 3 2.5 5(3 f f
a All FTIR data are from measurements in the transmission mode at normal incidence, except those indicated by (B), which designates the Brewster’s angle geometry. The estimated errors are mainly due to uncertainties in the determination of the baselines for area calculations and to noise. b Overall C-H stretch absorbance. c See Fig 11 for IR spectra of OTS/ZnSe taken before and after the X-ray irradiation. d Measurement too noisy. e XPS signal change not available, as the initial XPS data were acquired only after 15 min of exposure to the X-ray radiation. f Data inaccessible (interference by substrate absorption).
Table 2. Results of the Combined XPS-FTIR Measurements Performed on the Long Chain Acid Monolayersa related figures time under XPS conditions (min) IR CH absc area decrease (%) IR CH2 ν(as) peak blue shift (cm-1) XPS C 1s signal area decrease (%) IR top function CH3 abs area decrease (%) IR bottom function COO- abs area decrease (%)
AA/Gb
AA/Gb
AA/Gb
AA/Gb
AA/G
AA/ZnSe
6, 7 25 15 ( 2 0.0 13 ( 3 13 ( 5 d
6, 7 57 26 ( 4 0.2 22 ( 4 20 ( 5 d
6, 7 168 41 ( 4 0.6 33 ( 4 42 ( 6 d
6, 7 337 70 ( 5 6.4 40 ( 5 >90 d
10 180 40 ( 3 0.8 25 ( 4 43 ( 8 d
4, 5, 10 82 36 ( 3 3.5 20 ( 3 30 ( 10 28 ( 3
a All FTIR data are from measurements in the transmission mode at normal incidence. The estimated errors are mainly due to uncertainties in the determination of the baselines for area calculations and to noise. b Successive measurements on the same sample (the “intervals” experiment) (Figures 6 and 7). c Overall C-H stretch absorbance. d Data inaccessible (opaque spectral region of the substrate).
Figure 9. Integrated XPS C 1s line and overall IR C-H stretch absorbance (transmission at normal incidence) as a function of the XPS exposure time of OTS monolayers on various substrates. The data are normalized, as in Figure 7, with reference to the respective initial signals.
functionalized silane monolayer (NTSox), on silicon and glass, are summarized in Table 1 and Figure 12. Finally, examples of contact angles measured on unirradiated and X-ray exposed areas of two OTS monolayers, on glass and ZnSe, are given in Table 3. Similar XPS-FTIR-contact angle experiments performed with two MTS/Si samples indicate that this onecarbon-atom silane monolayer is remarkably more stable than OTS under prolonged exposures to the X-ray radiation. The XPS C 1s signal area decrease after 3 h of exposure to the XPS measurement conditions was within 3 ( 3% of its initial value, and the area decrease of the CH3 IR bending band at 1276 cm-1 was within 5 ( 3% of
Figure 10. XPS and IR data as in Figure 9, for the AA monolayers on glass and ZnSe (the “intervals” experiment data summarized in Figure 7 are not included here).
the initial value. No changes in contact angles could be detected following the exposure to the X-ray radiation. Attempts were also made in the course of the present study to apply AFM for the direct visualization of eventual morphological transformations characteristic of X-raydamaged monolayers. We tried to follow the gradual degradation of OTS/Si; however, distinct changes (in otherwise featureless images) could be observed only at very high levels of damage, as shown in Figure 13 for an OTS/Si monolayer irradiated for ca. 7.5 h under a beam flux approximately 1.5 times more intense than that usually employed in the present experiments. While the loss of carbon in this sample was of the order of ∼30%, the drop in IR C-H absorption intensity (transmission at
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Figure 11. FTIR spectra (transmission at normal incidence, corrected “one-side” spectrum) of OTS/ZnSe showing the C-H, Si-O-Si, and Si-OH bands before (full line) and after (dashed line) 88 min of exposure to the XPS measurement conditions. Note the modified band distribution in the Si-O-Si spectral region following the exposure to the X-ray beam. Table 3. Contact Angles (Static, Advancing, and Receding) Measured on Unexposed and X-ray-Exposed Monolayer Areas bicyclohexyl n-hexadecane monolayer irradiation samplea time (min) OTS/G OTS/ZnSe
180 88
sample areab unexposed exposed unexposed exposed
θadv θrec (deg) (deg) 56 49 55 46c
56 44 55 46
θadv (deg)
θrec (deg)
52 40 51 37
52 36 51 34
a Same samples as in Table 1. b Unexposed area refers to the monolayer surface before irradiation as well as the masked regions of the surface during irradiation. Irradiated spots could be easily located by examining the movement of drops of liquid on the surface. Impeded motion was found to coincide with the position of an irradiated spot. c Higher instantaneous angle, reaching this stable value after ca. 60 s.
Figure 12. Plot of the integrated XPS lines, including the total carbon contribution from the C 1s line at ca. 283.6 eV (XPS.C) and the curve-fitted carboxylic carbon (XPS.CO) contribution around ca. 288.6 eV, the O 1s (XPS.O) and Si 2p (XPS.Si) lines, the integrated overall IR C-H stretch absorbance measured in the transmission mode at normal incidence (IR.CH) and at the Brewster’s angle of incidence (IRB.CH), and the integrated IR absorbance of the COOH band around 1716 cm-1 measured at the Brewster’s angle of incidence (IRB.COOH), as a function of the XPS exposure time of the NTSox monolayer on silicon. The data are normalized with reference to the respective initial signals, as in Figures 7, 9, and 10. Note the opposite trends displayed by the normal incidence and Brewster’s angle CH infrared signals, ascribable to disordering effects, and the decrease in the XPS.O, XPS.CO, and IRB.COOH signals, due to the loss of carboxylic acid functions.
normal incidence) was more than 85%, indicating the formation of a carbonaceous material with less than 20% of the hydrogen content of the initial monolayer. Discussion The combined XPS-FTIR data show unequivocally that, in general, hydrocarbon monolayers are prone to substantial damage when exposed to routine XPS measuring conditions. Although the irradiation conditions used in the present work were particularly mild, substantial
degradation of the hydrocarbon monolayer core occurred in part of the studied systems within less than 1 h, and in some cases about half of the hydrocarbon material was affected within 3 h of exposure to the X-ray radiation. The damage caused to specific functional groups in the monolayer was even more drastic. In general, the present results point to substantial differences between the IR and XPS data with regard to the rate and extent of degradation on n-alkane monolayers, the decrease in the IR C-H absorption (measured in transmission at normal incidence) tending to be significantly larger than the decrease in the XPS C 1s signal. This difference is readily explained by the complementarity of the two techniques; a decrease in the XPS C 1s signal indicates loss of total carbon, while a decrease in the IR C-H absorption may arise from loss of CH2 and CH3 groups, plus degradation processes of the paraffinic tails (loss of hydrogen) and disorientation effects not necessarily involving loss of carbon atoms from the surface. Thus, the experimental data can be interpreted in terms of two different monolayer degradation paths: (1) Evaporation of molecular fragments or entire debonded molecular chains (following scission of monolayer molecules). This process is detected by both IR and XPS. (2) Chemical transformations in the monolayer accompanied by disordering effects, without loss of carbon, such as dehydrogenation followed by cross-linking, branch-
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Figure 13. Topography (left) and the corresponding friction (right) AFM images of an OTS/Si monolayer, taken before (top) and after (bottom) its exposure to the X-rays for ca. 7.5 h (see text). Characteristic cross-sectional profiles along a marked line are shown below each image. The images were moderately flattened without any other filtering being applied.
ing, double-bond formation, rearrangements, etc. These processes result in the formation of a hydrogen-depleted, nonvolatile carbonaceous material.9 Monolayer degradation following this path is detected mainly by IR, as it has only a minor effect on the overall XPS carbon line.9 These two degradation paths appear to be competitive, their relative importance depending on the nature of each
particular monolayer system and the extent of induced damage. In this respect, two main factors are to be considered: the mode of anchoring to the solid substrate (which is determined by both the chemical nature of the monolayer constituent molecules and that of the substrate) and the presence of specific functional groups within the monolayer. The experimental data also suggest that the
Monolayer Damage in XPS Measurements
rate of degradation, especially the rate of carbon loss (as indicated by the drop in the XPS C 1s signal), is higher at the onset of the irradiation and gradually decreases as irradiation continues, while the IR C-H absorption, indicative of dehydrogenation and disordering effects, continues to drop after the XPS carbon signal levels off (see Figures 7, 9, and 10 and Tables 1 and 2). Binding Mode Effects. The different behavior of monolayers with different modes of monolayer-substrate and intralayer coupling, on the same or a similar substrate, is well demonstrated by comparing the results obtained for AA/G monolayers with those of the OTS/G or OTS/Q monolayers. While in the case of AA, both the IR and XPS measurements indicate a similar extent of damage (35-40% material loss) caused to the monolayer hydrocarbon core during ca. 3 h of irradiation (Table 2, Figure 7), for OTS (Table 1, Figure 9), the IR-detected damage is considerably larger (=55%) than that detected by XPS (=11%) over the same period of time. Furthermore, in the case of AA/G, the remaining monolayer material largely preserves its initial high degree of order, in spite of the observed decrease in the total IR absorption intensity (Figure 6, Table 2), while in the OTS monolayers the damage manifests itself also in significant disordering of the remaining paraffinic tails (Figure 3, Table 1). Since disordering effects are negligible in the AA/G system, as indicated by the fact that both the band widths and the peak positions of the C-H stretch vibrations are largely conserved during the first 3 h of irradiation (Figure 6), the observed decrease in the integrated IR absorption provides, in this case, a straightforward measure of the fraction of monolayer material lost as a result of the exposure to the X-ray beam. Thus, comparing the IR and XPS results (Table 2), we can conclude that ca. 33% of the initial hydrocarbon material is lost through detachment from the surface and subsequent evaporation (XPS data), while ca. 8% of it (adding up to the 41% decrease in the IR C-H absorbance) is converted into a hydrogen-depleted carbonaceous material still present on the surface at the end of 168 min of irradiation. According to this interpretation, the AA/G sample irradiated for 180 min (Table 2) lost ca. 25% of the initial monolayer material through detachment from the surface, while 10%-15% of it was converted into a nonvolatile carbonaceous material. The analysis of the results obtained with the OTS samples is more complex, since, in this case, disordering effects contribute significantly to the IR data, as evidenced both by the blue shift and broadening of the C-H stretch bands and by the rather large differences between the apparent damage deduced from the normal incidence and that deduced from the corresponding Brewster’s angle data (Table 1). An estimation of the degradation mechanism prevailing in OTS monolayers may be obtained by the following simplified calculation. Considering, for example, the OTS/G sample irradiated for 180 min, the XPS data indicate that 89% of the initial carbon is still present on the surface (Table 1). Since the high loss of terminal CH3 groups (ca. 82%) accounts for ca. 5% of the total carbon content of the layer, this residual monolayer material represents ca. 94% of the carbon content of the initial paraffinic tails, excluding the top methyl groups. If this residual monolayer were totally disordered (random in-space orientation of its hydrocarbon tails), the IR C-H absorption should have decreased by a factor of 2/3 compared with that of a corresponding ordered film;37 i.e., it should have dropped to ca. 63% (2/3 × 94%) of that of the initial monolayer. Since the actual observed value is ca. 45% of the absorbance measured for the initial monolayer (55% decrease), at least 18% of the initial hydrocarbon tails must have been converted into a
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hydrogen-depleted carbonaceous material. In fact, the degree of such conversion must be considerably higher, as, contrary to the assumed total disordering of the paraffinic tails, the position of the CH2 ν(as) peak at 2920 cm-1 and the relatively high residual contact angles (Table 3) indicate retention of a non-negligible degree of structural order in the irradiated film. Thus, contrary to the AA/G system where a major fraction of the hydrocarbon material seems to be lost through desorption from the surface (with only minor disordering of the remaining hydrocarbon tails during the first 3 h of irradiation), the major path of damage in the OTS systems appears to involve chemical degradation (presumably cross-linking with the consequent loss of hydrogen atoms), accompanied by significant disordering of the remaining hydrocarbon tails, and only little loss of carbon from the surface. The observed differences between the two systems must obviously originate in their different modes of monolayer-to-surface and intralayer coupling (ionic/hydrogen bonding to the surface, in AA,42,45 and covalent/hydrogen bonding to both the surface and adjacent monolayer molecules, in OTS34,42,45), as otherwise both of them consist of similarly packed and similarly oriented long paraffinic tails, on the same or a similar substrate material (glass, quartz). The damage mechanism prevailing in the AA/G system during the first 3 h of irradiation can be explained by assuming that the fast radiation-induced decarboxylation of the fatty acid molecules12 (vide infra) is followed by subsequent evaporation into the ambient space of entire debonded paraffinic tails and rapid lateral reorganization of the remaining (unaffected) material into a partial film whose local molecular arrangement resembles that of the initial monolayer. The comparable loss of CH3 and CH2 groups in the AA/G system (Table 2, Figure 6) is consistent with the evaporation of entire decarboxylated paraffinic tails, which probably occurs in a manner similar to the observed evaporation of paraffinic solvent molecules entrapped into incomplete long chain monolayers.45,47 Acceleration of this process is to be expected under the UHV conditions prevailing during the XPS measurement. The lateral diffusion and clustering of the remaining AA molecules left on the surface finally leads to the formation of a depleted monolayer with molecular organization similar to that of the initial one, as suggested a long time ago by Bigelow and Brockway on the basis of results of electron diffraction experiments performed with various incomplete fatty acid monolayers on glass.48 This reorganization process appears to lose its effectiveness as the monolayer depletion approaches ca. 40% of the initial coverage, and accumulation of defects results in significant disordering of the remaining paraffinic tails. Enhanced disordering of the tails favors the transition to a chemical degradation path like that observed in the OTS monolayers (see sample irradiated for 337 min, in Figure 6 and Table 2). Enhancement of the relative importance of chemical degradation processes (without loss of carbon), presumably due to disordering effects caused by surface charging, is also apparent in the results obtained for AA/G when it is irradiated in the absence of a charge-compensating electron flood gun, where enhanced disorder in the hydrocarbon core is clearly indicated by the IR data. The analysis of the XPS carbon lines of the AA/G sample shown in Figure 8 (irradiated without the use of the electron flood gun) indicates that (i) not every decarboxylation event results in the evaporation of an entire debonded paraffinic tail and (ii) CdO species differing from the initial COOH (47) (a) Bartell, I. S.; Ruch, R. J. J. Phys. Chem. 1956, 60, 1231. (b) Bartell, I. S.; Ruch, R. J. J. Phys. Chem. 1959, 63, 1045. (48) Bigelow, W. C.; Brockway, L. O. J. Colloid Sci. 1956, 11, 60.
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function are generated in the irradiated film. These observations suggest that both hydrocarbon and oxygenated carbon radicals resulting from the initial cleavage of paraffinic tails or from primary decarboxylation events may participate in secondary reaction events provided their lifetimes within the film are sufficiently long. Disordering of the paraffinic tails would obviously contribute to a delay in the escape of such species from the film. Further evidence regarding the role of the radiationinduced decarboxylation as an important primary event in the degradation path of the AA monolayers comes from the analysis of AA/ZnSe and NTSox/Si (vide infra) systems, where both IR spectroscopy and XPS are informative with regard to the fate of the carboxylic groups. According to the IR data (Figure 5, Table 2), there is a comparable loss of COO-, CH3, and CH2 groups in AA/ZnSe (taking into account the significant disordering effect here, which contributes to a somewhat higher apparent loss of CH2). According to XPS (Figure 4, Table 2), the loss of total carbon is lower than what could be expected if the only damage mechanism were via evaporation of entire debonded tails. This would indicate that the evaporation of debonded paraffinic tails competes here less favorably with other chemical events that may take place within the film following decarboxylation. Similarly to the case of AA/G irradiated without the use of the electron flood gun, this may be attributed to the considerable disordering of the hydrocarbon core in AA/ZnSe (indicated by the broadening and large blue shift of the CH2 stretch band), which is expected to slow down the detachment from the surface of debonded tails and thus enhance the possibility of their participation in secondary cross-linking events. The weak CdO band apparent in the IR spectrum of the irradiated film around 1720 cm-1 (Figure 5) is suggestive of CdO species formed, as in the case of the AA/G sample discussed above, following initial decarboxylation events. This would also account for the observed difference between the IR-detected loss of COO- (ca. 28%) and the XPSdetected loss of carboxylic carbon (ca. 20%), as derived from the deconvolution of the weak carbon line around 288 eV (Figure 4). The larger disordering effect in AA/ZnSe, compared with that in AA/G (irradiated under normal conditions), is, most probably, a consequence of the lower lateral mobility of AA molecules bonded ionically to divalent Zn2+ ions on the ZnSe surface,44 as compared with the mobility of AA on glass, where the coupling to the surface involves weaker ionic bonds with monovalent ions and presumably also hydrogen bonding.42 A lower lateral mobility would not permit sufficiently fast reorganization of the depleted monolayer following evaporation of paraffinic tails from the surface, thus promoting conformational disorder in the residual film material. It thus appears that besides characteristic similarities in the behavior of AA/G and AA/ZnSe (Figure 10), we can also identify clear differences, ascribable to the different strengths of binding to the substrate and the different lateral mobilities in these two monolayer systems. The AA/ZnSe system may, therefore, be regarded as an intermediate case between the highly mobile AA/G system and the covalently immobilized OTS monolayer system. Differences associated with variations in the mode of binding to different substrates are also apparent in the OTS monolayers (Table 1, Figure 9). Samples of OTS on glass, quartz, and silicon show quite similar behavior, which could be expected in view of the fact that the outer surface of each of these substrates is essentially silicon oxide, with outer exposed Si-OH groups as the only binding sites. Significantly larger IR-detected damage
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(including disordering of the paraffinic tails) is observed in OTS/ZnSe (see also contact angles in Table 3), and the XPS-detected damage follows the same trend. As evident from Figure 11, the Si-O-Si bands around ca. 1100 cm-1 undergo marked transformations upon exposure to the X-ray beam, which would suggest a higher degree of polymerization, reorientation, and eventual cross-linking of the silane head groups. Since the remarkable stability of the MTS monolayers implies that the silicon-carbon bond is not cleaved under exposure to the X-ray radiation used in XPS measurements, the radiation-induced damage in the silane monolayers should occur primarily through scission of C-C and C-H bonds, followed by evaporation of molecular fragments and cross-linking events within the hydrocarbon core of the monolayer. The stability of MTS monolayers further suggests that no major changes are directly induced by the radiation in the silane head groups region, at least in silane monolayers anchored via covalent/hydrogen bonds34 to silicon oxide surfaces. It thus appears that the weaker bonding of OTS to the ZnSe surface49 allows transformations in the silane region that either are directly induced or arise as a consequence of the stress caused by the cross-linking of the paraffinic tails. However, as in silane monolayers on silicon oxide surfaces, loss of film material via detachment of entire silane molecules from the surface is not likely to make a significant contribution to the radiation-induced degradation of OTS/ZnSe, if one considers that the total carbon loss (ca. 15%) is lower and that the loss of terminal CH3 groups (ca. 65%) is much higher than the drop in the integrated IR absorption of the siloxane bands (ca. 20%). Functional Group Effects. NTSox is identical to OTS in practically all respects,34 except for the replacement of CH3 by COOH as terminal group. This is seen to have profound consequences on the behavior of this long chain silane monolayer under exposure to the X-ray radiation. The IR CH2 and the XPS C 1s results for the NTSox monolayers differ notably from those of the other long chain monolayer systems (Table 1), indicating considerably lower loss of hydrocarbon material. However, there is a substantial loss (ca. 70%) of the top carboxylic acid functions following 100 min of X-ray exposure (Table 1, Figure 12). Although a similar high loss was also observed for the terminal methyl group in OTS (as a result of the evaporation of cleaved molecular fragments), the degradation in the NTSox case seems to be restricted mainly to the top carboxylic acid group, as the depletion of the NTSox hydrocarbon core is much lower than that observed in the OTS case. A quantitative analysis of the NTSox/Si data (Table 1, Figure 12) reveals that nearly all of the observed decrease in the XPS C 1s signal (4%) may be accounted for by the extensive loss of the top carboxylic acid functions (70% loss, as estimated from the Brewster’s angle IR data and considering that the carboxylic carbon is one out of 18 carbon atoms in the NTSox molecule), implying that almost no paraffinic tail carbon was lost. Furthermore, according to the Brewster’s angle IR measurements, there is an actual increase in the C-H absorption, accompanied by considerable broadening and blue shift of the CH2 stretch bands, following irradiation (Table 1, Figure 12). Such behavior would be expected if significant disordering of the paraffinic tails occurred without significant loss of hydrocarbon material. A comparison of the NTSox/Si and OTS/Si IR data (both the normal incidence and the Brewster’s angle modes) shows that while the molecular order of both monolayer systems is affected by the (49) The anchoring of OTS to ZnSe is not yet fully understood. There may be covalent bonding42 to hydroxyl (ZnOH) and selenium surface sites as well as hydrogen bond formation34 with the ZnSe surface.
Monolayer Damage in XPS Measurements
irradiation, the drop in the C-H absorption is much higher in the OTS case, which is also confirmed by the XPS results. A careful inspection of the XPS C 1s line in NTSox/Si, by means of its peak fitting into carboxylic and unoxidized paraffinic carbon components (around ca. 289 and 284 eV, respectively) reveals that ca. 65% of the carboxylic carbon was lost upon irradiation (Figure 12), with almost no change in the unoxidized carbon component. The selective loss of carboxylic groups in NTSox is also reflected by the quite strong decrease of the XPS O 1s signal during the X-ray irradiation, in contrast to the slight increase in the Si signal (Figure 12). Usually, we observed a gradual slight increase in both Si and O signals during the irradiation of OTS samples on silicon oxide substrates, because of the gradual depletion of the organic overlayer. Additional insight into the nature of the damage induced to NTSox monolayers under the XPS measurement conditions is gained by comparing the results obtained for the NTSox/Si and the NTSox/G samples (Table 1). According to XPS, the carbon loss is quite similar (4% versus 5%) in both samples, while NTSox/G, which was irradiated for a longer time, shows a significantly higher IR C-H absorption drop (17% versus 8%). Thus, prolonged irradiation results in slow dehydrogenation with only very little further carbon loss. The behavior of NTSox monolayers under exposure to the X-ray radiation can be rationalized in terms of a mechanism whereby primary radiation-induced decarboxylation events (occurring with a significantly higher probability than the random scission of C-C bonds12) are followed by rapid secondary cross-linking events initiated by the free radical ends of decarboxylated paraffinic tails that maintain their perpendicular orientation in the densely packed monolayer, while being immobilized on the substrate surface through stable bonds provided by the silane head groups. Confinement of the cross-linking, under such conditions, at or near the outer extremities of the paraffinic tails, may be expected to stabilize the reacted monolayer with respect to further loss of carbon, since the formation of volatile molecule fragments in top-crosslinked tails would require the simultaneous breaking of two C-C bonds along the same hydrocarbon chain, which is obviously a much less probable event. Moreover, one may also expect that single C-C bond scission events occurring along a hydrocarbon chain immobilized at both its extremities would preferentially terminate through the reformation of the same C-C bonds, as the positions in space of such cleaved tails cannot change appreciably during the lifetimes of the free radicals involved in the process. This should thus slow down further cross-linking of the film as well as the accompanying release of hydrogen atoms. While loss of hydrogen may occur also via formation of unsaturated double bonds, the efficient blocking of those degradation paths involving evaporation of molecular fragments and radical chain reactions leading to cross-linking throughout the entire volume of the monolayer would result in a considerable reduction of the overall rate of degradation of the backbone of NTSox as compared with the case for OTS. The relatively large disordering effect in NTSox, in spite of the much lower overall damage caused to the NTSox hydrocarbon backbone compared to that for OTS, is presumably a consequence of the severe conformational distortions (kinks, bending, tilting) imposed by the preferential cross-linking of the paraffinic tails at their outer extremities. Thus, the apparent enhanced stability of NTSox is gained at the expense of sacrificial loss of its outer COOH functions and significant structural distortions occurring within the hydrocarbon core of the irradiated film.
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MTS Case. MTS is unique among the silane systems, as its “paraffinic tail” is composed of a single CH3 group whose position on the surface and orientation in space are fixed by the silane head group to which it is directly linked. Thus, provided the Si-C bond is not cleaved by the X-ray radiation and no changes are induced in the silane head groups region either, those radiation-induced processes considered to play major roles in the degradation of the long chain silane monolayers can no longer be operative in MTS; cross-linking of adjacent CH3 groups is not likely to occur because of both conformational and lateral distance considerations, and dehydrogenation via double-bond formation is obviously not possible in a singlecarbon-atom molecule. The observed stability of MTS monolayers can, therefore, be taken as a further indirect confirmation of the validity of the proposed mechanism of radiation-induced degradation in the silane monolayers. Morphological Transformations. AFM images of high-quality monolayers prepared on various substrates by the present methodology reveal rather smooth, defectfree, and featureless surfaces, following the contour of the underlying substrate (Figure 13).50 Good OTS monolayers are not damaged by the tip even at very high applied forces (up to 100 nN) and do not display molecularly resolved features either, presumably because of the relatively large mobility of individual silane molecules around their equilibrium positions.34 No significant differences could be identified between AFM images taken before and after the exposure of OTS/ Si monolayers to the X-rays, up to IR-detectable damage levels of the order of 50-60% (as indicated by the decrease in the normal incidence C-H absorption intensity). Only at a very high IR-detected damage (80-90%), was it finally possible to see distinct features in the AFM image of an irradiated film (Figure 13), which are not observable in less damaged films. Although the ≈50 nm wide bumps seen in the topographic image (Figure 13, bottom left) have an average height of less than 0.4 nm, which is not much different from the typical surface roughness of the unirradiated OTS/Si monolayer (Figure 13, top left) and obviously much less than the total thickness of its hydrocarbon core (2.3 nm),34 they are stable and can be clearly identified in both the height and friction (Figure 13, bottom right) images. As with unirradiated OTS monolayers, the images in Figure 13, bottom, could be reproduced after repeated scans in the same surface area and even after very high forces were applied to the tip, without any visible wear being caused to the imaged film area. It is thus quite clear that the bumps are stable and cannot be moved by the tip, most probably representing hard protrusions on a hard and continuous surface background. The fact that they are even better revealed in the friction image is also an indication that what we see are indeed hard asperities on a hard surface, causing torsional motions of the tip as it scans across them. This extensively irradiated monolayer is totally wetted (0° contact angle) by both bicyclohexyl and n-hexadecane (compare with data in Table 3), which confirms the complete loss of the initial OTS structure. However, its relatively high residual hydrophobicity (water contact angles: 93° (advancing), 91° (receding) points to a rather efficient shielding of the underlying hydrophilic substrate by the irradiated film. The appearance of the irradiated OTS monolayers and their wetting behavior are thus consistent with the proposed chemical degradation path without major loss of carbon, according to which one would not expect (50) Maoz, R.; Matlis, S.; DiMasi, E.; Ocko, B. M.; Sagiv, J. Nature 1996, 384, 150.
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Figure 14. Three-dimensional plot showing the XPS-induced damage in various monolayer/substrate systems, as deduced from the integrated intensities of the IR C-H stretch bands (measured in transmission at normal incidence) and the integrated intensity of the XPS C 1s line, as a function of the XPS exposure time. Each point in the plot represents a particular monolayer sample from which both IR and XPS data were collected. The vertical anchoring lines represent the extent of “IR-detected damage”, and the horizontal anchoring lines, the corresponding “XPS-detected damage” (the various substrates are denoted by G-glass, Q-quartz, S-silicon, and Z-ZnSe). Note the significant differences between vertical and horizontal lines, between OTS (open circles) and AA (rectangles), and between NTSox (filled circles) and OTS. Differences between ZnSe and glass (for AA) and between ZnSe and glass, quartz, or silicon (for OTS) are also evident.
formation of visible holes, cracks, or other detectable defects in the film, as a result of the irradiation. Highly cross-linked, hydrogen-depleted hydrocarbons and hydrogen-free amorphous carbon materials, generally referred to a “diamond-like” phases,51 are currently prepared by various sputtering techniques.51 The low hydrogen content of the extensively irradiated monolayer imaged in Figure 13 (less than 20% of the hydrogen content of unirradiated OTS) and its peculiar morphology (as evident from the AFM images) suggest the possible formation of such a “diamond-like” hydrocarbon/carbon film, in which the relief of stress resulting from cross-linking and loss of hydrogen51 is achieved by phase-separation into domains of very low hydrogen content dispersed within a less hydrogen-depleted, less strained phase. Development of crystalline order in such a system is also possible.51 Conclusions The present combined XPS-FTIR experimental data demonstrate that organic monolayers are prone to substantial chemical and structural transformations during routine XPS measurements and particularly when searching for elements that yield weak signals or during elaborate XPS experiments (such as in angle-resolved XPS) which require prolonged X-ray exposures. Because of the limited amount of material available in an ultrathin surface film, levels of damage affecting 30%-40% of the entire hydrocarbon backbone of a monolayer within ca. 1 h of exposure may be common, with even more extensive damage being caused to specific functional groups within the monolayer. This can result in significant interpretation errors, particularly in XPS measurements intended for quantitative or structural purposes. Since the degradation rates (51) See, for example: Angus, J. C.; Hayman, C. C. Science 1988, 241, 913.
tend to be higher at the onset of the irradiation, damage effects should be taken into account even when relatively short (10-30 min) measurement times are employed. The radiation-induced damage may manifest itself as loss of surface-attached material, as chemical/structural deterioration of specific functional groups or of the monolayer hydrocarbon core itself, without major loss of material from the surface,52 or as various combinations of each of these effects. The central new findings of this work, illustrated by the examples summarized in Figure 14, are as follows: (i) Because of the diversity and complexity of the possible damage paths, XPS data alone cannot be used as a sufficiently reliable indicator of the stability of a given monolayer sample under the XPS measurement conditions. Often, the observed invariance of the XPS signals may be misleading, since XPS is rather insensitive to significant chemical and structural radiation-induced transformations in a thin surface film that occur without major loss of material from the surface, thus grossly underestimating or overlooking the actual damage caused to the film. Furthermore, since much of the damage may already occur within the first 10-30 min of exposure to the X-rays, significant degradation of the measured film is frequently caused during the first few minutes of irradiation which are devoted to instrument tuning, even before the actual data acquisition begins. All this would explain much of the inaccuracies, discrepancies, and data (52) We thank Professor Elias I. Franses for bringing to our attention unpublished results obtained at Purdue University with octadecylamine LB films, which appear to be more stable in the XPS environment than corresponding cadmium stearate films.12 Although no significant loss of nitrogen was observed (according to XPS) in these amine films, FTIR indicated up to 50% loss of intensity for the NH2 group after ca. 60 min of exposure. These results again point to possible structural/chemical transformations involving the NH2 groups, which take place in the film without a net loss of material from the surface or major degradation of its hydrocarbon core.
Monolayer Damage in XPS Measurements
analysis difficulties reported in the literature, suggesting that the use, besides XPS, of other chemically and structurally sensitive methods of surface analysis, complementary to XPS (e.g. FTIR), should become the standard accepted practice for a more comprehensive evaluation of eventual transformations induced by the radiation in ultrathin surface films in the course of their examination by XPS. (ii) Different monolayer systems behave differently under identical XPS measurement conditions, the nature and extent of the induced damage depending on the chemical nature and structure of each particular system. Details of the monolayer-substrate and/or intralayer modes of binding and the presence of specific functional groups may have profound effects on the extent of damage, as on the characteristic degradation paths followed by monolayers with otherwise essentially identical hydrocarbon cores. Thus, monolayers of the same compound self-assembled on different solid substrates were found to behave differently under exposure to the X-rays, because of differences in the strength of binding and the lateral mobility of the monolayer-forming molecules on the respective substrates.53 Rather unexpectedly, replacing the terminal methyl of a long chain silane (OTS) by a more labile carboxylic acid function (NTSox) was found to completely alter the damage path of the entire monolayer, resulting in an overall stabilization effect at the expense of sacrificial loss of the top COOH groups. This implies that the responses of different molecular constituents of a monolayer to the X-ray radiation are interrelated, so that the higher sensitivity of a particular functional group (such as CF3CONH,7b Br,4,9 or COOH12) to the radiation not only manifests itself in the selective loss of that particular function but also may influence the behavior of other molecular constituents as well.52 The entire body of available evidence suggests a general damage mechanism whereby an initial radiation-induced excitation in the surface film may evolve via a number of kinetically controlled relaxation paths, the relative weights of which reflect competitive processes strongly dependent on the chemical and structural details of each particular film. Thus, the possible escape into the vacuum of volatile species produced in primary radiation-induced events should compete with secondary intralayer cross-linking events that would tend to inhibit further release of molecular fragments from the film. This is best exemplified by the opposite effects arising from the presence of COOH as either a top (NTSox) or binding (AA) function. The efficient radiation-induced decarboxylation of AA was seen to lead to rapid deterioration of the irradiated film as a result of what appears as evaporation of entire debonded tails, whereas the same primary decarboxylation event in NTSox led to the stabilization of the irradiated film through presumably enhanced lateral cross-linking of the surface-immobilized tails. The significant differences between the nature and extent of the radiation-induced damage in different film systems suggest that, at this stage of our understanding, we cannot safely predict the behavior of a new film system under the XPS measurement conditions on the basis of results obtained for another, similar system, even if the differences between the two may appear rather insignificant at a first look. It is, therefore, necessary that the effects of the radiation on each newly studied film system be carefully assessed applying methods of surface analysis complementary to XPS. This should enable a more (53) This substrate effect should not be confused with that arising from differences in the fluxes of damaging photoelectrons emitted by different substrates.7
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realistic evaluation of the nature of the induced damage and the consequent selection of improved measurement conditions, together with an appropriate data interpretation framework for each particular system of interest. For example, using a monochromatic small size X-ray beam, it was possible to collect satisfactory XPS data (reasonable signal-to-noise ratios, no detectable damage, by either XPS or FTIR) from some self-assembled bilayer films with CdO and nitrogen-rich interlayer functions, by repeatedly exposing fresh surface spots to the beam for at most 8-10 min each, with the actual relevant data in the energy range under a peak of interest being acquired within 2-3 min.54a This procedure took advantage of the good lateral homogeneity of the studied film samples (on polished Si) over areas of the order of several square centimeters. However, the same measurement protocol, with the same monochromatic X-ray beam, applied to a structurally related bilayer on Si differing in its mode of interlayer binding (with amino and carboxylic acid functions)54a or to a silane monolayer on ZnS,54b was found to result in unacceptable levels of chemical and structural damage, as evidenced by both IR spectra and contact angles taken before and after the XPS measurements. Continuing research efforts along these lines will thus be required in order to create a sufficiently wide data base that would enable us to predict the level of applicability of XPS to various types of ultrathin molecular films. A final interesting observation emerging from the analysis of combined AFM-XPS-FTIR-contact angle results obtained in the course of this study is that extensive X-ray irradiation of organic monolayers anchored to the underlying solid support via radiation-resistant bonds may lead to eventual generation of certain new types of carbonaceous “diamond-like”51 films. Further studies of this phenomenon are in progress in this laboratory. As the writing of these concluding lines was completed, we came across a recently published paper by Allara, Craighead, et al. on the electron-beam-induced damage in OTS monolayers on silicon.55 Since the nature of the experimental data reported in ref 55 led the authors to propose that the damage caused to monolayers in the course of XPS measurements is fundamentally identical to that induced by the exposure to an e-beam (the only suggested difference being the much higher efficiency of the latter compared to the X-rays), we consider it proper to draw attention to the fact that a comparison of our results with those reported in ref 55 points to a number of significant differences, perhaps the most notable being the following: (i) Unlike in the e-beam experiment,55 the exposure to X-rays did not produce any oxygenated species. On the contrary, oxygenated functional groups initially present in a monolayer (AA, NTSox) were preferentially lost during the X-ray irradiation. (ii) The loss of hydrogen in the XPS environment is, in general, considerably higher than that of the corresponding carbon, dehydrogenation processes persisting under the X-ray irradiation after the loss of carbon levels off. This was not reported to happen, or is much less pronounced, under the e-beam irradiation.55 Assuming that photogenerated electrons rather than the X-ray photons themselves are indeed the principal damaging species,7 these observed differences would suggest that the final damage product resulting from the irradiation of an organic monolayer does not uniquely correlate with the total electron dose supplied by a given source of electrons to the irradiated surface. Kinetics, i.e. the rate at which the damaging electrons are injected into (54) (a) Maoz, R.; Cohen, H.; Sagiv, J. Submitted. (b) Maoz, R.; Cohen, H.; Sagiv, J. To be published. (55) Seshadri, K.; Froyd, K.; Parikh, A. N.; Allara, D. L.; Lercel, M. J.; Craighead, H. G. J. Phys. Chem. 1996, 100, 15900.
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an ultrathin surface film, seems to play an additional important role (besides their energy distribution7,9) in the determination of the specific damage path followed by the film. Similarly to the chemical and structural factors discussed above, this is conceivably a consequence of the interplay of several competitive processes, including electron-induced primary and secondary free radical55 and ionization9 events, intralayer reorganization and/or dis(56) A high electron flux would enhance the probability of instantaneous multiple C-C bond cleavage events occurring along the same paraffinic tail, which should favor the escape of small volatile species from the surface compared with the rate of propagation of intralayer radical chain reactions. This would thus lead to a higher loss of carbon and a lower preferential loss of hydrogen, as a result of the enhanced rate of the former and the relative inhibition of the intralayer dehydrogenation events. Since our results clearly rule out the possibility that oxygenated surface species are produced via scavenging of radical species generated in the organic layer by O2 from the ambient,55 we suggest that such species could be formed by the reaction of monolayer molecules with oxygen radicals generated through the interaction of a highly intense e-beam with the silicon oxide substrate or with oxygencontaining neutral or ionized species that are carried into the film from the ambient. The expected high electrical charging of nonconducting surfaces upon exposure to an intense e-beam may play a significant role in such a process. Possible damage routes through the e-beaminduced ionization of monolayer molecules9 might also be strongly influenced by surface-charging effects.
Frydman et al.
ordering processes, detachment of volatile species from the surface, and processes dependent on surface-charging effects.56 A comprehensive understanding of all these factors and their interrelated action could ultimately lead to the rational design of monolayers with predictable performance under exposure to X-rays or e-beam radiation: for example, films displaying enhanced stability or degradation properties that can be tailored for specific purposes. Acknowledgment. We thank Dr. Kazufumi Ogawa of Matsushita Electric Ind. Co. Ltd. (Osaka) for the supply of the NTS employed in this study. The AFM imaging (Figure 13) was done by Dr. Sophie Matlis, at the Chemical Services Unit of the Weizmann Institute. This research was supported by a grant from the G.I.F., the GermanIsraeli Foundation for Scientific Research and Development, and by a financial contribution of the Chemical Services Unit of the Weizmann Institute. E.F. is recipient of the Eshkol fellowship, within the program of strategic scientific and technological development, the Ministry of Science and the Arts, Jerusalem. LA962058Q
