J. Phys. Chem. B 2000, 104, 3291-3297
3291
X-ray Induced Modification of Semifluorinated Organic Thin Films† Anthony J. Wagner, Keping Han, Amanda L. Vaught, and D. Howard Fairbrother* Department of Chemistry and Department of Materials Science and Engineering, 3400 North Charles Street, The Johns Hopkins UniVersity, Baltimore, Maryland 21218 ReceiVed: September 17, 1999; In Final Form: January 10, 2000
The X-ray induced modification/defluorination of semifluorinated self-assembled monolayers (SAMs), based on 1H,1H,2H,2H-perfluorodecanethiol (CF3(CF2)7(CH2)2SH) has been studied using X-ray photoelectron spectroscopy (XPS). At short irradiation times X-ray induced defluorination of the semifluorinated SAM exhibits first-order kinetics with respect to the film’s fluorine concentration. The evolution of the C(1s) region during modification is consistent with a kinetic model of defluorination involving consecutive C-F bond breaking events (e.g., CF2 f CF). Relative defluorination rate constants obtained for the different fluorinecontaining functionalities (e.g., CF2, CF) provide support for a stochastic fluorine loss process, where each individual C-F bond is initially equally labile toward dissociative electron attachment, independent of local chemical environment. Upon atmospheric exposure the modified film’s oxygen adsorption characteristics demonstrate that the density of radicals produced within the organic film during X-ray exposure exhibit a non-linear dependence on irradiation time due to the onset of radical-radical coupling reactions at longer X-ray exposures. The appearance of oxygen within the film is also responsible for further defluorination due to the reactivity of peroxy radicals (-CF(OO•)-). Results from this study support the idea that SAMs can be employed as model systems for developing a detailed understanding of the molecular level events associated with organic surface modification processes.
1. Introduction Surface modification of fluoropolymers is of great technological importance in tailoring interfacial characteristics and properties.1-3 However, fluoropolymers such as polytetrafluoroethylene (PTFE) (-CF2-CF2-)n are extremely stable toward chemical modification; fuming sulfuric acid, nitric acid, aqua regia, hydrofluoric acid, and hydrogen peroxide have no effect on PTFE. PTFE is also thermally stable up to 300 °C.4 Due to the inherent chemical inertness of fluoropolymers, increasing interest is being placed on surface modification using plasma treatments and ionizing radiation.5 For example, PTFE is susceptible to the effects of X-rays,6,7 manifested by changes in its resistance to chemical etching,8 melt viscosity,9 crystalline content,10 and adhesion characteristics.11,12 These changes also provide the basis for technologies such as X-ray lithography and radiation polymerization.13 The mechanism of X-ray induced damage/modification to fluorinated polymers is known to proceed with a loss of fluorine accompanied by C-C and C-F bond scission, a degree of chain-chain cross-linking,8,10 as well as changes in surface morphology.14 Since X-rays do not interact strongly with matter, photogenerated primary and secondary electrons are largely believed to be responsible for the modification process.15 In addition to the bond breaking events, ionizing radiation also produces radicals within the organic film that are stable under vacuum conditions.16 When exposed to atmospheric conditions, electron spin resonance (ESR) studies have shown that these species react rapidly with oxygen to produce peroxy radicals.17,18 †
Part of the special issue “Gabor Somorjai Festschrift”. * Author to whom correspondence should be addressed.
Despite the importance of X-ray induced surface modification of fluorinated polymers, a detailed picture of the evolving nature of the interfacial region under irradiation, the rate processes, and their dependence upon external conditions have yet to be elucidated. The development of accurate kinetic models for polymer modification/degradation processes19,20 are clearly important for tailoring interfacial properties. Furthermore, understanding the changes induced by ionizing radiation is also useful for predicting the effective operational lifetimes of fluorinated polymers used as coatings on spacecraft in low earth orbits,21 as well as quantifying the unwanted effects of excitation radiation present in certain surface analytical techniques.7 The present investigation details experiments carried out to determine the X-ray induced modification of semifluorinated self-assembled monolayers (SAMs). Results from these studies indicate that X-ray induced defluorination proceeds through a sequence of consecutive C-F bond breaking events. Oxygen uptake measurements on the X-ray modified SAM provided a means to titrate the density of radicals produced within the organic film during X-ray irradiation and reveal the importance of the F/C ratio as a convenient measure of film characteristics. A comparison of the results obtained in the present investigation with those on conventional polymers provides strong support for the idea that SAMs can be employed as model systems to study chemical and physical transformations at polymeric interfaces. 2. Experimental Section Sample Preparation. Gold substrates (99.95%, Goodfellow) were cleaned by sputtering with 1-2 kV Ar+ until the carbon
10.1021/jp9933368 CCC: $19.00 © 2000 American Chemical Society Published on Web 03/02/2000
3292 J. Phys. Chem. B, Vol. 104, No. 14, 2000
Wagner et al.
content was less than 1.0% as measured by Auger electron spectroscopy. Cleaned substrates were immersed in 1H,1H,2H,2H-perflourodecanethiol (CF3(CF2)7(CH2)2SH, Oakwood Products) (5 mmol in ethanol) for 10 to 24 hours, allowing the SAM to fully form.22 Substrates were rinsed with excess amounts of ethanol, hexanes, and finally distilled water and stored in a dessicator. Analysis of SAMs stored for several days before X-ray modification revealed no oxygen in the film, indicating that the native SAM was inert toward atmospheric degradation during this limited storage period. Surface Analysis. SAMs were introduced and analyzed in a UHV chamber (Pbase < 2 × 10-8 Torr) equipped with a fast entry load-lock cell. X-ray photoelectron spectroscopy (XPS) experiments were carried out using the Mg KR (1253.6 eV) anode of a Physical Electronics (Φ) 04-500 Dual Anode X-ray Source and a Φ 10-360 Precision Energy Analyzer. All XPS spectra were acquired at 15 kV and 300W using a 45° take-off angle, unless otherwise noted. Binding energy (BE) scales were referenced to the Au 4f7/2 peak at 83.8 eV. High-resolution C(ls) and O(ls) spectra were recorded using an analyzer pass energy of 44.75 eV, while elemental compositions (survey scans: 0-1000 eV) were obtained using a pass energy of 178.95 eV. Self Assembled Monolayer. Before irradiation the integrity of the SAM was determined using the fluorine (1s) to carbon (1s) ratio in the survey XPS spectra. The semifluorinated film was considered acceptable if the F(1s)/C(1s) ratio, hereafter denoted as the F/C ratio, was greater than 1.7. For each of the CF3(CF2)7(CH2)2SH-based SAMs, the initial F(1s):Au (4d3/2) was used to check the uniformity of adsorbate concentration prior to X-ray irradiation. The geometry of the initial SAM was confirmed by angle-resolved XPS measurements (not shown) that indicate the CF3/CF2 intensity is enhanced at glancing angles of detection, consistent with the oriented nature of the CF3(CF2)7(CH2)2SH adsorbate.22 Atmospheric Exposures. Following X-ray exposure, selected SAMs were removed from the surface analysis chamber and placed in the chamber’s fast-entry load-lock. The SAMs were then exposed to atmospheric conditions for a controlled period of time before reintroduction into the chamber and subsequent XPS analysis. Reaction rates toward oxygen were calculated on the basis of the average uptake over the first 200 min of atmospheric exposure. 3. Results X-ray Modification of Native SAMs. Figure 1 shows the ratio of fluorine to carbon, F/C, within the SAM as a function of X-ray exposure time (t). Upon irradiation the F/C XPS ratio is observed to decrease monotonically, consistent with an X-ray induced defluorination process. The dependence of defluorination on X-ray intensity was measured by following the variation in F/C ratio for SAMs exposed to different X-ray powers (50, 150, 300 W). In each case the experimentally measured variation in the F/C ratio was fit using eq 1:
F/Ct ) F/Ct)0 exp(-kdefl t) + A (A ) constant)
(1)
F/Ct)0 and F/Ct represent the fluorine-to-carbon ratio initially and after time t, respectively; kdefl is the defluorination rate constant. For each of the X-ray powers employed a value of A ≈ 0.42 was found. Results from this kinetic analysis are shown as dotted lines in Figure 1. Differences in initial F/C ratios
Figure 1. Time-dependent variation in F(1s)/C(1s) XPS ratio for a CF3(CF2)7(CH2)2SH based SAM on Au, exposed to X-ray irradiation of various intensities; 50 W (b), 150 W (0), 300 W (2).
Figure 2. Variation in first-order defluorination rate constant as a function of X-ray intensity. For each X-ray power the integrated Au 4f7/2 XPS signal was employed to provide an accurate measure of the X-ray intensity.
observed in Figure 1 can be attributed to small variations in the concentration of adventitious carbon present on the surface. The defluorination rate constant (kdefl) determined for each of the three X-ray powers used in Figure 1 can also be plotted as a function of X-ray intensity (Figure 2). Results from this analysis are shown in Figure 2 and illustrate that the first-order defluorination rate constant is directly proportional to the X-ray intensity. More detailed information on the chemical transformations that accompany X-ray induced modification of the semifluorinated SAM, obtained through a detailed analysis of the C(1s) XPS region, is shown in Figure 3. Initially the C(1s) region can be decomposed into three distinct peaks centered at 292.8, 290.5, and 284.5 eV, corresponding to CF3, CF2, and CH2 groups, respectively (Figure 3A).22 Upon exposure to X-ray irradiation, a monotonic loss in intensity associated with the CF3 and CF2 groups is observed, concurrent with a steady build up in intensity associated with fluorine-free hydrocarbon (CC/CH2) species. In addition, at short irradiation times (500 min) there is a downward shift of 0.4 eV in the peak position of both CF2 and CF peaks (Figure 3D). This effect can be ascribed to a decrease
in the electronegativity of the fluorine-containing carbon species, due to the extensive defluorination within the organic film. This decrease is consistent with previous literature assignments of the variation in the CF peak position as a function of local chemical environment.24,25 Complementary information on the nature of the overlayer is also provided by following the variation in Au(4d3/2):C(1s) XPS intensity under vacuum conditions during X-ray exposure. Results from this analysis are shown in Figure 5. The Au/C ratio can be seen to increase at short irradiation times (0-200 min) before decreasing upon further X-ray exposure. Reactivity of X-ray Modified SAMs with Oxygen. Following X-ray exposure, the modified semifluorinated organic film was observed to react with oxygen under atmospheric conditions. Figure 6 shows the variation in the oxygen (1s) uptake with the organic film, measured as a function of the F/C ratio. Due to variations in the X-ray intensity for these experiments, the F/C ratio rather than the irradiation time was used to provide a semi-quantitative measure of defluorination within the organic film. Figure 6 shows that the oxygen uptake
3294 J. Phys. Chem. B, Vol. 104, No. 14, 2000
Figure 7. Variation in the C(1s) region, of a CF3(CF2)7(CH2)2S-Au SAM initially modified by 120 min X-ray irradiation as a function of atmospheric exposure. The insert (top left) shows the correlation between the F(1s) and O(1s) signal intensities as a function of atmospheric exposure; F(1s) (9), and O(1s) (2).
rises sharply at low X-ray exposures until a F/C ratio of approximately 1.2 was attained. At lower F/C ratios, corresponding to longer irradiation times, the oxygen uptake decreases rapidly until a constant rate of oxygen uptake into the film is reached. Uptake of oxygen into the X-ray modified films was also responsible for further defluorination. Experimental evidence of defluorination is shown in Figure 7 for a modified semifluorinated SAM (F/C ratio ) 1.09) illustrated by a reduction in the CF2 peak intensity at 290.5 eV upon exposure to atmospheric conditions. The insert in Figure 7 shows the variation in F(1s) and O(1s) XPS signal intensities as a function of atmospheric exposure for the same sample. The fluorine content of the film was observed to decrease rapidly upon exposure to atmospheric conditions. In contrast, the oxygen content of the film increased until a limiting value was reached for atmospheric exposures greater than 200 min. Similar variations in the fluorine and oxygen content of X-ray modified SAMs as a function of atmospheric exposure were observed for SAMs with different initial F/C ratios, the latter corresponding to various X-ray exposures. 4. Discussion The results obtained in this study enable a detailed picture of the X-ray induced modification of fluorinated hydrocarbon films to be developed. Previous studies26 have provided convincing evidence that X-ray induced defluorination occurs through a dissociative electron attachment process mediated by secondary photoelectrons produced by the initial interaction of X-rays with the adsorbate and/or substrate:
CF3-CF2-CF2-CF2- + e- f CF3-C˙ F-CF2-CF2- + F- (2) The ability to model the defluorination kinetics with eq 1 provides strong support for a first-order decay process. In mechanistic terms this suggests that the defluorination rate depends only on the concentration of C-F bonds within the organic film. Figure 2 shows that the defluorination rate is directly proportional to the X-ray flux and thus by inference the secondary electron concentration. Results from Figure 1 and Figure 2 are consistent with an overall defluorination rate within
Wagner et al.
Figure 8. Experimental concentrations of CF3 (9), CF2 (O), CF (2), and CH2/CC (]) species were determined from the fitted C(1s) spectra (Figure 3). To compensate for slight variations in the XPS signal intensity, the integrated C(1s) area was maintained at a constant value between different spectra. Best fits (shown as solid lines) for the variation in the CFx (x ) 0-3) were determined from a kinetic model in which defluorination is assumed to occur through a series of consecutive C-F bond breaking events (e.g., CF2 f CF).
the organic thin film which can be described by the following relationship:
d[F] ) -kdefl [F] [X-ray intensity] dt
(3)
The presence of a constant in eq 1 indicates that defluorination of the organic thin film does not proceed to completion but leaves behind a residual, partially fluorinated, carbonaceous overlayer. The ability to describe the defluorination process using eq 1 suggests that the film is initially composed of equally labile C-F bonds. Upon longer irradiation times the decrease in defluorination rate indicates a reduced reaction rate. This effect could result from either the presence of a less reactive C-F bond produced during X-ray exposure or a reduction in the flux of secondary electrons within the film due to a change in film composition/structure. The variation in film composition during irradiation can be obtained through a detailed analysis of the C(1s) region shown in Figure 3. X-ray irradiation is observed to produce a steady decrease in the concentration of CF3 and CF2 groups within the film. The production of CF groups as intermediates in the defluorination process, shown in Figure 3, is also consistent with the single-step dissociative electron attachment process described by eq 2. Deconvolution of the C(1s) region shown in Figure 3 also enables the relative population of CFx (x ) 0-3) species within the film to be determined. This information can be then used to construct a time-dependent picture of the film composition, shown as a function of X-ray irradiation as symbols in Figure 8. Kinetic Model. Based on the first-order nature of the defluorination process shown in Figures 1 and 2, a simple kinetic model can be postulated, based on a consecutive series of singlestep C-F bond breaking events within the organic film: k1
k2
k3
CF3 98 CF2 98 CF 98 C
(4)
This model leads naturally to a series of coupled first-order
X-ray Induced Modification of Semifluorinated SAMs
J. Phys. Chem. B, Vol. 104, No. 14, 2000 3295
TABLE 1: Best-Fit First-Order Rate Constants Determined from the Kinetic Model (eq 4) kinetic process
rate constant (s-1) × 10-3
k1 (CF3 f CF2) k2 (CF2 f CF) k1 (CF f C)
17.6 7.7 2.1
differential equations:
d[CF3] d[CF3] ) -k1 dt dt
PTFE was exposed to ionizing radiation, the yield of the chain radical (-CF-) was an order of magnitude greater than that of the chain scission radical (-CF2•). This observation is consistent with those of the present study, and it appears that the C-F bond is broken more easily than the C-C bond under the influence of ionizing radiation. Evolution of the Carbonaceous Overlayer. ESR studies have shown that radicals are produced from the interaction of ionizing radiation with fluoropolymers via eq 2.16,17,27 Under atmospheric conditions these species are known to react rapidly with molecular oxygen to produce peroxy radicals.16,17 For example,
d[CF2] d[CF3] d[CF2] ) k1 - k2 dt dt dt d[CF2] d[CF] d[CF] ) k2 - k3 dt dt dt d[C] d[CF] ) k3 dt dt
(5)
The variation in film composition as a function of X-ray exposure shown in Figure 8 has been fitted with this kinetic model using the software package Scientist for Windows (v2.01 Micromath, Salt Lake City, UT). Results from this analysis are shown in Figure 8, while the best-fit values of k1, k2, and k3 are shown in Table 1. The excellent agreement between the concentration profiles obtained experimentally and those generated from the kinetic model provide strong support for the validity of a single step C-F bond breaking mechanism. The absolute magnitude of the rate constants k1, k2, and k3 is expected to be dominated by the flux of secondary electrons ejected from the Au substrate during irradiation. However, the relative magnitude of k1, k2, and k3 with respect to one another, as well as their dependence upon external variables such as incident power, can be used as the basis for a more detailed mechanistic understanding of the defluorination process than has previously been possible. For example, the best-fit values of k1, k2, and k3 can be seen to decrease in the order (see Table 1):
k 1 > k2 > k3
Consequently, the initial reaction rate of oxygen with the modified SAM can be used to titrate the density of radicals present within the film as a function of X-ray irradiation. Figure 6 shows that the density of radicals rises sharply for small changes in the initial F/C ratio, passing through a maximum at F/C ≈ 1.2 before falling again to a roughly constant level as the F/C ratio decreases under the influence of continued X-ray irradiation. The observation of a maximum in the oxygen affinity of the modified film as a function of X-ray exposure can be understood in terms of radical-radical reactions within the overlayer. At short irradiation times the majority of radical species formed within the film are trapped as isolated reactive sites on the C-C backbone, e.g.,
CF3-CF2-C˙ F-CF2CF3-CF2-CF2-CF2-
(9)
However, upon longer exposures, cross-linking between proximate alkyl radicals formed during irradiation become increasingly significant:
(6)
scaling with the number of C-F bonds within the respective chemical groups, i.e.,
CF3 > CF2 > CF
(7)
This result supports the idea of a stochastic X-ray induced defluorination mechanism within the semifluorinated film and suggests that each CF containing moiety (e.g., CF2) can initially be considered a collection of equally labile C-F bonds, independent of local chemical environment. However, at longer irradiation times the kinetic model underestimates the CF2 film content (Figure 8), consistent with the defluorination kinetics observed in Figure 1. C-F versus C-C Bond Breaking During X-ray Irradiation. Our ability to model the evolution of the overlayer by a series of single C-F bond breaking events, coupled with the lack of significant attenuation in the Au/C ratio after prolonged irradiation times (Figure 5) suggests that X-ray induced modification is dominated by C-F rather than C-C bond breaking. Siegel and Hedgpath16 have also reported that when
This effect reduces the density of chain radicals and consequently the film’s affinity for oxygen. As the extent of defluorination continues even further, these structures could then give rise to further cross-linked structures, e.g.,
The predicted decrease in radical concentration due to radicalradical reactions at longer irradiation times is consistent with the results shown in Figure 6 as well as ESR studies on the density of radicals produced during the UV degradation of fluorinated ethylene propylene.21 The importance of coupling reactions in limiting the density of alkyl radicals has
3296 J. Phys. Chem. B, Vol. 104, No. 14, 2000 also been noted by Mizuno et al.28 in a study of the formation of carboxylic acid groups at the surface of irradiated polymer films. Experiments on the reactivity of the X-ray modified SAM have shown that the C(1s) peak profile and area were constant with respect to storage times of several days under vacuum conditions, indicating a lack of reactivity between the modified interface and any carbonaceous species. Consequently, changes in the Au(4d3/2)/C(1s) ratio during the X-ray irradiation can be used to provide a qualitative measure of the changing density of the carbonaceous overlayer. The changes in the Au(4d3/2)/ C(1s) ratio during irradiation, shown in Figure 5, can be rationalized in the context of the film’s evolution determined from the oxygen reactivity studies. For short exposure times the dominant process within the film is loss of fluorine, leading to a reduction in the density of the organic film. This is expected to produce an increase in the inelastic mean free path of the Au substrate X-ray photoelectrons, leading to an increase in the Au(4d3/2) signal at short irradiation times, reflected in the increase in initial Au/C ratio observed in Figure 5. At longer irradiation times (>200 min) radical-radical reactions become more significant than defluorination, leading to an increase in film density, evidenced in Figure 5 by the decreasing Au/C ratio. Similar variations in C(1s) intensities have been observed during electron beam irradiation of polymers, and have also been interpreted on the basis of cross-linking between carbon chains.29,12 Additional support for the relationship between the Au(4d3/2)/C(1s) ratio and the nature of the overlayer is that the peak observed in Figure 5 corresponds to a F/C ratio of ≈1.2, corresponding to the organic film’s maximum reaction rate with oxygen. The onset of significant cross-linking within the overlayer for irradiation times greater than 200 min could also be responsible for the change in the defluorination kinetics upon prolonged X-ray exposure. Experiments on the variation in the F/C ratio for the film exposed to more than 200 min of 300 W X-ray irradiation suggest that defluorination is sustained at longer irradiation times, but at a significantly reduced rate. This observation of a reduced defluorination rate upon prolonged X-ray exposure is also evidenced by the fact that the kinetic model underestimates the concentration of CF2 within the film for irradiation times in excess of 200 min (see Figure 8). The underlying mechanistic reasons for the observed change in defluorination kinetics are unclear. However, one possibility is that the reduced defluorination rate results from the increase in film density caused by the onset of significant radical-radical crosslinking reactions upon longer irradiation times. An increase in film density will reduce the density of Au secondary electrons within the film, consequently decreasing the rate of dissociative electron attachment (eq 1). The insert in Figure 7 illustrates that oxygen uptake is also responsible for a degree of additional defluorination within the organic film. Specifically, analysis of the C(1s) region reveals that the addition of oxygen induces a loss in CF2 intensity. On the basis of the dominance of C-F bond breaking events during irradiation, the major reaction upon exposure to the atmosphere is expected to be oxygen capture by the chain radical,27 i.e.,
Subsequent degradation reactions of this peroxy radical, based
Wagner et al. on studies by Corelli et al.,10
are shown in reaction scheme (13). Scheme 13 illustrates that oxygen capture by the chain radical results in production of -CFO and subsequent C-C bond scission at the β-position to yield peroxy radicals. Reactions of the -C˙ F2 radical with oxygen results in defluorination, consistent with the decrease in CF2 peak intensity shown in Figure 7. A detailed analysis of the C(1s) region has not been attempted since the variety of chemical species anticipated in the film after oxygen exposure makes an unambiguous fitting of the C(1s) envelope impractical. However, based on reaction scheme 13 the C(1s) binding energies of the likely reaction products lie below the CF2 binding energy (290.5 eV), consistent with the changes observed in the C(1s) spectral region shown in Figure 7. The potential X-ray induced activation of peroxy radicals during XPS analysis was checked by following the variation in the C(1s) spectral region for limited X-ray exposures (
