5348
J. Phys. Chem. 1993, 97, 5348-5355
Surface Studies by Static Secondary Ion Mass Spectrometry: Adsorption of 3-Mercaptopropionic Acid and Cysteine onto Gold Surfaces Graham J. Leggett,' Martyn C. Davies,' David E. Jackson, and Saul J. B. Tendler The VG SPM Laboratory for Biological Applications, Department of Pharmaceutical Sciences, University of Nottingham, University Park, Nottingham NG7 2RD, UK Received: November 20, 1992; In Final Form: February 26, I993
Static secondary ion mass spectra have been compared for 3-mercaptopropionic acid and cysteine in the crystalline state and adsorbed onto gold surfaces. The fragmentation behavior observed for the solid-state material is found to be different from that observed for themolecular adsorbatesystems. In thecrystalline state, the fragmentation of 3-mercaptopropionic acid is dominated by scission a and @ to the sulfur atom and by OH loss. In the adsorbate system, the sulfur-gold bond is sufficiently strong for the dominant fragmentation steps to involve scission of the S-C bond in the adsorbate and ejection of gold atoms bound to the adsorbate molecules. Similar changes are observed for cysteine when adsorbed onto gold surfaces. The spectra provide clear evidence for a chemically determinate relationship between surface structure and secondary ion structure. Electronic deexcitation of sputtered species after ejection from the surface is thought to be an important mechanism of ionization, with the effect that radical cations are observed in the SIMS spectrum.
Introduction The phenomenon of molecular self-assembly, in which thiol compounds adsorb spontaneouslyonto metallic substratesforming monolayers (self-assembled monolayers or SAMs), has been the subject of growing Much of the published work relates to adsorptionon gold surfaces, although self-assembly onto silverE and copper9surfaces has been reported. Monolayer formation from solution is thought to involve chemisorption of the thiol onto thegold surface to yield a thiolate.I0 Becauseof thestrength of the resulting sulfur-gold bond, SAMs are chemically very stable. Furthermore, a wide variety of thiol compounds are now known to form SAMs on gold, providing a versatile and elegant method for the preparation of organic surfaces with controlled structure and hence a means for investigating the effects of molecular structure on surface properties (for example, wetting7JI ) . A number of techniques have been used to study SAMs, including X-ray photoelectron spectroscopy (XPS),z4 infrared spectroscopy,1,2s6ellipsometry,1v2v66 electron and electron diffra~tion.~ As yet, only Tarlov's studyI2has reported the characterization of SAMs by static SIMS. However, the application of static SIMS to studies of SAM formation is potentially valuable. In an important series of studies, Vickerman and co-workers have demonstrated the utility of SIMS for the study of adsorbate systemsI3-l8 and have shown the SIMS sampling depth to be, at most, of the order of 2-3 monolayers with the bulk of the observed ions being ejected from the topmost mon01ayer.I~This degree of surface sensitivity compares very favorably with that of other surface-analytical techniques. Combined with its high molecular specificity,this suggestsstrongly that static SIMS can offer valuable insights into the structure of complex organic adsorbate structures, and SIMS has successfully been used to study the structures of a range of organic film structures?O including Langmuir-Blodgett films.21*22 A further motivation is the possibility that SAMs may provide good model systems for exploring some of the physical processes involved in ion production from the surfaces of organic materials. We are interested in the coupling of proteins to surfaces prepared by the adsorption of thiol compounds onto gold and, in particular, the adsorption of thiol compounds which contain a carboxyl group. Liedberg and co-workers have recently reported studies of the adsorption from aqueous solution of cysteine23-25 0022-3654/93/2097-5348$04.00/0
and 3-mercaptopropionic acid (MPA)23*24 onto gold23-25and surfaces. Using reflection-absorption infrared spectroscopy and XPS, they showed that chemisorption of MPA occurred onto gold. They estimated that the thickness of the adsorbed layer was 2-6 A. This was qualitatively estimated to represent a monolayer of adsorbed material. The present work is a comparative study of the adsorption of MPA and cysteine ontogold,using static SIMS to characterizethe adsorbatesystem.
Experimental Section Glass disks (13-mm diameter; Agar, Stansted, Essex, UK) were placed in a stainless steel staining rack and cleaned by ultrasonicationin a 10%solution of Decon 90 (Agar) in deionized water. The disks were rinsed three times by ultrasonication in deionized water and dried by heating for 90 min at 130 OC.The cleaned,dried disks were sputter-coated with gold (film thickness estimated to be ca. 10 nm using quartz crystal film thickness monitor) and replaced in the staining rack. Although the glass disks were quite clean (as determined by SIMS analysis), and there was little difference in the SIMS spectra of cleaned and untreated disks followingsputter-coating, the gold film was found to adhere much better to the clean disks. For the adsorption experiments, the gold-covered disks were inserted into the adsorbate solution as quickly as possible. the time gap between sputter-coatingof the glass disks and immersion in the MPA solution was typically ca. 5 min. Solutions (1 "01) of MPA and cysteine (Sigma, Poole, Dorset, UK) were prepared in deionized water. Untreated gold surfaces were also retained for analysis and were inserted into the SIMS system following similarly short periods of exposure to the air. Following removal from the MPA solution, the glass disks were rinsed five times with deionized water and inserted immediately into the vacuum system for SIMS analysis. Crystallinecysteine was placed in a dish-type 13-mm-diameter stainless steel sample stub and inserted into the vacuum system. Solid MPA was prepared by placing 100 I.~Lonto a clean glass disk fixed to a stainless steel sample stub the bottom of which was then dipped briefly into liquid nitrogen. There was no contact between the sample and the liquid nitrogen. Cooling was reasonably rapid, and because its freezing point (18-19 "C)was close to room temperature, the MPA remained in the solid state for periods of ca. 15 min (long enough to record several SIMS 0 1993 American Chemical Society
Adsorption onto Gold Surfaces spectra). Some sublimation of the MPA sample led to small increases in the pressure inside the vacuum chamber during the experiment (with argon gas admitted to the atom source, the pressure inside the vacuum chamber is ca. l t 7 mbar; after insertion of the MPA, the pressure rose to ca. 5 X l t 7mbar). However, the white crystalline MPA was visible throughout the experiment. Ultimately (after some 30 min), vaporization of the sample led to the gradual loss of peaks due to the MPA and the observation of peaks due to the glass gubstrate (notably, Si+ at m / z 28 and S O H + at m / z 45). Although this procedure is relatively crude (employed because our instrument does not have a cooling stage), we believe that it is nevertheless valid in this instance because of the closeness of the freezing point of the MPA to the ambient temperature in our laboratory. Moderate random fluctuation of the ambient temperature was on one occasion sufficient to cause a sample of the neat liquid to solidify while in storage. Because of the rapidity with which the procedure was completed and the small change in temperature of the sample surface,relativelylittle water vapor condensed on the MPA surface (with the consequence that only a small peak was observed due to H 2 0 + in the SIMS spectrum). Some small degree of hydrocarboncontaminationmay have resulted, but again we think that this was relatively insignificant by comparison with contamination from other sources (most importantly, trace contaminants in the liquid MPA which migrate to the surface during solidification). SIMS analyses were performed on a VG SIMS Lab system (VG Scientific, East Grinstead, Sussex, UK), fitted with a quadrupole mass analyzer. The primary particle source was an argon atom gun operated with a source potential of 2 keV. A flux density of 1 5 X lo9 atoms cm-2 s-I was employed, and the analysis time was typically 1 1 0 min, giving a primary particle dose 1 3 X 10’2 atoms cm-2. The beam was defocused with a diameter qualitatively estimated to be of magnitude similar to that of the target. No charge compensation was required in positive ion mode, where charging due to the ejectionof secondary electrons results in a small positive equilibrium surface potential which may readily be compensatedfor by adjustment of the target bias potential by ca. 35 V. In negative ion mode, a negative surface potential was achieved by flooding the sample with electrons from a VG LEG 51 electron flood gun.
The Journal of Physical Chemistry, Vol. 97, No. 20, I993 5349 17000 7-
23
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Figure 1. Positive ion static SIMS spectrum of a clean gold surface.
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Results and Discussion (a) SIMS Spectrum of Clean Gold Substrate. Figure 1 shows a positive ion static SIMS spectrum of one of the sputter-coated glass disks. The spectrum is dominated by the peak at m / z 23, Na+. The appearance of this peak in the spectrum is due to the presence of small quantities of sodium as a trace contaminant in the gold; its dominance is the result of a very large sputtered ion yield Y+for sodium. Similarly, the peak at m / z 39 is due to K+, which again has a very high Y+value. The principal gold peak is the Au+ peak at m / z 197 which is weak in intensity because of gold’s comparatively low Y+value.26 The other peaks are due to the presence of small quantities of organic contaminants at the surface. The peaks at m / z 149 and 115 probably arise from small quantities of phthalate-related compounds, commonly encountered as surface contaminants. The source of these contaminants is not clear in this case. The negative ion SSIMS spectrum is shown in Figure 2. The dominant peak is at m / z 35, due to Wl-. The peak at m / z 37 is due to j7Cl-. The peaks at m / z 16 and 17 are due to 0-and OH-, and those at m / z 24,25 and 26 are due to Cz-, C*H-, and C2H2-. Smaller peaks at m / z 32 and 33 are probably due to Sand SH-. All of these ions are due to the presence at the surface of small quantities of adsorbed contaminants and are commonly observed in negative ion SIMS spectra. A few peaks at higher m / z also correspond to ions formed from inorganic trace contaminants (e.g., SO3- at m / z 80). The only peak due to gold
100
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Figure 2. Negative ion static SIMS spectrum of a clean gold surface.
is a weak peak at m / z 197 (Au-) in accordance with the wellknown small Y- value for gold.26 (b) SIMS Spectrum of Frozen MPA. MPA was frozen onto a clean glass disk and the SIMS spectrum was recorded (Figure 3). A number of hydrocarbon ions are observed, some of which are due to the presence in the sampleof hydrocarboncontaminants which have high secondary ion yields. For example, the peaks at m / z 39,41, and 43 (C3H,,+ where n = 3,5, or 7) and the peaks at m / z 55 and 57 (C4H7+and C4H9+respectively) are probably due to the presence of contaminants at the sample surface. Some of these contaminant moleculesmay have adsorbed onto the MPA surface prior to insertion into thevacuum system, but it is believed that the majority are due to trace contaminants in the liquid MPA which migrate to the surface on freezing. The observation of such hydrocarbon ions is common in the analysis of organic solids with SIMS. A number of other major peaks are observed, however, which characterize specificallythe fragmentation of MPA. These ions appear at m / z 45, 47, 61, 89, 105, and 107. A relatively weak
Leggett et al.
5350 The Journal of Physical Chemistry, Vol. 97, No. 20, 1993 800
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Figure 3. Positive ion static SIMS spectrum of frozen MPA.
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100 mlz 200 Figure 4. Positive ion static SIMS spectrum of crystalline cysteine.
peak is also observed at m/z 35. The peaks at m/z 105 and 107 probably correspond to [M - HI+ (a) and [M + HI+ (b),
relatively common during the fragmentation of primary thiol by analogy with the loss of H20 which m r s d u r i n g the fragmentation of primary alcohols, and often thep-34 peak H 0 2 C C H 2 C H d H H02CCH2CH,-6H2 is the base peak in the spectrum.28 Similarly, the MPA SIMS (a) (b) spectrum does not exhibit peaks corresponding t o H 6 and p-58, which might also be expected in the E1 mass spectrum of a thiol respectively. No molecular ion M*+is observed. Here, we employ compound. There may be some contribution from p S H to the the standard nomenclature of mass spectrometry, in which M is m/z 7 1 peak, which is slightly larger than would be expected if the molecular species (the MPA molecule). Thus [M + HI+ and it was solely due to the hydrocarbon contamination, but this is [M - HI are the molecular species plus and minus a hydrogen probably not a large contribution. atom, respectively. The m/z 89 ion, [M - 17]+probably results (c) SIMS Spectrum of Cryst.lline Cysteiae. Figure 4 shows from the loss of OH from the MPA molecule. The weak peak the SIMS spectrum of a sample of crystalline cysteine. There at m/z45 probablycorrespondsto a fragment from thecarboxylic is an [M HI+ peak, at m/z 122, and there is no Me+peak. acid group. The absence of a peak at m/z 74, 73 or 72 ([M However, there is also no [M -HI+ peak, in contrast to the frozen SI*+,[M- SH]+ and [M - H2S]*+,respectively) suggests that MPA spectrum. There is a hint of a peak at m/z 105, although fragmentation by initial expulsion of sulfur does not occur. Thus the intensity of this ion is barely greater than the noise intensity. we conclude that the rest of the MPA peaks are due to sulfurThis peak is probably due to the expulsion of ammonia, a process containing fragments. The observation of peaks at m/z 35,47, invoked in studies of the chemical ionization (CI) mass spectra 61, and 89 is characteristic of the mass spectra of alkanethiols of amino acids.29 Expulsion of ammonia is thought to lead to a obtained by conventionalelectronimpact (EI) i ~ n i z a t i o n .The ~~,~~ small peak at m/z 105 in the CI mass spectrum of cy~teine.2~ peaks at m / z 47 (c) and m/z 61 (d) are thought to result from Because there is now an amino group attached to the &carbon a and /3 scission respectively in the (EI) experiments, and the atom, &scission yields a product at m/z 76 (e) rather than at ratio of the m/z 47 and 61 ions is generally remarkably constant at a value of approximately2 for all primary straight-chain thiols + larger than C3,28 NH2CH--CH-iH2 HOOC-CH=NH2 (e) m/z 76 (0 mlz 74 +
+
U
+*
(dJm/z61
It seems likely that the ions observed at m/z 47 and 61 in the SIMS spectrum of MPA also have the structures (a) and (b). The small peak which appears at m/z 35 is so unusual that it must almost certainly be a fragment of the MPA molecule, SH3+. Although the ions observed are generally those which would be expected to be formed by E1 ionization of a thiol compound, there is no p-34 ion (where p indicates the mass-to-charge ratio of the parent species and p-34 indicates the mass-to-chargeratio of the parent species minus 34-in this case, m/z 106 and 72, respectively) due to loss of mass 34 (H2S). The loss of H2S is
m/z 6 1. Eliminationof ammonia from this fragment leads to the peak at m/z 59; there is only a weak peak at m/z 59 in the crystalline MPA spectrum. Scission of the C-C bond a to the carbonyl carbon atom will yield the ion observed at m/z 74 (f). The peaks at m/z 87, 88, and 89 are more difficult to assign. Cysteine exhibits a peak at m/z 88 in its CI mass spectrum29due totheexpulsionofH2S. ThepeakatmIz88 initsSIMSspectrum could be formed by the same process, with the weak m/z 89 peak being formed by the loss of SH. However, this still leaves the m/z 87 ion. One route for its formation is by loss of ammonia followed by loss of OH. (d) SIMSSpectrumof MPA Adsorbed on Gold. Figure 5 shows a SIMS spectrum of a gold-coated glass disk following reaction with the MPA solution. Therpectrum is complex and extends to high m/z values. In the region m/z 0-100, the principal organic ions are at m/z 41,43,55, and 57, possibly due to the presence of small quantities
The Journal of Physical Chemistry, Vol. 97, No. 20, 1993 5351
Adsorption onto Gold Surfaces
'in
I
I
The most structurally specific ion in this region of the spectrum is the peak at m / z 74, which probably corresponds to [M -SI'+. Clearly this represents a deviation from the fragmentation behavior of the frozen MPA and is illustrative of the strength of the S-Au bond. The frozen MPA exhibited no peak in its SIMS spectrumcorresponding top-32. This ion is the more remarkable for being a radical cation. Other structurally significant radical cations are observed in the MPA SIMS spectrum. These are notably the ions at m / z 46 and 60, although the latter of these is relatively weak. The mechanisticimplicationsoftheobservation of these ions are discussed under section ( f ) below. Peaks at m / z 46,60, and 74 characterize the mass spectra of aliphatic acids,m and these ions probably have the structures (g-i). Whereas the HCOOHl" CH3COOH1'+ CH3CH2COOHl'+ (g) m / z 46 (h) m/z 60 (i) m / z 74
'i
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odd-electron ions are not observed in the SIMS spectrum of frozen MPA, they are observed with intensities greater than those of their even-electron neighbors at m / z 47 and 6 1, which are present only as very weak peaks in the SIMS spectrum of chemisorbed MPA. However, the intensities of all three ions are very much lower than the intensities of the peaks at m / z 45 and 30 in the adsorbate SIMS spectrum; this is in contrast with the SIMS spectrum of frozen MPA, in which the peaks at m / z 47 and 61 are substantially larger than the m / z 45 and 30 peaks. The dominant peak in the region of m / z 100-200 is the ion at m / z 130 (p 24). Assignation of a structure for this peak is difficult. Given that no peak is observed at m / z 130 in the spectrum of frozen MPA, it seems unlikely to be due to a contaminant. This suggests that the ion is characteristic of the adsorbate system, and one possible formulation for this ion would be [NaM HI+. Cationization of organic molecules to metals (commonly the noble is well-known to substantially increasesputtered ion yields, and given the very large discrepancy in the sputtered ion yields for sodium and gold (sufficiently large to counter the opposite discrepancy in their surface concentrations), it would perhaps not be surprising if NaM-type peaks were relatively quite intense. The second largest peak is due to the [M - H]+ ion, at m / z 105. The peaks at higher masses are probably due to trace quantities of organic contaminants, with the exception of the weak peak at m / z 111, which again does not correspond to any likely contaminant. This ion is possibly [NaM - H20]+, but this assignment must remain more speculative. It is also difficult to ascribe a structure to the ion at m / z 102. Between m / z 200 and 300, the peaks at m / z 231,243, and 257 are probably fragments of the adsorbate. However, these peaks are weak relative to the peak at m / z 74. Because of the strength of the sulfur-gold bond and the observation of [AuM i H]+ ions (see below), these ions are likely to be composed of fragments of the adsorbate molecule bound to gold via the thiol sulfur atom. This would suggest that these ions are gold-containing analogues of the ions observed at m / z 35, 47, and 61 in the MPA SIMS spectrum, with the structures j-1.
Figure 5. Positive ion static SIMS spectrum of MPA chemisorbcd onto gold.
of hydrocarbon contaminants at the surface, as in the case of the clean gold (see above). The peaks at m / z 28 and 30 are much larger than would be expected for hydrocarbon contaminants alone and probably correspond to CO+ and H2CO+, fragments sputtered from the adsorbed MPA molecules. A further smaller peak at m / z 31 (probably H3CO+) is most unlikely to originate from typical hydrocarbon contaminants. Related ions at m / z 44 and 45 are also fragments of the carboxylic acid function.
AuS+H2 AuS+=CH2 AuS+-CHCH3 (j)m / z 231 (k) m/z 243 (1) m / z 257 The most distinctive ions in the spectrum are observed above m / z 300, including the peaks at m / z 301 and 303 [AuM HI+, m / z 354, m / z 408 [ A u M ~- H2]+, and m / z 427,465,493, and 499 [Au2M- HI+. The identities of these ions are not all clear, and they may be complex, containing fragments of the adsorbate molecule (by analogy with the ions at m / z 231, 243, and 257) bound to clusters of the type Au,M,. These ions are due to the ejection of gold atoms attached to entire adsorbate molecules and/or fragments of the adsorbate molecule. This suggeststhat the major modes of fragmentation of the adsorbate system involve
5352 The Journal of Physical Chemistry. Vol. 97,No. 20. 1993
the scission of bonds other than the sulfur-gold bond ( C X and C-O, and Au-Au bonds). Indeed,the only significant peak in the spectrum which involva scission of the sulfur-gold bond is that which appears at m/z 105. This means that the sulfur-gold bond is at least as strong as the covalent bonds of the MPA molecule, and these observations thus provide clear evidence for the formation of a stable covalent bond between adsorbate and substrate in this case. ThenegativeionSIMSspectrumofchemisorbed MPA (Figure 6) exhibits few peaks below m/z 200. Those which are observed are primarily due to trace quantities of inorganic contaminants. The Au- peak is clearly seen at m/z 197, and the one major change is the increase in the intensity of the peak at m/z 32, due to S..Above.m/z 200, a number of ions of the type AuS, are observed, including thawat m/z 229and 231 (AIS-and AuSHl ); m/z 263 (AIS2H2.); m/z 427 (AufiH.); m/z 459 (Aufi2H ) and m/z 493 (Aufi~~Hl-). There is no AuM-type peak, but the peak at m/z 335 possibly corresponds to AuMS-. A weak peak is observed at m/r 499 due to [AulM - HI-. However, no other peaks are observed which are due to the fragmentation of the adsorbed MPA moltcule. (e) SIMS Spectrum of Cyst& Adsorkd onto Cdd. Figure 7 shows the positive ion SIMS spectrum of cysteine adsorbed onto gold. As in the case of MPA. the fragmentation behavior has been altered by formation of the bond between theadsorbate and the gold surface, and the SIMS spectrum is dominated by peaks due to ions containing the carboxylic acid group, or fragmentsof it. Them/z 76 peak,due to the nitrogen-containing analogue of the m/z 61 ion, is substantially reduced in intensity, whereas them/z 74ion, whichcontains thecarboxylicacidgroup, is increased in intensity. The ratio of the peaks at m/z 76 and 74 changes from 1.7 for the crystalline cysteine to 0.3 for the adsorbate. The peak at m/z 59, ascribed above to the loss of ammonia from the m/z 76 ion, is also very much reduced in intensity. In the region of m/z 1 W 2 0 0 , the spectrum is weak and complicated by the presence of peaks thought to be due to contaminants. However, a very weak peak is discernible at m/z 120, due to [M- HI+, together with the peak due to ammonia expulsion, at m/z 105. An [AuM]+ion is observed at m/z 318; it too is accompanied by a peak at m/z 301 due to the expulsion ofammonia. A substantial increasein the probability of expulsion of ammonia from cysteine, when bound to gold, is reflected in the relative intensities of the ions at m/z 301 and 318: the peak at m/z 301 is substantially more intense than the peak at m/z 318. The observation of the radical cation at m/z 318 represents a differemascompared tothegold-MPAadsorbatesystem. With the exception of the peak at m/z 354, no other ions are observed at higher mass. While the negative ion spectra of both adsorbate systems are alike in being less complex than their positive ion counterparts, they nevertheless exbibit differences which are due to different modes of fragmentation. The negative ion SIMS spectrum of adsorbed cysteine (Figure 8) exhibits a number of peaks which are not observed in the spectrum of adsorbed MPA. The peak at m/r 59, which is possibly due to the acetate ion, CHJCOO , is the principal peak at low m/z. More structurally specific are the peaks at m/z 105, due to expulsion of ammonia, and at m/z 121-remarkable for being a radical anion (somewhat less common than the radical cation in SIMS). All of the peaks at high m/z values are quite weak in intensity. A peak at 317 is due to [AuM - HJ-; no such peak is observed for the MPA adsorbate system. There is a hint of a peak corresponding to the expulsion of ammonia from the molecular ion. at m/z 301. A small peak at m/z 350 is due to [AuM + SI-, and a small peak at m/z 335 probably corresponds to the expulsion of ammonia from this species. Other peaks. at m/z 229,263.421, and 459. are due to AuS, ions.
Leggatt et al. 3300
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Negative ion static SIMS spectrum of MPA chemisorbed onto
(0 Mcch.btlclrpacreOr. Thestrong S-Au bond radically changes the mode of fragmentation of the MPA molecule. In the crystalline state, fragmentation is praiominantly by cleavage u and B to the sulfur atom and by OH loss, with moat of the ions retaining the sulfur atom in their structure as, apparently, the primary charge site. In the case of the chemisorbed MPA, cleavage of the sulfur-carbon bond or of a carbon-rbon bond
The Journal of Physical Chemistry, Vol. 97, No.20, 1993 5353
Adsorption onto Gold Surfaces
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Figure 7. Positive ion static SIMS spectrum of cysteine chemisorbed onto gold.
predominates, and the majority of the ions smaller than the molecular ion do not contain sulfur but instead retain the carboxylic acid group. Presumably, in this case, the carboxylic acid group has become the primary charge site. These observations and similar observations for cysteine, in addition to the observation of sulfur-gold clusters, suggest strongly that chemisorption has occurred, as suggested by others on the basis of data from IR and XPS studies.23-25 More than this, it seems clear that static SIMS is a sufficiently sensitive probe to detect changes in molecular fragmentation behavior caused by the alteration of one chemical bond. This conclusion is of considerablerelevance to a longstanding debate concerningthe nature of the mechanism of ion formation in SIMS, and the data which we have presented lead as to draw some conclusions concerning the nature of the ion-formation mechanism, expanding the discussion which we have previously published in a preliminary study of MPA chemisorption on g0ld.3~ In an important series of experiments, Vickerman and coworkers demonstratethat changesin the structure of an adsorbate
Figure 8. Negative ion static SIMS spectrum of cysteine chemisorbed onto gold.
system resulted in changes in its SIMS spectrum. For example, the relative intensities of MICO+ and MCO+ ions in the SIMS spectrum of CO adsorbed onto metallic surfaces were found to correlate closely with the relative proportions of bridged and linearly adsorbed CO as determined by vibrational spectroscopies.16-’8 Dissociative adsorptionl4 and changes in coveragelb-I8,35were also found to be accompanied by changes in the
5354 The Journal of Physical Chemistry, Vol. 97, No. 20, 1993
Leggett et al.
SCHEME1 (a)
(m) m/z 73
m/z 74
(n)
m/z 74
SIMS spectrum. That such correlations exist is now widely accepted; what has remained morecontroversialis the explanation for them. The debate has not only concerned ion formation from adsorbate systems but, more generally, has centered on the question of whether the dependenceof SIMS spectral data upon surfacecomposition is a statistical oneor a chemically determinate one. The classic conflict is between theories which invoke the direct emission of intact (or rearranged) fragments from the surface, on the one hand, and theories which invoke the statistical recombination of not-necessarily next-neighbored sputtered atoms, on the other. In molecular dynamics studies of a wide variety of adsorbate systems, Garrison, Winograd, and co-workers have concluded that a recombinative mechanism seems to be the more likely. In particular, for sputtering from CO adsorbate systems,3638they conclude that there is no direct relationship between the ions observed in the SIMS spectra and the linearand bridge-bonded surface states of the adsorbate. The changes which are observed in the SIMS spectra are a statistical reflection of changes in surfacecomposition. Under such an interpretation, there is no chemically determinate route, or fragmentation pathway, by which the observed ions are formed from the original surface structure. It has been suggested in studies of polymer systems, that there is a direct and chemically determinate fragmentation route leading from the original surface structure to the secondary ion structure. It has been suggested that ions are formed by predominantly low-energy pathways which are govemed by rulesverysimilar to those which apply in conventional forms of mass ~pectrometry,39~* supporting the claim that static SIMS provides an authentic surface mass spectrometry. We conclude that our results for the MPA and cysteine adsorbate systems provide support for this claim. While it is true that the adsorbatemass spectra are complex, it is clear that the formation of a chemical bond between the thiol sulfur atom and the gold surface has fundamentally changed the mode of fragmentation of the MPA molecule. Put simply, the frozen MPA has a SIMS spectrum in which the principal ions are the products expected of thiol fragmentation, whereas the chemisorbed MPA has a SIMS spectrum in which the principal ions are the products expected of carboxylic acid fragmentation. While the situation is more complex for cysteine, it is still apparent that a change in fragmentation behavior has occurred following adsorption, and the products of the fragmentationof the crystalline molecule are markedly different from the products of the fragmentationof the adsorbed molecule. We believe that these observations provide very good confirmation for the hypothesis that secondary ion structure is dependent, in a chemically determinatefashion, upon the bonding of an adsorbate at a surface. The observation of radical cation formation during sputtering of the adsorbate system provides further significant mechanistic information. The importanceof the observationof radical cation
bTl I I
I
I
e- tr nsfer
Figure 9. Schematic illustration of formation mechanism for radical cations. A sputtered radical species (a) abstracts a hydrogen atom from the surface (b); subsequent ionization yields a radical cation which may be neutralized by surface-particle electron transfer (d) if it is ejected from a metal surface, with the result that the sputtered particle is ultimately uncharged. When the radical is ejected from an insulating surface, ionization is not followed by neutralization, and the sputtered particle passes into the vacuum where it may be detected as a radical cation.
formation during SIMS studies of organicsolidshas been discwsed e l s e ~ h e r e . 4 ~The * ~ work function of gold (ca. 4.4'eV) is sufficiently small to make electron exchange between the gold surface and the sputtered species a reasonably facile process. Any radical fragments ejected in the ionized state from a metallic surface would generally be expected to undergo neutralization by surface-to-particle resonant or Auger electron-transfer proc e s ~ e s , 4with ~ ~ ~the ~ consequence that few radical cations should reach the detector. In contrast, the first ionization potentials of organic molecules are much larger (for example, 9.6 eV for propionic acid) and electron exchange between the frozen MPA and sputtered fragmentswould be expected to be much less likely. Thus sputtered fragments might be expected to reach the detector in their original charge ~tate.4~ The observation of radical cations in the adsorbateSIMS spectra (both positive and negative) clearly suggests that the adsorbate alters the effective work function of the surface to such an extent that the probability of surfaceparticle electron transfer is substantially reduced. Thus fragments desorbed with the breaking of the S-C bond, instead of becoming neutralized, are able to reach the detector in the ionized state. There are two possible routes by which the m / z 74 positive ion could be formed in this case (see Scheme I). First, scission of the S-C bond would yield a negatively charged sulfur atom, bound to the gold substrate, and a positively charged ion (m)of m / z 73. This ion must acquire a hydrogen atom from the surface in order to achieve m / z 74. Second, ejection of a radical of mass 73 (n) is followed by abstraction of hydrogen while in the nearsurface region, forming a relatively more stable even-electron molecule. If this species still possesses excess internal energy, electronic deexcitation (for example, via Auger electron emis~ i o n ~may ~ ) lead to ionization. Hydrogen abstraction from surfacesis thought to be relatively facile, irrespective of electronic work function, and it has been suggested e l ~ e w h e r e ~ that ~ ~it' may be involved in the formation of ions sputtered from polymer surfaces. However, the formation of an odd-electron ion of m / z 74 from an even-electron ion of m / z 73 would necessitate the violation of the even-electron rule, and there are good grounds for believing that the even-electron rule is obeyed quite strictly for ions sputtered from the surfacesof organic material^.^' Thus the latter mechanism seems to be the more likely of the two. Figure 9 illustrates schematically the sequence of events. Step (d), neutralization by surface-particle electron transfer, is the step which may occur for a particle ejected from a clean surface;
Adsorption onto Gold Surfaces for an ion ejected from a SAM, the neutralization in (d) does not mcur and the sputtered species traverses the remainder of the near-surface region in the ionized state. Ionizationby electronic deexcitationof sputtered species is not necessarily expected to account for all of the positive ions observed, however, and there may be a contribution to the sputtered ion yield from preformed ions. Of particular interest in this respect is the observation that the only ions appearing in both the positive and the negative ion spectra of the adsorbates are the AuM-type ions, on which the charge may be located on the gold atom. If electrontransfer between the surface and the departing sputtered fragments is inhibited by the presence of the adsorbed monolayer (as we have suggested above), it would seem that the only realistic mechanism for negative ion formation would involve the dissociation of a neutral species, in accordance with the nascent-ionmolecule model of Plog and Gerhard48,49 or a variant of the bondbreaking model such as that of Yu.50-52 Very different factors would favor the stability of positive and negative ions. If fragmentation of the adsorbate leads to carboxylic acid-derived fragments in the positive ion spectrum, by (predominantly) C-S bond cleavage, then the deposition of sulfur at the surface is the likely concomitant result. Sulfur-containing clusters are likely to be capable of effectively stabilizing negative charge. If some of the carboxylic acid fragments are ejected as preformed ions, leaving negative charges in the form of S or Au ions at the surface, or are formed by the fragmentation of large intact fragments containing two or more gold atoms and adsorbate molecules, then the dominanceof Au,S,--type fragments in the negative ion spectrum is expected. Conclusions
Thedata presented suggest that staticSIMS providesavaluable means by which the bonding of self-assembledmonolayer systems may be studied. In particular, for thiol adsorption onto gold, comparison of the SIMS spectra of molecules in the crystalline state and adsorbed onto surfaces has provided clear evidence for covalent bond formation between the thiol sulfur atom and gold atoms at the surface. For the bifunctional adsorbates, spectra of the crystalline and adsorbed material exhibited fundamentally different fragmentationbehavior. For MPA, theSIMS spectrum of the crystalline material was characterized by ions normally associated with thiol fragmentation,whereas theSIMS spectrum of the adsorbed material was characterized by ions normally associated with the fragmentation of carboxylic acids. Similar changeswere observed for cysteine. These changes provide clear evidence that ion formation during sputtering is via direct, chemically determinate fragmentation pathways. There was, furthermore, evidence that electronic deexcitation of sputtered neutral species constitutes a major mechanism of ionization, with the consequence that a number of radical cations were observed in the SIMS spectrum. Acknowledgment. The authors are grateful to the SERC/ DTI Link Protein Engineering Scheme, Glaxo Group Research, and VG Microtech for financial support for this work. References and Notes (1) Nuzzo, R. G.;Allara, D. L. J . Am. Chem. Sot. 1983,105, 4481. (2) Nuzzo, R.G.; Fusco, F.A,; Allara, D. L. J. Am. Chem. Soc. 1987, 109,2358.
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