Anal. Chem. 1994,66, 3644-3650
Molecular Ion Modification of a Hexanethiolate Self-Assembled Monolayer during Surface- Induced Dissociation John A. Burroughs and Luke Hanley' Department of Chemistv, m/c 11 1, University of Illinois at Chicago, 845 West Taylor Street, 4500 SES, Chicago, Illinois 60607-706 1
The modificationof a hexanethiolate self-assembledmonolayer (CH3(CH2)&) adsorbed on Ag(ll1) by exposure to pyridine, thiophene, furan, and tert-butyl molecularions during surfaceinduced dissociation (SID)experimentsis probed using infrared reflection absorption and scattered ion spectra. As much as 60%of the monolayer is modified after a 2.9 X 1014ions/cm2 exposure at an energy of 32 eV. Ion-induced modification consists of C-H and C-C bond cleavage in the hexanethiolate adsorbate. Ion-induced adsorbate modification appears at an ion kinetic energy threshold of -20 eV and rises sharply at 25 and 32 eV but is independent of the species of the incident ion. Even after extensive ion bombardment of the hexanethiolate, the scattered ion spectra observed during SID remain virtually unchanged. Collected from gas phase prepared selfassembled monolayers at atypicallyhigh ion exposuresof >1014 ions/cm2, these results nonetheless indicate that SID spectra recorded under practical conditions (i.e., solution prepared selfassembled monolayers and lower ion doses) should be stable and reproducible. Surface-induced dissociation (SID) of polyatomic ions has been developed over the last several years into a useful method for fragmentation analysis in tandem mass spectrometry.'-l6 The choice of collision targets used for SID has evolved toward well-defined surfaces including alkanethiolate self-assembled monolayers (SAMs) bound to gold or silver.24.6J4,17In this paper we investigate the extent and nature of ion-induced (1) Cooks, R.G.; Ast, T.; Mabud, Md.A. Int. J. Mass Spectrom. Ion Processes 1990, 100, 209 and references therein. (2) Winger, B.E.; Julian,R. K., Jr.;Cooks,R.G.;Chidsey,C.E.D.J . Am. Cfiem. SOC.1991, 113, 8967. (3) WvsockLV. H.: Jones. J. L.:Dinp. J.-M. J. Am. Cfiem.Soc.1991.113.8969. (4j Simogyi; A,; Kane, T . E.; Ding,-J.-M.; Wysocki, V. H. J . Am. h e m . SOC.
modification of a hexanethiolate SAM (CHj(CH2)sS) adsorbed on Ag( 1 11) during SID experiments: this modification consists of C-H and C-C bond cleavage in the hexanethiolate adsorbate. We also show that the scattered ion spectra are relatively insensitive to even extensive ion bombardment of the hexanethiolate adsorbate, in agreement with previous studies on similar systems.14J7 The growing interest in SID as a viable alternative to gas phase collision-induced dissociation has led to the study of an assortment of polyatomic ions including those of small organics,1,6.14,'5.18,19 polypeptides,9.20,21 C60+,1622,23 silicon clusters,24 smaller carbon cluster^,^^,^^ and alkali halide clusters.27 From this body of work it is clear that SID can efficiently dissociate large or resilient ions with no apparent limitation on theupper mass range amenable to fragmentation. Tandem time-of-flight and tandem quadrupole instruments have been shown to be particularly suited to this method.1*6J0J8 With few ex~eptions5J~-19-~9J~ most polyatomic ion-surface experiments have only examined scattered ion spectra, neglecting to analyze the effect of the ion beam upon the surface itself. Nevertheless, surface analysis of these systems is justified to obtain an improved fundamental understanding of the ion-surface collision event31and because the chemical and morphological states of the surface can affect the SID spectra.47,18,19.32 One approach to preparing a well-defined and reproducible surface for SID is to use alkanethiolate SAMs adsorbed on gold substrates.24*6 Hydrogenated, deuterated, and fluori-
(17) Wainhaus,S. B.; Burroughs, J. A.; Wu,Q.;Hanley, L. Anal. Cfiem. 1994,66, 1038. (18) Wu, Q.; Hanley, L. J. Pfiys. Cfiem. 1993, 97, 2677. (19) Wu, Q.; Hanley, L. J. Pfiys. Cfiem. 1993, 97, 8021. 1993. -I I-S-. ,-5215. (20) McCormack, A. L.;Somogyi, A.; Dongre, A. R.; Wysocki, V. H. Anal. Cfiem. - ( 5 ) Wu, Q.; Hanley, L. J . Am. Cfiem. Soc. 1993, 115, 1191. 1993, 65, 2859. (6) Morris, M. R.; Riederer, D. E., Jr.; Winger, B. E.; Cooks, R. G.; Ast, T.; (21) Wright, A. D.;Despeyroux, D.; Jennings, K. R.; Evans, S.;Riddoch, A. Org. Chidsey, C. E. D. I n t . J . Mass Spectrom. Ion Processes 1992, 122, 181. Mass Spectrom. 1992, 27, 525. (7) Dagan, S.;Amirav, A. J . Am. SOC.Mass Spectrom. 1993,4,869and references (22) Yeretzian, C.; Hansen, K.; Beck, R. D.; Whetten, R. L. J. Cfiem.Pfiys. 1993, within. 98, 7480 and references therein. (8) Riederer, D. E., Jr.; Miller, S.A.; Ast, T.;Cooks, R. G. J . Am. SOC.Mass (23) Busmann, H.-G.; Lill, Th.; Reif, B.; Hertel, I. V.; Maguirc, H. G. J. Cfiem. Spectrom. 1993, 4, 938. Pfiys. 1993, 98, 7574 and references therein. (9) Cole, R. B.; LeMeillour, S.; Tabet, J.-C. Anal. Cfiem. 1992, 64, 365. (24) %John, P. M.; Whetten, R. L. Cfiem. Pfiys. Left. 1992, 196, 330. (10) Williams,E.R.;Jones,G.C.,Jr.;Fang,L.;Zare,R.N.;Garrison,B.J.;Brenner, (25) Moriwaki, T.; Matsuura, H.; Aihara, K.; Shiromaru, H.; Achiba, Y. J. Pfiys. D. W. J. Am. Cfiem.SOC.1992, 114 , 3207. Cfiem. 1992, 96, 9092. (11) Williams, E. R.; Henry, K. D.; McLafferty, F. W.; Shabanowitz, J.; Hunt, D. (26) %.John, P. M.; Yeretzian, C.; Whetten, R. L. J. Pfiys.Cfiem. 1992,96,9100. F. J . Am. SOC.Mass Spectrom. 1990, 1, 413. (27) Beck, R. D.; %John, P.; Homer, M. L.; Whetten, R. L. Science 1991, 253, (12) Castoro, J. A.; Nuwaysir, L. M.; Ijames, C. F.; Wilkins, C. L. Anal. Cfiem. 879. 1992, 64, 2238. (28) Wysocki, V. H.; Ding, J.-M.; Jones, J. L.; Callahan, J. H.; King, F. L. J. Am. (13) Aberth, W. Anal. Cfiem. 1990, 62, 609. SOC.Mass Spectrom. 1992, 3, 27. (14) Kane, T. E.; Somogyi, A.; Wysocki, V. H. Org. Mass Spectrom. 1993, 28, (29) Lill, Th.; Busmann, H.-G.; Reif, B.; Hertel, I. V. Appl. Pfiys.A 1992.55.461. 1665. (30) Lill, Th.; Busmann, H.-G.; Hertel, I. V. 2.Pfiys. B 1993, 91, 267. (15) Pradeep, T.; Miller, S.A.; Cooks, R. G. J. Am. SOC.Mass Spectrom. 1993, (31) Kasi, S.R.; Kang, H.; Sass, C. S.;Rabalais, J. W. SurJ Sci. Rep. 1989, IO, 4. 769. 1 and references therein. (16) Callahan, J. H.;Somogyi,A.;Wysocki,V. H. RapidCommun. MassSpectrom. (32) Despeyroux, D.; Wright, A. D.; Jennings, K. R.; Evans, S.; Riddoch, A. Int. 1993, 7 , 693. J. Mass Spectrom. Ion Processes 1992, 122, 133.
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0 1994 American Chemical Society
nated alkanethiolate SAMs can be readily prepared from I I solution, have relatively low sticking coefficients to background gases present in vacuum, and display long-term stability under 1 L various conditions.6J3 Fluorinated SAMs have proven par3L ticularly useful as SID collision targets because of their high 7 L scattered ion yields and high ion fragmentation effi~iencies.~.~J~ bp 0 15 L From a fundamental viewpoint, the behavior of hydrogeC 0 nated alkanethiolate SAMs under ion bombardment can serve .-5 as a model for SID off more complex organic layers8 and the E C ion beam modification of Such an approach e c is attractive not only because SAMs are more highly ordered and better characterized than most organic and polymeric thin films but also because they are more amenable to surface analysis. I 2914 I The present study is a continuation of previous work in which we probed the extent of pyridine ion beam modification 2000 2850 2900 2950 3000 of methanethiolate covered Ag( 111) during SID:I7 we used Wave nu mbe r (c rn- ) infrared reflection absorption spectroscopy (IRAS) to show Figure 1. Infrared reflection absorption spectra of hexanethiolate that while the methanethiolate adsorbate was severely modified adsorbed on Ag( 1 1 1) for severaldifferent hexanethbi exposures(given by the pyridine ion beam, little effect was observed in the in langmuirs), recorded at 130 K. scattered ion spectra. We examine here the longer chain hexanethiolate since it better approximates the monolayers typically used in SID experiments. We also probe the effect gauge readings). The hexanethiol (95%, Aldrich) had been of varying both the projectile ion and its kinetic energy on the dried over alumina beads and thoroughly degassed prior to hexanethiolate modification. We then examine the scattered introduction into the system. Each IRAS spectrum of ion spectra to determine both how it is affected by the hexanethiolate/Ag( 111) was summed over 512 scans taken hexanethiolate modification and how it reflects the nature of at 4 cm-1 resolution after the surface was cooled to 90% of the molecular ion (CsHsN+, C4HdS+, trometer for the detection of 3 f 2 eV ions scattered at 90' and C4H4O+, respectively). The tert-butyl ion beam comoff the Ag( 111) surface. Both retarding field Auger electron position was 50% C4H9+, 10% C4H7+, and 40% C4H8+. The and infrared reflection absorption spectroscopies are present ion beam currents and kinetic energies were measured with in the main chamber to allow for elemental and molecular a Faraday cup positioned on a linear manipulator. The ion analysis of the surface. The main chamber is connected to kinetic energies used for these studies were 20, 25, and 32 f a differentially pumped ion source chamber which generates 2 eV, and the currents used were 6 f 3 nA/cm2, unless molecular ion currents of several nanoamperes per square otherwise noted. Mass spectra of the scattered ions were centimeter. The background pressure in the main chamber recorded between 1 and 120 min of ion bombardment with is typically 4 X 10-'0Torr when the ion source is in operation. the surface temperature held constant at 300 K. SecondThe Ag(ll1) single crystal was mounted in the main order fast Fourier transform smoothing was used for all mass chamber on a liquid nitrogen cooled manipulator which was spectra in order to remove statistical noise. Additionally, all translatable in all three directions and rotatable around its data were reproduced on two or more separate days under axis. The Ag(ll1) was cleaned every morning before identical conditions. experiments were run by sputtering the surface with a 1 pA beam of 500 eV Ar+ ions followed by annealing at 720 K. This RESULTS process was repeated until no carbon or sulfur contamination A. Characterization of the Hexanethiolate Adsorbate on was detected in the Auger electron spectra. Once the purity Ag(ll1). Figure 1 exhibits the IRAS C-H stretching region of the Ag(ll1) was confirmed, a SAM of hexanethiolate was of hexanethiolate adsorbed on Ag( 111) for 1-20 langmuir prepared by exposing the Ag(ll1) held at 300 K to 20 exposures of hexanethiol. The four C-H stretching modes langmuirs of hexanethiol (measured using uncorrected ion which are plainly visible in these spectra are labeled with the (33) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437 and corresponding wavenumbers (see below for peakassignments). references therein. Figure 2 displays the IRAS peak area of the C-H stretch (34) Tepermeister, I.; Sawin, H. J . Vac. Sci. Techno/. 1991, A9, 790. (35) Jeong, H.-S.; White, R. C. J . Vac. Sci. Technol. 1993, A l l , 2308. integrated from 2850 to 2970 cm-* (hereafter referred to as (36) White, R. C.; Ho, P. S. In Handbook of Ion Beam Processing Technology; the IRAS peak area) versus hexanethiol exposure. The IRAS Cuomo, J. J., Rossnagel, S.M., Kaufmann, H. R., Eds; Noyes Publications: peak area versus hexanethiol exposure curve in Figure 2 clearly Park Ridge, NJ, 1989; pp 315-337.
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Table 1. Vibrational Asslgnments and Peak Frequencler' of C-H Stretchlng Modes for mAlkanethioiate Monolayersb vs
sample
CH2
CH3, FRI
2850 2848 2852 2850
2900 2877 2879 2877 2878 2878
Cg on Ag(ll1) (this study) c6 on Ag (ref 44) c16-c22
CISon Au(lO0) (ref 38) c6 on Au(ll1) (ref 45) c 1 6 on Au(ll1) (ref 46)
VP
CHI, FR
CH3, FR2
CHz
CH3, OP
2914
2954
2895-2907
2939 2935 2935 2939 2939 2938
2918 2916 2921 2918
2954 2957
2893
2953
CH3, ip 2964 2964 2964 2966 2965
0 Peak frequencies are in units of cm-I. Abbreviations used: vs, symmetric; va, asymmetric; ip, in plane; op, out of plane; FR,Fermi resonance splitting component.
12 T
v,
4i 2
01 0
5
10
15
Hexanethiol Exposure
20
I 25
(L)
Figure 2. Integrated IRAS peak area of the 2850-2970 cm-I region of hexanethlolate adsorbed on Ag( 1 1 1) versus hexanethiol exposure in langmuirs.
follows langmuir adsorption kinetics,37 and so we conclude that the IRAS peak area quantitatively represents the relative hexanethiolate coverage on the surface, as was previously found for methanethi01ate.l~ The error bar in Figure 2 represents one standard deviation of the absolute error in the IRAS peak area at 20 langmuir exposure measured over 300 min, due primarily to day-to-day fluctuations in IR source intensity and sample positioning. Figure 2 illustrates that a 20 langmuir dose of hexanethiol leads to the formation of a saturated hexanethiolate monolayer on Ag( 11l), consistent with the 10 langmuir hexanethiol exposures required to saturate Au( 1 11) (given that absolute ion gauge errors are near Temperature programmed desorption studies of the saturation coverage of hexanethiolate on Ag(ll1) performed at a heating rate of 2 K/s indicate that hexanethiolate desorbs near 450 K (data not shown). This correlates well with the 500 K desorption temperature of methanethiolate on Ag(1 1 1)17,39and clearly indicates that the S-H bond has been cleaved during adsorption of hexanethiol to form the Ag-S bond in hexanethiolate. It has been previously seen that thiols desorb near 230 K when the S-H bond remains intact, whereas (37) Zangwill, A. Physics at Surfaces; Cambridge University Press: Cambridge, U.K., 1988; pp 206, 364. (38) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J . Chem. Phys. 1993, 98,678. (39) Harris, A. L.; Rothberg, L.; Dhar, L.; Levinos, N. J.; Dubois, L. H. J . Chem. Phys. 1991, 94, 2438.
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thiolates desorb above 450 Ke40 Conversion of the thiol to the thiolate during adsorption is further confirmed by the absence of the S-H stretching peak from the IRAS (expected near 2575 c ~ - I ) . ~ The I similar hexanethiol exposures required for saturation coverage on Ag( 111) and Au( 111) are also consistent with S-H cleavage, given that thiolate formation from gas phase dosing has been previously confirmed in the latter case.38 Finally, Ag and Au exhibit similar activities to the formation of thiolate monolayers via the dissociation of S-H bonds in solution,42and there is no reason to expect any deviation from this trend for gas phase dosing.33,43 Table 1 displays the peak frequencies and vibrational mode assignments from our IRAS data displayed in Figure 1 as compared to IRAS results from other investigations of similar SAMs.38,42q44-46 The asymmetric C-H stretch of CH2 is the strongest peak, at 2914 cm-', whereas the symmetric C-H stretch of CH2, expected from the literature to appear near 2850 cm-I, is completely absent. These relative CH2 peak intensities can be ascribed to a smaller off normal tilt angle of the hexanethiolate carbon backbone on Ag( 11 1) compared to those of alkanethiolates prepared from s o l ~ t i o n . ~We ~*~~ also see broader IRAS peak widths for hexanethiolate/Ag(1 11) than are observed for solution prepared alkanethiolates, indicating that gas phase dosing leads to a higher degree of disorder in the a d ~ o r b a t e . ~ ~ , ~ ~ The two Fermi resonance components of the symmetric C-H stretch of CH3 appear at 2900 and 2939 cm-l. The peak at 2939 cm-I is in complete agreement with the literature value (see Table 1). However, the peak at 2900 cm-l is blue shifted far from theliteraturevalueof -2880cm-l, indicative of differences in the adsorbate structure here compared with that in the solution prepared thiolates cited in Table 1. The weak peak at 2954 cm-I has been assigned as the asymmetric C-H out of plane stretch of CH3. No other peaks are visible in the 1000-4000 cm-l region of the vibrational spectra. The anticipated CH2 scissors and CH3 deformation modes expected to appear in the 13001500 cm-' region are absent, presumably as a result of poor (40) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J . Am. Chem. SOC.1987,109, 733. (41) Bryant, M. A,; Pemberton, J. E. J . Am. Chem. SOC.1991, 113, 3629. (42) Laibinis, P. E.; Whitesides, G . M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J . Am. Chem. SOC.1991, 113, 7152. (43) Ulman, A.; Eilers, J. G.; Tillman, N. Lnngmuir 1989, 5, 1147. (44) Walczak, M. M.; Chung, C.;Stole, S.M.; Widrig, C. A,; Porter, M. A. J . Am. Chem. SOC.1991, 113, 2370. (45) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J . Am. Chem. SOC.1987, 109, 3559. (46) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J . Am. Chem. SOC.1987,109,2358. (47) Fenter, P.; Eisenberger, P.; Li, J.; Camillone, N., 111; Bernasek, S . ; Scoles, G.; Ramanarayanan, T. A,; Liang, K. S . Lnngmuir 1991, 7 , 2013.
90 h
8 v 0
e
2 75 0
2
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Y
r
Ir 60 > .-+
-0a, ct
‘“t
45
I
V
30
0
5
10
15
20
25
30
35
Number of Ions (xl O ” C ~ - ~ )
Flgurs 3. Relative IRAS peak area of the 2850-2970 cm-I region versus the Incident ion exposure for 32 eV pyrldlne, thlophene, furan, and rert-butyl ions (native surface, 100%).
instrumental sensitivity in this region and the low intensity of these peaks, as observed e l s e ~ h e r e . ~ ~ ~ ~ ~ In summary, we find that hexanethiol adsorbs from the gas phase onto Ag( 111) by means of cleavage of the S-H bond to form a saturated monolayer of hexanethiolate, in a fashion similar to the preparation of alkanethiolates from solution. However, themonolayers studied hereare apparently oriented with the hexanethiolate carbon backbone closer to the surface normal, with a different saturation coverage, and with a higher degree of disorder than is typical for alkanethiolates prepared from solution. B. Ion Scattering: Ion-Induced Modification and SID Spectra. The dependence of the ion-induced modification of the hexanethiolate during SID experiments is determined by examining the IRAS peak area before and after ion bombardment. Figures 3 and 4 summarize the effects of ion beam modification for the four ions examined. Figure 3 is a plot of the percentage of the original IRAS peak area after various 32 eV incident ion exposures. Pyridine, thiophene, furan, and tert-butyl ions are used in Figure 3 to test for a dependence of the ionic species on the degree of modification of the hexanethiolate surface. Figure 3 illustrates that the degree of hexanethiolate modification is independent of the projectile ion used, within the indicated experimental error bar. The same insensitivity to the ionic species is observed for 20 and 25 eV kinetic energies for pyridine, thiophene, and furan. The dependence of the adsorbate modification on the projectile ion kinetic energy is investigated in Figure 4, which displays the percentage of the original IRAS peak area remaining after 20, 25, and 32 eV ion exposures. The data for all ions have been plotted together in Figure 4, given the insensitivity of the adsorbate modification to the incident ion. Data for tert-butyl ions have been recorded only at 32 eV kinetic energies. As previously reported for methanethiolate/ Ag(l1 l), the degree of modification of the alkanethiolate monolayer is quite sensitive to the incident ion energy:” the incident ion energy threshold for hexanethiolate modification is -20 eV, whereas a 2.9 X 1014ions/cm2 exposure of 25 eV
0
\r
0 20 eV Ions V 25 eV Ions V 32 eV Ions
5
10
15
20
25
30
Number of Ions ( ~ l O ’ ~ c m - ~ )
Figure 4. Relative IRAS peak area of the 2850-2970 cm-’ region versus the Incident Ion exposure of 20, 25, and 32 eV lncldent ions (native surface, 100%). Data for pyridine, thiophene, and furan Ions are plotted together at each energy, and tert-butyl data are Included at 32 eV.
ions modifies 25% of the monolayer, and a similar exposure of 32 eV ions modifies 60% of the monolayer. Furthermore, a 5.1 X 1014ions/cm2 exposure of 32 eV furan ions leads to 75% adsorbate modification (data not shown). Control experiments were performed prior to these measurements to confirm that the reduction in IRAS peak area was indeed due to ion-induced modification and was not caused by the sharing of infrared intensity by coadsorbates. The hexanethiolate covered surface was positioned in front of the ion beam entrance and held at 300 K while the magnet on the Wien filter was set to deflect all ions away from the surface. IRAS spectra were taken over 120 min, during which time the surface was directly exposed to background gas effusing from the ion source (partial pressure 3 X 10-lOTorr). During this period, the IRAS peak areas showed no reduction in peak area (within 1% error). We therefore conclude that attenuation of the IRAS peak area must have resulted from ioninduced modification of the adsorbate layer. SID spectra of pyridine, thiophene, and furan ions scattered off the saturated hexanethiolate/Ag( 111) surface at 25 eV are shown in Figure 5 . Only the [M H]+ peak is observed in the molecular ion region for pyridine ions, whereas the M+ peak predominates for thiophene and furan ions.1J4J7-19At 25 eV ion kinetic energy, the [M H]+ peak at m / z 80 is the most intense peak in the pyridine ion SID spectrum. Unimolecular dissociation of C5HsNH+ gives rise mainly to peaks corresponding to loss of C2H3 or HCN ( m / z 53), C4H4 ( m / z 28), and C4H5 or C3H3N ( m / z 27).18 A greater extent of fragmentation is achieved when thiophene and furan ions collide with hexanethiolate at 25 eV. The thiophene SID spectrum exhibits the molecular ion ( m / z 84) and its dissociation products: loss of C2H2 ( m / z 5 8 ) , C3H3 ( m / z 45), and CHS ( m / z 39). Likewise, 25 eV collisions of furan ions give rise to the molecular ion ( m / z 68) and various fragment ions from the loss of C3H3 ( m / z 29), CHO ( m / z 39), CO ( m / z 40), C2H3 ( m / z 41), and C2H2 ( m / z 42). The amount of total scattered ion signal is approximately 4 times
-
+
+
Analytical Chemlstty, Vol. 66, No. 21, November 1, 1994
3647
.
'
.
* .
I
45
v)
Y
C
3
0 0 C
39
68
I 10 20
30
40
50
70
60
80
90 100
0
6
3
9
12
15
18
Number of Ions ( ~ l O ' ~ c m - ~ )
Mass ( m / z )
Flgure 5. SID spectra of 25 eV pyridine (C5H5N+,mlr 79), thiophene (C4H4S+,mlr 84), and furan ions (C4H40+, mlz 68) scattered off hexanethlolatelAg(111) held at 300 K.
Figure 7. Normalized SID signals of the major scattered ions from 32 eV pyridine ions scattered off hexanethlolatelAg(111) versus the Incident ion exposure. All slgnal Intensities are normalized to the mlz 53 ion intensity and then offset for visibility.
39
vr
Y C
3
0 0
+
C
-
Normal Ion
68
10
20
30
40
50
60
70
Mass (m/z)
Flgure0. SID spectra of 32 eV furan ions scattered off hexanethiolatel Ag( 11 1) at 300 K, recorded using normal and high primary ion currents of 6 (lower trace) and 15 nA/cm2 (upper trace).
greater for pyridine than for thiophene or furan for similar primary ion beam currents. Figure 6 displays 32 eV furan ion SID spectra recorded at the normal primary ion current of 6 nA/cm2 (lower trace) and a high primary ion current of 15 nA/cm2 (upper trace). The normal ion current spectrum in Figure 6 resembles that at 25 eV (see Figure 5, lower trace), except that the former has almost no molecular ion peak ( m / z 68), due to enhanced fragmentation at the elevated kinetic energy. However, the high ion current spectrum in Figure 6 displays a variety of additional peaks ( m / z 14, 15, 27, 43, 51, 55, 65) whose appearance can only be attributed to sputtering of the alkane chain of the hexanethiolate adsorbate. The normal and high ion current spectra were recorded after 6.9 X 1013and 5.1 X 1 014 ions/cm2 exposures, respectively. The total scattered and secondary ion signals in the high ion current spectrum of Figure 6 is roughly 3 times greater than that in the normal 3048
ion current spectrum, an enhancement equal to the primary ion current ratios. Figure 7 displays the dependence of the major scattered ions on the incident ion exposure during 32 eV pyridine ion SID. All ion signals in Figure 7 are normalized to m / z 53 ion intensity and then offset to enhance visibility. While the intensity of the [M H]+ peak at m / z 80 fluctuates until stabilizing above ion exposures of 1.0 X 1014 ions/cm*, the intensities for the other three scattered ions do not change. For comparison, none of the major scattered ion signals in the thiophene and furan ion SID spectra show any significant change for similar ion exposures (data not shown).
Analytical Chemistry, Vol. 66, No. 21, November 1, 1994
DISCUSSION Our results show that molecular ions dramatically modify hexanethiolate SAMs during the SID process. The incident ion kinetic energy threshold for modification is -20 eV, but the degree of adsorbate modification at all ion energies is independent of the primary ion species. Despite the high levels of ion-induced adsorbate modification, the SID spectra are relatively unaffected. By inference, similar ion modification and SID spectral stability are expected on solution prepared SAMs. The IRAS and sputtered ion data clearly indicate that the ion-induced adsorbate modification consists of the fragmentation of C-H and C-C bonds of the hexanethiolate. Previous experiments indicated that many of the surface species formed by ion bombardment are likely to spontaneously desorb at the 300 K surface temperature used Other studies have estimated that at least 15 eV of kinetic energy is transferred to the surface from collisions with 25 eV more than enough to break any of the 2-4 eV bonds of the hexanethi0 1 a t e . ~ ~ The , ~ * complex nature of the ion-induced adsorbate modification observed here is in agreement with similar results on polymer surfaces.36 (48) Weast, R. C.;Astle, M. J., Eds. CRC Handbook of Chemistry and Physics, 62nd ed.; CRC Press: Boca Raton, FL, 1981; pp F-191, F-194.
One of the advantages of utilizing SAMs for surfaceinduced dissociation experiments is that sputtered ion peaks are usually not ~bserved.~-"JQ~J~ While the appearance of sputtered ions in the high current SID spectrum of Figure 6 is an unusual case, it clearly supports the argument that ion bombardment cleaves C-H and C-C bonds in the adsorbate. We tentatively make the following assignments of the peaks unique to the high current spectrum in Figure 6: m / z 14 as CH2+, m / z 15 as CH3+, m / z 27 as C&+, m / z 43 as C3H7+, m / z 51 as C4H3+, m / z 55 as C4H7+, and m / z 65 as CsHs+. The increase in m / z 39 in the high current spectrum can be attributed to C3H3+ formation. All of these sputtered ions effectively derive from the ion-modified adsorbate layer where 75% of the hexanethiolate has been modified after exposure to 5.1 X 1014ions/cm2 of 32 eV furan ions. These sputtered ions are assigned as such because they are consistent with the fragmentation of C-C and C-H bonds of hexanethiolate by the incident ions and inconsistent with furan ion fragmentation. The observation and assignment of these sputtered ions agree with data from static secondary ion mass spectrometry studies of SAMs where C2H3+ ( m / z 27), C2Hs+ ( m / z 29), C3H3+ ( m / z 39), C3H7+ ( m / z 43), and C4H7+ ( m / z 55) secondary ions were detected from alkanethiolates adsorbed on gold.49 However, the sputtered ions observed here do not derive from single ion collision events (static conditions), or else they would be observed in the normal ion current spectra. That the sputtered ions appear only after large ion exposures and at high ion currents further indicates that these species are formed in at least one ion collision event and then desorbed in a separate event (dynamic conditions). The appearance of CH2+ and CH3+ ions is most readily explained as deriving from CH2 and CH3 species cleaved off the hexanethiolate chain. We have not considered the role that deposited furan ions may play in the formation of the sputtered ions, although deposition may be important since the surface has been exposed to roughly one monolayer of furan ions in the high current spectrum (Figure 7). While some of our sputtered ion peak assignments are open to challenge, it is clear that no other mechanism besides sputtering of the hexanethiolate can explain the appearance of all of these ions. From the sputtered ion data, we conclude that a variety of species reside on the surface after exposure to molecular ions. That we do not observe any of these species by IRAS is attributed to the low oscillator strength in any one species, as has been argued previ0us1y.l~ The absence of shifts in the relative intensities of the various C-H stretching modes or broadening of any IRAS peaks following ion bombardment indicates that reordering, tilt angle changes, and projectile adsorption are not major effects of ion b ~ m b a r d m e n t . ~ ~ The adsorbate modification measurements shown in Figures 3 and 4 provide clear evidence that the magnitude of the ioninduced modification is, to a first approximation, independent of the chemical nature of the incident ion. This independence implies that the adsorbate modification occurs predominantly via momentum transfer rather than by chemical or electronic (i.e., charge transfer) interactions, in analogy with the description of physical ~puttering.~'The -20 eV molecular ion-induced modification threshold of hexanethiolate is similar (49) Tarlov, M. J.; Newman, J. G. Langmuir 1992, 8, 1398.
to the 35 eV threshold for Ne+-induced disruption of graphite lattices.50 The sharp increase in the magnitude of hexanethiolate modification with incident ion energy is also consistent with a momentum transfer model such as has been employed to model previous SID experiments.'O However, it is conceivable that certain polyatomic ion-surface pairs are sufficiently reactive that their interaction may be dominated by chemicai sputtering processes, as can occur in atomic ion-surface collisions.31 In contrast to furan and thiophene ions, which tend to scatter intact off hydrogenated surfaces without picking up hydrogen atoms, pyridine ions usually reactively scatter off similar surfaces to form [M + H]+ (as shown in Figure 5 and in agreement with previous s t ~ d i e s ) . l - ~ J ~ Simple J ~ - l ~ thermodynamic predictions can explain these results: the heat of reaction for pyridine ions extracting a hydrogen atom from n-hexane (in lieu of hexanethiolate) is-21.1 kcal/mol, whereas those for the same reactions for thiophene and furan are 12.9 and + 16.4 kcal/mol, re~pectively.~~ However, such thermodynamic arguments are clearly inappropriate in a momentum transfer model of ion-induced surface modification. If reactively scattered ions selectively modify the surface, then conversion of the hexanethiolate's terminating methyl group to methylene would be expected to occur: the IRAS data show that neither this channel nor complete methyl group removal dominates the ion-induced surface chemistry. Furthermore, it is known that even-electron ions are less likely to abstract hydrogen or neutralize at a surface than are odd-electron It could therefore be argued that even-electron ions would modify an ads irbate to a different extent than would the odd-electron ions. The experimental results shown in Figure 3 do not support this argument: the tert-butyl ion beam has 50% composition of t-C4H9 even electron ions with a 2-3 eV lower ionization potential, yet it is no more or less effective in modifying the adsorbate than are odd-electron ions, within our experimental error. We conclude that the ion-induced adsorbate modification observed here is primarily a momentum transfer event and that chemical sputtering and/or charge transfer effects play no more than a secondary (undetected) role. Reactive scattering yields have been previously shown to depend upon the degree of ion-induced adsorbate modification.14J7J8 Our results (Figure 7) indicate a rather complicated behavior for the pyridine [M H]+ion intensity as a function of incident ion exposure. Thedriving force behind this behavior is not clear at this time. Changes in pyridine [M H]+ ion intensity during SID off fluorinated alkanethiolates were previously attributed to the formation of active centers (surface sites with enhanced sticking coefficients) at which adventitious background hydrocarbons (pump oil and ion source gases) could adsorb.14 By this argument, incoming ions would be expected to pickup hydrogen atoms from these newly adsorbed hydrocarbons. l 4 This argument is plausible but more difficult to apply in the current situation, where the surface is initially hydrogenated and likely remains so throughout the experiment. It is also important to note that little is presently known about
+
+
+
(50) Steffen, H. J.; Marton, D.; Rabalais, J. W. Phys. Reo. L e f f .1992, 68, 1726. (51) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. Gas-Phase Ion and Neutral Thermochemistry. J. Phys. Chem. Ref Data 1988, 17, Suppl. 1.
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the fundamental nature of reactive scattering beyond the simple thermodynamic predictions made above. Consistent with previous studies,14J7these results illustrate that SID spectra are relatively independent of the degree of ion exposure when SAMs are used as collision targets. Even the extraordinarily high ion currents and total ion exposures utilized here do not induce major fluctuations in SID yields. Since most SID experiments are expected to have much lower ion doses than those employed here, ion beam modification should not significantly deteriorate the quality of the scattered ion spectra, even over extended periods of use. Furthermore, ion exposure dependent fluctuations in SID spectra tend to manifest themselves first in the reactively scattered ions, which can therefore be used to indicate when the collision target must be replaced, as previously suggested.14 The list of practical SID collision targets has begun to expand beyond SAMs to include metal oxides' and organic liquid surfaces.* While the search for the ideal collision target is clearly still in progress, SAMs possess a few unique advantages which should be considered: (1) they presently constitute some of the best defined and most thoroughly examined organic surfaces, (2) they are relatively amenable to examination by traditional surface analysis methods, and (3) they can be prepared on atomically flat gold substrates. The first two advantages should facilitate the elucidation of any anomalies in SID spectra which may appear as the collisions of different primary ions are examined. The third feature should preclude irreproducibilities reported for SID off roughened ~urfaces.3~
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Analytical Chemistry, Vol. 66, No. 21, November 1, 1994
CONCLUSION In order to obtain further insight into the fundamental processes associated with surface-induced dissociation, previous results examining the modification to a CH,S/Ag( 111) surface have been extended here to encompass longer chain alkanethiolate SAMs. We have shown that pyridine, furan, thiophene, and tert-butyl ions are equally effective in damaging hexanethiolate adsorbed on Ag( 111) even though their respective chemical, structural, and electronic properties vary considerably: these data support the notion of using momentum transfer models in theoretical studies of SID.l0 We have also found that molecular ion modification leads to the cleavage of C-H and C-C bonds in the hexanethiolate adsorbate. Even significant levels of ion-induced adsorbate modification lead to only minor changes in the SID spectra, and these are apparently confined to reactively scattered ions. The stability of the scattered ion spectra is consistent with previous results obtained in this laboratory and o t h e r ~ . By ~~J~ inference, similar ion modification and SID spectral stability are expected on solution prepared SAMs, further indicating the feasibility of this method for practical use in mass spectrometry. Acknowledgment is made to the National Science Foundation (CHE-9220393) for financial support of this research. Received for review February 14, 1994. 1994." ~
Accepted July 20, ~~~
Abstract published in Aduance ACS Abstracts, September IS, 1994.
