Surface Modification Using a Commercial Triple Quadrupole Mass

Publication Date (Web): November 3, 2000. Copyright ... We report instrumental modifications to a commercial mass spectrometer that allow surface modi...
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Anal. Chem. 2000, 72, 5798-5803

Surface Modification Using a Commercial Triple Quadrupole Mass Spectrometer Jeff W. Denault, Chris Evans, Kim J. Koch, and R. Graham Cooks*

Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-1393

We report instrumental modifications to a commercial mass spectrometer that allow surface modification experiments to be performed using low-energy (electronvolt range) mass-selected ion beams. The design of the detector housing allows placement of the surface on the ion optical axis and some distance beyond the off-axis detector. Manipulation of the potentials applied to the final lens, detector housing, conversion dynode, and electron multiplier allow the ions to pass through the detector housing and impinge upon the surface without loss of the normal mode of detector operation. Ex situ analysis of the modified surface is performed using a home-built multisector mass spectrometer. The ability to modify organic thin films is demonstrated by a number of soft landing and surface modification experiments including (i) soft landing of (CH3)2SiNCS+ ions formed from trimethylsilyl isothiocyanate upon a fluorinated self-assembled monolayer (F-SAM) surface, (ii) soft landing and dissociative soft landing of the pseudomolecular cation of triphenylpyrylium tetrafluoroborate, viz. the triphenylpyrylium cation, upon an F-SAM surface, (iii) dissociative soft landing of 35ClCH2(CH3)2SiOSi(CH3)2+ formed from 1,3bis(chloromethyl)disiloxane upon an F-SAM surface, (iv) surface passivation by reaction of the trimethylsilyl cation, Si(CH3)3+, with a hydroxyl-terminated self-assembled monolayer (OH-SAM), and (v) transhalogenation by reaction of CCl3+ (m/z 119) with an F-SAM surface. Chemical and physical modifications of organic thin films constitute an area of research of technological importance. Basic information on interfacial gas-phase/surface interactions should lead to technological advances in areas such as coatings, semiconductor etching, micromachines, and biomedical devices. Mass spectrometry plays an important role in characterizing the products of interfacial gas-phase/surface interactions and can play a vital role in advancing and improving current technology for both surface analysis and surface modification. Among many important applications of mass spectrometry to surface analysis, time-of-flight secondary ion mass spectrometry (TOF-SIMS) uses kiloelectronvolt ion beams to compile elemental or molecular images of surfaces.1,2 In work from which the present study has evolved, hyperthermal ion beams with translational energy on the * Corresponding author: (e-mail) [email protected]. (1) Benninghoven, A.; Rudenauer, F. G.; Werner, H. W. Secondary Ion Mass Spectrometry; John Wiley & Sons: New York, 1987. (2) Winograd, N. Anal. Chem. 1993, 65, 622A.

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order of 1-100 eV are used for both surface analysis and surface modification. These experiments include surface-induced dissociation (SID) for ion characterization,3-5 chemical sputtering for surface analysis,6,7 and ion/surface reactions as a means to effect surface modifications.8-11 Chemical sputtering is a surface analysis tool based on lowenergy ion/surface collisions. In this experiment, ions are ejected from the surface as a result of dissociative charge exchange between the low-energy ion beam and the surface. Chemical sputtering differs from SIMS in that secondary ions in SIMS are released by momentum transfer in the course of the collision cascade produced by high-energy collisions.6,7 Chemical sputtering can be performed using either atomic (e.g., Xe•+) or molecular (e.g., CF3+) species. Ion/surface reactions have been performed using a number of different surfaces including metal surfaces covered with hydrocarbon films, self-assembled monolayers (SAMs), and Langmuir-Blodgett (L-B) films. Ion/surface reactions can involve the chemical transformation of the projectile ion and/or the surface, with the formation of new chemical bonds. Self-assembled monolayer surfaces composed of either functionalized or nonfunctionalized alkanethiols bound to a gold substrate provide a consistent and stable surface for performing a variety of interfacial experiments. Hydrocarbon (H-SAM), fluorinated (F-SAM), and other derivatized SAM surfaces have been used as substrates in ion/surface reactions. Transhalogenation is one well-characterized example of an ion/surface reaction at an F-SAM surface.12,13 Transhalogenation has been observed to involve an F atom of the F-SAM surface and the Cl (3) Mabud, M. A.; Dekrey, M. J.; Cooks, R. G. Int. J. Mass Spectrom. Ion Processes 1985, 67, 285. (4) Cooks, R. G.; Ast, T.; Pradeep, T.; Wysocki, V. Acc. Chem. Res. 1994, 27, 316. (5) Dongre, A. R.; Somogyi, A.; Wysocki, V. H. J. Mass Spectrom. 1996, 31, 339. (6) Vincenti, M.; Cooks, R. G. Org. Mass Spectrom. 1988, 23, 317. (7) Kasi, S. R.; Kang, H.; Sass, C. S.; Rabalais, J. W. Surf. Sci. Rep. 1989, 10, 1. (8) Mair, C.; Fiegele, T.; Worgotter, R.; Futrell, J. H.; Mark, T. D. Int. J. Mass Spectrom. 1998, 177, 105. (9) Shen, J. W.; Evans, C.; Wade, N.; Cooks, R. G. J. Am. Chem. Soc. 1999, 121, 9762. (10) Miller, S. A.; Luo, H.; Jiang, X.; Rohrs, H. W.; Cooks, R. G. Int. J. Mass Spectrom. Ion Processes 1997, 160, 83. (11) Gu, C. G.; Wysocki, V. H.; Harada, A.; Takaya, H.; Kumadaki, I. J. Am. Chem. Soc. 1999, 121, 10554. (12) Miller, S. A.; Luo, H.; Jiang, X.; Rohrs, H. W.; Cooks, R. G. Int. J. Mass Spectrom. Ion Processes 1997, 160, 83. (13) Shen, J. W.; Grill, V.; Evans, C.; Cooks, R. G. J. Mass Spectrom. 1999, 34, 354. 10.1021/ac0005823 CCC: $19.00

© 2000 American Chemical Society Published on Web 11/03/2000

of the projectile ion, C(CN)2Cl+. This exchange reaction is also known to occur for pseudo-halogens such as CN in the case of C(CN)2Cl+ as well as NCS+ and NCO+ derived from OCNCS+ and OCNCO+, respectively.13 Functionalization of the SAM terminus increases the range of reactions that can be performed in the course of ion/surface collisions. For instance, a carboxalate-terminated SAM surface (-OOCCH2(CH2)9-S-Au) in the form of the ammonium salt has been subjected to an interfacial Kolbe reaction. Upon bombardment with C6H5+, a C-C bond is formed between the projectile ion and the surface. Formation of a phenylalkyl derivative is shown by the appearance of C7H7+ (m/z 77) in the Xe•+ chemical sputtering spectrum.9 A new type of surface modification, termed soft landing, was first observed with F-SAM surfaces14 and has subsequently been shown to occur with H-SAM surfaces.15 Three different classes of the soft-landing experiment have been delineated: (i) soft landing defined as the deposition of intact polyatomic ions at a surface using low-energy ion beams, (ii) reactive soft landing defined as the deposition of polyatomic ions upon a surface in the course of which an ion/molecule reaction occurs at the interface, and (iii) dissociative soft landing defined as the deposition of polyatomic ions upon a surface during which the projectile ion fragments upon initial impact with the surface and the fragment ions become trapped at the interface.14,16 Soft-landing experiments have provided a new method with which to study novel chemistry. For instance, Feng et al.17 deposited modified DNA oligonucleotide cations into a membrane for retrieval and further enzymatic manipulation. Cowin et al.18 soft landed H3O+ on the surface of ice in order to follow the temperature dependence of migration and to estimate the activation energy associated with proton motion in ice. Typically, surface modifications by mass spectrometry are achieved using home-built instruments.3,19-21 Many of these instruments are equipped only with an electron impact (EI) ionization source, which severely limits the range of compounds that can be used to generate projectile ions. For surface modification experiments, the ease of use and additional ionization methods of commercial instruments make a number of biological and other novel ions available for use as projectile ions. Furthermore, simple modification of a commercial mass spectrometer, such as reported here, potentially allows surface modification experiments to be accessible to more investigators. Simple instrumental modifications to an existing commercial instrument that is capable of other ionization methods such as electrospray ionization (ESI), fast atom bombardment (FAB), and atmospheric pressure chemical ionization (APCI) were therefore undertaken. (14) Miller, S. A.; Luo, H.; Pachuta, S. J.; Cooks, R. G. Science 1997, 275, 1447. (15) Shen, J. W.; Yim, Y. H.; Feng, B. B.; Grill, V.; Evans, C.; Cooks, R. G. Int. J. Mass Spectrom. 1999, 183, 423. (16) Luo, H.; Miller, S.; Cooks, R. G.; Pachuta, S. Int. J. Mass Spectrom. Ion Processes 1998, 174, 193. (17) Feng, B. B.; Wunschel, D. S.; Masselon, C. D.; Pasa-Tolic, L.; Smith, R. D. J. Am. Chem. Soc. 1999, 121, 8961. (18) Cowin, J. P.; Tsekouras, A. A.; Iedema, M. J.; Wu, K.; Ellison, G. B. Nature 1999, 398, 405. (19) Bier, M. E.; Amy, J. W.; Cooks, R. G.; Syka, J. E. P.; Ceja, P.; Stafford, G. Int. J. Mass Spectrom. Ion Processes 1987, 77, 31. (20) Wysocki, V. H.; Ding, J.-M.; Jones, J. L.; Callahan, L. H.; King, F. L. J. J. Am. Soc. Mass Spcetrom. 1992, 3, 27. (21) Schey, K.; Cooks, R. G.; Grix, R.; Woellnik, H. Int. J. Mass Spectrom. Ion Processes 1987, 77, 49.

Projectile ions formed from nonvolatile compounds, in particular biological or otherwise interesting molecules, thereby become readily available. These ions could be used to modify a surface, or as shown by Feng et al.,17 they could be “captured” by the surface and upon retrieval used in further experiments. Geiger et al.22 reported modifications to a hybrid tandem mass spectrometer (EBqQ) in order to perform soft-landing experiments using novel cluster and noncovalent complexes as projectile ions. These authors reported the successful soft landing of metal-containing cluster ions upon a nominally stainless steel target, presumably covered with a hydrocarbon film. To confirm the deposition of the intact projectile ion upon the surface, the target was removed from the instrument and washed using a methanolic solution (5% HCl). The resulting wash was then analyzed by ESI-MS/MS. The spectra obtained from the wash was the same as the standard sample, confirming the soft-landing experiment. We report here instrumental modifications of a commercial triple quadrupole mass spectrometer for the purpose of performing chemical modifications of self-assembled monolayer surfaces. These capabilities are demonstrated by soft landing (CH3)2SiNCS+ (m/z 116) formed from trimethylsilyl isothiocyanate, (C6H5)3C5H2O+ (m/z 309) formed from 2,4,6-triphenylpyrylium tetrafluoroborate, and ClCH2(CH3)2SiOSi(CH3)2+ (m/z 181) formed from 1,3-bis(chloromethyl)tetramethyldisiloxane upon an F-SAM surface. In addition, we show surface modifications of a hydroxyl-terminated self-assembled monolayer (OH-SAM) surface and an F-SAM surface in the course of ion/surface reactions with Si(CH3)3+ (m/z 73) formed from tetramethylsilane and of CCl3+ formed from carbon tetrachloride, respectively. EXPERIMENTAL SECTION Surface Preparation. The F-SAM and OH-SAM surfaces23,24 were prepared by exposing ∼1 mM ethanolic solutions of 1,2tetrahydroperfluorodecane disulfide, (CF3(CF2)7(CH2)2S)2, and 11mercapto-1-undecanol, HO(CH2)11SH, respectively, to a gold substrate. The starting materials for the F-SAM surface and 11mercapto-1-undecanol, were purchased from Aldrich and used as received. 1,2-Tetrahydroperfluorodecane disulfide, (CF3(CF2)7(CH2)2S)2, was prepared using standard methods.24 The adsorbate chemisorbs to the gold film through a sulfur-gold bond. Prior to thin-film preparation, the gold surfaces were cleaned in boiling piranha solution for ∼2 min, rinsed with deionized water and then with ethanol, and then placed in the disulfide or mercaptan solution. The SAM was allowed to self-organize for a minimum of 5 days. A fresh SAM surface was used for each experiment. Surface Modification. Surface modification experiments were performed using a triple quadrupole mass spectrometer (TSQ 700 Finnigan MAT, San Jose, CA). A detailed description of the modifications made to this instrument will be presented in the Results and Discussion section. Briefly, projectile ions were formed by 70-eV EI and then extracted from the ion source into the first quadrupole assembly. The ions were passed through both the first quadrupole and the quadrupole collision cell with both (22) Geiger, R. J.; Melnyk, M. C.; Busch, K. L.; Bartlett, M. G. Int. J. Mass Spectrom. 1999, 183, 415. (23) Somogyi, A.; Kane, T. E.; Ding, J. M.; Wysocki, V. H. J. Am. Chem. Soc. 1993, 115, 5275. (24) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301.

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components operating in the broad-band rf-only mode. The projectile ions were then mass analyzed using the third quadrupole and, for the surface modification experiments, passed through the detector system and allowed to impinge upon the SAM surface. The projectile ions were passed through the detector housing via a hole at the back end of the housing and impinged upon the surface at 90° to the surface plane. The curvature of the collision cell in this instrument ensures that fast neutrals exiting the ion source do not follow the same line of flight as the ion beam. In order select the appropriate ion for the surface modification experiment, several mass spectra were recorded with the detector system in its standard operating condition. Once the appropriate conditions for ion transmission had been established, the external power supplies for the conversion dynode, electron multiplier, detector housing, and final lens were connected. The collision of the projectile ions upon the SAM surface at an appropriate energy was then achieved by applying potentials to these components in order to create a three-element lens stack in front of the surface (vide infra). The current generated at the surface through these ion/surface collisions was monitored using a Keithley 485 Autoranging Picoammeter (Cleveland, OH). Typical surface currents recorded were on the order of 0.04-0.20 nA and the deposition times chosen were on the order of 5 h, unless otherwise noted. The manifold pressure during the experiments was ∼1 × 10-6 Torr. Upon completion of the surface modification, the instrument was vented in order to remove the surface and ex situ surface analysis was performed. Surface Analysis. Analysis of the modified surface was performed using a home-built BEEQ (B ) magnetic sector, E ) electric sector, Q ) quadrupole mass filter) mass spectrometer, described in detail elsewhere,25 using low-energy (∼70 eV) Xe•+ or CF3+ sputtering, as denoted. Projectile ions were formed by 70-eV EI, accelerated to 2 keV, mass selected by the BE portion of the instrument, decelerated upon entering the main chamber, and then focused onto the target located in the surface analysis chamber, after being modified in the triple quadrupole mass spectrometer. In the experiments described below, the incident angle of the chemical sputtering ion beam was set at 45° and the scattering angle was set to 90°. The secondary ions resulting from chemical sputtering were accelerated from the surface and directed toward the electric sector/quadrupole analyzers for mass analysis. The individual surfaces used for each of the modification experiments were not examined prior to modification in the commercial triple quadrupole mass spectrometer, although other surfaces from the same batch were monitored. Changes in the surface were noted by comparing the modified surface sputtering spectra with that of a corresponding standard SAM. Reagents. The projectile ions, (CH3)2SiNCS+, ClCH2(CH3)2SiOSi(CH3)2+, Si(CH3)3+, (C6H5)3C5H2O+, and CCl3+ were formed by 70-eV EI upon trimethylsilyl isothiocyanate, 1,3-bis(chloromethyl)tetramethyldisiloxane, tetramethylsilane, 2,4,6-triphenylpyrylium tetrafluoroborate, and carbon tetrachloride, respectively. Trimethylsilyl isothiocyanate, 1,3-bis(chloromethyl)tetramethyldisiloxane, tetramethylsilane, and carbon tetrachloride were leaked into the ion source via a Granville Phillips valve attached to a sample probe. 2,4,6-Triphenylpyrylium tetrafluoroborate was in(25) Winger, B. E.; Laue, H.-J.; Horning, S. R.; Julian, R. K., Jr.; Lammert, S. A.; Riederer, D. E., Jr.; Cooks, R. G. Rev. Sci. Instrum. 1992, 63, 5613.

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troduced into the ion source via the solids probe. The source temperature was maintained at 150 °C and the solids probe was kept at 250 °C in order to obtain sufficient ion current from the solid sample. 2,4,6-Triphenylpyrylium tetrafluoroborate was used as received from Alfa Products (Danvers, MA). All other reagent chemicals were used as received from Aldrich Chemical Co. (Milwaukee, WI). RESULTS AND DISCUSSION Instrumental Modifications. A commercial triple quadrupole mass spectrometer was modified in order to perform ion/surface collision experiments. The simplicity of the necessary instrumental modifications results from the design of the detector system in this instrument. As can be seen from Figure 1, the detector was designed to allow X-rays and fast neutral particles to pass through the detector housing without impinging upon the detector thereby minimizing random noise. Capitalizing on this feature, we simply place the surface to be studied immediately behind the detector without making further mechanical changes to the instrument. The surface is held on a modified rail mount attached to the optical rail of the mass spectrometer. The position of the surface holder relative to the detector housing is also illustrated in Figure 1. The surface was floated relative to the ion source using a high-voltage power supply (model 205B-05R, Bertan High Voltage Power Supply, Hicksville, NY). The current resulting from ions impinging upon the surface was measured by a picoammeter. It was necessary to apply additional voltages to the surface, the final lens, and the detector housing and to modify the voltages applied to the conversion dynode and the electron multiplier, in order to pass ions through the detector housing. These voltages were necessary because both the final lens and the detector housing were grounded to the manifold in the standard commercial configuration, while the standard voltages applied to the conversion dynode and the electron multiplier were too high (kiloelectronvolt range) to allow the low-energy ions of interest to reach the surface. In the modified system, the voltages applied to both the surface and the final lens and detector housing were supplied by independent external power supplies via unused lens connections. During the surface modification experiments, the standard connections for the conversion dynode and the electron multiplier were removed and connections were made to an external power supply (Lambda LPD 422 FM, San Diego, CA). Typical voltages supplied to the final lens/detector housing and the conversion dynode/electron multiplier during the soft-landing experiments were negative 10 and negative 30 V relative to the ion source, respectively. Soft Landing. Soft landing of low-energy intact polyatomic ions was first observed as a result of a transhalogenation reaction using a pseudo-halogen projectile, the dimethylsilylisothiocyantosilyl cation, and an F-SAM surface.10 As shown in Figure 2a, the 70-eV Xe•+ chemical sputtering spectrum of a freshly prepared F-SAM shows no peak in the region of the major fragment ion of trimethylsilyl isothiocyanate (m/z 116). The 70-eV Xe•+ chemical sputtering spectrum of a surface modified after a 4-h treatment (surface current 0.38 nA) with the dimethylsilylisothiocyantosilyl cation (m/z 116) at 7-eV collision energy shows a sputtered ion peak corresponding to the intact projectile ion soft-landed onto the surface (Figure 2b). In the original work,16 the chemical sputtering mass spectrum collected from an F-SAM surface

Figure 1. Finnigan TSQ 700 detector showing the flight path of undeflected projectile ions, the position of the surface relative to the detector, and the external power supply connections. (Adapted from Finnigan TSQ Operators Manual.)

Figure 2. 70-eV Xe•+ (m/z 132) chemical sputtering spectra used to characterize the following surfaces (a) an unmodified F-SAM and (b) an F-SAM modified for 4 h at 7-eV collision energy by bombardment with (CH3)2SiNCS+ (m/z 116) in the commercial triple quadrupole mass spectrometer.

bombarded for 1 h at a collision energy of 20 eV by the dimethylsilylisothiocyantosilyl cation shows comparable abundance of the soft-landed species in these experiments, (m/z 116 is 29% of m/z 100 here, versus 13% in the original experiment). Longer times are necessary to achieve similar degrees of surface modification, and this suggests that the transmission of the ion beam to the surface in the commercial quadrupole instrument is quite low, due to either the divergence of the ion beam exiting the quadrupole or to a poorly focused beam. SIMION 3D26 calculations of the detector region confirm that, under the

experimental conditions used here, the ion beam is poorly focused and dispersed across the exposed surface. From the simulations, we estimate, as an upper limit, that 10% of the ions that enter the detector region actually hit surface. Further optimization of the ion optics might be achievable with further manipulation of the potentials applied to the detector housing and the conversion dynode and electron multiplier. Multiple individual spots on this modified surface were analyzed by Xe•+ chemical sputtering, and the results suggest that the deposition of the projectile ion is occurring over the entire exposed SAM surface (∼80 mm2). Note that, for 60-eV Xe•+ chemical sputtering and 10-eV soft landing on the BEEQ (conditions used for the original experiments), the areas of the impinging ion beams are estimated to be 3 and 50 mm2, respectively.16 The larger dispersion of the ion beam in the triple quadrupole mass spectrometer relative to that of the multisector mass spectrometer can be understood in light of (a) the angular distribution of the ion beam as it exits the last quadrupole and (b) the distance that the ions have to travel from the exit of the last quadrupole to the surface. The applied potentials that the ions travel through to the detector system, while sufficient to pass the ions through the detector housing unit, are not sufficient to focus the ion beam to a spot size comparable to that of the BEEQ. Soft Landing and Dissociative Soft Landing of the Triphenylpyrylium Cation. The 2,4,6-triphenylpyrylium cation is a relatively stable oxonium ion which has the unusual characteristic that it can be generated by 70-eV EI of the 2,4,6-triphenylpyrylium tetrafluoroborate salt. The gas-phase MS/MS spectrum of the pyrylium cation (m/z 309), shown in Figure 3a, reveals a prominent benzoyl ion at m/z 105. Modification of an F-SAM surface was carried out using low-energy 2,4,6-triphenylpyrylium cations and the results characterized by 70-eV Xe•+ chemical (26) Dahl, D. A. Proceedings of the 43rd ASMS Conference on Mass Spectrometry and Allied Topics, Atlanta, GA, 1995.

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Figure 4. Partial Xe•+ (m/z 132) chemical sputtering spectra of an F-SAM modified for 6 h at 5-eV collision energy by ClCH2(CH3)2SiOSi(CH3)2+ (m/z 181) in the triple quadrupole mass spectrometer. /, contaminant.

Figure 3. (a) Collision-induced dissociation (CID) of the 2,4,6triphenylpyrylium cation under single-collision conditions with argon at 50-eV collision energy. (b) 70-eV Xe•+ (m/z 132) chemical sputtering spectrum of an F-SAM modified at 10-eV collision energy by bombardment with (C6H6)C5H2O+ (m/z 309) in the commercial triple quadrupole mass spectrometer.

sputtering (Figure 3b). Both the intact projectile ion (m/z 309) and the benzoyl fragment ion (m/z 105) are observed, suggesting successful soft landing as well as a dissociative soft landing. The benzoyl ion (m/z 105), which can be seen in the full scan of the F-SAM surface, is 15% of the surface peak, C2H4•+ (m/z 100). Dissociative Soft Landing. The low-energy deposition of 35ClCH (CH ) SiOSi(CH ) + (m/z 181) upon an F-SAM surface has 2 3 2 3 2 been shown previously to result in dissociative soft landing with retention in the surface of fragment ions, most notably the fragment ion C3H10OSi235Cl (m/z 153).16 This dissociative softlanding modification was also demonstrated using the modified commercial triple quadrupole mass spectrometer. The 70-eV Xe•+ chemical sputtering spectrum of an F-SAM surface, modified by bombardment with 35ClCH2(CH3)2SiOSi(CH3)2+, reveals the presence of a sputtered ion at mass-to-charge 153 (Figure 4). The intensity of this fragment ion in these experiments is comparable to the intensities observed in the original work. Once again, it is noted that the deposition times for the two experiments are not the same, 0.5 h on the BEEQ and 6 h on the TSQ. Dissociative soft landing is likely to result from the same collision processes that operate in surface-induced dissociation (SID) but with retention, instead of emission, of the fragment ion. While it is possible that the intact projectile ion is deposited at the surface and that fragmentation occurs only during the chemical sputtering experiment, it has been shown that the abundance of the fragment ion observed in the sputtering spectrum does not depend on the kinetic energy of Xe•+.16 That is to say, lowering the Xe•+ beam 5802 Analytical Chemistry, Vol. 72, No. 23, December 1, 2000

Figure 5. 70-eV CF3+ (m/z 69) chemical sputtering spectra of (a) an unmodified OH-SAM and (b) an OH-SAM modified for 5 h at 10eV collision energy by bombardment with Si(CH3)3+ (m/z 73) in the triple quadrupole mass spectrometer.

energy to the onset energy of the fragmentation does not decrease the abundance of the fragment ion in the chemical sputtering spectrum. In this case, at least, the projectile ion fragments upon collision with the surface and the fragment ion is trapped within the SAM matrix. Surface Modification: Transhalogenation of an F-SAM Surface by CCl3+. As a further test of the capabilities of the triple quadrupole in surface modification experiments, a transhalogenation reaction was performed using CCl3+ (m/z 119) as the projectile ion and an F-SAM surface as the reaction partner. The surface was bombarded for 5.5 h with a surface current of ∼0.15

nA and a collision energy of 70 eV. The resulting 70-eV Xe•+ chemical sputtering mass spectrum reveals an ion having massto-charge 85, corresponding to CF2Cl+ in ∼19% the abundance of the surface sputtering peak, C2F4+, at mass-to-charge 100. Surface Modification: Surface Passivation of an OH-SAM Surface by Si(CH3)3+. A striking example of a surface modification is the deposition of the trimethylsilyl cation, Si(CH3)3+ (TMS), upon a hydroxyl-terminated self-assembled monolayer (OH-SAM) surface. Figure 5b shows the 70 eV CF3+ chemical sputtering mass spectrum of an OH-SAM which had been modified by reaction with the trimethylsilyl cation (m/z 73) for 4 h (surface current 0.15 nA) at a collision energy of 15 eV in the triple quadrupole mass spectrometer. The base peak corresponds to the release of the intact projectile ion used to modify the surface. The ion/ surface reaction results in the formation of an Si-O bond linking the projectile ion to the surface. Additional evidence which supports the claim that a chemical bond is formed between the trimethylsilyl cation and the OH-SAM surface is given elsewhere.27 Briefly, ions that soft land upon a surface in which a chemical bond is not formed between the projectile ion and the surface will be neutralized by long-term exposure to air as well as exposure to solvent. In this case, when the modified OH-SAM surface was exposed to air for as many as 10 days and the surface was rinsed with solvents such as diethyl ether and ethanol, the trimethylsilyl ion was still observed in the CF3+ chemical sputtering spectrum. Furthermore, a SAM surface has been synthesized with a silyl ether terminal group identical to that of the proposed modified surface. The CF3+ chemical sputtering spectrum of the synthesized surface is nearly identical to the CF3+ chemical sputtering spectrum of the OH-SAM surface modified by the trimethylsilyl cation. This surface modification experiment is of particular interest because the surface terminus is passivated as a result of the modification.

CONCLUSIONS We have shown that, with relatively minor changes to a commercial triple quadrupole mass spectrometer, surface modification of organic thin films can be effected by treatment with low-energy polyatomic ions. The modified surfaces are characterized independently by the low-energy experiment of chemical sputtering. The ability to perform surface modification experiments was demonstrated by soft landing the intact fragment ion, (CH3)2SiNCS+, which is the major fragment ion of trimethylsilyl isothiocyanate. Both soft landing and dissociative soft landing occur by soft landing the 2,4,6-triphenylpyrylium cation upon an F-SAM surface. The Xe•+ chemical sputtering spectrum reveals that some molecular ions land intact while others dissociate and the benzoyl fragment (m/z 105) is deposited on the surface. Dissociative soft landing was demonstrated using ClCH2(CH3)SiOSi(CH3)2+ as the projectile ion and observing the ion C3H10OSi235Cl+ (m/z 153) in the Xe•+ chemical sputtering spectra. Other surface modification experiments performed include transhalogenation between the projectile ion, CCl3+, and the F-SAM surface. Finally, a surface modification that passivates the surface was performed by reaction of the trimethylsilyl cation with an OH-SAM surface. These instrumental modifications provide capabilities to perform previously inaccessible experiments. Optimization of the ion beam impinging on the surface will allow such reactions to be performed.

(27) Evans, C.; Wade, N.; Pepi, F.; Schuerlein, T.; Cooks, R. G., in preparation.

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ACKNOWLEDGMENT This work was supported by the National Science Foundation (Grant CHE-9732670). Eli Lilly is acknowledged for the donation of the TSQ700 and their participation in the Purdue Chemistry Department’s Industrial Associate program. J.W.D. acknowledges a fellowship sponsored by Merck & Co.

Received for review May 22, 2000. Accepted September 22, 2000.

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