Langmuir 1991, 7, 2279-2286
2279
Surface Chemistry Effects and Sampling Depth of Molecular Ions from Static Secondary Ion Mass Spectrometry of Langmuir-Blodgett Fatty Acid Films Paula A. Cornelio-Clark+and Joseph A. Gardella, Jr.' Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14214 Received December 10, 1990. In Final Form: May 8, 1991
The effects of surface chemistry on the probability of formation of secondary molecular ions in static secondary ion mass spectrometry (SIMS)are studied. Langmuir-Blodgett (L-B) films and solution deposition of neutral fatty acids on polycrystalline silver substrates are chosen as model systemsto explore the effects of orientation, coverage, and primary ion beam dosage on the detection of molecular ions. Alternating layers within a five layer L-Bmultilayer system are used to explore the depth from which such molecular ions originate in a organized assembly. Results presented show the effectsof specificorientation on the formation of (M+ H)+,which requires intermolecular proton transfer in head to head configurations. Deprotonation of molecules to (M- H)- is a simple path to a stable structure. High signal intensity of this ion is observed from a variety of treatments and is shown to be related to proximity to the metal interface. Cationization to form (M + Ag)+ and (M- H + 2Ag)+ is very sensitive to the concentration of silver ions near the surface. The coverage of multiple layers attenuates detection of these ions. The sampling depth of molecular ions is very different. For (M + H)+, ions are detected from the first three molecular layers. Cationized molecular ions only originate from the topmost molecular layer. Deprotonation to form the carboxylate anion (M- HI- can occur from molecules within the first five layers. Ion beam dosage can severely restrict the detection of molecular ions. which are sensitive to orientation and surface chemistry.
Introduction The development of techniques for molecular surface analysis is extremely important for the complete characterization of a variety of technologically relevant materials.'+ The static SIMS (secondary ion mass spectrometry) technique offers analytical advantages such as the detection of all elements and isotopic sensitivity. In addition, static SIMS is capable of detecting molecular secondary ions from surface species in concentrations as low as 10-1' mol/cm2.7 For many elements and organic species, detection limits on the order of 1ppm of a monolayer have been reported.* Static SIMS has been demonstrated to be useful for the detection of a wide variety of organic compounds, including amino acids, peptides, vitamins, and pharmaceutical^.^ Finally, since the probability of secondary ion emission is dependent upon the matrix from which the ion originates, static SIMS has surface compositional sensitivity. However, to gain information about the chemistry and structure of surfaces using static SIMS, a better understanding of both qualitative and quantitative aspects of molecular ion formation is desired.
* Address correspondence to this author. t Current address: Air Products and Chemicals, Inc., 7201 Hamilton Blvd., Allentown, PA 18195-1501. (1) Gardella, J. A., Jr.; Pireaux, J. J. Anal. Chem. 1990,62, 645A. (2) Hercules,D. M. In CharacterizationofMetaZandPolymer Surfaces; Lee, L. H., Ed.;Academic Press: New York, 1977; Vol 1, p 399. (3)Wandass, J. H.; Schmitt, R. L.; Gardella,J. A., Jr. J. Appl. Surf. Sci. 1989, 40, 85. (4) Wandass, J. H.; Gardella, J. A., Jr. J. Am. Chem. Soe. 1985,107, 6192. (5) Clark, M. B., Jr.; Gardella, J. A., Jr. Anal. Chem. 1990, 62, 870. (6) Clark, M. B., Jr.; Gardella, J. A., Jr. Anal. Chem. 1990,62,949. (7) Gardella, J. A., Jr.; Wandass, J. A.; Comelio, P. A.; Schmitt, R. L. In Proceedings of the Fifth International Conference on Secondary Zon MassSpectrometry;BenNnghoven,A.,Colton, R.,Simons,D.S.,Werner, H.W., Eds.; Springer-Verlag: New York, 1985; p 534. (8)Secondary Ion Mass Spectrometry-Basic Concepts,Instrumental Aspects, Applications and Trends;Benninghoven, A., Rudenauer,R. G., Werner, H. W., Eds.;John Wiley and Sons: New York, 1987; p 685. (9) Delgass, W. N.; Cooks, R. G. Science 1987,235, 546.
0743-7463/91/2407-2279$02.50/0
Relevant to the present study is work by Benninghoven's Thin organic films of amino acids prepared by vacuum deposition onto polycrystalline Ag, Au, and Cu surfaces11J2were studied. The formation of the (M+ H)+ and (M - H)- molecular ions was examined for submonolayer, monolayer, and multilayer coverages. Of the systems studied, the highest intensity of (M + H)+was for amino acids deposited on Au. For example, the intensity of (M + H)+ and (M- H)- increased with glycine coverages up to a value of about l O I 5 molecules/cm2. The signal intensity was assumed to saturate when monolayer coverage was reached. In contrast to the Au surface, studies of intensity of (M - H)- from glycine showed much higher intensities than for the same preparation on Au. The (M - H)- ion intensity was found to increase until monolayer coverage had been reached (1015molecules/cm2). On the other hand, the (M + H)+ signal was not detected for submonolayer to monolayer coverages on Cu. Benninghoven suggested that this behavior was the result of two different mechanisms of film-substrate interfacial interaction. In the submonolayer to monolayer regime, the glycine molecule is chemically adsorbed to the Cu surface. The molecules become deprotonated by the "active" Cu surface and the (M - H)- ion is formed with higher probability. For reactive substrates, the (M + H)+ ion was only observed for multilayer coverages, in which physically adsorbed layers were formed on top of the chemisorbed monolayer. In this circumstance, Benninghoven suggested the (M + H)+ ion was formed as a result of intermolecular proton transfer between the glycine molecules. Both the (M+ H)+and the (M - H)-ion intensities were found to increase with increased film coverages on (10) Benninghoven, A. Surf. Sci. 1976, 53, 596. (11) Benninghoven,A.; Lange,W.; Jirikowsky,M.; Holtkamp, D. Surf. Sci. 1982, 123, L721. (12) Lange, W.; Jirikowski, M.; Benninghoven, A. Surf. Sci. 1976, 79, 549. (13) Tamaki. 5.:Sichtermann,. W.:. Benninahoven, J. Appl. . A. Jpn. __ Phys. '1984, 23 (5); 544. (14) Benninghoven,A.; Jaspers,D.;Sichtermann,W. Appl. Phys. 1976, 11.35.
0 1991 American Chemical Society
2280 Langmuir, Vol. 7, No. 10, 1991 Au. It was proposed that only physical adsorption takes place on the Au substrate. Moreover, this physically adsorbed layer promoted (M + H)+ formation, since the intensity of (M H)+ is greater than the intensity of (M - H)-. Tamaki and Benninghoven have also examined the formation of the (M + H)+ ion from arginine on polycrystalline Ag.13 The intensity was found to increase linearly from about 10'2 to 10'6 arginine molecules/cm2. Cationization is a general phenomenon in SIMS. For organic species on Ag substrates, the (M + Ag)+ ion is commonly 0bserved.3*~J4-16Typically, the mechanism of cationization is thought to involve the attachment of a Ag+ ion to a neutral organic molecule. This interpretation is supported by the appearance of a variety of substrate cluster ions in the SIMS spectrum, including Ag+, Agz+, Ag3+, AgzCl+, AgC12-, and Ag2Cls-.l6 The cationization mechanism is thought to take place in the selvedge, the reactive phase above the solid which is dominated by electrical forces and molecular collisions. In ordered organic films (e.g., L-B films),substrate ions, like Ag+, Agz+, etc., can be channeled into the selvedge. Such channeling mechanisms were reported by Standing17 who observed the emission of the divalent cation, Cd2+, from cadmium stearate L-B films. The latter case does not involve substrate related ions, but it is believed that in the present work, the alkyl chains of organic molecules direct the substrate ions into the selvedge. Channeling processes were originally proposed to explain sputtering in single crystals.* The incoming primary ion was channeled by the close-packedorder of the singlecrystal targets. In these cases, the path near the center of a channel was thought to have a certain degree of stability and, as a result, the ion would penetrate a large distance into the crystal. Fatty acid and polymeric L-B films have been used as model systems to study ion formation in static SIMS.3*4~7JeJs Wandass et al. have investigated the formation of molecular ions from stearic acid and barium stearate L-B films,8p4 supported by silver, gold, and germanium substrates. In particular, the formation of (M + H)+ was found to depend upon the film-substrate interfacial chemistry. The (M + H)+ion was observed for single stearic acid L-B monolayers on Au and Ge but not for L-B monolayers on Ag. Consistent with the previous work of Benninghoven" the reactive Ag surface was thought to promote chemical adsorption through the deprotonation of the fatty acid molecules. The (M- H)-, (M + HI+, and (M + H - H2O)+ ions were observed for 3,5, 7, and 15 L-B layers of stearic acid on Ag and Au. Also, the (M + H)+and (M + H - H2O)+ions were found to have similar damage cross sections. In general, molecular ion emission was observed for 20-50 min of sputter time under 1-5 nA/cm2 Ar+ ion bombardment. However, substrate ion emission was also detected through the L-B multilayer films. Wandass reported that (M + Ag)+ formation depended upon the substrate preparation.3 Solutiondeposition onto substrates prepared by using acid etch methods introduced by Benninghovenll-14 produced a high intensity of substrate-cationized quasi-molecular and parent For L-B prepared films, the (M + Ag)+ ion emission was only
+
(15) Pachuta, S. J.; Cooks, R. G. Chem. Rev. 1987,87,647. (16) Grade, H.; Winograd, N.; Cooks, R. G. J. Am. Chem. SOC.1977, 99,7725. (17)Bolbach, G.; B e a h , R.; E b , W.; Main, D. E.; Schueler, B.; Standing, K. Nucl. Instrum. Method8 Phys. Res., Sect. B 1988, B30,74. (18) Hook, K. J.; Hook, T. J.; Wand-, J. H.; Gardella,J. A., Jr. Appl. Surf.Scr. 1990, 44, 29. (19) Cornelio, P. A.; Gardella, J. A., Jr. J . Vac. Sci. Technol. A 1990, 8(3), 2283.
Cornelio-Clark and Gardella
observed for unsaturated acids.' From this observation, it was asserted that coordination of Ag+ to double bonds was responsible. However, the initial observations4were made before digital acquisition capabilitieswere available. The signal intensity of (M + Ag)+ emission was present at a much lower intensity for the saturated fatty acid than in the unsaturated acid case. It was then suggested that Ag cationization proceeded via attachment to the carbonyl group and that sufficient sensitivity had not been available previously to detect the low levels of Ag cationization that were present.20 In addition, the (M+ Ag)+ ion was detected for barium stearate L-B films. Wandass further proposed that the Ag oxidation state had been altered by the variation in subphase conditions used to prepare the barium substituted L-B films. These explanations were consistent with the work of Grade and Cooks21 who have studied the production of substrate cationized species as a function of the Ag oxidation state. The depth from which secondary ions can originate depends on the extent and intensity of the collision cascade. In general, the sampling depth of static SIMS is thought to be on the order of an atomic monolayer to greater than 20 A.ll Since SIMS involves the sputtering of material from the surface, depth profiling is possible. Several studies using L-B films have been reported which attempted to determine the sampling depth of organic molecular ions.22-24 Laxhuber et al. have investigated the sputtering characteristics of arachidic acid multilayer8 on Au backings under 2.5-keV Ar+ primary ion bombardment.22 Ion beam compositional analysis was reported to be feasible to a depth of 25 A. In addition, cadmium- and magnesiumsubstituted arachidate L-B films were studied.23 Depth profiling experiments were conducted in which the divalent cations served as depth markers. Only the emission of atomic ions was examined because of the high damage cross section of molecular species. The intensity of the Mg2+ was found to be higher when placed at the near surface region. The results were complicated by selective sputtering and a thickness-dependent erosion rate. In related work, Hakansson et al. examined the sputtering characteristics L-B films during "electronic" sputtering processes, using megaelectronvolt lS0and 12C i ~ n ~ The . ~ results ~ - ~indicated ~ the creation of a crater some 150 A deep and having a radius of 80 A. The molecular ions were thought to originate from this sputter volume. In addition, the number of sputtered ions was found to depend on the energy density along the primary ion track. The emission of molecular ions in static SIMS is also dependent upon the energy density of the primary ion. Save et alaz also examined alternating layer arrangements as studied in the present work. The focus was the use of the tag layers to probe sampling depth as a function of nuclear stopping power (primary ion energy and mass). In this study, the tag layers were behenic acid bilayers, some 50 A thick. A similar study was reported by Bol(20) Wandaes, J. H., 111 Ph.D. Thesis, Stab University of New York at Buffalo, Sept 1986. (21) Grade, H.; Cooks, R. G. J. Am. Chem. SOC.1978,100,5615. (22) Laxhuber, L.; Mohwald, H.; Hashmi, M. Int. J. Maas Spectrom. Ion Phys. 1983,61,93. (23) Wittmaack, K.; Laxhuber, L.; Mohwald, H. Nucl. Inetrum. Methods Phys. Res., Sect. B 1987, B18, 639. (24) Haknnsson, P.; Sundqvist, B. U. R. Vacuum 1989,39 (2-4),399. (25) Save, G.; H h m n , P.; Sundqvist, B. U. R.; Johnson, R. E.; Soderstrom,E.;Lindquist, S.E.;Berg, J. Appl. Phys. Lett. 1987,61,1379. (26) Save, G.; Hakanseon, P.; Sundqvist, B. U. R.; Soderstrom, E.; Lindquist, S.E.;Berg, J. Int. J. Mas8 Spectrom. Ion ROCeS8e8 1987,78, 259.
Static SIMS of L-B Fatty Acid F i l m
bach et al.n Yields of molecular ions were shown from 80 to lOOA depths,from L-B depositedoverlayers. Sampling depths for the electronic sputtering process range to hundreds of angstroms due to the crater formation. However, in contrast to electronic sputtering, molecular ions in static SIMS are thought t o originate at some distance away from the point of primary impact. The damage induced by the megaelectronvolt energy incident ion is very different than in SIMS. It is generally held that megaelectronvolt energy particles are less damaging to a material than kiloelectronvolt energy particles.28For example, Rutherford backscattering spectrometry, which makes use of 1-MeV He+, is usually considered a nondestructive technique when compared to SIMSor AES. This statement is generally true for metals and inorganic materials. However, the damage t o fragile organic materials can be extensive. Thus, sampling depths between the two methods should be very different. In this study, model thin films have been prepared by both solution casting and L-B techniques. Thus, the effects of molecular orientation, film substrate coverage, film-substrate interfacial chemistry, and primary ion beam damage on secondary ion formation have been examined. Secondly, experiments have been carried out to evaluate the depth from which different ions originate. Finally, the relationship between the lifetime and the stability of the secondary ion has been evaluated. ESCA and field-emissionscanning electron microscopy (FE-SEM)have been used to characterizethe homogeneity of the model films as well as to determine the cleanliness and morphology of the supporting substrate^.^^ ESCA has also provided information about the film-substrate interfacial bonding chemistry.
Experimental Section Sample Preparation. The L-B filmswere prepared in a KSV 2200 Langmuir trough (KSV Chemicals, Finland). The trough has been discussed elsewhereSmFor each preparation the empty trough was cleaned with detergent (Sparkleen, Fisher Scientific) and rinsed with copiousamounts of tap water, followed by distilled water. The Teflon surface was then dried and 'polished" with Kim wipes and filled with triple distilled water. The subphase was equilibrated for 20 min, allowingtime for surface-active agents in the water to surface segregate. These impurities could then be removed by sweeping the surfacewith a movable Teflon barrier. The purity of the gas/liquid interface was examined by allowing the barrier to move across the surface at 20 mm/min. This procedure was repeated until an increase in surface pressure of less than 0.2 dyn/cm was obtained. The fatty acid materials were chromatographicreferencegrade (>99!%purity, Sigma Chemical Co.). L-B films of octadecanoic acid (stearic acid, SA), eicosanoic acid (arachidic acid, AA), and docosanoic acid (behenic acid, BA) were prepared by casting a monomolecularfilm from 1mg/mL solutions in benzene (Baker Reagent Grade). The materials were installed into the trough by allowing the solution to glide down a glass slide ramp (50% HN03 (aq) acid cleaned and rinsed in triple distilled water). After solvent evaporation (15-20 min) the glass slide was gently pushed into the trough and the film allowed to reequilibrate for about 1 min. The films were compressed a t 10 mm/min to a maximum surface pressure of 25 dyn/cm. The fatty acid L-B films were transferred at a surface pressure of 25 dyn/cm at a rate of 4 mm/min. Single monomolecular layers were prepared by pulling a hydrophilic substrate upward through the monomolecular layer. In this way, the fatty acid carboxylic acid head (27) Bolbach, G.; Della-Negra,S.;Deprun, C.;Le Beyec, Y.; Standing, K.0. Rapid Commun. hfms Spectrom. 1987,l (2), 22-4. (28) Grant, W. A. Rutherford Backscattering Spectrometry. In Methods of Surface Analysis; Walls, J. M., Ed.;Cambridge University Press: Cambridge, U.K., 1989; Chapter 9, pp -337. (29) Cornelio, P. A. Ph.D. Thesis, State University of New York at Buffalo,Sept 1990.
Langmuir, Vol. 7, No.10, 1991 2281 group attaches to the substrate surface." Multilayer films were prepared by redepositing the monolayer-coatedsubstrate back through the monomolecular film in both directions. A negativemeniscus was formed at the film/gas interface, representative of aliphatic tail to tail attachment during multilayer preparations. An odd number of multilayers was deposited in a Y-type depositi0n.a This molecular multilayer arrangement gives the most stable conformation, since the low surface tension aliphatic tailis located at the air/solid interface. The quality of the transfer was evaluated by examining the meniscus formed at the film/ substrate interface on the upward and downward strokes. In addition, the deposition ratio for all fatty acid transfers was 1.00 f 0.08. All transfers outside of this range were rejected. The L-B layers were transferred to Ag and Au 99.999 % pure polycrystallinefoils (AlphaProducte). The substrates were first cleaned with detergent and sonicated 3 times in an organic solvent (n-hexaneor 2-propanol). After this cleaning step, the substrates were treated in a radio frequency glow discharge (RFGD).Si4The Harrick quartz plasma chamber utilized a 60-W, 13.56 MHz radio frequency induced plasma. The plasma treatment was carried out in air at a pressure of about 30 mTorr (mechanicalpumping). The quartz chamber was cleaned by dischargingfor 15min prior to introducing the substrates. The substrates were stored in boiled, triple distilled water for no longer than 30 min to 1 h before being installed into the Langmuir trough. Fatty acid thin films were also prepared by casting from dilute solution onto RFGD-treated silver substrates using a microsyringe method."-" Coverages equivalent to monolayer and multilayer L-B films were constructed by assuminga 20 &/molecule packing density. On the basis of the solution concentration and the geometric area of the RFGD Ag coupon, the equivalent of either one (5 X l0ls molecules/cm2)or five monolayers (25 X 1OI6 molecules/cma) of arachidic acid was deposited onto the Ag coupon. ESCA Experiments. ESCA experiments were conducted on a Surface ScienceInstrumenta Model 206 instrument fitted with a hemispherical analyzer and utilizing a monochromatized A1 Ka X-ray source (180 W, 10 kV, 18mA). The experiments were conductedat atakeoff angle of 35O with respect to surface normal. Base pressures of 6 X 1o-B Torr and operating pressures of lese. than 1 X 10-8 Torr were maintained. Instrumental diagnostics at a pass energy of 50 eV and 600 pm spot size yielded a Au 4 f ~ p peak at 84.0 eV with a full width at half maximum of 0.91 eV and a count rate of 360 OOO counts eV/s. For the L-B film analysis, low resolution survey spectra were taken at a pass energy of 150 eV and a spot size of 1000pm. High-resolutionscans were taken at a pass energy of 100 eV with a 300 Mm spot size. The data reduction was carried out with a Hewlett-Packard 9836CS computer using a standard SSI ESCA 8.01A software package. Fatty acid standard materials were studied to determine the carbon to oxygen percent atomic concentration ratios and the carbon 1s and oxygen 1s functional group distributions. The standard materials were analyzed as powder samples on double sided Scotch tape. In this case, an auxiliary flood gun (1eV) was used for charge compensation. The homogeneity of the L-B films was determined by examining three to six, 300-pm spots at various points across the sample. To avoid thermal induced damage, the L-B films were analyzed without the use of a flood gun. ESCA analysis was also carried out to examine the change in concentration of the surface contamination after rf plasma treatment of the Ag substrate. Field-Emission SEM. Electron microscopy was carried out on a Hitachi S-800 field emission scanning electron microscope. The electron micrographs were taken at 25 kV filament voltage. The tilt angle was 45O. The substrates were examined at magnifications of 6000 to 100 OOO without coating. SIMS Experiments. The static SIMS experiments were carried out on an in-house modified3 Leybold-Heraeus LHSlO SIMS 100 secondary ion mass spectrometer. The system had a base pressure of 5 X 1W10mbar and an operating pressure of 6 X 1o-B mbar (99.9995% pure Ar (Airco) or Xe (Air Products)). The ion gun was differentially pumped. Both 4-keV Ar+ and Xe+primary ions have been used. Sampleswere mounted directly (30) Gaines, G. L., Jr. Insoluble Monolayers at The Liquid-Gas Interface; Intemience: New York, 1966.
Cornelio-Clark and Gardella
2282 Langmuir, Vol. 7, No. 10, 1991 onto a sample introduction rod, which was biased at a potential of a13.6V, to improve the transmission of the prefilter lens. The mass spectrometer consisted of a 3 to lo00 mass/charge ( m / z ) Balzera quadrupole mass filter (QMF). The sample current was about 3 nA/cm*. The analysis area was 16 mm*. For collection times of 5-60 min, the resultant primary ion beam dosage is 6-70) X lo1*ions/cm2. The data were collected with an IBM PC through a custom-built interface system (Assmuth and Muelhoff, Inc., Wilkensburg, PA). This interface system also controlled the QMF.18 Low-resolution survey scans from 3 to 700 D were taken at 5 data points per mass unit (0.2 D sampling resolution) at a rate of 5 ma per mass unit. High-resolution scans were taken at 10 data points per mass unit (0.1 D sampling resolution) at a rate of 10 ma per maw unit. The total acquisition time for highresolution spectra was also 5-10 min. After cleaning, the resultant Ag surface, available for bonding to the L-B monolayer, is composed of 28% carbon, 50% silver, ~~~~ 70 75 and 22 7% oxygen from ESCA a n a l y ~ i s . 3Approximately of the carbon layer is composed of aliphatic species. Since aliphatic species have a low surface energy, they do not posses a thermodynamic driving force to participate in bond formation with the L-B layers. Thus, the L-B monolayer is expected to interact with the Ag surface through the silver and silver oxide sites. One disadvantage of the plasma treatment is the introduction of surface texture. SEM of the Ag foil as received from the manufacturer revealed the presence of semiparallel machining lines. These lines are probably the result of pressing the Ag into foil sheets. In addition, the lines appear to have different heights and the area between the machining lines contains holes. Several points across the original Ag foil were examined and the morphology was found to be heterogeneous. Within 10 min of plasma treatment, large changes in the surface morphology were observed. Plasmasputtering resulted in the formation of a pitted surface and the machining lines were removed. Using tunneling electron microscopy, Pireaux found the surface corrugations to be on the order of 400 A.92 The corrugated Ag surface may be incommensurate with the fabrication of continuous L-B layers. However, Ag is an important substrate in static SIMS, since only Ag forms metal cationized molecular ions? Furthermore, Gainesw has suggested that the deposition ratio can give a reasonable measure of L-B monolayer ability to bridge across substrate corrugations. The quality of the L-B film transfer can be improved when the depositions are made at high surface pressures. For L-B films in which the transfer ratio is very near 1, the monolayer is considered to "carpet" the voids of the supporting solid. The "carpet effect" can be further enhanced if the film is composed of molecules which demonstrate strong intermolecular forces. Of course, the fatty acid films are highly crystallinew and therefore present a good system for overcoming substrate roughness. However,Bikerma+ and QuinckeMhave taken a considerably less pristine view of the "built-up multilayer". The true surface area of a substrate will always be greater than the geometrical area. Thus, the L-B film which is transferred to solid cannot coat the actual surface as long as the transfer ratio is 1. Bikerman examined the transfer characteristics of stearate monolayers to grooved substrates and wire gauze under low surface pressures. The films were then examined under a light microscope. The multilayer films were found to stretch across the voids of the gauze substrate. However, after a few hours, the films dried, burst, and contracted to the wire. In order to minimize such roughness-induced defects, the fatty acid L-B films were transferred at a surface pressure of 25 dyn/ (31) Comelio,P. A.;Gardella,J. A,, Jr. Langmuir Blodgett Film-Metal Interfacee-Static Secondary Ion Maee Spectrometry and Electron Spectroscopy for Chemical Analysis. Chapter 28 In Metallization of Polymers; Sacher, E., Pireaux, J.-J., Kowalczyk, S. P., Eds.; ACS Symposium Series 440; American Chemical Society: Washington, DC, 1990: ChaDtar 20. DD 37S393. (32) H&m, B.;Giegoire, Ch.; Pireaux, J. J.; Cornelio, P. A.; Gardella, J. A., Jr. Appl. Surf. Sci. 1991, 47, 163-172. (33) Bikerman, J. J. Physical Surfaces;Academic Press: New York, 1979 _ _ .. cD 232. (34) Quincke, G. Philos. Mag. 1968,36 (4), 267. I
A
5
! J
I
(00
0
B
OD
u
u
I J
1
Id
1
Od
0 0
Figure 1. Static SIMS of 1 L-B AA/Ag: (A) positive ion spectrum; (B) negative ion spectrum. cm. In addition, only L-B films with deposition ratios of 1.00 f 0.08 were used for proceeding investigations. The deposition ratio was not taken as evidencethat the L-B films were continuous. Rather, this ratio simply served as a means of ensuring the reproducibility of each film preparation. Ongoing studies on preparation of atomically smooth silver substratesare underway and will be reported elsewhere.
Results and Discussion The positive and negative ion static SIMSsurvey spectra for a monolayer of arachidic acid on RFGD silver (1L-B AA/Ag) are shown in Figure 1. The characteristic secondary molecular ions which are observed for both solution cast and L-B monolayer of arachidic acid on Ag include (M + Ag)+, (M - H + 2Ag)+ (cationization by Ag), and (M - H)-.Wandass et al.,3+4?'initially reported that Ag cationization was not observed for 1L-B stearic acid (SA)/Ag. Signal due to (M + Ag)+ was only detected for unsaturated fatty acid L-B and solution cast monolayers on silver. Later, Wandass reported a weak (M + Ag)+ signal for 1 L-B SA/Ag, after the secondary ion mass spectrometer had been configured for signal averaging capability.20 I n the present work, the intensity of t h e silver cationized molecular ions was much higher, owing to lower overall primary ion dosages to obtain the necessary signal/ noise. As in previous work on SA,3 the intensity due to t h e (M + H)+cation was not observed for 1L-B SA/Ag, 1L-B AA/Ag, or 1L-B BA/Ag or for solution cast monolayers. Signal from t h e (M - H)-carboxylate anion is observed at high and steady intensity in the static SIMS spectra of both solution cast and L-B monolayers on Ag. However, ESCA analysis of the fatty acid L-B films has shown that t h e fatty acid monolayer is not completely deprotonated by the Ag surface; in other words, t h e fatty acid molecules
Static SIMS of L-B Fatty Acid Film
Langmuir, Vol. 7, NO. 10,1991 2283
Table I. Summary of Qualitative Static SIMS Data for Solution Cast and Langmuir-Blodgett Fatty Acid Thin
A
15
Ag+
lSa/Ag YES
5Sa/Ag YES lAA/Ag YES 5AA/Ag YES
lBA/Ag YES
5BA/Ag
YES
lAA/Ag YES
(M-H+
(M + H)+ (M + Ag)+
2&)+
(M- H)-
YES YES YES(w) NO YES(w) NO
YES
YES YES(w) YES NO
YES YES
YES YES
1
I
Films.
L-B Films NO YES NO YES NO YES
YES YES YES YES(w) YES NO
YES
Solution Cast Films
NO YES NO YES 5AA/Ag YES a (w), weak signal. 1 monolayer is -5
X 1016 molecules/cm2.
260
5
270
280
monolayers is ==25 X 10l6molecules/cm2.
in the L-B film on RFGD silver substrates do not exist as free carboxylate anions.29*31932 The presence of a single oxygen 1s peak in the ESCA spectrum would have indicated the formation of a carboxylate anion via the deprotonation of the fatty acid by the Ag surface. Such ESCA spectra would, however, were detected for barium Therefore, stearate or cadmium arachidate L-B the (M - H)-ion which appears in the static SIMS spectrum of 1 L-B AA/Ag is not a preformed carboxylate anion. The production of the (M - H)- ion for 1 L-B AA/Ag requires the deprotonation of the neutral fatty acid molecules. The intensity from signal due to (M - H)- ions from 1 L-B AA/Ag is not as strong as that from the preexisting (M - H)-anion from the arachidate salt L-B film. The arachidate salt would form a strong quasi-molecular (M - H)- anion via direct sputtering into the selvedge. Similar observations have been reported by Benninghoven;" the (M - H)- ion was formed preferentially over (M+ H)+ for a monolayer of glycine on Cu. Benninghoven has reported that less reactive substrates, like Au and Ge, set up weak physisorptive bonding with adsorbed organic overlayers.11 The Cu and Ag substrates are considered active substrates in static SIMS and as a result promote chemical adsorption. Static SIMS of Fatty Acid Multilayers. The static SIMS results for stearic, arachidic, and behenic acid monoand multilayer L-B films are summarized in Table I. The (M + H)+and the (M + H- HzO)+ionshave been observed in the static SIMS spectra of 3,5,7,9,and 11 L-B AA/Ag. Benninghoven has also reported similar results for multilayer coverages of glycine on Cu.l0 The formation of (M + H)+ was attributed to intermolecular proton transfer between the amino acids. An interesting result is that the (M+ H)+ion is not detected for the solution cast equivalent of five layers of arachidic acid on Ag (Figure 2). The solution cast multilayer is undoubtedly a film of random molecular orientation. Thus, some structural order may be required for (M + H)+formation in fatty acids. Furthermore, the head to head arrangement of the carboxylic acid group within the L-B multilayer may, in fact, promote intermolecular proton transfer. However, the solution-deposited multilayers probably do not form complete and uniform film coverage. In fact, the film probably consists of regions of monolayer coverage, regions of multilayer coverage, and regions in which the Ag substrate is exposed. This thesis is supported by the appearance of a very weak emission of Ag3+ (-321-327 D) in the mass spectrum of the solution cast multilayer. Therefore, (M+ H)+ ion emission may be precluded for solution deposited multilayers as a result of H+complexation with the bare sites of the Ag surface. The formation of (M + H)+may require that all active Ag
290
300
3tO
320
330
340
MASS Daltond
B
I
260
270
280
290
300
I
I
I
I,
310
320
330
340
MASS W t d
Figure 2. Static SIMS of five layers of AA on Ag: (A) no (M + HI+signal from the solution cast multilayer; (B) (M + H)+ and (M + H - HzO)+ ions from 5 L-B AA/Ag. surface sites be complexed by, for example, the formation of a chemisorbed first layer. Then, for a single fatty acid L-B monolayer on Ag, the (M - H)-ion is formed by deprotonation of the neutral fatty acid molecule by the active Ag surface sites. Upon multilayer formation, the first monolayer complexeswith the active Ag surface, covering the active Ag sites. Additional layers are physically adsorbed on top of the complexed/chemisorbed layer. Only in the multilayer L-B films can (M + H)+ formation be observed because of the better coverageswhich are attained with an ordered L-B multilayer. The intensity of the (M - H)- ion emission was found to decrease with increased layer coverage (Table I). Such attenuation effects were observed for both solution cast and L-B multilayers. Figure 3 shows the negative ion SIMS spectra of one and five solution cast arachidic acid layers on Ag. The Ag2Cls- substrate ion a t 320 D is clearly attenuated with increased material coverage. The loss of Ag2C13- ion intensity indicates the improved coverage of the L-B layers. Similar (M - H)- ion attenuation has been is reported by Benninghoven" and Standing.17 The attenuation is attributed to the fact that additional fatty acid layers are not in close proximity to the Ag substrate and that available active sites have already been complexed by the first monolayer. Table I also indicates that the attenuation of (M - H)- can be roughly correlated to the fatty acid chain length and coverage. The (M- HIanion was observed for 1 L-B AA/Ag and 1 L-B BA/Ag, but the anion was attenuated a t 5 L-B AA/Ag. Finally, a t 5 L-B BA/Ag the (M - H)- ion signal is lost completely. If the AA molecule is taken to be about 27 A in length and
2284 Langnuir, Vol. 7, No. 10, 1991
Cornelio-Clark and Gardella
A
c
so
'1
40
-B
30
h
20
io 0 300
2o
1s
'O S
0
306
310
Mss
316
320
125
306
310
31s
320
326
MASS mtcrr)
B 1
Mss
300
(D.1tU-d
(D.1tU-d
*
1
121
MASS bltaml
Figure 3. (M- HI-signal from (A) 1solution cast AA on Ag, (B) 5 solution cast AA on Ag, (C) 1L-B AA/Ag, and (D) 5 L-B AA/Ag.
the BA molecule about 30 A in length, then the sampling depth for (M - HI-is about 135-150 A. Table I shows that the loss of signal from substrate cationization also correlated with increased molecular chain length. Both the (M + Ag)+and (M- H + 2Ag)+ions have been observed for 1 L-B AA/Ag and 1 L-B BA/Ag, although the (M - H + 2Ag)+ ion is very weak. At 5 L-B AA/Ag, the (M + Ag)+ ion emission is weak and (M - H 2Ag)+is not detected. No Ag cationized molecular ions were observed for 5 L-B BA/Ag. The attenuation of substrate cationized species is directly related to the availability of Ag+. A progressive attenuation of Ag+ ion emission with respect to the low molecular weight fragmentation has been observed (Figure 4). If metal cationization is a selvedge gas-phase reaction, then the attenuation of substrate cationized molecular ions is commensurate with the loss of Ag+ ion intensity with increased film thickness. The lower probability of Ag+ emission is attributed to a decrease in the efficiency of Ag+ channeling through thicker overlayers. Previously reportedlo time of flight SIMS data have also supported the progressive attenuation of Ag+ cationized species with increased layer coverage. The (M + Ag)+ ion intensity was found to decrease from -600 counts/s for 5 L-B AA/Ag to =300 counts/s for 7 L-B AA/Ag. Similar attenuation effects were observed for (M - H + 2Ag)+. Furthermore, the intensity of (M- H + 2Ag)+ is weaker than the intensity of (M + Ag)+. On the basis of this attenuation pattern, it is postulated that the formation of the former requires the attachment of two silver ions to the (M- H)-ion, while the latter requires the attachment of only one silver ion to Mo. Static SIMS of 1BA-4AA Alternating Multilayera. The data presented so far indicate that the intensity of molecular ion emission is affected by film-substrate interfacial chemistry, film coverage,and primary ion beam damage. Effects of different primary ion beam energies
+
have been reported elsewhere.10*2eA series of alternating L-B multilayer films have been constructed to further characterize the effect of overlayer signal attenuation and ion beam damage on the formation of secondary molecular ions. The model consists of five L-B layers, one behenic acid layer and four arachidic acid layers. As discussed above, a similar approach has been taken by Save et a1.z and Bolbach et al?' to understand sampling depth and nuclear stopping power effects in the plasma desorption experiment. The molecular ion emission from the behenic acid layer was examined as a function of its position within the multilayer structure, so five different behenic acid layer positions are possible. The data have been used to determine the sampling depth of secondary molecular ions. The static SIMS data for the alternating layer model are summarized in Table 11. Molecular ion emission of (M + H)+, (M - HF,and (M + Ag)+ has been examined. The intensity of (M + H)+BA,(M - H)-BA,and (M + Ag)+BA ion emission has been examined as a function of the position of the behenic acid layer. The primary beam was 4-keV Xe+ at 3 nA/cm2 current density. The current dosages were (5-70) X 10'2 ions/cm2 (5-60 min sputter time). The (M+ H)+BAion is observed when the behenic acid layer is the outermost layer (i.e. the air interface) or when it is in the third (middle) layer. The (M + H)+BA ion is not observed when the behenic acid occupies the second layer position or when buried more than three layers deep. The former result, if generalizable, would suggest that secondary molecular ion emission is dependent on orientation, which could only result if the emission of these ions occurs in extremelylow vibrational energy state. Given that the kinetic energy of secondary molecular ions desorbed from surfaces is on the order of a few electronvolts, this is unlikely. We are currently investigating this result with a variety of other alternating layer experiments. With a conservative interpretation, these results suggest that
Langmuir, Vol. 7, No. 10, 1991 2206
Static SIMS of L-B Fatty Acid Films A
IUW4 lE4 *
4’
I
’-’
I
*o+
1-+
100
200
300
460
C
600
500 11
700
6
12
9
16
ION DCSAW(iEi3)
-a- iM+i4AA
+(Mcl-oBA
Figure 5. Intensity of the (M+ H)+ ion for the 2AA-1BA2AA/Ag alternatingL-B film as a functionof primary ion dosage.
-1 0
3
0
I
I
t
O W 250
“4
1 E4
200
50
-
-
n
0
-.w\ 4
8
12
16
20
.24
28
32
36
ION DOSAOU(iEi3) (id# cm)
0
too
200
i UK,
&
7MI
a00
YLII 0
Figure 4. Attenuation of the Ag+ ion with film coverage: (A) 1 L-B AA/Ag; (B) 5 L-B AA/Ag; (C)11 L-B AA/Ag. Table 11. Summary of the Alternating Layer Model Static SIMS Rerultr: The Molecular Ions from Behenic Acid as a Function of the Behenic Acid Layer Porition. film (M + H)+ (M - H)- (M + A d + 4 AA-1 BA/Ag NO YES(w) NO 3 AA-1 Be-1 AA/Ag NO YES(w) NO 2 AA-1 BA-2 AA/Ag YES YES(w) NO 1 AA-1 BA-3 AA/Ag NO YES NO 1 BA-4 AA/Ag YES YES YES a Note: Molecular emission from arachidic acid (AA) is observed for all films. (w), weak signal.
(M + H)+ is only formed from the first three layers of the L-B multilayer structure. Therefore each L-B layer of the multilayer structure does not contribute equally to the overall (M + H)+signal. Figure 5 shows that the (M + H)+ ion is extremely sensitive to primary ion beam damage. The lifetime of the signal from (M + H)+BAion under 4-keV Xe+ primary ion beam damage is about 90 min. The total primary ion dosage was 1 X 10“ ions/cm2. After 20 min of primary ion beam bombardment, the (M + H)+BAion signal was 56% of its original signal intensity. The formation of (M - H)-BAhas been observed for all behenic acid positions. The signal intensity decreaseswith increased arachidic acid layer coverage. This is consistent with the results for signal due to (M - HI- from 1 L-B AA, 1 L-B BA, 5 L-B AA, and 5 L-B BA. The (M- HI-ion intensity decreased with increased film coverage. Increasing the amount of fatty acid molecules on the surface will not lead to increased intensity of (M- H)-ion emission, since there are only a limited number of “active” sites on the Ag surface that can complex the H+ cation. In
-a- OUH-OAA
+ w
Figure 6. Intensity of the (M - H)-ion for the 2AA-1BA-BAAI Ag alternating L-B film as a function of primary ion doeage.
principle, only increasing the Ag surface area and thus the number of active sites, would allow for the formation of more (M - H)- ions. The sampling depth from which the (M - H)- ion can originate is greater than five monolayers. The signal from the (M- H)-BAion had a lifetime of greater than 640 min under 4-keV Xe+ primary ion beam bombardment (Figure 6). These conditions correspond to approximately 7.5 X 1014 ions/cm2 primary ion dose. After 20 min of primary ion beam damage, the (M - H ) B A signal was 79% of the initial signal intensity. The long lifetime of the signal from the (M - H)-anion is attributed to the increased stability of the carboxylate anion which can be resonance stabilized. The substrate cationization of the behenic acid molecule to form (M + Ag)+BA is observed only when behenic acid is the outermost layer. The (M + Ag)+BAmolecular ion is readily attenuated by increased arachidic acid layer coverage. The loss of substrate cationized molecules was attributed to decreased Ag+ emission with increased coverage. The alternating layer results now further suggest that both Ag+ and the organic molecule must be available in the selvedge for metal cationization to occur. The sampling depth of (M + Ag)+ ion is on the order of one monolayer. The lifetime of signal from the (M + Ag)+M ion is about 90 min under 4-keV Xe+ primary ion bombardment; the total ion dosage was 1X 10“ ions/cm2 (Figure 7). After 20 min of primary ion beam exposure, the (M +Ag)+usignalwas 78% of the initial intensity. The results suggests this ion is of moderate stability, but not as sensitive to ion beam induced damage aa (M + H)+ ion.
Conclusions The static SIMS experiments have provided important structural and chemical bonding information about fatty
Corrtelio-Clark and Gardella
2286 Langmuir, Vol. 7, No. 10,1991 32 r
1
\
0’ 0
3
6
9
.
., 12
.
I 15
ION DOSAGU(lE13) ( l d e q a)
Figure 7. Intensity of the (M + Ag)+Mion for the 2AA-1BA2AAJ Ag alternatingL-B film as a function of primary ion dosage.
acid L-B films on Ag substrates. ESCA analysis of 1L-B AA/Ag has shown that the fatty acid monolayer does not exist as a preformed carboxylate anion on the Ag surface. Stoichiometric single fatty acid monolayer L-B films could not be obtained due to contributions from the underlying substrate. In the single monolayer range, the formation of (M H)- dominates over (M + H)+ ion formation. The carboxylate anion was observed for stearic, arachidic, and behenic acid monolayers on Ag. The carboxylate anion results when the neutral fatty acid molecule is complexed by the active Ag surface. The active sites on the Ag complex the H+, preventing attachment to a neutral fatty acid molecule to form (M + H)+.The (M + H)+ion is only observed for L-B multilayers and not for five solution cast multilayers. The L-B multilayer results in an ordered molecular arrangement of physisorbed layers on top of a single chemisorbed layer. The (M + H)+ ion probably originates from intermolecular proton transfer between fatty acid molecules. The solution-deposited layers do not completely cover the Ag surface, therefore, active sites are available to deprotonate the fatty acid molecule. Under these conditions, the (M - H)-anion is formed preferentially over (M + H)+. Cationization by ions from the Ag substrate was observed. The cationization mechanism depended on the availability of both the Ag+ and the organic molecule at the near surface region. The (M+ Ag)+ and (M - H + 2Ag)+ ions were found to be attenuated by overlayer
coverage. The (M - H + 2Ag)+ion was attenuated more quickly than the (M + Ag)+ion, since the former requires the attachment of two Ag+ ions. These results support the hypothesis that cationization takes place as attachment in the selvedge.16pM Therefore, changes in film thickness have dramatic effects on the formation and detection of metal cationized molecular ions. The secondary molecular ion intensity has been examined as a function of depth. The results indicate that different ions come from different depths. The sampling depth of the (M H)+ion is about three monolayers. The lifetime of the signal from the (M + H)+ ion is about 90 min. After the primary ion dose was about 4 X ions/ cm2, the signal dropped to 56% of the initial intensity. The formation of the (M H)+ion has been found to be sensitive to ion beam damage. The (M - H)-anion is attenuated with increased layer coverage, but the sampling depth is greater than five monolayers. The molecular ion did not disappear after 10 h of primary ion beam bombardment at 3 nA/cm2 with 4-keV Xe+. After 4 X 1013ions/cm2 primary ion dose, the signal intensity was 78% of the initial intensity. The stability of the anion to primary ion beam damage is attributed to the resonance stabilization of the negative charge by the carboxylate anion. The (M+ Ag)+ ion is rapidly attenuated by increased film thickness and coverage. The sampling depth is about one monolayer. The lifetime of the signal from (M + Ag)+ ion is 90 min under 3 nA/cm2 current density with 4-keV Xe+. After 4 X 1013 ions/cm2 primary ion dose, the intensity was 79% of the initial intensity. The (M+ Ag)+ ion is of moderate stability. These data once more indicate the important role that film to substrate interfacial chemistry plays in molecular secondary ion formation in static SIMS.
+
+
Acknowledgment. We acknowledge support for this work from the National Science Foundation Polymers Program, Division of Materials Research (DMR 87-20650). Additional support was provided through a grant from the Exxon Educational Foundation to J.A.G. We are grateful for continuing advice and access to the Langmuir Adam trough in the laboratories of Professor Robert Baier, Department of Biomaterials, SUNY Buffalo. Registry No. Ag, 1440-22-4.
