Nanovolume Analysis with Secondary Ion Mass Spectrometry Using

Sep 28, 2006 - At the level of an individual projectile impact, the resulting SI emission will be from a nanovolume.19-23 The practicality of this app...
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Anal. Chem. 2006, 78, 7410-7416

Nanovolume Analysis with Secondary Ion Mass Spectrometry Using Massive Projectiles Zhen Li, Stanislav V. Verkhoturov, and Emile A. Schweikert*

Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255

Secondary ion mass spectrometry (SIMS) is well recognized as a microanalysis tool.1 A key factor affecting the performance of SIMS is the efficiency of secondary ion (SI) formation. In recent years, it has been shown that SI yields can be increased by 1-2 orders of magnitude when a surface is bombarded by polyatomic instead of atomic projectiles.2-18 As a result, commercial SIMS

instruments have become available that use beams of Au3+, Bi3+, or C60+.12-14,18 Current efforts with instruments operating in the microprobe mode aim to characterize molecular species with submicrometer lateral and nanometric vertical resolution.9 Further progress toward nanophase characterization will require cluster beams focused to ∼10 nm, a still elusive goal for large projectiles.9 We present here an alternative, which sidesteps the issue of focusing a beam, yet allows extracting chemical information from nanodomains. In this approach, the intensity of the primary ion beam is reduced to where single projectiles are resolved in time and space. At the level of an individual projectile impact, the resulting SI emission will be from a nanovolume.19-23 The practicality of this approach depends on the probability of SI emission from a projectile impact. Recent experiments with large energetic projectiles, e.g., C60q+, Aunq+ (where 100 e n e 400; q ) 1-4), have shown that analytes from neat targets may be detected with the SI signal accumulated from a few tens to hundreds of projectiles.15,16 In the study presented here, samples were bombarded with a sequence of single projectiles, specifically Au4004+. The SIs from each impact were recorded as an individual mass spectrum. The collected mass spectra were then examined for correlations among co-emitted SIs. Such correlations arise when, in the suite of volumes probed with successive projectiles, there are sites where the same chemical species are co-located. It must be noted that while a correlated SI emission reveals chemical composition within the volume perturbed by one projectile, the successive impacts occur in random order at

* To whom correspondence should be addressed. Phone: +1-979-845-2341. Fax: +1-979-845-1655. E-mail: [email protected]. (1) Benninghoven, A., Bertrand, P., Migeon, H. N., Werner, H. W., Eds. Secondary Ion Mass Spectrometry-SIMS XII; Elsevier: Amsterdam, 2000. (2) Blain, M. G.; Della-Negra, S.; Joret, H.; Le Beyec, Y.; Schweikert, E. A. Phys. Rev. Lett. 1989, 63, 1625-1628. (3) Appelhans, A. D.; Delmore, J. E. Anal. Chem. 1989, 61, 1087-1093. (4) Benguerba, M.; Brunelle, A.; Della-Negra, S.; Depauw, J.; Joret, H.; Le Beyec, Y.; Blain, M. G.; Schweikert, E. A.; Ben Assayag, G.; Sudreau, P. Nucl. Instrum. Methods Phys. Res. B 1991, 62, 8-22. (5) Szymczak, W.; Wittmaack, K. Nucl. Instrum. Methods Phys. Res. B 1994, 88, 149-153. (6) Van Stipdonk, M. J.; Harris, R. D.; Schweikert, E. A. Rapid Commun. Mass Spectrom. 1996, 10, 1987-1991. (7) Harris, R. D.; Van Stipdonk, M. J.; Schweikert, E. A. Int. J. Mass Spectrom. Ion Processes 1998, 174, 167-177. (8) Gillen, G.; King, L.; Freibaum, B.; Lareau, R.; Bennett, J.; Chmara, F. J. Vac. Sci. Technol., A 2001, 19, 568-575. (9) Weibel, D.; Wong, S.; Lockyer, N.; Blenkinsopp, P.; Hill, R.; Vickerman, J. C. Anal. Chem. 2003, 75, 1754-1764. (10) Rickman, R. D.; Verkhoturov, S. V.; Parilis, E. S.; Schweikert. E. A. Phys. Rev. Lett. 2004, 92, 047601-4. (11) Locklear, J. E.; Verkhoturov, S. V.; Schweikert, E. A. Int. J. Mass Spectrom. 2004, 238, 59-64.

(12) Xu, J.; Szakal, C. W.; Martin, S. E.; Peterson, B. R.; Wucher, A.; Winograd, N. J. Am. Chem. Soc. 2004, 126, 3902-3909. (13) Bryan, S. R.; Belu, A. M.; Hoshi, T.; Oiwa, R. Appl. Surf. Sci. 2004, 231/ 232, 201-206. (14) Touboul, D.; Kollmer, F.; Niehuis, E.; Brunelle, A.; Lapre´vote, O. J. Am. Soc. Mass Spectrom. 2005, 16, 1608-1618. (15) Hager, G. J.; Guillermier, C.; Verkhoturov, S. V.; Schweikert, E. A. Appl. Surf. Sci. 2006, 252, 6558-6561. (16) Verkhoturov, S. V.; Rickman, R. D.; Guillermier, C.; Hager, G. J.; Locklear, J. E.; Schweikert, E. A. Appl. Surf. Sci. 2006, 252, 6490-6493. (17) Rickman, R. D.; Verkhoturov, S. V.; Hager, G. J.; Schweikert, E. A. Int. J. Mass Spectrom. 2005, 245, 48-52. (18) Winograd, N. Anal. Chem. 2005, 7, 143A-149A. (19) Park. M. A.; Gibson, K. A.; Quinones, L.; Schweikert, E. A. Science 1990, 248, 988-990. (20) Boussofiane-Baudin, K.; Bolbach, G.; Brunelle, A.; Della-Negra, S.; Hakansson, P.; Le Beyec, Y. Nucl. Instrum. Methods Phys. Res. B 1994, 88, 160163. (21) Verkhoturov, S. V.; Schweikert, E. A.; Rizkalla, N. M. Langmuir 2002, 18, 8836-8840. (22) Rickman, R. D.; Verkhoturov, S. V.; Schweikert, E. A. Appl. Surf. Sci. 2004, 231/232, 54-58. (23) Wojciechowski, I. A.; Garrison, B. J. J. Phys. Chem. A 2006, 110, 13891392.

A variant of secondary ion mass spectrometry is presented where the surface is bombarded with individual gold nanoparticles each resolved in time and space with a corresponding event-by-event detection of the secondary ions (SIs). The projectile used, Au4004+, with impact energy of 136 keV, generates high SI yields. Typically, there is co-emission of multiple SIs from a single impact, i.e., emission of SIs from molecules co-located within a nanovolume with dimensions in the 10-nm range. The ability to detect co-located molecules was tested on samples consisting of alternating nanometric layers of oppositely charged polyions, poly(diallyldimethylammonium chloride), poly(styrenesulfonate) (PSS), and clay nanoplatelets. To achieve signal statistics, the chemical analysis was carried out with a sequence of stochastic impacts making this method suitable for characterization of similar nanoparticles or spots dispersed on a surface. Attomole detection sensitivity was achieved for PSS. The homogeneity of assembled layers could be assessed with ∼10-nm resolution.

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10.1021/ac0603779 CCC: $33.50

© 2006 American Chemical Society Published on Web 09/28/2006

random surface sites; thus, the method presented here is nonimaging. The feasibility of extracting spatially resolved chemical information from event-by-event bombardment/detection was tested on samples consisting of alternating nanometric layers of oppositely charged polyions, poly(diallyldimethylammonium chloride) (PDDA), poly(styrenesulfonate) (PSS), and clay nanoplatelets. Such assemblies can be readily produced by dipping a solid substrate (e.g., mica, silicon wafer, glass slide) alternatively into polycation and polyanion solutions.24-27 They can be fashioned to exhibit electrochemical and catalytic activities.28 Layer-by-layer films can accommodate a wide range of components; i.e., a threedimensional structure can be assembled with an “indicator” layer to probe the depth of SI emission and with successive distinct layers to test the planar homogeneity. The sophistication in spatially resolved chemical composition is a challenge for conventional surface characterization techniques, such as small-angle X-ray reflectivity, ellipsometry, and AFM. They can provide either chemical and physical information at the macroscopic level or a surface topography image with nanometric resolution.24-32 Reports of SIMS analysis of layer-by-layer thin films of polyelectrolytes are sparse.33-35 As noted already, the method presented here differs from customary SIMS in the projectile characteristics (Au4004+, impact energy of 136 keV) and, most importantly, in the mode of operation. The experiments described below have as prerequisite that all projectile impacts result in equivalent emission events; i.e., there is reproducibility in emission events occurring from nanovolumes of like composition.17 EXPERIMENTAL SECTION a. Materials. PDDA (MW ) 100 000-200 000, 20% solution) and PSS (MW ) 70 000) (Figure 1) were purchased from Aldrich (Milwaukee, WI). The polymer stock solutions for film assembling were 5 mg/mL with 0.5 M NaCl. Montmorillonite clay (STx-1, (Ca0.27Na0.04K0.01)[Al2.41Fe(III)0.09Mg0.71Ti0.03][Si8.00]O20(OH)4) was purchased from The Source Clays Repository (The Clay Minerals Society, Purdue University, West Lafayette, IN) and purified according to the literature.28 Stock clay solution for film assembling was 0.5 mg/mL. All water used was purified by Milli-Q system (Millipore, Billerica, MA) with a specific resistance of 18.2 MΩ cm. (24) Decher, G.; Schmitt, J. Thin Solid Films 1992, 210/211, 831-835. (25) Lvov, Y.; Decher, G.; Mohwald, H. Langmuir 1993, 9, 481-486. (26) Schlenoff, J. B.; Dubas, S. T. Macromolecules 2001, 34, 592-598. (27) Izquierdo, A.; Ono, S. S.; Voegel, J. C.; Schaaf, P.; Decher, G. Langmuir 2005, 21, 7558-7567. (28) Zhou, Y.; Li, Z.; Hu, N.; Zeng, Y.; Rusling, J. F. Langmuir 2002, 18, 85738579. (29) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. Thin Solid Films 1996, 284/ 285, 797-801. (30) Kim, D. W.; Blumstein, A.; Tripathy, S. K. Chem. Mater. 2001, 13, 19161922. (31) Mamedov, A.; Ostrander, J.; Aliev, F.; Kotov, N. A. Langmuir 2000, 16, 3941-3949. (32) Kim, D. W.; Choi, H.-S.; Lee, C.; Blumstein, A.; Kang, Y. Electrochim. Acta 2004, 50, 659-662. (33) Delcorte, A.; Bertrand, P.; Arys, X.; Jonas, A.; Wischerhoff, E.; Mayer, B.; Laschewsky, A. Surf. Sci. 1996, 366, 149-165. (34) Li, Z.; Rickman, R. D.; Verkhoturov, S. V.; Schweikert, E. A. Appl. Surf. Sci. 2003, 231/232, 328-331. (35) Lua, Y. Y.; Yang, L.; Pew, C. A.; Zhang, F.; Fillmore, W. J. J.; Bronson, R. T.; Sathyapalan, A.; Savage, P. B.; Whittaker, J. D.; Davis, R. C.; Linford, M. R. J. Am. Soc. Mass Spectrom. 2005, 16, 1575-1582.

Figure 1. Structure of film components and major m/z peaks in mass spectrum.

b. Film Assembly. Silicon wafers (12.7-cm diameter, Waferworld, West Palm Beach, FL) were cut into 1 cm × 1 cm pieces. The small Si pieces were washed with copious amount of ethanol, dried under a N2 stream, and cleaned with UV-ozone cleaner. The cleaned wafer was immediately dipped into PDDA stock solution for 10 min, rinsed with water, and dried under a gentle N2 stream; the resultant film was a 1-layer film. To obtain a 2-layer film, a 1-layer film was dipped into PSS stock solution for 10 min. For films with more than 3 layers, the wafer with a 2-layer film was dipped alternatively into PDDA and clay stock solutions followed by rinsing and drying until the desired number of layers were assembled. One- up to 12-layer films were assembled and tested. These films have been reported as stable and retaining their thickness in vacuum as in air.36 c. Ellipsometry. Ellipsometry experiments were performed on a Gaertner L2W26D ellipsometer (Gaertner Scientific Co., Skokie, IL) with a 632.8-nm laser beam and 70° incident angle. A cleaned blank Si wafer was measured prior to film assembling to obtain substrate information. After assembling the desired number of layers, the Si wafer was measured again to obtain thickness data. The refractive index of all films was fixed at n ) 1.54, 4-5 spots on the sample surface were measured, and an average thickness was obtained.26 The thicknesses of the films (Table 1) showed good reproducibility; a less than 10% variation was observed for three sets of films prepared. An illustration of a 12layer film is shown in Figure 2. d. Mass Spectrometer. A custom-designed Au liquid metal ion source (Au LIMS) TOF-SIMS mass spectrometer was used for the experiments (Figure 3). A detailed description of the instrumental setup is available in the literature.15-17,37 Briefly, a wide range of Aunq+ cluster projectiles (n ) 1-1000, q ) 1-4) can be produced with the Au LIMS. The projectiles are mass separated by a Wien filter. The selected m/z is pulsed across a 0.4-mm aperture to ensure an “event-by-event” mode operation. The pulsed Au projectiles impact the sample surface at 60° to the sample normal. Unless otherwise specified, the projectiles used in this report were 136-keV Au4004+. The negative SIs ejected by (36) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. Langmuir 1996, 12, 30383044. (37) Bouneau, S.; Della-Negra, S.; Depauw, J.; Jacquet, D.; Le Beyec, Y.; Mouffron, J. P.; Novikov, A.; Pautrat, M. Nucl. Instrum. Methods Phys. Res. B 2004, 225, 579-589.

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Table 1. Total Thickness and Net Thickness of the Films no. of layers total thickness (nm) net thickness (nm)

1 0.6 0.6

2 1.6 1.0

3 2.6 1.0

4 6.0 3.4

5 7.6 1.6

6 10.6 3.0

7 12.4 1.8

8 14.7 2.3

9 16.1 1.4

10 20.2 4.1

11 21.8 1.6

12 24.0 2.2

Figure 4. Probability distribution P(xA) of the number of ions xA detected per impact/emission event for SO3- (m/z ) 80) in a 2-layer film. Figure 2. Illustration of a 12-layer film. The thickness of each layer is drawn in accordance with thickness measured by ellipsometry.

for the probability of detection of multiple identical SIs. Thus, for a given ion A, the percentage yield YA is computed as follows:

YA (%) ) 100

∑x I(x )/N ) 100∑x P(x ) A

xA

Figure 3. Schematic of Au LIMS TOF-SIMS mass spectrometer.

the Au projectile impact were analyzed by a time-of-flight mass spectrometer equipped with an eight-anode detector, which can detect up to eight SIs with the same m/z simultaneously. The signals from each anode were converted to logic pulse and processed through a time-to-digital converter with a 250-ps time resolution (CTN-M4, obtained from Nuclear Physics Institute Orsay, France). e. Data Analysis. The SI data were collected and analyzed by Total Matrix of Event (TME) software developed for eventby-event detection.38 The TME acquisition method enables one to select events by the number of SIs, n, detected per event and obtain mass spectra corresponding to that specific n-ion emission events. For example, when selecting n ) 2, a mass spectrum containing only 2-ion emission events will be obtained. The experiments involved collection of at least 1 million events, which means that the targets were exposed to at least 1 million projectiles over a probed area of ∼1 mm2. Given the efficiency of the projectiles to generate SIs, the detected SI yield must account (38) Rickman, R. D. Ph.D. Dissertation, Texas A&M University, College Station, TX, 2004.

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A

A

A

xA

where xA is the number of the SIs of type A detected simultaneously per single impact/emission event (0 e xA e 8), I(x)A is the number of events where ions A are detected, and N is the total number of projectile impacts. P(xA) is the probability distribution of the number of ions A detected per impact/emission event. The above expression reduces to YA (%) ) 100P(1) when the most probable detection from an impact is at best a single SI; i.e., when xA ) 1. This latter case is the one prevalent in bombardment with atomic or small polyatomic projectiles. With the 136-keV Au4004+ projectile, emission of multiple identical ions is sufficiently frequent that the SI yields must be computed based on the probability P(xA). A typical distribution P(xA) for the ions of m/z ) 80 is shown in Figure 4. The contribution from multiple ion detection events to the SI yield becomes apparent in a comparison to the probability of single detection [P(1)]. Indeed, for m/z ) 80,

[

∑x P(x ) - P(1)]/P(1) ) 0.3 A

A

xA

Note that the corresponding yield for m/z ) 80 is 23%. RESULTS AND DISCUSSION a. Detection Sensitivity. A mass spectrum of a monolayer of PSS is shown in Figure 5. In this case, PSS was deposited on top of a layer of PDDA itself mounted on a Si substrate. The spectrum shows two major peaks at m/z ) 80 and 183. The lower mass signal is mainly due to SO3- from PSS with a small contribution from PDDA. PSS produces a unique signal at m/z ) 183 corresponding to CH2CHC6H4SO3-. A yield of 30% was measured for this species; i.e., 30 CH2CHC6H4SO3- ions were detected per 100 projectile impacts. The spectrum shown in Figure 5a was obtained with a total of ∼1 × 106 projectiles. Recalling that the

Figure 5. Mass spectra of 2-layer film from bombardment with 136-keV Au4004+ projectiles. (a) Total spectrum obtained with ∼1 million projectiles. (b) Spectrum obtained with the first ∼103 projectiles. The SI yields calculated from integrated peak areas were similar for the same SI in either mass spectrum. A modern time-to-digital converter was used to record flight time of the ions, and typically there are ∼100 channels associated with a certain m/z ion.

Figure 6. Evolution of the yield of m/z ) 183 (CH2CHC6H4SO3-) with number of layers. The x axis is drawn according to the thickness of each layer.

area exposed to projectile impact was ∼1 mm2, the bombardment occurred under “superstatic” conditions; i.e., each Au4004+ impacted a fresh area of the target. The detection sensitivity is illustrated in Figure 5b. The mass spectrum shown here was obtained on the same target with the first ∼1000 projectiles. Similar SI yields were obtained from the total spectrum and from the first ∼1000 projectiles. We infer from these data that, for the case at hand, i.e., for m/z ) 183, a few hundred projectiles are sufficient for a decision limit.39 Anticipating on depth and lateral resolution data provided below, the cumulative volume sampled by a few hundred Au4004+ contained ∼1 amol of PSS monomer. b. Depth and Volume Probed per Impact. The depth probing capability of the massive Au projectile was determined by monitoring the peak at m/z ) 183 from PSS with a monolayer of the latter covered by varying numbers of PDDA and clay layers. As the number of layers on top of the PSS indicator layer increased, the SI signal intensity from the PSS layer decreased (Figure 6). For the 2-layer film, i.e., with PSS as the topmost layer, the yield of m/z ) 183 was nearly 30%. For the 3-layer film, the yield of these ions was slightly higher although the PSS indicator (39) Currie, L. A. Anal. Chem. 1968, 40, 586-593.

layer was now covered by a 1-nm-thick PDDA layer. This increased signal intensity might be due to the fact that the positively charged PDDA layer could enhance the ionization of PSS layer and thus facilitate the ionization and emission of ions from the underlying PSS layer. In the case of a 4-layer film, that is, when the PSS indicator layer was covered by a 4.4-nm-thick PDDA/clay bilayer, the intensity of ions at m/z ) 183 decreased 9-fold compared with 2-layer film. However, for the 4- and 5-layer films, the ions at m/z ) 183 were still detectable, which meant the massive Au4004+ projectile could still penetrate the PDDA/clay layers on top of the PSS layer and lead to the emission of SIs from the PSS layer. Thus, the 6.0-nm distance from the top of the fifth layer to the top of the PSS layer was still within the SI emission depth range. When more layers were added onto the PSS layer, i.e., for a 6-layer film and beyond, the intensity of ions at m/z ) 183 dropped to background. This observation suggests that the SI emission depth for 136-keV Au4004+ on soft polymer film targets is about 6-9 nm (the distance between second layer and fifth layer is 6.0 nm, and the distance between second layer and sixth layer is 9.0 nm). This value is in agreement with MD simulation data.23,40,41 Analytical Chemistry, Vol. 78, No. 21, November 1, 2006

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Figure 7. Yields of ions m/z ) 77 SiO2OH- and m/z ) 179 (Al2O3)(SiO2)OH- and their relationship with the number of layers.

The layer-by-layer resolution of SI signal is demonstrated by the evolution of yields of ions m/z ) 77 (SiO2OH-) and 179 ((Al2O3)(SiO2)OH-) with the number of layers (Figure 7). A clear transition for m/z ) 77 was observed at the third layer. The yield decreased prior to that layer and increased and oscillated beyond. This observation suggests that m/z ) 77 has two origins. For 1-3-layer films (without clay layer), the emission of SiO2OH- from the Si wafer substrate is progressively blocked by the increasing thickness of the PDDA and PSS films. The intensity of ions at m/z ) 77 for a 3-layer film was only 25% of that of the 1-layer film. Experiments on thick films of PDDA and PSS (>1 µm in thickness) showed no distinguishable peak at m/z ) 77. Thus, the decrease in the yield of ions at m/z ) 77 from 1-layer film to 3-layer film shows that a 2.6-nm-thick PDDA/PSS film considerably decreases the emission of SIs from the Si wafer substrate. Nonetheless, the thickness of 2.6 nm is still within the SI emission depth of the Au4004+ projectile used here. When the fourth layer, which was a clay layer, was adsorbed onto the film, the contribution of clay to the yield of ions at m/z ) 77 was evident. Beyond a 4-layer film, the intensity of ions at m/z ) 77 increased with the addition of clay layers and tended to be stable beyond the 10layer film. The ions at m/z ) 179, i.e., (Al2O3)(SiO2)OH-, are mainly from clay, which contains 70% SiO2 and 16% Al2O3. Indeed, for 1-, 2-, or 3-layer films, i.e., without clay layers in the assemblies, the signal at m/z ) 179 was practically near zero. For films with 4 layers or more, a trend similar to ions at m/z ) 77 was observed. The similarity of the yields of ions m/z ) 77 and m/z ) 179 on 4-layer film and beyond confirms that, for films with more than 4 layers, the SiO2OH- ions were mostly contributed by the clay layers. The intensities of both ions (m/z ) 77 and 179) were higher on clay-topped layers, lower on PDDA-topped layers (oscillation effect), and tended to be stable beyond 10-layer film. For example, for 9-layer film (PDDA-topped), the intensities of both ions were ∼60% of those in the 8-layer film (clay-topped), while the intensities of these ions were similar for the 10- and 12-layer films. Thus, for the test case examined and the projectile characteristics used, the SI emission was in practice limited to the topmost 2-3 layers of the films.34 Most importantly, the characteristics of the topmost layer influence the type and intensity of the SIs emitted. We conclude that the SI emission depth is between 6 and 9 nm. (40) Postawa, Z.; Czerwinski, B.; Winograd, N.; Garrison, B. J. J. Phys. Chem. B 2005, 109, 11973-11979. (41) Anders, C.; Urbassek, H. M. Nucl. Instrum. Methods Phys. Res. B 2005, 228, 57-63.

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Figure 8. Yield of ions m/z ) 183 (CH2CHC6H4SO3-) for 1-12layer films with 34-keV Au3+, Au5+, and 136-keV Au4004+ projectiles.

The shape of SI emission volume is likely to vary with the nature of the target. For carbon and graphite targets, a cylindrical structure seems to best approximate the emission volume. A MD simulation of Au400 impacts at 100 eV/atom shows a virtually intact projectile at the bottom of a cylindrical crater.41 Recent experiments with impact energies close to ours show again implantation of virtually intact projectiles, thus suggesting also a cylindrical emission volume.42 Conversely, a MD simulation of C60 impacting a benzene layer at 250 eV/atom shows a semispherical crater with a diameter in excess of 10 nm.40 For the samples studied here, we assumed a semispherical emission volume with a 6-9-nm radius, which might be an overestimation. c. Comparison of Small Au Cluster Projectiles with Massive Au Cluster Projectiles. The ability to extract information from single impacts is a distinct feature of the massive Au4004+ projectiles. For comparison, we show in Figure 8 the results of massive and small Au clusters on the same set of layers. Clearly, the small Au cluster projectiles are less efficient in SI yields, even though the kinetic energy carried by each Au atom is much higher. Starting with the 4-layer film, ions with m/z ) 183 were barely detectable with small Au cluster projectiles. Thus, the SI emission depth is shallower for the smaller Au cluster projectiles tested here compared with the massive Au projectiles. d. Test of Nanovolume Homogeneity. As an example, let us look at a 4-layer film. The coincidental emission of SIs from the PSS indicator layer and the clay layer would indicate the emission depth is larger than the distance between the PSS layer and the clay layer. If, there is no co-emission of PSS-related ions with clay-related ions, one could conclude that, either the SI emission depth is less than the distance between the PSS layer and the clay layer or these two film components are segregated spatially beyond the probe volume of the projectile. Thus, by monitoring coincidentally emitted SIs, the relationship of film components and homogeneity of the film can be investigated. The physical relevance of co-emitted SIs is evaluated with a correlation coefficient, Q, which is defined as follows:

∑∑x x P(x x ) A B

QAB )

A B

xA xB

)

∑x P(x )∑x P(x ) A

xA

A

B

B

YAB YAYB

xB

where P(xAxB) is the probability distribution of the number of ions (42) Della-Negra, S. Private communication.

Table 2. Correlation Coefficients (Q) of Ions m/z ) 80 (SO3-) and 183 (CH2CHC6H4SO3-) for 2-5-Layer Films no. of layers Q

2 1.1

3 1.1

4 6.4

5 6.4

Table 4. Correlation Coefficients (Q) of Ions m/z ) 80 (SO3-) and 179 (Al2O3)(SiO2)OH-), and m/z ) 183 (CH2CHC6H4SO3-) and 179 (Al2O3)(SiO2)OH-) for 4-Layer Film ions (m/z) Q

80 and 179 0.73

183 and 179 0.76

Table 3. Correlation Coefficients (Q) of Ions m/z ) 119 (Al2O3OH-) and 179 ((Al2O3)(SiO2)OH-) for 4-8-Layer Films no. of layers Q

4 1.62

5 2.06

6 1.50

7 1.81

8 1.77

A and B detected simultaneously (emitted via single impact/ emission event) and YAB is the corresponding coincidental yield. If the emission of ions A and B is uncorrelated, i.e., A and B are emitted independently within the single impact/emission event, then the distribution P(xAxB) is equal to P(xA)P(xB), which results in QAB ) 1. When the emission of ions A and B is correlated, that is the emission of ion A or B enhances the emission of the other one, QAB > 1. If the emission of ion A or B suppresses the emission of the other, then the value of the correlation coefficient, QAB, will be lower than unity (anticorrelation). Let us, for example, consider the correlation coefficient for ions m/z ) 25 and 26 in the case of a 2-layer film. Ions with an m/z of 25 correspond to C2H-; they originate either from the polymer skeleton or from randomly adsorbed organic contaminants. Ions at m/z ) 26 are mainly due to CN- from either PDDA or contaminants. It may be assumed that the polymers together with contaminants cover the surface in a uniform manner; i.e., the molecules from which these two ions originate are evenly distributed on the films. Accordingly, the co-emission of these two ions should be uncorrelated; i.e., the Q value should be unity. The experimental value obtained from the coincidental spectrum is 1.1, which is in agreement with the assumption. The correlation coefficients for ions m/z ) 80 and 183 in the case of 2-5-layer films are presented in Table 2. For 2- and 3-layer films, the Q values were close to unity; this means the emission of SO3- and CH2CHC6H4SO3- is not correlated. Further, this result suggests that PSS is formed as a uniform layer on top of the Si wafer. Indeed, if the PSS did not form a uniform layer on the surface, the random impacts of Au projectiles over a probed area of ∼1 mm2 would result in two types of emissions, either with or without PSS-related ions. However, if the availability of PSS-related ions is equal at different locations of the surface, the emission of PSS-related ions follows a statistical distribution and the correlation coefficient will be unity. For 4- and 5-layer films, the correlation coefficient was higher than unity; this can be explained by the presence of the clay layer on top of the PSS layer. High correlation coefficient values (Q > 1) were also observed for clay-related ions on 4-layer film and beyond (Table 3). While low correlation coefficients (Q < 1) were observed between PSSrelated ions and clay-related ions for 4-layer film (Table 4). These observations reveal a lack of homogeneity in the clay layers within the film. Such observations are consistent with the dimension and properties of clay nanoplatelets. Inspection of clay nanoplatelets with TEM and XRD has shown that they are ∼50-100 nm in diameter and ∼1.5 nm in thickness. When these nanoplatelets are assembled into layer-by-layer thin films, they are not as

uniform as the PSS layer.30,43 Instead, the clay platelets tend to aggregate and stack; thus, in the clay layers, there are regions with multiple layers of clay platelets, while at other sites only one sheet of clay is adsorbed. The real thickness of a “stack” or “island” region is higher than the average thickness of the film measured by ellipsometry, while the thickness of a “base” region, where only a monolayer of clay platelets is adsorbed, is lower than the average thickness. Consider now for a 4-layer film, if a projectile impacts the island region, then due to the higher thickness at that region, it would be more difficult for the projectile to cause SI emission from the underlying PSS layer. Under such circumstance, most co-emitted SIs would be related to the clay in the topmost layer. However, if the base region is impacted by a projectile, SIs from the PSS layer would be more easily ejected and detected, because the clay cover layer is not as thick as that in the island region. In this case, one can expect co-emission of PSS-related SIs. This is also the only location where co-emission of PSS-related ions and clay-related ions can occur. Indeed, nearly all events contribute to the emission of clay-related ions, regardless whether an island or a base region is impacted, while only events from a base region impact contribute to the co-emission of PSS and clay-related ions. Thus, the co-emission of these two types of SIs will be anticorrelated. The observed Q values of ∼0.7-0.8 support this assumption (Table 4). In the case of co-emission solely from the PSS layer, only impacts in the base region contribute to emission and co-emission of PSS-related ions. Thus, the emission of PSS-related ions had to be correlated, and a high Q value (>6) was observed (Table 2). When clay was present within the top 2 layers, all impacts generate clay-related ions and hence coemission with clay-related ions. However, when hitting a clay island, co-emission of clay-related ions occurred with increased frequency. Consequently, a correlation coefficient larger than unity was expected. The experimental values ranged indeed from 1.5 to 2 for 4-layer to 8-layer films (Table 3). The examination of co-emitted SIs and the calculation of correlation coefficient amount to probing the sample surface with nanometric resolution. Even though the projectiles were not focused and they impacted the surface randomly, the nanometersize SI emission volume allows resolving spatially related coemitted SIs. The correlation coefficient calculation showed that with the emission volume probed (12-18 nm in diameter) the PSS layer was quite uniform in thickness, while the clay layer was not as uniform with overlap in adjacent clay platelets causing islands on the surface. CONCLUSION SIMS run with Au4004+ projectiles with event-by-event bombardment/detection mode generates chemical signal from volume with (43) Ariga, K.; Lvov, Y.; Ichinose, I.; Kunitake, T. Appl. Clay Sci. 1999, 15, 137152.

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dimensions in the 10-nm range. To achieve signal statistics, the analysis is carried out with a sequence of stochastic impacts, making this method suitable for characterizing a suite of similar nanoparticles or spots dispersed on a surface. The ability to examine co-emitted SIs and determine submicrometer homogeneity is a distinct feature for analytical accuracy. The correlation coefficient Q, as a measure of planar homogeneity, enhances the certainty of proper identification of a chemical species. Further, in revealing the environment from which a given SI originates, Q facilitates quantitative assessments since ionization probabilities depend on the surrounding material. Indeed, the detection sensitivities depend on the nature of the sample. The test case

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shown here illustrates the current performance of SIMS with Au4004+ projectiles. Further studies are warranted to determine whether different projectile characteristics will enable surface analysis with a still smaller number of bombardment events. ACKNOWLEDGMENT This work was supported by the NSF (CHE-0449312) and the R. A. Welch Foundation (A-1482). Received for review August 22, 2006. AC0603779

February

28,

2006.

Accepted