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Articles Molecular Depth Profiling of Sucrose Films: A Comparative Study of C60n+ Ions and Traditional Cs+ and O2+ Ions Zihua Zhu,* Ponnusamy Nachimuthu, and Alan S. Lea Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352 Depth profiling of sucrose thin films was investigated with time-of-flight secondary ion mass spectrometry (TOFSIMS) using 10 keV C60+, 20 keV C602+, and 30 keV C603+, and 250, 500, and 1000 eV Cs+ and O2+ as sputtering ions. With C60n+ ions, the molecular ion signal initially decreases and reaches a steady state that is about 38-51% of its original intensity, depending on the energy of the C60n+ ions. In contrast, with Cs+ and O2+ sputtering, molecular ion signals decrease quickly to the noise level, even using very low-energy (250 eV) ions. In addition, the measured width of the sucrose/Si interface is much narrower using C60+ ions than that using Cs+ or O2+ ions. To understand the mechanisms of sputtering-induced damage by these ions, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) were used to characterize the bottoms of these sputter craters. XPS data show very little chemical change in the C60+ sputter crater, while considerable amorphous carbon was found in the O2+ and Cs+ sputter craters, indicating extensive decomposition of the sucrose molecules. AFM images show a very flat bottom in the C60+ sputter crater, while the bottoms of the Cs+ and O2+ sputter craters are significantly rougher. We used the sputtering model developed by Wucher and co-workers to quantitatively analyze our C601-3+ data. The results show that low energy C60+ ions generate a relatively thin damage layer with a high molecular ion signal, suggesting that low energy C60+ may be the optimal choice for molecular depth profiling of sucrose films. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) has proven to be a very powerful surface analysis technique used in the semiconductor industry, nanomaterial characterization, catalyst surface characterization, environmental research, biological research, and many other areas.1-3 Obtaining depth dependent distribution of elements, depth profiling, is one of the major * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: 1-509-371-6240. Fax: 1-509-371-6242. (1) Pacholski, M. L.; Winograd, N. Chem. Rev. 1999, 99, 2977–3005.
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strengths of TOF-SIMS. Normally, a dual-beam depth profiling strategy is used, in which one beam is used for SIMS analysis and the other beam is used for sputtering. Cs+/O2+ dual sources have been the default choice for sputtering sources on SIMS instruments for many years. The Cs+ source is used for negative ion analysis because cesium is a strong electron donor, which enhances negative ion yield. The O2+ source is used for positive ion analysis because it oxidizes the sample surface to enhance positive ion yield. In recent years, C60n+ sources have been introduced for TOFSIMS analysis.4-6 It shows molecular depth profiling capability, which greatly facilitates characterization of biological samples,7-14 polymers,15-17 and organic materials.18,19 These samples are mostly composed of C, H, O, and N elements, so that elemental (2) Benninghoven, A.; Rudenauer, F.; Werner, W. Secondary Ion Mass Spectrometry; John Wiley & Sons Ltd.: Chichester, U.K., 1987. (3) Vickerman, J. C., Briggs, D., Eds. ToF-SIMS: Surface Analysis by Mass Spectrometry; IM Publications and SurfaceSpectra Limited: Manchester, U.K., 2001. (4) Weibel, D.; Wong, S.; Lockyer, N.; Blenkinsopp, P.; Hill, R.; Vickerman, J. C. Anal. Chem. 2003, 75, 1754–1764. (5) Winograd, N. Anal. Chem. 2005, 77, 142A–149A. (6) Wong, S. C. C.; Hill, R.; Blenkinsopp, P.; Lockyer, N. P.; Weibel, D. E.; Vickerman, J. C. Appl. Surf. Sci. 2003, 203, 219–222. (7) Breitenstein, D.; Rommel, C. E.; Mollers, R.; Wegener, J.; Hagenhoff, B. Angew. Chem., Int. Ed. 2007, 46, 5332–5335. (8) Debois, D.; Brunelle, A.; Laprevote, O. Int. J. Mass Spectrom. 2007, 260, 115–120. (9) Fletcher, J. S.; Lockyer, N. P.; Vaidyanathan, S.; Vickerman, J. C. Anal. Chem. 2007, 79, 2199–2206. (10) Jones, E. A.; Lockyer, N. P.; Vickerman, J. C. Int. J. Mass Spectrom. 2007, 260, 146–157. (11) Jones, E. A.; Lockyer, N. P.; Vickerman, J. C. Anal. Chem. 2008, 80, 2125– 2132. (12) Nygren, H.; Hagenhoff, B.; Malmberg, P.; Nilsson, M.; Richter, K. Microsc. Res. Tech. 2007, 70, 969–974. (13) Piehowski, P. D.; Kurczy, M. E.; Willingham, D.; Parry, S.; Heien, M. L.; Winograd, N.; Ewing, A. G. Langmuir 2008, 24, 7906–7911. (14) Vaidyanathan, S.; Fietcher, J. S.; Goodacre, R.; Lockyer, N. P.; Micklefield, J.; Vickerman, J. C. Anal. Chem. 2008, 80, 1942–1951. (15) Hinder, S. J.; Lowe, C.; Watts, J. F. Surf. Interface Anal. 2007, 39, 467– 475. (16) Hinder, S. J.; Watts, J. F.; Simmons, G. C.; Lowe, C. Surf. Interface Anal. 2008, 40, 436–440. (17) Szakal, C.; Sun, S.; Wucher, A.; Winograd, N. Appl. Surf. Sci. 2004, 2312, 183–185. (18) Shard, A. G.; Green, F. M.; Brewer, P. J.; Seah, M. P.; Gilmore, I. S. J. Phys. Chem. B 2008, 112, 2596–2605. 10.1021/ac900553z CCC: $40.75 2009 American Chemical Society Published on Web 09/21/2009
depth profiling or mapping can only provide very limited information. However, if 2-D or 3-D distribution of molecular species can be observed, such data will be greatly helpful for biological as well as organic and polymer material research. For example, Winograd and co-workers observed that C60+ ions can be used to obtain molecular depth profiles for a number of peptides in trehalose films.20 This result is important because trehalose has been used to fix cells or tissues in vacuum, so that 3-D molecular imaging of biological samples by TOF-SIMS is feasible with trehalose fixation.21 Therefore, the C60+ ion gun quickly became a common accessory for new TOF-SIMS instruments in recent years. It has been reported, however, that low-energy Cs+ ion sputtering can also be utilized to obtain molecular depth profiles. For example, low-energy (∼200 eV) Cs+ has been used for molecular depth profiling of polycarbonate.22 In practical applications, it is critical to select a suitable sputtering species, such that desirable depth information can be easily obtained. In order to select the best sputtering ion, comparative data sets with different sputtering ions on the same category of samples are necessary. Unfortunately, such data are far from adequate for the determination of the ideal sputtering ions so that, in most cases, personal experience is a major factor in the determination of sputtering ions. This situation obviously is irrational, and it may lead to difficulty in selection of the optimal sputtering species. In this work, we report a comparative study of depth profiling of sucrose films on silicon substrates using several different sputtering ions. The sputtering ions include 10-30 keV C60n+ and low-energy (0.25-1.0 keV) Cs+ and O2+ ions. Sucrose is selected here because it is a very common biomolecule. At the same time, it has a structure similar to the trehalose molecule, which has been studied by C60+ sputtering,20 and so, the experimental data can be easily compared in these two systems. In our study, we are especially interested in the molecular ion signal intensity during depth profiling as well as sputtering yield and depth resolution at the interface. To help understand the sputtering mechanism, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) were used to characterize the sputter craters. Finally, a simple schematic figure was developed for easy understanding of the key difference between C60+ sputtering and Cs+/O2+ sputtering. EXPERIMENTAL SECTION Sucrose Thin Film Preparation. Sucrose (purity >99.5%) was obtained from Sigma-Aldrich and used without any further purification. A 1 cm × 1 cm silicon (110) wafer was sonicated in methanol for 2 min and dried by high-purity N2. The wafer was then treated with UV/ozone for three minutes to remove surface hydrocarbon contamination and to produce a highly hydrophilic surface. Three microliters of a 5% (weight) sucrose solution (solvent: water/methanol 90:10) was placed on the clean silicon wafer. The sample was kept flat in ambient (19) Weibel, D. E.; Lockyer, N.; Vickerman, J. C. Appl. Surf. Sci. 2004, 231-2, 146–152. (20) Cheng, J.; Winograd, N. Anal. Chem. 2005, 77, 3651–3659. (21) Parry, S.; Winograd, N. Anal. Chem. 2005, 77, 7950–7957. (22) Mine, N.; Douhard, B.; Brison, J.; Houssiau, L. Rapid Commun. Mass Spectrom. 2007, 21, 2680–2684.
conditions until the solution dried. The center area of the film (normally >1 mm2) was very flat, and it was used for depth profiling experiments. This desirable area could be easily found under an optical microscope based on interference color strips. TOF-SIMS. A TOF-SIMS 5 spectrometer (IONTOF GmbH, Mu¨nster, Germany) was used for the TOF-SIMS experiments. It was equipped with a 25 keV bismuth cluster ion source, a 0.2-2 keV Cs+/O2+ sputtering source, and a 10 keV C60+ source. In particular, 250, 500, and 1000 eV of Cs+ and O2+, 10 keV C60+, 20 keV C602+, and 30 keV C603+ were used as sputter sources for this work. The sputtering ion beams were rastered over a 300 × 300 µm2 area with an incident angle of 45° during sputtering. Cs+ beams, with a diameter of ∼40 µm, had currents of 70, 50, and 20 nA for 1000, 500, and 250 eV energies, respectively. O2+ beams, with a diameter of ∼50-60 µm, had currents of 150, 80, and 20 nA for 1000, 500, and 250 eV energies, respectively. In all cases, the diameter of the C601-3+ beams was ∼3 µm, and the beam currents were ∼0.7 nA. Twenty-five keV pulsed Bi+ ions were used as the primary ions for the analysis and were focused into an ∼4 µm diameter with a pulse width smaller than 1 ns. The pulsed beam was rastered over a 100 × 100 µm2 area during data acquisition. Interlace mode depth profiling was used in all experiments. The total Bi+ dose was controlled at
