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Imaging the Composition of Oxide-Glass Surfaces by Friction Atomic Force Microscopy Nathalie Destouches,† Marie Foret,*,† Eric Courtens,† and Michel Ramonda‡ Laboratoire des Verres, UMR 5587 CNRS, Universite´ Montpellier 2, CC 069, Place Euge` ne Bataillon, F-34095 Montpellier, France, and Laboratoire de Microscopie en Champ Proche, Universite´ Montpellier 2, CC 082, Place Euge` ne Bataillon, F-34095 Montpellier, France Received March 22, 2003. In Final Form: July 2, 2003 Fracture surfaces of high-purity silica glasses, finely patterned with small amounts of either GeO2 or P2O5 substituted for SiO2, are investigated by atomic force microscopy under an electrolyte. The samples are cross sections of graded-index optical fibers of the type used for long-distance optical communications. While topography images are featureless, the frictional force is strikingly sensitive to the composition, down to ∼0.1% for the [P]/[Si] ratio. This unexpectedly large contrast depends on the pH of the electrolyte showing its relation to the acid-base properties of the oxides. This result holds considerable promise for the local exploration of acid-base properties of mixed oxides down to the nanometer scale.
1. Introduction Atomic force microscopy (AFM)1 became the choice tool to measure, beyond topography, forces at the nanometer scale on insulator or semiconducting surfaces. Many scanning probe microscopies (SPMs) were developed in the wake of AFM.2 For organic systems, so-called “chemical imaging” using adhesion or friction was achieved, often with considerable selectivity using chemically modified scanning tips.3-6 In comparison, for inorganic insulators, reports of composition-dependent imaging are scarce. Lin et al. suggested that one might construct an image from the local acid-base properties of oxide surfaces using AFM in the contact mode under appropriate electrolytes.7 They proposed to image the deviations from zero normal force caused by chemical domains that modify the mean isoelectric point (IEP) of oxide surfaces. Later, Marti et al. noticed that besides the normal force, the lateral or frictional force is strongly sensitive to IEP variations.8 The frictional force should follow the adhesion hysteresis (i.e., the dissipated energy) as predicted by Israelachvili et al.9 Such imaging of oxides was so far only demonstrated for samples assembled from clearly distinct materials.10 We show here that under electrolyte solutions of welltuned pH, lateral force microscopy (LFM) is capable of an arresting sensitivity on mixed oxides. Variations in * To whom correspondence should be addressed. E-mail:
[email protected]. † Laboratoire des Verres. ‡ Laboratoire de Microscopie en Champ Proche. (1) Binnig, G.; Quate, C. F.; Gerber, C. Phys. Rev. Lett. 1986, 56, 930. (2) For a recent survey, see the Proceedings of the 11th International Conference on Scanning Tunneling Microscopy/Spectroscopy and Related Techniques; University of British Columbia and National Research Council: Vancouver, Canada, 2001. (3) Frisbie, C. D.; Roznyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071. (4) McKendry, R.; Theoclitou, M.-H.; Rayment, T.; Abell, C. Nature 1998, 391, 566. (5) Wong, S. S.; Joselevich, E.; Woolley, A. T.; Cheung, C. L.; Lieber, C. M. Nature 1998, 394, 52. (6) Takano, H.; Kenseth, J. R.; Wong, S.-S.; O’Brien, J. C.; Porter, M. D. Chem. Rev. 1999, 99, 2845. (7) Lin, X.-Y.; Creuzet, F.; Arribart, H. J. Phys. Chem. 1993, 97, 7272. (8) Marti, A.; Ha¨hner, G.; Spencer, N. D. Langmuir 1995, 11, 4632. (9) Israelachvili, J. N.; Chen, Y.-L.; Yoshizawa, H. J. Adhes. Sci. Technol. 1994, 8 (11), 1231. (10) Ha¨hner, G.; Marti, A.; Spencer, N. D. Tribol. Lett. 1997, 3, 359.
Figure 1. Schematic view of a concentration modulated preform and its relation to the drawn fiber. The surfaces studied here are cross sections obtained by breaking fibers perpendicular to their length, as indicated.
concentration at the level of one part in a thousand can produce high-contrast LFM images. 2. Materials and Method The experiments are performed on cross sections of gradedindex SiO2-glass fibers. The fibers are drawn from cylindrical preforms fabricated by chemical vapor-phase deposition (CVD). In this standard process,11 a rotating SiO2 tube is filled with successive concentric glass layers. The glass contains a small amount of either GeO2 or P2O5 whose concentration is adjusted with the vapor composition. A quasi-continuous grading can thus be achieved. After sintering, this tube is the “preform”. Its typical diameter is of the order of 2 cm. The cross-sectional composition of preforms can be checked by various techniques. We use the results of local refractive index measurements which are routinely used to obtain composition profiles. The preforms are the source material from which fibers are drawn (see Figure 1). We use fibers of about 100 µm diameter with small modulations in either their GeO2 or P2O5 concentration. These constituents are of importance in silica-based optical devices. Ideally, the concentration profile of the drawn fiber is a homothetic image of the preform profile. This procedure provides us with samples of controlled composition variation at submicron length scales. To expose fresh surfaces, the fibers are carefully cleaved12 just before measurements, producing a so-called “mirror” region over most of the cross section defined in Figure 1. The measurements are performed with an atomic force microscope (Nanoscope IIIa, Dimension 3100) from Digital Instruments fitted with a liquid cell. The cell is filled with an aqueous electrolyte, NaCl (10-3 mol/L). The pH is adjusted with NaOH or HCl, keeping the ionic strength nearly constant. Topographic and lateral force images are recorded simultane(11) Schlutz, P. C. In Recent advances in fiber optics; Mitra, S. S., Bendow, B., Eds.; Plenum Press: New York, 1979. (12) FK 11 fiber optic cleaver from GN Nettest (U.K.), Ltd. http:// www.nettest.com/
10.1021/la034497c CCC: $25.00 © 2003 American Chemical Society Published on Web 07/23/2003
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Figure 2. Simultaneous topographic (a) and friction (b) images of a silica fiber containing some P2O5. The topographic image shows a rather flat surface. The friction image clearly reveals concentric ripples produced by the known radial modulation in composition. The orange line in (c) is the radial average of the friction signal over the entire image (left scale). The black line is an estimate of the radial variations of the atomic ratio [P]/[Si] based on refractive index measurements performed on the original preform and extrapolated to the fiber (right scale). ously in the contact mode using a standard V-shaped Si3N4 cantilever with a square pyramidal tip.13 The normal and lateral spring constants of the cantilever are kN ≈ 0.1 N/m and kL ≈ 5 N/m, respectively.14 In the friction mode, the instrument records the lateral deflection of the cantilever as the sample is displaced transversally, forward and backward. The difference between a forward and a backward scan is called the friction signal.15
3. Results and Discussion Typical 10 µm × 10 µm images obtained on P2O5- or GeO2-containing fibers at pH ≈ 3 are illustrated in Figures 2 and 3, respectively. These images were acquired at a scan rate of 24 µm s-1 and at a constant applied load of ∼40 nN. Lower scan rates down to 10 µm s-1 gave identical images. In each case, panel a shows the topographical signal whereas panel b shows the friction. The topographical images indicate rather flat and featureless surfaces. The root-mean-square roughness, calculated over the 10 µm × 10 µm area sampled with 5122 points, is around 0.55 nm for both samples. This is a typical value for the mirror zone of silica-based glasses.16 The friction images (b) disclose a considerable sensitivity to the chemical composition. Concentric ripples, unambiguously related to the radial concentration modulation, are clearly observed. Radial averages of the friction over the entire images are drawn in orange in panels c of both figures. They closely match the estimated radial variations of the concentration shown as black lines. These are obtained from refractive index measurements on the preforms and extrapolated to the fibers assuming a perfect homothetic scaling. This hypothesis, which is the usual assumption (13) Nanoprobe, SPM tips, Digital Instruments. (14) Bogdanovic, G.; Meurk, A.; Rutland, M. W. Colloids Surf., B 2000, 19, 397. (15) Meyer, G.; Amer, N. M. Appl. Phys. Lett. 1990, 57, 2089. (16) Ra¨dlein, E.; Frischat, G. H. J. Non-Cryst. Solids 1997, 222, 69.
Figure 3. The topographic (a) and friction (b) images obtained in the core region of a silica fiber containing GeO2. The topographic image shows a featureless surface. The friction image shows concentric ripples produced by the radial modulation in GeO2 concentration. The orange line in (c) is the radial average of the friction signal over the entire image (left scale). The black line shows the ratio [Ge]/[Si] obtained as in Figure 2 (right scale). Owing to the process related to the closure of the preform core, the index information on the preform is not usable to determine the [Ge]/[Si] ratio near the center.
in determining the refractive index profile of optical fibers, neglects possible irregularities such as an incomplete closure of the preform core.17 The shape of the concentration profile is fairly well-known, the concentration uncertainty being estimated to about (10% owing mainly to fluctuations in the preform composition along its length. The friction signal in Figure 2c agrees with the concentration profile, suggesting that the friction increases nearly linearly with P2O5 concentration. A variation of the [P]/[Si] molar ratio by 0.1% produces a detectable lateral force modulation of ∼0.2 nN. As shown in Figure 3c, a similar trend is observed for GeO2-substituted silica fibers, though the sensitivity is approximately 10 times lower. In both cases, the friction depends on the pH and on the applied load. The effect of the load is described in ref 8. We discuss now the likely origin of the observed sensitivity of friction to concentration and its dependence on pH. We did not apply an external field in this experiment, as in many other sensitive SPM schemes.2 Although one could possibly advance other ideas, we believe that in view of the great sensitivity to the electrolyte pH, a reasonable explanation must invoke the electrostatic forces related to the acid-base reactions occurring at the two interfaces in the glass-surface/electrolyte/Si3N4-tip system. Oxides, and in particular silica, form amphoteric surfaces in electrolyte solutions:18 as the pH decreases, surface hydroxyl groups change from proton donors to proton acceptors. Therefore the surface charge changes from negative to positive. When there are no adsorbed ions besides H+ and OH-, the pH value giving no average (17) Hammond, C. R.; Norman, S. R. Opt. Quantum Electron. 1977, 9, 399. (18) Feldman, K.; Fritz, M.; Ha¨hner, G.; Marti, A.; Spencer, N. D. Tribol. Int. 1998, 31, 99.
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decrease as the P2O5 concentration increases, whereas IEP2 remains unchanged. This must produce a friction dependence on concentration which should be highest at pH values where the slope of the friction versus pH curve is greatest, that is, slightly above IEP1. As shown in Figure 4, the frictional contrast to P2O5 concentration indeed decreases as the pH increases, and it disappears above ∼4.
Figure 4. Friction signals measured on the P2O5-substituted silica fiber as a function of the electrolyte pH and P concentration. All measurements are performed with the same tip keeping the applied load at ∼20 nN. Blue circles and red crosses are our measurements in the regions of 0.2% and 0.8% [P]/[Si] ratio, respectively (left scale). The error bars are derived from the root-mean-square friction signal over the entire images. These data are compared to measurements from ref 8 obtained on an oxidized Si wafer over a wide range of pH, shown as black dots (right scale). The line is a guide for the eyes. These data suggest that friction goes through a maximum in the interval between the isoelectric points of the materials in contact. The friction contrast is large near pH ≈ 3 and completely disappears above pH ≈ 4.
surface charge is called the isoelectric point. This IEP varies from one material to another. It is estimated to be IEP1 ∼ 2 for SiO2 and IEP2 ∼ 6 for a Si3N4 tip, depending somewhat on material purity.7,18 It follows that the SiO2Si3N4 couple has an attractive interaction for pH values between IEP1 and IEP2 and a repulsive one otherwise. Presumably these electrostatic interactions dominate adhesion and friction phenomena that are encountered in AFM experiments under electrolytes. It has actually been shown experimentally that the adhesion hysteresis and the friction force correlate extremely well in conditions that are similar to ours.8,18 In the constant deflection mode, the variation of the adhesion force is practically unseen on the topographic signal. The acidic character of P, Ge, and Si surface sites in the substituted silica is expected to vary in proportion with the cation electronegativity. The acidic character should be much larger for P than for Si, and about the same for Ge and Si.19 One thus expects a nonuniform charge distribution at the silica-electrolyte interface that will follow the P or Ge concentration. The P sites being much more negative than Si ones, at pH values between IEP1 and IEP2 the average attractive interaction is stronger in P2O5-richer areas. This qualitatively explains the appearance of a friction chemical contrast that coincides with the P2O5 concentration. However, the case of GeO2 substitution is more delicate as Ge and Si are isoelectronic. As shown in Figure 3, the friction increases with the [Ge]/ [Si] ratio, in apparent contradiction with the prediction of ref 19 concerning the relative acidity of Ge and Si sites in oxides. Unfortunately, we know of no experimental report that gives information about the relative electrochemical properties of Ge and Si on oxide surfaces. Figure 4 displays measurements by Ha¨hner et al. of friction versus pH for an oxidized Si wafer against a Si3N4 tip.10 These can be extrapolated to our situation. The main trend is that the friction goes through a maximum in the interval IEP1-IEP2. As explained above, IEP1 should (19) Parks, G. A. Chem. Rev. 1965, 65, 177.
4. Conclusion and Perspectives These studies will need to be pursued and extended in order to obtain a complete picture of the frictional interactions of AFM tips with oxide surfaces under liquid electrolytes. The use of LFM as a sensitive concentration probe for oxide surfaces promises to be of considerable practical interest. State-of-the-art scanning probe instruments, such as scanning electron microscopy (SEM) combined with X-ray photoelectron spectroscopy (XPS) or Auger electron spectroscopy (AES) and time-of-flight secondary ion mass spectrometry (TOF-SIMS), are limited either in their analysis capability or in their lateral resolution, especially on insulating oxide surfaces.20-22 The main advantage of LFM, in comparison with other techniques that measure composition, is its potential ability to map chemical variations with both high sensitivity and high lateral resolution. An upper bound of the latter should be of the order of the tip radius (∼30 nm in the present experiment). A more refined estimate of the effective area of interaction could depend on the depth of the double layer7 and thus on the electrolyte pH.23 LFM holds considerable promise for submicron concentration characterization on patterned oxide surfaces. Figure 3 provides an example of this for the core of a SiO2-GeO2 fiber. This core obviously differs from the cross-sectional analysis of the preform as seen in Figure 3c. Since the properties of such cores are of uppermost importance for the good operation of low-loss single mode fibers, a tool to characterize their structure is most useful. More fundamentally, one can now envisage comparison of local surface properties of mixed oxides, such as mixtures of basic ones (e.g., MgO, CaO, or Al2O3) with acidic ones (e.g., SiO2, ZrO2, or TiO2). Such mixtures find many applications in catalysis. Further, as mixed oxides are used in optoelectronic devices and in nanotechnologies, an understanding of their local acid-base properties, in particular with respect to the imaging of cross sections, is foreseen to be of high interest. Acknowledgment. The authors express their appreciation to R. Vacher and I. Campbell (Laboratoire des Verres, Montpellier, France) for their stimulating support. M.F. and E.C. thank Professors H.-J. Guntherodt and E. Meyer in Basle for their hospitality and for an excellent introduction to AFM under electrolytes. Dr. R. Bru¨ning is thanked for a careful reading of the manuscript. N.D. acknowledges the Centre National de la Recherche Scientifique for financial support. LA034497C (20) Diebold, C.; Lindley, P.; Viteralli, J.; Kingsley, J.; Liu, B. Y. H.; Woo, K. J. Vac. Sci. Technol., A 1998, 16, 1825. (21) Schenkel, T.; Hamza, A. V.; Barnes, A. V.; Schneider, D. H. Prog. Surf. Sci. 1999, 61, 23. (22) Schenkel, T.; Holder, J. P.; McDonald, J. W.; Meijer, J.; Persaud, A.; Schneider, D. H. Engineering thin films with ion beams, nanoscale diagnostics, and molecular manufacturing; Knystautas, E. J., Kirk, W. P., Browning, V., Eds.; Proceedings of SPIE, Vol. 4468; SPIE: Bellingham, WA, 2001; pp 35-46. (23) Israelachvili, J. N. Intermolecular and surface forces, 2nd ed.; Academic Press: London, 1991; Chapter 12.
