Tapping-Mode AFM in Comparison to Contact-Mode AFM as a Tool for

Institute of Analytical Chemistry, Vienna University of Technology, Getreidemarkt 9/151, A-1060 Vienna, Austria, and Institute of Chemistry, Academy o...
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Anal. Chem. 1997, 69, 1012-1018

Tapping-Mode AFM in Comparison to Contact-Mode AFM as a Tool for in Situ Investigations of Surface Reactions with Reference to Glass Corrosion I. Schmitz,†,‡ M. Schreiner,*,†,‡ G. Friedbacher,† and M. Grasserbauer†

Institute of Analytical Chemistry, Vienna University of Technology, Getreidemarkt 9/151, A-1060 Vienna, Austria, and Institute of Chemistry, Academy of Fine Arts, Schillerplatz 3, A-1010 Vienna, Austria

A critical comparison between atomic force microscopy operated in both contact mode (CM-AFM) and tapping mode (TM-AFM) is presented by means of imaging surface reactions in situ. Corrosion phenomena of potash-limesilica glass were studied under the ambient atmosphere, dry nitrogen, and nitrogen with a relative humidity of 50%, as well as under liquids by both techniques. The results show that surface reactions on sensitive samples can be imaged in situ without the need of UHV conditions. Furthermore, the initial stages of the corrosion process could be identified as an attack of humidity. The formation of swelled glass was observed. In the presence of corrosive gases from the ambient atmosphere, the formation of secondary corrosion products took place. During the in situ investigations, both CM-AFM and TM-AFM measurements influence the corrosion phenomena observed as well as the corrosion process. CM-AFM introduces relatively high lateral forces leading to plastic deformations of soft domains whereas TM-AFM activates surface processes by the oscillating tip transferring energy to the sample surface. These artifacts introduced by the different techniques are discussed critically. Commercial glass normally possesses excellent chemical durability in the ambient atmosphere. An optimized composition of modern glass and slow corrosion rates have led to the opinion that weathering (corrosion under the ambient atmosphere) does not take place or that it is more or less unrecognizable. Considering the long-term behavior of glass, e.g., as embedding material for high-level nuclear waste material, and looking at ancient and medieval art objects consisting of glass or containing other silicate materials such as glazes or enamels, it becomes obvious that corrosion has to be considered and endangers our cultural heritage.1-3 Generally, the chemical durability of a glass is closely related to its chemical composition. The corrosion resistance decreases as the percentage of the alkali oxides in the glass increases.4-13 In some cases, ancient glasses suffer a great deal of damage due to their high content of such network modifiers. High amounts †

Vienna University of Technology. Academy of Fine Arts. (1) Frenzel G. Sci. Am. 1985, 252 (5), 100-106. (2) Newton R. G. The deterioration and conservation of painted glass: a critical bibliography; Oxford University Press: Oxford, U.K., 1982. (3) Cox, G. A.; Heavens, O. S.; Newton, R. G.; Pollard, A. M. J. Glass Stud. 1979, 21, 54-75. (4) Peddle, C. J. J. Soc. Glass. Technol. 1920, 4, 299-366. (5) Peddle, C. J. J. Soc. Glass. Technol. 1921, 5, 195-268. ‡

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of alkalis have made the production easier with respect to the temperature necessary for melting the raw materials such as silica, chalk, soda, or potash. Potash-lime-silica glass used for medieval glass paintings in churches and cathedrals of the Romanesque and Gothic periods suffers from degradation by weathering due to its low chemical durability against the ambient atmosphere. Previous results obtained by scanning electron microscopy in combination with energy-dispersive X-ray analysis (SEM/EDX) and secondary ion mass spectrometry (SIMS) showed that the weathering process as well as the corrosion in aqueous solutions is initiated by an ion exchange mechanism with M ) K and to a certain extent also Ca:14,15

Si

O–M+ + H2O

Si

O–H+aq + M+ + OH–

(1)

Additionally, aqueous solutions with a pH >9 lead to a dissolution of the silicate network and a complete destruction of the glass: Si O

Si

+ OH–

Si

OH +

Si

O–

(2)

Secondary reactions with corrosive gases such as CO2, NO2, or SO2 from the ambient atmosphere can lead to the formation of corrosion products such as carbonates, nitrates, or sulfates (gypsum or syngenite).8,16 Above most of the other surface analytical techniques, atomic force microscopy (AFM) is able to study the topography of insulating and conducting samples down to the atomic scale under ambient conditions (e.g., ambient air, liquids).17-20 The structural (6) Dimbley, V.; Turner, W. E. S. J. Soc. Glass. Technol. 1926, 10, 304-358. (7) Morey, G. W. The Properties of Glass, 2nd ed.; Reinhold: New York, 1954; Chapter IV, pp 101-131. (8) Simpson, H. E. Am. Ceram. Soc. Bull. 1958, 41, (2), 43-49. (9) Rana, M. A.; Douglas, R. W. Phys. Chem. Glasses 1961, 2 (6), 179-195. (10) Bubb, S. M.; Frackiewicz J. Phys. Chem. Glasses 1962, 3 (4), 116-120. (11) Douglas, R. W.; El Shamy, T. M. M. J. Am. Ceram. Soc. 1967, 50 (1) 1-8. (12) Das, C. R. Glass Ind. 1969, 50, 483-485. (13) Clark, D. E.; Pantanao, C. G., Jr.; Hench, L. L. Corrosion of Glass; Books for Industry and the Glass Industry: New York, 1979. (14) Schreiner, M.; Scholze, H. Forschungsbericht 10608104 des Umweltbundesamtes, Berlin, Germany, 1985. (15) Schreiner, M. Glasstechnol. Ber. 1988, 61, 223. (16) Tichane, R. M. Glass Technol. 1966, 7 (1), 26-29. (17) Frommer, J. Angew. Chem. 1992, 104, 1325-1357. (18) Magonov, S. N. Appl. Spectrosc. Rev. 1993, 28, 1-121. (19) Binnig, G.; Quate, C. F.; Gerber, Ch. Phys. Rev. Lett. 1986, 56, 930-933. S0003-2700(96)00702-0 CCC: $14.00

© 1997 American Chemical Society

and morphological changes of silica glass due to ion bombardment21,22 or reaction with the ambient atmosphere23 have been observed successfully by this technique. However, the correlation of morphological changes with the underlying chemical reaction often remains uncertain especially, when experiments cannot be carried out in situ. This limitation could be overcome by in situ observation of the surface processes mentioned at a particular sample position throughout the whole experiment. Contact-mode AFM (CM-AFM) is a powerful technique for this kind of investigations and previous results of potash-lime-silica glass exposed to the ambient atmosphere and observed by CM-AFM showed significant changes of the topography. However, these first results still suffered from the scanning process, heavily influencing the reaction.24 In the presented work, AFM was applied to study the initial steps of glass corrosion at spatial resolutions in the lower nanometer range, and the combination of contact-mode AFM (CMAFM) and tapping-mode AFM (TM-AFM) allowed a better correlation of the observed topographic changes with known reaction mechanisms. It has to be kept in mind that when AFM is used the results are impaired by the principle of measurement. Especially soft structures and weakly bound particles are influenced by the tip. Though contact forces are generally small in CM-AFM, the sample experiences both compressive forces originating from the tip/sample contact and shear forces attributed to the lateral scan movement. Both forces could induce elastic and/or plastic sample deformation. Furthermore, poor image resolution can result due to the stick-slip motion of the tip attracted by the water absorbed at the surface. Additionally, chemical information is not provided directly by evaluating topographic data only. Therefore, one aim of this work is to access material properties by applying different environmental conditions to the sample and by using different techniques of measurement. The combination of the results obtained from potash-lime-silica glass exposed to 2-propanol with a varying water content and to nitrogen with a defined relative humidity should enable us to access more information about the corrosion of that type of glass. In order to overcome artifacts introduced by the measurements and thus to apply more appropriate conditions similar to natural weathering conditions, the influences of the probe have to be minimized. This can be achieved by a modified AFM technique, TM-AFM,25 where the tip is not in permanent contact with the sample surface. A vibrating tip that strikes the surface only in the lower part of each oscillation cycle is used to sense the surface. The vibrating tip has enough kinetic energy to escape out of the water layer which is present on surfaces under ambient conditions. When the tip strikes the surface, part of the energy in the vibrating system is transferred to the sample leading to a reduced vibrational amplitude. The control system maps the sample surface by adjusting the piezo height to maintain a constant vibrational amplitude. Here, the valuable figures of merit of both (20) Rugar, D.; Hansma, P. Phys. Today 1990, 43, 23-30. (21) Oyoshi, K.; Hishita, S.; Wada, K.; Suehara, S.; Aizawa, T. Appl. Surf. Sci. 1996, 100, 374-377. (22) Wong, T. K. S.; Wilson, I. H. Instrum. Methods. Phys. Res. 1994, B91, 639642. (23) Trens, P.; Denoyel, R.; Guilloteau, E. Langmuir 1996, 12, 1245-1250. (24) Schmitz, I.; Prohaska, T.; Friedbacher, G.; Schreiner, M.; Grasserbauer, M. Fresenius J. Anal. Chem. 1995, 353, 666-669. (25) Digital Instruments Inc., 520 E. Montecito St., Santa Barbara, CA 93193.

Figure 1. Cleaved glass seen with TM-AFM under dry nitrogen immediately after cleavage. The scan size is 10 × 10 µm2, the height range is 30 nm from black to white and the root mean square roughness is 1.1 nm. Chances in topography or root mean squareroughness could not be detected even after 1 h of exposure to dry nitrogen.

CM- and TM-AFM are utilized to access a wealth of information on the addressed corrosion processes. EXPERIMENTAL SECTION Samples. For the present investigations, a model glass with a low durability against chemical attack and even the ambient atmosphere was chosen. This glass has a chemical composition similar to medieval stained glass but with an even higher amount of alkali oxides (48.0 wt % SiO2, 25.5 wt % K2O, 15 wt % CaO, 4.0 wt % P2O5, 3.0 wt % MgO, 3.0 wt % Na2O, 1.5 wt % Al2O3), resulting in an even lower durability. The glass was prepared by the Fraunhofer-Institut fu¨r Silicatforschung, Wu¨rzburg/FRG from reagent-grade oxides and carbonates. The mixtures were melted in a Pt crucible in an electrically heated furnace. After heating to ∼1720 K and homogenizing for 3 h at this temperature, the silicate melt was poured into bars, annealed for 5 h at 720 K, and cooled to room temperature over 15 h. Platelets (approximately 30 × 10 × 1 mm3) were cut from the annealed bars and stored in a desiccator. In order to obtain a fresh surface, the glass samples were cleaved under dry nitrogen, either in the ambient atmosphere or under 2-propanol, and transferred in the corresponding medium to the microscope immediately before the measurement was started. The measurements were carried out on the cleaved area of 1 × 10 mm2. Instrumentation. In the present studies, a NanoScope III MultiMode AFM by Digital Instruments25 in combination with a home-built sample holder, which provides mechanical fixation capabilities,26,27 and commercially available liquid cells for contact mode or tapping mode, respectively, were used. The detection scheme of the AFM used is based on laser beam deflection off a microfabricated cantilever. Imaging under gases and liquids in contact mode was performed by means of Si3N4 cantilevers with a cantilever length of 200 µm and a spring constant of 0.06 N‚m-1. Imaging under gases in tapping mode was carried out by means of silicon cantilevers with a cantilever length of 125 µm and (26) Prohaska, T.; Friedbacher, G.; Grasserbauer, M. Fresenius J. Anal. Chem. 1994, 349, 190-194. (27) Prohaska, T.; Friedbacher, G.; Grasserbauer, M.; Nickel, H.; Lo¨sch, R.; Schlapp, W. Anal. Chem. 1995, 67, 1530-1534.

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Figure 2. Cleaved glass seen with CM-AFM under ambient atmosphere. The scan size is 10 × 10 µm2 [except (c)] and the height range is 30 nm from black to white: (a, upper left) After 150 min pits are visible; (b, upper right) these pits are deepened with time; (c, lower left) a larger scan of 13 × 13 µm2 shows corrosion products moved by the tip to the right border of the previously scanned area of 10 × 10 µm2; (d, lower right) after 8 h the entire surface is covered with products and scratches in the fast-scan direction are visible.

Figure 3. Cleaved glass seen with TM-AFM under the ambient atmosphere. The scan size is 5 × 5 µm2, and the height range is 50 nm from black to white: (a, upper left) After 2 h, the formation of raised round features is visible; (b, upper right) after 5 h, these features are grown; (c, lower left) a previously scanned area of 2 × 2 µm2 is visible in the center of the image due to a higher growth rate enhanced by the oscillating tip; (d, lower right) after 43 h, the formation of weathering products due to secondary reactions of the swelled glass matrix with corrosive gases from the ambient atmosphere is visible.

resonance frequencies of F0 ) 265-378 kHz. For imaging under liquids in tapping mode, oxide-sharpened Si3N4 cantilevers with a cantilever length of 200 µm and resonance frequencies of F0 ) 18-19 kHz in liquids were used. All cantilevers with integrated pyramidal tips are commercially available.25 All measurements 1014 Analytical Chemistry, Vol. 69, No. 6, March 15, 1997

were performed in constant-force mode or constant-amplitude mode. The scan rate was adjusted to 2 Hz, and a resolution of 256 × 256 pixels2 per image was selected. Quantitative evaluation of the image data was performed using the NanoScope III off-line analysis software.

Figure 4. Cleaved glass seen with CM-AFM under humid nitrogen (50% relative humidity) after 100 min: (a, left) The scan size is 12.5 × 12.5 µm2 and the height range is 30 nm from black to white. The previously scanned area of 10 × 10 µm2 is lowered by 5.5 nm in average. At the edges of the image, pits similar to the investigations under the ambient atmosphere (Figure 2) are visible. (b, right) The diagram shows an average line scan of the box marked in the image. The arrows denote the border.

A defined atmosphere was achieved by means of a steel tube which ends ∼2 mm above the sample surface without interfering with the laser beam of the detection system or other sensitive parts of the AFM. Further isolation of the optical head was not necessary in order to use this simple setup as a “climate chamber”. Defined relative humidity was achieved by mixing dry nitrogen gas with nitrogen saturated with water by means of flowmeters. Measurements under nitrogen were performed at a flow rate of 10 L·h-1. All measurements were carried out at room temperature. RESULTS AND DISCUSSION Figure 1 shows a representative TM-AFM image of the glass surface immediately after cleavage under pure nitrogen gas. The

scan size is 10 × 10 µm2. Besides the fine texture, other features are not visible and the root mean square roughness is 1.1 nm. Changes in the topography could not be detected after keeping the sample under dry nitrogen in the microscope for more than one hour. Comparable results have been obtained with CM-AFM. Investigations in the Ambient Atmosphere. The freshly prepared glass surface was exposed to the ambient atmosphere and imaged by CM-AFM. Already after 150 min pits with a diameter of 0.5-1 µm and ∼4.5 nm in depth were formed (Figure 2a), which were deepened with time. After 264 min, the average depth was 8.2 nm (Figure 2b). This seems to be the initial stage of pit corrosion, where craters are formed on the glass surface.1-3 Obviously the attack of the ambient atmosphere leads to the destruction or softening of round domains of the glass surface and the contacting tip is then able to remove the modified material. When a larger area was scanned (13 × 13 µm2 instead of 10 × 10 µm2 as in Figure 2b), the removed products were found at the right border of the previously scanned area (Figure 2c). After 8 h of exposure, the entire surface was covered with products and scratches in the fast-scan direction were visible (Figure 2d). The tip was no longer able to wipe away the softened glass compounds formed due to the attack of water and corrosive gases present in the ambient atmosphere. From these results, it is obvious that high contact forces lead to irreversible rearrangements and plastic deformations of the weathered glass surface. In particular, the consequences of high lateral forces in combination with compressive forces are visible in Figure 2d. In order to exclude these forces, the same experiment was carried out with TM-AFM on the freshly prepared glass surface exposed to the ambient air. Pit formation could not be observed by TM-AFM. In contrast to the results obtained by CM-AFM, round features, which are heightened with respect to the scanned surface were detected after 2 h (Figure 3a). These features were

Figure 5. Cleaved glass seen with TM-AFM under humid nitrogen (50% relative humidity). The scan size is 5 × 5 µm2 and the height range is 50 nm from black to white (except d, 10 × 10 µm2 and 200 nm from black to white): (a, upper left) After 30 min, the formation of raised round structures is observed; (b, upper right) after 156 min, these features have grown with time. In the center of the scanned area one feature is growing much faster compared to the others; (c, lower right) after 8.4 h, the centered feature is clearly distinguishable from the other features; (d, lower left) after 24 h, the formation of crystalline products could be observed.

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Figure 6. Cleaved glass seen with CM-AFM under 2-propanol. The scan size is 10 × 10 µm2 [except (d) and (e)] and the height range is 30 nm from black to white: (a, upper left) Under pure 2-propanol, the surface did not show distinct features but shows a fine substructure; (b, upper right) after 47 min, under 2-propanol with 1 vol % water, the substructure appeared blunted; c) after 213 minutes under 2-propanol with 1 vol % water the blurring continued; (d + e, middle right and lower left, respectively) when a larger area of 15 × 15 µm2 is imaged, the previously scanned area of 10 × 10 µm2 is raised by 5 nm in average with respect to the surrounding surface. The diagram shows an average horizontal line scan of the area denoted by the box. The arrows denote the border of the previously scanned area.

growing with time (Figure 3b). Comparing these images with the CM-AFM measurements, it can be assumed that the features in Figure 3b consist of swelled glass material according to reaction 1 or even degraded glass according to reaction 2. Due to the adsorption of water from the ambient atmosphere, an ion exchange process takes place increasing the pH in the surface layer dramatically. As a consequence, reaction 2 leads to a breakdown of the silicate network. TM-AFM is able to image these swelled surface areas while CM-AFM scrapes away the silicate residue. However, TM-AFM also can cause artifacts. By switching to a smaller scan size (2 × 2 µm2) and applying a high tapping load for one entire scan, it was possible to identify such a scanned area by the higher amount of products formed in this domain (Figure 3c). The features in the Figure 3a-c grew with exposure time, and after 43 h, the formation of secondary weathering products due to reactions of the swelled glass matrix with corrosive gases present in the ambient atmosphere were observed (Figure 3d). Further investigations under artificial atmospheres 1016 Analytical Chemistry, Vol. 69, No. 6, March 15, 1997

containing higher amounts of SO2 as corrosive gases yielded similar features which showed sulfur in the SEM/EDX investigations. Investigations under Nitrogen with 50% Relative Humidity. Experiments were also carried out excluding the ambient atmosphere during sample preparation as well as during the measurement procedure. This was achieved by cleaving the sample in dry nitrogen and transferring the sample to the microscope as already described. Changes of the topography could not be observed by CM-AFM under dry dynamic nitrogen atmosphere for more than 1 h. By switching to nitrogen with a relative humidity of 50% and permanently scanning an area of 10 × 10 µm2, an image shown in the center of Figure 4a was obtained after 100 min. By scanning a larger size of 12.5 × 12.5 µm2, the previously scanned domain is clearly visible in the center of the larger image. Compared to the surrounding glass material, where pits similar to the experiments in the ambient atmosphere occur, the previously scanned area does not show craters at all and is deepened by 5.5 nm on

Figure 7. Cleaved glass seen with TM-AFM under 2-propanol. The scan size is 5 × 5 µm2 [except (e) and (f) 10 × 10 µm2] and the height range is 50 nm from black to white: (a, upper left) After several minutes under pure 2-propanol only a few features are visible; (b, upper right) after 13 min under 2-propanol with 1 vol % water raised features appeared; (c, middle left) after 1 h under 2-propanol with 1 vol % water, the number of features is higher and features are grown in height and diameter; (d, middle right) after 17.4 h, a further growth of the features is visible; (e, lower left) after 18.4 h, the TM-AFM picture shows unstable features loosely bound to the surface; (f, lower right) after 18.5 h, products apparently seem to disappear due to dissolution induced by continued scanning of the sample.

average (Figure 4b). This can be explained by the scanning tip smearing the adsorbed water over the glass surface and leading to a homogeneous attack of the silicate material. On the unscanned area, distinct domains are attacked by the moisture due to the inhomogeneous water film formed or due to hygroscopic centers in the glass acting as nuclei for the corrosion and the formation of pits. From CM-AFM images, we can only conclude that the glass surface shows domains with swelled or even degraded silicate material according to reactions 1 and 2, which is removed completely by a single scanning process. Consequently, the small craters seen on the edges of Figure 4a occur. With TM-AFM, the formation of raised features could already be observed on the surface after 30 min of exposure to nitrogen with a relative humidity of 50% (Figure 5a). These features, which are comparable to those observed under ambient atmosphere (Figure 3a), were growing in height and diameter with exposure time, and after 156 min (Figure 5b), a more or less inhomogeneous distribution of the size of the features is observed. Furthermore, in the center of the scanned area, one feature was

growing much faster compared to all others (Figure 5c). This artifact can be explained by the oscillating tip. During force calibration, the sample is not moved laterally but is raised and lowered periodically against the oscillating tip, touching the sample in the center of the scanned area. As no feedback controlling is applied to maintain a constant vibrational amplitude of the cantilever, a high amount of energy is transferred to the center of the scanning area. Several force calibrations during the measurement have yielded the artifact shown in the center of Figure 5c. The transfer of energy by the oscillating tip to the glass surface also has to be considered during the scanning, although the feedback controller reduces the total amount of energy by maintaining a constant damping of the vibrational amplitude. The tapping load has to be adjusted as low as possible in order to minimize such artifacts. Finally, after 24 h of exposing the glass surface to nitrogen with 50% relative humidity, the formation of crystalline products could be observed (Figure 5d). Investigations under Liquids. Imaging a freshly prepared glass surface under dry 2-propanol by CM-AFM did not show any Analytical Chemistry, Vol. 69, No. 6, March 15, 1997

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surface alterations for more than 1 h (Figure 6a). Adding 10 vol % of water to 2-propanol led to a blunting of the surface structures with time in the scanned area (Figure 6b and c). Neither the formation of pits nor the appearance of additional features was observed. By switching to a larger scan area of 15 × 15 µm2, it could be proven that the previously scanned area of 10 × 10 µm2 was raised by 11.5 nm with respect to the surrounding surface (Figure 6d). Comparing these results with the CM-AFM measurements under the ambient atmosphere, there is strong evidence that reaction 1 dominates the glass corrosion process under aqueous solutions. The pH is kept below 9 due to the large liquid reservoir and thus reaction 2 is suppressed. Contrary to the experiments carried out in the ambient air and under moist nitrogen, the entire scanned area incorporates water at an even higher rate than the surrounding area, probably due to steering effects of the scanning tip permanently in contact with the surface. Use TM-AFM for imaging under dry 2-propanol yielded no surface alterations for more than 1 h. Again, by sample preparation and imaging under pure 2-propanol, it was possible to protect the sample from any attack by the ambient atmosphere without the need of UHV conditions (Figure 7a). Addition of 1 vol % of water to 2-propanol, corrosion products already occur after 13 min (Figure 7b). The bright horizontal lines sometimes visible in the image are evidence for the products loosely bound to the surface. These products are even removed from the glass by the scanning tip in TM-AFM. After 1 h of exposure in 2-propanol with 1% water, the corrosion products formed have increased in number and size (Figure 7c) and after 17 h some of the features already have a diameter of 400 nm (Figure 7d). However, even under these conditions an influence of the scanning tip on the ongoing corrosion reaction could be detected. An increase of the scan size from 5 × 5 (Figure 7d) to 10 × 10 µm2 (Figure 7e) reveals the previously scanned area in the center of the image covered by products smaller than the surrounding glass surface. Additionally, the corrosion products in the outer region imaged for the first time appear less stable, causing the tip to produce error lines in the fast-scan direction (Figure 7e). Continuous scanning led to a reduction of the size of the products in the outer region of the image whereas the products in the center of the image remained stable in size (Figure 7f). Here, a dynamic process has been observed which is heavily influenced even by using TMAFM. Previously formed products are dissolved by scanning the surface. Obviously a layer has been formed near the glass surface, where the water content is lower than in the liquid. In this layer, the formed products seem to be more stable. Unfortunately, this layer is disturbed by the moving tip, leading to dissolution of the products.

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CONCLUSION AFM proved to be well suited for in situ investigations of surface reactions on potash-lime-silica glass. The method is advantageous for imaging small spatial structures like pits or heights in the lower nanometer range. It has been impossible to study such features by other analytical techniques without any pretreatment of the specimen such as coating or conditioning in vacuum, which hinders or sometimes even falsifies the subsequent measurement. By sample preparation in dry nitrogen or 2-propanol, it was also possible to protect the sensitive freshly cleaved glass surface without any UHV during imaging with either CMor TM-AFM. This opens up a wide field for simulating corrosive conditions and carrying out weathering studies under controlled atmospheres. Here TM-AFM is more appropriate because it does not alter the surface by introducing shear forces. Generally, the experiments have shown that both AFM techniques used have special advantages but also disadvantages for studying the weathering of glass. CM-AFM was successfully applied for identifying soft structures since it introduces high lateral forces as well as compressive forces to the sample surface with the consequence of plastic deformations. On the other hand, swelled glass material or corrosion products weakly bound to the glass surface are scratched away by the tip and pits occur where heights have been formed due to the weathering process. For imaging such corrosion phenomena, TM-AFM is more suitable. However, the enhancement of surface reactions by the oscillating tip and the TM-AFM should always be considered. Additionally, stirring effects also occurred during the experiment under liquids, leading to a dissolution of corrosion products already formed by the diffusion-controlled process. Together with these results, we also have evidence from the CM-AFM and TM-AFM images that the first step of glass corrosion is the attack of water, leading to the formation of a reactive layer in 2-propanol/water or of liquids in humid nitrogen or moist air. Reactions of this swelled glass material with corrosive gases from the ambient atmosphere form secondary weathering products, which takes several hours or even days. ACKNOWLEDGMENT Financial support of this work by the Austrian Science Foundation (Project 9220-TEC and 11015-O ¨ PY) is gratefully acknowledged. Received for review July 18, 1996. Accepted January 7, 1997.X AC9607020 X

Abstract published in Advance ACS Abstracts, February 15, 1997.