Revealing Contamination on AFM Cantilevers by Microdrops and

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Revealing Contamination on AFM Cantilevers by Microdrops and Microbubbles Elmar Bonaccurso* and Graeme Gillies Max-Planck-Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany Received August 12, 2004. In Final Form: October 1, 2004

Introduction It is generally recognized that the resolution of atomic force microscopy (AFM1) strongly depends on the geometry and chemical composition of the cantilever tip. For example, scanning a surface with a double tip results in an image with a so-called ghost, while scanning a surface with a contaminated tip may result in a larger adhesion between tip and sample, thus increasing the contact area and lowering the resolution.2 AFM is used not only for imaging but also for friction measurements,3-6 surface force measurements in air or liquids,7,8 force spectroscopy between chemicals and biomolecules,9-13 adhesion,14,15 and elasticity and indentation measurements,16-19 just to cite the most popular. Therefore, to produce reliable and reproducible results, a clean, or at least a known, AFM tip chemistry is necessary. Cleanliness is important for the quality and effectiveness of further surface modifications of the AFM tip. Thin layers of contaminants may change the reactivity or adsorptivity of surfaces. Most researchers do not, or cannot, independently verify and characterize the wanted tip modifications with surfacesensitive spectroscopies. The same reasoning applies to the so-called “colloidal probe technique”,7,8 where, instead of the tip, a micrometer-sized particle is fixed to the end of the cantilever. * To whom correspondence should be addressed. Phone: +496131-379-112. Fax: +49-6131-379-310. E-mail: bonaccur@ mpip-mainz.mpg.de. (1) Binnig, G.; Quate, C. F.; Gerber, C. Phys. Rev. Lett. 1986, 56, 930-933. (2) Thundat, T.; Zheng, X.-Y.; Chen, G. Y.; Sharp, S. L.; Warmack, R. J. Appl. Phys. Lett. 1993, 63, 2150-2152. (3) Mate, C. M.; McClelland, G. M.; Erlandsson, R.; Chiang, S. Phys. Rev. Lett. 1987, 59, 1942-1946. (4) Marti, O.; Colchero, J.; Mlynek, J. Nanotechnology 1990, 1, 141144. (5) Meyer, E.; Lu¨thi, R.; Howald, L.; Bammerlin, M.; Guggisberg, M.; Gu¨ntherodt, H.-J. J. Vac. Sci. Technol., B 1996, 14, 1285-1288. (6) Tsukruk, V. V.; Bliznyuk, V. N. Langmuir 1998, 14, 446-455. (7) Butt, H.-J. Biophys. J. 1991, 60, 1438-1444. (8) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239. (9) Florin, E.-L.; Moy, V. T.; Gaub, H. E. Science 1994, 264, 415-417. (10) Hinterdorfer, P.; Baumgartner, W.; Gruber, H. J.; Schilcher, K.; Schindler, H. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 3477-3481. (11) Radmacher, M. Phys. World 1999, 12, 33-37. (12) Heinz, W. F.; Hoh, J. Nanotechnology 1999, 17, 143-150. (13) Lee, G. U.; Kidwell, D. A.; Colton, R. J. Langmuir 1994, 10, 354-357. (14) Hoh, J.; Cleveland, J. P.; Prater, C. B.; Revel, J.-P.; Hansma, P. K. J. Am. Chem. Soc. 1992, 114, 4917-4918. (15) Heim, L.-O.; Blum, J.; Preuss, M.; Butt, H.-J. Phys. Rev. Lett. 1999, 83, 3328-3331. (16) Stolz, M.; Raiteri, R.; Daniels, A. U.; VanLandingham, M. R.; Baschong, W.; Aebi, U. Biophys. J. 2004, 86, 3269-3283. (17) Vinckier, A.; Semenza, G. FEBS Lett. 1998, 430, 12-16. (18) Mathur, A. B.; Collinsworth, A. M.; Reichert, W. M.; Kraus, W. E.; Truskey, G. J. Biomech. 2001, 34, 1545-1553. (19) Cappella, B.; Sturm, H.; Schulz, E. J. Adhes. Sci. Technol. 2002, 16, 921-933.

Cantilevers without tips are used as force transducers and micromechanical stress sensors,20-24 above all for biochemical applications. In this case, the whole cantilever must be chemically modified and/or functionalized and a noncontaminated surface is of paramount importance. Because the AFM tip is so small, characterization of its surface chemistry is often difficult. One indirect approach is to characterize reference surfaces, that are made of the same materials as the AFM tips and treated with the same process as the tips, by means of macroscopic techniques, such as contact angle measurements25 or X-ray photoelectron spectroscopy (XPS).26 However, unless the reference surface was subjected to the same storage conditions, it might not be representative of the actual cantilever and tip surface chemistry. Surface-sensitive methods, such as scanning Auger microscopy (SAM27) and time-of-flight secondary-ion mass spectroscopy (TOFSIMS26), have been employed to perform surface microarea analyses (areas of hundreds of square micrometers) on the cantilever legs in the vicinity of the tip. These methods allow for a precise characterization of the contaminants; however, they do not allow for the characterization of the sole tip area and they are indeed time-consuming and expensive and require a vacuum. A simple and straightforward method for checking tip contamination was proposed by Thundat et al.2 They observed that sometimes during imaging the adhesion force between tip and sample increased, and they related this to tip contamination. Several procedures for cleaning the AFM tip-cantilever assembly have been discussed in the literature. Such procedures include ultraviolet ozone treatment,27,28 aggressive acid-based baths,10,26 and plasma etching.13,29,30 The majority of users, nevertheless, use as-received cantilevers, only applying simple cleaning methods such as rinsing with organic solvents. Some users perform no precleaning at all. The purpose of this note is to point out that, despite the fact that the AFM community has grown in the last years, apparently little is still known about the contamination of AFM cantilevers. Earlier attempts to draw the attention to this issue have been made by Thundat et al. and by Lo et al., who reported that contamination undoubtedly affects AFM experiments2 and that the main source of organic contamination on new cantilevers comes from their packaging.26 Nevertheless, little effort has been since then in looking for a straigthforward, low-cost, and (20) Berger, R.; Delamarche, E.; Lang, H. P.; Gerber, C.; Gimzewski, J. K.; Meyer, E.; Gu¨ntherodt, H.-J. Science 1997, 276, 2021-2024. (21) Fritz, J.; Baller, M. K.; Lang, H. P.; Rothuizen, H.; Vettiger, P.; Meyer, E.; Gu¨ntherodt, H.-J.; Gerber, C.; Gimzewski, J. K. Science 2000, 288, 316-318. (22) Raiteri, R.; Butt, H.-J.; Grattarola, M. Electrochim. Acta 2000, 46, 157-163. (23) McKendry, R.; Zhang, J.; Arntz, Y.; Strunz, T.; Hegner, M.; Lang, H. P.; Baller, M. K.; Certa, U.; Meyer, E.; Gu¨ntherodt, H.-J.; Gerber, C. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 9783-9788. (24) Lang, H. P.; Hegner, M.; Meyer, E.; Gerber, C. Nanotechnology 2002, 13, R29-R36. (25) Knapp, H. F.; Stemmer, A. Surf. Interface Anal. 1999, 27, 324331. (26) Lo, Y.; Huefner, N. D.; Chan, W. S.; Dryden, P.; Hagenhoff, B.; Beebe, T. P. Langmuir 1999, 15, 6522-6526. (27) Arai, T.; Tomitori, M. Appl. Phys. A 1998, 66, S319-S323. (28) Fujihira, M.; Okabe, Y.; Tani, Y.; Furugori, M.; Akiba, U. Ultramicroscopy 2000, 82, 181-191. (29) Senden, T. J.; Drummond, C. J. Colloids Surf., A 1995, 94, 2951. (30) Feiler, A.; Larson, I.; Jenkins, P.; Attard, P. Langmuir 2000, 16, 10269-10277.

10.1021/la047978k CCC: $27.50 © 2004 American Chemical Society Published on Web 11/18/2004

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effective way of characterizing the most simple, but representative, cantilever and tip property which is its wetting behavior. Therefore, we propose two methods, one for the characterization of the cantilever surface and the other for the characterization of the tip or particle. Both methods are based on microscopic contact angle measurements. Microscopic Contact Angle Measurements. In microscopic contact angle measurements, a small water drop is deposited directly on the cantilever. Upon deposition, the drop spreads depending on the hydrophobicity (contact angle) of the cantilever surface and finally attains a contact radius (a) and a contact angle (Θ), which are measured by video microscopy. The width (w) of the cantilever is required to be at least 2 times larger than the contact radius (a) of the drop; that is, if the drop touches the edges of the cantilever, border effects distort the shape of the drop, flawing the measurement of the contact angle. Microsphere Tensiometry Measurements with the Tip or the Particle. The interactions determined between a tip and a bubble will be qualitatively similar to those observed for a particle interacting with a bubble.31-37 Fielden et al.33 showed that a particle with a zero contact angle will not penetrate the air-water interface and that the interaction is monotonically repulsive with no adhesion, while particles with nonzero contact angles are known to show a snap in (large attractive force resulting in mechanical instability in the cantilever) associated with the capillary force as the particle penetrates the airwater interface. Fielden et al.33 observed a repulsive force owing to Derjaguin-Landau-Verwey-Overbeek (DLVO) interactions prior to the snap in; however, such forces will not likely be resolvable for a small tip. In particle bubble interactions, the lateral drive distance between the point of snap in and the subsequent point at zero force increases with increasing particle size and increasing contact angle.35,37 Determining the contact angle of a tip would require precise knowledge of the geometry and line tension effects;37 however, qualitative comparisons are possible by comparing the breadth of the snap-in region at zero force. The adhesion of the tip will depend on the advancing contact angle of the tip;38 however, the adhesion is also dependent on the applied load,33 resulting in small changes in contact angle being difficult to resolve. Experimental Section AFM Cantilevers. We used V-legged, gold-coated silicon nitride NanoProbes (Veeco Instruments Inc., NY) and rectangular silicon PointProbes (NanoWorld GmbH, Erlangen, Germany) as representatives for imaging cantilevers. Rectangular silicon Scentris probes (Veeco Instruments Inc., NY) were employed as a representative for stress-sensing cantilevers. The cantilevers were used as received for controlling the degree of contamination, afterward cleaned in a plasma reactor for controlling the efficiency of the method, and subsequently stored in a Gel-Box (Gel-Pak Inc., Hayward, CA) for controlling the rate of recontamination. Gel-Boxes are plastic containers standardly used for storing, handling, and shipping microelectronic components as well as (31) Butt, H.-J. J. Colloid Interface Sci. 1994, 166, 109-117. (32) Ducker, W. A.; Xu, Z.; Israelachvili, J. N. Langmuir 1994, 10, 3279-3289. (33) Fielden, M. L.; Hayes, R. A.; Ralston, J. Langmuir 1996, 12, 3721-3727. (34) Preuss, M.; Butt, H.-J. Langmuir 1998, 14, 3164-3174. (35) Preuss, M.; Butt, H.-J. J. Colloid Interface Sci. 1998, 208, 468477. (36) Preuss, M.; Butt, H.-J. Int. J. Miner. Process. 1999, 56, 99-115. (37) Yakubov, G. E.; Vinogradova, O. I.; Butt, H.-J. J. Adhes. Sci. Technol. 2000, 14, 1783-1799. (38) Scheludko, A.; Toshev, B. V.; Bojadjiev, D. T. J. Chem. Soc., Faraday Trans. 1 1976, 72, 2815-2828.

Figure 1. Sequence of two images showing a water drop deposited on a V-legged silicon nitride cantilever before (A) and after (B) plasma cleaning. Before plasma cleaning, the drop forms a contact angle of Θ ) 60° ((4°); after plasma cleaning, the drop forms a contact angle of Θ ) 14° ((6°). cantilevers. The sticky, elastomeric bottom coating is called GEL by the manufacturer, but its composition is not known. Cleaning Procedure. The cantilevers were placed in a plasma reactor (PDC-002, Harrick Scientific Corp., NY) in an argon gas atmosphere at 1 mbar and at medium power for 20 s. Microscopic Contact Angle Experiments. A microdrop system (Microdrop GmbH, Norderstedt, Germany) was used to generate and deposit water drops with a diameter of ≈40 µm onto the cantilevers. The contour of the drops on cantilevers was monitored with a video microscope (Precise Eye, Navitar Inc., NY) from the side in combination with a frame grabber and a charge-coupled device (CCD) camera (WAT-202D, Watec Co. Ltd., Japan). We used a long-distance 5× objective (Mitutoyo Corp, Kawasaki, Japan). In combination with the Navitar microscope, this was capable of a magnification up to ×2000. From video images, the contact radius and contact angle of the drops were obtained. Force-Separation Experiments. Force-separation curves were acquired with a molecular force probe (MFP-1D-SA, Asylum Research, Santa Barbara, CA). Interactions were determined at drive velocities of 1.0 µm/s over a trajectory of 4 µm. Hemispherical air bubbles ∼300 µm in radius were produced in water. Cleaned polystyrene Petri dishes possess a finite contact angle; thus, an air bubble is easily attached to the base of the Petri dish filled with water using a microsyringe. The bubble and the cantilever were visualized using the built-in optics and video camera of the standalone base. Thus, the tip, located at the end of the cantilever, could be positioned at the crest of the bubble within several microns. The MFP fluid cell and a 60 mm polystyrene (Sarstedt AG, Nu¨mbrecht, Germany) Petri dish, used as a receptacle for the water and the air bubble, were cleaned using concentrated solutions of potassium hydroxide and nitric acid followed by rinsing in copius amounts of water immediately prior to use. Solutions were prepared from the respective AnalaR grade chemicals (Sigma-Aldrich Chemie GmbH, Munich, Germany). Throughout this study, distilled water further purified by a Sartorius Arium 611 VF (Sartorius AG, Go¨ttingen, Germany) system was used exclusively. The resistivity of this is 18.2 MΩ‚ cm.

Results and Discussion Microscopic Contact Angle Measurements. Water drops are deposited on one of the most common types of cantilevers used for imaging: contact mode, V-legged, silicon nitride NanoProbes (Figure 1). The average contact angle of a series of as-received cantilevers was Θ ) 60° ((4°). The spherical cap form of the drop is clearly visible (Figure 1A). The drop rests on the area at the end of the cantilever where the two legs meet and does not spread

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Figure 2. Sequence of five images showing a water drop deposited on a rectangular silicon cantilever. Before plasma cleaning (A), the drop forms a contact angle of Θ ) 88° ((1°); after plasma cleaning (B), the drop forms a contact angle of Θ ) 11° ((2°); after storing the cantilever for 1 (C) , 3 (D) , and 24 (E) h in a Gel-Box, the drop forms contact angles of Θ ) 26° ((2°), 45° ((3°), and 72° ((3°), respectively.

to the legs. After plasma treatment, the value of the contact angle decreases to Θ ) 14° ((6°), though here the determination of the angle is difficult, since the drop wets also the two cantilever legs and is flattened (Figure 1B). Other commonly used contact mode cantilevers are the rectangular PointProbes, which have a length of l0 ) 470 µm, a width of w ) 50 µm, and a thickness of t ) 2 µm. The average contact angle of water drops deposited along the longitudinal axis on an as-received cantilever is Θ ≈ 88° ((1°) (Figure 2A). After plasma treatment, the value of the contact angle decreased to Θ ≈ 11° ((2°), but since the drop strongly wets the cantilever, its form is not hemispherical anymore (Figure 2B). To control the recontamination rate, the cantilever was stored in a GelBox, with contact angle measurements being performed after 1, 3, and 24 h (Figure 2C-E). The contact angle gradually increases, being Θ ≈ 72° ((3°) after 24 h, and the contact radius decreases, suggesting that the cantilever becomes more hydrophobic with time.

Notes

Figure 3. Force-separation curves on air bubbles, taken with a V-legged cantilever (A) and with a rectangular cantilever (B). On the left side of each diagram is presented the forceseparation curve for the as-received cantilever; on the right side, that for the plasma-cleaned cantilever is presented. In both diagrams, the approaching (upper) and retracting (lower) curves are offset by an arbitrary value with respect to the zero force axis for avoiding overlapping of parts of the curves.

Microtensiometry Measurements with the Tip. The interactions between an as-received cantilever tip and a bubble varied to the extent that a typical forceseparation curve cannot be given; instead, only example interactions are shown (Figure 3). Each interaction could however be characterized by an initial attractive force upon approach that corresponds to the tip penetrating the air-water interface followed by a repulsive force that can be associated with the bubble’s elasticity and movement of the three-phase contact (TPC) line.35 Upon retraction, interactions always exhibited adhesion, the magnitude of which is related to capillary force associated with the advancing contact angle. Force curves often showed secondary minima upon additional loading (labeled A in Figure 3B). The number, position, and size of these minima were completely random. We believe these secondary minima arise from an inhomogeneous surface chemistry, most likely small localized contaminated regions. When the tip is pressed against the bubble, the TPC recedes back toward the cantilever when the TPC encounters a region of contamination (which possesses a larger contact angle) and the tip is drawn further into the bubble. The apparent random nature of the position at which a secondary minimum is observed may arise from

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desorption and readsorption of contaminants as the interface advances and recedes over the tip. Upon retraction, the force curve reveals relatively large adhesion forces that sometimes possess random secondary maxima (labeled B in Figure 3B) akin to secondary minima in approach data, although these secondary maxima were less frequent than the secondary minima upon approach. After plasma cleaning, consecutive tip-bubble interactions were near identical. Each force curve shows a single minimum associated with the tip penetrating the airwater interface followed by a monotonically increasing repulsion. Furthermore, the snap-in distance of the approach curve was reduced considerably. Upon retraction, the adhesion decreased by an order of magnitude, with no observable secondary maxima. Both findings indicate that the plasma cleaning has significantly reduced the contact angle and has reduced inhomogeneities in surface chemistry, further suggesting that they were a result of contaminants. Conclusions From microscopic contact angle and microsphere tensiometry measurements, organic contaminants (e.g., silicone oils) were found to be present on as-received AFM cantilevers and tips that were shipped and stored in plastic boxes on silicone substrates by the manufacturers. The microscopic contact angle method allows us to directly measure the wettability of the surface of the cantilever. It cannot be used to analyze the contamination of the cantilever tip, since the microdrops are too large

compared to the size of the tip radius. Results of consecutive measurements showed that storing cantilevers in a plastic box after plasma cleaning, even if only for 1 h, causes a contamination layer to be redeposited on the cantilever. This suggests the necessity for further cleaning prior to a new use of the cantilever. Tip-bubble interactions give a qualitative indication on the hydrophobicity of a tip. Prior to cleaning, tips show snap-in distances that are comparatively large compared to the size of the tip, indicating a hydrophobic surface chemistry. Furthermore, the multiple snap-in events indicate an inhomogeneous surface chemistry that was not observed after plasma cleaning. Plasma-cleaned tips were hydrophilic, showing only a small affinity with the air bubble. This suggests that organic contamination is localized and hydrophobic. A well established cleaning procedure, plasma cleaning, was applied on the cantilevers. On the basis of the experimental results, we can conclude that plasma cleaning removes satisfactorily the silicone oil contamination, even from the tips. However, cleaned cantilevers are fully recontaminated if they are stored in plastic boxes for 24 h. Acknowledgment. We thank Hans-Ju¨rgen Butt for discussion and useful comments. This work was supported by a Marie Curie Fellowship of the European Community program Human Potential under Contract No. HPMFCT-2002-02160 (E.B.) LA047978K