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Adhesion Forces Measured by Atomic Force Microscopy in Humid Air Anal. Chem. 2000, 72, 2183-2189 Articles Adhesion Forces Measured by Atomic Force Microscopy in Humid Air Dana L. Sedin and Kathy L. Rowlen* Department of Chemistry and Biochemistry, University of Colorado, Boulder Colorado 80309 Adhesive forces measured with an atomic force micro- scope under ambient conditions are generally regarded to be dominated by non-surface-specific capillary force. In this study, the nature of the ªpull-offº force on a variety of surfaces was investigated as a function of relative humidity. The results indicate that even under the condi- tion where capillary condensation occurs there is chemical specificity in the measured pull-off force. Issues such as tip-surface contact time and surface roughness were ruled out as possible artifacts. A mathematical model of pull-off force as a function of relative humidity is proposed Figure 1. Graphical representation of a capillary bridge formed in which the chemical specificity is explained. between an AFM tip and surface. Rt is the radius of curvature for the AFM tip, ψ is the fill angle, r1 and r2 are the principal radii of curvature Accurate determination of adhesive forces on the molecular of the meniscus, θ1 is the contact angle for water on the AFM tip, - level has led to significant insight in many fields of science.1 3 and θ2 is the contact angle for water on the surface. Atomic force microscopy (AFM) provides a simple and sensitive (piconewton) means of determining adhesive forces with a high of environmental conditions including ambient, vacuum, controlled degree of spatial resolution.2 Numerous forces have been studied gas, and liquid. However, due to the large capillary force exerted by AFM. For example, by modifying AFM tips, colloidal forces,4 on the tip in the presence of water vapor, most measurements hydrophobic and hydrophilic forces,5 and chiral forces6 have been made with the AFM are conducted under liquid.4-7 The liquid studied. Single bond rupture forces have also been measured by environment typically yields adhesion forces that are 1 or 2 orders AFM.7 of magnitude less than the same measurements made in humid Using AFM, adhesive forces are probed by measuring the air.8 degree of distortion in a cantilever as the tip is retracted from Capillary force results from condensation of water vapor the surface. Interaction forces can be studied under a wide range between the surface and AFM tip during contact. When condensa- * To whom correspondence should be addressed: (phone) (303)492-3631; tion occurs, a capillary bridge (as shown in Figure 1) forms be- - (fax) (303)492 5894; (e-mail) [email protected]. tween the tip and surface. Capillary condensation tends to occur (1) Craig, V. Colloids Surf. A 1997, 129-130,75-93. (2) Prater, C. B.; Maivald, P. G.; Kjoller, K. J.; Heaton, M. G. Digital Instruments on hydrophilic surfaces. The equilibrium radius of the capillary Application Note, 1995. bridge meniscus has long been described by the Kelvin - (3) Capella, B.; Dietler, G. Surf. Sci. Rep. 1999, 34,1 105. equation:9,10 (4) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239-241. (5) Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science - 265 - 1 1 1 γLV 1994, , 2071 2074. + ) r ) (6) McKendry, R.; Theoclitou, M.; Rayment, T.; Abell, C. Nature 1998, 391, [ ] K (1) r1 r1 RT log(p/po) 566-568. (7) Williams, J. M.; Han, T.; Beebe, T. P. Langmuir 1996, 12, 1291-1295. (8) Weisenhorn, A. L.; Hansma, P. K.; Albrecht, T. R.; Quate, C.F. Appl. Phys. Lett. 1989, 26, 2651-2653. where r1 and r2 are the principal radii of curvature of the meniscus 10.1021/ac991198c CCC: $19.00 © 2000 American Chemical Society Analytical Chemistry, Vol. 72, No. 10, May 15, 2000 2183 Published on Web 04/13/2000 (as shown in Figure 1), rK is the Kelvin radius, γL is the surface deflection by its spring constant. The tips used were Si3N4 (Digital tension of the liquid, V is the molar volume of the liquid, R is the Instruments NP, 100 µm cantilever length, 20-40 nm radius of gas constant, T is the temperature, and p/po is the relative vapor curvature, 0.1-0.4 N/m spring constant). Before and after each pressure. The Kelvin equation has been experimentally verified experiment, the quality of the AFM tip was evaluated by imaging for radii as small as 4 nm.10 Burnham and co-workers estimated a vapor-deposited silver film (∼5 nm height) on mica. The tip was a capillary force by calculating the Laplace pressure acting over rejected if artifacts were observed in the silver film image (for an area.11 They approximated the surface area of the meniscus example, multiple tip contact points). Force curves were obtained between an AFM tip and a flat surface to be 2πRth, where Rt is under the following conditions: 1.97 Hz Z scan rate, 512 points the radius of the tip and h is the height of the meniscus. The collected per line, and the average count set to 1. Contact time 11 capillary force (Fcap) may then be estimated by was varied by adjusting the Z scan rate. The contact time was calculated by dividing the sample travel distance while the tip is ) Fcap 2πRthγL/rK (2) in contact by the average velocity of the sample. Muscovite mica and highly ordered pyrolitic graphic surfaces were cleaved prior to use. Quartz (ESCO, S1-UVB) and p-doped In this expression, F is nonsurface specific since its magnitude cap silicon 〈111〉 (Silicon Quest International) were cleaned by depends only on the characteristics of the liquid (i.e., γ ). L sequential sonication (∼5 min) in chloroform (Fisher Scientific, Several researchers have investigated the effect of relative ACS grade), acetone (Fisher Scientific, ACS grade), and methanol humidity on AFM.12-19 Early on, a study by Hansma and co- (Burdick and Jackson, HPLC grade) or treated in a boiling piranha workers demonstrated a 100-fold reduction in pull-off forces on solution of 70% H SO (Mallinckrodt) and 30% H O (Fisher, ACS mica when the experiment was conducted under liquid water 2 4 2 2 grade) for 15 min. The two different cleaning procedures resulted rather than in humid air.8 That study, combined with the fact that in different water contact angles on the surfaces. After cleaning, the Kelvin equation is regarded only as ªa useful guidelineº,20 may the quartz and silicon surfaces were dried under a flow of USP account for general statements in the literature that, in the regime nitrogen. of capillary condensation between an AFM tip and the surface, The AFM was enclosed in a Molecular Imaging CleanLoad little, if any, chemically specific information can be obtained by glovebox. Relative humidity within the glovebox was controlled pull-off force measurements.21 While such may be the case for by an adjustable ratio of dry to wet nitrogen (USP). The water certain surface/tip combinations, this investigation of pull-off used to humidify the chamber was distilled and deionized (18 forces as a function of relative humidity provides strong evidence MΩ). Humidity levels were monitored continuously by a Vaisla to the contrary. RS80 radiosonde. Prior to measurement, the humidity was allowed to reach equilibrium as indicated by a constant value for >3 min. EXPERIMENTAL SECTION For experiments conducted on two substrates, mica (∼0.5 cm × Deflection curves were obtained with a Molecular Imaging 1 cm) was glued (Digital Imaging adhesive tabs), or held down PicoSPM in contact mode with an S scanner (6 µm lateral range). using a Molecular Imaging liquid cell, to quartz or silicon (∼1 Cantilever spring constants were calculated from the resonance cm × 1 cm). A polonium source (Static Master, 1c200) was placed frequency of the cantilever.22 The effect of water vapor adsorption over both samples to reduce electrostatic effects between the AFM onto the spring constant of the cantilever was regarded as tip and surface. Surfaces were heated to 100 °C for 1 min, when negligible based on work by Thundat and co-workers.18 Cantilever the relative humidity in the chamber dropped below 10%, to drive deflection values were obtained offline using the offline/view/ off any water from the surface. The samples were not heated for graph subroutine in the Nanoscope software (4.23r2). Adhesive experiments at or above 50% relative humidity. Typically, three force was calculated by multiplying the appropriate cantilever force curves were obtained on both samples from approximately 0 to 80% relative humidity, in steps of 5-10%. (9) Israelachvili, J. Intermolecular and Surface Forces; Academic Press: London, 1991. Silicon samples were roughened in a solution of NH4OH 23,24 (10) Fisher, L. R.; Israelachvili, J. N. J. Colloid Interface Sci. 1981, 80, 528-541. (Mallinckrodt)/H2O (1/5) at 80 °C for varying amounts of time. (11) Burnham, N. A.; Colton, R. J.; Pollock, H. M. J. Vac. Sci. Technol. 1991, 9, To ensure that SiO2 was present on the surface of silicon after 2548-2856. (12) Fujihira, M.; Aoki, D.; Okabe, Y.; Takano, H.; Hokari, H.; Frommer, J.; roughening, the samples were placed in a boiling piranha solution 25,26 Nagatani, Y.; Sakai, F. Chem. Lett. 1996, 7, 499-500. for 15 min. The presence of SiO2 was experimentally verified (13) Fuji, M.; Machida, K.; Takei, T.; Watanabe, T.; Chikazawa, M. Langmuir using a Nicolet Impact 410 FT-IR. Tapping mode AFM images of 1999, 15, 4584-4589. (14) Sugawara, Y.; Ohta, M.; Konishi, T.; Morita, S.; Suzuki, M.; Enomoto, Y.
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