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2006, 110, 10231-10233 Published on Web 05/06/2006
Dynamic Meniscus Growth at a Scanning Probe Tip in Contact with a Gold Substrate Brandon L. Weeks*,† and James J. DeYoreo‡ Department of Chemical Engineering, Texas Tech, Lubbock, Texas 79409, and Lawrence LiVermore National Laboratory, LiVermore, California 94551 ReceiVed: March 14, 2006; In Final Form: April 21, 2006
Environmental scanning electron microscopy was used to investigate the dynamic meniscus growth at a cantilever in contact with a substrate. The meniscus was observed to take many minutes to reach an equilibrium state. The observed growth rate is similar to initial patterning rates observed from dip-pen nanolithography and suggest that the meniscus growth may be the rate-limiting step in initial pattering rates.
The atomic force microscope (AFM) has emerged as a powerful tool for direct patterning of materials on a variety of substrates with nanometer precision. Direct writing deposition of an organic material using an AFM was first demonstrated by Jaschke and Butt in 1995.1 In that work, octadecanethiols (ODT) were deposited on mica substrates, and the deposition was not well-controlled. Indeed, they concluded that ODT could not be deposited on gold, a substrate to which ODT is known to coordinate. By carefully controlling the environmental conditions, the Mirkin group learned to control deposition and form surface-immobilized stable nanostructures. This technique is known as dip-pen nanolithography (DPN).2 DPN has emerged as a very powerful tool with high special resolution primarily for patterning organic materials. By far, most of the studies of DPN have been done with alkanethiol inks on gold substrates. However, a wide range of inks can be patterned with DPN including metals, DNA, proteins, and polymers.3 The mechanism of transport is still a source of controversy. Initial studies suggested that a meniscus formed between the tip and the substrate aided in the transport of the ink. This model appears to hold true for soluble and slightly soluble inks such as DNA, 16-mercaptohexadecanoic acid (MHA), and metal salts, where the transport rate increases with increasing relative humidity. For insoluble materials, such as ODT, there is very little dependence on humidity, and the contribution from the meniscus, if any, cannot easily be explained.4 In all cases, the meniscus has been assumed to form instantaneously, and the condensation time required to form the meniscus has never been introduced into transport models. The rate of transport from the AFM tip can vary with a number of parameters including the ink composition, the surface energy of the substrate, and the environmental conditions (temperature and humidity). Recent work by Hampton et al. showed that the overall transport rate decreases significantly with increasing contact time.5 Interestingly, the effect was independent of ink, and both MHA and ODT showed a similar transition from high patterning rates to slow pattering rates around 300 s. The transition in the patterning rate was attributed *
[email protected]. † Texas Tech. ‡ Lawrence Livermore National Laboratory.
10.1021/jp0615914 CCC: $33.50
to the effects of initial patterning of inks on subsequent ink diffusion rates through changes in the surface energy of the substrate. An extension of their work demonstrates that changes in the surface energy can alter the rates of the inks being patterned significantly.6 Changing the hydrophobicity of the substrate, either intentionally or by contamination, affected both the surface diffusion of inks and the shape of the meniscus.6 Although Hampton et al. did not investigate the effect of humidity in their work, the role of the meniscus could explain the differences in patterning rates for both short and long contact times. Peterson et al. did investigate the effect of humidity on patterning rates over time with a constant dwell time.7 In that work, high patterning rates at the initial writing phase were attributed to depletion of ink near the cantilever tip. The relaxation time was dependent on methods of coating the tip and ranged from 20 min to nearly 2 h. A direct experiment investigating the dependence of relaxation time on relative humidity was not performed. Changes of patterning rates have also been observed by other groups, although on a much shorter time scale than observed by Hampton et al.8 A transition from fast to slow patterning rates at much shorter times than investigated by Hampton et al.6 was attributed to dissolution kinetics, but the meniscus was assumed to form quickly compared to the time scale for dissolution of the ink into the meniscus. In this work, we use environmental scanning electron microscopy (ESEM) to directly observe meniscus formation9 as a function of time. We show that the meniscus takes time to reach an equilibrium width and suggest that this may account for the higher transport rate at short contact times observed in previous studies. ESEM can image in various atmospheres (water vapor, nitrogen, air, etc.) at a relatively high pressure as compared to standard scanning electron microscopy. The instrument works at these higher pressures by utilizing a special detector such that the gas molecules amplify the secondary electrons emitted from the surface being imaged. By using a Peltier cooling stage and varying the pressure of water vapor within the instrument, the relative humidity (RH) can be controlled and calculated precisely, allowing images to be collected from 0% to ∼100% RH. Images collected using ESEM have shown that the © 2006 American Chemical Society
10232 J. Phys. Chem. B, Vol. 110, No. 21, 2006
Figure 1. ESEM series of meniscus formation on a cantilever tip at RH ) 98%. Each image takes approximately 30 s to collect. (A) t ) ∼80 s. (B) t ) ∼160 s. (C) t ) ∼450 s.
meniscus formed on an AFM tip in contact with a surface can clearly be observed at high humidity.10 The instrument used for these studies was an FEI XL-30 field emission ESEM. The imaged cantilevers were Thermo Microscopes11 model MSCT-AUHW Sharpened Microlevers with tip “A”. The cantilevers were mounted with their tips in contact with gold-coated silicon substrates by gluing the glass substrate holding the cantilever directly onto the silicon surface with epoxy. Care was taken to be sure no adhesive was near the tip, and this was confirmed by ESEM micrographs. Images were collected with the cantilever parallel to the electron gun allowing direct imaging of the meniscus formation at the tip/substrate interface. Images were obtained at 5 °C, and the relative humidity was controlled by varying the water vapor pressure within the ESEM chamber. The saturation point is clearly visible as water condenses over the entire surface of the sample. All images shown here were collected at 98% relative humidity in order to clearly observe the meniscus width and were collected over a period of time to determine the rate of growth of the meniscus. Each image took about 30 s to collect. The result was a dynamic series of images showing meniscus growth vs time. Figure 1 shows a series of images collected at 98% relative humidity over a total time of ∼6 min. After equilibrium, the meniscus is very stable and does not change over an extended period of time. The width of the meniscus was measured from the most obvious point of contact with the surface (where a change in contact angle is observed). The increase in the width of the meniscus is not an effect of gravity, since the substrate is vertical within the ESEM. The dynamics of meniscus growth in other systems have primarily been extensively studied by a variety of experimental and modeling techniques.12-14 The condensation times have been shown to strongly depend on the temperature, the relative humidity, the curvature of the meniscus, and the roughness of the surfaces in contact. Figure 2A shows the contact area of the meniscus with time. Initially, the meniscus grows rapidly and then reaches an equilibrium width after ∼500 s. For the initial growth phase of the meniscus, a surface coverage rate can be calculated by dividing by time. The initial rate is 0.03 µm2/s and slows to ∼0.01 µm2/s at 450 s prior to the meniscus reaching equilibrium. These rates match closely to the transport rates observed for patterning of both MHA and ODT (21% RH) at short contact times, which is
Letters shown in Figure 2B. The results from this work show a slightly higher surface coverage rate which may be attributed to the higher humidity used in the ESEM. AFM adhesion vs contact time experiments were performed to investigate the effect of contact time on the adhesive force.15,16 The results implied that the meniscus takes from many seconds to minutes to form, as deduced from the increased force required to remove the tip from the surface. The forces measured were dependent on both the relative humidity and the surface energy of the substrate. At a relative humidity below 10%, there was no effect of the contact time, suggesting that the formation of the meniscus is not relevant at low humidity. However, when the humidity is increased above 20%, the pull-off force increases with contact time dramatically. The adhesive force is a function of humidity and generally increases with higher humidity. This result supports our direct observation that the meniscus takes time to form and must be considered in the deposition process for DPN. An analytical solution for the meniscus growth dynamics in our system does not currently exist. The solution would require simultaneous solution of the equations for Fickian diffusion17 and Kelvin condensation.18 But one can make some estimate of the relative importance of the two processes in limiting the rate of meniscus growth. The volume of the meniscus at the equilibrium point can be calculated by treating the meniscus as a truncated cone with the volume occupied by the tip subtracted out. This gives a total volume of water of 3.9 µm3. By treating the water as an ideal gas in the vapor phase under the environmental conditions used for imaging, we calculate that a total vapor volume of 5e-8 L is required to provide sufficient water vapor to create a meniscus of that size. Taking the characteristic time τ for diffusion from that volume to the tip to be τ ≈ V2/3D-1, where D is the diffusivity of H2O in the ESEM chamber, gives τ ≈ 1 s for D ≈ 10-3cm2/s. This is 2-3 orders of magnitude less than the time scale for meniscus formation. Although this number is a rough calculation at best, it does suggest that the environmental boundary conditions can be treated as a source of constant concentration and that the rate of meniscus formation is dominated by the kinetics of Kelvin condensation. Since the Kelvin equation predicts the equilibrium shape of the meniscus, the complexity in the solution comes from the dynamics of the meniscus growth where at the initial growth phase there is a high curvature in the meniscus leading to a lower meniscus partial pressure. As the meniscus grows, the curvature decreases and the pressure within the meniscus eventually reaches equilibrium within the chamber.14 In the case of DPN, the meniscus growth may account for the change in patterning rate observed with increasing contact times. When the AFM tip is first brought into contact, the meniscus grows, and ink is spread rapidly on the basis of the rate of meniscus growth. Transport of ink through the meniscus
Figure 2. (A) Contact area of the meniscus with the substrate over time. After 600 s, the shape is stable. (B) Comparison of surface coverage rate of the meniscus growth compared to patterning rates of ODT and MHA during short contact times and RH ) 21% (ref 5). The results indicate that, during initial patterning, the meniscus growth may account for the faster patterning rates observed.
Letters may occur either through the bulk meniscus or on the surface of the meniscus. However, the mechanism of transport is not important as long as there is sufficient ink to produce adequate coverage on the substrate as the meniscus grows. When the meniscus reaches an equilibrium shape, the growth of features can only occur by surface diffusion across the substrate being patterned leading to patterns much larger than the meniscus. This proposed model would hold true only if the meniscus had to re-form for every feature. This would always be the case when the tip was removed from the surface between each feature patterned. However, most often the tip is in contact with the surface throughout the entire patterning process and is rapidly moved to subsequent positions to prevent writing between features. Depending on the translation speed, the meniscus could travel along with the tip or need to re-form at each writing point. To date, there has been no study of the dependence of deposition rate vs contact time on the translation speed between features. The work presented in these studies did not contain any adsorbed inks on the surface of the cantilever or the gold substrate. Clearly, ink on the surface may change the shape of the meniscus by changing the contact angle. In the case of ODT, the contact angle of the tip would be significantly lower than that of MHA. However, since the substrate is initially uncoated, the contact angle would be independent of the ink on the tip until a layer of material was formed. The final meniscus shape will be dependent on the tip shape and the surface energy of the substrate and tip.19 At short contact times, a meniscus would still form, independent of ink, and would explain why both ODT and MHA show a transition in patterning rates at similar times. But this still leaves the mechanism of transport involving the meniscus open to debate. This work demonstrates that the meniscus takes many minutes to form and confirms conclusions based on past experiments using force measurements as well as recent experiments measuring deposition rates for long contact times. The equilibrium time from the ESEM data is much longer than observed in DPN and can be attributed to the decreased pressure within
J. Phys. Chem. B, Vol. 110, No. 21, 2006 10233 the environmental stage, which impacts both mass transport and condensation kinetics. Clearly, the meniscus condensation times need to be accounted for in future DPN transport models. Acknowledgment. The authors thank Jose Saleta (UCSB) for ESEM assistance. This work was performed under the auspices of U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory, under contract #W-7405-Eng-48. References and Notes (1) Jaschke, M.; Butt, H. J. Langmuir 1995, 11, 1061. (2) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S. H.; Mirkin, C. A. Science 1999, 283, 661-663. (3) Ginger, D. S.; Zhang, H.; Mirkin, C. A. Angew. Chem., Int. Ed. 2004, 43, 30-45. (4) Sheehan, P. E.; Whitman, L. J. Phys. ReV. Lett. 2002, 88, 15. (5) Hampton J. R.; Dameron A. A.; Weiss P. S. J. Phys. Chem. B 2005, 109 (49), 23118-23120. (6) Hampton J. R.; Dameron A. A.; Weiss P. S. J. Am. Chem. Soc. 2006, 128 (5), 1648-1653. (7) Peterson, E. J.; Weeks, B. L.; De Yoreo, J. J.; Schwartz, P. V. J. Phys. Chem. B 2004, 108, 15206-15210. (8) Weeks, B. L.; Noy, A.; Miller, A. E.; De Yoreo, J. J. Phys. ReV. Lett. 2002, 88, 255505. (9) Schenk, M.; Fu¨ting, M.; Reichelt, R. J. Appl. Phys. 1998, 84, 4880. (10) Weeks, B. L.; Vaughn, M. W.; DeYoreo, J. J. Langmuir 2005, 21 (18), 8096-8098. (11) Now Veeco Instruments, Santa Barbara, CA. (12) Restagno, F.; Bocquet, L.; Biben, T. Phys. ReV. Lett. 2000, 84, 2433-2436. (13) Gokhale, S. J.; Plawsky, J. L.; Wayner, P. C., Jr. J. Colloid Interface Sci. 2003, 259, 354-366. (14) Sharma, A. Langmuir 1998, 14, 4915-4928. (15) He, M. Y.; Blum, A. S.; Aston, D. E.; Buenviaje C.; Overney R. M.; Luginbuhl R. J. Chem. Phys. 2001, 114, 1355-1360. (16) Sedin, D. L.; Rowlen, K. L. Anal. Chem. 2000, 72, 2183-2189. (17) Carslaw, H. S.; Jaeger, J. C. Conduction of Heat in Solids; Oxford University Press: Oxford, United Kingdom, 1959. (18) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces; Wiley: New York, 1997. (19) Jang, J.; Schatz, G. C.; Ratner, M. A. Phys. ReV. Lett. 2003, 90, 156104.
