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Nanografting of Alkanethiols by Tapping Mode Atomic Force Microscopy Jian Liang† and Giacinto Scoles*,†,‡ Department of Chemistry, Princeton UniVersity, Princeton, New Jersey 08544, and International School for AdVanced Studies and Elettra Synchrotron Laboratory, Trieste, Italy ReceiVed NoVember 20, 2006. In Final Form: March 1, 2007 Nanografting, an atomic force microscopy (AFM) based nanolithography technique, is becoming a popular method for patterning self-assembled monolayers (SAMs). In this technique, a nanoscale patch of a thiol-on-gold SAM is exchanged with a different thiol by the action of an AFM tip operated in contact mode at high load. The results are then imaged in topographic or lateral force microscopy again at low values of the load. One of the problems of contact mode nanografting is that monolayers of large molecules such as proteins are likely to be deformed, damaged, or even removed from the surface by contact mode imaging even when small loads are used. Furthermore, we need to note that the stiffness of the cantilevers used in contact mode is different than that of the cantilevers used in tapping mode and that tip changing in the course of an experiment can be quite inconvenient. Here, we show that a monolayer on a gold substrate can be nanografted using tapping mode AFM (also referred to as amplitude modulation AFM) rather than the commonly used contact mode. While the grafting parameters are somewhat trickier to choose, the results demonstrate that nanografting in tapping mode can make patches of the same quality as those made by contact mode, therefore allowing for gentle imaging of the grafted molecules and the whole SAM without changing the microscope tip.
Introduction Micro- or nanomanipulation of self-assembled monolayers (SAMs) have attracted a large amount of attention because patterned SAMs have many applications such as templates for pattern transfer in microcontact printing,1 components for making nanosensors,2 and building blocks for constructing 3-D supramolecular structures.3 Several scanning probe microscopy (SPM) based lithographic methods have been developed in recent years for patterning SAMs,4 and one of the promising techniques is nanografting.5 In nanografting, the AFM tip is used to scrape away the thiol molecules of the monolayer from the gold substrate by applying a relatively large force load (that depends on the tip radius) while scanning over a flat region of interest in the presence of molecules different than those of the initial SAM, which can be the solute in a neighboring solution or the adsorbate on the tip itself. As revealed by a lower force scan, a new patch of SAM composed of the new molecules is then formed on the exposed gold sites, surrounded by the initial SAM. Nanografting has been successfully applied for various purposes, for example, to make two-dimensional patterns within inorganic,6 organic,7,8 or biological materials;9,10 to detect DNA hybridization at the nanoscale;11-13 to confine De Novo proteins onto gold surfaces;14,15 to build three-dimensional surface-bound biological assemblies;16-21 and to explore nanoscopic elasticity,22 friction,23 and mechanical response to force modulation24 of * To whom correspondence should be addressed. Princeton University, Frick Laboratory, Room 10, Washington Rd., Princeton, NJ 08544. Tel: (609) 258-5570; Fax: (609) 258-6665; E-mail:
[email protected]. † Princeton University. ‡ International School for Advanced Studies and Elettra Synchrotron Laboratory. (1) Xia, Y. N.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. ReV. 1999, 99, 1823. (2) Revzin, A.; Russell, R. J.; Yadavalli, V. K.; Koh, W. G.; Deister, C.; Hile, D. D.; Mellott, M. B.; Pishko, M. V. Langmuir 2001, 17, 5440. (3) Yu, A. A.; Savas, T. A.; Taylor, G. S.; Guiseppe-Elie, A.; Smith, H. I.; Stellacci, F. Nano Lett. 2005, 5, 1061. (4) Kra¨mer, S.; Fuierer, R. R.; Gorman, C. B. Chem. ReV. 2003, 103, 4367. (5) Liu, G.-Y.; Xu, S.; Qian, Y. Acc. Chem. Res. 2000, 33, 457.
organic thin films. It has also been used to characterize the shape of AFM tips25 and to measure the accelerated kinetics of thiols under nanografting conditions.26-28 Finally, several groups have also tested nanografting on surface systems other than thiols on gold.29-31 (6) Garno, J. C.; Yang, Y.; Amro, N. A.; Cruchon-Dupeyrat, S.; Chen, S.; Liu, G.-Y. Nano Lett. 2003, 3, 389. (7) Liu, J.; Cruchon-Dupeyrat, S.; Garno, J. C.; Frommer, J.; Liu, G.-Y. Nano Lett. 2002, 2, 937. (8) Wang, X.; Zhou, D.; Rayment, T.; Abell, C. Chem. Commun. 2003, 474. (9) Garno, J. C.; Amro, N. A.; Wadu-Mesthrige, K.; Liu, G.-Y. Langmuir 2002, 18, 8186. (10) Jang, C.; Stevens, B. D.; Carlier, P. R.; Calter, M. A.; Ducker, W. A. J. Am. Chem. Soc. 2002, 124, 12114. (11) Zhou, D.; Sinniah, K.; Abell, C.; Rayment, T. Langmuir 2002, 18, 8278. (12) Zhou, D.; Sinniah, K.; Abell, C.; Rayment, T. Angew. Chem., Int. Ed. 2003, 42, 4934. (13) Liu, M.; Liu, G.-Y. Langmuir 2005, 21, 1972. (14) Case, M. A.; McLendon, G. L.; Hu, Y.; Vanderlick, T. K.; Scoles, G. Nano Lett. 2003, 3, 425. (15) Hu, Y.; Das, A.; Hecht, M. H.; Scoles, G. Langmuir 2005, 21, 9103. (16) Browning-Kelley, M. E.; Wadu-Mesthrige, K.; Hari, V.; Liu, G.-Y. Langmuir 1997, 13, 343. (17) Wadu-Mesthrige, K.; Xu, S.; Amro, N. A.; Liu, G.-Y. Langmuir 1999, 15, 8580. (18) Wadu-Mesthrige, K.; Amro, N. A.; Garno, J. C.; Xu, S.; Liu, G.-Y. Biophys. J. 2001, 80, 1891. (19) Liu, G.-Y.; Amro, N. A. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5165. (20) Zhou, D.; Bruckbauer, A.; Ying, L.; Abell, C.; Klenerman, D. Nano Lett. 2003, 3, 1517. (21) Zhou, D.; Wang, X.; Birch, L.; Rayment, T.; Abell, C. Langmuir 2003, 19, 10557. (22) Jourdan, J. S.; Cruchon-Dupeyra, S. J.; Huan, Y.; Kuo, P. K.; Liu, G.-Y. Langmuir 1999, 15, 6495. (23) Houston, J. E.; Doelling, C. M.; Vanderlick, T. K.; Hu, Y.; Scoles, G.; Wenzl, I.; Lee, T. R. Langmuir 2005, 21, 3926. (24) Price, W. J.; Kuo, P, K.; Lee. T. R.; Colorado, R., Jr.; Ying, Z. C.; Liu, G.-Y. Langmuir 2005, 21, 8422. (25) Xu, S.; Amro, N. A.; Liu, G.-Y. Appl. Surf. Sci. 2001, 175, 649. (26) Xu, S.; Laibinis, P. E.; Liu, G.-Y. J. Am. Chem. Soc. 1998, 120, 9356. (27) Yu, J.-j.; Tan, Y. H.; Li, X.; Kuo, P.-K.; Liu, G.-Y. J. Am. Chem. Soc. 2006, 128, 11574. (28) Ryu, S.; Schatz, G. C. J. Am. Chem. Soc. 2006, 128, 11563. (29) Horne, J. C.; Huang, Y.; Liu. G.-Y.; Blanchard, G. J. J. Am. Chem. Soc. 1999, 121, 4419. (30) Crampton, N.; Bonass, W. A.; Kirkham, J.; Thomson, N. H. Langmuir 2005, 21, 7884. (31) Headrick, J. E.; Armstrong, M.; Cratty, J.; Hammond, S.; Sheriff, B. A.; Berrie, C. L. Langmuir 2005, 21, 4117.
10.1021/la063385t CCC: $37.00 © 2007 American Chemical Society Published on Web 04/25/2007
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Table 1. Typical Parameters in Nanografting Using Tapping Mode AFMa small force (imaging)
nanografting in solution nanografting in air
large force (nanografting)
drive F (kHz)
drive A (mV)
A setpoint (mV)
estimate force (nN)
drive A (mV)
A setpoint (mV)
estimate force (nN)
170 300
2000 200
200 1000
∼0 2
8000 2000
200 200
11 25
a Different probes have slightly varied dimensions and sharpness, which may result in the diversity of the parameters. A: amplitude; F: frequency.
Among the many applications, there has been increasing interest in confining biomolecules to surfaces via nanografting for the purpose of biosensor fabrication or investigating their electrical and mechanical properties. All the previously published works utilize, however, contact mode AFM requiring next-to-impossible careful control of the normal load to avoid damaging the grafted biomolecules after a few scans. Particularly difficult (if at all possible) are experiments, such as when the properties of a surfaceconfined protein in a given patch are to be imaged many times in the same experimental run.14,15 Tapping mode AFM, in which the cantilever vibrates near its resonant frequency (typically 102 kHz), touching the surface only at the bottom of each oscillating cycle, offers a less-damaging imaging method. In this technique, the oscillation amplitude (and phase) is modulated by the tipsample interaction, and the modulation is detected and used as the feedback control for the piezo extension and retraction, so it is also (more technically) referred to as the amplitude modulation AFM. Tapping mode AFM has already become the routine method for imaging surfaces made of soft materials, including proteins,32,33 DNAs,34,35 polymers,36,37 and numerous other molecules and structures.38-40 Remarkably, the tapping mode AFM technique has also been successfully employed for manipulating nanoparticles and dissecting biomolecules,41 etching polymer thin layers,41,42 and performing nanolithography, such as nanooxidation of silicon surfaces43-47 and dip-pen nanopatterning.48 However, to the best of our knowledge, no attempts to apply tapping mode AFM to nanografting have been reported. Some researchers tried to circumvent the problem by nanografting a functional thiol into the nanopattern and subsequently immobilizing the biomolecules on top of it.10,17-21 However, in some cases, such as investigating the electron-transfer properties through biomolecules, adding a spacer between the biomolecule and the substrate adds unwelcome complications to the problem. Because of the different requirements of the stiffness and resonant (32) Fritz, M.; Radmacher, M.; Cleveland, J. P.; Allersma, M. W.; Stewart, R. J.; Gieselmann, R.; Janmey, P.; Schmidt, C. F.; Hansma, P. K. Langmuir 1995, 11, 3529. (33) Boussaad, S.; Tao, N. J. J. Am. Chem. Soc. 1999, 121, 4510. (34) Hansma, H. G.; Laney, D. E.; Bezanilla, M.; Sinsheimer, R. L.; Hansma, P. K. Biophys. J. 1995, 68, 1672. (35) Kasas, S.; Thomson, N. H.; Smith, B. L.; Hansma, H. G.; Zhu, X. S.; Guthold, M.; Bustamante, C.; Kool, E. T.; Kashlev, M.; Hansma, P. K. Biochemistry 1997, 36, 461. (36) Magonov, S. N.; Reneker, D. H. Annu. ReV. Mater. Sci. 1997, 27, 175. (37) Bar, G.; Thomann, Y.; Brandsch, R.; Cantow, H. J.; Whangbo, M. H. Langmuir 1997, 13, 3807. (38) Liu, Z. F.; Shen, Z. Y.; Zhu, T.; Hou, S. F.; Ying, L. Z.; Shi, Z. J.; Gu, Z. N. Langmuir 2000, 16, 3569. (39) Sauer, B. B.; McLean, R. S.; Thomas, R. R. Langmuir 1998, 14, 3045. (40) Vandamme, N.; Snauwaert, J.; Janssens, E.; Vandeweert, E.; Levens, P.; Van Haesendonck, C. Surf. Sci. 2004, 558, 57. (41) Liu, Z.; Li, Z.; Wei, G.; Song, Y.; Wang, L.; Sun, L. Microsc. Res. Tech. 2006, 69, 998. (42) Klehn, B.; Kunze, U. Superlattices Microstruct. 2002, 31, 19. (43) Garcı´a, R.; Calleja, M.; Rohrer, H. J. Appl. Phys. 1999, 86, 1898. (44) Bae, S.; Han, C.; Kim, M.-S.; Chung, C. C.; Lee, H. Nanotechnology 2005, 16, 2082. (45) Dubois, E.; Bubbendorff, J.-L. Solid-State Electron. 1999, 43, 1085. (46) Kim, B. I.; Pi, U. H.; Khim, Z. G.; Yoon, S. Appl. Phys. A 1998, 66, S95. (47) Fontaine, P. A.; Dubois, E.; Stie´venard, D. J. Appl. Phys. 1998, 84, 1776. (48) Agarwal, G.; Sowards, L. A.; Naik, R. R.; Stone, M. O. J. Am. Chem. Soc. 2003, 125, 580.
frequency of the AFM cantilevers used in contact mode and tapping mode, it is not easy to perform nanografting in contact mode and then image in tapping mode with the same probe.49 To avoid possible perturbations to the biomolecules, the cantilever used for contact mode imaging should have the smallest force constant (the softest). This type of cantilever, however, has the lowest resonant vibration frequency when used in tapping mode and consequently is more likely to be affected by the environmental noise and the limited bandwidth of the feedback loop of the microscope. This conundrum cannot be overcome without changing the probe. Especially, when using the same AFM tip for both contact mode nanografting and tapping mode imaging in liquid, the resonant frequency of the cantilever would be further damped by the medium and would make the problem worse. Another reason for using stiff cantilevers in tapping mode AFM is when operating in air, the adhesion force between the tip and the surface resulting from the meniscus (the water column from humidity) would stick the cantilever to the surface if its force constant is too low and, consequently, the feedback loop would not be able to respond properly. Switching the probe between nanografting and imaging would lose track of the nanoscale patches and consequently would increase substantially the complexity of the experiment. Thus, if we were able to perform nanografting directly using tapping mode, it would enable us, not only to deposit, but subsequently to image or manipulate biomolecules or other soft materials using a gentler mode of AFM. In this paper, we demonstrate the applicability of tapping mode AFM in nanografting by grafting 1-octadecanethiol (C18SH) into 1-decanethiol (C10SH) using tapping mode AFM both in solution and using a dip-pen technique. We focus on two basic issues of tapping mode nanografting. The first of these is if tapping mode nanografting can be applied to fabricating thiol monolayers with well-predictable configurations. The second is how to control the normal force during nanografting in tapping mode. Alkyl thiols are excellent candidates for experiments that aim to answer the first question because they have rather well-known configurations in the monolayers and can be used as reference systems. To evaluate the quality of the acquired patterns, the heights of the patches are statistically analyzed and are compared with the calculated height difference of the two molecules. The control of the normal force, which is a key issue in nanografting, is not trivial because, in tapping mode, the cantilever oscillates at high frequencies which makes it more difficult to experimentally adjust and theoretically calculate the average interaction between the tip and the surface. In this paper, we try to estimate the normal force semiquantitatively and the proper settings of the parameters to perform nanografting in tapping mode are discussed. Experimental Section Sample Preparation. The Au(111) surface was prepared by thermal evaporation on a mica substrate in a vacuum chamber (K. J. Lesker Co.) at a background pressure of 1 × 10-7 mbar. The (49) Hansen, L. T.; Ku¨hle, A.; Sorensen, A. H.; Bohr, J.; Lindelof, P. E. Nanotechnology 1998, 9, 337.
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Results and Discussions
Figure 1. Typical force plots of tapping mode AFM in air. (a) Amplitude vs Z. (b) Deflection vs Z. The black and the blue curves indicate the piezo extending and retracting directions, respectively. muscovite mica (S & J Trading, Inc.) had been heated up to 563 K for 5 h before and was kept at the same temperature during the evaporation. Typically, 1000 Å of gold (CERAC, 99.999% purity) was deposited on freshly cleaved mica at the rate of 0.2 Å/s. During gold deposition, the vacuum increased to and remained at 6∼8 × 10-7 mbar. After metallization, the Au-coated mica was allowed to cool down to room temperature. The chamber was then filled with nitrogen, and the sample was taken out and immediately was immersed into 0.1 mM 1-decanethiol (Sigma-Aldrich, 96% purity)/ 2-butanol (Sigma-Aldrich, 99.5% purity) solution. A compact monolayer was allowed to form on the Au(111) surface for at least 48 h. Before it was characterized by AFM, the sample was rinsed with absolute ethanol (AAPER Alcohol and Chemical Co.) and was dried by a gentle flow of nitrogen. AFM Experiments. As revealed by the AFM, the surface was composed of gold “mesas” with atomically flat tops as large as ∼300 nm in diameter. The flat top was the region on which we performed nanografting. All nanografting and imaging were made using a Digital Instruments MultiMode AFM (Santa Barbara, CA) with a Nanoscope IIIa controller. The scanner was calibrated in the Z direction by measuring atomically resolved gold steps. A singlebeam Si probe (TAP300, NanoDevices) was then used, in tapping mode, to image the surface morphology and to select a flat region. The process of nanografting in contact mode has been described extensively elsewhere.5,50 Tapping mode nanografting follows similar procedures but requires amplitude modulations which will be discussed in detail below. Basically, the drive amplitude and the amplitude setpoint were adjusted to control the normal “force” during nanografting and imaging. Nanografting in solution was carried out in a liquid cell filled with 0.1 mM C18SH/ethanol while nanografting in air was done under ambient conditions where the humidity was kept low at 23 ( 1% (T ) 24 ( 2 °C) by putting the AFM in an acoustic isolation box (Molecular Imaging) with desiccant inside. The typical scan rate for nanografting was 2 Hz. Finally, crosssectional height analysis was carried out on the acquired topographical images. (50) Amro, N. A.; Xu, S.; Liu, G.-Y. Langmuir 2000, 16, 3006.
Nanografting in solution is most commonly used because it offers the flexibility of working in different environments. The liquid cell can be filled with pure organic solvent just for dissolving and transporting the thiols51,52 or with an aqueous solution such as when using a buffer with antioxidants to stabilize the structure of a given biomolecule.14,15 In our experiments, the liquid cell is filled with 0.1 mM C18SH/ethanol. The ethanol can both help the elimination of C10SH from the surface and the transport of C18SH to the exposed gold sites during nanografting. The liquid medium also significantly moves to lower values the vibrational frequency and amplitude of the cantilever from their values in air (Table 1). (a) Force Calibration. To perform nanografting in contact mode, the normal load must exceed a certain threshold to allow the tip to penetrate the organic film and to induce the substitution reaction. However, there is no well-defined value of the “force” in tapping mode AFM, and the interaction between the tip and the surface should be considered an averaged effect. In actual tapping-mode AFM operation, the parameters one can control are the drive amplitude and the amplitude setpoint. The drive amplitude defines the amplitude of the voltage modulation applied to the piezo system that drives the cantilever vibration, while the amplitude setpoint is the root mean square (rms) value of the cantilever deflection as given by the voltage output of the photocell maintained by the feedback loop. Garcı´a and co-workers53,54 derived an analytical relationship between the oscillation amplitude and average tip-surface forces by using the virial theorem and energy conservation principles:
[ ( )]
A ≈ A0 1 - 4
〈Fts〉 F0
2 1/2
(1)
where A is the amplitude setpoint, A0 is the free oscillation amplitude of the cantilever which is proportional to the drive amplitude, 〈Fts〉 is the averaged tip-sample force of one oscillating cycle, and F0 is the driving force. A0 and F0 are related by the following equation:
A0 )
QF0 k
(2)
where Q and k are the quality factor and force constant of the cantilever, respectively. Combining eqs 1 and 2, we get
〈Fts〉 )
k A 2 - A2 2Qx 0
(3)
Equation 3 leads to the conclusion that to increase the averaged tip-sample force, we can either increase the drive amplitude or decrease the amplitude setpoint or both. Because the tip-sample interaction is the cause of the damping of the excited cantilever, increasing the difference between the oscillating amplitude of the cantilever in free air and after being engaged to the sample means that more energy is dissipated by the tip-sample interaction while the average value of the interaction force increases. Equation 3 can qualitatively clarify the amplitude modulation procedure during nanografting but cannot accurately calculate the averaged tip-sample force because of the approximations used during the deduction of the equation.53,54 Because of the complexity of the (51) Xu, S.; Liu, G.-Y. Langmuir 1997, 13, 127. (52) Xu, S.; Miller, S.; Laibinis, P. E.; Liu, G.-Y. Langmuir 1999, 15, 7244. (53) San Paulo, A Ä .; Garcı´a, R. Phys. ReV. B 2001, 64, 193411. (54) Garcı´a, R.; Pe´rez, R. Surf. Sci. Rep. 2002, 47, 197.
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Figure 2. Nanografting in solution using tapping mode AFM. (a) The height image of a typical C18SH patch (100 nm × 100 nm) in the C10SH matrix. (b) The corresponding line profile (∆h ) 1.0 ( 0.2 nm). (c) The histogram of height differences on the basis of 67 patches. The fitted Gaussian curve has the peak position at 0.90 nm.
theoretical analysis of the tip motion and the tip-surface interaction, the analytical description or numerical simulation of the force in tapping mode AFM is still a partially open question.55-58 However, using eq 3 to estimate the normal load is still possible, as demonstrated by the following. First, we take the k value as 40 N/m, as offered by the manufacturer. The Q value in solution is measured by tuning the cantilever in solution and dividing the resonant peak frequency by the fwhm of the peak. To deduce A0 and A, we obtained the force plots of the piezo extension versus amplitude and deflection as shown in Figure 1. Figure 1a demonstrates how the cantilever’s oscillating amplitude decreases as the tip is positioned closer to the sample. When the tip is clear of the surface, the oscillating amplitude is constant (section CD). When the tip is pressed tightly against the surface, it stops vibrating and the amplitude is 0 (section AB). The voltage difference between these two values is A0 in the unit of the voltage measured by the photodiode. Figure 1b shows that even as the tip’s oscillating amplitude is dampened by the surface, the average deflection is unchanged (section FG). Only after the oscillation fully ceases, pressing the tip further causes the average deflection to increase (section EF). This section relates the bending of the (55) Lee, M. H.; Jhe, W. H. Phys. ReV. Lett. 2006, 97, 036104. (56) Kowalewski, T.; Legleiter, J. J. Appl. Phys. 2006, 99, 064903. (57) kokavecz, J.; Horvath, Z.; Mechler, A. Appl. Phys. Lett. 2004, 85, 3232. (58) Stark, R. W.; Schitter, G.; Stemmer, A. Phys. ReV. B 2003, 68, 085401.
cantilever in the Z-direction with the signal of the photodiode, from which we can convert the unit of A0 to that of length. In a similar way, we can derive A at a certain setpoint. Then, we can use eq 3 to calculate the normal load at certain A0 and A, and the results are shown in Table 1. These semiquantitative data indicate that the nanografting force is greater than 10 nN, in agreement with previous results obtained in contact mode.5,14 Although there are two ways to increase the normal force, we found that increasing the drive amplitude can fabricate better patches than decreasing the amplitude setpoint. This phenomenon can be explained by the fact that if the amplitude setpoint is too low, the tip is driven so close to the surface that it almost stops vibrating (similar to contact mode). The contact force in this case would be very high because of the stiffness of the tapping mode cantilever. (b) Nanografting of C18SH Patches into a C10SH SAM in Solution Using Tapping Mode AFM. We chose long-chain alkanethiols in our nanografting experiments because it has been proven by various techniques that in a stable, ordered phase the hydrocarbon chains in the SAMs tilt by approximately 30° with respect to the surface normal.59,60 By measuring the height difference between the patch and the matrix SAM, the height of the grafted molecules can be determined and the conformation (59) Ulman, A. Chem. ReV. 1996, 96, 1533. (60) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103.
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Figure 3. Nanografting in air using tapping mode AFM. (a) The height image of a typical C18SH patch (100 nm × 100 nm) in the C10SH matrix. (b) The corresponding line profile (∆h ) 1.0 × 0.2 nm). (c) The histogram of height differences on the basis of 32 patches. The fitted Gaussian curve has the peak position at 0.86 nm. (d) Another C18SH patch showing the contaminations around (indicated by the arrows).
can be deduced. If the conformation agrees with the wellestablished model, it means that tapping mode nanografting, similarly to contact mode, can reliably fabricate thiol monolayer patches with predictable conformations. The height difference between C18SH and C10SH has been established, both by calculation and by nanografting in contact mode, to be 0.9 nm.14 Figure 2a shows a typical image of a C18SH patch in a C10SH SAM made by nanografting using tapping mode in solution. The cross-sectional analysis (Figure 2b), which makes direct height measurements of surface features by producing line profiles and averaging over them, indicates that the height difference between the C18SH and C10SH is 1.0 nm, in fair agreement with the calculated value (however, see below). Considering the atomic defects on the gold surface, the imperfection and the compressibility of the SAM, and the deviation of the calculated model from the real nature of the SAM, the error on the height measurement can reach 0.25 nm which is approximately the height of a step on a Au(111) surface. Thus, it is necessary to make multiple patches and to construct a histogram of the height difference as shown in Figure 2c. The Gaussian fit of the histogram clearly shows a peak at 0.9 nm. The statistics also indicates that the average height difference is 0.90 nm with standard deviation of 0.16 nm, which confirms that nanografting in tapping mode can fabricate patches of layers with well-predictable molecular configuration.
(c) Nanografting of C18SH Patches into a C10SH SAM in Air Using Tapping Mode AFM. Although nanografting in solution is common in most cases, (dip pen) nanografting in air is also useful because of its simplicity when applied to patterning nanodevices that are expected to function under ambient conditions. Tapping mode nanografting in air can be, therefore, a good supplement to nanografting in solution. Nanografting in air requires the preadsorption of the to-be-grafted molecules on the AFM tip.43 When the applied force exceeds the threshold, the molecules will transfer and bind to the surface, replacing the initial molecules. The parameters for controlling the oscillating amplitudes during nanografting in air, which are also listed in Table 1, are similar but not identical to those in solution. Table 1 shows, while imaging at small force using tapping mode AFM in air, relatively small drive amplitude can achieve large amplitude setpoint, indicating less damping by air than by liquid media. To calculate the normal load from eq 3, the k and Q values in air are taken as 40 N/m and 300, respectively, as offered by the manufacturer. A0 and A are derived following the same procedure as described in the previous sections, and the results are also shown in Table 1. The numbers indicate that the force needed for nanografting is greater than 10 nN, again in good agreement with previous results obtained in contact mode and proving the validity of this method for estimating the average force in tapping mode AFM. In our experiments, 32 C18SH patches were made
Nanografting of Alkanethiols by Tapping Mode AFM
into C10SH SAMs, and the statistics (Figure 3c) indicate that the average height difference is 0.86 nm with standard deviation of 0.15 nm, which suggests that nanografting in air using tapping mode can also make high-quality patches composed of a densely packed monolayer (Figure 3a and b). Interestingly, in some of the AFM images captured in air, we can see some “bumps” around the patch which might be the spilled molecules or contaminants from the atmosphere (Figure 3d). These contaminations are not seen in solution probably because they would be dissolved into the liquid phase or under contact mode AFM in air because they tend to be “swept” away by the frictional force of the probe. While the presence of these “impurities” cannot be totally removed when using tapping mode AFM, it can be reduced by lowering the concentration of the thiol “ink” used for dipping in the tip before nanografting.
Conclusions and Outlook We have demonstrated that tapping mode AFM can successfully be employed to perform nanografting on SAMs made of alkyl thiols on Au(111), both in solution and in air, by increasing the difference between the drive amplitude and the amplitude setpoint. A histogram of the height differences between the grafted C18SH patches and the C10SH matrix shows a peak at 0.9 nm, which agrees well with the calculated value for compact monolayers and previous nanografting results using contact mode AFM. Tapping mode nanografting can pattern the SAM as well as contact mode but does not exert damaging forces to the surface,
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which can deform the structure under examination. This technique opens up a new route to depositing and subsequently imaging biomolecules or other molecules that are not strongly anchored to the surface. It would be quite interesting now to compare the quality of grafting (i.e., the compactness) of two patches made using contact mode and tapping mode nanografting. This comparison could be carried out using lateral force imaging in contact mode or phase imaging in tapping mode on both patches using the monolayer matrix into which both would be grafted in as a common reference. Previous work in our and in other laboratories26,61 has shown that lateral deflections can give reliable information on the compactness of monolayers when all other conditions such as the applied load remain constant. This type of experiment, which is presently being attempted in our laboratory, is more difficult than the experiments reported here but would be very interesting because the literature correlating the phase modulation and the compactness of the monolayer is still scarcely available. Acknowledgment. This work was supported by the National Science Foundation (MRSEC Program) through the Princeton Center for Complex Materials (DMR 0213706) and was partially supported by DOE under Grant No. DE-FG02-93ER45503. We gratefully thank Dr. Cristian Staii for valuable discussions. LA063385T (61) Scaini, D.; Liang, J.; Casalis, L.; Scoles, G. to be published.
