Langmuir 1999, 15, 7897-7900
7897
A New Tool for Studying the in Situ Growth Processes for Self-Assembled Monolayers under Ambient Conditions Seunghun Hong, Jin Zhu, and Chad A. Mirkin* Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208 Received August 13, 1999 A new tool for studying self-assembled monolayer (SAM) nucleation and growth processes on faceted inorganic substrates under ambient conditions is reported herein. The methodology involves coating an atomic force microscope tip with molecules of interest and raster scanning across a substrate of interest with the modified tip. Large scale images of the deposition process provide kinetic information about the nucleation and growth of SAMs. The water meniscus, which is the transport medium in these experiments, is proposed to strongly influence the growth process for 1-octadecanethiol and 16-mercaptohexadecanoic acid. The former involves an island formation and subsequent growth behavior while the latter shows evidence of more random growth. Lattice-resolved images of both SAM structures have been obtained at the end of film growth, and in the case of the 1-octadecanethiol SAM, lattice-resolved images were obtained for the SAM islands. The virtues of this novel technique are its simplicity, adsorbate generality, and serial nature, which allows one to control monolayer growth in a step-by-step process with concomitant imaging capabilities.
Introduction We wish to report a new tool for studying self-assembled monolayer (SAM) nucleation and growth processes on faceted inorganic substrates. The methodology reported herein derives from a soft nanolithography process, termed dip-pen nanolithography (DPN), which was developed and reported recently by our group.1 DPN is a process that involves casting a molecule-based ink onto an atomic force microscope (AFM) cantilever and then transporting that ink, via raster scanning, to a substrate of interest through the water capillary that naturally forms between the tip and substrate when the experiment is conducted in air,1 Scheme 1A. In most cases, an ink that is capable of reacting with the substrate is used to help facilitate the process. Herein, we report data that suggests we can use this tool to monitor the nucleation and growth of self-assembled monolayers in situ under ambient conditions. This novel tool is complementary to some of the existing methods for studying nucleation and growth via synchrotron in-plane X-ray diffraction,2,3 helium diffraction,4 scanning probe microscopy (SPM),5-10 and other techniques;11 its virtues * To whom correspondence should be addressed.
[email protected].
Scheme 1
E-mail:
(1) (a) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S.; Mirkin, C. A. Science 1999, 283, 661. (b) For a discussion of water transport by AFM, see: Piner, R. D.; Mirkin, C. A. Langmuir 1997, 13, 6864. (2) (a) Fenter, P.; Schreiber, F.; Berman, L.; Scoles, G.; Eisenberger, P.; Bedzyk, M. J. Surf. Sci. 1999, 425, 138. (b) Schwartz, P.; Schreiber, F.; Eisenberger, P.; Scoles, G. Surf. Sci. 1999, 423, 208. (c) Fenter, P.; Schreiber, F.; Berman, L.; Scoles, G.; Eisenberger, P.; Bedzyk, M. J. Surf. Sci. 1998, 413, 213. (d) Eberhardt, A.; Fenter, P.; Eisenberger, P. Surf. Sci. 1998, 397, L285. (3) Wasserman, S. R.; Whitesides, G. M.; Tidswell, I. M.; Ocko B. M.; Pershan, P. S.; Axe, J. D. J. Am. Chem. Soc. 1989, 111, 5852. (4) Camillone, N.; Eisenberger, P.; Leung T. Y. B.; Schwartz P.; Scoles G.; Poirier G. E.; Tarlov M. J. J. J. Chem. Phys. 1994, 101, 11031. (5) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145. (6) Woodward, J. T.; Schwartz, D. K. J. Am. Chem. Soc. 1996, 118, 7861. (7) Yamada, R.; Uosaki, K. Langmuir 1998, 14, 855. (8) Tamada, K.; Hara, M.; Sasabe, H.; Knoll, W. Langmuir 1997, 13, 1558. (9) Larsen, N. B.; Biebuyck, H.; Delamarche, E.; Michel, B. J. Am. Chem. Soc. 1997, 119, 3017. (10) Xu, S.; Cruchon-Dupeyrat, S. J. N.; Garno, J. C.; Liu, G.-Y.; Jennings, G. K.; Yong, T.; Laibinis, P. E. J. Chem. Phys. 1998, 108, 5002.
are its simplicity, apparent generality, and serial nature, which allows one to control monolayer growth in a stepby-step process with concomitant imaging capabilities. It should be noted that, prior to our invention of DPN, attempts to use an AFM to transport alkanethiols to Au substrates on the nanometer length scale were unsuccessful.12 The importance of the meniscus in controlling the ink transport process was apparently not recognized. Results and Discussion For proof-of-concept, we provide two different examples of SAM nucleation growth as monitored by DPN. The first involves 1-octadecanethiol (ODT), which forms a hydrophobic SAM, while the second involves 16-mercaptohexa(11) Karpovich, D. S.; Blanchard, G. J. Langmuir 1994, 10, 3315. (12) Jaschke, M.; Butt, H. J. Langmuir 1995, 11, 1061.
10.1021/la991095p CCC: $18.00 © 1999 American Chemical Society Published on Web 10/13/1999
7898
Langmuir, Vol. 15, No. 23, 1999
Letters
Figure 1. Consecutive 1.5 µm × 1.5 µm LFM images showing the ODT SAM growth process. The lighter contrast areas represent areas of higher friction. The total painting requires 19 raster scannings over the scan area. Each frame required 51 s to acquire. Only the first, seventh, ninth, and nineteenth frames are presented. (A) The rectangular area, marked by the white dotted line, on the bottom has been prepainted with an ODT SAM. The remaining gray area is unmodified Au(111). The stark white, high friction areas are the deep valleys surrounding the Au(111) facets. (B) The seventh image after raster scanning, showing SAM nucleation sites. (C) The ninth image showing the formation of larger SAM islands. (D) The nineteenth frame showing a contiguous monolayer. (E) Lattice resolved, raw data image of an ODT SAM island (darker area) and bare Au(111) surface (white area), imaged with an uncoated tip. Inset shows the FFT from the ODT SAM island area of the image. (F) Kinetics of ODT SAM growth.
decanoic acid (MHA), which forms a hydrophilic SAM. ODT and MHA were chosen for these studies since their Au(111) surface modification properties are well-known
and because they can be easily differentiated from bare Au via lateral force microscopy.13 All experiments were performed on a ThermoMicroscopes CP AFM.
Letters
Langmuir, Vol. 15, No. 23, 1999 7899
Figure 2. Consecutive 2 µm × 2 µm LFM images showing the MHA SAM growth process. The lighter contrast areas represent areas of higher friction. The first, ninth, and twelfth frames out of a 12-frame experiment. (A) The rectangular area, denoted by the black dotted line, on the bottom has been prepainted with an MHA SAM. The remaining gray area is unmodified Au(111). (B) The ninth image after raster scanning, showing increased friction (whiter) as compared with Au(111). (C) The twelfth frame showing a contiguous monolayer. (D) Lattice-resolved image of MHA SAM deposited by DPN (raw data, imaged with a MHA-coated tip). Inset shows the FFT.
AFM cantilevers (ThermoMicroscopes, Microlever A) are loaded with ODT or MHA by immersing the cantilevers into acetonitrile solutions saturated with the adsorbate of interest. After being soaked for 1 min, the cantilevers are removed from the solutions and blown dry with compressed difluoroethane. Then, the alkanethiol-coated tips are used to cover one portion of an Au(111)/mica substrate with a SAM of adsorbate by raster scanning it with the ink-coated tip (5 Hz, 0.1 nN, relative humidity ) 30 ( 5%) several times over the designated area while monitoring the magnitude of the lateral force between tip and substrate via an oscilloscope. In other work, we have shown that there is a characteristic 1 order of magnitude reduction in lateral force after a SAM of ODT is deposited onto an Au substrate.1 This “prepainting” is done so that we have standard SAM and bare Au(111) areas for comparison throughout the SAM growth experiment. After (13) Wilbur, J. L.; Biebuyck, H. A.; MacDonald, J. C.; Whitesides, G. M. Langmuir 1995, 11, 825.
prepainting (denoted by the white dotted rectangles in Figure 1A-D), as ODT is transported to the Au by raster scanning over a 1.5 µm × 1.5 µm area (5 Hz, 0.1 nN), the lateral force decreases in a nonuniform fashion, which is recorded via lateral force microscopy (LFM) as a series of dark spots on the bare Au(111) substrate (lighter contrast areas), panels B and C of Figure 1. Images are taken in a serial fashion as the deposition process occurs, allowing one to arrest the growth process at any time during the experiment. As this is done, one can see dark sites forming and growing on the lighter Au(111) background. The dark areas, which we assign to ODT SAM nucleation sites, eventually grow into larger domains, which ultimately coalesce and form a contiguous monolayer structure after 19 passes over the scan area with the AFM tip, Figure 1D. (See Supporting Information for images of this process on a single Au(111) terrace.) High-resolution LFM of a monolayer-modified terrace shows that the contiguous monolayer is crystalline with
7900
Langmuir, Vol. 15, No. 23, 1999
the expected hexagonal lattice parameter of 5.0 ( 0.2 Å9 and no detectable large scale defects. Significantly, even the nucleation sites exhibit evidence of crystallinity and the correct lattice parameter for an ODT SAM, Figure 1E.5 Note that the bare Au areas, surrounding the SAM islands, exhibit the expected lattice parameter of 2.9 ( 0.2 Å (see Supporting Information). The quality and structures of the SAMs prepared via the DPN method are remarkably similar to those prepared by conventional solution and vapor deposition methods (at least as measured by AFM), which shows that the contact force AFM has minimal, if any, effect on the structure of the SAM generated via the DPN method. Finally, although we do not rule out the possibility of the so-called “striped phases” detected in ultrahigh vacuum by STM and X-ray methods at low adsorbate dosings, under the conditions of the experiments reported herein, we do not see evidence for them. By integrating the dark SAM-covered areas as a function of scan frame, one can extract information about the growth kinetics for a particular monolayer system. By doing this for the ODT system, we see three distinct growth periods, Figure 1F. There is a relatively slow, initial nucleation process followed by a fast island growth process and then finally a slow saturation process. This is similar to what others have observed via X-ray and atom diffraction techniques used to study the deposition of SAMs of shorter alkanethiols.2 Note that there is not a directional influence on the SAM island growth process arising from the lateral raster scanning of the ODT-coated AFM tip across the Au(111) substrate (i.e., the islands grow in isotropic fashion); indeed, although an analogous negative force (-0.1 nN; i.e., the tip is attracted to the surface) contact mode scanning experiment has been used to study such processes, it yields virtually identical data. This strongly suggests that these are measurements of the monolayer self-assembly process rather than tip-directed writing of a monolayer. In other words, the tip controls the general path and area for monolayer deposition, but the formation of the monolayer within this path area is driven by natural adsorbate diffusion from the tip through the water to the substrate. Interestingly, the MHA adsorbate exhibits strikingly different behavior from ODT. If we monitor the growth process for MHA on Au(111), we again can differentiate monolayer-modified areas from the Au(111) on the basis of relative friction, Figure 2A. Here, lighter contrast areas (higher friction) are spots where the hydrophilic SAM has been deposited onto the Au(111) while darker contrast areas are bare Au(111). Significantly, we see no evidence for distinct nucleation sites as observed in the ODT system. Instead, we see a uniform increase in friction as the MHA is deposited in the scan area. This continues until a full
Letters
monolayer is deposited and the terrace areas are uniformly white, Figure 2A-C. High-resolution LFM of a monolayermodified terrace shows that the SAM of MHA, like the SAM of ODT, has a (x3×x3)R30° structure, Figure 2D. There are several features of these experiments worth noting. First, the growth process for the MHA SAM is drastically different from the growth process for the ODT system. This difference is attributed to the very different hydrophobicities of the two SAMs and their interaction with the water meniscus, Scheme 1. In the case of MHA, the adsorbate molecules are hydrophobic, but the SAM is hydrophilic. In the case of ODT, both adsorbate molecule and SAM are hydrophobic. For the MHA system, the water meniscus can effectively wet both the Au surface and the areas with deposited molecules, Scheme 1B. Therefore, in the case of MHA the effective DPN contact area, which is controlled by the size of the meniscus and the surface tension of the water droplet with respect to the surface, grows as more molecules are deposited (i.e., the wetting properties of the substrate increase with MHA deposition), Scheme 1. In the case of ODT, the wettability of the surface decreases as the ODT is deposited from the tip to the Au, thereby localizing monolayer growth and establishing welldefined nucleation sites. Summary DPN provides a relatively simple but powerful way of studying monolayer nucleation and growth processes with a conventional AFM. The results reported herein suggest that this will be a general approach to obtaining such data for both hydrophobic and hydrophilic SAMs under ambient conditions. Finally, it is important to note that these data are a reflection of growth processes involving a water transport medium. However, DPN is not limited to water, and in principle, in a controlled environment one could study these processes as a function of a range of different solvent transport media. These studies are underway. Acknowledgment. The AFOSR is gratefully acknowledged for support of this work. Several instruments used in this study were supported by the Materials Research Center at NU, which is funded by the NSF through the MRSEC Program. We also wish to acknowledge R. P. Van Duyne for helpful discussions. Supporting Information Available: Single terrace LFM images showing ODT SAM island growth and a latticeresolved LFM image of an Au(111) area between ODT SAM islands. This material is available free of charge via the Internet at http://pubs.acs.org. LA991095P
