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Preparation and Characterization of Self-Assembled Monolayers of Octadecylamine on Mica Using Hydrophobic Solvents J. J. Benı´tez,*,† S. Kopta,‡ D. F. Ogletree,‡ and M. Salmeron‡ Instituto de Ciencia de Materiales de Sevilla, Centro Mixto CSIC, Universidad de Sevilla, Avda. Ame´ rico Vespuccio s/n, Isla de la Cartuja, Sevilla 41092, Spain, and Materials Science Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720 Received November 2, 2001. In Final Form: May 10, 2002 The formation of self-assembled monolayers of octadecylamine on mica from chloroform solutions has been studied ex situ with atomic force microscopy (AFM). Growth occurs in the form of islands with structure and properties that depend critically on preparation conditions. Inside the islands, the molecules are oriented such that they expose the terminal CH3 group and are tilted with respect to the normal direction of the surface. The weak bond between mica and the polar amino (-NH2) group results in weak mechanical stability of the islands, which are easily damaged by the AFM tip during repetitive scans. The influence of several preparation parameters on the islands’ structure has been analyzed and discussed on the basis of a model involving residual water molecules on the mica substrate.
1. Introduction The possibility of preparing monolayers of ordered molecules on a substrate by self-assembly from solutions is very interesting from the scientific and technological point of view. Layers with very specific properties can be obtained in this way and used in various applications such as optoelectronic devices,1 biosensors,2 and surface protection and lubrication additives.3-5 More recently, these layers have also been employed as models for biological systems.6,7 Self-assembled monolayers (SAMs) also provide excellent model systems with structures and compositions that can be designed through suitable preparation and can then be studied using high-resolution imaging techniques such as atomic force microscopy (AFM). Compared to Langmuir-Blodgett (LB) films, SAM films are much easier to prepare and require less instrumentation. The formation of a SAM is assumed to be driven by diffusion of molecules and competition to strongly adsorb on specific surface sites.8 The formation of compact layers is a result of attractive van der Waals cohesive forces between the molecules. The requirement of flat surfaces for efficient AFM operation and strong anchoring for monolayer stability has concentrated most of the research on systems such as alkanethiols on gold and alkylsilanes on silicon, glass, or * To whom correspondence should be addressed. E-mail:
[email protected]. † Universidad de Sevilla. ‡ University of California. (1) Ulman, A. Thin Films: Self-Assembled Monolayers of Thiols; Academic Press: New York, 1998. (2) Schierbaum, K.; Weiss, T.; van Velzen, E. T.; Engbersen, J.; Reinhoudt, D.; Go¨pel, W. Science 1994, 265, 1413. (3) Zamborini, F.; Crooks, R. Langmuir 1998, 14, 3279. (4) Xiao, X.; Hu, J.; Charych, D.; Salmeron, M. Langmuir 1996, 12, 235. (5) Maboudian, R. MRS Bull. 1998, 23, 47. (6) Motesharei, K.; Myles, D. J. Am. Chem. Soc. 1994, 116, 7413. (7) Mrksich, M.; Whitesides, G. M. Annu. Rev. Biophys. Biomol. Struct. 1996, 25, 55. (8) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: New York, 1991.
mica substrates. However, it will be interesting to expand this field into systems with weaker molecule-substrate interaction and analyze the influence of other events competing with molecular adsorption. In this paper, we study self-assembled monolayers of octadecylamine on mica and the influence of preparation conditions on their structure. Data about the preparation of ordered monolayers of octadecylamine is scarce in the literature. Lee9 reported that stable octadecylamine LB films are obtained only from alkaline solutions and stated that the stability of the monolayer increases with increasing surface pressure. Hazell et al.10 also reported that such LB films may be structured at the time of preparation but may not survive the drying process that occurs when ultrahigh vacuum analytical techniques are applied. We found that the nature of the solvent used in the preparation of the alkylamine SAM is crucial in determining the structure of the deposited films. We have therefore explored the effects of solvent type, particularly in regard to their hydrophobic (e.g., chloroform), hydrophilic, or amphiphilic character (such as alcohols). The present paper is the first of a series that summarizes the research performed in our laboratory on this subject. 2. Experimental Section Octadecylamine (Fluka, >99%) was used as received and dissolved in chloroform (Aldrich, 99.8%) to obtain 15 mM solutions. Samples (3 × 1 cm) of muscovite mica (KAl2(Si3AlO10)(OH)2, Mica New York Corp.) were cleaved on both sides at ambient conditions of 20 °C ((1) and 40-50% relative humidity (RH) and quickly immersed in the solution for periods of time ranging from seconds to days. The samples were subsequently removed and dried without rinsing under a N2 stream for several minutes. They were kept in a test tube until analyzed (usually after overnight ripening). The atomic force microscope head is home-built and operates inside a box that provides sound isolation and humidity control. It is controlled by RHK electronics and software. Contact Si3N4 (9) Lee, Y. Langmuir 1999, 15, 1796. (10) Hazell, L. B.; Rizvi, A. A.; Brown, I. S.; Ainsworth, S. Spectrochim. Acta 1985, 40B, 739.
10.1021/la011629y CCC: $22.00 © 2002 American Chemical Society Published on Web 07/04/2002
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Figure 1. Topographic and friction images of octadecylamine islands self-assembled on mica after immersion into a 15 mM solution in chloroform. The applied load is 0 nN. Dark regions in the bottom image indicate lower friction. The top graph on the right-hand side (A) shows an island profile. The middle graph (B) shows force-distance curves during retraction on top of the amine island and on the bare mica revealing adhesion forces of 18 and 47 nN, respectively. The bottom graph (C) shows a friction force loop over the mica and across an alkylamine island. cantilevers (NanoProbes, Digital Instruments, Santa Barbara, CA) with 0.12 N/m nominal force constant were employed. The z-distance scale (normal to the surface) was calibrated by using a commercial test silicon grating (TGZ01, Silicon-MDT, Moscow) with a nominal step height of 25.5 nm and also in situ by generating a 1 nm deep hole on the mica surface scanning under a large load.11 Special care was taken to electronically compensate the small cross-talk between vertical (topographic) and horizontal (friction) signals in the photodiode detector, a feature that is part of the capabilities of the electronic controller.
3. Results Adsorption and self-assembly of the octadecylamine molecules on mica under the preparation conditions described above for short immersion and long ripening times produces discrete, densely packed islands with rounded edges. The islands, typically 0.3-0.5 micron wide and 1.6 nm high, have low friction compared to the surrounding uncovered mica, as shown in Figure 1. Uncertainty in the height measurement is typically 0.1 nm. The friction and pull-off force values obtained on the islands are comparable with those obtained for silanes on mica,12 consistent with an orientation of the molecule where the terminal methyl (-CH3) group is exposed and the polar amino (-NH2) termination is facing the mica surface. The measured island height is shorter than the nominal length of the molecule calculated from bond (11) Kopta, S.; Salmeron, M. J. Chem. Phys. 2000, 13 (18), 8249. (12) Lio, A.; Charych, D. H.; Salmeron, M. J. Phys. Chem. B 1997, 101, 3800.
distances and angles (2.4 nm), indicative of a tilted orientation with an angle of 49° with respect to the normal direction. High-resolution imaging on the islands was attempted in an effort to determine the existence of periodicity, using the mica background as reference. Although low loads were used to avoid damage, no structure could be observed in either topography or friction images. Instead, after successive scans over the same area the periodic structure of mica was revealed. This was true even at applied loads of -15 nN (i.e., in the adhesive regime), 3-4 nN short of the pull-off point. Damage was progressively caused to the octadecylamine layer until the underlying mica was finally imaged, as shown in Figure 2. In parallel to this, the friction loop, shown in Figure 2, evolved from an initial low value to one that corresponds to bare mica under the same experimental conditions. The damage to the film was still observable after 12-24 h. The amine islands could also be removed by rinsing in chloroform, indicative of the weakness of the moleculesurface bond and in contrast to the case of thiols on gold and silanes on silicon, glass, or even mica. Besides their mechanical and chemical weakness, the amine films exhibited remarkable stability in air. No change in the height or morphology of the islands was observed for several months after preparation. We will now describe in detail the effects on the film morphology of changing the preparation conditions. Important parameters include immersion time in the solution, ripening time (outside the solution), humidity,
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Figure 2. Topographic image acquired on top of an octadecylamine island (top left) showing damage (holes) caused by successive scans at low loads (0 nN). The magnified friction images (6.5 × 6.5 nm) and their fast Fourier transforms (bottom) show no lattice resolution at first and the periodicity of the mica substrate several scans later. The graph on the top right shows the evolution of the friction loop upon successive scans over the same area of an octadecylamine island. The amplitude of the forward and backward traces, shown by the arrow bars, increases in successive scans.
Figure 3. Overview summarizing the influence of preparation conditions on the morphology of octadecylamine islands on mica obtained from chloroform solutions. The starting point is after a 30 s adsorption followed by ripening at room conditions (20 °C, 40-50% RH) for 30-60 min (center image). The changes observed are as follows: Top left, ripening for 48 h at room conditions leads to larger and more compact islands. Top right, rinsing by dipping in pure chloroform for 3-5 s leads to removal of most of the islands. Bottom right, adsorption time in the solution for 24 h produces numerous small islands. Bottom left, low humidity (below 1%) gives rise to small and defective islands.
and use of posterior rinses with the solvent. The images in Figure 3 illustrate and summarize the findings. 3.1. Ripening. Figure 4 shows images illustrating the time evolution of islands in air after preparation (ripening). Freshly prepared islands, with diameters below 0.5 µm, show fuzzy edges as well as many internal holes (Figure 4A). After ripening overnight at ambient conditions, the island edges become sharper, as shown in Figure 4B. In parallel with this, the friction contrast is also increased. The holes inside the islands are no longer visible at this scan range. The diameter of the larger islands increased, while the smaller ones disappeared, a result of molecular
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diffusion and incorporation into the larger ones, as in Ostwald ripening. The ripened islands, Figure 4C, are larger, more compact, and exhibit lower friction. In addition, there is a decrease in height of about 0.35 nm upon aging, as shown in the cursor profiles across the islands in the same figure. Such a height change, from 1.6 to 11.5 nm ((0.1 nm), is routinely reproduced. 3.2. Immersion Time. Another important parameter controlling the morphology of the islands formed by the octadecylamine molecules is the adsorption time, that is, the time the sample remains immersed in the chloroform solution. Densely packed, round islands are obtained after short adsorption times (seconds to a few minutes), as shown in Figure 5A. Unexpectedly, however, longer immersion times in the octadecylamine solution do not lead to a higher coverage (within the range of a few minutes). If the immersion time is increased from minutes to an hour, the number of islands is actually reduced and their borders lose their initial convex aspect, becoming rough instead, as shown in Figure 5B. After longer times of 2448 h, smaller and more numerous islands develop, as shown in Figure 5C. Occasionally, for adsorption times around an hour, islands with shapes as in Figure 5B,C coexist. 3.3. Humidity. Samples have been prepared both in air (40-50% RH) and under a dry N2 atmosphere. In the latter case, mica was cleaved, immersed, dried, and stored for several hours inside a glovebox with a RH value below 1% (the humidity sensor detection limit). The samples prepared under a dry environment were analyzed under normal room conditions a few minutes later and found to consist of loosely packed islands (=0.5 micron diameter) with defects, fuzzy edges, and weak friction contrast, as shown in Figure 6A. The height of the island is smaller than 1.6 nm and changes slightly from one to another, indicating that these loosely packed islands are easily deformed by pressure exerted by the tip. A few successive scans over the same area at relatively low loads (0 nN) are sufficient to displace them outside of the scanned region. Ripening under laboratory conditions (40-50% RH), on the other hand, results in growth and coalescence between islands. After 24-48 h, large islands (typically 1-1.5 micron wide) with sharp edges, still showing some regions with defects, are observed as shown in Figure 6B. These islands are more robust, as shown by their lower friction properties. In these same conditions, islands prepared in air already have rounded edges and no observable vacancies (see Figure 4C). 4. Discussion The first conclusion to be drawn from our experiments is that while bonding forces between alkylthiols and gold and alkylsilanes and mica are strong enough to permit contact imaging with AFM, the weak interaction between alkylamine molecules and mica makes these samples easy to damage, even at very low loads (near the pull-off point) by repetitive scanning. The second and important conclusion is the decisive role of water in the adsorption, aggregation, and stability of the alkylamine films. Mica is a very hydrophilic substrate, particularly when clean after cleavage. Previous studies in our laboratory have shown that a water layer forms readily when the RH is above 30-40% at room temperature. The water film saturates around 80% RH, and multilayers form above that.13 (13) Bluhm, H.; Inoue, T.; Salmeron, M. Surf. Sci. 2000, 462, L599.
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Figure 4. Effect of ripening in air on the structure and height of octadecylamine islands on mica: (A) 30 min after preparation, (B) overnight ripening, and (C) 2 days later. Cursor profiles over islands in each image are shown at the bottom. Notice the presence of numerous defects in (A) and the lower height (by 0.35 nm) of the islands in (C).
Figure 5. AFM images showing the influence of immersion time in the chloroform solution on the morphology and height of octadecylamine islands on mica: (A) 30 s, (B) 30 min, and (C) 24 h followed by overnight ripening. Cursor profiles are shown in the graphic panel.
After immersion in the hydrophobic chloroform solvent containing the octadecylamine molecules, only partially covered surfaces are obtained. The experimental observations are best explained by assuming that the water layer is displaced totally or partially from the mica/chloroform interface and that the amphiphilic octadecylamine molecules bind with their amino (-NH2) groups only on residual patches of water on the surface. The degree of coverage by the amine islands would then be directly correlated with the extent of the residual water patches. This is also likely determined by the relative kinetics of water displacement and amine adsorption on the immersed mica. The affinity of the amino group for water is then the driving force for bonding. Other contributions such as the coordination between amino groups and with
the K ions present on the mica surface might also play a role, although our data are presently insufficient to draw conclusions in this respect. Water appears also to increase the diffusivity of loosely bound molecules that are probably lying between islands and facilitates their aggregation into islands. This explains why exposure of the sample to a humid ambient for extended periods of time results in the growth of islands and the removal of point defects and aggregates of defects, such as holes in the islands. This ripening process also results in islands that are more stable to normal and shear forces applied by the tip. As mentioned above, however, even then the mechanical stability is weaker than that of alkylthiols on gold and alkylsilanes on oxide surfaces. The diffusivity of the molecules will depend of their
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agreement with two of the predicted tilted states with thicknesses of 1.66 (47°) and 1.29 nm (58°), respectively. Although the chemistry at the amine-water-mica interface is not known, it likely involves protonation of a fraction of the octadecylamine molecules (-NH2 f -NH3+). As a reference, using tabulated acid/base equilibrium constants, in an aqueous 15 mM solution of ethylamine (CH3-CH2-NH2) protonation should be about 15%. Because of the higher concentration of amine molecules in the island with respect to the solution and because the basic strength of alkylamines decreases with the length of the chain, it is likely that the value is lower for our octadecylamine SAM. Changes in the amount of water and/or in the structure of the water/amine network at the amine-mica interface during prolonged processes may lead to an increase of electrostatic repulsion between protonated groups. In this sense, molecular tilting could be a mechanism to reduce such repulsion, since it results in an increased separation between the headgroups.17 5. Conclusions
Figure 6. Topographic and friction images of octadecylamine islands prepared in a dry environment (A) and after 48 h of ripening in ambient air (40-50% RH) (B). Island growth and densification is observed from topographic images and in the increased friction contrast.
state of aggregation. Once they are part of a densely packed island, their diffusivity is reduced as observed from the permanent damage caused by the tip upon scanning. After reaching an initial and relatively rapid equilibrium with the chloroform solution, the continued immersion of the samples results in the disaggregation and/or removal of the amine film. This indicates that other reactions of molecular exchange between sample and solution take place with much slower kinetics. These reactions probably involve the loss of surface water into the dry chloroform solution. In the case of rinsing, of course, there is the additional effect of redissolution of the amine molecules into the solvent. The reason for the observed decrease in the height of the islands after prolonged ripening and immersion time is not clear at present. It could be due to the adoption by the molecules of different tilt angles, perhaps in response to the growth of a water structure between the molecules and mica. It is known that self-assembled layers of alkyl chain molecules can adopt several packing geometries with discrete tilt angles of the chains. Stepwise tilting of alkylsilanes on mica14 and alkanethiols on gold15 was observed by increasing the pressure exerted by the tip. Tilted phases can also be formed spontaneously.16 The heights of 1.6 and 1.25 nm observed in the present experiments for octadecylamine molecules are in good (14) Barrena, E.; Kopta, S.; Ogletree, D. F.; Charych, D. H.; Salmeron, M. Phys. Rev. Lett. 1999, 82, 2880. (15) Barrena, E.; Ocal, C.; Salmeron, M. J. Chem. Phys. 2000, 113, 2413. (16) Barrena, E.; Ocal, C.; Salmeron, M. J. Chem. Phys. 2001, 114, 4210.
Molecularly thin films of octadecylamine self-assembled on mica can be prepared by immersion of the sample in chloroform solutions. The films are always in the form of islands. Bonding between mica and octadecylamine is much weaker than that of alkylsilanes, and rinsing with the solvent easily removes the molecules. The molecular films are mechanically weak, and repetitive scanning with the AFM tip, even using loads close to the pull-off point, causes damage. The binding of the molecules to the mica surface is strongly dependent on the presence of water. The formation of islands is in fact attributed to the presence of residual water patches from the original film covering the mica that are not completely displaced by the hydrophobic chloroform solvent during immersion. Water also facilitates the diffusivity of the molecules on the surface and accelerates the ripening process leading to aggregation into large islands of densely packed molecules and the elimination of defects. The octadecylamine molecules are found to be tilted away from vertical and most likely expose their terminal -CH3 groups. Two different island heights are observed differing by about 0.35 nm, which likely correspond to different tilt angles. Their stability is probably related to the amount and/or arrangement of water molecules at the interface with mica. Acknowledgment. Dr. Jose´ J. Benı´tez gratefully acknowledges financial support from a Spanish Ministerio de Asuntos Exteriores and NATO Scientific Committee grant. This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Department of Energy, under Contract No. DE-AC03-76SF00098. LA011629Y (17) Salmeron, M.; Kopta, S.; Barrena, E.; Ocal, C. Atomic Scale Origin of Adhesion and Friction; NATO ASI; Kluwer Academic Publishers: Dordrecht, 2001.
