Breaking the Intermolecular Hydrogen Bonds in Organic Dimers with

CRYSTAL GROWTH & DESIGN 2001 VOL. 1, NO. 5 347-353

Articles Breaking the Intermolecular Hydrogen Bonds in Organic Dimers with Atomic Force Microscopy Liesbeth I. de Jong - van Steensel,*,‡ Margot M. E. Snel,‡ Gerrit J. K. van den Berg,‡ Leonardus W. Jenneskens,§ and Jan P. J. M. van der Eerden‡ Department of Interfaces and Department of Physical Organic Chemistry, Debye Institute, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands Received May 18, 2001;

Revised Manuscript Received July 10, 2001

ABSTRACT: Microcrystals of 1,1′-bicyclohexyliden-4-one oxime (oxime) were prepared with the spin-coating technique. In these crystals, the oxime molecules form dimers through intermolecular hydrogen bonding. In this orientation, the crystals have a hydrophobic surface. The crystals were imaged with atomic force microscopy (AFM) at three different temperatures, i.e., 16, 20, and 30 °C. We show that it is possible to break the dimeric hydrogen bond with the AFM tip. The force necessary to accomplish this is temperature dependent and increases with decreasing temperature; its magnitude is on the order of the strength of the dimer bond. Breaking of the dimeric hydrogen bonds only occurs near the step edges. In these regions, a clear contrast in friction imaging was observed with lateral force microscopy together with a reduction of the step height to half the original value. We interpreted this as the removal of one layer of oxime molecules resulting in an (oxime-terminated) hydrophilic surface with a higher friction contrast as compared to the normal hydrophobic surface. In this way, the surface properties of the crystals can be tuned. This concept may be applicable to other systems. Introduction Originally, scanning tunneling microscopy (STM), invented in 1982,1 and atomic force microscopy (AFM), developed in 1986,2 were used to determine the morphology of a sample surface without inflicting damage. However, soon after, researchers found that scanning could also alter the surface, and AFM/STM were rapidly used to modify surfaces at the micrometer to nanometer scale in a controlled way.3-7 For instance, it is possible to remove surface layers of a lead pyrophosphate crystal layer-by-layer.7 The AFM tip can also be used to displace material on a sample surface with dip-pen nanolithography.8,9 Apart from the huge experimental efforts to modify surfaces with scanning probe microscopy (SPM), a lot of theoretical research has been done to identify sensible experiments.10 Nowadays, controlled manipulation of atoms or molecules with SPM and its application to the development of novel devices or memory systems are being studied extensively. Manipulating surfaces need not always be accompanied by topographic changes. Therefore, various meth* To whom correspondence should be addressed. [email protected]. ‡ Department of Interfaces. § Department of Physical Organic Chemistry.

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ods have been developed to visualize surface changes based on other contrast mechanisms. One of these methods is lateral force microscopy (LFM) originally developed by Mate et al. in 1987.11 In LFM, the lateral twisting of the cantilever is sensed which is related to the amount of friction the cantilever experiences on the sample surface. Therefore, this technique is also called friction force microscopy or friction imaging. On slippery surfaces, the twisting will be less than on rough surfaces. Hence, LFM is sensitive to both chemical and physical material properties. For example, LFM can distinguish between OH and Br terminated self-assembled monolayers (SAMs) of alkanethiols on gold12 and is also capable of visualizing orientation differences of CH3 terminated SAMs.13 Noy et al.14 showed that high friction forces are always observed in the hydrophilic regions when scanning patterned hydrophobic/ hydrophilic substrates. This is even valid for measurements in air where one has to deal with capillary forces regardless whether the tip is coated with Au or modified with groups such as COOH. Besides sample properties, the lateral twisting of the cantilever is also dependent on many cantilever-specific factors including the exact tip shape (which does not have to be constant even during one measurement due to adhesion of material

10.1021/cg015525x CCC: $20.00 © 2001 American Chemical Society Published on Web 08/15/2001

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Figure 1. Schematic representation of the oxime molecule.

on the tip) and the (lateral) spring constant. This requires a precise calibration of each cantilever before every measurement for which several methods exist.15-17 For these reasons, LFM measurements are often used only qualitatively. An extensive molecular dynamics simulation study on the theoretical description of friction forces encountered in LFM was recently published by Fujihira and Ohzono.18 Among other things, they describe the influence of the chain length and the packing density of (models for) organic molecules in a monolayer on the appearance of friction images. In the past decades, much research has been done on constructing molecular building blocks to make supermolecular structures with specific properties that can be influenced by external stimuli. These “designer solids” might be applicable in many different fields such as microelectronics or in sensing systems. In this context, a new class of materials has been proposed based on oligo(cyclohexylidene) molecules that can be functionalized with various end groups.19 It has been shown that oligo(cyclohexylidenes) end-functionalized with oxime groups form microcrystals on solid supports after deposition by spin-coating.20-22 Microcrystals can be formed either during the deposition procedure or after the deposition procedure during a ripening period ranging from 1 day up to several days. The length of the ripening period depends on the length and degree of saturation of the compound used. During the ripening period, Ostwald ripening occurs, and the thin amorphous layer originally covering the substrate disappears, meanwhile showing microcrystal formation. One of the compounds that forms microcrystals during the spin-coating process is 1,1′-bicyclohexyliden-4-one oxime (Figure 1), briefly called oxime. Oxime is an amphiphilic molecule with the hydrophobic part consisting of two cyclohexylidene rings and the hydrophilic part of an oxime (NOH) group, and it has an overall rodlike geometry.23-25 The molecules form dimers via intermolecular hydrogen bonding between the oxime groups. Oxime can form crystals in which these dimers are roughly aligned along the crystallographic direction c ) [001]. Consequently, the interplanar distance d001 is always almost equal to the dimer length. AFM measurements showed that the step height between the crystal terraces is 1.9 ( 0.1 nm. The microcrystals were rigid enough to enable molecular resolution imaging. These AFM images show that the surface structure is comparable to the bulk structure obtained by X-ray diffraction on single crystals, so no surface relaxations were found.20,21 Further comparison of these two techniques indicated that the microcrystals are oriented with the (001) planes parallel to the substrate. The step height between the crystal terraces corresponds to the interplanar distance d001. In this arrangement, the microcrystals have a hydrophobic surface consisting of the hydrophobic parts of the dimers. If one oxime monolayer could be removed (hence breaking the dimeric hydrogen bond) and the remaining layer of monomers would be stable, then the surface of

de Jong - van Steensel et al.

the crystals could be modified from hydrophobic to hydrophilic. In this study, we report on the breaking of the dimeric hydrogen bond of oxime with AFM at different temperatures. Breaking this bond is only possible at the step edges present on the microcrystal surface. Instead of the usual hydrophobic (001) terraces, a new (oximeterminated) hydrophilic (001) terrace is formed at a height that is a half d001 lower than the usual terraces. We use LFM to distinguish between the hydrophobic and hydrophilic parts. The hydrophilic terraces are stable enough to be visualized with AFM, although they change in time. The force necessary to break the dimeric hydrogen bond is strongly dependent on the temperature. Experimental Section Microcrystals of oxime on a hydrophilic silicon wafer were prepared with the spin coating technique and studied with AFM. To this end, a one-side polished hydrophobic p-type (100) silicon wafer (Wacker Siltronic AG, Burghausen, Germany or Siltronix, Vernier, Switserland) was made hydrophilic by boiling it for 15 min in a mixture of H2O2/NH4OH/H2O 1:1:5 (v/v). After rinsing it with distilled water and drying in a flow of nitrogen gas, the wafer was placed in a home-built spin coater with the polished side facing up. At a rotation frequency of 3000 rpm, 0.75 mL of a solution of oxime in chloroform with a concentration of 1 mg mL-1 was injected onto the middle of the wafer. After evaporation of the solvent (within seconds), the wafer was cut into pieces, which were glued to sample pucks. The AFM measurements were performed with a Digital Instruments Nanoscope III Multimode (Santa Barbara, California USA) scanning probe microscope mounted with a J-scanner (maximum lateral scan size 125 µm) or E-scanner (maximum lateral scan size 10 µm). Measurements were done in contact mode with oxide-sharpened tips with a spring constant of 0.12 N/m as specified by the manufacturer (Digital Instruments, Santa Barbara, California, USA). The scan angle was set to 90°. To explain the results, knowledge of the movement of the tip across the sample surface is essential. Therefore, in this section, a brief description of the tip movement during the scanning process is given. In this AFM setup, the images are recorded in a zigzag scan pattern. The scan line direction where the tip relative to the sample scans from left to right is called the trace scan. Scanning in the other direction is called the retrace scan. When scanning a sample surface, it is possible to display/record both the trace and the retrace scan direction or display/record only one of them. In either way, both scans are carried out because the tip stays on the sample during the measurement. Besides the standard height images, both the deflection and the friction images were recorded. The deflection image depicts the derivative of the height and provides a sensitive edgedetection method.26 To minimize the force applied to the sample, care was taken to adjust the set point voltage to the lowest value possible for which tip-sample contact was still maintained during scanning. When needed, the force exerted on the sample was increased by increasing the set point value. The force exerted on the sample was calculated from the force curves measured at the applied set point value. The samples were studied at three different temperatures. Because of the heat production of the laser and the electronics in the AFM setup, a sample temperature of 30 ( 1 °C can be reached without any modification to the instrument. A sample temperature of 16 ( 1 °C and 20 ( 1 °C was reached with a homebuilt cooling stage.27

Results General Description of the Samples. AFM imaging at various temperatures showed that spin coating

Splitting Organic Dimers with AFM

Figure 2. AFM height image showing irregular shaped oxime crystals (A) with the corresponding deflection image (B). In panel C, an enlarged height image is given, revealing the existence of steps on the crystal surface. A cross section profile across the steps is given in panel D. The lateral scale bar is 1 µm for (A) and (B) and 150 nm for (C). The vertical scale is 500 nm for (A) and 10 nm for (C).

a solution of oxime led to the formation of micrometer sized crystals on the hydrophilic silicon substrate. A representative AFM height image of such a crystal is depicted in Figure 2A. The crystals are rather flat; the height is about one tenth of the length or even less. In the remainder of this paper, deflection images instead of height images will be depicted, to provide a clearer view of the crystal surface (compare Figure 2A to 2B). Cross section profile lines are inserted in the deflection (and friction) images at the position they were measured in the height images. The existence of steps on the surface (Figure 2C) clearly demonstrated that crystals were formed indeed, despite their irregular overall shapes. The step height, measured in the cross section profile (Figure 2D), is 1.9 ( 0.1 nm, corresponding to the full interplanar distance d001. Measurements performed at various temperatures (16, 20, and 30 °C) displayed the same step height. As reported earlier, the microcrystals spontaneously sublimate.21 The time necessary for complete sublimation is dependent on the temperature and varies from less than 1 h at 40 °C to more than 5 days at 4 °C. We found that AFM imaging of the crystals at minimum applied force had no effect on the sublimation time. Measurements at 30 °C. When measuring at minimum force (∼ 6 nN), the appearance of the step patterns changed in time as is shown by comparison of Figure 3A and 3C. Moreover, due to the spontaneous sublimation the upper terrace visible in Figure 3A has disappeared in Figure 3C. These images were captured 10 min after each other. In the friction images, no contrast on a terrace or between terraces was observed (Figure 3B and 3D), indicating that the crystal surface has the same physical structure and chemical composition on all positions at any time.

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Figure 3. AFM deflection image showing several terraces on the crystal surface (A) with corresponding friction image (B) measured at 30 °C. The same crystal after a 10 min time interval is depicted in the deflection image (C) with its corresponding friction image (D). For all images, the scale bar is 150 nm.

Figure 4A and 4B show another example of a deflection and a friction image of a step pattern on a crystal measured at minimum force (∼ 8 nN). The measured step height along the line depicted in Figure 4A and 4B is 1.9 ( 0.1 nm (Figure 4C) and corresponds to the full interplanar distance d001. When scanning the same area at a force of 27 nN, a drastic change in friction contrast appeared near the step edges (Figure 4E); concomitantly granular areas in the deflection images are observed at the same positions (Figure 4D). The light areas in Figure 4E are regions with higher lateral forces (and thus larger friction coefficients) than the dark areas i.e., light areas are less slippery than dark areas. Apart from the changes in deflection and friction images, the height profile changed. At minimum force, the step height was 1.9 ( 0.1 nm (Figure 4C), but at a high force a step height of 0.95 ( 0.1 nm was measured (Figure 4F and 4I). The position, shape, and quantity of the altered areas changed from scan to scan as is illustrated by comparison of Figure 4D and 4E, and Figure 4G and 4H. These images show two successive deflection and friction scans at the same location, measured at the same applied force. The changes at high applied force described above were observed for many microcrystals; changing the scan direction did not affect the results. Measurements at Lower Temperatures. To investigate the effect of temperature on the behavior of the step pattern, measurements at lower temperatures, 20 and 16 °C, were performed. Measurements at a minimum applied force (∼ 8 nN) show a similar behavior to the measurements at 30 °C (see Figure 3), performed at a minimum applied force. Also, in this case, changing of the step pattern and disappearance of terraces were observed (data not shown) but considerably slower. This is attributed to a decrease in the rate of spontaneous sublimation at lower temperatures. A deflection image of a representative crystal measured

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Figure 4. AFM deflection image measured at 30 °C displaying several terraces on the crystal surface (A) with corresponding friction image (B) and cross section profile (C). The deflection image in panel D with corresponding friction image (E) and section profile (F) show the same crystal surface after increasing the force exerted on the sample. Panels G-I show the situation in the subsequent scan. For all images, the scale bar is 1 µm.

at 20 °C and its accompanying friction image are given in Figure 5A and 5B. Similar to the measurements at 30 °C, at 20 °C the friction image does not show contrast when measuring at low force. The step height as determined from the cross section profile is 1.9 ( 0.1 nm (Figure 5C). After the force was gradually increased to 12 nN, areas with a higher friction became visible on the crystals at 20 °C (Figure 5E). These areas appeared granular in the deflection image (Figure 5D). A cross section profile shows that the step height in this case was 0.95 ( 0.1 nm. After the scanning force was decreased, the higher friction areas are not discernible anymore, and only step heights of 1.9 nm were found: the areas with higher friction reorganize. In contrast to the measurements at 20 and 30 °C, at 16 °C, it was impossible to induce areas with higher friction accompanied by granular areas in the deflection images and by halving of the step height, even when scanning with a force up to 29 nN. At 16 °C, the surface of the microcrystals is too stable to be altered at this scanning force. Friction Contrast at Low Scanning Force. On several occasions at various temperatures, friction contrast was observed when scanning at low force. In these cases, the higher friction areas were not accompanied with granular areas in the deflection images. In Figure 6A (deflection) and 6B (friction), an example

of this phenomenon is depicted measured at 16 °C. No height difference was found between the areas with higher and lower friction (Figure 6C). The step height is 1.9 nm as usual (Figure 6C). The contrast in the friction images remained when the scanning force was increased. Discussion Breaking the Dimeric Hydrogen Bond. Measurements performed at 30 °C showed that the step pattern changes spontaneously in time (Figure 3). This was visible in both the deflection and friction images. However, no contrast in friction accompanied this type of change in step pattern. Taking into account that scanning the crystals with minimal force did not enhance the sublimation rate, this observation is ascribed to the spontaneous sublimation of the crystals. Because of the intermolecular hydrogen bonding between the oxime molecules presumably dimers rather than monomers detach from the crystal surface during the sublimation process. This is consistent with the absence of contrast in the friction images because only hydrophobic terrace surfaces occur. Moreover, the step height of 1.9 nm corresponds to the interplanar distance and hence to the dimer length of oxime. After the (normal) force exerted on the sample was increased to 27 nN, the step



Figure 5. AFM deflection image measured at 20 °C showing a terrace on the crystal surface (A) with corresponding friction image (B) and cross section profile (C). The deflection image in panel D with corresponding friction image (E) and section profile (F) show the same crystal surface after increasing the force exerted on the sample. For all images, the scale bar is 250 nm.

Figure 6. AFM deflection image measured at 16 °C showing several terraces on the crystal surface (A) with corresponding friction image (B) and cross section profile (C). For both images, the scale bar is 75 nm.

pattern changed dramatically, both in the deflection and friction images. In the deflection images granular areas were observed, these areas have high friction contrast in the LFM images and the step height is 0.95 nm (Figure 4). This suggests that due to the high force exerted by the AFM tip on the crystal surface, the AFM tip is capable of breaking the noncovalent hydrogen bond between the oxime molecules within the dimers. In these areas, it is expected that a change in friction contrast should accompany this breaking because terrace surfaces are formed where the dipolar head of the oxime molecules, rather than the hydrophobic part, forms the upper layer. LFM is sensitive to the chemical

composition of the sample, and hence, it can distinguish between hydrophobic and hydrophilic surfaces since the lateral twisting of the cantilever will differ.14 Indeed, areas with a different friction force were observed after increasing the force exerted on the sample as revealed by the presence of light and dark areas. The light areas have a higher friction force than the “normal” dark areas. We ascribe these light areas to be the hydrophilic part of the oxime molecules and the dark areas to be the hydrophobic part. This is consistent with the findings of Noy et al.14 who measured friction contrast on a patterned SAMs only differing in terminal groups. They observed higher friction forces on hydrophilic areas as compared to the hydrophobic areas. These measurements were done in air with a standard Si3N4 tip, like we did. The observation that the areas with higher friction appear granular in the deflection images can be caused by disruption of the layer by the tip and is not unexpected for thermodynamically less stable surfaces. Alhough the position, shape, and quantity of the altered areas changed from scan to scan as is illustrated by comparison of Figure 4D and 4E, and Figure 4G and 4H, the altered areas are at least stable enough to be imaged with AFM. The monomers on the crystal surfaces, created by breaking the dimeric hydrogen bonds are less stable than the dimers and sublimate rapidly. When the sample could have been cooled rapidly after breaking the dimeric hydrogen bonds an enhanced stability of the monomeric layer may be observed. This is not feasible with the current AFM setup. Breaking the dimeric hydrogen bond may not be the only possible explanation for the friction contrast observed after increasing the scanning force. Another, less likely, possibility could be that the AFM tip is capable of bending the dimers, i.e., disrupt the lateral organization of the dimers. Also, in this case, friction contrast could be expected, although probably less pronounced. Disruption of the lateral organization of the dimers

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would disturb the step height locally but would leave the terrace distance at d001. At various temperatures sometimes contrast in friction images was observed even at low forces (but was also visible at high scanning force) as is illustrated in Figure 6B measured at 16 °C. However, the contrast in the friction images was accompanied neither with granular areas in the deflection images nor with a reduction of the step height (Figure 6A and 6C). Consequently, this type of friction contrast is not related to breaking of the dimeric hydrogen bond. A possible explanation for the friction contrast observed could be the existence of stacking faults in the crystals. These stacking faults can be formed during the spin coating process by coalescence of two nuclei with a different orientation of the dimers. Thus, both friction contrast and a reduction of the step height should be considered to be sure that bond breaking has occurred. Influence of Scan Direction. Although the scan direction could influence the breaking of the dimeric hydrogen bond, this is not the case. In the AFM used, the images are recorded in a zigzag scan pattern and the tip stays continuously in contact with the sample surface. Because of the zigzag scan pattern and to the fact that the crystal steps are not straight the AFM tip will always encounter step edges. Breaking of the noncovalent dimeric hydrogen bonds can only start at the edges of the terraces where the dimers do not have the maximum amount of neighbors and thus will behave differently from dimers in the bulk crystal. Suppose that on a trace scan line the tip first scans a terrace and then goes down across the step to the next terrace. In this case, the AFM tip cannot disorder the step edge because it first encounters bulk dimers. However, in the retrace scan it will first encounter the step edge and then it will move up to the terrace where it is possible to remove one or several monomers from the terrace. How many monomers are removed depends on the force exerted on the sample and on the size of the tip. When scanning at a scan size of 3 × 3 µm with 512 scan lines each consisting of 512 measurements (as was used when capturing Figure 4), the pixel size is about 34 nm2. The area of an oxime molecule determined by X-ray measurements is 28.6 Å2.21 To get an estimate for the tip contact area, the area of the tip at 2 nm from the tip end is calculated and is 240 nm2 (assuming a spherical tip with a nominal radius of 20 nm). This means that the tip is touching several hundred monomers at the same time and only 14% of the touching area is recorded in the image (this is the pixel size). After measuring the retrace scan in the subsequent trace scan a part of the area disturbed in the retrace scan is visualized. Because of the disrupted ordering in this part of the terrace it is possible that the AFM tip is now capable of breaking dimer bonds in the trace direction before reaching the step edge. There is no possibility for switching off either the trace or retrace scan line. This explains the independency of scan direction to the breaking of the dimeric hydrogen bond. Strength of the Dimeric Hydrogen Bond. To gain more insight into the minimal force necessary to break the dimeric hydrogen bond, measurements at different temperatures were performed at which the force exerted on the sample was gradually increased. At 20 °C, a force


of 12 nN was sufficient for contrast in friction imaging to appear concomitantly with a reduction of the step height to 0.95 nm (Figure 5) and with the appearance of granular areas in the deflection image. These observations were explained by breaking of the dimer bonding by the AFM tip. By changing the force exerted on the sample, it was possible to switch the bond breaking on and off. At 16 °C, even at a force of 29 nN, no breaking of the dimeric hydrogen bond took place. This shows that the force necessary to break the dimeric hydrogen bond is temperature dependent. At 30 °C, we focused on the breaking morphology, and therefore we used a force strong enough to induce breaking. At 20 °C, the force was gradually increased to find a critical value. We found that a force of 12 nN is the onset for breaking the dimeric hydrogen bond at this temperature. To validate this statement, an estimate of the energy needed to break the dimer bond can be obtained as follows. In our measurements, a force (F) of about 10 nN was necessary to break the dimer bond. Assuming that a bond is broken when it is elongated over a distance (s) of 0.1 Å, the work (W) associated with this bond breaking is the product of force and distance (W ) Fs) and is 10-19 J per bond. Per mole of bonds, this comes down to 60 kJ mol-1. The hydrogen bonding between the oxime molecules is about 20 kJ mol-1 (∼4.8 kcal mol-1).19 This estimate shows that the exerted force is large enough indeed to break several dimer bonds. It can be expected that the onset for breaking the dimer bond is temperature dependent. This is supported by the observation that at 20 °C at a force of 12 nN breaking was induced easily while at 16 °C a force of 29 nN was not sufficient to break the dimeric hydrogen bond. After breaking the bond, the crystal surface will be partly saturated by water from the air. Thus, an additional dependence of the minimal force on air humidity should be anticipated. This will be studied in further work. Although at first sight the temperature range of 16-30 °C is rather small, these experiments clearly demonstrate that there are large differences between the temperatures used. Clearly, the vapor pressure of oxime is high enough to ensure sublimation of the crystals, enhanced by the open AFM system.20 The vapor pressure of a compound can change significantly within only a few degrees Celsius. Moreover, the hydrogen bond strength is temperature dependent. The fact that the step edges are not straight but quite strongly meandering, suggests that the roughening temperature of oxime surfaces is not far above room temperature. Under such circumstances, the molecular scale surface structure usually is strongly temperature dependent, and this could be reflected in the temperature dependence of the onset force for breaking the dimeric hydrogen bonds. Force Measurements. To elaborate on the nature of the broken steps one ideally would perform force measurements on the broken (hydrophilic terrace regions) and nonbroken (hydrophobic terrace regions) dimers in the crystals, but attempts were unsuccessful for several reasons. First, the amount and position of areas consisting of broken dimers change from scan to scan (Figure 4). This makes it impossible to unambiguously locate these areas in the subsequent scan. Drift in the AFM system enhances this problem. Further-


more, with our temperature regulation, it is not possible to decrease the temperature rapidly over a temperature range of tens of degrees Celsius to freeze the broken dimer structure. Finally, although force volume measurements are possible with the AFM setup used, i.e., it is possible to measure topography and force curves at the same time, these measurements are too slow for this dynamic system and also have a low resolution. Moreover, it is not sure whether bond breaking is to be expected in the force volume mode due to a different motion of the tip across the sample surface. In force volume measurements, the AFM tip has to measure the topography of one pixel size followed by a force measurement during which the tip has to be removed from the surface, then topography is measured again for the next pixel and so on. It should be noted that the strength of the forces mentioned above is only a rough estimation, and it is the normal force exerted on the sample and not the lateral force. The actual force exerted on the sample will be dependent on several parameters such as the exact tip shape, which is unknown and needs not to be the same within one measurement due to the adhesion of material to the tip. We also noticed that due to vertical drift the scan parameters to ensure a minimum force exerted on the sample surface even changed during one scan by 25%. Outlook. These measurements show that the AFM is capable of locally changing the surface structure from hydrophobic to hydrophilic by breaking the dimeric hydrogen bond between two amphiphilic organic molecules. The hydrophilic parts could be used for further modification. It should be possible to attach other molecules with specific properties on these parts of the microcrystals. Notwithstanding several drawbacks of our present system (the microcrystals sublimate, limited stability of the hydrophilic layers), the concept could be extended to other (oxime) systems. Microcrystals of longer oligo(cyclohexylidene) oximes are stable in time. Moreover, SAMs of modified oligo(cyclohexylidene) oximes are stable on gold and form dimers through intermolecular hydrogen bonding as well.28 It is anticipated that breaking of the dimeric hydrogen bond in those systems is also possible, perhaps with a better stability of the hydrophilic layers. The stability of the hydrophilic layers could be enlarged by a rapid decrease of the temperature after the formation stage. An additional advantage of using SAMs as compared to microcrystals is that with SAMs the whole substrate is covered with material. Breaking of the dimeric hydrogen bonds in organic molecules as observed in the oxime crystals used may be extended to other organic systems (not necessarily having oxime groups) with better stability. In these systems, the created hydrophilic layer can be used for further modifications. Using this concept, tailor-made surfaces could be constructed. Conclusions Controlled manipulation with scanning probe techniques such as AFM is a hot topic in current research. We used the AFM for locally modifying a crystal surface from hydrophobic to hydrophilic by breaking the dimer bond between two amphiphilic molecules. The force necessary to accomplish this is strongly dependent on the temperature: the higher the temperature, the lower


the force necessary. The magnitude of the force is on the order of the strength of the hydrogen bonding. LFM imaging of surfaces is used to distinguish between hydrophobic and hydrophilic surfaces. However, to prove that bond breaking has occurred, a combination of LFM measurements and cross section profiles is required. The concept of breaking dimer bonds between molecules with an AFM tip could be extended to other systems and may be of use in fields where controlling surface properties is of major importance. References (1) Binnig, G.; Rohrer, H. Helv. Phys. Acta 1982, 55, 726-735. (2) Binnig, G.; Quate, C. F.; Gerber, C. Phys. Rev. Lett. 1986, 56, 930-933. (3) Nyffenegger, R. M.; Penner, R. M. Chem. Rev. 1997, 97, 1195-1230. (4) Schoer, J. K.; Crooks, R. M. Langmuir 1997, 13, 2323-2332. (5) Lyding, J. W.; Shen, T.-C.; Hubacek, J. S.; Tucker, J. R.; Abeln, G. C. Appl. Phys. Lett. 1994, 64, 2010-2012. (6) Chi, L. F.; Eng, L. M.; Graf, K.; Fuchs, H. Langmuir 1992, 8, 2255-2261. (7) Thundat, T.; Sales, B. C.; Chakoumakos, B. C.; Boatner, L. A.; Allison, D. P.; Warmach, R. J. Surf. Sci. Lett. 1993, 293, L863-L869. (8) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S.; Mirkin, C. A. Science 1999, 283, 661-663. (9) Hong, S.; Zhu, J.; Mirkin, C. A. Science 1999, 286, 523525. (10) Cho, K.; Joannaopulos, J. D. Phys. Rev. B 1996, 53, 45534556. (11) Mate, C. M.; McClelland, G. M.; Erlandsson, R.; Chiang, S. Phys. Rev. Lett. 1987, 59, 1942-1945. (12) Kim, Y.; Kim, K.-S.; Park, M.; Jeong, J. Thin Solid Films 1999, 341, 91-93. (13) Wong, S.-S.; Takano, H.; Porter, M. D. Anal. Chem. 1998, 70, 5209-5212. (14) Noy, A.; Frisbie, D.; Rozsnyai, L. F.; Wrigthon, M. S.; Lieber, C. M. J. Am. Chem. Soc. 1995, 117, 7943-7951. (15) Ogletree, D. F.; Carpick, R. W.; Salmeron, M. Rev. Sci. Instrum. 1996, 67, 3298-3306. (16) Liu, E.; Blanpain, B.; Celis, C. P. Wear 1996, 192, 141150. (17) Gibson, G. T.; Watson, G. S.; Myhra, S. Wear 1997, 213, 72-79. (18) Fujihira, M.; Ohzono, T. Jpn. J. Appl. Phys. 1999, 38, 39183931. (19) Hoogesteger, F. J.; Havenith, R. W. A.; Zwikker, J. W.; Jenneskens, L. W.; Kooijman, H.; Veldman, N.; Spek, A. L. J. Org. Chem. 1995, 60, 4375-4384. (20) ten Grotenhuis, E.; Marsman, A. W.; Hoogesteger, F. J.; van Miltenburg, J. C.; van der Eerden, J. P.; Jenneskens, L. W.; Smeets, W. J. J.; Spek, A. L. J. Cryst. Growth 1998, 191, 834-845. (21) ten Grotenhuis, E.; van der Eerden, J. P.; Hoogesteger, F. J.; Jenneskens, L. W. Chem. Phys. Lett. 1996, 250, 549554. (22) ten Grotenhuis, E.; van der Eerden, J. P.; Hoogesteger, F. J.; Jenneskens, L. W. Adv. Mater. 1996, 8, 666-668. (23) Hoogesteger, F. J.; Kroon, J. M.; Jenneskens, L. W.; Sudho¨lter, E. J. R.; de Bruin, T. J. M.; Zwikker, J. W.; ten Grotenhuis, E.; Mare´e, C. H. M.; Veldman, N.; Spek, A. L. Langmuir 1996, 12, 4760-4767. (24) Hoogesteger, F. J.; Jenneskens, L. W.; Kooijman, H.; Veldman, N.; Spek, A. L. Tetrahedron 1996, 52, 1773-1784. (25) Marsman, A. W.; Leusink, E. D.; Zwikker, J. W.; Jenneskens, L. W.; Smeets, W. J. J.; Veldman, N.; Spek, A. L. Chem. Mater. 1999, 11, 1484-1491. (26) Putman, C. A. J.; van der Werf, K. O.; de Grooth, B. G.; van Hulst, N. F.; Greve, J.; Hansma, P. K. Scanning Probe Microscopies 1992, SPIE1639, 198-204. (27) de Jong-van Steensel, L. I. et al., to be published. (28) Reincke, F.; Hickey, S. G.; Kelly, J. J.; Jenneskens, L. W.; Braam, T. W.; Vanmaekelbergh, D. submitted to J. Electroanal. Chem.

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Breaking the Intermolecular Hydrogen Bonds in Organic Dimers with

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