Friction force silicon patterns

Figure Bl.19.36. Image of the frictional force distribution of a pattern consisting of areas of CH -tenuinated and areas of COOH-tenninated molecules attached to gold-coated silicon. The tip was also fiinctionalized in (a) with CH3 species and in (b) with COOH species. The bright regions correspond to the higher friction force, which in (a) is observed on the CH areas and in (b) on the COOH areas. (Taken from [187], figure 3.)...

Figure Bl.19.36. Image of the frictional force distribution of a pattern consisting of areas of CH -terminated and areas of COOH-terminated molecules attached to gold-coated silicon. The tip was also functionalized in...

Figure 2.14b shows the friction force measured on platinum patterns and the averaged scan pull-off force measured before and after each friction measurement for the same scanned area. In fig. 2.14b, both the friction and pull-off forces decreased as the asperity height increased, but not as rapidly as the forces with silicon groove depth (fig. 2.10b). The difference in contact conditions, as mentioned previously, possibly caused the differences in rates of decrease in the friction and pull-off forces. 

Figure 2.17 shows the relation between the friction force and pull-off force, from the data in fig. 2.16b. The friction force was proportional to the pull-off force except for the point (open circle) measured on the silicon surface having no platinum asperity. This difference was caused by the difference in the material. Therefore, the friction force was proportional to the sum of the normal load and the pull-off force for the platinum patterns. 

It was difficult to precisely compare the friction forces of different materials on different substrates (figs. 2.13 and 2.17) because the sensitivity of detecting the torsion angle was not always the same for each measurement. Platinum and silicon patterns were made on the same plate, and the friction and pull-off forces were measured to compare the friction coefficients calculated by dividing the friction forces by the pull-off forces for the different materials. Figure 2.18 shows the friction and scanned pull-off forces as a function of groove depth, and these forces were measured for... 

Various patterns of two-dimensional asperity arrays were created by using FIB to deposit platinum asperities and to mill patterns on silicon plates and on a platinum layer deposited on the silicon plate. The pull-off and friction forces between the respective patterns and a flat scanning probe of an AFM were measured. Our findings are as follows ... 

The friction force was more proportional to the pnll-off force than to the cnrvature radins. The friction coefficient (which was calcnlated by dividing the friction force by the pull-off force) for the silicon pattern was abont twice that for the platinnm pattern. These findings indicate that the adhesion force (pnll-off force) did not directly affect the friction bnt, rather, indirectly affected friction, similarly to the effect of an external load. 

For the friction force between a silicon AFM tip and a NaCl(OOl) surface in UHV, such a transition was in fact observed [997] (Figure 9.16). At an applied normal load of 4.7 nN, stick-slip was observed with a clear hysteresis between forward and backward scan on the same line. By reducing the applied load to 3.3 nN, the stick-slip amplitude stays almost constant but the hysteresis, which is proportional to friction loss, is clearly reduced. When changing the applied force to —0.47 nN to compensate pardy the adhesion force of 0.7 nN, the stick-slip pattern changed into a continuous modulation with no detectable hysteresis between trace and retrace. This corresponds to a frictionless sliding at least within the force resolution of the experiment. This effect of vanishing friction due to very small loads is called static superlubricity. 

Fig. 2 Atomic force microscopy (AFM) friction images and schematic illustrations of the patterning processes of a microcontact printed SAMs (mercaptoethanol dots in oc-tadecanethiol matrix, scale bar 10 xm) b patterned molecular printboards fabricated by supramolecular dip-pen nanolithography (DPN) (reprinted with permission from [92] Copyright 2004. WUey VCH) e locally hydrolyzed tert-butyl acrylate-terminated polymer film on oxidized silicon (soft lithography scale bar 3 xm) (Feng CL, Vancso GJ, SchOn-herr H, manuscript submitted to Langmuir) d photopatterned bilayer of diacetylene lipid (scale bar 10 xm). Reprinted in part with permission from [93], copyright (1999), American Chemical Society...

In this study, we clarified these friction-reduction effects under microload conditions by measuring the friction and pull-off forces for two-dimensional asperity arrays on silicon plates. First, two-dimensional asperity arrays were created using a focused ion bean (FIB) system to mill patterns on single-crystal silicon plates. Each silicon plate had several different patterns of equally spaced asperities. Then, the friction and pull-off forces were measured using an atomic force microscope (AFM) that had a square, flat probe. This report describes the geometry effects of creating asperity arrays and the chanical effects of depositing LB films or SAMs on the friction and puU-off forces. 

FIGURE 2.10 Friction and pull-off forces measured on a silicon periodic asperity array, (a) Pull-off forces were measured on each pattern without surface scanning between the measurements. (b) Pull-off forces were measured before and after each friction measurement at the same scanning area and were averaged. 

Figure 5. Lateral force and SIMS images of patterned ODTS on silicon, showing lower friction in the area that contains die silane film.

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