Ultramicroscopy 110 (2010) 826 830 Contents lists available at ScienceDirect Ultramicroscopy journal homepage: www.elsevier.com/locate/ultramic Real-time atomic force microscopy in lubrication condition Hyunsoo Lee a, Donghyeok Lee a, K.B. Kim a, Yongho Seo a,, Hyunsook Kim b, Haiwon Lee b a Faculty of Nanotechnology and Advanced Material Engineering, Sejong University, Seoul 143-747, South Korea b Department of Chemistry, Hanyang University, Seoul 133-791, South Korea article info Article history: Received 24 November 2009 Received in revised form 16 February 2010 Accepted 26 February 2010 Keywords: AFM Fast scanning Real-time Atomic force microscopy Lubrication condition tip modification abstract We have studied frictional force and wear problem in real-time atomic force microscopy in contactmode using a resonator type mechanical scanner allegedly reported. The fast scanning may cause wear in the sample surface or the tip, and may deteriorate the image quality. Mineral oil was used to make a lubricious surface on a polycarbonate sample, and it was found that the interfacial frictional force was decreased. A Si tip which was coated with a hydrophobic film by means of chemical modification was confirmed to diminish the frictional force in the fast scanning process. The resultant image quality was improved due to reduced friction and wear. & 2010 Elsevier B.V. All rights reserved. 1. Introduction Since the atomic force microscope (AFM) was developed in 1986 by Binnig et al. [1], the technique has been maturing as the imaging rate grows as much as to capture a moving object. In this context, the high-speed AFM is required to get several tens frames per second, which was achieved by Miles group using a combined tuning fork and flexure-stage scanning system [2,3]. In order to maximize scanning speed, several components are required to be considered in terms of mechanical properties of the hardware; small size and high resonance frequency of cantilever, fast data acquisition electronics, fast scanner with high resonance frequency, and low friction. Among them, the low friction could be essential issue, especially for soft samples. It is natural that noncontact-mode AFM (NC-AFM) cannot be faster than contactmode AFM (C-AFM) in terms of conventional instrumentation. While the scanning speed of NC-AFM is dependent on the resonance frequency and quality factor of the cantilever, for C-AFM acceleration of the cantilever motion is dominant factor, which depends on the mass and elastic force acting on it [4]. The most striking advantage of the NC-AFM is that it makes no scratch on soft samples such as biological and polymer materials. In general, the C-AFM may not avoid making scratch on the sample. On the other hand, Humphris et al., claimed that the fast scanning C-AFM may not cause serious scratch problem, because Corresponding author. E-mail address: yseo@sejong.ac.kr (Y. Seo). the tip touches sample surface instantaneously [2]. Therefore, a real-time C-AFM for soft sample was accomplished with nanometer resolution in air [3]. However, in order to achieve high resolution imaging in C-AFM, problems related with friction and abrasive wear should be addressed [5,6]. Gotsmann and Lantz studied the abrasive wear of sliding tips on a polymeric surface and suggested a model in which the barrier for breaking an atomic bond is lowered by the frictional stress [7]. The wear of tip has an effect on frictional force in the scanning process of C-AFM [8]. Bhushan et al. [9] demonstrated that the quantity of wear is proportional to the energy dissipated by frictional force and suggested the relationship between the wear and the dissipated energy as VpFd, where V is the quantity of wear, d the sliding distance of tip, and F the frictional force. As F has a logarithmic dependence on scanning velocity, the amount of wear is logarithmically proportional to the scanning velocity. To decrease adhesion and wear, the lubricative surface modification of the tip and sample could be a possible candidate. Maw et al. [10] studied tribochemical wear of silicon nitride by measuring the wear of AFM tips in aqueous solutions. They showed that significant wear is related to metal-hydroxide bonds. The blunt tip due to wear has increased radius so that the enlargement of the effective dimensions gives rise to image distortion [11]. Knapp et al. [12] asserted that by using hydrophobic coated tip in conventional AFM equipment, tip wear and abrasion of sample can be reduced. Recently, our group has reported a real-time AFM (RT-AFM) using a mechanical resonator type scanner [13]. In order to improve image resolution and prevent wear of tip which could be 0304-3991/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ultramic.2010.02.045
H. Lee et al. / Ultramicroscopy 110 (2010) 826 830 827 a serious problem for soft sample measurement, we studied the effectiveness of hydrophobic coated tip and lubrication environment in the high speed AFM system. 2. Experimental The hydrophobic film coating on Si tip was prepared by selfassembly using n-octadecyltrichlorosilane (OTS, CH 3 (CH 2 ) 17 SiCl 3, Aldrich). The Si tip was first treated by UV-ozone for 1 h to obtain the silicon hydroxide layer and to remove the contaminant on the tip surface. The Si tip modified by hydroxyl group was immersed in a 3 mm OTS solution in toluene for 1 h at room temperature. By using a homemade contact-mode RT-AFM previously reported by our group [13], fast imaging experiments were performed in lubrication environment. The main body of the RT-AFM was composed of a head, a PZT tube scanner, optical microscope, and coarse approach mechanism (Picomotor, New focus Inc.). In the head, a position-sensitive photodiode (PSPD), preamplifier with 2 MHz bandwidth and the microscanner are mounted [13]. The microscanner is a mechanical resonator type scanner which was suggested by Humpris et al. [2]. The microscanner has a resonance frequency of 5 khz and an amplitude of 1 3 mm depending on the driving voltage. It has 15 mm long bar at the center vibrating in shear direction and two side bars which serves as supporter and holder. A PZT actuator is located in between the vibrating bar and one of side bars to excite the resonator scanner. Our RT-AFM imaging rate was demonstrated as 30 frames/s with 200 200 pixels and 1 cm/s scanning speed [13]. Two different types of cantilever were used, according to experimental purposes. The first one is Si 3 N 4 cantilever (biolever, Olympus) which has spring constant of 0.03 N/m and 13 KHz resonance frequency. The other one is Si cantilever (CSC38, MikroMasch) having dimension of 350 35 mm 2, resonance frequency of 10 khz, and spring constant of 0.03 N/m. A standard grating sample (TDG01, NT-MDT) was used for spatial resolution analysis and distance calibration of the microscanner. The sample has aluminium coating on top surface and grating pattern with 50 nm height and 278 nm period. Also, a freshly cleaved surface of recorded layer in a commercial compact disk (CD) made from polycarbonate was used for abrasion and wear measurement. As a liquid lubricant, pure mineral oil (paraffin oil, acros organics) was prepared which was clear and colorless appearance, has density of 872 882 kg/m 3 (at 15 3 C) and kinetic viscosity of 65 75 mm 2 /s (at 40 3 C). 3. Results and discussion The pull-off force in AFM spectroscopy implies the adhesive force between the tip and sample, which can be an indirect indicator showing the bluntness of the tip. In order to confirm the relationship between the pull-off force and the blunt tip, the pull-off forces were measured repeatedly after strong mechanical contacts ð100 nnþ with the coarse approach mechanism, as shown in Fig. 1(a). The tip mounted on Si 3 N 4 cantilever and Si grating sample were used. Two different tips (fresh and blunt) were measured, and they showed the same tendency, almost linear dependence except the offset. This means that the amount of wear of tip can be estimated from the pulloff force [8]. Fig. 1(b) shows relationship between the pull-off force and duration time of fast scanning with vibration amplitude of 1 mm (corresponding to the excitation voltage 3 V PP for microscanner actuator) and normal force of 1.0 nn loaded on the cantilever constantly. The scanning rate of the microscanner was 5.5 khz, which was the resonance frequency of the microscanner. These data also show nearly linear dependence, which means that continuous Fig. 1. (a) The pull-off forces were measured as a function of the number of hard contacts. Two different condition tips (fresh and blunt) were used. (b) It shows relationship between the pull-off force and duration time of fast-scanning with vibration amplitude of 1 mm. (c) The pull-off forces were measured as a function of the vibration amplitude of the tip. The fast-scanning duration time was 1 min and the normal force 1.0 nn in z-direction. The lines are for eye guides.
828 H. Lee et al. / Ultramicroscopy 110 (2010) 826 830 Fig. 2. The topographic images were measured after fast-scanning by RT-AFM with the normal forces: (a) 0 nn, (b) 2.0 nn, (c) 4.0 nn, (d) 6.0 nn, (e) 8.0 nn, and (f) 10.0 nn in z-direction. The vibration amplitude was 1 mm and Si 3 N 4 cantilever was used. The sample was a CD surface made of polycarbonate. The scan ranges were 4:5 4:5 mm 2. Fig. 3. The force distance curves and RT-AFM images were measured by (a) hydrophobic coated tip and (b) uncoated (normal) tip. The RT-AFM imaging rate was 30 frames/s and the scan size was 1 1 mm. The sample was a standard grating sample with 278 nm pitch. wear occurred as long as the fast scanning was performed. In addition, the behavior of dependence on amplitude of fast scanning motion was measured as shown in Fig. 1(c). In each data point measurement, scanning duration time was 1 min and 1.0 nn normal force was applied constantly. This result shows that the amount of wear is proportional to the scanning velocity of the tip, but it is difficult to make sure that the dependence was linear or logarithmic due to insufficient data points. In order to investigate the quantity of abrasion wear on sample in RT-AFM, we measured slow scan image with a conventional scanning rate (0.5 1 Hz) after fast-scan (1.2 cm/s) imaging with the normal forces of (a) 0 nn, (b) 2.0 nn, (c) 4.0 nn, (d) 6.0 nn, (e) 8.0 nn, and (f) 10.0 nn, as shown in Fig. 2. The CD sample made of polycarbonate was appropriate to measure abrasion wear on the surface, because of its softness. When the normal force was o4 nn (a c), no evidence of scratch was found. When the normal force 48:0 nn (e f) was applied, scratches were found at lower left corner where the RT-AFM scanned in scanning rate of 5.5 khz. In order to reduce the frictional force and tip wear, the Si tip was modified by coating with a hydrophobic film. In C-AFM measurement, the adsorbed water layer between tip and sample can cause the adhesive force, which increase friction and tip wear [14]. The
H. Lee et al. / Ultramicroscopy 110 (2010) 826 830 829 Fig. 4. The topographic images were measured with slow-scanning (conventional) mode (a) before and (b) after the fast-scanning with the hydrophobic coated tip. For comparison purpose, the topography measurements with the same condition were performed (c) before and (d) after the fast-scanning with an uncoated tip. The scan ranges were 4:5 4:5 mm 2. Fig. 5. The fast topographic images with RT-AFM were measured (a) in air and (b) in mineral oil as a lubricant liquid, which was located in between the tip and sample. The Si 3 N 4 cantilever was used, and the sample was a polycarbonate surface of a CD. The image size was 1 1 mm, and total scanning time was 30 ms. topography images of the grating sample were measured with the RT- AFM of 30 frames/s imaging rate, as shown in the inset of Fig. 3. The topographic image in Fig. 3(a) was measured by hydrophobic coated tip, and (b) measured by uncoated tip. One can see that the resolution of the image measured with the hydrophobic coated tip is better than that with the uncoated tip. The force spectroscopies were performed before and after the fast scanning, as shown in graphs in Fig. 3(a and b). The black line is for the approach process and the red line is for the
830 H. Lee et al. / Ultramicroscopy 110 (2010) 826 830 Fig. 6. The high speed images with RT-AFM were measured (a) in air and (b) in water. The scanned size of (a) was 1 1 mm, and (b) was 0:5 0:5 mm, and the imaging time was 30 ms. retract one. The pull-off force difference between before and after imaging implies the tip wear. The difference of pull-off force with the hydrophobic coated cantilever (a) was much less than that by uncoated tip (b). This means that the friction is reduced by preventing the adsorbed water layer due to the hydrophobic coating. In order to compare the amounts of wear on sample with coated and uncoated tips, the sample surfaces before and after fast scanning were imaged by conventional slow scanning. A constant normal force (1 nn) was applied and the tip was scanned with an amplitude of 2 mm in the fast scanning. The CD surface made with polycarbonate was scanned with the stiff Si cantilever, and abrasion wear may occur on this soft surface. The topography images of slow scanning are shown in Fig. 4(a) before and (b) after the fast-scanning with the hydrophobic coated cantilever. The abrasion wear was not made judging from that no distinguishable difference is seen between (a) and (b). However, when the uncoated tip was used for fast-scanning, a serious damage was made on the surface as shown in (d), differently from the image before fast scanning (c). Therefore, it was confirmed that the hydrophobic coated tip could be used for the RT-AFM to suppress damage on soft sample. Furthermore, the hydrophobic coated tip was inspected by using a scanning electron microscope after the RT-AFM measurement, and no noticeable change was found (not shown in figure). As a result, the image quality can be improved by reducing the frictional force and the sample damage. In addition, we demonstrated that the higher image resolution can be acquired by using lubricant liquid. Tiny oil droplets (pure mineral oil) were distributed on the CD sample, and the cantilever tip was contacted with the sample. Then, the oil meniscus was formed in between the tip and the sample. In this lubricant liquid environment, the tip and sample surface are not exposed in air or humidity during RT-AFM measurement. Fig. 5 shows the topography images ð1 1 mm 2 Þ takenbyrt-afmwithanuncoatedtip(a)inairand(b)inoil environment. While the image taken in air shows low signal to noise ratio due to stiction, the image in oil shows higher resolution with reduced noise. It implies that more high quality image can be obtained in lubrication environment. To measure the RT-AFM in water environment, we prepared a water container and glass window, which contain the microscanner, cantilever, and sample. The resonance frequency of the microscanner was decreased by 5% in the water, and the amplitude was decreased slightly. Through the glass window, the optical signal was transmitted, and real-time imaging in water environment was performed successfully, as shown in Fig. 6. With the same cantilever, the grating sample was scanned (a) in air and (b) in water for comparison purpose. The scanned size of (a) was 1 1 mm 2, and (b) was 0:5 0:5 mm 2, and the imaging time was 30 ms. In case of image in air, there are some wiggles, which is suspected to be mechanical noise generated by bouncing motion of the cantilever due to its elasticity. However, in case of imaging in water, no noise feature can be seen and it looks more realistic. In the water environment, there is no meniscus force and the cantilever motion become stable, supposedly. 4. Conclusions We confirmed that the frictional force causes scratch on soft sample surface and abrasion wear of tip in fast-scanning RT-AFM. In order to lessen frictional force in the RT-AFM, a hydrophobic coated tip can be used, and also lubricant liquid environment can be an alternative. We demonstrated that the wear of tip and abrasion of sample were decreased in lubricant environment, because there is no meniscus force due to adsorbed water layer. Therefore, the RT-AFM in contact mode which was designed to image a moving object in nanoscale is expected to be improved with better image quality in lubrication condition. The RT-AFM imaging in the water environment was demonstrated, and the image taken in water showed better quality than that in air. This aqueous RT-AFM is expected to be useful in biological study. Acknowledgements This work was supported by the National Program for Teralevel Nanodevices of the Ministry of Education, Science and Technology as one of the 21st century Frontier Programs. References [1] G. Binnig, C.F. Quate, Ch. Gerber, Phys. Rev. Lett. 56 (1986) 930. [2] A.D.L. Humphris, M.J. Miles, J.K. 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