- Email: [email protected]
Comparison of silicon and OH-modified AFM tips for adhesion force analysis on functionalised surfaces and natural polymers
Colloids and Surfaces A 529 (2017) 363–372
Contents lists available at ScienceDirect
Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Comparison of silicon and OH-modified AFM tips for adhesion force analysis on functionalised surfaces and natural polymers
MARK
⁎
Jérôme Colsona, ,1, Laurin Andorfera,1, Tiina Elina Nypelöa, Bernd Lütkemeierb, Frank Stöckelc, Johannes Konnertha a University of Natural Resources and Life Sciences Vienna, Department of Materials Sciences and Process Engineering, Institute of Wood Technology and Renewable Materials, Konrad-Lorenz-Straße 24, 3430 Tulln, Austria b Georg-August-Universität Göttingen, Department of Wood Biology & Wood Products, Büsgenweg 4, 37077 Göttingen, Germany c Technical University Braunschweig, Institute for Building Materials, Solid Construction and Fire Protection (IBMB), Department of Organic and Wood-Based Materials, Hopfengarten 20, 38102 Braunschweig, Germany
G RA P H I C A L AB S T R A C T
A R T I C L E I N F O
A B S T R A C T
Keywords: Atomic force microscopy Silicon tip Functionalised tip Polarity Model surface Wood
In this paper, the position resolved adhesion behaviour of AFM cantilevers with standard silicon tips, rounded silicon tips and OH-modified tips was compared. Surfaces of a flat functionalised microscopy glass slide (hydrophilic background with hydrophobic spots) and of a comparably rough wood/wax sample were scanned in gaseous atmosphere. These two samples were chosen because both of them contain polar and non-polar regions within an area small enough to be scanned at once by the AFM without relevant quality loss due high scan speeds. Moreover, OH and CH3 modified cantilever chips providing very flat functionalised surfaces were scanned. Except for the wood/wax sample, measurements were performed at different humidity levels – with only very little influence on the measured adhesion. Both silicon and OH modified tips showed a higher adhesion on the polar regions of each sample than on the non-polar ones. The difference between the adhesion values on the polar and non-polar surfaces was however systematically higher when standard silicon tips were used. This was also true on the wood/wax sample. As silicon tips are relatively cheap, robust and have a much smaller
⁎
Corresponding author. E-mail addresses: [email protected] (J. Colson), [email protected] (L. Andorfer), [email protected] (T.E. Nypelö), [email protected] (B. Lütkemeier), [email protected] (F. Stöckel), [email protected] (J. Konnerth). 1 These authors contributed equally. http://dx.doi.org/10.1016/j.colsurfa.2017.06.017 Received 22 March 2017; Received in revised form 7 June 2017; Accepted 8 June 2017 Available online 10 June 2017 0927-7757/ © 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Colloids and Surfaces A 529 (2017) 363–372
J. Colson et al.
radius than typical functionalised tips, they allow simple high resolution differentiation of polar and non-polar domains even when the sample surface is relatively rough, as it is the case for natural wood-based polymers.
photodegradation [20], surface aging [21], or adhesion forces on fibres [22,23] can be found in literature. The adhesion forces on bulk wood were also investigated [24], however this study was operated in contact mode, which is damaging to both the sample and the tip. Taking all this into account we want to show that peak force tapping measurements with chemical specificity (i.e. contrast in polarity) can be performed in ambient conditions with bare silicon AFM tips without any modification. Indeed, silicon surfaces have been shown to be quickly covered by a thin oxide layer [25–27], itself covered by a few water monolayers in ambient conditions [28]. In the case of silicon AFM tips, this process is often unwanted, but for some purposes it can also be considered as a useful “natural functionalisation” of the scanning probe. Obviously the interaction with the sample is expected to be less pronounced as it is expected with intentionally OH-modified tips, but the silicon tips come with a couple of advantages: They are cheap, sturdy and available with relatively small radii and thus preferable in terms of spatial resolution when compared to the chemically modified tips used for CFM, where chromium and gold layers are typically applied prior to chemical functionalization, thus increasing the tip radius. In order to prove this assertion OH-modified and bare silicon tips have been compared on several samples in ambient and controlled humidity.
1. Introduction The atomic force microscope (AFM) was invented in the 1980′s [1] and has since then proven to be a powerful tool for high-resolution surface analysis. Its capabilities have been greatly enhanced over the years, making it even able to resolve single atoms if the right conditions are given [2]. One of these enhancements is the atomic force spectroscopy (AFS) [3] in which the interaction between AFM tip and sample on approach and retract is continuously recorded while the cantilever is oscillated and the sample is raster-scanned. This method provides the possibility to simultaneously image numerous physicochemical properties of a surface at the nanometre scale and even in molecular resolution in some cases [4–6]. For this purpose, the cantilever is excited far below its resonance frequency, resulting in an approximately sinusoidal oscillation. When brought into close proximity of a surface, the tip attached to the cantilever will tap this surface at each oscillation, thereby causing disturbances in the periodic motion. For each tapping cycle, superposition of these disturbances with the original motion results in a so-called force curve. Amongst others, information as stiffness, adhesion forces, friction forces or topography can be extracted from these curves and from the other recorded signals (e.g. piezo displacement) [4]. The exact shape of the force-distance curves depends on numerous interacting forces: electrostatic forces, van der Waals forces and capillary forces between the tip and the scanned surface [7]. Tip and sample characteristics (chemical composition, sample roughness, tip radius) as well as environmental parameters (relative humidity, type of surrounding medium, temperature) determine the nature and the magnitude of these interactions [8–12]. This interplay of a high amount of parameters influencing the final results is one of the major drawbacks of the method, as most of the obtained values are specific to the single tip-sample configuration used. Variation of tip radius during the measurements due to tip wear as well as the need for accurate calibration of cantilever stiffness and deflection sensitivity make it difficult to obtain trustworthy absolute values. That is why relative values obtained by comparing several measurements done with the same tip are often preferred. A closer look on these issues will be taken in the discussion. When performing an AFS measurement, a chemically modified tip can be used instead of a native silicon tip. These measurements are usually performed in various liquids biasing the tip to sample interaction. This method, called chemical force microscopy (CFM), results in the tip being particularly sensitive to specific regions of the sample surface thus allowing the distinction of functional groups exhibiting different chemical properties [13]. For example, Frisbie et al. [14] observed significant differences in the frictional and adhesive contrast in agreement with chemical intuition between regions on a pattern covered with more or less polar functional groups. With an adequately sophisticated experimental setup, local chemical interactions between functionalised AFM tips and sample surfaces were even shown to be measureable down to the scale of single molecules [15]. To limit the number of parameters influencing the measurement, model surfaces are often scanned. These surfaces can be created by microcontact printing (μCP) [16] and have been extensively studied in the past, e.g. with chemically modified tips [17] or with carbon nanotubes attached to the tip apex [18]. The tip functionalisation generally results in a better contrast than with standard silicon tips. Similar measurements were also performed on natural materials like cellulose. In this case, flat surfaces can be produced by layer by layer (LbL) deposition [19]. Even rough natural materials like wood are getting into the focus of some research groups in the past years: Studies about
2. Materials and methods 2.1. Materials 2.1.1. R-OH and R-CH3 functionalised sample By the courtesy of Innopsys (Carbonne, France) a pattern of CH3functionalised spots (about 2.5 μm in diameter, about 10 μm distance from centre to centre) on OH-functionalised background was provided. The pattern had been spread on an optical microscopy glass slide by magnetic field assisted μCP [29]. 2.1.2. AFM cantilevers AFM cantilever chips with OH-modified and CH3-modified surfaces (tip radius around 15 nm) were acquired from Nanocraft (Nanocraft Coating GmbH, Engen, Germany). Sharp silicon tips (Scanasyst Air, nominal tip radius 2 nm) were purchased from Bruker, USA. Rounded silicon tips (Special Developments Rounded Tips R30, tip radius 30 nm) were purchased from Nanosensors (Neuchatel, Switzerland). The silicon tips were stored in closed boxes on poly(dimethylsiloxane) (PDMS), which may lead to contamination of the tips [30]. However, it was decided to work with the cantilevers without applying an aggressive cleaning which could result in larger tip radii. The OH-modified and CH3-modified cantilevers were stored in closed glass boxes in Argon atmosphere. The OH-modified cantilevers from Nanocraft were used both as scanning probes and as sample surfaces. To avoid confusions, they will be called “flat OH-functionalised surface” in the case they serve as a sample to be scanned, while “OH-modified tip” in the text will stand for the scanning probe. 2.1.3. Wood/wax sample A wood sample (pinus radiata) was cut into boards of 1000 × 150 × 30 mm3. These boards were immersed in wax (Sasol, Sasolwax C105, Hamburg, Germany) and subjected to a vacuum of 6 kPa and a temperature of 120 °C for one hour. In the following step the wax was pressed into the wood with a pressure of 1.2 MPa and a temperature of 120 °C. After 14 h under these conditions, most of the lumina were filled with wax and the wood had gained 110% of weight. 364
Colloids and Surfaces A 529 (2017) 363–372
J. Colson et al.
discarded. For the measurements on the flat OH and CH3-functionalised surfaces, humidity steps of 45%, 55%, and 65% were chosen. Measurements on the wood/wax sample were only done in ambient conditions (50% RH, 22 °C). Evaluation of the output data from the AFM was done with the Gwyddion 2.39 freeware (www.gwyddion.net).
The wood was then further disintegrated into cross sections of approximately 1 × 3 × 2 mm3 which were glued on AFM specimen discs with epoxy adhesive (Uhu, Uhu plus sofortfest, Bühl/Baden, Germany) and finally microtomed (Leica, Ultracut R, Wetzlar, Germany). The final cuts were done with an ultra AFM diamond blade (Diatome, Ultra AFM, Biel, Switzerland) reducing the thickness of the cut off sections in steps down to approximately 10 nm for smooth surfaces. After preparation the sample was stored in ambient air (23 °C, 55% relative humidity) for three days to reduce the effect of dwindling adhesion due to wood aging [21] between subsequent measurements.
2.2.2. Matlab code The correlation between topography and adhesion data from the AFM was determined with a short Matlab routine. The routine as well as an explanatory text about it can be found in supplementary information. Care was taken to select the exact same pixels on the adhesion and the topography image as routine input.
2.2. Methods 2.2.1. Atomic force microscopy (AFM) All measurements were performed with a Dimension Icon scanning probe microscope (Bruker AXS, France; formerly Veeco) operated in quantitative nanomechanical mapping (QNM) mode with cantilevers as described above. Amongst others, topography and adhesion force maps were recorded. The silicon probes deflection sensitivities were calibrated by taking a force-distance curve on a sapphire sample (from the peakforce QNM sample kit provided by Bruker). The calculated values were all found to be close to 60 nm/V. The deflection sensitivities of the OH-modified probes were generously estimated in order to protect the OH-functionalisation. As mainly relative adhesion force differences are in focus, the results should not be affected by such an approach. The cantilevers spring constants were determined by thermal tuning (Lorentzian fit, frequency range from 1 kHz to 100 kHz) and were in the range between 0.3 N/m and 0.5 N/m. By combining the scan size and the scan frequency properly the tips movement speed was fixed at 2.5 μm/s for all measurements. Excitation frequency was set to 1 kHz and peak force to 200 pN to minimize tip wear. As a result of keeping the mentioned parameters constant the contact time of tip and sample was kept constant for each tapping cycle as well, which should lead to similar nanomechanical interaction behaviour and therefore consistently comparable adhesion forces for measurements done with the same tip. On the OHeCH3 functionalised sample QNM measurements were taken in ambient conditions (50% RH, 22 °C) as well as in a defined controlled humidity range (40–70% RH, 22 °C) using a humidity generator (Wetsys, Setaram Instrumentation, France) with Argon as carrier medium. The humid Argon was transported through small pipes into a special environment chamber (Bruker) implemented into the AFM and containing the sample, the AFM head as well as a temperature and humidity sensor (Sensirion, SHT7x, Stäfa, Switzerland). Equilibrium time was around 15 min for each humidity level. Measurements at humidity levels below 40% and above 70% were also preliminary performed, but they did not lead to reproducible results and were
3. Results and discussion 3.1. Micro-contact printed sample In case of the micro-contact printed sample, the scanning area of the AFM was chosen in a way that both CH3 modified spot and OH background would be recorded on the final image. The adhesion signal obtained using two different tip types is shown in Fig. 1. Only parts of the pictures containing both compared surfaces within the same measurement lines were examined. On the figure, this corresponds to the area in-between the two horizontal white lines. Transition regions between these compared surfaces were excluded from evaluation. At a first glance, it can be seen that the signal recorded with a sharp silicon tip is much noisier than the one recorded with an OH-modified tip. This can be easily explained by the difference in the size of these two kinds of tips. The OH-modified tip being much larger, its resolution power is lower as it provides data averaged on a larger contact area. This results in a “smoother” final picture. In both cases, a clear contrast in adhesion forces between the two different sample regions can be noticed: the CH3 modified spot is clearly visible in the centre of the image. Large variations in the adhesion force are also apparent. In Fig. 1A, small dark spots (i.e. low adhesion force) randomly distributed over the image can be observed. After scanning the same area several times, these spots could always be found at the same location: Undefined contaminations on the sample seem to be responsible for these artefacts. They were not included in further evaluation steps involving averaging. The same applies to Fig. 1B, where these spots are also present but less obvious because of the noise. The horizontal black lines which can be seen in Fig. 1 represent the location of the adhesion force and topography profiles shown in Fig. 2. For both tip types these profiles seem to show an interdependent behaviour between topography and adhesion. This may be related to the Fig. 1. Adhesion force pictures taken on the microcontact printed sample with an OH-modified tip (A) and a silicon tip (B). The white arrow represents the scanning direction. The area between the horizontal white lines was used for further processing, except between the circles representing the transition zones.
365
Colloids and Surfaces A 529 (2017) 363–372
J. Colson et al.
Fig. 2. Topography (black, plain) and adhesion profiles (red, dashed) corresponding to the black lines in Fig. 1 on the microcontact printed sample with an OH-modified (A, C) and a native silicon tip (B, D). (C) and (D) each include the transitions from OH to CH3 covered surfaces and correspond to scaled-up sections of (A) and (B), respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
influence of several factors as the asperities on the surface, its roughness as well as their relation to the tip size on the adhesion, since these parameters are influencing the actual contact area [31–33]. However,
the magnified views show that the interdependence between topography and adhesion is not absolute. To assess the correlation between changes in topography and
Table 1 Correlation between adhesion and topography signals on different areas of the micro-contact printed sample with OH and Silicon tips. Correlation coefficients between specific areas are indicated right below the pictures. The numbers in square brackets represent the correlation coefficients obtained when comparing the whole scanning area (see Fig. 1).
366
Colloids and Surfaces A 529 (2017) 363–372
J. Colson et al.
tips (Fig. 3 B). The markedness of the differences between the adhesion force values on the OH and on the CH3 modified surface seems to be mainly dependent on the individual tip, especially in the case of bare silicon tips. As humidity should have a biasing effect on the adhesion force [35,36], the measurements described above were also performed at several relative humidity levels. Results are plotted in Fig. 4. In these graphs, all output values were divided by the mean adhesion value for the CH3 modified surface of the respective measurement. In that way, the relative adhesion force on the CH3 modified surface is automatically set to 1 and the relation between adhesion force values on the CH3 and OH modified surfaces can be clearly seen. It also allows the different experiments to be compared with each other in a straightforward way. Mean adhesion force values are consistently higher for the OHmodified surface. Unexpectedly, OH-modified tips result in lower mean value relations than silicon tips. On the other hand, standard deviations are considerably smaller with OH-modified tips, implying less variation in the measurement itself. As the used silicon tips have a much smaller radius, their adhesion to the sample is probably more affected by its nanoscale roughness [31,32] and the local tip to sample interaction. The relation of adhesion values between CH3- and OH-modified surfaces does not show a significant dependence on relative humidity.
adhesion in a more precise way, a Matlab routine was written. Selected areas of the micro-contact printed sample as well as the whole scanning area were compared. This was done with unprocessed images straight from the AFM and with contamination corrected images. The correction was performed by selecting each contaminated spot and applying the “remove spots” tool available in Gwyddion, using the “hyperbolic flatten” interpolation method. This algorithm uses data from the boundaries of the selected area to interpolate the information inside the area [34]. The results are displayed in Table 1. With the OH tip, the correlation between adhesion and topography is strong on the uncorrected images. It decreases on both surfaces after levelling out the contaminations. Concerning the Silicon tip, the correlation coefficient is relatively low on the uncorrected images and decreases to values close to zero after image correction, implying that the adhesion and topography signals are almost uncorrelated. The silicon tip is therefore more suited than its OH modified counterpart when it comes to assessing adhesion and topography independently, which may be related to its much smaller contact area with the sample. The distribution of adhesion force values over the whole scanning area is shown in Fig. 3. The histograms all were normalised by setting the area under each curve to 1. They include the values obtained after several subsequent measurements (4–6) on the same area of the OH/ CH3-sample with each of the tips. Moreover, the measurement was repeated with two different tips of the same type (OH modified tip in Fig. 3A and C, Silicon tip in Fig. 3B and D) to assess the influence of the individual tip itself. In each histogram, isolated peaks at very low adhesion force values are present due to contaminants on the sample. Furthermore, most of the peaks are located as a group at higher adhesion forces. In these groups, consistently higher relative adhesion force values for the OH than for the CH3 modified surfaces can be observed, although this effect is not very pronounced for one of the silicon
3.2. Influence of the tip radius on flat OH and CH3-functionalised surfaces A flat OH-functionalised surface and a flat CH3 functionalised surface were placed into the AFM sample chamber. The aim was on one hand to assess the differences in adhesion force values on very flat surfaces with different polarities, and on the other hand to evaluate the reproducibility of such measurements. Moreover, the relative humidity in the AFM sample chamber was raised from 45% to 55% after the fifth
Fig. 3. Normalised histograms of adhesion values on OH and CH3 modified regions of the microcontact printed sample measured with two different OH-modified (A, C) and silicon tips (B, D) each in ambient conditions (22 °C, 50% relative humidity). To achieve a better visibility, y-axis was reversed for the CH3 modified surfaces.
367
Colloids and Surfaces A 529 (2017) 363–372
J. Colson et al.
Fig. 4. Relation of adhesion force values on different regions of the OH/CH3-sample measured with two different OH-modified (A, C) and sharp silicon tips (B, D) in controlled humidity (stepwise increase) in argon atmosphere. Obviously contaminated spots on the sample (see Fig. 1) were excluded from evaluation. Error bars represent the standard deviation.
Fig. 5. Relative adhesion on OH (red filled triangles) and CH3 (black hollow triangles) functionalised surfaces and the corresponding roughness on these OH (red filled squares) and CH3 (black hollow squares) surfaces in a series of subsequently performed measurements with a sharp silicon tip (A) and a rounded silicon tip (B). Error bars represent the standard deviation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
368
Colloids and Surfaces A 529 (2017) 363–372
J. Colson et al.
and from 55% to 65% after the tenth measurement. The first and the last point of every quintet were done on the same sample for comparison. The mean adhesion force and the mean roughness obtained in a series of subsequent measurements with the same Scanasyst Air tip are shown in Fig. 5A. To facilitate comparison, the output adhesion values were divided by the value obtained from the first measurement on CH3 for each series. Fig. 5B shows the results obtained under the exact same conditions with a rounded silicon tip. In all cases, the relative adhesion force on the R-CH3 surface is lower than on the R-OH surface. With the sharp silicon tip, the relative adhesion force slightly but steadily increases from 2.2 to 2.6 and from 1 to 1.5 on the R-OH and the R-CH3 surface, respectively. With the rounded silicon tip, the relative adhesion force is decreasing from 2.1 to 1.1 on the R-CH3 surface, whereas no trend is observable on the R-OH surface. Concerning the roughness, the pattern followed by a measurement quintet at the same humidity level is always nearly the same, but no explicit trend can be seen. An obvious contrast in the relative adhesion force between the OH and the CH3 functionalised surfaces is observable for both silicon tips at all tested humidity levels. The expected interdependence between roughness and adhesion force is visible. However, this is not the only parameter which should be taken into account. For the sharp silicon tip (Scanasyst Air), the mean adhesion force on both surfaces seems to increase with the humidity level. As chronologically the measurements were performed by raising the humidity level, a variation with time may also be involved. Moreover, an increase is also observable within one humidity level (for instance, compare the values obtained on the OH-modified surface at 55% RH in Fig. 5A). This might indicate that the tip wear plays a more important role than the humidity variation. Moreover, this trend is not observable anymore with the rounded silicon tip, which seems to be less prone to wear.
However, samples used in materials sciences tend to be much rougher. It would therefore not be reasonable to extend the validity of the obtained results to more realistic samples without additional measurements. That is why a wood/wax sample was also studied. Hydrophilicity of the wood and hydrophobicity of the wax provide a polarity contrast which should be recognizable in the AFM measurements. Topography and adhesion force signals from the wood/wax sample are shown in Fig. 6. The topography was levelled by using the plane substraction tool available in Gwyddion and shifting the minimum height value to zero, while the adhesion picture is unprocessed. Wax and wood secondary S2 cell wall layer are marked on the figure. With a silicon tip, the adhesion force is much lower in the wax than in the S2, as can be clearly seen from the corresponding signal (Fig. 6B). When an OH-modified tip is used, the clear contrast between the wax and the S2 cell wall disappears. Moreover the image quality is lower, showing numerous horizontal “scratches” on the adhesion signal (Fig. 6 D). Topography signals yield similar results for both kinds of tips (Fig. 6A and C) whereas adhesion signals differ. Fig. 7 shows topography and adhesion force profiles located at the horizontal lines inserted into the previous figure. The two vertical green lines delimit the transition from the wax to the S2 cell wall. Even if the sample preparation involved the use of a microtome equipped with a very sharp diamond blade, the height differences are still in the range of tens of nanometres (on the model surfaces, these differences reached at most 2 nm). With a silicon tip, the mean adhesion force on the wax is clearly lower than on the S2 cell wall, with a larger variation on the wax. In the transition zone between the green lines, both topography and adhesion increase. However, the topography then decreases again whereas the adhesion value stays more elevated than on the wax (Fig. 7A). With an OH-modified tip, the topography signal behaves similarly than with a Silicon tip. Indeed, the height is measured through recording cantilever deflection, which is mostly independent from the tip coating. The adhesion signal on Fig. 7B shows similar values on the wax and on the S2 cell wall, thus making it impossible to differentiate between two differently polar areas with the modified tip.
3.3. Wood/wax sample As described and discussed in the previous sections differently polar areas can be distinguished with bare silicon tips on flat model surfaces.
Fig. 6. Topography and adhesion force pictures obtained on a wood/wax sample (pine, cross section) containing wax and different parts of the wood cell wall. Topography recorded with a silicon tip (A) and corresponding adhesion force signal (B). Topography recorded with an OH-modified tip (C) and corresponding adhesion force signal (D). Horizontal black lines correspond to the locations used for profile extraction (see Fig. 7).
369
Colloids and Surfaces A 529 (2017) 363–372
J. Colson et al.
Fig. 7. Topography (black, plain) and adhesion force (red, dashed) profiles on the wood/wax-sample measured with a native silicon tip (A) and an OH-modified tip (B). Profiles were extracted at the position of the corresponding black line in Fig. 6. The vertical green lines represent the transition between the wax and the S2 cell wall. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
As for the micro-contact printed sample, the correlation between adhesion and topography was assessed more precisely on the wood/ wax sample by using the above mentioned Matlab routine (Table 2).
The correlation is low on the whole picture (r = 0.310). On the S2 cell wall, it becomes even lower (r = −0.214), while the signals are uncorrelated on the wax (r = −0.029). This indicates that even on
Table 2 Correlation between adhesion and topography on the wood/wax sample with a sharp silicon tip. The correlation coefficient on the whole picture (see Fig. 6) was 0.310.
370
Colloids and Surfaces A 529 (2017) 363–372
J. Colson et al.
polymer surfaces such as wood can be performed with the cheapest tips available, were a small tip radius combined with robustness is important. Acknowledgments Financial support was provided by the Austrian Science Fund FWF (P 27344). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa.2017.06.017. References [1] G. Binnig, C.F. Quate, Atomic force microscope, Phys. Rev. Lett. 56 (1986) 930–933, http://dx.doi.org/10.1103/PhysRevLett.56.930. [2] Y. Sugimoto, P. Pou, M. Abe, P. Jelinek, R. Pérez, S. Morita, et al., Chemical identification of individual surface atoms by atomic force microscopy, Nature 446 (2007) 64, http://dx.doi.org/10.1038/nature05530. [3] F. Leite, P. Herrmann, Application of atomic force spectroscopy (AFS) to studies of adhesion phenomena: a review, At. Force Microsc. Adhes. Stud. 19 (2005) 3–42, http://dx.doi.org/10.1163/1568561054352667. [4] M. Radmacher, J.P. Cleveland, M. Fritz, H.G. Hansma, P.K. Hansma, Mapping interaction forces with the atomic force microscope, Biophys. J. 66 (1994) 2159–2165, http://dx.doi.org/10.1016/S0006-3495(94)81011-2. [5] F.L. Leite, L.H.C. Mattoso, O.N. Oliveira Jr, P.S.P. Herrmann Jr, The atomic force spectroscopy as a tool to investigate surface forces: basic principles and applications, Mod. Res. Educ. Top. Microsc. (2007) 747–757. [6] J. Hoh, J. Cleveland, C. Prater, J. Revel, P. Hansma, Quantized adhesion forces detected with the atomic force microscope, J. Am. Chem. Soc. 114 (1992) 4917–4918, http://dx.doi.org/10.1021/ja00038a075. [7] H.J. Butt, B. Cappella, M. Kappl, Force measurements with the atomic force microscope: technique, interpretation and applications, Surf. Sci. Rep. 59 (2005) 1–152, http://dx.doi.org/10.1016/j.surfrep.2005.08.003. [8] A. Noy, S. Zepeda, C.A. Orme, Y. Yeh, J.J. De Yoreo, Entropic barriers in nanoscale adhesion studied by variable temperature chemical force microscopy, J. Am. Chem. Soc. 125 (2003) 1356–1362, http://dx.doi.org/10.1021/ja026865p. [9] A. Çolak, H. Wormeester, H.J.W. Zandvliet, B. Poelsema, Surface adhesion and its dependence on surface roughness and humidity measured with a flat tip, Appl. Surf. Sci. 258 (2012) 6938–6942, http://dx.doi.org/10.1016/j.apsusc.2012.03.138. [10] A. Ata, Y. Rabinovich, R. Singh, Role of surface roughness in capillary adhesion, J. Adhes. Sci. Technol. 16 (2002) 337–346, http://dx.doi.org/10.1163/ 156856102760067145. [11] L. Zitzler, S. Herminghaus, F. Mugele, Capillary forces in tapping mode atomic force microscopy, Phys. Rev. B 66 (2002) 1–8, http://dx.doi.org/10.1103/PhysRevB.66. 155436. [12] Y. Zou, S. Jayasuriya, C.W. Manke, G. Mao, Influence of nanoscale surface roughness on colloidal force measurements, Langmuir 31 (2015) 10341–10350, http:// dx.doi.org/10.1021/acs.langmuir.5b02672. [13] A. Noy, D.V. Vezenov, C.M. Lieber, Chemical force microscopy, Annu. Rev. Mater. Sci. 27 (1997) 381–421, http://dx.doi.org/10.1146/annurev.matsci.27.1.381. [14] C.D. Frisbie, L.F. Rozsnyai, A. Noy, M.S. Wrighton, C.M. Lieber, Functional-group imaging by chemical force microscopy, Science 80 (265) (1994) 2071–2074, http:// dx.doi.org/10.1126/science.265.5181.2071. [15] T. Ito, S. Ibrahim, I. Grabowska, Chemical-force microscopy for materials characterization, TrAC – Trends Anal. Chem. 29 (2010) 225–233, http://dx.doi.org/10. 1016/j.trac.2009.12.008. [16] A. Kumar, G.M. Whitesides, Features of gold having micrometer to centimeter dimensions can be formed through a combination of stamping with an elastomeric stamp and an alkanethiol ink followed by chemical etching, Appl. Phys. Lett. 63 (1993) 2002–2004, http://dx.doi.org/10.1063/1.110628. [17] E.W. van der Vegte, G. Hadziioannou, Scanning force microscopy with chemical specificity: an extensive study of chemically specific tip-surface interactions and the chemical imaging of surface functional groups, Langmuir 13 (1997) 4357–4368, http://dx.doi.org/10.1021/la970025k. [18] H. Azehara, Y. Kasanuma, K. Ide, K. Hidaka, H. Tokumoto, Distinct chemical contrast in adhesion force images of hydrophobic- hydrophilic patterned surfaces using multiwalled carbon nanotube probe tips, Jpn. J. Appl. Phys. 47 (2008) 3594–3599, http://dx.doi.org/10.1143/JJAP.47.3594. [19] E. Gustafsson, E. Johansson, L. Wågberg, T. Pettersson, Direct adhesive measurements between wood biopolymer model surfaces, Biomacromolecules 13 (2012) 3046–3053, http://dx.doi.org/10.1021/bm300762e. [20] M. Meincken, P.D. Evans, Nanoscale characterization of wood photodegradation using atomic force microscopy, Eur. J. Wood Wood Prod. 67 (2009) 229–231, http://dx.doi.org/10.1007/s00107-008-0305-x. [21] S. Frybort, M. Obersriebnig, U. Müller, W. Gindl-Altmutter, J. Konnerth, Variability in surface polarity of wood by means of AFM adhesion force mapping, Colloids Surf. A Physicochem. Eng. Asp. 457 (2014) 82–87, http://dx.doi.org/10.1016/j.colsurfa. 2014.05.055.
Fig. 8. Relation of normalised adhesion values on different sample regions of the wood/ wax sample measured with OH-modified and silicon tips in controlled ambient conditions. Error bars represent the standard deviation.
comparably rough natural materials regions having different polarities can be differentiated by analysing the adhesion signal obtained after scanning the sample with a standard silicon tip. This is confirmed after several repetitions of the measurements with silicon and OH modified tips. The summary of the results, presented in Fig. 8, shows the adhesion values obtained on the wax and on the S2 cell wall. Output values where divided by the mean value of the adhesion force on the wax for each measurement. The figure represents averaged values from several measurements (3–6) for each tip. The by far best contrast in adhesion force between the two sample regions, wax and S2 cell wall, is generally given by unmodified silicon tips. Considering that these tips were 9 times cheaper than the modified ones, this result may substantially impact the choices made with regard to scanning probes to be used in further studies of this kind. Only for the first silicon tip the two values can barely be differentiated, which may be related to a flaw in the tip shape or a tip contamination present even before the start of the measurement. Thus we recommend verifying the capability of each single silicon tip to provide a good contrast on a known polarity contrast rich surface prior to use on an unknown surface. Results obtained with rounded silicon tips and OH modified tips are very similar, indicating that the tip radius plays an important role in the reachable resolution (the larger the tip radius, the larger the tip/ sample contact area and hence the higher the influence of asperities on adhesion). Once again we conclude that the actual performance relating to the contrast in adhesion force depends on the single tip used. 4. Conclusions Silicon tips and tips with hydrophilic functionalisation were compared with regard to their adhesion behaviour on 3 different kinds of samples in humidity controlled conditions. In all cases, adhesion was higher on hydrophilic than on hydrophobic surfaces. Even though the influence of sample roughness on the adhesion signal (caused by the changing contact area) could not be totally eliminated, it appears that a small tip radius results in a high adhesion force contrast. As a consequence, the unmodified (and therefore comparably sharp) standard silicon tips were very well suited when it comes to their ability to distinguish polarity changes on a sample surface – even if they were not originally designed for this purpose! Neither rounded silicon tips nor OH-modified tips could reach a similarly high contrast. OH-modified tips could be assumed to perform better, but this was counterbalanced by the influence of roughness and tip size. Moreover, as standard silicon tips are sturdier than their OH-modified counterparts, they can be used on relatively rough samples (e.g. wood) without losing their sensitivity. As a conclusion it may be stated that polarity differentiation on natural 371
Colloids and Surfaces A 529 (2017) 363–372
J. Colson et al.
03.164. [30] Y.S. Lo, N.D. Huefner, W.S. Chan, P. Dryden, B. Hagenhoff, T.P. Beebe, Organic and inorganic contamination on commercial AFM cantilevers, Langmuir 15 (1999) 6522–6526, http://dx.doi.org/10.1021/la990371x. [31] A. Mendez-Vilas, M.L. Gonzalez-Martin, L. Labajos-Broncano, M.J. Nuevo, Experimental analysis of the influence of surface topography on the adhesion force as measured by an AFM, J. Adhes. Sci. Technol. 16 (2002) 1737–1747, http://dx. doi.org/10.1163/156856102320396111. [32] N. Islam, P. Stewart, I. Larson, P. Hartley, Surface roughness contribution to the adhesion force distribution of salmeterol xinafoate on lactose carriers by atomic force microscopy, J. Pharm. Sci. 94 (2005) 1500–1511, http://dx.doi.org/10.1002/ jps.20381. [33] Y.I. Rabinovich, J.J. Adler, A. Ata, R.K. Singh, B.M. Moudgil, Adhesion between nanoscale rough surfaces – I. Role of asperity geometry, J. Colloid Interface Sci. 232 (2000) 10–16, http://dx.doi.org/10.1006/jcis.2000.7167. [34] P. Klapetek, D. Nečas, C. Anderson, Gwyddion User Guide, (2017). [35] D.L. Sedin, K.L. Rowlen, Adhesion forces measured by atomic force microscopy in humid air, Anal. Chem. 72 (2000) 2183–2189, http://dx.doi.org/10.1021/ ac991198c. [36] R. Jones, H.M. Pollock, J.A.S. Cleaver, C.S. Hodgest, Adhesion forces between glass and silicon surfaces in air studied by AFM: effects of relative hunidity, particle size, roughness, and surface treatment, Langmuir 18 (2002) 8045–8055.
[22] X. Tan, K. Li, Adhesion forces between ligno-cellulose surfaces by atomic force microscopy, J. Pulp Pap. Sci. 34 (2008) 77–85. [23] A.A. Kafi, K. Magniez, B.L. Fox, Measuring the adhesion force on natural fibre surface using scanning probe microscopy, J. Adhes. Sci. Technol. 26 (2012) 175–187, http://dx.doi.org/10.1163/016942411X569381. [24] X. Jin, B. Kasal, Adhesion force mapping on wood by atomic force microscopy: influence of surface roughness and tip geometry, R. Soc. Open Sci. 3 (2016), http:// dx.doi.org/10.1098/rsos.160248. [25] A.H. Al-Bayati, K.G. Orrman-Rossiter, J.A. van den Berg, D.G. Armour, Composition and structure of the native Si oxide by high depth resolution medium energy ion scatering, Surf. Sci. 241 (1991) 91–102, http://dx.doi.org/10.1016/0039-6028(91) 90214-d. [26] M. Morita, T. Ohmi, E. Hasegawa, M. Kawakami, M. Ohwada, Growth of native oxide on a silicon surface, J. Appl. Phys. 68 (1990) 1272–1281, http://dx.doi.org/ 10.1063/1.347181. [27] A. Redondo, W.A. Goddard, C.A. Swarts, T.C. McGill, Oxidation of silicon surfaces, J. Vac. Sci. Technol. 19 (1981) 498–501. [28] A. Verdaguer, C. Weis, G. Oncins, G. Ketteler, H. Bluhm, M. Salmeron, Growth and Structure of water on SiO2 films on Si investigated by Kelvin probe microscopy and in situ X-ray Spectroscopies, Langmuir 23 (2008) 9699–9703. [29] J.C. Cau, L. Lafforgue, M. Nogues, A. Lagraulet, V. Paveau, Magnetic field assisted microcontact printing: a new concept of fully automated and calibrated process, Microelectron. Eng. 110 (2013) 207–214, http://dx.doi.org/10.1016/j.mee.2013.
372
Contents lists available at ScienceDirect
Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Comparison of silicon and OH-modified AFM tips for adhesion force analysis on functionalised surfaces and natural polymers
MARK
⁎
Jérôme Colsona, ,1, Laurin Andorfera,1, Tiina Elina Nypelöa, Bernd Lütkemeierb, Frank Stöckelc, Johannes Konnertha a University of Natural Resources and Life Sciences Vienna, Department of Materials Sciences and Process Engineering, Institute of Wood Technology and Renewable Materials, Konrad-Lorenz-Straße 24, 3430 Tulln, Austria b Georg-August-Universität Göttingen, Department of Wood Biology & Wood Products, Büsgenweg 4, 37077 Göttingen, Germany c Technical University Braunschweig, Institute for Building Materials, Solid Construction and Fire Protection (IBMB), Department of Organic and Wood-Based Materials, Hopfengarten 20, 38102 Braunschweig, Germany
G RA P H I C A L AB S T R A C T
A R T I C L E I N F O
A B S T R A C T
Keywords: Atomic force microscopy Silicon tip Functionalised tip Polarity Model surface Wood
In this paper, the position resolved adhesion behaviour of AFM cantilevers with standard silicon tips, rounded silicon tips and OH-modified tips was compared. Surfaces of a flat functionalised microscopy glass slide (hydrophilic background with hydrophobic spots) and of a comparably rough wood/wax sample were scanned in gaseous atmosphere. These two samples were chosen because both of them contain polar and non-polar regions within an area small enough to be scanned at once by the AFM without relevant quality loss due high scan speeds. Moreover, OH and CH3 modified cantilever chips providing very flat functionalised surfaces were scanned. Except for the wood/wax sample, measurements were performed at different humidity levels – with only very little influence on the measured adhesion. Both silicon and OH modified tips showed a higher adhesion on the polar regions of each sample than on the non-polar ones. The difference between the adhesion values on the polar and non-polar surfaces was however systematically higher when standard silicon tips were used. This was also true on the wood/wax sample. As silicon tips are relatively cheap, robust and have a much smaller
⁎
Corresponding author. E-mail addresses: [email protected] (J. Colson), [email protected] (L. Andorfer), [email protected] (T.E. Nypelö), [email protected] (B. Lütkemeier), [email protected] (F. Stöckel), [email protected] (J. Konnerth). 1 These authors contributed equally. http://dx.doi.org/10.1016/j.colsurfa.2017.06.017 Received 22 March 2017; Received in revised form 7 June 2017; Accepted 8 June 2017 Available online 10 June 2017 0927-7757/ © 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Colloids and Surfaces A 529 (2017) 363–372
J. Colson et al.
radius than typical functionalised tips, they allow simple high resolution differentiation of polar and non-polar domains even when the sample surface is relatively rough, as it is the case for natural wood-based polymers.
photodegradation [20], surface aging [21], or adhesion forces on fibres [22,23] can be found in literature. The adhesion forces on bulk wood were also investigated [24], however this study was operated in contact mode, which is damaging to both the sample and the tip. Taking all this into account we want to show that peak force tapping measurements with chemical specificity (i.e. contrast in polarity) can be performed in ambient conditions with bare silicon AFM tips without any modification. Indeed, silicon surfaces have been shown to be quickly covered by a thin oxide layer [25–27], itself covered by a few water monolayers in ambient conditions [28]. In the case of silicon AFM tips, this process is often unwanted, but for some purposes it can also be considered as a useful “natural functionalisation” of the scanning probe. Obviously the interaction with the sample is expected to be less pronounced as it is expected with intentionally OH-modified tips, but the silicon tips come with a couple of advantages: They are cheap, sturdy and available with relatively small radii and thus preferable in terms of spatial resolution when compared to the chemically modified tips used for CFM, where chromium and gold layers are typically applied prior to chemical functionalization, thus increasing the tip radius. In order to prove this assertion OH-modified and bare silicon tips have been compared on several samples in ambient and controlled humidity.
1. Introduction The atomic force microscope (AFM) was invented in the 1980′s [1] and has since then proven to be a powerful tool for high-resolution surface analysis. Its capabilities have been greatly enhanced over the years, making it even able to resolve single atoms if the right conditions are given [2]. One of these enhancements is the atomic force spectroscopy (AFS) [3] in which the interaction between AFM tip and sample on approach and retract is continuously recorded while the cantilever is oscillated and the sample is raster-scanned. This method provides the possibility to simultaneously image numerous physicochemical properties of a surface at the nanometre scale and even in molecular resolution in some cases [4–6]. For this purpose, the cantilever is excited far below its resonance frequency, resulting in an approximately sinusoidal oscillation. When brought into close proximity of a surface, the tip attached to the cantilever will tap this surface at each oscillation, thereby causing disturbances in the periodic motion. For each tapping cycle, superposition of these disturbances with the original motion results in a so-called force curve. Amongst others, information as stiffness, adhesion forces, friction forces or topography can be extracted from these curves and from the other recorded signals (e.g. piezo displacement) [4]. The exact shape of the force-distance curves depends on numerous interacting forces: electrostatic forces, van der Waals forces and capillary forces between the tip and the scanned surface [7]. Tip and sample characteristics (chemical composition, sample roughness, tip radius) as well as environmental parameters (relative humidity, type of surrounding medium, temperature) determine the nature and the magnitude of these interactions [8–12]. This interplay of a high amount of parameters influencing the final results is one of the major drawbacks of the method, as most of the obtained values are specific to the single tip-sample configuration used. Variation of tip radius during the measurements due to tip wear as well as the need for accurate calibration of cantilever stiffness and deflection sensitivity make it difficult to obtain trustworthy absolute values. That is why relative values obtained by comparing several measurements done with the same tip are often preferred. A closer look on these issues will be taken in the discussion. When performing an AFS measurement, a chemically modified tip can be used instead of a native silicon tip. These measurements are usually performed in various liquids biasing the tip to sample interaction. This method, called chemical force microscopy (CFM), results in the tip being particularly sensitive to specific regions of the sample surface thus allowing the distinction of functional groups exhibiting different chemical properties [13]. For example, Frisbie et al. [14] observed significant differences in the frictional and adhesive contrast in agreement with chemical intuition between regions on a pattern covered with more or less polar functional groups. With an adequately sophisticated experimental setup, local chemical interactions between functionalised AFM tips and sample surfaces were even shown to be measureable down to the scale of single molecules [15]. To limit the number of parameters influencing the measurement, model surfaces are often scanned. These surfaces can be created by microcontact printing (μCP) [16] and have been extensively studied in the past, e.g. with chemically modified tips [17] or with carbon nanotubes attached to the tip apex [18]. The tip functionalisation generally results in a better contrast than with standard silicon tips. Similar measurements were also performed on natural materials like cellulose. In this case, flat surfaces can be produced by layer by layer (LbL) deposition [19]. Even rough natural materials like wood are getting into the focus of some research groups in the past years: Studies about
2. Materials and methods 2.1. Materials 2.1.1. R-OH and R-CH3 functionalised sample By the courtesy of Innopsys (Carbonne, France) a pattern of CH3functionalised spots (about 2.5 μm in diameter, about 10 μm distance from centre to centre) on OH-functionalised background was provided. The pattern had been spread on an optical microscopy glass slide by magnetic field assisted μCP [29]. 2.1.2. AFM cantilevers AFM cantilever chips with OH-modified and CH3-modified surfaces (tip radius around 15 nm) were acquired from Nanocraft (Nanocraft Coating GmbH, Engen, Germany). Sharp silicon tips (Scanasyst Air, nominal tip radius 2 nm) were purchased from Bruker, USA. Rounded silicon tips (Special Developments Rounded Tips R30, tip radius 30 nm) were purchased from Nanosensors (Neuchatel, Switzerland). The silicon tips were stored in closed boxes on poly(dimethylsiloxane) (PDMS), which may lead to contamination of the tips [30]. However, it was decided to work with the cantilevers without applying an aggressive cleaning which could result in larger tip radii. The OH-modified and CH3-modified cantilevers were stored in closed glass boxes in Argon atmosphere. The OH-modified cantilevers from Nanocraft were used both as scanning probes and as sample surfaces. To avoid confusions, they will be called “flat OH-functionalised surface” in the case they serve as a sample to be scanned, while “OH-modified tip” in the text will stand for the scanning probe. 2.1.3. Wood/wax sample A wood sample (pinus radiata) was cut into boards of 1000 × 150 × 30 mm3. These boards were immersed in wax (Sasol, Sasolwax C105, Hamburg, Germany) and subjected to a vacuum of 6 kPa and a temperature of 120 °C for one hour. In the following step the wax was pressed into the wood with a pressure of 1.2 MPa and a temperature of 120 °C. After 14 h under these conditions, most of the lumina were filled with wax and the wood had gained 110% of weight. 364
Colloids and Surfaces A 529 (2017) 363–372
J. Colson et al.
discarded. For the measurements on the flat OH and CH3-functionalised surfaces, humidity steps of 45%, 55%, and 65% were chosen. Measurements on the wood/wax sample were only done in ambient conditions (50% RH, 22 °C). Evaluation of the output data from the AFM was done with the Gwyddion 2.39 freeware (www.gwyddion.net).
The wood was then further disintegrated into cross sections of approximately 1 × 3 × 2 mm3 which were glued on AFM specimen discs with epoxy adhesive (Uhu, Uhu plus sofortfest, Bühl/Baden, Germany) and finally microtomed (Leica, Ultracut R, Wetzlar, Germany). The final cuts were done with an ultra AFM diamond blade (Diatome, Ultra AFM, Biel, Switzerland) reducing the thickness of the cut off sections in steps down to approximately 10 nm for smooth surfaces. After preparation the sample was stored in ambient air (23 °C, 55% relative humidity) for three days to reduce the effect of dwindling adhesion due to wood aging [21] between subsequent measurements.
2.2.2. Matlab code The correlation between topography and adhesion data from the AFM was determined with a short Matlab routine. The routine as well as an explanatory text about it can be found in supplementary information. Care was taken to select the exact same pixels on the adhesion and the topography image as routine input.
2.2. Methods 2.2.1. Atomic force microscopy (AFM) All measurements were performed with a Dimension Icon scanning probe microscope (Bruker AXS, France; formerly Veeco) operated in quantitative nanomechanical mapping (QNM) mode with cantilevers as described above. Amongst others, topography and adhesion force maps were recorded. The silicon probes deflection sensitivities were calibrated by taking a force-distance curve on a sapphire sample (from the peakforce QNM sample kit provided by Bruker). The calculated values were all found to be close to 60 nm/V. The deflection sensitivities of the OH-modified probes were generously estimated in order to protect the OH-functionalisation. As mainly relative adhesion force differences are in focus, the results should not be affected by such an approach. The cantilevers spring constants were determined by thermal tuning (Lorentzian fit, frequency range from 1 kHz to 100 kHz) and were in the range between 0.3 N/m and 0.5 N/m. By combining the scan size and the scan frequency properly the tips movement speed was fixed at 2.5 μm/s for all measurements. Excitation frequency was set to 1 kHz and peak force to 200 pN to minimize tip wear. As a result of keeping the mentioned parameters constant the contact time of tip and sample was kept constant for each tapping cycle as well, which should lead to similar nanomechanical interaction behaviour and therefore consistently comparable adhesion forces for measurements done with the same tip. On the OHeCH3 functionalised sample QNM measurements were taken in ambient conditions (50% RH, 22 °C) as well as in a defined controlled humidity range (40–70% RH, 22 °C) using a humidity generator (Wetsys, Setaram Instrumentation, France) with Argon as carrier medium. The humid Argon was transported through small pipes into a special environment chamber (Bruker) implemented into the AFM and containing the sample, the AFM head as well as a temperature and humidity sensor (Sensirion, SHT7x, Stäfa, Switzerland). Equilibrium time was around 15 min for each humidity level. Measurements at humidity levels below 40% and above 70% were also preliminary performed, but they did not lead to reproducible results and were
3. Results and discussion 3.1. Micro-contact printed sample In case of the micro-contact printed sample, the scanning area of the AFM was chosen in a way that both CH3 modified spot and OH background would be recorded on the final image. The adhesion signal obtained using two different tip types is shown in Fig. 1. Only parts of the pictures containing both compared surfaces within the same measurement lines were examined. On the figure, this corresponds to the area in-between the two horizontal white lines. Transition regions between these compared surfaces were excluded from evaluation. At a first glance, it can be seen that the signal recorded with a sharp silicon tip is much noisier than the one recorded with an OH-modified tip. This can be easily explained by the difference in the size of these two kinds of tips. The OH-modified tip being much larger, its resolution power is lower as it provides data averaged on a larger contact area. This results in a “smoother” final picture. In both cases, a clear contrast in adhesion forces between the two different sample regions can be noticed: the CH3 modified spot is clearly visible in the centre of the image. Large variations in the adhesion force are also apparent. In Fig. 1A, small dark spots (i.e. low adhesion force) randomly distributed over the image can be observed. After scanning the same area several times, these spots could always be found at the same location: Undefined contaminations on the sample seem to be responsible for these artefacts. They were not included in further evaluation steps involving averaging. The same applies to Fig. 1B, where these spots are also present but less obvious because of the noise. The horizontal black lines which can be seen in Fig. 1 represent the location of the adhesion force and topography profiles shown in Fig. 2. For both tip types these profiles seem to show an interdependent behaviour between topography and adhesion. This may be related to the Fig. 1. Adhesion force pictures taken on the microcontact printed sample with an OH-modified tip (A) and a silicon tip (B). The white arrow represents the scanning direction. The area between the horizontal white lines was used for further processing, except between the circles representing the transition zones.
365
Colloids and Surfaces A 529 (2017) 363–372
J. Colson et al.
Fig. 2. Topography (black, plain) and adhesion profiles (red, dashed) corresponding to the black lines in Fig. 1 on the microcontact printed sample with an OH-modified (A, C) and a native silicon tip (B, D). (C) and (D) each include the transitions from OH to CH3 covered surfaces and correspond to scaled-up sections of (A) and (B), respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
influence of several factors as the asperities on the surface, its roughness as well as their relation to the tip size on the adhesion, since these parameters are influencing the actual contact area [31–33]. However,
the magnified views show that the interdependence between topography and adhesion is not absolute. To assess the correlation between changes in topography and
Table 1 Correlation between adhesion and topography signals on different areas of the micro-contact printed sample with OH and Silicon tips. Correlation coefficients between specific areas are indicated right below the pictures. The numbers in square brackets represent the correlation coefficients obtained when comparing the whole scanning area (see Fig. 1).
366
Colloids and Surfaces A 529 (2017) 363–372
J. Colson et al.
tips (Fig. 3 B). The markedness of the differences between the adhesion force values on the OH and on the CH3 modified surface seems to be mainly dependent on the individual tip, especially in the case of bare silicon tips. As humidity should have a biasing effect on the adhesion force [35,36], the measurements described above were also performed at several relative humidity levels. Results are plotted in Fig. 4. In these graphs, all output values were divided by the mean adhesion value for the CH3 modified surface of the respective measurement. In that way, the relative adhesion force on the CH3 modified surface is automatically set to 1 and the relation between adhesion force values on the CH3 and OH modified surfaces can be clearly seen. It also allows the different experiments to be compared with each other in a straightforward way. Mean adhesion force values are consistently higher for the OHmodified surface. Unexpectedly, OH-modified tips result in lower mean value relations than silicon tips. On the other hand, standard deviations are considerably smaller with OH-modified tips, implying less variation in the measurement itself. As the used silicon tips have a much smaller radius, their adhesion to the sample is probably more affected by its nanoscale roughness [31,32] and the local tip to sample interaction. The relation of adhesion values between CH3- and OH-modified surfaces does not show a significant dependence on relative humidity.
adhesion in a more precise way, a Matlab routine was written. Selected areas of the micro-contact printed sample as well as the whole scanning area were compared. This was done with unprocessed images straight from the AFM and with contamination corrected images. The correction was performed by selecting each contaminated spot and applying the “remove spots” tool available in Gwyddion, using the “hyperbolic flatten” interpolation method. This algorithm uses data from the boundaries of the selected area to interpolate the information inside the area [34]. The results are displayed in Table 1. With the OH tip, the correlation between adhesion and topography is strong on the uncorrected images. It decreases on both surfaces after levelling out the contaminations. Concerning the Silicon tip, the correlation coefficient is relatively low on the uncorrected images and decreases to values close to zero after image correction, implying that the adhesion and topography signals are almost uncorrelated. The silicon tip is therefore more suited than its OH modified counterpart when it comes to assessing adhesion and topography independently, which may be related to its much smaller contact area with the sample. The distribution of adhesion force values over the whole scanning area is shown in Fig. 3. The histograms all were normalised by setting the area under each curve to 1. They include the values obtained after several subsequent measurements (4–6) on the same area of the OH/ CH3-sample with each of the tips. Moreover, the measurement was repeated with two different tips of the same type (OH modified tip in Fig. 3A and C, Silicon tip in Fig. 3B and D) to assess the influence of the individual tip itself. In each histogram, isolated peaks at very low adhesion force values are present due to contaminants on the sample. Furthermore, most of the peaks are located as a group at higher adhesion forces. In these groups, consistently higher relative adhesion force values for the OH than for the CH3 modified surfaces can be observed, although this effect is not very pronounced for one of the silicon
3.2. Influence of the tip radius on flat OH and CH3-functionalised surfaces A flat OH-functionalised surface and a flat CH3 functionalised surface were placed into the AFM sample chamber. The aim was on one hand to assess the differences in adhesion force values on very flat surfaces with different polarities, and on the other hand to evaluate the reproducibility of such measurements. Moreover, the relative humidity in the AFM sample chamber was raised from 45% to 55% after the fifth
Fig. 3. Normalised histograms of adhesion values on OH and CH3 modified regions of the microcontact printed sample measured with two different OH-modified (A, C) and silicon tips (B, D) each in ambient conditions (22 °C, 50% relative humidity). To achieve a better visibility, y-axis was reversed for the CH3 modified surfaces.
367
Colloids and Surfaces A 529 (2017) 363–372
J. Colson et al.
Fig. 4. Relation of adhesion force values on different regions of the OH/CH3-sample measured with two different OH-modified (A, C) and sharp silicon tips (B, D) in controlled humidity (stepwise increase) in argon atmosphere. Obviously contaminated spots on the sample (see Fig. 1) were excluded from evaluation. Error bars represent the standard deviation.
Fig. 5. Relative adhesion on OH (red filled triangles) and CH3 (black hollow triangles) functionalised surfaces and the corresponding roughness on these OH (red filled squares) and CH3 (black hollow squares) surfaces in a series of subsequently performed measurements with a sharp silicon tip (A) and a rounded silicon tip (B). Error bars represent the standard deviation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
368
Colloids and Surfaces A 529 (2017) 363–372
J. Colson et al.
and from 55% to 65% after the tenth measurement. The first and the last point of every quintet were done on the same sample for comparison. The mean adhesion force and the mean roughness obtained in a series of subsequent measurements with the same Scanasyst Air tip are shown in Fig. 5A. To facilitate comparison, the output adhesion values were divided by the value obtained from the first measurement on CH3 for each series. Fig. 5B shows the results obtained under the exact same conditions with a rounded silicon tip. In all cases, the relative adhesion force on the R-CH3 surface is lower than on the R-OH surface. With the sharp silicon tip, the relative adhesion force slightly but steadily increases from 2.2 to 2.6 and from 1 to 1.5 on the R-OH and the R-CH3 surface, respectively. With the rounded silicon tip, the relative adhesion force is decreasing from 2.1 to 1.1 on the R-CH3 surface, whereas no trend is observable on the R-OH surface. Concerning the roughness, the pattern followed by a measurement quintet at the same humidity level is always nearly the same, but no explicit trend can be seen. An obvious contrast in the relative adhesion force between the OH and the CH3 functionalised surfaces is observable for both silicon tips at all tested humidity levels. The expected interdependence between roughness and adhesion force is visible. However, this is not the only parameter which should be taken into account. For the sharp silicon tip (Scanasyst Air), the mean adhesion force on both surfaces seems to increase with the humidity level. As chronologically the measurements were performed by raising the humidity level, a variation with time may also be involved. Moreover, an increase is also observable within one humidity level (for instance, compare the values obtained on the OH-modified surface at 55% RH in Fig. 5A). This might indicate that the tip wear plays a more important role than the humidity variation. Moreover, this trend is not observable anymore with the rounded silicon tip, which seems to be less prone to wear.
However, samples used in materials sciences tend to be much rougher. It would therefore not be reasonable to extend the validity of the obtained results to more realistic samples without additional measurements. That is why a wood/wax sample was also studied. Hydrophilicity of the wood and hydrophobicity of the wax provide a polarity contrast which should be recognizable in the AFM measurements. Topography and adhesion force signals from the wood/wax sample are shown in Fig. 6. The topography was levelled by using the plane substraction tool available in Gwyddion and shifting the minimum height value to zero, while the adhesion picture is unprocessed. Wax and wood secondary S2 cell wall layer are marked on the figure. With a silicon tip, the adhesion force is much lower in the wax than in the S2, as can be clearly seen from the corresponding signal (Fig. 6B). When an OH-modified tip is used, the clear contrast between the wax and the S2 cell wall disappears. Moreover the image quality is lower, showing numerous horizontal “scratches” on the adhesion signal (Fig. 6 D). Topography signals yield similar results for both kinds of tips (Fig. 6A and C) whereas adhesion signals differ. Fig. 7 shows topography and adhesion force profiles located at the horizontal lines inserted into the previous figure. The two vertical green lines delimit the transition from the wax to the S2 cell wall. Even if the sample preparation involved the use of a microtome equipped with a very sharp diamond blade, the height differences are still in the range of tens of nanometres (on the model surfaces, these differences reached at most 2 nm). With a silicon tip, the mean adhesion force on the wax is clearly lower than on the S2 cell wall, with a larger variation on the wax. In the transition zone between the green lines, both topography and adhesion increase. However, the topography then decreases again whereas the adhesion value stays more elevated than on the wax (Fig. 7A). With an OH-modified tip, the topography signal behaves similarly than with a Silicon tip. Indeed, the height is measured through recording cantilever deflection, which is mostly independent from the tip coating. The adhesion signal on Fig. 7B shows similar values on the wax and on the S2 cell wall, thus making it impossible to differentiate between two differently polar areas with the modified tip.
3.3. Wood/wax sample As described and discussed in the previous sections differently polar areas can be distinguished with bare silicon tips on flat model surfaces.
Fig. 6. Topography and adhesion force pictures obtained on a wood/wax sample (pine, cross section) containing wax and different parts of the wood cell wall. Topography recorded with a silicon tip (A) and corresponding adhesion force signal (B). Topography recorded with an OH-modified tip (C) and corresponding adhesion force signal (D). Horizontal black lines correspond to the locations used for profile extraction (see Fig. 7).
369
Colloids and Surfaces A 529 (2017) 363–372
J. Colson et al.
Fig. 7. Topography (black, plain) and adhesion force (red, dashed) profiles on the wood/wax-sample measured with a native silicon tip (A) and an OH-modified tip (B). Profiles were extracted at the position of the corresponding black line in Fig. 6. The vertical green lines represent the transition between the wax and the S2 cell wall. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
As for the micro-contact printed sample, the correlation between adhesion and topography was assessed more precisely on the wood/ wax sample by using the above mentioned Matlab routine (Table 2).
The correlation is low on the whole picture (r = 0.310). On the S2 cell wall, it becomes even lower (r = −0.214), while the signals are uncorrelated on the wax (r = −0.029). This indicates that even on
Table 2 Correlation between adhesion and topography on the wood/wax sample with a sharp silicon tip. The correlation coefficient on the whole picture (see Fig. 6) was 0.310.
370
Colloids and Surfaces A 529 (2017) 363–372
J. Colson et al.
polymer surfaces such as wood can be performed with the cheapest tips available, were a small tip radius combined with robustness is important. Acknowledgments Financial support was provided by the Austrian Science Fund FWF (P 27344). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa.2017.06.017. References [1] G. Binnig, C.F. Quate, Atomic force microscope, Phys. Rev. Lett. 56 (1986) 930–933, http://dx.doi.org/10.1103/PhysRevLett.56.930. [2] Y. Sugimoto, P. Pou, M. Abe, P. Jelinek, R. Pérez, S. Morita, et al., Chemical identification of individual surface atoms by atomic force microscopy, Nature 446 (2007) 64, http://dx.doi.org/10.1038/nature05530. [3] F. Leite, P. Herrmann, Application of atomic force spectroscopy (AFS) to studies of adhesion phenomena: a review, At. Force Microsc. Adhes. Stud. 19 (2005) 3–42, http://dx.doi.org/10.1163/1568561054352667. [4] M. Radmacher, J.P. Cleveland, M. Fritz, H.G. Hansma, P.K. Hansma, Mapping interaction forces with the atomic force microscope, Biophys. J. 66 (1994) 2159–2165, http://dx.doi.org/10.1016/S0006-3495(94)81011-2. [5] F.L. Leite, L.H.C. Mattoso, O.N. Oliveira Jr, P.S.P. Herrmann Jr, The atomic force spectroscopy as a tool to investigate surface forces: basic principles and applications, Mod. Res. Educ. Top. Microsc. (2007) 747–757. [6] J. Hoh, J. Cleveland, C. Prater, J. Revel, P. Hansma, Quantized adhesion forces detected with the atomic force microscope, J. Am. Chem. Soc. 114 (1992) 4917–4918, http://dx.doi.org/10.1021/ja00038a075. [7] H.J. Butt, B. Cappella, M. Kappl, Force measurements with the atomic force microscope: technique, interpretation and applications, Surf. Sci. Rep. 59 (2005) 1–152, http://dx.doi.org/10.1016/j.surfrep.2005.08.003. [8] A. Noy, S. Zepeda, C.A. Orme, Y. Yeh, J.J. De Yoreo, Entropic barriers in nanoscale adhesion studied by variable temperature chemical force microscopy, J. Am. Chem. Soc. 125 (2003) 1356–1362, http://dx.doi.org/10.1021/ja026865p. [9] A. Çolak, H. Wormeester, H.J.W. Zandvliet, B. Poelsema, Surface adhesion and its dependence on surface roughness and humidity measured with a flat tip, Appl. Surf. Sci. 258 (2012) 6938–6942, http://dx.doi.org/10.1016/j.apsusc.2012.03.138. [10] A. Ata, Y. Rabinovich, R. Singh, Role of surface roughness in capillary adhesion, J. Adhes. Sci. Technol. 16 (2002) 337–346, http://dx.doi.org/10.1163/ 156856102760067145. [11] L. Zitzler, S. Herminghaus, F. Mugele, Capillary forces in tapping mode atomic force microscopy, Phys. Rev. B 66 (2002) 1–8, http://dx.doi.org/10.1103/PhysRevB.66. 155436. [12] Y. Zou, S. Jayasuriya, C.W. Manke, G. Mao, Influence of nanoscale surface roughness on colloidal force measurements, Langmuir 31 (2015) 10341–10350, http:// dx.doi.org/10.1021/acs.langmuir.5b02672. [13] A. Noy, D.V. Vezenov, C.M. Lieber, Chemical force microscopy, Annu. Rev. Mater. Sci. 27 (1997) 381–421, http://dx.doi.org/10.1146/annurev.matsci.27.1.381. [14] C.D. Frisbie, L.F. Rozsnyai, A. Noy, M.S. Wrighton, C.M. Lieber, Functional-group imaging by chemical force microscopy, Science 80 (265) (1994) 2071–2074, http:// dx.doi.org/10.1126/science.265.5181.2071. [15] T. Ito, S. Ibrahim, I. Grabowska, Chemical-force microscopy for materials characterization, TrAC – Trends Anal. Chem. 29 (2010) 225–233, http://dx.doi.org/10. 1016/j.trac.2009.12.008. [16] A. Kumar, G.M. Whitesides, Features of gold having micrometer to centimeter dimensions can be formed through a combination of stamping with an elastomeric stamp and an alkanethiol ink followed by chemical etching, Appl. Phys. Lett. 63 (1993) 2002–2004, http://dx.doi.org/10.1063/1.110628. [17] E.W. van der Vegte, G. Hadziioannou, Scanning force microscopy with chemical specificity: an extensive study of chemically specific tip-surface interactions and the chemical imaging of surface functional groups, Langmuir 13 (1997) 4357–4368, http://dx.doi.org/10.1021/la970025k. [18] H. Azehara, Y. Kasanuma, K. Ide, K. Hidaka, H. Tokumoto, Distinct chemical contrast in adhesion force images of hydrophobic- hydrophilic patterned surfaces using multiwalled carbon nanotube probe tips, Jpn. J. Appl. Phys. 47 (2008) 3594–3599, http://dx.doi.org/10.1143/JJAP.47.3594. [19] E. Gustafsson, E. Johansson, L. Wågberg, T. Pettersson, Direct adhesive measurements between wood biopolymer model surfaces, Biomacromolecules 13 (2012) 3046–3053, http://dx.doi.org/10.1021/bm300762e. [20] M. Meincken, P.D. Evans, Nanoscale characterization of wood photodegradation using atomic force microscopy, Eur. J. Wood Wood Prod. 67 (2009) 229–231, http://dx.doi.org/10.1007/s00107-008-0305-x. [21] S. Frybort, M. Obersriebnig, U. Müller, W. Gindl-Altmutter, J. Konnerth, Variability in surface polarity of wood by means of AFM adhesion force mapping, Colloids Surf. A Physicochem. Eng. Asp. 457 (2014) 82–87, http://dx.doi.org/10.1016/j.colsurfa. 2014.05.055.
Fig. 8. Relation of normalised adhesion values on different sample regions of the wood/ wax sample measured with OH-modified and silicon tips in controlled ambient conditions. Error bars represent the standard deviation.
comparably rough natural materials regions having different polarities can be differentiated by analysing the adhesion signal obtained after scanning the sample with a standard silicon tip. This is confirmed after several repetitions of the measurements with silicon and OH modified tips. The summary of the results, presented in Fig. 8, shows the adhesion values obtained on the wax and on the S2 cell wall. Output values where divided by the mean value of the adhesion force on the wax for each measurement. The figure represents averaged values from several measurements (3–6) for each tip. The by far best contrast in adhesion force between the two sample regions, wax and S2 cell wall, is generally given by unmodified silicon tips. Considering that these tips were 9 times cheaper than the modified ones, this result may substantially impact the choices made with regard to scanning probes to be used in further studies of this kind. Only for the first silicon tip the two values can barely be differentiated, which may be related to a flaw in the tip shape or a tip contamination present even before the start of the measurement. Thus we recommend verifying the capability of each single silicon tip to provide a good contrast on a known polarity contrast rich surface prior to use on an unknown surface. Results obtained with rounded silicon tips and OH modified tips are very similar, indicating that the tip radius plays an important role in the reachable resolution (the larger the tip radius, the larger the tip/ sample contact area and hence the higher the influence of asperities on adhesion). Once again we conclude that the actual performance relating to the contrast in adhesion force depends on the single tip used. 4. Conclusions Silicon tips and tips with hydrophilic functionalisation were compared with regard to their adhesion behaviour on 3 different kinds of samples in humidity controlled conditions. In all cases, adhesion was higher on hydrophilic than on hydrophobic surfaces. Even though the influence of sample roughness on the adhesion signal (caused by the changing contact area) could not be totally eliminated, it appears that a small tip radius results in a high adhesion force contrast. As a consequence, the unmodified (and therefore comparably sharp) standard silicon tips were very well suited when it comes to their ability to distinguish polarity changes on a sample surface – even if they were not originally designed for this purpose! Neither rounded silicon tips nor OH-modified tips could reach a similarly high contrast. OH-modified tips could be assumed to perform better, but this was counterbalanced by the influence of roughness and tip size. Moreover, as standard silicon tips are sturdier than their OH-modified counterparts, they can be used on relatively rough samples (e.g. wood) without losing their sensitivity. As a conclusion it may be stated that polarity differentiation on natural 371
Colloids and Surfaces A 529 (2017) 363–372
J. Colson et al.
03.164. [30] Y.S. Lo, N.D. Huefner, W.S. Chan, P. Dryden, B. Hagenhoff, T.P. Beebe, Organic and inorganic contamination on commercial AFM cantilevers, Langmuir 15 (1999) 6522–6526, http://dx.doi.org/10.1021/la990371x. [31] A. Mendez-Vilas, M.L. Gonzalez-Martin, L. Labajos-Broncano, M.J. Nuevo, Experimental analysis of the influence of surface topography on the adhesion force as measured by an AFM, J. Adhes. Sci. Technol. 16 (2002) 1737–1747, http://dx. doi.org/10.1163/156856102320396111. [32] N. Islam, P. Stewart, I. Larson, P. Hartley, Surface roughness contribution to the adhesion force distribution of salmeterol xinafoate on lactose carriers by atomic force microscopy, J. Pharm. Sci. 94 (2005) 1500–1511, http://dx.doi.org/10.1002/ jps.20381. [33] Y.I. Rabinovich, J.J. Adler, A. Ata, R.K. Singh, B.M. Moudgil, Adhesion between nanoscale rough surfaces – I. Role of asperity geometry, J. Colloid Interface Sci. 232 (2000) 10–16, http://dx.doi.org/10.1006/jcis.2000.7167. [34] P. Klapetek, D. Nečas, C. Anderson, Gwyddion User Guide, (2017). [35] D.L. Sedin, K.L. Rowlen, Adhesion forces measured by atomic force microscopy in humid air, Anal. Chem. 72 (2000) 2183–2189, http://dx.doi.org/10.1021/ ac991198c. [36] R. Jones, H.M. Pollock, J.A.S. Cleaver, C.S. Hodgest, Adhesion forces between glass and silicon surfaces in air studied by AFM: effects of relative hunidity, particle size, roughness, and surface treatment, Langmuir 18 (2002) 8045–8055.
[22] X. Tan, K. Li, Adhesion forces between ligno-cellulose surfaces by atomic force microscopy, J. Pulp Pap. Sci. 34 (2008) 77–85. [23] A.A. Kafi, K. Magniez, B.L. Fox, Measuring the adhesion force on natural fibre surface using scanning probe microscopy, J. Adhes. Sci. Technol. 26 (2012) 175–187, http://dx.doi.org/10.1163/016942411X569381. [24] X. Jin, B. Kasal, Adhesion force mapping on wood by atomic force microscopy: influence of surface roughness and tip geometry, R. Soc. Open Sci. 3 (2016), http:// dx.doi.org/10.1098/rsos.160248. [25] A.H. Al-Bayati, K.G. Orrman-Rossiter, J.A. van den Berg, D.G. Armour, Composition and structure of the native Si oxide by high depth resolution medium energy ion scatering, Surf. Sci. 241 (1991) 91–102, http://dx.doi.org/10.1016/0039-6028(91) 90214-d. [26] M. Morita, T. Ohmi, E. Hasegawa, M. Kawakami, M. Ohwada, Growth of native oxide on a silicon surface, J. Appl. Phys. 68 (1990) 1272–1281, http://dx.doi.org/ 10.1063/1.347181. [27] A. Redondo, W.A. Goddard, C.A. Swarts, T.C. McGill, Oxidation of silicon surfaces, J. Vac. Sci. Technol. 19 (1981) 498–501. [28] A. Verdaguer, C. Weis, G. Oncins, G. Ketteler, H. Bluhm, M. Salmeron, Growth and Structure of water on SiO2 films on Si investigated by Kelvin probe microscopy and in situ X-ray Spectroscopies, Langmuir 23 (2008) 9699–9703. [29] J.C. Cau, L. Lafforgue, M. Nogues, A. Lagraulet, V. Paveau, Magnetic field assisted microcontact printing: a new concept of fully automated and calibrated process, Microelectron. Eng. 110 (2013) 207–214, http://dx.doi.org/10.1016/j.mee.2013.
372