J. Phys. IV France 124 (2005) 129 134 C EDP Sciences, Les Ulis DOI: 10.1051/jp4:2005124020 Use of chemically modified AFM tips as a powerful tool for the determination of surface energy of functionalised surfaces H. Awada, G. Castelein and M. Brogly Institut de Chimie des Surfaces et Interfaces (ICSI) UPR 9069, 15 rue Jean Starcky, 68057 Mulhouse Cedex, France, e-mail: h.awada@uha.fr Abstract. Atomic Force Microscopy (AFM) has been used to determine the surface energy of chemically modified surfaces at a local scale. To achieve this goal, it was necessary to graft the AFM tip with the same chemical functional groups as our analysed surface. Two different chemical functionalities were used: hydrophilic and hydrophobic. The grafting of tip is well developed in the literature [1-5], it consists in depositing a layer of titanium from 3 to 4 nm followed by a 30 nm layer of Au. The thickness of this layer deposited on the tip is of the same order of magnitude than the radius of the AFM tip. On the other hand, the deposition of Ti increases the gold surface roughness. To avoid the use of Ti and to decrease the thickness of the gold layer, we developed a new way of grafting by using organic molecules such as the (3 mercaptopropyl)-triethoxysilane (MPS) as coupling agent [6]. Then we checked this way of grafting and finally we carried out Deflection-Distance curves (DD) with grafted tips in static mode by using AFM. Calibration of the various parts of the apparatus [7] and especially of the cantilever (spring constant and radius) was primordial to reach quantitative data. Finally, by applying a suitable theory of contact [8,9], we were able to determine the surface energy of our system. 1. INTRODUCTION Let us consider a material, the thermodynamic work of adhesion W necessary to create two surfaces when separating a volume into two is worth 2 γ (γ: surface energy). This surface energy can be measured on a macroscopic scale by using wettability. Experimentally to measure this work W at a local scale, we used the AFM. The tip-surface contact in this case is considered as a sphere-plane contact. The DMT and JKR contact theories, allow to calculate the work of adhesion W by knowing the adhesion force F (F=nπRW; n = 1.5 for JKR and 2 for DMT). The surface energy could be then related to the work of adhesion by the relation: W = γ tip + γ surface γ tip/surface. If the tip and the surface are chemically identical then γ tip/surface is equal to zero. In this case we can deduce surface energy from the experimental measured W (γ = W/2). In order to determine the surface energy in AFM it was necessary to graft the tip and the surface with identical terminated functional groups. Two organothiols with different chemical functionalities were used: hydrophilic and hydrophobic functionalities. 2. EXPERIMENTAL 2.1 Methods and materials The deflection distance curves (DD) were performed with a commercial apparatus (NanoscopeIIIa D3000, DI) and using triangular silicon nitride levers (Si 3 N 4 ). In order to carry out a quantitative analysis we gauged the various parts of the AFM and especially the tip-lever (cantilever) part. For the photodiode we worked in its linear zone 10 located between ±2 V to avoid the non-linearity and for the hysteresis of the piezoelectric we worked in its linear zone (0V) and with a frequency cycle of 10 Hz. For the cantilever unit, we gauged the lever in order to determine its spring constant K. Several methods have been described in the literature [11-14] to measure the force constant of a cantilever. We have chosen the Article published by EDP Sciences and available at http://www.edpsciences.org/jp4 or http://dx.doi.org/10.1051/jp4:2005124020
130 JOURNAL DE PHYSIQUE IV method proposed by Torii for its simplicity. The method requires the use of a cantilever of known force constant as a reference cantilever. The radius of the tip was measured by electron scanning microscopy SEM. For our measurement we used 20 tips. These 20 tips made from a same Silicon wafer have the same spring constant but they do not have the same tip radius, which remains pledge of an isotropic acid attack during fabrication [15] and thus we do not control it. For each grafted tip we measured the apex radius. Chemically modified plates of silicon wafers (100) (supplied by MAT Technology, France) represent our system: hydrophilic surface finished by hydroxyl functionalities (OH) and hydrophobic surface finished by methyl function (CH 3 ) [16]. A piranha treatment (70% H 2 SO 4 and 30% H 2 O 2 ) is typical to obtain the hydrophilic surface (50 C for about 30 minutes). Surface covered by CH 3 SAMs is obtained by immersion of the piranha pretreated silicon wafers in the graft solution (hexadecyltrichlorosilane C 16 H 42 O 3 Si). During this study, a native wafer is considered as a reference surface. Our systems were characterised by wettability using the contact angle measurements. 2.2 Grafting of the tips The tip to be grafted has undergone an UV/Ozone treatment for 30 minutes in order to clean it and activate it in order to get a good MPS adsorption. Then the tip is immersed in the MPS solution during 24 hours. Then we deposited a layer of Gold of approximately 10 nm. The tip is then immersed in the organothiol solution during one night. First of all, we comparisons between the geometry of the tip prepared using the MPS as a coupling agent and prepared using Ti, were done. To check that the layer of gold holds on the tip, we carried out friction experiments using 10 nm gold covered tip and a reference surface. The protocol consists in measuring DD curves between the tip and the reference surface at the beginning and then to carry out friction for 5 minutes and then to reproduce the same protocol by increasing the set point. To check the tip surface grafting we carried out force curves in liquid phase and then compared the adhesion forces. For a reproductible study we select tips who have close adhesion forces. Forces were carried out with grafted tips after removing the solvent (one hour in a dessicator). The reference surface is used to check the form and chemistry of tip. Images in AFM intermittent resonance mode (TM-AFM) were taken for gold coated wafers under the same experimental conditions of gold coated tips in order to analyse the gold morphology in both cases. 3. RESULT AND DISCUSSION Table 1 shows adhesion forces between 20 tips and the reference surface. As we mentioned before, the tips having comparable forces, were selected for the quantitative study. These different values of F observed in table 1 are attributed to the radius differences between the studied tips. Table 1. Adhesion force between tips and reference surface. 33 26 17 17 19 17 15 16 15 17 15 16 15 16 16 12 16 18 22 P1 P2 P3 P4 P5 P6 P7 P8 P9 P10 P11 P12 P13 P14 P15 p17 p18 p19 p20
CSM4 131 The next figure (1) shows two SEM images of two tips prepared with Ti (b) and MPS (a) as coupling agents. The presence of artefacts observed in the case of the tip prepared with Ti compared to the good quality of the other one prepared with MPS highlights the advantage of this last. In fact, these artefacts present on the tip surface are the consequence of the Ti migration into gold. Moreover, it is obvious that the surface roughness of the gold layer decreases when using MPS as coupling agent. a) Tip/MPS/Au b) Tip/Ti/15 nm Au Figure 1. SEM images of AFM Tips. Table 2. Adhesion force versus in function of set point and number of DD curve between gold coated tip and a reference surface. In order to check the resistance of the tip coatd gold layer, we carried out friction experiments between a gold tip covered and a reference surface, by increasing progressively the normal load during the friction of a gold coated tip onto a reference surface. Table 2 shows that, the adhesion force does not vary neither during consecutive measurements, neither with the setpoint. As a consequence, we deduce that we have a very good adsorption of the gold coating on the tip. To verify the grafting quality, we conducted the same measurements in water (liquid phase). Compared to the measurements taken in the air, we noticed an increase of the adhesion force in the case of CH 3 tip - CH 3 surface, and a decrease in the case of OH-OH surface. In table 3, we give the spring constant of the tips before and after grafting. The difference between the value given by the manufacturer (0.58 N.m 1 ) and the experimental value shows the interest of this stage for quantitative analysis. Table 3. Spring constant of untreated and grafted tips. Untreated tip T4 T5 T14 0.20 ± 0.02 0.20 ± 0.02 0.20 ± 0.02 Grafted tip T3 T4 T18 0.20 ± 0.02 0.19 ± 0.02 0.19 ± 0.02
132 JOURNAL DE PHYSIQUE IV On the other hand, this table shows that the chemical modification as well as the gold coating layer does not modify the mechanical properties of the tip. In addition these values confirm that the difference in the adhesion force between the various tips results from the tip radius and not from the spring constant. Tables 4 gives the adhesion forces respectively between a CH 3 - tip and a CH 3 -surface, and between an OH-tip and OH surface. Table 4. Adhesion forces respectively between a CH 3 -tip and a CH 3 -surface, and between an OH-tip and OH surface. adhesion force (nn) 30 25 20 15 10 5 0 6±2 CH3-tip CH3-surface 22±4 OH-tip OH-surface The calculation of the adhesion work from adhesion measured by AFM could be done after tip radius determination and by choosing an appropriate contact model. Concerning the tip radius it was measured by SEM as shown in Figure 2 for a) OH-tip and b) CH 3 -tip. Cliché 3 MPS-Au-OH Cliché 4 MPS-Au-CH 3 Figure 2. SEM images of a) OH-tip and b) CH 3 -tip. Table 5. Thermodynamic work of adhesion (W) and surface energy (γ) by AFM and by wettability. tip OH tip CH 3 tip Si 3 N 4 Surface OH Surface CH 3 Surface Si 3 N 4 W (mj.m 2 ) 78 17 26 γ(mj.m 2 ) 39 8.5 13 γ(mj.m 2 ) 76 22 Macroscopic Concerning contact theory, the DMT model is applied since the system is rigid. The relation between W and Fadh is established as following:f=2πr W. Table 6 shows the values of W as well as the surface energy γ.
γ mj.m 2 Échelle Locale 31 51.5 CSM4 133 Table 6. Surface energy after correction. Surface CH 3 OH γ (mj.m 2 ) 31 116 Figure 3. Picture of the gold layer adsorbed on a reference surface, treated by the same treatment than the tip before grafting (Silicon wafer-10 nm Au). The obtained values on a local scale are smaller than macroscopic ones. To verify our model and the calculated values we show, in picture 3, a Tapping mode image of the gold surface (on silicon wafer). Observing this image, we can notice that the gold is adsorbed as 30 nm spherical islands. Now, it will be possible to adjust our model by using 15 nm as tip radius. New values of γ are gathered in table 6. In the case of CH 3 surface, the new values (31 mj.m 2 ) are in good agreement with the macroscopic one. Oppositely, the value of the OH surface is much higher than the macroscopic one. This difference in the last case if explained in terms of the capillarity effect that could be measured by using: F cap = 2πRγ(cos θ 1 + cos θ 2 ) [17], with R the radius of contact, γ the surface energy = 72.6 mj.m 2, θ 1 and θ 2 respectively for the contact angles between water and the tip, water and surface (in our case θ 1 = θ 2 ). For θ = 26, Fcap is of 12.3 nn, thus Fadh will be 9.7nN. The value of γ OH becomes equal to 51.5 mj.m 2 that is more coherent that the 116 mj.m 2 value when comparing with the macroscopic value of 76 mj.m 2. Table 7 shows the corrected values of surface energy at a local scale. Table 7. Surface energy (γ) by AFM and by wettability after correction. Surface CH 3 OH Échelle macroscopique 22 76 4. CONCLUSION In this work, we developed a new way of tip grafting. After calibration of the various parts of the AFM and the optimisation of the grafting conditions, we measured the adhesion forces between chemically modified tips and chemically modified surfaces. By applying the suitable model of contact to our system, we have calculated the surface energy of hydrophilic and hydrophobic surface.
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