pubs.acs.org/Langmuir © 2010 American Chemical Society
AFM Force Mapping for Characterizing Patterns of Electrostatic Charges on SiO2 Electrets Yiheng Zhang,† Dan Zhao,‡ Xinxin Tan,† Tingbing Cao,*,‡ and Xi Zhang*,† †
Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, P.R. China, and ‡Department of Chemistry, Renmin University of China, Beijing 100872, P.R. China Received April 1, 2010. Revised Manuscript Received May 6, 2010 To characterize patterns of charges on electrets, Kelvin probe force microscopy (KFM) usually serves as a very useful tool to measure the electrostatic potential through an electric cycle; however, it is limited to electrets supported on conductive substrates. In this article, we demonstrate the use of atomic force microscopy (AFM) force mapping to visualize the pattern of charges on SiO2 electrets. In contrast to KFM, AFM force mapping can be used for characterizing electrets that are formed not only on conductive substrates but also on nonconductive substrates. Functional groups can be introduced to the AFM tip to achieve the force mapping and to improve the resolution. Our study clearly indicates that AFM force mapping can serve as an optional method for the characterization of electrets.
Introduction An electret is a material that has a permanent electric field maintained either by trapping net electrostatic charge (spacecharge electret) or by holding macroscopic electric dipole moment (dipolar electret).1-3 To characterize the pattern of charges on electrets, the Kelvin probe force microscopy (KFM) usually serves as the sole and powerful tool to measure the surface potential difference between a probe and a substrate through an electric cycle.4-6 KFM, also known as scanning Kelvin probe microscopy (SKPM), has been derived from macroscopic Kelvin Method with the atomic force microscope (AFM).4 It is essential for KFM to use a conductive probe as an electrode to realize the electric cycle, and use the lift mode, i.e., lift the probe over a sample surface for several nanometers, during detection of electric signals. Then the potential differences can be measured to construct a KFM image, except when the surface potential is really homogeneous. Considering that there are a lot of nonconductive electrets, it should be very useful and important to develop a new method to characterize electrets regardless of the conductivity of the probes and substrates. AFM is not only an imaging method, but also a versatile platform for direct measurements of intramolecular and *To whom correspondence should be addressed. (T.C.) Tel.: þ86-1062514332. Fax: þ86-10-62516444. E-mail:
[email protected]; (X.Z.) Tel.: þ86-10-62796283. Fax: þ86-10-62771149. E-mail:
[email protected]. edu.cn.
(1) Jacobs, H. O.; Whitesides, G. M. Science 2001, 291, 1763. (2) McCarty, L. S.; Whitesides, G. M. Angew. Chem., Int. Ed. 2008, 47, 2188. (3) Zhao, D.; Duan, L.; Xue, M.; Ni, W.; Cao, T. Angew. Chem., Int. Ed. 2009, 48, 6699. (4) Nonnenmacher, M.; O’Boyle, M. P.; Wickramasinghe, H. K. Appl. Phys. Lett. 1991, 58, 2921. (5) Jacobs, H. O.; Knapp, H. F.; Stemmer, A. Rev. Sci. Instrum. 1999, 70, 1756. (6) Liscio, A.; Palermo, V.; Samori, P. Acc. Chem. Res. 2010, 43, 541. (7) Noy, A.; Vezenov, D. V.; Lieber, C. M. Annu. Rev. Mater. Sci. 1997, 27, 381. (8) Janshoff, A.; Neitzert, M.; Oberd€orfer, Y.; Fuchs, H. Angew. Chem., Int. Ed. 2000, 39, 3212. (9) Zhang, W.; Zhang, X. Prog. Polym. Sci. 2003, 28, 1271. (10) Butt, H. J.; Cappella, B.; Kappl, M. Surf. Sci. Rep. 2005, 59, 1. (11) Zhang, X.; Liu, C.; Wang, Z. Polymer 2008, 49, 3353. (12) M€uller, D. J.; Dufr^ene, Y. F. Nat. Nanotechnol. 2008, 3, 261. (13) Cao, Y.; Li, H. Nat. Nanotechnol. 2008, 3, 512.
11958 DOI: 10.1021/la101290r
intermolecular forces.7-14 Until now, various intra- and intermolecular interactions have been detected by AFM, including covalent bonding,15 ligand-receptor pairing,16 hydrophobic interactions,17,18 host-guest interactions,19,20 coordination bonding,21,22 multiple hydrogen bonding,23 charge-transfer interactions,24,25 π-π interactions,26 intercalation interaction,27 multivalent interaction,28 as well as the interactions between dendrimers.29 These forces detection can provide useful information of the measured interaction, and also importantly, the measured forces can be used to realize the AFM force mapping of the surface, therefore visualizing the special molecules or structures on surface.30-34 AFM Force mapping (also known as Force Volume) is constructed by the adhesion forces (14) Yu, Y.; Zhang, Y.; Jiang, Z.; Zhang, X.; Zhang, H.; Wang, X. Langmuir 2009, 25, 10002. (15) Grandbois, M.; Beyer, M.; Rief., M.; Clausen-Schaumann, H.; Gaub, H. E. Science 1999, 283, 1727. (16) Zlatanova, J.; Lindsay, S. M.; Leuba, S. H. Prog. Biophys. Mol. Biol. 2000, 74, 37. (17) Cui, S.; Liu, C.; Zhang, W.; Zhang, X.; Wu, C. Macromolecules 2003, 36, 3779. (18) Gu, C.; Ray, C.; Guo, S.; Akhremitchev, B. B. J. Phys. Chem. B 2007, 111, 12898. (19) Sch€onherr, H.; Beulen, M. W. J.; Bugler, J.; Huskens, J.; van Veggel, F. C. J. M.; Reinhoudt, D. N.; Vancso, G. J. J. Am. Chem. Soc. 2000, 122, 4963. (20) Eckel, R.; Ros, R.; Decker, B.; Mattay, J.; Anselmetti, D. Angew. Chem., Int. Ed. 2005, 44, 484. (21) Conti, M.; Falini, G.; Samori, B. Angew. Chem., Int. Ed. 2000, 39, 215. (22) Kersey, F.; Yount, W.; Craig, S. J. Am. Chem. Soc. 2006, 128, 3886. (23) Zou, S.; Sch€onherr, H.; Vancso, G. J. J. Am. Chem. Soc. 2005, 127, 11230. (24) Skulason, H.; Frisbie, C. D. J. Am. Chem. Soc. 2002, 124, 15125. (25) Yu, Y.; Yao, Y.; Wang, L.; Li, Z. Langmuir 2010, 26, 3275. (26) Zhang, Y.; Liu, C.; Shi, W.; Wang, Z.; Dai, L.; Zhang, X. Langmuir 2007, 23, 7911. (27) Liu, C.; Jiang, Z.; Zhang, Y.; Wang, Z.; Zhang, X.; Feng, F.; Wang, S. Langmuir 2007, 23, 9140. (28) Zhang, Y.; Yu, Y.; Jiang, Z.; Xu, H.; Wang, Z.; Zhang, X.; Oda, M.; Ishizuka, T.; Jiang, D.; Chi, L.; Fuchs, H. Langmuir 2009, 25, 6627. (29) Shi, W.; Zhang, Y.; Liu, C.; Wang, Z.; Zhang, X. Langmuir 2008, 24, 1318. (30) Eaton, P.; Smith, J. R.; Graham, P.; Smart, J. D.; Nevell, T. G.; Tsibouklis, J. Langmuir 2002, 18, 3387. (31) Song, J.; Duval, J. F. L.; Stuart, M. A. C.; Hillborg, H.; Gunst, U.; Arlinghaus, H. F.; Vancso, G. J. Langmuir 2007, 23, 5430. (32) Qiu, D.; Xiang, J.; Li, Z.; Krishnamoorthy, A.; Chen, L.; Wang, R. Biochem. Biophys. Res. Commun. 2008, 369, 735. (33) Sullan, R. M. A.; Li, J. K.; Zou, S. Langmuir 2009, 25, 7471. (34) Gilbert, Y.; Deghorain, M.; Wang, L.; Xu, B.; Pollheimer, P. D.; Gruber, H. J.; Errington, J.; Hallet, B.; Haulot, X.; Verbelen, C.; Hols, P.; Dufr^ene, Y. F. Nano Lett. 2007, 7, 796.
Published on Web 05/17/2010
Langmuir 2010, 26(14), 11958–11962
Zhang et al.
Article
between the tip and substrate measured by point-by-point force measurements on the surface. Considering that AFM force mapping is possible regardless of the conductivity and that AFM tips can be chemically modified by different functional groups, we are wondering whether the AFM force mapping can be utilized to characterize the electrets. In this article, we have demonstrated successfully that AFM force mapping enables visualization of pattern of electrostatic charges on SiO2 electrets. It shows clearly that this method can serve as an optional choice for the characterization of electrets.
Scheme 1. AFM Force Mapping for Characterization of Pattern of Charges on an Electret; the Adhesion Forces of the Retrace Force-Displacement Curve Are Used to Construct a Force Map
Experimental Section Materials. All chemicals were purchased commercially and used as received. SiO2 (300 nm) supported on silicon wafer was commercially purchased. All solvents were freshly distilled before use. Electrets. The patterning of electrostatic charges on SiO2 electrets was conducted using the same procedure as reported.1 The patterned surface of the poly(dimethylsiloxane) (PDMS) stamp was made electrically conducting by thermal evaporation of 7 nm of Cr (as an adhesion promoter) and 80 nm of Au onto it. The silicon wafer with SiO2 thin film was cut into 0.5 cm2 squares. The surface of SiO2 film was contacted with metal-coated PDMS stamp. A Keithley 2400 source-meter was used to apply pulse current. The voltage was kept at 10 kV/cm for 20 s. Then the silicon wafer was removed from the power source, and patterns of charge were achieved on the SiO2 film supported by silicon wafer. Fabrication of Hydrogel Stamp. A hot, degassed 5% w/w solution of agarose (Biowest agarose) in deionized water is cast against a PDMS stamp with desired pattern on its surface. Cooled at room temperature until it is well gelated, the agarose stamp is gently peeled off. The hydrogel stamp is patterned with the negative of the array of features in the PDMS stamp. AFM Tip Preparation. For KFM and AFM measurements, the AFM cantilevers were used as received. For force mapping measurements, AFM tips were treated by air plasma (PDC-32G Plasma Cleaner, Harrick Plasma, NY) for 1 min using medium level, and then used for the measurement if not specified. A piranha solution-treated tip was obtained by immersing the AFM tip in the piranha solution (7:3 v/v 98% sulfuric acid/30 wt % H2O2) for 10 min. Caution: piranha solution is highly corrosive and reacts violently with organic materials. It should be handled with great care. Upon removal, the tips were rinsed with large amounts of water and then dried in vacuum. An amino-functionalized tip was obtained by the following procedure: the plasma-treated AFM tips were immersed in a solution of 3-aminopropyldimethylethoxysilane (APDES) in dry toluene (1% v/v) for 15 min at room temperature, and then washed with CH2Cl2 and methanol for 3 times, followed by drying against a filter paper every time after washing. Measurements. All the KFM images were recorded using a Veeco D3100 instrument using conductive probes. All AFM height imaging (AC mode) and force mapping were obtained using Molecular Force Probe 3D (Asylum Research, Santa Barbara, CA) and V-shaped Si3N4 cantilevers (Veeco, Santa Barbara, CA). In force mapping measurement, the spring constants of the Si3N4 cantilevers were in the range of 0.03-0.10 N/m according to the measurement of their thermal fluctuation.35 All the KFM, AFM, and AFM force mapping are performed in air at room temperature. The characterization of electrets by AFM force mapping is schematically shown in Scheme 1. To obtain a force map, a set of force-displacement curves were collected over an area of an electret sample. This area was divided into a grid pattern (64 64 pixels), and then the scanner performed a single force spectroscopy measurement at the center of every pixel, each time collecting trace and retrace force-displacement curves. The contact forces in the range of 1-2 nN were used. The collected forcedisplacement curves were analyzed by IGOR Pro 6 (Wavemetrics, (35) Butt, H. J.; Jaschke, M. Nanotechnology 1995, 6, 1.
Langmuir 2010, 26(14), 11958–11962
Portland, OR) software. For each curve, the adhesion force Fadhesion was calculated by returning the difference of the average of the last 10 points and the minimum of the retrace curve. The obtained adhesion forces were used to construct a force map. The scan rate is about 2 Hz, the Z velocity is about 3-4 μm/s, the XY velocity is about 6-8 μm/s.
Results and Discussion To examine whether the AFM force mapping can become a possible method for characterizing electrets, we have fabricated the electret samples on the SiO2 film using silicon wafer as the substrate, and then employed the AFM height image, KFM, and force mapping, respectively, to comparatively study the same sample. Silica is a kind of nonpolar electret that could trap space charges, and can be either positive or negative charged using electrostatic microcontact printing. It should be noted that silica is nonconductive and therefore can not be directly studied by KFM. To realize the KFM measurement and achieve the comparative study, we have used the sample with silicon substrate as a conductive layer and silica electret as a rather thin layer. Figure 1a shows the AFM height image and the corresponding section analysis, and it is clear that the surface is rather flat, and there is no obvious morphology pattern on the surface. In contrast, the KFM observation clearly indicates that there are patterns of charges on the SiO2 electret (Figure 1b). The potential difference between the bright round spot and dark area is about 3.5 V, as shown in the section analysis in Figure 1b. Interestingly, when we conducted the force mapping measurement on the same electret sample using plasma treated AFM tip, the similar patterns have been observed, as shown in Figure 1c. The difference of the adhesion forces between the bright round spot and dark area is about 2.5 nN, as shown in the section analysis in Figure 1c. The relative ratio of this difference to the force in dark area (relative difference) is ∼86%. Herein, the force map has the resolution of 64 64 pixels, and about 30 min is needed to obtain it. The difference in the noise level between KFM and force map is due to the difference in the resolution. It is difficult to further improve the resolution of the force map because it needs to increase the experimental time exponentially and to keep the instrument stable at the same time. In addition, it should be pointed out that the absolute value of adhesion force between the AFM tip and the electret surface can be obtained from force mapping, while the relative difference of potential can be obtained from KFM. For example, the absolute adhesion force of pixel (A) and (B) is 2.9 nN and 5.4 nN, respectively. The corresponding force-displacement curves of pixel (A) and (B) are shown in Figure 1d. The adhesion force of (B) is bigger than (A), shown as the brighter pixel in the force map. This fact shows that there are different interactions between the AFM tip and the electret with different surface potentials. These results have demonstrated that the AFM force mapping can serve as an alternative way to characterize the pattern of charges on electrets by taking advantage of the adhesion force between the AFM tip and the electret surface. DOI: 10.1021/la101290r
11959
Article
Zhang et al.
Figure 1. Electret characterizations. Before releasing charges on the electret: (a) AFM height image; (b) KFM; (c) AFM force map using plasma treated AFM tip; (d) corresponding retrace force-displacement curves of points A and B in panel (c). After releasing charges on the electret: (e) AFM height image; (f ) KFM; (g) AFM force map using plasma treated AFM tip; (h) The corresponding retrace forcedisplacement curves of points C and D in panel (g). All the image sizes are 30 μm 30 μm.
The characterization of patterns of charges on the electret surface has also been examined after releasing the charges on the electret. One hydrogel stamp has been brought into contact with the electret surface to remove the charges on the electret, and then the corresponding experiments have been performed to obtain the AFM height image, KFM image, and AFM force map. For comparison, the electret before and after releasing the charges has been characterized in the same experiment using the same AFM tip. The AFM height images of Figure 1a and Figure 1e have no significant differences. The potential difference between the bright round spot and dark area has fallen down to 1.5 V, as shown in Figure 1f. The corresponding force map is shown as Figure 1g, which has a pattern similar to that of the KFM image. The average difference of the adhesion forces between the bright round spot and dark area has dropped to 0.9 nN (relative difference ∼50%). Two representative pixels (C) and (D) are used as examples for the corresponding retrace force-displacement curves. As shown in Figure 1h, the adhesion force of point (C) of 2.8 nN is greater than that of point (D) (1.8 nN), reflecting as the difference of brightness in the force map. It should be noticed that the absolute value rather than the relative value of the adhesion force is measured by force mapping, so that we may get the direct comparison of the difference of the charges by comparing the difference of the adhesion forces in the same experiment. Herein, the adhesion forces of points (C) and (D) in Figure 1h are smaller than that of points (B) and (A) in Figure 1d, respectively. Compared with the electret before releasing the charges, the only difference is that there are fewer charges on the electret after releasing the charges, which may be the reason why the adhesion forces become smaller. 11960 DOI: 10.1021/la101290r
Figure 2. Electret characterizations. (a) KFM image and (b) the corresponding force map using a piranha solution-treated AFM tip. All the image sizes are 25 μm 25 μm.
The adhesion force between the AFM tip and the electret substrate can be influenced either by altering the charges on the electret or by varying the surfaces of the AFM tip. It is supposed that the nature of the interaction between the tip and the sample is mainly the electrostatic and electrostatic-dipole interaction. To find how we can change the adhesion force by changing the surface of the AFM tip, we have done the following experiments. At the beginning, we have used the untreated AFM tip to obtain force maps. The force mapping sometimes has shown patterns on the electret, but the imaging quality should be improved because of the possible contamination of the AFM tip without any cleaning process. Given that there are only a few polar groups on the untreated tip, we have tried to introduce more polar groups Langmuir 2010, 26(14), 11958–11962
Zhang et al.
Article
Figure 3. Electret characterizations. (a) KFM image. (b) AFM force map using amino-terminated AFM tip. (c) AFM force map using amino-terminated AFM tip after flattening. All the image sizes are 10 μm 10 μm.
to the AFM tip, which may change the adhesion force between the AFM tip and the electret. First, we have treated the AFM tip with air plasma, which is a popular way to clean the AFM tip and yield more polar groups to the tip. Air plasma leads to the oxidation of surfaces and to the formation of polar surface groups (-OH, CdO, O-CdO).36-38 As the time of the plasma treatment of the AFM tip increases, the adhesion force increases. However, it should be pointed out that it is difficult to give a quantitative relationship between the adhesion force and the plasma treatment time, because of the variations in the shape of the AFM tip and the environmental conditions, including humidity.39 Second, we have used the piranha solution to treat the tip, which is an effective way to produce hydrophilic surfaces covered by polar OH groups. Figure 2a shows the KFM image of another SiO2 electret surface, and the potential difference between the bright and dark area is about 1.0 V. When an AFM tip after immersion for 10 min in a piranha solution is used, we have obtained the force map of Figure 2b. From section analysis of Figure 2b, it can be seen that the adhesion forces are in the range of 100-140 nN, and the difference between the bright and dark area is about 30 nN (relative difference ∼33%). The spot area of the force map of Figure 2b is a little bigger than that in the KFM image of Figure 2a, reflecting that there are still charges outside the round spots of the KFM image that still can interact with the AFM tip. As the immersion time of the AFM tip in the piranha solution decreases, the adhesion force decreases. These experiments indicate that the adhesion forces can be increased by introducing more polar groups on the AFM tips. Furthermore, functional groups can be introduced to the AFM tip by chemical modification and can be utilized to achieve the force mapping of electrets. The amino groups have been introduced to the AFM tip by immersing the tip into the toluene solution of APDES, an amino-terminated silane reagent. Then, this modified tip has been used to characterize the electret, the KFM image of which shows that there are bright and dark strips with the potential difference of ∼0.5 V, as shown in Figure 3a. The corresponding force map, Figure 3b, shows patterns similar to (36) (37) 1005. (38) (39)
Langowski, B. A.; Uhrich, K. E. Langmuir 2005, 21, 6366. Poncin-Epaillard, F.; Legeay, G. J. Biomater. Sci., Polym. Ed. 2003, 14, Rosso, M.; Giesbers, M.; Schron, K.; Zuilhof, H. Langmuir 2010, 26, 866. Gouveia, R. F.; Galembeck, F. J. Am. Chem. Soc. 2009, 131, 11381.
Langmuir 2010, 26(14), 11958–11962
Figure 4. Electret characterizations: (a) 3D KFM image, (b) 3D force map using a plasma-treated AFM tip. All the image sizes are 30 μm 30 μm.
those of the KFM image. The adhesion forces between the aminomodified tip and the electret are in the range of 11.5-13.5 nN, and the difference between the bright and dark strip is ∼1.5 nN (relative difference ∼13%). These results imply that, by introducing different functional groups to the AFM tip, one can realize the force map of an electret, as well as obtain the interacting strength between the functional groups and the electret surfaces. To improve the image quality of the force map, the methods for dealing with AFM height image can also be used in force map data. Figure 3c shows the force map after flattening (first order) the force map of Figure 3b. It can be seen that the contrast of this force map after flattening is clearer than that in Figure 3b, and thus the image quality is improved to some extent. It should be noted that, after flattening, the adhesion forces become the relative forces rather that the absolute value. However, the difference between the bright and dark strip is similar to that before flattening, as indicated by the section analysis. We can also give the 3D KFM image and 3D force map, which can give 3D visualization for the patterns of charges on electret. One example of characterizing the electret has been shown in panels a and b of Figure 4, which show similar patterns. Moreover, when contrasting the results of KFM with the force map, there is some inconsistency in additon to the similarity of both images, which may be due to the different imaging mechanisms. For example, the spot area of the force map of Figure 2b is a bit different from that in the KFM image of Figure 2a. The effects of the defects on the electret sample are also different in both measurements. Figure 5b shows some defects on the force map, while the KFM shown in Figure 5a are more perfect. Figure 5c is DOI: 10.1021/la101290r
11961
Article
Zhang et al.
Figure 5. Electret characterizations. (a) KFM image, (b) AFM force map with defects, and (c) AFM force mapping of the electret after storage in air for one week by using a plasma-treated AFM tip. All the image sizes are 40 μm 40 μm.
the force map of the electret after storing the electret sample in air for one week, showing that there are more defects on the sample. One plausible reason is that KFM is performed in a lift mode, while AFM force mapping is performed in a contact mode and thus is more sensitive to the defects on the surface.
Conclusion In conclusion, we have demonstrated that the characterization of patterns of electrostatic charges on an SiO2 electret has been realized by AFM force mapping. Besides KFM, AFM force mapping is certainly an alternative method in characterizing electrets due to the following advantages: (1) it is regardless of the conductivity of the substrate or the tip; (2) the AFM tip can be modified and introduced with functional species. However, the scanning velocity of force mapping is slower than KFM using our instrument, which should be improved. Furthermore, when the
11962 DOI: 10.1021/la101290r
AFM tip is brought into contact with the electret surface, we can obtain not only the adhesion force but also the quantitative information of modulus, deformation, and energy dissipation. It is very promising to investigate these factors to give a better view of electrets. In addition, it is not possible to operate the KFM experiment in biologically relevant conditions (aqueous solutions), while AFM force mapping does make it possible. We hope that the AFM force mapping method can open a new avenue for studying the electrets. Acknowledgment. This work was funded by the Natural Science Foundation of China (20834003, 20674096, 50773092), the NSFC-DFG joint transregion SFB (TRR61), Tsinghua University Initiative Scientific Research Program (2009THZ02230), and the National Basic Research Program of China (2007CB808000).
Langmuir 2010, 26(14), 11958–11962
