In the Laboratory
Chemical Analysis Using Scanning Force Microscopy
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An Undergraduate Laboratory Experiment Mathew M. Maye, Jin Luo, Li Han, and Chuan-Jian Zhong* Department of Chemistry, State University of New York at Binghamton, Binghamton, NY 13902-6016; *
[email protected] eral Chemistry) to show how SFM provides analytical inforWhile there is growing interest in introducing scanning mation on surface chemical composition through interatomic probe microscopy (SPM) to the undergraduate curriculum, force interactions. This experiment is suitable for the underthe construction of molecular structures on surfaces and their graduate chemistry laboratory. subsequent chemical analysis using SPM have been largely limited to research settings. With NSF-CCLI support, we Chemical Mapping at Patterned Self-Assembly developed an SPM laboratory for undergraduate chemistry In a typical commercial AFM instrument (15), the laboratory courses at SUNY-Binghamton (1). The goal is to probe, usually a pyramidal-shaped silicone nitride tip (Si3N4) create inquiry-based hands-on learning settings, which would secured on a cantilever end for contact-mode measurement, serve as a model for chemistry educators who are interested senses the tip–sample interaction forces via cantilever deflection in introducing this exciting instrumentation to undergraduate upon scanning it across the surface. The tip–sample interaction chemistry students. Our experience has demonstrated that involves a wide range of forces such as van der Waals, electrothis type of experiment indeed captures students’ interest and static, and chemical binding (ca. 10᎑9–10᎑6 N). Figure 1A imagination about molecular- and atomic-scale concepts. illustrates cantilever deflections in a typical setting for topoThe major SPM techniques include scanning tunneling graphical imaging and an idealized force–distance curve for microscopy (STM) and atomic force microscopy (AFM). As the tip–sample interaction. The cantilever deflections due to hands-on experiments or demonstrations in the chemistry the tip–sample interaction forces are monitored by the vertical laboratory, we have found that both STM and AFM experisegments of a position-sensitive photodetector (PSPD). The ments enhance learning of chemical concepts at atomic and tip–sample interaction induces a positive deflection of the molecular levels (1). Because interatomic and intermolecular forces constitute the basis of AFM imaging, AFM is also known as scanning force microscopy (SFM) (2). In addition to providing information on atomic or molecular arrangements, SFM has the capability of chemical mapping (2–6 ) because molecular interaction forces depend on chemical composition at atomic levels. A recent survey among students in our analytical chemistry classes indicated a lack of understanding of molecular interactions and their correlation with properties of materials. However, while our own work (1) and a number of other reports (7–13) have demonstrated the enhancement of learning by STMbased laboratories, there has been little discussion of such chemical mapping capability in the laboratory. Since the invention of STM and AFM (14), there have been remarkable innovations (2) in the visualization of surface compositions at high spatial resolution unattainable by other surface analysis techniques or physical measurements of bulk wetting phenomena. The learning of atomic or molecular interaction concepts could be greatly enhanced by introducing this capability into the laboratory. The SFM chemical analysis experiment described herein engages students in critical thinking about tip–sample interaction forces or frictions at self-assembled monolayer surfaces with different functional groups. The objective Figure 1. (A) An idealized force–distance curve for a tip–probe contact. The left illusis to provide hands-on experience for upper-di- trates the positive and negative deflections of the cantilever and the corresponding vision chemistry courses (e.g., Instrumental laser beam reflection pathway. (B) An idealized friction loop for friction scanning. Analysis) or demonstrations for lower-level chem- The left illustrates the torsional deflection of the cantilever and the corresponding laistry courses (e.g., Introductory Analytical, Gen- ser beam reflection pathway. JChemEd.chem.wisc.edu • Vol. 79 No. 2 February 2002 • Journal of Chemical Education
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Figure 2. (a) A schematic of µP-stamping and backfilling processes. (b) The pattern in the stamp is an array of square depressions. (c) The stamped ODT monolayer and (d) the backfilled MHA monolayer are highly idealized.
cantilever in the contact region (C), whereas a negative deflection of the cantilever is detected in the adhesion region (D and E). The standard contact-mode topographical imaging relies on the positive/negative deflections with respect to the surface normal. The measurement of lateral force detects relative differences in surface frictional characteristics related to the surface composition. In comparison with the topographical imaging, the friction imaging relies on the torsion force of the cantilever generated as the tip is moved laterally across the surface, which is monitored by the lateral segments of the PSPD (Fig. 1B). The trace–retrace loop is related to energy dissipation in the frictional process. The four quadrants of PSPD detector can simultaneously collect topographic and friction images. In this experiment, we chose two-component monolayer assemblies as a chemical-mapping platform, which was created by patterning a gold surface with alkyl thiolate monolayers (16, 17 ). The dense packing and orientation (average ~30°) structure and the functionalized end-groups define the surface chemical composition and the frictional property. In this experiment, we focused on two types of functional groups, methyl groups (–CH3) and carboxylic acid (–CO2H), largely in view of their distinctive differences in molecular interaction. Octyldecanethiol (ODT) and 15-mercaptohexadecanoic acid (MHA) provided these two functions, respectively. We used a microcontact printing (µP) technique (18) to prepare the monolayer patterns of different functional groups to facilitate a direct composition contrast with an internal
Figure 3. Images of an ODT-patterned monolayer after backfilling with MHA: (A) SFM friction and (B) topography. The cross-section below each image corresponds to the line drawn in the image. (Cantilever k = 0.12 nN/m).
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In the Laboratory
analytical standard. Other patterning techniques (2, 19) should also work. A prefabricated polydimethylsiloxane (PDMS) stamp was used to deposit the desired thiols onto localized areas of a surface. Figure 2a illustrates key processes in this approach (18). It involved exposing the PDMS stamp (an array of square depressions in Fig. 2b) to an ethanolic solution of 1–10 mM ODT (or MHA) (inking). The inked ODT was then transferred to the gold surface, creating an ODT monolayer only in the regions of contact (stamping). The uninked portion was then backfilled with MHA (or ODT) by immersion into a dilute thiol solution (backfilling). The procedure was relatively easy and there was a close correspondence of the features of the stamp to those on the patterned surface, as illustrated in Figure 2 (c, d) in a highly-idealized fashion.
Results and Discussion Some typical friction and topographical images are described in this section to illustrate the correlation with surface chemical compositions. Figure 3 presents (A) the friction and (B) the topography images for a patterned sample prepared by ODT stamping
and MHA backfilling. In the friction image, a sharp contrast between the squares (MHA) and the domains surrounding them (ODT) is evident. In comparison, the topography image displays barely any height contrast between the two different domains, which is consistent with the nearly identical chain lengths of ODT and MHA (~2 nm). Depending on imaging force it was sometimes possible to observe a slight height difference, but the cross-section view revealed hardly any contrast above the noise level. The sharp contrast in the friction image reflects the difference in friction force between the tip and the different end-group domains. The fact that the –CO2H domain has a friction force greater than that for the –CH3 domain can be correlated with the microscopic molecular interaction or the surface free energy. The surface of an uncontaminated Si3N4 tip consists largely of hydroxyl (–OH) or amine (–NH2) (2), which are hydrophilic and capable of hydrogen bonding. The interaction at the tip–MHA-domain contact thus involves both hydrophilic and hydrogen-bonding interactions, which are stronger than the hydrophilic–hydrophobic interaction at the tip–ODT-domain contact. Clearly, the tip–surface molecular interaction is the basis for the differentiation of surface chemical composition defined by the functional groups. The ability to detect the friction difference was further demonstrated in the laboratory by comparing samples prepared by different stamping and backfilling sequences. Figures 4 and 5 compare two friction images of the patterned samples comprising ODT and MHA domains prepared in two different
Figure 4. SFM friction image of an ODT-patterned monolayer after backfilling with MHA. The cross-section below the image corresponds to the line drawn in the image. (Cantilever k = 0.06 nN/m).
Figure 5. SFM friction image of an MHA-patterned monolayer after backfilling with ODT. The cross-section below the image corresponds to the line drawn in the image. (Cantilever k = 0.06 nN/m).
Hazards There are no significant chemical hazards involved in the actual student experiment, except for cleaning used gold film substrates using a 1:3 H2O2 (30%)–H2SO4 (conc.) solution. This cleaning process was completed by the teaching assistant before the experiment.
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sequences: ODT stamping and MHA backfilling (Fig. 4) and MHA stamping and ODT backfilling (Fig. 5). The images show the opposite friction contrast, with a greater friction for the MHA-domains than for the ODT-domains. The crosssection view of the image corresponding to each case also reveals a reversed pattern of the friction voltage, which is proportional to friction force. The topographical images revealed barely any height contrast above the noise level. In these images, the surface composition or functional group identity plays a key role in determining the tip–sample interaction force. The consideration of surface free energy is one way to correlate the force difference with known physical and chemical properties. The surface free energy for –CH3 surface is 19 mJ/m, whereas the CO2H surface has ~50 mJ/m (20). The microcontact at the higher surface-free-energy domains of MHA exhibits a larger friction than that at the lower surfacefree-energy domains of ODT. Another qualitative measure of the interaction forces involves chemical binding forces. In comparison with the weak van der Waals interaction at contact between the tip (– OH or NH2) and the sample (–CH3), hydrogen-bondingtype interaction is expected for the tip (–OH or NH2) and sample (–CO2H) contact, which has a larger adhesive force. The differences of surface-free-energy and hydrophobic or hydrogen-bonding interactions thus constitute the molecular forces in the friction-based chemical mapping. How to assess these forces quantitatively is an area of active research (2–6 ) that serves as an inquiry-based problem to engage students in further systematic and theoretical correlation at atomic and molecular scales. Conclusion The SFM-based chemical analysis at chemically designed platforms has been developed as a hands-on experiment or demonstration for the undergraduate chemistry laboratory. The measurement of tip–sample interactions at the patterned monolayers enhances the learning of chemical interaction concepts at atomic and molecular levels. Because of the simplicity of sample preparation, the relatively easy operation of the microscope, and the many applications of SFM chemical mapping capability in analytical, biological, and materials fields (2, 16, 21), we encourage others to consider incorporating this experiment into their undergraduate chemistry curriculum. Importantly, open-ended questions generated from analyzing hydrophobic, hydrophilic, electrostatic, van der Waals, and hydrogen-bonding interactions between tip and sample are an ideal vehicle for stimulating inquiry-based learning activities. Experiments using chemically modified probe tips and other functionalized molecular assemblies are currently being developed in our undergraduate laboratory and student research projects. Acknowledgments Financial support of this work from the National Science Foundation CCLI program (EDU-9952628) is gratefully acknowledged. Partial support from ACS-PRF and 3M funds for some of the instrumentation components is also gratefully acknowledged. We appreciate helpful discussions with Marc
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Supplemental Material
Further details of SFM instrumentation, principles, and procedures are available in this issue of JCE Online. Literature Cited 1. For our recent work on STM and AFM imaging, please visit our Web sites http://matlabs.clt.binghamton.edu/spm/ matspmindex.htm and http://www.chem.binghamton.edu/ ZHONG/spm/spmmain.htm (both accessed Nov 2001). 2. Takano, H.; Kenseth, J. R.; Wong, S.-S.; O’Brien, J. C.; Porter, M. D. Chem. Rev. 1999, 99, 2845. 3. Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071. Noy, A.; Frisbie, C. D.; Rozsnyai, L. F.; Wrighton, M. S.; Lieber, C. M. J. Am. Chem. Soc. 1995, 117, 7943. 4. Finot, M. O.; McDermott, M. T. J. Am. Chem. Soc. 1997, 119, 8564. 5. Green, J.-B. D.; McDermott, M. T.; Porter, M. D.; Siperko, L. M. J. Phys. Chem. 1995, 99, 10960. Wong, S.-S.; Takano, H.; Porter, M. D. Anal. Chem. 1998, 70, 5209. Jones, V. W.; Kenseth, J. R.; Porter, M. D.; Mosher, C. L.; Henderson, E. Anal. Chem. 1998, 70, 1233. 6. van der Vegte, E. W.; Hadziioannou, G. Langmuir 1997, 13, 4357. 7. Rapp, C. S. J. Chem. Educ. 1997, 74, 1087. 8. Glaunsinger, W. S.; Ramakrishna, B. L.; Garcia, A. A.; Pizziconi, V. J. Chem. Educ. 1997, 7, 310. 9. Poler, J. C. J. Chem. Educ. 2000, 77, 1198. 10. Giancarlo, L. C.; Fang, H.; Avila, L.; Fine, L. W.; Flynn, G. W. J. Chem. Educ. 2000, 77, 66. 11. Lorenz, J. K.; Olson, J. A.; Campbell, D. J.; Lisensky, G. C.; Ellis, A. B. J. Chem. Educ. 1997, 74, 1032A. 12. Coury, L. A. Jr.; Johnson, M.; Murphy, T. J. J. Chem. Educ. 1995, 72, 1088. 13. Skolnik, A. M.; Hughes, W. C.; Augustine, B. H. Chem. Educator 2000, 5, 8. 14. Binning, G.; Roher, H.; Gerber, C.; Weibel, E. Phys. Rev. Lett. 1982, 49, 57. 15. Magonov, S. N.; Whangbo, M.-H. Surface Analysis with STM and AFM; Wiley: New York, 1996. Scanning Tunneling Microscopy and Spectroscopy. Theory, Techniques, and Applications; Bonnell, D. A., Ed.; VCH: New York, 1993. 16. Zhong, C. J.; Porter, M. D. Anal. Chem. 1995, 67, 709A. 17. Ulman, A. Introduction of Thin Organic Films: From LangmuirBlodgett to Self-Assembly; Academic: Boston, 1991. 18. Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498. Delamarche, E.; Schmid, H.; Bietsch, A.; Larsen, N. B.; Rothuizen, H.; Michel, B.; Biebuyck, H. J. Phys. Chem. B 1998, 102, 3324. 19. Tarlov, M. J.; Burgess, D. R. F. Jr.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305. 20. Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. 21. Zheng, W. X.; Maye, M. M.; Leibowitz, F. L.; Zhong, C. J. Anal. Chem. 2000, 72, 2190.
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