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On the Factors Affecting the Contrast of Height and Phase Images in Tapping Mode Atomic Force Microscopy R. Brandsch and G. Bar* Freiburger Materialforschungszentrum, Stefan-Meier-Strasse 21, D-79104 Freiburg, Germany
M.-H. Whangbo* Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204 Received July 23, 1997. In Final Form: September 23, 1997X Tapping mode atomic force microscopy measurements were performed for patterned self-assembled monolayers (SAMs) of -S(CH2)15CH3 and -S(CH2)15COOH groups on a polycrystalline Au substrate. Height and phase images of these SAMs were obtained as a function of the driving amplitude A0 and the set-point amplitude Asp. Factors influencing the contrasts of these images are discussed in terms of the simple approximation that the essential consequence of bringing the tip closer to the sample surface is to change the force constant of the cantilever.
1. Introduction Tapping mode atomic force microscopy (TMAFM)1 greatly reduces irreversible destruction of sample surfaces, so it has been widely used for the study of soft materials such as polymers and biological samples.2 Important experimental parameters of TMAFM are the amplitude A0 of the free oscillation and the set-point amplitude ratio rsp ) Asp/A0, where Asp is the set-point amplitude. During scanning the observed amplitude of oscillation is maintained at Asp by adjusting the vertical position of the sample. As the tip is brought close to the sample surface, the vibrational characteristics (i.e., the force constant, resonance frequency, phase angle, and amplitude) of the cantilever change due to the tip-sample interaction and the contamination layer on the sample surface. In TMAFM a surface region of larger amplitude damping is recorded as higher in topography and hence brighter in height image. It is a crucial issue of TMAFM studies how one can relate contrasts of height and phase images recorded at various A0 and rsp to tip-sample interactions and hence to physical properties of samples.3-8 X
Abstract published in Advance ACS Abstracts, October 15, 1997.
(1) Zhong, Q.; Innis, D.; Kjoller, K.; Elings, V. B. Surf. Sci. Lett. 1993, 290, L688. (2) (a) Radmacher, M.; Tillmann, R. W.; Fritz, M.; Gaub, H. E. Science 1992, 257, 1900. (b) Umemura, K.; Arakawa, H.; Ikai, A. Jpn. J. Appl. Phys. 1993, 32, L1711. (c) Stocker, W.; Beckmann, J.; Stadler, R.; Rabe, J. P. Macromolecules 1996, 29, 7502. (d) Bustamante, C.; Keller, D. Phys. Today 1995 (Dec), 32. (e) Howard, A. J.; Rye, R. R.; Houston, J. E. J. Appl. Phys. 1996, 79, 1885. (f) Ho¨per, R.; Workman, R. K.; Chen, D.; Sarid, D.; Yadav, T.; Withers, J. C.; Loutfy, R. O. Surf. Sci. 1994, 311, L731. (3) Magonov, S. N.; Elings, V.; Papkov, V. S. Polymer 1997, 38, 297. (4) (a) Magonov, S. N.; Elings, V.; Whangbo, M.-H. Surf. Sci. 1997, 375, L385. (b) Whangbo, M.-H.; Magonov, S. N.; Bengel, H. Probe Microsc. 1977, 1, 23. (c) Magonov, S. N.; Cleveland, J.; Elings, V.; Denley, D.; Whangbo, M.-H. Surf. Sci., in press. (d) Bar, G.; Thomann, Y.; Brandsch, R.; Cantow, H.-J.; Whangbo, M.-H. Langmuir 1997, 13, 3807. (e) Whangbo, M.-H.; Bar, G.; Brandsch, R. Appl. Phys. A, in press. (f) Whangbo, M.-H.; Bar, G. Surf. Sci. Lett., submitted for publication. (5) (a) Spatz, J. P.; Sheiko, S.; Mo¨ller, M.; Winkler, R. G.; Reineker, P.; Marti, O. Nanotechnology 1995, 6, 40. (b) Winkler, R. G.; Spatz, J. P.; Sheiko, S.; Mo¨ller, M.; Reineker, P.; Marti, O. Phys. Rev. B 1996, 54, 8908. (6) (a) Sarid, D.; Ruskell, T. G.; Workman, R. K.; Chen, D. J. Vac. Sci. Technol., B 1996, 14, 864. (b) Sarid, D.; Chen, D.; Workman, R. K. Computat. Mater. Sci. 1995, 3, 475. (c) Chen, D.; Workman, R. K.; Sarid, D.; Ho¨per, R. Nanotechnology 1994, 5, 1999. (7) Anczykowski, B.; Kru¨ger, D.; Fuchs, H. Phys. Rev. B 1996, 53, 15485.
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In recent years, a number of TMAFM studies have reported anomalous height images.9-14 For example, a considerable height reduction was found for biomolecules 9,10 and for inorganic materials (e.g., Cu clusters on SiOx and Au clusters on Al2O312) whose compressibility is much smaller than that of biomolecules. In some cases, even a reversal in height image occurs (e.g., a partially-removed monolayer of Au on mica13 and InAs islands on GaAs(001)14). Recently, Van Noort et al.13 showed that adhesion dampens the amplitude of the cantilever oscillation, and the associated energy loss decreases the average amplitude of the cantilever thereby causing anomalous height images. In the present work, we carry out TMAFM studies of several different patterned self-assembled monolayers (SAMs) on a polycrystalline Au substrate as a function of A0 and rsp in order to investigate factors affecting the contrasts of height and phase images. 2. Experimental Section Three types of patterned SAMs were prepared on a polycrystalline Au substrate using the microcontact printing method described elsewhere:15 (1) samples possessing circular patterns of -S(CH2)15CH3 groups in the surrounding background of Au; (2) samples possessing circular patterns of -S(CH2)15CH3 groups in the surrounding background of the -S(CH2)15COOH groups; (3) samples possessing circular patterns of -S(CH2)15CH3 groups in the surrounding background of -S(CH2)7CH3 groups. For convenience, the SAMs (1), (2), and (3) will be referred to as CH3/Au, CH3/COOH, and C16/C8, respectively. The CH3/Au and CH3/COOH samples have two surface regions of very different (8) Burnham, N. A.; Behrend, O. P.; Oulevey, F.; Gremaud, G.; Gallo, P.-J.; Gourdon, D.; Dupas, E.; Kulik, A. J.; Pollock, H. M.; Briggs, G. A. D. Nanotechnology 1997, 8, 67. (9) Vinckier, A.; Dumortier, C.; Engelborghs, Y.; Hellemans, L., J. Vac. Sci. Technol., B 1996, 14, 1427. (10) Li, M.-Q.; Xu, L.; Ikai, A. J. Vac. Sci. Technol., B 1996, 14, 1410. (11) Fritz, M.; Radmacher, M.; Cleveland, J. P.; Allersma, M. W.; Steward, R. J.; Gieselmann, R.; Janmey, P.; Schmidt, C. F.; Hansma, P. K. Langmuir 1995, 11, 3531. (12) (a) Ku¨hle, A.; Sørensen, A. H.; Bohr, J. J. Appl. Phys. 1997, 81, 6562. (b) Mahoney, W.; Schaefer, D. M.; Patil, A.; Andres, R. P.; Reifenberger, R. Surf. Sci. 1994, 316, 383. (13) Van Noort, S. J.; Van der Werf, K. O.; De Grooth, B. G.; Van Hulst, N. F.; Greve, J. Ultramicroscopy, in press. (14) Ramachandran, T. R.; Kobayashi, N. P.; Heitz, R.; Chen, P.; Madhukar, A. Proceedings of the MRS Society Meeting, Boston, Fall 1996, in press. (15) (a) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498. (b) Bar, G.; Parikh, A. N.; Rubin, S.; Swanson, B. I.; Zawodzinski, T. A., Jr.; Whangbo, M.-H. Langmuir 1997, 13, 373.
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6350 Langmuir, Vol. 13, No. 24, 1997 hydrophilicity, while the C16/C8 sample has two surface regions of identical hydrophilicity. In CH3/Au the CH3 region is about 1.9 nm higher in topography than the bare Au substrate according to the ellipsometric data.16 In CH3/COOH the CH3 and COOH regions are practically the same in topography. According to the measurements of contact angles,17 the hydrophilic character of the CH3, Au, and COOH regions increases in the order CH3 < Au < COOH. The prepared samples were investigated within a few hours of preparation by TMAFM. These experiments were performed with a Nanoscope III scanning probe microscope as a function of rsp at several different A0 values under ambient conditions and also after purging the air with dry N2 for 12 h. Commercial Si cantilevers with force constants of 13-70 N/m and their fundamental resonance frequencies around 160 kHz were used for measurements. Images were recorded with typical scan speeds of 0.5-1 line/s using a scan head with a maximum range of 170 × 170 µm.
3. Results The height images of CH3/Au obtained for A0 ) 20, 60, and 140 nm at rsp ) 0.8 under ambient conditions are shown in parts a, c, and e of Figure 1, respectively, and the corresponding phase images are shown in parts b, d, and f of Figure 1, respectively. For A0 ) 20 and 140 nm the height images show no contrast difference between the CH3 and Au regions. In the height image for A0 ) 60 nm the CH3 regions appear darker (i.e., lower lying by about 2 nm) than the Au surrounding, in contradiction to the real topography of CH3/Au. In the phase images the CH3 regions are darker than the Au surrounding for A0 ) 20 nm, and the opposite is the case for A0 ) 60 and 140 nm. To understand such variations of image contrasts, the dependence of the phase and height image contrasts on rsp was examined systematically. The dependence of the phase angle shift ∆Φ on rsp for A0 ) 20, 60, and 140 nm is summarized in Figure 2a-c, where every ∆Φ-versusrsp plot was obtained by averaging ∆Φ versus rsp plots recorded at several different places on the CH3 or Au region. Though not shown for lack of space, the height images in general show always a brighter contrast on the lower-lying Au region when the phase shift on the Au region is negative and is considerably lower than that on the CH3 region. Only when the phase shift on the Au region is positive, was it possible occasionally to obtain the correct height contrast (i.e., brighter on the CH3 region). However, the reproducibility was poor, and the height image shows no contrast mostly. The amplitude of the freely vibrating cantilever is increased by the N2 purging, most probably because the removal of moisture reduces the viscous damping of the cantilever. The height and phase images (not shown) recorded under N2 for A0 ) 40 nm and rsp ) 0.8 were similar to those obtained under ambient conditions for A0 ) 60 nm and rsp ) 0.8 (Figure 1c,d), except that the contrast difference between the Au and CH3 regions is reduced. Figure 2d shows the phase shifts of the CH3 and Au regions as a function of rsp. With respect to the ambient condition data (Figure 2b), the phase shift difference between the CH3 and Au regions is reduced under N2 atmosphere. Under N2 atmosphere height images of CH3/Au show the correct height contrast reproducibly, when A0 > 40 nm, ∆Φ > 0, and rsp < 0.5. Parts a and b in Figure 3 show the height and phase images of CH3/COOH recorded for A0 ) 60 nm and rsp ) 0.6, respectively. The phase image shows a darker (16) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (17) For example, contact angle measurements yield θ(H2O) ≈ 110° for the CH3 region, θ(H2O) ≈ 55°-80° for Au region, and θ(H2O) < 10° for the COOH region.
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contrast on the COOH region (∆Φ ) -26° and -39° on the CH3 and COOH regions, respectively), and the height image is darker (about 3.5 nm lower-lying) on the CH3 region. This situation is similar to the case of CH3/Au for A0 ) 60 nm and rsp ) 0.8 (Figure 1c,d). In the case of CH3/Au, the phase shift is more negative on the less hydrophilic region (i.e., CH3) for small A0 and large rsp (Figure 2a). The same is found for CH3/COOH also when A0 is small and rsp is large (e.g., A0 ) 10 nm, rsp ) 0.450.75). 4. Qualitative Relationships Affecting Image Contrasts For qualitative purposes, height and phase images of TMAFM can be interpreted by considering the change in the force constant of the cantilever when it is brought close to the sample surface.4 For the free cantilever with force constant k and resonance frequency ω0, its amplitudeversus-frequency plot A(ω) has a peak at ω0 with the peak height A0.18 The force constant of the interacting cantilever can be written as keff ) k + σ,4,19 where σ is the sum of the force derivatives of all attractive and repulsive forces acting on the cantilever. In this approximation, the resonance frequency of the interacting cantilever is shifted to a new resonance frequency ωeff, and the phase angleversus-frequency and amplitude-versus-frequency plots of the interacting cantilever are obtained by shifting the centers of the corresponding plots of the free cantilever from ω0 to ωeff. As a result, when |σ| , k, the phase shift ∆Φ and the amplitude damping ∆A measured at ω0 are given by 4a,d,f
∆Φ ≈ Qσ/k ∆A ≈ A0
(1)
[(Qσk ) - (2kσ )r ] 2
2
sp
(2)
The amplitude damping shown in eq 2 was derived4f by considering the maximum kinetic energy the cantilever has when it strikes the sample surface. At each oscillation the maximum kinetic energy available for the freely oscillating cantilever is given by W ) 0.5kA02.18 The corresponding energy for the interacting cantilever can be written as 0.5keffAsp2 because when the cantilever reaches the maximum amplitude Asp, its force constant is keff. Thus the cantilever’s energy loss ∆W at each oscillation can be expressed as
σ ∆W ) 1 - rsp2 - rsp2 W k
()
(3)
Namely, the energy loss of the cantilever is enhanced when σ < 0, i.e., when the cantilever is dominated by attractive interactions such as adhesion and capillary forces. Such an energy loss enhances the amplitude damping, as expressed in eq 2.4f This supports the conclusion of Van Noort et al.13 In the discussion of TMAFM results under ambient conditions, it is convenient to write the force constant change as σ ) σts + σcl, where σts is caused by the tipsample interaction and σcl by the contamination layer. This distinction is important because it is not always the σts term that dominates σ. For small A0 and large (18) Rao, S. S. Mechanical Vibrations, 3rd ed.; Addison-Wesley: New York, 1995. (19) (a) Martin, Y.; Williams, C. C.; Wickramasinghe, H. K. J. Appl. Phys. 1987, 61, 4723. (b) Du¨rig, U.; Zu¨ger, O.; Stalder, A. J. Appl. Phys. 1992, 72, 1778. (c) Babcock, K.; Dugas, M.; Manalis, S.; Elings, V. Mater. Res. Soc. Symp. Proc. 1995, 355, 311. (d) Pethica, J. B.; Oliver, W. C. Phyis. Scr., 1987, T19, 61.
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Figure 1. Height images of CH3/Au obtained under ambient condition with rsp ) 0.8 for (a) A0 ) 20 nm, (c) A0 ) 60 nm, and (e) A0 ) 140 nm. Phase images of CH3/Au obtained under ambient condition with rsp ) 0.8 for (b) A0 ) 20 nm, (d) A0 ) 60 nm, and (f) A0 ) 140 nm. The scale bar corresponds to 5 µm in all images. In the height images the contrast covers height variations in the 20 nm range in (a), (c), and (e). In the phase images the contrast covers phase angle variations in the 20° range in (b), in the 100° range in (d), and in the 50° range in (f).
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Figure 2. Phase shifts of the CH3 and Au regions as a function of rsp for (a) A0 ) 20 nm, (b) A0 ) 60 nm, and (c) A0 ) 140 nm. (d) Phase shifts of the CH3 and Au regions as a function of rsp after the air was purged with dry N2.
Figure 3. (a) Height and (b) phase image of CH3/COOH obtained under ambient condition with rsp ) 0.6 for A0 ) 60 nm. The scale bar corresponds to 10 µm. In the height image the contrast covers height variations in the 10 nm range and in the phase image the contrast covers phase angle variations in the 50° range.
5. Image Analysis
of the Si cantilever is covered with SiO2,20 so it is significantly more hydrophilic than the CH3 surface. Thus in CH3/Au the hydrophilicity difference between the tip and the CH3 region should be much larger than that between the tip and the Au region. Then it is reasonable
On the basis of the theoretical discussion of the previous section, we first analyze the phase shift data under light tapping conditions, when σ is dominated by σcl. The tip
(20) Contact angle measurements yield θ(H2O) < 10° for clean Si and SiO2: Dulcey, C. S.; George, J. H.; Chen, M.-S.; McElvany, S. W.; O’Ferrall, C. E.; Benezru, V. I.; Calvert, J. M. Langmuir 1996, 12, 1638.
rsp (i.e., light tapping), the tip-sample interaction is weak so that σ is expected to be well approximated by σcl.
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to speculate that the tip will be covered with more water and debris (from the contamination layer) on the less hydrophilic surface, so that σcl (on CH3) < σcl (on Au) < 0. This explains why the phase shift is more negative on the less hydrophilic region at light tapping.21 The same reasoning holds true for why in CH3/COOH the less hydrophilic region has a more negative phase shift at light tapping. With an increase of A0 the tip penetrates deeper into the contamination layer and hence becomes strongly affected by the capillary force and also by the tip-sample van der Waals (VDW) attraction. In this interaction regime, the phase shift is more negative on the more hydrophilic region (i.e., the Au region in CH3/Au due to the capillary force and VDW attraction and the COOH region in CH3/COOH due to the capillary force and hydrogen bonding interaction). With a further increase of A0, the repulsive tipindentation force also contributes to σts. Eventually the σts term becomes positive, and so does σ. Figure 2c shows that ∆Φ (on Au) < ∆Φ (on CH3) even for large A0 and small rsp (i.e., hard tapping) although the ∆Φ values are both positive. This means that the indentation forces on the CH3 and Au regions are nearly the same, which in turn indicates that the indentation force on the CH3 region largely comes from the Au substrate lying underneath. When the height images of CH3/Au show a contrast difference between the CH3 and Au regions, the phase shift on the Au region is considerably more negative so that σ is considerably more negative (eq 1) i.e., the tipsample interaction is considerably more attractive on the Au than on the CH3 region. The frequency shift ωeff - ω0 increases with increasing |σ|, and the amplitude damping ∆A increases with increasing |σ| (eq 2). Therefore, when the height images show a contrast difference, the amplitude damping ∆A is significantly larger on the Au region. This makes the Au region brighter in height image, although the actual height of the Au region is about 1.9 nm lower than that of the CH3 regions. Similarly, in the height image of CH3/COOH (Figure 3a), the COOH region appears lower-lying because the tip-sample attraction is
much stronger on the COOH region (i.e., ∆Φ (on COOH) < ∆Φ (on CH3) < 0). In agreement with the study of Van Noort et al.,13 our work shows that a reversal in height image is found for the case of A0 and rsp values when the more hydrophilic region of the SAMs is dominated by the attractive interaction. The height reversal is explained in terms of the cantilever’s amplitude damping, which is caused by a change in the cantilever’s force constant and by the cantilever’s energy loss during tapping. Our results on CH3/Au with N2 purging show that on a surface of two different components anomalous heights can be obtained even if the contamination layer is reduced, as long as the tip-sample attraction is much stronger on one component. Nevertheless, reducing the effect of the contamination layer facilitates the observation of height images with correct image contrast. According to this result and the above discussion, it is predicted that on patterned SAMs composed of methyl-terminated alkanethiols of different chain length (e.g., C16/C8), one should observe the correct height image contrast (i.e., the C16 region is brighter) at all A0 and rsp values. Our measurements at various A0 and rsp values indicate that this is indeed the case.
(21) It is of interest to consider our observation from the perspective of Sinniah et al.’s recent study. They carried out a systematic examination of the adhesive forces between tips and samples coated with alkanethiolate monolayers with different terminal groups in various solvents. In water the highest adhesion forces are found for the most hydrophobic combination of interfaces. See: Sinniah, S. K.; Steel, A. B.; Miller, C. J.; Reutt-Robey, J. E. J. Am. Chem. Soc. 1996, 118, 8925.
Acknowledgment. Work at North Carolina State University was supported by the Office of Basic Energy Sciences, Division of Materials Sciences, U.S. Department of Energy, under Grant DE-FG05-86ER45259.
6. Concluding Remarks Our study shows that anomalous height images are obtained when the phase shift on the more hydrophilic is negative and is considerably lower than that on the less hydrophilic region. On regions with attractive tip-sample interactions, the amplitude damping can be large because the frequency lowering is large and because the cantilever’s energy loss is large. Since the feedback mechanism of tapping mode AFM registers amplitude damping as a height increase, the lower-lying regions of a heterogeneous surface can appear higher lying in height images when these regions cause strong tip-sample attractive interactions. Our study implies that the anomalous heights observed for biomolecules can occur without the tip squeezing or collapsing the molecules on the substrate. It also suggests that at light tapping when σ is dominated by the contamination layer, the σcl term is more negative on a less hydrophilic region.
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