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Atomic Force Microscopy: Imaging with Electrical Double Layer Interactions Tim J. Senden,*ftCalum J. Drummond,**tJand Patrick K6kichefPtt Department of Applied Mathematics, Research School of Physical Sciences, The Australian National University, Canberra, ACT 0200, Australia, and CSXRO,Division of Chemicals and Polymers, Private Bag 10, Rosebank MDC, Clayton, Victoria 3169, Australia Received July 29, 1992. In Final Form: December 3, 1993"
The relationshipbetween atomic force microscope (AFM)surface images and tip-sample surface forces in aqueous electrolyte solutions is examined. Two novel AFM imaging modes, designated as electrical double layer and "hydration", are compared with the conventionalBorn "contact" mode of imaging. Fourier analysis suggests that AFM images may reveal correlation lengths and order parameters characteristicof surface forces. Many scientific disciplines can benefit from having a nondestructive method of measuring surface topography with suboptical resolution. The atomic force microscope (AFM)' now routinely provides images of a variety of hard periodic surfaces with better than nanometer resolution.24 AFM images of fragile organic samples, including DNA, proteins, blood cells, Langmuir-Blodgett films, and polymers, have alsobeen reported.6-14 For many fragileorganic materials the routine acquisition of AFM images in aqueous solution has, however, proved illusive.16 This situation is due, at least partially, to a limited understanding of the AFM tip-sample interactions. In this Letter the relationship between AFM surface images and tip-sample interactions in aqueous electrolyte solutions is explored. The AFM generally operates by scanning a relatively sharp tip, atached to a weak cantilevered beam, across the plane of a sample. A common procedure is to maintain a constant applied load while the tip is scanned. In principle any repulsive tip-sample interaction can be employed to obtain an AFM image. To date only Born repulsion,arising from electronic overlap,has been utilized. ~
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* Authors to whom correspondence may be addressed at ANU or CSIRO. T h e Australian National University. CSIRO. 0 Abstract published in Advance ACS Abstracts, January 1,1994. (1)Binnig, G.; Quate, C. F.; Gerber, Ch. Phys. Reu. Lett. 1986,56, 930-933. (2)Marti, 0.; Drake, B.; Hansma, P. K. Appl. Phys. Lett. 1987,51, 484-486. (3)Erlandsson, R.; Hadziioannou, G.; Mate, C. M.; McClelland, G. M.; Chiang, S.J. Chem. Phys. 1988,89,5190-5193. (4)Alexander, S.; Hellemans, L.; Marti, 0.; Schneir, J.; Elings, V.; Hansma, P. K.J. Appl. Phys. 1989,65,164-167. (5)Friedbacher, G.; Hansma, P. K.; Ramli, E.; Stucky, G. D. Science 1991,253,1261-1263. (6)Weisenhorn, A. L.; Hansma, P. K.; Albrecht, T. R.; Quate, C. F. Appl. Phys. Lett. 1989,54,2651-2653. (7)Weisenhom.A.L.:Drake.B.:Prater.C.B.:Gould.S.A.C.:Hansma, P. K:;Ohnesorge,'F.; Egger, hi.;Heyn, S.-P.; Gaub, H. E. Biophys. Ji 1990,58,1251-1268. (8)Egger, M.;Ohnesorge, F.;Weisenhorn, A. L.; Heyn, S. P.; Drake, B.; Prater, C. B.; Gould, S. A. C.; Hansma, P. K.; Gaub, H. E. J.Struct. Biol. 1990,103,89-94. (9)Butt, H.-J.; Downing, K. H.; Hansma, P. K. Biophys. J. 1990,58, 1473-1480. (10)Erlandsson, R.; Wigren, R.; Olsson, L. Microsc. Microanal. Microstruct. 1990,1, 471-480. (11)Meyer,E.;Howald,L.;Overney,R. M.;Heinzelmann,H.;Frommer, J.; GGntherodt, H.-J.; Wagner, T.; Schier, H.; Roth, S. Nature (London) 1991,349,398-400. (12)Tsao, Y.-H.; Yang, S. X.;Evans,D. F.;Wennerstrbm,H.Langmuir 1991,7,3154-3159. (13)Marchant, R. E.; Lea, A. S.; Andrade, J. D.; Bockenstedt, P. J. Colloid Interface Sci. 1992,148,261-272. (14)Leung, 0.M.; Goh, M. C. Science 1992,255,6446. (15)Lea, A. S.;Pungor, A.; Hlady, V.; Andrade, J. D.; Herron, J. N.; Voss, E. W., Jr. Langmuir 1992,8,68-73. f
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The utilization of other repulsive surface forces,that decay less rapidly with surface separation, may provide the opportunity to minimize tip-sample interactions and aid the development of nondestructive imaging strategies. As a result of commercial availability, many groups have been imaging surfaces with microfabricated siliconnitride tips.16 Therefore, the focus throughout this report is on the interaction between a silicon nitride tip and a silicon nitride sample. The choice of a symmetric system avoids complications that may arise from heterosurface interaction. The results refer to the same tip and sample so that direct comparisons can be made. Similar behavior was observed in repeat experiments with different tips. In aqueous solutions, the surface of silicon nitride is hydrolyzed and is composed of ionizableamphotericsilanol (Si-OH) and basic silazane and silylamine (Si2-NH and Si-NH2, respectively; the latter at low surface concentration) groups.17J8 Dependent upon the pH and electrolyte concentration, the surface charge of silicon nitride can be either positive, zwitterionic (zero net charge), or negative. Solution conditions have been chosen so as to compartmentalize and highlight the presence of specific tip-sample interactions. An in-depth discussion of the silicon nitride surface chemistry and the construction of the electrical double layer will be presented e1~ewhere.l~ In water (pH 6),the total force between tip and sample is clearly attractive for separations below ca 11nm (Figure 1). This is indicative of a surface with either a very low or zero net charge,where the van der Waals forcedominates the total interaction. Only regionsof the force curve where the gradient is less than the spring constant can be measured; thus all nonequilibrium data have been removed. On approach, the surface jump from ca. 4nm into "contact" ( D = 0), where Born repulsion dominates the total interaction and it is at this "zero of separation" that contact mode imaging is normally conducted. On separation, the surfaces initially adhere then jump out to ca. 17nm (*, see Figure la) which corresponds to a normalized pull-off force,FIR,ff, of ca. 13mN.m-l. The tip was treated as a sphere with an effective radius, Rem of 220 nm and the LPCVD silicon nitride sample as a flat plate. Effective (16)Albrecht, T. R.;Akamine, S.; Carver, T. E.; Quate, C. F. J. Vuc. Sci. Technol. 1990,A8, 3386-3396. (17)Harame, D. L.; Bouese, L. J.; Shott, J. D.; Meindl, J. D. IEEE Tram. Electron Devices 1987, ED-34,1700-1707. (18)Bergstrbm, L.; Bostedt, E. Colloids Surf. 1990,49,183-197. (19)Senden, T. J.; Dnunmond, C. J. Submitted to Colloids Surf. (20)Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature (London) 1991,353,239-241. (21)Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992,8, 1831-1836.
0 1994 American Chemical Society
Langmuir, Vol. 10, No.2,1994 359
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5 10 15 20 25 30 Tip-Sample Separation (nm) Figure 1. Force (F)as a function of separation (D)between a microfabicated silicon nitride AFM tip (Digital Instruments, Inc., Santa 0
Barbara, CA) and a silicon nitride sample (Silica-Source Technology Corp., Tempe, AZ) immersed in aqueous electrolyte solutions (inwardruns only are shown). The silicon nitride sample was a polished silicon wafer, coated via low pressure chemical vapor deposition, with a 500 nm layer of silicon nitrde. Both silicon nitride surfaces were treated with a water plasma (18 MHz, 10 W for 30 s, PH = 0.065 Torr, Ph = 0.02 Torr) immediately prior to use. Ultrahigh purity water was used (conductivity < 0.9 &cm-l a t 25 OC). NaOg and NaCl (both AnalaR grade) were used as supplied. Measurements were made with a Nanoscope I1 AFM (Digital Instruments Inc.). The F-D curves were obtained by converting the photodiode signal (proportional to the cantilever deflection; spring constant K = 0.11 i 0.01 Nom-') versus the sample displacement data that are shown in the figure insets (both an inward and an outward run are shown).w$l As the driving velocity is less than 200 n m 4 , the hydrodynamic contribution to the total force is negligible. Both the figure insets and the main parts show the distance the cantilever jumps upon separation of the surfaces. This is denoted by an asterisk u*n. Note the occurrence of intrinsic unstable regions where aF/dD 1 K and so the data have been ommitted from the F-D profiles as they do not represent equilibrium measurement. Assuming the long range effective radius, R (calibrated as explained in the text) to be valid, the normalized force FIR is equivalent to the free energy, E, per unit area of two interacting flat plates according to the Derjaguin approximationU (FIR = 2rE). Hollow lines are DLVO fits as a sum of the attractive van der Waals force and of the repulsive double-layer force, obtained by numerical solution of the nonlinear Poisson-Boltzmann equation. The onset of the dispersion forces is assumed to be at the Born contact (D= 0) while the diffuse layer boundary is located either a t this contact or shifted by 0.9 nm per each surface. (a, top left) pH = 6 water. The hollow line is the nonretarded van der Waals attraction calculated for two silicon nitride surfaces immersed in water. (b, top right) pH = 10 with 10-9 M NaC1. The lowest and the highest hollow lines describe the interaction (9= -112 mV; surface charge density l/a = 1.1 X 1013 cm-2) a t constant potential and constant charge boundary condition respectively for the case where the onset of the diffuse layer has been shifted to the O.H.P. The shaded region is bounded by the unshifted case (9= -140 mV; l / a = 2.0 X 1013 cm-2) of each constant potential and constant charge conditions. (c, bottom) pH 10 with le2 M NaCl. Description of the DLVO fits as in b); shifted case: 9 = -85 mV; l / a = 2.1 X 1013 cm-2; unshifted case: 9 = -140 mV; l/a = 6.5 X 1013 cm-2.
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tip radii were estimated by comparisonwith a well-defined sphere-flat system under identical solution and surface condition^.^^^^ The hollow line in Figure l a represents the van der Waals interaction where retardation effects have been ignored (F/R,ft = -A/6D2 using the Derjaguin appro~imation~~; with a nonretarded Hamaker constant A = 6.1 X lem J; this value can be derived23from optical properties of bulk CVD silicon nitride within Lifshitz theory25). It should be noted that the method for determiningReftinvolves an analysisof long-rangesurface (22) Senden, T. J. Ph.D. Thesis, Australian National University, Canberra, Australia, 1993. (23) Drummond, C. J.; Senden, T. J. Submitted to Colloids. Surf. (24) Derjaguin, B. V. Kolloid 2. 1934,69, 155-164. (25) Lifshitz, E. M. J. Exp. Theor. Phys. 1965,29,94(Sou.Phys.JETP 1966,2,73); Dzyaloshinskii, I. E.; Lifshitz, E. M.; Pitaevskii, L. P. Adu. Phys. 1961, IO, 165-209. Mahanty, J.; Ninham, B. W. Dispersion Forces; Academic Press: London, 1976.
interactions.22~23Consequently, Refi is an effective radius calibrated at large separations and does not necessarily reflect the small scale curvature of the tip. This needs to be kept in mind when normalizing the interaction at short separations and estimatingthe adhesion. In addition the pull-off force may be influenced by contact through asperities and by lateral tip movement, which may be problematic with the use of a single cantilever beam.26 At pH = 10silicon nitride is negatively charged.la The total interaction is governed by the competition between the attractivevan der Waals and repulsive electrical double layer (edl) forces. At large separations,the van der Waals contribution is negligible and the edl interaction is exponentially repulsive with a range decreasing upon addition of 1:lelectrolyte (Figure lb,c) in accordancewith (26) Christenson,H. K. J. Colloid Interface Sci. 1987,121,170-178.
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360 Langmuir, Vol. 10, No. 2, 1994
the DLVO theory.27928 The decay lengths are in reasonable agreement with the calculated Debye lengths ( d ) ; the M experimental K-1are 9.0 and 2.7 nm in 103 and NaC1, respectively. In this distance regime the normalized force curves can be fitted by assigning an apparent surface potential and by using a numerical solution of the nonlinear Poisson-Boltzmann equation.29 There are ambiguities associated with defining the “zero of separation”and, therefore, the absolute separation scale in force microscopy. This situation can lead to complications in the description of the charge formation at surfaces. In this work, it is assumed that the same primary contact is attained between silicon nitride surfaces, independent of solution condition~.~~s~3 In M NaC1, this “contact” is achieved after the surfaces have jumped together when, at short separations (2-3 nm) the van der Waals attraction overcomes the electrostatic repulsion (Figure lb; normalised pull-off force 13 mN.m-l). Conversely, at higher ionic strengths, it appears that the Born “contact” can only be realized if surface adsorbed layers of hydrated counterions are displaced under high applied loads. Due to the high degree of curvature, the tip may exert pressures well exceeding thousands of atmospheres at the area of contact. The pressure is certainly high enough to overcome the repulsive barrier of ca. 11 mN0m-l in M NaCl (Figure IC),thereby depleting the surfaces of their adsorbed hydrated ions, as indicated by a reproducible inward jump of ca. 1.2 nm (Le. one hydrated layer of ca. 0.6 nm per surface; by comparing the data for a number of tips the jump-in distances were found to be independent of both R,H and ionic strength above 1P2M NaC1). When both the van der Waals forces and the electrical diffuse layer are given the same onset, located a t the experimental Born “contact” (Figure lb,c) the DLVO analysis provides an effective surface potential of 140 mV for both 103and 1C2M NaC1. Nevertheless, in treatments of the electrical double layer, it is common to locate the onset of the diffuse layer at the plane of closest approach of hydrated diffuse-layer i~ns.~Oal This outer Helmholtz plane (O.H.P.) may be situated at three hydrated radii (0.9 nm) out from each surface. This location for the O.H.P. now gives effective diffuse layer potentials of 112 M NaC1, respectively. and 85 mV at 104 and The two DLVO fits presented in each of Figure l b and Figure ICprovide the limits for the diffuse layer potentials. When the onset of the diffuse layer is assumed to originate at “contact” both experimental force curves at 103 and M NaCl lie below the lowest limit (constant potential boundary condition) predicted by DLVO theory. This is misleading as the contribution of the electrostatics is misrepresented. Indeed, the data can now be reasonably well fitted at constant surface potential boundary conditions over the entire separation range when the onset of the diffuse layer is shifted away from contact to the O.H.P. Nevertheless, concerns over variations in the short-range tip radius severely limit any conclusions about the total interaction at short separations. Thus, no assessment can be made regarding the presence or absence of any (27) Derjaguin, B. V.; Landau, L. Acta Physicochim. URSS 1941,14, 633-632. (28) Verwey, E. J. W.;Overbeek, J. Th. G. Theory of Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948. (29) Chan,D. Y.C.; Pashley,R. M.; White, L. R. J. Colloid Interface Sci. 1980. 77.283-285. (30) Yata, D. E.; Levine, S.; Healy, T. W.J. Chem. SOC.,Faraday Trans. 1 1974, 70,1807-1818. (31) Shubin, V. E.; Klkicheff, P. J. Colloid Interface Sci. 1993,155, 108-123.
Table 1. Micrograph Characterization
B. B h Eh’ Eh“ Be microgrephrotation(deg) 90 180 180 180 90 90 90 surface roughness (nm) 14.0 12.1 7.3 8.4 12.6 14.7 9.9 2.1 1.5 stdmean height dev (nm) 2.3 1.5 2.7 2.1 1.7
additional short-range forces for silicon nitride surfaces in 1:lelectrolytes,and in particular the “hydration“ force.32 Figure 2 shows images obtained under different applied loads. Micrographs B,, Bb, and B, are examples of Born “contact” mode images. The term “Born contact” is used to distinguish this mode of imaging,which relies upon Born repulsion, from other modes which utilize different repulsive surface forces. Images acquired by using the Born mode appear to be independent of applied load. They exhibit very similar surface features, as indicated by surface roughness (Table 1). The main features are broad, flat convex mounds. These mounds are a result of the LPCVD manufacturing process and are also seen when imaging is conducted in dry air. Nevertheless, Born images may be a function of the surface chemistry of the tip and the sample. Note that the average x-y dimensions of the mounds estimated from the Born “contact” image when the surfaces have either zero or low net charge (B3 become larger when the surfaces bear a significant net charge (Bb and B,). The apparent larger dimensions may be an artefact due to lateral edl interactions between the side of the tip and neighbouring mounds. Micrographs Eb, Eb’, Eb”,, and E, all utilize the edl repulsion and show surface topography similar in nature. edl mode image definition improves as the surface separation is reduced, e.g., Eb’ may be approached reversibly from point Eb” with concomitant image improvement. Micrograph H, is taken at a load which is on the verge of displacingthe hydrated adsorbed counterions. €&shows some sections where the tip temporarily adheres to a particular region on the surface. Generally, when the tipFigure IC)the tip sample adhesion is weak (ca. 6 “em-’; may flip in and out of the minimum if the scan is operated at separations very close to the maximum of the hydration layer. Micrograph H, is comprised of both Born and hydration sections. Interestingly, the tip does not randomly flip in and out of Born “contact” along a single scan line. On repeat scans, both the Born and hydration sections appear at the same surface sites. This suggesta the presence of subtle surface site differences, possibly hydrated ion domains along the surface rather than random fluctuations in the adsorption of the hydrated counterions. When the adhesion is large (Figure lb) there is no image flipping because the tip becomes trapped in the primary minimum (Figure 2, micrograph Eb). The term “hydration”mode is meant to convey only the sense that a hydration wall is present, it is beyond the data to comment on the presence of any forces additional to those treated to DLVO. Scans of the same area under different applied loads show an increase in surface feature definition in the order edl < “hydration” < Born mode. Improvement in image resolution is linked to an increase in the gradient of the surface force being utilized. Small scale roughness effects are essentially smoothed over when imaging is conducted with long range repulsive forces. As the extent of the force is diminished the small scale roughness plays an increasing part in the interaction, until the image is generated predominantly by atomic scale (32)See for instance: Proceedings of the Nobel Conference on Hydration Forces and the Molecular Aspects of Solvation Chemistry. Chem. Scripta 1985, 25, 3-107.
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Langmuir, VoZ. 10, No. 2, 1994 361
Figure 2. AFM images (1064 nm)2 of the silicon nitride surface taken with different applied loads that are identified by the labels on the F-D curves in Figure 1. The vertical scale is the same for all figures (bar = 30 nm, BJ. Images were neither flattened nor filtered and, for visual purposes alone, only one out of every four data lines have been plotted. Images refer to the same area on the sample only for a given solution condition. Note that micrograph Eb is an example where the applied load has been allowed to drift slowly upward until dF/dD eventually exceeds the cantilever spring constant thereby causing the tip to jump into “contact” with the _ _ Key: Ba, top left; Bb, top middle; B,, top right; Eb, middle left; Eb’, middle center; Eb”, middle right; E,,bottom left; H,, bottom surface. right.
asperities. It is important to recognize that the image is generated by the integration of the entire force of interaction (including both solution extended and Born “contact” components where relevant) over the tip and sample surface geometries. A direct consequence of this complex process of image generation is that the edl and Born modes are fundamentally different. In the edl mode the tip experiences only a repulsive force. In the Born mode the tip apex experiences a Born repulsion, and depending upon solution conditions, the next section of the tip outward from the sample surface may experience an attractive van der Waals force (discussed by Abraham et u Z . ~ ~ ) ,while the next section of the tip may experience a repulsive double layer force (as shown in this Letter). The Born “contact” mode is further distinguished from ~
~~
(33) Abraham, F. F.; Batra, I. P.; Ciraci, S. Phys. Rev. Lett. 1989,60, 1314-17.
the other modes in that the nanomechanical properties, for instance friction, on the interacting surfaces must be considered an important component of imaging.34 However, in the edl and “hydration” modes the tip may glide fricti~nlesslf~ above the surface at separationsjust beyond the height of any adsorbed molecule. Solution viscous forces may be considered to be acting in a constant manner over the tip regardless of the imaging mode. The possibility of the detection of surface structure giving rise to an interaction is explored in this section. The deconvolution of surface geometry and surface force is complex and must be explored; however a different tact may be employed in this first instance. Noise or the effects (34) Burnham, N. A.; Colton, R. J. J. Vac. Sci. Technol. 1989, A7, 2906-2913.
(35) O’Shea, S. J.; Welland, M. E.; Rayment, T. Appl. Phys. Lett. 1992,61,2240-2242.
362 Langmuir, Vol. 10,No. 2, 1994 of time scale ean often obscure the correlation of objects. In real space this leads to the loss of interpreted information. Comparison in reciprocal space, however, at the loss of positional information, offers quantitative analysis of the correlation of objects with each other. Fourier analysis of the three types of repulsive imaging modes was conducted to study such relationships. Sections of the images shown in Figure 2 were enlarged so as to increase the spacial resolution before Fourier transformation. Each image comprised (40012pixels giving a Nyquist cut-off of less than 1.25 nm. Images were transferred to mainframe based image processing software. AFM images are produced by raster scanningand have associated frequencies, together with the usual zero frequency noise. These components were masked in recipracal space with a narrow cross ("+") shapedfdter. These 2 - 0 data were then radially regrouped. In 2-D reciprocal space the resulting patterns are isotropic in every imaging mode. The spectrum for image H,,however, reveals a bump centered at about Q = 3nm-', which is absent from the spectra for both the Born and edl images at this concentration. The bump indicates a lateral correlation length of about 2 nm ( 2 6 3 ) with no apparent long range order. Because this feature is not seen in the edl spectrum, it is unlikely that the observed correlation is due to surface charge density. It is also unlikely to be a result of the surface roughness as a similar result has been obtained in the case of amolecularly smooth mica surface (to be reported in a forthcoming article%). At this stage, an unequivocal statement about the origin of the feature cannot be made; our current work may suggest that the correlation length characterizes some (36) KBkicheff, P.; et al. In preparation.
Letters
structural ordering along the surfaces that is present only in the "hydration" mode of imaging. In summary, the effects that imaging with edl, "hydration", and Born repulsion has on AMF micrographs have been pointed out. Notably, imaging can be conducted with forces less than 0.1 nN and at tip-sample separations that are controllable through the charge densities of the surfaces and the properties of the bulk solution. This may be used to great advantage in nondestructive AFM imaging (indeed this methodology has recently been employed to image mica adsorbed virus proteinas'). In principle, the adsorbate may then be imaged based on either its disturbance of the edl or a temporary jump into the Born "contact" mode with reversion to the edl mode after leaving the molecule. The latter occurrence will naturally depend on the relative binding energies of the adsorbate to the tip and substrate. There is the potential to circumvent disruptive lateral interactions between the tip and adsorbates. There may be an added 'degree of uncertainty" in an edl mode image, but an image with "reduced" resolution may be all that can be obtained for some fragile systems. AFM imaging conducted with long range repulsive surface forces may also provide an insight into how "rough", charged surfaces actually "view" one another upon approach. Acknowledgment. C.J.D. is the recipient of a Queen Elizabeth I1 Fellowship and an Australian National University Visiting Fellowship. We thank William Ducker for introducing us to AFM force measuremepts. We also are grateful to Barry Ninham and Stjepan Marcelja for useful comments. (37) Senden, T. J.; et al. In preparation.
