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In-Situ Atomic Force Microscopy (AFM) Imaging: Influence of AFM Probe Geometry on Diffusion to Microscopic Surfaces David P. Burt, Neil R. Wilson, Ulrich Janus, Julie V. Macpherson,* and Patrick R. Unwin* Department of Chemistry, UniVersity of Warwick, CoVentry CV4 7AL, U.K. ReceiVed January 30, 2008. ReVised Manuscript ReceiVed April 25, 2008 The effect of AFM probe geometry on diffusion to micrometer-scale reactive (electrode) interfaces is considered. A disk-shaped substrate electrode was held at a potential to reduce a species of interest (aqueous Ru(NH3)63+) at a diffusion-controlled rate and the current response during AFM imaging provided information on local mass transport to the interface. This approach reveals how the AFM probe influences diffusion to a reactive surface, which is of importance in more clearly delineating the conditions under which in-situ AFM can be treated as a noninvasive probe of surface processes involving mass transport (e.g., electrode reactions and crystal dissolution and growth). An assessment has been made of three types of probes: V-shaped silicon nitride contact mode probes; single beam silicon probes; and batch-fabricated scanning electrochemical-atomic force microscopy (SECM-AFM) probes. Two disk electrodes, (6.1 µm and 1.6 µm diameter) have been considered as substrates. The results indicate that conventional V-shaped contact mode probes are the most invasive and that the batch-fabricated SECM-AFM probes are the least invasive to diffusion at both of the substrates used herein. The experimental data are complemented by the development of simulations based on a simple 2D model of the AFM probe and active surface site. The importance of probe parameters such as the cantilever size, tip cone height, and cone angle is discussed, and the implications of the results for studies in other areas, such as growth and dissolution processes, are considered briefly.
Introduction Atomic force microscopy (AFM) has revolutionized the study of those reactions at solid/liquid interfaces1 that involve a change in surface topography, such as crystal dissolution/growth,2,3 metal electrodeposition,4 and molecular adsorption.5 However, although reactions at solid/liquid interfaces involve surface processes coupled to the mass transport of reactants and products to and from the interface,6 the nature of mass transport under AFM conditions is often neglected or is poorly defined. Most AFM studies of solid/liquid interfaces have employed a commercially available fluid cell operated under either stagnant solution conditions7,8 or with continuous flow.9–11 In stagnant solution, the surface reaction may deplete the concentration of reactant adjacent to the interface, causing a notable reduction in the rate of the process.12 Under continuous flow, investigations have usually increased the flow rate to a level where there is apparently no effect on the rate of the surface process (as judged by AFM imaging).10 This approach seeks to eliminate the influence of mass transport so that it can be assumed that the * Corresponding authors.
[email protected].
E-mail:
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(1) Macpherson; J. V. In Encyclopedia of Electrochemistry; Bard, A. J., Stratmann, M., Unwin, P. R. Ed.; Wiley-VCH: Weinheim, Germany, 2003; Vol. 3, pp 413-443. (2) Hillner, P. E.; Manne, S.; Gratz, A. J.; Hansma, P. K. Ultramicroscopy 1992, 42-44, 1387–1393. (3) Hillier, A. C.; Ward, M. D. Science 1994, 263, 1261–1264. (4) Manne, S.; Hansma, P. K.; Massie, J.; Elings, V. B.; Gewirth, A. A. Science 1991, 251, 183–186. (5) Xu, S.; Cruchon-Dupeyrat, S. J. N.; Garno, J. C.; Liu, G.-Y.; Jennings, G. K.; Yong, T.-H.; Laibinis, P. E. J. Chem. Phys. 1998, 108, 5002–5012. (6) Unwin, P. R. J. Chem. Soc., Faraday Trans. 1998, 94, 3183–3195. (7) Macpherson, J. V.; Unwin, P. R.; Hillier, A. C.; Bard, A. J. J. Am. Chem. Soc. 1996, 118, 6445–6452. (8) Hay, M. B.; Workman, R. K.; Manne, S. Langmuir 2003, 19, 3727–3740. (9) Teng, H. H.; Dove, P. M.; De Yoreo, J. J. Geochim. Cosmochim. Acta 2000, 64, 2255–2266. (10) Teng, H. H.; Dove, P. M.; Orme, C. A.; De Yoreo, J. J. Science 1998, 282, 724–727. (11) Davis, K. J.; Dove, P. M.; De Yoreo, J. J. Science 2000, 290, 1134–1137. (12) Jung, T.; Sheng, X.; Choi, C. K.; Kim, W.-S.; Wesson; J.A.; Ward, M. D. Langmuir 2004, 20, 8587–8596.
solution composition close to the interface is close to that in bulk solution. However, recent simulations indicate that the hydrodynamics in a conventional fluid cell are complex13 and attaining this condition may not be as straightforward as previously envisaged. To address the issue of poorly defined mass transport in AFM, a fluid flow cell has been designed in which the hydrodynamics are readily treated.14 This cell has found application in several systems and allows interfacial fluxes to be measured quantitatively.15,16 For example, in the study of rapid processes such as the proton-promoted dissolution of calcite, this flow cell was able to deliver sufficiently high mass transport rates to compete with the surface process16 and yielded rate constants similar to those deduced from channel flow measurements.17 An alternative approach to generating high mass transport rates is to employ micrometer-scale interfaces in static solution for which diffusion is very well defined. This approach is exemplified by the wide-ranging application of ultramicroelectrodes (UMEs) in the study of the kinetics of fast heterogeneous electron transfer and coupled solution reactions.18 It follows that other types of microscale interfaces, such as single microcrystals, should display well-defined mass transport and could thus be attractive for the study of surface reaction kinetics using AFM imaging in quiescent solutions, with conventional fluid cells.19,20 When the size of the AFM tip apex becomes comparable to the area of the active site on a surface, the effect of the physical (13) Gasperino, D.; Yeckel, A.; Olmsted, B. K.; Ward, M. D.; Derby, J. J. Langmuir 2006, 22, 6578–6586. (14) Coles, B. A.; Compton, R. G.; Booth, J.; Hong, Q.; Sanders, G. H. W. J. Chem. Soc., Chem. Commun. 1997, 619–620. (15) Hong, Q.; Sua´rez, M. F.; Coles, B. A.; Compton, R. G. J. Phys. Chem. B 1997, 101, 5557–5564. (16) Coles, B. A.; Compton, R. G.; Sua´rez, M.; Booth, J.; Hong, Q.; Sanders, G. H. W. Langmuir 1998, 14, 218–225. (17) Compton, R. G.; Unwin, P. R. Philos. Trans. R. Soc. London, Ser. A 1990, 330, 1–45. (18) See, for example, Forster, R. J. In Encyclopedia of Electrochemistry; Bard, A. J., Stratmann, M., Unwin, P. R. Ed.; Wiley-VCH: Weinheim, Germany, 2003; Vol. 3, pp 160-195.
10.1021/la8003323 CCC: $40.75 2008 American Chemical Society Published on Web 06/18/2008
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presence of the probe on diffusion to and from the active region may become important; this is an issue that does not appear to have been treated sufficiently. By employing disk UMEs as model substrates, we show in this article that it is possible to assess the influence of AFM probe geometry on diffusion to micrometersized interfaces. The use of UMEs allows the electrode reaction driving force to be controlled precisely via the applied potential, whereas the current gives a direct measure of the diffusion rate under transport-limited conditions. The investigations herein, which deal with the effect of imaging probes on diffusion to microscale interfaces, are not only of benefit to AFM19,20 but also have implications for quantitative imaging applications of other techniques, such as scanning electrochemical microscopy (SECM)21–25 and combined SECM-AFM.26–37 The studies are particularly pertinent, for example, to porous membrane transport,30,38 the detection of electroactive regions on oxide-passivated films,39 and imaging variations in surface activity where physically noninvasive probes are desirable. We consider the effect of three types of AFM probes on mass transport to UMEs: V-shaped silicon nitride probes (a common design for contact mode AFM) and single-beam probes (employed for contact mode and tapping mode imaging), which are available commercially, and a microfabricated probe design that has been used for thermal AFM40 and combined SECM-AFM34,35 imaging. To support the experimental studies, finite element modeling has been employed to provide insight into the nature of diffusion to disk-shaped active sites in the presence of an AFM probe tip.
Simulations We consider the n-electron reduction of species Ox to species Red at a substrate UME
Ox + ne- f Red
(1)
with only Ox initially present in bulk solution. For an inlaid disk (19) Dobson, P. S.; Bindley, L. A.; Macpherson, J. V.; Unwin, P. R. Langmuir 2005, 21, 1255–1260. (20) Dobson, P. S.; Bindley, L. A.; Macpherson, J. V.; Unwin, P. R. Chem. Phys. Chem. 2006, 7, 1019–1021. (21) Bard, A. J.; Fan, F. F.; Pierce, D. T.; Unwin, P. R.; Wipf, D. O.; Zhou; F, Science 1991, 254, 68–74. (22) Arca, M.; Bard, A. J.; Horrocks, B.; Richards, T. C.; Treichel, D. A. Analyst 1994, 119, 719–726. (23) Mirkin, M. V. Anal. Chem. 1996, 68, 177A–182A. (24) Baltes, N.; Thouin, L.; Amatore, C.; Heinze, J. Angew. Chem., Int. Ed. 2004, 43, 1431–1435. (25) Amemiya, S.; Guo, J.; Xiong, H.; Gross, D. A. Anal. Bioanal. Chem. 2006, 386, 458–471. (26) Macpherson, J. V.; Unwin, P. R. Anal. Chem. 2000, 72, 276–285. (27) Macpherson, J. V.; Unwin, P. R. Anal. Chem. 2001, 73, 550–557. (28) Kranz, C.; Friedbacher, G.; Mizaikoff, B.; Lugstein, A.; Smoliner, J.; Bertagnolli, E. Anal. Chem. 2001, 73, 2491–2500. (29) Holder, M. N.; Gardner, C. E.; Macpherson, J. V.; Unwin, P. R. J. Electroanal. Chem. 2005, 585, 8–18. (30) Gardner, C. E.; Unwin, P. R.; Macpherson, J. V. Electrochem. Commun. 2005, 7, 612–618. (31) Abbou, J.; Anne, A.; Demaille, C. J. Phys. Chem. B 2006, 110, 22664– 22675. (32) Gullo, M. R.; Frederix, P.L.T.M.; Akiyama, T.; Engel, A.; deRooij, N. F.; Staufer, U. Anal. Chem. 2006, 78, 5436–5442. (33) Fasching, R. J.; Tao, Y.; Prinz, F. B. Sens. Actuators, B 2005, 108, 964– 972. (34) Dobson, P. S.; Weaver, J. M. R.; Holder, M. N.; Unwin, P. R.; Macpherson, J. V. Anal. Chem. 2005, 77, 424–434. (35) Dobson, P. S.; Weaver, J. M. R.; Burt, D. P.; Holder, M. N.; Wilson, N. R.; Unwin, P. R.; Macpherson, J. V. Phys. Chem. Chem. Phys. 2006, 8, 3909– 3914. (36) Burt, D. P.; Wilson, N. R.; Weaver, J. M. R.; Dobson, P. S.; Macpherson, J. V. Nano Lett. 2005, 5, 639–643. (37) Shin, H.; Hesketh, P. J.; Mizaikoff, B.; Kranz, C. Anal. Chem. 2007, 79, 4769–4777. (38) Bath, B. D.; White, H. S.; Scott, E. R. Anal. Chem. 1998, 70, 1047–1058. (39) Basame, S. B.; White, H. S. J. Phys. Chem. B 1998, 102, 9812–9819. (40) Mills, G.; Zhou, H.; Midha, A.; Donaldson, L.; Weaver, J. M. R. Appl. Phys. Lett. 1998, 72, 2900–2902.
Figure 1. Schematic showing the influence of (a) an inert AFM tip and (b) a conducting AFM tip on the diffusion of redox-active species to an UME.
UME of radius a, the steady-state diffusion-limited current is given by
i ) 4nFaDc*
(2)
where F is Faraday’s constant, D is the diffusion coefficient of Ox, and c* is the concentration of Ox in bulk solution. This equation applies provided that there is a sufficient excess of supporting electrolyte to suppress the migration of Ox and/or Red, if charged. One way the current measured at the electrode will deviate from the value given by eq 2 is if an object is positioned within the region of the hemispherical diffusion field of the UME. The presence of an insulating body (in this case an electrically inert AFM tip) within close proximity of the electrode will cause a hindrance of diffusion to the electrode and, in turn, reduce the current,41 whereas a conducting body, placed close to the electrode, may cause diffusional feedback in which Ox is regenerated, so increasing the UME current, as in SECM positive feedback.41 These two scenarios are depicted in Figure 1. The question posed herein is to what extent do AFM probes modify diffusion to the UME surface? To address this question, we investigate the influence of probe geometry on mass transfer to and from an UME by first developing a model to simulate the steady-state diffusion of redox-active species to the electrode surface. The assumption of steady state is reasonable because of the fast response time of the micrometer-scale disk substrate electrodes that are of interest. Simulations utilized the finite element method (FEM) with the commercially available package FEMlab 3.1 (COMSOL Ltd., Oxford, U.K.) using the chemical engineering module and Matlab 7 (The MathsWorks, Inc.) extensions. All simulations were carried out in real space using experimentally realistic parameters to allow the comparison of simulations and experiment. The diffusion coefficient and bulk concentration of species Ox (i.e., Ru(NH3)63+) were 8.5 × 10-6 cm2 s-1 and 10 mM, respectively. Cantilever and tip geometries used in the model were derived for the probes employed in the experimental measurements using field emission-scanning electron microscopy (FE-SEM). Images of typical probes used in the experiments described herein are shown in Figure 2. Because we were interested in identifying general effects from the simulations, the problem was reduced to 2D axisymmetric geometry. Thus, the probe apex was approximated as a cone, upon which the cantilever, (41) Kwak, J.; Bard, A. J. Anal. Chem. 1989, 61, 1221–1227.
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The geometric parameters of the V-shaped and single-beam probes determined by FE-SEM and the equivalent manufacturer values are given in Table 1. Note that the angled tip of the batchfabricated SECM-AFM probe, shown in Figure 2g,h, does not allow the probe to be modeled readily in two dimensions. The geometry of this probe has been fully characterized elsewhere,34,35 and it is included in these studies to provide a useful comparison with more traditional probe geometries. Both inert and conducting AFM probes are used in the investigation of solid/liquid interfaces,7,19,42,43 and hence the effect of each on diffusion to micrometer-scale interfaces was considered. The following steady-state diffusion equation, appropriate to the 2D axisymmetric geometry, was solved
(
0)D
∂2c 1 ∂c ∂2c + + ∂r2 r ∂r ∂z2
)
(3)
where c denotes the concentration of the species of interest and r and z are the coordinates in the directions radial and normal to the electrode surface, respectively. For simplicity, the diffusion coefficients of Ox and Red are considered to be equal, which allows a mass balance to be applied at any point in space, thus reducing the problem to the consideration of one species (Ox). The axisymmetric system used is shown in Figure 3, where the boundaries have been labeled for clarity. The simulation domain was separated into three subdomains (SD1, SD2, and SD3) to aid in the mesh generation. The reduction of Ox at the electrode surface is not subject to kinetic limitation, and thus we may write Figure 2. FE-SEM images of the AFM probes used in this study. The probes shown are single-beam silicon probes supplied by MicroMasch (a, b) and Nanodevices (c, d); contact mode V-shaped probes from Digital Instruments (e, f); and batch-fabricated SECM-AFM probes provided by the University of Glasgow (g, h). The geometric parameters Ct, Th, and Ta are shown in b, and the values of Cr used in the simulations are shown (to scale) in a, c, and e.
considered to be a disk, was mounted. The probe geometric parameters influenced by the 2D simplification (i.e., the tip halfcone angle, Ta, and cantilever radius, Cr, (Figure 2b)) were approximated, respectively, by taking a mean value of the four half-cone angles of the tip pyramid (viewed along and perpendicular to the cantilever) and by an estimation of an effective cantilever radius by visual inspection. The AFM probes and the effective cantilever radii (drawn to scale) used in the simulations are shown in Figure 2a,c,e. For the three probe types used in the simulation, an estimation of the effective cantilever radius was chosen as the most suitable method to account for the variation in tip position relative to the cantilever. The effective cantilever radius scaled with the width of the cantilever and also increased the further the tip was positioned from the end of the cantilever. The size of Cr was further guided by fits of approach simulations to experiment, as discussed later. An example of the method used to assign the tip half-cone angle, Ta, tip height, Th, and cantilever thickness, Ct (perpendicular to the cantilever direction in this case), is shown in Figure 2b. The consequences of these approximations in relation to real AFM tips are considered herein. Ct and Th were unaffected by the model constraints and could be measured directly by FESEM. This generalization results in a considerable efficiency of the simulations compared to a full 3D simulation and allows key effects to be elucidated, thereby providing a useful insight into the extent to which diffusion to UMEs is affected by the presence of an AFM probe.
boundary(1) : c ) 0
(4)
Because the insulating glass around the UME and the AFM probe (for most situations) is inert, a null flux condition is made in the normal direction, n, to each of these boundaries.
( ∂n∂c ) ) 0
boundary(2, 8, 9, 10, 11) : D
(5)
For boundaries that define the cylindrical symmetry, a null radial flux condition is set.
( ∂c∂r ) ) 0
boundary(7, 12) : D
(6)
At the edge of the simulation domain, the concentration of Ox was set to the bulk value
boundary(3, 4, 5, 6) : c ) c*
(7)
and interior boundaries (13, 14) were set as continua. For the cases where regions of the AFM probes were coated in a metal film (i.e., single-beam silicon probe (apex side) and V-shaped contact mode probe back side or both sides), the boundary conditions changed as follows: metal-coated single beam silicon probe (apex side)
boundary(10, 11) : c ) c*
(8)
V-shaped silicon nitride contact mode probe (back side)
boundary(8, 9) : c ) c*
(9)
V-shaped silicon nitride contact mode probe (both sides)
boundary(8, 9, 10, 11) : c ) c*
(10)
The conditions on boundaries 8-11 arise because they are largely bathed in species Ox (present in solution), which fixes (42) Jones, C. E.; Macpherson, J. V.; Unwin, P. R. J. Phys. Chem. B 2000, 104, 2351–2359.
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Table 1. Geometric Parameters of AFM Probes as Measured and Used for the Simulation of Experimental Approach Curve Dataa V-shaped silicon nitride probes probe
single beam silicon probes
tip half-cone angle, Ta (deg)
Type 1 (Digital Instruments) (contact NP) 40 (45-35)
Type 2 (MikroMasch) (NSC18/no A1) 20 (15)
tip cone height, Th (µm) cantilever thickness, Ct (µm) cantilever half-width, Cr (µm)
3.3 (3) 0.7 (0.4-0.7) 6 (not provided)
18 (22.5 ( 2.5) 5 (3 ( 0.5) 10 (20 ( 1.5)
a
Type 3 (Nanodevices Inc.) (MPP-21100) 24 (front angle 15 ( 2) (back angle 25 ( 2) (side angle 17.5 ( 2) 15 (17.5 ( 2.5) 3.5 (4 ( 0.5) 25 (side 17.5 ( 2.5) (front 15 ( 10)
The manufacturer values are shown in parentheses.
to 0.4 and 1 × 10-10 cm, respectively, and this gave converged results to a precision of better than 0.1%. An exponential expansion of the element spacing was made from all geometric boundaries with the exception of those at the extremes of the simulation domain (boundaries 3-6). In addition, expanding grids were generated on either side of the electrode/ glass division (boundaries 1 and 2). The current at the UME was simulated with the AFM probe positioned directly above the electrode center at distances in the range of 10 nm to 100 µm.
Simulation Results and Discussion
Figure 3. Schematic of the axisymmetric cylindrical geometry used in the simulations, showing the three subdomains (SD1, SD2, and SD3) and the variable parameters of the tip-substrate separation (d), tip cone height (Th), tip half-cone angle (Ta), cantilever thickness (Ct), cantilever radius (Cr), and electrode radius (a).
the potential (and hence concentration) of these boundaries, similar to the situation of positive feedback with an unbiased macroscopic conductor in SECM.41,44,45 The mesh used in the simulations was based on exponentially expanding grids in the z and r directions, where the grid elements, xg, are defined by the general equation
xg ) xg - 1 + Em exp Egf(g - 1)
(11)
where Em and Egf are the minimum element size and growth factor, respectively, and g takes integer values between 1 and gmax. The value of gmax depended on the length of the boundary over which the grid was generated. Grids using an exponential expansion of this kind have been used previously for simulations of the SECM geometry41,46–48 and lead to more efficient simulations by concentrating the density of mesh points in regions where the steepest concentration gradients are anticipated. For the present situation, this approach allowed a larger simulation domain to be used, thus minimizing any effects of the semiinfinite boundary condition applied at the exterior boundaries of the simulation domain. After initial tests of the grid size and spacing, the growth factor and minimum element size were set (43) McEvoy, A. L.; Stevens, F.; Langford, S. G.; Dickinson, J. T. Langmuir 2006, 22, 6931–6938. (44) Martin, R. D.; Unwin, P. R. J. Electroanal. Chem. 1997, 439, 123–136. (45) Wipf, D. O.; Bard, A. J. J. Electrochem. Soc. 1991, 138, 469–474. (46) Bard, A. J.; Crayston, J. A.; Kittlesen, G. P.; Shea, T. V.; Wrighton, M. S. Anal. Chem. 1986, 58, 2321–2331. (47) Amphlett, J. L.; Denuault, G. D. J. Phys. Chem. B 1998, 102, 9946–9951. (48) Unwin, P. R.; Bard, A. J. J. Phys. Chem. 1991, 95, 7814–7824.
We first consider the effect of an inert probe on diffusion to an UME. Figure 4 shows typical concentration profiles of Ox for a probe (parameters defined in the Figure caption) positioned 5 µm (a) and 50 nm (b) above a 1 µm radius disk electrode. Two important points can be ascertained from these initial images. First, a close examination of the concentration profile around the electrode region indicates that the mesh generated was appropriate for the simulation geometry. The concentration distribution is smooth with no distortion due to the mesh used. Second, it can be seen that the probe has some influence on diffusion to the UME, with the effect most pronounced at the closest tip-UME separation. This aspect is covered later in simulations of the UME current. We next consider the influence of the tip cone geometry on the current response for the same 1 µm radius UME as a function of distance between the apex of the cone and the underlying surface. For clarity, the substrate current has been normalized by the theoretical steady-state substrate current given by eq 2 and confirmed by numerical simulation with the probe absent. Figure 5a shows the effect of the tip cone height, Th, on the normalized current at the substrate electrode for a tip half-cone angle, Ta, of 12 ° (with Ct ) 0.7 µm and Cr ) 6 µm). The simulation demonstrates clearly that in all situations the probe diminishes the current at the electrode by blocking diffusion, but as the cone height increases and/or the tip-substrate separation increases (z displacement), the current tends toward that with the probe in bulk solution (i.e., a normalized current of 1). It was found that cone heights exceeding 5 µm gave rise to a 40% at the very closest tip-substrate separation. The inset in Figure 5a shows the same data but presents the z displacement on a log10 scale to focus on the situation when the probe is closest to the substrate surface. It can be seen that when the probe is essentially at the electrode surface (z displacement 10 nm) the current is very sensitive to the cone height, with a small cone height causing the greatest reduction in current. This is as expected, given that for an unblocked electrode diffusion occurs predominantly over a distance of a few electrode radii. Clearly, the longer the tip, the less the effect on diffusion, and for a sufficiently long tip, the probe becomes essentially noninvasive.
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Figure 4. Simulated concentration maps for a probe positioned 5 µm (a) and 50 nm (b) above a 1 µm radius disk electrode. The cantilever radius (Cr) is 6 µm, the tip height (Th) is 3.3 µm, the tip half-cone angle (Ta) is 40°, and the cantilever thickness (Ct) is 0.7 µm.
Figure 5. (a) Effect of varying Th on the substrate electrode current, where Th is (from top to bottom) 10, 7, 5, 4, 3, 2, and 1 µm. Ta ) 12°, a ) 1 µm, Ct ) 0.7 µm, and Cr ) 6 µm. (b) Effect of Ta on the substrate electrode current, where Ta takes values of (from top to bottom) 16.9, 31.2, 42.3, and 50.5°. Th ) 3.3 µm, a ) 1 µm, Ct ) 0.7 µm, and Cr ) 6 µm. Insets show the data replotted with z displacement on a log10 scale to highlight the closest distances.
It would be expected that the larger the cone angle, the greater the hindrance of diffusion to the electrode; this effect is evident in the data in Figure 5b. The distance at which the substrate current deviates from the bulk value (without the probe present) occurs at z < 10 µm (i.e., z < 10a), as clear from the inset to Figure 5b. This is the distance at which the solution recovers its bulk composition (to within 95% of the bulk value).38,49 When the probe is within the UME diffusion field (z < 10 µm), it is observed that increasing the half-cone angle from ∼17° to ∼50° results in a decrease in substrate current, which at the closest distance changes from ∼91% of the bulk value to ∼77% as the half-cone angle varies across the range indicated. This is a relatively small effect for a considerable change in geometry but indicates the benefit of using probes with smaller cone angles if the blocking effect on mass transport to the UME is to be minimized. (49) Saito, Y. ReV. Polarogr. 1968, 15, 177–187.
Figure 6. (a) Effect of cantilever radius on substrate UME current (a ) 1 µm). The probe is defined by Th ) 3.3 µm, Ta ) 40°, and Ct ) 0.7 µm, with Cr taking values of (from top to bottom) 5, 10, 15, 20, 30, 50, and 100 µm. (b) Effect of substrate UME radius on the substrate current, where a takes values of (from top to bottom) 1, 2, 3, 4, 5, and 6 µm. The probe is defined by Th ) 3.3 µm, Ta ) 40 °, Ct ) 0.7 µm, and Cr ) 6 µm. Insets show the data replotted with z displacement on a log10 scale.
The effect of the cantilever radius on the substrate current is considered in Figure 6a. As expected, increasing the radius (width) of the cantilever leads to a decrease in the substrate current. The deviation from the bulk current value occurs at greater distances of the tip from the substrate surface as the cantilever radius increases. In the case of a large cantilever, for which the cone height is small, the hindering behavior tends toward that of an UME adjacent to a planar substrate, and diffusion to the substrate UME is similar to that for a disk electrode adjacent to an inert surface in SECM (negative feedback).41 It is evident from these results that the cantilever plays a large role in blocking diffusion to and from the active site (UME) region. This effect becomes increasingly important as the substrate size increases, as evident from the data in Figure 6b, which shows the effect of an inert probe of a fixed geometry (see Figure caption for details) translated toward disk-shaped electrodes of
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different sizes. The deviation of the current from the bulk value occurs at larger z displacement the greater the electrode radius. This is due to the scaling of the diffusion field at the UME with electrode radius49 so that the probe cone and cantilever disturb diffusion to the substrate UME at a greater z displacement of the AFM tip.
Experimental Section AFM Probes. In light of the simulation results, four different AFM probes were used to examine the widest range of geometrical factors that may influence diffusion to a reactive UME substrate and to identify the probe geometry and conditions giving rise to the least invasive AFM imaging: (a) Type 1: Silicon nitride contact mode probes with V-shaped cantilevers (Contact NP, Digital Instruments, CA). Cantilevers with a nominal spring constant of 0.06 N m-1 were used for all studies herein. Probes were used as received or modified by either removing the reflective Au coating (see below) or sputter coating the apex side of the probe with a thin metal film (see below). (b) Type 2: Single-beam silicon AFM probes (NSC18/no Al, MikroMasch, Madrid, Spain) with a spring constant of 2-5.5 N m-1 were either insulated by dry thermal oxidation (see below) or metal coated on the apex side of the probe (as for Type 1 probes). (c) Type 3: Single-beam silicon AFM probes (Multi 75, MPP21100, Nanodevices Inc. CA) with a spring constant of 1-5 N m-1 were insulated by dry thermal oxidation (as for Type 2 probes). (d) Type 4: Batch-fabricated SECM-AFM probes35 (supplied by Dr. P. S. Dobson and Professor J.M.R. Weaver, University of Glasgow, U.K.) were modified by removing the Au tip electrode (see below). Further details of the probe geometries are provided in Table 1. Methods of Probe Modification. Removal of Au from the probes to render them fully insulating was achieved using an aqueous etch solution of I2/KI/H2O (1:4:40 weight ratio).50 Au removal involved bathing the probes in the etchant for approximately 3 min or until the Au had been visibly removed. Probes were then bathed for a further 5 min in fresh solution to ensure the complete removal of any remaining Au. The probes were subsequently rinsed in Milli-Q reagent water (Millipore Corp., resistivity g18 MΩ cm) and dried using nitrogen to remove any salt residues. Silicon probes were dry thermally oxidized for 9 h (three runs of 3 h duration) at 1100 °C in a 1 in. diameter tube furnace on a quartz boat under a 200 standard cubic centimeters per minute flow of oxygen to electrically insulate the probe.36 Probes used in the positive feedback experiments (V-shaped contact mode probes, Type 1, and single beam silicon probes, Type 2) were made conducting by sputter coating an ∼100 nm thick layer of Pt onto the apex side of the probe. A 5 nm Ti anchor layer was deposited first to increase the Pt film adhesion and uniformity. UME Substrate Preparation. The two substrates used were diskshaped UMEs with electrode diameters of 1.6 and 6.1 µm. The dimensions were determined by AFM and cyclic voltammetry; additional information on this basic characterization can be found in Supporting Information. The 1.6 µm diameter substrate was fabricated by heat sealing an electrochemically etched 50 µm diameter Pt microwire (Goodfellow, Cambridge, U.K.) in a glass capillary. The central metal wire for the 6.1 µm diameter disk electrode substrate was obtained by etching Wollaston microwire to remove the outer Ag layer.51 Electrical contact to the UMEs was made as described elsewhere.27 The UMEs were polished, cut to a length of ∼2 mm, and each sealed into an epoxy resin base. The substrates were then polished on moistened polishing cloths (Buehler, Coventry, U.K.), initially with 0.05 µm alumina and then without alumina to obtain a mirrorlike finish. The substrates were rinsed thoroughly in Milli-Q water and dried in a nitrogen stream prior to use. (50) Williams, K. R.; Gupta, K.; Wasilik, M. J. Micromech. Syst. 2003, 12, 761–778. (51) Wehmeyer, K. R.; Wightman, R. M. Anal. Chem. 1985, 57, 1989–1993.
Burt et al. Materials. All chemicals were used as received, and aqueous solutions were made using Milli-Q reagent water. Hexaammine ruthenium(III) chloride (99%) was purchased from Strem Chemicals, and potassium nitrate (99.999%) was obtained from Sigma-Aldrich. Instrumentation. A Veeco MultiMode AFM with a Nanoscope IIIa controller and electronics extender was used for this study. For experiments with the 6.1 µm UME electrode, a J scanner with a maximum scan range of 150 µm × 150 µm was employed. A Veeco Picoforce scanner and controller with a maximum z-scan range of (10 µm and a lateral range of 50 µm × 50 µm was used for the 1.6 µm diameter UME. The AFM was situated inside a Faraday cage and positioned on an air table. AFM probes were electrically connected to the Au clasp of the fluid cell using Ag paint (Agar Scientific, Stanstead, Essex, U.K.) and then sealed into the fluid cell with a 1:1 nail varnish/superglue mixture. A bipotentiostat (CH Instruments, Austin, TX, model 750A) was used for cyclic voltammetry and chronoamperometric measurements. A PC equipped with an external data acquisition card (Data Translation GmbH, model DT 16DI-DT3010), with software written in-house (LabView, National Instruments), was used in conjunction with a virtual earth preamplifier (DL Instruments, model 1211, Ithaca, NY) in approach curve measurements and electrochemical AFM imaging. Procedures. The electrochemical AFM experiments were carried out in two-electrode mode with the UME substrate serving as the working electrode and a Ag/AgCl (saturated AgCl) reference electrode.52 All potentials are stated with respect to this electrode. The electrolyte solution comprised 10 mM Ru(NH3)63+ as the redox species and 0.2 M KNO3 as a supporting electrolyte. The substrate UME was held at -0.45 V to reduce Ru(NH3)63+ to Ru(NH3)62+ at a diffusion-limited rate, and the current was measured through the reference electrode. The diffusion-limited substrate UME current was measured as a function of AFM probe position for three situations: (i) As the AFM probe was translated in an x-y plane in contact with the sample surface (contact mode topographical imaging); (ii) As a function of AFM x displacement across the center of the electrode, with the probe orientation depicted in Figure 7a. To achieve this, the electrode location was identified by imaging the substrate surface and positioning the probe in the desired location. The slow scan axis movement was then disabled, and the probe was repeatedly scanned along the line of interest across the center of the electrode. (iii) As the substrate electrode current was measured as a function of AFM probe z displacement from the electrode surface using the AFM force mode, as shown in Figure 7b. The point of zero displacement of the tip from the substrate was identified by monitoring the deflection of the cantilever simultaneously with the substrate current, as shown in Figure 7c. As can be seen in Figure 7c, there was good agreement between the two measurements, with the current decreasing as the probe was brought toward the substrate and then reaching a constant value when the tip encountered, and was pushed against, the substrate (increased deflection of the AFM probe). For cases (i) and (ii), the substrate UME current was normalized by the steady-state diffusion-limited current measured when the AFM probe was in bulk solution, far from the UME. For the approach curve measurements, the current was normalized (theory and experiment) by the value when the probe was retracted far from the electrode surface (8 µm), which was close to the bulk value. To ensure that the probe was positioned directly over the center of the electrode, the scan area was reduced to 1 µm × 1 µm to focus on the electrode surface. The electrode surface was continuously imaged until no lateral drift was observed before engaging the force mode.
Experimental Results and Discussion Effect of AFM Probe Geometry on Diffusion to a 6.1 µm Diameter UME. Figure 8 shows a typical set of data of electrode topography from AFM height (a) and electrode current recorded (52) Macpherson, J. V.; Unwin, P. R. J. Phys. Chem. 1995, 99, 14824–14831.
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Figure 7. Probe orientation while measuring the substrate UME current as a function of (a) x displacement (0° scan angle shown) and (b) z displacement. (c) Typical data for the normalized substrate UME current (upper plot) and cantilever deflection (lower plot) in the extend (black line) and retract (red line) directions as an AFM probe (single-beam Si probe, Nanodevices Inc.) was brought into contact (extend) and retracted from the UME (a ) 0.8 µm) surface (retract).
Figure 8. AFM height (a) and substrate current images recorded at scan angles of 90° (b) and 0° (c) for a 6.1 µm diameter UME sealed in glass. The substrate potential was set to -0.45 V (vs Ag/AgCl) in an electrolyte containing 10 mM Ru(NH3)63+/ 0.2 M KNO3. The scan rate was 0.1 Hz. The black dotted line in image (b) represents the scan line across the substrate electrode center line, and the corresponding cross sections are shown in trace (black) and retrace (red) directions. The dotted green line in image (b) shows the line where maximum hindrance (as reflected in the UME current) was observed, and the corresponding cross-sectional signals recorded in the trace (green line) and retrace (blue line) directions are shown.
simultaneously (b, c) for the 6.1 µm diameter disk electrode substrate biased at a potential to reduce Ru(NH3)63+ to Ru(NH3)62+ at a diffusion-controlled rate while imaging with a V-shaped contact mode probe. The probe orientation and fast scan axis direction are shown in the top right corner of the two current images. It can be seen that the current images reflect the probe geometry (but as mirror images) and change according to the scan angle. Clearly, when in the vicinity of the substrate electrode, the probe influences diffusion to and from the UME. However, the substrate current recovers toward the bottom and left of the images in Figure 8b,c, respectively, as the probe cantilever passes over the electrode. This is due to the angle (typically 11°) at which the cantilever meets the substrate surface causing diffusion to be more hindered toward the tip cone and less hindered at lateral distances removed from the probe.
It is interesting to observe that although the primary effect of the probe and cantilever is to block diffusion to the UME an enhancement in the current above the bulk value occurs when the UME is coaligned with the end of the cantilever as a result of positive feedback from the reflective Au coating on the back of the cantilever (see below). The cross sections in Figure 8b show that the current minimum (region of maximum hindrance) does not occur when the tip line scan coincides with the center of the electrode (black dotted line) but rather at a line that is at a distance of 4.3 µm from this position with respect to the AFM chip (green dotted line). Again, it is important to note that when the scan angle was changed to 0° (Figure 8c) the maximum enhancement of the current and most significant hindrance of diffusion was found to be consistent with the observations in Figure 8b. The slight asymmetry in the trace and retrace scan
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Figure 9. Substrate current images recorded at the 6.1 µm diameter UME during AFM imaging of the substrate topography with (a) a V-shaped contact mode probe (with the reflective Au coating removed), (b) an oxidized single-beam silicon probe (Nanodevices Inc.), and (c) a batch-fabricated SECM-AFM probe. The cross section in part (a) is for a contact mode probe with (green) and without (black) the Au back-coating for the trace direction is given. Cross sections below the current images (b, c) show trace (black) and retrace (red) directions. The scan rate was 0.1 Hz, and the scan angle was 90°. The probe orientation is shown in the inset motif.
directions shown in each cross section is most likely due to a small hydrodynamic effect brought about by the motion of the cantilever (effective tip velocity of 10 µm s-1). By removing the reflective Au film (on the reverse side of the contact mode V-shaped probes), we were able to investigate its effect on the substrate current (Figure 9a). The cross section below the image in Figure 9a compares the substrate current measured at probes with (green line) and without (black line) a reflective Au coating for the scan line showing the greatest hindrance of diffusion to the electrode. For the probe with the Au removed, there is no enhancement of the substrate current because of positive diffusional feedback between the substrate UME and the tip, and the current enhancement observed in Figure 8b,c is eliminated. This confirms that the enhanced current evident previously is due to positive feedback from the Au coating, rather than convection or transient diffusion effects as a partially blocked electrode becomes unblocked and vice versa. Of additional significance is the observation that the lateral distance over which diffusion to the UME is modified (as reflected in the current) is reduced from ∼80 µm (with the Au coating) to ∼25 µm (without the Au coating). Figure 9b shows the electrochemical image obtained when the substrate was imaged with an oxidized insulating singlebeam silicon probe (Nanodevices Inc.). The influence of the single-beam cantilever can be seen in the electrochemical current image. Significantly, because of the large cone height and smaller cone angle, the maximum hindrance measured as the probe scans over the electrode is considerably less than found with the V-shaped contact mode probe. Figure 9c shows the substrate current image for a batch-fabricated electrically inert SECMAFM probe (the electrode sensor was removed by etching). This image shows that the cantilever and probe have very little influence on the diffusion field, as evidenced by the UME current, and only the apex of the angled tip interferes with the diffusion of species to the electrode. The cross section below the image shows that the region where the probe disturbs the diffusion field is localized to a 10 µm lateral region and that the current decreases at most by ca. 6%. In comparison to the other probes investigated, this is the least physically invasive.
Effect of AFM Probe Geometry on Diffusion to a 1.6 µm Diameter UME. To investigate the influence of the AFM probes on diffusion to a smaller electrode as a model active site, an UME with a diameter of 1.6 µm was employed as the substrate. The diffusion-controlled current images for a V-shaped contact mode probe (with a Au coating on the back of the cantilever), oxidized Si probe (Type 3), and a batch-fabricated SECM-AFM probe (Au removed) recorded while imaging the UME in contact mode are shown in Figure 10. As before, the substrate electrode potential was biased at -0.45 V in a solution containing 10 mM Ru(NH3)63+ and 0.2 M KNO3 so that Ru(NH3)63+ was reduced to Ru(NH3)62+ at a diffusion-controlled rate. An inspection of Figure 10 clearly shows that the V-shaped contact mode probe in part (a) is the most invasive. The SECM-AFM probe in part (c) is clearly the least invasive. Note that although a scan angle of 90° was used for the image in part (c) the conclusion that this probe is the least invasive is true for all scan orientations. The reproducibility between the trace and retrace scan directions shown in the cross section below each plot demonstrates that the substrate electrode reaction is operating under steadystate conditions and that there are no appreciable hydrodynamic (or transient diffusion) effects due to the passing probe. The hornlike current profile in the vicinity of the electrode surface in all three images, which is not really apparent in data for the 6.1 µm diameter electrode, is interesting. It suggests that as the tip moves across the insulating support of the substrate toward the active electrode, the current first decreases, but as the tip passes over the electrode edge and moves to the center, the current actually recovers slightly before decreasing at the other electrode edge and then increasing as the probe moves away from the electrode. This, of course, reflects the nonuniform current distribution at an UME,53 with the highest current density at the edges due to significant edge effects. Clearly, when diffusion to these regions of the substrate is blocked, the current is diminished most. The images in Figure 10a,b were obtained at a scan angle of 0°. However, the two types of probe (Type 1 (Au present) and Type 3) have strikingly different effects on the substrate current. Because of the short cone height of the V-shaped contact mode (53) Aoki, K. Electroanalysis 1993, 5, 627–639.
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Figure 10. Substrate (1.6 µm diameter disk) current images recorded during topographical imaging with (a) a V-shaped contact mode probe (with a reflective Au coating), (b) an oxidized single-beam silicon probe (Nanodevices), and (c) a batch-fabricated SECM-AFM probe (Au removed). Scan angles for a-c are 0, 0, and 90°, respectively. Cross sections are shown below each image for trace (black) and retrace (red) scan directions. Probe orientation and fast scan axis direction are shown in the motif inset. The scan rate in each case was 0.25 Hz.
Figure 11. Substrate (1.6 µm diameter disk) current images obtained during topographical imaging (a) in metal-coated V-shaped contact mode and (b) with single-beam probes (MikroMasch). The scan rate was 0.25 Hz, and the scan angle was 0°. (c) Substrate current cross sections for the (a) V-shaped (red line) and (b) single-beam (black line) probes with a Pt coating on the apex side. Trace (solid line) and retrace (dotted line) scan directions are shown for each probe.
probe, the cantilever causes a reduction in the current of ∼17% when it is directly over the electrode surface, and even when the probe tip is imaging a region 20 µm beyond the electrode (x displacement ) 0 µm in Figure 10a), the current is still 6% below the unhindered current. This latter value (6% current decrease) is comparable to the maximum effect on substrate current caused by the inert single-beam probe (Nanodevices Inc.) in Figure 10b as it images the UME surface. To investigate further the effect of positive feedback between the UME substrate and AFM probe, measurements were made with probes that had been deliberately metal coated with a 100 nm thick layer of Pt. Figure 11a shows the substrate current image when a metal-coated contact mode V-shaped probe was
scanned across the 1.6 µm diameter substrate electrode surface. Note that electrical contact between the metal-coated probe and the substrate electrode was eliminated by deliberately removing Pt from the tip apex by scanning the substrate surface in contact mode with a large applied force until the current (with the tip and substrate electrode in contact) decreased to zero. As the probe encountered the substrate electrode, a relatively large increase in current (∼60% for V-shaped probes) resulted from positive feedback between the substrate UME and AFM tip. Although the single-beam probe produced a positive feedback effect (Figure 11b,c), it was considerably smaller (∼20% enhancement). In this case, the image shows that feedback occurred predominantly from the tip and that the cantilever is relatively noninvasive, confirming the results with the nonconducting AFM probe presented above (vide supra). Effect of AFM Probe Geometry on Diffusion to a 1.6 µm Diameter UME as a Function of z Displacement. The influence of the AFM probes on diffusion to the 1.6 µm diameter substrate electrode was investigated further by monitoring the substrate current as a probe, positioned directly above the electrode surface, approached and retreated from the surface. Except for the batchfabricated SECM-AFM probe, these experiments can be approximated reasonably well to 2D axisymmetric cylindrical geometry, thus permitting an analysis of the data with the model presented earlier in the article. Figure 12 shows experimental and simulated data for the different probes (see Table 1 for the parameters defining the geometry of each type of probe used), except for the SECM-AFM probe, where no simulations were carried out. The experimental data presented are representative of numerous (more than 10) approach curves recorded for each probe. In each case, the current response recorded showed high reproducibility between the approach and retract scans, and the point of contact was determined from the cantilever deflection response recorded simultaneously (but not shown) as explained above. Figure 12a shows the approach curve recorded for an inert (red line) and a metal-coated (black line) single-beam probe (MikroMasch), clearly showing that an inert probe leads to blocked diffusion whereas a metal-coated probe enhances the current as a result of positive feedback. In the latter case, the substrate current signal showed an enhancement of ∼30% above the bulk current value when the probe contacted the substrate.
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Figure 12. Normalized current as a function of z displacement for (a) a metal-coated (black) and insulated (red) single-beam probe (MikroMasch) and (b) a V-shaped probe with a reflective Au coating (black), an inert single-beam probe (Nanodevices) (red), and a batch-fabricated SECM-AFM probe with the Au removed (green). The scan rate towards the surface was 0.1 Hz. Corresponding simulated approach curves are shown as dotted lines. For probe geometries used in the simulation, refer to Table 1.
This is of a similar magnitude to the enhancement shown in Figure 11c. The enhancement is lower than predicted by theory, which is likely due to the deliberate removal of metal from the tip apex during imaging, while locating the electrode. This was evidenced by the absence of electrical shorting as the tip contacted the electrode surface. There was also reasonably good agreement between the simulated and experimental approach curves for the small decrease in substrate current (maximum 4%) brought about by the inert single-beam probe, shown in red in Figure 12a. These data again highlight the relatively noninvasive nature of this type of probe. Figure 12b shows the effect of the V-shaped contact mode probe (reflective Au coating present), oxidized Si probe (Nanodevices Inc.), and batch-fabricated SECM-AFM probe (Au removed) on the substrate current. It can be seen that the batchfabricated SECM-AFM probe (green line) is the least invasive, causing a reduction of only 3% in the substrate current. The V-shaped contact mode probe, with the reflective Au film (black line), disturbs the diffusion field around the electrode to the greatest extent (13% decrease in current). This is again due to the relatively small tip cone height of the V-shaped contact mode probes. It can be seen that experiment and simulation are in good agreement. The inert single-beam probe (Nanodevices Inc.) results in the substrate current decreasing by a maximum value of 6%. This small decrease is due to the large tip cone height, and it can again be seen that the simulation results closely agree with experiment. Taken together, the data in Figure 12 highlight that the maximum blocking effect of AFM tips on mass transport to a microscopic reactive (UME) interface is readily evaluated with approach curve measurements. The effects of different tips are readily compared, and those that are least perturbing are identified. Moreover, most commercially available AFM probes employed for this type of measurement can be approximated by a 2D representation of the tip and cantilever.
Conclusions It has been demonstrated that diffusion to an UME is influenced by AFM probes during imaging and approach curve measurements, and the effects observed depend critically on the geometry and electrical conductivity of the probe. The studies herein have
clearly illustrated that the extent to which diffusion is perturbed can be minimized by a careful choice of the probe. The probes considered in this study were found to hinder diffusion to an UME (1.6 µm diameter) in the following order: V-shaped silicon nitride contact mode probe . single beam silicon probe (Nanodevices Inc.) > single beam silicon probe (MikroMasch) . batch-fabricated SECM-AFM probe. Finite element simulations have supported the experimental deductions, highlighting that probes with a large cone height and small cone angle lead to only a small perturbation in diffusion to the underlying interface. It is worth noting that although the simulation and experiment conditions employed herein were conducted with no kinetic limitation on the electrode reaction and under diffusion-limited mass transport, the relative perturbation is independent of the absolute flux. The studies in this article provide confidence that reactions at microscale interfaces can be studied with AFM under conditions where the effect of the probe on mass transport can be accounted for by simulation. This provides a foundation for the use of AFM in the study of processes not only at UMEs but also at other small-scale interfaces, such as microcrystals.19,20 This approach would lead to well-defined and modelable mass transport regimes that would allow mass transport and surface kinetic effects to be readily resolved. Acknowledgment. We thank Mr. Steve York (Department of Physics, University of Warwick) for help with electron microscopy and Mr. Martin Edwards (Department of Chemistry, University of Warwick) for assistance with simulations. We are grateful to Dr. Phil Dobson and Professor John Weaver (Department of Electronics and Electrical Engineering, University of Glasgow) for providing batch-fabricated SECM-AFM probes. J.V.M. thanks the Royal Society for the award of a university research fellowship. We also thank the Leverhulme Trust (PDRA for N.R.W.) and the EPSRC for funding. Supporting Information Available: AFM and voltammetric characterization of UMEs. This material is available free of charge via the Internet at http://pubs.acs.org. LA8003323
