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High resolution in-situ imaging of reactive heterogeneous surfaces
Electrochimica Acta 44 (1999) 3651±3657
High resolution in-situ imaging of reactive heterogeneous surfaces L.F. Gar®as-Mesias, W.H. Smyrl* Corrosion Research Center, Department of Chemical Engineering and Materials Science, University of Minnesota, 221 Church Street SE, Minneapolis, MN 55455, USA Received 9 September 1998
Abstract A modi®ed near-®eld optical microscope (NSOM) was used to locate reactive sites on polycrystalline titanium immersed in sulfuric acid. Photoelectrochemical imaging was carried out on the NSOM by using a new shear-force feedback method. PEM with simultaneous independent topography (using the tuning fork method) resolved submicron features at reactive sites at inclusions and grain boundaries. Both PEM and topographic images were obtained in-situ with high lateral resolution by using specially fabricated tips with 100 nm aperture. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction As the critical dimensions shrink in microelectronic and magnetic storage structures and assemblies, corrosion issues assume a greater importance. Thus, even uniform corrosion may become critical, and because the relevant materials are often small domains embedded in insulating or dielectric matrices, this forces one to use microscopic probes to study the corrosion processes. Many of the functional domains are now smaller than 100 nm, so microscopy resolution must be at least of the same magnitude. As the functional domains shrink to smaller sizes, the microscopic techniques are also being pushed to higher resolution. A novel technique that will be described in more detail
* Corresponding author. Tel.: +1-612-625-4048; fax: +1612-626-7246. E-mail address: [email protected] (W.H. Smyrl)
below promises to meet the needs outlined here. The open architecture of the technique will provide a testbed to develop and evaluate dierent functional insitu microscopies. As an example, scanning photoelectrochemical microscopy with topography can be used to map the properties of surface oxides and to correlate the release rates with morphology and surface structure. The ultimate resolution of the technique appears to be about 5±10 nm. The range of study between 5 and 100 nm should be rich with new observations. Since the pioneering work of Butler [1±3], who used Photoelectrochemical Microscopy (PEM) to study the passive ®lm on Ti, a number of dierent studies have used the technique as a viable method to study the passive ®lms of some metals and alloys [4±10]. The PEM technique uses a focused laser beam to measure locally the photocurrent on semiconducting oxide ®lms grown on the parent metal substrates [1±4,7±9]. By using this approach, the resolution of the images obtained varied from 5 mm [8,9] to 3 mm [7]. In order
0013-4686/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 9 ) 0 0 0 6 8 - 7
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L.F. Gar®as-Mesias, W.H. Smyrl / Electrochimica Acta 44 (1999) 3651±3657
Fig. 1. Experimental arrangement for Photoelectrochemical Microscopy with concurrent independent topography.
to obtain high resolution images, a 1-mm size optical ®ber has been used (instead of the focused laser beam) and the ®ber was rastered across the surface of the sample yielding an image with a modest increase in the lateral resolution [11]. It is clear that PEM images obtained with the optical ®ber method were limited by either the size of the probe (1 mm) or by the tip-tosample distance (normally tens of microns). In either case it was not possible to obtain images with submicron resolution. A recent paper by Williams et al. [12], reported resolution of about 0.5 mm on pitting sites on stainless steels based on a combined photoelectrochemical and electrochemical technique with a confocal microscope and a modi®ed Atomic Force Microscope (AFM). By comparison, we have approached 0.1 mm resolution in the present work and resolutions of 0.01 mm appear to be possible in the future. In the present paper we will describe a novel technique using shear force feedback, that utilizes a tuning
fork method in order to measure concurrently the topography and PEM on TiO2 covered polycrystalline Ti samples. The technique is based on a modi®cation of a Near-Field Scanning Optical Microscope (NSOM). The modi®cation was reported in a recent paper [14], in which we described its use for Scanning Electrochemical Microscopy (SECM). In a previous publication [13] we identi®ed electrochemically active sites on polycrystalline titanium by using SECM. Previous studies by Casillas et al. [15,16] found precursor sites on polycrystalline Ti foil materials that had no inclusions. The purpose of the present research is to demonstrate that by using the shear force feedback method it is possible to image in-situ in liquids both the particles and other active sites (i.e. grain boundaries) with high resolution. Full details of the experimental setup, nanoprobe preparation and experimental ®ndings will be published elsewhere.
L.F. Gar®as-Mesias, W.H. Smyrl / Electrochimica Acta 44 (1999) 3651±3657
2. Experimental 2.1. Tip preparation All probes were prepared from type F-AS optical ®bers (3.7 mm in core diameter with 125 2 2 mm of cladding and 2452 15 mm of polymer coating), which were obtained from Newport Corporation. The nanoprobes were made from optical ®bers of 1-m length. One end was then cleaned with isopropanol and etched in 20% HF (v/v) solution, prepared from a concentrated 48% HF solution (Mallinckrodt). After completion of the etching, the ®bers were observed under a 35X stereoscope and if the size of the etched ®ber was found be to 100 nm or less, the ®ber was selected for the studies. Those ®bers with larger tip diameter were discarded. We found that the time required to etch a ®ber tip to a size of approximately 100 nm diameter was about 24 h. 2.2. Sample preparation All samples used in the present research were cut from commercially pure plate titanium (99.99% pure) which had been specially heat treated to achieve large grains (approximately 1 mm grain size). We have previously determined that pitting precursors were located both inside the grains and at some grain boundaries [12] in these materials. Therefore, our goal was to study photoelectrochemical activity of precursor sites for pitting with high resolution images. The samples were prepared by successive polishing with 6, 1 and 0.25 mm diamond paste. After polishing, the samples were lightly etched in a solution containing 2% HF, 4% HNO3 and 94% H2O (volume %). 2.3. Oxide growth Before starting the PEM tests, an oxide ®lm was anodically grown on the samples by using a three-electrode cell con®guration inside the PEM cell and using a 0.05 M H2SO4 solution. The oxide ®lm was grown at a potential sweep rate of 0.1 mVsÿ1 starting from ÿ0.25 to +6 VSCE. 2.4. Photoelectrochemical Microscopy with concurrent topography All the PEM tests were performed on a Topometrix Aurora Near-Field Scanning Optical Microscope (NSOM) with a custom modi®ed head incorporating a tuning fork transducer, which has been brie¯y described in an earlier publication [14] and is shown in Fig. 1. An optical ®ber nanoprobe (described above) was used to monitor the surface topography and to focus the 351 nm laser beam onto the surface of the
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TiO2 covered Ti. The nanoprobe was glued onto one of the arms of the tuning fork following the feedback procedure developed by Karrai and Grober [17,18]. The shear-force feedback method (which will be discussed in the following section) was then used to obtain the topography of the sample concurrently with the photocurrent as a function of the tip position. All the PEM tests were obtained with the sample and the nanoprobe immersed in 0.05 M H2SO4 solutions. To perform PEM+TOPOGRAPHY measurements, the shaped ®ber was attached to a tuning fork which was mounted on a modi®ed NSOM head to control the tip-to-sample distance. 2.5. Shear-force feedback with the tuning fork Shear-force feedback is commonly used to maintain a constant distance between the probe and the surface in a conventional NSOM. By using this method, the probe is kept at a constant height above the sample [19]. The distance is normally regulated by monitoring the damping of the oscillations of a resonating probe as it encounters viscous damping near the sample surface. The method is basically non-contact with a typical interaction distance between 10±30 nm in air [20]. There are two well established methods to achieve feedback: (1) The optical detection method and (2) the tuning fork method. In the optical feedback method, light (from a second laser) is re¯ected o the probe and onto a split diode where lock-in techniques can discern the small oscillations of the tip [19]. In our experience, the optical feedback method works well in gases, but in liquids and at re¯ective metal surfaces the feedback is stable only for tens of minutes and the experimental setup becomes more restrictive. Since most of the electrochemical processes we are interested in studying occur in liquids, optical feedback is not ideal for these purposes. However, Karrai and Grober recently developed an alternative method that utilizes a tuning fork on the transducer for detecting the shearforce damping [17,18]. That method also has the advantage of working in liquids, being robust and normally remaining stable for hours. Fig. 1 shows a schematic representation of the system. The ®rst and crucial component is the optical ®ber nanoprobe (or tip), which is glued onto one arm of the quartz crystal tuning fork. The fork is rigidly attached to a piezoelectric vibrator (the driving piezo or dither ) which vibrates at a frequency of 33 kHz with an amplitude of nearly 0.01 nm [17,18]. To engage the system in feedback, the surface is moved towards the tip ®rst manually (to within a few microns) and then by means of four piezos that control the scanner (the sample is ®xed rigidly to the top of the scanner). As the sample is moved towards the tip, it reaches a position at which the amplitude of the vibration of the nanoprobe is
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L.F. Gar®as-Mesias, W.H. Smyrl / Electrochimica Acta 44 (1999) 3651±3657
Fig. 2. Photoelectrochemical Microscopy (PEM) and Topography (using the shear-force method) of an inclusion particle on polycrystalline Ti obtained with an etched optical ®ber. The images (21 21 mm) were obtained in 0.05 M H2SO4.
reduced by damping and consequently the piezoelectric signal of the fork is reduced. This reduction is large when the tip-to-sample separation is in the range of 40 nm [17,18]. By means of the electronic feedback loop, the system prevents any further movement of the sample towards the tip. The changes in the feedback signal can then be used and plotted as a function of the lateral position to generate a surface topography map (as shown in Figs. 2 and 3).
3. Results and discussion 3.1. Inclusions as precursor sites for pitting In the past, SECM was used for polycrystalline titanium to identify precursor sites for pit initiation [13,15,16]. The precursor sites were associated with particles containing Al and Si or grain boundaries which apparently were particle free [13]. The sites were
Fig. 3. Photoelectrochemical Microscopy (PEM) and Topography (using the shear-force method) of an inclusion particle on polycrystalline Ti obtained with an etched optical ®ber. The images (35 35 mm) were obtained in 0.05 M H2SO4.
L.F. Gar®as-Mesias, W.H. Smyrl / Electrochimica Acta 44 (1999) 3651±3657
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Fig. 4. Optical image of an inclusion on polycrystalline Ti sample (left) and contact AFM imaging (right) of the same particle. The bar next to the AFM image corresponds to the height pro®le in the image. The particle was approximately 30 mm in width.
found to be conductive and were able to support electrochemical reactions [11,13] like oxidation of ferrocyanide and bromide. In both cases (particles or grain boundaries), pitting could be induced at the sites at around +2.5 VSCE in bromide solutions at room temperature. We propose that the initiation sites on polycrystalline Ti can be attributed to two distinct causes; inclusions or defective oxide grown over active grain boundaries or other surface features. In the case of the inclusions, they appear to be complex particles that contain Al and Si (but that may have other impurities in lower concentrations). On the other hand, the substrate grain boundaries can be particle-free or can have particles that are too small to be detected with the techniques used previously. 3.2. PEM and Topography study In order to study submicron features, we adapted an NSOM instrument to perform such tests along with insitu topography in liquids. In order to be able to achieve high resolution (submicron) topography and PEM, we had to overcome three well de®ned problems: (1) tip preparation; (2) sample preparation and (3) to be able to perform the test in liquids. 3.2.1. Tip preparation By choosing an appropriate probe, one can achieve high resolution topography and also obtain a better signal to noise ratio while performing PEM. The maximum photocurrent obtained with the pulled tips (1350 pA for a new tip) was lower than the value obtained with an etched ®ber (3300 pA). The results are in com-
plete agreement with previous NSOM studies [21,22], which suggest that etched tips have higher light output than pulled tips. 3.2.2. Sample preparation As suggested by Tyler and coworkers [8], the oxide was grown at a low sweep rate (0.1 mVsÿ1), and this increased the photocurrent measured subsequently on the oxide covered surface. Second, the ®nal growth voltage was chosen to form a thick oxide, and this in turn increased not only the photocurrent measured, but also the signal to noise ratio (by making a more stable oxide ®lm). In the case of both Figs. 2 and 3, the oxide was grown to +6 VSCE at a sweep rate of 0.1 mVsÿ1. It is important to mention that because of the very low intensity of light focused onto the sample, it was not possible to obtain PEM maps when the probe was not in feedback (about 6 mm away from the sample). The main reason was the dissipation of light from the laser beam which reduced the photocurrent below detection limits when the tip is far from the surface. 3.2.3. PEM and Topography in liquids In our experiments, the length of the nanoprobe glued to the leg of the fork was sucient to guarantee that the system could operate in solution without wetting the legs of the fork. It is our observation that increasing the length of the nanoprobe increases its amplitude of oscillation and this can lead to loss of feedback. This is because the end of the nanoprobe (the apex of the tip) is further from the legs and therefore will have larger oscillations. We conclude that it is important to control the length of the nanoprobe.
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An optical microscope and an Atomic Force Microscope were used to image a particular inclusion on the polycrystalline Ti surface. The images were compared with the topography obtained with the tuning fork method. Fig. 4 shows both images, the optical image which includes most of the grain boundaries surrounding the particle and a more detailed image obtained by AFM showing details of the particles at a higher magni®cation. NSOM images of the same features are shown in Fig. 2. The lateral resolution of the topography of the inclusion (Fig. 2) is better than 100 nm, that is, small features can be resolved within the particle and around it. The inclusions seem to be comprised of two dierent parts apparently broken in the middle. The PEM image, on the other hand, is very distinctive; the right side of the particle has a lower photocurrent compared to the left side which is more photoactive. This ®nding may explain the observation made previously that the precursor site areas seen by using the conventional PEM method do not have the same size as the particles seen with topography. By using the tuning fork method and obtaining high resolution PEM with concurrent topography, the dierences can be clearly observed and inhomogeneities within the inclusions can be seen. A dierent kind of surface is shown in Fig. 3. Here, contrast from grain to grain is demonstrated, and small particles can also be resolved. In this case, it is clearly seen that the three Ti grains present in this area of the surface have distinct photocurrents. The lateral resolution in these images is probably better than 100 nm, even though the contrast between grains is generated by a very small change in the photocurrent (about 5 pA). 3.3. Future work The results presented here encourage us to continue the development of the new technique, and also to explore new problems that can be solved by using the novel technique described here. Future work will focus on the fabrication of smaller size nanoprobes (around 10 nm) that can increase the lateral resolution of the images, and that will require improved sensitivity to measure the photocurrent in these samples. With higher lateral resolution images, one can locate smaller precursor sites in other advanced materials, particularly those used in the semiconductor and microelectronics industry. 4. Conclusions A novel technique to perform both PEM and topography concurrently in liquids has been developed. The lateral resolution and accuracy of the topographic
information in liquid is retained (compared to conventional methods like AFM in air). At the present time the lateral resolution is around 100 nm, and at the same time one can characterize the local chemical behavior of the system by using PEM. The resulting blend has yielded high resolution topography and photoelectrochemical information on polycrystalline Ti. The new method can be combined with other techniques (SECM, PEM, Fluorescence and others), to characterize heterogeneous reactive systems. The precursor sites observed here for polycrystalline Ti samples were associated with particles containing Al and Si and with grain boundaries that are particle free. The precursors are apparently inhomogeneous and support photoelectrochemical reactions. Acknowledgements This study was supported by NSF grant DMR9509766. Early developments of the new technique were made by Drs PS Moyer and PI James. References [1] M.A. Butler, J. Electrochem. Soc. 130 (1983) 2358. [2] M.A. Butler, J. Electrochem. Soc. 131 (1984) 2185. [3] M.A. Butler, W.H. Smyrl, in: Proceedings of the 82 ECS Spring Meeting held in Montreal Canada, Abstract 7, 1982, p. 15. [4] M. Kozlowski, W.H. Smyrl, Lj Atanasoska, R. Atanasoski, Electrochimica Acta 34 (1989) 1763. [5] N. Casillas, P. James, W.H. Smyrl, J. Electrochem. Soc. 142 (L16) (1995). [6] P. James, N. Casillas, W.H. Smyrl, J. Electrochem. Soc. 143 (1996) 3853. [7] S. Kudelka, J.W. Schultze, Electrochimica Acta 42 (1997) 2817. [8] P.S. Tyler, M.R. Kozlowski, W.H. Smyrl, R.T. Atanasoski, J. Electroanal. Chem. 237 (1987) 295. [9] M.R. Kozlowski, P.S. Tyler, W.H. Smyrl, R.T. Atanasoski, J. Electrochem. Soc. 136 (1989) 442. [10] A.R. Kucernak, R. Peat, D.E. Williams, J. Electrochem. Soc. 138 (1991) 1645. [11] G. Shi, L.F. Gar®as-Mesias, W.H. Smyrl, J. Electrochem. Soc. 145 (1998) 2011. [12] D.E. Williams, T.F. Mohiuddin, Y.Y. Zhu, J. Electrochem Soc. 145 (1998) 2664. [13] F. Gar®as-Mesias, M. Alodan, P.I. James, W.H. Smyrl, J. Electrochem. Soc. 145 (1998) 2005. [14] P.I. James, L.F. Gar®as-Mesias, P.J. Moyer, W.H. Smyrl, J. Electrochem. Soc. 145 (L64) (1998). [15] N. Casillas, S.J. Charlesbois, W.H. Smyrl, H.S. White, J. Electrochem. Soc. 140 (L142) (1993). [16] N. Casillas, S. Charlesbois, W.H. Smyrl, H.S. White, J. Electrochem. Soc. 141 (1994) 636. [17] K. Karrai, R.D. Grober, J. Electrochem. Soc. 66 (1994) 1842.
L.F. Gar®as-Mesias, W.H. Smyrl / Electrochimica Acta 44 (1999) 3651±3657 [18] K. Karrai, R.D. Grober, Ultramicroscopy 61 (1995) 197. [19] E. Betzig, P.L. Finn, J.S. Weiner, Appl. Phys. Lett. 60 (1992) 2484. [20] M.A. Paesler, P.J. Moyer, Near-Field Optics: Theory,
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Instrumentation and Applications, John Wiley and Sons, Inc, New York, 1996. [21] P. Homann, B. Dutoit, R. Salathe, Ultramicroscopy 61 (1995) 165. [22] D.R. Turner, US-Patent, 4,469,554, 1983.
High resolution in-situ imaging of reactive heterogeneous surfaces L.F. Gar®as-Mesias, W.H. Smyrl* Corrosion Research Center, Department of Chemical Engineering and Materials Science, University of Minnesota, 221 Church Street SE, Minneapolis, MN 55455, USA Received 9 September 1998
Abstract A modi®ed near-®eld optical microscope (NSOM) was used to locate reactive sites on polycrystalline titanium immersed in sulfuric acid. Photoelectrochemical imaging was carried out on the NSOM by using a new shear-force feedback method. PEM with simultaneous independent topography (using the tuning fork method) resolved submicron features at reactive sites at inclusions and grain boundaries. Both PEM and topographic images were obtained in-situ with high lateral resolution by using specially fabricated tips with 100 nm aperture. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction As the critical dimensions shrink in microelectronic and magnetic storage structures and assemblies, corrosion issues assume a greater importance. Thus, even uniform corrosion may become critical, and because the relevant materials are often small domains embedded in insulating or dielectric matrices, this forces one to use microscopic probes to study the corrosion processes. Many of the functional domains are now smaller than 100 nm, so microscopy resolution must be at least of the same magnitude. As the functional domains shrink to smaller sizes, the microscopic techniques are also being pushed to higher resolution. A novel technique that will be described in more detail
* Corresponding author. Tel.: +1-612-625-4048; fax: +1612-626-7246. E-mail address: [email protected] (W.H. Smyrl)
below promises to meet the needs outlined here. The open architecture of the technique will provide a testbed to develop and evaluate dierent functional insitu microscopies. As an example, scanning photoelectrochemical microscopy with topography can be used to map the properties of surface oxides and to correlate the release rates with morphology and surface structure. The ultimate resolution of the technique appears to be about 5±10 nm. The range of study between 5 and 100 nm should be rich with new observations. Since the pioneering work of Butler [1±3], who used Photoelectrochemical Microscopy (PEM) to study the passive ®lm on Ti, a number of dierent studies have used the technique as a viable method to study the passive ®lms of some metals and alloys [4±10]. The PEM technique uses a focused laser beam to measure locally the photocurrent on semiconducting oxide ®lms grown on the parent metal substrates [1±4,7±9]. By using this approach, the resolution of the images obtained varied from 5 mm [8,9] to 3 mm [7]. In order
0013-4686/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 9 ) 0 0 0 6 8 - 7
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L.F. Gar®as-Mesias, W.H. Smyrl / Electrochimica Acta 44 (1999) 3651±3657
Fig. 1. Experimental arrangement for Photoelectrochemical Microscopy with concurrent independent topography.
to obtain high resolution images, a 1-mm size optical ®ber has been used (instead of the focused laser beam) and the ®ber was rastered across the surface of the sample yielding an image with a modest increase in the lateral resolution [11]. It is clear that PEM images obtained with the optical ®ber method were limited by either the size of the probe (1 mm) or by the tip-tosample distance (normally tens of microns). In either case it was not possible to obtain images with submicron resolution. A recent paper by Williams et al. [12], reported resolution of about 0.5 mm on pitting sites on stainless steels based on a combined photoelectrochemical and electrochemical technique with a confocal microscope and a modi®ed Atomic Force Microscope (AFM). By comparison, we have approached 0.1 mm resolution in the present work and resolutions of 0.01 mm appear to be possible in the future. In the present paper we will describe a novel technique using shear force feedback, that utilizes a tuning
fork method in order to measure concurrently the topography and PEM on TiO2 covered polycrystalline Ti samples. The technique is based on a modi®cation of a Near-Field Scanning Optical Microscope (NSOM). The modi®cation was reported in a recent paper [14], in which we described its use for Scanning Electrochemical Microscopy (SECM). In a previous publication [13] we identi®ed electrochemically active sites on polycrystalline titanium by using SECM. Previous studies by Casillas et al. [15,16] found precursor sites on polycrystalline Ti foil materials that had no inclusions. The purpose of the present research is to demonstrate that by using the shear force feedback method it is possible to image in-situ in liquids both the particles and other active sites (i.e. grain boundaries) with high resolution. Full details of the experimental setup, nanoprobe preparation and experimental ®ndings will be published elsewhere.
L.F. Gar®as-Mesias, W.H. Smyrl / Electrochimica Acta 44 (1999) 3651±3657
2. Experimental 2.1. Tip preparation All probes were prepared from type F-AS optical ®bers (3.7 mm in core diameter with 125 2 2 mm of cladding and 2452 15 mm of polymer coating), which were obtained from Newport Corporation. The nanoprobes were made from optical ®bers of 1-m length. One end was then cleaned with isopropanol and etched in 20% HF (v/v) solution, prepared from a concentrated 48% HF solution (Mallinckrodt). After completion of the etching, the ®bers were observed under a 35X stereoscope and if the size of the etched ®ber was found be to 100 nm or less, the ®ber was selected for the studies. Those ®bers with larger tip diameter were discarded. We found that the time required to etch a ®ber tip to a size of approximately 100 nm diameter was about 24 h. 2.2. Sample preparation All samples used in the present research were cut from commercially pure plate titanium (99.99% pure) which had been specially heat treated to achieve large grains (approximately 1 mm grain size). We have previously determined that pitting precursors were located both inside the grains and at some grain boundaries [12] in these materials. Therefore, our goal was to study photoelectrochemical activity of precursor sites for pitting with high resolution images. The samples were prepared by successive polishing with 6, 1 and 0.25 mm diamond paste. After polishing, the samples were lightly etched in a solution containing 2% HF, 4% HNO3 and 94% H2O (volume %). 2.3. Oxide growth Before starting the PEM tests, an oxide ®lm was anodically grown on the samples by using a three-electrode cell con®guration inside the PEM cell and using a 0.05 M H2SO4 solution. The oxide ®lm was grown at a potential sweep rate of 0.1 mVsÿ1 starting from ÿ0.25 to +6 VSCE. 2.4. Photoelectrochemical Microscopy with concurrent topography All the PEM tests were performed on a Topometrix Aurora Near-Field Scanning Optical Microscope (NSOM) with a custom modi®ed head incorporating a tuning fork transducer, which has been brie¯y described in an earlier publication [14] and is shown in Fig. 1. An optical ®ber nanoprobe (described above) was used to monitor the surface topography and to focus the 351 nm laser beam onto the surface of the
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TiO2 covered Ti. The nanoprobe was glued onto one of the arms of the tuning fork following the feedback procedure developed by Karrai and Grober [17,18]. The shear-force feedback method (which will be discussed in the following section) was then used to obtain the topography of the sample concurrently with the photocurrent as a function of the tip position. All the PEM tests were obtained with the sample and the nanoprobe immersed in 0.05 M H2SO4 solutions. To perform PEM+TOPOGRAPHY measurements, the shaped ®ber was attached to a tuning fork which was mounted on a modi®ed NSOM head to control the tip-to-sample distance. 2.5. Shear-force feedback with the tuning fork Shear-force feedback is commonly used to maintain a constant distance between the probe and the surface in a conventional NSOM. By using this method, the probe is kept at a constant height above the sample [19]. The distance is normally regulated by monitoring the damping of the oscillations of a resonating probe as it encounters viscous damping near the sample surface. The method is basically non-contact with a typical interaction distance between 10±30 nm in air [20]. There are two well established methods to achieve feedback: (1) The optical detection method and (2) the tuning fork method. In the optical feedback method, light (from a second laser) is re¯ected o the probe and onto a split diode where lock-in techniques can discern the small oscillations of the tip [19]. In our experience, the optical feedback method works well in gases, but in liquids and at re¯ective metal surfaces the feedback is stable only for tens of minutes and the experimental setup becomes more restrictive. Since most of the electrochemical processes we are interested in studying occur in liquids, optical feedback is not ideal for these purposes. However, Karrai and Grober recently developed an alternative method that utilizes a tuning fork on the transducer for detecting the shearforce damping [17,18]. That method also has the advantage of working in liquids, being robust and normally remaining stable for hours. Fig. 1 shows a schematic representation of the system. The ®rst and crucial component is the optical ®ber nanoprobe (or tip), which is glued onto one arm of the quartz crystal tuning fork. The fork is rigidly attached to a piezoelectric vibrator (the driving piezo or dither ) which vibrates at a frequency of 33 kHz with an amplitude of nearly 0.01 nm [17,18]. To engage the system in feedback, the surface is moved towards the tip ®rst manually (to within a few microns) and then by means of four piezos that control the scanner (the sample is ®xed rigidly to the top of the scanner). As the sample is moved towards the tip, it reaches a position at which the amplitude of the vibration of the nanoprobe is
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L.F. Gar®as-Mesias, W.H. Smyrl / Electrochimica Acta 44 (1999) 3651±3657
Fig. 2. Photoelectrochemical Microscopy (PEM) and Topography (using the shear-force method) of an inclusion particle on polycrystalline Ti obtained with an etched optical ®ber. The images (21 21 mm) were obtained in 0.05 M H2SO4.
reduced by damping and consequently the piezoelectric signal of the fork is reduced. This reduction is large when the tip-to-sample separation is in the range of 40 nm [17,18]. By means of the electronic feedback loop, the system prevents any further movement of the sample towards the tip. The changes in the feedback signal can then be used and plotted as a function of the lateral position to generate a surface topography map (as shown in Figs. 2 and 3).
3. Results and discussion 3.1. Inclusions as precursor sites for pitting In the past, SECM was used for polycrystalline titanium to identify precursor sites for pit initiation [13,15,16]. The precursor sites were associated with particles containing Al and Si or grain boundaries which apparently were particle free [13]. The sites were
Fig. 3. Photoelectrochemical Microscopy (PEM) and Topography (using the shear-force method) of an inclusion particle on polycrystalline Ti obtained with an etched optical ®ber. The images (35 35 mm) were obtained in 0.05 M H2SO4.
L.F. Gar®as-Mesias, W.H. Smyrl / Electrochimica Acta 44 (1999) 3651±3657
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Fig. 4. Optical image of an inclusion on polycrystalline Ti sample (left) and contact AFM imaging (right) of the same particle. The bar next to the AFM image corresponds to the height pro®le in the image. The particle was approximately 30 mm in width.
found to be conductive and were able to support electrochemical reactions [11,13] like oxidation of ferrocyanide and bromide. In both cases (particles or grain boundaries), pitting could be induced at the sites at around +2.5 VSCE in bromide solutions at room temperature. We propose that the initiation sites on polycrystalline Ti can be attributed to two distinct causes; inclusions or defective oxide grown over active grain boundaries or other surface features. In the case of the inclusions, they appear to be complex particles that contain Al and Si (but that may have other impurities in lower concentrations). On the other hand, the substrate grain boundaries can be particle-free or can have particles that are too small to be detected with the techniques used previously. 3.2. PEM and Topography study In order to study submicron features, we adapted an NSOM instrument to perform such tests along with insitu topography in liquids. In order to be able to achieve high resolution (submicron) topography and PEM, we had to overcome three well de®ned problems: (1) tip preparation; (2) sample preparation and (3) to be able to perform the test in liquids. 3.2.1. Tip preparation By choosing an appropriate probe, one can achieve high resolution topography and also obtain a better signal to noise ratio while performing PEM. The maximum photocurrent obtained with the pulled tips (1350 pA for a new tip) was lower than the value obtained with an etched ®ber (3300 pA). The results are in com-
plete agreement with previous NSOM studies [21,22], which suggest that etched tips have higher light output than pulled tips. 3.2.2. Sample preparation As suggested by Tyler and coworkers [8], the oxide was grown at a low sweep rate (0.1 mVsÿ1), and this increased the photocurrent measured subsequently on the oxide covered surface. Second, the ®nal growth voltage was chosen to form a thick oxide, and this in turn increased not only the photocurrent measured, but also the signal to noise ratio (by making a more stable oxide ®lm). In the case of both Figs. 2 and 3, the oxide was grown to +6 VSCE at a sweep rate of 0.1 mVsÿ1. It is important to mention that because of the very low intensity of light focused onto the sample, it was not possible to obtain PEM maps when the probe was not in feedback (about 6 mm away from the sample). The main reason was the dissipation of light from the laser beam which reduced the photocurrent below detection limits when the tip is far from the surface. 3.2.3. PEM and Topography in liquids In our experiments, the length of the nanoprobe glued to the leg of the fork was sucient to guarantee that the system could operate in solution without wetting the legs of the fork. It is our observation that increasing the length of the nanoprobe increases its amplitude of oscillation and this can lead to loss of feedback. This is because the end of the nanoprobe (the apex of the tip) is further from the legs and therefore will have larger oscillations. We conclude that it is important to control the length of the nanoprobe.
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An optical microscope and an Atomic Force Microscope were used to image a particular inclusion on the polycrystalline Ti surface. The images were compared with the topography obtained with the tuning fork method. Fig. 4 shows both images, the optical image which includes most of the grain boundaries surrounding the particle and a more detailed image obtained by AFM showing details of the particles at a higher magni®cation. NSOM images of the same features are shown in Fig. 2. The lateral resolution of the topography of the inclusion (Fig. 2) is better than 100 nm, that is, small features can be resolved within the particle and around it. The inclusions seem to be comprised of two dierent parts apparently broken in the middle. The PEM image, on the other hand, is very distinctive; the right side of the particle has a lower photocurrent compared to the left side which is more photoactive. This ®nding may explain the observation made previously that the precursor site areas seen by using the conventional PEM method do not have the same size as the particles seen with topography. By using the tuning fork method and obtaining high resolution PEM with concurrent topography, the dierences can be clearly observed and inhomogeneities within the inclusions can be seen. A dierent kind of surface is shown in Fig. 3. Here, contrast from grain to grain is demonstrated, and small particles can also be resolved. In this case, it is clearly seen that the three Ti grains present in this area of the surface have distinct photocurrents. The lateral resolution in these images is probably better than 100 nm, even though the contrast between grains is generated by a very small change in the photocurrent (about 5 pA). 3.3. Future work The results presented here encourage us to continue the development of the new technique, and also to explore new problems that can be solved by using the novel technique described here. Future work will focus on the fabrication of smaller size nanoprobes (around 10 nm) that can increase the lateral resolution of the images, and that will require improved sensitivity to measure the photocurrent in these samples. With higher lateral resolution images, one can locate smaller precursor sites in other advanced materials, particularly those used in the semiconductor and microelectronics industry. 4. Conclusions A novel technique to perform both PEM and topography concurrently in liquids has been developed. The lateral resolution and accuracy of the topographic
information in liquid is retained (compared to conventional methods like AFM in air). At the present time the lateral resolution is around 100 nm, and at the same time one can characterize the local chemical behavior of the system by using PEM. The resulting blend has yielded high resolution topography and photoelectrochemical information on polycrystalline Ti. The new method can be combined with other techniques (SECM, PEM, Fluorescence and others), to characterize heterogeneous reactive systems. The precursor sites observed here for polycrystalline Ti samples were associated with particles containing Al and Si and with grain boundaries that are particle free. The precursors are apparently inhomogeneous and support photoelectrochemical reactions. Acknowledgements This study was supported by NSF grant DMR9509766. Early developments of the new technique were made by Drs PS Moyer and PI James. References [1] M.A. Butler, J. Electrochem. Soc. 130 (1983) 2358. [2] M.A. Butler, J. Electrochem. Soc. 131 (1984) 2185. [3] M.A. Butler, W.H. Smyrl, in: Proceedings of the 82 ECS Spring Meeting held in Montreal Canada, Abstract 7, 1982, p. 15. [4] M. Kozlowski, W.H. Smyrl, Lj Atanasoska, R. Atanasoski, Electrochimica Acta 34 (1989) 1763. [5] N. Casillas, P. James, W.H. Smyrl, J. Electrochem. Soc. 142 (L16) (1995). [6] P. James, N. Casillas, W.H. Smyrl, J. Electrochem. Soc. 143 (1996) 3853. [7] S. Kudelka, J.W. Schultze, Electrochimica Acta 42 (1997) 2817. [8] P.S. Tyler, M.R. Kozlowski, W.H. Smyrl, R.T. Atanasoski, J. Electroanal. Chem. 237 (1987) 295. [9] M.R. Kozlowski, P.S. Tyler, W.H. Smyrl, R.T. Atanasoski, J. Electrochem. Soc. 136 (1989) 442. [10] A.R. Kucernak, R. Peat, D.E. Williams, J. Electrochem. Soc. 138 (1991) 1645. [11] G. Shi, L.F. Gar®as-Mesias, W.H. Smyrl, J. Electrochem. Soc. 145 (1998) 2011. [12] D.E. Williams, T.F. Mohiuddin, Y.Y. Zhu, J. Electrochem Soc. 145 (1998) 2664. [13] F. Gar®as-Mesias, M. Alodan, P.I. James, W.H. Smyrl, J. Electrochem. Soc. 145 (1998) 2005. [14] P.I. James, L.F. Gar®as-Mesias, P.J. Moyer, W.H. Smyrl, J. Electrochem. Soc. 145 (L64) (1998). [15] N. Casillas, S.J. Charlesbois, W.H. Smyrl, H.S. White, J. Electrochem. Soc. 140 (L142) (1993). [16] N. Casillas, S. Charlesbois, W.H. Smyrl, H.S. White, J. Electrochem. Soc. 141 (1994) 636. [17] K. Karrai, R.D. Grober, J. Electrochem. Soc. 66 (1994) 1842.
L.F. Gar®as-Mesias, W.H. Smyrl / Electrochimica Acta 44 (1999) 3651±3657 [18] K. Karrai, R.D. Grober, Ultramicroscopy 61 (1995) 197. [19] E. Betzig, P.L. Finn, J.S. Weiner, Appl. Phys. Lett. 60 (1992) 2484. [20] M.A. Paesler, P.J. Moyer, Near-Field Optics: Theory,
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Instrumentation and Applications, John Wiley and Sons, Inc, New York, 1996. [21] P. Homann, B. Dutoit, R. Salathe, Ultramicroscopy 61 (1995) 165. [22] D.R. Turner, US-Patent, 4,469,554, 1983.