Charge Transfer through Thin Layers of Water Investigated by STM

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Charge Transfer through Thin Layers of Water Investigated by STM, AFM, and QCM Moon-Bong Song, Jai-Man Jang, Sang-Eun Bae, and Chi-Woo Lee* Department of Advanced Materials Chemistry, Korea University, Jochiwon, Choongnam 339-700, Korea Received July 30, 2001. In Final Form: December 18, 2001 Charge transfer through thin layers of water between a Pt/Ir tip and a gold surface has been investigated by using a scanning tunneling microscopy (STM) technique at the temperature of 21 ( 1 °C. The amount of the water layer inside the STM or an atomic force microscopy (AFM) junction was controlled by relative humidity. It was found from STM, AFM, and quartz crystal microbalance experiments that the thin water film was formed on the gold sample when relative humidity was in the range up to 80%. Charge transfer across the interfacial water layer was found to originate mostly from electron tunneling. The value of the barrier height of the electron tunneling was determined to be 0.95 eV from the current vs distance curve, which was independent of the tip-sample distance and the bias voltage. At relative humidity above 90%, the surface was covered with a thick water layer. In such a case the decay for charge transfer was strongly dependent on the bias voltage. For a low bias of 0.03 V charge transfer was similar to that at low relative humidity. On the contrary, the current was found to decay nonexponentially in the bias range of |0.1|-|0.5| V, where the plateau currents were observed in the long distance. The plateau current at 0.5 V was observed to flow up to several hundred nanometers. The magnitude of the plateau currents depended on the applied electrode potential as well as its polarity. The results were rationalized by means of electron tunneling and electrochemical process through the thick water layer.

Introduction Charge transfer is of fundamental importance to many processes in surface science and electrochemistry.1-23 A characteristic of direct electron tunneling among charge * Corresponding author. Fax: 82-41-867-6823. Phone: 82-41-8601333. E-mail: [email protected]. (1) Wiesendanger, R. Scanning probe microscopy and spectroscopy methods and applications; Cambridge: New York, 1994. (2) Schuster, R.; Barth, J. V.; Wintterlin, J.; Behm, R. J.; Etrl, G. Ultramicroscopy 1992, 42-44, 533. (3) Biscarini, F.; Kenkre, V. M. Surf. Sci. 1999, 426, 336. (4) Olesen, L.; Lægsgaard, E.; Stensgaard, I.; Besenbacher, F.; Schiøtz, J.; Stoltze, P.; Jacobsen, K. W.; Nørskov, J. K. Phys. Rev. Lett. 1994, 72, 2251. (5) Mamin, H. J.; Ganz, E.; Abraham, D. W.; Thomson, R. E.; Clarke, J. Phys. Rev. 1986, B34, 9015. (6) Hong, Y. A.; Hahn, J. R.; Kang, H. J. Chem. Phys. 1998, 108, 4367. (7) Hahn, J. R.; Hong, Y. A.; Kang, H. Appl. Phys. 1998, A66, S467. (8) Fan, F.-R. F.; Bard, A. J. Science 1995, 270, 1849. (9) Heim, M.; Eschrich, R.; Hillebrand, A.; Knapp, H. F.; Guckenberger, R.; Cevc, G. J. Vac. Sci. Technol. 1996, B42, 1498. (10) Schmickler, W. Surf. Sci. 1995, 335, 416. (11) Pan, J.; Jing, T. W.; Lindsay, S. M. J. Phys. Chem. 1994, 98, 4205. (12) Meepagala, S. C.; Real, F. Phys. Rev. 1994, B49, 10761. (13) Guckenberger, R.; Heim, M.; Cevc, G.; Knapp, H. F.; Weigrabe, W.; Hillebrand, A. Science 1994, 266, 1538. (14) Vaught, A.; Jing, T. W.; Lindsay, S. M. Chem. Phys. Lett. 1995, 236, 306. (15) McCarley, R. L.; Hendricks, S. A.; Bard, A. J. J. Phys. Chem. 1992, 96, 10089. (16) Sass, J. K.; Gimzewski, J. K. J. Electroanal. Chem. 1991, 308, 333. (17) Toney, M. F.; Howard, J. N.; Richer, J.; Borges, G. L.; Gordon, J. G.; Melroy, O. R.; Wiesler, D. G.; Yee, D.; Sorensen, L. B. Nature 1994, 368, 444. (18) Toney, M. F.; Howard, J. N.; Richer, J.; Borges, G. L.; Gordon, J. G.; Melroy, O. R.; Wiesler, D. G.; Yee, D.; Sorensen, L. B. Surf. Sci. 1995, 335, 326. (19) Lang, N. D.Phys. Rev. 1987, B36, 8173. (20) Ahn, J. H.; Pyo, M. H. Bull. Korean Chem. Soc. 2000, 21, 644. (21) Lindsay, S. M.; Barris, B. J. Vac. Sci. Technol. 1988, A6, 544. (22) Song, M.-B.; Yoshimi, K.; Ito, M. Chem. Phys. Lett. 1996, 263, 585. (23) Lee, S.-Y.; Bae, S.-E. Paek, S.-H.; Lee, C.-W. Synth. Met. 2001, 117, 105.

transfers has been extensively studied using scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) circumstances.2-5 The value of an apparent barrier height for electron tunneling related to the local wave function is easily determined from conductance variation as a function of tip-surface distance. According to a simple one-dimensional tunneling model as well as experimental studies, the barrier height was found to be independent of both the bias voltage applied between an STM tip and a surface and the surface-tip spacing at longer distances than that coming into a quantum-point contact.2-5 In a similar way, attempts to measure STM barriers have been made in humid atmosphere, pure water, or electrochemical environment.5-16,20,21 The results showed that the electron tunneling distance is longer and the value of the apparent barrier height for electron tunneling is lower than that in UHV.6,7,9,14,16,20,21 The interfacial water (or contaminant) inside the STM junction was concluded to facilitate electron tunneling in the applied electric field. However, some results revealed that the apparent barrier height depends on the applied electrode potential as well as the tip-surface distance.6,7,9,14,16,20,21 On the other hand, electrochemical processes were found to contribute to current decay in humid atmosphere.8 Therefore, charge transfer across interfacial water layer is not so simple as compared with that observed in UHV and is still not well understood. In this work, we examined the charge transfer through the interfacial water layer (varying thickness) inside STM junction by measuring current behaviors as functions of the applied bias voltage and the tip-surface distance at relative humidity range between 5 and 97% for the model system of a gold surface and a Pt/Ir tip. Experimental Section The experiments were carried out using an SPM (Molecular Imaging Corp.) and a quartz crystal microbalance (EG & G QCA 917) at the temperature of 21 ( 1 °C. We used an Au film deposited onto mica (for STM and AFM) or deposited onto quartz (for QCM)

10.1021/la011189a CCC: $22.00 © 2002 American Chemical Society Published on Web 03/07/2002

Charge Transfer through Thin Layers of Water

Figure 1. STM image (90 × 72 nm2) of a reconstructed Au(111) surface with the herringbone structure observed in humid atmosphere. Tunneling condition: sample bias voltage ) 0.05 V; tunneling current ) 1.542 nA. as a sample. A sample stage was kept in an environmental chamber. The chamber was flushed with dry nitrogen gas. The relative humidity inside the chamber was controlled by silica desiccant or wet paper. The system was left for 1-10 h to stabilize relative humidity and to avoid thermal drift before the STM and AFM measurements. Platinum/iridium tips were prepared mechanically from Pt/Ir wire for the STM experiment. Microcantilevers of silicon nitride for the AFM experiment were from Park Scientific Instruments (“V”-shaped, length ) 85 µm, width ) 18 µm, thickness ) 0.6 µm, force constant ) 0.5 N/m). Current vs distance characteristics were measured in the following way: An initial current was held at 5 nA by using a constant current feedback for the optimum detection of changes in current with the distance. The bias voltage with respect to the tip was adjusted to the fixed value that was selected in the range between -0.5 and 0.5 V. To make a measurement, the tip was pushed by 0.3 nm toward the sample after the feedback was switched off. Then, the tip was rapidly withdrawn from the sample and the resulting current in the range between 10 and 0.001 nA was recorded as a function of the distance perpendicular to the surface. Afterward, the tip was moved back to its original position, and the feedback loop was enabled again. Under these measurement conditions current changes with the smaller standard deviations were obtained. For each bias voltage value, this process was repeated 20-200 times, and then the data were averaged. The z scale, which is perpendicular to the surface, was calibrated using monatomic steps on the Au surface. In a similar way, force curves were recorded as a function of the displacement of the cantilever from the sample. QCM measurement was performed using an Au film on an AT-cut quartz crystal with a resonance frequency of 9 MHz. After an introduction of water the resonance frequency change was measured as a function of time and was converted to change in water condensed on an Au film.23

Results and Discussion Figure 1 shows an STM image (90 × 72 nm2) of an Au(111) surface obtained in humid atmosphere after annealing a gold film deposited onto mica in a hydrogen flame for a few minutes. The “herringbone” pattern of reconstructed Au(111) was observed in the STM image. The herringbone structure did not disappear after repeating cycles of approaching and withdrawing of a Pt/Ir STM tip to obtain current versus tip distance curves. It is wellknown that such a herringbone structure is easily lifted to a (1 × 1) structure by chemisorption on the surface at room temperature.24 Therefore, the presence of the herringbone structure reveals that there are few contaminants bonded strongly to the surface, being, if any, physisorbed molecules such as water. (24) Hallmark, V. M.; Chiang, S.; Rabolt, J. F.; Swalen, J. D.; Wilson, R. J. Phys. Rev. Lett. 1987, 59, 2879.

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Figure 2. Tunneling current versus distance characteristic measured on an Au(111) surface with a Pt/Ir tip at relative humidity near 5% and a bias voltage of 0.1 V. The tunneling current is shown on a logarithmic scale. The zero distance corresponds to the initial position. The solid line is exponential fitted with the tunnel barrier height of 0.95 eV.

Figure 2 shows current vs distance characteristics measured on the Au(111) surface with a Pt/Ir tip at relative humidity near 5% and at a bias voltage of 0.1 V. The current is displayed on a logarithmic scale. The zero point in the horizontal axis refers to the equilibrium position (5 nA). It was observed that for higher currents than 3 pA the data are well fitted with a single-exponential decay curve (indicated by the solid line) as a function of the tip distance. In a simple one-dimensional tunneling model,1 the electron tunneling current (I) inside STM junction can be written as I ) I0 exp(-1.025xφs), where φ is the apparent barrier height in electronvolts and s the distance in angstroms. The apparent barrier height of 0.95 eV was calculated from the current curve. The value 0.95 eV was found to be almost constant, irrespective of the applied bias voltage over the range between -0.5 and 0.5 V as well as of relative humidity extending from 5 to 80%. The value of the apparent barrier height (0.95 eV) at relative humidity extending from 5 to 80% is low compared to that in ultrahigh vacuum. The difference in the tunneling barrier height can be explained in terms of the difference in tunneling environment.6,7,9,14,16 That is, the electron tunneling under ultrahigh vacuum and humid atmosphere takes place directly through space and across the thin layer of water on the surface, respectively. It is well-known that the apparent barrier height for electron tunneling is independent of the tip-surface separation.1 Therefore, the width of a tunnel gap can be estimated by extrapolating the conductance to the expected value at quantum-point contact (77.52 µΩ-1).11,14 In this work the tunnel distance for the model system of the gold surface and the Pt/Ir tip was calculated to be 12 ( 1 Å from the tunneling current data in Figure 2. The values for the apparent barrier height and the tunnel distance are consistent with the previous STM results6,7 studied under ambient atmosphere with 50-80% relative humidity. Also, independence of the apparent barrier height on the applied bias voltage and its polarity in the tunnel current range less than 10 nA is in good agreement with the STM results.6,7 For saturated relative humidity (about 97%), current decay data were also measured as shown in Figure 3. Solid circles, open circles, triangles, and squares were obtained at the bias voltages of 0.03, 0.1, 0.4, and 0.5 V, respectively. There are two remarkable differences between low and high relative humidity. First, each current,

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Figure 3. Current versus distance data for various bias voltages at relative humidity as high as about 97%. Solid circles, open circles, triangles, and squares were obtained at bias voltages of 0.03, 0.1, 0.4, and 0.5 V, respectively.

at separations larger than 5 Å where the current decay stops, has a certain constant value that depends on the applied bias voltage. The plateau currents were always observed at relative humidity above 90% when an applied bias became high (>0.05 V). The plateau currents presented hysteresis in the direction of the tip movement. The plateau at 0.5 V bias was extended to several hundred nanometers during the tip retraction. However, the width of the plateau on the approaching movement was substantially narrower. Second, the value of the apparent barrier height was also a function of the applied bias voltage. Then we found that the value of the plateau current was larger and that the current curve decayed more slowly as the applied bias voltage became higher. The plateau current in Figure 3 is believed to be caused by electrochemical processes through a thick water film formed inside STM junction, because it appeared up to long distance at high relative humidity and its value depended on the bias voltage. QCM and AFM experiments were performed to monitor the growth of water on a sample surface as a function of relative humidity.25,26 Figure 4 shows a typical resonance frequency change on a gold film deposited onto quartz as a function of time after introduction of water into an environmental chamber (near 5%) prepurged with dry nitrogen gas. The horizontal axis was displayed as time instead of relative humidity due to the difference in response time between near the Au surface and near the hygrometer. The QCM experiment was carried out on the surface without a probe tip. Therefore, even though we can make no direct comparison between STM and QCM results, it is possible to qualitatively estimate the thickness of water condensed on the gold surface. The resonance frequency was constant after 80 min and decreased by around 40 Hz. Such a resonance frequency change can be explained to originate from the contribution from changes in mass, density, and viscosity of water condensed on the surface. However, we may ignore the influences of water density and viscosity on the frequency change, because the condensed water is pure and the hydrophobic Au surface is not biased, respectively. The change in resonant resistance was not significantly large. The measured resonance frequency change was simply converted to change in water layer on the Au film using the Sauerbrey equation, shown also at the right(25) Cappella, B.; Dietler, D. Surf. Sci. Rep. 1999, 34, 1. (26) Thundat, T.; Zheng, X.-Y.; Chen, G. Y.; Sharp, S. L.; Warmack, R. J.; Schowalter, L. J. Appl. Phys. Lett. 1993, 63, 2150.

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Figure 4. Typical resonance frequency change and thickness of water bilayer condensed on a gold film deposited onto quartz as a function of time after introduction of water inside an environmental chamber.

Figure 5. Typical normal force versus displacement curves obtained on an Au(111) surface with a cantilever of silicon nitride at relative humidity of (a) 5% and (b) 97%.

hand side. It was found that 5-8 bilayers (1 bilayer is assumed to be 0.67 monolayer of x3 × x3 structure27) of water were formed on the gold sample at a saturated relative humidity. An STM experimental condition with a probe tip is similar to AFM rather than QCM. Figure 5a,b shows two typical normal force versus displacement curves obtained on the Au(111) surface with a tip of silicon nitride for two extremes of relative humidity, near 5% and 97%, respectively. The tip was moved in and out of contact. Two jumps of jump-to-contact and jump-off-contact were observed during the approach and withdrawal of the tip, respectively, as was observed with the electrical surface force balance.28 The jump-off-contact is related to adhesion force which consists of a mixture of meniscus force, Coulomb force, van der Waals force, and so forth.25,26 The average value of adhesion force was obtained from the force-curve measurements more than 200 times at controlled relative humidity, and the results are shown in Figure 6. Adhesion force was almost constant (1.8 ( 0.3 nN) up to 75% and then becomes larger (about 4 nN) at 97%. The meniscus force almost equally contributed to the adhesion force in air due to formation of a water bridge.25,26 The cleanliness of a tip and a surface were reported to make a large effect (27) Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1987, 7, 211. (28) Lee, C.-W.; Bard, A. J. J. Electrochem. Soc. 1988, 135, 1599.

Charge Transfer through Thin Layers of Water

Figure 6. Plots of adhesion force as a function of relative humidity. Adhesion force was almost constant (1.8 ( 0.3 nN) up to 75% and then becomes larger (about 4 nN) at 97%.

on adhesion.26 Adhesion force was measured as a function of relative humidity for freshly cleaved mica substrate using both a clean and a contaminated tip of silicon nitride.26 Adhesion force for the clean tip changed from 5 to 10 nN with increasing relative humidity, while that for the contaminated tip changed from 40 to 70 nN.26 Thus, our value of the adhesion force at the gold surface is compatible with the AFM result using the clean tip for the mica surface. Considering the three STM, QCM, and AFM results, it clearly indicates that an amount of pure water layer condensed on the surface depends on relative humidity. When the STM or AFM tip approached the surface covered with a water layer having a thickness above 1 nm at a saturated relative humidity, a water bridge was concluded to be formed between the tip and the surface. The present STM result is compatible with the previous work.9 The plateau current was reported to be observed with the hydrophilic surfaces such as Pt/C and mica at relative humidity of 50-80% and above 65%, respectively.9 However, it was not detected on a gold surface up to relative humidity of 75% because of its hydrophobicity,9 as was found in this work. The presence of the plateau was generally interpreted by the formation of the thick layer of water with conductivity.9 When the tip is approached toward the sample surface, it comes into contact with the conductive water layer. This results in an electrochemical current being dependent on the applied voltage. On the contrary, the water bridge formed between the tip and the sample is elongated due to the water meniscus adhered to the tip during the tip retracting. Consequently, such a phenomenon leads to the different widths of the plateau for the direction of the tip movement. Figure 7 shows the values of the plateau currents through water bridge as a function of the bias voltage at the relative humidity of 96 ( 1%. Each value is the average (with the standard deviations) of many experiments over different locations. Somewhat large deviations are probably due to the different shapes of tips. The plateau current reveals the bias voltage and polarity dependence. That is, the plateau currents increased with the increasing bias voltage. The magnitude of the plateau current observed at the bias voltages larger than 0.3 V was higher than that at the bias voltages smaller than -0.3 V. However, no polarity dependence appears at lower biases (