Water Vapor Adsorption Effect on Silica Surface ... - ACS Publications

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J. Phys. Chem. C 2008, 112, 17193–17199

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Water Vapor Adsorption Effect on Silica Surface Electrostatic Patterning Rubia F. Gouveia, Carlos A. R. Costa, and Fernando Galembeck* Institute of Chemistry, UniVersidade Estadual de Campinas - UNICAMP, P.O. Box 6154, CEP 13084-971, Campinas - SP, Brazil ReceiVed: April 30, 2008; ReVised Manuscript ReceiVed: August 23, 2008

This work verifies a model for the creation and dissipation of reproducible electric potential patterns on silica surfaces, based on water adsorption, ionization, and ion migration under applied electric potential. Samples were thin silica films grown on silicon wafers and partially covered with sets of parallel gold stripe interdigitated electrodes that are normally used for Kelvin force microscope calibration. Noncontact electric potential measurements with a 20 nm spatial resolution were done using the Kelvin method under controlled atmosphere, in an atomic force microscope (AFM) with a Kelvin force attachment (KFM) mounted within an environmental chamber. Patterns were observed in micrographs acquired while one electrode set was biased and the other was grounded and when both were short-circuited and grounded. Electrostatic charging and discharging are much faster at high relative humidity, showing that the charged or discharged silica states are both changed faster under high humidity, while pattern preservation is effective under low humidity. The results are explained considering surface conductance and the partitioning of water cluster ions both in the solid-gas interfaces and the atmosphere, under the biased electrode potential. Introduction Electrostatic phenomena1 are widely used in many important technologies, such as photocopying,2,3 electrostatic painting,4 and electrospinning,5 but they are also the source of many practical problems, including serious recent industrial accidents.6,7 Even after a few centuries of research by distinguished authors, electrostatic charging of insulators is still poorly known, and it often goes out of control because the identity of charge carriers is not known, in nearly every case in the laboratory or in a practical situation.1,8 A persistent problem is the difficulty to produce repeatable and previsible electric potential patterns9,10 that cannot be solved unless the underlying charge-bearing species, transport phenomena, and chemical reactions are wellunderstood. This undesirable situation is well-documented in the literature,9-16 and it requires new efforts for the speciation of charge carriers in insulators, as well as their quantification.16 This problem has been addressed recently, and new proposals have been put forward by different authors.16-22 Three different mechanisms for contact electrification were recently emphasized by McCarty and Whitesides: electron transfer for contact between metals or semiconductors, ion transfer for contact involving materials that contain mobile ions, and asymmetric partitioning of hydroxide ions between adsorbed layers of water for contact involving nonionic and insulating materials.16 According to the same authors, a fundamental unanswered question is the chemistry of materials that bear a net electrostatic charge. Great progress in the study of electrostatic patterns on dielectrics and other solids was achieved following the introduction of Kelvin scanning electrostatic voltmeters with various degrees of spatial resolution.23-26 The systems based on Kelvin force scanning microscopy (KFM) or scanning electric potential microscopy (SEPM) are specially useful because of their 10 * Corresponding author. Phone: +55-19-3521-3080. Fax: +55-19-35213023. E-mail: [email protected].

nm lateral resolution that is within the macromolecular or nanoparticle size range.27-29 Most work using these techniques has been done on semiconductors or metals,30,31 but they were also used to investigate the charge pattern imaged on thin film electrets32 and nanopatterned surfaces.33 The authors’ laboratory has been using this technique for the examination of dielectric solids, especially colloid polymers or latexes,34,35 in association with analytical electron microscopy (electron energy-loss spectroscopy in the transmission electron microscope, EELS-TEM), what allowed the unequivocal identification of polymer charge carriers with ionic constituents such as K+ and RSO4- ions introduced during the polymerization process.36 Theoretical results show important effects of various gas adsorbates and adsorption of water on the charge patterns of carbon nanotubes.37 In a recent work,38 silica-on-wafer surfaces were electrostatically patterned in a submicrometer scale and in a reproducible way, using an experimental setup based on overlaying the silica surfaces with two sets of interdigitated gold stripe electrodes, biasing one electrode set and observing the electric potential maps. This setup is also widely used for calibration of scanning Kelvin force microscopes so that the initial results are made during calibration work that produce some unexpected observations. Systematic and extensive experiments were then done, leading to a reproducible procedure for electrostatic patterning down to the nanometer scale. The observed phenomena were interpreted using a model based on the following assumptions: (i) water is chemisorbed at the silica surface, forming a silanol layer covered with water molecules;39 (ii) when the interdigitated electrodes are polarized, H+ ions migrate to the catode, and they are discharged to a greater extent than negative silicate ions in the surface, that acquires an excess negative charge; and (iii) the excess charge accumulated on silica is dissipated following further water adsorption. A crucial test for this model is the determination of the effect of water content in the atmosphere surrounding the sample, as

10.1021/jp803812p CCC: $40.75  2008 American Chemical Society Published on Web 10/11/2008

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Figure 1. Illustration drawn over an AFM image of a section of the sample used. Clear areas are coated with gold while the dark areas are bare silica. External wiring is connected to the bus gold stripes at both sides of the figure. One electrode set is always grounded, and the other is biased, positive, or negative.

measured by the relative humidity (RH), on the silica charging and discharging phenomena. For this reason, silica electrostatic patterning was examined in a systematic way but under controlled relative humidity, and the results thus obtained are described in this paper. Experimental Section Sample. Samples used were thin silica-on-wafer films prepared in the microelectronics facilities in this university (CCS), following designs that are widely used for KFM calibration.40,41 Si wafers were oxidized in a furnace at 1000 °C, when a thin silica layer was obtained. This was covered with interdigitated Ti stripes connected to bus stripes that were in turn coated with a thin gold film, using microlithography techniques. Ti is used to improve gold adhesion to silica. The bus gold-coated stripes were then connected to wires for external connection, as shown in Figure 1. Kelvin Force Microscopy (KFM). KFM experiments were performed in a Shimadzu WET-SPM 9500J3 instrument. This instrument is mounted within an environmental chamber that allows control of ambient pressure, temperature, relative humidity, and atmosphere composition. Images were obtained during sample exposure to different humidity conditions under 25 ( 1 °C constant temperature and 1 bar argon pressure. The noncontact AFM42 mode was used to obtain topographic information on the silica surfaces covered with gold stripes. The KFM43 technique uses the standard noncontact AFM setup, but the sample is scanned with Pt-coated Si tips with a 20 nm nominal radius. Tips were changed whenever measurement instabilities were observed.40 An AC signal is fed 10-20 kHz below the frequency of the normal AFM oscillator, which matches the natural frequency of mechanical oscillation of the cantilever-tip system (40-70 kHz). The principle is analogous to the Kelvin method,23 except that forces are measured instead of current. The image is built using the DC voltage fed to the tip, at every pixel, thus detecting electric potential gradients throughout the scanned area. Electrode Biasing and KFM Imaging. The following image acquisition procedure was used: (1) The sample was shortcircuited and stored under 70% RH, to approach equilibrium. (2) RH within the environmental chamber was adjusted to the desired value, and one electrode set was biased by connecting to a battery stack. The potential differences applied to the electrodes were thus 1.5 ( 0.1 V, 3.0 ( 0.1 V, 4.5 ( 0.1 V, and 6.0 ( 0.1V while one electrode set was kept connected to ground. (3) Sample scanning was started and continued until successive lines did not show significant changes. Scanning was always run from top to bottom so that the y axis in the images

Gouveia et al. does also represent an increase in time, from top to bottom. Scanning rates were 2.5 µm/s. (4) When successive scanned lines appeared identical, the electrodes were short-circuited and grounded. Sample scanning was restarted until successive lines were again identical, within experimental error. From each image, potential versus position can be read at any pixel. This was done by taking line-scans, this means, potential versus x coordinate (or y coordinate) plots out of which potential gradients can be calculated. Image processing was performed in a PC microcomputer using the Shimadzu analysis software. Charge Determinations with a Faraday Cup. Faraday cup experiments were used for electrostatic charge determinations.15,48,49 The apparatus was built using two aluminum concentric cylinders (electrically insulated from each other by using polyethylene foam) connected by a coaxial cable to a Keithley instrument model 610C electrometer that was used in the charge measurement mode. Figure S1 in Supporting Information shows a picture of the apparatus. Model. For the sake of clarity, this section introduces a model for the changes that can take place when the electrodes shown in Figure 1 are biased. This model considers the existence of atmospheric water and thus of ions formed by water dissociation. It also considers information on water films on gold surfaces under various conditions.44-46 Even though there is disagreement in the detailed results of different authors, they agree in one point: there are significant amounts of water deposited in gold surfaces, even at relative humidity as low as 35%. Above 50% RH, the gold surface is coated with a film with thickness in the nanometer range. Ions formed by water molecule dissociation are thus expected to occur in these films, also. Beyond, the model considers other ions in the atmosphere and their concentration. Under an applied potential, ion distribution among water molecules and clusters is changed according to the equation for the electrochemical potential:

µi ) µi0 + RT ln ai + ZiFV Regions under a positive potential are thus enriched in negative ions and vice-versa, both in the gas phase and in the adsorbed layer in the silica and gold surfaces. The concentration of ions formed by water dissociation is expected to increase with the overall amount of water in any phase or interface, and the local changes of ion concentration affect the local electric potential gradients, according to Poisson equation (in the case of fixed charges) or Poisson-Boltzmann equation (for freely moving charges). This is shown in Figure 2. As for the relative amounts of ions in the atmosphere (including those formed by water vapor molecules) and adsorbed water, they can be compared as follows. Considering an electrode area of 5 × 5 µm2 covered with a 1 nm water layer,44-46 the number of adsorbed water molecules is approximately 109. On the other hand, by considering the gas volume over this same area, between the electrode surface and the tip at 10 nm distance and at 50% RH under 25 °C, the number of water molecules is 105. Thus, adsorbed water can contribute 104 times as much as water vapor can contribute to excess ion concentration under an applied potential. To evaluate the contribution of atmospheric ions not associated to local potentials, including those generated by background radiation, we observe that surface continental atmospheric ion concentration47 is 100-2000 cm-3. Thus, the average number of ions in the 25 × 10-14 cm3 volume above 5 × 5 µm2 electrode area is much less than 10-10. Thus, the only immediate relevant source

Water Vapor Adsorption Effect

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Figure 2. Schematic representation of the model for the behavior of water molecules in the electrodes and silica surface, as well as of the silanol groups on silica. (a) In the initial state, the water film is neutral. At the gas-solid interfaces, (b) when an electrode set is biased, surface ions migrate to electrodes carrying potential of opposite signal; (c) silanol groups are slowly converted to silicate while H+ ions are discharged at the grounded electrode; (d) when the electrodes are all grounded, ions at the surface film migrate reforming a neutral water layer; (e) silicate groups bind H+ ions from water and they are thus neutralized.

of ions in this system is adsorbed water, since its amount is 104 greater than that in the surrounding gas phase, and other ions are all but negligible. However, considering the high mobility of gas ions, there is always the possibility for a contribution from gas phase ions migrating fast from more distant points in the atmosphere. Under neutral conditions (zero potential and assuming that the dissociation constant Kw for adsorbed water is the same as in bulk water, there are approximately 2 hidronium and 2 hydroxyl ions within each 5 × 5 µm2 electrode surface area. Under a few volts local positive potential, only hidronium ions should be in excess in most areas, and the resulting net charge should be at least one ion per square micrometer. Results KFM images acquired from the samples depend largely not only on the applied potentials, as expected, but also, (i) on the sample recent history, that is, the changes in externally biased electrode, (ii) on the relative humidity, and (iii) on the respective times after changing RH, as seen in the electric maps shown in Figure 3. The upper part of each micrograph shows images taken while one electrode set was externally biased, while the lower part shows short-circuited and grounded electrodes. Many changes are observed, from one to another micrograph as well as within the micrographs recorded at the higher RH values. These are more easily understood by examining the linescans measured from the micrographs, as shown in Figure 4. From these, we can observe that there are RH-dependent potential changes at high humidity, but potentials are steady at low RH. The lines measured at different times while the electrodes were biased at 10 and 30% RH are coincident, while the lines measured at 50 and especially 70% show large changes with time. Electrode bias produces fast changes within times well-below 1 s followed by slow changes extending for many minutes. According to the model presented in Figure 2, fast changes are

Figure 3. KFM micrograph of a silica-on-wafer thin film partially covered with interdigitated electrodes. Successive changes in the state of electrode polarization and relative humidity of the surrounding atmosphere were made while the image was acquired, as indicated at the sides of the figures. Brighter areas are positive; dark areas are negative.

due to electrode connection to the power supply followed by the formation of excess negative ion concentration in the surrounding atmosphere and adsorption of molecular cluster ions [OH(H2O)n]- together with a minor amount of other atmospheric ions.50 Rates of charge accumulation and dissipation are thus dependent on water adsorbed at the solid-vapor interfaces. Potential versus distance plots were also acquired after the electrodes were short-circuited and grounded, at each RH. These are shown in Figure 5, and they are also strongly dependent on the relative humidity. Line-scans measured at 70 and 50% RH show local potentials down to -1.2 V over silica and up to 0.3 V at the metal borders, forming regular, persistent patterns. The curves recorded under low RH also show deviations from zero but much smaller. This confirms that fixed charges are produced on silica while the electrodes are biased,38 and this is increasingly more pronounced at high RH. Excess negative charge on the sample following electrode biasing was verified by making Faraday cup measurements on a sample that was placed within a closed container, above a water layer under room temperature. After equilibrating for 1 h, the sample was biased under 3 V (ungrounded) for 10 min, then placed within the Faraday cup under air (55% RH) and under

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Figure 4. Potential vs distance along horizontal lines drawn in Figure 3. Solid line: profiles acquired immediately after electrode set bias. Dash line: profiles acquired 10 min after electrode bias. (a) Profiles 1,2 (solid line) and 3,4 (dash line); (b) profiles 9,10 (solid line) and 11,12 (dash line); (c) profiles 17,18 (solid line) and 19,20 (dash line) and (d) profiles 27,28 (solid line) and 29,30 (dash line), under 10%, 30%, 50%, and 70% RH, respectively.

argon. Sample charge was then determined, equal to -0.80 ( 0.05 pC and -0.72 ( 0.05 pC, respectively, corresponding to an average charge -0.002 fC/µm2 throughout the overall silica area in between electrodes, as expected from the model presented in Figure 2. In other experiments, the sample was introduced in the cup under air at 55% RH, the electrodes were biased, and the charge was measured right after biasing. In this case, no charge was detected, within the sensitivity limit of the apparatus (0.01 pC). There is also a large RH effect on the rate of charge dissipation that is clearly evidenced in Figure 6. This shows electric potential over silica versus time along vertical lines in Figure 3, while electrodes were biased and also after they were short-circuited. The curve measured under 70% humidity shows large variations both before and after the electrodes were shortcircuited. The electric potential over silica surface changes toward negative potentials at longer times, as shown in Figure 5. We explained these slow potential changes38 by using a mechanism based on silanol group dissociation followed by proton migration to the negative electrodes followed by discharge. The relative humidity effect that is now observed verifies that mechanism: discharge is more pronounced under higher humidity, when proton migration is faster. The effect of the voltage applied to the electrodes on the potentials read at different sample points was determined by successively applying a potential, then short-circuiting and grounding followed by application of a higher voltage. Electric potentials were measured at the center of strips of a biased electrode, a grounded electrode, and silica, under 10% and 70% RH (Figure 7).

Gouveia et al.

Figure 5. Potential vs distance along horizontal lines drawn in Figure 3. Solid line: profiles acquired immediately after electrodes were grounded. Dash line: profiles acquired 10 min after electrodes grounding. Dash dot line: profiles acquired 30 min after electrodes grounding. (a) Profiles 5,6 (solid line) and 7,8 (dash line); (b) profiles 13,14 (solid line) and 15,16 (dash line); (c) profiles 21,22 (solid line), 23,24 (dash line) and 25,26 (dash dot line) and (d) profiles 31,32 (solid line) and 33,34 (dash line) and 35,36 (dash dot line), under 10%, 30%, 50%, and 70% RH, respectively.

Figure 6. Electric potential over silica vs time, measured along vertical lines in Figure 3. Dot line, 10% RH; dash line, 30% RH; dash dot line, 50% RH; solid line, 70% RH. In every case, the electrodes were initially biased, and they were short-circuited after ca. 10 min biasing.

Figure 8 is another plot of data from Figure 7, showing the electric potentials measured over the biased electrodes, grounded electrodes, and silica versus voltage applied to the electrodes under (a) 10% and (b) 70% RH. The potentials measured over the positive electrodes are always lower than the applied voltage; the electric potential over silica is always positive when the biased electrodes are positive, as well as the electric potential measured over the grounded electrodes, and these effects are strongly dependent on the humidity.

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Figure 7. (a) Potential difference applied to the metal electrodes. (b) Measured KFM potential over the biased electrode surface. (c) Measured potential over the grounded electrode. (d) Measured potential over the silica insulator in between the two electrodes. Left: 10% relatively humidity. Right: 70% relatively humidity.

The lower-than-expected potentials over the biased electrode are due to the adsorption of negative ions, while the positive potentials over silica and the grounded electrode can in turn be explained by the adsorption of positive ions, right after the negative ions were attracted to the positive electrode. These processes are expected to be very fast since their rates are only limited by diffusion under very short distances. The slow changes in Figure 7 are much more pronounced at 70% relative humidity when the electrodes are grounded, but the largest changes are observed on silica, because of the conversion of silanol to silicate. A very interesting observation is that the center of the biased electrode shows a small negative potential right after short-circuiting, as expected, considering that it was coated with a film carrying adsorbed negative ions. This shows that part of the adsorbed negative ions is retained for some seconds. This can be seen in Figure 8b, by observing the difference between measurements made right after electrode biasing and measurements made 10 min later. The potential changes measured over the biased electrodes are fast and reversible, and this is independent of the polarity and magnitude of the external bias voltage as seen in Figure 9. On the other hand, the electric potential measured over the grounded electrode is positive when the polarized electrode is positively biased, but it is barely negative when that electrode is biased negative. This is an intriguing asymmetry that can be assigned to stronger adsorption of positive ions as compared with negative ions, at the gold surface. This is confirmed by comparing measurements made under positive and negative bias, shown in Figures 3 and 7 in a previous work.38 Discussion The results presented in this paper show many unexpected and intriguing observations that were fully explained using a fairly simple model based on standard physicochemical knowledge.

Previous observations38 that were made using a completely different apparatus under room atmosphere are confirmed, and the previous hypothesis based on water adsorption is now verified. On the other hand, new observations did require further elaboration of the previous hypothesis but maintaining its essential feature and preserving the role of the atmosphere as a source and sink for solid surface electrostatic charging. Charge build-up and dissipation associated with adsorbed water were also used to explain results from another recent work from this group but in a completely different context, on charge induction in cellulose and in glass sheets.50 Summing up, we can explain all of the observations described in the results section and also some previously published observations: (1) The potential difference measured by the Kelvin electrode adjacent to the gold electrodes is always less than the applied potential, and it changes with relative humidity, because of ions adsorbed at the metal surface, according to the superposition principle.51 (2) The adsorbed ions are formed mostly from neutral adsorbed water molecules under an applied potential: water layer over positive electrodes contain excess negative ions and vice versa. (3) During accumulation of excess negative (positive) ions at the positive (negative) electrode surface, positive (negative) ions are released in the surrounding surfaces and atmosphere. (4) The RH effect on the difference between applied and measured potentials on the gold electrodes is easily understood considering that at higher RH the concentration of molecular cluster ions in the surfaces should also be larger thus increasing their effect on the measured potentials. (5) Whenever one electrode is biased and the other is grounded, the electrostatic potentials measured over silica are closer to the potential of the positive electrode than to that of the negative electrodes. This is also observed when the biased electrode is negative relative to ground38 and is explained by ion migration as in 3 above.

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Figure 8. Electric potentials measured over the biased electrodes, grounded electrodes, and silica vs voltage applied to the electrodes, plotted from data in Figure 7. (a) Under 10% RH; (b) 70% RH. 9 0 @, biased electrode; bOY, silica; 2∆6, grounded electrode. Closed symbols, measurement made immediately after electrode polarization; open symbols, 10 min after electrode polarization; half-open symbols, immediately after the electrodes were short-circuited and grounded.

Gouveia et al. The many observations described in the present work cannot be explained by surface conductance phenomena only, except for the faster decrease of silica surface potential under higher humidity. However, neither charge formation over silica, electrode potentials different from applied potentials, nor all other intriguing observations that were described in detail and explained by using adsorption-desorption phenomena are explained by surface conduction phenomena. A detailed quantitative model for the observed phenomena is desirable, but it depends on a number of hitherto unavailable adsorption and electrochemical parameters. Among these, the most important are those related to the partitioning of ions between the atmosphere and the solid surfaces. This is a topic that has been ignored in the literature: reference books on solid surfaces do not consider electrification of insulator solid-gas interfaces, and the extensive information available for neutral solid-liquid interfaces cannot be simply transferred to the present context. In this work, we privileged the role of adsorbed water. However, the adsorption of mobile gas ions which can be attracted toward biased electrodes cannot be excluded as they may result in the formation of charged adsorbates which impact the recorded surface potential. While the concentration of gas ions is known to be low compared with water soluble surface ions, the higher mobility of gas ions could partially compensate to yield a net flux of charged species toward biased electrodes. Whether or not gas ions play a role is left to be determined in the future. The authors hope that the present results will stimulate research in this area, and this is certainly necessary for a clear understanding of electrification in dielectrics. The need for new ideas in this area was recently demonstrated by McCarty and Whitesides,16 when they emphasized selective OH- adsorption as a new factor for ionic electret formation. A conflicting view was latter presented by Liu and Bard, and it seems that this debate will further heat up in the near future.52 The present observations lead to a practical conclusion: effective measures for countering unwanted electrostatic discharges should include the avoidance of sample charging events at high humidity followed by sample transfer to a low humidity environment. On the other hand, electrostatic charge build-up is very slow at low relative humidity, meaning that this condition is convenient for preserving silica and probably other dielectrics from electrostatic charging, at least under situations akin to the experiments described in this paper. From these results, we also learn that Kelvin probe microscopy should better be done under controlled RH, and the effects of atmospheric ion adsorption should be taken into account to achieve quantitative interpretation of measurements. These precautions are to be seen as a complement to recommendations made by Stemmer and colleagues.40,41 Conclusions

Figure 9. (a) Voltages applied to the electrodes and electric potentials measured over (b) the polarized electrodes and (c) grounded electrodes vs time, under 10% RH.

Water adsorption plays a decisive role in silica surface electrostatic charge build-up, beyond its widely acknowledged role on charge dissipation by conduction. Pattern formation and erasing are much faster at high relative humidity while the charged and discharged silica states are both more stable at low humidity. Thus, pattern preservation is effective under low humidity, but surface writing is best done under high humidity. Beyond their interest for writing on silica, these findings can lead to improved control of unwanted electrostatic discharges as well as to new applications of electrostatic patterning phenomena.

Water Vapor Adsorption Effect Acknowledgment. The authors thank Dr. Andre´ Galembeck (UFPE) for important suggestions concerning the interpretation of the experimental results and funding from FAPESP, Pronex/ Finep/MCT, and PADCT/CNPq. This is a contribution from the Millenium Institute for Complex Materials. Supporting Information Available: Photograph of the device used to measure charge on silica-on-wafer. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Schein, L. B. Science 2007, 316, 1572. (2) Schein, L. B. J. Electrost. 2007, 65, 613. (3) Crowley, J. M. Fundamentals of applied electrostatics; Laplacian Press: Morgan Hill, CA, 1999; p 40. (4) Taylor, D. M.; Secker, P. E. Industrial electrostatics: fundamentals and measurements; Research Studies Press: England, 1994; p 6. (5) Frenot, A.; Chronakis, I. S. Curr. Opin. Colloid Interface Sci. 2003, 8, 64. (6) Available at http://www.esdjournal.com. (7) Chang, J. I.; Cheng-Chung, L. J. Loss. PreV. Process Indust. 2006, 19, 51. (8) Bailey, A. G. J. Electrost. 2001, 51, 82. (9) Castle, G. S. P. J. Electrost. 1997, 40, 13. (10) Ne´meth, E.; Albrecht, V.; Schubert, G.; Simon, F. J. Electrost. 2003, 58, 3. (11) Masuda, H.; Yasuda, D.; Ema, A.; Tanoue, K. Kona 2004, 22, 168. (12) Davidson, J. L.; Williams, T. J; Bailey, A. G.; Hearn, G. L. J. Electrost. 2001, 51, 374. (13) Castle, G. S. P.; Schein, L. B. J. Electrost. 1995, 36, 165. (14) Chen, G.; Tanaka, Y.; Takada, T.; Zhong, L. IEEE Trans. Dielectr. Electr. Insul. 2004, 11, 113. (15) McCarty, L. S.; Winkleman, A.; Whitesides, G. M. J. Am. Chem. Soc. 2007, 129, 4075. (16) McCarty, L. S.; Whitesides, G. M. Angew. Chem., Int. Ed. 2008, 47, 2188. (17) Hogue, M. D.; Buhler, C. R.; Calle, C. I.; Matsuyama, T.; Luo, W.; Groop, E. E. J. Electrost. 2004, 61, 259. (18) Chen, G.; Tay, T. Y. G.; Davies, A. E.; Tanaka, Y.; Takada, T. IEEE Trans. Dielectr. Electr. Insul. 2001, 8, 867. (19) Bigarre´, J.; Hourquebie, P. J. Appl. Phys. 1999, 85, 7443. (20) Duff, N.; Lacks, D. J. J. Electrost. 2008, 66, 51. (21) Choi, K. S.; Yamaguma, M.; Ohsawa, A. Jpn. J. Appl. Phys. 2007, 46, 7861. (22) Park, A. A.; Fan, L. S. Chem. Eng. Sci. 2007, 62, 371. (23) Nonnenmcher, M.; O’Boyle, M. P.; Wickramasinghe, H. K. Appl. Phys. Lett. 1991, 58, 2921. (24) He, T.; Ding, H.; Peor, N.; Naama, P.; Lu, M.; Corley, D. A.; Chen, B.; Ofir, Y.; Gao, Y.; Yitzchaik, S.; Tour, J. M. J. Am. Chem. Soc. 2008, 130, 1699.

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