Electrostatic Patterning of a Silica Surface: A New Model for Charge

The polarization of interdigitated gold electrodes mounted over a silica thin film formed by oxidation of a Si wafer produces reproducible electrostat...
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J. Phys. Chem. B 2005, 109, 4631-4637

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Electrostatic Patterning of a Silica Surface: A New Model for Charge Build-Up on a Dielectric Solid Rubia F. Gouveia, Carlos A. R. Costa, and Fernando Galembeck* Institute of Chemistry, UniVersidade Estadual de CampinassUNICAMP, P.O. Box 6154, CEP 13084-971, Campinas - SP, Brazil ReceiVed: September 17, 2004; In Final Form: December 20, 2004

The polarization of interdigitated gold electrodes mounted over a silica thin film formed by oxidation of a Si wafer produces reproducible electrostatic patterns with overall excess negative charge, as observed by scanning electric potential microscopy. Domain charge concentrations as high as 76 charge units per square micrometer are obtained when a 5 V difference is applied to the electrodes thus producing fields in the 106 V m-1 range. These patterns vanish when the electrodes are short-circuited and grounded. Characteristic times for pattern formation and relaxation are in the order of 10 min. The results are consistent with a model based on the discharge of H+ ions at the negative electrodes, leaving behind immobile surface-bound SiO- groups and thus showing that chemisorption phenomena are decisive for electrostatic charging of insulators.

Introduction Electrostatic phenomena were discovered 25 centuries1 ago, they were intensively studied since the 16th century, and they are widely used in many important technologies, such as photocopying, electrostatic painting and electrospinning.2 However, these phenomena are still the source of many practical problems, including serious recent industrial accidents, for a very simple reason: electrostatic charging of insulators is poorly known and it often goes out of control because the charge carriers are not known, in nearly every case. This undesirable situation is well documented in the literature,3-6 and it poses a serious challenge: how can charge carriers in insulators be detected, identified, and quantified? This question has been addressed recently, and new proposals have been put forward by different authors.7-11 However, a persistent problem is the difficulty to produce repeatable and previsible electric potential patterns5 and this cannot be solved unless the underlying transport phenomena and chemical reactions are well understood. A great help for the study of electrostatic patterns on dielectrics and other solids has been the introdution of Kelvin scanning electrostatic voltmeters with various degrees of spatial resolution, because they allow the detection of electrostatic patterns adjacent to an insulator surface.12-14 The systems based on Kelvin force scanning microscopy (KFSM) or scanning electric potential microscopy (SEPM) are specially useful because they allow measurements with a 10-nm resolution that is close to the macromolecular or nanoparticle size. However, the majority of published work using these techniques has been done on semiconductors or metals. This laboratory used this technique for the examination of dielectric solids, specially those made out of colloid polymers or latexes.15-18 The association between SEPM and analytical electron microscopy (electron energy-loss spectroscopy in the transmission electron microscope, EELS-TEM) allowed the unequivocal identification of * To whom correspondence should be addressed. Phone: +55-19-3788-3080. Fax: +55-19-3788-3023. E-mail: fernagal@ iqm.unicamp.br.

polymer charge carriers with ionic constituents such as K+ and RSO4- ions introduced during the polymerization process.19,20 However, charge patterns were also identified in many thermoplastics that do not contain ionic constituents introduced during the synthesis, thus showing that other types of charge carriers should be considered. Theoretical results show important effects of various gas adsorbates and adsorption of water on the charge patterns of carbon nanotubes, but similar work has not yet been performed on dielectric solids.21 Calibration of KFSM or SEPM microscopes is done using arrays of interdigitating electrode stripes on top of a silica layer formed by limited oxidation of a silicon wafer. Although calibration samples were examined in the microscope, a number of observations showed the existence and mobility of electrical charges on the insulating silica. An extensive literature search showed that these observations are unprecedented and they could yield completely new information on electrostatic charging of dieletrics. They were thus repeated systematically producing reproducible charge patterns on silica, that are reported in this paper. Experimental Section Sample Preparation. Samples were prepared in the microelectronics facilities in this university (CCS). Si wafers were oxidized in a furnace at 1000 °C, when a thin silica layer was obtained. This was covered with interdigitated TiO2 stripes that were in turn coated with a thin gold film, using microlithography techniques (TiO2 is used to improve gold adhesion to silica). The interdigitated gold stripes were then connected to wires for external connection, as shown in Figure 1. Atomic Force (AFM) and Scanning Electric Potential Microscopy (SEPM). AFM and SEPM experiments were performed simultaneously in a Discoverer TMX 2010 (TopoMetrix) instrument, and images of the two types were obtained from the same area, at the same time. The noncontact AFM mode was used to obtain topographic information on the silica surfaces covered with gold stripes. The SEPM technique uses the standard noncontact AFM setup, but the sample is scanned with Pt-coated Si tips with a

10.1021/jp0457601 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/15/2005

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Figure 1. Optical micrograph 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 the right and left of the figure.

20 nm nominal radius. An AC signal is fed 10 kHz below the frequency of the normal AFM oscillator, which matches the natural frequency of mechanical oscillation of the cantilevertip system (40-70 kHz). During a measurement, the mechanical oscillation of the tip is tracked by the four-quadrant photodetector and analyzed by two feedback loops. The first loop is used in the conventional way to control the distance between tip and sample surface, while scanning the sample at constant oscillation amplitude. The second loop is used to minimize the electric field between tip and sample, as follows: the second lock-in amplifier measures the tip vibration at the AC frequency oscillation while scanning, and adds a DC bias to the tip, to cancel the phase displacement in the AC oscillation. The principle is analogous to the Kelvin method, except that forces are measured instead of current.12-14 The image is built using the DC voltage fed to the tip, at every pixel, thus detecting electric potential gradients throughout the scanned area. Image processing was performed in a PC microcomputer using the TopoMetrix software. Electrode Polarization. AFM and SEPM images were acquired simultaneously while one set of electrodes was grounded and the other was biased at -5 V. Image acquisition started immediately after electrode polarization and each image took ca. 10 min. Thus, scanning for 50 min produced five successive images. All electrodes were then grounded (and shortcircuited) when a new set of four consecutive AFM and SEPM images was acquired. The whole procedure was then repeated but biasing one electrode set at +5V, acquiring five images, grounding the electrodes and acquiring other four images. Model Calculations. The electric potential created at any point close to an electrostatically patterned surface can be calculated, using the principle of superposition.22 Electric potential maps were then calculated for given (or else, assumed) electric charge distributions over the silica, using virtual objects carrying definite numbers of charges per pixel that were created using MS Excel. The potential created at any pixel adjacent to the object was calculated using a code written in C++, thus producing an electrostatic potential map that can be compared to readings extracted from the experimental SEPM micrographs. Results When the sample electrodes are connected to a power supply and imaged, the results obtained are as shown in Figure 2. The AFM image shows the uniform metal stripes (bright) on the silica surface (dark). Many sample defects are also observed but these were very helpful in ascertaining that the same area was been examined in every scan. A number of interesting observations are made from the successive SEPM images from the same area. First, one set of electrodes acquires a positive potential relative to the other set, as expected. The electrodes are arranged as a parallel-plate capacitor, with very thin plates and the potential

Figure 2. AFM (A) and SEPM (B) images from the same area of a silica surface covered with an array of parallel gold electrodes. Electrodes are alternately biased at -5 and 0 V (grounded), positive electrodes appear as bright stripes while the negative electrodes are dark. Image acquisition started immediately after biasing and five consecutive SEPM images were acquired, out of which the image shown was the first.

Figure 3. Line-scans from the five consecutive SEPM images acquired under electrode bias. The scanned lines are 10 microns below the top of each image and the times given are those elapsed from the moment of electrode polarization to the start of the scanned line.

gradients between each successive pair of electrodes should be established instantly. However, the electric potential changes slowly during at least 1 h, as seen in the electric potential vs position plots (line-scans) shown in Figure 3. Each line-scan was taken 10 microns beneath the top of the respective SEPM image, and they show a number of interesting features. First, the electric potentials measured at each pixel over the electrode stripes are not uniform, showing that they

Electrostatic Patterning of a Silica Surface are affected also by neighbor charged domains. On the other hand, the potentials at points 10 nm (that is a normal height for SEPM) above the silica surface change slowly: they form initially a shoulder at negative potentials but at voltages closer to the grounded electrodes potential than to the negatively biased electrodes, changing later into quasilinear gradients between each pair of electrodes. Silica domains adjacent to each negative electrode become negative, relative to the domains adjacent to the grounded electrodes. The potentials measured over the metal stripes are not those expected, since the figures obtained are ca. -0.7 and -3.3 V instead of 0 and -5 V. To check the accuracy of voltage measurements, -5 V was applied to both sets of electrodes while keeping the supporting sample metal base connected to ground; in this case, the potentials measured at any point over the sample surface were (-5 ( 0.1) V, showing that the observed differences between the voltages applied to the interdigitated electrodes and those read over them are not the result of poor calibration or power supply malfunctioning. Indeed, they are the result of the interplay between fixed charges on silica and the potential fed to the electrodes. Calibration made using other fixed potentials confirmed this result. On the other hand, the slow change of the initial electric potential profile producing a linear gradient shows that there is a slow change in the charge distribution across the silica film that may be due to one or both factors: dipole orientation in the silica or charge migration in the silica surface or bulk. Assuming that charge migration is the main factor, the mobility of these charges can be estimated observing that they are displaced for a few microns within some tens of minutes, typically 2 × 10-7 cm s-1. Considering that this motion takes place under a 5V/5 micrometer field or 1 × 104 V cm-1, the charge carrier mobility is exceedingly slow, in the 10-11 cm2/V s range, which is orders of magnitude lower than electron or hole mobility in silicon semiconductor, respectively 1.4 × 103 and 5 × 102 cm2/V s23 or ionic mobility in aqueous solutions, H+ ) 3.25 × 10-3, OH- ) 1.76 × 10-3, Al3+ ) 4.1 × 10-4, F- ) 4.8 × 10-4 cm2/V s.24 On the other hand, it is 1 order of magnitude higher than carrier surface mobility estimated for PTFE and polystyrene (10-12 cm2/V s) and 5 orders lower than in the case of PET (10-6 cm2/V s).25 The potential gradients in Figure 3 always change monotonically between the positive and negative electrodes thus giving no indication for the formation of spatial structures such as an electric double layer. This means that ions moving toward oppositely charged electrodes are discharged, since they do not accumulate at the electrode vicinity. Figure 3 also shows small but consistent features of the potential distribution across each electrode. Potentials measured over the electrodes are expected to be very uniform since the electrodes are gold stripes, but these potentials are not completely uniform: the corresponding lines are curved upward over the negative electrodes and they are flatter over the positive electrodes. Consequently, the negative potential is extreme at the negative electrode centers. Moreover, the potentials read over the metal stripes decrease slightly with time: this is a regular trend toward lower potentials over the positive (grounded) electrodes but not so much at the negative (-5 V) electrodes. Immediately after acquiring the last image in Figure 2, the two sets of electrodes were connected and grounded. Then, a new set of consecutive images was acquired, and these are shown in Figure 4. Image contrast is observed but it is now more marked within the silica surface than over the electrodes. Line-scans were

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Figure 4. AFM (A) and SEPM (B) images from the same area of a silica surface with all electrodes grounded. Acquisition of these images started immediately after short-circuiting and grounding the electrodes. Three other consecutive SEPM images were recorded from the same area (not shown). Note the decrease in the SEPM (but not AFM) contrast from top to bottom.

Figure 5. Line-scans from the four consecutive SEPM images acquired when electrodes are short-circuited and grounded. The scanned lines are 10 microns below the top of each image and the times given are those elapsed from the moment of electrode grounding to the start of the scanned line.

measured from the SEPM images, and they are presented in Figure 5, showing a significant electric potential contrast between domains collinear with the metal stripes. Electric potential gradients as large as 1.9 × 105 V m-1 are observed even after the electrodes are short-circuited. Another observation is related to the difference in the potential patterns in the electrode vicinity: the electrodes that were initially grounded and thus positive in the biasing experiment (shown in Figure 2) are those surrounded by silica domains with

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Figure 7. Line-scans from the five consecutive SEPM images acquired under +5V - 0V electrode bias. The scanned lines are 10 microns below the top of each image and the times given are those elapsed from the moment of electrode polarization to the start of the scanned line.

Figure 6. AFM (A) and SEPM (B) images from the same area of a silica surface covered with an array of parallel gold electrodes. Electrodes are alternately biased at +5V and 0V (grounded), positive electrodes appear as bright stripes while the negative electrodes are dark. Image acquisition started immediately after biasing and five consecutive SEPM images were acquired, out of which the image shown was the first.

the highest positive potentials in Figure 4, and vice-versa. On the other hand, the negative domains in Figures 4 and 5 show a greater departure from the ground potential than the positive domains. This pattern changes qualitatively with time: the silica domains adjacent to the electrodes all appear positive, in Figure 4B and in the curves in Figure 5. This is also observed in the change of the symmetry of the V-shaped valleys in Figure 5: in the first scan after grounding, the Vs are strongly asymmetrical but they become more symmetrical in later plots. The depths of the negative peaks in Figure 5 decay faster than the positive peaks reaching a situation in which every metal stripe is adjacent only to positive silica domains while the negative domains persist in the central parts of the insulator stripes, only. Nevertheless, the silica negative domains still display more pronounced potentials than the positive domains, at the last recorded image. After allowing the electric potentials to decay for 40 min, the electrodes were polarized again but one set was grounded and the other set was biased at +5 V. Consecutive images were then acquired and are shown in Figure 6, whereas the corresponding line-scans are in Figure 7. Comparison of these figures with Figures 2 and 3 shows changes in the electric potential in the silica insulator, symmetrical to those observed when one electrode set was biased at -5 V: the differences between the (relative) positive and negative electrodes are as in Figure 3 as well as the slow disappearance of the shoulders observed in the first scan, soon after biasing the electrodes.

Figure 8. AFM (A) and SEPM (B) images from the same area of a silica surface with all electrodes grounded. Acquisition of these images started immediately after short-circuiting and grounding the electrodes. Three other consecutive SEPM images were recorded from the same area (not shown). Note the decrease in the SEPM (but not AFM) contrast from top to bottom.

The electrodes were then short-circuited and grounded, and a new set of images was acquired, as shown in Figure 8. The corresponding line-scans are in Figure 9, showing features analogous to the observations previously made in Figures 4 and 5. The potential data in Figures 5 and 9 can also yield the time evolution of the electric potentials measured on different pixels over the silica insulator. Figure 10 shows results from pixels in

Electrostatic Patterning of a Silica Surface

Figure 9. Line-scans from the four consecutive SEPM images acquired when electrodes are short-circuited and grounded. The scanned lines are 10 microns below the top of each image and the times given are those elapsed from the moment of electrode grounding to the start of the scanned line.

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Figure 11. Potential vs distance along a line on the sample (dotted line, calculated; solid line, experimental curve from Figure 5).

Figure 12. Potential vs distance along a line on the sample (dotted line, calculated; solid line, experimental curve from Figure 9).

Figure 10. Electric potentials measured over the silica insulator. (a) Data from Figure 5; (b) Data from Figure 9. Circles: points adjacent to positive electrode; triangles: points adjacent to negative electrode; squares: points in the central area of the insulator stripes.

three different positions. In every point over the sample, the electric potential increases toward slightly positive values. Calculated Potential Patterns. To understand the potential vs distance curves over the silica stripes shown in Figures 5 and 9, the electric potential maps generated by charge distributions over a model surface were calculated. In these calculations, the number of charges per pixel was adjusted by trial and error

to reproduce the main features of the experimental curves. The results are shown in Figures 11 and 12. The charge patterns producing the calculated curves shown in Figures 11 and 12 were built as follows: a square was divided in 100 × 100 pixels. All of the 100 pixels within a column contain the same number of excess charges, and consequently, all of the 100 lines are identical. The number of charges in each pixel of a line leading to the calculated curve in Figure 11 is the following (from left to right): 0.7, 0.8, 0.8, 0.9, 1, -3.8, -3, -2.5, -1.5, -1, -0.5, 0, 0.5, 1, 1.5, 0.7, 0.6, 0.6, 0.6, 0.5, 0.5, 0.6, 0.6, 0.6, 0.7, 1.5, 1, 0.5, 0, -0.5, -1, -1.5, -2.5, -3, -3.8, 1, 0.9, 0.8, 0.8, 0.7, 0.7, 0.8, 0.8, 0.9, 1, -3.8, -3, -2.5, -1.5, -1, -0.5, 0, 0.5, 1, 1.5, 0.7, 0.6, 0.6, 0.6, 0.5, 0.5, 0.6, 0.6, 0.6, 0.7, 1.5, 1, 0.5, 0, -0.5, -1, -1.5, -2.5, -3, -3.8, 1, 0.9, 0.8, 0.8, 0.7, 0.7, 0.8, 0.8, 0.9, 1, -3.8, -3, -2.5, -1.5, -1, -0.5, 0, 0.5, 1, 1.5, 0.7, 0.6, 0.6, 0.6, and 0.5. This means the pixels over the more positive gold stripe contain respectively 1, 0.9 0.8, 0.8, 0.7, 0.7, 0.8, 0.8, 0.9, and 1 charges; pixels over silica contain -3.8, -3, -2.5, -1.5, -1, -0.5, 0, 0.5, 1, and 1.5, and the pixels over the less positive electrodes contain 0.7, 0.6, 0.6, 0.6, 0.5, 0.5, 0.6, 0.6, 0.6, and 0.7 charges. For grater clarity, each line can be represented as a succession of domains, each divided into 10 pixels:

Au I, SiO2 I, Au II, SiO2 II, Au I, SiO2 I, Au II, SiO2 II, Au I, SiO2 I

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Figure 13. AFM micrograph acquired from the border of an electrode at higher magnification. The square shows one of the areas used for measurements of silica surface roughness.

The number of excess charges per pixel in each domain leading to the calculated curve in Figure 12 is the following:

Au I: 0.95, 0.9, 0.9, 0.9, 0.8, 0.8, 0.9, 0.9, 0.9, 0.95 SiO2 I: 0.8, 0.5, 0.3, 0, -1, -1.5, -2, -3, -4, -5.5 Au II: 1.25, 1.2, 1.1, 1.1, 1, 1, 1.1, 1.1, 1.2, 1.25 SiO2 II: -5.5, -4, -3, -2, -1.5, -1, 0, 0.3, 0.5, 0.8 Thus, the charge maps leading to the observed electric potential maps are similar but they show some significant numerical differences. The elucidation of these differences will probably require much longer cyclic experiments under rigorously controlled conditions that we expect to be doing in this laboratory during the next months. To verify if silica surface charging produces detectabletopography changes, AFM micrographs were acquired from the same sample used in this work but at a higher magnification, as shown in Figure 13. This figure shows that gold electrode borders have rough features as large as many tens of nanometers. On the other hand, silica surface rugosity measurements yield root mean squares (rms) roughness in the 1.4-5 nm range, as determined in many small (ca. 0.25 µm2) chosen areas. This roughness is much larger than the height difference resulting from the removal or introduction of any likely charge carriers (e.g., H+, see the Discussion) from the silica surface. Discussion Applying a potential difference between the electrode sets laid over a silica surface produces complex and slow-decaying, reproducible electric potential patterns on these surfaces and the relaxation of these patterns can be followed for more than half-hour. A remarkable feature of the patterns is the predominance of negative domains, showing that electrode biasing leads to the consumption of positive charge carriers to a greater extent than the negative charge carriers. This may be explained following a simple model: it is wellknown that the silica surface under air adsorbs water vapor forming silanol groups that are Bro¨nsted acids.26 When the electrodes are biased, protons from silanol groups are discharged due to electron injection from the negative electrode forming H2 and leaving behind immobile negative SiO- groups. The insulator surface thus acquires a net negative charge that causes

Gouveia et al. a decrease in the potential measured in the vicinity of all metal stripes, seen in Figures 3 and 7 (polarized electrodes), and an increase observed in Figures 5 and 9 (short-circuited electrodes). There is not a corresponding extensive reaction at the positive electrodes and electrons or other negative charges are thus not withdrawn from the silica surface, at least to the same scale as protons. This model has some similarities with that recently proposed by Eder and Kramer for ionic conduction on titania surfaces.27 When the metal stripes are all short-circuited, the dominating negative charges in the silica surface disappear slowly, due to SiO- protonation concurrent to electron transfer from adsorbed water molecules to gold electrodes. An independent observation in support of electron abstraction from water by grounded gold electrodes is the following: all of the insulator surfaces tend toward slightly positive values in the 0.05-0.l5 V range, whenever they are allowed to equilibrate for long periods. This was observed not only when the electrode pairs were biased and then grounded as in the figures shown but also when all of the electrodes were biased to +5 and -5 V while the metal base of the sample holder was grounded: the insulator then showed a positive potential in the 0.05-0.15 range relative to the ground. The disappearance of the shoulders in the electric potential curves measured over the silica surface in Figures 3 and 7 also demonstrates that the charge distribution over the insulator changes when the electrodes are polarized and positive charges are eliminated during electrode biasing. Charge patterns are thus formed over the silica insulator and charge surface concentrations can be calculated, as shown in Figures 11 and 12, assuming that the observed potentials in the grounded samples are all due to charges located over the surface plane. This does not provide any information on the nature of the charge carriers but it is certainly a fundamental step toward planning further analytical experiments aimed at elucidating the nature of these charges, since their concentration is now known. The surface charge concentrations in the charge distribution maps in Figures 11 and 12 are -2.1 × 103 and -5.3 × 103 charge units per 70 square micrometers. This means that the total number of charge carriers in 1 µm2 is -30 and -76 charge units, respectively, and the speciation of such small concentrations of charge carriers is certainly a big challenge, especially because the charge carriers considered under the present model are very simple species. The low concentration of charge carriers coupled to the intrinsic roughness of the silica surface has prevented direct observation of adsorbed species by AFM, so far. However, this may be eventually achieved but using an atomically smooth silica film surface. In principle, we should also consider an alternative explanation for these results, based on using the work functions of gold and silica. This alternative can be eliminated, at least as the dominating polarization factor, due to the similarity of the results obtained in this work using opposite electrode bias (+5 or -5 V vs a grounded electrode). On the other hand, the model used in this work may help to understand why the patterns of charge transfer in a metal-insulator interface are still a matter of open debate, as in the case of charging at the gold-Teflon interface:5 the role of adsorbed species is usually neglected but they seem to have paramount importance. Moreover, if we consider that silica in contact with grounded gold electrodes becomes positive due to the abstraction of silica electrons by gold, this does not explain why the inner part of the silica stripes remains pronouncedly negative when the

Electrostatic Patterning of a Silica Surface electrodes are short-circuited after being polarized, while the insulator adjacent to the metal becomes positive. These results bring some answers to important and persistent questions in the literature: first, we can say for sure that charge carriers in the silica surface are not electrons, since the domains with excess charges are not short-circuited by the gold electrodes. Charge carriers are thus ions formed at the silica surface and perhaps also within the silica bulk or silica layers at the gold-silica interface. To sum up, this work describes a new approach to the study of the electrification of insulator surfaces that can be applied to many other cases. On the other hand, it points toward a new mechanism for the participation of chemisorption phenomena in electrostatic charging. This means that each surface behaves in its own special way due to its chemical composition features, surrounding atmosphere and other factors adding to the complexity of insulator electrostatic behavior, especially the recent history of the surface. Conclusions Reproducible electrostatic patterns are obtained on the surface of silica-on-wafer thin films and their formation/relaxation behavior is interpreted using well-known surface electrochemistry phenomena. This shows that the current challenge of reproducing and understanding electrostatic behavior of insulators may be overcome by considering the participation of chemisorption of atmospheric substances in electrostatic charging. Acknowledgment. The authors thank FAPESP, Pronex/ Finep/MCT and PADCT/CNPq. This is a contribution from the Millenium Institute for Complex Materials. The C++ code was written by R. Galembeck. References and Notes (1) Bailey, A. G. J. Electrost. 2001, 51, 82.

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