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Figure 2 Pit density as a function of pit depth on HOPG bombarded with 2.62 × 1010 ions cm-2 at various Cs+ ion energies and subsequently oxidized at...
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7632

J. Phys. Chem. B 2001, 105, 7632-7638

Nanometer-Size Monolayer and Multilayer Molecule Corrals on HOPG: A Depth-Resolved Mechanistic Study by STM Ying-Jie Zhu, Terra A. Hansen, Sven Ammermann,† Jennifer D. McBride, and Thomas P. Beebe, Jr.* Department of Chemistry, and Surface Analysis Facility, UniVersity of Utah, Salt Lake City, Utah 84112 ReceiVed: April 12, 2001; In Final Form: June 9, 2001

Surface defects on highly oriented pyrolytic graphite (HOPG) were controllably produced by bombardment with Cs+ ions at various incident kinetic energies ranging from 0.3 to 10 keV and at various dose densities. Defects in the HOPG created by Cs+ ion impacts were subsequently oxidized at 650 °C in air to produce nanometer-size monolayer and multilayer molecule corrals (pits). The controlled production of both monolayer and multilayer pits on HOPG bombarded with energetic Cs+ ions was realized and studied by scanning tunneling microscopy (STM). The pit density, pit yield, pit diameter, and pit depth can be well controlled by varying the experimental conditions. Multilayer pits can be controllably produced using Cs+ ion bombardment at higher kinetic energies, and monolayer pits can be produced using low-energy Cs+ ion bombardment. The presence of both monolayer and multilayer pits on the same HOPG samples makes the direct comparison of pit growth rates possible under exactly the same conditions. The measured depth-resolved pit growth rates for multilayer pits are in good agreement with a model of the pit growth rate “acceleration” by adjacent layers, and the separate contributions to the pit growth rate of surface diffusion and collision were extracted.

Introduction Highly oriented pyrolytic graphite (HOPG) is a twodimensional solid, with strong covalent bonding in carbon basal planes and only weak van der Waals interaction between the layers. HOPG is ideal for STM molecular imaging studies because it is a conducting and weakly interacting substrate. In recent years, this group has used etch pits produced by the oxidation of HOPG as “molecule corrals” for the study of the ordering phenomena of self-assembled molecular monolayers.1-5 We have used molecule corrals to confine molecule ensembles within small regions, and have studied molecular behavior and several important properties by statistical methods. Molecule corrals were also recently used in this group and by others as templates for the formation of metal and semiconductor nanostructures.6-8 The pits formed by the oxidation reaction on unbombarded HOPG are usually exclusively monolayer in depth, and the pit density is normally low (from less than 1.0 × 108 to 2.5 × 109 pits cm-2).9-11 The pit depth and pit density are difficult to control because they are determined by the intrinsic nature and history of individual HOPG samples. Multilayer pits are especially difficult to produce because line defects that cut through a number of graphite layers are usually needed.12 In the past, multilayer pits have been observed so infrequently that it has been difficult to collect statistically meaningful data on them. This fact makes the study of multilayer pits difficult, and little data have been available on them in the literature. Reported growth rates for multilayer pits range from equal to 100 times greater than monolayer pits. Evans13 reported that * Corresponding author address: Department of Chemistry, HEB Room 3416, University of Utah, 315 South, 1400 East, Salt Lake City, UT 84112. Phone: 801-581-5383. Fax: 801-581-8433. E-mail: [email protected]. † Visiting from Institute for Physical and Theoretical Chemistry, Technische Universita¨t Braunschweig, Braunschweig 38106, Germany

multilayer pits grow 100 times faster than monolayer pits at 0.01 Torr of O2 and 840 °C. Others have reported that multilayer pits grow faster than monolayer pits by factors of 4.3,9,10 20 (5 Torr),10 8 (700 °C, 150 Torr),14 3.4 (650 °C, 121 Torr)12 and “much larger and deeper than the monolayer etch pits.”11 Tracz15 reported that the growth rates of monolayer and multilayer pits were nearly the same at 670 °C and 150 Torr of O2. The discrepancy of these reported data may be due to many experimental variables including etch temperatures, pressures, and samples. The growth rates reported above were for all observed multilayer pits, and not for specific depth-resolved multilayer pits. A recent in-depth study by this group predicted the growth rate as a function of pit depth, based on a simple model of pit growth rate “acceleration” by adjacent layers.12 However, this model has not yet been experimentally tested. Ion bombardment is an effective method for inducing new defects in HOPG in a controlled fashion.16 Varying the ion dose density can control defect density, as well as the subsequent pit density. More importantly, the depth of etch pits can be controlled using ion bombardment of HOPG at various ion energies. Thermal oxidation of the ion-bombarded HOPG leads to the controlled formation of monolayer pits as well as multilayer pits.16-20 Although we used a higher grade (ZYB) of HOPG with relatively few intrinsic defects, we were able to image a large number of multilayer pits produced by Cs+ ion bombardment and to determine their depth-resolved growth rates during subsequent oxidation. By producing monolayer and multilayer pits on the same HOPG samples, the pit growth rate, pit density, and pit depth were studied under the same experimental conditions. This work will aid in the examination of theoretical models for the oxidation of the carbon basal planes on HOPG and improve our understanding of the mechanism of the graphite oxidation reaction.

10.1021/jp011377+ CCC: $20.00 © 2001 American Chemical Society Published on Web 07/20/2001

Surface Defects on HOPG

J. Phys. Chem. B, Vol. 105, No. 32, 2001 7633

Experimental Section HOPG samples, grade ZYB, were generously supplied by Dr. Arthur W. Moore of Union Carbide/Advanced Ceramics. The HOPG samples were cleaved with adhesive tape in ambient conditions. Immediately after cleaving, the HOPG samples were loaded into a TOF-SIMS ultrahigh vacuum (UHV) analysis chamber through the vacuum sample transfer system. The base pressure of the UHV analysis chamber was maintained at 10-10 mbar. To produce defects for molecule corrals, the HOPG samples were bombarded with a Cs+ ion beam generated from a Cs+ ion gun of a time-of-flight secondary ion mass spectrometry (TOF-SIMS) instrument (TOF-SIMS IV, ION-TOF, Mu¨nster, Germany). The Cs+ ion beam was rastered over a 1 × 1 mm2 sample surface area to achieve uniform ion exposure. High-resolution TOF-SIMS spectra were performed by a 25 keV monoisotopic 69Ga+ primary ion beam generated by a Ga+ ion gun on the same TOF-SIMS instrument. The details of TOF-SIMS measurements have been described elsewhere.16 The Cs+-ion-bombarded HOPG samples were then removed from the TOF-SIMS instrument, and thermally oxidized by heating the samples at 650 °C (as measured by a chromelalumel thermocouple) in a Lindberg Hevi-Duty tube furnace. HOPG samples were placed in the center of a 2.5-cm diameter × 60-cm long horizontal quartz tube. Both ends of the tube were left open, and no effort was made to cause or prevent air flow. The STM instrument used in these studies is custom built and has been described elsewhere.21 Typical scanning conditions used were - 0.60 V (bias voltage), 160 pA (tunneling current). Images were acquired in constant-current (topographic) mode, using mechanically cut tips of 80% Pt/20% Ir. All images were acquired at room temperature under ambient conditions. Results and Discussion Defects Induced by Cs+ Ion Bombardment. In this work, two-step experiments were performed to produce and study nanometer-size molecule corrals on HOPG. Defects for producing molecule corrals were first created by Cs+ bombardment at controlled energies and dose densities in UHV. The defects were subsequently oxidized at 650 °C in air to create nanometersize molecule corrals for STM study. The surface chemistry of HOPG bombarded with energetic Cs+ ions was studied using the combined surface analysis techniques of TOF-SIMS, XPS (X-ray photoelectron spectroscopy), and STM in this group. These results16 have been previously published and will not be discussed in this paper. The defects induced by Cs+ bombardment include vacancy defects as well as Cs interstitial defects.16,17 We have used STM to examine the defects induced by Cs+ ion bombardment of HOPG without thermal oxidation. STM results show that isolated protrusions separated by flat terraces form on Cs+ bombarded HOPG. These randomly distributed protrusions are defects induced by the Cs+ ion bombardment. Such protrusionlike features were not observed on the unbombarded HOPG surface. STM images similar to these obtained here have been reported in previous STM studies17 and are not presented here. Experimental results16,17 show that etch pits initiate both at the vacancy defects and at Cs interstitial defects created by Cs+ bombardment. To study the contribution from different defects to the formation of pits, we performed the following experiments. HOPG samples were bombarded with 1-keV Cs+ ions, followed by annealing of bombarded HOPG at 500 °C in UHV to remove implanted Cs+ ions. Heating of Cs+-bombarded HOPG was performed in UHV (∼10-9 mbar) in order to prevent

Figure 1. (a) STM image of HOPG bombarded with 1-keV Cs+ at a dose density of 2.62 × 1010 ions cm-2 and subsequently oxidized at 650 °C for 7 min in air. The field of view is 500 × 500 nm2. (b) Line profile of pits along line A from left to right in Figure 1a.

oxidation during heating. TOF-SIMS high-resolution mass spectra were acquired on HOPG to examine if Cs+ ions in HOPG were removed. No Cs+ ion peak was found in TOFSIMS spectra of Cs+ bombarded HOPG after heating in UHV, indicating all Cs+ ions were removed. After oxidation at 650 °C in air, STM analysis of HOPG samples shows that both pit density and pit yield decreased compared with the sample bombarded with 1-keV Cs+ ions at the same dose density and left on the surface during thermal oxidation. The decreases in pit density and pit yield are due to the removal of Cs+ ions in HOPG before oxidation. After removal of Cs+ ions from HOPG, pits form only from vacancy defects. A more detailed study is in progress and will be discussed in a forthcoming publication. STM Images of Molecule Corrals. Figure 1a shows a typical STM image of molecule corrals formed on HOPG bombarded with 1-keV Cs+ and subsequently oxidized at 650 °C for 7 min in air. Most of the observed pits were hexagonal in shape. Multilayer pits with depths up to six monolayers, as well as monolayer pits, were observed by STM on the same samples. The diameter of the monolayer and multilayer pits, which can be controlled by the etch time and etch temperature, ranged from 25 to 50 nm in Figure 1a. This diameter distribution for all pits is considerably broader than what is normally observed

7634 J. Phys. Chem. B, Vol. 105, No. 32, 2001 for monolayer pits that result from intrinsic defects,8 for reasons that will be discussed later. Figure 1b shows a line profile across selected pits (line A) in Figure 1a. Pit depth and pit diameter can be obtained from these line profiles. One can see that line A passes through three pits (from left to right: pit 1, pit 2, and pit 3). Pit 1 has a depth of two layers and a diameter of 38 nm. Pit 2 is a monolayer pit with a diameter of 29 nm. Pit 3 is two layers deep and 27 nm in diameter. Most pits are flat-bottomed with steep edges. Deeper, smaller pits that formed inside larger, shallower pits were also observed by STM, as seen in Figure 1a. However, they represent only a small fraction of all pits on HOPG. The oxidation also occurred at noncorral steps on HOPG, as observed by STM. Oxidation at steps (whether at corral edges or linear steps) is faster than on the terraces because carbon atoms at step edges are more reactive due to the presence of dangling bonds. Formation of Multilayer Pits. Intrinsic defects for producing multilayer pits, although very infrequent, presumably occur at line defects that cut through a number of graphite layers.12 While some intrinsic multilayer pits have been shown to occur at screw defects, others have not.12 Higher energy Cs+ ions penetrate deeper into HOPG and produce line defects that cut through more layers. Experimental results support that multilayer pits originate from line defects that cut through a number of graphite layers caused by the penetrating energetic Cs+ ions. If the defects induced by ion bombardment are not in-line defects (for example, if there are no defects in the upper layers just above Cs+ ions), these buried defect sites are not accessible to the thermal oxidation by oxygen. Therefore, STM pit depth measurements only give us information about the depth population profiles of defects that are accessible to thermal oxidation. In a few cases, smaller, deeper pits were formed at the center of larger, shallower pits, as observed by STM (see the center of Figure 1a). In these less frequent cases, oxidation at the defect in the nth layer started at a later time than that in the (n -1)th layer (layer-by-layer etching), essentially forming a pit within a pit. In contrast, the most commonly observed multilayer pits are flat-bottomed and have steep edges, such that the diameter of all layer edges of a pit is essentially the same. These multilayer pits form at line defects that cut through a number of graphite layers when etching in all n layers is initiated essentially simultaneously (simultaneous etching) and proceeds at the same rate in all layers, leading to the formation of steepedged and flat-bottomed multilayer pits. Because the etching process begins at the same time for a number of graphite layers, the etch rate will therefore be faster than for the case of monolayer pits. The explanations will be given below. Control of Pit Depth. The topographic and depth population profiles of defects that are accessible to thermal oxidation can be examined by performing statistical pit depth measurements (counting the numbers of pits with different depths) using STM. Here we present the results of STM statistical measurements of the pit depth distribution on HOPG bombarded with Cs+ ions at various kinetic energies and a fixed Cs+ dose density of 2.62 × 1010 cm-2, as shown in Figure 2. This dose density corresponds to an ion collision with approximately one of every 105 surface carbon atom sites. Figure 2 demonstrates that Cs+ ion kinetic energy has a significant effect on the depth of pits formed on HOPG. The pit depth can be controlled by adjusting the Cs+ ion kinetic energy. Increasing the Cs+ kinetic energy results in the formation of deeper pits in general. For example, monolayer pits were exclusively observed on low energy (80 eV) Cs+-bombarded HOPG.17 From Figure 2, one can see that

Zhu et al.

Figure 2. Pit density as a function of pit depth on HOPG bombarded with 2.62 × 1010 ions cm-2 at various Cs+ ion energies and subsequently oxidized at 650 °C for 7 min in air. The lines are not meant to imply detailed trends, but rather are meant to guide the eye to the general trend.

Figure 3. Weighted average pit depth davg as a function of Cs+ ion bombardment energy. The slope of a fitted line in the Cs+ energy range from 0.3 to 5 keV indicates an average increase of 0.42 ML in depth per keV. Error bars correspond to the standard deviation of pit depth distributions at each Cs+ kinetic energy. Number of pits measured: 2744.

for a Cs+ kinetic energy range from 0.3 to 3 keV, monolayer pits have the highest pit density, and that deeper pits become more probable as the ion energy increases. As the Cs+ kinetic energy increases from 3 to 5 keV, pits of the highest density change from monolayer pits to three-layer pits (Figure 2). The pit density also increases with increasing Cs+ ion energy for depth-resolved multilayer pits (depth g three monolayers). Pits as deep as 13 layers have been formed at a Cs+ kinetic energy of 10 keV. Figure 3 shows the weighted average pit depth (number of layers, ML) as a function of Cs+ kinetic energy. The weighted average pit depth davg at each kinetic energy was calculated using the equation n

davg )

diai ∑ i)1

where di is the pit depth (in number of layers, integers from 1

Surface Defects on HOPG

Figure 4. Pit density and pit yield as a function of Cs+ bombardment energy at a Cs+ dose density of 2.62 × 1010 ions cm-2. HOPG was oxidized at 650 °C in air for 7 min after Cs+ bombardment. (a) All pits; and (b) pits with various depths. The lines are not meant to imply detailed trends (i.e., trends are expected to be monotonic), but rather are meant to guide the eye.

to n), ai is the percentage of pits with a depth of di. These pits were produced by oxidation of HOPG at 650 °C in air for 7 min after Cs+ bombardment at a fixed Cs+ dose density of 2.62 × 1010 ions cm-2. One can see that in the Cs+ kinetic energy range from 0.3 to 5 keV, the weighted average pit depth increases with increasing Cs+ kinetic energy. The slope of a fitted line (by a linear least-squares fit) in this Cs+ kinetic energy range is (0.42 ( 0.06) ML per keV, indicating that the average increase in depth is 0.42 layers per keV. These results are in good agreement with the data reported by Bra¨uchle et al.20 who studied fullerene cluster ion bombarded HOPG. When the Cs+ kinetic energy was higher than 5 keV, the weighted average pit depth showed a tendency to level off with Cs+ ion energy. We have no pit depth data for higher Cs+ kinetic energies (>10 keV) due to the energy limitations of our Cs+ ion gun. Bra¨uchle et al.20 reported that the average pit depth increases linearly with C60+ kinetic energy up to 23 keV. Control of Pit Density and Pit Yield. In a previous paper,16 we showed that the pit density and pit yield could be well controlled by controlling the Cs+ dose density. The pit density increases with increasing Cs+ dose density. The pit yield is almost the same at about 0.5 pits per Cs+ impact and drops when the pit density is high. Here we show that Cs+ kinetic energy also has an effect on pit density and pit yield (Figure 4). At a fixed Cs+ dose density of 2.62 × 1010 ions cm-2 corresponding to less than 10-5 ML, both total pit density and total pit yield increased with increasing Cs+ kinetic energy ranging from 0.3 to 10 keV (Figure 4a). In the process of bombardment, Cs+ ion energy is transferred to HOPG carbon atoms via atomic collisions and a collision cascade is generated.22,23 Part of the energy is then transported back to the surface, allowing surface atoms to overcome the surface binding energy and to subsequently leave the surface and produce vacancy defects. Other types of etchable defects may also be

J. Phys. Chem. B, Vol. 105, No. 32, 2001 7635 produced. The energy transfer to the surface, and subsequent defect production, increases with increasing Cs+ bombardment energy in this energy range. It is therefore expected, and observed, that more defects are produced by bombardment with a higher Cs+ energy at a fixed Cs+ dose density, leading to an increase in both total pit density and total pit yield. By controlling both Cs+ kinetic energy and Cs+ dose density, the pit density and pit yield can be accurately controlled. Pit densities as high as about 5 × 1011 pits cm-2 can be produced on HOPG by this method. This is in contrast with the very low pit density on HOPG (from less than 1 × 108 to 2.5 × 109 pits cm-2) without ion bombardment.9-11 It should be noted that the pits will eventually overlap if the pit density and/or diameter are too large. On the other hand, the pit yield, which is a measure of how efficiently pits are produced, can be greatly increased by increasing Cs+ kinetic energy at a fixed Cs+ dose density. For example, the total pit yield increases from 0.28 to 0.81 when Cs+ kinetic energy increases from 0.3 to 10 keV at a fixed Cs+ dose density of 2.62 × 1010 cm-2. We know from previous studies16 that at a fixed Cs+ kinetic energy, the pit yield is almost constant below a certain Cs+ dose density and drops at higher Cs+ dose densities. To produce pits efficiently, the Cs+ dose density should therefore be low enough that the pits produced by thermal oxidation do not overlap with each other. Figure 4b shows the depth-resolved pit density and pit yield as a function of Cs+ kinetic energy at a fixed Cs+ dose density of 2.62 × 1010 cm-2. For monolayer pits, the pit density and pit yield tend to decrease with Cs+ bombardment energy, because higher energy Cs+ ions penetrate deeper into HOPG and create line defects that cut through more graphite layers, leading to the formation of deeper pits and fewer monolayer pits. At very low Cs+ kinetic energies (for example, 80 eV), only monolayer pits were produced.17 Increasing the Cs+ kinetic energy to 0.3 keV, 4.8% two-layer pits were formed. This percentage of two-layer pits increased to 38.8% at a Cs+ kinetic energy of 1 keV. For pits with a depth from three to seven layers, the pit density and pit yield increase with increasing Cs+ kinetic energy in the range from 0.3 to 10 keV. One can see that the three-layer pits have the highest pit density and pit yield in the Cs+ energy range of 5 to 10 keV. Growth Rates of Monolayer and Multilayer Pits. Controlling the oxidation time at elevated temperatures can control the size of monolayer pits.8,12,24 The growth rate for multilayer pits is defined here and elsewhere as the increase in pit diameter per unit time,12 even though the total number of carbon atoms removed per unit time is much larger for a deeper pit than for a shallower pit of the same diameter. From Figure 1a one can see that the diameters of monolayer pits are typically smaller than those of two-layer pits, and diameters of two-layer pits are smaller than those of three-layer pits, and so on. This was true in general for all samples. Figure 5 shows the size distributions of pits produced on HOPG bombarded with 10-keV Cs+ ions at a dose density of 2.62 × 1010 cm-2 and subsequently oxidized at 650 °C in air for 7 min. Figure 5a shows the size distribution for all pits, where diameters mostly ranged from 50 to 325 Å. The average diameter for all pits was (196 ( 20) Å. Monolayer pits have a much narrower size distribution, ranging mostly from 75 to 175 Å (Figure 5b) and with an average diameter of (141 ( 4) Å. From Figures 5b to 5e, one can see that the size distribution becomes wider and shifts to higher values for deeper pits. The average diameters for two-layer, three-layer, and four-layer pits are (162 ( 14), (188 ( 19), and (237 ( 31) Å, respectively, indicating a higher

7636 J. Phys. Chem. B, Vol. 105, No. 32, 2001

Figure 5. Size distribution of monolayer and multilayer etch pits on HOPG bombarded with 10-keV Cs+ ions at a dose density of 2.62 × 1010 ions cm-2 and subsequently oxidized at 650 °C for 7 min in air.

growth rate for deeper pits, since the reaction time was the same for all pits. Because multilayer pits and monolayer pits were formed on the same HOPG samples bombarded with energetic Cs+ ions and subsequently oxidized in air at 650 °C, this enabled us to measure and compare the depth-resolved growth rates for both monolayer and multilayer pits under the same conditions. The measured depth-resolved pit growth rate as a function of pit depth is shown in Figure 6. Figure 6a shows the pit growth rate data for two HOPG samples with different pit densities. The top curve in Figure 6a was obtained on HOPG with a lower pit density of 3.0 × 109 pits cm-2, which is close to that on unbombarded HOPG. One can see that the growth rate for monolayer pits is (32.9 ( 0.2) Å min-1, consistent with a recently reported value for monolayer pits on unbombarded HOPG.8 The depth-resolved growth rate for multilayer pits increases with increasing pit depth up to four or five layers. For pits deeper than four or five layers, the growth rate nearly levels off and is almost independent of pit depth. The maximum growth rate for n-layer pits (n g 5) is about two times of that of monolayer pits. The change of pit growth rate as a function of pit depth is in good agreement with the model of growth rate “acceleration” by adjacent layers.12 This model assumes that carbon oxides (CO and CO2) form from the reaction of carbon with adjacent chemisorbed oxygen atoms and explains why the growth rate of multilayer pits is faster than that of monolayer pits. For a monolayer pit, edge-carbon atoms can react only with adjacent oxygen atoms supplied from either side along the monolayer edge. For a multilayer pit, a carbon atom can react with adjacent oxygen atoms on either side along the same layer edge, as well as with oxygen atoms on adjacent layer edges above or below the layer edge in question. This increases the possibilities for the carbon oxidation reaction for a multilayer pit. This model also provides an explanation for why we observed a diminishing depth dependence on the growth rate for deeper pits (g4 or 5 layers in depth). Because a carbon atom reacts with oxygen atoms on the top or bottom adjacent layer edges, the presence

Zhu et al.

Figure 6. (a) Pit growth rate as a function of pit depth. Two HOPG samples with different pit densities were measured. Top curve: pit density 3.0 × 109 pits cm-2. Bottom curve: pit density 2.1 × 1010 pits cm-2. (b) Predicted growth rate as a function of pit depth on the basis of the model of growth rate “acceleration” by adjacent layers. Curve 1 (circles) is the experimentally measured data. Curve 2 (squares) and curve 3 (triangles) are predicted using xn and x1 in eq 1, respectively (see text).

or absence of additional edges beyond the nearest edge will have only a diminishing nth-order effect on the growth rate, aside from being a source for more oxygen atoms to “hop up” or “hop down” the carbon layer edges. The predicted growth rate based on the model of growth rate “acceleration” by adjacent layers, together with the experimental data, as a function of pit depth are shown in Figure 6b. We used the observed experimental pit growth rate on HOPG with a lower pit density (3.0 × 109 pits cm-2) because the pit density was close to that on unbombarded HOPG. Curve 1 in Figure 6b (circles) is the experimentally measured data, and curves 2 (squares) and 3 (triangles) are two predicted growth rates, to be explained below. Suppose the increase in growth rate of a multilayer pit is due to the reaction between sites on adjacent layer edges, where xn is the etch rate of a single nth-layer edge and y is the increase in etch rate caused by one adjacent layer edge. The average growth rate Rn of a pit n layers deep will thus be12

Rn ) xn +

2(n - 1)y n

(1)

For example, the growth rate of a monolayer pit in the topmost layer (n ) 1) is R1 ) x1, and the growth rate for a pit two layers deep (n ) 2) and for a pit three layers deep (n ) 3) is R2 ) x2 + y and R3 ) x3 + 4/3 y, respectively. Here, xn is the net etch rate of a deeper single-layer edge (n g 2), which is newly exposed to oxygen during thermal oxidation, and xn does not include the increase in etch rate caused by adjacent layers. x1 is the etch rate of a monolayer pit on the topmost layer exposed to oxygen before and during thermal oxidation. Therefore, xn * x1. This model is based on the fact that all layer edges of a multilayer pit etch at the same rate, as supported by the STM

Surface Defects on HOPG

J. Phys. Chem. B, Vol. 105, No. 32, 2001 7637

Figure 7. Schematic diagrams of the role of oxygen surface diffusion in etch rate of a monolayer edge. (a) Monolayer edge in the topmost layer. (b) Second single-layer edge.

results. The etch rate calculated by the rate eq 1 is for all layer edges of a multilayer pit. For any deeper nth single-layer edge (n g 2) of a multilayer pit, the etch rate is determined by both xn and the rate acceleration effect by adjacent layers. The etch rate is essentially the same for all deeper nth single layer edges (n g 2) of a multilayer pit. For the topmost layer edge of a multilayer pit, although it has only one adjacent layer (secondlayer edge), and therefore the rate acceleration effect is smaller than for deeper layer edges, oxygen surface diffusion across the terrace compensates for the decreased etch rate of the firstlayer edge. The validity of the rate eq 1 derived from this model is essentially not affected by whether oxygen atoms chemisorbed to HOPG layer edges are fixed or mobile. Whether or not adsorbed oxygen atoms diffuse up and/or down the layer edges, this effect is reflected by y in the rate eq 1 and determined experimentally. However, it is possible that adsorbed oxygen atoms at layer edges of a multilayer pit “hop up” and “hop down” the carbon layer edges, and this will help to explain the same etch rate (manifested by steep edges of multiplayer pits) for all layer edges of a multilayer pit. The effect of increased sticking probability of oxygen at layer edges of pits relative to across the terrace is reflected by both xn and y in the rate eq 1 and determined experimentally. Based on our measured growth rates of monolayer and multilayer pits, x1 is determined to be 32.9 Å min-1. As a starting point, if we incorrectly use x1 ) 32.9 Å min-1 to replace xn in eq 1, and then calculate the growth rate of multilayer pits, y is found to be 11.6 Å min-1. The predicted data according to these values are shown in curve 3 (triangles) in Figure 6b. One can see that (as expected, since x1 * xn) this predicted curve does not fit well with experimental data (curve 1, circles), and values lie at the bottom end of the 1σ (σ is the standard deviation) experimental values. The predicted values are smaller than experimental values if the pit depth is larger than two layers. This is because we used x1 to replace xn in eq 1, and in fact xn * x1. To properly fit the experimental data, xn, where n

g 2, should be used instead of x1 in eq 1. We found that by solving the rate eq 1 for n g 2, the growth rate of a single nth layer edge (xn) is 20.4 Å min-1. Using xn ) 20.4 Å min-1 in eq 1, the increase in growth rate caused by one adjacent layer edge, y, is 24.1 Å min-1 for HOPG with a pit density of 3.0 × 109 pits cm-2 (oxidized at 650 °C in air), and the predicted growth rates (curve 2, squares) are in good agreement with the experimental data, as shown in Figure 6b. Now we discuss the physical meanings of xn and x1. x1 is the growth rate for monolayer pits on the topmost graphite layer. xn is the etch rate of a single-layer edge in a deeper layer which is newly exposed (for example, the second layer edge), as shown in Figure 7b. In this case, the etching of a deeper, newly exposed single-layer edge results only from the collision of oxygen from the gas phase with edge-carbon atoms (xcollision), or from oxygen adsorbed on adjacent layer edges above and below, but not from oxygen surface diffusion on the topmost layer (xdiffusion). Note that xn only considers the etch rate for a deeper, single-layer edge, without taking into account the rate increase caused by adjacent layers (this effect is covered by y in eq 1). Therefore, xn ) xcollision. The difference between x1 and xn indicates that the etch rate of a monolayer edge in the topmost graphite layer is significantly different from that of a single-layer edge in a deeper layer. This underlines the importance of oxygen surface diffusion on the topmost terrace surface in the overall kinetics of graphite oxidation, as shown schematically in Figure 7. The involvement of surface diffusion of oxygen adatoms in the carbon oxidation reaction on a carbon surface has also been reported.25-28 For a monolayer edge in the topmost layer, the oxidation of edge carbon atoms results from two sources of oxygen. One source is the oxygen atoms arriving by surface diffusion across the topmost layer terrace (xdiffusion). Another source is the reaction of edge-carbon atoms with oxygen from the gas phase by collision (xcollision). We showed in a previous paper that the collision rate alone is not sufficient to explain the experimentally observed rates.12 Therefore, the overall etch rate for a monolayer

7638 J. Phys. Chem. B, Vol. 105, No. 32, 2001 edge in the topmost layer is x1 ) xdiffusion + xcollision. For a deeper single-layer edge (deeper than two layers), the etching essentially results only from the collision of oxygen from the gas phase with edge-carbon atoms. Oxygen surface diffusion from the relatively small area of basal plane inside the pit (newly exposed) is much smaller than that from the topmost layer terrace, as supported by the work from Yang et al.,25 and it is neglected here. Therefore, the etch rate for a deeper, singlelayer edge is xn ) xcollision. In the case discussed above, the etch rate of a deeper singlelayer edge (n g 2) without oxygen surface diffusion effects is xn ) 20.4 Å min-1. When oxygen surface diffusion is taken into account, the monolayer pit (in the topmost layer) growth rate increases to x1 ) 32.9 Å min-1. The difference between xn and x1 (12.5 Å min-1) is the fraction (38%) of the overall growth rate of monolayer pits in the topmost layer due to oxygen surface diffusion. Therefore, the independent effects of the two contributions (xdiffusion and xcollision) on the overall growth rate of monolayer pits in the topmost layer has been determined through depth-resolved rate measurements. The bottom curve in Figure 6a is the growth rate for pits on HOPG with a higher pit density (2.1 × 1010 pits cm-2). It has a similar trend in growth rate as for the case of lower pit density (upper curve); however, the growth rate is lower. One can see that the growth rate for both monolayer and multilayer pits is dependent on pit density on HOPG. This dependence of growth rate on pit density was also observed for monolayer pits by Yang et al.25 The growth rate for both monolayer and multilayer pits decreases with increasing pit density. This trend may be explained by competition for oxygen among different etch pits as well as in the same pit. The pit growth rate may be affected by the number of reactive oxygen atoms available per unit time per unit area at the reaction sites, and further insights into this mechanism will be forthcoming. Conclusions Nanometer-size monolayer and multilayer molecule corrals can be controllably produced on HOPG by controlling defect formation using Cs+ ion bombardment. Cs+ ion bombardment of HOPG induces defects that act as starting points for the oxidation of carbon atoms. The density and depth of molecule corrals can be well controlled by varying Cs+ dose density and Cs+ bombardment energy. Multilayer pits can be produced using Cs+ ion bombardment at higher energies, and monolayer pits can be produced using low-energy Cs+ ion bombardment. The pit growth rate is dependent on pit density if the pit density is high. The growth rate for both monolayer and multilayer pits decreases with increasing pit density. The growth rate of multilayer pits increases with increasing depth up to four or five layers, and then levels off and is essentially the same for all deeper pits. The pit growth rate as a function of pit depth is in good agreement with that predicted based on the model of growth rate “acceleration” by adjacent layers. The role of oxygen surface diffusion in the oxidation of HOPG is important for monolayer pits in the topmost layer. The etch rate of a single-layer edge is lower in a newly exposed deeper layer (n g 2), where the role of oxygen surface diffusion

Zhu et al. is not significant. For a monolayer edge in the topmost layer, the oxidation of edge carbon atoms results from both oxygen arriving by surface diffusion of adsorbed oxygen atoms from neighboring sites and by collision of oxygen molecules from the gas phase. For a deeper single-layer edge (deeper than two layers), the etching results only from the collision of oxygen from the gas phase with edge carbon, and oxygen terrace surface diffusion does not play a large role. At our experimental conditions, the contribution of oxygen surface diffusion is 38% of the overall growth rate of monolayer pits in the topmost layer, as determined by depth-resolved rate measurements by STM. Acknowledgment. Financial support from the National Science Foundation (CHE-9814477, DMR-9724307) and the Alfred P. Sloan Foundation is gratefully acknowledged. T.A.H. is an undergraduate scholar supported by the ACCESS program of the College of Science of the University of Utah. The authors thank Dr. Arthur Moore from Advanced Ceramics Corporation for his generous provision of HOPG samples. References and Notes (1) Patrick, D. L.; Cee, V. J.; Beebe, T. P., Jr. Science 1994, 256, 231234. (2) Patrick, D. L.; Cee, V. J.; Beebe T. P., Jr. J. Phys. Chem. 1996, 100, 8478-8481. (3) Schulze, J.; Stevens, F.; Beebe T. P., Jr. J. Phys. Chem. B 1998, 102 (27), 5298-5302. (4) Stevens, F.; Beebe T. P., Jr. Langmuir 1999, 15 (20), 6884-6889. (5) Patrick, D. L.; Cee, V. J.; Morse, M. D.; Beebe T. P., Jr. J. Phys. Chem. B 1999, 103 (39), 8328-8336. (6) Ho¨vel, H.; Becker, Th.; Bettac, A.; Reihl, B.; Tschudy, M.; Williams, E. J. J. Appl. Phys. 1997, 81 (1), 154-158. (7) Ho¨vel, H.; Becker, Th.; Bettac, A.; Reihl, B.; Tschudy, M.; Williams, E. J. Appl. Surf. Sci. 1997, 115, 124-127. (8) McBride, J. D.; Van Tassell, B.; Jachmann, R. C.; Beebe, T. P., Jr. J. Phys. Chem. B 2001, 105, 3972-3980. (9) Chu, X.; Schmidt, L. D. Carbon 1991, 29, 1251-1255. (10) Chu, X.; Schmidt, L. D. Surf. Sci. 1992, 268, 325-332. (11) Chang, H.; Bard, A. J. J. Am. Chem. Soc. 1991, 113, 5588-5596. (12) Stevens, F.; Kolodny, L. A.; Beebe, T. P., Jr. J. Phys. Chem. B 1998, 102, 10799-10804. (13) Evans, E. L.; Griffiths, R. J. M.; Thomas, J. M. Science 1971, 171, 174-175. (14) Wong, C.; Yang, R. T. Carbon 1982, 20, 253-254. (15) Tracz, A.; Wegner, G.; Rabe, J. P. Langmuir 1993, 9 (11), 30333038. (16) Zhu, Y. J.; McBride, J. D.; Hansen, T. A.; Beebe, T. P., Jr. J. Phys. Chem. B 2001, 105, 2010-2018. (17) Hahn, J. R. Surf. Sci. 1999, 423, L216-L221. (18) Bra¨uchle, G.; Richard-Schneider, S.; Illig, D.; Rockenberger, J.; Beck, R. D.; Kappes, M. M. Appl. Phys. Lett. 1995, 67 (1), 52-54. (19) Reimann, C. T.; Sullivan, P. A.; Tu¨rpitz, A.; Altmann, S.; Quist, A. P.; Bergman, A.; Oscarsson, S. O.; Sundqvist, B. U. R.; Hakansson, P. Surf. Sci. 1995, 341, L1019-L1024. (20) Bra¨uchle, G.; Richard-Schneider, S.; Illig, D.; Beck, R. D.; Schreiber, H.; Kappes, M. M. Nucl. Instrum. Methods B 1996, 112, 105108. (21) Zeglinski, D. M.; Ogletree, D. F.; Beebe, T. P., Jr.; Hwang, R. Q.; Somorjai, G. A.; Salmeron, M. B. ReV. Sci. Instrum. 1990, 61, 3769-3774. (22) Sigmund, P. Phys. ReV. 1969, 184, 383-416. (23) Sigmund, P. Phys. ReV. 1969, 187, 768. (24) Chang, H.; Bard, A. J. J. Am. Chem. Soc. 1990, 112, 4598-4599. (25) Yang, R. T.; Wong, C. J. Chem. Phys. 1981, 75 (9), 4471-4476. (26) Olander, D. R.; Siekhaus, W.; Jones, R.; Schwarz, J. A. J. Chem. Phys. 1972, 57, 408-420. (27) Marsh, H.; Foord, A. D. Carbon 1973, 11, 421-424. (28) Yang, R. T.; Wong, C. J. Phys. Chem. 1980, 84, 678-679.