Directed Long-Range Migratory Reaction of Benzene on Si(100) - The

Oct 10, 2011 - Electron impact on chemisorbed benzene at Si(100) is shown by scanning tunneling microscopy to cause long-range molecular recoil in the...
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Directed Long-Range Migratory Reaction of Benzene on Si(100) Krishnan R. Harikumar, John C. Polanyi,* and Amir Zabet-Khosousi Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada, M5S3H6 ABSTRACT: Electron impact on chemisorbed benzene at Si(100) is shown by scanning tunneling microscopy to cause long-range molecular recoil in the plane of the surface, followed by chemisorptive reattachment at a substantial distance (on average, 48 Å) from the originating event. Several indicators—directionality of migration, persistance of motion over obstacles, and insignificant probability of desorption—all point to a rolling mechanism in which the recoiling molecule cartwheels across the surface in a physisorbed state. In distinction to our previous report of long-range migration of alkenes on the same surface [Nat. Chem. 2011, 3, 400 408], in the present case, the benzene molecules migrate substantially further and favor motion along the dimer rows rather than across them. Ab initio theory provides a qualitive model that satisfactorily explains these experimental observations.

’ INTRODUCTION Excitation of molecules at a surface can induce molecular motion in three possible directions: (1) toward the surface, as in “localized atomic reaction” (LAR);1,2 (2) away from the surface, as in “desorption induced by electronic transitions” (DIET);3,4 or (3) parallel to the surface, as in surface migration.5 8 The first two categories have, so far, dominated the study of surfacereaction dynamics. Surface migration could, however, prove to be of considerable interest as a means of inducing reaction at a distance from the point of excitation. This requires a mechanism that can couple to a lateral coordinate, thereby inducing motion across the surface.9 An example of such mode-coupling has been recently demonstrated in the case of laser-induced surface diffusion of chemisorbed CO on Pt(533),8 where rocking motion (hindered rotation) preceded lateral translation, and diffusion occurred via short-range molecular hopping of less than 10 Å, since the hindered rotation was never fully released. Recently, we showed that molecular cartwheeling induced by surface reaction can lead to long-range in-plane recoil of alkene molecules across the rough surface of Si(100).10 The recoil distance averaged 29 Å and occurred preferentially across the dimer rows of Si(100). This was in clear constrast to surface diffusion, which is short-range (averaging 5 Å) and random in direction.11 From experiment and theory, we showed that cartwheeling rotation in a mobile physisorbed state, followed by chemisorptive reattachment, constituted the most likely explanation of the observed long-range migration. Evidence for cartwheeling was provided by the observation of molecular tumbling in the case of propene molecules, and by the observation of persistent and directed motion over surface obstacles, such as defects and raised steps. Here, we extend our earlier findings to a new adsorbate, benzene, and present a further striking example of the phenomenon of r 2011 American Chemical Society

directed long-range migration. We show that, upon excitation by the tip of a scanning tunneling microscope (STM), chemisorbed benzene molecules migrate, on average, 66% further than did the alkene ones, and this time preferentially along the direction of Si dimer rows rather than perpendicular to the dimer rows. In addition, we show that the range of recoil strongly depends on the polarity of the excitation voltage, thereby offering, for the first time, a means to control surface migration. We discuss the origin of the torque and the consequent cartwheeling motion in the recoiling benzene molecule in the context of a model analogous to the Menzel Gomer Redhead description of DIET.12 Ab initio calculations of the ground and ionic states of benzene on Si(100) show that the molecule experiences a substantial tilt and large asymmetric forces that could cause rolling, following excitation to the ionic state and subsequent reversion to the ground state. The calculations, performed for both cationic and anionic states, are in qualitative accord with the experiments.

’ METHODS The experiments were carried out in ultrahigh vacuum (base pressure ∼ 3  10 11 Torr) employing an RHK-400 STM at room temperature. Images were recorded in a constant-current mode. STM tips and silicon samples were prepared as described previously.13 Spectrograde benzene, 99.95% purity, was obtained from ACP Chemicals Inc., and treated by several freeze pump thaw cycles before dosing into the vacuum chamber. Benzene was background-dosed for 75 s through a leak valve at a base pressure of 1  10 9 Torr (uncorrected ion-gauge value, Received: July 13, 2011 Revised: September 28, 2011 Published: October 10, 2011 22409

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calibrated for nitrogen). For electron-induced experiments, the electron pulse was performed during a normal imaging scan. Once the tip reached the point of interest, it was stopped and a pulse was applied. No rapid movement of the tip was required. The STM tip was positioned over an adsorbate molecule, and then, with feedback loop still engaged, the bias was linearly increased over a time interval of 5 ms to the desired pulse voltage. The bias and set current were maintained until a reaction occurred. The signature of reaction was an abrupt change in the tip height. The rate and yield of reaction were obtained from the average time and average number of electrons required for reaction. Ab initio calculations were performed using Gaussian 03, and a basis set of D3LYP/3-21g*. The Si(100) surface was modeled by a Si33H28 cluster, passivated with hydrogen, consisting of four Si layers and five Si dimers in a single row. For geometry optimizations, all atoms were allowed to relax (freezing the bottom layer gave similar results). Ionic states were calculated by modifying charge (+1 or 1) and spin multiplicity (2).

Figure 1. Electron-induced migration of benzene on Si(100). (a) STM image ( 1.5 V, 0.2 nA) of Si(100) after dosing with benzene at room temperature. Two different chemisorption configurations of benzene are indicated: “butterfly” or BF (oval) and “tight-bridge” or TB (square). Insets show the corresponding chemisorption configurations. (b) STM images ( 1.4 V, 0.2 nA) of identical areas before and after an electron pulse ( 2 V, 0.2 nA, 1 s) on a TB benzene. A lightening bolt shows the location of the pulse. The white arrow shows the direction of migration. (c) STM images ( 1.5 V, 0.2 nA) of identical areas before and after an electron pulse ( 2.4 V, 0.2 nA, 2 s) on a BF benzene. The pulsed molecule migrated along the dimer row by 74 Å at a 5° angle onto a defect-free area. Dashed lines above and below the images indicate the middle of Si dimer rows.

’ RESULTS AND DISCUSSION Figure 1a shows an STM image of Si(100) after dosing with benzene at room temperature. The Si(100) surface consists of parallel rows of Si dimers, one being indicated by dashed lines. Benzene can chemisorb in several possible configurations,14 of which the two dominant—termed “butterfly” (BF) and “tightbridge” (TB)—are shown in Figure 1a, along with schematics of their configurations. The BF adsorbate appears as a symmetric bright feature; it involves the di-σ-attachment of the para C-atoms of benzene with a single Si dimer. This bonding configuration leaves two CdC double bonds in the benzene, which appear bright under STM. The TB configuration, by contrast, has an asymmetric dark-and-bright appearance; it involves four σ-bonds between benzene and two adjacent Si dimers of the same row. TB has only one CdC double bond remaining, which appears bright in the STM image, while the reacted part appears dark. Previous experimental and theoretical studies15 19 have shown that BF is a metastable state, thermally converting to the more stable TB configuration across an activation barrier of 0.94 eV. In the present work, we found that electronic excitation of both TB and BF could lead to long-range migration of benzene. Figure 1b shows an example of electron-induced migration starting from the TB configuration. Following an electron pulse at 2 V on a TB benzene, the molecule disappeared from its initial location and, after traveling a distance of 79 Å, reacted with the surface in the BF configuration. An example of migration starting from the BF configuration is shown in Figure 1c. In this instance, an electron pulse at 1.8 V caused the BF benzene to leave its initial location and travel a distance of 77 Å at a 5° angle with respect to the dimer row before chemisorptive reattachment to the surface as BF. In ∼16% of the cases, the recoiled molecules could not be found in typically a 300  300 Å2 image area and were presumed to have desorbed. This gives an upper limit to desorption of benzene due to an electron pulse. In the remaining cases, the migrating benzene invariably reattached by reaction with the surface to form BF, regardless of the starting configuration. The resulting BF would later convert thermally to TB over a period of a few minutes, as expected for the metastable state BF. Previous experimental work on this system reported in ref 20 proposed mainly desorption of benzene. It is noteworthy that Figure 1 of ref 20 shows more “reattachment” (i.e., migratory) events than desorption (the ratio being 3:2 in favor of migration). 22410

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Figure 2. Migration over surface obstacles. (a) STM images ( 1.5 V, 0.2 nA) of identical areas before and after an electron pulse ( 2.0 V, 0.2 nA, 1 s) on a BF benzene. The pulsed molecule migrated by 35 Å along the dimer row and over a missing dimer defect. (b) STM images ( 1.5 V, 0.2 nA) of identical areas before and after an electron pulse ( 2.0 V, 0.2 nA, 1 s) on a BF benzene. The pulsed molecule migrated by 54 Å onto an upper terrace.

No percentages of the two types of behavior were reported. These earlier experiments were performed by scanning an area of adsorbate at higher bias than normal. In this case, migration would frequently be obscured by the departure of molecules from the area under observation. In the present study, directed toward the identification of migratory events, we examined the behavior of individual benzene molecules located, in most cases, near the center of a large 300 Å  300 Å area so that reattachment would more often occur within the field of view. Additionally, we applied our current pulses one molecule at a time to the benzene adsorbate so that we could more readily correlate the disappearance of a single molecule with a distant reattachment. That the migratory species is a physisorbed molecule is evidenced by the observation of persistent motion over raised surface obstacles. In Figure 1b, the direct migration trajectory between initial TB and final BF involves traversal of a missing dimer defect as well as another chemisorbed TB. Another two examples are shown in Figure 2, which illustrates BF benzene migration over a surface defect (Figure 2a) or across an upward terrace step (Figure 2b), without deflection. The estimated height of the physisorbed benzene from the Si(100) surface, from our calculations, is ∼3 Å, which is higher than the height of terrace steps (∼1.4 Å) or chemisorbed benzene (∼2.5 Å).

Figure 3. Characteristics of the electron-induced processes. (a) Tip height vs time for a typical electron pulse ( 2.25 V, 0.05 nA) on a TB benzene. The spectrum shows three regions characteristic of TB, BF, and Si, indicating initial conversion of TB to BF, followed by migration of BF from under the tip. (b) Yield of electron-induced events vs surface bias for BF (blue circle) and TB (red square). Thresholds are 1.8 and 2.0 V for BF and TB, respectively. (c) Rate of electron-induced events at 2.25 V vs tunneling current for BF (blue circle) and TB (red square). Solid lines represent best linear fits to the data.

Reattachment does not necessarily occur at a defect. Figure 1c shows a common example of termination in the absence of an adjacent defect. We interpret the observed termination in such 22411

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Figure 4. Radial and angular distributions of migration. (a) Radial distribution and (b) angular distribution of benzene migration at negative (blue) and positive (red) surface biases. N gives the total number of samples, the average distance, and the average angle. In (b), the zero degrees corresponds to the direction of Si dimer rows.

cases as being due to the fact that the cartwheeling physisorbed molecule has lost a sufficient fraction of its rotational energy so that it lingers and reacts. The effect of increased rotational energy in decreasing reaction probability is discussed in our earlier paper on migration in alkenes.10 In the present work, the minority of cases in which migration terminated at a defect ended at a C-type defect resulting from traces of adsorbed water. This C-type termination occurred in 20% of some 150 migratory events studied. Figure 3 shows characteristics of the electron-induced processes. Figure 3a shows a typical time spectrum obtained during an electron pulse on a TB benzene. The spectrum consists of two steps, the first of which corresponds to a switching of TB to BF directly under the tip. The switching to BF could be confirmed by interrupting the electron pulse immediately after the first step and examining the adsorbate by STM. The subsequent step involves migration of the newly formed BF from under the tip. Bias and current dependencies of the yield and rate of both steps are shown in Figure 3b,c. The bias dependencies give thresholds of 2.0 V for the electron-induced switching of TB, and 1.8 V for the electron-induced migration of BF, both at negative surface biases. The rate current plots yield linear relationships, which

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indicate that both switching and migration are single-electron processes. The electron-induced events were also observed at positive surface biases; however, the thresholds were significantly higher, estimated to be +3.2 V for the conversion of TB to BF, and +3.0 V for the electron-induced migration of BF. Radial and angular distributions of the electron-induced migration at both negative and positive biases are shown in Figure 4. The distance distribution at the negative surface bias is greater, exhibiting a maximum of 180 Å and an average of 48 ( 2 Å, indicating extremely long-range migration. In contrast, at the positive bias, the distance distribution is narrower, peaking at 7.5 Å, corresponding to two dimer spacings along the dimer row. The angular distributions, however, are similar at both bias polarities, exhibiting sharp peaks at 0°, that is, migration directed along the dimer row. It should be noted that migration across terraces was a rare event, comprising 3 4 instances out of approximately 150. This would not significantly influence the angular distribution. In such cases, we assigned the angle with respect to the original terrace. We note that directed long-range migration was previously observed in this laboratory for the case of chemisorbed alkenes (ethylene, propene, and trans-2-butene) on the same surface.10 However, in the previous case, the direction of migration was markedly different, being preferentially across the dimer rows. This difference in the direction of migration signifies that the directionality is dictated by the molecule itself and is not due to an STM artifact, such as scattering from the tip, which would be random, or pick-up by the tip, which would be along the scanning direction. For the case of alkenes, we previously concluded that the directed long-range motion was due to cartwheeling rotation, as evidenced by traversal of substantial obstacles with retention of direction (see above) and independently by the observation of molecular tumbling for propene molecules.10 The involvement of molecular rotation was supported by the theoretical finding of asymmetric forces on the recoiling alkene. The asymmetric forces on the alkenes were consistent with the concept of rolling recoil and with the observation of motion preferentially along the CdC bond axis, that is, perpendicular to the direction of the dimer rows. To investigate the mechanism of rolling in the present case, and to understand the difference in the migratory behavior at opposite bias polarites, we carried out similar ab initio calculations. Because migration always began from the BF configuration, we consider only this configuration. To simulate the surface, we used a cluster model of five Si dimers. This number of dimers was necessary to reproduce interaction of the adsorbed BF with adjacent Si dimers and to minimize artifacts due to cluster edge effects. Previous studies using only a single-dimer cluster could only reproduce desorption.20 22 To verify the validity of our cluster model, we compared the calculated molecular orbital (MO) energies of the BF configuration with the experimental thresholds. Figure 5 shows the result of a ground-state calculation of the BF configuration. The highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals are shown in Figure 5c,b. The HOMO and LUMO are calculated to be at energy levels of 1 and +1 eV with respect to the Fermi level, which is presumed to be midway of the HOMO LUMO gap. At negative surface biases, electroninduced events occur by withdrawal of electrons from the occupied MOs. In our cluster model, the occupied MO that has substantial charge localization on the molecule is not the 22412

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Figure 5. Molecular orbitals of the ground state of butterfly benzene. Top and side views of (a) LUMO+6, (b) LUMO, (c) HOMO, and (d) HOMO-5 of BF benzene. Energies are reported in reference to the Fermi level, which is taken to be in the middle of the HOMO LUMO gap. Orbitals are visualized with an isosurface value of 0.035.

HOMO (which is mainly localized on unreacted Si dimers) but is HOMO-5. The energy of this MO is 1.9 eV relative to the Fermi level, in good agreement with the observed threshold of 1.8 eV and previous photoelectron spectroscopy studies of chemisorbed benzene on Si(100).23 Similarly, for positive surface biases, the unoccupied MO localized on the molecule is LUMO+6 with an energy of +3.3 eV, consistent with the experimental threshold of +3.0 eV. Next, we calculated energies of the BF cluster with one electron removed (cationic state, negative surface bias) or one electron added (anionic state, positive surface bias). Results for the case of the cationic state are shown in Figure 6a. A vertical transition from the optimized neutral state (stage 1) to the cationic state (stage 2) introduces small forces on BF toward the surface. Relaxation of this excited cationic state (stage 3) causes the BF to tilt by 8° from its initial upright configuration. This tilt in the excited state implies a lateral interaction between one of the CdC wings of the BF structure and a neighboring Si dimer, as evidenced by flattening of the neighboring Si dimer (indicated by the black arrow in panel 3 of Figure 6a). Upon reversion to the neutral state (stage 4), the molecule experiences large asymmetric forces of up to 0.48 eV/Å that tend to bend the molecule in the direction of the dimer row. A similar analysis for the case of

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Figure 6. Origin of torque. (a) Schematic representation of neutral and cationic potential energy paths for electron-induced migration of butterfly benzene. The energy, structure, and forces are calculated for stages 1 5 and are visualized in the insets. A black arrow in panel 3 indicates a Si dimer that has become flat. Horizontal dashed lines in the insets indicate the dimer row direction. (b) Calculated structures corresponding to stages 4 and 5 for the case of excitation via the anionic state.

the anionic state gives a smaller tilt angle of 2° and a smaller force of 0.15 eV/Å, following a round trip to the anionic state. These results are in qualitative agreement with the experimental observation of migration directed along the dimer rows and are in accord with the marked greater migration distance observed for the removal of an electron (cationic excited state). From the results of our DFT calculations, we conclude that the tilt of the molecule in the excited state holds the key to whether a molecule after an electronic excitation will migrate or desorb. If a molecule spends long enough time in the excited state to experience a large tilt angle, upon return to the ground state, it would be expected to experience a torque that could lead to in-plane rolling. If, however, the lifetime of the excited molecule is so short that the tilt is insignificant, then, upon return to the ground state, the repulsion would be in a direction normal to the surface and could cause desorption, as reported by Alavi et al.21,22

’ CONCLUSIONS Directed long-range migration was observed due to electroninduced recoil of chemisorbed benzene on Si(100). The migration was directed along the dimer rows, carrying the molecules over raised surface obstacles. Electron impact at negative and positive surface biases yielded markedly different ranges of migration: on average, 48 Å at negative biases and 7.5 Å at positive biases. Ab initio calculations suggested a qualitative picture of the 22413

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The Journal of Physical Chemistry C origins of cartwheeling motion in conformity with the experimental observations. Besides the directed long-range recoil, another important finding of this study is the observation of surface reaction at a substantial distance from the originating event. The majority of the in-plane recoil trajectories terminated in the surface reattachement by chemisorption of the migratory benzene. Given the high mobility of the recoiling molecule, this finding can open up a new category of “migratory” surface reaction,24 in distinction to “localized” reaction (so-called LAR).

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), Ontario Centre of Excellence (OCE), Photonics Research Ontario (PRO), the Xerox Research Centre Canada (XRCC), and the Canadian Institute for Advanced Research (CIFAR). A.Z.-K. is supported by an Ontario Post-Doctoral Fellowship. ’ REFERENCES (1) McNab, I. R.; Polanyi, J. C. Patterned atomic reaction at surfaces. Chem. Rev. 2006, 106, 4321–4354. (2) Harikumar, K. R.; McNab, I. R.; Polanyi, J. C.; Zabet-Khosousi, A.; Hofer, W. A. Imprinting self-assembled patterns of lines at a semiconductor surface, using heat, light, or electrons. Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 950–955. (3) Avouris, P.; Walkup, R. E. Fundamental mechanisms of desorption and fragmentation induced by electronic transitions at surfaces. Annu. Rev. Phys. Chem. 1989, 40, 173–206. (4) Mayne, A. J.; Dujardin, G.; Comtet, G.; Riedel, D. Electronic control of single-molecule dynamics. Chem. Rev. 2006, 106, 4355–4378. (5) Komeda, T.; Kim, Y.; Kawai, M.; Persson, B. N. J.; Ueba, H. Lateral hopping of molecules induced by internal vibration mode. Science 2002, 295, 2055–2058. (6) Pascual, J. I.; Lorente, N.; Song, Z.; Conrad, H.; Rust, H.-P. Selectivity in vibrationally mediated single-molecule chemistry. Nature 2003, 423, 525–528. (7) Bartels, L.; Wang, F.; M€oller, D.; Knoesel, E.; Heinz, T. F. Realspace observation of molecular motion induced by femtosecond laser pulses. Science 2004, 305, 648–651. (8) Backus, E. H. G.; Eichler, A.; Kleyn, A. W.; Bonn, M. Real-time observation of molecular motion on a surface. Science 2005, 310, 1790–1793. (9) Ueba, H. Adsorbate motions induced by vibrational mode coupling. Surf. Sci. 2007, 601, 5212–5219. (10) Harikumar, K. R.; Polanyi, J. C.; Zabet-Khosousi, A.; Czekala, P.; Lin, H.; Hofer, W. A. Directed long-range molecular migration energized by surface reaction. Nat. Chem. 2011, 3, 400–408. (11) Barth, J. V. Transport of adsorbates at metal surfaces: From thermal migration to hot precursors. Surf. Sci. Rep. 2000, 40, 75–149. (12) (a) Menzel, D.; Gomer, R. Desorption from metal surfaces by low-energy electrons. J. Chem. Phys. 1964, 41, 3311–3328. (b) Redhead, P. A. Interaction of slow electrons with chemisorbed oxygen. Can. J. Phys. 1964, 64, 886–905. (13) Harikumar, K. R.; Leung, L.; McNab, I. R.; Polanyi, J. C.; Lin, H.; Hofer, W. A. Cooperative molecular dynamics in surface reactions. Nat. Chem. 2009, 1, 716–721. (14) Wolkow, R. A. Controlled molecular adsorption on silicon: Laying a foundation for molecular devices. Annu. Rev. Phys. Chem. 1999, 50, 413–441.

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