directed long-range molecular migration

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Directed long-range molecular migration

energized by surface reactionK. R. Harikumar1, John C. Polanyi1*, Amir Zabet-Khosousi1, Piotr Czekala2, Haiping Lin2

and Werner A. Hofer2

The recoil of adsorbates away (desorption) and towards (reaction) surfaces is well known. Here, we describe the long-rangerecoil of adsorbates in the plane of a surface, and accordingly the novel phenomenon of reactions occurring at asubstantial distance from the originating event. Three thermal and three electron-induced surface reactions are shown byscanning tunnelling microscopy to propel their physisorbed ethylenic products across the rough surface of Si(100) over adistance of up to 200 before an attachment reaction. The recoil energy in the ethylenic products comes from thermalexoergicity or from electronic excitation of chemisorbed alkenes. We propose that the mechanism of migration is a rollingmotion, because the recoiling molecule overcomes raised surface obstacles. Electronic excitation of propene causesdirectional recoil and often end-to-end inversion, suggesting cartwheeling. Ab initio calculations of the halogenation andelectron-induced reactions support a model in which asymmetric forces between the molecule and the surface inducerotation and therefore migration.

Adsorbate migration is a fundamental step in many surfacephenomena, including self-assembly, material growth,phase separation and heterogeneous catalysis. Migration

has been described previously in the work of other laboratories asdiffusion, via sequential random hopping1. Real-space and evenreal-time observations of surface diffusion have been achievedusing scanning tunnelling microscopy (STM)25 and ultrafast laserspectroscopy57. In all these cases of diffusion, the direction ofmotion is random, and it is also short-range (5 ), even onsmooth metal surfaces210.

Here, we provide evidence of directed long-range migration of

ethylenic molecules on the rough surface of Si(100). We show,using STM, this long-range migration for six cases; three involvingthe thermal reaction of 1,2-dihaloethanes and three involving theelectron-induced reaction of chemisorbed alkenes (ethylene,propene, trans-2-butene). In all cases, the migratory trajectoriesterminate with the reaction of the recoiling molecules with theunderlying silicon surface. The observed migration over a distanceof up to 200 is clearly different to hopping diffusion. Instead,we interpret the motion as being a result of cartwheeling across theSi(100), as suggested by the following observations. First, themigration occurs over raised surface obstacles such as surfacedefects and steps, but the direction of travel is maintained. Second,directed motion is observed along the bound CC axis of the initialchemisorbed state. Third, the directional migration of propene andtrans-2-butene is accompanied, significantly, by end-to-end inversion.

Finally, for both thermal and electron-induced reactions, ab initiocalculations show asymmetric forces on the carbon atoms at eitherend of the ethylenic bond, giving rise to torque.

Recoil away from surfaces has been widely studied under therubric of DIET (desorption induced by electronic transitions)11.Recoil towards surfaces is implied by the frequent observation inrecent studies of electron-induced surface reactions12. Directedlong-range recoil in the plane of the surface as reported here,however, is a novel phenomenon. The novelty resides not only inthe long-range in-plane recoil, but also in the observation of

reactions at a substantial distance from the originating events.This contrasts with frequent observations, in previous work, oflocalized chemical reactions1315. This difference arises from thefact that in the present instance the exoergic process at first yieldsa mobile physisorbed species, which only later chemically reacts,whereas in the case of a localized reaction a chemical bond formsimmediately adjacent to the reagent molecule.

ResultsThermal reaction and migration. Molecular migration as a result ofsurface reactionwasobservedin the room-temperaturedihalogenation

by 1,2-dihaloethanes (XCH2CH2X, X F,Cl,Br)onSi(100).Figure1ashows a typical large-area STM image of Si(100) dosed with 1,2-dibromoethane (DBE) at 25 8C. The Si(100)-2 1 surface consistsof parallel rows of silicon dimers. On dosing with dihaloethane, twonew surface features were observed: one characterized as achemisorbed halogen pair bound to adjacent silicon atoms (greensquares), and the other identified as chemisorbed ethylene (blackcircles). The STM images show no features attributable to intactphysisorbed dihaloethane molecules, indicating that all dihaloethanehas reacted at the room-temperature surface.

The three dihalogenation reactions are highly exoergic, withenergy releases calculated as23.6,22.6 and22.5 eV (heats of reac-tion calculated using the Vienna ab-initio simulation package(VASP)) for difluoroethane (DFE), dichloroethane (DCE) anddibromoethane (DBE), respectively, to yield two chemisorbed X-

atoms and one chemisorbed ethylene. The ethylene products wereseen in these experiments to adsorb on the room-temperaturesurface, but, surprisingly, at locations up to 80 from thenearest halogen pairs. There was no thermal diffusion of the chemi-sorbed halogen pairs or ethylene in these room-temperature exper-iments, as determined by successive STM traces, and we thereforeattribute the observed separation between the ethylene and thehalogen pairs to the surface migration of hot ethylene products,before accommodation at the surface. This phenomenon has beentermed transient mobility for dissociative adsorption of diatomic

1Department of Chemistry and Institute for Optical Sciences, University of Toronto, 80 St George Street, Toronto, Ontario, M5S 3H6, Canada, 2SurfaceScience Research Centre, University of Liverpool, Liverpool L69 3BX, UK. *e-mail: [email protected]

ARTICLESPUBLISHED ONLINE: 17 APRIL 2011 | DOI: 10.1038/NCHEM.1029

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molecules on metals16,17. Figure 1a shows a total of eleven cases in anarea of 300 230 2. In all eleven, the ethylene and bromine pairswere formed on separate dimer rows, in contrast to conventionaldiffusion, which involves preferential motion along the dimer row.Additional evidence of directed migration comes from the obser-

vation in case 1 of the ethylene having traversed an upwardterrace step, and in case 3 of having passed over a surface defectwithout being deflected.

An approximate comparison of the abundance of chemisorbedethylene with that of adsorbed X-pairs gave a ratio of1.52 (forhigh and low coverages, respectively). It appears that as much ashalf of the chemisorbed ethylene was formed with the loss of twohalogen atoms as X

2to the gas. This process is only1.5 eV less

exoergic than the observed formation of two chemisorbed X withchemisorbed ethylene.

The approximate distributions of ethylenedihalogen separ-ations obtained for the three dihaloethane cases are shown inFig. 1bd. The separation distances were obtained by assigningeach ethylene to the nearest unassigned halogen pair and, therefore,

do not always represent the actual migration distances. For DFEand DCE, the halogen pairs tended to form clusters, but forDBE, individual halogen pairs were observed. This introduces abias into the distance distributions of DFE/DCE and DBE.Nevertheless, the mean distance of 2530 travelled by recoiling

ethylene across the room-temperature surface was similar for allthree thermal dihaloethane reactions. The energy release, asnoted, decreases by1 eV along the series DFE, DCE, DBE, butthe effect of this on ethylene recoil distance is undetectable.This may be because the recoil energy arises from the exoergicp-bond formation, which is common to all three dihaloethanecases (see below).

Electron-induced reaction and migration. To shed further light onthe surface migration, we studied the electron-induced reaction ofchemisorbed ethylene at surface temperatures of 25, 100 and150 8C. Figure 2a shows an STM image of an ethylene-dosedsilicon surface at 25 8C (for bias-dependent STM images seeSupplementary Fig. S1). Chemisorption of ethylene on Si(100) has

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Figure 1 | Thermal dissociation of dihaloethanes. a, STM image (2.0 V, 0.2 nA) of Si(100) after dosing with DBE (1 1029 torr, 30 s) at 25 8C. Green

squares and black circles indicate chemisorbed bromine atoms and ethylene, respectively. White ovals encircle the nearest bromineethylene pairs. The

direction of ethylene migration is shown in case 4 by a white arrow. A white rectangular box highlights a single C-type defect, although many are visible.

Dashed black lines at the sides of the image indicate the middle of a silicon dimer row. bd, Distributions of ethylenehalogen separations (bin size, 7.7 )

obtained from surface reactions of DFE (d), DCE (e) and DBE (f) on Si(100). N gives the total number of counts. Error bars represent the standard error: the

square root of count.

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previously been the subject of extensive experimental1822 andtheoretical23,24 studies. Ethylene attaches to the surface by openingits p-bond and forming two CSi s-bonds, either with one silicondimer (intra-dimer configuration) or bridging two adjacentsilicon dimers of one dimer row (inter-dimer configuration). Weobserved 90% intra-dimer chemisorption configuration at roomtemperature, in agreement with a recent STM study25.

Figure 2b shows the same area as in Fig. 2a after a23 V electronpulse is applied to an intra-dimer chemisorbed ethylene. The mol-ecule disappears from its original chemisorption site and re-attaches

to the surface 41 away, in the same di-s-bound intra-dimer con-figuration. Note that the direct path between the initial and finalsites is hindered by two chemisorbed ethylene species and onesurface defect. Figure 2c,d presents another example, with ethylenemigrating 104 from a lower terrace onto an upper terrace across astep height of 1.4 .

Ethylene migration was observed in 67% of room-temperatureelectron-induced events (N 1,400). The migrated ethyleneappeared predominantly as an intra-dimer chemisorbed product(only10% of cases were inter-dimer).

e pulse

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Figure 2 | Electron-induced ethylene migration. a, STM image (2.0 V, 0.2 nA, 25 8C) showing three intra-dimer (circle) and one inter-dimer (oval)

chemisorbed ethylene on Si(100). b, STM image of the same area after an electron pulse (23 V, 0.2 nA, 0.5 s). The pulse location is marked by a lightning

bolt. White arrow shows migration of the pulsed ethylene by 41 over a few surface features. Dashed lines above and below the images indicate the middle

of silicon dimer rows. c,d, STM images (21.7 V, 0.1 nA, 25 8C) of identical areas before (c) and after (d) an electron pulse (23.5 V, 0.1 nA, 2 s) on a

chemisorbed ethylene. White arrow shows ethylene migration by 104 onto an upper terrace. e, Yield of electron-induced migration as a function of surface

bias. A threshold of 23.0 V is obtained assuming a linear threshold law. f, Rate of electron-induced migration at 23.2 V as a function of current. A linearrelationship is observed, indicative of a single-electron process. Red solid lines represent best linear fits to the data. Error bars represent the standard error:

standard deviation divided by the square root of count.

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A further 28% of electron-induced events resulted not inmigration but in switching of the ethylene configuration, byrotation on one of its two silicons to convert from an intra-dimerarrangement to inter-dimer (breaking one CSi s-bond, and rotat-ing around the other; see Supplementary Fig. S2). In90% of theseswitching events the CSi bond-breaking occurred at the locationnearest to the STM tip where an electron pulse had been applied,indicating localization of the electronic excitation26. In the remain-

ing 5% of electron-induced events, ethylene could not be foundwithin the 300 300 2 imaging areas, and was presumed tohave desorbed. Electron-induced migration and switching werealso observed for chemisorbed ethylene starting from an inter-dimer (rather than intra-dimer) configuration. Analyses of theyield and rate of electron-induced events (intra-dimer or inter-dimer) as a function of pulse bias and current revealed a single-elec-tron process with a threshold of 23.0 V (Fig. 2e,f). Similarthresholds for electron-induced reactions have been reported for anumber of di-s-bound adsorbates on Si(100)10,2628.

Increasing the surface temperature altered the outcome of theelectron-induced events. The ratio of migration to switchingremained constant, but the desorption probability increased to10% and then to 15% at 100 and 150 8C, respectively. More impor-tantly, the radial and angular distributions of the migrating ethylenechanged significantly, as shown in Fig. 3ad. At 25 8C, the radial dis-tribution exhibited an exponential decay, with an average distance of29+2 (Fig. 3c). A three-dimensional histogram of migrationacross a room-temperature surface is shown in SupplementaryFig. S3. As the surface temperature was raised, the distributionbecame broader and the average distance increased progressively to70+7 and 87+11 at 100 and 150 8C. At the same time, as thetemperature was raised, the angular distribution became sharper.At 25 8C, the angular distribution exhibited broad peaks at 08 and638. At 100 8C, these peaks moved towards larger angles, and at150 8C a pronounced peak was observed at 908. On the hotsurface, therefore, the ethylene migrated predominantly along thedirection of the CC bond of its initial chemisorbed configuration.

To explore the generality of the ethylenic surface migration, we

studied the electron-induced recoil of two other alkenes; propeneand trans-2-butene29,30. For both molecules, migration and switch-ing were induced using electron pulses with the same threshold of23.0 V. However, shorter migration distances averaging 19+3 ,and higher desorption probabilities of 32% for propene and 62%for trans-2-butene, were observed at the 25 8C surface. Theseshorter travel distances are likely to be due to the increasednumber of degrees of freedom in the adsorbate and hence increasedinelasticity of the adsorbed molecules.

Electron-induced migration of propene is demonstrated in Fig. 4.Figure 4a shows an STM image of propene chemisorbed on a singlesilicon dimer, and Fig. 4b shows the corresponding chemisorptionconfiguration. The non-reactive methyl group of propene appearsin the STM image as a bright protrusion at one side of the silicondimer on which the propene is chemisorbed29, and therefore can

serve as a reference for determining the alignment of the molecule.Significantly, we found that in 70% of cases of propene migration,the direction of recoil was away from this bright methyl group(Fig. 4c,d). In addition, in 62% of cases the methyl group appearedin the final state at the opposite end of the molecule from its initiallocation. Recoil away from the methyl end and flipping of the mol-ecule are evident in Fig. 4c,d. Flipping strongly suggests rotation ofthe migrating propene.

Radial and angular distributions of propene migration are shownin Fig. 4e,f. Theangular distributionof propene migration was broad,exhibiting two peaks at 08 and 908. As in the case of ethylene, the 908peak was more pronounced. This suggests directed migration ofpropene along its bound CC axis, as indicated by black arrows inFig. 4b. (For images oftrans-2-butene see Supplementary Fig. S4.)

The directionality of propene migration, away from the methyl,was observed irrespective of which CSi bond was pulsed. Forethylene, no preferred recoil direction with respect to the pulseposition was observed. The observed directionality in the case ofpropene suggests that the methyl group enhances electronic

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Figure 3 | Radial and angular distributions of electron-induced ethylene

migration at various temperatures. ac, Radial distributions (bin size, 15 )

at 150 8C (a), 100 8C (b) and 25 8C (c). Red solid line in c is an exponential

fit to the data. d, Angular distribution at 150 8C (dashed red), 100 8C

(dashed green) and 25 8C (solid blue). Zero degrees corresponds to the

dimer-row direction. Initial orientation of chemisorbed ethylene is shown in

blue. Error bars represent the square root of count.




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excitation at the neighbouring CSi bond. This is in agreementwith the observation of markedly increased brightness at themethyl end of absorbed propene in the STM images of this andprevious work26.

DiscussionThe observation that recoiling ethylene (i) migrated over distancesas great as 200 , (ii) surmounted raised surface obstacles, and(iii) exhibited directionality along the initial CC molecular axis,suggests that it moved ballistically on the surface rather than diffu-sively31. Diffusive motion involves sequential short-range hopping

in random directions, scattering the adsorbate from its initialrecoil direction. On Si(100), moreover, diffusion is typicallyfavoured along the direction of dimer rows because of the lower dif-fusion barrier32,33, in marked contrast to our observation ofmigration preferentially across the rows.

We can rule out scattering of molecules from the STM tip, ortransfer of molecules to the tip followed by re-deposition on thesurface, as possible mechanisms for migration on the basis ofthree factors. First, scattering from the STM tip would be arandom event, but we observed directionality in the migration.Second, attachment to the tip would carry the molecule along the

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Figure 4 | Electron-induced migration of chemisorbed propene. a, STM image (22.0 V, 0.2 nA, 25 8C) showing a chemisorbed propene (circle) bound to a

single silicon dimer. The methyl group appears bright and is indicated by black rays. b, Schematic of the propene chemisorption configuration. Black rays

indicate the methyl groups, and black arrows show the preferred direction of migration. c,d, STM images (22.0 V, 0.2 nA, 25 8C) of identical areas before (c)

and after (d) an electron pulse (23.5 V, 0.2 nA, 2 s) on a chemisorbed propene. White arrow shows migration by 15 . The propene following migration

appears with its methyl group flipped with respect to the dimer rows. e, Radial distribution (bin size, 15 ) of electron-induced propene migration at 25 8C.

f, Angular distribution (bin size, 158) of electron-induced propene migration at 25 8C. Initial orientation of the bound CC axis is shown in blue. Error bars

represent the square root of count.




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scanning direction, which was not found to be the case. Third,transfer of molecules to the tip would strongly affect the imagingconditions, and no such changes were observed.

We can also rule out the electric field present in the STM junc-tion as a source of recoil. The effect of the electric field gradient onelectron-induced hopping of polar molecules at metal surfaces hasbeen demonstrated34. A characteristic of this field effect is that thehopping direction is controllable, because it depends on the pos-

ition of the STM tip relative to the centre of the molecule. Asalready noted, in a systematic study we found no such dependence;the direction of recoil of the ethylene was shown to be independentof the placing of the STM tip at the left- or right-hand sides of themolecule. For propene, 70% of the time the molecule recoiled awayfrom the methyl end, again irrespective of the tip position, whichwas deliberately altered from one end of the molecule to theother. We attributed the observed preference in the direction ofrecoil of propene to the enhanced electronic excitation at the pro-minent methyl end of the molecule. The direction of recoil wasdetermined by the molecule, and was independent of thefield gradient.

To test further regarding the influence of the electric field, weexamined the effect on migration distance of varying the fieldstrength35. Figure 5 shows distance distributions at various surfacebiases, ranging from 23 V to 24 V, corresponding to 33%increase in the electric field. We could detect no significantchange in the distance distribution; the average migration distanceat all biases examined, indicated by black arrows, remainedconstant at 30 . Our observations, taken together, indicate thatthe electric field has little effect on either the direction or therange of migration.

Mechanism of migration. To account for our observations, wepropose a rolling mechanism in which the recoiling ethylenecartwheels, rotating in a plane perpendicular to the surface in aphysisorbed state. Rotation is suggested by the observed inversion ofapproximately one-half of the propene molecules in the course oftheir migration. As with a cartwheel, the adsorbate travels in the

direction that the wheel turns. Assuming a picosecond per rotationand sonic velocity, the ethylene molecule would turn approximatelyonce per angstrom travelled, in other words, a few rotations perdimer row traversed. Evidence for frustrated molecular rotationduring diffusion of CO on Pt(111) has been presented recently,where CO hopped from step edges to terrace sites6.

The ethylene cartwheeling rotation can be attributed to a torquearising from asymmetric forces on the carbon atoms during therelease of ethylene from its dihaloethane parent in the threeinstances of thermal reaction cited, or from its chemisorbedsurface adduct in the three examples of electron-induced migration.As noted above, it appears most probable that the migration is ener-gized in all cases by the formation of the CC p-bond. Previousmeasurements indicate that the energy release in going from thedi-s-bound chemisorbed state to a physisorbed state of ethylene

is 1.3 eV (ref. 21). If the torque causes preferential channellingof the reaction energy into internal rotation (cartwheeling) thiswould reduce the probability of desorption. The distance ofmigration will depend on the rate of energy loss through transferto the surface. This inelasticity is expected to be greatestas the rotor crosses successive dimer rows, where it undergoesabortive reactive encounters, until with diminished rotationalenergy it falls into a reactive potential well from which itcannot escape.

Earlier work3638 has shown that highly rotationally excited mol-ecules would be unreactive, rotating out of their reactive configur-ations before reaction could take place. This rotational hinderingprovides a rationale for the observation in the present work of thelong-range migration of the rotationally hot physisorbed ethylenic

molecules. Following rotational cooling in the course of migration,the ethylenic double bond opens and chemical attachment to thesurface ensues.

Molecular-beam scattering experiments using NO on Pt(111)have established that the rate of rotational energy transfer to thesurface decreases as temperature rises39. This effect could explain

the increase in migration distance at higher temperatures that wehave observed.

Theory. To explore the origin of the torque, we performed extensivedensity functional theory (DFT) calculations. Figure 6a shows acalculated minimum-energy path for the dissociation of DFE onSi(100) using a nudged elastic band (NEB) algorithm. The NEBcalculations yielded a relatively low activation barrier of0.8 eV(step 6). By step 8, the fluorine atoms were fully transferred to thesurface, and by step 10, formation of ethylene was complete.Repulsive forces on the carbon atoms of the ethylene recoilingfrom the fluorinated surface are evident in steps 69. The largestrepulsive force (1.5 eV 21) was observed at step 8 following thetransfer of fluorine atoms. This force acted predominantly on onecarbon atom, causing torque that could give rise to cartwheeling

rotation of the ethylene.Figure 6b shows the results of calculations for an excited state of

chemisorbed ethylene following a FrankCondon transition; that is,without relaxation of the nuclei. The energy of the excited state iscalculated to be 3.1 eV above the ground state, in excellent agree-ment with the observed threshold for electron-induced reaction.The charge distribution in the excited state shows asymmetriccharge on the two CSi bonds. The higher charge density on oneCSi bond is accompanied by a larger repulsive force (0.17 eV 21)on the carbon atom, creating a net torque that could initiatemotion along the direction of the CC bond axis. The observedoutcome depends, however, on the integral of this and subsequentforces in the excited state (shown), as well as forces on the groundpotential-energy surface following reversion to that state.

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Figure 5 | Bias dependence of distance distribution. Histograms of distance

distributions at surface biases ranging from 23 to 24 V. Black arrows

indicate the mean of the distribution.




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ConclusionsIn conclusion, we demonstrate long-range migration of ethylenicmolecules on Si(100), induced by six surface reactions involvingCC p-bond formation. We show the generality of migrationfor (i) the thermal reaction of three related di-halo-alkanes and(ii) the electron-induced reaction of a series of three related chemi-sorbed alkenes at the same surface. Each migratory event led at itsterminus to a di-s-bound ethylene (90% intra-dimer, 10% inter-dimer). Migration distances averaged 29 for the room-tempera-ture surface, increasing to 87 for the 150 8C surface and extend-ing up to 200 . This long-range migration occurs despite the well-known roughness of this surface that makes possible STM obser-vation of adsorbates on Si(100) at room temperature. The observed

migration is energized by the formation of a CC p-bond. Forelectron-induced recoil of ethylene the migration is directedalong the initial CC bond axis in the chemisorbed state. Themechanism for the migration gives evidence of being rolling,rather than translation, of recoiling physisorbed molecules acrossthe surface, because obstacles are surmounted and end-to-endinversion is exhibited. The rotation is induced by torque duringthe recoil of ethylene following surface dihalogenation (in threeexamples examined) or during the electron-induced recoil of theethylenic molecules from their surface counterparts (in threefurther examples). Evidence of this torque comes from DFT calcu-lations performed for both the thermal and electron-induced reactions.

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Figure 6 | DFT calculations of thermal and electron-induced recoil of ethylene. a, Calculated minimum-energy path for thermal dissociation of DFE on

Si(100). Twelve NEB configurations from the initial state (step 0) to the final state (step 11) were calculated. Insets show how the reaction proceeds. White

arrows show differential forces on carbon atoms. b, Calculated charge distribution in the first excited state of chemisorbed ethylene on Si(100). White arrows

represent forces on carbon atoms due to a FrankCondon transition from the ground to the excited state.




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These findings open up possibilities for the use of chemicalenergy to direct long-range adsorbate migration across surfaces,and also offer a new means with which to study adsorbatesurfaceinteractions during the course of surface migration. Directed mol-ecular movement over many tens of angstroms followed by chemicalreaction offers a means to promote reaction at a distance. To date,localized reaction has dominated in the discussion of surfacedynamics1315. Additionally, the understanding of long-range

migration should assist in applications that require substantialmovement across a surface, such as molecular nanomachines40,41.

MethodsThe experiments were carried out in ultrahigh vacuum (base pressure3 10211 torr) by an RHK-400 STM. Images were recorded in constant-currentmode. STM tips and silicon samples were prepared as described previously42. Theadsorbates were background dosed through a leak valve. The dosing pressure andduration were as follows. DFE: 5 1029 torr and 160 s; DCE: 1 1029 torr and250 s; DBE: 5 1029 torr and 60 s; ethylene at 25 8C: 1 1029 torr and 30 s;ethylene at 100 8C: 1 1029 torr and 60 s; ethylene at 150 8C: 1 1029 torr and200 s; propene: 1 1029 torr and 60 s; trans-2-butene: 1 1029 torr and 120 s.Exposures were measured using an ion gauge calibrated for nitrogen. For high-temperature experiments, the sample was radiatively heated by a bulb heater and thetemperature was measured by a C-type thermocouple directly beneath the sample.For electron-induced experiments, the STM tip was positioned over an adsorbate,then, with feedback loop still engaged, the bias was linearly increased or decreased

over a time interval of 5 ms, to the desired pulse voltage. The bias and set currentwere maintained until a reaction occurred. The signature of reaction was an abruptchange in the tip height. Rate and yield of reaction were obtained from the averagetime and the average number of electrons required for reaction.

Theoretical simulations of the ground state of ethylene, the ground state of DFEphysisorbed on the surface, and the final state of a reacted DFE molecule wereperformed using VASP43,44. A 4 4 supercell of Si(100) was simulated using a slabof eight layers with the bottom layer passivated with hydrogen. The adsorbatemolecules and three surface layers were relaxed until forces on individual atoms werebelow 0.02 eV 21. NEB simulations45 were undertaken with the same code. Thesimulations used generalized gradient approximation (GGA) exchange correlationpotentials46 and projector augmented waves to obtain the electronic ground state,and typically integrated the surface Brillouin zones of the system with only thegamma point. The excited state of the ethylene molecule was simulated by the DeltaSelf-Consistent-Field method47, as implemented in grid-based projector-augmentedwave (GPAW) (https://wiki.fysik.dtu.dk/gpaw/). We used the same exchangecorrelation potentials in both codes, so that energy differences were comparable.

Received 10 January 2011; accepted 15 March 2011;published online 17 April 2011

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AcknowledgementsThe authors thank I. R. McNab for critical discussions and for DFT calculations. J.C.P.thanks the Natural Sciences and Engineering Research Council of Canada (NSERC),

Photonics Research Ontario (PRO), an Ontario Centre of Excellence (OCE) and the XeroxResearch Centre Canada (XRCC) for their support for this work. W.A.H. thanks the RoyalSociety of London for support. J.C.P. and W.A.H. also thank the Canadian Institute forAdvanced Research (CIFAR) for support. A.Z.-K. is supported by an OntarioPost-Doctoral Fellowship.

Author contributionsK.R.H., J.C.P., and A.Z.-K. designed the research. K.R.H. and A.Z.-K. collected andanalysed the experimental data. P.C., H.L. and W.A.H. performed the DFT and NEBcalculations. All authors contributed to the manuscript.

Additional informationThe authors declare no competing financial interests. Supplementary informationaccompanies this paper at www.nature.com/naturechemistry. Reprints and permissioninformation is available online at http://npg.nature.com/reprintsandpermissions/.

Correspondence and requests for materials should be addressed to J.C.P.


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