SURFACE CHEMISTRY

Regulating the femtosecondexcited-state lifetime of asingle moleculeK. R. Rusimova1, R. M. Purkiss1, R. Howes1, F. Lee1, S. Crampin1,2, P. A. Sloan1,2*

The key to controlling reactions of molecules induced with the current of ascanning tunneling microscope (STM) tip is the ultrashort intermediate excitedionic state. The initial condition of the excited state is set by the energy andposition of the injected current; thereafter, its dynamics determines the reactionoutcome. We show that a STM can directly and controllably influence theexcited-state dynamics. For the STM-induced desorption of toluene moleculesfrom the Si(111)-7x7 surface, as the tip approaches the molecule, the probabilityof manipulation drops by two orders of magnitude. A two-channel quenchingof the excited state is proposed, consisting of an invariant surface channeland a tip height–dependent channel. We conclude that picometer tip proximityregulates the lifetime of the excited state from 10 femtoseconds to less than0.1 femtoseconds.

Using the tip of a scanning tunneling micro-scope (STM) to initiate chemical reactionsoffers a route to controllable single-moleculechemistry (1, 2). Through the mechanicalinteraction between tip and target mol-

ecule, or by the electric field in the gap, the STMcan induce molecular change across a ground-state potential energy landscape (3). The STMtunneling current, however, can generate excitedstates of a molecule and hence give enhancedspecificity, and more varied outcomes, to themanipulation action [e.g., bond dissociation(4, 5), isomerization (6), or tautomerization (7)].The specificity arises by controlling the energy(5) or position (7, 8) of the single electron (orhole) excitation within a single molecule. Theensuingmolecular dynamics, and hence the finaloutcome, evolve naturally from that point. Havingthe ability to control and influence the dynam-ics of the excited state itself would open newpaths to control matter, and its reactions, at themolecular level.Here, we found that the lifetime of the positive

ion of single toluene molecules on the Si(111)-7×7surface can be directly controlled by the STM.By bringing the tip close to the molecule (600 to800 pm), we regulated the excited state–mediatedreaction outcome (molecular desorption) bymorethan two orders of magnitude. We correlate thisto a reduction of the excited-state lifetime byapproximately two orders of magnitude. We pro-pose that a new electronic state is generated bythe tip-molecule interaction that provides an ad-ditional decay channel for the excited state, thusquenching the excited state before its naturalsurface-limited lifetime elapses. We anticipate

this work to be a starting point for other morecomplex molecular systems that yield multipleexcited-state outcomeswhere this technique couldbe used to instigate, probe, and control them. Thequenching process relies on fundamental quan-tum processes and should be applicable to a wideclass of molecule/surface systems.Multiple molecular adsorbates have been

shown to react to the STM tunneling current (1),especially benzene and derivatives (5, 9). Broadly,the probability per electron of inducing a molec-ular reaction is higher on the Si(111)-7×7 surfacethan on the Si(100)-2×1 surface and is orders ofmagnitude higher than on metal surfaces (7–9).On metals, lifetimes of molecular ion states areas low as 0.1 fs (10), but the reduced density ofstates in semiconductors leads to longer excited-state lifetimes. The theory of dynamics inducedby electronic transition (DIET) links greaterlifetimes of excited states to higher probabilitiesof reaction (such as bond breaking or desorp-tion) (11). Benzene, chlorobenzene, and tolueneon Si(111)-7×7 have all been extensively stud-ied (5, 9) and are highly sensitive to tunnelingcurrent.Figure 1 shows a series of STM images chart-

ing the positive ion resonance (or negative-bias“hole”)–induced manipulation of a single chem-isorbed toluene molecule on the Si(111)-7×7surface. At the imaging conditions used (+1 V,100 pA), chemisorbed toluene molecules wereunperturbed by the STM (12) and appeared asdark features against the bright spots that werethe adatoms of the silicon surface. Chemisorbedtoluene molecules formed a 2,5-di-s bondingconfiguration with the surface, forming one co-valent bond to a silicon adatom (colored red) andone to a neighboring silicon rest atom (second-layer atoms with dangling bonds) (Fig. 1A). Tomanipulate the molecule, during a raster scanfrom bottom to top, we halted the tip atop the

molecule and performed current injection (–1.3 V,900 pA) for 8 s.Figure 1D shows the tip height variation dur-

ing this process. In step i, the tip was haltedabove the molecule. The feedback loop wasdisabled, and the tip retracted by 1 nm beforethe voltage was ramped to the desired manipu-lation value with the set-point current at 20 pA.In step ii, the feedback loop was then reengaged.In step iii, the set-point current changed to therequired injection current, resulting in the tipapproaching the surface closer than its initialvalue by an amount DzI. Charge injection con-tinued, and in this particular case, after 0.35 s ofcharge injection, the molecule-adatom bond wasbroken, leading to desorption. The underlying(bright) silicon adatom was exposed, causing thetip to withdraw by DzM to restore the set-pointcurrent (step iv). Resuming the interrupted imagescan of Fig. 1B resulted in a “half-moon” fea-ture at the molecular adsorption site, typical ofa manipulation event occurring mid-scan. Sub-sequent image scans (Fig. 1C) had the conven-tional Si(111)-7×7 surface appearance, includingthe silicon dangling bond at the original locationof the toluene molecule.From the fraction of ~120 toluene molecules

that were manipulated after an injection time t,we deduced a time-dependent probability of ma-nipulation P(t) for a single molecule, consist-ent with the first-order rate equation dP(t)/dt =k[1 – P(t)], where k is the rate of manipulation.Figure 2A illustrates this for injection parametersof +1.6 V, 450 pA. Figure 2B illustrates how themanipulation rate k varied with tunneling cur-rent for electron injection at +1.6 V. Figure 2Cpresents data for hole injections at –1.3 V and at–1.0 V. For electron injection, the rate increasedlinearly with injection current (see fit to Fig. 2B).For hole injection at low current (2 to 10 pA), weagain found a linear dependence, but beyond10 pA the rate ofmanipulationwas approximatelyconstant; the fitting function of Fig. 2C is dis-cussed below.The numbern of electrons (or holes) that drive

a single-molecule manipulation (4) leads to apower-law dependence of the rate kwith currentI, kº In. Hence, for electron injection, the near-linear dependence n = 0.8 ± 0.1 indicates a one-electron process (9). Similarly, at low currents, aone-hole process is responsible for desorption.However, for hole injection at currents above10 pA, the near-constant rate implies a largelycurrent-independent process. If the current isnot driving the manipulation, what does?Themanipulation rates observedwere a factor

of 104 greater than those occurring in purelythermally driven desorption of toluene fromSi(111)-7×7 (12). Hence, the presence of the STMtip is required for this manipulation to take place.Possible tip-molecule interactions that mightdrive manipulation are mechanical (i.e., a short-range chemical interaction between tip andmolecule) or result from the electric field of thetunnel junction. However, we can rule outboth. Figure 3A shows the tip height z duringthe electron and hole injections performed at

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1Department of Physics, University of Bath, Bath BA2 7AY,UK. 2Centre for Nanoscience and Nanotechnology, Universityof Bath, Bath BA2 7AY, UK.*Corresponding author. Email: p.sloan@bath.ac.uk

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different currents. The tip height z is the distancefrom the center of the bonding Si adatom to thecenter of the leading atom of the STM tip (seesupplementary materials). In all cases, z exceeds600 pm. To identify possible mechanical mani-pulation, we modified the manipulation experi-ments by setting the bias during step ii to 0 V,disabling the feedback loop, and setting z to aspecific value (Fig. 3B, schematic). For each zvalue, we then measured the outcome of ~90single-molecule manipulation experiments withan 8-s “exposure” of each target molecule. Littleor no desorption was observed for z at or above600 pm (Fig. 3B, shaded portion). Thus, in theheight regime of the current-manipulation ex-periments, nomechanicalmanipulation occurred,and the desorption that did occur in Fig. 3B wasconsistent with that expected for a thermallydriven process (see supplementary materials forz < 600 pm discussion).We eliminated the possibility of an electric

field–inducedmanipulationmechanism bymod-ifying step ii so that, with feedback disabled, thetip retracted an additional distance from thesurface. We applied a –10 V bias to generate anelectric field E ≈ V/z in the junction comparableto that in the current-injection experiments, andwhose magnitudes are shown in Fig. 3C. In thiscase, however, there was no current. As shown bythe data in Fig. 3D, without the current, therewas little or no manipulation.A similar linear to constant rate crossover ap-

pears in two previous studies (13, 14). There, tip-induced band bending (TIBB) was put forwardas a possible explanation. Since then, detailedtheoretical work and scanning tunneling spec-troscopy show that TIBB only occurs if the semi-conductor is in depletion (15, 16). For our workwith n-type Si, this would be for electron injec-tion. Therefore, TIBB cannot explain our hole in-jection results, nor the results of (13). The dopinglevel here and in (14) also precludes anymeasur-able TIBB even if it occurs in the depletion re-gime (17). Instead, the model proposed here isconsistent with all three reports.The final outcome of the molecular manipula-

tion can be either that the molecule completelyleaves the surface (desorption) or that it reat-taches to the surface elsewhere (diffusion). Welabel an outcome as diffusion if, in an “after”STM image (e.g., Fig. 1C), the manipulated mol-ecule appeared at an adjacent binding site. Allother manipulation outcomes are classified asdesorption. For all injection currents used, wefound a branching ratio B of the probabilityof desorption to diffusion that was constantthroughout the hole-injection experiments, Bh =0.037 ± 0.004. It was also constant for electroninjections with Be = 0.24 ± 0.03 over the reportedrange of currents. Furthermore, there was no evi-dence of other forms of manipulation, such asintramolecular bond dissociation (5), in eithercurrent regime.Recasting the rate of manipulation in terms of

the probability per injected charge of manipula-tion (electron or hole), Pe = ke/I (where e is themagnitude of the electron charge), yields Pe as a

function of the tip height z during the currentinjections. For electron injection, as expected fora one-electron process (Fig. 4A), Pe was fairlyconstant over the range of z studied. For –1.3 Vhole injection (Fig. 4B), Pe exponentially increasedwith z (i.e., decreasing current) until at ~800 pm,we found a near-constant region. Figure 4C showsdata for –1.0 V hole injections (10 to 900 pA),where for all injections, we found a similar ex-ponential increase in the manipulation probabil-ity as the tip withdrew from the surface.The desorption of toluene, via a DIET process,

occurs in three steps (18): (i) excitation by cap-ture of the injected charge by the toluene mol-ecule; (ii) dynamics, the evolution of the ionicmolecule on its excited-state potential; and (iii)detachment, with decay of the state (neutral-ization) leaving a vibrationally excited neutralmolecule and leading to molecule-surface bond

breaking and the final outcome of desorption ordiffusion. Given that for the same z change, Pewas constant for electron injection, we concludethat step i was also constant for hole injection.That is, the change in the “spot size” of the tun-neling current caused by the change in thevalue of z did not change the fraction of the cur-rent captured by the molecule. Given the invar-iance of the branching ratio Bh, we furtherconclude that step iii was the same for all theexperiments shown in Fig. 4B. Within a DIETmodel, what remains to influence the proba-bility of manipulation is step ii, specifically thelifetime of the excited state (19).For the similar system of benzene on Si(100),

Alavi et al. (20) identified a hole excited-statelifetime of ~10 fs and a probability per hole in-jection similar to our findings. Further, in linewith theoretical predictions for long-lived excited

Rusimova et al., Science 361, 1012–1016 (2018) 7 September 2018 2 of 5

Fig. 1. STM imaging and time trace of single-molecule manipulation. (A to C) High-resolutionSTM images and corresponding schematic diagrams of the manipulation procedure (imagingparameters: +1 V, 100 pA, 3 nm × 3 nm). (A) Before manipulation. A half unit cell of Si(111)-7×7 isoutlined; the white circle atop the missing adatom–like dark spot location indicates the positionof a single toluene molecule. (B) During manipulation. (C) After manipulation. (D) Time trace of thetip height during charge injection.

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states (19), Alavi et al. reported a monotonic andnear-linear dependence of the manipulationprobability on the hole excited-state lifetime.Therefore, for our hole injections, we relate ourmaximum (i.e., constant region) manipulationprobability per hole of 320 (±10) × 10–9, with anexcited-state lifetime of 10 fs, and use the lineardependence Pe = bt (where b = 32 × 10–9 fs–1) tomap our measured probability of manipulationto an excited-state lifetime t. The result is anexcited-state lifetime that changes by two ordersofmagnitude, from 10 fs to <0.1 fs (Fig. 4, B andC,right-side y axes). A value on the order of 0.1 fsis more typical of that of adsorbates on metal

surfaces (10), indicating that the proximity ofthe tip transforms the molecule-semiconductorsystem into a metal-molecule-semiconductorsystem.Studies of cyclohexadiene on Si(100) (21, 22)

have shown the creation of an interface elec-tronic state at the location of the molecule as anSTM tip approaches. The new state lies near theFermi level, and in tandem with its creation, thehighest occupied molecular orbital (HOMO) at–1.5 V broadens and decreases in intensity as thetip approaches closer. Given that our system alsocontains a p-bonding orbital on a six-membercarbon ring that is di–s-bonded to a Si substrate,

we propose that at our higher currents (closestapproach) a similar interfacial electronic stateresults in the reduced probability per hole ofmanipulation by providing a new decay channelfor the excited state, which reduces its lifetimeand concomitantly the probability ofmanipulation.The lifetime of an excited state is the inverse of

its relaxation rate R = 1/t. We propose two com-ponents for the relaxation of the positive ionstate: (i) a fixed rate arising from the presence ofthe surface, RS = 1/tS, with tS = 10 fs; and (ii) az-dependent rate accounting for the effect of thetip,RT(z) = 1/tT(z), giving R =RS +RT(z). This tip-mediated relaxation channel will be related tothe density of states of the interface state, ri,through Fermi’s Golden Rule. An analogousscheme is used to describe the STM excitation,direct measurement, and z-dependent quench-ing of the millisecond spin excitation in singleatoms (23).Figure 4D presents schematic energy level

diagrams for three regimes of tip height: (I) largetip-molecule separation with surface-dominatedexcited-state relaxation and thus reaction, (II)intermediate-z rangewith onset of (assumednearthe Fermi level) tip-dependent interface statequenching, and (III) small-z separationwith tip-dependent interface state quenching dominat-ing the excited-state relaxation. These threeregimes are indicated in the rate dependencies ofFig. 4, A to C.For a tip-molecule system with localized elec-

tronic structure, the force between tip and mol-ecule has been calculated as F º Im with mbetween 1 and 2 (24). This calculation invoked awave function overlap argument and should bebroadly similar to the perturbative physics of theinitial generation of an interface state by ourSTM tip. Thus, wemake the connectionRT(z)ºri(z) º exp(−2kz)m, leading to a z-dependenceof Pe of

PeðzÞ ¼ btS1þ exp½�2kmðz � z0Þ� ð1Þ

where tT(z0) = tS and k = 1.17 ± 0.06 A–1, asfound from Fig. 3A. A surface-limitedmodel, Pe =btS, has a constant lifetime and therefore a con-stantmanipulation probability. This surface-onlymodel fits the +1.6 V electron injection in Fig. 4A.Below ~800 pm, there is a possible slight de-crease in Pe, which suggests that the negative-ion state is also perturbed by the interface state.For –1.0 V hole injection shown in Fig. 4C, the fitis purely exponential,Pe(z)º exp(2kz), correspond-ing to a tip-dominated dynamics. For –1.0 V, atall currents, the tip was near themolecule, hencethe excited-state dynamics were always tip-limited.At –1.3 V, the tip was slightly farther removedfrom the surface. Thus, Fig. 4B shows a fit toEq. 1 with m = 1.1 ± 0.1, and demonstrates acrossover at z0 = 830 ± 20 pm from a tip-limitedto a surface-limited regime.Our initial finding of a near-invariant rate of

manipulation can therefore be reconciled with aone-hole process. For a one-hole process, the rateis defined as k = PeI/e. Combining this with thetip-dependent manipulation probability Pe of

Rusimova et al., Science 361, 1012–1016 (2018) 7 September 2018 3 of 5

Fig. 2. Rate of manipulation. (A) Time dependence of the fractional manipulated moleculepopulation (injection parameters: +1.6 V and 450 pA; 117 molecules). The dashed line shows the fitto P(t) = 1 – exp(–kt). (B) Rate of manipulation for electron injection at +1.6 V with a linear fit.(C) Rate of manipulation for hole injection at –1.3 V (solid circles) and at –1.0 V (open triangles).See text for fit details of tip-dependent model (dashed line). Error bars indicate SD.

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Rusimova et al., Science 361, 1012–1016 (2018) 7 September 2018 4 of 5

Fig. 3. Mechanical and electric-field tip-induced interactions. (A) Tip-surface separation as a function of tunneling current during charge injection.Solid circles, hole injection at –1.3 V; open diamonds, electron injection at+1.6 V. (B) Probability of manipulation after 8 s in the mechanical presence ofthe tip (0 V, 0 pA). (C) Estimated electric field (magnitude) in the junction

between tip and surface as a function of the current I during the chargeinjection manipulation experiments. Solid circles, hole injection at –1.3 V;open diamonds, electron injection at +1.6 V. (D) Probability of manipulationafter 8 s with only the electric field interaction (–10 V, 0 pA). Error barsindicate SD; some error bars are too small to see.

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Fig. 4. Manipulation suppression at close tip proximity. (A) +1.6 Velectron injection data from Fig. 2B recast as probability per electron as afunction of the tip height. (B)–1.3 V hole injection data fromFig. 2C, recast as in(A). (C) –1.0 V hole injection. Blue lines in (A) and (B) show surface-limitedexcited-state dynamics; dashed red lines in (B) and (C) model tip-limited

dynamics. Black curve in (B) is a fit to Eq. 1. Right-side y axes of (B) and(C) show the inferred excited-state lifetime t of the positive ion with a valueof 10 fs for purely surface-limited dynamics. (D) Schematic energy leveldiagrams depicting three regimes of tip manipulation suppression. VB is thebias voltage applied to the tip relative to the sample Fermi level EF.

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Eq. 1 gives the fit to the rate of manipulation(dashed line) in Fig. 2C. At large tip-moleculeseparation (low current), the traditional linearkº I dependence is evident. For higher currentsthat led to a closer tip and quenching of themolecular excited state, the rate of manipulationbecame k = I/Im = I –0.1±0.1, giving the largelyrate-invariant region of manipulation (Fig. 2C).Molecules that requiremore than one electron

(or hole) for manipulation, such as C-Cl dissoci-ation of chlorobenzene (5) or diffusion of NH3

(25), should naturally bemore sensitive to any tipmodification of the excited state. Alongside othersemiconducting molecule/surface systems, wewould also expect any molecule/surface systemthat displays a long-lived excited state—for ex-ample, molecule/single–atomic layer insulator/metal systems (26)—to be sensitive to tip-inducedmodification of the excited state.

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ACKNOWLEDGMENTS

We thank R. Palmer, D. Bird, and D. Wolverson for fruitfuldiscussions. Funding: Supported by EPSRC grant EP/K00137X/1(P.A.S.), a University of Bath studentship (K.R.R.), and EPSRCCDT CMP grant EP/L015544/1 (R.M.P.). Author contributions:K.R.R. was the primary experimentalist and performed the analysis.R.M.P., R.H., and F.L. performed subsets of the experiments;S.C. provided theoretic support and data interpretation; P.A.S. ledthe team, designed the experiment and the analysis; and K.R.R.,S.C., and P.A.S. wrote the manuscript. Competing interests:Authors declare no competing interests. Data and materialsavailability: All data supporting this study are openly availablefrom the University of Bath data archive at https://doi.org/10.15125/BATH-00527.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/361/6406/1012/suppl/DC1Materials and MethodsSupplementary TextReferences (27–37)

30 April 2018; accepted 9 July 201810.1126/science.aat9688

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Regulating the femtosecond excited-state lifetime of a single moleculeK. R. Rusimova, R. M. Purkiss, R. Howes, F. Lee, S. Crampin and P. A. Sloan

DOI: 10.1126/science.aat9688 (6406), 1012-1016.361Science 

, this issue p. 1012Sciencedesorption probability.When the tip is very close to the molecule, it can quench the excitation. The decreased lifetime, in turn, decreases thesurface, two activation channels exist: One is invariant, but the other depends on the height of the tip above the surface.

show that for the tip-induced desorption of toluene molecules from a siliconet al.tunneling microscope. Rusimova Reactions of molecules adsorbed on surfaces can be induced by injecting electrons from the tip of a scanning

Selectively exciting desorption

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