Detection of heating in current-carryingmolecular junctions by Raman scattering

ZVI IOFFE†, TAMAR SHAMAI†, AYELET OPHIR, GILAD NOY, ILAN YUTSIS, KOBI KFIR,ORI CHESHNOVSKY* AND YORAM SELZER*School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel†These authors contributed equally to this work.

*e-mail: selzer@post.tau.ac.il; orich@chemsg1.tau.ac.il

Published online: 26 October 2008; doi:10.1038/nnano.2008.304

As the scaling of electronic components continues, local heatingwill have an increasing influence on the stability andperformance of nanoscale electronic devices. In particular,the low heat capacity of molecular junctions means thatit will be essential to understand local heating and heatconduction in these junctions1–4. Here we report a methodfor directly monitoring the effective temperature ofcurrent-carrying junctions with surface enhanced Ramanspectroscopy (SERS) that involves measuring both theStokes and anti-Stokes components of the Raman scattering.All the Raman-active modes in our system show similarheating as a function of bias at room temperature, whichsuggests fast vibrational relaxation processes inside thejunctions. These results demonstrate the power of directspectroscopic probing of heating and cooling processesin nanostructures.

The effective temperature of current-carrying molecularjunctions is a result of the equilibrium between heating and heatdissipation out of the junctions. The former results from inelasticscattering of conducting electrons and is dominated by thecoupling of electronic molecular states with molecularvibrations1,3,4. The latter depends on the vibrational couplingbetween the ‘hot’ molecular vibrations and the degrees offreedom of the ‘cold’ electrode leads2.

Previous experiments indirectly evaluated the effectivelocal temperature of nanoscale junctions by measuringeither the breaking rate of metal atomic chains as a function ofbias5, changes in the apparent energy barrier between twomicro-configurations of atomic point contacts6, or the forcerequired to break molecule–metal bonds under bias7.Heat conduction through molecules was recently observedby probing surface thermal disorder using sum frequencygeneration spectroscopy8.

Surface-enhanced Raman scattering (SERS) is an obviouspossibility for probing the temperature of junctions9, in particularbecause gaps of a few nanometres between metal particles havebeen implicated as possible ‘hot spots’ with particularly strongRaman enhancement9–12. Former Raman scattering measurementsfrom molecular junctions probed their characteristics only forthe Stokes (S) components13–16. Because we are able to also(simultaneously) probe the anti-Stokes (AS) components, it ispossible to determine the AS/S ratio of each Raman activevibrational mode, n, which directly represents its steady-state

non-equilibrium population in the presence of inelastic tunnellingcurrent. This ratio can be translated into a mode-specific effectivetemperature, Teff(n) (ref. 17).

Raman measurements of ‘on-edge’ junctions18 of the formsilver–SAMBPDT–silver (SAMBPDT, self-assembled monolayerof 4,40-biphenyldithiol) were performed using two excitationwave lengths: 532 and 671 nm (see Fig. 1 and Methods).

Of interest are the modes of BPDT at 1,083, 1,280 and1,585 cm21, which have been observed to be excited in 4 Kinelastic electron tunnelling spectroscopy (IETS) measurements(Fig. 1). A spatial map of the 1,585 cm21 Raman scatteringshows strongly localized enhanced scattering from a junction(Fig. 2). The ratio of the integrated signals at the junctionand at an arbitrary spot along the silver electrode is �5,corresponding to a factor of �800 in the effective Ramanenhancement per molecule (see Methods), which is inagreement with previous reports on field enhancementat ‘hot spots’9–12.

Figure 3a,b depicts plots of the apparent Teff(n) of severalvibrational modes in two representative junctions as a functionof bias potential, based on their AS/S ratio measured with 532and 671 nm Raman lasers, respectively. For detailed discussionon the statistics of the measurements see the Methods and theSupplementary Information. Apparent Teff (n) was calculated foreach mode at each bias according to equation (1) assumingsASA2

AS/sSA2S ¼ 1

IAS

IS

¼ ðnL þ nvÞ4

ðnL � nvÞ4 expð�hnv=kBTeff ðnÞÞ

sAS

sS

A2AS

A2S

ð1Þ

where IAS(S) is the intensity of the anti-Stokes (Stokes) Ramanmode, nL(n) is the frequency of the laser (Raman mode), sAS(S) isthe anti-Stokes (Stokes) scattering cross-section of the adsorbedmolecules and AAS(S) is the average local field enhancement at themolecules at the anti-Stokes (Stokes) frequency.

Equation (1) is thermodynamically well defined only underequilibrium conditions, that is, at zero bias. At this bias, weassume that the ratio sASA2

AS/sSA2S is close to unity. This

assumption is experimentally supported (see later discussion) bythe fact that Teff(n) at zero bias is close to the measuredtemperature under the beam as determined by a microfabricatedthermocouple. Theoretical calculations also show that for the

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geometry of an ‘on-edge’ junction the plasmon resonances in thevisible region constitute a continuous band, with relatively smallchanges in (wavelength-dependent) local field enhancement19.More intricate effects of apparent temperature changes due to thedependence of chemical enhancement on bias are discussed inthe following.

Once a bias is applied and current flows through themolecules, the occupancy of vibrational levels may change, butnot necessarily obeying a Bose–Einstein distribution2,4. However,because the ratio sASA2

AS/sSA2S is in principle bias dependent20,

it is essential to determine its functional dependence beforeevaluating bias-induced changes in Teff(n), as discussed inthe following.

The apparent dependence of Teff (n) on the applied bias(Fig. 3a) reveals two modalities: (1) Between 0 and �j0.2 Vj inboth polarities an apparent cooling process is observed; (2) at biasvalues higher than j0.2 Vj, heating of the vibrational modestakes place.

The apparent cooling in the first zone can be effectivelyaccounted for by the dependence of the enhancementratio sASA2

AS/sSA2S on bias by distinct contributions of

‘chemical enhancement’ Raman processes. This assumption issupported by the fact that the Stokes signal decreases uponthe increase of bias up to j0.2 Vj, although the ground-state population of the vibrational modes hardly changes(Fig. 3a, inset).

0.010

0.008

0.006

0.004dI/d

V (A

V –1)

0.002

0.000–2 –1

Bias voltage (V)

0 1 2

HS SH

AgAg

Si/SiO2

Junction

15

10

5

0

–5

–10

–15 1,58

5 cm

–1

1,38

0 cm

–1

1,28

0 cm

–1

1,08

3 cm

–1

1,08

3 cm

–1

1,20

5 cm

–1

1,38

0 cm

–1

1,58

5 cm

–1

dI2 /d

V 2

(µA

V–2)

–0.3 –0.2

Bias voltage (V)

–0.1 0.0 0.1 0.2 0.3

Figure 1 Experimental setup and conductance characteristics of the junctions. a, Scheme of an ‘on-edge’ junction under the Raman setup. Inset: the structure

of BPDT and a typical dI/dV curve measured at 300 K. b, Inelastic electron tunnelling spectroscopy (IETS) of a junction at cryogenic temperatures. The 1,585, 1,380

and 1,205 cm21 modes are associated with the phenyl rings, 1,083 and 1,280 cm21 correspond to the C–S bond and to the C–C bond between the rings,

respectively. All Raman active vibrational modes are revealed in the IETS measurements, thus enabling determination of the effective temperature of modes in the

conducting junctions.

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The theory of the charge-transfer ‘chemical enhancement’process has been developed in considerable detail20–23.Briefly, when an electron is transferred from the metal toan empty anti-bonding p*-orbital of the aromatic rings, the net

p character is reduced and skeletal vibrations, such as the onesthat are monitored by the Raman in this study, should beenhanced. The preferential enhancement of either the S or ASscattering must be assigned to the relative position of the

20,000

15,000

10,000Inte

nsity

Inte

nsity

5,000

1,200

1,300

1,400

1,500

2,000 1,500

Wavenumber (cm–1)

1,000

1,083

1,280

1,585

2,0001,500

Wavelength (cm–1)

1,000

1,083

1,2801,585

2 µm

Figure 2 Raman spectra and maps of a junction. a, Raman spectrum in the Stokes (S) regime (671 nm laser) of BPDT molecules in a junction (black) and as a

monolayer at an arbitrary spot on the silver electrode (red). b, The corresponding anti-Stokes (AS) spectrum. c, Optical microscope top view of a junction. d, Raman

map of a junction based on the 1,585 cm21 Stokes line. The Raman enhancement at the junction is shown as brighter pixels. Raman spectra for a were taken at the

points indicated by the arrows. e, Raman map of the 1,585 cm21 AS line of the same junction.

380

360

340

Tem

pera

ture

(K)

320

400

420

380

360

Tem

pera

ture

(K)

340–0.4 –0.2 0.0

Bias voltage (V)

0.2 0.4

140

120

100

20

–0.4 –0.2 0.0Bias voltage (V)

Bias voltage (V)0.2 0.4

0.60.40.20.0–0.2–0.4–0.6

40

60

100

80

Inte

nsity

(tho

usan

dsof

cou

nts)

Inte

nsity

(tho

usan

dsof

cou

nts)

–0.4 –0.2 0.0

Bias voltage (V)

0.2 0.4

Figure 3 Teff(n) as a function of bias for two representative junctions. a, Plot of Teff(n) as a function of bias voltage for the 1,585 cm21 (black triangles) and

1,083 cm21 (red squares) modes (532 nm laser). Inset: the Stokes signal of this junction is bias dependent, suggesting an underlying resonance model. b, Plot of

Teff(n) as a function of bias voltage for the 1,585 cm21 (black triangles), 1,280 cm21 (blue circles) and 1,083 cm21 (red squares) modes (671 nm laser). Inset: the

Stokes signal is bias independent. Heating of the order of 30 K for all modes is observed in both bias polarities. (See Supplementary Information for a discussion of

the 3 K error bars.)

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absolute energy of the Raman-shifted photon to the metal–molecule electronic transition (Fig. 4a).

We have modified the theoretical model described in ref. 20 toevaluate the AS/S ratio as a function of bias (see Methods):

IAS

IS

¼ ðvL þ vvÞ4

ðvL � vvÞ4 expð�h� vv=kBTeff Þ

� ð½nL þ V=2� � ne þ nnÞ2 þ G 2

e

ð½nL þ V=2� � ne � nnÞ2 þ G 2

e

"

þð½nL � V=2� � ne þ nnÞ2 þ G 2

e

ð½nL � V=2� � ne � nnÞ2 þ G 2

e

#ð2Þ

where nL, nv, ne, Ge and V are the energies (in eV) for the excitationlaser, the vibrational mode of the SERS band, the electronic metal-to-molecule transition, the half-width of the molecular electroniclevel and the applied bias, respectively. Note that equation (2),containing a Boltzman thermal factor, oversimplifies the complexnon-equilibrium vibrational distribution in the system.

The two green curves in Fig. 4c plot the AS/S ratiobased on equation (2) using nL ¼ 2.33 eV (532 nm greenlaser), nv ¼ 0.196 eV (lower curve) and 0.133 eV (upper curve),ne ¼ 2.6 eV and Ge ¼ 0.4 eV. The value of ne is based on previousexperimental results24. It corresponds to a broad continuum ofunoccupied states of the adsorbed BPDT as determined byprevious theoretical calculation of silver–SAMBPDT–silverjunctions25. The damping factor (resonance half-width) Ge isbased on previously reported values20,26,27. Although the modelcalculation depicted in Fig. 4c is not exact, it does reproduce twoessential characteristics of the experimental results shown inFig. 3a, namely apparent cooling at lower than j0.2 Vj bias, anddistinct apparent temperatures for the two vibrational modes atzero bias with a higher apparent temperature for the 1,083 cm21

(0.133 eV) mode. All these are a direct consequence of theresonance model.

Once the bias voltage is larger than j0.2 Vj, heating of theRaman-monitored modes is observed. Because calculationsshow that above 0 K hot electrons at the tail of the Fermidistribution can heat adsorbed molecules28, the observed heatingis probably already commencing at bias values lower than�j0.2 Vj, overlaid in our case by apparent ‘cooling’ caused byresonance (Fig. 3a).

The above resonance model indicates that with the sameparameters a 671 nm (1.84 eV) Raman laser produces an AS/Sratio enhancement that is practically bias-independent (Fig. 4c, redline). Therefore, any observation of change is expected to be dueto pure heating or cooling. The experimental results (Fig. 3b)support this assumption in three ways: (1) the Stokes signal (seeinset) is bias-independent; (2) Teff (n) at zero bias is essentiallysimilar for all vibrational modes; (3) Teff(n) at zero bias is inagreement with the actual temperature under the laser beam, asdetermined by a microfabricated calibrated gold/platinumthermocouple with an ‘on-edge’ structure. The increase of Teff (n)with bias appears to be comparable (��2) to previously reportedresults on alkyl chains7. Assuming that heat dissipation is similarin both cases (a monolayer in our case versus individual moleculessurrounded by a solvent in ref. 7), the higher temperature can beattributed to the higher current density in BPDT junctions.

We do not expect a sharp threshold for heating in our room-temperature measurements; however, progressive heating isclearly observed and, unlike IETS, the evidence for inelastictunnelling is also discernible in our spectroscopic observations,above the thresholds, for this process.

Note in Fig. 3 that heating of the junction is observed for bothbias polarities. The fact that the Teff(n) of all vibrational modes isquite similar, changing with bias in a similar way, suggests fastintramolecular vibrational relaxation or redistribution (IVR)within the junction.

The experimental results reveal a rich heating/coolingbehaviour that is inexplicable using existing models. Recenttheoretical discussions on bias dependence of heat dissipation inmolecular junctions2–4,29,30 do not include the additional

420

400

380

360

340

0.0 0.2 0.4 0.6Bias voltage (V)

0.8 1.0

Tem

pera

ture

(a.u

.)

V V

ASAS

ASS

S

S

V L V L

E F1 E F2E F2

E F1

Metal 1Metal 2Metal 1 Metal 2

Vbias

Figure 4 Schematic model of a charge-transfer Raman scattering process. Photons with nL energy excite an electron to a level above the Fermi energy, having

a certain weight in the unoccupied part of the resonance leading to partial nuclear excitation of the molecule. The electron is de-excited by emitting a Raman photon

with energy (nL+nn). a, At zero bias the contribution of the two electrodes to the resonance process is similar. b, This contribution changes under an applied bias.

c, The dependence of the AS/S ratio on bias (based on equation (2)) assuming a resonant level 2.6 eV above the Fermi level with 0.4 eV half-width. The behaviour

under a 532 nm laser at Teff(n) ¼ 320 K is shown as green curves, with the upper curve corresponding to the 1,083 cm21 mode and the lower curve to the

1,585 cm21 mode. The behaviour for the same parameters under a 671 nm Raman laser is shown as a red curve, with Teff(n) ¼ 345 K. In this case the AS/S ratio

appears to be bias independent for all vibrational modes.

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intermolecular dissipation channel prevailing in our monolayerjunctions, Also, dissipation into the electrodes depends on thedetailed phonon density of states at the interface between themolecules and the electrodes. Exact calculation of the non-equilibrium distribution of phonons would require a self-consistent calculation of coupled phonon and electron dynamicsthat is beyond the scope of this letter.

To conclude, we demonstrate the use of SERS as a tool tospectroscopically monitor heating (and cooling) processes inconducting molecular junctions. The method allows directobservation of the non-equilibrium occupancy of vibrationalmodes at any bias and current conditions. Combined with IETS,the method enables monitoring of the fate of molecularvibrations at any given temperature. Further work to enhance thecapabilities of the method and to explore other molecularjunctions and processes is in progress.

METHODS

FABRICATION OF JUNCTIONS

The fabrication of these junctions has been outlined in detail in previouspublications18. Briefly, 20-nm-thick and �2-mm-wide silver lines were patternedperpendicularly by optical lithography, all the way to the edge of a cleaved siliconwafer covered with 100 nm of oxide. The patterned sample was placed in a 1 mMethanol solution of BPDT for 12 h of self-assembly at room temperature under anitrogen blanket. After rinsing and drying under nitrogen, a 60-nm silver film wasvapour deposited onto the side of the sample to realize the ‘on-edge’ structure.

The yield of junctions stable enough for extended Raman measurementswas �1%. To maintain stability, the bias potential on the junctions was notallowed to exceed j0.5 Vj under the laser beam.

In order to impart ‘hot-spot’ characteristics to the junctions, that is, toincrease their SERS characteristics relative to their surroundings, the sampleswere cooled to 90 K during the last metal deposition step. In this way, theresulting (top contact) silver films were rough, consisting of large aggregatesof nanoparticles a few tens of nanometres in diameter, leading to a stronglocal electromagnetic field across the interconnecting molecules atthe junctions9–12.

RAMAN MEASUREMENTS

Raman measurements of the junctions under ambient conditions were taken by ahome-built scanning Raman confocal microscope operating in reflection mode,with through the objective illumination from fibre-coupled 532 and 671 nmsolid-state lasers with an effective power on the samples of �5 mW. Thereflected Raman scattered light was collected with a 0.7 NA �100 objective andanalysed by a spectrometer equipped with an electron multiplying CCD camera(Andor Newton EM, DU971N-BV). Bias-dependent Raman spectra weremeasured using an integration time of 10 s for each bias value. The alignment ofsamples to maximize the Raman signal was performed once on each junctionbefore a full set of bias-dependent measurements. All reported AS/S ratios takeinto account the sensitivity of the spectrometer as a function of wavenumber. Inmost junctions, the signal to noise ratio of the 1,280 cm21 mode was too low forproper calculations.

Raman maps were collected by raster scanning the conducting junctions in300-nm steps using an x–y piezoelectric stage. Full Stokes and antiStokes spectrawere collected at each pixel, enabling the generation of Raman maps of anyarbitrary Raman line.

Two representative plots of Teff (n) as a function of bias are presented inFig. 3. The average results from all junctions (15 altogether) are not presented, asthey are shifted relative to one another by their temperature at zero bias. Thistemperature variation results from the variations in the exact position of the laserbeam on the sample as well as from variations in the number of Raman-activemolecules in each junction. Nevertheless, all junctions reveal thesame heating/cooling trends that are depicted in Fig. 3, justifying thesemi-quantitative discussion presented in the text. Further details anddiscussion on the statistical significance of our results can be found in theSupplementary Information.

CALCULATION OF ENHANCEMENT FACTOR AT A JUNCTION

With a laser beam diameter at the surface of �500 nm, the measured Ramansignal at a junction has two contributions: one from the �40,000 molecules in the

junction itself (500 nm beam diameter � 20 nm top silver film thickness �4 molecules nm22) and the other from the �800,000 molecules adsorbed as amonolayer on the top silver electrode (p(250 nm)2 � 4 molecules nm22). Finite-difference time-domain (FDTD) modelling of the junctions suggests, however,that only a small fraction (conservatively �10%) of the molecules (located at thejunction side adjacent to the laser beam) is interacting with the incoming light.Thus, effectively, the Raman signal from �4,000 molecules in a junction is 4 timesstronger than the Raman signal from �800,000 molecules in a monolayer; that is,the effective enhancement per molecule is of the order of �800.

The reported AS/S ratios are not corrected for the fact that �20% of thecollected signal is coming from outside of the junctions. At zero bias, nocorrection is needed because there is no heating current. At the highest appliedbias (0.5 V) with the 30 K reported heating, the corrected Teff should have been5 K higher assuming 90% inactive molecules in the junction. The reported Teff isessentially a lower bound to the real heating effect.

RESONANCE MODEL

The AS/S ratio of equation (2) is a modification of a ‘chemical enhancement’resonance model that is applicable to a molecule adsorbed on anunbiased surface20:

IAS

IS

/ðvL þ vvÞ

4

ðvL � vvÞ4 expð�h� vv=kBTeff Þ �

ðnL � ne þ nnÞ2 þ G 2

e

ðnL � ne � nnÞ2 þ G 2

e

" #ð3Þ

This equation shows how the ratio is changed as a function of nL, the laser’swavelength. We have extended the above theoretical model to include theinfluence of bias on the AS/S intensity ratio by using three simplisticassumptions: (1) the bias merely shifts the energy levels of the metal relative tothe molecular levels, namely the sum of the bias potential and the laser energy aredetermining the transition probabilities of the electron transfer; (2) the junctionpotential V is assumed here to be equally distributed between the two contactsrelative to the molecules so that the metal junction levels are shifted by +V/2relative to the molecular levels, respectively; (3) each electrode contributesindependently to the chemical enhancement. Modification of the above equationby these three assumptions results in equation (2).

Mild variations in the values of ne and Ge (within the order of j0.2 Vj) do notchange the qualitative behaviour described above as long as ne . nL. The widthof the resonance 2Ge affects the rate of apparent ‘cooling’ as a function of bias.

Received 14 July 2008; accepted 19 September 2008; published 26 October 2008.

References1. Montgomery, M. J., Todorov, T. N. & Sutton, A. P. Power dissipation in nanoscale conductors.

J. Phys. Condens. Matter 14, 5377–5389 (2002).2. Galperin, M., Nitzan, A. & Ratner, M. A. Heat conduction in molecular transport junctions. Phys.

Rev. B 75, 155312 (2007).3. Chen, Y. C., Zwolak, M. & Di Ventra, M. Local heating in nanoscale conductors. Nano Lett. 3,

1691–1694 (2003).4. Pecchia, A., Romano, G. & Di Carlo, A. Theory of heat dissipation in molecular electronics. Phys. Rev.

B 75, 035401 (2007).5. Smit, R. H. M., Untiedt, C. & van Ruitenbeek, J. M. The high-bias stability of monatomic chains.

Nanotechnology 15, S472–S478 (2004).6. Tsutsui, M., Kurokawa, S. & Sakai, A. Bias-induced local heating in Au atom-sized contacts.

Nanotechnology 17, 5334–5338 (2006).7. Huang, Z. et al. Local ionic and electron heating in single-molecule junctions. Nature Nanotech. 2,

698–703 (2007).8. Wang, Z. H. et al. Ultrafast flash thermal conductance of molecular chains. Science 317,

787–790 (2007).9. Moskovits, M. Surface-enhanced Raman spectroscopy: a brief retrospective. J. Raman Spectrosc. 36,

485–496 (2005).10. Jiang, J., Bosnick, K., Maillard, M. & Brus, L. Single molecule Raman spectroscopy at the junctions of

large Ag nanocrystals. J. Phys. Chem. B 107, 9964–9972 (2003).11. Fromm, D. P. et al. Exploring the chemical enhancement for surface-enhanced Raman scattering

with Au bowtie nanoantennas. J. Chem. Phys. 124, 061101 (2006).12. Zhou, Q., Li, X., Fan, Q., Zhang, X. & Zheng, J. Charge transfer between metal nanoparticles

interconnected with a functionalized molecule probed by surface enhanced Raman spectroscopy.Angew. Chem. Int. Ed. 45, 3970–3973 (2006).

13. Ward, D. R. et al. Electromigrated nanoscale gaps for surface-enhanced Raman spectroscopy.Nano Lett. 7, 1396–1400 (2007).

14. Ward, D. R. et al. Simultaneous measurements of electronic conduction and Raman response inmolecular junctions. Nano Lett. 8, 919–924 (2008).

15. Tian, J. H. et al. Study of molecular junctions with a combined surface-enhanced Raman andmechanically controllable break junction method. J. Am. Chem. Soc. 128, 14748–14749 (2006).

16. Nowak, A. M. & McCreery, R. L. In situ Raman spectroscopy of bias-induced structural changes innitroazobenzene molecular electronic junctions. J. Am. Chem. Soc. 126, 16621–16631 (2004).

17. Oron-Carl, M. & Krupke, R. Raman spectroscopic evidence for hot-phonon generation in electricallybiased carbon nanotubes. Phys. Rev. Lett. 100, 127401 (2008).

18. Shamai, T., Ophir, A. & Selzer, Y. Fabrication and characterization of ‘on-edge’ molecular junctionsfor molecular electronics. Appl. Phys. Lett. 91, 102108 (2007).

19. Nordlander, P. & Le, F. Plasmonic structure and electromagnetic field enhancements in the metallicnanoparticle-film system. Appl. Phys. B. 84, 35–41 (2006).

LETTERS

nature nanotechnology | VOL 3 | DECEMBER 2008 | www.nature.com/naturenanotechnology 731

© 2008 Macmillan Publishers Limited. All rights reserved.

20. Brolo, A. G., Sanderson, A. C. & Smith, A. P. Ratio of the surface-enhanced anti-Stokes scattering tothe surface-enhanced Stokes–Raman scattering for molecules adsorbed on a silver electrode. Phys.Rev. B. 69, 045424 (2004).

21. Persson, B. N. J. On the theory of surface-enhanced Raman scattering. Chem. Phys. Lett. 82,561–565 (1981).

22. Adrian, F. J. Charge transfer effects in surface enhanced Raman scattering. J. Chem. Phys. 77,5302–5314 (1982).

23. Lombardi, J. R., Birke, R. L., Lu, T. & Xu, J. Charge transfer theory of surface enhanced Ramanspectroscopy: Herzberg–Teller contributions. J. Chem. Phys. 84, 4174–4180 (1986).

24. Demuth, J. E. & Sanda, P. N. Observation of charge transfer states for pyridine chamisorbed on Ag.Phys. Rev. Lett. 47, 57–60 (1981).

25. Heimel, G., Romaner, L., Zojer, E. & Bredas, J. L. Toward control of the metal–organic interfacialelectronic structure in molecular electronics: A first-principles study on self-assembled monolayersof p-conjugated molecules on noble metals. Nano Lett. 7, 932–940 (2007).

26. Marinyuk, V. V., Lazorenko-Manevich, R. M. & Kolotyrkin, Y. M. Nature of the interaction ofadsorbate molecules with metal ad-atoms. J. Electroanal. Chem. 110, 111–118 (1980).

27. Kambhampati, P. & Campion, A. Surface enhanced Raman scattering as a probe of adsorbate–substrate charge-transfer excitations. Surf. Sci. 427, 115–125 (1999).

28. Todorov, T. N. Local heating in ballistic atomic scale contacts. Phil. Mag. B 9, 965–973 (1998).29. Schulze, G. et al. Resonant electron heating and molecular phonon cooling in single C60 junctions.

Phys. Rev. Lett. 100, 136801 (2008).30. D’Agosta, R., Sai, N. & Di Ventra, M. Local electron heating in nanoscale conductors. Nano Lett.

6, 2935–2938 (2006).

Supplementary Information accompanies this paper at www.nature.com/naturenanotechnology.

AcknowledgementsWe thank A. Nitzan from TAU for very insightful discussions. Support by the GIF young scientistprogram for Y.S. is gratefully acknowledged. T.S. thanks the Israeli Ministry of Science and Technologyfor an Eshkol fellowship. The research was supported by the Israel Science foundation under grant no.987/05 (OC).

Author informationReprints and permission information is available online at http://npg.nature.com/reprintsandpermissions/.Correspondence and requests for materials should be addressed to Y.S. and O.C.

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