Vol 46 4/ eemer 9 doi:.8/nture869 LETTER Oservtion of moleulr oritl gting Hyunwook ong,, Youngsng Kim {, Yun Hee Jng, Heejun Jeong, Mrk A. Reed 4 & Tkhee Lee, The ontrol of hrge trnsport in n tive eletroni devie depends intimtely on the modultion of the internl hrge density y n externl node. For exmple, field-effet trnsistor relies on the gted eletrostti modultion of the hnnel hrge produed y hnging the reltive position of the ondution nd vlene nds with respet to the eletrodes. In moleulr-sle devies, longstnding hllenge hs een to rete true three-terminl devie tht opertes in this mnner (tht is, y modifying oritl energy). Here we report the oservtion of suh solid-stte moleulr devie, in whih trnsport urrent is diretly modulted y n externl gte voltge. Resonne-enhned oupling to the nerest moleulr oritl is reveled y eletron tunnelling spetrosopy, demonstrting diret moleulr oritl gting in n eletroni devie. Our findings demonstrte tht true moleulr trnsistors n e reted, nd so enhne the prospets for moleulrly engineered eletroni devies. The demonstrtion of true three-terminl moleulr devie, one tht depends on externl modultion of moleulr oritls, hs een the outstnding hllenge of the field of moleulr eletronis sine soon fter its ineption 5, ut until now it hs defied experimentl efforts. Theoretil proposls indite tht field-effet gting of moleulr juntion is possile 6,7. Crrier modultion in moleulr juntions n e hieved y lterntive non-moleulr mesosopi mehnisms suh s Coulom lokde nd Kondo effets 8, nd hs een limed to e oserved in n eletrohemil rek juntion, ut until now proof of diret oritl gte ontrol of solid-stte devie hs een elusive. inglemoleule oritl modultion hs een demonstrted in two-terminl, non-devie snning tunnelling mirosopy experiments,lending enourgement to the development of three-terminl devie implementtions, notwithstnding onerns of the degree of gte ontrol tht n e hieved in field-effet trnsistor (FET) onfigurtion 6,.Here we demonstrte diret eletrostti modultion of oritls in solidstte moleulr FET onfigurtion, with oth effetive gte ontrol nd enhned resonnt oupling of the moleulr oritls to the soure nd drin eletrodes. We show this using multiproe pproh omining vriety of trnsport tehniques tht give self-onsistent hrteriztion of the moleulr juntion. As illustrted in the inset of Fig., individul moleules re onneted to soure nd drin eletrodes with ottom-gte ontrol eletrode in FET onfigurtion. In suh devies, the energies of the moleulr oritls with respet to the Fermi level of the eletrodes n e diretly tuned y djusting gte voltge, V. We mke suh devies using the well-known eletromigrtion tehnique of frturing ontinuous gold wire (oted with the desired moleules, in vuum, t 4. K) tht is pled over n oxidized luminium gte eletrode 8,9 (Methods ummry). This produes soure nd drin eletrodes with nnometre-sle gp, whih re often ridged y single or very few moleules, reting moleulr juntions. We hve exmined smple size of 48 devies ontining two prototype moleules: the ontrol,,8-otnedithiol (OT) with n lkyl kone s s-sturted liphti moleule; nd the tive devie,,4-enzenedithiol (BT) with delolized romti ring s p-onjugted moleule. We found 5 devies with sigmoidl nd signifintly gte-dependent soure drin urrent (I) voltge (V) hrteristis. The mesured low-is ondutne is in good greement with single-moleule ondutne vlues of the moleules of interest (upplementry Fig. 4). Coherent, off-resonne tunnelling with strong moleule metl ontt oupling (for exmple, the thiol gold ond) n e resonly expeted s ondution mehnism for these juntions, in ordne with the form of the symmetri I(V) urves 4,5, the lk of signifint ondutne gp 4,5, the exponentil dependene of ondutne on moleulr length 5 7 (upplementry Fig. 6) nd the temperture-independent I(V) ehviour 7 (upplementry Fig. 5). Figure shows representtive I(V) urves for Au OT Au (s-sturted liphti) juntion mesured t different V vlues. The tunnelling urrent pssing through the OT juntion inreses s V eomes inresingly negtive. The orresponding nlysis of ln(i/v ) versus /V, the grph of whih is known s Fowler Nordheim plot, is shown in Fig.. Here two distint trnsport regimes re evident; the oundry etween them, whih ours t the trnsition voltge, V trns, is indited with n rrow nd exhiits ler gte dependene. In the low-is region, the logrithmi dependene is hrteristi of diret tunnelling (V, W B /e, where W B is the rrier height nd e is the elementry hrge), wheres in the high-is region ove V trns, the urves yield liner reltion with negtive slope, inditive of Fowler Nordheim tunnelling or field emission (V. W B /e). The mesurement of V trns llows n experimentl determintion of the height of the energy rrier ssoited with the tunnelling trnsport in moleulr juntions, given y the differene etween the eletrode Fermi energy, E F, nd the energy of the nerest moleulr oritl (the highest oupied moleulr oritl () or the lowest unoupied moleulr oritl (LUMO) 8 ; see upplementry Informtion for detils). We oserve ontrollle gte-voltge dependene of V trns in the moleulr juntion. For Au OT Au juntions, V trns shifts to lower is s more negtive V is pplied (Fig. ). To explore this further, we plot the mesured V trns vlues ginst V in Fig.. We find tht V trns sles linerly nd reversily s funtion of V. The slope, 5 V trns /V, is the gte effiieny ftor, whih desries the effetiveness of moleulr oritl gting; for exmple, if ejj 5.5 V (from the liner fit in Fig. ) then the moleulr oritl energy hnges y.5 when V is pplied to the gte eletrode. This lrge gte oupling proly indites tht the moleule resides lose to the gte metl dieletri interfe to redue sreening y the eletrodes; future improvements to the devie performne my e mde y using tpered-eletrode pproh.we n lso determine the tul mount of moleulr oritl shift produed y the pplied gte voltge in terms of n effetive moleulr oritl gting energy,,eff 5 ejjv. eprtment of Nnoio Mterils nd Eletronis, eprtment of Mterils iene nd Engineering, wngju Institute of iene nd Tehnology, wngju 5-7, outh Kore. eprtment of Applied Physis, Hnyng University, Ansn 46-79, outh Kore. 4 eprtments of Eletril Engineering nd Applied Physis, Yle University, New Hven, Connetiut 65, UA. {Present ddress: eprtment of Physis, University of Konstnz, -78457 Konstnz, ermny. 9 Mmilln Pulishers Limited. All rights reserved 9
LETTER NATURE Vol 46 4/ eemer 9 I (na) 5.7 V trns (V) Al O /Al V =.8 V 5 V =.6 V 4 V =. V V =. V V =.6 V 6 V =.8 V 5 V V =.6 V =. V V =. V 8 V =. V V =.6 V V =. V V =. V 5.6.5.4... Au,eff Φ B In(I/V ) 8 V =. V /V (V ) E F Au α = +.5 d.6 FN..8.4..6..8.6.4. V (V),eff ().5.4.. C B T A.5 C B A din(i/v )/d(/v). Figure te-ontrolled hrge trnsport hrteristis of Au-OT- Au juntion., Representtive I(V) urves mesured t 4. K for different vlues of V. Inset, the devie struture nd shemti., soure;, drin;, gte. le r, nm., Fowler Nordheim plots orresponding to the I(V) urves in, exhiiting the trnsition from diret to Fowler Nordheim tunnelling with ler gte dependene. The plots re offset vertilly for lrity. The rrows indite the oundries etween trnsport regimes (orresponding to V trns )., Liner sling of V trns in terms of V. The error rs denote the s.d. of individul mesurements for severl devies nd the solid line represents liner fit. Inset, the shemti of the energy nd for -medited hole tunnelling, where,eff desries the tul mount of moleulr oritl shift produed y gting. d, Two-dimensionl olour mp of dln(i/v )/d(/v) (from Fowler Nordheim plots). Energy-nd digrms orresponding to four different regions (points A ) re lso shown. FN, Fowler Nordheim tunnelling; T, diret tunnelling. The gte-ontrolled tunnelling trnsport is fully illustrted in Fig. d, where we show two-dimensionl olour mp of dln(i/v )/d(/v) (from Fowler Nordheim plots) in the trnsition region, s funtion of V nd,eff. The dshed line inditing the liner fit of V trns versus V defines the oundry etween two distint trnsport regimes (diret nd Fowler Nordheim tunnelling). In Fig. d, the trnsport t point A shows typil diret tunnelling ross the trpezoidl rrier t given vlues of V nd,eff. If the pplied is inreses further, to reh V trns (point B), trnsition from trpezoidl to tringulr rrier shpe ours. This indites the onset of Fowler Nordheim tunnelling, orresponding to n infletion point in the plot of ln(i/v )ginst/v. Point C shows V trns to e t lower is thn t point B, whihis ttriuted to derese in the je F E j offset (where E is the energy of the level), rising from more negtive V vlue. Finlly, t point, the rrier is ompletely tringulr shpe nd the hrge trnsport is dominted y Fowler Nordheim tunnelling. In three-terminl devie, negtive or positive gte voltge would respetively rise or lower the oritl energies in the moleules reltive to E F (refs, 6). Hene, positive vlue of indites -medited hole tunnelling (p-type-like; Fig., inset). Conversely, would e negtive for LUMO-medited eletron tunnelling (n-type-like); for exmple, Au,4-enzenediynide Au juntion (upplementry Fig. 8). By extrpolting the y interept from the liner fit in Fig., we otin the zero-gte trnsition voltge, V trns, 5.9 6.6 V, whih provides n estimte of the originl position (t V 5 V) of the level reltive to E F in Au OT Au juntions. 4 9 Mmilln Pulishers Limited. All rights reserved Now let us exmine p-onjugted BT moleule, the prototypil moleulr trnsport juntion 4,6,4. Figure shows the gte-modulted trnsport of Au BT Au juntion. The tunnelling urrent flowing through the BT juntion is enhned when negtive V is pplied, wheres positive V suppresses the urrent level (p-type-like; see lso upplementry Fig. 7). Figure nd Fig. respetively disply plots of ln(i/v ) versus /V nd V trns versus V for this moleulr system. The positive sign of 5. in the BT juntion expliitly indites tht -medited tunnelling is the dominnt trnsport hnnel. The olour mp of dln(i/v )/d(/v) (from Fowler Nordheim plots) is shown s funtion of V nd,eff (Fig., inset); the solid line nd the dotted rrow indite the liner fit nd the y interept otined from the plot of V trns versus V, respetively. We find tht V trns, 5.4 6.4 V for the BT juntion, whih is muh less thn the vlue for the OT juntion owing to the p-onjugted BT moleule hving smller LUMO gp. These results demonstrte reltive movement of the moleulr oritls with respet to E F, omplished y gting. This should give rise to modified oupling of the moleulr oritls to the eletrodes, depending oth on the oritl Fermi level sping nd on the hrter of the moleulr oritl (tht is, s-sturted or p-onjugted). An unmiguous method of investigting this in urrent-rrying moleulr juntions is inelsti eletron tunnelling (IET) spetrosopy, whih is powerful tool for investigting the moleulr role in hrge trnsport, speifilly the oupling etween hrge rriers nd moleulr virtions,9. We performed IET spetrosopy
NATURE Vol 46 4/ eemer 9 LETTER 6 4 I (µa) 4 6. V = V 8 V = V V = V V = V V = V.6 V = V V = V α = +... V = V..4 FN 4. 6.8 V = V..8 T.5..5 8.4,eff () 5 5 /V (V ) V (V) In(I/V ) V trns (V) Figure te-ontrolled hrge trnsport hrteristis of Au-BT- Au juntion., Representtive I(V) urves mesured t 4. K for different vlues of V., Fowler Nordheim plots demonstrting the gte-vrile trnsition from diret to Fowler Nordheim tunnelling (olour-oded s in ). The plots re offset vertilly for lrity. Also shown re drwings of the mesurements on the two types of moleules (OT nd BT). Figures nd 4 show the IET spetr of OT nd BT juntions mesured t vrious vlues of,eff, respetively. A stndrd.. modultion tehnique with lok-in mplifier is rried out t 4. K to diretly quire the first (di/dv) nd seond (d I/dV ) hrmoni signls (see upplementry Informtion for detils). We ssign the oserved spetrl fetures to speifi moleulr virtions y omprison with previously reported infrred, Rmn nd IET spetrosopy mesurements, nd lso y density funtionl theory (FT) lultions (upplementry Tle ). All of the spetrl fetures re rrier shpe with inresing is., Plot of V trns versus V. The solid line is liner fit nd the error rs denote the s.d. of the individul mesurements. Inset, the olour mp of dln(i/v )/d(/v) (from Fowler Nordheim plots) with liner fit (solid line) otined from the plot of V trns versus V. The zero-gte trnsition voltge is indited y the dshed rrow. ttriutle to virtionl modes ssoited with the moleulr speies, whih provides dditionl verifition of the onstituent moleules. A well-estlished vlidtion method for IET spetrosopy is to exmine the linewidth rodening s funtion of modultion voltge nd temperture,. Figure shows the modultion rodening of representtive IET spetrosopy feture (the 57-mV n(c H) strething mode in the OT juntion) t onstnt temperture of 4. K. The dt points show the FWHM of the experimentl pek. Tking into ount the known therml rodening (5.4k B T, FWHM (mv) Wvenumer (m ) 5,,5,,5,,5.5.7.8,eff =.75,eff =.5.4,eff =.5,eff =.. 5 5 5 5 K 7.8 mv 4 K 7. mv 4 K 6. mv 4.9 mv K 4. mv K.8 mv 4. K.4.6.8.4.6.8 ν(au ) ν(c ) δ r (CH ) ν(c C) γ w (CH ) δ s (CH ) d FWHM (mv). δ s (CH ) γ.5 w (CH ) ν(c C) δ. r (CH ) ν(c ) ν(au ).......4..5.5.75,eff () 4. K 5 4 5 6 7 8.. modultion (r.m.s. vlue) (mv) Figure ted IET spetr nd linewidth rodening of Au-OT-Au juntion., IET spetr mesured t 4. K for different vlues of,eff, with virtion modes ssigned., Two-dimensionl olour mp of the gted IET spetr, inditing ner independene with respet to,eff., d, Fullwidth t hlf-mximum (FWHM) of the pek orresponding to the n(c H) strething mode (,57 mv) s funtion of.. modultion voltge () nd 9 Mmilln Pulishers Limited. All rights reserved 7.8 mv 4 5 Temperture (K) temperture (d). The irles indite experimentl dt, nd the solid line () nd squres (d) show theoretil vlues. The error rs re determined y the ussin fitting. Insets, suessive IET spetrosopy sns for the n(c H) mode under inresing.. modultion voltge () nd inresing temperture (d), s indited. r.m.s., root men squred. 4
( LETTER NATURE Vol 46 4/ eemer 9 Wvenumer (m ) 5,,5,,5 ν(au ) γ (C H) ν(8) ν(8) Ω,eff =... V,eff 6 V,eff,eff =. 4 V,eff,eff =.66. η (%).. 5 4 5. 6.5 Pek Pek Pek ip ip ip.5.4.45,eff () ν(8) d I/dV (.u.). ν(8) ν(8) γ (C H),eff =..66..4.5.6,eff ().6 Figure 4 Resonntly enhned IET spetr of Au-BT-Au juntion., IET spetr mesured t 4. K for different vlues of,eff, with virtion modes ssigned. The left-hnd y xis orresponds to the grey shded region of the spetr, nd the vrious right-hnd y xes (with different sles) orrespond to the relted (olour-oded) spetr in the non-shded region. The vertil dotted line orresponds to V 5 45 mv (6 m ). ignifint modifition in the spetrl intensity nd line shpe for the enzene ring modes, (C H), n(8) nd n(8), ws oserved for different vlues of,eff, s indited. Insets, energy digrms illustrting inelsti tunnelling s the position of the resonne shifts s result of gting., Twodimensionl olour mp of the gted IET spetr, showing tht IET spetrosopy intensity nd line shpe vry signifintly s funtions of,eff., The reltive hnge, g, in the normlized ondutne for the n(8) mode s funtion of,eff. The irles show the experimentl dt nd the solid urve represents the theoretil fit lulted from eqution (). Inset, the gte-vrile IET spetr for the n(8) mode, simulted using eqution ()..u., ritrry units. where k B is Boltzmnn s onstnt nd T denotes temperture) nd modultion rodening (.7V, where V is the.. modultion voltge), the intrinsi linewidth, W I, n e determined from fit to the modultion rodening dt (Fig., solid line), giving W I 5 4.94 6.89 m. Figure d shows the therml rodening of the sme n(c H) pek t fixed modultion, demonstrting exellent greement etween experimentl FWHM vlues (irles) nd theoretil vlues (squres) (see upplementry Fig. 9 for the BT juntion). We now fous on the gte-voltge dependene of the IET spetr mesured in the two types of moleulr juntion. The spetr of the OT juntion remin essentilly unhnged t different vlues of,eff, s shown in Fig.. This onstny with respet to,eff n e lso identified in Fig., whih shows olour mp of the gted IET spetr. This result implies tht, lthough the totl urrent is sensitively dependent on the je F E j rrier height (euse it is tunnelling), the inelsti tunnelling fetures tht orrespond to virtionl exittions of the moleule re nerly independent of the energeti prmeters in the s-sturted OT juntion, inditing little oritl oupling. However, the IET spetr of the p-onjugted BT juntion re drstilly modified y gting (Fig. 4, ), exhiiting n enhned spetrl intensity nd hnge in line shpe for speifi virtionl modes. As,eff eomes more negtive in Fig. 4 (tht is, s E is rought loser to E F ), the normlized mplitude (d I/dV )/(di/dv) of some IET spetrosopy fetures in the BT juntion inrese in mgnitude y ftor of more thn nd pek shpes of the zero-gte spetrum hnge to distint pekderivtive-like (or Fno-shped) fetures. The differene etween the Fermi energy nd the energy of the moleulr oritl hs een shown to e n importnt ftor in 4 9 Mmilln Pulishers Limited. All rights reserved determining the IET spetrosopy intensity nd line shpe,4.as illustrted in Figs nd, the level for OT is energetilly muh frther wy from E F thn it is for BT. Owing to the fr-offresonnt nture of the OT juntion, the,eff vlues oserved in the devie (Fig. ) re predited not to produe ny signifint hnges in the IET spetr 5. However, in the opposite limit of ner resonne s in the BT se, the dominnt trnsport oritl (tht is, the ) will e strongly oupled to the internl virtionl modes (Fig. 4, insets), resulting in resonntly enhned IET spetr 6 8. In ddition, strong oupling etween the moleulr resonne nd the virtionl modes is expeted only when the energy sttes of the moleulr oritls re distriuted round the ond diretly relted to the virtionl modes 8,9, whih hs een shown to e prominent for p p oritls (for exmple for BT) nd miniml for s s oritls (for exmple for OT). This is orroorted y FT lultions showing tht the of BT hs signifint ontriution from the enzene ring, wheres there is essentilly no ontriution from the lkyl hin in the of OT (upplementry Fig. ). We note tht the virtionl modes ssoited with the enzene ring re the only modes enhned y moleulr oritl gting. The zero-is fetures within,4 mv, inluding the shoulder fetures ssigned s n(au ) modes, re ommonly ttriuted to phonon intertions in the metl leds,,9, nd we oserve tht this metlli phonon regime is not ffeted y gting, s shown in Fig. 4 (grey shded region). For the enhned resonnt oupling in ner-resonne IET, it hs een shown tht ner the exittion threshold, jj 5 V, for moleulr virtion, the orresponding hnge in the totl tunnelling ondutne (normlized to its elsti kground) is 7
NATURE Vol 46 4/ eemer 9 LETTER de (E M {E F {V) {(C=) g~ (E M {E F ) zc= (E M {E F {V) h( {V) z(c=) { (E M {E F {V)C {V p (E M {E F {V) ln ðþ z(c=) Eqution () implies tht the IET spetrosopy intensity nd line shpe n e vried, depending on the oritl energy, E M, nd the width, C, of the moleulr resonne nd its oupling, de, to prtiulr moleulr virtion (where is the ut-off energy, whih is muh smller thn C, nd E F 5 E F ). Reently, theoretil models sed on the non-equilirium reen s funtion formlism hve lso desried suh vritions in the IET spetrosopy fetures s the energy of the resonnt level shifts,4,6. Using eqution (), we n reprodue the experimentl results for the gte-vrile IET spetr (see upplementry Informtion for detils). In Fig. 4, we show the greement etween the experimentl vlues (dt points) nd lulted vlues (solid urve) of g for the,8-mv n(8) streth mode of the BT juntion s funtion of,eff, whih suggests tht eqution () n provide useful estimtes of the ontriution from the resonnt oupling to the gte-vrile IET spetr. We lso quntittively fit the oserved IET spetrosopy line shpes for the,eff vlues shown in Fig. 4. The greement etween theory nd experiment is good for oth the spetrl line shpe nd the reltive mgnitude (Fig. 4, inset). The sme nlysis for other virtion modes in the spetr genertes essentilly similr results to tht for the n(8) mode. Our results demonstrte diret gte modultion of moleulr oritls. The IET spetr revel whih oritls re resonntly enhned, nd drmti differenes re seen in the omprison etween nerresonnt nd fr-from-resonnt systems. This demonstrtion vlidtes the onept of moleulr-oritl-modulted rrier trnsport, llowing new designs of moleulr-sed devies nd eluidting oth rrier trnsport mehnisms nd the eletroni struture of moleulr juntions. METHO UMMARY We used eletron-em lithogrphy to pttern ontinuous thin gold wires with minimum widths of pproximtely nm on top of n luminium gte eletrode, previously oxidized in fresh oxygen tmosphere to form n Al O lyer typilly, nm thik tht ts s gte dieletri. Moleulr deposition on the gold surfe ws performed in dilute ethnol solution ( mm) of eh moleule inside nitrogen-filled glove ox. Before use, eh smple ws rinsed in ethnol nd gently lown dry in nitrogen strem. Then we immeditely ooled the smples oted with the moleules to 4. K (in vuum ryostt), nd used eletromigrtion to form eletrode pirs with nnometre-sle seprtion y rmping up d.. voltge ross the wire while monitoring the resistne. Full detils on methods re presented in the upplementry Informtion. Reeived July; epted 5 Novemer 9.. Ahn, C. H. et l. Eletrostti modifition of novel mterils. Rev. Mod. Phys. 78, 85 (6).. Johim, C., imzewski, J. K. & Avirm, A. Eletronis using hyrid-moleulr nd mono-moleulr devies. Nture 48, 54 548 ().. lperin, M., Rtner, M. A., Nitzn, A. & Troisi, A. Nuler oupling nd polriztion in moleulr trnsport juntions: eyond tunneling to funtion. iene 9, 56 6 (8). 4. Reed, M. A., Zhou, C., Muller, C. J., Burgin, T. P. & Tour, J. M. Condutne of moleulr juntion. iene 78, 5 54 (997). 5. Avirm, A. & Rtner, M. A. Moleulr retifiers. Chem. Phys. Lett. 9, 77 8 (974). 6. hosh, A. W., Rkshit, T. & tt,. ting of moleulr trnsistor: eletrostti nd onformtionl. Nno Lett. 4, 565 568 (4). 7. Andrews,. Q., olomon,. C., Vn uyne, R. P. & Rtner, M. A. ingle moleule eletronis: inresing dynmi rnge nd swithing speed using rossonjugted speies. J. Am. Chem. o., 79 79 (8). 8. Prk, J. et l. Coulom lokde nd the Kondo effet in single-tom trnsistors. Nture 47, 7 75 (). 9. Ling, W., hores, M. P., Bokrth, M., Long, J. R. & Prk, H. Kondo resonne in single-moleule trnsistor. Nture 47, 75 79 ().. Kutkin,. et l. ingle-eletron trnsistor of single orgni moleule with ess to severl redox sttes. Nture 45, 698 7 ().. Xu, B., Xio, X., Yng, X., Zng, L. & To, N. Lrge gte modultion in the urrent of room temperture single moleule trnsistor. J. Am. Chem. o. 7, 86 87 (5).. Piv, P.. et l. Field regultion of single-moleule ondutivity y hrged surfe tom. Nture 45, 658 66 (5).. tt,.., trhn,. R. & Johnson, A. T. C. te oupling to nnosle eletronis. Phys. Rev. B 79, 544 (9). 4. hosh,. et l. evie struture for eletroni trnsport through individul moleules using nnoeletrodes. Appl. Phys. Lett. 87, 59 (5). 5. Xu, B. & To, N. J. Mesurement of single-moleule resistne y repeted formtion of moleulr juntions. iene, (). 6. Venktrmn, L., Klre, J. E., Nukolls, C., Hyertsen, M.. & teigerwld, M. L. ependene of single-moleule juntion ondutne on moleulr onformtion. Nture 44, 94 97 (6). 7. Wng, W., Lee, T. & Reed, M. A. Mehnism of eletron ondution in selfssemled lknethiol monolyer devies. Phys. Rev. B 68, 546 (). 8. Beee, J. M., Kim, B., dzuk, J. W., Frisie, C.. & Kushmerik, J.. Trnsition from diret tunneling to field emission in metl-moleule-metl juntions. Phys. Rev. Lett. 97, 68 (6). 9. Jklevi, R. C. & Lme, J. Moleulr virtion spetr y eletron tunneling. Phys. Rev. Lett. 7, 9 4 (966).. tipe, B. C., Rezei, M. A. & Ho, W. ingle-moleule virtionl spetrosopy nd mirosopy. iene 8, 7 75 (998).. Wng, W., Lee, T., Kretzshmr, I. & Reed, M. A. Inelsti eletron tunneling spetrosopy of n lknedithiol self-ssemled monolyer. Nno Lett. 4, 64 646 (4).. Kushmerik, J.. et l. Vironi ontriutions to hrge trnsport ross moleulr juntions. Nno Lett. 4, 69 64 (4).. lperin, M., Rtner, M. A. & Nitzn, A. On the line widths of virtionl fetures in inelsti eletron tunneling spetrosopy. Nno Lett. 4, 65 6 (4). 4. himzki, T. & Asi, Y. Theoretil study of the lineshpe of inelsti eletron tunneling spetrosopy. Phys. Rev. B 77, 548 (8). 5. Troisi, A., Rtner, M. A. & Nitzn, A. Vironi effets in off-resonnt moleulr wire ondution. J. Chem. Phys. 8, 67 68 (). 6. lperin, M., Rtner, M. A. & Nitzn, A. Inelsti eletron tunneling spetrosopy in moleulr juntions: peks nd dips. J. Chem. Phys., 965 979 (4). 7. Persson, B. N. J. & Brtoff, A. Inelsti eletron tunneling from metl tip: the ontriution from resonnt proesses. Phys. Rev. Lett. 59, 9 4 (987). 8. Komed, T. Chemil identifition nd mnipultion of moleules y virtionl exittion vi inelsti tunneling proess with snning tunneling mirosopy. Prog. urf. i. 78, 4 85 (5). 9. Yu, L. H., Zngmeister, C.. & Kushmerik, J.. Origin of disrepnies in inelsti eletron tunneling spetr of moleulr juntions. Phys. Rev. Lett. 98, 68 (7).. Troisi, A. & Rtner, M. A. Propensity rules for inelsti eletron tunneling spetrosopy of single-moleule trnsport juntions. J. Chem. Phys. 5, 479 (6). upplementry Informtion is linked to the online version of the pper t www.nture.om/nture. Aknowledgements This work ws supported y the Koren Ntionl Reserh Lortory progrmme; Koren Ntionl Core Reserh Center grnt; the World Clss University progrmme of the Koren Ministry of Edution, iene nd Tehnology; the Progrm for Integrted Moleulr ystem t the wngju Institute of iene nd Tehnology; the ystemic projet of the Koren Ministry of Knowledge Eonomy; the U Army Reserh Offie (W9NF-8--65); nd the Cndin Institute for Advned Reserh. Author Contriutions T.L. plnned nd supervised the projet; H.. designed nd performed the experiments; H.., T.L. nd M.A.R. nlysed nd interpreted the dt nd wrote the mnusript; H.J. designed the eletril mesurement systems; Y.K. ssisted in low-temperture eletril mesurements; nd Y.H.J. performed FT lultions. Author Informtion Reprints nd permissions informtion is ville t www.nture.om/reprints. The uthors delre no ompeting finnil interests. Correspondene nd requests for mterils should e ddressed to T.L. (tlee@gist..kr) or M.A.R. (mrk.reed@yle.edu). 9 Mmilln Pulishers Limited. All rights reserved 4