Reeived 23 Nov 2010 Aepted 13 Apr 2011 Pulished 10 My 2011 DOI: 10.1038/nomms1310 Reveling the moleulr struture of single-moleule juntions in different ondutne sttes y fishing-mode tip-enhned Rmn spetrosopy Zheng Liu 1, Song-Yun Ding 1, Zho-Bin Chen 1, Xing Wng 1, Jing-Hu Tin 1, Json R. Anem 1, Xio-Shun Zhou 1, De-Yin Wu 1, Bing-Wei Mo 1, Xin Xu 1, Bin Ren 1 & Zhong-Qun Tin 1 The ondutne of single-moleule juntions my e governed y the struture of the moleule in the gp or y the wy it onds with the leds, nd the informtion ontined in Rmn spetrum is idel for exmining oth. Here we demonstrte tht moleule-tosurfe onding my e hrterized during eletron trnsport y fishing-mode tipenhned Rmn spetrosopy (FM-TERS). This tehnique llows mutully verifile singlemoleule ondutne nd Rmn signls with single-moleule ontriutions to e quired simultneously t room temperture. Density funtionl theory lultions revel tht the most signifint spetrl hnge seen for gold-4,4 -ipyridine-gold juntion results from the deformtion of the pyridine ring in ontt with the drin eletrode t high voltge, nd these lultions suggest tht stronger onding intertion etween the moleule nd the drin my ount for the nonliner dependene of ondutne on is voltge. FM-TERS will led to etter understnding of eletron-trnsport proesses in moleulr juntions. 1 Stte Key Lortory of Physil Chemistry of Solid Surfes nd Key Lortory of Anlytil Sienes, College of Chemistry nd Chemil Engineering, Ximen University, Ximen 361005, Chin. Correspondene nd requests for mterils should e ddressed to B.R. (emil: ren@xmu.edu.n).
nture ommunitions DOI: 10.1038/nomms1310 In moleulr eletronis, individul moleules with unique properties serve s the si eletril omponents. This field hs ttrted inresing interest in reent yers 1 3. Vrious methods hve een estlished to onstrut eletron-trnsport juntions, whih serve s the min investigtive tool in moleulr eletronis, nd to hrterize different properties of the tive region 4 11. Inelsti eletron tunnelling spetrosopy is ommonly used to provide struturl informtion out the moleules there, ut this tehnique requires low-temperture nd vuum onditions 12 15. Conventionl optil spetrosopi methods pper to e the most promising ones with whih to exmine the orreltion etween struture nd ondutne under mient onditions 16 23 ; however, their low sptil resolution hs led to interferene y moleules other thn the one in the juntion 1,16. Tip-enhned Rmn spetrosopy (TERS), type of pertureless snning ner-field optil mirosopy, offers omintion of single-moleule sensitivity nd ultrhigh sptil resolution 24 28. The eletromgneti field-enhning struture is gold (or silver) tip, s shown in the entre of Figure 1. When the tip is rought suffiiently lose to the sustrte to mke ontt with moleules dsored there, single-moleule juntion will spontneously form. In generl, the tip will e free of moleules exept for the one t its pex tht forms the juntion. Other moleules tht re on the sustrte nd diretly underneth the tip will experiene the enhned eletromgneti field to some extent, ut the moleule forming the juntion will experiene the gretest enhnement. Therefore, interferene from moleules outside the juntion is minimized nd spetrosopi informtion from the juntion is predominnt. For this reson, TERS would pper to e n idel tool for studying the struture of moleulr juntion. In this work, we introdue fishing-mode snning tunnelling mirosopy (FM-STM), whih n e used to otin high-qulity ondutne histogrms from moleulr juntions with gret effiieny. We then omine our FM-STM method with TERS to yield fishing-mode TERS (FM-TERS). This n e used to otin mutully verifile single-moleule ondutne nd Rmn signls with single-moleule ontriutions simultneously t room temperture. We then perform FM-TERS mesurements on moleulr juntions t different is voltges, nd use theoretil methods to determine struture in the juntion t eh voltge from the spetrl fetures oserved. We re onfident tht our FM-TERS tehnique will led to etter understnding of eletron-trnsport proesses in moleulr juntions. Results Priniple of FM-STM for mesuring single-moleule ondutne. To otin relile TERS signl while monitoring single-moleule ondutne, it is neessry to keep the distne etween the tip nd the sustrte within suitle rnge for onsiderly long period of time. For this purpose, we developed new ondutne-mesuring method. It is sed on trditionl snning tunnelling mirosopy (STM) ut hs ritilly ltered feedk stte. Experimentlly, we first pre-dsor gold(111) surfe with moleules. We pproh it with the STM tip until n pproprite juntion distne is rehed (Fig. 1); then we derese the response of the STM y lowering the proportionl gin nd the integrl gin to out 0.03% of their norml vlues (we lower it from ~3 to ~0.001). The ondutne of the system is ontinuously reorded. When moleulr juntion forms (Fig. 1), urrent flowing through the STM will inrese ording to the tul eletron-trnsport properties of the moleule. Under the low-feedk ondition, the tip will retrt to derese the urrent with rte tht is signifintly redued in omprison to the norml feedk-ondition rte. As the tip moves wy from the gold(111) surfe, the juntion will rek (Fig. 1). Then, the urrent will drop nd the feedk system will fore the tip to return to the sustrte OFF Breking Fishing e Au(111) ON ON Au tip Rmn photons Fishing Drifting nd reking (Fig. 1d) to regin the tunnelling urrent (Fig. 1e). As result, the tip is onfined within few ngstroms of the juntion distne, nd the instrument n remin stle for severl minutes t time (Supplementry Fig. S1). Thermlly driven movement of moleules on the surfe will led to formtion nd reking of moleulr juntions in the gp with high frequeny, nd this my e oserved s set of urrent jumps. The proility of trpping moleule in the juntion nd the lifetime of eh moleulr ridge re oth inresed y our method in omprison with the STM rek-juntion nd lternting-urrent modultion methods 6,7. Menwhile, the low-feedk stte remins le to ompenste ny slow therml or mehnil instrument drift 7, in ontrst to the feedk-disled method 9,29. These two dvntges llow ondutne to e mesured ontinuously over long period of time with miniml instrument drift interferene. We ll this fishing-mode STM (FM-STM). To demonstrte the reliility nd effiieny of FM-STM for mesuring single-moleule ondutne, we used 1,8-otnedithiol (1,8-ODT) s proe moleule, n eletrohemilly ethed gold wire s the STM tip, nd gold(111) surfe s the sustrte 30. Hundreds of urrent jumps were esily otined in very short period of time (400 ms), with eh urrent jump on the time sle of frtion of ms (Fig. 2,). These urrent jumps hve een sried to formtion (the ON-ondution stte) nd reking (the OFFondution stte) of moleulr juntions 9,29. We performed sttistil nlysis on the urrent jumps nd otined hrteristi histogrm for ondutne with modl vlue of out 5.6 10 5 G 0 (Fig. 2), where G 0 = 2e 2 /h orresponds to the ondutne of gold t single-tom to single-tom point of ontt 6. This vlue is omprle with the ones otined using other methods 6,7,11,29,31. In ddition to demonstrting its reliility, Figure 2 lso demonstrtes the effiieny of our FM-STM method: it n provide high-qulity d OFF OFF Feedk effets Tip pprohes Figure 1 Shemti digrm of fishing-mode TERS (FM-TERS). An illustrtion of the FM-TERS system for simultneous ondutne nd TERS mesurement of single-moleule juntions. A gold(111) surfe is pre-dsored with moleules. The distne etween the gold tip nd the gold(111) surfe is then ontrolled y n STM equipped with ondutne-mesuring iruit. The red ontour indites the distriution of eletromgneti field strength etween the tip nd the sustrte. Wvy rrows indite Rmn sttering y the moleules. () The tip nd the sustrte re lose enough for moleulr juntion to form etween them, ut the moleule is not yet in ontt with the tip. () Formtion of moleulr juntion. () Therml or mehnil drift uses the juntion to rek. (d) STM feedk tkes effet, nd it rings the tip k to the previous distne to fish for nother moleule. (e) A new juntion is formed nd the yle ontinues.
nture ommunitions DOI: 10.1038/nomms1310 ARTICLE 3 3 2 1 2 1 Counts 0 0 100 200 300 400 Time (ms) 0 80 82 84 86 88 90 92 94 Time (ms) 0 2 4 6 G (10 5 G 0 ) Figure 2 Current tres nd ondutne histogrm otined y FM-STM for 1,8-ODT. () A time-sequene ondutne urve for 1,8-ODT in the juntion etween the gold tip nd the gold(111) sustrte (is voltge = 600 mv, urrent set-point = 1 na; the urrent ws kground orreted). () An expnded view of the region mrked with red outline in (). () The ondutne histogrm for 1,8-ODT. 30 25 20 15 10 5 I t (na) 10 5 I t 77.5 77.6(s) Counts 4 10 4 G 0 0 1 (G/10 3 G 0 ) 3 na 50 s~51 s 56 s~57 s On Off 0 10 20 30 40 50 60 70 80 90 Time (s) 0.0 0.2 0.4 0.6 0.8 1.0 Time (s) 1,700 1,550 1,350 1,150 950 10 20 30 40 50 60 70 80 90 Time (s) 20 ps 350 ps d Intensity 100 ps 51 s 57 s 800 1,200 1,600 On Off Figure 3 Simultneous ondutne nd TERS mesurement of 4ipy y FM-TERS. () Time sequene of ondutne for the gold tip-4ipy-gold(111) system (is voltge = 200 mv, tunnelling urrent (I t ) = 5 na). Left inset: expnded view of (); right inset: ondutne histogrm for the gold tip-4ipygold(111) system. () Time sequene of 4ipy TERS spetr otined simultneously with the ondutne dt in (). () Seleted ondutne urves for 1 s time period. (d) TERS spetr tht orrespond to the ondutne urves in (). ondutne histogrm in very short period of time down to hlf seond (see Supplementry Fig. S2 for the omprison) 31. Moreover, the low-feedk stte employed is suffiient to onfine the tip within few ngstroms of the pre-set vlue during the whole proess. This is essentil for omintion with TERS. Simultneous ondutne nd TERS mesurement. We then omined the ility to mesure single-moleule ondutne y our FM-STM method, with the ility to quire Rmn spetr hving single-moleule ontriutions y TERS. We ll this seond tehnique fishing-mode TERS (FM-TERS). We used 4,4 -ipyridine (4ipy), ommon proe in oth moleulr eletronis nd surfe-enhned Rmn sttering (SERS), s the juntion-forming moleule 6,10,17, nd we otined ondutne dt (Fig. 3) nd TERS spetr (Fig. 3) from the gold tip-4ipy-gold(111) juntion system t the sme time. Agin, the key feture of our FM-TERS method is tht it llows oth of these dt types to e quired from single-moleule juntion simultneously. An expnded view of the ondutne urve (left inset of Fig. 3) shows some hrteristi urrent jumps. A sttistil nlysis of the ON-stte ondutne vlues produed typil ondutne histogrm with modl vlue of (4.0 ± 1.0) 10 4 G 0 (right inset of Fig. 3) 10,31. Interestingly, the TERS signl from period of time with high proility of ON sttes shows inresed intensities nd nd widths, nd some shifting of nd positions (see Fig. 3 nd the wterfll plot in Fig. 3). To fully understnd the orreltion etween the ondutne nd the TERS signl, we seleted some 1-s ondutne urves (with nd without urrent jumps, Fig. 3) nd their orresponding TERS spetr (Fig. 3d) for nlysis. Impressively, the TERS signl flututes, when the ondutne
nture ommunitions DOI: 10.1038/nomms1310 On/Off Intensity 1 X 200 X Experiment 1,011 1,227 Clultion 1,609 1,291 1,631 1,510 I,II On Off 1,000 1,200 1,400 1,600 III IV V VI VII 1.5 I (µa) 1,600 0 0.4 0.8 Bis (V) 1,800 0.01 0.1 0.3 0.5 0.8 0.5 0.3 0.1 0.01 Low voltge Drin (+) 1,609 m 1 1,609 m 1 ( ) Soure High voltge 1,631 m 1 1,609 m 1 Figure 4 FM-TERS of 4ipy in moleulr juntion nd its onding onfigurtion. () I is n verge of ll 190 ON-stte TERS spetr nd II is typil OFF-stte TERS spetrum. Eh of III, IV nd V orrespond to n verge of 20 ON-stte spetr. VI nd VII re lulted ON- nd OFF-stte spetr, respetively. () The ν 8 nd of 4ipy (1609 m 1 ) is seen to hnge with the is voltge (urrent set-point = 5 na). Inset: urrent versus is urve for 4ipy otined with mehnil rek-juntion setup. () This digrm illustrtes why 1,609 m 1 nd splitting is proportionl to the pplied is voltge. swithes etween the single-moleule juntion (ON) stte nd the disonneted (OFF) stte, inditing tht the hnge in the TERS signl is due to single-moleule events. This result demonstrtes the dvntge of FM-TERS over the existing orrelted ondutne nd SERS methods 20,21,32. Spetrl flututions re often used s n inditor for singlemoleule SERS nd TERS, ut they my lso e used y degrdtion of proe moleules in the enhned eletromgneti field. A simple test hs een proposed to distinguish etween the different origins of these flututions 33 : if the spetrl hnges re due to nondestrutive proesses, the signl qulity will e mintined over time nd spetrl verging will give the fr-field ensemle spetrum of the moleule; if, however, the hnges re due to photoindued degrdtion, spetrl verging will yield typil ron spetrum with rod peks t round 1,350 nd 1,580 m 1. We therefore used sttistis to help us understnd the gold tip- 4ipy-gold(111) juntion system. When we verged over ll 190 ON-stte spetr, we otined spetrum I (red) in Figure 4. An OFFstte spetrum is given for omprison (spetrum II, lk). We lso present three typil spetr, eh of whih is tully the verge of t lest 20 ON-stte spetr, in the sme figure (they re lelled III, IV nd V). All five of these spetr disply fetures of 4ipy; however, nd intensity, width nd position ll vry. The sene of ron nds t 1,350 nd 1,580 m 1 indites tht the spetrl flututions re not due to photo-indued deomposition of the proe moleule, ut reflet dynmi single-moleule events in the juntion. We note tht euse of differenes etween the wy our TERS system nd most SERS sustrtes re prepred, our TERS system is muh lener initilly (Supplementry Note 1 nd Fig. S3), nd this should help redue the mount of ron formed during irrdition. Compring spetrum I (ON stte) to spetrum II (OFF stte), we find tht some of the virtionl nds of 4ipy (1,011, 1,227 nd 1,510 m 1 ) roden or shift in the ON stte. Moreover, when we inresed the is voltge ross the moleulr juntion from 10 to 800 mv, we found tht one pek (1,609 m 1 ) hnged reversily to doulet ove 100 mv (Fig. 4). This splitting ours in the nonliner region of the urrent versus voltge urve (inset of Fig. 4). When we reversed the is voltge, the splitting disppered, inditing tht the splitting of the pek ours reversily with the is voltge. Interestingly, when we redued the tunnelling urrent to 300 pa nd returned the feedk to its norml setting, the hne of forming moleulr juntion ws signifintly redued (Supplementry Fig. S4), nd we did not oserve splitting of the 1,609-m 1 nd (Supplementry Fig. S4). This result demonstrtes tht the splitting of the nd n only e oserved t the moleulr juntion t high is voltge nd tht FM-TERS n revel sutle hnges in the juntion tht re undetetle y other methods. TERS spetr in the OFF nd ON sttes. To understnd these orrelted spetrl nd moleulr ondutne hnges, nd the underlying hnges in struture nd onding tht our during swithing, we used luster-model sed density funtionl theory to qulittively simulte TERS spetr quired for the ove sttes (Supplementry Methods). Our pproh is shown shemtilly in Fig. 4, nd detiled in Supplementry Fig. S5. The 4ipy moleule is omposed of two pyridine rings: the one in ontt with the positively ised tip is designted s the drin ring, nd the one in ontt with the negtively ised gold(111) sustrte is the soure ring. The lulted Rmn spetr for the moleulr juntion in the ON (Fig. 4, spetrum VI) nd OFF (Fig. 4, spetrum VII) sttes gree well with the mesured spetr (I nd II). In the ON stte, inresing the is voltge uses the Fermi level energy to derese nd the surfe hrge density on the drin eletrode to inrese. This leds to stronger hemil onding etween the drin eletrode nd the pyridine ring in ontt with it. The polrizility of 4ipy inreses, nd Rmn sttering is enhned y n dditionl two or three orders of mgnitude (Supplementry Tle S1) 34. The signifine of this dditionl enhnement, whih ours for the ON stte ut not the OFF stte, is tht the signl from one moleule in the ON stte n e omprle with the signl from hundreds or thousnds of moleules in the OFF stte whih re dsored on the sustrte in the viinity of the moleulr juntion. This dvntge exists even when the ON-stte lifetime is out fifth of the OFFstte lifetime, s in the gold-4ipy-gold system studied here (Fig. 3). It thus improves the signl-to-noise rtio hievle y FM-TERS even further, nd llows the quisition of distint Rmn signl from the single-moleule in the juntion despite the kground signl from lrge numer of surrounding moleules whih do not mke ontt with the tip nd re therefore in the OFF stte. Voltge-dependent pek splitting in the ON stte n e understood in terms of the different onding intertions whih our
nture ommunitions DOI: 10.1038/nomms1310 ARTICLE etween the gold tip nd 4ipy, nd etween 4ipy nd the gold(111) sustrte. At low voltge, the two pyridine rings intert with the two eletrodes in similr wy. As the is voltge inreses, however, the onding intertion on the drin side will inrese signifintly. In our high-is model, the gold-nitrogen ond is 2.35 Å long on the soure side ut only 2.15 Å long on the drin side (Supplementry Tle S2). The nitrogen-ron onds eome weker, the prllel ron-ron onds eome stronger, nd deformtion of the drin ring ours s result (Supplementry Tle S2). Therefore the frequeny of the ν 8 C-N/C-C dephsing streth inreses to 1,631 m 1 for the drin ring wheres it remins t 1,609 m 1 for the soure ring (Supplementry Methods nd Tle S3). This is the first time tht voltge-dependent hnges in struture nd onding hve een oserved for moleulr-eletroni juntion, nd we hope tht it will invoke further study. Disussion The ility to quire virtionl informtion from moleulr juntion during eletron trnsport y FM-TERS will open up new venues of reserh, oth experimentlly nd theoretilly. The orrelted dt permits eletron trnsport to e understood in wy tht hs not een possile efore: y use of virtionl informtion otined dominntly from the single moleule in the juntion. Our FM-TERS tehnique is highly effiient, simple nd flexile; it my soon e developed into routine tool for this field. The ility to perform FM-TERS under mient onditions is quite signifint euse these re the prtil ones under whih moleulr eletronis must funtion in the future. When it is omined with Stokes/nti-Stokes pility, FM-TERS my e used to exmine heting effets on the moleule forming the juntion 2,35,32. It is rodly pplile nd it n e used to investigte the reltionships etween struture, eletri properties nd optil properties of vrious mterils inluding grphene, quntum dots, nd those used for solr energy onversion. Methods Experimentl setup of the FM-TERS instrument. The setup inludes Veeo STM with n dditionl home-mde iruit for ontrolling the XYZ signl nd is voltge (Supplementry Fig. S6, left pnel). The tip urrent is otined y Digitl Instruments signl ess module ontroller nd olleted vi 16 it digitl-to-nlog nd nlog-to-digitl rds t very high rte of quisition: 100,000 points s 1 to llow the plotting of relile ondutne histogrms. Bis nd feedk sensitivities re djusted using Veeo softwre. The optil onfigurtion of our TERS instrument is desried in detil in ref. 30, nd lso shown in Supplementry Fig. S6 (right pnel). It onsists of home-uilt Rmn optil fire hed, Veeo nnosope-e STM, nd n optil fire oupled Aton spetrometer equipped with liquid nitrogen ooled k-illuminted CCD detetor. A helium-neon lser is used to provide light with wvelength of 632.8 nm. The em is foused y 50 ultrlong working distne mirosope ojetive (Olympus, numeril perture = 0.45) onto the tip-to-sustrte gp with n inident ngle of 60 from the surfe norml in the p-polrized mode, power of 5 mw, nd spot dimeter of out 2 µm. Tip preprtion. Our gold tips re prepred y eletrohemilly ething gold wire in mixture of fuming hydrohlori id nd nhydrous ethnol (1:1/v:v) (ref.30). Typilly, 2 mm of 12-mm-long stright gold wire (dimeter = 0.25 mm) is immersed in the ething solution t the entre of gold wire ring (wire dimeter = 1 mm, ring dimeter 2 m) pled just elow the surfe of the solution. The gold wire nd the gold ring serve s the node nd the thode, respetively. A potentiostt is used to fix the ething potentil t 2.2 V. The eletrohemil urrent is monitored to evlute the qulity of the tip nd indite the end of the ething proess. Afterwrds, the tip is rinsed thoroughly with ultrpure wter (18.2 MΩ m) or ethnol to remove ething produts. A snning eletron mirosope imge of typil TERS gold tip is shown in Supplementry Fig. S7 in this se, the dimeter of the pex is out 10 nm. Eh tip is soked in onentrted sulphuri id for 15 min efore use 36. The min purpose of this step is to remove ny ontminnts dsored from the tip-ething solution or from the ir fterwrds. It is then soked in ultrpure wter to remove the sulphuri id, nd fterwrds in ethnol to remove the wter. Eh tip is used for TERS immeditely fter drying to insure its lenliness. Smple preprtion nd TERS mesurement. Our gold(111) single rystl sustrtes re prepred ording to the method ommonly used in surfe siene. The single rystl is lened nd rinsed with pirnh solution nd wter respetively, then nneled with hydrogen or utne flme. The resulting single rystl is soked in 1 mm queous solution of 4ipy for 10 min. It is then rinsed with ultrpure wter to remove physisored speies nd yield len monolyer. Finlly, the eletrode is lown dry with nitrogen. After the tip is in ple ove the gold(111) sustrte t the tunnelling distne, we derese the STM feedk setting from ~3 to ~0.001 nd reord the timesequene of ondutne nd Rmn spetr. All mesurements re performed under mient onditions, exept tht the whole setup is in len room. Theoretil methods. We dopted n ion modified luster-model sed density funtionl theory method to simulte struture/onding of 4ipy with gold in the OFF nd ON sttes, nd to simulte the orresponding Rmn spetr (Supplementry Methods). A Br Au 4 luster ws used to represent the negtively hrged gold(111) sustrte soure eletrode. The gold tip drin eletrode ws represented y Au 4 F nd Au 4 Cl in two low-is ses, nd y Au 4 in high-is se. Thus, our model implied stti eletri field pointing from the tip ( + ) to the sustrte ( ) with severl pproprite is voltges. Referenes 1. Glperin, M., Rtner, M. A., Nitzn, A. & Troisi, A. Nuler oupling nd polriztion in moleulr trnsport juntions: Beyond tunneling to funtion. Siene 319, 1056 1060 (2008). 2. To, N. J. Eletron trnsport in moleulr juntions. Nture Nnoteh. 1, 173 181 (2006). 3. Ulgut, B. & Arun, H. D. Eletron trnsfer through moleules nd ssemlies t eletrode surfes. Chem. Rev. 108, 2721 2736 (2008). 4. Reed, M. A., Zhou, C., Muller, C. J., Burgin, T. P. & Tour, J. M. Condutne of moleulr juntion. Siene 278, 252 254 (1997). 5. Cui, X. D. et l. Reproduile mesurement of single-moleule ondutivity. Siene 294, 571 574 (2001). 6. Xu, B. Q. & To, N. J. Mesurement of single-moleule resistne y repeted formtion of moleulr juntions. Siene 301, 1221 1223 (2003). 7. Xi, J. L., Diez-Perez, I. & To, N. J. Eletron Trnsport in Single Moleules Mesured y Distne-Modultion Assisted Brek Juntion Method. Nno Lett. 8, 1960 1964 (2008). 8. Ddosh, T. et l. Mesurement of the ondutne of single onjugted moleules. Nture 436, 677 680 (2005). 9. Hiss, W. et l. Preision ontrol of single-moleule eletril juntions. Nture Mter. 5, 995 1002 (2006). 10. Quek, S. Y. et l. Mehnilly ontrolled inry ondutne swithing of single-moleule juntion. Nture Nnoteh. 4, 230 234 (2009). 11. Li, C. et l. Chrge Trnsport in Single Au Alknedithiol Au Juntions: Coordintion Geometries nd Conformtionl Degrees of Freedom. J. Am. Chem. So. 130, 318 326 (2008). 12. Stipe, B. C., Rezei, M. A. & Ho, W. Single-moleule virtionl spetrosopy nd mirosopy. Siene 280, 1732 1735 (1998). 13. Wng, W. Y., Lee, T., Kretzshmr, I. & Reed, M. A. Inelsti eletron tunneling spetrosopy of n lknedithiol self-ssemled monolyer. Nno Lett. 4, 643 646 (2004). 14. Kushmerik, J. G. et l. Vironi ontriutions to hrge trnsport ross moleulr juntions. Nno Lett. 4, 639 642 (2004). 15. Song, H. et l. Oservtion of moleulr oritl gting. Nture 462, 1039 1043 (2009). 16. MCreery, R. L. Anlytil hllenges in moleulr eletronis. Anl. Chem. 78, 3490 3497 (2006). 17. Tin, Z. Q., Ren, B., Li, J. F. & Yng, Z. L. Expnding generlity of surfeenhned Rmn spetrosopy with orrowing SERS tivity strtegy. Chem. Commun. 3514 3534 (2007). 18. Nie, S. M. & Emory, S. R. Proing single moleules nd single nnoprtiles y surfe-enhned Rmn sttering. Siene 275, 1102 1106 (1997). 19. Kneipp, K. et l. Single moleule detetion using surfe-enhned Rmn sttering (SERS). Phys. Rev. Lett. 78, 1667 1670 (1997). 20. Tin, J. H. et l. Study of moleulr juntions with omined surfeenhned Rmn nd mehnilly ontrollle rek juntion method. J. Am. Chem. So. 128, 14748 14749 (2006). 21. Wrd, D. R. et l. Simultneous mesurements of eletroni ondution nd Rmn response in moleulr juntions. Nno Lett. 8, 919 924 (2008). 22. Chen, X. D. et l. Chemil frition of heterometlli nnogps for moleulr trnsport juntions. Nno Lett. 9, 3974 3979 (2009). 23. Yoon, H.P. et l. Crossed-nnowire moleulr Juntions: new multispetrosopy pltform for ondution-struture orreltions. Nno Lett. 10, 2897 2902 (2010). 24. Bilo, E. & Dekert, V. Tip-enhned Rmn sttering. Chem. So. Rev. 37, 921 930 (2008). 25. Hrtshuh, A., Snhez, E. J., Xie, X. S. & Novotny, L. High-resolution nerfield Rmn mirosopy of single-wlled ron nnotues. Phys. Rev. Lett. 90, 095503 (2003).
nture ommunitions DOI: 10.1038/nomms1310 26. Steidtner, J. & Pettinger, B. Tip-enhned Rmn spetrosopy nd mirosopy on single dye moleules with 15 nm resolution. Phys. Rev. Lett. 100, 236101 (2008). 27. Yno, T., Verm, P., Sito, Y., Ihimur, T. & Kwt, S. Pressure-ssisted tip-enhned Rmn imging t resolution of few nnometres. Nture Photonis 3, 473 477 (2009). 28. Berweger, S. et l. Optil nnorystllogrphy with tip-enhned phonon Rmn spetrosopy. Nture Nnoteh. 4, 496 499 (2009). 29. Hiss, W. et l. Impt of Juntion Formtion Method nd Surfe Roughness on Single Moleule Condutne. J. Phys. Chem. C 113, 5823 5833 (2009). 30. Wng, X. et l. Tip-enhned Rmn spetrosopy for investigting dsored speies on single-rystl surfe using eletrohemilly prepred Au tips. Appl. Phys. Lett. 91, 101105 (2007). 31. Zhou, X. S. et l. Single moleule ondutne of dipyridines with onjugted ethene nd nononjugted ethne ridging group. J. Phys. Chem. C 112, 3935 3940 (2008). 32. Wrd, D. R., Corley, D. A., Tour, J. M. & Ntelson, D. Virtionl nd eletroni heting in nnosle juntions. Nture Nnoteh. 6, 33 38 (2011). 33. Domke, K. F., Zhng, D. & Pettinger, B. Enhned Rmn spetrosopy: single moleules or ron? J. Phys. Chem. C 111, 8611 8616 (2007). 34. Zho, L. L., Jensen, L. & Shtz, G. C. Surfe-enhned Rmn sttering of pyrzine t the juntion etween two Ag-20 nnolusters. Nno Lett. 6, 1229 1234 (2006). 35. Ioffe, Z. et l. Detetion of heting in urrent-rrying moleulr juntions y Rmn sttering. Nture Nnoteh. 3, 727 732 (2008). 36. Liu, Z. et l. Tip-enhned Rmn spetrosopy for investigting dsored nonresonnt moleules on single-rystl surfes: tip regenertion, proe moleule, nd enhnement effet. J. Rmn Spetros. 40, 1400 1406 (2009). Aknowledgments We thnk Ying Zhng nd Yi-Fn Hung for helpful disussion regrding the theoretil lultions. This work ws supported y Ntionl Nturl Siene Foundtion of Chin (20825313, 20827003, 21021002 nd 21061120456) nd MOST of Chin under the 973 progrms (2007CB935603 nd 2009CB930703). Author ontriutions B.R., Z.Q.T. nd Z.L. oneived nd designed the experiments, nd nlysed the results. B.R., Z.Q.T., Z.L., J.R.A., B.W.M. nd X.X. prtiipted in writing the mnusript. Z.L., X.W. nd J.H.T. exeuted ll of the experiments reported. Z.B.C., X.S.Z. nd B.W.M. designed nd onstruted the eletroni iruits nd the ontrol softwre for ondutne mesurements. S.Y.D., D.Y.W. nd X.X. ontriuted to the theoretil lultions. Additionl informtion Supplementry Informtion ompnies this pper t http://www.nture.om/ ntureommunitions Competing finnil interests: The uthors delre no ompeting finnil interests. Reprints nd permission informtion is ville online t http://npg.nture.om/ reprintsndpermissions/ How to ite this rtile: Liu, Z. et l. Reveling the moleulr struture of singlemoleule juntions in different ondutne sttes y fishing-mode tip-enhned Rmn spetrosopy. Nt. Commun. 2:305 doi: 10.1038/nomms1310 (2011). Liense: This work is liensed under Cretive Commons Attriution-NonCommeril- Shre Alike 3.0 Unported Liense. To view opy of this liense, visit http:// retiveommons.org/lienses/y-n-s/3.0/