Ultrafast charge transfer in atomically thin MoS 2 /WS 2 heterostructures

Transcription

1 PUBLISHED ONLINE: 24 AUGUST 214 DOI: 1.18/NNANO Ultrfst hrge trnsfer in tomilly thin /WS 2 heterostrutures Xioping Hong 1, Jonghwn Kim 1, Su-Fei Shi 1,2, Yu Zhng, Chenho Jin 1, Yinghui Sun 1, Sefttin Tongy 2,4,5, Junqio Wu 2,4, Ynfeng Zhng nd Feng Wng 1,2,6 * Vn der Wls heterostrutures hve reently emerged s new lss of mterils, where quntum oupling etween stked tomilly thin two-dimensionl lyers, inluding grphene, hexgonl-oron nitride nd trnsition-metl dihlogenides (MX 2 ), give rise to fsinting new phenomen 1 1. MX 2 heterostrutures re prtiulrly exiting for novel optoeletroni nd photovolti pplitions, euse two-dimensionl MX 2 monolyers n hve n optil ndgp in the ner-infrred to visile spetrl rnge nd exhiit extremely strong light mtter intertions 2,,11. Theory predits tht mny stked MX 2 heterostrutures form type II semiondutor heterojuntions tht filitte effiient eletron hole seprtion for light detetion nd hrvesting Here, we report the first experimentl oservtion of ultrfst hrge trnsfer in photoexited /WS 2 heterostrutures using oth photoluminesene mpping nd femtoseond pump proe spetrosopy. We show tht hole trnsfer from the lyer to the WS 2 lyer tkes ple within 5 fs fter optil exittion, remrkle rte for vn der Wls oupled two-dimensionl lyers. Suh ultrfst hrge trnsfer in vn der Wls heterostrutures n enle novel two-dimensionl devies for optoeletronis nd light hrvesting. Atomilly thin two-dimensionl rystls onstitute rih fmily of mterils rnging from insultors nd semiondutors to semimetls nd superondutors 1. Heterostrutures from these twodimensionl mterils offer new pltform for exploring new physis (for exmple, superlttie Dir points 4 nd Hofstdter utterfly pttern 5 7 ) nd new devies (suh s tunnelling trnsistors 8, memory devies 9 nd ultrthin photodetetors 2, ). Vn der Wls heterostrutures of semionduting MX 2 lyers re prtiulrly exiting for optoeletroni nd light-hrvesting pplitions, euse mny MX 2 monolyers re diret-ndgp semiondutors 17,18 with remrkly strong light mtter intertions 2,,11. Importntly, MX 2 heterostrutures re predited to form type II heterojuntions, whih n ssist in the effiient seprtion of photoexited eletrons nd holes In type II heterojuntions, the ondution nd minimum nd vlene nd mximum reside in two seprte mterils. Photoexited eletrons nd holes therefore prefer to sty t seprte lotions. Figure 1 illustrtes the lignment of eletroni nds of nd WS 2 monolyers s predited y reent theory 12. It shows tht monolyer nd WS 2 hve ndgps of 2.9 ev nd 2.1 ev, respetively, nd the vlene nd mximum is 5 mev lower thn tht of WS 2. Consequently, the /WS 2 heterostruture forms type II heterojuntion (if we neglet the hyridiztion of eletroni sttes in the nd WS 2 lyers), with the ondution nd minimum residing in nd the vlene nd mximum in WS 2, respetively (Supplementry Setions 1 nd 2). In the single-prtile piture this heterojuntion struture will led to effiient hrge trnsfer, with seprted eletrons nd holes residing in two lyers upon optil exittion (Fig. 1), senrio tht n hve dominting effet on oth light emission nd photovolti responses in /WS 2 heterostrutures. However, there re two outstnding questions regrding hrge trnsfer proesses in the tomilly thin nd vn der Wlsoupled /WS 2 heterostruture: (1) How do strong eletron eletron intertions nd exitoni effets ffet hrge trnsfer proesses? nd (2) How fst n hrge trnsfer tke ple etween vn der Wls-oupled lyers? Eletron eletron intertions re drmtilly enhned in two-dimensionl mterils due oth to size onfinement nd ineffiient sreening. Theoretil studies 19,2 hve predited n exiton inding energy from 5 mev to 1 ev in MX 2 monolyers, whih is lrger thn the expeted nd displement of 5 mev in the /WS 2 heterostruture. Aordingly, the exiton nnot dissoite into free eletron nd free hole in two seprte lyers. Will this lrge exiton inding energy then prevent hrge trnsfer proesses nd keep the exiton in one lyer, or will new ound stte of lyer-seprted eletron nd hole pir e generted? In ddition, vn der Wls oupling is rther wek ompred to ovlent onding. Will tht led to muh slower hrge trnsfer proess in vn der Wls heterostrutures thn in their ovlent ounterprts? Previous studies in orgni photovoltis (OPV) hve shown tht ultrfst hrge trnsfer nd seprtion n tke ple in orgni/orgni vn der Wls oupled interfes. However, the two-dimensionl MX 2 heterostrutures possess twodimensionl rystlline strutures nd tomilly shrp interfes, whih is fundmentlly different from OPV moleulr systems. So fr, little is known out the ultrfst hrge trnsfer dynmis in these new two-dimensionl heterostrutures. In this Letter, we study hrge trnsfer dynmis in /WS 2 heterolyers experimentlly. Through omined photoluminesene spetrosopy nd optil pump proe spetrosopy, we demonstrte tht ultrfst hrge trnsfer tkes ple very effiiently in /WS 2 heterostrutures. In prtiulr, holes in the lyer n seprte into the WS 2 lyer within 5 fs upon photoexittion. Figure 1 shemtilly shows the smple onfigurtion. In rief, monolyers were grown on 285 nm SiO 2 /Si sustrtes using the hemil vpour deposition (CVD) method 25. They were 1 Deprtment of Physis, University of Cliforni t Berkeley, Berkeley, Cliforni 9472, USA, 2 Mterils Siene Division, Lwrene Berkeley Ntionl Lortory, Berkeley, Cliforni 9472, USA, Deprtment of Mterils Siene nd Engineering, College of Engineering, Peking University, Beijing 1871, Chin, 4 Deprtment of Mterils Siene nd Engineering, University of Cliforni, Berkeley, Cliforni , USA, 5 Shool for Engineering of Mtter, Trnsport nd Energy, Arizon Stte University, Tempe, Arizon 85287, USA, 6 Kvli Energy NnoSienes Institute t the University of Cliforni, Berkeley nd the Lwrene Berkeley Ntionl Lortory, Berkeley, Cliforni 9472, USA, These uthors ontriuted eqully to this work. *e-mil: fengwng76@erkeley.edu 682 NATURE NANOTECHNOLOGY VOL 9 SEPTEMBER Mmilln Pulishers Limited. All rights reserved

2 NATURE NANOTECHNOLOGY LETTERS DOI: 1.18/NNANO e 6 7 Rmn ount (.u.) 4 h+ h+ A1ʹ Eʹ 5, A1ʹ 2,5 / Eʹ 7,5 e Rmn shift (m 1) Figure 1 Bnd lignment nd struture of / heterostrutures., Shemti of the theoretilly predited nd lignment of / heterostruture, whih forms type II heterojuntion. Optil exittion of the A-exiton will led to lyer-seprted eletron (e ) nd hole (h+) rriers., Illustrtion of / heterostruture with monolyer lying on top of monolyer. Eletrons nd holes reted y light re shown to seprte into different lyers., Rmn spetr of n isolted monolyer (lue tre), n isolted monolyer (red tre) nd / heterostruture (lk tre). 5 4 Photoluminesene (.u.) 2 (x.5) 9 6 / Figure 2 Photoluminesene spetr nd mpping of / heterostrutures t 77 K., Optil mirosope imge of typil / heterostruture smple. The lyer overs the entire imge nd right res orrespond to / heterostrutures. Sle r, 5 µm., Photoluminesene mpping dt tken in the re within the dshed retngle in. The olour sle represents photoluminesene intensity t the A-exiton resonne (1.9 ev). It lerly shows tht photoluminesene is strongly quenhed in the heterostruture. Sle r, 5 µm., Typil photoluminesene spetr of n isolted monolyer, n isolted monolyer nd / heterostruture. The isolted nd monolyers show strong photoluminesene t 1.9 ev nd 2.6 ev, respetively, orresponding to their A-exiton resonnes. Both exiton photoluminesene signls re strongly quenhed in the / heterostruture, suggesting n effiient hrge trnsfer proess exists in the heterostruture. susequently trnsferred on top of s-grown CVD flkes on spphire sustrtes26 to form / heterostrutures. Rmn spetr (Fig. 1) from isolted nd films onfirm tht oth re monolyers, euse the energy seprtion etween Rmn tive modes grees well with previously reported vlues for monolyer nd (refs 27 ). The Rmn spetrum of / heterostruture (Fig. 1) ppers to omprise the ddition of Rmn modes from the onstituent lyers (see Supplementry Setion for omprison of efore nd fter nneling). One sensitive proe of hrge trnsfer in MX2 heterostrutures is photoluminesene spetrosopy, euse n eletron nd hole pir sptilly seprted in two MX2 lyers nnot emit effiiently. We performed photoluminesene spetrosopy nd mpping on multiple / heterostruture smples. Figure 2 presents n optil imge of one smple, in whih lrge ontinuous piee (overing the entire imge) hs een trnsferred on top of flkes (the right res). Figure 2 shows the photoluminesene intensity mp t the A-exiton resonne (1.9 ev) t 77 K when the smple is exited y 2. ev photons. We oserved strong photoluminesene signls in the -only region, ut the photoluminesene is signifintly quenhed in the / heterostruture region. Figure 2 further displys typil photoluminesene spetr for / heterostrutures, isolted nd isolted lyers with 2. ev exittion. It is pprent tht nd monolyers show strong photoluminesene t their respetive A-exiton resonnes (1.9 ev nd 2.6 ev), ut oth photoluminesene signls re effiiently quenhed in / heterostrutures. Room-temperture photoluminesene spetr lso exhiit similr ehviour (Supplementry Setion 4). In priniple, photoluminesene signls n e quenhed y two mehnisms in heterostruture: energy trnsfer nd hrge trnsfer. However, energy trnsfer quenhes only the photoluminesene from higher energy trnsition (tht is, 2.6 ev resonne in ), nd tends to enhne luminesene from the lower energy trnsition (1.9 ev resonne in ). On the other hnd, hrge trnsfer will quenh light emission from ll trnsitions. Aordingly, the oservtion of redued photoluminesene from oth nd exiton resonnes in / heterostrutures demonstrtes tht effiient hrge trnsfer tkes ple in this type II heterojuntion. To diretly proe the hrge trnsfer proess nd its ultrfst dynmis, we mesured trnsient sorption spetr of / heterostrutures using resonnt pump proe spetrosopy. A femtoseond pulse first exites the heterostruture, nd the photoindued hnges in the refletion spetrum (ΔR/R) re proed y lser-generted superontinuum light fter ontrolled time delys. For tomilly thin heterostrutures on trnsprent spphire sustrte, the refletion hnge ΔR/R is diretly proportionl to the hnge in sorption oeffiient1,2. nd NATURE NANOTECHNOLOGY VOL 9 SEPTEMBER Mmilln Pulishers Limited. All rights reserved 68

3 NATURE NANOTECHNOLOGY DOI: 1.18/NNANO /WS 2 (x.5) K Dely (ps) Dely (ps) x d.1..1 e 2 1 Asorption (.u.) 1 ps 2 ps Pump /WS 2 (x.5) WS Proe energy (ev) Figure Trnsient sorption spetr of /WS 2 heterostrutures.,, Two-dimensionl plots of trnsient sorption spetr t 77 K from /WS 2 heterostruture () nd n isolted monolyer () upon exittion of the A-exiton trnsitions. The horizontl xis, vertil xis nd olour sle represent the proe photon energy, pump proe time dely nd the trnsient sorption signl, respetively. Positive signls indite pumpindued derese in sorption.,d, Trnsient sorption spetr for /WS 2 (red irles) nd (green squres) t 1 ps nd 2 ps pump proe delys, respetively. e, Liner sorption spetr of monolyers of (mgent line) nd WS 2 (lue line). Although only A-exiton trnsitions re optilly exited, trnsient sorption spetr in the /WS 2 heterostruture re dominted y resonne feture (red irles in nd d) orresponding to the WS 2 A-exiton trnsition (lue line in e), whih is lerly distinguishle from the resonne feture orresponding to the B-exiton trnsition in n isolted monolyer (green squres in nd d nd mgent line in e). This unmiguously demonstrtes effiient hole trnsfer from the photoexited lyer to the WS 2 lyer in /WS 2 heterostrutures. monolyers hve distintly different exiton trnsitions. We n therefore seletively exite the or WS 2 lyers using speifi resonnt optil exittions nd proe the umultion of eletrons nd holes in different lyers through photo-indued hnges in their respetive exiton trnsitions. Speifilly, we hose pump photon energy t 1.86 ev to exite exlusively the A-exiton trnsition of. This pump nnot exite WS 2 diretly euse the photon energy is fr elow the sorption threshold of WS 2.We then exmined the photo-indued hnges of oth WS 2 nd exiton resonnes in trnsient sorption spetr from 2. to 2.5 ev to proe the hrge distriution in the heterostrutures. Using pump fluene of 85 µj m 2, A-exitons in with density m 2 re generted immeditely fter photoexittion. Figure presents two-dimensionl plot of trnsient sorption spetr in /WS 2 heterostruture t 77 K, where the olour sle, the horizontl xis nd the vertil xis represent the mgnitude of ΔR/R, the proe photon energy nd the pump proe time dely, respetively. The figure shows prominent resonnt fetures in trnsient sorption entred on 2.6 ev nd 2.46 ev, with the higher energy feture severl times weker thn the lower energy one. On ompring this with the liner sorption spetr of isolted WS 2 nd monolyers in Fig. e, we n ttriute these two resonnt fetures, respetively, to the A- nd B-exiton trnsitions in WS 2, lthough the WS 2 lyer is not exited y the pump. To etter understnd the trnsient sorption spetr in /WS 2 heterostrutures, we lso performed ontrol experiments for isolted WS 2 nd monolyers. In re WS 2 monolyers no pump-indued signl n e oserved ove the noise level, onsistent with the ft tht no diret sorption n tke ple in WS 2 (Supplementry Setion 5). In isolted monolyers, pump-indued sorption hnges in our spetrl rnge re entred t 2.11 ev (Fig. ), orresponding to the B-exiton trnsition of. Figure,d presents detiled omprisons of the trnsient sorption spetr in /WS 2 heterostruture nd n isolted monolyer t pump proe time delys of 1 ps (Fig. ) nd 2 ps (Fig. d). Although the resonnt fetures t 2.6 ev for the heterostruture nd t 2.11 ev for monolyer re lose in energy, they re lerly distinguishle nd mth well with the A-exiton in WS 2 nd B-exiton in in the sorption spetr (Fig. e), respetively. In ddition, the trnsient sorption signl t the WS 2 A-exiton trnsition in the heterostruture is stronger in mgnitude nd hs nrrower spetrl width nd slower dey time onstnt. Our trnsient sorption mesurements of /WS 2 heterostrutures estlish unmiguously tht optil exittion in leds to strong modifition of exiton trnsitions in WS 2, whih hs lrger optil ndgp. This provides diret evidene of effiient hrge seprtion in photoexited /WS 2 heterostrutures (Fig. 1): eletron hole pirs re initilly reted in the lyer, ut holes quikly trnsfer to the WS 2 lyer due to the type II nd lignment, while eletrons sty in the lyer. The photoexited eletrons in nd holes in WS 2 led to strong trnsient sorption signl for exiton trnsitions in oth nd WS 2. Trnsient sorption signls re strongest for the A-exitons due to their shrper resonnes nd effiient photolehing effets from Puli loking, ut B-exiton trnsitions re lso ffeted. Consequently, the trnsient sorption spetr in /WS 2 heterostrutures re dominted y the A-exiton trnsition in WS 2. Photo-indued hnges of B-exiton trnsitions in the /WS 2 heterostruture (Fig. ) nd in the monolyer (Fig. ) n lso e identified, ut they re signifintly weker thn tht of A-exiton trnsitions. Room-temperture dt show similr trends (Supplementry Setion 6). The rise time of the WS 2 A-exiton trnsient sorption signl diretly proes the hole trnsfer dynmis from the lyer, euse this signl exists only fter hole trnsfer, nd not right t 684 NATURE NANOTECHNOLOGY VOL 9 SEPTEMBER Mmilln Pulishers Limited. All rights reserved

4 NATURE NANOTECHNOLOGY DOI: 1.18/NNANO LETTERS /WS 2.59 ev, 77 K Dely 2.1 ev, 77 K Dely (ps) Figure 4 Ultrfst hole trnsfer dynmis from vertil uts in Fig.,., Evolution of trnsient sorption signls t the WS 2 A-exiton resonne in the /WS 2 heterostruture., Dynmi evolution of trnsient sorption signls t the B-exiton resonne in the isolted monolyer. Both signls show lmost identil ultrfst rise times, limited y the lser pulse durtion of 25 fs. By onvoluting the instrument response funtion (lue dshed line in ) nd n instntneous response in,we n reprodue the ultrfst dynmis in the monolyer (red tre in ). Similr onvolution shows tht the rise time in the /WS 2 monolyer is 25 fs (red tre in ) nd hs n upper limit of 5 fs. This demonstrtes tht holes n trnsfer from the photoexited lyer to the WS 2 lyer in the /WS 2 heterostruture within 5 fs. the exittion of. Figure 4 presents the dynmi evolution of the WS 2 A-exiton resonne in the /WS 2 heterostruture (Fig. 4), whih n e ompred to the trnsient sorption signl for the B-exiton resonne in n isolted monolyer (Fig. 4). We found tht the rise times in oth signls re lmost identil, limited y the lser pulse durtion of 25 fs. In Fig. 4, the monolyer is diretly pumped nd the photo-indued signl should pper instntneously. (The rise times of pump-indued A- nd B-exiton signls in hve indistinguishle ehviour, s shown in Supplementry Setion 8.) We ould reprodue the ultrfst dynmis in the monolyer in Fig. 4 y onvoluting the instrument response funtion (lue dshed urve in Fig. 4) with n instntneous response in. Using the sme instrument response funtion for time onvolution, we n then reprodue the experimentlly oserved signl in the heterostruture with rise time shorter thn 5 fs (red line in Fig. 4). Our results therefore show tht holes re trnsferred from the lyer to the WS 2 lyer within 5 fs fter optil exittion of the /WS 2 heterostruture, remrkly fst rte. Similr ultrfst hole trnsfer lso tkes ple t room temperture, s shown in Supplementry Setion 7. This hole trnsfer time is muh shorter thn the exiton lifetime nd most other dynmi proesses in MX 2 monolyers, whih re on the order of severl to tens of pioseonds. Eletrons nd holes n therefore e effiiently seprted into different lyers immeditely fter their genertion. Consequently, photoluminesene from nd WS 2 exiton resonnes will e strongly quenhed, s we oserved previously. Our experimentl dt estlish tht hrge seprtion in /WS 2 heterostrutures is very effiient, lthough the nd offset etween nd WS 2 is smller thn the predited exiton inding energy in monolyer MX 2. Energetilly unorrelted free eletrons nd holes in seprted nd WS 2 lyers nnot e produed through the exittion of A-exitons. However, the nd WS 2 lyers re only seprted from eh other y.62 nm (ref. 1), suggesting tht even for lyer-seprted eletrons nd holes, strong Coulom intertions n led to ound exiton sttes. These exiton sttes with eletrons nd holes residing in different lyers n e energetilly fvourle ompred to n exiton onfined to only the lyer, nd re likely to e responsile for the effiient hrge seprtion oserved in /WS 2 heterostrutures. Suh ounded exitons with n eletron nd hole in different mterils, known s hrge trnsfer exitons (CTCs), hve lso een investigted in other type II heterojuntions, suh s moleulr donor/eptor interfes in the ontext of orgni photovoltis 21 24,4. The oserved su-5 fs hole trnsfer time is remrkly short onsidering tht the nd WS 2 lyers re twisted reltive to eh other nd re oupled y reltively wek vn der Wls intertions. One ftor ontriuting to the ultrfst hrge trnsfer rte in tomilly thin heterostrutures is the lose proximity of the two heterolyers, euse eletrons or holes only need to move less thn 1 nm vertilly for the hrge trnsfer proess to hppen. Still, the 5 fs hole trnsfer time for vn der Wls heterostrutures is fst. A mirosopi understnding of this ultrfst hole trnsfer in MX 2 heterostrutures requires detiled theoretil studies to exmine the hyridiztion of eletroni sttes in twisted heterolyers nd the dynmi evolution of photoexited sttes due to eletron phonon nd eletron eletron intertions. For exmple, euse /WS 2 heterostrutures re extended rystlline two-dimensionl lyers, resonnt hrge trnsfer hs to stisfy oth energy nd momentum onservtions, nd eletroni oupling etween sttes with different moment in the Brillouin zone n vry signifintly. It is known tht for ilyers, eletroni oupling t the K point in the Brillouin zone is wek. Eletron wvefuntion hyridiztion t the Γ point, however, is muh stronger, whih leds to rise in Γ point vlene nd nd n indiret ndgp in ilyer (refs 17,18). Eletroni oupling etween inommensurte nd WS 2 n ply n importnt role in the hrge trnsfer dynmis of twisted /WS 2 heterostrutures, the ehviour of whih hs een little studied to dte. Beuse vn der Wls heterostrutures hve tomilly shrp interfes with no dngling onds nd well-defined optil resonnes, they provide n idel model system for further experimentl nd theoretil investigtions of interfil hrge trnsfer proesses nd hrge trnsfer exiton sttes. The ultrfst hrge trnsfer proess in tomilly thin MX 2 heterostrutures hs importnt implitions for photoni nd optoeletroni pplitions. MX 2 semiondutors hve extremely strong optil sorption, nd hve een onsidered previously for photodetetors 2,,11, photovoltis 5 nd phototlysis 6. Compred with orgni photovolti mterils, these two-dimensionl lyers hve rystlline struture nd etter eletril trnsport properties. Our studies here show tht the type II MX 2 heterostrutures lso exhiit femtoseond hrge trnsfer rte, whih provides n idel wy to sptilly seprte eletrons nd holes for eletril olletion nd utiliztion. In summry, we hve demonstrted, for the first time, effiient hrge trnsfer in /WS 2 heterostrutures through omined photoluminesene mpping nd trnsient sorption mesurements. We hve quntittively determined the ultrfst hole trnsfer time to e less thn 5 fs. Our study suggests tht MX 2 heterostrutures, with their remrkle eletril nd optil properties nd the rpid development of lrge-re synthesis, hold gret promise for future optoeletroni nd photovolti pplitions. Methods MX 2 monolyer growth. Monolyer ws grown y CVD on 285 nm SiO 2 /Si sustrtes 25. Sustrtes were loded into 1-inh CVD furne nd pled fe down ove ermi ot ontining 4.2 mg of MoO ( 99.5%, Sigm-Aldrih). A ruile ontining 15 mg of sulphur ( 99.5%, Sigm-Aldrih) ws pled upstrem. CVD growth ws performed t tmospheri pressure with flowing ultrhigh-purity nitrogen. Tuning the sulphur onentrtion n roughly modify the nuletion density nd ontrol the trnsition of tringulr single rystls to lrge-re monolyer. Lrge-re WS 2 monolyer ws grown on spphire sustrtes y CVD 26.A multi-temperture-zone tue furne (Linderg/Blue M) equipped with 1-inhdimeter qurtz tue ws used for growth. Sulphur powder ws mildly sulimted t 1 C nd pled outside the hot zone. WO powder (Alf Aesr, purity 99.9%) NATURE NANOTECHNOLOGY VOL 9 SEPTEMBER Mmilln Pulishers Limited. All rights reserved

5 NATURE NANOTECHNOLOGY DOI: 1.18/NNANO nd spphire sustrtes ( 1 oriented single rystls) were suessively pled in the hot entre. We used rgon (flow rte 8 s...m.) or mixed rgon nd hydrogen gs (flow rtes of 8 nd 1 s...m., respetively) to rry WO x vpour speies to the downstrem sustrtes. The growth pressure ws set t P. Growth temperture ws set t 9 C nd growth time t 6 min. Heterostruture preprtion. The heterostruture ws prepred y trnsferring 25 monolyer onto monolyer WS 2 on spphire. The CVD-grown single lyer (desried ove) on SiO 2 /Si ws spin-oted with poly(methyl methrylte) (PMMA) (A4) t 4, r.p.m. for 6 s. The PMMA/ film ws seprted from the sustrte (SiO 2 /Si) y KOH ething (1 mol l 1 ) t 8 C. The film ws trnsferred to deionized wter ekers to dilute KOH residue under. It ws then trnsferred onto CVD-grown WS 2 on spphire sustrte (desried ove) nd soked in etone to dissolve the PMMA. Finlly, the heterostruture smple ws nneled t n elevted temperture in vuum (Supplementry Setion ). Note there is no polymer etween the nd WS 2 lyers in the smple fter PMMA trnsfer (PMMA ws on top of the top lyer), so they n form firly good ontt. Photoluminesene nd Rmn mesurements. For photoluminesene mpping we used 52 nm lser (photon energy of 2. ev) to exite the isolted monolyers of nd WS 2 nd the /WS 2 heterostrutures. The lser em ws foused to diffrtion-limited spot (dimeter, 1 µm) nd the photoluminesene olleted in refletion geometry with onfol mirosope. A monohrometer nd liquid-nitrogen-ooled hrge-oupled devie (CCD) were used to reord the photoluminesene spetr. Two-dimensionl photoluminesene mpping ws rried out y snning the omputer-ontrolled piezoeletri stge. For Rmn mesurements we used 488 nm exittion lser. Liner sorption spetr. A superontinuum lser (Finium SC45) ws used s rodnd light soure. The lser ws foused t the smple with 2 µm em size nd the refletion signl R olleted vi onfol mirosopy nd nlysed y spetrometer equipped with one-dimensionl CCD rry. Referene spetrum R ws tken on the spphire sustrte ner the smple (isolted, isolted WS 2 nd heterostruture). The normlized differene signl (R R )/R is diretly proportionl to the liner sorption from tomilly thin lyers on spphire 1,2. Pump proe mesurement. Femtoseond pulses t 1,26 nm were generted y regenertive mplifier seeded y mode-loked osilltor (Light Conversion PHAROS). The femtoseond pulses (t repetition rte of 15 khz nd pulse durtion of 25 fs) were split into two prts. One ws used to pump n optil prmetri mplifier to generte tunle exittion lser pulses, nd the other ws foused into spphire rystl to generte superontinuum white light ( 5 9 nm) for proe pulses. The pump nd proe ems were foused t the smple with dimeters of 5 µm nd 25 µm, respetively. The proe light ws deteted y high-sensitivity photomultiplier fter wvelength seletion through monohrometer with spetrl resolution of 1 nm. The pump proe time dely ws ontrolled y motorized dely stge nd the pump proe signl ws reorded using lok-in detetion with hopping frequeny of 1.6 khz. Reeived 6 Ferury 214; epted 16 July 214; pulished online 24 August 214 Referenes 1. Geim, A. K. & Grigoriev, I. V. Vn der Wls heterostrutures. Nture 499, (21). 2. Britnell, L. et l. Strong light mtter intertions in heterostrutures of tomilly thin films. Siene 4, (21).. Yu, W. J. et l. Highly effiient gte-tunle photourrent genertion in vertil heterostrutures of lyered mterils. Nture Nnoteh. 8, (21). 4. Ynkowitz, M. et l. Emergene of superlttie Dir points in grphene on hexgonl oron nitride. Nture Phys. 8, (212). 5. Ponomrenko, L. et l. Cloning of Dir fermions in grphene superltties. Nture 497, (21). 6. Hunt, B. et l. Mssive Dir fermions nd Hofstdter utterfly in vn der Wls heterostruture. Siene 4, (21). 7. Den, C. R. et l. Hofstdter s utterfly nd the frtl quntum Hll effet in moiré superltties. Nture 497, (21). 8. Britnell, L. et l. Field-effet tunneling trnsistor sed on vertil grphene heterostrutures. Siene 5, (212). 9. Choi, M. S. et l. Controlled hrge trpping y molydenum disulphide nd grphene in ultrthin heterostrutured memory devies. Nture Commun. 4, 1624 (21). 1. Jones, A. M. et l. Spin lyer loking effets in optil orienttion of exiton spin in ilyer WSe 2. Nture Phys. 1, 1 5 (214). 11. Lopez-Snhez, O., Lemke, D., Kyi, M., Rdenovi, A. & Kis, A. Ultrsensitive photodetetors sed on monolyer. Nture Nnoteh. 8, (21). 12. Gong, C. et l. Bnd lignment of two-dimensionl trnsition metl dihlogenides: pplition in tunnel field effet trnsistors. Appl. Phys. Lett. 1, 551 (21). 1. Koms, H. & Krsheninnikov, A. Eletroni strutures nd optil properties of relisti trnsition metl dihlogenide heterostrutures from first priniples. Phys. Rev. B 88, 8518 (21). 14. Kng, J., Tongy, S., Zhou, J., Li, J. & Wu, J. Bnd offsets nd heterostrutures of two-dimensionl semiondutors. Appl. Phys. Lett. 12, (21). 15. Terrones, H., López-Urís, F. & Terrones, M. Novel hetero-lyered mterils with tunle diret nd gps y sndwihing different metl disulfides nd diselenides. Si. Rep., 1549 (21). 16. Kosmider, K. & Fernndez-Rossier, J. Eletroni properties of the -WS 2 heterojuntion. Phys. Rev. B 87, (21). 17. Mk, K. F., Lee, C., Hone, J., Shn, J. & Heinz, T. F. Atomilly thin : new diret-gp semiondutor. Phys. Rev. Lett. 15, 1685 (21). 18. Splendini, A. et l. Emerging photoluminesene in monolyer. Nno Lett. 1, (21). 19. Berkelh, T. C., Hyertsen, M. S. & Reihmn, D. R. Theory of neutrl nd hrged exitons in monolyer trnsition metl dihlogenides. Phys. Rev. B 88, 4518 (21). 2. Qiu, D. Y., d Jornd, F. H. & Louie, S. G. Optil spetrum of : mnyody effets nd diversity of exiton sttes. Phys. Rev. Lett. 111, (21). 21. Grnini, G., Miuri, M. & Fzzi, D. Hot exiton dissoition in polymer solr ells. Nture Mter. 12, 29 (21). 22. Jiluekov, A. E. et l. Hot hrge-trnsfer exitons set the time limit for hrge seprtion t donor/eptor interfes in orgni photovoltis. Nture Mter. 12, 66 7 (21). 2. Kke, L. G., Moses, D. & Heeger, A. J. Coherene nd unertinty in nnostrutured orgni photovoltis. J. Phys. Chem. Lett. 4, (21). 24. Gélins, S. et l. Ultrfst long-rnge hrge seprtion in orgni semiondutor photovolti diodes. Siene 4, (214). 25. Vn der Znde, A. M. et l Grins nd grin oundries in highly rystlline monolyer molydenum disulphide. Nture Mter. 12, (21). 26. Zhng, Y. et l. Controlled growth of high-qulity monolyer WS 2 lyers on spphire nd imging its grin oundry. ACS Nno 7, (21). 27. Lee, C. et l. Anomlous lttie virtions of single-nd few-lyer. ACS Nno 4, (21). 28. Berkdemir, A. et l. Identifition of individul nd few lyers of WS 2 using Rmn spetrosopy. Si. Rep., 1755 (21). 29. Luo, X. et l. Effets of lower symmetry nd dimensionlity on Rmn spetr in two-dimensionl WSe 2. Phys. Rev. B 88, 1951 (21).. Terrones, H. et l. New first order Rmn-tive modes in few lyered trnsition metl dihlogenides. Si. Rep. 4, 4215 (214). 1. Wng, F. et l. Gte-vrile optil trnsitions in grphene. Siene 2, (28). 2. Mk, K. F. et l. Mesurement of the optil ondutivity of grphene. Phys. Rev. Lett. 11, (28).. Shi, H. et l. Exiton dynmis in suspended monolyer nd few-lyer ; 2D rystls. ACS Nno 7, (21). 4. Zhu, X., Yng, Q. & Muntwiler, M. Chrge-trnsfer exitons t orgni semiondutor surfes nd interfes. A. Chem. Res. 42, (29). 5. Gourmelon, E. et l. MS 2 (M = W, Mo) photosensitive thin films for solr ells. Sol. Energy Mter. Sol. Cells 46, (1997). 6. Ho, W., Yu, J. C., Lin, J., Yu, J. & Li, P. Preprtion nd phototlyti ehvior of nd WS 2 nnoluster sensitized TiO 2. Lngmuir 2, (24). Aknowledgements Optil mesurements nd growth were supported y the Offie of Bsi Energy Siene, Deprtment of Energy (ontrt no. DE-SC949, Erly Creer Awrd; ontrt no. DE-AC2-5CH1121, Mterils Siene Division). The WS 2 growth prt ws supported finnilly y the Ntionl Nturl Siene Foundtion of Chin (grnts nos , ) nd the Ministry of Siene nd Tehnology of Chin (grnt no. 211CB9219). F.W. knowledges support from Dvid nd Luile Pkrd fellowship. The uthors thnk K. Liu nd Y. Chen for help in smple hrteriztion nd L. Ju for providing the evportion msk. Author ontriutions F.W. oneived nd supervised the experiment. X.H., J.K. nd S-F.S. rried out photoluminesene nd pump proe mesurements. Y.S., S.T. nd J.W. grew CVD monolyer. Y.Z. nd Y.F.Z. grew CVD monolyer WS 2. J.K., X.H. nd S-F.S prepred the heterostruture smple. X.H., J.K., S-F.S. nd C.J. performed dt nlysis. All uthors disussed the results nd wrote the mnusript. Additionl informtion Supplementry informtion is ville in the online version of the pper. Reprints nd permissions informtion is ville online t Correspondene nd requests for mterils should e ddressed to F.W. Competing finnil interests The uthors delre no ompeting finnil interests. 686 NATURE NANOTECHNOLOGY VOL 9 SEPTEMBER Mmilln Pulishers Limited. All rights reserved