LETTER. Two types of luminescence blinking revealed by spectroelectrochemistry of single quantum dots

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1 LETTER doi:./nture69 Two types of luminescence linking reveled y spectroelectrochemistry of single quntum dots Christophe Gllnd,, Ygnseni Ghosh, Andre Steinrück, Miln Sykor, Jennifer A. Hollingsworth, Victor I. Klimov, & Hn Htoon,, Photoluminescence linking rndom switching etween sttes of high () nd low () emissivities is universl property of moleculr emitters found in dyes, polymers, iologicl molecules nd rtificil nnostructures such s nnocrystl quntum dots, cron nnotues nd nnowires 6. For the pst yers, colloidl nnocrystls hve een used s model system to study this phenomenon,6. The occurrence of periods in nnocrystl emission hs een commonly ttriuted to the presence of n dditionl chrge 7, which leds to photoluminescence quenching y nonrditive recomintion (the Auger mechnism). However, this chrging model ws recently chllenged in severl reports 9,. Here we report time-resolved photoluminescence studies of individul nnocrystl quntum dots performed while electrochemiclly controlling the degree of their chrging, with the gol of clrifying the role of chrging in linking. We find tht two distinct types of linking re possile: conventionl (A-type) linking due to chrging nd dischrging of the nnocrystl core, in which lower photoluminescence intensities correlte with shorter photoluminescence lifetimes; nd second sort (B-type), in which lrge chnges in the emission intensity re not ccompnied y significnt chnges in emission dynmics. We ttriute B-type linking to chrge fluctutions in the electron-ccepting surfce sites. When unoccupied, these sites intercept hot electrons efore they relx into emitting core sttes. Both linking mechnisms cn e electrochemiclly controlled nd completely suppressed y ppliction of n pproprite potentil. In the conventionl linking model (Fig. ), nd periods correspond to neutrl nnocrystl nd chrged nnocrystl, respectively, nd photo-ssisted chrging/dischrging cuses rndom switching etween these two sttes. The dynmics of the right stte is dominted y rditive recomintion of the neutrl exciton, X (Fig. ), which is chrcterized y long, mono-exponentil decy ( ns in CdSe nnocrystls ). For chrged exciton (trion), X, three-prticle Auger recomintion opens fst, non-rditive chnnel, resulting in shorter lifetime ( few nnoseconds or less) nd, consequently, reduced photoluminescence quntum yield. As illustrted in Figs c, d, this model predicts correlted fluctutions of the photoluminescence intensity nd lifetime (referred to here s A-type linking) tht hve indeed een oserved experimentlly,,. Photochrging cn led to inry switching etween the nd the sttes (Fig. c) when the timescle of chrge fluctutions is longer thn the experimentl inning time (typiclly t lest tens of milliseconds). As we discuss elow, the sme discrete chrging process cn lso produce qusi-continuous photoluminescence fluctutions, referred to s flickering (Fig. d). In this cse, the dt within ech in represents n verge over the neutrl nd chrged nnocrystl sttes, which results in photoluminescence intensities nd lifetimes tht vry continuously ccording to the reltive times spent y the nnocrystl in ech chrge stte. A convenient tool for the nlysis of correltions etween photoluminescence intensities nd lifetimes is fluorescence lifetimeintensity distriution (FLID) representtion. In this representtion, the proility of occupying given stte in the two-dimensionl lifetime intensity spce is shown y flse colour, which chnges from lue to red s the proility increses. As illustrted in the insets of Figs c, d, the use of FLIDs llows redy identifiction of different types of chrge stte s well s different types of linking ehviour (for exmple inry linking versus flickering). In our work, to verify the vlidity of the chrging model of photoluminescence intermittency, we comine single-nnocrystl spectroscopy with n electrochemicl pproch for controlling the extent of nnocrystl chrging 7. Specificlly, we conduct single-nnocrystl, time-tgged, time-resolved, single-photon counting studies of smples incorported into three-electrode electrochemicl cell (Fig. ). We investigte coreshell CdSe/CdS nnocrystls synthesized ccording to ref.. In the cse of exceptionlly thick, 69-monolyer shells, they show nerly complete suppression of linking. Here we use nnocrystls with intermedite shell thicknesses (79 monolyers) tht show typicl linking ehviour ut hve quntum yields during c PL lifetime X Blinking X Time Intensity Time X X Lifetime PL X γ r γ A d PL lifetime Flickering X Time dely Intensity Time X γ r X Lifetime Figure Conventionl chrging model: A-type linking nd flickering., In the conventionl photoluminescence (PL) linking model, nd periods correspond to neutrl nnocrystl (X ) nd chrged nnocrystl (X ), respectively., Schemtic photoluminescence decy of the nd the sttes on logrithmic scle. The dynmics of the stte is dominted y the rditive rte c r. In the chrged stte, the increse in the numer of recomintion pthwys leds to higher rditive rte, c r, responsile for the higher emission intensity t short delys. Simultneously, the onset of threeprticle Auger recomintion with the rte c A? c r opens new, non-rditive, chnnel, leding to fster photoluminescence decy nd reduced photoluminescence quntum yield. c, When the timescle of chrging nd dischrging is longer thn the experimentl inning time, inry linking is oserved. d, For fluctutions much fster thn the in size, continuous distriution of intensities nd lifetimes is otined, often referred to s flickering. The insets in c nd d show corresponding schemtic fluorescence lifetimeintensity distriutions (FLIDs). Chemistry Division, Los Almos Ntionl Lortory, Los Almos, New Mexico 7, USA. Center for Advnced Solr Photophysics, Los Almos Ntionl Lortory, Los Almos, New Mexico 7, USA. Mterils Physics & Applictions: Center for Integrted Nnotechnologies, Los Almos Ntionl Lortory, Los Almos, New Mexico 7, USA. NOVEMBER VOL 79 NATURE Mcmilln Pulishers Limited. All rights reserved

2 RESEARCH LETTER 6 Silver reference electrode Pltinum counterelectrode PL (counts s ) ITO working electrode Ojective Dichroic mirror. V.7 V. V. V V Electrolyte Pulsed lser Potentiostt Filter APDs 6 Time dely (ns) periods tht re considerly higher thn those of stndrd nnocrystls. This llows us to investigte in detil the properties of the stte nd the effect of controlled chrging on its emissivity nd dynmics. All chnges in photoluminescence intensity nd dynmics induced y the pplied potentil re reversile, indicting no permnent chemicl or photochemicl modifictions of the nnocrystls nd suggesting tht such chnges re due to controlled chrging/dischrging. To verify tht the oserved photoluminescence origintes from single nnocrystl, we mesure the second-order intensity correltion function 9, g, nd ensure tht g (),. (Methods Summry; Fig., inset). All experiments re performed under mient conditions t room temperture. All potentils re reported with respect to silver wire qusi-reference (see Methods for further experimentl detils). Electrochemicl control of emission intensity from individul nnocrystls hs een demonstrted previously 7. In this study, we nlyse the effect of chrging on oth photoluminescence intensity nd photoluminescence dynmics. Figure shows photoluminescence time trnsients recorded for single nnocrystl under incresing negtive potentil, V, which corresponds to electron injection. The photoluminescence decy ecomes progressively fster s V ecomes more negtive. All decys cn e fitted glolly to triple-exponentil function (Fig., grey lines) The high fidelity of the fit (see residuls in the lower inset) suggests tht only three distinct emitting sttes re involved, ech hving well-defined photoluminescence lifetime: t d ns,t s nsndt n ns. As V is decresed from to.7 V, the weight of the t s component grdully increses reltive to tht of the t n component. At more-negtive vlues of V, the component with the fstest decy (corresponding to t d ) emerges nd ecomes dominnt t Coincidences Residul (counts s ).. BS TCSPC electronics. Time dely (ns) 6 Figure Experimentl set-up nd electrochemicl chrging of n individul nnocrystl., Set-up of single-nnocrystl spectroelectrochemicl experiment. APD, vlnche photodiode; BS, / em splitter; ITO, indium tin oxide; TCSPC, time-correlted single-photon counting., Series of photoluminescence decys for single nnocrystl for incresingly negtive pplied potentils. The thin grey lines show the est glol triple-exponentil fits with the shred time constnts, yielding the lifetimes t d ns, t s ns nd t n ns. Top inset: the second-order photoluminescence intensity correltion function mesured for this nnocrystl indictes tht g ().. Bottom inset: residuls of the glol fit indicte very high fidelity of the fitting procedure, with devitions within the noise level nd elow % of the mximum photoluminescence signl.. V. We ssign the lifetimes t n, t s nd t d to three distinct sttes, respectively neutrl excitons (X ) nd singly (X ) nd douly (X ) chrged excitons. Owing to incresed rtes of rditive decy, singly chrged excitons (negtive trions) show incresed signl t short times reltive to neutrl excitons (Fig. ); however, the time-integrted photoluminescence signl is reduced ecuse of Auger recomintion. The oserved quickening of photoluminescence decy on chrging is due to enhncement in oth the rditive decy rte nd the nonrditive decy rte. To confirm the ove ssignments nd investigte the reltionship etween chrging nd linking, we nlyse the correltions in the temporl vritions of photoluminescence decy time nd intensity. In Fig., we plot photoluminescence intensity nd verge lifetime trjectories (clculted for -ms in size; Supplementry Informtion) long with corresponding FLIDs for the nnocrystl shown in Fig.. To illustrte the vriility in linking ehviours, we present the dt collected for this nnocrystl on two different dys. These dt, representing exmples of inry switching (Fig. ) nd flickering (Fig. ), indicte strong correltion etween the photoluminescence intensity nd the photoluminescence lifetime during the fluctutions, in greement with the conventionl chrging model. We cll this A-type linking. At V (Fig., middle), the nnocrystl shows inry linking etween the neutrl stte (X ) nd the singly chrged stte (X ). The Averge lifetime (ns) (counts ms ) Averge lifetime (ns) (counts ms ) Averge lifetime (ns) V =.6 V V = V X X Time: s X V = V Time: s V =.7 V Averge lifetime (ns) X X V =.6 V V =. V Figure Correlted photoluminescence intensity nd lifetime fluctutions: A-type linking nd flickering., Photoluminescence intensities (lck lines) nd verge lifetimes (red lines), nd corresponding FLIDs, for the nnocrystl shown in Fig. t three different potentils. Binry linking seen t V V is lrgely suppressed t V.6 V, wheres electron injection is chieved t V.6 V. In the FLID colour scle, red corresponds to the most frequently occurring lifetimeintensity pir, nd proilities less thn % of this mximum re indicted y drk lue. A liner scling from lue to red is used etween these extremes., Dt from the sme nnocrystl, cquired on different dy, disply continuous photoluminescence intensity nd lifetime fluctutions, typicl of flickering. At V. V, we oserve emission from douly chrged exciton, X. All dt were nlysed with in size of ms. Full time trjectories for nd re shown in Supplementry Fig.. (counts ms ) (counts ms ) NATURE VOL 79 NOVEMBER Mcmilln Pulishers Limited. All rights reserved

3 LETTER RESEARCH verge photoluminescence lifetime of X (the stte) is t n < ns, which corresponds to rditive lifetime of,6 ns (Supplementry Informtion) nd is in greement with previous ensemle studies of this type of nnocrystl. Appliction of positive potentil, V.6 V (Fig., left), drsticlly suppresses chrge fluctutions nd results in lmost non-linking emission from the neutrl exciton (see corresponding FLID nd Supplementry Figs nd ; nother exmple is shown in Supplementry Fig. ). At negtive potentil, V.6 V (Fig., right), the pek of the photoluminescence distriution shifts to the lower-emissivity stte, X, chrcterized y lifetime of, ns. Assuming sttisticl scling of recomintion rtes with the numer of chrges (Supplementry Informtion nd ref. ), we deduce the Auger lifetime for X to e,. ns. This is much shorter thn the rditive lifetime of X (, ns; Supplementry Informtion), which explins the reltively low photoluminescence quntum yield of the negtive trion. The existence of fluctutions etween X nd X is indicted y well-resolved trce in the FLID connecting the two sttes. We simulte the FLID dt ssuming tht the photoluminescence intensity during given time in is determined y the reltive times spent y the nnocrystl in the sttes X nd X (Supplementry Informtion). A very good greement, without ny djustle prmeters, etween the simulted trce (Fig., white lines) nd the mesured FLID provides strong support for oth the ssignment of emitting sttes nd the model used in the nlysis. We note tht the sme nnocrystl mesured on different dy (Fig. ) shows more continuous distriution of photoluminescence intensities nd lifetimes, typiclly referred to s flickering. This chnge in the linking ehviour proly occurs s result of the shortening of time spent y the nnocrystl in given chrge stte, which leds to fst switching etween X nd X within the in time used in the mesurements. Photoluminescence from X ecomes dominnt t V.7 V (Fig., middle FLID). By pplying more negtive potentil, V. V, we detect new stte with lifetime t d < ns, ssocited with the formtion of douly chrged exciton, X, with n Auger lifetime of,. ns. Judging from the FLID t this potentil (Fig., right), fluctutions occur lso etween the sttes X nd X. Figure shows dt from different nnocrystl, which hs distinct linking ehviour tht we refer to s B-type linking. Specificlly, t V V (Supplementry Fig. ) nd V. V (Fig., left), we oserve periods of low photoluminescence intensity tht re not ccompnied y significnt shortening of photoluminescence lifetimes. In fct, the photoluminescence time constnt mesured for the B-type stte is identicl to tht of the stte (X ). These B-type linking events were oserved in of the dots we studied (Supplementry Tle ) nd usully coexisted with A-type fluctutions (Supplementry Fig. ). Notly, t V V there is complete suppression of linking ut the long photoluminescence lifetime (,6 ns) typicl of neutrl exciton is preserved. This suppression could e chieved in the mjority of the nnocrystls with B-type linking; however, the potentil required to otin the suppression vried widely from dot to dot (from.6 to. V; Supplementry Tle ). For some nnocrystls, the elimintion of B-type linking occurred simultneously with the onset of A-type fluctutions etween X nd X (see elow). At more negtive potentil (V. V; Fig., right), we oserve cler signtures of electron injection into the nnocrystl. The photoluminescence decy ecomes i-exponentil, with n incresing contriution from the negtive trion, which in this quntum dot hs lifetime of,6 ns (Supplementry Fig. ). In this cse, switching etween X nd X occurs on much shorter timescle thn the in time, which gives rise to nrrow photoluminescence lifetimeintensity distriution. As with the dt in Fig., we cn closely reproduce this pttern using the chrging model (simulted white lines in FLID). To explin B-type linking, we invoke the ctivtion nd dectivtion of non-rditive recomintion centres (denoted R) tht efficiently cpture hot electrons efore they relx into the lowest-energy emitting Averge lifetime (ns) (counts ms ) E R E F 6 6 V =. V X B-type P e γ B << γ D S e S h R V =. V V = V Averge lifetime (ns) E F E R Time: s γ D V = V stte (Fig. ). Such processes of hot-electron trpping hve een recently oserved for oth nnocrystls in solutions nd surfcedispersed prticles,. In this picture, photoluminescence dynmics during the periods should e similr to tht of neutrl exciton wheres the emission intensity will e reduced ccording to the rtio etween the rtes of intrnd relxtion, c B, nd hot-electron cpture y the recomintion centre, c D. Becuse the frequency of B-type linking events is controlled y the electrochemicl potentil, the ctivtion nd dectivtion of the ypss chnnel re proly ssocited with emptying nd, respectively, filling of the corresponding surfce trp stte. For positive potentil (V. V; Fig., left), the Fermi level decreses in energy, which increses the reltive time spent y the trp in the unoccupied (tht is, ctive) stte nd leds to incresed occurrence of B-type events (Fig., left). The trpped electron cn recomine non-rditively with vlence-nd hole efore the next photoexcittion event, leving ehind neutrl dot. Occsionlly, photon sorption occurs efore reneutrliztion of the dot, resulting in positive trion, X ; Auger decy of X could explin oservtions of shorter photoluminescence lifetimes within the B-type periods illustrted in Supplementry Fig.. For n incresingly negtive potentil, the Fermi level increses in energy nd eventully regime is reched where the trp sttes ecome populted nd c D R owing to Coulom lockde (V V; Fig.,, middle). In this cse, B-type linking is completely suppressed. Appliction of n even more negtive potentil leds to chrging of the nnocrystl core with n extr electron nd emission from negtive trions (V. V; Fig.,, right). Blinking suppression due to filling of electron-ccepting trp sites is consistent with previous oservtions tht electron-donting thioltes X EF Bin, ms V =. V X X 6 6 V =. V Figure Photoluminescence intensity fluctutions without lifetime chnges: B-type linking., Photoluminescence intensities (lck lines) nd verge lifetimes (red lines), nd corresponding FLIDs, for nnocrystl showing the B-type stte; nlysis done with -ms in. Full time trjectories re shown in Supplementry Fig. 6., The model of B-type linking. The B-type stte is due to the ctivtion of recomintion centres (R) tht cpture hot electrons t rte, c D, tht is higher thn the intrnd relxtion rte, c B (the ground nd the excited electron sttes re shown s S e nd P e, respectively; S h is the nd-edge hole stte). The position of the Fermi level, E F, reltive to the trp energy, E R, is determined y the electrochemicl potentil nd controls the occupncy of the surfce trp R. This, in turn, llows for electrochemicl control of B-type linking. (counts ms ) NOVEMBER VOL 79 NATURE Mcmilln Pulishers Limited. All rights reserved

4 RESEARCH LETTER enhnce ensemle photoluminescence emission nd reduce linking. Similr phenomen were oserved for other electron-donting molecules 6 s well s n-doped sustrtes 7. These oservtions of significnt effect of surfce species on photoluminescence intensity nd intermittency imply tht the trp sites responsile for B-type linking re proly of surfce origin. Recent ultrfst studies of crrier surfce trpping in ensemles of CdSe nnocrystls lso suggest tht this process is directly relevnt to the prolem of nnocrystl linking. Finlly, our model of B-type hot-electron surfce trps provides n explntion of previously reported properties of the nnocrystl stte, such s low emission quntum yields 9 nd the lck of systemtic size dependence of photoluminescence lifetimes, tht could not e explined y the trditionl chrging model (Supplementry Informtion, section IV). The distinct nture of the processes responsile for A- nd B-type linking is evident from the effect of incresing shell thickness on photoluminescence intermittency. Specificlly, we oserve tht s the outer shell gets thicker, the B-type type linking events ecome less frequent until they re completely eliminted for shells with or more CdS monolyers. By contrst, the A-type linking cn still e oserved even in the cse of the extremely thick 9-monolyer shells. The nlysis of photoluminescence intermittency in more thn twenty nnocrystls with -monolyer shells (Supplementry Tle nd Supplementry Figs 7 nd ) indictes tht,7% of these dots re non-linking nd tht the rest hve A-type linking ehviour; none of the nnocrystls showed ny detectle B-type linking. By contrst, B-type linking is clerly the dominnt ehviour in nnocrystls with 79-monolyer shells (Supplementry Tle ). The fct tht B-type linking is quickly suppressed s shell thickness increses is consistent with the proposed mechnism of hot-electron tunnelling outside the nnocrystl, ecuse this process is expected to e extremely (in fct exponentilly) sensitive to the thickness of the tunnelling rrier. The studies of sttistics of nd times lso indicte cler distinction etween the A-type nd B-type linking mechnisms. In (counts ms ) Numer of events.. V =. V V = V.. B-type A-type. Averge lifetime (ns). V: B -type V: A -type Event durtion (ms) Figure Electrochemiclly controlled switching etween distinct sttistics for nd times in the sme nnocrystl, ccompnying the trnsition from B-type to A-type linking., FLIDs indicting nnocrystl switching from B-type linking t. V (left) to A-type linking t V (right). Detils of the nlysis re given in Supplementry Fig. 9., Sttistics for (red circles) nd (lck squres) times for the FLIDs in, in the log log representtion. At. V (B-type linking), the dt cn e fitted to power-lw distriution, / t, with.7 for the times (red line) nd. for the times (lck line). At V (A-type linking), this description is no longer vlid; however, the dt cn e closely fitted y introducing n exponentil cut-off such tht the distriution is / t exp(t/ t c ), where. nd t c 7. ms for the times (red line) nd.7 nd t c 7. ms for the times (lck line). Fig., we show nnocrystl with B-type linking t. V, which switches to A-type linking t V. Remrkly, wheres the B-type nd times oth follow power-lw distriution over lmost three decdes, the distriutions of nd times in the A-type linking regime re qusi-exponentil with cut-off time of,7 ms (Fig. ). This electrochemiclly controlled switching etween different linking regimes in the sme nnocrystl is nother strong indiction tht the difference etween A-type nd B-type linking is linked to the distinct nture of the underlying physicl mechnisms ut not to dot-to-dot vritions. Furthermore, the fct tht the cut-off time mesured in the cse of A-type linking is close to typicl in size used in the mesurements suggests tht reltively smll chnges in the timescle of chrge fluctutions cn result in switching etween inry linking nd flickering s seen, for exmple, in Fig.. METHODS SUMMARY We used home-uilt electrochemicl cell with three-electrode configurtion. The nnocrystls were directly deposited onto n ITO-coted trnsprent working electrode from very dilute hexne or wter solution. As counterelectrode, we used pltinum guze ttched to pltinum wire. All potentils reported in the min text re mesured reltive to silver wire qusi-reference. The electrochemicl experiments were performed using severl comintions of solvents (cetonitrile nd propylene cronte) nd supporting electrolytes (ll concentrtions,. M): tetrutylmmonium hexfluorophosphte (TBAPF 6 ), tetrutylmmonium perchlorte (TBAClO ) nd lithium perchlorte (LiClO ). We note tht the results presented here re not dependent on the identities of the solvent, supporting electrolyte or surfce lignds used. The nnocrystls were excited y pulsed diode lser t wvelength of nm using low fluences (the verge numer of excitons per nnocrystl per pulse, ÆNæ #.) to void multiexcitonic effects nd to limit photochrging. The photoluminescence ws collected confoclly nd sent to Hnury Brown/Twiss set-up (time resolution, ps) to mesure the second-order intensity correltion function, g. The re of the centrl pek normlized to the re of side pek is mesure of multiphoton emission proility during single excittion cycle. Any g () vlue less thn. implies tht the mesured signl origintes from single quntum emitter ( single nnocrystl). For lifetime nd linking nlyses, we used time-tgged, time-resolved mode, in which we recorded the dely time of ech photoluminescence photon with regrd to the lser pulse. These dt were nlysed with the SYMPHOTIME softwre. All susequent nlysis nd plotting were performed in ORIGIN.. Full Methods nd ny ssocited references re ville in the online version of the pper t Received April; ccepted 9 Septemer.. Hoogenoom, J. P., Hernndo, J., vn Dijk, E. M. H. P., vn Hulst, N. F. & Grcí- Prjó, M. F. Power-lw linking in the fluorescence of single orgnic molecules. ChemPhysChem, (7).. Bout, D. A. V. et l. Discrete intensity jumps nd intrmoleculr electronic energy trnsfer inthe spectroscopy of singleconjugtedpolymer molecules. Science 77, 777 (997).. Riley, E. A., Binghm, C., Bott, E. D., Khr, B. & Reid, P. J. Two mechnisms for fluorescence intermittency of single violmine R molecules. Phys. Chem. Chem. Phys., 797 ().. Frntsuzov, P., Kuno, M., Jnko, B. & Mrcus, R. A. Universl emission intermittency in quntum dots, nnorods nd nnowires. Nture Phys., 9 ().. Nirml, M. et l. Fluorescence intermittency in single cdmium selenide nnocrystls. Nture, (996). 6. Fernndo, D. Stefni, J. P. H. & Brki, E. Beyond quntum jumps: linking nnoscle light emitters. Phys. Tody 6, 9 (9). 7. Efros, A. L. & Rosen, M. Rndom telegrph signl in the photoluminescence intensity of single quntum dot. Phys. Rev. Lett. 7, (997).. Klimov, V. I., Mikhilovsky, A. A., McBrnch, D. W., Letherdle, C. A. & Bwendi, M. G. Quntiztion of multiprticle Auger rtes in semiconductor quntum dots. Science 7, (). 9. Zho, J., Nir, G., Fisher, B. R. & Bwendi, M. G. Chllenge to the chrging model of semiconductor-nnocrystl fluorescence intermittency from off-stte quntum yields nd multiexciton linking. Phys. Rev. Lett., 7 ().. Rosen, S., Schwrtz, O. & Oron, D. Trnsient fluorescence of the off stte in linking CdSe/CdS/ZnS semiconductor nnocrystls is not governed y Auger recomintion. Phys. Rev. Lett., 7 ().. Fisher, B. R., Eisler, H.-J., Stott, N. E. & Bwendi, M. G. Emission intensity dependence nd single-exponentil ehvior in single colloidl quntum dot fluorescence lifetimes. J. Phys. Chem. B, ().. Zhng, K., Chng, H., Fu, A., Alivistos, A. P. & Yng, H. Continuous distriution of emission sttes fromsingle CdSe/ZnS quntum dots.nno Lett. 6, 7 (6). 6 NATURE VOL 79 NOVEMBER Mcmilln Pulishers Limited. All rights reserved

5 LETTER RESEARCH. Grcí-Sntmrí, F. et l. Brekdown of volume scling in Auger recomintion in CdSe/CdS heteronnocrystls: the role of the coreshell interfce. Nno Lett., 6769 ().. Gómez, D. E., vn Emden, J., Mulvney, P., Fernee, M. J. & Ruinsztein-Dunlop, H. Excitontrion trnsitions in single CdSeCdS coreshell nnocrystls. ACS Nno, 7 (9).. Jh, P. P. & Guyot-Sionnest, P. Trion decy in colloidl quntum dots. ACS Nno, (9). 6. Houtepen, A. J. & Vnmekelergh, D. Oritl occuption in electron-chrged CdSe quntum-dot solids. J. Phys. Chem. B 9, 9696 (). 7. Jh, P. P. & Guyot-Sionnest, P. Electrochemicl switching of the photoluminescence of single quntum dots. J. Phys. Chem. C, ().. Chen, Y. et l. Gint multishell CdSe nnocrystl quntum dots with suppressed linking. J. Am. Chem. Soc., 67 (). 9. Wng, X. et l. Non-linking semiconductor nnocrystls. Nture 9, 6669 (9).. Klimov, V. I., McGuire, J. A., Schller, R. D. & Rupsov, V. I. Scling of multiexciton lifetimes in semiconductor nnocrystls. Phys. Rev. B 77, 9 ().. McGuire, J. A. et l. Spectroscopic signtures of photochrging due to hot-crrier trnsfer in solutions of semiconductor nnocrystls under low-intensity ultrviolet excittion. ACS Nno, ().. Tisdle, W. A. et l. Hot-electron trnsfer from semiconductor nnocrystls. Science, 7 ().. Li, S., Steigerwld, M. L. & Brus, L. E. Surfce sttes in the photoioniztion of highqulity CdSe core/shell nnocrystls. ACS Nno, 677 (9).. Jeong, S. et l. Effect of the thiolthiolte equilirium on the photophysicl properties of queous CdSe/ZnS nnocrystl quntum dots. J. Am. Chem. Soc. 7, 67 ().. Hohng, S. & H, T. Ner-complete suppression of quntum dot linking in mient conditions. J. Am. Chem. Soc. 6, (). 6. Fomenko, V. & Nesitt, D. J. Solution control of rditive nd nonrditive lifetimes: novel contriution to quntum dot linking suppression. Nno Lett., 79 (). 7. Jin, S., Song, N. & Lin, T. Suppressed linking dynmics of single QDs on ITO. ACS Nno, ().. Tygi, P. & Kmhmpti, P. Flse multiple exciton recomintion nd multiple exciton genertion signls in semiconductor quntum dots rise from surfce chrge trpping. J. Chem. Phys., 976 (). Supplementry Informtion is linked to the online version of the pper t Acknowledgements C.G. nd V.I.K. cknowledge support of the Center for Advnced Solr Photophysics, n Energy Frontier Reserch Center funded y the US Deprtment of Energy (DOE), Office of Science, Office of Bsic Energy Sciences (BES). Y.G. nd A.S. re supported y Los Almos NtionlLortoryDirectedReserchndDevelopment Fund. M.S., J.A.H. nd H.H. re supported y NIH-NIGMS grnt RGM7. This work ws conducted, in prt, t the Center for Integrted Nnotechnologies, DOE/BES user fcility. Author Contriutions C.G., M.S., J.A.H., V.I.K. nd H.H. conceived the study. C.G., M.S. nd H.H. designed the experiments. C.G. constructed the experimentl set-up nd performed the mesurements under the guidnce of M.S., V.I.K. nd H.H. Y.G. synthesized nd A.S. modified quntumdotmterils under theguidnceof J.A.H. C.G., V.I.K. nd H.H. nlysed nd interpreted the dt, nd wrote the mnuscript with the ssistnce of ll other co-uthors. Author Informtion Reprints nd permissions informtion is ville t The uthors declre no competing finncil interests. Reders re welcome to comment on the online version of this rticle t Correspondence nd requests for mterils should e ddressed to V.I.K. (klimov@lnl.gov), H.H. (htoon@lnl.gov) or M.S. (sykorm@lnl.gov). NOVEMBER VOL 79 NATURE 7 Mcmilln Pulishers Limited. All rights reserved

6 RESEARCH LETTER METHODS Mterils. Cdmium oxide, oleic cid (9%), -octdecene (ODE, 9%), -octdecne (OD, 9%), oleylmine, sulphur, selenium pellet nd trioctylphosphine (TOP) were purchsed from Aldrich nd used without further purifiction. Trioctylphosphine oxide (TOPO, 9%) ws purchsed from Strem nd used without further purifiction. Nnocrystl synthesis. A -ml round-ottomed flsk equipped with reflux condenser nd thermocouple proe ws chrged with g TOPO, ml ODE nd. mmol Cd-olete under stndrd ir-free conditions. The rection system ws evcuted for min t room temperture (uc) nd min t uc, nd then the temperture ws rised to uc under rgon. Following this, mixture of mmol of TOP-Se, ml oleylmine nd ml of ODE ws quickly injected into the rection system. The temperture ws then lowered to 7 uc for CdSe nnocrystl growth. After severl minutes, the solution ws cooled to room temperture nd CdSe nnocrystls (dimeter, d nm) were collected y precipittion with ethnol nd centrifugtion. The CdSe nnocrystls were redispersed in hexne. The synthesis of coreshell CdSe/nCdS nnocrystls followed the successive ionic lyer dsorption nd rection (SILAR) pproch,9, with modifictions. A -ml round-ottomed flsk ws chrged with, 7 mol pre-wshed CdSe cores, ml oleylmine nd ml OD. Here OD ws chosen s the solvent ecuse it llevited the prolem of precipittion oserved during lter stges of thick-shell growth. Elementl sulphur (. M) dissolved in OD nd. M Cdolete in ODE were used s precursors for shell growth. The quntity of precursor used for ech ddition of shell monolyer ws clculted to ccount for the successive increses in prticle volume s function of incresing shell thickness. The rection temperture ws uc nd growth times were h for sulphur nd. h for Cd precursors. Rections were continued until desired shell thickness ws chieved. The coreshell nnocrystls were wshed in similr fshion s the CdSe cores, y precipitting two to three times with ethnol nd redispersing in hexne. Reltive photoluminescence quntum yields were determined y comprison with stndrd dye (rhodmine 6G, 99%; Acros) nd were oserved to vry s function of shell thickness. For the CdSe/9CdS nnocrystls used in the present study, the photoluminescence quntum yield ws,%. The purified coreshell nnocrystls were studied using trnsmission electron microscopy to determine their shpes nd sizes nd to confirm the growth of thick, high-qulity CdS shell over the CdSe core. Lignd exchnge. Coreshell nnocrystls were precipitted with ethnol then centrifuged for pproximtely min (, r.p.m., or,g). The resulting pellet ws redispersed in toluene. This procedure ws repeted twice. The nnocrystl concentrtions were clculted ccording to ref.. An mount of lignd (mercptoundecnoic cid) equivlent to twice the numer of moles of Cdchlcogenide in the smple ws dded to the toluene solution. After h, solution of tetrmethylmmonium hydroxide in wter (four times the numer of moles of Cd-chlcogenide) ws dded dropwise. The nnocrystls were trnsferred from the toluene phse to the wter phse. The wter phse ws seprted from the toluene phse nd precipitted with isopropnol, followed y centrifugtion (, min t, r.p.m., or,g). Finlly, the pellet ws redispersed in distilled wter. Electrochemicl cell. A home-uilt electrochemicl cell with three-electrode configurtion ws used. As working electrode, we used n ITO-coted glss slide with sheet resistnce of, V (SPI Supplies). Before use, the electrode ws sonicted in cetone nd isopropnol ths, rinsed with deionized wter, dried nd plsm-etched for min. The nnocrystls were directly deposited onto the electrode from very dilute hexne or wter solution. We note tht plsm etching significntly improves the ttchment of wter-solule nnocrystls y providing hydrophilic surfce. As counterelectrode, we used pltinum guze ttched to pltinum wire. The high-surfce-re guze ws used to chieve uniform current density cross the working electrode. A silver wire ws used s qusi-reference electrode. This electrode ws clirted using the Ru / redox-couple of [Ru(py) ](PF 6 ) (refs, ). By comprison of the hlf-wve potentils otined with the silver wire, stndrd clomel electrode nd Ag/Ag(NO ) reference electrodes, we found tht the silver qusi-reference is offset from the norml hydrogen electrode y. 6. V. All potentils reported in the min text re mesured reltive to the silver qusi-reference. The electrochemicl experiments were performed using severl comintions of solvents (cetonitrile nd propylene cronte) nd supporting electrolytes (ll concentrtions,. M): tetrutylmmonium hexfluorophosphte (TBAPF 6 ), tetrutylmmonium perchlorte (TBAClO ) nd lithium perchlorte (LiClO ). The results presented here re not dependent on the identities of the solvent, supporting electrolyte or surfce lignds used. Opticl set-up. The excittion source ws PicoQunt pulsed diode lser producing,-ps pulses t nm with repetition rte of. MHz. Most of the experiments were performed t. MHz, which corresponds to pulse-to-pulse seprtion of ns, n order of mgnitude greter thn the longest photoluminescence lifetimes. This llows us to minimize pile-up effects nd prsitic chrge ccumultion due to possile photochrging. The verge nnocrystl excitonic occupncies generted per pulse, ÆNæ, were estimted from sorption crosssections clculted using nnocrystl sizes derived from trnsmission electron microscopy dt nd were independently verified y photoluminescence sturtion nd intensity-dependent g mesurements. Photoluminescence ws excited nd collected through n oil-immersion Olympus ojective with numericl perture of.. After reflection from dichroic mirror (Semrock), photoluminescence then went through long-pss or nd-pss filter (Semrock). A flip mirror ws used to send emission to -mm spectrometer equipped with liquidnitrogen-cooled silicon chrge-coupled device. Emission from the nnocrystls typiclly peked round 6 nm with full-width of, nm t hlf mximum. A Hnury Brown/Twiss set-up ws relized using / em splitter nd two vlnche photodiodes (APDs; SPCM-AQRH-, Perkin Elmer) with quntum efficiency of,% t the photoluminescence wvelength, time jitter of, ps nd drk count rte of, Hz. The single-photon counting device ws PicoHrp stnd-lone module (PicoQunt). Two APDs were used to produce strt nd stop signls in the mesurements of the second-order intensity correltion function, wheres the synchroniztion pulse of the lser provided the strt signl in the time-tgged, time-resolved mode. Photon rrivl times were recorded from one of the APDs (stop signl). Anlysis. For the nlysis of rw time-tgged, time-resolved dt, we used the SYMPHOTIME softwre. All susequent nlysis nd plotting were performed in ORIGIN.. For the dynmicl correlted lifetimeintensity nlysis, we chose in time, corresponding to more thn photons per in on verge, to ensure relile i-exponentil fitting for ech decy curve. We fixed the lifetimes on the sis of the vlues produced y the glol fit procedure nd constrined the mplitudes to e positive numers. To enhnce the precision, we used Poissonin mximum-likelihood estimtor. To confirm the vlidity of the multi-exponentil pproch, we lso constructed FLIDs for which the lifetime for ech in ws clculted s weighted verge of photoluminescence photon rrivl times, tht is, without ny fitting procedure. The resulting FLIDs were similr to those produced y multi-exponentil fit, s illustrted in Supplementry Fig.. 9. Li, J. J. et l. Lrge-scle synthesis of nerly monodisperse CdSe/CdS core/shell nnocrystls using ir-stle regents vi successive ion lyer dsorption nd rection. J. Am. Chem. Soc., 677 ().. Vel, J. et l. Effect of shell thickness nd composition on linking suppression nd linking mechnism in gint CdSe/CdS nnocrystl quntum dots. J. Biophoton., 7677 ().. Yu, W. W., Qu, L., Guo, W. & Peng, X. Experimentl determintion of the extinction coefficient of CdTe, CdSe, nd CdS nnocrystls. Chem. Mter., 6 ().. Juris, A. et l. Ru(II) polypyridine complexes: photophysics, photochemistry, eletrochemistry, nd chemiluminescence. Coord. Chem. Rev., 77 (9).. DeLive, P. J., Foremn, T. K., Ginnotti, C. & Whitten, D. G. Photoinduced electron trnsfer rections of trnsition-metl complexes with mines. Mechnistic studies of lternte pthwys to ck electron trnsfer. J. Am. Chem. Soc., 676 (9).. Prk, Y. S. et l. Ner-unity quntum yields of iexciton emission from CdSe/CdS nnocrystls mesured using single-prticle spectroscopy. Phys. Rev. Lett. 6, 7 (). Mcmilln Pulishers Limited. All rights reserved