thermal activation of non-radiative auger recombination in …cxz124830/clubarticle... · 2013. 5....

Thermal activation of non-radiative Augerrecombination in charged colloidal nanocrystalsC. Javaux1, B. Mahler1, B. Dubertret1*, A. Shabaev2, A. V. Rodina3, Al. L. Efros4*, D. R. Yakovlev3,5,

F. Liu5, M. Bayer5, G. Camps6, L. Biadala6, S. Buil6, X. Quelin6 and J-P. Hermier6*

Applications of semiconductor nanocrystals such as biomarkers and light-emitting optoelectronic devices require that theirfluorescence quantum yield be close to 100%. However, such quantum yields have not been obtained yet, in part, becausenon-radiative Auger recombination in charged nanocrystals could not be suppressed completely. Here, we synthesizecolloidal core/thick-shell CdSe/CdS nanocrystals with 100% quantum yield and completely quenched Auger processes atlow temperatures, although the nanocrystals are negatively photocharged. Single particle and ensemble spectroscopy inthe temperature range 30–300 K shows that the non-radiative Auger recombination is thermally activated around 200 K.Experimental results are well described by a model suggesting a temperature-dependent delocalization of one of the trionelectrons from the CdSe core and enhanced Auger recombination at the abrupt CdS outer surface. These results point to aroute for the design of core/shell structures with 100% quantum yield at room temperature.

Since the landmark syntheses of core1 and core/shell2 nanocrys-tals, the quest for the perfect semiconductor colloidal nano-crystal with 100% quantum yield has been stymied by

efficient non-radiative exciton recombination processes. Theseresult in heat production, reduced quantum yield and a fluorescenceemission intensity that flickers in time3. One of the main sources ofnon-radiative processes in nanocrystals is Auger recombination4–6,whereby a photo-excited electron–hole pair transfers its energynon-radiatively to a third charge. Although the Auger process hasbeen identified as the main reason for the decrease of photolumines-cence (PL) quantum efficiency in light-emitting diodes7 and for theincrease in the stimulated emission threshold in lasers8, it is not theonly reason for PL degradation and blinking. Indeed, evidence forother mechanisms of PL quenching has been found in nanocrys-tals9,10, connected with the presence of defects at or near thesurface of the nanoparticles11, and strongly depending on thesurface state and composition of the nanocrystals12. This mixtureof several non-radiative recombination processes, and their depen-dence on the type of material under study, has led to a large bodyof experimental work and various theoretical interpretations13,especially in the field of single-particle blinking studies14. As a con-sequence, a clear direction for the design of colloidal nanocrystalswith complete suppression of non-radiative processes and 100%quantum yield at room temperature has not yet emerged, in spiteof recent interesting advances in the synthesis of non-blinkingnanocrystals either with CdZnSe15 alloyed nanocrystals or thick-shell CdSe/CdS nanocrystals16,17.

In this Article, we investigate the PL properties of core/thick-shell CdSe/CdS nanocrystals in the 30–300 K temperature range,and discover a thermal activation of non-radiative Auger recombi-nation. This activation of the Auger process was observed for allnanocrystals analysed in this study: �150 nanocrystals from fourbatches (Fig. 1a; Supplementary Figs S1 and S2) with two CdSecore radii (1.5 nm and 2.5 nm) and two CdS shell thicknesses

(6 nm and 10 nm). In agreement with previous studies18,19, ournanocrystals oscillate between a photocharged state and a neutralstate at 300 K. As the temperature decreases from 300 K to 30 K,however, the PL intensity and lifetime analysis of single nanocrystalsreveals several interesting and unique features. First, below 200 K,the nanocrystals become permanently negatively charged. Thesign of the charge was unambiguously obtained using polariz-ation-resolved PL measurements in high magnetic fields. Second,the nanocrystal quantum yield increases continuously with decreas-ing temperature, reaching 100% quantum yield at 30 K, while the PLradiative lifetime decreases steadily to values lower than 8 ns. Todescribe these data, we performed a theoretical analysis of theenergy states of the charge excitons in the negatively chargedCdSe/CdS nanocrystals that takes into account the decrease of theconduction band offset at the CdSe/CdS interface when the temp-erature increases20. We established that the activation of Augerrecombination for the negatively charged trion results from thethermal delocalization of one of the electrons from the CdSe coreinto the CdS shell. This electron starts to interact with the abruptnanocrystal outer surface, which stimulates non-radiativeAuger recombination.

The core/shell nanocrystals we used in this study are referred tohereafter as ‘C nm/S nm nanocrystals’, where C and S are the CdSecore radius and the CdS shell thickness, respectively. We firststudied the PL properties of 2.5 nm/6 nm nanocrystals using a con-tinuous-flow helium cryostat incorporated into a confocal micro-scope with a Hanbury Brown and Twiss setup (see Methods). Werecorded the fluorescence emission intensity of single nanocrystalsunder three experimental conditions: in air at 300 K (Fig. 1b), undervacuum at 300 K (Fig. 1c) and under vacuum at 30 K (Fig. 1d).

In Fig. 1b, left panel, we show the PL intensity of an individualCdSe/CdS nanocrystal measured at 300 K, in air. The PL intensityjumps between two values, leading to a double-peak intensity histo-gram (Fig. 1b, middle panel). The high intensity state corresponds

1Laboratoire de Physique et d’Etude des Materiaux, CNRS, ESPCI, 10 rue Vauquelin 75231 Paris, France, 2George Mason University, Fairfax, Virginia 22030,USA, 3Ioffe Physical-Technical Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia, 4Naval Research Laboratory, Washington DC,Washington 20375, USA, 5Experimentelle Physik 2, TU Dortmund University, 44227 Dortmund, Germany, 6Groupe d’Etude de la Matiere Condensee,Universite de Versailles-Saint-Quentin-en-Yvelines, CNRS UMR8635, 45 avenue des Etats-Unis, 78035 Versailles, France. *e-mail: [email protected];[email protected]; [email protected]

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to excitonic recombination in the neutral state (X) of the nanocrys-tal, while the low intensity state is associated with recombination inthe charged state (X*) of the nanocrystal18,19, as schematically shownin Fig. 1b, right panel. When the nanocrystal is in the neutral state

(X), we measure a PL quantum yield close to 100%, using a methodsimilar to the one reported in refs 18 and 21. We then deduce a valueof 38% for the quantum yield of the nanocrystal in the charged state(X*). Further evidence of the two emitting states comes from

C = 2.5 nmS = 6 nm

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Figure 1 | TEM images of nanocrystals and single-nanocrystal fluorescent traces. a, TEM images of CdSe/CdS core/thick-shell nanocrystals with 1.5 nm or

2.5 nm core radius (denoted C) and 6 nm or 10 nm shell thickness (denoted S). b–d, Time traces of a single 2.5 nm/6 nm nanocrystal. Left panels:

fluorescence intensity trace of the nanocrystal (black trace) at 300 K in air (b), at 300 K in vacuum (c) and at 30 K in vacuum (d), with a 10 ms binning

time. The grey trace is the background noise intensity. Middle panels: intensity histograms (black crosses). The red line is a fit with Poisson distributions with

mean values of 44.5 and 22 (b), 23.7 (c) and 42 (d). The grey open circles are histograms of the noise intensity, fitted with a Poisson distribution (in blue)

with mean values of 8 (b), 9.3 (c) and 7.3 (d). To correct for the change of focus and/or fluctuations of the laser diode, the corrected fluorescence intensity

I of a given nanocrystal is defined as I¼ (INC 2 IBGD)/IBGD, where INC is the nanocrystal PL intensity and IBGD is the background fluorescence intensity of a

spot close to the nanocrystal but far enough away that the recorded signal is due solely to the background fluorescence. The corrected intensity mean values

are 4.56 and 1.75 (b), 1.55 (c) and 4.75 (d). Right panels: schematic representations of the nanocrystal charging state. In b, the nanocrystal oscillates

between the neutral and charged states. In c and d, it is permanently charged and its lifetime (t) and quantum yield (QY) depend on temperature. e,

Fluorescence intensity trace (black) of a single 1.5 nm/6 nm nanocrystal at 30 K in vacuum. The noise is in grey. f, Temperature dependence of the

2.5 nm/10 nm nanocrystal quantum yield under low, continuous excitation intensity. Data are averaged over 120 single nanocrystals.

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lifetime analysis of the intensity time trace. The decay curve is wellfitted by a bi-exponential decay with lifetimes of 60 ns and 11 ns(Supplementary Fig. S4a). These lifetimes correspond to theneutral (X) and charged (X*) states, respectively18. The (X) lifetimeis long compared to regular CdSe/CdS nanocrystals and points to astrong delocalization of the electron in the thick CdS shell, and thusto a quasi type II core/shell structure22.

In Fig. 1c, left panel, we show the PL intensity of the same nano-crystal recorded at 300 K, but under vacuum. The PL intensity isstable over the entire experiment, and is well fitted by a singlePoisson distribution (Fig. 1c, middle panel) with a mean valuesimilar to the intensity of the charged state (X*) in air. The intensi-ties are compared after correction for possible changes in focusand/or laser diode fluctuations (see Methods). We thus concludethat, under vacuum, the nanocrystal is mostly in a charged state(X*) (Fig. 1c, right panel). Additional support for this conclusioncomes from the PL lifetime of this nanocrystal—that is, 11 ns(Supplementary Fig. 4b)—a value similar to the lifetime of the (X*)state in air. These measurements confirm the role of water and/orair in the emission intermittency of individual nanocrystals19,23.

In Fig. 1d, left panel, the same 2.5 nm/6 nm nanocrystal isobserved at 30 K. The PL intensity is steady, and the intensity histo-gram is a perfect Poisson distribution (Fig. 1d, middle panel andSupplementary Fig. S5). We measured an emission intensitysimilar to the bright state in air. The nanocrystal quantum yield isthus close to 100% at 30 K with a perfectly steady PL intensity.For the PL lifetime, we measured a value close to 8 ns(Supplementary Fig. S4c). Rapid fluorescence decay times(,10 ns) at low temperatures (,30 K) are very unusual in colloidalnanocrystals. Previous studies both on CdSe/CdS core/thick-shellnanocrystals24 and on large (5 nm diameter) CdSe nanocrystals25

have reported PL lifetimes ten times longer at similar temperatures.These long lifetimes are connected, respectively, with the thick CdSshell and with the presence of a low-energy dark state in the finestructure of the band-edge exciton26. In our experiments, the 8 nslifetime observed at 30 K demonstrates the absence of a low-energy dark state, which can be explained only if our nanocrystalsare charged27. When photoexcited, a charged exciton, commonlytermed a trion, forms in these nanocrystals. It has two electronsin the case of negatively charged nanocrystals, or two holes in thecase of positively charged nanocrystals, with opposite spins occupy-ing the same ground quantum size level (singlet trion state) at cryo-genic temperatures. The trion does not have a low-energy spin-forbidden dark state, and at low temperatures its fluorescence life-time is orders of magnitude faster than that of a neutral exciton.The observation of PL with 100% quantum yield in the chargednanocrystals leads us to the striking conclusion that the non-radia-tive Auger recombination is completely suppressed at cryogenictemperatures, in spite of the presence of a charge. Although wepresent only the trace of one nanocrystal in Fig. 1d, an identicaltrend is observed with all the nanocrystals we have studied, regard-less of their core size and shell thickness (Fig. 1e). Retrospectively,this study validates the hypothesis we have asserted in earlierwork, where we suggested that at 300 K in air, single nanocrystalsoscillate between a charged state and a neutral state18.

For better statistics regarding the evolution of CdSe/CdSquantum yield with temperature, we analysed the emission intensityof 120 individual nanocrystals simultaneously excited continuouslywith a mercury lamp at temperatures ranging from 30 K to 300 Kunder vacuum (Fig. 1f). Although at room temperature the emissionintensity varies from one nanocrystal to another due to their slightdispersion in size and shape or in ligand coverage on the surface,they all exhibit the same behaviour with respect to temperature.We observe that the averaged quantum yield of the charged nano-crystal decreases slowly from 100% at 30 K to 80% at 200 K andthen starts to decrease more rapidly, reaching 40% at 300 K. This

value is in good agreement with the single-nanocrystal measure-ments (Fig. 1c). The evolution of quantum yield from 100% to40% suggests thermal activation of Auger recombination.

To better understand how the behaviour of our nanocrystalsdepends on temperature, we recorded the PL lifetime of single2.5 nm/10 nm nanocrystals for temperatures ranging from 30 Kto 300 K in steps of �25 K (Fig. 2a). We chose to focus on the PLlifetime instead of PL intensity, because two emissive states withsimilar PL intensities can be distinguished easily using a fluor-escence decay analysis if the decays are different, as in our case(Supplementary Figs S6 and S7). Such a technique has recentlybeen used to study the oscillation between charging and dischar-ging18 or the lifetime blinking in CdSe/CdS quantum dots at300 K (ref. 28). In Fig. 2a, we present the typical fluorescencedecay of a single 2.5 nm/10 nm nanocrystal measured at varioustemperatures under vacuum. The fluorescence decay time shortenscontinuously when the temperature decreases from 300 K to 30 K.Each decay curve recorded in this temperature range can be fittedby a sum of two or three exponentials that can be attributed tothe exciton (X), the trion (X*) and the neutral or charged biexciton(XX/XX*) states. At 300 K under vacuum, all three states contributeto fluorescence decay (Fig. 2b), but the probability of the nanocrystalbeing neutral is small (Fig. 2b, inset). The absence of a measurablecontribution of the neutral exciton (X) to the PL below 200 K indi-cates that the nanocrystal becomes permanently charged at thistemperature. For T , 200 K, the PL decay of the nanocrystal is bi-exponential, with a contribution from the trion (X*) and from thecharged biexciton (XX*). The trion lifetime decreases by a factor 5(from 35 ns down to 7 ns) when the temperature decreases from300 K to 30 K (Fig. 2b).

To understand the unusual properties of these thick-shell nano-crystals, we investigated whether they were charged with anadditional electron or hole. To this end, we measured the polariz-ation-resolved PL spectra of 2.5 nm/10 nm nanocrystals at lowtemperatures in magnetic fields (Fig. 2c,d). In these experiments,the sign of the circular polarization degree, defined as P¼[I(sþ) 2 I(s2)]/[I(sþ)þ I(s2)], is unambiguously related to thesign of the resident charge (Supplementary Fig. S8 and ref. 29). Anegative P, observed for the nanocrystals we studied, implies thatthe resident carrier is an electron and consequently that the nano-crystal is negatively charged. The corresponding scheme for thespin structure and the optical transitions is shown in the inset ofFig. 2d. In contrast, if P were positive it would correspond to a posi-tively charged nanocrystal with a resident hole in the CdSe core.

The measurements reported in Fig. 2a,b point to two interestingfeatures. First, at high temperatures the very thick-shell CdSe/CdSnanocrystals oscillate between a negatively charged state and aneutral state, even if these oscillations are not visible on the PLintensity trace. They become permanently negatively charged forT , 200 K. Second, the negative trion lifetime decreases steadilywith decreasing temperature, despite the fact that the nanocrystalquantum yield increases (Fig. 1f ).

We now turn to a model to explain the experimental datareported in Figs 1 and 2. We need to explain two seemingly contra-dictory observations: first, the shortening of the negative trion PLlifetime when the temperature decreases; second, the increase ofthe PL quantum yield from 40% to 100% over the same temperaturerange. We will show that the PL lifetime shortening results from thetemperature dependence of the conduction band offset (CBO)between the CdSe core and the CdS shell. As for the quantumyield change with temperature, this is due to activation of Augernon-radiative recombination when one of the trion electrons is ther-mally delocalized from the core into the shell and reaches the abruptCdS outer surface. All the calculations for the nanocrystal energystructure presented below were conducted as reported in ref. 30,supposing an infinitely thick CdS shell.

ARTICLES NATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2012.260





The conduction and valence band offsets between CdSe and CdSand their temperature dependence are not precisely known.However, the valence band offsets reported in the literature arealways positive, and in general large, at least 400 meV (refs 31–33),so the hole is always confined in the CdSe core regardless of temp-erature. For the CBO, the situation is not as clear. CBO valuesranging from 2300 meV (refs 31,32) to �0 meV (refs 33,34) havebeen reported, and this value depends on crystal structure33, nano-particle size35, lattice strain36 and temperature20. At 300 K, in ourCdSe/CdS nanocrystals, we observed an increase of the exciton flu-orescence lifetime with the thickness of the shell16, in agreementwith a quasi type II behaviour. We thus expect that in our systemthe CBO is close to zero at 300 K.

The strong localization of a hole in the CdSe core has importantconsequences. The hole rapidly moving in the CdSe core30 averagesout the Coulomb interaction between the hole and the electrons andcreates an additional steady-state confined potential acting on theelectrons37. This smooth adiabatic potential adds up to the sharpconfinement potential that the electrons feel in the CdSe core(Fig. 3a). Furthermore, the long-range Coulomb potential attractsthe electrons to the CdSe core and prevents them from reachingthe CdS outer surface. Even when the CBO vanishes, an electronremains bound to the hole with an energy of 55 meV (rightscheme in Fig. 3a). The binding energy of the second electron to

the exciton, called the trion binding energy (and denoted 1tr), isdefined as the difference between the ground states of the excitonand of the trion (Fig. 3b, inset). At small CBO, 1tr is very small,but it increases significantly when the CBO increases, creatingtype I CdSe/CdS nanocrystals (negative CBO).

At low temperatures, when the CdSe/CdS nanocrystals are well-defined type I structures, the strong confinement allows the two elec-trons to occupy a ground-state level with two opposite spins, leadingto a large trion binding energy (Fig. 3a, left). As the temperatureincreases, the confinement connected with the CBO decreases andthe trion binding energy is mainly controlled by the electron–electronand electron–hole Coulomb interactions. The Coulomb repulsionbetween two electrons forces them to occupy different orbits, whichreduces the repulsion and leads to a small, but positive bindingenergy for the trion. The orbits are characterized by the two radialwavefunctions ce1(r) and ce2(r), shown in the inset of Fig. 3c for aCBO of 2120 meV. One orbit is mainly localized in the CdSe core,while the other explores the CdS shell, but with a probability closeto zero to reach the outer CdS surface for a shell thickness of10 nm. This demonstrates the importance of the thick CdS shell forthe suppression of surface effects regarding the intrinsic propertiesof our CdSe/CdS nanocrystals.

The binding energy of the trion decreases with CBO, as one cansee in the inset of Fig. 3b, and finally becomes smaller than the

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Figure 2 | Single nanocrystal lifetime evolution versus temperature, and charge determination. a, Time-resolved PL of a single CdSe/CdS 2.5 nm/10 nm

nanocrystal for temperatures ranging from 30 K to 300 K under vacuum. b, Lifetimes extracted with a tri- or bi-exponential fit of the decay curves of a for

each temperature. X, X* and XX*/XX refer to the exciton, the charged exciton and the charged or neutral biexciton, respectively. Error bars are computed as

described in the Supplementary Information. Inset: temperature evolution of the weight of each lifetime extracted from the fit of the decay curve. c,

Polarization-resolved PL spectra of an ensemble of 2.5 nm/10 nm CdSe/CdS nanocrystals at magnetic field of B¼ 8 T and

T¼ 4.2 K. The red and blue lines are sþ and s2 polarized PL, respectively. d, Magnetic field dependence of circular polarization degree of 2.5 nm/10 nm

CdSe/CdS nanocrystals measured under continuous-wave excitation condition at T¼ 4.2 K. lexc¼ 372 nm. Inset: schematic representation of the emitted

polarization in the case of a negative trion with heavy hole.






thermal energy. At this point, the second electron reaches the firstconfined level of the CdS (E1S-CdS), explores the CdS shell, andbegins to be able to touch the outer CdS surface (Fig. 3a, right).The total activation energy necessary for the delocalization of oneof the electrons is 1trþ E1S-CdS. The first confined level of the elec-trons in CdS (E1S-CdS) in our 2.5 nm/10 nm CdSe/CdS nanocrystalswas evaluated to be 10 meV. However, the activation energy variesfrom nanocrystal to nanocrystal due to shell size and shape disper-sions, which affect the energy of the 1S-CdS electron level.

The radiative decay rate for excitons and trions is proportional tothe square of the electron–hole wavefunction overlap38. The calcu-lation of this wavefunction overlap of the negatively charged trionis detailed in Supplementary Section S4, and the results of such cal-culations are shown in Fig. 3c as a function of the CdSe/CdS CBO.The square of the overlap integral increases when the CBOincreases, creating a type I structure, which stimulates electron local-ization in the CdSe core, in agreement with our time decay measure-ments. We also established that the overlap integral decreases withthe core radius (Supplementary Fig. S9), which is consistent withour measurements of longer decay times in nanocrystals with acore radius of 1.5 nm (Supplementary Fig. S10).

These calculations of the overlap integral for the negativelycharged trion explain the shortening of the radiative decay timewith lowering temperature (Supplementary Fig. S11). They do notexplain, however, the temperature evolution of the PL quantumyield (Fig. 1f ). When the temperature rises from 30 K to 150 K,the trion quantum yield decreases from 100% to 80%. This onsetof Auger recombination, the only source of non-radiative recombi-nation in these nanocrystals, can be explained by the thermal delo-calization of one of the trion electrons from the CdSe core to theCdS shell. This electron may then interact with the sharp potentialof the outer surface, which is a source of enhanced Auger recombi-nation39. For large enough CBO (,2120 meV), the spatial exten-sion of the wavefunction of the external electron is significantlysmaller than the 10 nm shell thickness (Fig. 3c, inset), and noneof the two electrons can reach the outer CdS surface. At the sametime, a CBO in the range 120–250 meV is not deep enough topermit Auger recombinations of the trion, because the trion wave-function in such a shallow potential does not have the large enoughFourier components necessary for Auger process acceleration39.

The analysis of the temperature dependence of the radiativeand non-radiative decay time led us to estimate that, at 200 K, the

Auger suppressed Auger allowed

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Figure 3 | Model of temperature-dependent electron localization. a, Schematic representation of the band alignment for the negatively charged trion in a

CdSe/CdS nanocrystal. Auger processes are suppressed when the trion is in its ground state in a type I structure (left), but they are allowed when one of the

electrons is delocalized in the shell and interacts with the surface in a quasi type II structure (right). b, Ground-state energy of the exciton (dashed line) and

of the trion (solid line) versus the conduction band offset (CBO) between CdSe and CdS, for core radius 2.5 nm. Inset: binding energy of the trion versus the

CBO. c, Trion overlap squared, K, versus CBO in a CdSe/CdS nanocrystal with core radius of 2.5 nm. Inset: wavefunctions of the two electrons of the negative

trion for a core radius of 2.5 nm and CBO of –120 meV. The radial dimension is normalized with respect to the core radius a.






CBO between the CdSe core and the CdS shell is 250 meV(Supplementary Fig. S12). This suggests that, at lower temperatures,the trion binding energy is large enough to keep the two electronsin the core.

In summary, we have studied negatively charged colloidal thick-shell CdSe/CdS nanocrystals with, for the first time, a 100%quantum yield at cryogenic temperatures and short fluorescencelifetimes. These colloidal nanocrystals are thus approaching thecharacteristics of some epitaxially grown nanocrystals. We demon-strated a thermo-activation of non-radiative Auger recombinationin these CdSe/CdS core/thick-shell nanocrystals that seems to beconnected with the temperature dependence of the electron localiz-ation around the CdSe core. When delocalized, an electron reachesthe abrupt CdS shell surface, which accelerates the rate of non-radiative Auger decay. Within this interpretation of our results, wepredict that the Auger activation energy of the negativelycharged trion could be increased by increasing the CBO,resulting in nanocrystals with 100% PL quantum yield at 300 K.This could be achieved using a higher-bandgap thick shellwith a graded composition at the core/shell interface. Anotherroute to obtain 100% quantum yield nanocrystals at roomtemperature would be to end the CdS shell with a gradedcomposition so that the abrupt outer interface becomes smoothand eliminates Auger recombinations at this interface. Althoughthese different possibilities are theoretically attractive, they need tobe implemented practically and optimized. This may take sometime, and some synthetic effort, but there are some clear directionsfor exploration.

MethodsWe synthesized CdSe/CdS core/thick-shell nanocrystals using a method similar tothat used in ref. 16. The nanocrystals’ emission spectra are shown in SupplementaryFig. S1. Their absorption cross-section is �9 × 10214 cm2 at 405 nm for thenanocrystals with a shell thickness of 6 nm, and �3.6 × 10213 cm2 at 405 nm for thenanocrystals with 10 nm shell thickness. Transmission electron microscopy (TEM)images were acquired on a JEOL 2010 field electron gun microscope operated at200 keV. Supplementary Fig. S2 shows TEM images for 1.5 nm/10 nm nanocrystals.We used zinc-blende CdSe cores, and obtained wurtzite core/shell nanocrystals(Supplementary Fig. S3) by using primary amines for CdS shell growth, as describedin ref. 40. For single nanocrystal measurements, the nanocrystals were diluted in amixture of hexane/octane (9:1) and dropcast on a sapphire substrate. The samplewas mounted in a continuous-flow helium cryostat (Microstat-HiResII, OxfordInstruments) and observed with an inverted microscope (IX 71, Olympus). Singlenanocrystals were excited with a pulsed laser diode emitting at 405 nm (LDH-D-C-405, Picoquant; pulse duration, 70 ps; tunable frequency from 1 to 20 MHz) suchthat the average number of excitons created per pulse in a nanocrystal was �0.1 forthe 2.5 nm/6 nm nanocrystals and �1 for the 2.5 nm/10 nm nanocrystals. Thenanocrystal fluorescence was collected with a ×60, 0.7 NA objective and sent into aconfocal microscope (Microtime 200, Picoquant) with a Hanbury Brown and Twisssetup (SPAD PDM; time resolution, 50 ps). The signal was recorded using aHydraHarp 400 module (Picoquant) in a time-tagged, time-resolved mode. Thehistogram of photon arrival times (called PL decay curves) was built with aresolution of 128 ps. The noise intensity (open circles in the middle panels ofFig. 1b–d) varies linearly with the out-of-focus distance when almost perfectly infocus, and can be used as a proportionality factor to correct for slight changes offocus and/or intensity excitation. For the statistical intensity measurements, thenanocrystals were excited with a mercury lamp, combined with a 550 nm short-passfilter, at 10 mW cm22. The average delay time between two consecutive excitoncreations was �100 ms. The nanocrystal emission was collected through a 590 nmlong-pass filter on an electron multiplying CCD (EMCCD) camera (Cascade 512B,Roper Scientific) at 1 Hz for 3 min. The data were analysed with homemadesoftware. For ensemble measurements, the samples were prepared by dropcastingthe core/shell CdSe/CdS nanocrystal solution on a quartz plate. The sample wasmounted on a fibre-coupled holder within a helium-gas-filled tube inserted in acryostat equipped with a 15 T superconducting magnet. The sample was excitedthrough a multimode optical fibre by a continuous-wave diode laser (lexc¼ 372 nm).The PL, collected through another multimode optical fibre, was sent to a 0.55 mspectrometer and recorded by a liquid-nitrogen-cooled CCD. A circular polarizerinserted between the sample and the detection fibre allowed the measurement of sþ

and s2 polarized PL.

Received 30 July 2012; accepted 17 December 2012;published online 10 February 2013

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AcknowledgementsAl.L.E. acknowledges financial support from the Office of Naval Research and Alexander-von-Humboldt Foundation. A.S. acknowledges support from the Center for Advanced

Solar Photophysics (CASP), an Energy Frontier Research Center founded by the Office ofBasic Energy Sciences (OBES), Office of Science (OS), US Department of Energy (USDOE). C.J., B.D. and J-P.H. acknowledge support from the Agence Nationale de laRecherche, and the Region Ile-de-France. B.D. acknowledges support from the ESPCI andJ-P.H acknowledges support from the Institut Universitaire de France. F.L. and D.R.Y.acknowledge support from the EU Seventh Framework Programme (grant no. 237252,Spin-optronics). The authors thank T. Pons, N. Lequeux, E. Cassette, M. Tessier,I. Maksimovic, N. Bergeal and R. Lobo for stimulating discussions and advice. The authorsare also grateful to Xiangzhen Xu for expert help with TEM measurements.

Author contributionsC.J. and B.M. synthesized the nanocrystals. C.J. and G.C. performed the single particlemeasurements under the guidance of B.D., S.B., X.Q. and J-P.H. L.B and F.L. performed themeasurements in magnetic fields under the guidance of D.Y. and M.B. Al.L.E., A.R. and A.S.developed the theoretical model. C.J., B.D., J-P.H., L.B., D.Y. and Al.L.E. analysed andinterpreted the data. C.J., B.D. and Al.L.E. wrote the manuscript with the assistance of allother co-authors.

Additional informationSupplementary information is available in the online version of the paper. Reprints andpermission information is available online at http://www.nature.com/reprints. Correspondenceand requests for materials should be addressed to B.D., Al.L.E. and J.P.H.

Competing financial interestsThe authors declare no competing financial interests.





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