REPORT◥

QUANTUM DOTS

Redefining near-unity luminescencein quantum dots with photothermalthreshold quantum yieldDavid A. Hanifi1*, Noah D. Bronstein2*, Brent A. Koscher2,3*, Zach Nett2,Joseph K. Swabeck2,3, Kaori Takano4, Adam M. Schwartzberg5, Lorenzo Maserati5†,Koen Vandewal6, Yoeri van de Burgt7, Alberto Salleo1‡, A. Paul Alivisatos2,3,8‡

A variety of optical applications rely on the absorption and reemission of light.The quantumyield of this process often plays an essential role.When the quantum yield deviates fromunity by significantly less than 1%, applications such as luminescent concentrators and opticalrefrigerators become possible.To evaluate such high performance, we develop a measurementtechnique for luminescence efficiency with sufficient accuracy below one part per thousand.Photothermal threshold quantum yield is based on the quantization of light to minimizeoverall measurement uncertainty.This technique is used to guide a procedure capable of makingensembles of near-unity emitting cadmium selenide/cadmium sulfide (CdSe/CdS) core-shellquantum dots.We obtain a photothermal threshold quantum yield luminescence efficiency of99.6 ± 0.2%, indicating nearly complete suppression of nonradiative decay channels.

Photoluminescence (PL), the absorption andreemission of light, is an essential featureof numerous dyes and semiconductingmaterials. Recent commercial applica-tions include solid-state lighting (1), high-

efficiency color displays, and bioimaging (2).Moreover, there are ongoing efforts to com-mercialize luminescent solar concentrators (LSCs)(3) and spectrum-shifting greenhouses (4). Eachof these applications has specific performancerequirements, which are assessed by a standardset of measurement techniques. An essential per-formancemetric is the photoluminescence quan-tum yield (PLQY), which is determined by thecompetition between radiative relaxation of theexcited material and nonradiative losses, typi-cally due to defects. The PLQY is directly re-

lated to the electronic and structural quality ofmaterials, with the highest values recorded todate of 99.5 and 99.7%, respectively, in rare earth–doped high-bandgap single crystals (5) andepitaxially deposited thin films (6). However,in many commercial applications relying onPL, nanocrystals have important potential ad-vantages over their molecular or bulk/thin-filmcounterparts, which include stability; low cost;ability to be selectively placed inside a variety ofcomposites, fluids, polymers, and biological en-vironments; large-area processing compatibil-ity; and absorption and emission tunability. ThePLQY for CdSe/CdS, the prototypical core-shellquantum dot in research labs, routinely exceeds95% (7). But this is insufficient for applicationsthat require an absoluteminimum amount of pho-ton energy to be lost as heat. For example, the lightconcentration of LSCs increases logarithmicallyas the nonradiative losses of the luminophorevanish (8). Optical refrigeration (9), thermophoto-voltaic engines (10), and thermal energy storagein optical cavities (11, 12) all require materialswith PLQY ≥ 99% with negligible nonradiativelosses (13–15).Inspired by the photon recycling needs of

LSCs (13), we aim in this study to reduce the re-absorption losses by keeping the single-pass PLQYas high as possible while retaining a narrowemission linewidth. We achieve this throughthe growth of a 4- to 11-monolayer CdS shellaround a highly monodisperse CdSe core. Oursynthesis technique is inspired by the work ofChen et al. (7), with some noteworthy modifica-tions (supplementary text SM1 and SM2). Themost notable change is the complete lack ofoleylamine, which Chen et al. use to keep a

narrow size distribution. We omit amines, asthey can aid in the desorption of the Z-typeCd(oleate)2 surface ligand (16). This synthesisstrategy helps maintain a high ligand surfacecoverage, reducing possible surface traps whilepreserving high radiative efficiency. The result-ing PLQY, as measured in an integrating sphere,is plotted in Fig. 1A as a function of CdS shellthickness for different batches of quantum dotsgrown with similar-sized cores (~3.3 to 3.6 nm).For a particle with a 2-nm-thick shell, the PLQYconsistently exceeds 90 ± 2%, and after 4 nm ofshell is grown, the PLQY tends to exceed 97 ± 3%.Figure 1B shows standard characterization ofrepresentative samples, including the absorbancespectra, the PL spectra, the photoluminescenceexcitation (PLE) spectra scaled to core absorbance,and transmission electron micrographs. The PLspectrum has a full width at half maximum of28 to 32 nm for all shown batches of particles.Critical for LSCs, the Stokes ratio increases asthe shell thickness increases (17), reaching valuesof ~100. The wavelength-dependent PLQYs ofthese samples were measured in an integratingsphere (Fig. 1C) and are indistinguishable withinerror regardless of excitation wavelength [100 ±3% and 97 ± 3% for 8 and 10 monolayers (ML),respectively].Once the PLQY is within a couple of percent of

unity, existing experimental measurements donot carry the accuracy necessary to provide feed-back toward the materials’ quality to further im-prove synthetic design. The most common andwell-characterized PLQY measurement tech-niques, the relative dye method and the inte-grating sphere technique, are both radiometrytechniques at their core. Both rely on spectralsensitivity calibrations with a spectral radiancetransfer standard, introducing at least 2 to 5%uncertainty into the measurement through theuncertainty of the spectral radiance transferstandard itself (17, 18).In response to these optical sensitivity lim-

itations, we developed a measurement of thePLQY that does not rely on photon flux mea-surement but rather uses the quantization oflight in a process analogous to the photoelectriceffect. Instead of measuring the excitation en-ergy threshold for electron emission, this tech-nique measures the excitation energy thresholdthat results in net positive heat generation.Therefore, we call our technique photothermalthreshold quantum yield (PTQY).In a typical PL event (Fig. 2A), an absorbed

photon excites an electron from a ground stateinto a higher-energy excited state, leaving behinda hole; both charge carriers quickly (approximatelypicoseconds) relax to the band edges by emittingthermal phonons into the host crystal. At a latertime (approximately nanoseconds), the thermal-ized electron and hole recombine, bringing thematerial back to its ground state. This final tran-sition can either be radiative ormediated by defectsby emitting more heat, i.e., a nonradiative loss.Therefore, the average amount of heat emittedper absorption event is a combination of theintrinsic band-edge thermalization process (often

RESEARCH

Hanifi et al., Science 363, 1199–1202 (2019) 15 March 2019 1 of 4

1Department of Materials Science and Engineering, StanfordUniversity, Stanford, CA 94305, USA. 2Materials SciencesDivision, Lawrence Berkeley National Laboratory, Berkeley,CA 94720, USA. 3Department of Chemistry and Departmentof Materials Science and Engineering, University of California,Berkeley, CA 94720, USA. 4Materials R&D Group, HPMResearch and Development Department, High PerformanceMaterials Company, JXTG Nippon Oil & Energy Corporation,8 Chidori-Cho, Naka-ku, Yokohama, Kanagawa, 231-0815,Japan. 5The Molecular Foundry, Lawrence Berkeley NationalLaboratory, Berkeley, CA 94720, USA. 6IMO-IMOMECand Institute for Materials Research, Hasselt University,Wetenschapspark 1, 3590 Diepenbeek, Belgium.7Microsystems, Institute for Complex Molecular Systems,Eindhoven University of Technology, 5612AJ Eindhoven,Netherlands. 8Kavli Energy NanoScience Institute, Berkeley,CA 94720, USA.*These authors contributed equally to this work. †Present address:Center for Nano Science and Technology@PoliMi, Istituto Italianodi Tecnologia, Via Giovanni Pascoli 70/3, 20133, Milano, Italy.‡Corresponding author: asalleo@stanford.edu (A.S.);paul.alivisatos@berkeley.edu (A.P.A.)

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called “blue loss” in the photovoltaic literature)and nonradiative band-to-band relaxation (“non-radiative loss”). Because the blue loss representsheat generated by carrier thermalization, an ex-citation energy–dependent measurement of theheat per luminescence event allows us to deter-mine the fraction of emitted heat that originatesfromnonradiative band-to-band relaxation. Deter-mination of the photon energy at which the netemitted heat vanishes (threshold energy, E0)allows us to precisely determine the PLQY (Fig.2B and SM6).We implemented the PTQY technique (Fig.

2C and SM5) using a transverse photothermaldeflection spectrometer (19) to measure the non-radiative heat and a PL spectrometer to mea-sure the above-gap PL excitation spectrum. Bymeasuring the ratio of nonradiative to radiativeemission components, this technique enablesremoval of the spectral radiance dependencein the PLQY measurement (figs. S10 to S16). Byextrapolating a thermal limit, the experimental

technique relies solely on the measurement ofphoton energies [i.e., excitation energy (Eexc)and photoluminescence energy (EPL)], valuesthat can be determined considerably more ac-curately than quantifying spectral radiance. Aspectral radiance transfer standard is requiredonly to determine the correct shape of the PLspectrum, bringing an uncertainty limit of 6 partspermillion (Fig. 3D). Our apparatus has an experi-mental uncertainty of 0.02%, limited by the de-tection spectrometer wavelength accuracy, whichwas used both to characterize the excitation lampenergy and average emission energies. Typicalmeasurements are further limited by finite aver-aging, commonly yielding uncertainties of 0.1to 0.2% (Fig. 3D), whereas the best uncertaintywe achieved was 0.04%. Our method has a100× improvement in uncertainty comparedwith 4% for relative and integrating spheremeasurements (18). Measurement of heat gen-eration can be performed very sensitively; evenoptically heterogeneous thermally thin samples

(<50 mm) present little complication (see SM3and SM7 and figs. S26 to S33 for details ensuringmeasurement fidelity).The sensitivity of PTQY enables us to investi-

gate high-quality CdSe/CdS quantum dots wherePLQY surpasses 90% (Figs. 1A and 3C). Thehighest PTQY value, shown in Fig. 3A, is 99.6 ±0.2%. This value was measured for excitationsoriginating in the CdS shell of an 8.5-ML (corediameter: 3.6 ± 0.2 nm; shell thickness: 3.7 ±0.3 nm) CdSe/CdS core-shell sample. This PL ef-ficiency compares favorably with the internalradiative efficiency estimated indirectly in well-passivated GaAs [99.7 ± 0.2% extrapolated fromphoton recycling (6)] and single-crystalNd:Y3Al5O12

[99.5 ± 0.2%measured via laser cooling (5)]. Onesample, shown in Fig. 3B, exhibited small radia-tive variations between the shell and core thatare barely discernible by traditional methods. Inthis sample (Fig. 3B, inset), the PLQY of coreexcitations is 96.2 ± 0.1%, whereas the PLQYwhen exciting the shell is 93.6 ± 0.2%.We ascribe

Hanifi et al., Science 363, 1199–1202 (2019) 15 March 2019 2 of 4

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Fig. 1. Characterization of highly emissive CdSe/CdS quantum dots.(A) PLQY for many samples with different shell thicknesses grown withour synthesis procedure. (B) Absorption factor (Abs) (dotted lines), PLE(shaded regions), and PL spectra (solid lines) for 5-, 8-, 10-, and 11-ML CdS

shells, respectively. A.u., arbitrary units. (Inset) Representative transmis-sion electron micrographs. Scale bars, 25 nm. (C) Wavelength-resolvedPLQY of the 8- and 10-ML samples shown in Fig. 1B. Full characterization(SM4) dataset in figs. S1 to S9.

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Fig. 2. Measurement scheme for the PTQY technique. (A) Excitationlight is absorbed by an electron, which moves from a low energy state toa higher energy state. The excited electron and hole emit heat Q to relaxto their lowest energy excited states and then can recombine throughPL or nonradiative processes (the latter of which emits heat QNR).(B) A plot of the heat emitted per excitation versus the excitation energyshows how the PLQY can be determined from the threshold energy E0

above which heat is emitted and the photoluminescence energy EPL.A near-perfect emitter at the thermal limit (Q + QNR = 0) where thedeviation of EPL ≥ E0 represents the deviation in PTQY (i.e., PLQY ≤ 1).(C) The instrumental realization of the PTQY technique uses aphotothermal deflection spectrometer to measure heat, synchronizedto a PL spectrometer, which determines both EPL and the number ofexcitation events (SM5, SM6, and figs. S17 to S25).

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Hanifi et al., Science 363, 1199–1202 (2019) 15 March 2019 3 of 4

Fig. 3. Detailed optical characterization of CdSe/CdS quantum dots andPTQYmeasurement uncertainty. (A) PTQYof the best CdSe/CdS (core-shell)particles (8.5 ML).The inset displays probability distributions and uncertaintiesassociatedwithPTQYuncertainty (blue andblack shading), and integrating spherePLQY (Gaussian, red shading). (B) High-resolution PTQYof high-quality CdSe core(96.2 ± 0.1%) but with poorly passivated CdS shell (93.6 ± 0.2%) with inset ofrespective integrating sphere PLQYandPTQYuncertainty (for detailed uncertainty,see SM6 and figs. S10 to S16). (C) Top plot shows PL lifetimes of CdSe/CdS coreand shell excited lifetimes, and bottom plot shows PLQYand PTQYof the samenear-unity particles with measurement uncertainty plotted against particle Feret

diameter.The circle markers represent a constant CdSe core size and varied CdSshell thickness, and the inset diamond markers represent the highest PTQYparticlesmeasured thus far,with a slightly larger 3.6 ± 0.2 nmCdSe core and 3.7 ±0.3 nm (8.5ML) CdS shell. (D) Table representing themain sources of uncertaintyand the general uncertainty budget for the calibrated PTQYspectrometerdependent on photon energy (figs. S17 to S26). CCD, charge-coupled device.(E) TRPL data (log-log scale) taken at 407.1-nm excitation. Despite near-unityPLQYof 4- to 11-ML samples, significant differences in long lifetime componentsarise across the shelled series. (F) PL lifetime excitation wavelength dependencewith varied shell thickness normalized to each particle’s respective core lifetime.

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this decrease to shell excitations naturally lead-ing to more trapping and nonradiative processes(see SM8 for further discussion).We present a series of particles with the same

size CdSe core but with different CdS shell thick-nesses (4 to 11 ML). We can systematically de-termine the origin of the peak in PLQY at 8 ±0.5 ML using PTQY and time-resolved PL (TRPL)decay measurements. With shell thicknesses lessthan 8 ML, insufficient passivation, consistentwith surface hole traps, is displayed (20). Thiseffect is apparent in TRPL decays as a tail past~100 ns (Fig. 3E), resulting in significantly lowerefficiencies (Fig. 3C). At shell thicknesses ≥8 ML,the excited-state decays are well-described by abiexponential decay, but >8ML, the excited-statedecays have substantially longer excited-statelifetimes. The increasing excited-state lifetimecan be understood by considering the quasi–type II band alignment whereby a thicker shellresults in reduced overlap between a delocalizedelectron in the shell and core-localized hole (SM8and SM9). Despite this extended lifetime, en-sembles still display PTQYs of 99.6%, suggestingvery slow nonradiative processes, possibly as slowas 2 to 20 ms (figs. S34 to S36). Moreover, ex-citations of larger shells allow the electron to bemore easily trapped at the CdS surface in shallowtraps (Fig. 3F). Thus core-shell particles cantolerate shallow surface traps that only affectone charge carrier (21). These results suggestan alternative, defect-tolerant path toward thefuture development of quantum dots with 99.9%or better quantum yields, in addition to the well-known but challenging approach of eliminatingall defects and trap levels in nanocrystalline sam-ples, which necessarily have an enormous surfacearea. The combination of more-accurate PLQYmeasurements and increased TRPL dynamicrange allows us to better track rare events thatoccur in quantum dot ensembles at excitationintensities that single particle luminescence ex-periments cannot achieve (fig. S37). Such rareevents under low flux excitation, hNi≪ 1, wherehNi is the average number of excitons created

per pulse per particle, are relevant to furtheroptimization of the particles for numerousenergy-related applications.The true limit of colloidal quantum dot lu-

minescence efficiency is currently unknown. Im-provements in both process engineering andluminescence efficiency quantification can helpdrive the design, synthesis, and understandingof optimized quantum dots, opening the doorto tests of fundamental theory and applica-tions of these important materials. In thiswork, we have demonstrated a synthetic methodthat produces particles with external lumi-nescent efficiencies >99.5% and a techniquethat is capable of measuring the efficienciesof these ensembles with nearly 100× less un-certainty than traditional measurement tech-niques. Our results demonstrate that there is nofundamental impediment to synthesize nano-crystals with nonradiative decay channels atleast as low as, if not lower than, those of thebest single-crystal semiconductor materials, andprovide an important platform for future devel-opment of near-lossless materials at the thermo-dynamic limit.

REFERENCES AND NOTES

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M. G. El-Shaarawy, Desalination 209, 244–250 (2007).5. T. Kushida, J. E. Geusic, Phys. Rev. Lett. 21, 1172–1175 (1968).6. I. Schnitzer, E. Yablonovitch, C. Caneau, T. J. Gmitter,

Appl. Phys. Lett. 62, 131–133 (1993).7. O. Chen et al., Nat. Mater. 12, 445–451 (2013).8. D. R. Needell et al., IEEE J. Photovoltaics 8, 1560–1567 (2018).9. M. Sheik-Bahae, R. I. Epstein, Nat. Photonics 1, 693–699

(2007).10. R. M. Swanson, Recent developments in thermophotovoltaic

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11. T. Zhong et al., Science 357, 1392–1395 (2017).12. T. P. Xiao, K. Chen, P. Santhanam, S. Fan, E. Yablonovitch,

J. Appl. Phys. 123, 173104 (2018).13. H.-J. Song et al., Nano Lett. 18, 395–404 (2018).14. D. A. Bender, J. G. Cederberg, C. Wang, M. Sheik-Bahae,

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J. Am. Chem. Soc. 135, 18536–18548 (2013).17. N. D. Bronstein et al., ACS Photonics 2, 1576–1583 (2015).18. C. Würth, M. Grabolle, J. Pauli, M. Spieles, U. Resch-Genger,

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A. P. Alivisatos, J. Am. Chem. Soc. 137, 15567–15575 (2015).22. D. Hanifi et al., Software and Data From: Redefining

Near-Unity Luminescence in Quantum Dots with PhotothermalThreshold Quantum Yield, UC Berkeley Dash (2019);https://doi.org/10.6078/D1XM30.

ACKNOWLEDGMENTS

We thank C. J. Takacs, V. Klimov, W. Jackson, R. Prasanna,and B. Guzelturk for valuable advice, discussions, or reading of themanuscript. Funding: This work is part of the ‘Photonics atThermodynamic Limits’ Energy Frontier Research Center fundedby the U.S. Department of Energy (DOE), Office of Science,Office of Basic Energy Sciences, under award DE-SC0019140. Workat the Molecular Foundry by A.M.S. and L.M. (transient absorptionmeasurements) was supported by the DOE, Office of Science, Office ofBasic Energy Sciences, under contract DE-AC02-05CH11231. Y.v.d.B.(modeling and analysis) acknowledges funding from the EuropeanResearch Council under the European Union’s Horizon 2020 researchand innovation program (grant 802615). K.T. (synthesis methodology)acknowledges funding from JXTG Nippon Oil & Energy. Authorcontributions: D.A.H., N.D.B., and B.A.K. contributed equally to thiswork. D.A.H. and N.D.B. conceived of the idea and directed the team.N.D.B., D.A.H., B.A.K., and Y.v.d.B. co-wrote the manuscript. D.A.H.,N.D.B., Y.v.d.B., and K.V. co-designed the PTQY experimental setup.B.A.K., N.D.B., and K.T. designed and developed the new syntheticmethod for CdSe/CdS nanoparticles used in this manuscript. B.A.K.and Z.N. synthesized the near-unity nanocrystals used in thismanuscript. D.A.H., N.D.B., B.A.K., Z.N., J.K.S., A.M.S., L.M., and Y.v.d.B.all contributed to characterization of the samples in this paper. Allauthors contributed to the discussion and analysis of the results.Competing interests: The authors declare no competing interests.Data and materials availability: All processed data are availablein the main text or in the supplementary materials. All software andcalibrations datasets are archived in the Dash repository (22).

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/363/6432/1199/suppl/DC1Materials and MethodsSupplementary TextFigs. S1 to S37Tables S1 and S2References (23–28)

13 November 2018; accepted 19 February 201910.1126/science.aat3803

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yieldRedefining near-unity luminescence in quantum dots with photothermal threshold quantum

Lorenzo Maserati, Koen Vandewal, Yoeri van de Burgt, Alberto Salleo and A. Paul AlivisatosDavid A. Hanifi, Noah D. Bronstein, Brent A. Koscher, Zach Nett, Joseph K. Swabeck, Kaori Takano, Adam M. Schwartzberg,

DOI: 10.1126/science.aat3803 (6432), 1199-1202.363Science 

, this issue p. 1199Sciencesuch as photovoltaic luminescent concentrators.99.5%. This is important for applications that require an absolute minimum amount of photon energy to be lost as heat, allowed them to tune the synthesis of CdSe/CdS quantum dots so that the external luminescent efficiencies exceededspectroscopy to measure very small nonradiative decay components in quantum dot photoluminescence. The method

used photothermal deflectionet al.current analytical methods cannot measure efficiencies above 99%. Hanifi A challenge to improving synthesis methods for superefficient light-emitting semiconductor nanoparticles is that

Superefficient light emission

ARTICLE TOOLS http://science.sciencemag.org/content/363/6432/1199

MATERIALSSUPPLEMENTARY http://science.sciencemag.org/content/suppl/2019/03/13/363.6432.1199.DC1

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