Photoluminescence Up-Conversion in CsPbBr3 Nanocrystals Yurii V. Morozov,† Shubin Zhang,‡ Michael C. Brennan,† Boldizsar Janko,‡ and Masaru Kuno*,† †
Department of Chemistry and Biochemistry, University of Notre Dame, 251 Nieuwland Science Hall, Notre Dame, Indiana 46556, United States ‡ Department of Physics, University of Notre Dame, 225 Nieuwland Science Hall, Notre Dame, Indiana 46556, United States S Supporting Information *
mission electron microscopy (TEM) images of resulting NCs, which possess an average edge length of l = 9.2 ± 1.3 nm (see Figure S1c for the NC size distribution). NCs were subsequently subjected to a thiocyanate salt treatment9 to increase their EQEs. Following treatment, near unity EQE values (EQE ∼ 85%) were obtained. Additional details regarding the CsPbBr3 NC synthesis, their postsynthetic processing, and subsequent PL EQE measurements can be found in the Supporting Information (SI). Figure 1a first shows the room-temperature linear absorption spectrum of a NC ensemble along with its corresponding above gap Stokes (λexc = 460 nm) PL spectrum (solid blue line). When the ensemble is excited below gap (λexc = 536 nm), up-converted anti-Stokes photoluminescence (ASPL) is readily observed (dashed red line). Both ASPL and PL spectra occur from the same NC band edge emitting state10,11 with the ASPL spectrum slightly red-shifted (∼5 meV). This stems from a residual 14% size distribution in the NC ensemble (Figure S1c) and relates to size-dependent differences in NC up-conversion efficiencies.5 Observed ASPL stems from single-photon/phonon upconversion7 and does not result from two-photon excitation. This has been established using excitation intensity (Iexc)dependent ASPL intensity (IASPL) measurements wherein the order of the power law emission growth (i.e., IASPL ∝ Ibexc) establishes the one- or two-photon nature of the upconversion.6,7 For CsPbBr3 NCs, a near linear dependence of IASPL with Iexc (i.e., b = 0.993, see Figure S2) illustrates the singlephoton nature of the up-conversion. Details of these measurements have been outlined in the SI. Corresponding up-conversion efficiencies (ηASPL) have been estimated by comparing both above gap (IPL exc) and below gap (IASPL exc ) excitation values required to make equivalent IASPL and the associated PL emission intensity (Iem). Specifically, by taking into account different absorptances for above and below gap excitation, the up-conversion efficiency can be defined as
ABSTRACT: We report the first observation of phonon-assisted photoluminescence up-conversion from CsPbBr3 nanocrystals (NCs) at both the ensemble and single-NC levels. Ensemble up-conversion efficiencies are estimated to be on the order of 75% for a ΔE = 23 meV excitation detuning into the nanocrystal gap. fficient phonon-assisted photoluminescence (PL) upconversion and near unity external quantum efficiencies (EQEs) are crucial to achieving condensed phase optical cooling.1 Achieving semiconductor-based laser cooling opens the door to a number of exciting applications such as solid-state optical refrigeration below 10 K.2 Realizing net laser cooling in semiconductors, however, remains problematic because of stringent materials requirements required to overcome competing nonradiative recombination processes which lead to heating. As an illustration, achieving laser cooling in GaAs requires material EQE values that exceed 99%.3 To date, most research on condensed phase laser cooling has involved bulk semiconductors.4 Very few studies investigate requisite PL up-conversion and/or cooling in semiconductor nanostructures.5−7 Here we demonstrate that highly emissive CsPbBr3 NC ensembles with near-unity EQEs possess high PL up-conversion efficiencies. We additionally demonstrate that PL up-conversion can be observed in individual NCs. Together, these results suggest that low-dimensional CsPbBr3 may represent a potential materials system for achieving practical semiconductor-based laser cooling. Colloidal CsPbBr3 NCs were synthesized using a previously described literature procedure.8 Figure S1a,b illustrates trans-
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© 2017 American Chemical Society
ηASPL ≈
PL Iexc
( )( ), ASPL Iexc
APL AASPL
where A P L and A ASPL are NC
absorptances for above and below gap excitation, respectively. See the SI for more details. These measurements yield ηASPL = 75% (32%) at λexc = 514 nm (532 nm), a ΔE = 23 meV (102 meV) excitation detuning into the CsPbBr3 NC gap. Received: September 20, 2017 Accepted: October 3, 2017 Published: October 3, 2017 2514
DOI: 10.1021/acsenergylett.7b00902 ACS Energy Lett. 2017, 2, 2514−2515
Energy Express
http://pubs.acs.org/journal/aelccp
Energy Express
ACS Energy Letters
Figure 1. (a) Absorption (solid green line) and band-edge emission (solid blue line) spectra of a CsPbBr3 NC ensemble when excited above gap at λexc = 460 nm. Corresponding ASPL spectrum (dashed red line) when excited below gap at λexc = 536 nm. (b) Raster-scanned PL image of an individual CsPbBr3 NC excited above gap at λexc = 460 nm. (c) Raster-scanned ASPL image of the same area, excited below gap at λexc = 536 nm. (d) Corresponding single-NC PL (solid blue line) and ASPL spectrum (red circles). All measurements conducted at 298 K.
Large ηASPL and PL EQE values mean that ASPL can be observed from individual NCs. Panels b and c of Figure 1 show raster scanned PL (λexc = 460 nm) and ASPL (λexc = 536 nm) images of a single CsPbBr3 NC, respectively. Figure 1d shows corresponding PL and ASPL spectra. The SI provides additional details of these single-NC measurements. As with Figure 1a, both single-NC PL and ASPL spectra are near coincident in energy. This demonstrates again that PL and ASPL involve the same final emitting state.10,11 CsPbBr3 NCs are one of the few semiconductor systems to show near unity PL EQEs.4,12−14 In this study, we have demonstrated that they efficiently up-convert at both the ensemble and single-NC levels. These exceptional optical properties together with existent scalable syntheses8 make CsPbBr3 NCs attractive candidate materials for eventually realizing practical semiconductor-based laser cooling.
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in part, by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy under Award DE-SC0014334. We thank the Notre Dame Integrated Imaging Facility (NDIIF), the ND Energy Materials Characterization Facility as well as the Center for Sustainable Energy at Notre Dame (cSEND) for use of their equipment and for partial financial support. We also thank the Notre Dame Radiation Laboratory (NDRL) for use of its facilities. The NDRL is supported by the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences under Award Number DE-FC0204ER15533. This is NDRL manuscript 5189.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00902. TEM images of CsPbBr3 NCs, details of PL EQE measurements, additional details regarding up-conversion efficiency measurements, and details regarding the singleNC PL/ASPL raster scan imaging (PDF)
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REFERENCES
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Masaru Kuno: 0000-0003-4210-8514 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS M.K. thanks the MURI:MARBLe project under the auspices of the Air Force Office of Scientific Research (Award No. FA955016-1-0362) for financial support. This work was also supported, 2515
DOI: 10.1021/acsenergylett.7b00902 ACS Energy Lett. 2017, 2, 2514−2515
