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Existence of a Size-Dependent Stokes Shift in CsPbBr Perovskite Nanocrystals 3
Michael C Brennan, Jessica Zinna, and Ken Kuno ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 31 May 2017 Downloaded from http://pubs.acs.org on May 31, 2017
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Existence of a Size-Dependent Stokes Shift in CsPbBr3 Perovskite Nanocrystals Michael C. Brennan1, Jessica Zinna1, and Masaru Kuno1* 1
Department of Chemistry and Biochemistry, University of Notre Dame, 251 Nieuwland Science Hall, Notre Dame, Indiana 46556, USA
Corresponding Author *E-mail:
[email protected] ABSTRACT: The existence of a size-dependent Stokes shift is observed in CsPbBr3 perovskite nanocrystals for the first time. Stokes shifts range from ~100 to 30 meV for particles with edge lengths between ~4 and 12 nm respectively.
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Colloidal perovskite
hybrid (APbX3;
and A
all-inorganic =
CH3NH3+,
CH(NH2)2+, or Cs+; X = Cl-, Br-, or
I-)
nanocrystals (NCs) are excellent candidates for a number of next-generation photovoltaic and light emitting applications due to exceptional optoelectronic
properties
such
as
high
Figure 1. (a) Representative room temperature photoluminescence quantum yields (40-90%), absorption (solid black lines) and emission spectra (dashed red lines) of four representative narrow emission linewidths (70-140 meV), and CsPbBr3 NC ensembles. Corresponding edge lengths are indicated adjacent to the spectra. (b) size-/composition-tunable band gaps across the Size-dependent Stokes shifts are summarized for this work (blue diamonds) as well as data 1,2,3 extracted from the literature (purple triangles).1 visible. However, a better understanding of their underlying photophysics, specifically the nature of the emitting state, is needed to realize their eventual implementation into devices. In this regard, a Stokes shift exists between the emission and the band edge absorbing state which is universal among hybrid and all-inorganic NCs1,2,3 as well as their thin film counterparts.4 The presence of a Stokes shift is interesting because it suggests that the absorbing and emitting states are not necessarily the same. We report for the first time a size-dependent Stokes shift for CsPbBr3 NCs. In this study, 8 NC ensembles with edge lengths (l) ranging from l ~ 4 to 12 nm were synthesized using a technique previously described in the literature.1 Transmission electron microscopy images and sizing histograms for the samples can be found in Figure S1 and Figure S2 respectively. Figure 1a shows representative room temperature linear absorption and emission spectra for four different
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sizes of NC ensembles. Evident is that the emission is redshifted (or Stokes shifted) relative to the absorption band edge state. This Stokes shift increases with decreasing edge length, as summarized in Figure 1b. In order to quantify the absorption band edge state, absorption spectra were fit with a series of Gaussians, where the lowest energy Gaussian describes the band edge state. Representative fits can be found in Figure S3. Stokes shifts were measured as the energy difference between the emission maximum and the absorption band edge state. Resulting Stokes shift values range from ~100-30 meV for NCs with l = 4.1 to 11.7 nm respectively. In addition to our work, data extracted from fits to literature spectra1 corroborate our observation of a sizedependent Stokes shift. Despite recent studies on the spectral properties,
5
excited state dynamics,
6
and electronic
structure of CsPbBr3 NCs, 7,8 the origin of a size-dependent Stokes shift has yet to be explained. Of note, size-dependent Stokes shifts have previously been reported in CdSe quantum dots (QDs), where Stokes shifts range from ~100 to 10 meV for particles with diameters between ~1 and
5
nm
respectively.9,10
Through detailed theoretical modeling the size-dependent Stokes shift in CdSe QDs was shown to originate from the existence of band edge excitonic fine structure.9,10 This work has greatly increased the community’s understanding of the optoelectronic properties of this important model system.11 Similarly, revealing the origin of a size-dependent Stokes shift in CsPbBr3 NCs will provide significant insight into the photophysics and electronic structure of all-inorganic/hybrid lead halide perovskite nanostructures and thin films, in turn, better enabling their applications. ASSOCIATED CONTENT The Supporting Information is available on the ACS publication website.
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Transmission electron micrographs of nanocrystal ensembles, Sizing histograms for all ensemble samples, representative absorption fits, and experimental methods. AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGEMENTS This work was supported by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy under award DE-SC0014334. J.Z acknowledges support from the Arthur J. Schmitt Fellowship Foundation. REFERENCES (1) Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V. Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X= Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Lett. 2015, 15, 3692–3696. (2) Protesescu, L.; Yakunin, S.; Bodnarchuk, M.; Bertolotti, F.; Masciocchi, N.; Guagliardi, A.; Kovalenko, M. Monodisperse Formamidinium Lead Bromide Nanocrystals with Bright and Stable Green Photoluminescence. J. Am. Chem. Soc. 2016, 138, 14202-14205. (3) Zhang, F.; Zhong, H.; Chen, C.; Wu, X. G.; Hu, X.; Huang, H.; Han, J., Zou; B.; Dong, Y. Brightly Luminescent and Color-Tunable Colloidal CH3NH3PbX3 (X= Br, I, Cl) Quantum Dots: Potential Alternatives for Display Technology. ACS Nano. 2015, 9, 4533-4542.
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(4) D’Innocenzo, V.; Srimath-Kandada, A. R.; De Bastiani, M.; Gandini, M.; Petrozza, A. Tuning the Light Emission Properties by Band Gap Engineering in Hybrid Lead Halide Perovskite. J. Am. Chem. Soc. 2014, 136, 17730–17733. (5) Makarov, N.; Guo, S.; Isaienko, O.; Liu, W.; Robel, I.; Klimov, V. Spectral and Dynamical Properties of Single Excitons, Biexcitons, and Trions in Cesium Lead Halide Perovskite Quantum Dots. Nano Lett. 2016, 16, 2349–2362. (6) Castaeda, J. A.; Nagamine, G.; Yassitepe, E.; Bonato, L. G.; Voznyy, O.; Hoogland, S.; Nogueira, A. F.; Sargent, E. H.; Cruz, C. H. B.; Padilha, L. A. Efficient Biexciton Interaction in Perovskite Quantum Dots Under Weak and Strong Confinement. ACS Nano. 2016, 10, 8603– 8609. (7)
Ten
Brinck,
S.;
Infante,
I.
Surface
Termination,
Morphology,
and
Bright
Photoluminescence of Cesium Lead Halide Perovskite Nanocrystals. ACS Energy Lett. 2016, 1, 1266-1272. (8) Kang, J.; Wang, L.W. High Defect Tolerance in Lead Halide Perovskite CsPbBr3. J. Phys. Chem. Lett. 2017, 8, 489–493. (9) Kuno, M.; Lee, J.; Dabbousi, B.; Mikulec, F.; Bawendi, M. The Band Edge Luminescence of Surface Modified CdSe Nanocrystallites: Probing the Luminescing State. J. Chem. Phys. 1997, 106, 9869–9882. (10) Efros, A.; Rosen, M.; Kuno, M.; Nirmal, M.; Norris, D.; Bawendi, M. Band-Edge Exciton in Quantum Dots of Semiconductors with a Degenerate Valence Band: Dark and Bright Exciton States. Phys. Rev. B. 1996, 54, 4843–4856.
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(11) Kuno, M. Colloidal Quantum Dots: A Model Nanoscience System. J. Phys. Chem. Lett. 2013, 4, 680.
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