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Letter
Enhanced Size Selection in Two-Photon Excitation for CsPbBr Perovskite Nanocrystals 3
Junsheng Chen, Pavel Chabera, Torbjörn Pascher, Maria E Messing, Richard D. Schaller, Sophie Canton, Kaibo Zheng, and Tõnu Pullerits J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02178 • Publication Date (Web): 04 Oct 2017 Downloaded from http://pubs.acs.org on October 5, 2017
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Enhanced Size Selection in Two-Photon Excitation for CsPbBr3 Perovskite Nanocrystals Junsheng Chen,1, 2 Pavel Chábera,1 Torbjörn Pascher,1 Maria E. Messing,3 Richard Schaller,4
，5
Sophie Canton,6,7 Kaibo Zheng, *, 1,8 and Tõnu Pullerits *, 1 1
Department of Chemical Physics and NanoLund, Lund University, P.O. Box 124, 22100
Lund, Sweden 2
State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical
Physics, Chinese Academy of Sciences Dalian, 116023, China 3
Solid State Physics and NanoLund, Lund University, Box 118, 22100 Lund, Sweden
4
Center for Nanoscale Materials, Argonne National Laboratory, 9700 Cass Avenue, Argonne,
Illinois 60439, United States 5
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois
60208, United States 6
ELI-ALPS, ELI-HU Non-Profit Ltd., Dugonics ter 13, Szeged 6720, Hungary
7
Attoscience Group, Deutsche Elektronen Synchrotron (DESY), Notkestrasse 85, D-22607
Hamburg, Germany 8
Gas Processing Center, College of Engineering, Qatar University, PO Box 2713, Doha,
Qatar
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Abstract Cesium lead bromide (CsPbBr3) perovskite nanocrystals (NCs), with large two-photon absorption (TPA) cross-section and bright photoluminescence (PL), have been demonstrated as stable twophoton-pumped lasing medium. With two-photon excitation, red-shifted PL spectrum and increased PL lifetime is observed compared to one-photon excitation. We have investigated the origin of such difference using time-resolved laser spectroscopies. We ascribe the difference to the enhanced size selection of NCs by two-photon excitation. Due to inherent nonlinearity the size-dependence of absorption cross-section under TPA is stronger. Consequently, larger size NCs are preferably excited leading to longer excited-state lifetime and red-shifted PL emission. In a broad view, the enhanced size selection in two-photon excitation of CsPbBr3 NCs is likely a general feature of the perovskite NCs and can be tuned via NCs size distribution to influence their performance within NCs based nonlinear optical materials and devices. TOC
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In two-photon absorption (TPA) materials coherently absorb two photons without involving any one-photon resonance. Because of high nonlinearity, the process allows three-dimensional localization of the excitation at tight focal point and long excitation penetration depth. Consequently, TPA materials are currently attracting considerable attentions in as the application of bio-imaging1, 2, data storage3, 4, frequency upconversion lasing and amplification5, optical power limiting6, photodynamic therapy7, three-dimensional material micro-fabrication,3 and information technology8. In order to develop those TPA based applications, understanding the photo-physics of the materials is highly essential. Inorganic semiconductor NCs with high photo-chemical stability9, 10 show superior TPA compared to traditional organic TPA molecules. 11-16 The recently reported all-inorganic perovskite CsPbBr3 NCs exhibit TPA cross-section which is similar to those of conventional CdSe and CdTe NCs of the same size.13-16 Since the typical CsPbBr3 NCs are much larger than the above conventional NCs, the resulting TPA cross section is usually orders of magnitude higher.17 Prototype two-photon pumped lasers based on these NCs have been reported with outstanding performance.13,
14
We have
systematically characterized the TPA cross-section of CsPbBr3 NCs in our previous studies17. However the details of the excited state dynamics under TPA are yet to be investigated. It has been reported that perovskite NC photoluminescence (PL) shows different kinetics under TPA and one-photon linear absorption (OPLA) excitations.14,
18
This indicates that TPA induced
excitation may decay in different electronic relaxation process compared to OPLA. However, it has been also observed that PL has very similar kinetics under TPA and OPLA excitation for this material.13, 18 The apparent controversy has not been resolved so far. Moreover, several articles have reported that the PL spectra from TPA show a red-shift compared with that in OPLA.14, 17-20 The redshift in TPA excited PL spectra, similar to CH3NH3PbI3 single crystals and conventional CdSe NCs, has been attributed to size inhomogeneity and reabsorption.13, 21-24 However, the reabsorption effect in solution or thin film should not be as significant as in a bulk single crystal (larger than several µm).14 Furthermore, the narrow PL emission line width indicates a narrow size distribution of synthesized CsPbBr3 NCs.25 Hence, the red-shifted PL spectra and different PL decay kinetics were related to the 3 / 18 ACS Paragon Plus Environment
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selection rules for TPA and OPLA processes leading to different initially excited states in these two cases.14, 20 Contrary to that, de Weerd et al., suggested that the narrow PL emission line width is mostly caused by the efficient energy transfer (Förster resonance energy transfer and/or re-absorption) from smaller to larger NCs.26 In this work we study excited state dynamics after the TPA and OPLA by utilizing time resolved PL and femtosecond transient absorption (TA) spectroscopies. We exclude the possibility of reabsorption induced PL red-shift by using very low NCs concentration. We found that both PL and TA decays show strong pump intensity dependence, due to the fast Auger recombination. This is the most likely origin for the different conclusions drawn from the TPA and OPLA excited PL decays in previous reports. Under low pump intensity where only monomolecular recombination takes place, TPA induced excitation still shows slower relaxation process compared with OPLA. The excited state relaxation difference between TPA and OPLA excitation is diminished when the size of NCs increases. Considering the different size dependence of TPA and OPLA cross-section, we conclude that under TPA larger NCs are preferably excited compared to the OPLA excitation. Since larger NCs have longer excited state lifetime, the differences in the PL dynamics in the two different excitation regimes are ascribed to the size selectivity of the excitation. CsPbBr3 colloidal NCs were prepared by using a method developed by Kovalenko and coworkers.25,
27
0.814 g Cs2CO3 (Sigma-Aldrich, 99%) was mixed with 40 mL 1-octadecene (ODE,
Sigma-Aldrich, 90%) and 2.5 mL oleic acid (OA, Sigma-Aldrich, 90%), and dried for 1 hour at 120 ℃. The mixture was heated up to 150 ℃ for 30 mins under N2 atmosphere. The obtained Cs-oleate was kept in glove box and heated up to 100 ℃ before using. 0.689 g PbBr2 (Sigma-Aldrich, 99.999%) and 10 mL ODE were heated up to 120 ℃ under vacuum for 1 hour, afterwards 0.5 mL dry oleylamine (OAm, Sigma-Aldrich, 80−90%) and 0.5mL OA were added and heated up to 120 ℃ under N2 atmosphere. The temperature was increased to different levels to control the NCs size. Then 0.4 mL Cs-oleate solution was rapidly injected. After injection the mixture solution was immediately cooled in an ice-water bath. The injection temperature was set at 140 ℃, 180 ℃ and 200 ℃ to obtain particles with mean size of 4.6, 9.4 and 11.4 nm, respectively. The detailed information about isolation, 4 / 18 ACS Paragon Plus Environment
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purification has been discussed in our previous work.17 The size of CsPbBr3 NCs was characterized by high-resolution analytical transmission electron microscopy (TEM, Jeol 3000F). The sizes of CsPbBr3 NCs were measured along their edges (see Figure S1). Due to deviation from cubic shapes, the lengths of both the long edge and the short edge were measured for each NC. The corresponding distribution histograms are shown in Figure S2. The mean lengths of the long edge and the short edge of CsPbBr3 NCs were obtained by fitting the size distribution with a Gaussian. We use the average of these lengths to define the size of CsPbBr3 NCs (see Table S1). UV-Vis absorption spectra were obtained using a PerkinElmer Lambda 1050 spectrophotometer. The steady-state fluorescence spectra were recorded using a Horiba Jobin Yvon Fluorolog-3 spectrofluorometer with excitation at 430 nm. Time-resolved photoluminescence (TRPL) spectra were obtained using a streak camera (Hamamatsu, C6860). The laser source is a Ti:sapphire passively mode-locked femtosecond laser (Spectra-Physics, Tsunami), emitting at 800 nm with 2 kHz repetition rate and 150 fs pulse length. For TPA excitation, the laser pulses were used directly. For OPLA excitation, the 400 nm laser pulses were generated by a BBO crystal. TA experiments were performed by using a femtosecond pump-probe setup.28 Laser pulses (800 nm, 80 fs pulse length, 1 kHz repetition rate) were generated by a regenerative amplifier (Spitfire XP Pro) seeded by a femtosecond oscillator (Mai Tai SP, both Spectra Physics). For the OPLA experiments, the pump pulses at 400 nm were generated by a BBO crystal as a second harmonic of the laser. For the TPA experiments the pump pulses at 800 nm were obtained directly from the regenerative amplifier. For the probe we used either the super-continuum generation from a thin CaF2 plate (TA spectra measurements) or the Topas C (Light Conversion) to obtain pulses with central wavelength tuned from 470 to 513 nm (TA kinetic measurements). The mutual polarization between pump and probe beams was set to the magic angle (54.7°) by placing a Berek compensator in the pump beam. In order to avoid photo-damage, the sample was moved to a fresh spot after each time delay point. The kinetic of the different scans stay the same for both OPLA and TPA experiment showing no sign of degradation. The absorption spectrum of the sample was recorded after each scan. No changes were detected.
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Excitation power and spot size measurements were used to determine the excitation fluence with uncertainty within 20% of the fluence value.
Figure 1. Steady-state absorption spectrum (black dots curve), one-photon excited PL spectrum (dark yellow curve), two-photon excited PL spectrum (magenta curve) and the difference (Diff.) between two- and one-photon PL spectra (blue dots curve) to highlight the difference between two PL spectra from solution of CsPbBr3 NCs (~15 nmol/L). The inset shows the TEM image of the NCs. Cubic-shaped CsPbBr3 NCs with mean size of 9.4 nm (inset of Figure 1) were prepared using reported hot injection methods. The 1S exciton peak is located at 2.49eV (498 nm). The NCs show band edge emission, under both one-(400 nm) and two-photon excitation (800 nm). The two-photon excited PL spectrum is red-shifted compared to the one-photon counterpart (Figure 1). Such red6 / 18 ACS Paragon Plus Environment
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shifted PL spectrum has been observed in thin film of CsPbBr3 NCs and attributed to reabsorption.13 The effect occurs at high concentration (>22 nmol/L, see Figure S3), but can be avoided by measuring PL spectra at low concentration (
