Reversible Concentration-Dependent Photoluminescence Quenching

Letter pubs.acs.org/JPCL

Reversible Concentration-Dependent Photoluminescence Quenching and Change of Emission Color in CsPbBr3 Nanowires and Nanoplatelets Francesco Di Stasio,† Muhammad Imran,†,§ Quinten A. Akkerman,†,§ Mirko Prato,‡ Liberato Manna,† and Roman Krahne*,† †

Nanochemistry Department, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy Materials Characterization Facility, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy § Dipartimento di Chimica e Chimica Industriale, Università degli Studi di Genova, Via Dodecaneso, 31, 16146 Genova, Italy ‡

S Supporting Information *

ABSTRACT: We discuss the photoluminescence (PL) of quantum-confined CsPbBr3 colloidal nanocrystals of two different shapes (nanowires and nanoplatelets) at different concentrations in solution and in solid-state films. Upon increasing the nanocrystal concentration in solution, a constant drop in photoluminescence quantum yield is observed, accompanied by a significant PL red shift. This effect is reversible, and the original PL can be restored by diluting to the original concentration. We show that this effect can be in part attributed to self-absorption and partly to aggregation. In particular, for nanoplatelets, where the aggregation is mostly irreversible, while the self-absorption effect is reversible, the two contributions can be well separated. Finally, when dry solidstate films are prepared, the emission band is shifted into the green spectral region, close to the bulk CsPbBr3 band gap, thus preventing blue emission from such films.

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ybrid1,2 and all-inorganic3 halide perovskites in the form of films, bulk crystals, and colloidal nanocrystals (NCs) have unquestionably transformed the optoelectronics research field since 2012.4 Considering that perovskite-based solar cells have now reached efficiencies above 22%,5 it comes as no surprise that the development of light sources6 is progressing at a fast pace as well. High photoluminescence quantum yield (PLQY), color purity, and color tunability of perovskite materials have driven the development of light-emitting diodes (LEDs),7 white phosphors,8,9 and lasers.10,11 In the case of CsPbX3 or CH3NH3PbX3 (where X is Br, I, or Cl) NCs, the emission color can be tuned during synthesis by selecting the desired halide precursor or through anion exchange reactions.3,12 NCs present a further venue to control the emission wavelength compared with bulk perovskites: quantum-confinement. Only recently has it been possible to achieve size and shape control for CsPbX3 NCs thanks to the development of novel synthetic methods, enabling the preparation of nanoplatelets,13,14 sheets,15 and wires,16,17 yet many research groups have observed relatively broad emission that consists of several peaks from such quantum-confined NCs and a reduction of the PLQY in comparison with bulk-like NCs.15,17−20 Both phenomena have been observed already in solution and were attributed to NC stacking, coalescence, or arising from a size distribution effect.15,17,19,20 Here we demonstrate that the PL properties of colloidal, strongly confined CsPbBr3 nanowires (NWs) and nanoplatelets (NPLs) depend on their concentrations in solution. While © XXXX American Chemical Society

diluted solutions of NWs and NPLs mainly manifest quantum confinement, increasing concentration of the NCs in solution results in red shifts of the PL maximum up to tens of nanometers, accompanied by a drop in PLQY up to 50%. However, over a broad range of NC concentrations in solution (in our study from 0.08 to 9 mM Pb concentration) the PL remains at shorter wavelength than the bulk emission of CsPbBr3. Interestingly, for the NWs the process is fully reversible, and by diluting the solutions again to the original conditions, we could restore the quantum-confined PL wavelength and higher quantum yield. On the contrary, in NPLs we observe a partially irreversible stacking, with bulk-like emission that is still present after redilution. Films fabricated from both NWs and NPLs show green PL that can be associated with the bulk band gap of CsPbBr3 and therefore to a significant decrease (and in some cases a complete loss) of quantum confinement. These findings are important for the application of quantum-confined CsPbBr3 NCs in light sources, where blue emission is desired, because the fabrication of such devices is based on highly concentrated solutions that are necessary for spin coating homogeneous films. Our study was carried out on solutions of CsPbBr3 NWs of different lengths and on NPLs. Blue-emitting NWs with Received: May 24, 2017 Accepted: June 5, 2017

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The Journal of Physical Chemistry Letters uniform diameter of 3.5 ± 0.5 nm were synthesized using our previously reported method.17 By using octanoic acid (Oac) along with alkyl amines (octylamine and oleylamine), the NW length could be tuned up to 1 μm, while with hexanoic acid (Hac) the length of the NWs was significantly increased to 5 μm. The CsPbBr3 NPLs were prepared following the synthesis protocol in ref 3; see the experimental section in the Supporting Information (SI). Figure 1a−c shows the PL and

dependent PL properties, we have performed PL, PLQY, and time-resolved PL decay measurements at different concentrations of a set of NW and NPL solutions. Here we took the as-synthesized solutions and concentrated them by reducing their volume to 100 μL. These concentrated solutions were then stepwise diluted and measured. Figure 2a,b reports the

Figure 1. (a−c) Optical absorption (dashed line) and PL spectra (solid line) of CsPbBr3 NWs prepared with Oac ligands (a), NWs with Hac ligands (b), and NPLs (c) and the respective TEM images (d−f). Spectra and TEM images were collected after synthesis from diluted toluene solutions of the NCs.

Figure 2. (a−c) PL spectra of CsPbBr3 NWs with Oac ligands (a), NWs with Hac ligands (b), and NPLs (c). PL has been recorded from solutions at increasing dilution and for drop-cast films on soda-lime glass. (d−f) Time-resolved PL recorded at the PL peak for solutions and films of CsPbBr3 NWs prepared with Oac ligands (d), Hac ligands (e), and NPLs (f). The NW PL decays were fitted with a threeexponential function: I = I0 + I1e(−t/τ1) + I2e(−t/τ2) + I3e(−t/τ3), while a two-exponential one was used for the NPLs: I = I0 + I1e(−t/τ1) + I2e(−t/τ2).

optical absorption spectra of two NW samples (a,b) and the NPL (c) sample recorded in solution. The NW solutions manifest a main PL peak centered at 464 and 466 nm for the samples with Oac and Hac surfactants, respectively, and a lowenergy shoulder around 480 nm. This low-energy shoulder can be attributed to partial aggregation;17 see Figure SI1 in the SI that reports spectra recorded from a highly diluted NW solution, where the low-energy shoulder is almost completely suppressed. The NPLs have a single distinctive emission peak centered at 468 nm, indicating a thickness of six monolayers.13 PLQY measurements carried out on the same solutions reveal a value of 59 and 40% for the Oac and Hac NWs and 70% for NPLs. The optical absorption spectra show a distinctive excitonic peak in all three samples at 457, 453, and 457 nm for Oac, Hac NWs, and NPLs, respectively. For the NWs we observe additional peaks at longer wavelength, similar to the PL spectra. Furthermore, the NW samples display a long absorption tail that extends to energies below the bulk band gap, which can be related to scattering induced by their considerable length. Typical transmission electron microscopy (TEM) images of the three samples are displayed in Figure 1d−f, where the difference in length between the two NW samples can be observed. To study the concentration-

steady-state PL spectra for the NW samples upon dilution. The concentrated solutions have a main PL peak centered at 484 and 480 nm for Oac and Hac NWs. Upon dilution, the initial PL peaks observed at shorter wavelengths (464 and 466 nm) start reappearing, and the overall contribution of the PL peaks at 484 and 480 nm decreases. Fitting of the PL spectra using a sum of two Gaussian curves (see Figure SI2) reveals the decrease in contribution of the longer wavelength peak and an increase in the blue component upon dilution (see Tables SI1 and SI2). Importantly, the concentrated NW solutions show a reduced PLQY (8−9%, Figures SI2 and SI3 and Table SI1), and the increase in blue emission upon dilution leads to a PLQY recovery up to the original value that was measured directly after synthesis. In Figure 3 we report TEM images of carboncoated 200 mesh copper grids obtained from concentrated solutions of NWs (i.e., at ∼3 mM Pb concentration). In the case of Oac NWs (a), single NWs can only be barely resolved due to their parallel stacking, but the most striking effect is seen in Hac NWs (b), where coils are formed. These two different conformations demonstrate that CsPbBr3 NW aggregation can take place in different ways, depending on their length. 2726

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properties that is further confirmed by a PL lifetime increase from 7 ns in solution to above 300 ns in the film, accompanied by a drop in PLQY from 38 to 10%. This observation indicates the formation of bulk like aggregates in NPL films with a large amount of trap states that cannot be redissolved in toluene. Figure 4a,b demonstrates the reversibility of the process by showing PL spectra recorded at different concentrations from a

Figure 3. TEM images of Oac (a) and Hac (b) NWs obtained from concentrated solutions (2.55 to 3 mM Pb concentration). Both types of NWs show a different conformation compared with Figure 1, where the carbon-coated grids were prepared with much more diluted solutions.

The CsPbBr3 NPLs have a different PL behavior, as depicted in Figure 2c: With increasing concentration, the PL peak is redshifted up to 8 nm. This red shift cannot be ascribed to an increase in the number of monolayer forming the NPLs.13 Furthermore, an additional weak PL peak at ∼522 nm is observed, indicating the presence of bulk-like emission.3 A similar phenomenon has already been reported for five monolayer NPLs upon storage of the sample for 1 month. However, in our case, we do not observe the formation of a precipitate. Dilution of the NPL solution induces a continuous blue shift of the PL to the original emission wavelength (468 nm, see Figure SI4 and Table SI3),13 yet the bulk-like emission is never completely suppressed, indicating that the formation of aggregates with non-quantum-confined emission is partially irreversible. The PLQY of the NPLs follows a trend similar to the NWs, with a sharp reduction to 38% upon concentration and a complete recovery upon dilution (Figure SI4 and Table SI3). In the case of NPLs, their stacking is already evident in Figure 1f and well known from literature, similar to other types of 2D NCs.21 To gain further insight into the photophysics of the concentration-dependent PL shift, we have performed timeresolved PL measurements for all samples and dilutions discussed in Figure 2. In the NW samples with different ligands, we have monitored the time decay of the blue peak, and Figure 2d−e shows that the decrease in NW concentration in solution leads to a significant decrease in lifetime (Tables SI1 and SI2). Similarly, the NPLs show a reduction of lifetime as well, although less pronounced (Table SI3). Considering the increase in PLQY upon dilution, the faster PL decay at low concentration indicates an increase in the radiative rate of the system, which rules out energy transfer as a quenching mechanism of the high-energy emission.22,23 Solid-state NC films fabricated by drop-casting on soda-lime glass substrates manifest an even more dramatic change in PL properties, with a stronger red shift toward the green part of the spectrum (Figure 2). The Oac NW film shows two emission peaks at 491 and 518 nm, while the Hac one has a single peak centered at 502 nm. For both films an increase in PL lifetime is observed (Tables SI1 and SI2), and their PLQY is close to the value measured in diluted solutions (44% for Oac and 50% for Hac NWs). The NPL film (Figure 2c,f) instead has the main PL peak at 522 nm, which is 7 nm red-shifted from the weak green peak that was observed in solution. The blue NPL emission is strongly suppressed, indicating a dramatic change in the material

Figure 4. (a) Photoluminescence spectra of CsPbBr3 NWs with Hac ligands in toluene solution upon concentration from 3.5 mL (0.1 mM) to 0.5 mL (0.7 mM) and (b) vice versa (starting solution volume of 0.5 mL and diluting back to 3.5 mL), demonstrating complete reversibility of the process. (c−e) PL spectra from concentrated (green) and diluted (blue) nanocrystal solutions, plotted together with a correction for self-absorption applied to the PL spectra of the diluted solution for Oac NWs (c), Hac NWs (d), and NPLs (e).

toluene solution of NWs with Hac ligands (different sample then the one in Figure 2), covering a full cycle of concentration and dilution from 0.1 mM solution to 0.7 mM and back. The solution was concentrated by evaporating the solvent under a nitrogen flux and diluted by adding toluene. Despite the PL shoulder at ∼522 nm, indicating bulk-like emission, we note that the peak wavelength of 494 nm at highest concentration still proves the presence of quantum confinement (see Figure 2727

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S5). Diluting the solution again enabled us to reproduce the blue emission from the original solution. We now discuss possible reasons for the reversible and concentration-dependent red shift of the PL. We have already ruled out energy transfer due to the PL lifetime behavior described above. Another possibility could be reabsorption that is likely to occur when the onset in the absorption spectrum overlaps with the emission band, as is true in our case. To evaluate if this effect can be responsible for such large red shifts, we have convoluted the emission spectra from the diluted solution with the respective transmission spectrum, which results in corrected PL spectra displayed in Figure 4c−e that account for self-absorption. Here we calculated the absorption spectrum for a given density by Ac = 1/2 Aref c/cref, where Aref is the absorption of the diluted solution, and c/cref is the ratio or the respective concentration to the diluted one. The factor 1/2 accounts for the shorter path length in the PL measurement. We note that in this correction we neglected the reduction in PLQY with increasing concentration; therefore, our correction represents an upper limit of the possible red shift due to selfabsorption. The corrected spectra manifest a notable red shift of the PL that might explain the behavior of the NPL solutions. However, for the NWs, it cannot reproduce the full red shift; therefore, other mechanisms must be at work. In organic conjugated systems, where moieties can give rise to various types of molecular aggregates upon concentration, large PL red shift, quenching, and other phenomena have been observed.24 Also, for colloidal NCs, aggregation can be expected to affect the emission wavelength and PLQY. Considering the dynamic ligand binding on the CsPbBr3 NCs surface,25 a possible scenario is that at high concentrations in solution the NWs have partially exposed surfaces/facets and that this leads to aggregation. Such aggregation can reduce the confinement of the excitons and modify the dielectric environment of the NCs. Concerning dielectric environment, Royo et al.26 demonstrated that low dielectric constant media surrounding CdSe- or CdTebased NCs can blue-shift the PL up to tens of millielectronvolts and increase the recombination rates, which, in reverse, means that increasing the dielectric constant of the environment can lead to red-shifting of the PL. In our case, we can expect an increase in the effective dielectric constant with increasing NC concentration; moreover, the static dielectric contrast of CsPbBr3 can exceed the one of CdTe and CdSe,27 which will increase the shift in PL. The reason that for NPLs the red shift due to self-absorption is more dominant should be related to their small size and therefore much larger number of particles in solution, which is likely to enhance self-absorption. Furthermore, the small but strongly red-shifted peak at 522 nm in the spectrum of the concentrated NPLs evidences irreversible aggregation and, consequently, also for NPLs, both effects, self-absorption and aggregation, occur, only the aggregation is not reversible as it is for the NWs. In conclusion, we demonstrated that self-absorption and aggregation effects in colloidal CsPbBr3 NCs have a strong impact on their optical properties and that the emission color depends critically on the NC concentration in solution. Because the fabrication of light-emitting, sensing, and photovoltaic devices relies on processing of concentrated NC solutions, strategies to preserve blue emission from quantum confinement, for example, by more robust surface passivation, have to be developed.

Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b01305. Experimental details of the various syntheses, additional optical characterization data, and TEM images. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Mirko Prato: 0000-0002-2188-8059 Liberato Manna: 0000-0003-4386-7985 Roman Krahne: 0000-0003-0066-7019 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research leading to these results has received funding from the European Union seventh Framework Programme under Grant Agreement No. 614897 (ERC Consolidator Grant “TRANSNANO”) Horizon 2020 under the Marie Skłodowska-Curie Grant Agreement COMPASS No. 691185. We thank Iwan Moreels from IIT and Juan Ignacio Climente from Universitat Jaume I for helpful discussions.

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