Article pubs.acs.org/cm
The Evolution of Quantum Confinement in CsPbBr3 Perovskite Nanocrystals Justinas Butkus,† Parth Vashishtha,† Kai Chen,† Joseph K. Gallaher,† Shyamal K. K. Prasad,† Dani Z. Metin,‡ Geoffry Laufersky,† Nicola Gaston,‡ Jonathan E. Halpert,*,† and Justin M. Hodgkiss*,† †
MacDiarmid Institute for Advanced Materials and Nanotechnology, and School of Chemical and Physical Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand ‡ MacDiarmid Institute for Advanced Materials and Nanotechnology, and Department of Physics, University of Auckland, Private Bag 92019, Auckland, New Zealand S Supporting Information *
ABSTRACT: Colloidal nanocrystals (NCs) of lead halide perovskites are considered highly promising materials that combine the exceptional optoelectronic properties of lead halide perovskites with tunability from quantum confinement. But can we assume that these materials are in the strong confinement regime? Here, we report an ultrafast transient absorption study of cubic CsPbBr3 NCs as a function of size, compared with the bulk material. For NCs above ∼7 nm edge length, spectral signatures are similar to the bulk material−characterized by state-filling with uncorrelated charges−but discrete new kinetic components emerge at high fluence due to bimolecular recombination occurring in a discrete volume. Only for the smallest NCs (∼4 nm edge length) are strong quantum confinement effects manifest in TA spectral dynamics; focusing toward discrete energy states, enhanced bandgap renormalization energy, and departure from a Boltzmann statistical carrier cooling. At high fluence, we find that a hot-phonon bottleneck effect slows carrier cooling, but this appears to be intrinsic to the material, rather than size dependent. Overall, we find that the smallest NCs are understood in the framework of quantum confinement, however for the widely used NCs with edge lengths >7 nm the photophysics of bulk lead halide perovskites are a better point of reference.
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INTRODUCTION
ΔE =
Lead halide perovskites have emerged in the past five years as one of the most promising materials for solar cell applications.1−3 The properties for which perovskites were initially synthesized in the late 90s have come back into focus recently, namely, their ability to exhibit bright, stable emission with wavelength tunability, ideal for photo- and electroluminescence applications.4 In addition to thin film and bulk perovskites, nanosized perovskite materials have also shown great promise in this area with their quantum confinement tunable properties ideal for such applications as electrically pumped lasers.5,6 Recently, Protesescu et al. reported a synthetic route to composition and size-tuned perovskite nanocrystals (NCs) made from CsPbX3, (X = Cl, Br, I).5 This material family retains the lead-halide motif at the heart of the semiconducting band structure of organometal halide perovskites, but with the organic cation replaced by cesium. CsPbBr3 particles with edge lengths ranging from ∼12−4 nm were synthesized with tunable bandgaps up to ∼0.4 eV (∼15%) higher than the bulk. This variation is in agreement with the predicted confinement energy in eq 1, where r is the particle radius and m* is the exciton reduced mass, which relates to the Bohr radius through the effective mass approximation.5 © 2017 American Chemical Society
ℏ2π 2 2m*r 2
(1)
The bandgap tuning achieved with these materials invokes the transformative progress seen for other colloidal inorganic quantum dots over the past 20 years.7 In a prototypical example, CdSe, the bandgap tunability throughout the visible spectrum has been exploited for LEDs, PVs, lasers, biological labeling, optical filters, and sensors.8−14 However, it is worth observing that currently CdSe NCs can be synthesized in comparatively smaller sizes than CsPbBr3, relative to their respective Bohr radii, leading to CdSe quantum dot bandgaps higher than the bulk by over 2 eV (∼115% of the bulk value).15,16 The photophysics of such strongly confined quantum dots is dominated by discrete hydrogen-like excitonic states. These states contribute to phenomena like well-defined excitonic peaks in the ensemble absorption spectrum of monodisperse samples, molecular-like intraband and multiexcitonic transitions, including strong photobleaching effects in the latter case, strong polarization effects, faster radiative Received: February 5, 2017 Revised: March 30, 2017 Published: March 31, 2017 3644
DOI: 10.1021/acs.chemmater.7b00478 Chem. Mater. 2017, 29, 3644−3652
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Chemistry of Materials
Figure 1. (a,d,e) High resolution transmission electron micrographs (HR-TEMs) (see SI for wider angle), including (b) imaging of lattice planes of a CsPbBr3 nanocrystal and (c) a fast Fourier transform (FFT) image from (b) showing the crystallinity of the nanocrystals. Histograms (overlaid a,d,e) of particle size, measured as mean length of the edge of the cube, are also included for three samples with average edge length in Table 1. (f) Absorbance (broad peak, left) and photoluminescence (Gaussian peak, right) spectra of the four samples showing emission peaks at 522 nm (red curves), 516 nm (green curves), 512 nm (blue curves), and 495 nm (purple curves) for the different samples.
were later reassessed by Castañeda et al.23 By resolving how decay kinetics probed at the band edge scale with particle volume, and also by resolving the low energy emission from bound biexcitons, the authors concluded that biexcitons only form for the smallest particles that are in the strong quantum confinement regime. Whereas the previous investigations focused on (single wavelength) decay kinetics as a function of particle size, the evolution of quantum confinement effects in NCs should clearly manifest in transient absorption (TA) spectral dynamics.22,23 Herein, we present extensive transient absorption (TA) spectroscopy analysis for bulk CsPbBr3 and a sizetuned series of CsPbBr3 NCs spanning the weak to strong quantum confinement regimes. We resolve a clear evolution in excited state behavior; larger NCs with edges over ∼8 nm (relevant to most previous perovskite NC studies) exhibit bulklike state-filling dynamics, bandgap renormalization, carrier cooling, and hot-phonon bottleneck effects, whereas strong confinement effects such as state focusing, state-to-state transitions, and strong bandgap renormalization are only evident in the TA spectra of smallest ∼4 nm NCs.
lifetimes, and increased Auger recombination than is observed in bulk materials. Changes in carrier phonon coupling, such as a phonon bottleneck have also been predicted, though rarely observed.17 Such signatures of strong confinement may not be expected in CsPbBr3 NCs, for which monodisperse samples can be synthesized at only modestly smaller sizes than the Bohr diameter dB (∼7 nm in bulk CsPbBr3).18 In cases where physical nanosized particle dimensions are larger than the Bohr diameter, the confinement effect does not produce discrete states but rather redistributes the bulk density of states (DOS) into semidiscretized bands of states that appear to be an intermediate between a single band and molecular-like states. The degree to which this occurs is determined by the degree to which the system is confined.19 Given that bulk samples of related organometal halide perovskites are characterized by free charge carriers filling a continuum of states, and that only modest optical absorption and emission shifts are seen in CsPbX3 NCs, it remains to be seen how confinement manifests in the photophysics of perovskite NCs as a function of size.5,20,21 Do discrete excitonic and biexcitonic states emerge producing strong changes in excited state dynamics, or does confinement in perovskite NCs merely induce a small perturbation of the free carrier photophysics established for the bulk materials? Makarov et al. recently examined CsPbX3 NCs, where X is Br, I, or a mixture of the two halides.22 Using steady-state and time-resolved absorption and emission spectroscopy, the authors concluded that the photophysics of CsPbX3 NCs is largely similar to traditional quantum dots such as CdSe and PbSe in the weak confinement regime. Dynamics in the perovskite NCs were attributed to excitons, with faster decay contributions at higher fluence attributed to biexcitons and unusually strong Auger recombination. However, even statistical confinement of uncorrelated carriers in the same volume will lead to new kinetic components. The photophysics
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EXPERIMENTAL SECTION
Materials. All materials were used as received without further purification unless otherwise indicated. Size Controlled Synthesis of CsPbBr3. CsPbBr3 nanocrystal synthesis procedures were adapted from those reported by Protesescu et al. (see Supporting Information (SI) for complete details on synthetic procedure).5 In brief, oleic acid was mixed with cesium carbonate to form cesium oleate. Cesium oleate was then injected into a 3-neck round-bottom flask containing a degassed solution of lead bromide in oleic acid, oleylamine and octadecene at 140−180 °C. The solution was mixed vigorously for 10 s prior to cooling by removing the heating mantle and, while continuing stirring, immersing the round-bottom flask in a solution of ice water. Nanocrystals in their native solution were diluted with hexane to the desired optical density for use in spectroscopic experiments. For characterization purposes, 3645
DOI: 10.1021/acs.chemmater.7b00478 Chem. Mater. 2017, 29, 3644−3652
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Chemistry of Materials nanocrystals were purified from the growth solution 2−3 times via solvent-antisolvent methods using isopropanol to aggregate the nanocrystals and hexane to redisperse them. Up to 25 mg of dried, purified NCs could be produced in one batch. For microcrystalline thin film fabrication, an equivalent molar ratio of CsBr and PbBr2 was dissolved in dimethyl sulfoxide (DMSO) and spin coated onto a TiO2 coated glass substrate at 2000 rpm for 60 s followed by annealing at 100 °C in N2 glovebox. Nanocrystal Characterization. Purified nanocrystals on a carbon coated copper grid (Ted Pella) were characterized by a JEOL 2100 high-resolution transmission electron microscope (HR-TEM) equipped with energy-dispersive X-ray spectroscopy (EDS) and by scanning tip electron microscopy (STEM). Transient Absorption Spectroscopy. Excited state dynamics were monitored using ultrafast transient absorption (TA) spectroscopy, in which 400 nm excitation (pump) pulses were generated from the second harmonic of an amplified Ti-sapphire 800 nm laser (Spectra-Physics Spitfire, 100 fs pulsewidth, 3 kHz) and were chopped at half of the amplifier rep-rate. The pump was filtered with a variable ND filter to required excitation intensities and sample excitation densities were estimated via concentration dependent optical absorption cross section calculation (SI). A portion of the 800 nm output was focused in to linearly translated 3 mm CaF2 crystal window and the generated white light supercontinuum polarized at the magic angle with respect to the pump was used as a probe. After transmission through a sample the probe was spatially dispersed using an optical glass prism and read out at 3 kHz using a linear CMOS photodiode array (Imaging Solutions Group). The pump−probe delay was varied using a retroreflector mounted on a computer controlled mechanical delay stage providing a delay range of up to 6 ns. Temporal experimental resolution was limited by the ∼200 fs total instrument response function. Approximately 6000 shots were averaged each time point and repeated for at least four scans. Transient absorption (ΔT/ T) spectra were obtained according to [T* (λ,t) − To(λ)]/To(λ), where T* and To are transmitted probe intensities measured with the excitation beam unblocked and blocked by the chopper, respectively. NC dispersions for room temperature spectroscopic measurements were contained in a 1 mm path length fused quartz cuvette sealed in inert N2 environment which was continuously translated on a linear stage during the measurement in order to avoid sample charging effects.
This is most likely due to the speed of the reaction, occurring within just a few seconds.25 Very long reaction times (1−10 min) were found to generally lead to very wide particle size distributions as many large (>20 nm) particles are formed through fusion of smaller particles, as has been observed previously in nanoplates and nanowires.26 Static Absorption and Photoluminescence Spectroscopy. Consistent with earlier reports,5,27 the photoluminescence (PL) peaks of these four samples were found to blue-shift with decreasing NC size (Figure 1). The PL peaks, listed in Table 1, correspond to the band gaps (extracted from Tauc plots) increasing from 2.37 eV for the bulk to 2.5 eV for the smallest NCs in agreement with the theoretical band gap estimates from eq 1 shown in Figure 2. Dominant contributions
RESULTS AND DISCUSSION Physical Characterization of Samples. Three separate populations of cubic nanoparticles of CsPbBr3 of varying dimensions were synthesized using methods similar to those reported by Protesescu et al.5 The size tuning of the particles was achieved by varying the reaction time and temperature to produce relatively homogeneous distributions of nanocrystals. High-resolution transmission electron micrographs of these samples (Figure 1) show them to be primarily composed of single crystals with fast-Fourier transform (FFT) and XRD patterns matching the cubic phase of CsPbBr3 (see SI). The nanocrystals in each of the samples are referred to as NC-8.6, NC-7.3, and NC-4.1 with mean edge dimensions and low size variance referred in Table 1, making them monodisperse.24
to the valence band maximum and conduction band minimum were determined to come from Br-p and Pb-p orbitals respectively through density functional theory calculations of the bulk material (SI). Quantum yields of ∼34% were measured after processing the NC-7.3 material, which was observed to be optically bright under UV excitation (SI). No broad defect or trap based emission at lower energies were observed, indicating that the NC surfaces were well passivated by ligands with few emissive defects.28,29 Absorption onsets for the series of NCs correlated with blue shifting emission peak for smaller particles. We observe a pronounced band edge absorption peak in bulk material which diminishes in nanocrystal absorption spectra. Similar behavior was observed in polycrystalline perovskite films with different grain sizes as well as other nanocrystals, and was attributed to reduction of exciton binding energy due to surface effects.30,31 This is a competing effect to quantum confinement in nanocrystals. Moreover, new, higher energy, absorption peaks were observed in the smallest NCs (4.1 nm edge length). We note that the reported absorption spectra of these materials often varies in the literature.5,32−36 These new features can be understood in the context of different confinement regimes that are defined based on NC dimensions relative to the exciton Bohr diameter.37 Structures with dimensions larger than ∼1 dB fall under the weak quantum confinement regime, while
Figure 2. Experimental versus theoretical (effective mass approximation) size dependence of the bandgap energy with quantum confinement regimes noted in relation to the Bohr diameter (db = 7 nm).
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Table 1. NC Size Estimates, Steady State Measurement Data and Derived Values
edge length (nm) PL peak (nm) PL fwhm (nm) BG (eV) BG/BGbulk
film
NC-8.6
NC-7.3
NC-4.1
522 20 2.37 1.00
8.59 ± 1.0 516 22 2.38 1.00
7.32 ± 0.74 512 21 2.40 1.01
4.14 ± 0.57 495 32 2.50 1.06 3646
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Chemistry of Materials particles with dimensions smaller than about ∼1 db belong to the strong quantum confinement regime, with an intermediate quantum confinement regime around ∼1 dB between these limits, assuming similar effective carrier masses.37 It is worth noting that the degree of quantum confinement may also vary along different directions of anisotropically structured particles, giving rise to 1-D and 2-D excitonic materials.38 Taking the literature estimate of dB ∼ 7 nm for CsPbBr3, the NC-8.6 and NC-7.3 samples should fall into the weak (and around intermediate) quantum confinement range, whereas NC-4.1 sample should exhibit strong quantum confinement (with its longest dimension, the diagonal length, extending to ∼1 db).18 Taken together, these steady-state spectroscopy results are consistent with confinement effects previously reported in a number of nanocrystalline materials and confirm that our samples span the weak to strong confinement regimes.5,39,40 Assignment of TA Features. TA spectra were taken of bulk CsPbBr3 films and dilute nanocrystal suspensions in hexane, as shown in Figure 3, making sure the measurements
(CH3NH3)PbI3 leading us to suggest the same spectral assignment here.41−43 The dominant positive differential transmission feature reflects the bleaching of band-to-band transitions by photocarriers filling states at the edges of the valence and conduction band.20,41,44 The monomodal GSB shape of the CsPbBr3 film is characteristic of the Burstein− Moss effect observed in polycrystalline (CH 3 NH 3 )PbI 3 samples, where the effective density for electron and hole states follow parabolic bands according to eq 2. ⎛ me,h k bT ⎞3/2 NC,V = 2⎜ ⎟ ⎝ 2π ℏ ⎠
(2)
The short-lived sub-bandgap feature, once thought to originate from bound excitons, instead most likely derives from a band gap renormalization leading to a momentary increase in unoccupied states at energies slightly below the ground state band gap.41,43−45 These states are quickly reoccupied by relaxation of hot carriers, observed via the high energy broadening of the bleaching feature at early times.41 Finally, the broad negative differential transmission feature above the bandgap was recently shown to correspond to photoinduced reflection in films, which is also observed in the NC samples and likely corresponds to increased scattering for these NC suspensions from the same photorefractive effect.41 The TA spectra of the NC-8.6 (Figure 3b) closely mirror the typical bulk perovskite behavior described above, suggesting that the weak quantum confinement has little effect on the bulk band-filling behavior. Smaller NC samples exhibit several similarities with the bulk and largest NC sample spectral profiles, while exhibiting some pronounced differences that reflect quantum confinement. Each spectral series in Figure 3 is dominated by the GSB feature, but as expected from the linear absorption spectra (Figure 1f), the GSB peak blue-shifts with decreasing NC size. The second obvious trend is that the GSB feature develops a multimodal structure which is most pronounced for the smallest (NC-4.1) particles, but also emerging for the NC-7.3 particles that are near the strong confinement regime. The evolution of GSB structure in the strong confinement regime will be explored further in the next section. Finally, each of TA spectra include the signature subgap peak at early times from bandgap renormalization. This peak has previously been described as arising from exciton−exciton interaction, where the presence of an exciton reduces the energy of subsequent exciton absorption.22,41 While conceptually similar to bandgap renormalization, we keep the bandgap renormalization description because the peak is present in bulk films and, like in organometal halide perovskites, it rapidly disappears on the subpicosecond time scale of carrier cooling.28,41 By comparing the energy of the bandgap renormalization PIA peak with the main GSB position, we are able to quantify the bandgap renormalization energy as a function of NC size. As shown in Table 2, we find that the bandgap renormalization energy increases with decreasing
Figure 3. Normalized transient absorption (TA) spectra of four samples measured at various times after above bandgap photoexcitation with 100 fs pulses centered at 400 nm. Excitation densities were kept ≪1 excitation/nanoparticle to avoid higher order processes.
were carried out with densities of ≪1 excitation per NC and corresponding densities in the bulk sample. The bulk material (Figure 3a) exhibits three distinct spectral features: (i) a longlived ground state bleaching (GSB) feature (positive differential transmission) centered close to the average band gap energy of the sample, (ii) a short-lived (7 nm (which are in the range of most previous studies) exhibit similar spectral dynamics as the bulk material; TA bleaching spectra reflect free carriers filling uniform density of states, and accelerated decay of multiply excited particles can be understood as bimolecular recombination of uncorrelated charges simply confined to the same volume. Only the smallest NCs reveal TA spectral features consistent with strong quantum confinement; focusing toward discrete energy states, high bandgap renormalization energy, and departure from a Boltzmann statistical carrier cooling. Subpicosecond carrier cooling appears to be rather insensitive to NC size. Carrier cooling dynamics on the order of 10 ps were observed at high fluence due to hot phonon bottleneck or Auger effects. However, across all NC samples and the bulk material, hot carrier lifetimes were found to scale with volumetric excitation density rather than particle occupation number, suggesting that the hot carrier dynamics were not strongly affected by quantum confinement in this case. Finally, we resolved a band edge state degeneracy of 2 for the strongly confined 4.1 nm NC, in agreement with models of discrete state formation for strongly confined NCs. Our results highlight that quantum confinement effects are only important for the smallest CsPbBr3 NCs that are currently available (∼4 nm edge length). For larger NCs (>7 nm) - in the weak confinement regime - photophysics is better described as a small perturbation on the free carrier photophysics established for the bulk material.
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CONCLUSION We have used broadband ultrafast TA spectroscopy to examine the onset of quantum confinement in size-tuned cubic CsPbBr3 NCs compared with the bulk material. We find that larger NCs 3650
DOI: 10.1021/acs.chemmater.7b00478 Chem. Mater. 2017, 29, 3644−3652
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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/acs.chemmater.7b00478. Synthesis details, spectroscopic analysis and methods, TA data, computational analysis (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Shyamal K. K. Prasad: 0000-0003-2365-2610 Justin M. Hodgkiss: 0000-0002-9629-8213 Author Contributions
All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS J.M.H. and J.E.H. each acknowledge funding support from Rutherford Discovery Fellowships, and JEH also acknowledges support from the Marsden Fund of New Zealand.
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ABBREVIATIONS OMH, organometal halide; PV, photovoltaic; NC, nanocrystal; LED, light-emitting diode device; QD, quantum dot; DOS, density of states; HR-TEM, High resolution transmission electron micrograph; EDS, energy-dispersive X-ray spectroscopy; STEM, scanning tip electron microscopy; TA, transient absorption; FFT, fast-Fourier transform; XRD, X-ray diffraction; fwhm, full-width at half-maximum; UV, ultraviolet; GSB, ground state bleaching
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REFERENCES
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DOI: 10.1021/acs.chemmater.7b00478 Chem. Mater. 2017, 29, 3644−3652
