Article pubs.acs.org/crystal
Optical Properties of Colloidal CH3NH3PbBr3 Nanocrystals by Controlled Growth of Lateral Dimension Artavazd Kirakosyan, Jiye Kim, Sung Woo Lee, Ippili Swathi, Soon-Gil Yoon,* and Jihoon Choi* Department of Materials Science and Engineering, Chungnam National University, Daejeon, South Korea
Crystal Growth & Design 2017.17:794-799. Downloaded from pubs.acs.org by UNIV OF SUSSEX on 08/13/18. For personal use only.
S Supporting Information *
ABSTRACT: Organometal halide perovskites become important in the photovoltaic and light emitting devices due to the compositional flexibility with AMX3 formula (A is a monovalent organic ammonium cation; M is a metal ion; X is a halogen atom), imposing a significant demand to develop a synthetic route toward new types of nanocrystals. Although chemical pathways for perovskites nanoparticles were developed on the basis of the reprecipitation method, poor control of the nucleation and growth process results in a large size polydispersity that induces the ambiguities associated with a quantum confinement effect depending on their size. Here, a modified reprecipitation method is presented for the synthesis of CH3NH3PbBr3 perovskite nanoparticles with a controlled nanoparticle size by systematically tuning the feed ratio of the precursors. Fine control of the nanocrystal size provides new insights into the quantum confinement effect observed in microscale and nanoscale perovskite materials, where their energy bandgap is associated with the thickness of nanoparticles and invariant to preferential growth in a lateral dimension.
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double-, triple-, and multilayer of the unit cell).7−13 Especially, PL emission of the nanoplatelets can be adjusted from 427 to 519 nm by tuning the number of nanoplatelet layers.7−13 Further interesting features are observed in the perovskite nanocrystal materials with the reduced lateral crystal size (i.e., colloidal nanoparticles),6,14−20,24 leading to many efforts to control their nanocrystal dimension and size. The CH3NH3PbX3 perovskite nanocrystals with a diameter of ∼5 nm obtained by porous Al2O3 templates emit an intense green light at 523 nm.14 Schmidt et al. reported that the perovskite nanocrystals with a size of 6.2 ± 1.1 nm synthesized by a hot injection method exhibit the quantum confinement with an absorption edge 16 nm blue-shifted from that of the bulk counterpart. PL emission was also blue-shifted from 532 to 520 nm with decreasing the average size of nanocrystals (160 to 30 nm).16 Similarly, Zhang et al. reported that the PL emission of the CH3NH3PbBr3 perovskite nanocrystals (3.3 nm of diameter) was blue-shifted by 30 nm.6 Further blue-shift of PL emission (475−520 nm) was observed for much smaller perovskite nanocrystals (1.8−3.6 nm).20 Emulsion synthesis method adopted by Huang et al. provided precise particle size control from the 2.5 to 7 nm range where PL emission is observed at 521 nm for all sizes of particles.18 In particular, a controlled shape of the perovskite nanocrystals from squares to spherical geometry exhibits significant blue-shift of PL emission (450−505 nm).25 Here, it should be noted that the exciton Bohr radius of CH3NH3PbBr3 perovskite is only 2 nm,26 imposing the criterion for the observation of the size-dependent
INTRODUCTION Colloidal semiconductor nanocrystals have attracted considerable attention in the fields of optoelectronic and energy conversion applications due to their promising physical and chemical properties (i.e., quantum-size effects, enhanced optical properties, versatile surface chemistry, etc.), which have led to recent research efforts to develop a synthetic route toward new types of nanocrystals.1−25 Among these optoelectronic nanocrystals, organometal halide perovskites (CH3NH3PbX3, where X = Br, I, Cl) are of great interest due to their excellent performance in photovoltaic and light emitting devices. In particular, the compositional characteristics of the perovskite nanocrystals with AMX3 formula (A is a monovalent organic ammonium cation such as methylammonium, ethylammonium, etc.; Mis a metal ion such as Pb, Sn, etc.; X is a halogen atom) provide a unique approach to tune their electronic and optical properties by (i) organic cation exchange,1,2 (ii) metal ion exchange,3,4 (iii) halogen ion exchange,5,6 as well as (iv) control of a crystal size and dimensionality.6−25 Despite such efforts devoted to the application of organometal halide perovskite materials in the fields of optoelectronics and photovoltaics, most studies were focused on the compositional tunability of the perovskite materials as mentioned above (i−iii) to control their physical properties such as energy bandgap, photoluminescence (PL), quantum yield, etc. However, recent studies have shown that a control of the crystal dimensions (i.e., nanoscale colloidal or low-dimensional perovskite systems) holds the promise to realize the potential of organometal halide perovskites. For example, nanoplatelets (NPLs) with lateral dimensions up to hundreds of nanometers exhibit a well-defined quantum confinement (QC) effect when their thickness is reduced to a few nanometers (e.g., single-, © 2017 American Chemical Society
Received: November 14, 2016 Revised: December 23, 2016 Published: January 6, 2017 794
DOI: 10.1021/acs.cgd.6b01648 Cryst. Growth Des. 2017, 17, 794−799
Crystal Growth & Design
Article
limited to terminate the growth process when the desired particle size is achieved. Briefly, the precursor solution was prepared by dissolving the appropriate amount of PbBr2, CH3NH3Br, n-octylamine, and oleic acid in N-dimethylformamide (DMF). Colloidal CH3NH3PbBr3 nanocrystals were obtained by precipitation of the precursor solution into a poor solvent (typically, toluene; however, some other solvents are also available such as dichloromethane, benzene, etc.) under vigorous stirring at room temperature. Here, the precursor solution was added dropwise with a fixed periodicity (∼45 drops/min), and their amount was determined by counting the number of droplets (i.e., each droplet ∼60 mg). Six different samples were prepared by adding 1, 3, 5, 8, 12, and 20 drops (i.e., 60 mg ∼1.2 g of the precursor solution, see Supporting Information Table S1), which yields 0.07−1.3 mg/mL of perovskite nanocrystals in toluene. Precipitation of the precursor solution (1 droplet ∼60 mg) in the toluene (∼10 mL) produces approximately 0.692 mg of CH3NH3PbBr3 nanocrystals, indicating that it already exceeds the limitation of their solubility. Starting from the very first droplet, the solution changed its color from transparent to bluish green, suggesting a formation of nanocrystals that emit a bright blue-green light under UV irradiation (λ = 365 nm) as shown in Figure 2a. Figure 2b
PL characteristics of the perovskite nanocrystals. However, despite the recent efforts to explore the quantum confinement effect in the perovskite nanocrystals, most of the nanocrystal systems have a much larger size than the exciton Bohr radius, thereby tackling the ambiguities associated with size-dependent PL characteristics. In this work, we systematically control the geometric size responsible for the quantum confinement of CH3NH3PbBr3 nanocrystals to reveal their size-dependent optical properties. In particular, a careful control of the nucleation and growth process for perovskite nanocrystals provides a comprehensive understanding that unifies the seemingly contradictory observation of optical absorption and PL emission of perovskite nanocrystals with different geometric shapes and sizes.
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RESULTS AND DISCUSSION We adopted a ligand-assisted reprecipitation (LARP) technique with a certain modification.6 Here, the precipitation occurs through the reduced solubility via a solvent mixing in the presence of capping ligands localized on the surface, while it prevents further particle growth and their aggregation. Thus, the nanocrystals form stable colloids with a desired size and morphology. In homogeneous nucleation and growth, a colloid formation proceeds according to the LaMer diagram (Figure 1), where the reduction of precursors occurs to generate a
Figure 1. La Mer diagram for the generation of atoms, nucleation, and subsequent growth of colloidal systems (LaMer and Dinegar, 195027).
critical concentration of atomic species in solution. Above this critical concentration, nucleation results in a rapid depletion of the reactants such that all subsequent growth occurs on the preexisting nuclei.28 However, in the case of the organometal halide perovskites nanocrystal, the reduction of precursor materials is not required because the atomic and molecular species are already dissolved in a good solvent. Furthermore, the solubility of these species is extremely low in the solvent for reprecipitation, and thus an introduction of the starting materials immediately exceeds the critical concentration for nucleation, as indicated by “adding point” in Figure 1. To the best of our knowledge, the previous experimental works on the synthesis of perovskite nanocrystals involve excess amounts of starting materials, which definitely results in significant size polydispersity (i.e., both nanoscale and microscale perovskite particles coexist in the same batch). Nanocrystals are further separated from the microscale particles by centrifugation that is not appropriate to obtain uniform size of nanocrystals. Thus, in our approach the amount of starting materials is carefully
Figure 2. (a) Photo images of CH3NH3PbBr3 nanocrystal solution under ambient light (left) and under 365 nm UV illumination (right). (b) UV−vis absorption and PL emission spectra with excitation wavelength (λex) of 365 nm. (c) Transmission electron micrograph of colloidal CH3NH3PbBr3 nanocrystals obtained by 1 droplet of the precursor solution (denoted by 1 day). (d) High resolution transmission electron micrograph. (e) Simulated and experimental X-ray diffraction patterns of CH3NH3PbBr3 nanocrystal. Corresponding Miller indexes are labeled at the top of the diffraction peaks.
shows the absorption (red) and emission (black) spectra of colloidal CH3NH3PbBr3 nanocrystals. In the PL spectrum, a sharp emission peak at 489 nm (2.53 eV) with a full width at half-maximum (fwhm) value of 36 nm was observed, which is blue-shifted by ∼45 nm compared to the bulk counterpart consistent with that in the literature.29,30 The UV−vis 795
DOI: 10.1021/acs.cgd.6b01648 Cryst. Growth Des. 2017, 17, 794−799
Crystal Growth & Design
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Figure 3. (a) CH3NH3PbBr3 particle size as a function of the droplet count denoting the corresponding minimum and maximum values, as well as standard deviations. The average particle size was determined from at least 50 particles per sample from which the experimental error was determined by the standard deviation of the measurements (see Supporting Information, Figure S1 and Table S2). Inset exhibits an electron micrograph of the 8d, showing uniform nanocrystals. (b) Transmission electron micrograph of the 20d shows two distinct morphologies (i.e., rounds and squares shapes coexist in one batch), in which significant contrast difference of the CH3NH3PbBr3 nanocrystals is not observed. (c) UV−vis absorbance and (d, e) photoluminescence emission spectra depending on the particle size and dilution degree. (f) Time-resolved PL decay curves of the CH3NH3PbBr3 nanocrystals with the different particle size. PL decay curve of the CH3NH3PbBr3 nanocrystals that are not capped with noctylammine (1d) is used for reference.
Careful analysis of the electron micrographs reveals that the particle size is still uniform at higher contents of the precursor (i.e., longer duration of the precursor addition, > ∼20 s), which indicates that the growth of the pre-existing CH3NH3PbBr3 particles is preferred rather than additional nucleation. Since the solubility of reactants is significantly low in the toluene, further addition of the precursor leads to the supersaturated environment, and the CH3NH3PbBr3 particles experience coarsening from Ostwald ripening, in which large particles grow at the expense of small particles.31 At much higher contents of the precursor (>8d), the CH3NH3PbBr3 particles get closer and form aggregates of a few micrometers and twodimensional nanoplatelets (Figure 3b), which is consistent with the previous observation involving excess amounts of the precursor in the literature.7,9 Figure 3c−e shows the UV−vis absorption and PL emission spectra of CH3NH3PbBr3 particles depending on the nanocrystal size and dilution. The absorption spectra have a band edge in the range of 490−499 nm (i.e., 2.52−2.48 eV) with the particle size of 3.6 to 8 nm (see Supporting Information S4 for the Tauc plot). PL emission was measured in the dilute solution to exclude the reabsorption phenomena, and the degree of dilution was experimentally determined until no blue shift is observed (Figure 3d). PL spectra of all samples exhibit sharp emission peaks in the range of 489−493 nm with narrow FWHMs (32−36 nm), slightly depending on the particle size (Figure 3e). The emission peaks are all blue-shifted by about ∼40 nm, compared to those in the literature for CH3NH3PbBr3 bulk materials.29,30 PL quantum yield (QY) was 49, 51, 53, 53, 45, and 65% for 1d−20d samples. Figure 3f exhibits the time-
absorption spectrum of CH3NH3PbBr3 nanocrystal has a band edge at 490 nm (2.52 eV) that is also blue-shifted by ∼55 nm (250 meV) from that of the bulk CH3NH3PbBr3 (Figure S3).29 The observed Stokes shift (∼10 meV) is an obvious excitonic feature implying that the PL emission originates from excitonic recombination.30 A bluish-green solution consists of uniform and well dispersed nanometer-sized CH3NH3PbBr3 particles with a diameter of 3.6 ± 1.3 nm, as shown in the electron micrographs (Figure 2c,d). The X-ray diffraction pattern (Figure 2e) indicates that the crystal structure of CH3NH3PbBr3 nanocrystals belongs to the space group of Pm3̅m with a lattice spacing, a = 5.9896 Å. The broadening of the diffraction peaks on the X-ray diffraction (XRD) pattern corresponds to the size of colloidal nanocrystals (4.5 ± 0.7 nm) by application of the Scherrer formula, which is comparable to that obtained by TEM. Moreover, two strong and sharp diffraction peaks of (100) and (200) crystalline planes are dominantly observed, which suggest a platelet rather than spherical structure. No significant change to interlayer spacing and only slight improvements to layer orientation were observed from XRD. Further addition of the precursor solution (i.e., the second droplet and even more) leads to further growth of the CH3NH3PbBr3 particle rather than a formation of new particles, as discussed below. At this step, the size of the CH3NH3PbBr3 particles was controlled by adding a predefined amount of the precursor (1d−20d denote the count of droplet of the precursor solution, as shown in Table S1). The average size of the CH3NH3PbBr3 particles increases from 3.6 to 8 nm with the amount of the precursor solution (1d−8d, Figure 3a). 796
DOI: 10.1021/acs.cgd.6b01648 Cryst. Growth Des. 2017, 17, 794−799
Crystal Growth & Design
Article
resolved PL decay data and fitting curves for the CH3NH3PbBr3 nanocrystals to understand a size dependence of the exciton recombination dynamic (Table S3). As a reference, CH3NH3PbBr3 nanocrystals (1d) that are not capped with n-octylammine show very fast decay time (∼2.2 ns). In contrast, n-octylammine capped CH3NH3PbBr3 nanocrystals (1d) exhibits that the fluorescent decay dramatically gets slower, and the PL lifetime significantly increases, which reveals that the surface defects acting as energy trapping sites are successfully passivated with the capping agent. PL lifetime becomes longer and decay rate decreases with the particle size (1d−20d). Longer average decay time (5.8−7.0 ns) imply that the exciton radiative recombination is mostly responsible for PL decay in colloidal CH 3 NH3 PbBr 3 nanocrystals. Slower fluorescent decay arises from several factors, e.g., a reduction of surface defects as the surface-to-volume ratio of nanocrystals becomes smaller with increasing particle size, as reported in the literature.19 This trend is further supported by enhanced QYs with the particle size, indicating the reduction of nonradiative decay in high-quality CH3NH3PbBr3 nanocrystals. Although the absorption spectrum of the CH3NH3PbBr3 particles (1d−8d) shows a significant blue-shift (∼55 nm) compared to the bulk counterpart consistent to number of works reported previously, extremely small variation in the energy bandgap (
