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Research Paper

Applied Science and Convergence Technology 2024; 33(5): 140-143

Published online September 30, 2024

https://doi.org/10.5757/ASCT.2024.33.5.140

Copyright © The Korean Vacuum Society.

Room-Temperature Synthesis and Tuning Optical Properties of CsPbBr3 Nanocrystals via Ligand-Assisted Reprecipitation

Jun Yeong Heo , Duy Hoang Nguyen , Dong Gwon Heo , Sung Hun Kim , and Hong Seok Lee

Department of Physics, Research Institute of Physics and Chemistry, Jeonbuk National University, Jeonju 54896, Republic of Korea

Correspondence to:hslee1@jbnu.ac.kr

Received: August 26, 2024; Revised: September 13, 2024; Accepted: September 23, 2024

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (https://creativecommons.org/licenses/by-nc-nd/4.0/) which permits non-commercial use, distribution and reproduction in any medium without alteration, provided that the original work is properly cited.

Perovskite nanocrystals (NCs) have attracted significant attention owing to their unique properties such as high photoluminescence (PL) quantum yield and color tunability. They can be easily synthesized at room temperature via ligand-assisted reprecipitation (LARP). The bandgap energy of the NCs could be tuned by controlling the growth conditions of the LARP method. Therefore, we synthesized the CsPbBr3 perovskite NCs with the precursor dissolution time sequentially increased (10, 15, 20, and 25 min) so as to increase the precursor concentration in the solution. The PL spectra showed that the PL peak positions of the samples shifted from 509.5 to 518.0 nm with increasing dissolution time owing to the decreased ligand-to-precursor ratio. The full-width at half-maximum and Stokes shift decreased with increasing dissolution time because of size-defocusing and the confined hole states near the valance band, respectively. In addition, the bandgap and size estimations were investigated using the Tauc plot and effective mass approximation. With increasing the dissolution time, the estimated bandgap of the NCs varied from 2.41 to 2.37 eV, while the calculated size increased from 5.7 to 6.6 nm.

Keywords: Perovskite nanocrystals, CsPbBr3, Ligand-assisted reprecipitation, Dissolution time, Optical properties

Lead halide perovskite (LHP) nanocrystals (NCs) have attracted significant attention because of their unique optical properties such as high photoluminescence (PL) quantum yield and narrow full-width at half maximum (FWHM) [1,2]. Moreover, owing to the quantum size effect, the bandgap energy of LHP NCs can be tuned by varying the halide component and size of the NCs [2,3]. Given these properties, perovskite NCs have applicability in lighting, displays, photovoltaics, and laser devices [47]. Hot injection (HI) and ligand-assisted reprecipitation (LARP) methods are typically used to synthesize perovskite NCs. Among these methods, HI requires a high-temperature vacuum process, making the synthesis process relatively challenging [1]. In addressing this issue, the LARP method has attracted considerable interest because of it entails facile synthesis at room temperature while maintaining excellent optical properties [1,8]. The LARP method harnesses solubility differences between polar and nonpolar solvents to synthesize perovskite NCs [8,9]. Because of the synthesis temperature difference and presence of a polar solvent during the LARP method, the growth conditions of the NCs in the LARP method differ from those in the typical HI method, which does not use polar solvents [10,11]. Various growth conditions such as adjusting the cation ratio, adding water or ligand, and modifying the ligand-to-precursor ratio have been investigated to control the optical properties of the LHP NCs synthesized via the LARP method [1215]. However, the influence of the precursor dissolution time on the optical properties of the LHP NCs via the LARP method has not been thoroughly investigated. Furthermore, the clarification of the growth kinetics based on the precursor dissolution time is crucial for the synthesis of LHP NCs.

In this study, we investigate the effect of the dissolution time of the precursor powder in a polar solvent on the optical properties of CsPbBr3 NCs prepared via the LARP method. By varying the dissolution time, we could control the precursor concentration of the solution; this helped tune the bandgap of the as-prepared CsPbBr3 NCs. Further, we investigated the growth kinetics of the LARP method. The optical properties were measured using ultraviolet–visible (UV–Vis) absorption spectroscopy and PL spectroscopy. The bandgap energy and size of the NCs were estimated using the Tauc plots and effective mass approximation (EMA).

Cesium bromide (CsBr, 99.999 %), lead bromide (PbBr2, ≥98 %), N, N-Dimethylformamide (DMF, 99.8 %), toluene (99.8 %), and oleylamine (OAm, 70 %) were purchased from Sigma-Aldrich. Oleic acid (OA, 95 %) was purchased from Daejung Chemicals. All chemicals were used as received without purification. CsPbBr3 NCs were synthesized at room temperature and in ambient atmosphere as follows. The precursor solution was prepared by mixing CsBr (0.085 g, 0.4 mmol), PbBr2 (0.147 g, 0.4 mmol), and DMF (10 mL), followed by stirring at 1,000 rpm for each dissolution time (10, 15, 20, and 25 min). OA (1 mL) and OAm (0.5 mL) were injected as the organic ligands after the desired dissolution time and allowed to react for 10 min. After the reaction, the precursor solution (0.5 mL) was rapidly injected into toluene (10 mL) with vigorous stirring for 5 s. The optical properties of the as-synthesized CsPbBr3 NCs were investigated via UV–Vis absorption and PL measurements using a FLAME-S spectrophotometer (Ocean Optics Inc., Largo, FL, USA).

A schematic of the synthesis process for different dissolution times is presented in Fig. 1(a). The precursor CsBr and PbBr2 were dissolved in DMF for 10, 15, 20, and 25 min; subsequently, the organic ligands were added to each solution [1]. The concentration of the dissolved precursor in the solution increases with increasing dissolution time. Simultaneously, the reaction time for the ligands was maintained at 10 min for each sample. Toluene exhibits a lower solubility than does the DMF precursor solution; therefore, the precursors rapidly precipitated into the CsPbBr3 NCs after injection because of the decreased solubility of the mixed solution [1]. The LaMer model can be used to explain this precipitation process: when the amount of monomer exceeds the critical monomer concentration, burst nucleation of the NCs occurs, following which the NCs would consume the monomers to grow [10]. After the monomers were depleted, the growth of the NCs will be terminated, resulting in the specific size. However, this growth process can be altered by adjusting the ratio and amounts of precursors and ligands [9,15]. The ligand-to-precursor ratio decreased with increasing dissolution time under fixed ligand concentration conditions. This is because of the increased precursor concentration in the solution. A higher ligand-to-precursor ratio inhibits monomer addition for NC formation as ligands occupy the NC surfaces [16]. Conversely, a lower ligand concentration results in more dynamic ligands on the NC surfaces, thereby promoting NC growth [17]. Therefore, reduction of the ligand-to-precursor ratio by increasing the dissolution time promotes monomer addition into the NCs, thus increasing their size [9,15,16]. A sample image of the as-prepared CsPbBr3 NCs is shown in Fig. 1(b), and the PL intensity of the as-prepared CsPbBr3 NCs with increasing dissolution time is shown in Fig. 1(c). As the dissolution time increases, the emission color changes from cyan to green.

Figure 1. (a) Schematic of the synthesis process of the CsPbBr3 NCs via the LARP method. (b) Image of the CsPbBr3 NCs under a UV light source with the different precursor dissolution times. (c) the PL spectra of the CsPbBr3 NCs with different precursor dissolution times.

These color changes were also observed in the PL and absorption spectra of CsPbBr3 at different dissolution times [Figs. 2(a) and 2(b), respectively]. With increasing dissolution time, redshifts were observed in the PL and absorption peaks of the CsPbBr3 NCs owing to the quantum size effect [2]. Figure 2(c) shows the chromatic changes in the Commission Internationale de l’Eclairage (CIE) chromaticity diagram. The corresponding (x, y) coordinates of the diagram are (0.0581, 0.6911), (0.0671, 0.7219), (0.0848, 0.7508), and (0.0969, 0.7716) with increasing dissolution times. This change is attributable to the increased size resulting from prolonged dissolution time.

Figure 2. (a) PL and (b) absorption spectra of the CsPbBr3 NCs with different precursor dissolution times. (c) CIE chromaticity diagram of the CsPbBr3 NCs with different precursor dissolution times. The inset shows the enlarged view of the specified area within the CIE chromaticity diagram.

The PL peaks were located at 509.5, 511.9, 515.3, and 518.0 nm, and the absorption peaks were located at 475.7, 487.6, 497.1, and 502.3 nm, for dissolution times of 10, 15, 20, and 25 min, respectively [Fig. 3(a)]. During the growth of the NCs, size-focusing behavior occurs, narrowing the size distribution of the NCs. This is because smaller NCs exhibit a faster growth rate than do larger ones [18,19]. However, in the case of CsPbBr3, the growth process ends within a few seconds because of its rapid growth rate caused by the high diffusion coefficient of bromine ions [19]. This rapid growth of CsPbBr3 NCs rapidly depletes the monomers in the mixed solution. Over time, the monomer depletion can lead to size defocusing, also known as Ostwald ripening. During this process, NCs smaller than the critical radius tend to dissolve, while those larger than the critical radius tend to grow. Consequently, this broadened the size distribution of the NCs [19]. As evident from Fig. 3(b), the FWHM decreased from 103 to 91 meV as the dissolution time increased; this is attributable to the narrower size distribution with time. This phenomenon is related to the increased precursor concentration caused by an increase in the dissolution time, which prevents size defocusing. Prior studies have indicated that higher monomer concentrations can prevent the size-defocusing behavior, resulting in a narrower size distribution; this is consistent with our results [19,20]. The Stokes shift decreased significantly from 171.65 to 73.91 meV as the dissolution time increased, as shown in Fig. 3(b). This size-dependent Stokes shift is related to the confined hole state (CHS) near the valence band edge (VBE) state. The CHS is bright in the case of emission and dark in the case of absorption because of its low density of states. Therefore, emissions occur between the conduction band edge and CHS, whereas absorption occurs between the band edges. Additionally, the CHS became increasingly separated from the VBE as its size decreased. Consequently, these combined effects led to the size-dependent Stokes shift [21].

Figure 3. (a) PL and absorption peak position of the CsPbBr3 NCs as a function of precursor dissolution time. (b) the Stokes shift and the FWHM of the CsPbBr3 NCs as a function of precursor dissolution time.

The confined bandgap energy of the CsPbBr3 NCs was calculated using a Tauc plot [Fig. 4(a)]. The confined size of the NCs changes their bandgap structure [22]. Calculation of their bandgap energies is crucial for analyzing the properties and further applications of the NCs. The Tauc plot is a useful tool for calculating the bandgap energy of NCs. In the Tauc plot, the absorption coefficient (a) and the incident photon energy (hv) are used to calculate the bandgap energy. The bandgap energy was obtained by plotting hv on the x-axis and (ahv)2 on the y-axis. The linear region of the Tauc plot was extrapolated to the x-axis to determine the bandgap energy [23]. The obtained bandgap energy values are 2.41, 2.39, 2.38, and 2.37 eV for the dissolution times of 10, 15, 20, and 25 min, respectively. Therefore, the bandgap energy was confirmed to have decreased as the dissolution time increased. The sizes of the CsPbBr3 NCs and the corresponding dissolution times are shown in Fig. 4(b). The size of the CsPbBr3 NCs was calculated using the EMA, as shown in Eq. (1) [2224].

Figure 4. (a) Tauc plot of the CsPbBr3 NCs absorption spectra. The dotted line indicates the optical bandgap of the CsPbBr3 NCs with different precursor dissolution times. (b) Size estimation of the CsPbBr3 NCs with different precursor dissolution times. The size estimation was performed using the calculated energy spacing and the EMA.

ΔE=EgEbulk=h28m*r2=h28r21me*+1mh*,

where Eg is the energy gap of the CsPbBr3 NCs; Ebulk is the energy gap of the bulk CsPbBr3 crystals; h is Planck’s constant; m* is the reduced mass of the exciton; r is the radius of the NCs; and me* and mh* are the effective electron and hole masses, respectively. The energy spacing (ΔE) was calculated on the basis of the following parameters: Ebulk = 2.25 eV, me* = 0.15 eV, and mh* = 0.14 eV [2,24]. As the confined energy bandgap was obtained from the Tauc plot, the EMA equation can now be solved as a function of the radius. The estimated radii of the perovskite NCs were 5.7, 6.0, 6.2, and 6.6 nm for dissolution times of 10, 15, 20, and 25 min, respectively; evidently, the radius was smaller than the exciton Bohr radius of CsPbBr3 (~7 nm) [2]. This result indicates that increasing the dissolution time leads to increased NC sizes and lowered bandgap energy owing to the quantum size effect.

We investigated the optical properties of CsPbBr3 NCs synthesized using the LARP method with different precursor dissolution times. As the dissolution time increased, we observed a redshift in both the PL and absorption spectra; this was attributed to the change in the ligand-to-precursor ratio with extended dissolution time. The FWHM was reduced because the increased precursor concentration helps prevent the size defocusing of the NCs. Moreover, a size-dependent Stokes shift was observed due to the difference between CHS and VBE state. We estimated the size of the CsPbBr3 NCs by using the Tauc plot and EMA. The bandgap was found to decrease from 2.41 to 2.37 eV as the size increased from 5.7 to 6.6 nm. These results afford valuable insights into the growth kinetics of the LARP method.

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2021R1A2C1003074, 2022R1A4A1033358).

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