All-Inorganic CsPbBr3 Perovskite Nanocrystals Synthesized with Olive Oil and Oleylamine at Room Temperature

Room temperature (RT) synthesis of the ternary cesium lead bromide CsPbBr3 quantum dots with oleic acid and oleylamine ligands was developed by Zeng and coworkers in 2016. In their works, the supersaturated recrystallization (SR) was adopted as a processing method without requiring inert gas and high-temperature injection. However, the oleic acid ligand for haloplumbate is known to be relatively unstable. Hence, in this work, we employed the eco-friendly olive oil to replace the oleic acid portion for the SR process at RT. Resultantly, we found that the cube-shaped nanocrystal has a size of ~40–42 nm and an optical bandgap of ~2.3 eV independent of the surface ligands, but the photoluminescence lifetime (τav) and crystal packing are dependent on the ligand species, e.g., τav = 3.228 ns (olive oil and oleylamine; here less ordered) vs. 1.167 ns (oleic acid and oleylamine). Importantly, we explain the SR mechanism from the viewpoint of the classical LaMer model combined with the solvent engineering technique in details.


Synthesis of CsPbBr 3 Nanocrystals
In the SR method [52], the perovskite precursors and ligands, CsBr (0.2 mmol, 43 mg), PbBr 2 (0.2 mmol, 73 mg), OA or OO (0.5 mL), and 0.25 mL of oleylamine, were dissolved into 5 mL of a polar DMF solvent and stirred using magnetic stirrers at RT for two hours. Then, 1 mL of the perovskite precursor solution was titrated into 10 mL of a nonpolar toluene antisolvent and stirred for 60 s. Then, the CsPbBr 3 NC dispersion exhibited a green-light emission under 365 nm UV illuminations, confirming that the reaction was successful through the SR method. Then, this colloidal dispersion was transferred to a centrifuge tube for centrifugations, and it was centrifuged at 8000 rpm for 10 min. The supernatant was discarded, and the resulting precipitate was re-dispersed in the nonpolar hexane. Then, the final CsPbBr 3 sample was dried under a vacuum oven at 60 • C overnight. Finally, the CsPbBr 3 samples were kept for further characterization at RT.

Characterization
The structural properties of CsPbBr 3 (OA and OAm) and CsPbBr 3 (OO and OAm) were determined by the X-ray diffraction (XRD) measurements measured using the Rigaku mini flex-300/600 diffractometer (Tokyo, Japan). The irradiation used CuKα, λ = 1.5406 Å at 40 kV and 15 mA for the XRD analysis. The CsPbBr 3 NC's size and shape were determined by using the high-resolution transmission electron microscopy (HR-TEM) (Model: JEOL, JEM-2100) with an operating voltage of 200 kV. The optical absorption was characterized by using the Ultraviolet-visible (UV-vis) absorption spectroscopy (SHIMADZU UV-2600, Kyoto, Japan), whereas the photoluminescence (PL) spectra were measured using a spectrophotometer (SHIMADZU RF-6000, Kyoto, Japan) at the excitation wavelength of 375 nm. The Fourier transform infrared spectroscopy (FT-IR) analysis was performed by using the PerkinElmer Spectrum Two FT-IR Spectrometer. For this purpose, the attenuated total reflection (ATR) was employed as a sampling technique to record the FT-IR spectra of the drop-cast CsPbBr 3 thin film in the range 4000-400 cm −1 with resolution of 4 cm −1 . Here, for the drop-cast thin films, the glass substrate was washed in detergent soap, followed by ultra-sonication using distilled water, acetone, and isopropanol, respectively, for 15 min per each process. Then, the cleaned substrate was dried at 80 • C for 1 h in vacuum oven and subsequently cooled down at room temperature. The prepared sample solution (10 mg perovskite precursors in 1 mL of toluene) was drop cast on the clean glass substrate and dried in ambient conditions. The PL decay curves were recorded by using a time-correlated single photon counting (TCSPC) (Model: Fluorolog 3 TCSPC, Horiba, Irvine, CA, USA).

Computational Methods
The electronic band structures of the CsPbBr 3 unit cell were calculated using the Cambridge Serial Total Energy Package (CASTEP, Materials Studio 2017) software with the density functional theory. The Perdew-Burke-Ernzerhof (PBE) parametrization of the General Gradient Approximation (GGA) was used to describe the exchange correla-tion functional. The unit cell in the Brillouin zone was applied to evaluate the electronic band structures. Energy of 5 × 10 −5 eV/atom, maximum force of 0.01 eV/Å, maximum displacement of 5 × 10 −4 Å, and maximum stress of 0.02 GPa were used for all geometry optimization. Figure 1a shows the orthorhombic structure (space group, Pbnm) of CsPbBr 3 crystal. However, when temperature increased, CsPbBr 3 undergoes a phase transition from orthorhombic to tetragonal (P4/mbm) at 88 • C and from tetragonal to cubic (Pm3m) at 130 • C through a tiny rearrangement of the unit cell atoms [60,61]. On the other hand, in the case of NCs, the unit cell structure could be affected not only by crystal size/shape, but also by the Pb 2+ /ligand ratio [34]. For example, the cubic phase was observed at RT if the crystal size is in nanoscale, indicating the metastable state of NCs. In the case of the electronic structures of CsPbBr 3 unit cell, the information could be found in the Supplementary Materials (SM), Figure S1, displaying that the bandgap (E g ) is reduced with from 2.40 eV (cubic) to 2.31 eV (orthorhombic) via 2.37 eV (tetragonal).

Computational Methods
The electronic band structures of the CsPbBr3 unit cell were calculated using the Cambridge Serial Total Energy Package (CASTEP, Materials Studio 2017) software with the density functional theory. The Perdew-Burke-Ernzerhof (PBE) parametrization of the General Gradient Approximation (GGA) was used to describe the exchange correlation functional. The unit cell in the Brillouin zone was applied to evaluate the electronic band structures. Energy of 5 × 10 −5 eV/atom, maximum force of 0.01 eV/Å, maximum displacement of 5 × 10 −4 Å, and maximum stress of 0.02 GPa were used for all geometry optimization. Figure 1a shows the orthorhombic structure (space group, Pbnm ) of CsPbBr3 crystal. However, when temperature increased, CsPbBr3 undergoes a phase transition from orthorhombic to tetragonal ( 4 P mbm) at 88 °C and from tetragonal to cubic ( 3 Pm m ) at 130 °C through a tiny rearrangement of the unit cell atoms [60,61]. On the other hand, in the case of NCs, the unit cell structure could be affected not only by crystal size/shape, but also by the Pb 2+ /ligand ratio [34]. For example, the cubic phase was observed at RT if the crystal size is in nanoscale, indicating the metastable state of NCs. In the case of the electronic structures of CsPbBr3 unit cell, the information could be found in the Supplementary Materials (SM), Figure S1, displaying that the bandgap (Eg) is reduced with from 2.40 eV (cubic) to 2.31 eV (orthorhombic) via 2.37 eV (tetragonal).  Figure 2 shows the chemical structure of (a) OO, (b) OAm, and (c) DMF and toluene, among which both organic OO and OAm surface ligands were employed as a mixed ligand for the synthesis of CsPbBr3 at RT using the SR process [52]. As shown in Figure 2a, the OO's glycerol fractions (~98% of total OO) are composed of oleic acid, linoleic acid, palmitic acid, and palmitoleic acid, but it also contains small amounts of non-glycerol fraction (~1-2% of the total OO by weight), such as biophenols and triterpenic acids [62]. Here, because of the conformational freedom (e.g., a chain folding) of the C-C single-bond rotation, the high carbon ligand may undergo a chain folding in a typical solvent medium, DMF. Importantly, it is noteworthy that the compositional engineering (or mixing strategy) has been commonly used for perovskite electronics for enhancing stability, e.g., cation mixtures (MA + /FA + ), metal ion mixtures (Pb 2+ /Sn 2+ or Ge 2+ /Sn 2+ ), halogen mixtures (Cl − /Br − /I − ), perovskite dimensional mixtures (2D/3D), etc.,  Figure 2 shows the chemical structure of (a) OO, (b) OAm, and (c) DMF and toluene, among which both organic OO and OAm surface ligands were employed as a mixed ligand for the synthesis of CsPbBr 3 at RT using the SR process [52]. As shown in Figure 2a, the OO's glycerol fractions (~98% of total OO) are composed of oleic acid, linoleic acid, palmitic acid, and palmitoleic acid, but it also contains small amounts of non-glycerol fraction (~1-2% of the total OO by weight), such as biophenols and triterpenic acids [62]. Here, because of the conformational freedom (e.g., a chain folding) of the C-C single-bond rotation, the high carbon ligand may undergo a chain folding in a typical solvent medium, DMF. Importantly, it is noteworthy that the compositional engineering (or mixing strategy) has been commonly used for perovskite electronics for enhancing stability, e.g., cation mixtures (MA + /FA + ), metal ion mixtures (Pb 2+ /Sn 2+ or Ge 2+ /Sn 2+ ), halogen mixtures (Cl − /Br − /I − ), perovskite dimensional mixtures (2D/3D), etc., which resulting in the enhancement of stability of materials and devices [62][63][64][65][66][67]. In the same vein, when we use the edible OO as a surface ligand, it might be useful in stability with a guaranteed benefit of eco-friendliness, which is a desirable characteristic for a mass production process in future. Figure 3a shows the free energy change (∆G) as a function of the radius (r) when the nucleus is assumed to be a spherical according to the LaMer model, a classical nucleation theory [56,57]. Here, the free energy change (∆G hom ) for homogenous nucleation and that (∆G het ) for heterogeneous nucleation could be defined as follows, which resulting in the enhancement of stability of materials and devices [62][63][64][65][66][67]. In the same vein, when we use the edible OO as a surface ligand, it might be useful in stability with a guaranteed benefit of eco-friendliness, which is a desirable characteristic for a mass production process in future.  Figure 3a shows the free energy change ( G Δ ) as a function of the radius ( r ) when the nucleus is assumed to be a spherical according to the LaMer model, a classical nucleation theory [56,57]. Here, the free energy change ( hom G Δ ) for homogenous nucleation and that ( het G Δ ) for heterogeneous nucleation could be defined as follows,

Results and Discussion
where ∆G v is the free energy change associated with the formation of a small volume of solid, whereas γ SL is the solid/liquid interfacial free energy. Furthermore, ∆G het = ∆G hom · S(θ), where S(θ) = (2 + cos θ)(1 − cos θ) 2 /4 is a shape factor at a wetting angle θ. According to the LaMer model [57], when a solution is supersaturated by undercooling, a nucleus could be formed. If this nucleus is larger than the critical size (r * ), the spontaneous growth is available because ∆G is decreasing as shown in Figure 3a (see the blue solid line). In the case of the SR process, a supersaturated condition is directly available when the perovskite precursor solution (a colloidal dispersion containing bromide plumbate, PbBr 2−n n with n = 2-6) is dropped into the antisolvent medium, e.g., toluene with Gutmann's number (D N )~0.1 kcal/mol, i.e., Lewis basicity (see Table 1 for details) [68]. Here, it is noteworthy that a nonpolar toluene with solubility parameter, δ = 8.9 (cal/cm 3 ) 1/2 is antisolvent for the bromide plumbate but it is still miscible with the polar DMF solvent (δ = 12.1 (cal/cm 3 ) 1/2 ) [69], indicating that the perovskite precursor ions (Cs + , Pb 2+ , Br − ) will be directly exposed to the antisolvent toluene molecules, resulting in the nucleation and growth of perovskite NCs as explained through the critical point (r*-G*) in the LaMer model. However, it is notable that in the case of SR process, the supersaturation was reached simply by adding the precursor solution into the antisolvent (not by undercooling like in the LaMer model). Interestingly, the SR process is similar to the well-known Solvent Engineering (SE) (i.e., one-step antisolvent method) in the sense that the wet precursor solution is directly exposed to the antisolvent molecules [58,59]. However, the difference is that the SE method is processed during the spin-coating step by dispensing the antisolvent on the top of a wet precursor solution. Furthermore, SE allows for the formation of intermediate phase when a polar solvent DMSO is employed, whereas SR may not allow for it because the colloidal dispersion may undergo a direct crystallization (at least without a co-solvent such as DMSO). Importantly, the SR process corresponds to the heterogeneous nucleation and growth of CsPbBr 3 perovskite because the nanoscale crystallization was processed through the surface of haloplumbate dispersed in the DMF medium. Recall that the perovskite precursor solution is a typical colloidal dispersion [70], i.e., haloplumbate is dispersed in a solvent medium, herein DMF with D N = 26.6 kcal/mol. In the case of Br-, its D N is 33.7 kcal/mol [71], indicating that Pb 2+ (Lewis acid) may have a stronger coordination bonding with Br − ions (Lewis base) than DMF to form the acid-base adducts in spite that DMF is a good solvent for the perovskite precursor materials. See Figure    , respectively, when the X-ray wavelength (λ) is 1.5406 Å. Probably, the CsPbBr3 NC may prefer to exist in cubic phase due to its high surface energy and capping ligand effects. Furthermore, all the drop-cast CsPbBr3 nanocrystalline thin films on the glass substrate display two intense peaks at 2θ = 16° and 32°, corresponding to (100) and (200) crystallographic planes, respectively, indicating that there is orientational order to the (100) direction. However, when the OO and OAm ligands were incorporated as a ligand system for CsPbBr3, the additional (110) peak is clearly observed, indi-    (200) are more corresponding to that of cubic (JCPDS: PDF#96-153-0682: space group Pm3m) instead of orthorhombic in spite that it was synthesized at RT [29,55,72,73]. Note that the lattice constant is 5.6050 Å based on the above JCPDS data, which was confirmed by the d-spacing of~5.478 Å or 5.418 Å = λ/(2 · sin θ) at θ = 8.08 • (OA) or 8.17 • (OO), respectively, when the X-ray wavelength (λ) is 1.5406 Å. Probably, the CsPbBr 3 NC may prefer to exist in cubic phase due to its high surface energy and capping ligand effects. Furthermore, all the drop-cast CsPbBr 3 nanocrystalline thin films on the glass substrate display two intense peaks at 2θ = 16 • and 32 • , corresponding to (100) and (200) crystallographic planes, respectively, indicating that there is orientational order to the (100) direction. However, when the OO and OAm ligands were incorporated as a ligand system for CsPbBr 3 , the additional (110) peak is clearly observed, indicating that the mixed ligand system by OO's glycerol fraction makes the orientational order be reduced, resulting in a partial amorphous halo (see the baseline shape centered around (110) peak). However, when we estimated the crystallite size (t) by Scherrer equation (Equation (3)), it was in the range of~40-42 nm for all the samples ( Table 2).
where λ (= 0.154 nm) is the X-ray wavelength, and B is the full width at half maximum (FWHM) at the angle θ. Here, it is noteworthy is that the estimated crystallite size is a direct particle size of this single crystal in nanoscale, which will be further addressed in the below section.      Figure 5 shows HR-TEM images for CsPbBr 3 depending on the surface ligand system. Figure 5a,b show the case of OA and OAm, whereas Figure 5c,d show the case of OO and OAm. As shown in Figure 5, the TEM images show roughly a nano cubic, which is in line with the XRD data exhibiting the cubic structure.
Micromachines 2023, 14, x FOR PEER REVIEW 9 of 15 Figure 5 shows HR-TEM images for CsPbBr3 depending on the surface ligand system. Figure 5a,b show the case of OA and OAm, whereas Figure 5c,d show the case of OO and OAm. As shown in Figure 5, the TEM images show roughly a nano cubic, which is in line with the XRD data exhibiting the cubic structure.  Here, it is noteworthy that the CsPbBr3 perovskite is a direct semiconductor. In addition, the synthesized CsPbBr3 NCs do not display any grain boundary in Figure 5, indicating a single crystalline nature in nanoscale although there might be some surface defects. Therefore, the CsPbBr3 NCs are different from the disordered conjugated polymers showing a localized state. For disordered system, Mott defined a mobility edge between localized and delocalized states [74]. However, in the case of CsPbBr3 NCs, the sharp absorption edge is expected, affording the decision of optical bandgap by using a  Here, it is noteworthy that the CsPbBr 3 perovskite is a direct semiconductor. In addition, the synthesized CsPbBr 3 NCs do not display any grain boundary in Figure 5, indicating a single crystalline nature in nanoscale although there might be some surface defects. Therefore, the CsPbBr 3 NCs are different from the disordered conjugated polymers showing a localized state. For disordered system, Mott defined a mobility edge between localized and delocalized states [74]. However, in the case of CsPbBr 3 NCs, the sharp absorption edge is expected, affording the decision of optical bandgap by using a simple tangential slope based on the equation, E = hc/λ, where E is energy, h is Planck's constant, c is the speed of light, and λ is the wavelength of light. Therefore, according to the UV-Vis absorption edge (530 nm vs. 533 nm) in Figure 6a, the corresponding optical bandgap (E g ) is 2.34 eV = 1240/530 (for OA and OAm) and 2.33 eV = 1240/533 (for OO and OAm), respectively. See Equation (4) for the conversion factor, 1240, between the energy (bandgap) and the wavelength (absorption edge). 34 8 -9 19 6.62607 10 J s 3 10 m s 10 nm 1m 1 nm 1240 eV eV 1.60218 10 J 1 eV This result indicates that the nanoparticle size might be slightly larger in CsPbBr3 synthesized with OO and OAm. This result is in line with the estimated crystallite size trend according to the sharpest peak at the (200) crystallographic plane in Figure 4a and Table 2. On the other hand, the PL spectra exhibit the PL peak at 517 nm for CsPbBr3 with oleic acid and oleylamine and at 526 nm for CsPbBr3 with OO and OAm. Here, the shift of PL peak from 517 nm to 526 nm (a red shift) indicates that the effective crystal size (or aggregation) of CsPbBr3 NCs is slightly larger in the colloidal dispersion state when OO and OAm were employed as the surface ligands, although the apparent size ( Table 2) is similar for both ligand systems in the solid state. Furthermore, it is believed that the surface state of NCs is different depending on the NC-ligand complexations. Hence, when OO and OAm were complexed with CsPbBr3 NCs, there might be more surface defects, resulting in more non-radiative recombination (i.e., smaller PL intensity) in Figure 6b. Furthermore, FWHM is 21 nm for the former, but 19 nm for the latter. According to our finding, the narrow emission spectra were obtained below 25 nm, which falls in the required range of 12-42 nm [51] for the optoelectronic applications such as lighting and display technology.   Figure 7b shows the FT-IR spectra for the drop-cast CsPbBr3 nanocrystalline thin film. First of all, the IR spectrum for CsPbBr3 with OA and OAm is relatively simpler than that for CsPbBr3 with OO and OAm. Second, the FT-IR spectrum for OO and OAm shows a high absorption at 3020-2850 cm −1 , whereas the corresponding spectrum for OA and OAm is very weak at those wavenumbers, which correspond to the symmetric and asymmetric C-H stretching modes [53,54]. The functional groups of other organic components can be attributed to the absorption peaks at 3781 cm −1 [54]  This result indicates that the nanoparticle size might be slightly larger in CsPbBr 3 synthesized with OO and OAm. This result is in line with the estimated crystallite size trend according to the sharpest peak at the (200) crystallographic plane in Figure 4a and Table 2. On the other hand, the PL spectra exhibit the PL peak at 517 nm for CsPbBr 3 with oleic acid and oleylamine and at 526 nm for CsPbBr 3 with OO and OAm. Here, the shift of PL peak from 517 nm to 526 nm (a red shift) indicates that the effective crystal size (or aggregation) of CsPbBr 3 NCs is slightly larger in the colloidal dispersion state when OO and OAm were employed as the surface ligands, although the apparent size (Table 2) is similar for both ligand systems in the solid state. Furthermore, it is believed that the surface state of NCs is different depending on the NC-ligand complexations. Hence, when OO and OAm were complexed with CsPbBr 3 NCs, there might be more surface defects, resulting in more non-radiative recombination (i.e., smaller PL intensity) in Figure 6b. Furthermore, FWHM is 21 nm for the former, but 19 nm for the latter. According to our finding, the narrow emission spectra were obtained below 25 nm, which falls in the required range of 12-42 nm [51] for the optoelectronic applications such as lighting and display technology. Figure 7a shows PL decay curve for the colloidal CsPbBr 3 NC dispersion as a function of ligand species. The average lifetime of electron-hole pairs (Wannier-Mott excitons) is 1.167 ns for CsPbBr 3 with OA and OAm, whereas it is 3.228 ns for CsPbBr 3 with OO and OAm, indicating that a slightly larger CsPbBr 3 with OO and OAm displays a longer life time of excitons. Figure 7b shows the FT-IR spectra for the drop-cast CsPbBr 3 nanocrystalline thin film. First of all, the IR spectrum for CsPbBr 3 with OA and OAm is relatively simpler than that for CsPbBr 3 with OO and OAm. Second, the FT-IR spectrum for OO and OAm shows a high absorption at 3020-2850 cm −1 , whereas the corresponding spectrum for OA and OAm is very weak at those wavenumbers, which correspond to the symmetric and asymmetric C-H stretching modes [53,54]. The functional groups of other organic components can be attributed to the absorption peaks at 3781 cm −1 [54] and 3510 cm −1 [53], which originate from the N-H characteristic absorption. The IR peaks at around 1800-1600 cm −1 are related to carbonyl (C=O) mode of stretching vibration which can be shift to lower wavenumber due to any perturbation. The shift of wavenumber may be ascribed to the presence of hydrogen bonding. The peaks at 1538 cm −1 and 1462 cm −1 are related to C-CH 3 asymmetric bending. On the other hand, the peaks at 1377 cm −1 and 1262 cm −1 correspond to C-CH 3 symmetric bending vibration and C-N stretching vibration, respectively. Absorption peaks in the range of 1000-500 cm −1 indicate that the existence of aromatic bending vibrations [75], which might originate from OO components, trapped toluene (C 6 H 5 CH 3 ), and/or aromatic impurities in samples. The peak at 477-447 cm −1 is associated with CsPbBr 3 -OAm complexes [76]. and 3510 cm −1 [53], which originate from the N-H characteristic absorption. The IR peaks at around 1800-1600 cm −1 are related to carbonyl (C=O) mode of stretching vibration which can be shift to lower wavenumber due to any perturbation. The shift of wavenumber may be ascribed to the presence of hydrogen bonding. The peaks at 1538 cm −1 and 1462 cm −1 are related to C-CH3 asymmetric bending. On the other hand, the peaks at 1377 cm −1 and 1262 cm −1 correspond to C-CH3 symmetric bending vibration and C-N stretching vibration, respectively. Absorption peaks in the range of 1000-500 cm −1 indicate that the existence of aromatic bending vibrations [75], which might originate from OO components, trapped toluene (C6H5CH3), and/or aromatic impurities in samples. The peak at 477-447 cm −1 is associated with CsPbBr3-OAm complexes [76].

Conclusions
In this work, we demonstrated the usefulness of olive oil and oleylamine as the surface ligand system (vs. the typical oleic acid and oleylamine mixtures) for the synthesis of CsPbBr3 nanocrystals at room temperature. For this purpose, the supersaturated recrystallization process was employed, enabling a direct synthesis of perovskite nanocrystals. First, according to XRD, the crystallite size is ~40-42 nm in both conditions (oleic acid and oleylamine vs. olive oil and oleylamine), whereas the latter system allows for a less orientational order than the former by showing a partial amorphous halo. The TEM images confirmed that the CsPbBr3 nanocrystals have a cuboid shape in line with XRD data exhibiting a cubic phase. The UV-Vis absorption data exhibited that the optical bandgap of CsPbBr3 is about 2.3 eV in both ligand systems. The PL emission spectra indicated that FWHM is about 19 nm (olive oil and oleylamine) vs. 21 nm (oleic acid and oleylamine), which falls into the desirable FWHM values, i.e., ~12-42 nm for technical applications. Finally, the average PL lifetime is 3.228 ns for olive oil and oleylamine and 1.167 ns for oleic acid and oleylamine. Future work may include the application of CsP-bBr3 nanocrystals to the biosensors and optoelectronic devices.
Supplementary Materials: The following are available online at: www.mdpi.com/xxx/s1, Figure  S1: The electronic structures of CsPbBr3 unit cells displaying polymorphism: (a) orthorhombic, (b) tetragonal, and (c) cubic phase. Here, the bandgap energy (Eg) of each unit cell is 2.31 eV for orthorhombic, 2.37 eV for tetragonal, and 2.40 eV for cubic phase, respectively. Figure S2: Vials con-

Conclusions
In this work, we demonstrated the usefulness of olive oil and oleylamine as the surface ligand system (vs. the typical oleic acid and oleylamine mixtures) for the synthesis of CsPbBr 3 nanocrystals at room temperature. For this purpose, the supersaturated recrystallization process was employed, enabling a direct synthesis of perovskite nanocrystals. First, according to XRD, the crystallite size is~40-42 nm in both conditions (oleic acid and oleylamine vs. olive oil and oleylamine), whereas the latter system allows for a less orientational order than the former by showing a partial amorphous halo. The TEM images confirmed that the CsPbBr 3 nanocrystals have a cuboid shape in line with XRD data exhibiting a cubic phase. The UV-Vis absorption data exhibited that the optical bandgap of CsPbBr 3 is about 2.3 eV in both ligand systems. The PL emission spectra indicated that FWHM is about 19 nm (olive oil and oleylamine) vs. 21 nm (oleic acid and oleylamine), which falls into the desirable FWHM values, i.e.,~12-42 nm for technical applications. Finally, the average PL lifetime is 3.228 ns for olive oil and oleylamine and 1.167 ns for oleic acid and oleylamine. Future work may include the application of CsPbBr 3 nanocrystals to the biosensors and optoelectronic devices.

Supplementary Materials:
The following are available online at: https://www.mdpi.com/article/ 10.3390/mi14071332/s1, Figure S1: The electronic structures of CsPbBr 3 unit cells displaying polymorphism: (a) orthorhombic, (b) tetragonal, and (c) cubic phase. Here, the bandgap energy (E g ) of each unit cell is 2.31 eV for orthorhombic, 2.37 eV for tetragonal, and 2.40 eV for cubic phase, respectively. Figure

Data Availability Statement:
The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.