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Abstract
Organic-inorganic hybrid perovskite nanocrystals have been widely studied for their excellent photoelectric properties. However, the irregular morphologies of organic-inorganic hybrid perovskite nanocrystals have limited application in the field of lighting and display. From this, the regular morphologies of nanospheres, nanorods, nanoplatelets and MAPbBr3 (MA = CH3NH3+) nanocrystals have been synthesized by regulating the type and proportion of auxiliary ligands. The phase evolution, morphology and fluorescent properties were systematically studied by the various instruments of XRD, TEM, PL/UV-vis spectroscopy and fluorescence decay analysis. With the morphologies changing from nanospheres to nanoplatelets, the emission peaks of MAPbBr3 nanocrystals red-shifted, and the lifetimes have increased gradually. The underlying mechanisms were thoroughly investigated and elucidated. On this basis, the role of acid and amine in the synthesis of MAPbBr3 nanocrystals was systematically studied by regulating the ratio of oleic acid and N-octylamine. The fluorescence kinetics of MAPbBr3 nanocrystals were studied by femtosecond transient absorption spectroscopy, and the charge carrier relaxation mechanism was clarified. Furthermore, the effect of temperature on the fluorescence properties of the nanocrystal was investigated in detail. Organic-inorganic hybrid perovskite nanocrystals with morphologies-controlled and excellent fluorescence properties are expected to be widely used in lighting and display fields.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Organic-inorganic hybrid perovskite nanocrystal, as a kind of fluorescent material with great application prospect in the field of solar cell and lighting [1–5], has been well studied due to its excellent properties of higher carrier mobility [6,7], longer lifetime, stronger light absorption capacity [8], narrow half peak width and higher photoluminescence quantum yield [9]. The organic-inorganic hybrid perovskite nanocrystal was a composite with direct band gap semiconductors. The diversity of organic molecules can effectively adjust the band gap at the molecular level for organic-inorganic hybrid perovskite nanocrystal [10]. As the result, it can meet the different requirements of light-emitting diode, laser and optical sensor fabrication.
Compared to the traditional quantum dot materials (eg. CdSe and PbS), MAPbX3 (X = Cl-, Br- and I-) perovskite nanocrystals have attached great attention in the field of lighting and display due to its unique photoelectric properties [11–13]. However, the uncontrolled morphologies and their poor chemical stability of MAPbX3 nanocrystals still limited the application [14,15]. The geometry shape and size of nanocrystal directly affect their physical properties, and the ability to control the shapes and sizes can effectively adjust the photoelectric properties due to change the band gap width [16]. For instance, spherical perovskite quantum dots have the same luminescence properties in all directions owing to three-dimensional isotropy in three dimensional spaces, while the nanorod nanocrystals exhibit polarization emission to a certain extent [17,18]. Therefore, it is urgent to synthesize nanocrystal with controllable morphology and uniform size.
Here, the MAPbBr3 nanocrystals can be synthetized by auxiliary ligand precipitation method [19]. Compared to hot injection method [20] and evaporative solvent method [21], they have such advantages as simple operation and low energy consumption. Although MAPbBr3 nanocrystals can be prepared relatively simply by solution processing, they exhibit irregular shapes and sizes, which prevent organic-inorganic hybrid perovskite nanocrystals from being put into the market. On the basis of the above, the MAPbBr3 nanocrystals with the morphologies of nanospheres, nanorods and nanoplatelets were synthesized by changing the type and proportion of ligands [22–24]. The phase evolution, morphologies and fluorescent properties of samples were investigated by XRD, TEM, PL/Uv-vis spectroscopy and fluorescence decay analysis. In the following, the results of MAPbBr3 nanocrystals have been systematically discussed about the phase evolution, morphologies and fluorescent properties.
2. Experiment procedure
2.1 Materials
In a study, methylamine alcohol solution (CH3NH2, AR, Sinopharm Chemical Reagent Co. Ltd., Shanghai, China), hydrobromic acid (HBr, AR, 40%, Macklin), lead bromide (PbBr2, 99.999%, Aladdin), oleic acid (OA, 90%, Macklin), caproic acid (CH3(CH2)4COOH, 90%, Macklin), acetic acid (CH3COOH, 90%, Macklin) N-octylamine (CH3(CH2)6NH2, 90%, Macklin), laurylamine (CH3(CH2)11NH2, 90%, Macklin), N,N-dimethylformamide (DMF, AR, Sinopharm Chemical Reagent Co. Ltd., Shanghai, China), toluene (C7H8, 99%, Sinopharm Chemical Reagent Co. Ltd., Shanghai, China), methanol (CH3OH, AR, Tianjin City Fuyu Fine Chemical Co. Ltd., Tianjin, China), acetone (CH3COCH3, AR, Tianjin City Fuyu Fine Chemical Co. Ltd., Tianjin, China). All reagents do not need purification and can be used directly.
2.2 Preparation procedure
2.2.1 Synthesis of MABr precursor
Methylamine alcohol solution (17.4 mL) was added in methanol (30 mL) for dilution in 100 mL three-mouthed flask. The mixture was cooled to 0∼5 °C while the hydrobromic acid (13.6 mL) was mixed with the rate of 0.5 mL/min. The mixture solution continually reacted for 2 h with the same environment. After the reaction was over, the target products were removed from the mixture solutions by rotary evaporation. Lastly, the depositions were washed thrice with methanol and acetone, and dried at 80 °C for 24 h to obtain white products.
2.2.2 Synthesis of different morphologies of MAPbBr3 nanocrystals
The MABr precursor (0.16 mmol) and PbBr2 (0.2 mmol) were dissolved in 5 mL N, N-dimethylformamide, and the caproic acid (0.05 mL) and N-octylamine (0.066 mL) were added. Then, the mixture solution was taken out 1 mL and added in 10 mL toluene severely stirring. Subsequently, the colloid solution was centrifuged at a rate of 7000 rpm/min to separate solution and precipitation. Lastly, the precipitation was washed by toluene and centrifuged to obtain the nanospheres MAPbBr3 nanocrystals. The nanorods and nanoplatelets MAPbBr3 nanocrystals were synthesized by the same methods, and the different was that the caproic acid (0.05 mL) and N-octylamine (0.066 mL) were changed to acetic acid (0.177 mL), laurylamine (0.04 mL), oleic acid (1.33 mL) and laurylamine (0.04 mL), respectively. On the above basis, in order to further explore the role of the adjuvant ligand in the synthesis of MAPbBr3 nanocrystals, the oleic acid and N-octylamine were set to 0.47 mL, 0.03 mL, 0.37 mL and 0.09 mL, respectively, which obtained the morphologies of nanospheres and nanorods MAPbBr3 nanocrystals.
2.3. Characterization
The phase composition analyses were performed by XRD. And the patterns were recorded with room temperature using nickel-filtered CuKα radiation in the 2θ range 10-50° at a scanning speed of 4.0° 2θ/min (Model D8 ADVANCE, BRUKER Co., Germany). The morphology of resultant products were collected via the HR-Transmission Electron Microscope (HR-TEM) (JEM-2100F, JEOL, Japan). The UV-vis absorption spectra were performed by UV-vis spectrophotometer (UV-3600, Shimadzu Corporation). The photoluminescence (PL) spectra were obtained using a Fluorescence Spectrophotometer (FP-6500, JASCO Co., Japan) at room temperature equipped with a Φ60-mm integrating sphere (ISF-513, JASCO, Tokyo, Japan) and a 150-W Xe-lamp was used as the excitation source. The optical performances for all samples were conducted under identical conditions with the slit breadth of 5 nm. The phosphor powder was excited with a selected wavelength and the intensity of the intended emission was recorded as a function of elapsed time after the excitation light was automatically cut-off using a shutter.
3. Results and discussion
Figures 1(a)-(d) show the different morphologies of MAPbBr3 nanocrystals, which were synthetized by changing the type of ligand. When the types of ligand are caproic acid and N-octylamine, the MAPbBr3 nanocrystals display uniform nanospheres morphology (Fig. 1(a)) and the average sizes are ∼5 nm. The HR-TEM micrograph of nanospheres MAPbBr3 nanocrystals is shown in Fig. 1(b) with the interplanar spacing of 0.187 nm, which corresponds to the (211) lattice plane. The SAED pattern of nanospheres MAPbBr3 nanocrystals possess the (211) plane (in inset of Fig. 1(b)). In like manner, the nanorods (Fig. 1(c)) and nanoplatelets (Fig. 1(d)) of MAPbBr3 nanocrystals were obtained when the ligands are acetic acid, oleic acid and laurylamine, respectively, regardless of containing little of quantum dots.
Fig. 1. The TEM morphologies of MAPbBr3 nanocrystals with auxiliary ligand of caproic acid and N-octylamine (a), acetic acid and laurylamine (c), oleic acid and laurylamine (d), respectively, and Fig. 1(b) is the HR-TEM micrograph of MAPbBr3 nanocrystals with the auxiliary ligand of caproic acid and N-octylamine. The insets of the Figs. 1(a), (c), and (d) show the digital pictures under 365 nm UV excitations from a hand-held UV lamp, and inset of Fig. 1(b) is the SAED pattern of MAPbBr3 nanocrystals. Figure 1(e) shows the effect of the auxiliary ligand type on the XRD patterns of MAPbBr3 nanocrystals.
The XRD patterns of MAPbBr3 nanocrystals with the morphologies of nanospheres, nanorods and nanoplatelets are shown in Fig. 1(e). The MAPbBr3 nanocrystals display the characteristic absorption band of (100), (110), (200), (210), (211), (300) and (310) peaks, which are similar with XRD diffraction behavior in publish reports [25–27]. It is worth noting that the change of morphology does not affect the crystal structure of nanocrystal, and the octahedral perovskite crystal structures are still maintained in three-dimensional space.
Figure 2(a) shows the photoluminescence (PL) and ultraviolet–visible absorption (UV-vis) spectra of nanospheres, nanorods and nanoplatelets MAPbBr3 nanocrystals, among which the PL spectra was obtained via monitoring at 400 nm excitation. It can be seen that the emission peaks and UV absorption peaks of MAPbBr3 nanocrystals have red shift with the morphologies changing from nanospheres to nanoplatelets, and the luminous colors change from cyan to green. This phenomenon is mainly attributed to the changes in the band gap. Owing to the different polarity of ligands, the sizes of nanocrystal increase gradually with the changes of morphologies, resulting in the increase of specific surface area and the decrease of band gap. Thus, the red shifts of emission peak and ultraviolet absorption peak can be observed. Further observation is that the Stokes shifts of MAPbBr3 nanocrystals with the morphologies of nanospheres, nanorods and nanoplatelets are small (5-10 nm). It can be distinguished from the fluorescent dye with large Stokes shift and the background fluorescence. In practical application, the background can be eliminated, and the sensitivity can be improved by adjusting the wavelength of the excited light or using the filter.
Fig. 2. (a) shows the PL behavior and UV-vis spectra as function of the types of auxiliary ligand, among which the PL spectra were obtained via monitoring at 400 nm excitation. (b) is the CIE chromaticity diagram for the different morphologies of MAPbBr3 nanocrystals under 400 nm excitation.
The Commission International de L'Eclairage (CIE) chromaticity coordinates can be calculated in detail, and the results are shown in Fig. 2(b). It can be seen that the color coordinates of MAPbBr3 nanocrystals with the morphologies of nanospheres, nanorods and nanoplatelets are (0.11, 0.28), (0.15, 0.57) and (0.15, 0.67), respectively, and they are consistent with the digital picture (the inset of Figs. 1(a), (c) and (d)) under 365 nm UV excitation from a hand-held UV lamp and the results of emission peaks in Fig. 2(a).
On the basis of the above, the color purity can be calculated by formulas (1) [28].
In order to study the effect of the MAPbBr3 nanocrystal with different morphologies on fluorescence decay behavior, the decay curves of nanospheres, nanorods and nanoplatelets MAPbBr3 nanocrystals are shown in Fig. 3, which are fitted with two-exponent exponential decay functions. The decay times of MAPbBr3 nanocrystals with the nanospheres, nanorods and nanoplatelets are 5.51 ns (Fig. 3(a)), 112.32 ns (Fig. 3(b)) and 215.31 ns (Fig. 3(c)), respectively, which are similar with the reporter in previous papers [4,14,23]. Moreover, the lifetimes gradually increase with the morphologies changing from nanospheres to nanoplatelets. This phenomenon can be attributed to the reasons that the PL attenuation of the MAPbBr3 colloidal nanocrystals mainly occurs in the exciton radiation recombination of the small nanocrystals. When the morphologies changing from nanospheres to nanoplatelets, the specific surface areas increase gradually to make it easier to combine exciton radiation, and thus the fluorescence lifetimes increase. Furthermore, the nanoplatelets nanocrystals are generally less than other shapes of the nano-crystals in defects and surface state emission, so its fluorescence lifetime is the longest.
Fig. 3. Decay curves of the nanospheres (a), nanorods (b) and nanoplatelets (c) MAPbBr3 nanocrystals under 400 nm excitations as function of the types of auxiliary ligand.
On the basis of the above research, in order to further explore the role of the adjuvant ligand in the synthesis of MAPbBr3 nanocrystals, the effect of the proportion of auxiliary ligands on the shape/size of MAPbBr3 nanocrystals is shown in Fig. 4. From which it can be seen that the particles possess good dispersion and uniform morphology. However, the shape can be changed from nanospheres to nanorods as the ratio of oleic acid and octylamine changing from 9:1 to 3:1. The particle sizes increase with the N-octylamine increasing gradually. The reasons can be explained that the dimensional control process of MAPbBr3 nanocrystal dots was mainly attributed to the anisotropic growth of crystals, which caused by the kinetic mechanism. The ligands tend to bind selectively to a crystal facet of MAPbBr3 nanocrystals, resulting in different growth rates on different crystal faces. When the dosage of N-octylamine is high, the N-octylamine easily wraps the surface of MAPbBr3 nanocrystals, and binds to a crystal surface by electrostatic action to inhibit the growth of the crystal surface. Thus, the morphology of MAPbBr3 nanocrystals is easier to form nanorods structure. In contrast, when the amount of N-octylamine is small, the surface of the MAPbBr3 nanocrystals will not be completely covered by the ligand, which is beneficial to the growth of nanospheres. Thus, the shape/size of MAPbBr3 nanocrystals is controlled.
Fig. 4. The TEM images of MAPbBr3 nanocrystals with the ratio of oleic acid and N-octylamine changing from 9:1 (a) to 3:1 (b).
Figure 5(a) shows the curves of photoluminescence and ultraviolet–visible absorption as function of the different ratios of oleic acid and N-octylamine. As shown in Fig. 5(a), the red shifts of emission and ultraviolet absorption peak are observed as the ratio of oleic acid and N-octylamine changing from 9:1 to 3:1, and the reasons stem from quantum size effect which is similar with the result of Fig. 2(a). The color coordinates (x, y) of nanospheres and nanorods MAPbBr3 nanocrystals are analyzed, and the results are (0.10, 0.16) and (0.12, 0.77) for the ratios of oleic acid and N-octylamine at 9:1 to 3:1, respectively, which are derived from calculations in Fig. 5(b). The color purity can be calculated via formulas (1), and they are 96.32% and 97.16%, respectively. The colors are consistent with the digital picture (the inset of Fig. 4) under 365 nm UV excitation from a hand-held UV lamp.
Fig. 5. (a) is the PL behavior and UV-vis spectra as function of the proportion of auxiliary ligand, among which the PL spectra were obtained via monitoring at 400 nm excitation. (b) is the CIE chromaticity diagram for the different morphologies of MAPbBr3 nanocrystals under 400 nm excitation with regulating the ratio of oleic acid and N-octylamine.
The decay curves of MAPbBr3 nanocrystals as function of the different ratios of oleic acid and N-octylamine are shown in Fig. 6. It can be seen that the decay time of nanorods (132.31 ns, Fig. 6(b)) MAPbBr3 nanocrystals is far longer than nanospheres (3.71 ns, Fig. 6(a)), and it is attributable to the increase of geometric structure of in-plane single crystal [29], which are similar with the results of the above analysis in Figs. 3(a) and (b).
Fig. 6. Decay curves of the nanospheres (a) and nanorods (b) MAPbBr3 nanocrystals under 400 nm excitations as function of the proportion of oleic acid and N-octylamine.
In order to better clarify its luminous mechanism, the photon induced fluorescence kinetics have been further studied using the femtosecond transient absorption spectroscopy, which explain the radiative and non-radiative processes. The femtosecond time-resolved transient absorption spectroscopy is a transient absorption spectrum technique and a femtosecond laser-detection technique, which is mainly used to study the superfast kinetics of the excited state in the liquid phase environment. The MAPbBr3 nanocrystals, which were excited via femtosecond-pulsed laser at 400 nm, jumped from ground state to excited state. Meanwhile, another beam of quasi-continuous white light is introduced into the excited state molecule under different time delay to release photons from the excited state back to the ground state, and the change of fluorescence emission intensity under different time delay is investigated. The transient absorption spectra of MAPbBr3 nanocrystals are shown in Fig. 7(a). From which it can be seen that a strong and broad photo bleaching signal (PIB) band center at ∼520 nm at the range of 500-540 nm, which stem from the photo-induced radiative emission. The variation of optical density for the wavelength band of 480-660 nm with the delay times from 28 ps to 30 ps is illustrated in Fig. 7(b). It can be seen that the ground-state bleach (GSB) absorption peak red shifts and the GSB signal increased while the photo-induced absorption (PIA) signal decreased, when the delay times change from 28 ps to 30 ps. These phenomena are similar with the previous reporters [30–32]. The reason can be explained that the long wavelength optical absorption life is longer, and the excited transient signal is easier to be captured by the probe. In addition, the TA spectra show the phenomenon of the band edge transition red shift in the early time due to the double exciton effect. The signal manifested as the PIA at lower energy, and bleaching absorption at higher energy. With the accumulation of carriers in the lowest energy state, the PIA signal will eventually be replaced by strong band edge bleaching via state filling for a long time [33].
Fig. 7. (a) the femtosecond time-resolved transient absorption spectra of MAPbBr3 nanocrystals, excited by a femtosecond-pulsed laser at 400 nm with a pump-light intensity of 2.4 mW. (b) is the TA spectra with delay time from 28 ps to 30 ps (400 pump)
The temperature-dependent PL spectra for MAPbBr3 nanocrystals are shown in Fig. 8. With the temperatures varying from 60 K to 300 K, the emission peaks blue shift and the emission intensities of sample decrease gradually. Moreover, the half-maximum (FWHM) of the sample increases gradually. The main reason can be attributed to lattice shrinkage and electron-phonon coupling. At low temperature, the lattice shrinkage of nanocrystals is severe. With the temperatures increasing from 60 K to 300 K, the electron-phonon coupling in nanocrystals intensifies, and the lattice shrinkage is gradually alleviated, which make the luminous peak of nanocrystal blue shift and its half peak width increases gradually. Furthermore, the defect state or carrier thermal escaping on the surface of nanocrystals increased, leading to thermal quenched. These phenomena are consistent with the temperature-dependent of CdSe and PbS quantum dots [34,35].
Fig. 8. (a) the temperature-dependent PL spectra for MAPbBr3 nanocrystals, (b) the relationship between ln(I0/I−1) and 1/kT for the thermal quenching of MAPbBr3 nanocrystals.
In order to further discuss the thermal quenching, the results can be explained by the Arrhenius equation [36]:
where Ea, T, A and k refer to activation energy, temperature (K), constant and Boltzmann constant, respectively. The I0 and I represent the emission intensity at room temperature and differently operated temperature, respectively. The relationship between ln(I0-I/I) and 1/kT for the thermal quenching of MAPbBr3 nanocrystals is depicted in Fig. 8(b). From which it can be seen that the slope of fitting curve is −0.0431, and thus the Ea can be calculated to be 43.1 meV. The activation energy indicates that the MAPbBr3 nanocrystals have good thermal stability, and are expected to be widely used in lighting and display areas.4. Conclusions
In general, the morphologies-controlled MAPbBr3 nanocrystals have been synthetized via regulating the type and proportion of auxiliary ligands. The fluorescence properties are systematically studied via using the instruments (XRD, TEM, PL and UV-vis, etc.), and the conclusions can be summarized as follows:
- (1) The nanospheres, nanorods and nanoplatelets MAPbBr3 nanocrystals have been obtained by changing the types of auxiliary ligand. The red shifts of the PL emission peaks and ultraviolet absorption peaks occur, and the lifetimes increased gradually with the morphologies changing from nanospheres to nanoplatelets. The phenomena are attributed to quantum size effect and surface effect.
- (2) On the basis of the above, the particles regulated from nanospheres to nanorods with the ratio of oleic acid and N-octylamine changing from 9:1 to 3:1. The reasons can be explained the dimensional control of MAPbBr3 nanocrystals is mainly attributed to the anisotropic growth of crystals, which caused by the kinetic mechanism.
- (3) The transient absorption spectra indicated the radiative and non-radiative processes of MAPbBr3 nanocrystals, when was excited by the pump light at 400 nm. Moreover, the fluorescence properties of the MAPbBr3 nanocrystals manifested as stronger temperature dependence.
Funding
National Natural Science Foundation of China (51402125); China Postdoctoral Science Foundation (2017M612175); Research Fund for the Doctoral Program of University of Jinan (XBS1447); Natural Science Foundation of University of Jinan (XKY1515); Science Foundation for Post Doctorate Research from the University of Jinan (XBH1607); Special Fund of Postdoctoral innovation project in Shandong province (201603061).
Acknowledgments
“Conceptualization, J.K. Li; software, M. Ji; validation, G.B. Duan; formal analysis, B. Liu; investigation, B. Liu; resources, Z.M. Liu; data curation, M. Ji; writing—original draft preparation, B. Liu; writing—review and editing, J.K. Li.; visualization, T. Yan and J.K. Li; supervision, J.K. Li, Y.Z. Lu and Z.M. Liu; project administration, Y.Z. Lu.
Disclosures
The authors declare no conflicts of interest.
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