Probing molecule-like isolated octahedra via phase stabilization of zero-dimensional cesium lead halide nanocrystals

Abstract

Zero-dimensional (0D) inorganic perovskites have recently emerged as an interesting class of material owing to their intrinsic Pb²⁺ emission, polaron formation, and large exciton binding energy. They have a unique quantum-confined structure, originating from the complete isolation of octahedra exhibiting single-molecule behavior. Herein, we probe the optical behavior of single-molecule-like isolated octahedra in 0D Cesium lead halide (Cs₄PbX₆, X = Cl, Br/Cl, Br) nanocrystals through isovalent manganese doping at lead sites. The incorporation of manganese induced phase stabilization of 0D Cs₄PbX₆ over CsPbX₃ by lowering the symmetry of PbX₆ via enhanced octahedral distortion. This approach enables the synthesis of CsPbX₃ free Cs₄PbX₆ nanocrystals. A high photoluminescence quantum yield for manganese emission was obtained in colloidal (29%) and solid (21%, powder) forms. These performances can be attributed to structure-induced confinement effects, which enhance the energy transfer from localized host exciton states to Mn²⁺ dopant within the isolated octahedra.

Introduction

Over the past few years, inorganic lead halide perovskites have attracted great attention as a promising optoelectronic material for light-emitting devices, photodetectors, and low-threshold lasers, owing to their high photoluminescence quantum yield (PL QY), narrow emission, and tunable band gap^1,2,3,4. The basic building block in these material is PbX₆ octahedra (where X is a halogen), whose diverse connectivity can produce structures with various dimensionalities, ranging from three-dimensional (3D) to zero-dimensional (0D)^5,6,7. The 3D inorganic perovskites, with general formula APbX₃ (A = Cs, Rb, and X = Cl, Br, or I), consist of an extended network of corner-sharing PbX₆ octahedra with cavities occupied by A ions^8,9. Despite being the most explored material, the poor chemical stability of 3D perovskites against moisture, inherent phase transformation, and ion migration make their low-dimensional counterparts more favorable for optoelectronic applications^4,6,10.

In particular, 0D inorganic perovskite-like Cs₄PbX₆ system have crystal structure in which the PbX₆ octahedra are decoupled from each other by the surrounding Cs⁺ ions. The complete isolation of octahedra leads to strong quantum confinement and exciton–phonon interactions, which in turn can result in exciton localization, self-trapping, and polaron formation¹⁰. The optical features of 0D Cs₄PbX₆ are governed by transitions between the electronic states of Pb²⁺ ions, and its broad ultraviolet (UV) emission has been assigned to the radiative decay of Frenkel excitons at Pb²⁺ sites^{11,12,13,14,15}. However, the origin of their PL in the visible range is still under debate because of mixed views on the efficient green luminescence (QY of 45%) of Cs₄PbBr₆, which some studies have been attributed to the minor 3D CsPbBr₃ nanoscale impurity^3,16,17,18.

Nikl et al.¹² reported a UV emission band at 355 nm for Cs₄PbCl₆ single crystals, assigning to Pb²⁺ ion emission, originating from the optical transitions of ³P₀,₁ → ¹S₀ in the isolated PbCl₆ octahedra, similar to Pb²⁺ doping in the alkali halide hosts¹². In addition, the coexistence of 3D CsPbCl₃-like impurity was noted by 414 nm emission in Cs₄PbCl₆. While, Mohammed’s group¹⁹ reported Cs₄PbBr₆ characteristic with two broad UV emissions at 340 nm (high-energy) and 400 nm (low-energy) attributing to Pb²⁺ ion and charge-transfer band denoted as “D-state”, respectively. D-state emission originates via the transfer of excited electrons from (PbBr₆)^4– octahedra to the Pb²⁺ occupying Cs sites (D-states), through their strong coupling in Cs₄PbX₆ host¹⁹. Therefore, 0D Cs₄PbX₆ behave as an ideal host–guest system, with periodically doped individual PbX₆ species in a wide band gap matrix. However, though many reports attribute the green emission (512 nm) to the intrinsic property of Cs₄PbBr_6, we believe it arise from the coexistence of CsPbBr₃ impurity, similar to 414 nm emission of Cs₄PbCl₆¹². Nevertheless, 0D cesium lead halide perovskites still represent largely unexplored and intriguing perovskite-like material, expected to exhibit interesting optoelectronic properties, due to their strongly localized excitons and high exciton binding energies (150 to 380 meV)^3,20,21.

The coexistence of 3D CsPbX₃ impurity has precluded a detailed understanding on the optical properties of Cs₄PbX₆, leading to a growing demand for their synthesis in pure form and their phase stabilization. Solvent washing has been recently reported for the synthesis of Cs₄PbBr₆ free from CsPbX₃ impurity, which, however, is inadequate to treat the chloride compositions due to their poor solubility³. As the inevitable coexistence of CsPbX₃ phase represents a major drawback, new synthetic strategies are of utmost interest for obtaining pure Cs₄PbX₆ nanocrystals with reduced CsPbX₃ impurity.

Recently, the molecule-like behavior of Cs₄PbBr₆ has been discussed in terms of charge carrier transport and polaron formation, arising from weak interactions between isolated octahedra¹⁰. Thus, understanding the fundamental optoelectronic behavior of 0D Cs₄PbX₆ would promote the development of bulk perovskite-based devices. Incorporation of transition metal ions such as manganese (Mn²⁺) in the semiconductor nanocrystal leads to interesting optoelectronic properties by modulating the electronic properties of the host, providing new routes for designing solid-state lighting and light-harvesting devices^22,23. Hence, Mn²⁺ ions can serve as the sensitive probe for investigating the local structure of the host and altering its optical and electronic behavior. Thus far, the potential effect of isovalent cation doping on the isolated octahedral units of Cs₄PbX₆ remains unexplored, particularly in terms of phase stabilization and optical properties.

In this work, we probe the optical behavior of isolated octahedra in the Cs₄PbX₆ (X = Br, Cl, Br/Cl) by introducing Mn²⁺ at the octahedral sites and explore its phase stabilization over commonly coexisting CsPbX₃ impurity. The luminescence measurements show that Mn²⁺ dopant significantly alter the optical properties of different host emissive states in the Cs₄PbX₆. The dopant emission is controlled by adjusting the band gap of Cs₄PbX₆ through halide identity, facilitating energy transfer for enhancing the dopant emission efficiency in both colloidal and solid forms. The quantum-confined structure of 0D Cs₄PbX₆ further improves the stability of the host and efficiency of the dopant emission, particularly in their solid form.

Results

Mn²⁺-doped Cs₄PbX₆ nanocrystals

A series of 0D Cs₄PbX₆ (X = Br, Br/Cl, and Cl) nanocrystals, doped with manganese, were synthesized via modified reverse microemulsion method (Supplementary Figure 1 and Supplementary Note 1)²⁰. A short-chain alkylamine (octylamine) surfactant was employed instead of the previously reported long-chain oleylamine to control size and morphology²⁰. The exact composition of the synthesized undoped and Mn²⁺-doped Cs₄Pb(Br/Cl)₆ (denoted as mixed halide) analogs are Cs₄PbBr₂Cl₄ and Cs₄Pb_1–xMn_xBr_2–2xCl_4+2x, respectively, with x varying from 0.05 (5% Mn) to 0.80 (80% Mn). The Pb/Mn ratios were estimated from inductively coupled plasma−optical emission spectroscopy (Supplementary Table 1). For simplicity, the synthesized samples were represented as Cs₄PbX₆:x% Mn, where X = (Br/Cl), Cl, and Br; and x = percentage of Mn concentration with respect to displaced lead ions.

Local and bulk structure

The crystal structure of Cs₄PbX₆ consists of caged PbX₆ octahedra, isolated by interspersed Cs–X bridges²¹. Two types of Cs sites are present: the Cs(1) site form an alternating octahedra with PbX₆, whereas the Cs(2) trigonal prisms share one triangular face to form infinite [CsPbX₆]_n^3– chains along the [001] direction²⁴. The synthesized samples were highly crystalline matching with the rhombohedral Cs₄PbX₆ phase (X = Br, Br/Cl, and Cl; space group: R$\bar 3$c) as presented in Fig. 1a. However, in the undoped Cs₄PbX₆, the presence of 3D CsPbX₃ impurity were clearly apparent in all the halide analogs (Fig. 1a, Supplementary Figure 2, and Supplementary Note 2). Despite using high Cesium content (Cs:Pb = 5:1), formation of undesired CsPbX₃ phase were observed in the undoped Cs₄PbX₆ probably due to localized occurrence of Cs-deficient regions.

Fig. 1

Local and bulk structures of Mn²⁺-doped Cs₄PbX₆. a X-ray diffraction patterns of Mn²⁺-doped Cs₄PbX₆ series with different doping concentrations showing the rhombohedral structure is preserved after Mn-doping. Selected 2θ range of (22 to 24° and 29 to 31°) XRD patterns were presented. A monotonic shift of the XRD peak at Bragg’s angle at 23.4° towards higher 2θ is the result of progressive lattice contraction as the Mn²⁺ concentration increases, due to the substitutional replacement of Pb²⁺ with isovalent but smaller Mn²⁺ ions. The additional impurity cubic phases of 3D CsPb(Br/Cl)₃, 3D CsPbCl₃, CsBr, Cs(Br/Cl), and CsCl were denoted as hash, asterisk, triangle, open circle, and closed circle, respectively. It is to be noted that the Cs:Pb precursor ratio was fixed at 5:1 for all the samples of Cs₄PbX₆ (Br, Cl, and Br/Cl). b Phase fraction of the desired 0D Cs₄PbX₆ perovskite with respect to 3D CsPbX₃ perovskite phase and segregation of CsX, with varying Mn concentration in the Cs₄PbX₆ synthesis. c Octahedral tilting/distortion without and with Mn doping (10% Mn) causing the phase stabilization of Cs₄Pb(Br/Cl)₆ structure. Electron density distribution profile derived from Maximum Entropy method for the d undoped Cs₄PbCl₆, e Cs₄PbCl₆:10% Mn, and f Cs₄PbCl₆:20% Mn, along the (010) plane. g Schematic representation of phase stabilization of Cs₄PbX₆ perovskite structure upon Mn²⁺ incorporation

Upon Mn incorporation, the Bragg’s peak at 23° monotonically shifts to higher 2θ, suggesting the substitution of Pb²⁺ with smaller Mn²⁺ ion in Cs₄PbX₆ (X = Br, Br/Cl, Cl), which is also evidenced from the decrease in lattice parameters (Table 1, Supplementary Tables 2 to 4)^24,25. Lattice contraction of 0.3% and volume change of 2%, after Mn (20% Mn) doping, resulted in the shortening of lead halide (Pb–Br/Cl) bond lengths and similar feature was also noted in chloride analog (Supplementary Table 5, 6). The strong bonding nature upon Mn doping is evidenced from high bond-dissociation energy of Mn–Cl (361 kJ mol⁻¹) than Pb–Br bonds (247 kJ mol⁻¹)²⁶. The valence states and substitution of Mn²⁺ ions at the isolated octahedral PbX₆ sites was reaffirmed from X-ray photoelectron spectroscopy and electron spin resonance results, respectively (Supplementary Figures 3a to 3f and Supplementary Note 3)^22,27,28. Quantification of multiple crystalline phases (Cs₄PbX₆ and CsPbX₃) in the synthesized samples were estimated using Rietveld refinement of X-ray diffraction (XRD) results. Interestingly, Mn doping resulted in a decrease in the phase fraction (mass%) of 3D CsPbX₃ impurity, which further reduced with Mn content in both Br/Cl and Cl analogs, as shown in Fig. 1b. In other words, the phase fraction of 0D Cs₄Pb(Br/Cl)₆ with respect to the total (0D+3D) phase gradually increased from 86 to 100% upon increasing Mn concentration from 0 to 20%. This increase in the phase fraction of Cs₄PbX₆ was also apparent in the pure chloride and bromide analog (Supplementary Figure 2), suggesting that Mn²⁺ incorporation in the framework favors the formation of Cs₄PbX₆ phase, while preventing the undesirable CsPbX₃ phase. A CsPbX₃ impurity-free Cs₄Pb(Br/Cl)₆ phase was obtained with 10% Mn content. Despite the complete suppression of CsPbX₃, additional CsX (8%) phase tends to precipitate at higher Mn (above 10% Mn) content in both Cs₄Pb(Br/Cl)₆ and Cs₄PbCl₆ (Fig. 1b). Since the presence of Mn²⁺ in the lattice favors the Cs₄PbX₆ formation, the excess Cs precursor (5:1) which is used to favor the 0D phase, tend to segregate as CsX at higher Mn concentration. The CsX formation could be further prevented by decreasing the Cs precursor (Cs:Pb = 4.5:1) in the Mn²⁺-substituted samples, leading to pure Cs₄PbX₆ nanocrystals (Supplementary Figure 4). Our attempt to incorporate higher Mn content above 20% Mn, resulted in an additional hexagonal CsMnCl₃ phase, along with Cs₄Pb(Br/Cl)₆ (Supplementary Figure 5 and Supplementary Note 4).

Table 1 Structural parameters of Cs₄Pb(Br/Cl)₆ with varying concentration of Mn, calculated using the Rietveld refinement

Phase stabilization of Cs₄PbX₆

The versatility of the Cs–Pb–X ternary compounds (particularly Cs₄PbX₆ and CsPbX₃) lies in their ability to interconversion via post-synthetic physical and chemical treatments²⁹. The post-synthetic conversion of cubic CsPbX₃ to rhombohedral Cs₄PbX₆ has been demonstrated via amine- and thiol-mediated extraction of PbBr₂³⁰, while the reverse conversion from Cs₄PbX₆ to CsPbX₃ has been reported by the insertion of PbX₂, stripping of CsX, excess oleic acid, and by heat treatment (90 to 180 °C)¹⁸. These post-synthetic conversion mechanisms are inadequate to explain the simultaneous formation of Cs₄PbX₆ and CsPbX₃ phases, during synthesis. It has been demonstrated that the phase stabilization of perovskite semiconductors could be achieved by controlling the octahedral tilting via partially substituting Pb²⁺ with other metal ions. For instance, substitution of smaller Mn²⁺ stabilizes the cubic α-CsPbI₃ phase by reducing the octahedral rotation or tilting via a decrease in the bond angle of Pb–I–Pb below 180°, which is along the interconnected neighboring PbX₆ octahedra through the bridging halide ion³¹. In the present case, a similar octahedral tilting (enhanced) results in the phase stabilization of low-symmetry Cs₄Pb(Br/Cl)₆ upon Mn²⁺ incorporation. A decrease in the Pb–X bond length after Mn doping (10% Mn) to 2.950 Å, than undoped Cs₄Pb(Br/Cl)₆ (2.991 Å), resulted in an enhanced octahedral tilting, preventing the undesired high-symmetry cubic CsPbX₃ phase (Fig. 1c). Here, the octahedral tilting in the Cs₄PbX₆ (X = Br/Cl) is described based on the X–Pb–Cs₁ bond angle along c axis of the unit cell, which changes from 54.94° (undoped) to 54.76° upon Mn incorporation. This octahedral tilting would distort the PbX₆ octahedra, which subsequently lowers the symmetry of crystal structure. It is worth mentioning that only two stable compositions exist in the phase diagram of mixed CsX-PbX₂, under Cs-rich or Pb-deficient conditions viz., low-symmetry rhombohedral Cs₄PbX₆ and high-symmetry cubic CsPbX₃³². Therefore, enhanced octahedral tilting upon Mn substitution, result in the formation of only possible low-symmetry Cs₄PbX₆ phase; at the same time, destabilizing the cubic CsPbX₃ phase under Cs-rich condition. This phase stabilization of 0D Cs₄PbX₆ via Mn substitution, might favor deeper investigation to their unexplored and intriguing properties.

Electronic structure

The changes in the electronic structure namely bonding nature and electron density distribution in the Cs₄PbCl₆ were analyzed with Mn incorporation via maximum entropy method (MEM) using XRD results. In the Cs₄PbCl₆, the electron density around Pb and Cl is anisotropic and highly resolved with no feature of electron sharing between Pb and Cl, resembling electrostatic attractive/repulsive forces (Fig. 1d). Upon Mn substitution a significant change in the local charge density around Pb atom was apparent, suggesting that Mn modulates the electronic structure of the 0D Cs₄PbCl₆ (Fig. 1e, f). With increasing Mn content, high electron density distribution between Pb and Cl atoms, suggests covalent bonding nature of (Pb/Mn)–Cl bonds (Supplementary Figure 6). This also supports the stronger (Pb/Mn)–Cl bonds or shorter bond lengths of Pb–Cl upon Mn doping, as observed from XRD. The change in local electron density after Mn-doping results in the lattice distortion (also evidenced from octahedral tilting, Fig. 1c) and could lead to electron-phonon (lattice) coupling and formation of polaron³³. Recently, the formation of polarons in the Cs₄PbBr₆ has been reported from density functional theory calculation and transient absorption measurements¹⁰. In the present case, at variance with the undoped Cs₄PbX₆, a strong lattice distortion upon Mn incorporation could eventually enhance the electron-phonon coupling, leading to polaron formation. Therefore, it could be concluded that Mn²⁺ incorporation modulates the electronic structure and favors the phase stabilization of Cs₄PbX₆, preventing the formation of undesirable CsPbX₃ impurity (Fig. 1g). Though the phase stabilization of Cs₄PbX₆ were concluded based on XRD results, this technique has its own limitation of insensitivity to detect any minor crystalline phase below 5%³⁴. Henceforth, to further support the phase stabilization, optical characterization was exploited owing to its high sensitivity to detect even trace of fluorescent CsPbX₃ impurity.

The excitation and photoluminescence (PL) properties of Cs₄PbX₆:Mn colloids were presented in Fig. 2. The undoped Cs₄Pb(Br/Cl)₆ exhibited broad emission at 432 nm (ranging 415 to 470 nm) under 365 nm excitation (Fig. 2a). The broad emission arises from the contribution from “D-states” of the host and 3D CsPb(Br/Cl)₃ impurity (443 nm emission, Supplementary Figure 7)_, where the former dominates the latter. The presence of CsPb(Br/Cl₃) impurity was also evident from the broad absorption ranging 360 to 400 nm in the undoped sample (Fig. 2b). It is to be noted that D-state emission arise from the charge transfer from halide ions of the PbX₆ octahedra to Pb²⁺ ion occupying mismatched neighboring Cs⁺ sites, resulting in the creation of Frenkel excitons localized at the vicinity of Pb ions^19,35. Upon Mn incorporation (5% Mn), a narrow emission band at 420 nm was observed, corresponding to the D-state of Cs₄Pb(Br/Cl)₆ phase, with feeble emission from CsPb(Br/Cl)₃ impurity. The inclusion of chlorine during Mn doping (MnCl₂ precursor) may contribute meagerly to the D-state emission compared to undoped one. Further increase in Mn content (above 10% Mn), eliminated the CsPb(Br/Cl)₃ impurity as evidenced from its negligible absorption (360–400 nm), absorption narrowing, and unaltered D-state emission (Fig. 2a, b). The above results reaffirm the strong phase stabilization of Cs₄PbX₆ in presence of Mn, further corroborating the XRD results (Fig. 1a)^36,37. The additional orange emission at 599 nm, is assigned to the forbidden Mn²⁺ d–d transitions²², indicating the presence of exchange coupling between charge carriers of Cs₄Pb(Br/Cl)₆ and Mn²⁺ ions. Further, the single-halide Cs₄PbX₆ analogs were investigated to understand the role of Mn²⁺ on the optoelectronic properties and distinguish them from halide effect, which is a limitation in the mixed halides. The undoped Cs₄PbBr₆ exhibited two UV emission bands and intense green emission (512 nm) arising from Pb²⁺/D-state emission and CsPbBr₃ impurity, respectively (Supplementary Figure 8 and Supplementary Note 5). While in the undoped Cs₄PbCl₆, the 414 nm emission corresponds to the band-edge emission of CsPbCl₃ impurity¹² and the hump ranging 450 to 480 nm is assigned to the quantum size effect of CsPbCl₃. Upon Mn incorporation, a new emission at 400 nm arises from the D-state of Cs₄PbCl₆, with less contribution from CsPbCl₃, compared to undoped one, which completely disappeared with higher Mn (20% Mn) content (Supplementary Figure 4), reaffirming the phase stabilization of Cs₄PbX₆ upon Mn incorporation.

Fig. 2

Luminescent properties of Mn²⁺-doped Cs₄PbX₆. a PL spectra of Mn²⁺-doped Cs₄PbX₆ colloids were measured under 365 nm excitation, where X = Br/Cl and Cl, with Mn contents denoted as 0% Mn, 5% Mn, 10% Mn, and 20% Mn and the insets shows the respective samples under 365 nm UV lamp. b UV-Vis spectra of Mn²⁺-doped Cs₄PbX₆ colloids. c PL spectra of Mn²⁺-doped Cs₄PbX₆ colloids were measured under 290 nm excitation and the insets shows the respective samples under 254 nm UV lamp. The PL spectra presented are the PL intensities corrected using optical density. The asterix mark denote the 3D CsPb(Br/Cl)₃ phases. FE-SEM images of d undoped Cs₄Pb(Br/Cl)₆ and e Cs₄Pb(Br/Cl)₆:10% Mn, and f, g HR-TEM images of Cs₄Pb(Br/Cl)₆:10% Mn. A lattice spacing of 0.31 nm corresponding to (131) plane of rhombohedral structure of Cs₄Pb(Br/Cl)₆

The undoped and Mn²⁺-doped Cs₄PbCl₆ displayed blue and pinkish (combination of D-state ‘blue’ and Mn²⁺ ‘orange’) emission under 365 nm, respectively (Fig. 2a, inset). The dual UV absorption bands at 218 and 308 nm, are the characteristic features of Cs₄Pb(Br/Cl)₆. The band gaps estimated from the absorption maxima are 3.83, 4.02, and 4.25 eV, for Cs₄PbBr₆, Cs₄Pb(Br/Cl)_6, and Cs₄PbCl₆, respectively (Supplementary Figure 8a), which agrees with the earlier reports¹⁸. It is interesting to note the increase in Cs₄PbX₆ host emission after Mn doping (Fig. 2a–c) in both mixed halide and chloride analog, similar to the perovskite nanocrystals³⁸. The host emission increment is more prominent in the mixed halide (Br/Cl) analog than pure chloride, which is presumed to the passivation effect of chloride ion, on the pre-existing defects of the host³⁸. Therefore, Mn²⁺ doping significantly alters the competitive kinetics between the radiative and non-radiative relaxation process of the Cs₄PbX₆ host.

The PL properties were investigated, by exciting at energies higher than band gap of host (290 nm), due to their strong absorption. Mn²⁺-doped (Br/Cl) samples exhibited dual UV emissions at 372 nm and a hump at 324 nm, arising from optical ³P_0,1,2 → ¹S₀ transitions of Pb²⁺ ion^35,39 and henceforth denoted as ‘Pb²⁺ ion’ emission (Fig. 2c) . Unlike at 365 nm excitation, the D-state (420 nm) emission of both undoped and Mn²⁺-doped was either absent or buried in the broad 300 to 450 nm spectral range under 290 nm excitation. A similar, broad Pb²⁺ emission (354 nm) was observed for Cs₄PbCl₆¹², without the D-state emission, when excited at 290 nm. With Mn doping, an apparent drop in the 3D CsPb(Br/Cl)₃ emission (ranging 430 to 450 nm) further demonstrates the phase stabilization of Cs₄PbX₆ (Fig. 2c). In addition, despite the unaltered Pb²⁺ emission, an enhanced Mn²⁺ emission in the mixed halides, was observed with increasing Mn concentration, due to efficient energy transfer from Pb²⁺ to Mn²⁺. The unusual red-shifted emission from 599 to 635 nm for Cs₄Pb(Br/Cl)₆:20% Mn sample, is attributed to the formation of Mn-to-Mn pairs at higher Mn concentration, which reduces the ⁴T₁ - ⁶A₁ energy gap_, resulting in the red shift of Mn²⁺ emission⁴⁰. Hence, the phase stabilization of Cs₄PbX₆ (X = Cl, Br/Cl) with Mn incorporation were demonstrated in the chlorine-dominant system (above 60%), through XRD and optical results.

The Mn incorporation was also investigated in bromide analog (Cs₄PbBr₆) for monitoring phase stabilization via XRD and PL results (Supplementary Figures 2 and 8). With increasing Mn, the green emission (512 nm) of CsPbBr₃ impurity decreased gradually, which completely disappeared above 70% Mn, suggesting the prevention of CsPbBr₃ impurity in presence of Mn, agreeing with the XRD results. However, unlike Br/Cl and Cl analogs, the bromide counterpart necessitates higher Mn²⁺ concentration (above 50% Mn) to stabilize the Cs₄PbBr₆ structure, which is ascribed to the low substitution ratio of Mn²⁺ observed in the bromine-dominated Cs–Pb–Br perovskite-like system⁴¹. In addition, this leads to residual segregation of CsBr, at higher Mn concentration (above 50% Mn). The above results further validate the phase stabilization of zero-dimensional Cs₄PbX₆ upon Mn incorporation in all the bromine, Br/Cl, and chlorine analogs.

The synthesized Cs₄Pb(Br/Cl)₆:10% Mn exhibited cubic morphology with particle size ranging between 20 and 40 nm (Fig. 2d–g) which is much smaller than undoped one (50 to 120 nm). Further increasing the Mn content did not affect the particle size. It is worthy to note that the emission property of Cs₄PbX₆ host is independent of the particle size due to its intrinsic structural confinement and extremely localized emissive centers (isolated octahedra). However, the dependence of size and morphology of Cs₄PbX₆ on the length of alkyl chains of amine surfactant were clearly apparent. Unlike long-chain amines (oleylamine), which lead to a narrow particle size distribution in the 26 ± 4 nm range (mixture of spherical, hexagonal, and cubic morphologies)²⁰, the use of shorter-chain octylamine in the present work favored a cubic shape with a relatively wide size distribution.

Probing the host emissive states

The excitation-dependent PL properties of Cs₄Pb(Br/Cl)₆ were explored to get insight of multiple luminescent states of the host. A range of excitation wavelengths from 280 to 365 nm (4.40 to 3.40 eV) were employed including excitation energies higher than the band gap of Cs₄Pb(Br/Cl)₆ host (4.00 eV). At higher excitation energies (λ_ex of 280 nm), dominant D-state emission (432 nm) was accompanied by Pb²⁺ emission (324 and 375 nm) for undoped Cs₄Pb(Br/Cl)₆ (Fig. 3a). It is to be noted that 432 nm emission is denoted here as D-state emission, disregarding the less contribution from CsPb(Br/Cl)₃ impurity (Fig. 2a), for the ease of emphasizing on dependence of host emissions (Pb²⁺ and D-state) with varying excitation energies. As excitation wavelength increased from 280 to 310 nm, the D-state emission gradually decreased, where Pb²⁺ emission remains dominant. While above 310 nm excitation, Pb²⁺ emission disappeared due to insufficient energy to excite its ³P₁ levels and only D-state is retained. Upon Mn doping, the D-state emission completely disappeared in the excitation wavelengths ranging, 280 to 310 nm, exhibiting only Pb²⁺ emission (Fig. 3b, c). The absence of D-state emission after Mn²⁺ doping is probably due to suppression of charge-transfer process from PbX₆ octahedra to Pb²⁺ occupying Cs⁺ site, primarily at high excitation energies (above 4.0 eV), where both Pb²⁺ and D-states are excited. Therefore, the presence of Mn²⁺ plays a key role in regulating the optical behavior of molecule-like isolated PbX₆ octahedra in Cs₄PbX₆ nanocrystals.

Fig. 3

Probing the host states at the isolated octahedron with Mn²⁺ ion. Colloidal PL spectra measured at different excitation wavelengths ranging 280 to 365 nm for the a undoped Cs₄Pb(Br/Cl)₆ and b Cs₄Pb(Br/Cl)₆:5% Mn. c Emission intensities of Pb²⁺ and D-state emission with varying excitation energies for the undoped and Mn-doped Cs₄Pb(Br/Cl)₆:5% Mn, d excitation spectra of Cs₄Pb(Br/Cl)₆:5% Mn colloids by monitoring at 375 nm (Pb²⁺ ion), and 420 nm (D-state), e PL intensity ratio of Mn²⁺ to the host emission with respect to varying excitation energies, f PL intensity ratio of Mn²⁺ to the host states with varying Mn concentration for Cs₄PbX₆ colloids, where X = (Br/Cl) and Cl, g origin of host emissions namely Pb²⁺ and D-state under high excitation energies, without and with Mn doping at high excitation energies. The solid, dashed, and curved arrows are denoted for optical excitation, emission, and energy transfer processes, respectively. h Strength of energy transfer from the host states namely Pb²⁺ and D-state to the Mn²⁺ emission are depicted. The upward, downward, and curved arrows are denoted for the optical excitation, emission, and energy transfer processes, respectively

The energy transfer and direct excitation of ³P₁ levels of Pb²⁺ (375 nm) and D-states (420 nm), were explored from excitation spectrum of 5% Mn²⁺-doped Cs₄Pb(Br/Cl)₆ (Fig. 3d). The spectrum reveals Pb²⁺ ion can be excited at high excitation energies above the band gap of Cs₄Pb(Br/Cl)₆ host (below 310 nm excitation); while the D-state is excited at lower excitation energies typically below the host band gap, (above 310 nm excitation). A direct excitation of D-state at 280 nm, is also apparent along with Pb²⁺ ion³⁵. However, the energy transfer between the Pb²⁺ and D-state of the host cannot be ruled out for the undoped sample, due to the occurrence of D-state emission in the excitation wavelengths (290 to 310 nm) where only Pb²⁺ state is excited, as reported earlier³⁵, and is consistent with Cs₄PbX₆ solids (Supplementary Figure 9 and Supplementary Note 6).

Moreover, Mn²⁺ emission is well-known to be sensitized by the host via energy transfer, where Pb²⁺ and D-state sensitizes at high and low excitation energies, respectively. Therefore, Mn²⁺ emission at varying excitation energies is determined from the absorption of the corresponding host state and efficiency of energy transfer from host states. The Mn²⁺ emission relative to the total host emission (i.e., Pb²⁺ + D-state) was the highest at 280 nm, attributing to the efficient energy transfer from the Pb²⁺ state, which then gradually decreased at lower excitation energies, apart from 320 nm (Fig. 3e). Although the direct excitation of both Pb²⁺ and D-states is possible at 280 nm, Mn²⁺ is sensitized only through Pb²⁺ state, owing to the suppression of D-state in presence of Mn. The, dominant Mn²⁺ emission at 320 nm excitation, ascribed to the Mn²⁺ sensitization from both D-state and partially from Pb²⁺ ion. In addition, energy transfer from Pb²⁺ ion to Mn²⁺ was more efficient than from D-state (Fig. 3f, Supplementary Figure 10, and Supplementary Note 7). Moreover, the presence of Mn²⁺ considerably suppresses the D-state emission by blocking the charge-transfer process from isolated octahedra to the D-state, especially at high excitation energies (about 4.3 eV, 280 nm excitation), where both the D-state and Pb²⁺ could be directly excited (Fig. 3g). This may be caused by the weak coupling between electronically decoupled (Pb/Mn)X₆ octahedra and D-state (Pb²⁺ at Cs site) which deteriorates the charge transfer between them, due to strong Mn–Cl bond formation. The sensitization of Mn²⁺ is predominantly occurs from Pb²⁺ than the D-state, due to larger energy difference between Mn²⁺ and Pb²⁺ states (Fig. 3h). Moreover, the efficient energy transfer to Mn²⁺ dopant in the wide band gap host, due to large energy difference between host and Mn²⁺ levels, is well documented³⁸. The above results demonstrate that the Mn²⁺ incorporation significantly affects the optical properties of host state in Cs₄PbX₆ nanocrystals.

Luminescence efficiency

The potential application of perovskite-like Cs₄PbX₆ as emitting layer in color-converting devices has been explored in colloidal and solid forms (thin films and powders)¹⁶ and is expected to have high conversion efficiency similar to other color converter viz. phosphor⁴². High PL QY has been reported in perovskite colloids by employing ligands to stabilize the nanocrystals^1,43, and thin films (solids) of micron-sized grains^44,45. Here, although PL QY of undoped Cs₄Pb(Br/Cl)₆ nanocrystal is low (5%), the overall QY substantially increased to 32% upon Mn²⁺ doping, under 365 nm excitation (Table 2, Supplementary Figure 11a, and Supplementary Note 8). Highest PL QY of 29% was obtained for Cs₄Pb(Br/Cl)₆:10% Mn and Cs₄PbCl₆:10% Mn (Table 2), which is among the best values reported for Mn²⁺ emission (Supplementary Table 8)⁴⁶. In addition, Mn²⁺ luminescence was the highest at lower (365 nm) and higher excitation energy (290 nm), for Cs₄Pb(Br/Cl)₆ and Cs₄PbCl₆, respectively, due to their suitable band gaps (determined by halide species), enabling efficient energy transfer to Mn²⁺. The overall superior PL QY of Mn²⁺-doped Cs₄PbX₆ colloids (X = Br/Cl, Cl) is attributed to the spatially confined zero-dimensional structure, and presence of dopant at the isolated octahedra favoring enhanced dopant emission through the localized excitons and high exciton binding energy (Supplementary Figure 11b to 11e). The reported exciton binding energies of Cs₄PbBr₆ and Cs₄PbCl₆ are 353³ and 153 eV⁴⁷, respectively, compared to CsPbBr₃ (19 to 62 meV) and CsPbCl₃ (72 meV)^48,49,50. These results suggest that the high luminescent Cs₄PbX₆ nanocrystals doped with Mn²⁺ could represent promising emitting materials for efficient solid-state lighting applications.

Table 2 PL quantum yield of the Cs₄PbX₆ colloids in n-octane medium

In the Cs₄PbX₆ solids, a similar low PL QY was observed for undoped sample, but the overall PL QY increased markedly upon Mn doping in both mixed halide and chloride samples (Supplementary Figure 11e). PL QY of 21% was obtained for Mn²⁺ emission in Cs₄PbCl₆ solids, under 290 nm excitation (Supplementary Table 9). The bright orange emission of Mn²⁺-doped Cs₄PbX₆ in their solid form, is a unique feature among Mn²⁺-doped perovskite solids (Fig. 4c). This remarkable PL QY and retention of dopant emission in solid form is the attributes of spatial confinement of low-dimensional Cs₄PbX₆ induced by isolated octahedra, eventually favoring a stable dopant emission in solids.

Fig. 4

Lifetime of Mn²⁺-doped Cs₄PbX₆. Time-resolved PL spectra of Cs₄Pb(Br/Cl)₆ colloids excited at 375 nm and monitoring the D-state and Mn²⁺ emission at a 420 and b 600 nm, respectively. c Images of Cs₄PbX₆ perovskite solids under 254 and 365 nm UV lamp, which are denoted by their respective halides and Mn concentrations

To further elucidate the emission properties of Cs₄PbX₆ host and Mn²⁺, time-resolved PL results were performed by monitoring the D-state (420 nm) and Mn²⁺ emission (600 nm), exciting at 375 nm (Fig. 4a, b, Supplementary Figures 12 to 14, and Supplementary Note 9). A nominal decrease in the average lifetime of D-state emission in Cs₄PbX₆ (X = Br/Cl and Cl) was observed with increasing Mn content (Supplementary Figure 13). For instance, the average lifetime of D-state decreased from 1.08 ns (undoped) to 0.82 ns (20% Mn), corroborating the Mn²⁺ sensitization by the Cs₄Pb(Br_/Cl)₆ host (Table 3). A long lifetime was obtained for the Mn²⁺ emission in Cs₄Pb(Br/Cl)₆ and Cs₄PbCl₆ of 1.20 ms and 1.39 ms, respectively (Fig. 4b and Supplementary Figure 13b). The lifetime of D-states of Cs₄Pb(Br/Cl)₆ converged with three components namely 3.7 (slow), 2.6, and 0.40 ns (last two combined is fast), similar to the previous report³. The first slow (radiative) component is assigned to D-state emission and the fast (non-radiative) component to the energy transfer process to Mn²⁺. A rise in the contribution of slow component with increasing Mn²⁺ content, agrees with the increase in D-state emission (Supplementary Figure 12). This suggests that the inclusion of chlorine species with Mn-doping (MnCl₂ precursor), passivates the pre-existing defects in the Cs₄Pb(Br/Cl)₆ host, resulting to an enhanced host PL efficiency³⁸. Hence, the passivation effect of chlorine in the Cs₄Pb(Br/Cl)₆, would contribute greatly to the PL efficiency of D-state rather than energy transfer to Mn²⁺ at 375 nm excitation. On the contrary, superior contribution of fast component in Cs₄PbCl₆, illustrates the dominant energy transfer from D-state to Mn²⁺ (Supplementary Figures 13 and 14) at 375 nm excitation. The chloride passivation effect in the Cs₄PbCl₆ may be negligible as chloride content remain unchanged with increasing dopant concentration. Therefore, the D-state emission in mixed halide Cs₄Pb(Br/Cl)₆, is predominated by the passivation effect of chlorine over energy transfer to Mn²⁺, while in the Cs₄PbCl₆, energy transfer to Mn²⁺ dominated over passivation effect.

Table 3 Average lifetime of D-state emission of Cs₄Pb(Br/Cl)₆ and Cs₄PbCl₆ monitored at 420 nm and 400 nm, respectively

The mechanism of energy transfer from host states (Pb²⁺ and D-state) to Mn²⁺ in the mixed halide perovskite is further illustrated in the Fig. 5. The high PL QY of Mn²⁺-doped Cs₄PbX₆ is the attributes of structure-induced quantum confinement with high exciton binding energy favoring enhanced emission, compared to 3D CsPbX₃ perovskite (Fig. 5a). The superior PL QY of Mn²⁺-doped Cs₄Pb(Br/Cl)₆ and Cs₄PbCl₆ were observed at low (365 nm) and high excitation energy (290 nm), due to high absorption in their respective band gaps arising from halide identity. The Pb²⁺ ions are excited at high excitation energies (above 4.0 eV), leading to 375 nm emission, while D-states are excited at low excitation energies (below 4.0 eV), and both these states eventually generate Mn²⁺ emission through energy transfer (Fig. 5b). The presence of Mn²⁺ suppresses the charge transfer from isolated octahedra to the D-state, due to the weak coupling with the regular crystal lattice sites particularly at higher excitation energy. The enhanced overall PL QY of Mn²⁺-doped Cs₄PbX₆ in both colloidal and solid form is attributed to the synergistic effect of structural quantum confinement effect from the isolated octahedra and high exciton binding energies, facilitating dopant emission through enhanced energy transfer process.

Fig. 5

Mechanism of energy transfer from host states to Mn²⁺. a Illustration of high exciton binding energy (at above 150 meV) due to the localized excitons formed in the isolated octahedra of 0D Cs₄PbX₆ (X = Cl, Br/Cl) perovskite, compared to low binding energies (less than 60 meV) of 3D CsPbX₃ perovskite, where filled and hollow circles represent electrons and holes, respectively. b Configurational coordinate representation of Cs₄Pb(Br/Cl)₆ perovskite depicting Pb²⁺-based (³P_0,1  →¹S₀ transition) and D-state emissions, and their energy transfer to the Mn²⁺ energy levels. The long dash and dotted arrows depicts the excitation and energy transfer process leading to emission (solid arrows)

Discussion

In conclusion, we rationalized the exceptional optical behavior of isolated octahedra in the zero-dimensional cesium lead halide structure by introducing Mn²⁺ dopants. Structural, PL, and lifetime results confirmed the incorporation of Mn²⁺ in the 0D Cs₄PbX₆ lattice. The Mn²⁺ incorporation stabilized the Cs₄PbX₆ structure via enhanced octahedral tilting and limited compositional variation of Cs–Pb salts, suppressing the formation of CsPbX₃ impurity phase. The PL QY at a given excitation energy was determined by the energy transfer from the host states to Mn²⁺ and band gap of the host. A high PL QY for Mn²⁺ emission was achieved in the colloidal (29%) and solid (21%, powder) forms; making the solid form of Cs₄PbX₆ as a promising emissive layer for light-emitting devices. The enhanced PL QY of the Mn²⁺ emission was attributed to the synergistic effect of structure-induced spatial confinement of 0D Cs₄PbX₆ and electronically decoupled PbX₆ octahedra favoring dopant emission via localized exciton. The present work provides deep insight into the structure and molecular behavior of 0D perovskite-like Cs₄PbX₆ material and opens an avenue to design low-dimensional perovskites with photo and chemical stability for high-performance optoelectronic applications.

Methods

Materials

All the reagents were used without any purification. Cesium carbonate (Cs₂CO₃, 99.999% Kojundo), lead (II) bromide (PbBr₂, 99%, Sigma-Aldrich), lead (II) chloride (PbCl₂, 98%, Sigma-Aldrich), manganese (II) bromide (MnBr₂, 98%, Sigma-Aldrich) manganese (II) chloride tetrahydrate (MnCl₂.4H₂O, 99%, Sigma-Aldrich), oleic acid (OA, 90%, Fluka), octylamine (OcA, 97%, Sigma-Aldrich), N,N-dimethylformamide (DMF, 99.8%, Sigma-Aldrich), n-hexane (99.8%, Junsei, Japan), n-octane (90%, Sigma-Aldrich), 1-octadecene (95%, Sigma-Aldrich), hydrochloric acid (37%, Sigma-Aldrich), and hydrobromic acid (48%, Sigma-Aldrich) were employed for the synthesis.

Synthesis of undoped and Mn²⁺-doped Cs₄Pb(Br/Cl)₆ colloids

The exact compositions of the synthesized undoped and Mn²⁺-doped Cs₄Pb(Br/Cl)₆ series are Cs₄PbBr₂Cl₄ and Cs₄Pb_1–xMn_xBr_2–2xCl_4+2x with x = 0.05 (5% Mn), 0.10 (10% Mn), and 0.20 (20% Mn), respectively. Mn²⁺-doped Cs₄Pb(Br/Cl)₆ nanocrystals were synthesized by a reverse microemulsion method at room temperature, as reported earlier²⁰. PbBr₂ precursor was used as the lead precursor. A simultaneous anion (Cl^–) and cation (Mn²⁺) inclusion were effected by employing MnCl₂.4H₂O precursor for the synthesis of Mn²⁺-doped Cs₄Pb(Br/Cl)₆. An optimal concentration of octylamine (OcA) and oleic acid (OA) surfactants were used in facilitating the solubilization of Cs, Pb, and Mn precursors. The mixed MnCl₂.4H₂O and PbBr₂ precursors, and the Cs-oleate precursor were synthesized separately. First, a mixture of 2.25 g of Cs₂CO₃ and 21.5 mL of OA was stirred and degassed at 130 °C, which is further maintained under vacuum for 1 h to generate a yellowish stock of Cs-oleate precursor. Second, 0.2 mL Cs-oleate precursor, 10 mL of n-hexane, 5 mL of OA were loaded into a 50-mL three-neck flask, followed by mild degassing (vacuum) for 5 min and Argon purging for 5 min. Third, a mixture of PbBr₂ and MnCl₂.4H₂O (DMF, 1 mL), HCl (38 wt%, 19 μL), 0.10 mL OA, and 0.05 mL OcA was swiftly injected into the flask, under vigorous stirring. Upon stirring for 10 min, white color crystals were formed suggesting the formation of Mn²⁺-doped Cs₄Pb(Br/Cl)_6. The as-synthesized nanocrystals were collected via centrifugation at 7000 rpm for 5 min followed by dispersion in 2 mL of n-octane for further characterization. The contents of Cs-oleate, HCl, and surfactants (OA, OcA) were maintained the same for varying concentration of Mn precursors.

The undoped Cs₄Pb(Br/Cl)₆ was synthesized following the same procedure, in the absence of MnCl₂.4H₂O. A color change from a pale-white to green was observed in 10 min upon the addition of Pb precursor into the Cs-oleate, which is then centrifuged and dispersed in n-octane for further characterization. N-octane was employed as the solvent for the Mn²⁺-doped colloids for its superior dispersibility over hexane and toluene.

Synthesis of undoped and Mn²⁺-doped Cs₄PbX₆ (X = Br, Cl)

Cs₄PbX₆ samples were synthesized using stoichiometric quantity of PbX₂ and HX (15 μL) as the lead and halide precursors along with Cs-oleate, where X = Br, Cl. The limited solubility of PbCl₂ in DMF solvent at room temperature was overcome by warming the contents until it gets dissolved. Mn²⁺-doped Cs₄PbX₆ samples were synthesized following the same procedure along with manganese precursors of MnCl₂.4H₂O and MnBr₂, for the chloride and bromide analog, respectively. 

Synthesis of CsPbX₃ (X = Br, Br/Cl, Cl) perovskites

CsPbX₃ (X = Br, Br/Cl, Cl) nanocrystals were synthesized as described by Palazon et al.²⁹. In a typical synthesis, PbX₂ (0.2 mmol; X = Br, Cl, Br/Cl), 0.47 mL oleic acid, 5.0 mL octadecene, and 0.50 mL oleylamine were loaded in a 25 mL 3-neck flask and dried under vacuum at 100 °C for 15 min. The temperature was raised to 190 °C, after degassing and 0.5 mL of Cs-oleate solution (made from 0.4 g of Cs₂CO₃ dissolved in 11.75 g of octadecene, and 1.55 g oleic acid at 100 °C under vacuum), which was pre-heated on a hot-plate at 100 °C was swiftly injected. Five seconds after the injection, the nanocrystal solution was quickly cooled to room temperature with an ice bath and by the addition of 5 mL of toluene (at 100 °C). The nanocrystals were collected by centrifugation (2500 rpm/2 min.) and re-dispersed in 5 mL toluene. The composition of (Br/Cl) analog is CsPb(Br_0.4/Cl_0.6)₃, where a mix of 0.08 mmol of PbBr₂ and 0.12 mmol of PbCl₂ precursor were employed for the synthesis.

Synthesis of zero-dimensional Cs₄PbX₆ solids

The as obtained colloidal samples dispersed in n-octane were centrifuged and re-dispersed in tert-butanol and centrifuged at 7000 rpm for 5 min. The samples were then dried at 80 °C overnight to get respective Cs₄PbX₆  solids. The body color of the zero-dimensional solids were pure white for the Cs₄PbCl₆, Cs₄PbCl₆:Mn, and Cs₄Pb(Br/Cl)₆:Mn_, except Cs₄Pb(Br/Cl)₆ (pale greenish white), and Cs₄PbBr₆ (bright yellow).

Characterization

The synthesized zero-dimensional Cs₄PbX₆ nanocrystals were characterized by powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electron spin resonance spectra (ESR), inductive coupled plasma-optical emission spectroscopy (ICP-OES), field-emission scanning electron microscope (FE-SEM), and high-resolution tunneling electron microscope (HR-TEM). The XRD was performed using a Philips X’Pert diffractometer with Cu Kα radiation, in the range of 10° to 120° with a step size of 0.026° with the synthesized powders. The structural information was derived from Rietveld refinement using the General Structure Analysis System (GSAS) software suite. The VESTA program was used to draw the crystal structure⁵¹. The phase fractions of multiple phases were estimated using Rietveld refinement of XRD results considering full refinement of crystallographic and instrumental parameters as implemented in the GSAS program suite. The FE-SEM and HR-TEM images were recorded using a Hitachi S-4700 and FEI Tecnai F20 (200 kV), respectively, at Korea Basic Science Institute (KBSI), Gwangju, South Korea. The sample for TEM measurements were prepared by dispersing the nanocrystals in the ethanol medium. It is to be noted that the undoped and Mn²⁺-doped Cs₄PbX₆ perovskite were unstable upon exposure to high-energy electron beam and tend to damage within short period of time. XPS was performed on the powder samples using VG Multilab 2000 instruments to identify the chemical state of the elements on the sample surface (below 10 nm), employing Al as the target anode for the X-ray generation. The energy of the X-ray photon (Al Kα) is 1480 eV with a line width of 0.5 eV. The chemical compositions of the synthesized samples were analyzed using ICP-OES (Perkin-Elmer, OPTIMA 8300), at KBSI, Gwangju, South Korea. ESR measurement was carried out using JEOL JES-FA200 with an X-band microwave frequency of 9.17 GHz, with sample dispersion in n-octane. The liquid samples were taken in an ESR tube and frozen in liquid nitrogen at 173 K. All the spectra were recorded using the following ESR parameters: microwave power, 1 mW; modulation amplitude, 0.2 mT; modulation frequency, 100 kHz; and sweep time, 120 s.

Photoluminescence was measured using a Hitachi F-4500 fluorescence spectrophotometer over the wavelength range of 200 to 750 nm, using n-octane dispersions in a 1×1 cm quartz cuvette. UV-vis spectra were obtained using Optizen POP UV/Vis spectrophotometer. The lifetime was measured using a time-correlated single photon counting (TCSPC) system on FL920 Edinburgh instruments at Korean Advance Institute of Science and Technology (KAIST), South Korea. Pulsed laser irradiation with 300 nm was used as the excitation source. Internal quantum efficiency was measured with 365 and 290 nm excitation using a xenon laser (Hamamatsu C9920-02) at the Korea Photonics Technology Institute (KOPTI), South Korea. Time-resolved photoluminescence measurement for Mn²⁺ (600 nm) and D-state emission (420 nm for Cs₄Pb(Br/Cl)₆ and 400 nm for Cs₄PbCl₆) was carried out using single-mode pulsed diode laser as the excitation source (λ_max = 375 nm) at Korea Basic Science Institute (KBSI), Daegu Center, South Korea.