Mn(II)-Activated Zero-Dimensional Zinc(II)-Based Metal Halide Hybrids with Near-Unity Photoluminescence Quantum Yield

Abstract

As derivatives of metal halide perovskite materials, low-dimensional metal halide materials have become important materials that have attracted much attention in recent years. As one branch, zinc-based metal halides have the potential for practical applications due to their lead-free, low-toxicity and high-stability characteristics. However, pure zinc-based metal halide materials are still limited by their poor optical properties and cannot achieve large-scale practical applications. Therefore, in this work, we report an organic–inorganic hybrid zero-dimensional zinc bromide, (TDMP)ZnBr₄, using transition metal Mn²⁺ ions as dopants and incorporating them into the (TDMP)ZnBr₄ lattice. The original non-emissive (TDMP)ZnBr₄ exhibits bright green emission under the excitation of external UV light after the introduction of Mn²⁺ ions with a PL peak position located at 538 nm and a PLQY of up to 91.2%. Through the characterization of relevant photophysical properties and the results of theoretical calculations, we confirm that this green emission in Mn²⁺:(TDMP)ZnBr₄ originates from the ⁴T₁ → ⁶A₁ optical transition process of Mn²⁺ ions in the lattice structure, and the near-unity PLQY benefits from highly localized electrons generated by the unique zero-dimensional structure of the host material (TDMP)ZnBr₄. This work provides theoretical guidance and reference for expanding the family of zinc-based metal halide materials and improving and controlling their optical properties through ion doping.

1. Introduction

Lead-based metal halide perovskites with the general structural formula ABX₃ have received widespread attention from the scientific community in recent years due to their excellent optoelectronic properties, and they have been used in solar cells, light-emitting diodes, photodetectors and other fields [1,2,3,4,5,6,7,8,9,10,11,12,13,14]. Although remarkable achievements have been made in the field of optoelectronic applications, the inherent toxicity and instability of lead-based materials limit their further practical applications [15,16]. How to improve the stability of materials and reduce the environmental toxicity of Pb²⁺ ions while maintaining excellent optoelectronic properties has become an urgent problem to be solved.

Low-dimensional metal halide materials developed based on ABX₃ metal halide perovskites are ideal materials to replace lead-based metal halide materials [17,18]. In recent years, low-dimensional metal halide luminescent materials have been widely reported; in particular, Sb(III)-based, In(III)-based and Mn(II)-based materials have become typical representatives due to their high-efficiency luminescent properties and low toxicity [19,20,21,22,23,24]. In addition, low-dimensional Zn(II)-based materials based on Zn with a d¹⁰ electron configuration have gradually attracted the attention of researchers in the field in recent years due to their variable crystal structure and low-toxicity properties [25,26]. McWhorter et al. reported a Zn-based zero-dimensional metal halide, (C₇H₁₁N₂)₂ZnBr₄, with white light emission with a PLQY of up to 7.35% [25]. At the same time, they also used single crystals of the compound to prepare X-ray detectors and explored future practical applications of Zn-based low-dimensional halide materials. Furthermore, they reported another Zn-based organic–inorganic hybrid metal halide, (P-xd) ZnCl₄, with white light emission. This compound reached the current maximum white light emission PLQY in Zn-based materials of 23.06% [27]. In addition to broadband white light emission, Zn-based low-dimensional halide materials can also produce other colors of light. For example, R₂ZnCl₄ (R = (E)-4-styrylpyridinium, C₁₃H₁₂N⁺) with bright green emission, [N-EPD]₂ZnBr₄ with broadband yellow emission and (Ph₃S)₂ZnCl₄ with blue emission have been reported [28,29,30]. Unfortunately, the optical properties of these materials are poor, which is not conducive to future practical applications. In addition to single-mode light emission, some Zn-based materials also have the characteristic of tunable light emission. Ma et al. reported a kind of (BPP)ZnX₂ (X = Cl, Br) with a tunable emission color and ultralong room-temperature phosphorescence. Combining the results of experiments and theoretical calculations, they found that the dynamic polychromatic afterglow at room temperature can be attributed to the emission of the ligand BPP and its interaction with halogen atoms [31]. In addition, Wei et al. also reported a BAPPZn_{2 × 8} material with ultralong room-temperature phosphorescence, whose luminescence color is adjustable with the excitation wavelength and applied to the field of optical anti-counterfeiting [32]. Li et al. used a large-sized cyclic organic molecule, 2,5-bis(4-pyridinium)thiazolo [5,4-d]thiazole, as the A-site organic ligand and combined it with zinc halide to prepare a zero-dimensional structure of (H₂TTz)ZnX₄·MeOH (X = Cl, Br); the emission wavelength could switch from 462 nm to 512 nm, and they used Br to replace Cl in the lattice structure. Further, they explored the application of these two materials in the field of optical waveguides and found that both compounds had excellent optical waveguide properties. The optical loss coefficient R of (H₂TTz)ZnCl₄·MeOH at 462 nm was 0.01577 dB·μm⁻¹, and that of (H₂TTz)·ZnBr₄·MeOH at 515 nm was 0.01182 dB·μm⁻¹ [33]. These results revealed the great potential of low-dimensional Zn-based metal halide materials in photonic technology and provided new inspiration for the practical application of low-loss optical waveguides.

In summary, not only have many Zn-based halide materials been designed, developed and reported, but also a preliminary exploration of future practical applications has been carried out. However, the optical properties of the pure Zn-based metal halides reported so far still have inherent flaws and need to be further improved. How to improve the optical properties of Zn-based materials is of great significance to promoting their future commercial applications. Ion doping is an effective strategy to improve the optical properties of materials [34,35]. By introducing specific optically active ions into a non-luminescent matrix material, efficient luminescence can be achieved without changing the crystal structure of the matrix material. Taking the most widely reported and most intensively studied all-inorganic Cs-Zn-X (Cs₂ZnX₄ and Cs₃ZnX₅, X = Cl, Br, I) system as an example, many optically active ions have been doped into Cs-Zn-X and achieved a variety of efficient luminescent colors. The incorporation of Cu⁺ ions into Cs-Zn-X can achieve violet-to-blue light emission [36,37,38]. Cu⁺ ions usually exhibit blue emission after doping in chloride and bromide, while in iodide, they exhibit violet emission. Divalent Mn²⁺ ions usually form tetrahedrons in Zn-based halide systems and exhibit green light emission, and Sn²⁺ or Sb³⁺ exhibit deep red to near-infrared emission [39,40,41,42]. In addition to single doping, Cs-Zn-X co-doped with different luminescent ions has also received widespread attention. Wang et al. reported the tunable emission of Mn²⁺/Cu⁺ co-doped Cs₂ZnBr₄ and explored the application in the field of optical anti-counterfeiting [43]. Guo et al. reported Sb³⁺/Mn²⁺ co-doped Cs₂ZnCl₄ with dual-band emission. This material showed different luminescence responses at different temperatures. They built tunable thermochromic luminescent materials that can be used in applications such as thermal sensing and optical encryption [44]. Zeng et al. also reported Pb²⁺/Mn²⁺ co-doped Cs₂ZnBr₄ microcrystals with single-matrix white light emission. White light comes from the cooperative emission of Pb²⁺ ions and Mn³⁺ ions. At the same time, this white light emission can also be tuned from cold white to warm white by controlling the doping concentration of Pb²⁺ ions [45].

Inspired by ion doping, we used the protonated large-size cyclic organic molecule trans-2,5-dimethylpiperazine (C₆H₁₄N₂, TDMP) as the A-site cation and combined it with zinc bromide to prepare a zero-dimensional organic–inorganic hybrid zinc bromide, (TDMP)ZnBr₄. Optically active Mn²⁺ ions were introduced into the crystal lattice of non-luminescent (TDMP)ZnBr₄ as dopants, and bright green light emission (λ_em = 538 nm) with a PLQY of up to 91.2% under UV light excitation was achieved. Through optical property characterization and first-principles calculations, it was confirmed that the bright green emission in Mn²⁺:(TDMP)ZnBr₄ comes from the d-d transition of Mn²⁺ ions, and near-unity PLQY is related to the zero-dimensional structure of (TDMP)ZnBr₄. The zero-dimensional structure is conducive to the generation of a strong quantum confinement effect and dielectric confinement effect, which can have a strong binding effect on photogenerated excitons and ensure an efficient radiation recombination process, thereby obtaining an ultra-high PLQY. This work provides a certain reference for improving the optical properties of Zn-based metal halide materials and lays a theoretical foundation for their future commercial applications.

2. Results and Discussion

2.1. Structure and Morphology

The crystal structure of the host material (TDMP)ZnBr₄ is shown on the left side of Figure 1a. The (TDMP)ZnBr₄ crystalline in the C2/c space group is as previously reported [46]. A (TDMP)ZnBr₄ unit cell consists of four [ZnBr₄]²⁻ tetrahedrons and fifteen protonated TDMP molecules. [ZnBr₄]²⁻ tetrahedrons are periodically arranged in the structural skeleton composed of protonated TDMP molecules. Adjacent [ZnBr₄]²⁻ tetrahedrons are fully separated by protonated TDMP molecules and exhibit isolated distribution at the molecular level. Therefore, there are no interactions between adjacent [ZnBr₄]²⁻ tetrahedrons, which is typical of zero-dimensional structures. There are two types of Zn-Br bonds in each [ZnBr₄]²⁻ tetrahedron, with bond lengths of 2.3964 Å and 2.4248 Å. These two bond lengths are very similar, indicating that the structural distortion of the [ZnBr₄]²⁻ tetrahedron is very small. It is worth noting that, when Mn²⁺ ions are incorporated into the (TDMP)ZnBr₄ lattice, they replace the lattice positions of Zn²⁺ ions to form [MnBr₄]²⁻ tetrahedrons, which indicates that the doping of Mn²⁺ ions does not change the crystal structure of (TDMP)ZnBr₄. As shown on the right side of Figure 1a, a unit cell of Mn²⁺:(TDMP)ZnBr₄ contains the metal–halogen tetrahedron, with Zn and Mn occupying the site of the tetrahedron center and protonated TDMP molecule. In Mn²⁺:(TDMP)ZnBr₄, the metal–Br bond length of the tetrahedron is the same as the undoped system. There are two types of metal–Br bonds, with bond lengths of 2.5375 Å and 2.5675 Å. The deformation of tetrahedrons is also very small in Mn²⁺:(TDMP)ZnBr₄, which indicates that the doping of Mn²⁺ ions does not cause local lattice distortion of the host material. In addition, the bond length of the metal–Br bond becomes larger after Mn²⁺ doping, indicating that the doping of Mn²⁺ ions causes the expansion of the tetrahedron. This is due to the different ionic radii of Mn²⁺ and Zn²⁺. In the case of four coordination, the ionic radius of Mn²⁺ is 0.66 Å, and the ionic radius of Zn²⁺ is 0.60 Å [47]. The incorporation of larger Mn²⁺ ions into the crystal structure will inevitably lead to the expansion of the lattice, which will increase the bond length of the metal–Br in the tetrahedron. The XRD patterns of (TDMP)ZnBr₄ samples with different Mn²⁺ ion feed ratios are shown in Figure 1b. The undoped (TDMP)ZnBr₄ lattice structure files used for simulation calculations were obtained from the Cambridge Crystallographic Data Center (CCDC-1424478). The XRD patterns of all (TDMP)ZnBr₄ samples are in good agreement with those calculated using simulations, and no redundant diffraction peaks appear, indicating that Mn²⁺:(TDMP)ZnBr₄ were successfully synthesized and that the samples were pure and free of impurities. At the same time, the diffraction peak of the sample moves to a low angle direction as the Mn²⁺ ion feed ratio increases. This result indicates the successful doping of Mn²⁺ ions, because the incorporation of Mn²⁺ ions with a larger radius will cause the lattice to expand and shift the diffraction peak to a lower angle direction. The surface morphology of Mn²⁺:(TDMP)ZnBr₄ microcrystals and their distribution of elements were observed using a scanning electron microscope (SEM) and energy-dispersive X-ray spectrometer attachment (EDS), which obtain by HITACHI SU8020 (HITACHI, Tokyo, Japen). Figure 1c shows the surface morphology of the Mn²⁺:(TDMP)ZnBr₄ microcrystals and their corresponding element distribution. The surface of rod-shaped Mn²⁺:(TDMP)ZnBr₄ microcrystals is smooth, indicating that the microcrystalline samples synthesized through solvothermal synthesis have good crystallinity and high crystal quality. The results of the elemental analysis show that Zn, Mn and Br elements are evenly distributed in the crystal, and the atomic ratio of the three elements is Zn:Mn:Br = 20.4:0.95:78.65 = 1:0.047:3.855 (Figure S1). The atomic ratio of Zn to Br is close to the stoichiometric ratio of 1:4 in (TDMP)ZnBr₄, which further demonstrates the successful synthesis of (TDMP)ZnBr₄ and the doping of Mn²⁺ ions. In addition, the Raman spectrum of undoped (TDMP)ZnBr₄ and Mn²⁺:(TDMP)ZnBr₄ obtained with a 633 nm continuous laser as the excitation source was also collected, as shown in Figure S2. Figure S2a shows the inorganic part of the Raman spectra. The Raman mode in the low wavenumber (<400 cm⁻¹) range originates from the vibrations of inorganic [ZnBr₄]²⁻ and [MnBr₄]²⁻ tetrahedrons [48]. The Raman mode of the Mn²⁺ ion-doped sample shifts to a low wavenumber direction compared with the undoped sample, which is consistent with the XRD pattern. This result also proves that Mn²⁺ ions were successfully doped in the structure of (TDMP)ZnBr₄, because the expansion or contraction of the crystal lattice causes a shift in the Raman peak [49]. The Raman spectrum of the organic part demonstrates that the protonated organic TDMP molecules were successfully inserted into the lattice of inorganic ZnBr₂ (Figure S2b).

2.2. Optical Properties

The optical properties of Mn²⁺:(TDMP)ZnBr₄ were characterized to explore their underlying photophysical mechanisms. First, a Lambda-750 UV-vis-NIR spectrophotometer was used to collect the absorption spectra of (TDMP)ZnBr₄ with different Mn²⁺ ion feed ratios. As shown in Figure 2a, undoped (TDMP)ZnBr₄ absorbs in the range of 250 nm to 550 nm, and the absorption intensity gradually increases as the wavelength decreases. Three new absorption bands appear at 267 nm, 285 nm and 353 nm when Mn ions are incorporated, and the intensity of these three absorption bands increases with the increase in the Mn²⁺ ion feed ratio. Obviously, these absorption bands come from the d-d transition absorption of Mn²⁺ ions. According to the elemental analysis results, the actual Mn²⁺ ions incorporated into the (TDMP)ZnBr₄ lattice are still quite limited, even though the feed ratio of Mn²⁺ ions reaches 40% or even 60%. The following formula was used to calculate the bandgap value of (TDMP)ZnBr₄ with different Mn²⁺ ion feed ratios [50]:

[F(R_∞)hυ]ⁿ = A(hυ − E_g)

where F(R_∞) is the Kubelka–Munk (K-M) equation, hυ is the photon energy, A is the constant, and E_g is the bandgap. The value of n depends on the bandgap properties of the semiconductor. n = 2 for the direct bandgap semiconductor, and n = 1/2 for the indirect bandgap semiconductor. Since (TDMP)ZnBr₄ is a direct bandgap semiconductor, the bandgap values of the (TDMP)ZnBr₄ samples with different Mn²⁺ ion feed ratios were calculated according to the above formula, and the results are shown in Figure S3. The bandgap of the undoped (TDMP)ZnBr₄ sample is 3.71 eV, and the bandgap will slightly reduce while Mn²⁺ ions are introduced.

The photoluminescence excitation spectrum (PLE) and photoluminescence (PL) spectrum of Mn²⁺:(TDMP)ZnBr₄ are shown in Figure 2b. Mn²⁺:(TDMP)ZnBr₄ exhibits bright green emission under the excitation of 365 nm UV light, with the PL peak located at 538 nm; the full width of half maximum (FWHM) is 51.71 nm, and the Stokes shift is 173 nm. It is worth noting that the green emission of Mn²⁺:(TDMP)ZnBr₄ is not completely standard green. The CIE coordinate of this green emission is (0.30, 0.67) and located at the edge of the green light area, as shown in Figure S4. The PLE spectrum of Mn²⁺:(TDMP)ZnBr₄ was collected at the PL position of 538 nm. Obvious excitation bands can be observed at 275 nm, 291 nm, 360 nm, 375 nm, 388 nm and 462 nm, which belong to the absorption transitions of ⁶A₁ to ⁴E(D), ⁴T₂(D), ⁴A₁(G), ⁴E(G), ⁴T₂(G) and ⁴T₁(D) of Mn²⁺ ions, respectively [51,52]. The excitation band in the PLE spectrum does not observe the contribution of the host material (TDMP)ZnBr₄ to the 538 nm green emission. All the excitation bands come from the absorption transition of Mn²⁺ ions, which shows that the optical transition process of Mn²⁺:(TDMP)ZnBr₄ is dominated by the Mn²⁺ ions doped in the crystal lattice. The PL spectra of (TDMP)ZnBr₄ with different Mn²⁺ ion feed ratios are shown in Figure S5. As the feed ratio increases, the PL intensity of (TDMP)ZnBr₄ first increases and reaches the maximum when the Mn²⁺ feed ratio is up to 20%. Then, the PL intensity of (TDMP)ZnBr₄ decreases as the feed ratio of Mn²⁺ ions is further increased, which is caused by the concentration quenching effect under high-concentration doping. The PLQY of the Mn²⁺-doped sample was collected using 365 nm UV light as the excitation source. The measurement results show that Mn²⁺:(TDMP)ZnBr₄ reaches the maximum of 91.2% when the Mn²⁺ ion feed ratio is 20%, as shown in Figure S6. In order to further explore the origin of green emission in Mn²⁺:(TDMP)ZnBr₄, we collected the PL spectra under different excitation wavelengths and the PLE spectra under different emission wavelengths, as shown in Figure S7. The PLE spectra of Mn²⁺:(TDMP)ZnBr₄ collected at different emission wavelengths were normalized. We found that the obtained PLE spectra all have the same profile as the emission wavelength increases from 520 nm to 550 nm, as shown in Figure S7a. At the same time, the PL spectra obtained at different excitation wavelengths also show the same profile (Figure S7b). This indicates that the 538 nm green emission in Mn²⁺:(TDMP)ZnBr₄ originates from the same excited state.

The PL lifetime decay curves of (TDMP)ZnBr₄ under different Mn²⁺ ion feed ratios were collected to confirm the source of green emission. As shown in Figure 2c, all Mn²⁺:(TDMP)ZnBr₄ samples exhibit hundreds of microseconds PL decay lifetimes. This long lifetime of hundreds of microseconds to tens of milliseconds is a typical feature of the Mn²⁺ ions’ d-d transition (⁴T₁ → ⁶A₁) [51,53,54]. Combining the PLE spectrum and PL spectrum results, we confirmed that the 538 nm green emission in Mn²⁺:(TDMP)ZnBr₄ originates from the ⁴T₁ → ⁶A₁ radiation transition of Mn²⁺ ions. Therefore, the photophysical processes in Mn²⁺:(TDMP)ZnBr₄ can be summarized as follows: Optically active Mn²⁺ ions are introduced into non-luminescent (TDMP)ZnBr₄. Mn²⁺ ions replace the lattice site of Zn²⁺ ions and combine with the surrounding four Br⁻ ions to form [MnBr₄]²⁻ tetrahedrons when they enter the (TDMP)ZnBr₄ lattice. According to the crystal field theory, the crystal field intensity experienced by the four-coordinated Mn²⁺ ions is weak. The energy level difference between the ⁴T₁ and ⁶A₁ energy levels is large, leading to high-energy green emission under the excitation of UV light. According to the results of the PLE spectrum, the host material does not contribute to the emission; hence, the excitation–emission process in Mn²⁺:(TDMP)ZnBr₄ occurs entirely at the d energy level of the Mn²⁺ ions. As shown in Figure 2d, under UV light excitation, electrons at the ground-state energy level ⁶A₁ absorb the energy of UV light and transition to excited states of higher energy levels. Subsequently, the electrons in the high-energy excited state will return to the lowest energy level ⁴T₁ excited state through non-radiative transition and, finally, transition back to the ground state through radiative recombination producing 538 nm green emission.

In order to deeply study the relationship between the crystal structure and photophysical properties, PL spectra of Mn²⁺:(TDMP)ZnBr₄ in the temperature range of 80–400 K were collected. As shown in Figure 3a, Mn²⁺:(TDMP)ZnBr₄ exhibits single-band emission in the entire temperature range from 80 K to 400 K, without additional emission bands appearing with temperature changes. This indicates that Mn²⁺:(TDMP)ZnBr₄ has a high structural stability and that the crystal structure will not undergo phase changes with drastic changes in temperature, because phase changes may generate new emission bands. The PL intensity at each temperature was extracted from the temperature-dependent PL spectrum and used as the vertical axis with the reciprocal of temperature as the horizontal axis, and a scatter plot of the changing trend of PL intensity with the reciprocal of temperature was drawn. The scatter plot was fitted using the Arrhenius formula to obtain the activation energy E_b of Mn²⁺:(TDMP)ZnBr₄ [52]:

$$I\left(T\right)=\frac{I_0}{\left(1+A{e}^{\frac{-E_b}{k_{b}T}}\right)}$$

where I₀ is the PL intensity at 0 K, I(T) is the PL intensity at temperature K, and k_b is the Boltzmann constant. The activation energy E_b obtained by fitting is 43.75 meV, which is much higher than the room-temperature thermal energy (~26 meV) [55]. This indicates that the emission of Mn²⁺:(TDMP)ZnBr₄ has strong resistance to thermal quenching. A larger activation energy means that the dissociation process requires more energy, which also indicates that the excitons are more stable and have stronger resistance to thermal quenching [56]. This means that, when the temperature reaches 400 K, Mn²⁺:(TDMP)ZnBr₄ can still maintain a certain intensity of emission, as shown in the pseudo-color image of the temperature-dependent PL spectrum (Figure 3c). In addition, we also extracted the FWHM of the Mn²⁺:(TDMP)ZnBr₄ spectrum at each temperature and fitted the FWHM–temperature change curve using the following formula [57]:

$$\mathrm{FWHM}\left(T\right)=2.36\sqrt{S}\hslash {\mathsf{\omega }}_{\mathrm{phonon}}\sqrt{\coth \frac{\hslash {\mathsf{\omega }}_{\mathrm{phonon}}}{2k_{b}T}}$$

where S is the Huang–Rhys factor, ω_phonon is the phonon frequency, T is the temperature, FWHM is the full-width half maximum of the PL spectra, and k_b is the Boltzmann constant. The fitting results are shown in Figure 3d, and Huang–Rhys factor S obtained by fitting is 6.65. Such a small S value indicates that the emission of Mn²⁺:(TDMP)ZnBr₄ comes from the d-d transition of Mn²⁺ ions rather than self-trapped exciton (STE) emission caused by local lattice distortion because STE emission requires a certain intensity of the electron–phonon coupling effect, and the S value obtained by fitting is larger [58]. In addition, the measurement results of the Raman spectrum also confirm this result. Strong Raman scattering at room temperature is an important indicator of strong coupling between excited-state electrons and lattice vibrations [59]. Therefore, strong scattering interactions reflect the tendency of local fluctuations in the lattice. This propensity to form localized charges plays a role in charge transport, as free charge carriers cause lattice deformation, which moves with the carriers and forms self-trapped excitons. The Raman spectrum in Figure S2 shows a very weak Raman scattering intensity, which indicates that the coupling between its electrons and lattice vibration is too weak to produce STE emission. At the same time, cyclic organic molecules with greater structural rigidity are also an important reason for the smaller S value, because the lattice vibration produced by rigid organic molecules is smaller [60].

2.3. Theoretical Calculation

In order to study the relationship between the Mn²⁺:(TDMP)ZnBr₄ electronic structure and luminescence properties, we performed first-principles calculations on (TDMP)ZnBr₄ before and after Mn²⁺ ion doping. Figure 4a,c present the energy band structure of (TDMP)ZnBr₄ before and after Mn²⁺ ion doping, respectively. The pristine (TDMP)ZnBr₄ bandgap value obtained by first-principles calculations is 3.76 eV, while the bandgap value of Mn²⁺:(TDMP)ZnBr₄ is 3.12 eV. The calculation results show that the doping of Mn²⁺ ions shrinks the bandgap of the host material, which is consistent with the actual bandgap change in the samples obtained using the tauc plot method. Figure 4b,d show the density of state (DOS) of (TDMP)ZnBr₄ and Mn²⁺:(TDMP)ZnBr₄, respectively. In undoped (TDMP)ZnBr₄, the conduction band minimum (CBM) is formed by Br-4p orbitals and Zn-5s orbitals, while the valence band maximum (VBM) is mainly formed by Br-5p orbitals, as shown in Figure 4b. The VBM of Mn²⁺:(TDMP)ZnBr₄ is formed by Br-4p orbitals and Mn-3d orbitals, while the CBM is mainly formed by Br-4p orbitals, as shown in Figure 4d. The theoretical calculation results indicate that the Zn-5s orbital involved in band-edge formation is replaced by the Mn-3d and Br-4p orbitals when Mn²⁺ ions are incorporated. This indicates that [MnBr₄]²⁻ tetrahedrons dominate the main optical transition process in Mn²⁺:(TDMP)ZnBr₄ after doping. The UV light directly excites Mn²⁺ ions in the lattice structure, and all optical processes occur between the d orbital energy levels of Mn²⁺ ions. This also explains why the contribution of the host material to 538 nm green emission cannot be observed in the PLE spectrum. Organic cations do not contribute to the formation of frontier orbits but could affect the optical processes by producing spatial confinement effects and dielectric confinement effects on [MnBr₄]²⁻ tetrahedrons. At the same time, Mn²⁺:(TDMP)ZnBr₄ shows a relatively flat conduction band and valence band with very small dispersion, which indicates that the electrons are highly localized (Figure 4c). This band structure is unique to the 0D structure [22]. Each [MnBr₄]²⁻ tetrahedron is separated by organic cations to form the highly localized electrons in Mn²⁺:(TDMP)ZnBr₄, thus generating the isolated luminescent centers. This highly localized electron is a key factor in producing efficient luminescence [22,61].

2.4. Stability

The stability of a material is an important indicator of whether it has practical application value. Here, we determined the structure, luminescence and thermal stability of Mn²⁺:(TDMP)ZnBr₄ to evaluate its potential for practical applications in the future. As shown in Figure 5a, the XRD diffraction pattern of Mn²⁺:(TDMP)ZnBr₄ after being placed under ambient conditions for 60 days is basically consistent with the diffraction pattern of the as-synthesized sample, and no additional impurity diffraction peaks were observed, indicating that this compound can retain a stable structure without decomposition for a long time. Meanwhile, Mn²⁺:(TDMP)ZnBr₄ also has high PL stability. The PL intensity of Mn²⁺:(TDMP)ZnBr₄ does not decrease significantly and remains at 90–95% of the initial PL intensity after being placed under ambient conditions for 60 days (Figure 5b), and it can exhibit bright green emission under UV light excitation. Finally, the results of the thermogravimetric analysis show that Mn²⁺:(TDMP)ZnBr₄ also has high thermal stability. As shown in Figure 5c, the decomposition temperature of Mn²⁺:(TDMP)ZnBr₄ is about 342 °C. All experiments, including synthesis, characterization and storage, were performed under ambient conditions, with a relative humidity of 50–70% without any inert gas protection. The stability test results show that Mn²⁺:(TDMP)ZnBr₄ has great structural, luminescence and thermal stability, and it has application potential in the field of future optoelectronic devices.

3. Conclusions

In summary, we selected transition metal Mn²⁺ ions as sensitizers and introduced them into a non-luminescent (TDMP)ZnBr₄ lattice in this work. Under the excitation of 365 nm UV light, Mn²⁺:(TDMP)ZnBr₄ exhibits bright green light emission (λ_em = 538 nm) with a PLQY of up to 91.2%. Combining optical property characterization and first-principles calculations, we confirmed that the 538 nm green light emission in Mn²⁺:(TDMP)ZnBr₄ originates from the ⁴T₁ → ⁶A₁ transition of Mn²⁺ ions in the lattice structure. The unique 0D structure of the host material can generate a strong quantum confinement effect and dielectric confinement effect, which is very beneficial to the generation of highly localized excitons. The photogenerated excitons will be bound within the tetrahedron by the strong confinement effect of the crystal lattice once they are formed by outside excitation, and they will radiate photons outward through radiative recombination, ultimately producing efficient green light emission. Otherwise, Mn²⁺:(TDMP)ZnBr₄ also has high structural, luminescence and thermal stability, which further enhances the competitiveness of this compound in future practical applications. This work provides theoretical guidance and references for expanding the zinc-based metal halide family and improving its optical properties via ion doping.