Advances of Low-Dimensional Organic-Inorganic Hybrid Metal Halide Luminescent Materials: A Review

Abstract

Low-dimensional organic–inorganic hybrid metal halides (OIMHs) have garnered significant research attention due to their remarkable optical, electrical, and mechanical properties. These materials feature tunable optoelectronic characteristics, high photovoltaic efficiency, exceptional scalability and processability and ease of fabrication. By selecting appropriate organic and inorganic components, it is possible to achieve molecular-level dimensional control of the metal halides. Here, this review provides an in-depth analysis of the structure and synthesis methods of OIMHs materials, explores their optical properties, and summarizes their current applications in areas such as white-light LEDs, X-ray detectors, sensors, and solar cells. Finally, we also discuss the challenges faced by these materials and offer a perspective on their future development, aiming to serve as a reference for advancing research in OIMHs.

1. Introduction

Three-dimensional (3D) metal halides, as a type of organic–inorganic hybrid luminescent material, have attracted widespread attention due to their superior absorption coefficients, tunable bandgaps, high charge carrier mobility, exceptional ionic tolerance, and cost-effectiveness [1,2,3,4,5]. These properties make them highly applicable in a range of fields, including photovoltaic cells, light-emitting diodes (LEDs), photodetectors, lasers, and scintillators [6,7,8]. Although oxide perovskites were discovered as early as 1839, it was not until 2009 that Tsutomu Miyasaka’s research group incorporated MAPbBr₃ and MAPbI₃ into dye-sensitized solar cells, achieving a power conversion efficiency of 3.8%, thereby demonstrating the potential use of metal halides in photovoltaics [9,10]. To date, the conversion efficiency of laboratory 3D metal halide solar cells has surpassed 26% [11,12]. In 2014, Friend and colleagues further validated the potential use of 3D metal halides in electroluminescent devices, sparking significant enthusiasm among researchers seeking to explore these materials [13].

The general structural formula of common 3D metal halides is A^IB^IIX₃, characterized by a typical 3D perovskite framework. In this structure, the A-site is occupied by small monovalent cations (e.g., Cs⁺, Rb⁺, MA⁺, CH (NH₂)⁺), the B-site by divalent metal cations (e.g., Pb²⁺, Cu²⁺, Mn²⁺, Cr²⁺), and the X-site by halide anions (e.g., F⁻, Cl⁻, Br⁻, and I⁻). CsPbX₃ (X = Cl, Br, I), a representative of fully inorganic 3D metal halides, exhibits tunable emission colors and high photoluminescence quantum yields (PLQY), making it a promising material for use in lighting applications [14,15,16]. Although the narrow-band emission resulting from bimolecular recombination in 3D metal halides offers high PLQY and adjustable optical properties, their severe air instability often leads to performance degradation [17,18,19,20]. Additionally, the environmental and biological hazards associated with Pb limit their large-scale application in optoelectronic devices [21,22,23]. In the 3D perovskite structure, the BX₆ octahedra are typically connected via corner-sharing. By replacing the small monovalent cations at the A-site with larger organic ammonium cations, the structure can be dimensionally reduced, resulting in the formation of low-dimensional OIMHs, including two-dimensional (2D), one-dimensional (1D), and zero-dimensional (0D) structures (Figure 1) [24,25,26].

Low-dimensional OIMHs have gained significant attention due to their superior luminescence properties. In low-dimensional OIMHs, the reduced spatial confinement leads to quantum effects, such as discrete energy levels, which improve the emission efficiency and tunability of the material’s luminescence [27,28]. In addition, excitons (electron–hole pairs) are more strongly confined, leading to intense emission peaks, as well as better PLQY [29]. Compared to their 3D counterparts, the optical and electrical properties of the low-dimensional OIMHs can be tuned by controlling their structures, compositions and size [30]. For example, the band gap of low-dimensional OIMHs can be tuned by adjusting the composition and thickness of the perovskite layers [31]. Usually, doping with different metal ions or controlled defect engineering in low-dimensional OIMHs can introduce additional luminescent centers, modifying the emission spectra to achieve desired optical characteristics [32]. Such tunability makes them versatile for various optoelectronic applications. The reduced dimensionality can also limit the movement of ions and help prevent degradation, which is a major issue for 3D perovskites; therefore, low-dimensional OIMHs generally exhibit better stability against moisture, heat, and ultraviolet radiation [33]. Moreover, by modifying the dimensionality, low-dimensional OIMHs can reduce the amount of lead or other toxic elements, which is a critical concern for 3D perovskites, especially in large-scale production and applications.

Low-dimensional OIMHs exhibit structural diversity, ease of synthesis, and suitability for large-scale production [31,34]. Notably, most low-dimensional OIMHs feature strong quantum confinement, tunable bandgaps and highly efficient emissions, making them highly competitive luminescent materials for next-generation optoelectronic applications [35,36]. Consequently, the design and synthesis of novel low-dimensional OIMHs with efficient light emission have become key areas of research interest [37]. The field of organic–inorganic hybrid metal halides has witnessed significant advancements in recent years [38,39,40,41,42]. There are a number of review articles available for 3D or 0D metal halide perovskites, which summarize their properties and applications in optoelectronic devices [43,44,45,46]. However, little attention has been directed towards the rapid development of innovative studies for low-dimensional OIMHs [47,48]. This review provides a systematic summary of the crystal structures and synthesis methods of low-dimensional OIMHs, along with an analysis of their optical properties, luminescence mechanisms, and application fields. Finally, it offers insights into the future challenges and opportunities for these materials, aiming to provide a valuable reference for ongoing research in the field.

2. Structure of Low-Dimensional OIMHs

Figure 2 presents a schematic diagram of the structures of metal halide materials across different dimensions [49,50,51]. The early 3D metal halide perovskites (formula A^IB^IIX₃) consist of a simple cubic arrangement of divalent B-site cations bridged by halide anions (X = F⁻, Cl⁻, Br⁻, I⁻), which create a 3D anionic network of corner-sharing octahedra. Only the organic cation has a relatively small volume, and the inorganic octahedra grow along the metal–halogen coordination bonds, forming a 3D network structure (Figure 2a) [52]. In contrast, 2D metal halides can be considered as sheets or layers split from a 3D structure along specific crystallographic directions (Figure 2b) [53]. In 2D structures, the inorganic metal halide octahedra are corner-connected and extend within the crystal plane. These layers are interspersed with organic cations, forming an alternating inorganic–organic–inorganic layered structure. The 2D perovskite structure allows for substitutions at the B and X sites, while a much greater diversity of A-site cations can be accommodated between the inorganic layers, yielding a richer platform for chemical tuning. Based on the bonding modes of the spacer cations, 2D metal halides are typically categorized into three types. They are Ruddlesden–Popper (RP) type (R₂A_n−1B_nX_3n+1), Dion–Jacobson (DJ) type (RA_n−1B_nX_3n+1), and Alternating Cation Interlayer (ACI) type [(GA)A_nB_nX_3n+1, GA-guanidinium] [54]. In these three types, R mainly represents the organic ammonium cations, which may be aromatic or aliphatic alkyl amines, such as butylamine (BA) or 2-phenylethylamine (PEA) [55]. n represents the number of inorganic BX₆ octahedral layers between two large organic ammonium cation layers. Depending on whether n = 1 or another integer, these structures can be further classified as purely 2D (for n = 1) or quasi-2D (for n > 1) [56,57]. This fine-tuning of n enables tailored optical and electronic properties suitable for various applications [58,59,60].

Here, 1D metal halide structures can be derived by slicing perpendicular to the 2D inorganic sheets (Figure 2c) [61]. The structure can adopt linear or zigzag geometries, with the variety of organic cations and bonding modes contributing to significant chemical diversity and strong structural stability in ambient environments [62]. 1D metal halides usually feature better stability against desorption and moisture than their 3D counterparts. Usually, the BX₆ octahedra in 1D metal halides can connect via corner-sharing, edge-sharing, or face-sharing, forming 1D nanowires encapsulated by organic ammonium cations at the molecular level. The discontinuity between adjacent metal halide building blocks results in variations in electronic structures, enabling 1D metal halides to exhibit properties analogous to macroscopic nanowires. Then, these 1D metal halide materials can be considered as core–shell nanostructured assemblies with organic cations as the shell and negatively charged octahedra chains as the core. Thus, this structural stability and unique electronic characteristics make 1D metal halides a promising candidate for advanced optoelectronic applications [63,64,65].

Further slicing perpendicular to the 1D structure results in 0D metal halides composed of non-connected metal halide octahedra (Figure 2d) [66]. In this structure, individual BX₆ octahedra are spatially isolated and surrounded by organic components, forming single-core 0D metal halides. It must be mentioned that the outer electronic configuration of B-site cations is crucial for 0D metal halide materials, exerting influence on both their structural and photoelectric properties. In addition to octahedral units, other types of metal halide polyhedra, such as pyramids, tetrahedra, and seesaw structures, can also serve as the inorganic units in 0D metal halides [67]. Also, multinuclear metal halide clusters have been reported in 0D metal halides, such as (CH₃NH₃)₃Bi₂I₉, which contains the binuclear [Bi₂I₉]³⁻ cluster [68,69,70,71]. This cluster is separated by CH₃NH₃⁺ groups, and such a unique structure is beneficial for enhancing device stability and longevity. In addition, organic cation substitution engineering enlarges the atomic distance between isolated luminescent centers, forming a perfect host–guest system with unprecedented luminescent performance. The diverse ion selection provides infinite possibilities for designing the components of 0D metal halides, enhancing both the structural diversity and the tunability of their properties, thus significantly expanding the variety of metal halides [72]. Therefore, the tunability in the context of the composition and crystal structure of 0D metal halides results in their abundant and tailorable optoelectronic properties in metal halide perovskite materials.

Density functional theory (DFT) is often used to calculate the electronic structure of low-dimensional OIMHs. These calculations can provide insights into band gaps, charge carrier mobility, and stability under different environmental conditions, and they can even predict how defects (such as vacancies, interstitials, and antisites) affect the stability and performance of these materials, helping guide the design of more robust compounds. Moreover, theoretical models are employed to predict the absorption spectra, photoluminescence, and excitonic effects. Also, one of the attractive features of low-dimensional OIMHs is their tunable band gap. Through theoretical calculations, it is possible to predict how different organic cations, halide anions, or metal ions can be combined to control the band gap, providing pathways for designing low-dimensional OIMHs with optimized optoelectronic properties for specific applications.

3. Preparation Strategies of Low-Dimensional OIMHs

Common synthesis methods for low-dimensional OIMHs crystals include temperature-induced crystallization, anti-solvent vapor diffusion, solvent evaporation, and mechanical grinding [73,74,75,76]. These methods can be further categorized into solvent-based and solvent-free approaches (

Table 1. Comparison of solvent-based methods and solvent-free methods.

Aspect	Solvent-Based Methods	Solvent-Free Methods
Scalability	High, but solvent management can be challenging	Moderate, often requiring specialized equipment
Morphology control	Excellent (via solubility, temperature, surfactants)	Limited, mainly controlled by reaction parameters
Low-dimensional control	Effective for nanostructure synthesis	Less precise, but advancing in mechanochemistry
Environmental impact	Solvent waste, energy-intensive drying steps	Green chemistry-friendly, minimal waste
Efficiency	High reaction rates, better dispersion	May require high energy input (e.g., milling)
Process Simplicity	Complex (multi-step synthesis)	Simple, direct solid-state reactions

). A detailed introduction of these methods can be given as follows.

Solvent-free methods are commonly used for synthesizing low-dimensional OIMHs. Among these, the most representative is the mechanical grinding method, which offers several advantages such as simplicity and cost-effectiveness over solution-based methods [77,78]. These include simplicity of operation, the elimination of organic solvents, and controllable stoichiometry. The mechanical grinding method generates fewer by-products compared to solution-based methods, making it particularly suitable for the large-scale production of metal halides [79]. For example, Xu developed a series of 2D M₂CdCl₄:x%Mn (M = CH₃NH₃⁺, C₂H₈N⁺, C₃H₁₀N⁺) materials via mechanical grinding, where the PLQY of MA₂CdCl₄:20%Mn approached 90% [80]. He et al. synthesized 0D (TMS)₂MnBr₄ powder with a PLQY of up to 69.8% using ball milling [81]. Jiang et al. combined mechanical grinding and solution methods to synthesize polycrystalline powder EDBEMnBr₄, which exhibited efficient green emission [82]. Despite its advantages, such as a simple, fast, environmentally friendly process and low preparation costs, the mechanical grinding method cannot achieve precise control over the crystal shape and size. In a recent study, Umemoto et al. proposed a novel and straightforward ultrasound-assisted bead milling process, leveraging the characteristics of facile crystallization (Figure 3a) [83]. During milling, ultrasonic irradiation disperses the milling beads, enhancing intense collisions between the beads and the perovskite precursors. As a result, this bead milling method eliminates the need for planetary ball milling, offering a simple approach to producing brightly luminescent metal halide with high PLQY. However, the mechanical grinding method always leads to a broad distribution of crystal sizes, which can negatively impact the uniformity and quality of the low-dimensional OIMHs. The limited degree of morphology control is due to the absence of a medium that influences nucleation and growth processes. The high shear forces and friction involved in grinding can cause the degradation of the low-dimensional OIMHs, potentially leading to the formation of defects or unwanted phases that can degrade the material’s luminescent properties. It also has difficulties in the control of stoichiometry. Despite these drawbacks, the method remains a popular choice due to its simplicity and low cost, but alternative techniques like solution-based synthesis or vapor deposition are increasingly being explored to address these limitations [77,78].

Solvent-based synthesis methods are more scalable due to ease of mixing, heat transfer, and reaction control in a liquid medium, which provide superior control over particle morphology, size distribution, and crystallinity due to the tenability of parameters like concentration, reaction time, and temperature. Solvent-based synthesis methods are widely used for synthesizing large-sized crystals [84,85]. This method can be further categorized into cooling-induced crystallization and inverse temperature crystallization, with common solvents including methanol, ethanol, tetrahydrofuran (THF), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and hydrogen halides [86,87]. In cooling crystallization, the precursor is completely dissolved in a solvent, and then, as the system cools slowly and the solubility of the precursor decreases, metal halide crystals are formed. By adjusting the cooling interval and cooling rate, the shape and size of the crystals can be precisely controlled. For example, Yue’s team successfully prepared centimeter-sized 2D BDAPbI₄ single crystals by cooling the precursor solution from 90 °C to room temperature at a rate of 1 °C/h [88]. In contrast to cooling crystallization, inverse temperature crystallization is suitable for solvents whose solubility is negatively correlated with temperature [89]. Zhumekenov and colleagues utilized inverse temperature crystallization to prepare centimeter-sized metal halide single-crystal films [90]. Zhuang et al. prepared centimeter-sized 0D (NH4)₃Bi₂I₉ single crystals using inverse temperature crystallization from 110 °C to 60 °C, which demonstrated an anisotropic response according to a highly sensitive X-ray detector, as shown in Figure 3b [91].

The solvent evaporation method is very similar to temperature-induced crystallization [92,93]. In this method, the precursor is first dissolved in a solvent, and then the solvent is evaporated. As the system reaches a highly supersaturated state, crystals precipitate from the solution. Zhang et al. achieved nucleation and crystal growth by evaporating the solvent, successfully preparing ultra-large-sized (>200 mm²) 2D (PEA)₂PbBr₄ crystals [94]. The anti-solvent diffusion method utilizes the solubility differences between substances in different solvents to induce crystallization. By controlling the diffusion rate, single crystals and powders can be obtained. Yang used a rapid anti-solvent method to prepare 0D (PP)₃MnBr₅ and (CP)₂MnBr₄, with a PLQY of up to 73%, demonstrating high environmental and thermal stability [95]. Guan also used the anti-solvent method to prepare centimeter-sized DMA₄[InCl₆]Br single crystals, which exhibited excellent thermal stability, good circularly polarized luminescence, and second harmonic generation properties [96].

A recent study reported a novel process, named the liquid phase diffusion method, which was applied to prepare large-sized 1D hybrid metal halide single crystals (Figure 3c) [97]. In brief, the process is conducted in an H-shaped test tube consisting of three sections (A, B, and C). Initially, HI solutions containing BiI₅ and 1,5-PDAI₂ are added to test tubes A and C, respectively. HI, acting as a buffer, is then carefully injected into test tube B to prevent the rapid formation of (1,5-PDA)BiI₅ crystallites. Over time, the 1,5-PDAI₂ precursor from test tube A and BiI₅ from test tube C continuously diffuse into the central section of test tube B. Once the concentration of (1,5-PDA) BiI₅ in the middle section reaches supersaturation, crystal nuclei form. This nucleation lowers the local concentration of BiI₅ and 1,5-PDAI₂ around the nuclei, creating a concentration gradient. Consequently, 1,5-PDAI₂ and BiI₅ continue to diffuse toward the growing nuclei, and crystal growth proceeds synchronously with the diffusion of the precursors. By adopting this method, centimeter-sized (1,5-PDA)BiI₅ single crystals were successfully harvested, which exhibit a low hole trap density and high carrier mobility.

Figure 3. Various synthesis methods used for preparing low-dimensional OIMHs using (a) the ultrasound-assisted bead milling method, (b) inverse temperature crystallization, (c) the liquid-phase diffusion method, and (d) the hot-injection method. Reprinted with permission from Ref. [83], Copyright 2020, American Chemical Society; Ref. [91], Copyright 2019, Springer Nature; Ref. [97], Copyright 2023, Royal Society of Chemistry; Ref. [98], Copyright 2022, American Chemical Society.

Figure 3. Various synthesis methods used for preparing low-dimensional OIMHs using (a) the ultrasound-assisted bead milling method, (b) inverse temperature crystallization, (c) the liquid-phase diffusion method, and (d) the hot-injection method. Reprinted with permission from Ref. [83], Copyright 2020, American Chemical Society; Ref. [91], Copyright 2019, Springer Nature; Ref. [97], Copyright 2023, Royal Society of Chemistry; Ref. [98], Copyright 2022, American Chemical Society.

Hot-injection, as a popular synthesis method for preparing quantum dots, can also be applied to grow low-dimensional OIMHs, as shown in Figure 3d [99,100,101]. Ghimire et al. conducted a hot-injection method and obtained 2D RP halide with a nanosheet morphology [98]. Slow precipitation during cooling is expected to occur through the gradual release of iodine from the alkyl iodide, which provides sufficient time for the nuclei to grow into large sheets. Additionally, the substantial presence of oleic acid in the reaction mixture guides the templated growth of 2D structures. Interestingly, red-colored nanosheets were formed when tin oleate was dried under a high vacuum of approximately 10⁻² mbar, while white-colored samples were obtained at a vacuum of 1 mbar. Thus, the presence of trace amounts of polar species in the reaction mixture induces structural reconstruction.

Besides this, to fabricate high-performance optoelectronic devices based on low-dimensional organic–inorganic hybrid metal halides, it is very important to improve the mechanical properties and machinability. Thus, researchers have explored different strategies. For example, they can use the physical mixing method involving blending and filling the already-synthesized low-dimensional organic–inorganic hybrid metal halides crystals into the polymer or coating the crystals directly on the polymer substrate with a polymer matrix.

4. Properties of Low-Dimensional OIMHs

Low-dimensional OIMHs exhibit diverse compositions, controllable structures, and rich chemical compositions and morphologies, which contribute to their excellent optoelectronic properties [102,103,104]. In 2D, 1D, and 0D OIMHs, the dielectric mismatch between inorganic and organic layers localizes excitons, and sometimes results in highly localized Frenkel-like excitons and self-trapped excitons (STEs) due to strong exciton–phonon coupling. The luminescent behavior of low-dimensional OIMHs can be classified into the four following categories based on the origin of the emission. The first one is intrinsic emission from electron–hole pair recombination [105]. This emission occurs due to the direct recombination of electron–hole pairs in the conduction band and valence band (Figure 4a) [49]. This type of emission is a characteristic feature of materials with efficient bandgap properties, and it typically results in sharp, well-defined emission peaks. The second one is self-trapped exciton (STE) emission [106]. STE emission arises from transient defects within the material that cause the exciton (the bound state of an electron and a hole) to become localized at specific sites in the lattice (Figure 4b) [107]. This type of emission is often broad and can lead to high-efficiency, broadband light emission, making it particularly useful in lighting applications. The third one is defect emission, arising from permanent defects in the material, such as vacancies or impurity states, referred to as defect emission [108]. These defects can act as localized states in the bandgap, emitting light when electrons recombine with holes at these defect sites (Figure 4c) [109]. This emission typically has lower efficiency compared to intrinsic or STE emission, but can be significant in materials with a high concentration of defects [110]. The last one is emission from metal cation or organic cation centers (Figure 4d) [111]. This type of emission can also originate from the metal cations or organic cations that act as the luminescent center [112]. These emissions are often related to specific transitions between electronic states in the cations, and the emission characteristics can be tuned by altering the cation composition, which allows for color-tunable luminescence. In summary, the emission behavior of low-dimensional OIMHs is highly diverse and can be engineered through the manipulation of material composition, structure, and defect states. These features provide great flexibility in designing materials with specific optical and luminescent properties tailored to various optoelectronic applications.

In 2D metal halides, the significant difference in dielectric constants between the organic and inorganic components leads to a strong quantum confinement effect, causing highly localized excitons to be strongly confined within the inorganic component [113,114,115]. This results in an exciton binding energy as high as 200–500 meV, which greatly facilitates the radiative recombination process and improves the luminescence efficiency. The emission of these excitons exhibits characteristics such as high PLQY and narrow full width at half maximum (FWHM). Moreover, as the thickness of the inorganic layers (n) increases, the quantum confinement effect gradually decreases [116]. Varying the number of inorganic layers of 2D metal halides can tune the bandgap energy, exciton binding energy and emission wavelength. Li’s team reported that for 2D (BA)₂(MA)_n−1Pb_nBr_3n+1, the bandgap decreases as the number of n increases, causing the single-crystal emission color to transition from white to orange, and finally to black, due to the decreased confinement [117]. Moreover, the dielectric constraint of the organic cations reduces the exciton binding energy, leading to a decrease in the material’s bandgap [118]. This results in narrow-band emission and tunable luminescent colors. Additionally, the absorption edge and emission peak gradually redshift with increasing n, which can be attributed to the weakening of the quantum confinement effect, leading to a reduction in the bandgap, and the decreased dielectric confinement of the organic cations over the inorganic layers, resulting in a further reduction in the exciton binding energy.

Defect emission is an important optoelectronic property [119,120]. In certain solid materials, the presence of defects or impurities on the material’s surface can lead to visible light emission. Defects can be classified into transient defects, which are generated by photoexcitation, electron beam excitation and other similar processes, and permanent defects, which arise due to vacancies or other structural irregularities [121,122]. Transient defect emission in low-dimensional OIMHs occurs due to the soft lattice characteristics of these materials, where the interaction between electrons and phonons is more pronounced. Specifically, during photoexcitation, the interaction between electrons and the lattice alters the internal electron distribution, disrupting the balance of interatomic forces mediated by electrons, leading to lattice distortion [123]. When this distortion is sufficiently large, it creates potential wells with a certain attractive force, causing electrons to spontaneously fall into these distorted lattice-generated wells. This results in energy transfer and redistribution during the emission process, ultimately leading to large Stokes shifts and long-lived broadband emission [124]. In low-temperature spectroscopy, it is observed that the spectra of these materials broaden as the temperature increases. In lifetime spectra, the time it takes for the material to transition from the excited state to the ground state is also longer. The Huang–Rhys factor (S) is often used to assess whether a material exhibits soft lattice characteristics, and its value can be calculated using the FWHM formula [125],

$$FWHM=2.36\sqrt{S}\hslash {\omega }_{phonon}\sqrt{\coth {\displaystyle \frac{\hslash {\omega }_{phonon}}{2k_{B}T}}}$$

where $\hslash$ represents the reduced Planck constant, ω is the phonon frequency, k_B is the Boltzmann constant, and T is the temperature. From the formula, it can be seen that S is positively correlated with FWHM. A larger S value indicates a greater likelihood of lattice distortion occurring. Thus, this correlation suggests that materials with a larger S value are more prone to exhibiting soft lattice characteristics, which in turn facilitates the occurrence of transient defect emissions, characterized by broad spectral features and longer emission lifetimes [126,127]. The stronger electron–phonon interaction and lattice distortion enhance the efficiency of the energy transfer and redistribution processes, resulting in notable optical properties such as large Stokes shifts.

Some researchers attribute the aforementioned type of emission to STE emission [128,129,130]. According to the formation process of STE, they can be categorized into intrinsic STE, defect-related STE, and non-intrinsic STE [131,132,133]. Karunadasa et al. used a model wherein hard balls represent electrons, holes, and excitons, while a rubber sheet represents a deformable lattice, to illustrate three processes. The first one is intrinsic STE, which occurs when the rubber sheet does not have any initial indentations, but the application of pressure from the hard balls deforms the rubber sheet, creating a potential well. The hard balls are trapped in this potential well and cannot move. Once the hard balls are removed, the rubber sheet reversibly returns to its original state. The second one is defect-related STE, which occurs when the lattice itself has permanent defects. As the hard balls move on the rubber sheet, they fall into the existing defects (indentations) and are confined by these potential traps. The third one is non-intrinsic STE, which happens when the rubber sheet has local indentations from the start, and as the hard balls move, they fall into these indentations [107].

In contrast to the narrow peak emissions of 3D and 2D metal halides, 1D and 0D structures often exhibit strong structural distortion and significant quantum confinement effects, which are conducive to exciton self-trapping and thus result in highly efficient broadband STE emission [134,135,136]. Research has shown that the broadband emission from STE is related to the degree of octahedral distortion in the inorganic layers, as well as the internal distortion of individual octahedra [137]. Mao et al. synthesized three novel 2D materials, α-(DMEN)PbBr₄, (DMAPA)PbBr₄, and (DMABA)PbBr₄, and found a strong correlation between the distortion of the PbBr₄ octahedra and the width of the emission peaks. The structures with the most severe distortion exhibited the widest emission peaks and the longest fluorescence lifetimes [138]. England’s team synthesized two (100)-oriented 2D Pb-Br halides, both of which had significant internal octahedral distortion, influencing the exciton radiative recombination process and leading to the formation of multiple photoluminescent centers [139].

Levine et al. proposed that the broadband emission in 2D metal halides, such as BA₂PbI₄, originates from the radiative recombination of permanent defects [140]. They used Density Functional Theory (DFT) calculations to demonstrate the presence of defect states, which aligned well with experimental results. For instance, PEPI crystals typically show narrow-band green emission, but as the iodine ion concentration increases, the crystals exhibit pale yellow broadband emission. Furthermore, the emission intensity increases with iodine concentration, suggesting that the emission arises from iodine defects. Additionally, Park’s team found that the intensity of broadband emission in (BuA)₂PbBr₄ crystals is proportional to their thickness. They attributed this broadband emission to defect radiative recombination caused by organic cation vacancies, which are created when external water molecules enter the crystal and induce these vacancies [141].

In addition to exhibiting STE emissions similar to 1D structures, 0D metal halides further isolate the emission centers through the A-site organic cations, forming a host–guest-like system. The energy transfer between the isolated octahedra in the structure requires overcoming significant energy barriers, leading to a higher degree of exciton localization in 0D metal halides compared to high-dimensional derivatives. Since electrons and holes are strongly confined within the same octahedron, 0D metal halides exhibit light physical properties similar to those of individual metal halide ions in solution, theoretically enhancing radiative recombination efficiency and improving their emission performance. Therefore, the emission of these materials relies on the inherent photo-physical properties of the B-site metal ions, and is easily influenced by the outer electron configuration. Typical metal ion emission centers include ns² lone-pair electron ions such as Pb²⁺, Bi³⁺, Sn²⁺, Sb³⁺, Te⁴⁺, as well as d¹⁰ series metal cations like Ag⁺, Cu⁺, Cd²⁺, Zn²⁺, In³⁺, Sn⁴⁺, d⁵ series Mn²⁺ ions, and rare-earth ions [142,143,144,145,146,147,148].

Pb-based hybrid halides have garnered significant attention in fields such as solar cells and optoelectronic detectors due to their excellent optical properties. However, their toxicity limits further development. In 2019, Kanatzidis’s team synthesized a series of 2D hybrid halides, (xAMPY) (MA)_n−1Pb_nI_3n+1 (x = 3 or 4, n = 1–4), which exhibited strong photoluminescence at room temperature [149]. Zhao and colleagues synthesized 1D (C₄N₂H₁₄)₂Pb_1-xMn_xBr₄ microcrystals, and through Mn doping, they successfully tuned the emission color from blue-white to white and orange-red [150].

Ma’s group reported two 0D octahedral Sn (II) metal halides, [(C₄N₂H₁₄Br)₄]-SnBr₆ and [(C₄N₂H₁₄I)₄]SnI₆, which originated from STEs emission. [(C₄N₂H₁₄Br)₄]SnBr₆ exhibited a bright yellow light emission, with its emission peak at 570 nm, and the corresponding PLQY was as high as 95%. For [(C₄N₂H₁₄I)₄]SnI₆, it emitted bright red light under the irradiation of a 365 nm ultraviolet lamp, with its emission peak at 620 nm, and relatively low PLQY, at 75%. 2D (OCTAm)₂SnX₄ (OCTAm = octylammonium cation, X = Br, I, or mixtures thereof) displayed emissions centered at 600 nm, with a broad bandwidth (136 nm) and high PLQY of near-unity in the solid-state.

Sb³⁺, due to its isolated ns² luminescent center and strong electron–phonon coupling, exhibits strong emission. Liu and colleagues synthesized three 0D metal halides: (PPA)₆InBr₉ (PPA = [C₆H₅(CH₂)₃NH₃]⁺), (PBA)₂SbBr₅, and (PBA)₂SbI₆ (PBA= [C₆H₅(CH₂)₄NH₃]⁺). After doping Sb³⁺ into (PPA)₆InBr₉, the fluorescence emission was greatly enhanced, with the PLQY increasing from 9.2% in the original sample to 53.0% in (PPA)₆In_0.99Sb_0.01Br₉ [151]. In addition to being used as a doping ion, Kovalenko’s team also reported on the Sb³⁺-based 0D hybrid metal halide TPP₂SbBr₅ phosphor and its related applications [152]. Efficient NIR emission under blue light excitation is achieved in Sb³⁺-doped 0D (ETPP)₂ZnCl_4xBr_4-4x (x = 0–1) (ETPP+ = (Ethyl) triphenylphosphonium) through coordination structure modulation and halogen substitution. Furthermore, research on Bi³⁺-based low-dimensional OIMHs is limited. Bi³⁺ typically exists in the form of octahedra [BiX₆]³⁻, and the PLQY values of currently reported compounds remain low [153]. Even when forming dimers such as [Bi₂X₉]³⁻, [Bi₂X₁₀]⁴⁻, and [Bi₂X₁₁]⁵⁻, the PLQY do not show significant improvements [154,155,156].

Mn²⁺ ions form various coordination polyhedra, including the weak crystal-field tetrahedron [MnX₄]²⁻ that emits green light, the strong crystal-field octahedron [MnX₆]⁴⁻ that emits red light, and the trimeric structure [Mn₃X₁₂]⁶⁻. The emission mechanisms for all of these structures are based on the ⁴T₁→⁶A₁ transition. Studies have shown that choosing appropriate organic cations to increase the Mn–Mn bond length can reduce energy transfer, leading to more efficient emission. For instance, Xu’s team recently reported 0D PrPP₂MnBr₄ single crystals that exhibited green emission with a high PLQY of 90.3% and a narrow FWHM of 44 nm [157]. Xu et al. also reported 0D (Ph₄P)₂[MnBr₄], which exhibited green emission from the [MnBr]²⁻ tetrahedron, originating from the d-d transition of outer electrons in Mn²⁺. The coordination environment of Mn²⁺ also affects its emission properties [158]. Ju’s team synthesized 0D C₄H₁₂NMnCl₃ and (C₈H₂₀N)₂MnBr₄ single crystals, where Mn²⁺ was coordinated in octahedral and tetrahedral environments, respectively. They observed red and green emission peaks at 635 nm and 515 nm due to the ⁴T₁→⁶A₁ transition of Mn²⁺. The special crystal structures resulted in PLQY values of 91.8% and 85.1%, respectively [159].

Liu et al. introduced two Ce³⁺-based 0D hybrid compounds, (DFPD)₄CeX₇ (X = Cl, Br) and (DFPD)CeCl₄·2MeOH, whose emissions are attributed to the radiative transitions between the ²F_5/2 and ²F_7/2 energy levels of Ce³⁺ ions [160]. These emissions are characterized by a narrow bandwidth, with FWHM of only 30–40 nm, and a Stokes shift of 30–50 nm. Similarly, Yu et al. synthesized a series of 0D (DFPD)LnX₇ (Ln³⁺ = Nd³⁺, Eu³⁺, Ho³⁺, Sm³⁺, Tm³⁺, Tb³⁺, Yb³⁺, Er³⁺; X = Cl⁻, Br⁻) compounds, using DFPD⁺ as the organic spacer cation [161]. For the Tb³⁺ and Eu³⁺ crystals, the PLQY can reach close to 100% and 90%, respectively.

Doping photoactive metal ions (such as Mn²⁺, Sb³⁺, etc.) or forming polymeric emission centers can adjust the photo-physical properties of low-dimensional OIMHs without altering their crystal structure, especially significantly improving the PLQY. Xia et al. doped Sb³⁺ ions into ATPP₂SnCl₆, achieving efficient energy transfer and improving the PLQY from 5.1% to 73.8% [162]. Lv et al. doped Mn²⁺ ions into different 2D Cd-based halides, tuning the room-temperature phosphorescence (RTP) color from green to red, with a long duration of up to 4 s, and improving PLQY from 14.8% to 44.11% [163].

2D metal halides possess a unique organic–inorganic dual-component combination, where the inorganic layers serve as natural templates to guide the regular arrangement and conformation of organic amine cations. Additionally, the mutual restrictions imposed by the organic amine molecules themselves effectively limit their rotation and vibration, thereby suppressing the non-radiative recombination of triplet excitons and achieving high-efficiency phosphorescence. Recently, it has been reported that the transfer of inorganic excitons from inorganic frameworks to organic cations triplet states can induce RTP, and such excitons transfer has been demonstrated in 2D, 1D, and 0D bulk MHs, which enables them to obtain efficient RTP. Ema et al. used time-resolved photoluminescence measurements to confirm that the transfer is triplet–triplet Dexter-type energy transfer from Wannier excitons in the inorganic well to the triplet state of naphthalene molecules in the organic barrier [164,165]. In 2018, Lam et al. designed and synthesized 2D (TTMA)₂PbBr₄, realizing multicolor RTP originating from the organic component [166]. They also achieved long-lifetime emission by modifying the metal halide composition. In 2019, Yan’s team combined the 2D structure with the heavy atom effect to design a 2D CdCl₂-4HP hybrid halide with ultralong blue thermally activated delayed fluorescence (TADF) and high PLQY [167]. The TADF lifetime reached 103.12 ms, and the PLQY was as high as 63.55%. Lv’s group synthesized two single-component 2D metal halides, B-MACC and B-EACC, both exhibiting ultralong RTP emission [168]. Among them, B-EACC demonstrated stable yellow-green phosphorescent emission at room temperature, with a lifetime of 579 ms and a PLQY of 14.86%. It has been certified that this phosphorescence emission is attributable to B-EA⁺.

In addition to the previously discussed single emission modes, various other luminescent behaviors also exist in low-dimensional OIMHs. By integrating different metal halides into a single crystal to form a multi-component system, multifunctionality can be achieved in a single-phase material, allowing a single metal halide to exhibit multiple properties simultaneously. Xia et al. discovered that 0D (C₉NH₂₀)_9[Pb₃Cl₁₁](ZnCl₄):Sb³⁺ can display dual emission from the Pb₃Cl₁₁⁵⁻ cluster and Sb³⁺ ions. By adjusting the concentration of Sb³⁺ ions, the emission color can be tuned from green to yellow and orange [169]. Ma’s team reported 0D (bmpy)₉[SbCl₅]₂[Pb₃Cl₁₁], which shows green emission from Pb₃Cl₁₁^5- and orange emission from SbCl₅²⁻, with a PLQY of 70% [170]. Since the emission spectra of the two materials almost do not overlap, no energy transfer occurs between the different luminescent substances, allowing for precise color tuning by varying the excitation wavelength. Ma’s group also synthesized (HMTA)₄PbMn_0.69Sn_0.31Br₈, which incorporates PbBr₄²⁻, MnBr₄²⁻, and SnBr₄²⁻ monomer structures to achieve white-light emission with a PLQY of 73%. Li et al. reported 0D (C₉NH₂₀)₉Pb₃Br₁₁(MnBr₄)₂, which exhibits two emission centers at 565 nm and 528 nm, attributed to STE emission and the ⁴D-⁶A₁ transition of Mn²⁺ ions, respectively [171]. In 2022, Quan’s team integrated RTP and STE emissions in 0D hybrid metal halide (Ph₃S)₂Sn_1−xTe_xCl₆ (x = 0–1) by adjusting the ratio of non-emissive octahedra [SnCl₆]²⁻ and emissive octahedra [TeCl₆]²⁻ [172].

Recently, Lv’s group designed and synthesized two 2D Cd-based metal halides, B-MACC and B-EACC, by varying the alkyl lengths of the organic units, both exhibiting typical RTP characteristics [168]. It was found that the phosphorescence spectra of B-MACC or B-EACC originate from the triplet-state emission of the organic components B-MA⁺ or B-EA⁺ cations and STE emissions from the inorganic component Cd-Cl. This is due to the molecular packing of the B-EA⁺ cations, which forms strong hydrogen bonds and π-π stacking interactions, reducing triplet exciton quenching. Recently, Lei’s team achieved dual emissions from organic and inorganic components in 0D [BTPP]₂ZnBr₄, where the heavy atom effect and rigid crystal structure significantly enhanced spin-orbit coupling and spin transitions, suppressing non-radiative recombination [173].

5. Optoelectronic Devices Based on Low-Dimensional OIMHs

Low-dimensional OIMHs exhibit excellent semiconductor luminescent properties, making them ideal candidates for use as light-emitting layers in white light-emitting diodes (WLEDs), electroluminescent devices, active layers in solar cells, channel materials in thin-film field-effect transistors, and various other optoelectronic devices [174,175,176,177]. In this section, we will explore the current applications and future development prospects of low-dimensional OIMHs in these fields (Figure 5).

5.1. White Light-Emitting Diodes

Low-dimensional OIMHs enable tunable emission spanning from blue-violet to red and even near-infrared regions, with FWHM typically greater than 60 nm. This allows them to cover the entire visible spectrum while exhibiting high PLQY, low self-absorption, and excellent optical/thermal stability, making them highly promising for WLEDs applications. Ma et al. integrated 0D (C₄N₂H₁₄Br)₄SnBr_xI_6−x, which emits in the yellow to red spectral range, with commercially available blue-emitting phosphor BaMgAl₁₀O₁₇: Eu²⁺ for use as a down-converter in UV-pumped WLEDs [181]. This combination addresses the red-emission deficit often seen in most yellow phosphors, achieving an exceptional color rendering index (CRI) of up to 85. Compared to UV chips, blue light sources are more cost-effective and have higher luminous efficiency, making it particularly important to explore luminescent materials that can be excited by blue light chips. Zou’s team reported a (CH₆N₃)₂MnCl₄:8%Zn²⁺ material with strong red emission at 650 nm and a PLQY exceeding 55.9% [182]. When combined with Y₃Al₅O₁₂:Ce³⁺ yellow phosphor, they successfully fabricated WLEDs excited by blue light chips, achieving a CRI of up to 93.7. Hu’s group presented a series of (100)-oriented 2D metal halides, the structures of which are templated by organic cations [178]. By manipulating the tilting of the inorganic octahedra, they successfully tuned the broadband emission from blue to white. Detailed photo-physical analysis further indicated that the coexistence of STE and free excitons is responsible for the observed double-peak broad emission, spanning the entire visible spectrum. Leveraging these broad-emitting perovskites as down-converting phosphors, WLEDs were fabricated, which exhibited a correlated color temperature (CCT) of 6600 K and a high CRI of 86 (Figure 5a). The energy distribution of the WLED spectra is broader than those of traditional YAG-LED, as shown in Figure 5a.

The preparation process of rare-earth-doped phosphors is relatively complex, leading to an increasing focus on WLEDs based on entirely low-dimensional OIMHs. Zhang and colleagues designed novel WLEDs based on a blue light chip, using green-emitting (C₅H₁₄N₃)₂MnBr₄ and red-emitting (CH₆N₃)₂MnBr₄, achieving a high CRI of 90.8 at a CCT of 3709 K, with CIE coordinates of (0.39, 0.39) [183]. Li’s group synthesized a 1D copper-based halide, [KC₂]₂[Cu₄I₆] (C = 12-crown-4 ether), which exhibited greenish-yellow emission with a high PLQY of 97.8%. DFT calculations, in conjunction with comprehensive spectroscopic data, elucidate the characteristics of STE emission. The potential application of [KC2]₂[Cu₄I₆] as a phosphor was further confirmed by fabricating WLEDs using commercially available blue LEDs (450 nm) as the excitation source (Figure 5b). Commercial red-emitting phosphors (Ca, Sr)AlSiN₃:Eu²⁺ and the sample were mixed to form a film as the emission source. The WLEDs were obtained with the high color rendering index (CRI) of 86.5, a CCT of 5834 K, and a CIE color coordinate of (0.32,0.33) [179]. Similarly, Xia et al. reported 0D (18-crown-6)₂Na₂(H₂O)₃Cu₄I₆ (CNCI), which exhibited green emission with a near-unity PLQY upon excitation at 450 nm. WLEDs were then fabricated, demonstrating high luminous efficiency of 156 lm/W and CRI of 89.6 (Figure 5c) [180]. Recently, multi-component 0D metal halides with white light emission have also been explored as down-converting luminescent layers for WLEDs. For instance, Ma’s team reported (bmpy)₉[SbCl₅]₂[Pb₃Cl₁₁], which produces warm white light emission, with a CCT range from 3599 to 1272 K and a CRI of up to 90 [170]. Kuang’s team reported BAPPIn_1.996Sb_0.004Cl₁₀, which exhibited yellow and bright white light emission upon excitation at 320 nm and 365 nm, respectively, with PLQY values of 100% and 44.0% [184]. They subsequently fabricated WLEDs excited by ultraviolet light chips, achieving a CRI of 73.9 and a CCT of 6206 K.

5.2. X-Ray Detectors

X-ray detectors demonstrate immense potential across a wide range of fields, including medical imaging, space exploration, non-destructive testing, and public safety [185,186,187]. Based on their detection mechanisms, X-ray detectors can be categorized into direct and indirect detectors. Compared to semiconductor-based direct detectors, scintillator-based indirect detectors offer advantages such as higher detection efficiency, lower manufacturing costs, and better stability [188]. However, traditional scintillators like CsI:Tl crystals suffer from drawbacks such as complex fabrication processes, high production costs, long decay times, and low radiation hardness. The polycrystalline nature of Gd₂O₂S:Tb (GOS) leads to optical crosstalk and reduced spatial resolution, while its long fluorescence lifetime results in afterglow and a slow X-ray response. Although single crystals like Y₃Al₅O₁₂:Ce and Be₄Ge₃O₁₂ offer transparency, they suffer from relatively low light output. Low-dimensional OIMHs, on the other hand, are ideal candidates for high-performance scintillators due to their low manufacturing costs, tunable band gaps, structural diversity, and excellent optoelectronic properties [189,190,191,192]. Additionally, their high atomic numbers provide a heavy atom effect, high X-ray absorption coefficients, and high PLQY, making them promising for use in scintillation materials.

Figure 6. Low-dimensional OIMHs scintillators including (a) Sb-based flexible halides, (b) ink writing Pb-based halides, (c) chiral Mn-based halides, and (d) 0D Cu-based halides. Reprinted with permission from Ref. [193], Copyright 2025, Wiley-VCH GmbH; Ref. [194], Copyright 2024, Wiley-VCH GmbH; Ref. [195], Copyright 2024, Wiley-VCH GmbH; Ref. [196], Copyright 2024, Wiley-VCH GmbH.

Figure 6. Low-dimensional OIMHs scintillators including (a) Sb-based flexible halides, (b) ink writing Pb-based halides, (c) chiral Mn-based halides, and (d) 0D Cu-based halides. Reprinted with permission from Ref. [193], Copyright 2025, Wiley-VCH GmbH; Ref. [194], Copyright 2024, Wiley-VCH GmbH; Ref. [195], Copyright 2024, Wiley-VCH GmbH; Ref. [196], Copyright 2024, Wiley-VCH GmbH.

Xia’s team reported blue-emitting 0D (C₈H₂₀N)₂Cu₂Br₄ single crystals with a PLQY as high as 99.7%, excellent stability, and strong X-ray absorption capabilities [197]. This scintillator demonstrated an impressive light output of 91,300 photons MeV⁻¹ and a low detection limit of 52.1 μGyair s⁻¹, which is two orders of magnitude lower than the typical dose required for medical diagnostics. Meng et al. reported that large quantities of high-quality C₃₈H₃₆P₂Sb₂C_l8 single crystals were synthesized using a straightforward solution-based approach [193]. The resulting single crystals, featuring a dimeric [Sb₂C_l8]²⁻ structure, exhibited yellow emission with an exceptional near-unity PLQY of 99.8%. In non-planar X-ray imaging, the flexible scintillator screen can be easily attached to the imaging object, and the bending of the spring can be clearly observed, effectively alleviating the problem of vignetting and obtaining better imaging quality. Building upon this, a large-scale, ultra-flexible scintillator for X-ray imaging was fabricated via a template-assisted method. As expected, this team demonstrated a high spatial resolution of 8.15 lp mm⁻¹, an outstanding light yield of 41,300 photons MeV⁻¹ and a remarkably low detection limit of 45.6 nGyair s⁻¹ (Figure 6a). Lv’s group reported 0D (4BTP)₂MnBr₄, which emits strong green fluorescence at 524 nm and exhibits outstanding optical properties as an X-ray scintillator, including a light output of 98,000 photons MeV⁻¹ and a sensitivity detection limit as low as 37.4 nGy s⁻¹ [198]. It also demonstrated excellent radiation damage resistance and successfully achieved high-resolution X-ray imaging with a resolution of 21.3 lp mm⁻¹. By engineering an ink with optimized printability and shape fidelity, direct ink writing has been developed by Wang et al. as an innovative method for fabricating a unique single-crystal-assembled perovskite thick film (Figure 6b) [194]. Unlike polycrystalline materials, which consist of randomly oriented crystal domains, this film was composed of densely packed crystals with well-defined facets, exhibiting trap density that was three to four orders of magnitude lower. As a result, the corresponding X-ray detectors demonstrate state-of-the-art detection performance, with a sensitivity-to-dark current ratio of 1.26 × 10¹¹ μC Gyair⁻¹ A⁻¹, a low detection limit of 114.2 nGyair s⁻¹, and negligible baseline drift of 0.27 fA cm⁻¹ s⁻¹ V⁻¹.

Table 2. Properties of low-dimensional OIMHs scintillators.

Materials	Crystal System	Dim	Photo Yield (Photons/MeV × 103)	Lifetime	Peak Position (nm)	Resolution (lp/mm)	Detection Limit (nGy/s)	PLQY (%)	Refs.
(C6H13NH3)2PbI4	Monoclinic orthorhombic	2D	-	3.8 ns	558	-	-	-	[199]
PEA2PbBr4	Triclinic	2D	38.8	11.9 ns	430	-	-	-	[200]
C4H12NMnCl3	Hexagonal	1D	50.5	758.95 μs	635	-	24.2	91.8	[159]
TPPen2MnBr4	Monoclinic	0D	43	298.04 μs	515	4.6	696.9	97.3	[201]
TPPen2Mn0.9Zn0.1Br4	Monoclinic	0D	68	296.34 μs	515	11.2	204.1	97.7	[201]
HTP2MnBr4	Monoclinic	0D	38	318.11 μs	520	17.3	130	98.66	[202]
MTP2MnBr4	Trigonal	0D	67	331 ms	516	6.2	82.4	99.5	[203]
(4BTP)2MnBr4	Monoclinic	0D	98	292.31 μs	524	21.3	37.4	96.26%	[198]
(TBA)CuCl2	Monoclinic	0D	23.4	28.7 μs	510		-	92.8	[204]
(BzTPP)2Cu2I4	Monoclinic	0D	27.7	1.93 μs	558	4.9	352	44.2	[190]
(C8H20N)2Cu2Br4	Monoclinic	0D	91.3	33.4 μs	468	9.6	52.1	99.7	[197]
(PPN)2SbCl5	Monoclinic	0D	49	4.1 μs	635	-	191.4	-	[205]

summarizes the properties of low-dimensional OIMHS scintillators reported in recent years.

Chiral scintillators, which emit circularly polarized light, present a promising strategy for controlling the direction of light propagation, thereby enhancing the performance of X-ray detection. Guo’s group reported novel chiral metal–organic polymers, which utilized chiral polymeric monomers and 0D Mn (II)-based organic–metal halide hybrid scintillators [195]. These materials achieve luminescence dissymmetry factors of 5.823 × 10⁻² and −2.877 × 10⁻². Impressively, this prototype yielded a spatial resolution value as high as 14.54 lp mm⁻¹ for chiral metal–organic polymer films, demonstrating their potential for advanced optical and detection applications (Figure 6c). 0D β-(MePh₃P)₂CuI₃ was synthesized by Qi et al., which exhibited cyan emissions with an exceptional near-unity PLQY [196]. Additionally, β-(MePh₃P)₂CuI₃ can be excited via X-ray radioluminescence, demonstrating a high scintillation light yield of 16,193 photons MeV⁻¹ and an ultra-low detection limit of 47.97 nGy s⁻¹—only 0.87% of the standard medical diagnostic limit of 5.5 μGy s⁻¹. Under 365 nm UV excitation, the scintillator screen exhibits a bright cyan emission. Under natural light, the metal nib of the ball-point pen was not visible. However, under X-ray irradiation, the nib was visible through the fabricated scintillator screen (Figure 6d).

Furthermore, Ma’s team developed 0D (PPN)₂SbCl₅, which exhibits a good linear relationship across a broad range of X-ray doses [205]. Its light output of 49,000 photons MeV⁻¹ is comparable to that of commercial CsI (Tl) scintillators, with a detection limit of 191.4 nGy s⁻¹, well below the required values for conventional medical diagnostics. Additionally, this scintillator showed remarkable stability, with its properties remaining nearly unchanged even after two years of storage at room temperature. Wei’s team reported on 0D (C₃₈H₃₄P₂)MnBr₄ gel, which has self-healing properties and flexibility, enabling X-ray imaging underwater [206]. It retains strong radioluminescence intensity even when stretched up to 1300%, offering a promising reference for portable X-ray detectors in harsh conditions.

5.3. Sensors

Low-dimensional OIMHs exhibit significant changes in their photo-physical properties when exposed to stimuli such as heat, volatile organic solvents, and humidity, making them promising candidates for various types of sensors [49,207]. Huang et al. presented a millimeter-scale single crystal of mixed low-dimensional organic lead iodide, exhibiting a well-defined crystallinity [208]. A single-crystal device was employed to evaluate the X-ray response performance, as shown in Figure 7a. The fabricated single-crystal device exhibited exceptional photoresponse sensitivity and X-ray detection capabilities. By spatially segregating organic molecules to form the hybrid 1D–0D crystal structure, ion migration is effectively suppressed, leading to a significant reduction in dark current by three orders of magnitude (56.4 pA at 200 V). Additionally, by optimizing the background characteristics, they achieved an impressive X-ray detection limit of 154.5 nGy s⁻¹ in the single-crystal device.

Zhou et al. reported that the distinctions in bonding between Sb³⁺/In³⁺ and Cl⁻ in (Me₂NH₂)₄MCl₆·Cl (M = Sb, In) can be highlighted with respect to electronic excitation and hybridization. Additionally, the mechanisms of electron transition were delineated based on the PL spectra observed at the extreme temperatures ranging from 5 K to 305 K, alongside supporting theoretical calculations. They also photographed the (Me₂NH₂)₄MCl₆·Cl (M = Sb, In) under UV light of 365 nm or 300 nm in the temperature range of 80–305 K, showing that the temperature-triggered color changes from cyan, to yellow, and finally to orange, and it can be used as a temperature testing indicator strip to detect ultra-low temperatures (Figure 7b). Therefore, this work illuminates the principles governing the photoluminescent behavior of hybrid metal halides, while also exploring their potential optical-functional applications in aerospace temperature sensors and access control systems [209].

Bu’s group synthesized 1D hybrid metal halide piezoelectrics R/SMPCdCl₄, where R/SMP denotes R/S-2-methylpiperazine, and their subsequent applications in piezoelectric energy harvesting and sensing are presented [210]. These chiral piezoelectrics exhibit low elastic properties coupled with high piezoelectric constants of 16.71, 8.39, and 7.35 pC N⁻¹. Devices composed of the aforementioned composite films are fabricated for piezoelectric energy harvesting and sensing applications. As shown in Figure 7c, a 15 wt. % RMPCdCl4 piezoelectric energy harvester was utilized as the sensor to detect human motions (tapping, finger bending, walking, and running). The piezoelectric energy harvesters demonstrated not only exceptional performance—featuring an open-circuit voltage of 2.57 V, a short-circuit current of 0.37 μA, and a power density of 0.55 μW·cm⁻²—but also remarkable stability, maintaining performance over more than 3500 cycles. Additionally, these piezoelectric energy harvesters demonstrated outstanding sensing capabilities for detecting activities such as tapping, finger bending, walking, and running.

Moreover, Zhang’s team discovered that the luminescent properties of 0D (Me₂NH₂)₄MCl₆·Cl (Me = −CH₃, M = Sb, In) exhibited excellent temperature and compositional dependence [209]. They explored the application of this material in aerospace temperature sensors, and found that it can monitor extreme temperatures in space ranging from 5 to 80 K. Gao and colleagues utilized 2D PEA₂MnBr₄, which exhibited green emissions in a dehydrated state and pink emissions in a humidified state, for the detection of water content in toluene. This sensor achieved an ultra-low detection limit of 0.02–0.05 vol. % [211].

5.4. Solar Cells

Low-dimensional OIMHs also show immense potential use in solar cells and optoelectronic applications [212,213,214]. In the past, the design of low-dimensional capping materials has traditionally been limited to tuning the A-site organic cation, with Pb²⁺ and Sn²⁺ being the sole options for the metal cation. In Ye’s work, they expanded the possibilities by introducing a diverse range of low-dimensional capping materials with metal cations beyond Pb²⁺/Sn²⁺, achieved by processing a precursor solution containing both metal and ammonium halides. Specifically, the solvent of the HP (half precursor) solution is generally prepared using an antisolvent such as isopropanol, whereas a slightly more polar solvent (acetonitrile, ACN) is used to dissolve the FP (full precursor) components, especially the metal halide (Figure 8a) [214]. This approach facilitates more precise control over the synthesis of the low-dimensional capping layer and enhances flexibility for interface engineering. It was demonstrated that 0D PEA₂ZnX₄ provided superior surface passivation and stronger n–N isotype 3D/low-dimensional heterojunctions compared to its Pb-based counterparts. As a result, the p–i–n solar cells achieved an impressive power conversion efficiency (PCE) of 24.1% (certified 23.25%), maintaining 94.5% of their initial efficiency after over 1000 h of operation at the maximum power point.

The development of additives capable of effectively regulating the crystallization kinetics of perovskite films is crucial for achieving optimized optoelectronic properties, which in turn are essential for the development of high-efficiency and stable solar cells. Liu et al. proposed a novel additive, which was synthesized directly within the perovskite precursor solution through an addition reaction between but-3-yn-1-amine hydrochloride (BAH) and formamidinium iodide [215]. The resulting product is found to regulate the intermediate precursor phase, facilitating perovskite nucleation and promoting the formation of a beneficial 2D perovskite layer (Figure 8b). This layer not only reduces lattice strain but also strongly interacts with the perovskite to passivate surface defects. Leveraging these synergistic effects, the optimized solar cells achieved a high PCE of 25.19% and a high open-circuit voltage of 1.22 V. Furthermore, the devices exhibited impressive stability, maintaining over 90% of their initial efficiency under ambient conditions for 60 days, at elevated temperatures (85 °C) for 200 h, or during maximum power point tracking for 500 h.

Recently, Jo’s team improved the stability of devices by employing self-assembled 2D metal halides and ion-exchange-mediated cation tuning to enhance ion conductivity, reducing halide separation and charge non-uniformity [216]. Devices made using this strategy exhibited a high PCE of up to 24.38% and maintained 91.87% of their initial PCE after 2070 h. Mao’s team developed high-performance optoelectronic thin-film detectors based on BA₂FA_n−1Sn_nI_3n+1 (n = 1,2) components. These detectors demonstrated stable operation for two to three hours under ambient conditions, with dark currents of 2.1 × 10⁻⁹ A (n = 1) and 2.6 × 10⁻⁹ A (n = 2), an on/off ratio of 458 (n = 1) and 1108 (n = 2), and detection rates of 1.46 × 1013 Jones (n = 1) and 6.23 × 10¹² Jones (n = 2). While 1D metal halides are typically considered unfavorable for photovoltaic applications due to their high exciton binding energy, Lee’s team developed a novel low-dimensional hybrid halide solar cell based on 1D BDAPbI₄, achieving a PCE of up to 14% [217]. This device also demonstrated exceptional stability under continuous illumination and heating conditions.

5.5. Anti-Counterfeiting

Fluorescent marking anti-counterfeiting technology has garnered significant attention due to its advantages, including straightforward decryption, simple equipment requirements, and low production costs [218,219,220]. The structure of low-dimensional OIMHs allows for the independent tuning of both organic and inorganic components’ properties. Moreover, the rigid structure formed by the inorganic component provides a platform for regulating organic luminescence. Therefore, combining organic components with RTP properties and inorganic metal halides offers an effective pathway for developing high-performance, multifunctional RTP materials for anti-counterfeiting applications [221,222,223].

Zhang’ group reported the 1D hybrid halide (C₅H₁₁N₃)MnCl₂Br₂·H₂O, which exhibited water-molecule-induced reversible photoluminescence transformation (Figure 9a) [224]. Upon heating for a certain period, a dynamic transition occurs from red-emissive (C₅H₁₁N₃)MnCl₂Br₂·H₂O to green-emissive (C₅H₁₁N₃)MnCl₂Br₂. Additionally, the green emission can progressively revert to red emission during the cooling process in a humid environment, demonstrating excellent reversibility. Therefore, water molecules acted as an external stimulus, driving the reversible transition between red and green emission, which remained stable throughout repeated cycles. The powder samples with 2 mL of a methanol solution were prepared in designed patterns, including a four-leaf clover and a fish. After the printed samples had been subjected to crystallization under heating−cooling treatments, the reversible emissions could be observed, from red to green and, finally, back to red emission. Moreover, with the aid of heating and cooling, a complex digital encryption–decryption system with optical “AND” logical gate was successfully implemented, advancing the development of anti-counterfeiting and information security technologies (Figure 9a).

The achievement of significant PL in lanthanide ions (Ln³⁺) has traditionally relied on host sensitization, where energy is transferred from the excited host material to the Ln³⁺ ions. However, this mechanism involves a single optical antenna, namely, the host material, limiting the accessibility of excitation wavelength-dependent (Ex-De) PL. As a result, the broader application of Ln³⁺ ions in light-emitting devices has been constrained. In a recent study, Zhao et al. introduced low-dimensional hybrid halide (DMA)₄LnCl₇ (Ln³⁺ = Ce³⁺, Tb³⁺), which acts as an independent host lattice material, enabling efficient Ex-De emission upon doping with Sb³⁺ [225]. The pristine (DMA)₄LnCl₇ compounds exhibited strong luminescence, maintaining the characteristic sharp emission bands of Ln³⁺ ions and demonstrating a high PLQY of 90–100%. Upon Sb³⁺ doping, the compound exhibited distinct Ex-De emission with tunable colors. A flower pattern was fabricated using 0.8 mol % Sb-doped (DMA)₄TbCl₇ compounds as encryption powders. Using the excitation wavelength-responsive property, multilevel information encryption can be achieved. Besides this, the information erasure function could be activated by spraying a small amount of water onto the pattern (Figure 9b). It was confirmed that the prominent energy transfer process typical in traditional host-sensitized systems is absent in these materials. Instead, the system features two independent emission centers from Ln³⁺ and Sb³⁺, each with unique luminous color and radiative lifetime characteristics. As a result, a novel excitation wavelength-dependent anti-counterfeiting technique was proposed as an alternative strategy in the field information encryption.

Lei’s group introduced a photo-physical tuning strategy within 0D hybrid zinc halides (BTPP)₂ZnX₄ (X = Cl, Br) [173]. Uniquely, the synergistic combination of organic and inorganic components enables this family to exhibit multiple ultralong green afterglow and efficient STE-associated cyan phosphorescence (Figure 9c). Compared to the inert luminescence of the [BTPP]⁺ cation, the incorporation of the anionic [ZnX₄]²⁻ group effectively enhances the spin–orbit coupling, significantly improving the PLQY to 30.66% for afterglow and 54.62% for phosphorescence. Simultaneously, the corresponding luminescence lifetimes extend to 143.94 ms and 0.308 μs, surpassing the barely detectable phosphorescence of [BTPP]X salts. Notably, this halide family demonstrates robust RTP emission with nearly unchanged PLQY in water and under harsh conditions, such as acidic and basic aqueous solutions, for over six months. The highly efficient integrated afterglow and STE phosphorescence, coupled with the ultrahigh aqueous state RTP, enable diverse anti-counterfeiting applications across a wide range of chemical environments.

Moreover, Kuang’s team synthesized a series of 0D BAPPZn₂ (Cl_yBr_1−y)₈ (y = 0–1) compounds and tuned the RTP lifetime by adjusting the heavy-atom effect caused by halogen substitution [226]. These materials can be recognized either by the naked eye or using a simple machine vision system, with their RTP properties being applied for spatiotemporal dual-resolution anti-counterfeiting. Wu’s team reported the design and synthesis of a series of metal halide materials featuring tunable phosphorescence and fluorescence dual-emission characteristics [221]. Via halogen substitution synthesis, they could easily adjust the ratio of dual emission and RTP lifetime. Based on their tunable dual emission and afterglow properties, the team developed a time-resolved anti-counterfeiting application, providing a feasible design strategy for advanced portable anti-counterfeiting technology. Zhao’s team reported a 0D ZnCl₂·R/S-2-MP enantiomer, which exhibits wavelength-dependent emission and RTP characteristics [227]. The organic ligand induces structural chirality through N–H···Cl hydrogen bonding, enabling information encoding based on the excellent photo-physical properties of ZnCl₂·R/S-2-MP. Recently, Zhang and colleagues discovered that under thermal stimulation, 0D TMA₂SbCl₅·X (X = DMSO and DMF) exhibits a photoluminescence “switching” transition, while TMA₂SbCl₅·MeCN shows a change in photoluminescence color from yellow to orange due to a structural phase transition [228]. This remarkable stimulus–response performance renders these materials applicable for anti-counterfeiting and encryption applications.

5.6. Other Applications

Low-dimensional OIMHs, due to their excellent optical transmission and optical amplification properties, are promising candidates for use in optical communication devices, particularly as optoelectronic converters or optical amplifiers [229]. These materials can enhance the transmission efficiency of fiber optic communication systems. Additionally, low-dimensional OIMHs can function as photo-catalysts, facilitating chemical reactions under light, especially in environmental protection applications, such as photocatalytic hydrogen production from water and the photocatalytic reduction of carbon dioxide [230,231]. The broad emission wavelength ranges and high optical gain characteristics of these materials make them ideal for manufacturing solid-state lasers, suitable for a wide range of laser applications, including communications, medical, and industrial uses [232].

6. Conclusions and Perspectives

In summary, low-dimensional OIMHs hold broad application prospects in the optoelectronic field. Despite significant progress made in the research of these materials, the controlled synthesis and emission mechanisms of this class of materials remain in the early stages. Beyond photoluminescence, other properties such as electronic and magnetic characteristics require further multidisciplinary studies. Future research on low-dimensional OIMHs materials should focus on the following challenges.

Achieving both high light yield and short fluorescence lifetimes remains a major challenge for all scintillators. Many traditional scintillators, with doping luminescent centers as the light source, rely on ion radiative emission channels, and still show a low light yield or hygroscopic nature. While light yield has been improved, these materials still suffer from inherently long lifetimes. Although low-dimensional OIMHs may not replace existing inorganic scintillators in the short term, their low cost and high performance provide a breakthrough opportunity to overcome these challenges and find a balance between light yield and lifetime.

The optical properties of low-dimensional OIMHs are influenced by their components. Organic cations currently primarily serve as structural components, isolating the inorganic metal halide polyhedral units. However, the optical properties of low-dimensional OIMHs dominated by organic cations, dual organic cations, or even more complex systems have yet to be thoroughly explored.

Large-area and flexible imaging issues remain unsolved. While significant breakthroughs have been made in the preparation of large-sized low-dimensional OIMHs single crystals, some specific applications, such as high-energy radiation detection, require even larger, thicker, and more flexible scintillation screens. Currently, the performances of devices based on low-dimensional OIMHs materials is far from meeting the practical needs in these scenarios.

Low-dimensional OIMHs, which have many attractive advantages as light-emitting materials for white light sources, still exhibit low PLQY when used in WLEDs, which is often limited by several intrinsic and extrinsic factors. For example, the halide vacancies, interstitial defects, and organic ligand disorder can create trap states that promote nonradiative recombination. The quasi-2D and 1D structures often suffer from moisture sensitivity, ion migration, and phase segregation under operation. Moreover, under continuous LED operation, heating and UV exposure degrade the OIMH structure, leading to a drop in PLQY. Additionally, the emission spectrum of OIMHs can shift due to ion migration, external stress, or phase changes, impacting white LED performance. Thus, the luminous quantum yield of WLEDs is still insufficient for practical use. Significant research efforts are still required to improve the performances of WLEDs based on low-dimensional OIMHs in the future. For instance, using small molecules, polymers, or additional halide sources can reduce defect density and enhance radiative recombination. Moreover, approaches for the development of practical white LEDs may include adopting encapsulation and protective coatings, which can enhance environmental stability. Also, engineering mixed-halide compositions with suppressed phase segregation ensures stable emissions.