Perovskite Quantum Dots: A New Generation of Promising Scintillator Materials

Abstract

Scintillator materials are essential parts of X-ray imaging/detection for medical diagnostics, non-destructive detection, security inspection, and space exploration. Traditional commercial scintillators are limited by low luminescence efficiency and afterglow effect. Perovskite (ABX₃) quantum dots (PQDs), with the ability to convert X-ray radiation into multi-color visible light, has emerged as a new class of competitive scintillators, based on the advantages of high X-ray absorption coefficients, fast luminescence, high luminescence efficiency, and low-cost wet chemical facile synthesis on various substrates. Understanding the relationship between the structure and luminescence of PQDs is essential for scintillator discovery and optimization. This chapter first reviewed the work principle, materials selection, and key parameters of indirect X-ray detectors, followed by the basic crystal structure, optoelectronic properties, and synthesis methods for PQDs. Then the recent advances in perovskite scintillators are comprehensively reviewed. Finally, we summarize the current challenge in perovskite scintillators with promising solutions and provide a perspective on the future direction of this emerging scintillator.

1. Introduction

X-Ray radiation, uncovered by Wilhelm Röntgen circa 1895, comprises a genre of high-intensity electromagnetic radiation with extraordinary penetrative properties, offering provisions for non-destructive inspection [1]. Operating in medical radiography, photodynamic therapy (PDT), nuclear deterrence, and security audits, X-rays have become an indispensable tool [2]. The operative principle of X-ray detection hinges on tracking and disclosing the post-penetration diminishing of incident X-ray radiation on specific subjects. For instance, the inspection of in-vivo organs renders high-resolution imaging instrumental in clinical diagnostics, as does the exploration of non-living matter for industrial and security purposes [3].

The current X-ray detection machinations are bifurcated into two central categories: direct conversion, where X-ray quantum is translated to an electrical signal via a semiconductor, or to a chemical signal via a film, and indirect conversion, where the transformation is to low-energy photons (visible light) mediated by scintillating materials [4, 5]. Presently, for X-ray imaging based on integrating detectors, there are limited materials that can replace nano-scintillators in the consumer market, outside of the selenium (Se) detector purposed for mammography. Even leading candidates for direct conversion materials, such as CdTe and CdZnTe (CZT), wrestle with issues such as a delay in response rate, coupled with temporal maladies like persistent afterglow incited by electron entrapment. Consequently, manufacturing large-sized wafers with diminished noise current yet high mobility-lifetime product (μτ) remains an arduous task, rendering the achievement of an acceptable price-performance ratio for commercial applications a challenge (Figure 1).

Figure 1.

Application of scintillator in X-ray detectors [6].

Conversely, scintillators pave the way to convert X-rays into visible light, known as down-conversion, engendering the commercial utilization of affordable sensing arrays such as photomultiplier tubes (PMT), avalanche photodiodes (APD), amorphous-Si photodiode matrices, and charge-coupled devices. This manipulation has generated a surge of research interest toward indirect conversion scintillating materials for X-ray detection.

However, established scintillators such as NaI:Tl, CsI:Tl, CdWO₄ (CWO), Bi₄Ge₃O₁₂ (BGO), Lu₂SiO₅ (LSO), and Lu_2(1 − X)Y_2XSiO₅ (LYSO) necessitate further enhancements. Despite their potent stopping power and heightened light emission, qualifying them for commercial application in medical imaging devices, there is a demand to streamline the manufacturing processes, circumvent persistent afterglow, and discover novel electron-transition energy levels for adaptable scintillation. Particularly in diagnostic radiology, the attainment of superior spatial resolution while minimizing radiation exposure is a critical priority.

Thus, lead-halide perovskite nanocrystals-based X-ray sensors are an emerging technology projected to have considerable application prospects in future radiography, thanks to their accelerated fabrication, rapid response, and elevated spatial resolution. Such halide perovskites typically present unique electronic characteristics, including a diminished trap density, heightened charge-carrier mobility, and an extended minority-carrier diffusion length, specifically in nanocrystal morphology.

CsPbCl₃, despite demonstrating robust luminescence via free excitons upon exposure to radiation with prompt nanosecond response, was marred by issues such as intense self-absorption, decreased photoluminescence quantum yield (PLQY), Pb toxicity, and overall fragility. However, following decades of research, efficacious Pb-free scintillators, like double-perovskite, Cu, and Bi-based all-inorganic halide perovskites, have shown encouraging scintillation behaviors.

This review outlines the fundamentals of scintillators, their quintessential luminescence mechanisms, primary parameter performance, and the essential considerations for selecting optimal scintillator materials. A robust comparison is made between emerging perovskite quantum dot scintillators and traditional variants, emphasizing the merits, demerits, performance, and applications inherent to perovskite quantum dot systems. Finally, the obstacles encountered by perovskite quantum dots as a formidable contender in the realm of scintillator materials are stressed.

2. Fundamentals and progress of scintillators

2.1 Scintillators and their luminescence mechanism

Scintillators are defined as materials capable of emitting ultraviolet or visible light during the de-excitation phase having undergone stimulation by high-energy photons or irradiated particles. Presently, a broad array of inorganic scintillators is available, each embodying a distinct property. Concurrently, for Computed Tomography (CT) applications, it is imperative that the corresponding inorganic scintillators possess elevated density, augmented light yield, and minimal detection limits.

The process of scintillator luminescence is considerably intricate [7, 8], an aspect that has piqued extensive scientific curiosity, leading to persistent advancements. During the interactions between high-energy photons and the electrons within the lattice configuration, the conduction band experiences excitation, eliciting the integration of high-energy electrons and concurrently leaving hole states within the valence band. Upon surpassing the ionization threshold, the electron’s energy provokes collision ionization, occasioning the generation of secondary electrons. If the energy is inadequate to foster ionization, the electrons and holes will implement energy reduction via lattice relaxation until they respectively migrate to the base of the conduction band and the apex of the valence band. Consequently, these can be bound to form excitons or traverse to the luminescent center to facilitate recombination. Explicitly, the luminescence process in scintillators can be classified into three operative phases: conversion, transfer, and lu-minescence, [9, 10, 11] as shown in Figure 2.

Figure 2.

The luminescence mechanism of an inorganic scintillator is divided into three stages: conversion, transfer, and luminescence [11].

2.2 Criteria for selecting scintillator materials

An ideal scintillator will have essential features such as a high light yield, acute energy resolution, and swift time resolution. However, the reality demands that the chosen scintillator must cater to specific application requirements. In the context of CT imaging, the scintillator should exhibit strong X-ray absorption, minimal self-absorption, excellent light yield, and a short lifetime, optimally paired with the capacity for large-scale, arrayed production. This combination allows for the realization of superior quality imaging under low-dosage conditions. The probable principles for selecting scintillators can be tabulated as follows:

1. Potent absorption capability: a larger quantity of incident X-ray photons absorbed by a scintillator translates to a higher conversion of hot electron-holes. X-ray absorption coefficient depends on the material’s effective atomic number (Z_eff) and the density (ρ), hence, materials with a higher density and heavy elemental composition are deemed desirable.

2. Substantial light yield: light yield (LY), indicative of the scintillator’s luminous efficacy, is quantified as the number of photons emitted per MeV of ionizing radiation [12]:

   $$LY=\frac{1000000}{\beta E_g}SQ.$$E1

   Where E_g symbolizes the bandgap, β is a constant range between 2 and 3, S represents the transfer efficiency in the transfer process, and Q is the luminescence efficiency. As per its definition, light yield hinges on the bandgap E_g, the quality of the crystal, and the photoluminescence quantum efficiency (PLQY). Aiming for a high light yield mandates the chosen scintillator to possess a relatively narrow E_g value, superior crystal quality, and a PLQY value as high as conceivable. An enhanced light yield is also beneficial for attaining a lower detection limit and a broader linear response, thereby improving the sensing efficiency.

3. Trivial self-absorption: self-absorption denotes a phenomenon where the scintillation photons are reabsorbed by the crystal—preventing their emission. Reducing self-absorption is a significant challenge in the field of scintillators. Minimized self-absorption implies a larger Stokes shift, and self-trapped exciton luminescent scintillators in this scenario possess an innate superiority.

4. Radioluminescence wavelength: the emission wavelength of a scintillator determines the photodetection coupling devices that can be selected. Ensuring minimal loss in collecting scintillation photons requires that the spectral detection range of the photodetector should overlap the emission range of the scintillation light.

5. Rapid decay: a faster decay lifetime signifies a sharp photoelectric conversion speed, imposing elevated time resolution of the scintillator detector. However, faster decay of light emission, at times, becomes unfavorable in obtaining a higher light yield. For CT imaging applications, a decay time (t) of less than 0.1 ms should suffice for this application requisites.

6. High stability: the stability of a scintillator pegs its usable life and serves as a prerequisite for its application. It mainly encompasses physicochemical stability, luminescent stability, and irradiation stability, which would accordingly impact the structural stability of the scintillator, its light yield, and irradiation hardness parameters.

7. Other aspects, such as low detection limit, broader linear response, economical, weak background radiation, upscaling potential, suitability for arrayed production, and more. Lower detection limits imply enhanced sensitivity of the detector for low-dose X-rays. Also, cost efficiency remains a significant consideration for practical applications.

Image quality parameters include spatial resolution, presenting the image’s contrast and clarity within a specific spatial frequency range, generally quantified by the modulation transfer function (MTF) deploying the edge method. Image quality is generally regulated by the intrinsic properties of the scintillator, the detector, and their coupling. We will principally discuss the scintillator’s influence.

3. Perovskite as promising scintillator materials

Perovskite is a promising material that is used in the field of solar cells, light-emitting diodes, lasers, and photodetectors. This material has caught the attention of scientists for its potential application as high-energy radiation detectors and scintillators due to their excellent light yield, mobility-lifetime product (μτ), and X-ray sensitivity [13]. The unique properties of perovskites come from their special structure.

3.1 The structure and stability of perovskite

Perovskite is a family of compounds that share the same crystal structure as CaTiO₃ and are often represented by the ideal formula AMX₃. In the area of photoelectric, the cation A is typically an organic ammonium ion (CH₃NH₃⁺), a fluor ammonium ion (CH(NH₂)²⁺), or a cesium ion (Cs⁺), while the halide anion X is represented by Cl⁻, Br⁻, I⁻, and the metal cation B by Pb²⁺, Ge²⁺, Sn²⁺, Cd²⁺, Mn²⁺, and others [14]. These compounds form BX₆⁴⁻ octahedral structures by coordinating metal cations B and halide anions X, which are interconnected through shared vertices to create a three-dimensional network structure [15]. Halide ions X can also be partially replaced by other halogen atoms, resulting in mixed perovskites like ABCl_xBr_3−x, ABBr_xI_3−x, and others. The band structure of perovskite materials varies depending on the chemical composition of ABX₃, with the A-site cation being highly ionized and contributing little to the band edge [16]. In contrast, the BX₆⁴⁻ octahedral structure directly determines the band structure of perovskites [17].

The crystal structure of perovskite is highly flexible, but it needs to meet certain conditions to keep stable. The definitions of Goldschmidt tolerance factor (t) and the octahedral factor (μ) are shown in the Figure 3; both are used to describe the stability of the perovskite structure [19]. Among them, t represents the degree of distortion of the perovskite structure and is related to the ionic radii r of A, B, X, satisfying the formula (1–1), μ is directly calculated of the octahedral complex BX₆⁴⁻ as shown in the formula (1–2). Atomic packing fraction (η) can be described by formula (1–3), where V_A, V_B, and V_X are atomic volumes of A, B, and X and a is the lattice constant of the cubic cell. Wan-Jian Yin’s team studied 138 kinds of perovskite compounds using first-principles calculations. They found a linear correlation between decomposition energies and the descriptor (μ + t)^η, indicating a notable thermodynamic stability trend that can be used to predict stable perovskite structures [18].

Figure 3.

Definitions of the structural factors t, μ, η in a cubic phase perovskite crystal structure [18].

3.2 Synthesis method of fully inorganic perovskite quantum dots

The crystal quality of perovskite quantum dots is the most critical factor determining the efficiency of scintillator devices. Optimizing synthesis methods to achieve control over the morphology, size, and structure of nanocrystals is also crucial. Currently, there are several main methods for preparing perovskite quantum dots:

The hot-injection is the most classic synthesis method for quantum dots, which involves rapidly injecting one type of precursor into a high-temperature solvent containing other precursors and necessary ligands. After the injection of the precursor, nucleation and growth occur rapidly. This method can produce high-quality crystals with a narrow size distribution. The hot-injection synthesis method allows for the control of the size, morphology, and structure of the growing nanocrystals, mainly by controlling: (1) the reaction temperature at injection; (2) reaction time; (3) ligands or activators; (4) precursor concentration; (5) the ratio of precursor components. As illustrated in Figure 4a [21], the hot-injection method was first used in 2015 by the Protesescu research group to synthesize perovskite CsPbX₃ [22]. The primary method involved synthesizing cesium oleate as a precursor, which was then injected into a high-boiling solvent containing PbX₂ at temperatures ranging from 140 to 200°C. The final nanocrystal size could be controlled by adjusting the temperature, thus tuning the luminescence peak position based on the quantum size effect. Additionally, by simply changing the proportion of halogens in PbX₂, mixed halogen perovskite nanocrystals could be synthesized. Therefore, this method allows for complete and fine-tuned coverage across the entire visible light spectrum.

Figure 4.

Diagram of (a) high-temperature hot-injection method; (b) room-temperature recrystallization method [20].

When ions dissolve in a solvent, their concentration increases with the dissolution of more ions until reaching equilibrium concentration. If the solution becomes supersaturated and non-equilibrium, precipitation and crystallization will spontaneously occur in the solution to restore balance. Moving the system from an equilibrium to a supersaturated non-equilibrium state can be achieved by controlling temperature, evaporating some of the solvent, or mixing in another solvent with lower ion solubility. In chemical synthesis, ligands are often used to further control the crystallization process, allowing for nanoscale control of crystal growth, hence this method is known as ligand-assisted reprecipitation in chemical synthesis. As shown in Figure 4(b), the ligand-assisted reprecipitation method is widely used in the synthesis of perovskites. Typically, precursors (like CsX, PbX₂) and ligands are dissolved in a highly polar solvent such as DMSO or DMF, thoroughly mixed and dissolved. Then, a small amount is dropped into a poor solvent for perovskites (like toluene), causing nucleation and growth of perovskites due to the instantaneous shift from a supersaturated non-equilibrium state to equilibrium. This method can be proceeded in the air and is easier for scaleup. However, the size distribution of quantum dots synthesized by this method is larger.

3.3 Classification and applications of perovskite scintillator materials

Traditional scintillators have consistently served as the gold standard in performance evaluation for nascent scintillating materials. Herein, they are classified into three cardinal sub-categories: single-crystal, oxide-based, and organic scintillators. Single-crystal scintillators and oxide scintillators normally require high vacuum and high-temperature processing. Organic scintillators offer advantages such as ease of synthesis, low cost, and shorter decay times of approximately 1 ns [23, 24]. However, complexities emerge due to their low density (around 1 g cm⁻³) and low effective atomic number (Z), leading to a drastically reduced detection efficiency for high-energy X-rays [25]. Furthermore, organic dyes exhibit a heightened susceptibility to photobleaching and quenching effects from oxygen, significantly impeding their utilization across various applications.

Over the past few years, perovskite materials, due to their distinctive attributes such as a high photoluminescence quantum yield (PLQY), swift decay time, substantial atomic number, easily tunable luminescence color, and straightforward, cost-effective preparation methods, have been identified as potential scintillator materials [26, 27].

Based on the composition and morphological properties of perovskite, perovskite scintillators can be categorized into the following groups: single-crystal scintillators, two-dimensional (2D) scintillators, and perovskite quantum dots (PQDs) scintillators.

3.3.1 Single crystal (SCs) scintillators

As previously stated, the quality of the scintillating material is a critical determinant in gauging the sensitivity of X-ray detectors. Therefore, excellent scintillators should ideally exhibit characteristics such as significant volume resistivity, diminished dark current, superior crystallinity, and fewer trapping states, which will enhance the μτ value. Notably, an optimal perovskite SC demonstrates small trap states and no grain boundaries, subsequently reducing the likelihood of carrier scattering and thus accelerating the carrier migration rate, ultimately enhancing the overall performance of the device [7].

Additionally, beneficial attributes such as considerable thickness, high density, effective energy response, and a broad absorption cross-section of perovskite SC scintillators have demonstrated enormous potential in X-ray detection. However, the implementation of traditional high-temperature furnace growth techniques for SC scintillators presents challenges including not only high production costs but also inducing doping gradient that leads to uneven light output and a decrease in resolution, precluding large-scale production.

3.3.2 Two-dimensional scintillators

Two-dimensional scintillators represent nanoscale materials, facilitating free-electron movement across only two dimensions. The transition from 3D to 2D structures boosts the quantum confinement effect within perovskite scintillators, producing several unique features [3]:

1. Efficacious scintillation under ambient conditions. Conventionally, substantial 3D perovskites barely facilitate impactful room-temperature scintillation owing to low exciton binding energy engendering prominent thermal quenching. In contrast, for identical materials, 2D structures typically possess higher exciton binding energy than their 3D counterparts, thereby mitigating luminescence efficiency interferences from phonons. Deep exciton energy levels consequently endow them with thermal quenching resistance at room temperature.

2. Swift response and minimal decay. Quantum wells intrinsic to 2D perovskites confine electrons and holes instigated by ionizing radiation, augmenting the overlap of the electron-hole wave function. This sole electronic characteristic amplifies exciton oscillator strength while mitigating 2D perovskites’ exciton radiation lifetime.

3. Exceptional environmental stability against ionizing radiation. Layered 2D halide perovskites’ stability lies in their unique architecture, featuring inorganic semiconductor layers enveloped by copious insulating organic ones. The existence of potent hydrogen bonds among inorganic and organic units and hydrophobicity characteristic to organic spacers bestows onto scintillators superior resistance against humidity and radiation intensity. Additionally, 2D scintillator synthesis can achieve exceptional thinness to meet X-ray micro-imaging requirements, assisting in mitigating response time delays and efficiency losses provoked by the self-absorption effect. 2D scintillators are envisaged for future extensive applications [28].

3.3.3 Quantum dots scintillators

Standing in contrast to previously mentioned material structures, PQDs manifest amplified quantum confinement effects, due to unique electronic structures, resulting in superior scintillation performance. Moreover, due to quantum confinement effects, nanocrystal scintillators generate excellent luminescent efficiency. In recent years, Halide PQDs scintillators have been extensively studied for their easy-to-manufacture nature, tunable bandgap within the visible light spectrum, and high X-ray absorption coefficients [1].

The wet chemical solution processibility and controllable thickness of PQDs scintillators make them ideal candidates for flexible or large-area panel detectors within imaging technologies. Furthermore, PQDs solutions can function as inks within manufacturing processes. Regarding X-ray scintillation applications, materials’ tolerance to high-power X-ray radiations is fundamental in constructing medical detectors. Various all-inorganic halide perovskites have steered an increasing research interest in scintillation applications, attributed to their swift response and high detection efficacy toward X-rays.

The emerging nanoscale perovskites possess commendable manufacturability when fabricating scintillating thin films, favoring the construction of flexible devices. Furthermore, owing to quantum and small-size effects, perovskite PQDs scintillators showcase a unique set of electromagnetic and extensively extended optical characteristics. It is notable that while material downsizing to the nanoscale can augment scintillation performance immensely, it might concurrently induce a reduction in effective mass density, thereby curtailing the X-ray stopping power. Fortunately, researchers have devised effective solutions to this predicament through novel manufacturing methodologies. For instance, integrating nanocrystals with glassy media has realized high stability and high-resolution X-ray imaging.

Although PQDs scintillators have observed widespread use in detection and imaging, several leaps persist toward achieving practical applications. Numerous technical issues await solutions—structural optimization, detection efficiency improvement, and cost reduction to name a few. Typically, thicker active materials facilitate effective X-ray photon absorption; however, inefficient carrier collection could compromise device sensitivity. Furthermore, despite PQDs scintillators’ potential for conjunction with silicon detectors, strong re-absorption phenomena inherent between them result in decreased silicon detector efficiency (Figure 5) [30].

Figure 5.

Scintillator materials and application fields used in X-ray indirect detectors [29].

4. Progress in the development of perovskite quantum dot scintillators

Perovskite single crystals’ luminescent yield at ambient conditions is relatively low (less than 1000 photons/MeV) [31, 32], gaining merit solely at lowered temperatures [33]. Research postulates that perovskite single crystals undergo considerable thermal quenching at room temperature, leading to non-radiative recombination of most carriers rendering low luminescent yield [34].

Leveraging quantum confinement effects, Asai’s research team [28, 35] fabricated two-dimensional perovskites. This approach enhanced the exciton binding energy of the crystals, subdued thermal quenching, and led to the creation of various “quantum scintillators” exhibiting ultrafast responses. Koshimizu’s team further advanced these two-dimensional perovskite scintillators by synthesizing the quintessential (Phe)₂PbBr₄ crystals, Phe = C₆H₅(CH₂)₂NH₃. They realized sub-nanosecond time resolutions scintillator with decay lifetime of 7.4 ns [36], and a luminescent yield of 14,000 photons/MeV [37].

The team also explored the influence of disparate organic groups and inorganic ions on the scintillator attributes [3]. Noteworthy members of the two-dimensional perovskite scintillators family include (EDBE)PbCl₄ [31], (PEA)₂PbBr₄ [38], and (BA)₂PbBr₄ [39], all exceeding a luminescent yield of 10,000 photons/MeV and exhibiting nanosecond decay lifetimes. Some exploratory studies leveraged the incorporation of fluoride atoms [40], organic groups [41], or alternative metallic ions [42, 43] to enhance these two-dimensional perovskite scintillators with satisfactory progress. Lead-free two-dimensional perovskite scintillators have surfaced as a novel research direction. Successful material synthesis [43] followed the replacement of Pb²⁺ with Sn²⁺, effectively demonstrating imaging [4].

In 2015, Protesescu et al. [22] first pioneered the synthesis of inorganic perovskite nanocrystals (CsPbX₃, X refers to halide) with near 90% photoluminescence quantum efficiency (PLQY). This material structure reinforced the quantum confinement effects, displaying high luminescence rates and minimal non-radiative recombination, thus serving as a highly efficient light-emitting material. Research teams recognized the material’s potential for scintillation and studied CsPbX₃ inorganic perovskite nanocrystals for X-ray imaging applications, resulting in elementary indirect X-ray imagers [11]. Exemplarily, the green-emitting CsPbBr₃ demonstrated the best performance, prompting researchers to explore X-ray imaging performances based on this scintillator (Figure 6) [44].

Figure 6.

(a) Ultrasensitive X-ray sensing and radi-ography using CsPbBr₃ nanocrystals. (b) Image of the sample under X-ray [6].

PQDs scintillators exhibit several significant challenges: (1) A pervasive deficiency of methodologies allowing for the large-scale fabrication of scintillator films; (2) compromised stability, which manifests in performance degradation when subjected to heat and humidity in the ambient environment; (3) limited imaging resolution with the inherent opacity of thick films persistently hampering their overall imaging efficacy.

To solve these challenges, researchers have studied various solutions. Wang et al. [45] disclosed a room-temperature process for fabricating high-content CsPbBr₃ nanosheet colloidal scintillators by directly mixing varied amount of CsPbBr₃ QDs with acrylate-based resin, the resulting 200 um thick CsPbBr₃ QD-resin layers were laminated with 100 μm thick barrier films on both sides as a water vapor barrier (as shown in Figure 7), varied polymer and resins have been applied as matrix for PQDs follows this advancement. Similarly, based on the “emitter-in-matrix” design principle, Cao et al. [1] prepared CsPbBr₃@Cs₄PbBr₆ with emissive CsPbBr₃ QDs embedded inside a solid-state Cs₄PbBr₆ host is subjected to X-ray sensing and imaging. The Cs₄PbBr₆ matrix not only enhances X-ray absorption but also dramatically improves the stability of CsPbBr₃ QDs. The low absorption of Cs₄PbBr₆ matrix to the emission from CsPbBr₃ NCs enables efficient light output, as shown in Figure 8.

Figure 7.

(a) Schematic representation of the sample structure with the CsPbBr₃ QDs mixed in resin between two barrier films. Image of 15% CsPbBr₃ QD concentration in resin under (b) visible light and (c) UV-light excitation [46].

Figure 8.

Schematic view of the band gap of the CsPbBr₃ core and the Cs₄PbBr₆ matrix [1].

In recent years, an innovative research trend has surfaced, involving the in situ growth of perovskite nanocrystals within vitreous matrices [45, 47], thus presenting an exciting potential for X-ray scintillators. Such an approach considerably bolsters nanocrystal longevity and attains a commendable imaging resolution, approximately 15 lp/mm. Taking this a step further, a notable innovative solution was proposed by a research team [48] who dissolved CsPbBr₃ nanocrystals in an organic solvent, the resultant transparent solution was then used for imaging, representing significant strides in the field, as shown in Figure 9.

Figure 9.

Schematic diagram of CsPbBr₃ liquid scintillators and corresponding imaging demonstration [48].

Recently, Wu et al. [3] successfully implemented a suction filter method for the preparation of an ultra-thin transparent perovskite nanocrystal scintillator film. Incorporating Ce³⁺ ions as dopants leads to a considerable surge in light yield, enabling a resolution as high as 862 nm for X-ray micro-imaging. With continuous investigations being carried out in the field, some scientists [49] have identified a critical deficit impeding the efficacy of direct band gap perovskite scintillators—self-absorption. Introducing efficient energy transfer in PQDs film could successfully restrain self-absorption to a very low degree. PQDs with varied particle size have varied conduction band (CB) and valence band (VB) positions, an X-ray absorption layere composed of a wider sized distribution of QDs was prepared by Li et al. [26]. The scintillator performance of PQDs mixture with different sizes but at an appropriate ratio was studied. The successful engineering of absorption-emission overlapping of varied size CsPbBr₃ QDs avoids the emission loss by reabsorption essentially and ensures the maintenance of high quantum yields of CsPbBr₃ QDs. The engineered CsPbBr₃ QDs also show more efficient radiative recombination than conventional QDs due to more efficient charge transfer between varied size CsPbBr₃ QDs. Instead of using varied size QDs, Gandini et al. [50] fabricated a nanocomposite scintillator by embedding CsPbBr₃ QDs together with conjugated organic dyes into the PMMA matrix. As shown in Figure 10, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the organic dye and CsPbBr₃ QDs were perfectly matched, providing an effective energy/charge transfer channel. The emission spectrum of CsPbBr₃ QDs is directly overlapped with the main absorption spectrum of the dye, where the emitted photons from CsPbBr₃ QDs could further excite the dye molecule, thus reducing the self-absorption of CsPbBr₃ QDs. This induced energy transfer between organic molecules and perovskite can effectively increase the utilization efficiency of photons.

Figure 10.

(a) Self-absorption effect of CsPbBr_{3 [49]}; (b) CsPbBr₃ is coupled with organic luminescent materials to form plastic scintillators [50].

Recently, a research team [51] conducted an analysis of CsPbBr₃ nanocrystal scintillator films, aiming to determine the optimal thickness to minimize the influence of self-absorption effects. This strategic move found a harmonious balance between X-ray absorption, light yield, and imaging resolution.

Pioneering luminescent principles beyond band gap recombination has motivated significant academic inquiries. A luminary example is a rare-earth ion orbital electron transition-based double perovskite scintillator; Cs₂NaTbCl₆ as reported by Hu et al. [52]. Despite recording an exceptional light yield of up to 46,600 photons/MeV, the prolonged decay period and severe afterglow may preclude a direct application within real-time X-ray or CT imaging systems.

Furthermore, consequential exploration involves applying diverse element doping to establish multi-modal luminescent scintillators [53] and cerium-doped double perovskite scintillators [54]. Besides this, Zhu and colleagues [55] reported on a lead-free double perovskite scintillator [Cs₂Ag_0.6Na_0.4In_1-yBi_yCl₆], based on the promising principle of self-trapped exciton (STE) luminescence. Fueling high light yield coupled with a sizeable Stokes shift, the decay duration of this variety of scintillators is markedly shorter, substantially lower than the luminous lifetime of rare-earth ions.

Finally, diminutive halide scintillators leveraging STE luminescence are gradually capturing research attention. They include, prominently, Rb₂CuBr₃ [56] and Cs₃Cu₂I₅ [57], both of which demonstrate an enormously high light emission power, marking them out as a vanguard family of scintillators.

5. Challenges and remedies in perovskite QDs scintillators implementation

Over recent years, extensive research has been carried out on metal halide perovskite scintillators for application in X-ray detection and imagery. Despite this, a multitude of technical hurdles persist, preventing these from practical utilization. These obstacles predominantly encompass reducing self-absorption and amplifying luminescent efficacy, bolstering stability, curtailing production expenses, optimizing production on a larger scale, curbing light scattering, and augmenting imaging resolution [58, 59].

1. Improving light yield: the light yield of the scintillator is inversely proportional to the band gap, so adjusting the bandgap by halogen replacement or B site doping is an efficient way to increase the light yield. Other strategies, including modulation of the exciton confinement effect, minimization of self-absorption in PQDs, constructing energy-transfer channel, and defect modification are also widely studied to improve the light yield.

   Many strategies could be adopted to restrain the self-absorption of scintillators. For instance, reducing the scintillator’s thickness may considerably mitigate the efficiency losses and response time due to self-absorption. Rare-earth ions (e.g., Eu³⁺, Yb³⁺, Tb³⁺) exhibit intrinsic 4f–4f transitions with large Stokes shifts, introducing these ions into perovskite structures could effectively enlarge the Stokes shift of perovskite scintillators, which leads to a decent scintillation efficiency. Another strategy to lower the self-absorption of the perovskite scintillators is to mix them with molecules with matched energy levels. During the charge transfer of the scintillating processes, the transport charge carriers can be captured by both defects and impurities in the crystal lattices, which can lead to an increase in non-radiative recombination. Therefore, improving crystal quality and reducing defects in the scintillator (such as impurity doping and surface passivation) are also important strategies to enhance the scintillating performance. Furthermore, an appropriate integration of halogen elements could serve to proliferate light transmission and minimize self-absorption effects [60, 61].

2. Elevating stability. The stability quandaries of metal halide perovskite scintillators include both chemical resilience and tolerance to radiation. The primary issue of chemical stability pertains to the sensitivities of perovskites to environmental stimuli such as water, light, and heat. As for radiation tolerance, it refers to the ability of perovskites to endure high-energy radiation. Given the good defect tolerance of perovskites, disruptions in their ionic characterization could easily lead to surface defects. Hence, attempts to enhance perovskite stability may focus on two fronts—augmenting innate structural stability by amending the composition of perovskites and improving crystal development, and deploying chemical strategies to passivate surface anomalies.

3. Mitigating costs and enabling large-scale production. The inherent solubility of metal halide perovskites makes them amenable to mass synthesis. Yet, many issues, such as compatibility, efficiency retention upon large-scale expansion, and alignment with extant processing and manufacturing apparatus, linger before they can be adapted to practical usage. Prior literature suggests that merging perovskite nanocrystals with polymeric materials—using polymers as substrates, coating materials, or as a growth medium—presents as an efficacious strategy to realize large-scale fabrication.

4. Restraining light scattering to augment imaging resolution. A challenge that metal halide perovskites confront pertains to the diminution of imaging resolution as a consequence of light scattering. This phenomenon prompts a photon cross-talk among neighboring luminescent centers, marking a decrement in resolution [62]. Moreover, internal light scattering within the scintillator can result in impaired transparency, hence impacting the imaging resolution. Varied strategy such as waveguide effect and circularly polarized radioluminescence could better confine the propagation directions of emission photons. In addition, resourceful avenues must be explored to suppress light scattering in nanocrystals—for example, by enhancing the crystallinity and dispersion of nanocrystals, obviating nanocrystal congregation, or regulating the size distribution of nanocrystals to facilitate homogeneous crystallization.

Fabricating all-inorganic perovskite quantum dots into composite materials is an effective method to enhance their performance. As early as 2012, Kojima et al. synthesized MAPbBr₃ nanocrystals in situ within mesoporous Al₂O₃ or ZrO₂ templates, utilizing the mesoporous confinement to limit the growth of the nanocrystals, eventually producing highly luminescent and stable PQDs with a particle size of about 5 nm [63]. It is precisely because of the solubility processing characteristics of PQDs that they can be easily combined with mesoporous inorganic templates. Combining metal halide perovskites with inorganic mesoporous oxides can significantly enhance their stability, but the preparation process is usually complex, and the manufacturing cost is high. Moreover, the optical performance of PQDs is impaired, hindering their further practical application [64].

Besides inorganic encapsulation, organics include polymers and metal-organic frameworks (MOFs). Polymers, with their network structure built on polymer chains, can provide a natural level of nanoscaffold for the growth of nanocrystals, so in situ crystallization processes can be used to prepare PQDs nanocomposite as shown in Figure 11. However, when selecting polymers, the following points should be considered: they should have the same solvent solubility as the perovskite precursors to ensure effective and thorough mixing; strong interactions should be established between the polymer and perovskite; the polymer should have compatible dielectric properties with the device to be assembled; and it should have a certain size of pore structure.

Figure 11.

HAADF-STEM image of (a) CsPbBr₃@MOF-PCN-33, (b) TEM image of CsPbBr₃@PCN-33(Fe) with the dark dots are CsPbBr₃ QDs. Scale bars: a, 25 nm; b, 10 nm [65].

6. Summary and outlook

This comprehensive review has examined the development and current state of perovskite quantum dot scintillators, speculating also on the potential for future advancements. Traditional scintillators, though foundational to the field, display shortcomings that perovskite counterparts have the potential to amend, including cost-efficiency, moisture resistance, and detection capabilities.

The advent of perovskite single-crystal, two-dimensional, and nanocrystalline scintillators has opened up new avenues in the realm of X-ray detection and imaging, attributable to their excellent photoluminescent quantum yield, efficient luminescence, achievable fabrication, and an adjustable bandgap in the visible light spectrum. Considerations regarding high uτ value maintenance and structural optimization, however, remain paramount.

Significant progress and breakthroughs have been seen in the development of various types of perovskite quantum dot scintillators, from the pioneering fabrication of two-dimensional perovskites, demonstrating distinct photoluminescent properties, to the potential of lead-free two-dimensional perovskite scintillators, exhibiting promising synthesis capabilities and photoluminescent quantum efficiency.

Despite the significant progress made, there are still numerous technical challenges hindering the practical utilization of metal halide perovskite scintillators. These primarily include achieving high luminescent efficiency while minimizing self-absorption, ensuring ultra-fast response times and attenuation, maintaining stability, managing production costs, adapting for large-scale production, reducing light scattering, and enhancing imaging resolution. Innovative solutions are thus necessitated to address these challenges, with potential leadways including thickness reduction of the scintillator, the introduction of defect passivation and interface modification, halogen elements, structural stability enhancements, and the utilization of polymeric materials for scalable production.

In conclusion, the exploration and optimization of perovskite quantum dot scintillators for X-ray imaging and dose monitoring represent an emerging research field. Flexible scintillator films broaden the application of bendable X-ray imaging, releasing the frequency and related risk for radiation damages, enabling more accurate real-time dose monitoring for radiotherapy and tomography.

Notes/thanks/other declarations

The authors acknowledge support from the Cities partnerships Programme (Rome seed fund) from UCL Global Engagement Office. The European Commission supported this work under the H2020 HI-ACCURACY project (grant agreement ID: 862410).