Fantastic Photons and Where to Excite Them: Revolutionizing Upconversion with KY₃F₁₀-Based Compounds

Abstract

This review delves into the forefront of upconversion luminescence (UCL) research, focusing on KY₃F₁₀-based compounds, particularly their cubic α-phase. These materials are renowned for their exceptional luminescent properties and structural stability, making them prime candidates for advanced photonic applications. The synthesis methods and structural characteristics of the existing works in the literature are meticulously analyzed alongside the transformative effects of various doping strategies on UCL efficiency. Incorporating rare earth (RE) sensitizer ions such as Yb³⁺, along with activator ions like Er³⁺, Ho³⁺, Nd³⁺, or Tm³⁺, researchers have achieved remarkable enhancements in emission intensity and spectral control. Recent and past breakthroughs in understanding the local structure and phase transitions of single-, double-, and triple-RE³⁺-doped KY₃F₁₀ nanocrystals are highlighted, revealing their pivotal role in fine-tuning luminescent properties. Furthermore, the review underscores the untapped potential of lesser-known crystal structures, such as the metastable δ-phase of KY₃F₁₀, which offers promising avenues for future exploration. By presenting a comprehensive analysis and proposing innovative research directions, this review aims to inspire continued advancements in the field of upconversion materials, unlocking new potentials in photonic technologies.

1. Introduction

Upconversion luminescence (UCL) refers to the process by which lower-energy photons are converted into higher-energy light through multiphoton absorption mechanisms [1,2]. This phenomenon allows for the transformation of infrared or visible radiation into visible or ultraviolet light, which can be harnessed for a variety of applications, spanning from bioimaging and photovoltaics to laser technology and security printing [3]. The efficiency of UCL depends on the complex interplay of energy transfer processes among the different energy levels of lanthanide ions (Ln³⁺). Near-infrared (NIR) lasers are commonly used as excitation sources for UCL, making this technique particularly valuable in fields like bioimaging, where their deep tissue penetration and reduced photodamage are advantageous [4].

The upconversion (UC) phenomenon has been observed in various transition metals and actinides, but it is most prominently noted in the rare earth (RE) elements, which encompass the lanthanide (Ln) series, as well as yttrium and scandium [5]. In UC materials, lanthanide ions like Er³⁺, Ho³⁺, and Tm³⁺ are commonly used as activators, often co-doped with sensitizer ions like Yb³⁺ to improve the overall performance of the upconversion process. The interplay between these dopants and the host lattice determines the effectiveness and efficiency of the UCL.

To date, host materials like NaREF₄, REF₃, and RE₂O₃, have been well renowned for their efficacy in the UCL process [6]. Within this realm, compounds with low phonon energies, such as inorganic fluorides (typically <550 cm⁻¹), are preferred because they minimize non-radiative losses and enhance UC efficiency [7].

Among the diverse array of fluoride-based host lattices [8], β-NaYF₄ stands out due to its excellent UCL properties [9,10,11,12,13], yet the underlying reasons for this enhanced performance are still less understood. Some plausible explanations in the literature are founded in the consideration of β-NaYF₄ as a flexible lattice framework, especially in nano-sized materials, with intricate characteristics and structural disorders caused by atomic displacements within the lattice [11].

Over the past few decades, significant efforts have been devoted to optimizing the UCL efficiency of β-NaYF₄, particularly through precise control over the sizes, shapes, and crystallinity of nanoparticles [14,15,16,17]. Therefore, if researchers are asked where to excite photons to achieve a fantastic UCL response, the β-NaYF₄ host lattice always emerges as the first potential candidate.

However, some studies have suggested that alternative host lattices, such as LiYF₄ or Y₂O₂S, could be more suitable for UC-based applications. This is due to their superior chemical and thermal stability, as seen with Y₂O₂S, or their potential for achieving better optical resolution, UCL efficiency, and color purity in emission [18,19,20,21,22]. This raises the question of whether a rival material might eventually surpass β-NaYF₄.

All of this brings us to the conclusion that further investigation into uncharted crystal structures is necessary. Within the fluoride family, KY₃F₁₀ has emerged as another promising material [23,24,25,26,27]. Despite its potential, the volume of research on UC materials based on KY₃F₁₀ is vastly distant from that on β-NaYF₄.

The primary aim of this review is to comprehensively summarize the existing literature on UC in KY₃F₁₀-based compounds and to provide readers with an extensive overview of the potential applications of this host lattice. It aims to spark interest in this relatively unexplored material that, perhaps, could revolutionize the field. Notably, all UC studies discussed herein (and in the literature) focused exclusively on α-KY₃F₁₀ while the δ-KY₃F₁₀·xH₂O variant remains largely neglected by the scientific community.

2. Crystal Phases of KY₃F₁₀

KY₃F₁₀ is known to exist in four distinct crystal phases: α-KY₃F₁₀, which is the thermodynamically most stable phase; a high-pressure β-KY₃F₁₀ phase; a tetragonal γ-KY₃F₁₀ phase (whose stability and assignment are subjects of much controversy); and δ-KY₃F₁₀·xH₂O, identified as a metastable hydrated phase and with great potential for optical applications. It should be clarified that the labels “β” and “γ” used here follow the conventional Greek notation for polymorphs in Materials Science as no specific names for these phases exist in the current literature. These labels have been intentionally introduced in this article to provide a more coherent narrative.

Traditionally, the different structures can be described using prisms formed by YF₈ units whose assemblage results in a [Y₆F₂₀]²⁻ cluster [28]. As an illustrative scheme, the local distribution of the YF₈ units comprising the [Y₆F₂₀]²⁻ clusters within the unit cell (highlighted in gray) is shown in Figure 1.

For a better distinction of the various crystal phases, the corresponding crystallographic parameters are detailed in

Table 1. Crystallographic information on the different structures of KY₃F₁₀: crystal system, space group (SG), formula units (Z), lattice parameter (a), cell volume (V), local symmetry of Y³⁺ ions (LS), and coordination number of Y³⁺ ions (CN).

Compound	Crystal System	SG	Z	a (Å)	V (Å3)	LS (Y3+)	CN (Y3+)
α-KY3F10	Cubic	$Fm\overline{3}m$ (No. 225)	8	11.543	1538.06	C4v	8
β-KY3F10	Cubic	$Pm\overline{3}m$ (No. 221)	1	5.705	185.67	D4h	8
γ-KY3F10	Tetragonal	$I4/mmm$ (No. 139)	4	8.161 1	768.52	C4v/C2v	8
δ-KY3F10·xH2O	Cubic	$Fd\overline{3}m$ (No. 227)	16	15.492	3717.90	C2v	8

¹ For the tetragonal system, c = 11.539 Å.

[29,30,31]. The local symmetry (LS) and coordination number (CN) of Y³⁺ ions are also indicated as this information can be of interest because when doping the material with RE ions, they substitute yttrium cations in the host lattice.

The following subsections provide a detailed exploration of these diverse crystal structures and their suitability as host lattices for practical purposes.

2.1. Cubic α-KY₃F₁₀

Under ambient conditions, the α-phase is the thermodynamically most stable and well-studied form of potassium triyttrium decafluoride. Indeed, a δ→α phase transformation is observed around 440 °C [31], thus corroborating the higher thermodynamic stability of α-KY₃F₁₀. It adopts a fluorite-type structure, distinguished by its cubic symmetry, which simplifies its crystallographic analysis [32]. Its synthesis process is well documented and relatively easy to perform, enabling consistent production for use in optical materials, laser hosts, and phosphors [33,34].

2.2. Cubic High-Pressure β-KY₃F₁₀

The β-phase is quite uncommon because it requires specific synthesis parameters that can only be achieved under high-pressure conditions. The α→β transition occurs when compressing α-KY₃F₁₀ above 7.5 GPa at room temperature (298 K). However, as stated in the work of Grzechnik et al. [29], this high-pressure polymorph can also be obtained at lower pressures combined with high temperatures (e.g., 4.5 GPa and 793 K). Be that as it may, the extreme, delicate, and difficult experimental conditions required to obtain this crystal structure prevent it from having practical and broader applications.

2.3. Tetragonal γ-KY₃F₁₀

The existence of a tetragonal phase of KY₃F₁₀ has been a subject of discussion and analysis based on various crystal structure reports. The indexation of an X-ray powder diffraction (XRD) pattern of KY₃F₁₀ prepared at ambient conditions in an early study (which dates back to 1971 [30]) indicated a tetragonal unit cell, which was recorded in the International Centre for Diffraction Data (ICDD) database as entry No. 00-027-0465. However, subsequent investigations using single crystal data and XRD have cast doubt on this initial finding [28].

Recent research has provided new insights into the local structure of nanoscale Tb: KY₃F₁₀ [35]. The average structure was examined using the Rietveld method and compared with findings from pair distribution function (PDF) analysis, which detected subtle local structural distortions. PDF analysis revealed the presence of small tetragonal domains within the nanoparticles. Nevertheless, these distortions were so minor that the overall structure could still be accurately described by the cubic symmetry of the α-phase [35].

Moreover, if we compare the theoretical XRD patterns of both α-KY₃F₁₀ (cubic) and γ-KY₃F₁₀ (tetragonal), it is quite complicated to experimentally discriminate between both of them since the reflections’ intensity and position are virtually the same, except for one peak that appears at ≈28 °2θ (CuK_α radiation) but tends to have a low intensity (as theoretically calculated). Consequently, inaccurate conclusions in the literature can sometimes arise because it is difficult to distinguish this reflection peak from the main peak (100% intensity) that appears at ≈26.5 °2θ. This main peak is associated with α-KY₃F₁₀, but it also exists for γ-KY₃F₁₀. Thus, in cases with broad XRD peaks, as commonly found in nanoparticulate systems, it becomes impossible to discriminate the reflection appearing at 28 °2θ.

Therefore, although some local distortions might exist in cubic KY₃F₁₀, it would be more prudent not to attribute them to the γ-KY₃F₁₀ phase unless this can be clearly proven crystallographically by advanced methods or techniques, such as those mentioned above, which use alternative radiation sources (e.g., synchrotron radiation) rather than conventional CuK_α. Even so, the current literature suggests that for nano-sized materials, where local distortions are more likely due to the small size regime, the global structure should be described by the cubic polymorph rather than the tetragonal one.

2.4. Cubic δ-KY₃F₁₀·xH₂O

The δ-phase is metastable and more challenging to synthesize. It requires the careful control of the synthesis environment, often involving specific kinetic conditions that favor its formation [36,37]. It has a more complex structure compared to the α-phase, characterized by a super-diamond cubic arrangement and also by the presence of crystalline water molecules. The number of water molecules has been established to be x = 1.8–2.8, depending on the synthesized material [32,38,39]. In fact, the presence of these water molecules enables mobility through the channels, resulting in a zeolitic behavior in this distinctive phase [38]. This compound has recently gained attention for its unique luminescent properties, particularly when doped with RE ions like Eu³⁺, which enhances its optical applications due to the increased emission efficiency compared to the α-phase (up to 20 times higher) [39].

According to the aforementioned points, it is evident that among the crystal phases of KY₃F₁₀, the α-phase emerges as the most stable and extensively studied. Therefore, it will be the primary focus of this review. In contrast, the β- and γ-phases do not garner as much attention. However, the δ-phase presents a promising yet underexplored direction for future research and necessitates further development.

3. Upconversion: Features and Mechanisms

The UC process is characterized by several distinctive features. Unlike conventional Stokes luminescence, where emitted photons are of lower energy than the absorbed ones, UCL results in the emission of photons with higher energy [40]. This anti-Stokes emission is achieved through nonlinear multiphoton interactions, where the intensity of the upconverted emission typically scales with the square or higher power of the excitation intensity [1]. Additionally, UCL emissions are often sharp and well defined, stemming from the intra-4f transitions of RE³⁺ ions, which are shielded by outer 5s and 5p orbitals, leading to minimal crystal field splitting. The long-lived excited states of these ions also provide an extended timeframe for the sequential absorption and energy transfer processes that are crucial for effective upconversion [41]. In addition, by carefully choosing the specific RE³⁺ dopants, it is possible to fine-tune the UC emission, allowing for precise control across the entire visible spectrum [42], as highlighted in Figure 2.

Several UC mechanisms have been proposed, each involving different pathways and interactions between electronic states. The primary mechanisms include excited-state absorption (ESA), energy transfer upconversion (ETU), cooperative energy transfer (CET), photon avalanche (PA), and energy-migration-mediated upconversion (EMU). Given that the UC mechanisms of RE ions, depicted in Figure 3, have been extensively explored and documented in numerous studies [43,44,45], only a brief overview will be provided here to refresh readers’ understanding of the key processes involved in UCL.

3.1. Excited-State Absorption (ESA)

ESA involves the sequential absorption of two or more photons by a single ion. An ion in the ground state absorbs a photon and is excited to a higher energy state. This excited ion can then absorb another photon, promoting it to an even higher energy state, from which it can emit a photon of higher energy than the original incident photons [44]. ESA is typically observed in systems where the energy levels of the dopant ions are well matched to the photon energies involved [46].

3.2. Energy Transfer Upconversion (ETU)

ETU occurs through interactions between neighboring ions. In this mechanism, an excited ion transfers its energy to a neighboring ion in the ground state, exciting it to a higher energy level. This process can repeat, with multiple energy transfers leading to the emission of a higher-energy photon [45].

3.3. Cooperative Energy Transfer (CET)

CET involves the simultaneous absorption of photons by multiple ions, which cooperatively transfer their energy to a single ion, resulting in upconversion. This process is less common than ESA and ETU, and as a three-body process, it takes place with a much lower probability but can be significant in systems with appropriate ion pairs or clusters [43].

3.4. Photon Avalanche (PA)

PA is a highly nonlinear process characterized by a rapid increase in the population of excited states once a critical threshold of excitation is reached [45,47]. This mechanism involves a feedback loop where initial excitation leads to increased absorption and further excitation. PA is usually observed under intense excitation conditions and can result in dramatic upconversion efficiency [48].

3.5. Energy-Migration-Mediated Upconversion (EMU)

In this process, energy is transferred from one sensitizer ion to another through non-radiative energy migration until it reaches an activator ion, which then emits upconverted photons. EMU represents a novel pathway that can enhance upconversion efficiency in certain materials [49].

4. Upconversion in Single-RE³⁺-Doped KY₃F₁₀

In single-doped systems, the UC process relies solely on the intrinsic properties of the dopant ion without the influence of other RE³⁺ ions. While typically less efficient than co-doped systems, single-doped UC materials are crucial for refining theoretical models and for investigating direct excitation pathways and energy migration processes that can contribute to designing new materials with optimized luminescent properties [50]. In light of the current literature, the following subsections summarize the different studies comprising single-doped KY₃F₁₀ with trivalent erbium, holmium, neodymium, samarium, and thulium as RE³⁺ ions.

4.1. KY₃F₁₀:Er³⁺

Existing macroscopic and microscopic models frequently encounter challenges when analyzing systems with finite energy migration or time-dependent energy transfer (ET) rates [51]. Therefore, theoretical UC models are essential for advancing our understanding of luminescence dynamics and in-depth studies of Er³⁺ energy levels are necessary to overcome these challenges.

In that sense, Solanki et al. [52] conducted a detailed study on the upconversion luminescence of KY₃F₁₀ nanoparticles (NPs) doped with 2 mol% Er³⁺. The NPs were synthesized via a hydrothermal method (200 °C, 6 h) using KF as a fluoride source. Transmission electron microscope (TEM) images revealed the presence of NPs with an average size of 68 ± 16 nm.

The research involved high-resolution absorption and laser-excited fluorescence spectroscopy, identifying 49 crystal-field energy levels of the Er³⁺ ion within the C_4v symmetry of the α-KY₃F₁₀ matrix (see Table 1), extending up to the ²P_3/2 multiplet at 32,000 cm⁻¹. A single-electron crystal-field calculation provided a close match to the experimental data, with parameters consistent with bulk KY₃F₁₀:Er³⁺ crystals.

The wavefunctions derived from these calculations were then used to simulate the absorption spectra, which demonstrated good agreement with experimental observations. This meticulous modeling effort confirmed that excited-state absorption (ESA) is the predominant upconversion mechanism in KY₃F₁₀:Er³⁺ NPs. This finding was further supported by the simulated spectra and was consistent with previously published upconversion excitation spectra for KY₃F₁₀:Er³⁺/Yb³⁺ NPs [53].

4.2. KY₃F₁₀:Ho³⁺

Mujaji and Wells [54] studied KY₃F₁₀ crystals doped with 0.1 mol% Ho³⁺ ions grown using the Bridgman–Stockbarger technique in a two-zone resistance heating furnace. Their work involved examining absorption, fluorescence, and excitation spectra at temperatures ranging from 8 to 77 K utilizing laser selective excitation and infrared absorption techniques, which allowed them to determine 51 crystal-field energy levels for seven Ho³⁺ multiplets (⁵I₈, ⁵I₇, ⁵I₆, ⁵F₅, ⁵S₂, ⁵F₄, ⁵F₃). They found that these levels closely matched those predicted by their crystal-field fitting routine.

According to the authors, KY₃F₁₀:Ho³⁺ is particularly intriguing due to its high symmetry environment and the presence of a nearly degenerate singlet pair in its ⁵I₈ ground state, akin to the behavior observed in another common fluoride-based host lattice, as it is CaF₂:Ho³⁺ [55]. The results underscored the need for precise wavefunctions for accurate hyperfine structure studies.

4.3. KY₃F₁₀:Nd³⁺

Near-infrared (NIR) light is usually categorized into two regions based on wavelength: the NIR-I region (or biological window I, 650–950 nm) and the NIR-II region (or biological window II, 1000–1450 nm) [56]. The NIR-I region has minimal absorption by biological tissues due to its strong penetration, making it ideal for use in imaging applications [57,58]. Recent research has demonstrated that light in the NIR-II region penetrates tissues more effectively than NIR-I, leading to improved biological detection and imaging [59,60,61,62]. Consequently, the research focus is increasingly shifting from NIR-I to NIR-II; see further details in Figure 4.

Unlike other systems that usually use 980 nm excitation and suffer from limitations like water absorption and overheating, using an 808 nm laser provides a more favorable option. Moreover, the Nd³⁺ ions are noted for their large absorption cross-section around 800 nm, which enhances the efficiency of the 808 nm laser diode.

On account of that, Lin et al. [63] explored the NIR luminescent properties of KY₃F₁₀ doped with 1 mol% Nd³⁺. The samples were prepared by a conventional solid-state reaction route using anhydrous KF, YF₃, and NdF₃ powders. The reagents were preheated at 250 °C under a nitrogen atmosphere for 2 h and finally sintered at 750 °C for 10 h. The study emphasized the potential of these phosphors for bioimaging applications due to their emissions falling within the dual biological windows. It was observed that under 808 nm excitation, KY₃F₁₀:Nd³⁺ exhibits simultaneous upconversion and down-shifting (DS) emissions, dominated at 737 nm and 1055 nm, respectively. The energy transfer mechanisms involve cross-relaxation processes, with NIR-I-to-NIR-I UC and NIR-I-to-NIR-II DS emissions being controlled by the concentration of Nd³⁺ ions.

4.4. KY₃F₁₀:Sm³⁺

Wells et al. investigated KY₃F₁₀ crystals doped with 0.1 mol% Sm³⁺ ions using laser-selective excitation and fluorescence methods. The crystals were grown using the Bridgman–Stockbarger technique in a two-zone resistance heating furnace and the authors identified 46 energy levels of the Sm³⁺ center, which aligned well with the parameters observed for other rare-earth ions in KY₃F₁₀.

Notably, the researchers also identified three distinct Sm²⁺ centers within the material. Two of these centers exhibited efficient UC fluorescence, facilitated by the presence of high-lying 4f⁵5d states and a metastable intermediate state that enabled the absorption of a second photon. This observation was particularly surprising as, based on ionic size considerations, samarium should typically substitute for Y³⁺ ions, and therefore, the trivalent oxidation state would only be expected. The authors suggested that this uncommon case could be explained through the existence of fluoride (F^–) vacancies, which would act as charge compensators for the divalent samarium ions.

4.5. KY₃F₁₀:Tm³⁺

Regarding the system KY₃F₁₀:Tm³⁺, the published works put their focus on the development of novel mid-infrared (mid-IR) lasers. Morova et al. [64] made significant advances in upconversion laser technology by investigating KY₃F₁₀:Tm³⁺ crystals doped with 8 at. % Tm³⁺ grown by the Czochralski method using an induction heating furnace and prepared from 5N purity raw materials. Their research highlighted the use of a 1064 nm ytterbium (Yb) fiber laser to efficiently pump these crystals, achieving lasing in both the 1.9 µm and 2.3 µm wavelength regions. Initially, they demonstrated that this setup effectively excites the ³H₄−³H₅ transition, resulting in continuous-wave (CW) lasing at 2344 nm. The system produced up to 124 mW of output power from 604 mW of absorbed pump power, with tunability across a broad range from 2268 to 2373 nm. The lasing efficiency reached an impressive 32% value, with a nearly complete conversion of absorbed 1064 nm photons into 2.3 µm laser photons.

Building on this success, Morova et al. [65] further explored the ability of the 1064 nm Yb fiber laser to pump the ³F₄−³H₆ and ³H₄−³H₅ transitions, aiming to achieve lasing near 1.9 µm. They successfully generated up to 142 mW of output power at 1943 nm with an incident pump power of 1.9 W. The laser demonstrated tunability from 1849 to 1994 nm and achieved a slope efficiency of 29%. This research showed that 1 µm pumping offers a viable alternative to traditional 800 nm pumping methods, expanding the utility of Tm³⁺ lasers in the mid-IR range. To explore lasing at 1.9 µm and 2.3 µm, corresponding to the ³F₄−³H₆ and ³H₄−³H₅ transitions, respectively, two distinct sets of cavity optics were employed, as showcased in Figure 5.

Finally, it must be noted that Guillemot et al. [66] significantly advanced the understanding of KY₃F₁₀:Tm³⁺ crystals (5 at. % Tm³⁺, grown in a Czochralski furnace) by demonstrating their high efficiency (≈50%) and power scalability for laser operation around 2.3 µm, leveraging the ³H₄−³H₅ transition. The efficient energy transfer upconversion (ETU) at moderate doping levels enhances the pump quantum efficiency and the intense energy absorption (ESA) aids in effective upconversion pumping.

5. Upconversion in Double-RE³⁺-Doped KY₃F₁₀

The double-RE³⁺-doped UC strategy is one of the most widely used and studied approaches in the field of upconversion luminescent materials. This method is highly regarded for its ability to significantly enhance upconversion efficiency through optimized energy transfer processes between the sensitizer and activator ions.

The sensitizer absorbs low-energy photons, usually in the NIR region, and transfers the energy non-radiatively to the activator ion, which subsequently emits photons at higher energies in the visible or ultraviolet regions. The double-doping strategy mitigates the limitations of single-doped systems, such as low-absorption cross-sections and inefficient energy transfer, by optimizing the overlap of energy levels and promoting multiphoton absorption processes [67,68]. This results in significantly improved UCL intensity and spectral tunability.

The UC efficiency in co-doped systems can be enhanced by one to several orders of magnitude compared to single-doped materials. However, it is important to recall that both single-doped and co-doped systems are affected by factors such as the doping concentration, host crystal structure, nanocrystal size, and core-shell configurations [67]. While co-doped systems typically exhibit higher efficiency, there can be instances where single-doped materials may achieve superior performance because of the specific material properties [69].

Considering the existing literature, the following subsections summarize the different studies comprising double-RE³⁺-doped KY₃F₁₀. In all the studies, the Yb³⁺ ion acted as a sensitizer while the activators were Er³⁺, Ho³⁺, Pr³⁺, Tb³⁺, or Tm³⁺ ions.

5.1. KY₃F₁₀:Er³⁺/Yb³⁺

Using the co-dopant system with Er³⁺/Yb³⁺ has been the most common strategy used for UCL in KY₃F₁₀. In this system, the Yb³⁺ ion acts as the most widely used sensitizer for 980 nm light due to its energy levels. The energy separation of the ²F_7/2 ground state and ²F_5/2 excited state of Yb³⁺ matches well with the transition energies between the ⁴I_11/2 and ⁴I_15/2 states, as well as the ⁴F_7/2 and ⁴I_11/2 states of Er³⁺, enabling efficient quasi-resonant energy transfer [70]. Typically, Yb³⁺ is co-doped in high concentrations (≈20 mol%) to optimize this energy transfer while Er³⁺ is introduced in lower concentrations [71].

Table 2 summarizes the published works related to KY₃F₁₀:Er³⁺/Yb³⁺ compounds. To better understand the various studies, a summary is provided that includes the relative molar concentration of activator-sensitizer pairs; the synthesis method, particle size, excitation wavelength, and temperature measurement conditions; and the main applications of the work or compounds.

As an example, Xue et al. [72] successfully synthesized Er³⁺/Yb³⁺ co-doped KY₃F₁₀ nanocrystals using a simple hydrothermal method with an average size of about 60 nm. These nanocrystals, when excited by a 976 nm laser, displayed strong visibility—see Figure 6a—and NIR emission centered at 1539 nm. Figure 6b depicts a scheme of the different energy levels to appreciate, at first glance, the energy transfers occurring between the sensitizers and activators ions, as well as the most probable emissions that take place.

The researchers optimized Er³⁺ concentrations to enhance the quantum yield (QY), finding that the maximum QY of 14.4% occurred with 0.5 mol% Er³⁺ and 5 mol% Yb³⁺. While increasing Er³⁺ concentration initially improved the QY, excessive Er³⁺ led to concentration quenching, reducing the QY, as shown in Figure 6c. Notably, they also embedded these nanocrystals in a polymer matrix to create laser-written waveguides, which demonstrated amplified spontaneous emission at 1539 nm when pumped at 976 nm.

Table 2. Details of the different studies comprising KY₃F₁₀:Er³⁺/Yb³⁺ materials. The activator (Er³⁺)-sensitizer (Yb³⁺) proportions are also expressed, as well as the synthetic methods employed and experimental conditions (EDTA: ethylenediaminetetraacetic acid; PVP: polyvinylpyrrolidone), the particle size distributions, the excitation wavelengths, the temperatures of the measurements (RT: room temperature), and the main applications of the materials discussed in the studies.

Er3+: Yb3+ (mol%)	Synthesis Method	Experimental Conditions	Particle Size (nm)	${\mathit{\lambda }}_{\mathbf{e}\mathbf{x}\mathbf{c}}$(nm)	T (K)	Application	Ref.
2:20	Thermal decomposition	300 °C, 1 h	15 ± 5	978	RT	Fundamental investigation	[73]
(0.2–5):5	Hydrothermal	180 °C, 12 h	60	976	RT	Self-written waveguides	[72]
0.5:20	Coprecipitation + thermal annealing	140–550 °C, 6 h Ar	8 (as grown), 85 (annealed)	972	RT	Er3+ distribution and thermal influence	[74]
0.2:1	Melt quenching (glass ceramics)	(1550 °C, 20 min air) + (400 °C, 3 h) + (polished coupons: 750 °C, 5 h)	60	980	RT	Transparent nano-glass ceramics	[75]
5:20	Hydrothermal	180 °C, 12 h, use of EDTA	38 ± 7	980	RT	Theranostic	[76]
2:2	Hydrothermal	200 °C, 6 h, use of PVP	58 ± 17	940–980	4.2, 10	Magnetic-field influence	[77]
2:20	Hydrothermal	200 °C, 3 h, use of PVP	57 ± 10	975	10, (297–366)	Nanothermometry	[53]
(0.5–1.5):10	Hydrothermal + thermal annealing	(180 °C, 12 h) + (500 °C, 12/5 h)	50–100 (5 h), 200–500 (12 h)	980	RT	Color tunability	[78]

Table 2. Details of the different studies comprising KY₃F₁₀:Er³⁺/Yb³⁺ materials. The activator (Er³⁺)-sensitizer (Yb³⁺) proportions are also expressed, as well as the synthetic methods employed and experimental conditions (EDTA: ethylenediaminetetraacetic acid; PVP: polyvinylpyrrolidone), the particle size distributions, the excitation wavelengths, the temperatures of the measurements (RT: room temperature), and the main applications of the materials discussed in the studies.

Er3+: Yb3+ (mol%)	Synthesis Method	Experimental Conditions	Particle Size (nm)	${\mathit{\lambda }}_{\mathbf{e}\mathbf{x}\mathbf{c}}$(nm)	T (K)	Application	Ref.
2:20	Thermal decomposition	300 °C, 1 h	15 ± 5	978	RT	Fundamental investigation	[73]
(0.2–5):5	Hydrothermal	180 °C, 12 h	60	976	RT	Self-written waveguides	[72]
0.5:20	Coprecipitation + thermal annealing	140–550 °C, 6 h Ar	8 (as grown), 85 (annealed)	972	RT	Er3+ distribution and thermal influence	[74]
0.2:1	Melt quenching (glass ceramics)	(1550 °C, 20 min air) + (400 °C, 3 h) + (polished coupons: 750 °C, 5 h)	60	980	RT	Transparent nano-glass ceramics	[75]
5:20	Hydrothermal	180 °C, 12 h, use of EDTA	38 ± 7	980	RT	Theranostic	[76]
2:2	Hydrothermal	200 °C, 6 h, use of PVP	58 ± 17	940–980	4.2, 10	Magnetic-field influence	[77]
2:20	Hydrothermal	200 °C, 3 h, use of PVP	57 ± 10	975	10, (297–366)	Nanothermometry	[53]
(0.5–1.5):10	Hydrothermal + thermal annealing	(180 °C, 12 h) + (500 °C, 12/5 h)	50–100 (5 h), 200–500 (12 h)	980	RT	Color tunability	[78]

Going ahead with the influence of the particle size on the optical properties, the subsequent thermal treatments of KY₃F₁₀:Er³⁺/Yb³⁺ NPs prepared by Gomes et al. were shown to significantly impact their UCL efficiency [74]. The as-synthesized nanocrystals at room temperature exhibited remarkably low luminescence efficiency, particularly in the green emission from the Er^{3+ 4}S_3/2 state, with an efficiency as low as 1.6%. However, this efficiency drastically improved, reaching 99% relative to the bulk crystal when the nanopowder was subjected to heat treatment at 550 °C. This trend was similarly observed in the red emission associated with the Er^{3+ 4}F_9/2 state. The study identified that the non-uniform distribution of Er³⁺ and Yb³⁺ ions in the nanocrystals, particularly a concentration gradient towards the NP surface, played a significant role in reducing the luminescence efficiency. Thermal treatment was found to mitigate this issue by enabling the dopant ions to migrate within the lattice, leading to a more uniform distribution. Consequently, the luminescence efficiency improved significantly.

In another interesting work, Martin et al. [77] studied, both experimentally and theoretically, the influence of applying a magnetic field on the UCL response. Historically, it has been established that magnetic fields can significantly affect energy transfer efficiencies and the nature of the energy transfer processes in bulk lanthanide-doped crystals by tuning resonances between electronic and phonon states [79,80]. Given that, the authors extended this understanding to nanoparticle systems by reporting Zeeman infrared absorption spectra for KY₃F₁₀ NPs doped with 2 mol% Er³⁺ and 2 mol% Yb³⁺. They employed a geometric model to interpret the magnetic field splittings of these ions, using wavefunctions derived from conventional crystal-field calculations.

Figure 7 illustrates the measured Zeeman infrared absorption spectra for the Er³⁺/Yb³⁺ co-doped sample. The visible spectral region, Figure 7a, is crucial because applying a magnetic field can enhance the spectral overlap between the energy donor and acceptor ions, thereby influencing the rate of inter-ionic energy transfer. Despite the randomness in nanoparticle orientation, distinct splittings are clearly shown, highlighting the impact of the magnetic field. Additionally, Figure 7b presents the Zeeman infrared absorption spectrum for the 1.5 μm Er³⁺ transition, which is significant for applications in telecommunications and biosensing, also being able to appreciate the splitting.

As a seminal study, Solanki et al. [53] underscored the optimal adequacy of KY₃F₁₀ as a host lattice for luminescence thermometry. In the study, KY₃F₁₀:Er³⁺/Yb³⁺ core-only and (KY₃F₁₀:Er³⁺/Yb³⁺)@KY₃F₁₀ core-shell NPs were synthesized with approximate particle sizes of 57 and 70 nm, respectively. The KY₃F₁₀:Er³⁺/Yb³⁺ NPs were produced by a hydrothermal method (3 h at 200 °C) using PVP (polyvinylpyrrolidone) as the surfactant. For the shell synthesis, the as-obtained core KY₃F₁₀:Er³⁺/Yb³⁺ NPs were dispersed and the same synthesis protocol was used, except for the addition of lanthanide ions, i.e., a subsequent hydrothermal treatment was implemented into the synthetic route.

UC emission spectra of the NPs were recorded by observing the typical Er^{3+ 2}H_11/2→⁴I_15/2 (517−527 nm), ⁴S_3/2→⁴I_15/2 (535−560 nm, green), and ⁴F_9/2→⁴I_15/2 (640−680 nm, red) bands. The resonant excitation (with optical transitions of the Yb³⁺ ion) at 975 nm increased the Er³⁺ fluorescence intensity five-fold compared to 980 nm; see Figure 8a. On the other hand, Figure 8b showcases that core-shell NPs exhibit a two-fold increase in the overall fluorescence intensity in comparison with the core-only NPs (see, for instance, the highlighted numbers of the vertical axis). This effect can be linked to the mitigation of surface quenching and cross-relaxation processes. Therefore, the core-shell synthesis strategy has a profound impact on the resulting optical response.

Exceptional relative temperature sensitivities, S_r, were achieved with maximum S_r values of (1.510 ± 0.015) % K⁻¹ at 300 K and (1.245 ± 0.013) % K⁻¹ at 316 K for the core-only and core−shell NPs, respectively. Moreover, the minimum temperature uncertainties, δT, were also extremely low: 0.112 and 0.118 K, respectively, thus highlighting the potential of these NPs as primary thermometers.

5.2. KY₃F₁₀:Ho³⁺/Yb³⁺

Following the exploration of luminescence thermometry in the previous subsection, the study by Pang et al. [81] delved into the practical applications of UCL in temperature sensing and photo-thermal therapy employing KY₃F₁₀ NPs co-doped with 15 mol% Yb³⁺ and 1 mol% Ho³⁺ ions. The authors synthesized the NPs using a common hydrothermal method (200 °C, 24 h) that yielded particles of an average size of about 38 nm.

The underlying luminescence mechanisms were elucidated through steady-state rate equations and an analysis of the power dependence of the UCL when pumping with a 980 nm laser. It was determined that the green emission results from a two-step energy transfer process from Yb³⁺ to Ho³⁺ while the red emission arises from the multi-phonon relaxation process of the ⁵S₂/⁵F₄ states to the ⁵F₅ state and the promotion of Ho³⁺ from the ⁵I₇ state to the ⁵F₅ state.

Additionally, the study highlighted the significant thermal effects associated with prolonged laser irradiation, as evidenced by the temperature-sensing behavior of the UCL. This capability to convert light to heat effectively positions these materials as promising candidates for photo-thermal therapy applications, combining efficient luminescence, thermal management, and precise temperature sensing in a single system.

5.3. KY₃F₁₀:Pr³⁺/Yb³⁺

More from a fundamental point of view, Kim et al.’s work [82] provided significant insights into the use of IR laser diodes for UC processes in KY₃F₁₀:Pr³⁺/Yb³⁺ systems, highlighting the potential for efficient laser emission and paving the way for further advancements in the development of UC materials for laser applications. For that purpose, single crystals were grown using the micro-pulling-down method, fixing the dopant concentration to 0.5 and 1 mol% of Pr³⁺, and using several percentages of Yb³⁺: 5, 8, and 10 mol%. The resulting crystals were cut and polished (diameter = 2.5 mm, thickness = 1.36 mm) for optical characterization.

Detailed luminescence measurements were conducted, including the investigation of the decay kinetics of the Pr³⁺ visible emissions using time-resolved spectra at room temperature. Specifically, under IR pumping (975 nm) to the ²F_5/2 excited state of Yb³⁺, the decay time, $\tau$, of Pr³⁺ was calculated to be approximately $\tau$ = 30.2 μs, which was notably three times longer than the decay time obtained under the pulsed-laser selective excitation of Pr³⁺ ions at 446 nm (³P₀) and $\tau$ = 9.3 μs. This considerable difference in decay times indicates a robust interaction and energy transfer between the Yb³⁺ and Pr³⁺ ions, enhancing the UC efficiency.

5.4. KY₃F₁₀:Tb³⁺/Yb³⁺

Xue et al. [83] investigated the UCL properties of Tb³⁺/Yb³⁺ co-doped KY₃F₁₀ NPs, identifying them as promising hosts for UC emissions and laser applications. The NPs, with an average size of about 45 nm, were obtained using a common hydrothermal method (180 °C, 12 h). Unlike in previous studies with NaYF₄, which had achieved only strong green emissions from the ⁵D₄ level of Tb³⁺ [84,85], Xue et al. succeeded in obtaining intense ultraviolet (UV) emissions from the ⁵D₃ level [83].

The emission spectrum of these NPs revealed radiative transitions corresponding to the (⁵D₃,⁵G₆)→⁷F_J (J = 4–6) and ⁵D₄→⁷F_J (J = 3–6) levels of Tb³⁺ ions. Notably, the UV emission at 381 nm was observed to be significantly stronger than the green emission at 544 nm, a novel finding in Tb³⁺/Yb³⁺ co-doped NPs. The relative intensity of the UV UC emission was approximately 100 times higher than in previously reported results.

The study also explored possible UC mechanisms. It was proposed that two adjacent Yb³⁺ ions absorb 976 nm photons and transfer the excited energy to one Tb³⁺ ion through cooperative sensitization. The UCL also integrated ETU and ESA mechanisms. Additionally, the cross-relaxation process involving ⁵D₃ (Tb) + ⁷F₆ (Tb) → ⁵D₄ (Tb) + ⁷F₀ (Tb) was examined in samples with varying Tb³⁺ concentrations. These findings were especially significant for controlling multi-wavelength emissions of Tb³⁺ ions extending from the UV to the visible region.

5.5. KY₃F₁₀:Tm³⁺/Yb³⁺

Braud et al. [86] quantitatively analyzed the energy transfer processes in a range of fluoride crystals (KY₃F₁₀, LiYF₄, and BaY₂F₈) doped with trivalent thulium and ytterbium ions. The authors selected these fluorides due to their status as single-site host materials for RE³⁺ ions substituting Y³⁺ (C_4v symmetry in KY₃F₁₀, S₄ in LiYF₄, and C₂ in BaY₂F₈), which guaranteed homogeneously broadened absorption and emission spectra—a critical factor for exploring multipolar interactions between ions. Crystals of these compounds were produced using the Czochralski technique in a custom-made pulling apparatus specifically engineered for fluoride materials. The crystal growth process involved the meticulous control of the fluorinating atmosphere, a mixture of Ar and CF₄, to prevent contamination of the melt with oxygen or hydroxide (T ≈ 1000 °C).

The authors experimentally measured energy transfer parameters and compared them with theoretical models, finding good agreement. They calculated laser thresholds for key Tm³⁺ ion transitions and validated these calculations through laser experiments. This allowed them to determine optimal Yb³⁺ and Tm³⁺ concentrations to minimize pump power requirements for laser transitions around 1.5 and 2.3 µm. Among the three compounds, KY₃F₁₀ presented the highest efficiency of Yb³⁺→Tm³⁺ energy transfers, thus underscoring its great potential [86].

In another interesting work, Rapaport et al. [87] analyzed the UCL properties of a (0.4 mol% Tm³⁺):(20 mol% Yb³⁺) co-doped KY₃F₁₀ single crystal, grown using a hydrofluorination process to purify the materials, along with additional proprietary techniques developed by AC Materials. YLiF₄, which is considered one of the most efficient host lattices for blue emission, requires being pumped near 960 nm, which can be difficult to assess by some lasers. In contrast, the authors reported that KY₃F₁₀ offers a more accessible exciting wavelength of 974.5 nm, addressing previous challenges associated with less available diode laser sources and solidifying its role as a superior blue UC emitter in advanced photonic display applications.

More recently, Runowski [88] delved into the investigation of multifunctional luminescent–plasmonic (KY₃F₁₀:Tm³⁺/Yb³⁺)@SiO₂-NH₂@Au NPs of the core-shell type. For this purpose, KY₃F₁₀:Tm³⁺/Yb³⁺ NPs were first synthesized using a hydrothermal method (20 h at 180 °C). EDTA (ethylenediaminetetraacetic acid) was employed as both a surfactant and chelating agent in a basic medium (pH ≈ 10), with KBF₄ serving as the fluoride source. For the shell synthesis, a modified Stöber procedure was employed. The as-obtained core KY₃F₁₀:Er³⁺/Yb³⁺ colloidal NPs were added to a solution containing water, ethanol, and ammonia solution. Then, TEOS (tetraethylorthosilicate) and APTES (3-aminopropyltriethoxysilane) were dropwise added to form the SiO₂-NH₂ shell. Finally, the colloidal core-shell NPs were mixed with Au NPs previously prepared by a common citrate-reducing approach.

The key innovation of this study lay in the interaction between the plasmonic gold NPs (4–7 nm) and the luminescent core (20–40 nm), which significantly affected the upconversion emission of the NPs (average size of 100–200 nm after the shell coating). The presence of gold NPs introduced strong plasmonic absorption in the visible range (450–650 nm), which interacted with the Tm³⁺ ion emissions, particularly affecting the visible range emission bands (¹G₄→³H₆ and ¹G₄→³F₄) while leaving the near-infrared band (³H₄→³H₆) relatively unchanged. The ratio and relative intensity of these visible range emission bands varied with the number of gold NPs on the surface, leading to notable changes in the emission spectra’s shapes and colors. Specifically, the emission color shifted from bright blue to blue–violet as the gold NPs concentration increased.

6. Upconversion in Triple-RE³⁺-Doped KY₃F₁₀

Doping a material simultaneously with three different RE³⁺ ions can significantly enhance UCL compared to double-doped systems. One of the primary advantages of tri-doping is the enhanced energy transfer efficiency. In double-doped systems, energy transfer typically occurs between two ions, limiting the pathways for non-radiative relaxation and potentially leading to lower upconversion efficiencies. In contrast, tri-doped systems introduce an additional dopant ion, providing more pathways for energy transfer and reducing the likelihood of non-radiative losses [89].

In addition, the emission wavelengths can be precisely controlled by adjusting the relative concentrations and types of dopant ions. For example, in a system containing Er³⁺, Yb³⁺, and Tm³⁺, the combination of these ions can produce emissions in the blue, green, and red regions of the spectrum, depending on the excitation wavelength and dopant ratios [90,91]. This tunability is more challenging to achieve with double-doped systems, which are limited to fewer emission pathways.

In contrast with other materials, this strategy has not been explored extensively using KY₃F₁₀ as a host lattice. The following two subsections discuss the limited number of studies on triple-RE³⁺-doped KY₃F₁₀. In all the works, the Yb³⁺ ion acted as a sensitizer while the activators were Er³⁺-Tm³⁺ or Nd³⁺-Tm³⁺ ions.

6.1. KY₃F₁₀:Er³⁺/Yb³⁺/Tm³⁺

In the study by Pang et al. [92], the team addressed a significant challenge in UCL by achieving stable and controllable white emission. Traditional approaches had struggled with the sensitivity of blue emissions from Tm³⁺ compared to the green and red emissions from Er³⁺, resulting in a narrow power range for effective white light emission. To overcome this, the authors designed and synthesized KY₃F₁₀ NPs co-doped with (0.2 mol% Er³⁺):(30 mol% Yb³⁺):(0.1–0.3 mol% Tm³⁺) using a hydrothermal method (220 °C for 12 h). The TEM analysis revealed that the particles exhibited a quasi-spherical morphology with an average size of 60 nm.

Upon excitation at 980 nm, the NPs produced a bright white emission with high brightness and a favorable color balance composed of blue, green, and red components. Remarkably, this white emission demonstrated low sensitivity to variations in pumping power, with the color coordinates shifting only slightly from (0.339, 0.356) to (0.306, 0.363) as the power increased from 146.7 to 742 mW. Note that the chromaticity coordinates of equal-energy white light (light having an equal mixture of all wavelengths) were (0.333, 0.333).

The paper also delved into the mechanism behind the UC white emission in these tri-doped KY₃F₁₀ NPs. The energy level diagram presented in Figure 9 illustrates the possible UC emission and excitation pathways for Tm³⁺, Yb³⁺, and Er³⁺ ions [93]. In this tri-doped system, energy transfers from Yb³⁺ to Tm³⁺_, and Er³⁺ ions predominantly populate the excited states of Tm³⁺ and Er³⁺. However, the authors also concluded that a notable cross-relaxation process occurred between the ions, specifically ¹G₄ (Tm³⁺) + ⁴I_11/2 (Er³⁺) → ³H₄ (Tm³⁺) + ⁴S_3/2 (Er³⁺), which played a crucial role in the emission process. This stability in color balance under varying excitation conditions marked a significant advancement, highlighting the potential of these nanocrystals for applications requiring consistent white light emission.

6.2. KY₃F₁₀:Nd³⁺/Yb³⁺/Tm³⁺

Regarding the tri-doped Nd³⁺/Yb³⁺/Tm³⁺ system, da Silva et al. [94] investigated, remarkably, from a physical point of view, the mechanisms behind blue and ultraviolet UC emissions in RE³⁺-doped KY₃F₁₀ single crystals. For that purpose, they grew several crystals based on the following dopant concentrations: (i) (1 mol% Nd³⁺):(5–30 mol% Yb³⁺):(0.5 mol% Tm³⁺), (ii) (5–20 mol% Yb³⁺):(0.5 mol% Tm³⁺), and (iii) (1 mol% Nd³⁺):(0.5 mol% Tm³⁺). The crystals were obtained by mixing the fluoride reagents (KF, REF₃) in an open cylindrical platinum boat under an HF atmosphere. The mixture was melted at approximately 990 °C and then cooled at a rate of about 10 °C/h.

The authors employed time-resolved luminescence spectroscopy to measure decay times and identify the key processes populating the ¹G₄ and ¹D₂ excited states of Tm³⁺. Their findings revealed that energy transfer from Nd³⁺ to Yb³⁺ significantly enhances blue UC efficiency in the tri-doped Nd³⁺/Yb³⁺/Tm³⁺ system compared to the single Yb³⁺-doped one. This efficiency is due to a sequence of energy transfers initiated by 797 nm excitation of Nd³⁺ and Tm³⁺, with a fast Nd³⁺→Yb³⁺ transfer rate leading to effective Tm³⁺ excitation.

The study showed that the optimal Yb³⁺ concentration for achieving blue laser emission at 483.1 nm under continuous wave pumping at 797 nm lies between 40 and 50 mol%. Numerical simulations indicated a population inversion threshold at a pumping rate of 98 s⁻¹, corresponding to an intensity of 3.3 kW cm⁻². However, this population inversion was not observed for 960 nm pumping. Additionally, the research highlighted the significance of Nd³⁺ doping (1 mol%) for achieving a gain in blue emission as simulations showed a negative gain without Nd³⁺.

Continuing the investigation of the UC dynamics yielding to blue emission, Gomes et al. [95] presented an in-depth study of the spectroscopic properties in this tri-doped system. They synthesized KY₃F₁₀ NPs co-doped with (1.3 mol% Nd³⁺):(10 mol% Yb³⁺):(0.5 mol% Tm³⁺) by a coprecipitation method and also submitted them to different thermal treatments from 150 °C to 550 °C under an argon atmosphere.

Using time-resolved luminescence spectroscopy, the decay times were extracted and it was possible to identify the different mechanisms contributing to the population of the ¹G₄ excited state of Tm³⁺. When tri-doped samples are excited within the 797–802 nm range, the following competing processes (U₁ and U₂) are observed for yielding blue emission:

U₁ (Nd³⁺→Tm³⁺):

Nd³⁺ (⁴F_3/2) + Tm³⁺ (³H₄) → Nd³⁺ (⁴I_11/2) + Tm³⁺ (¹G₄)

(1)

U₂ (Nd³⁺→Yb³⁺→Tm³⁺):

Nd³⁺ (⁴F_3/2) + Yb³⁺ (²F_7/2) → Nd³⁺ (⁴I_11/2) + Yb³⁺ (²F_5/2)

(2)

Yb³⁺ (²F_5/2) + Tm³⁺ (³H₄) → Yb³⁺ (²F_7/2) + Tm³⁺ (¹G₄)

(3)

Figure 10 presents a schematic energy level diagram for Nd³⁺/Yb³⁺/Tm³⁺, illustrating the proposed UC mechanism leading to the Tm³⁺ (¹G₄) state, from which the deexcitation yields the blue emission.

They also analyzed the UCL dependence on the thermal treatment and crystallite size. Their findings showed that the luminescence efficiency of the ¹G₄ state in Tm³⁺ increased from 0.38% in as-grown nanocrystals (12 nm) to 97% in post-thermal treated crystals (550 °C, 198 nm). Moreover, the contributions of UC processes were also altered. The contribution of the direct U₁ process changed from 78% to 33% while the contribution of the sequential U₂ processes evolved from 22% to 67%.

7. Conclusions and Prominent Prospects

KY₃F₁₀-based upconversion materials have shown significant advancements, particularly in their cubic α-phase, which is ideal for efficient upconversion due to its stability and structure. Different doping strategies with rare earth ions have greatly enhanced upconversion efficiency and allowed for tunable emission spectra. Controlled synthesis and thermal treatments have also been demonstrated to be crucial for optimizing luminescent performance.

The review has also highlighted that the synthesis methods primarily yield nanoparticles. As discussed, thermal treatments have been applied in some cases to improve crystallinity, control the particle size, and enhance the optical response as a result. These processes collectively contribute to achieving the desired crystallite sizes, which are crucial for optimizing luminescent performance and studying dopant distribution within the host lattice.

Recent insights into the local structures and phase transitions of KY₃F₁₀ nanocrystals further emphasize their essential role in tailoring luminescent properties. These advancements highlight the continuous evolution in the design and application of upconversion materials, fostering innovative applications in fields such as bioimaging, photovoltaics, and security printing.

Furthermore, while the α-phase remains the most extensively studied, the metastable δ-phase presents promising opportunities for future research due to its unique structural and luminescent properties. Investigating this phase could lead to breakthroughs in achieving higher upconversion efficiencies and novel applications. Future studies should aim at exploring uncharted crystal structures and innovative doping strategies to further elevate the capabilities of upconversion luminescence.

Then, one might reasonably ask, “After all this time, can we still find new opportunities in upconversion materials?” The answer is, undoubtedly, “Always”. A fantastic quantum world awaits, ready to be excited and to yield unprecedented results, poised to shift our current understanding of photoluminescence.