Room-temperature and one-minute synthesis of alcohol-soluble and ultra-small Eu-doped CaMoO₄ nanoparticles

Abstract

Lanthanide (Ln³⁺) doped alkaline earth metal molybdates (AMoO₄, A = Ca, Sr, Mg) have been widely investigated owing to their excellent photoluminescence properties. Alcohol-soluble and ultra-small Eu³⁺-doped CaMoO₄ nanocrystals are prepared at room temperature within one minute using a propionic acid/propylamine-assisted method. Short-chain propionic acid and propylamine are employed as the capping agents, which is crucial for alcohol-soluble ability, controlling particle size and passivating surface defects. X-ray diffraction and transmission electron microscopy characterization revealed that the Eu³⁺-doped CaMoO₄ nanoparticles have a pure tetragonal scheelite structure and an average particle size of 3.4 nm. Under UV light or high-energy X-ray excitation, Eu³⁺-doped CaMoO₄ nanocrystals exhibit the intense characteristic emission of Eu³⁺ ions, which show a great potential application in high-quality displays, bio-imaging, and radiation detection.

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Introduction

Rare earth ion doped alkaline earth metal molybdates (AMoO₄, A = Ca, Sr, Mg) have attracted extensive research interest due to their outstanding photoluminescence properties, rendering them promising candidates for a variety of applications such as high-quality display devices,^1,2 light-emitting diodes (LEDs),^3–6 scintillators,⁷ and solar cells.^8,9 Among trivalent lanthanide ions (Ln³⁺), the narrow emission bands originating from 4f–4f transitions make them ideal activators in luminescent materials.¹⁰ Calcium molybdate (CaMoO₄), as an important member of the metal molybdate family, exhibits excellent physicochemical stability and serves as an efficient host lattice for luminescence.¹¹ Notably, CaMoO₄ itself shows intrinsic luminescence, where energy transfer from the host to dopant ions can yield blue to green emission under UV excitation.^11,12

Conventional synthesis of Ln³⁺-doped CaMoO₄ often relies on high-temperature solid-state reactions,^13,14 which not only consume significant energy but also introduce undesirable oxygen vacancies, thereby adversely affecting the luminescence performance. In recent years, several wet-chemical approaches, such as co-precipitation,^15,16 sol–gel processing,^17,18 and hydrothermal methods,^19–21 have been developed to prepare Ln³⁺-doped CaMoO₄ nanocrystals. However, the luminescence efficiency of such nanocrystals remains considerably lower than that of their bulk counterparts sintered at high temperature,^13,14 highlighting the need for a synthetic strategy that combines high crystallinity, strong luminescence, and avoidance of high-temperature thermal treatment.

Our group has recently developed a room-temperature and short-chain ligand-assisted co-precipitation synthetic method for producing highly crystalline and strongly luminescent Ln³⁺-doped nanocrystals, including Eu³⁺-doped YVO₄,²² Eu³⁺-doped CaMoO₄,²³ Ce/Tb-codoped NaYF₄,²⁴ and Yb³⁺/Nd³⁺-doped CaMoO₄ nanocrystals.²⁵ These nanomaterials could be readily dispersed in chloroform for further solution processing. Nevertheless, the use of chloroform poses health and environmental concerns due to its toxicity. To address this issue, we report here a propionic acid/propylamine-assisted approach that enables rapid and room-temperature synthesis of alcohol-soluble Eu³⁺-doped CaMoO₄ nanocrystals. The short-chain propionic acid and propylamine act as effective capping ligands, conferring ethanol solubility, controlling particle size and passivating surface defects. As a result, we have successfully obtained highly luminescent, alcohol-soluble Eu³⁺-doped CaMoO₄ nanocrystals with a maximum photoluminescence quantum yield (PLQY) of 46.12%, demonstrating their potential for practical applications in eco-friendly optoelectronic devices.

Experimental section

Chemicals

Ammonium molybdate ((NH₄)₆Mo₇O₂₄, 99%) was purchased from Beijing Chemical Works. Calcium nitrate (Ca(NO₃)₂·4H₂O, A.R.), europium nitrate (Eu(NO₃)₃·6H₂O, 99.99%), propionic acid (CH₃CH₂COOH, 99%), propylamine (CH₃(CH₂)₂NH₂, 99%), ethanol (CH₃CH₂OH, A.R.), and methanol (CH₃OH) were purchased from Aladdin Inc. All the chemicals are original without any purification.

Synthesis of Eu³⁺-doped CaMoO₄ nanocrystals

First, 1.0 mL of propionic acid, 1.0 mL of propylamine, 1.0 mmol of Ca(NO₃)₂·4H₂O and Eu(NO₃)₃·6H₂O (x = Eu/(Eu + Ca), 0 ≤ x ≤ 0.25), and 10.0 mL ethanol were mixed in a glass beaker (referred to as Solution A). Ammonium molybdate (1.0 mmol of MoO₄²⁻) was dissolved in a mixed solution containing 4.0 mL of methanol and 1.0 mL of propylamine under sonication, forming Solution B. Solution A and Solution B were mixed and reacted under magnetic stirring at room temperature for 1 min to form a Ca_1−xMoO₄:Eu_x³⁺ nanoparticle solution. An appropriate amount of acetonitrile was added to the precipitate and a white solid was obtained after centrifugation. Finally, Ca_1−xMoO₄:Eu_x³⁺ nanocrystals were dispersed in ethanol after washing with acetonitrile.

Characterization

The Ca_1−xMoO₄:xEu nanocrystals were characterized by XRD (Bruker D8 Discover) in the 2θ range from 10° to 70° with a scanning rate of 0.02° s⁻¹. Electron microscopy images were obtained using a transmission electron microscope (FEI TECNAIG2 F30). The chemical composition was determined using a Hitachi S-3400N scanning electron microscope equipped with an energy dispersive X-ray spectroscopy system. The chemical valence states were determined using X-ray photoelectron spectroscopy (XPS, Thermo Fisher ESCALAB 250Xi). Excitation and photoluminescence (PL) spectra, PL quantum yields (PLQYs), and PL lifetimes were measured using a Horiba Fluoromax-4 spectrometer and an Edinburgh FLS-1000 fluorescence spectrometer. Radioluminescence (RL) spectra were recorded using the Edinburgh FLS-1000 fluorescence spectrometer equipped with an X-ray tube (Tungsten target, Moxtek). The commercial LuAG:Ce crystal was employed to compare the light yields of the samples. The UV-vis diffuse reflectance spectrum was recorded using a UV-vis-near infrared spectrophotometer (Shimadzu, UV-3600 Plus). The Fourier transform infrared (FT-IR) spectrum was acquired using a Bruker Vertex 70 FT-IR spectrophotometer. The I–V–L curve of light emitting diodes was recorded using a Minolta LS-150 luminance meter and Keithley 2400.

Results and discussion

Recently, hydrophobic Eu³⁺-doped CaMoO₄ nanocrystals were synthesized by a ligand-assisted co-precipitation method.²³ In this work, the alcohol-soluble Eu³⁺ doped CaMoO₄ nanocrystals were rapidly synthesized at room temperature within one minute, employing short-chain propionic acid and propylamine as capping ligands. It has been reported that the conversion rate from Mo₇O₂₄⁶⁻ to MoO₄²⁻ is pH-dependent and accelerates under high pH conditions.²⁶ The addition of propylamine facilitates this transformation. The resulting MoO₄²⁻ precursor solution was then rapidly introduced into the Ca²⁺ solution, thereby inducing the instant nucleation and crystallization of CaMoO₄ nanoparticles. The entire reaction process can be completed within 5 seconds (see Fig. S1, SI). The acid–base reaction between propionic acid and propylamine generates acid–base ion pairs (CH₃CH₂COO⁻H₃N⁺CH₂CH₂CH₃), which function as capping ligands. This ligand system not only enables the nanocrystals to be highly dispersed in ethanol but also effectively controls particle size and passivates surface defects.²⁴ Purification was achieved by adding acetonitrile as an anti-solvent, followed by centrifugation. The successful coordination of propionic acid and propylamine on the surface of the Eu³⁺ doped CaMoO₄ nanocrystals was confirmed by FT-IR spectroscopy (Fig. 1). The characteristic absorption peaks observed for the C=O stretch from the carboxylate group and the N–H stretches from the amine group provide direct evidence of the presence of both capping agents.

Fig. 1 FT-IR spectrum of propionic acid/propylamine-capped Ca_0.85MoO₄:Eu_0.15³⁺ nanocrystals.

The crystal structure of CaMoO₄, illustrated in Fig. 2a, features a scheelite-type tetragonal framework composed of MoO₄ tetrahedra and CaO₈ polyhedral units. Each MoO₄ tetrahedron shares its corners with four adjacent CaO₈ polyhedra via oxygen bridges.²⁷Fig. 2b displays a series of X-ray diffraction (XRD) patterns of pristine CaMoO₄ and Ca_1−xMoO₄:Eu_x³⁺ nanocrystals (x = 0.01–0.4). All diffraction peaks can be accurately indexed to the pure tetragonal scheelite structure of CaMoO₄ (JCPDS No. 07-0212), confirming the phase purity of the as-synthesized samples. No impurity-related peaks are detected. We found a systematic shift of the diffraction peaks toward lower 2θ angles with increasing Eu³⁺ concentration, as notably exemplified by the peak around 28.8°. This shift indicates an expansion of the unit cell volume – a phenomenon that appears counterintuitive given the smaller ionic radius of Eu³⁺ ions (1.066 Å) compared to that of Ca²⁺ ions (1.12 Å). However, such anomalous peak shifting has been documented in previous studies.^28–30 The substitution of Ca²⁺ by Eu³⁺ introduces a difference in electronegativity (1.132 for Ca²⁺vs. 1.377 for Eu³⁺).³¹ The higher electronegativity of Eu³⁺ draws oxygen ions away from the central Mo⁶⁺ cation, leading to an expansion of the MoO₄ tetrahedra. This local structural distortion subsequently propagates, resulting in an overall increase in the unit cell volume.²⁸

Fig. 2 (a) Polyhedral representation of a CaMoO₄ unit cell. (b) Powder X-ray diffraction (XRD) of Ca_1−xMoO₄:Eu_x³⁺ (x = 0.01, 0.05, 0.1, 0.15, 0.2, 0.25, and 0.4) nanocrystals with different Eu doping concentrations.

The morphology and microstructure of the as-synthesized Eu³⁺-doped CaMoO₄ nanocrystals were examined by transmission electron microscopy (TEM). As shown in Fig. 3a, the nanocrystals are uniformly distributed and well-dispersed. A corresponding high-resolution TEM (HR-TTEM) image (Fig. 3b) reveals clear lattice fringes with an interplanar spacing of 0.201 nm, which matches the (204) crystallographic planes of the tetragonal CaMoO₄ structure. Furthermore, statistical analysis of the particle size distribution (Fig. 3c) indicates an average nanocrystal diameter of 3.4 nm and a narrow size distribution.

Fig. 3 (a) TEM and (b) HR-TEM images of Ca_0.85MoO₄:Eu_0.15³⁺ nanocrystals and (c) the size distribution of Ca_0.85MoO₄:Eu_0.15³⁺ nanocrystals.

Since Eu³⁺-doped CaMoO₄ nanocrystals are capped by propionic acid and propylamine, they are highly soluble in ethanol, forming a transparent and homogeneous colloidal solution. Fig. 4a and b display the excitation and emission spectra of Ca_0.85MoO₄:Eu_0.15³⁺ nanocrystal powder and nanocrystal solution prepared at room temperature. In the excitation spectrum of the Ca_0.85MoO₄:Eu_0.15³⁺ nanocrystal powder, a broad band ranging from 240 to 350 nm with a maximum at 280 nm is observed, which is ascribed to the charge transfer (CT) transitions of O²⁻ → Mo⁶⁺ and O²⁻ → Eu³⁺ groups.¹² Several weaker and sharper peaks are also present at 362 nm (⁷F₀ → ⁵D₄), 382 nm (⁷F₀ → ⁵L₇), 395 nm (⁷F₀ → ⁵L₆), 416 nm (⁷F₀ → ⁵D₃), and 465 nm (⁷F₀ → ⁵D₂), corresponding to the characteristic 4f–4f transitions of Eu³⁺ ions.^31,32 Compared to the powder sample, the excitation spectrum of the solution sample exhibits a slight red shift in the CT band (260–350 nm, centered at 300 nm), along with a series of weaker 4f–4f transition peaks in the 350–500 nm region. This behavior can be attributed to the dispersed state of the nanocrystals in ethanol, as opposed to the aggregated powder form.²³ The ultrasmall size of the nanocrystals also results in reduced light scattering in solution. Furthermore, the isolation of individual nanocrystals in the solution leads to a lower density of luminescent centers per unit volume and a decrease in energy transfer efficiency. As shown in Fig. 4a and b, the characteristic emission lines of Eu³⁺ corresponding to the ⁵D₀ → ⁷F_J (J = 1–4) transitions are clearly observed. The most intense peak at 615 nm is assigned to the ⁵D₀ → ⁷F₂ electric dipole transition, which is highly sensitive to the local crystal field environment around the Eu³⁺ ions. Other emission peaks at 592 nm (⁵D₀ → ⁷F₁), 652 nm (⁵D₀ → ⁷F₃), and 700 nm (⁵D₀ → ⁷F₄) are attributed to magnetic dipole transitions. The dominance of the ⁵D₀ → ⁷F₂ transition over the ⁵D₀ → ⁷F₁ transition in both powder and solution samples indicates that the Eu³⁺ ions occupy low-symmetry sites lacking an inversion center.³³ This can be quantitatively confirmed by the asymmetry ratio, R = I(⁵D₀ → ⁷F₂)/I(⁵D₀ → ⁷F₁), which is widely used to probe the local environment of Eu³⁺ ions.^34,35 The calculated R values of 16.7 for the powder and 18.2 for the solution support the incorporation of Eu³⁺ ions into low-symmetry sites, further demonstrating that Ca²⁺ ions on the surface and inner region of CaMoO₄ nanocrystals can be replaced by Eu³⁺ ions. Fig. 4c and d compare the photoluminescence intensities of 1.0 mol% Eu³⁺-doped CaMoO₄ nanocrystals and undoped CaMoO₄ nanocrystals under 280 nm excitation. The intrinsic emission from the MoO₄²⁻ groups is significantly weaker than the Eu³⁺-related emission, indicating the efficient energy transfer from the MoO₄²⁻ host to the Eu³⁺ ions, even at low doping levels (1.0 mol%).³⁶ This efficient energy transfer also confirms the successful incorporation of Eu³⁺ into the CaMoO₄ lattice, in agreement with the XRD analysis.

Fig. 4 Excitation and emission spectra of (a) the Ca_0.85MoO₄:Eu_0.15³⁺ nanocrystal powder and (b) Ca_0.85MoO₄:Eu_0.15³⁺ solution in ethanol. PL spectra of (c) the Ca_0.99MoO₄:Eu_0.01³⁺ nanocrystal powder and (d) pristine CaMoO₄ nanocrystal powder.

In addition, the UV-vis diffuse reflectance spectrum of Ca_0.85MoO₄:Eu_0.15³⁺ nanocrystals is shown in Fig. 5a, revealing strong optical absorption in the 200–350 nm range. The Tauc plot method was used to estimate the optical band gap energy (E_g) from the absorption data,³⁷ yielding a band gap value of 3.79 eV (Fig. 5b). The obtained E_g is approximately 0.38 eV larger than previously reported theoretical values,³⁸ which can be reasonably attributed to the quantum size effect in the ultrasmall nanocrystals.³⁹ To investigate the influence of Eu³⁺ doping concentration on the PL intensity, a series of Ca_1−xMoO₄:Eu_x³⁺ nanocrystals with varying x values were synthesized. The photoluminescence quantum yield (PLQY) of each sample was measured under 280 nm excitation, as summarized in Fig. 5c. It was found that the PLQY initially increases with rising Eu³⁺ concentration, reaching a maximum value of 46.12% at x = 0.15 (15 mol%). Beyond this optimal doping level, the PLQY gradually decreases, though it remains above 40% even at x = 0.25. This phenomenon results from the concentration quenching effect.^40,41 To eliminate measurement uncertainty, three different batch tests were conducted on the PLQYs of Eu³⁺-doped CaMoO₄ nanocrystals (Table S1, SI). At lower concentrations, increasing dopant density enhances the energy transfer efficiency from the CaMoO₄ host to Eu³⁺ ions. However, beyond a critical threshold, the average distance between Eu³⁺ ions becomes sufficiently small to promote non-radiative energy migration among them, leading to a reduction in overall emission efficiency.¹² Additionally, the presence of propionic acid and propylamine capping ligands may help mitigate interparticle energy transfer by isolating individual nanocrystals, thereby partially alleviating the concentration quenching effect.²³ The comparisons between alcohol-soluble Eu³⁺-doped CaMoO₄ nanocrystals and our previously reported hydrophobic nanocrystals are summarized in Table S2 (SI). It is evident that the capping agents have a vastly important influence on PLQY and solvent compatibility. Besides, the comparisons of PLQY and synthesis time with those of previously reported Eu-doped CaMoO₄ phosphors prepared with different synthetic methods are presented in Table S3 (SI).

Fig. 5 (a) UV-vis diffuse reflectance spectrum and (b) the optical band gap of Ca_0.85MoO₄:Eu_0.15³⁺ nanocrystals. (c) PLQYs and PL lifetimes of Ca_1−xMoO₄:Eu_x³⁺ nanocrystals.

The luminescence decay dynamics of Ca_1−xMoO₄:Eu_x³⁺ nanocrystals were investigated by monitoring the ⁵D₀ → ⁷F₂ transition at 615 nm under 280 nm excitation (Fig. 6). Consistent with previous reports,^11,12 all decay curves exhibit biexponential behavior, which were well fitted using the function:

where I(t) and I₀ represent the luminescence intensities at time t and 0, τ₁ and τ₂ are the decay time constants, and A₁ and A₂ denote their respective amplitudes. In luminescent materials, the decay kinetics are strongly influenced by the nature of luminescent centers, energy transfer processes, and defect-related interactions.^42,43 A biexponential decay profile often arises when Eu³⁺ ions occupy multiple distinct crystallographic sites within the host lattice, forming different luminescent centers. In the present case, Eu³⁺ ions are expected to substitute for Ca²⁺ ions in low-symmetry sites, which could support more than one type of local coordination environment. Additionally, the room-temperature synthesis used here is known to yield nanocrystals with relatively low oxygen vacancy concentrations,²³ thereby reducing the defect-related nonradiative decay. Therefore, the observed biexponential decay is likely attributable to energy transfer from the CaMoO₄ host to the Eu³⁺ activators and/or the presence of multiple luminescent centers.^36,43

Fig. 6 PL decay curves of Eu³⁺-doped CaMoO₄ nanocrystals with different Eu³⁺ doping concentrations.

The average lifetime (τ_av) was calculated using the expression:

As summarized in Fig. 5c, the average lifetime of Ca_1−xMoO₄:Eu_x³⁺ nanocrystals gradually decreases with increasing Eu³⁺ doping concentration. Specifically, τ_av declines from 0.796 ms to 0.652 ms as the doping level rises from 1 mol% to 25 mol%. This reduction in lifetime is a clear signature of concentration quenching, wherein higher dopant densities promote nonradiative energy migration among neighboring Eu³⁺ ions,⁴⁴ thereby shortening the observed luminescence decay time.

To evaluate the practical application potential of the luminescent nanocrystals, the thermal stability of CaMoO₄:Eu³⁺ was systematically investigated. The temperature-dependent PL spectra of Ca_0.85MoO₄:Eu_0.15³⁺ nanocrystals were measured over a temperature range of 20–200 °C. As shown in Fig. 7a, the position and shape of the emission profiles remain largely unchanged with increasing temperature. However, the PL intensity exhibits a significant decrease, demonstrating typical thermal quenching behavior.^45,46 This sensitive temperature-dependent luminescence suggests that the Ca_0.85MoO₄:Eu_0.15³⁺ nanocrystals hold promise as an optical temperature-sensing material. The chemical compositions and actual Eu³⁺ doping concentration in Ca_1−xMoO₄:Eu_x³⁺ nanocrystals were determined by energy-dispersive X-ray spectroscopy (EDS). From Fig. 7b, it can be seen that actual Eu³⁺ doping concentrations are close to the feeding concentration of Eu³⁺ ions.

Fig. 7 (a) Temperature-dependent PL spectra of Ca_0.85MoO₄:Eu_0.15³⁺ nanocrystals. (b) Plot of the actual doping ratio and feeding concentration of Eu³⁺ ions in Ca_1−xMoO₄:Eu_x³⁺ nanocrystals detected by EDS.

The oxidation states of constituent elements in the 10 mol% Eu³⁺-doped CaMoO₄ nanocrystals were analyzed by X-ray photoelectron spectroscopy (XPS), with the C 1s peak at 284.8 eV used as a reference. As shown in Fig. 8a, the Ca 2p spectrum exhibits two peaks at 347.1 eV (2p_3/2) and 350.8 eV (2p_1/2), consistent with the +2 oxidation state of calcium. The Mo 3d core-level spectrum (Fig. 8b) displays doublets at 232.7 eV (3d_5/2) and 235.7 eV (3d_3/2), confirming the +6 oxidation state of molybdenum. Fig. 8c shows two XPS peaks of Eu 3d at 1134.3 eV for 3d_5/2 and 1164.8 eV for 3d_3/2, indicating the presence of Eu³⁺ ions. The O 1s spectrum (Fig. 8d) is asymmetric and can be deconvoluted into two components: the lower binding energy peak at 530.5 eV is assigned to lattice oxygen in CaMoO₄, while the higher binding energy peak at 532.4 eV is attributed to surface-adsorbed organic ligands.^12,47

Fig. 8 XPS spectra of Ca_0.9MoO₄:Eu_0.1³⁺ nanocrystals; (a) Ca 2p; (b) Mo 3d; (c) Eu 3d; and (d) O 1s.

Additionally, the Ca_1−xMoO₄:Eu_x³⁺ nanocrystals can also be excited by high-energy X-rays to produce luminescence. The proposed scintillation mechanism in Eu³⁺-doped CaMoO₄ nanocrystals is illustrated in Fig. 9a.^48–50 Upon X-ray irradiation, high-energy photons interact with the nanocrystals via the photoelectric effect and Compton scattering, generating hot electrons and deep holes. These primary carriers subsequently produce a large number of secondary electrons through Auger processes and electron–electron scattering, resulting in numerous electron–hole pairs with lower kinetic energy. These charge carriers then migrate through the lattice and eventually transfer their energy to Eu³⁺ ions, exciting them and leading to characteristic visible emission. Fig. 9b shows the radioluminescence (RL) spectra of undoped and Eu³⁺ doped CaMoO₄ nanocrystals under X-ray excitation. The doped samples show the characteristic emission of Eu³⁺ ions and the peaks are labelled in the emission spectra. The ⁵D₀ → ⁷F_J (J = 1–4) transitions of Eu³⁺ ions provide sharp peaks at 592, 615, 654, and 702 nm. The RL intensity initially increases with Eu³⁺ concentration and reaches a maximum at 15 mol%, beyond which concentration quenching likely occurs. Therefore, the 15 mol% doped sample (Ca_0.85MoO₄:Eu_0.15³⁺) was selected for further investigation. As shown in Fig. 9c, the RL intensity increases steadily with rising X-ray dose, and notably, the material shows detectable sensitivity even at a low dose rate of 0.38 μGy s⁻¹. Furthermore, a comparison of the RL intensity between the Ca_0.85MoO₄:Eu_0.15³⁺ nanocrystals and a commercial LuAG:Ce scintillator is provided in Fig. 9d, demonstrating the promising potential of the developed Eu³⁺-doped CaMoO₄ nanocrystals for scintillation applications. The RL spectra of both our sample and the reference LuAG:Ce scintillator were recorded separately but sequentially under an identical instrumental setup and X-ray excitation parameters. The integrated area in each respective RL spectrum was then calculated. The comparison of these integrated intensities forms the basis for assessing their relative RL efficiency. This method offers a straightforward and experimentally robust means for a head-to-head comparison. It is widely used in the field to quickly rank the performance of new scintillator materials against established benchmarks under equivalent conditions. However, this comparative approach has some limitations. First, the result indicates which material emits more light under the specific test conditions used but does not yield an absolute light yield value. Second, minor intensity fluctuations in the X-ray beam between the two sequential measurements could introduce a small degree of uncertainty into the direct intensity comparison.

Fig. 9 (a) Radioluminescence (RL) mechanism of Eu³⁺-doped CaMoO₄ nanocrystals. (b) RL spectra of the Eu³⁺-doped CaMoO₄ nanocrystals with different Eu³⁺ doping concentrations. (c) RL spectra of the Ca_0.85MoO₄:Eu_0.15³⁺ nanocrystals under different X-ray doses. (d) Comparison of the RL intensity between Ca_0.85MoO₄:Eu_0.15³⁺ nanocrystals and commercial LuAG:Ce scintillator crystals.

This study focuses on the potential application of alcohol-soluble Eu³⁺-doped CaMoO₄ luminescent nanocrystals, which form a transparent colloidal solution in ethanol. Using the propionic acid/propylamine-assisted method, Ca_0.85MoO₄:Eu_0.15³⁺ nanocrystals were dispersed in ethanol with a concentration of 10 mg mL⁻¹. The transparent nanocrystal solution can remain stable in air for at least 24 hours (Fig. S2, SI). The uniform luminescent thin film can be deposited on a glass substrate via spin-coating (inset of Fig. 10a). Furthermore, a deep-UV LED chip was coated with the nanocrystal film; when excited by 280 nm UV light, the LED can emit bright red light, as shown in the inset of Fig. 10b. Fig. 10a displays the current–voltage–luminance (I–V–L) characteristics of the resulting red-emitting LED. The device achieves a maximum luminance of 14 503 cd m⁻². In addition, Fig. 10b presents the current efficiency and power efficiency as functions of voltage. The LED exhibits a maximum current efficiency of 1.25 cd A⁻¹ and a maximum power efficiency of 0.65 lm W⁻¹. These results demonstrate that the highly luminescent and alcohol-soluble Eu³⁺-doped CaMoO₄ nanocrystals are promising candidates for use in high-quality display technologies.

Fig. 10 (a) Current–voltage–luminance (I–V–L) curves of a red-emitting LED; inset: the picture of Ca_0.85MoO₄:Eu_0.15³⁺ nanocrystal solution and thin film. (b) Current efficiency–voltage–power efficiency curves of the red-emitting LED, inset: the photograph of the red-emitting LED.

Conclusions

In conclusion, we report a room-temperature synthetic route utilizing propionic acid/propylamine as co-ligands to rapidly produce alcohol-soluble Eu³⁺-doped CaMoO₄ nanocrystals. This approach strikes a balance between speed, operational simplicity, and eco-friendliness. Notably, the lattice expansion observed at higher Eu³⁺ doping levels is explained by a local structural distortion mechanism driven by the higher electronegativity of Eu³⁺. The obtained nanocrystals are ultrasmall (∼3.4 nm) and highly efficient (PLQY = 46.12%), and exhibit scintillation properties comparable to those of LuAG:Ce. These findings reveal that our Eu³⁺-doped CaMoO₄ nanocrystals are highly promising for advancing applications in high-quality displays and radiation detection.

Conflicts of interest

The authors declare no competing financial interest.

Data availability

Data provided in the article and the supplementary information (SI) are available on request from the corresponding author.

Supplementary information (SI): photographs of nanocrystal crude solutions at diffrent reaction times; detailed PLQY values; photographs of nanocrystal solution after different storage times. See DOI: https://doi.org/10.1039/d5dt03028e.

Acknowledgements

This work was supported by Guangxi Provincial Science and Technology Development Program (No. 2023GXNSFDA026056), Guangxi Science and Technology Major Project (No. ZY24212006 and No. AA23073018), and the Special Fund for Science and Technology Development of Guangxi (Grant No. AD25069078).

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