Shape-controlled synthesis and self-assembly of highly uniform upconverting calcium fluoride nanocrystals

Abstract

Herein, the size- and shape-controlled synthesis of nearly-monodisperse, lanthanide-doped calcium fluoride (CaF₂) nanocrystals (NCs) is reported. The sizes and shapes of CaF₂ NCs are controlled by tailoring the reaction conditions, such as the concentration of lithium fluoride precursors, reaction time and temperature, and the procedure for adding the calcium trifluoroacetate precursors in the reaction mixture. Highly uniform CaF₂ NCs are synthesized with several morphologies, such as nanospheres, truncated octahedra, nanoplates, and nanowires. The shape-controlled CaF₂ NCs self-assemble into NC superlattices with long-range orientational and positional order forming crystalline and liquid crystalline structures. The near-infrared-to-visible upconversion luminescence properties are investigated by varying the types of dopants as well as the sizes and shapes of the CaF₂ NCs.

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Introduction

Lanthanide-based luminescent nanocrystals (NCs) offer immense potential in many emerging applications, such as biomedical imaging,^1,2 optical sensors,^3,4 spectral converters for photovoltaics^5,6 and agriculture,⁷ and displays.⁸ Lanthanides are f-block elements with partially-filled and well shielded f-orbitals. Their ions are responsible for the characteristic atomic optical absorption and emission spectra, the long, several microsecond excited-state lifetimes, and the magnetic properties of the NCs. Optically-active, lanthanide-doped NCs show unique optical properties, such as upconversion luminescence (UC),⁹ quantum cutting,¹⁰ and downshifting¹¹ are observed as energy is transferred between excited-state atomic levels in the dopants.¹² The composition and crystal structure of the host material plays a critical role in influencing the NC luminescence properties.^13,14 Utilization of lanthanide materials in large-scale industrial applications requires the development of host materials using inexpensive and environmentally friendly elements, while maintaining their desired physical and chemical properties.

To date, various synthetic procedures for the preparation of lanthanide-based NCs have been reported, which include hydro- and solvo-thermal syntheses,^15,16 as well as co-precipitation¹⁷ and thermal decomposition methods.^18,19 Among these, thermal decomposition has been widely investigated to afford highly uniform NCs, which has also been utilized for the synthesis of semiconducting quantum dots²⁰ and metal oxide NCs.^21,22 The thermolysis of organometallic precursors at high temperature enables the formation of uniform NCs with controlled crystal structures. The sizes, shapes, and compositions of the NCs can be easily tailored by controlling the reaction parameters, such as time, temperature, precursor ratio, and temperature ramping time. In lanthanide-doped NCs, the size^23,24 and shape²⁵ of hosts, distance between dopants,^26,27 and the spatial position of dopants²⁸ can be tailored during the synthesis, allowing their UC properties to be precisely controlled. Therefore, the development of a synthetic method to precisely control the sizes and morphologies of lanthanide-doped NCs is critical to optimizing the UC properties.

Several types of host materials, such as sodium lanthanide fluorides,^14,29–31 lanthanide oxides,^32–35 lanthanide fluorides,^18,36 lanthanide oxyfluorides,^37,38 and alkali earth fluorides,^39,40 have been reported for lanthanide-doped NCs. Among these, CaF₂ is a promising candidate host material, as the rare earth elements common in other host compositions are substituted by inexpensive and abundant calcium. CaF₂ is an inert and non-hygroscopic material showing high chemical and thermal resistance. In addition, CaF₂ shows broadband transmittance in the ultraviolet, visible, near-infrared (NIR), and even mid-IR regions. CaF₂ is a biocompatible material exhibiting low toxicity, which allows its application in biomedicine.⁴¹ Lanthanide-doped CaF₂ is used as a scintillating material for the detection of gamma rays and charged particles,^42,43 allowing the utilization of rare-earth-doped NCs as spectral converters for high-energy radiation in bio-imaging applications⁴⁴ and radiation detection.^45,46 Although many literature reports have described the synthesis of uniform CaF₂ NCs,^16,47,48 the development of a synthetic procedure to precisely control the sizes and shapes of highly uniform lanthanide-doped CaF₂ NCs in a reproducible manner is still challenging.

Herein, a size- and shape-controlled synthesis of upconverting CaF₂ NCs is reported. Nearly-monodisperse CaF₂ NCs are synthesized via the high-temperature thermal decomposition of calcium trifluoroacetate (CaTFA₂) precursors in a mixture of lithium fluoride (LiF), oleic acid (OA), and 1-octadecene (ODE). The addition of LiF in the reaction mixture and the control of the relative ratio of OA to ODE affords CaF₂ NCs with tunable morphologies including nanospheres (NSs), truncated octahedra (NOs), nanoplates (NPs), and nanowires (NWs). Highly uniform CaF₂ NCs self-assemble into close-packed superlattices over large areas via liquid–air interfacial assembly. CaF₂ NWs self-assemble into liquid-crystalline superlattices with long-range orientational and positional order. The optical properties of upconverting CaF₂ NCs are investigated. First, the NIR-to-visible two-photon upconversion luminescence is studied using Er³⁺/Yb³⁺, Tm³⁺/Yb³⁺, and Tm³⁺/Er³⁺/Yb³⁺-doped CaF₂ NCs, which exhibit characteristic blue, green, red, and NIR emission peaks upon excitation at 980 nm depending on the types of dopants. Next, the UC properties of the Er³⁺/Yb³⁺-doped CaF₂ NCs with varying sizes and shapes are investigated using highly uniform CaF₂ NCs.

Results and discussion

The CaF₂ NCs were synthesized by adding CaTFA₂ and LiF to a 50 : 50 (v/v) OA/ODE mixture. The CaTFA₂ precursors were synthesized by dissolving CaCO₃ in trifluoroacetic acid at room temperature, followed by solvent evaporation. The reaction solution was preheated at 125 °C under vacuum for 1 h to remove water and volatile solvents. The reaction mixture was then heated to 300 °C and maintained at this reaction temperature for 30 min. During the synthesis, LiF does not completely dissolve in the solvent mixtures and non-polar solvent, whereas CaTFA₂ and the CaF₂ NC_S show complete dissolution. The residual LiF was thus removed by dissolving the CaF₂ NCs in a non-polar solvent, such as hexane, toluene, or chloroform, followed by centrifugation.

Fig. 1a and b respectively show representative low- and high-magnification transmission electron microscopy (TEM) images of Er³⁺,Yb³⁺-co-doped CaF₂ NCs synthesized at 300 °C; a 1 : 3 molar ratio of CaTFA₂ to LiF was used in this reaction. The low-magnification TEM image indicates the high uniformity in size and shape of the CaF₂ NCs achieved by this method. The high-magnification TEM image shows the NCs have pseudo-spherical morphologies. Fig. 1c–f show high-resolution TEM (HRTEM) images of single CaF₂ NCs. The single-crystal structures are observed from the fringe interference patterns of the isolated NCs. The HRTEM images show CaF₂ NCs oriented along the [110] and [112] zone axes, respectively, which are further validated by the fast Fourier transforms (FFTs) of the HRTEM data (Fig. 1d and f) and simulated electron diffraction patterns (Fig. S1†). The average size and size distribution of the NCs are obtained from the small-angle X-ray scattering (SAXS) data (Fig. 1g). The simulated small-angle X-ray patterns are well-matched to the experimental results with an input parameter of 9.2 ± 0.8 nm diameter, which represents the high size uniformity of the CaF₂ NCs. The powder X-ray diffraction data reveals the face-centered cubic crystal structures of the CaF₂ NCs (JCPDS No. 87-0971, space group: Fm3m).

Fig. 1 (a) Low-magnification and (b) high-magnification transmission electron microscopy (TEM) images of upconverting CaF₂:2%Er³⁺,20%Yb³⁺ NCs. High-resolution TEM (HRTEM) images of an isolated CaF₂ nanocrystal (NC) along the (c) [110] and (e) [112] projections, and (d and f) fast Fourier transform of the respective HRTEM images. (g) Small-angle X-ray scattering data with simulated pattern and (h) wide-angle X-ray diffraction spectrum of Er³⁺,Yb³⁺-co-doped CaF₂ NCs.

The sizes of the CaF₂ NCs are easily tuned by changing the reaction temperature. When the reaction is performed at a temperature of 320 °C, the average size of the CaF₂ NCs increases to 10.8 ± 0.8 nm in diameter (Fig. S2†). The sizes of the CaF₂ NCs are increased further by modifying the method for adding the metal precursors in the reaction solution. When the CaTFA₂ precursor in the OA and ODE solution is slowly injected into the solution mixtures containing LiF at 320 °C, truncated octahedral CaF₂ NCs are obtained as the final product (Fig. 2). For the reaction, two reaction vessels are prepared with LiF on one flask and CaTFA₂ precursors in OA and ODE (50 : 50 vol%) on the other side. Both reaction mixtures are degassed at 125 °C for one hour to remove water and volatile solvent. After degassing, the temperature of the reactor containing LiF in 40 mL of the OA/ODE mixture is increased to 320 °C, and the solution containing CaTFA₂ in 20 mL of the solvent mixture is slowly added at 1.33 mL min⁻¹ using a syringe pump. The solution with the CaTFA₂ precursor is pumped into the flask over 15 min, which is then held for additional 15 min for the further growth of NCs. Fig. 2 shows TEM images of the truncated CaF₂ octahedra at low and high magnification to show their uniformity and the top and side views of the truncated CaF₂ octahedral NCs (Fig. 2b and c). The tip-to-tip distance of the truncated CaF₂ octahedra is approximately 25.3 ± 2.6 nm, as calculated by statistical analysis from TEM images. During the slow injection of the metal precursors, few nuclei are formed in the reaction mixture during the early stage of injection, and the growth of the NCs occurs from these nuclei at a high temperature, which results in the formation of larger NCs.⁴⁹

Fig. 2 TEM images of CaF₂ NCs showing truncated octahedral morphologies. (a) Low-magnification, (b) top-view, and (c) side-view images of the truncated octahedra.

Synthesis of the NCs without the addition of LiF in the reaction mixture yields CaF₂ nanoplates with rectangular plate-like morphologies, as reported in prior literature.⁴⁰ The morphologies are further tuned by changing the relative ratio of OA to ODE added into the reaction mixture. When the relative amount of OA to ODE is decreased from 50 vol% to 1.6 vol%, CaF₂ grows in a uniaxial direction to form NWs with high aspect ratios. As shown in Fig. 3c, the widths of the NWs are ≤3 nm and the lengths are up to a micrometer. Some NWs are thin in the middle, while a plate-like shape is observed at the end of the NWs with approximately 10 nm widths (Fig. 3d and S3†). During the synthesis of the CaF₂ NWs, carried out at 280 °C, the viscosity of the reaction solution increases 15 min into the reaction, which may indicate the formation of the CaF₂ NWs.^50,51 When the reaction time or reaction temperature is increased, the NWs dissociate into smaller NWs and nanorods, eventually forming two-dimensional nanoplate morphologies (Fig. S4†). These results indicate that the kinetically stable NWs are formed at the beginning, which eventually transform into nanoplates to minimize the surface energy by decreasing the relative surface area. With the same solvent ratio of OA to ODE (1.6 vol% OA), when the reaction is performed with LiF salts, isotropic spherical NCs are obtained as the final product, indicating that LiF induces the isotropic growth of CaF₂ NCs.

Fig. 3 TEM images of (a and b) CaF₂ nanoplates and (c and d) CaF₂ nanowires.

The self-assembly of the colloidal NCs into ordered nanostructures allows the utilization of their unique size-, shape-, and orientation-dependent properties, which is of considerable interest for photonic,⁵² plasmonic,⁵³ electronic,⁵⁴ and electrochemical applications.⁵⁵ We used liquid interfacial assembly to organize superlattices of CaF₂ NCs with varying shapes. Liquid interfacial assembly is a facile, robust, and reproducible technique to fabricate uniform single and binary superlattices from isotropic and anisotropic NC building blocks.^56–58 First, we self-assembled the spherical CaF₂ NCs as shown in Fig. 1a and b. TEM images reveal that CaF₂ NC superlattices self-assembled over a large area (Fig. 4a). The sharp, hexagonal small-angle selected area electron diffraction (SAED) pattern reveals the long-range ordering of two-dimensional close-packed hexagonal arrays of CaF₂ NCs stacked into bilayer superlattices. The wide-angle SAED pattern of the self-assembled superlattice also shows relatively intense single-crystal-like diffraction spots with six-fold symmetry (Fig. 4b, inset). This is attributed to the preferential alignment and perpendicular orientation of the [111] axis of all NCs on the substrate, owing to the faceting of the CaF₂ NCs.

Fig. 4 Assemblies of CaF₂ NCs forming (a) bilayer and (b) trilayer superlattices. Inset images show the (a) small-angle and (b) wide-angle SAED patterns, exhibiting the CaF₂ lattice in the [111] plane.

The upconverting CaF₂ NWs, as exemplified for Er³⁺,Yb³⁺-co-doped CaF₂ NWs, self-assemble into superlattices with liquid crystalline order (Fig. 5). The domain sizes of the superlattices are up to a micrometer in scale with the preservation of long-range orientational order. The single-crystalline SAED patterns indicate that the NWs are oriented in the same crystallographic direction, not only along their long axes, but also their short axes. The SAED patterns show two sets of diffraction patterns. The NWs are projected from the [110] zone axis in the cubic crystal lattices, resulting in an angle of 54.7° between the diffraction spots of the (100) and (111) planes (Fig. S5†). Another set of diffraction data corresponding to the (110) plane with four-fold symmetry is also seen in the SAED pattern of the self-assembled structures, representing the diffraction spots projected along the [100] zone axis. This indicates that the cross-sections of the NWs are faceted with (100) and (110) planes, allowing the preferential orientation of NWs along either the (100) or (110) plane parallel to the substrate, as shown in Fig. S5.†

Fig. 5 (a and b) Low-magnification and (c) high-magnification TEM images of liquid-crystalline superlattices of Er³⁺,Yb³⁺-co-doped CaF₂ NWs. (d) The wide-angle electron diffraction data.

The UC properties of the lanthanide-doped CaF₂ NCs are investigated by incorporating optically active lanthanide ions in the CaF₂ NC host. First, the two-photon UC properties are characterized in Er³⁺/Yb³⁺, Tm³⁺/Yb³⁺, and Tm³⁺/Er³⁺/Yb³⁺-doped CaF₂ NCs. Fig. 6a and b show a photograph and the photoluminescence (PL) spectra for dispersions of the upconverting CaF₂ NCs illuminated with 980 nm radiation. Upon excitation at 980 nm, blue, orange, and red emissions are observed for CaF₂ doped with Tm³⁺/Yb³⁺, Er³⁺/Yb³⁺, and Tm³⁺/Er³⁺/Yb³⁺, respectively. The 0.2%Tm³⁺/20%Yb³⁺-doped CaF₂ NCs show characteristic blue and NIR peaks at 480 and 800 nm. The weak blue emission is attributed to the ¹D₂→³F₄ and ¹G₄→²H₆ transitions, and the NIR emission at 800 nm corresponds to the radiative transition from the ³H₄→³H₆ energy levels of the Tm³⁺ dopants. With the 2%Er³⁺/20%Yb³⁺ dopants, green (521 and 539 nm) and red (656 nm) peaks are observed, which are the characteristic emission lines of Er³⁺ dopants for ²H_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2 (green) and ⁴F_9/2→⁴I_15/2 (red) transitions. Triply-doped CaF₂ NCs with 0.5%Tm³⁺/2%Er³⁺/20%Yb³⁺ ions exhibit emission peaks at 480, 539, 656, and 800 nm, which are attributed to emission from both Tm³⁺ and Er³⁺. In addition, the relative intensity of NIR peaks (800 nm) to visible peaks (480, 539, 656 nm) increases with an increase in the relative ratio of Tm³⁺ in the compositions of the dopants (Fig. S6†), attributed to the relaxation from higher flevels in Er³⁺ to lower ³H₄-levels in Tm³⁺ with increasing concentration of Tm³⁺ ions in the host materials.

Fig. 6 (a) Photograph of upconverting CaF₂ doped with 0.2%Tm³⁺/20%Yb³⁺ (left), 2%Er³⁺/20%Yb³⁺ (middle), and 0.5%Tm³⁺/2%Er³⁺/20%Yb³⁺ (right) NCs under 980 nm irradiation. UC spectra of CaF₂ NCs (b) with varying dopant compositions and (c) different sizes and shapes.

In addition, the size and shape dependence of the UC properties is demonstrated using CaF₂ NSs, NOs, and NWs. Fig. S7† shows the TEM images of the CaF₂ NCs used for the UC measurements. The sizes of CaF₂ NSs and NOs are measured to be 12.1 ± 1.1 nm and 17.3 ± 1.2 nm, respectively, exhibiting high size uniformity. All NCs are doped with 2%Er³⁺ and 20%Yb³⁺ ions as experimentally confirmed by energy dispersive X-ray spectroscopy in the scanning transmission electron microscope (Table S1†). The larger-sized CaF₂ NOs exhibit the highest UC intensity, whereas the CaF₂ NWs show the weakest UC intensities under identical measurement conditions (Fig. 6c). This difference might be attributed to the surface-to-volume ratio of CaF₂ NCs, where smaller-sized NCs exhibit a larger surface area, resulting in a higher population of surface ions. The surface lanthanide ions can potentially introduce defects, inducing nonradiative quenching, subsequently reducing the UC intensities.⁵⁹Fig. 6c describes the UC spectra of CaF₂ NSs, NOs, and NWs normalized to the red emission peaks at 656 nm. The green-to-red intensity ratio in the UC spectra is influenced by several factors, such as the particle size,²³ shape,⁶⁰ the excitation power density,⁶¹ and solvents.⁶² When other factors are kept constant, the green-to-red ratio decreases with decreasing NC size owing to the larger surface-to-volume ratio.⁶³ Non-radiative relaxation is mediated by nanocrystal phonons, the vibration of surface ligands, and surface defects.⁶⁴ Consequently, the large surface-to-volume ratio of smaller-sized CaF₂ NSs enhances multiphonon relaxation between f levels, favoring the population of the ⁴F_9/2 state over ²H_11/2 and ⁴S₃ states.⁵⁹ In contrast, the green-to-red intensity ratio of CaF₂ NWs is nearly identical to that of NSs. Shan et al. reported shape effects on the green-to-red intensity ratio of hexagonal-phase Er³⁺,Yb³⁺-doped β-phase (hexagonal) NaGdF₄ NCs, attributed to the crystal anisotropy between the a-axis and c-axis in β-phase NaGdF₄ structures, influencing the UC intensities and green-to-red intensity ratio. In contrast, the green-to-red intensity ratio of Er³⁺ in CaF₂ NCs was relatively insensitive to the shape compared to the β-phase NaGdF₄ attributed to the highly symmetric crystal structure of the CaF₂ hosts.

Conclusions

In summary, the shape-controlled synthesis and self-assembly of luminescent CaF₂ NCs are described. Highly uniform CaF₂ NCs are synthesized via high-temperature thermal decomposition. The shapes of the CaF₂ NCs are tunable by changing the reaction conditions, such as the addition of LiF salts, method of precursor injection, and relative ratio of OA to ODE. In the presence of LiF, highly uniform pseudo-spherical CaF₂ NCs are obtained, and their sizes can be further tuned by changing the reaction temperature. The CaF₂ NCs with truncated octahedral shapes are synthesized by slowly injecting the metal precursor solution at high temperature. In the absence of LiF, the shapes of CaF₂ NCs are tailored from square nanoplates to NWs, depending on the relative ratio of OA to ODE. CaF₂ NCs are self-assembled into ordered arrays via liquid interfacial self-assembly with a long-range order over a large area. The UC luminescence properties are investigated using Er³⁺/Yb³⁺, Tm³⁺/Yb³⁺, and Tm³⁺/Er³⁺/Yb³⁺-doped CaF₂ NCs as well as Er³⁺/Yb³⁺-doped CaF₂ NCs with different sizes and shapes. The uniformity of the sizes and shapes of CaF₂ NCs comprising the inexpensive and abundant calcium can allow the utilization of upconverting NCs for large-scale applications.

Author contributions

Taejong Paik: conceptualization, methodology, investigation, funding acquisition, supervision, writing – original draft, and writing – review & editing. Nicholas J. Greybush: methodology, investigation, data curation, and formal analysis. Ho Young Woo: investigation, data curation, and formal analysis. Seong Vin Hong: data curation and formal analysis. Seung Hyeon Kim: data curation and formal analysis. Jennifer D. Lee: data curation and formal analysis. Cherie R. Kagan: conceptualization, funding acquisition, supervision, and writing – review & editing. Christopher B. Murray: conceptualization, funding acquisition, supervision, and writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by the Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) grants funded by the Korean government (2018M3D1A1059001, 2021M3H2A1038042, 2021M3H4A3A01062960, 2021R1A2C1013604, and 2022R1A4A2000776). C. R. K. thanks for the support of Stephen J. Angello Professorship. C. B. M. is grateful for the support of Richard Perry University Professorship.

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Footnote

† Electronic supplementary information (ESI) available: Experimental section and supplementary figures. See DOI: https://doi.org/10.1039/d3qi01864d

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