Introduction

Organic ligands have played an irreplaceable role in myriads of nanomaterials synthesis towards control of size1,2, morphology3,4, crystal structure5, and functionality6,7. Meanwhile, rational selection of the chemical groups on ligand molecules is also critical for post-modulations such as assembly, hybridization, and fabrication of micro or macroscaled architectures from individual nanoparticles (NPs) aiming at unprecedented chemical and physical properties8,9, particularly when other technologies such as Langmuir-Blodgett10,11, layer-by-layer12, and optical or electron-beam lithography are combined13,14.

Moreover, development of a library of ligand exchange methods through nanoscale chemical reactions has facilitated easy conversion of nanomaterials from hydrophobic to hydrophilic or vice versa15,16,17, which generates additional functionalities to realize various purposes such as QLED display18, trace analysis19, disease diagnosis20, bioimaging21, as well as photodynamic therapy22, and so on. As a typical example, replacement of citrate ligand on gold NPs with thiol-ended single chain DNA has enabled successful assembly of gold NPs possessing complementary DNA sequences, which leads to precise fabrication of supercrystals with varied lattice structures23 and remarkable features that neither individual component possesses24,25.

However, ligand molecules may also affect or even prevent functionality expression. For example, insulating molecules are unwanted for semiconducting quantum dots when used for nanodevice fabrication26, chemical molecules surrounding perovskite nanocrystals usually exacerbate the optoelectronic performance in therein fabricated solar cells27, and catalytic noble metal NPs get poisoned by organic molecules used as protecting ligands and so do for the photo- and/or electrochemical catalysis28. On the other side, aggregation of nanomaterials occurs once ligands are removed, accompanied with irreversible functionality loss. Thus, it remains an urgent challenge to find an optimal balance between this dilemma, that is, how to fully exploit the functionalities of nanomaterials that are prone to aggregation once without ligand and meanwhile avoid the influence of ligand molecules, particularly for tiny NPs (<5 nm) that are very active due to the high surface energy29.

Herein, we have developed a clickable assembly strategy that facilitates reliable fabrication of variously structured lanthanide nanoclusters (NCs) with synergized multi-functionalities. The formation of LnF3 NCs starts with conversion of LnOF NPs to LnF3 NPs after organic ligand removal, followed by the assembly process. This HCl-triggered click-reaction allows arbitrary fabrication of the compositions and structures of target NCs such as single-component, dimeric, core-shelled/core-shell-shelled, and reversely core-shelled/core-shell-shelled. The embedded dual luminescent probes via either dynamic energy transfer (Gd3+–Eu3+) or direct excitation (of Eu3+) enables sensitive track of tiny variations in structure and energy transfer pathways.

Results

Assembly of EuOF NPs into EuF3 NCs

Specifically, addition of certain amount of diluted HCl into the stock solution of EuOF NPs (Fig. 1a and Supplementary Fig. 1) strips off majority of the oleic acid anion (OA−) ligands and almost simultaneously triggers the conversion to small EuF3 NPs (Fig. 1b). The coordination between surface Eu3+ on EuF3 NPs and solvent molecules (H2O or C2H5OH)30 improves the solubility. Although these consequent nanoscaled-rare earth chemical reactions complete within 5 mins, they describe exactly why the solution changes from opaque to transparent after HCl addition (Supplementary Fig. 2a, b). However, EuF3 NPs are metastable due to high surface energy and finally assemble into stable NCs through a random attachment mechanism (Fig. 1c, d, Supplementary Fig. 3 and Supplementary Fig. 4). The reason we speculate that the final solution turned back from transparent to opaque (Supplementary Fig. 2c) is that, the trace amount of OA− left on each EuF3 NP gradually accumulated on NCs surface when assembly progresses, which leads to increased hydrophobicity and thus decreased solubility in ethanol (Fig. 1c).

Fig. 1: Self-assembly mechanism of NCs.
figure 1

High-resolution transmission electron microscope (HRTEM) images of a EuOF NPs and b EuF3 NPs. c Schematic diagram illustrating the structural details, random attachment process, and interaction forces responsible for NCs formation. d HRTEM image taken at assembly time of 15 min. The insets in a and b enlighten the difference of the number of ligand molecules after H+ introduction. TEM structures of (i), (ii), and (iii) in d correspond exactly to those in c.

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Based on systematic analysis of TEM (Fig. 2a–h) and FT-IR (Fig. 2i) results of the samples collected at different assembly times, the growth process of EuF3 NCs can be divided into the following three stages:

Fig. 2: Different assembly stages of NCs.
figure 2

TEM images of the products obtained at assembly time of a 0, b 1, c 15, d 30, e 60, f 120, g 240, and h 360 min, respectively. i FT-IR spectra of the products obtained at the assembly time of 0, 5, 30, and 360 min, respectively. j Time-dependent size evolution of NCs. White dotted-circles in c show the NCs formed at very early stage.

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Stage 1: Conversion of EuOF NPs to EuF3 NPs. This is the most critical step, which forms the foundation for subsequent assembly and growth of EuF3 NCs. TEM images (Fig. 2a–c and Supplementary Fig. 5) suggest that EuOF NPs at diameter of 3.2 nm turned into semi-dissociated morphologies after 1 min of H+ addition, which is caused by the ligands (OA−) removal induced by the post-added H+30,31. Then, chemical conversion of EuOF NPs to EuF3 NPs occurred, which can be explained by the theory of hard and soft acids and bases (HSAB)32. The so called “hard” acids and “hard” bases prefer to bond with each other, and so do the “soft” acids and “soft” bases. In our reaction system, F− ion as a base is harder than O2− and similarly H+ as an acid is harder than Eu3+. Therefore, the Eu-O bond was broken under the attack of H+, which results in the semi-dissociation of EuOF NPs and release of non-bonded Eu3+ and F− ions. Then Eu3+ preferentially bonds with F− rather than O2− by forming tiny EuF3 NPs33. The chemical equation can be expressed as:

$${\rm{EuOF}}+{\rm{HCl}}\to {{\rm{EuF}}}_{3}+{{\rm{EuCl}}}_{3}+{{\rm{H}}}_{2}{\rm{O}}$$
(1)

Moreover, disappearance of FT-IR peaks at ~2923, 2852, 1560 and 1466 cm−1 clearly indicates debond of OA− ligands from surface Eu3+. The conversion of EuOF NPs to EuF3 NPs completes at reaction time of 5 min (Supplementary Fig. 5). This is also consistent with the change of solution from opaque to transparent at 5 min (Supplementary Fig. 2), which is due to the improved solubility of EuF3 NPs through solvent molecule (H2O or C2H5OH) coordination, as proved by the broad hydroxyl peak at ~3400 cm−1 (ref. 34) (Fig. 2i).

Stage 2: Initiation of the assembly. Presence of OA- improves solubility and mitigates collision probability of NPs in solution, which allows long-term storage of LnOF NPs precursors without worrying about aggregation (Supplementary Fig. 1). However, surface ions become exposed once OA− is removed, causing sharply risen surface energy of EuF3 NPs29 and initiates the assembly (Fig. 2c).

Stage 3: Growth of EuF3 NCs. The size of EuF3 NCs grew from 3.1 to 21.8, 29.2, 35.7, 38.4, and 40.1 nm at the assembly time of 15, 30, 60, 120, 240, and 360 min, respectively (Fig. 2c–h, j and Supplementary Fig. 6). Parallelly, the FT-IR peaks at 2852 and 2923 cm−1 corresponding to –CH3 stretching vibration intensified from almost nil at 5 min to that of 30 min and further at 360 min (Fig. 2i). This is reasonable because trace amount of OA− on the surface of individual EuF3 NP can’t produce sufficient FT-IR signal at stages 1 and 2. More and more ligands are accumulated (Fig. 1c, d) when NCs grow larger as assembly progresses, resulting in stronger signals. This also well agrees with the solubility change in ethanol solution from transparent to opaque (Supplementary Fig. 2b, c). Thus, the intensity variation of FT-IR peaks at 2852 and 2923 cm−1 (Fig. 2i) could qualitatively unveil the structural details of NCs formed at different assembly stages.

Driving force for EuF3 NCs assembly

As previously reported35, multiple interactions usually work together to make for the final structure of assemblies. In our case, removal of surface ligands from NPs not only exposes a large number of uncoordinated Eu3+, but also leads to increased surface energy. In this situation, unstable EuF3 NPs tend to contact and attach with each other to decrease the total energy through interparticle interactions, and eventually assemble into NCs. The FT-IR peak at 3400 cm−1 implies the existence of hydrogen bond among solvent molecules, while the partially fused crystal lattices (Fig. 1d) suggest the formation of Eu-F chemical bond among EuF3 NPs, both of which are the main interactions responsible for the assembly of EuF3 NCs through a random attachment process (Fig. 1c, d). Moreover, as widely reported36,37, dipole-induced interaction is a strong force at the nanoscale to drive self-assembly, which usually occurs in NPs with asymmetric crystal lattice. Considering the trigonal crystal structure of EuF3 NPs with relatively low symmetry among the seven crystal systems and the big difference in electronegativity between F− and Eu3+, there might exist electric dipole moment originated from the polarity of crystal lattice of EuF3 NPs, which may generate dipole-dipole interactions between EuF3 NPs and act as a driving force for the self-assembly process.

More intriguingly, although the as-assembled EuF3 NCs are polycrystalline, they are readily to be converted into single crystals when annealed at temperatures as low as 50 °C (Supplementary Fig. 7). This principle might be instructive to the synthesis of nanocrystals or nanoalloys that are not possible at nearly room temperature. Moreover, NCs of NdF3, SmF3, and GdF3 were also successfully constructed by following the same strategy (Supplementary Figs. 8 and 9).

Working principle of Eu3+ as luminescent probe

As two major photoluminescence (PL) emissions in Eu3+, the intensity at 612 nm corresponding to the 5D0→7F2 electric dipole transition (IE) is highly sensitive to the local site symmetry while that of the 5D0→7F1 magnetic dipole transition (IM) at 591 nm is not38 (Fig. 3a). Considering that local site symmetry of luminescent ions, here means Eu3+, usually links with surface coordination environment, evolution of the value of IE/IM may be used as a luminescent probe to track the changes of the proportion of surface Eu3+ in different assemblies (Fig. 3a, b and Supplementary Table 1). Indeed, when excited at the characteristic working wavelength of Eu3+ (395 nm), the calculated IE/IM decreases gradually from 3.80 in EuF3 NPs to 1.14 in EuF3 NCs, indicating that the amount of surface Eu3+ keeps reducing with the size increase of NCs (Fig. 3b, c)39. This well-agrees with the mergence of NPs when forming NCs, which causes reduced surface area (Figs. 1d and 3a).

Fig. 3: Working principle of Eu3+ used as luminescent probe.
figure 3

a Illustration of different electron transition pathways in Eu3+ and intensity evolutions of electric-dipole (612 nm) and magnetic-dipole (591 nm) transitions during NCs growth. b Evolution of PL spectra and c IE/IM value of Eu3+ in NCs with different sizes. 395 nm light was used as excitation source.

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It is worth pointing out that the gradually decrescent full-width at half maximum (FWHM) at 591 nm is due to the increased site symmetry of Eu3+ during NCs growth38. Generally, the crystallographic site symmetry significantly affects the spectral line-width of Eu3+, and a minor structural distortion may alter the crystal field symmetry of Eu3+, which will be reflected by the gradual change of FWHM from 12.1 to 7.2 nm as the NCs grow from 3.1 to 40.1 nm40,41.

Construction of arbitrarily designed NCs

Clear understanding of the NC formation mechanisms, particularly the information extracted from luminescent probe has inspired us to further fabricate more complicated NCs and explore their structural relationships with the evolution of IE/IM. Instead of the-above uninterrupted NC assembly (Fig. 2), the conversion of LnOF NPs to and the following assembly of LnF3 NPs were purposely separated, which offers more freedom to grow NCs with widely varied structures and pre-designed chemical compositions. For example, dimeric NCs of EuF3&GdF3 and EuF3&2GdF3 could be reliably constructed by simply mixing NPs of EuF3 and GdF3 that were pre-converted from EuOF and GdOF NPs via a H+-induced clickable reaction, at molar ratios of 1:1 and 1:2, respectively (Fig. 4a–c). Similarly, core-shelled NCs of EuF3@GdF3 and EuF3@GdF3@GdF3 could also be fabricated by controlling the number of GdF3 shells grown on pre-formed core of EuF3 NCs (Fig. 4d–f) and alternatively, the reversely core-shelled NCs of GdF3@EuF3 and GdF3@EuF3@EuF3 were further grown by reversing the assembly sequence of respective components (Fig. 4g–i).

Fig. 4: Schematic illustration of the structures and energy transfer pathways of different NCs.
figure 4

The structures of a dimeric, b EuF3&GdF3, and c EuF3&2GdF3 NCs. d Core-shelled, e EuF3@GdF3, and f EuF3@GdF3@GdF3 NCs. g Reversely core-shelled, h GdF3@EuF3, and i GdF3@EuF3@EuF3 NCs, as well as their respective energy transfer pathways. b, c, e, f and h, i illustrate the energy transfer details in differently structured NCs using 273 nm light to excite Gd3+, which transfers energy to Eu3+ (Supplementary Fig. 11). Note: for simplicity, energy migrations between Gd3+–Gd3+ and Eu3+–Eu3+ were not shown.

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Luminescent probes for tracking of structural variations and energy transfer pathways

Compared with previous assembly mechanism studies where HRTEM, XRD, and synchrotron characterizations were heavily relied42,43, information provided by luminescent probes can be in situ, dynamic, precise, and particularly sensitive to its surrounding environment. Indeed, as depicted in Fig. 4, different Gd3+–Eu3+ energy transfer modes were clearly distinguished in their respective PL spectral evolution, which could be utilized to track the distribution variation of surface Eu3+ on NCs with different structures.

For pure EuF3 NCs, there is only very weak PL because Eu3+ has almost no absorption at 273 nm (Supplementary Fig. 10a, b). However, a 70-fold emission enhancement (Fig. 5a) was seen in EuF3&GdF3 NCs, which is rational because 273 nm is the characteristic working wavelength of Gd3+ and more energy is transferred to Eu3+ in the co-assembled NCs (Fig. 4b and Supplementary Fig. 11). Subsequently, a 103-fold PL enhancement was obtained in EuF3&2GdF3 NCs due to further increased energy transfer from Gd3+ to Eu3+ (Fig. 4c). Consistently, the characteristic absorption peak of Gd3+ at 273 nm appeared in EuF3&GdF3 and became stronger in EuF3&2GdF3, proving the successful assembly of GdF3 NPs at increased amount (Supplementary Fig. 10a). More excitingly, such PL evolution also agrees with the gradually intensified optical images of respective samples (Supplementary Fig. 12a) under 273 nm light excitation where EuF3&2GdF3 is the brightest. Correspondingly, the lifetime of Eu3+ (Fig. 5a and Supplementary Fig. 13a) shows similar trend of increase from EuF3 NPs (0.59 ms) to EuF3&GdF3 (1.46 ms) as that from EuF3&GdF3 (1.46 ms) to EuF3&2GdF3 (2.12 ms). As a stark contrast, neither emission intensity nor lifetime change was observed in Eu3+ under 273 nm light excitation (Supplementary Fig. 14) when solutions containing pre-formed NCs of GdF3 and EuF3 were physically mixed at the atomic ratios of 2:1, 1:1, and 0:1, respectively. This proves straightforwardly that Gd3+–Eu3+ energy transfer can hardly occur if without uniform co-assembly between EuF3 and GdF3 NPs (Fig. 4a–c).

Fig. 5: The PL evolution in different-structured NCs.
figure 5

a–c Variation of luminescence enhancement factor (orange lines) and lifetime (baby blue lines) of Eu3+ in different NCs under 273 nm light excitation, through Gd3+–Eu3+ energy transfer. d–f Luminescence enhancement factor (purple lines) and lifetime (baby blue lines) of Eu3+ in different NCs under 395 nm light excitation. g–i Normalized IE/IM value of Eu3+ in different NCs.

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Similarly, Gd3+ in GdF3 shell keeps transferring energy to internal Eu3+, resulting in enhanced PL in EuF3@GdF3 NCs (Fig. 5b and Supplementary Fig. 10c, d). However, it is reasonable to see only 2.5-fold enhancement because there exists partial energy loss from surface Gd3+ on GdF3 shell (Fig. 4e). Indeed, an enhancement of 6.5 folds is seen in EuF3@GdF3@GdF3 NCs, which is due to: 1) increased amount of Gd3+ that transfers more energy to Eu3+ and meanwhile, 2) decreased amount of Gd3+ energy loss due to the growth of the second GdF3 shell (Fig. 4f). Accordingly, there is a larger Eu3+ lifetime increase (Fig. 5b and Supplementary Fig. 13b) after growth of the second GdF3 shell (from 1.07 to 2.43 ms) compared with that of the first GdF3 shell (from 0.59 to 1.07 ms).

Interestingly, more enhanced (7.3-fold) PL is seen in GdF3@EuF3 NCs than that of 2.5-fold in EuF3@GdF3 NCs (Fig. 5b, c and Supplementary Fig. 10e, f). This is due to more efficient energy transfer from internal Gd3+ to exterior Eu3+ where energy loss in Fig. 4e was largely impressed in Fig. 4h. However, the reason that only 8.1-fold PL enhancement is gained after growth of a second EuF3 shell is because the amount of energy transferred from Gd3+ is fixed so that the second layer of Eu3+ can only absorb the extra energy that passes through the first Eu3+ shell (Fig. 4i). Rationally, the lifetime increase of Eu3+ (Fig. 5c and Supplementary Fig. 13c) is not as much after growth of the second EuF3 shell (from 1.33 to 1.61 ms) compared with that of the first (from 0.59 to 1.33 ms). It is worth emphasizing that the calculated quantum yields39,44 of EuF3 (Supplementary Equation (1) and Supplementary Table 2) are consistent with above-discussed PL intensity evolution, and variation of the calculated GdF3-EuF3 energy transfer rate45,46,47,48 in different NCs based on simplified models (Supplementary Fig. 15) provides further support to our schematic illustrations in Fig. 4.

To dig out more structural information from another point of view, the PL evolution in different NCs was investigated by directly exciting Eu3+ at 395 nm (Supplementary Fig. 16). For easy comparison, the PL intensity in pure EuF3 NCs was set at 1.0 and the calculated enhancement factors in different NCs were shown in Fig. 5d–f. It is reasonable to see enhanced PL in EuF3&GdF3 and EuF3&2GdF3 because there exists a Eu3+–Eu3+ energy migration loop (Supplementary Fig. 16a and Supplementary Fig. 17a–c) in pure EuF3 NCs49 that deteriorates the emission while insertion of co-assembled GdF3 NPs breaks the loop and leads to enhanced PL in EuF3&GdF3. It is also reasonable to see a 2.6- and 2.0-fold enhancement because the unit number of Eu3+ is diluted 2 and 3 times in EuF3&GdF3 and EuF3&2GdF3 (Fig. 5d), respectively. In another word, there shall be a 5.2- and 6-fold absolute PL enhancement in EuF3&GdF3 and EuF3&2GdF3, respectively. This phenomenon is quite instructive for materials design towards maximized PL intensity while minimizing the amount of activators used. Parallelly, the lifetime of Eu3+ also increases from 0.62 to 1.49, and 2.08 ms (Supplementary Fig. 16b).

Growth of a GdF3 shell suppresses the energy loss of surface Eu3+ similar to that of Gd3+ in Fig. 4e and results in 3.6-fold PL enhancement in EuF3@GdF3, while the 6.0-fold PL enhancement in EuF3@GdF3@GdF3 suggests that the second shell has even better energy loss suppression (Supplementary Fig. 16c). This is also consistent with the lifetime increase of Eu3+ from 0.62 to 1.05, and 2.40 ms (Supplementary Fig. 16d). Alternatively, growth of a EuF3 shell around pre-formed core of GdF3 NCs results in larger inter-particle distance, which generates similar (3.8-fold) effect as that in EuF3&GdF3 (Supplementary Fig. 17a–c), that is, the broken energy migration loop and enhanced PL (Supplementary Fig. 17a, d and e). Growth of the second EuF3 shell not only suppresses the energy loss of Eu3+ in the first EuF3 shell, the increased amount of Eu3+ generates extra PL so that a total enhancement of 6.7-fold is obtained in GdF3@EuF3@EuF3 NCs (Supplementary Fig. 16e). Accordingly, the lifetime of Eu3+ increases from 0.62 to 1.35 and 1.65 ms (Fig. 5f and Supplementary Fig. 16f).

Moreover, based on the principle discussed in Fig. 3a, evolution of IE/IM value directly links with the variation of surface Eu3+ proportion in different NCs where tiny structural difference may be precisely probed. Indeed, for EuF3&GdF3 NCs with different EuF3/GdF3 molar ratios, the IE/IM remains almost constant (Fig. 5g), which is reasonable because EuF3 and GdF3 NPs are mixed uniformly during co-assembly process. So, no matter what the EuF3/GdF3 molar ratio is, the proportion of surface Eu3+ compared to internal Eu3+ remains unchanged, which agrees exactly with the proposed structure in Fig. 4a.

Similarly, compared with pure EuF3 NCs, growth of external GdF3 shell sharply reduces the proportion of surface Eu3+, so IE/IM value declined rapidly from 1.10 in EuF3 NCs to 0.87 in EuF3@GdF3 and further to 0.78 in EuF3@GdF3@GdF3 (Fig. 5h). The IE/IM value of Eu3+ also declined (Fig. 5i) from 1.10 in EuF3 NCs to 1.00 in GdF3@EuF3 NCs and 0.92 in GdF3@EuF3@EuF3 NCs because enlarged NCs shows lower proportion of surface Eu3+.

Although TEM images only show morphological and dimensional differences of the above NCs (Supplementary Figs. 18, 19 and 20), the elemental mapping results provide unambiguous evidence for the structural differences of the purposely fabricated NCs illustrated in Fig. 4. For example, uniform distribution of Eu and Gd signals in the structure of EuF3&2GdF3 (Fig. 6a) suggests the complete co-assembly of EuF3 and GdF3 NPs, as depicted in Fig. 4c. Similarly, the opposite Eu and Gd signal distribution in Fig. 6b and c perfectly matches the core-shelled structure of EuF3@GdF3@GdF3 (Fig. 4d) and reversely core-shelled structure of GdF3@EuF3@EuF3 (Fig. 4g), respectively. These results are also highly consistent with both our principle designs and the structural details extracted from PL signals generated by the embedded dual luminescent probes of Gd3+ and Eu3+.

Fig. 6: Structural details of different NCs.
figure 6

High-angle annular dark-field scanning TEM images and corresponding energy-dispersive X-ray elemental mapping results of a EuF3&2GdF3 dimeric, b EuF3@GdF3@GdF3 core-shelled, and c GdF3@EuF3@EuF3 reversely core-shelled NCs.

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Discussion

We have developed a clickable assembly strategy that allows facile construction of arbitrary NCs from individual LnF3 NPs. As a complement to widely used one-step assembly, this two-step method enables fabrication of more versatile nanoarchitectures such as clusters, dimers, core-shells, and more importantly with multiple components and tunable functionalities without worrying about aggregation of precursors. It is worth emphasizing that the in-situ, sensitive, and dynamic PL signal generated by the embedded luminescent probes offers unparalleled superiority in precisely tracking not only tiny structural variations of but also energy transfer pathways in complex nanoarchitectures. Considering the large family of OA-stabilized NPs and principally other systems, this work has paved a potential avenue to the fabrication of widely diversified NCs with promising functionalities in the areas of catalysis, optoelectronics, energy conversion/harvesting, nanoalloys, bioimaging, as well as theranostics.

Methods

Materials

Oxides of Ln2O3 (99.99%, Ln = Eu, Gd, Sm, and Nd) were purchased from Beijing HWRK Chem. Co. LTD. Oleic acid (OA, 90%) and oleylamine (OM, 70%) were purchased from Sigma-Aldrich. Trifluoroacetic acid (TFA) and hydrochloric acid (HCl) was purchased from Shanghai Lingfeng Chemical Company, China. All the chemicals were used as received without further purification.

Preparation of Ln(CF3COO)3

In a typical synthesis, lanthanide oxide was added into the aqueous solution containing slightly excessive trichloroacetic acid with continuous stirring, which was then kept refluxing so as to form an optically transparent solution. The resulting solution was filtered to remove the insoluble materials (if any), and the following solution was dried in an oven at 85 °C for 24 h to obtain Ln(CF3COO)3.

Synthesis of LnOF NPs

Typical procedure: a given amount of Ln(CF3COO)3 (2 mmol) was added into the mixture of OA (20 mmol) and OM (20 mmol) in a three-necked flask (100 mL) at room temperature. The slurry was heated to 100 °C with vigorous magnetic stirring under vacuum for 10 min in a temperature-controlled electromantle to remove water and oxygen, thereby forming an optically transparent solution, which was then ramped to 310 °C at a heating rate of 10 °C/min and maintained for 2 h under N2 atmosphere. When cooling down to room temperature, the NPs were precipitated by adding excessive absolute ethanol into the reaction solution, followed by washing with ethanol for three times. Then the as-prepared NPs were dispersed into cyclohexane for further use.

Self-assembly of LnOF NPs into LnF3 NCs

The NCs assembled from LnOF NPs were obtained through an acid treatment process. Typically, a certain amount of LnOF NPs were added into 4 mL ethanol, which was then poured into a 20 mL beaker and stirred for 5 min. 200 μL diluted hydrochloric acid solution (1 M) was added into the above solution and stirred at around 1000 r/min. After 6 h, the assembled NCs were generated in the solution, which was centrifuged at a speed of 8000 r/min for 4 min to obtain the NC samples, and then washed twice with ethanol under this condition to obtain the final NC samples.

Self-assembly of EuF3&GdF3 dimeric NC

Take the EuF3&GdF3 dimeric NC (molar ratio of EuF3:GdF3 = 1:1) as an example: typically, a certain amount of EuOF NPs and GdOF NPs were transferred into 4 mL ethanol solution, respectively, which was poured into 20 mL beaker, and stirred for 5 min for complete dispersion. Then, 200 μL diluted hydrochloric acid solution (1 M) was added respectively into the above EuOF and GdOF solutions and kept stirring at around 500 r/min. After 10 min, the EuOF NPs and GdOF NPs totally converted into EuF3 NPs and GdF3 NPs, respectively. Then, solutions of EuF3 NPs and GdF3 NPs were mixed together into 20 mL beaker, and stirred at 1000 r/min. After 6 h, the above solution was centrifuged at a speed of 8000 r/min for 4 min to obtain the EuF3&GdF3 dimeric NCs, which were then washed with ethanol twice under this condition to obtain the final products.

Self-assembly of core-shelled NCs

Take the EuF3@GdF3 core-shelled NCs (molar ratio of EuF3:GdF3 = 1:1) as an example: typically, a certain amount of EuOF NPs was transferred into 4 mL ethanol solution, poured into 20 mL beaker, and stirred for 5 min. 200 μL diluted hydrochloric acid solution (1 M) was then added into the above solution and kept stirring for 6 h, until the assembled EuF3 NCs were formed. The such-obtained EuF3 NCs were further used as core for EuF3@GdF3 core-shell structured NCs growth. Meanwhile, a certain amount of GdOF NPs was transferred into 4 mL ethanol solution, poured into 20 mL beaker, and stirred for 5 min. Subsequently, 200 μL diluted hydrochloric acid solution (1 M) was added into the above GdOF solution and kept stirring for 10 min, until GdOF NPs totally converted into GdF3 NPs. Then, solutions containing GdF3 NPs and EuF3 NCs were mixed together at certain ratio, and stirred for 6 h. The solution was centrifuged at 8000 r/min for 4 min to obtain the EuF3@GdF3 core-shelled NCs, which were then washed twice with ethanol for future analysis.

Characterization

Powder X-ray diffraction (XRD) analysis was carried out on a Rigaku Smartlab (3 kW) X-ray diffractometer (Rigaku Corporation, Japan) using Cu Kα radiation (λ = 1.5406 Å) and the measurement was performed at ambient temperature in the range of 2θ = 10-60 degree with 0.02 degree/step. Transmission electron microscope (TEM) images were recorded on a JEOL-1400 Plus (JEOL Ltd, Japan) at an acceleration voltage of 120 kV. High-resolution TEM (HRTEM) images and Energy-dispersive X-ray (EDX) spectra were obtained on a Tecnai G2 F20 TEM (FEI, America) at an acceleration voltage of 200 kV.

Fourier transform infrared (FT-IR) spectra were collected on a Nicolet 380 FT-IR spectrometer (Nicolet, America). Photoluminescence (PL) emission spectra were recorded on an Edinburg FLS1000 Photoemission Spectrometer (Edinburgh Instruments, UK). All luminescent characterizations including PL emission spectra, excitation spectra, and luminescence decay curves were collected at 25 °C. Excitation and emission spectra were measured by using a 450 W continuous xenon arc lamp for sample excitation, and detailed parameters are shown in Supplementary Table 1. Luminescence decay curves were measured by using a compact 60 W xenon flash lamp (μF2) as excitation source with the frequency of 40 Hz.