Bright upconversion over extended temperatures enabled by an organic surface layer

Suo, Hao; Zhao, Peihang; Zhang, Xin; Guo, Yang; Guo, Dongxu; Chang, Jiwen; Chen, Jiangkun; Li, Panlai; Wang, Zhijun; Wei, Hanlin; Zheng, Weilin; Wang, Feng

doi:10.1038/s41467-025-58587-8

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Published: 05 April 2025

Bright upconversion over extended temperatures enabled by an organic surface layer

Nature Communications volume 16, Article number: 3249 (2025) Cite this article

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Abstract

Lanthanide-doped nanocrystals are promising for photon frequency upconversion with substantial spectrum tunability. However, the utilization of the upconversion process has been constrained by low luminescence efficiency, which may further attenuate at elevated temperatures due to thermal quenching. Herein, we report a versatile strategy to boost upconversion luminescence across a wide temperature range by surface coordination of small organic molecules. Mechanistic investigations affirm that the organic surface layer passivates defects and isolates high-energy surface oscillators, thereby preventing the dissipation of excitation energy. The energy preserving effect becomes more prominent with increasing temperatures, especially in a humid environment. Accordingly, the upconversion emission of NaGdF₄:Yb³⁺/Tm³⁺ nanocrystals is substantially enhanced in the ambient environment after ligand coordination, accompanied by an additional emission augmentation with increasing temperature to 443 K. By leveraging this anomalous optical response to thermal stimuli, we further establish full-color thermochromic upconversion switching for advanced anti-counterfeiting and logic encryption technologies.

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Introduction

Upconversion is a promising luminescence approach to generate high-energy photon emission by sequential absorption of multiple low-energy photons^1,2,3. This nonlinear optical process is most efficiently realized through successive energy transfer in lanthanide-doped crystals^4,5,6,7,8. Electronic transitions within the unique 4 f configuration of lanthanide ions can render tunable upconversion luminescence across a wide spectrum spanning deep ultraviolet (UV) and near-infrared (NIR), with the features of sharp emission lines, long excited-state lifetimes, large anti-Stokes shifts, non-blinking and exceptional photostability^9,10,11. Over the past decades, the development of lanthanide-doped upconversion nanoparticles (UCNPs) has enabled a diversity of cutting-edge applications in solar energy harvesting, volumetric display, optical cryptography and particularly in the realm of biophotonics^{12,13,14,15,16,17,18}. However, the practical application of these small-sized nanoparticles has been hindered by the low light conversion efficiencies due to surface-related energy losses, which may further deteriorate under thermal stimulation^19,20,21.

The ongoing attempts to attain bright upconversion emission have only met with limited success by eliminating interfacial energy losses at the nanoscale. For example, inert shell passivation has been conclusively demonstrated to improve the upconversion brightness at room temperature (RT) by isolating the luminescent centers from surface quenchers and maximizing the utilization of excitation energy by activators^{22,23,24,25,26}. However, core–shell construction inevitably increases the particle size. Besides, core–shell nanoparticles can hardly sustain high brightness at elevated temperatures due to thermally promoted non-radiative deactivation^27,28,29. Although anti-thermal quenching of upconversion luminescence has been recently observed, such processes typically rely on small-sized nanoparticles (<15 nm) without surface protection^30,31,32. This kind of thermal enhancement essentially stems from the partial recovery of radiative emission in the naked core nanoparticles by the thermal removal of surface-adsorbed water molecules, which induces intensive quenching of upconversion luminescence at RT^33,34,35. To date, achieving bright upconversion luminescence in lanthanide-doped nanocrystals at both RT and high temperatures remains unfulfilled research.

Herein, we present a ligand-coordination approach that substantially enhances upconversion luminescence across a wide temperature range. In complementary to the mechanism of energy-level reconstruction proposed by Liu and co-workers³⁶, our study unveils that the strong coordination of bidentate picolinic acid ligands to surface lanthanide ions can form an organic surface layer to suppress energy dissipation. Our experiment investigations, corroborated by theoretical calculations, affirm that the organic surface layer can effectively passivate defects and isolate high-energy oscillations of surface water molecules, thereby preventing the excitation energy of nanoparticles from being quenched (Fig. 1 and Supplementary Fig. 1). Furthermore, we reveal that the effect of the ligand-coordinated surface layer on preserving energy is more prominent at elevated temperatures, especially in a humid environment, offering an excellent opportunity for combating thermal quenching. We show that the ligand-coordination approach can be extended to construct thermochromic upconversion across a wide color gamut, enabling multi-dimensional anti-counterfeiting and logic encryption technologies.

**Fig. 1: Schematic illustration of upconversion enhancement by an organic surface layer.**

Results

Nanocrystal synthesis and modification

This study employed a pyridine-2-carboxylic acid (2PA)-capped nanostructure developed by Liu and co-workers for luminescence enhancement³⁶. To maximize the 2PA-mediated effect, we first optimized the synthetic protocol by refining the reaction conditions to enable the firm bonding between the nanoparticles and capped ligand molecules. Since the direct exchange of OA with 2PA ligand is not feasible, our synthesis involves the preparation of ligand-free UCNPs followed by treatment with 2PA ligands in ethanol (Fig. 2a). In a typical procedure, hexagonal-phase NaGdF₄:Yb³⁺/Tm³⁺ nanoparticles were synthesized through a co-precipitation method as a model platform³⁷, which were subsequently treated in concentrated HCl to obtain the ligand-free UCNPs (Supplementary Fig. 2). The ligand-free nanoparticles were positively charged and favored the adsorption of negatively charged 2PA molecules in the weakly basic condition (Supplementary Fig. 3)³⁸. Accordingly, we dissolved the 2PA ligands in ethanol solution with sodium hydroxide at pH = 9 to facilitate the deprotonation of –COOH, rendering the ligands more accessible to the nanoparticles. It is worth noting that the 2PA coating process hardly affected the spherical shape and uniform size of the original nanoparticles (Fig. 2a).

**Fig. 2: Upconversion enhancement by surface modification.**

The successful coordination of 2PA ligands on the nanoparticle surface was confirmed by Fourier transform infrared (FT-IR) spectroscopy (Fig. 2b). The apparent peak shifts related to ring deformation (676 → 704 cm^-1, 749 → 756 cm^-1), heterocyclic skeleton vibration (1452 → 1445 cm^-1, 1524 → 1476 cm^-1), C = N and C = O stretching vibration (1593 → 1566 cm^-1, 1654 → 1625 cm^-1) indicated the involvement of the pyridine ring and –COOH group in the coordination between 2PA and surface lanthanide ions³⁹. By contrast, the ligands showed ineffective attachment to the nanoparticles in the absence of pH adjustment due to their limited ionization (Supplementary Fig. 3c). The slightly suppressed O–H stretching vibration at around 3400 cm^-1 upon ligand coating suggested that the strong coordination of rigid ligands can partially obstruct the adsorption of water molecules to the nanoparticle surface.

The reaction temperature is crucial for efficient capping of ligands on nanoparticles. A high temperature may favor the reaction kinetics by facilitating molecular diffusion and collision frequency, thereby contributing to a more efficient and robust bonding between the surface lanthanide ions and capping ligands. This assumption was supported by thermogravimetric (TG) analysis of 2PA-capped nanoparticles obtained at varied reaction conditions (Fig. 2c and Supplementary Fig. 4). As expected, we found an increased detachment of 2PA molecules from nanoparticles prepared at elevated reaction temperatures. The stronger intensity of FT-IR peaks related to the ligand groups further revealed a higher concentration of surface-capped 2PA molecules under a higher reaction temperature (Supplementary Fig. 3c). In addition, magnetic stirring was preferred over ultrasonic treatment as it offers better mixing homogeneity and thus enhanced ligand accessibility, enabling a higher ligand-capping efficiency.

Upconversion enhancement at RT

Upon surface modification with 2PA ligands, the upconversion luminescence of NaGdF₄:Yb³⁺/Tm³⁺ nanoparticles dispersed in ethanol was substantially enhanced under 980 nm excitation (Fig. 2d, Supplementary Fig. 5 and Supplementary Note 1). Through optimizing the 2PA concentration and capping conditions (i.e., 0.08 mmol of 2PA under stirring for 8 hours at 78 ^oC), we recorded a remarkable enhancement factor of about 2100 and 160 folds for the UV emission (¹D₂ → ³H₆) using ligand-free and OA-coated nanoparticles as controls, respectively (Fig. 2e, Supplementary Figs. 6–9 and Supplementary Table 1). Note that a high loading density of 2PA ligands on the nanoparticle surface typically rendered a significant upconversion enhancement effect (Supplementary Fig. 10 and Supplementary Table 2). Under the optimized 2PA coating conditions, the enhancement phenomenon of the nanoparticles was well preserved even in the powder form, indicating the excellent stability and effectiveness of the nanoparticle-ligand interaction (Supplementary Figs. 11–12). Similar improvement of upconversion luminescence spanning visible and NIR regions was also observed in other lanthanide activators (e.g., Er³⁺, Ho³⁺ and Nd³⁺), demonstrating the versatility of the 2PA modification in enhancing upconversion (Supplementary Figs. 13–15).

Previously, the 2PA-induced upconversion enhancement was primarily ascribed to the energy-level reconstruction of surface Yb³⁺ sensitizers, which improves energy resonance between surface and inner Yb³⁺ ions, thereby facilitating energy migration within the sensitizer sublattice and subsequent energy transfer to activators³⁶. However, our investigation suggests that the effective preservation of interfacial quenching effect of Yb³⁺-harvested excitation energy after 2PA coordination may dominate the upconversion enhancement. As shown in Fig. 3a, the upconversion emission of ligand-free nanocrystals was boosted by 43 folds if the nanocrystals were subjected to a prior solution annealing process in OA/ODE containing Y³⁺-OA complexes, which mainly passivates surface defects of the nanoparticles without altering their size and morphology⁴⁰ (Supplementary Figs. 16, 17). Further removal of high-energy surface oscillations (i.e., O–H) in a dry argon atmosphere could induce another 42-fold enhancement of the upconversion emission, rendering even higher emission intensity than 2PA-capped counterparts in ambient air.

**Fig. 3: Mechanistic investigation of the ligand-mediated upconversion enhancement.**

After the surface quenching effect was minimized beforehand, the 2PA-mediated upconversion enhancement is largely compromised. As summarized in Fig. 3b, ligand coordination on solution-annealed nanoparticles induced a minor enhancement of upconversion emission by 13 folds, in contrast to 92 folds for untreated nanoparticles. Likewise, we only recorded a 10-fold emission enhancement of nanoparticles due to 2PA coordination in dry argon flow (Supplementary Fig. 18 and Supplementary Note 2). Notably, the solution-annealed nanoparticles in a dry environment can merely be marginally enhanced by 2.3 folds upon 2PA coating (Supplementary Fig. 19). Similar diminishing of 2PA-induced enhancement effect was also detected in the absence of Yb³⁺ ion, stemming from suppressed long-distance energy migration and thus alleviated surface quenching (Supplementary Fig. 20). These observations validate that 2PA achieves its major role in enhancing upconversion by preventing interfacial quenching of the excitation energy, that is, passivating surface defects and isolating high-energy oscillations of surface water molecules. In the absence of surface quenching, 2PA lost most of its power to boost the upconversion emission.

The effect of the 2PA-coordinated surface layer on preserving energy was further supported by time decay studies, which revealed a positive correlation of the upconversion enhancement factor with the increase in Yb³⁺ lifetimes (Supplementary Fig. 21). The reduced quenching of excitation energy facilitates the utilization of the absorbed photons, thereby lowering the power density threshold of intensity saturation⁴¹ (Fig. 3c). This saturation effect resulted in the attenuation of 2PA-induced enhancement factors under high excitation power densities (>20 W/cm², Supplementary Fig. 22). Accordingly, the 2PA ligand offers a significant enhancement effect at a relatively low power density before saturation occurs. Moreover, the preservation of excitation energy in Yb³⁺ ions also favours multiphoton processes, resulting in a greater enhancement factor of a higher-order upconversion luminescence (Fig. 2e). It is worth noting that 2PA coordination induced negligible changes in the absorption band of Yb³⁺, indicating that the local crystal structure was minimally altered (Supplementary Fig. 23)⁴². Therefore, the effective protection of excitation energy at the inorganic-organic interface played the major role in enhancing the upconversion process, which was backed by improved quantum efficiency (QY, Supplementary Table 3).

The comparison in Fig. 3b implies that 2PA coordination is highly effective in rejecting water-induced quenching. Mechanistic investigations by FT-IR and TG analysis revealed a suppression in O–H stretching vibration and a slight reduction in water adsorption upon 2PA modification (Fig. 3d). Therefore, partial rejection of water adsorption could be identified as a contributing factor to the mitigated surface quenching. However, the minor reduction of surface water content (~ 16%) is unlikely to take full responsibility for the nearly two orders of magnitude enhancement of upconversion emission. We thus speculate that the 2PA-coordinated surface layer may shield the surface lanthanide ions from the vibrations of water molecules.

To gain mechanistic insight into the role of the organic surface layer in mediating nanoparticle-water interaction, we calculated the adsorption energy (E_ads) of different surface configurations using the density functional theory (DFT) through the following equation⁴³:

$${E}_{{ads}}={E}_{{slab}+{molecule}}-{E}_{{slab}}-{E}_{{molecule}}$$

(1)

where E_{slab+molecule}, E_slab and E_molecule indicate the energy of the nanohybrid system, bare surface and the isolated ligand molecule, respectively. The theoretical results reveal a strong interaction between 2PA and surface metal ions due to the formation of a stable five-membered chelate ring (Fig. 3e and Supplementary Fig. 24). Consequently, the attachment of water molecules on the nanoparticle is mainly bridged by 2PA ligands through weak intermolecular interactions³⁹, resulting in increased water-nanoparticle separation. In line with calculation results, the 2PA-capped nanoparticles displayed appreciably higher resistance to water-induced quenching than the ligand-free counterparts (Fig. 3f and Supplementary Fig. 25). It is worth noting that, despite being less effective than OA ligand in isolating surface water molecules (Supplementary Fig. 4), 2PA performs much better than OA in defect passivation (vide infra), thereby offering a superior overall upconversion enhancement effect (Supplementary Fig. 9). As an added benefit, the 2PA ligands significantly improved the dispersibility, luminescence intensity and stability of the nanoparticles in diverse polar solvents (Supplementary Figs. 26, 27), greatly expanding their utility in advanced photonic applications.

Bright upconversion in a thermal field

The main mechanism of thermal quenching is the promotion of multiphonon relaxation and phonon-assisted energy transfer to quenching centers at elevated temperatures. Accordingly, the 2PA-coordinated surface layer with an energy-preserving effect is potentially useful for combating thermal quenching. To test our hypothesis, we first measured the thermal stability of the upconverson luminescence in dry argon flow (Supplementary Figs. 28, 29), which could exclude dehydration-induced anti-thermal quenching effect widely observed in upconversion nanoparticles^34,35. As anticipated, the upconversion emission of 2PA-capped nanoparticles exhibited greatly improved thermal resistance compared to the uncoated counterparts (Fig. 4a). These observations are ascribed to the reduced defect formation in the 2PA-coordinated surface layer, which effectively suppresses non-radiative dissipation of excitation energy in a thermal field (Fig. 4b)⁴⁴. As a result, we observed a significant amplification of the 2PA-induced upconversion enhancement effect as the temperature increased (Fig. 4c). On a separate note, in-situ powder X-ray diffraction (XRD) measurements revealed an unaltered thermal expansion coefficient of the nanoparticles after 2PA modification (Supplementary Fig. 30), which ruled out the effect of anomalous lattice expansion on the thermal-resistant upconversion.

**Fig. 4: Ligand-mediated anti-thermal quenching of upconversion.**

We next assessed the temperature-dependent upconversion emissions under ambient air conditions, which is the typical operational environment for many applications. Due to the involvement of moisture, thermally enhanced emission was detected for both ligand-free and 2PA-coated nanoparticles through thermal dehydration³⁴ (Fig. 4d and Supplementary Note 3). Intriguingly, we observed a more prominent thermal enhancement effect of upconversion emissions for 2PA-capped nanoparticles compared to the ligand-free counterparts (25 versus 7.2 folds for overall intensity and 240 versus 2.0 folds for 450 nm emission, Fig. 4e and Supplementary Fig. 31). Similar reinforced enhancement of multicolor luminescence was also recorded at the elevated temperature for Er³⁺, Ho³⁺ and Nd³⁺ activators after 2PA coordination (Supplementary Fig. 32). Although the well-established core–shell structures can render slightly brighter upconversion than the 2PA-coated nanoparticles at RT due to more effective shielding against surface water molecules, they fail to maintain the initial emission intensity as the temperature increased (Fig. 4f and Supplementary Figs. 33–35). Notably, we detected much stronger luminescence in 2PA-capped nanoparticles than core–shell samples after dehydration at elevated temperatures or in dry argon (Supplementary Figs. 36, 37). This phenomenon likely arises from the combined effects of reduced defect formation, minimized light scattering and improved energy-level alignment upon 2PA modification³⁶. Therefore, surface coordination with 2PA ligand provides a general approach for enhancing upconversion across a wide temperature range, with the advantages of unaltered particle size and convenient synthesis.

We ascribe the significant thermal enhancement effect in 2PA-capped nanoparticles to effective surface dehydration mediated by the organic interfacial layer that weakens the water-nanoparticle interactions. Notably, we recorded a progressive increment of the emission intensity in the ligand-free nanoparticles by continuously heating at 443 K, which was better pronounced under a lower humidity condition (Fig. 5a and Supplementary Fig. 38). In sharp contrast, 2PA-coated nanoparticles quickly attained intensity maximum and then remained stable under continuous laser irradiation for 5 hours at the high temperature, regardless of the humidity. These observations aligned well with the thermal evolutions of decay times (Fig. 5b). Specifically, the lifetime of naked nanoparticles kept prolonging upon continuous heating at 443 K, while the lifetime maximum was rapidly established in the 2PA-coated counterparts (Supplementary Fig. 39). These results demonstrated that the ligand-coordinated surface layer greatly promotes the thermal desorption of surface-adsorbed water molecules.

**Fig. 5: Ligand-coordination effects on thermal enhancement performance.**

In-situ FT-IR spectroscopy was employed to shed more light on the ligand-mediated dehydration process. As compiled in Fig. 5c, the O–H absorption peak in the 2PA-capped nanoparticles quickly diminished as the temperature increased from 303 to 443 K. By contrast, the absorption peak was largely preserved at elevated temperatures in the ligand-free counterpart. The results confirmed that the 2PA-coordinated surface layer expedited the thermal desorption process. Moreover, the FT-IR intensity of vibration peaks related to 2PA ligands hardly weakened as the temperature increased, suggesting the negligible thermal desorption of surface 2PA ligands. This exceptional thermal stability of the 2PA-coordinated layer is likely due to the formation of a stable five-membered chelate ring between the ligand and nanoparticle surface.

2PA ligands also favor the re-adsorption of water molecules onto the nanoparticle surface upon cooling, leading to a highly reversible modulation of upconversion by thermal stimuli. During the natural cooling process, ligand-free nanocrystals featured incomplete recovery to the initial intensity of upconversion emission and O–H stretching vibration within 3 h (Fig. 5d). This observation stemmed from the inefficient surface re-adsorption of surrounding water molecules, resulting in poor repeatability over successive thermal cycles (Fig. 5e). Upon surface modification with 2PA ligands, the emission intensity rapidly returned to the original level within 40 min after the temperature fell back to RT, in consistency with the FT-IR results (Supplementary Fig. 40). These results suggest that the strong coordination of high-rigidity 2PA ligands to surface lanthanide ions can facilitate the water re-adsorption process (Supplementary Fig. 41). Accordingly, we realized excellent repeatability of thermal enhancement performance in 2PA-capped nanocrystals under different humidity conditions (Fig. 5f).

The effect of the 2PA-coordinated surface layer on enhancing upconversion is strongly dependent on dopant concentration and nanoparticle size. Typically, an increase in Yb³⁺ concentration or a decrease in particle size resulted in a notable amplification of the enhancement effect mediated by 2PA ligands, which was more pronounced at elevated temperatures (Fig. 5g and Supplementary Figs. 42, 43). The observations can be ascribed to prevalent surface quenching in small-sized and heavily doped nanoparticles, stemming from a large surface-to-volume ratio and fast energy hopping through the crystal lattice²³. Consequently, the utilization of the 2PA-coordinated surface layer could enable a more significant recovery of radiative emissions. Remarkably, we observed an approximately 4000-fold enhancement of the 450 nm emission for 10 nm nanocrystals in the powder form at RT upon 2PA coordination (Supplementary Fig. 44). Further increase of temperature to 443 K induced additional thermal reinforcement of the blue emission by about 1,200 folds in the powder form, which was over six orders of magnitudes higher than that of the ligand-free counterparts at RT (Fig. 5h and Supplementary Fig. 45).

Full-color upconversion switching by thermal stimuli

The ligand-mediated upconversion enhancement with dimensional dependence provides tremendous opportunities for full-color tuning by thermal stimuli. As a proof-of-concept, we devised a 2PA-capped core–shell–shell nanostructure for thermochromic upconversion switching. In detail, the inner core and outmost layer with distinct accessibility to surface ligands are designed to show thermally quenched and enhanced upconversion, respectively (Fig. 6a). An inert interlayer is incorporated to precisely control the impact of the 2PA-coordinated surface layer and the luminescent shell on emission behavior of the core. As a result, thermal fine-tuning of upconversion emissions across the full spectrum could be realized by using core/shell combinations with red-green-blue (RGB) emission features (Supplementary Figs. 46–49 and Supplementary Table 4). Apart from the thermo-responsive emissions, these UCNPs also preserved the inherent dependence on excitation power and wavelength. The highly variable upconversion emission holds great potential for applications in advanced information security. In an exemplary illustration, we patterned a graphic on a copper substrate using four types of 2PA-capped UCNPs, which could reveal a series of optical codes with distinguishable graphic and color features by controlling the temperature and excitation power/wavelength (Fig. 6b).

**Fig. 6: Thermal modulation of upconversion for information security.**

In a further set of experiments, we established a multi-level pattern for logic encryption by integrating the stimuli-responsive and excitation wavelength-dependent features of 2PA-capped UCNPs. In our design, photonic color output codes are programmable by controlling the input parameters of temperature and excitation wavelength, and the cryptographic data can be decoded via prescribed logical operations⁴⁵. As illustrated in Fig. 6c, we devised an array of hybrid textual information consisting of four sets of alphabetical codes using different UCNPs. The encrypted information of “UCNP” can only be deciphered by a specific combination of excitation wavelength and thermal conditions followed by an AND logical operation. These results substantiated the great superiorities of thermochromic upconversion for high-security data storage, with the advantages of direct visualization and easy authentication. As an added benefit, the thermal control over the upconversion emissions can also be harnessed for high-performance ratiometric thermometry, enabling superior thermal sensitivity and resolution (S_r-max = 12.0% K^-1 and δT_min = 0.06 K, Supplementary Fig. 50 and Supplementary Table 5).

Discussion

In summary, our investigation of the ligand-coordination effect highlights a versatile approach for upconversion enhancement across a wide temperature range. The strong coordination of rigid organic molecules can form an organic surface layer to suppress energy dissipation by passivating surface defects and isolating high-energy oscillations of surface water molecules. Importantly, this ligand-coordinated interfacial layer can provide a more prominent effect on energy preservation at elevated temperatures to combat thermal quenching. As a result, we attain bright upconversion luminescence in lanthanide-doped nanocrystals at both RT and high temperatures in ambient air. These advances may inspire new ideas in constructing high-efficiency upconversion nanoparticles, going beyond the conventional core–shell engineering and dye-sensitization method, for advanced photonic applications.

Methods

Raw materials and reagents

Oleic acid (OA; 90%), 1-octadecene (ODE; 90%), gadolinium (III) acetate hydrate (Gd(CH₃CO₂)₃·xH₂O; 99.9%), ytterbium (III) acetate hydrate (Yb(CH₃CO₂)₃·xH₂O; 99.9%), thulium (III) acetate hydrate (Tm(CH₃CO₂)₃·xH₂O; 99.9%), erbium (III) acetate hydrate (Er(CH₃CO₂)₃·xH₂O; 99.9%), holmium (III) acetate hydrate (Ho(CH₃CO₂)₃·xH₂O; 99.9%), neodymium (III) acetate hydrate (Nd(CH₃CO₂)₃·xH₂O; 99.9%), yttrium (III) acetate hydrate (Y(CH₃CO₂)₃·xH₂O; 99.9%), sodium hydroxide (NaOH; >98%), ammonium fluoride (NH₄F; >98%), sodium trifluoroacetate (CF₃COONa, Na-TFA; 97%), 2-Picolinic acid (2PA; 99%) and benzoic acid (BA; 99%) were all purchased from Sigma-Aldrich. Absolute ethanol (99.85%), methyl alcohol (99.99%), cyclohexane (99.9%) and hydrochloric acid (37%) were purchased from Aladdin Company. All chemicals were used as received without further purification.

Preparation of OA-capped core nanoparticles

Lanthanide-doped NaGdF₄ core nanoparticles were synthesized through our previously established protocol³⁷. In a typical synthetic procedure of 15 nm NaGdF₄:Yb³⁺/Tm³⁺ (49/1%), 4.0 mL of RE(CH₃CO₂)₃ (0.2 M, RE = Gd, Yb and Tm) aqueous solution was firstly added to a binary solvent mixture of OA (8.0 mL) and ODE (12.0 mL) in a 50 mL flask. The mixture was heated at 150 °C for 60 min and then naturally cooled down to 45 °C. Subsequently, 6.6 mL of NH₄F (0.4 M) and 2.0 mL of NaOH (1.0 M) in methanol solution were added with continuous stirring for 90 min. After evaporating the volatile molecules (methanol and water) at 105 °C, the resultant solution was heated to 280 °C under argon protection for 1.5 h, followed by cooling down to RT. The resulting nanoparticles were precipitated by the addition of ethanol, collected by centrifugation at 8300 × g for 5 min, washed with ethanol and methanol for several times, and finally re-dispersed in 4.0 mL cyclohexane for further use. The particle size can be finely tuned by adjusting the volume ratio of NH₄F and NaOH in a methanol solution. The synthetic procedure of NaGdF₄:Yb³⁺/RE³⁺ (RE = Er, Ho and Nd) was similar except for the use of corresponding rare-earth precursors. Note that the ratio of NH₄F and NaOH was set as 3.9:1 for preparing NaErF₄:Tm³⁺ (0.5%) nanoparticles.

Preparation of core–shell nanoparticles

The core–shell nanoparticles were synthesized through the established layer-by-layer strategy using RE-OA and NA-TFA-OA as shell precursor solutions⁴⁶. Prior to the synthetic procedure, a clear RE-OA precursor solution (0.1 M) was first obtained by heating a mixture of Ln(CH₃CO₂)₃ (2.5 mmol), OA (10.0 mL) and ODE (15.0 mL) at 150 °C for 60 min. Then, a mixture of Na-TFA (4.0 mmol) and OA (10.0 mL) was added into a flask and heated at RT under vacuum for 60 min to get a clear Na-TFA-OA precursor solution (0.4 M). In a typical procedure for synthesizing NaGdF₄:Yb³⁺/Tm³⁺@NaYF₄, 2.0 mL of colloidal core nanoparticles (0.2 M) was first added to a binary solvent mixture of OA (4.0 mL) and ODE (6.0 mL) in a 50 mL flask. After evaporating the volatile molecules (cyclohexane and water) at 105 °C, the resultant solution was heated to 280 °C under argon protection, followed by dropwise adding the mixture of Y-OA (0.1 M, 6.0 mL) and Na-TFA-OA (0.4 M, 3.0 mL) precursor solutions (1.0 mL/min). After cooling down to RT, the resulting nanoparticles were precipitated by the addition of ethanol, collected by centrifugation at 8300 × g for 5 min, washed with ethanol and methanol for several times and finally re-dispersed in 2.0 mL cyclohexane for further use. The synthetic procedure of other core–shell nanoparticles, including NaGdF₄:Yb³⁺/Tm³⁺ (49/1%)@NaYF₄:Yb³⁺ (49%), NaGdF₄:Yb³⁺/Er³⁺ (20/2%)@NaYF₄ and NaErF₄:Tm³⁺ (0.5%)@NaYF₄ was similar except for the use of corresponding rare-earth shell precursors.

Preparation of core–shell–shell nanoparticles

The core–shell–shell nanoparticles, including NaErF₄:Tm³⁺ (0.5%)@NaYF₄@NaGdF₄:Yb³⁺/Er³⁺ (20/2%), NaErF₄:Tm³⁺ (0.5%)@NaYF₄@NaYbF₄:Tm³⁺ (1%), NaGdF₄:Yb³⁺/Er³⁺ (20/2%)@NaYF₄@NaYbF₄:Tm³⁺ (1%) and NaGdF₄:Yb³⁺/Er³⁺ (20/2%)@NaGdF₄:Yb³⁺/Tm³⁺ (49/1%) were synthesized using the protocol similar to that of core–shell nanoparticles, except for using the corresponding rare-earth shell precursors and core–shell nanoparticles as the seeds.

Wet chemical post-annealing of core nanocrystals

In a typical procedure, 5 mL of Y(CH₃COO)₃ (0.2 M) aqueous solution was first added to a binary solvent mixture of OA (3.0 mL) and ODE (7.0 mL) in a 50 mL flask. The mixture was heated at 150 °C for 60 min and then naturally cooled down to 45 °C. Shortly after that, 1.0 mL of colloidal NaGdF₄:Yb³⁺/Tm³⁺ (49/1%) nanoparticle solution (0.2 M) was added, followed by evaporating the volatile molecules (methanol and water) at 105 °C. The resultant solution was heated to 300 °C under argon protection for 1 hour, and then cooled down to RT. The resulting nanoparticles were precipitated by the addition of ethanol, collected by centrifugation at 8300 × g for 5 min, washed with ethanol and methanol for several times and finally re-dispersed in 1.0 mL cyclohexane for further use.

Preparation of ligand-free nanoparticles

The OA ligands were stripped from the nanoparticles using previously reported methods³⁸. First, 2.0 mL of as-prepared nanoparticles (0.2 M) in cyclohexane were precipitated by adding 2.0 mL of ethanol, followed by centrifugation at 8300 × g for 10 min. The resulting pellets were re-dispersed in 3.0 mL of dilute HCl solution (0.05 M) and then sonicated at RT for 1 h to remove the surface OA ligands. After centrifugation at 16,000 × g for 10 min, the supernatant was discarded and the resulting precipitation was re-dispersed in ethanol by ultrasonication. The washing process was repeated twice and the colorless colloidal nanoparticles were finally re-dispersed in 2.0 mL of ethanol for further use.

Preparation of 2PA-capped nanoparticles

2PA-Na ligand solution (0.2 M, pH = 9) was first obtained by dispersing 2PA and NaOH (1:1) in ethanol under stirring for 30 min. Subsequently, 0.4 mL of 2PA-Na ligand solution was added dropwise to 0.5 mL of ligand-free nanoparticle solution (0.2 M) under vigorous stirring at 78 ^oC for 8 h. Note that the solution was maintained at approximately 1.0 mL by continuously supplementing with ethanol against evaporation loss. Finally, the resulting clear solution was centrifuged and re-dispersed in 1.0 mL of ethanol for further use. In addition, 2PA-capped nanoparticles were centrifuged and dried at 70 ^oC for 1 h to get the white powder samples for temperature-dependent spectral measurements.

Fabrication of nanoparticle composite films

The film was composed of transparent polydimethylsiloxane (PDMS, 4.0 g), curing agent (0.4 g) and solution-dispersed nanoparticles (2.0 mL, 0.06 g) at a weight ratio of 10:1:0.15. First, the curing agent was premixed with SYLGARD 184 silicone elastomer base, to which the nanoparticle solution was added under vigorous stirring. Subsequently, the jelly-like mixture was transferred into a petri dish for casting, followed by degassing in a vacuum oven to remove air bubbles. Finally, the uncured film was dried at 80 °C for 2 h to get the PDMS film with a thickness of 1 mm.

Physical measurements

XRD data were recorded using a Bruker D8 Advance powder diffractometer with Cu-Kα radiation. The microstructural characterizations, including element mapping, high-resolution and high-angle annular dark-field scanning transmission electron microscopy (HR-TEM & HAADF-STEM), were conducted on a JEOL-2100Plus transmission electron microscope operated at an acceleration voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Fisher ESCALAB 250Xi. FT-IR and absorption spectra were carried out by a Bruker EQUINOX55 spectrometer and Hitachi 4100 UV-vis-NIR spectroscopy, respectively. The dynamic light scattering (DLS) and Zeta potentials of as-prepared nanoparticles were determined using a Zeta Plus zeta potential analyzer (Malvern Zetasizer Nano ZS90). TG analysis curves were performed on a Hitachi STA7300 at a heating rate of 10 ^oC/min. Photoluminescence spectra and decay curves at different temperatures were recorded using a Horiba FL3 fluorescence spectrometer equipped with an external temperature controlling system (Hotz instrument, RTL 550) and power-controllable laser diodes (980, 808 & 1530 nm). The pulsed laser was achieved by coupling a laser diode with transistor-transistor logic (TTL) pulse mode and a digital pulse generator (DG, Rigol technologies, Beijing, China). The bolometric thermograms were recorded using a Fluke Ti300 infrared camera. The absolute QY measurements were conducted at RT by an Edinburgh FLS980 spectrometer equipped with an integrating sphere. The upconversion QY can be calculated as the ratio of photons emitted to photons absorbed. In this measurement, the power density of 980 nm excitation laser was set as about 50 W/cm², where an attenuation filter (~1% for 980 nm) was employed to improve the measurement limits. Note that the QY value of widely studied NaGdF₄:Yb³⁺/Er³⁺ (20/2%)@NaYF₄ with a particle size of around 28 nm was measured as the reference⁴⁷.

Computation details

The interactions between the nanoparticles and the 2PA or H₂O molecules were described on the stable (0001) facet of the NaGd_0.5Yb_0.5F₄ supercell. The First-principles calculation was performed in the framework of DFT combined with the projector augmented wave method, as implemented in the Vienna Ab Initio Simulation Package (VASP) package. Perdew-Burke-Ernzerh (PBE) function is utilized to approximate the exchange-correlation interactions. The cutoff energy of the plane wave was set as 550 eV throughout the simulation and 2 × 2 × 1 k-point mesh in the Brillouin zone was employed. The structure was optimized until the force between each atom was smaller than 0.015 eV/Å. The GdF-teminated and NaGdF-teminated atomic slabs were constructed with 25 Å vacuum layer.

Data availability

All data supporting the results of this study are available in the Article and its Supplementary Information. Source data are provided with this paper.

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Acknowledgements

This work was supported by the National Natural Science Foundation of China (Nos. 12474402 and 12004093, H.S.), the Research Grants Council of Hong Kong (C5110-20G, F.W.), the Central Government to Guide Local Scientific and Technological Development (236Z1013G, H.S.), the Hebei Yanzhao Golden Platform Talent Gathering Programme Core Talent Project (Education Platform, HJYB202503, H.S.), and the Hong Kong Scholars Program (No. XJ2020011, H.S.).

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These authors contributed equally: Hao Suo, Peihang Zhao.

Authors and Affiliations

Hebei Key Laboratory of Optic-electronic Information and Materials, College of Physics Science & Technology, Hebei University, Baoding, China
Hao Suo, Peihang Zhao, Dongxu Guo, Jiwen Chang, Panlai Li & Zhijun Wang
Department of Materials Science and Engineering, City University of Hong Kong, Kowloon, Hong Kong SAR, China
Hao Suo, Xin Zhang, Yang Guo, Jiangkun Chen, Hanlin Wei, Weilin Zheng & Feng Wang
Hong Kong Institute for Clean Energy, City University of Hong Kong, Kowloon, Hong Kong SAR, China
Feng Wang

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Hao Suo
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Contributions

H.S. and F.W. initiated the project. H.S., P.H.Z., and F.W. designed the experiments. H.S. and P.H.Z. performed the experiments and analysed the data. X.Z., Y.G., D.X.G., J.W.C., J.K.C., P.L.L., Z.J.W., H.L.W. and W.L.Z. contributed to the theoretical calculations and experimental measurements. H.S. and F.W. wrote the manuscript. All authors contributed to the analysis of this manuscript.

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Correspondence to Hao Suo or Feng Wang.

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Suo, H., Zhao, P., Zhang, X. et al. Bright upconversion over extended temperatures enabled by an organic surface layer. Nat Commun 16, 3249 (2025). https://doi.org/10.1038/s41467-025-58587-8

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Received: 11 March 2024
Accepted: 24 March 2025
Published: 05 April 2025
DOI: https://doi.org/10.1038/s41467-025-58587-8