1 Introduction

Over the past decade, lanthanide-doped nanoparticles have left their mark as efficient luminescent nanomaterials [1]. In contrast to conventional luminescent materials, such as fluorescent organic dyes and semiconducting nanoparticles, lanthanide-doped nanoparticles generally offer narrow emission bands, long luminescence lifetimes (micro- to millisecond range), low toxicity, as well as high resistance to photobleaching, blinking, and photochemical degradation [2]. In particular, the lanthanide-doped nanoparticles comprising proper host–dopant combinations can convert near infrared long-wavelength excitation radiation into shorter-wavelength visible emissions [3]. These unique anti-Stokes emitters, now widely known as upconversion nanoparticles, have evolved as a rapidly growing field and opened up the opportunity for creating new applications in diverse fields such as displays, solar cells, and biological assays [4].

Upconversion nanoparticles are typically composed of an inorganic host lattice and lanthanide dopant ions embedded in the host lattice [3]. Although the upconversion process primarily makes use of the ladder-like arranged 4f energy levels of the lanthanide dopant ions, the host lattice can significantly alter the upconversion processes by exerting a crystal field around the dopant ion and subtracting excitation energy of the dopant ion through lattice vibration. Therefore, a selection of appropriate host materials is essential in the synthesis of lanthanide-doped nanoparticles with favorable optical properties such as high upconversion efficiency and controllable emission profile. Upconversion emissions have been observed for a wide variety of host materials including oxides, bromides, chlorides, and fluorides [5]. Fluorides are commonly studied due to their low phonon energy and relatively high chemical stability [6]. For example, NaYF₄ has been generally regarded as the most efficient upconversion host for practical applications. To this regard, considerable efforts have been devoted to the synthesis and multicolor tuning of lanthanide-doped NaYF₄ upconversion nanoparticles [7].

As one of the most efficient host materials for upconversion process, LiYF₄ has also been frequently employed to fabricate lanthanide-doped upconversion crystals [8]. In contrast to NaYF₄ host matrix, LiYF₄ not only offers comparable emission intensity, but also generates additional emission lines [9]. Thus, LiYF₄ can provide us with a complementary host matrix to NaYF₄ for technical applications such as scintillation and tunable upconversion lasers [9b,10]. Despite the phenomenal growth in nanoparticle synthesis, there are relatively few reports on the synthesis of LiYF₄ nanoparticles due to formidable synthetic challenges. To the best of our knowledge, most LiYF₄ nanoparticles known to date have been synthesized by thermal decomposition [9]. Although this method enables the synthesis of monodisperse nanoparticles with high dispersability in organic solvents, the requirement of toxic rare-earth trifluoroacetate precursors and release of toxic byproducts raises issues of substantial environmental concern, thereby limiting its use for commercial purposes.

In this paper, we present a facile and relatively environmental-benign approach for the synthesis of monodisperse lanthanide-doped LiYF₄ nanoparticles. The synthetic strategy is based on the interaction of metal oleate with ammonium fluoride at room temperature and subsequent growth of the nanoparticles at elevated temperatures. We also show that multicolor tuning of the upconversion emission can be obtained by varying doping composition and concentration, while the size of the nanoparticles could be tuned by controlling the ratio of the fluoride to metal ions.

2 Experimental section

3 Results and discussion

The size and morphology of the lanthanide-doped LiYF₄ nanoparticles were first studied by transmission electron microscopy (TEM). Fig. 1a shows a representative TEM micrograph of Yb/Er (18/2 mol %) co-doped nanoparticles with an average diameter of 21 nm. High-resolution TEM (HRTEM) image reveals the highly crystalline nature of the as-synthesized LiYF₄ nanoparticles (Fig. 1b). The HRTEM image also shows lattice fringes with observed d-spacings of 0.46 nm and 0.30 nm that respectively correspond to lattice spacings in the (011) and (103) planes of tetragonal-phase LiYF₄ (Fig. 1b). The electron diffraction pattern shown in Fig. 1c obtained from Fourier transform of the HRTEM image confirms the single-crystalline tetragonal-phase of the nanoparticle. Compositional analysis of an individual particle by energy-dispersive X-ray spectroscopy (EDX) reveals the presence of the doped elemental Yb and Er (Fig. 1d). The samples were further studied by XRD and shown in Fig. 1e. All diffraction peaks can be well indexed in accordance with tetragonal LiYF₄ crystal (JCPDS file No. 81-2254), indicating the formation of pure tetragonal-phase of the nanoparticles. Importantly, these nanoparticles are well dispersed in a variety of nonpolar solvents including hexane, cyclohexane, dichloromethane, and toluene.

The same synthetic procedure was further used to synthesize LiYF₄ nanoparticles doped with other lanthanide ions such as Yb/Tm (20/0.2 mol %) and Yb/Ho (18/2 mol %). TEM images in Fig. 2 a and b show that the size and morphology of the nanoparticles closely resemble to those of Yb/Er co-doped LiYF₄ nanoparticle counterpart. These results demonstrate that the variation of dopant ions in low concentrations does not alter the particle growth process. However, under identical experimental conditions, the size of the nanoparticles is found to be dependent on the ratio of fluoride (F⁻) to rare-earth metal (RE³⁺) ions. As shown in Fig. 2c and 2d, large nanoparticles of about 40 nm were obtained as the ratio of F⁻ to RE³⁺ reached 8/3. With increase in the F⁻ to RE³⁺ ratio, a decrease in particle size was observed. Ultra small nanoparticles of about 12 nm can be obtained when the F⁻ to RE³⁺ ratio reached 6. The particle size variation as a function of F to rare-earth metal ratio can be explained by a crystallization speed-controlled model [11]. Because the particle growth was initiated with the reaction between the F⁻ ion and rare-earth oleate complex, a high concentration of F⁻ can significantly enhance particle crystallization rate. The crystallization acceleration effect related to high monomer concentration eventually leads to decreased critical size of the product [12].

Each lanthanide ion possesses a distinct set of energy levels that result in characteristic emissions at particular wavelengths. The upconversion emission from these nanoparticles can be readily manipulated by modifying doping compositions. Under excitation at 980 nm, the LiYF₄:Yb/Er (18/2 mol %) nanoparticles exhibit characteristic sharp emission peaks resulting from ²H_9/2 → ⁴I_15/2 (410 nm), ²H_11/2,⁴S_3/2 → ⁴I_15/2 (525–540 nm), and ⁴F_9/2 → ⁴I_15/2 (660 nm) transitions of Er³⁺ (Fig. 3a). These emissions lead to an overall green color light (Fig. 3a, insert). In contrast, the LiYF₄:Yb/Tm (20/0.2 mol %) nanoparticles exhibit a blue color emission resulting from ¹D₂ → ³F₄ (450 nm), ¹G₄ → ³H₆ (475 nm), and ¹G₄ → ³F₄ (650 nm) transitions of Tm³⁺ while the LiYF₄: Yb/Ho (18/2 mol %) nanoparticles display a yellow color emission due to ⁵F₄ → ⁵I₈ (525 nm), ⁵F₅ → ⁵I₈ (650 nm) transitions of Ho³⁺ (Fig. 3 b and c).

It should be noted that the Yb³⁺ in the host lattice does not emit visible light. However, the Yb³⁺ makes a significant contribution to the upconversion emissions by acting as a sensitizer that strongly absorbs the excitation irradiation and then transfers the excitation energy to the emitters (Fig. 4a). The energy transfer process finds reflection as a rising edge in the luminescence decay curves (Fig. 4 b–d). The decay curves also showed a nearly single-exponential decay in the tail. By fitting with a single exponential function I(t) = I₀ exp(−t/τ), the lifetimes for ²H_11/2, ⁴S_3/2, ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ are determined to be around 51, 54, and 56 μs, respectively. As expected, the luminescence lifetimes of ²H_11/2 and ⁴S_3/2 are the same since their populations are thermally coupled. The observed lifetimes for ²H_11/2, ⁴S_3/2 → ⁴I_15/2 transitions are basically consistent with those in the NaYF₄ counterparts, whereas much shorter lifetime for ⁴F_9/2 → ⁴I_15/2 transition is observed relative to that in NaYF₄ (∼141 μs) [13]. The significant difference in lifetime for ⁴F_9/2 → ⁴I_15/2 transition may be ascribed to dissimilar crystal-field surroundings of Er³⁺ ions embedded in LiYF₄ and NaYF₄.

In a further set of experiments, we demonstrated that upconversion color tuning can be achieved by utilizing a three-component doping system in a dual emission process. Control of the doping concentration in the host lattice enables precise modulation of the relative intensities of upconversion emissions and thus the color output. For example, the LiYF₄ nanoparticles triply-doped with Yb/Er/Tm show dual emissions of Er³⁺ and Tm³⁺. The relative intensity ratio of Er³⁺ and Tm³⁺ emissions can be manipulated by varying the doping concentration of Er³⁺ in the host lattice. In the absence of Er³⁺ dopant, the Yb/Tm co-doped nanoparticle system exhibits a blue color emission as previously discussed. Upon addition of a second emitter of Er³⁺ with increased dopant concentrations (0–1 mol %), the nanoparticle system exhibits significant changes in the green (²H_11/2,⁴S_3/2 → ⁴I_15/2) and red (⁴F_9/2 → ⁴I_15/2) spectral region (Fig. 5a), resulting in tunable color output from blue to green (Fig. 5c–g).

Dual emission can also be achieved by simultaneously doping Tm and Ho in the host lattice. In comparison with Yb/Tm co-doped sample, the Yb/Tm/Ho triply-doped system exhibits additional emission peaks of Ho in the green and red spectral region (Fig. 5b). This dual emission process provides an alternative route towards upconversion multicolor tuning. As the doping concentration of Ho³⁺ increases, the colloidal solution of the triply-doped nanoparticles shows a notable color change (Fig. 5 h and i). The absence of quenching in the emission intensity as evidenced by the examination through the naked eye in all the triply-doped samples indicates that there is negligible energy transfer and cross relaxation between different types of activators. This phenomenon can be attributed to the low concentration (< 1 mol) of dopant emitter ions in the host lattice [14].

4 Conclusions

In this work, tetragonal-phase LiYF₄ nanoparticles doped with upconverting lanthanide ions were synthesized by a novel solution-based method. Monodisperse nanoparticles with a tunable size can be readily obtained by varying the ratio of fluoride to rare-earth metal ions. The as-synthesized nanoparticles show intense upconversion emissions upon excitation at 980 nm.

The color output of the upconversion emission can be precisely manipulated by varying doping composition and concentration, providing potential applications for multiplex biological labeling and imaging.

Acknowledgements

X.L. acknowledges the Defence Science & Technology Agency (DSTA), the Singapore-MIT Alliance (SMA), and the Agency for Science, Technology and Research (A*STAR) for supporting this work. H.C. thanks the support from MOE (Grant No. ARC 27/07). X.C. acknowledges the support from the Hundreds of Talents Program of the Chinese Academy of Sciences (CAS), Instrument Developing Project of the CAS (Grant No. YZ200712), and the Key Project of International Cooperation of Fujian Province (Grant No. 2007I0024). We acknowledge Xiao Huang for high-resolution TEM Characterizations.