1. Introduction

Luminescent materials based on lanthanide (Ln³⁺) ions, which can convert the incident irradiation into useful wavelengths, have been extensively studied due to their wide ranges of applications, such as solid-state lasers, displays, fluorescent lamps, white light-emitting diodes, optical communications, and sensors [1–11]. Ln³⁺ ions possess rich energy levels arising from the 4fⁿ configurations, and the electronic transitions with these energy levels can induce various emissions with wavelength spanning from ultraviolet to infrared regions. In recent years, Ln³⁺-doped upconversion nanoparticles have received renewed attentions because they hold great promise for solar energy conversion and bioimaging [12–19]. Upconversion is an amazing nonlinear luminescence phenomenon that can generate one high-energy photon by absorbing two or more lower-energy photons. In the case of bioimaging application, upconversion nanoparticles exhibit distinctive advantages over conventional biomarkers such as quantum dots and organic fluorescent dyes, owing to their unique optical properties, including narrow bandwidth, long-lived emission, large anti-Stokes shifts, and near-infrared (NIR) radiation. Both quantum dots and organic dyes are generally excited with ultraviolet and visible light, which could cause autofluorescence and photo-damage of biological tissues. Moreover, organic dyes often suffer from intrinsic photobleaching and low emission efficiency, whereas the toxicity of quantum dots is another big concern. In sharp contrast, upconversion nanoparticles are excited by NIR light, and thus they have many merits such as the absence of photodamage to live organisms, low autofluorescence background, high signal-to-noise ratio and high tissue penetration depth [20,21]. Furthermore, upconversion nanoparticles have good photostability and biocompatibility, and consequently, they are considered as promising bioprobes for in vitro and in vivo optical imaging.

In upconversion nanoparticles, Ln³⁺ ions such as Er³⁺, Tm³⁺ and Ho³⁺ are typically used as activator ions, because they possess ladder-like energy levels, where consecutive absorption of low-energy light pushes electrons up to higher energies. To enhance upconversion luminescence efficiency, Yb³⁺ ions with larger absorption cross-sections in the NIR region are frequently co-doped as sensitizer ions. The luminescence efficiency of upconversion nanoparticles depends strongly on the host materials. It has been demonstrated that fluorides, in particular, NaREF4 (RE = rare earth) materials, generally possess higher upconversion efficiency than other host materials (such as oxides), since their low phonon energies (< 400 cm⁻¹) can reduces the nonradiative decay multiphonon relaxations [22–33]. Among fluorides, NaYbF₄ is regarded as an important host material for producing upconversion luminescence [34,35].

In this paper, we present an effective strategy to simultaneously tune the particle size and the upconversion luminescence of NaYbF₄:Er³⁺/Tm³⁺ nanoparticles through varying the sensitizer ytterbium concentration via Y³⁺ or Gd³⁺ ions doping. Various amounts of Y³⁺ or Gd³⁺ ions from 20 to 40 mol% were doped into the fluoride nanoparticles, and the particle size was found to be decreased continuously from 250 nm to a minimum of 15.8 nm without phase transformation. For NaYbF₄:2%Er³⁺ nanoparticles, the intensity ratio of red emission to green emission decreases monotonically with increasing Y³⁺ or Gd³⁺ doping concentrations, resulting in tunable multicolor upconversion emissions. Particularly, compared with NaYbF₄:2%Er³⁺ nanoparticles, the green upconversion emission was enhanced by 21-fold for NaYbF₄:2%Er³⁺/30%Gd³⁺ nanoparticles. For NaYbF₄:2%Tm³⁺ nanoparticles, the luminescence intensity ratio of the NIR emission at 800 nm to the blue emission at 475 nm was greatly enhanced by 36 and 17 times for doping of 40 mol%Y³⁺ and 20 mol%Gd³⁺ ions, respectively. These results presented here provide a paradigm for simultaneous control of the size and luminescence properties of Ln³⁺-doped upconversion nanoparticles.

2. Experimental

Yb(CH₃CO₂)₃•4H₂O (99.9%), Y(CH₃CO₂)₃•xH₂O (99.9%), Gd(CH₃CO₂)₃•xH₂O (99.9%), Er(CH₃CO₂)₃•xH₂O (99.9%), Tm(CH₃CO₂)₃•xH₂O (99.9%), Ho(CH₃CO₂)₃•xH₂O (99.9%), NaOH (98 + %), NH₄F (98 + %), 1-octadecene (90%), and oleic acid (90%) were purchased from Sigma-Aldrich and were used directly as the raw materials without further purification. Distilled water was used throughout.

For example, for synthesis of NaYbF₄:2%Er³⁺ nanoparticles, in a typical procedure, the water solution (2 mL, total 0.4 mmol) of Yb(CH₃CO₂)₃ (0.392 mmol) and Er(CH₃CO₂)₃ (0.008 mmol) was firstly added together to a three-neck round-bottom flask (50 mL). Then, oleic acid (3 mL) and 1-octadecene (7 mL) were also added. Subsequently, the resulting mixture solution was heated at 150 °C for 1 h under stirring for forming the lanthanide-oleate complexes. After cooled down to room temperature, methanol solution (6 mL) containing NH₄F (1.6 mmol) and NaOH (1 mmol) was added and the obtained solution was stirred at 50 °C for 30 min. Subsequently, the reaction temperature was elevated to 100 °C for 30 min in order to remove the methanol from the reaction mixture. After the methanol was evaporated, the solution was heated at 290 °C under argon for 1.5 h and then cooled down to room temperature. Finally, the obtained nanoparticles were precipitated by addition of ethanol, collected by centrifugation at 6000 rpm for 5 min, washed with ethanol for several times, and then dispersed in cyclohexane (4 mL).

The crystalline phases of the of the synthesized samples were examined by x-ray diffraction (XRD) patterns recorded on a Bruker D8 Advance diffractometer using graphite monochromatized Cu Kα radiation (40 kV, 40 mA, λ = 1.5406 Å). Transmission electron microscopy (TEM) images were recorded using a JEOL-JEM 2010F field emission transmission electron microscope operated at an acceleration voltage of 200 kV. The upconversion emission spectra were obtained by an Edinburgh FLS920 spectrometer equipped with a 980 nm continuous-wave diode laser as the excitation source. All the measurements were carried out at room temperature.

3. Results and discussion

The crystal structures and the phase purity of the nanoparticle samples were identified by XRD. Figure 1 shows the XRD patterns of NaYbF₄:2%Er³⁺, NaYbF₄:2%Er³⁺/x%Y³⁺, and NaYbF₄:2%Er³⁺/x%Gd³⁺ (x = 20, 30, and 40) nanoparticles. All of the XRD patterns could clearly be indexed to the pure hexagonal phased NaYbF₄ (JCPDS 27-1427), and no trace of other phases or impurities were observed, indicating all the Y³⁺ or Gd³⁺ ions are incorporated into the NaYbF₄ host matrix and formed a solid solution structure. Furthermore, it can be observed that with increasing doping contents of Y³⁺ and Gd³⁺ ions, the diffraction peaks gradually broaden, which can be attributed to the reduction of particle size. This result can be verified by the TEM images of these samples (see Fig. 2). In addition, due to the substitution of Yb³⁺ ions (radius = 0.868 Å) by the larger Y³⁺ ions (radius = 0.893 Å) or Gd³⁺ ions (radius = 0.938 Å), the lattice constant and unitcell volume were found to increase with the increasing the doping concentrations of Y³⁺ or Gd³⁺ ions. The values of the lattice constant and unit-cell volumes were determined on basis of the XRD results and are presented in Table 1.

 

Fig. 1 XRD patterns of the (a) hexagonal NaYbF₄ (JCPDS No. 27-1427), (b) NaYbF₄:2%Er³⁺, (c) NaYbF₄:2%Er³⁺/20%Y³⁺, (d) NaYbF₄:2%Er³⁺/30%Y³⁺, (e) NaYbF₄:2%Er³⁺/40%Y³⁺, (f) NaYbF₄:2%Er³⁺/20%Gd³⁺, (g) NaYbF₄:2%Er³⁺/30%Gd³⁺, and (h) NaYbF₄:2%Er³⁺/40%Gd³⁺ nanoparticles.

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Fig. 2 TEM images of the as-prepared (a) NaYbF₄:2%Er³⁺, (b) NaYbF₄:2%Er³⁺/20%Y³⁺, (c) NaYbF₄:2%Er³⁺/30%Y³⁺, (d) NaYbF₄:2%Er³⁺/40%Y³⁺, (e) NaYbF₄:2%Er³⁺/20%Gd³⁺, (f) NaYbF₄:2%Er³⁺/30%Gd³⁺, and (g) NaYbF₄:2%Er³⁺/40%Gd³⁺ nanoparticles.

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Table 1. The lattice constants, unit-cell volumes and the NCs diameters of the NaYbF₄:2%Er³⁺ nanoparticles doped with different concentrations of Y³⁺ or Gd³⁺ ions

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The particle size and morphology of the as-prepared nanoparticles were investigated by TEM. Figure 2 shows the TEM images of NaYbF₄:2%Er³⁺, NaYbF₄:2%Er³⁺/x%Y³⁺, and NaYbF₄:2%Er³⁺/x%Gd³⁺ (x = 20, 30, and 40) nanoparticles with different doping concentrations of Y³⁺ and Gd³⁺ ions. As can be seen in Fig. 2(a), the pure NaYbF₄:2%Er³⁺ nanoparticles are large monodisperse hexagonal plates with an average diameter of about 250 nm. In sharp contrast, with the introduction of Y³⁺ and Gd³⁺ ions into NaYbF₄:2%Er³⁺ nanoparticles, the resulting nanoparticles appear nearly spherical in shape and much more uniform, and the particle size of Na(Yb,Y/Gd)F₄:2%Er³⁺ nanoparticles reduces obviously upon increasing Y³⁺ or Gd³⁺ contents. By randomly measuring particle size of 80 nanoparticles from the TEM images in Fig. 2(b–f), size distributions of Na(Yb,Y/Gd)F₄:2%Er³⁺ nanoparticles were determined, as shown in Fig. 3. The average diameter was dramatically decreases from 250 nm for pure NaYbF₄:2%Er³⁺ nanoparticles to 34.8 nm for NaYbF₄:2%Er³⁺/40%Y³⁺ nanoparticles. Compared with Y³⁺ doping, the particle size decreases more rapidly via Gd³⁺ doping. Particularly, the sample of NaYbF₄:2%Er³⁺/40%Gd³⁺ exhibits an average diameter of 15.8 nm. These results obviously indicate that the Y³⁺ and Gd³⁺ doping indeed alters the particle growth process. The variation of particle size can be attributed to the effect of the Y³⁺ and Gd³⁺ dopants on crystal growth rate through surface charge modification. In a previous study by Wang et al. [36], the authors found that for NaYF₄ nanoparticles, the substitution of Y³⁺ ions (radius = 0.893 Å) by the larger Gd³⁺ ions (radius = 0.938 Å) resulted in reduction of particle size. Wang et al. demonstrated that the electron charge density of the crystal surface was increased with Gd³⁺ doping, thus repelling the negatively charged F⁻ ions to produce a smaller nanoparticle size [36]. Similarly, in our case, when Yb³⁺ ions (radius = 0.868 Å) was replaced by larger Y³⁺ ions (radius = 0.893 Å) or Gd³⁺ ions (radius = 0.938 Å), a tunable reduction of the particle size was observed.

 

Fig. 3 Size distributions of the (a) NaYbF₄:2%Er³⁺/20%Y³⁺, (b) NaYbF₄:2%Er³⁺/30%Y³⁺, (c) NaYbF₄:2%Er³⁺/40%Y³⁺, (d) NaYbF₄:2%Er³⁺/20%Gd³⁺, (e) NaYbF₄:2%Er³⁺/30%Gd³⁺, and (f) NaYbF₄:2%Er³⁺/40%Gd³⁺ nanoparticles.

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Efficient tuning of the upconversion emissions in Ln³⁺-doped nanoparticles is very important for multicolor imaging, multiplexed detection, and multicolor displays [37–40]. An effective strategy for manipulating upconversion color output is to vary the dopant species and concentrations of Ln³⁺ ions [41,42]. Figure 4 shows the upconversion emission spectra of the NaYbF₄:2%Er³⁺, NaYbF₄:2%Ho³⁺, and NaYbF₄:0.5%Tm³⁺ nanoparticles under 980 nm excitation. For NaYbF₄:2%Er³⁺, two green emissions at 524 and 543 nm, which can be attributed to the ²H_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2 transitions of Er³⁺, and a red emission at 656 nm, which can be assigned to the ⁴F_9/2→⁴I_15/2 transition of Er³⁺, were observed. The luminescence intensity ratio of red emission to green emission is 5.57, and thus the NaYbF₄:2%Er³⁺ nanoparticles show bright orange-color upconversion emission. The NaYbF₄:2%Ho³⁺ nanoparticles exhibit green color emission, due to a combination of a strong green emission at 539 nm (⁵F₄→⁵I₈ transition) and a relatively weak red emission at 645 nm (⁵F₅→⁵I₈ transition). The NaYbF₄:0.5%Tm³⁺ nanoparticles display pure blue color emissions, corresponding to the ¹D₂→³F₄ (451 nm), ¹G₄→³H₆ (475 nm), and ¹G₄→³F₄ (646 nm) transitions of Tm³⁺ ions, respectively.

 

Fig. 4 Upconversion emission spectra of the (a) NaYbF₄:2%Er³⁺, (b) NaYbF₄:2%Ho³⁺, and (c) NaYbF₄:0.5%Tm³⁺ nanoparticles under 980 nm excitation.

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Furthermore, multicolor upconversion tuning can be realized in the Er³⁺/Ho³⁺, Tm³⁺/Er³⁺ and Tm³⁺/Ho³⁺ co-doped NaYbF₄ nanoparticles. Figure 5(a) shows the upconversion emission spectra of the NaYbF₄:1%Tm³⁺/1%Ho³⁺, NaYbF₄:1%Tm³⁺/1%Er³⁺ and NaYbF₄:1%Er³⁺/1%Ho³⁺ nanoparticles under 980 nm excitation. Due to the dual emission process, the NaYbF₄:1%Tm³⁺/1%Ho³⁺ and NaYbF₄:1%Tm³⁺/1%Er³⁺ nanoparticles exhibit white light emission, while the NaYbF₄:1%Er³⁺/1%Ho³⁺ nanoparticles show green-orange color emission. According to the upconversion emission spectra in Fig. 4 and Fig. 5(a), the Commission International del’Eclairage (CIE) chromaticity coordinates of these nanoparticles were calculated. The values are (0.4979, 0.4633), (0.2843, 0.6854), (0.1463, 0.0607), (0.1669, 0.1503), (0.3311, 0.3354), and (0.3736, 0.6068) for NaYbF₄ nanoparticles doped with 2%Er³⁺, 2%Ho³⁺, 0.5%Tm³⁺, 1%Tm³⁺/1%Ho³⁺, 1%Tm³⁺/1%Er³⁺, and 1%Er³⁺/1%Ho³⁺, respectively. The corresponding CIE chromaticity diagram is depicted in Fig. 5(b).

 

Fig. 5 (a) Upconversion emission spectra of the (a) NaYbF₄:1%Tm³⁺/1%Ho³⁺, (b) NaYbF₄:1%Tm³⁺/1%Er³⁺, and (c) NaYbF₄:1%Er³⁺/1%Ho³⁺ nanoparticles under 980 nm excitation. (B) The CIE chromaticity diagram of (a) NaYbF₄:2%Er³⁺, (b) NaYbF₄:2%Ho³⁺, (c) NaYbF₄:0.5%Tm³⁺, (d) NaYbF₄:1%Er³⁺/1%Ho³⁺, (e) NaYbF₄:1%Tm³⁺/1%Er³⁺, and (f) NaYbF₄:1%Tm³⁺/1%Ho³⁺ nanoparticles upon excitation at 980 nm.

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Figure 6 shows the normalized upconversion emission spectra of NaYbF₄:2%Er³⁺/x%Y³⁺ and NaYbF₄:2%Er³⁺/x%Gd³⁺ (x = 20, 30, and 40) nanoparticles with different doping concentrations of Y³⁺ and Gd³⁺ ions. As can been seen obviously in Fig. 6(a) and Fig. 6(b), the addition of Y³⁺ and Gd³⁺ ions into NaYbF₄:2%Er³⁺ nanoparticles resulted in a gradual increase in green emissions of Er³⁺ ions. With the increasing Y³⁺ concentration from 0 to 40 mol%, the luminescence intensity ratio of red emission to green emission of NaYbF₄:2%Er³⁺/x%Y³⁺ nanoparticles was decreased from 5.57 (x = 0 mol%) to 2.21 (x = 40 mol%), as shown in Fig. 6(c). In the case of Gd³⁺ doping, the change on the upconversion emission spectra profile are much more larger compared with Y³⁺ doping. The luminescence intensity ratio of red emission to green emission in NaYbF₄:2%Er³⁺/x%Gd³⁺ (x = 20, 30, and 40) nanoparticles was found to be 0.7, 0.26, and 0.27 for x = 20, 30, and 40, respectively. This means the introduction of 30 mol%Gd³⁺ into NaYbF₄:2%Er³⁺ nanoparticles can give rise to a maximum enhancement of 21-fold in the green emissions. Based on the upconversion emission spectra in Fig. 6, their corresponding CIE coordinates were determined and plotted in Fig. 7. As can be seen clearly in Fig. 7, multicolor upconversion emission has been realized in NaYbF₄:Er³⁺ nanoparticles through Y³⁺ and Gd³⁺ doping, suggesting these nanoparticles have promising applications as bioprobes for multicolor imaging and multiplexed detection in view of their small particle size [43,44].

 

Fig. 6 Upconversion emission spectra of (a) NaYbF₄:2%Er³⁺/x%Y³⁺ and (b) NaYbF₄:2%Er³⁺/x%Gd³⁺ (x = 20, 30, and 40) nanoparticles with different doping concentrations of Y³⁺ and Gd³⁺ ions. All the emission spectra were normalized to the red emission of Er³⁺ ions at 655 nm. The corresponding diagrams of R/G values (intensity ratio of red to green emission) versus doping concentrations of (c) Y³⁺ and (d) Gd³⁺ ions.

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Fig. 7 The CIE chromaticity diagram of (a) NaYbF₄:2%Er³⁺, (b) NaYbF₄:2%Er³⁺/20%Y³⁺, (c) NaYbF₄:2%Er³⁺/40%Y³⁺, (d) NaYbF₄:2%Er³⁺/20%Gd³⁺, and (e) NaYbF₄:2%Er³⁺/40%Gd³⁺ nanoparticles upon excitation at 980 nm.

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NaYbF₄ is different from other fluoride hosts because NaYbF₄ itself contains Yb³⁺ ions, which service as sensitizers. For example, NaYF₄:Yb³⁺/Er³⁺ and NaGdF₄:Yb³⁺/Er³⁺ nanoparticles are well-known to give green-emitting upconversion emissions when excited at 980 nm. In sharp contrast, the NaYbF₄:2%Er³⁺ nanoparticles show red emission. The possible upconversion mechanisms for the NaYbF₄:2%Er³⁺ nanoparticles are proposed on the basis of the simplified energy level diagram, as illustrated in Fig. 8. Under 980 nm laser excitation, the Yb³⁺ ion can be excited to the ²F_5/2 state by ground state absorption. Then, the excited Yb³⁺ ion transfers its energy to the nearby Er³⁺ ion through successive two-step energy-transfer processes, resulting in the population of ⁴I_11/2 and ⁴F_7/2 intermediate states of Er³⁺ ion. The Er³⁺ ion populated in the ⁴F_7/2 level will decay non-radiatively to the ²H_11/2, ⁴S_3/2 and ⁴F_9/2 level by multi-phonon relaxation process and subsequently give rise to green (524 and 543 nm) and red (656 nm) emissions. This intense red emission was attributed to the efficient cross-relaxation process (⁴F_7/2 + ⁴I_11/2 →□⁴F_9/2 + ⁴F_9/2) of Er³⁺, which consequently leads to the dominant population of the Er³⁺:⁴F_9/2 excited state and then makes the ⁴F_9/2 →□⁴I_15/2 transition of Er³⁺ more efficient [45,46]. However, with the introduction of Y³⁺ and Gd³⁺ ions into NaYbF₄:2%Er³⁺ nanoparticles, the Yb³⁺ concentration decreases, thus resulting in reduced energy transfer from Yb³⁺ to Er³⁺ ions and less population in the ⁴I_11/2 or ⁴F_7/2 states of the Er³⁺ ions. Finally, the cross-relaxation process of (⁴F_7/2 + ⁴I_11/2 →□⁴F_9/2 + ⁴F_9/2) between two nearby Er³⁺ ions will be weaken, which induces less population of the ⁴F_9/2 state and consequently a weaker red emission.

 

Fig. 8 Schematic energy-level diagram of Yb³⁺ and Er³⁺, and the proposed mechanism of upconversion luminescence. CR denotes cross-relaxation.

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In order to validate the general applicability of Y³⁺ and Gd³⁺ doping in Ln³⁺-doped NaYbF₄ nanoparticles for realizing upconversion emission tuning, we further investigated the upconversion luminescence properties of NaYbF₄:Tm³⁺/Y³⁺/Gd³⁺ nanoparticles. Figure 9 shows the normalized upconversion emission spectra of NaYbF₄:x%Tm³⁺ (x = 0.5 and 2), NaYbF₄:2%Tm³⁺/x%Y³⁺ (x = 0 and 40), and NaYbF₄:2%Tm³⁺/x%Gd³⁺ (x = 0 and 20) nanoparticles under 980 nm excitation. Apparently, the introduction of Y³⁺ and Gd³⁺ ions into NaYbF₄:2%Tm³⁺ nanoparticles induced great enhancement in the NIR emission at 800 nm (³H₄→³H₆ transition), as shown in Fig. 9(b) and Fig. 9(c). The luminescence enhancement factor reaches 36- and 17-fold for 40%Y³⁺ doping and 20%Gd³⁺ doping, respectively. The intense NIR upconversion emissions make these NaYbF₄:2%Tm³⁺/40%Y³⁺ nanoparticles promising for high-contrast deep tissue bioimaging [47,48]. Previous studies have demonstrated that increasing Tm³⁺ concentration is a facile approach to enhance the NIR-to-NIR upconversion luminescence (980 nm excitation and 800 nm emission), owing to the efficient cross-relaxation process of ¹G₄ + ³F₄→ ³H₄ + ³F₂ [49,50]. As shown in Fig. 9(a), the luminescence intensity of NIR emission at 800 nm of NaYbF₄:2%Tm³⁺ nanoparticles is about 3 times higher than that of NaYbF₄:0.5%Tm³⁺ nanoparticles. Obviously, compared with elevating the Tm³⁺ contents, Y³⁺ doping is a more effective strategy for achieving NIR-to-NIR upconversion emission enhancement in NaYbF₄:Tm³⁺ nanoparticles.

 

Fig. 9 Upconversion emission spectra of different Tm³⁺-doped nanoparticles under 980 nm excitation. (a) NaYbF₄:x%Tm³⁺ (x = 0.5 and 2), (b) NaYbF₄:2%Tm³⁺/x%Y³⁺ (x = 0 and 40), and (c) NaYbF₄:2%Tm³⁺/x%Gd³⁺ (x = 0 and 20). All the emission spectra were normalized to the blue emission of Tm³⁺ ions at 475 nm.

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Figure 10 shows the energy level diagram of Yb³⁺ and Tm³⁺ ions and the possible upconversion mechanism for NaYbF₄:Tm³⁺ nanoparticles. Under a 980 nm excitation, the Yb³⁺ ions are excited from the ²F_7/2 ground state to the ²F_5/2 state then transfer their excited energies to the neighbouring Tm³⁺ ions, and then four successive energy-transfer processes from Yb³⁺ to Tm³⁺ ions result in population of the ³H₅, ³F₂, ¹G₄, and ¹D₂ states of Tm³⁺ ions. Subsequently, a series of emissions are generated, which can be ascribed to the ¹D₂→³F₄ (451 nm), ¹G₄→³H₆ (475 nm), ¹G₄→³F₄ (646 nm), and ³H₄→³H₆ (800 nm) transitions of Tm³⁺ ions. At elevated Tm³⁺ concentration, cross-relaxation process of ¹G₄ + ³F₄→ ³H₄ + ³F₂ becomes efficient, leading to enhanced population of the ³H₄ state. As a result, improved intensity of NIR emission (³H₄→³H₆ at 800 nm) and suppressed visible emissions (¹G₄→³H₆ at 475 nm, ¹G₄→³F₄ at 646 nm) were observed, as shown in Fig. 9(a). In NaYbF₄:Tm³⁺ nanoparticles, the luminescence intensity of 800 nm NIR emission was lower compared to the blue emissions, which can be attributed to the back-energy-transfer from the ³H₄ state of Tm³⁺ ion to the ²F_5/2 state of a nearby Yb³⁺ ion, as illustrated in Fig. 10. The energy gap between the ³H₄ state of Tm³⁺ ion to the ²F_5/2 state of Yb³⁺ ion is about 2460 cm⁻¹, which can be compensated with the assistance of phonons or ligand groups with high energy vibrations. Such back-energy-transfer from Tm³⁺ to Yb³⁺ ions in NaYbF₄ host will cause the reduced population of the ³H₄ state via long-distance energy migration that takes excitation energy to the lattice or surface defects by non-radiative relaxation. Thus, relatively weak NIR-to-NIR upconversion luminescence was observed in NaYbF₄:Tm³⁺ nanoparticles. Notably, doping of Y³⁺ and Gd³⁺ ions into NaYbF₄:2%Tm³⁺ nanoparticles greatly enhanced the 800 nm NIR emission intensity. The possible reason is that: introducing Y³⁺ or Gd³⁺ into the framework of the NaYbF₄ host could induce structural inhomogeneity since the smaller rare earth ion (Yb³⁺) was replaced by the larger rare earth ion (Y³⁺ or Gd³⁺) [51], which effectively prevents the above-mentioned energy-loss through the back-energy-transfer from the Tm³⁺:³H₄ state to the Yb³⁺:²F_5/2 state and subsequent long-distance energy migration among Yb³⁺ ions.

 

Fig. 10 Schematic energy-level diagram of Yb³⁺ and Tm³⁺, and the proposed mechanism of upconversion luminescence. CR denotes cross-relaxation, BET represents back-energy-transfer.

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4. Conclusion

In summary, monodispsered NaYbF₄:Ln³⁺ (Ln = Er, Tm, Ho) upconversion nanoparticles with diameter of about 250 nm were synthesized by a facile chemical coprecipitation method. The tunable multicolor upconversion emissions, including green, orange, blue, and white emissions, can be readily achieved from these nanoparticles under the excitation of a 980 nm diode laser, depending on the dopant species and concentrations. More importantly, tunable upconversion luminescence with particle size decrease have been realized in NaYbF₄:Er³⁺/Tm³⁺ nanoparticles through doping with Y³⁺ or Gd³⁺ ions. There was no phase transformation during the Y³⁺ or Gd³⁺ doping. It was found that with increasing Y³⁺ or Gd³⁺ doping concentrations from 20 to 40 mol%, the NaYbF₄:2%Er³⁺ nanoparticles size continuously decreases while the luminescence intensity ratio of red emission to green emission also decreases monotonically. Compared with NaYbF₄:2%Er³⁺ nanoparticles, the green upconversion emission was enhanced by a factor of 21 for NaYbF₄:2%Er³⁺/30%Gd³⁺ nanoparticles. For NaYbF₄:2%Tm³⁺ nanoparticles, the luminescence intensity ratio of the NIR emission at 800 nm to the blue emission at 475 nm was greatly enhanced via doping of Y³⁺ or Gd³⁺ ions. For example, doping 40 mol% Y³⁺ ions into NaYbF₄:2%Tm³⁺ nanoparticles resulted in 36-fold enhancement in the 800 nm NIR emission. These Ln³⁺-doped NaYbF₄ upconversion nanoparticles have promising applications in optical bioimaging and multicolor displays.