1. Introduction

The field of photon upconversion has recently inspired the design of near-infrared (NIR) upconverters to generate visible light suitable for a variety of applications, such as solar cells, three-dimensional (3D) displays, biological imaging, medical therapy and light-emitting diodes [1–6]. These materials have been found to have a unique anti-stokes emission [7–13]. They possess a unique 4f electronic configuration, which enables the transition of the 4f electrons between the f-f and f-d orbitals. These materials normally possess high photochemical stability, low potential cytotoxicity and lack tissue auto-fluorescence which has attracted attention in the field of biomedical science. For instance, an upconverting phosphor technology-based lateral flow (UPT-LF) assay has been developed that uses a nanometer-sized luminous upconvering phosphor particles as the biolabel [14] and exhibits a 10-fold improvement over conventional assays. Among all of these UC materials Yb, Er, co-doped fluorides have been reported to be the most efficient upconversion materials for green emission due to their low phonon energy (~350 cm⁻¹), low non-radiative probability and enhanced luminescence intensity [15–17].

Upconversion luminescence intensities of rare earth ions are mainly dependent on electronic transition probabilities, which can be disrupted by the local crystal field of the rare-earth ions. Consequently, tailoring the local crystal field of the rare-earth ions can be an effective strategy to increase the UC emissions of various host materials doped with rare-earth ions [18, 19]. For example, Zhang’s group reported an enhancement of the visible UC radiation in Y₂O₃:Yb³⁺/Er³⁺ nanocrystals by codoping with Li⁺ ions [20]. Wang et al found that Cr³⁺ ion co-doped β-NaYF₄:Yb³⁺/Er³⁺ nanoparticles exhibited enhanced UC luminescence [21]. Our group has also co-doped K⁺ in β-NaYF₄:Yb³⁺, Er³⁺ nanoparticles to enhance its upconversion fluorescence [22].

Inspired by these studies, the upconversion luminescence of NaGdF₄:Yb³⁺/Er³⁺ codoped with Mo³⁺ ions was investigated in this reported study. The effects of the doping concentration of Mo³⁺ ions on the size and UC luminescence properties of NaGdF₄:Yb³⁺/Er³⁺ was systematically investigated and a possible mechanism for the resulting UC behavior is proposed.

2. Experimental

NaGdF₄ UCNPs codoped with 20 mol%Yb³⁺ and 2 mol% Er³⁺ and x mol% (x = 0, 5, 10, 15, 20) Mo³⁺ ions were synthesized using the solvothermal method following the protocols reported previously [23, 24]. In brief, 1 mmol of GdCl₃, YbCl₃, ErCl₃ and MoCl₃ in a ratio of (78-x):20:2 was mixed with 6 ml of OA and 15 ml of ODE were added to a 50 ml three-neck flask and heated to 160 °C until a homogeneous solution formed. After cooling to room temperature, 10 ml of a methanol solution containing a total of 2.5 mmol NaOH and 4 mmol NH₄F were slowly added into the flask, which resulted in the rapid formation of solid-state precipitates in the solution. The solution was stirred for 30 min and then heated slowly to 100 °C for another 30 min to remove the methanol steam and residual water. Subsequently, the solution was quickly heated up to 300 °C and maintained at this temperature for 1.5 h. The entire reaction process was carried out in an argon atmosphere. As the solution was cooled to room temperature the desired nanoparticle product was precipitated from ethanol and then washed three times with ethanol/deionized water (1:1 in volume). A portion of the resulting NPs were redispersed in 1 mol% in cyclohexane. The rest of the product was dried in a vacuum at 60 °C for several hours.

X-ray power diffraction (XRD) was performed on the product using a D/max 2200v X-ray powder diffractometer equipped with Cu Ka radiation (λ = 1.540 Å). Transmission electron microscopy (TEM) images were recorded on a HEOL-3000F TEM operated at an acceleration voltage of 200 kV. The upconversion (UC) luminescence spectra were recorded on a ZOLIX fluorescence spectrometer system, while the excitation source used was an external 980 nm semiconductor laser (ZOLIX INSTRUMENTS CO.LTD) with an optic fiber accessory. All spectral measurements were performed at room temperature.

3. Results and discussion

The crystallinity and phase transformation of the synthesized nanoparticles with various Mo³⁺ dopant concentrations were determined from the XRD patterns of the products (Fig. 1). All of the synthesized nanoparticles exhibited well-defined peaks, confirming that they were highly crystalline and had sharp diffraction peaks that were indexed to the standard XRD pattern of hexagonal NaGdF₄ (JCPDS.No.27-0699). No extra peaks or significant differences were identified even with the increase of the Mo³⁺ ions concentration to 20 mol%, which suggested that pure β-NaGdF₄ had been fabricated. This result indicated that the Mo³⁺ doping had no influence on the crystalline phase of the nanocrystals and Mo³⁺ had been successfully incorporated into the host lattice. In addition, as a result of the substitution of Gd³⁺ by the Mo³⁺ ions, the lattice constant and unit-cell volume increased with increasing Mo³⁺ ion concentration, which caused the diffraction peaks to shift to larger angles. This shift may have been caused by the alterations in the crystal cell volume following the Mo³⁺ doping.

 

Fig. 1 XRD patterns of NaGdF₄:20 mol%Yb³⁺/2 mol% Er³⁺xmol% Mo³⁺ (x = 0, 5, 10, 15, and 20 mol%) nanoparticles and amplified XRD patterns of the main diffraction peaks

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The shapes of the NaGdF₄:20 mol% Yb³⁺/2 mol% Er³⁺ nanocrystals prepared with different amounts of Mo³⁺ is shown in Figs. 2(a)-2(e). Elliptical nanocrystals with a width of 11 nm and a length of 14 nm were obtained by doping with various amountsof Mo³⁺ to 5 mol% in the experiments as shown in Fig. 2(b). The size of the nanocrystals was not uniform, but the range of particle size decreased as the Mo³⁺ concentration was increased. When the dopant concentration of Mo³⁺ ions was 10 mol% the resulting nanocrystals were reasonably well dispersed as shown by dynamic light scattering measurements indicating an average hydrodynamic diameter of approximately 13 nm (Figs. 2(c)-2(d)). In addition, as the dopant Mo³⁺ concentration was increased to 20 mol%, the corresponding TEM image shown in Fig. 2(e) reveals that the resulting nanocrystals had a hexagonal plate-like shape. In the EDS spectrum shown in Fig. 2(g) and 2(h), Yb, Gd, Er and Mo peaks are clearly evident. It was found that the peak intensity of Gd³⁺ decreased while the intensity of Mo³⁺ increased as the concentration of Mo³⁺ was increase, which further indicated that Gd³⁺ ions were replaced by Mo³⁺ in β-NaGdF₄ host lattice.

 

Fig. 2 (a)-(e) TEM images of NaGdF₄:20 mol%Yb³⁺/2 mol% Er³⁺ nanocrystals with xmol% Mo³⁺ ions (x = 0, 5, 10, 15, and 20 mol%); (f) the resulting diameter distribution; (g)-(h)EDS images with 5% mol and 10% mol Mo³⁺

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To analyze the luminescence properties of these prepared nanocrystals quantitatively, the nanocrystals were dispersed in cyclohexane and excited by a 980 nm·cw laser at a power density of 500 mW·cm⁻². Figure 3 shows the room-temperature upconversion emission spectra of the various NaGdF₄:Yb/Er nanocrystals. The UC emission intensity initially increased with the increase in Mo³⁺ dopant content from 0 to 10 mol% and then decreased when the Mo³⁺ content was increased to 15%. Notably, the UC emission intensity of the NaGdF₄:Yb/Er doped with 10 mol% Mo³⁺ was about 9 times higher than the NaGdF₄:Yb/Er without Mo³⁺ doping.

 

Fig. 3 (a) UC luminescence spectra of the NaGdF₄:20mol% Yb³⁺/2mol% Er³⁺ nanocrystals codoped with 0-20mol% Mo³⁺ions. (b) integral intensity of green and red emissions as a function of Mo³⁺ ion concentration.

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The emission decay curves of the ⁴S_3/2/⁴I_15/2 and ⁴F_9/2/⁴I_15/2 were measured under 980 nm excitation (Fig. 4). The lifetimes of the ⁴S_3/2 and ⁴F_9/2 states of the Er³⁺ ions in the β-NaGdF₄:Yb³⁺/Er³⁺ crystals codoped with Mo³⁺ were calculated by fitting the curve to a single exponential function. The lifetimes of emission 540 nm and 655 nm are prolonged with the increasing of Mo³⁺ ions concentration from 0 mol% to 10 mol%. Then lifetimes are shortened with the concentration of Mo³⁺ ions further increasing. As is well known, the inverse of the lifetime is equal to the sum of the nonradiative transition and radiative transition. So it can be concluded that doping with 10 mol% Mo³⁺ can tailor the local crystal field around Er³⁺ and reduce the nonradiative transition. These results are consistent with the effects of Mo³⁺ ions on UC luminescence properties of β-NaGdF₄:Yb³⁺/Er³⁺ crystals (Fig. 3).

 

Fig. 4 Lifetimes of the (a) ⁴S_3/2→⁴I_15/2 and (b)⁴F_9/2→⁴I_15/2 states in NaGdF₄:Yb³⁺/Er³⁺ crystals codoped with 0-20 mol% Mo³⁺ ions.

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To understand the UC process in more detail, the excitation power dependence of the UC luminescence intensities was investigated. The dependence of the green and red emissions on the excitation power were calculated according to Auzel’s method, IPⁿ, where P is the pumping laser power and n is the number of laser photons required to populate the upper emitting state. Figures 5(a) and 5(b) show the typical pump-power dependence of NaGdF₄:Yb/Er with 0 mol% Mo³⁺ and 10 mol% Mo³⁺. The slope n, in this plot is close to 2, which indicates that a two-photon process is primarily responsible for the green and red upconversion emissions.

 

Fig. 5 Power dependence of UC emissions of NaGdF₄:Yb,Er of (a) 10 mol% and (b) 0 mol% dispersions in cyclohexane with a 980 nm semiconductor laser.

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The two-photon UC process is shown schematically in Fig. 6 to help explain the emissions. In the two-photon green UC luminescence process, the Er³⁺ ions in the ground state ⁴I_15/2 are excited to the ⁴I_11/2 level by the energy from the Yb³⁺ ion energy transfer. Again, this ion is excited to the ⁴F_7/2 level by Yb³⁺ ion energy transfer. From the ⁴F_7/2 level the ion relaxes to the ²H_11/2/⁴S_3/2 level by nonradiative relaxation processes. The transitions from these states to the ground state yield the green UC emission. For the two-photon red UC luminescence process, the Er³⁺ ion in the excited state ⁴I_11/2 relaxes to the ⁴I_13/2 level by nonradiative relaxation processes. Then, this ion is excited to the ⁴F_9/2 level by the Yb³⁺ ion energy transfer. The transition from the ⁴F_9/2 level to the ground state yields the red UC emission.

 

Fig. 6 Energy transfer mechanism diagram of NaGdF₄:Yb,Er

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4. Conclusions

In conclusion, Mo³⁺ ions in low concentration were doped into hexagonal NaGdF₄ nanocrystals for the first time. The influence of the doping concentration of Mo³⁺ ions on the growth and upconversion luminescence properties of β-NaGdF₄:Yb³⁺/Er³⁺ nanocrystals were investigated in detail. The morphology and size of β-NaGdF₄:Yb³⁺/Er³⁺ nanocrystals were influenced by concentration of the Mo³⁺ dopant ions. It was found that the upconversion luminescence intensities of the green emission of upconversion NPs co-doped with 10% Mo³⁺ ions was enhanced 9 fold and the red emission was enhanced 4 fold. These results suggest that the enhanced UC luminescence from the Mo³⁺ doped β-NaGdF₄:Yb³⁺/Er³⁺ nanocrystals may have potential pratical applications.