β-NaGdF4:Yb3+/Er3+ nanoparticles (NPs) co-doped with Mo3+ ions were prepared for the first time using a solvothermal process at 300 °C. The influence of the doping concentration of Mo3+ ions on the growth and upconversion (UC) luminescence properties of the resulting β-NaGdF4:Yb3+/Er3+ nanocrystals were investigated in detail. The morphology and size of the β-NaGdF4:Yb3+/Er3+ nanocrystals were influenced by the concentration of the Mo3+ dopant ions. A possible mechanism for this behavior is proposed in this report. It was found that the upconversion luminescence intensity of the green emissions of the upconversion NPs co-doped with 10% Mo3+ ions was enhanced 9 fold and that the red emission was enhanced 4 fold. The experimental results suggest that the enhanced UC luminescence from the Mo3+ doped β-NaGdF4:Yb3+/Er3+ nanocrystals may have potential practical applications.
© 2016 Optical Society of America
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  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 . Wang et al found that Cr3+ ion co-doped β-NaYF4:Yb3+/Er3+ nanoparticles exhibited enhanced UC luminescence . Our group has also co-doped K+ in β-NaYF4:Yb3+, Er3+ nanoparticles to enhance its upconversion fluorescence .
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.
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.
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.
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.
The emission decay curves of the 4S3/2/4I15/2 and 4F9/2/4I15/2 were measured under 980 nm excitation (Fig. 4). The lifetimes of the 4S3/2 and 4F9/2 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).
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.
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 4I15/2 are excited to the 4I11/2 level by the energy from the Yb3+ ion energy transfer. Again, this ion is excited to the 4F7/2 level by Yb3+ ion energy transfer. From the 4F7/2 level the ion relaxes to the 2H11/2/4S3/2 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 4I11/2 relaxes to the 4I13/2 level by nonradiative relaxation processes. Then, this ion is excited to the 4F9/2 level by the Yb3+ ion energy transfer. The transition from the 4F9/2 level to the ground state yields the red UC emission.
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.
We express thanks to: the National High Technology Research and Development Program of China (863 Program) under Grant No. 2013AA032205; the National Natural Science Foundation of China under Grant No. 51272022.; the Research Fund for the Doctoral Program of Higher Education Grant No. 20120009130005 and the Fundamental Research Funds for the Central Universities under Grant No.2012JBZ001.
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