1. Introduction

Lanthanide-doped fluorides have been widely studied in frequency upconversion for potential applications such as 3-D display and bio-labeling. More interesting application of upconversion is to achieve laser operation at various waveguides in the (ultraviolet) UV and visible region under pumping of near-infrared laser. To enhance the upconversion emissions of lanthanide activators, Yb³⁺ is widely applied as the sensitizer which absorbs the energy of pumping near-infrared laser, typically at 976 nm, and transfers the energy to the activators. Tm³⁺, Er³⁺, and Ho³⁺ are well known activators for upconversion in the UV and visible region under excitation of near-infrared laser. High order upconversion processes, such as four- and five-photon upconversion, can occur in these activators. [1–4] To achieve high optical gain for the visible upconversion emissions which are generated by two- or three-photon upconversion process, those high order upconversion processes which can depopulate the electrons on the low lying energy levels to reduce the optical gain should be avoided. Tb³⁺ is an excellent activator for green emissions under UV even X-ray radiation. [5–8] By a two-photon upconversion process, intense green emissions of Tb³⁺ (radiation from the ⁵D₄ level) have been widely studied in Tb³⁺–Yb³⁺ co-doped crystals and glass. [9–12] Generally, green emissions from the ⁵D₄ level is the main emission for Tb³⁺, which is excited by a co-operative sensitizing of two Yb³⁺ ions. Because of the existence of the cross relaxation ⁵D₃ +⁷F₆ → ⁵D₄ +⁷F₀ between Tb³⁺ pairs, when the concentrations are carefully controlled, the emissions from the ⁵D₃ level of Tb³⁺ can be observed in the UV and blue region. [13,14] The electrons on the ⁵D₃ level are populated by nonradiative relaxation from the ⁵D₁ level which is populated by a three-photon upconversion process. [14] There are many energy levels lying between the ⁵D₁ and ⁵D₃ level of Tb³⁺ in the 4f⁸ configuration. [15,16] However, the observed upconversion emissions are merely generated from the ⁵D₃ and ⁵D₄ levels. According to the theory of multiphonon relaxation [17], the probability of nonradiative transition of electrons on energy levels higher than the ⁵D₃ level is high, since the gaps between these levels are too small (several hundreds wavenumber). It is quite difficult to observe the emissions from high energy levels of Tb³⁺. Lights of high energy, e.g. 172, 215, 225, 250, 274, 284, and 325 nm laser, even X-ray were utilized to pump Tb³⁺-doped various crystals, but only emissions from the ⁵D₃ and ⁵D₄ level can be observed. [5,12,18–23] Due to the specific energy level structure of Tb³⁺, CW laser operation at 543 nm has been achieved based on the emissions from the ⁵D₄ level [24], and the optical gain of the emissions from the ⁵D₃ level could be expected. Investigation of the high energy levels is helpful to understand the process of populating the ⁵D₃ for UV gain media. To the best of our knowledge, there are rare reports about the emissions from the high energy levels of Tb³⁺.

Here, we reported the observation of upconversion emissions from high energy levels of Tb³⁺ in NaYF₄ microcrystals. NaYF₄ is known as one of the most efficiency host for lanthanide-doped upconversion luminescence. Under excitation by a 976 nm laser diode, the UV emissions at 325, 339, 351, 361, and 371 nm were generated from the ⁵D₁, ⁵L₈, ⁵L₉, ⁵G₅, and ⁵L₁₀ levels. To the best of our knowledge, it is the first time to observe upconversion emissions of Tb³⁺ from high energy levels (higher than the ⁵D₃), and 325 nm is the shortest wavelength of Tb³⁺ upconversion emission ever reported.

2. Experimental section

2.1. Synthesis of NaYF₄: Tb³⁺, Yb³⁺

All chemicals were of analytical grade and used without further purification. Y(NO₃)₃, Tb(NO₃)₃, and Yb(NO₃)₃ were supplied by Sigma-Aldrich. NaF and tetraacetic acid (EDTA) were purchased from Kishida Chemical. The NaYF₄: Tb³⁺, Yb³⁺ microcrystals were synthesized by a hydrothermal method as previously reported. [2] Typically, 0.5 mmol of RE(NO₃)₃ (RE=79%Y, 1%Tb, and 20%Yb) was added into 7 mL aqueous solution containing 0.5 mmol of EDTA. After stirring 1 h, rare earth complexes (RE-complex) were formed. Then 7 mL deionized water containing 6 mmol of NaF was added into the RE-complex. The mixture was kept stirring for 1 h, then transferred into a 25 mL autoclave and heated at 180 °C for 12 h. After cooling to room temperature, the precipitates were collected by centrifugation, washed three times with deionized water, and finally dried at 50 °C for 12 h. The procedure of synthesizing NaYF₄ with various concentrations of Tb³⁺ were similar as the above method, except carefully controlling the concentration of rare earth ions. The samples in this experiment were used directly after drying procedure. There were no annealing and pulverization processes.

2.2. Characterizations

Crystal phase was examined by X-ray powder diffraction (XRD) a LabX XRD-6100 X-ray diffractometer with a Cu Kα radiation source (λ=1.5405 Å) operated at 40 kV and 30 mA. The scan was performed in the arrange from 2θ = 10° to 80° with a scan speed of 0.02°/s in steps of 0.02°. The morphology of sample was characterized by a field emission scanning electron microscope (JSM-7000F). Emission spectra were recorded on a monochromator (JASCO, CT-25C) equipped with a photomultiplier tube (Hamamatsu, R636-10). Spectral resolution was about 1.5 nm. A fiber coupled laser diode with tunable output power at 976 nm was used as a pump source. A 300–900 nm band-pass filter was used to prevent the scattered pump laser. Decay curves of emissions at various wavelength were recorded on a digital oscilloscope (Yokogawa, DL1620). Excitation spectrum was measured on a spectrometer (Perkin Elmer, LS55). All measurements were performed at room temperature.

3. Results and discussion

Figure 1(a) shows the XRD pattern the as-prepared sample. The diffraction peaks can be indexed to the standard data of hexagonal phase NaYF₄ (JCPDS 16-0334). It reveals that the as-prepared sample was hexagonal phase NaYF₄. The field-emission scanning electron microscopy (FE-SEM) in Fig. 1(b) shows that the Tb³⁺-Yb³⁺ co-doped NaYF₄ was microcrystal rods with the length about 9 µm and diameter about 3.6 µm.

 

Fig. 1 (a) XRD pattern and (b) FE-SEM image of the as-prepared NaYF₄:1%Tb³⁺, 20%Yb³⁺ sample.

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Figure 2(a) shows the upconversion emission spectra of the NaYF₄:Tb³⁺, Yb³⁺ microcrystals under the 976 nm excitation. It exhibits intense UV and visible upconversion emissions. The emissions at 381, 414, 437, 490, 544, 582, 622 nm are assigned to the radiative transition of ⁵D₃→⁷F_J (J=6, 5, and 4) and ⁵D₄→⁷F_J (J=6, 5, 4 and 3) of Tb³⁺, respectively. [13, 14] The intensity of UV emission at 381 nm is comparable to that of green emission (⁵D₄ → ⁷F₅). It was found that the ⁵D₃ level was not directly pumped under excitation by a 976 nm laser in an upconversion process. Electrons were pumped to the ⁵D₁ level then relaxed to the ⁵D₃ level via a nonradiative relaxation. Due to the high population of the ⁵D₃ level, we could observe some weak emissions shorter than 381 nm, for example an emission at 371 nm, as shown in Fig. 2(a). The weak emission at 371 nm was assigned to the transition from the ⁵D₃ to ⁷F₆ level of Tb³⁺. [15]

 

Fig. 2 Upconversion emission spectra in the region of (a) 300–700 and (b) 310–370 nm excited at 976 nm with a power of 200 mW, and (c) excitation spectrum monitored at 544 nm in the as-prepared NaYF₄:1%Tb³⁺, 20%Yb³⁺ sample. (d) Energy level diagrams of Tb³⁺ and Yb³⁺ and possible upconversion processes.

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Some weak emission peaks (< 371 nm) were also observed. Figure 2(b) shows these emission peaks in detail in the range of 310–370 nm. Four UV upconversion emissions of Tb³⁺ at 325, 339, 351, and 361 nm were observed at room temperature for the first time. They were assigned to the radiative transition from the ⁵D₁, ⁵L₈, ⁵L₉, ⁵G₅, ⁵L₁₀, and ⁵D₃ to ⁷F₆ level of Tb³⁺, respectively. It is worth mentioning that 325 nm is the shortest wavelength of upconversion emission of Tb³⁺ ever reported. The upconversion UV emissions of Er³⁺ were reported around 316, 355, and 390 nm. [3, 25] Obviously, they were different compared with the emissions shown in Fig. 2. It indicates that the observed upconversion UV emissions in NaYF₄:1%Tb³⁺, 20%Yb³⁺ microcrystals were generated from the energy levels of Tb³⁺.

Excitation spectrum of as-prepared NaYF₄:1%Tb³⁺,20%Yb³⁺ monitored at 544 nm is shown in Fig. 2(c). Excitation peaks associated with transitions from the ground state of Tb³⁺ (⁷F₆) to high lying energy levels within the 4f⁸ configuration. The f – f transitions of Tb³⁺ around 319 (⁷F₅ → ⁵H₇ ⁵D₀), 327 (⁷F₆ → ⁵D₁), 341 (⁷F₆ → ⁵L₈), 353 (⁷F₆ → ⁵L₉, ⁵G₄, ⁵D₂), 360 (⁷F₆ → ⁵G₅), 371 (⁷F₆ → ⁵L₁₀), and 380 nm (⁷F₆ → ⁵D₃) were clearly observed in the region of 310–370 nm. It was similar as the observation of absorption and excitation spectra in Tb³⁺ doped materials that confirmed the position of high lying energy levels. [6,26,27]

Figure 2(d) schematically describes the energy level in Tb³⁺ and Yb³⁺ and possible upconversion energy transfer processes. Due to the large energy mismatch between the ²F_5/2 (Yb³⁺) and ⁷F₀ (Tb³⁺) level, Yb³⁺ can not directly transfer energy to populate ⁷F₀ level after absorbing photons from pump laser. The ⁵D₄ level of Tb³⁺ can be populated by energy transfer from two adjacent Yb³⁺ ions via co-operative sensitization. Then, electrons can be pumped from the ⁵D₄ to ⁵D₁ level by energy transfer from Yb³⁺ or excited state absorption. There are several energy levels between the ⁵D₁ and ⁵D₃ levels. Since the energy gaps were small, multiphonon relaxation process became significant. As a result, most of the electrons relaxed to the ⁵D₃ level. Therefore, the emission of ⁵D₃ level (about 381 nm) is widely observed.

In the theory of multiphonon relaxation, large phonon energy results in a high probability of nonradiative relaxation. [17] Theoretically, the emissions from energy levels higher than ⁵D₃ can be observed, if the nonradiative relaxation is weak. Two main factors can affect nonradiative relaxation: phonon energy of matrix and quenching effect from surface. [17, 28] If energy gap is fixed, phonon of low energy will have weak quenching effect, compared with large energy phonon. Fluorides have low phonon energy, for example, 450 cm⁻¹ in NaYF₄, which is smaller than that in oxide crystals. [29] It will have relatively low quenching caused by multiphonon relaxation. The NaYF₄ microcrystals in our experiments has a low surface to volume ratio (0.0018 nm⁻¹) calculated from the shape and size of particles. It was inferred that the quenching effect would be weak if the surface to volume ratio was low like this. Therefore, there is a probability of observing the emissions from higher energy levels, although the intensity would be quite weak. As shown in Fig. 2(b), the intensities of emissions from high energy levels are 100 times weaker than the emission from ⁵D₃ level.

Decay curves in Fig. 3 shows the temporal evolution of the emissions. Single exponential function I = I₀exp(−t/τ) was used to fit decay curves for the analysis of lifetime of emission at various wavelengths. The obtained fluorescent lifetime was about 309 ± 9 µs, 350 ± 15 µs, 457 ± 10 µs, and 571 ± 11 µs for 325, 339, 351, and 361 nm emissions, as shown in Table 1. The difference of fluorescent lifetime indicates emissions were generated from the different energy levels.

 

Fig. 3 Decay curves of upconversion UV emissions (< 400 nm) in NaYF₄:1%Tb³⁺, 20%Yb³⁺ excited at 976 nm with a power of 200 mW.

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Table 1. Averaged fluorescence liftimes of upconversion emissions in NaYF₄:1% Tb³⁺, 20%Yb³⁺ samples calculated from fluorescent decay curves.

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Figure 4 depicts the dependence of upconversion emission intensity on the power of pump laser in the NaYF₄:1%Tb³⁺, 20%Yb³⁺ sample. The relation was shown in log-log plot. In the case of lower power pumping, the relation between emission intensity and power of pump laser can be express by I_f = Pⁿ, where I_f is the integrated intensity of measured upconversion emission, P is the power of incident pump laser, and n is the number of photons required in the corresponding upconversion process. [30] In the double-logarithmic plotting of emission intensity and power of pump laser, the slope value of a linear fitting indicates the number of photons (n) involved in the certain upconversion process, when pump power is low. In the case of high pump power, the plotting seems to be saturated with the slope value close to 1, due to the competition between upconversion process and linear decay. As shown in Fig. 4(a), the obtained n value was about 3.00 ± 0.06, 2.98 ± 0.02, 2.96 ± 0.10, and 3.01 ± 0.12 for the emissions at 325, 339, 351, and 361 nm. It indicates that the energy levels to generate these emissions were populated by three-photon upconversion process. The power dependence for the emissions at 371 and 381 nm was measured under different sensitivity compared with emissions shorter than 371 nm. Figure 4(b) shows that the fitted n value of 371 and 381 nm emission was about 2.99 ± 0.06 and 3.06 ± 0.08, respectively. It indicates the upconversion process to populate the ⁵L₁₀ and ⁵D₃ levels also requires three photons of 976 nm as previously reported. [13]

 

Fig. 4 Excitation power dependence of emissions (a) shorter than 370 nm and (b) 370–400 nm in the NaYF₄: 1%Tb³⁺, 20%Yb³⁺ sample.

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The UV upconversion emissions in samples with different concentrations were measured to investigate the effect of cross relaxation on the electron population of high energy levels. Samples with various concentration (1%Tb³⁺/20%Yb³⁺, 10%Tb³⁺/20%Yb³⁺ and 15%Tb³⁺/20%Yb³⁺) were synthesized under the same heating conditions, as shown in Fig. 5. The samples were excited at 976 nm with a fixed power about 200 mW. The spectra were normalized by the peak intensity at 381 nm to compare the relative intensities of emissions from high energy levels of Tb³⁺ in different samples. The relative intensities of emissions at 381, 414, and 437 nm were similar. Moreover, the intensity of emissions from the high energy levels (⁵D₁, ⁵L₈, ⁵L₉, ⁵G₅, ⁵L₁₀,) were similar. It reveals that the cross relaxation process between Tb³⁺ ions caused by high concentration has little influence on the electron population on the high energy levels. Although the small energy gaps between the high energy levels can be easily matched with the gaps between the ⁷F₀ –⁷F₅ level. However, the ⁷F_J (J = 0–5) multiplets are difficult to own large electron populations. Concentration quenching effect on the high energy levels is difficult to occur. The strong nonradiative transition will be the main process of depopulating the electrons according to the energy gap law. [17] As a result, the ratios of the emissions intensities from high energy levels to those from the ⁵D₃ level in samples with various Tb³⁺ concentration were similar, as shown in Fig. 5. As a result, the Tb³⁺ concentrations just influence the ratio of three- to two-photon upconversion process. The intensity ratios of emissions generated from the same upconversion process are independent on the Tb³⁺ concentrations.

 

Fig. 5 Upconversion emission spectra in 1%Tb³⁺/20%Yb³⁺, 10%Tb³⁺/20%Yb³⁺, and 15%Tb³⁺/20%Yb³⁺ doped NaYF₄ samples in the UV and blue region excited at 976 nm with a power of 200 mW. The subgraph represents the magnified part of the spectra in the range of 300–370 nm.

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4. Conclusions

In summary, Tb³⁺–Yb³⁺ co-doped NaYF₄ microcrystals that prepared by a hydrothermal method showed intense UV and visible upconversion emissions under excitation by 976 nm laser. The UV upconversion emissions from the high-energy levels (⁵D₁, ⁵L₈, ⁵L₉, and ⁵G₅) of Tb³⁺ in the region of 310–370 nm were observed for the first time. The power dependence measurements reveals that these high energy levels were populated by a three-photon upconversion process. Decay curves were measured to estimate fluorescence lifetimes of high energy levels. It is helpful to deeply understand the emission properties of high energy levels of Tb³⁺ for designing UV gain media in the future.