English

• 0.   Introduction

  Over the past few decades, lanthanide-doped luminescence materials exhibited promising prospects for application in upconversion lasers^[1-2], display tech-nologies^[3-4], medical treatment^[5-6], anti-counterfeit tech-nologies^[7-8] and solar cells^[9-10]. In particular, the appli-cation of up- and down-conversion phosphors to solar cells has attracted the attention of a large number of scientific researchers^[11-12]. In past study, ultraviolet (UV) and infrared light conversion to visible light were realized by doping of different lanthanide ions in different materials^[13-16]. Yu et al.^[17] prepared Er³⁺-Yb³⁺ co-doped TiO_2-xF_x upconversion luminescence powder via hydrothermal method. They applied the upconversion-TiO_2-xF_x into dye sensitized solar cell (DSSC) and obtained an overall energy conversion efficiency of 7.08%. Zhang et al.^[18] prepared SrAl₂O₄:Eu²⁺, Dy³⁺ powder via a combustion method, spectral characterization showed that it can convert ultraviolet (200~400 nm) to visible luminescence (520 nm). The solar conversion efficiency for a DSSC with SrAl₂O₄:Eu²⁺, Dy³⁺ doping (weight ratio of phosphor powder to TiO₂ was 7:100) reached 7.938%. However, research on the fluorescence properties of bidirectional conversion luminescent materials that simultaneously convert ultraviolet and infrared light into visible light have rarely been reported.

  Compared with other matrix materials, such as oxysulfide and NaYF₄, lead fluoride (PbF₂) have the advantage of excellent fluorescence response under 1 064 and 1 550 nm excitation, and are a fair good host material for bidirectional conversion lumine-scence^[19-20]. In this work, we reported a bidirectional conversion PbF₂:Er³⁺, Yb³⁺ phosphors, which could convert both UV light (378 nm) and infrared light (808, 980, 1 064 and 1 550 nm) into visible light (blue, green and red). Therefore, the as-prepared phosphors could improve conversion efficiency of solar cells and be widely used in solar cells. The bidirectional conversion PbF₂:Er³⁺, Yb³⁺ phosphors were synthesized by the high temperature solid-state reaction method. The luminescence properties of the phosphors were analyzed. In addition, the down-conversion (DC) and up-conversion (UC) multi-wavelength sensitive lumine-scence mechanisms of the samples were discussed in detail.

  1.   Experimental

  1.1   Preparation

  PbF₂:Er³⁺, Yb³⁺ bidirectional conversion phosphors were prepared via the high temperature solid-state reaction method. PbF₂ (99.99%), Na₂SiF₆ (99.99%) were purchased from Shanghai Sinopharm Chemical Reagent Company. Er₂O₃ (99.99%), YbF₃ (99.99%) were purchased from Hawei Ruike Chemical Reagent Company, HNO₃ (65%~68% (w/w)), HF (40% (w/w)) and anhydrous ethanol were purchased from Beijing Chemical Company. Firsly, excess diluted HNO₃ was added into the Er₂O₃ and stirred on a magnetic stirrer until the oxides were completely dissolved. After cooling, poured the mixed solution into a plastic cup and HF was added into the solution to obtain ErF₃ precipitates, and washed several times. Then, the molar ratio of PbF₂:ErF₃:YbF₃ was 80:2:18 according to the composition of the compound. Reactants were weighed according to a predetermined molar ratio and thoroughly ground in an agate mortar. Finally, the base materials were placed in a crucible covered with NaSiF₆, heated to predetermined temperature (650 ℃) under a fluoride atmosphere provided by the decom-position of NaSiF₆ and sintered for a certain time (1.5 h) to get the (Pb_0.80Er_0.02Yb_0.18)F₂ phosphors.

  1.2   Measurements

  The XRD patterns were performed using a Rigaku D/max IIB diffractometer which recorded diffraction angel (2θ) from 20° to 80°, Cu Kα₁ (λ=0.154 06 nm) was used as the radiation source with an accelerating voltage of 40 kV and a working current of 20 mA. The structure refinement data was collected through using the general structure analysis system (GSAS) software. The emission spectra and the intensity-power curves were measured with UV-Vis spectrophotometry using RF-5301PC coupled 808, 980, 1 064 and 1 550 nm laser. All the measurement data were performed at room temperature.

  2.   Results and discussion

  2.1   Phase identification and crystal structure

  Fig. 1(a) gives XRD pattern of the (Pb_0.80Er_0.02Yb_0.18)F₂ phosphors. It is seen that all peak positions were consistent with the standard PDF No.06-0251 of cubic PbF₂. The XRD results indicated that the synthesized sample was a cubic PbF₂ structure with the space group of Fm3m (225). The Er³⁺ and Yb³⁺ ions have successfully entered PbF₂ host lattice by occupying the Pb²⁺ sites; there was no impurity phase, which demonstrated that the synthetic crystal was pure phase. Fig. 1(b) displays the refinement pattern of the (Pb_0.80Er_0.02Yb_0.18)F₂ phosphor. The final refinement converged with indicator of goodness of fit χ²=1.858, with weighted profile factor R_wp=9.63%, and with profile factor R_p=7.76%, The comparison between experimental and calculated results illustrated that sample could better crystallize in the cubic. Fig. 1(c) gives the crystal structure of PbF₂ and 8 coordination of Pb²⁺, which are distributed in face-centered cubic (FCC) crystal structure.

  Figure 1

  

  Figure 1.  (a) XRD pattern of the (Pb_0.80Er_0.02Yb_0.18)F₂ sample; (b) Rietveld refinement of the XRD pattern of (Pb_0.80Er_0.02Yb_0.18)F₂ sample; (c) Crystal structure representation of PbF₂

  下载: 全尺寸图片 幻灯片

  Table 1 presents the refinement result and structure parameter for (Pb_0.80Er_0.02Yb_0.18)F₂ phosphors. According to the data acquired by refinement, the occupation ratio of the Er³⁺/Yb³⁺ ions was 14.08% of the Pb²⁺ position, indicating that the doped Er³⁺/Yb³⁺ ions could effectively enter the PbF₂ lattice cell. The lattice parameters are a=b=c=0.585 7 nm, α=β=γ=90°. The crystal structure of resultant sample shifted to high angle in comparison with the peak positions of the standard PbF₂ crystallographic data (PDF No.06-0251, a=b=c=0.594 nm, α=β=γ=90°), which was mainly cause via bigger radius (Pb²⁺: 0.143 nm) replaced by smaller radius (Er³⁺: 0.117 nm, Yb³⁺: 0.113 nm).

  Table 1

  

  Table 1.  Refinement results and structure parameters for (Pb_0.80Er_0.02Yb_0.18)F₂

  下载: 导出CSV

  Formula	(Pb0.80Er0.02Yb0.18)F2
  Crystal system	Cubic
  Space group	Fm3m(225)
  Z	4
  a=b=c / nm	0.585 7
  χ2	1.858
  Rp	7.76%
  Rwp	9.63%
  Atom	X	Y	Z	Occupation ratio
  Pb1	0.000	0.000	0.000	0.859 2
  Er3+/Yb3+	0.000	0.000	0.000	0.140 8
  F1	0.250	0.250	0.250	1

  The Er³⁺ and Yb³⁺ ions entered PbF₂ lattice to replace Pb²⁺ ions to form Er_Pb and Yb_Pb· and F_F, and the F^- ions in ErF₃ and YbF₃ entered PbF₂ lattice to replace F^- ions to form F_F. The excess F^- ions entered [F₈] interspace in unit cell, forming an interstitial F_i′. Because the valence of both Er³⁺ and Yb³⁺ ions are higher than that of Pb²⁺ ions, charge imbalance is produced. The imbalance of charge between replacement and substituted ions leads to formation of vacant ions or interstitial ions^[21-22]. The possible reactions when Er³⁺ and Yb³⁺ ions entered into lattice sites of Pb²⁺ ions were displayed below:

  $ \text{Er}{{\text{F}}_{3}}\xrightarrow{\text{Pb}{{\text{F}}_{2}}}\text{ErP}{{\text{b}}^{\centerdot }}+2{{\text{F}}_{\text{F}}}+{{{\text{F}}}_{\text{i}}}' $

  (1)

  $ \text{Yb}{{\text{F}}_{3}}\xrightarrow{\text{Pb}{{\text{F}}_{2}}}\text{ErP}{{\text{b}}^{\centerdot }}+2{{\text{F}}_{\text{F}}}+{{{\text{F}}}_{\text{i}}}' $

  (2)

  The relative difference between the ionic radius of Er³⁺ and Yb³⁺ ions and the ionic radius of Pb²⁺ ions is not more than 15%, which improves the tendency of replacement. More importantly, the presence of [F₈] interspace in PbF₂ cell creates favorable conditions for the entry of F^- (0.133 nm) into the lattice either in size (the length of [F₈] interspace is 0.297 nm) or energy, so that the interstitial ion is in well coordination with interspace position.

  2.2   Luminescent properties under excitation of 378 nm

  Fig. 2 exhibits the emission and excitation spectrum of the optimal sample. Fig. 2a shows the excitation spectrum of the PbF₂:Er³⁺, Yb³⁺ sample at 550 nm, which could be excited from the UV to the visible region, and highest peak intensity of the excitation spectrum at 378 nm. Therefore, to obtain a higher DC efficiency, the DC luminescence property of PbF₂:Er³⁺, Yb³⁺ system at 378 nm excitation was studied. Fig. 2b presents the DC spectrum of the sample excited at 378 nm. It could be divided into three parts: (1) A blue emission band with an emission peak near 409 nm; (2) Both peaks are located at the green light emission band near 521 and 550 nm; (3) A red-light emission center around 650 nm. The emission peaks at ~409, ~521, ~550 and ~650 nm correspond to the ²H_9/2→⁴I_15/2, ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 transitions, respectively.

  Figure 2

  

  Figure 2.  Emission and excitation spectra of (Pb_0.80Er_0.02Yb_0.18)F₂ sample

  (a) λ_em=550 nm; (b) λ_ex=378 nm

  下载: 全尺寸图片 幻灯片

  Fig. 3 describes the Yb³⁺-Er³⁺ energy-level diagram of the sample excited at 378 nm. First, the electrons could be pumped from ⁴I_15/2 to ⁴G_11/2 by absorbing the energy of near-ultraviolet light at 378 nm, and Er³⁺ ions transfered part of the energy to Yb³⁺ ions, produc-ing Yb³⁺:²F_5/2→²F_7/2 transition. Then, because of the narrow energy gap between the ⁴G_11/2 and ²H_9/2, the ⁴G_11/2 energy level in Er³⁺ could easily relax to the ²H_9/2 by non-radiation relaxation. Subsequently, particles in ²H_9/2 returned to ⁴I_15/2 generating blue luminescence around 409 nm. At this time, Er³⁺ ions at the ²H_9/2 level might relax to the ²H_11/2, ⁴S_3/2, ⁴F_9/2 levels by the fast multiphoton process^[23-24]. In the end, Er³⁺ ions decayed from ²H_11/2, ⁴S_3/2 and ⁴F_9//2 to the ground state ⁴I_15/2 and emitted 521, 550 and 650 nm red-green light emission, respectively.

  Figure 3

  

  Figure 3.  Er³⁺-Yb³⁺ energy level diagram excited at 378 nm

  下载: 全尺寸图片 幻灯片

  The cross relaxation (CR₁) process results in particles significant reduction at the Er³⁺ ion ⁴S_3/2 level. Consequently, the emission intensity at 550 nm reduced. The CR₂ process leads to a significant increase in the population of the ⁴F_9/2 level of Er³⁺ ions. As a result, the 650 nm luminescence intensity was relatively enhanced.

  $ {\rm{C}}{{\rm{R}}_1}{\rm{:E}}{{\rm{r}}^{3 + }}\left( {^4{S_{3/2}}} \right) + {\rm{Y}}{{\rm{b}}^{3 + }}\left( {^2{F_{7/2}}} \right) \to {\rm{ E}}{{\rm{r}}^{3 + }}{(^4}{I_{11/2}}) + {\rm{Y}}{{\rm{b}}^{3 + }}\left( {^2{F_{5/2}}} \right) $

  (3)

  $ {\rm{C}}{{\rm{R}}_2}{\rm{:E}}{{\rm{r}}^{3 + }}\left( {^4{\mathit{G}_{11/2}}} \right) + {\rm{Y}}{{\rm{b}}^{3 + }}\left( {^2{F_{7/2}}} \right) \to {\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{F_{9/2}}} \right) + {\rm{Y}}{{\rm{b}}^{3 + }}\left( {^2{F_{5/2}}} \right){\rm{ }} $

  (4)

  2.3   Upconversion spectral analysis

  Fig. 4 depicts a series of UC emission spectra of optimal sample excited by different wavelength infrared light. Under excitation of 1 064, 980 and 808 nm lasers respectively, the spectra were presented an intense green emission at 540~550 nm as the result of the ⁴S_3/2→⁴I_15/2 transition of Er³⁺ ions, the weak green emission at 521 nm derived from the ²H_11/2→⁴I_15/2 transition while the weak red emission at 650~665 nm originated from the ⁴F_9/2→⁴I_15/2 transition. Compared with other luminescence emission spectra, the phosphor presented an intense red emission under 1 550 nm excitation.

  Figure 4

  

  Figure 4.  Emission spectra of the (Pb_0.80Er_0.02Yb_0.18)F₂ excited at different wavelengths

  (a) 808, (b) 980, (c) 1 064 and (d) 1 550 nm

  下载: 全尺寸图片 幻灯片

  2.3.1   Luminescent mechanism analysis under 1 064 nm excitation

  It is known that the emission spectrum of the trivalent rear earth (RE) ions doped UC material varies with excitation power^[25]. To identify the relative mechanism of the three UC emissions of Er³⁺ ions excited at 1 064 nm at room temperature for the (Pb_0.80Er_0.02Yb_0.18)F₂ sample. The dependence of emission intensities versus pump power is presented in Fig. 5. The result is illustrated in the intensity-power plots, and the values of slopes for the ~521 nm (green), ~550 nm (green) and ~650 nm (red) were approximately to 2, indicating that the two-photon process was mainly responsible for the observed UC emission under 1 064 nm excitation.

  Figure 5

  

  Figure 5.  Intensity-power plots of the green and red emissions versus excitation power at 1064 nm

  下载: 全尺寸图片 幻灯片

  Fig. 6 illustrates the simplified Er³⁺-Yb³⁺ energy level diagram of sample under excitation of 1 064 nm. Er³⁺ ion acted as an activator and was the luminescent center of the sample. It is observed that the ²F_5/2 state of Yb³⁺ ions was very close to the ⁴I_11/2 state of Er³⁺ ions, therefore, an effective energy transmission process could take place between Er³⁺ and Yb³⁺ ions. Firstly, the ²F_5/2 state of Yb³⁺ ions could be populated through absorbing 1 064 nm infrared light. And the Er³⁺ ion transited from ⁴I_15/2 to ⁴I_11/2 level by ground state absorption (GSA) and energy transfer (ET₁) from Yb³⁺ ions. Then, the electrons could be pumped from ⁴I_11/2 state of Er³⁺ ions to ⁴F_7/2 state through ET₂ process:

  $ \text{GSA:E}{{\text{r}}^{3+}}\left( ^{4}{{\mathit{I}}_{15/2}} \right)+\text{a}\ \text{1}\ \text{064 nm}\ \text{photon}\to \text{E}{{\text{r}}^{3+}}\left( ^{4}{{I}_{11/2}} \right)~ $

  (5)

  $ {\rm{E}}{{\rm{T}}_1}:{\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{I_{15/2}}} \right) + {\rm{Y}}{{\rm{b}}^{3 + }}\left( {^2{F_{5/2}}} \right) \to {\rm{ E}}{{\rm{r}}^{3 + }}{(^4}{I_{11/2}}) + {\rm{Y}}{{\rm{b}}^{3 + }}\left( {^2{F_{7/2}}} \right){\rm{ }} $

  (6)

  $ {\rm{E}}{{\rm{T}}_2}:{\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{I_{11/2}}} \right) + {\rm{Y}}{{\rm{b}}^{3 + }}\left( {^2{F_{5/2}}} \right) \to {\rm{ E}}{{\rm{r}}^{3 + }}{(^4}{I_{7/2}}) + {\rm{Y}}{{\rm{b}}^{3 + }}\left( {^2{F_{7/2}}} \right){\rm{ }} $

  (7)

  Figure 6

  

  Figure 6.  Er³⁺-Yb³⁺ energy level diagram excited at 1 064 nm

  下载: 全尺寸图片 幻灯片

  Furthermore, Er³⁺ ions at the ⁴F_7/2 level might relax to the ²H_11/2 and ⁴S_3/2 levels by the fast-multiphoton process. Subsequently, Er³⁺ions at the ²H_11/2 and ⁴S_3/2 energy level returned to the ground state ⁴I_15/2 while emitting green light at 521 and 550 nm, respectively. In addition, Er³⁺ ions at the ⁴I_11/2 level possibly underwent a non-radiative transition to reach the ⁴I_13/2 levels, and the Er³⁺ ion transited from the excited state ⁴I_13/2 to ⁴F_9/2 level by excited state absorption (ESA) and ET₃:

  $ \text{GSA:E}{{\text{r}}^{3+}}\left( ^{4}{{\mathit{I}}_{13/2}} \right)+\text{a}\ \text{1}\ \text{064 nm}\ \text{photon}\to \text{E}{{\text{r}}^{3+}}\left( ^{4}{{F}_{9/2}} \right)~ $

  (8)

  $ {\rm{E}}{{\rm{T}}_3}:{\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{I_{13/2}}} \right) + {\rm{Y}}{{\rm{b}}^{3 + }}\left( {^2{F_{5/2}}} \right) \to {\rm{ E}}{{\rm{r}}^{3 + }}{(^4}{F_{9/2}}) + {\rm{Y}}{{\rm{b}}^{3 + }}\left( {^2{F_{7/2}}} \right){\rm{ }} $

  (9)

  Finally, the electron at the ⁴F_9/2 level returns to ⁴I_15/2 ground state and emitted weak red fluorescence at 650 nm. Furthermore, the CR₁ and CR₂ process among Er³⁺ ions enhanced the UC luminescence efficiency of the sample^[26]:

  $ {\rm{C}}{{\rm{R}}_1}{\rm{:E}}{{\rm{r}}^{3 + }}\left( {^4{I_{13/2}}} \right) + {\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{I_{11/2}}} \right) \to {\rm{ E}}{{\rm{r}}^{3 + }}{(^4}{F_{9/2}}) + {\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{I_{15/2}}} \right){\rm{ }} $

  (10)

  $ {\rm{C}}{{\rm{R}}_2}{\rm{:E}}{{\rm{r}}^{3 + }}\left( {^4{I_{11/2}}} \right) + {\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{I_{11/2}}} \right) \to {\rm{ E}}{{\rm{r}}^{3 + }}{(^4}{F_{7/2}}) + {\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{I_{15/2}}} \right){\rm{ }} $

  (11)

  2.3.2   Luminescent mechanism analysis under 1 550 nm excitation

  Fig. 7 depicts the measured power dependence of the three UC emissions of Er³⁺ ions excited by 1 550 nm at room temperature for the (Pb_0.80Er_0.02Yb_0.18)F₂ sample. The result is illustrated in the intensity-power plots, the green and red emissions exhibited a sub-cubic (2.675, 2.60 and 2.873) power-law behavior for the excitation at 1 550 nm, which indicated that three pumping photons around 1 550 nm participate in the UC excitation process.

  Figure 7

  

  Figure 7.  Intensity-power plots of the green and red emissions versus excitation power at 1 550 nm

  下载: 全尺寸图片 幻灯片

  Fig. 8 presents the energy-level diagram of the sample excited by 1 550 nm. Yb³⁺ ions could not absorb 1 550 nm infrared light, hence the UC luminescence should depend on the absorption process of Er³⁺ ions and the energy transfer up-conversion (ETU) process among Er³⁺ ions. When excited by 1 550 nm infrared light, Er³⁺ ions were promoted from the ground state ⁴I_15/2 to the first excited state ⁴I_13/2 by GSA. Subse-quently, the Er³⁺ ion transited from the excited state ⁴I_13/2 to ⁴I_9/2 level by ESA₁ and ETU₁^[27-28]:

  $ \text{GSA:E}{{\text{r}}^{3+}}\left( ^{4}{{I}_{15/2}} \right)+\text{a}\ 1\ 550\text{ nm photon}\to \text{E}{{\text{r}}^{3+}}\left( ^{4}{{I}_{13/2}} \right)~ $

  (12)

  $ \text{ES}{{\text{A}}_{\text{1}}}\text{:E}{{\text{r}}^{3+}}\left( ^{4}{{I}_{13/2}} \right)+\text{a}\ 1\ 550\text{ nm photon}\to \text{E}{{\text{r}}^{3+}}\left( ^{4}{{I}_{9/2}} \right)~ $

  (13)

  $ {\rm{ET}}{{\rm{U}}_{\rm{1}}}{\rm{:E}}{{\rm{r}}^{3 + }}\left( {^4{I_{13/2}}} \right) + {\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{I_{13/2}}} \right) \to {\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{I_{11/2}}} \right) + {\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{I_{15/2}}} \right){\rm{ }} $

  (14)

  Figure 8

  

  Figure 8.  Er³⁺-Yb³⁺ energy level diagram excited at 1 550 nm

  下载: 全尺寸图片 幻灯片

  Then, the electrons at ⁴I_9/2 state could transit to ²H_11/2 state through ESA₂ and transit to ⁴I_11/2 level by the no-radiation relaxation process, and through the ETU2 process transitions to the higher excited state ²H_11/2:

  $ \text{ES}{{\text{A}}_{2}}\text{:E}{{\text{r}}^{3+}}\left( ^{4}{{I}_{9/2}} \right)+\text{a}\ 1\ 550\text{ nm photon}\to \text{E}{{\text{r}}^{3+}}\left( ^{2}{{H}_{11/2}} \right)~ $

  (15)

  $ {\rm{ET}}{{\rm{U}}_2}{\rm{:E}}{{\rm{r}}^{3 + }}\left( {^4{I_{13/2}}} \right) + {\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{I_{9/2}}} \right) \to {\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{I_{15/2}}} \right) + {\rm{E}}{{\rm{r}}^{3 + }}\left( {^2{H_{11/2}}} \right) $

  (16)

  Finally, the Er³⁺ions at the ²H_11/2 state relaxed to the ⁴S_3/2 and ⁴F_9/2 level by the fast-multiphoton process. The radiation decayed from the ²H_11/2, ⁴S_3/2 and ⁴F_9/2 states to the ground state and generated UC luminescence at 521, 540~550 nm and 650~665 nm, respectively.

  Compared with other luminescence emission spectra, the red emission of the sample under 1 550 nm excitation was increased. The increase of the red component in the sample was mainly due to the ETU₃ between the Er³⁺ ion at the ⁴I_13/2 and ⁴I_11/2 level^[29-30], resulting in an increase of Er³⁺ ions at the ⁴F_9/2 level.

  $ {\rm{ET}}{{\rm{U}}_3}{\rm{:E}}{{\rm{r}}^{3 + }}\left( {^4{I_{13/2}}} \right) + {\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{I_{11/2}}} \right) \to {\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{I_{15/2}}} \right) + {\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{F_{9/2}}} \right) $

  (17)

  2.3.3   Luminescent mechanism analysis under 980 nm excitation

  The dependence excited by 980 nm is illustrated in the intensity-power plots in Fig. 9. The green and red emissions exhibited a sub-square power-law behavior for the excitation at 980 nm, the values of slopes for the ~521 nm green emission, ~550 nm green emission and ~650 nm red emission are 1.857, 1.920 and 1.831, respectively. It is suggested that two pumping photons around 980 nm participate in the UC excitation process.

  Figure 9

  

  Figure 9.  Intensity-power plots of the green and red emissions versus excitation power at 980 nm

  下载: 全尺寸图片 幻灯片

  Fig. 10 describes the simplified Yb³⁺-Er³⁺ energy-level diagram of the sample under excitation of 980 nm. First, when the sample was irradiated by 980 nm laser, Yb³⁺ ions transited from the ground state ²F_7/2 to ²F_5/2 level. Then, the Er³⁺ ion transited from the ground state to ⁴I_11/2 was through either GSA or ET₁ process between Yb³⁺ and Er³⁺ ions:

  $ \text{GSA:E}{{\text{r}}^{3+}}\left( ^{4}{{I}_{15/2}} \right)+\text{a}\ 980\text{ nm photon}\to \text{E}{{\text{r}}^{3+}}\left( ^{4}{{I}_{11/2}} \right) $

  (18)

  $ {\rm{E}}{{\rm{T}}_1}:{\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{I_{15/2}}} \right) + {\rm{Y}}{{\rm{b}}^{3 + }}\left( {^2{F_{5/2}}} \right) \to {\rm{ E}}{{\rm{r}}^{3 + }}{(^4}{I_{11/2}}) + {\rm{Y}}{{\rm{b}}^{3 + }}\left( {^2{F_{7/2}}} \right) $

  (19)

  Similarly, the Er³⁺ ion transited from the excited state ⁴I_11/2 to ⁴F_7/2 level by ESA₁, CR₁, ET₂:

  $ \text{ES}{{\text{A}}_{\text{1}}}\text{:E}{{\text{r}}^{3+}}\left( ^{4}{{I}_{11/2}} \right)+\text{a}\ 980\text{ nm photon}\to \text{E}{{\text{r}}^{3+}}\left( ^{4}{{F}_{7/2}} \right) $

  (20)

  $ {\rm{E}}{{\rm{T}}_2}:{\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{I_{11/2}}} \right) + {\rm{Y}}{{\rm{b}}^{3 + }}\left( {^2{F_{5/2}}} \right) \to {\rm{ E}}{{\rm{r}}^{3 + }}{(^4}{F_{7/2}}) + {\rm{Y}}{{\rm{b}}^{3 + }}\left( {^2{F_{7/2}}} \right) $

  (21)

  $ {\rm{C}}{{\rm{R}}_1}{\rm{:E}}{{\rm{r}}^{3 + }}\left( {^4{I_{11/2}}} \right) + {\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{{\rm{I}}_{11/2}}} \right) \to {\rm{ E}}{{\rm{r}}^{3 + }}{(^4}{{\rm{I}}_{15/2}}) + {\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{F_{7/2}}} \right) $

  (22)

  Figure 10

  

  Figure 10.  Er³⁺-Yb³⁺ energy level diagram excited at 980 nm

  下载: 全尺寸图片 幻灯片

  In addition, Er³⁺ ions at the ⁴F_7/2 level relaxed to the ²H_11/2 and ⁴S_3/2 levels by the fast-multiphoton process. Then, the radiation decayed from the ²H_11/2 and ⁴S_3/2 levels to the ⁴I_15/2 ground state and generated green UC emission at 521 and 550 nm, respectively. Red UC emission mainly came from the population of ⁴F_9/2 level of Er³⁺ ions^[31-32]. The Er³⁺ ions at ⁴I_11/2 relaxed to the ⁴I_13/2 by the fast-multiphoton process and then transit to ⁴F_9/2 level by ESA₂, CR₂, CR₃ and ET₃:

  $ \text{ES}{{\text{A}}_{2}}\text{:E}{{\text{r}}^{3+}}\left( ^{4}{{I}_{13/2}} \right)+\text{a}\ 980\text{ nm photon}\to \text{E}{{\text{r}}^{3+}}\left( ^{4}{{F}_{9/2}} \right)~ $

  (23)

  $ {\rm{C}}{{\rm{R}}_2}{\rm{:E}}{{\rm{r}}^{3 + }}\left( {^4{I_{13/2}}} \right) + {\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{\mathit{I}_{11/2}}} \right) \to {\rm{ E}}{{\rm{r}}^{3 + }}{(^4}{\mathit{I}_{15/2}}) + {\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{F_{9/2}}} \right) $

  (24)

  $ {\rm{C}}{{\rm{R}}_3}{\rm{:E}}{{\rm{r}}^{3 + }}\left( {^4{I_{7/2}}} \right) + {\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{\mathit{I}_{11/2}}} \right) \to {\rm{ E}}{{\rm{r}}^{3 + }}{(^4}{F_{9/2}}) + {\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{F_{9/2}}} \right) $

  (25)

  $ {\rm{E}}{{\rm{T}}_3}:{\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{I_{13/2}}} \right) + {\rm{Y}}{{\rm{b}}^{3 + }}\left( {^2{F_{5/2}}} \right) \to {\rm{ E}}{{\rm{r}}^{3 + }}{(^4}{F_{9/2}}) + {\rm{Y}}{{\rm{b}}^{3 + }}\left( {^2{F_{7/2}}} \right){\rm{ }} $

  (26)

  Finally, the electronic at ⁴F_9/2 level decayed to ⁴I_15/2 ground state and emitted red fluorescence at 650 nm. In summary, the red UC luminescence intensity was not as strong as the green emission, which was due to the large energy gap between the ⁴S_3/2 and ⁴F_9/2 energy levels, making it difficult to relax without radiation.

  2.3.4   Luminescent mechanism analysis under 808 nm excitation

  Fig. 11 presents the energy-level diagram of the sample excited at 808 nm. Firstly, Er³⁺ ions are promoted from ⁴I_15/2 to ⁴I_9/2 by absorb an 808 nm photon (GSA). Since ⁴I_9/2, ⁴I_11/2 and ⁴I_13/2 belonged to different spectral branches of the same spectral term, Er³⁺ ions at the ⁴F_9/2 level relaxed to the ⁴I_11/2 and ⁴I_13/2 levels by no-radiation transitions process. Subsequently, the Er³⁺ ions at ⁴I_13/2 could absorb an 808 nm photon to transit to ⁴F_7/2 and ⁴S_3/2 level (ESA₁, ESA₂). Then, the radiation decayed from the ⁴S_3/2 levels to the ⁴I_15/2 and generated green UC emission 550 nm. In addition, the CR₂ phenomenon caused energy transfer between ions at the ⁴I_11/2 level, Er³⁺ ions at the ⁴F_7/2 level might relax to ²H_11/2 by the fast-multiphoton process, the radiation decayed from the ²H_11/2 levels to the ⁴I_15/2 generateed weak green UC emission at 521 nm. Finally, another CR₁ between ⁴I_11/2 and ⁴I_13/2 level resulted in a large accumulation of ions at the ⁴F_9/2 level, the electronic at ⁴F_9/2 level decayed to ⁴I_15/2 and emitted red fluorescence at 650 nm^[33].

  $ {\rm{C}}{{\rm{R}}_1}{\rm{:E}}{{\rm{r}}^{3 + }}\left( {^4{I_{13/2}}} \right) + {\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{\mathit{I}_{11/2}}} \right) \to {\rm{ E}}{{\rm{r}}^{3 + }}{(^4}{F_{9/2}}) + {\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{I_{15/2}}} \right){\rm{ }} $

  (27)

  $ {\rm{C}}{{\rm{R}}_2}{\rm{:E}}{{\rm{r}}^{3 + }}\left( {^4{F_{11/2}}} \right) + {\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{\mathit{I}_{11/2}}} \right) \to {\rm{ E}}{{\rm{r}}^{3 + }}{(^4}{F_{7/2}}) + {\rm{E}}{{\rm{r}}^{3 + }}\left( {^4{F_{15/2}}} \right) $

  (28)

  Figure 11

  

  Figure 11.  Er³⁺-Yb³⁺ energy level diagram excited at 808 nm

  下载: 全尺寸图片 幻灯片

  3.   Conclusions

  In summary, we report on the bidirectional conv-ersion properties of Er³⁺-Yb³⁺ co-doped PbF₂ prepared by high temperature solid-state reaction method, which may be used for enhancement of the conversion efficiency of solar cells. XRD and structure refinement data showed the Er³⁺/Yb³⁺ occupied solely Pb²⁺ sites in the crystal host. Through analysis the DC and UC multi-wavelength sensitive luminescence mechanisms of the phosphor, it is found that the resultant phosphor could simultaneously convert both UV light (378 nm) and infrared light (808, 980, 1 064 and 1 550 nm) into visible light (blue, green and red) which could improve conversion efficiency of solar cells.

• 1. [1]

     Cheng Y Y, Fuckel B, Khoury T, et al. Energy Environ. Sci., 2012, 5(5):6953-6959 doi: 10.1039/c2ee21136j

  2. [2]

     Yuan C Z, Chen G Y, Li L, et al. ACS Appl. Mater. Interfaces., 2014, 6(20):18018-18025 doi: 10.1021/am504866g

  3. [3]

     Cho J Y, Ko K Y, Do Y R. Thin Solid Films., 2007, 515(7):3373-3379 https://www.sciencedirect.com/science/article/pii/S0040609006010807

  4. [4]

     Rapaport A, Milliez J, Bass M, et al. J. Disp. Technol., 2006, 2(1):68-78 doi: 10.1109/JDT.2005.863781

  5. [5]

     Chen G Y, Liu Y, Zhang Y G, et al. Appl. Phys. Lett., 2007, 91(13):133103 doi: 10.1063/1.2787893

  6. [6]

     Wang G F, Peng Q, Li Y D. J. Am. Chem. Soc., 2009, 131(40):14200-14201 doi: 10.1021/ja906732y

  7. [7]

     张小青, 林祥, 乔旭升.材料科学与工程学报, 2010, 28(5):663-666 http://www.cqvip.com/QK/71135X/201107/35645375.htmlZHANG Xiao-qing, LIN Xiang, QIAO Xu-Sheng. J. Mater. Sci. Eng., 2010, 28(5):663-666 http://www.cqvip.com/QK/71135X/201107/35645375.html

  8. [8]

     You M L, Zhong J J, Hong Y, et al. Nanoscale., 2015, 7(10):4423-4431 doi: 10.1039/C4NR06944G

  9. [9]

     Yuan C Z, Chen G Y, Prasad P N, et al. J. Mater. Chem., 2012, 22(33):16709-16713 doi: 10.1039/c2jm16127c

  10. [10]

      Ramasamy P, Manivasakan P, Kim J. RSC Adv., 2014, 4(66):34873-34895 doi: 10.1039/C4RA03919J

  11. [11]

      Shockley W, Queisser H J. J. Appl. Phys., 1961, 32(3):510-519 doi: 10.1063/1.1736034

  12. [12]

      Yella A, Lee H W, Tsao H N, et al. Science., 2011, 334(6056):629-634 doi: 10.1126/science.1209688

  13. [13]

      Hirai T, Orikoshi T. J. Colloid Interface Sci., 2004, 273(2):470-477 doi: 10.1016/j.jcis.2003.12.013

  14. [14]

      Hosseini Z, Taghavinia N, Duang E W. Mater. Lett., 2016, 188(104):92-94 https://www.sciencedirect.com/science/article/abs/pii/S0167577X16317013

  15. [15]

      Kumar G A, Pokhrel M, Sardar D K. Mater. Lett., 2012, 68(1):395-398 https://www.sciencedirect.com/science/article/abs/pii/S0167577X11012535

  16. [16]

      Kumar P, Kanika, Singh S, et al. J. Lumin., 2017, 196(12):207-213 https://www.sciencedirect.com/science/article/pii/S0022231317317015

  17. [17]

      Yu J, Yang Y L, Fan R Q, et al. J. Power Sources., 2013, 243:436-443 doi: 10.1016/j.jpowsour.2013.06.014

  18. [18]

      Zhang J F, Lin J M, Wu J, et al. J. Mater. Sci.-Mater. Electron., 2016, 27(2):1350-1356 doi: 10.1007/s10854-015-3896-0

  19. [19]

      Wang H Q, Batentschuk M, Osvet A, et al. Adv. Mater., 2011, 23(22/23):2675-2680 https://www.ncbi.nlm.nih.gov/pubmed/21823249

  20. [20]

      Ma W, Yu W S, Dong X T, et al. Chem. Eng. J., 2014, 244(7):531-539

  21. [21]

      Lorbeer C, Behrends F, Cybinska J, et al. J. Mater. Chem. C, 2014, 2(44):9439-9450 doi: 10.1039/C4TC01214C

  22. [22]

      Sinha S, Mahata M K, Swart H C, et al. New J. Chem., 2017, 41(13):5362-5372 doi: 10.1039/C7NJ00086C

  23. [23]

      Luo X X, Cao W H. Sci. China Chem., 2017, 50(4):505-513 http://www.wanfangdata.com.cn/details/detail.do?_type=perio&id=zgkx-eb200704009

  24. [24]

      Xiao S, Yang X, Ding J W. Appl. Phys. B, 2010, 99(4):769-773 doi: 10.1007/s00340-010-3960-7

  25. [25]

      Luo W J, Wang Y G, Chen Y P, et al. J. Mater. Chem. C, 2013, 1(36):5711-5717 doi: 10.1039/c3tc31016g

  26. [26]

      钱静娴, 柏朝晖, 张希艳.硅酸盐学报, 2013, 41(12):1725-1729 https://www.ingentaconnect.com/content/ccs/jccs/2013/00000041/00000012/art00022QIAN Jing-Xian, BAI Zhao-Hui, ZHANG Xi-Yan. Journal of the Chinese Ceramic Society, 2013, 41(12):1725-1729 https://www.ingentaconnect.com/content/ccs/jccs/2013/00000041/00000012/art00022

  27. [27]

      柏朝晖, 李秀贤, 张希艳, 等.无机化学学报, 2012, 28(4):674-678BAI Zhao-Hui, LI Xiu-Xian, ZHANG Xi-Yan, et al. Chinese J. Inorg. Chem., 2012, 28(4):674-678

  28. [28]

      Fan W, Zhang X Y, Chen L X, et al. CrystEngComm, 2015, 17(8):1881-1889 doi: 10.1039/C4CE02243B

  29. [29]

      周禾丰, 张树全, 王华, 等.光谱学与光谱分析, 2013, 33(1):23-26 doi: 10.3964/j.issn.1000-0593(2013)01-0023-04ZHOU He-Feng, ZHANG Shu-Quan, WANG Hua, et al. Spectrosc. Spectr. Anal., 2013, 33(1):23-26 doi: 10.3964/j.issn.1000-0593(2013)01-0023-04

  30. [30]

      Tai Y, Wang H Y, Wang H, et al. RSC Adv., 2015, 6(5):4085-4089

  31. [31]

      Georgobiani A N, Bogatyreva A A, Ishchenko V M, et al. Inorg. Mater., 2007, 43(10):1073-1079 doi: 10.1134/S0020168507100093

  32. [32]

      杨魁胜, 翟海青, 崔光, 等.无机化学学报, 2009, 25(5):855-859 doi: 10.3321/j.issn:1001-4861.2009.05.017YANG Kui-Sheng, ZHAI Hai-Qing, CUI Guang, et al. Chinese J. Inorg. Chem., 2009, 25(5):855-859 doi: 10.3321/j.issn:1001-4861.2009.05.017

  33. [33]

      Tian L J, Zheng X, Zhao S L, et al. Materials, 2014, 7(11):7289-7303 doi: 10.3390/ma7117289

• 

  Figure 1  (a) XRD pattern of the (Pb_0.80Er_0.02Yb_0.18)F₂ sample; (b) Rietveld refinement of the XRD pattern of (Pb_0.80Er_0.02Yb_0.18)F₂ sample; (c) Crystal structure representation of PbF₂

  下载: 全尺寸图片 幻灯片
  

  Figure 2  Emission and excitation spectra of (Pb_0.80Er_0.02Yb_0.18)F₂ sample

  (a) λ_em=550 nm; (b) λ_ex=378 nm

  下载: 全尺寸图片 幻灯片
  

  Figure 3  Er³⁺-Yb³⁺ energy level diagram excited at 378 nm

  下载: 全尺寸图片 幻灯片
  

  Figure 4  Emission spectra of the (Pb_0.80Er_0.02Yb_0.18)F₂ excited at different wavelengths

  (a) 808, (b) 980, (c) 1 064 and (d) 1 550 nm

  下载: 全尺寸图片 幻灯片
  

  Figure 5  Intensity-power plots of the green and red emissions versus excitation power at 1064 nm

  下载: 全尺寸图片 幻灯片
  

  Figure 6  Er³⁺-Yb³⁺ energy level diagram excited at 1 064 nm

  下载: 全尺寸图片 幻灯片
  

  Figure 7  Intensity-power plots of the green and red emissions versus excitation power at 1 550 nm

  下载: 全尺寸图片 幻灯片
  

  Figure 8  Er³⁺-Yb³⁺ energy level diagram excited at 1 550 nm

  下载: 全尺寸图片 幻灯片
  

  Figure 9  Intensity-power plots of the green and red emissions versus excitation power at 980 nm

  下载: 全尺寸图片 幻灯片
  

  Figure 10  Er³⁺-Yb³⁺ energy level diagram excited at 980 nm

  下载: 全尺寸图片 幻灯片
  

  Figure 11  Er³⁺-Yb³⁺ energy level diagram excited at 808 nm

  下载: 全尺寸图片 幻灯片

  Table 1.  Refinement results and structure parameters for (Pb_0.80Er_0.02Yb_0.18)F₂

  Formula	(Pb0.80Er0.02Yb0.18)F2
  Crystal system	Cubic
  Space group	Fm3m(225)
  Z	4
  a=b=c / nm	0.585 7
  χ2	1.858
  Rp	7.76%
  Rwp	9.63%
  Atom	X	Y	Z	Occupation ratio
  Pb1	0.000	0.000	0.000	0.859 2
  Er3+/Yb3+	0.000	0.000	0.000	0.140 8
  F1	0.250	0.250	0.250	1

  下载: 导出CSV
•