Gd3+-Doping Effect on Upconversion Emission of NaYF4: Yb3+, Er3+/Tm3+ Microparticles

β-NaYF4 microcrystals co-doped with Yb3+, Er3+/Tm3+, and Gd3+ ions were synthesized via a hydrothermal method using rare-earth chlorides as the precursors. The SEM and XRD data show that the doped β-NaYF4 form uniform hexagonal prisms with an approximate size of 600–800 nm. The partial substitution of Y by Gd results in size reduction of microcrystals. Upconversion luminescence spectra of microcrystals upon 980 nm excitation contain characteristic intra-configurational ff bands of Er3+/Tm3+ ions. An addition of Gd3+ ions leads to a significant enhancement of upconversion luminescence intensity with maxima at 5 mol % of dopant.


Introduction
Rare earth-based materials are known to demonstrate efficient upconversion properties and are able to transform near-infrared (NIR) light to visible or even UV light via multiphoton processes [1][2][3]. NaYF 4 doped by rare earth ions is one of the most efficient upconversion phosphors among numerous luminescent materials due to the low phonon energy of host lattice, which reduces the amount of nonradiative transitions [4,5]. Lanthanide elements have attracted intense attention in recent years in numerous fields, such as photodynamic therapy [6,7], flat-panel displays [8], solid-state lasers [9][10][11], bio-imaging [4,12,13], and biosensing [14]. NaYF 4 : Yb 3+ , Tm 3+ /Er 3+ upconversion microcrystals are known to have the best luminescence property of all fluorescent materials [15]. Different methods for the synthesis of NaYF 4 : Yb 3+ , Tm 3+ /Er 3+ have been recently reported, including hydrothermal and solvothermal methods [16][17][18][19]. Using various synthetic approaches, particles of different sizes can be obtained. In solvothermal synthesis with oleic acid/octadecene solvent, hexagonal nanoparticles of a small size (<100 nm) are obtained. Microcrystals of a larger size (>500 nm), which can be fabricated by hydrothermal synthesis, usually have higher luminescence intensity. The Tm 3+ and Er 3+ ions act as optical active centers; the Yb 3+ ion is a sensitizer that absorbs NIR light and then transfers energy to Tm 3+ or Er 3+ .
In our work, we partially substituted Yb 3+ by Gd 3+ ions in NaYF 4 : Yb 3+ , Tm 3+ /Er 3+ materials to improve upconverting properties. It was previously demonstrated that Gd 3+ co-doping improves the luminescent properties of rare earth-based materials [17,20]. By introducing Gd 3+ ions into the NaYF 4 crystal lattice, it is possible to change local symmetry, thus increasing the probability of energy transfer processes, which could increase luminescence intensity. We studied the structure and upconverting luminescent properties of NaYF 4 : Gd 3+ /Yb 3+ /Tm 3+ and NaYF 4 : Gd 3+ /Yb 3+ /Er 3+ The diffraction maxima positions of all our samples matched the standard values for pure hexagonal β-NaYF4 (JCPDS No. . No diffraction peaks attributed to impurities were observed. We found that the addition of Gd did not lead to a phase transformation. The XRD data of all the samples were the same; therefore, only several XRD patterns are given here for simplicity.
Scanning electron microscope (SEM) was used to analyze the shape and size of the microcrystals. SEM images of various composition microcrystals are shown in Figure 2. -NaYF4: 20% Yb, 1% Tm, 5% Gd, and (d) -NaYF4: 20% Yb, 1% Er, 5% Gd. All the samples were synthesized with the same reaction time (24 h). The diffraction maxima positions of all our samples matched the standard values for pure hexagonal β-NaYF 4 (JCPDS No. . No diffraction peaks attributed to impurities were observed. We found that the addition of Gd did not lead to a phase transformation. The XRD data of all the samples were the same; therefore, only several XRD patterns are given here for simplicity.
Scanning electron microscope (SEM) was used to analyze the shape and size of the microcrystals. SEM images of various composition microcrystals are shown in Figure 2. The diffraction maxima positions of all our samples matched the standard values for pure hexagonal β-NaYF4 (JCPDS No. . No diffraction peaks attributed to impurities were observed. We found that the addition of Gd did not lead to a phase transformation. The XRD data of all the samples were the same; therefore, only several XRD patterns are given here for simplicity. Scanning electron microscope (SEM) was used to analyze the shape and size of the microcrystals. SEM images of various composition microcrystals are shown in Figure 2.   All the samples consisted of sub-micron-sized uniform hexagonal prism-shaped particles (Figure 2a-d). The morphology of the microcrystals obtained by SEM agreed with that obtained by AFM ( Figure 3).
Materials 2020, 13, x FOR PEER REVIEW 4 of 12 All the samples consisted of sub-micron-sized uniform hexagonal prism-shaped particles (Figure 2a-d). The morphology of the microcrystals obtained by SEM agreed with that obtained by AFM ( Figure 3). Earlier studies demonstrated that substitution of yttrium ion by larger gadolinium(III) ion (ionic radii of Y 3+ and Gd 3+ are 1.159 and 1.193 Å, respectively) results in an increase in the electron charge density of the crystal surface [31,32]. Therefore, the larger electron charge density in the Gd 3+ -containing crystal nucleus slows the diffusion of negatively charged fluoride ions, which leads to a reduction in the crystal growth rate and a smaller final size of Gd 3+ co-doped microcrystals. Furthermore, the difference in charge density inside the crystal can result in a minor change of local symmetry of rare earth ions and surface structural defects. The composition of microcrystals was roughly estimated by energy dispersive X-ray analysis (EDX). The EDX spectra ( Figure 4) indicated the presence of all elements (Y, Yb, F, Na, Gd, and Er/Tm) in the synthesized materials. Earlier studies demonstrated that substitution of yttrium ion by larger gadolinium(III) ion (ionic radii of Y 3+ and Gd 3+ are 1.159 and 1.193 Å, respectively) results in an increase in the electron charge density of the crystal surface [31,32]. Therefore, the larger electron charge density in the Gd 3+ -containing crystal nucleus slows the diffusion of negatively charged fluoride ions, which leads to a reduction in the crystal growth rate and a smaller final size of Gd 3+ co-doped microcrystals. Furthermore, the difference in charge density inside the crystal can result in a minor change of local symmetry of rare earth ions and surface structural defects. The composition of microcrystals was roughly estimated by energy dispersive X-ray analysis (EDX). The EDX spectra ( Figure 4) indicated the presence of all elements (Y, Yb, F, Na, Gd, and Er/Tm) in the synthesized materials.
Upconversion intensity enhancement by Gd 3+ co-doping of NaYF4: Yb, Er or NaYF4: Yb, Tm is usually explained by host phase transition from cubic to hexagonal, which would significantly improve luminescence intensity [17,35]. However, in our case, hexagonal phase formed even in the case of Gd 3+ -free powders. Introduction of Gd 3+ ions in the NaYF4 host leads to the formation of crystal lattice defects, as shown in Figure 2c,d, which change the symmetry of the surroundings of ytterbium, Upconversion intensity enhancement by Gd 3+ co-doping of NaYF 4 : Yb, Er or NaYF 4 : Yb, Tm is usually explained by host phase transition from cubic to hexagonal, which would significantly improve luminescence intensity [17,35]. However, in our case, hexagonal phase formed even in the case of Gd 3+ -free powders. Introduction of Gd 3+ ions in the NaYF 4 host leads to the formation of crystal lattice defects, as shown in Figure 2c,d, which change the symmetry of the surroundings of ytterbium, thulium, and erbium ions. Thereby, energy transfer processes and/or radiative transitions become more possible from the symmetry point of view, which leads to an increase in luminescence intensity [36]. This suggestion is confirmed by comparison of Gd 3+ (r = 1.193 Å) and Y 3+ (r = 1.159 Å) ionic radii [31] displaying possible appearance of crystal lattice defects as a result of gadolinium co-doping. The addition of a large amount of Gd 3+ ions reduced Er 3+ and Tm 3+ luminescence due to two co-directional processes. Firstly, large numbers of crystal lattice defects enhance nonradiative decay rate, which decreases luminescence intensity. Secondly, high Gd 3+ co-doping concentration promotes energy transfer from high excited states of thulium and erbium to gadolinium ions [36].
To study the mechanism of upconversion processes in NaYF 4 : Yb, Er, Gd and NaYF 4 : Yb, Tm, Gd phosphors, we measured the emission intensity dependence on pump power. The upconversion emission intensity (I UC ) increased proportionally to the pumping power (p) of the excitation source according to I UC -P n , where n is the number of photons that pump the population in a particular energy level [26,36]. Therefore, n, the number of photons involved in the upconversion emission, can be obtained from the logarithmic plot of the integral emission intensity vs. the incident laser power. Figure 6a-c show the plot of the integral emission intensity of the green and red emission lines as a function of the pump laser power for NaYF 4 : Yb, Er, Gd powders. emission intensity (IUC) increased proportionally to the pumping power (p) of the excitation source according to IUC-P n , where n is the number of photons that pump the population in a particular energy level [26,36]. Therefore, n, the number of photons involved in the upconversion emission, can be obtained from the logarithmic plot of the integral emission intensity vs. the incident laser power. Figure 6a-c show the plot of the integral emission intensity of the green and red emission lines as a function of the pump laser power for NaYF4: Yb, Er, Gd powders. Figure 6. Dependence of integral upconversion luminescence on laser power of (a) NaYF4: 20% Yb, 1% Er; (b) NaYF4: 20% Yb, 1% Er, 5% Gd; (c) NaYF4: 20% Yb, 1% Er, 20% Gd; (d) NaYF4: 20% Yb, 1% Tm; (e) NaYF4: 20% Yb, 1% Tm, 5% Gd; and (f) NaYF4: 20% Yb, 1% Tm, 20% Gd microparticles.
Based on the obtained experimental data, the energy level diagrams of Yb 3+ , Er 3+ , and Tm 3+ ions, as well as the possible energy transfer mechanisms for upconversion emissions in NaYF4 host upon 980 nm excitation, are shown in Figure 7. All experimental data can be perfectly fitted using linear function with the slopes of 1.79-2.22 on a log-log plot giving n ≈ 2. We concluded that the observed 2 H 11/2 -4 I 15/2 , 4 S 3/2 -4 I 15/2 and 4 S 3/2 -4 I 15/2 transitions in NaYF 4 : Yb, Er, and Gd samples originated from two-photon process [37] irrespective of Gd 3+ co-doping concentration. Figure 4d-f present integral emission intensity of the blue and red emission lines as a function of the pump laser power for NaYF 4 : Yb, Tm, and Gd powders. Similar to NaYF 4 : Yb, Er, and Gd samples, the amount of Gd 3+ ions did not affect the number of photons needed to excite certain transition. 1 D 2 -3 F 4 , 1 G 4 -3 H 6 , and 3 F 2,3 -3 H 6 transitions require absorption of 4, 3, and 2 photons, respectively.

Conclusions
We synthesized hexagonal NaYF 4 microcrystals co-doped with different rare earth ions Yb 3+ , Tm 3+ /Er 3+ , and Gd 3+ via a hydrothermal method: NaY 0.79−х Yb 0,20 Er 0.01 Gd x F 4 and NaY 0.79−х Yb 0,20 Tm 0.01 Gd x F 4 (x = 0-0.2). The size of the synthesized particles was determined to be about 800 nm for NaY 079 Yb 0,20 Tm 0.01 F 4 and NaY 0.79 Yb 0,20 Er 0.01 F 4 , and about 600 nm for NaY 0.79−х Yb 0,20 Er 0.01 Gd x F 4 and NaY 0.79−х Yb 0,20 Tm 0.01 Gd x F 4. The decrease in particle size when co-doped with Gd 3+ ions is explained by the slower crystal growth rates due to an increase in the electron charge density of the crystal surface in Gd 3+ -co-doped microcrystals. XRD showed that all the samples consisted of hexagonal phase and the addition of Gd 3+ did not lead to phase transformation.
All synthesized materials demonstrated prominent upconversion luminescence upon 980 nm excitation. The addition of gadolinium enhances upconversion luminescence. This is probably associated with the appearance of crystal lattice defects, which change the symmetry of the surroundings of ytterbium, thulium, and erbium ions. Thus, energy transfer processes and/or radiative transitions become enabled from the symmetry point of view, which results in an increase in luminescence intensity. Larger numbers of Gd 3+ ions promote quenching of Er 3+ and Tm 3+ emission through depopulation of their upper excited levels. We found an optimal composition of the particles for the maximum intensity luminescence: NaYF 4 : 20% Yb, 1% Er, 5% Gd and NaYF 4 : 20% Yb, 1% Tm, 5% Gd. Possible energy transfer mechanisms for upconversion emissions in NaYF 4 host co-doped with different rare earth ions Yb, Tm, Er, and Gd upon 980 nm excitation were proposed.