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

Abstract

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.

1. 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]. NaYF₄ 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].

NaYF₄: Yb³⁺, Tm³⁺/Er³⁺ upconversion microcrystals are known to have the best luminescence property of all fluorescent materials [15]. Different methods for the synthesis of NaYF₄: Yb³⁺, Tm³⁺/Er³⁺ 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 Tm³⁺ and Er³⁺ ions act as optical active centers; the Yb³⁺ ion is a sensitizer that absorbs NIR light and then transfers energy to Tm³⁺ or Er³⁺.

In our work, we partially substituted Yb³⁺ by Gd³⁺ ions in NaYF₄: Yb³⁺, Tm³⁺/Er³⁺ materials to improve upconverting properties. It was previously demonstrated that Gd³⁺ co-doping improves the luminescent properties of rare earth-based materials [17,20]. By introducing Gd³⁺ ions into the NaYF₄ 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 NaYF₄: Gd³⁺/Yb³⁺/Tm³⁺ and NaYF₄: Gd³⁺/Yb³⁺/Er³⁺ microparticles synthesized via a hydrothermal method. We found that co-doping of 5% Gd³⁺ ions in NaYF₄: Yb³⁺, Tm³⁺/Er³⁺ increases the upconversion luminescence intensity in the visible range by 2–5 times upon 980 nm excitation.

2. Materials and Methods

Anhydrous chlorides of the rare earth elements (YCl₃, ErCl₃, GdCl₃, YbCl₃, and TmCl₃, 99.999%) were purchased from Chemcraft (Russia), NaOH, NH₄F, sodium citrate, and ethanol were purchased from Sigma-Aldrich Pty Ltd. (Germany), and used without additional purification.

Microcrystalline β-NaYF₄ samples co-doped with Yb³⁺, Er³⁺, Tm³⁺, and Gd³⁺ were synthesized by the hydrothermal method using citric acid as a stabilizing agent. We redesigned the previously reported hydrothermal method of synthesis [16,17,18,19]. In the typical synthesis, yttrium, ytterbium(III), gadolinium(III), and thulium(III)/erbium(III) chlorides (total amount of rare earth chlorides was 0.75 mmol) with 3 mmol of citric acid were dissolved in distilled water to obtain 5 mL solution in total. Chlorides of rare earth elements were taken in stoichiometric amounts. Then, 2.5 mL of aqueous solution containing 9 mmol of NaOH was added to the flask of the previous solution. After vigorous stirring for 30 min, 8 mL of aqueous solution containing 11 mmol of NaOH and 11 mmol of NH₄F was introduced into the above solution. The solution was maintained after vigorous stirring for 30 min at room temperature before being transferred to a Teflon-lined autoclave with an internal volume of 20 mL. The hydrothermal syntheses were conducted in an electric oven at 180 °C for 24 h. After that, the precipitate was separated from the reaction mixture by centrifugation, washed with ethanol and deionized water, and dried at 60 °C for 24 h. The desired microstructure materials were obtained in a form of white powders.

Dopant concentration, particle size, and crystallite phase are known to significantly influence the efficiency of upconversion luminescence [21,22]. Earlier studies demonstrated that the Yb³⁺ optimal concentration is about 20 at % [23,24,25]. Our preliminary experiments demonstrated that in NaY_0.8-yYb_0.20Tm_yF₄ and NaY_0.8-zYb_0.20Er_zF₄, the optimal concentration of Tm³⁺ and Er³⁺ is in the range of 0.5–1 at %, which agrees with earlier studies where the optimal dopant concentration for Tm³⁺ and Er³⁺ ions varied from 1% to 2% [18,20,22,23]. Therefore, in this work, we synthesized and studied the two Gd³⁺ co-doping series of upconverting microcrystals with 1% Tm³⁺/Er³⁺ concentration: NaY_0.79−xYb_0,20Tm_0.01Gd_xF₄ and NaY_0.79−xYb_0,20Er_0.01Gd_xF₄ (x = 0–0.2).

The morphologies of microstructures of the synthesized samples were characterized using scanning electron microscopy (SEM) with a Zeiss Merlin electron microscope (Zeiss, Germany) with energy-dispersive X-ray spectroscopy (EDX) module (Oxford Instruments INCAx-act, UK) and confirmed by atomic force microscopy (AFM) using a Nanoeductor II microscope (NT-MDT Spectrum Instruments, Moscow, Russia); the AFM measurements were performed in a semi-contact regime. X-ray powder diffraction (XRD) measurements were performed on a D2 Phaser (Bruker, Billerica, MA, USA) X-ray diffractometer using (Cu Kα radiation, λ = 1.54056 Å) radiation. The upconversion luminescence emission spectra were recorded with an Fluorolog-3 fluorescence spectrometer (Horiba Jobin Yvon, Japan) with diode laser (wavelength 980 nm, power 320 mW, and beam diameter 2 mm) as an excitation source for upconversion luminescence. Lifetime measurements were performed with the same spectrometer using pulsed Xe lamp (pulse duration 3 µs).

3. Results and Discussion

3.1. Crystal Structure and Morfology

NaYF₄ exists in two phases [5,26,27,28]: cubic α-NaYF₄ phase and hexagonal β-NaYF₄ phase. The upconverting efficiency of the hexagonal phase NaYF₄: Yb,Tm/Er materials are significantly higher [19,29,30]. X-ray powder diffraction (XRD) patterns of the synthesized samples are given in Figure 1.

The diffraction maxima positions of all our samples matched the standard values for pure hexagonal β-NaYF₄ (JCPDS No. 16-0334). 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).

The microcrystals without Gd³⁺ dopant (Figure 2a,b) had a uniform morphology and an average length along the diagonal direction of about 800 nm. Notably, the addition of the Gd³⁺ dopant (Figure 2c,d) led to a decrease in the size of the microcrystals that is clearly seen from the SEM images. Addition of Gd³⁺ ions in NaYF₄: Yb, Tm/Er also resulted in the formation of surface defects (Figure 2c,d), such as cracks and chips. The size of the crystals is probably guided by crystal growth rates. Earlier studies demonstrated that substitution of yttrium ion by larger gadolinium(III) ion (ionic radii of Y³⁺ and Gd³⁺ 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 Gd³⁺-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 Gd³⁺ 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.

3.2. Luminescence Properties

Upconversion spectra of NaYF₄: 20% Yb, 1% Er microcrystals with different Gd³⁺ concentration upon 980 nm excitation are shown in Figure 5a.

Emission spectra measured in the spectral range 500–700 nm consisted of characteristic sharp lines corresponding to the intra-configurational 4f transitions of erbium ions. The observed emission peaks are assigned to ²H_11/2–⁴I_15/2 (522 and 529 nm), ⁴S_3/2–⁴I_15/2 (541 and 548 nm), and the most prominent ⁴F_9/2–⁴I_15/2 (655 and 661 nm) transitions [17,33]. Note, concentration of Gd³⁺ ions non-monotonically affected emission intensity. Optimal Gd³⁺ co-doping concentration was 5% for green emission, whereas red emission showed equal intensities for 5% and 10% Gd³⁺ co-doped samples. An example of Gd³⁺ concentration dependence of emission intensity (⁴S_3/2–⁴I_15/2 integral intensity) of NaYF₄: 20% Yb, 1% Er, Gd phosphors is shown in Figure 5b. At first, luminescence intensity increased along with Gd³⁺ ions growth, reaching a maximum at 5%. Further increase in Gd³⁺ ions resulted in concentration quenching.

Figure 5c presents upconversion spectra of NaYF₄:20% Yb, 1% Tm powders with different Gd concentration upon 980 nm excitation. The obtained emission spectra include the following transitions: ¹D₂–³F₄ (452 nm), ¹G₄–³H₆ (477 nm), ¹G₄–³F₄ (648 nm and 656 nm), and ³F_2,3–³H₆ (697 nm) [32,34]. Similar to NaYF₄: Yb, Er, Gd samples, the addition of gadolinium ions in NaYF₄: Yb, Tm phosphors significantly affected emission intensity. Evolution of ¹G₄–³H₆ integral intensity as a function of Gd³⁺ concentration is presented in Figure 5d. Analyzing the obtained experimental data, we concluded that the best luminescence intensity enhancement was achieved for 5% Gd³⁺-co-doped powder.

Upconversion intensity enhancement by Gd³⁺ 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 Gd³⁺-free powders. Introduction of Gd³⁺ 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, 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 Gd³⁺ (r = 1.193 Å) and Y³⁺ (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 Gd³⁺ ions reduced Er³⁺ and Tm³⁺ luminescence due to two co-directional processes. Firstly, large numbers of crystal lattice defects enhance nonradiative decay rate, which decreases luminescence intensity. Secondly, high Gd³⁺ 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 NaYF₄: Yb, Er, Gd and NaYF₄: 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–Pⁿ, 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.

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 ²H_11/2–⁴I_15/2, ⁴S_3/2–⁴I_15/2 and ⁴S_3/2–⁴I_15/2 transitions in NaYF₄: Yb, Er, and Gd samples originated from two-photon process [37] irrespective of Gd³⁺ 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 NaYF₄: Yb, Tm, and Gd powders. Similar to NaYF₄: Yb, Er, and Gd samples, the amount of Gd³⁺ ions did not affect the number of photons needed to excite certain transition. ¹D₂–³F₄, ¹G₄–³H₆, and ³F_2,3–³H₆ transitions require absorption of 4, 3, and 2 photons, respectively.

Based on the obtained experimental data, the energy level diagrams of Yb³⁺, Er³⁺, and Tm³⁺ ions, as well as the possible energy transfer mechanisms for upconversion emissions in NaYF₄ host upon 980 nm excitation, are shown in Figure 7.

Gd³⁺ ions have a very large energy gap between ground ⁸S_7/2 and first-excited ⁶P_7/2 states (>30,000 cm⁻¹). Therefore, gadolinium ions could participate in energy transfer processes in highly-doped NaYF₄: Yb, Er, Gd and NaYF₄: Yb, Tm, Gd samples. Large numbers of Gd³⁺ ions promote quenching of Er³⁺ and Tm³⁺ emission through depopulation of their upper excited levels by following energy transfers: ⁴G_9/2–⁴I_15/2 (Er³⁺):⁸S_7/2–⁶I_J (Gd³⁺), ⁴G_7/2 and ²K_13/2–⁴I_15/2 (Er³⁺):⁸S_7/2–⁶P_J (Gd³⁺); ³P_0,1,2–³H₆ (Tm³⁺):⁸S_7/2–⁶I_J (Gd³⁺); and ¹I₆–³H₆ (Tm³⁺):⁸S_7/2–⁶P_7/2,5/2 (Gd³⁺) [36,37,38]. When upper energy levels of Er³⁺ and Tm³⁺ are populated (even minor amounts), there are two possibilities of energy dissipation: (1) internal conversion to lower levels of Er³⁺ and Tm³⁺ followed by luminescence and (2) the energy transfer to Gd³⁺. Therefore, a large concentration of Gd³⁺ ions significantly decreases the population of upper energy levels of Er³⁺ and Tm³⁺, leading to some decrease in the population of the states from which luminescence occurs.

To provide a more detailed study of the Gd³⁺ co-doping effect on luminescence properties of NaYF₄: Yb, Er, Gd and NaYF₄: Yb, Tm, and Gd powders, we carried out kinetics measurements. Decay curves of NaYF₄: 20% Yb, 1% Er/1% Tm (without Gd³⁺ co-doping); NaYF₄: 20% Yb, 1% Er/1% Tm, 5% Gd (the most prominent sample); and NaYF₄: 20% Yb, 1% Er/1% Tm, 20% Gd (highly Gd³⁺ co-doped sample) were recorded (Figure 8).

Notably, the kinetics studies were performed upon Stokes excitation (λ_ex = 375 and 355 nm for Er³⁺ and Tm³⁺-doped phosphors, respectively). ⁴S_3/2–⁴I_15/2 (541 nm) and ⁴F_9/2–⁴I_15/2 (655 nm) transitions were monitored in Er³⁺-doped samples, and ¹G₄–³H₆ (477 nm) transition was measured in Tm³⁺-doped powders. All experimental decay curves displayed non-single exponential behavior and two exponential models were applied for fitting. Average luminescence lifetime (τ_av) was calculated according to the following equation to simplify comparison [35,36]:

$${\mathsf{\tau }}_{\mathrm{av}}=\frac{{\mathrm{A}}_1{\mathsf{\tau }}_1^2+{\mathrm{A}}_2{\mathsf{\tau }}_2^2}{{\mathrm{A}}_1{\mathsf{\tau }}_1+{\mathrm{A}}_2{\mathsf{\tau }}_2}$$

(1)

where A₁ and A₂ are pre-exponential constants, and τ₁ and τ₂ are fitting lifetimes (Table S1, Supplementary Materials). The calculated lifetimes of NaYF₄: Yb, Er, Gd and NaYF₄: Yb, Tm, Gd powders are listed in

Table 1. Average luminescence lifetimes of Yb, 1% Er/1% Tm, Gd microparticles.

Sample	Transition	τav, ms
NaYF4: 20% Yb, 1% Er	4S3/2–4I15/2 (541 nm)	0.34
NaYF4: 20% Yb, 1% Er, 5% Gd		0.25
NaYF4: 20% Yb, 1% Er, 20% Gd		0.33
NaYF4: 20% Yb, 1% Er	4F9/2–4I15/2 (655 nm)	0.46
NaYF4: 20% Yb, 1% Er, 5% Gd		0.48
NaYF4: 20% Yb, 1% Er, 20% Gd		0.44
NaYF4: 20% Yb, 1% Tm	1G4–3H6 (477 nm)	0.19
NaYF4: 20% Yb, 1% Tm, 5% Gd		0.23
NaYF4: 20% Yb, 1% Tm, 20%Gd		0.18

. The obtained lifetimes are in agreement with the previous studies, where the typical luminescence lifetimes are in the range of 0.1–0.5 ms depending on the morphology and composition [38,39,40,41].

The introduction of 5% Gd³⁺ ions affected the average lifetime more profoundly compared with 20% Gd³⁺ doped sample, which is consistent with earlier the observed concentration dependence of emission intensity. Nonmonotonic lifetime changes in Er³⁺ and Tm³⁺-doped phosphors may be due to different mechanisms of Gd doping on the monitored emission transitions.

4. Conclusions

We synthesized hexagonal NaYF₄ microcrystals co-doped with different rare earth ions Yb³⁺, Tm³⁺/Er³⁺, and Gd³⁺ via a hydrothermal method: NaY_0.79−хYb_0,20Er_0.01Gd_xF₄ and NaY_0.79−хYb_0,20Tm_0.01Gd_xF₄ (x = 0–0.2). The size of the synthesized particles was determined to be about 800 nm for NaY₀₇₉Yb_0,20Tm_0.01F₄ and NaY_0.79Yb_0,20Er_0.01F₄, and about 600 nm for NaY_0.79−хYb_0,20Er_0.01Gd_xF₄ and NaY_0.79−хYb_0,20Tm_0.01Gd_xF_4. The decrease in particle size when co-doped with Gd³⁺ ions is explained by the slower crystal growth rates due to an increase in the electron charge density of the crystal surface in Gd³⁺-co-doped microcrystals. XRD showed that all the samples consisted of hexagonal phase and the addition of Gd³⁺ 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 Gd³⁺ ions promote quenching of Er³⁺ and Tm³⁺ emission through depopulation of their upper excited levels. We found an optimal composition of the particles for the maximum intensity luminescence: NaYF₄: 20% Yb, 1% Er, 5% Gd and NaYF₄: 20% Yb, 1% Tm, 5% Gd. Possible energy transfer mechanisms for upconversion emissions in NaYF₄ host co-doped with different rare earth ions Yb, Tm, Er, and Gd upon 980 nm excitation were proposed.