Effect of Pr3+ concentration in luminescence properties & upconversion mechanism of triple doped NaYF4: Yb3+, Er3+, Pr3+

Lanthanide-doped fluoride nanocrystals (NCs) exhibit excellent optical features, including upconversion and downconversion luminescence (UCL and DCL), that can be utilized in a variety of applications. In this study, we have successfully demonstrated the photoluminescence behavior of triple-doped NaYF4: Yb3+, Er3+, Pr3+ NCs in the Vis-NIR region. Herein, highly monodisperse hexagonal phase NaYF4: Yb0.2, Er0.02, Prx nanocrystals in various Pr3+ (x = 0, 0.1, 0.5, and 1 mol %) concentration with ∼22 nm diameter synthesized by thermal decomposition technique. The photoluminescence studies for all samples were performed under 980 nm laser excitation. The luminescence intensity of Er3+ including blue (407 nm), green (520 and 540 nm), red (654 nm), and near-infrared (845 nm and 1530 nm) emissions was significantly quenched and Pr3+ emission intensity at 1290 nm (Pr3+:1G4→3H5) changes irregularly upon doping with Pr3+ ions. Furthermore, we performed the excitation power dependence and decay time analysis to investigate the energy transfer and upconversion mechanisms of samples. These findings indicate that the presence of praseodymium strongly reduces emission intensities due to abundant cross-relaxation channels. In addition, particle size is an efficient factor, shedding light on the influence of Pr3+ on the energy transfer and upconversion mechanisms of the fluorides.


Introduction
Near-infrared emitted upconversion nanoparticles (UCNPs) are remarkably focused on today's technologies, specifically in optical and biotechnological applications.UCNP nanoparticles, when exposed to low energy radiation (NIR), such as low-cost and readily available lasers, usually release higher energy in the visible and NIR region.The near-infrared (NIR) spectrum (700-1700 nm) has captivated considerable interest owing to having extraordinary features, including low autofluorescence and scattering, significant anti-stoke shift, long photoluminescence lifetime, minimum toxicity, and high penetrability into various materials [1,2].Owning to defined characteristics, UCNPs have the potential to include lasers [3], displays [4], solar cells [5], optical fiber amplifiers [6], and biological applications such as intracellular signaling, and NIR-II-based deep-tissue bioimaging [7,8].
Despite huge studies in UCNPs, a big challenge for nanoparticles remains to achieve high-intensity emission in the NIR region.Clarifying this issue requires a deep investigation of various factors such as the host material, crystal structure, dopant type, and dopant concentration present in nanoparticles.Among the host materials, the beta phase of NaYF 4 is well known to be an efficient host due to its low phonon energy and has been extensively studied in numerous upconversion luminescence applications [9].To achieve NIR emission, dopant ions like Yb 3+ , Er 3+ , Nd 3+ , and Pr 3+ are frequently employed.Notably, Ytterbium ions exhibit an impressive absorption cross-section under NIR radiation (980 nm); therefore, many studies utilized Yb 3+ as a promising choice for sensitized luminescence [10,11].For instance, Dai et al successfully developed ultra-bright nanoparticles doped with Er 3+ for non-invasive imaging of cancer immunotherapy in a second near-infrared window [12].In another study, Nd 3+ was used as a doping ion in β-NaYF 4 microcrystals, and near-infrared emission was observed at 1058 nm with 4 F 2/3 → 4 I 11/2 transition [13].In addition to the studies carried out with Er 3+ and Nd 3+ ions to obtain near-infrared emission, Pr 3+ is another alternative dopant ion as reported by Ruiz-Caridad et al who demonstrated a near-infrared emission at 1350 nm emission under 1020 nm excitation in Prdoped Y 2 O 3 -ZrO 2 crystals [14].Although many studies focused on a single type of lanthanide doping, as mentioned above, many researchers have investigated the optical properties of co-doped or triple-doped materials to overcome the quenching effect encountered at high doping concentrations [15].
Among the rare-earth elements, (Pr 3+ ) and (Er 3+ ) are attractive candidates for emission in the NIR region, especially in the 1300 and 1550 nm ranges, respectively [16].However, due to the electron configuration of the Pr 3+ , enhancing the emission of f-f transitions is often challenging [17].Therefore, Er 3+ can be a feasible alternative for decreasing the cross relaxation-induced quenching of Pr 3+ near-infrared emission . Mostly glasses and crystals co-doped with Pr 3+ /Er 3+ appear to be relatively suitable candidates for broadband near-IR luminescence.In particular, the Pr 3+ and Er 3+ ions can emit light in the range of 1200 to 1650 nm, via transitions such as 1 G 4 → 3 H 5 (Pr 3+ ), 1 D 2 → 1 G 4 (Pr 3+ ), and 4 I 13/2 → 4 I 15/2 (Er 3+ ) in fluoridated glass [19].Furthermore, Pr 3+ and Yb 3+ co-doped nanoparticles have been applied for near-infrared bioimaging and antibacterial effects [20].In another application, a highly efficient quantum-cutting process was observed from the Pr 3+ /Yb 3+ co-doped NaYF 4 nanocrystals, converting one high-energy visible photon into two or more low energy near-infrared photons [21].
So far, trivalent Pr 3+ and Er 3+ ions have been codoped with different host structures to obtain a broad emission band in the near-infrared region.Se Ho Park et al introduced Pr 3+ /Er 3+ co-doped Ge-As-Ga-S glasses, investigating fluorescence emissions in the 1.31 and 1.55 μm communication windows.Their work showed that the lifetime of the Pr 3+ : 1 G 4 level rose with increasing Er 3+ concentration, whereas that of the Er 3+ : 4 I 11/2 level declined.Doped Ge-As-Ga-S glasses, containing Pr 3+ and Er 3+ exhibit great promise as amplifier materials [22].Ho Kim Dan et al examined energy transfer (ET) and NIR emission from Pr 3+ , Er 3+ , Yb 3+ tri-doped niobate tellurite glass (TeO 2 -Nb 2 O 5 -BaO-La 2 O 3 ), resulting in broadband NIR spectrum from 1250 to 1650 nm, with a fullwidth half maximum (FWHM) of ∼ 300 nm.It was found that enhancing Yb 3+ concentration strengthens the NIR emissions.These findings suggest that the niobate tellurite glass (TeO 2 -Nb 2 O 5 -BaO-La 2 O 3 ) material has the potential to be used in optical fiber amplifiers (OFAs) [23].Furthermore, Bo Zhou et al, demonstrated for the first time that praseodymium (Pr 3+ ), and erbium (Er 3+ ) codoped fluorotellurite glasses emit in a super broadband range, covering 1.30 to 1.68 μm under 488 nm illumination.The results indicate that fluoro tellurite glass doped with Pr 3+ and Er 3+ is a promising material for applications in optical amplification, tunable lasers, and super broadband amplified spontaneous emission sources [16].Additionally, triple-doped NaYF 4 nanoparticles with the Yb 3+ , Er 3+ , and Pr 3+ combination were reported to probe the white light upconversion luminescence [18].Under 980 nm excitation, a nanoparticle co-doped with Yb 3+ , Er 3+ , and Pr 3+ exhibited a combination of emissive modes peak at 1320 nm, and 1530 nm belonging to Pr 3+ and Er 3+ emission, respectively.Meanwhile, Er 3+ was also used to promote near-infrared emission which may be employed for NIR DCL bioimaging in the NIR-II window of biological tissue transparency [24].These studies collectively highlight the potential of various doping combinations and host materials in achieving efficient near-infrared emission and advancing the development of optical amplification and other related applications.
The present research aims to evaluate the influence of Pr 3+ dopants on photoluminescence properties under a 980 nm excitation laser.We employed NaYF 4 nanocrystals doped with Er 3+ and Pr 3+ concentrations (1 mmol %).The Pr 3+ ion was selected as the Er 3+ and Pr 3+ energy levels are in resonance with one another, leading to an enhanced probability of crossrelaxation due to the broad range of energy levels.To the best of our knowledge, our study represents the photoluminescence behaviour of NaYF 4 : Yb 3+ , Er 3+ , Pr 3+ in various Pr 3+ concentrations that resulted in emissions in both the VIS and NIR regions.The required analysis was performed on samples such as absorbance spectra, decay time measurements, and power dependence studies for the first time.3+ , and Yb 3+ ions were synthesized following the standard thermal decomposition technique based on a protocol that has been reported in detail [25].First, (1 mmol) of aqueous lanthanide acetate Ln (CH 3 CO 2 ) 3 (Ln= Y 3+ , Yb 3+ , Er 3+ , Pr 3+ ) was prepared and dissolved in 6 ml oleic acid (OA), and 15 ml octadecene (OCDE) under 150 °C heating for 30 min while stirring to form a clear, homogenous solution.After the reaction was completed the reaction temperature was lowered to 60 °C then a methanol solution of ammonium fluoride (2.5 mmol) and sodium hydroxide (4 mmol) was added slowly to the solution.In the following process, the reaction temperature was increased to 110 °C and the obtained mixture was maintained at 110 °C under a vacuum for the completion of methanol evaporation.In the final step, the reaction temperature increased to 300 °C and kept for 90 min under an inert atmosphere.After the process is completed, the mixture is cooled to room temperature and nanocrystals are precipitated followed by ethanol and cyclohexane several times and collected by centrifugation.After the washing process was completed, the obtained pellets were dispersed in cyclohexane.This procedure was performed for the synthesis of hexagonal structure nanoparticles; the samples were prepared with the different mole percentages NaYF 4 : 20%Yb 3+ , 2%Er 3+ , x % (x = 0, 0.1, 0.5, 1) Pr 3+ .

Measurement and characterization
The x-ray powder diffraction (XRD) BRUKER D-8 ADVANCE, employing Cu Kα radiation (λ = 1.54 A°), 40 kV-40 mA at room temperature, is utilized for evaluation of crystal structures and phase purities of samples.Transmission electron microscopy (TEM) was performed with a device model JEM-2100-PLUS to characterize the size and morphology of samples.The high-resolution TEM (HRTEM) was recorded using an instrument model Talos L120C.The optical absorption spectra of samples were carried out with a UV/VIS spectrometer model PerkinElmer model LAMBDA35 with a wavelength range between 190-1100 nm at room temperature.Photoluminescence (PL) spectra of the synthesized UCNPs were characterized by the Horiba FluoroLog-QM spectrofluorometer with a continuous wave NIR diode laser (980 nm).The spectrofluorometer featured a 75 W xenon arc lamp, an R928P photomultiplier tube (PMT) with a detection limit of 250-850 nm, and an InGaAs detector (measurement range, 800-1700 nm).The fluorescence decay curves were recorded, using a pulsed light source (980 nm) incorporated into a regular Fluorolog-QM and utilizing the standard SSTD mode to measure phosphorescence lifetimes.

Structural analysis
In this study, the NaYF 4 :Yb 3+ , Er 3+ UCNPs were synthesized by thermal decomposition technique.The UCNPs were then characterized by XRD and TEM, respectively.The XRD patterns for different dopant concentrations of Pr 3+ in the NaYF 4 :Yb 3+ , Er 3+ matrix demonstrated the pure single-phase hexagonal structure nanoparticles.For the sake of brevity and clarity, the experimental parameter such as dopant ion concentration is listed in table 1 and the samples have been appropriately labelled in the figures.
The normalized XRD pattern of samples is shown in figure 1.The diffraction peaks of samples matched with a slight shift to the standard card of hexagonal Na (Y 0.57 Yb 0.39 Er 0.04 ) F 4 (reference code: 00-028-1192) with the lattice constants of a = b = 5.960 Å, c = 3.510 Å, α = β = 90 °C, γ = 120 °C.The XRD results confirmed the crystallinity and phase purity of samples that remained unchanged despite various doping concentrations of Pr 3+ ions.
Studies also reported lanthanides due to their comparable ionic radii can be substituted for one another in crystalline host matrices; they can also be added in various combinations and introduced in varying concentrations without significantly altering the crystal structure [26].Since the ionic diameters of Pr 3+ (1.82 Å) and Y 3+ (1.80 Å) are similar, and the valence of Pr 3+ and Y 3+ are the same, Pr 3+ can substitute for the Y 3+ sites [27].
In order to reveal the subtle differences caused by Pr 3+ doping, a selected region of diffraction peaks is magnified and marked by a rectangle in figure 1.The magnified XRD pattern of the NaYF 4 : Yb 3+ , Er 3+ , Pr 3+ samples exhibited peak shift towards smaller angle compared to the reference pattern, which can be attributed to the smaller particle size.The XRD peaks widened and shifted slightly; the broadening concerns the decrease in particle size, whereas the shift can be attributed to alter in the crystal lattice [28].The difference in particle size observed in the 0.1% Pr 3+ sample compared to the other samples can be attributed to the potential differences in the colloidal synthesis process across different experiments.
Moreover, the Scherer formula can be used to determine the size of the grain based on the diffraction width peak, though the average nanocrystalline sizes were determined by applying the Scherrer formula (equation ( 1)) to the XRD spectra shown in table 2. Furthermore, no correlation was found between the Pr 3+ concentration and the average particle size of the samples.

Morphology and size distribution
The Morphology and size distribution of samples (NaYF 4 : Yb 0.2 , Er 0.02 , Pr x ) were studied by transmission electron microscope (TEM).In figure 2, the TEM image and size distribution related to NaYF 4 : Yb 3+ , Er 3+ doped nanoparticles are demonstrated with a different dopant ion concentration of Pr 3+ , as shown in figures 2(a)-(h).The average size of nanoparticles was achieved at around 22 nm.The results confirmed that the TEM analysis is roughly consistent with the XRD average particle size calculations.Moreover, the high-resolution TEM (HRTEM) image and the Fast Fourier-transformation (FFT) of the nanoparticle result proves the pure single-crystalline structure with a uniform lattice fringe and a d-spacing of 0.53 nm, which corresponds to the d-spacing for the (110) and (100) lattice planes of the beta phase NaYF 4 , as depicted in figure 2(i).In particular, the formation of a perfect hexagonal close-packed (HCP) structure of triple-doped NaYF 4 nanoparticles was confirmed by the selected area electron diffraction (SAED), shown in figure 2(j).
The comparison in particle sizes calculated by the Scherer formula from the XRD result and size distribution of TEM observations are given in table 2. Scherrer equation provides an estimate of the crystallite size in the sample, which is smaller than the nanoparticle size because nanoparticles are often agglomerations of many crystallites.Therefore, the size distribution of the nanoparticles observed by TEM is larger than the crystallite size calculated by the Scherrer equation [29].

Optical properties 3.3.1. Absorption spectra
The absorption spectra were utilized to confirm the presence of pure Yb 3+ , Er 3+ , and Pr 3+ dopants and to understand the energy levels and transitions in Yb 3+ , Er 3+ , Pr 3+ triple-doped NaYF 4 samples, thereby providing valuable information about the electronic structure of the material.A Perklin-Elmer UV-vis-NIR Lambda 35 spectrophotometer was used to collect the optical absorption spectra.The powder samples were placed and examined in a sample holder with a quartz window.The range of wavelengths that were measured fell within the spectrum of 190 nm to 1100 nm.The absorption spectroscopy for the Yb 3+ , Er 3+ , Pr 3+ triple doped NaYF 4 samples were investigated with and without Er 3+ dopant.The absorption spectra of samples 20Yb1Pr and 20Yb2 Er1Pr are shown in figure 3.
The intense peak at 976 nm is common for both spectra and originates from 2 F 5/2 → 2 F 7/2 absorption of Yb 3+ .For the blue lined spectrum (20Yb1Pr), the absorption bands located at 445, 469, 483, 587 nm  correspond to the transitions from the ground state 3 H 4 to specific excited states 3 P 2 , 1 I 6 + 3 P 1 , 3 P 0 and 1 D 2 of Pr 3+ , respectively.For the red lined spectrum (20Yb2Er1Pr), in addition to the aforementioned transitions belonging to the Pr 3+ the absorption bands located at 520, 540, and 654 have been observed,  which can be assigned to  4).When Pr 3+ ions were doped into the 20Yb2Er sample, all peak intensities were quenched with the increase in the Pr 3+ dopant concentration, shown in figure 4.However, no emission peaks belonging to Pr 3+ ions were observed at the visible region originating from the 3 P 0,1 → 3 H 4-6 , 3 F 2-4 , 1 D 2 → 3 H 4 transitions [31].
In the NIR region, depicted in figure 5, the luminescence intensity at 1530 nm (Er 3+ : 4 I 13/2 → 4 I 15/2 ) was significantly quenched and weak emission intensity at 1290 nm (Pr 3+ : 1 G 4 → 3 H 5 ) changes irregularly  upon doping with Pr 3+ ions.It is evident that in the population of 4 I 13/2 and 1 G 4 levels, cross relaxations seem to be the main process affecting the emission from these levels.

Power dependence of upconversion luminescence
In order to comprehend the upconversion mechanism of our samples, we analyzed the excitation power dependence of UC emission intensities for NaYF 4 : Yb 3+ , Er 3+ , Pr 3+ samples excited at various pump powers using the 980 nm diode laser and the pump power dependent upconversion luminescence spectra are recorded, either for emissions in visible or NIR regions, as depicted in figures 6 and 7, respectively.The relation between upconversion emission intensity and excitation power was established using equation (2), Where I is the emission intensity, P is the power of excitation and n is the number of photons involved in the upconversion process.The value of n can be ascertained by analyzing the slope value of a linear fitting in a logarithmic-logarithmic plot of equation (2).
In the visible region, nanoparticles under a 980 nm laser diode indicated that the number of photons involved in the upconversion process was found to be closely related to the concentration of dopant ion, as the increment in concentration can either promote or impede the upconversion process.This relationship is illustrated in figures 6(a) and (b), which shows the impact of increasing the Pr 3+ concentration from 0.1% to 1%, respectively.
Figure 6(a) shows that for the 20Yb2Er0.1Prsample, the slope values for UC emission peaks at 407, 488, 520, 540, and 654 nm were 3.11, 2.53, 2.54, 2.53, and 2.66, respectively.Since the slope values are approximately three, the upconversion mechanism should be a three-photon process.Subsequently, upon adding 1 mol% of Pr 3+ ions, the slope values decreased to 2.82, 2.81, 2.25, 2.23, and 2.23 for the 407, 488, 520, 540, and 654 nm emissions, respectively, as demonstrated in figure 6(b).The slope value for 407 and 488 nm emission is close to 3, indicating that three-photon processes are involved in populating the 2 H 9/2 and 4 F 7/2 levels.For the 520, 540, and 654 nm emissions, the slope values are close to 2, indicating two-photon processes are involved in populating the 2 H 11/2 , 4 S 3/2 , and 4 F 9/2 , respectively.The decrease in slope values for 520, 540, and 654 nm emissions from three to two photons can be attributed to the transitions 4 F 7/2 → 3 P 0 and the following multiphonon relaxations (MPRs) and cross-relaxation (CR) processes.Additionally, competition between the energy transfer from 4 I 13/2 (Er 3+ ) to 3 F 3,4 (Pr 3+ ) level and upconversion processes pumping to 4 F 7/2 where the depletion of intermediate excited states of 4 I 13/2 level might be effective [32].
In the NIR region, 0.1% and 1% Pr 3+ doped NaYF 4 : Yb, Er samples were examined for 1290 nm and 1530 nm emissions at different pump powers of the 980 nm diode laser and the pump power-dependent upconversion luminescence spectra is recorded as shown in figures 7(a), (c).The log-log plots for 1290 nm and 1530 nm emissions, logarithmic upconversion emission intensity versus logarithmic excitation power density graphs, are illustrated in figures 7(b), (d).
For the 1290 nm emissions, the slope value of the 20Yb2Er0.1Prsample was found to be 0.58, as shown in figure 7(a).As Pr 3+ ion concentrations increased in the system, the slope value of the 20Yb2Er1Pr sample decreased to ∼0.08, as depicted in figure 7(d).These

Fluorescence decay time
Decay time analysis was conducted on the samples to achieve a better understanding of the energy transfer and upconversion mechanism.Samples with increased concentration of Pr 3+ dopant were examined under 980 nm pulsed laser excitation.To demonstrate the concentration-dependent photoluminescence dynamics of the Pr 3+ dopant, decay time analysis was monitored at 520 nm (figure 8(a)), 540 nm (figure 8(b)), 1290 nm (figure 8(c)), and 1530 nm (figure 8(d)).The emission lifetimes (τ) were calculated using a single exponential decay function (equation ( 3)) and results presented in table 3.
where I(t) represent the photoluminescence intensity at a specific time point corresponding to the on and off of the pulsed laser.τ is the decay lifetime of the upconversion luminescence.
The decay time at 520 nm for the sample without Pr 3+ ions was calculated to be 111.98 μs, which originated from the 2 H 11/2 energy level of the Er 3+ ion.As the Pr 3+ concentration was increased to 0.1% in the second sample, decay time decreased to 92.61 μs.With further increases in the Pr 3+ concentration to 0.5% and 1%, decay times decreased to 72.42 μs and 62.55 μs, respectively.The decay time at 540 nm presents similar behavior with increasing Pr 3+ concentration, as shown in table 3. It is expected that the decrease in the lifetime of 2 H 11/2 and 4 S 3/2 levels might be caused by an energy transfer of Er 3+ : 4 F 7/2 → Pr 3+ : 3 P 0 .
In the NIR region, decay time was studied as well.The decay time for the samples without Pr 3+ and with 0.1% Pr 3+ was not detected at 1290 nm.By increasing the Pr 3+ concentration to 0.5% Pr 3+ and 1% Pr 3+ , the decay time is determined to be 32.63 μs and 40.52 μs, respectively.
This increment in decay times at 1290 nm for the samples with varying Pr 3+ concentrations demonstrates the existence of energy transfer ( 4 I 11/2 → 1 G 4 ).On the other hand, the decay time at 1530 nm ( 4 I 13/2 → 4 I 15/2 ) for all samples demonstrated an irregular behavior as Pr 3+ concentration increased (table 3).Hence, there should be another mechanism explaining this irregularity in lifetime at 1530 nm.
Based on these findings, the energy transfer from Er 3+ to Pr 3+ can be estimated.As deduced from decay    F 7/2 state can be de-excited via ET4 and transfer its energy to the 3 P 0 state.It is expected that, if an energy transfer ET4 exists there should be emissions stemming from transitions due to the 3 P 0,1 → 3 H 4-6 , 3 F 2-4 , and 1 D 2 → 3 H 4 .However, no visible emissions are observed related to those transitions.Even if ET4 exists, visible emissions could be hampered through numerous cross-relaxations (CR1, CR2, and CR3) and MPR processes [31,33].
Herein, the quenching in blue (407 nm), green (520 and 540 nm), red (654 nm) and near-infrared (845 nm) emissions can be attributed to the decrease in the 4 I 13/2 population.The decrease in the 4 I 13/2 population leads to the decrease in the 4 F 9/2 population through ET2 and subsequently followed by the decrease in the 4 G 11/2 population through ET3.Two mechanisms affect the population of the 4 I 13/2 level.One of the mechanisms is the CR2 process that is highly affected by the particle size and Pr 3+ concentration.The other mechanism is the ET6 energy transfer process affected by only Pr 3+ concentration.The probability of the CR2 relaxations in smaller particles is quite high due to the random distribution of rare earth ions.While the probability of CR2 relaxation is quite less in the larger particles due to the strict distribution of rare earth ions [35].To better understand the CR2 mechanism, populating 4 I 13/2 , we have analyzed the intensity ratio of R/G, as shown in table 4.
Comparing 20Yb2Er (23.66 nm) and 20Yb2Er0.1Pr(31.08 nm), as particle size increases R/G ratio decreases from 0.407 to 0.102, indicating the decrease in CR2 relaxations depopulating the 4 I 13/2 level.Pr 3+ concentration is another effect that populates 4 I 13/2 level through the CR2 process.However, the CR2 processes that are populating 4 I 13/2 level are negligible for small concentrations (0.1 mol % Pr 3+ doped sample) [36].Notably, the cross relaxations stemming from Er 3+ ions are ignored as the Er 3+ concentration is constant for all samples.The ET6 process mitigating emission intensity due to non-radiative decays is negligible for the 0.1 mol % Pr doped sample as well.Overall, the less likely CR2 relaxation process results in a depopulation of the 4 I 13/2 level, giving rise to a shorter decay time from 34.89 μs to 19.83 μs.
By increasing the Pr 3+ concentration to 0.5 mol% (sample 20Yb2Er0.5Pr),both decrease in particle size to 21.18 nm and increase in Pr 3+ concentration are effective on the increase in CR2 relaxations populating 4 I 13/2 level.Although the rise in ET6 process in parallel with the Pr 3+ concentration depopulates 4 I 13/2 level.Yet, the CR2 processes populating 4 I 13/2 levels are dominant leading to an increase in intensity ratio of R/G to 0.144.Hence, a longer decay time was observed from 19.83 μs to 31.93 μs.
Further increasing Pr 3+ concentration to 1 mol% (sample 20Yb2Er1Pr), particle size increased to 22.48 nm leading to a decrease in CR2 relaxations but their contribution is negligible as particle sizes are almost equal.The increase in Pr 3+ concentration causes an increase in CR2 relaxations, which populate the 4 I 13/2 level.However, due to the decrease in the distance between the lanthanide ions, ET6 processes increase and, as a result, 4 I 13/2 is depopulated.The population of 4 I 13/2 with CR2 process is estimated to be dominant over the depopulation of 4 I 13/2 with ET6 process, causing a further increase in the intensity ratio of R/G to 0.18 and, Hence, a longer decay time observed from 31.93 μs to 38.04 μs.
Meanwhile, an energy transfer ET5 may occur from Er 3+ ( 4 I 11/2 ) to Pr 3+ ( 1 G 4 ) level due to their wellalignment.This transfer leads to a weak emission at 1290 nm, corresponding to the transitions of 1 G 4 → 3 H 5 .The observed emission at 1290 nm seems to be affected by CR2 relaxations enhancing the population of 1 G 4 level as well.As stated before, particle size and Pr 3+ concentrations are the factore that highly affect the CR2 relaxation process [35,36].Herein, it is estimated that within the CR2 process, particle size is more effective than Pr concentration.Even if Pr 3+ concentration is a significant factor in the population of 1 G 4 level through CR2, the gradual increment in 1290 nm would be anticipated in parallel with Pr 3+ concentration.Nevertheless, the results show that the enhancement of 1 G 4 → 3 H 5 transitions is more associated with particle size rather than Pr 3+ concentrations.

Conclusion
In conclusion, nanoparticles with an average size of around 22 nm were synthesized using thermal decomposition technique.XRD and TEM characterization techniques were utilized to characterize the morphology and crystal structure of the nanoparticles.The luminescence properties of UCNPs were examined by co-doping various amounts of Pr 3+ in NaYF 4 : Yb 3+ , Er 3+ to understand the upconversion luminescence properties.The luminescence intensity of Er 3+ including blue (407 nm), green (520 and 540 nm), red (654 nm), and near-infrared (845 nm and 1530 nm) emissions was significantly quenched and Pr 3+ emission intensity at 1290 nm (Pr 3+ : 1 G 4 → 3 H 5 ) changes irregularly upon doping with Pr 3+ ions.Powerdependent analysis revealed a gradual decrease in the intensity in the visible region.The slope value decreased from three to two, which can be attributed to the competition between the energy transfer from 4 I 13/2 (Er 3+ ) to 3 F 3,4 (Pr 3+ ) level and upconversion processes for the depletion of intermediate excited states of 4 I 13/2 level.A similar trend was observed in the near-infrared region as well.The rate of decreasing slope value at 1530 nm is greater than 1290 nm, indicating a faster depletion for the 4 I 13/2 level compared to the 4 I 11/2 level.Consequently, the energy transfer rate of 4 I 13/2 → 3 F 3,4 is faster than 4 I 11/2 → 1 G 4 .The decay time measurements at the 4 I 13/2 level of Er 3+ ions change irregularly with the Pr 3+ concentrations due to the competition between CR2 and ET6 energy transfer processes.From the results, it is estimated that particle size is more effective than Pr 3+ concentration in the population of 4 I 13/2 through the CR2 process.Simultaneously, the population of 1 G 4 level enhanced with the CR2 process as well.Therefore emission at 1290 nm (Pr 3+ : 1 G 4 → 3 H 5 ) is also more affected by the particle size than Pr 3+ concentration.The results presented in this work serve as valuable literature for researchers interested in this field, providing a foundation for further investigations and advancements in the utilization of Pr 3+ ions in upconversion applications.

Figure 4 .
Figure 4. Upconversion photoluminescence emission spectra of nanoparticles under 980 nm laser excitation in the visible region, where all discernible peaks are exclusively attributed to Er 3 .(Inset presented high-resolution region).

Figure 7 .
Figure 7. Near-infrared photoluminescence spectra of UCNPs under different excitation power of 980 nm diode laser (a), (c) and accompanied by Logarithmic plot (b), (d).The slope value comparison versus excitation power at 1290 and 1530 nm emission for samples (e).
time and power-dependent analysis, it is possible to figure out the upconversion mechanism, as shown in Scheme 1.The energy level diagram, Scheme 1, presents possible energy transfer mechanisms of triple doped NaYF 4 : Yb 3+ , Er 3+ , Pr 3+ systems.Upon 980 nm excitation, emissions observed in visible regions including blue (407, 465 and 488 nm), green (520 and 540 nm), red (654 nm), and near-infrared (845, 1290, and 1530 nm) region can be attributed to the population of 4 G 11/2 and

Table 1 .
Labelled experimental parameters for dopant ion concentration.

Table 2 .
The average particle size of samples in different Pr 3+ concentrations doped in NaYF 4 .Using the Scherrer equation and TEM observations.

Table 3 .
Lifetime measurements of samples at different emission peaks.

Table 4 .
The decay time and R/G ratio analysis of samples associated with Pr 3+ concentration and particle size.Prof.Murat Erdem for the invaluable support in PL analysis and Pınar Akkuş Süt for TEM observations.