Understanding the effect of Mn2+ on Yb3+/Er3+ co-doped NaYF4 upconversion and obtaining the optimal combination of these tridoping

In this work, we investigated in detail the upconversion properties of several types of nanoparticles, including NaYF4:5%Yb3+/30%Mn2+, NaYF4:40%Mn2+/x%Yb3+ (x% = 1, 5, 10, 20, 30, and 40), NaYF4:2%Er3+/x%Mn2+ (x% = 20, 30, 40, 50, 60, and 70), NaYF4:40%Mn2+/x%Er3+ (x% = 1, 2, 5, and 10), and NaYF4:40%Mn2+/1%Yb3+/x%Er3+ (x% = 0, 2, 5, and 10). We studied their upconversion emission under 980 nm excitation in both pulsed and continuous wave modes at different synthesis temperatures. The nanoparticles were characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD), and photoluminescence (PL) spectroscopy. The doping of Yb3+ and Mn2+ ions resulted in the nanoparticles assuming cubic and hexagonal crystal structures. The emission intensity increased (106.4 (a.u.*103) to 334.4(a.u.*103)) with increasing synthesis temperature from 120 to 140 °C, while a sharp decrease was observed when the synthesis temperature was increased to 200 °C. The gradual decrease in peak intensity with increasing Mn2+ concentration from 20 to 70% was attributed to energy transfer from Mn2+ to Yb3+. In NaYF4:Mn2+/Yb3+/Er3+ UCNPs, increasing the Er3+ concentration from 0 to 10% led to the disappearance of the blue, orange, and green emission bands. The intense upconversion luminescence pattern with high spatial resolution indicates excellent potential for applications in displays, biological sensors, photodetectors, and solar energy converters.

www.nature.com/scientificreports/with Mn 2+ ions can lead to strong red fluorescence in UCNPs, making them useful for in vivo imaging and drug delivery 29 .
In our study, we investigated the upconversion properties of several types of UCNPs to better understand the energy transfer mechanisms among Yb 3+ /Mn 2+ , Mn 2+ /Er 3+ , and Mn 2+ /Er 3+ /Yb 3+ ions.We found that NaYF 4 is an efficient host material for red, green, and blue UC phosphors with cubic and hexagonal structures.Er 3+ -doped NaYF 4 is recognized as one of the most stable UCNPs upon NIR excitations, while Mn 2+ -doped NaYF 4 can lead to strong red fluorescence in UCNPs.Our results suggest that the doping of Mn 2+ can decrease the non-radiative transition probability, thus improving the intensity of UCNP emission.
Overall, our findings help to shed further light on the energy transfer mechanisms in UCNPs and highlight the potential applications of these materials in various fields, such as biological sensing, photonics, and solar energy conversion.

Nanoparticle synthesis
Synthesis of NaYF 4 :5%Yb 3+ /30%Mn 2+ nanoparticles Yb 3+ and Mn 2+ co-doped NaYF 4 nanoparticles were synthesized using a well-established hydrothermal method.NaOH (0.2 M, 1.2 ml) was added to a 50 ml centrifuge tube and stirred for 10 min.A mixture of ethanol (5 ml) and OA (5 ml) was added to NaOH and magnetically stirred for 15 min at room temperature until the solution became uniform and clear.A Ln solution was added to the centrifuge tube (Ln in total is 0.4 mmol): YCl 3 (0.26 mmol), YbCl 3 (0.02 mmol) and MnCl 3 (0.12 mmol).The solution was kept under magnetic stirring for 15 min.Subsequently, 59.26 mg of NH 4 F dissolved in 1.6 ml of deionized water was added to the solution.The final solution was stirred magnetically for 1 h at a temperature of 40 °C.Thereafter, the solution was transferred into a Teflon (PTFE) reactor and sealed with Argon (Ar) gas.The PTFE reactor was inserted into an autoclave and heated at 120 °C, 140 °C, 160 °C, 180 °C, and 200 °C for 8 h.After 8 h, the autoclave was emptied, and the material in the PTFE reactor was cooled to room temperature.The solution was precipitated with ethanol (20 ml), collected by centrifugation (temperature: room temperature, time: 5 min, spin speed: 7500 rpm) washed several times with ethanol (10 ml), and dispersed in cyclohexane (10 ml) for further characterization.Further details of the synthesis of nanoparticles are given in the supplementary file.

Characterization
The size, shape, structure, and morphological characterization of the as-prepared UCNPs were characterized using a transmission electron microscope (TEM, JEOL, JEM-1400).The optical absorption spectrum in the wavelength range of 300-1000 nm was measured using an Edinburgh FS5 spectrophotometer.UC luminescence (UCL) spectra and decay curves were obtained using an Edinburgh FS5 spectrophotometer equipped with a 980 nm diode laser, operating in pulsed and continuous-wave (CW) mode.The pulse duration for the impact mode was 3 min, the power density of the 980 nm laser was 1 W/cm 2 , and the spot width was 3 mm.1000 µl of the solution was poured into the cuvette and the cuvette was placed inside the measuring device.

Structure and morphology characterization
TEM was used to characterize the morphologies of the as-prepared UCNPs.Figure 1 displays TEM images of non-doped NaYF 4 UCNPs and NaYF 4 : 30% Mn 2+ , 5%Yb 3+ UCNPs at various synthesis temperatures (120 °C, 140 °C, 160 °C, 180 °C, and 200 °C).The non-doped NaYF 4 UCNP crystal structure appeared completely hexagonal, while with the addition of Yb 3+ and Mn 2+ ions as dopants, the nanoparticles exhibited both cubic and hexagonal crystal structures.The average width of the nanoparticles was determined using the Digimizer software (version 4. 1. 1. 0, MedCalc software) (Fig. 1), and only a minor dispersion of the nanoparticles' diameters was observed.To estimate the mean diameter of the nanoparticles, the obtained data were fitted with the Log-Normal distribution equation 35 .www.nature.com/scientificreports/Equations (1-3) were used to fit the data and obtain the fitting parameters D 0 and σ.Additionally, the standard deviation (σ D ) and mean diameter of the nanoparticles <D> were calculated using the results obtained from the data fitting.As shown in Fig. 1, the diameter of the nanoparticles increased from 19 to 36 nm as the temperature increased from 120 to 200 °C.
The TEM micrographs in Fig. 2 reveal that the NaYF 4 :5%Yb 3+ /x%Mn 2+ (x% = 30, 40, 50, 60, 70) nanoparticles are polyhedral in shape with a uniform size.The average diameter of the NaYF4:5%Yb 3+ /x%Mn 2+ (x% = 30, 40, 50, 60, and 70) nanoparticles was found to be 28 nm.These results suggest that the crystal size of the nanoparticles increases and undergoes continuous and regular changes with the increase in Mn 2+ content.As Mn 2+ concentration increases from 30 to 70%, the number and size of the nanoparticles increases, as calculated using Eq. ( 2).The data demonstrate that the size of the nanoparticles increases from 21 to 30 nm as the Mn 2+ concentration increases from 30 to 70%. Figure 2 also shows the appearance of several hexagonal hollow nanoparticles as Mn 2+ concentration increases up to 70%.These observations are consistent with earlier reports 36,37 .
In summary, XRD is a powerful technique that allows for the analysis of crystal structure and purity.The XRD patterns of NaYF 4 : 5%Yb 3+ /30%Mn 2+ UCNPs, NaYF 4 : 5%Yb 3+ /x%Mn 2+ (x% = 30, 40, 50, 60, 70) UCNPs, and NaYF 4 UCNPs at different Yb 3+ concentrations were analyzed, and the results indicate mixed-phase NaYF 4 crystals with standard patterns of cubic and hexagonal phases.The crystallinity and phase composition were found to be dependent on temperature and dopant concentration, which is consistent with the TEM images.

Determination of optimum synthesis temperature for NaYF 4 : 5% Yb 3+ /30% Mn 2+ nanoparticles
The UCL spectra of the different synthesis temperatures (120 °C, 140 °C, 160 °C, 180 °C, 200 °C) of NaYF 4 : 5% Yb 3+ /30% Mn 2+ nanoparticles were measured using CW 980 nm excitation.Figure 5a displays the UCL emission spectra of these nanoparticles, revealing a relatively strong emission band at 583 nm and a weaker emission band at 575 nm under 980 nm excitation, corresponding to Mn 2+ : 4 A 1 ( 4 G) → 6 A 1 ( 6 S) and Mn 2+ : 4 T 1 ( 4 G) → 6 A 1 ( 6 S) respectively, as shown in Fig. 5a.Our objective was to determine the optimal synthesis temperature for subsequent experiments, and thus, the relationship between reaction temperature and emission intensity was crucial.As depicted in Fig. 5a, the emission intensity increased as the synthesis temperature increased from 120 to 140 °C, but significantly decreased as the synthesis temperature increased to 200 °C.Figure 5b illustrates the log intensity different synthesis temperatures (120 °C, 140 °C, 160 °C, 180 °C, 200 °C) nanoparticles, indicating that the synthesis temperature of 140 °C resulted in the highest emission intensity among all samples, which was www.nature.com/scientificreports/considered the optimum synthesis temperature.These observations were attributed to the shrinking host lattice and the decreased non-radiative relaxation processes, consistent with a previous study 38 .

Optical properties of NaYF 4 : Yb 3+ /Mn 2+ under 980 nm excitation
Figure 5c and e present the UCL yellow emission spectra of NaYF 4 : Yb 3+ /Mn 2+ nanoparticles at their optimum synthesis temperature (140 °C).The UCL spectra were measured using CW 980 nm excitation, revealing a visible area to yellow UCL emission.The pulse duration for the impact mode was 3 min, the power density of the 980 nm laser was 1 W/cm 2 , and the spot width was 3 mm.1000 µl of the solution was poured into the cuvette and the cuvette was placed inside the measuring device.The UCL spectrum consisted of a weak emission peak at 487 nm, corresponding to the d-d transition [Mn 2+ : 4 A 1 ( 4 G) → 6 A 1 ( 6 S)] and a broadband emission peak at approximately 590 nm, corresponding to Mn 2+ : 4 T 1 ( 4 G) → 6 A 1 ( 6 S). Figure 5c illustrates the concentration-dependent UCL emission of NaYF 4 : 5Yb 3+ /x%Mn 2+ (x% = 20, 30, 40, 50, 60 and 70), with the optimal doping concentration of Mn 2+ determined to be x = 40%.Figure 5d displays the log intensity versus Mn 2+ concentration, indicating an  www.nature.com/scientificreports/increase in emission intensity as the Mn 2+ percentage increased from 20 to 40%, followed by a sharp decrease as the Mn 2+ percentage increased to 70%.The gradual decrease in peak intensity with increasing Mn 2+ concentration was attributed to energy transfer from Mn 2+ to Yb 3+39 .Additionally, with increasing Mn 2+ concentration, the peak position shifted from 563 nm (yellow) to 593 nm (orange), confirming this energy transfer.Figure 5e shows the PL intensity versus Yb 3+ concentration, indicating a decrease in emission intensity as the Yb 3+ percentage increased from 1 to 40%, with the optimal doping concentration of Yb 3+ determined to be x = 1% 33 .The decrease in peak intensity with increasing Yb 3+ concentration was attributed to an energy transfer from Yb 3+ to Mn 2+39 .Furthermore, with increasing Yb 3+ concentration, the peak position shifted from 599 nm (orange) to 573 nm (yellow), confirming this energy transfer (Fig. 5f).Visible photons from the excited Yb 3+ -Mn 2+ pairs were ) nanoparticles and (f) log intensity versus Yb 3+ concentration under 980 nm CW excitation.The pulse duration for the impact mode was 3 min, the power density of the 980 nm laser was 1 W/cm 2 , and the spot width was 3 mm.1000 µl of the solution was poured into the cuvette and the cuvette was placed inside the measuring device.

Luminescence decay time of NaYF 4 : Yb 3+ /Mn 2+ nanoparticles under 980 nm excitation
To provide further evidence for the role that temperature plays in enhancing UC emission, decay curves of the UCNPs are presented in Fig. 6a-f.Decay curves were fitted with the following formula proposed by Nakazawa 40 : where τ m is the effective decay time constant, and I(t) is the intensity at time t.
Figures 6a and b show that the luminescence lifetimes increase from 96 to 115 µs as the synthesis temperature rises from 120 to 140 °C, while as the synthesis temperature increases from 140 to 200 °C, the luminescence lifetimes decrease.Therefore, the NaYF4:5%Yb 3+ /30%Mn 2+ nanoparticles with a synthesis temperature of 140 °C, showing the longest luminescence lifetime, were selected as the main sample.As previously demonstrated, a synthesis temperature of 140 °C is the optimal synthesis temperature for producing samples.Figure 6c and d reveal that by increasing the Mn 2+ concentration from 20 to 70%, the luminescence lifetimes of the Yb 3+ : 2 F 5/2 level first increase and then decrease significantly.The luminescence lifetimes for Yb 3+ were found to decrease monotonically with increasing Mn 2+ concentration, providing evidence for efficient energy transfer from Yb 3+ to Mn 2+ ions 37 .These time-decay measurements are consistent with the UCL emission spectra.Figure 6e and f illustrate that by increasing Yb 3+ concentration from 1 to 40%, the luminescence lifetimes of the Yb 3+ : 2 F 5/2 level decrease significantly from 116 to 15 µs.The luminescence lifetimes for NaYF4: 40%Mn 2+ /x%Yb 3+ (x% = 1, 5, 10, 20, 30 and 40) UCNPs were found to decrease monotonically with increasing Yb 3+ concentration, providing evidence for efficient energy transfer from Yb 3+ to Mn 2+ ions 37 .Our main objective was to find a combination of Yb 3+ and Mn 2+ that had the longest luminescence lifetime.Therefore, a sample of NaYF4: 40%Mn 2+ /1% Yb 3+ was selected as the main sample and used in the subsequent steps.These results are in good agreement with the UCL emission spectra and observed structural properties.

Optical properties of NaYF 4 :Er 3+ /Mn 2+ nanoparticles under 980 nm excitation
To investigate the energy transfer between Mn 2+ and Er 3+ and the luminescence properties of NaYF4: Er 3+ /Mn , the UCL emission spectra of synthetic samples were measured under CW 980 nm excitation.The pulse duration for the impact mode was 3 min, the power density of the 980 nm laser was 1 W/cm 2 , and the spot width was 3 mm.1000 µl of the solution was poured into the cuvette and the cuvette was placed inside the measuring device.Figure 7a displays the UCL emission spectra of NaYF 4 : 5%Er 3+ /x%Mn 2+ (x% = 20, 30, 40, 50, 60 and 70) nanoparticles.The UCL spectra consist of four emission bands centered at 486 nm (blue), 524 (green), 549 nm (green), and 654 nm (red), corresponding to the transitions of 4 A 1 ( 4 G) → 6 A 1 ( 6 S), 2 H 11/2 → 4 I 15/2 , 2 S 3/2 → 4 I 15/2 and 2 F → 4 I 15/2 , respectively.The UCL spectra in Fig. 7a demonstrate that the green and blue emissions increase when the Mn 2+ concentration increases to 40%, but then decrease when it increases to 70%.Additionally, the red emission decreases when the Mn 2+ concentration increases from 20 to 70% 41 .These results suggest that increasing the Mn 2+ concentration leads to a continuous decrease in the emission intensity of Er 3+ , indicating the possibility of energy transfer from Er 3+ ions to Mn 2+42 .
To gain further insight into the mechanism of Mn 2+ doped NaYF 4 : 2%Er 3+ nanoparticles, the log intensity versus Mn 2+ concentration for 524 nm, 549 nm, and 654 nm was calculated and presented in Fig. 7b.The green emission intensity increases as the Mn 2+ concentration increases from 20 to 40%, but then decreases as it increases to 70%.Moreover, the red emission intensity decreases as the Mn 2+ concentration increases from 20 to 70%, confirming the role of Mn 2+ in enhancing the green and blue emission and suppressing the red emission in the NaYF 4 : Er 3+ /Mn 2+ system 41 .
To further investigate the effect of Mn 2+ on the upconversion luminescence (UCL) properties of Yb 3+ /Mn 2+ / Er 3+ triply-doped NaYF 4 nanoparticles, a diagram of energy levels and corresponding energy transfer mechanisms is presented in Fig. 8a.When Mn 2+ ions are introduced into NaYF 4 : Yb 3+ /Er 3+ , a new energy transfer process between Er 3+ and Mn 2+ is induced under the excitation of 980 nm CW laser.This process leads to a decrease in the radiative transition rate of Er 3+ :H 11/2 and Er 3+ : S 3/2 levels to the ground state, while the population density of Mn 2+ : 4 T 1 increases due to the resonance energy transfer.Subsequently, a back-energy transfer from 4 T 1 of Mn 2+ to the 4 F 7/2 level of Er 3+ leads to an enhancement in the red emission.Here, the direct multi-phonon relaxation process and the indirect energy transfer process of (F 5/2 (Yb 3+ ), 4 I 13/2 (Er 3+ )) → ( 2 F 7/2 (Yb 3+ ), 7 I 9/2 (Er 3+ )) are expected to have a minor contribution to the population of 4 S 3/2 energy level in Er 3+ ions.It is noteworthy that the Er 3+ : F 9/2 lifetime is shorter than that of Mn 2+ : 4 T 1 , which explains why no orange luminescence is observed corresponding to the Mn 2+ : 4 T 1 → 6 A 1 transition.A similar mechanism has been discussed in NaYF 4 by Zhangyu Huang et al. 34 .The Schematic of energy level diagrams of Mn 2+ -Yb 3+ dimer is shown in Fig. 8b.For the Mn 2+ -Yb 3+ dimer, the sensitization through the Mn 2+ -Yb 3+ dimer complex entails both ground state absorption (GSA) and excited state absorption (ESA).The Mn 2+ -Yb 3+ dimer ground state is represented by | 2 F 7/2 , 6   To provide further evidence of the role played by Er +3 in enhancing UC emission, Fig. 9a and b presents the decay curves of the UCNPs.The pulse duration for the impact mode was 3 min, the power density of the 980 nm laser was 10 W/cm 2 , and the spot width was 3 mm.1000 µl of the solution was poured into the cuvette and the cuvette was placed inside the measuring device.Figure 9a shows the lifetime spectra of NaYF 4 : 1% Yb 3+ /40% Mn 2+ /x% Er 3+ (x% = 0, 2, 5, and 10) nanoparticles, while Fig. 9b shows the amount of decay under 980 nm pulsed excitation.The luminescence lifetimes of NaYF 4 : Yb 3+ /Mn 2+ /Er 3+ were found to decrease monotonically with increasing Er 3+ concentration.This provides evidence for efficient energy transfer from Yb 3+ to Mn 2+ ions 38 .The decrease in lifetime is likely due to a competitive effect between the energy transfer of Er 3+ and Mn 2+ ions and the decrease in radiative transition probability resulting from increased local symmetry after Er 3+ doping.The former prolongs the lifetime, while the latter decreases it.

Figure 8 .
Figure 8.(a) Schematic of energy level diagrams of NaYF4:Yb 3+ /Mn 2+ /Er 3+ and the proposed mechanism of the UC process under the excitation of 980 nm.(b) Schematic of energy level diagrams of Mn 2+ -Yb 3+ dimer.