Introduction

Rare earth ions doped up-conversion luminescent nano-materials, benefiting from the unique 4f energy level of trivalent rare earth ions1, have draw extensive attention in many research fields, like bio-imaging2,3, solid-state lasers4,5, optical temperature sensing6,7 and anti-counterfeiting8,9. Due to the remarkable physical and chemical stability, large anti-Stokes shift, multiple emission span and long emission lifetime, the rare earth ions doped materials show a broad application prospects in colorful displaying10,11,12,13,14,15,16,17,18,19,20. In up-conversion luminescent nano-materials, Er3+ (2H11/2/4S3/2 → 4I15/2; 4F9/2 → 4I15/2) and Ho3+ (5F4/5S2 → 5I8; 5F5 → 5I8) are used to achieve green and red emission colors under infrared light excitation. Tm3+ (1G4 → 3H6; 1G4 → 3F4) is used to obtain the blue and red emission colors10,11. While, the up-conversion emission colors, like green and red emissions radiated from Er3+, are accompanied with each other. The single emission from up-conversion phosphor is rare. Therefore, it is important and necessary to obtain a pure emission color, especially red emission color, for colorful displaying. Generally, the pure emission color can be tuned through by selecting proper ratio of co-doping rare ions and engineering local structure13,14,15,16,17. Guo et al. reported that the pure green, red and blue emissions are obtained in Yb3+/Ln3+ (Ln = Er, Ho, Tm) co-doped Gd2O3 up-conversion phosphors by adjusting the doping concentration of Er3+, Ho3+ and Tm3+, which is attributed to the strengthened cross relaxation processes14. To obtain broader range emission colors, the core–shell structured nano-materials are rational design in the past few years11,18,19,20,21,22. Jang et al. reported that the emission colors from heavily doped NaErF4:Tm-based core@ multi-shell nano-materials were fine tuned through changing the excitation laser from 980 to 808 and 1550 nm and the full-color emissions, including green, red and blue, were achieved via combination effects of elemental migration and photon blocking19. These researches indicate that the wide range emission color could be possibly achieved via rational designing core–shell structured nano-materials and tailoring the up-conversion processes. What’s more, other methods, like combining localized surface plasmon resonance (LSPR) and modifying quantum dots or dyes were also utilized to tune the emission color23,24,25,26.

In this work, the pure red and blue emissions are realized trough the simple double-layer structured NaYF4:Er/Ho@NaYF4 and NaYbF4:Tm@NaYF4 nanocrystals under excitation of 980 nm laser and the color can also be fine tuned from blue to red via tuning the mass ratio of the two samples, with the corresponding CIE chromatic coordinates changing from (0.1599, 0.0388) to (0.7010, 0.2813). The red to green ratios (R/Gs) reach 23.3 and 25 of NaYF4:Er/Ho@NaYF4 nanocrystals under excitation of 980 and 1550 nm lasers, respectively, which is originated to the energy transfer processes between Er3+ and Ho3+.

Experimental

Synthesis of NaYF4:Er/Ho@NaYF4 and NaYbF4:Tm@NaYF4 core–shell structured nanocrystals

Synthesis of NaYF4:Er/Ho nanocrystals

NaYF4:Er/Ho up-conversion nanocrystals were prepared through co-precipitation of the lanthanide chloride with oleic acid and 1-octadecene27, where YCl3·6H2O (99.9%), ErCl3·6H2O (99.9%) and HoCl3·6H2O (99.9%) were used as original materials. 1 mmol LnCl3·6H2O (Ln = 86.3%Y, 13.5% Er, 0.2% Ho), 6 ml of oleic acid and 15 ml of 1-octadecene were added into a 50 mL three-necked flask simultaneously. Heated the mixture to 150 ℃ and kept it at this temperature for 40 min. After cooling to 50 ℃, a methanol mixture of 2.5 mmol NaOH and 4 mmol NH4F was added to the three-necked flask and kept the reaction at this temperature for 40 min. Subsequently, the mixture was heated to 120 ℃ for 20 min to eliminate remaining water and methanol. Finally, the temperature of the mixture was increased to 310 ℃ for 1 h. The obtained nanocrystals were dispersed in 10 ml cyclohexane as the precursor solution of core–shell structure after washing with cyclohexane and ethanol in a ratio of 1:3.

Synthesis of NaYF4:Er/Ho@NaYF4 nanocrystals

NaYF4:Er/Ho@NaYF4 nanocrystals were prepared through the similar procedure. 1 mmol YCl3·6H2O were used as original materials. The methanol mixture of 2.5 mmol NaOH, 4 mmol NH4F and the precursor solution (NaYF4:Er/Ho) were added to the three-necked flask simultaneously. The obtained nanocrystals were washed and dried at 60 ℃ in air for 12 h for up-conversion luminescence tested.

Synthesis of NaYbF4:Tm and NaYbF4:Tm@NaYF4 nanocrystals

NaYbF4:0.5Tm and NaYbF4:0.5Tm@NaYF4 nanocrystals were prepared through the above procedure. Only the rare earth ions and the doped ratio differed from the previous samples.

Measurements and characterization

The X-ray powder diffraction (XRD) patterns were recorded using a Bruker D8 diffractometer to investigate the phase purity and phase structure of the samples. The transmission electron microscope (TEM) images were recorded by a Talos F200X G2 field emission electron microscope to investigate the morphologies of the samples. The 980 nm laser (EC31439), using to excite the sample, was purchased from Changchun New Industries Optoelectronics Tech Co., Ltd. The 1550 nm laser (BTW DS2-21312110), using to excite the sample, was purchased from Beijing Kipling Photoelectric technology Co., Ltd. The up-conversion emission spectra of the samples were measured through the fiber optic spectrometer purchased from Chen Xu instrument Co., Ltd (Type: ST4000). The time-dependent emission profiles of the samples were recorded using iHR550 grating spectrometer with a DSO5032A Digital Storage Oscilloscope.

Results and discussion

Structure and morphological characterization

XRD patterns of all samples are illustrated in Fig. 1a. Compared with the two kinds of standard hexagonal phase NaYF4 (JCPDS No.16–0334) and NaYbF4 (JCPDS No.27–1427), NaYF4:Er/Ho and NaYF4:Er/Ho@NaYF4 nanocrystals are pure hexagonal phase NaYF4. NaYbF4:Tm and NaYbF4:Tm@NaYF4 nanocrystals are pure hexagonal phase NaYbF4. Figure 1b–e show TEM images of NaYbF4:Tm, NaYbF4:Tm@NaYF4, NaYF4:Er/Ho and NaYF4:Er/Ho@NaYF4 nanocrystals, respectively. As shown in Fig. 1b, the obtained NaYbF4:Tm nanocrystals is composed of monodisperse sphere and the average diameter is ~ 28.5 nm. After coating by inert shell NaYF4, the NaYbF4:Tm@NaYF4 nanocrystals are prepared, where the nanospheres become ellipsoid and the average size increases to ~ 41.1 × 31.9 nm. As revealed in Fig. 1d, the average size of single core NaYF4:Er/Ho nanocrystals is ~ 26.1 nm and the nanocrystals distribute homogeneously. The morphology of NaYF4:Er/Ho@NaYF4 nanocrystals is similar to NaYbF4:Tm@NaYF4 nanocrystals and the average size is ~ 39.9 × 30.3 nm. The size distribution diagrams of these nanocrystals are shown in Fig. S1.

Figure 1
figure 1

(a) XRD patterns NaYF4:Er/Ho, NaYF4:Er/Ho@NaYF4, NaYbF4:Tm and NaYbF4:Tm@NaYF4; TEM mages of (b) NaYbF4:Tm (c) NaYbF4:Tm@NaYF4 (d) NaYF4:Er/Ho (e) NaYF4:Er/Ho@NaYF4 nanocrystals.

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Up-conversion luminescent properties

Up-conversion emission spectra and emission color of NaYF4:Er/Ho, NaYF4:Er/Ho@NaYF4 nanocrystals

The Er3+ and Ho3+ co-doping NaYF4 nanocrystals are responsive to the excitation wavelengths of 980 and 1550 nm. Figure 2a shows the up-conversion emission spectra of the NaYF4:Er/Ho nanocrystals under 980 nm laser excitation (0.35W, 0.45W, 1.05W), where the emission spectra were normalized at 654 nm. The typical emission bands of Er3+ located at 524, 540 and 655 nm are observed, which corresponding to the radiated transitions of 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 respectively. What more, comparing with the emission spectrum of NaYF4:Er (shown in Fig. S2), part of the emissions from the NaYF4:Er/Ho nanocrystals belong to the green-emitted 5F4, 5S2 → 5I8 transitions and the red-emitted 5F4 → 5I8 transition in Ho3+. The R/G ratio are 18.7, 7.6 and 3.6 as the power of 980 nm laser changes to 0.35, 0.45 and 1.05, respectively. Comparing to Er3+ doped NaYF4 nanocrystals, the value of R/G increases obviously in Er3+ and Ho3+ co-doped NaYF4 nanocrystals, which is due to the new energy transfer processes between Er3+ and Ho3+ ions28. In the Er3+/Ho3+ co-doped system, Er3+ ions can absorb the energy of 980 nm laser as a kind of sensitizer and transfer part of energy to the co-doped Ho3+. Based on the well energy level overlap between the Er3+ and Ho3+, the energy transfer ET1 (Er3+: 4F9/2 → Ho3+: 5F5), ET2 (Er3+: 2H11/2/4S3/2 → Ho3+: 5F4/ 5S2) and the ground state absorption (Er3+: 4I15/2 → 4I11/2), excited state absorption (Er3+: 4I11/2 → 2H11/2/4S3/2 and 4I13/2 → 4F9/2) are shown in Fig. 2b. As shown in Fig. 2c, the R/G decreases from 18.7 to 3.6 with the rise of power. The reason for the decrease could be explanation as follows. The R/G value mainly depends on the depletion of the energy level 4I11/2 (Er3+). There are two channels for the depletion of 4I11/2, including excited state absorption 4I11/2 → 4F7/2 and non-radiative relaxation 4I11/2 → I13/2. When the pump power is very small, the up-conversion process of 4I11/2 mainly contributes to the population of the energy level I13/2 due to the non-radiative relaxation and then populate the red light-emitting level through excited state absorption I13/2 → 4F9/2. With the increase of pumping power, a considerable part of the electrons on 4I11/2 will populate the green light-emitting level through the up-conversion process, which in turn reduces the proportion of red light-emitting level29,30. As shown in Fig. 2d, the corresponding CIE chromatic coordinate changes from red to yellow as the power increases and the detail CIE chromatic coordinates are displayed in Table S1.

Figure 2
figure 2

(a) Normalized emission spectra of NaYF4:Er/Ho nanocrystals under 980 nm laser excitation; (b) The proposed up-conversion and energy transfer processes; (c) R/G variation and (d) the corresponding CIE chromatic coordinates at different powers.

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Figure 3a shows the emission intensity enhancement factor of NaYF4:Er/Ho@NaYF4 nanocrystals. The enhancement factors of green (539 nm) and red (654 nm) emissions show a slow downward trend as laser power increases. And the enhancement factor of green emissions decreases from 1.29 to 1.18. The enhancement factor of red emission decreases from 2.33 to 1.88. The enhancement factor of the red declines faster with increasing power than that of the green emissions. The emission enhancement is due to the suppression of surface quenching, as shown in Fig. 3b. The emission lifetimes of green and red emissions change longer as the NaYF4 shell are coated (as shown in Fig. S3), which confirms the decline of surface quenching31. And the more obvious enhancement of red emission might be originated to the increased ET1 process. It is worth to mention that the R/G decreases from 23.3 to 5.8 with the increasing of power from 0.35 to 1.05 W (as shown in Fig. 3c). As a result, comparing to NaYF4:Er/Ho nanocrystals, the corresponding CIE chromatic coordinate changes to deep red region (as shown in Fig. 3d and Table S2).

Figure 3
figure 3

(a) The enhancement factor (c) dependence of R/G and (d) the corresponding CIE chromatic coordinates of NaYF4:Er/Ho@NaYF4 nanocrystals under 980 nm laser excitation with different powers; (b) The proposed up-conversion processes and surface quenching.

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To investigate the non-steady up-conversion processes, the up-conversion emission spectra of NaYF4:Er/Ho@NaYF4 at different pulse widths from 500 to 1300 μs were tested (the pulse frequency was fixed at 600 Hz). As shown in Fig. 4a, the green emissions are weak and become obviously as the pulse width enlarges. And the value of R/G ratio decreases from 9.3 to 6.5 with the pulse duration times increasing from 500 to 1300 μs, as shown in Fig. 4b. This phenomenon is different to the tendency of other rare ions doped materials, like Ho(Er)/Yb, where the R/G rises as the pulse widths increase32,33. To explain the reason why the R/G declines with pulse width rise, we investigated the non-steady state behavior of the sample under 980 nm laser excitation. As shown in Fig. 4c, the intensity of green and red emissions rise slowly under excitation. And the rise time is longer than the nanocrystals without NaYF4 shell (as shown in Fig. S4), indicating that the NaYF4 shell intensifies the ET1 and ET2. The similar rise tendency of green and red emissions, unlike the shorter rise time of green emissions in other reports32,33,34, make the different R/G change tendency with pulse width increasing. The reason of the slower rise time of this sample is that the ET1, ET2, back-ET1 (BET1), back-ET2 (BET1) and non-radiative relaxation processes, as shown in Fig. 3b, repopulate the energy levels of 4F9/2, 2H11/2, 4S3/2, 5F5, 5F4/ 5S2.

Figure 4
figure 4

(a) Normalized emission spectra and (b) the R/G of NaYF4:Er/Ho@NaYF4 nanocrystals under 980 nm laser excitation at different pulse width; (c) Time-dependent green and red emission profiles of NaYF4:Er/Ho@NaYF4 nanocrystals; (d) The up-conversion emission spectra of NaYF4:Er/Ho@NaYF4 nanocrystals under 1550 nm laser excitation, the insert show the R/G with different excitation powers.

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Upon changing the excitation wavelength to 1550 nm, the emission spectra of NaYF4:Er/Ho@NaYF4 nanocrystals are detected under 1550 nm laser. As shown in Fig. 4d, the value of R/G reaches 25 under low power excitation and show the similarity decrease tendency as laser power increases. Comparing with sample under 980 nm laser excitation, the larger value of R/G is obtained under 1550 nm laser excitation. This phenomenon can be interpreted with the original populations of energy levels of Er3+: 4F9/2, 2H11/2, 4S3/2, which can be deduced from the up-conversion emission spectrum of NaYF4:Er under 1550 nm laser excitation (as shown in Fig. S5). The large R/G value indicates that the NaYF4:Er/Ho@NaYF4 nanocrystals can be used as red phosphors under 980 and 1550 nm laser excitation.

Up-conversion emission spectra and emission color of NaYbF4:Tm and NaYbF4:Tm@NaYF4 nanocrystals

To obtain the pure blue phosphors, the bare core NaYbF4:Tm and core–shell NaYbF4:Tm@NaYF4 structured nanocrystals were prepared. It can be observed in Fig. 5a, after coating the inert shell with NaYbF4:Tm, the emission intensity of NaYbF4:Tm@NaYF4 increases by 58.3 fold. It should be mentioned that the NaYF4 shell plays an important role in inhibiting surface quenching and increasing emission intensity. The intense blue emissions at 450 and 472 nm makes the emission color display pure blue, as shown in the insert of Fig. 5a (the detail CIE chromatic coordinates are displayed in Table S3). The relevant up-conversion processes are displayed in Fig. 5b. The efficient energy transferred from Yb3+ can be used to populated the energy levels of 1D2, 1G4 and then radiated intense blue emissions. What’s more, the emission color almost unchanges with the increasing of laser power, as shown in Fig. S6, which provides the possibility for the mixed materials to regulate emission color.

Figure 5
figure 5

(a) The up-conversion emission spectra of NaYbF4:Tm and NaYbF4:Tm@NaYF4 nanocrystals, the insert show the corresponding CIE chromatic coordinates; (b) The proposed up-conversion luminescent mechanism.

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Broad bange upconversion emission spectra and emission color

In order to realize the broad domain multicolor up-conversion luminescence, these two types of distinct phosphors, NaYbF4:Tm@NaYF4 and NaYF4:Er/Ho@NaYF4 nanocrystals, were dissolve in alcohol and grind in accordance with fixed mass ratio: ① only NaYbF4:Tm@NaYF4 nanocrystals, ② 1:10, ③ 1:5, ④ 1:2 and ⑤ only NaYF4:Er/Ho@NaYF4 nanocrystals. The emission spectra of the mixed samples are shown in the Fig. 6a, and the corresponding CIE chromatic coordinates are presented in Fig. 6b. As expected, Fig. 6b shows a wide range of color diversity from blue to red, including blue (0.1613, 0.0421), purple (0.2604, 0.0872), magenta (0.3722, 0.1489), crimson (0.5506, 0.2474) and red (0.6700, 0.3202). We also investigated the luminescence properties of these samples under excitation with different 980 nm laser power, as shown in Fig. S7. As the power increases, the CIE chromatic coordinates go to the red and green region (the detail CIE chromatic coordinates are displayed in Table S4), which eventually occupy over one-third of the entire chromaticity diagram. This result indicates that the composites may find applications in colorful displaying and anti-counterfeiting.

Figure 6
figure 6

(a) Normalized emission spectra (b) the corresponding CIE chromatic coordinates of samples with fixed mass ratio under 980 nm laser excitation.

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Conclusions

In summary, we designed double-layer core–shell structure to investigate the effect of excitation condition on up-conversion emission spectra and emission color. The pure red emission can be realized by the designed NaYF4:Er/Ho@NaYF4 nanocrystals under 980 or 1550 nm laser excitation. And the R/G declines as the power of 980 nm laser increases, with the emission color changing from red to yellow, which can be interpreted by the quick saturation of the energy levels, radiating red emissions. Because of the ET, BET and non-radiative relaxation processes among Ho3+ and Er3+, the R/G also decreases as the pulse width rises. Meanwhile, the up-conversion luminescence of NaYbF4:Tm@NaYF4 phosphors under 980 nm laser excitation were also studied. After encasing the inert shell NaYF4, the emission intensity from NaYbF4:Tm@NaYF4 nanocrystals increases by 58.3 folds. A wide range emission colors from blue to red, including blue, purple, magenta, crimson and red are realized through tuning the mass ratio of two samples. As the power increasing, the CIE chromatic coordinates go to the red and green region and eventually occupy over one-third of the entire chromaticity diagram. These results indicate the potential applications of these materials in various fields, including colorful displaying, security anti-counterfeiting and information coding.