Construction of active-inert core–shell structured nanocrystals for broad range multicolor upconversion luminescence

Rare earth doped up-conversion luminescent nano-materials exhibit abundant emission colors under suitable excitation condition. In this work, NaYF4:Er/Ho@NaYF4 and NaYbF4:Tm@NaYF4 nanoparticles were synthesized by co-precipitation method. The pure red emission can be realized by the designed NaYF4:Er/Ho@NaYF4 nanocrystals and the R/Gs reach 23.3 and 25 under excitations of 980 and 1550 nm lasers, respectively. The R/G declines as the power increasing with the emission color changing from red to yellow, which is due to the quick saturation of the energy levels, radiating red emissions. Meanwhile, the emission intensity of NaYbF4:Tm@NaYF4 nanocrystals increases by 58.3 folds after encasing the inert shell NaYF4 and the CIE color coordinate reaches (0.1646, 0.0602) under 980 nm laser excitation. Furthermore, broad range multicolor from blue to red and yellow up-conversion emissions is achieved by mixing NaYF4:Er/Ho@NaYF4 and NaYbF4:Tm@NaYF4 nanocrystals, which could be applied to colorful displaying, security anti-counterfeiting and information coding.


Experimental
Synthesis of NaYF 4 :Er/Ho@NaYF 4 and NaYbF 4 :Tm@NaYF 4 core-shell structured nanocrystals Synthesis of NaYF 4 :Er/Ho nanocrystals NaYF 4 :Er/Ho up-conversion nanocrystals were prepared through co-precipitation of the lanthanide chloride with oleic acid and 1-octadecene 27 , where YCl 3 •6H 2 O (99.9%), ErCl 3 •6H 2 O (99.9%) and HoCl 3 •6H 2 O (99.9%) were used as original materials.1 mmol LnCl 3 •6H 2 O (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 NH 4 F 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 NaYF 4 :Er/Ho@NaYF 4 nanocrystals NaYF 4 :Er/Ho@NaYF 4 nanocrystals were prepared through the similar procedure.1 mmol YCl 3 •6H 2 O were used as original materials.The methanol mixture of 2.5 mmol NaOH, 4 mmol NH 4 F and the precursor solution (NaYF 4 :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 NaYbF 4 :Tm and NaYbF 4 :Tm@NaYF 4 nanocrystals NaYbF 4 :0.5Tm and NaYbF 4 :0.5Tm@NaYF 4 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.

Up-conversion luminescent properties
Up-conversion emission spectra and emission color of NaYF 4 :Er/Ho, NaYF 4 :Er/Ho@NaYF 4 nanocrystals The Er 3+ and Ho 3+ co-doping NaYF 4 nanocrystals are responsive to the excitation wavelengths of 980 and 1550 nm. Figure 2a shows the up-conversion emission spectra of the NaYF 4 :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 Er 3+ located at 524, 540 and 655 nm are observed, which corresponding to the radiated transitions of 2 H 11/2 → 4 I 15/2 , 4 S 3/2 → 4 I 15/2 and 4 F 9/2 → 4 I 15/2 respectively.What more, comparing with the emission spectrum of NaYF 4 :Er (shown in Fig. S2), part of the emissions from the NaYF 4 :Er/Ho nanocrystals belong to the greenemitted 5 F 4 , 5 S 2 → 5 I 8 transitions and the red-emitted 5 F 4 → 5 I 8 transition in Ho 3+ .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 Er 3+ doped NaYF 4 nanocrystals, the value of R/G increases obviously in Er 3+ and Ho 3+ co-doped NaYF 4 nanocrystals, which is due to the new energy transfer processes between Er 3+ and Ho 3+ ions 28 .In the Er 3+ /Ho 3+ co-doped system, Er 3+ ions can absorb the energy of 980 nm laser as a kind of sensitizer and transfer part of energy to the co-doped Ho 3+ .Based on the well energy level overlap between the Er 3+ and Ho 3+ , the energy transfer ET1 (Er 3+ : 4 F 9/2 → Ho 3+ : 5  www.nature.com/scientificreports/due to the non-radiative relaxation and then populate the red light-emitting level through excited state absorption I 13/2 → 4 F 9/2 .With the increase of pumping power, a considerable part of the electrons on 4 I 11/2 will populate the green light-emitting level through the up-conversion process, which in turn reduces the proportion of red light-emitting level 29,30 .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 3a shows the emission intensity enhancement factor of NaYF 4 :Er/Ho@NaYF 4 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 NaYF 4 shell are coated (as shown in Fig. S3), which confirms the decline of surface quenching 31 .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 NaYF 4 :Er/Ho nanocrystals, the corresponding CIE chromatic coordinate changes to deep red region (as shown in Fig. 3d and Table S2).
To investigate the non-steady up-conversion processes, the up-conversion emission spectra of NaYF 4 :Er/ Ho@NaYF 4 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 increase 32,33 .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 NaYF 4 shell (as shown in Fig. S4), indicating that the NaYF 4 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 reports [32][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 4 F 9/2 , 2 H 11/2 , 4 S 3/2 , 5 F 5 , 5 F 4 / 5 S 2 .
Upon changing the excitation wavelength to 1550 nm, the emission spectra of NaYF 4 :Er/Ho@NaYF 4 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 Er 3+ : 4 F 9/2 , 2 H 11/2 , 4 S 3/2 , which can be deduced from the up-conversion emission spectrum of NaYF 4 :Er under 1550 nm laser excitation (as shown in Fig. S5).The large R/G value indicates that the NaYF 4 :Er/Ho@NaYF 4 nanocrystals can be used as red phosphors under 980 and 1550 nm laser excitation.

Up-conversion emission spectra and emission color of NaYbF 4 :Tm and NaYbF 4 :Tm@NaYF 4 nanocrystals
To obtain the pure blue phosphors, the bare core NaYbF 4 :Tm and core-shell NaYbF 4 :Tm@NaYF 4 structured nanocrystals were prepared.It can be observed in Fig. 5a, after coating the inert shell with NaYbF 4 :Tm, the emission intensity of NaYbF 4 :Tm@NaYF 4 increases by 58.3 fold.It should be mentioned that the NaYF 4 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 Yb 3+ can be used to populated the energy levels of 1 D 2 , 1 G 4 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.

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, NaYbF 4 :Tm@NaYF 4 and NaYF 4 :Er/Ho@NaYF 4 nanocrystals, were dissolve in alcohol and grind in accordance with fixed mass ratio: ① only NaYbF 4 :Tm@NaYF 4 nanocrystals, ② 1:10, ③ 1:5, ④ 1:2 and ⑤ only NaYF 4 :Er/Ho@NaYF 4 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  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.

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 NaYF 4 :Er/Ho@NaYF 4 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 Ho 3+ and Er 3+ , the R/G also decreases as the pulse width rises.Meanwhile, the up-conversion luminescence of NaYbF 4 :Tm@NaYF 4 phosphors under 980 nm laser excitation were also studied.After encasing the inert shell NaYF 4 , the emission intensity from NaYbF 4 :Tm@NaYF 4 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.

2 Figure 2 .
Figure 2. (a) Normalized emission spectra of NaYF 4 :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.

Figure 3 .
Figure 3. (a) The enhancement factor (c) dependence of R/G and (d) the corresponding CIE chromatic coordinates of NaYF 4 :Er/Ho@NaYF 4 nanocrystals under 980 nm laser excitation with different powers; (b) The proposed up-conversion processes and surface quenching.

Figure 4 .
Figure 4. (a) Normalized emission spectra and (b) the R/G of NaYF 4 :Er/Ho@NaYF 4 nanocrystals under 980 nm laser excitation at different pulse width; (c) Time-dependent green and red emission profiles of NaYF 4 :Er/ Ho@NaYF 4 nanocrystals; (d) The up-conversion emission spectra of NaYF 4 :Er/Ho@NaYF 4 nanocrystals under 1550 nm laser excitation, the insert show the R/G with different excitation powers.

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.