Giant enhancement of upconversion emission in NaYF₄:Er³⁺@NaYF₄:Yb³⁺ active-core/active-shell nanoparticles

Abstract

Varying the type and the doping concentration of rare-earth ions was reported as a conventional method to tune upconversion emissions of rare-earth ions doped nanoparticles (NPs). In this work, an effective method to enhance greatly the infrared-visible upconversion emissions of Er³⁺ ions is demonstrated through coating an optimized active NaYF₄:Yb³⁺ shell around NaYF₄:Er³⁺ core nanoparticles. Phase, shape, and size of the resulting core–shell NPs are studied by X-ray diffraction and high-resolution transmission electron microscopy. Compared with the core-only NaYF₄:Er³⁺ NPs, a maximum 97.56-fold overall enhancement in the emission intensity of Er³⁺ ions in the core–shell NPs is achieved under 980 nm infrared excitation, and a maximum 38.6-fold overall enhancement is achieved under 1540 nm infrared excitation. The luminescence enhancement effect and color output exhibits a strong dependence on the doping concentrations of Yb³⁺ in the NaYF₄:Yb³⁺ active-shell. By analyzing the decay lifetimes of the emission bands, the giant luminescence enhancement is due to the significant increase in the near-infrared absorption and efficient energy transfer from Yb³⁺ primary-sensitizers to Er³⁺ activators via Yb³⁺ bridging sensitizers.

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1. Introduction

Near 99% of the solar radiation energy in the ground solar spectrum is located in the 220–2500 nm wide emission band,^1,2 where visible light (400–700 nm) and infrared light (700–2500 nm) account for 71% and 22% of the amount of solar radiation, respectively. Visible light is widely used in the lighting industry, three-dimensional displays, photovoltaic industry, biomedical imaging, and photosynthesis.^3–12 However, there is a lack of applications for infrared light in above research fields. Infrared light in the solar radiation energy contains two main infrared emission bands centered at 980 nm and 1540 nm. Recently, lanthanide-doped nanoparticles (NPs) exhibited Stokes-shifting that can convert low-energy infrared excitation radiation into higher-energy visible emissions.^13,14 The Yb³⁺ ion was reported to have wide absorption cross-section of 980 nm infrared emission in fluoride host.^15,16 In order to enlarge photo-absorption cross section of 980 nm infrared light, the Yb³⁺ ion is chosen to be as a sensitizer for the Re³⁺ (Re = Er, Tm, and Ho) ions,^17–22 due to the fact that the Yb³⁺ ion consists of only two levels, and has only one electronic excited state ²F_5/2 that is located in the near infrared region about 980 nm. Moreover, high-power InGaAs diode lasers are available to directly pump the Yb³⁺ absorption band around 980 nm. The doping content of Yb³⁺ was usually kept at 18% or higher, because the large energy gap between the excited state ²F_5/2 and ground state ²F_7/2 blocks multiphoton cross-relaxations.^13,17 However, among the rare-earth ions, only the Er³⁺ ion owns an excited state level that can absorb the 1540 nm infrared light. Unlike the Yb³⁺, absorption cross-section of the Er³⁺ ion for infrared 1540 nm emission is relative narrow, which limits the emission intensity in the Er³⁺ up-conversion process.^23,24 Additionally, the doping contrition of Er³⁺ ion is usually low (about 2%), due to luminescence quenching induced by the cross-relaxation energy transfer between excited states.¹³ It is necessary to explore a new method to enhance the efficiency of up-conversion emissions of Er³⁺ ion from infrared to visible.

Recently, Liu and coworkers proposed the energy migration-mediated upconversion (EMU) in rare-earth ions doped NaGdF₄ core–shell NPs to enhance the intensity of UC emissions from activators without proper intermediate energy levels.²⁵ In the EMU process, four types of luminescent centers, such as sensitizer (Yb³⁺), accumulator (Tm³⁺), migratory (Gd³⁺) and activator (Tb³⁺, Eu³⁺, Dy³⁺ or Sm³⁺), are incorporated into four separate layers with precisely defined concentrations. Under 980 nm excitation, the higher excited state of Tm³⁺ is populated by the Yb³⁺, migrates its energy to the Gd³⁺, and then this energy is transferred to Tb³⁺, Eu³⁺, Dy³⁺ or Sm³⁺ ions through the core–shell interface within the migrator ions, emitting multicolor luminescence. Similarly, the EMU was found by Huang in NaYF₄:Nd³⁺/Yb³⁺/Ln³⁺@NaYF₄:Nd³⁺/Yb³⁺ (Ln = Ho, Er, Tm) and NaGdF₄:Nd³⁺/Yb³⁺/Tm³⁺@NaGdF₄:Eu³⁺/Tb³⁺ core/shell NPs.^26–28 However, at present, giant luminescence enhancement has not been achieved under 1540 nm excitation. There have been few systematic theoretical and experimental studies focusing on it. In this work, we present an effective method to enhance infrared-visible upconversion emissions of Er³⁺ ions through synthesizing the colloidal cubic-phase NaYF₄:Er³⁺@NaYF₄:Yb³⁺ active-core/active-shell NPs. The giant overall enhancement in the emission intensity of Er³⁺ ions in core–shell NPs is achieved with the 980 nm and 1540 nm infrared laser as two excitation sources. The mechanism on interfacial energy transfer is discussed.

2. Experimental section

2.1. Materials

All chemical materials were used without further purification. The starting materials were reagent grade 99% pure NaCF₃COO, 99.95% pure trifluoroacetic acid (CF₃COOH), sodium carbonate (AR), 99.99% pure Y₂O₃, 99.99% pure Yb₂O₃, and 99.99% pure Er₂O₃. Analytical grade cyclohexane (99.5%), oleic acid (OA), oleylamine (OM), and ethanol were used. Deionized water was used throughout.

2.2. Synthesis of NaYF₄:Er³⁺ NPs

Rare earth trifluoroacetates (RE(CF₃COO)₃, RE = Y, Yb, Er) were prepared at the beginning by dissolving rare earth oxides in the CF₃COOH. According to the molar ratio of NaYF₄:2%Er³⁺, a mixture of NaCF₃COO, Y(CF₃COO)₃, and Er(CF₃COO)₃ powder were dissolved in 8 mL OM and 16 mL OA. Under vigorous stirring in a three-neck flask, the mixture was then heated to 120 °C under the protection of argon atmosphere and maintained at the same temperature for 30 min to remove the oxygen and residual water. At this end, the solution was totally clear with a slight yellow color. The mixture was then heated slowly to 275 °C in the presence of argon atmosphere and maintained at 275 °C for 30 min. After then, the mixture was cooled down rapidly to room temperature. Finally, NPs were then precipitated using ethanol and isolated via centrifugation for at least three times. The resulting nanocrystals were dried in vacuum at 70 °C for a minimum of 24 h.

2.3. Synthesis of NaYF₄:Er³⁺@NaYF₄:Yb³⁺ NPs

To prepare the core–shell NPs, 1 mmol core NPs of NaYF₄:2%Er³⁺ were added to 5 mL cyclohexane, then using ultrasonic homogenizer to shatter the core NPs for 30 min. According to the molar ratio of NaYF₄:2%Er³⁺@NaYF₄:x%Yb³⁺ (x = 0.5, 1, 2, 3, 5), a mixture of NaCF₃COO, Y(CF₃COO)₃, and Yb(CF₃COO)₃ powder were dissolved in 8 mL OM and 16 mL OA. After that, the totally shattered core NPs were added to the mixture solution. Under vigorous stirring in a three-neck flask, the mixture was then heated to 120 °C under the protection of argon atmosphere and maintained at the same temperature for 30 min to remove the oxygen and residual water. At this end, the solution was totally clear with a slight yellow color. Until the mixture was cooled down rapidly to room temperature, the mixture was then heated slowly to 275 °C in the presence of argon atmosphere and maintained at 275 °C for 30 min. Finally, NPs were then precipitated using ethanol and isolated via centrifugation for at least three times. The resulting nanocrystals were dried in vacuum at 70 °C for a minimum of 24 h.

Structures of the samples were investigated by X-ray diffraction (XRD) using X'TRA (Switzerland ARL) equipment provided with a Cu tube with Kα radiation at 1.54056 Å. The size and shape of the samples were observed by a JEM-2100 transmission electron microscope (JEOL Ltd., Tokyo, Japan). Luminescence spectra were obtained by the Acton SpectraPro Sp-2300 Spectrophotometer with a photomultiplier tube equipped with 980 nm and 1540 nm infrared laser as the excitation sources. The fluorescence decay curves in visible region were recorded on a FLSP920 fluorescence spectrophotometer and using a Shimidazu R9287 photomultiplier (200–900 nm) as the detectors. All measurements were performed at room temperature.

3. Results and discussion

Fig. 1 shows the TEM images of NaYF₄:2%Er³⁺ core, NaYF₄:2%Er³⁺@NaYF₄ active-core/inactive-shell, and NaYF₄:2%Er³⁺@NaYF₄:2%Yb³⁺ active-core/active-shell NPs. In order to further study the structure of NPs, the histograms of size distributions of the corresponding NPs were shown in Fig. 1(d). A typical TEM image of the NaYF₄:2%Er³⁺ core NPs in Fig. 1(a) shows that the core NPs are well dispersed hexagon and uniform in size with an average diameter of ∼12 nm. After surface modification by the NaYF₄ inactive shell, the average size of the NaYF₄:2%Er³⁺@NaYF₄ core–shell NPs increased to ∼20 nm, and their shapes remain almost unchanged, as shown in Fig. 1(b) and (d). The same result for the NaYF₄:2%Er³⁺@NaYF₄:2%Yb³⁺ core–shell NPs is observed, which are shown in Fig. 1(c) and (d). The increase in the particle size indicates that the shell has been successfully grown around the core. Typical high-resolution TEM (HRTEM) images of the core–shell NPs in the insets reveal their highly crystalline nature and core–shell structure with the clear crystal fringes. The powder X-ray diffraction patterns of NaYF₄:2%Er³⁺ core, NaYF₄:2%Er³⁺@NaYF₄ active-core/inactive-shell, and NaYF₄:2%Er³⁺@NaYF₄:2%Yb³⁺ active-core/active-shell NPs are shown in Fig. 2. The diffraction peaks of all the UCNPs can be indexed as a pure cubic NaYF₄ phase (JCPDS, Card No. 06-0342.). The well defined peaks indicate the high crystallinity of the synthesized UCNPs. The diffraction peak shifts slightly to the lower-angle side is observed. The similar shifts of diffraction peaks were also observed in Mn³⁺–Yb³⁺–Er³⁺ doped and Li⁺–Yb³⁺–Tm³⁺ doped NaYF₄ UCNPs,^29,30 which is ascribed to decrease of the unit-cell volumes induced by the substitution of Y³⁺ ions by smaller other ions in the host lattice.

Fig. 1 TEM micrographic images of (a) NaYF₄:2%Er³⁺ core, (b) NaYF₄:2%Er³⁺@NaYF₄ active core/inactive shell, and (c) NaYF₄:2%Er³⁺@NaYF₄:2%Yb³⁺ active-core/active-shell. The insets are HRTEM images. (d) Histograms of size distributions of the corresponding NPs.

Fig. 2 XRD patterns of (a) NaYF₄:2%Er³⁺ core, (b) NaYF₄:2%Er³⁺@NaYF₄ active core/inactive shell, and (c) NaYF₄:2%Er³⁺@NaYF₄:2%Yb³⁺ active-core/active-shell NPs. (d) The standard data of JCPDS card No. 06-0342.

Fig. 3(a) shows the upconversion emission spectra of the core-only, active-core/inactive-shell and active-core/active-shell NPs under 980 nm excitation. Under 980 nm excitation, green, red, and infrared upconversion emissions centered at around 524, 543, 653 nm, and 800 nm are observed in the core-only NPs, corresponding to the ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2, ⁴F_9/2 → ⁴I_15/2, and ⁴I_9/2 → ⁴I_15/2 transitions of Er³⁺ ions, respectively. Compared with the core-only NPs, an obvious upconversion luminescence enhancement can be observed after coating the NaYF₄ and NaYF₄:Yb³⁺ shell. The enhancement of NaYF₄:2%Er³⁺@NaYF₄ active-core/inactive-shell NPs is up to 9.35 times. By changing the doping concentrations of Yb³⁺ in the NaYF₄:Yb³⁺ active-shell, the largest enhancement about ∼97.56 times is observed in the NaYF₄:2%Er³⁺@NaYF₄:2%Yb³⁺ active-core/active-shell NPs. This giant enhancement is originated from significant increase in the near-infrared absorption and efficient energy transfer from Yb³⁺ primary-sensitizers to Er³⁺ activators via Yb³⁺ bridging sensitizers.^13,26

Fig. 3 (a) Upconversion emission spectra of NaYF₄:2%Er³⁺ core, NaYF₄:2%Er³⁺@NaYF₄ active-core/inactive-shell, and the NaYF₄:2%Er³⁺@NaYF₄:x%Yb³⁺ (x = 0.5, 1, 2, 3, 5) active-core/active-shell NPs under 980 nm excitation. (b) Upconversion emission spectra of the NaYF₄:2%Er³⁺@NaYF₄:2%Yb³⁺ active-core/active-shell NPs and NaYF₄:2%Er³⁺,2%Yb³⁺ NPs under 980 nm excitation.

As reported in literature, Yb³⁺–Er³⁺ codoped NaYF₄ NPs is one of the effective upconversion materials.^15,31 In this paper, we study the difference between Yb³⁺–Er³⁺ codoped NaYF₄ NPs and NaYF₄:2%Er³⁺@NaYF₄:2%Yb³⁺ active-core/active-shell NPs on their optical properties. The upconversion emission spectra of NaYF₄:2%Er³⁺@NaYF₄:2%Yb³⁺ and NaYF₄:2%Er³⁺,2%Yb³⁺ NPs under excitation of 980 nm is shown in Fig. 3(b). From Fig. 3(b), we can see that the emission intensity of the NaYF₄:2%Er³⁺,2%Yb³⁺ is much higher than that of NaYF₄:2%Er³⁺@NaYF₄:2%Yb³⁺ active-core/active-shell under the excitation of 980 nm. It means that energy transfer between Yb³⁺ and Er³⁺ ions in in situ substitutional Y site is more effective than energy transfer between Yb³⁺ and Er³⁺ ions in the active-core/active-shell structure.

Expect for the 980 nm infrared light, the infrared band centered at 1540 nm corresponding to the ⁴I_15/2 → ⁴I_13/2 transition, is an effected excitation light for the Er³⁺.

To gain more insight, we also study the photoluminescence of the core-only, active-core/inactive-shell and active-core/active-shell NPs under 1540 nm excitation.

Fig. 4(a) shows the upconversion emission spectra of the core-only, active-core/inactive-shell and active-core/active-shell NPs under 1540 nm excitation. Green, red, and infrared upconversion emissions centered at around 523, 542, 667, and 801 nm are observed in the core-only NPs, corresponding to the ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2, ⁴F_9/2 → ⁴I_15/2, and ⁴I_9/2 → ⁴I_15/2 transitions of Er³⁺ ions, respectively. Compared with the core-only NPs, an obvious upconversion luminescence enhancement can be observed after coating the NaYF₄ and NaYF₄:Yb³⁺ shell. The largest enhancement of NaYF₄:2%Er³⁺@NaYF₄ active-core/inactive-shell NPs is up to 11.3 times, and the largest enhancement of the NaYF₄:2%Er³⁺@NaYF₄:2%Yb³⁺ active-core/active-shell NPs is up to about ∼38.6 times.

Fig. 4 (a) Upconversion emission spectra of NaYF₄:2%Er³⁺ core, NaYF₄:2%Er³⁺@NaYF₄ active-core/inactive-shell, and the NaYF₄:2%Er³⁺@NaYF₄:2%Yb³⁺ active-core/active-shell NPs under 1540 nm excitation. (b) Upconversion emission spectra of the NaYF₄:2%Er³⁺@NaYF₄:2%Yb³⁺ active-core/active-shell and NaYF₄:2%Er³⁺,2%Yb³⁺ NPs under 1540 nm excitation.

For comparison, we also study the difference on optical properties of Yb³⁺–Er³⁺ co-doped NaYF₄ NPs and NaYF₄:2%Er³⁺@NaYF₄:2%Yb³⁺ active-core/active-shell NPs. Upconversion emission spectra of the NaYF₄:2%Er³⁺,2%Yb³⁺ NPs and NaYF₄:2%Er³⁺@NaYF₄:2%Yb³⁺ active-core/active-shell are given in Fig. 4(b). One can find that the emission intensity of the NaYF₄:2%Er³⁺@NaYF₄:2%Yb³⁺ active-core/active-shell is much stronger than that of NaYF₄:2%Er³⁺,2%Yb³⁺ under 1540 nm excitation with the same excitation power. It means that the giant emission enhancement induced by the Yb³⁺ is more obvious in the active-core/active-shell structure than that in the substitutional doping single crystal.

In order to investigate the effect of emission enhancement induced by shell structure, the enhancement factor is calculated. The enhancement factor A is expressed as

A = I_core/shell/I_core, (1)

where I_core/shell and I_core are corresponding to the emission intensity of core–shell NPs and the emission intensity of core-only NPs, respectively. Fig. 5 shows the factors of emission intensities of NaYF₄:2%Er³⁺ core, NaYF₄:2%Er³⁺@NaYF₄ active-core/inactive-shell, and the NaYF₄:2%Er³⁺@NaYF₄:x%Yb³⁺ (x = 0.5, 1, 2, 3, 5) active-core/active-shell NPs under 1540 nm excitation. It can be seen that the 523 nm, 542 nm, 667 nm and 801 nm emission bands are enhanced sharply in NaYF₄:2%Er³⁺@NaYF₄ active-core/inactive-shell, and the NaYF₄:2%Er³⁺@NaYF₄:Yb³⁺ active-core/active-shell NPs. Especially, the biggest enhancement of about 118 times for the 667 nm red emission is achieved in the NaYF₄:2%Er³⁺@NaYF₄:2%Yb³⁺ active-core/active-shell NPs. The emission enhancement factor is about 32 times for the 667 nm emission of NaYF₄:2%Er³⁺@NaYF₄ NPs, which is much weaker than the enhancement factor of NaYF₄:2%Er³⁺@NaYF₄:2%Yb³⁺ NPs. The observed emission enhancement is originated from the presence of strain-induced modification of the local crystal field in the shell host lattice.¹³ The Yb³⁺ dependent total emission intensities of NaYF₄:2%Er³⁺@NaYF₄:x%Yb³⁺ (x = 0, 0.5, 1, 2, 3, 5) active-core/active-shell NPs are given in Fig. 5(c). One can find that the luminescence enhancement effect exhibits the inhomogeneous increase and a strong dependence on the doping concentrations of Yb³⁺ in the NaYF₄:Yb³⁺ active-shell. This inhomogeneous increase of total emission intensities was also observed in NaYF₄:Nd³⁺/Yb³⁺/Ho³⁺@NaYF₄:Nd³⁺/Yb³⁺ core/shell NPs under 808 nm excitation,²⁶ which was ascribed to the trade-off between energy transfer benefit and surface quenching effect of core–shell structures. With the increase of Yb³⁺ concentration, the enhancement factor reaches the maximum values for the NaYF₄:2%Er³⁺@NaYF₄:2%Yb³⁺ NPs. Furthermore, the effect of the shell structures on the intensity ratio of the green/red emission (I_G/I_R) is studies in Fig. 5(b). One can find that the I_G/I_R is tuned by varying the Yb³⁺ concentration in the NaYF₄:Yb³⁺ shell. The intensity of green emission is much stronger than that of red emission in the NaYF₄:2%Er³⁺ core NPs, while the intensity of red emission is much stronger than that green of emission in the core–shell NPs.

Fig. 5 (a) Enhancement factors of emission intensities of core, active-core/inactive-shell, and active-core/active-shell NPs under 1540 nm excitation. (b) The effect of core–shell composition on the intensity ratio between the green and the red emissions. (c) The Yb³⁺ dependent total emission intensity of NaYF₄:2%Er³⁺@NaYF₄:x%Yb³⁺ (x = 0, 0.5, 1, 2, 3, 5) active-core/active-shell NPs.

To our knowledge, under 1540 nm excitation the giant enhancement of Er³⁺ upconversion emission induced by the active-core/active-shell construction has not been reported. Notably, the energy of the ⁴I_11/2 level of Er³⁺ is almost equal to the energy of the excited state Yb³⁺.³¹ The resonant energy transfer ⁴I_11/2 (Er³⁺) + ²F_7/2 (Yb³⁺) → ⁴I_15/2 (Er³⁺) + ²F_5/2 (Yb³⁺) is possible to occur. In order to explore energy transfer from Er³⁺ to Yb³⁺, the decay behavior of 523 nm, 542 nm, 667 nm, and 801 nm emissions of NaYF₄:2%Er³⁺ core NPs and the NaYF₄:2%Er³⁺@NaYF₄:2%Yb³⁺ active-core/active-shell NPs is studied in Fig. 6. These curves with non-exponential nature reveal the existence of energy transfer from Er³⁺ to Yb³⁺.^32,33 The corresponding decay lifetimes can be calculated by the second order exponential curve fitting:

I = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂), (2)

where I is the luminescence intensity, A₁ and A₂ are constants, t is the time, τ₁ and τ₂ are the short and long lifetimes for exponential components, respectively. The average decay lifetimes τ are determined by the formula

τ = (A₁τ₁² + A₁τ₂²)/(A₁τ₁ + A₁τ₂). (3)

Fig. 6 Decay profiles of (a) 523 nm, (b) 542 nm, (c) 667 nm, and (d) 801 nm of NaYF₄:2%Er³⁺ core NPs and the NaYF₄:2%Er³⁺@NaYF₄:2%Yb³⁺ active-core/active-shell NPs under 1540 nm excitation.

Compared with the NaYF₄:2%Er³⁺ core NPs, the obvious prolonged lifetimes are experimentally observed for the NaYF₄:2%Er³⁺@NaYF₄:2%Yb³⁺ core–shell NPs, substantiating that the excited states ⁴H_11/2 (523 nm), ⁴S_3/2 (542 nm), ⁴H_11/2 (667 nm), and ⁴S_3/2 (801 nm) are populated again through the back energy transfer from Yb³⁺ to Er³⁺.^34,35 As is shown in Fig. 6, no signal rising edge is observed for the 523 nm, 542 nm, and 801 nm emissions, while the long rising edges are observed for the 667 nm emission. The excited states ⁴H_11/2 (523 nm), ⁴S_3/2 (542 nm), and ⁴S_3/2 (801 nm) are populated by non-radiative relaxation from the ⁴F_7/2 state, since no signal rising edge is observed.³⁶ The excited state ⁴F_9/2 (667 nm) is populated by the back energy transfer from Yb³⁺ to Er³⁺, since the long signal rising edge is observed.³¹ As reported in literature, the Er³⁺ emitted 980 nm upconversion emission under 1540 nm excitation.¹³ In order to analyse the energy-transfer process in NaYF₄ : Er³⁺@NaYF₄ : Yb³⁺ active-core/active-shell NPs, the 980 nm emission spectrum of NaYF₄ : Er³⁺@NaYF₄ is given in Fig. 7. The emission intensity of 980 nm is found to decrease and then increase with increase of the Yb³⁺ ions, shown in the inset of Fig. 7. It means that the energy transfer from Yb³⁺ to Er³⁺occurs when the Yb³⁺ concentration is less than 2 mol%, while the back energy transfer from Er³⁺ to Yb³⁺ occurs when the Yb³⁺ concentration is more than 2 mol%.¹³

Fig. 7 The 980 nm emission spectrum for NaYF₄:2%Er³⁺@NaYF₄ NPs under 1540 nm excitation. The insert shows the Yb³⁺ concentration dependent on the intensity of 980 nm emission in NaYF₄:2%Er³⁺@NaYF₄:x%Yb³⁺ (x = 0, 0.5, 1, 2, 3, 5) active-core/active-shell NPs.

By analyzing the decay lifetimes and spectrum of active-core/active-shell NPs, the photoluminescence mechanism is illustrated in Fig. 8. Under 1540 nm excitation, the absorption of 1540 nm photons results in direct excitation of Er³⁺ ions from the ground ⁴I_15/2 state to the long-lived ⁴I_13/2 state through a ground state absorption (GSA) process.¹³ Then, Er³⁺ ions in the ⁴I_11/2 state are promoted to higher ⁴F_7/2 state through two successive processes of excited state absorption. The 523 nm, 542 nm, 667 nm, 801 nm emission bands are observed through the transitions from ²H_11/2, ⁴S_3/2, ⁴F_9/2, and ⁴I_9/2 states to ⁴I_15/2 state. After coating NaYF₄:2%Yb³⁺ active-shell onto the NaYF₄:2%Er³⁺ active-core, the back energy transfer from Er³⁺ (⁴I_11/2) to Yb³⁺ (²F_5/2) occurs, promoting the Yb³⁺ ion from ²F_7/2 to ²F_5/2. Two cross-relaxation processes between Er³⁺ and Yb³⁺, such as ²F_5/2 (Yb³⁺) + ⁴I_13/2 (Er³⁺) → ²F_7/2 (Yb³⁺) + ⁴F_9/2 (Er³⁺) and ²F_5/2 (Yb³⁺) + ⁴I_11/2 (Er³⁺) → ²F_7/2 (Yb³⁺) + ²H_11/2 (Er³⁺), promote the Er³⁺ ions in ⁴I_13/2 and ⁴I_11/2 states to the ⁴F_9/2 and ²H_11/2 states. An energy loop chain, Er³⁺ → Yb³⁺ → Er³⁺, is established to populate ²H_11/2, ⁴S_3/2, ⁴F_9/2, and ⁴I_9/2 states for two times through capturing non-radiative relaxation energy from the ⁴I_13/2 and ⁴I_11/2 states. As a result, the decay lifetimes of ²H_11/2, ⁴S_3/2, ⁴F_9/2, and ⁴I_9/2 states are prolonged and the corresponding emission intensities are increased sharply.

Fig. 8 Schematic design of the active-core/active-shell NPs for greatly enhanced upconversion luminescence of Er³⁺ through the core ↔ shell two-way energy transfer under 1540 nm excitation.

4. Conclusion

In summary, colloidal cubic NaYF₄:Er³⁺ core, NaYF₄:Er³⁺@NaYF₄ active-core/inactive-shell, and NaYF₄:Er³⁺@NaYF₄:Yb³⁺ active-core/active-shell nanoparticles were controlled by thermal decomposition of lanthanide trifluoroacetate precursors. It is found that the upconversion emission of Er³⁺ ions can be greatly enhanced by a factor up to 97.56-fold and 38.6-fold under 980 nm and 1540 nm excitations. Under 980 nm excitation, the luminescence enhancement effect induced by core–shell structure is lower than that induced by in situ substitutional doping of the Yb³⁺. Under 1540 nm excitation, the effect of core–shell structure on luminescence enhancement is prior to that induced by the substitutional Yb³⁺-doping. The giant enhancement of upconversion luminescence under 1540 nm excitation can be attributed to capturing non-radiative relaxation energy from the ⁴I_13/2 and ⁴I_11/2 states by the Yb³⁺ in active shell. The dependence of upconversion emission enhancement on the doping concentrations of Yb³⁺ ions in the active-shell layer were investigated, and the optimal doping concentrations of Yb³⁺ ions was found to be 2 mol%. This work provides a new paradigm for tuning luminescence intensity and color output, having implications for a range of biophotonic and infrared laser display applications.

Acknowledgements

This work was supported by National Natural Science Foundation of China (11404171), Natural Science Youth Foundation of Jiangsu Province (BK20130865), the Six Categories of Summit Talents of Jiangsu Province of China (2014-XCL-021), and the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (14KJB430020).

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