Simultaneous size manipulation and red upconversion luminescence enhancement of CaF₂:Yb³⁺/Ho³⁺ nanoparticles by doping with Ce³⁺ ions

Abstract

Harnessing the color tuning capability of upconversion nanoparticles (UCNPs) is of great significance in the field of advanced bioimaging and color display. Here, we report the tunable size and upconversion luminescence (UCL) multicolor in CaF₂:Yb³⁺/Ho³⁺/Ce³⁺ UCNPs, which were synthesized by a facile hydrothermal method. It was found that the size of these UCNPs could be controlled (from 600 to 30 nm) by varying the concentration of Ce³⁺ ions. Under the excitation of a 980 nm continuous-wave (CW) laser, the UCL color of these UCNPs can be tuned from green to red as the doped Ce³⁺ ions gradually increase from 0 to 10 mol% and the red-to-green (R/G) ratio is enhanced remarkably. It is suggested that the cross-relaxation (CR) processes between Ho³⁺ and Ce³⁺ ions contribute to the tunable multicolor and enhancement of the R/G ratio. The mechanism of these processes is well supported by the time-resolved decay and near infrared (NIR) emission measurements.

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1. Introduction

UCNPs doped with lanthanide ions are a unique category of luminescent materials featuring abundant electronic transition in 4f electron shells, possessing remarkable optical properties, such as a sharp emission peak, long luminescence decay time and extremely low susceptibility to the chemical environment. These materials have the capability to upconvert two or more low energy photons into one high energy photon, with various emission bands ranging from violet to infrared.^1–3 Over the past decade, this anti-Stokes optical property has advanced a broad range of applications, including bioimaging,^4,5 lasers,⁶ anti-counterfeiting,⁷ photovoltaics,^8,9 drug delivery,^10,11 and solar energy harvesting.¹² In particular, manipulating the upconversion (UC) color of UCNPs has gained tremendous attention throughout the past few years due to its promise for applications in multiplex biological labelling,^13,14 color display,^15,16 and imaging.^17,18 Moreover, owing to being located in the “optical window” of bio-tissues and cells, red UC emission centered at 650 nm is favored by application in the biomedicine area.^19,20 In addition, when the luminescence color is tuned from green to red (even a single-red-band), it presents much better chromatic purity, resulting in a much higher spatial resolution in bio-medical field applications (e.g. bioimaging).^11,21 Thus, it is of great significance to achieve enhancing red UCL in lanthanide doped nanoparticles through color manipulation.

Up to now, effective and controllable approaches have been implemented to tune the UCL color, including controlling the power density of excitation laser,²² varying the excitation pulse width and wavelength,^23,24 changing temperature,²⁵ modulating the doping concentration and introducing suitable doping ions.^26–29 However, except the last two methods, most of them could be hindered by extra requirements for laser sources and experimental environment in specific applications. So far, many UC materials have easily realized red emission, for example, NaYF₄:Yb³⁺/Er³⁺ nanoparticles facilitating red UC emission by increasing the concentration of Yb³⁺ ions, NaYF₄:Yb³⁺/Er³⁺ (Yb³⁺/Ho³⁺) nanoparticles emitting red UC emission via codoping with Mn²⁺, Fe³⁺ or Pb²⁺.^11,18,30,31

Yb³⁺/Ho³⁺ codoped UCNPs are one of the most efficient UC materials and have been widely studied. Generally, under the excitation of 980 nm CW laser, Yb³⁺/Ho³⁺ codoped UCNPs mainly exhibit green (⁵S₂/⁵F₄ → ⁵I₈, 540 nm) and red (⁵F₅ → ⁵I₈, 650 nm) UC emissions. It should be mentioned that the red UC emission closely associated with two extra non-radiative relaxation (NR) processes (⁵I₆ → ⁵I₇ and ⁵S₂/⁵F₄ → ⁵F₅). Consequently, altering these two NR processes could effectively change the red UC emission radiative probability and modulate the luminescence color. Based on the analysis above, Zhang et al. reported the single-red-band UC emission in NaYF₄:Yb³⁺/Ho³⁺ nanoparticles by codoping with Ce³⁺ ions for the first time.³² Moreover, similar phenomenon has been demonstrated in NaGdF₄, NaLuF₄, LiYbF₄, AgLa(MoO₄)₂ and Sr₂GdF₇ host lattices.^33–37 Actually, CaF₂ is also an important yet under-studied UCNPs due to its low phonon energy, easy substitution by lanthanide ions and non-toxicity to biological tissues, which has been widely applied in biological fields such as biological labelling^38,39 and drug delivery.⁴⁰ Besides, the Ce³⁺ ion has a larger ionic radius than Ca²⁺ ion. This indicates that the incorporation of Ce³⁺ ions in CaF₂:Yb³⁺/Ho³⁺ UCNPs would vary the particle size of the CaF₂ host lattice. The simultaneous size manipulation and UCL multicolor tunability, especially dominant red emission of UCNPs could meet the growing demand in biological applications. However, relevant studies are still challenging and rarely reported. Until now, this has been realized in Yb³⁺/Er³⁺ codoped UCNPs by doping with Fe³⁺ and Mn²⁺ ions.^18,41 To the best of our knowledge, Ce³⁺-induced UCL multicolor and size manipulation of Yb³⁺/Ho³⁺ codoped UCNPs has never been reported so far.

In this work, we synthesized the Ce³⁺ doped CaF₂:Yb³⁺/Ho³⁺ UCNPs through a hydrothermal method. The influence of Ce³⁺ concentration on the size and phase of CaF₂ nanoparticles was studied in detail. Under the excitation of 980 nm CW laser, the UCL color of these UCNPs can be tuned from green to red as the doped Ce³⁺ ions increase from 0 to 10 mol%. Moreover, the mechanism of the enhancement of red UC emission has been demonstrated by the measurements of fluorescence lifetime, NIR emission as well as the dependence of luminescence intensity on the excitation power.

2. Experimental details

2.1. Synthesis of CaF₂:Yb³⁺/Ho³⁺/Ce³⁺ (20/2/x mol%) UCNPs

The raw materials were purchased from Aladdin (China), including YbCl₃·6H₂O (99.9% metals basis), HoCl₃·6H₂O (99.9% metals basis), CeCl₃·7H₂O (99.9% metals basis), CaCl₂·2H₂O (99.99% metals basis), ethylenediaminetetraacetic acid (EDTA) (99% analytical grade) and NaBF₄ (99.99% metals basis). All the chemicals were used as received without further purification.

The CaF₂:Yb³⁺/Ho³⁺/Ce³⁺ UCNPs were synthesized by a modified hydrothermal procedure. The molar ratio of (Ln³⁺ & Ca²⁺)/EDTA/NaBF₄ was fixed to 1 : 1 : 2. In a typical procedure, 2 mmol of chloride salts and 2 mmol EDTA were dissolved in 20 mL of deionized (DI) water and the mixtures were stirred vigorously for 1 h. Then, 20 mL of aqueous solution containing 4 mmol NaBF₄ was transferred to the aqueous solution prepared above. By stirring for another 1 h, a milky colloidal solution was obtained. Subsequently, the mixtures were transferred into a 50 mL Teflon-lined autoclave and heated at 200 °C for 30 h, and then slowly cooled down to room temperature. The precipitates were collected by centrifugation at 6000 rpm for 4 min, and washed with DI water and ethanol for several times, and dried at 40 °C for 12 h in air. Different dopant contents of UCNPs were synthesized by varying the composition of the RECl₃ and CaCl₂ while keeping the total RE³⁺ and Ca²⁺ ions constant at 2 mmol.

2.2. Physical characterization

The powder X-ray diffraction (XRD) patterns of the UCNPs were measured by an X-ray diffractometer with Cu Kα radiation at 40 kV and 200 mA (TTR III system, Rigaku), with the angular 2θ ranging from 20° to 80°. The scanning rate of the 2θ angle of the XRD spectra was 10° min⁻¹. Meanwhile, the size and morphology of these UCNPs were characterized by transmission electron microscopy (TEM) (Tecnai G2 F20, FEI). Before the TEM test, all the samples were dispersed in ethanol and sonicated for 10 min, and a drop of the solution of each sample was evaporated on a copper mesh grid supported by a carbon film.

2.3. Photoluminescence measurements

The UCNPs were dispersed in ethanol and irradiated by 980 nm CW laser with a focus diameter of 4 mm. The UCL was collected by a lens coupled grating monochromator (Omni-λ3072i, Zolix) with an integrated photomultiplier tube (PMTH-S1-R928). For the NIR emission measurements, a NIR spectrometer (NIR 1700, ideaoptics) was utilized. The decay profiles of UC emissions were recorded by a digital oscilloscope (1 GHz, InfiniiVision DSOX6002A, KEYSIGHT) and a nanosecond pulsed 980 nm laser used as the excitation source (the repetition rate is 10 Hz and the pulse duration is 20 ns). All the above experiments were carried out at room temperature.

3. Results and discussion

The morphology and phase of the as-prepared CaF₂:Yb³⁺/Ho³⁺/Ce³⁺ UCNPs were characterized by TEM and XRD. Fig. 1(a–f) present the TEM micrographs of the CaF₂:Yb³⁺/Ho³⁺ (20/2 mol%) UCNPs doped with 0, 2, 4, 6, 8 and 10 mol% Ce³⁺ ions, respectively. It could be seen that these UCNPs are nearly monodispersed with uniform size distribution in each sample. The mean size of UCNPs decreases from 600 to 30 nm with the doping Ce³⁺ ions increasing from 0 to 10 mol%. Notably, the size of the UCNPs is independent on the doping Ce³⁺ ions (when the doping Ce³⁺ ions larger than 6 mol%), which always keep the approximate average size of 30 nm. The tunable size reduction of the UCNPs might attribute to the fact that the larger Ce³⁺ ions enter the CaF₂ host lattice by substituting relatively smaller Ca²⁺ ions.^3,42

Fig. 1 (a–f) Typical TEM images of CaF₂:Yb³⁺/Ho³⁺ (20/2 mol%) UCNPs doped with 0, 2, 4, 6, 8, 10 mol% Ce³⁺ ions, respectively.

Fig. 2 displays the XRD patterns of the CaF₂:Yb³⁺/Ho³⁺ UCNPs doping with different concentrations of Ce³⁺ ions. All the diffraction peaks match well with the standard peak positions of the cubic phase of CaF₂ materials (JCPDS no. 87-0976), which indicates that the as-prepared UCNPs are highly crystallized. It should be noted that the diffraction peaks shifted slightly towards lower angle, which is ascribed to the substitution of smaller Ca²⁺ ions by the relatively larger Ce³⁺ ions.

Fig. 2 XRD patterns of CaF₂:Yb³⁺/Ho³⁺ UCNPs doped with Ce³⁺ ions of 0–10 mol%, and its local magnification.

Fig. 3(a) illustrates the UC emission spectra of CaF₂:Yb³⁺/Ho³⁺ UCNPs doping with different Ce³⁺ ions (0, 4 and 10 mol%), and the insets exhibit the corresponding UCL color. Two typical UC emissions of Ho³⁺ ion can be observed under the excitation of 980 nm CW laser: green (541 nm) and red (650 nm) emission bands, attributing to the transitions of ⁵S₂/⁵F₄ → ⁵I₈, and ⁵F₅ → ⁵I₈, respectively. For Ce³⁺-free UCNPs, it is found that the green emission (541 nm) is stronger than the red emission (650 nm), which exhibits a green luminescence color. By increasing the doping Ce³⁺ ions up to 4 mol%, the red emission is further enhanced, leading to the color changing from green to yellow. If the doping Ce³⁺ ions further increase to 10 mol%, the green UC emission is efficiently suppressed and the red UC emission is significantly enhanced, resulting in the luminescence color tuning from yellow to red. As presented in Fig. 3(b), we have calculated the CIE chromaticity coordinates of CaF₂:Yb³⁺/Ho³⁺ UCNPs doping with different concentrations of Ce³⁺ ions (0–10 mol%) based on their UCL spectra. The result reveals that a wide range of multicolor can be acquired by adjusting the concentrations of Ce³⁺ ions. This means that these UCNPs could be suitable for different applications.

Fig. 3 (a) UC emission spectra of CaF₂:Yb³⁺/Ho³⁺ UCNPs doping with different concentrations of Ce³⁺ ions (0, 4 and 10 mol%) at the power density of 31.8 W cm⁻². The insets show the corresponding luminescence color of the UCNPs. (b) CIE chromaticity coordinates for the CaF₂:Yb³⁺/Ho³⁺ UCNPs doping with different concentrations of Ce³⁺ ions. All excitation wavelengths are at 980 nm.

To figure out the color tuning capability of CaF₂:Yb³⁺/Ho³⁺/Ce³⁺ UCNPs in detail, UC samples with Ce³⁺ ions contents ranging from 0 to 10 mol% were prepared, and the corresponding UC spectra were measured as well. The R/G intensity ratio is calculated as exhibited in Fig. 4. When the Ce³⁺ concentration varies from 0 to 10 mol%, the R/G ratio can be promoted from 0.17 to 7.44. It indicates that the doping of Ce³⁺ ions plays an important role in tuning the luminescence color. To understand the mechanism of the UC emission, the population processes in CaF₂:Yb³⁺/Ho³⁺/Ce³⁺ UCNPs are schematically demonstrated. As shown in Fig. 5(a), a proposed energy level diagram of Yb³⁺, Ho³⁺, Ce³⁺ ions and the relevant ET processes are also displayed. Under the excitation of 980 nm CW laser, the Yb³⁺ ions absorb the laser energy and the ground state (²F_7/2) can be excited to the excited state (²F_5/2). Next, Ho³⁺ ions are excited from ground state ⁵I₈ to ⁵I₆ through the efficient ET process between Yb³⁺ and Ho³⁺ ions, and a NR process occurs in the ⁵I₆ state, which leads to a population in the ⁵I₇ state. Similarly, ⁵F₅ and ⁵S₂/⁵F₄ state of Ho³⁺ ions can be populated by the utilization the ET processes from the excited Yb³⁺ ions as well. Therefore, once these excited states are populated, the efficient UC emissions will generate, including the green (⁵S₂/⁵F₄ → ⁵I₈) and red (⁵F₅ → ⁵I₈) emission, as well as a NIR (⁵I₆ → ⁵I₈) emission. It should be mentioned that the NR process from ⁵S₂/⁵F₄ to the ⁵F₅ state also makes contribution to the red emission. As discussed in the section of introduction, the two NR processes (⁵I₆ → ⁵I₇ and ⁵S₂/⁵F₄ → ⁵F₅) could involve in the modulation of red emission of Ho³⁺ ions. It should be noted that the phonon energy of CaF₂ host lattice (∼350 cm⁻¹) is much lower than the energy gaps of ⁵S₂/⁵F₄ → ⁵F₅ and ⁵I₆ → ⁵I₇ (∼3000 cm⁻¹). This means that these two NR processes in CaF₂ UCNPs should occur inefficiently. However, the energy gap between the ground and excited states (²F_5/2 and ²F_7/2) of Ce³⁺ ions is about 3000 cm⁻¹, which matches well with the value of energy gap of the above two NR processes. This results in a fact that these two NR processes are replaced by the following two CR processes: ⁵S₂/⁵F₄(Ho³⁺) + ²F_5/2(Ce³⁺) → ⁵F₅(Ho³⁺) + ²F_7/2(Ce³⁺) (CR1) and ⁵I₆(Ho³⁺) + ²F_5/2(Ce³⁺) → ⁵I₇(Ho³⁺) + ²F_7/2(Ce³⁺) (CR2). Hence, the introduction of Ce³⁺ ions into CaF₂:Yb³⁺/Ho³⁺ UCNPs would significantly change the NR probability. These two CR processes result in the population of red emitting level ⁵F₅ and its intermediate level ⁵I₇, together with the depopulation of green emitting level ⁵S₂/⁵F₄ and its intermediate level ⁵I₆. Thus, the efficient CR processes contribute to the remarkable enhancement of red emission and the suppressed green emission.

Fig. 4 The R/G ratio of CaF₂:Yb³⁺/Ho³⁺ UCNPs doped with different Ce³⁺ concentrations (0–10 mol%). (R and G represent red and green UC emissions, respectively.)

Fig. 5 (a) Schematic energy level diagram and proposed UC mechanism of CaF₂:Yb³⁺/Ho³⁺/Ce³⁺ UCNPs. (b) Pump power dependence of UC emission intensity of Ce³⁺-free and 10 mol% Ce³⁺ doped CaF₂:Yb³⁺/Ho³⁺ UCNPs under the excitation of 980 nm CW laser. (c) Decay profiles of CaF₂:Yb³⁺/Ho³⁺ UCNPs doped with different Ce³⁺ concentrations (0 and 10 mol%) monitored at 541 nm under 980 nm pulse laser excitation. (d) Measured NIR emission spectra of CaF₂:Yb³⁺/Ho³⁺/Ce³⁺ UCNPs with different Ce³⁺ concentrations (0, 4 and 10 mol%) under the excitation of 980 nm CW laser.

Fig. 5(b) displays the dependence of the red and green UC emission as a function of the pump density. Generally, the number of photons required for UC emission is determined by the following formula: I ∝ Pⁿ, where I represents the UC emission intensity, P is pump power density, and n is the number of photons required for UCL. Thus, the slope of the plot of UC emission intensity as a function of pump density determines the number n in the logarithmic coordinate. In Fig. 5(b), the slopes of green and red UC emissions are all close to 2. It suggests that the red and green UC emissions of these two samples are both two-photon processes. Noted that the number of photons required for green and red emissions of CaF₂:Yb³⁺/Ho³⁺/Ce³⁺ UCNPs is slightly lower than those of CaF₂:Yb³⁺/Ho³⁺ counterparts. This is attributed to the fact that the population of intermediate level of the red UC emission in CaF₂:Yb³⁺/Ho³⁺ is cancelled in CaF₂:Yb³⁺/Ho³⁺ doped with 10 mol% Ce³⁺ ions due to the quenching of the green UC emission.

As illustrated in Fig. 5(c), the decay curves of green UC emission for Ce³⁺-free and 10 mol% Ce³⁺ doped CaF₂:Yb³⁺/Ho³⁺ UCNPs are performed under the excitation of 980 nm pulsed laser. The lifetime of the green UC emission was measured to be 127.1 and 108.0 μs for the UCNPs doping with 0 and 10 mol% Ce³⁺ ions, respectively. The decrease of lifetime confirms the existence of the CR1 process. Fig. 5(d) shows the NIR emission (1187 nm, attributed to the transition of ⁵I₆ → ⁵I₈ of Ho³⁺ ions) spectra of CaF₂:Yb³⁺/Ho³⁺ UCNPs doping with different concentrations of Ce³⁺ ions (0, 4 and 10 mol%). It can be found that the NIR emission intensity decreases as the doped Ce³⁺ ions increase, verifying the occurrence of CR2 process. As discussed above, the addition of Ce³⁺ ions in Yb³⁺/Ho³⁺ codoped materials will lead to the occurrence of CR1 and CR2 processes between the Ce³⁺ and Ho³⁺ ions, resulting in the enhancing red and suppressing green UC emissions.

4. Conclusions

In conclusion, CaF₂:Yb³⁺/Ho³⁺ (20/2 mol%) UCNPs codoped with different concentrations of Ce³⁺ ions were successfully synthesized by a simple hydrothermal method. Size manipulation was achieved in the CaF₂:Yb³⁺/Ho³⁺ UCNPs by doping with different concentrations of Ce³⁺ ions. In addition, the introduction of Ce³⁺ ion can greatly suppress the green and enhance the red UC emission, resulting in the luminescence color changing from green to red. The R/G ratios can be varied from 0.17 to 7.44 as the doped Ce³⁺ ions increase from 0 to 10 mol%. The mechanism of the enhancement of red emission has also been demonstrated based on the two CR processes between Ho³⁺ and Ce³⁺ ions, which was supported by the time-resolved decay curves and NIR emissions. The tunability of multicolor and enhancement of red emission make these UCNPs suitable for applications in bioimaging and color display.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We would like to gratefully acknowledge the financial support from the State Key Laboratory of Laser Interaction with Matter Foundation (SKLLIM1708).

Notes and references

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