Tuning the size and upconversion emission of NaYF₄:Yb³⁺/Pr³⁺ nanoparticles through Yb³⁺ doping

Abstract

We introduce a simple method to tune the resulting size as well as the upconversion luminescence of NaYF₄:Yb³⁺/Pr³⁺ nanoparticles through varying the sensitizer ytterbium concentration. Various amounts of ytterbium from 10–70% were doped into the fluoride nanoparticles, producing a tunable size from 29 to 153 nm. Meanwhile, the blue upconversion luminescence intensity of NaYF₄:Yb³⁺/Pr³⁺ nanoparticles was monotonously enhanced, reaching a maximum ∼3.4 fold enhancement at an ytterbium concentration of 70% owing to an improved energy transfer from the ytterbium to the praseodymium ions. Moreover, the luminescence intensity ratio of the blue to the green upconversion was tailored by the composition-dependent cross relaxation process. The result here provides a paradigm for simultaneous control of the physical dimensions as well as the luminescence properties of lanthanide-doped upconversion nanoparticles.

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Introduction

Lanthanide-doped upconversion nanoparticles (UCNPs) have attracted a great deal of consideration due to their intriguing optical properties that promise their potential applications in fields as diverse as solid-state lasers,¹ optical data storage,² solar energy conversion,^3–6 biological imaging,^7–10 and photodynamic therapy.^11,12 UCNPs are able to convert two or more long wavelength light photons into short wavelength emissions through the use of energy level of trivalent lanthanide ion that is embedded into an inorganic host lattice. In particular, lanthanide doped fluoride UCNPs generally exhibit the highest upconversion (UC) or downconversion (DC) efficiency,³ as the fluoride materials have high physicochemical stabilities, as well as intrinsic low phonon energies (<350 cm⁻¹) that are able to minimize energy losses at the intermediate states of lanthanide ions. Typically, UCNPs of NaYF₄ doped with a sensitizer Yb³⁺ and an activator of Er³⁺, Ho³⁺ or Tm³⁺ has been of extensive study, and considered to be one of the most efficient UC systems.¹³ This is not only because the sensitizer Yb³⁺ has a unique one excited energy ²F_5/2 (∼10 000 cm⁻¹) which possesses a significantly higher extinction coefficient (typically ∼10 l mol⁻¹ cm⁻¹) and which matches the ladder-like energy gaps of the Er³⁺, Ho³⁺ or Tm³⁺ ions to empower efficient resonant energy transfers to produce efficient UC emissions.^14–18 However, limited success has been met in NaYF₄-based UCNPs doped with other lanthanide ions like Pr³⁺ ion that has a unique pattern of energy levels.

The Pr³⁺ ion has been of great use as multi-wavelength laser activator or colored emitting ions, as it offers the possibility to achieve a simultaneous blue, green or red emission lasing or other purposes.¹⁹ Visible UC lasers at multiple wavelengths have been realized in Pr³⁺ doped ZBLAN (Zr–Ba–La–Al–Na) bulk glass under an infrared laser pump at ∼840 nm.²⁰ Laser action of the blue UC from the transition of the excited ³P₀ state to the ground ³H₄ state has been demonstrated in various bulk crystals, involving the frequency conversion mechanisms of either excited state absorption or photon avalanche.^1,21–23 Moreover, yellow-to-blue frequency UC processes in Pr³⁺ ions have also been extensively studied in a range of low phonon crystals and glass,^24,25 while intense white light emitting were developed in Pr³⁺/Er³⁺ co-doped tellurite glass sensitized by Yb³⁺ ion.²⁷ Despite successes on Pr³⁺-doped bulk materials, till this point, limited investigation has been devoted to Pr³⁺-doped nanoparticles. The relative work in literature is on visible UC in Yb³⁺/Pr³⁺ co-doped Y₂O₃ nanoparticles.²⁶ However, the employed low Yb³⁺ concentration of 1–2 mol% produces a low UC efficiency owing to the large Yb³⁺–Pr³⁺ ion–ion distance that limits the efficiency of energy transfer from the Yb³⁺ to the Pr³⁺. It is noted that the sensitizer Yb³⁺ ion has an exclusive excited state; increase of its concentration, therefore, exclude the possibility producing cross relaxation process induced quenching effect. Varying Yb³⁺ concentration to shorten the sensitizer–activator distance is an important approach to enhance UC luminescence; this conclusion has been verified by Duan et al., Han et al. as well as by our group in Yb³⁺/Tm³⁺-codoped NaYF₄ and YF₃ UCNPs.^28–34 On the other hand, doping of trivalent lanthanide ions into nanomaterials can influence its growth dynamics due to the dopant-induced transient electric dipole on the surface of the growing nanoparticles.^35–37 For example, our group showed that varying dopant concentration of Yb³⁺ or Gd³⁺ can tune the size and phase of the resulting multifunctional CeO₂ oxide nanoparticles.³⁷ Here, we show that varying the sensitizer Yb³⁺ concentration in Yb³⁺/Pr³⁺-codoped NaYF₄ nanoparticles not only can produce a size-tunable UNCPs, but also can produce enhanced and tailored upconversion emissions from the Pr³⁺ ion under ∼980 nm NIR light excitation.

Results and discussion

The ionic radii of Yb³⁺ (0.868 Å) and Y³⁺ (0.893 Å) are different, which may result in the variation of size and morphology in the formation of NaREF₄ UCNPs. We prepared NaYF₄:Yb³⁺/Pr³⁺ UCNPs doped with varied Yb³⁺ ion concentrations of 10–70 mol%. During the synthesis, all synthetic parameters were kept exactly the same, except for varying the doping concentration of Yb³⁺ ions. Fig. 1(a–d) displays the transmission electron microscopy (TEM) images of NaYF₄ UCNPs doped with 10, 30, 50, 70 mol% Yb³⁺, respectively. High resolution TEM imaging and selected area electron diffraction patterns confirmed the high crystallinity and hexagonal phase of all resulting nanoparticles (Fig. S1, ESI†). The energy dispersive X-ray spectra (EDAX) indicate that the elemental content of the resulting NaYF₄:Yb³⁺/Pr³⁺ UCNPs is in general agreement with the mixture of cationic precursor, suggesting stoichiometric doping of Yb³⁺ ions at a precisely defined concentration (Fig. S2, ESI†). The results demonstrate that all the rare earth ions can be effectively incorporated into the final UCNPs with our synthetic conditions. Therefore, the Yb³⁺ and Pr³⁺ doping concentration can be readily controlled by varying the initial reactant ratio.

Fig. 1 Transmission electron images for NaYF₄ nanocrystals codoped with 0.5% Pr³⁺ and various concentration of (a) 10% Yb³⁺, (b) 30% Yb³⁺, (c) 50% Yb³⁺, and (d) 70% Yb³⁺. (e) Schematic representation of the hexagonal structure of NaYF₄ or NaYbF₄ nanoparticles (a = b = 6.245 Å, c = 4.392 Å for NaYF₄ and a = b = 5.912 Å, c = 4.653 Å for NaYbF₄).

As one can see in Fig. 1(a), very small ∼29 nm sphere-like UCNPs were formed at Yb³⁺ concentration of 10%. However, it evolves into larger ∼62 nm hexagonal-shape UCNPs when Yb³⁺ concentration of 30% were doped [Fig. 1(b)]. For higher Yb³⁺ concentration of 50 and 70%, the hexagonal shape remains unchanged, yet the size increased further to 103 and 153 nm, respectively [Fig. 1(c and d)]. These results agree with the corresponding histogram of the average size distribution (ESI, Fig. S3†). The size evolution can be attributed to the Yb³⁺-varied crystal growth rate via the modification of electron charge density on the nanoparticle surface. Liu et al. calculated, based on density functional theory (DFT), that the electron charge density of the crystal surface was increased when a Y³⁺ (0.893 Å) ion in NaYF₄ was replaced by a larger Gd³⁺ (0.938 Å) ion, which thus repelled the anion F⁻ to produce a smaller nanoparticle size.³⁵ Therefore, when Y³⁺ (0.893 Å) is replaced by a smaller Yb³⁺ (0.868 Å) ion replacing, the electron charge density on the surface of growing UCNPs will be decreased, thus allowing more attraction of F⁻ ion to the particle surface to form a larger size UCNPs.

The crystalline phases of as-prepared samples were further determined by powder X-ray diffraction (XRD); the result is shown in Fig. 2. The XRD pattern for the sample doped with Yb³⁺ of 10 mol% is in good agreement with the standard hexagonal phase NaYF₄ (JCPDS 16-0334), confirming the formation of a hexagonal crystal phase. A sequential shifting of the XRD peaks is observed towards high-angle when higher Yb³⁺ concentration were doped. This peak shifting indicates the decrease of a unit-cell volume due to the replacement of Y³⁺ by smaller ion radii Yb³⁺. In other words, the hexagonal phase NaYF₄ gradually transforms to hexagonal phase NaYbF₄ when elevated Yb³⁺ concentration was doped.

Fig. 2 The XRD patterns of NaYF₄ nanoparticles codoped with 0.5% Pr³⁺ and various concentration of Yb³⁺ ions of (a) 10% Yb³⁺, (b) 30% Yb³⁺, (c) 50% Yb³⁺, and (d) 70% Yb³⁺. The standard XRD patterns of hexagonal phase NaYF₄ and NaYbF₄ are also plotted as a reference.

It is interesting to note that doping of varied Yb³⁺ concentration also produced important influence on the UC luminescence. Fig. 3 shows the emission spectra of NaYF₄ UCNPs codoped with 0.5% Pr³⁺ and varied Yb³⁺ ion concentrations of (a) 10%, (b) 30%, (c) 50%, (d) 70%, respectively, under ∼980 nm laser excitation. All UCNPs exhibit the typical blue emission band at ∼486 nm, the green emission band at ∼523 nm, the green emission band at ∼540 nm, the red emission band at ∼605 nm, as well as the red emission band at ∼639 nm, being ascribed to the ³P₀→³H₄, ¹I₆→³H₅, ³P₁→³H₅, ³P₀→³H₆ and ³P₀→³F₂ transitions of Pr³⁺ ions, respectively. As one can see in Fig. 3, the blue emission at 486 nm is increased gradually when elevating the Yb³⁺ concentration, and enhances about ∼3.4 times for Yb³⁺ of 70%. We also compared the luminescence intensity of NaYF₄:70%Yb³⁺/0.5%Pr³⁺ with that of well-investigated hexagonal NaYF₄:20%Yb³⁺/0.5%Er³⁺ and NaYF₄:20%Yb³⁺/0.5%Tm³⁺ nanocrystals (size ∼30 nm) (ESI, Fig. S4†); it is lower than that of the typical NaYF₄:20%Yb³⁺/0.5%Er³⁺ and NaYF₄: 20%Yb³⁺/0.5%Tm³⁺ systems. This result indicates that a further design of this Yb³⁺/Pr³⁺ codoped system is in need to increase its upconversion efficiency to compete with currently well-known Yb³⁺/Er³⁺ and Yb³⁺/Tm³⁺ UC systems.

Fig. 3 UC emission spectra of colloidal NaYF₄ nanocrystals codoped with 0.5% Pr³⁺ and various concentrations of Yb³⁺ ions (10–70%) under diode laser excitation at 980 nm.

To shed light on the enhancement mechanism of blue UC emission as well as the population process of upper excited ³P₀ levels of Pr³⁺ ions, the dependence of the intensities of various UC emission peaks on laser power for NaYF₄:10%Yb³⁺/0.5%Pr³⁺ UCNPs were measured and displayed in Fig. 4. For an unsaturated UC process, the number of photons that are involved to populate the upper emitting state can be obtained by the relation.

I_UC ∝ Pⁿ, (1)

where I_UC is the UC intensity, P is the pump laser power, and n is the number of laser photons involved to populate the upper emitting levels. The value of n can be determined by the slope value of a linear fitting in a logarithmic–logarithmic plot of eqn (1). As shown in Fig. 4, slope values of 1.98, 1.91, 1.89, and 1.75 were observed for UC emissions peaked at 486, 523, 540, and 639 nm, respectively, for NaYF₄:10%Yb³⁺/0.5% Pr³⁺ UCNPs. These slope values agree well with the previous results in Yb³⁺–Pr³⁺ codoped bulk materials under continuous wave laser excitation at ∼980 nm.³⁸ This result illustrates that two-photon processes are involved to populate the ¹I₆, ³P₁ and ³P₀ states, respectively (consult Fig. 5). It is noted that the blue emission at 486 nm and the green emissions at 540 nm have the same slope value of ∼2 (see Fig. 5). The mechanisms for blue, green, and red UC generation are presented in Fig. 5.

Fig. 4 log–log plots of the intensities of various UC emission bands in Fig. 3 on the excitation density in NaYF₄ nanoparticles codoped with 10 mol% Yb³⁺ and 0.5 mol% Pr³⁺. The slope values of the linear fits (solid line) are presented in the inset together with the peak wavelength of each UC band.

Fig. 5 Energy level diagram of the Pr³⁺ and Yb³⁺ ions as well as the proposed UC mechanism in NaYF₄:Yb³⁺/Pr³⁺ nanocrystals via varying content of Yb³⁺ ions under a 980 nm laser excitation.

Fig. 5 depicts the pertinent energy levels of the Pr³⁺ and Yb³⁺ ions as well as the proposed UC mechanisms.²⁷ Laser photons at ∼980 nm match the ²F_7/2→²F_5/2 transition of Yb³⁺ ion, resonantly exciting Yb³⁺ ion from the ground to its excited state. Energy transfer process from the Yb³⁺ ions to a neighboring Pr³⁺ ion can promote the Pr³⁺ ions in the ³H₄ state to ¹G₄ state (ET1). The population in the ¹G₄ level can be promoted to the ³P₀ levels either by energy transfer (ET2) from another excited Yb³⁺ ion or absorbing the energy of pumping photon. Once the ³P₀ level is populated, the excited electron can release its energy by emitting visible emissions. The blue emission at 486 nm can be produced by radiative decay to the ground state from the ³P₀ state. Generally, the pumping energy of 980 nm is very difficult to population in ³P₁ and higher states of Pr³⁺ ions. But the cross relaxation between ³P₀→³P₂ and ³H₅→³H₄ in Pr³⁺ ions promotes the occupation in the ³P₂ state, which through nonradiative relaxation process populates ¹I₆ and ³P₁ states. Excited Pr³⁺ ions in ¹I₆ and ³P₁ states decay to ³H₅ state, leading to green emission centered at 523 and 540 nm. The red emissions at 605 and 639 nm originate from the ³P₀ state to ³H₆ and ³F₂ transitions, respectively.

It is noted that the efficiency of cross relaxation processes strongly depends on the Pr³⁺ ion's concentration, while the efficiency of ET1 and ET2 processes are insensitive to Pr³⁺ ion concentration in the host lattice when varying in a small amount range.

As a result, we further prepared NaYF₄ nanoparticles codoped with Yb³⁺ of 10%, and various Pr³⁺ concentrations of 0.1%, 0.25%, 0.5%, and 1% Pr³⁺. The UC spectra from them are displayed in Fig. 6(a). The intensity increases slightly when the Pr³⁺ concentration is increased from 0.1 to 0.5% molar fraction. This augment of emission intensity can be ascribed to the increased number of emitting Pr³⁺ centers. However, a further increase in the Pr³⁺ concentration results in a decrease in the emission intensity. This is partly because, for a given doping content of Yb³⁺ and overall excitation energy, the excitation received by individual Pr³⁺ ion decreased with the increase of Pr³⁺ ion concentration. More importantly, the high concentration of Pr³⁺ ions leads to a decrease in distance between Pr³⁺ ions, which enhances the probability of cross-relaxation, and thus suppresses the efficient energy transfer processes from Yb³⁺ ions to Pr³⁺ ions for population of ³P₀ state, resulting in an increase in green/blue ratio with the increase of Pr³⁺ concentration [Fig. 6(b)]. It is noted that the intensity ratio of the band 540 nm to the band at 639 nm remain almost unchanged. This is because they arise from the same upper excited energy level of the ³P₀ state.

Fig. 6 (a) UC emission spectra of NaYF₄:Yb³⁺/Pr³⁺ nanoparticles codoped with Yb³⁺ of 10%, and various concentrations of Pr³⁺ ions (0.1–1%) under diode laser excitation at 980 nm. (b) The intensity ratios of emissions at 486 nm (from the ³P₀ state to the ³H₄ state), at 540 nm (from the ³P₁ state to the ³H₅ state), to the emission at 639 nm (from the ³P₀ state to the ³F₂ state) versus Pr³⁺ concentration.

Fig. 7 shows the variation of UC emissions from NaYF₄:10%Yb³⁺/0.5%Pr³⁺ UCNPs when irradiated with a varied pump power. For low pump power the intensity for emission at 486 nm is lower than that of the emission at 540 nm, while a reverse result is obtained under high pump power. This switching result illustrates that high laser irradiance also facilitates the increase of blue UC emission in NaYF₄:10%Yb³⁺/0.5%Pr³⁺ nanoparticles. It is reasonable to assume that, for the given Yb³⁺ concentration in the samples, the excitation received by individual Pr³⁺ ions increased with the enhancement of laser irradiance.

Fig. 7 (a) Pump-power-dependent UC 4f emission spectra of NaYF₄:10%Yb³⁺/0.5%Pr³⁺ UCNPs. All spectra were recorded at room temperature under excitation of ∼980 nm diode laser at a power density of 5 W cm⁻². (b) Emission intensities for the ³P₀→³H₄ transition and the ³P₁→³H₅ transition from NaYF₄: 10%Yb³⁺/0.5%Pr³⁺ UCNPs as a function of laser pump power.

Conclusions

In summary, we have demonstrated that varying the sensitizer Yb³⁺ concentration can simultaneously tune the resulting size as well as the upconversion luminescence of NaYF₄:Yb³⁺/Pr³⁺ UNCPs. The intensity of the blue UC emission was enhanced by ∼3.4 fold when elevating the sensitizer concentration from 10 to 70%, being ascribed to the improved energy transfer from the Yb³⁺ to Pr³⁺ ions due to the shorter distance between them. The intensity ratio of the blue to the green upconversion was also varied by the composition-dependent cross relaxation process. The reported result might have important implications in other lanthanide-doped UCNPs that involve the use of Yb³⁺ as sensitizer ions.

Acknowledgements

This work is supported by Natural Science Foundation of China (51102066 and 51402071), the Fundamental Research Funds for the Central Universities (Grant no. HIT. NSRIF.2015048 and AUGA5710052614), international scientific and technological cooperation projects (Grant no. 2014DFA50740), the Program for Basic Research Excellent Talents in Harbin Institute of Technology (BRETIII 2012018) and the National Science Fund for Distinguished Young Scholars (Grant no. 51325201).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra11357h

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