Nonlinear spectral and lifetime management in upconversion nanoparticles by controlling energy distribution

Abstract

Optical tuning of lanthanide-doped upconversion nanoparticles has attracted considerable attention over the past decade because this development allows the advance of new frontiers in energy conversion, materials science, and biological imaging. Here we present a rational approach to manipulating the spectral profile and lifetime of lanthanide emission in upconversion nanoparticles by tailoring their nonlinear optical properties. We demonstrate that the incorporation of energy distributors, such as surface defects or an extra amount of dopants, into a rare-earth-based host lattice alters the decay behavior of excited sensitizers, thus markedly improving the emitters’ sensitivity to excitation power. This work provides insight into mechanistic understanding of upconversion phenomena in nanoparticles and also enables exciting new opportunities of using these nanomaterials for photonic applications.

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Introduction

Lanthanide-doped photon upconversion materials have been studied for several decades owing to their distinct optical properties such as large anti-Stokes’ spectral shift, long luminescence lifetime, and sharp emission band. These attributes make the upconversion materials particularly attractive for use in lighting, lasing, quantum cutting, etc.^1,2 In recent years, the rapid development of nanotechnology has enabled immense interest in upconversion nanoparticles (UCNPs) because of their fascinating optical properties and technological potential in many fields of research.^3–37 Notably, many strategies, including doping composition/concentration variation, surface ligand coordination and pulse-duration modulation, have been developed to tune the emission profile of the UCNPs.^38–45 An important and facile route to control upconversion emission, based on the nonlinear power response of photon upconversion, is to vary the excitation power externally.^46–48

In a typical photon upconversion process, the excitation energy is absorbed by sensitizers, such as Yb³⁺ ions doped in a host matrix, and then transferred to neighboring emitters (or activators), such as Er³⁺ or Tm³⁺ featuring ladder-like energy states (Fig. S1 in the ESI†). The sequential photon absorption by the sensitizers permits the emitters to be excited to higher energy levels through energy transfer. It is widely accepted that for multi-step (n) energy transfer, the luminescence intensity I is proportional to n power of the excitation density P.^49,50 Thus, the number of energy transfer upconversion (ETU) steps can be derived from the slope (I–P slope) of the luminescence intensity curve plotted against pump power density in a double-logarithmic representation (Fig. S2†). However, the upconversion emission intensity I may not be strictly proportional to n power of the excitation density P. The n value is likely to be affected by a multitude of factors related to energy distribution in UCNPs, such as a crystalline structure, particle size, and dopant concentration. Moreover, the variation of the n value is typically accompanied by a concurrent change in luminescence decay lifetime, meaning that n is highly relevant to the spectral properties and the luminescence dynamics of upconversion materials.

In this work, we investigate the spectroscopic properties and decay behaviors of UCNPs under the influence of different types of energy distributors through surface defect engineering or doping (Fig. 1). Our measured n value correlates well with upconversion emission characteristics in that large numbers of energy distributing pathways increase the sensitivity of the UCNPs to excitation power. Importantly, the decay lifetimes of both sensitizer and emitter ions could be tuned by manipulating the nature of the energy distributors, providing new opportunities for photonic applications in optical switching and security encoding.

Fig. 1 Illustration of typical energy distribution processes involving Yb³⁺-sensitized upconversion. Note that for a particular system at the excited state, possible energy distributors include sensitizer ions, codoped emitters, surface defects, and solvent molecules with high energy vibrational groups.

Results and discussion

We prepared NaYF₄:Yb/Er (20/2%) UCNPs as a model sample by a well-established co-precipitation method.⁵¹ As seen under a transmission electron microscope (TEM), the as-synthesized nanoparticles are 15.3 ± 1.1 nm in diameter with a narrow size distribution (Fig. 2a and b). A pure hexagonal phase structure of the nanoparticles was confirmed by X-ray diffraction studies (Fig. 2c). Under the excitation of a 975 nm CW laser, we observed visible emission bands at 540 and 654 nm, typically attributable to the ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ions (Fig. 2d). We considered the 540 nm emission as an example to study the excitation power response and decay behaviors of the UCNPs. The power dependent luminescence measurement indicates that in the power density range of 29–70 W cm⁻², the 540 nm emission is generated via a typical 2-photon upconversion process (Fig. 2e). On 975 nm pulsed laser excitation, we observed a distinct rise edge in the temporal luminescence curve of the 540 nm emission due to the time consumed by the multi-step energy transfer process.⁵² We focus on the decay lifetime of the luminescence which is critical to the application of upconversion tuning in the time domain for optical multiplexing.^9,23 Notably, the emission of Yb³⁺ (²F_5/2 → ²F_7/2 transition) at 985 nm has a decay lifetime of 69 μs, while the decay of Er³⁺ emission at 540 nm registers a lifetime of 88 μs (Fig. 2f).

Fig. 2 (a) A typical TEM image of the as-prepared NaYF₄:Yb/Er (20/2%) nanoparticles. Scale bar is 50 nm. (b) Size histogram of the nanoparticles. (c) X-ray diffraction patterns of the nanoparticles, which are in good agreement with the literature reference for hexagonal NaYF₄ crystal (Joint Committee on Powder Diffraction Standards file number 16-0334). (d) Emission spectra of the UCNPs recorded with a 975 nm CW laser excitation at different power densities (29–70 W cm⁻²). (e) The 540 nm (⁴S_3/2 → ⁴I_15/2 transition of Er³⁺) emission intensity versus pump power density in double-logarithmic representation. (f) Decay curves of 540 nm emissions from Er³⁺ as well as 985 nm from Yb³⁺ obtained upon excitation of the nanoparticles at 975 nm.

To study the change in the I–P slope and emission lifetime in the presence of varied energy distributors, we adopted different strategies to alter the upconversion luminescence of NaYF₄:Yb/Er (20/2%) UCNPs: (i) surface passivation by coating the particles with different thicknesses of a NaYF₄ inert shell (Fig. S3†);^53–60 (ii) controlling the size of UCNPs (Fig. S4†);⁶¹ (iii) coating the particles with a NaYF₄:Yb active shell containing different amounts of Yb³⁺ (Fig. S6†);^62,63 and (iv) changing the Er³⁺ doping concentrations (Fig. S7†). By investigating the luminescence decay lifetime and the pump-power-dependent luminescence intensity as shown in Table 1, it is evident that the I–P slopes of the 540 nm emission enlarge with increasing numbers of energy distributors, indicating the high sensitivity of the luminescence to the excitation power density. Notably, the luminescence decay lifetimes of both Yb³⁺ and Er³⁺ are found to diminish as the number of energy distributors increases.

Table 1 Comparative investigations of emission profiles for different types of nanoparticles. Note that decay lifetimes were measured at 985 and 540 nm, and the I–P slope tuning was realized by modulating energy distributors through a variety of strategies, including inert and active shell coating, particle size control, and doping concentration adjustment. All 540 nm I–P slopes were measured under CW laser excitation at 975 nm with a power density range of 20–80 W cm⁻²

Structure		Core composition	Core diameter (nm)	Shell composition	Shell thickness (nm)	985 nm (μs)	540 nm (μs)	540 nm I–P slope
UCNPs with different inert shells		NaYF4:Yb/Er (20/2%)	15.3	—	—	69	88	1.89
		NaYF4:Yb/Er (20/2%)	15.3	NaYF4	1.3	304	187	1.77
		NaYF4:Yb/Er (20/2%)	15.3	NaYF4	1.8	403	251	1.73
		NaYF4:Yb/Er (20/2%)	15.3	NaYF4	2.8	783	301	1.63
UCNPs with different sizes		NaYF4:Yb/Er (20/2%)	15.3	—	—	69	88	1.89
		NaYF4:Yb/Er (20/2%)	23.8	—	—	327	245	1.76
		NaYF4:Yb/Er (20/2%)	27.2	—	—	506	333	1.65
UCNPs with different active shells		NaYF4:Yb/Er (20/2%)	15.3	NaYF4	1.6	379	231	1.74
		NaYF4:Yb/Er (20/2%)	15.3	NaYF4:Yb (10%)	1.9	274	180	1.78
		NaYF4:Yb/Er (20/2%)	15.3	NaYF4:Yb (20%)	1.7	131	128	1.82
UCNPs with different emitter concentrations		NaYF4:Yb/Er (20/0.5%)	15.3	—	—	101	123	1.83
		NaYF4:Yb/Er (20/1%)	15.5	—	—	97	103	1.88
		NaYF4:Yb/Er (20/2%)	15.3	—	—	69	88	1.89
		NaYF4:Yb/Er (20/4%)	15.7	—	—	50	65	1.92
		NaYF4:Yb/Er (20/8%)	15.2	—	—	35	34	1.92
		NaYF4:Yb/Er (20/16%)	15.2	—	—	19	14	1.98
		NaYF4:Yb/Er (20/32%)	15.4	—	—	13	7	2.09

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In addition, high levels of vibration energy in solvent molecules are considered to be a dominant factor governing the luminescence profiles of UCNPs. As can be seen in Fig. S8,† ligand-free NaYF₄:Yb/Tm (30/0.5%) nanoparticles showed a short decay lifetime (346 μs) in water and a large I–P slope for Tm³⁺ emission at 800 nm. Indeed, the high vibration nature of water molecules allows the multiphonon relaxation process to be more accessible.⁶⁴ It is worth noting that with the coating of NaYF₄ the core–shell nanoparticles showed essentially the same decay lifetime and I–P slope in cyclohexane and water. Taken together, our results suggest that the vast combinations of different energy distributors (such as surface defects, dopants and solvents) enable precise depletion of the excited states of Yb³⁺ ions, thereby allowing the emission decay lifetime of the nanoparticles to be controlled within a wide range (Table 1). These strategies could be utilized in generating distinct time-domain codes for optical multiplexing and security applications. Another important note is that without the sensitizer the influence of energy distributors on the luminescence properties of the emitter is virtually negligible (Fig. S9†).

The effect of energy distributors on the measured decay lifetime of Yb³⁺ is investigated using the rate equations derived from a sensitization upconversion model (Fig. 3):

where N_i (i = 0, 1, 2…) is the population density of the emitter on level i and W_i is the corresponding energy transfer rate from the sensitizer to the emitter. W_D is the energy transfer rate from the sensitizer to the energy distributor. N_D represents the population density of effective energy distributors. R_b is the radiative decay rate of the sensitizer from the excited state. From eqn (1), one can conclude that increasing the number of energy distributors accelerates the depletion of excited Yb³⁺ ions and thus shortens the measured Yb³⁺ decay lifetime.

Fig. 3 Simplified scheme of the energy distributor effect on energy transfer processes involving a sensitization upconversion mechanism.

We next carried out theoretical investigations to explore the fundamental principle underlying the spectral and lifetime management of UCNPs through the control of energy distribution within the particles. From the analysis of the rate equations of energy levels in a simplified two-photon upconversion process (see the ESI†), the population density at the emitting level N₂, which is proportional to the emission intensity I, can be expressed by eqn (2):

where A and B are constants, N_b denotes the population density of the excited states of the Yb³⁺ sensitizer, which determines the population density of the emitting level. From eqn (2), the N₂–N_b slope value in double-logarithmic representation is calculated in between 1 and 2.⁵⁰ Upon excitation with a power density of P, N_b can be further derived from eqn (3):where C and D are constants. Notably, N_b is not simply proportional to the excitation power density P, as indicated in eqn (3). We conducted numerical simulations on NaYF₄:Yb/Er (20/2%) UCNPs to validate the relationship of N_b with the incident photon flux by introducing the rate equations of a two-photon upconversion process, as shown in Fig. 4a. We consider that the population density of Yb³⁺ at the excited state is nonlinear to the excitation photon flux due to the excitation saturation of Yb³⁺. This phenomenon further affects the relationship of N₂–P, resulting in the deviation of the I–P slope away from the normal range. Our numerical simulations on both NaYF₄:Yb/Er (20/2%) and NaYF₄:Yb/Er (20/2%)@NaYF₄ (∼2.8 nm in shell thickness) UCNPs indicated that the slope of N₂–P diminishes with increasing excitation photons. Compared with the core particles, the shell-coated UCNPs showed a lower value of the I–P slope under identical incident photon flux (Fig. 4b). Remarkably, the simulated value of the I–P slope could be reduced to less than 1 at a high power excitation, which agrees well with our experimental results (Fig. 4c).

Fig. 4 (a) Numerical simulations on the population density of the excited state of Yb³⁺ ions versus the number of excitation photon flux in linear representation using NaYF₄:Yb/Er (20/2%) UCNPs. (b) Numerical simulations on the population density of the 540 nm emission level of Er³⁺ ions versus the number of excitation photon flux in double-logarithmic representation using NaYF₄:Yb/Er (20/2%) and NaYF₄:Yb/Er (20/2%)@NaYF₄ (∼2.8 nm in shell thickness) core–shell UCNPs. (c) Power-density-dependent emission profiles of NaYF₄:Yb/Er (20/2%) and NaYF₄:Yb/Er (20/2%)@NaYF₄ core–shell UCNPs at 540 nm. The excitation power density under study is in the range of 1–10⁴ W cm⁻².

From published work, it can be found that in Yb³⁺/Er³⁺/Tm³⁺ triply-doped upconversion systems, increasing the excitation power density of UCNPs give rise to a larger rate of increase in the blue emission intensity than that of the green emission intensity.^46,65–68 This can be attributed to the fact that blue emission from Tm³⁺ (¹G₄ → ³H₆ and ¹D₂ → ³F₄ transitions) requires more excited photons than green emission from Er³⁺ (²H_11/2/⁴S_3/2 → ⁴I_15/2 transitions) (Fig. S1†). As a result, the blue emission typically features a large I–P slope n, thus its intensity increases rapidly with increasing excitation power density. On the basis of the findings described in Table 1, we reason that placing the emitters in regions close to energy distributors, for example, at the particle surface, might be able to improve the particle's sensitivity to the excitation power density (Fig. 5).

Fig. 5 Schematic representation showing pump-power-dependent luminescence of a sensitizer/emitter codoped nanoparticle in the presence or absence of an energy distributor.

To verify our hypothesis, we prepared two sets of multi-shell structured UCNPs comprising NaYF₄:Yb/Tm@NaYF₄@NaYF₄:Yb/Er@NaYF₄:Yb and NaYF₄:Yb/Er@NaYF₄@NaYF₄:Yb/Tm@NaYF₄:Yb (Fig. 6a, d and S10†). Note that Tm³⁺ and Er³⁺ ions are doped at different layers for direct comparison of their power-dependent emission profiles. An inert layer of NaYF₄ was inserted between the two emitting layers to minimize cross-relaxation between Er³⁺ and Tm³⁺ (Fig. S12 and S13†).^69,70 The outermost NaYF₄:Yb shell was utilized to enhance the emission of the outer emitting layer to a level comparable to that obtained from the inner emitting layer (Fig. S14†). As expected, from the obtained emission spectra (Fig. 6b and e), it can be seen that the changes in the blue-to-green emission ratio of these two samples are dramatically different as a function of excitation power density (Fig. 6c, f, and S15a and c†). Consequently, the two colloidal samples exhibited different CIE color profiles with varied excitation power densities (Fig. S15b and d†). As control experiments, we synthesized two similar sets of UCNPs except for replacing the outermost layer of NaYF₄:Yb with NaYF₄. Interestingly, we observed that both the samples exhibit a similar trend in the blue-to-green emission intensity ratio as measured in response to different excitation powers (Fig. 7). This observation was largely attributed to the elimination of the surface quenching effect through the use of an optically inert NaYF₄ layer.

Fig. 6 (a and d) TEM images of NaYF₄:Yb/Tm@NaYF₄@NaYF₄:Yb/Er@NaYF₄:Yb and NaYF₄:Yb/Er@NaYF₄@NaYF₄:Yb/Tm@NaYF₄:Yb nanoparticles, respectively. Insets showing the design layout of the core–shell structures. (b and e) Corresponding power-dependent upconversion emission spectra of the two samples under 975 nm excitation. (c and f) Corresponding pump-power-dependence of the blue/green emission ratio obtained from the two samples.

Fig. 7 Power dependent blue-to-green upconversion emission intensity ratio of NaYF₄:Yb/Tm@NaYF₄@NaYF₄:Yb/Er@NaYF₄ and NaYF₄:Yb/Er@NaYF₄@NaYF₄:Yb/Tm@NaYF₄ UCNPs upon 975 nm laser irradiation.

Conclusions

We have demonstrated a general approach for the design of upconversion nanocrystals with emitters amenable to nonlinear spectral and lifetime management. By controlling the proximity of energy distributors to specific emitters in core–shell structured nanocrystals, power dependent upconversion processes are demonstrated with high temporal tunability. The luminescence lifetime of upconversion nanocrystals can be modified in a wide range, which has potential wide-ranging implications for time-domain optical multiplexing and security applications.

Experimental section

Materials

Yttrium(III) acetate hydrate (99.9%), ytterbium(III) acetate hydrate (99.9%), thulium acetate hydrate (99.9%), erbium(III) acetate hydrate (99.9%), sodium hydroxide (NaOH; >98%), ammonium fluoride (NH₄F; >98%), 1-octadecene (90%), oleic acid (90%), and hydrochloric acid were all purchased from Sigma-Aldrich and used as received without further purification. Analytical grades of ethanol and cyclohexane were purchased from Sigma-Aldrich. Water used in the experiment was purified to have a resistivity of 18.2 MΩ.

Synthesis of NaREF₄ (RE = Y, Yb, Tm, Er) core nanoparticles

In a typical experiment,⁵¹ to a 50 mL flask charged with 3 mL of oleic acid (OA) and 7 mL of 1-octadecene (ODE) was added 2 mL of an aqueous solution containing RE(CH₃CO₂)₃ precursors with a total lanthanide amount of 0.4 mmol. The resulting mixture was heated at 150 °C for 1 h to form a transparent solution and then cooled down to 50 °C. Subsequently, a methanol solution (6 mL) of NH₄F (1.6 mmol) and NaOH (1 mmol) was added and stirred for 30 min. The reaction temperature was then increased to 110 °C for another 30 min to remove the methanol from the reaction mixture. After that, the solution was heated to 290 °C and maintained at this temperature under an argon atmosphere for 1.5 h. Then the mixture was cooled down to room temperature. The resulting nanoparticles were precipitated out through addition of ethanol, collected by centrifugation (5000 rpm, 10 min), washed with ethanol twice, and finally dispersed in 4 mL of cyclohexane.

Synthesis of NaREF₄@NaREF₄ (RE = Y, Yb, Tm, Er) core–shell nanoparticles

The shell precursor was prepared by mixing RE(CH₃CO₂)₃ (0.2 mmol, RE = Y, Yb, Er, Tm), 3 mL OA and 7 mL ODE in a 50 mL flask, followed by heating at 150 °C for 1 h. After cooling down to 80 °C, 0.2 mmol of NaREF₄ core nanoparticles dispersed in 2 mL of cyclohexane was added. The mixture was slowly cooled down to 50 °C for 30 min, and a methanol solution (3 mL) of NH₄F (0.8 mmol) and NaOH (0.5 mmol) was added. Subsequently, the reaction temperature was increased to 110 °C and kept for 30 min to remove the methanol. After that, the solution was heated to 290 °C and maintained at this temperature under an argon flow for 1.5 h. After cooling to room temperature, the nanoparticles were precipitated out by addition of ethanol, collected by centrifugation, washed with ethanol, and redispersed in 4 mL of cyclohexane. The preparation of multi-shell nanoparticles was carried out by repeating the same procedure except for changing the composition ratio of the lanthanides.

Synthesis of ligand-free nanoparticles

The as-prepared OA-capped nanoparticles were dispersed in the mixture containing 1 mL of ethanol and 1 mL of 0.2 M HCl solution. After removal of the surface ligands by sonication for 1 min, the resulting products were collected by centrifugation (14 000 rpm, 20 min) and were re-dispersed in H₂O after washing with a mixed ethanol–H₂O solution (1 : 1, v/v) 3 times.

Characterization

Transmission Electron Microscopy (TEM) measurements were carried out on a JEOL-1400 transmission electron microscope (JEOL) operating at an acceleration voltage of 100 kV. Powder X-ray Diffraction (XRD) data were obtained on a Siemens D5005 X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å). The upconversion luminescence spectra under 975 nm excitation were recorded using an Edinburgh FSP920-C spectrometer equipped with a photomultiplier (PMT), in conjunction with a 975 nm diode laser (1 W). The CW laser beam was applied onto UCNP samples loaded in a quartz cuvette with a path length of 1 cm. The emission from the samples was collected at an angle of 90° to the excitation beam by using a pair of lenses. The decay curves of upconversion emission were measured by using a customized phosphorescence lifetime spectrometer (FSP920-C, Edinburgh) equipped with a nanosecond optical parametric oscillator (OPO) pumped by using a 3.8-ns-pulsed Nd:YAG laser as the excitation source (Ekspla, NT352).

Acknowledgements

The work was supported in part by the Singapore Ministry of Education (grant no. R143000627112), the GSK (Singapore) Research Fund (R143000492592), the National Natural Science Foundation of China (grant no. 21471109 and 21371095), and the Natural Science Foundation of Jiangsu Province (Key Project: BE2015699, BK20131404, and BL2014075).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6nr00812g

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