Investigation of energy transfer mechanisms in rare-earth doped amorphous silica films embedded with tin oxide nanocrystals

Xiaowei Zhang; Ruowang Chen; Pengjun Wang; Zhixing Gan; Yuejun Zhang; Han Jin; Jiawen Jian; Jun Xu

doi:10.1364/OE.27.002783

1. Introduction

With abundant f-orbital configurations, rare earth (RE³⁺) ions-doped amorphous silica host can exhibit sharp and long-lived fluorescent emissions via the intra-4f transitions. Recent startling interest for RE³⁺ ions-doped silica is stimulated by the continuously expanding need for fluorescent emissions meeting the stringent requirements of optical telecommunications, lighting devices, bio-sensors and bio-imaging set-ups [1–3]. On the other hand, due to the Laporte-forbidden character and intra-configurational nature of intra-4f transitions, the optical absorption cross-sections of RE³⁺ ions-doped pure amorphous silica is only ~10⁻²¹ cm² and the corresponding molar absorption coefficient is typically less than 10 M⁻¹ cm⁻¹, which indicates the direct excitation of RE³⁺ ions is an extremely inefficient process [4,5]. So far, it still remains a great challenge to obtain high-efficiency characteristic emissions from RE³⁺ ions doped pure amorphous silica hosts. One of effective strategies to improve the characteristic emission efficiency of RE³⁺ ions is co-doping size-tunable quantum dots (QDs) with large absorption cross-sections, which can transfer the absorbed energy to surrounding RE³⁺ ions and then improve the characteristic emission efficiency from RE³⁺ ions in silica hosts. Most of early reports focus on Si QDs as sensitizers of RE³⁺ ions [6]. Due to the narrow bandgap of Si QDs, the back energy transfer process from RE³⁺ ions to Si QDs severely hinders the improvement of emission efficiency [7]. A proven more effective way is co-doping with wider bandgap semiconductor QDs rather than Si, such as ZnO QDs [8], In₂O₃ QDs [9] or TiO₂ QDs [10], taking full advantages of their sensitizing ability to populate the excited state of RE³⁺ ions. Generally, these co-doped wide bandgap semiconductor QDs have relatively large absorption cross-sections to absorb the incident light and can effectively improve the density of sensitizing RE³⁺ ions. However, up to now, as for the energy transfer efficiency and UV excitation range, the overall optical performance of RE³⁺ ions-doped amorphous silica still remains inferior to that of the best organic dyes [11]. Besides, the fundamental question of how the local environments affect the energy transfer efficiency and mechanism remains under debate until now [12–16]. A more in-depth insight into the energy transfer processes may be very important to optimize optical performance of RE³⁺ ions-doped silica for technological applications in future.

Here, we fabricate three different types of RE³⁺ ions and SnO₂ QDs co-doped silica thin films by a soft chemistry-based method. In marked contrast with PVD or PLD technique, this soft chemistry method appears as a straightforward approach to prepare RE³⁺ ions-activated silica thin films with flexibility to regulate the size of QDs and local environment of activated RE³⁺ ions [17–20]. By introducing SnO₂ QDs with suitable size as sensitizers, the characteristic emission intensity of RE³⁺ ions can be enhanced by more than two orders of magnitude via the energy transfer process. Further, Eu³⁺ ions are selected as the representative sensitizers of RE³⁺ ions to elucidate the possible energy transfer mechanism because it is a well-known structural probe used to investigate the surrounding crystal-field symmetry. Selective PLE measurements confirm that energy transfer process occurs between SnO₂ QDs and Eu³⁺ ions. PL measurements support the surrounding crystal-field and site symmetry of Eu³⁺ ions can be readily controlled by changing the annealing temperatures. Quantitative studies of PL decay lifetime and photoluminescence temperature-dependence suggest that the partial incorporation of Eu³⁺ ions into SnO₂ sites gives rises to the change of crystal-field symmetry and the significant enhancement of energy transfer efficiency from SnO₂ QDs to Eu³⁺ ions. Typical analytical EDS mapping results prove that part of Eu³⁺ ions doped into the SnO₂ sites after annealing at 1000 °C. We anticipate that our results will be a starting point for the further research on the energy transfer mechanisms involved in RE³⁺ ions doped silica under different local environments.

2. Material and methods

The sol-gel method and spin-coating technique was used to fabricate the RE³⁺ ions (Eu³⁺ ions, Tb³⁺ ions, Er³⁺ ions) and SnO₂ QDs co-doped amorphous silica thin films. Firstly, a mixture of tetraethyl orthosilicate (TEOS), ethanol, and de-ionized water was used to form a pure silica sol by the gradual hydrolysis of TEOS. Tin (IV) chloride (SnCl₄) and lanthanide nitrates were added into the starting solution as precursors of SnO₂ QDs and RE³⁺ ions, respectively. These precursors were selected for their high solubility in the aqueous sol. Then, the dilute hydrochloric acid (HCl) was added drop-wise to the mixed solution until the pH value reached to 2.0. Subsequently, the mixed solution was stirred constantly at 65°C water-bath for complete hydrolysis of TEOS. After a series of hydrolysis and condensation reactions, the viscosity of the transparent solution gradually increased when the sol interconnected to form gel. Subsequently, the as-prepared gel was spin-coated onto Si substrates with a constant speed of 4000 rpm, following by different post-annealing temperatures from 500 °C to 1100 °C. During the annealing process, the rapid evaporation of ethanol and de-ionized water caused the silica network to shrink gradually. The rigid silica network would confine the final dimensions of SnO₂ crystals and prevent them from agglomeration. Finally, the SnO₂ QDs with a specific size formed inside amorphous silica thin films during the post-annealing process. The final average size of the SnO₂ QDs as sensitizers can be tunable by precisely controlling the amounts of Sn⁴⁺ ions in precursors [21–23]. All chemical products were used as received without further purification.

Microstructures of silica thin film were confirmed by the FEI TECHNAI-F20 field emission transmission electron microscope (TEM) operated at 200 kV. The cross-sectional thin films for TEM observations were fabricated by the GATAN 691 precision ion polishing system. The elemental analysis of samples were obtained by the FEI TitanX with the EDS mapping. Each EDS map was collected with no more than 1 min to minimize the electron beam radiation damage. The Cliff-Lorimers method provided by the Espirit software was used to quantify the atomic percentage in each spectrum.

Stead-state PL and PLE spectra were obtained by using the Horiba Jobin Yvon Fluorolog-3 system equipped with a 30 mW He-Cd laser and a 450W wavelength-tunable Xe lamp as the excitation sources. A Hamamatsu R928 PMT and a liquid-nitrogen-cooled InGaAs photodetector with the standard lock-in amplifying technique were used for visible and near-infrared luminescence detection, respectively. Time-resolved PL spectra were obtained by using the Edinburgh Photonics FLS 980 fluorescence spectrophotometer based on the time-correlated single photon counting technology. Temperature-dependent PL spectra were obtained by placing the thin films into a closed-cycle helium cryostat equipped with four quartz glasses as the optical windows. All the spectra were corrected for the system response and thin film thickness.

3. Results and discussion

Figure 1(a)-1(c) show the characteristic emissions of three different types of RE³⁺ ions (Eu³⁺ ions, Tb³⁺ ions, Er³⁺ ions, respectively) doped in silica thin films. A series of sharp characteristic emission peaks correspond to the intra-4f transitions of different RE³⁺ ions in amorphous silica. By co-doping SnO₂ QDs with suitable average size as sensitizers, these sharp characteristic emission peaks of all RE³⁺ ions-doped silica thin films can be strikingly enhanced by more than two orders of magnitude, which can be explained as the effective resonance energy transfer process [24]. The formation of SnO₂ QDs can be confirmed by the high-resolution cross-sectional TEM image of silica thin films as shown in Fig. 2(d). All these SnO₂ QDs show the clear-cut crystalline features inside the QDs and the measured inter-planar spacing corresponds to the (110) facets of tetragonal phase SnO₂ crystals. These crystalline QDs uniformly distribute in the amorphous silica thin film and no aggregation of particles is found as shown in Fig. 1(d). Figure 1(e) is the corresponding size distribution histogram of SnO₂ QDs inside amorphous silica. The average size of SnO₂ QDs in diameter is 5.2 nm and the low variance of the estimate is 0.8 nm, indicating the uniformed size distribution.

Fig. 1 (a)-(c) PL spectra of three different types of RE³⁺ ions (Eu³⁺ ions, Tb³⁺ ions, Er³⁺ ions, respectively) and Sn⁴⁺ ions co-doped in silica thin films (red lines) and PL spectra of Sn⁴⁺ ions-free samples (black lines, multiplied by a factor of 10 to allow the comparison). All the samples are excited by the He-Cd laser working at 325 nm. (d) Cross-sectional TEM image of SnO₂ QDs-doped amorphous silica thin film. Scale bar equals to 10 nm. (e) Size distribution histogram of SnO₂ QDs. (f) PLE spectra of Eu³⁺ ions-doped silica thin films with/without SnO₂ QDs as sensitizers. The detected wavelength is kept at 613 nm.

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Fig. 2 (a) Normalized PL spectra of Eu³⁺ ions and SnO₂ QDs co-doped silica thin films after different annealing temperatures from 500°C to 1100°C. (b) Symmetry ratios of Eu³⁺ ions emission from electric dipole transition and magnetic dipole transition as a function of annealing temperatures. (c) Characteristic emission intensity of Eu³⁺ ions as a function of annealing temperatures.

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As the representative sensitizers of RE³⁺ ions, the Eu³⁺ ions have unique narrow emission and absorption bands, which are either of pure magnetic or electric dipole moment origin. Therefore, Eu³⁺ ions are known as the most sensitive probe to survey the local structure in various hosts [25]. In order to evaluate the possible energy transfer affected by the local environment with the surrounding crystal-field, throughout the remainder of this paper, we will select Eu³⁺ ions as the representative RE³⁺ ions. Figure 1(f) shows the PLE spectra of Eu³⁺ ions-doped silica thin films with/without SnO₂ QDs as sensitizers. The detected wavelength is kept at 613 nm, corresponding with the ⁵D₀-⁷F₂ electric dipole transition of Eu³⁺ ions. Two wider and stronger excitation bands centered at 310 nm and 562 nm are ascribed to the band-gap and defect state-related energy levels of SnO₂ QDs, respectively. During the fabrication process, along with the detachment of organic species, oxygen vacancies will be gradually produced on the surface of SnO₂ QDs. The silica matrix surrounding the SnO₂ QDs gradually slows down the oxygen diffusion, which contributes to the recombination of photo-generated carriers trapped by interface states with deep trapped holes, leading to the strong excitation band centered at 562 nm. Meanwhile, we estimated the average size of SnO₂ QDs by using the effective mass theory [26]. The average size of SnO₂ QDs corresponding to the band-gap excitation peak at 310 nm is 5.0 nm, which is consistent with the TEM observations. Another two sharp PLE peaks located at 392 nm and 462 nm are assigned to the direct transitions from the ⁷F₀ ground state to the ⁵L₆ and ⁵D₂ excited state of Eu³⁺ ions, respectively. According to the above two types of PLE peaks, we suggest two types of possible excitation channels for Eu³⁺ ions. One is the direct excitation of Eu³⁺ ions from the ground state to an excited one based on the sharp excitation peaks, which can be confirmed by the two sharp excitation peaks. The other is the indirect sensitization process through the defect state energy level to the value band transition or from the inter-band transition of the SnO₂ QDs to the surrounding Eu³⁺ ions, which can be confirmed by the two wide excitation bands. By introducing the SnO₂ QDs as sensitizers, the indirect energy transfer processes contribute to the enhanced characteristic PL emissions of RE³⁺ ions by more than two orders of magnitude.

Figure 2(a) shows the normalized PL spectra of Eu³⁺ ions and SnO₂ QDs co-doped silica thin films after different annealing temperatures from 500°C to 1100°C. Obviously, with the increasing annealing temperatures, the spectral features of characteristic emissions from Eu³⁺ ions undergo great changes. Among the intra-4f transitions of Eu³⁺ ions, the ⁵D₀-⁷F₂ is an electric dipole transition which is sensitive to the local symmetry while the ⁵D₀-⁷F₁ is a magnetic dipole one which is hard to vary with the crystal field strength around Eu³⁺ ions. Therefore, the spectral feature of ⁵D₀-⁷F_j(0-4) transitions from Eu³⁺ ions reflects the local environment of luminescence centers. Here, we define the relative PL emission intensity ratio of the ⁵D₀-⁷F₁ transition to the ⁵D₀-⁷F₂ transition as the “symmetry ratio”. The symmetry ratio can be the quantitative indicative of how far is the Eu³⁺ ions local environment to be centrosymmetric. The smaller the value of the symmetry ratio, the weaker the average symmetry of the local environment. As shown in Fig. 2(b), with the increasing annealing temperatures from 500°C to 1100°C, the symmetry ratio of Eu³⁺ ions increases from 0.25 to 1.23, indicating the ratio of Eu³⁺ ions occupied in the symmetry sites gradually increases with the increasing annealing temperatures. After 1000°C annealing temperature, the Eu³⁺ ions and SnO₂ QDs co-doped silica thin film shows a well-defined spectrum with three strong peaks at 587 nm, 593 nm, 598 nm and one relatively weak peak at 613 nm. These three strong emission peaks correspond to the allowed magnetic dipolar transition (⁵D₀-⁷F₁) while the one weak emission peak corresponds to the electric dipolar transition (⁵D₀-⁷F₂). This PL spectral features are similar to that of the Eu³⁺ ions in the SnO₂ crystals [27], which indicates Eu³⁺ ions partially incorporated into SnO₂ QDs in the amorphous silica thin films. On the other hand, as shown in Fig. 2(c), with the increasing annealing temperatures from 500°C to 1100°C, the characteristic emission of Eu³⁺ ions is greatly enhanced by a factor of ~550. Combined with the increased symmetry ratio and greatly enhanced emission intensity with the increasing annealing temperatures, we speculate the greatly enhanced emission intensity should be assigned to the improved energy transfer efficiency due to the shrinking donor-to-acceptor separation distance, which is induced by Eu³⁺ ions partially incorporate into SnO₂ QDs.

Sn⁴⁺ ions in the stoichiometric tetragonal phase SnO₂ QDs have the D_2h or C_2h point symmetry [28,29]. If the partial Eu³⁺ ions incorporate into SnO₂ QDs and occupy the sites of Sn⁴⁺ ions, the local environment of the Eu³⁺ ions should be similar to that of the Sn⁴⁺ ions. In order to evaluate the local environment of the Eu³⁺ ions in silica thin films co-doped with SnO₂ QDs, the time-resolved PL spectra of Eu³⁺ ions’ characteristic emission at 613 nm are measured after different annealing temperatures as shown in Fig. 3. The average decay lifetimes of the emission at 613 nm are obtained from the following equation,

τ = \int I (t) \times d (t) / I_{\max},

where

I (t)

is the time-dependent PL intensity at 613 nm and

I_{\max}

is the maximal PL intensity at the initial time. Clearly, the lifetime (

τ

) shows a monotonically increased from 0.33 ms to 4.20 ms with the increasing annealing temperature from 500°C to 1100°C. On one hand, with the increasing annealing temperatures, the elimination of hydroxyl groups existed in amorphous silica leads to the lengthening of the PL lifetime. On the other hand, it is worth noting that the phonon energy of SnO₂ crystals is quite small (< 700 cm⁻¹), which is lower than the one of amorphous silica (> 1000 cm⁻¹). Therefore, if Eu³⁺ ions are partially partitioned into SnO₂ QDs, the non-radiative recombination rate corresponding to phonon-assisted energy transfer to host vibrations will be much lower and the PL lifetime will be lengthened greatly.

Fig. 3 Time-resolved PL spectra of Eu³⁺ ions’ characteristic emission at 613 nm with the different annealing temperatures from 500 °C to 1100 °C.

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Temperature-dependent PL emission spectra of co-doped samples are measured to further understand the improvement of energy transfer efficiency. Theoretically, the high vibration levels are populated with the increasing test temperature due to the increased electron-phonon interactions. As shown in Fig. 4(a), the integrated PL intensities are found to decrease with the increasing test temperature from 9.6 K to 300 K. The thermally-activated excited luminescent centers release photons via the crossover processes between the exited and ground states of Eu³⁺ ions. Therefore, with the increasing test temperature, the increased population density of phonons causes the quenching in the PL emission intensities via a series of non-radiative processes. It is observable that the integrated PL intensities from 9.6 K to 300 K come down to 24.31% for the co-doped samples. Meanwhile, the symmetry ratio increases with the test temperatures from 0.65 to 1.34, as shown in Fig. 4(b). At the low test temperature, the emission intensity from ⁵D₀-⁷F₁ transition is lower than that from ⁵D₀-⁷F₂ transition, which can be explained as the Eu³⁺ ions embedded in the random glassy hosts have much larger probability to release their excited energy through phonon-assisted non-radiative recombination than the ones embedded in the crystalline SnO₂. Further, it is found that the ⁵D₀-⁷F₂ emission intensity mainly contributed by the Eu³⁺ ions embedded in the silica matrix is more sensitive to surrounding temperature than ⁵D₀-⁷F₁ emission intensity mainly contributed by the Eu³⁺ ions incorporated into the SnO₂ QDs. If the thermally activated quenching process is caused by only one dominant phonon-assisted non-radiative recombination process, the activation energy for thermal quenching can be calculated with the following Arrhenius equation [30,31],

\frac{I_{0}}{I (T)} - 1 = a \exp (- \frac{E_{a}}{K T}),

where

a = k_{n r} / k_{r}

,

k_{n r}

and

k_{r}

are the non-radiative and radiative probabilities,

E_{a}

is the activation energy for non-radiative recombination, K is the Boltzmann constant, and I₀ is the low temperature PL intensity. As shown in the inset of Fig. 4(c), the plot of

I_{0} / I (T) - 1

vs.

1000 / T

is nonlinear and the slight upward curve in logarithmic coordinate suggests two or more non-radiative recombination pathways should be available to excited Eu³⁺ ions. Based on this assumption, the Arrhenius equation is modified by adding a second term representing the existence of another non-radiative pathway.

\frac{I_{0}}{I (T)} - 1 = a_{1} \exp (- \frac{E_{a 1}}{K T}) + a_{2} \exp (- \frac{E_{a 2}}{K T}),

where

E_{a 1}

and

E_{a 2}

are the activation energies for different non-radiative recombination processes, respectively. As shown in Fig. 4(c), for the samples co-doped with SnO₂ QDs, the activation energy derived from the temperature-dependent PL spectra is 76.5 meV and 13.2 meV, which can be ascribed to bridge the energy variety between the stretching vibrations of hydroxyl groups and the excited states of Eu³⁺ ions existed in different local environments. Consideration of these spectral features proves that Eu³⁺ ions existed in two types of sites, one embedded in the vicinity of SnO₂ QDs and the other within the amorphous silica host.

Fig. 4 (a) Temperature-dependent PL spectra of Eu³⁺ ions and SnO₂ QDs co-doped silica thin films versus the reciprocal temperature. (b) Symmetry ratios of Eu³⁺ ions emission from electric dipole transition and magnetic dipole transition as a function of temperatures. (c) PL integrated intensity of Eu³⁺ ions versus the reciprocal temperature.

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Given the fact that high concentration of oxygen presents inside silica thin films, the characteristic EDS map of oxygen does not define the QDs as clearly as Sn element. Hence, we captured the characteristic EDS spectra of Sn and Eu in a certain region which correlates with the HAADF image. According to Fig. 5(a)-5(c), the observed contrast is indicative of variations in the chemical composition. STEM energy-dispersive EDS maps show evidence of Sn elements in SnO₂ QD arrays while the majority of Eu elements distribute uniformly in the amorphous silica matrix and others are incorporated at the interstitial sites of SnO₂ QDs. Though the present results need to further investigation, we consider that the partial incorporation of Eu³⁺ ions into the SnO₂ sites plays a significant role in the change of crystal-field symmetry and the enhancement of energy transfer efficiency.

Fig. 5 (a) HADDF image of Eu³⁺ ions and SnO₂ QDs co-doped silica thin films. (b) and (c) STEM-EDS elemental mappings of samples. Maps shown here have been smoothed by convoluting a Gaussian function with a standard deviation of 3 pixels.

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4. Conclusions

In summary, three different types of RE³⁺ ions and SnO₂ QDs co-doped silica thin films are fabricated by a soft chemistry-based method. By introducing SnO₂ QDs with suitable size as sensitizers, the characteristic emission intensity of RE³⁺ ions can be enhanced by more than two orders of magnitude via the energy transfer process. Eu³⁺ ions are selected as the representative sensitizers of RE³⁺ ions to elucidate the possible energy transfer mechanisms under different local environment. Quantitative studies of PL decay lifetime and temperature-dependence PL spectra suggest that the partial incorporation of Eu³⁺ ions into SnO₂ sites should give rises to the change of crystal-field symmetry and the significant enhancement of energy transfer efficiency from SnO₂ QDs to Eu³⁺ ions. Typical analytical EDS mapping results prove that part of Eu³⁺ ions doped into the SnO₂ sites after annealing at 1000 °C. We anticipate that our results would shed light on the future research on the energy transfer mechanisms under different local environments of RE³⁺ ions.

Funding

National Natural Science Foundation of China (NSFC) (61704094, 61735008, 61771267, 61471210, 61474068); Program 973 (2013CB632101); K. C. Wong Magna Fund in Ningbo University.

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Investigation of energy transfer mechanisms in rare-earth doped amorphous silica films embedded with tin oxide nanocrystals

Abstract

1. Introduction

2. Material and methods

3. Results and discussion

4. Conclusions

Funding

References

Cited By

Figures (5)

Equations (3)

Optics Express