Highly efficient near-infrared emission in Er<sup>3+</sup> doped silica films containing size-tunable SnO<sub>2</sub> nanocrystals

Xiaowei Zhang; Tao Lin; Pei Zhang; Jun Xu; Shaobing Lin; Ling Xu; Kunji Chen

doi:10.1364/OE.22.000369

1. Introduction

Erbium-ion-doped silica materials have been extensively investigated since first application of Er-doped fiber amplifier at the end of last century [1]. Indeed, the unique 1540 nm emission by intra-4f transition of trivalent erbium (Er³⁺) ions coincides with the minimum absorption band of silica, bringing various applications in optical communication field [2]. Moreover, Er³⁺ ions doped silica thin films can be developed as efficient light source for Si-based monolithic optoelectronic integration [3] and up-conversion layer for thin-film solar cells [4]. However, the small optical absorption cross-section of Er³⁺ ions results in low emission efficiency [5]. As one of the strategies to solve this problem, co-doping with Si nanocrystals has been reported to evidence the Förster resonance energy transfer (FRET) process which leads to enhancement of Er-related near-infrared emission [6]. Unfortunately, due to small energy gap of Si nanocrystals, the back energy transfer (BET) from Er³⁺ ions to Si nanocrystals may inevitably lead to the quenching of luminescence [7]. In order to circumvent this problem, metal oxide nanocrystals with wider band-gaps, such as In₂O₃, ZnO, have been attempted to replace Si in previous works [8,9]. Nonetheless, efficient near-infrared emission from Er³⁺ ions in SiO₂ matrix remains a challenging goal for their maximum utilization in optoelectronics.

In the present work, we fabricated SiO₂ thin films co-doped with Er³⁺ ions and SnO₂ nanocrystals. The SnO₂ nanocrystals were used as sensitizers on the purpose of avoiding the BET effect, for the large mismatch between SnO₂ band-gap and Er³⁺ excited state (⁴F_13/2) level. Several works [10–12] have proved that SnO₂ nanocrystals were capable to incorporate rare-earth ions during in situ crystallization and form rare-earth-doped SnO₂ nanocrystals in SiO₂ matrix. These incorporated rare-earth ions demonstrate different photoluminescence (PL) properties from free rare-earth ions in amorphous SiO₂ host, leading to positive enhancements of energy transfer efficiency. Moreover, SnO₂ nanocrystals show better thermal stability in SiO₂ than other wide-band-gap semiconductor. These enable the necessary post-annealing at around 900 °C to form compact SiO₂ matrix [13]. The size of prepared SnO₂ nanocrystals can be precisely controlled by varying the Sn concentrations in precursors [14]. Here, we show that the luminescence intensity at 1540 nm was significantly enhanced by up to three orders of magnitude after co-doping with SnO₂ nanocrystals on optimum condition. The mechanism of luminescence enhancement was discussed based on quantitatively analysis to the selective photoluminescence excitation (PLE) and temperature-dependent PL spectra. Furthermore, the efficiency of energy transfer process up to 63.4% indicated well spectral overlapping of SnO₂ nanocrystals and Er³⁺ ions as the result of incorporation of Er³⁺ ions into SnO₂ nanocrystals.

2. Experimental

A modified sol-gel process was employed to fabricate SnO₂ nanocrystals and Er³⁺ ions co-doped SiO₂ films. Tetraethyl orthosilicate (TEOS), de-ionized water and ethanol were mixed (8:9:18 in volume) as silica sol. Hydrochloric acid was then added to the mixture dropwise under rigorous stirring to adjust pH value to 2.0. Different amounts of tin tetrachloride (SnCl₄) and erbium nitride (Er(NO₃)₃) salts were dissolved in the solution to obtain desired concentration. In our case, the amount of Er³⁺ ions added into the precursor solution was fixed at 5% (molar ratio, the same as follows) of Si amount, while the amount of Sn was changed from 0 to 60%. We use these values to label the final samples with different Sn or Er concentrations. Then the precursor was heated to 60 °C to complete hydrolysis and aged for one day. The as-prepared gel was spin-coated onto Si substrates to form SiO₂ films followed by the post-annealing at 1000 °C to eliminate hydroxyl groups (-OH), which may lead to the quenching of rare-earth-related emission as nonradiative carrier-recombination centers [9].

The prepared thin films were characterized by using a TECHNAI-F20 field emission transmission electron microscope (TEM) operated at 200 kV. The cross-sectional samples for TEM observation were fabricated by GATAN 656 dimple-grinder and GATAN 691 precision ion polishing system. Steady state PL and PLE spectra were obtained by using a model Fluorolog-3 system equipped with a 30 mW He-Cd laser working at 325 nm and a 450 W Xe lamp as the excitation sources. A photomultiplier and a liquid-nitrogen-cooled InGaAs photo-detector with lock-in techniques were used during the measurements. Time-resolved PL spectra were measured by FLS920 fluorescence spectrophotometer with an EPL picosecond pulsed laser working at 405 nm based on the TCSPC technology. The instrumental response of the system was measured by using Ludox elastic scattering. The fluorescence decay was obtained by using a deconvolution method based on the Levenberg-Marquard algorithm. For the temperature-dependent PL measurements, the samples were placed in a closed-cycle helium cryostat equipped with four optical windows.

3. Results and discussion

As shown in Fig. 1(a), amorphous SiO₂ layer with smooth surface can be identified on Si substrate and its thickness is around 110 nm. Discrete nanoparticles can be clearly distinguished from amorphous film. As shown in the following high-resolution TEM images [Figs. 1(b)–1(e)], the average size of these particles increases with increasing Sn concentrations. No aggregation of particles is found in the images. All particles show clear-cut crystalline features and the measured inter-planar spacing is 0.33 nm, corresponding to the (110) inter-planar spacing of SnO₂ tetragonal phase.

Fig. 1 (a). Cross-sectional TEM image of 20% Sn doped SiO₂ film annealed at 1000 °C. The related concentration of Sn is measured from the molar ratio of Sn molecules to Si molecules in the sol precursor, the same as follows. (b)-(e). The high-resolution TEM images of the SiO₂ films doped with different Sn concentrations (5%, 10%, 20%, 50%), and the average sizes are 2.9 nm, 3.8 nm, 5.2 nm, 11.2 nm, respectively.

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Figure 2 shows the room temperature PL and PLE spectra of the SiO₂ thin films doped with size-tunable SnO₂ nanocrystals and Er³⁺ ions. In Fig. 2(a), the Er-free film containing 20% Sn exhibits a broad emission band centered at 576 nm under He-Cd laser excitation. This emission is attributed to the radiative recombination in the defect-states, such as the oxygen vacancies on the SiO₂-SnO₂ interface [14,15]. Co-doping with 5% Er partially quenches this broad emission band and additionally produces narrow emission peak at 547 nm corresponding to the transition from ⁴S_3/2 to ⁴I_15/2 of Er³⁺ ions under the same excitation. The Sn amount dependence of PL emission intensities of samples with fixed 5% Er concentration are shown in Fig. 2(b). Sn-free film shows a characteristic peak at 1540 nm under Xe lamp excitation at 382 nm. With increasing Sn concentration to 20%, the near-infrared emission intensity at 1540 nm is gradually enhanced as revealed by the PL measurements. It can be as ascribed to the effective energy transfer process resulted by optimizing the size and density of SnO₂ nanocrystals. PLE spectra in Fig. 2(d) are used to trace the origin of this enhanced emission with fixing the detected wavelength at 1540 nm. Only two sharp PLE peaks at 382 nm and 525 nm which correspond to the transitions from ⁴I_15/2 ground state of Er³⁺ ions to ⁴G_11/2 and ²H_11/2 state, respectively, appear for the Sn-free sample with 5% Er. Besides these two sharp peaks with fixed intensities, the Sn and Er co-doped samples demonstrate another two broad PLE bands whose intensities are dependent on Sn concentrations. One is located at UV region and the other is located around 576 nm. The peak energy of UV band is larger than energy of bulk SnO₂ crystal and is red-shifted with increasing Sn concentration. We ascribe these UV bands to the band-to-band transition in size-confined SnO₂ nanocrystals due to the quantum confinement effect. The red-shifts are led by band-gap energy reduction with increasing crystal size and the changes of energy value versus the cluster radius estimated by TEM analysis well agrees with the effective mass theory [16]. The other PLE band has the fixed energy close to SnO₂-defect-related emission. We ascribe this visible PLE band to the transition from valance band of SnO₂ directly to the defect-state-band on the crystal surface [14]. Both of these Sn-related PLE bands evidence an energy transfer process from SnO₂ nanocrystals to Er³⁺ ions.

Fig. 2 (a). PL spectra of thin films co-doped with 20% Sn and different Er concentrations excited at 325 nm. (b). PL spectra of thin films with 5% Er and different Sn concentration (0-20%) excited by Xe lamp under 382 nm, 307 nm, 318 nm, 326 nm wavelength, respectively. (c). Sn amount dependence of characteristic emission intensity of Er³⁺ ions at 1540 nm. (d). PLE spectra of samples with 5% Er and different Sn concentration detected at 1540 nm.

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As shown in the Fig. 2(c), PL intensity of infrared emission increases monotonously with increasing Sn concentration from 0 to 20%, indicating the increases of the number of SnO₂ nanocrystals working as light absorbers. Higher density of SnO₂ nanocrystals also shortens the average distance between SnO₂ nanocrystals and Er³⁺ ions leading to higher energy transfer efficiency. As a result, the PL intensity reaches its maximum at 20%, which is more than three orders of magnitude to the Sn-free sample. However, further increasing Sn concentration results in luminescent quenching effect. Based on TEM results, superfluous Sn concentration beyond 20% results in formation of oversized nanocrystals and smaller number density. The decrease in surface-to-volume ratio induces a decline of energy transfer efficiency and PL intensity. Based on the above results, we deduce the whole luminescence process like this: The sharp PLE peaks indicate the direct, intrinsic excitation, relaxation and recombination process inside Er³⁺ ions. Besides, Er³⁺ ions can be also “excited” by nearby SnO₂ nanocrystals through energy transfer process, as the SnO₂ nanocrystals firstly collect excitation energy of incident light by either band-to-band transition or band-to-defect-state transition. Spectral overlapping between SnO₂ defect-state-related emission and Er³⁺ ions-related excitation provides the precondition of energy transfer process. Its efficiency is strongly dependent on the average distance between SnO₂ nanocrystals and Er³⁺ ions. This explains the PL enhancement by three orders of magnitude by co-doping SnO₂ nanocrystals with optimal size and density.

In order to evaluate the efficiency of energy transfer process from SnO₂ nanocrystals to Er³⁺ ions, we measure the time-resolved PL spectra by using an EPL picosecond pulsed laser working at 405 nm. The luminescence decays of samples doped with different Er³⁺ concentrations are shown in Fig. 3. Clearly, lifetime of 576 nm emission band decreases with the inclusion of additional 5% Er, indicating the occurrence of energy transfer process which contributes to the increase of nonradiative recombination rate. The mean decay lifetimes of the emission at 576 nm are obtained from the following equation:

τ = \int \frac{I (t)}{I_{m a x}} d t,

where I(t) is the time-dependent PL intensity at 576 nm. I_max is the maximal PL intensity at the initial time. Normally, defect-state-related emission decays in a few nanoseconds due to the rapid recombination of electrons and holes trapped in the defect-state level. In our experiments, the lifetime value derived from the fitting by Eq. (1) is 3.42 ns for Sn 20%, Er-free sample and 1.25 ns for Sn 20%, Er 5% sample, respectively. These results reveal the fact that introduction of Er³⁺ ions leads to decline of Sn-related emission decay lifetime. This decline indicates the increase of nonradiative decay rate of Sn-related emission, which is ascribable to the energy transfer process from SnO₂ nanocrystals (donors) to Er³⁺ ions (acceptors), for the reason that it acts as an additional nonradiative recombination pathway to Sn-related emission. Based on the distance limit of energy transfer, we believe that most of the energy transfer process occurs between SnO₂ nanoparticles and the Er³⁺ ions very close to their interface and we assume that they share the similar average distance to the SnO₂ nanoparticles. If we define the energy transfer efficiency (ETE) as the fraction of donors that are depopulated by energy transfer process to acceptors over the total number of donors being excited, it can be obtained as a function of the PL decay lifetimes [17]. By applying the values of lifetimes to Eq. (2),

η_{E T} = 1 - \frac{τ_{E r 5 %}}{τ_{E r - f r e e}},

the ETE is calculated as 63.4% for Sn 20%, Er 5% sample. As a comparison, The ETE for ZnO nanocrystal sensitized Er³⁺ ions emission obtained from similar method is 43.9% [18], which may be explained as that the optimum annealing temperature for forming ZnO nanocrystals with suitable size is about 500 °C and the -OH bonds cannot be completely removed at this temperature, which may deteriorate the near-infrared emission of Er³⁺ ions.

Fig. 3 PL intensity decay traces from time-resolved PL measurements of the SnO₂ emission at 576 nm for the SiO₂ films with different concentrations of Er³⁺ (0 and 5%) under 405 nm picosecond pulsed laser excitation.

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Temperature-dependent PL spectra are measured to further understand the mechanism of PL enhancement. The integrated PL intensities at 1540 nm are found to decrease with increasing temperature due to the domination of nonradiative recombination process. If we propose a thermally activated quenching model with only one dominant nonradiative channel, the variation of the PL emission intensities with temperatures can be described by the following Arrhenius equation [19]:

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

where

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

,

k_{n r}

and

k_{r}

are the nonradiative and radiative probabilities,

E_{a}

is the activation energy for nonradiative recombination, K is the Boltzmann constant, and I₀ is the low temperature PL intensity. As shown in the inset of Fig. 4, the above Arrhenius model well describes PL quenching of the Er-only sample (red squares). The activation energy derived from the fitting is 4.9 meV. Based on the knowledge that activated energy transfer from Er³⁺ ions to the vibrations of -OH dominates the nonradiative recombination process in a sol-gel-processed silica material [20], these obtained activation energy can be ascribed to bridge the energy variety between two -OH vibrations and ⁴I_13/2→⁴I_15/2 transition. However, the Eq. (3) does not fit well the plots of Sn and Er co-doped sample, implying that, unlike the Er-only sample, two or more nonradiative recombination channels are competitive in the Sn-Er-co-doped sample. Based on this assumption, the Arrhenius fitting is modified by adding a second term representing the existence of another nonradiative channel in the system [21].

Fig. 4 Temperature-dependence of PL intensity at 1540nm is plotted versus the reciprocal temperature. The insert illustrates the (I₀/I(t)-1) -T⁻¹ plots to guide the Arrhenius fitting.

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

The activation energy E_a₁ and E_a₂ derived from the temperature-dependent PL spectrum of 20% Sn and 5% Er co-doped sample is 4.9 meV and 100 meV, respectively. E_a₁ corresponds to the same channel of Er-only sample, while E_a₂ is ascribable to the energy variety (114 meV) between excited state level ⁴S_3/2 of Er³⁺ and defect-state-level of SnO₂, which provides the necessary energy to achieve BET. The similar quenching mechanism has been reported, for instance, in Yb³⁺ ions doped InP nanocrystals by Taguchi et al. [22] and Er³⁺ ions doped Si nanocrystals by Coffa et al. [23]. As the quenching of Er³⁺ ions emission is strongly affected by surrounding crystalline structure, our present result indicates that parts of Er³⁺ ions remain in the amorphous SiO₂ matrix, and the others are trapped in the SnO₂ nanocrystals. For these Er³⁺ ions in SnO₂ nanocrystals, Dexter energy transfer process, or charge exchange mechanism may happen [11]. Its energy transfer efficiency depends on the distance between donor and acceptor, which means that it is much more sensitive than FRET at a very short distance, typically of the order 15 to 20 Å. In fact, the Er³⁺ ions dopant concentration in SnO₂ nanocrystals is extremely low by substituting Sn⁴⁺ at the C_2h or D_2h sites due to the large radius mismatch (0.088 nm for Er³⁺, 0.071 nm for Sn⁴⁺) and the charge imbalance between Er³⁺ and Sn⁴⁺. However, it has been reported that the Er³⁺ ions were successfully doped into SnO₂ nanocrystals with Er³⁺ doping concentration of the order of 10¹⁹cm⁻³ by Kong et al [11]. In our previous work [24], we studied the trivalent Europium (Eu³⁺) ions co-doped with SnO₂ films and Eu³⁺ ions can act as “probe” ions as its characteristic emission intensity at 590 nm and 613 nm reflects different symmetries of Eu³⁺ ions surrounding. In most cases, the Eu-related emission intensity at 590 nm was found much weaker than the one at 613 nm. However, in the case of using SnO₂ nanocrystals as the co-dopant showed a totally different phenomenon. The dominant emission at 613 nm was gradually replaced by emission at 590 nm with increasing Sn concentration and annealing temperature. At the same time, The 590 nm emission split into three sharp peaks. These phenomena implied that Eu³⁺ ions partially incorporated into SnO₂ nanocrystals [10]. Therefore, it is reasonable to hypothesis that Er³⁺ ions can be partially incorporated into SnO₂ nanocrystals after annealing at 1000 °C based on the knowledge that Er³⁺ ions and Eu³⁺ ions share very similar physical and chemical properties, for example, their atomic radii and solubility in metal oxides. Furthermore, it is worth noting that the phonon energy of SnO₂ is quite small (< 700 cm⁻¹), which is lower than the one of amorphous SiO₂ (> 1000 cm⁻¹) [25]. Consequently, the nonradiative recombination rate corresponding to phonon-assisted energy transfer to host vibrations is much lower if Er³⁺ ions are partially partitioned into SnO₂ sites, which will also contribute to the enhanced near-infrared emission.

4. Conclusion

In summary, SnO₂ nanocrystals and Er³⁺ ions co-doped SiO₂ films are fabricated via a facile sol-gel method and spin-coating technique. By precisely controlling the Sn concentrations, the SnO₂ nanocrystals can be formed and grow up to a controlled size and suitable density. PLE results monitored Er-related infrared emission demonstrate two excitation both related to co-doped SnO₂ nanocrystals, which indicates energy transfer process occurs between the tunable-sized SnO₂ nanocrystals and the nearby Er³⁺ ions, leading to the enhancement of characteristic emissions of Er³⁺ ions. Furthermore, the co-doped sample demonstrates shorter lifetime for Sn-related emission than the Sn-only sample, which also supports the energy transfer process. The corresponding energy transfer efficiency is about 63.4%. The results of temperature-dependent PL spectra for Er-related infrared emission suggest that Er³⁺ ions are partially partitioned into SnO₂ nanocrystals. Both the high ETE and the incorporation of Er³⁺ into SnO₂ nanocrystals contribute to near-infrared emission enhancement of Er³⁺ ions by three orders of magnitude.

Acknowledgments

This work was financially supported by “973” Project (No. 2013CB632101), NSFC (No. 61036001 and 11274155), and PAPD.

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Highly efficient near-infrared emission in Er³⁺ doped silica films containing size-tunable SnO₂ nanocrystals

Abstract

1. Introduction

2. Experimental

3. Results and discussion

4. Conclusion

Acknowledgments

References and links

Cited By

Figures (4)

Equations (4)

Optics Express