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Abstract
This work studied the luminescent properties of Er3+/Ag-codoped bismuthate glass nanocomposites (BGN). Surface plasmon resonance (SPR) peaks characteristic of Ag nanoparticles (NPs) were present at 575–590 nm. Transmission electron microscopy revealed that Ag NPs with different sizes were distributed in the glass matrix. The fluorescence intensity of Er3+ ions at 2.7 μm first increased and then decreased with increasing Ag content, and the maximum fluorescence intensity was obtained under the addition of 1.5 mol% AgCl. Meanwhile, the fluorescence lifetime at 2.7 μm (4I13/2) was extended through the addition of Ag NPs and achieved the maximum value under the addition of 1.5 mol% AgCl. This phenomenon was caused by the local field enhancement of Ag NPs and Ag0→Er3+ energy transfer. The maximum stimulated emission cross-section σem of the Er3+: 4I11/2→4I13/2 transition at 2.7 μm was 1.36 × 10−19 cm2. All the above results indicated that Er3+/Ag-codoped BGN is a promising gain medium for lasers, optical displays, and optical memory devices.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Nanoparticle (NP) research has emerged as an advanced research field in the twenty-first century. The conduction-band electrons of NPs interact with various forms of energy (i.e., light, electrical, and magnetic energy) and exhibit interesting characteristics that are not exhibited by their bulk forms. The surface plasmon resonance (SPR) of a NP occurs when a particular wavelength of electromagnetic radiation interacts with a NP to induce the coherent oscillation of conduction-band electrons [1–3]. The plasmonic properties of noble-metal NPs have attracted considerable interest from various researchers. Over the past several decades, nanometal particle composite materials doped with Er3+ ions have been widely studied because of their potential applications in color displays and fiber amplifiers [4].
Bismuthate glass is an ideal substrate for fiber amplifier fabrication given its wide infrared (IR) transmission area, strong ability to dissolve rare-earth ions, low phonon energy, and high refractive index [5, 6]. Its application as a matrix material for Er3+ ions should be studied given these properties. Mid-infrared (MIR) wavelength lasers, particularly fiber lasers with wavelength of 3 μm, have been applied as eye-safe laser radars in the military and medical fields [7, 8]. Er3+-doped glass has been extensively investigated because of the existence of the Er3+: 4I11/2→4I13/2 transition at 2.7 μm [9]. The luminous efficiency of rare-earth ions in a glass matrix is considerably improved by the introduction of metal NPs. Eichelbaum et al. reported that luminous enhancement results from classical energy transfer between small noble-metal particles and lanthanide ions but not from plasmonic field enhancement [10]. Recently, many studies have focused on the effect of Ag NPs on the emission properties of rare-earth-ion-doped glass under visible and near-IR wavelengths. Wu et al. reported that Ag NPs significantly affect the fluorescence intensity of rare-earth-ion-doped glass under green (527 and 548 nm) and red (661 nm) upconversion (UC) emissions and at 1.53 μm emission bands [11, 12]. However, few studies have focused on the enhancement of MIR emission by metal NPs. Hence, studying the effect of Ag NPs on the emission properties of Er3+-doped glass under 2.7 μm emission is essential.
In this paper, Er3+-/Ag-codoped bismuth germanate glass nanocomposites (BGN) were prepared through the conventional melt-quenching method. Ag NPs were precipitated during glass annealing. Understanding the effect of Ag NPs on the spectral properties and fluorescence lifetime of BGN samples can guide the development of materials with high luminous efficiency under the wavelength of 2.7 μm.
2. Experimental
2.1 Sample preparation
The investigated glass samples had a composition of 40Bi2O3-50GeO2-10Na2O-1Er2O3-xAgCl (in mol%). BGN0, BGN1.5, BGN2, and BGN2.5 denote samples with x = 0, 1.5, 2, and 2.5, respectively. High-purity Bi2O3, GeO2, Na2CO3, Er2O3, and AgCl powders were weighed and thoroughly mixed. Ten grams of raw materials were placed in a quartz crucible and melted at 1200 °C for 30 min in an oxygen atmosphere. The melts were quickly poured onto a preheated stainless-steel mold and annealed for 2 h at near the glass-transition temperature (Tg). The annealed samples were cut and polished to sizes of 20 mm × 10 mm × 1 mm for optical property measurements.
2.2 Measurements
The Tg, crystallization onset temperature Tx, and crystallization peak temperature Tp of BGN samples were characterized by using a differential scanning calorimeter (TA Q2000) at a heating rate of 10 K/min. The X-ray diffraction (XRD) patterns of the BGN samples were collected using a Bruker D2 X-ray diffractometer. Refractive indices were determined with an IR variable-angle spectroscopic ellipsometer (IR-VASE Mark II, J. A. Woollam). Transmission spectra were collected by using a spectrometer (Nicolet 380 FTIR, Thermo Scientific) over the wavelength range of 1500–4000 cm−1. Absorption spectra were recorded over 300–2000 nm by using a spectrophotometer (Lambda 900 UV/VIS/NIR, Perkin-Elmer) with 2 nm steps. Fluorescence spectra and fluorescence lifetime were obtained using a FLS 980 fluorescence spectrometer equipped with an InSb detector and cooled with liquid nitrogen. A 980 nm laser diode (LD) was used as the excitation source. A 200 kV transmission electron microscope was used to investigate the nucleation of Ag NPs (TEM, FEI Tecnai G2 F30).
3. Results and discussion
3.1 Absorption spectra and IR transmittance
Figure 1(a) shows the room-temperature absorption spectra of the BGN glass samples. The spectra were collected over the wavelength range of 350–1700 nm. The spectra exhibit several absorption bands that are attributed to the 4f–4f transitions of Er3+ from the ground to the excited state. In addition, a distinct and broad SPR absorption band is present. The characteristic SPR peak positions (>570 nm) of Ag NPs in BGN glasses are considerably higher than those of Ag NPs in calcium sodium silicate glasses (approximately 410 nm) [13, 14]. The position of the SPR peak is influenced by the refractive index of the glass matrix [15]. The refractive indices of BGN glasses exceed 1.8 (as listed in Table 1) and are higher than those of silicate glasses (n = 1.45–1.5).
Fig. 1 (a) Absorption spectra of Er3+/Ag-codoped BGN glass samples and (b) IR transmittance spectrum of BGN0.
Table 1. Refractive indices and SPR peak wavelengths of Er3+--doped BGN glasses.
The OH− content of a glass is related to the emission efficiency of rare-earth ions because residual OH− groups will participate in the energy transfer of rare-earth ions and reduce emission intensity. The IR transmittance spectrum of BGN0 is shown in Fig. 1(b) and indicates that the sample has low OH− content. The maximum transmittance of BGN0 from 2500 to 2900 nm is as high as 82.9%. The absorption cut-off edge is 6 μm. The small absorption band at approximately 3250 cm−1 is assigned to the stretching vibration of free OH− groups. The absorption coefficient α(OH) of BGN0 is 0.43 cm−1, which is lower than that of other glasses [16–21]. The MIR spectroscopic properties of BGN0 will be improved by its good IR transmission.
3.2 XRD, TEM, high-resolution transmission electron microscopy, and selected area electron diffraction pattern images
Figure 2(a) shows the XRD patterns of the BGN2 and BGN0 samples. Both patterns show amorphous features, and only the pattern of BGN2 glass exhibits a weak peak at 2θ = 43.862°. This weak peak is the characteristic peak of Ag crystals (JCPDS file No. 3-921), which were reduced from Ag+ ions through autothermo-reduction during annealing under 420 °C. To further verify that Ag NPs have precipitated in the glass, the BGN2 sample was subjected to TEM. The results are shown in Fig. 2(b). Spherical particles with sizes of ~10 nm are distributed in the glass. Lattice-resolved high-resolution transmission electron microscopy (HRTEM) images show that crystals with d111 = 2.360 Å and d200 = 2.030 Å planes have precipitated in the BGN2 sample (Fig. 2(c)). Figure 2(d) shows the selected area electron diffraction pattern (SAED) of the crystals in the BGN2 sample. The figure shows the distinct hkl crystalline planes of <111> and <200> of Ag NPs. Lattice plane analysis revealed that these planes belong to Ag NPs (JCPDS file No. 3-921).
3.3 Fluorescence spectra
The MIR emission spectra of Er3+/Ag-codoped BGN samples excited under 980 nm LD were recorded and are shown in Fig. 3(a). The intensity of the samples under 2710 nm emission increases with increasing AgCl content, and negligible concentration quenching occurs when AgCl content is increased to ~1.5 mol%. The corresponding emission intensity of the Er3+-doped BGN samples in the presence of Ag NPs is 1.6 times higher than that in the absence of Ag NPs. Under increasing AgCl content, the fluorescence intensity of Er3+-doped BGN samples decreases dramatically but remains higher than that of undoped glass. The full-width at half-maximum (FWHM) under 2.7 µm emission is approximately 170 nm. The 4I11/2→4I13/2 transition of Er3+ ions in the low-symmetry coordination field after Stark splitting resulted in three fluorescence peaks, which are shown in Fig. 3(a). To understand the luminescent mechanism of the glasses, the visible and near-IR emission spectra of the glasses were acquired. As shown in Fig. 3(b), the glasses exhibit strong green emissions (527 and 548 nm) and red emission (661 nm) under 980 nm excitation.
Fig. 3 Fluorescence spectra of BGN2 samples. The spectra were acquired under excitation at (a) 2.7 μm; (b) 527, 548, and 661 nm; and (c) 1.5 μm. (d) Energy level diagram of Er3+ ions. The Ag SPR band and putative luminescence mechanism are indicated in the diagram.
As shown in Fig. 3(c), fluorescence intensity is visibly enhanced by the addition of AgCl and reaches the maximum value under 1.54 µm excitation. The FWHM at 1.54 µm is approximately 112 nm. The two distinct emission peaks at 1499 and 1534 nm are caused by the 4I13/2→4I15/2 transition of Er3+ ions in the low-symmetry coordination field after Stark splitting. The luminescence intensity of the samples rapidly increases with increasing Ag NP content, and the BGN2 sample exhibits the maximum luminescence intensity among all samples. Interestingly, the relative integrated intensity of the emission bands of BGN1.5 at 2710, 1540, and 548 nm have increased by 1.6-, 4.8-, and 9.6-folds, respectively, relative to that of the host BGN0 glass. These behaviors are illustrated in Table 2. Wavelengths close to the IR correspond to a low magnitude of enhancement. Green light (548 nm) can match the SPR peak and induce the SPR of Ag NPs. Near-IR and MIR bands gradually move away from the SPR peak, the plasma resonance effect weakens, and the magnitude of enhancement decreases.
Table 2. Increase in the integrated emission intensity and fluorescence lifetimes of BGN glass samples.
Fluorescence enhancement is attributed to LFE induced by localized surface plasmon resonance and the lightning rod effect, and energy transfer from Ag NPs to rare earth is useless in samples lacking photoluminescence emission [22]. Therefore, the possible mechanisms underlying the enhancement of Er3+ fluorescence can be explained as follows:
(1) LFE induces the SPR of Ag NPs
The energy diagram of Er3+ ions is shown in Fig. 3(d). Under 980 nm excitation, the electrons are first excited from the 4I15/2 ground level to the 4I11/2 level. Thereafter, a second photon promotes the transfer of Er3+ electrons from the 4I11/2 level to the 4F7/2 level. Another possible excitation source for the luminescence process is the energy transfer between two Er3+ ions: 4I11/2 + 4I11/2→4I15/2 + 4F7/2. In this process, the electrons of the populated Er3+:4F7/2 level rapidly relax and nonradiatively transfer to the 4S3/2 level. The energy gap between the 4S3/2 and 4F9/2 levels is approximately 3070 cm−1. The MPR rate of the 4S3/2 level is sufficiently low to produce green emission centered at 548 nm. The 2H11/2 level is populated from the 4S3/2 level via the thermal equilibrium between these two levels. Thermal equilibrium between the 2H11/2 and 4S3/2 levels is relatively small under the experimental temperature, and the energy gap between these two levels is approximately 770 cm−1. Consequently, the band at 527 nm, which is attributed to the 2H11/2→4I15/2 transition, has weak intensity. The partial electrons of Er3+ ions in the 4I11/2 level de-excite radiatively to the 4I13/2 level to produce 2.7 µm emission. The large amount of electrons in the 4I11/2 level nonradiatively transfer to the 4I13/2 level and then relaxes to the 4I15/2 level, thereby yielding 1.5 µm emission. For the glasses containing Ag NPs, the SPR absorption bands cover the 4S3/2 level (green emission) and the 4I11/2 level (excitation wavelength at 980 nm). Therefore, the 4S3/2 level is resonated by LFE, and the number of electrons at this level will rapidly increase. In this case, green emission is remarkably enhanced. Additionally, the 980 nm excitation light is resonated because of the LFE of Ag NPs and therefore enhances pumping efficiency. However, the relatively weak resonance will increase near- and MIR emission to levels lower than those of green emission.
(2) Ag NPs transfer energy to Er3+ ions
Ag NPs may absorb photons under the 980 nm excitation and transfer energy to electrons of Er3+ ions because the SPR bands of the Ag NPs extend to the near-IR regions. The energy transfer of metal NPs and rare-earth ions is often characterized by changes in the lifetime of rare-earth ions. Table 2 shows the fluorescence lifetime of BGNs with different Ag contents. As AgCl content is increased from 0 mol% to 2.5 mol%, the fluorescence lifetime correspondingly increases from 0.525 ms to 0.902 ms and then decreases to 0.714 ms. These photons of Ag NPs energetically transfer to the Er3+: 4I11/2 energy level by absorbing 980 nm pump light, providing additional electron population of 4I11/2 level. As shown in Fig. 3, the strongest fluorescence intensity is found when Ag content is 1.5 mol% [22–27].
3.4 Lifetime
The fluorescence photon efficiency of the Er-doped material is determined by the lifetime of photon fluorescence, which is an effective index for assessing the effect of Ag codoping. The decay curves of the Er3+ ions doped in BGN0, BGN1.5, BGN2, and BGN2.5 were experimentally measured at the wavelength of 2.7 µm and are shown in Fig. 4. The fluorescence lifetime was obtained by fitting the curves with a single exponential function. The decay time of BGN glasses with increasing AgCl content are 0.525, 0.902, 0.853, and 0.714 ms, as listed in Table 2. The results indicated that codoping with Ag NPs can effectively produce resonant excitation photons, which enhance the local electric field [28]. High Ag content, however, may shorten fluorescence lifetime because it reduces the efficiency of energy transfer between Er3+ ions.
The stimulated emission is calculated using the Füchtbauer–Landenburg formula,
where the λ represents the wavelength of the emission peak; g(λ) represents the normalized emission spectrum; n, τrad, and Δλ represent the refractive index, the fluorescence lifetime, and FWHM of the glass matrix, respectively; and c is the speed of light. The λ, Δλ, n, and τrad of BGN1.5 are 2710 nm, 170 nm, 1.85, and 0.902 ms, respectively. Therefore, the calculated maximum emission cross section is 1.36 × 10−19 cm2.4. Conclusion
BGN samples codoped with Er3+ and Ag NPs were fabricated through the conventional melt-quenching method. The bismuthate glass matrix has an important role in the precipitation of Ag NPs. Absorption spectra showed that the SPR peaks of Ag NPs range from 575 nm to 590 nm. The effect of metal Ag NPs on the 2.7 μm luminescence of Er3+/Ag-codoped BGN samples was investigated. The 2.7 μm fluorescence intensity of Er3+ reached its maximum value in glass with 1.5 mol% AgCl and is 1.6 times higher than that in glass without AgCl. The fluorescence lifetime of the glass with 1.5 mol% AgCl content is 0.902 ms. The local field enhancement effect induced by Ag0 SPR and energy transfer from Ag0 NPs to Er3+ ions are responsible for luminescence enhancement. The maximum stimulated emission cross-section σem of Er3+: 4I11/2→4I13/2 transition at 2.7 μm is 1.36 × 10−19 cm2. The prepared nanocomposites are promising candidate host matrices for lasers, optical displays, and optical memory devices given their facile preparation and 2.7 μm luminescence, which is significantly enhanced by the addition of Ag NPs.
Funding
Zhejiang Provincial Natural Science Foundation of China (LY18F050004); the Natural Science Foundation of Ningbo City (2017A610006); National Natural Science Foundation of China (61435009, 61627815); K. C. Wong Magna Fund in Ningbo University.
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