Ag-Sensitized NIR-Emitting Yb3+-Doped Glass-Ceramics

The optical photoluminescent (PL) emission of Yb3+ ions in the near infrared (NIR) spectral region at about 950–1100 nm has many potential applications, from photovoltaics to lasers and visual devices. However, due to their simple energy-level structure, Yb3+ ions cannot directly absorb UV or visible light, putting serious limits on their use as light emitters. In this paper we describe a broadband and efficient strategy for sensitizing Yb3+ ions by Ag codoping, resulting in a strong 980 nm PL emission under UV and violet-blue light excitation. Yb-doped silica–zirconia–soda glass–ceramic films were synthesized by sol-gel and dip-coating, followed by annealing at 1000 ◦C. Ag was then introduced by ion-exchange in a molten salt bath for 1 h at 350 ◦C. Different post-exchange annealing temperatures for 1 h in air at 380 ◦C and 430 ◦C were compared to investigate the possibility of migration/aggregation of the metal ions. Studies of composition showed about 1–2 wt% Ag in the exchanged samples, not modified by annealing. Structural analysis reported the stabilization of cubic zirconia by Yb-doping. Optical measurements showed that, in particular for the highest annealing temperature of 430 ◦C, the potential improvement of the material’s quality, which would increase the PL emission, is less relevant than Ag-aggregation, which decreases the sensitizers number, resulting in a net reduction of the PL intensity. However, all the Ag-exchanged samples showed a broadband Yb3+ sensitization by energy transfer from Ag aggregates, clearly attested by a broad photoluminescence excitation spectra after Ag-exchange, paving the way for applications in various fields, such as solar cells and NIR-emitting devices.


Introduction
Rare earth ions (RE 3+ ) have many optical applications due to their unique spectral properties, in relation to the features of their electronic energy levels, covering UV, visible and IR [1]. This makes them excellent candidates for lighting [2][3][4], displays [5], biosensing [6][7][8], optical amplification [9], anticounterfeiting [10,11] and solar cells [12][13][14]. Among them, Yb 3+ ions provide near infrared (NIR) emission around 950-1100 nm, which is interesting for many of the previously cited applications. However, the absorption and emission of Yb 3+ ions are due to transitions between only two levels: 2 F 7/2 (ground state) and 2 F 5/2 (excited state). Therefore, Yb 3+ ions are not able to directly absorb UV or visible photons, and other lanthanides are often added as codopants to provide an alternative excitation path by energy transfer, and sometimes to offer the additional possibility of photon multiplication by quantum cutting [15][16][17][18][19]. Nevertheless, codoping with other RE 3+ ions still do not solve the limited excitation and absorption bandwidths and their small excitation cross sections, which are major limitations for their implementation in thin film downconverting devices and other specific applications.
In the past, silicon [20][21][22][23] or silver aggregates [24][25][26][27] were proven as broadband efficient sensitizers for Er 3+ ions. Ag sensitization was successfully observed also for other RE 3+ ions (Eu, Tb, Yb, Dy, Sm) [28][29][30][31][32][33][34][35][36]. In this paper, we will further investigate the interaction between Ag aggregates and Yb 3+ , analyzing the role of the glass-ceramic (GC) matrix and its crystalline structure on the optical properties of the composite material. A GC is a homogeneous dispersion of ceramic nanocrystals in a glass. EXAFS studies suggest that RE 3+ ions should be preferentially located in the nanocrystals [37], resulting in better spectroscopic properties. Additionally, zirconia has lower phonon energy and higher refractive index than silica [38]. By combining Ag enhancement with Yb 3+ -doped silica-zirconia GC, the possibility to obtain a more efficient optical device is discussed in this paper. Compositional, structural and optical measurements were combined for retrieving information on the role and nature of Ag-aggregates controlled by Ag + ↔Na + ion exchange, followed by annealing at different temperatures, and their interaction with Yb 3+ ions for obtaining efficient sensitization and photoluminescence boosting.
Undoped (label GC0) and 4 mol% Yb-doped (label GC4) films were then immersed for 1 h in a molten salt bath (1 mol% AgNO 3 in NaNO 3 ) at 350 • C to obtain Ag + Appl. Sci. 2020, 10, x FOR PEER REVIEW 2 of 8 emission around 950-1100 nm, which is interesting for many of the previously cited applications.

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However, the absorption and emission of Yb 3+ ions are due to transitions between only two levels: visible photons, and other lanthanides are often added as codopants to provide an alternative excitation path by energy transfer, and sometimes to offer the additional possibility of photon 49 multiplication by quantum cutting [15][16][17][18][19]. Nevertheless, codoping with other RE 3+ ions still do not

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Yb, Dy, Sm) [28][29][30][31][32][33][34][35][36]. In this paper, we will further investigate the interaction between Ag aggregates 56 and Yb 3+ , analyzing the role of the glass-ceramic (GC) matrix and its crystalline structure on the 57 optical properties of the composite material. A GC is a homogeneous dispersion of ceramic 58 nanocrystals in a glass. EXAFS studies suggest that RE 3+ ions should be preferentially located in the 59 nanocrystals [37], resulting in better spectroscopic properties. Additionally, zirconia has lower 60 phonon energy and higher refractive index than silica [38]. By combining Ag enhancement with Yb 3+ -61 doped silica-zirconia GC, the possibility to obtain a more efficient optical device is discussed in this

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Silica-zirconia (70%-30%) GC films were synthesized by following a sol-gel and dip-coating 68 procedure, starting from Tetraethyl orthosilicate Si(OC2H5)4 (TEOS) and zirconium propoxide 69 Zr(OC3H7)4 (ZPO) precursors, as described in [32]. As usual in sol-gel synthesis, all dopants were 70 calculated and put in addition to this composition: 5 mol% Na was added as sodium acetate for 71 allowing the introduction of Ag by ion exchange; 4 mol% YbNO3 was added for Yb-doped samples.

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In short, three solutions were prepared as follows: YbNO3 for Yb-doped samples;

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and GC4C) were used for controlling Ag migration and aggregation [32].

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The film composition was studied by Rutherford backscattering spectrometry (RBS), by 2.2 MeV 86 4 He + beam at 160° backscattering angle in IBM geometry, and simulating the experimental spectra by 87 the RUMP code [39]. Areal density (the natural unit of measurement for RBS) to film thickness 88 conversion was obtained, considering a molar density of the film equal to a weighted average 89 between silica (2.00 g cm -3 ) and zirconia (5.68 g cm -3 ), according to the nominal stoichiometric 90 composition of the matrix (70 SiO2 -30 ZrO2), which was confirmed by RBS analysis.

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The crystal phase identification was obtained by X-ray diffraction (XRD) at room temperature, Na + ion exchange (label GC0A and GC4A). Finally, post-exchange annealing in air at 380 • C (GC0B and GC4B) and 430 • C (GC0C and GC4C) were used for controlling Ag migration and aggregation [32].
The film composition was studied by Rutherford backscattering spectrometry (RBS), by 2.2 MeV 4 He + beam at 160 • backscattering angle in IBM geometry, and simulating the experimental spectra by the RUMP code [39]. Areal density (the natural unit of measurement for RBS) to film thickness conversion was obtained, considering a molar density of the film equal to a weighted average between silica (2.00 g cm -3 ) and zirconia (5.68 g cm -3 ), according to the nominal stoichiometric composition of the matrix (70 SiO 2 -30 ZrO 2 ), which was confirmed by RBS analysis.
The crystal phase identification was obtained by X-ray diffraction (XRD) at room temperature, with an X'Pert PRO diffractometer (Panalytical) using a Cu anode (Kα, λ = 1.54056 Å). Diffractograms were collected in Bragg-Brentano geometry in the 2θ range 10 • -100 • . Nanocrystal size was determined by Line Broadening Analysis (LBA) [40] by the Warren-Averbach method.
Photoluminescence excitation (PLE) and emission (PL) spectra were recorded by a FLS980 (Edinburgh Instruments). The excitation was provided by a xenon lamp coupled to a double-grating monochromator, while the emission from the sample was analyzed by a double-grating monochromator and recorded by a photon counting R928P (Hamamatsu) cooled at -20 • C (visible region) or by a photon counting R5509-73 (Hamamatsu) cooled at −80 • C (NIR region). The excitation wavelength for recording PL emission spectra was 330 nm, which is not absorbed by Yb 3+ ions, while the emission wavelength for acquiring PL excitation spectra was set at 975 nm, the peak wavelength for Yb 3+ ions 2 F 5/2 → 2 F 7/2 transition.

Results and Discussion
The film composition was confirmed by RBS analysis in agreement with the nominal values for Si, Zr and Yb. The Ag concentration depth profile (see examples in Figure 1) revealed a reduction from about 2 mol% (at surface) to 1 mol% (inner part of the film) both for the Ag-exchanged and the annealed samples. This indicates a small silver mobility in the glass ceramic matrix. Figure 1 presents the RBS spectra for the undoped samples before and after Ag-exchange, and the RUMP simulation of the latter (Figure 1a). In Figure 1b, a selected region corresponding to the Zr and Ag signals is reported, including a depth profile scale for the Zr signal, obtained from the RBS signal combined with the weighted average molar density between silica and zirconia, as described in the Experimental section. In Figure 1c, the calculated Ag depth profiling is reported for the samples GC0A, GC0B and GC0C. Post-exchange heat treatment changes only slightly the Ag profile, with no difference between sample GC0B and GC0C, within the experimental uncertainty of the RBS.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 9 were collected in Bragg-Brentano geometry in the 2θ range 10°-100°. Nanocrystal size was determined by Line Broadening Analysis (LBA) [40] by the Warren-Averbach method. Photoluminescence excitation (PLE) and emission (PL) spectra were recorded by a FLS980 (Edinburgh Instruments). The excitation was provided by a xenon lamp coupled to a double-grating monochromator, while the emission from the sample was analyzed by a double-grating monochromator and recorded by a photon counting R928P (Hamamatsu) cooled at -20 °C (visible region) or by a photon counting R5509-73 (Hamamatsu) cooled at −80 °C (NIR region). The excitation wavelength for recording PL emission spectra was 330 nm, which is not absorbed by Yb 3+ ions, while the emission wavelength for acquiring PL excitation spectra was set at 975 nm, the peak wavelength for Yb 3+ ions 2 F5/2  2 F7/2 transition.

Results and discussion
The film composition was confirmed by RBS analysis in agreement with the nominal values for Si, Zr and Yb. The Ag concentration depth profile (see examples in Figure 1) revealed a reduction from about 2 mol% (at surface) to 1 mol% (inner part of the film) both for the Ag-exchanged and the annealed samples. This indicates a small silver mobility in the glass ceramic matrix. Figure 1 presents the RBS spectra for the undoped samples before and after Ag-exchange, and the RUMP simulation of the latter (Figure 1a). In Figure 1b, a selected region corresponding to the Zr and Ag signals is reported, including a depth profile scale for the Zr signal, obtained from the RBS signal combined with the weighted average molar density between silica and zirconia, as described in the Experimental section. In Figure 1c, the calculated Ag depth profiling is reported for the samples GC0A, GC0B and GC0C. Post-exchange heat treatment changes only slightly the Ag profile, with no difference between sample GC0B and GC0C, within the experimental uncertainty of the RBS.  XRD reflections before Ag + ↔Na + ion exchange are reported in Figure 2, showing the presence of ZrO 2 tetragonal nanocrystals in the undoped samples and cubic nanocrystals in Yb-doped films, attested by a double peak (tetragonal) or a single peak (cubic) at 2θ ≈ 35.5 • and 2θ ≈ 75 • . Noteworthy, deeper analysis which goes beyond the scope of this paper can be further exploited to corroborate the observed structural behavior and to quantify the relative phases [41]. The effect of Ag-exchange and subsequent annealing was not observed on XRD spectra. Indeed, it should be noted that crystal formation in similar materials occurs at 1000 • C. Previous studies highlighted that below that temperature, the sol-gel remains in a glassy state (see, for example, Zur et al. [19]). Ag-exchange and the following annealing processes were done at 350 • C and up to 430 • C. Therefore, it can be reasonably concluded that the energies involved at these treatment temperatures are not enough for modifying the crystalline features of the glass-ceramic material. Finally, proper line broadening analysis (LBA) of XRD diffraction peaks [40] obtained by the Warren-Averbach method showed that nanocrystal size was about 14 nm for tetragonal zirconia in our undoped samples (GC0), and about 12 nm for cubic zirconia in Yb-doped samples (GC4). XRD reflections before Ag + ↔Na + ion exchange are reported in Figure 2, showing the presence of ZrO2 tetragonal nanocrystals in the undoped samples and cubic nanocrystals in Yb-doped films, attested by a double peak (tetragonal) or a single peak (cubic) at 2θ ≈ 35.5° and 2θ ≈ 75°. Noteworthy, deeper analysis which goes beyond the scope of this paper can be further exploited to corroborate the observed structural behavior and to quantify the relative phases [41]. The effect of Ag-exchange and subsequent annealing was not observed on XRD spectra. Indeed, it should be noted that crystal formation in similar materials occurs at 1000 °C. Previous studies highlighted that below that temperature, the sol-gel remains in a glassy state (see, for example, Zur et al. [19]). Ag-exchange and the following annealing processes were done at 350 °C and up to 430 °C. Therefore, it can be reasonably concluded that the energies involved at these treatment temperatures are not enough for modifying the crystalline features of the glass-ceramic material. Finally, proper line broadening analysis (LBA) of XRD diffraction peaks [40] obtained by the Warren-Averbach method showed that nanocrystal size was about 14 nm for tetragonal zirconia in our undoped samples (GC0), and about 12 nm for cubic zirconia in Yb-doped samples (GC4).
It is known from previous studies that the optical properties of silver-doped silicate glasses have broad luminescence emissions under UV light [42][43][44][45] related to isolated Ag + ions (Em. 330-370 nm), Ag + -Ag + pairs (Em. 430-450 nm), formation of (Ag3) 2+ trimers, multimers, and aggregates (Em. 550-650 nm and above). Studies of the PL emission under 280 nm excitation before (GC0) and after Agexchange (GC0A) and 430 °C annealing (GC0C) have been reported elsewhere [32], showing a main peak at 425 nm, reasonably related to Ag+-Ag+ pairs, with a broad emission extending to the red spectral region, attributed to trimers and multimers. The modification of the curves after annealing It is known from previous studies that the optical properties of silver-doped silicate glasses have broad luminescence emissions under UV light [42][43][44][45] related to isolated Ag + ions (Em. 330-370 nm), Ag + -Ag + pairs (Em. 430-450 nm), formation of (Ag 3 ) 2+ trimers, multimers, and aggregates (Em. 550-650 nm and above). Studies of the PL emission under 280 nm excitation before (GC0) and after Ag-exchange (GC0A) and 430 • C annealing (GC0C) have been reported elsewhere [32], showing a main peak at 425 nm, reasonably related to Ag+-Ag+ pairs, with a broad emission extending to the red spectral region, attributed to trimers and multimers. The modification of the curves after annealing is related to the evolution of the Ag species within the material. After annealing, the maximum peak at 425 nm slightly decreases, while the red emission contribution increases as a consequence of the decreasing of the number of dimers towards the formation of multimers and small aggregates. The optical properties of these Ag-aggregates can be better investigated by 330 nm excitation, which is reported in Figure 3. Additionally, there is also higher interest in analyzing near UV and blue excitation because sources like lasers, LEDs, or lamps for these wavelengths are much more available and cheaper than deeper UV light sources. As expected, the PL spectra of Ag-exchanged GC samples under 330 nm excitation, before (GC0A) and after annealing at 380 • C (GC0B) and 430 • C (GC0C), show a broadband emission related to multimers and aggregates, covering up to 750 nm. As previously observed [32], the intensity of this emission increases by annealing, as a possible consequence of matrix recovery with the removal of non-radiative defects, and of the increase of the number of multimers at the expense of single ions and dimers, at least for 380 • C annealing. A further thermal treatment at 430 • C seems detrimental with respect to 380 • C annealing. The reduction of PL intensity suggests a decrease in the effective number of emitting species, which is corroborated by the following PL analysis in the NIR spectral range.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 9 consequence of matrix recovery with the removal of non-radiative defects, and of the increase of the number of multimers at the expense of single ions and dimers, at least for 380 °C annealing. A further thermal treatment at 430 °C seems detrimental with respect to 380 °C annealing. The reduction of PL intensity suggests a decrease in the effective number of emitting species, which is corroborated by the following PL analysis in the NIR spectral range. The NIR PL emission under 330 nm excitation is reported in Figure 4b for Yb 3+ -doped GC samples before Ag-exchange (GC4), after exchange (GC4A), and after annealing at 380 °C (GC4B) and 430 °C (GC4C). The direct excitation of Yb 3+ ions by 330 nm light is completely absent. After Ag ion exchange, a strong Yb 3+ PL emission peak at 975 nm is detected, possibly due to dimers, multimers or small aggregates formed during the ion exchange process, which act as efficient sensitizers. In agreement with the previous PL analysis of Ag aggregates, annealing is expected to improve the quality of the matrix, which should increase the PL intensity. However, annealing also decreases the number of sensitizers due to the formation of multimers and bigger aggregates, and this additionally increases the average distance between them and Yb 3+ ions, reducing the efficiency of energy transfer. The overall combination of these positive and negative effects is almost balanced at 380 °C, while it results in a significant decrease in the NIR PL signal at 430 °C. Therefore, Ag as-exchanged samples are actually the best performing NIR emitters.
Deeper investigation of the excitation behavior of Yb 3+ -doped GC samples, before and after Agexchange and annealing, is reported in Figure 4a. The intensity of the PLE curves agrees well with the PL analysis described above, with a further remarkable feature: the possibility for efficient and broadband excitation on Ag-exchanged samples, covering the whole spectral range from UV to blue, up to about 500 nm. On the contrary, the PLE spectrum of Yb 3+ -doped samples before Ag-exchange (GC4) gives no signal above 300 nm excitation. Below that wavelength, the possible excitation of Yb 3+ ions is due to the occurrence of charge-transfer processes from the matrix itself. The NIR PL emission under 330 nm excitation is reported in Figure 4b for Yb 3+ -doped GC samples before Ag-exchange (GC4), after exchange (GC4A), and after annealing at 380 • C (GC4B) and 430 • C (GC4C). The direct excitation of Yb 3+ ions by 330 nm light is completely absent. After Ag ion exchange, a strong Yb 3+ PL emission peak at 975 nm is detected, possibly due to dimers, multimers or small aggregates formed during the ion exchange process, which act as efficient sensitizers. In agreement with the previous PL analysis of Ag aggregates, annealing is expected to improve the quality of the matrix, which should increase the PL intensity. However, annealing also decreases the number of sensitizers due to the formation of multimers and bigger aggregates, and this additionally increases the average distance between them and Yb 3+ ions, reducing the efficiency of energy transfer. The overall combination of these positive and negative effects is almost balanced at 380 • C, while it results in a significant decrease in the NIR PL signal at 430 • C. Therefore, Ag as-exchanged samples are actually the best performing NIR emitters.

Conclusions
The synthesis and characterization of efficient NIR emitting glass-ceramic films is presented and Deeper investigation of the excitation behavior of Yb 3+ -doped GC samples, before and after Ag-exchange and annealing, is reported in Figure 4a. The intensity of the PLE curves agrees well with the PL analysis described above, with a further remarkable feature: the possibility for efficient and broadband excitation on Ag-exchanged samples, covering the whole spectral range from UV to blue, up to about 500 nm. On the contrary, the PLE spectrum of Yb 3+ -doped samples before Ag-exchange (GC4) gives no signal above 300 nm excitation. Below that wavelength, the possible excitation of Yb 3+ ions is due to the occurrence of charge-transfer processes from the matrix itself.

Conclusions
The synthesis and characterization of efficient NIR emitting glass-ceramic films is presented and discussed. Yb 3+ -doped silica-zirconia-soda materials are prepared by sol-gel and dip-coating. Annealing in air at 1000 • C induces the precipitation of zirconia nanocrystals. Different crystalline phases have been observed, depending on the presence of the RE dopant: tetragonal for undoped, and cubic for Yb 3+ -doped glass-ceramics. The following Ag + ↔Na + ion exchange allows the introduction of Ag species as dimers, multimers, and aggregates in the film, which can be excited in the UV spectral region, and act as sensitizers for Yb 3+ ions. As a consequence, Yb 3+ NIR emission around 950-1100 nm is significantly enhanced after Ag ion exchange, not only in intensity, but also in the broadness of the excitation band, covering spectral regions not allowed for the direct absorption of Yb 3+ ions. Ag-sensitized Yb 3+ -doped films could have applications as spectral downshifters for photovoltaic (PV) solar cells and NIR light-emitting sources. Furthermore, the high versatility and optical quality of sol-gel glass-ceramic waveguides make these materials also suitable for realizing solar concentrators and integrated optical devices, such as optical amplifiers and lasers.