1. Introduction

Luminescent Ag nanoclusters, consisting from a few Ag atoms, have been attracting major interest because of a variety of novel applications, such as nanolabels [1,2], white light generation [3], UV-driven white and color phosphors, luminescent lamps, flexible monitors, down-shifting of the solar spectrum for enhanced solar cells, [see 1-13 and references therein]. These nanoclusters have unique optical properties that cover the range between bulk Ag metal and single Ag atoms [1–13].

The Ag nanoclusters, dispersed homogeneously across the bulk of the oxyfluoride glass host, have been reported only recently, e.g. in [4-6 and refs therein]; spectra of their luminescence and transmission electron microscopy images have been presented in [4,5,9,13]. The quantum yield of the luminescence of Ag nanoclusters may reach values up to 70% [2,10], which is comparable to the quantum yield of some commercial phosphors, such as semiconductor quantum dots and rare-earth ions.

For example, recently the Pb/CdF₂ [4,10], BaF₂ [5] and CaF₂ [6] fluoride glass formers were used together with the SiO₂ oxide glass former, and in combination with other glass modifiers, to obtain oxyfluoride glasses doped with the Ag nanoclusters homogeneously dispersed across the bulk of the oxyfluoride glass host. The fluoride component was argued to be very suitable for dispersion of the Ag nanoclusters [4–6], as the Ag nanoclusters do not disperse within the bulk of pure oxide glass host (though they may be dispersed at the surface of the oxide glass at the depth of the order of 1 μm if one uses special techniques, such as ion exchange, ion beam implantation, X-ray irradiation and so on [1–3,10–12]). However, some concern might be raised regarding toxicity of a Cd component; despite the fact that Cd atoms were sealed within the bulk of the corresponding oxyfluoride glass which strongly reduces toxicity of the final glass product.

We propose here a simpler chemical composition of glass host based on silica: namely, SiO₂-PbF₂ and show that this host is still capable to provide a homogeneous dispersion of luminescent Ag nanoclusters, obviously thanks to the presence of the fluorite component [4]. Melting of this glass is performed at temperature 1000°C during only 5 minutes. Co-doping of this glass with about 0.1 mol% of Tm³⁺, which has an absorption band ³H₆→¹D₂ around commercial LED wavelength of 365 nm [14,15], can adjust the colour of the emission band towards the white colour. The tint of this white emission can be tuned by a slightly varying excitation wavelength near to 365 nm.

It is important to note that silica (SiO₂) is a well-known low cost glass former, and that Pb has been incorporated in silica for centuries already aiming at lowering melting temperature of silica based glasses [16]. Glass formation in the (SiO₂):(PbF₂):(CdF₂) system was investigated in [17], and in particular the 50(SiO₂):50(PbF₂) glass has been explored in refs [17,18]. and found to be a good glass former. In this work we demonstrate that the bulk 50 ÷ 60(SiO₂):50 ÷ 40(PbF₂)mol% oxyfluoride glasses can be homogeneously doped with dispersed luminescent Ag nanoclusters, and when being co-doped with a minor admixtures of Tm³⁺, their luminescence becomes of white colour; the tint of this white luminescence can be tuned when exciting with varying excitation wavelength around 365 nm, i.e. a wavelength of commercial LED [19]. The emission of this glass is of interest for applications in UV-driven low cost lighting devices, flexible monitors and as down-shifting layers for enhanced solar cells.

Earlier, we proposed Tm³⁺-Tb³⁺-Eu³⁺ co-doped oxyfluoride glasses for white light generation under excitation at around 365 nm [14], comparing to that work Ag, Tm³⁺- doped glasses have the following advantageous. The glass host composition in our present work is much simpler than the one reported before, as the present host consists only of SiO₂ and PbF₂. The glasses do not contain Cd²⁺ ions making them more environmentally friendly. As the luminescent spectrum of Ag, Tm³⁺ co-doped glasses covers the whole visible range, it also results in a higher colour rendering index comparing to Tm³⁺-Tb³⁺-Eu³⁺ co-doped glasses [20]. The combination of Ag nanocluster and Tm³⁺ ion luminescent spectra allows doping of the glasses with as low amounts of Tm³⁺ ions as 0.1 mol.%. For instance, white light generation by the system Tm³⁺-Tb³⁺-Eu³⁺ ions required Tm³⁺ doping level as high as 3.0 mol%. Comparing to the currently most widespread white light emitting diode system based on YAG: Ce³⁺ excited by blue-emitting diode, we may consider the following: UV excited phosphor can be more efficient due to high efficiency of UV-light emitting diodes and more stable with respect to the driving current and temperature of UV-light emitting chip [20]. It is worth mentioning that a quantum yield of about 30% has been reported for Ag-doped glasses with similar composition [3], and this value is close to the values of phosphors based on oxynitrides, nitrides, sulfides and zeolites [20,21].

2. Experimental

The (SiO₂):(PbF₂):(CdF₂) glass system was studied in detail in [17]. and the glass formation region was already identified for this system. The possibility to avoid using the CdF₂ component was addressed also in [18], where it was found that a region of good glass stability exists near the composition 50(SiO₂):50(PbF₂). In this work, we have found that the simple oxyfluoride glass system is capable to dissolve luminescent Ag nanoclusters, due to fluorite type component PbF₂, following previous arguments regarding more complicated oxyfluoride glasses [4–7].

In this work, 99.9% purity SiO₂ and PbF₂ components were melted at 1000°C in a Pt crucible for only 5 minutes. AgNO₃ (of the same purity 99.9%) was added, in different wt%, when preparing the batch. The melt was quenched in air to room temperature cold mould, according to procedure described in [14, 22]. On the whole, the preparation procedure looks substantially easier and cheaper than that which was used earlier while dealing with bulk oxyfluoride glasses homogeneously doped with Ag nanoclusters [4–7]. Some of the glass samples studied in this work were just left for cooling in a Pt crucible in plain air after being taken from the furnace and the resulting glass pieces were removed from the Pt-crucible manually.

3. Results

Figure 1 shows the absorption spectrum of the undoped glass host (55SiO₂-45PbF₂) and the effect of doping with AgNO₃ on the absorption spectra. The doping with large amounts (1 to 10 wt%) AgNO₃ results in a considerable shift of the UV edge to longer wavelengths. In [4, 9], it has been demonstrated that the introduction of AgNO₃ into the glass gives rise to the growth of Ag nanoclusters. The Ag nanoclusters absorb UV light what causes the UV edge to shift. Likewise the broad band centred at 460 nm has already been revealed, and according to [22], it can be explained by the formation of tiny amorphous Ag nanoparticles (~1-2 nm) in the glass host.

 

Fig. 1 Absorption spectra of the glass host 55SiO₂-45PbF₂ doped with 1, 2.5, 5 and 10 wt% AgNO₃.

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The photoluminescence measurements were carried out for glasses with two different host compositions (55SiO₂-45PbF₂, Fig. 2(a), and 60SiO₂-40PbF₂, Fig. 2(b)), and for a doping level of AgNO₃ (1-5 wt%), Fig. 2(c). The excitation and detection wavelengths were 360 or 380 nm, 550 or 600 nm, respectively. Note that the undoped glasses did not show any appreciable luminescence, particularly when excited in the UV. Therefore all further luminescence spectra are due to the presence of Ag nanoclusters and/or Tm³⁺ dopants. The photoluminescence and excitation spectra cover a broad range from 400 to 900 nm and from 320 to 500 nm, corresponding to the spectra of Ag nanoclusters [4]. These spectra are not affected by the change of the composition ratio of SiO₂ and PbF₂, and by varying AgNO₃ doping level. This invariance indicates that the site of the Ag nanoclusters does not depend on the specific ratio (60/40 or 55/45) of these two glasses.

 

Fig. 2 Emission and excitation spectra of Ag nanoclusters embedded in a glass host 55SiO₂-45PbF₂ (a) and 60SiO₂-40PbF₂ (b), comparison of excitation and emission spectra of Ag nanoclusters in the both hosts and with varying AgNO₃ content (c).

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The photoluminescence excitation and emission spectra of Tm³⁺ doped glass (a, b) and Ag, Tm³⁺ -co-doped glass (c, d), both cooled down in the mould, are shown in Fig. 3.Under excitation of 360 nm, high efficient blue luminescence of Tm³⁺ doped glass can be clearly seen, the corresponding emission spectrum is shown in Fig. 3(a). The emission bands of Tm³⁺ ions have been assigned according to [23]. The major blue emission band of Tm³⁺ ion can be observed at around 455 nm, corresponding to the radiative transition from the ¹G₄ excited state to the ³H₆ ground state. The excitation spectra of Tm³⁺ doped sample are given in Fig. 3(b). These spectra exhibit two different bands at around 360 and 450-500 nm. The bands at 360 and 450-500 nm are due to direct excitation of Tm³⁺ ions through ³H₆→¹D₂, ¹G₄ transitions.

 

Fig. 3 Excitation and emission spectra of the pure (no silver) 0.1 mol% Tm³⁺ doped glass (a,b) and the Ag and Tm³⁺ co-doped glass (c,d), respectively.

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Figures 3(c) and 3(d) demonstrate that co-doping of the glasses with Ag nanoclusters and Tm³⁺ ions results in combining the emission bands of the Ag nanoclusters and the Tm³⁺ ions, under ~360 nm excitation. The excitation spectrum detected at 455 nm, associated with ³H₆→¹D₂ transition in Tm³⁺ ions, is consistent with the spectrum shown for glass doped only with Tm³⁺, thus indicating a direct excitation of Tm³⁺ ions at ~360 nm. The excitation spectra detected at other wavelengths correspond to the absorption spectra of Ag nanoclusters and Tm³⁺ ions, pointing to the energy transfer from the Ag nanoclusters to the Tm³⁺ ions. Excitation of the co-doped glasses at 340 or 380 nm results in a typical emission band of Ag nanoclusters in combination with the emission band of Tm³⁺ ions at 800 nm. This additional band is due to energy transfer from the Ag nanoclusters to the Tm³⁺ ions; the red part of Ag nanoclusters spectrum overlaps with the red emission band of Tm³⁺ ions ensuring the efficient transfer.

Figure 4 shows the CIE (Commission Internationaled’Eclairage) chromaticity diagrams for Ag, Tm³⁺ co-doped glasses cooled down in the mould (a) and in the Pt crucible (b). This diagram presents all chromaticity visible to the human eye after correction to sensitivity of blue, green and red receptors of the eye [24]. The x and y-axis of CIE diagram are the respective projected coordinates of the total visible luminescence. The white zone on the diagrams indicates a white light gamut area. As it is seen from Fig. 4(a) and 4(b), white colour emission with varying tint can be generated with excitation wavelength near to 365 nm. The chromaticity coordinates of our samples depend strongly on the excitation wavelength and cooling conditions. The inserts in Fig. 4(a) and 4(b) show the photographs of white light luminescent spots seen on the surface of the glasses excited in the range from 350 to 380 nm.

 

Fig. 4 CIE chromaticity diagrams for Ag, Tm³⁺ - co-doped glasses cooled down in room temperature mould (a) and in a Pt crucible (b), excited in the range from 350 to 380 nm. Inserts in (a, b) show photographs of these white-luminescent Ag, Tm³⁺ -co-doped glasses. Excitation wavelengths are also indicated in the figure.

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Figure 5 presents emission and excitation spectra of Ag, Tm³⁺-co-doped oxyfluoride glasses cooled down in the mould and the crucible. Cool down of the glasses in the different environments results in different cooling rates for the glasses. The emission spectra of Ag,Tm³⁺ -co-doped glasses, excited at 360 and 380 nm, are shown on Fig. 5(a) and 5(b). As can be seen from Fig. 5(a) and 5(b), different cooling rates affect intensity ratio of Tm³⁺ bands and broadness of Ag nanocluster luminescent bands. Excitation spectra of the glasses cooled down in different conditions are shown in Fig. 5(c) and 5(d). In this case, cool down in the crucible results in broadening of the excitation spectrum of Ag nanocluster band and changing of the excitation ratio of the Tm³⁺ ion bands. These changes indicate a modification of the Tm³⁺ ions environment [25]. According to paper [26], the Ag nanoclusters emitting in blue, green and red visible range are responsible for broad emission band of Ag doped glasses. The broadening of the emission and excitation spectra points to an increase of concentration of “blue” Ag nanoclusters in the glass cooled down in the crucible comparing to one cooled down in the mould [26].

 

Fig. 5 Emission (a,c) and excitation (b,d) spectra of Ag, Tm^3+-co-doped glasses cooled down in the mould and the crucible.

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4. Discussion

As it was argued in [27, 28], the luminescence of Ag nanoclusters doped glasses arises mostly due to emission from the Ag₄²⁺ tetramers dispersed in a fluorite host. This point of view is based upon experimental excitation and emission luminescence spectra, electron spin resonance measurements and quantum chemistry calculations. Likewise, transmission electron microscopy results demonstrated that the Ag nanoclusters were dispersed homogeneously in a fluorite-type lattice [4]. According to density functional theory quantum chemistry calculations reported in [27], the Ag₄²⁺nanoclusters are rhombic shaped with a long and short diagonals. The proposed energy level diagram of the Ag₄²⁺ tetramer has been calculated in the references [27, 28]. This diagram is shown in Fig. 6(a) and viewed along the short diagonal of the rhombus. When an excitation occurs with UV light (shown by the violet up-arrow) an electron undergoes a transition from the ground S₀ to the excited singlet state S₁. The intersystem crossing process (curved orange arrow) occurs between the excited singlet S₁ and triplet T₂ states resulting in population of the T₂ state with electrons. The optical transitions from the excited singlet (blue down-arrow) and triplet (green down-arrow) states give rise to the broad band emission of Ag₄²⁺ tetramers dispersed in the glass.

 

Fig. 6 Energy level diagram of the Ag nanoclusters (left) and Tm³⁺ ions (right) [23] (a), and decay kinetics detected at 795 nm (b).

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The energy level diagram of Tm³⁺ ions, based on the results reported in [23], is shown in Fig. 6(a). The red down arrows show the observed emission transitions in the Tm³⁺ doped glasses. The purple and blue up arrows correspond to the ³H₆→¹D₂ and ³H₆→¹G₄ excitation transitions in Tm³⁺ ions. The minimum energy of the Ag nanocluster ground state and ³H₆ ground energy level of Tm³⁺ ion are assumed to be close to each other in order to explain the energy transfer from the Ag nanoclusters to the Tm³⁺ ion (see red lines). Two black curves show energy transfer from the excited singlet and triplet states of Ag nanoclusters to ³H₅ energy level of Tm³⁺ ion resulting in emission at around 800 nm. The microsecond kinetics of photoluminescence has been detected at 750 nm under excitation at 380 nm (Fig. 5(b)). The co-doping of Ag doped glasses with Tm³⁺ results in quenching of the visible emission due to energy transfer from Ag nanoclusters to Tm³⁺ ions. The kinetics of luminescence has been fitted with double exponential function:

An external quantum yield of Ag, Tm³⁺ co-doped glasses have been measured using an integrating sphere by the method described in [21]. Under excitation at 360 nm, the quantum yield for the glasses cooled down in the mould and the crucible was estimated to be 2.3 and 2.0%, respectively. It is worth noting that the glasses have been prepared using 99.9% purity SiO₂ and PbF₂ instead of 99.999% purity SiO₂ and PbF₂ as it was in our previous papers where the quantum yield up to 30% was achieved [3]. The impurities of initial batch compounds may lead to formation of different non-luminescent centres in the glass host resulting in low external quantum yields [21]. Subsequently, the purification of initial batch compounds may result in rise of the quantum yield up to values reported earlier.

The Ag nanoclusters doped glasses have a promising future for application in light sources pumped with a commercial UV-LED at about 365 nm, e.g. in [3,10,13], especially because an emission spectrum of Ag nanoclusters is reminiscent of the solar spectrum and capable of achieving a quantum yield up to 30% at room temperature, e.g. in review [3].

The absorption spectrum of Ag nanoclusters falls into the part of solar spectrum where existing solar cells, like Si solar cells, or organics solar cells, do not absorb much energy, and therefore down-conversion of the solar spectrum is required to the green (organics solar cells) or red in and near-infrared part of the spectrum.

5. Conclusion

Ag, Tm³⁺ co-doped oxyfluoride glasses have been prepared using conventional melt-quenching technique. White light luminescence is excited in this glass by UV light from 340 to 400 nm, which is at about the same wavelength as in commercial LEDs. We believe that the glass proposed it this work, either co-doped with Tm³⁺or only doped with Ag nanoclusters could be a low cost, UV-driven, alternative for white light sources. These glasses also exhibit a potential as active luminescent layers for flexible screen monitors and down-conversion layers for enhancing efficiency in solar cells.