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Abstract
BGG -Bi2O3 glasses with composition 40GeO2–20Ga2O3–40BaO: x Bi2O3 (x = 0, 1, 2, 4, 6, 8 - 1.0 mol %) were analyzed in terms of optical, thermal properties and structure. The dependence of Bi2O3 content on thermal, structural, and optical properties was investigated by thermal analysis (DSC), Raman spectroscopy, UV–visible and near-infrared absorption, and the M-Line technique to access refractive index values. The results show that with the increase of Bi2O3, the density, the glass transition temperature (Tg), the refractive index, and the optical band gap energy decrease. Different melting atmospheres were added during the syntheses to measure their impact on the photoluminescence of bismuth around 1300 nm. From XPS analysis, photoluminescence measurements indicate that the Bi3+ oxidation state is the main one responsible for the broad near-infrared band from 1000 nm to 1600 nm in these glasses.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Nowadays a large amount of information must be transmitted with high-speed high-capacity. Since the silica-based transmission fibers have a wide-band operating window ranging from 1250-1650 nm [1], the development of new functional materials that can be potentially used for amplifiers and lasers operating in the entire telecommunication window with a wider amplification bandwidth are required to cover ultra-wide broadband optical communication. Rare-earth doped glass optical fibers are the most efficient active medium for the near-infrared (NIR) spectral region, but it is not possible to construct rare-earth-doped efficient optical amplifiers for the indicated spectral bands. As for now, traditional erbium-doped fiber amplifiers have reached their limit in bandwidth (1530-1610 nm) and no efficient rare earth-doped fiber lasers exist.
Since the discovery of an exceptionally wide region NIR luminescence (1100-1600 nm) range in bismuth-silica glasses [2], great attention has been focused on Bi-based glasses as an interesting alternative where the traditional rare earths-based glasses have a rather narrow operating spectral range [3,4]. There are several reports on infrared luminescence from bismuth-doped glasses using the melt-quenching method, such as silicate [2,5,6], borate [7], germanate [5], phosphate [8], and chalcohalide [9] glasses. However, even though NIR emission has been observed in a variety of bismuth-doped glasses there is no consensus among researchers regarding the nature of NIR PL making the exact origin of this luminescence unclear. As an example, some authors assigned that reduced or metallic Bi ions will generate the NIR emission [9,10] while others claim that it is caused by subvalent Bi centers with bismuth nominal valence state (below than 3+) [11,12].
One of the problems that prevent the understanding of the nature of the emitting centers in Bi-related IR is related to the different oxidation state of the bismuth element itself such as Bi5+, Bi3+, Bi2+ and Bi+. The polyvalent ions in molten glass are in reduction/oxidation (redox) equilibrium, and these equilibria depend strongly on the different synthesis parameters: glass composition, melting temperature, atmosphere, and concentration of polyvalent elements [13]. Synthesizing to Bi-doped glasses having single oxidation state of Bi, is a challenging task in terms of the optimization of the synthesis conditions and maintaining the large and intense photoluminescence.
This present study, bismuth-doped BaO-Ga2O3-Ge2O3 (BGG) glasses were considered, and the impact of melting atmosphere during syntheses of these glasses on their luminescence properties was studied in detail. BGG glass is chosen as the host matrix due to its combination of good thermal stability, chemical durability, low phonon energy, and high transparency in a wide wavelength range. An exploration of the effect of the melting atmosphere will be examined using several characterization techniques concerning their structure and chemical state.
2. Material and methods
BGG-Bi2O3 glass having molar composition of 40GeO2 – 20Ga2O3 – 40BaO – x Bi2O3 (x = 0, 1, 2, 4, 6, 8 mol %) were prepared from Bi2O3 (3N), GeO2 (5N), Ga2O3 (3N) powder, and BaO (2N). For each glass, well-mixed raw materials (10 g) were placed in an alumina crucible and melted at 1500°C for 30 min under a first reducing atmosphere (N2), a second reducing atmosphere (5% H2+ 95% N2), or under dry air (oxidizing). Thus, the samples are labeled as xBiy, where x is the Bi concentration, and y the atmosphere. The atmosphere flow was set at 1L/min and a purge of 20 minutes was maintained before the melting. Subsequently, the melt was kept in the furnace chamber without casting to prevent exposition to the ambient atmosphere. The furnace was brought down to 670°C at a rate of 10°C/min to avoid crystallization. Then, the samples are annealed at 670°C for 3 hours in the same alumina crucible. The annealed samples were cut and polished for optical property measurements.
Optical characterizations were performed on glass bulks with parallel and optically polished faces. Their optical absorption spectra were recorded in the 300–1000 nm wavelength range using an UV-visible spectrophotometer (CARY 5000 from VARIAN). The absorption spectra were used to measure the glasses band gap energy (Eg).
The linear refractive index was recorded by the prism coupling method, using a Metricon Model 2010 with precision of ± 0.01 operating at 532 nm, 633 nm, 972 nm, 1308 nm, and 1538 nm.
The characteristics temperature of the glasses Tg, glass transition, and Tx, crystallization was measured with a Differential scanning calorimetric (DSC) by using a Netzsch DSC Pegasus 404F3 apparatus on glass pieces in PtRh pans at a heating rate of 10°C/min.
The density has been measured using Archimedes principle as (metre formula):
where Dx is the density of water, Wa is the weight of sample in air and Wx is weight in water. The molar volume (Vm) thus has been calculated as Vm=M/D with M as the molecular weight of the glass samples.Raman spectra were recorded with a Renishaw inVia spectrometer coupled to a Leica DM2700 microscope. A back-scattering geometry was used in the frequency range of 100–1200 cm-1. The excitation light source was a vertically polarized He-Ne laser with a wavelength of 633 nm and a power of 17 mW. The frequency uncertainty was estimated to be ± 2 cm-1. The deconvolution of Raman spectra was performed using the curve fit function of Wire 4.1 software.
NIR luminescence spectra were obtained by a Horiba Jobin Yvon Nanolog spectrofluorometer equipped with an R928 photomultiplier tube (PMT) multi-alkali PMT ambient-cooled visible detector coupled with an InGaAs photodiode. The measurements were performed over the 480 to 850 nm wavelength range by maintaining the same conditions for all samples. The method of measurement chosen here was the ‘front face detection mode’ where emission is collected at an angle of 30° respective to the excitation beam to minimize stray light reflection.
The XPS measurements were carried out with a Kratos AXIS Ultra DLD electron spectrometer which has Al conical anode for charge control. Non-monochromatic 150 W MgKa X-ray provided the excitation radiation. During experiment, the pressure inside the analyzer chamber was about 10−9 torr. The drift of the electron binding energy due to the surface charge effect was calibrated by utilizing C1s peak of the contamination of adsorption from air (Eb = 284.8 eV).
3. Results and discussion
Figure 1 shows the impact of the Bi2O3 doping on the color of the 40GeO2 - 20Ga2O3 - 40BaO glasses system. As can be observed in the optical micrographs, all glasses show high optical quality free of bubbles and visible striae. The color change observed in the studied glasses depends on their composition and the melting atmosphere selected.
Fig. 1. Optical micrographs of the BGG glasses doped with different concentrations of Bi2O3 (increasing from the left to the right) and melted at different atmospheres (increasing from the bottom to the top).
With increasing the content of Bi2O3 as well the melting atmosphere (oxidant or reducing the visual coloration changed from colorless and transparent to deep brown which can be assigned to reduction of Bi3+ ions to bismuth metal (Bi◦) during the melting process, called auto-thermo reduction. This process will be explained in details in the next section.
3.1 Physical properties
It was noticed that changing the melting atmosphere do not affect the main physical characteristics Thus, the results presented below will be evaluated only with the oxidizing atmosphere O2N2.
Table 1 shows the glass transition temperature (Tg), the density, and the molar volume of BGG-Bi2O3 glasses using O2N2 atmosphere.
Table 1. Composition dependence of glass transition temperature (Tg), density (D) and molar volume (V) of 40GeO2-20Ga2O3-40BaO-xBi2O3 (x=0,1,2,4,6,8 mol %) prepared at O2N2 melting atmosphere. Tg was measured at a heating rate of 10 °C/min
Table 1 shows that with increasing concentration of Bi2O3, the Tg of the glasses decreased from 699°C to 618°C suggesting that the inclusion of bismuth oxide into the glass decrease the network connectivity. In this case, Bi2O3 acts like as glass network modifier increasing the ionic character of the chemical bonds in the glass structure. Bi2O3 will be incorporated into the glass structure by breaking up some Ge(Ga)-O bounds and open the chains formed by the network formers [14]. So, the replacement of Ge-O bonds with Bi-O bonds whose difference on the binding energy may explain the decrease in the value of Tg. Indeed, the Ge-O bond strength (157.6 kcal ∕mol) is much stronger than the Bi-O bond strength (80.6 kcal/mol) [15]. The formation of new structural bonds will be showed by Raman spectroscopy in section 3.5 to support our hypothesis.
The density is one of the tools to reveal the degree of structural changes of the glass network with composition. Table 1 also displays the variation of density (D) and molar volume (Vm) with Bi2O3 content.
The density increases from 4.869 to 5.336 g/cm3 and the molar volume also increases from 28.89 to 33.35 cm3/mol as the Bi2O3 content increases at the expense of the other glass compounds. The change in density is related to the atomic mass of the elements present in the composition. So, the increase in the density of the glasses is explained by the higher molecular weight of Bi2O3 compared to the other compounds. The molecular weight of Bi2O3 (336.48 g/mol) is heavier than the molecular weight of the other compounds (the molecular weight of Ga2O3, GeO2, and BaO are 187.44 g/mol and 153.33 g/mol, respectively) and hence, the overall molecular weight of the glasses has increased when Bi3+ ions are added into the glass network.
A monotonic evolution of molar volume depending on the composition is observed. The increase in the molar volume of the samples may be due to the formation of new Bi-O bounds and the non-bridging oxygen (NBO) which could expand the structure of the network of BaO-GeO2-Ga2O3 glass. Similar behaviors were observed in other glass system [16].
3.2 Refractive index
The refractive index (n) is considered to be one of the most important optical parameters of materials which is related to the electronic polarizability of ions and the local field of the material [17].
Figure 2 shows the refractive index dispersion of the 40GeO2-20Ga2O3-40BaO-xBi2O3 (x=0,1,2,4,6,8 mol %) glasses as a function of wavelength and Bi2O3 concentration. The values reported here were obtained for the transverse-electric (TE) mode of the incident laser radiation while no significant difference was observed in the transverse-magnetic (TM) mode, confirming the absence of birefringence, as expected in isotropic glass materials.
Fig. 2. Measured linear refractive index (n) of the undoped and BGG-Bi2O3 glasses as a function of wavelength. The influence of the melting atmosphere is presented and shows no impact on the refractive index.
First, one can observe in Fig. 2(a) the decrease of the refractive index with increasing the wavelength for each glass sample, showing thus their respective chromatic dispersion. Moreover, all glasses have refractive indices >1.75 in the visible region. The BGG-Bi2O3 glasses show high refractive index that can be explained by the high refractive index of individual Bi2O3 ∼2.5) [18]. We note that with increasing concentration of Bi2O3 (oxidation state 3+), the refractive index increases (Fig. 2(b)). This can be explained by the fact that the addition of Bi2O3 acts like a glass network modifier provide non-bridging oxygen [NBO] to the glass structure. From a structural point of view, the Bi3+ ion, which has a high polarity, can break the bridging oxygen [BO] to non-bridging oxygen [NBO]. Therefore, the observed increase in glass refractive index can be ascribed to the higher polarity of non-bridging oxygen (NBO). [14]. Figure 2(b) shows that the refractive is not influenced severely by the different melting atmosphere used during the synthesis process.
3.3 IR absorption spectra
To determine the position of the IR edge and the OH content of the glasses, we measured the absorption spectra in the mid-infrared in the 2–7 µm range.
Figure 3(a) presents the absorption spectra of BGG-Bi2O3 glasses from 2 to 7 µm. These glasses are transparent from 0.4 µm up to 5.5 µm, much higher than those of silicate glass (2 µm) and fluorophosphate glass (4.5 µm) [19] showing great potential for the development of new tunable laser sources in the IR region. However, a broad absorption band at 3.1 µm is present on the spectra which is assigned to stretching vibration of free OH groups. It can be seen that the OH− content decreases gradually with an increase in Bi2O3 concentration up to 2% and remains almost constant above 4% Bi2O3.The OH- impurities in glass are undesirable as they play a role of quenching centers in the energy transfer process [20,21]. One of the strategies to reduce the amount of hydroxyl groups is to add fluoride into IR glasses besides working under a controlled atmosphere in a glove box [22].
Fig. 3. Absorption spectrum for BGG-Bi2O3 glasses prepared under O2N2 atmosphere. The main peak at 3.1 µm is caused by OH- absorption. b) Evolution of OH- absorption under different melting atmospheres. As expected, OH- bonds are more important in the most reducing atmosphere (H2N2), and less present in the oxidizing atmosphere (O2N2).
Figure 3(b) shows the evolution of OH- absorption among the samples. All the absorption curves were normalized in accordance to each samples’ thickness. The addition of Bi in the BGG glass matrix decreases the OH- absorption for glass samples up to 2 at% of Bi. Above this value, the OH absorption remains almost stable. Under oxidizing atmosphere, it was reported that the hydroxyl group tends to be lower [23,24]. As expected, the OH- bonds are least present under O2N2 atm. ((oxidizing) and are far more important under H2N2 atm. (reducing).
3.4 UV-Vis absorption spectra and optical bandgap
Figures 4(a)–4(c) show absorption spectra of BGG-Bi2O3 glasses prepared at different melting atmospheres the impact of Bi2O3 on the UV-Vis spectrum of the samples from 300 nm to 1000 nm.As shown in Figs. 4(a)–4(c) a broad absorption band at 470 nm appears as Bi concentration reaches 4% for all melting atmosphere. As the bismuth concentration increases, the position of this peak remains unchanged, but the absorption coefficient (peak area) gradually increases from 1 to 10 cm−1, implying increasing concentration of the active center(s).
Fig. 4. a) UV–visible absorption spectra in BGG- Bi2O3 glasses, (b) relationship between (αhω)1∕2 and photon energy (hω) and (c) evolution of the band-gap energy Eg with bismuth oxide as a dopant.
The band at 470 nm maybe explained based on two hypothesis: one is to assign to Bi related species reduced from Bi3+ ions to lower valence state, namely Bi2+, Bi+, Bi° and Bi colloids [25] by the auto-thermo reduction [26] which can be a particular species, or a mixture of those species. On the other hand, some researchers have interpreted the 470 nm band as a result of the direct formation of Bi metallic [27,28] which could be associate to the broad surface plasmon resonance (SPR) due to the formation of the Bi° NPs. Khonthon et al. [29] have also reported the broadband due to SPR of Bi° NPs at 470 nm. In that case, the direct reduction of Bi2O3 to Bi° is supposed to occurs through the following thermal dissociation reaction [30]:
Equation (2) shows that the equilibrium reaction mechanism is reached when oxygen is released and the metallic Bi° is formed as the melting temperature increases. This equilibrium depends on the melting temperature and on the oxygen partial pressure in the glass melt. However, in our case, the melting temperature is kept the same during all the synthesis in BGG-Bi2O3 glasses prepared. For this reason, it is more plausible to consider a mix of other oxidation states which could originate the broad absorption band at ∼470 nm. Further assignments will be discussed later where the exact contribution of bismuth chemical oxidation states will be analyzed through XPS measurements.
The optical band gap energy (Eg) from an optical absorption spectrum employ the Tauc’s relationship:
where h is Planck’s constant, ν is the frequency of light, and Eg is the bandgap energy. The value of the exponent n is related to the electronic nature of the bandgap in our case is 2, corresponding to indirect allowed transitions. The bandgap energy, Eg, is typically determined through inspection of (αhν)1/n vs hν plots (also called Tauc plots) [31], where the linear trend given by Eq. (3) is modeled as the tangent of the Tauc’s plot near the point of maximum slope if the Tauc’s plot contains a sufficiently linear region [32]. This linear tangent is then extrapolated to the point where (αhν)1/n is 0. The values of bandgap energy (Eg) obtained from the plots are show in Fig. 4(b).It is seen from Fig. 4(b) that with the addition of Bi2O3, the values of Eg decrease from 4.03 eV to 3.03 eV for 0% mol. and 8% mol. of Bi2O3, respectively, regardless of the atmosphere used. Since the absorption bandgap is due to electronic transitions from the conduction band to the valence band of the material, it can be inferred that Bi2O3 incorporation progressively decreases the bandgap energy between these bands. This can be understood in terms of the structural changes that are taking place in the glass systems due to more stable configuration play by bismuth pyramidal units [33]. Since BiO bond length (≅2.01 Å) is greater than that of Ge-O bond (≅1.84 Å) [34], the addition of Bi2O3 increases the average bond length which narrows the optical band gap. The redshift in the cut-off with the Bi2O3 concentration was also observed for other heavy-metal glasses, for example, lead gallobismuttes glasses [35].
3.5 Raman spectra and structural characterization
Figure 5 presents the deconvoluted Raman spectra of BGG-Bi2O3 glasses samples to follow the structural changes in the glasses as well the representation of the main vibrational modes.
Fig. 5. (a) Deconvolution of Raman spectra of BGG- Bi2O3 glasses samples prepared under O2N2 showing the glass structure evolution as the amount of bismuth increase from 0% to 8% mol. For the sake of clarity, spectra have been vertically displaced. (b) Basic structural units of BGG glasses, (c) basic structural units of bismuth, (d) scheme of the 4-membered rings tetrahedral units, containing non-bridging oxygen (NBO) (e) Schema of structural units forming Bi2O3-BGG glasses. Germaniun (
The Raman spectra of undoped BBG glasses consist of mainly four main bands at 270, 335, 500 cm-1 and 800 cm-1. The main band in the range centered at 500 cm-1 is assigned to symmetric stretching of bond vibrations of type X-O-X (with X = Ga or Ge), in a polymerized network. According to McKeown et al. [36], which focuses on the study on the germanium oxide-rich system BaO-GeO2-Ga2O3, these bonds can be in a polymerized network of [GeO4] and [GaO4] tetrahedra which are located in 4-membered rings. Deconvolution of the broad band centered at around 800 cm-1 resulted in the fitting of three peaks centered at 730, 800, 810 cm-1 which are ascribed to the symmetric vibration of [Ge(Ga)O4] tetrahedron with one non-bridging oxygen (GeO- bond) derived from the breakdown of tetrahedral [GeO4] units [37]. In the low-frequency region, the two weak bands at ∼270 cm-1 and 335 cm-1 are related to the bending modes of Ge-O-Ge bonds derived from stretching vibrations of GeO6 connected units [38].
As is seen, the main band at 500 cm-1 became broader and is progressively shift at lower energy reaching 460 cm-1 as the content of Bi2O3 increases reflecting changes in the GeO2 network. In Raman spectra, shifting of peaks towards higher wavenumber is related to the chemical bond length of molecules. As the length of Bi–O bonds is higher than that of Ge–O, we suppose that this band corresponds to the Bi–O–Ge linkage vibration. It means that with the addition of Bi2O3, the number of non-bridging oxygen atoms (O-) increases inducing an increase of the network disorder. This suggestion is consistent with the observed decrease in the glass-transition temperature. Simultaneously, the bands at 800 cm-1 [GeO-], 270 cm-1 and 335 cm-1 [GeO6] decrease with the increase of the Bi2O3 concentration.
As Bi2O3 concentration increases up to 4%, one remark that the structural arrangement of the [GeO4] tetrahedra is strongly distorted by the increases of the GeO2 3-membered rings and the appearance of a new broad band at 420 cm-1 is observed. Lin [39] and Lines et al. [40] have reported classification of the bands in heavy metal-doped germanate glasses. Thus, the bridging anion bands (BA) which appear in the region 300–500 cm−1 are due to the symmetric-stretch anion motion in cation (Bi)–anion (O)–cation (Bi) configurations and the non-bridging anion bands (NBA) which appear at higher wavenumbers are due to asymmetric stretch cation (Bi)–anion(O)–cation (Bi) motions. Bi3+ being a cation which has strong glass-forming tendency support both bridging (BA) and non-bridging anion (NBA) modes, but favor BA due to angularly constrained anion bridges. According to their data, the band that appears at 420 cm−1 can be attributed to the stretch Bi–O–Bi vibration of the distorted [BiO6] octahedral units. We can also notice that a new band at 140 cm-1 appears above 4% Bi2O3 concentration. The oscillations in the region ω < 150 cm-1 in the Raman spectrum of α- Bi2O3 are related to the external oscillations of the Bi atom. It should be mentioned that the Raman bands that arise in the spectral region between 50 and 200 cm-1 are usually related to vibrations involving motions of the Bi3+ cations or Bi metal vibrations [41,42]. Also, a theoretical assignment of this band points out that vibrations could be assigned to heavy metal (Bi) [43]. However, some authors assigned the band around 135 cm-1 to the existence of [BiO3] and [BiO6] polyhedra in the glass structure [44,45].
On the basis of studies on BGG glasses [36,46,47], we know that the glass matrix structure is preponderantly formed of germania tetrahedra bonded either to other GeO4 units or GaO4 units. Ba2+ ion acts as a compensator for the excess negative charge on two GaO4 tetrahedra (Fig. 5(d)). On doping of Bi metal ions in the host glass, the BGG glass matrix is highly influenced.
Although Bi2O3 is not a classical glass former due to high polarizability and small field strengths of ions, in the presence of other oxides such as GeO2, it assists to form a glass network made up of [BiO3] and [BiO6] pyramids [48]. Therefore, the Bi2O3 may enter in the glass network as modifier as well as former [49] which further depends on the concentration of the Bi2O3 itself. In our case, when the Bi2O3 concentration is lower than 4%, Bi2O3 play a role of the glass network modifier [BiO3] entering into the glasses by sharing some of its oxygen with germanium which is responsible for the progressive decrease of NBOs. As suggested by Sokolov et al., Bi atoms occupying, can form substitutional centers of two types, threefold coordinated Bi atoms bonded by three bridging O atoms with Ge atoms, and fourfold coordinated Bi atoms in bi-pyramidal configuration bounded by four bridging O atoms Ge atoms [50].
As the content of Bi2O3 increases up to 4%, pyramidal [BiO3] units gradually reach saturation and the network becomes less connected which implies partial 4-GeO4-membered rings have transformed into the three-membered rings (Fig. 5(e)). Consequently, the Raman band at 140 cm-1 increases, some kinds of a small network consisting of Bi° atom and Bi+ ion started to occur as interstitial centers in 3-member ring sites in BGG glass matrix. This could explain the formation of bismuth metallic in BGG glasses as well the coexistence of Bi+ ion.
3.6 Photoluminescence (PL) measurements
The typical emission spectra of Bi-doped glasses, as reported in the references [51,52,53,54] overlapped from 1000 to 1600 nm. Figures 6(a)–6(c) presents NIR PL spectra of BGG- Bi2O3 glasses (0 to 8 mol. % Bi) excited by 808 nm and melted in dry air (O2N2), N2, and H2N2 atmospheres, respectively.
Fig. 6. a)-c) Emission spectra of BGG-Bi2O3 glass samples with various bismuth concentrations (0, 1, 2, 4, 6, 8 mol. %) upon 800 nm excitation; (d) Full width half max of the 1300 nm emission. (e) Dependence of the intensity on Bi2O3 concentration and melting atmosphere.
We confirmed that glass samples without Bi doping show no apparent visible and NIR emission. The BGG-Bi2O3 glasses exhibit the typical broadband emission which covers the spectral range from 1000 to 1600 nm, with a maximum value at 1300 nm. Figure 6(b) shows the full-width at half-maximum (FWHM) of the emission bands. The FWHM of Bi NIR emission is broadened from 309 nm to 327 nm for O2N2 atmosphere with different Bi2O3 concentrations but is still much wider than those of rare-earth ions. Cao [55] suggests that this broadening is related to a reduction of high valence Bi ions to low valence Bi. Also, it can be seen that the FWHM increases with the increase of Bi2O3 and reaches a maximum FWHM bandwidth of 316 nm when Bi2O3 concentration is equal to 6.0 mol% under O2N2 atmosphere. Bearing in mind that the addition of bismuth in BGG glass matrix lead to change in the glass structure, it will also induce great variation in Bi NIR emission. Thus, the great change in the FWHM may originate from the change in glass structure due to the high sensitivity of Bi ions on crystal field or the conversion of Bi active centers from the high valence to low valence states [56].
Figure 6(e) shows the dependence of emission intensity on bismuth concentration and melting atmosphere. It is noticed that, despite of the melting atmosphere the PL, intensity of BGG-Bi2O3 glasses increases initially until the Bi2O3 content reaches up to x 6% Bi2O3, and decreases with further increasing the Bi2O3 concentration. There are two possible reasons for the decline of Bi3+ emission: one is the typical concentration quenching effect caused by cross-relaxation between the Bi3+ ions that are closer together, and the other one is the increasing extent of the conversion of Bi3+ into low valent Bi species in the glass matrix, which matches reasonably with the increased absorption around 470 nm observed in Fig. 3(a). Moreover, the influence of the melting atmosphere on the PL intensity is observed. Compared with the 6 mol.% Bi2O3-BGG glass melted in O2N2, when melted in 95%N2/5%H2, the intensity of the peak at around 1300 nm increases and becomes dominant. The BGG-Bi2O3 glass melted in N2 atmosphere presents an intermediary behavior.
It is well known that bismuth has several valence states existing simultaneously in glasses materials. The various emission centers in the bismuth glass arise due to various ionic species of bismuth (e.g., Bi3+, Bi2+ and Bi+) as shown earlier. In Bi2O3–BGG glasses, these low valence state bismuth ions could be formed through the thermo-atmosphere decomposition of Bi2O3 during the glass melting process forming the intermediate valence states (see Eq. (2)).
However, there is still no direct evidence to determine the exact valence state of bismuth responsible for the NIR emission, due to some artifacts caused by interaction of Bi ions with traditional instrumental approaches. In fact, electron energy loss spectroscopy (EELS), X-ray absorption near edge structure (XANES) or X-ray photoelectron spectroscopy(XPS), can change the valence state of bismuth species when irradiated with high energy beams such as electrons, X-ray, or fs laser leading a controversy regarding the mechanism of NIR luminescence [27,57]. Rao et al. [58] demonstrate that NIR luminescence in Bi-doped glasses is observed because of the reduction of Bi3+ ions to a lower oxidation state. On the other hand, some researchers agree that the valence should be lower than +2 [9,10,59].
3.7 X-ray photoelectron spectroscopy (XPS)
To further reveal the bismuth species present in BGG glasses, we performed XPS measurements which give direct information on the charge state of atoms. As an example, the XPS spectra of Bi 4f in 40GeO2-20Ga2O3-20BaO–1Bi2O3 glass prepared under O2N2 atmosphere is shown in Fig. 7.
Fig. 7. a) Example of XPS data from BGG-Bi2O3 samples. The two bands with two shoulders at specific binding energies of 157.2 eV, 159.4 eV, 162.6 eV, and 164.7 eV indicate there are only two oxidation states are present within the glass, namely Bi3+ and Bi°. b) Evolution of both oxidation states with increasing bismuth oxide quantity. Bi3+ content increases almost linearly, and Bi° content stays relatively constant.
The BGG-Bi2O3 samples exhibit two characteristic peaks at 159.4 and164.7 eV, corresponding to the core lines of Bi 4f7/2 and 4f5/2 of Bi3+, respectively, which reveals that 3+ is the dominant valence state of Bi in these samples. Moreover, the presence of two shoulders at 157.2 e V and 162.6 eV are assigned to Bi metal (Bi°) and suggest that the Bi was in doublet oxidation states [60]. In order to ensure the validity of the measurements, a gap between each pair corresponding to either Bi° or Bi3+, the gap must be 5.3 eV [61,61,62]. In all the BGG-Bi2O3 glass samples prepared, this condition was satisfied. The graphs also show that there are no other oxidation state present in the glass’s matrix, which means there is no trace of Bi2+ or Bi+.In order to calculate the proportion of each oxidation state, the peak area is divided by the sum of each area of a set (4f 7/2 or 4f 5/2). This is valid for both sets, so the values calculated for a set must be identical to the other. Here again, this condition is satisfied with the BGG-Bi2O3 glass samples prepared. Hence, the concentration of Bi° and Bi3+ for each sample can be calculated and their evolution is represented in Fig. 7(b).
The bismuth ion concentration per unit volume can be calculated using the ratio of the number of atoms of bismuth in a volume. The number of atoms of bismuth is calculated by Eq. (4).
where mBi is the mass of bismuth, MBi is the molar mass of bismuth, and NA is the Avogadro number.The mass of bismuth is given by Eq. (5),
The molar volume (Vm) of glass samples is calculated using the relative molecular mass (M) and density (ρ) Combining molar volume (Vm) relation and Eqs. (4) and (5), the concentration of bismuth ion in the glass can be calculated following Eq. (6).
The mass and molar mass of bismuth are no longer needed, only those of bismuth oxide. Since XPS also establishes the proportions in which oxidation states occur, multiply by the proportions will lead to the concentration of an ion per unit volume.
Figure 7(b) shows the concentrations of each oxidation state as a function of the amount of bismuth added. Note that, no correlation can be found concerning to the addition of Bi2O3 to BGG glass matrix and metallic bismuth concentration. In contrast, the calculated concentration of Bi3+ of these glasses shows a linear increase with the Bi2O3 content and it is almost identical for all three melting atmospheres used.
Figure 8 shows the relationship between PL intensity of the band at 1300 nm with the Bi2O3 concentration in BGG glasses.
Fig. 8. Photoluminescence (PL) intensity evolution of BGG-Bi2O3 samples under different melting atmospheres. The oxidizing atm. O2N2 provides the highest intensity.
It is interesting to note that Fig. 8 follows the same tendency as Fig. 6(e), ie, PL intensity increases with an increase in Bi3+ concentration from x=1% to x=6% mol. Thereafter, it decreases with further increase in the Bi3+ concentration up to 8% at/cm3. This decrease in PL intensity could be related to concentration quenching. In general, the Bi3+ ions in the glass network have to increase with increase in Bi2O3 concentration, but the decrease in PL intensity (up 6% mol Bi2O3) indicates the decrease of Bi3+ ions at higher concentrations. This might be due to either reduction or oxidation of Bi3+ ions in the glass matrix. This argument can be clarified by a considerable increase in the intensity of optical bands correspond to Bi° particles. Hence, at a higher concentration of Bi2O3 reduction of Bi3+ ions into lower oxidation states could also take place.
Moreover, PL intensity depends on the melting atmosphere. One of the reasons is assigned to OHbonds which can attenuate the PL emission. Indeed, UV-Vis measurements show that OH bonds are more pronounced under H2N2 melting atmosphere whereas is less important under O2N2 melting atmosphere (see Fig. 3(b)). It would still be beneficial to reduce the hydroxyl bonds to see if they have a direct impact on fluorescence intensity.
4. Conclusion
Bismuth-doped heavy metal oxide glasses with varying Bi2O3 concentrations were successfully prepared in reducing atmosphere (H2N2 (5%H2, 95% N2), and N2) and oxidizing atmosphere (O2N2) to be used as promising materials for optical amplifier. Optical, thermal and structural properties were investigated in 40GeO2-20Ga2O3-40BaO (BGG) glass to understand the vitreous network and the dependence of the optical properties as a function of the relative amount of Bi2O3 in the glass composition and melting atmosphere. The melting atmosphere does no influence the main physical and structural characteristics of the BGG- Bi2O3 glasses.
DSC measurements revealed the decrease in Tg on the addition of Bi2O3 due to the breaking-up of the glass network structure which has been further confirmed by Raman measurements where the 4-GeO4-membered rings have transformed into the three-membered rings. This transformation confirms the role of Bi2O3 as a strong structural modifier. Besides, the increase of Bi–O–Bi bonds in the samples containing more than 4% mol % of Bi2O3 with the vibrations involving Bi3+ or Bi metal vibrations, is reflected in the appearance of a new band at low Raman shift.
The density and molar volume of the glass sample increase since the atomic mass of bismuth ions is higher compared to the other ions on the glass matrix, and the atomic radius of bismuth is also greater than that of germanium ions. Additionally, the refractive index increases due to the increase of polarity of the Bi3+ ion content in BGG based glasses. NIR PL of these samples was measured as a function of the melting atmosphere. It was found that the PL intensity reaches a maximum in the oxidizing atmosphere, and the lowest in the strongest reducing atmosphere (H2N2). The FWHM of the sample with the highest intensity in the O2N2 atmosphere was measured at 316 nm. By combining the information obtained from PL measurements with that of XPS study, it was found that increasing the bismuth concentration in BGG glasses increase the concentration of Bi3+ which acts as NIR emission centers.
The reported observations concerning their structure and chemical state allow the conclusion that Bi3+ ions are responsible for photoluminescence in the NIR spectral range in BGG glasses. The broadband NIR emission shown in BGG-Bi2O3 glasses and its high transparency over a wide wavelength range is very promising for their application in ultra-broadband fiber amplifiers for telecommunications purposes.
Funding
Ministère du Développement Économique, de l’Innovation et de l’Exportation; Canada Excellence Research Chairs, Government of Canada; Natural Sciences and Engineering Research Council of Canada; Canada Foundation for Innovation.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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), Gallium (
), Barium (
), bismuth (
) and oxygen n (
).