Abstract
Physical, optical, and luminescence properties of Nd3+ ions in bismuth barium borate glass system were studied. The glasses prepared by a melt quenching method were doped at various Nd2O3 concentrations in compositions (40-x)B2O3 : 40Bi2O3 : 20BaO : xNd2O3 (where x = 0.00, 0.50, 1.00, 1.50, 2.00, and 2.50 in mol%). Luminescence properties of the glasses were studied under two excitations of 585 and 750 nm for downconversion. From both excitations, the results show emission bands in NIR region corresponding to the transitions between 4F3/2 → 4I9/2 (900 nm), 4F3/2 → 4I11/2 (1,060 nm), and 4F3/2 → 4I13/2 (1,345 nm). The luminescence intensity obtained with 585 nm excitation was stronger than 750 nm, with the strongest NIR emission at 1,060 nm. The upconversion emission spectrum exhibits strong fluorescence bands in the UV region at 394 nm ( nm). The processes are associated with excited state absorption (ESA) from 4F3/2 level to 4D3/2 level and it is the radiative decay from the 4D3/2 to ground levels (4D3/2 → 4I13/2) which are responsible for the emission at 394 nm.
1. Introduction
The first demonstration of laser action in a neodymium doped glass was done by Snitzer in 1961 and since then considerable progress has been made in evaluating the effects of amorphous host materials on the lasing properties of various rare earth ions. A large variety of laser glasses doped with Nd3+ ions have been investigated with the purpose of generating efficient broadband laser emission around 1060 nm [1–3]. Recently, glass-ceramics containing neodymium oxides have been found in applications for several different purposes. First of such application has been Nd2O3 in special glasses for halogen lamps to absorb ultraviolet rays, harmful emissions to human. In application as refractory glasses, the glasses containing Nd2O3 can have high hardness and excellent chemical durability. Lastly, neodymium contained in glass can be used as a band rejection filter for image display devices, owing to absorption originating in the intertransition within the 4f shell of the Nd3+ ion [4, 5].
In addition to the rare-earth ion characteristics, the glass host matrices also play a fundamental role in determining the performance of photonic devices since the stimulated emission characteristics of a trivalent rare-earth ion depend on the surrounding host matrix in which the ions are incorporated. The surrounding ligand field can have a considerable influence on the optical absorption cross-section, stimulated emission cross-section and fluorescence decay, and the quantum efficiency of the system [5]. Bi2O3 containing glass possesses higher refractive index and exhibits high optical basicity, large polarizability, and large nonlinear optical susceptibility [6]. It was also reported that glasses containing Bi2O3 have been developed for nuclear engineering applications because they accomplish the double task of allowing visibility while absorbing radiations like gamma rays and neutrons [7]. Bi2O3 cannot be considered as actual network former due to small field strength of Bi3+ ion. However, in the presence of conventional glass-forming cations such as P5+, Si4+, and B3+ it may have this property [8, 9]. Borate glasses are structurally more intricate as compared to silicate or phosphate glasses due to two types of coordination of boron atoms with oxygen (3 and 4) and the structure of vitreous B2O3 consisting of a random network of boroxyl rings and BO3 triangles connected by B–O–B linkages. In addition, the additional modifier oxide causes a progressive change of some BO3 triangles to BO4 tetrahedra, resulting in the formation of various cyclic units including diborate, triborate, tetraborate, or pentaborate groups [9]. The presence of two network forming oxides, the classical B2O3 and the conditional Bi2O3 glass former, the possible participation in the glass structure of both boron and bismuth ions with more than one stable coordination (and, thus, the presence of several structural units namely, BO3, BO4, BiO3, and BiO6), the capability of the bismuth polyhedra and of the borate structural groups to form independent interconnected networks [4]. Over the last several years, bismuth barium borate glasses have been also useful for variety of optical applications such as radiation shielding window, gamma rays shielding materials, and scintillation counters [10, 11].
Many literatures have been studied on Bi2O3–BaO–B2O3 glass system and its related properties [12–16]. However, study on rare earth ion doped to this glass structure and its luminescence properties are very lacking. Only one literature doped with Eu3+ has been studied by Egorysheva et al., [17] and good luminescence property was obtained. So, the studies of other rare earth ions doped in this glass are needed. In this present work has been to study the effect of Nd3+ content on physical, optical, and luminescence properties from excitations and their responding emission spectra of the glass system. The variety of the emission from Nd3+ ions in the glasses system could be potential applications to be developed for various fields such as new light sources, display devices, UV-sensors, and interestingly tunable visible lasers. This work is the first report on luminescence properties of Nd3+ on Bi2O3–BaO–B2O3 glass system.
2. Experimental Detail
2.1. Glass Preparation
The glasses with their chemical compositions (40-x)B2O3 : 40Bi2O3 : 20BaO : xNd2O3 (where x is the mol% of Nd2O3 content in the glass systems which varied between 0.0, 0.5, 1.0, 1.5, 2.0, and 2.5) were prepared by the normal melt-quenching technique and the glass compositions in mole percent for different Nd2O3 dopings are given in Table 1. For each batch composition, 20 g of homogeneous mixture of starting chemicals was melted in high purity alumina crucibles by an electric furnace at a temperature of 1,100°C for 3 hours. The melts were quenched by pouring into preheated stainless steel molds. The glasses were then annealed at 500°C for about 3 hours to remove thermal strains. Finally, the as-prepared glass samples were cut and then finely polished to a dimension of 1.0 cm × 1.5 cm × 0.3 cm.
2.2. Measurements
The density () at room temperature of the glass samples was determined by a method based on Archimedes’ method principle. The weights of the prepared glass samples were measured in air and xylene immersion by using a 4-digit sensitive microbalance (AND, HR-200). The molar volume () was calculated using the relation , where is the total molecular weight of the multicomponent glass system. The optical absorption spectra of the prepared glass samples in the UV-VIS-NIR region from 300 to 1,800 nm were recorded at room temperature using UV-VIS-NIR spectrophotometer (UV-3600, Shimadzu). In the NIR luminescence measurement, the Nd3+ doped in glass sample was excited by two wavelengths at 585 and 750 nm. The emission spectra were recorded at room temperature using a Quanta Master 3 luminescence spectrometer from Photon Technology International (PTI). For the upconversion luminescence studies, the Nd3+ doped glass samples were excited with a 591 nm wavelength. The emission spectra were recorded at room temperature using a spectrofluorophotometer (Shimadzu RF-5301PC, Japan) with a 150 watts Xenon lamp as a light source.
3. Results and Discussions
3.1. Physical Properties
The density and molar volume of the glasses determined with the described methods are shown in Figure 1. As can be seen from the figure, by adding of Nd2O3 into the Bi2O3–BaO–B2O3 glass network, the density of the glass is increased with the increasing of Nd2O3 content. This indicates the increase of the molecular weight by the replacement of B2O3 with a heavier Nd2O3 oxide in the glass and this as expected increases the density of these glasses. However, the variation of density is nonlinearly increasing with composition of Nd2O3. This is may be due to different loss rates of Bi2O3 at high melting temperature process (melting point of Bi2O3 is 817°C [18].) The molar volume of the glass systems, however, shows a decrease trend with increasing of Nd2O3 content. The molar volumes of glasses were decreased with increasing of Nd3+ ions concentration, reflecting increase compactness of the glass structure with higher concentration of Nd3+, that is, the decrease of average atomic separation. The variations of density and molar volume with concentration of Nd2O3 are presented in Figure 1.
3.2. Absorption Spectra
The absorption spectra of Nd3+ doped bismuth barium borate glasses in the range of 300–1,800 nm at room temperature are shown in Figure 2. It is clearly observed that the height of the absorption bands increase with the increase of Nd2O3 concentration. Seven absorption bands were observed and assigned to transitions from the 4I9/2 ground state to 4F3/2, 4F5/2 + 4H9/2, 4F7/2 + 4S3/2, 4F9/2, 2H11/2, 4G7/2, and 4G9/2 levels [19, 20]. The absorptions in the UV-visible and NIR regions were slightly increased with increasing composition of Nd3+ in the glass system.
3.3. Emission Spectra
3.3.1. Downconversion Spectra
The downconversion emission spectra of Nd3+ doped bismuth barium borate glass recorded when excited by 585 and 750 nm wavelengths at room temperature are shown in Figure 3. The emission bands in NIR region were assigned to the corresponding transitions: 4F3/2 → 4I9/2 (900 nm), 4F3/2 → 4I11/2 (1,060 nm), and 4F3/2 → 4I13/2 (1,345 nm) [19, 20]. The emission for the 4F3/2 → 4I9/2 transition was not observable, probably due to its low intensity. Owing to its importance as a high-power and high-energy laser line, the 4F3/2 → 4I11/2 (1,060 nm) emission is probably the most thoroughly characterized transition for both glass and crystalline hosts. All the luminescence peaks (900, 1,060, and 1,345 nm) obtained when excited with the 585 nm have been shown with stronger emissions than when excited by the 750 nm wavelength. The luminescence peak intensity was decrease with higher Nd2O3 concentration with the strongest NIR emission intensity is at 1,060 nm and then 1,345 and 900 nm respectively.
(a)
(b)
3.3.2. Upconversion Spectra
Figure 4 shows upconversion emission spectra of bismuth barium borate glass doped Nd3+ ions under 591 nm excitation. The emission spectra exhibit strong fluorescence bands in the UV region at 394 nm. The upconversion process may be associated with the excited state absorption (ESA) from 4F3/2 to 4D3/2 levels. Figure 5 shows the possible upconversion mechanisms. In the first step, Nd3+ in the ground level (4I9/2) was excited directly by absorbing the excitation wavelength 591 nm photons to the excited 4G5/2 level and then these ions relax nonradiatively to the metastable 4F3/2 level. The cross-relaxation process between the 4G5/2 and 4F3/2 levels causes the population of the 4D3/2 level by absorbing a second photon. Finally, the excited Nd3+ at 4D3/2 state can decay radiatively to ground levels and cause the upconversion emission at 394 nm (4D3/2 → 4I13/2 or 2P3/2 → 4I11/2). Through a nonradiative process Nd3+ ions at the 4D3/2 level can relax to the lower (2P3/2) levels due to a small energy gap between the two levels. This nonradiative decay has the possibility to cause upconversion emissions from the levels at 467 nm (2P3/2 → 4I15/2), but this was not observed in these glass structures.
4. Conclusions
The physical and luminescence properties of Nd3+ doped B2O3–Bi2O3–BaO–Nd2O3 glasses prepared by a conventional melt-quenching method were studied for Nd2O3 compositions. Density of glasses was found to increase with increasing of Nd2O3 composition. The behavior of molar volume mainly depends on the density of glasses, but it follows an opposite trend with density. The molar volume of glasses was decreased with increasing of Nd2O3 concentration, reflecting more compact of the glass structure. The optical absorption spectra in UV-VIS-NIR region were observed with seven absorption peaks assigned to 4I9/2 ground state transition to 4F3/2, 4F5/2 + 4H9/2, 4F7/2 + 4S3/2, 4F9/2, 2H11/2, 4G7/2, and 4G9/2 levels. Downconversion of Nd3+ in the glasses was studied under two excitations at 585 and 750 nm. The result shows luminescence peaks at 900, 1,060, and 1,345 nm. Stronger luminescence peak intensity was observed when excited by 585 nm in comparison to 750 nm excitation wavelength. Upconversion observed with 591 nm excitation showed emission spectra at 394 nm. The mechanisms for the excited state absorption (ESA) were purposed as the possible emission pathways from 4D3/2 → 4I13/2 or 2P3/2 → 4I11/2 transitions. These results showed that the Nd3+ doped B2O3–Bi2O3–BaO glass system can be used in laser or optical devices design.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
P. Limsuwan would like to thank King Mongkut’s University of Technology Thonburi (KMUTT) for partially funding the project under the National Research University. J. Kaewkhao, N. Chanthima, and S. Ruengsri would like to thank National Research Council of Thailand (NRCT) for a partial financial support. R. Ruamnikhom would like to thank Institute for the Promotion of Teaching Science and Technology (IPST) for his Ph.D. grant.