Comprehensive comparison on optical properties of samarium oxide (micro/nano) particles doped tellurite glass for optoelectronics applications

. Rare-earth oxides microparticles doped tellurite-based glass have been studied extensively to improve the capability of optoelectronic devices. We report a detailed comparison between two sets of glass series containing samarium microparticles and nanoparticles denoted as ZBTSm-MPs and ZBTSm-NPs respectively. The two sets of glass have been successfully fabricated via melt-quenching technique with chemical formula {[(TeO 2 ) 0.70 (B 2 O 3 ) 0.30 ] 0.7 (ZnO) 0.3 } 1-y (Sm 2 O 3 (MPs/NPs)) y with y = 0.005, 0.01, 0.02, 0.03, 0.04 and 0.05 mol fraction. The TEM analysis confirmed the existence and formation of nanoparticles in ZBTSm-NPs glasses. The density of ZBTSm-NPs glasses was found higher than ZBTSm-MPs glasses due to the distributions of nano-scale particles in tellurite glass network. There was a linear trend of increment in the refractive index in both sets of glass series along with the concentrations of dopants. The refractive index of ZBTSm-NPs glasses was found higher than ZBTSm-MPs glasses due to the shift in compactness of glass structure with nano-scale particles. In comparison, the absorption peaks of ZBTSm-MPs glasses were greater than ZBTSm-NPs glasses which were mainly due to the restriction of electrons mobility in glass network with nano-scale particles. The optical band gap energy in ZBTSm-NPs glasses was found greater than ZBTSm-MPs glasses which correspond to the widening of forbidden gap with nano-scale particles. The polarizability of ZBTSm-NPs and ZBTSm-MPs was found in non-linear trend along with dopant concentrations. Based on these findings, the improvement of optical properties have been made by introducing samarium oxide nanoparticles in tellurite glass which is beneficial for optoelectronic devices.


Introduction
Among chalcogenide oxide groups, tellurite oxide, TeO2 is the most stable oxide. The effectiveness and usefulness of tellurite oxide for optoelectronics applications have motivated researchers around the world [1 -4]. The recent study by Peng et al. proposed that tellurite oxide has been shown as a future material for a visible-band conversion laser fiber [5].
Furthermore, tellurite oxide is often transferred to many other glass oxides that support multiple compositions. Fares et al., reported that tellurite oxide, TeO2 consists of a lone pair electron at TeO4 equatorial positions [6]. This occurrence will lead to the limitation of structural rearrangement and the formation of glass. As a result, pure tellurite oxide, TeO2 glass is unstable and tend to crystallize [7]. However, in order to stabilize the formation of tellurite glass, it is necessary to incorporate modifiers such as alkali, alkali earth and metal oxides in tellurite glass network [8].
The best glass additive to be incorporated in tellurite glass network is borate oxide, B2O3. The hygroscopic characteristic of a glass system may be reduced by combining the tellurite oxide and borate oxide [9]. Moreover, borotellurite glasses have broader infrared transmittance which is beneficial for optoelectronic devices. Meera et al., observed that the borate glasses were made up of two tetrahedral (BO4) and trigonal (BO3) units [10]. The blending of these units would establish groupings of diborate, triborate, tetraborate and pentaborate [11]. In addition, Manara et al., suggested that the inclusion of borate oxide in tellurite glass network may contribute to a stable structural unit as applied in the borosilicate glass system [12]. Tellurite glasses with small amount of borate oxide are composed of TeO4, BO4, and BO3 groups. Such groups may result in a stable structure of tellurite glass and hence, improve its optical properties.
The improvement of mechanical strength, chemical resistance and the thermal expansion of the glass system can be made by introducing the Zinc oxide, ZnO in borotellurite glass network.
Khattak and Salim stated that zinc oxide transforms TeO4 (trigonal bipyramidal) to TeO3+1 polyhedra and TeO3 (trigonal bipyramidal) coordination in tellurite glass network [13]. The lone pair in TeO4 (trigonal bipyramidal) restricts the free movement of trigonal bipyramidal during cooling and melting processes. Zinc borotellurite glasses are stable in structure and contribute to the low crystal field of rare earth ions in the glass network [14].
Samarium oxide microparticles is frequently utilized in optical and photonic applications. A large number of studies were conducted to develop novel laser materials with the addition of samarium oxide. The establishment of zinc borotellurite glass doped with samarium oxide will therefore introduce alternative glass materials for possible uses in optoelectronic devices. As of now, several findings on samarium oxide microparticles doped tellurite glass are already researched [15,16,17]. Besides that, there are limited number of researches appears to be published on samarium oxide nanoparticles doped tellurite glass. Samarium oxide micro/nanoparticles vary in particle size. Samarium oxide nanoparticles comprises nano -size particles (< 100 nm), while samarium oxide forms micron-size particles. Samarium oxide nanoparticles have special features with respect to their composition, size and shape. These special features have an impact on the optical properties.
The contribution of this research is the development of novel glass materials for the improvement of optoelectronic devices. The aim of this study is to draw comparisons between the impact of samarium oxide microparticles (>100 nm) and samarium oxide nanoparticles (20 -30 nm) inclusions in the tellurite glass system on their optical properties. Optical properties such as optical band gap, Urbach energy, refractive index, molar refraction, metallization criterion, electronic polarization and optical basicity of the glass system have been analyzed.

Methodology
The sets of glasses named as ZBTSm-MPs and ZBTSm-NPs were fabricated via melt- In the meantime, ZBTSm-NPs glasses comprise with samarium oxide nanoparticles with particles size of ~30 nm. The mass of raw materials was weighed at 10 g and mixed thoroughly in platinum crucible. The platinum crucible containing the raw materials was heated in the electric furnace at 400 ° C for 30 minutes. The raw materials were melted at 900 °C for 2 hours in the second furnace. The molten of raw materials was quenched into stainless-steel moulds which was pre-heated at 400 °C to prevent thermal stress. The obtained glass along with the stainless-steel moulds were annealed at 400 °C for 60 minutes to improve the mechanical strength. The glass sample was allowed to cool down at room temperature for 24 hours. The glass sample was polished with different kinds of sandpapers, 1500 grids, 1200 grids and 1000 grids to achieve the thickness of 2mm and smooth surfaces.  demonstrate the morphological structure of ZBTSm-NPs glass. The micrograph image of samarium oxide microparticles is not able to be displayed due to restriction in the TEM instrument that disallows the analysis of micro-size particles.

X-ray Diffraction and Transmission Electron Microscopy
The shape of samarium oxide nanoparticles in raw materials is in three-dimensional shape. Meanwhile, the shape of samarium oxide nanoparticles is unchanged after the glass formation as shown in Fig. 2. The average particle size of raw materials for samarium oxide nanoparticles is 12.54 nm. After the glass formation, the size of samarium oxide nanoparticles is slightly enhanced with a diameter of approximately 23.53 nm. The growing size of nanoparticles in the glass structure is due to the Ostwald ripening effect via the dissolution of particles with small radius and re-precipitation with a large radius [18]. In addition, the size of the particles in the glass network may be increased due to the following factors: 1. Coagulation process: small particles may disappear by collisions as the nanoparticles migrate within the glass system, resulting in larger particles [19].

Density and molar volume
The density of ZBTSm-MPs and ZBTSm-NPs glasses are listed in Table 1 and Table 2 and shown in Fig. 3. The major difference between the two sets of glass series is that the ZBTSm- The comparison in molar volume between the sets of glasses can be seen in Fig. 4 and tabulated in Table 1 and Table 2. Fig. 4 shows that the molar volume of ZBTSm-MPs glasses is higher

Refractive index
Refractive index is extremely important parameter to develop the optoelectronics applications such as optical waveguides, optical filters, optical adhesives and optical fiber.   Table 3. Refractive index, molar refraction and polarizability for ZBTSm-MPs glass series

Optical absorption and band gap energy
The optical absorption versus wavelength spectra for ZBTSm-MPs and ZBTSm-NPs glasses are shown in Fig. 6 and  Fig. 8, Fig. 9, Fig. 10 and Fig. 11 respectively. The optical band gap values depend on the structural variations in the glass matrix and the type of dopants.

Molar refraction and polarizability
The estimation of the non-linear optical response for glass materials can be made by computing ……………. (6) Where Vm is the molar volume and n is the refractive index.
Polarizability and molar refraction values for ZBTSm-MPs and ZBTSm-NPs glass series are illustrated in and Fig. 14 and Fig. 15 and tabulated in Table 4 and Table 5 respectively. It can be seen from the Fig. 14 and

Conclusions
The two sets of glass series denoted as ZBTSm-MPs and ZBTSm-NPs were fabricated by using conventional melt-quenching method. The significant outcomes on structural, physical and optical properties between ZBTSm-MPs and ZBTSm-NPs glasses are as follows:  The average size of nanoparticles in ZBTSm-NPs was found in the range ~23.53 nm.  The absorption peaks of a ZBTSm-MPs glasses are two times intense than ZBTSm-NPs glasses which correspond to the restriction of electrons in nano-scale particles.
 The optical band gap of ZBTSm-NPs glasses is found greater than ZBTSm-MPs glasses which is mainly due to widening of forbidden gap with nano-scale particles  The non-linear trend of polarizability is found in both set of glasses due to the role of zinc oxide in tellurite glass system Hence, based from these results the proposed glasses might be useful to develop the optoelectronic devices.