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 {[(TeO2)0.70 (B2O3)0.30]0.7 (ZnO)0.3}1−y (Sm2O3 (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 trends along with dopant concentrations. Based on these findings, the improvement of optical properties has been made by introducing samarium oxide nanoparticles in tellurite glass which is beneficial for optoelectronic devices.


Introduction
Tellurite oxide (TeO 2 ) is the most stable oxide among the chalcogenide groups. The effectiveness and usefulness of tellurite oxide for optoelectronics applications have motivated researchers around the world [1][2][3][4]. A recent study by Peng et al. proposed that tellurite oxide has been shown as a future material for a visible-band conversion fiber laser [5]. Furthermore, tellurite oxide is often transferred to many other glass oxides that support multiple compositions. Fares et al. reported that tellurite oxide, TeO 2 consists of a lone pair electron at TeO 4 equatorial positions [6]. This occurrence will lead to the limitation of structural rearrangement and the formation of glass. As a result, pure tellurite oxide, TeO 2 glass is unstable and tends to crystallize [7]. However, to stabilize the formation of tellurite glass, it is necessary to incorporate modifiers, such as alkali, alkali earth and metal oxides in the tellurite glass network [8].
The best glass additive to be incorporated in the tellurite glass network is borate oxide, B 2 O 3 . 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 (BO 4 ) and trigonal (BO 3 ) 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 the tellurite glass network may contribute to a stable structural unit as applied in the borosilicate glass system [12]. Tellurite glasses with a small amount of borate oxide are composed of TeO 4 , BO 4 and BO 3 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 TeO 4 (trigonal bipyramidal) to TeO 3?1 polyhedra and TeO 3 (trigonal bipyramidal) coordination in tellurite glass network [13]. The lone pair in TeO 4 (trigonal bipyramidal) restricts the free movement of trigonal bipyramidal during the 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 are 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 glasses have been extensively investigated [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 study aims 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-quenching technique. The compositions of the sets of glasses are as follows: ZBTSm ZBTSm-MPs glasses consist of samarium oxide microparticles with particles size of [ 100 nm. In the meantime, ZBTSm-NPs glasses comprise samarium oxide nanoparticles with particles size of * 30 nm. The mass of raw materials was weighed at 10 g and mixed thoroughly in a platinum crucible. The platinum crucible containing the raw materials was heated in the electric furnace at 400°C for 30 min. The raw materials were melted at 900°C for 2 h in the second furnace. The molten 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 was annealed at 400°C for 60 min to improve the mechanical strength. The glass sample was allowed to cool down at room temperature for 24 h. The glass sample was polished with different kinds of sandpapers, 1500 grids, 1200 grids and 1000 grids to achieve the thickness of 2 mm and smooth surfaces.
Some of the glass samples were crushed into a powder form to perform XRD analysis (2h \ 1.5%) and high-resolution transmission electron microscopy ((HRTEM) JEOL JSM-IT-100) with the uncertainty of ± 0.02 nm. The glass samples with a thickness of * 2 mm were sent for optical absorption measurement by using a UV-1650PC UV-Vis Spectrophotometer (Shimadzu). The range of wavelengths for UV-Vis measurement is 200-900 nm with the uncertainty of ± 0.3 nm. Meanwhile, the density and molar volume of the glass samples were measured by utilizing Archimedes' principle with distilled water as immersion liquid with the uncertainty of ± 0.001 g/cm 3 . The refractive index of the prepared glass samples was measured using EL X-02 C high precision Eilipsometer where the wavelength of the light source is 632.80 nm and the angle of incident 70°.

X-ray diffraction and transmission electron microscopy
The X-ray diffraction spectra for ZBTSm-MPs and ZBTSm-NPs glass series are plotted in the range of 4°B O -B 80°as shown in Fig. 1a and b, respectively.
The results indicate that the XRD pattern of ZBTSm-MPs and ZBTSm-NPs glass series reveals wide diffusion at lower scattering angles. This pattern proved the existence of long-range structural disorder which corresponds to the amorphous nature of the glass system. The absence of crystalline peaks shows that the sets of glasses are completely in an amorphous arrangement. The image in Fig. 2 demonstrates 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 microsize particles. 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 a 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 Tables 1 and 2 and shown in Fig. 3. The major difference between the two sets of glass series is that the ZBTSm-MPs glasses are denser than the ZBTSm-NPs glasses. Greenwood stated that the small size of particles has a high tendency to disperse throughout the materials with a high degree of homogeneity and solubility [20]. Moreover, Toy et al.
confirmed that the small size of particles affects the distribution of the particles by reducing the density of materials [21]. Nanda et al. proposed that the size of particles affects the cohesive energy by lowering its number if the particles are in small size [22]. Hence, the reduction in cohesive energy may increase the number of density of the glass system. Moreover, the small size particles have low number of volume in unit cells which contributes to the rise of density. The similarity between the sets of glasses can be found in the average trend of density along with the dopant concentration. It is clearly seen that the density increases with the increasing number of dopant concentrations. The formation of non-bridging oxygen in tellurite glass network is the main reason for such trend. Samarium consists of trivalent ions which produce three non-bridging oxygens by breaking the chain of bridging oxygen. The crosslinking of tellurite glass network will be degraded by the formation of non-bridging oxygen which leads to the increment of the density. Hence, the concentrations of samarium ions have major role in determining the density of the glass system.  The comparison in molar volume between the sets of glasses can be seen in Fig. 4 and tabulated in Tables 1 and 2. Figure 4 shows that the molar volume of ZBTSm-MPs glasses is higher than ZBTSm-NPs. The nanoparticles distribution in tellurite glass network leads to the decrease of molar volume as the glass network become more compact [23]. Moreover, the molar volume is the reciprocal of the density which contributes to the shift of molar volume in both sets of glasses. The two sets of glass series show similarities in the trend of molar volume along with the dopant concentrations. The increasing values of molar volume for both ZBTSm-MPs and ZBTSm-NPs glasses are mainly due to the difference in atomic radius of samarium (r = 175 pm) which is much higher than tellurium (r = 140 pm). Hence, the molar volume of tellurite glass will be expanded along with dopant concentrations. The decrease in molar volume at 0.05 mol fraction of both sets of glasses may be due to the structural rearrangement during the glass formation.

Refractive index
Refractive index is an extremely important parameter to develop the optoelectronics applications such as optical waveguides, optical filters, optical adhesives and optical fiber. The shift in refractive index can be explained by the following factors: (a) density, (b) non-bridging oxygen, (c) polarizability and (d) coordinate number [24]. The high value of refractive index is due to the high compactness in ZBTSm-NPs glasses structure than ZBTSm-MPs glasses. The small particles reduce the interstitial spaces between the atoms and limit the    propagation of photon energy in the glass network [25]. Meanwhile, both ZBTSm-MPs and ZBTSm-NPs glass series are found similar in the trend of refractive index along with dopant concentrations. The rareearth ions increase the number of non-bridging oxygen in the glass structure that affects the change in refractive index [26]. It is known that the non-bridging oxygen consists of lone pair electrons which are less tightly bound to the nuclear charge. Hence, the high polarizability of lone pair electrons enhances the number of refractive index. Moreover, the presence of rare-earth ions may reduce the average cross-linking density that enhances the value of the refractive index [27]. Large polarizability of the glass system minimizes the velocity of light propagation in a medium which, in turn, generates a high refractive index [28]. Hence, the increase in the refractive index has also corresponded to the high value of polarizability in tellurite glass.

Optical absorption and band gap energy
The optical absorption versus wavelength spectra for ZBTSm-MPs and ZBTSm-NPs glasses are shown in Figs. 6 and 7, respectively. The non-existence of sharp  [29].
The sharp peaks of ZBTSm-MPs glasses are found more intense than ZBTSm-NPs glasses. The reduction of absorption intensity in ZBTSm-NPs glasses may be due to the restriction of valence electrons which is closer to the nuclear charge [30]. The small particles may have a smaller distance of valence electrons to the nuclear charge which reduces the mobility of electrons. The absorption coefficient values can be calculated based on the absorption edge by the following formula: where A corresponds to the absorbance and d is the thickness of the glass samples. From the above formula, it is clear that the thickness of the glass sample affects the absorption coefficient value. Hence, the thickness of the glass samples is set to 2 mm for all glass samples to prevent errors in the data. The absorbance of the glass sample influences the absorption coefficient with direct proportional behavior to the value of the absorption coefficient. It can be seen from the figure that the absorption edge shifts to a longer wavelength along with dopant concentrations. This trend may be due to the less rigidity in the glass system [31]. The optical band gap energy is determined by applying Mott-Davis equation as follows [32]: B is the constant, hm defined the photon energy, E opt correspond to the energy band gap and n determines the type of transitions.
The obtained values of a belong to the high absorption region; a C 10 4 (cm) -1 B = 4pr min /ncDE; r min is the minimum metallic conductivity, n is the refractive index, c is the light-velocity and DE = DEc-DEv represents the band tailing [33]. The indirect band gap is determined by applying r = 2 for E opt gi while the direct band gap is when r = for E opt gd . Direct transition is a process where the electrons are transmitted to conduction band directly from the valance band. This transition is done by transition-dipole moments and surface electric fields [34]. On the other hand, the indirect transition is a process where the photo-excited electrons are excited into an intermediate state and transferred to the conduction band. E opt gi can be calculated from the linear part of the relation between (ahm) 1/2 and (ht), i.e. (aht) 1/2 = 0 according to equation [33]: E opt gd can be calculated from the linear part of the relation between (ahm) 2 and (ht), i.e. (aht) 2 = 0 according to [32]: The extrapolation of the direct and indirect graph for ZBTSm-MPs and ZBTSm-NPs is shown in Figs. 8, 9, 10 and 11 respectively. The optical band gap values depend on the structural variations in the glass matrix and the type of dopants. Figures 12 and 13 illustrate the direct and indirect optical band gap for ZBTSm-MPs and ZBTSm-NPs glasses, respectively, meanwhile, Table 5 listed the specific values of direct and indirect optical band gap. It is clearly seen from Fig. 10 Fig. 8 Plot of (a hx) 2 against photon energy hx of ZBTSm-MPs for direct band gap measurement band gap of ZBTSm-NPs is higher than ZBTSm-MPs. The small particles restrict the mobility of valence electrons and hence, widen the optical band gap. Gupta et al. proposed that the optical properties of materials are highly dependent on the particle size [35]. The previous reports proved that the optical band gap is size dependent and there is an increase in the band gap of the semiconductor with a reduction in the particle size [36,37]. The small particles of samarium oxide reduce the number of overlapping orbitals and hence, widen the gap between valence and conduction bands.

Molar refraction and polarizability
The estimation of the non-linear optical response for glass materials can be made by computing the value of polarizability. The exposure of an intense light beam in glass materials leads to polarization of ions and optical non-linearity. Polarization of rare-earth ions in glass materials affects greatly on optical properties such as absorption, refractive index and electro-optical effect. The computation of polarizability, a e can be made by considering the refractive index values as follows [38]: Here V m is the molar volume, N corresponds to the Avogadro number and a e denotes the polarizability. Eq. (5) can be altered by introducing the density of the glass system as follows [38]: Here, R M is the refractivity of glass materials. M is the molecular weight and M q is the molar volume of glass materials. Based on Eqs. 6 and 7, the value of refractivity, R M and refractive index, n depends on the polarizability of glass materials. Molar refractivity is proportional to the polarizability of the samarium ions. Molar refractivity, R M can be obtained by the following expression: where V m 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 Figs. 14 and 15 and tabulated in Tables 4 and  5, respectively. It can be seen from Figs. 14 and 15 that there seem to be a sudden decrease in the molar refractivity and polarizability at 0.02 mol% and 0.05 mol% for ZBTSm-MPs glass series. Meanwhile, it is found that the molar refractivity and polarizability for ZBTSm-NPs glass series decrease at 0.01 mol%. The sudden decrease in molar refractivity and polarizability for both glass series can be due to the structural rearrangement in the glass network at a  certain amount of dopant. The increasing trend of molar refractivity and polarizability in both series is related to the formation of non-bridging oxygen in glass network. The high number of non-bridging oxygen increases the ionicity of glass materials and reduces the bond energy. The non-bridging oxygen is more likely to polarize compare to bridging oxygen. Besides that, the dual nature of zinc oxide may lead to the shifts of molar refraction and polarizability at 0.02 mol% and 0.05 mol% of samarium oxide. The high coordination number in samarium ions may result in the structural shifts from symmetric (TeO 4 ) 4to asymmetric (TeO 3 ) 2which gives a significant effect on the optical properties [39]. Furthermore, the Sm 2? ions are changed to Sm 3? ions after the glass formation by getting one electron from the oxygen via redox reaction [40]. This trend will subsequently increase the number of outer electrons and creation of new bonds with oxygen atoms. Based on these variations, it can be justified that the overall data are in non-linear trend but slightly increases. The variations of molar refractivity and polarizability are related to the role of zinc oxide which breaks the Te-O-Te bridging oxygen and thus, form the Te-O-Zn 2 ? non-bridging oxygen [41].

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 density of ZBTSm-NPs glasses is found less than ZBTSm-MPs glasses due to the increasing compactness in glass structure with the existence of nano-scale particles. • The refractive index of ZBTSm-NPs glasses is found higher than ZBTSm-MPs glasses which is caused by the shift in density. • The absorption peaks of a ZBTSm-MPs glasses are two times intense than ZBTSm-NPs glasses which correspond to the restriction of electrons in nanoscale 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 refractive index, optical band gap and polarizability results, the proposed glasses might be useful to develop the optoelectronic devices.