Structural, optical, magnetic, and photoluminescence properties of Sn0.7−xMo0.3 NdxO2+δ (0.0 ≤ x ≤ 0.3)

In this study, the properties of a series of (Sn0.7−xMo0.3 NdxO2+δ) (0.0 ≤ x ≤ 0.3) Nd3+ thin films prepared by sol–gel/spin coating technique were examined. The XRD analysis revealed the formation of all thin films in the form of Cassiterite structure. According to the FTIR investigation, when Nd3+ was substituted for Sn4+ ions in the crystal lattice, the absorption peaks migrated to the lower wavenumber side. This could be related to variations in bond length that occurs when Sn4+ ions in the crystal lattice are replaced with lighter Nd3+ ions. The morphology of the films was examined by using scanning electron microscope (SEM). In terms of Nd content, optical properties such as optical band gap, refractive index (n), and extinction coefficient (k) were investigated. The magnetic characteristics indicated diamagnetic behavior of Sn0.7Mo0.3O2+δ, paramagnetic behavior of Sn0.6Nd0.1Mo0.3O2+δ, and ferromagnetic behavior of samples with a high concentration of Nd, (Sn0.5Nd0.2Mo0.3O2+δ, Sn0.4Nd0.3Mo0.3O2+δ). The presence of active Nd3+ successfully introduced into the Sn:Mo host matrix is confirmed by the excitation dependent (PL) observed in the 350–700 nm range. PL measurements reveal two large bands located at 425 and 466 nm.


Introduction
Transparent conducting oxides (TCOs) have received a lot of attention in recent years due to their high transmittance in the visible range and outstanding electrical conductivity [1]. TCOs have been used in a variety of applications for these reasons, including photovoltaics, energy-efficient windows [2], electroluminescent devices [3], gas sensors [4], electrocatalysis [5], photocatalysis, laser diodes, and lightemitting diodes (LEDs) [6]. Many materials, such as ZnO, In 2 O 3 -based films, and SnO 2 -based films, are transparent conducting oxides. [7]. Tin oxide and related compounds, in particular, have attracted a lot of attention because they have natural n-type conductivity with a large band gap of nearly 3.5-4.6 eV, a stable structure, and the ability to alter electrical properties based on doping concentration [8,9].
They also have strong electrical conductivity (10 -4 S cm), optical transmittance ([ 85), and electrochemical stability [10,11]. However, because of its limited stability and surface area, SnO 2 has some disadvantages. Due to its high exciton binding energy (130 meV) [12] compared to ZnO (60 meV) [6], SnO 2 nanocrystal is a promising material for short-wavelength optoelectronic systems and is a feasible candidate for UV luminescence devices. The observed efficient excitonic photoluminescence emissions at room temperature are due to this high exciton energy. Due to its exceptional optical, electrical, electrochemical, and photocatalytic capabilities, SnO 2 (Tin oxide), an n-type wide band gap transparent oxide semiconducting material, has piqued interest in a variety of applications. Because of its excellent gas detecting performance and low cost, it is used in gas sensing. Photocatalytic applications, solar cells, lithium-ion batteries, optical data storage, gas-discharge displays, flat panel displays, transparent conducting electrodes, and other applications are all good candidates for SnO 2 [13][14][15].
However, because of its limited stability and surface area, SnO 2 has some disadvantages. However, because they are highly reactive and have a large surface area, synthesizing stable nanostructured materials with the desired properties is difficult. This results in the formation of secondary phases. As a result, choosing the right synthesis process is crucial for achieving the specified restrictions, such as homogeneity, shape, crystallite size, and so on. To obtain stable SnO 2 nanomaterials, Molybdenum trioxide (MoO 3 ) was chosen. Photochromic [16], thermochromic [17], gasochromic [18], and electrochromic [19] materials and/or devices, various solar cells as hole transport layer or back contact with high work function pseudocapacitive as electrode, photocatalytic system, and organic light-emitting diodes are just a few of the applications. In addition, to enhance the optical properties of materials, rare earth neodymium oxide was used. Rare earth oxides having one-dimensional structures, such as La 2 O 3 , Sm 2 O 3 , Gd 2 O 3 , and Nd 2 O 3 , have been widely used in many functional devices due to their unique electrical, optical, magnetic, catalytic, and chemical features [20]. Nd 2 O 3 has sparked a surge of interest in recent years, opening up a slew of new possibilities in a variety of fields. It is one of the most fascinating oxides in the industrial world because it has been widely used in a variety of applications such as ceramic capacitors, ultraviolet absorbents, color television tubes, coloring glass, catalyst, and carbon-arclight electrodes [21]. Due to its significant luminance properties, it is used in Nd-layer applications and Nd 3? ions of particular interest for silicon-based solar cells. The main objective of this study is to synthesize transparent conductive materials to modulate their physical properties to be used in spintronics and optoelectronic applications. The nanosized tin:molybdenum oxide films were mainly successfully prepared via lower cost and temperature sol-gel technique [22,23]. Thin films obtained by a sol-gel technique which has been effectively adapted for production because of its high throughput, controllable thickness, as well as high uniformity, high purity, lower time preparation, and higher dopant concentrations. The study includes synthesis of a series corresponding to this formula oxide (Sn 0.7-x- 3) by sol-gel method, to enhance its electrical and optical properties of materials. Another important objective is to use a simple and low cost method for achieving this target.  [24].

Experimental work
XRD data of the thin films were collected at ambient conditions on an Empyrean diffractometer by Panalytical (Almelo, The Netherlands), and filtered CuKa radiation, tube operated at 30 mA and 45 kV, and using a Ni filter to eliminate K b . The crystal structure of the prepared samples was investigated based on XRD patterns, the scanning range was 20 to 80 (2h), step scan mode with step size of 0.026 (2h), and counting time of 20 s/step. Instrumental broadening was corrected using quartz standard sample. FTIR studies were carried out with JASCO 460 PLUS, FTIR spectrometer range from 400 to 2000 cm -1 . Morphological properties were studied by using Scanning electron microscope. Transmittance and reflectance were measured in the wavelength range 300-1800 nm by using a double-beam spectrophotometer; JASCO V-570 model. Photoluminescence properties were measured using JASCO Spectrometer/data system, at the excitation wavelength 230 nm. The vibrating sample magnetometer was used to determine the magnetic characteristics.

XRD analysis
The purity of the prepared films admitted out in PXRD investigation. Figure (231), respectively. These assimilated into the Cassiterite, tetragonal SnO 2 rutile structure. All existed diffraction peaks perfectly matched with ICSD card no. 98-3-9175. The absence of any other phase such as MoO 3 or Nd 2 O 3 or impurity peaks revealed that Mo and Nd dopants properly incorporated into pure SnO 2 lattice sites through the sol-gel synthesis. The incorporation of Nd into the SnO 2 matrix could be confirmed by the shift in the 110 peak positions. Moreover, with increased Nd doping, the intensity of the diffraction peaks diminishes, which may be due to impurities that counteract the growth of SnO 2 as shown in Fig. 1b. The same behavior for the decrease in the intensity of the diffracted peaks with the increase of dopant level was obtained before by Lekshmy et al. [25] during their studies on the effect of Mn-doped SnO 2 thin films prepared by the Sol-Gel Coating.
The crystallite size was determined using the Scherrer equation from the most intense peak plane 110 using the following equation: where k is the shape factor, k is the wavelength of X-Ray, h hkl is the Bragg angle, and b hkl is the corrected full width at half maximum (FWHM) after subtracting the instrumental broadening.  [26,27]: where D is dislocation density, n is a factor which equals unity, giving minimum dislocation density, and D is the crystallite size. It is clear that the dislocation density decreases with the increase in Nd doping.

FTIR study
FTIR is a technique for obtaining information about a material's chemical bonds and functional groups.   The same behavior in the shift of the peak position due to doping was obtained by Inderan et al. [31] during their studies of Ni-doped SnO 2 prepared by the hydrothermal method. All thin films presented a dominant band at 1401 cm -1 which corresponds to the interaction of Sn with hydrogen lined Mo-O, as a weak acid [32,33]. The band at 1613 cm -1 appeared in both nanosized Sn:Mo and doped with Nd films; the intensity and the shape of the band suggest that it may correspond to the deformation mode of -OH stretching vibration, as humidity in the films [34]. From the FTIR spectra it can be obvious that these peaks 1650-800 cm -1 become stronger with the increase of Nd ratio in the Sn:Mo structure, which denotes the absorbing ability of films to -OH decreases. This might be due to the substitution of Sn with Nd ions in the Sn:Mo lattice, which supports the formation of Sn-O-Nd and eliminates -OH groups.  (Fig. 3c, d) images for films showed that the grain size changes from 50 to 500 nm, where the image region was heavily aggregate as is observed in Fig. 3a, b.

Optical properties
The optical properties of all samples were studied in the wavelength range of 300-1800 nm. Figure 4 shows the optical transmission (T) spectrum of Sn 0 . 7-x Mo 0 . 3 Fig. 5. Absorption coefficient is estimated by the following Beer-Lambert's law [36]: where d is the films thickness and T is the transmittance. The thickness of the film without Nd equals 500 nm and the thickness of the films doped with different Nd contents is 530 nm. It is observed that (a) increases with increasing Nd content and decreases with increasing wavelength (k). The direct allowed energy band gap (E g ) of Sn 0 . 7-x Mo 0 . 3 Nd x O 2?d samples have been calculated from the correlation between the incident photon energy (ht) and the absorption coefficient (a) as is given below [37]: The band gap energy has been evaluated by extrapolating the straight line portion of the plot to the (ht) axis to get the value of optical band gap (E g ) as is shown in Fig. 6a-d. The results show that when Nd doping levels rise, the optical band gap widens, which is consistent with the changes in crystallite size. In addition to when the degree of doping rises, the Burstein-Moss effect on band gap broadening, a well-known quantum confinement phenomena, causes the band gap to widen by shrinking the crystallite size [38,39]. Also, structural disorder in the lattice may cause changes in the intermediate energy level distribution inside the band gap, resulting in variations in E g values. The band gap is also affected by the strain caused by the Nd dopant as ensured by XRD data. The increase in E g indicates that Sn 0 . 70x Mo 0 . 3 Nd x O 2?d films could be used in optoelectronic devices.  The extinction coefficient (k) is a ratio that describes how rapidly the intensity of light diminishes when it travels through a substance. A detailed examination of the relationship between k and wavelength reveals that k rose as both the wavelength and the Nd content increased as shown in Fig. 7. The refractive index (n) is an important optical constant that explains the function of the incident photon in beginning particle polarization, as (n) is unmistakably influenced by the material's packing density and polarization [40,41]. Also the refractive index (n) is an important parameter for optical material and their applications; they consider it as the main parameter for device design. The refractive index can be calculated by using these relations [42]: The dependence of the reflectance on the wavelength is shown in Fig. 8. The relation between the refractive index and the wavelength is shown in Fig. 9. It is obvious that raising the Nd content leads to an increase in n.

Photoluminescence
The photoluminescence spectra of (a) nanosized 0.7Sn:0.3Mo film doped with various molar ratios of Nd to Sn (b) 0.2 and (c) 0.3 are illustrated in Fig. 10. The spectra of 0.7Sn:0.3Mo display major two emission peaks located at wavelengths of 425 and 466 nm. Besides, there are two less intense emission bands located at 562 and 622 nm. These emission peaks are decreased with the introduction of the Nd element without any shift in the position of the peaks. By the increase of Nd ions, the distance between Nd ions decreases. Owing to this decrease in distances between Nd ions, the phonons are forced by Nd ions to exchange energy to the electrons which are close to Nd ions resulting in the decrease in emission intensity. This phenomenon is known as concentration quenching. However, the fundamental edge, located at wavelengths less than 400 nm, is found to show a slight shift towards higher wavelengths with the introduction of Nd dopant. It was an expected behavior due to increasing magnetic ions content in the materials leads to increase the magnetic order in these materials; this behavior is appeared in increasing the saturation magnetization in Fig. 11a. The saturation magnetization (M s ) is increased from 0.723 to 1.0238 (emu/g) with increasing Nd ions concentration from 0.2 to 0.3. The obtained magnetic parameters are listed in Table 2.

Conclusion
The series of (Sn 0.   The morphology of the thin film studied by scanning electron microscope shows aggregates of smaller individual nanoparticles, foam-like structure. The energy band gaps (E g ) are estimated by using the optical data. It was found that E g increases with the increase in the Nd content. Moreover, the calculated extinction coefficient (k) was increased as both the wavelength and the Nd content increased. The refractive index (n) was found to increase with increasing the Nd content.

Data availability
The data in support of our findings of this study are available within the paper. All authors confirmed that all data and materials as well as software application or custom code support their published claims and comply with field standards.

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