Structural, optical, photoluminescence, and magnetic properties of Mo0.6−xTi0.3Zn0.1ErxO3 nanorods films fabricated by sol–gel/spin coating technique

Er3+-incorporated Mo0.6−xTi0.3Zn0.1 nanorods thin films were prepared on glass substrates via controlled sol–gel method. The Mo, Zn, Ti, and Er solutions were prepared using ammonium dimolybdate, titanium isopropoxide, zinc acetate dehydrate, and erbium nitrate as the starting material. Ethylene glycol, monoethanolamine, and HCl acid are solvents and solution stabilizers in the sol–gel process. The effect of the Er3+ concentration (0 ≤ x ≤ 0.3 mol%) on the films structure, optical, photoluminescence, and magnetic properties of the nanorods films was investigated by XRD, SEM, Pl, and magnetic measurement. XRD analysis proved that the samples with Er doping = 0.0 and 0.1 consists of single-phase MoO3. By increasing erbium doping to 0.2 and 0.3, bi-phasic were obtained, one of them MoO3 and the other was related to Ti6O11. These results indicate that MoO3 (two-dimensional) can control the internal growth of the Mo0.6−xTi0.3Zn0.1ErxO3 film's structure as supported by SEM and FTIR results. The reflectance of doped films exhibits high values that are increasing with the Er ratio, which adapted an increase in the Eg values from 2.85 to 3.25 eV. The presence of Er3+ in the films is found to sense the photoluminescence process that reveals two emission lines at 1477 and 1543 nm for Er ions. Magnetization behavior for samples exhibits antiferromagnetic behavior with weak ferromagnetic and unsaturated characteristics, where the magnetization at the maximum field (Mmax) increases with increase in Er content.


Introduction
In light of their potential for scientific and technological advancement, transition metal oxides have received a lot of attention recently. These compounds have a variety of structures, characteristics, and properties. The transition metal oxides, including MoO 3 , have intriguing structural, chemical, electrical, and optical characteristics. MoO 3 is used as a cathode material in the creation of solid-state microbatteries with a high energy density. As a result of its electro, photo, and gasochromic properties, it is regarded as a potential chromogenic material and is of great interest for the development of electrochromic display devices, optical switching coatings, and smart window technologies. Additionally, sensors and lubricants use molybdenum oxide films and nanocrystals [1]. Given its extensive breadth of stoichiometry and intriguing behavior, including chromogenic and catalytic characteristics, MoO 3 is a prospective material. This results in applications for lithium batteries, optical memory, gas sensors, and electrochromic display technology. Molybdenum has diverse oxidation states from (2 ? -6 ? ), its oxides exist predominantly in two forms: molybdenum (IV) and molybdenum (VI) oxide. The thermodynamically stable orthorhombic a-phase and the metastable monoclinic b-phase [2] are the two most prevalent crystal phases of MoO 3 , both of which are built using distinct methods on the MoO 6 octahedron building block [1].
The refractive indices, bandgap energies, and mechanical hardness of both these phases are extremely distinctive in terms of their physical and chemical characteristics. Due to its unique physicochemical characteristics and uses, titanium dioxide is frequently used in a variety of products, including pigments, cosmetics, fine ceramics, photocatalysts, and catalytic supports [3][4][5][6][7][8]. TiO 2 has also attracted a lot of scientific attention, with an emphasis on its distinct properties. Anatase, brookite, and rutile are the three natural crystallographic forms in which it is most frequently found [9]. However, TiO 2 is frequently employed in optics and has a large energy band gap (3.2 eV), making it an excellent photocatalytic material for the conversion of solar energy and photodegradation [10,11]. Therefore, additional study is thought to be required to comprehend the impact of metal-ion doping on the optical and structural characteristics of TiO 2 . Doping with metals or metal oxides is, in this sense can result in two different outcomes: first, the metals can lower the bandgap energy, causing the absorption band to shift toward the visible region [12][13][14][15][16][17][18], and second, the metals can slow down the rate of hole-electron recombination, acting as electron traps. The type of doping applied to TiO 2 thin films may significantly affect the bandgap energy and crystalline structure. Furthermore, by synthesizing these materials as composites, physical parameters such as optical transmittance, resistivity, and energy gap can be tailored to the desired value.
Researchers have created varied composites, including MoO 3 -WO 3 [19], MoO 3 -V 2 O 5 [20], WO 3 -V 2 O 5 [21], and TiO 2 -MoO 3 [22], which have been experimentally proved to modify and improve various physical properties such as electrochromism, photochromism, gas sensitivity, and water treatment [23]. At high temperatures, such as above 700 K, the MoO 3 -WO 3 combination did not demonstrate any increase in conductivity or stability. Similarly, forming a MoO 3 -V 2 O 5 composite film is challenging, and the film is highly unstable. Additionally, lines WO 3 -V 2 O 5 have been applied in a number of fields, including bolometers which is a wide bandgap n-type semiconductor, is of interest for applications. Due to its outstanding stability, MoO 3 is a strong contender for photocatalysis applications [24,25]. MoO 3 can be created and synthesized using a variety of techniques, including MOCVD [26], chemical vapor deposition (CVD) [27], sputtering [28], pulsed laser ablation (PLD), vacuum evaporation [29], and spray pyrolysis [30]. Because it is a less expensive and simpler process, spray pyrolysis has frequently been used to create MoO 3 films [31]. To enhance some physical qualities, MoO 3 was doped with a variety of elements, including tin, cobalt, tungsten, erbium [32], zinc, europium [32], and cadmium. It has been determined that the TiO 2 -MoO 3 composite is a strong contender for enhancing electrical conductivity, producing a stable compound, and maintaining the composite condition at high temperatures [33]. On the other hand, the mixing of the rare-earth ions with wide band gap semiconductors is considered as part of a new class of materials that have a high potential for the production of several optical devices such as fiber amplifiers, lasers, fluorescent lamps, optical memory devices, LCD, and field emission displays in telecommunications. Additionally, MoO 3 has been doped with REs such Er, Er-Yb, and Nd ions [34][35][36] in order to study up-conversion emission and find potential uses in the areas of color displays, nearinfrared detectors, biological diagnostics, laser cooling, and temperature sensors [37][38][39][40][41].
Mo 0.6-x Ti 0.3 Zn 0.1 Er x O 3 (0 B x B 0.3) thin films with different Er contents were deposited onto glass substrates using the sol-gel method and calcined at 450°C. The main aim of this work is to improve the physical properties and investigate the role of Er on structural, optical, photoluminescence, and magnetic properties of films containing molybdenum that have diverse oxidation states. Also investigate the optical, electrical, photoluminescence, magnetic, and FTIR properties of the samples. XRD data of the thin films were collected on an Empyrean diffractometer by Panalytical using CuKa radiation. Based on XRD patterns, the crystal structure of the synthesized samples was assessed. The scanning range was 20°to 80°(2h), step scan mode, step size was 0.026(2h), and counting time was 20 s/ step. Quartz standard sample was employed to correct for instrumental widening. A double-beam spectrophotometer, JASCO V-570 model, was used to measure transmittance and reflectance in the wavelength range of 500-2000 nm. Using the JASCO Spectrometer/Data System, photoluminescence characteristics were evaluated at ExBW 10 and 970 nm. The vibrating sample magnetometer was used to determine the magnetic characteristics. FTIR studies were carried out with JASCO 460 PLUS FTIR spectrometer from 400 to 2000 cm -1 . Morphological properties were studied by using Scanning electron microscope. From the figure, we can see that increasing the amount of Er more than 0.1 not only leads to the formation of the secondary phase of Ti 6 O 11 , but also leads to a change in the relative intensities of the peaks of the major phase MoO 3 , i.e., enhanced the intensity of the plane (021) and decreased the intensity of the plane 120 (peak 100%). The crystal structure of orthorhombic MoO 3 is based on a succession of bilayers that are perpendicular to the (010) y-axis and held together by Van der Waals forces. In each bilayer, MoO 6 octahedra are bent into two sublayers, forming three crystallographically in equivalent oxygen sites. These include singly coordinated (terminal) oxygen O (1), two-coordinate oxygen O (2), and three-coordinate oxygen O (3). With a very brief Mo-O bond length of 1.67, each O (1) oxygen is connected to just one Mo atom. O (2) is asymmetrically positioned between two Mo centers with bond lengths of 1.73 and 2.25 and is twofold coordinated. Last but not least, because of its enormous ionic radius, the O (3) oxygen does not replace the Mo at its six coordination sites despite being symmetrically positioned between two Mo centers in one sub-layer and connected to another Mo center in the other sublayer with a bond length of 2.33. The disappearance of the Er 2 O 3 phase in the XRD pattern may be due to its lower content, i.e., below the detection limit of XRD equipment.  Fig. 2. The pure and Er-doped films exhibited regularly united nanorods and fewer cracks; while there was more irregularity for the higher Er contents films. However, the 30 Er/Mo 0.6-Zn 0.1 Ti 0.3 O 3 nanowires films include the appearance of some nano-crystallized with a size of around 30 nm. Also, the SEM image revealed that, with increase in Er content in the Mo 0.6 Zn 0.1 Ti 0.3 O 3 matrix, the size of nanorods becomes larger, while the number of nanocrystallites is increased.

Optical analysis
The optical transmittance (T) and reflectance (R) spectra of the pure films with thickness 350 nm and 435 nm for doped with Er were studied in the range (500-2000 nm) to define the optical quality and type of transition of the samples. Figure 3a, b depicts the T and R of Mo 0.6-x Ti 0.3 Zn 0.1 Er x O 3 (0 B x B 0.3) films deposited at 300 K. T of the films is 60%, rises with increase in Er at.%, and then decreases. The absorption coefficient (a) as a function of photon energy (hm) is shown in Fig. 4, evaluated from T and R spectra as follows: where d is the film thickness of these films. The data from a were used to calculate the energy gap, E g , using the relationship: The optical band gap values of Mo 0.6-x Ti 0.3 Zn 0.1-Er x O 3 films were calculated as shown in Fig. 5a-  change in the values of E g . However, after Er doping, the band gap is increased, may be due to the nanosize effect as appeared in the SEM and the secondary phase as shown in Fig. 1. However, a dopant effect on E g depends on important factors such as phase structure, and doped component type [42,43]. Accurate knowledge of the refractive index and its frequency dependence is critical for many devices. The extinction coefficient (k) is a ratio that describes  how fast the intensity of light decreases when it passes through a substance, k for the prepared films were evaluated using the relation: Values of k are low, indicating that these films are transparent. A closer examination of the k-photon energy relationship is shown in Fig. 6; (k) varies as both photon energy and Er concentration increase. The decrease/increase in k values as the (hm) varied demonstrated the success/failure of moving electrons from the valence and conduction bands. The refractive index (n) is a critical parameter for optical materials and applications; it is regarded as the primary parameter for device design. These relationships can be used to calculate the refractive index [44]: The observed variations in the increase and decrease in the refractive index can be due to the change in the number of nonbridging oxygens in the Mo 0.6-x Ti 0.3 Zn 0.1 network and changes in the grain size with Er content. Figure 7 reveals the relation between n and hm. It is shown that (n) increases with increase in the Er content. Dielectric function is a complex quantity that has both real (e 1 ) and imaginary (e 2 ) components. The real part of a dielectric constant describes how much it slows the speed of light in the material, while the imaginary part describes how a dielectric material absorbs energy from an electric field due to dipole motion. The dielectric constant of a solid material is important for optoelectronic applications because a change in optical E g causes a change in, which indirectly changes the ionization energies of impurity atoms and the binding energy of the excision.
The values of n and k were used to determine the real (e 1 ) and imaginary (e 2 ) parts of the dielectric constant as follows [45]: A dissipation factor tan indicates the rate of energy loss in a mode of oscillation and is denoted by [46]:  [47,48] can be used to calculate refractive index in the normal dispersion region n: where E d and E O are the dispersion energy and single-oscillator energy. Furthermore, E d denoted dispersion energy, which can be thought of as a parameter related to the charge distribution within a unit cell and chemical bonding, and energy of effective dispersion oscillator [49]. The slope (E o E d ) -1 and intercept (E o /E d ) of the (n 2 -1) -1 versus (hv) 2 plot, shown in Fig. 9, can be used to calculate the E o and E d values. Dielectric constant at infinite wavelength (e ? ) can be derived from the following relations: The parameters E o and E d are related to the imaginary part of the complex dielectric constant (e i ) and the M -1 and M -3 (first and third order of moments) for optical spectrum [50] can be derived from the following relationships: N/m* can be calculated by using this equation: where e, c, and m* are the electronic charge, velocity of light, and electron effective mass. Static refractive index (n) related to e ? by a relation: n ¼ ffiffiffiffiffiffi ffi n 1 p . Figure 10 shows the relationship between n 2 and k 2 , and the lattice dielectric constant (e L ) can be calculated by extrapolating the straight line to n 2 . All results of E o , E d, e ? , M -1 , M -3 , e L , n and N/m* are tabulated in Tables 1 and 2. From Table 1, it is noticed that E d , E o values rise with increase in Er content. An increase in dispersion energy (E d ) indicates an increase in bond strength, which leads to an increase in disorder. The calculated n and e ? prove that the potential application of optoelectronics.

FTIR analysis
The FTIR spectra of Mo 0.6-x Ti 0. Fig. 11. Between 400 and 800 cm -1 , a broad band is seen in the low wavenumber region, Ti-O-Ti vibration was attributed to this band [51]. Additionally, the peak of the Ti-O-Ti vibration may overlap with the stretching vibration of the Er-O bond, which is centered at 557-565 cm -1 , assuring that no further peaks can be seen due to the Er doping. More specifically, the distorted splitting of bridging m as Mo-O in MoO 3 is connected to the bands at 858 and 868 cm -1 [52]. The peaks at 1608-1622 cm -1 and 1400 cm -1 ascribed to Ti-OH bonds of TiO 2 molecules, respectively [53]. Moreover, Ti-O stretching bonds were recognized for the peak between 417 and 800 cm    Er/Mo 0.6 Z 0.1 Ti 0.3 O 3 significance detailed evaluation. Hence, the sol-gel method allows good chemistry flexibility to introduce higher Er contents and lower process temperatures. Figure 12 appears two emission lines at 1477 nm and 1543 nm for Er: Mo 0.6 Z 0.1 Ti 0.3 O 3 ; herein, the insertion loss was very low. Increasing Er contents is achieved, confirming the erbium absorption in Mo 0.6 Z 0.1 Ti 0.3 O 3 films is significantly higher and intensities of the bands vary with the content of (Er 3? ) ions. The two bands assignment corresponds to the transitions from upper level originating from the Er ground state ( 4 I 15/2 ) to ( 4 I 13/2 ) excited ones [53]. These results confirm that Mo 0.6 Z 0.1 Ti 0.3 O 3 is successful in incorporating Er compared with using silicate-based systems, which can incorporate the other of dopants that needed to modify the Er domain [54,55]. All samples give similar absorption line shapes with no shift in their absorption peaks, which specifies that the deliberate Mo 0.6 Z 0.1 Ti 0.3 O 3 have higher Er 3? solubility and good homogeneity Er/Mo 0.6 Z 0.1 Ti 0.3 O 3 films.

Magnetic properties
Magnetic hysteresis loops of Mo 0.6-x Ti 0.3 Zn 0.1 Er x O 3 have been measured at room temperature and their plots are shown in Fig. 13, sample at x = 0 shows diamagnetic behavior as shown in the inset of Fig. 13. The behavior of magnetization for samples (x = 0.1, 0.2, and 0.3) shows antiferromagnetic behavior with weak ferromagnetic with unsaturated characteristic, where the magnetization at the maximum field (M max ) increases with increase in Er content as shown in Fig. 14, also we can note that the coercivity decreases with Er content. X-ray diffraction pattern showed sample x = 0 is pure MoO 3 with Er doping  secondary phase appeared as Ti 6 O 11 and its intensity increased with Er doping as shown in Fig. 1a. Previous reports demonstrated that the MoO 3 and Ti 6 O 11 have diamagnetic properties [56,57]. So the magnetism in samples arises from Er doping. The peculiar electronic configurations of the rare-earth element erbium, in which the outer 4f electrons are shielded by the 5d and 6 s electrons, as well as the quantity of unpaired 4f electrons and their ionic radii, are key factors in determining the material's various functional properties [58,59]. The magnetization behavior can be explained by the typical antiferromagnetic spin-canted process, which combines antiferromagnetic in-plane exchange interactions with ferromagnetic inter-plane interactions to increase magnetization with increasing structural distortion [60,61]. The diminution in coercivity (H c ) with ascending Er content is seen in Fig. 10, this behavior in H c may be due to the reduction of the magnetocrystalline anisotropy with increasing Er content [62].

Conclusion
Samples  B x B 0.3) indicted that, the increase in the optical energy band gaps enables us to use it in the optoelectronic applications. It is a material that is being developed for use on large areas, and its development can be used with

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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