Role of Cobalt Doping on the Physical Properties of CdO Nanocrystalline Thin Films for Optoelectronic Applications

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Introduction
Transparent conducting oxides (TCOs) are widely used in optoelectronic devices such as fat panel displays, organic light emitting diodes, photovoltaics, heat refectors, and energy-efcient windows because they are electrically conductive and optically transparent [1][2][3]. CdO is a promising transparent conducting oxide (TCO) due to its high electrical conductivity (<10 3 Ω −1 ·cm −1 ) and its direct band gap of 2.2 eV. It also has a nonstoichiometric composition, which is because of the presence of cadmium interstitial oxygen vacancies acting as donors [4]. Te introduction of cobalt, which is a transition magnetic metal element, to CdO led to astonishing optical, electrical, and magnetic properties. Tis is mainly because of the interaction between the band electrons and the cobalt ion within the CdO lattice.
A review of the literature on pure and doped CdO flms reveals a huge array of fabrication studies. In addition to vacuum evaporation, successive ionic layer adsorption and reaction technique, sol-gel technique, magnetron sputter, organic chemical vapour deposition system, chemical spray pyrolysis, chemical bath coating, successive ionic layer adsorption and reaction technique, pulsed laser deposition, and others, thin flms deposition methods have been reported to produce undoped and metal-doped CdO thin flms. A key consideration for selecting appropriate contributing materials is the ionic radius [5]. (Co 2+ ) is projected to be the optimal doping candidate for CdO flms because it substitutes the Cd 2+ sites in the lattice and contributes electrons to serve as charge carriers [6]. Structural, optical, and NLO properties of CdO flm could be controlled with Co-doping because the ionic radius of Co being smaller than that of cadmium ions [7].
From the literature review, we found Al: CdO flms and N: CdO flms deposited by the SP technique withlinear and nonlinear optical properties [8,9]. As a result, we attempted to prepare and study the Cd 1 − x Co x O flms using this versatile technique. Furthermore, dilute concentration was chosen because we need homogeneous solutions in the SP technique so that during spray, chemical reactions take place in proportion, resulting in a homogeneous thin flm in its entire volume. So far, there has been no discussion of the detailed report on NLO studies of Cd 1 − x Co x O flms. Te present study aimed to prepare pure and Cd 1 − x Co x O thin flms using the SP technique by varying the various contents of Co from 0 to 10 wt. % by volume and focused more on the enhancement of the structural, linear, and 3 rd -order NLO properties by the Z-scan technique for optoelectronic device applications.

Undoped and Co-Doped CdO Tin Films Preparation.
Tin flms of Cd 1 − x Co x O (with "x" wt.% Co of 0, 0.01, 0.05, and 0.1) were fabricated on glass substrates by the SP technique. To remove contaminants from the surface of the glass substrates, they were dipped in a chromic oxide solution for 24 hours. Tese were then cleaned with detergent and acetone and rinsed. To fabricate the undoped CdO thin flms, the cadium acetate dehydrate of 2.66 g is dissolved in 100 ml of double-distilled water to obtain the precursor standard solution of 0.1 M cadmium acetate. Te thin flms of CdO with a diferent doping concentration of Co are fabricated by mixing the proper ratio by volume of cadmium acetate dehydrate and cobalt chloride (0.1 M) solutions, which are properly mixed and loaded to an SP instrument and sprayed on a well-cleaned glass substrate. Nozzle to spray distance was kept at 15 cm, and the spray interval was 3 mins. Te total duration of flm coating was adjusted to get a flm thickness of ±450 nm. During the deposition, the substrate temperature was preserved at 573 K (±2%). Te reaction for the formation of Cd 1 − x Co x O flms can be written as (1) Te thickness of the as-prepared thin flms was determined using SEM cross-sectional analysis and also confrmed using the gravimetric method, and it ranged from 500 nm-530 nm. Te Carl Zeiss FESEM instrument was used to examine the surface morphology of the grown thin flms. By using a powder X-ray difractometer (Rigaku Minifex 600) and nickel-fltered copper Ka radiation with a wavelength of 1.5418Å, the structural characterization of the flms was carried out. Using the double-beam spectrophotometer (Shimadzu 3600 UV-Viz), the absorbance and transmittance of the prepared thin flms were measured in the spectral range of 400-800 nm. Te NLO properties were determined using the Z-scan technique by the diodepumped solid-state CW laser (200 mw) at an excitation wavelength of 532 nm.

Results and Discussions
3.1. Structural Properties. Figure 1 reveals XRD patterns of thin flms of Cd 1 − x Co x O with various Co concentrations (0, 1, 5, and 10 wt. %). Te patterns in XRD confrm the nature and correspond to the planes (1 1 1), (2 0 0), and (2 2 0) that ft with the pure CdO polycrystalline structure. Te P-XRD patterns show that the pure flm has a strong (1 1 1) superior orientation, which increases as the concentration of Co-doping increases. Te factor f (h, k, and l) values were determined using the method proposed by Jin et al. [10]. As the concentration of Co increases, the 2θ-values of the peaks (1 1 1) and (2 0 0) change to a lower 2θ-value, favouring an expansion in the lattice volumes of the doped flms. Te peaks on the graph plotted are in the planes of (111) and (200), which are the major peaks, whereas, the (220) and (311) planes are minor peaks. Te peaks obtained from the P-XRD graph, which match JCPDS card no. 05-0640, confrm the cubic structure of the samples. By increasing the Co-doping from 1% to 10 wt. %, the broadening of the peaks along the preferential directions of the (111) and (200) planes can be observed. Te crystallite size (D avg ) was calculated using the Scherer equation (1) [11].
where k is the Scherer's constant and is equal to 0.9 for spherical crystals (wurtzite/cubic), β is the full width at half maximum (FWHM), θ is the Bragg's angle, which is given in radians, and the calculated values are shown in Table 1. Te difraction patterns were indexed into a polycrystalline lattice, and the lattice parameters were calculated using the following formula: nλ � 2d sin θ.

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Te lattice constant (a) was determined using the following formula [12]: (4) It is observed that with an increase in the lattice-parameter values excessive accumulation of Co-doping as predicted. Te enhancement in lattice parameter values may be due to the strain caused by the replacement of Co 2+ in the host CdO lattice, which has an ionic radius of 1.2Å, i.e., more than that of Cd 2+ (0.97Å). Also, c/a values remained constant; indicating Co-doping has no efect on the ultimate crystal structure of CdO. Microstructural parameters such as strain (ε), density, and density of dislocation (d) were determined using the following formula [13]: and Te decreased ε and δ values obtained strongly support the Co-doping on CdO nanostructures. Te value of "D avg " was found to be in the range of 10 nm-20 nm, which indicates the flms are composed of nanocrystallites, and the variations are shown in Figure 2.
Te value of "d" of the undoped CdO flm was found to be a � 0.4658 nm, which is a bit less than the reported value of a � 0.4694 nm, which is due to the lattice contraction or to the presence of O vacancies [14]. Te lattice constant "a" remains nearly constant for 1% (0.4566 nm) of cobalt (Co) doping in CdO crystal, but for 5% (0.4578 nm) and 10 wt. % (0.4652 nm) cobalt (Co) doping, it increases. Tis is due to the fact that the covalent atomic radius of the Co 2+ ion (0.160 nm) is greater than that of the Cd 2+ (0.149 nm). Te calculated dislocation δ is increased for the (111) and (200) planes. By raising the dopant concentration, this supports the hypothesis that the number of crystallographic defects per unit area varies asymmetrically.

Surface Morphological Studies.
Te FESEM images of undoped and Co-doped CdO thin flms are displayed in Figure 3. Te micrographs show a distinct reduction in grain size and a shift in the growth direction of CdO flms as Co concentrations increase.
As a consequence, the FESEM characteristics support the powder X-ray fndings that, as Co-doping concentration rises, crystallite size reduces and particle growth direction changes. Similar outcomes for MOCVD-prepared Ga-doped CdO flms have been reported [15].  Table 2 depicts the chemical composition of undoped and Co-doped CdO nanostructured thin flms.

Optical Properties.
Te absorbance spectra of the CdO: Co thin flms are shown in Figure 5(a). Te spectra for all of the flms have the same shape, and the absorbency of CdO flms coated with 1, 5, and 10 wt. % Co-doping concentrations is higher than that of the undoped flm. Te variation can be correlated with the Co-doping concentration in the flms. It is well known that the result of increased doping is an increase in the number of atoms, and thus more states are present. As a result, absorption is increased [16] for the energy to be absorbed.
In the inset of Figure 5(b), the transmittance spectra of Co: CdO nanostructures are shown. In the visible region, all of the flms have an average transparency of 90%. It is discovered that CdO flms coated with 1, 5, and 10 wt.% of Co-doping concentration exhibit lower transparency, while flms with undoped CdO flms exhibit higher transparency when compared to the doped flm. Tis could be attributed to higher thickness values, which increase light scattering losses [17].
Te absorption coefcient (α) of all thin flms is calculated using the transmission, refection, and thickness measurements obtained using the equation [18] and is found to be on the order of 10 4 cm −1 as follows: Te absorption index (k) was calculated using the formula k � αλ/4π. Te larger k value ( Figure 5(c)) indicate the defect in the flm. Using the value of refectance (R) and α, the refractive index ( Figure 5(d)  n � where "t" is the thickness of the flms. Te absorption coefcient is proportional to the energy of the incident photon (h]) as [20] αh] A plot of (αhc) 2 vs. E (h] in eV allows an estimate of the "Eg" and is shown in Figure 5(e). Te Eg value of the undoped CdO flm is 2.52 eV, which decreases to 2.05 eV for the flm fabricated with 10 wt. % of Co-doping. Similar bandbowing results have previously been reported for Al-doped CdO flms prepared using the SP technique [21]. Tis may be explained by their greater thickness and stoichiometry, as the decrease in band gap with Co content can also be explained by sp-d exchange interactions between the band electrons in CdO and the localized d electrons of the Co 2+ [22]. Te Burstein-Moss (BM) efect [23], in which the dopant Co-2p ions cause an enhancement in free carrier concentration, which lifts the E F up to the CB and results in a decrease in Eg value, can be used to explain the red shift in the Eg value of the flms coated with 10 wt. % Co-doping concentration.

Tird Order Nonlinear Optical Studies.
Semiconducting materials with nonlinear operations are used in valuable applications in the modern technological world of laser devices. Te NLO properties of thin flms are caused by nonlinear polarisation, which occurs when the material is subjected to a strong electric feld. Even in weak NLO materials, the laser is a high-intensity source powerful enough to cause NLO mechanisms such as 2 nd and 3 rd -order NLO effects. Te charge distribution changes as a result of the strong electric felds, resulting in a net dipole moment. Te NLO phenomenon is important in a variety of devices, including electro-optic modulators and frequency converters [24][25][26][27][28][29][30][31][32][33][34][35]. Te induced dipole moment per unit volume, also known as electrical polarisation (P), plays an important role in the NLO phenomenon. In a nonlinear medium, the polarisation of light is given by (P NL ) [26] P � X (1) · E + P NL .
In the abovementioned equation, P NL � χ (2) E + χ (3) E. P is the polarizability, χ (1) depicts the conventional linear response, χ (2) represents the 2 nd , and χ (3) is the 3 rd -order NLO susceptibilities, respectively. NLO materials are extremely important in photonic devices. Te open aperture (OA) and closed aperture (CA) Z-scan methods were performed with an I o ≈ of 8.48 × 10 7 W/m 2 (input intensity) and are illustrated in Figure 6.
Te Z-scan technique established and developed by Bahae et al. [27] is an efcient tool for probing the third-order NLO that dwells up in the sample when the incident beam of light is of sufcient intensity and frequency. Te samples were placed at the focus Z � 0 th position, and the output intensity of the transmitted beam was successively recorded using the far feld photo-detector. Te beam profle and sample thickness are the most important factors in this approach. In order to maintain a continuous beam profle, the thickness of the sample should always be kept lower than the Rayleigh range. Te pure and doped CdO flms possess good NLO characteristics. As a result, it appears necessary and justifable to investigate the NLO properties of Co: CdO flms in terms of laser applications. Te OA measurements for thin flms of Cd 1 − x Co x O are shown in Figure 7. When the input intensity (I in ) is exceptionally large, there is a substantial increase in the likelihood that a material will absorb more than one photon before going into the ground state. Tere are many diferent types of NLA processes. Te RSA efect is caused by the fact that ground-state linear absorption dominates excited-state absorption (ESA) in the sample.

Nonlinear Optical Absorption.
Te absorption ratio (β) improves with optical intensity in NLA, and the enhancement of (β) occurs in RSA. Tese phenomena appear optically as reduced (SA) or enhanced (RS) absorption [28]. Te transmission curve reveals a standardised transmittance valley with reference to the Z � 0 focus, where there is low transmission, indicating that the samples comprise induced absorption. By ftting the standard transmission data to the OA formula, the values of may be obtained from the experimental OA Z-scan fndings using the following equation [29]:

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In the abovementioned equation, is the sample efective thickness and Z o � πω 2 o /λ is a beamwaist.
Te normalised transmittance valley deepens, with a signifcant upgrade in β values observed with Co-doping concentrations in CdO. Te TPA mechanism occurred in CdO: Co flms because the energy "E" was less than the "Eg" but greater than Eg/2. Tus, electrons are absorbed and stimulated at the higher energy levels before they reach the ground state [30]. As the Co-doping was increased, there was a noticeable improvement in the RSA mechanism and the relative worth of the produced thin flms.
In the current scenario, as a source of excitation, CW lasers have been used, and the source of nonlinearity can also be thermal, with the sample also acting as a thermal lens [31]. Furthermore, the flms found current leakage and nanostructural features that support the lattice defects. Table 3 shows the calculated β values of the Cd 1 − x Co x O nanostructures.

Nonlinear Refractive Index.
Te CA Z-scan technique was used to calculate the n 2 and χ (3) of the synthesised samples. In this confguration, an aperture after the sample limits the transmitted light to the detector. Nonlinear optical materials' self-focusing or self-defocusing efects cause changes in the intensity of light received by the detector. Te NLR index of samples varies as the sample scans the z-axis and the beam transmittance changes. Te Z-scan CA experimental data show a postfocal peak followed by a prefocal valley, indicating a negative sign of n 2 due to the samples' self-defocusing nature [32]. Te experimental results (scattered pattern) agree well with the theoretical results (solid line) suggested by Sheik-Bahae. Te functions ftted to the experimental CA data are displayed in Figure 8. Te increased polarizabilities and n 2 are due to the atoms' larger atomic radius [33]. From the CA, the parameters have been calculated by the following standard formulae: x � z/z o and Δϕ o � Kn 2 I o L eff , the value of n 2 is calculated. Te inclusion of Co-material in the fabricated CdO: Co flms raises the linear and nonlinear refractive indexes. Te increasing behaviour of the Co-doping concentration can be explainedin terms of enhanced crystallinity. Because Co 2+ ions have large polarizability and very little cat-ionic feld intensity, the value of n 2 in CdO can be amplifed by the signifcant polarizability produced by Co-ions. Te structural properties and surface morphology are important factors in light-intensity scattering to achieve the expected NLO efect [34]. In fact, the theoretical and experimental normalised transmittances are very close. Tis results in a thermal lens and severe phase distortion of the propagating beam [35]. In summary, thermal nonlinearity causes the defocusing efect. It is worth noting that particle size infuences optical nonlinearity. In the synthesised samples, there is a clear increasing trend for nonlinearity with particle size as the Co concentration in CdO increases. Tis observation is consistent with what has been reported by others [36].
All nonlinear optical parameters of the Cd 1 − x Co x O nanoparticles are given in Table 3. Te structural symmetry of the material is directly related to the higher order χ (3) of the Cd 1 − x Co x O deposited thin flms ( Table 4). Variations of NLO susceptibility (e.s.u.) with crystallite size (nm) for the diferent doping concentrations of Co in the Cd 1 − x Co x O thin flms are shown in Figure 9. Te improved NLO behaviour of the synthesised samples is associated with improved polarizability and an enlarged carrier concentration for higher Co-content [38].
Te NLO properties of synthesised samples can be directly caused by their structural properties [39]. Actually, the much higher polarizability observed for Co-doped CdO samples compared to pristine CdO samples resulted from the higher atomic mass of the substituted Co ions for the Cd ones. Table 4 shows the current and some of the reported values of a few metal sulphide/oxide flms for β, n 2 and χ (3) [40,41] and [42]. Nevertheless, for optical switching applications, the actual potency of these materials is heavily dependent on n 2 .

Conclusion
Tin flms of Co-doped CdO with diferent contents of Co were prepared on the glass substrates at 300°C using the SP technique. To understand the structural and morphological changes in the flms, XRD and FESEM were studied for each sample. Te P-XRD patterns revealed an increase in the crystalline behavior of the sample in preferentially (111) and (200) plane directions strongly. FESEM images revealed the surface morphology of the prepared samples, with spherically shaped grains and smoothening with increased Codoping content. With the increasing Co-doping, the UV-Visible double-beam spectra confrm the increase of absorption in the visible and UV region. Te band gap of the fabricated flms decreases by increasing the Co-doping concentration. Te third-order NLO studies are carried out using the Z-scan technique, which revealed the TPA in NLO behavior. Te value of M nN_2, β, and χ (3) was enhanced by increasing the Co-doping concentration. Tese results suggest that the prepared Cd 1 − x Co x O thin flm samples enriching NLO behavior are more applicable as modulators in optical devices.

Data Availability
No data were used to support this study.

Conflicts of Interest
Te authors declare that there are no conficts of interest.