A. Structural analysis

XRD diffractogram of pristine and MZO thin films is shown in Fig. 1(a). The respective positions of ZnO major peaks (100), (002), (001), and (110) (Zincite, JCPDS 36-1451) were observed from the recorded XRD patterns, which show that all the synthesized thin films were polycrystalline. The XRD peak (002) at 34.4° is dominant plane along with hexagonal wurtzite structure. Apart from the above observed peaks, supplementary peaks (110), (200), and (211) were observed at 2θ = 26.7°, 37°, and 52°, which correspond to the FTO substrate such as tin dioxide phase (JCPDS# 00-041-1445), the intensity of the FTO substrate peak increases with increasing Mg⁺² doping concentrations.

FIG. 1.

(a) XRD patterns and (b) vibrational spectra for all deposited ZnO films.

The grain sizes for the prepared pristine/FTO and MZO/FTO films were determined form the recorded XRD patterns by applying Debye Scherer formula^{Reference Shkir and AlFaify18–Reference Shkir, Yahia, Ganesh, Algarni and AlFaify20}: $D = {{k\lambda } \over {{\beta _{2\theta }}\cos \theta }}$, where β_2θ is the FWHM, k is the coupling constant taken to be 0.9, λ is the wavelength used (1.5406 Å), and θ is the Bragg’s angle. The calculated values of lattice parameters and crystallite size of pristine/FTO and MZO/FTO films are found to be varying with Mg concentration and are given in Table I. This difference is attributed to diversity in relative ionic size of Mg²⁺ (0.72 Å) and Zn²⁺ (0.60 Å). Moreover, the density of dislocations and microstrain were also estimated through well-known relations^{Reference Shkir, Kilany and Yahia21,Reference Yahia, Shkir, AlFaify, Ganesh, Zahran and Kilany22} and are tabulated in Table I. The highest value of D was estimated for 15% MZO, and also values of dislocation density and lattice strain are reduced, which indicates good quality of deposited films.

TABLE I.

POWDERX refined structural parameters of pristine/FTO and MZO/FTO films.

B. FT Raman analysis

FT-Raman spectra of pristine/FTO and MZO/FTO films are depicted in Fig. 1(b). The presence of vibrational bands at ∼1095 ± 2 cm⁻¹, 950 ± 5 cm⁻¹, and 790 ± 3 cm⁻¹ are might be for the substrate. The Raman peak situated at ∼557 ± 3 cm⁻¹ is allocated to A₁ (LO) asymmetric mode, and this might be recognized to the contributions of lower and higher wavenumbers.^{Reference Bundesmann, Ashkenov, Schubert, Spemann, Butz, Kaidashev, Lorenz and Grundmann23,Reference Decremps, Pellicer-Porres, Saitta, Chervin and Polian24} However, this Raman peak is slightly shifted to the low wave number region compare to the bulk one. The Raman peak situated at ∼470 ± 5 cm⁻¹ (E₂ mode) is the fundamental mode of vibration for the ZnO hexagonal phase and also have shift in its position to higher wave number in contrast to films and bulk ZnO.^{Reference Decremps, Pellicer-Porres, Saitta, Chervin and Polian24,Reference Kumar, Valanarasu, Kathalingam, Ganesh, Shkir and AlFaify25} The possible reason behind this shift in position is the development of stress in films when deposited on the substrate, or may be due to variation in grain size due to doping. It can be visibly seen in the figure that the intensity and sharpness of Raman peaks are increased with Mg doping concentration, which indicates high crystallinity of films. All Raman peaks are found to be in well-agreement with previous reports.^{Reference Wei, Man, Liu, Xue, Zhuang and Yang26–Reference Ganesh, Yahia, AlFaify and Shkir28}

C. Microstructure and EDX analysis

Figures 2(a)–2(e) exhibit the SEM micrographs of all pristine/FTO and MZO/FTO films. SEM images showed that the whole FTO substrate is roofed by uniformly grown smaller grains, revealing the smoothness of the surface for all pure and MZO/FTO thin films. The low-magnification image clearly shows the uniformity of the microstructure without any creaks and holes. On the other hand, at higher magnification, it can be detected that thin films are self-possessed of strongly packed and randomly arranged wire-like structures. Conferring to EDX elemental mapping results [see Fig. S1 (Supplementary Material)], magnesium is uniformly distributed over the entire surface of the prepared thin films on the FTO substrate.

FIG. 2.

SEM micrographs of (a) pristine/FTO, (b) 1%, (c) 5%, (d) 10%, and (e) 15% MZO/FTO films.

D. Linear optical properties

Figures 3(a)–3(c) exhibit absorption, transmittance, and reflectance plots of pristine/FTO and MZO/FTO films. The nanostructure films hold higher transparency in the visible range. Pristine ZnO displays transmittance ∼80% in the visible region, whereas this value is decreased for MZO films. Correspondingly, the absorption for pure ZnO film on FTO was found to be lower than that of MZO films. The transmittance spectra of films consist of interference fringes in the higher wavelength region, which might be due to manifold reflections at two film edges deposited on different substrates.^{Reference Springer, Rech, Reetz, Müller and Vanecek29} The films exhibited strong absorption in the UV, which is beneficial for various device fabrications. Analogous behaviors are detected in many metal oxide nanostructured films along with diverse dopants.^{Reference Shkir, Ganesh, AlFaify, Yahia and Zahran30}

FIG. 3.

(a) Absorbance, (b) transmittance, and (c) reflectance of pristine and MZO films.

Direct and indirect band gap values of pristine/FTO and MZO/FTO films were obtained through Tauc’s relation^{Reference Tauc, Grigorovici and Vancu31,Reference Shakir, Kushwaha, Maurya, Bhagavannarayana and Wahab32} : (αhν)^1/m = A(hν − E _g), where A is the fundamental constant, h is the plank’s constant, ν is the frequency, α is the absorbance coefficient, E _g is the energy gap, and m characterizes the transition process involved. Here, α was estimated from absorbance (A) and thickness (t) by equation^{Reference Shkir, AlFaify, Ganesh and Yahia33} α = 2.303A/t. The value of m equals to 2 corresponds to an allowed direct transition and 1/2 for an indirect transition.

Figures 4(a) and 4(b) represent the graphs of direct and indirect energy transitions of pristine and MZO/FTO nanostructured films. The E _g values are calculated by inducing the rectilinear area of (αhν)^m versus hν plot. Energy gap values corresponding to both the transitions were found to be decreasing with Mg doping (5, 10, and 15%). The measured direct and indirect energy gaps are found between 3.103 and 3.283 eV for pristine and 2.423 and 2.968 eV for MZO/FTO films and are in line with earlier studies.^{Reference Ajili, Castagné and Turki34–Reference Shkir, Arif, Ganesh, Manthrammel, Singh, Yahia, Maidur, Patil and AlFaify38}

FIG. 4.

(a) Direct and (b) indirect transition, (c) absorption index, and (d) refractive index of MZO/FTO films.

Decisive optical characteristics such as index of absorption and refraction, k, and n films are work out using k = αλ/4π and $n = {{\left( {1 + R} \right)} \over {\left( {1 - R} \right)}} + \sqrt {{{4R} \over {{{\left( {1 - R} \right)}^2}}}} - {k^2}$, where R is the function of refractive index (n) ^{Reference Shkir, Ganesh, AlFaify and Yahia39} and plotted in Figs. 4(c) and 4(d), respectively.

It has been observed from these plots that both n and k are displaying analogous tendency with wavelength, i.e., on increasing the wavelength, and the values of n, k are decreasing first and reach the steady state at higher wavelengths. This kind of tendency is a clear indication of defect-free growth of the synthesized nanostructure thin films. Some trivial increases in refractive index (n) are observed after 400 nm and these increases in the value of n at higher wavelengths are attributed to the consequence of magnesium doping. Moreover, similar behaviors are also reported for other metal oxide semiconductor nanostructured thin films with different dopants.^{Reference Ali Omar40}

In an application point of view, the variation of polarization with the energy of incident wavelength is very significant for the fabrication and optimization of an optoelectronic device. Dielectric correlated factors such as real (ε₁) and imaginary (ε₂) components of dielectric constant describe the electronic excitation spectra of any nanocrystalline films. In view of this, ε₁ and ε₂ for pristine/FTO and MZO/FTO nanostructured films are calculated by applying formulae^{Reference Khatibani and Rozati41,Reference Yahia, Ganesh, Shkir, AlFaify, Zahran, Algarni, Abutalib, Al-Ghamdi, El-Naggar and AlBassam42} : ε_{1 =} n ² − k ² and ε₂ = 2nk, respectively, and depicted in Figs. S2(a) and S2(b) [see the Supplementary Material]. Through these plots, it was seen that ε₁ and ε₂ are varying from 2 to 10 and 0.1 to 0.9 in the energy range of 1.25–3.75 eV with the Mg doping, indicating the substantial influence of Mg on dielectric properties of nanostructured ZnO. These values of MZO/FTO are better than those previously reported on MZO/Glass in the mentioned energy region.^{Reference Shkir, Khan, Ganesh, Yahia, Ul Haq, Almohammedi, Patil, Maidur and AlFaify43} Fig. S2(b) [see the Supplementary Material] also denotes the dielectric loss with respect to the energy of incident photon. From the figure, the trend of dielectric loss with Mg doping in ZnO is similar to that of dielectric constant and indicates that the addition of Mg helps to reduce the defects in the film.

Figures S2(c) and S2(d) [see the Supplementary Material] display the plots for volume energy loss (VELF) and surface energy loss (SELF) functions of pristine and MZO films with respect to photon energy (hν). These are key parameters to estimate energy decay rate occurring when electrons pass over any material and are obtained from ε₁ and ε₂ using the following equation^{Reference Sussman44}:

$${\rm{VELF}} = - {\mathop{\rm Im}\nolimits} \left( {{1 \over {{\varepsilon ^ * }}}} \right) = {{{\varepsilon _2}} \over {\varepsilon _1^2 + \varepsilon _2^2}}\;{\rm{and}}\;{\rm{SELF}} = - {\mathop{\rm Im}\nolimits} \left( {{1 \over {{\varepsilon ^ * } + 1}}} \right) = {{{\varepsilon _2}} \over {{{\left( {{\varepsilon _1} + 1} \right)}^2} + \varepsilon _2^2}}\quad .$$

From the graphs, the VELF and SELF plots follow almost similar trend without much noteworthy differences at higher and lower energies except that the VELF values are comparatively higher than that of SELF in the entire energy level, giving a sign about the journey of radiation from inner and upper levels is the same.

E. Nonlinear studies using optical and Z-scan measurements

The inference of light rays on doped materials having high susceptibilities is very significant in several optoelectronic device applications due to the great nonlinear susceptibilities they possess, which in turn provides extensive details of the preferred sample to defy high laser beats in the course of second- and third-order effects.^{Reference Shkir, Ganesh, AlFaify and Yahia39} Many research teams have been trying to apply various methodologies to assess the polarizability and refractive index and a reliable formulation is achieved as follows^{Reference Shkir, Ganesh, AlFaify, Yahia and Zahran30,Reference Frumar, Jedelský, Frumarova, Wagner and Hrdlička45} : polarizability, P = χ⁽¹⁾E ⁽¹⁾ + P _NL, where P _NL = χ⁽²⁾E ² + χ⁽³⁾E ³, where χ⁽¹⁾, χ⁽²⁾, & χ⁽³⁾ are termed as susceptibilities as linear, second-, and third-order nonlinear, respectively.

Congruently, linear refractive index n(λ) is obtained using^{Reference Xia, Liu, Qiu, Mao, He and Chen46} n(λ) = n ₀(λ) + n ₂(E ²), where n ₀ and n ₂ are the components of n and E ² is the square mean of electric field, with n ₀(λ) ≫ n ₂(λ) eventually resulting in n(λ) = n ₀(λ). The χ⁽¹⁾ for a given medium follows the relation^{Reference Ticha and Tichy47}: χ⁽¹⁾ = (n ² − 1)/4π, while the χ⁽³⁾ follows^{Reference Yahia, Ganesh, Shkir, AlFaify, Zahran, Algarni, Abutalib, Al-Ghamdi, El-Naggar and AlBassam42,Reference Adair, Chase and Payne48,Reference Farag49} : χ⁽³⁾ = Aχ⁽¹⁾. On the other hand, ${\chi ^{\left( 3 \right)}} = {A \over {{{\left( {4\pi } \right)}^4}}}{\left( {n_0^2 - 1} \right)^4}$, where A = 1.7 × 10⁻¹⁰ (in e.s.u).^{Reference Farag49} The value of n ₂ is governed by^{Reference Shkir, Ganesh, AlFaify and Yahia39} ${n_2} = {{12\pi {\chi ^{\left( 3 \right)}}} \over {{n_0}}}$.

Figures 5(a) and 5(b) display the trends of χ⁽¹⁾ and χ⁽³⁾ of the pristine/FTO as well as MZO/FTO films in terms of wavelength. From these graphs, it has been found that χ⁽¹⁾ and χ⁽³⁾ values increase with growing concentration of Mg, particularly at higher wt%. This shows that doping of Mg increased the crystallinity of ZnO and also reduced the defects. At higher concentration, the overall particles’ interaction with the incident photons improves and thereby improves the optical susceptibilities. The values of χ⁽¹⁾ and χ⁽³⁾ are found to be from 0.1 to 0.4 and 0.023 × 10⁻¹² to 4.59 × 10⁻¹², respectively, in the wavelength region 400–1000 nm. The stable values are observed compared to the previous report on MZO/glass films.^{Reference Shkir, Arif, Ganesh, Manthrammel, Singh, Yahia, Maidur, Patil and AlFaify38}

FIG. 5.

(a) χ⁽¹⁾, (b) χ⁽³⁾, and (c) n ₂ for pristine/FTO and MZO/FTO films.

Nonlinear refractive index (n ₂) is also considered as a very important quantity of the material, which gives significant information regarding the capacity of the material to gather enough light when it interacts with light. Figure 5(c) represents the plot of n ₂ versus wavelength for pristine/FTO and MZO/FTO thin films. The graph clearly shows an analogous trend of n ₂ with χ⁽³⁾ and demonstrates the strong dependency of n ₂ and χ⁽³⁾.

F. Z-scan measurement

The measured absorption of nonlinearity and limiting of optic performance of all films are demonstrated in Figs. 6(a) and 6(b). The open aperture plot displays a reverse saturation absorption process by means of a stout nonlinear absorption at the origin (focal point). This is accredited to a 2 PA nonlinear absorption procedure occurring in films. By growing the doping concentration, nonlinear absorption increased. The theoretical fitting for the experimental data was done using $T\left( Z \right) = 1 - {{\beta {I_0}{L_{{\rm{eff}}}}} \over {2\sqrt 2 \left( {{{1 + {Z^2}} / {Z_0^2}}} \right)}}$, where I ₀ is the intensity of the CW laser at the focal Point and L _eff is the effective span of the material^{Reference Patil, Shkir, Maidur, AlFaify, Arora, Rao, Abbas and Ganesh50} used to obtain the nonlinear absorption coefficient (β). The β values are found in the range 0.35–123 (×10⁻⁵) cm/W, and these values are comparatively higher than ZnO/glass films reported previously.^{Reference Shkir, Khan, Ganesh, Yahia, Ul Haq, Almohammedi, Patil, Maidur and AlFaify43}

FIG. 6.

Plots of Z-scan measurements for (a) absorption and (b) optical limiting.

A direct relation has been found to exist between χ⁽³⁾ with β and n ₂ via the relation^{Reference Maidur, Patil, Rao, Shkir and Dharmaprakash51} ${\mathop{\rm Im}\nolimits} {\chi ^{\left( 3 \right)}}\left( {{\rm{esu}}} \right) = {{{{10}^{ - 2}}\left( {{\varepsilon _0}{C^2}{n_0}\beta \lambda } \right)} \over {4{\pi ^2}}}\left( {{{{\rm{c}}{{\rm{m}}^2}} \over {\rm{W}}}} \right)$, where n ₀ is the linear refractive index and C, λ, and ε₀ have their usual meaning. The calculated values of χ⁽³⁾ are found to be enhanced from 0.084 to 29.7 (×10⁻⁸) when doped 15% Mg in ZnO. These values are comparatively higher than ZnO/glass films reported previously^{Reference Shkir, Khan, Ganesh, Yahia, Ul Haq, Almohammedi, Patil, Maidur and AlFaify43} and comparable to other reports of MZO films.^{Reference Agrawal, Ahmad Dar, Solanki, Phase and Sen52} All these values are tabulated in Table II.

TABLE II.

Obtained NLO parameters from Z-scan study for pristine/FTO and MZO/FTO films.

Figure 6(b) illustrates the limiting behavior of films at 532 nm DPSS CW laser. The normalized transmission is drawn against input fluence of the laser, and limiting threshold (LT) was estimated at 50% of linear transmission.^{Reference Shkir, Arif, Ganesh, Manthrammel, Singh, Yahia, Maidur, Patil and AlFaify38,Reference Maidur, Patil, Rao, Shkir and Dharmaprakash51} LT values were calculated to be 6.34 for pristine, 4.51, 3.91, 3.28, and 2.57 kJ/cm² for 1%, 5%, 10%, and 15% MZO films, respectively. The values are comparable as well higher than previous reports on MZO films.^{Reference Shkir, Khan, Ganesh, Yahia, Ul Haq, Almohammedi, Patil, Maidur and AlFaify43,Reference Agrawal, Ahmad Dar, Solanki, Phase and Sen52} The reduction of LT with an increase in Mg doping indicates that the MZO/FTO films are of better optical activity and could be used to shield the sensor devices from intense lights.