Comparative investigation of some selected properties of Mn₃O₄/PbS and CuO/PbS composites thin films

In recent years, semiconductor nanocrystalline films have been widely investigated due to the novel size dependent properties [1]. The main advantage of using nanomaterial is that it consists of very small particles. As they are small, nanostructures can be packed very closely together. As a result, on a given unit area one can locate more functional nanodevices, which is very important for nanoelectronics [2]. Their high packing density has the potential to bring high area and volume capacity to information storage and higher speed of information processing [3]. In view of technical applications, the optical properties of nanoparticles and nanocomposites are of major interest. Besides their economic, the scientific background of these properties is of fundamental importance in order to understand the behaviour of nanomaterials [4].

The survey of literature showed that the synthesis of core/shell heterostructures has gained recognition due to the synergetic properties or complementary behaviours offered by the composite nanostructures. There are many studies on the synthesis of various core/shell heterostructures such as semiconductor/semiconductor [5], semiconductor/metal [6], metal/metal oxide [7], metal/metal [8], metal oxide/metal oxide [9], metal oxide/conductive polymer [10] have been explored, and enhanced properties have been demonstrated. However, research work on oxide/sulphide core–shell thin films is limited [11–15]. In our previous work, we studied the effect of annealing temperature on the structural, optical and solid state properties of Mn₃O₄/PbS and CuO/PbS double layer thin films [16, 17]. Here, we report on the comparative investigation of the structural, optical and solid state properties of Mn₃O₄/PbS and CuO/PbS composite films deposited on plane glass substrates using chemical bath deposition technique.

The chemical bath for the deposition of Mn₃O₄ was made up of a mixture of 12 ml of 1 M MnCl₄, 12 ml of 1 M NH₄Cl, 12 ml of 100% NH₃ and 24 ml of water. Five (5) clean glass slides were then inserted vertically into the solution. The deposition was allowed to proceed at temperature of 80 °C for 5 h after which the coated substrates were removed, washed with distilled water and allowed to dry. The deposited Mn₃O₄ was inserted in a mixture containing 5 ml of 0.2 M Pb(NO₃)₂, 5 ml of 1 M SC (NH₂)₂, 5 ml of 1 M NaOH and 35 ml of distilled water put in that order in 100 ml cleaned and dried beaker for 50 min to form Mn₃O₄/PbS core–shell thin film. One sample each of the deposited films was selected for characterization.

The chemical bath for the deposition of CuO was made up of a mixture of 4 ml of 1 M CuSO₄, 4 ml of 1 M KCl, 2 ml of 100% NH₃ and 13 ml of distilled water. The deposition took place at 80 °C bath temperature for 3 h. To deposit CuO/PbS films, the deposited CuO films were inserted into a mixture containing 5 ml of 0.2 M Pb(NO₃)₂, 5 ml of 1 M SC (NH₂)₂, 5 ml of 1 M NaOH and 35 ml of distilled water put in that order in 50 ml beaker for 50 min.

Optical absorption data were obtained with a thermo scientific GENESYS 10 S model UV–vis spectrophotometer at normal incidence of light in the wavelength range of 300–1000 nm. Proton Induced x-ray Emission (PIXE) scans on the samples from a Tandem Accelerator Model 55DH 1.7 MV Pellaton by National Electrostatic Corporation (NEC), USA which effectively performed Rutherford Back Scattering (RBS) elemental characterizations on both deposits and substrates. The RBS also deciphered the thicknesses of film deposits. The crystal structure and phase analysis of the deposited film was carried out at room temperature with an x-ray diffractrometer Rigaku Ultima IV model, using gracing incident at 30 mA, 40 KV with CuKα radiation of wavelength λ = 0.154 06 nm. Scanning Electron Micrscope (Tescan model) was used to observe the morphology of the deposited composites thin films.

The grown composites thin films were characterized by powder x-ray diffractometer. Figure 1 shows the x-ray diffraction spectra of Mn₃O₄/PbS and CuO/PbS composite thin films. The strong peak intensity indicates the high degree of crystallinity of the films. The presence of more than one peak indicates that both films are polycrystalline. Different diffraction patterns can be observed for both composites. Comparative study of the XRD patterns of both films showed that Mn₃O₄/PbS films exhibited more intense peaks compared to CuO/PbS films. The average grain sizes of 34.86 nm and 19.14 nm were observed for Mn₃O₄/PbS and CuO/PbS films respectively. The micro strain of both films which is related to the lattice misfit and depends on the deposition conditions was calculated as 0.1020 and 0.1039 for Mn₃O₄/PbS and CuO/PbS films respectively. The peaks at 2θ values of 25.964°, 30.075°, 43.059° and 50.978° are attributed to galena PbS (JCPDS 00-005-0592). These were assigned to the diffraction line produced by (111), (200), (220) and (311) planes. The peaks at angles of 18°, 32.316°, 36.450° and 50.710° are identified to be hausmannite Mn₃O₄ (JCPDS 00-024-0734) and are assigned diffraction lines produced by (101), (103), (202) and (105) planes. However, additional peaks of 28.082°, 29.615° and 30.645° are identified to be Pb₂MnO₄ (JCPDS 00-036-0844) phase and assigned diffraction lines produced by (131), (330) and (231) planes while peak of 35.583° corresponds to (212) plane of Copper hydroxide sulphate phase (JCPDS 00-011-0653).

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The average crystallites sizes D in both composites was deduced from the scherrer's equation [18, 19].

$$\begin{eqnarray}&&D=\displaystyle \frac{{\rm K}\lambda }{\beta \,\cos \,\theta }\end{eqnarray} \tag{ 1 }$$

The microstrain (ε) produced in the composites was calculated using the equation below [20].

$$\begin{eqnarray}&&\varepsilon =\displaystyle \frac{\beta }{4\,\tan \,\theta }\end{eqnarray} \tag{ 2 }$$

Where $\beta$ is the peak width at mid height, θ is the diffraction angle, K is a constant and is equal to 0.9, the constant $\lambda$ = 0.15406 nm, D is the grain size and ε represent strains in the film.

The SEM images of Mn₃O₄/PbS and CuO/PbS composites thin films are shown in figures 2 and 3 respectively. Figure 2 depicts non-uniform irregular spherical grains scattered across the substrate surface with few pinholes while figure 3 shows particles engaged in a flower-like rod structure indicating the nanocrystalline nature of CuO/PbS thin films. The observed long network of structure ordering makes the CuO/PbS film good material for design of light-trapping configuration for solar cells [21].

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The elemental composition of the films was carried out using Rutherford backscattering technique. The percentage elemental composition of Mn₃O₄/PbS and CuO/PbS composites thin films are presented in tables 1 and 2 respectively. The thicknesses of the films as deciphered by RBS are 1026 nm and 650 nm for Mn₃O₄/PbS and CuO/PbS films respectively. The equation below [22] was also used to calculate the thickness of the film in order to compare with that deciphered by RBS technique. The calculated thicknesses are 1067 nm and 698 nm for Mn₃O₄/PbS and CuO/PbS films respectively. The calculated thicknesses of Mn₃O₄/PbS and CuO/PbS films are in conformity with the RBS results. The RBS micrographs of Mn₃O₄/PbS and CuO/PbS thin films are shown in figures 4 and 5 respectively.

$$\begin{eqnarray}&&t=\displaystyle \frac{\left[{\tan }^{-1}{\left[\displaystyle \frac{{\left({n}_{o}+{n}_{g}\right)}^{2}R-{\left({n}_{o}-{n}_{g}\right)}^{2}}{{\left(\displaystyle \frac{{n}_{o}{n}_{g}}{n}-n\right)}^{2}-{\left(\displaystyle \frac{{n}_{o}{n}_{g}}{n}+n\right)}^{2}R}\right]}^{\displaystyle \frac{1}{2}}\lambda \right]}{2\pi n}\end{eqnarray} \tag{ 3 }$$

Where t is thickness, n_o is the refractive index of the medium of the incident light, which in this study is air, n_g is the refractive index of the substrate (glass in this case); and n is the refractive index of the thin film, R is reflectance and λ is wavelength.

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The optical absorption and transmission spectra of Mn₃O₄/PbS and CuO/PbS composites deposited on transparent glass recorded in wavelength range 300–1000 nm are shown in figures 6 and 7 respectively. Mn₃O₄/PbS film exhibited peak absorbance of about 1.40 (a.u) whereas CuO/PbS exhibited peak absorbance of about 1.70 (a.u). Both peaks occur in the UV region. A significant difference in the absorption peak is observed. The absorption edge for Mn₃O₄/PbS films occurred at about 0.40 (a.u) corresponding to about 318 nm whereas CuO/PbS films exhibited absorption edge at about 0.65 (a.u) corresponding to about 314 nm. The absorbance of both films are higher in the UV region with a sharp decrease in the visible and near infrared regions. The absorbance of CuO/PbS composite is generally higher in the UV region compared to Mn₃O₄/PbS composite. From figure 8, Mn₃O₄/PbS films transmit more than CuO/PbS films in the UV region. However, in the visible and infrared regions, CuO/PbS films transmit higher than Mn₃O₄/PbS films. Generally, Mn₃O₄/PbS composite transmit below 50%, whereas CuO/PbS composite transmit above 50%. Figure 9 depicts the plots of reflectance against wavelength for Mn₃O₄/PbS and CuO/PbS composites thin films. It is observed that the reflectance of both films vary in similar manner. However, Mn₃O₄/PbS composite films shows higher reflectance in the entire electromagnetic region compared to CuO/PbS composite films.

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The transmittance (T), reflectance (R), absorption coefficient (α) and band gap (E_g) were calculated from the following relations [23]:

$$\begin{eqnarray}&&{\rm T}=\left(1-{R}^{2}\right)\exp (-A)\end{eqnarray} \tag{ 4 }$$

$$\begin{eqnarray}&&R=1-{\left[{\rm T}\exp (A)\right]}^{\displaystyle \frac{1}{2}}\end{eqnarray} \tag{ 5 }$$

$$\begin{eqnarray}&&\alpha =\displaystyle \frac{1}{t}\,\mathrm{ln}\,\displaystyle \frac{(1-{R}^{2})}{{\rm T}}\end{eqnarray} \tag{ 6 }$$

$$\begin{eqnarray}&&{\left(\alpha hv\right)}^{n}=A\left(hv-{E}_{g}\right)\end{eqnarray} \tag{ 7 }$$

Where A is band edge parameter and value of n determines the nature of optical transition (n = ½ indicates direct transition and n = 2 indicates indirect transition).

The plots of absorption coefficient as a function of photon energy for both composites are presented in figure 9. The absorption coefficients of Mn₃O₄/PbS and CuO/PbS composites films vary in a similar manner, increasing from 0.75 × 10⁶ m⁻¹ at 1.23 eV to a maximum of 2.75 × 10⁶ m⁻¹ at 4.25 eV for Mn₃O₄/PbS composite films, and 1.1 × 10⁶ m⁻¹ at 1.23 eV to a maximum of 6.25 × 10⁶ m⁻¹ at 4.25 eV for CuO/PbS composite films. In general, Mn₃O₄/PbS composite films exhibited lower absorption coefficient values compared to CuO/PbS composite thin films. The absorption coefficients of both films increased with increasing photon energy (decreasing wavelength).

Figure 10 depicts the plots of the product of absorption coefficient and photon energy square (αhν)² against photon energy (hv). The extrapolation of straight line portion of (αhν)² against hv to energy axis for zero absorption coefficient gave optical band gap energy value as 3.90 eV and 3.95 eV for Mn₃O₄/PbS and CuO/PbS composites respectively. Values for Mn₃O₄ as widely reported is 3.07–3.55 eV [24, 25] while that of CuO is 1.90–2.91 eV [26, 27]. It can be seen that the formation of Mn₃O₄/PbS and CuO/PbS composites films shifted the fundamental absorption edge of core binary origin, thus providing tuning effect to the band gap for special applications. The primary function of a window layer in a heterojunction as contained in the literature is to form a junction with the absorber layer while admitting a maximum amount of light to the junction region and the absorber layer. The wide direct band gaps of the films placed them as suitable materials for window layers in solar cell fabrication.

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The refractive index (n), extinction coefficient (k), optical conductivity (σ_o), real dielectric constant (${\varepsilon }_{r}$), imaginary dielectric constant (${\varepsilon }_{i}$), electrical conductivity (σ_e) and thermal conductivity (σ_t) were calculated using the relations available in literature [28, 29].

$$\begin{eqnarray}&&n=\displaystyle \frac{1+R}{1-R}+\sqrt{\displaystyle \frac{4R}{\left(1+{R}^{2}\right)}-{k}^{2}}\end{eqnarray} \tag{ 8 }$$

$$\begin{eqnarray}&&k=\sqrt{\left(\displaystyle \frac{1}{2}{\left\{{\varepsilon }_{r}^{2}+{\varepsilon }_{r}^{2}\right\}}^{\displaystyle \frac{1}{2}}-\displaystyle \frac{{\varepsilon }_{r}}{2}\right)}\end{eqnarray} \tag{ 9 }$$

$$\begin{eqnarray}&&{\sigma }_{o}\left({s}^{-1}\right)=\displaystyle \frac{\alpha nc}{4\pi }\end{eqnarray} \tag{ 10 }$$

$$\begin{eqnarray}&&{\varepsilon }_{r}={n}^{2}-{k}^{2}\end{eqnarray} \tag{ 11 }$$

$$\begin{eqnarray}&&{\varepsilon }_{i}=2ink\end{eqnarray} \tag{ 12 }$$

$$\begin{eqnarray}&&{\sigma }_{e}{\left({\rm{\Omega }}{\rm{m}}\right)}^{-1}=\displaystyle \frac{2\pi }{\lambda nc}\end{eqnarray} \tag{ 13 }$$

$$\begin{eqnarray}&&{\sigma }_{t}=\displaystyle \frac{{\sigma }_{e}{\pi }^{2}}{3{\left(\displaystyle \frac{{K}_{B}}{e}\right)}^{2}T}\end{eqnarray} \tag{ 14 }$$

Figure 11 shows the plots of refractive index as a function of photon energy for Mn₃O₄/PbS and CuO/PbS composites thin films while figure 12 depicts the extinction coefficient plots against photon energy. From figure 11, we observed that the refractive index increased with increasing photon energy (decreasing wavelength) with a sharp decrease at 3.75 eV and 3.00 eV for Mn₃O₄/PbS and CuO/PbS composites respectively. The extinction coefficients of both films vary in the same manner in the VU and infrared regions. However, different behaviour is observed in the visible region. It decreased with photon energy in the visible region for Mn₃O₄/PbS film, whereas it increased with photon energy in the same region for CuO/PbS films. Generally, Mn₃O₄/PbS composite films showed lower values of extinction coefficients compared to CuO/PbS composite thin films.

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The plots of optical conductivity as a function of photon energy (figure 13) shows that Mn₃O₄/PbS and CuO/PbS composites films behaviour differently in the infrared region. i.e it increased with photon energy for Mn₃O₄/PbS composite films, whereas it decreased with photon energy for CuO/PbS composite films. However, in UV and visible regions, both films exhibited similar behaviour. We observed that the real dielectric constant (figure 14) and refractive index (figure 11) vary in a similar manner with photon energy. Such behaviour has been reported by other authors [22, 30–32]. The imaginary dielectric plots as a function of photon energy is shown in figure 15. Mn₃O₄/PbS films has higher values of imaginary dielectric compared to CuO/PbS films. However, both films showed similar behaviour in the UV and infrared regions. Figure 16 depicts the plots of thermal conductivity as a function of photon energy of Mn₃O₄/PbS and CuO/PbS composites thin films. Both films exhibited similar behaviour across the entire spectrum reaching a maximum in the UV region. An increasing trend of thermal conductivity with increasing photon energy (decreasing wavelength) was observed for both films.

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Mn₃O₄/PbS and CuO/PbS composites thin films were successfully coated on the glass substrate using chemical bath deposition technique. The structural, morphological, optical and solid state properties of the prepared films were analyzed. Both composites varied considerably with the structural, morphological, optical and solid state parameters. Both composites are semiconductors with different wide direct band gaps suitable for use as window materials in heterojunction solar cell, optoelectronic devices and high frequency applications.