Optical and Structural Properties of Natural MnSeO₄ Mineral Thin Film

Abstract

Manganese selenite (MnSeO₄) crystalline thin film has been produced with chemical bath deposition on substrates (commercial glass). Properties of the thin film, such as transmittance, absorption, and optical band gap and refraction index have been investigated via UV/VIS Spectrum. The structural properties of orthorhombic form have been observed in XRD. The structural and optical properties of MnSeO₄ thin films, deposited at different pH levels were analyzed. Some properties of the films have been changed with the change of pH level, which has been deeply investigated. The grain size of MnSeO₄ thin film has reached its highest value at pH 9. The refraction index and extinction coefficient of MnSeO₄ thin films were measured to be 1.53, 2.86, 2.07, 1.53 (refraction index) and 0.005, 0.029, 0.014, 0.005 (extinction coefficient) for grain sizes 21, 13, 26, and 5 nm respectively. The band gaps (Eg) of the films were measured to be 2.06, 2.57, 2.04, and 2.76 eV for the grain sizes mentioned above. The value of dielectric constant at pH 10 was calculated as 1.575.

Keywords:
Crystal growth; thin films; chemical synthesis; X-ray diffraction; optical spectroscopy

1. Introduction

Selenium is one of the most important elements of 6A group in periodical table in terms of utilization areas. This element and his compounds, including selenite, have various characteristics. Selenate can be obtained from a salt or ester by replacing one or both hydrogen of selenic acid with metal ions or with an organic group. Selenate, unlike sulfate, is a good oxidizer and can be easily reduced to selenite or selenium. Selenium's valence can be reduced from +6 to +4 and -2 by removing the oxygen atoms, but it is really difficult to reduce its valance from +6 to -2. Some chemical properties of selenate are similar to sulfate. But, unlike sulfate, metallic compounds of selenate are not soluble in water. Cadmium, copper, nickel and manganese selenite are analogues of each other. These are known as metallic salts of selenic acid. Metals are bond to the oxygen atoms of selenite anion^1,2.

Researchers have worked on the thermo-gravimetric analysis of some divalent metals' selenite, obtained by the hydration and decomposition of manganese (II), cobalt, nickel, copper (II), and zinc cadmium and cobalt selenite³. Other researchers investigated metal selenite as open-framework materials which are showing interesting physical properties⁴. Moreover, some of them worked on the biologic effects of selenite on cells⁵. But, nobody examined optical properties of metal selenite (especially manganese selenite) up to now.

On the other hand, being easy to produce, low cost and not harmful for human health, natural mineral oxide thin films have become a focus of subject of the researchers. These kind of metal oxide materials may be very useful in technology because they are used as p-type absorber in solar batteries, detectors and sensors. P-type absorbing materials, which have a high production cost, are much-needed in the technology; thus the manufacturing of them, where there is a shortcoming, has a particular significance. The most common example of them is Cu₂O thin films^6-10. This mineral, which is widely present in the nature, is a quite stable compound. Therefore, with the addition of required chemicals, copper can be easily transformed into its oxide in aqueous media. On the other hand, heating at high temperatures and adding much more chemicals are needed for the production of other metal oxide materials. Therefore, the production of natural mineral thin films poses a particular importance for technology^11-14.

In this study, structural and optical properties natural MnSeO₄ mineral thin films, which were produced via chemical bath deposition method, have been investigated. The pH value of the bath has been used to control structural and optical properties of MnSeO₄ thin films. Chemical bath deposition method has been preferred because it is a simple and cheap method, and as far as we know nobody has produced natural MnSeO₄ mineral thin films up to now neither with this method, nor using other ones.

2. Experimental

The preparation and production of selenite anions is the most important stage of this method. 0.01 mol solid selenium was dissolved in the water with 0.04 mole potassium hydroxide, which was four times more concentrated than selenium. But, since the pH level of the bath was very high for the deposition, 5 mL HCl solution was added in order to decrease the pH value to 11. As another component of the bath, 0.01 M manganese nitrate solution was prepared. Then, 20 mL selenite solution and 20 mL manganese solution (with equal concentration) were mixed in a beaker. Glass substrates of 76x26 mm, soaped with detergent and rinsed with distilled water, were dipped into this bath.

The pH levels of the baths were set as 10, 9 and 8, by adding 2, 4 and 8 mL of 8% HCl solution. The temperature and deposition time was set as 4 hours and 50°C for all baths. The pH levels of the chemical baths were measured using a pH meter (Lenko mark 6230N).

The crystalline structure of the MnSeO₄ was confirmed by X-ray diffraction (XRD) with a CuKα₁ radiation source (Rikagu RadB model, λ=1.5406 Å) over the range 10º <2θ<90ºat a speed of 3º min^-1 with a step size of 0.02º. The optical measurements were determined by Hach Lange 500 Spectrophotometer at room temperature by placing an uncoated identical commercial glass substrate in the reference beam. The optical spectrum of thin films was recorded in the wavelength range of 300-1100 nm.

3. Results and discussion

The chemical reactions occurred during the bath deposition are summarized below. When manganese ions (Mn⁺²) is added to the solution containing selenate (SeO₄^-2) the precipitation of manganese selenite (MnSeO₄), which is not soluble in water, is observed.

(1) ${\mathrm{Se}}_{(k)}\stackrel{\mathrm{KOH}/H2O}{\to }\mathrm{SeO}_4^{-2}{}_{(\mathrm{aq})}$

(2) $\mathrm{Mn}^{+2}{}_{(\mathrm{aq})}+\mathrm{SeO}_4^{-2}{}_{(\mathrm{aq})}\stackrel{}{\to }{\mathrm{MnSeO}}_{4\left(k\right)}$

Equations (1) and (2) show the formation of SeO₄^2- and MnSeO₄. In an aqueous environment with a basic solution, selenium is dissolved in the form of selenite anions.

Figure 1 displays XRD patterns of various MnSeO₄ thin films, produced at different pH levels, using chemical bath deposition method (a: pH 11, b: pH 10, c: pH 9, d: pH 8). In addition, the comparisons of these peaks with ASTM values are given in Table 1. Nearly, amorphous structures were observed at pH 11, 9 and 8, but it was comparatively lower at pH 10 where the highest peak was observed. Thus, the method and the bath prepared at pH 10 can be set as a specific methodology for the production of MnSeO₄ thin films. An orthorhombic structure was observed with the indexing of the structure on XRD EVA program (a= 4.93, b= 9.13, c= 7.02 Ǻ; Z: 4). The comparison of XRD results with ASTM values fitted well with standard values.

Figure 1
X-ray patterns of MnSeO4 films deposited at various pH levels: (a) pH 11, (b) pH 10, (c) pH 9 and (d) pH 8

Table 1
XRD datas of ASTM values versus films

pH	ASTM Data File	ASTM Value	Observed Value	Miller İndice
11	017-0840	25.42	26.43	MnSeO4 (020)
10	017-0840	19.49	19.60	MnSeO4 (110)
	017-0840	25.42	25.88	MnSeO4 (021)
	017-0841	32.12	31.60	MnSeO4 (110)
9	011-0683	28.29	27.72	MnSeO4 (111)
8	011-0683	32.75	32.33	MnSeO4 (022)

For all films, grain size (D), which is the structural parameter, were evaluated by XRD patterns and presented in Figure 2. The grain size of the thin films was calculated from XRD patterns using Debye Scherrer's formula¹⁵,

(3) $\mathit{D}=\frac{0.9\mathit{\lambda }}{\mathit{B}\cos \mathit{\theta }}$

Figure 2
The grain size (D) of MnSeO₄ thin films deposited at various pH levels

where D is the grain size, λ is the X-ray wavelength used, β is the angular line width at half-maximum intensity in radians and θ is Bragg's angle. The grain size of the films is calculated using the peaks of FWHM (020) at pH 11, (021) at pH 10, (111) at pH 9 and (022) at pH 8, obtained through the Scherrer's method which takes peaks of XRD patterns.

The highest average grain size of MnSeO₄ thin films has been found at pH 9. The average grain sizes of the thin films are 21, 13, 26, and 5 nm for pH 11, 10, 9 and 8 respectively. The reason of the low grain sizes at pH 10 and 8 is the strength of crystallization peaks. The dominance of crystal structure to amorphous structure is another reason. When the average grain sizes get smaller, the grains stack more stringently, which causes the spaces between grains to decrease. Thus, the deposition of the film on such structures leads to more orderly structures. This can be seen on XRD patterns as well. So, the peaks that we have observed at pH 10 are in line with the obtained results.

The optic band gap energy (Eg) was calculated from the absorption spectra of the films using the following relation^16,17:

(4) $\left(\mathit{\alpha }\mathit{hv}\right)=\mathit{A}{\left(\mathit{hv}-\mathit{Eg}\right)}^{\mathit{n}}$

Where A is a constant, α is absorption coefficient, hν is the photon energy and n is a constant, equal to 1/2 for direct band gap semiconductor. The plot of (αhν)² versus hν is drawn in figure 3 (a: pH 11, b: pH 10, c: pH 9, d: pH 8).

Plot of (αhυ)² vs. hυ for MnSeO₄ films at different pH levels (a) pH 11, (b) pH 10, (c) pH 9, (d) pH 8 is displayed in figure 3. As mentioned before, we were able to control the structures of the films by controlling pH levels of the baths. This caused variations on the grain sizes for each pH level and optic band gaps have been varied accordingly. Optic band gaps of the films are 2.76, 2.57, 2.06 and 2.04 eV for average grain sizes 5, 13, 21, 26 nm, respectively. As can be seen, the grain size is inversely correlated with optical band gap, which were also observed by other researchers before. The quantum size has affected optic band gap¹⁸.

Figure 3
Plot of (αhυ)² vs. hυ for MnSeO₄ films deposited at various pH levels: (a) pH 11, (b) pH 10, (c) pH 9, (d) pH 8

The transmittance (T) of MnSeO₄ thin film can be calculated using reflectivity (R) and absorbance (A) by the following expression^16,17:

(5) $\mathit{T}={\left(1-\mathit{R}\right)}^2{\mathit{e}}^{-\mathit{A}}$

Figures 4 and 5, show the transmittance, reflectance and the absorbance of MnSeO₄ thin films obtained from baths with different pH levels. The lowest transmittance and the highest reflectivity values were observed at pH 10. This is due to the fact that the absorption of light by crystal structure is higher. The absorption curve behaved as the reflectance curve and showed an increase with the decrease of the wavelength¹⁹. The refraction indexes and extinction coefficients of the films are calculated by the formulas^16,17:

(6) $\mathit{n}=\frac{\left(1+\mathit{R}\right)}{\left(1-\mathit{R}\right)}+\sqrt{\frac{4\mathit{R}}{{\left(1-\mathit{R}\right)}^2}-{\mathit{k}}^2}$

(7) $\mathit{k}=\frac{\mathit{\alpha }\mathit{\lambda }}{4\mathit{\pi }}$

Figure 4
Transmittance and reflectance of MnSeO₄ thin films deposited at various pH levels

Figure 5
Absorbance of MnSeO₄ thin films deposited at various pH levels

The refraction index is shown in Figure 6. The refraction index and extinction coefficient of MnSeO₄ thin films were parallel with average grain size. The refraction indexes and extinction coefficients of MnSeO₄ films deposited at various pH levels were calculated as 1.53, 2.86, 2.07, 1.53 (refraction index) - 0.005, 0.029, 0.014, 0.005 (extinction coefficient) at 550 nm wavelength, for average grain sizes of 21, 13, 26, and 5 nm. The values of refraction index and extinction coefficient show a natural behavior. Since the refraction of the light is bigger in scattered structure, the refraction index is higher for amorphous films.

Figure 6
Plot of the refraction index and extinction coefficient of MnSeO₄ films deposited at various pH levels

The dielectric constant (ε) can be calculated using the following equation:

(8) $\epsilon \epsilon =\frac{4\mathit{R}}{1-{\mathit{R}}^2}-{\mathit{k}}^2$

Dielectric constant also varied with the pH level of deposition bath and calculated to be 0.195, 1.575, 0.633 and 0.198 at 550 nm wavelengths (shown at Figure 7). Dielectric constant is a measure of vulnerability to the optical field. At 550 nm wavelengths, the highest dielectric constant was calculated for pH 10, where the highest peaks have also been observed. This fact was expected considering the orientation of the grains around the regular axis. The sum of the dipole moment vectors is higher at the film deposited at pH 10 compared to the other films, so the interaction of the light with the particles was higher in this regular structure.

Figure 7
Plot of dielectric constant of MnSeO4 films deposited at various pH levels

Table 2 shows EDX analysis of empty glass and MnSeO₄ thin film, which is a natural mineral, deposited on this glass. It will be a mistake to comment on the amount of oxygen due to the empty glass used for deposition. On the other hand, we cannot use the percentages of Mn and Se elements directly for other elements because of the contribution of the substrate, however we can calculate Se/Mn ratio theoretically from the chemical formulation of MnSeO₄. Theoretically Se/Mn ratio of this compound (in other words the ratio of the mass of the atomic weights in one mole) is 1.43. In our experimental analysis, this ratio was found to be 1.44, which is quite close to the theoretic ratio.

Table 2
EDX Analysis

% Element
Element	Empty Glass	MnSeO4 Thin Film
Na	9,02	-
Mg	2,26	-
Al	0,72	-
Si	35,10	3,25
K	0,21	-
Cl	-	6,40
Ca	5,63	-
O	47,06	22,82
Se	-	32,53
Mn	-	32,59

Natural MnSeO₄ mineral thin films may be very good alternatives for microelectronics world, where the materials with low dielectric constant are valuable (pH 8 and 11)²⁰. A near-field optical technique, using a new type of solid immersion lens (using a SIL made from n=1.83 glass), applied to the writing and reading of domains in magneto-optic material (written and read 350 nm diameter magnetic domains), serves to increase the numerical aperture of the optical system by n, where n is the index of refraction of the lens material²¹. Because of the unavailability of optical materials with very low refraction indices that closely match the refraction index of air, such broadband antireflection coatings have not been realizable (pH: 8 and 11). Also, they demonstrate their potential for antireflection coatings by virtually eliminating Fresnel reflection from an AlN-air interface over a broad range of wavelengths²². The film of pH: 11 may be useful IR detectors because of higher transmission after the 700 nm wavelength (% T > 90)²³.

4. Conclusion

In this study, we tried to produce and investigate MnSeO₄ thin films, which were not studied by any researcher up to now. The films were produced at various pH levels, such as 11, 10, 9 and 8. Acidic values were not tried concerning about the formation of selenic acids in acidic environments. Optical properties of the films were investigated for various deposition pH's. Characteristics, such as optic band gap and refraction index were found to be in line with the average grain size and crystal structure of the films, moreover some of the results were compared with the literature. Most importantly, we do believe that a specific method has been discovered for the production of MnSeO₄ thin films. Considering their characteristics, these films are good candidates for the optical materials. In addition, it has been found that natural MnSeO4 mineral thin films have potential of being used as lens, magnetic-optic material and IR detector. The production methods of these kinds of different materials may be enlightening for the innovative researchers.

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Publication Dates

• Publication in this collection
  13 Mar 2017
• Date of issue
  May-Jun 2017

History

• Received
  01 Dec 2015
• Reviewed
  26 Jan 2017
• Accepted
  18 Feb 2017