UV region supercapacitor: Bi-doped natural MgO rock salt thin film

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Abstract

This study is based on MgO thin films, on which different amounts of bismuth were doped. XRD, SEM and EDX were performed for the characterization of thin films, whereas capacitance of the films was examined through current-voltage measurements. In addition, capacitance of these films was measured in the environments with violet, green, red and yellow lighting. It was found that under violet light, in other words around the UV region, the capacitance of Bi-doped thin films increased by 15%. SEM and EDX analysis revealed that the percentage of bismuth on the surface was dramatically increased with the increase of doped bismuth amount. On the other hand, various differences were observed on the surface images of doped and undoped films, serious differentiations were especially observed on the morphologic structure of 15% Bi-doped films.

Introduction

Magnesium oxide (MgO) is formed with the hybridization of magnesium atom's s-orbitals, which is the valence shell of magnesium atom, with p-orbitals of oxygen atom. MgO, which is known as a natural rock salt, has NaCl-type cubic structure. MgO thin films' dielectric constant with high ferroelectric property (9.8), broad bandwidth (7.8 eV), higher breakdown field (12 MV/cm), and capacitance [1] properties provide them a broad area of utilization in infrared optic, gas detector, reflector, heat and magnetic device areas [[2], [3], [4]].

Many methods have been tried to produce MgO thin films, which have a wide range from chemical methods to production with devices, including: spin coating [3], rf/dc sputtering [4], cathodic vacuum arc deposition [5], thermal oxidation [6], sol-gel [7], atomic reduction [8], metal-organic decomposition [9], spray pyrolysis [10], ion beam assisted deposition [11]. In each of these methods, magnesium is first reduced to its hydroxide in alkaline medium, then it is converted into its hydroxide by annealing at high temperature. Annealing is not required for the devices operating at high temperatures, however it is obligatory for chemical methods.MgCl(aq) + 2NaOH(aq) → Mg(OH)2(s) + 2NaCl(aq)

The biggest advantage of the chemical method defined in Equity (1) is, the NaCl salt formed while producing Mg(OH) in solid form as thin film, automatically passes to aqueous environment without forming any contamination in the environment.Mg(OH)2(s)HeatingMgO(s)+H2O

The water on the substrate will move off at high temperature, allowing the production of high-quality MgO films.

Even though MgO thin films produced through chemical bath method are quite rare in the literature, as we mentioned earlier MgO thin films have been successfully produced several times through sol-gel [7], spin coating [3] and SILAR [12] methods, which are sub-methods of chemical bath. Researchers started to identify MgO XRD peaks in the films produced at 400 °C [12]. They found that MgO thin films’ crystallites produced at 400 °C have orientations such as (002) and (220) [12], however these orientations are transformed into (200), (111) and (222) over 400–500 °C [13,14].

Especially after 2000s, researchers have remarked that observable changes occur on the structural, optical and electrical properties of MgO thin films by doping. In addition to Mu metal doping, elemental doping such as chromium [15], aluminum [16] and iron [17] doping was experimented. For example, it was found that with the doping of 4% Aluminum into MgO thin film, optical transmittance increased from 90% to 99%, refractive index from 1.453 to 1.454 and energy band gap from 2.30 eV up to 3.55 eV [16]. Serious changes have been observed on the morphology and crystalline structure of the films by doping chromium into MgO thin films [15]. As a result of iron doping, cell parameters were changed, magnetic susceptibility was increased (went up to 10 μB/f.u. from 4 μB/f.u.) and the polarization augmented by approximately 60% [17].

In this study, we doped different amounts of bismuth to MgO thin films that we produced at 450 °C, an experiment which has not been tried before, and we examined both structural and capacitance properties of the films. In addition, we tried to understand how capacitance properties of bismuth-doped MgO thin films vary under lightings at different wavelengths. We obtained very interesting results at the end of the study.

Section snippets

Materials and measurements

All chemicals were purchased in analytical purity (Sigma Aldrich). Amorphous glasses were used as the substrate material in the deposition of the films. These glasses were cleaned before use and washed with de-ionized water.

Crystalline structure of magnesium oxide was confirmed by X-ray diffraction (XRD) with a CuKα1 radiation source (Rikagu RadB model, λ = 1.5406 Å) over the range 10° <2θ < 90° at a speed of 3° min−1 with a step size of 0.02°. Surface properties of the films were measured

Results and discussion

XRD graphs and data of the films are given in Fig. 3 and Table 1. Even though undoped MgO thin films exhibit an almost amorphous structure, a broad and shallow peak belonging to MgO4 compound was identified. When bismuth was doped into the films, peaks with similar intensity belonging to MgO4 and Bi4O7 were identified, especially for 5–10% doping. However, very strong MgO4 and Bi4O7 peaks were identified in 15% bismuth-doped thin films. The doped bismuth has been converted into Bi4O7 as a

Conclusion

As a result, we found that the capacitance of MgO thin films that we doped different percentages of bismuth increases as doped Bi amount increases. We also demonstrated that the capacitance can change with different light sources, in other words with energy sources having different energy level, which made this study a work that can be used in space researches and in the design of electronic devices that will be used in space. Because space is the place that we benefit from UV light the most.

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