Structural and Optical Dispersion Characterisation of Sprayed Nickel Oxide Thin Films

Safwat A. Mahmoud, Alshomer Shereen, Mou’ad A. Tarawnh
DOI: 10.4236/jmp.2011.210147                

Abstract

Crystalline and non-crystalline nickel oxide (NiO) thin films were obtained by spray pyrolysis technique (SPT) using nickel acetate tetrahydrate solutions onto glass substrates at different temperatures from 225 to 350℃. Structure of the as-deposited NiO thin films have been examined by X-ray diffraction (XRD) and atomic force microscope (AFM). The results showed that an amorphous structure of the films at low substrate temperature (T_s = 225℃), while at higher T_s ≥ 275℃, a cubic single phase structure of NiO film is formed. The refractive index (n) and the extinction coefficient (k) have been calculated from the corrected transmittance and reflectance measurements over the spectral range from 250 to 2400 nm. Some of the optical absorption parameters, such as optical dispersion energies, E_o and E_d, dielectric constant, ε, the average values of oscillator strength, S_o, wavelength of single oscillator λ_o and plasma frequency, ω_p, have been evaluated.

Keywords

Nickel Oxide Thin Films, Nickel Acetate, Optical Dispersion, Spray Pyrolysis Technique, AFM, XRD

1. Introduction

The optical properties of thin films are very important for many applications, including interference devices, such as antireflection coatings, laser mirrors and monochromatic filters, as well as optoelectronics, integrated optics, solar power engineering, microelectronics and optical sensor technology depending on the reflectance and transmittance properties of the films during their preparation.

Dispersion models parameterize the spectral dependencies on the dielectric functions of films or corresponding functions, such the band gap, optical dispersion energies, E_o, E_d, dielectric constant, ε, ratio between the number of charge carriers and the effective mass, N/m^*, wavelength of the single oscillator, λ_o, plasma frequency…etc., can be determined by data treatment. The optical constants of thin films provide us with information concerning with microscopic characteristics of the material.

Transition metal oxides like nickel oxides have found wide applications due to these anti-ferromagnetic semiconductor with wide gab ≈ 3.6 eV and cubic rock saltlike crystal structure [1,2]. It offers promising candidature for many application such as electrocatalysis [3], positive electrode in batteries [4], fuel cell [5], electrochromic devices [6], solar thermal absorber [7], catalyst for oxygen evolution [8] and photo electrolysis [9]. Several physical and chemical methods, such as sputtering [10], pulsed laser deposition [11], chemical bath deposition [10,12] and sol-gel [13] have been used to obtain nickel oxide films. All these methods offer different advantages depending on the application of interest and many efforts have been conduced to obtain films with the desirable physical and/or chemical properties. Among the different methods for film deposition, the relative simplicity of the spray pyrolysis method and its potential application for large area deposition make it very attractive, low cost and feasible for mass production processes.

This paper reports the influence of substrate temperature (T_s) as an important parameter on the preparation of nickel oxide (NiO) thin films by spray pyrolysis technique (SPT) using nickel acetate tetrahydrate solutions onto glass substrates. The accurate determination of the optical constants of these materials is important, not only in order to know the basic mechanisms underlying these phenomena, but also to exploit and develop their interesting technological applications. Therefore, the optical absorption parameters, such as optical dispersion energies, E_o, and E_d, dielectric constant, ε, the average values of oscillator strength S_o, wavelength of single oscillator λ_o, and plasma frequency, ω_p, have been evaluated under the effect of substrate temperature.

2. Experimental Details

2.1. Film Preparation

Nickel acetate tetrahydrate (Ni(C₂H₃O₂)₂·4H₂O) in a concentration of 5 × 10^–2 M in ethanol was used as a starting solution for deposition of nickel oxide films. Nickel acetate tetrahydrate decomposes in a two stepprocess, firstly dehydration from 95˚C to 150˚C and secondly decomposition of the acetate between 300˚C to 350˚C [14,15]. The solution was passed through a pneumatic nebulizer with a nozzle diameter of 0.7 mm. The spraying process lasted for about 15 s. The period between spraying processes was about 3 min; this period is enough to avoid excessive cooling of glass substrate. The overall reaction process can be expressed as heat decomposition of nickel acetate to clusters of nickel oxide in the presence of water and air oxygen.

In order to explore the influence of the ultrasonically cleaned preheated glass substrate temperature (Ts), different deposition temperatures ranged between 225 to 350˚C were used to prepare these films. In order to get uniform thin films, the height of spraying nozzle, solution molarity and the rate of spray process were kept constant during the deposition process at 35 cm, 5 × 10^–2 M and 15 cm³/min, respectively. A thermocouple was fixed to the substrate surface using silver past and the temperature was measured at the four corners of the glass substrate surface, then the results were averaged and the standard deviation was calculated (±5˚C).

2.2. Thickness Measurements

Thickness of the as-deposited films prepared at different substrate temperatures was determined by multiple-beam Fizeau fringes at reflection using either white light or monochromatic light (Hg, λg = 546 nm). A glass substrate was partially masked during deposition of the film to obtain a sharp edge. Then, a thermally evaporated aluminum layer coated the total surface of the substrate. The colored interference fringes enabled the determination of the order of magnitude of the fringes shift, while the monochromatic fringe shift, as a fraction of the order separation was measured using an eyepiece micrometer.

2.3. Structural Investigation

Crystallinity and the phases of the as-deposited films were characterized using JEOL X-ray diffractometer (XRD) (Model JSDX-60PA) with monochromatic highintensity Cu K_α radiation (λ = 0.154184 nm). Continuous scanning was applied with a slow scanning speed (1˚/min) and a small time constant (1 s).

2.4. Microstructure Analysis

Atomic force microscope (AFM) was used to analyze microstructures by molecular imaging in twoand three-dimensions. The atomic cross-section and surface roughness were determined. Each sample before AFM investigation was washed carefully with distilled water and dried. A Nanoscope III instrument with a 100 μm long silicon tip with repulsive and constant interaction force less than a few nano-Newtons were used [16].

2.5. Optical Measurements

Optical transmission and reflection of the prepared films were recorded over the wavelength range from 250 - 2400 nm using Shimadzu UV 3101 PC; UV-Vis-NIR double-beam spectrophotometer with reflection attachment based on V-N geometry (incident angle 5˚). The spectral variation of transmission and reflection obtained in this work are used to provide a qualitative guide to the film quality. A laboratory developed computer program [16] was used to calculate the absorption coefficient, energy gaps, optical constants and dielectric constants.

3. Results and discussion

3.1. Variation of the Film Thicknesses with Substrate Temperature

Thickness of the obtained films was investigated as a function of the substrate temperatures ranged between 225˚C and 350˚C and the data are graphically represented in Figure 1. It was found that the film thickness decreases with increasing the substrate temperature despite the fact that, the kinetics of the NiO forming reaction should increase with temperature. The observed dependence can be explained by the diminished mass transport to the substrate at higher temperature due to gas convection from bath pushing the droplets of the precursor, as well as the rate of re-evaporation was increased at

Figure 1. Variation of film thicknesses with substrate temperatures.

higher temperatures. In addition, the decrease in the film thickness may be attributed to water loss [17] or removal of interlayer water with consequent formation of compact NiO films.

3.2. Structural Characterization

Figure 2 shows the variation of the diffraction patterns as a function of the substrate temperature. The Bragg geometry is employed for analysis and the results of XRD for the as-deposited NiO films at T_s = 225˚C – 250˚C show that the film structure is non-crystalline structure. Further increasing in the substrate temperature up to ≥ 275˚C, the films were fully transformed to polycrystalline phase of NiO (JCPDS 47-1049) with cubic structural [9,18,19].

Conflicts of Interest

The authors declare no conflicts of interest.

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