Effect of Cd Doping on Structural and Some Optical Studies of Nano CuO Films Prepared by Sol–Gel Technique

WSN 64 (2017) 69-83 EISSN 2392-2192

Effect of Cd Doping on Structural and Some Optical Studies of Nano CuO Films Prepared by Sol–Gel

Technique

Raheem G. Kadhim and Ban Raheem Saleh Kzar

Department of Physics, Collage of Science, University of Babylon, Babylon, Iraq E-mail address: [email protected] , [email protected]

ABSTRACT

This research deals with the study of the structural and optical properties of CuO and Cd doped CuO nano films in thickness 100nm were deposited on a glass substrate using sol-gel spin coating technique. XRD results indicated that the films are polycrystalline and have monoclinic structure with a preferred orientation along ̅̅̅̅̅̅̅̅ and the grain size increase with increasing concentration Cd. Data of AFM indicate that the surface of films is smooth. From SEM image observed the particles are smallest an even particle distribution on a smooth surface. The optical transition in the CuO and (Cd–

doped CuO) thin films are observed to be allowed direct transition. The value of the optical energy gap decreases with increasing of (Cd) for all samples.

Keywords: CuO; Cd; Thin Film; Sol–Gel Spin Coating Technique

1. INTRODUCTION

Different transition metal oxides (copper, iron, nickel, zinc and cobalt) have a numerous applications. One of these is cupric oxide (CuO) as an important P-type transition metal semiconductor oxide has been extensively studied. CuO has been established as a number of applications like gas sensors [1-3]. solar photovoltaic [4,5], lithium ion electrode [6] etc.

This is an attractive system for many researchers due to its photoconductive and photo- thermal applications and for studies of the transport mechanism in cuprates with high-TC superconductivity [7]. CuO is considered to be a p-type transition metal semiconductor with a narrow band gap of 1.2-1.5 eV [7], Doped copper oxide thin films have found applications such as in the fabrication of p-type transparent conductors [8].

There are various established ways of fabricating CuO thin films like spray pyrolysis [9], spin coating [10,11], dip coating [12,13], SILAR [7,14] to name a few. Among all these Spray pyrolysis technique has stoichiometry in multi-component system and splendid control of chemical uniformity As the presence of acceptor levels attributed to copper vacancies, CuO have several advantages: (i) non-toxic nature (ii)availability and abundance of the starting materials, (iii) low production cost, (iv) p-type conductivity and (v) and band gaps lie in an acceptable range for solar energy conversion [15-16].

We have prepared thin films from undoped CuO and Cd- doped CuO using sol-gel spin coating technique. The effects of Cd doping on the structure and some optical properties of CuO films were studied.

2. EXPERIMENTAL

CuO and (Cd–doped CuO) thin films were deposited onto glass substrates by the sol–

gel method using a spin–coating method. The CuO and (Cd–doped CuO) precursor solutions were prepared starting from:

1. Copper (II) Chloride dehydrate (CCD) (2 g).

2. Methanol (20 mL).

3. Glycerol (185 μL).

4. Triethylamine (870 μL).

5. Hydrochloric acid (1 μ).

At first, the CuO precursor solution was prepared by the following procedure:

First solution: the Copper (II) Chloride dehydrates was dissolved in half of the methanol (10 mL) for each g of (CCD) at constant magnetic stirring until a transparent solution was obtained. The glycerol was added to the solution. Second solution: the trimethylamine previously dissolved in the other half of the methanol (10 mL) was also incorporated and the (HCl) was added in order easy dissolved process to the solution. Storing the mixture of the two solution for 24 hours at room temperature.

The resulting solution is completely yellow-Green. The glass substrates were first cleaned by detergent, and then in methanol and acetone each for 10 min by using ultrasonic cleaner. At last, the substrates were rinsed with deionized water and dried with nitrogen.The dissolving of (0.1 g) of Cadmium acetate dehydrate Cd(CH₃COO)₂·2H2O nitrate in (25 ml) of methanol to prepare Cadmium and then doping the original solution with Cadmium in three ratios of (1%, 2% and 4%).

The coating solution was dropped into the glass substrate, which was rotated at 3000 rpm for (40 s) using LAURELL WS-400B-6NPP/LITE spin coater. Figure 1 shows flow chart of CuO : Cd synthesis and the structural and optical measurements.

3. RESULT AND DISCUSSION 3. 1. Structural studies

X-ray diffraction patterns (XRD) of prepared for CuO and (Cd–doped CuO) thin films are recorded at annealing temperatures (500) °C (1h) as shown in the Figure (2). The CuO (Cd) doped CuO thin films are found to have a polycrystalline structure for all samples with a direction in the ( ̅̅̅̅̅̅, (111) (020), (111)*, (220)* (311)*, and (222)* the intensity Of ( ̅̅̅̅̅) reflections increase with increase doped and grown in the monoclinic phase with strongly preferred orientation. and it is noticed that the crystallite size increases with increasing of concentration of (Cd), The crystallite size (D) was calculated using the Scherrer’s formula [17].

Figure 1.Flow chart of CuO : Cd synthesis and the structural and optical measurements.

SEM AFM

XRD Optical spectrophotometer

Annealing at 500 °C for 2h

Spin coating (at Ts 298K and 3000 rpm for 40 s) store the mixture for 24 hours

CuCl₂+Cd(CH₃COO)₂·2H2O) + methanol Trimethylamine + methanol + HCl

Mixing both solutions with 1 h

clean the substrate by aceotone and ethanol

Drying by 50 °C for 2 min (gel)

0.94 D cos 

 



where λ (1.54056 Ǻ) is the X-ray wavelength, θ is the Bragg’s angle and β is the full width at half maximum (FWHM) of the diffraction peak in radians From the Table (1) it has been shown that the values of FWHM of peaks increases with the increasing concentration of (Cd) but the crystalline quality become healthy compared to low doped and The standard interplanar distances (d-values) of CuO thin films are (1.70, 2.34, and 2.52) Å for diffraction peaks ( ̅̅̅̅̅), (111) and (020) respectively, and the d -values are shown in Table (1).

Table 1. The calculated crystalline size (G.s), lattice spacing (d), full width at half maximum (FWHM) and angle for undoped CuO and different concentrations (Cd-doped films at

thickness of (120) nm.

Sample 2θ (Deg.) d_Exp.(Å) FWHM

(Deg.) G.S (nm) hkl

Pure

35.5 2.52 0.2272 38.38 (111)

38.3 2.34 0.3409 25.79 (111)

53.7 1.70 0.4545 20.46 (202)

Cd 2%

35.4 2.532 0.3409 25.54 (111)

37.25 2.414 0.4545 19.25 (111)*

39 2.30 0.284 30.98 (111)

53.71 1.704 0.4545 20.455 (020)

56 1.640 0.1136 20.69 (220)*

66.5 1.404 0.3977 24.94 (331)*

69 1.359 0.4545 22.14 (222)*

Cd 4%

35.5 2.525 0.4545 19.16 (111)

37.5 2.414 0.2272 38.55 (111)*

39 2.306 0.2840 30.98 (111)

53.7 1.704 0.2262 40.92 (020)

56 1.640 0.2830 33.08 (220)*

66.5 1.404 0.2257 43.65 (331)*

69 1.359 0.2267 44.30 (222)*

Figure 2. X-ray diffraction spectra of CuO film doped with (a) 0% Cd, (b) 2% Cd and (c) 4% Cd.

a

b

c

3 .2. Surface Morphological Studies

Figure 3. AFM images for undoped CuO and various Cd –doped CuO films.

(-1- for 2D, and -2-for 3D)

2D CuO: 4% mol Cd 3D 2D CuO: 2% mol Cd 3D

2D CuO: Pure 3D

The surface morphology of undoped CuO and Cd doping by 2%, and 4% mol, was studied measured at thickness of 100 nm, It was observed with AFM micrographs as shown in Figure 3. It can be notice that The grains were columnar, with Avg. diameter of (74.96-93.78) nm for Cd-doped and a root mean square (RMS) roughness ranged of 1.42-2.13 nm, Data above indicate that the surface of films is highly smooth as can be seen from the results listed in Table 2.

Table 2. AFM data for undoped CuO and various Cd –doped CuO films measured at thickness of 100 nm films.

3. 3. Scanning electron micrographs (SEM)

Figure (6) show scanning electron micrographs (SEM) of the undoped CuO and Cd- doped CuO films grown on glass substrates at temperature 055 °C. SEM micrograph reveals the formation of particles with different shapes and sizes. From the image observed the particles are smallest an even particle distribution on a smooth surface From this image we revealed that the film is highly dense with uniform surface The Cluster of nanocrystals was observed for pure CuO film. From the Figure (4) we find that CuO thin films nanoparticles size increases with proportional relationship with the doped. There are some bright spots on the surface of CuO thin films which are considered to be microparticles.

Item Avg. Diameter(nm) Roughness average Root mean square (rms)

Pure 74.96 1.42 1.71

Cd 2% 93.78 1.65 1.94

Cd 4% 75.73 2.13 2.53

a

Figure 4. Scanning electron microscopy images of (a) undoped CuO thin film, and CuO doped with (b) 2% Cd, and (c) 4% Cd.

3. 4. Some Optical Studies

The optical properties of deposited CuO and (Cd) doping by 1%, 2% and 4 mol thin films on glass, substrates have been determined by using visible transmittance spectrum in the region of (300-1100) nm, also calculate the optical energy gap for direct transition, absorption coefficient and optical constants like refractive index and extinction coefficient.

b

c

3. 4. 1. Absorbance

The optical absorbance dependence on the wavelength (λ) in the spectra range (300- 1100) nm for undoped CuO and (Cd) doped CuO thin films measured at thicknesses of (100) nm respectively are shown in Figure (5) films higher absorption on the shorter wavelength side (ultraviolet region), and low absorption on the higher wavelength side (visible region).

The spectra reveal that the absorbance was decreased by the increase in doping.

Figure 5. Optical absorbance VS. wavelength for CuO and various Cd-doped CuO nano films

3. 4. 2. Transmittance

Figure 6.Optical transmittance spectra VS wavelength for CuO and various Cd-doped CuO.

The optical transmittance dependence on the wavelength (λ) in the spectra range (300- 1100) nm for undoped CuO and (Cd) doped CuO thin films measured at thicknesses of (100) nm respectively. are shown in Figure (6). It is clear from this Figure that the transmittance spectrum of all deposited thin films increases with the increasing of wavelength (λ). The

optical transmittance value of nano CuO film reaches to 94% in the VIS and NIR regions, which is important for its applications as window layers in solar cells This behavior may be attributed to microstructural features of prepared films, as the photon scattering increases by crystal defects [18].

3. 4. 3. The Absorption Coefficient (α)

The absorption coefficient (α) of the CuO and (Cd) doped CuO thin films deposited on glass substrate at annealing 500 °C are shown in Figure (7). The absorption coefficient exhibits high values (α > 104) which means that there is a large probability of the direct transition [19], The absorption coefficient dependence on the wavelength in the spectral range (300-1100) nm, and then (α) decreases with the increasing of wavelength .It is observed that the absorption coefficient (α) decrease with increasing the concentration of Cd. the absorption coefficient (α) for the prepared thin film which calculated from eq. (1.2).

Figure 7. The absorption coefficient for CuO and various Cd-doped CuO nano films

2.303A



The refractive index of the CuO and (Cd) doped CuO thin films deposited on glass substrate at annealing (500) °C are shown in Figure (8). It can be noticed that the refractive index decreases with the increasing of doped ration and the refractive index decreasing with increasing of wavelength. The refractive index (n) was calculated from relation (1.3).

1/2 2 2

4 ( 1)

( 1) ( 1)

  

     

R R

n k

R R (1.3)

where R represents the reflectance which can be calculated from the following equation (1.4)

1

R T    A

Figure 8. The Refraction index versus wavelength for CuO and various Cd-doped CuO nano films

3. 4. 5. Optical Energy Gap

The values of optical energy gap (Egopt

) for CuO and (Cd) doped CuO thin films deposited on glass substrate at annealing (500) °C, have been determined using Tauc (Egopt

) is determined by the extrapolation of the portion at (αhυ)² from the relations between) (αhυ)² the photon energy (hυ), as shown in Figure (9) and Table (3). The direct optical energy gap (Eg) was calculated by using the relation (1.6)

hv A hv Eg

  

In general, the values of direct optical energy gap decreasing with increasing concentration of Cd for all samples. The direct (Egopt

) decrease from (1.6 to 1.4) eV.

Table 3. Show the values of Egopt

for CuO and (Cd-doped CuO) thin films

Sample Eg (eV)

CuO pure 1.6

Cd 1% 1.4

Cd 2% 1.48

Cd 4% 1.0

(a)

(b)

(c)

(d)

Figure 9(a-d). (αhυ)² as a function of photon energy (hυ) for for CuO and (Cd-doped CuO) thin films

4. CONCLUSIONS

Monoclinic structure is the CuO phase for CuO and (Cd-doped CuO) thin films and orientated along ( ̅̅̅̅̅̅). The AFM results revealed that the surface of films is highly smooth.

With increase Cd content an the roughness of film surface increases. SEM image observed the particles are smallest an even particle distribution on a smooth surface .The transmittance value of undoped CuO film reaches to 94% in the visible and NIR range, which is important for its applications as window layers in solar cells. The optical energy gap of CuO with concentration (1%, 2%, 3%and 4%) mol of Cd thin films were equal to1.6,1.4,1.48 and 1.5eV.

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( Received 20 December 2016; accepted 08 January 2017 )