Electrochemical synthesis and physical properties of Sn-doped CdO nanostructures

Accepted Manuscript Electrochemical Synthesis and Physical Properties of Sn-doped CdO Nanostructures

Zahra Portaghvaei, Farid Jamali-Sheini, Ramin Yousefi PII:

S0749-6036(16)31273-3

DOI:

10.1016/j.spmi.2016.10.064

Reference:

YSPMI 4609

To appear in:

Superlattices and Microstructures

Received Date:

20 October 2016

Accepted Date:

24 October 2016

Please cite this article as: Zahra Portaghvaei, Farid Jamali-Sheini, Ramin Yousefi, Electrochemical Synthesis and Physical Properties of Sn-doped CdO Nanostructures , Superlattices and Microstructures (2016), doi: 10.1016/j.spmi.2016.10.064

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Highlights:  Un- and Sn-doped CdO thin films deposited using electrodeposition method

 Optical and electrical properties of CdO thin films were studied.

 Oxygen vacancy has an important role in electrical and photo-switching

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behavior of Sn-doped CdO thin films.

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Electrochemical Synthesis and Physical Properties of Sn-

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doped CdO Nanostructures

Zahra Portaghvaei1,2, Farid Jamali-Sheini3,*, Ramin Yousefi4

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Department of Physics, Science and Research Branch, Islamic Azad University, Khuzestan, Iran

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Department of Physics, Ahvaz Branch, Islamic Azad University, Ahvaz, Iran 3

Advanced Surface Engineering and Nano Materials Research Center,

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Department of Physics, Ahvaz Branch, Islamic Azad University, Ahvaz, Iran Department of Physics, Masjed-Soleiman Branch, Islamic Azad University

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(I.A.U), Masjed-Soleiman, Iran

*Corresponding author: E-mail: [email protected], [email protected] Telephone No: +98 - 61 - 33348420 - 24 Fax No: +98 - 61 - 33329200

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Abstract

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Un- and Sn-doped CdO thin films were deposited on FTO-coated glass substrates using electrodeposition method. Crystalline study revealed a cubic phase for all samples. SEM images showed flower- and rod-like morphologies for the Sndoped samples in nano-dimensions. Optical energy band gaps of 2.70, 2.79, and 2.84 eV were obtained for Sn0, Sn1, and Sn2 samples, respectively. Electrical investigations indicted that CdO thin films revealed n-type conductivity with high density of donor levels, which exceeded by corporation of Sn cations in the

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crystalline structures of CdO thin films.

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Keywords: CdO thin films; Sn doping; Electrodeposition; Optical and electrical

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properties; Photocurrent applications

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ACCEPTED MANUSCRIPT Introduction A number of the metal-oxide semiconductors have recently received researchers’ due attention as a result of possessing suitable properties and

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remarkable advantageous such as high energy band gap, non-toxicity [1], high transparency [2], good electrical conductivity [3, 4], ease of large-scale synthesis [5], and higher technological applications such as photocatalytic materials [6], gas sensing materials [7, 8], antibacterial materials [9, 10], photovoltaic conversion [11, 12], field emission and photodetectors [13, 14]. Cadmium oxide (CdO) is one of the well-recognized metal-oxide semiconductors with n-type conductivity and direct and indirect band gap energy within the ranges of 2.2-2.5 eV and 1.36-1.98 eV, respectively [15].

Generally, Undoped CdO materials reveal high n-type conductivity due to

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presence of interstitial Cd atoms and/or oxygen vacancies, which act as donors in crystal structures [16]. High mobility of electrons at CdO is one of the significant parameters in CdO applications as a transparent conductive oxide (TCO). Hence,

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examination of electrical properties is of great value for researchers. Doping is one of the simplest and most well-known ways to engineer electrical and optical

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properties of semiconductors. Gupta et al. deposited In-doped CdO thin films on SiO2 substrates to examine the effect of substrate temperature on electrical

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properties with application of pulsed laser deposition technique [4]. The results of the mentioned study revealed that by increasing the substrate temperature of

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In-doped thin films, resistivity decreased and carrier concentrations and mobility increased. It was the first time that Un- and Pb-doped CdO films were prepared on glass substrates by successive ionic layer adsorption and reaction (SILAR) method by Gulen et al [17]. Optical studies indicated that by adding Pb as a dopant and increasing Pb concentrations, optical energy band gap values of CdO films increased. They attributed the observed increase in optical energy band gaps of Pb-doped films to the changes occurring in the crystal structures and incident 3

ACCEPTED MANUSCRIPT of quantum size effects. Benhaliliba et al. examined the growth of Un- and copper (Cu)-doped CdO thin films by sol-gel method [18]. Electrical and optical experiments showed that Cu as a cationic dopant decreases the electrical

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resistance and increases the optical energy band gaps of CdO thin films. So far, a good number of studies addressing the growth of tin (Sn)-doped CdO structures with application of different methods have investigated their various aspects and properties for a wide range of applications such as microwave irradiation technique in gas sensor applications [19], sol–gel technique in electrical applications [20], chemical vapor deposition (CVD) and pulse laser deposition (PLD) in optoelectronic applications [3, 21, 22], and successive ionic layer adsorption and reaction (SILAR) [23] and chemical bath deposition (CBD) methods [24].

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Among the various methods suggested for growth of CdO thin films, elctrodeposition is a simple and cost-effective method leading to deposition of thin films with nano-scale surface morphologies. In this method, a good control

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can be exerted on the growth parameter to obtain desired properties. To the best of researchers’ knowledge, an appropriate and thorough study addressing the

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electrical and photocurrent properties of Sn-doped CdO thin films grown by electrodeposition method has not been reported, yet. Hence, in the present study

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Un- and Sn (various concentrations)-doped CdO thin films were grown by electrodeposition method. Furthermore, optical, electrical, and photocurrent

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properties of Un- and Sn-doped CdO thin films were examined to facilitate their applications in optoelectronics devices.

Experimental section Electrochemical cells were provided by the following details for deposition of Un- and Sn-doped thin films. An electrolyte solution containing 5 mM of 4

ACCEPTED MANUSCRIPT cadmium sulfate (CdSO4, Merck, high purity), 1 mM of hydrogen peroxide (H2O2, Merck, high purity), and 40 mLit of distilled water were mixed by a stirrer for 5 min. Then, pH of solution fixed on 8 using 1 mM solution of potassium chloride (KCl, Merck, high purity). FTO-coated glass substrates with 2×1 cm

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size, platinum sheet, and saturated calomel electrode (SCE, E0 = 0.244 V vs. NHE) serving as the working, counter, and reference electrodes, respectively were prepared. The deposition growth process was performed in an electrodeposition cell with three electrodes. For deposition of doped samples, an aqueous solution of tin chloride (SnCl2, Merck, high purity, 5mM) as a source of dopant was prepared by adding different quantities (2 cc and 5 cc) of this solution to different beakers. The samples were called Sn0, Sn1, and Sn2 for the undoped and doped samples with different amounts of the solution, respectively.

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Substrates and platinum electrode were washed with acetone and ethanol solutions for 10 minutes in an ultrasonic bath to ensure no pollution was left on them. A computer-controlled electrochemical analyzer (Potentiostat, Autolab,

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A3ut71167, Netherlands) was used to create the cathodic polarization conditions at a voltage of -0.75 volts in the sequence of SCE (SCE, E0 = 0.244 V vs NHE).

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The deposition duration was 40 minutes, and the temperature of electrolyte solution was keeps at 80 ◦C for all samples. After deposition and removal of the samples from the electrolyte solution, they were gently washed with distilled

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water and then dried. To obtain higher structural phase of CdO films, one-step heat treatments at 200 ◦C for 2 h were administered to all the samples, and then

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the characterization was carried out. The crystal structure of the obtained CdO thin films was studied with

application of an X-ray diffraction (XRD) system (model X'Pert-Philips) using a Cu anode with wavelength of 1.5406 Å. In order to study the surface morphology and elemental composition of deposited films, FESEM imaging and EDS analysis were performed using scanning electron microscope FESEM, Zeiss ΣIGMA VP. 5

ACCEPTED MANUSCRIPT Optical properties of thin films were also analyzed using the photoluminescence (PL) analyses and the absorption spectra. PL analysis was performed at room temperature with a UniRam system, which had a He-Cd laser operating at the excitation wavelength of 320 nm and power of 20 mW. The absorption spectrum

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was also recorded at room temperature using a UV-Vis Perkin Elmer Lambda 900 spectrophotometer.

I-V and photocurrent cell were assembled using the deposited films as working electrodes and Pt foil as the counter electrodes, which were sealed in a sandwich-type cell that was filled with I-/I-3electrolyte. The photocurrent response was measured by a potentiostat (Autolab, A3ut71167, Netherlands) using a xenon lamp (1.5 Air Mass) serving as the light source provided by a solar cell simulator (solar cell simulator IIIS-200+, Nanosat Co., Iran). With respect to C–V plot, an

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LCR meter (LCR-8000G Series Gw-instek) was employed to measure the capacitance to obtain Mott–Schottky plot. This measurement was carried out at a

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Results and discussion

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frequency of 1 kHz and voltage range of 10 to 1000 mVolt.

Fig. 1 presents XRD patterns of Un- and Sn-doped CdO thin films. As

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revealed in this figure, the obtained materials were crystalline in nature, and diffraction peaks thoroughly matched with bulk and cubic CdO phases (JCPDS Card No.01-078-0653) [25]. Inset of this figure shows a shift to higher positions

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for (200) peak. This shift was observed for other diffraction peaks, as well. These shifts are due to lower effective ionic radius of Sn (Sn2+ ~ 0.69 Å) compared with Cd (Cd2+ ~ 0.95 Å) [26]. SEM images and EDS spectra of Un- and Sn-doped CdO films are shown in Fig. 2. The SEM images presented a relatively smooth surface for undoped CdO films. However, by adding Sn (Sn1, Fig. 2c), the surface morphology 6

ACCEPTED MANUSCRIPT changed into the flower-like morphology with the average diameter of 35 ± 4 nm. By increasing the concentrations of dopants, surface morphology changed into a rod-like (Sn2, Fig. 2e) morphology with the average diameter of 156 ± 4 nm. The surface of Sn-doped CdO thin films was dense and devoid of any gaps. EDS

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spectra (Fig. 2) indicated increased Wt% (Wight percentage) of Sn, which increased in response to increasing the concentrations of dopants. This Wt% of Sn is equal to 1.51 and 3.77 At% (Atomic percentage) for Sn1 and Sn2 samples, respectively.

To examine the effect of Sn doping on optical properties of CdO thin films, photo-excited emission and absorption spectra of Un- and Sn-doped CdO thin films were recorded. Fig. 3 indicates the PL spectra of Un- and Sn-doped CdO thin films at room temperature (ambient conditions). The films demonstrated

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intense broad emission peaks at 515 nm (~ 2.41 eV), 512 nm (~ 2.43 eV), and 513 nm (~ 2.42 eV) for Sn0, Sn1, and Sn2 samples, respectively. These emission bands were observed in the green region of the visible spectra of electromagnetic

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waves and were well-known as near band edge (NBE) emissions. The reasons behind observation of this type of emission for CdO are the carriers transported

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between conduction band and valence band [27]. It is well-recognized that the emission band in the visible region is associated with defects originated from the interstitials of ions in crystal structures produced by transition of excited carriers

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from deep to valence band [13]. Hence, increasing the intensity of emission band with addition of Sn dopants reveals a significant increase in the defect levels and

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surface recombination rate of Sn-doped CdO films. A blue shift in position of band emissions was also observed by adding Sn dopant. The mentioned process returned the partial substitution of Sn atoms into the crystal lattice of CdO instead of Cd atoms. The band emissions of doped samples within the range of 690 nm to 740 nm could be attributed to a number of defects such as O vacancy, Cd interstitial, and etc [28]. 7

ACCEPTED MANUSCRIPT Absorption spectra of Un- and Sn-doped CdO films are presented in Fig. 4. CdO thin films showed high absorption in the ultra violet and visible regions of electromagnetic wavelength. Sn-doped films indicated lower absorption in comparison with the undoped films. An absorption edge of about 400 nm can be

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conjectured for all samples. To estimate the optical band gap energy of Un- and Sn-doped CdO films, the Kubbelka-Munke model and Tauc plots were used [29]. The mentioned method can be considered as a recognized and effective method for assessment of optical energy band gaps of semiconductors in micron- and nano-dimensions. Tauc plot of CdO thin films is shown in Fig. 5. This plot indicates higher optical energy band gaps for Sn-doped films in comparison with the undoped films. The observed increase in optical energy band gaps (blue shift in the absorption edge) is in correspondence with the PL results and is related to

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the substitutions of Sn atom in Cd sites of CdO crystal structures.

Electrical properties of CdO thin films were investigated addressing voltage-current and Mott-Schottky plots. Fig. 6 reveals the voltage-current

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characteristics of Un- and Sn-doped thin films in dark and xenon irradiation under ambient conditions. All samples presented a Schottky behavior and in

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illumination conditions showed lower current in the same voltage, which was the feature of n-type semiconductors. Furthermore, lower conductivity of CdO thin films was observed in the case of their exposure to the xenon radiations.

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Comparison of I-V characteristic of Un- and Sn-doped CdO thin films revealed that Sn as a dopant increased the electrical resistivity of CdO thin films. Similar

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results, i.e. high Wt% of Sn concentrations, have been reported by other research studies [22]. The mentioned variation of the electrical resistivity of Sn-doped CdO samples can be explained by resorting to defects in the doped samples, which is verified by PL results, as well. Hence, different concentrations caused various effects in the electrical properties which are originated from the substitutions and/or interstitial ions in the crystal [30]. 8

ACCEPTED MANUSCRIPT Mott-Schottky characteristic of CdO thin films were obtained by measuring the capacitance of films in the range of 0 to 1 volt. It would be definitely expressed that Mott-schottky method is one of the most accurate methods for investigation of electrical properties of thin films [29]. Fig. 7

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represents Mott-schottky plot of Un- and Sn-doped CdO thin films. The positive slope of Mott–Schottky plot confirms the n-type conductivity of all samples [31]. Moreover, it confirms the obtained results from the I-V plot. Hence, it can be certainly stated that obtaining the p-type conductivity of CdO is almost impossible [32]. The intersection of fitted line on the Mott-Schottky plot with horizontal axis determined flat band voltage. The flat band voltage as well as other electrical parameters of CdO thin films is presented in Table 1, which indicates that high donor concentration for all samples. This high donor

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concentration reflects merging of donor levels with conduction band of CdO thin films [33].

Fig. 8 displays photoresponse of Un- and Sn-doped thin films. Generations

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of photocurrents in the CdO cell is assumed to be a process occurring in the space charge region (SCR) of CdO [34]. This plot shows a high photocurrent for Sn1

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sample compared with that of the other samples. The measurements of the present study showed that the raise times of photocurrent for all samples are less than the fall times. The photocurrent generation mechanism in the samples can be

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described addressing the point that in the case of photos with an equal or larger energy than the optical band gap energy of semiconductor incident upon their

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surface, electron-hole pairs are generated. The generated holes are near the neutral surface of the chemisorbed oxygen; the cases that narrow the depletion region of the semiconductor resulting in increased conductivity. At the same time, the photo-generated electrons contribute to increasing the concentration of electrons flowing between the electrodes [35]. In this case, after exposure of Unand Sn-doped CdO thin films to light, the current is generated as a result of 9

ACCEPTED MANUSCRIPT generated electron-hole pairs in the interface or depletion layer of semiconductor films and electrolyte solution (iodide). For all samples, decay was observed as long as the light was off. This decay was proportional with the carrier recombination in each sample. The obtained results are significant as the

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recombination rate in Sn1 sample was higher than the other samples, which was due to existence of the highest level of defect such as O vacancy and Cd interstitials in the crystal structures of this sample in comparison with the other two samples.

Conclusion

To investigate the optical and electrical properties, Sn-doped CdO thin films were

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deposited by an electrodeposition method for the first time. The analyses showed formation of cubic structures with flower- and rod-like morphologies in nanodimension with a broad band emission in the visible regions (blue) of

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electromagnetic wavelength. These band emissions, which were the result of crystal defects, revealed a blue shift with Sn dopants indicating substitution and interstitial of Sn cation in CdO crystalline structures. I-V, Mott-Schottky, and

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photoresponse results revealed high defect concentrations such as O vacancy and the most recombination states in Sn1 sample, which were due to high

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concentration of Sn cation dopants that were interstitial in crystal structures of

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CdO. Furthermore, EDS and PL results are consistent with the mentioned results.

Acknowledgments F. Jamali-Sheini gratefully acknowledges Islamic Azad University, Ahvaz, for their financial supporting in this research work. He also thanks to Advanced

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ACCEPTED MANUSCRIPT Surface Engineering and Nano Materials Research Center, Islamic Azad University, Ahvaz Branch, Ahvaz, Iran, for their instrumentation support.

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ACCEPTED MANUSCRIPT Table and figures captions: Table. 1. Electrical parameters obtained from Mott–Schottky plot of CdO thin films. Fig. 1. XRD patterns of Un- and Sn-doped thin films.

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Fig. 2. SEM images and EDS spectra of a, b) Sn0, c, d) Sn1 and e, f) Sn2, CdO thin films. Fig. 3. PL spectra of Un- and Sn-doped thin films.

Fig. 4. Absorption spectra of Un- and Sn-doped thin films.

Fig. 5. Tauc plot of a) Sn0, b) Sn1 and c) Sn2, CdO thin films for estimation of optical energy band gap. Fig. 6. Dark and illumination current-voltage characterization of a) Sn0, b) Sn1 and c) Sn2, CdO thin films.

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Fig. 7. Mott-Schottky plots of Un- and Sn-doped thin films.

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Fig. 8. Photoresponse of Un- and Sn-doped thin films in exposure of Xenon light radiations.

Table. 1 Sn0 n 94 0.14 0.21

Sn1 n 130 3.38 0.38

Sn2 n 108 1.56 0.34

0.19 149.63

1.80 15.68

1.24 22.92

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Parameters/Samples Carrier type Vfb : Flat band voltage (mVolt) ND : Donor concentration (10+23/cm3) Vb : Built in voltage (band bending) (Volt) W : Depletion width (µm) Cv : Depletion Capacitance (nF/cm2)

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Electrochemical Synthesis and Physical Properties of Sn-

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doped CdO Nanostructures

Zahra Portaghvaei1,2, Farid Jamali-Sheini3,*, Ramin Yousefi4

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Department of Physics, Science and Research Branch, Islamic Azad University, Khuzestan, Iran

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Department of Physics, Ahvaz Branch, Islamic Azad University, Ahvaz, Iran 3

Advanced Surface Engineering and Nano Materials Research Center,

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Department of Physics, Ahvaz Branch, Islamic Azad University, Ahvaz, Iran Department of Physics, Masjed-Soleiman Branch, Islamic Azad University

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(I.A.U), Masjed-Soleiman, Iran

*Corresponding author: E-mail: [email protected], [email protected] Telephone No: +98 - 61 - 33348420 - 24 Fax No: +98 - 61 - 33329200

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Abstract

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Un- and Sn-doped CdO thin films were deposited on FTO-coated glass substrates using electrodeposition method. Crystalline study revealed a cubic phase for all samples. SEM images showed flower- and rod-like morphologies for the Sndoped samples in nano-dimensions. Optical energy band gaps of 2.70, 2.79, and 2.84 eV were obtained for Sn0, Sn1, and Sn2 samples, respectively. Electrical investigations indicted that CdO thin films revealed n-type conductivity with high density of donor levels, which exceeded by corporation of Sn cations in the

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crystalline structures of CdO thin films.

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Keywords: CdO thin films; Sn doping; Electrodeposition; Optical and electrical

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properties; Photocurrent applications

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ACCEPTED MANUSCRIPT Introduction A number of the metal-oxide semiconductors have recently received researchers’ due attention as a result of possessing suitable properties and

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remarkable advantageous such as high energy band gap, non-toxicity [1], high transparency [2], good electrical conductivity [3, 4], ease of large-scale synthesis [5], and higher technological applications such as photocatalytic materials [6], gas sensing materials [7, 8], antibacterial materials [9, 10], photovoltaic conversion [11, 12], field emission and photodetectors [13, 14]. Cadmium oxide (CdO) is one of the well-recognized metal-oxide semiconductors with n-type conductivity and direct and indirect band gap energy within the ranges of 2.2-2.5 eV and 1.36-1.98 eV, respectively [15].

Generally, Undoped CdO materials reveal high n-type conductivity due to

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presence of interstitial Cd atoms and/or oxygen vacancies, which act as donors in crystal structures [16]. High mobility of electrons at CdO is one of the significant parameters in CdO applications as a transparent conductive oxide (TCO). Hence,

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examination of electrical properties is of great value for researchers. Doping is one of the simplest and most well-known ways to engineer electrical and optical

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properties of semiconductors. Gupta et al. deposited In-doped CdO thin films on SiO2 substrates to examine the effect of substrate temperature on electrical

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properties with application of pulsed laser deposition technique [4]. The results of the mentioned study revealed that by increasing the substrate temperature of

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In-doped thin films, resistivity decreased and carrier concentrations and mobility increased. It was the first time that Un- and Pb-doped CdO films were prepared on glass substrates by successive ionic layer adsorption and reaction (SILAR) method by Gulen et al [17]. Optical studies indicated that by adding Pb as a dopant and increasing Pb concentrations, optical energy band gap values of CdO films increased. They attributed the observed increase in optical energy band gaps of Pb-doped films to the changes occurring in the crystal structures and incident 3

ACCEPTED MANUSCRIPT of quantum size effects. Benhaliliba et al. examined the growth of Un- and copper (Cu)-doped CdO thin films by sol-gel method [18]. Electrical and optical experiments showed that Cu as a cationic dopant decreases the electrical

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resistance and increases the optical energy band gaps of CdO thin films. So far, a good number of studies addressing the growth of tin (Sn)-doped CdO structures with application of different methods have investigated their various aspects and properties for a wide range of applications such as microwave irradiation technique in gas sensor applications [19], sol–gel technique in electrical applications [20], chemical vapor deposition (CVD) and pulse laser deposition (PLD) in optoelectronic applications [3, 21, 22], and successive ionic layer adsorption and reaction (SILAR) [23] and chemical bath deposition (CBD) methods [24].

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Among the various methods suggested for growth of CdO thin films, elctrodeposition is a simple and cost-effective method leading to deposition of thin films with nano-scale surface morphologies. In this method, a good control

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can be exerted on the growth parameter to obtain desired properties. To the best of researchers’ knowledge, an appropriate and thorough study addressing the

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electrical and photocurrent properties of Sn-doped CdO thin films grown by electrodeposition method has not been reported, yet. Hence, in the present study

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Un- and Sn (various concentrations)-doped CdO thin films were grown by electrodeposition method. Furthermore, optical, electrical, and photocurrent

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properties of Un- and Sn-doped CdO thin films were examined to facilitate their applications in optoelectronics devices.

Experimental section Electrochemical cells were provided by the following details for deposition of Un- and Sn-doped thin films. An electrolyte solution containing 5 mM of 4

ACCEPTED MANUSCRIPT cadmium sulfate (CdSO4, Merck, high purity), 1 mM of hydrogen peroxide (H2O2, Merck, high purity), and 40 mLit of distilled water were mixed by a stirrer for 5 min. Then, pH of solution fixed on 8 using 1 mM solution of potassium chloride (KCl, Merck, high purity). FTO-coated glass substrates with 2×1 cm

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size, platinum sheet, and saturated calomel electrode (SCE, E0 = 0.244 V vs. NHE) serving as the working, counter, and reference electrodes, respectively were prepared. The deposition growth process was performed in an electrodeposition cell with three electrodes. For deposition of doped samples, an aqueous solution of tin chloride (SnCl2, Merck, high purity, 5mM) as a source of dopant was prepared by adding different quantities (2 cc and 5 cc) of this solution to different beakers. The samples were called Sn0, Sn1, and Sn2 for the undoped and doped samples with different amounts of the solution, respectively.

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Substrates and platinum electrode were washed with acetone and ethanol solutions for 10 minutes in an ultrasonic bath to ensure no pollution was left on them. A computer-controlled electrochemical analyzer (Potentiostat, Autolab,

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A3ut71167, Netherlands) was used to create the cathodic polarization conditions at a voltage of -0.75 volts in the sequence of SCE (SCE, E0 = 0.244 V vs NHE).

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The deposition duration was 40 minutes, and the temperature of electrolyte solution was keeps at 80 ◦C for all samples. After deposition and removal of the samples from the electrolyte solution, they were gently washed with distilled

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water and then dried. To obtain higher structural phase of CdO films, one-step heat treatments at 200 ◦C for 2 h were administered to all the samples, and then

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the characterization was carried out. The crystal structure of the obtained CdO thin films was studied with

application of an X-ray diffraction (XRD) system (model X'Pert-Philips) using a Cu anode with wavelength of 1.5406 Å. In order to study the surface morphology and elemental composition of deposited films, FESEM imaging and EDS analysis were performed using scanning electron microscope FESEM, Zeiss ΣIGMA VP. 5

ACCEPTED MANUSCRIPT Optical properties of thin films were also analyzed using the photoluminescence (PL) analyses and the absorption spectra. PL analysis was performed at room temperature with a UniRam system, which had a He-Cd laser operating at the excitation wavelength of 320 nm and power of 20 mW. The absorption spectrum

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was also recorded at room temperature using a UV-Vis Perkin Elmer Lambda 900 spectrophotometer.

I-V and photocurrent cell were assembled using the deposited films as working electrodes and Pt foil as the counter electrodes, which were sealed in a sandwich-type cell that was filled with I-/I-3electrolyte. The photocurrent response was measured by a potentiostat (Autolab, A3ut71167, Netherlands) using a xenon lamp (1.5 Air Mass) serving as the light source provided by a solar cell simulator (solar cell simulator IIIS-200+, Nanosat Co., Iran). With respect to C–V plot, an

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LCR meter (LCR-8000G Series Gw-instek) was employed to measure the capacitance to obtain Mott–Schottky plot. This measurement was carried out at a

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Results and discussion

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frequency of 1 kHz and voltage range of 10 to 1000 mVolt.

Fig. 1 presents XRD patterns of Un- and Sn-doped CdO thin films. As

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revealed in this figure, the obtained materials were crystalline in nature, and diffraction peaks thoroughly matched with bulk and cubic CdO phases (JCPDS Card No.01-078-0653) [25]. Inset of this figure shows a shift to higher positions

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for (200) peak. This shift was observed for other diffraction peaks, as well. These shifts are due to lower effective ionic radius of Sn (Sn2+ ~ 0.69 Å) compared with Cd (Cd2+ ~ 0.95 Å) [26]. SEM images and EDS spectra of Un- and Sn-doped CdO films are shown in Fig. 2. The SEM images presented a relatively smooth surface for undoped CdO films. However, by adding Sn (Sn1, Fig. 2c), the surface morphology 6

ACCEPTED MANUSCRIPT changed into the flower-like morphology with the average diameter of 35 ± 4 nm. By increasing the concentrations of dopants, surface morphology changed into a rod-like (Sn2, Fig. 2e) morphology with the average diameter of 156 ± 4 nm. The surface of Sn-doped CdO thin films was dense and devoid of any gaps. EDS

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spectra (Fig. 2) indicated increased Wt% (Wight percentage) of Sn, which increased in response to increasing the concentrations of dopants. This Wt% of Sn is equal to 1.51 and 3.77 At% (Atomic percentage) for Sn1 and Sn2 samples, respectively.

To examine the effect of Sn doping on optical properties of CdO thin films, photo-excited emission and absorption spectra of Un- and Sn-doped CdO thin films were recorded. Fig. 3 indicates the PL spectra of Un- and Sn-doped CdO thin films at room temperature (ambient conditions). The films demonstrated

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intense broad emission peaks at 515 nm (~ 2.41 eV), 512 nm (~ 2.43 eV), and 513 nm (~ 2.42 eV) for Sn0, Sn1, and Sn2 samples, respectively. These emission bands were observed in the green region of the visible spectra of electromagnetic

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waves and were well-known as near band edge (NBE) emissions. The reasons behind observation of this type of emission for CdO are the carriers transported

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between conduction band and valence band [27]. It is well-recognized that the emission band in the visible region is associated with defects originated from the interstitials of ions in crystal structures produced by transition of excited carriers

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from deep to valence band [13]. Hence, increasing the intensity of emission band with addition of Sn dopants reveals a significant increase in the defect levels and

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surface recombination rate of Sn-doped CdO films. A blue shift in position of band emissions was also observed by adding Sn dopant. The mentioned process returned the partial substitution of Sn atoms into the crystal lattice of CdO instead of Cd atoms. The band emissions of doped samples within the range of 690 nm to 740 nm could be attributed to a number of defects such as O vacancy, Cd interstitial, and etc [28]. 7

ACCEPTED MANUSCRIPT Absorption spectra of Un- and Sn-doped CdO films are presented in Fig. 4. CdO thin films showed high absorption in the ultra violet and visible regions of electromagnetic wavelength. Sn-doped films indicated lower absorption in comparison with the undoped films. An absorption edge of about 400 nm can be

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conjectured for all samples. To estimate the optical band gap energy of Un- and Sn-doped CdO films, the Kubbelka-Munke model and Tauc plots were used [29]. The mentioned method can be considered as a recognized and effective method for assessment of optical energy band gaps of semiconductors in micron- and nano-dimensions. Tauc plot of CdO thin films is shown in Fig. 5. This plot indicates higher optical energy band gaps for Sn-doped films in comparison with the undoped films. The observed increase in optical energy band gaps (blue shift in the absorption edge) is in correspondence with the PL results and is related to

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the substitutions of Sn atom in Cd sites of CdO crystal structures.

Electrical properties of CdO thin films were investigated addressing voltage-current and Mott-Schottky plots. Fig. 6 reveals the voltage-current

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characteristics of Un- and Sn-doped thin films in dark and xenon irradiation under ambient conditions. All samples presented a Schottky behavior and in

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illumination conditions showed lower current in the same voltage, which was the feature of n-type semiconductors. Furthermore, lower conductivity of CdO thin films was observed in the case of their exposure to the xenon radiations.

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Comparison of I-V characteristic of Un- and Sn-doped CdO thin films revealed that Sn as a dopant increased the electrical resistivity of CdO thin films. Similar

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results, i.e. high Wt% of Sn concentrations, have been reported by other research studies [22]. The mentioned variation of the electrical resistivity of Sn-doped CdO samples can be explained by resorting to defects in the doped samples, which is verified by PL results, as well. Hence, different concentrations caused various effects in the electrical properties which are originated from the substitutions and/or interstitial ions in the crystal [30]. 8

ACCEPTED MANUSCRIPT Mott-Schottky characteristic of CdO thin films were obtained by measuring the capacitance of films in the range of 0 to 1 volt. It would be definitely expressed that Mott-schottky method is one of the most accurate methods for investigation of electrical properties of thin films [29]. Fig. 7

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represents Mott-schottky plot of Un- and Sn-doped CdO thin films. The positive slope of Mott–Schottky plot confirms the n-type conductivity of all samples [31]. Moreover, it confirms the obtained results from the I-V plot. Hence, it can be certainly stated that obtaining the p-type conductivity of CdO is almost impossible [32]. The intersection of fitted line on the Mott-Schottky plot with horizontal axis determined flat band voltage. The flat band voltage as well as other electrical parameters of CdO thin films is presented in Table 1, which indicates that high donor concentration for all samples. This high donor

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concentration reflects merging of donor levels with conduction band of CdO thin films [33].

Fig. 8 displays photoresponse of Un- and Sn-doped thin films. Generations

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of photocurrents in the CdO cell is assumed to be a process occurring in the space charge region (SCR) of CdO [34]. This plot shows a high photocurrent for Sn1

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sample compared with that of the other samples. The measurements of the present study showed that the raise times of photocurrent for all samples are less than the fall times. The photocurrent generation mechanism in the samples can be

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described addressing the point that in the case of photos with an equal or larger energy than the optical band gap energy of semiconductor incident upon their

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surface, electron-hole pairs are generated. The generated holes are near the neutral surface of the chemisorbed oxygen; the cases that narrow the depletion region of the semiconductor resulting in increased conductivity. At the same time, the photo-generated electrons contribute to increasing the concentration of electrons flowing between the electrodes [35]. In this case, after exposure of Unand Sn-doped CdO thin films to light, the current is generated as a result of 9

ACCEPTED MANUSCRIPT generated electron-hole pairs in the interface or depletion layer of semiconductor films and electrolyte solution (iodide). For all samples, decay was observed as long as the light was off. This decay was proportional with the carrier recombination in each sample. The obtained results are significant as the

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recombination rate in Sn1 sample was higher than the other samples, which was due to existence of the highest level of defect such as O vacancy and Cd interstitials in the crystal structures of this sample in comparison with the other two samples.

Conclusion

To investigate the optical and electrical properties, Sn-doped CdO thin films were

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deposited by an electrodeposition method for the first time. The analyses showed formation of cubic structures with flower- and rod-like morphologies in nanodimension with a broad band emission in the visible regions (blue) of

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electromagnetic wavelength. These band emissions, which were the result of crystal defects, revealed a blue shift with Sn dopants indicating substitution and interstitial of Sn cation in CdO crystalline structures. I-V, Mott-Schottky, and

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photoresponse results revealed high defect concentrations such as O vacancy and the most recombination states in Sn1 sample, which were due to high

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concentration of Sn cation dopants that were interstitial in crystal structures of

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CdO. Furthermore, EDS and PL results are consistent with the mentioned results.

Acknowledgments F. Jamali-Sheini gratefully acknowledges Islamic Azad University, Ahvaz, for their financial supporting in this research work. He also thanks to Advanced

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ACCEPTED MANUSCRIPT Surface Engineering and Nano Materials Research Center, Islamic Azad University, Ahvaz Branch, Ahvaz, Iran, for their instrumentation support.

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ACCEPTED MANUSCRIPT Table and figures captions: Table. 1. Electrical parameters obtained from Mott–Schottky plot of CdO thin films. Fig. 1. XRD patterns of Un- and Sn-doped thin films.

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Fig. 2. SEM images and EDS spectra of a, b) Sn0, c, d) Sn1 and e, f) Sn2, CdO thin films. Fig. 3. PL spectra of Un- and Sn-doped thin films.

Fig. 4. Absorption spectra of Un- and Sn-doped thin films.

Fig. 5. Tauc plot of a) Sn0, b) Sn1 and c) Sn2, CdO thin films for estimation of optical energy band gap. Fig. 6. Dark and illumination current-voltage characterization of a) Sn0, b) Sn1 and c) Sn2, CdO thin films.

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Fig. 7. Mott-Schottky plots of Un- and Sn-doped thin films.

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Fig. 8. Photoresponse of Un- and Sn-doped thin films in exposure of Xenon light radiations.

Table. 1 Sn0 n 0.12 0.14 0.21

Sn1 n 0.15 3.38 0.38

Sn2 n 0.14 1.56 0.34

0.19 149.63

1.80 15.68

1.24 22.92

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Parameters/Samples Carrier type Vfb : Flat band voltage (Volt) ND : Donor concentration (10+23/cm3) Vb : Built in voltage (band bending) (Volt) W : Depletion width (µm) Cv : Depletion Capacitance (nF/cm2)

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