Effects of Pb-Doping on Phase Structural and Optical Properties of CdZnS Nano-Powders

Mahdi Ghasemifard, Somayeh Tohidi, and Roholah Karimzadeh

Effects of Pb-Doping on Phase Structural and Optical Properties of CdZnS Nano-Powders

Mahdi Ghasemifard, Somayeh Tohidi, and Roholah Karimzadeh

—Cadmiumzincleadsulfide [Cd0.8(Zn1-x,Pbx)0.2S] nano-powders were prepared by an improved coprecipitation method. The effect of Pb2+concentration at 500°C on the phase and crystalline structure of the Cd0.8(Zn1-x,Pbx)0.2S powders were investigated by X-ray diffraction (XRD). According to the transmission electron microscopy (TEM) images, the particles size are in the range of 58nm to 72nm. In addition, optical band gap energy and optical constants of nano-powders were determined using the ultraviolet (UV)spectrum,Fourniertransforminfrared spectroscopy (FTIR), and Kramers-Kronig analysis, respectively. We calculate the refractive indexn, extinction coefficientk, and dielectric functionεas a function of the wavenumber. The experiment results demonstrate that the amount of Pb+2has been playing an increasingly important role on optical properties of CZPS nanocrystals.

Index Terms—Coprecipitation, Kramers-Kronig analysis, optical constants, X-ray diffraction.

1. Introduction

Cadmium Sulfide (CdS) nano-powders are especially attractive because of their applications in some novel electronic and optoelectronic devices. Cadmium zinc sulfide (CdZnS) has great potential applications, especially in solar cells and photo voltaic devices which are based on the structure of CdZnS[1]?[3]. The replacement of cadmium telluride (CdTe) with the lower band gap alloys in solar cell systems, leading to an increase in window absorption and an increase in the short circuit current[4].

Up to now, the Kramers-Kronig (KK) method has been used to analyze the reflection spectra to calculate optical constants of materials[5]?[8]. In fact, with this method we can analyze the optical properties of a wide variety of materials. However, its complexity of the integration limits the application of the KK method[9]?[13]. In the literature, there are few related works reporting optical constants using infrared spectroscopy. In the last years, there were only some works related written by our team published. Recently, the great interest on material characterization especially semiconductor was illustrated by Fourier transforms infrared (FTIR) spectroscopy. The transmission spectra and FTIR technique used to determine the refractive index (n) and extinction coefficient (k) as a function of the wavenumber. The wide-band gap, high refractive index (n), and low extinction coefficient (k) are important characteristics for electro-optical devices[14]. Hence, the measurement of these parameters is helpful in determining the relevance of the particle size to possible device applications. The optical properties were studied in the wavenumber range of 1000 cm?1to 2400 cm?1.

The present study focuses on optical properties of Cd0.8(Zn1-x, Pbx)0.2S nano-powders withx=0.1, 0.2, and 0.3 compositions. This procedure is a simple way to synthesis CZPS nano-powders with inexpensive materials. However, in this paper, the optical constants of CZPS have been presented and discussed in detail.

2. Experimental

Raw materials used in this experiment consist of cadmium acetate [Cd(CH3COO)2.2H2O], zinc acetate [Zn(CH3COO)2.2H2O], lead acetate [Pb(CH3COO)2.3H2O], and thiourea [SC(NH2)2] as a source of sulfide. The aqueous solution of each single cation and anion (i.e. Cd+2, Zn+2, Pb+2and S-2) was prepared by dissolving all of them in distilled water. The solutions of zinc, lead, and sulfide were added dropwise to the aqueous solution of cadmium under continuous stirring at room temperature. The ammonium hydroxide solution is added to sol to form the cadmium tetraamine ions [Cd(NH3)4], zinc tetraamine ions [Zn(NH3)4], and lead tetraamine ions [Pb(NH3)4]. The flow

3.2 TEM Analysis

The typical transmission electron microscopy (TEM) image of the CZPS nano-powder calcinated at 500°C prepared by the coprecipitation method is shown in Fig. 3. From TEM analysis, the primary particle size of the nano-powders can be determined. The primary particles size of the nano powder is approximately 58 nm to 72 nm in diameter. According to Fig. 3, it can be observed that at 500°C the effects of Pb-doping lead to the particle size increase.

3.3 Band Gap Energy

In order to calculate the optical band gap of samples, the Uv-Vis (Ultraviolet and Visible) absorption spectrum was analyzed. The Uv-Vis of the Cd0.8(Zn1-x,Pbx)0.2S nano-powders is shown in Fig. 4. From Fig. 4, the absorption coefficient increases with the increase of the Pb concentrations. The absorption coefficient of nano-powder is calculated by

whereAis the amount of optical absorption of nano-powders andDis the crystallite size with the unit nanometer (the Sherer relationship). The optical band gap,Eg, is obtained by fitting the optical absorption coefficientαto Tauc’s relation[21]:

whereα,hν,α0, andEgare absorption coefficient, photon energy, a constant, and optical band gap energy, respectively. Thenrefers to these categorizes, ifn=2 then it relates to the direct optical transitions and forn=0.5 it relates to the indirect optical transitions. The results of the fi rst-principle calculation show that the ZnCdTe has a indirect energy band gap[22]. Therefore, by plotting, we evaluatedEgfrom the extrapolated linear portion of the graph. The graphs of (αhv)2versus the photon energy (hv) for CZPS are shown in Fig. 5.

As a result from Fig. 5, for the obtained optical band gap for Cd0.8(Zn1-x,Pbx)0.2S at 500°C for different Pb concentrations, as increasing the Pb concentration fromx=0.0 tox=0.3, the optical band gap energy (Eg) of the nanoparticle decreases significantly. In other words, it is notable when the Pb concentration rises to 0.3, itsEgwill be reduced about 1.52 eV.

Fig. 3. TEM micrograph of Cd0.8(Zn1-x,Pbx)0.2S: (a)x=0.0 and (b)x=0.2.

Fig. 4. Uv-Vis absorption spectrum of Cd0.8(Zn1-x,Pbx)0.2S annealed at 500°C.

Fig. 5. Dependence of the absorption coefficients (αhν)2on the photon energy for Cd0.8(Zn1-x,Pbx)0.2S nano-powders.

3.4 Evaluation of the Optical Constants

A. FTIR Spectroscopy

FTIR spectroscopy was used in order to monitor the transformation of precursor solutions during the change of Pb concentration. By making the pallet of CZPS nano-powders in potassium bromide (KBr) the FTIR was prepared. The FTIR spectra of the CZPS powders in the range of 4000 cm?1to 250 cm?1treated at 500°C for a period of 2 h are shown in Fig. 6. In this frequency interval, a broad band was observed for each spectrum from 1257 cm?1to 986 cm?1with a maximum absorbance in thevicinities of 1115 cm?1. This peak has been associated with the vibration of M–O (M=Zn and Pb) bonds in the systems. Bands associated with Pb ions were not clearly observed in the mid-infrared spectra because of their heavy masses.

Fig. 6. FTIR spectra of CZPS powders treated at 500°C.

The FTIR spectrum for CZPS is similar to the most other CdZnS compounds that present four distinct vibration modes[23]. As can be seen from Fig. 6, clearly the transmittance bands’ intensities decrease with the increase of the molar ratios of the lead.

Fig. 7. Reflectance spectrum for Cd0.8(Zn1-x,Pbx)0.2S powder: (a)x=0.0, (b)x=0.1, (c)x=0.2, and (d)x=0.3.

B. Reflectance

The reflectance (R) spectrum from 750 cm?1to 1800 cm?1for CZPS powder calcinated at 500°C is shown in Fig. 7. From Fig. 7 the peak values and the peak width ofRincrease slowly when the Pb concentration increases.

Fig. 8. Plot of the refractive index (n) and extinction coefficient (k) as a function of the wavenumber: (a)x=0.0, (b)x=0.1, (c)x=0.2, and (d)x=0.3.

C. Refractive Index and Extinction Coefficient

At the next step of the present study, we used the transmission spectrum to determine the complex refractive index (n~) as a function of the wavenumber (K) using the KK analysis as stated previously. The complex refractive index can be calculated by[24]?[27]

wherenis the real andkis the imaginary parts of the complex refractive index. The refractive index and extinction coefficient can be calculated by

whereRis the reflectance andφis the phase change at a particular wavenumber between the incidences and the reflected signal, which is obtained by

For calculatingφ(K), several extrapolation approaches have been evaluated and reported[28],[29]. We have calculatedφ(K) by Maclaurin’s method[31]as (7). This method is rather accurate, but it needs double Fourier transform and the calculation takes a relatively longer time.

whereh=Ki+1–Kiand if data intervalgis an odd number theni=2, 4, 6, ???,g?1,g+1, ???, while ifgis an even number theni=1, 3, 5, ???,g?1,g+1, ???.

D. Dielectric Function

With the knowledge aboutnandk, the real (ε') and imaginary (ε") parts of the complex dielectric functioncan be calculated by

With regard to (8) and (9) and havingnandk, the real and imaginary parts of the complex dielectric function can be easily calculated and plotted as a function of the wavenumber. The real and the imaginary parts of the frequency dependent dielectric function for the CZPS powder calcinated at 500°C and those for different Pb concentration are shown in Fig. 9.

Based on data presented in Figs. 6, 7, 8, and 9, it is easy to find that a few meaningful differences inε'andε"take place. The curves are flat in the long wavenumber region and rapidly increase towards the shorter wavenumber at 1360 cm?1. This characteristic is related to the near electronic inter band transition. The rise that is rapid in the refractive index is associated with the fundamental absorption[30],[31].

Fig. 9. Plot of the extinction coeffitiont as a function of the wavenumber at 500°C for: (a)x=0.0, (b)x=0.1, (c)x=0.2, and (d)x=0.3.

4. Conclusions

The Cd0.8(Zn1-x,Pbx)0.2S (x=0.0, 0.1, 0.2, and 0.3) nano-powder has been synthesized by the coprecipitation method using salt and metal precursor. The XRD patterns indicate the presentation of hexagonal and cubic phases forx=0.2 at 500°C. On the other hand, by increasing the annealing temperature from 400°C to 500°C, the percent of hexagonal phase is decreased. According to TEM images, the average particles size was estimated to be 65 nm. The optical properties of the CZPS have been investigated by transmittance measurements in the range of 250 cm?1to 4000 cm?1. We have presented a detailed description of using the KK method to analyze the normal incidence infrared reflectance spectra. From the achieved optical data, it can be drawn that the growth of Pb concentration and the alteration of the structure have an increasingly important role on the optical constant.

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Mahdi Ghasemifardwas born in Esfarayen, Iran in 1980. He received the B.S., M.S., and Ph.D. degrees from the Ferdowsi University of Mashhad, Iran in 2004, 2006, and 2009, respectively, all in solid state physics. Now, He works with the Nano Technology Lab, Esfarayen University, Esfarayen, Iran as a faculty member. His research interests include nano-electroceramic and thin film.

Somayeh Tohidiwas born in Maragheh, Iran in 1987. She received the B.S. degree from the Urmiyeh University, Iran in 2009, the M.S. degree from the Shahide Beheshti University of Tehran, Iran in 2012, both in solid state physics. Her research interests include solar cell.

Roholah Karimzadeh’s photograph and biography are not available at the time of publication

t

November 15, 2012; revised January 25, 2013.

M. Ghasemifard is with the Nano Technology Lab, Esfarayen University, Esfarayen +98-585, Iran (Corresponding author e-mail: mahdi.ghasemifard@gmail.com).

S. Tohidi and R. Karimzadeh are with the Department of physics, Shahid Beheshti University, Tehran +98-21, Iran (e-mail: somayeh_tohidi@yahoo.com; r_karimzadeh@sbu.ac.ir).

Color versions of one or more of the figures in this paper are available online at http://www.intl-jest.com/

Digital Object Identifier: 10.3969/j.issn.1674-862X.2013.03.016