A study of the characterization of CdS/PMMA nanocomposite thin film

Nanocomposites of polymer material based on CdS as fillermaterial and poly methyl methacrylate (PMMA) as host matrix havebeen fabricated by chemical spray pyrolysis method on glasssubstrate. CdS particles synthesized by co-precipitation route usingcadimium chloride and thioacetamide as starting materials andammonium hydroxide as precipitating agent. The structure isexamined by X-ray diffraction (XRD), the resultant film hasamorphous structure. The optical energy gap is found to be (4.5,4.06) eV before and after CdS addition, respectively. Electricalactivation energy for CdS/PMMA has two regions with values of0.079 and 0.433 eV.


Introduction
The study of polymer nanocomposites (PNC) is a fast growing area of research. The fabrication of polymer nano-composites (PNC) is an integral aspect of polymer nanotechnology. These PNC are composed of two main parts, filler and host matrix. The inorganic particles, having at least one dimension less than 100 nm, are acted as filler and these particles dispersed in polymer, which acts as host matrix. Recently, many efforts have been devoted to the synthesis of polymer nano-composite materials due to their synergistic and hybrid properties [1]. The properties of polymer nano-composite (PNC) mainly depend upon filler material and host matrix. The PNC's show almost all properties of their parent materials together with some additional properties like chemical stability, improved moldability etc.
These polymer nanocomposites (PNC's) are better than conventional composites because conventional composites require high content of filler. But PNC's achieve the same properties with a much smaller amount of filler and producing materials of lower density and higher moldability.
The functions of PNC's depend upon filler phase. In the present study (CdS) nanoparticles were used as filler material. CdS has cubic structure. The polymer nano-composite materials containing semiconductor as filler attract much attention due to their promising engineering applications like sensor technology [2,3]. In the present paper we synthesized CdS nanoparticles and impregnated them in poly methyl methacrylate (PMMA) matrix. The films were prepared by spray pyrolysis. The aim of this work is to investigate the optical and electrical behavior of polymer nanocomposites.

Experimental part 1. Synthesis of CdS nanoparticles
CdS nanoparticles were synthesized by co-precipitation method.
First, CdCl 2 and thioacetamide (TAA) at 0.1 M for both materials were dissolved separately in distilled water, with vigorous stirring for 30min. Then they were mixed together and stirred with vigorous stirring for 1hour. Finally NH 4 OH was added drop wise with stirring for 30min. Afterward, the resulting yallow suspension was further stirred for 30 min. Then the reactor was cooled up to room temperature. The resulting precipitate was filtered and washed several times with water. Finally it was dried in an oven at 80 o C.

Synthesis of CdS/PMMA composite thin film
In order to disperse CdS particles in polymer matrix (PMMA), we used solvent evaporation method. As the second step of fabrication of composite material we dissolved 500 mg PMMA in DCM and left the sealed vial overnight. Afterward, the mixture was sonicated for 30 min. When all PMMA dissolved completely, 50 mg CdS filler particles were added in the solution and sonicated the suspension for further 45min. In the last step, this solution was sprayed on glass substrate.

Characterization of CdS/PMMA composite thin film
X-ray powder diffraction (XRD) data were taken by X-ray diffractometer (SHIMADZU-XRD 6000) using Cu-Ka radiation (λ=1.54056 Å) at room temperature. The optical absorbance and transmittance spectra were measured using UV/ Visible SP -8001 spectrophotometer over the range 190-1000 nm to calculate the optical energy gap and optical constants. The energy gap was calculated by using Tauc's relation [4] {αh =A(h −E ) } where A is a constant, which is different for different material, α is the absorption coefficient, h is the energy of incident photon, E g is the band gap energy. The optical constant such as the extinction coefficient, which is related to the exponential decay of the wave as it passes through the medium, that defined as [5] {k αλ 4π ⁄ }. Also, the refractive index is determined by using the formula [6] where R is the reflectance, and can be expressed by the relation [7]: R n 1 k n 1 k ⁄ The real (ε r ) and imaginary (ε i ) parts of the dielectric constant of thin films are determined by using equations: [8] { (n-ik) 2 Fig.3 represents the transmission spectrum of CdS/PMMA composite. The spectrum is recorded in the range of 190-1100 nm at room temperature. High transmission is appeared from 360 nm till 1100 nm and it is more than 80%.

The optical energy gap
CdS/PMMA has direct band gap material where the exponent m value equals to ½ that indicates the type of transition. The linear part of the graph was extrapolated to {(αh ) 1/m ~ 0} to determine the bandgap. From Fig.4, it has been determined that the optical band gap of CdS/PMMA is 4.5 and 4.06 eV without and with CdS nanoparticles.

Optical constants a) Extinction coefficient
It is clear from the equation mentioned before that k depends on (α) and has a similar behavior to it. It can be noted that k increases highly at the absorption edge region. This increase is attributed to the increase of the absorption coefficient due to the direct electronic transitions. The extinction coefficient later reaches its maximum value at the high absorption region corresponding to the increment in the photon's energy and the increase in the absorption coefficient with the decrease in the wavelength.

b) Refractive Index
It is necessary to give attention to the refractive index (n) in order to complete the fundamental study of the optical properties and the optical behavior of the material. The variation of the refractive index as a function of the wavelength for CdS/PMMA thin film is illustrated in Fig. 6. It is clear from the figure that the refractive index decreases with the increase in the wavelength of the incident photon.

c) Dielectric constant
The real part of the dielectric constant (ε r ) depends mainly on the value of (n 2 ), because of the smaller values of (k 2 ) comparison with (n 2 ), whereas the imaginary part of the dielectric constant (ε i ) depends mainly on the (k) values which are related to the variations of the absorption coefficient. Figs. 7 and 8 illustrate the variation of the real part of the dielectric constant as a function of the wavelength for CdS/PMMA thin film. The variation of the imaginary part of the dielectric constant as a function of the wavelength for CdS/PMMA thin film is illustrated in Fig. 8.
The optical properties parameters including absorption coefficient and optical constants which include refractive index, extinction coefficient, real and imaginary parts of the dielectric constant at the wavelength which is equal to 550 nm for CdS/PMMA thin film deposited by chemical spray pyrolysis method on a glass substrate at 100°C with thickness about (100) nm are listed in Table 1.

The electrical properties of CdS/PMMA thin film
The electrical properties of CdS/PMMA thin Film deposited by chemical spray pyrolysis method on a glass substrate with thickness about (100) nm are studied. These properties include the D.C. conductivity from which the transport mechanism of the charge carriers can be estimated.  The (d.c) conductivity (σ d.c ) for CdS/PMMA film is studied as a function of (1000/T) with the range of (298-483) K, is shown in Fig. 9. It can be deduced from this figure that there is an increase in conductivity with the increase in the temperature that prove the semiconductor behavior for CdS/PMMA nanocomposite. As well as, it can be observed two separated regions throughout the heating temperature range, the first region is at low temperature and the second region is at higher temperature, indicating different conduction mechanisms dominating at specific temperature intervals [9].
These two conduction mechanisms mean that the conductivity is nonlinear with temperature. The first activation energy (Ea 1 ) occurs at low temperatures, in which the conduction mechanism is due to charge carriers' transport (hopping) to localized states near the conduction band [10,11], in this temperature region (298-363) K. The second activation energy (E a2 ) occurs at high temperatures, in which the conduction mechanism is attributed to the thermal excitation of charge carriers from grain boundaries to neutral region of the grains [10]. It is specifically due to carriers excited into the extended states beyond the mobility edge [9]. In this temperature region (363-483)K, the variation of (ln σ) with (1000/T) is pronounced and increases sharply with high activation energies relative to first activation energies, as shown in the Fig. 9 and Table 2.

Conclusions
CdS/PMMA nanocomposite has been successfully synthesized via chemical spray pyrolysis method. The XRD results indicate that the composite is in amorphous phase. The optical transition in the CdS/PMMA films is observed to allow direct transition. The refractive index, and extinction coefficient and dielectric constant (real and imaginary parts) decrease with the increasing of wavelength in the UV-Vis-NIR range. There are two transport mechanisms of the charge carrier of the d.c conductivity at temperature range (298-473) K.