1. Introduction
Cadmium sulfide (CdS) is an important chalcogenide semiconductor with a direct band gap energy of 2.42 eV, which falls in the visible spectrum region, and with a large absorption coefficient of 4
CdS thin film deposition has already attracted great attention to develop synthesis techniques with the advantages of green, low cost and easy fabrication on a large scale. Various techniques such as chemical bath deposition (CBD)[10-12], chemical vapor deposition[13], electrodeposition[14], molecular beam epitaxy[15], a successive ionic layer adsorption and reaction (SILAR) method[16] and spray pyrolysis[17], have been reported for the preparation of CdS thin films. Most of the aforementioned methods are effective. However, there are few reports about the synthesis of CdS thin films by the polyol method.
It is well known that diethylene glycol (DEG) has excellent water solubility, biocompatible lubricity, and thermal stability; furthermore, many researchers have reported the preparation of nanomaterials using DEG, exploiting its non-toxic, nonirritating and moisturizing properties. For example, Feldmann[18] used DEG in the preparation of nanoscale MS particles (M
The aim of this study is to confirm the effect of deposition temperature on the structure, morphology and optical properties of CdS thin film by the polyol method. The thin films were deposited in high-boiling polyol (DEG, bp 246 ℃) solution by a chemical reaction between dissolved precursors composed of cadmium salt as a source of Cd and thiourea as a source of S. The polyol itself was used for a stabilizer, limiting particle growth and prohibiting agglomeration[21]. Highly crystalline sulfides were yielded as a result of the application of high temperatures (
2. Experimental
CdS thin films were prepared by the simple polyol method in which cadmium acetate (Cd(CH
CdS thin films were deposited on microscope glass substrates of 40
The crystal phase of the films was determined by X-ray diffraction (XRD, Panalytical X'pert PRO, Netherlands), using Cu Ka radiation. The morphology of the films was observed through scanning electron microscopy (SEM, Fei Quanta 200, USA) and atomic force microscopy (AFM, CSPM4000, China). Additionally, the optical property of the film was examined by a UV-vis 2550 spectrophotometer (Shimadzu 2550, Japan) ranging from 300 nm to 800 nm.
3. Results and discussion
3.1 Structural characterization of CdS thin films
Figure 1 shows the X-ray diffraction spectra of the CdS thin films deposited at the different temperatures. The diffraction peaks at 2
d=0.94λBcosθ, |
(1) |
where
3.2 The morphology of CdS thin films
Figure 2 shows the photo of CdS thin films deposited at 180 ℃. To the naked eye, it can be observed that the film is yellow, uniform, complete and well-adhered to the substrate. The microtopography of CdS films is later observed through SEM and AFM. Figure 3 shows the SEM morphology of the CdS thin films deposited at 120, 140, 160, 180, and 200 ℃, respectively. It is found that when the deposition temperature is 120 or 140 ℃, the film displays a rougher, inhomogeneous surface with some overgrowth grains. When the deposition temperature rises to 160-200 ℃, the surface becomes much smoother. It indicates that the deposition temperature is a crucial factor affecting the morphology of the CdS films.
In order to further find out the structure and the grain size of CdS thin films, an AFM morphology study is carried out and the films deposited at temperature of 140 and 180 ℃ are chosen. Typical contact mode AFM images are shown in Fig. 4. The employed surfaces morphology at the micro scale shows the relatively uniform, dense and homogeneous circular grains distribution over the substrates. It is seen that the grain boundaries of CdS films can be distinguished clearly, so the grain size of the films could be estimated by Nano Measurer 1.2.5 Software (Fudan University) from AFM images[26]. It can be seen that the substrate is covered with granular structures of different sizes, mainly ranging from 50 to 90 nm (see Fig. 5) and the average grain sizes of CdS nanoparticles are about 77.16 and 76.61 nm, the values are similar to the results obtained by the CBD method[11]. The grain size observed by the AFM is larger than that by the Scherrer relation, which might be attributed to the agglomeration of small size grains, called polygonizations of the crystallites[27].
The surface topology, values of average roughness (Ra) and root mean squared (RMS) roughness are analyzed and presented in Table 1; the values are lower than the result obtained by electrodeposition[14]. The table indicates that the surface of the sample has relatively smaller roughness values at higher deposition temperatures, which agrees with the results of the SEM. This trend is found by many authors[11, 28]. However, the real reason is unclear, but we deduce that at higher deposition temperatures, due to the high rate of the chemical reaction and ion diffusion to the grains, the surface grains grow rapidly to coalesce with each other to shape a continuous film. On the other hand, higher deposition temperatures may lead to a slight annealing effect, which results in the smoother surface compared to films deposited at lower temperatures.
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3.3 Optical properties of CdS thin films
Optical properties of CdS films are measured with UV-vis spectrophotometer. Figure 6 shows the optical transmittance spectra of CdS thin films deposited at different temperatures in the wavelength range of 300-800 nm. It can be found that the CdS thin films present a higher transmission in the longer wavelength range and the average transmission in the wavelength range 300-800 nm is 81.3%, 66.9%, 69.1%, 68.0%, and 74.3% from 120 to 200 ℃, respectively. The absorption edge is between 470 nm and 510 nm for all films and exhibits slightly a red shift with the increase of deposition temperature. This indicates that the deposition temperature can indeed affect the band gap value of CdS thin films.
The optical band gap of CdS thin films can be estimated from the plot of (
(αhν)2=A(hν−Eg), |
(2) |
where
4. Conclusions
In this study, we have presented an easy and low cost synthetic process of CdS films in the polyhydric alcohol solution and prepared a set of samples with various deposition temperatures in the same solution. The crystal structure, morphology, and optical properties of CdS films depend strongly on the deposition temperature. With the increasing deposition temperature, the crystallinity of the samples was improved and the surface of CdS thin films becomes smoother. At 140 and 180 ℃, the average grain sizes of CdS thin films are about 77.16 and 76.61 nm, and the values of Ra and RMS are 4.6 nm and 5.94 nm, and 4.38 nm and 5.43 nm, respectively. These values are relatively small, which might be due to the films at the micro scale being relatively smooth and uniform. Films that are fabricated at different temperatures from 120 to 200 ℃ show good transmittance. The band gaps reduce from 2.55 to 2.45 eV with the increase of deposition temperature. Although the exact formation mechanism of the CdS thin films is unclear, we think that temperature is an important factor in determining not only the phase composition but also the morphology and optical properties of the CdS thin films. Some correlative studies are needed to know the exact mechanism.