Transformation of the structural surface morphology and electronic structure of CdS nanostructured films prepared in glycerol-containing solution, under ultrasonic irradiation

So far, interest in cadmium sulfide (CdS), a typical II–VI semiconductor, having an optical band gap of 2.42 eV, has increased due to its application in various strategic fields, such as in photovoltaic solar cells and electronic and optoelectronic devices [1–4]. Among several deposit techniques, chemical bath deposition (CBD) is one of the most promising and has proved advantageous because it is easy to process, cost effective and suitable for large area and high quality thin films. Of course the physical properties of the CdS film depend on different fabrication methods with different additives, the relative concentrations of reactants, filming growth temperatures and pH value. In CBD, the nucleation and particle growth to form CdS film on a substrate is limited with two processes involved in hydrolysis and deposition of material on the substrate immersed in the solution. To overcome the harmful influences of hydrolysis, and for better quality surface CdS thin films, optimal processing parameters and additive complexing agents for metal ions should be chosen. Indeed, the CdS films that were prepared via the solution route have a nanostructured surface with grain dimensions less than 100 nm in diameter [5] or are nanocrystalline CdS thin films consisting of needle-shaped grains [6]. Beside this, films that are flat, homogeneous, green-yellowish, transparent with very good adherence to the substrate, and possess a preferential orientation, were also reported [7]. Recently, filming in CBD was carried out under the application of electromagnetic irradiations [8, 9] in order to improve the successive growth of nuclei and homogeneity of the films. In this work an effective method is suggested to prepare CdS films consisting of either CdS fibrils or monodispersed CdS grains in addition to glycerol.

All chemicals involving ^CdSO ₄, ^CS(^NH ₂)₂, ^NH ₄ ^NO ₃ and glycerol were purchased from Merck and are AR grade and used without further purification. The molar ^Cd/^S/^NH ₃ ratio is kept constant at 1 : 5 : 5. The CdS films were deposited on 18×18 ^mm glass slides (Superior), at ^pH=8.5 and room temperature, under ultrasonic irradiation for one hour, using a H8890 ultrasonic cleaner (Cole-parmer). The samples are known as Sp-1, Sp1-SA, Sp-2, Sp2-SA and Sp-3, Sp3-SA (SA is for samples prepared under ultrasonic irradiation), corresponding to concentrations of glycerol of 0.05, 0.01 and 0.005 M. The obtained yellow-reddish films were washed with mixed 50/50 alcohol–water solution several times to remove excessive thiourea and ions possibly remaining in the films. The films were dried and heated in a vacuum oven, at 230 °^C. A D5005 diffractometer with Cu-Kα radiation (Siemens) was used to study the crystalline characterization of CdS films, at an x-ray operating incident angle kept constantly at 1°. The surface morphology of the CdS films was investigated with a JSM 5410LV scanning electron microscope (Jeol). A Shimadzu UV 3101PC spectrophotometer was used to record optical absorption spectra of CdS films in the region of 300 to 800 nm, with a resolution setting of 2 nm.

It is evident from the scanning electron microscope (SEM) photographs that at a very low glycerol concentration, the CdS film consists of close packed round grains and aggregates of grains that are heaped up together (figure 1(e)). The size of the grains ranges from 35 to 80 nm. It reveals the considerable contribution of cluster-by-cluster deposition during filming. Under ultrasonic irradiation the accumulation of clusters of CdS into aggregates of CdS was suppressed. The CdS species are constrained in oriented interconnects. Thus, the surface morphology of the film (figure 1(f)) just consists of close linked fibrils and some slabs of CdS covered between them. At a glycerol concentration up to 0.01 M, the morphology of the CdS film is in the form of a smoother and more homogeneous grain structure, but less dense (figure 1(c)).

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The aggregates of clusters of CdS disappear. The film appears to be of a single layer of CdS grains. Compared to figure 1(f), under ultrasonic irradiation, the morphology of the CdS film is in the form of denser and more homogeneous fibrous structures. The average diameter of the fibrils is about 52 nm (figure 1(d)), slightly less than for the figure 1(f) sample, about 60 nm. The film consists of several networks of CdS fibrils laid on together. The number of slabs of CdS is obviously reduced. At a concentration of glycerol up to 0.05 M, under ultrasonic irradiations the morphology of the CdS film consists of a single layer of regular-shaped grains with a high degree of crystallinity and average diameter of about 45 nm (figure 1(b)), quite different from that shown in figures 1(d) and 1(f). These observations evidence the existence of various linkages of cadmium species and glycerol, of course, affecting the mechanism of two deposition processes, cluster-by-cluster and ion-by-ion [10, 11].

At a low concentration of glycerol (figure 1(e)) the part of the cadmium species linked with glycerol acts as the center for aggregation of the remaining cadmium species. However, the aggregation of cadmium species is significantly decreased due to the increase of the glycerol concentration. The increase of most cadmium species is linked with glycerol. Then the close packed round grains of CdS are deposited (figure 1(c)). At a higher concentration of glycerol (0.05 M), the glycerol molecules may be polymerized into chains and a matrix. The clusters of CdS appear to be decreased in size, and just interconnect together in accordance with the chains or matrix of the polymer, into a form either chain-like or aggregate (figure 1(a)). Under ultrasonic irradiation, the polymerization appears to be reduced, but there may still exist some glycerol species. Also the linkages of CdS species and glycerol are reduced. Glycerol species now play the role of a shield to aggregation of CdS species. Then the CdS is deposited into regular-shaped grains (figure 1(b)). The crystallographic properties of the CdS films have been investigated by the x-ray diffraction technique (figure 2). CdS may exist in two crystalline modifications—hexagonal and cubic phases—that are characterized by several diffraction peaks positioned at 2θ≈ 23.4°, 27.1° and 47.8°, as reported in the literature for CdS thin films [11–13]. A highest intensity peak at 2θ≈ 27.1° here is characteristic of the (002) reflection of the hexagonal CdS lattice and the (111) reflection of the cubic. The XRD patterns also reveal several peaks at 38.6° and 55.6°, respectively, corresponding to (200) and (220) reflections of the CdO cubic lattice, which agree fairly well with those reported [5].

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The increase of glycerol concentration gives rise to the most intense peak of CdS, indicating the stronger preferred orientation and higher crystallinity of the CdS films. It reveals the lower the formation rate of CdS, the higher the concentration of glycerol. Oppositely, all the diffraction peaks of CdO were reduced at a concentration of glycerol up to 0.05 M. Aiming at glycerol and ultrasonic irradiation, UV-visible absorption spectroscopy was employed on the CdS films prepared under different conditions. Figure 3(a) shows the optical absorption spectra of the CdS films. As seen (figure 3(a)), the features at 488, 482 and 470 nm, respectively, for Sp-3, Sp-2 and Sp-1, reveal considerable blue-shift relative to the absorption peak of bulk CdS, at 515 nm, indicating the quantum size effect. At high concentrations of glycerol, the absorption spectra also reveal a steadily increasing absorption tail as the wavelength is smaller, except for a low concentration of glycerol (0.005 M). These observations reveal that the presence of glycerol has a positive effect at grain boundaries, leading to detrimental reduction of band bending towards the absorber surface and increasing the population of optical absorptions. Under ultrasonic irradiations, all the optical absorptions are the same in shape (figure 3(b)). It again evidences that the linkages of CdS species and glycerol appear to be suppressed. So the positive effect of glycerol at grain boundaries is significantly reduced. The optical absorptions disappeared at wavelengths less than about 290 nm. Further studies on the role of glycerol are currently in progress.

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Due to the presence of glycerol and under ultrasonic irradiation, the CBD technique was revealed as an effective one to prepare CdS films of various size and structure. The morphology of the film can be transformed from a fibrous structure to regular shaped grains.

The CdS films may exist in two crystalline modifications—a hexagonal and a cubic phase—that are characterized by several diffraction peaks positioned at 2θ≈23.4°, 27.1° and 47.8°, as reported in the literature for CdS thin films [11–13].

The UV-Vis investigations revealed the positive effect of glycerol at grains boundaries, leading to detrimental reduction of band bending towards the absorber surface and increasing the population of optical absorptions.

This work was supported in part by KHCB5-033-06, Research Grant of the Ministry of Science and Technology, Vietnam.