Aerosol assisted chemical vapor deposition (AACVD) of CdS thin films from heterocyclic cadmium(II) complexes
Graphical abstract
The cadmium piperidine and tetrahydroquinoline dithiocarbamate complexes were used to deposit CdS thin films using aerosol assisted chemical vapor deposition (AACVD) method.
Introduction
Binary metal chalcogenides as thin films, or particulates in mono-dispersed form, especially II–VI semiconductors and in particular cadmium or zinc chalcogenides (CdS, CdSe, CdTe, ZnS, ZnSe) have attracted considerable attention [1], [2], [3], [4], [5]. They have high symmetry axis distinguished from all the other axes which serves as the directional axis for the asymmetric growth for the synthesis of nanorods [6], [7], [8]. Their properties and potential for applications in photovoltaic technologies which employ the use of cheap and widely available elements is important [10], [11], [12].
Cadmium sulfide is a group II–VI semiconductor with a direct band gap of 2.42 eV. CdS thin films have been used for a wide variety of technological applications, including photovoltaic cells, electro-optic modulators, sensors, electroluminescent and photo luminescent devices, and antireflection coatings [13], [14], [15]. The route used to prepare or deposit a material can profoundly affect the phase composition, thermal stability, and morphology, which in turn can influence the functional behavior of the material. Many techniques such as spray pyrolysis [16], chemical bath deposition [17], sol–gel [18], and metal–organic chemical vapor deposition (MOCVD) [19], [20], [21], [22], [23], has been used to deposit phase pure films of CdS.
The MOCVD technique employing precursors is a well established giving high-quality thin films. The use of single-source precursors (SSPs) can potentially provide several key advantages over other routes due to the existence of preformed bonds which can lead to a material with fewer defects and better stoichiometry [24]. In this work, the aerosol-assisted CVD (AACVD) technique was used because it could lead to the deposition of nanosized thin films under soft processing conditions where the volatility of the precursor is no longer crucial as in the case conventional CVD [25].
The nature, morphology and composition of the final product can be affected by the single source precursor used to deposit the materials [26], [27]. As initially reported the use of single source precursor for CdS has also provided an efficient route to high quality crystalline monodispersed nanoparticles of semiconductors [28], [29]. Various classes of single source precursors have been identified and developed including dialkyldithiocarbamates [21], [20], [30], [31], xanthates [19], [32], N-alkyl thioureas [33], and dithioimidodiphosphinates [34], [35], for the preparation of CdS thin films and nanoparticles.
We have recently demonstrated that heterocyclic piperidine and tetrahydroquinoline dithiocarbamato metal complexes can be used as single source precursors for the preparation of high quality ZnS, PbS and CdS nanoparticles [8], [9], [10], [36], [37], [38]. The variation of some reaction parameters such as the temperature, the monomer concentration and the capping molecule showed an effect on the size and the morphology of the synthesized nanoparticles. The precursors used for the decomposition were air and moisture stable at room temperature. The AACVD of CdS thin films using the pyridine adduct of bis(piperidinedithiocarbamato)cadmium(II) complex was also reported [39]. In this work, we report the use of piperidine (1) and tetrahydroquinoline (2) dithiocarbamato cadmium(II) complexes as single source precursors for the deposition of CdS thin films at different temperatures by AACVD. The single crystal X-ray structure of the bis(piperidinedithiocarbamato)cadmium(II) complex is also reported.
Section snippets
Chemicals
Cadmium chloride 99%, acetonitrile, 1,2,3,4, tetrahydroquinoline 98% (Aldrich). Piperidine 99% (Sigma–Aldrich). Petroleum ether, methanol 99.5%, dichloromethane, carbon disulfide 99.5%, chloroform, sodium hydroxide 98%, and acetone (Merck) were used as purchased without any further purification.
Instrumentation
Microanalysis was performed on a Perkin-Elmer automated model 2400 series II CHNS/O analyser. Infrared spectra were recorded on a Bruker FT-IR tensor 27 spectrophotometer directly on small samples of the
Characterization of the ligand and the complexes
The purity of piperidine/tetrahydroquinoline dithiocarbamate ligands and the corresponding cadmium complexes were confirmed by elemental analysis, IR, NMR and TGA. The complexes were obtained in good yield with a simple reaction route. The piperidine dithiocarbamate and its complex were white while tetrahydroquinoline dithiocarbamate and its complex were yellow. The complexes are air and moisture stable at room temperature for periods of several months.
The TGA thermograms show decomposition in
Conclusion
Piperidine and tetrahydroquinoline cadmium dithiocarbamato complexes have been synthesized and the single crystal X-ray structure of [Cd(pip-dtc)2] elucidated. Deposition of thin films by AACVD at various temperatures showed that the growth of CdS was influenced by deposition temperature. The morphologies of CdS films obtained showed that the thin films become increasingly dense with increase in temperature. The UV–Vis spectra of the films showed blue shift in absorption edge compared to bulk
Acknowledgements
The authors thank the National Research Foundation (NRF), South Africa through the South African Research Chair Initiative (SARChI) and Royal Society-DFID program for financial support. Dr. LD Nyamen also acknowledges the Organization for Women Scientists for the Developing World (OWSDW) formerly TWOWS for a Sandwich Postgraduate Fellowship.
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