Aerosol assisted chemical vapor deposition (AACVD) of CdS thin films from heterocyclic cadmium(II) complexes

Highlights

• Cadmium dithiocarbamato complexes of piperidine and tetrahydroquinoline were synthesized.

• Single X-ray crystal structure of cadmium(II)piperidine dithiocarbamato complex elucidated.

• AACVD of the complexes formed nanocrystalline, compact CdS thin films.

• Optical properties of the CdS films exhibited quantum confinement effect.

Abstract

Cadmium dithiocarbamato complexes of piperidine (1) and tetrahydroquinoline (2) were synthesized and characterized by elemental analysis, FT-IR spectroscopy, NMR and TGA analyses. The X-ray single crystal structure of bis(piperidinedithiocarbamato)cadmium(II) complex (1) was determined. The synthesized compounds were used as single source precursors to deposit CdS films on glass substrates at 350, 400 and 450 °C using the aerosol assisted chemical vapor deposition (AACVD) method. The surface morphology and the size of the CdS films were determined by AFM and SEM analyses; the crystalline phases by p-XRD analyses and the elemental composition by EDX measurements. The particle sizes were found to be in the range between 50–110 nm and 100–220 nm for complex (1) and (2), respectively. The electronic properties of the CdS films were analyzed by UV–Vis and Raman spectroscopic methods. Results revealed that the morphology, size and composition of the films are influenced by the deposition temperature.

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.