Synthesis of CdS Nanocrystals Using Polymer Metal Complex as a Single Source Precursor and their Physicochemical Properties

Polymeric ligand (TSCF) have been synthesized via polycondansation of thiosemicarbazide with formaldehyde and its polymer metal complex [TSCF-Cd (II)] has been prepared with Cd (II) ion. Two different batches of CdS chalcogenide, semiconductor nanocrystals have been synthesized by hydrothermal treatment of TSCF-Cd (II)] using a glass Q tube reactor at 200 °C and 250 °C, and marked as (CdS)1 and (CdS)2 respectively. The resultant nanocrystals were characterized by a variety of methods for thier physochemcial properties. The TEM micrographs showed well-defined, close to hexagonal particles, and the lattice fringes in the HRTEM images confirmed their crystalline nature. The sizes of (CdS)1 and (CdS)2 were 40 and 50 nm respectively but their morphologies were similar. The optical band gap energies (2.52 eV/ 2.48 eV) and photoluminescence peaks (532/534 nm) of the synthesized CdS nanocrystals suggest that they can be promising photocatalysts. The conductivities and the dielectric constants of the CdS nanocrystals were also studied and the activation energy value of (CdS)1 and (CdS)2 nanocrystals were found to be 0.64 eV and 0.70 eV.


Introduction
Development of semiconductor nanocrystals has been intensively pursued because of their sizedependent characteristics, and their novel electronic, magnetic, optical, chemical, and mechanical properties that cannot be obtained in their bulk counterparts. Cadmium sulfide (CdS) is one of the most important chalcogenide semiconductor materials because of its direct band gap (2.42 eV at 300 K), which leads to many valuable properties such as photoluminescence, electroluminescence, and photocatalytic effects [1]. Considerable effort has been devoted to prepare monodisperse nanocrystals with controlled size and shape [2,3]. A wide range of approaches for synthesis of CdS nanoparticles has been reported, such as the hydrothermal method, solvothermal methods, low temperature synthesis and liquid ammonia synthesis etc. [4][5]. However, the product obtained in some cases suffers from particle agglomeration and a broad final particle size distribution [6,7].
Xie et al. [ 6 ] have reported branch-like CdS micropatterns, using thiosemicarbazide as both a sulfur source and a capping ligand in a methanol/water system. The work is important for understanding the formation of complicated CdS fractals and their potential applications in microelectronic devices. However, the shape produced was irregular, and the electron diffraction from the CdS architectures showed that the fractals were polycrystalline in nature [6]. W h en u s ed in combination with capping agents such as oleic acid or oleyl amine, polyvinyl alcohol, polyethylene glycol, etc, this procedure results in the formation of monodisperse nanoparticles with good crystallinity and uniform size [7,8]. The use of single source precursors for the synthesis of nanoparticles has proven to be efficient routes for the synthesis of high quality nanocrystals.
Ligands properties of the metal complexes used as precursor could be used in the modification of the size and shape of the nanoparticles. For example, alkyl-phenyl dithiocarbamate complexes have been used as single source precursors to prepare ZnS, CdS, and HgS semiconductor nanoparticles [9]. However to date, there is no published, controlled, research data on synthesis of CdS nanoparticles using polymer metal complexes as single-source precursor. In this study, we have synthesized a polymer metal complex TSCF-Cd(II) as a single source precursor by the reaction of    confirming the wurtzite phase [13]. Average particle size of nanocrystal is estimated according to Scherrer equation [14], 50 Nano Hybrids Vol. 1 Where K is a constant taken to be 0.94, λ is the X-ray wavelength and β 2θ is the full width at half maximum of the XRD selected diffraction peak on the 2θ scale and θ is diffraction angle. The particle size of the synthesized nanocrystal are found to be 39 nm and 48 nm for (CdS) 1 and (CdS) 2 respectively.

Fig. 3. (a) Optical spectra of CdS nanocrystals (b) Photoluminescence spectra
The UV-Vis absorption spectra of dispersed (CdS) 1 and (CdS) 2 were measured in the region 250-550 nm at room temperature and shown in Fig. 3a. The optical spectra can be used to calculate an approximate direct band gap using the Tauc relation [15].
Where α is absorption coefficient, E g is the absorption band gap, and A is a constant.
Hexagonal (CdS) 1 and (CdS) 2 have optical band gaps of 2.52 eV (484 nm) and 2.48 eV (496 nm), respectively, at room temperature. The band edges for both t h e samples are blue-shifted in relation to the bulk material [16].

Nano Hybrids Vol. 1 51
The particle size of nanocrystals is also calculated using Brus equation [17]-And , where m e * is the effective mass of the electron (0.19 me), m h * is the effective mass of hole (0.8 mh), R is the radius of the particle, e is the dielectric constant (5.7) and ε 0 is the permittivity of free space. The particle size of the CdS nanoparticles as estimated using the above equation are found to be 39 nm and 48 nm for (CdS) 1 and (CdS) 2 , nearly similar to that obtained from the XRD and TEM analysis. The electrical conductivity (σdc and σac) of CdS nanocrystals as a function of temperature is shown in Fig. 4a. The variation of log σdc with temperature T(K) shows that there are two distinct temperature zones with two characteristic regions. The first region from room temperature up to 475 K that indicating a slow increase of dc conductivity (σdc) with temperature and the second region above 475 K, indicating a rapid increase in dc conductivity. The ac conductivity (σac) of CdS nanocrystalsis found to be increases slowly in the temperature range between room temperature up to 400 K and above this temperature increased rapidly up to 450 K. The CdS nanocrystals show a maximum around almost the same σac above 450 K temperature, it may be attributed to the large band gap (nanosize), which means that the charges are not released from the particles by thermionic emission and are therefore not available for tunneling [19]. It is seen that the plot of the σdc v/s T(K) is almost a straight line at lower temperature range up to 475 K, showing an Arrhenius behavior in which the electrical conductivity is proportional to exp(∆Ea /KT ), where ∆Ea is the activation energy of the electrical conduction. The activation energy has been determined using slop of the plot between ln(σdc) and T(K). The activation energy value of (CdS) 1 and (CdS) 2 nanocrystals have been found to be 0.64 ev and 0.70 eV respectively.
All the physicochemical properties of the prepraed CdS naocrystals are tabulated in Table-1.
Because of the small size of the (CdS) 1 particles, the charge carriers reach the surface of the particles more easily enabling, easy electron transfer by thermionic emission, or tunneling or both, and resulting in a slightly enhanced conductivity compared to that of (CdS) 2 . The dielectric constant values of CdS nanoparticles are found to be larger than the corresponding values for bulk CdS crystals [20]. The dielectric constant (Є) of (CdS) 1 nanoparticles was slightly greater than that of (CdS) 2 . Also σac of CdS1 is slightly larger than that of the (CdS) 2 crystals as shown in Fig. 4b.