Inuence of Phosphine-Free Solvents On the Structural and Optical Properties of Oleylamine Assisted Synthesis of CdS Nanocrystals

Synthesis of cadmium sulphide (CdS) nanocrystals was carried out using hot-injection method through oleylamine as solvent and capping ligand at 220 o C. From the UV-visible absorption spectra, the band gap of the synthesized nanocrystals was estimated as 2.5 eV. X-ray diffraction pattern conrms the presence of wurtzite phase with their characteristic reections. TEM analysis of the synthesized nanocrystals show that the nanocrystals are having nearly spherical shape with the diameter (cid:0)5-10 nm. For the comparison, different solvent mixtures were employed to prepare the CdS nanocrystals and the structural and optical results show that the solvents have signicant inuence on the optical properties of the prepared nanocrystals. The results are discussed in detail.


Introduction
Colloidally prepared semiconductor nanocrystals are widely applied for different kind of promising sectors such as solar cell, photodetector, photocatalysis and photodiodes [1]. Preparation of semiconductor nanocrystals using colloidal method has emerged as one of the versatile approaches to deliver nanocrystals with different morphologies. Speci cally, hot-injection method has been used to prepare variety of chalcogenide nanocrystalline materials for the solution processed applications [2,3]. In this method, easily decomposable precursors are actively reacting in hot-coordinating solvents under high temperature and result highly monodispersed, crystalline nanomaterials. The contribution of hot-injection method to the preparation of cadmium chalcogenide nanomaterials is remarkable and several research ndings are dealing about their preparation under different solvents [4,5,6]. By varying the physical parameters, the size and shape of the nanocrystals are cleverly managed in order to use them for different applications. Out of others, cadmium chalcogenides nanomaterials are commonly explored for several promising applications including solar cells, photocatalysis, light emitting diode (LED), bioimaging etc. For the core-shell preparation also CdS nanocrystals are playing important role in reducing surface defects of nanocrystalline materials such as cadmium selenide (CdSe), cadmium telluride (CdTe). Although several research ndings are dealing about the synthesis of CdS nanocrystals under different conditions [7,8,9,10] higher boiling point solvent assisted colloidal synthesis always has some advantages [11]. Higher boiling point solvents such as oleylamine (OLAm), oleic acid (OA) and 1octadecene (1-ODE) have been extensively used in recent years to produce monodispersed, highly crystalline semiconductor nanocrystals for the solution processed fabrication of optoelectronic devices [12,13]. Importantly, the morphology of the resultant nanocrystals is hardly in uenced by these solvents under the controlled conditions and it is also greatly in uenced by their binding ability. The composition, phase and optical properties of the semiconductor nanocrystals are often in uenced by these solvents.
With different kind of amines and acids, single molecular cadmium precursor such as dithiocarbamate, thiosemicarbazide, thiosemicarbazone etc. have shown formation crystalline CdS nanocrystals with different morphologies [14,15,16]. Having the boiling point over 300 o C, OLAm is enormously used to produce variety of metal chalcogenide and metal oxide nanocrystals by decomposing simple molecular precursors under optimized conditions [17,18]. The reducing ability of OLAm is additionally favour to achieve semiconductor nanocrystals with desired phase [19]. Other than functioning as solvent and ligand, OLAm also in uencing on the disproportionation of the precursors in the reaction medium in order to control the growth of the nanocrystals [20]. Furthermore, the activity of OLAm together with other coordinating and non-coordinating solvents such as OA and 1-ODE is found to be different and this critically in uence on the different facets of the nanocrystals which results in different morphologies [21,22]. Hence, it is essential to explore the role of OLAm in synthesizing metal sulphide nanocrystals and its activity with other solvents. In this present work, we investigate on the synthesis of CdS nanocrystals using OLAm as solvent and in uence of different solvents with OLAm on the structural and optical properties of the synthesized CdS nanocrystals. Lattice strain and crystallite size of the nanocrystals were calculated from the diffraction data using Williamson-Hall plot (W-H). W-H plot is drawn between sinθ and βcosθ in x and y-axis respectively ( gure 3) using the equation (1).

Experimental Section
Where, β is full width at half-maximum, λ is the wavelength of the X-ray, D is the average crystallite size and ε is the lattice strain of the compound. The obtained values are tabulated and given in Table 1. From the table, it is found that nanocrystals synthesized using OLAm are smaller than the nanocrystals obtained using OLAm/OA or OLAm/1-DDT. It should be noted that the size of the nanocrystals synthesized using OLAm calculated by this method is approximately equal with the TEM result of the same. In addition, the W-H plots show that nanocrystals synthesized using pure OLAm and OLAm/OA ( gure 3a and b) have negative slopes from the linear t, it construes that the nanocrystals have experienced a compressive strain in its lattice. Meanwhile, plot of the nanocrystals prepared using OLAm/1-DDT shows positive slope, hence, its lattice is experienced tensile stress ( gure 3c). The tensile strain in OLAm/1-DDT might be due to the elongation in the morphology. Furthermore, it is observed that this strain has ensued peaks shift in the XRD patterns. Chen et al. have found that because of the metalthiol interaction effect in 1-DDT, larger nanocrystals with lesser thickness are formed [23,24]. Hence, compressive strain is averted and the lattice experiences tensile strain. Similar kind of effect can also be correlated in the present case. Moreover, from the XRD analysis, it is observed that the metal-thiol interaction leads change in the predominant plane growth. Transmission electron microscopy (TEM) analysis of the CdS nanocrystals prepared by pure OLAm is given in gure 4 (a, b). The images clearly show that the prepared nanocrystals exhibit nearly spherical in shape. The diameter of the nanocrystals is estimated as 5-10 nm. Since the intention of the present study is to explore the primary role and effect of OLAm in the synthesis of CdS nanocrystals, the analysis is especially focused on the exploring the structural, optical and morphological properties of CdS nanocrystals prepared by pure OLAm. The TEM images of the prepared nanocrystals in show that the nanocrystals are assembled together quite closely, which may be due to the ligand (i.e. OLAm) induced self-assembly. The lattice spacing of the prepared CdS nanocrystals is calculated as 0.34 nm which further ensure the formed nanocrystals with wurtzite phase ( gure 4c). The selected area diffraction pattern (SAED) of the prepared nanocrystals in gure 4 (d) show that highly crystalline nanocrystals are formed with OLAm capping and this is also resembled with the obtained results from XRD analysis.
The formation mechanism of the nearly spherical shaped CdS nanocrystals could be described by the following theory. Because of the long chain (i.e bulky nature), OLAm is signi cantly in uencing on the CdS nanocrystals surface. When OLAm is re uxed with sulphur powder, it forms oleylammonium sulphide which functionally act as the sulphide precursor [25,26,27]. Upon injecting into the Cd-oleate complex mixture, formation of CdS nanocrystals rapidly happen through the nuclei formation. According to the Ostwald ripening process, in this situation, growth of nanocrystals takes place under prolonged reaction conditions. The non-spherical nature of the formed nanocrystals in the present study could be due to the steric hindrance provided by the OLAm capping which strongly in uence on the growth of facets of CdS nanocrystals. The schematic diagram of the synthesis scheme of CdS nanocrystals through hot-injection method in the presence of OLAm is represented in gure 5.

Conclusion
Page 6/12 Oleylamine capped CdS nanocrystals were synthesized using hot-injection method and their structural and optical properties were analysed. The nanocrystals are possessed nearly-spherical shape with the bandgap 2.5 eV. Although there is no phase transformation observed under the in uence of different solvents, the obtained results show that solvents have signi cantly in uence on the optical and morphological properties. Hence, it is possible to tailor the functional properties of CdS nanocrystals using suitable ligand chemistry. The results also show that the prepared nanocrystals are suitable for the solution-processed solar cell fabrication applications. Figure 1 XRD patterns of the CdS nanocrystals synthesized in the presence of different solvents