Studies on cadmium sulphide nanoparticles formed by the Langmuir-Blodgett technique

• Main Author: Iwantono, M.
• Other Authors: Nabok, Aleksey ; Ray, Asim
• Published: Sheffield Hallam University 2003
• Subjects: 621; Semiconductors
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.273592

In this project, the formation and characterisation of CdS nanoparticles embedded within Langmuir-Blodgett (LB) films have been studied systematically. CdS nanoparticles were formed within calix[8]arene (CA) and stearic acid (SA) Cd-salt LB films by exposure to H[2]S gas at room temperature. The AFM images of the treated SA and CA bilayers show CdS clusters with lateral dimensions in the range of 20-30 nm and 10-15 nm, respectively. These particles are pseudo two-dimensional and have a shape of hexagonal platelets which is most likely consequence of their wurtzite structure. Calculations of the cluster thickness Lz yield the value of 2 nm (Photoluminescence excitation-PLE) and 1.9 nm (UV-visible) for CdS clusters in SA matrix and two values of 1.2 nm and 1.6 nm (PLE) and 1.08 nm and 1.42 nm (UV-visible) for CA LB films. The PL spectra demonstrate a large Stokes shift, indicating the formation of "dark excitons" in the platelet CdS clusters. The transformations of the absorption spectra caused by ageing yield a monotonic increase in CdS cluster size with the tendency of saturation. This behaviour can be explained by the model of two dimensional (2D) diffusion. According to this model, CdS molecules in the SA films, initially evenly distributed within each LB bilayer, aggregate and form "large" particles with an average size of 1.9 nm and smaller particles with their size distributed over a wide range down to single molecules. The Surface Plasmon Resonance (SPR) and ellipsometry measurements confirmed that the H2S treatment to the LB films results both in the increase in thickness and refractive index of the LB films due to formation of CdS nanoparticles inside. SPR kinetics measurements during H[2]S treatment show that the formation of CdS clusters happened very fast and almost reached the saturation in about 30 minutes. The I-V characteristics of the sandwich structures of Aluminium/LB films/ Aluminium show exponential behaviour and weak temperature dependence, indicating a combination of electron-tunnelling and hopping mechanism. The formation of CdS clusters in the film matrix did not result in either an enhancement of the electron transfer or an additional polarisation at a small (50 mV) AC signal. Most likely, the observed changes in DC conductivity and AC impedance are caused by the film disorder introduced by CdS nanoparticles.