Studies on Organic/Inorganic Nanocomposites of Lead Sulphide Quantum Dots in Solution- Processed Phthalocyanine Films.

Abstract

A unique organic/inorganic nanocomposite of lead sulphide (PbS) quantum dots (QDs) embedded in substituted metal-free phthalocyanine (C6H2Pc) has been prepared by a simple and low-cost method. The preparation procedure consists of exposure of a thin spun film of non-peripherally octa-hexyl lead phthalocyanine to hydrogen sulphide atmosphere. The formation of the PbS QDs has been verified using X-ray diffraction and transmission electron microscopy techniques. From the transmission electron microscopic measurements, the average size of the PbS QDs is found to be 4.5 nm, which is smaller than the exciton Bohr radius. Independent Xray diffraction and optical absorption studies provide supportive evidence for the size of QDs. Quantum confinement gives rise to a clear blue shift in the absorption spectrum with respect to the bulk PbS. The QDs band gap has been estimated to be 1.95 eV from Tauc's law and the frontier energy levels of the PbS QDs has been derived. About two orders of magnitude increase in ohmic conductivity, from 6.0×10−12 for C6H2Pc to 3.1×10−10 for the nanocomposite, is observed by steady-state electrical measurements in sandwich structure between indium tin oxide and aluminium. Temperature-dependence of the electrical conduction is studied aimed to calculate the activation energy and determine the type of conductivity. The incorporation of the PbS QDs decreases the activation energy by about 0.5 eV at temperatures higher than 240 K. It is found that the Poole-Frenkel mechanism is in good consistency with the superlinear electrical behaviour of the nanocomposite. The frequency response of alternating current (AC) conduction is found to obey the universal power-law. The cryogenic study of AC conduction reveals that the correlated barrier hopping (CBH) model closely fits to the experimental data at temperatures below 240 K. The parameters obtained by fitting the CBH model point out that the hopping process cannot take place directly between neighbouring PbS QDs but involves the localised states within the matrix.