Photoconductivity of Poly-p-Xylylene + CdS Nanocomposite Films over a Wide Temperature Range

摘要

For poly-p-xylylene + CdS (PPX + CdS) nanocomposite films, the dependences of the photo- conductivity σ_(ph)(T) on the concentration C of CdS nanoparticles, intensity and wavelength of exciting light, and temperature T within 15–300 K are examined. An appreciable photocurrent appears at C ≥ 10 vol %, when a large percolation cluster of CdS nanoparticles is formed. The photocurrent spectrum is compared to the absorption spectrum of the film. The photocurrent I_(ph)(P) increases with the intensity of light flux P in a wavelength range near 435 nm according to the I_(ph)(P) ~ P~n power law, where n < 1. At 15 K, the photocon- ductivity of films with C ≈ 11.5 and 13.5 vol % is higher than that of a pure CdS film (C = 100 vol %) by factors of ~100 and ~30, respectively. For films with C > 11.5 vol %, the σ_(ph)(T) dependence at low T exhibits a metal- like character (σ_(ph)(T) decreases with increasing temperature). Atomic force microscopy is used to examine the surface topography of PPX + CdS films, which is found to be strongly dependent on the concentration of nanoparticles. The dark conductivity and photoconductivity of nanocomposite films arise due to the thermo- and photoexcitation transfer of electrons from the CdS nanoparticles to the PPX matrix with the formation of an electronic double layer at the PPX matrix–large percolation CdS cluster interface, a process that pop- ulates the phenyl rings of the adjacent PPX layer with excess electrons. As a result, various mechanisms of electron transfer in the polymer matrix can be realized: Mott’s hopping conduction mechanism with vari- able-range hopping in the matrix between CdS clusters and the metal-like behavior of the conductivity in the polymer shell of the large cluster at low temperatures. The polymer shell contains excess electrons on the phe- nyl rings-C_6H_4 in the composition of anion-resonances -C_6H~-_4-.