Optical study on II-VI semiconductor nanoparticles in Langmuir-Blodgett films

Cadmium sulfide (CdS) nanoparticles formed within Langmuir-Blodgett (LB) films of stearic acid and calix[8]arene were studied with different optical methods including surface plasmon resonance (SPR), ellipsometry and UV-visible absorption and fluorescence spectroscopies. For the first time, the process of formation of CdS nanoparticles within LB films was monitored in-situ with SPR. The results of ellipsometry, SPR and UV-vis absorption spectroscopy were analyzed to evaluate simultaneously the thickness, refractive index and extinction coefficient of LB films. It was shown that all three parameters increase as a result of formation of CdS nanoparticles. Photoluminescence measurements provided direct confirmation directly for previous observation with UV-vis absorption spectroscopy of the blue spectral shift caused by CdS particles formation. The observed large Stoke\´s shift of the luminescence band is discussed in terms of the formation of "dark excitons" in the platelet-type CdS nanoclusters. AFM study shows the formation of pseudo-two dimensional platelets of CdS with the lateral dimensions in the range of 20-30 nm.