Evaporation and decomposition of acrylic acid grafted luminescent silicon quantum dots in ultrahigh vacuum

A thin film of silicon quantum dots (Si-QDs) is a potential thermoelectric material with a high figure of merit. Evaporation and deposition in ultrahigh vacuum is a novel method to produce such a thin film of high quality. Acrylic acid grafted luminescent Si-QDs have been synthesized by a simplified method. Their surface electronic core levels were investigated by X-ray photoelectron spectroscopy (XPS) at various temperatures. At room temperature, three components at 100.4, 102, and 104.4 eV were identified, corresponding to Si core, Si-C, and silicon oxide on surface, respectively. The appearance of Si-C component showed that acrylic acid has been successfully grafted onto Si-QDs surface. At 200 deg. C, intact evaporation took place and the ratios of silicon to carbon in core levels remained constant as the annealing time increased. Decomposition occurred at 340 deg. C, where the ratios of Si2p to C1s began to change. The XPS results are in agreement with thermogravimetric analysis (TGA) measurements which showed a sharp weight loss of 80% at 200 deg. C, which is the intact evaporation temperature. Another slow weight loss of 14% took place between 300 deg. C and 500 deg. C, which is a footprint of surface Si-C decomposition.