Synthesis of w-CdS quantum dots and discovery of intense sub band emission owing to longitudinal optical phonons

Cadmium sulfide (CdS) quantum dots (QDs), capped with cetyltrimethylammonium bromide (CTAB), and was synthesized as stable, aqueous, colloidal nanofluid. A series of nine intense, well-resolved emission lines between 400 and 750 nm were observed for the first time when exciting the CdS QDs nanofluid with a 355-nm high energy pulsed Nd:YAG laser radiation. The energy separation between any two successive emission lines equals to the characteristic overtone energy of 295 cm(-1) of the longitudinal optical phonon of CdS QDs. In addition, recording the PL spectrum by using a xenon broad band light source resulted in the observation of this characteristic overtone energy of 295 cm(-1). In agreement with this photoluminescence characteristic, Raman spectrum exhibited four prominent Stokes lines with Raman shift equal to and multiple of 295 cm(-1). Transmission electron microscopy investigation showed that the CdS QDs were spherical with hexagonal wurtzite structure and had a size in the range of 5-10 nm.