Raman spectroscopy of ZnS quantum dots

Highlights: • ZnS nanoparticles were synthesized mechanochemically by high-energy milling. • Optical properties were investigated applying Raman spectroscopy. • Continuum medium model of optical phonon confinement in ZnS QD. • Second-order Raman modes in ZnS QDs. - Abstract: ZnS nanoparticles were synthesized mechanochemically by high-energy milling. In order to investigate influence off milling time to sample properties, samples were produced in three different milling times (5 min, 10 min and 20 min). The morphology of samples has been investigated by scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). X-ray diffraction (XRD) investigation of synthesized nanocrystals identified cubic structure. From XRD, ZnS size of crystallites was estimated as 1.9 nm (after 5 min milling time), 2.3 nm (10 min) and 2.4 nm (20 min), implying that we are in strong confinement regime. The optical properties were studied by Raman spectroscopy, in spectral region 100–500 cm⁻¹, excitation source was 514.5 nm (E_L = 2.41 eV), which means that we are in off resonant regime. Dominant spectral structures, of comparable intensity, are registered in spectral region 130–180 cm⁻¹, around 265 cm⁻¹ and around 345 cm⁻¹. First two are assigned as second-order ZnS modes. A theoretical model of continuum medium was used to calculate frequencies of the confined optical phonons in ZnS. Satisfactory agreement with experimental results was found and mode at 345 cm⁻¹ is assigned as LO type phonon confined in ZnS nanocrystal