Off-Resonant Raman Spectroscopy of ZnS Quantum Dots

Abstract

ZnS nanoparticles were synthesized mechanochemically by high-energy milling. Samples were produced in three different milling times. The morphology of samples has been investigated by scanning electron microscopy (SEM). X-ray diffraction (XRD) investigation of synthesized nanocrystals identified cubic structure, and crystallite size was estimated to 1.9 nm (5 min milling), 2.3 nm (10 min) and 2.4 nm (20 min). These dimensions ensure strong confinement regime. Raman spectroscopy studies (100–500 cm⁻¹) have been performed. Excitation source was 514.5 nm (E_L = 2.41 eV), implying that we are in off-resonance regime. Dominant spectral structures are registered in spectral region 130–180 cm⁻¹, around 265 cm⁻¹ and around 345 cm⁻¹. First two are assigned as combination modes and mode at 345 cm⁻¹ as confined ZnS LO type phonon. Absence of TO mode with visible excitation is consequence of poor scattering efficiency and anti-resonant behavior. We report relatively strong, compared to confined ZnS LO type phonon, Raman activities of combination modes away from the resonance in the strong confinement regime in ZnS quantum dots (QD). We find that off-resonance Raman spectroscopy can be used for quick estimation of the dimension of produced ZnS QDs. Sum of second-order Raman active modes centered at 265 cm⁻¹ dominates over LO-like mode at 345 cm⁻¹ in strong confinement regime.