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Lead sulfide (PbS) and Au-PbS core-shell nanoparticles were successfully synthesized using the sonochemical method at room temperature. The morphology of the synthesized particles was characterized by FESEM and TEM images. Pure fcc phase of PbS and Au crystal structures was examined and confirmed by XRD patterns. The quantum confinement effect plays a crucial role in blue-shifting the absorption edge and the band gap energy of both solid PbS nanoparticles and a thin spherical PbS shell toward shorter wavelength region in comparison to those of PbS bulk. | VNU Journal of Science: Mathematics – Physics, Vol. 33, No. 3 (2017) 11-16 Optical Properties of PbS and Au-PbS Core-Shell Nanoparticles Sai Cong Doanh, Pham Nguyen Hai, Ngac An Bang* Faculty of Physics, VNU University of Science, 334 Nguyen Trai, Hanoi, Vietnam Received 09 August 2017 Revised 30 August 2017; Accepted 20 September 2017 Abstract: Lead sulfide (PbS) and Au-PbS core-shell nanoparticles were successfully synthesized using the sonochemical method at room temperature. The morphology of the synthesized particles was characterized by FESEM and TEM images. Pure fcc phase of PbS and Au crystal structures was examined and confirmed by XRD patterns. The quantum confinement effect plays a crucial role in blue-shifting the absorption edge and the band gap energy of both solid PbS nanoparticles and a thin spherical PbS shell toward shorter wavelength region in comparison to those of PbS bulk. Due to the high refractive index of PbS shell, Surface Plasmon Resonance (SPR) peak of Au nanocores is significantly red-shifted by roughly 80 nm toward the longer wavelength region. More sophisticate experimental data and some adequate theoretical models are needed to fully explain the matters. Keywords: PbS nanoparticle, Au-PbS core-shell nanoparticle, quantum confinement, Surface Plasmon Resonance (SPR). 1. Introduction Lead sulfide (PbS) is classified to be in a class of IV-VI semiconductors with a narrow direct band gap energy of 0.41 eV at 300 K [1]. Due to the strong quantum confinement effect, the band gap energy of PbS nanomaterils can be tuned in the near infrared and even in the visible regions leading them to be employed in a lot of applications such as IR detectors, glucose sensor [2, 3], phototransitors [4], solar absorber [5] or materials for luminescent display device [6], recently. Furthermore, metal-semiconductor heterostructures such as Au-PbS, Au-Cu2O, Ag-Cu2O and AuSnO2 core-shell nanoparticles or TiO2-Ag and ZnO-Au composites have been finding .