Synthesis and investigation of band energy variations in PbS nanocrystallites via optical route

In this study, ultrafine PbS nanoparticles were synthesized through wet-chemical synthesis, using 2-mercaptoethanol as a capping agent. It is for the first time that mercaptoenthanol is being used during synthesis of PbS. The structural, electronic/optical and morphological properties of the samples were studied by XRD (X-ray diffraction), UV-Vis spectroscopy and STM (scanning tunneling microscopy). It was found that, the size of the particles is highly dependent on the molarity of the capping agents. Excitonic shoulders along with a large blue shift of the absorption peaks in the optical spectra (due to quantum size effect) were observed to appear when the capping agent (2-mercaptoethanol) concentration increased from 0.1M (on set) to 1M (saturated state). The X-ray diffraction patterns exhibited the PbS characteristic peaks and also specified high purity of the samples. STM measurements were performed to observe the size and morphology of particles, which suggested that the particles are about 1.5-2 nm in size and spherical in shape. Band gap of the obtained PbS nanoparticles was calculated from the optical spectra and was found to be 4.1 eV. The merit of this study is that extremely fine PbS nanoparticles could be achieved with an approximate band gap of 3.5 eV which has not been reported so far.