Growth and photoluminescence properties of PbS nanocubes

This paper reports the synthesis of semiconductor nanocrystals of PbS through a facile and inexpensive synthetic route between lead acetate [Pb(Ac)2·3H2O] and dithioglycol (HSCH2CH2SH) without any surfactant. The products were characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), Raman spectroscopy, x-ray photelectron spectroscopy (XPS) and infrared (IR) spectroscopy. The size of cube-shaped PbS nanocrystals can be controlled by changing the relative amount of Pb(Ac)2·3H2O and dithioglycol. Smaller cube-shaped PbS nanocrystals were obtained at higher sulfur content in the reaction system. The nanometre-sized single crystals of PbS measure ∼83 nm x 59 nm x 22 nm and 39 nm x 10 nm x 7 nm, for lead acetate:dithioglycol ratios of 1:0.375 (denoted as PbS-1) and 1:1.5 (denoted as PbS-2), respectively, when a mixture of NaOH solution of lead acetate and ethyl alcohol solution of dithioglycol is maintained at 220 0C for 4 h in a Teflon-lined autoclave. XRD patterns and HRTEM images confirm the crystalline structure of the PbS crystals. TEM helped to determine the dimension of the single crystals. Raman, XPS and IR helped to further demonstrate the purity of the as-synthesized samples. The optical absorption spectra of suspensions of the these crystals in glycol show shifts in absorption band edge for as-prepared sample PbS-1 from 246 to 271 nm and for PbS-2 from 246 to 275 nm, after four days of storage, respectively. The photoluminescence peaks from the two samples are located at 434 and 433 nm for photoexcitation radiation of 315-355 nm, thus exhibiting a Stokes shift compared with band edge energy. The principal contribution of this work is that the nanometre-size PbS single crystals are prepared without using a surfactant or templating agent. The formation of a five-membered ring via the chelation between the Pb2+ ion and dithioglycol is considered to be crucial in this mechanism