The nanometer particles of lead sulfide, PbS, have shown an increased demand in the last decade due to their unique physical properties. PbS nanoparticles have been prepared in colloidal solutions or embedded in transparent solid media such as glass, zeolite and polymer. Bulk PbS has a cubic (rock salt) crystal structure and a narrow direct band gap (0.41ev) at the L point of the Brillouin zone. This point is well separated from all the other bands; thus size quantization will influence mainly the valence and conduction band edges. Moreover, the high dielectric constant (17.3), as well as the narrow band and small electron effective mass (<0.1,m), create an excition with large effective Bohr radius (180Å) and relatively weak binding energy. The aforementioned properties suggest that size quantization possesses strong influence on the electronic properties of PbS nanoparticles. On the other hand, a key objective of researches in the field of small semiconductor clusters is to produce such species in a very narrow size dispersion range. Such a material would permit the examination of intrinsic optical and other physical properties of a given cluster dimension unclouded by averaging effects of having an heterogeneous distribution of cluster size. The results reported for almost all developed techniques so far, show a quite wide significant dispersion. Among those, previous work on extremely small PbS clusters in zeolite Y , is one of the very few ones having a narrow cluster size range with a unique observed optical properties.
A range of colloidal technique has readily prepared small particles of lead sulfide for the over 17 years. In this research, based upon procedure taken from ref., preparation of lead sulfide nanoparticles solutions were carried out. The main step of this procedure, which has been schematically shown in the block diagram in fig.(1), is as follow. 30 µl of 0.1M Pb(NO3)2 aqueous solution was added into 30 ml of water containing 2.29×10-3 g sulfur ions (S2-). Some 30 µl of 0.1M ethylenediamine tetraaciticacid (EDTA) sodium salt aqueous solution were added to this provided solution in order to form a complex with Pb2+ in order to retard the rate of the subsequent reaction. The solution was then stirred for a few seconds. In order to have a controlled chemical reaction, H2S gas purging usually has been used. In this work for this purpose, H2S gas dissolved in water was used and the amounts of S2- in water were determined by back titration process (titration with Na3AsO3.12H2O), and direct method. H2S gas was prepared from reaction between ferrous sulfide and hydrochloric acid in a Kipps apparatus. Absorption spectra of the colloidal solutions in the visible range were taken using a Hitachi U-3410 spectrophotometer. Most common solvents absorb light strongly in the near infrared, therefore, in order to measure the absorption of the PbS in this region it was necessary to use 200-800 nm wavelength rang of photospectroscopy. MgCl2 was added to solution to induce fluctuation of the nanoparticles and to increase stability of the colloids. X- ray diffraction was carried out on the sample provided by above-mentioned procedure. The diffraction pattern obtained confirmed that PbS was the dominant phase presents. No significant diffraction peak due to MgCl2 was observed. The X- ray diffraction data indicated that nanoparticles of PbS, preserve their rock salt crystal structure with relatively high degree of crystallinity. Since the clusters in colloidal solution are too small, it is difficult to be characterized by either an ordinary X-ray diffraction apparatus or transmission electron microscopy. Therefore, it was decided to use a theoretical estimation method to do so. For this reason, it seems that the 'Tight Bonding Cluster Model', developed and presented by Wang, et.al., is a reasonable way to determine the size of the cluster. The calculation shows the average particle size is about 5 Å.
Chen.W, et.al.,'New observation on the formation of PbS clusters in zeolite-Y', Appl.Phys.Lett., 68 (14), pp 1990-92 (1996)
Bhat.J.C, et.al.,'Lead sulfide quantum-size particles with absorption band onset in the near infrared', Mat.Res.Symp.Proc., 421, pp 145-150 (1996)
Wang.Y, et.al.,'PbS in polymers. From molecules to balk solids', J.Phys. Chem, 87 (12), pp 7315-7322 (1987)
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