Controlled Synthesis, Growth Mechanism And Photoconductivity Of PbSe,PbS And Their Ternary Solutions

This dissertation is intended to develop the controllable synthesis and growth mechanism of hexapod-like PbSe and PbS as well as its ternary PbSexS1-x nanostructures. By designing different reaction routes, well defined hexapod-like PbSe as well as its ternary PbSexS1-x nanostructures with various compositions and morphologies were fabricated, the related growth mechanism and morphologies based structural characters were discussed. Meanwhile, the photoconductivity of the ternary PbSexS1-x (x=0.2) had also been studied and a possible mechanism was also proposed. The details are summarized as follows:1. A facile organometallic synthetic route had been developed for the preparation of well-defined star-shaped PbSe nanostructures, different morphologies of PbSe, including star-shape, cube, octahedron and side-cut cube were produced. Lead selenide (PbSe) nanostructures with well-defined star-shaped morphology were successfully fabricated via a facile organometallic synthetic route from the reaction of Pr4Pb with Ph3PSe in DBA with the assistance of OA and OAm at220℃for30min. The structure and shape of the nanocrystals were investigated by TEM, SEM, XRD, SEAD and EDX and found that the obtained PbSe nanostars presented Pb-rich features although the PbSe nanostars were still in typical rock salt phase. Experimental investigations demonstrated that the media of DBA, OA and OAm played important role in the morphologies evolutions of different PbSe nanostructures and found an order of OA> DAB> OAm in the growth of the PbSe nanostars with well-defined shapes. ATR-FTIR studies also showed that the media could change their morphologies as well as their surface features. By manipulating the absorption on exposed{111} faces and other relevant reaction conditions, different morphologies with cubic, side-cut cubic, and octahedral shapes can be fabricated. This study not only provided an insight into the controllable growth mechanism of different morphologies of PbSe nanostructures, but also gave a feasible synthesis route for the materials with specific faces and morphology-related characters.2. On the basis of the preliminary work, we expanded this organometallic synthetic route to the synthesis of PbSe, PbS and their ternary PbSexS1-x nanostructures. By effective selection of Se-and S-sources of Ph3PSe and DBDS that had similar reactivity in the current reaction system, hexapod-like ternary PbSexS1-x had been fabricated from the reaction of PhsPSe and DBDS in DBA with the addition of OA at260℃for5min. The shape, structure and composition of the nano structured hexapods was investigated by XRD, SEAD, EDX and XPS and found that the obtained ternary nano structured hexapods were of typical rock salt phase with Pb-rich features and their compositions could be systematically regulated by facile variations of reaction parameters. Investigations revealed that the successful fabrication of the ternary hexapods with tunable compositions was resulted from the similar reactivity of Se-and S-sources along with small lattice mismatch between the two endmembers of PbSe and PbS. In general, the relations between the composition and lattice parameters for the ternaries obtained in DBA with varied addition of OA exhibited linear slops that were consistent well with Vegard’s law. Meanwhile, the growth mechanism and procedure were also discussed, and found that the as-grown ternary PbSexS1-x hexapods were mainly gradient alloyed nanostructures with somewhat chalcogen-element segregations or disorders rather than homogeneously alloyed solid-state solutions due to kinetic limitation for short reaction time even though thermodynamics was feasible in the system, and also, high concentration of S element in the feedstocks tended to relative high density of disorders in the ternaries. Based on the reveal of the growth mechanism for the ternary hexapods with varied micro structures, the ternary PbSexS1-x hexapods could be tuned from gradient alloys with segregations to approximately homogeneous nanostructures via enlongating reaction time.3. The photoconductivity of the ternary PbSexS1-x (x=0.2) had been investigated and found its photocurrents could reached820nA under light illumination of a typical hexapod-like ternary PbSexS1-x (x=0.2) nanostructure and its signal-to-noise ratio could reach27times, better than its two binary materials. Different technical methods, like XRD, TEM, SEM, SEAD and EDX, were carried out and found that the obtained ternary PbSexS1-x (x=0.2) nano structured hexapods were of typical rock salt phase and obviously gradiently nanostructures with relatively small size distribution. XRD, Raman, XPS and EDX presented the Pb-rich features of the as-grown ternary PbSexS1-x (x=0.2) hexapods and different degree of chalcogen-element segregations, disordered or defected microstructures existed in them, namely, the concentration of S decreased gradually from the tips of arms to its central region. The photodetectors of the ternary PbSexS1-x (x=0.2) nanocrystal and its two endmembers PbSe and PbS were also fabricated and found there was an obviously enhanced photoconductivity feature between the ternary nanostructure and its two endmembers. A mechanism related to defected and vacancy nanostructure in ternary was proposed which demonstrated that these disordered nanostructure greatly increased concentration of carrier density and the photoconductivity enhanced.