Long-standing interest in understanding and ultimately controlling crystal growth has recently materialized as studies into new routes for making high quality metal and semiconductor nanocrystals (NCs), nanorods (NRs), nanowires (NWs), as well as other higher order nanostructures. The discovery that metal NCs have catalytic properties for promoting asymmetric crystal growth has motivated studies into making 1D semiconductor NWs. Recent investigations have led to the development of synthetic techniques that include variations of vapor-liquid-solid (VLS) growth, wherein chemically synthesized or laser ablated metal NCs are used as catalyst particles. Other approaches include complete solution phase analogues of VLS growth such as solution-liquid-solid (SLS) growth. Here the solution phase synthesis of narrow diameter (< 10 nm) straight and branched CdSe and PbSe NWs is described. Crystalline NWs with lengths between 1-10 microns are obtained using a seeded solution approach, whereby NW growth is initiated using Au/Bi core/shell NCs. A gold biphasic reduction step results in the formation of small 1.5 nm diameter Au NCs followed by the thermolysis of trialkylbismuthines to yield low melting, bimetallic, core/shell particles. The resulting NWs have diameters less than 10 nm and may exhibit unique quantum confinement effects given that the bulk exciton Bohr radius of CdSe (PbSe) is 5.6 nm (46 nm). Manipulating the reaction conditions allows one to transition from straight to branched nanowires yielding tripod, v-shaped, y-shaped and t-shaped NWs. Further variations in the preparation lead to higher order NWs that exhibit multiple branching points. In all cases, the presence of surface binding surfactants yields soluble straight and branched NWs opening up intriguing opportunities for future surface modification and/or surface functionalization chemistries. Such branched wires also provide the distinct possibility of studying not only size dependent optical and electrical properties of NWs but their shape dependent properties as well.
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