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Colloidal methods to synthesize Ag nanocrystals include citrate reduction \cite{Wu_2008,Lee_1982}, silver mirror reaction \cite{Yin_2002}, polyol synthesis \cite{Wiley_2008,Sun_2002}, seed-mediated growth \cite{Pietrobon_2009,Sun_2002,Zhang_2010}, and light-mediated synthesis \cite{Pietrobon_2008,Jin_2003,Zhou_2008}.  The polyol synthesis is a common and successful synthesis route, in which nanocrystals with well-defined shapes and sizes can be obtained.  % Talk about different self-seeding  polyol synthesis In a self-seeding polyol synthesis, AgNO_3 and PVP are dissolved in ethylene glycol.  The mixture is typically refluxed at 150$^{\circ}$C.  The morphology and dimensions of the product were shown to depend on different aspects of  the reaction conditions \cite{Sun_2002}. Irregular nanoparticles were obtained when the temperature is below 120$^{\circ}$C or above 190$^{\circ}$C.  At the initial concentration of AgNO_3 higher than 0.1 M nanocubes were formed, otherwise nanowires were the major product.  Multiply twinned particles were obtained instead of single crystal cubes when the molar ratio of PVP to AgNO_3 was increased from 1.5 to 3.  It was shown that nanowires with higher aspect ratios can be obtained by reducing the molar ratio of PVP to AgNO_3 \cite{Sun_2002}.  The optimal molar ratio was found to be 1.5:1, noting that the concentration of AgNO_3 is below 0.1 M.  Although at a molar ratio of less than 1:1, the yield and monodispersity of nanowires were too unacceptably low.  Higher degree of polymerization of PVP can also promote the formation of nanowires with high aspect ratios \cite{Sun_2002}.  Irregular nanoparticles are formed when 1-ethyl-2-pyrrolidone, the monomer of PVP, is used in place of PVP.  This observation demonstrates the importance of the steric effect of PVP that prevents aggregation between nanoparticles.  % Introducing foreign species  Slight modifications of the polyol synthesis protocol, such as introducing foreign species, enable different nanocrystal shapes to be produced. 
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