Methods for forming samples of noble metal bipyramid nanocrystals having very low size and shape polydispersities from samples of mixed noble metal nanocrystals are provided. The samples include those comprising high purity, substantially monodisperse, plasmonic gold bipyramid nanocrystals. Also provided are methods of growing secondary twinned metal nanocrystals using the noble metal bipyramid nanocrystals as seed particles. Like the seed bipyramid nanocrystals from which they are grown, the secondary nanocrystals are twinned nanocrystals and may also be characterized by very low size and shape polydispersities. Secondary twinned nanocrystals grown by these methods include enlarged metal bipyramid nanocrystals and nanocrystals with anisotropic “dumbbell” shapes having a variety of tip geometries. Methods for using noble metal bipyramid nanocrystals as plasmonic heaters to heat reaction solutions via plasmonic-photothermal radiation-to-heat conversion are also provided.