Pressed Bi nanocrystals synthesized by a simple colloidal method exhibited very high electrical conductivity of 104-105 S m -1 and extremely low thermal conductivity of 0.35 W m-1 K-1. The synthetic method can be used for the inexpensive preparation of highly efficient thermoelectric materials by engineering the size of nanocrystals (see picture; scale bar 50 nm; ZT=figure of merit).