A new Enhanced High Pressure Sintering (EHPS) method for making three-dimensional fully dense nanostructures and nano-heterostructures formed from nanoparticle powders, and three-dimensional fully dense nanostructures and nano-heterostructures formed using that method. A nanoparticle powder is placed into a reaction chamber and is treated at an elevated temperature under a gas flow to produce a cleaned powder. The cleaned powder is formed into a low density green compact which is then sintered at a temperature below conventional sintering temperatures to produce a fully dense bulk material having a retained nanostructure or nano-heterostructure corresponding to the nanostructure of the constituent nanoparticles. All steps are performed without exposing the nanoparticle powder to the ambient.