Unlike typical Au used as a catalyst for the synthesis of silicon nanowires via the vapour-liquid-solid mechanism, Cu has been found to induce a synthesis process governed by the vapour-solid-solid mechanism. Moreover, the temperature window for obtaining high-quality wires with Cu has been found to be relatively smaller than that shown by the Au: from 600 to 650 degrees C. However, high-resolution transmission electron microscopy analysis reveals significant new properties of the nanowires obtained. They have the peculiarity of successively switching the silicon structure from diamond to the wurtzite phase along the growth direction. This change of the crystalline structure implies that it has an important impact on the transport properties and characteristics of electronic devices. The results will be important for the future integration and application of silicon, where electrical and thermal transport properties play a significant role.