Metallic alpha-FeSi2 nanowires (NWs) are epitaxially grown on Si(110) at 650 degrees C Their evolution as a function of annealing temperature has been studied in situ by scanning tunneling microscopy. The NWs are stable up to 750 degrees C, which is much lower than that of the bulk alpha-FeSi2. With further increasing the annealing temperature, some NWs begin to shrink in length and transform into wider and higher semiconducting beta-FeSi2 nanorods or three-dimensional (3D) islands at 925 degrees C. The phase transformation is driven by the reduction in surface energy. On the other hand, some beta-FeSi2 NWs begin to dissolve and become thinner until disappearing. The growth of the beta-FeSi2 nanorods or 3D nanocrystals follows the Ostwald ripening mechanism, i.e., the large islands grow in size at the expense of the small ones. X-ray photoelectron spectroscopy study shows that the Fe 2p peaks of beta-FeSi2 nanocrystals exhibit a negative shift of 0.2 eV with respect to the alpha-FeSi2 NWs. (C) 2016 Elsevier B.V. All rights reserved.