The formation of CoSi2 nanowires (NWs) via the annealing of Si NWs conformally coated with atomic layer deposited (ALD) Co layers was investigated. Co ALD was carried out using Co(iPr-AMD)(2) and NH3 as a precursor and a reactant, respectively. Rapid thermal annealing (RTA) of Co/Si core-shell NWs produced Co oxide instead of CoSi2 due to oxygen contamination during annealing. To prevent oxygen contamination, a highly conformal ALD Ru layer was used as a capping layer. X-ray diffraction showed the formation of CoSi2 when the Co/Si core-shell NWs were annealed at over 800 degrees C. To investigate the deposition and silicidation process on a nanometer scale, high resolution C-s-corrected scanning transmission electron microscopy was utilized with electron energy loss spectroscopy and energy dispersion spectroscopy. In contrast to previous reports on silicidation of NWs, the dominant diffusion species was found to be Si instead of Co based on a nucleation-controlled reaction. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.078205jes]