The behavior of Co atoms on Si (111)-7 x 7 surfaces at low temperatures was studied by using a variable-temperature scanning tunneling microscopy (VT-STM). Co atoms deposited on Si (111)-7 x 7 surfaces are randomly adsorbed at 100 K. Co atoms start to react with adatoms of Si (111)-7 x 7 surfaces at temperatures between 126 K and 130 K. The reaction transfers the bright dots of Co atoms to dark dots under the STM observation of negative bias. Analysis of the reaction occurrence sites and comparing with the results of room temperature deposition shows that the Co atoms tend to diffuse and react with the adatoms of Si (11 l)-7 x 7 surfaces at the center sites of unfaulted half unit cell (UHUC) at higher temperatures. (c) 2007 Elsevier B.V. All rights reserved.