The shape transformation of platinum (Pt) nanosheets with a uniform thickness of as thin as 3.5 +/- 1 nm supported on graphite was investigated by in situ atomic force microscopy (AFM). The AFM observations revealed the shape transformation and the coalescence in preferred directions for the Pt nanosheets at room temperature (25 degrees C), which is much lower than the melting point of bulk metallic platinum (1769 degrees C). The behavior may be attributed to the high surface energy for the edge parts of Pt nanosheets with the small curvature of the nanometer scale. (c) 2006 Elsevier B.V. All rights reserved.