Origin of peculiar STM molecular contrast in C60/Ag (100)

In the c(6 x 4) C-60 layer on Ag(I 00), a strong "bright" and "dim" molecular contrast has recently been observed in scanning tunneling microscopy. While the nature of such contrast is still under debate, substrate reconstruction is considered unlikely due to the rather complicated local ordering in the C-60 layer. Here we use variable temperature STM to study the temperature evolution Of C-60 Contrast. We clearly show that the bright C-60 turns into the dim C-60, starting at approximately similar to280 K. This phenomenon, along with the simulated contrast evolution of multi-layer C60 film and the molecular resolved non-contact AFM (NCAFM) measurements, are consistent with a topographic interpretation of the bright-dim contrast, in contrary to what is previously believed. (C) 2002 Elsevier Science B.V. All rights reserved.