Growth of ${\mathrm{C}}_{60}$ thin films on GeS(001) studied by scanning force microscopy

The epitaxial growth of ${\mathrm{C}}_{60}$ thin films sublimed onto GeS(001) substrates under ultrahigh-vacuum conditions was investigated in the range of 0.7–11 monolayers coverage by scanning force microscopy in air. The growth process follows basically a layer-by-layer mechanism if evaporated in the narrow temperature range from 180 to 200 °C, but was found to be very sensitive to even small changes in temperature. At substrate temperatures above 200 °C, the second and higher layers are not stable. For temperatures close to 200 °C, our studies indicate pure layer-by-layer-type growth. Somewhat lower substrate temperatures lead to the formation of two different types of triangular-shaped islands which originate from a different stacking of the molecules, beginning with the second layer. Triangular islands belonging to the same type tend to coalesce, whereas islands of different types are separated by grooves. At substrate temperatures close to 180 °C during evaporation, dendritic islands are formed due to the limited mobility of the ${\mathrm{C}}_{60}$ molecules at this temperature. Furthermore, the growth mode changes to a nonideal layer-by-layer growth where the second and even higher layers start to grow before the first layer has been completed. This behavior can well be explained by a two-dimensional diffusion-limited growth mechanism. Finally, substrate temperatures significantly lower than 180 °C result in films without long-range order.