A scanning tunnelling microscopy investigation into the initial stages of copper phthalocyanine growth on passivated silicon surfaces

Different growth modes were identified using scanning tunnelling microscopy (STM) in the initial stages of ultra-thin films growth of copper phthalocyanine (CuPc) on Si(0 0 1) surfaces terminated with ammonia (Si(0 0 1):NH3) and hydrogen (Si(0 0 1):H), depending on the surface chemistry. At the onset of the growth, defect sites are saturated with strongly-pinned flat-lying CuPc molecules. Subsequently, molecules arrange in a standing manner, with two macroscopic island directions along the [1 1 0] and [11¯0] directions. The Si surface termination influences the shape and coverage of the molecular layer before the onset of multilayer formation: on NH3 terminated surfaces, large elongated islands are formed, and nucleation of the second layer begins after the first layer is only 50% complete. This is in contrast to the H passivated surfaces, where a high density of smaller islands form and a higher coverage is reached before second layer nucleation, which we have attributed to a difference in molecular mobility on the substrate. STM shows that the islands contain columns of molecules, which on both surfaces are aligned by 64° with respect to the macroscopic island direction. From these observations, we have derived a model in which the molecular orientation observed in these studies is very similar to the bulk α-phase CuPc. This rationalises the reduced crystallite size characteristic of thicker phthalocyanine films.