LEED and STM investigations of organic–organic heterostructures grown by molecular beam epitaxy

For the first time, heterostructures of 3,4,9,10-perylenetetracarboxylic-3,4,9,10-dianhydride (PTCDA) on hexa-peri-hexabenzocoronene (HBC) on reconstructed Au(1 1 1) have been prepared with molecularly perfect interfaces by organic molecular beam epitaxy. First, we investigate the growth of HBC on reconstructed Au(1 1 1) surfaces. Large domains which are defect free on the molecular scale are observed by scanning tunneling microscopy (STM). While a surface reconstruction of the substrate can still be seen through the molecular film, the periodicity has changed as compared to the bare substrate. The low energy electron diffraction (LEED) images show a hexagonal pattern for the HBC films. From our simulations, it is concluded that HBC grows on such modified reconstructed surfaces in a point-on-line coincident superstructure with a hexagonal unit cell. Second, a monolayer of PTCDA is deposited on HBC and structurally characterized. Despite the inhomogeneous nature of the sample we are still able to obtain molecularly resolved STM images for the PTCDA. The LEED and STM measurements reveal the well-known herringbone alignment of the PTCDA molecules with two molecules in a rectangular unit cell, similar to that of the (1 0 2) bulk plane of PTCDA. The LEED measurements and the pronounced two-dimensional moiré pattern in the STM images are used to determine precisely the distinctive relative orientation of the PTCDA unit cell with respect to the underlying HBC lattice.