Stable alignment of 4,4″-diamino-p-terphenyl chemically adsorbed on a Si(001)-(2 × 1) surface observed by scanning tunneling microscopy

Low coverages of 4,4″-diamino-p-terphenyl (DAT) molecules deposited on a Si(001)-(2 × 1) surface in ultrahigh vacuum at room temperature were observed by scanning tunneling microscopy (STM). The linear framework of DAT, consisting of a central benzene ring, two phenyl rings (terphenyl) and two amino groups at both ends, mostly lay down laterally on the surface. The majority of DATs were tilted at about 17° with respect to the direction of a Si dimer row on the surface, though a variety of DAT configurations with different angles was found by STM. The histograms of the tilted angles showed that the most frequent angle was 17°. The apparent height of DAT tilted at 17° looked hollow at the center and lower than that of other configurations of DAT in STM images. This indicates that the DAT tends to take a double arched shape at the tilted angle of 17° in a stable conformation with butterfly-like bonding through the central benzene ring to a Si dimer as well as through the two amino groups bonded to respective Si atoms on the dimer row.