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

Low coverages of 4,4 ''-diamino-p-terphenyl (DAT) molecules deposited on a Si(001)-(2 x 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 degrees 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 degrees. The apparent height of DAT tilted at 17 degrees looked hollow at the center and lower than that of other configurations of DAT in STM images. This indicates that the DAT tends to lake a double arched shape at the tilted angle of 17 degrees 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. (C) 2014 Elsevier B.V. All sights reserved.