Pentacene Molecules on the Si(111)-(7x7) Surface

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Pentacene Molecules on the Si(111)-(7x7)
Surface Kevin Kimberlin Kelly Roos, Bradley
University DMR-0320908 and DMR-0215899
One of the most technologically promising organic
semiconductors is pentacene (C22H14), an aromatic
molecule consisting of five benzene rings.
Semiconductor technology requires high quality
thin films, and it is therefore important to
study and understand how organic semiconductor
thin films progress from the initial arrival of
molecules on the substrate to the completion of a
film that actually covers the surface. We have
obtained the first Scanning Tunneling Microscopy
(STM) images of pentacene molecules on the
Si(111)-(7x7)surface. Fig. 1 shows a series of
STM images with increasing pentacene coverage
from left to right. In the left image pairs of
distinct bright spots are visible as well as
apparently missing Si adatoms. We interpret the
pairs as intact pentacene molecules and attribute
the apparently missing adatoms to the presence of
fragments of some pentacene molecules that have
dissociated upon adsorption. In the image on the
right the first layer of pentacene has been
completed and layer number two has commenced in
the form of an island that has nucleated near a
surface defect. Because of the extremely high
degree of mobility of the pentacene molecules on
top of the first pentacene layer, second layer
nucleation predominantly takes place at
inhomogeneous defect sites. Fig. 2 shows the
adsorption geometry of an isolated pentacene
molecule.
Figure 2 The adsorption geometry of an isolated
pentacene molecule on Si(111)-(7x7). An sp3
hybridization of one of the outer carbon atoms
of the middle ring allows the molecule to bond to
the surface. The molecule lies flat on the
surface, can rotate about this bond axis and
align within the unit cell. The inset shows a
scale drawing of the five benzene rings of the
pentacene molecule in the same orientation with
respect to the underlying Si(111)-(7x7) unit cell
as in the STM image.
Figure 1 A series of STM images of pentacene
molecules on the Si(111)-(7x7) surface. The
pentacene coverage increases from left to right.
The field of view is 40 nm x 40 nm in each. In
the right-most image a triangular-shaped
second-layer island, that has nucleated near a
surface defect, can be seen.