Page 93 - Organic Electronics in Sensors and Biotechnology

70 Cha pte r T w o

π-systems, independently from the structural packing of the D3ANT
molecules in the solid active layer. However, as a bulk technique,
GIXRD is unable to provide structural information on molecular
arrangement of the first monolayer at ultrathin organic semiconductor/
SiO interface where field effect takes place. XRD studies only allow
2
the interpretation of oligomer chain packing up to the resolution of
several monolayers.
To shed more light on the two-dimensional self-organization of
D3ANT, we investigated monolayers on highly organized pyrolitic
graphite (HOPG) and Au(111) by scanning tunneling microscopy
(STM) in a liquid environment of n-tetradecane. 135–137 The STM study
reveals 2D organization of the molecular architectures induced by the
substrate-molecule and molecule-molecule interactions on conduct-
ing and atomically flat substrates.
Two-dimensional layer consisting of highly ordered monodo-
mains that extend over distances of 100 to 400 nm on highly oriented
pyrolytic graphite (HOPG) was formed, as seen in Fig. 2.6b. Each
domain consists of a closed-packed arrangement of linear and parallel
rows of molecules. The distance between adjacent rows is 1.3 nm (in
dark contrast) corresponding to the decyl chains in a slightly tilted posi-
tion. The bright contrast areas correspond to the main π-conjugated
backbone of D3ANT with a length of ~2.0 nm, in good agreement
with the calculated core length (1.9 nm) and the profile of frontier wave
functions, which is mainly defined by three anthracene subunits. The
2D unit cell parameters were measured to be a = 3.1 nm, b = 1.2 nm,
α= 94°. Moreover, from the high-resolution STM figure, the three
anthracene subunits of each individual molecule are clearly visible
on HOPG surface. Such evidence proves a nearly planar adsorption
of the entire molecule on HOPG with no twist angle and full conjuga-
tion between the anthracene subunits.
D3ANT molecules adsorbed on reconstructed Au(111) (Fig. 2.6c)
also form self-organized domains with linear and parallel row 2D
structure very similar to that on HOPG. However, noticeable differ-
ences in the organizing behavior of the D3ANT can be observed by
comparing both images. The self-organized domains on Au(111)
surface do not extend over distances as large as on HOPG, and the
distance between adjacent D3ANT rows on Au(111) (~0.3 nm) is
markedly smaller than that on HOPG. On Au surface, D3ANT cannot
accommodate decyl chains which are probably folded below or above
the anthracene subunits, and this results in a more densely packed 2D
structure on Au(111) as compared to HOPG. Again, the individual
anthracene subunits of each molecule are clearly visible.
As seen, for spin-coated and annealed thin films of D3ANT, the
GIXRD analysis indicated the lack of bulk ordered, but at the same
time the STM revealed the strong tendency to form high-order 2D
self-assembled monolayer on both HOPG and Au(111) surface.