| dc.description.abstract | Thiophene and its derivatives have been important constituents in the study of
organic semiconductors and solar cell research. Their spatial structures and adsorption
configuration can affect the charge transport between molecules and gold electrode,
which is an important process in the operation of light-emitting diodes, organic thinfilm transistors (OTFTs), etc. Polythiophenes have been prepared by anodization in
organic or aqueous phases containing thiophenes or oligothiophenes. Cyclic
voltammetry (CV) and scanning tunneling microscopy (STM) were used to study the
adsorption and polymerization of thiophene and its derivatives on gold (111)
electrode.
First, the adsorption of terthiophene (TT) on the Au (111) electrode was
investigated as a function of potential and anion coadsorption. In H2SO4, we obtained
molecular resolution STM images to show that the pre-deposited TT layer became
more compact with more positive potential. Judged from the STM appearance of TT
admolecule and intermolecular spacing, TT molecule lay horizontally. In contrast, in
perchloric acid, TT admolecules were poorly ordered. This contrast also manifested in
the CV results, showing multiple sharp peaks in H2SO4, but featureless in HClO4.
STM also showed the oxidative polymerization of TT monomers on Au(111)
electrodes, producing ribbon-like oligothiophenes with molecular sizes varying
between 1 and 20 nm in pH 1 and 5 sulfate media. The oligothiophenes could be
linear or curved, which suggests the possible trans and cis configurations of
thiophenes in the molecular chains. High pH value appeared to benefit the formation
of oligothiophenes.
In 0.1 M H2SO4 and HClO4, DNT-Ph molecule was adsorbed in highly ordered
arrays on the reconstructed Au(111). Potential control affected greatly the adsorption
structure of DNT-Ph, as ordered DNT-Ph structures were only observed between −0.1
and 0.4 V. DNT-Ph was desorbed at -0.15 V, whereas the ordered DNT-Ph structures
were destroyed, as Au(111) surface became (1 1) at positive potential. DNT-Ph was
mainly adsorbed with its molecular plane parallel to the surface of the gold electrode.
High resolution STM imaging revealed details in the molecular conformations of
DNT-Ph, as two geometric isomers resulting from rotating the single bond connecting
the thiophene were distinguished by the STM.
In 0.1 M perchloric acid and sulfuric acid, the adsorption of Ph(DTP)2 molecule on
Au(111) electrode was also examined by using in situ STM. Ordered molecular
adlayers were found on the reconstructed and (1 1) phases of the Au(111) electrode
in a wide potential region. The spatial structure of Ph(DTP)2 was formed faster and
more compact on the reconstructed Au(111) than (1 1) phase in sulfuric acid at
negative potential. Local bilayer structure of was observed on the reconstructed lines. | en_US |