Scanning tunneling microscopy of the formation, transformation, and property of oligothiophene self-organizations on graphite and gold surfaces

Abstract

Two alkyl-substituted dual oligothiophenes, quarterthiophene (4T)-trimethylene (tm)-octithiophene (8T) and 4T-tm-4T, were used to fabricate molecular structures on highly oriented pyrolytic graphite and Au(111) surfaces. The resulted structures were investigated by scanning tunneling microscopy. The 4T-tm-8T and 4T-tm-4T molecules self-organize into long-range ordered structures with linear and/or quasi-hexagonal patterns on highly oriented pyrolytic graphite at ambient temperature. Thermal annealing induced a phase transformation from quasi-hexagonal to linear in 4T-tm-8T adlayer. The molecules adsorbed on Au(111) surface in randomly folded and linear conformation. Based on scanning tunneling microscopy results, the structural models for different self-organizations were proposed. Scanning tunneling spectroscopy measurement showed the electronic property of individual molecules in the patterns. These results are significant in understanding the chemistry of molecular structure, including its formation, transformation, and electronic properties. They also help to fabricate oligothiophene assemblies with desired structures for future molecular devices.phase transition ͉ self-assembly ͉ molecular structure T hiophene derivatives, including oligothiophene (1-5), cyclothiophene (6-8), and polythiophene (9), are currently receiving considerable attention because of their well defined chemical structures, improved solubilities, and various electronic properties (10, 11). They are promising materials in electronic and optical devices such as Schottky diodes (12), organic lightemitting diodes (OLEDs) (13, 14), field-effect transistors (15), and organic thin film transistors (11,16). For example, the transistors fabricated with thiophene derivatives shows high charge-carrier mobility, high on/off modulation ratio, and long life (15). By modifying molecular structures with functional groups, alkyl chains and thiophene rings, the emitting color of an OLED can be regulated from blue to red (17).The property of a molecular assembly is governed by not only the property of individual molecules but also the spatial arrangement of the molecules in the assembly (10,18,19). Therefore, understanding the molecular arrangement and single molecule's property on substrate surface is very important in fabricating thiophene-based devices. Previous studies have demonstrated that most thiophene molecules can self-organize and form adlayer structures when they adsorb on certain substrates. With fine balance between intermolecular interactions and molecule/ substrate interactions, the molecular structures can be varied, resulting in desirable functionalities (20).The direct observation and investigation of molecular nanostructures at solid surfaces is a constant challenge in molecular science. Scanning probe microscopy, in particular, scanning tunneling microscopy (STM), is the most potent tool for studying the molecules adsorbed onto a solid substrate (20)(21)(22)(23)(24). With recent developments in this field, it is now possible to obtain...

Structural selection of graphene supramolecular assembly oriented by molecular conformation and alkyl chain

“…STS is used to study the electronic properties of FTBC on HOPG at ambient condition. Using the resonance between Fermi energy of the substrate and certain molecular orbitals of adsorbate when applying an appropriate bias on substrate, the value of highest occupied molecular orbital (HOMO), LUMO, and energy gap can be obtained from the I-V and dI/dV-V curve (31,32). Fig.…”

Molecular adlayer and photo-induced structural transformation of a diarylethene derivative on Au(111) investigated with scanning tunneling microscopy

“…The calculated HOMO and LUMO orbitals are also in agreement with the round shape feature as shown in the STM images. It is well known that different molecule/substrate interactions can modulate the self-assembled structures [49][50][51][52]. Excessive molecule/substrate interaction can drive the molecules to form random structures on surface [50][51][52].…”

Adsorption and thermal decomposition of 2-octylthieno[3,4-b]thiophene on Au(1 1 1)

“…Pentacene, however, suffers from poor stability, insolubility, and chemical liability. Meanwhile, thiophene based materials have shown strong intermolecular interactions originating from the high polarizability of sulfur atoms in the thiophene rings, but they easily twist from planarity, thus disrupting conjugation and potentially affecting the band gap in the solid state [11][12][13]. To circumvent these disadvantages, highly stable materials based on fused thiophenes have been proposed as alternative organic semiconductors [14][15][16][17][18].…”

“…Pentacene and thiophene based materials have been proposed as promising for utilization in high performance electronic devices [7][8][9][10][11]. The extended p system of pentacene enhances the intermolecular overlap of p-p orbitals in the solid state and thus allows for high carrier mobility.…”