Dynamics of Poly(3-hexylthiophene) Monolayers at Solution/Graphite Interfaces.

We have studied the dynamics of poly(3-hexylthiophene) (P3HT) monolayers at the interfaces between highly oriented pyrolytic graphite (HOPG) and P3HT solutions in 1,2,4-trichlorobenzene, using scanning tunneling microscopy (STM). Real-time STM observation at room temperature reveals that P3HT molecules adsorbed on graphite are substantially mobile even in densely packed conditions, causing significant fluctuations of the self-organized monolayer of P3HT. We find that in the monolayers, the orientational order is limited to a short range comparable to the polymer chain length. We show that our observations can be understood based on a 2D semiflexible lattice polymer model by performing kinetic Monte Carlo simulations.

Scanning probe analysis of twisted graphene grown on a graphene/silicon carbide template.

Overlayer growth of graphene on an epitaxial graphene/silicon carbide (SiC) as a solid template by ethanol chemical vapor deposition is performed over a wide growth temperature range from 900 °C to 1450 °C. Structural analysis using atomic force and scanning tunneling microscopies reveal that graphene islands grown at 1300 °C form hexagonal twisted bilayer graphene as a single crystal. When the growth temperature exceeds 1400 °C, the grown graphene islands show a circular shape. Moreover, moiré patterns with different periods are observed in a single graphene island. This means that the graphene islands grown at high temperature are composed of several graphene domains with different twist angles. From these results, we conclude that graphene overlayer growth on the epitaxial graphene/SiC solid at 1300 °C effectively synthesizes the twisted few-layer graphene with a high crystallinity.

Impact of the Air Atmosphere on Photoinduced Chain Polymerization in Self-Assembled Monolayers of Diacetylene on Graphite.

We have studied the effect of the reaction environment on the photoinduced chain polymerization in self-assembled monolayers of a diacetylene compound 10,12-pentacosadiyn-1-ol on graphite, using scanning tunneling microscopy. Comparing the polymerization behaviors in air and in vacuum, we show that the polymer generation efficiency is considerably enhanced under vacuum because of suppressed collisional quenching of the photoexcited radicals by oxygen molecules. We also find that the polymer chain length tends to increase in vacuum as a result of the inhibition of deactivation of reactive species for the polymer chain growth due to oxidation.

Mechanism of Chain Polymerization in Self-Assembled Monolayers of Diacetylene on the Graphite Surface.

We have studied the mechanism of photoinduced chain polymerization in the self-assembled monolayer of a diacetylene compound, 10,12-pentacosadiyn-1-ol, on the graphite surface. We statistically analyze the polymerization degree of the polydiacetylene chains formed at different temperatures using scanning tunneling microscopy. The distributions of the polymerization degree agree well with the prediction from a simple probabilistic model, allowing for addition reaction and deactivation at both the ends of the chain as stochastic events. The estimated activation energies of the addition reaction and deactivation are noticeably different from those of the conventional solid-state polymerization in the bulk crystals of diacetylene.

Two-dimensional solid-state topochemical reactions of 10,12-pentacosadiyn-1-ol adsorbed on graphite.

The photoinduced topochemical reaction of the diacetylene (DA) compound 10,12-pentacosadiyn-1-ol in the two-dimensional (2D) crystal phase adsorbed on graphite was examined by scanning tunneling microscopy (STM). The reaction efficiency and the structure of the generated polydiacetylene depend on its polymorphic forms, i.e., the "herringbone" or the "parallel" monomer arrangements. The reaction efficiency of the herringbone arrangement is lower than that of the parallel arrangement because the distance R between the probable reactant acetylenic carbon atoms of the herringbone arrangement is longer than that of the parallel arrangement. However, the fact that polydiacetylenes form from the herringbone arrangement (R = 0.58 nm) is contrary to the geometric criteria for the polymerization of three-dimensional (3D) crystals (the reaction was only observed if R < 0.4 nm). The polymerization criterion for the 2D phase differs from that of the 3D phase. In addition, the STM cross-section profiles of the polydiacetylenes reveal that the "lifted-up" and "in-plane" conformations form from the parallel and herringbone arrangements, respectively.

Synthesis, electronic, and morphological properties of tetrahedral oligothiophenes with n-hexyl terminal groups.

A series of tetrahedral oligothiophenes bearing n-hexyl groups at the α-positions of the terminal thiophene rings, (n-C(6)H(13)(C(4)H(2)S)(n))(4)C (Hex-TnTM; n=1-4), has been synthesized by Kosugi-Migita-Stille coupling as a key reaction. Thanks to the improved solubility afforded by the terminal n-hexyl groups, the largest homologue (n=4) was successfully obtained. Whereas the smaller derivatives (n=1, 2) were obtained as liquid substances, the larger derivatives (n=3, 4) were obtained as solids. Hex-T3 TM partially adopts syn conformations between the adjacent thiophene rings in the crystal, probably owing to the packing force. Hex-T3 TM not only appeared in the crystalline state but also the amorphous state, which was stable to up to 80 °C. Regardless of the terminal groups, the derivatives of n=2 exhibited a broad fluorescence with large Stokes shifts compared to the corresponding linear analogues, thereby suggesting the presence of intramolecular interactions between the bithiophene moieties. Interactions between terthiophene branches was also suggested in the radical cations of Hex-T3 TM by cyclic voltammetry measurements.

Atomic force microscopy and theoretical investigation of the lifted-up conformation of polydiacetylene on a graphite substrate.

The structure of a single polydiacetylene compound on a graphite substrate was investigated using atomic force microscopy (AFM). The linear conjugated polydiacetylenes were obtained through chain polymerization of a monomolecular layer of diacetylene compound on a graphite substrate under ultraviolet light irradiation. AFM observations revealed that the polydiacetylenes were imaged higher than the unpolymerized monomer rows. This result supports the 'lifted-up' conformation model, in which the polydiacetylene backbone is geometrically raised. To investigate why the polymer backbone is lifted, we also carried out first-principles density-functional calculations in the local density approximation. These calculations suggested that the steric hindrance between the alkyl side-chains of the monomers and the oligomer caused the lifted-up conformation.

Chain polymerization of diacetylene compound multilayer films on the topmost surface initiated by a scanning tunneling microscope tip.

Chain polymerizations of diacetylene compound multilayer films on graphite substrates were examined with a scanning tunneling microscope (STM) at the liquid/solid interface of the phenyloctane solution. The first layer grew very quickly into many small domains. This was followed by the slow formation of the piled up layers into much larger domains. Chain polymerization on the topmost surface layer could be initiated by applying a pulsed voltage between the STM tip and the substrate, usually producing a long polymer of submicrometer length. In contrast, polymerizations on the underlying layer were never observed. This can be explained by a conformation model in which the polymer backbone is lifted up.