Danqin Feng

Comparison of the electronic structure of two polymers with strong dipole ordering

Two different polymers, with large local electric dipoles, are compared: copolymers of polyvinylidene fluoride with trifluoroethylene [P(VDF-TrFE, 70%:30%)] and polymethylvinylidenecyanide (PMVC). While the different local point group symmetries play a key role, both crystalline polymers exhibit intra-molecular band structure, though the Brillouin zone critical points differ.

Photofragmentation of closo-Carboranes Part 1: Energetics of Decomposition

The ionic fragmentation following B 1s and C 1s excitation of three isomeric carborane cage compounds [closo-dicarbadodecaboranes: orthocarborane (1,2-C2B10H12), metacarborane (1,7-C2B10H12), and paracarborane (1,12-C2B10H12)] is compared with the energetics of decomposition. The fragmentation yields for all three molecules are quite similar. Thermodynamic cycles are constructed for neutral and ionic species in an attempt to systemically characterize single-ion closo-carborane creation and fragmentation processes. Lower energy decomposition processes are favored. Among the ionic species, the photon-induced decomposition is dominated by BH+ and BH2(+) fragment loss. Changes in ion yield associated with core to bound excitations are observed.

Engineering of the electronic structure in an aromatic dithiol monomolecular organic insulator

Cross-linking, leading to an insulating quasi-polymeric material, occurs with the initial irradiation-induced modification in films of the monomolecular insulator (1,1′;4′,1″-terphenyl)-4,4″-dimethanethiol, although this is accompanied by a decrease in the gap between the highest occupied molecular orbital and lowest unoccupied molecular orbital.

The anomalous "stiffness" of biphenydimethyldithiol

Although organic adsorbates and thin films are generally regarded as “soft” materials, the effective Debye temperature, indicative of the dynamic motion of lattice normal to the surface, can be very high. For biphenyldimethyldithiol, the effective Debye temperature, determined from core level photoemission from the all carbon arene rings, is comparable to that of graphite. We associate this rigidity to the stiffness of the benzene rings, and the ordering in the molecular thin film. Measurements on the sulfur of poly(hexylthiophene) show that sulfur in an arene ring is less dominated by soft modes than the pendant sulfur of biphenyldimethyldithiol on the time scale of photoemission.

Intermolecular Band Dispersion for Self-Assembly Monolayers of A Polyphenyl Thiol

We find evidence of intermolecular interactions for a self-assembled monolayer (SAM) formed from a large molecular adsorbate, [1,1′;4′,1″-terphenyl]-4,4″-dimethanethiol, from the dispersion of the molecular orbitals with changing the wave vector k. Both unoccupied and occupied molecular orbitals hybridize to electronic bands, with significant band dispersion that appears to be dependent on structure. The results demonstrate that Blochs theorem can apply to the wave vector dependence of the electronic band structure for monomolecular films formed from a relatively large polyphenyl-based species.