Optical Properties of Solid Pentacene

We have prepared pentacene films on Si, Al₂O₃, and glass substrates by thermal evaporation and have investigated their optical properties at room temperature over a wide range of frequencies from infrared to ultraviolet (5 meV to 6.5 eV). A series of optical phonon modes and electronic transitions have been observed. The internal vibrational modes in the infrared region match well their molecular counterparts but the electronic transitions show substantial changes from those of a free pentacene molecule. The HOMO–LUMO gap energy of the pentacene films deposited on Al₂O₃ and glass substrates is 1.85 eV.     

Theoretical investigations of oligo(phenylene ethylene) molecular wire: Effects from substituents and external electric field

Theoretical investigation of the substituent effect on the molecular wire, oligo(phenylene ethynylene) (OPE), has been performed with density functional calculations by considering the influence from the external electric field (EF). Compared to the electron-donating –NH₂ group, the electron-withdrawing –NO₂ group plays more important roles, such as increase of molecular dipole moment, decrease of LUMO–HOMO gap, and localization of LUMO. Both the geometric and electronic structures of the model molecular wires are sensitive to the external EF. In particularly, the –NO₂ substituted OPE yields obvious asymmetrical evolutions of both the frontier molecular orbital energies and their spatial distribution, which could be used to intuitively interpret the asymmetrical current–voltage behaviors of molecules.     

Self-organized dye particles: Role of dewetting process in molecular ordering

Self-organized rhodamine 6G particles prepared by wetting/dewetting process of an ethanol solution on a hydrophilic glass surface exhibited fluorescence without quenching, showing a sharp linewidth of 2 nm with a large redshift, which indicates an existence of dye aggregates, similar to J aggregates, inside the particle. Polarized evanescent field excitation showed that the dye molecule's transition moment along the π-conjugation was oriented unidirectionally within particles and parallel to the substrate surface. This deduced dye orientation showed correlation between adjacent, however separated, particles, and pointed roughly 45° off the dewetting direction. In contrast, another π-conjugated NK1420 dye, J aggregates of which grow easily from an oversaturated solution, yielded particles with constituent dyes oriented along the dewetting direction preferably, still indicating the effect of self-organization, however based on a different mechanism.     

A molecular dynamics study on the impact of defects and functionalization on the Young modulus of boron–nitride nanotubes

In this article, we examine the Young modulus of (6,m) boron–nitride nanotubes with vacancy and functionalization defects. We employ molecular dynamics simulations using the Parrinello–Rahman approach. To this end, all systems are modeled with a reactive many-body bond order Tersoff potential with parameters due to Matsunaga et al. [K. Matsunaga, Y. Iwamoto, Molecular dynamics study of atomic structure and diffusion behavior in amorphous silicon nitride containing boron, Journal of American Ceramics Society 84(10) (2001) 2213–2219], which is able to accurately describe covalent bonding. We apply external stress to periodically repeated tubes in vacuum and derive stress–strain curves for various tensile loads at standard temperature and pressure. In addition to the Young modulus, we study visualized stress-per-atom snapshots of the simulation runs. Our results show that the decrease in Young modulus with increasing defect concentration is independent of the chirality of the tube for vacancy defects. Also, we observe that functionalization does not weaken the tube. There is even indication of a relative strengthening for armchair types.     

Numerical algorithms for prediction of mechanical properties of single-walled carbon nanotubes based on molecular mechanics model

The objective of this paper is to develop the numerical algorithms for the prediction of mechanical properties of single-walled carbon nanotubes (SWCNTs). By using the energy method, the analytical expressions are obtained and the five independent variables algorithm is developed for the prediction of the elastic properties of SWCNTs via a molecular mechanics model in which the geometrical relationship of carbon nanotube is introduced. It can be found that due to the introduction of the geometrical approximate conditions some errors may exist in the calculation of mechanical properties of SWCNTs in terms of the five independent variables algorithm. Therefore, two improved algorithms, i.e., eigenvalues modified method (EMM) and eigenvalues and eigenvectors modified method (EEMM) are proposed to analyze the possible errors in the numerical results. It is found that the results obtained by the three kinds of algorithms are almost consistent with one another, but EMM and EEMM are preferred to be used because they have properties similar to those of the finite element method, where the consistent equation works just as the constraint condition to void the singularity of the element stiffness matrix. The computational results also reveal that both the surface Young’s modulus and Poisson’s ratio depend on the diameter of carbon nanotubes, and finally converge to the values of the graphite sheet with an increase in the tube diameter in the inverse trends. For SWCNTs with approximately the same diameters, the surface Young’s modulus is in direct and Poisson’s ratio is in inverse proportion to chiral angles, respectively.