Electromechanical Characteristics by a Vertical Flip of C70 Fullerene Prolate Spheroid in a Single-Electron Transistor: Hybrid Density Functional Methods.

In this study, the B3LYP hybrid density functional theory was used to investigate the electromechanical characteristics of C70 fullerene with and without point charges to model the effect of the surface of the gate electrode in a C70 single-electron transistor (SET). To understand electron tunneling through C70 fullerene species in a single-C70 transistor, descriptors of geometrical atomic structures and frontier molecular orbitals were analyzed. The findings regarding the node planes of the lowest unoccupied molecular orbitals (LUMOs) of C70 and both the highest occupied molecular orbitals (HOMOs) and the LUMO of the C70 anion suggest that electron tunneling of pristine C70 prolate spheroidal fullerene could be better in the major axis orientation when facing the gate electrode than in the major (longer) axis orientation when facing the Au source and drain electrodes. In addition, we explored the effect on the geometrical atomic structure of C70 by a single-electron addition, in which the maximum change for the distance between two carbon sites of C70 is 0.02 Å.

Snapshots of the Fragmentation for C70@Single-Walled Carbon Nanotube: Tight-Binding Molecular Dynamics Simulations.

In previously reported experimental studies, a yield of double-walled carbon nanotubes (DWCNTs) at C70@Single-walled carbon nanotubes (SWCNTs) is higher than C60@SWCNTs due to the higher sensitivity to photolysis of the former. From the perspective of pyrolysis dynamics, we would like to understand whether C70@SWCNT is more sensitive to thermal decomposition than C60@SWCNT, and the starting point of DWCNT formation, which can be obtained through the decomposition fragmentation of the nanopeapods, which appears in the early stages. We have studied the fragmentation of C70@SWCNT nanopeapods, using molecular dynamics simulations together with the empirical tight-binding total energy calculation method. We got the snapshots of the fragmentation structure of carbon nano-peapods (CNPs) composed of SWCNT and C70 fullerene molecules and the geometric spatial positioning structure of C70 within the SWCNT as a function of dynamics time (for 2 picoseconds) at the temperatures of 4000 K, 5000 K, and 6000 K. In conclusion, the scenario in which C70@SWCNT transforms to a DWCNT would be followed by the fragmentation of C70, after C70, and the SWCNT have been chemically bonding in the early stages. The relative stability of fullerenes in CNPs could be reversed, compared to the ranking of the relative stability of the encapsulated molecules themselves.

Predicting the Number of Graphene-Like Layers on Surface for Commercial Fumed Nanodiamonds with Raman Spectra and Model Calculations.

Nanoscale carbon materials have a broad range of applications in the field of surface and material sciences. Each vibration mode of a Raman and Fourier transform infrared (FT-IR) spectra corresponds to a specific frequency of a bond in the core and surface of the crystal, thus it is highly sensitive to morphology, implying that every band is sensitive to the orientation of the bonds and the atomic weight at either end of the bond. Accordingly, in this study we apply transmission electron microscope (TEM), Raman spectroscopies, and model calculations to study the relative content of carbon-carbon (C-C) bonds to the anhydrous and weakly aggregated elementary nanoscale carbon particles of detonation nanodiamonds. One point of the Raman bands at approximately 1300 cm-1 established that there are highly uniform C-C bonds in a tetrahedral crystal field environment not unlike that of diamonds. Another point at approximately 1600 cm-1 would be a hexagonal graphene-like sheet. By analyzing the relative content of carbon bonds using the area of intensity of the Raman peaks and a simulation of crystal morphology, we suggest that the number of graphene surface layers would be monolayers in nanodiamonds, comprising two kinds of C-C bonds, one being sp3 bonds of diamond in the core and the other being sp2 bonds of graphene on the surface.

Symmetry-dependent strong reduction of the spin exchange interactions in Cs2CuCl4 by the 6p orbitals of Cs+ ions.

Despite a three-dimensional arrangement of its CuCl(4)(2-) ions, the magnetic properties of Cs(2)CuCl(4) are explained by a two-dimensional frustrated triangular antiferromagnetic spin-lattice. The origin of this low-dimensional magnetism was explored by evaluating the spin exchange interactions of A(2)CuCl(4) (A = Cs, Rb, K, Na) on the basis of first principles density functional calculations. The calculated spin exchange parameters agree with experiment only when the Cs(+) ions located between the CuCl(4)(2-) ions are not neglected. The antiferromagnetic spin exchange interaction between adjacent CuCl(4)(2-) ions is strongly reduced by the 6p orbitals of the intervening Cs(+) ions when the arrangement of the CuCl(4)(2-) and Cs(+) ions has either mirror-plane or inversion symmetry. The observed magnetism of Cs(2)CuCl(4) arises from this symmetry-dependent participation of the 6p orbitals of the Cs(+) ions in the spin exchange interactions between CuCl(4)(2-) ions.

Preferential site of attack on fullerene cations: frontier orbitals and rate coefficients.

An analysis of reaction efficiency is presented for reactions of carbonaceous ions and molecules. Our results show that the combination of experimental rate-coefficient measurements and computations of the condensed Fukui functions of frontier molecular orbitals and pyramidal angles of pi orbitals is very useful for elucidating the reactive sites on fullerene carbon clusters in the gas phase.

Oxygen interstitials in superconducting La2CuO4: their valence state and role.

The nature of the much debated valence state of an interstitial oxygen atom in oxygen-doped La2CuO4 is the subject of this paper. In model cluster calculations, we studied the position, charge, and spin state of the interstitial oxygen atoms in this superconductor. The models considered allow the interstitial oxygen to move off a symmetrical position, to have varying spin and charge, and to be surrounded by various magnetic environments. UB3LYP calculations show that a model having an interstitial oxygen atom with a total spin of 1 is lowest in energy; the interstitial oxygen atoms here act as stable radicals with a net charge of -1. These results agree with experimental evidence for the paramagnetic behavior for interstitial atoms. The energy associated with a spin flip at a Cu site in our models is lower if interstitial oxygen has a local electron spin density, compared to the case when it does not. We provide a possible explanation for the increase of the doping concentrations of interstitial oxygen with a decrease of the Néel temperature of this system. The relative stability of the models we consider depends on their spin states, accompanied by structural changes; this explains indirectly the experimental change of the slope (from 2 to 1.3) of the linear relationship between the hole concentration and the oxygen content. Our results support a stripe phase in high temperature superconductivity; in our calculations, hole doping to the copper oxide layer comes only through the formation of an oxygen interstitial pair, not from any change of the local structural environment and magnetic field around the single interstitial.

Investigation of the scanning tunneling microscopy image, the stacking pattern, and the bias-voltage-dependent structural instability of 1,10'-phenanthroline molecules adsorbed on Au(111) in terms of electronic structure calculations.

A self-assembled monolayer of 1,10'-phenanthroline (phen) molecules on Au(111) was found to undergo a structural phase transition when the bias voltage is switched in scanning tunneling microscopy (STM) experiments (Phys. Rev. Lett. 1995, 75, 2376; Surf. Sci. 1997, 389, 19). The nature of two bright spots representing each phen molecule in the high-resolution STM images of phen molecules on Au(111) was identified by calculating the partial density plots for a monolayer of phen molecules adsorbed on Au(111) with tight-binding electronic structure calculations. The stacking pattern of chains of phen molecules on Au(111) was explained by studying the intermolecular interactions between phen molecules on the basis of first-principles electronic structure calculations for a phen dimer, (phen)(2). The structural instability of phen molecule arrangement caused by the bias-voltage switch was probed by estimating the adsorbate-surface interaction energy with the point-charge approximation for Au(111).