New Cap-Holed AlP, GaP, and InP Nanotubes.

The structural, vibrational, and electronic properties of new inorganic X-phosphide nanotubes (ch-XPNT), with X = Al, Ga, or In and chirality of (5,5), are investigated. These new NTs display cap-hole ends, with the cap-hole features induced by the nonpassivated ends. Studies are based on density functional theory (DFT) using the M06-2X, PBE, and B3LYP functionals together with the LanL2DZ basis set. All nanostructures have been relaxed by minimizing the total energy, assuming a nonmagnetic nature and a total neutral charge. Note that the cap-hole NTs are terminated by a 10-atom ring, which in turn favors the geometrical ordering and yields stable structures. The (5,5) ch-XPNT are highly electrophilic and nonpolar, in addition to having high solvation energy values. Let us remark that solvation energies are produced by the intermolecular forces that involve the induced dipoles. Structural and vibrational results show that the X-P bonds are single bonds. Finally, results suggest that the inorganic nanotubes are structurally stable with semiconductor features, which means that their functionalization may yield interesting future applications.

Band Alignment of the CuGaS2 Chalcopyrite Interfaces Studied by First-Principles Calculations.

The valence and conduction band offsets for both polar and nonpolar CuGaS2/CuAlSe2 and CuGaS2/ZnSe interfaces were studied here by the state-of-the-art first-principles calculations. Using the hybrid functional calculations, we show that the CuGaS2/CuAlSe2 and CuGaS2/ZnSe heterostructures in all interfaces form type II band alignment. The difference of valence and conduction band offsets is mainly due to lattice mismatch, generating stress in the interface and affecting the electronic properties of each material; meanwhile, the polarity configuration does not play an important role in these values. From the local density of states and the charge density, we can determine how the nature of the band alignments changes when the semiconductor conforms to each interface. This allows us to localize the electrons and holes at different sites of the interface.

Static dielectric constant of water within a bilayer using recent water models: a molecular dynamics study.

The water confined within a surfactant bilayer is studied using different water models via molecular dynamics simulations. We considered four representative rigid models of water: the SPC/E and the TIP4P/2005, which are commonly used in numerical calculations and the more recent TIP4Q and SPC/ε models, developed to reproduce the dielectric behaviour of pure water. The static dielectric constant of the confined water was analyzed as a function of the temperature for the four models. In all cases it decreases as the temperature increases. Additionally, the static dielectric constant of the bilayer-water system was estimated through its expression in terms of the fluctuations in the total dipole moment, usually applied for isotropic systems. The estimated dielectric was compared with the available experimental data. We found that the TIP4Q and the SPC/ε produce closer values to the experimental data than the other models, particularly at room temperature. It was found that the probability of finding the sodium ion close to the head of the surfactant decreases as the temperature increases, thus the head of the surfactant is more exposed to the interaction with water when the temperature is higher.

The structure and interaction mechanism of a polyelectrolyte complex: a dissipative particle dynamics study.

The mechanism of complex formation of two oppositely charged linear polyelectrolytes dispersed in a solvent is investigated by using dissipative particle dynamics (DPD) simulation. In the polyelectrolyte solution, the size of the cationic polyelectrolyte remains constant while the size of the anionic chain increases. We analyze the influence of the anionic polyelectrolyte size and salt effect (ionic strength) on the conformational changes of the chains during complex formation. The behavior of the radial distribution function, the end-to-end distance and the radius of gyration of each polyelectrolyte is examined. These results showed that the effectiveness of complex formation is strongly influenced by the process of counterion release from the polyelectrolyte chains. The radius of gyration of the complex is estimated using the Fox-Flory equation for a wormlike polymer in a theta solvent. The addition of salts in the medium accelerates the complex formation process, affecting its radius of gyration. Depending on the ratio of chain lengths a compact complex or a loosely bound elongated structure can be formed.