RESUMO

In this work, the structural, electronic, and optical stability properties of the chitosan monomer (M-Ch) and atomic silver complex are reported, as well as a unitary cell of a silver cluster in the gas phase and acetic acid. The generalized gradient approximation HSEh1PBE/def2-TZVPP50 results established the structures' anionic charge (Q = -1|e|) and the doublet state (M = 2). The high cohesive energy indicates structural stability, and the quantum-mechanical descriptors show a high polarity and low chemical reactivity. Also, the quantum-mechanical descriptors present a low work function that shows the structures are suitable for applications in light-emitting diodes. Finally, the electronic behavior observed by the |HOMO-LUMO| gap energy changes depending on the atomic silver incorporated into the complex.

RESUMO

The effect of chemical order in the structural and physicochemical properties of B12N12 [4,6]-fullerene (BNF) isomers was evaluated using density functional theory and molecular dynamic calculations. The feasibility to find stable BNF isomers with atomic arrangement other than the well-known octahedral Th-symmetry was explored. In this study, the number of homonuclear bonds in the modeled nanostructures was used as categorical parameter to describe and quantify the degree of structural order. The BNF without homonuclear bonds was identified as the most energetically favorable isomer. However, a variety of BNF arrays departing from Th-symmetry was determined as stable structures also. The calculated vibrational spectra suggest that isomers with chemical disorder can be identified by infrared spectroscopy. In general, formation of homonuclear bonds is possible meanwhile the entropy of the system increases, but at expense of cohesive energy. It is proposed that formation of phase-segregated regions stablishes an apparent limit to the number of homonuclear bonds in stable B12N12 fullerenes. It was found that formation of homonuclear bonds decreases substantially the chemical hardness of BNF isomers and generates zones with large charge density, which might act as reactive sites. Moreover, chemical disorder endows BNF isomers with a permanent electric dipole moment as large as 3.28 D. The obtained results suggest that by manipulating their chemical order, the interaction of BNF's with other molecular entities can be controlled, making them potential candidates for drug delivery, catalysis and sensing.

RESUMO

Using the density functional theory (DFT) we study the structural and electronic properties of functionalized (5,5) chirality single wall beryllium oxide nanotubes (SW-BeONTs), i.e. armchair nanotubes. The nanotube surface and ends are functionalized by the hydroxyl (OH) functional group. Our calculations consider the Hamprecht-Cohen-Tozer-Handy functional in the generalized gradient approximation (HCTH-GGA) to deal with the exchange-correlation energies, and the base function with double polarization (DNP). The geometry optimization of both defects free and with point defects nanotubes is done applying the criterion of minimum energy. Six configurations are considered: The OH oriented toward the Be (on the surface and at the end), toward the O (on the surface and at the end) and placed at the nanotube ends. Simulation results show that the nanotube functionalization takes place at the nanotube ends with the BeO bond displaying hydrogen-like bridge bonds. Moreover the nanotube semiconductor behavior remains unchanged. The polarity is high (it shows a transition from covalent to ionic) favoring solvatation. On the other hand, the work function low value suggests this to be a good candidate for the device fabrication. When the nanotube contains surface point defects the work function is reduced which provides excellent possibilities for the use of this material in the electronic industry.

Assuntos

RESUMO

The influence of vacancies and substitutional defects on the structural and electronic properties of graphene, graphene oxide, hexagonal boron nitride, and boron nitride oxide two-dimensional molecular models was studied using density functional theory (DFT) at the level of local density approximation (LDA). Bond length, dipole moment, HOMO-LUMO energy gap, and binding energy were calculated for each system with and without point defects. The results obtained indicate that the formation of a point defect does not necessary lead to structural instability; nevertheless, surface distortions and reconstruction processes were observed, mainly when a vacancy-type defect is generated. For graphene, it was found that incorporation of a point defect results in a semiconductor-semimetal transition and also increases notably its polar character. As with graphene, the formation of a point defect in a hexagonal boron nitride sheet reduces its energy gap, although its influence on the resulting dipole moment is not as dramatic as in graphene. The influence of point defects on the structural and electronic properties of graphene oxide and boron nitride oxide sheets were found to be mediated by the chemisorbed species.

RESUMO

The electrical and chemical properties of graphene (C(24)H(12)), graphane (C(24)H(24)) and graphene oxide (C(54)H(17)+O+(OH)(3)+COOH) were studied through the density functional theory (DFT) at level of Local Density Approximation (LDA) using a model C(n)H(m) like. The optimized geometry, energy gap and chemical reactivity for the proposed carbon 2D models are reported. It was found that while the graphene and graphane structures have semiconductor behavior, the graphene oxide behaves as semi-metal. However, a transition from semi-mental to semiconductor is predicted if the carboxyl group (COOH) is removed from such structure. The chemically active sites are analyzed on the basis of the electrophilic Fukui functions for each structure.

Assuntos