J. D. Santos

Model for zinc oxide varistor

Zinc oxide varistors are very complex systems, and the dominant mechanism of voltage barrier formation in these systems has not been well established. Yet the MNDO quantum mechanical theoretical calculation was used in this work to determine the most probable defect type at the surface of a ZnO cluster. The proposed model represents well the semiconducting nature as well as the defects at the ZnO bulk and surface. The model also shows that the main adsorption species that provide stability at the ZnO surface are O - , O 2 - , and O 2 .

Interaction between Li+ and C60 molecules

Abstract MNDO calculations with complete geometry optimization were used to calculate the interaction between Li+ and C60, C−60 and C2−60 molecules. In order to determine the more stable configuration, Li+ was approximated to the C60 molecule or to their reduced forms from 10 A up to the geometric center of the molecule. From the simulations we determine that Li+ remains close to the surface at 5 A from the geometric center of the C60 molecules.

An investigation of oxidation and reduction of C60, the excited states, energy gaps and stability using semi-empirical and ab initio methods

We have used semi-empirical and ab initio methods to investigate the various oxidation and reduction states of C60 with their respective more energetically stable excited states resulting from variations of the charge from 112 to 212 electrons in the system with singlet, doublet, triplet and quadruplet multiplicities. We have analysed the various conformations, energy variations, uHOMO 2 LUMOu, system charges, average interatomic distances as well as regions of minimum energies resulting from the simulation of the possible reaction routes. q 2001 Elsevier Science B.V. All rights reserved.

A theoretical study of lithium ion interaction with tin oxide

Abstract Semiempirical MNDO calculations were used to calculate the interaction between Li + and (SnO 2 ) 15 clusters. Our calculations predicted that Li + adsorbs preferentially on mono-coordinate oxygen (A-type sites). The higher energy barrier to intercalation (−7.41 eV) suggests that the ion remains adsorbed at the oxide surface, but that the presence of oxygen vacancies probably decreases this barrier, favouring the Li + intercalation.

MNDO theoretical study of ethanol decomposition process on SnO2 surfaces

Abstract An MNDO study has been carried out to analyze the decomposition process of the ethanol molecule on a SnO 2 surface. A (SnO 2 ) 7 (110) model has been selected to represent the surface. The decomposition process has been monitored by selection of a hydrogen-α-carbon distance of the ethanol molecule as reaction coordinate. This minimum energy profile shows a maximum of 186 kJ mol -1 , and in the transition state there is a transfer of hydrogen-α-carbon to the SnO 2 surface. There is also the interaction between the alcohol hydroxyls and the two oxygens of the oxide.

Stability of rolled-up GaAs nanotubes

This work presents a theoretical study of gallium arsenide (GaAs) nanotubes obtained from the (100), (110) and (111) crystal planes of zincblende structure in order to evaluate the electronic properties. The DFT/B3LYP/6-31G method was used to predict structures and stabilities. It was found that nanotubes from the (110) crystal plane tended to be the most stable. The results for average diameter and bond length obtained for optimized nanotube geometries show that nanotubes constructed from the (100) plane have a hyperbolic format, while (110) or (111) nanotubes have a conical format. This difference in relation to geometry introduces regions with different charge concentrations along the tube. From the calculated values for the gap it follows that increasing the number of atoms per layer causes a displacement of the frontier orbitals with a reduction in the gap, yielding characteristics of a semiconductor material.