A. L. Rosa

Structural and electronic properties of ZnO nanotubes from density functional calculations

The structural and electronic properties of armchair and zigzag ZnO nanotubes were studied using density functional theory with the generalized gradient approximation. It was found that the strain energy required for rolling a ZnO graphitic sheet into a tube is lower than those for BN and GaN nanotubes. Both the armchair and zigzag ZnO nanotubes were found to be direct gap semiconductors with the gaps decreasing with the diameter increase.

Covalent functionalization of ZnO surfaces: A density functional tight binding study

We have demonstrated the covalent functionalization of 1010-ZnO surfaces with carboxylic acids by employing self-consistent charge density functional tight binding (SCC-DFTB) calculations. We have found two thermodynamically stable surface configurations: (i) a monolayer coverage with a bidentate chelating ligand and (ii) a half-monolayer coverage with a bidentate bridging ligand. In both cases, the electronic band structures show the presence of covalent surface/adsorbate interactions. Besides, a nonbonding carboxylate character is verified for the bidentate adsorbate. Our results are consistent with infrared spectroscopy experiments on functionalized ZnO nanostructures, and open possibilities for further investigations on functionalized ZnO-based materials for bio/chemical sensing.

Density-functional theory calculations of bare and passivated triangular-shaped ZnO nanowires

The authors employ density functional theory within the generalized-gradient approximation to investigate infinitely long [0001] ZnO nanowires. The authors report on atomic relaxations, formation energies, and electronic structure of bare and hydrogen passivated ZnO wires with triangular cross sections. The authors find that surface reconstruction plays an important role in stabilizing the nanowires. The authors have shown that the band gap can be tuned by changing the wire diameter and by passivating with hydrogen. While bare and completely passivated wires are semiconducting, wires with intermediate hydrogen passivation exhibit metallic behavior.

Self-Consistent-Charge Density-Functional Tight-Binding Parameters for Cd-X (X = S, Se, Te) Compounds and Their Interaction with H, O, C, and N.

Parameters for CdX, SeX, and TeX (X = H, C, N, O, S, Se, Te, and Cd) have been generated within the self-consistent-charge density-functional tight-binding (SCC-DFTB) framework. The approach has been tested against ab initio density-functional theory calculations for the relevant bulk phases, surfaces, nanowires, and small molecular systems. The SCC-DFTB approach reproduces structural, electronic, and energetic properties very well, demonstrating that the developed parameters are fully transferable among different chemical environments.

Intense Intrashell Luminescence of Eu-Doped Single ZnO Nanowires at Room Temperature by Implantation Created Eu–Oi Complexes

Successful doping and excellent optical activation of Eu(3+) ions in ZnO nanowires were achieved by ion implantation. We identified and assigned the origin of the intra-4f luminescence of Eu(3+) ions in ZnO by first-principles calculations to Eu-Oi complexes, which are formed during the nonequilibrium ion implantation process and subsequent annealing at 700 °C in air. Our targeted defect engineering resulted in intense intrashell luminescence of single ZnO:Eu nanowires dominating the photoluminescence spectrum even at room temperature. The high intensity enabled us to study the luminescence of single ZnO nanowires in detail, their behavior as a function of excitation power, waveguiding properties, and the decay time of the transition.

Band gap engineering of GaN nanowires by surface functionalization

We investigated [0001] bare and functionalized gallium nitride (GaN) nanowires by using the density-functional theory. Passivation of GaN nanowires with various functional groups (H, NH2, OH, and SH) show distinct electronic properties. We found that the band gap for the nanowires with partial surface coverage is dependent on the coverage ratio and adsorption sites. In view of the importance of surface states to the properties of nanowires, we suggest that the electronic and optical properties can be modulated by controlling the surface states of nanowires by functionalization.

A complete set of self‐consistent charge density‐functional tight‐binding parametrization of zinc chalcogenides (ZnX; X=O, S, Se, and Te)

We have developed a complete set of self‐consistent charge density‐functional tight‐binding parameters for ZnX (X = Zn, O, S, Se, Te, Cd, H, C, and N). The transferability of the derived parameters has been tested against Pseudo Potential‐Perdew, Burke and Ernzerhof (PP‐PBE) calculations and experimental values (whenever available) for corresponding bulk systems (e.g., hexagonal close packing, zinc‐blende, and wurtzite(wz)), various kinds of nanostructures (such as nanowires, surfaces, and nanoclusters), and also some small molecular systems. Our results show that the derived parameters reproduce the structural and energetic properties of the above‐mentioned systems very well. With the derived parameter set, one can study zinc‐chalcogenide nanostructures of relatively large size which was otherwise prohibited by other methods. The Zn‐Cd parametrization developed in this article will help in studying large semiconductor hetero‐nanostructures of Zn and Cd chalcogenides such as ZnX/CdX core/shell nanoparticles, nanotubes, nanowires, and nanoalloys.

First principles investigations on the electronic structure of anchor groups on ZnO nanowires and surfaces

We report on density functional theory investigations of the electronic properties of monofunctional ligands adsorbed on ZnO-(1010) surfaces and ZnO nanowires using semi-local and hybrid exchange-correlation functionals. We consider three anchor groups, namely thiol, amino, and carboxyl groups. Our results indicate that neither the carboxyl nor the amino group modify the transport and conductivity properties of ZnO. In contrast, the modification of the ZnO surface and nanostructure with thiol leads to insertion of molecular states in the band gap, thus suggesting that functionalization with this moiety may customize the optical properties of ZnO nanomaterials.

Water adsorption on ZnO(101̄0): The role of intrinsic defects

Density functional theory (DFT) calculations have been performed to investigate the interaction of water molecules with bare and defective surfaces. We show that at high coverages water molecules avoid adsorption close to defect sites, whereas at low coverages adsorption on defective surfaces show a similar adsorption pattern to those adsorbed on the defect-free surface, adsorbing in a molecular fashion. Finally we show that the electronic structure of the defective non-polar surface is not much affected by the adsorption of water, with exception of the O-defect surfaces.

Structural Evolution of Cu/ZnO Active Sites: From Reactive Environment to Ultrahigh Vacuum

By using first‐principles thermodynamics calculations, we investigate the structural evolution of active sites on Cu/ZnO surfaces from reactive environment to ultrahigh vacuum conditions. Under O‐rich conditions, the formation of active oxygen vacancies on various ZnO surfaces is unfavorable. However, addition of Cu dopants can significantly improve the reducibility of the ZnO nonpolar and polar surfaces, to an extent that ZnO(0 0 0 1)–O polar surface can be fully reduced. The formed oxygen vacancies in turn enhance the charging of Cu active sites on the highly dispersed metallic monolayers containing Cu and Zn. This is believed to be a factor contributing to the synergetic effects of Cu/ZnO catalysts. Irreversible reconstruction of the active metallic monolayers would take place upon removal from the reactive environment and exposure to ultrahigh vacuum condition, resulting in less active Cu overlayers. Therefore, experiments performed directly under UHV conditions may underestimate the activity of the actual catalysts under reactive environment.