Juarez L. F. Da Silva

Adsorption of Xe atoms on metal surfaces: New insights from first-principles calculations

The adsorption of rare gases on metal surfaces serves as the paradigm of weak adsorption where it is typically assumed that the adsorbate occupies maximally coordinated hollow sites. Density-functional theory calculations using the full-potential linearized augmented plane wave method for Xe adatoms on Mg(0001), Al(111), Ti(0001), Cu(111), Pd(111), and Pt(111), show, however, that Xe prefers low coordination on-top sites in all cases. We identify the importance of polarization and a site-dependent Pauli repulsion in actuating the site preference and the principle nature of the rare-gas atom-metal surface interaction.

Stability and electronic structures of CuxTe

In this work, the authors study the stability and electronic properties of CuxTe as a function of the Cu∕Te ratio using first-principles calculations. They find that CuxTe shows bistability with energy minimum at x≈1.25 and x≈1.75. At low x, CuxTe is more stable in the tetragonal-based structures, whereas for x>1.47, it is more stable in the trigonal-based structures. The valence-band maximum of Cu2Te is found to be 0.7eV higher than that of CdTe, indicating that it can easily be doped p type. The Cu diffusion barriers and hole density decrease with increasing Cu concentration, indicating that for x∼2, ionic conductivity may play a role.

Atomic structure of In2O3–ZnO systems

The authors present first-principles density-functional theory studies on the atomic structure of the In2O3–ZnO system. They find that the In2O3–ZnO system has a polytypoid structure, which consists of wurtzite (Zn∕In)O slabs separated by single In–O octahedral layers that are inversion boundaries. Another type of boundary with polarity inversion formed by fivefold coordinated In and Zn atoms exists inside the (Zn∕In)O slabs. The authors find that these polarity-inversion boundaries inside the (Zn∕In)O slabs prefer a low-energy modulated structure.

Optical properties of (GeTe, Sb2Te3) pseudobinary thin films studied with spectroscopic ellipsometry

The authors measure the dielectric functions of (GeTe, Sb2Te3) pseudobinary thin films by using spectroscopic ellipsometry. By using standard critical point model, they obtained the optical transition (critical point) energies of the amorphous (crystalline) thin films. The optical (indirect band) gap energies of the amorphous (crystalline) phase are estimated from the linear extrapolation of the absorption coefficients. The band structure calculations show that GeTe, Ge2Sb2Te5, and Ge1Sb2Te4 have indirect gap whereas Ge1Sb4Te7 and Sb2Te3 have direct gap. The measured indirect band gap energies match well with electronic band structure calculations.

Atomistic origins of the phase transition mechanism in Ge2Sb2Te5

The fast and reversible phase transition mechanism between crystalline and amorphous phases of Ge2Sb2Te5 has been in debate for several years. Through employing first-principles density functional theory calculations, we identify a direct structural link between the metastable crystalline and amorphous phases. The phase transition is driven by the displacement of Ge atoms along the rocksalt [111] direction from stable octahedron to high energy unstable tetrahedron sites close to the intrinsic vacancy regions, which generates a high energy intermediate phase between metastable and amorphous phases. Due to the instability of Ge at the tetrahedron sites, the Ge atoms naturally shift away from those sites, giving rise to the formation of local-ordered fourfold motifs and the long-range structural disorder. Intrinsic vacancies, which originate from Sb2Te3, lower the energy barrier for Ge displacements, and hence, their distribution plays an important role in the phase transition. The high energy intermediate c...

Ethanol and Water Adsorption on Transition-Metal 13-Atom Clusters: A Density Functional Theory Investigation within van der Waals Corrections

Transition-metal (TM) nanoparticles supported on oxides or carbon black have attracted much attention as potential catalysts for ethanol steam reforming reactions for hydrogen production. To improve the performance of nanocatalysts, a fundamental understanding of the interaction mechanism between water and ethanol with finite TM particles is required. In this article, we employed first-principles density functional theory with van der Waals (vdW) corrections to investigate the interaction of ethanol and water with TM13 clusters, where TM = Ni, Cu, Pd, Ag, Pt, and Au. We found that both water and ethanol bind via the anionic O atom to onefold TM sites, while at higher-energy structures, ethanol binds also via the H atom from the CH2 group to the TM sites, which can play an important role at real catalysts. The putative global minimum TM13 configurations are only slightly affected upon the adsorption of water or ethanol; however, for few systems, the compact higher-energy icosahedron structure changes its configuration upon ethanol or water adsorption. That is, those configurations are only shallow local minimums in the phase space. Except few deviations, we found similar trends for the magnitude of the adsorption energies of water and ethanol, that is, Ni13 > Pt13 > Pd13 and Cu13 > Au13 > Ag13, which is enhanced by the addition of the vdW correction (i.e., from 4% to 62%); however, the trend is the same. We found that the magnitude of the adsorption energy increases by shifting the center of gravity of the d-states toward the highest occupied molecular orbital. On the basis of the Mulliken and Hirshfeld charge analysis, as well as electron density differences, we identified the location of the charge redistribution and a tiny charge transfer (from 0.01 e to 0.19 e) from the molecules to the TM13 clusters. Our vibrational analysis indicates the red shifts in the OH modes upon binding of both water and ethanol molecules to the TM13 clusters, suggesting a weakening of the O-H bonding.

Theoretical Investigation of the Adsorption Properties of CO, NO, and OH on Monometallic and Bimetallic 13-Atom Clusters: The Example of Cu13, Pt7Cu6, and Pt13.

We report a density functional theory investigation of the adsorption properties of CO, NO, and OH on the Cu13, Pt7Cu6, and Pt13 clusters in the cationic, neutral, and anionic states with the aim to improve our atomistic understanding of the adsorption properties on bimetallic clusters compared with monometallic clusters. The adsorption energy of CO and NO are substantially stronger on Pt13 than on Cu13, and hence, CO and NO bind preferentially on Pt sites on Pt7Cu6. Thus, it can contribute to drive the migration of the Pt atoms from the core to the surface region in large PtCu nanoalloys. The CO and NO adsorption energies on the bimetallic cluster are enhanced by a few percent compared with the energies of the monometallic clusters, which shows that the Pt-Cu interaction can contribute to an increase in the adsorption energy. In contrast with CO and NO trends, the OH adsorption energies on Cu13, Pt7Cu6, and Pt13 deviates only up to 0.31 eV, and hence, there is no clear preference for Cu or Pt sites on Pt7Cu6 or an enhancement of the adsorption energy on the bimetallic systems. We found a reduction of the CO and NO vibrational frequencies upon adsorption, which indicates a weakening of the CO and NO binding energies, and it is supported by a slight increase in the bond lengths. However, the OH vibrational frequency increases upon adsorption, which indicates an enhancement of the OH binding energy, which is supported by a slight decrease in the bond length by about 0.01 Å. It can be explained by the large charge transfer from the clusters to the O atom, which enhances the electrostatic interaction in the O-H bonding.