Efficient DFT+U calculations of ballistic electron transport: Application to Au monatomic chains with a CO impurity

An efficient method for computing the Landauer-Buttiker conductance of an open quantum system within DFT + U is presented. The Hubbard potential is included in electronic-structure and transport calculations as a simple renormalization of the nonlocal pseudopotential coefficients by restricting the integration for the onsite occupations within the cutoff spheres of the pseudopotential. We apply the methodology to the case of an Au monatomic chain in the presence of a CO molecule adsorbed on it. We show that the Hubbard U correction removes the spurious magnetization in the pristine Au chain at the equilibrium spacing, as well as the unphysical contribution of d electrons to the conductance, resulting in a single (spin-degenerate) transmission channel and a more realistic conductance of 1 G(0). We find that the conductance reduction due to CO adsorption is much larger for the atop site than for the bridge site, so that the general picture of electron transport in stretched Au chains given by the local density approximation remains valid at the equilibrium Au-Au spacing within DFT + U. DOI: 10.1103/PhysRevB.87.085108

First-principles molecular-dynamics study of metal-supported nanosystems

Massimiliano Stengel

This thesis is concerned with the theoretical study of the adsorption of molecules and thin films to single-crystal metal surfaces. First-principles electronic calculations are performed in the framework of density functional theory in the local density approximation (LDA-DFT), by using a plane-wave formalism, norm-conserving pseudopotentials and the supercell method. For the calculation of the electronic ground state and the subsequent structural optimizations we use the Car-Parrinello technique, modified for the optimal treatment of metallic systems. This method has been improved in this work with the introduction of a Lagrangian formalism and a mathematically conserved constant of motion in the presence of variable fractional occupancies. Complex mechanisms involving substantial reconstructions of the substrate are elucidated by the study of the C60/Al(111) system. We find that the interaction of C60 molecules with the Al(111) surface is predominantly covalent, and that the adsorbates bind optimally to the surface if an Al vacancy is created directly underneath. The removed Al atoms form a (6 x 6) array of ad-dimers in the interstices of the C60 overlayer, to which they strongly bind. Large-scale structural relaxations performed directly on a surface unit cell containing three C60 molecules lead to a reconstructed structure with two vacancies and an Al ad-dimer which is significantly more stable than the unreconstructed one. This spontaneous local process, rather than the compression state of the C60 overlayer, explains why one C60 molecule out of three protrudes from the surface upon reconstruction. The delicate interplay of many competing factors occurring at the interface between different media is investigated through the study of ultrathin MgO films deposited on the Ag(100) surface. We consider pseudomorphic MgO films of thickness ranging from one to three atomic layers, and we study the evolution of the surface electronic properties as a function of coverage. We find that already three MgO monolayers are sufficient to fully develop the electronic structure of the single-crystal MgO surface, in agreement with experimental findings. We discuss the reliability of the LDA-DFT results for the band structure by using simple 1D models for the dielectric properties of the system.

EPFL2004

Finite-Size Supercell Correction for Charged Defects at Surfaces and Interfaces

Hannu-Pekka Komsa, Alfredo Pasquarello

A finite-size supercell correction scheme is introduced for the formation energy of charged defects at surfaces and interfaces. The scheme combines classical electrostatics with the dielectric profile and the electrostatic potential extracted from the electronic-structure calculation. Spurious electrostatic interactions are removed while retaining the dielectric and quantum-mechanical features of the system of interest, which may have no interface (bulk), a single interface or surface, or two interfaces. A pertinent extrapolation scheme validates the proposed corrections. Applications to the charged Cl vacancy at the surface of NaCl and to the dangling bond at the Si(100) surface show that the corrected formation energies are largely independent of the supercell dimensions and of the size of the vacuum region. DOI: 10.1103/PhysRevLett.110.095505

American Physical Society2013

Alignment of energy levels in amorphous oxides and at semiconductor-water interfaces

Zhendong Guo

We conduct a detailed investigation of defects in two representative amorphous oxides: amorphous Al2O3 (am-Al2O3) and TiO2 (am-TiO2), by combining ab initio molecular dynamics (MD) simulations and hybrid functional calculations. Our results indicate that oxygen vacancies and interstitials occur neither in am-Al2O3 nor in am-TiO2 due to structural rearrangements which assimilate the defect structure, and that the injection of extra holes can lead to the formation of O-O peroxy linkages. In am-Al2O3, hydrogen is found to be amphoteric. Based on localized Wannier functions, we identify the nominal charge state and the composition of the defect core units related to C and N impurities in am-Al2O3, which are found to depend on the total charge set in the simulation cell. Through the adopted electron counting rule, we assess that carbon and nitrogen impurities are only found in neutral and in singly positive charge states, respectively, indicating that none of them gives charge transition levels in am-Al2O3. In addition, those defect core units are shown to incorporate a varying number of oxygen atoms, by which their formation energy is dependent on the oxygen chemical potential. In addition, we propose an exchange mechanism for hole transport in am-TiO2 that relies on the simultaneous breaking and forming of O-O peroxy linkages that share one O atom. Through the use of nudged-elastic-band calculations, a hopping path as long as 1.2 nm with barriers of 0.3-0.5 eV is demonstrated, suggesting that hole diffusion through O-O peroxy linkages is viable in am-TiO2. In this work, we also determine the band alignment between various semiconductors and liquid water by combining MD simulations of atomistic interface models, electronic-structure calculations at the hybrid-functional and GW levels, and a computational standard hydrogen electrode (SHE). Our study comprises GaAs, GaP, GaN, CdS, ZnO, SnO2, rutile and anatase TiO2. For each semiconductor, we generate atomistic interface models with liquid water at the pH corresponding to the point of zero charge. The MD simulations are started from initial configurations, in which the water molecules are either molecularly (m) or dissociatively (d) adsorbed on the semiconductor surface. The calculated band offsets are found to be strongly influenced by the adsorption mode at the semiconductor-water interface, leading to differences larger than 1 eV between m and d models of the same semiconductor. We then assess the accuracy of various ab initio electronic-structure schemes in determining the band alignment. In the last part, we try to evaluate photocatalysts for water-splitting by considering the surface coverage and the energy alignment. We determine surface concentrations of water molecules, protons, and hydroxyl ions adsorbed at the semiconductor-water interfaces mentioned above as a function of pH. This is achieved through the calculation of the acidity constants at the surface sites, which are derived from ab initio MD simulations and a grand-canonical formulation of adsorbates. It is finally shown how the potential of a semiconductor material as photocatalysts for water splitting can be inferred by combining the nature of the surface coverage and the alignment of the band edges to the relevant redox levels.