Publications by Alfredo Pasquarello

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The energy levels of the As-As dimer defect at GaAs/oxide interfaces are accurately determined within a hybrid functional scheme and aligned with respect to the GaAs band gap. An As-As dimer is constructed at a model interface between GaAs and kappa-Al2O3, ...

A comprehensive microscopic description of thermally induceddistortions in lead halide perovskites is crucial for their realistic applications, yetstill unclear. Here, we quantify the effects of thermal activation in CsPbBr3nanocrystals across length scales with atomic-level precision, and we provide aframework for the description of phase transitions therein, beyond the simplisticpicture of unit-cell symmetry increase upon heating. The temperature increasesignificantly enhances the short-range structural d

We introduce mixed Wannier-Bloch functions for studying electronic and vibrational spectra of periodic systems. These functions carry both spatial localization and limited spectral broadening, thereby combining the advantages of descriptions based on energy eigenstates (Bloch states) and position eigenstates (Wannier states). For the analysis of vibrational modes, a lattice position operator is introduced, analogous to the electronic Berry-phase position operator. Application to vitreous SiO2 demonstrates t

We have modeled the decomposition of nonstoichiometric amorphous SiOx upon annealing into silicon and stoichiometric silica, using a new method based on mapping Metropolis Monte Carlo simulations onto rate equations. The concentrations of all oxidation states of silicon are derived as a function of time and found to attain steady-state values at long times dependent on temperature T and oxygen content x. The degree of phase separation and the sizes of Si particles are predicted as a function of T and x, ena

The stability of oxygen vacancies across the Ge-HfO2 interface is studied through semilocal and hybrid density-functional calculations. On the semiconductor side, the formation energies are obtained for substoichiometric GeOx of varying x through the use o ...

The correlation between Si-29 chemical shifts and Si-O-Si bond angles in SiO2 is determined within density-functional theory for the full range of angles present in vitreous silica. This relation closely reproduces measured shifts of crystalline polymorphs. The knowledge of the correlation allows us to reliably extract from the experimental NMR spectrum the mean (151 degrees) and the standard deviation (11 degrees) of the Si-O-Si angular distribution of vitreous silica. In particular, we show that the Mozzi

Through the sequential use of classical molecular dynamics and first-principles relaxation methods, we generate an abrupt model interface for the 4H(0001)SiC-SiO2 interface showing regular structural parameters without any coordination defect. The bond density reduction at the interface is achieved through the use of two interfacial structural units which connect the disordered oxide to the abrupt crystalline substrate. The present model demonstrates that the sole topological constraints at SiC substrates d

We carried out an ab initio investigation of the charged states of hydrogen in amorphous SiO2 using a previously generated model structure. We found a large variety of equilibrium configurations as a result of the disordered atomic structure in our model system. The formation energies of H-0, H+ and H- remain nevertheless close to the energetics found for a-quartz, with the charged species always more stable than the neutral one irrespective of the electron Fermi level.

We present an efficient procedure for constructing nonempirical hybrid functionals to accurately predict band gaps of extended systems. We determine mixing parameters by enforcing the generalized Koopmans’ condition on localized electron states, which are ...

In this topical review, we discuss recent progress in electronic-structure methods for calculating defect energy levels in semiconductors and insulators. We concentrate mainly on two advanced electronic-structure schemes, namely hybrid density functional t ...

By analyzing the vibrational eigenmodes of a model structure obtained previously by quantum molecular dynamics, the two sharp features appearing in the Raman spectrum of vitreous silica could conclusively be assigned to three- and four-membered ring structures embedded in the network. The projections of the eigenmodes onto oxygen breathing motions in these rings yield well defined peaks at frequencies in striking correspondence to the positions of the Raman lines. The good agreement between calculated and m

First-principles calculations, classical molecular dynamics, and Monte-Carlo simulations are used in sequence to describe at the atomic scale the long-range oxygen migration through the amorphous SiO 2 layer during silicon oxidation. The O 2 molecule is identified as the transported oxygen species and is found to percolate through interstices without exchanging oxygen atoms with the network. The effective activation energy for diffusion is found in agreement with experimental values.

Experimental characterization is facing severe difficulties in elucidating the nature of defects at interfaces between silicon and amorphous high-kappa oxides. This motivates recourse to first principle simulation approaches based on density functional theory. A major issue for these approaches is the choice of the model structure. indeed, the most trivial choice of parent crystalline structures may fail in describing the amorphous nature of the oxide or the specific bonding arrangements occurring at the in

We analyze the principal vibrational spectra of vitreous GeO2 and derive therefrom structural properties referring to length scales beyond the basic tetrahedral unit. We generate a model structure that yields a neutron structure factor in accord with experiment. The inelastic-neutron, the infrared, and the Raman spectra, calculated within a density-functional approach, also agree with respective experimental spectra. The accord for the Raman spectrum supports a Ge-O-Ge angle distribution centered at 135 deg

Using density-functional theory, we investigate the structural, vibrational, and dielectric properties of Hf and Zr oxides and silicates which have drawn considerable attention as alternative high-kappa materials. For the silicates, we consider hafnon HfSiO4 and zircon ZrSiO4; while for the oxides, we study the cubic and tetragonal phases of HfO2 and ZrO2. Special emphasis is put on the analysis of the differences and similarities between Hf and Zr in these materials. In particular, we discuss the Born effe

Structural assignments are determined for Se-77 and Ge-73 chemical shifts through density-functional NMR calculations for GeSe2, Ge4Se9, and GeSe crystals. In particular, a very good agreement between calculated and measured Se-77 isotropic chemical shifts ...

Based on constant Fermi-level molecular dynamics and a proper alignment scheme, we perform simulations of the Pt(111)/water interface under variable bias potential referenced to the standard hydrogen electrode (SHE). Our scheme yields a potential of zero c ...

We discuss various decomposition schemes for analysing the dielectric constants of Zr silicates in terms of local properties. Such schemes serve the purpose of predicting the dielectric constants of amorphous alloys, when their system size precludes the possibility of performing accurate first-principles calculations. In particular, we compare two decomposition schemes which have found application in the recent literature. The first scheme is based on a decomposition into basic structural units characterize

The outstanding performance of NiOOH/FeOOH-based oxygen evolution reaction (OER) catalysts is rationalized in terms of a bifunctional mechanism involving two distinct active sites. In this mechanism, the OOHads reaction intermediate, which unfavorably affe ...

QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The ac ...

Alfredo Pasquarello

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Accurate determination of charge transition levels of the As-As dimer defect at GaAs/oxide interfaces through hybrid functionals

Atomic-Level Description of Thermal Fluctuations in Inorganic LeadHalide Perovskites

Mixed Wannier-Bloch functions for electrons and phonons in periodic systems

Modeling phase separation in nonstoichiometric silica

Formation of substoichiometric GeOx at the Ge-HfO2 interface

Si-O-Si bond-angle distribution in vitreous silica from first-principles Si-29 NMR analysis

Abrupt model interface for the 4H(1000)SiC-SiO2 interface

Ab initio study of charged states of H in amorphous SiO2

One-Shot Approach for Enforcing Piecewise Linearity on Hybrid Functionals: Application to Band Gap Predictions

First-principles determination of defect energy levels through hybrid density functionals and GW

Identification of Raman defect lines as signatures of ring structures in vitreous silica

Atomic processes during silicon oxidation: Oxygen diffusion through the oxide layer

First principles investigation of defects at interfaces between silicon and amorphous high-kappa oxides

Medium-range structural properties of vitreous germania obtained through first-principles analysis of vibrational spectra

First-principles investigation of high-kappa dielectrics: Comparison between the silicates and oxides of hafnium and zirconium

Structural assignments of NMR chemical shifts in GexSe1-x glasses via first-principles calculations for GeSe2, Ge4Se9, and GeSe crystals

Atomic-Scale Simulation of Electrochemical Processes at Electrode/Water Interfaces under Referenced Bias Potential

Modelling of dielectric constants of amorphous Zr silicates

High-performance NiOOH/FeOOH electrode for OER catalysis

QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials