Rita MAGRI - Academia.edu (original) (raw)

Papers by Rita MAGRI

Research paper thumbnail of Structural and Dynamic Characterization of Li–Ionic Liquid Electrolyte Solutions for Application in Li-Ion Batteries: A Molecular Dynamics Approach

Batteries

Pyrrolidinium-based (Pyr) ionic liquids (ILs) have been proposed as electrolyte components in lit... more Pyrrolidinium-based (Pyr) ionic liquids (ILs) have been proposed as electrolyte components in lithium-ion batteries (LiBs), mainly due to their higher electrochemical stability and wider electrochemical window. Since they are not naturally electroactive, in order to allow their use in LiBs, it is necessary to mix the ionic liquids with lithium salts (Li). Li–PF6, Li–BF4, and Li–TFSI are among the lithium salts more frequently used in LiBs, and each anion, namely PF6 (hexafluorophosphate), BF4 (tetrafluoroborate), and TFSI (bis(trifluoromethanesulfonyl)azanide), has its own solvation characteristics and interaction profile with the pyrrolidinium ions. The size of Pyr cations, the anion size and symmetry, and cation–anion combinations influence the Li-ion solvation properties. In this work, we used molecular dynamics calculations to achieve a comprehensive view of the role of each cation–anion combination and of different fractions of lithium in the solutions to assess their relative ...

Research paper thumbnail of Evolution of the bandgap and band-edge energies of the lattice-matched Ga In As Sb

Using atomistic pseudopotential calculations we predict the evolution of the valence-band maximum... more Using atomistic pseudopotential calculations we predict the evolution of the valence-band maximum energy E ͑x , y͒ and conduction-band minimum energy E c ͑x , y͒ for a compositionally graded quaternary Ga 1−y In y As x Sb 1−x alloy lattice matched to GaSb or InAs as a function of ͑x , y͒ or, equivalently, as a function of distance from the substrate. We find upward-concave bowing for both E c and E , in contradiction with simple interpolative models. A transition from staggered ͑type II͒ to broken-gap ͑type III͒ lineup relative to GaSb is predicted to occur at x = 0.81 and y = 0.92 on a GaSb substrate, and at x = 0.59 and y = 0.62 on an InAs substrate. In the latter case, the quaternary alloy has a minimum gap at x = 0.85 and y = 0.87.

Research paper thumbnail of Surface Reactivity of Ag-Modified Ceria to Hydrogen: A Combined Experimental and Theoretical Investigation

We investigate the mechanism of H2 activation on Ag-modified cerium oxide surfaces, of interest f... more We investigate the mechanism of H2 activation on Ag-modified cerium oxide surfaces, of interest for different catalytic applications. The study is performed on thin epitaxial cerium oxide films, investigated by X-ray photoemission spectroscopy to assess the changes of both the Ag oxidation state and the concentration of Ce 3+ ions, O vacancies and hydroxyl groups on the surface during thermal reduction cycles in vacuum and under hydrogen exposure. The results are interpreted using density functional theory calculations to model pristine and Ag-modified ceria surfaces. Although the reactivity of ceria towards H2 oxidation improves when a fraction of Ce cations is substituted with Ag, the concentration of reduced Ce 3+ ions in Ag-modified ceria is found to be lower than in pure ceria under the same conditions. This behavior is observed even though the number of surface oxygen vacancies caused by the thermal treatment under hydrogen exposure is larger for the Ag-modified surface. These results are explained in terms of a change of the oxidation state of the surface Ag, which is able to acquire some of the extra surface electrons created by the oxygen vacancies and the adsorbed hydrogen atoms. Our findings provide new insights into the reactivity of Ag-modified ceria, which has been proposed as a promising alternative to platinum electrodes in electro-chemical devices.

Research paper thumbnail of Attractive interactions between like-oriented surface steps from an ab initio perspective: Role of the elastic and electrostatic contributions

Physical Review B, 2019

In this work, we show how using the density functional approach, in which the electronic degrees ... more In this work, we show how using the density functional approach, in which the electronic degrees of freedom are separated by the ionic ones, it is possible to individuate and separately study the elastic and electrostatic step interactions, traditionally introduced in the literature as the only two kinds of step interaction expected at T = 0. We have applied the method to the technologically important GaAs(001) surface and found some unexpected results for the relatively short step distances accessible to the ab-initio approaches, contradicting those of the continuum models so far employed for the study of the elastic step interactions: (i) the sign of the step interaction depends on the step termination and is due to the electrostatic interaction; (ii) the elastic interaction does not contribute to the step interaction, contrary to the common belief of a strong elastic repulsive interaction between like-oriented steps. We show that this is due to the electron behavior. When considering only ion displacements and point-like steps as in the continuum theories, we recover the classical results and repulsive step elastic interactions; (iii) the experimentally observed Ab step termination shows a weakly attractive step interaction whereby attractive step interactions between like-oriented steps on an unstrained surface are believed not to exist. The proposed method of separating elastic and electrostatic interactions for further analysis of their dependence on the configurational degrees of freedom can be extended to other defective situations.

Research paper thumbnail of Si and Ge based metallic core/shell nanowires for nano-electronic device applications

Scientific Reports, 2018

One dimensional heterostructure nanowires (NWs) have attracted a large attention due to the possi... more One dimensional heterostructure nanowires (NWs) have attracted a large attention due to the possibility of easily tuning their energy gap, a useful property for application to next generation electronic devices. In this work, we propose new core/shell NW systems where Ge and Si shells are built around very thin As and Sb cores. The modification in the electronic properties arises due to the induced compressive strain experienced by the metal core region which is attributed to the lattice-mismatch with the shell region. As/Ge and As/Si nanowires undergo a semiconducting-to-metal transition on increasing the diameter of the shell. The current-voltage (I-V) characteristics of the nanowires show a negative differential conductance (NDC) effect for small diameters that could lead to their application in atomic scale device(s) for fast switching. In addition, an ohmic behavior and upto 300% increment of the current value is achieved on just doubling the shell region. The resistivity of na...

Research paper thumbnail of Ordering inBxC1−xcompounds with the graphite structure

Physical Review B, 1994

We present a theoretical study of the structural stability of ordered configurations of B Cl soli... more We present a theoretical study of the structural stability of ordered configurations of B Cl solid solutions with the graphite structure. The calculations were carried out self-consistently using normconserving pseudopotentials in the local-density approximation. The cohesive energies of a number of ordered structures at different compositions x have been subjected to a cluster expansion, which enables us to predict the cohesive energy of the random alloy at the corresponding compositions and to gain insight on the tendency of the system towards ordering. We find that, at T=O K, some ordered configurations have a cohesive energy higher than the corresponding random distribution at the same compositions. The inspection of the cluster interactions leads moreover to the conclusion that segregation of boron in graphite is highly discouraged and long-range order is likely to occur.

Research paper thumbnail of Ground-state structures and the random-state energy of the Madelung lattice

Research paper thumbnail of Theory of optical properties of segregated InAs/GaSb superlattices

IEE Proceedings - Optoelectronics, 2003

The authors study the effects of interfacial atomic segregation on the electronic and optical pro... more The authors study the effects of interfacial atomic segregation on the electronic and optical properties of InAs/GaSb superlattices. They describe their atomistic empirical pseudopotential method and test its performance against the available experimental data. They show its ability to predict the band structure dependence on the detailed atomic configuration, and thus to properly account for the effects of interfacial atomic segregation and structural disorder. They also show how their method avoids the approximations underlying the pseudopotential method of Dente and Tilton, which gives different results. The application of the proposed method to the InAs/GaSb superlattices allows the explanation of some observed experimental results, such as: the bandgap difference between ðInAsÞ 8 =ðGaSbÞ 8 superlattices with almost pure InSb-like or GaAs-like interfaces; the large blue shift of the bandgap when the growth temperature of the superlattice increases; and the blue shift of the bandgap of ðInAsÞ 8 =ðGaSbÞ n superlattices with increasing GaSb period n. They present a detailed comparison of their predicted blue shift with that obtained by other theories.

Research paper thumbnail of Real-space description of semiconducting band gaps in substitutional systems

Physical Review B, 1991

The goal of "band-gap engineering' in substitutional lattices is to identify atomic configuration... more The goal of "band-gap engineering' in substitutional lattices is to identify atomic configurations that would give rise to a desired value of the band gap. Yet, current theoretical approaches to the problems, based largely on compilations of band structures for various latice configurations, have not yielded simple rules relating structural motifs to band gaps. We show that the band gap of substitutional A1As/GaAs lattices can be usefully expanded in terms of a hierarchy of contributions from real-space "atomic figures" (pairs, triplets, quadruplets) detemined from first-principles band-structure calculations. Pair figures (up to fourth neighbors) and three-body figures are dominant. In analogy with similar cluster expansions of the total energy, this permits a systematic search among all lattice configurations for those having "special" band gaps. This approach enables the design of substitutional systems with certain band-gap properties by assembling atomic figures. As an illustration, we predict that the [012joriented (AlAs) l/(GaAs)4/(A1As) &/(GaAs)2 superlattice has the largest band gap among all Alo 25Gao 75As lattices with a maximum of ten cations per unit cell.

Research paper thumbnail of Electronic structure and density of states of the randomAl0.5Ga0.5As,GaAs0.5P0.5, andGa0.5In0.5As semiconductor alloys

Physical Review B, 1991

The electronic density of states (DOS), charge densities, equilibrium bond lengths, and optical b... more The electronic density of states (DOS), charge densities, equilibrium bond lengths, and optical bowing of the direct band gaps are calculated for three perfectly random semiconductor alloys within the firstprinciples pseudopotential method using the concept of "special quasirandom structures" (SQS's). The SQS's are periodic structures with moderately large unit cells whose sites are occupied by atoms in a way designed to reproduce the structural features of the infinite, perfectly random substitutional alloys. In avoiding averaging over atoms (as in the virtual-crystal approximation) or over atomic environments (as in the site-coherent-potential approximation), this approach is capable of revealing the multisite nature of chemical disorder, as well as atomic-relaxation effects. We show how the existence of different local environments about chemically identical sites leads to splittings and fine structures in the density of states, and how atomic relaxations are induced by such nonsymmetric environments and lead to significant modifications in these DOS features. The calculated alloy bond lengths and optical-bowing coefficients are found to be in good agreement with experiment. Relaxation-induced splittings in the DOS are offered as predictions for future photoemission studies.

Research paper thumbnail of Predicting structural energies of atomic lattices

Physical Review B, 1991

The complexity of current ab initio quantum-mechanical calculations of the total energy of given ... more The complexity of current ab initio quantum-mechanical calculations of the total energy of given distributions of atoms on a periodic lattice often limits explorations to just a few configurations. We show how such a small number of calculations can be used instead to compute the interaction energies of a generalized Ising model, which then readily provides predicted energies of many more interesting configurations. This is illustrated for A1As/GaAs systems.

Research paper thumbnail of Low-dimensional In-based semiconductor systems

ecent advances in the epitaxial growth of low-dimensional semiconductor systems with techniques s... more ecent advances in the epitaxial growth of low-dimensional semiconductor systems with techniques such as molecular beam epitaxy (MBE) make it possible to form structures of various geometries. The composition profile and the vertical transport properties of self-assembled InAs/GaAs quantum dots (QDs) and quantum rings (QRs) are investigated by X-ray photoemission electron microscopy and conductive atomic force microscopy (1-3). The resulting

Research paper thumbnail of Surface compositional profiles of self-assembled InAs∕GaAs quantum rings

AIP Conference Proceedings, 2010

The composition profile of self-assembled InAs/GaAs quantum rings (QR) is studied both experiment... more The composition profile of self-assembled InAs/GaAs quantum rings (QR) is studied both experimentally and theoretically. 2D surface maps obtained by X-ray photoemission electron microscopy (XPEEM) reveal a non-uniform profile with an In-rich core, corresponding to the central hole of the QR, surrounded by a rim with

Research paper thumbnail of Role of surface structural motifs on the stability and reflectance anisotropy spectra of Sb-rich GaSb(001) reconstructions

Physical Review B, 2011

The structure of the technologically important-but still mostly unknown-GaSb(001)-c(2 × 6) surfac... more The structure of the technologically important-but still mostly unknown-GaSb(001)-c(2 × 6) surface reconstruction is investigated by means of ab initio simulations of reflectance anisotropy spectroscopy (RAS) and total energy calculations. A large number of reconstruction models for the GaSb(001) surface in the Sb-rich coverage regime are considered. The influence of each single surface structural motif on the RAS spectra is studied in detail, as well as their role in the surface stability with regard to application of the electron counting rule (ECR). We interpret the features of the RAS data measured for this reconstruction and suggest a new model for the c(2 × 6) phase. In this model a few Sb atoms in the second layer are randomly substituted by Ga, forming surface antisite defects. When used to fulfill the ECR, this "doping" effect considerably lowers the total energy of the long chain c(2 × 6) reconstruction model, making it competitive with the more stable short-chain (4 × 3) reconstructions. Formation of the surface antisites occurs spontaneously in the presence of dynamical negative charge fluctuations and is favored by the excellent matching between GaSb(001) and metallic Sb and by the natural softness of the Ga-Sb bonds. Calculations of the reflectance anisotropy spectra confirm that this structure is a major component of a largely disordered surface, where motifs of the stable (4 × 3) reconstructions are also present.

Research paper thumbnail of Silicon nanocrystallites in aSiO2matrix: Role of disorder and size

Research paper thumbnail of Excitons in silicon nanocrystallites: The nature of luminescence

Research paper thumbnail of Engineering silicon nanocrystals: Theoretical study of the effect of codoping with boron and phosphorus

Physical Review B, 2007

We show that the optical and electronic properties of nanocrystalline silicon can be efficiently ... more We show that the optical and electronic properties of nanocrystalline silicon can be efficiently tuned using impurity doping. In particular, we give evidence, by means of ab-initio calculations, that by properly controlling the doping with either one or two atomic species, a significant modification of both the absorption and the emission of light can be achieved. We have considered impurities, either boron or phosphorous (doping) or both (codoping), located at different substitutional sites of silicon nanocrystals with size ranging from 1.1 nm to 1.8 nm in diameter. We have found that the codoped nanocrystals have the lowest impurity formation energies when the two impurities occupy nearest neighbor sites near the surface. In addition, such systems present band-edge states localized on the impurities giving rise to a red-shift of the absorption thresholds with respect to that of undoped nanocrystals. Our detailed theoretical analysis shows that the creation of an electron-hole pair due to light absorption determines a geometry distortion that in turn results in a Stokes shift between adsorption and emission spectra. In order to give a deeper insight in this effect, in one case we have calculated the absorption and emission spectra going beyond the single-particle approach showing the important role played by many-body effects. The entire set of results we have collected in this work give a strong indication that with the doping it is possible to tune the optical properties of silicon nanocrystals.

Research paper thumbnail of Adsorption of indium on an InAs wetting layer deposited on the GaAs(001) surface

Physical Review B, 2008

In this work we perform a first-principles study of the adsorption properties of an In adatom dep... more In this work we perform a first-principles study of the adsorption properties of an In adatom deposited on 1.75 monolayers (ML) InAs, forming a wetting layer on GaAs(001) with the α 2 (2 × 4) or β 2 (2×4) reconstruction. The structural properties of these reconstructions have been studied: we determine the equilibrium geometry of the surfaces and their stability for various growth conditions. We have then carried out a detailed study of the potential energy surface (PES) for an In adsorbate, finding the minima and the saddle points. The main characteristics of the PES and the bonding configurations of the In adatom on the surface are analyzed by comparing with analogous studies reported in the literature, trying to extract the effects due to: (i) the compressive strain to which the InAs adlayer is subjected, (ii) the particular surface reconstruction, and (iii) the wetting layer composition. We found that, in general, stable adsorption sites are located at: (i) locations besides the As in-dimers, (ii) positions bridging two As in-dimers, (iii) between two adjacent ad-dimers (only in β 2), and (iv) locations bridging two As ad-dimers. We find also other shallower adsorption sites which are more reconstruction specific due to the lower symmetry of the α 2 reconstruction compared to the β 2 reconstruction.

Research paper thumbnail of Ab-Initio Investigation of the Polarization Anisotropy of the Optical Absorption in (InGa)As–InP Superlattices

physica status solidi (a), 1998

ABSTRACT

Research paper thumbnail of Surface Effects on the Atomic and Electronic Structure of Unpassivated GaAs Nanowires

ACS Nano, 2010

On the basis of accurate ab initio calculations, we propose a model for predicting the stability ... more On the basis of accurate ab initio calculations, we propose a model for predicting the stability of III؊V nanowires (NW) having different side walls and ridge configurations. The model allows us to obtain the NW formation energies by performing calculations only on relatively "small" systems, small diameter NWs and flat surfaces, to extract the contributions to the stability of each structural motif. Despite the idea illustrated here for the case of hexagonally shaped GaAs NWs grown along the [111]/[0001] direction, the method can also be applied generally to other differently shaped and oriented III؊V NWs. The model shows that NW surfaces (side walls plus ridges) mainly determine the NW stability, so the changes to the surface structure (e.g., induced by defects or different growth conditions) would modify the final NW structure in a remarkable way. We find that wurtzite and zinc blende nanowires have similar energies over a wide diameter range, thus explaining the observed polytypism. Furthermore, new more stable ridge reconstructions are proposed for zinc blende nanowires. The surface-related structural motifs also have clear fingerprints on the NW electronic structure. We find that the more stable nanowires are all semiconducting. The band gaps are ruled by surface states and do not follow the trend mandated by the quantum confinement effect. Small diameter wurtzite nanowires have an indirect band gap, but for some of them, an indirect to direct transition can be foreseen to occur at larger diameters. Surface states have a larger impact on the zinc blende NW band gaps than on the wurtzite NW ones. Zinc blende nanowire band gaps reduce significantly with increasing nanowire radius, reaching the bulk value at a diameter of about 30 Å. The surface structure and the high surface related DOS below the conduction band are going to affect the nanowire dopant incorporation and efficiency when doping is carried out during the NW growth.

Research paper thumbnail of Structural and Dynamic Characterization of Li–Ionic Liquid Electrolyte Solutions for Application in Li-Ion Batteries: A Molecular Dynamics Approach

Batteries

Pyrrolidinium-based (Pyr) ionic liquids (ILs) have been proposed as electrolyte components in lit... more Pyrrolidinium-based (Pyr) ionic liquids (ILs) have been proposed as electrolyte components in lithium-ion batteries (LiBs), mainly due to their higher electrochemical stability and wider electrochemical window. Since they are not naturally electroactive, in order to allow their use in LiBs, it is necessary to mix the ionic liquids with lithium salts (Li). Li–PF6, Li–BF4, and Li–TFSI are among the lithium salts more frequently used in LiBs, and each anion, namely PF6 (hexafluorophosphate), BF4 (tetrafluoroborate), and TFSI (bis(trifluoromethanesulfonyl)azanide), has its own solvation characteristics and interaction profile with the pyrrolidinium ions. The size of Pyr cations, the anion size and symmetry, and cation–anion combinations influence the Li-ion solvation properties. In this work, we used molecular dynamics calculations to achieve a comprehensive view of the role of each cation–anion combination and of different fractions of lithium in the solutions to assess their relative ...

Research paper thumbnail of Evolution of the bandgap and band-edge energies of the lattice-matched Ga In As Sb

Using atomistic pseudopotential calculations we predict the evolution of the valence-band maximum... more Using atomistic pseudopotential calculations we predict the evolution of the valence-band maximum energy E ͑x , y͒ and conduction-band minimum energy E c ͑x , y͒ for a compositionally graded quaternary Ga 1−y In y As x Sb 1−x alloy lattice matched to GaSb or InAs as a function of ͑x , y͒ or, equivalently, as a function of distance from the substrate. We find upward-concave bowing for both E c and E , in contradiction with simple interpolative models. A transition from staggered ͑type II͒ to broken-gap ͑type III͒ lineup relative to GaSb is predicted to occur at x = 0.81 and y = 0.92 on a GaSb substrate, and at x = 0.59 and y = 0.62 on an InAs substrate. In the latter case, the quaternary alloy has a minimum gap at x = 0.85 and y = 0.87.

Research paper thumbnail of Surface Reactivity of Ag-Modified Ceria to Hydrogen: A Combined Experimental and Theoretical Investigation

We investigate the mechanism of H2 activation on Ag-modified cerium oxide surfaces, of interest f... more We investigate the mechanism of H2 activation on Ag-modified cerium oxide surfaces, of interest for different catalytic applications. The study is performed on thin epitaxial cerium oxide films, investigated by X-ray photoemission spectroscopy to assess the changes of both the Ag oxidation state and the concentration of Ce 3+ ions, O vacancies and hydroxyl groups on the surface during thermal reduction cycles in vacuum and under hydrogen exposure. The results are interpreted using density functional theory calculations to model pristine and Ag-modified ceria surfaces. Although the reactivity of ceria towards H2 oxidation improves when a fraction of Ce cations is substituted with Ag, the concentration of reduced Ce 3+ ions in Ag-modified ceria is found to be lower than in pure ceria under the same conditions. This behavior is observed even though the number of surface oxygen vacancies caused by the thermal treatment under hydrogen exposure is larger for the Ag-modified surface. These results are explained in terms of a change of the oxidation state of the surface Ag, which is able to acquire some of the extra surface electrons created by the oxygen vacancies and the adsorbed hydrogen atoms. Our findings provide new insights into the reactivity of Ag-modified ceria, which has been proposed as a promising alternative to platinum electrodes in electro-chemical devices.

Research paper thumbnail of Attractive interactions between like-oriented surface steps from an ab initio perspective: Role of the elastic and electrostatic contributions

Physical Review B, 2019

In this work, we show how using the density functional approach, in which the electronic degrees ... more In this work, we show how using the density functional approach, in which the electronic degrees of freedom are separated by the ionic ones, it is possible to individuate and separately study the elastic and electrostatic step interactions, traditionally introduced in the literature as the only two kinds of step interaction expected at T = 0. We have applied the method to the technologically important GaAs(001) surface and found some unexpected results for the relatively short step distances accessible to the ab-initio approaches, contradicting those of the continuum models so far employed for the study of the elastic step interactions: (i) the sign of the step interaction depends on the step termination and is due to the electrostatic interaction; (ii) the elastic interaction does not contribute to the step interaction, contrary to the common belief of a strong elastic repulsive interaction between like-oriented steps. We show that this is due to the electron behavior. When considering only ion displacements and point-like steps as in the continuum theories, we recover the classical results and repulsive step elastic interactions; (iii) the experimentally observed Ab step termination shows a weakly attractive step interaction whereby attractive step interactions between like-oriented steps on an unstrained surface are believed not to exist. The proposed method of separating elastic and electrostatic interactions for further analysis of their dependence on the configurational degrees of freedom can be extended to other defective situations.

Research paper thumbnail of Si and Ge based metallic core/shell nanowires for nano-electronic device applications

Scientific Reports, 2018

One dimensional heterostructure nanowires (NWs) have attracted a large attention due to the possi... more One dimensional heterostructure nanowires (NWs) have attracted a large attention due to the possibility of easily tuning their energy gap, a useful property for application to next generation electronic devices. In this work, we propose new core/shell NW systems where Ge and Si shells are built around very thin As and Sb cores. The modification in the electronic properties arises due to the induced compressive strain experienced by the metal core region which is attributed to the lattice-mismatch with the shell region. As/Ge and As/Si nanowires undergo a semiconducting-to-metal transition on increasing the diameter of the shell. The current-voltage (I-V) characteristics of the nanowires show a negative differential conductance (NDC) effect for small diameters that could lead to their application in atomic scale device(s) for fast switching. In addition, an ohmic behavior and upto 300% increment of the current value is achieved on just doubling the shell region. The resistivity of na...

Research paper thumbnail of Ordering inBxC1−xcompounds with the graphite structure

Physical Review B, 1994

We present a theoretical study of the structural stability of ordered configurations of B Cl soli... more We present a theoretical study of the structural stability of ordered configurations of B Cl solid solutions with the graphite structure. The calculations were carried out self-consistently using normconserving pseudopotentials in the local-density approximation. The cohesive energies of a number of ordered structures at different compositions x have been subjected to a cluster expansion, which enables us to predict the cohesive energy of the random alloy at the corresponding compositions and to gain insight on the tendency of the system towards ordering. We find that, at T=O K, some ordered configurations have a cohesive energy higher than the corresponding random distribution at the same compositions. The inspection of the cluster interactions leads moreover to the conclusion that segregation of boron in graphite is highly discouraged and long-range order is likely to occur.

Research paper thumbnail of Ground-state structures and the random-state energy of the Madelung lattice

Research paper thumbnail of Theory of optical properties of segregated InAs/GaSb superlattices

IEE Proceedings - Optoelectronics, 2003

The authors study the effects of interfacial atomic segregation on the electronic and optical pro... more The authors study the effects of interfacial atomic segregation on the electronic and optical properties of InAs/GaSb superlattices. They describe their atomistic empirical pseudopotential method and test its performance against the available experimental data. They show its ability to predict the band structure dependence on the detailed atomic configuration, and thus to properly account for the effects of interfacial atomic segregation and structural disorder. They also show how their method avoids the approximations underlying the pseudopotential method of Dente and Tilton, which gives different results. The application of the proposed method to the InAs/GaSb superlattices allows the explanation of some observed experimental results, such as: the bandgap difference between ðInAsÞ 8 =ðGaSbÞ 8 superlattices with almost pure InSb-like or GaAs-like interfaces; the large blue shift of the bandgap when the growth temperature of the superlattice increases; and the blue shift of the bandgap of ðInAsÞ 8 =ðGaSbÞ n superlattices with increasing GaSb period n. They present a detailed comparison of their predicted blue shift with that obtained by other theories.

Research paper thumbnail of Real-space description of semiconducting band gaps in substitutional systems

Physical Review B, 1991

The goal of "band-gap engineering' in substitutional lattices is to identify atomic configuration... more The goal of "band-gap engineering' in substitutional lattices is to identify atomic configurations that would give rise to a desired value of the band gap. Yet, current theoretical approaches to the problems, based largely on compilations of band structures for various latice configurations, have not yielded simple rules relating structural motifs to band gaps. We show that the band gap of substitutional A1As/GaAs lattices can be usefully expanded in terms of a hierarchy of contributions from real-space "atomic figures" (pairs, triplets, quadruplets) detemined from first-principles band-structure calculations. Pair figures (up to fourth neighbors) and three-body figures are dominant. In analogy with similar cluster expansions of the total energy, this permits a systematic search among all lattice configurations for those having "special" band gaps. This approach enables the design of substitutional systems with certain band-gap properties by assembling atomic figures. As an illustration, we predict that the [012joriented (AlAs) l/(GaAs)4/(A1As) &/(GaAs)2 superlattice has the largest band gap among all Alo 25Gao 75As lattices with a maximum of ten cations per unit cell.

Research paper thumbnail of Electronic structure and density of states of the randomAl0.5Ga0.5As,GaAs0.5P0.5, andGa0.5In0.5As semiconductor alloys

Physical Review B, 1991

The electronic density of states (DOS), charge densities, equilibrium bond lengths, and optical b... more The electronic density of states (DOS), charge densities, equilibrium bond lengths, and optical bowing of the direct band gaps are calculated for three perfectly random semiconductor alloys within the firstprinciples pseudopotential method using the concept of "special quasirandom structures" (SQS's). The SQS's are periodic structures with moderately large unit cells whose sites are occupied by atoms in a way designed to reproduce the structural features of the infinite, perfectly random substitutional alloys. In avoiding averaging over atoms (as in the virtual-crystal approximation) or over atomic environments (as in the site-coherent-potential approximation), this approach is capable of revealing the multisite nature of chemical disorder, as well as atomic-relaxation effects. We show how the existence of different local environments about chemically identical sites leads to splittings and fine structures in the density of states, and how atomic relaxations are induced by such nonsymmetric environments and lead to significant modifications in these DOS features. The calculated alloy bond lengths and optical-bowing coefficients are found to be in good agreement with experiment. Relaxation-induced splittings in the DOS are offered as predictions for future photoemission studies.

Research paper thumbnail of Predicting structural energies of atomic lattices

Physical Review B, 1991

The complexity of current ab initio quantum-mechanical calculations of the total energy of given ... more The complexity of current ab initio quantum-mechanical calculations of the total energy of given distributions of atoms on a periodic lattice often limits explorations to just a few configurations. We show how such a small number of calculations can be used instead to compute the interaction energies of a generalized Ising model, which then readily provides predicted energies of many more interesting configurations. This is illustrated for A1As/GaAs systems.

Research paper thumbnail of Low-dimensional In-based semiconductor systems

ecent advances in the epitaxial growth of low-dimensional semiconductor systems with techniques s... more ecent advances in the epitaxial growth of low-dimensional semiconductor systems with techniques such as molecular beam epitaxy (MBE) make it possible to form structures of various geometries. The composition profile and the vertical transport properties of self-assembled InAs/GaAs quantum dots (QDs) and quantum rings (QRs) are investigated by X-ray photoemission electron microscopy and conductive atomic force microscopy (1-3). The resulting

Research paper thumbnail of Surface compositional profiles of self-assembled InAs∕GaAs quantum rings

AIP Conference Proceedings, 2010

The composition profile of self-assembled InAs/GaAs quantum rings (QR) is studied both experiment... more The composition profile of self-assembled InAs/GaAs quantum rings (QR) is studied both experimentally and theoretically. 2D surface maps obtained by X-ray photoemission electron microscopy (XPEEM) reveal a non-uniform profile with an In-rich core, corresponding to the central hole of the QR, surrounded by a rim with

Research paper thumbnail of Role of surface structural motifs on the stability and reflectance anisotropy spectra of Sb-rich GaSb(001) reconstructions

Physical Review B, 2011

The structure of the technologically important-but still mostly unknown-GaSb(001)-c(2 × 6) surfac... more The structure of the technologically important-but still mostly unknown-GaSb(001)-c(2 × 6) surface reconstruction is investigated by means of ab initio simulations of reflectance anisotropy spectroscopy (RAS) and total energy calculations. A large number of reconstruction models for the GaSb(001) surface in the Sb-rich coverage regime are considered. The influence of each single surface structural motif on the RAS spectra is studied in detail, as well as their role in the surface stability with regard to application of the electron counting rule (ECR). We interpret the features of the RAS data measured for this reconstruction and suggest a new model for the c(2 × 6) phase. In this model a few Sb atoms in the second layer are randomly substituted by Ga, forming surface antisite defects. When used to fulfill the ECR, this "doping" effect considerably lowers the total energy of the long chain c(2 × 6) reconstruction model, making it competitive with the more stable short-chain (4 × 3) reconstructions. Formation of the surface antisites occurs spontaneously in the presence of dynamical negative charge fluctuations and is favored by the excellent matching between GaSb(001) and metallic Sb and by the natural softness of the Ga-Sb bonds. Calculations of the reflectance anisotropy spectra confirm that this structure is a major component of a largely disordered surface, where motifs of the stable (4 × 3) reconstructions are also present.

Research paper thumbnail of Silicon nanocrystallites in aSiO2matrix: Role of disorder and size

Research paper thumbnail of Excitons in silicon nanocrystallites: The nature of luminescence

Research paper thumbnail of Engineering silicon nanocrystals: Theoretical study of the effect of codoping with boron and phosphorus

Physical Review B, 2007

We show that the optical and electronic properties of nanocrystalline silicon can be efficiently ... more We show that the optical and electronic properties of nanocrystalline silicon can be efficiently tuned using impurity doping. In particular, we give evidence, by means of ab-initio calculations, that by properly controlling the doping with either one or two atomic species, a significant modification of both the absorption and the emission of light can be achieved. We have considered impurities, either boron or phosphorous (doping) or both (codoping), located at different substitutional sites of silicon nanocrystals with size ranging from 1.1 nm to 1.8 nm in diameter. We have found that the codoped nanocrystals have the lowest impurity formation energies when the two impurities occupy nearest neighbor sites near the surface. In addition, such systems present band-edge states localized on the impurities giving rise to a red-shift of the absorption thresholds with respect to that of undoped nanocrystals. Our detailed theoretical analysis shows that the creation of an electron-hole pair due to light absorption determines a geometry distortion that in turn results in a Stokes shift between adsorption and emission spectra. In order to give a deeper insight in this effect, in one case we have calculated the absorption and emission spectra going beyond the single-particle approach showing the important role played by many-body effects. The entire set of results we have collected in this work give a strong indication that with the doping it is possible to tune the optical properties of silicon nanocrystals.

Research paper thumbnail of Adsorption of indium on an InAs wetting layer deposited on the GaAs(001) surface

Physical Review B, 2008

In this work we perform a first-principles study of the adsorption properties of an In adatom dep... more In this work we perform a first-principles study of the adsorption properties of an In adatom deposited on 1.75 monolayers (ML) InAs, forming a wetting layer on GaAs(001) with the α 2 (2 × 4) or β 2 (2×4) reconstruction. The structural properties of these reconstructions have been studied: we determine the equilibrium geometry of the surfaces and their stability for various growth conditions. We have then carried out a detailed study of the potential energy surface (PES) for an In adsorbate, finding the minima and the saddle points. The main characteristics of the PES and the bonding configurations of the In adatom on the surface are analyzed by comparing with analogous studies reported in the literature, trying to extract the effects due to: (i) the compressive strain to which the InAs adlayer is subjected, (ii) the particular surface reconstruction, and (iii) the wetting layer composition. We found that, in general, stable adsorption sites are located at: (i) locations besides the As in-dimers, (ii) positions bridging two As in-dimers, (iii) between two adjacent ad-dimers (only in β 2), and (iv) locations bridging two As ad-dimers. We find also other shallower adsorption sites which are more reconstruction specific due to the lower symmetry of the α 2 reconstruction compared to the β 2 reconstruction.

Research paper thumbnail of Ab-Initio Investigation of the Polarization Anisotropy of the Optical Absorption in (InGa)As–InP Superlattices

physica status solidi (a), 1998

ABSTRACT

Research paper thumbnail of Surface Effects on the Atomic and Electronic Structure of Unpassivated GaAs Nanowires

ACS Nano, 2010

On the basis of accurate ab initio calculations, we propose a model for predicting the stability ... more On the basis of accurate ab initio calculations, we propose a model for predicting the stability of III؊V nanowires (NW) having different side walls and ridge configurations. The model allows us to obtain the NW formation energies by performing calculations only on relatively "small" systems, small diameter NWs and flat surfaces, to extract the contributions to the stability of each structural motif. Despite the idea illustrated here for the case of hexagonally shaped GaAs NWs grown along the [111]/[0001] direction, the method can also be applied generally to other differently shaped and oriented III؊V NWs. The model shows that NW surfaces (side walls plus ridges) mainly determine the NW stability, so the changes to the surface structure (e.g., induced by defects or different growth conditions) would modify the final NW structure in a remarkable way. We find that wurtzite and zinc blende nanowires have similar energies over a wide diameter range, thus explaining the observed polytypism. Furthermore, new more stable ridge reconstructions are proposed for zinc blende nanowires. The surface-related structural motifs also have clear fingerprints on the NW electronic structure. We find that the more stable nanowires are all semiconducting. The band gaps are ruled by surface states and do not follow the trend mandated by the quantum confinement effect. Small diameter wurtzite nanowires have an indirect band gap, but for some of them, an indirect to direct transition can be foreseen to occur at larger diameters. Surface states have a larger impact on the zinc blende NW band gaps than on the wurtzite NW ones. Zinc blende nanowire band gaps reduce significantly with increasing nanowire radius, reaching the bulk value at a diameter of about 30 Å. The surface structure and the high surface related DOS below the conduction band are going to affect the nanowire dopant incorporation and efficiency when doping is carried out during the NW growth.