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Ivan MARRI

Professore Associato
Dipartimento di Scienze e Metodi dell'Ingegneria

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Pubblicazioni

2023 - Interplay of Quantum Confinement and Strain Effects in Type I to Type II Transition in GeSi Core-Shell Nanocrystals [Articolo su rivista]
Marri, Ivan; Grillo, Simone; Amato, Michele; Ossicini, Stefano; Pulci, Olivia
abstract

The electronic properties of hydrogenated, spherical SiGe and GeSi core-shell nanocrystals, with a diameter ranging from 1.8 to 4.0 nm, are studied within density functional theory. Effects induced by quantum confinement and strain on the near band-edge state localization, as well as the band-offset properties between Si and Ge regions, are investigated in detail. On the one hand, we prove that SiGe core-shell nanocrystals always show a type II band-offset alignment, with the HOMO mainly localized on the Ge shell region and the LUMO mainly localized on the Si core region. On the other hand, our results point out that a type II offset cannot be observed in small (diameter less than 3 nm) GeSi core- shell nanocrystals. In these systems, quantum confinement and strain drive the near-band-edge states to be mainly localized on Ge atoms, i.e., in the core region. In larger GeSi core-shell nanocrystals, instead, the formation of a type II offset can be engineered by playing with both core and shell thickness. The factors which determine the band-offset character at the Ge/Si interface are discussed in detail.

2022 - Evolution of the Electronic and Optical Properties of Meta-Stable Allotropic Forms of 2D Tellurium for Increasing Number of Layers [Articolo su rivista]
Grillo, Simone; Pulci, Olivia; Marri, Ivan
abstract

In this work, ab initio Density Functional Theory calculations are performed to investigate the evolution of the electronic and optical properties of 2D Tellurium—called Tellurene—for three different allotropic forms (α-, β- and γ-phase), as a function of the number of layers. We estimate the exciton binding energies and radii of the studied systems, using a 2D analytical model. Our results point out that these quantities are strongly dependent on the allotropic form, as well as on the number of layers. Remarkably, we show that the adopted method is suitable for reliably predicting, also in the case of Tellurene, the exciton binding energy, without the need of computationally demanding calculations, possibly suggesting interesting insights into the features of the system. Finally, we inspect the nature of the mechanisms ruling the interaction of neighbouring Tellurium atoms helical chains (characteristic of the bulk and α-phase crystal structures). We show that the interaction between helical chains is strong and cannot be explained by solely considering the van der Waals interaction.

2021 - Electronic and Optical Properties of Si, Ge and Sige Low Dimensional Systems: Ab-Initio Results [Abstract in Rivista]
Marri, Ivan; Ossicini, Stefano
abstract

In recent years,considerable efforts have been done to study silicon, germanium, and silicon/germanium slabs, nanocrystals, and nanowires for optoelectronics and photovoltaic solar energy applications. These zero- one- and two-dimensional systems, with sizes ranging from few to some tenths of nanometers, show unique electronic, optical, and transport properties that are intrinsically associated with their low dimensionality and to the quantum confinement effect. The possibility of understanding the microscopic properties of these systems and modulating their characteristics by doping and passivation can open new perspectives in the development of new, advanced, photovoltaics and optoelectronics devices. In this talk, we will discuss ab-initio theoretical results obtained by our group in the study of electronic, optical, and transport properties of silicon, germanium, and silicon/germanium low dimensional systems. The role played by size, passivation, and doping will be discussed. Moreover, we will show how the interaction between different nanostructures is a promising route to foster the establishment of third-generation photovoltaics.

2021 - Multiple exciton generation in isolated and interacting silicon nanocrystals [Articolo su rivista]
Marri, I.; Ossicini, S.
abstract

An important challenge in the field of renewable energy is the development of novel nanostructured solar cell devices which implement low-dimensional materials to overcome the limits of traditional photovoltaic systems. For optimal energy conversion in photovoltaic devices, one important requirement is that the full energy of the solar spectrum is effectively used. In this context, the possibility of exploiting features and functionalities induced by the reduced dimensionality of the nanocrystalline phase, in particular by the quantum confinement of the electronic density, can lead to a better use of the carrier excess energy and thus to an increment of the thermodynamic conversion efficiency of the system. Carrier multiplication, i.e. the generation of multiple electron-hole pairs after absorption of one single high-energy photon (with energy at least twice the energy gap of the system), can be exploited to maximize cell performance, promoting a net reduction of loss mechanisms. Over the past fifteen years, carrier multiplication has been recorded in a large variety of semiconductor nanocrystals and other nanostructures. Owing to the role of silicon in solar cell applications, the mission of this review is to summarize the progress in this fascinating research field considering carrier multiplication in Si-based low-dimensional systems, in particular Si nanocrystals, both from the experimental and theoretical point of view, with special attention given to the results obtained by ab initio calculations.

2020 - Ab initio studies of the optoelectronic structure of undoped and doped silicon nanocrystals and nanowires: the role of size, passivation, symmetry and phase [Articolo su rivista]
Ossicini, S.; Marri, I.; Amato, M.; Palummo, M.; Canadell, E.; Rurali, R.
abstract

Silicon nanocrystals and nanowires have been extensively studied because of their novel properties and their applications in electronic, optoelectronic, photovoltaic, thermoelectric and biological devices. Here we discuss results from ab initio calculations for undoped and doped Si nanocrystals and nanowires, showing how theory can aid and improve comprehension of the structural, electronic and optical properties of these systems.

2020 - Surface chemistry effects on work function, ionization potential and electronic affinity of Si(100), Ge(100) surfaces and SiGe heterostructures [Articolo su rivista]
Marri, Ivan; Amato, Michele; Bertocchi, Matteo; Ferretti, Andrea; Varsano, Daniele; Ossicini, Stefano
abstract

We combine density functional theory and many body perturbation theory to investigate the electronic properties of Si(100) and Ge(100) surfaces terminated with halogen atoms (–I, –Br, –Cl, –F) and other chemical functionalizations (–H, –OH, –CH3) addressing the absolute values of their work function, electronic affinity and ionization potential. Our results point out that electronic properties of functionalized surfaces strongly depend on the chemisorbed species and much less on the surface crystal orientation. The presence of halogens at the surface always leads to an increment of the work function, ionization potential and electronic affinity with respect to fully hydrogenated surfaces. On the contrary, the presence of polar –OH and –CH3 groups at the surface leads to a reduction of the aforementioned quantities with respect to the H-terminated system. Starting from the work functions calculated for the Si and Ge passivated surfaces, we apply a simple model to estimate the properties of functionalized SiGe surfaces. The possibility of modulating the work function by changing the chemisorbed species and composition is predicted. The effects induced by different terminations on the band energy line-up profile of SiGe surfaces are then analyzed. Interestingly, our calculations predict a type-II band offset for the H-terminated systems and a type-I band offset for the other cases.

2019 - Many-body perturbation theory calculations using the yambo code [Articolo su rivista]
Sangalli, D.; Ferretti, A.; Miranda, H.; Attaccalite, C.; Marri, I.; Cannuccia, E.; Melo, P.; Marsili, M.; Paleari, F.; Marrazzo, A.; Prandini, G.; Bonfa, P.; Atambo, M. O.; Affinito, F.; Palummo, M.; Molina-Sanchez, A.; Hogan, C.; Gruning, M.; Varsano, D.; Marini, A.
abstract

yambo is an open source project aimed at studying excited state properties of condensed matter systems from first principles using many-body methods. As input, yambo requires ground state electronic structure data as computed by density functional theory codes such as Quantum ESPRESSO and Abinit. yambo's capabilities include the calculation of linear response quantities (both independent-particle and including electron-hole interactions), quasi-particle corrections based on the GW formalism, optical absorption, and other spectroscopic quantities. Here we describe recent developments ranging from the inclusion of important but oft-neglected physical effects such as electron-phonon interactions to the implementation of a real-time propagation scheme for simulating linear and non-linear optical properties. Improvements to numerical algorithms and the user interface are outlined. Particular emphasis is given to the new and efficient parallel structure that makes it possible to exploit modern high performance computing architectures. Finally, we demonstrate the possibility to automate workflows by interfacing with the yambopy and AiiDA software tools.

2018 - First Principle Studies of B and P Doped Si Nanocrystals [Articolo su rivista]
Marri, Ivan; Degoli, Elena; Ossicini, Stefano
abstract

The properties of n- and p-doped silicon nanocrystals obtained through ab initio calculations are reviewed here. The aim is the understanding of the effects induced by substitutional doping on the structural, electronic and optical properties of free-standing and matrix-embedded Si nanocrystals. The preferential positioning of the dopants and their effects on the structural properties with respect to the undoped case, as a function of the nanocrystals diameter and termination, are identified through total-energy considerations. The localization of the acceptor and donor related levels in the band gap of the Si nanocrystals, together with the impurity activation energy, are discussed as a function of the nanocrystals size. The dopant induced differences in the optical properties with respect to the undoped case are presented. Finally, the case of B and P co-doped nanocrystals is discussed showing that if carriers are perfectly compensated, the Si nanocrystals undergo a minor structural distortion around the impurities inducing a significant decrease of the impurities formation energies with respect to the single doped case. Due to co-doping, additional peaks are introduced in the absorption spectra, giving rise to a size-dependent red shift of the absorption spectra.

2018 - First Principles Modeling of Si/Ge Nanostructures for Photovoltaic and Optoelectronic Applications [Articolo su rivista]
Marri, I.; Amato, M.; Guerra, R.; Ossicini, S.
abstract

We discuss results of ab initio calculations for Si, Ge, and Si/Ge nanowires and nanocrystals showing that theory can improve the comprehension of the properties of these systems. First, we consider doped and undoped freestanding hydrogenated nanowires and we explore their properties as a function of the size, geometry, and composition. Secondly, we focus the discussion on the electronic properties of matrix embedded Si, Ge, and Si/Ge nanocrystals by pointing out the role played by composition, quantum confinement, and strain. The discussed results show that, for Si/Ge nanowires, the interface between Si and Ge region plays an important role determining, in some case, the formation of a type II band offset, which is essential for photovoltaic applications. Moreover, for Si/Ge core–shell nanowires, it is shown that: i) selective doping results in the formation of hole or electron accumulation, with interesting consequences for the use of these materials in thermoelectrics and ii) through compensated doping, it is possible to tune the optical properties of these systems. For the embedded nanocrystals, the outcomes suggest that Ge nanocrystals can be suitable as optical absorption centers and that Si/Ge nanocrystals, owing to the localization of the band edge states, are interesting for photovoltaic cells.

2018 - First-principle investigations of carrier multiplication in Si nanocrystals: A short review [Relazione in Atti di Convegno]
Marri, Ivan; Ossicini, Stefano
abstract

Carrier Multiplication (CM) is a Coulomb-driven non-radiative recombination mechanism which leads to the generation of multiple electron-hole pairs after absorption of a single high-energy photon. Recently a new CM process, termed space separated quantum cutting, was introduced to explain a set of new experiments conducted in dense arrays of silicon nanocrystals. The occurrence of this effect was hypothesized to generate the formation of Auger unaffected multiexciton configurations constituted by single electron-hole pairs distributed on different interacting naocrystals. In this work we discuss ab-initio results obtained by our group in the study of CM effects in systems of strongly interacting silicon nanocrystals. By solving a set of rate equations, we simulate the time evolution of the number of electron-hole pairs generated in dense arrays of silicon nanocrystals after absorption of high energy photons, by describing the circumstances under which CM dynamics can lead to the generation of Auger unaffected multiexciton configurations.

2017 - Carrier Multiplication in Silicon Nanocrystals: Theoretical Methodologies and Role of the Passivation [Articolo su rivista]
Marri, I.; Govoni, M.; Ossicini, S.
abstract

Carrier multiplication is a non-radiative recombination mechanism that leads to the generation of two or more electron–hole pairs after absorption of a single photon. By reducing the occurrence of dissipative effects, this process can be exploited to increase solar cell performance. In this work, we introduce two different theoretical fully ab initio tools that can be adopted to study carrier multiplication in nanocrystals. The tools are described in detail and compared. Subsequently, we calculate carrier multiplication lifetimes in H- and OH-terminated silicon nanocrystals, pointed out the role played by the passivation on the carrier multiplication processes.

2017 - Doped and codoped silicon nanocrystals: The role of surfaces and interfaces [Articolo su rivista]
Marri, Ivan; Degoli, Elena; Ossicini, Stefano
abstract

Si nanocrystals have been extensively studied because of their novel properties and their potential applications in electronic, optoelectronic, photovoltaic, thermoelectric and biological devices. These new properties are achieved through the combination of the quantum confinement of carriers and the strong influence of surface chemistry. As in the case of bulk Si the tuning of the electronic, optical and transport properties is related to the possibility of doping, in a controlled way, the nanocrystals. This is a big challenge since several studies have revealed that doping in Si nanocrystals differs from the one of the bulk. Theory and experiments have underlined that doping and codoping are influenced by a large number of parameters such as size, shape, passivation and chemical environment of the silicon nanocrystals. However, the connection between these parameters and dopant localization as well as the occurrence of self-purification effects are still not clear. In this review we summarize the latest progress in this fascinating research field considering free-standing and matrix-embedded Si nanocrystals both from the theoretical and experimental point of view, with special attention given to the results obtained by ab-initio calculations and to size-, surface- and interface-induced effects.

2017 - Multiple excitation generation in silicon nanocrystals [Relazione in Atti di Convegno]
Marri, Ivan; Ossicini, Stefano; Govoni, M.
abstract

We present density functional theory calculations of carrier multiplication processes in a system of strongly coupled silicon nanocrystals

2017 - TEACHING SILICON NEW POSSIBILITIES [Articolo su rivista]
Ossicini, S.; Marri, I.
abstract

Although silicon is the most widely used material in microelectronics, photonics and photovoltaics, in recent years concerns about the evolu-tion of these sectors have been raised which seem related to funda-mental materials and processing aspects. Example are the limitations of the operating speed of microelectronic devices due to the intercon-nect, the lack of optical second-order nonlinearities in bulk silicon, the necessity to improve the efficiency in solar cells. Silicon based systems with new functionalities able to solve these and other prob-lems are highly desirable. In the last years, our group has developed new theoretical tools to study structural, electronic and optical proper-ties of silicon based systems, in particular silicon nanostructures. Our analyses have permitted to predict new properties of silicon, thus opening new possibilities for applications of this material in different technological fields. Here we review our main results.

2017 - Tuning the Work Function of Si(100) Surface by Halogen Absorption: A DFT Study [Articolo su rivista]
Bertocchi, M.; Amato, M.; Marri, I.; Ossicini, S.
abstract

First-principles calculations of work function tuning induced by different chemical terminations on Si(100) surface are presented and discussed. We find that the presence of halogen atoms (I, Br, Cl, and F) leads to an increase of the work function if compared to the fully hydrogenated surface. This is a quite general effect and is directly linked to the chemisorbed atoms electronegativity as well as to the charge redistribution at the interface. All these results are examined with respect to previous theoretical works and experimental data obtained for the (100) as well as other Si surface orientations. Based on this analysis, we argue that the changes in the electronic properties caused by variations of the interfacial chemistry strongly depend on the chemisorbed species and much less on the surface crystal orientation.

2016 - First-principles calculations of electronic coupling effects in silicon nanocrystals: Influence on near band-edge states and on carrier multiplication processes [Articolo su rivista]
Marri, Ivan; Govoni, Marco; Ossicini, Stefano
abstract

Arrays of closely packed nanocrystals show interesting properties that can be exploited to induce new features in nanostructured optoelectronic devices. In this work we study, by first principles calculations, effects induced on near band-edge states and on carrier multiplication by nanocrystals interplay. By considering both hydrogenated and oxygenated structures, we prove that interaction between silicon nanocrystals can alter both the energy gap of the system and dynamics of excited states with a relevance that depends on the nanocrystal-nanocrystal separation, on nanocrystals orientation and on nanocrystals surface properties.

2016 - Silicon Nanocrystals for Photonics and Photovoltaics: Ab-initio Results [Capitolo/Saggio]
Ossicini, Stefano; Govoni, Marco; Guerra, Roberto; Marri, Ivan
abstract

Review Article on electronic and optical properties of silcon nanocrystals for photonic and photovoltaic applications

2015 - Carrier Multiplication in Isolated and Interacting Silicon Nanocrystals [Capitolo/Saggio]
Marri, Ivan; Ossicini, Stefano; Govoni, M.
abstract

n/a

2015 - Carrier multiplication in silicon nanocrystals: ab initio results [Articolo su rivista]
Marri, Ivan; Govoni, Marco; Ossicini, Stefano
abstract

One of the most important goals in the field of renewable energy is the development of original solar cell schemes employing new materials to overcome the performance limitations of traditional solar cell devices. Among such innovative materials, nanostructures have emerged as an important class of materials that can be used to realize efficient photovoltaic devices. When these systems are implemented into solar cells, new effects can be exploited to maximize the harvest of solar radiation and to minimize the loss factors. In this context, carrier multiplication seems one promising way to minimize the effects induced by thermalization loss processes thereby significantly increasing the solar cell power conversion. In this work we analyze and quantify different types of carrier multiplication decay dynamics by analyzing systems of isolated and coupled silicon nanocrystals. The effects on carrier multiplication dynamics by energy and charge transfer processes are also discussed.

2014 - Red-shifted carrier multiplication energy threshold and exciton recycling mechanisms in strongly interacting silicon nanocrystals. [Articolo su rivista]
Marri, Ivan; Govoni, Marco; Ossicini, Stefano
abstract

We present density functional theory calculations of carrier multiplication properties in a system of strongly coupled silicon nanocrystals. Our results suggest that nanocrystal-nanocrystal interaction can lead to a reduction of the carrier multiplication energy threshold without altering the carrier multiplication efficiency at high energies, in agreement with experiments. The time evolution of the number of electron-hole pairs generated in a system of strongly interacting nanocrystals upon absorption of high-energy photons is analyzed by solving a system of coupled rate equations, where exciton recycling mechanisms are implemented. We reconsider the role played by Auger recombination which is here accounted also as an active, nondetrimental process.

2012 - Carrier multiplication between interacting nanocrystals for fostering silicon-based photovoltaics [Articolo su rivista]
Govoni, Marco; Marri, Ivan; Ossicini, Stefano
abstract

The conversion of solar radiation into electric current with high efficiency is one of the most important topics of modern scientific research, as it holds great potential as a source of clean and renewable energy. Exploitation of interaction between nanocrystals seems to be a promising route to the establishment of third-generation photovoltaics. Here, we adopt a fully ab initio scheme to estimate the role of nanoparticle interplay in the carrier multiplication dynamics of interacting silicon nanocrystals. Energy and charge transfer-based carrier multiplication events are studied as a function of nanocrystal separation, demonstrating the benefits induced by the wavefunction sharing regime. We prove the relevance of these recombinative mechanisms for photovoltaic applications in the case of silicon nanocrystals arranged in dense arrays, quantifying at an atomic scale which conditions maximize the outcome.

2011 - Auger recombination in Si and GaAs semiconductors : Ab initio results [Articolo su rivista]
Govoni, Marco; Marri, Ivan; Ossicini, Stefano
abstract

A detailed description, at the atomistic scale, of the dynamics of excess electrons and holes is fundamentalin order to improve the performance of many optoelectronic devices. Among all recombination processes,nonradiative decay paths play a fundamental role in most semiconductor devices, such as optoelectronic devicesand solar cells, limiting their efficiency. In this work, a precise ab initio analysis of the direct Auger recombinationprocesses in both n- and p-type Si and GaAs crystals is presented. Our simulations of minority carrier Augerlifetimes rely on an accurate electronic band structure, calculated using density functional theory with theinclusion of quasiparticle corrections. The results obtained are in good agreement with experimental data forboth n-Si and p-GaAs, proving the importance of the direct Auger recombination mechanism in such systems. Onthe contrary, we show that different nonradiative recombination paths are necessary to explain the experimentalresults for both p-Si and n-GaAs.

2011 - The NASCEnT project [Relazione in Atti di Convegno]
Janz, S.; Loper, P.; Schnabel, M.; Zacharias, M.; Hiller, D.; Gutsch, S.; Hartel, A. M.; Summonte, C.; Xanino M., Allegrezza; Ossicini, Stefano; Guerra, Roberto; Marri, Ivan; Garrido, B.; Hernandez, S.; Lopez Vidrier, J.; Valenta, J.; Kubera, T.; Foti, M.; Gerardi, C.
abstract

NAscent Project

2009 - Ab-initio calculations of luminescence and optical gain properties in silicon nanostructures [Articolo su rivista]
Degoli, Elena; Guerra, Roberto; Iori, Federico; Magri, Rita; Marri, Ivan; O., Pulci; Bisi, Olmes; Ossicini, Stefano
abstract

Density-functional and many body perturbation theory calculations have been carried out in order to study the optical properties both in the ground and excited state configurations, of silicon nanocrystals in different conditions of surface passivation. Starting from hydrogenated clusters, we have considered different Si/O bonding geometries at the interface. We provide strong evidence that not only the quantum confinement effect but also the chemistry at the interface has to be taken into account in order to understand the physical properties of these systems. In particular, we show that only the presence of a surface Si–O–Si bridge bond induces an excitonic peak in the emission-related spectra, redshifted with respect to the absorption onset, able to provide an explanation for both the observed Stokes shift and the near-visible PL experimentally observed in Si-nc. For the silicon nanocrystals embedded in a SiO2 matrix, the optical properties are discussed in detail. The strong interplay between the nanocrystal and the surrounding host environment and the active role of the interface region between them is pointed out, in very good agreement with the experimental results. For each system considered, optical gain calculations have been carried out giving some insights on the system characteristics necessary to optimize the gain performance of Si-nc.

2009 - Optical properties of silicon nanocrystallites in SiO2 matrix:Crystalline vs. amorphous case [Articolo su rivista]
Guerra, Roberto; Marri, Ivan; Magri, Rita; L., Martin Samos; O., Pulci; Degoli, Elena; Ossicini, Stefano
abstract

Within a first-principles framework we show the dependenceof the optical properties of silicon nanocrystallites embeddedin a silica matrix on the crystalline vs. amorphous order ofthe system. Moreover we calculate how many-body effectsmodify the electronic and optical properties of the embeddedsilicon nanodots. A discussion about the different roles playedby dimensionality, interface properties, disorder and excitoniceffects on the electronic and optical properties of the confinednanostructures is presented.

2009 - Silicon nanocrystallites in a SiO2 matrix: Role of disorder and size [Articolo su rivista]
Guerra, Roberto; Marri, Ivan; Magri, Rita; L., Martin Samos; O., Pulci; Degoli, Elena; Ossicini, Stefano
abstract

We compare, through first-principles pseudopotential calculations, the structural, electronic, and optical properties of different size silicon nanoclusters embedded in a SiO2 crystalline or amorphous matrix with that of freestanding, hydrogenated, and hydroxided silicon nanoclusters of corresponding size and shape. We findthat the largest effect on the optoelectronic behavior is due to the amorphization of the embedded nanocluster. In that, the amorphization reduces the fundamental gap while increasing the absorption strength in the visible range. Increasing the nanocluster size does not change substantially this picture but only leads to the reduction in the absorption threshold, following the quantum confinement rule. Finally, through the calculation of the optical absorption spectra both in an independent-particle and a many-body approach, we show that the effect of local fields is crucial for describing properly the optical behavior of the crystalline case while it is of minor importance for amorphous systems.

2009 - The Role of the Surface Coverage on the Structural and the Electronic Properties of TiO2 Nanocrystals [Articolo su rivista]
A., Iacomino; G., Cantele; F., Trani; D., Ninno; Marri, Ivan; Ossicini, Stefano
abstract

We present here a characterization of TiO2 0D nanoclusters and 1D nanowires in the framework of ab initio density functional theory (DFT) calculations. We analyze the effect of the surface coverage by functionalizing dangling bonds with simple adsorbates modeling the basical interactions of TiO2 nanosystems with the hydration sphere. We thus address the electronic reorganization and the surface role in determining the overall properties of the nanostructures. The structural reconstruction is found to depend on the surface coverage and the experimental evidences on the structural variations can be explained by a topological analysis of the Ti-O bonds. Q-size effects are observed through the bandgap widening, but the surface competes to determine the energy distribution of the electronic levels. The hydrogenated nanocrystals do show occupied levels at the bottom of the conduction bands, which can enhance the conductive properties of the nanowires. In the hydrogenated cluster such levels present a localized charge distribution with strong similarities (orbital character, energy position) to the defect states arising after oxygens desorption. From the analysis of the electronic density of states we found that Ti-H bonds induce in-gap states above the valence bands, whereas hydration leads to occupied states that shift the valence bands to lower binding energies.

2008 - First-Principles Study of Silicon Nanocrystals: Structural and Electronic Properties, Absorption, Emission, and Doping. [Articolo su rivista]
Ossicini, Stefano; Bisi, Olmes; Degoli, Elena; Marri, Ivan; F., Iori; E., Luppi; Magri, Rita; R., Poli; G., Cantele; D., Ninno; F., Trani; M., Marsili; O., Pulci; V., Olevano; M., Gatti; K., GAAL NAGY; A., Incze; G., Onida
abstract

Total energy calculations within the Density Functional Theory have been carried out in order to investigate the structural, electronic, and optical properties of un-doped and doped silicon nanostructures of different size and different surface terminations. In particular the effects induced by the creation of an electron-hole pair on the properties of hydrogenated silicon nanoclusters as a function of dimension are discussed in detail showing the strong interplay between the structural andoptical properties of the system. The distortion induced on the structure by an electronic excitation of the cluster is analyzed and considered in the evaluation of the Stokes shift between absorption and emission energies. Besides we show how many-body effects crucially modify the absorption and emission spectra of the silicon nanocrystals. Starting from the hydrogenated clusters, different Si/O bonding at the cluster surface have been considered. We found that the presence of a Si-O-Si bridge bond originates significative excitonic luminescence features in the near-visible range. Concerning the doping, we consider B and P single- and co-doped Si nanoclusters. The neutral impurities formation energies are calculated and their dependence on the impurity position within the nanocrystal is discussed. In the case of co-doping the formation energy is strongly reduced,favoring this process with respect to the single doping. Moreover the band gap and the optical threshold are clearly red-shifted with respect to that of the pure crystals showing the possibility ofan impurity based engineering of the absorption and luminescence properties of Si nanocrystals.

2008 - Oxygen vacancy effects on the Schottky barrier height at the Au/TiO2(100) interface: a first principle study [Articolo su rivista]
Marri, Ivan; Ossicini, Stefano
abstract

Motivated by the pioneering work of McFarland and Tang on multilayer photovoltaic devices, we discuss here structural and electronic properties of the Au/TiO2(110) interface for a coverage of 1 monolayer (1 ML) of gold, both for a stoichiometric and a reduced (Ti-rich) rutile surface. A detailed analysis of theSchottky barrier height for such systems is presented and the effects generated on this barrier by the presence of an oxygen vacancy (localized on the rutile support) are discussed.

2008 - Structural, Electronic and Surface Properties of Anatase TiO2 Nanocrystals from First Principles [Articolo su rivista]
A., Iacomino; G., Cantele; D., Ninno; Marri, Ivan; Ossicini, Stefano
abstract

The structural and electronic properties of anatase TiO2 nanocrystals (NCs) are investigated through firstprinciplescalculations. The dependence of the structural properties,e.g., NC volume variations on the surface chemistry is discussed by considering two different surface coverages (dissociated water and hydrogens). Both prevent a pronounced reconstruction of the surface, thus ensuring a better crystalline organization of the atoms with respect to the bare NC. In particular, the results for the hydrated NC do show the largest overlap with theexperimental findings. The band-gap blueshift with respect to the bulk shows up for both the bare and the hydrated NC, whereas hydrogen coverage or oxygen desorption from the bare NCs induce occupied electronic states below the conduction levels thus hindering the gap opening due to quantum confinement. These states are spatially localized in a restricted region and can be progressively annihilated by oxygen adsorption onundercoordinated surface titanium atoms. Formation energy calculations reveal that surface hydration leads to the most stable NC, in agreement with the experimental finding that the truncated bipyramidal morphology is typical of the moderate acidic environment. Oxygen desorption from the bare NC is unfavorable, thus highlighting the stabilizing role of surface oxygen stoichiometry for TiO2. Available experimental data on theelectronic and structural properties of TiO2 NCs are summarized and compared with our results.

2008 - THEORETICAL STUDIES OF ABSORPTION, EMISSION AND GAIN IN SILICON NANOSTRUCTURES [Capitolo/Saggio]
Degoli, Elena; Guerra, Roberto; Iori, Federico; Magri, Rita; Marri, Ivan; Ossicini, Stefano
abstract

Density-functional and many body perturbation theory calculations have been carried out in order to study the structural, electronic, and opti- cal properties both in ground and excited state configuration, of silicon nanocrystals in different conditions of surface passivation and doping. Starting from hydrogenated clusters, we have considered different Si/O bonding geometries at the interface. We provide strong evidences that not only the quantum confinement effect but also the chemistry at the interface has to be taken into account in order to understand the phys- ical properties of these systems. In particular we show that only the presence of a surface Si-O-Si bridge bond induce an excitonic peak in the emission-related spectra, redshifted with respect to the absorption onset, able to provide an explanation for both the observed Stokes shift and the near-visible photoluminescence (PL) experimentally observed in Si-nc. For the silicon nanocrystals embedded in a SiO2 matrix, the electronic and optical properties are discussed in detail. The strong in- terplay between the nanocrystal and the surrounding host environment and the active role of the interface region between them is pointed out, in very good agreement with the experimental results. Finally, concerning doping, we will show that, thanks to electronic transitions between donor and acceptor states present at the band edges and considering also the effect of quantum confinement it is possible to engineer the absorp- tion and emission spectra of Si nanocrystals. For each considered system optical gain calculations have been carried out giving some insights on the system characteristics necessary to optimize the gain performance of Si-nc.

2007 - Engineering silicon nanocrystals: Theoretical study of the effect of codoping with boron and phosphorus [Articolo su rivista]
Iori, F.; Degoli, E.; Magri, R.; Marri, I.; Cantele, G.; Ninno, D.; Trani, F.; Pulci, O.; Ossicini, S.
abstract

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 ele...

2006 - Angular and polarization dependence of x-ray resonant elastic scattering in transition metals [Articolo su rivista]
Marri, Ivan; Bertoni, Carlo Maria; Ferriani, P; Joly, Y.
abstract

We present a comparison of the x-ray elastic scattering at the 2p threshold of Ni calculated both in a single-ion picture with a full description of the multiplet manifolds of the electron configurations and in a one-electron model using a multiple scattering approach for the excited intermediate state. We study the case of perpendicular geometry of the circularly polarized incoming light and analyze the variation of the intensity of the emitted light along a conical scan. We discuss the results obtained in the two approaches for the total and dichroic signals, that confirm the presence of a significant anisotropy as previously predicted in the inelastic case.

2006 - Doping in silicon nanocrystals: An ab initio study of the structural, electronic and optical properties [Articolo su rivista]
Iori, Federico; Degoli, Elena; Luppi, Eleonora; Magri, Rita; Marri, Ivan; G., Cantele; D., Ninno; F., Trani; Ossicini, Stefano
abstract

There are experimental evidences that doping control at the nanoscale can significantly modify the optical properties with respect to the pure systems. This is the case of silicon nanocrystals (Si-nc), for which it has been shown that the photoluminescence (PL) peak can be tuned also below the bulk Si band gap by properly controlling the impurities, for example by boron (B) and phosphorus (P) codoping. In this work, we report on an ab initio study of impurity states in Si-nc. We consider B and P substitutional impurities for Si-nc with a diameter up to 2.2 nm. Formation energies (FEs), electronic, optical and structural properties have been determined as a function of the cluster dimension. For both B-doped and P-doped Si-nc the FE increases on decreasing the dimension, showing that the substitutional doping gets progressively more difficult for the smaller nanocrystals. Moreover, subsurface impurity positions result to be the most stable ones. The codoping reduces the FE strongly favoring this process with respect to the simple n-doping or p-doping. Such an effect can be attributed to charge compensation between the donor and the acceptor atoms. Moreover, smaller structural deformations, with respect to n-doped and p-doped cases, localized only around the impurity sites are observed. The band gap and the optical threshold are largely reduced with respect to the undoped Si-nc showing the possibility of an impurity-based engineering of the Si-nc PL properties. (c) 2006 Elsevier B.V. All rights reserved.

2006 - Optical dichroism: E1-M1 integral relations [Articolo su rivista]
Marri, Ivan; P., Carra; Bertoni, Carlo Maria
abstract

The present paper discusses optical dichroism in noncentrosymmetric systems. The cases of circular and linear polarizations are considered. Integrated spectra are interpreted using effective two-electron operators, which are derived within a localized (atomic) model. As a consequence, our theory is not suitable for quantitative predictions; nevertheless, it identifies microscopic origins of natural, nonreciprocal and Jones' dichroisms.

2005 - Magneto-Electric Interactions Probed by X-ray Optical Activity [Articolo su rivista]
J., Goulon; A., Rogalev; F., Wilhelm; N., Jaouen; C., Goulon Ginet; P., Carra; Marri, Ivan; Brouder, C. h.
abstract

X-ray Optical Activity (XOA) is caused mainly by E1E2 interference terms in systems with odd space Parity (P). Extra emphasis is put below on nonreciprocal XOA effects in Magneto-Electric (ME) solids in which both Parity and Time- reversal (ð) symmetries are broken whereas the magnetic structure remains invariant in the product Pð. Two types of dichroism related to non-reciprocal XOA have been observed experimentally: (i) The X-ray Magnetochiral dichroism (XMðD) detected in the antiferromagnetic (AFM) phase of Cr2O3 is associated with the (unpolarized) Stokes component S0 ; (ii) A non-reciprocal X-ray Magnetic Linear Dichroism (XMLD) was also observed in the low temperature AFM phase a (V1−xCrx)2O3 single crystal and is associated with the Stokes components S1 or S2. Edge selective XOA sum rules were derived recently which give a unique access to a whole family of spherical ME operators mixing orbitals of different parities in what is assumed to be the ground state. It is proved that the orbital anapole moment ð(1) is primarily responsible for the XMðD signal measured both 0 with a single crystal or a powdered sample of Cr2O3. It is suggested that the non- reciprocal XOA in both Cr2O3 and (V1−xCrx)2 O3 is due to partially unquenched angular moments that had a different quantization axis from the spins.

2004 - Scattering operators for E1-E2 x-ray resonant diffraction [Articolo su rivista]
Marri, Ivan; P., Carra
abstract

Resonant x-ray diffraction in noncentrosymmetric crystals is studied by considering E1-E2 processes in the fast-collision approximation. The scattering amplitude is expressed in terms of polar and magnetoelectric operators of the valence states, which are involved in the resonance. Near-edge Bragg peaks from ferroelectric, antiferroelectric, and magnetoelectric structures are predicted.

2003 - X-ray Optical Activity: Applications of Sum Rules [Articolo su rivista]
J., Goulon; A., Rogalev; F., Wilhelma; C., Goulon Ginet; P., Carra; Marri, Ivan; Brouder, C. h.
abstract

Edge-selective sum rules are proposed for a variety of X-ray dichroisms related to natural or non- reciprocal optical activity. Four spherical operators are identified that mix orbitals of different parities in what is assumed to be the ground state. The orbital anapole moment W(1) is primarily responsible for the magne- tochiral dichroism; the time-even rank-2 tensor N(2) = [L, W](2) for natural circular dichroism; the time-odd rank-2 tensor W(2) = [L, n](2) for nonreciprocal magnetic linear dichroisms. At higher orders, the time-odd rank- 3 tensor G(3) = [L, L, W](3) can also contribute to all nonreciprocal dichroisms. The physical content of these operators is analyzed. For every magnetoelectric group, one can predict which dichroic effect can be measured with either a single crystal or a powdered sample. Experimental spectra are produced to illustrate the value of the sum rules and the practical conditions of their application. Regarding nonreciprocal activity, one should be cautious about discussing magnetic symmetry because the deep core hole can couple the true ground state with low-lying excited states.

2003 - X-ray dichroism in noncentrosymmetric crystals [Articolo su rivista]
P., Carra; A., Jerez; Marri, Ivan
abstract

In this paper the authors analyze near-edge absorption of x rays in noncentrosymmetric crystals. The work is motivated by recent observations of x-ray dichroic effects which stem from parity-nonconserving electron interactions. We provide a theoretical description of these experiments and show that they are sensitive to microscopic polar and magnetoelectric properties of the sample. Our derivation extends previous theoretical work on centrosymmetric systems and identifies interesting directions in the microscopic analysis of crystalline materials using x-ray-absorption spectroscopy.

2003 - X-ray optical activity: Application of sum rules [Articolo su rivista]
Goulon, J.; Rogalev, A.; Wilhelm, F.; Goulon-Ginet, C.; Carra, P.; Marri, I.; Brouder, Ch.
abstract

2000 - Resonant x-ray magnetic scattering from U1-xNpxRu2Si2 alloys [Articolo su rivista]
E., Lidström; D., Mannix; A., Hiess; J., Rebizant; F., Wastin; G. H., Lander; Marri, Ivan; P., Carra; C., Vettier; M. J., Longfield
abstract

We have studied U1-xNpxRu2Si2 alloys with x=0.1, 0.5, and 1.0 using resonant x-ray magnetic scattering. For the x=1 neptunium compound we have confirmed previous neutron scattering results, but with much higher count rates and improved q resolution. Using the element specificity of the method, we have found that the temperature dependence of the uranium and the neptunium moments differ in the mixed U1-xNpxRu2Si2 solid solutions and we present some tentative explanations for this behavior. In principle, by measuring the responses at the individual M edges we are able to determine the ratio of the magnetic moments on the two magnetic species in the random alloys. The observed variation of intensity versus energy is compared to a calculation of a x=0.50 alloy using a localized model and a coherent superposition of U4+ and Np3+ ions. The agreement between theory and experiment is reasonable, suggesting a ratio μU/μNp∼0.25 in this alloy. Since μNp is known to be 1.5μB for 0.10<~x<~1, the uranium moment is ∼0.4μB. This is much larger than 0.02μB known to exist in URu2Si2 (x=0). The increase is a consequence of the molecular field of the ordered Np3+ moments and is consistent with the crystal-field model proposed for the U4+ ground state.