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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A detailed description, at the atomistic scale, of the dynamics of excess electrons and holes is fundamental in 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 devices and solar cells, limiting their efficiency. In this work, a precise ab initio analysis of the direct Auger recombination processes in both n- and p-type Si and GaAs crystals is presented. Our simulations of minority carrier Auger lifetimes rely on an accurate electronic band structure, calculated using density functional theory with the inclusion of quasiparticle corrections. The results obtained are in good agreement with experimental data for both n-Si and p-GaAs, proving the importance of the direct Auger recombination mechanism in such systems. On the contrary, we show that different nonradiative recombination paths are necessary to explain the experimental results for both p-Si and n-GaAs. [ABSTRACT FROM AUTHOR]

Abstract: Within a first-principles framework we show the dependence of the optical properties of silicon nanocrystallites embedded in a silica matrix on the crystalline vs. amorphous order of the system. Moreover we calculate how many-body effects modify the electronic and optical properties of the embedded silicon nanodots. A discussion about the different roles played by dimensionality, interface properties, disorder and excitonic effects on the electronic and optical properties of the confined nanostructures is presented. [Copyright &y& Elsevier]

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 the Schottky 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. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

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. In this work, Marri et al. discuss first‐principles calculations concerning the microscopic properties of Si, Ge, and Si/Ge nanowires (NWs) and nanocrystals (NCs). These systems are very interesting for their possible technological applications. The authors prove that electronic, optical, and transport properties of NWs can be tuned by changing composition and geometry. Moreover, the authors deeply analyze the role played by quantum confinement and by strain in Si, Ge, and Si/Ge NCs. The novelty and importance of these results may support further experimental investigation for the design of advanced Si‐ and Ge‐based nanodevices. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. To cite this article: E. Degoli et al., C. R. Physique 10 (2009). [Copyright &y& Elsevier]

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 2.2nm. 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. [Copyright &y& Elsevier]