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The interlayer van der Waals interaction in twisted bilayer graphene (tBLG) induces both in-plane and out-of-plane atomic displacements showing complex patterns that depend on the twist angle. In particular, for small twist angles, within each graphene layer, the relaxations give rise to a vortex-like displacement pattern which is known to affect the dispersion of the flat bands. Here, we focus on yet another structural property, the chirality of the twisted bilayer. We perform first-principles calculations based on density functional theory to investigate the properties induced by twist chirality in both real and momentum space. In real space, we study the interplay between twist chirality and atomic relaxation patterns. In momentum space, we investigate the spin textures around the $K$ points of the Brillouin zone, showing that alternating vortex-like textures are correlated with the chirality of tBLG. Interestingly, the helicity of each vortex is inverted by changing the chirality while the different twist angles also modify the spin textures. We discuss the origin of the spin textures by calculating the layer weights and using plot regression models., 12 pages and 7 figures in main text, 11 pages and 12 figures in supplemental material; revised according to referees' suggestions; Accepted in Phys. Rev. B

The structural and electronic properties of twisted bilayer graphene are investigated from first principles and tight binding approach as a function of the twist angle (ranging from the first "magic" angle $\theta=1.08^\circ$ to $\theta=3.89^\circ$, with the former corresponding to the largest unit cell, comprising 11164 carbon atoms). By properly taking into account the long-range van der Waals interaction, we provide the patterns for the atomic displacements (with respect to the ideal twisted bilayer). The out-of-plane relaxation shows an oscillating ("buckling") behavior, very evident for the smallest angles, with the atoms around the AA stacking regions interested by the largest displacements. The out-of-plane displacements are accompanied by a significant in-plane relaxation, showing a vortex-like pattern, where the vorticity (intended as curl of the displacement field) is reverted when moving from the top to the bottom plane and viceversa. Overall, the atomic relaxation results in the shrinking of the AA stacking regions in favor of the more energetically favorable AB/BA stacking domains. The measured flat bands emerging at the first magic angle can be accurately described only if the atomic relaxations are taken into account. Quite importantly, the experimental gaps separating the flat band manifold from the higher and lower energy bands cannot be reproduced if only in-plane or only out-of-plane relaxations are considered. The stability of the relaxed bilayer at the first magic angle is estimated to be of the order of 0.5-0.9 meV per atom (or 7-10 K). Our calculations shed light on the importance of an accurate description of the vdW interaction and of the resulting atomic relaxation to envisage the electronic structure of this really peculiar kind of vdW bilayers., Comment: 10 pages, 5 figures

The electronic properties of twisted bilayer graphene are being intensely investigated. The flat bands emerging at the first ``magic angle'' are responsible for anomalous insulating behavior at half-filling and superconductivity. Using state-of-the-art $a\phantom{\rule{0}{0ex}}b\phantom{\rule{0.333em}{0ex}}i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ calculations, it is shown that the energy gaps and band dispersion are captured correctly only when the atomic corrugation, arising as a result of the interlayer interaction, is considered. The atomic corrugation shows a wave profile with maximum atomic out-of-plane displacements of about 0.1 angstrom.

We report on a theoretical study of the nonpolar ZnO (101̅0) and (112̅0) surfaces carried out in the framework of density functional theory, aiming to elucidate the thermodynamic and kinetic stability of the clean surface against the formation and diffusion of oxygen vacancies. At variance with other oxide materials and ZnO surfaces with different orientation, we show that, under exposure to molecular oxygen in the gas phase, no significant amounts of oxygen vacancies can be sustained by the surface, in agreement with recent Scanning Tunnelling Microscope (STM) observations. However, our calculations show also that under ultrahigh vacuum and high-temperature conditions the observation of oxygen vacancies might be possible, as reported in earlier experiments of Göpel and Lampe.(1) We characterize the defected surfaces electronic and structural properties as a function of the position of the defect with respect to the surface and discuss the diffusion paths of such defects both parallel and across the surface.

We show that the central finite difference formula for the first and the second derivative of a function can be derived, in the context of quantum mechanics, as matrix elements of the momentum and kinetic energy operators on discrete coordinate eigenkets |xn〉 defined on a uniform grid. Starting from the discretization of integrals involving canonical commutations, simple closed-form expressions of the matrix elements are obtained. A detailed analysis of the convergence toward the continuum limit with respect to both the grid spacing and the derivative approximation order is presented. It is shown that the convergence from below of the eigenvalues in electronic structure calculations is an intrinsic feature of the finite difference method. © 2018 Wiley Periodicals, Inc.

Few layer bismuth nanofilms with (111) orientation have shown striking electronic properties, especially as building blocks of novel two-dimensional heterostructures. In this paper we present state-of-the-art first principles calculations, based on both density functional theory and maximally localized Wannier functions, that encompass electronic and structural properties of free-standing Bi(111) nanofilms. We accurately evaluate both the in-plane lattice constant and, by including the van der Waals interaction between bismuth bilayers, the intra/interlayer distances. Interestingly and somehow unexpectedly, the in-plane lattice constant is predicted to shrink by about 5% going from the thickest investigated nanofilm $(\ensuremath{\sim}80\phantom{\rule{0.16em}{0ex}}\AA{})$ to single bilayer Bi(111), entailing a thickness dependent lattice mismatch in complex heterostructures involving ultrathin Bi(111). Moreover, quantum confinement effects, that would be expected to rule the electronic structure at this size range, compete with surface states that appear close to and across the Fermi level. The implication is that not only all but the thinnest films have a metallic band structure but also that such surface states might play a role in either the formation of interfaces with other materials or for sensing applications. Finally, the calculated electronic structure compares extremely well with ARPES measurements.

We have performed first-principles calculations on anatase TiO2 nanowires (NWs) to investigate the dependence of their structural and electronic properties on the size, the surface coverage, and the morphology. We have found that the overall crystallinity of the NWs increases on increasing the diameter size or equivalently upon surface coverage with simple water-derived adsorbates. The NWs grown along the [010] direction are found to be more ordered with respect to the NWs in the [001] direction of the same size, thus highlighting the dependence of the crystallinity on the choice of the morphology. The bare and hydrated NWs do show the band gap blue shift due to the size confinement, but deviations from an ideal trend with the size are found and ascribed to the morphology and the crystallinity. Through the analysis of the valence band maximum and conduction band minimum energies we found that the electrochemical potential variations of the TiO2 NWs profit from the confined size, for example, by favoring the water splitting process. Moreover, the availability of internal channels for an efficient charge transport can be tuned by the surface coverage. The terminal hydroxyl groups of the hydrated NWs cannot be considered as deep hole traps since their related electronic states have binding energies in the same range of the NW oxygen states. The hydrogenated NWs grown along the [001] direction show occupied states at the bottom of the conduction bands, thus we expect that TiO2 NWs can be used as efficient hydrogen sensors. Finally, the surface hydration leads to the most stable NWs among the considered surface configurations with formation energies that are even close to the bulk limit.

The absorption and, for the first time, the emission spectra of doped silicon nanocrystals have been calculated within a first-principles framework including geometry optimization. Starting from hydrogenated silicon nanocrystals, simultaneous n- and p-type doping with boron and phosphorous impurities have been considered. We found that the B-P co-doping results to be easier than simple 13- or P-doping and that the two impurities tend to occupy nearest neighbours sites inside the nanocrystal itself. The co-doped nanocrystals bandstructure presents band edge states that are localized on the impurities and are responsible of the red-shifted absorption threshold with respect to that of pure un-doped nanocrystals in fair agreement with the experimental outcorne. The emission spectra show a Stokes shift with respect to the absorption due to the structural relaxation after the creation of the electron-hole pair. Moreover, the absorption and emission spectra have been calculated for a small co-doped nanocrystal beyond the single particle approach by introducing the self-energy correction and solving the Bethe-Salpeter equation scheme. Our procedure shows the important role played by the many-body effects. (C) 2006 Elsevier B.V. All rights reserved.

The electronic and optical properties of silicon nanocrystals passivated with hydrogen and oxygen have been investigated both in the ground- and in an excited-state configuration, through different ab-initio techniques. The presence of an electron–hole pair leads to a strong interplay between the structural and optical properties of the system. The structural distortion of the nanocrystals induced by an electronic excitation is analysed together with the role of the symmetry constraint during the relaxation. The structural distortion can account for the experimentally observed Stokes Shift. Size-related aspects are also analysed and discussed.

We address the structural and electronic properties of graphene nanoribbons (GNRs) covalently immobilized on a metallic substrate by means of an organic layer. The GNR–organic layer and organic layer–metal interfaces can be thought of as constituents of a nanodevice and have been accurately studied using large-scale density functional theory calculations. Our results demonstrate the possibility of combining nanopatterned metal–organic layer substrates with selected GNRs to obtain well ordered and stable structures while preserving the GNR energy band gap, an essential requirement for any switching nanodevice.

A novel class of heteroacene molecules, benzodifuroxazin-4-ones, has been effectively synthesized by means of a thermally activated double cyclization reaction starting from amidic precursors. The new molecules were thermally and optically characterized, revealing an outstanding thermal stability to oxidation and an uncommon enhancement of fluorescent properties in solid state as compared to solution. As shown by single-crystal XRD analysis, the molecules crystallize in a face-to-face (π-stack) arrangement instead of the herringbone structure typical of the most acene derivatives. The electronic properties of both molecules and crystals have been investigated by means of a detailed Density Functional Theory computational analysis: very stable HOMO energies have been calculated and, from the band structure analysis, it is possible to suggest a preferential direction of charge transport along the π-stacking direction. All the reported properties indicate this new class of heteroacene derivatives as interesting candidates for a possible application in organic electronics.

The quantum confinement for a conduction-band electron within a spherical quantum dot is investigated in the framework of the effective-mass approximation. The conduction-band effective-mass anisotropy is fully taken into account. The exact solution for the electronic spectrum is provided, showing how the 'isotropic' mass levels map into the new spectrum which accounts for the anisotropy. The removal of degeneracies typical of the spherical dot, as well as its effects on the infrared transition energies, are discussed.

In this paper we study the electron-electron correlation in silicon anisotropic quantum dots. A variational approach to the problem of the quantum confinement of two electrons in an ellipsoidal quantum dot is presented. All the calculations are done as a function of the dot aspect ratio, showing how the Coulomb repulsion and dielectric effects (related to the dielectric mismatch between the dot and the surrounding medium) give rise to shape-dependent correlation effects. The energy needed to add a second electron to the ellipsoidal quantum dot is calculated and discussed as well.

The single particle spectrum of a charged particle in a nanometric ellipsoidal quantum dot is calculated taking also into account the surface polarization effects. Furthermore the optical properties of the absorption coefficient also depending on the light polarisation are discussed and compared with the experimental data.

We report on our effective mass calculations for a particle confined in an ellipsoidal quantum dot. Upon using a curvilinear coordinates system, the single particle exact eigenvalues and eigenfunctions have been numerically calculated. Dielectric image effects have been properly taken into account. It is shown that on keeping the volume constant, the confinement energies depend only on the dot shape. The implications of this shape dependence are analyzed for the infrared optical properties.

We study the motion of a particle confined in an ellipsoidal quantum dot, solving the corresponding Schrodinger equation both numerically, using the appropriate coordinate system, and variationally. The results from the two methods are compared, varying the ellipsoid semi-axes. We find that the confined-state energies split with respect to those of the spherical quantum dot and this can be explained as a consequence of both a volume-induced deformation effect and a geometry-induced one. The role of the dot geometry is shown to be relevant also for the formation of topological surface states.

Aluminium-doped ZnO thin films, with a doping level in the range 2–2.8 at.%, were deposited by RF magnetron sputtering. Sputtering processes with pure Ar and Ar/H 2 gas mixtures have been explored. Electrical conductivity and Hall mobility of ZnO:Al films were measured in a wide temperature range. It has been found that the addition of hydrogen to the sputtering gas is an effective method to modify the morphological, structural and electrical properties of the ZnO:Al films. A low hydrogen dilution is able to produce a noticeable improvement of the conductivity by means of a better effectiveness of the Al doping. ZnO:Al films deposited at low hydrogen dilution showed a columnar structure whereas at high hydrogen dilution spherical shaped domains were present, formed by many stacked crystallites. Carrier mobility for the former structure was limited by bulk mechanisms, particularly by acoustical phonon and ionized impurity scattering. Carrier mobility for the latter structure was limited by grain boundary mechanisms, particularly by tunnelling effect between neighbouring spherical macroaggregates.

The electronic state of a system of spatially confined atoms is first studied within a single-particle Hamiltonian approach, starting from atomic orbitals with suited boundary conditions. Then, the case of a macroscopic crystal is obtained after determining the limiting behavior of the confined system while the number of atoms becomes infinitely large. As an alternative to the atomic orbitals, the same electronic states are as well determined starting from a different basis set, that is composed of free-particle wavefunctions. The Bloch theorem is introduced, as the fundamental tool establishing the form assumed by the wavefunctions representing the electronic states, it being a consequence of lattice translational invariance and periodicity. Specific methods suited to the computation of electronic states are thus illustrated, that are applied to selected semiconductors, graphene, and carbon nanotubes. Then, a few elementary concepts of many-particle physics are introduced, useful to describe the interactions of electrons in crystals and nanostructures and to introduce different types of correlated ground states. Indeed, interacting ground states are first investigated within the Hartree and Hartree-Fock methods and excitonic effects are considered. Eventually, ground states related to spin-related magnetic phenomena and to superfluidity and superconductivity are described at an elementary level.

The structure of crystals is here described after introducing Bravais and reciprocal lattices. Experimental techniques like X-ray elastic scattering and electron spectroscopy are briefly discussed, along with techniques specifically suited to nanostructures. The concept of cohesive energy is introduced and a few related bond mechanisms are reviewed: Van der Waals, ionic, covalent, and metallic.

Two related issues are here discussed: a) deformations of continuous bodies responding to applied external forces, their dynamics within a classical approach and applications of related concepts to both macroscopic and nanoscopic systems; b) classical dynamics of systems made of lattice points with finite and infinite size, with varying boundary conditions. Issue b) provides a microscopical framework to understand the macroscopic phenomena discussed in a). Classical dynamics represented in terms of normal modes is shown to be an useful framework to pass to the quantum description, and introduce the phonons as vibrational quanta. Applications to specific heats of insulators are illustrated, including the effects of anharmonic vibrations. Finally, experimental methods and techniques are discussed, related to the determination of vibrational spectra in solids.

The Density Functional Theory (DFT) and its extensions to time-dependent systems and to special types of ground states is formulated, as a powerful framework to perform explicit and accurate calculations for atoms, molecules, crystals and nanosystems and obtain good agreement in comparison with experimental findings. Concepts, methods, and applications are illustrated. The discussion around the DFT represents a framework where many puzzle pieces found in previous Chapters fit together: the approach adopted within approximate models such Thomas- Fermi and Hartree and Hartree-Fock methods, the density and current-density response theories used to understand the transport in Chap. 4 and optical in Chap. 5 behaviors, the crystal, electron and phonon structures investigated in Chaps. 1, 2 and 3.

The equations of motion for electrons and holes are derived within the semiclassical approximation for the treatment of external forces. The motion of electrons and holes in the presence of either electric or magnetic fields or both is investigated, with reference to their charge and spin properties. Conducting, semiconducting and insulating materials are characterized according to their transport behavior. The results obtained in this way are compared with the experimental data observing significant incompatibilities, so that scattering mechanisms of the carriers (electrons or holes) with phonons and impurities are introduced in order to explain the observations. Thus, electric and thermal conductivity are discussed with some length for metals and semiconductors, also referring to p–n junctions. Then, selected characteristic transport properties of superconductors are briefly pointed out. Finally, electric conductivity in nanostructures is briefly discussed.

We propose a microscopic model for a nanoelectromechanical system made by a radio-frequency driven suspended carbon nanotube (CNT) in the presence of an external magnetic field perpendicular to the current. As a main result, we show that, when the device is driven far from equilibrium, one can tune the CNT mechanical properties by varying the external magnetic field. Indeed, the magnetic field affects the CNT bending mode dynamics inducing an enhanced damping as well as a noise term due to the electronic phase fluctuations. The quality factor, as observed experimentally, exhibits a quadratic dependence on external magnetic field strength. Finally, CNT resonance frequencies as a function of gate voltage acquire, increasing the magnetic field strength, a peculiar dip-peak structure that should be experimentally observed., 15 pages, 9 figures, submitted to Phys. Rev. B

The propagation of the acoustic and the electromagnetic signals is studied in materials with tetragonal and hexagonal symmetries that are piezoelectric, piezomagnetic, or magnetoelectric. Three magnetic spatial symmetries are considered: 622 for hexagonal crystals and 422 and 4(')22(') for tetragonal crystals. The equations for the five modes (three mainly acoustic and two mainly electromagnetic) are solved both in the general case and in specific cases in which the material is only piezoelectric, piezomagnetic, or magnetoelectric. It is found that the piezomagnetic and the magnetoelectric coefficients and the magnetic permeability renormalize the elastic constants, the piezoelectric coefficients, and the dielectric tensor. The acoustic frequencies depend on the angle theta that the component of the wave vector in the a-b plane forms with the a axis. The main contribution to the electromagnetic modes derives only from the dielectric tensor and the magnetoelectric coefficients and it is independent of theta and of the piezoelectric and the piezomagnetic coefficients. Starting from the general equations, a method has been devised to study separately the acoustic and the electromagnetic solutions. It is found that a number of relations must be verified in order to have stability of the electromagnetic and the acoustic modes.

The impurity screening in silicon nanocrystals is analyzed using a first-principles approach based on density functional theory. The electron density induced by a positively charged impurity is evaluated as a function of the nanocrystal size. From our calculations we found that the impurity is responsible for an electron density accumulation around the impurity site, fully compensated by a positive charge accumulation at the surface (electron depletion). The results are sound and shed new light on the most recent findings in this field. On the basis of the present first-principles results, we propose a Thomas–Fermi model of the impurity screening in silicon nanocrystals. The model gives reliable estimations of the screening function, that well compares to recent ab-initio calculations.

Nowadays we live in the so-called "Silicon Era", in which devices based on the silicon technology permeate all aspects of our daily life. One can simply think how much silicon is in the everyday household objects, gadgets and appliances. The impact of silicon technology has been very relevant in photodetection as well. it enables designing large or very large-scale integration devices, in particular microchips and pixelled detectors like the Silicon Photo Multiplier made of micrometric channels grouped in mm(2) pixels. However, on the horizon, the recent development of nanotechnologies is opening a new direction in the design of sub-micron photodevices, owing to the capability to deal with individual molecules of compounds or to chemically grow various kinds of materials. Among them, carbon compounds appear to be the most promising materials being chemically very similar to silicon, abundant and easy to handle. In particular, carbon nanotubes (CNT) are a very intriguing new form of material, whose properties are being studied worldwide providing important results. The photoelectric effects observed on carbon nanotubes indicate the possibility to build photodetectors based on CNTs inducing many people to claim that we are at the beginning of a Post Silicon Era or of the Carbon Era. In this paper, we report on the most important achievements obtained on the application of nanotechnologies to photodetection and medical imaging, as well as to the development of radiation detectors for astro-particle physics experiments.

Molecular materials with a polar arrangement of the constituent dipoles are good candidates for exhibiting piezoelectric properties directly related to the strain-induced polarization. We assess, from first principles, the properties of the metal-organic molecular crystal (4-dimethylaminopyridyl)bis(acetylacetato)zinc(II) in presence of strain. The spontaneous polarization and the piezoelectric properties are studied by means of the modern theory of polarization. The cooperative interaction among molecular chains is shown to lead to a collective polarization enhancement effect. Some theoretical issues concerning the multivalued behavior of the Berry phase are also illustrated.

The titanium dioxide (TiO2) complexes are widely investigated for their striking and multipurpose capabilities. The TiO2 key feature lies in its photocatalytic activity for several reactions of social (bioengineering, environmental and artistic protection, pollution containment) and commercial (photovoltaic, alternative-energy, gas sensing) interests. The possibility to enhance specific reactions at the nanoscale by a fine tuning of the nano-sized single crystals properties boosted in the last decade the scientific research. Thus a theoretical understanding of the fundamental properties of TiO2 nanocrystals became necessary to predict and expedite the experimental effords. We present here a characterization of TiO2 0D nanoclusters and 1D nanowires in the framework of ab initio DFT calculations. Based on both theoretical and experimental evidences we defined a stoichiometric TiO2 NC by modifying a perfect bipyramidal morphology and then used this NC as a chain repetition unit in the NW. We analyzed the effect of the surface coverage by functionalizing dangling bonds with simple adsorbates (dissociated water and hydrogens) modeling two acidic environments. These terminations are important to model the basical interactions of TiO2 nanosystems with the hydration sphere, which is always found to surround the nanosamples and toaffect their photocatalytic activity. We thus address the electronic reorganization and the surface weight in determining the global features 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. Quantum confinement effects in the electronic properties are observed through the bandgap widening and the discretization of the energy distribution, but the surface competes to determine the energy dispersion of the electronic levels. The hydrogenated nanocrystals do show occupied levels at the bottom of the coduction bands, thus leading to metallic nanowires in one dimension. Whereas in the hydrogenated cluster such levels present a localized charge distribution with respect to the whole structure and they are also similar for the atomic orbital character and energy position to the defect states obtained by 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. Formation energy calculations reveal that surface hydration leads to the most stable nanocrystals, in agreement with the experimental findings that water coverage stabilizes the surface.

The effects of the incorporation of group-III (B and Al), group-IV (C and Ge), and group-V (N and P) impurities on the formation energies, electronic density of states, optical absorption spectra, and radiative lifetimes of Si nanocrystallites of different shape and with diameters up to 2 nm are studied by means of an ab initio pseudopotential method that takes into account spin polarization. The single doping with group-III or group-V impurities leads to significant changes on the onsets of the absorption spectra that are related to the minority-spin states. In contrast to the optical absorption spectra, the radiative lifetimes are sensitively influenced by the shape of the nanocrystallites, though this influence tends to disappear as the size of the nanocrystallites increase. Codoping is investigated for pairs of group-III and group-V impurities. We show that the impurity formation energies decrease significantly when the nanocrystallites are codoped with B and P or with Al and P. Additional peaks are introduced in the absorption spectra due to codoping, giving rise to a redshift of the absorption onset with respect to the undoped nanocrystallites. Those additional peaks are more intense when codoping is performed with two different species either of the group III or of the group V. The values of radiative lifetimes for the codoped nanocrystallites are mostly in between the values for the nanocrystallites doped with the impurities separately.

The structural and electronic properties of anatase ${\text{TiO}}_{2}$ nanocrystals (NCs) are investigated through first-principles calculations. 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 the experimental 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 on undercoordinated 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 ${\text{TiO}}_{2}$. Available experimental data on the electronic and structural properties of ${\text{TiO}}_{2}$ NCs are summarized and compared with our results.

Oxygen vacancies at the SnO2(110) and (101) surface and subsurface sites have been studied in the framework of density functional theory by using both all-electron Gaussian and pseudopotential plane-wave methods. The all-electron calculations have been performed using the B3LYP exchange-correlation functional with accurate estimations of energy gaps and density of states. We show that bulk oxygen vacancies are responsible for the appearance of a fully occupied flat energy level lying at about 1 eV above the top valence band, and an empty level resonant with the conduction band. Surface oxygen vacancies strongly modify the surface band structures with the appearance of intragap states covering most of the forbidden energy window, or only a small part of it, depending on the vacancy depth from the surface. Oxygen vacancies can account for electron affinity variations with respect to the stoichiometric surfaces as well. A significant support to the present results is found by comparing them to the available experimental data.

Well-oriented thin films of methyl-ammonium trihalogenostannates $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Sn}{X}_{3}$ compounds $(X=\mathrm{Cl},\mathrm{Br})$ are grown by the single source thermal ablation technique. Optical absorption and structural analysis of $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Sn}{X}_{3}$ thin films are combined with ab initio calculations in order to clarify their electronic properties and the role of the organic cation. The structural calculations reproduce well the measured lattice parameters, and optical measurements reveal a direct gap absorption onset for both compounds, in agreement with the calculated band structures. It turns out from projected density of states analysis that the valence and conduction bands take negligible contribution from the methylammonium. It is inferred that the band gap mainly depends on the ionicity and size of the inorganic cage $\mathrm{Sn}{X}_{3}$.

A first-principles calculation of the impurity screening in Si and Ge nanocrystals is presented. We show that isocoric screening gives results in agreement with both the linear response and the point-charge approximations. Based on the present ab initio results, and by comparison with previous calculations, we propose a physical real-space interpretation of the several contributions to the screening. Combining the Thomas-Fermi theory and simple electrostatics, we show that it is possible to construct a model screening function that has the merit of being of simple physical interpretation. The main point upon which the model is based is that, up to distances of the order of a bond length from the perturbation, the charge response does not depend on the nanocrystal size. We show in a very clear way that the link between the screening at the nanoscale and in the bulk is given by the surface polarization. A detailed discussion is devoted to the importance of local field effects in the screening. Our first-principles calculations and the Thomas-Fermi theory clearly show that in Si and Ge nanocrystals, local field effects are dominated by surface polarization, which causes a reduction of the screening in going from the bulk down to the nanoscale. Finally, the model screening function is compared with recent state-of-the-art ab initio calculations and tested with impurity activation energies.

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 nano-structures 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 and optical 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 of an impurity based engineering of the absorption and luminescence properties of Si nanocrystals.

The structural and electronic properties of both inorganic and hybrid organic-inorganic perovskites based on tin halides are investigated from the first principles. In particular, we contrast the inorganic ${\text{CsSnCl}}_{3}$ and ${\text{CsSnI}}_{3}$ to their hybrid counterparts $({\text{CH}}_{3}{\text{NH}}_{3}){\text{SnCl}}_{3}$, $({\text{CH}}_{3}{\text{NH}}_{3}){\text{SnI}}_{3}$, and $({\text{NH}}_{2}\text{CH}={\text{NH}}_{2}){\text{SnI}}_{3}$, which were obtained by substituting the inorganic Cs cation with the methylammonium ${\text{CH}}_{3}{\text{NH}}_{3}$ and the formamidinium ${\text{NH}}_{2}\text{CH}={\text{NH}}_{2}$ cations. The comparison between the hybrid perovskites and the inorganic counterparts sheds light on the effects of the filling molecule on the structural and electronic properties of the compound. We show that the stability against the distortion of the perovskitic cage strongly depends on the embedded cation. The electronic properties (in particular, the band gap) can be tuned by a suitable choice of the organic molecule, and, in particular, of its size.

We introduce a very simple method to study the phase transition from the homogeneous dilute large polaron gas toward the formation of a cubic crystal within the framework of the Vlasov [Many-Particle Theory and its Applications to Plasma (Gordon and Breach, New York, 1962)] nonlinear kinetic equation and the equations for the Bogolyubov [Problems of Dynamic Theory in Statistical Physics. Selected Works on Statistical Physics (Moscow State University, Moscow, 1979)] distribution functions. Two different critical temperatures ${T}_{1}^{\mathrm{cr}}$ and ${T}_{2}^{\mathrm{cr}}$ are introduced, defining the range of stability of the crystal phase $({T}_{2}^{\mathrm{cr}}lTl{T}_{1}^{\mathrm{cr}})$. The existence and properties of the crystal phase are discussed as a function of the ionicity parameters of the medium, the electron-phonon Fr\"ohlich coupling constant and the density of the dilute polaron gas. In particular, the lattice parameter is found to increase when the ionicity of the medium decreases whereas it is independent of the Fr\"ohlich constant $\ensuremath{\alpha}$ and the polaron density. Furthermore, it increases upon decreasing the temperature from ${T}_{1}^{\mathrm{cr}}$ toward ${T}_{2}^{\mathrm{cr}}$, showing a behavior different from that observed for common solids. We find also that a drift velocity of the polaron gas tends to reduce the stability of the crystal. To illustrate the simplicity of the calculation scheme, we apply the same technique to study the formation of the argon (Ar) crystal starting from the atomic gas interacting through a reliable Lennard-Jones potential. We calculate the crystallization temperature and the Ar atom distribution. The lattice parameter is an increasing function of the temperature and the second critical temperature is not defined. We try to relate the different dependence on the temperature of the lattice parameter to the features of the polaron-polaron and atom-atom interactions.

The absorption and the emission spectra of undoped and doped silicon nanocrystals of different size and surface terminations have been calculated within a first-principles framework. The effects induced by the creation of an electron-hole pair on the atomic structure and on the optical spectra of hydrogenated silicon nanoclusters as a function of dimension are discussed showing the strong interplay between the structural and optical properties of the system. Starting from the hydrogenated clusters, (i) different Si/O bonding at the cluster surface and (ii) different doping configurations have been considered. We have found that the presence of a Si-O-Si bridge bond at the nanocrystal surface gives rise to significant excitonic luminescence features in the near-visible range that are in fair agreement with photoluminescence (PL) measurements on oxidized and SiO$_{2}$ embedded nanocrystals. The study of the structural, electronic and optical properties of simultaneously n- and p-type doped hydrogenated silicon nanocrystals with boron and phosphorous impurities have shown that B-P co-doping is energetically favorable with respect to single B- or P-doping and that the two impurities tend to occupy nearest neighbors sites. The co-doped nanocrystals present band edge states localized on the impurities that are responsible of a red-shifted absorption threshold with respect to that of pure un-doped nanocrystals in agreement with the experiment.

The effect of organic adsorbates on the silicon (001) surface is investigated using first-principles calculations. Ethylene and a class of cyclopentene derivatives, containing different functional groups, are considered, all anchoring to the surface through the same $[2+2]$ cycloaddition mechanism. Because they all show similar bonding properties, any variation in the surface properties must be related to the functional group. The structural relaxation induced by the adsorption is discussed, elucidating the effect of both the adsorbate species and coverage. It turns out that different distortions occur in the molecular geometry, depending on both the species and the surface coverage, while molecule-to-surface bonding does show very similar features for all the considered molecules. We show that the presence of the adsorbate can modify the surface charge density, thus giving rise to an induced dipolar layer that modifies the electrostatic potential outside the surface. Such a dipole layer can, in turn, be related to surface electron affinity and work function changes. A careful analysis of the dipole moment and of the electrostatic potential changes is carried out discussing the correlations with the properties of the isolated molecules. All the results indicate how the surface properties can be tuned through a suitable choice of the adsorbate.

The effects of both single doping and simultaneous codoping on the structural, electronic, and optical properties of Si nanocrystals are calculated by the first-principles method. We show that the amount of the nanocrystal relaxation around the impurity is directly related to the impurity valence. Moreover, both the neutral impurity formation energies and the impurity activation energies scale with the reciprocal radius. Interestingly, no significant variation of the activation energy on the impurity species is found, and the cluster relaxation gives a minor contribution to it. The role of the impurity position within the nanocrystal has also been elucidated showing that the subsurface positions are the most stable ones. We show that, if the carriers in the Si nanocrystals are perfectly compensated by simultaneous doping with the n- and p-type impurities, the nanocrystals undergo a minor structural distortion around the impurities. The formation energies are always smaller than that for the corresponding single-doped cases. Moreover, in the case of codoping, the bandgap is strongly reduced with respect to the gap of the pure crystals showing the possibility of an impurity-based engineering of the photoluminescence properties of the Si nanocrystals.

We investigate the effects of constraining the motion of atoms in finite slabs used to simulate the rutile TiO2 (110) surface in first-principles calculations. We show that an appropriate choice of fixing atoms in a slab eliminates spurious effects due to the finite size of the slabs, leading to a considerable improvement in the simulation of the (110) surface. The method thus allows for a systematic improvement in convergence in calculating both geometrical and electronic properties. The advantages of this approach are illustrated by presenting the first theoretical results on the displacement of the surface atoms in agreement with experiment. (c) 2006 American Institute of Physics.