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Polar surfaces are known to be unstable due to the divergence of the surface electrostatic energy. Here we report on the experimental determination, by grazing incidence x-ray diffraction, of the surface structure of polar Ti-terminated (111) ${\mathrm{SrTiO}}_{3}$ single crystals. We find that the polar instability of the $1\ifmmode\times\else\texttimes\fi{}1$ surface is solved by a pure electronic reconstruction mechanism, which induces out-of-plane ionic displacements typical of the polar response of ${\mathrm{SrTiO}}_{3}$ layers to an electron confining potential. On the other hand, the surface instability can be also eliminated by a structural reconstruction driven by a change in the surface stoichiometry, which induces a variety of $3\ifmmode\times\else\texttimes\fi{}3$ (111) ${\mathrm{SrTiO}}_{3}$ surfaces consisting in an incomplete Ti (surface)--${\mathrm{O}}_{2}$ (subsurface) layer covering the $1\ifmmode\times\else\texttimes\fi{}1$ Ti-terminated (111) ${\mathrm{SrTiO}}_{3}$ truncated crystal. In both cases, the ${\mathrm{TiO}}_{6}$ octahedra are characterized by trigonal distortions affecting the structural and the electronic symmetry of several unit cells from the surface. These findings show that the stabilization of the polar (111) ${\mathrm{SrTiO}}_{3}$ surface can lead to the formation of quasi-two-dimensional electron systems characterized by radically different ground states which depend on the surface reconstructions.

We report on an ab initio study of the structural and electronic properties of B- and P-doped Si nanoclusters. The neutral impurities formation energies are calculated. We show that they are higher in smaller nanoclusters and that this is not related to the structural relaxation around the impurity. Their dependence on the impurity position within the nanocluster is also discussed. Finally, we have calculated the B and P activation energies showing the existence of a nearly linear scaling with the nanocluster inverse 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.

Gas chromatographic/mass spectrometric (GC/MS) and nuclear magnetic resonance (NMR) investigations were carried out to verify the possibility of identifying the presence of perdeuterated ethyl alcohol added to beet sugar to increase the alcoholic grade of wine must. Data were obtained on the limits of significance of the EEC analytical method (based on measurement of the D/H ratio and of δ13C‰) used to reveal such sophistication. The results demonstrated that the percentages of beet saccharose in the order of 20–25% (calculated on the total sugars), appropriately corrected for perdeuterated ethyl alcohol, cannot be revealed with the SNIF-NMR method as published in its present form. The minimum limit of detection of the addition of perdeuterated ethyl alcohol for GC/MS is instead about four times greater than that revealed by the SNIF-NMR method.

Unsere Untersuchungen zeigen demnach beim Kaninchen einen Einflus von Sympathikusreizung und-lahmung auf die vestibulare Erregbarkeit: 1. Einseitige Durchschneidung des Halssympathikus bewirkt eine deutliche Verminderung der Nystagmusfrequenz bei Kaltspulung auf der operierten Seite. 2. Auf der operierten Seite ist die Dauer des Kaltnystagmus regelmasig herabgesetzt, die Latenzzeit angedeutet verlangert. 3. Bei Heisspulung ist ein der Kaltspulung entgegengesetzter Reaktionstyp zu erkennen. 4. Die Dreherregung zeigt ohne Bevorzugung einer Seite eine allgemeine Herabsetzung der Erregbarkeit. 5. Sympathikusreizung bewirkt Abnahme der Zahl der Nystagmusschlage auf der normalen Seite. 6. Auf derselben Seite ist dabei die Nystagmusdauer und Latenzzeit angedeutet herabgesetzt. 7. Heisspulung zeigt bei Sympathikusreizung keinen einheitlichen Reaktionstyp. 8. Die Untersuchungen sprechen demnach gegen eine vasomotorische Erklarung des kalorischen Nystagmus.

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.

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.

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 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.

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.

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., 9 pages, 10 figures; corrected typos

The structural and electronic properties of anatase TiO2 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 TiO2. Available experimental data on the electronic and structural properties of TiO2 NCs are summarized and compared with our results.

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 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.

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.

Using first-principle density-functional theory in the GGA approximation we have studied the electronic screening in semiconductor nanocrystals. Combining simple electrostatics and the Thomas-Fermi theory it is shown that an analytical and general form of a model position-dependent screening function can be obtained. Taking as a case study silicon nanocrystals, the relative weights of the nanocrystal core and surface polarization contribution to the screening are thoroughly discussed. The connection between the screening at the nanoscale and in the bulk is clarified.

In this paper we report on a comparative ab initio study of the adsorption of ethylene, cyclopentene and 1-amino-3-cyclopentene on the silicon (100) surface. Accurate calculations of the reaction path have been carried out using a cluster model for the surface dimer (Si9H12) and Gaussian-type basis functions. The dependence of the computed reaction path on the theoretical method is investigated-, activation energies turn out to be quite independent of the method, and general trends can be found for the three systems studied: the larger difference is found between linear and cyclic alkenes. Periodic calculations with plane waves are also performed on periodic slabs, finding an adsorption energy in fair agreement with the cluster model. The Surface band structure is carefully studied: a strong dispersion is found along some directions for highly covered surfaces, in good agreement with the experimental data available for ethylene. Also in this case, differences arise when passing from linear to cyclic alkenes, while the second substituent on the cycle has very limited effects.

In this paper we establish criteria of instability of the homogeneous large polaron gas versus the periodic state. We study, in the framework of the Bogolyubov approach of the equilibrium distribution functions, how this transition depends on the parameters of the polar medium (dielectric parameters, electron-phonon coupling constant, polaron density). The temperature dependence of the transition is analyzed, indicating that the stability of the periodic phase can be achieved in a given range of temperatures, depending on the material and the polaron density. The comparison of the total energies of the periodic and homogeneous phases does confirm that the stability of the periodic phase corresponds to the lowest-energy configuration.

Electronic and optical properties of silicon nanocrystals are calculated and discussed within a semiempirical tight-binding approach, which allows to study systems composed of thousands of atoms. Oscillator strengths, frequency-dependent optical absorption cross sections, and static dielectric constants are investigated for both spherical and ellipsoidal nanocrystals, with the aim of pointing out their size- and shape-dependent features. We show that the anisotropy of the optical functions follows the nanocrystal shape, and a comparison is discussed between very elongated structures and quantum wires.

The adsorption of 1-amino-3-cyclopentene on the (100) silicon surface has been studied by methods rooted in the density-functional theory using both delocalized (plane waves, PWs) and localized (Gaussian-type orbitals, GTOs) basis functions. The results obtained by modeling the surface by silicon clusters of different sizes are quite similar, thus confirming that the reaction is quite localized. Furthermore, PW and GTO computations give comparable results, provided that the same density functional and carefully chosen computational parameters (contraction of GTO, pseudopotentials, etc.) are used. Slab computations performed in the PW framework show that the cluster results are retrieved when low-coverage adsorption on the surface is considered. On these grounds, we are quite confident that reaction parameters obtained by the more reliable hybrid density functional (PBE0) are essentially converged, our best estimates of reaction and activation free energies are thus -40 and 6 kcal/mol, respectively. (c) 2005 American Institute of Physics.

Silicon ellipsoidal nanocrystals have been studied within a Tight Binding approximation. Transition energies and optical properties have been analyzed varying the shape and the number of atoms of the structures. We have investigated how the polarization of the absorbed radiation depends on the geometrical anisotropy. Our results can give a useful contribution in explaining recent measurements of polarized light emission from porous silicon and may give new insights on the silicon nanostructure physics. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The structural properties of small silicon nanoclusters as a function of dimension and surface passivation are studied from ab initio technique. The formation energies are calculated and the relative stability of the considered clusters is predicted and discussed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The effects of B and P codoping on the impurity formation energies and electronic properties of Si nanocrystals (Si-nc) are calculated by a first-principles method. We show that, if carriers in the Si-nc are perfectly compensated by simultaneous doping with n- and p-type impurities, the Si-nc undergo a minor structural distortion around the impurities and that the formation energies are always smaller than those for the corresponding single-doped cases. The band gap of the codoped Si-nc is strongly reduced with respect to the gap of the pure ones showing the possibility of an impurity based engineering of the photoluminescence properties of Si-nc.

Electronic and structural properties of small hydrogenated silicon nanoclusters as a function of dimension are calculated from ab initio technique. The effects induced by the creation of an electron-hole pair are discussed in detail, showing the strong interplay between the structural and optical properties of the system. The distortion induced on the structure after an electronic excitation of the cluster is analyzed together with the role of the symmetry constraint during the relaxation. We point out how the overall effect is that of significantly changing the electronic spectrum if no symmetry constraint is imposed to the system. Such distortion can account for the Stokes shift and provides a possible structural model to be linked to the four-level scheme invoked in the literature to explain recent results for the optical gain in silicon nanoclusters. Finally, formation energies for clusters with increasing dimension are calculated and their relative stability discussed.

The optical properties of anisotropic quantum dots with ellipsoidal shape are investigated and discussed as a function of the dot aspect ratio. The energy spectrum ~ground and many excited states! is calculated and analyzed in comparison with the spherical quantum dot. The optical matrix elements in the dipole approximation and the oscillator strengths for the infrared transitions are shown. The main result is that optical processes significantly depend on the radiation polarization in contrast to nonpolarized processes observed for spherical quantum dots. The dot anisotropy is shown to play a fundamental role in determining the dot properties.

First principles calculations were performed to study the interface electronic structure and the Schottky barrier heights (SBHs) of ZnO???metal interfaces. Different kinds of metals were considered with different chemistries on the polar (0 0 0 1) and (0 0 0 ¯1) ZnO surfaces. The projection of the density of states on the atomic orbitals of the interface atoms reveals that two kinds of interface electronic states appear: states due to the chemical bonding which appear at well defined energies and conventional metal-induced gap states associated with a smooth density of states in the bulk ZnO band gap region. The relative weight and distribution of the two classes of states depend on both the ZnO substrate termination and on the metal species. SBHs are found to be very sensitive to the specific interface chemical bonding. In particular, it is possible to note the occurrence of either Schottky barriers or Ohmic contacts. Our results have been compared with experiments and with available phenomenological theories, which estimate the SBH from few characteristic material parameters. Finally, the electronic and structural contributions to the SBH have been singled out and related to the different charge transfers occurring at the different interfaces.

"A method is proposed to calculate both acoustic and electromagnetic modes in piezoelectric hexagonal ceramics. In the infinite system, the five characteristic modes ( three acoustic and two electromagnetic ) are divided into two classes: the first one includes two modes ( one acoustic and one electromagnetic ) that are slightly interacting through the piezoelectric coupling; the second class comprises three modes ( two acoustic and one electromagnetic ) that show relevant effects due to piezoelectricity in their dispersion relations. Finally, acoustic and electromagnetic modes are analyzed in a slab surrounded by vacuum taking fully into account the effects of piezoelectric coupling."

We investigate the incorporation of group-III ( B and Al), group-IV ( C and Ge), and group-V ( N and P) impurities in Si nanocrystallites. The structural features and electronic properties of doped Si nanocrystallites, which are faceted or spherical-like, are studied by means of an ab initio pseudopotential method including spin polarization. Jahn-Teller distortions occur in the neighborhood of the impurity sites and the bond lengths show a dependence on size and shape of the nanocrystallites. We find that the acceptor ( group-III) and donor ( group-V) levels become deep as the nanocrystallites become small. The energy difference between the spin-up and spin-down levels of group-III and group-V impurities decreases as the size of the Si nanocrystallite increases and tends to the value calculated for Si bulk. Doping with carbon introduces an impurity-related level in the energy gap of the Si nanocrystallites.