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Electronic and structural properties of a hypothetical material, silicon nanotubes, are examined through first-principles calculations based on density functional theory. Even considering that Si nanotubes have never been observed, this paper attempts to establish the theoretical similarities between Si and C, like band structures and density of states, as well as the main differences, especially associated with cohesive energies. The band-structure calculations for silicon nanotubes show that, similar to carbon structures, depending on their chiralities, they may present metallic (armchair) or semiconductor (zigzag and mixed) behaviors.

We have investigated the magnetic properties of non-doped ZnO nanostructures by using {\it ab initio} total energy calculations. Contrary to many proposals that ferromagnetism in non-doped semiconductors should be induced by intrinsic point defects, we show that ferromagnetism in nanostructured materials should be mediated by extended defects such as surfaces and grain boundaries. This kind of defects create delocalized, spin polarized states that should be able to warrant long-range magnetic interactions., 8 pages, 3 figures

We report a highly efficient switch built from an organic molecule assembled between single-wall carbon nanotube electrodes. We theoretically show that changes in the distance between the electrodes alter the molecular conformation within the gap, affecting in a dramatic way the electronic and charge transport properties, with an on/off ratio larger than 300. This opens up the perspective of combining molecular electronics with carbon nanotubes, bringing great possibilities for the design of nanodevices.

We have investigated the electronic and transport properties of zigzag Ni-adsorbed graphene nanoribbons (Ni/GNRs) using ab initio calculations. We find that the Ni adatoms lying along the edge of zigzag GNRs represent the energetically most stable configuration, with an energy difference of approximately 0.3 eV when compared to the adsorption in the middle of the ribbon. The carbon atoms at the ribbon edges still present nonzero magnetic moments as in the pristine GNR even though there is a quenching by a factor of almost five in the value of the local magnetic moments at the C atoms bonded to the Ni. This quenching decays relatively fast and at approximately $9\text{ }\text{\AA{}}$ from the Ni adsorption site the magnetic moments have already values close to the pristine ribbon. At the opposite edge and at the central carbon atoms the changes in the magnetic moments are negligible. The energetic preference for the antiparallel alignment between the magnetization at the opposite edges of the ribbon is still maintained upon Ni adsorption. We find many $\text{Ni}\text{ }d$-related states within an energy window of 1 eV above and below the Fermi energy, which gives rise to a spin-dependent charge transport. These results suggest the possibility of manufacturing spin devices based on GNRs doped with Ni atoms.

Using ab initio methods, we propose a simple and effective way to substitutionally dope graphene sheets with boron. The method consists of selectively exposing each side of the graphene sheet to different elements. We first expose one side of the membrane to boron while the other side is exposed to nitrogen. Proceeding this way, the B atoms will be spontaneously incorporated into the graphene membrane without any activation barrier. In a second step, the system should be exposed to a H-rich environment, which will remove the CN radical from the layer and form HCN, leading to a perfect substitutional doping.

We investigate the electronic structure of superconductor YBa2Cu3O7 through a molecular cluster approach. The calculations are performed self-consistently through a semi-empirical LCAO technique, and correlation effects are taken into account by a configuration interaction procedure (INDO-CI). Our results for the larger clusters yield a density of states (in the valence band) that is in good agreement with the experimental data. We obtain for the ground-state a strongp-d covalency, resulting in a width of around 8 eV for the valence band. The interactions of Cu(1) and (2) with O(1),(2),(3) and (4) are analysed, showing how hybridization occurs. In all cases we calculate the Mott-Hubbard energy U taking into account many-body effects.

We investigated the interaction of intrinsic and extrinsic point defects with stacking faults in silicon. The calculations were carried out using ab initio total energy methods. The results show that the formation energies of intrinsic defects and impurities (P, As, and Al) are lower at the stacking fault as compared to the respective defects in crystalline environment. Therefore, stacking faults should have a large concentration of defects, and they should play an important role on the mechanisms of dislocation motion.

We studied the effect of quantum confinement in Mn-doped InAs nanocrystals using theoretical methods. We observe that the stability of the impurities decreases with the size of the nanocrystals, making doping more difficult in small nanoparticles. Substitutional impurities are always more stable than interstitial ones, independent of the size of the nanocrystal. There is also a decrease in the energy difference between the high and low spin configurations, indicating that the critical temperature should decrease with the size of the nanoparticles, in agreement with experimental observations and in detriment to the development of functional spintronic devices with doped nanocrystals. Codoping with acceptors or saturating the nanocrystals with molecules that insert partially empty levels in the energy gap should be an efficient way to increase ${T}_{C}$.

Using a combination of density functional theory and recursive Green's functions techniques, we present a full description of a large scale sensor, accounting for disorder and different coverages. Here, we use this method to demonstrate the functionality of nitrogen-rich carbon nanotubes as ammonia sensors as an example. We show how the molecules one wishes to detect bind to the most relevant defects on the nanotube, describe how these interactions lead to changes in the electronic transport properties of each isolated defect, and demonstrate that there are significative resistance changes even in the presence of disorder, elucidating how a realistic nanosensor works.

We present density of states and electronic transport calculations of single vacancies in carbon nanotubes. We confirm that the defect reconstructs into a pentagon and a nonagon, following the removal of a single carbon atom. This leads to the formation of a dangling bond. Finally, we demonstrate that care must be taken when calculating the density of states of impurities in one-dimensional systems in general. Traditional treatments of these systems using periodic boundary conditions leads to the formation of minigaps even in the limit of large unit cells.

The multiple scattering-Xa molecular cluster model is used to study the A center in silicon. We show that this center can be created by Jahn-Teller effects over substitutional oxygen impurity in the lattice. Our defect model provides a consistent explanation for the EPR experimental results obtained for the Si-A center.

The adsorption of atomic and molecular hydrogen on armchair and zigzag boron carbonitride nanotubes is investigated within the ab initio density functional theory. The adsorption of atomic H on the $\mathrm{B}{\mathrm{C}}_{2}\mathrm{N}$ nanotubes presents properties which are promising for nanoelectronic applications. Depending on the adsorption site for the H, the Fermi energy moves toward the bottom of the conduction band or toward the top of the valence band, leading the system to exhibit donor or acceptor characteristics, respectively. The ${\mathrm{H}}_{2}$ molecules are physisorbed on the $\mathrm{B}{\mathrm{C}}_{2}\mathrm{N}$ surface for both chiralities. The binding energies for the ${\mathrm{H}}_{2}$ molecules are slightly dependent on the adsorption site, and they are near to the range to work as a hydrogen storage medium.

We report ab initio calculations of total energies and electronic structures of P, As, and Sb donors in GaAs and InP. In the Td geometry, all these defects exhibit two donor states in the forbidden gap: an internal optical excitation energy of the order of 1 eV, and a Franck-Condon shift of the order of 0.1 eV. All these defects possess a metastable geometry of the electrically neutral vacancy-interstitial pair with no donor states in the band gap. We discuss the differences between the six systems and explain why an optically inducible transition to the metastable state is inefficient for GaAs:SbGa.

We present results of selfconsistent, first-principles calculations of total energies for AsGa and SbGa in GaAs. We confirm that both impurities in the substitutional Td site behave as double donors, and the first internal excitation appears at around 1 eV. For the neutral systems we obtain a metastable minimum in the total energy surface in a configuration with the impurity atom displaced toward the interstitial site; the transformation to this metastable configuration, however, is not expected to be operative for the SbGa defect.

The authors present results of electronic structure calculations for complex defect centres-chalcogen impurity pairs-in silicon: O2, S2 and SO, in different geometrical configurations. Treatment of many-electron interactions is included through configuration-interaction, and is shown to be relevant to the correct prediction of ground-state properties. They also report their results for excited states and hyperfine terms.

We investigate, using ab initio molecular dynamics simulations, the effect of temperature on the stability of pure, C and H doped, atomically thin gold nanowires. We show how thermal fluctuations lead to local instabilities that result in the rupture of the nanowires, and how appropriate impurities may help to stabilize these wires. We also show that when light impurity atoms, such as hydrogen, are incorporated in the Au nanowires, quantum effects will affect the nuclear dynamical evolution.

The electronic and structural properties of (i) boron doped graphene sheets, and (ii) the chemisorption processes of hydrogen adatoms on the boron doped graphene sheets have been examined by {\it ab initio} total energy calculations., 7 pages, 4 figures

We report a spin polarized density functional theory study of the electronic and transport properties of graphene nanoribbons doped with boron atoms. We considered hydrogen terminated graphene (nano)ribbons with width up to 3.2 nm. The substitutional boron atoms at the nanoribbon edges (sites of lower energy) suppress the metallic bands near the Fermi level, giving rise to a semiconducting system. These substitutional boron atoms act as scattering centers for the electronic transport along the nanoribbons. We find that the electronic scattering process is spin-anisotropic; namely, the spin-down (up) transmittance channels are weakly (strongly) reduced by the presence of boron atoms. Such anisotropic character can be controlled by the width of the nanoribbon; thus, the spin-up and spin-down transmittance can be tuned along the boron-doped nanoribbons.

Using ab initio total energy density-functional theory calculations, we investigated the electronic, structural, and magnetic properties of manganese-doped germanium nanowires. The nanowires have been constructed along the [110] direction and the dangling bonds on the surface have been saturated by hydrogen atoms. We observed that the Mn has lower formation energy at the center of the wire when compared to regions close to the surface. The Mn-Mn coupling has lower energy for a high-spin configuration except when they are first nearest neighbors. These results show that Ge:Mn nanowires are potential candidates for ferromagnetic quasi-one-dimensional systems.

We investigate, through ab initio density-functional theory calculations, the electronic and structural properties of neutral Mn impurities at tetrahedral interstitial and substitutional sites in both Si and Ge layers of a $\mathrm{Si}∕\mathrm{Ge}$ heterostructure. We conclude that substitutional Mn at the Ge layers is more stable than interstitial Mn at the Si layers by approximately $0.45\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, and we estimate an energy barrier of at least $1.12\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ to diffuse away from these most stable substitutional sites. Mn has a magnetic moment in the heterostructure that is similar to that in the bulk, and for the compressed Ge layer the Mn-Mn exchange interaction is always weakly antiferromagnetic. Varying the lattice constant of the substrate, the Mn-Mn ground state becomes ferromagnetic. This result opens up the possibility of manipulating the interaction between Mn impurities at Ge layers grown over a ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ substrate by changing $x$.

The interaction of 2,3,7,8-tetrachlorinated dibenzo-p-dioxin (dioxin) with carbon nanotubes is analyzed by ab initio methods. The structural and electronic properties of the dioxin interacting with the pristine, defective and B-, N-, and Si-doped SWNTs are considered by studying different geometries. We have found that the dioxin interacts with a pure carbon nanotube although it depends on the geometry of the molecule approximation, increasing if the tube is defective., The authors acknowledge CENAPAD-SP for computer time and financial support from Brazilian agencies CNPq (Grant 307417/2004-2, 51732/2005-9 and 555183/2005-0), FAPERGS (Grants 0511570/05 and 05/2096.2), FAPESP and FUNCAP (Grant PPP-985/03).

The adsorption of atomic and molecular hydrogen on carbon-doped boron nitride nanotubes is investigated within the ab initio density functional theory. The binding energy of adsorbed hydrogen on carbon-doped boron nitride nanotube is substantially increased when compared with hydrogen on nondoped nanotube. These results are in agreement with experimental results for boron nitride nanotubes (BNNT) where dangling bonds are present. The atomic hydrogen makes a chemical covalent bond with carbon substitution, while a physisorption occurs for the molecular hydrogen. For the H(2) molecule adsorbed on the top of a carbon atom in a boron site (BNNT + C(B)-H(2)), a donor defect level is present, while for the H(2) molecule adsorbed on the top of a carbon atom in a nitrogen site (BNNT + C(N)-H(2)), an acceptor defect level is present. The binding energies of H(2) molecules absorbed on carbon-doped boron nitride nanotubes are in the optimal range to work as a hydrogen storage medium.

We have investigated, using ab initio total energy density-functional calculations, the electronic, structural, and magnetic properties of Mn doped InP nanowires. The nanowires have been constructed along the [111] direction and the dangling bonds on the surface have been saturated by hydrogen atoms. We found that the most energetically favorable position for the Mn atom is near the surface. When the Mn atoms are in ``bulklike'' positions, the magnetic Mn-Mn coupling in the nanowire is ferromagnetic. In these cases, our results are consistent with a host-$p\text{\ensuremath{-}}\mathrm{Mn}\text{\ensuremath{-}}d$ coupling mechanism. On the other hand, if at least one of the Mn atoms is located near the surface, the coupling is antiferromagnetic.

The role of atomic oxygen, O2, and NO molecules in the nitrogen and oxygen removal processes observed during the thermal growth of oxynitride SiOxNy films is addressed by spin density functional calculations. Our results show that the energetically most favorable N-desorption mechanism would be induced by O2 reacting with the SiNO -Si structure in SiO2, where nitrogen is released from the network as an interstitial NO molecule, leaving a peroxy linkage Si-O-O-Si in its place. Atomic O diffusing via peroxy linkage also removes nitrogen from the SiNO -Si structure. However, this reaction has an occurrence rate of about 104 times smaller that the most favorable one. We also identify an oxygen exchange mechanism occurring through the annihilation of two peroxyl O atoms when they approach each other, where oxygen is released from the network as an O2 molecule leaving a perfect SiO2 structure. The energetics of other possible scenarios for atomic desorptions in SiOxNy are also discussed.

Using ab initio calculations within the density functional theory we investigate the stability and the electronic confinement of free-standing $\mathrm{InP}$ nanowires, grown along the [111] direction, as a function of the diameter $(d)$. We have found a small reduction of the cohesive energy for wires with diameters smaller than $5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, compared to the bulk value. Even for very thin wires the cohesive energy diminishes only a few percents. Also, we verified that, for $d\ensuremath{\sim}2\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, the energy gap enhancement scales as $1∕{d}^{2}$. However, for a very small diameter $(d\ensuremath{\sim}1.3\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$, this scaling is no longer valid. Concerning the electronic properties, we show that the top of the valence band level, as well as the bottom of the conduction band one are mainly localized in the central region of the wires.

We present a first-principles study of the structure and energetics of Bjerrum defects in ice Ih and compare the results to experimental electrical conductivity data. While the DFT result for the activation energy is in good agreement with experiment, we find that its two components have quite different values. Aside from providing new insight into the fundamental parameters of the microscopic electrical theory of ice, our results suggest the activity of traps in doped ice in the temperature regime typically assumed to be controlled by the free migration of L defects., Comment: 4 pages, 4 Figures, 1 Table

We investigate how the insertion of an oxygen atom in an atomically thin gold nanowire can affect its rupture. We find, using ab initio total energy density functional theory calculations, that O atoms when inserted in gold nanowires form not only stable but also very strong bonds, in such a way that they can extract atoms from a stable tip, serving in this way as a clamp that could be used to pull a string of gold atoms., Comment: 4 pages; 4 figures

Texto completo: acesso restrito. p.295–297 Submitted by Suelen Reis (suelen_suzane@hotmail.com) on 2013-02-15T14:55:54Z No. of bitstreams: 1 Silva.pdf: 241130 bytes, checksum: 96b4030adfa26d68798e7cb63327b999 (MD5) Approved for entry into archive by Fatima Cleômenis Botelho Maria (botelho@ufba.br) on 2013-02-18T16:22:53Z (GMT) No. of bitstreams: 1 Silva.pdf: 241130 bytes, checksum: 96b4030adfa26d68798e7cb63327b999 (MD5) Made available in DSpace on 2013-02-18T16:22:53Z (GMT). No. of bitstreams: 1 Silva.pdf: 241130 bytes, checksum: 96b4030adfa26d68798e7cb63327b999 (MD5) Previous issue date: 1996 We investigate impurity pair and triad formations in the narrow band-gap semiconductor n-doped InSb. It is found that in the region of the band-gap energy, Eg = 0.23 eV, at 0 K, the effects of such clusters play a relevant role in the optical properties around and below the ionization energy of an isolated impurity (i.e., Ei = 0.64 meV). The transition for the uncompensated system is obtained as the concentration of impurities exceeds Nc = 4.7 × 1013 cm−3.

formation of very thin gold NWs. We present realistic simulations for the breaking of these NWs, whose main features are very similar to the experimental results. We show how defects lead to the formation of one-atom constrictions in the Au NW, which evolves into a one-atom-thick necklace chain. Similarly to the experimental results, we obtain that these necklaces can get as long as five-atoms from apex to apex. Before breaking, we obtain relatively large Au-Au bond distances, of the order of 3.0‐3.1 A. A further pull of the wire causes a sudden increase of one of the bond distances, indicating the breaking of the NW. To get some more insight into the electronic structure aspects of this problem, we considered several of our tight-binding structures before breaking and studied them in detail using an ab initio method based on the DFT. By pulling the wire quasistatically in this case, we also observed the breaking of the wire at similar distances as in the TBMD. This result was independent of the exchange-correlation potential used—either the local density approximation ~LDA! or the generalized gradient approximation ~GGA!. The pulling force before rupture was obtained as 2.4 nN for the LDA, and 1.9 nN for the GGA. Finally, we also present a detailed analysis of the electronic structure properties for the Au neck atoms, such as the density of states and charge densities, for some configurations before the rupture.

We study the carbon substitutional impurity in GaAs at both cation and anion sites. We use first‐principle nonlocal pseudopotentials, and we find for the acceptor CAs impurity the expected shallow‐level defect behavior. The CGa impurity, however, behaves as a deep amphoteric impurity, not as a shallow donor, and in the neutral and negative charge states should cause virtually no lattice shrinking. We expect that this unusual behavior is the reason why the center was not directly detected up to now, although compensating for the shallow acceptor is known to occur with high carbon concentrations.

We report an ab initio calculation of periodically Si-\ensuremath{\delta}-doped GaAs, solving the Schr\"odinger equation in a three-dimensional (3D) system. We show, in the regime of high donor concentration, that the subband \ensuremath{\delta}, even for small period (p=45 \AA{}), exhibits small dispersion as does a 2D system. Also, we discuss the stability of the Si sheet in the bulk, and the electronic properties of the clustering ${\mathrm{Si}}_{\mathrm{Ga}\mathrm{\ensuremath{-}}}$${\mathrm{Si}}_{\mathrm{As}}$.

This contribution is concerned with the occurrence of metallic character in K-doped systems and the non-occurrence in Na-doped systems. An N-fold degenerate Hubbard model in the Gutzwiller approximation, previously presented by Jian Ping Lu, is utilized to discuss the metal–insulator transitions observed in these alkali-doped fullerides. The parameters used are derived from INDO calculations in the molecular cluster approach. © 1995 John Wiley & Sons, Inc.

In this work we used an empirical potential model in order to describe the bulk structure of the oxygen-rich Si-O-N amorphous system. From the atomic configuration obtained by Monte Carlo simulations, the local order structure and the pair correlation function are determined. The results show that the basic structure of the amorphous is well described by a random distribution of tetrahedra with a central Si atom bonded to nitrogen and oxygen atoms. The Fourier transform of the extended x-ray-absorption fine structure (EXAFS) signal obtained by ab initio multiple-scattering calculations, using the theoretical atomic structure as input, shows good agreement with the experimental EXAFS analysis performed at the silicon K edge.

The interaction of an iron atom with a single-wall carbon nanotube is investigated using spin-polarized total-energy first-principles calculations. A systematic study for the atom approaching the tube surface, both from outside and inside, is presented for several configurations to determine the equilibrium distances and the binding energies. It is shown that when the atom interacts with the tube from outside, a 3d 7 4s 1 effective configuration is obtained and the total magnetization is close to the atomic value. For the inside case, as a consequence of higher hybridization and a confinement effect, the magnetization decreases and the finally obtained effective configuration is 3d 8 4 s 0.

We report first-principles calculations on the adsorption of a metallic (6,6) single-walled carbon nanotube (SWCN) on the Si(001) surface. We find stable geometries for the nanotube between two consecutive dimer rows where C-Si chemical bonds are formed. The binding energy in the most stable geometry is found to be 0.2 eV/A. Concerning the electronic properties, the most stable structure shows an increase in the density of states near the Fermi level due to the formation of C-Si bonds enhancing the metallic character of the nanotube by the contact with the surface. These properties may lead one to consider metallic SWCNs adsorbed on Si substrates for interconnections and contacts on future nanoscale devices. Finally, the nature of the nanotube-surface interaction for nanotubes of larger diameters is also discussed.

Antisites, carbon substitutionals, and vacancy defects in BN nanotubes have been investigated by first-principles total energy calculations, based on a density functional spin-polarized method. All defects introduce localized energy levels inside the band gap. For some open-shell systems (carbon substitutionals and vacancy defects) the levels in the gap region present an exchange splitting greater than 0.5 eV. The ${\mathrm{C}}_{\mathrm{B}}$ and ${\mathrm{N}}_{\mathrm{B}}$ defects, in N-rich growth condition, and ${\mathrm{C}}_{\mathrm{N}}$ in B-rich growth condition, are the ones that present the lower formation energies.

Experimentally obtained atomically thin gold nanowires have presented exceedingly large Au-Au interatomic distances before they break. Since no theoretical calculations of pure gold nanowires have been able to produce such large distances, we have investigated, through ab initio calculations, how impurities could affect them. We have studied the effect of H, B, C, N, O, and S impurities on the nanowire electronic and structural properties, in particular how they affect the maximum Au-Au bond length. We find that the most likely candidates to explain the distances in the range of 3.6 A and 4.8 A are H and S impurity atoms, respectively.

We show, using first-principles spin-polarized total-energy calculations, that depending on the spin configuration of the system, the reaction of an O 2 molecule with a Si-Si bond in a suboxidized region might result either in a peroxy linkage defect (for a singlet spin state) or in a perfect Si-O-Si bond plus an interstitial O atom (for a triplet spin state). Even though the singlet has a lower energy than the triplet configuration, we find a rather small probability for triplet to singlet conversion. Therefore, as the O 2 in an SiO 2 interstitial site has a triplet configuration, this reaction spin dependence may have a strong influence on the high quality of the Si/SiO 2 interface.

Suspended gold nanowires have recently been made in an ultrahigh vacuum and were imaged by electron microscopy. Using realistic molecular dynamics simulation, we study the mechanisms of formation, evolution, and breaking of these atomically thin Au nanowires under stress. We show how defects induce the formation of constrictions that eventually will form the one-atom chains. We find that these chains, before breaking, are five atoms long, which is in excellent agreement with experimental results. After the nanowire’s rupture, we analyze the structure of the Au tip, which we believe will be universally present due to its highly symmetric nature. Miniaturization challenges science to understand the behavior of materials under circumstances other than those encountered in bulk solids. Gold nanowires suspended between electrodes have attracted a great deal of interest in the past few years, because of their great importance in fundamental physics as well as in technology, since miniaturization of devices needs metallic contacts. Au nanowires have been experimentally produced by contact of a metal surface with a scanning tunneling microscope [1 –4] and also from thin films [5 –7]. Surprises such as helical nanowire structures have been theoretically predicted [8] and experimentally observed [5] in gold. Under stress, these wires, before breaking, can get as thin as one-atom chains, and as long as five suspended atoms, with quite large interatomic distances between them [3,6,7]. Here we use realistic molecular dynamics (MD) simulations to understand the formation mechanisms of these atomically thin Au nanowires. We study in detail the evolution of a Au nanowire under stress, and show how defects induce the formation of necks that eventually will form the one-atom chains. After the nanowire’s rupture, we analyze the structure of the Au tip, which we believe will be universally present due to its highly symmetric nature.

Predictions of the electronic and structural properties of silicon substitutional doping in carbon nanotubes are presented using first-principles calculations based on the density-functional theory. A large outward displacement of the Si atom and its nearest-neighbor carbon atoms is observed. For the two tubes studied [metallic $(6,6)$ and semiconducting $(10,0)]$ the formation energies of the substitutional defects are obtained around 3.1 eV/atom. In the doped metallic nanotube case a resonant state appears about 0.7 eV above the Fermi level, whereas for the semiconductor tube, the silicon introduces an empty level at approximately 0.6 eV above the top of the valence band.

Free volume Monte Carlo simulation of the melting and quenching of a Si nanosystem is used to produce an amorphous layer on top of a nanocrystal, and a completely amorphous system. The interface is kept free of any temperature gradient during the melting. We find that the crystal-amorphous transition layer is thicker than previous studies indicated, and keeps memory from the crystal structure. In amorphous bulk, the concentration of fivefold coordinated atoms is substantial, and is associated with a more open structure than fourfold-coordinated regions. All calculations were done with an empirical Tersoff potential.

We report the electronic and structural properties of silicon doped carbon nanotubes using first principles calculations based on the density-functional theory. In the doped metallic nanotube a resonant state appears about 0.7 eV above the Fermi level and for the semiconductor tube the Si introduces an empty level at approximately 0.6 eV above the top of the valence band.

The electronic and structural properties of finite junctions produced by connecting two C nanotubes, saturated with hydrogens in both edges, are investigated using first-principles calculations, through self-consistent field Hartree-Fock-Roothaan method. The main target of our study is a molecular junction, which connects (10,0) and (6,6) tubes by the introduction of pentagon-heptagon pair defects diametrically opposed in the structure. The charge distributions, the character of the highest occupied molecular orbitals and the densities of states are determined for the finite (10,0) and (6,6) nanotubes and for the formed junction. An energetic analysis is also performed using ab-initio approach and empirical Tersoff potential.

Using two different approaches: Monte Carlo simulations with Tersoff empirical potential and first principles calculations, the energetics and the structural properties of double-walled carbon and silicon nanotubes are investigated. Through Tersoff potential, the changes on cohesive energies for the Si and C systems are determined for several outer tubules for a fixed inner tube. Adopting first principles calculations, based on density functional theory, the trends, in terms of the cohesive energies, are compared with the corresponding obtained results using Tersoff empirical potential. The structures, specially of the most stable double-walled nanotubes, are discussed.