Search

A theoretical study of the effects of disorder on the Mn-Mn exchange interactions for Ga1-xMnxAs diluted magnetic semiconductors is presented. The disorder is intrinsically considered in the calculations, which are performed using an ab initio total energy density-functional approach, for a 128 atoms supercell, and by considering a variety of configurations with 2, 3 and 4 Mn atoms. Results are obtained for the effective J$^{Mn-Mn}_n$, from first (n = 1) all the way up to sixth (n = 6) neighbors via a Heisenberg Hamiltonian used to map the magnetic excitations from ab initio total energy calculations. One then obtains a clear dependence in the magnitudes of the J$^{Mn-Mn}_n$ with the Mn concentration x. Moreover, we show that, in the case of fixed Mn concentration, configurational disorder and/or clustering effects lead to large dispersions in the Mn-Mn exchange interactions. Also, calculations for the ground-state total energies for several configurations suggest that a proper consideration of disorder is needed when one is interested in treating temperature and annealing effects.

Metal nanowires and in particular gold nanowires have received a great deal of attention in the past few years. Experiments on gold nanowires have prompted theory and simulation to help answer questions posed by these studies. Here we present results of computer simulations for the formation, evolution and breaking of very thin Au nanowires. We also discuss the influence of contaminants, such as atoms and small molecules, and their effect on the structural and mechanical properties of these nanowires.

A unified band structure model is proposed to explain the magnetic ordering in Mn-doped semiconductors. This model is based on the p–d and d–d level repulsions between the Mn ions and host elements and can successfully explain magnetic ordering observed in all Mn doped II–VI and III–V semiconductors such as CdTe, GaAs, ZnO, and GaN. The model can also be used to explain the interesting behavior of GaMnN, which changes from ferromagnetic ordering to antiferromagnetic ordering as the Mn concentration increases. This model, therefore, is useful to provide a simple guideline for future band structure engineering of magnetic semiconductors.

In this work a theoretical study of Ti monomers and wires interacting with an (8, 0) semiconductor single-wall carbon nanotube (SWNT), by inside as well as outside faces, is presented. Spin-polarized total-energy ab initio calculations, based on the density functional theory, are used to describe the structural, electronic and magnetic properties of the studied systems. The most stable configurations for monomers are found to be over the centre of a C-C bond for inside and over the midpoint of the centre of the hexagonal site for outside. Considering that the Ti atoms on the tube surface tend to form continuous wires, to allow a comprehensive view of the interaction of the Ti wires with the SWNT surface, we present a complete understanding, both from inside and outside the nanotube. Our calculations have shown that the most stable configuration is with the wire inside the tube, with the resulting electronic structures showing a metallic system with high hybridization between the Ti and C atoms and a large charge transfer from Ti to C atoms. For Ti wire adsorbed inside the tube the low spin configuration is shown to be more stable than high spin configuration and the opposite behaviour is observed for the corresponding outside case. These novel results are relevant for the understanding of Ti atoms covering and filling SWNTs, demonstrating the high stability of these systems and suggesting that they can be useful for future use in nanodevices, in particular for spintronics and nanosensors.

Suspended gold nanowires have recently been made in an ultra-high vacuum ambient and were imaged by electron microscopy. Two puzzles were presented: one atom thick wires are produced and some of the atomic distances between these atoms before their breaking were too large. Simulations using realistic molecular dynamics method were able to unveil some processes to explain the mechanisms of formation, evolution, and breaking of these atomically thin Au nanowires under stress. The calculations showed how defects induce the formation of constrictions that eventually will form the one-atom chains. Atomically thin chains, five atoms long were obtained, before breaking. The results were in excellent agreement with experimental results except for the large Au-Au distances. In fact no theoretical calculation of pure gold nanowires have been able to produce such large distances. Light impurities that cannot be imaged in these experiments may be responsible for these large Au-Au distances. Using ab initio total energy calculations based on the density functional theory, we have studied the effect of H, C, O, N, B, S, CH, CH2, and H2 impurities on the nanowire’s electronic and structural properties, in particular how they affect the rupture of the nanowire. We find that the impurities tend to locally increase the nanowire’s strength, in such a way that its rupture always occurs at an Au-Au bond and never at an Au-X bond (X being an impurity). In particular, oxygen seems to form very stable bonds that may be used to pull longer Au chains. Regarding the observed large Au-Au bond lengths, it was found, based on quasi-static calculations, that the best candidate to explain the large distances is H. However, some particular experimental conditions may lead to different results.

The structural and electronic properties of a metallic single-walled carbon nanotube (CNT), adsorbed on hydrogenated Si(0 0 1) surfaces, have been investigated by first-principles calculations. We find that the electronic properties of the adsorbed CNT can be ruled by the H concentration along the CNT–H/Si(0 0 1) contact region. (i) On the fully hydrogenated Si(0 0 1), the CNT is physisorbed, preserving almost unchanged its metallic character. (ii) Removing half the H atoms along the adsorption site, we find an enhancement on the metallicity of the adsorbed CNT. (iii) When all the H atoms along the adsorption site are removed, the adsorbed CNT becomes semiconducting, exhibiting an energy gap. These results suggest that metallic CNTs adsorbed on H/Si(0 0 1) could be transformed into metal–semiconductor junctions by grading the H concentration along the CNT–surface interface.

Electronic and structural properties of several charge states of vacancies, antisites and carbon substitutional impurities in a (10, 0) BN nanotube are investigated through density functional theory calculations. The formation energies indicate that neutral and simply charged states occur in the range of allowable electronic chemical potential. For carbon substitutional impurities, the most probable states are, besides the neutrals, the positively charged state for carbon at a boron site (CB+), and the negatively charged state for carbon at a nitrogen site (CN−). The charge compensation between neighbouring pairs of CB+ and CN− defects is suggested to explain the successful experimentally obtained boron carbonitride nanotubes. Vacancies always present high formation energies. The neutral and positively charged states of the nitrogen antisite show low formation energies. The calculated formation energies for all defects studied here can be interpreted as due to two main effects: a tendency to recover the number of electrons of the defect-free BN nanotube and the screening effects due to the perturbative potential of the defects.

We propose the use of the Si atom in the C 5 9 Si molecule as a possible way to controllably anchor fullerene molecules on an Si surface, due to the formation of a strong bond to one of the Si surface atoms. We considered a monohydride Si(100) surface with the H removed from one of the Si surface atoms, and the C 5 9 Si was adsorbed at this site. Through ab initio calculations based on Density Functional Theory, we obtained a final Si s u r f a c e -Si C 5 9 S i bond distance of 2.37 A, and a binding energy of -3.0 eV. Considering the reaction H s u r f a c e + C 5 9 SiH → (C 5 9 Si) a d s o r b e d + (H 2 ) g a s , we find this process to be exothermic by 0.41 eV. Upon adsorption of an isolated molecule, we find four levels in the gap, three empty and one singly occupied, all of them being basically localized at the C 5 9 Si molecule.

We report first-principles calculations on the adsorption of an armchair (6,6) single-walled carbon nanotube (CNT) on the Si(0 0 1) surface. We study several well-ordered adsorption configurations for the nanotube on the Si surface. Our results show stable geometries between two consecutive Si-dimer rows (the surface trench). The binding energy per tube length for the CNT in the most stable geometry is calculated to be 0.2 eV/� In this geometry, we observe the formation of C–Si chemical bonds. The density of states along the dimer rows for the lowest-energy adsorbed configuration shows an increase in the number of states at the Fermi level. This suggests an enhancement of the nanotube metallic character throughout the contact with the Si surface due to the formation of the C–Si bonds. These properties may lead to consider metallic CNTs as one-dimensional wires on the silicon surface with promising applications for contact and interconnections of future nanoscale electronic devices. � 2004 Elsevier B.V. All rights reserved.

Using ab initio calculations, based on the density functional theory, we have investigated the electronic, magnetic and structural properties of a Mn neutral impurity on the bare and hydrogenated Si(1 0 0) surface, as well as on bulk Si. We have observed that the most stable site for adsorption on the bare surface is near the second layer, below a substrate dimer, whereas for the hydrogenated surface it is in the through, between two dimer rows. Hydrogenation makes the surface less reactive and increases the formation energy of the impurity in the surface. In the bare surface, we have found a substitutional site that has the same formation energy as an interstitial one, and this can be an important route for the development of dilute magnetic semiconductors based on Si.

Using ab initio density functional theory total energy calculations, we study the influence of H, B, C, N, O, and S in the rupture of a gold nanowire. In particular, using an as realistic as possible model for a suspended gold nanowire under stress, we observe that the Au wire always breaks at an Au–Au bond, with a maximum bond length between 3.0 and 3.1 A. Therefore, the experimentally observed large Au–Au bonds before the rupture of the nanowire (≈3.6 A) are probably due to the presence of light impurities (X) forming Au–X–Au bonds. We obtain that the maximum Au–Au distance, for X equals C or N, is of the order of 3.9 A, whereas for B and O it is of the order of 4.1 A. On the other hand, for H this maximum distance before the rupture of the wire is approximately 3.6 A, being the best candidate to explain the experimental results. For both C and H impurities, we present a detailed analysis of the neck atoms electronic structures, and compare them with similar results for the pure nanowire.

We performed a theoretical investigation on the effects of hydrogenation and dehydrogenation in the structural properties of amorphous silicon carbide (a-SiC). The calculations were carried out combining interatomic potentials with free volume Monte Carlo simulations. Our prototype amorphous silicon carbide systems contained as much as 85 000 atoms, allowing for a good statistical sampling. We addressed the role of hydrogen on the local and mid-range order of the material. We found that carbon atoms segregate, forming small clusters, embedded in an extensive silicon network. Ring statistics showed that hydrogenation increases the concentration of micro-voids in the structure and reduces its mid-range order. On the other hand, dehydrogenation recovers most of the original structural order.

A systematic study of Mn monomers, dimers, trimers, and wires interacting with an (8, 0) semiconductor single-wall carbon nanotube (SWCN) is presented. Spin-polarized total-energy ab initio calculations based on the density functional theory are used to describe the structural, electronic and magnetic properties of all studied systems. For Mn monomers, either outside or inside the nanotube, the most stable configuration is found to be over the centre of the hexagonal site. The most stable geometry for outside dimers presents the Mn atoms adsorbed directly on top of C atoms, when in a high-spin configuration, whereas for a low-spin configuration the Mn atoms are adsorbed on bond-centred sites at opposite sides of a hexagonal ring, with the Mn?Mn bond aligned in a diagonal direction relative to the tube axis in both cases. There are many low energy configurations (at above the lowest energy ones) at distinct orientations, for both the low and high energy configurations. For trimers, two kinds of Mn structures are investigated: compact or open. The compact trimers are found to be more stable than the open systems by approximately 1?eV/Mn atom. A monoatomic wire in a zig-zag configuration has a binding energy that is intermediate between the open and the compact trimers, independently of the spin configuration. For all the investigated Mn structures adsorbed on the SWCN, either high-spin (HS) or low-spin (LS), the interactions between Mn atoms and between Mn and C atoms become stronger as the Mn coordination number increases. The resulting magnetic moments for all adsorbed systems are found to be close to their original values for the corresponding free Mn structures.

We have investigated the formation evolution and breaking of a gold nanowire using molecular dynamics simulation. Results reproduce many features observed experimentally. Suspended gold nanowires have recently been produced in an ultra-high vacuum and were imaged by electron microscopy. Here we simulate gold nanowires under stress. In particular, we discuss the mechanisms of formation, evolution and breaking of these atomically thin Au nanowires.

We study the structural, electronic and magnetic properties of Fe and Mn monomers and dimers adsorbed on a semiconductor (8,0) single-wall carbon nanotube. Using spin-polarized total-energy ab initio calculations based on the density-functional theory, we find for both Fe and Mn monomers the hexagonal center as the most stable adsorption site. For Fe dimers in the most stable configuration, the atoms are on opposite C–C bond-centered positions (BC–BC), with an Fe–Fe distance of 2.23 A. For Mn dimers, on the other hand, the most stable configurations are found to be with the Mn atoms adsorbed on top of C atoms, for a high-spin state (Mn–Mn distance of 2.6 A), and a slightly lower-energy BC–BC configuration for the low-spin state (Mn–Mn distance of 2.53 A). These results show that the dimer configurations are not simply related to the isolated monomer structures. The resulting magnetic moments for the adsorbed dimers are found to be similar to their corresponding values in the free diatomic molecules.

We perform a detailed theoretical study, within the density-functional theory, of the electronic structure and magnetic properties of Ga1−xMnxAs diluted semiconductors. Ab initio total energy results provide evidence that the appearance of a ferromagnetic state in these materials is due to an exchange coupling between the localized ↑ S= 5 2 Mn spins mediated by quasi-localized ↓ holes, with strong p-like character, surrounding the fully polarized Mn d5-electrons. From total energy differences, we find the effective Mn–Mn coupling always ferromagnetic, with the Mn–Mn interaction intermediated by an antiferromagnetic coupling of each Mn spin to the holes.

In this work the structural, electronic and magnetic properties of one-dimensional Fe chains interacting with a semiconductor single-wall carbon nanotube are investigated using first-principles spin-polarized calculations. A systematic study of several configurations of a Fe wire interacting with a nanotube both from outside as well as inside is presented, and the resulting equilibrium distances, binding energies and magnetizations are discussed. The most stable configurations are compared with previous results obtained for a single Fe atom interacting with the tube.

The energetics and structural properties of atoms and molecules on a substitutional Si atom in single wall carbon nanotubes (SWCN) are investigated using first principle calculations based on density-functional theory. A detailed analysis is performed for the geometry and the electronic structures of a Si-doped semiconducting (10,0) carbon nanotube interacting with F, Cl, H, CH3, and SiH3. A common feature for the systems with these atoms or molecules is the presence of one half-filled level close to the top of the valence band. The specific position of this level in the gap depends on the chemisorbed species and the binding energy between this species and the Si atom.

We studied the effects of arsenic on properties of dislocations in silicon. The theoretical investigation was carried out using ab initio total energy methods, based on the density functional theory. We find that the interaction of an arsenic impurity in the crystal with a dislocation results in a charge exchange, driving the dislocation core to a negative charge state. This interaction is essentially electrostatic and attractive, and leads to arsenic segregation. Although arsenic segregation to the core is energetically favourable, formation of arsenic pairs inside the core is energetically unfavourable. We also investigated the role of vacancies in arsenic diffusion inside the dislocation core.

We investigated through ab initio total energy calculations the interaction of arsenic impurities with the core of a 30° partial dislocation in silicon. It was found that when an arsenic atom sits in a crystalline position near the dislocation core, there is charge transfer from the arsenic towards the dislocation core. As a result, the arsenic becomes positively charged and the core negatively charged. The results indicate that the structural changes around the impurity are very small in both environments, namely, the crystal and the dislocation core. In this scenario, the interaction between arsenic and the core is essentially electrostatic, which eventually leads to arsenic segregation. The segregation energy was found to be as large as 0.5 eV/atom. Additionally, it was found that arsenic pairing inside the core is not energetically favorable.

Van der Waals (vdW) metal/semiconductor heterostructures have been investigated through first-principles calculations. We have considered the recently synthesized borophene (Mannix et al 2015 Science 350 1513), and the planar boron sheets (S1 and S2) (Feng et al 2016 Nat. Chem. 8 563) as the 2D metal layer, and the transition metal dichalcogenides (TMDCs) MoSe2, and WSe2 as the semiconductor monolayer. We find that the energetic stability of those 2D metal/semiconductor heterojunctions is mostly ruled by the vdW interactions; however, chemical interactions also take place in borophene/TMDC. The electronic charge transfer at the metal/semiconductor interface has been mapped, where we find a a net charge transfer from the TMDCs to the boron sheets. Further electronic structure calculations reveal that the metal/semiconductor interfaces, composed by planar boron sheets S1 and S2, present a p-type Schottky barrier which can be tuned to a p-type ohmic contact by an external electric field.

We perform an ab initio study of the electronic structure and magnetic properties of 3d, 4d and 5d transition metals (TM) adsorbed on freestanding and Ag(1 1 1)–supported –borophene. The stability of TM adsorption is high for all atoms and increases with the period. For the 3d TM adsorption we observed strong exchange effects. The Ag(1 1 1)–surface induced small effects on the calculated properties. Studying the magnetic interaction between TMs, VIB atoms showed direct exchange, while VIIB and Fe showed 2p(B)–mediated indirect exchange. In the ultimate case of a one-dimensional TM array, Ru and Os also show direct exchange effects.

Crystalline 2D boron sheets, known as borophene, are the most recently-discovered type of 2D materials, and very little is known about them. Different configurations of borophene have been reported stable when grown on Ag(1 1 1) surface under well controlled conditions. One of this configurations is partially oxidized while the other one remains quite inert to oxidation when exposed to ambient conditions. In this work, the oxidation process of the free-standing and Ag(1 1 1)-supported borophene is modeled using first-principles calculations based on density functional theory (DFT). On the free-standing case, the oxygen molecule may go through a triplet to singlet transition, followed by a barrierless oxidation process. This transition is not observed upon the presence of the Ag(1 1 1) surface, what accelerates the oxidation process. We also propose that the different structures of borophene reported in the literature can be understood by this oxidation process. Oxidation of the buckled structure could induce the planar structure with an ordered distribution of vacancies.

The observation of the quantum spin Hall effect without the need of an external magnetic field in HgTe/CdTe heterostructures triggered the study of materials exhibiting persistent spin-polarized electronic currents at their interfaces. These Dirac-like spin states are predicted to be topologically protected against perturbations preserving time-reversal symmetry. However, nonmagnetic (time-reversal preserving) perturbations will certainly affect these interface states. In this work, the density functional theory is used to characterize the topologically protected states of the (001) HgTe/CdTe heterostructure as well as to understand the influence of external adiabatic parameters as pressure and electric fields on these states. The intrasite Hubbard U term is seen to be important to correctly describe the HgTe bulk band structure. It is shown that, differently from the three-dimensional topological insulators, the HgTe/CdTe interface states present fully in-plane Rashba-like spin texture. Further, biaxial external pressures and electric fields perpendicular to the interfaces are seen to change the energetics and dispersion of the protected states, modifying the energy ordering of the crossing of the polarized interface states inside the band structure, and altering their Fermi velocities while not changing the topological quantum phase. These adiabatic variables can then be used to tune the topologically protected states with respect to the Fermi level.

In this Letter, we study the structural and electronic properties of single-layer and bilayer phosphorene with graphene. We show that both the properties of graphene and phosphorene are preserved in the composed heterostructure. We also show that via the application of a perpendicular electric field, it is possible to tune the position of the band structure of phosphorene with respect to that of graphene. This leads to control of the Schottky barrier height and doping of phosphorene, which are important features in the design of new devices based on van der Waals heterostructures.

Topological insulators are a new class of matter characterized by the unique electronic properties of an insulating bulk and metallic boundaries arising from non-trivial bulk band topology. While the surfaces of topological insulators have been well studied, the interface between topological insulators and semiconductors may not only be more technologically relevant, but the interaction with non-topological states may fundamentally alter the physics. Here, we present a general model to show that this type of interaction can lead to vertical twinning of the Dirac cone, whereby the hybridized non-spin-degenerate interfacial states cross twice as they span the bulk bandgap. This hybridization leads to spin-momentum locking of non-topological states with either helical (clockwise or anticlockwise) or even anti-helical (negative winding number) spin orientation depending on the parametization of the interaction. Model results are corroborated by first-principles calculations of the technologically relevant Bi2Se3 film van der Waals bound to a Se-treated GaAs substrate.

We show that germanene nanoroads embedded in a completely hydrogenated germanene (germanane) exhibits a quantum spin Hall effect (QSHE). These nanoroads can be obtained experimentally by local hydrogen dissociation from germanane. Using first principle calculations we predict that germanene nanoroads with zigzag interfaces show dissipationless conducting channels with in-plane and out-of-plane spin textures., 5 pages, 5 figures

The fundamental properties that compose the thermoelectric figure of merit are investigated in the confined systems of PbSe and PbTe nanowires, with the goal to improve the thermoelectric efficiency. Using the Landauer electronic transport theory, we verify that the figure of merit can be several times larger than the bulk value for nanowires with diameters down to the one nanometer scale. This enhancement in the thermoelectric efficiency is primarily due to the reduction of the thermal conductivity and an increase in the power factor. The origin of these desireable properties, that enable the transformation of heat into electricity, comes from the confinement effect which increases the density of states around the Fermi level, either for an n- or p-type system.

We have performed an ab initio investigation of the electronic properties of the graphene sheet adsorbed by Ru adatoms (Ru/graphene). For a particular set of triangular arrays of Ru adatoms, we find the formation of four (spin-polarized) Dirac cones attributed to a suitable overlap between two hexagonal lattices: one composed by the C sites of the graphene sheet, and the other formed by the surface potential induced by the Ru adatoms. Upon the presence of spin-orbit coupling (SOC) nontrivial band gaps take place at the Dirac cones promoting several topological phases. Depending on the Ru concentration, the system can be topologically characterized among the phases i) Quantum Spin Hall (QSH), ii) Quantum Anomalous Hall (QAH), iii) metal iv) or trivial insulator. For each concentration, the topological phase is characterized by the ab-initio calculation of the Chern number., Comment: 8 pages, 6 figures

The adsorption of the Ge monomer on the Si(1 0 0) surface is studied through first principles calculations based on the density-functional theory. We have observed that, for a given position of the monomer on the surface, there are many local minima which differ in the substrate local configuration of the buckling of the silicon dimers. We show that this local configuration may also play an important role for the diffusion of these ad-atoms.

Using ab initio calculations, we computed the core reconstruction energies of {111} 30° partial dislocations in zinc-blende semiconductors. Our results show a direct correlation between core reconstruction energies and the experimental activation energies for the velocity of 60° dislocations. The electronic structure of unreconstructed dislocation cores comprises a half-filled band, which splits up in bonding and antibonding levels upon reconstruction. The levels in the electronic gap come from the core of β dislocations, while the levels related to α dislocations lie on the valence band.

We study, through first principles calculations based on the density functional theory, the adsorption of Ge structures on the Si(100) surface. We calculate a variety of STM images of monomers and trimers, and we show that when the monomers are isolated they can be seen in both empty and filled-state images, whereas when they are adsorbed close to other structures, as in the trimers, they are almost invisible to filled-state images.

Even though silicon nanotubes have never been observed, this paper attempts to establish the theoretical similarities and differences between Si and C structures. Through the use of two alternative theoretical approaches, the first principles calculations and empirical potential, the electronic and structural properties of this hypothetical material are examined. The first principles calculations are based on the density-functional theory and it is shown that depending on their chiralities and diameters, the silicon nanotubes may present metallic (armchair) or semiconductor (zigzag and mixed) behaviors, similar to carbon structures. It is shown that the gap decreases in inverse proportion to the diameter, thus approaching zero for planar graphite, as was expected. In the second alternative approach, the Monte Carlo simulations are used with the Tersoff's empirical potential to present a systematic study on the thermal behavior of these new structures. It is shown that similarities like band structures and density of states are observed between the C and Si nanotubes. Nevertheless, there are relevant discrepancies in the thermal stabilities and energy differences between the cohesive energies per atom for the two tubes, compared with the corresponding bulks, implying the very improbable structure of the silicon nanotubes.

We performed a theoretical investigation on the interaction of dopant atoms with extended defects in semiconductors using ab initio total energy calculations. Dopant atoms (donors and acceptors) were found to segregate in stacking faults in silicon and in gallium arsenide, with segregation energies as large as 0.2 eV. A general trend on the electronic properties of segregated dopants in stacking faults is that the donor (or acceptor) electronic energy levels become deeper in the electronic gap, as compared to those levels for the defect in a crystalline site.

We investigated core properties of dislocations in zinc-blende semiconductors using ab initio total energy calculations. The core reconstruction energy of partial dislocations was found to scale almost linearly with the experimental dislocation activation energy. The electronic band structure related to dislocation cores was also determined. In an unreconstructed core, the gap states comprise a half-filled one-dimensional band, which splits up in bonding and antibonding states upon reconstruction. The energy states which lie in the electronic gap come from the cores of β partials, while those related to α partials remain resonant in the valence band.

We study, using first principles calculations, the electronic and structural properties of an isolated vacancy and self-interstitial defects in germanium. For the vacancy we analyze the lattice relaxations for the charge states (++), (+), and (0), and show that the atomic displacements, albeit small, are not completely negligible for the second-nearest-neighbor atoms to the vacancy site. Moreover, we show that the relaxations decay slowly along the zigzags that go through the vacancy site. For the self-interstitial we present an analysis of the charge density, which supports the description of this defect as a four-atoms ring strongly attached to the lattice, which we have named a kite-defect.

Using ab initio total-energy calculations, we computed core reconstruction energies of partial dislocations in zinc-blende semiconductors. The reconstruction energy of 30° partials was found to scale almost linearly with the experimental activation energy of 60° dislocations. The electronic structure of a dislocation shows that in an unreconstructed core, the gap states comprise a half-filled one-dimensional band, which splits up into bonding and antibonding states upon reconstruction. The energy states which lie in the electronic gap come from the core of β-partials, while those related to α-partials remain resonant in the valence band.

We performed a theoretical investigation on the effects of extended defects on the structural and electronic properties of dopant atoms in gallium arsenide. We observed that silicon impurities segregate at GaAs stacking faults. A Si atom at a Ga site in a stacking fault in either a neutral or a negatively charged state is energetically favourable as compared to a Si atom at a Ga site in a crystalline environment by as much as 0.2 eV. Additionally, a substitutional Si impurity in the negative charge state in a stacking fault has a distinct structure as compared to the same impurity in a crystal. The results suggest that the stacking fault may prevent the formation of metastable defects, such as DX centres.

A complete microscopic picture of the germanium vacancy is presented, and our results are compared with recent measurements. We analyze, through first principles calculations, the structural relaxations, Jahn-Teller distortions, and orbitals for the charge states $(++,+,0,\ensuremath{-},\ensuremath{-}\ensuremath{-}).$ The formation energies for the different charge states are presented, as well as the positions of the $(++/+),$ $(+/0),$ $(0/\ensuremath{-}),$ and $(\ensuremath{-}/\ensuremath{-}\ensuremath{-})$ levels, and we obtain that the vacancy in Ge is not an Anderson negative-$U$ system, as opposed to the silicon vacancy. We propose as an explanation a much smaller electron-lattice coupling for the E mode in germanium than in silicon.

The electronic and structural properties of an isolated vacancy and self-interstitial defects in germanium are studied through ‘parameter free’ calculations. We analyze the lattice relaxations and Jahn–Teller distortions for several charge states. Our results for ionization levels of the Ge-vacancy are in fair agreement with experimental results. In contrast to the silicon vacancy we obtain that the germanium vacancy does not present an Anderson negative-U. For the self-interstitial defect we obtain, similarly to silicon, that the lowest-energy configuration is the split dumbbell configuration.

We study, using ab initio calculations, small structures that are present at the early stages of growth of Ge on the Si(1 0 0) surface. Ad-atoms, ad-dimers, and ad-trimers are investigated. We consider different configurations of the adsorbed structures, and present results for their relaxed geometries and relative energies.

We have developed an interatomic potential to investigate structural properties of hydrogenated amorphous silicon nitride. The interatomic potential used the Tersoff functional form to describe the Si–Si, Si–N, Si–H, N–H, and H–H interactions. The fitting parameters for all these interactions were found with a set of ab initio and experimental results of the silicon nitride crystalline phase, and of molecules involving hydrogen. We investigated the structural properties of unhydrogenated and hydrogenated amorphous silicon nitride through Monte Carlo simulations. The results show that depending on the nitrogen content, hydrogen has a different chemical preference to bind to either nitrogen or silicon, which is corroborated by experimental findings. Besides, hydrogen incorporation reduced considerably the concentration of undercoordinated atoms in the material, and consequently the concentration of dangling bonds.

We show via first principle calculations why the antistructure-pair (GaAs–ASGa) in the nearest neighbor (nn) configuration is not experimentally detected. A low energy barrier for the concerted exchange mechanism is seen to be responsible for the recovery of the ideal crystal structure. This explains the fact that, although energetically more favorable when compared with the next nearest neighbor (nnn) case, the nn antistructure-pair is not observed at room temperature.

In this work, the electronic properties of the system composed by the CO molecules adsorbed on Ti-coated single-wall carbon nanotubes (SWNTs) are studied through first principles calculations. The changes in the electronic properties of the interaction of the CO molecules with a linear Ti wire covering an (8, 0) semiconductor SWNT are analyzed for different CO concentrations. A strong interaction between CO molecules and the SWCT/Ti system is observed, which decreases when the concentration of CO molecules increases. The resulting system are shown to be very sensitive to the CO concentration adsorbed on the tube/Ti system, making that the SWNT, which is originally semiconductor and becomes metallic after Ti covering, to recover the semiconductor behavior again when enough high concentrations of CO molecules is adsorbed on the SWNT/Ti system. These three distinct steps (semiconductor/metallic/semiconductor) constitute the basis for a feasible, flexible and efficient sensor device for CO molecule recognition.

First principles calculations of a substitutional chromium impurity in Al2O3 and analysis of its multiplet structure reveal that the major effect of the corundum to Rh2O3 (II) phase transformation in ruby’s optical transitions should be a redshift of the broad U and Y absorption lines across the transformation. A similar phenomenon is experimentally detected above 30 GPa, which is too low a pressure to attribute this phenomenon to a phase transition in the host. However, an anomalous relaxation around the impurity in corundum could explain the observed frequency shifts. The underlying cause of such abnormal relaxation could be a similar transformation in Cr2O3 taking place below 30 GPa. The centroids of the R, R0, and B lines are quite insensitive to the transformation. [S0031-9007(98) 07269-X]

An ab initio calculation based on pseudopotential density-functional theory is used. A supercell with 128 atoms is adopted in Car-Parrinello scheme. We study the antistructure pairs (GaAS-AsGa, bot...

The structural properties of N, P, and As doped a-Si are investigated through ab initio total energy calculations. We show that these impurities are stable in three-fold coordinated sites. In four-fold coordinated sites, the P and As impurities are metastable and the N impurity is unstable . The lattice relaxation processes lead to a very small Mott–Hubbard energy, U .

Using the Hartree-Fock method followed by second-order perturbation theory, the structural and electronic properties of the Ga n As m (n + m ⪯ 8) clusters and its positive and negative ions are examined. The ground state structures are obtained through geometry optimization calculations without spatial symmetry constraints. Electronic and structural properties are then calculated. In particular it is verified an alternating behavior in the ionization potential beyond three-atom clusters. A structural pattern is noticed where the embryonic forms of these clusters are observed to present highly symmetrical configurations formed by the central As atoms. The Ga atoms appear at positions that enhance a hybridization and chemical ordering which tends to that presented by the bulk. The change for a layer-type structure is observed to occur already in the eight-atom stoichiometric cluster. A theoretical explanation of previously obtained mass spectrometry results is suggested.