M. Buongiorno Nardelli

Mn Interstitial Diffusion in (Ga, Mn)As

We present a combined theoretical and experimental study of the ferromagnetic semiconductor (Ga,Mn)As which explains the remarkably large changes observed on low-temperature annealing. Careful control of the annealing conditions allows us to obtain samples with ferromagnetic transition temperatures up to 159 K. Ab initio calculations, in situ Auger spectroscopy, and resistivity measurements during annealing show that the observed changes are due to out diffusion of Mn interstitials towards the surface, governed by an energy barrier of 0.7-0.8 eV. Electric fields induced by Mn acceptors have a significant effect on the diffusion.

Mechanical properties, defects and electronic behavior of carbon nanotubes

Abstract Using state-of-the-art classical and quantum simulations, we have studied the mechanical and electronic response of carbon nanotubes to external deformations, such as strain and bending. In strained nanotubes the spontaneous formation of double pentagon–heptagon defect pairs is observed. Tubes containing these defects are energetically preferred to uniformly stretched tubes at strains greater than 5%. These defects act as nucleation centers for the formation of dislocations in the originally ideal graphitic network and constitute the onset of further deformations of the carbon nanotube. In particular, plastic or brittle behaviors can occur depending upon the external conditions and tube symmetry. We have also investigated the effects that the presence of addimers has on strained carbon nanotubes. The main result is the formation of a new class of defects that wrap themselves about the circumference of the nanotube. These defects are shown to modify the geometrical structure and to induce the formation of nanotube-based quantum dots. Finally, we computed transport properties for various ideal and mechanically deformed carbon nanotubes. High defect densities are shown to greatly affect transport in individual nanotubes, while small diameter bent armchair nanotubes mantam thier basic electrical properties even in presence of large deformations with no defects involved.

Spontaneous polarization and piezoelectricity in boron nitride nanotubes

Ab initio calculations of the spontaneous polarization and piezoelectric properties of boron nitride nanotubes show that they are excellent piezoelectric systems with response values larger than those of piezoelectric polymers. The intrinsic chiral symmetry of the nanotubes induces an exact cancellation of the total spontaneous polarization in ideal, isolated nanotubes of arbitrary indices. Breaking of this symmetry by intertube interaction or elastic deformations induces spontaneous polarization comparable to those of wurtzite semiconductors. order of magnitude weaker than those of PZT. 3 In this paper, we examine spontaneous polarization and piezoelectricity in boron nitride nanotubes ~BNNTs! in order to estimate their potential usefulness in various pyroelectric and piezoelectric device applications, and to understand the interplay between symmetry and polarization in nanotubular systems. BNNTs, broadly investigated since their initial predic- tion 4 and succeeding discovery, 5 are already well known for their excellent mechanical properties. 6 However, unlike car- bon nanotubes ~CNTs !, most of BN structures are noncen- trosymmetric and polar, which might suggest the existence of nonzero spontaneous polarization fields. Recently, these properties have been partially explored by Mele and Kral, using a model electronic Hamiltonian. 7 They predicted that BNNTs are piezoelectric and pyroelectric, with the direction of the spontaneous electric field that changes with the index of the tubes. The ab initio calculations presented in this pa- per provide a much fuller description and show that BNNT systems are indeed excellent lightweight piezoelectrics, with comparable or better piezoelectric response and superior me- chanical properties than in piezoelectric polymers. However, contrary to the conclusions of Ref. 7, our combined Berry phase and Wannier function ~WF! analysis demonstrates that electronic polarization in BNNTs does not change its direc- tion but rather grows monotonically with the increasing di- ameter of the tube. Furthermore, the electronic and ionic spontaneous polarizations in BNNTs cancel exactly and these systems are pyroelectric only if their intrinsic helical symmetry is broken by, e.g., intertube interactions or elastic distortions. The rest of this paper is organized as follows: Sec. II briefly reviews the formulation of the modern polarization theory in terms of Berry phases or Wannier functions. It also presents the details of the numerical techniques that were used to compute polarization. In Sec. III we discuss the re- sults and the complementary nature of the two techniques to compute the spontaneous polarization. Finally, Sec. IV pre- sents the summary and conclusions.

First-principles analysis of electron-phonon interactions in graphene

The electron-phonon interaction in monolayer graphene is investigated using density-functional perturbation theory. The results indicate that the electron-phonon interaction strength is of comparable magnitude for all four in-plane phonon branches and must be considered simultaneously. Moreover, the calculated scattering rates suggest an acoustic-phonon contribution that is much weaker than previously thought, revealing an important role of optical phonons even at low energies. Accordingly it is predicted, in good agreement with a recent measurement, that the intrinsic mobility of graphene may be more than an order of magnitude larger than the already high values reported in suspended samples.

Thermoelectric properties of graphene nanoribbons, junctions and superlattices.

Using model interaction Hamiltonians for both electrons and phonons and Greens function formalism for ballistic transport, we have studied the thermal conductance and the thermoelectric properties of graphene nanoribbons (GNR), GNR junctions and periodic superlattices. Among our findings we have established the role that interfaces play in determining the thermoelectric response of GNR systems both across single junctions and in periodic superlattices. In general, increasing the number of interfaces in a single GNR system increases the peak ZT values that are thus maximized in a periodic superlattice. Moreover, we proved that the thermoelectric behavior is largely controlled by the width of the narrower component of the junction. Finally, we have demonstrated that chevron-type GNRs recently synthesized should display superior thermoelectric properties.

Surface polar phonon dominated electron transport in graphene

The effects of surface polar phonons on the electronic transport properties of monolayer graphene are studied by using a Monte Carlo simulation. Specifically, the low-field electron mobility and saturation velocity are examined for different substrates (SiC, SiO2, and HfO2) in comparison to the intrinsic case. While the results show that the low-field mobility can be substantially reduced by the introduction of surface polar phonon scattering, corresponding degradation of the saturation velocity is not observed for all three substrates at room temperature. It is also found that surface polar phonons can influence graphene’s electrical resistivity even at low temperature, leading potentially to inaccurate estimation of the acoustic phonon deformation potential constant.

Influence of electron-electron scattering on transport characteristics in monolayer graphene

The influence of electron-electron scattering on the distribution function and transport characteristics of intrinsic monolayer graphene is investigated via an ensemble Monte Carlo simulation. Due to the linear dispersion relation in the vicinity of the Dirac points, it is found that pair-wise collisions in graphene do not conserve the ensemble average velocity in contrast to conventional semiconductors with parabolic energy bands. Numerical results indicate that electron-electron scattering can lead to a decrease in the low field mobility by more than a factor of 2 for moderate electron densities. The corresponding degradation in the saturation velocity is more modest at around 15%. At high densities, the impact gradually diminishes due to increased degeneracy.

Mixed finite element-tight-binding electromechanical analysis of carbon nanotubes

Electrical transport properties of carbon nanotubes can be dramatically changed by mechanical deformations that alter tube shape and the corresponding positions of the atoms comprising the tube wall. In principle, detailed atomic/electronic calculations can provide both the deformed configuration and the resulting electrical transport behavior of the tube. Here we simplify the process by refining a previously-developed nonlinear structural mechanics finite-element-based procedure for modeling mechanical behavior of carbon nanotubes to account explicitly for tube chirality. A quadrilateral element overlay procedure provides an isotropic finite element model of hexagonal cells within a graphene sheet, with the only nodal positions coincident with those of the atoms. Mechanical deformation of the nanotube structure is simulated with finite elements, and the evolving atomic [nodal] coordinates are processed within the finite element (FE) program by using a tight-binding (TB) code to calculate deformation-indu...

Phonon engineering in nanostructures: Controlling interfacial thermal resistance in multilayer-graphene/dielectric heterojunctions

Article discussing phonon engineering in nanostructures and controlling interfacial thermal resistance in multilayer-graphene/dielectric heterojunctions.

Electron transport properties of bilayer graphene

Electron transport in bilayer graphene is studied by using a first principles analysis and theMonte Carlo simulation under conditions relevant to potential applications. While the intrinsic properties are found to be much less desirable in bilayer than in monolayer graphene, with significantly reduced mobilities and saturation velocities, the calculation also reveals the dominant influence of extrinsic factors such as the substrate and impurities. Accordingly, the difference between two graphene forms are more muted in realistic settings although the velocity-field characteristics remain substantially lower in the bilayer. When bilayer graphene is subject to an interlayer bias, the resulting changes in the energy dispersion lead to stronger electron scattering at the bottom of the conduction band. The mobility decreases significantly with the size of the generated bandgap, whereas the saturation velocity remains largely unaffected.