S. A. Vyrko

Uniaxially deformed (5,5) carbon nanotube: Structural transitions

The Kekule structure of the ground state of (5,5) armchair carbon nanotube is revealed by semiempirical molecular orbital calculations. This structure has bonds with two different bond lengths, differing by 0.003 nm. The ground state has tripled (compared to undistorted case) translational period due to Peierls distortions. Two first order structural phase transitions controlled by the tension are predicted at zero temperature. These transitions correspond to 5% and 13% elongations of a uniaxially deformed (5,5) nanotube. The narrow gap semiconductor to metal transition is predicted at 5% elongation of the nanotube.

Electrostatic models of insulator-metal and metal-insulator concentration phase transitions in Ge and Si crystals doped by hydrogen-like impurities

Two electrostatic models have been developed that allow calculation of the critical concentration of hydrogen-like impurities in three-dimensional crystalline semiconductors corresponding to the insulator-metal and metal-insulator transition in the zero temperature limit. The insulator-metal transition manifests itself as a divergence of the static permittivity observed in lightly compensated semiconductors as the concentration of polarizable impurities increases to the critical level. The metal-insulator transition is signaled by the divergence of the dc electrical resistivity in heavily doped semiconductors as the compensation of the majority impurity increases (or its concentration decreases). The critical impurity concentration corresponds to the coincidence of the percolation level for the majority carriers with the Fermi level. The results of the calculations made with these models fit the experimental data obtained for n-and p-type silicon and germanium within a broad range of their doping levels and impurity compensation.

Graphene-Based Nanodynamometer

A new concept of an electromechanical nanodynamometer based on the relative displacement of layers of bilayer graphene is proposed. In this nanodynamometer, force acting on one of the graphene layers causes the relative displacement of this layer and related change of conductance between the layers. Such a force can be determined by measurements of the tunneling conductance between the layers. Dependences of the interlayer interaction energy and the conductance between the graphene layers on their relative position are calculated within the first-principles approach corrected for van der Waals interactions and the Bardeen method, respectively. The characteristics of the nanodynamometer are determined and its possible applications are discussed.

A semiclassical approach to Coulomb scattering of conduction electrons on ionized impurities in nondegenerate semiconductors

In the proposed model of mobility, the time of electron–ion interaction equals the time taken by the conduction electron to pass a spherical region, corresponding to one impurity ion in crystal, and the minimum scattering angle is determined after Conwell–Weisskopf. We consider the acts of electron scattering on ions as independent and incompatible events. It is shown in the approximation of quasimomentum relaxation time, that for nondegenerate semiconductors, the mobility μi, limited by the elastic scattering by impurity ions with the concentration Ni, is proportional to T/Ni2/3; the Hall factor equals 1.4. The calculated dependences of the mobility of the majority charge carriers upon their concentration for different temperatures T agree well with known experimental data. It is shown, that the Brooks–Herring formula μBH∝T3/2/Ni gives overestimated values of mobility. Comparison of the calculations of mobility in degenerate semiconductors with experimental data also yields μi

Magnetically operated nanorelay based on two single-walled carbon nanotubes filled with endofullerenes Fe@C20

Structural and energy characteristics of the smallest magnetic endofullerene Fe@C20 were calculated using the density functional theory. The ground state of Fe@C20 was found to be a septet state, and the magnetic moment of Fe@C20 was estimated to be 8 Bohr magnetons. The characteristics of an (8,8) carbon nanotube with a single Fe@C20 inside were studied with a semiempirical approach. The scheme of a magnetic nanorelay based on cantilevered nanotubes filled with magnetic endofullerenes was examined. This nanorelay is turned on as a result of bending of nanotubes by a magnetic force. The operational characteristics of such a nanorelay based on (8,8) and (21,21) nanotubes fully filled with Fe@C20 were estimated and compared to the ones of a nanorelay made of a (21,21) nanotube fully filled with experimentally observed (Ho3N)@C80 with the magnetic moment of 21 Bohr magnetons. The room-temperature opera- tion of (21,21) nanotube-based nanorelays was demonstrated.

Model of hopping dc conductivity via nearest neighbor boron atoms in moderately compensated diamond crystals

Abstract Expressions for dependences of the pre-exponential factor σ 3 and the thermal activation energy e 3 of hopping electric conductivity of holes via boron atoms on the boron atom concentration N and the compensation ratio K are obtained in the quasiclassical approximation. It is assumed that the acceptors (boron atoms) in charge states (0) and (−1) and the donors that compensate them in the charge state ( + 1 ) form a nonstoichiometric simple cubic lattice with translational period R h = [ ( 1 + K ) N ] − 1 / 3 within the crystalline matrix. A hopping event occurs only over the distance R h at a thermally activated accidental coincidence of the acceptor levels in charge states (0) and (−1). Donors block the fraction K / ( 1 − K ) of impurity lattice sites. The hole hopping conductivity is averaged over all possible orientations of the lattice with respect to the external electric field direction. It is supposed that an acceptor band is formed by Gaussian fluctuations of the potential energy of boron atoms in charge state (−1) due to Coulomb interaction only between the ions at distance R h . The shift of the acceptor band towards the top of the valence band with increasing N due to screening (in the Debye–Huckel approximation) of the impurity ions by holes hopping via acceptor states was taken into account. The calculated values of σ 3 ( N ) and e 3 ( N ) for K ≈ 0.25 agree well with known experimental data at the insulator side of the insulator–metal phase transition. The calculation is carried out at a temperature two times lower than the transition temperature from hole transport in v -band of diamond to hopping conductance via boron atoms.

Electronic energy band structure of uniaxially deformed (5,5) armchair carbon nanotube

Semiempirical molecular orbital calculations of the (5,5) armchair carbon nanotube give the Kekule structure in its ground state with two essentially different bonds (the bond lengths difference is 0.003 nm). This is a result of the Peierls distortions leading to tripled (compared with undistorted case) translational period. When the armchair nanotube is elongated, two first order deformational structural phase transitions are predicted. The first one at the elongation of 5% leads to doubling of a translational period (instead of tripling at smaller elongations). The second one at the elongation of 13% leads to the quinoid type structure. The dependence of the electronic energy-band structure of the (5,5) carbon nanotube on elongation is investigated using the tight binding approximation. The transition from narrow-gap semiconductor to metal is predicted at the elongation of 5%, indicating that the uniaxially deformed armchair carbon nanotube at greater elongation (more than 5%) remains metallic at all temperatures.

Calculation of capacitance of self-compensated semiconductors with intercenter hops of one and two electrons (by the example of silicon with radiation defects)

Low-frequency electrical capacitance of silicon crystals in the case of hopping migration of both electrons and bipolarons (electron pairs) via the defects of one type, which stabilizes the Fermi level near the midgap, is calculated. The crystals with two-level defects in three charge states (+1, 0, or −1) with a negative correlation energy are considered. It is shown that, as the absolute value of the external potential is increased, the capacitance of silicon containing defects with positive correlation energy increases, while that with defects with negative correlation energy decreases. The expression for the drift and diffusion components of current density for bipolarons hopping from defects with the charge state −1 to defects with the charge state +1 was derived for the first time.

Force and magnetic field sensor based on measurement of tunneling conductance between ends of coaxial carbon nanotubes

The interaction and tunneling conductance between oppositely located ends of coaxial carbon nanotubes are studied by the example of two (11, 11) nanotubes with open ends terminated by hydrogen atoms. The Green function formalism is applied to determine the tunneling current through the nanotube ends as a function of the distance between the ends, relative orientation of the nanotubes and voltage applied. The energy favorable configuration of the coaxial nanotubes is obtained by the analysis of their interaction energy at different distances between the nanotube ends and angles of their relative rotation. Using these calculations, a general scheme of the force sensor based on the interaction between ends of coaxial nanotubes is proposed and the relation between the tunneling conductance and measured force is established for the considered nanotubes. The operational characteristics of this device as a magnetic field sensor based on measurements of the magnetic force acting on the coaxial nanotubes filled with magnetic endofullerenes are estimated.

AA stacking, tribological and electronic properties of double-layer graphene with krypton spacer

Structural, energetic, and tribological characteristics of double-layer graphene with commensurate and incommensurate krypton spacers of nearly monolayer coverage are studied within the van der Waals-corrected density functional theory. It is shown that when the spacer is in the commensurate phase, the graphene layers have the AA stacking. For this phase, the barriers to relative in-plane translational and rotational motion and the shear mode frequency of the graphene layers are calculated. For the incommensurate phase, both of the barriers are found to be negligibly small. A considerable change of tunneling conductance between the graphene layers separated by the commensurate krypton spacer at their relative subangstrom displacement is revealed by the use of the Bardeen method. The possibility of nanoelectromechanical systems based on the studied tribological and electronic properties of the considered heterostructures is discussed.