Tubular Nanowires Research Articles

Effect of impurities on charge and heat transport in tubular nanowires

We calculate the charge and heat currents carried by electrons, originating from a temperature gradient and a chemical potential difference between the two ends of tubular nanowires with different geometries of the cross-sectional areas: circular, square, triangular, and hexagonal. We consider nanowires based on InAs semiconductor material, and use the Landauer-Büttiker approach to calculate the transport quantities. We include impurities in the form of delta scatterers and compare their effect for different geometries. The results depend on the quantum localization of the electrons along the edges of the tubular prismatic shell. For example, the effect of impurities on the charge and heat transport is weaker in the triangular shell than in the hexagonal shell, and the thermoelectric current in the triangular case is several times larger than in the hexagonal case, for the same temperature gradient.

Effects of transverse geometry on the thermal conductivity of Si and Ge nanowires

We explore the effects of geometry on the thermal conductivity (κ) of silicon and germanium nanowires, with lengths between 10–120 nm and diameters up to 5–6 nm. To this end we perform molecular dynamics simulations with the LAMMPS software, using Tersoff interatomic potentials. We consider nanowires with polygonal cross section and we discuss the effect of the transverse geometry on the thermal conductivity. We also consider tubular (hollow) nanowires and core/shell combinations of Si/Ge and Ge/Si, and we compare the heat transport of the core/shell structure with that of the separated core and shell components.

Morphologies evolution and mechanical behaviors of SiC nanowires reinforced C/(PyC-SiC) multilayered matrix composites

• The bionic (PyC-SiC) n multilayered structure was used to improve flexural properties. • The SiC nanowires formed in-situ on the surface of multilayered structure played an important role in micro-zone reinforcements. • The (PyC-SiC) n multilayered structure and SiC nanowires were prepared at the same time, and they could form a multiscale reinforcing system. • The flexural strengths and flexural modulus of modified composites were up to 62.36% and 102.20% respectively higher than C/C composites. SiC nanowires reinforced C/(PyC-SiC) n multilayered matrix composites (SM-CS for short) were prepared by combined with sol-gel and chemical vapor infiltration (CVI) method. Firstly, (PyC-SiOC) n multilayered structure was formed by cycles of impregnation and deposition. Then SiOC was transformed into SiC by heat-treatment, and (PyC-SiC) n multilayered structure would be obtained. At the same time, the PyC layer which was designed as the outmost layer could decrease gas supersaturation to form in-situ tubular SiC nanowires on the surface of multilayered structure. The results of three-point bending test showed that the maximum force of SM-CS composites was increased by the number of cycles of the preparation process, which were up to enhanced by 74.38% compared with C/C composite materials. The fracture surface showed that the improvement was due to the multiscale reinforcing system of (PyC-SiC) n multilayered structure and SiC nanowires. Multilayered structure can protect carbon fibers and release stress concentration by induction of cracks. And the mechanical interlocking effect of SiC nanowires could reinforce bonding force of the remaining matrix.

Electromagnetic field emitted by core–shell semiconductor nanowires driven by an alternating current

We consider tubular nanowires with a polygonal cross section. In this geometry, the lowest energy states are separated into two sets, one the corner and the other side-localized states. The presence of an external magnetic field transverse to the nanowire imposes an additional localization mechanism: the electrons being pushed sideways relatively to the direction of the field. This effect has important implications on the current density as it creates current loops induced by the Lorentz force. We calculate numerically the electromagnetic field radiated by hexagonal, square, and triangular nanowires. We demonstrate that because of the aforementioned localization properties, the radiated field can have a complex distribution determined by the internal geometry of the nanowire. We suggest that measuring the field in the neighborhood of the nanowire could be the basic idea of the tomography of the electron distribution inside it if a smaller receiver antenna could be placed in that zone.

Theory of magnetotransport in shaped topological insulator nanowires

We show that shaped topological insulator (TI) nanowires, i.e. such that their cross-section radius varies along the wire length, can be tuned into a number of different transport regimes when immersed in a homogeneous coaxial magnetic field. This is in contrast with widely studied tubular nanowires with constant cross-section, and is due to magnetic confinement of Dirac surface carriers. In flat 2D systems such a confinement requires non-homogeneous magnetic fields, while for shaped nanowires of standard size homogeneous fields of the order of $B\sim\,1$T are sufficient. We put recent work [Kozlovsky et al., Phys. Rev. Lett. 124, 126804 (2020)] into broader context and extend it to deal with axially symmetric wire geometries with arbitrary radial profile. A dumbbell-shaped TI nanowire is used as a paradigmatic example for transport through a constriction and shown to be tunable into five different transport regimes: (i) conductance steps, (ii) resonant transmission, (iii) current suppression, (iv) Coulomb blockade, and (v) transport through a triple quantum dot. Switching between regimes is achieved by modulating the strength of a coaxial magnetic field and does not require strict axial symmetry of the wire cross-section. As such, it should be observable in TI nanowires fabricated with available experimental techniques.

Thermoelectric properties of tubular nanowires in the presence of a transverse magnetic field

We calculate the charge and heat current associate with electrons, generated by a temperature gradient and chemical potential difference between two ends of a tubular nanowire of 30 nm radius in the presence of an external magnetic field perpendicular to its axis. We consider a nanowire based on a semiconductor material, and use the Landauer-Büttiker approach to calculate the transport quantities. We obtain the variation of the Seebeck coefficient (S), thermal conductivity (κ), and the figure of merit (ZT), with respect to the temperature up to 20 K, and with the magnetic field up to 3 T. In particular we show that the Seebeck coefficient can change sign in this domain of parameters. In addition κ and ZT have oscillations when the magnetic field increases. These oscillations are determined by the energy spectrum of the electrons.

Conductance features of core-shell nanowires determined by their internal geometry

We consider electrons in tubular nanowires with prismatic geometry and infinite length. Such a model corresponds to a core-shell nanowire with an insulating core and a conductive shell. In a prismatic shell the lowest energy states are localized along the edges (corners) of the prism and are separated by a considerable energy gap from the states localized on the prism facets. The corner localization is robust in the presence of a magnetic field longitudinal to the wire. If the magnetic field is transversal to the wire the lowest states can be shifted to the lateral regions of the shell, relatively to the direction of the field. These localization effects should be observable in transport experiments on semiconductor core-shell nanowires, typically with hexagonal geometry. We show that the conductance of the prismatic structures considerably differs from the one of circular nanowires. The effects are observed for sufficiently thin hexagonal wires and become much more pronounced for square and triangular shells. To the best of our knowledge, the internal geometry of such nanowires is not revealed in experimental studies. We show that with properly designed nanowires these localization effects may become an important resource of interesting phenomenology.

Magnetoconductance correction in zinc-blende semiconductor nanowires with spin-orbit coupling

We study the effects of spin-orbit coupling on the magnetoconductivity in diffusive cylindrical semiconductor nanowires. Following up on our former study on tubular semiconductor nanowires, we focus in this paper on nanowire systems where no surface accumulation layer is formed but instead the electron wave function extends over the entire cross section. We take into account the Dresselhaus spin-orbit coupling resulting from a zinc-blende lattice and the Rashba spin-orbit coupling, which is controlled by a lateral gate electrode. The spin relaxation rate due to Dresselhaus spin-orbit coupling is found to depend neither on the spin density component nor on the wire growth direction and is unaffected by the radial boundary. In contrast, the Rashba spin relaxation rate is strongly reduced for a wire radius that is smaller than the spin precession length. The derived model is fitted to the data of magnetoconductance measurements of a heavily doped back-gated InAs nanowire and transport parameters are extracted. At last, we compare our results to previous theoretical and experimental studies and discuss the occurring discrepancies.

Thermoelectric current in tubular nanowires in transverse electric and magnetic fields

In the presence of a transverse magnetic field, the charge current in nanowires can flow from the hot to the cold reservoir, but also backwards. The sign change can be obtained by increasing the temperature bias or the magnetic field. This behavior occurs when the magnetic field is sufficiently strong. Here, we will investigate how the size of the anomalous backward-flowing current is affected by an electric field perpendicular to the nanowire. The interplay of the electric and magnetic field modifies the dispersion curves, which will show up in the transport properties. We will also investigate how the presence of impurities affects the anomalous current. The electric field affects backscattering due to impurities, and thus the thermoelectric current reversal. Preliminary results show that the current reversal can survive in the presence of impurities.

Reversal of Thermoelectric Current in Tubular Nanowires

We calculate the charge current generated by a temperature bias between the two ends of a tubular nanowire. We show that in the presence of a transversal magnetic field the current can change sign; i.e., electrons can either flow from the hot to the cold reservoir, or in the opposite direction, when the temperature bias increases. This behavior occurs when the magnetic field is sufficiently strong, such that Landau and snaking states are created, and the energy dispersion is nonmonotonic with respect to the longitudinal wave vector. The sign reversal can survive in the presence of impurities. We predict this result for core-shell nanowires, for uniform nanowires with surface states due to the Fermi level pinning, and for topological insulator nanowires.

Self-Assembled Dehydro[24]annulene Monolayers at the Liquid/Solid Interface: Toward On-Surface Synthesis of Tubular π-Conjugated Nanowires.

We have studied the self-assembly behavior of dehydro[24]annulene (D24A) derivatives 1, 2a-2d, and 3a-3c at the liquid/solid interface using scanning tunneling microscopy (STM). Both the relative placement and the nature of the four D24A substituents strongly influence the self-assembly pattern. Overall, the eight D24A derivatives examined in this study display seven types of 2D packing patterns. The D24A derivatives 1, 2a, and 3a have either two or four stearate groups and adopt face-on configurations of their macrocyclic cores with respect to the highly oriented pyrolytic graphite (HOPG) surface. Their 2D packing pattern is determined by the interchain spacings and number of stearate substituents. The D24A derivatives 2b-2d and 3b-3c bear hydrogen-bonding carbamate groups to further strengthen intermolecular interactions. Face-on patterns were also observed for most of these compounds, while an unstable edge-on self-assembly was observed in the case of 2b at room temperature. Stable edge-on self-assemblies of D24A derivatives were sought for this work as an important stepping stone to achieving the on-surface topochemical polymerization of these carbon-rich macrocycles into tubular π-conjugated nanowires. The overall factors determining the 2D packing patterns of D24As at the liquid/solid interface are discussed on the basis of theoretical simulations, providing useful guidelines for controlling the self-assembly pattern of future D24A macrocycles.

Weak (anti)localization in tubular semiconductor nanowires with spin-orbit coupling

We compute analytically the weak (anti)localization correction to the Drude conductivity for electrons in tubular semiconductor systems of zinc blende type. We include linear Rashba and Dresselhaus spin-orbit coupling (SOC) and compare wires of standard growth directions $\langle100\rangle$, $\langle111\rangle$, and $\langle110\rangle$. The motion on the quasi-two-dimensional surface is considered diffusive in both directions: transversal as well as along the cylinder axis. It is shown that Dresselhaus and Rashba SOC similarly affect the spin relaxation rates. For the $\langle110\rangle$ growth direction, the long-lived spin states are of helical nature. We detect a crossover from weak localization to weak anti-localization depending on spin-orbit coupling strength as well as dephasing and scattering rate. The theory is fitted to experimental data of an undoped $\langle111\rangle$ InAs nanowire device which exhibits a top-gate-controlled crossover from positive to negative magnetoconductivity. Thereby, we extract transport parameters where we quantify the distinct types of SOC individually.

Three-Dimensional Tubular MoS2/PANI Hybrid Electrode for High Rate Performance Supercapacitor.

By using three-dimensional (3D) tubular molybdenum disulfide (MoS2) as both an active material in electrochemical reaction and a framework to provide more paths for insertion and extraction of ions, PANI nanowire arrays with a diameter of 10-20 nm can be controllably grown on both the external and internal surface of 3D tubular MoS2 by in situ oxidative polymerization of aniline monomers and 3D tubular MoS2/PANI hybrid materials with different amounts of PANI are prepared. A controllable growth of PANI nanowire arrays on the tubular MoS2 surface provides an opportunity to optimize the capacitive performance of the obtained electrodes. When the loading amount of PANI is 60%, the obtained MoS2/PANI-60 hybrid electrode not only shows a high specific capacitance of 552 F/g at a current density of 0.5 A/g, but also gives excellent rate capability of 82% from 0.5 to 30 A/g. The remarkable rate performance can be mainly attributed to the architecture with synergistic effect between 3D tubular MoS2 and PANI nanowire arrays. Moreover, the MoS2/PANI-60 based symmetric supercapacitor also exhibits the excellent rate performance and good cycling stability. The specific capacitance based on the total mass of the two electrodes is 124 F/g at a current density of 1 A/g and 79% of its initial capacitance is remained after 6000 cycles. The 3D tubular structure provides a good and favorable method for improving the capacitance retention of PANI electrode.

Thermal conductivity of tubular nanowire composites based on a thermodynamical model

A formula for the effective heat conductivity of a nanocomposite with cylindrical nanowire inclusions is derived. Both transversal and longitudinal heating along the wires are investigated. Several effects are examined: the volume fraction and sizes of the nanowires, the type of scattering at the particle-matrix interface and temperature. As illustration, silicon nanowires inclusions in a germanium matrix is considered; the results are shown to be in good agreement with other models and numerical solutions of the Boltzmann transport equation. Our main contribution consists of using extended irreversible thermodynamics to cope with the nano dimensions of the wires.

Role of anisotropy on the domain wall properties of ferromagnetic nanotubes

In this paper we investigate the role of magneto-crystalline anisotropy on the domain wall (DW) properties of tubular magnetic nanostructures. Based on a theoretical model and micromagnetic simulations, we show that either cubic or uniaxial magneto-crystalline anisotropies have some influence on the domain wall properties (wall size, propagation velocity and energy barrier) and then on the overall magnetization reversal mechanism. Besides the characterization of the transverse and vortex domain wall sizes for different anisotropies, we predict an anisotropy dependent transition between the occurrence of transverse and vortex domain walls in tubular nanowires. We also discuss the dynamics of the vortex DW propagation gradually increasing the uniaxial anisotropy constant and we found that the average velocity is considerably reduced. Our results show that different anisotropies can be considered in real samples in order to manipulate the domain wall behavior and the magnetization reversal process.

Analytical model for the boundary scattering phonon mean free path and thermal conductivity of nanowire heterostructures

Boundary scattering phonon mean free path (MFP) is an important parameter for thermal conductivity calculation of nanocomposites. In this work, a simple approximate model is proposed to predict boundary/interface scattering MFP and thermal conductivity of nanowire heterostructures (NWHSs) based on Casimir formalism. Calculated thermal conductivities of Si tubular nanowires and Si/Ge NWHSs agree well with the numerical and analytical solutions of Boltzmann transport equation. It is demonstrated that core/shell layer thickness plays a significant role on tuning NWHS thermal conductivity. The results indicate the approximate model of thermal conductivity can be used for quickly evaluating the thermal behavior of nanocomposites.

Longitudinal thermal conductivity of radial nanowire heterostructures

Thermal conductivity of tubular nanowires and radial nanowire heterostructures is analytically modeled along the longitudinal direction by using Boltzmann transport equation. This work is on the basis of Dingle [Proc. R. Soc. London, Ser. A 201, 545 (1950)] and Lucas [J. Appl. Phys. 36, 1632 (1965)] formalisms on thin wires and films, respectively. To investigate the thermal conductivity dependence on the interface conditions, we have generalized Prasher’s analytical solution [Appl. Phys. Lett. 89, 063121 (2006)] to cover the case where the scattering events at the interfaces are not totally diffuse scattering. The calculation of the size-dependent thermal conductivity includes the partly diffuse and partly specular scatterings at both internal and external interfaces of the tubular nanowires. It is found that the calculated thermal conductivities are in good agreement with the numerical solution of Yang et al. [Nano Lett. 5, 1111 (2005)]. Comparison is also made with the thermal conductivity of thin films and solid nanowires with the same dimensions. Results show that the thermal conductivity of the structures can be modulated by changing the radius ratio between the shell layer and the core layer of the radial nanowire heterostructures. The obtained results may serve as a possible way for tuning the thermal conductivity in nanostructures.

Conductance of tubular nanowires with disorder

We calculate the conductance of tubular-shaped nanowires having many potential scatterers at random positions. Our approach is based on the scattering matrix formalism and our results analyzed within the scaling theory of disordered conductors. When increasing the energy the conductance for a big enough number of impurities in the tube manifests a systematic evolution from the localized to the metallic regimes. Nevertheless, a conspicuous drop in conductance is predicted whenever a new transverse channel is open. Comparison with the semiclassical calculation leading to purely ohmic behavior is made.

Thermal conductivity of tubular and core/shell nanowires

Analytical solution of the Boltzmann transport equation (BTE) for phonon transport in tubular and core-shell nanowire is obtained. Thermal conductivity calculated from the analytical solution of BTE is in excellent agreement with a recently reported numerical model [R. Yang et al., Nano Lett. 5, 1111 (2005)]. Results show that thermal conductivity of tubular and core-shell nanowire can be significantly smaller than the bulk thermal conductivity.

Thermal conductivity of simple and tubular nanowire composites in the longitudinal direction

This work establishes a generic model to study phonon transport and the thermal conductivity of periodic two-dimensional nanocomposites in the longitudinal direction (along the wire axis direction). More specifically, the generic model is applied to study the thermal conductivity of silicon-germanium composites with simple silicon nanowire and tubular silicon nanowire inclusions in a germanium matrix, and cylindrical nanoporous silicon materials. The results show that the effective thermal conductivity changes not only with the volumetric fraction of the constituents but also with the radius of the nanowires and cylindrical pores due to the nature of the ballistic phonon transport. The smaller the wire/pore diameter, the smaller is the thermal conductivity of the periodic two-dimensional nanocomposites for a given volumetric fraction. Composites with tubular nanowire inclusions have a lower effective thermal conductivity than simple nanowire composites due to the introduction of additional surface scattering through the pores associated with tubular nanowires. Results of this study can be used to direct the development of both high-efficiency thermoelectric materials and thermal interface material containing high-thermal-conductivity particle or wire inclusions.