Superlattice Nanowires Research Articles

P3-1 Azimuthally symmetric torsional modes in nanowire superlattices

P3-1 Azimuthally symmetric torsional modes in nanowire superlattices

Heterostructured Bi2Se3 Nanowires with Periodic Phase Boundaries

Single and multijunction nanowires with diameters of 10-40 nm were synthesized by an electrically assisted sonochemical method in aqueous solution at room temperature. By properly controlling the reaction conditions and reactants, this method opens a new strategy towards the fabrication of one-dimensional superlattices in nanowires under ambient conditions. Such a chemical approach provides a promising and simple way to synthesize high-performance building blocks for device applications.

Fabrication of conducting Si nanowire arrays

The recent development of the superlattice nanowire pattern transfer technique allows for the fabrication of arrays of nanowires at a diameter, pitch, aspect ratio, and regularity beyond competing approaches. Here, we report the fabrication of conducting Si nanowire arrays with wire widths and pitches of 10–20 and 40–50nm, respectively, and resistivity values comparable to the bulk through the selection of appropriate silicon-on-insulator substrates, careful reactive-ion etching, and spin-on glass doping. These results promise the realization of interesting nanoelectronic circuits and devices, including chemical and biological sensors, nanoscale mosaics for electronics, and ultradense field-effect transistor arrays.

Acoustic phonon propagation in nanowire superlattices

Vibrational states in a nanowire superlattice consisting of an alternate stacking of two cylindrical layers are theoretically studied. For azimuthally symmetric torsional modes, the expressions for phonon dispersion relations and transmittance are derived analytically. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Synthesis and Characterization of ZnO:In Nanowires with Superlattice Structure

ZnO:In nanowires with superlattice structure have been successfully synthesized by evaporating ZnO, In2O3, and Co2O3 mixed powders. There are two types of superlattice nanowires in the product, one with longitudinal superlattice structure and another with transverse superlattice structure. Structures and components of the superlattice nanowires were investigated. It is found that they consist of In−O and In-doped Zn−O layers (In/Zn−O layers) stacked alternately perpendicular to the c axis and they have approximate In2O3(ZnO)m composition. In addition, zigzag-modulated structure was also observed and considered as the ordered arrangement of In atoms within In/Zn−O layers. A growth model based on the vapor−liquid−solid mechanism is proposed for interpreting the growth of nanowires in our work. Due to the heavy doping of In, the emission peak in photoluminescence spectra has red-shifted as well as broadened seriously.

Magnetic Bloch Oscillations in Nanowire Superlattice Rings

The recent growth of semiconductor nanowire superlattices encourages hope that Bloch-like oscillations in such structures formed into rings may soon be observed in the presence of a time-dependent magnetic flux threading the ring. These magnetic Bloch oscillations are a consequence of Faraday's law; the time-dependent flux produces an electromotive force around the ring, thus leading to the Bloch-like oscillations. In the spectroscopic domain, generalized Wannier-Stark states are found that are manifestations of the emf-induced localization of the states.

Molecular dynamics study of the lattice thermal conductivity of Kr/Ar superlattice nanowires

The nonequilibrium molecular dynamics (NEMD) method has been used to calculate the lattice thermal conductivities of Ar and Kr/Ar nanostructures in order to study the effects of interface scattering, boundary scattering, and elastic strain on lattice thermal conductivity. Results show that interface scattering poses significant resistance to phonon transport in superlattices and superlattice nanowires. The thermal conductivity of the Kr/Ar superlattice nanowire is only about 1 3 of that for pure Ar nanowires with the same cross-sectional area and total length due to the additional interfacial thermal resistance. It is found that nanowire boundary scattering provides significant resistance to phonon transport. As the cross-sectional area increases, the nanowire boundary scattering decreases, which leads to increased nanowire thermal conductivity. The ratio of the interfacial thermal resistance to the total effective thermal resistance increases from 30% for the superlattice nanowire to 42% for the superlattice film. Period length is another important factor affecting the effective thermal conductivity of the nanostructures. Increasing the period length will lead to increased acoustic mismatch between the adjacent layers, and hence increased interfacial thermal resistance. However, if the total length of the superlattice nanowire is fixed, reducing the period length will lead to decreased effective thermal conductivity due to the increased number of interfaces. Finally, it is found that the interfacial thermal resistance decreases as the reference temperature increases, which might be due to the inelastic interface scattering.

Quantum confinement phenomena in nanowire superlattice structures

Nanowire superlattices (NWSLs) are objects with a wide range of potential applications in nanoscale electronics and photonics. These objects are already grown experimentally in laboratories and studies of their properties represent an important task in nanotechnology research. Tools of mathematical modelling provide a powerful means for studying these fascinating structures with an ultimate goal of predicting their properties and understanding their limits. In this paper, we analyse finite nanowire GaAs/Al0.3Ga0.7As superlattice structures with a cylindrical cross-section and determine their electronic eigenstates and energy eigenvalues. We analyse in detail the qualitative differences, in terms of wavefunctions and energy eigenvalues, between structures containing the same number of barriers and wells (asymmetrical) and structures where the number of barrier layers is one above the number of well layers (symmetrical). We demonstrate that asymmetrical NWSL structures have well-pronounced qualitative and quantitative differences as compared to both symmetrical NWSL structures and infinite periodic NWSL structures. We show also that there exists a critical radius Rc for quantum confinement, that is the NWSL ground state is confined in GaAs (Al0.3Ga0.7As) for a nanowire of radius R above (below) Rc. In the specific case where R = Rc, the ground state is smeared out over the entire NWSL structure.

Theoretical phonon thermal conductivity of Si/Ge superlattice nanowires

An incoherent particle model has been developed to calculate the phonon thermal conductivity of superlattice nanowires. This is an extension of the photon net-radiation method and Schuster–Schwarzschild approximation to dispersive acoustic phonons in a gray medium. By comparing the roughness and geometric variations of typical nanowires to the characteristic phonon wavelength (∼1 nm at 300 K), diffuse scattering and incoherent three-dimensional dispersion are justified. An isotropic sine-type (Born–von Karman) dispersion is used, which requires only the sound velocity, atomic number density, and bulk conductivity to fully describe a material. A simple picture is also given in terms of Matthiessen’s rule and three effective mean free paths. Agreement with available experimental data is poor at the smallest diameters, but good above 30 nm diameter. Compared to a conventional superlattice, calculations show that the additional sidewall scattering in a superlattice nanowire can reduce the thermal conductivity by a factor of 2 or more.

Modeling a nanowire superlattice using the finite difference method in cylindrical polar coordinates

A new procedure for handling periodic boundary conditions within the finite difference method and in cylindrical polar coordinates is presented and applied to modeling a nanowire superlattice. The method is compared for accuracy and efficiency with two other formulations of the same problem: the finite difference method applied to a finite number of unit cells, and the exact solution of the equivalent Kronig–Penney model. The technique is then shown to reproduce a novel physical state, and is applied to an embedded nanowire for which there are no analytical solutions.

Thermal conductivity of Si/SiGe superlattice nanowires

The thermal conductivities of individual single crystalline Si/SiGe superlattice nanowires with diameters of 58 and 83 nm were measured over a temperature range from 20 to 320 K. The observed thermal conductivity shows similar temperature dependence as that of two-dimensional Si/SiGe superlattice films. Comparison with the thermal conductivity data of intrinsic Si nanowires suggests that alloy scattering of phonons in the Si–Ge segments is the dominant scattering mechanism in these superlattice nanowires. In addition, boundary scattering also contributes to thermal conductivity reduction.

Nanowires and nanorings at the atomic level.

The step-flow growth mode is used to fabricate Si and Ge nanowires with a width of 3.5 nm and a thickness of one atomic layer (0.3 nm) by self-assembly. Alternating deposition of Ge and Si results in the formation of a nanowire superlattice covering the whole surface. One atomic layer of Bi terminating the surface is used to distinguish between the elements Si and Ge. A difference in apparent height is measured in scanning tunneling microscopy images for Si and Ge. Also, different kinds of two-dimensional Si/Ge nanostructures like alternating Si and Ge nanorings having a width of 5-10 nm were grown.

Thermoelectric properties of superlattice nanowires

We report here on a theoretical model for the electronic structure and transport properties of superlattice nanowires, considering their cylindrical wire boundary and multiple anisotropic carrier pockets. The thermoelectric properties of superlattice nanowires made of various lead salts (PbS, PbSe, and PbTe) are investigated as a function of the segment length, wire diameter, crystal orientation along the wire axis, and the length ratio of the constituent nanodots of the superlattice, based on the Kronig-Penney potential for each one-dimensional (1D) subband and on the 1D Boltzmann transport equations. A potential barrier--well inversion induced by quantum confinement, which is a unique phenomenon in superlattice nanowires, is predicted as the wire diameter decreases. $\mathrm{ZT}$ values higher than 4 and 6 are predicted for 5-nm-diameter PbSe/PbS and PbTe/PbSe superlattice nanowires at 77 K, respectively. These ZT values are significantly larger than those of their corresponding alloy nanowires, indicating that superlattice nanowires are promising systems for thermoelectric applications.

Magnetic interactions in Ni–Cu/Cu superlattice nanowire arrays

The remanent magnetization of Ni–Cu/Cu superlattice nanowires was investigated. Arrays of superlattice nanowires were prepared by template deposition through polycarbonate nanoporous membranes using a single electrolyte bath. The thicknesses of the nickel-rich layers (tNi) and copper layers (tCu) were independently controlled by monitoring the current during deposition. The wire diameter was determined by transmission electron microscopy imaging to be 80 nm. A study of the remanent magnetization at 20 K for a range of values of tNi and tCu reveals the existence of magnetic interactions within each array. It is noted that for an array with tNi of 200 Å, the strength of the interactions decreases with increasing tCu, indicating that the interactions are taking place between nickel layers. However the interaction strength appears to reach a minimum level, beyond which an increase in tCu does not correspond to a decrease in the interaction strength observed. A minimum interaction level is also observed for an array with tNi of 50 Å, for all values of tCu investigated.

Phonon Thermal Conductivity of Superlattice Nanowires for Thermoelectric Applications

ABSTRACTAn incoherent particle model has been developed to predict the phonon thermal conductivity of nanowires and superlattice nanowires. It is argued that the surface roughness of most real nanowires prevents the formation of idealized confined dispersion relations for typical temperatures and diameters. Instead, the three-dimensional bulk dispersion is used, thus addressing only classical size effects. Four adjustable parameters capture the effects of diameter, superlattice, Umklapp, impurity, and alloy scattering. Predictions are compared with experimental data for nanowires and superlattice nanowires down to 22 nm diameter and 20 K, and are in good agreement above ∼40 nm diameter. The analysis suggests that ideal low thermal conductivity nanowires for thermoelectric applications would have small-diameter, alternating alloy segments that are acoustically dissimilar but electrically similar.

Molecular dynamics simulation of thermal conductivities of superlattice nanowires

Nonequilibrium molecular dynamics simulations were carried out to investigate heat transfer in superlattice nanowires. Results show that for fixed period length superlattice nanowires, the ratio of the total interfacial thermal resistance to the total thermal resistance and the effective thermal conductivities are invariant with the changes in interface numbers. Increasing the period length leads to an increase in the average interfacial thermal resistance, which indicates that the interfacial thermal resistance depends not only on the materials that constitute the alternating segments of superlattice nanowires, but also on the lattice strain throughout the segments. The modification of the lattice structure due to the lattice mismatch should be taken into account in the acoustic mismatch model. Simulation results also demonstrated the size confinement effect on the thermal conductivities for low dimensional structures, i.e. the thermal conductivities and the interfacial thermal resistance increase as the nanowire cross-sectional area increases.

New Directions for Low Dimensional Thermoelectricity

ABSTRACTLow dimensionality provides opportunities to modify the properties of bulk materials dramatically and to control materials properties independently in a manner that is not possible for bulk materials. The special characteristics of low dimensional materials to enhance thermoelectric performance have already been demonstrated in quantum wells, quantum wires and quantum dots. The main focus of this review is a summary of advances made in the modeling of quantum dot superlattice nanowires. Several new research directions for low dimensional thermoelectricity or inspired by this research are briefly mentioned.

Growth of nanowire superlattice structures for nanoscale photonics and electronics.

The assembly of semiconductor nanowires and carbon nanotubes into nanoscale devices and circuits could enable diverse applications in nanoelectronics and photonics. Individual semiconducting nanowires have already been configured as field-effect transistors, photodetectors and bio/chemical sensors. More sophisticated light-emitting diodes (LEDs) and complementary and diode logic devices have been realized using both n- and p-type semiconducting nanowires or nanotubes. The n- and p-type materials have been incorporated in these latter devices either by crossing p- and n-type nanowires or by lithographically defining distinct p- and n-type regions in nanotubes, although both strategies limit device complexity. In the planar semiconductor industry, intricate n- and p-type and more generally compositionally modulated (that is, superlattice) structures are used to enable versatile electronic and photonic functions. Here we demonstrate the synthesis of semiconductor nanowire superlattices from group III-V and group IV materials. (The superlattices are created within the nanowires by repeated modulation of the vapour-phase semiconductor reactants during growth of the wires.) Compositionally modulated superlattices consisting of 2 to 21 layers of GaAs and GaP have been prepared. Furthermore, n-Si/p-Si and n-InP/p-InP modulation doped nanowires have been synthesized. Single-nanowire photoluminescence, electrical transport and electroluminescence measurements show the unique photonic and electronic properties of these nanowire superlattices, and suggest potential applications ranging from nano-barcodes to polarized nanoscale LEDs.

Transport Properties of Superlattice Nanowires and Their Potential for Thermoelectric Applications

ABSTRACTA theoretical model for the electronic structure and transport properties of superlattice (SL) nanowires is presented, based on the electronic tunneling between quantum dots. Due to the periodic potential perturbation, SL nanowires exhibit unusual features in the electronic density of states that are absent in homogeneous nanowires. Transport property calculations of PbSe/PbS SL nanowires are presented, showing improved thermoelectric performance compared to homogeneous nanowires because of a lower lattice thermal conductivity and an enhanced Seebeck coefficient, indicating that SL nanowires are promising systems for thermoelectric applications.

Lateral correlation of InAs/AlInAs nanowire superlattices on InP(001)

The appearance of InAs quantum-wire-like morphology on an AlInAs buffer layer grown by molecular-beam epitaxy on nominal InP(001) surfaces is investigated. Lateral composition modulation in the AlInAs buffer layer is suggested to play an important role in the formation of InAs nanowires. For InAs/AlInAs nanowire superlattices, the InAs nanowires are laterally correlated with respect to growth directions. By changing the spacer thickness, no evidence of vertical correlation is observed. The lack of vertical correlation is ascribed to the asymmetrical cross-sectional shape of the nanowires.