Superlattice Nanowires Research Articles

The nucleation site selection of vapour–liquid–solid nanowires

Adopting twinning superlattice nanowires and III–V nanowires with sidewalls, whose cross-sections are irregular hexagons, the precise catalyst droplet surface area variation for the calculation of the nuclei formation enthalpy are obtained for possible nucleation positions at the nanowire growth front. Regular nanowires with hexagonal cross-section are also calculated. The nucleation site selection rules are drawn from the fact that the favorable site corresponds to a smaller nucleation barrier, which is not deducible from the spherical-cap shaped droplet approximation. The influence of droplet volume and nanowire geometry on the nucleation site selection should also be taken into consideration when calculations of the nanowire nucleation properties are involved.

Large-scale synthesis and characterisation of Ag/Bi 2 Te 3 superlattice nanowires via pulse electrodeposition

Ag/Bi2Te3 superlattice nanowire arrays have been fabricated by pulse electrodeposition using porous anodic aluminium oxide as the template at large-scale quantity. The nanowire arrays were characterised by X-ray diffraction, a scanning electron microscope, a transmission electron microscope and high-resolution transmission electron microscopy (HRTEM). The results indicate that the nanowires are composed of the Bi2Te3 rhombohedral lattice phase and the Ag cubic lattice phase with alternately ordered multi-segment characters. The feature of dark segments alternated with bright segments can be easily distinguished because of the obvious heterogeneous contrast. The length of each segment varies from 25 to 45 nm. The HRTEM result shows that the composition of one segment is Ag and the neighbouring segment is Bi2Te3.

Thermal transport in SiGe superlattice thin films and nanowires: Effects of specimen and periodic lengths

We compute the thermal conductivity of superlattice (SL) thin films and nanowires for various SL periods and total specimen lengths using nonequilibrium molecular dynamics. Both types of materials exhibit similar behaviors with respect to SL period but the thermal conductivity of the thin films exhibits a significantly higher sensitivity to the specimen length. Notably, the thermal conductivity of SL thin films is smaller than those of the corresponding nanowires for specimen lengths below approximately 35 nm. These results arise from the complex dependence of the conductivities of the interfaces and the SL components on the specimen size and period. These trends and observations are explained using a simple phonon model that builds on the relationship between the cumulative thermal conductivity and the phonon wavelength.

Gigahertz Coherent Guided Acoustic Phonons in AlN/GaN Nanowire Superlattices

The generation of guided acoustic phonons in the GHz range in GaN/AlN superlattices grown atop a GaN nanowire is presented. Combined with a femtosecond laser, ultrafast pump-probe spectroscopy allows the generation and detection of guided acoustic phonons at different frequencies in the nanowire superlattices. The capability of the nanowire superlattices to be excellent detectors of acoustic phonons at specific frequencies is then used to observe the strong dispersion, as a result of nanoconfinement, of guided acoustic phonons after their propagation in the nanowire. The generation of high frequency coherent guided acoustic phonons could be useful not only to realize an acoustic transducer with a nanolateral size but also as a source to understand the thermal behavior of nanowires.

Orientation-dependent growth rate of crystalline plane study in electrodeposited Ni/Cu superlattice nanowires

A method of using the superlattice nanowire with epitaxial growth to study the relationship between the crystal orientation and corresponding growth rate for the electrodeposited nanowire is proposed. The relationship between growth rate and orientation of two metals Ni and Cu was studied through analysis of the epitaxial growth of Ni/Cu superlattice nanowires. Three samples with different periodic lengths were prepared by an electrodeposition method and analyzed to compare the segment lengths of nanowires with different crystalline orientations. The quantitative analysis result indicates that the growth rate of different planes exist ν(220) : ν(111) = 1.25–1.31 for Ni, and ν(220) : ν(111) = 1.32–1.41 for Cu, which qualitatively follow the Bravais rule. The quantitative analysis for the relationship between growth rate and orientation is important for the effective control of the uniform growth of the electrodeposited nanowires.

High thermoelectric figure of merit in silicon-germanium superlattice structured nanowires

By using a combination of the first-principles density functional theory and nonequilibrium Green's function for electron and phonon transport, we investigate the thermoelectric properties of silicon-germanium superlattice nanowires (NWs). Our results show that introducing superlattice structures always increases thermoelectric figure of merit, ZT, which depends on the periodic length of the superlattice NWs. For n-type superlattice NWs, the achievable maximum ZT is 4.7, which is 5-fold increase as compared to the equivalent pristine silicon NWs. For p-type wires, the achieved maximum ZT is 2.74, which is 4.6-fold increase as compared to the pristine silicon NWs.

Si/Ge Superlattice Nanowires with Ultralow Thermal Conductivity

The engineering of nanostructured materials with very low thermal conductivity is a necessary step toward the realization of efficient thermoelectric devices. We report here the main results of an investigation with nonequilibrium molecular dynamics simulations on thermal transport in Si/Ge superlattice nanowires aiming at taking advantage of the inherent one dimensionality and the combined presence of surface and interfacial phonon scattering to yield ultralow values for their thermal conductivity. Our calculations revealed that the thermal conductivity of a Si/Ge superlattice nanowire varies nonmonotonically with both the Si/Ge lattice periodic length and the nanowire cross-sectional width. The optimal periodic length corresponds to an order of magnitude (92%) decrease in thermal conductivity at room temperature, compared to pristine single-crystalline Si nanowires. We also identified two competing mechanisms governing the thermal transport in superlattice nanowires, responsible for this nonmonotonic behavior: interface modulation in the longitudinal direction significantly depressing the phonon group velocities and hindering heat conduction, and coherent phonons occurring at extremely short periodic lengths counteracting the interface effect and facilitating thermal transport. Our results show trends for superlattice nanowire design for efficient thermoelectrics.

Optical properties of Ni and Cu nanowire arrays and Ni/Cu superlattice nanowire arrays

In this study, Ni and Cu nanowire arrays and Ni/Cu superlattice nanowire arrays are fabricated using standard techniques such as electrochemical deposition of metals into porous anodic alumina oxide templates having pore diameters of about 50 nm. We perform optical measurements on these nanowire array structures. Optical reflectance (OR) of the as-prepared samples is recorded using an imaging spectrometer in the wavelength range from 400 to 2,000 nm (i.e., from visible to near-infrared bandwidth). The measurements are carried out at temperatures set to be 4.2, 70, 150, and 200 K and at room temperature. We find that the intensity of the OR spectrum for nanowire arrays depends strongly on the temperature. The strongest OR can be observed at about T = 200 K for all samples in visible regime. The OR spectra for these samples show different features in the visible and near-infrared bandwidths. We discuss the physical mechanisms responsible for these interesting experimental findings. This study is relevant to the application of metal nanowire arrays as optical and optoelectronic devices.

Acoustic phonon modes and phononic bandgaps in GaN/AlN nanowire superlattices

We study numerically the phonon dispersion relations and corresponding displacement fields for a circular cross-section nanowire superlattices consisting of anisotropic GaN and AlN. We determine a set of parameters which gives complete phononic bandgaps. The results suggest the potential for manipulating phonons in the micro/nano electromechanical systems.

Model for the Mass Transport during Metal-Assisted Chemical Etching with Contiguous Metal Films As Catalysts

Metal-assisted chemical etching is a relatively new top-down approach allowing a highly controlled and precise fabrication of Si and Si/Ge superlattice nanowires. It is a simple method with the ability to tailor diverse nanowire parameters like diameter, length, density, orientation, doping level, doping type, and morphology. In a typical metal-assisted chemical etching procedure, a Si substrate is covered by a lithographic noble metal film and etched in a solution containing HF and an oxidant (typically H2O2). In general, the function of the metal is to catalyze the reduction of H2O2, which delivers electronic holes necessary for the oxidation and subsequent dissolution of the Si oxide by HF. However, the details of the etching process using contiguous metal thin films, especially the mass transport of reactants and byproducts are still not well understood. In this study, the etching mechanism was systematically investigated. Several models of metal-assisted chemical etching using a contiguous metal film...

Erratum: “Analyzing the growth of InxGa1−xN/GaN superlattices in self-induced GaN nanowires by x-ray diffraction” [Appl. Phys. Lett. 98, 261907 (2011)

an underestimation of the In content by approximately 30% of the given values, otherwise our findings remain valid. We derive here the correct expression that replaces Eq. (2) of the original paper. Symmetric Bragg x-ray diffraction probes the vertical (normal to the layers) lattice constants of strained layers in a heterostructure. For coherent epitaxial growth without plastic relaxation, all layers attain a common in-plane lattice parameter. In case of a planar heterostructure on a bulk substrate, all layers have the same lateral lattice parameter as the substrate. The resulting average vertical lattice parameter of the heterostructure is governed by the Poisson effect, and the well-known result is given below in Eq. (9) .I n a nanowire, however, the heterostructure is free to relax laterally, and the in-plane lattice parameter is obtained from a balance between layers. If the requirement of the stressfree side surface is satisfied on average, the Poisson effect is compensated and the average vertical lattice parameter is given by Eq. (11) below. To derive this not obvious result, let us consider a periodic superlattice stack made from two materials, 1 and 2. Material 1 is taken as a reference for all strain values and material 2 possesses an eigenstrain e0 with respect to it. We take the lateral and the vertical eigenstrain components equal to each other. Thus, the lattice constants c 0 and c 0 in the fully relaxed state are related by e0 ¼ð c 0 � c 0 Þ=c 0 . For the coherent growth considered here, the in-plane lattice parameter can be approximated to assume a constant value throughout the whole stack, so that the in-plane total strain e1xx ¼ e2xx ¼ exx. The same condition holds for the yy components, the axes x and y are in the lateral plane. For the lateral relaxation of the SL, we distinguish two cases

GaN/ZnO superlattice nanowires as photocatalyst for hydrogen generation: A first-principles study on electronic and magnetic properties

In hetero-nanostructures of solid state materials, the reconstruction of charge and spin at the interface can strongly affect the electronic and magnetic properties of the system. The nanomaterials with diverse functions can be achieved by precisely controlling the atomic structure and composition at the interface. Here, we present that the ferromagnetism and bandgap can be controlled by engineering the interfaces in non-magnetic semiconductor (GaN/ZnO) superlattice nanowires. We show that ferromagnetic p-type Zn–N interface can be developed by creating defects at the n-type Ga–O interface because of the defect-induced polar discontinuity. We further show that the bandgaps of the superlattice nanowires are narrowed with the formation of intermediate bands within the bandgaps, resulting in the enhancement of visible-light absorption. These nanostructures may find applications in photocatalyst for water splitting because of the improvement of carrier mobility and light absorption.

First-principles study of (InAs)1/(GaSb)1 superlattice nanowires

As the active areas and the connection part, semiconductor nanowires have ideal shapes which are beneficial to restricting the electron motion and atomic periodicity to one one-dimensional structure. The effective selection of material components in nanowires can enhance the advantages of low-dimensional structures by analyzing the features of bulk materials. Furthermore, the nanowire properties can also be tailored by controlling the internal or intrinsic characteristics such as diameters, crystallographic growth direction, structural phase, surface crystallographic plane or saturation degree, and by applying external influences such as electric, magnetic, thermal and force fields. The bulk InAs and GaSb have approximate lattice constants, thereby resulting in small lattice mismatch for InAs/GaSb heterostructures that can finally be grown into excellent infrared optoelectronic materials. Moreover, the bulk InAs has the lowest electron effective mass in binary III-V compound semiconductors, leading to high transport features for electrons distributing most in InAs layers of InAs/GaSb superlattices. In the present work, the zinc-blend (InAs)1/(GaSb)1 superlattice nanowires (subscript denotes the number of molecular or double-atomic layers) with [001] and [111] crystallographic wire-axes have been studied by first-principles calculations for their structural, electronic and mechanical properties together with the rule of different nanowire diameters (from ～0.5 to ～2.0 nm). We also analyze the stress effects from external forces and examine the electronic band-edge changes with strain in wire-axis direction to determine the deformation potentials.

First-principles studies on structural and electronic properties of GaN–AlN heterostructure nanowires

The structural and electronic properties of core-shell, eutectic, biaxial and superlattice GaN-AlN nanowires were studied through density functional theory computations. Due to more surface dangling bonds, nanowires with smaller diameters are energetically unfavorable. For the GaN-AlN heterostructure nanowires, their electronic properties highly depend on the GaN content, axial strain, configuration, and size. The valence bands are less affected by the GaN content, while the conduction bands depend on it. Hydrogen-passivated nanowires have much larger band gaps than their counterparts, since the surface states are removed by saturating the dangling bonds with hydrogen atoms. Moreover, due to multiple quantum-well structures, the confined electrons (holes) of superlattice nanowires become more localized and the difference of the mobility between the electron and hole becomes less apparent if the width of the barrier is larger. These findings are of value for better understanding heterostructure nanowires and their potential utilization.

Growth and properties of coherent twinning superlattice nanowires

Although coherent twin boundaries require little energy to form in nanoscale single crystals, their influence on properties can be dramatic. In recent years, some important steps forward have been made in understanding and controlling twinning processes at the nanoscale, making possible the fabrication of nanoengineered twinning superlattices in crystalline nanowires. These advances have opened new possibilities for properties and functionalities at the atomic and quantum scales by modulating twin densities. This article presents a brief overview of recent theoretical and experimental progress in growth mechanisms and promising properties of coherent twinning superlattice nanowires with special emphasis toward cubic systems in semiconductor and metallic materials. In particular, we show how nanoscale growth twins can considerably enhance bandgap engineering and mechanical behaviour in quasi-one-dimensional materials. Opportunities for future research in this emerging area are also discussed.

Atomistic Design of High Thermoelectricity on Si/Ge Superlattice Nanowires

We present predictions of thermoelectric figure of merit (ZT) on Si/Ge superlattice nanowires (SLNWs) based on ab initio electronic structure calculations and Boltzmann transport theory. Our results suggest that Si/Ge SLNW is a promising candidate for thermoelectric applications, much better than the known Si nanowires. We find that a ZT value higher than 1 at 300 K is achievable for 1.7 nm diameter Si/Ge SLNW in case of a 3-order reduction on lattice thermal conductivity with respect to bulk Si. The high thermoelectric performance in Si/Ge SLNWs can be further improved by tailoring the electrical transport (S2σ), which strongly depends on their unique electronic structures. We also discuss how the structural properties such as nanowire diameter, superlattice period, constituent length ratio, and the chemical potential (or doping levels) can be tuned to derive an optimal ZT for Si/Ge SLNWs.

Quantum restricted effects on ballistic thermal conductance associated with six types of vibration modes in nanowire superlattice

We study ballistic phonon transport and thermal conductance associated with six low-lying vibrational modes in a quadrate nanowire superlattice at low temperatures. The results show that the transmission rates of the six vibrational modes display periodic forbidden regions of frequencies and have different relations with geometrical details of the nanowire superlattice. The thermal conductances contributed from different vibrational modes show different characteristics. On the basis of the theoretical analysis and calculated simulation, quantum restricted effects on ballistic thermal conductance in the quadrate nanowire superlattice are revealed.

Spin Filtering in a Nanowire Superlattice by Dresselhause Spin-Orbit Coupling

An InAs/GaSb nanowire superlattice using GaAs for the impure layers is proposed. Dresselhaus spin-orbit coupling eliminates spin degeneracy, induces one miniband in the superlattices to split into two minibands and leads to complete spin polarization and excellent filtering by optimizing the well and barrier widths and GaAs layer distances.

Fabrication of ultrahigh-density nanowires by electrochemical nanolithography

An approach has been developed to produce silver nanoparticles (AgNPs) rapidly on semiconductor wafers using electrochemical deposition. The closely packed AgNPs have a density of up to 1.4 × 1011 cm-2 with good size uniformity. AgNPs retain their shape and position on the substrate when used as nanomasks for producing ultrahigh-density vertical nanowire arrays with controllable size, making it a one-step nanolithography technique. We demonstrate this method on Si/SiGe multilayer superlattices using electrochemical nanopatterning and plasma etching to obtain high-density Si/SiGe multilayer superlattice nanowires.

Novel photoluminescence properties of InAlO3(ZnO)m superlattice nanowires

One-dimension InAlO3(ZnO)m superlattice nanowires were successfully synthesized via chemical vapor deposition. Transmission electron microscopy measurements reveal that the nanowires have a periodic layered structure along the ⟨0001⟩ direction. The photoluminescence properties of InAlO3(ZnO)m superlattice nanowires are studied for the first time. The near-band-edge emissions exhibit an obvious red shift due to the formation of the localized tail states. The two peaks centered at 3.348 eV and 3.299 eV indicate a lever phenomenon at the low-temperature region. A new luminescence mechanism is proposed, combined with the special energy band structure of InAlO3(ZnO)m.