Nanowire Cross-section Research Articles

Optical and transport properties correlation driven by amorphous/crystalline disorder in InP nanowires

Indium phosphide nanowires with a single crystalline zinc-blend core and polycrystalline/amorphous shell were grown from a reliable route without the use of hazardous precursors. The nanowires are composed by a crystalline core covered by a polycrystalline shell, presenting typical lengths larger than 10 μm and diameters of 80–90 nm. Raman spectra taken from as-grown nanowires exhibited asymmetric line shapes with broadening towards higher wave numbers which can be attributed to phonon localization effects. It was found that optical phonons in the nanowires are localized in regions with average size of 3 nm, which seems to have the same order of magnitude of grain sizes in the polycrystalline shell. Regardless of the fact that the nanowires exhibit a crystalline core, any considerable degree of disorder can lead to a localized behaviour of carriers. In consequence, the variable range hopping was observed as the main transport instead of the usual thermal excitation mechanisms. Furthermore the hopping length was ten times smaller than nanowire cross-sections, confirming that the nanostructures do behave as a 3D system. Accordingly, the V-shape observed in PL spectra clearly demonstrates a very strong influence of the potential fluctuations on the exciton optical recombination. Such fluctuations can still be observed at low temperature regime, confirming that the amorphous/polycrystalline shell of the nanowires affects the exciton recombination in every laser power regime tested.

Tensile manipulation of ultrathin gold nanowires at different sizes and atomic vacancies

The fractures of ultrathin metallic nanowires usually exhibit their uncertainties at small scales. Here, statistics was used to study the uniaxial tension-induced deformation of ultrathin gold nanowires. With the same cross section of gold nanowires (5a × 5a × Ha), different sizes show various deformation mechanisms due to the moving styles of slipped crystalline planes. However, the deformations at different sizes (5a × 5a × 5a) and (5a × 5a × 25a) both show the sensitivity to one atomic vacancy, attributed to the dominant role of the same cross section. The statistical broken position distributions further provide that the deformation fracture is size dependent and sensitive to atomic vacancies, which is explained with the relationship between broken bonds and tensile wave propagation. For the size dependence of mechanical property, the nanowire height (H) of 10a is observed to be a transitional point, when the height is less than 10a, the mechanical strength is unstable, while above this transitional point, mechanical strengths decrease with the nanowire size increasing. Our work provides mechanistic insights into enhancing the reliability of metallic nanostructures by engineering the internal atomic imperfection and structural dimensions.

Effect of Ions and Ionic Strength on Surface Plasmon Absorption of Single Gold Nanowires.

The local temperature change from a single optically excited gold nanowire, lithographically prepared on Al0.94Ga0.06N embedded with Er(3+) ions, is measured in air, pure water, and various concentrations of aqueous solutions of ionic solutes of NaCl, Na2SO4, and MgSO4. The absorption cross section of the nanowire under pure water (2.25 × 10(-14) m(2)) and different solution ionic strength is measured from the slopes of temperature change versus laser intensity plots. Addition of charges into the solution decreases the amount of heat generated during optical excitation of the gold nanostructures because the absorption cross section of the gold nanowire is attenuated. A Langmuir-type behavior of the absorption cross section with ionic strength is observed that is identified with an increase in the occupancy of screened interfacial charges. The absorption cross section of the nanowire decreases with ionic strength until a saturation value of 9 × 10(-15) m(2), where saturation in the occupancy of screened interfacial charge occurs. Dynamic measurements of temperature for a single gold nanowire immersed in a microchannel flow cell show a sharp and fast temperature drop for the flow of ionic solution compared to the pure (deionized) water, suggesting that the technique can be developed as a sensor probe to detect the presence of ions in solution.

Analysis of ballistic and quasi-ballistic hole transport properties in germanium nanowires based on an extended “Top of the Barrier” model

The ballistic hole transport properties in rectangular cross-sectional germanium nanowire transistors with various geometries were studied based on the “Top of the Barrier” model. Then, by an extension of this model, the quasi-ballistic hole transport was discussed taking into account phonon and surface roughness scattering in the channel and source-to-drain direct tunneling. Among several nanowire geometries targeted in this study, the [110]-oriented nanowire with large height along [11¯0] ([110]/(11¯0) NW) exhibited the largest ballistic current. This was understood from its large density of states and resulting high hole density. Large density of states, however, enhances backscattering in the channel. An approximation analysis of quasi-ballistic transport suggested that the [110]/(001) NW with higher mobility can outperform [110]/(11¯0) NW when scattering and tunneling are considered.

Size and temperature dependence of the electron–phonon scattering by donors in nanowire transistors

Due to the constant size reduction, single-donor-based nanowire transistors receive an increasing interest from the semi-conductor industry. In this work we theoretically investigate the coupled influence of electron–phonon scattering, temperature and size (cross-section and channel length) on the properties of such systems. The aim is to determine under what conditions the localized character of the donor has a remarkable impact on the current characteristics. We use a quantum non-equilibrium Green’s function approach in which the acoustic electron–phonon scattering is treated through local self-energies. We first show how this widely used approach, valid at high temperatures, can be extended to lower temperatures. Our simulations predict a hysteresis in the current when reducing the temperature down to 150K. We also find that acoustic phonons degrade the current characteristics while their optical counterparts might have a beneficial impact with an increase of the ON-current. Finally we discuss the influence of nanowire length and cross-section and emphasize the complexity of precisely controlling the dopant level at room temperature.

Aspect-ratio dependence of magnetization reversal in cylindrical ferromagnetic nanowires

The magnetization reversal behavior in isolated cylindrical and square cross-section Ni81Fe19 nanowires was systematically studied as a function of nanowire cross-section dimensions from 10 up to 200 nm using micromagnetic simulations. This approach provides access to the switching field, remanence ratio and most significantly the magnetization structures during reversal, which allows the evolution of magnetization processes to be studied with scaling of the cross-sectional dimensions. The dimensional trends in reversal behavior for both square and circular cross-section were comparable throughout the range of dimensions studied. The thinnest nanowires showed simple square switching and 100% remanence. With increasing diameter the switching field reduces and above 40 nm the reversal behavior shows an increasing rotational component prior to sharp switching of the magnetization. The magnitude of the reversible component increases with increasing dimensions up to 150 nm, above which the magnetization reversal process is more complicated and the hysteresis loops are no longer bistable. The micromagnetic structures evolve from simple uniform parallel single domain states in the thinnest wires through the formation of vortex-like end states in thicker wires to complex multidomain structures during the reversal of the thickest wires. In the later cases the reversal is not simple curling-like behavior, although the angular switching field dependence was comparable with curling.

Continuum model of the potential of charge carriers in a bent piezoelectric ZnO nanowire: analytic and numerical study

The piezoelectric potential of a slightly n-doped ZnO nanowire is analytically and numerically solved. We approximated the dopant concentration $$(N_d^{+})$$(Nd+) to shift slightly from the intrinsic level toward an n-typed region by increasing the donor concentration. Correspondingly, the electron density is also approximated. The analytic solution of the continuum model indicates that the derived solution is dependent on the lateral force, Bessel function, and Meijer G-function. As per the potential of the nanowire's cross section, the potential in the tensile side is considered to have screening effects, and the polarity of the potential between the tensile and the compressive is antisymmetric such that the compressive is negative and the tensile is positive. The diameter in the ZnO nanowire used in the model is 200 nm, and the lateral force used is approximately $$80 \times 10^{-5}$$80×10-5 N. Although the resulting piezoelectric potential of the NW that is derived from the simplified coefficient form is not accurately estimated, it shows a trend with the scaled values. The difference in the potential between the maximum tensile and compressive value is approximately one order that is consistent with others (Wang and Song, Science 312:242---246, 2006;Gao and Wang, Nano Lett 9:1103---1110, 2009).

Determination of Young's modulus of Sb2S3 nanowires by in situ resonance and bending methods.

In this study we address the mechanical properties of Sb2S3 nanowires and determine their Young’s modulus using in situ electric-field-induced mechanical resonance and static bending tests on individual Sb2S3 nanowires with cross-sectional areas ranging from 1.1·104 nm2 to 7.8·104 nm2. Mutually orthogonal resonances are observed and their origin explained by asymmetric cross section of nanowires. The results obtained from the two methods are consistent and show that nanowires exhibit Young’s moduli comparable to the value for macroscopic material. An increasing trend of measured values of Young’s modulus is observed for smaller thickness samples.

A deep etching mechanism for trench-bridging silicon nanowires

Introducing a single silicon nanowire with a known orientation and dimensions to a specific layout location constitutes a major challenge. The challenge becomes even more formidable, if one chooses to realize the task in a monolithic fashion with an extreme topography, a characteristic of microsystems. The need for such a monolithic integration is fueled by the recent surge in the use of silicon nanowires as functional building blocks in various electromechanical and optoelectronic applications. This challenge is addressed in this work by introducing a top-down, silicon-on-insulator technology. The technology provides a pathway for obtaining well-controlled silicon nanowires along with the surrounding microscale features up to a three-order-of-magnitude scale difference. A two-step etching process is developed, where the first shallow etch defines a nanoscale protrusion on the wafer surface. After applying a conformal protection on the protrusion, a deep etch step is carried out forming the surrounding microscale features. A minimum nanowire cross-section of 35 nm by 168 nm is demonstrated in the presence of an etch depth of 10 μm. Nanowire cross-sectional features are characterized via transmission electron microscopy and linked to specific process steps. The technology allows control on all dimensional aspects along with the exact location and orientation of the silicon nanowire. The adoption of the technology in the fabrication of micro and nanosystems can potentially lead to a significant reduction in process complexity by facilitating direct access to the nanowire during surface processes such as contact formation and doping.

Effects of cross-section on mechanical properties of Au nanowire

The aim of this paper is study of the effects of multiple cross-section of Au nanowire on mechanical properties. Different cross-section models of Au nanowires including circular, hexagonal, pentagonal and rectangular were simulated by finite element modeling using ABAQUS. In this study, the bending technique was applied so that both ends of the model were clamped with mid-span under loading condition. The cross-sections had the length of 400 nm and the diameter of 40 nm, except the circular cross-section while the rest of the cross-sections had an equivalent diameter. Von Misses stresses distribution were used to define the stress distribution in the cross-section under loading condition, and elastic deformation was analyzed by the beam theory. The results disclosed that the circular and the rectangular models had highest and lowest strengths against plastic deformation, respectively.

Intrinsic polarization control in rectangular GaN nanowire lasers.

We demonstrate intrinsic, linearly polarized lasing from single GaN nanowires using cross-sectional shape control. A two-step top-down fabrication approach was employed to create straight nanowires with controllable rectangular cross-sections. A clear lasing threshold of 444 kW cm(-2) and a narrow spectral line width of 0.16 nm were observed under optical pumping at room temperature, indicating the onset of lasing. The polarization was along the short dimension (y-direction) of the nanowire due to the higher transverse confinement factors for y-polarized transverse modes resulting from the rectangular nanowire cross-section. The results show that cross-sectioned shape control can enable inherent control over the polarization of nanowire lasers without additional environment requirements, such as placement onto lossy substrates.

Transport in selectively magnetically doped topological insulator wires

We study the electronic and transport properties of a topological insulator nanowire including selective magnetic doping of its surfaces. We use a model which is appropriate to describe materials like Bi$_2$Se$_3$ within a k.p approximation and consider nanowires with a rectangular geometry. Within this model the magnetic doping at the (111) surfaces induces a Zeeman field which opens a gap at the Dirac cones corresponding to the surface states. For obtaining the transport properties in a two terminal configuration we use a recursive Green function method based on a tight-binding model which is obtained by discretizing the original continuous model. For the case of uniform magnetization of two opposite nanowire (111) surfaces we show that the conductance can switch from a quantized value of $e^2/h$ (when the magnetizations are equal) to a very small value (when they are opposite). We also analyze the case of non-uniform magnetizations in which the Zeeman field on the two opposite surfaces change sign at the middle of the wire. For this case we find that conduction by resonant tunneling through a chiral state bound at the middle of the wire is possible. The resonant level position can be tuned by imposing an Aharonov-Bohm flux through the nanowire cross section.

Electromagnetic absorption of quasi‐1D Majorana nanowires

AbstractWe calculate the electromagnetic absorption cross section of long and narrow nanowires, in the so‐called quasi‐1D limit. We consider only two transverse bands and compute the dipole absorption cross section taking into account quasiparticle transitions from negative to positive energy eigenstates of the Bogoliubov‐de Gennes Hamiltonian. The presence of the zero energy (Majorana) state manifests in the different absorption spectra for x (parallel) and y (transverse) polarizations of the electromagnetic field.In the y ‐polarized case, the Majorana state causes a low energy absorption plateau extending from mid‐gap up to full‐gap energy. Increasing further the energy, the plateau is followed by a region of enhanced absorption due to transitions across the full gap. For x polarization the low energy absorption plateau is not observed. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Vapor Phase Synthesis of Organometal Halide Perovskite Nanowires for Tunable Room-Temperature Nanolasers

Semiconductor nanowires have received considerable attention in the past decade driven by both unprecedented physics derived from the quantum size effect and strong isotropy and advanced applications as potential building blocks for nanoscale electronics and optoelectronic devices. Recently, organic-inorganic hybrid perovskites have been shown to exhibit high optical absorption coefficient, optimal direct band gap, and long electron/hole diffusion lengths, leading to high-performance photovoltaic devices. Herein, we present the vapor phase synthesis free-standing CH3NH3PbI3, CH3NH3PbBr3, and CH3NH3PbIxCl3(-x) perovskite nanowires with high crystallinity. These rectangular cross-sectional perovskite nanowires have good optical properties and long electron hole diffusion length, which ensure adequate gain and efficient optical feedback. Indeed, we have demonstrated optical-pumped room-temperature CH3NH3PbI3 nanowire lasers with near-infrared wavelength of 777 nm, low threshold of 11 μJ/cm(2), and a quality factor as high as 405. Our research advocates the promise of optoelectronic devices based on organic-inorganic perovskite nanowires.

Fano resonances in high energy electron transport in nanowires of variable cross-section

Electron ballistic transport in 2D quantum waveguide with two narrowings is considered. For longitudinal electron motion such narrowings play the role of effective potential barriers and conditions for resonant tunneling arise. If the electron energy is sufficiently high the electron wave can scatter into different quantum states (transverse channels of the leads) which results in complicated E-dependence of the scattering amplitudes. Numerical simulations have shown that the scattering amplitudes resonances are of Fano type. The form of the transmission probability curve is conditioned by interference of the quantum states into which the electron wave is scattered by the narrowings. The suggested interference model makes possible to find the resonance parameters with high precision and to link them to the closed resonator eigenvalues.

Scale effects and the formation of polarization vortices in tetragonal ferroelectrics

Vortices consisting of 90° quadrant domains are rarely observed in ferroelectrics. Although experiments show polarization flux closures with stripe domains, it is as yet unclear why pure single vortices are not commonly observed. Here, we model and explore the energy of polarization patterns with vortex and stripe domains, formed on the square cross-section of a barium titanate nanowire. Using phase-field simulations, we calculate the associated energy of polarization patterns as a function of nanowire width. Further, we demonstrate the effects of surface energy and electrical boundary conditions on equilibrium polarization patterns. The minimum energy equilibrium polarization pattern for each combination of surface energy and nanowire width is mapped for both open- and short-circuit boundary conditions. The results indicate a narrow range of conditions where single vortices are energetically favorable: nanowire widths less than about 30 nm, open-circuit boundary condition, and surface energy of less than 4 N/m. Short-circuit boundary conditions tend to favor the formation of a monodomain, while surface energy greater than 4 N/m can lead to the formation of complex domain patterns or loss of ferroelectricity. The length scale at which a polarization vortex is energetically favorable is smaller than the typical size of nanoparticle in recent experimental studies. The present work provides insight into the effects of scaling, surface energy, and electrical boundary conditions on the formation of polarization patterns.

An exact surface-integral approach for accurate interferometric microscopy of single nanoparticles.

We present a half-plane surface-integral equation (SIE) approach for modeling the optical phase response of a single nanowire under phase-stepping interferometric (PSI) microscopy. This approach calculates scattered fields exactly from the Helmholtz equation in this 2D problem, obviating the need for ray-optic approximations. It is demonstrated that refractive index metrology is enabled by this method, with precision as low as 7 × 10(-5) possible for current state-of-the-art PSI microscopes. For nanowires of known refractive index, radii as small as 0.001λ are shown to yield a measurable phase signal and are therefore potentially measurable by this approach. Measurements are also demonstrated to be relatively insensitive to the spectral and coherence characteristics of the light source, the illumination conditions, and variations in nanowire cross-section shape. Prospects for measuring both the radius and refractive index simultaneously, and scope for generalizing this approach to arbitrary nanoparticle shapes are discussed.

Tunable quantum confinement in ultrathin, optically active semiconductor nanowires via reverse-reaction growth.

A unique growth scheme is demonstrated to realize ultrathin GaAs nanowires on Si with sizes down to the sub-10 nm regime. While this scheme preserves the bulk-like crystal properties, correlated optical experiments reveal huge blueshifted photo-luminescence (up to ≈100 meV) with decreasing nanowire cross-section, demonstrating very strong quantum confinement effects.

Analytical theory of the dipole-exchange oscillations in long ferromagnetic nanowires of elliptical cross-section in a transverse external magnetic field

The analytical theory of the dipole-exchange spin-wave excitations of ferromagnetic cylinders of elliptical cross section with the bias magnetic field perpendicular to the axis of the cylinder is presented. Explicit analytical expressions for the frequency spectrum and the magnetization spatial distribution are derived. Their evolution as a function of cross-section's aspect ratios has been investigated. The developed formalism allows successful interpretation of experimental Brillouin Light Scattering spectra obtained previously for Ni nano-wires both qualitatively and quantitatively. It also provides a tool for comprehensive analysis of phenomena, in which spatial distribution and polarization of high-frequency magnetization play crucial role.

Atomistic study on dopant-distributions in realistically sized, highly P-doped Si nanowires.

The dependency of dopant-distributions on channel diameters in realistically sized, highly phosphorus-doped silicon nanowires is investigated with an atomistic tight-binding approach coupled to self-consistent Schrödinger-Poisson simulations. By overcoming the limit in channel sizes and doping densities of previous studies, this work examines electronic structures and electrostatics of free-standing circular silicon nanowires that are phosphorus-doped with a high density of ∼ 2 × 10(19) cm(-3) and have 12 nm-28 nm cross-sections. Results of analysis on the channel energy indicate that the uniformly distributed dopant profile would be hardly obtained when the nanowire cross-section is smaller than 20 nm. Insufficient room to screen donor ions and shallower impurity bands are the primary reasons of the nonuniform dopant-distributions in smaller nanowires. Being firmly connected to the recent experimental study (Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 15254-15258), this work establishes the first theoretical framework for understanding dopant-distributions in over-10 nm highly doped silicon nanowires.