Nanowire Cross-section Research Articles

Imaging the magnetic nanowire cross section and magnetic ordering within a suspended 3D artificial spin-ice

Artificial spin-ice systems are patterned arrays of magnetic nanoislands arranged into frustrated geometries and provide insight into the physics of ordering and emergence. The majority of these systems have been realized in two-dimensions, mainly due to the ease of fabrication, but with recent developments in advanced nanolithography, three-dimensional artificial spin ice (ASI) structures have become possible, providing a new paradigm in their study. Such artificially engineered 3D systems provide new opportunities in realizing tunable ground states, new domain wall topologies, monopole propagation, and advanced device concepts, such as magnetic racetrack memory. Direct imaging of 3DASI structures with magnetic force microscopy has thus far been key to probing the physics of these systems but is limited in both the depth of measurement and resolution, ultimately restricting measurement to the uppermost layers of the system. In this work, a method is developed to fabricate 3DASI lattices over an aperture using two-photon lithography, thermal evaporation, and oxygen plasma exposure, allowing the probe of element-specific structural and magnetic information using soft x-ray microscopy with x-ray magnetic circular dichroism (XMCD) as magnetic contrast. The suspended polymer–permalloy lattices are found to be stable under repeated soft x-ray exposure. Analysis of the x-ray absorption signal allows the complex cross section of the magnetic nanowires to be reconstructed and demonstrates a crescent-shaped geometry. Measurement of the XMCD images after the application of an in-plane field suggests a decrease in magnetic moment on the lattice surface due to oxidation, while a measurable signal is retained on sub-lattices below the surface.

Defect Location in Laterally Aligned Tin Oxide Nanowires: The Role of Growth Direction, Interface Dimensionality, and Catalyst

Laterally aligned SnO2 nanowires, which exhibit planar growth along crystallographically defined directions of a sapphire substrate surface, are superior for bio- and gas-sensing applications. Little is known about their cross-sectional geometry and the defect distribution within the nanowire cross section, although this substantially determines the electronic properties of the nanowires. In this study, SnO2 nanowires were grown on r-plane sapphire substrates. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) reveal the highly oriented growth of the SnO2 nanowires toward the substrate edges. High-resolution transmission electron microscopy (HRTEM) of the NW cross section and strain mapping allowed for analyzing the crystallographic alignment of the SnO2 NW to the sapphire substrate and the defect distribution within the SnO2 nanowire cross section. These techniques revealed a defect-free SnO2–Al2O3 interface and a high alignment of the SnO2 NW lattice toward the sapphire substrate along the NW width. The determined high defect density close to the nanowire surface will be discussed in comparison to freestanding SnO2 nanowires and SnO2 thin films on sapphire substrates considering the differences in the growth direction and the interface dimensionality.

Retrieving the subwavelength cross-section of dielectric nanowires with asymmetric excitation of Bloch surface waves.

Optical microscopy with a diffraction limit cannot distinguish nanowires with sectional dimensions close to or smaller than the optical resolution. Here, we propose a scheme to retrieve the subwavelength cross-section of nanowires based on the asymmetric excitation of Bloch surface waves (BSWs). Leakage radiation microscopy is used to observe the propagation of BSWs at the surface and to collect far-field scattering patterns in the substrate. A model of linear dipoles induced by tilted incident light is built to explain the directional imbalance of BSWs. It shows the potential capability in precisely resolving the subwavelength cross-section of nanowires from far-field scattering without the need for complex algorithms. Through comparing the nanowire widths measured by this method and those measured by scanning electron microscopy (SEM), the transverse resolutions of the widths of two series of nanowires with heights 55 nm and 80 nm are about 4.38 nm and 6.83 nm. All results in this work demonstrate that the new non-resonant far-field optical technology has potential application in metrology measurements with high precision by taking care of the inverse process of light-matter interaction.

Thermal Conductivity of Nanostructure in Microelectronic Equipment and the Enhancement of Its Thermo-Physical Properties

To optimize the heat dissipation performance of microelectronic equipment, optical instrument, and optoelectronic devices, it’s necessary to explore the heat transfer mechanism of the nanostructure in them, however, due to the limitation of the research objects, there isn’t a uniform simulation method for the thermo-physical properties of such nanostructure. Therefore, this paper aims to study the thermo-physical properties of nanostructure in microelectronic equipment. At first, by structuring Si super lattice nanowires in a specific direction and then using the Ge atoms to replace the Si atoms in the nanowires, this paper built a model for the periodic Si/Ge super lattice nanowires. Then, this paper performed integral operation on the dynamic equations of the two types of atoms using the time integration algorithm to attain the trajectories of Si and Ge atoms in the nanowire structure of the microelectronic equipment, and discussed the influence of the size and geometrical shape of the cross section of nanowires on thermal conductivity. After that, this paper simulated the structure of the nano-scale SiC material, selected appropriate cut-off radius, and optimized the structure of the nanostructure model based on the Discover Minimization module. At last, the thermal conductivity of the nanostructure was calculated and its thermo-physical properties were analyzed.

Analytical description of nanowires III: regular cross sections for wurtzite structures.

Setting out from König & Smith [Acta Cryst. (2019), B75, 788-802; Acta Cryst. (2021), B77, 861], we present an analytic description of nominal wurtzite-structure nanowire (NWire) cross sections, focusing on the underlying geometric-crystallographic description and on the associated number theory. For NWires with diameter dWire[i], we predict the number of NWire atoms NWire[i], the bonds between these Nbnd[i] and NWire interface bonds NIF[i] for a slab of unit-cell length, along with basic geometric variables, such as the specific length of interface facets, as well as widths, heights and total area of the cross section. These areas, the ratios of internal bonds per NWire atom, of internal-to-interface bonds and of interface bonds per NWire atom present fundamental tools to interpret any spectroscopic data which depend on the diameter and cross section shape of NWires. Our work paves the way for a fourth publication which - in analogy to König & Smith [Acta Cryst. (2022). B78, 643-664] - will provide adaptive number series to allow for arbitrary morphing of nominal w-structure NWire cross sections treated herein.

Magnetic Properties of Nanostructured Material with the Effect of Dimension

In this Study, Qi model, Jiang model and Lu model, are used to predict the variation of magnetic properties with size and shape in nanosolids. Magnetic properties of nanosolids such as Curie Temperature (Tcn), Magnetization (Msn) and Neel Temperature (Tnn) of Cu, Ag are studied. It is predicted that as the size of nanosolids is decreased, all these three properties decrease with decreasing size. These magnetic properties are also studied with different shapes of nanosolids, such as thin films, cylindrical nanowires, spherical, regular tetrahedral nanoparticles and regular triangular cross-section nanowires. The computational results are compared with the available experimental and simulated data to validate our theory.

A simulation study of polarization characteristics of ultrathin CsPbBr3 nanowires with different cross-section shapes and sizes

The polarization characteristics of ultrathin CsPbBr3 nanowires are investigated. Especially, for the height of cross-section of nanowires between 2 nm and 25 nm, the normalized intensity and polarization ratio ρ of CsPbBr3 nanowires with triangular, square and hexagonal cross-section shapes are compared. The results show that, along with the increase of the height of cross-section, the polarization ratios of these three nanowires decrease until T = 15 nm, and increase afterwards. Also, along with the increase of the cross-section area up to 100 nm2, the polarization ratios of these three nanowires increase too. In general, for the same height or area, the polarization ratio ρ of these nanowires follows ρ hexagon > ρ square > ρ triangle. Therefore, the nanowire with the hexagonal cross-section should be chosen, where for a cross-section height of 2 nm and a length-height ratio of 20:1, the maximal polarization ratio is 0.951 at the longitudinal center of the NW. Further, for the hexagonal NW with a cross-section height of 10 nm, the hexagonal NW with a length-height ratio of 45:1 exhibits the maximal polarization ratio at the longitudinal center of the NW. These simulation results predict the feasible size and shape of CsPbBr3 nanowire devices with high polarization ratios.

Ultra-thin broadband solar absorber based on stadium-shaped silicon nanowire arrays

This paper investigates how the dimensions and arrangements of stadium silicon nanowires (NWs) affect their absorption properties. Compared to other NWs, the structure proposed here has a simple geometry, while its absorption rate is comparable to that of very complex structures. It is shown that changing the cross-section of NW from circular (or rectangular) to a stadium shape leads to change in the position and the number of absorption modes of the NW. In a special case, these modes result in the maximum absorption inside NWs. Another method used in this paper to attain broadband absorption is utilization of multiple NWs which have different geometries. However, the maximum enhancement is achieved using non-close packed NW. These structures can support more cavity modes, while NW scattering leads to broadening of the absorption spectra. All the structures are optimized using particle swarm optimizations. Using these optimized structures, it is viable to enhance the absorption by solar cells without introducing more absorbent materials.Graphical

Chromium nanostructures for enhancing light trapping in a thin-film solar cell

In this work, a thin-film solar cell consists of metallic nanowires, semiconductor, and antireflection layers is investigated. The absorption of the structure is obtained numerically in the broadband wavelength range of 300–1700 nm. The influences of the layer thicknesses and the cross-section shape of metallic nanowires are studied. Results indicate that the absorption of the rectangular-nanowire system is over 90% in the range of 560 − 1180 n m . Also, the absorption in the range of 300 − 1200 nm is over 90% when the cross-section of nanowires is semi-circular. This high absorption in the visible region could be helpful in designing plasmonic solar cells. • The light trapping in a thin-film plasmonic solar cell is studied. • Impacts of the layer thicknesses and the cross-section of Chromium nanowires are investigated. • The absorption of the circular-nanowire structure in the range of 300–1200 nm is over 90%. • The absorption of the rectangular-nanowire system is over 90% in the range of 560–1180 nm.

Polarized Raman spectroscopy of GaP nanowires under 5% elastic strain

Optical porperties of highly-strained gallium phosphide nanowires were investigated via polarized Raman spectroscopy. 5% elastic strain was created in individual nanowire lying on nickel substrate by the means of atomic force microscopy. Micro-Raman mapping along the nanowire cross section in parallel and perpendicular polarization was carried out. Strain-induced effects on transverse optical mode position and shape were analyzed. The pronounced splitting of the mode due to high level of strain was observed. It was found that in parallel polarization the mode shape is sensitive to the position of the pumping spot which can be attributed to enhanced light-nanowire coupling effects.

Analytical description of nanowires. I. Regular cross sections for zincblende and diamond structures. Corrigendum

In the paper by König & Smith [Acta Cryst. (2019), B75, 788–802], several equations had minor errors in their coefficients defining characteristic lengths and area of zinc-blende nanowire cross sections, thereby deviating from the exact analytical solution by 1.4 ± 0.8 %. A fully corrected version of the paper is provided.

Polarization properties of Raman scattering by surface phonon polaritons in GaAsP nanowires

Strong resonant enhancement of Raman scattering on photonic resonance was observed in GaAsP semiconductor nanowires. The enhancement allowed for detailed studies of the surface phonon polariton (SPhP) scattering peak on individual nanowires. In particular, for the first time, the effect of the nanowire cross section shape on SPhP properties has been investigated. It was found that the cross section flattening induces a strong polarisation and a spectral shift of SPhPs supported by such nanowire. The assisting numerical simulations allowed to link the induced polarisation effect to a splitting of the resonant HE11 mode in the flattened nanowire. The observed spectral shift of SPhP has been also theoretically reproduced in elliptical approximation for the flattened cross section. The obtained results pave a ground for engineering of SPhP polarisation response and accurate spectral control of SPhPs in applications utilising the nanowire morphology.

Orientation and size effects on electronic structure of rectangular cross-sectional Sn nanowires

The band structures of Sn nanowires with various cross-sectional shapes and orientations were calculated by a tight-binding method. The wave functions of bulk Sn and Sn nanowires were also analyzed. The calculation revealed that the conduction band minimum of some Sn nanowires originates from the electronic states in the valence band of bulk Sn. This behavior was discussed in terms of the dependence on the orientation and cross-sectional shape of Sn nanowires. In addition, we clarified that the [110]-oriented nanowires with a narrow (001) sidewall and [111]-oriented nanowires have the lightest effective mass among the considered nanowires.

Impact of Electrical Current on Single GaAs Nanowire Structure

The impact of electrical current on the structure of single free‐standing Be‐doped GaAs nanowires grown on a Si 111 substrate is investigated. Single nanowires have been structurally analyzed by X‐ray nanodiffraction using synchrotron radiation before and after the application of an electrical current. The conductivity measurements on single nanowires in their as‐grown geometry have been realized via W‐probes installed inside a dual‐beam focused ion beam/scanning electron microscopy chamber. Comparing reciprocal space maps of the 111 Bragg reflection, extracted perpendicular to the nanowire growth axis before and after the conductivity measurement, the structural impact of the electrical current is evidenced, including deformation of the hexagonal nanowire cross section, tilting, and bending with respect to the substrate normal. For electrical current densities below 30 A mm−2, the induced changes in the reciprocal space maps are negligible. However, for a current density of 347 A mm−2, the diffraction pattern is completely distorted. The mean cross section of the illuminated nanowire volume is reconstructed from the reciprocal space maps before and after the application of electrical current. Interestingly, the elongation of two pairs of opposing side facets accompanied by shrinkage of the third pair of facets is found. The variations in the nanowire diameter, as well as their tilt and bending, are confirmed by scanning electron microscopy. To explain these findings, material melting due to Joule heating during voltage/current application accompanied by anisotropic deformations induced by the W‐probe is suggested.

Low-symmetry nanowire cross-sections for enhanced Dresselhaus spin-orbit interaction

We study theoretically the spin-orbit interaction of low-energy electrons in semiconducting nanowires with a zinc-blende lattice. The effective Dresselhaus term is derived for various growth directions, including <11(-2)>-oriented nanowires. While a specific configuration exists where the Dresselhaus spin-orbit coupling is suppressed even at confinement potentials of low symmetry, many configurations allow for a strong Dresselhaus coupling. In particular, we discuss qualitative and quantitative results for nanowire cross-sections modeled after sectors of rings or circles. The parameter dependence is analyzed in detail, enabling predictions for a large variety of setups. For example, we gain insight into the spin-orbit coupling in recently fabricated GaAs-InAs nanomembrane-nanowire structures. By combining the effective Dresselhaus and Rashba terms, we find that such structures are promising platforms for applications where an electrically controllable spin-orbit interaction is needed. If the nanowire cross-section is scaled down and InAs replaced by InSb, remarkably high Dresselhaus-based spin-orbit energies of the order of millielectronvolt are expected. A Rashba term that is similar to the effective Dresselhaus term can be induced via electric gates, providing means to switch the spin-orbit interaction on and off. By varying the central angle of the circular sector, we find, among other things, that particularly strong Dresselhaus couplings are possible when nanowire cross-sections resemble half-disks.

Turning Low-Nanoscale Intrinsic Silicon Highly Electron-Conductive by SiO2 Coating.

Impurity doping in silicon (Si) ultra-large-scale integration is one of the key challenges which prevent further device miniaturization. Using ultraviolet photoelectron spectroscopy and X-ray absorption spectroscopy in the total fluorescence yield mode, we show that the lowest unoccupied and highest occupied electronic states of ≤3 nm thick SiO2-coated Si nanowells shift by up to 0.2 eV below the conduction band and ca. 0.7 eV below the valence band edge of bulk silicon, respectively. This nanoscale electronic structure shift induced by anions at surfaces (NESSIAS) provides the means for low-nanoscale intrinsic Si (i-Si) to be flooded by electrons from an external (bigger, metallic) reservoir, thereby getting highly electron- (n-) conductive. While our findings deviate from the behavior commonly believed to govern the properties of silicon nanowells, they are further confirmed by the fundamental energy gap as per nanowell thickness when compared against published experimental data. Supporting our findings further with hybrid density functional theory calculations, we show that other group IV semiconductors (diamond, Ge) do respond to the NESSIAS effect in accord with Si. We predict adequate nanowire cross-sections (X-sections) from experimental nanowell data with a recently established crystallographic analysis, paving the way to undoped ultrasmall silicon electronic devices with significantly reduced gate lengths, using complementary metal-oxide-semiconductor-compatible materials.

Effect of Phonon Focusing on the Drag Thermopower in Single-Crystal Potassium Nanowires at Low Temperatures

The role of shear waves and the effect of phonon focusing on the anisotropy of the thermopower of the electron–phonon drag in single-crystal potassium nanowires at low temperatures are analyzed in this work. The deformation potential theory is used for the longitudinal components of elastic modes. For the shear components of vibrational modes, the electron–phonon interaction constant is used, which was determined previously from comparing the results of calculating the thermoelectric power with experimental data for bulk potassium crystals. It is shown that shear waves make a significant contribution to the drag thermopower of nanowires. In the regime of the Knudsen phonon gas flow for nanowires with a cross-section side D = 5 × 10–6 cm, the contribution of the slow transverse mode t2 with allowance for only the longitudinal component in the [111] directions turned out to be 32% smaller and, with allowance for the shear component of the t2 mode, 12% greater than the contribution of longitudinal phonons. Directions corresponding to the maximum and minimum values of the drag thermopower of nanowires are determined. It is shown that, under conditions of competition between the boundary and bulk mechanisms of phonon relaxation with an increase in the cross section of nanowires, the anisotropy of the drag thermopower changes nonmonotonically. It exceeds 30% not only in the Knudsen phonon gas flow regime, but also when the sample thickness is two orders of magnitude greater. This makes the anisotropy of thermopower available for experimental studies.

Tuning domain wall dynamics by shaping nanowires cross-sections

The understanding of the domain wall (DW) dynamics along magnetic nanowires is crucial for spintronic applications. In this work, we perform a detailed analysis of the transverse DW motion along nanowires with polygonal cross-sections. If the DW displaces under a magnetic field above the Walker limit, the oscillatory motion of the DW is observed. The amplitude, the frequency of oscillations, and the DW velocity depend on the number of sides of the nanowire cross-section, being the DW velocity in a wire with a triangular cross-section one order of magnitude larger than that in a circular nanowire. The decrease in the nanowire cross-section area yields a DW behavior similar to the one presented in a cylindrical nanowire, which is explained using an analytical model based on the general kinetic momentum theorem. Micromagnetic simulations reveal that the oscillatory behavior of the DW comes from energy changes due to deformations of the DW shape during the rotation around the nanowire.

Interplay between Mesoscale Architecture and Catalytic Output in CO2 Gas-Diffusion Electrolyzers

With the anticipated effects of global climate change upon us, induced by egregious CO2 emissions, it is imperative to implement technologies to close the carbon cycle. One potential technique to do this is storing the energy from intermittent sources (e.g. solar, wind, hydro) in the form of chemical bonds, such as the electrochemical reduction of CO2. By reducing CO2 to energetically and chemically useful products of CO2 (i.e. CO and C2H4) we can both reduce reliance on fossil fuels and mitigate atmospheric CO2 levels. However, controlling the selectivity towards which particular product is formed and preventing the parasitic hydrogen evolution reaction (HER) is a crucial challenge that must be overcome.1 Exploring fundamental mechanisms and new catalyst materials to overcome this challenge, can be skewed due to the low diffusivity (≈2x10-9 m2/s)2 of CO2 in the liquid phase causing inefficient utilization of catalyst active sites. Unlike the oxygen reduction reaction, use of a rotating disc electrode does not resolve the issue due to the competitive HER reaction. Therefore, it becomes imperative to study physical structure-function relations (e.g. catalyst layer thickness) in a cell that provides ample flux of solely CO2, such as a gas-diffusion electrolyzer (GDE) which greatly increases the diffusivity of CO2 and gas products by enabling transport through gas channels (≈1.6x10-5 m2/s). By filtering nanowires through a gas-diffusion electrode (Figure 1 a-b) we fabricated electrodes of varying thicknesses that were then applied as catalysts in a gas-diffusion electrolyzer (Figure 1c). Results show that even under high CO2 flux conditions there is an interplay between selectivity (Figure 1d), activity (Figure 1e) and catalyst layer thickness. Further exploration of this interplay in commercially relevant electrolyzers can expedite optimized implementation of catalysts in the industrial sector and hopefully aid in solving the global climate crisis at hand.(1) Verma, S.; Kim, B.; Jhong, H.-R. “Molly”; Ma, S.; Kenis, P. J. A. A Gross-Margin Model for Defining Technoeconomic Benchmarks in the Electroreduction of CO2. ChemSusChem 2016, 9 (15), 1972–1979. https://doi.org/10.1002/cssc.201600394.(2) Raciti, D.; Mao, M.; Ha Park, J.; Wang, C. Mass Transfer Effects in CO 2 Reduction on Cu Nanowire Electrocatalysts. Catalysis Science & Technology 2018, 8 (9), 2364–2369. https://doi.org/10.1039/C8CY00372F.Figure 1: Electron microscopy images of Ag nanowires (a) top-down and (b) cross-section, on a gas-diffusion electrode. This electrode is than applied as a CO2 reduction catalyst in a (c) gas-diffusion electrolyzer. The thickness of the Ag nanowire catalyst layer influences both the (d) selectivity and (e) mass activity of the reaction. Figure 1

Mechanical properties of Cu nanowires: Effects of cross-sectional area and temperature

Nanostructured materials likely exhibit different mechanical properties from the corresponding bulk materials and have potential applications in a variety of areas. In this work, we use molecular dynamics simulation with embedded-atom potential to investigate the dependence of the mechanical properties of Cu nanowires of square cross-section, including Young's modulus, yield stress and surface force, on cross-sectional area and temperature for the Cu nanowires with cross-sectional areas in a range of 13.1 to 117.6 nm2 in a temperature range of 100 to 500 K. The simulation results reveal that the Young's modulus increases with the increase of the cross-sectional area at the same temperature and decreases with the increase of temperature for the same cross-sectional area. The yield stress decreases with the increase of the cross-sectional area at the same temperature in the range of 100 to 400 K and increases with the increase of the cross-sectional area at 500 K, while it decreases with the increase of temperature for the same cross-sectional area. The surface force decreases with the increase of the cross-sectional area at the same temperature and with the increase of temperature for the same cross-sectional area. Semi-empirical expressions relating the mechanical properties to temperature are proposed. Using the semi-empirical expressions, the Young's modulus at 0 K is found to decrease with the decrease of the cross-sectional area, and the yield stress at 0 K is found to decrease linearly with the increase of the cross-sectional area.