Rectangular Nanowires Research Articles

Preparation and Characterization of Conducting Mixed-Valence 9,9′-Dimethyl-3,3′-bicarbazyl Rectangular Nanowires

The facile synthesis of an organic electric conducting nanowire is described. The simple oxidation of 9-methylcarbazole by iron(III) perchlorate in a methanol/acetonitrile mixture under atmospheric pressure and temperature produces abundant nanowires without using a template. The nanowire consists of 9,9'-dimethyl-3,3'-dicarbazyl and has a rectangular nanowire shape with an average diameter of 397 ± 50 nm and length of 17 ± 5 μm. The results of the elemental analysis, (1)H NMR, FT-IR, XPS, and ESR measurements revealed that the chemical composition of the nanowire is (dicarbazyl)(0.12)(dicarbazylium·ClO(4)(-))(0.88)·H(2)O. This result, combined with the UV-vis-NIR measurement, demonstrates that 9,9'-dimethyl-3,3'-dicarbazyl stacks in a mixed valence state. The nanowire is electroactive and has an electric conductivity of 3.0 × 10(-5) S cm(-1).

Electronic and vibrational properties of vanadium-carbide nanowires

We have made an effort to understand the properties of transition metal carbide nanowires (NWs) and studied vanadium-carbide (VC) nanowires as a specific case. Different structures have been considered and their electronic and vibrational properties studied employing density functional theory. The effect of dimensionality is very well brought forth by these NWs, narrow/thinner structures have clear preference for magnetic state with sizeable magnetic moment at the V sites. As the thickness/width increases, the margin decreases and the magnetic moment disappears altogether for structures like square and rectangular NWs. The cohesive energy per atom increases with the increase in lateral dimensions of the NW, and it is about 88% of the bulk value for the rectangular NW, while it is only 50% for the linear chain. All the wires are conducting in nature, with the linear and zigzag wires having half-metallic character. Our calculations show that the V atoms decide the electronic and magnetic properties in these while compressibility, a mechanical property, is governed by the C atoms. The electron localization function beautifully illustrates the closeness of thicker/wider NWs to the bulk. It also reveals that electrons are highly localized around C atoms; however, the amount of charge transferred depends strongly on the structure of wire. The optical properties unfurl the impact of different spatial expanse in the cross section of NW in a nice way, e.g., ε2xx > ε2yy (ε2 is imaginary part of dielectric function) for all those with a larger expanse along X compared to Y and vice-versa. Thicker nanowires seem to be more suitable for optical applications. Site-resolved phonon density of states shows that presence of C atoms is responsible for high frequency branches. The heat capacity variation for various structures closely follows the magnitude of respective phonon density of states.

Tuning Light Absorption in Core/Shell Silicon Nanowire Photovoltaic Devices through Morphological Design

Subwavelength diameter semiconductor nanowires can support optical resonances with anomalously large absorption cross sections, and thus tailoring these resonances to specific frequencies could enable a number of nanophotonic applications. Here, we report the design and synthesis of core/shell p-type/intrinsic/n-type (p/i/n) Si nanowires (NWs) with different sizes and cross-sectional morphologies as well as measurement and simulation of photocurrent spectra from single-NW devices fabricated from these NW building blocks. Approximately hexagonal cross-section p/i/n coaxial NWs of various diameters (170-380 nm) were controllably synthesized by changing the Au catalyst diameter, which determines core diameter, as well as shell deposition time, which determines shell thickness. Measured polarization-resolved photocurrent spectra exhibit well-defined diameter-dependent peaks. The corresponding external quantum efficiency (EQE) spectra calculated from these data show good quantitative agreement with finite-difference time-domain (FDTD) simulations and allow assignment of the observed peaks to Fabry-Perot, whispering-gallery, and complex high-order resonant absorption modes. This comparison revealed a systematic red-shift of equivalent modes as a function of increasing NW diameter and a progressive increase in the number of resonances. In addition, tuning shell synthetic conditions to enable enhanced growth on select facets yielded NWs with approximately rectangular cross sections; analysis of transmission electron microscopy and scanning electron microscopy images demonstrate that growth of the n-type shell at 860 °C in the presence of phosphine leads to enhanced relative Si growth rates on the four {113} facets. Notably, polarization-resolved photocurrent spectra demonstrate that at longer wavelengths the rectangular cross-section NWs have narrow and significantly larger amplitude peaks with respect to similar size hexagonal NWs. A rectangular NW with a diameter of 260 nm yields a dominant mode centered at 570 nm with near-unity EQE in the transverse-electric polarized spectrum. Quantitative comparisons with FDTD simulations demonstrate that these new peaks arise from cavity modes with high symmetry that conform to the cross-sectional morphology of the rectangular NW, resulting in low optical loss of the mode. The ability to modulate absorption with changes in nanoscale morphology by controlled synthesis represents a promising route for developing new photovoltaic and optoelectronic devices.

Periodic plasmonic nanoantennas in a piecewise homogeneous background

Periodic rectangular gold nanomonopoles and nanodipoles in a piecewise inhomogeneous background, consisting of a silicon substrate and a dielectric (aqueous) cover, have been investigated extensively via 3D finite-difference time-domain simulations. The transmittance, reflectance and absorptance response of the nanoantennas were studied as a function of their geometry (length, width, thickness, gap) and found to vary very strongly. The nanoantennas were found to resonate in a single surface plasmon mode supported by the corresponding rectangular cross-section nanowire waveguide, identified as the sa(b)(0) mode [Phys. Rev. B 63, 125417 (2001)]. We determine the propagation characteristics of this mode as a function of nanowire cross-section and wavelength, and we relate the modal results to the performance of the nanoantennas. An approximate expression resting on modal results is proposed for the resonant length of nanomonopoles, and a simple equivalent circuit, also resting on modal results, but involving transmission lines and a capacitor (modelling the gap) is proposed to determine the resonant wavelength of nanodipoles. The expression and the circuit yield results that are in good agreement with the full computations, and thus will prove useful in the design of nanoantennas.

Phononic and structural response to strain in wurtzite-gallium nitride nanowires

Gallium nitride (GaN) nanowires exist in a myriad of cross-sectional shapes. In this study, a series of classical molecular dynamics simulations is performed to investigate the strain-phononics-structure relationship in rectangular and triangular wurtzite-GaN nanowires. The thermal conductivity of the nanowires is linearly dependent on the uniaxial strain in both compressive and tensile regimes, and shows no significant dissimilitude for the same amount of strain exerted on the two types of nanowire. This is coherent with an analytical approach using the Boltzmann transport theory. However, the thermomechanical behaviour at the vertex regions shows palpable differences between the two subfamilies, relative to the non-vertex faceted regions, as the structural morphology is most disparate at the vertices. Furthermore, the degree of strain asymmetry is a strong determinant of the vibrational response and consequently thermal conductance.

Vibrating modes and dispersive spectra of full polar optical phonons in wurtzite GaN/AlGaN rectangular nanowires

By employing the dielectric continuum model and Loudon’s uniaxial crystal model, the full polar optical phonon modes including the propagating (PR) modes, the quasi-confined (QC) modes, the half-space (HS) modes, and the interface optical (IO) modes in a quasi-one-dimensional (Q1D) wurtzite rectangular nanowire (NW) structure are deduced and analyzed. The analytical phonon states, their dispersion equations and polarization eigenvectors are derived. Numerical calculations for the dispersive spectra of these phonon modes are performed on a wurtzite GaN/Al0.15Ga0.85N rectangular wurtzite NW. The behavior of the PR modes reducing to the HS modes with the same order are observed clearly in the dispersive curves of these modes, which reveals that the present theories of phonon modes are self-consistency and correct for the description of phonon modes in wurtzite Q1D rectangular NW. Moreover, there are a series of yielding points in the dispersive curves of the PR modes. These observations and results reveal that the confined dimensionality and cross-section shape influence greatly the dispersive properties of phonon modes in wurtzite quantum confined systems.

Dispersive spectra and Fröhlich electron–phonon interaction Hamiltonians of full polar optical phonon states in a wurtzite nitride nanowire: quantum size effect

Based on the dielectric continuum model and Loudon's uniaxial crystal model, the full polar optical phonon modes including the propagating (PR) modes, quasi-confined modes, half-space (HS) modes and interface optical modes in a quasi-one-dimensional (Q1D) wurtzite rectangular nanowire (NW) structure are deduced and analyzed. The analytical phonon states, their dispersion equations, polar polarization eigenvectors and Fröhlich electron–phonon interaction Hamiltonians are derived. Numerical calculations of quantum size effect on the dispersive spectra for these modes are performed on a wurtzite GaN/Al0.15Ga0.85N rectangular wurtzite NW. The dispersive features of the four types of phonon modes are analyzed. The behavior of the PR modes reducing to the corresponding HS modes is observed clearly in the dispersive curves of these modes, which reveals that the present theories of phonon modes are self-consistency and correct for the description of phonon modes in wurtzite Q1D rectangular NW. Moreover, there are some yielding points in the dispersive curves of the PR modes. These observations and results reveal that the confined dimensionality and cross-sectional shape influence greatly the dispersive properties of phonon modes in wurtzite quantum confined systems.

Geometrical dependence of thermal conductivity in elliptical and rectangular nanowires

The aim of this work is to analyze the shape dependence of the effective thermal conductivity of nanowires, based on the model of phonon hydrodynamics. In particular, we consider elliptical and rectangular nanowires, and we compare their respective thermal conductivities with those of circular nanowires and of wide thin plates.

Rectangular Polysilicon Nanowires by Top-Down Lithography, Dry Etch and Metal-Induced Lateral Crystallization

In this work, we demonstrate a low temperature polysilicon nanowire fabrication process using amorphous silicon deposition over an oxide pillar, anisotropic reactive ion etch and metal-induced lateral crystallization (MILC). The fabricated nanowires are rectangular, with a width and height of around 100 nm. MILC is successfully achieved at temperatures down to 450oC, making the process compatible with glass substrates and hence suitable for low cost, disposable biosensors. Crystallisation lengths of 4.1 µm and 0.8 µm are obtained for 15 hour anneals at 480oC and 450oC, respectively.

Reduction of lattice thermal conductivity in one-dimensional quantum-dot superlattices due to phonon filtering

One dimensional quantum-dot superlattices (1D-QDSLs) consisting of acoustically mismatched materials are demonstrated theoretically to possess sub-1 W m${}^{\ensuremath{-}1}$ K${}^{\ensuremath{-}1}$ thermal conductivity in the 50--400 K range of temperatures. We consider coherent Si/Ge 1D-QDSLs, as well as model Si/plastic, Si/SiO${}_{2}$ and Si/SiC 1D-QDSLs. The phonon energy spectra and group velocities are obtained in the framework of the face-centered cubic cell model of lattice dynamics. On this basis, lattice thermal conductivity is calculated. A strong reduction of lattice thermal conductivity in 1D-QDSL structures in comparison with homogeneous rectangular Si nanowires is explained by the exclusion of phonon modes folded in superlattice segments from the heat flow and by the decelerating action of Ge, SiO${}_{2}$, or plastic materials. Thus, the 1D-QDSL structures act as effective phonon filters, eliminating a significant number of phonon modes from thermal transport. The obtained results imply a perspective of quantum-dot superlattices as thermoelectric materials and thermal insulators.

Corner effects on phonon-limited mobility in rectangular silicon nanowire metal-oxide-semiconductor field-effect transistors based on spatially resolved mobility analysis

We investigated the phonon-limited electron mobility in rectangular silicon nanowire metal-oxide-semiconductor field-effect transistors (Si NW MOSFETs) using the Kubo-Greenwood formula. Although cross-sectional electrostatics has been actively investigated, the cross-sectional distribution of the spatially resolved mobility has not been studied yet. In this paper, we discuss the corner effects based on spatially resolved mobility analysis. Taking into account the subband composition for local electrons, we could study the cross-sectional spatially resolved carrier transport. A strongly confined Si NW MOSFET showed that mobility hardly fluctuated in the cross-sectional distribution with strong volume inversion. On the other hand, a less-confined Si NW MOSFET showed that the corner mobility was lower than the side mobility; therefore, our result could imply that the corner effects were not positive for improvement in mobility.

Quantum-confinement effect on holes in silicon nanowires: Relationship between wave function and band structure

The authors theoretically studied the valence band structure and hole effective mass of rectangular cross-sectional Si nanowires (NWs) with the crystal orientation of [110], [111], and [001]. The E–k dispersion and the wave function were calculated using an sp3d5s∗ tight-binding method and analyzed with the focus on the nature of p orbitals constituting the subbands. In [110] and [111] nanowires, longitudinal/transverse p orbitals are well separated and longitudinal component makes light (top) subbands and transverse component makes heavy subbands. The heavy subbands are located far below the top light band when NW has square cross-section, but they gain their energy with the increase in the NW width and come near the band edge. This energy shift of heavy bands in [110] NWs shows strong anisotropy to the direction of quantum confinement whereas that in [111] NWs does not have such anisotropy. This anisotropic behavior and the difference among orientations are understandable by the character of the wave function of heavy subbands. Regarding the [001] nanowires, the top valence state is formed by the mixture of longitudinal/transverse p orbitals, which results in heavy effective mass and large susceptibility to lateral-size variation. The correlation of the wave function of hole states between nanowires and bulk is also discussed briefly.

Characterization of silicon nanophotonic devices using the finite element method

The Poynting vector and the full-vectorial H and E-field profiles are considered for use in nanoscale silicon waveguides in this article. This paper reveals that the mode profile of a circular silicon nanowire is not circular and also has a strong axial field component. From the analysis, the characteristics of single mode operation and the vector field profiles of both circular and planar silicon nanowires are presented. The modal birefringence of rectangular silicon nanowires and power density in low-index region of a slot-type waveguide and designs of a compact polarization rotator are also presented in this work.

Fundamental study of mechanical energy harvesting using piezoelectric nanostructures

This paper numerically estimates the potential, the output power and the energy conversion efficiency of piezoelectric nanostructures, including rectangular nanowires (NWs), hexagonal NWs, and two-dimensional vertical thin films (the nanofins). Static analysis studies the maximum piezoelectric potential that can be produced by a BaTiO3 NW, a ZnO NW, and a ZnO nanofin when they are subjected to a constant external force. Dynamic analysis is performed to study the power generation ability via the vibration of these nanostructures agitated by ambient vibration energy. ZnO NW and nanofin are selected as two representative nanogenerator elements. Their dynamic responses are modeled using a single-degree of freedom system with a series of damping ratios. Combining the transfer functions of mechanical vibration and piezoelectric charge generation, we define the output power and efficiencies as functions of the vibration frequency and the sizes. The optimal size for constructing a high efficiency and high-power nanogenerator is suggested. The material dependence of a dynamic system is also studied based on different piezoelectric and ferroelectric material systems, including ZnO, BaTiO3, and (1−x)Pb(Mg1/3Nb2/3)O3−xPbTiO3. This research reveals a comprehensive relationship between the mechanical energy harvesting ability and the nanomaterials’ morphologies, dimensions, and properties. It provides a guideline for the design of high-power nanogenerators and the development of piezoelectric nanodevices in general.

In Situ Doped Source/Drain for Performance Enhancement of Double-Gated Poly-Si Nanowire Transistors

A poly-Si nanowire (NW) thin-film transistor configured with the double-gated scheme was fabricated and characterized. The fabrication process features the clever use of selective plasma etching to form a rectangular NW underneath a hard mask. In this paper, we show that replacing the original ion-implanted poly-Si with <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> doped poly-Si for the source/drain significantly enhances the device performance, including steeper subthreshold swing (SS), larger on/off current ratio, and reduced series resistance. In particular, the SS is improved to a record-breaking low value of 73 mV/dec, which, to the best of our knowledge, is the most ideal ever reported for a poly-Si based device. The new NW transistors with such excellent switching properties are highly promising for reducing power consumption and operational voltage in practical circuit applications.

On the bandstructure velocity and ballistic current of ultra-narrow silicon nanowire transistors as a function of cross section size, orientation, and bias

A 20 band sp3d5s∗ spin-orbit-coupled, semiempirical, atomistic tight-binding model is used with a semiclassical, ballistic field-effect-transistor model, to theoretically examine the bandstructure carrier velocity and ballistic current in silicon nanowire (NW) transistors. Infinitely long, uniform, cylindrical, and rectangular NWs, of cross sectional diameters/sides ranging from 3–12 nm are considered. For a comprehensive analysis, n-type and p-type metal-oxide semiconductor (NMOS and PMOS) NWs in [100], [110], and [111] transport orientations are examined. In general, physical cross section reduction increases velocities, either by lifting the heavy mass valleys or significantly changing the curvature of the bands. The carrier velocities of PMOS [110] and [111] NWs are a strong function of diameter, with the narrower D=3 nm wires having twice the velocities of the D=12 nm NWs. The velocity in the rest of the NW categories shows only minor diameter dependence. This behavior is explained through features in the electronic structure of the silicon host material. The ballistic current, on the other hand, shows the least sensitivity with cross section in the cases where the velocity has large variations. Since the carrier velocity is a measure of the effective mass and reflects on the channel mobility, these results can provide insight into the design of NW devices with enhanced performance and performance tolerant to structure geometry variations. In the case of ballistic transport in high performance devices, the [110] NWs are the ones with both high NMOS and PMOS performance as well as low on-current variations with cross section geometry variations.

Vibrational properties of Cu nanowires

We have calculated local vibrational density of states (LDOS) of a center atom (CA) of a rectangular Cu nanowire with the axial orientation of 〈 100 〉 , using a real space Green’s function approach with force constants extracted from interaction potentials based on the embedded atom method. The LDOS of a center atom of a strained nanowire is found to exhibit quite distinctive characteristics as compared to that of a strain-free nanowire, the most striking feature of which is the existence of high frequency modes above the bulk band even for small strain of 2.5%. We further find that these high frequency modes are shifted to even higher frequencies when the nanowire is exposed to an increasing axial strain.

Highly ordered rectangular silvernanowire monolayers: water-assisted synthesis and galvanic replacement reaction with HAuCl4

We demonstrate a simple water washing process for large-scale fabrication of highly ordered and aligned rectangular silver nanowire monolayers from polyol systems. These monolayers exhibit unique structure-dependent chemical reactivities for the galvanic replacement reaction with HAuCl(4), and novel gold nanoneedle arrays with sharp tips and a uniform structure have been produced.

Magnetization reversal in single ferromagnetic rectangular nanowires

We report on the magnetoresistance (MR) investigation of the magnetization reversal processes in single rectangular nanowire of Permalloy. A set of nanowires with lengths ranging from 6 to 20 μm, thicknesses fixed in 10 nm, and widths between 250 nm and 1.2 μm, was fabricated by means of AFM local anodic oxidation lithography. Magnetoresistive hysteresis loops show an abrupt jump corresponding to the magnetization reversal that depends on the angle between the wire axis and the applied magnetic field direction. The field value corresponding to the abrupt jump of the MR was associated to the nucleation field deduced from the Brown equations. By the angular dependence of this magnetization reversal field we were able to identify the nucleation mode as the magnetization buckling. We have investigated the temperature dependence of the switching field as well as its stochastic nature as a function of the in-plane angle.

Thermal conductivity modeling of copper and tungsten damascene structures

Thermal conductivity of copper and tungsten nanowires, copper damascene structures are modeled theoretically on the basis of Boltzmann transport equation. The size-dependent thermal conductivities of the nanowires are obtained by taking into account interface scattering, grain boundary scattering, and interface roughness effect. Expressions for the thermal conductivity of thin films including the above mechanisms are extended to the case of rectangular cross-sectional nanowires. The fractal model is also used to predict the contribution of interface roughness to the thermal conductivity of nanowires. It is shown that the interface roughness is important for the nanowires with lateral dimensions down to the sub-30 nm region. Calculated thermal conductivities are in good agreement with the experimental data for copper and tungsten nanowires. Furthermore, it is found that the titanium nitride barrier layer obviously leads to further thermal conductivity reduction in the copper damascene structures.