Shape Of Nanowires Research Articles

Junction-Tuned SnO2 Nanowires and Their Sensing Properties

On the basis of a selective growth of SnO2 nanowires by the vapor–liquid–solid growth method, networked SnO2 nanowire sensors were fabricated. Then, their sensing properties were systematically investigated in terms of NO2. The density of junctions was controlled by altering the spacing of the patterned-interdigital electrodes (PIEs), on which SnO2 nanowires selectively grew and touched the nanowires grown on neighboring PIEs, eventually producing junctions. The sensing mechanism was attributed to the change not only in the width of the space charge region along the length direction of each nanowire but also in the height of the built-in potential at the junctions during adsorption and desorption of gaseous species. Narrower spacings of PIEs led to an increasing the density of junctions projected to the plane and, consequently, superior properties for gas sensing. Importantly, a general principle to prepare networked nanowires of superior sensing capabilities was suggested from the point of view of nanowire shape and electrode configuration.

Thermodynamic and electronic study of Ga 1 − x Mn xN films. A theoretical study

Periodic slab calculations based on density functional theory were performed at the B3LYP level to gain insight into the surfaces of wurtzite GaN nanostructures. The (10 1 ¯ 0) and (11 2 ¯ 0) GaN surfaces are the most thermodynamically stable surfaces, the energy of the former being slightly smaller than that of the latter. The thermodynamic stability associated with the equilibrium shape of nanowires was determined using the calculated values. Doping with Mn further decreases the surface energy of (10 1 ¯ 0) and (11 2 ¯ 0). The minimum surface energy of Ga 1 − x Mn xN (0.04 ≤ x ≤ 0.17) is found at x ~ 0.08, for (10 1 ¯ 0) and (11 2 ¯ 0) slab models. Substitution of Ga with Mn in different positions relative to the surface shows that the total energy increases as the Mn atoms move from the surface layer to the interior sites of the slabs. Mn doping is also responsible for decreases in the band gap energy: the minimum calculated band gap in the Ga 1 − x Mn xN (10 1 ¯ 0) slab was found at x ~ 0.17, whereas the (11 2 ¯ 0) surface presented the corresponding minimum at x ~ 0.04. The magnetic moments associated with Mn were observed to increase as the ion positions moved closer to the surfaces.

Synthesis of vertically‐aligned GaAs nanowires on GaAs/(111)Si hetero‐substrates by metalorganic vapour phase epitaxy

AbstractWe report on the Au‐catalysed synthesis of GaAs nanowires on hetero‐structured GaAs/(111)Si substrates by metalorganic vapour phase epitaxy. It is demonstrated that the deposition of a 40‐50 nm thin GaAs epilayer onto Si guarantees a high percentage of straight and vertically‐aligned GaAs nanowires. GaAs epilayers were grown at 400 °C and subsequently annealed at 700 °C. Growth experiments performed on 4°‐miscut and exactly‐oriented (111)Si substrates show that a higher yield (close to 90%) of vertical nanowires is obtained using miscut substrates, an effect ascribed to the smoother surface morphology of GaAs epilayers on these substrates. Comparison between the cross‐sectional shape of nanowires grown on GaAs/(111)Si hetero‐substrates and those on (111)A‐GaAs and (111)B‐GaAs substrates demonstrates that both GaAs epilayers and over‐grown nanowires are (111)B‐oriented. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Absorption enhancement by matching the cross-section of plasmonic nanowires to the field structure of tightly focused beams

Nanostructured materials, designed for enhanced light absorption, are receiving increased scientific and technological interest. In this paper we propose a physical criterion for designing the cross-sectional shape of plasmonic nanowires for improved absorption of a given tightly focused illumination. The idea is to design a shape which increases the matching between the nanowire plasmon resonance field and the incident field. As examples, we design nanowire shapes for two illumination cases: a tightly focused plane wave and a tightly focused beam containing a line singularity. We show that properly shaped and positioned silver nanowires that occupy a relatively small portion of the beam-waist area can absorb up to 65% of the total power of the incident beam.

Silicon nanowires prepared by thermal evaporation and their photoluminescence properties measured at low temperatures

In the present work, silicon nanowires were prepared by a thermal evaporation method. The evaporating source was a mixture of silicon and carbon nanopowders. Surface morphology, structural characteristics and emission properties of the silicon nanowires were investigated by several techniques. The results showed that the obtained products have the shape of nanowires with diameters ranging from 30 to 120 nm and lengths from 300 to 400 nm. The x-ray diffraction (XRD) patterns confirmed the presence of crystalline silicon. Transmission electron microscope (TEM) images revealed the core-shell structure of the wires. In the photoluminescence (PL) spectra recorded at room temperature, only a broad emission band peaking at about 650 nm was observed. In addition to the red emission, two other bands centered at around 455 nm and 510 nm appeared when measured at low temperatures. The origin and emission mechanism of these bands are discussed.

Effects of Morphology on Stability, Electronic, and Optical Properties of Rutile TiO2 Nanowires

By means of first principles calculations we show that morphology of TiO2 nanowires in the rutile phase turns out to be important in their stability and band gap engineering. The most stable ⟨001⟩-oriented TiO2 nanowires are found to display the {110} facets while the most thermodynamically favorable nanowires with the ⟨110⟩ axis are characterized by the {110} and {001} facets. Our results clearly demonstrate that appearance of the {100} facets in the shape of TiO2 nanowires in the ⟨001⟩ orientation leads to a sizable reduction of band gaps, however, it does not cause the shift of the absorption edge to the lower energy range. Presence of flat bands similar to localized band gap states in the gap region of the ⟨110⟩-oriented TiO2 nanowires is predicted to be an intrinsic feature, which is not connected with O vacancies. We also discuss how our findings can be used for optimization of photovoltaic and photocatalytic cells.

High-Output Nanogenerator by Rational Unipolar Assembly of Conical Nanowires and Its Application for Driving a Small Liquid Crystal Display

We present a simple, cost-effective, robust, and scalable approach for fabricating a nanogenerator that gives an output power strong enough to continuously drive a commercial liquid crystal display. Utilizing the conical shape of the as-grown ZnO nanowires, a nanogenerator is fabricated by simply dispersing them onto a flat polymer film to form a rational "composite" structure. It is suggested that the geometry induced unipolar assembly of the conical nanowires in such a composite structure results in a macroscopic piezoelectric potential across its thickness by introducing a mechanical deformation, which may be responsible for driving the flow of the inductive charges between the top and bottom electrodes. A compressive strain of 0.11% at a straining rate of 3.67% s(-1) produces an output voltage up to 2 V (equivalent open circuit voltage of 3.3 V). This is a practical and versatile technology with the potential for powering small size personal electronics.

MOCVD 방법에 의한 Si 기판위 GaN 나노선의 성장

We have grown GaN nanowires by the low pressure MOCVD method on Ni deposited oxidized Si surface and have established optimum conditions by observing surface microstructure and its photoluminescence. Optimum growth temperature of , growth time of 30 min, TMG source flow rate of 10 sccm have resulted in dense nanowires on the surface, however further increase of growth time or TMG flow rate has not increased the length of nanowire but has formed nanocrystals. On the contrary, the increase of ammonia flow has increased the length of nanowires and the coverage of nanowire over the surface. The shape of nanowire is needle-like with a Ni droplet at its tip; the length is tens of micron with more than 40 nm in diameter. Low temperature photoluminescence obtained from the sample at optimum growth condition has revealed several peaks related to exciton decay near band-edge, but does not show any characteristic originated from one dimensional quantum confinement. Strong and broad luminescence at 2.2 eV is observed from dense nanowire samples and this suggests that the broad band is related to e-h recombination at the surface state in a nanowire. The current result is implemented to the nanowire device fabrication by nanowire bridging between micro-patterned neighboring Ni catalysis islands.

Stress-induced Sn Nanowires from Si–Sn Nanocomposite Coatings

The growth of stress-induced tin (Sn) whiskers has been considered responsible for the failure of many electronic devices and many approaches have been developed to mitigate their growth. In this report, however, we describe a simple approach based on the same mechanism to promote the growth of Sn nanowires. The thermal expansion induced stress was utilized as the driving force to initiate the growth of Sn nanowires from Si–Sn phase-separated nanocomposite coatings. The nanostructure of the Si–Sn matrix was the key to controlling the shape and diameter of Sn nanowires. This approach provides additional flexibility for making desirable metallic nanowires with controlled dimensions.

Magnetic properties of ordered nanowires of the quasi-two-dimensional antiferromagnet SpFeMn(C2O4)3

Ordered arrays of nanowires of the photochromic antiferromagnet SpFeMn(C2O4)3 (where Sp is 1-{(1′,3′,3′-trimethyl-6-nitro-5′-chlorospiro[2H-1-benzopyran-2,2′-indolin]-8-yl)methyl}pyridinium) have been fabricated in anodized aluminum oxide pores with diameters of 20 and 200 nm. It has been revealed that the growth of the spin-glass phase with noncollinear ordering of spins in nanowires is suppressed in favor of the uniaxial antiferromagnetic phase. A decrease in the nanowire diameter leads to an increase in the anisotropy of the magnetic resonance spectra. This is associated with the magnetocrystalline anisotropy that considerably exceeds the anisotropy of the nanowire shape.

Three‐Dimensional Carrier Profiling of Individual Si Nanowires by Scanning Spreading Resistance Microscopy

Three-dimensional profiling of nanowires is carried out by repeatedly scanning the same nanowire with controlled force of the probing tip. This method abrades material from the measured cross section during each scan, allowing three-dimensional profiling. The typical shape of an individual Si nanowire, measured by SEM, and the respective resistivity images of several cross sections obtained by scanning spreading resistance microscopy are shown.

Preparation and Characterization of Ni or Co/Cu Multilayer Nanowires

AbstractHighly ordered nanowires with multilayer structure of Ni/Cu, have been successfully prepared by pulsed chemical electrodeposition into nanoporous templates and the diameter (35 and 200 nm) of wires can be easily controlled by pore size of template. The nanowires have been characterized by transmission electron microscopy (TEM), magnetic force microscopy (MFM), and vibrating sample magnetometer (VSM) and high magnetic perpendicular anisotropy can be achieved by modulating the shape of nanowires. It was found that the coercivity (Hc) and squarness (S: Mr/Ms) of the nanowires are strongly dependent on the diameter and aspect ratio as well as on dipolar coupling between the ferromagnetic segments and the nanowires (aspect ratio: 11.4) can be obtained high perpendicular magnetic anisotropy with Hc: 895 Oe and S: 0.77 of Ni/Cu nanowires. The magnetoresistance ratios along the parallel and perpendicular axes of nanowires are 23.4% and 7.2% respectively. Owing to its high bit density, the structure may be suitable for ultrahigh‐density magnetic recording media.

Electrical field induced direct-to-indirect bandgap transition in ZnO nanowires

As a wide bandgap (3.37 eV) compound semiconductor with the exciton binding energy of 60 meV, ZnO nanowires have attracted considerable interest because of great potential applications in micro-optoelectronics and nano-optoelectronics. However, ZnO nanowires as basic units of microelectronic and optoelectronic nanodevices usually work in the circumstance of the electrical field. Therefore, the influence of electrical field on the electronic structure of ZnO nanowires partly determines the performance of nanodevices. For this issue, we inhere have studied the effects of the applied electrical field on the electronic structure of ZnO nanowires using the density functional theory. It was found that the applied electrical field cannot only effectively modulate the bandgap of ZnO nanowires but also leads to the bandgap transition from direct to indirect. Meanwhile, the above effects of electrical field depend on the size and shape of ZnO nanowires. The physical understanding of the electrical field effects of the energy band structure of ZnO nanowires is attributed to the bond length changes in lattice and the charge transfer between Zn and O atoms. Thus, these valuable investigations are important for designing the microelectronic and optoelectronic nanodevices based on ZnO nanowires.

Modeling of Stress-Retarded Thermal Oxidation of Nonplanar Silicon Structures for Realization of Nanoscale Devices

Accurate modeling of stress-retarded orientation-dependent 2-D oxidation is carried out by matching the experimental and simulated oxide thicknesses of silicon FIN nanostructures over a wide range of temperatures and times in dry oxygen. Experimentally observed initial oxidation rate enhancement, orientation-dependent stress retardation, and self-limiting phenomena are modeled, and a new universal stress retardation parameter set is obtained for the first time. The new parameter set has been validated against oxidation experiments presented here and those reported in the literature. Furthermore, the new model is used to explore silicon nanowire shape engineering.

Fabrication of Numerous Ferromagnetic Metal Nanowires Using Electrodeposition Technique

To synthesize an array of numerous ferromagnetic nanowires, iron-group metals such as Ni, Co, Fe, Ni-Fe and Co-Fe alloys were electrodeposited from aqueous solution into a nanoporous template with numerical cylindrical nanochannels . The shape of nanowires was precisely transferred from the nanochannel template and the aspect ratio reached to around 150. Magnetic hysteresis loops revealed that Ni, Co and Fe nanowires were spontaneously magnetized to the long axis direction. Coercive force of the nanowires with 6000 nm in length was increased in decreasing the pore-diameter. The coercive force of Co nanowires with 40 nm in diameter has increased up to 1084 Oe.

Tailoring the Morphology and Structure of Nanosized Zn2SiO4: Mn2+ Phosphors Using the Hydrothermal Method and Their Luminescence Properties

Mn2+-doped Zn2SiO4 phosphors with nanoellipsoid or nanowire morphologies were synthesized at low temperature <200 °C by a hydrothermal method without any surfactants. The morphologies of the phosphors were easily tailored by varying the amount of hydroxide ions in the precursor solution before the hydrothermal reaction. Absolute ethanol was used as a solvent instead of distilled water to explore the effects of hydroxide ions on the morphology and crystal structure. We adjusted the amount of hydroxide ions by changing the pH of the precursor solution. Sheaves of powders with an ellipsoid shape were synthesized at low pH values of 7 and 9, at which only a few hydroxide ions were present, whereas powders with a nanowire shape were produced at a high pH of 11, at which many hydroxide ions were present. In addition to its morphology, the hydroxide ions also affect the crystal structure of the synthesized powder. Whereas a Zn2SiO4 phase with a willemite structure was formed at pH 7 and 9, a Zn4Si2O7(OH)2·H2O ph...

One-step hydrothermal synthesis of CdTe nanowires with amorphous carbon sheaths

CdTe nanowires with carbon sheaths were synthesized via a one-step hydrothermal process using ascorbic acid as a reducing agent and carbonization source. The average diameter of CdTe core nanowires and thickness of amorphous carbon sheaths are about 20 nm and 10 nm, respectively. During this process, carbon-coated Te nanowires were formed and then reacted with Cd 2+ ions, resulting in the formation of carbon-coated CdTe nanowires. Additionally, the size and shape of the CdTe nanowires were determined during the initial stage of Te formation.

Shape Effects on the Performance of Si and Ge Nanowire Field-Effect Transistors Based on Size Dependent Bandstructure

In this paper, we evaulated the shape and size effects of Si and Ge nanowire (NW) field-effect transistors (FETs) on device performance using sp3d5s* tight-binding (TB) model and semi-classical top-of-barrier ballistic transport model. Our simulation results show that smaller cross-sectional area is desirable for high frequency device applications and for larger ON-state currents, square cross-section may be desirable due to larger cross-sectional area and insulator capacitance. Furthermore, it is also observed that due to quantum effects, the gate capacitance to gate oxide capacitance (Cg/Cox) ratio for the small size NW FETs could be much less than one, rendering the classical assumptions and calculations invalid for nano scale FETs. In this sub-nano region, therefore, a new set of assumptions and calculations in terms of effective mass, bandgap, and one-dimensional density-of-states should be implemented as quantum effects start to play an important role in device performance.

Analysis of strain and stacking faults in single nanowires using Bragg coherent diffraction imaging

Coherent diffraction imaging (CDI) on Bragg reflections is a promising technique for the study of three-dimensional (3D) composition and strain fields in nanostructures, which can be recovered directly from the coherent diffraction data recorded on single objects. In this paper, we report results obtained for single homogeneous and heterogeneous nanowires with a diameter smaller than 100 nm, for which we used CDI to retrieve information about deformation and faults existing in these wires. We also discuss the influence of stacking faults, which can create artefacts during the reconstruction of the nanowire shape and deformation.

Influence of substrate temperature on the shape of GaAs nanowires grown by Au-assisted MOVPE

GaAs nanowires (NWs) are grown on the GaAs(1 1 1)B substrates by the Au-assisted metal–organic vapor phase epitaxy (MOVPE). The NW shape is found to be strongly dependent on the substrate temperature during the growth. With increase in the growth temperature, the NW shape modifies from prismatic to conical. The observed temperature behavior is studied within the frame of a theoretical model. It is shown that the key process responsible for the lateral growth is the decomposition of MOVPE precursors at the NW sidewalls and the substrate. Theoretical results are in a good agreement with experimental findings and can be used for the numerical estimates of some important growth parameters as well as for the controlled fabrication of NWs with the desired shape.