Horizontally Aligned Research Articles

Capacitive nanoelectromechanical switch based on suspended carbon nanotube array

We present the fabrication and high frequency characterization of a capacitive nanoelectromechanical system (NEMS) switch using a dense array of horizontally aligned single-wall carbon nanotubes (CNTs). The nanotubes are directly grown onto metal layers with prepatterned catalysts with horizontal alignment in the gas flow direction. Subsequent wetting-induced compaction by isopropanol increases the nanotube density by one order of magnitude. The actuation voltage of 6 V is low for a NEMS device, and corresponds to CNT arrays with an equivalent Young’s modulus of 4.5–8.5 GPa, and resistivity of under 0.0077 Ω⋅cm. The high frequency characterization shows an isolation of −10 dB at 5 GHz.

Quality of horizontally aligned single-walled carbon nanotubes: Is methane as carbon source better than ethanol?

Chemical vapor deposition (CVD) growth of horizontally aligned single-walled carbon nanotubes (SWNTs) was studied using two representative carbon source sources: ethanol and methane. The resulting SWNTs were compared for similar reaction conditions which were based on the formation of Ni metal nanoparticles selective electrochemical deposition (SED) on the defect sites of SWNTs. The products were analyzed by Raman spectroscopy and SEM. The results demonstrate that methane was much better carbon source for growing high quality horizontal alignment of SWNTs than ethanol due to the etching effects of OH radicals on the SWNTs.

High-aspect ratio and horizontally oriented carbon nanotubes synthesized by RF-cCVD

Horizontally aligned long carbon nanotubes were efficiently synthesized on Si substrates by using a radio-frequency catalytic chemical vapor deposition method. The morphological as well as the growth properties of these nanotubes were systematically investigated with various analytical techniques including microscopy and Raman spectroscopy. Different reaction parameters such as temperature, type of hydrocarbon gas and catalyst amount were varied and their effects on the nanotube size, quality and alignment are reported. High-aspect ratio and horizontally oriented nanotubes were found to grow following the “tip growth” mechanism. The fast and localized heating rate produced by the RF generator helps nanotubes to separate and lift the nano-particles away from the support and hence contributes to the growth of CNTs with a very high-aspect ratio. Carbon nanotubes synthesized with methane show a better horizontal alignment compared to those synthesized with acetylene, which might be due to the flow rate of the hydrocarbon gas.

Nonuniform Compressive Strain in Horizontally Aligned Single-Walled Carbon Nanotubes Grown on Single Crystal Quartz

Interactions with the substrate that allow near perfect horizontal alignment in combination with large difference in the coefficient of thermal expansion are shown to lead to uniaxial compressive strain in as-grown single-walled carbon nanotubes on single crystal quartz. Temperature dependence of Raman G-band spectra along the length of individual nanotubes reveals that the compressive strain is nonuniform and can be larger than 1% locally at room temperature. A response of 27 cm(-1) upshift per % compressive strain is estimated for the G-band longitudinal optical phonon mode of semiconducting nanotubes. Comparison of Raman and atomic force microscope images suggests that the nonuniformity of the compression arises from the surface roughness induced by polishing. Effects of device fabrication steps on the nonuniform strain are also examined and implications on electrical performance are discussed.

Recent Developments in Technology for Horizontally Aligned Growth of Single-Walled Carbon Nanotubes

Recent Developments in Technology for Horizontally Aligned Growth of Single-Walled Carbon Nanotubes

Optical‐microphysical cirrus model

A model is presented that permits the simulation of the optical properties of cirrus clouds as measured with depolarization Raman lidars. It comprises a one‐dimensional cirrus model with explicit microphysics and an optical module that transforms the microphysical model output to cloud and particle optical properties. The optical model takes into account scattering by randomly oriented or horizontally aligned planar and columnar monocrystals and polycrystals. Key cloud properties such as the fraction of plate‐like particles and the number of basic crystals per polycrystal are parameterized in terms of the ambient temperature, the nucleation temperature, or the mass of the particles. The optical‐microphysical model is used to simulate the lidar measurement of a synoptically forced cirrostratus in a first case study. It turns out that a cirrus cloud consisting of only monocrystals in random orientation is too simple a model scenario to explain the observations. However, good agreement between simulation and observation is reached when the formation of polycrystals or the horizontal alignment of monocrystals is permitted. Moreover, the model results show that plate fraction and morphological complexity are best parameterized in terms of particle mass, or ambient temperature which indicates that the ambient conditions affect cirrus optical properties more than those during particle formation. Furthermore, the modeled profiles of particle shape and size are in excellent agreement with in situ and laboratory studies, i.e., (partly oriented) polycrystalline particles with mainly planar basic crystals in the cloud bottom layer, and monocrystals above, with the fraction of columns increasing and the shape and size of the particles changing from large thin plates and long columns to small, more isometric crystals from cloud center to top. The findings of this case study corroborate the microphysical interpretation of cirrus measurements with lidar as suggested previously.

Horizontally aligned Cu5Si polycrystalline nanorods on Si

In this letter, we report a method for the growth of copper silicide (Cu5Si) nanorods with a horizontal alignment via chemical vapor deposition. The Cu5Si nanorods with an oxygen-rich shell grew in the [110] direction of Si on a (001) Si substrate. A growth model that explains the formation mechanism of the horizontally aligned, polycrystalline nanorods is proposed.

Orientation-induced molecular rectification and nonlinear optical properties of a squaraine derivative

Self-assembled monolayers (SAMs) obtained via the chemisorption of 2,4-bis-[N-methyl-N-(3-acetylsulfanylpropyl)-aminophenyl]-squaraine on gold are disordered with the molecules adopting vertical alignments via a single C3H6–S–Au anchoring group and horizontal alignments via two C3H6–S–Au groups located at opposite ends of the donor–acceptor–donor chromophore. The films exhibit asymmetric current–voltage characteristics with a rectification ratio of ca. 6 at ±1 V, this unexpected behaviour being induced by different molecular orientations at the interfaces with the electrodes: the donor groups are adjacent both top and bottom whereas the long axis of the D–A–D moiety is only in contact with the gold-coated substrate. The higher forward bias current invariably relates to electron flow from the substrate electrode adjacent to the horizontally aligned chromophores to the contact adjacent to the terminal donors of the vertically aligned molecules. In contrast, Langmuir–Blodgett (LB) films of the squaraine exhibit symmetrical I–V curves and aggregation-induced second-harmonic generation (SHG) with an effective second-order susceptibility of ca. 80 pm V−1 at 1.064 µm.

Interpretation of Polarization Features in Ground-Based Microwave Observations as Caused by Horizontally Aligned Oblate Raindrops

Abstract Based on a comparison of ground-based radiometer measurements with microwave radiative transfer calculations, it is shown that raindrops with an oblate shape and a preferred horizontal orientation have a significant effect on microwave polarization signals when compared with spherical particle shape. Measurements with a dual-polarized 19-GHz radiometer reveal a polarization difference of as much as −18 K in the downwelling microwave radiation at 30° elevation angle. Averaging all rain observations within 19 months leads to a signal of −6 K. Model calculations covering roughly the same range of weather conditions as that inferred from the meteorological data recorded with the radiometer measurements were carried out with spherical raindrop shape and an oblate particle shape with a fixed horizontal alignment. From the model results, positive polarization difference is expected for spherical particles. This signal was never observed in the recorded data. For oblate drops, the averaged model results ...

Scattering from ice crystals at 94 and 220 GHz millimeter wave frequencies

Polarimetric scattering from cloud ice crystals modeled as hexagonal columns, hexagonal plates and stellar crystals are calculated at 94 and 220 GHz frequencies using the finite difference time domain (FDTD) method. Two orientation models are considered, In the first model the long axes of columns and broad surfaces of plates and stellar crystals are parallel to the horizontal plane and randomly oriented on this plane. The second model assumes uniform random orientation in three dimensions (3-D). The ratio of the backscattering cross sections at 220 and 94 GHz exhibits a steady decrease with increasing size up to about 1000 /spl mu/m (2000 /spl mu/m for columns) for horizontally aligned ice crystals at side incidence as well as for 3-D random orientation. This dual frequency ratio (DFR) may be useful in gauging the size of ice crystals, DFR may also be useful in discriminating between ice crystal types since it shows major differences between columns and planar crystals. The linear depolarization ratio (LDR), i.e. the ratio of the cross-polar to co-polar backscattering cross sections, at vertical incidence for the horizontal alignment model is much higher for columns (over 20 dB) compared to plates and stellar crystals. As a result LDR also has the potential for discriminating columns from planar crystals, Furthermore, the normalized Mueller matrices for horizontally aligned columns at vertical incidence are shown to be diagonal and a function of only LDR. This same result applies to the 3-D random orientation model for all three crystal types. >

The effects of stimulus alignment on children’s performance in a conservation of length problem

In the usual conservation-of-length task, one of two horizontally aligned sticks is displaced to the left and S is asked whether or not the sticks, previously judged equal in length, are now unequal. On the possibility that this procedure confounds a left-right discrimination task with the conservation task per se, 18 nonconserving first-grade children were each given four trials with the sticks in horizontal alignment and four in vertical alignment. Contrary to prediction, the incidence of conservation judgments did not increase in the vertical alignment condition, though speed of judgment increased slightly.