Tuning the thermal conductivity of silicon nanowires by surface passivation

Using large scale molecular dynamics simulations, we study the thermal conductivity of bare and surface passivated silicon nanowires (SiNWs). For the smaller cross-sectional widths $w$, SiNWs become unstable because of the surface amorphousization and even evaporation of a certain fraction of Si atoms when $w \leq 2$ nm. Our results suggest that the surface (in--)stability is related to a large excess energy $\Delta$ of the surface Si atoms with respect to the bulk Si. This is because the surface Si atoms being less coordinated and having dangling bonds. As a first step of our study, we propose a practically relevant method that uses $\Delta$ as a guiding tool to passivate these dangling bonds and thus stabilizes SiNWs. The surface stabilization is achieved by passivation of Si atoms by hydrogen or oxygen. These passivated SiNWs are then used for the calculation of the thermal conductivity coefficient $\kappa$. While the expected trend of $\kappa \propto w$ is observed for all SiNWs, surface passivation provides an added flexibility of tuning $\kappa$ with the surface coverage concentration $c$ of passivated atoms. Analyzing the phonon band structures via spectral energy density, we discuss separate contributions from the surface and the core to $\kappa$. The effect of passivation on SiNW stiffness is also discussed

Electroluminescence of metamorphic In x Al 1-x As / In x Ga 1-x As HEMTs ON GaAs substrate

We present the first impact ionization investigation by electroluminescence of InxAl1-xAs /InxGa1-xAs metamorphic High Electron Mobility transistors on GaAs. Two Indium content are investigated. First we observe the decrease of the detrimental effect of impact ionization with the decrease of the Indium content. Second, the electroluminescence measurements illuminate the functional relationship between impact ionization and the kink effect. In these metamorphic HEMT’s, we suggest that both kink effect and impact ionization threshold are originated to detrapping process of deep levels in the large band gap layer

Effect of solution conductivity and electrode shape on the deposition of carbon nanotubes from solution using dielectrophoresis

Dielectrophoresis (DEP) is a popular technique for fabricating carbon nanotube (CNT) devices. The electric current passing through the solution during DEP creates a temperature gradient, which results in electrothermal fluid flow because of the presence of the electric field. CNT solutions prepared with various methods can have different conductivities and the motion of the solution because of the electrothermal phenomenon can affect the DEP deposition differently in each case. We investigated the effect of this movement in solutions with various levels of conductivity through experiments as well as numerical modeling. Our results show that electrothermal motion in the solution can alter the deposition pattern of the nanotubes drastically for high conductivity solutions, while DEP remains the dominant force when a low conductivity (surfactant-free) solution is used. The extent of effectiveness of each force is discussed in the various cases and the fluid movement model is investigated using two- and three-dimensional finite element simulations.Applied Science, Faculty ofElectrical and Computer Engineering, Department ofUnreviewedFacult

First-principles study of field-emission from carbon nanotubes in the presence of methane

Carbon nanotubes are promising candidates for field-emitters. It has been shown that the presence of various gases can enhance or degrade the performance of nanotube emitters. Small hydrocarbons are of particular interest because of their ability to enhance the emission properties. The authors report a simulation study of field-emission from a carbon nanotube exposed to methane in various configurations with an emphasis on calculating the emission current. The Hartree–Fock theory combined with a Green’s functions approach was used for the simulations. It was observed that the change in the emission current strongly depends on the particular arrangement of the methane molecules on the nanotube.Applied Science, Faculty ofElectrical and Computer Engineering, Department ofReviewedFacult

Parameters and mechanisms governing image contrast in scanning electron microscopy of single-walled carbon nanotubes

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The synthesis of vertically aligned multi-walled carbon nanotube forests by thermal chemical vapor deposition

Despite the many potential applications and attractive characteristics of carbon nanotubes (CNTs), a major challenge on the way toward their commercialization remains their controlled synthesis. In this chapter, we present a parametric study of the synthesis of vertically-aligned CNT forests using an ethylene-based chemical vapor deposition system without any oxidizing agent. We also provide an overview of the complex effects of hydrogen and an interpretation of the non-monotonic effect of the hydrogen flow rate on the resulting carbon nanotube forests. Even though hydrogen plays a partial role as a carrier gas (other than acting as a reducing agent), variations in the hydrogen and argon flow rates lead to distinct trends in the synthesis outcome, pointing to the diverse roles for the reducing and carrier gases: adjusting the proportions of hydrogen and argon for a given total flow rate had a drastic impact. The gas flow profile was optimal at an ethylene-to-total flow ratio of 8% and ethylene-to-hydrogen ratio of 25%, respectively. A strong temperature dependence was also observed: while the growth was similarly successful at 675 C and 725 C, a temperature of 650 C led to severely suppressed yield. According to transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy analysis, the tip-growth mechanism dominates in this synthesis method

The synthesis of vertically aligned multi-walled carbon nanotube forests by thermal chemical vapor deposition

Despite the many potential applications and attractive characteristics of carbon nanotubes (CNTs), a major challenge on the way toward their commercialization remains their controlled synthesis. In this chapter, we present a parametric study of the synthesis of vertically-aligned CNT forests using an ethylene-based chemical vapor deposition system without any oxidizing agent. We also provide an overview of the complex effects of hydrogen and an interpretation of the non-monotonic effect of the hydrogen flow rate on the resulting carbon nanotube forests. Even though hydrogen plays a partial role as a carrier gas (other than acting as a reducing agent), variations in the hydrogen and argon flow rates lead to distinct trends in the synthesis outcome, pointing to the diverse roles for the reducing and carrier gases: adjusting the proportions of hydrogen and argon for a given total flow rate had a drastic impact. The gas flow profile was optimal at an ethylene-to-total flow ratio of 8% and ethylene-to-hydrogen ratio of 25%, respectively. A strong temperature dependence was also observed: while the growth was similarly successful at 675 C and 725 C, a temperature of 650 C led to severely suppressed yield. According to transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy analysis, the tip-growth mechanism dominates in this synthesis method

High-aspect-ratio, 3-D micromachining of carbon-nanotube forests by micro-electro-discharge machining in air

This paper reports micro-electro-discharge machining of vertically aligned carbon nanotube forests for the formation of high-aspect-ratio, three-dimensional microstructures in the material. The developed forest machining method is a dry process performed in air, generating high-frequency pulses of electrical discharge to locally machine the nanotubes in order to create target shapes in a forest. With this approach, forest microstructures can be fabricated to have varying shapes along their height, unachievable with conventional pre-patterned chemical vapor deposition growth techniques. The use of the pulses with a minimized discharge energy defined with 35 V and 10 pF in the discharge generation circuit leads to an aspect ratio of 20 with the smallest feature of 5 µm in forests without disordering the vertical orientation of the nanotubes. Micromachining of multilayer geometries as well as arrayed needle-like microstructures with angled surfaces is demonstrated