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Catalyst-free and catalyst-assisted nanowire (NW) syntheses are increasingly carried out by mechanism(s) other than the well-known VLS (vapor-liquid-solid) mechanism. Yet these growths are not fully understood. An in-depth investigation has been carried out to understand the mechanism of the catalyst-free and catalyst-mediated non-VLS NW growths. Various chemical and physical processes involved in these growths have been studied to formulate general principles. Phase transitions, synthesis routes, and the fundamentals underlying these routes have been explored. Nanoparticle surfaces conducive to NW syntheses have been examined. The role of surface treatment, such as oxidation, oxygenation, doping, acid treatment, plasma treatment, etc., in creating such surfaces has been elucidated. Surface treatment and phase transition under appropriate growth conditions (temperature, pressure, ambient, and the presence of contaminants) have been found to be important. They play a crucial role in creating diffusion paths for the diffusion of the growth species for NW growths. Interdiffusion of the catalyst and the growth species on the nanoparticle surface has been found also to add a new dimension to the growth kinetics. When integrated together, they create a unified platform versatile enough to explain essentially all catalyst-free and catalyst-mediated non-eutectic NW growths. The platform uncovers numerous growth-related problems never understood before. Available experiments extensively support this platform. These experiments suggest that it is based on solid foundation and has broad and probably universal appeal. It pertains to the vapor-quasiliquid-solid, vapor-quasi-solid-solid mechanism proposed some six years ago. © 2016 Author(s).

Mohammad S.N.,Sciencotech | Mohammad S.N.,U.S. Navy
Journal of Physical Chemistry C | Year: 2012

Fundamental physics and chemistry underlying nanotube synthesis and characteristics have not been fully understood. To facilitate this understanding, the concept of component seed, component droplet, and component nanowire for nanotube synthesis and characteristics has been introduced. This concept generalizes the shell model for nanotubes. It vastly broadens our ability to explain nanotube materials characteristics that could not otherwise be explained. Experiments widely corroborate with the present findings. They lend support to the concept of component seeds and component droplets. Size-dependent and solubility-dependent melting point depressions have been studied. They provide new insight and uncover the basic causes of melting (nonmelting) of the catalyst nanoparticles. They also elucidate nanotube growth, employing metal nanoparticles at temperatures lower than their melting points. The concept of component seed (droplet) also successfully explains nanotube branching. In light of this concept, growth mechanisms available in the literature have been modified. © 2012 American Chemical Society.

Fundamental physico-chemical mechanisms underlying the synthesis of nanotubes wereinvestigated, including conventional, doped, and bamboo-shaped nanotubes. The mechanisms are examined from the viewpoint of the well-known base growth (root growth) and tip growth mechanisms. The analysis of the surface characteristics of nanoparticles is key to the present approach. Surface and interface melting, surface and bulk diffusion through nanoparticle, and the formation of a hill due to over-segregation of the source species to the nanoparticle peripheral surface have also been investigated. The study may have led to an understanding of the basics and the differences between the base growth and the tip growths of nanotubes, and also of the formation of nanotube diaphragms (caps), if any. The proposed mechanisms have been used to attempt to explain various prior observations on the conventional, doped, and bamboo-shaped nanotubes. Experimental results available in the literature have been extensively employed to justify the validity of the mechanisms, and to highlight the possible appeal of these mechanisms. © 2014 Elsevier Ltd. All rights reserved.

Universally true rules constitute the very foundation of modern science and engineering. They are, in fact, the backbones of modern science and engineering. Nanotubes are promising materials, and nanotube science and engineering do not yet, to our knowledge, have these rules. Attempts have been made to explore if nanotube syntheses and characteristics follow any rule. Simple theoretical calculations were performed. Results of these calculations suggest that there may indeed be well-defined rules for nanotubes. The theoretical predictions are widely supported by available experiments. They indicate that the proposed rules may have broad appeal. They may have implication on exploring mechanisms most useful for growths of vertically aligned nanotubes of narrow chirality distributions. © 2014 Elsevier Ltd. All rights reserved.

A distinctly new route for the design, modeling and electrical behavior of very short-channel (5-10 nm in channel length) nanowire field-effect transistors (FETs) has been presented. Essential elements of the approach entail a drain current determined by thermionic emission, but not by carrier mobility in the channel of the transistor. A basic understanding of the fundamental physics and the concepts of Schottky-barrier-based design for the proposed route have been described. Quantum confinement in the nanowire channel together with Schottky barrier tailing and temperature-dependent fluctuations of applied biases has been taken into account for the development of the model. Both current-voltage characteristics and transconductance of FETs have been studied. The calculated results are in near-quantitative agreement with the available experiments. Measured data show very diverse (e.g., exponential, linear, saturating, and non-linear non-exponential non-saturating) nanowire transistor characteristics. The model explains these characteristics well and reveals a number of new transistor actions. It highlights the impacts of quantum confinement and Schottky contacts for these new transistor actions. It also quantifies the significant enhancement of the drain-source current and transconductance. With new findings thus achieved, suggestions for the realization of very high-performance, small-diameter (preferably 2 nm), small-Schottky-barrier- height, high-operating temperature, ultra-short-channel-length, nanowire transistors have been made. Optimized design of these transistors has been suggested. And the range (in terms of device and technological parameters) of the proposed model has been elucidated. © 2013 IOP Publishing Ltd.

Mohammad N.S.,Sciencotech
Journal of Physics Condensed Matter | Year: 2014

A comprehensive investigation of quantum confinement in nanowires has been carried out. Though applied to silicon nanowires (SiNWs), it is general and applicable to all nanowires. Fundamentals and applications of quantum confinement in nanowires and possible laws obeyed by these nanowires, have been investigated. These laws may serve as backbones of nanowire science and technology. The relationship between energy band gap and nanowire diameter has been studied. This relationship appears to be universal. A thorough review indicates that the first principles results for quantum confinement vary widely. The possible cause of this variation has been examined. Surface passivation and surface reconstruction of nanowires have been elucidated. It has been found that quantum confinement owes its origin to surface strain resulting from surface passivation and surface reconstruction and hence thin nanowires may actually be crystalline-core/amorphous-shell (c-Si/a-Si) nanowires. Experimental data available in the literature corroborate with the suggestion. The study also reveals an intrinsic relationship between quantum confinement and the surface amorphicity of nanowires. It demonstrates that surface amorphicity may be an important tool to investigate the electronic, optoelectronic and sensorial properties of quantum-confined nanowires. © 2014 IOP Publishing Ltd.

Extensive analyses of thermodynamic imbalance, surface energy, and segregation of nanotubes on nanoparticle surfaces are performed. A model for surface energy i developed. In addition, nanotube growth both by vapor-phase and solid-phase mechanisms is described. Segregation of the nanotube species to the periphery of the nanoparticle, the creation of an amorphous shell at this periphery, a droplet created in this shell, and the mediation of this droplet for supersaturation and nucleation of the nanotube species may be the true causes of nanotube growth. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Why vapor species land on the surface of the nanoparticle seed for nanotube synthesis is a vital question. An investigation has been carried out to find an answer to it. For this, a model of the dipole moment has been developed. A bimetallic alloy (non-alloy, solid solution) exhibiting the shape of a cap has been assumed to function as the nanoparticle seed. Various features of the dipole moment have been examined. The influence of the dipole moment on nanotube synthesis, alignment, chirality, and characteristics has also been studied. Available experiments on the synthesis of carbon nanotubes employing bimetallic catalysts have been compared with the results from calculations. Close correspondence between the two demonstrates that the catalysts may exhibit a dipole moment and have a crucial role in nanotube synthesis and characteristics. The dipole moment has also been employed to determine why some nanotubes grow vertically, while others are bent. Calculated results appear to explain the basic causes for this. These results suggest that the electric field resulting from the dipole moment of catalysts may be important for the vertical alignment of nanotubes. They may attest to the validity of the model and to the existence of a dipole moment in seeds. Although considered for nanotube syntheses, the results may be applicable to other nanomaterials (nanotubes, nanowires, nanodots). © 2012 IOP Publishing Ltd.

Nanoparticles of foreign element catalytic agents (FECAs) are widely employed for nanotube synthesis. Today these FECA nanoparticles include metals, semiconductors, oxides, clusters, polymers, and ceramics. Despite their very diverse characteristics, they all mediate nanotube growth. It is not known how they do it; their catalytic role has not been really understood. Attempts have been made to address this in some detail. For this, a wide variety of metals (transition metals, noble metals, alkali metals, etc.), oxides (SiO 2, GeO 2, MgO, CaO, etc.), semiconductors (Si, Ge, etc.), and even clusters, polymers, and ceramics (e.g., SiC), have been considered. The surface energy, chemical reactivity, and thermodynamic imbalance of FECAs; solubility of the nanotube source species in FECAs; supersaturation of the nanotube source species in FECAs; and the porosity of shells at the FECA peripheral surface have been studied. The fluctuations of FECAs resulting from thermodynamic imbalance and the dissociation of the precursors of the nanotube growth species on the FECA surface have also been examined. Together they appear to reveal a common platform that governs the chemical potential and catalytic activity of all possible FECAs. They explain why supersaturation needed, for example, for single-walled carbon nanotube growth, is possible even for very small FECAs. They elucidate why bimetallic and polymetallic FECAs have higher chemical reactivity for nanotube synthesis than monometallic FECAs and reveal the basic reasons why nanotubes of very small diameter may not be produced at all. Based on the science thus uncovered, some general rules for nanotube synthesis have been proposed. Attempts to quantify the validity of various findings and rules are substantiated by available experiments. The solutions thus obtained may have a significant impact on nanotube science and technology. © 2012 The Royal Society of Chemistry.

Mohammad S.N.,Sciencotech
Nanotechnology | Year: 2012

Electrical transport in semiconductor nanowires taking quantum confinement and dielectric confinement into account has been studied. A distinctly new route has been employed for the study. The fundamental science underlying the model is based on a relationship between the quantum confinement and the structural disorder of the nanowire surface. The role of surface energy and thermodynamic imbalance in nanowire structural disorder has been described. A model for the diameter dependence of energy bandgap of nanowires has been developed. Ionized impurity scattering, dislocation scattering and acoustic phonon scattering have been taken into account to study carrier mobility. A series of calculations on silicon nanowires show that carrier mobility in nanowires can be greatly enhanced by quantum confinement and dielectric confinement. The electron mobility can, for example, be a factor of 210 higher at room temperature than the mobility in a free-standing silicon nanowire. The calculated results agree well with almost all experimental and theoretical results available in the literature. They successfully explain experimental observations not understood before. The model is general and applicable to nanowires from all possible semiconductors. It is perhaps the first physical model highlighting the impact of both quantum confinement and dielectric confinement on carrier transport. It underscores the basic causes of thin, lowly doped nanowires in the temperature range 200KT500K yielding very high carrier mobility. It suggests that the scattering by dislocations (stacking faults) can be very detrimental for carrier mobility. © 2012 IOP Publishing Ltd.

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