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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 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.

Noor Mohammad S.,Sciencotech | Noor Mohammad S.,U.S. Navy
Journal of Applied Physics | Year: 2011

Nanowires, nanotubes, and nanodots (quantum dots) are nanomaterials (NMTs). While nanodots are miniaturized nanowires, nanotubes are hollow nanowires. A universal model for basic science of the synthesis and characteristics of NMTs must be established. To achieve this goal, a general hypothesis has been presented. This hypothesis makes use of the concept of droplets from seeds, the fundamentals of the adhesive properties of droplets, and a set of droplet characteristics. Fundamentals underlying the droplet formation from nanoparticle seeds under various physicochemical and thermodynamic conditions have been articulated. A model of thermodynamic imbalance of seeds at the growth temperature has been formulated. The dependence of thermodynamic imbalance on parameters such as surface energy, temperature, seed dimension, etc. has been described. The role of thermodynamic imbalance of seeds and of the foreign element catalytic agent (FECA) on NMT growth has been examined. Three different NMT growths, namely, FECA-free NMT growth; FECA-mediated non-eutectic NMT growth; and FECA-mediated eutectic NMT growth, have been considered. FECA-free NMT growth, and non-eutectic but FECA-mediated NMT growth, have been assumed to involve nanopores, grains, and grain boundaries in the seed. The basic science of all the NMT growths utilizes the concept of the creation of tiny component droplets (CODs). Extensive evidential (experimental and theoretical) demonstration of the hypothesis has been put forth. Both theoretical and experimental results lend support to the hypothesis. Calculated results address the roles of both the FECA-mediated and FECA-free droplets for NMT growths. The basics of multiple nucleation and biphasic structures have been spelled out. Possible relationship between the activation energy and the precursor decomposition on the droplet surface at the lowest possible temperature has been elucidated. The differences between the eutectic and no-eutectic seeds, the importance of thermodynamic imbalances in the creation of nanopores inside seeds, and the physicochemical reasons of nanowire growth at temperatures far below the seed's eutectic temperature (and/or melting temperature) have been revealed. Experimental evidences, particularly for CODs, droplets, dipole moment of the seeds (droplets), immovability of droplets, multiple nucleation, biphasic structures, etc., quantify the validity of the hypothesis. © 2011 American Institute of Physics.

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.

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