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Chapter 3: Nanowires. Lecture by: Rose Farahiyan Munawar , PhD . Models of 3-D nanostructures made from DNA. Pre-Quiz. Nanowires.
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Chapter 3: Nanowires Lecture by: Rose FarahiyanMunawar , PhD
Nanowires quantum wire, metallic nanowire, semiconductor nanowire, insulating nanowire, molecular nanowire, nanowire arrays, nanowire, alumina nanowire, bismuth nanowire, boron nanowire, cadmium selenidenanowire, copper nanowire, gallium nitride nanowire, gold nanowire, gallium phosphidenanowire, germanium nanowire, indium phosphidenanowire, magnesium oxide nanowire, manganese oxide nanowire, nickel nanowire, palladium nanowire, platinum nanowire, silicon nanowire, silicon carbide nanowire, silicon nitride nanowire, titanium dioxide nanowire, zinc oxide nanowire, gold microwire, silicon microwire,
Why Nanowires? The nanowires could be used, in the near future, to link tiny components into extremely small circuits. Using nanotechnology, such components could be created out of chemical compounds.
Advantages of NWs: • NW devices can be assembled in a rational and predictable because: – NWs can be precisely controlled during synthesis, – chemical composition, –diameter, –length, – doping/electronic properties • Reliable methods exist for their parallel assembly.
Advantages of NWs: • It is possible to combine distinct NW building blocks in ways not possible in conventional electronics. • NWs thus represent the best-defined class of nanoscale building blocks, and this precise control over key variables correspondingly enabled a wide range of devices and integration strategies to be pursued
Structure of NWs • Whiskers, fibers:1D structures ranging from several nanometers to several hundred microns • Nanowires: Wires with large aspect ratios (e.g.>20), • Nanorods: Wires with small aspect ratios. • NanoContacts: short wires bridged between two larger electrodes.
Structure of NWs A nanowire is a nanostructure, with the diameter of the order of a nanometer (10-9 meters). Alternatively, nanowires can be defined as structures that have a thickness or diameter constrained aroundtens of nanometers or less and an unconstrained length.
Structure of NWs At these scales, quantum mechanical effects are important — hence such wires are also known as "quantum wires". Presently diameters as small as 12 nanometers
Structure of NWs Typical nanowires exhibit aspect ratios (length-to-width ratio) of 1000 or more. As such they are often referred to as one-dimensional (1-D) materials.
Nanowires Structure The nanowires can show peculiar shapes. Single crystal formation- common crustallographic orientation along the nanowire axis Sometimes they can show noncrystalline order, assuming e.g. a pentagonal symmetry or a helicoidal (spiral) shape.
Nanowires Structure The lack of crystalline order is due to the fact that a nanowire is periodic only in one dimension (along its axis). Minimal defects within wire Minimal irregularities within nanowire.
Nanowires Structure Electrons zigzag along pentagonal tubes and spiral along helicoidal tubes. Hence it can assume any order in the other directions (in plane) if this is energetically favorable. thin, brittle, can be electrically conductive, quantum effects can be important
Structure of NWs • Hence it can assume any order in the other direction NWs are observed spontaneously in nature. • Nanowires can be either suspended, deposited or synthesized from the elements.
Properties of NWs Nanowires have many interesting properties that are not seen in bulk or 3-D materials. This is because electrons in nanowires are quantum confined laterally and thus occupy energy levels that are different from the traditional continuum of energy levels or bands found in bulk materials.
NW Properties Depending on what it's made from, a NW can have the properties of an insulator, a semiconductor or a metal.
SEM characterization of as-synthesized silicon oxidenanowires.
NW Properties Insulators won't carry an electric charge While metals carry electric charges very well. Semiconductors fall between the two, carrying a charge under the right conditions.
NW Properties By arranging semiconductor wires in the proper configuration, engineers can create transistors, which either acts as a switch or an amplifier Semiconductors are most useful in making transistors for computers.
NW Properties Optical properties • Controlling the flow of optically encoded information with nanometer-scale accuracy over distances of many microns, which may find applications in future high-density optical computing . • Silicon NWs coated with SiC show stable photoluminescence at room temperature
How do we make NWs? There is no single fabrication method for NWs All the materials (metallic, semiconductor etc) hane been grown as 2D nanomaterials (thin films) in the last three decades
How do we make NWs? NW fabrication is challenging Challenging is to grow 1D NWs Alignment is a critical first step for developing devices that use NWs
Methods • Spontaneous growth: Evaporation condensation Dissolution condensation Vapor-Liquid-Solid growth (VLS) Stress induced re-crystallization • Electro-spinning • Solution Synthesis
Methods • Template-based synthesis: Electrochemical deposition Electrophoretic deposition Colloid dispersion, melt, or solution filling Conversion with chemical reaction • Lithography (top-down)
General Idea of Spontaneous Growth A growth driven by reduction of Gibbs free energy or chemical potential. This can be from either recrystallization or a decrease in supersaturation. Anisotropic growth is required → growth along a certain orientation faster than other direction.
General Idea of Spontaneous Growth Crystal growth proceeds along one direction, where as there is no growth along other direction. Uniformly sized NWs (i.e. the same diameter along the longitudinal direction of a given NW)
Fundamentals of evaporation (dissolution)- condensation growth
Fundamentals of evaporation (dissolution)- condensation growth (1) Diffusion of growth species from the bulk (such as vapor or liquid phase) to the growing surface, which, in general, is considered to proceed rapid enough and, thus, not at a rate limiting process. (2) Adsorption and desorption of growth species onto and from the growing surface. This process can be rate limiting, if the supersaturation or concentration of growth species is low. (3) Surface diffusion of adsorbed growth species. During surface diffusion, an adsorbed species may either be incorporated into a growth site, which contributes to crystal growth, or escape from the surface.
Fundamentals of evaporation (dissolution)- condensation growth (4) Surface growth by irreversibly incorporating the adsorbed growth species into the crystal structure. When a sufficient supersaturation or a high concentration of growth species is present, this step will be the rate-limiting process and determines the growth rate. (5) If by-product chemicals were generated on the surface during the growth, by-products would desorb from the growth surface, so that growth species can adsorb onto the surface and the process can continue. (6) By-product chemicals diffuse away from the surface so as to vacate the growth sites for continuing growth.
Evaporation condensation Nanowires and nanorods grown by this method are commonly single crystals with fewer Imperfections The formation of nanowires or nanorods is due to the anisotropic growth.
Evaporation condensation The general idea is that the different facets in a crystal have different growth rates There is no control on the direction of growth of nanowire in this method
Dissolution condensation Differs from Evaporation-condensation The growth species first dissolve into a solvent or a solution, and then diffuse through the solvent or solution and deposit onto the surface resulting in the growth of nanorods or nanowires. The nanowires in this method can have a mean length of <500 nm and a mean diameter of ~60 nm
