Lecture by:Rose Farahiyan Munawar , PhD

Chapter 3: Nanowires

Models of 3-D nanostructures made from DNA.

Pre-Quiz

Nanowires

quantum wire, metallic nanowire, semiconductor nanowire, insulating nanowire, molecular nanowire, nanowire

arrays, nanowire, alumina nanowire, bismuth nanowire, boron nanowire, cadmium selenide nanowire, copper

nanowire, gallium nitride nanowire, gold nanowire, gallium phosphide nanowire, germanium nanowire, indium phosphide nanowire, 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,

Nanowire (Nw)

Why Nanowires?

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

• Whiskers, fibers:1D structures ranging from several nanometers to seve ral 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

Structure of NWs

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 around tens of nanometers or less and an unconstrained length.

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

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.

Helical Nanowire

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

• 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.

Structure of NWs

Types of nanowires (diameter)

1

•Classical Nanowires

2

•Quantum Nanowires

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.

NW Properties

SEM characterization of as-synthesized silicon oxidenanowires.

Indium arsenide (InAs) nanowires grown by the VLS technique

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

Building Blocks Synthesis

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 condensationDissolution condensation

Vapor-Liquid-Solid growth (VLS)Stress induced re-crystallization

• Electro-spinning

• Solution Synthesis

Methods

• Template-based synthesis:Electrochemical depositionElectrophoretic deposition

Colloid dispersion, melt, or solution fillingConversion 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.

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)

General Idea of Spontaneous 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.

Fundamentals of evaporation (dissolution)- condensation 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.

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

Evaporation condensation

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

E-Beam Lithography

Nanowires Typical Applications

• in electronic, opto-electronic and devices• as additives in advanced composites• for metallic interconnects in nanoscale

quantum devices• as field-emittors and as leads for biomolecular

nanosensors.• also optical, sensing, solar cells, magnetic, and

electronic device applications

Applications in Electronic

Applications in Electronic

Applications in Biomedical Engineering

Applications in Structural, Mechanical

Applications in Sensors

Conclusion

Challenges:The insufficient control of the properties of

individual building blocks

Low device-to-device reproducibility

Lack of reliable methods for assembling and integrating building blocks into circuits

Conclusion

Advances:• Synthesis of nanoscale building blocks with

precisely controlled chemical composition, physical dimension, and electronic, optical properties

• Some strategies for the assembly of building blocks into increasingly complex structures

• New nanodevice concepts that can be implemented in high yield by assembly approaches

References

• Synthesis, Characterization, and Manipulation of Helical SiO2 Nanosprings, Hai-Feng Zhang et al.

• One-Dimensional Nanostructures, Sharif Hussein Sharif Zain

• An Introduction to NanoWires And Their Applications, Amir Dindar and Shoeb Roman

• Nanostructures, Raul J. Martin-Palma et al.

References

• Nanostructures and Nanomaterials, GuoZhong Chao

• Synthesis and applications of one-dimensional Semiconductors, Sven Barth et al.

• Nanomaterials, nanotechnology and design: an introduction for engineer, M. F. Ashby et al.

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