Tài liệu Nano and Microelectromechanical Systems P3 doc

CHAPTER 3

STRUCTURAL DESIGN, MODELING, AND SIMULATION

3.1. NANO- AND MICROELECTROMECHANICAL SYSTEMS

3.1.1. Carbon Nanotubes and Nanodevices

Carbon nanotubes, discovered in 1991, are molecular structures which

consist of graphene cylinders closed at either end with caps containing

pentagonal rings. Carbon nanotubes are produced by vaporizing carbon

graphite with an electric arc under an inert atmosphere. The carbon

molecules organize a perfect network of hexagonal graphite rolled up onto

itself to form a hollow tube. Buckytubes are extremely strong and flexible

and can be single- or multi-walled. The standard arc-evaporation method

produces only multilayered tubes, and the single-layer uniform nanotubes

(constant diameter) were synthesis only a couple years ago. One can fill

nanotubes with any media, including biological molecules. The carbon

nanotubes can be conducting or insulating medium depending upon their

structure.

A single-walled carbon nanotube (one atom thick), which consists of

carbon molecules, is illustrated in Figure 3.1.1. The application of these

nanotubes, formed with a few carbon atoms in diameter, provides the

possibility to fabricate devices on an atomic and molecular scale. The

diameter of nanotube is 100000 times less that the diameter of the sawing

needle. The carbon nanotubes, which are much stronger than steel wire, are

the perfect conductor (better than silver), and have thermal conductivity

better than diamond. The carbon nanotubes, manufactured using the carbon

vapor technology, and carbon atoms bond together forming the pattern.

Single-wall carbon nanotubes are manufactured using laser vaporization, arc

technology, vapor growth, as well as other methods. Figure 3.1.2. illustrates

the carbon ring with six atoms. When such a sheet rolls itself into a tube so

that its edges join seamlessly together, a nanotube is formed.

Figure 3.1.1. Single-walled carbon nanotube

Figure 3.1.2. Single carbon nanotube ring with six atoms

Carbon nanotubes, which allow one to implement the molecular wire

technology in nanoscale ICs, are used in NEMS and MEMS. Two slightly

displaced (twisted) nanotube molecules, joined end to end, act as the diode.

Molecular-scale transistors can be manufactured using different alignments.

There are strong relationships between the nanotube electromagnetic

properties and its diameter and degree of the molecule twist. In fact, the

electromagnetic properties of the carbon nanotubes depend on the molecule's

twist, and Figures 3.1.3 illustrate possible configurations. If the graphite

sheet forming the single-wall carbon nanotube is rolled up perfectly (all its

hexagons line up along the molecules axis), the nanotube is a perfect

conductor. If the graphite sheet rolls up at a twisted angle, the nanotube

exhibits the semiconductor properties. The carbon nanotubes, which are

much stronger than steel wire, can be added to the plastic to make the

conductive composite materials.

Figure 3.1.3. Carbon nanotubes

The vapor grown carbon nanotubes with N layers are illustrated in

Figure 3.1.4, and the industrially manufactured nanotubes have ∆ngstroms

diameter and length.

Figure 3.1.4. N-layer carbon nanotube

The carbon nanotubes can be organized as large-scale complex neural

networks to perform computing and data storage, sensing and actuation, etc.

The density of ICs designed and manufactured using the carbon nanotube

technology thousands time exceed the density of ICs developed using

convention silicon and silicon-carbide technologies.

Metallic solids (conductor, for example copper, silver, and iron) consist

of metal atoms. These metallic solids usually have hexagonal, cubic, or body￾centered cubic close-packed structures (see Figure 3.1.5). Each atom has 8 or

12 adjacent atoms. The bonding is due to valence electrons that are

delocalized thought the entire solid. The mobility of electrons is examined to

study the conductivity properties.

(a) (b) (c)

Figure 3.1.5. Close packing of metal atoms: a) cubic packing;

b) hexagonal packing; c) body-centered cubic

More than two electrons can fit in an orbital. Furthermore, these two

electrons must have two opposite spin states (spin-up and spin-down).

Therefore, the spins are said to be paired. Two opposite directions in which

the electron spins (up + 2

1 and down – 2

1 ) produce oppositely directed

magnetic fields. For an atom with two electrons, the spin may be either

parallel (S = 1) or opposed and thus cancel (S = 0). Because of spin pairing,

most molecules have no net magnetic field, and these molecules are called

diamagnetic (in the absence of the external magnetic field, the net magnetic

field produced by the magnetic fields of the orbiting electrons and the

magnetic fields produced by the electron spins is zero). The external

magnetic field will produce no torque on the diamagnetic atom as well as no

realignment of the dipole fields. Accurate quantitative analysis can be

performed using the quantum theory. Using the simplest atomic model, we

assume that a positive nucleus is surrounded by electrons which orbit in

various circular orbits (an electron on the orbit can be studied as a current

loop, and the direction of current is opposite to the direction of the electron

rotation). The torque tends to align the magnetic field, produced by the

orbiting electron, with the external magnetic field. The electron can have a

spin magnetic moment of 24 9 ´± 10 A-m2

. The plus and minus signs that

there are two possible electron alignments; in particular, aiding or opposing

to the external magnetic field. The atom has many electrons, and only the

spins of those electrons in shells which are not completely filed contribute to

the atom magnetic moment. The nuclear spin negligible contributes to the

atom moment. The magnetic properties of the media (diamagnetic,

paramagnetic, superparamagnetic, ferromagnetic, antiferromagnetic,

ferrimagnetic) result due to the combination of the listed atom moments