Lecture Notes On NanoTechnology

Mahesh Lohith K.S,VVIET, Mysore

1 Introduction to Nanotechnology

Nanotechnology is a branch of physics which deals with the design constructionand utilization of functional structures with atleast one characteristic dimensionmeasured in nanometer. Such materials and systems can exhibit novel andimproved remarkable properties, phenomenon and processes as a result of theirlimited size constitutent particles or molecules. This is due to the intermediatebehavior in an extent between individual particles and the bulk material calledmesoscopic behavior.Richard Feynmann in the year 1959 promoted the ideaof Nanotechnology. The “Nanotechnology” was named so in the year 1974 byNorio Taniguchi.

2 Nano Materials

It has been observed that the values of some of the physical quantities likeYoung’s Modulus and thermal conductivity are independent of the size forbulk materials.This notion holds good only to a certain extent of limiting size(nanoscale) is reached, below which the physical properties are size dependent.Thus the material exhibits a remarkable interesting behavior in this state andis called the mesoscopic state. A conductor exhibits semiconducting behaviorwhen the bulk material is reduced to nanometer dimension (Cluster of metalatoms). The cluster of atoms is called Nanoparticle . A nano-materials aremade of nano-structures like Quantum Dots, Quantum wires,Carbon Nanotubesand Fullerenes. Nano materials are of two types.

2.1 Inorganic Nanomaterials

The organic nano materials made of nano structures formed by inorganic mate-rials. Gold nano clusters, Fullerenes, and Carbon nanotubes etc., are classifiedinto this type.

2.1.1 Gold Nano Particle

It is a cluster of gold atoms and its dimension is of few nanometer. This couldalso be referred to as Quantum Dot because the electron has no degrees offreedom due to 3-D confinement. Gold nano particle absorbs energy and emitsvisible wavelength which depends on the size of the partcile. Thus, in oldendays, it is used to pigment the glass which were used for window panes.

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Figure 1: Gold Nano Particles

2.1.2 Fullerene

It is an allotropic form of carbon in which 60,50 or 40 carbon atoms are arrangedforming a spherical structure similar to the foot ball. Fullerene as itself behaveslike a single intact entity and is found to be extremely useful in Drug DeliverySystem(DDS) and in the formation of Carbon Nanotubes.

Figure 2: Fullerene

2.1.3 Carbon Nanotube (CNT)

It is also an allotropic form of carbon and a single entity having novel proper-ties which make it a very useful nano structure in nano electronics and nanocomposites. The carbon nanotube is a cylyndrical structure made of carbonand is of dimension around 1.5 nm. This could be formed either by folding thecarbon sheets of graphite or using fullerenes. It has electrical properties betterthan copper and it has a very high Young’s modulus. Thus it is being used asa nano wire and also in the manufacturing of composites which are light,strongand tough. It can replace certain circuit elements like MOSFET because CNTscan be used in FETs(Field Effect Transistor). Thus the miniaturization frommicro to nano dimensions made possible. CNTs are also used in DDS. CNTscan also be used as sensing nano devices. CNT’s are of two types

1. Single-Walled Carbon Nanotube (SWCNT) is a just single cylinder

2. Multi-Walled Carbon Nanotube (MWCNT) consisting concentric nan-otube cylinders.

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CNTs have different types of arrangements of carbon atoms and they are

1. Armchair

2. Zigzag

3. Chiral

Figure 3: Carbon Nanotube

2.2 Organic Nanomaterials

They are the nanomaterials made of carbon compounds. Self Organization is atechnique using which the molecules can be assembled to form nano structures.Self Assembly is achieved by providing the suitable Physical and Chemical envi-ronment The modification of DNA molecule using this technique has remarkableimprovement in genetic evolution. The DNAs are the genetic codes which carrymessage through generations can be modified to make the life resistant for cer-tain hereditric diseases. Self organisation of organic nanomaterials results in thegrowth artificial layers of skin, liver tissues and other organs.

3 Molecular Manufacturing

Molecular manufacturing is a future technology that will allow us to build largeobjects to atomic precision, quickly and cheaply, with virtually no defects. Thisinvolves chemical reactions controlled by a type of machinery called Molecu-lar Machinery. Robotic mechanisms will position and react molecules to buildsystems to complex atomic specification. The act of controlling and guidinga chemical reaction mechanically during a synthesis is called Mechanosyntheis.The theoretical capabilities and performance of these systems have been an-alyzed for over many years. Some of the molecular machine components arebeing built. The molecular manufacturing could mature within the next fewyears. When it becomes available, it will enable immensely powerful computers,abundant and high quality consumer goods, and devices able to cure diseasesby repairing the body at the molecular level.

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4 Fabrication Technology

The fabrication of nanostructures and materials could be achieved through twoapproaches

1. Top-Down approach

2. Bottom-Up approach

The molecular nanotechnology makes use of the Bottom-up approach for thefabrication of nanosystems.

4.1 Top-Down appaorach

This is the method of reducing the dimension of the material of bulk scale toa nano scale. Lithograhic and etchig techniques are used to construct nanostructures and devices.

4.2 Bottom-up approach

Molecular manufacturing is an anticipated future technology based on Feyn-man’s vision of factories using nanomachines to build complex products, includ-ing additional nanomachines. The basic idea is to mix molecules in solution,allowing them to wander and bump together at random, nanomachines will in-stead position molecules, placing them in specific locations in a carefully chosensequence. Letting molecules bump at random leads to unwanted reactions anda problem that grows worse as products get larger. By holding and positioningmolecules, nanomachines will control how the molecules react, building up com-plex structures with atomically precise control. This Self organization or Selfassembly is a bottom up apparoach used in building nano machines.

5 Nano-Mechanical Bearings

Figure 4: Nano-Mechanical Bearing

Nano-Mechanical Bearings are the Nano devices formed to reduce friction innanomachines. They are realised using the polycyclic ring structure of atoms

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as shown in the sketch and roughly resemble the mechanical bearigs of bulkmachines. The bearing action is based on the internal motions of atoms inthe molecules such as vibration and rotation due to temperature. But at thegiven temperature different components or molecules exhibit internal motionsto different extents. The componets or the layers of the bearing are so formedat a given temperarure some are stiff and some are free to move. This resultsin bearing action. These are also referred to as molecular bearings.

Apart form the type of bearing shown in the sketch there are other typesof bearings like telescopic bearings formed using two carbon nanotubes onerotating inside the other and also having the property of telescopic sliding.

6 Scaling of Classical Mechanical Systems

6.1 Classical Mechanical Systems

Approximations are very much required since accurate physical models are com-putationally hard to deal with. Engineers use approximations of classical me-chanics in the design of macromechanical systems by neglecting quantum me-chanics. Since the macromechanical systems blend into the nanomechanicalsystems, the approximations encroch even into nanomechanical systems. Themechanical systems which does not obeys law of quantization of energy ex-changes and the system for which the heisenberg’s uncertainty principle is notapplicable is called Classical Mechanical system. The wave nature of matterhas no role to play. Thus the measurements are considered to be accurate anddependent only on the accuracies of the measuring instrument. The system com-pletely follows classical equations of motion and energy exchanges This providesan adequate basis for the design and analysis of the nanoscale systems.

6.2 Basic Assumptions

For the scaling of classical mechanical systems the fields and currents are ne-glected. The mechanical properties like strengths, moduli, densities and co-efficient of friction are held to be constant.

6.3 Scaling Laws

The characterization of variation of measures of physical quantities of a systemusing the relationships with respect to their dimensions are the scaling laws.

6.3.1 Magnitudes and Scaling

1. If the stress and material strength are held constant then both the strengthof a structure and the force it exerts scale with its cross-sectional area.TotalStrength ∝ Force ∝ Area ∝ L2

2. Similarly the shearing stiffness depends directly on area and dependsinversly on Length height. Therefore ShearingStiffness ∝ Area

Length ∝Length ∝ L1

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3. If the Density is assumed to be constant then the mass is proportional tovolume. Hence mass ∝ volume ∝ L3. This expression yields the scaling ofaccleration and the relation ship is given by Acceleration ∝ Force

Mass ∝ L−1

since Force ∝ Area ∝ L2 under constant stress

4. Characteristic frequencies are inversly proportional to Characteristic timesFrequency ∝ 1

time ∝ L−1

5. Characteristic times are inversely proportional to characteristic frequen-cies TimePeriod ∝ 1

Frequency ∝ L1

6. The problems of liquid lubrication motivate consideration of dry bearings.Assuming a constant coefficient of friction Frictionforce ∝ Force ∝ L2

6.4 Major Corrections

In addition to molecular structure of matter the major corrections to the resultssuggested by these scaling laws include uncertainties in position and velocityresulting from statistical and quantum mechanics

7 Scaling of Classical ElectroMagnetic Systems

The electromaganetic systems that obey classical laws are called Classical Elec-troMagnetic systems. Even in these systems the quantization effects and uncer-tainty effects have no role to play.

7.1 Basic Assumptios

It is convenient to assume that electrostatic field strengths and hence electro-static stresses are independent of scale, for the scaling of electronagnetic sys-tems. The magnetic effects are ignored. Such scaling is referred to as Constantfield scaling. Then for an electro-mechanical system the same assumptions ofclassical electrostatic and classical mechanical scaling hold good.

7.2 Major Corrections

The important corrections to the assumptions which neglect quantum effectsare

1. Many electromechanical systems use nano wires and insulating layers forwhich electrical conductivity is an important consideration. The conduc-tion occurs through a phenomenon called Quantum mechanical tunnel-ing.Thus Corrections to classical continuum models are more importantin electromagnetic systems than in mechanical systems. The quantum ef-fects become dominant and at small scales can render classical continuummodels useless even as crude approximations.

2. Electromagnetic systems on a nanometer scale commonly have extremelyhigh frequencies. Thus the molecules undergoing electronic transitionstypically absorb and emit light in the visible to ultraviolet range, ratherthan the infrared range characteristic of thermal excitation at room tem-perature.

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3. At high frequencies, the inertial effects of electron mass become significant,but these are neglected in the usual macroscopic expressions for electricalcircuits.

7.3 Steady state and Time varying electromagnetic sys-tems

• The electromagnetic systems that do not under go any change with respectto time are called steady state electromagnetic systems. Electrostatic andMagnetostatic systems are the steady state systems. Charged capacitorthat does not have any surrounding to discarge and a wire carrying con-stant current are the examples for steady state systems.

• If the fields and currents are subjected to variations then the system iscalled time varying electromagnetic system. For example oscillations inLCR circuit.

7.4 Magnitude and Scaling of Electromagnetic systems

Following examples are for the steady state electromagnetic systems.

1. Given a scale-invariant electrostatic field strength, V oltage ∝ ElectrostaticF ield×Length ∝ L

2. A scale-invariant field strength implies a Electrostaticforce ∝ area ×Electrostaticfield2 ∝ L2

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