Lecture Notes On NanoTechnology. Mahesh Lohith K.S .... tion occurs through a phenomenon called Quantum mechanical tunnel- ing.Thus Corrections to ...
Lecture Notes On NanoTechnology Mahesh Lohith K.S,AIEMS, Bangalore
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Introduction to Nanotechnology
Nanotechnology is a branch of physics which deals with the design construction and utilization of functional structures with atleast one characteristic dimension measured in nanometer. Such materials and systems can exhibit novel and improved remarkable properties, phenomenon and processes as a result of their limited size constitutent particles or molecules. This is due to the intermediate behavior in an extent between individual particles and the bulk material called mesoscopic behavior.Richard Feynmann in the year 1959 promoted the idea of Nanotechnology. The “Nanotechnology” was named so in the year 1974 by Norio Taniguchi.
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Nano Materials
It has been observed that the values of some of the physical quantities like Young’s Modulus and thermal conductivity are independent of the size for bulk 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 and is called the mesoscopic state. A conductor exhibits semiconducting behavior when the bulk material is reduced to nanometer dimension (Cluster of metal atoms). The cluster of atoms is called Nanoparticle. Nano-materials are made of nano-structures like Quantum Dots, Quantum wires,Carbon Nanotubes and Fullerenes. Nano materials are of two types.
2.1
Inorganic Nanomaterials
The organic nano materials made of nano structures formed by inorganic materials. Gold nano clusters, Fullerenes, and Carbon nanotubes etc., are classified into this type. 2.1.1
Gold Nano Particle
It is a cluster of gold atoms and its dimension is of few nanometer. This could also be referred to as Quantum Dot because the electron has no degrees of freedom due to 3-D confinement. Gold nano particle absorbs energy and emits visible wavelength which depends on the size of the partcile. Thus, in olden days, 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 arranged forming a spherical structure similar to the foot ball. Fullerene as itself behaves like a single intact entity and is found to be extremely useful in Drug Delivery System(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 properties which make it a very useful nano structure in nano electronics and nano composites. The carbon nanotube is a cylyndrical structure made of carbon and is of dimension around 1.5 nm. This could be formed either by folding the carbon sheets of graphite or using fullerenes. It has electrical properties better than copper and it has a very high Young’s modulus. Thus it is being used as a nano wire and also in the manufacturing of composites which are light,strong and tough. It can replace certain circuit elements like MOSFET because CNTs can be used in FETs(Field Effect Transistor). Thus the miniaturization from micro to nano dimensions made possible. CNTs are also used in DDS. CNTs can 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 nanotube cylinders. 2
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 a technique using which the molecules can be assembled to form nano structures. Self Assembly is achieved by providing the suitable Physical and Chemical environment The modification of DNA molecule using this technique has remarkable improvement in genetic evolution. The DNAs are the genetic codes which carry message through generations can be modified to make the life resistant for certain hereditric diseases. Self organisation of organic nanomaterials results in the growth artificial layers of skin, liver tissues and other organs.
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Molecular Manufacturing
Molecular manufacturing is a future technology that will allow us to build large objects to atomic precision, quickly and cheaply, with virtually no defects. This involves chemical reactions controlled by a type of machinery called Molecular Machinery. Robotic mechanisms will position and react molecules to build systems to complex atomic specification. The act of controlling and guiding a chemical reaction mechanically during a synthesis is called Mechanosyntheis. The theoretical capabilities and performance of these systems have been analyzed for over many years. Some of the molecular machine components are being built. The molecular manufacturing could mature within the next few years. When it becomes available, it will enable immensely powerful computers, abundant and high quality consumer goods, and devices able to cure diseases by repairing the body at the molecular level.
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Fabrication Technology
The fabrication of nanostructures and materials could be achieved through two approaches 1. Top-Down approach 2. Bottom-Up approach The molecular nanotechnology makes use of the Bottom-up approach for the fabrication of nanosystems.
4.1
Top-Down appaorach
This is the method of reducing the dimension of the material of bulk scale to a nano scale. Lithograhic and etchig techniques are used to construct nano structures and devices.
4.2
Bottom-up approach
Molecular manufacturing is an anticipated future technology based on Feynman’s vision of factories using nanomachines to build complex products, including additional nanomachines. The basic idea is to mix molecules in solution, allowing them to wander and bump together at random, nanomachines will instead position molecules, placing them in specific locations in a carefully chosen sequence. Letting molecules bump at random leads to unwanted reactions and a problem that grows worse as products get larger. By holding and positioning molecules, nanomachines will control how the molecules react, building up complex structures with atomically precise control. This Self organization or Self assembly is a bottom up apparoach used in building nano machines.
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Nano-Mechanical Bearings
Figure 4: Nano-Mechanical Bearing Nano-Mechanical Bearings are the Nano devices formed to reduce friction in nanomachines. They are realised using the polycyclic ring structure of atoms 4
as shown in the sketch and roughly resemble the mechanical bearigs of bulk machines. The bearing action is based on the internal motions of atoms in the molecules such as vibration and rotation due to temperature. But at the given temperature different components or molecules exhibit internal motions to different extents. The componets or the layers of the bearing are so formed at a given temperarure some are stiff and some are free to move. This results in bearing action. These are also referred to as molecular bearings. Apart form the type of bearing shown in the sketch there are other types of bearings like telescopic bearings formed using two carbon nanotubes one rotating inside the other and also having the property of telescopic sliding.
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Scaling of Classical Mechanical Systems
6.1
Classical Mechanical Systems
Approximations are very much required since accurate physical models are computationally hard to deal with. Engineers use approximations of classical mechanics in the design of macromechanical systems by neglecting quantum mechanics. Since the macromechanical systems blend into the nanomechanical systems, the approximations encroch even into nanomechanical systems. The mechanical systems which does not obeys law of quantization of energy exchanges and the system for which the heisenberg’s uncertainty principle is not applicable is called Classical Mechanical system. The wave nature of matter has no role to play. Thus the measurements are considered to be accurate and dependent only on the accuracies of the measuring instrument. The system completely follows classical equations of motion and energy exchanges This provides an 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 neglected. The mechanical properties like strengths, moduli, densities and coefficient of friction are held to be constant.
6.3
Scaling Laws
The characterization of variation of measures of physical quantities of a system using 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 strength of a structure and the force it exerts scale with its cross-sectional area. T otalStrength ∝ F orce ∝ Area ∝ L2 2. Similarly the shearing stiffness depends directly on area and depends Area ∝ inversly on Length height. Therefore ShearingStif f ness ∝ Length 1 Length ∝ L
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3. If the Density is assumed to be constant then the mass is proportional to volume. Hence mass ∝ volume ∝ L3 . This expression yields the scaling of −1 accleration and the relation ship is given by Acceleration ∝ FMorce ass ∝ L since F orce ∝ Area ∝ L2 under constant stress 4. Characteristic frequencies are inversly proportional to Characteristic times 1 F requency ∝ time ∝ L−1 5. Characteristic times are inversely proportional to characteristic frequen1 ∝ L1 cies T imeP eriod ∝ F requency 6. The problems of liquid lubrication motivate consideration of dry bearings. Assuming a constant coefficient of friction F rictionf orce ∝ F orce ∝ L2
6.4
Major Corrections
In addition to molecular structure of matter the major corrections to the results suggested by these scaling laws include uncertainties in position and velocity resulting from statistical and quantum mechanics
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Scaling of Classical ElectroMagnetic Systems
The electromaganetic systems that obey classical laws are called Classical ElectroMagnetic systems. Even in these systems the quantization effects and uncertainty effects have no role to play.
7.1
Basic Assumptios
It is convenient to assume that electrostatic field strengths and hence electrostatic stresses are independent of scale, for the scaling of electronagnetic systems. The magnetic effects are ignored. Such scaling is referred to as Constant field scaling. Then for an electro-mechanical system the same assumptions of classical electrostatic and classical mechanical scaling hold good.
7.2
Major Corrections
The important corrections to the assumptions which neglect quantum effects are 1. Many electromechanical systems use nano wires and insulating layers for which electrical conductivity is an important consideration. The conduction occurs through a phenomenon called Quantum mechanical tunneling.Thus Corrections to classical continuum models are more important in electromagnetic systems than in mechanical systems. The quantum effects become dominant and at small scales can render classical continuum models useless even as crude approximations. 2. Electromagnetic systems on a nanometer scale commonly have extremely high frequencies. Thus the molecules undergoing electronic transitions typically absorb and emit light in the visible to ultraviolet range, rather than the infrared range characteristic of thermal excitation at room temperature. 6
3. At high frequencies, the inertial effects of electron mass become significant, but these are neglected in the usual macroscopic expressions for electrical circuits.
7.3
Steady state and Time varying electromagnetic systems
• The electromagnetic systems that do not under go any change with respect to time are called steady state electromagnetic systems. Electrostatic and Magnetostatic systems are the steady state systems. Charged capacitor that does not have any surrounding to discarge and a wire carrying constant current are the examples for steady state systems. • If the fields and currents are subjected to variations then the system is called time varying electromagnetic system. For example oscillations in LCR 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 Electrostaticf orce ∝ area × Electrostaticf ield2 ∝ L2
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