PHYSICS OF NANO MATERIALS Origin of Nanotechnology: The process of nanotechnology was first described by Richard Feynman as a process by which the ability to manipulate the individual atoms and molecules can be developed by making use of a set of precise tools to build and operate a proportionally smaller scale. The scaling issues arise due to the changing magnitude of physical phenomenon such as gravity, surface tension, Vander Waal‟s attraction etc. The term “Nanotechnology” was defined by professor Norio Taniguchi and Dr. K. Eric Dexler explored the definition inn depth and promoted the technological significance of nano scale through speeches and books due to which the term got its current sense. In the early 1980‟s, Nanotechnology and nanoscience got stated with two major developments i.e. Birth of cluster science and invention of Scanning Tunneling Microscope (STM). This led to the discovery of Fullerene‟s and Carbon nanotubes. In the later stages, synthesis and properties of semiconductor nano crystals was studied leading to a rise of metal oxide nano particles of quantum dots. Nanoscale: Nanotechnology uses the basic measuring unit called nanometer abbreviated as nm. It is derived from Greek word „midget‟. “Nano” is a metric prefix which indicates a billionth part i.e., 10-9. There are one billion nm‟s to meter where which „nm‟ is three to five atoms wide. They are very tiny about 40,000 times smaller than the width of an average human hair. A nanometer is one thousandth of a micron which has a width of six bounded carbon atoms. Nanometers are applicable to science, technology, manufacturing, chemistry, space programs, health science material science and engineering. Quantum structures: When the reduction from the bulk material (3 dimension) is in one direction, it results in a structure in 2-Dimension and is called film. If the reduction is in two dimensions, obviously the resultant structure will be in one dimension which is called Quantum wire. If the reduction is in all the three dimensions, the material reduces to a point which is well known as a Quantum dot. It is also called a nano particle or cluster.

As we can expect, the charge carriers which were able to move in all directions in a 3 dimensional material, will be confined to a plane in a film and to only one direction in quantum wire. But in a zero dimension structure they will remain confined to a very small space. It may be noted that, though we talk of film, wire and dots which are of dimensions lower than the bulk material, they must always posses certain thickness for the material along the direction where we say the corresponding dimension is absent. However along Prepared by SHANKER RAO.G, (9949435575)

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these directions, it is assumed that the thickness is lesser than mean free path for the electron in the material. This thickness will be in nanometer range. Thus along these directions, the material exhibits mesoscopic properties. Density of states for various Quantum structures: The density of states for the various quantum structures as function of energy. For 3-D structures it is denoted by g(E) density of states curve is parabolic in shape. It is varies continuously. The density will be typically of the order of 1026/m3eV. For 2-D structure, it is denoted by D(E) varies as a step function i.e., there will be sudden rises in D(E) at certain energy values such as E1, E2…. It happens so because the energy progress (in terms of energy states) of the first sub band continues even when the second sub band starts. So, in the second sub band, apart from to its own energy states, there will be additional contribution from the continuation of energy states of first sub band. Hence the density of states rises at once at the beginning of second sub band. Again, the combined energy progress of first and second sub bands enter into the third sub band and density of states rises further at the commencement of third sub band. Thus the overall variation takes a stair case shape. However, the locus of all the corners for the steps will be a parabola. If the thickness of the 2-D material is increased, more sub bands will be created which results in decrease of the step width. If the thickness increase continues, then in the limit, the material reaches 3-D at which time, the innumerable steps would be seen merged into an envelope of parabolic variation for density of states. The density is typically of the order of 1018/m2eV for 2-D structure. m* D( E )dE  2  H ( E  Ei )dE  * Where m is the effective mass of electron in the structure and H ( E  Ei ) is a step function called Heaviside function. Its values are zero for E< Ei and E  Ei . Ei is the ith energy level in the sub band. For a 1-D structure, i.e., for quantum wire, the density of states variation is not smooth as in case of 3-D structure not even constant over the sub band also. The density hits peaks at energy values E1, E2…. And decreases rapidly in the range in between . the density of states in this case is given by  ni H ( E  E i )  2m ( E )dE    E  E dE  i   Where H ( E  Ei ) is Heaviside function and ni is degeneracy factor. The density is typically of the order of 109/m eV. For a 0-D structure, i.e., for nano particles, permitted energy values are not continuous but form discrete bunches of varying

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densities. This is because of the confined condition for the electrons. Thus, densities of energy levels appear as discrete lines. In real quantum dots, however, the size distribution leads to broadening of the lines. SYNTHESIS OF NANO MATERIALS: The nano materials can be synthesized in two broad ways, namely top-down and bottom-up approaches. In top-down method the bulk solids are dis-assembled into finer pieces until the particles are in the order of nanometers. Ex: Ball milling, Nano lithography ect. In bottom-up method, the nano materials are synthesized by assembling the atoms or molecules together to form the Nanomaterials. Ex: Sol-gel, Plasma arcing, Chemical vapoure deposition ect.

Top – down Fabrication: Ball milling: The ball milling method also called mechanical crushing, small balls are allowed to rotate around the inside of a drum and then fall on a solid with gravity force and crush the solid into nano crystallites. Ball milling can be used to prepare a wide range of elemental and oxide powders. For example, iron with grain sizes of 10-30 nm can be formed. Other crystallites, such as iron nitrides, can be made using ammonia gas. A variety of inter metallic compounds based on nickel and aluminium can be formed. Ball milling is the preferred method for preparing metal oxides. Advantages: 1. This method is suitable for large scale production at low cost. 2. It can be used to grind material irrespective of hardness. Prepared by SHANKER RAO.G, (9949435575)

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Disadvantages: Because of the nature of use, the purity of the material is effected. Bottom – up Fabrication: Sol-Gel: Sol-Gel is a process in which, precipitated tiny solid particles agglomerate to form long networks which are spread continuously throughout a liquid in the form of Gel. Sols are solid particles suspended in liquid medium. Gels comprise of long networks of particles like polymers in which, the interspaces from pores that contain liquid. In the Gel phase, both the liquid and the solid are dispersed in each other so that, the material possesses the character of both the solid and the liquid phases.

In the Sol-Gel method, precursors which have a tendency to form Gel are chosen. A solution of the precursor is obtained by dissolving it in a suitable solvent. The precursors are generally inorganic material salts or alkoxides which undergo hydrolysis. By poly condensation process, nucleation of solid particles starts and Sols are formed. Then the Sols undergo polymerization (i.e. forming continuous network of particles) which turns the solution into a Gel. In this process sedimentation also takes place at which time, the liquid part is separated out by decantation. The Sol-Gel is then centrifugated from which a form of Gel called Xerogel, which has zero or only traces of the dispersion medium, is obtained. The Xerogel is then dried by heating it up to a temperature 800oC during which time, the pores of the Gel network collapse. This is called densification after which we obtain the desired nano material. Advantages: (1) Highly pure and uniform nano structures can be obtained in this process. (2) It is an inexpensive technique with fine control of the product‟s chemical composition. (3) With this method: powder, fibre, thin film coating can be made. Disadvantages: (1) Controlling the growth of the particles is difficult. (2) Stopping the newly formed particles from agglomeration is also difficult. Carbon nano tubes (CNTs): The carbon nano tubs are formed by rolling the graphite sheet into tubes with the bonds at the end of the sheet, these bonds are used to close the tube. Generally, the CNTs are formed in the range of 2 to 10 nm in diameter and 100  m in length. During the formation of CNT, it gets capped at both ends with hemispheres of fullerenes.

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Types of CNTs: During the folding of graphite sheet, concentric cylinders can be formed as a nano tube. These concentric nano tubes are known as multi wall carbon nano tubes (MWCNT). In MWCNTs walls are separated by a small distance of 0.3 nm. The MWCNT is the most common and easily formed. One can obtained the single wall carbon nano tubes (SWCNT) under specified conditions.

When the graphite sheet is rolled up about an axis T, the CNT is formed. The circumferential vector is at the right angles to T. by rolling the graphite sheet about the T vector at different orientations, three different orientations, three different CNT structures can be obtained. When the vector T is perpendicular to the C-C bonds of the carbon hexagons, a structure known as armchair structure is obtained. If n and m are the pair of integers representing the possible tube structures. C is given as; C = na1 + ma2. a1 and a2 are the unit cell vectors of the open graphite sheet with n greater than or equal to m. When m = 0 we obtain 'zig-zag' tubes, when n = m (n, n) we have 'arm-chair' nanotubes. All other tubes are chiral

Properties of CNT: (i) Mechanical Properties: The tensile strength of the CNTs is much higher than steel or Kevlar. They are around 100 times stronger than steel. In addition to it, the weight of CNTs is about only one sixth that of steel. They have highest tensile strength of all known materials so far, they are also highly elastic. (ii) Thermal conductivity: They possess thermal conductivity which is twice that of diamonds. They retain their physical structure in vacuum even upto 2800oC, i.e., they are thermally very stable. Prepared by SHANKER RAO.G, (9949435575)

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(iii) Electrical properties: They can be metallic or semiconducting depending on their structure and size. Their current carrying capacity is 1000 that of copper. Synthesis of Carbon Nanotubes: (i) Arc Discharge method: It consists of a vacuum chamber in which two graphite rods are mounted on two supports. There is a gap of about 1-2 mm between the two tips. The chamber is evacuated by using a vacuum pump and methane gas at certain pressure is introduced into the chamber. The two rods are maintained at a suitable dc potential difference (70 A at 18 V dc) to enable the discharge. On the application of voltage, the arc discharge starts. Carbon evaporates from the anode. Some part of the evaporated carbon, deposits on the cathode tip layer by layer. This is called hard deposit and the rest condenses on the other parts of cathode (cathode soot) and on the walls of the chamber (chamber soot). Both the cathode soot and the chamber soot yield, either single walled or multi walled carbon nano tubes whereas, the hard deposit does not yield any. Though this method enables production of large quantities of nano tubes, it involves purification of the soot by oxidation, centrifugation, filtration and acid treatment. However, the resulting products will be highly impure, as, 60-70% of it comprises of metal particles and amorphous carbon. (ii) Pyrolysis method: Pyrolysis is a decomposition of a chemical compound of higher molecular weight into simpler compounds by heating in the absence of oxygen so that, no oxidation occurs. It takes place usually at a temperature in the range of 400 oC to 800oC. In this method, a hydro carbon gas such as methane is passed through a heated quartz tube in which a catalyst is present. Due to Pyrolysis, the gas decomposes. Carbon atoms are freed, which bind with the catalyst such as Ni, Fe or Co. CNTs grow on the catalyst and the same is collected after cooling. Pyrolysis is a bottom-up method. Applications of CNTs: (i) CNTs can store Lithium due to which they can be used in batteries. These can also store Hydrogen and hence find potential applications in fuel cells. (ii) They are used in the tips for atomic force microscope probes. (iii) They are being used to develop flat panel displays for television and computer monitors. Prepared by SHANKER RAO.G, (9949435575)

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(iv) They are being used to develop light shield for electromagnetic radiation. (v) Filed effect transistors are being developed using semiconducting CNTs which can be used to build faster process for computers. It is estimated that these processors will be 104 times faster than the present processors. (vi) CNTs are being used to produce light weight materials with higher strength than steel. These can be used in automobile, aircraft and rocket parts. (vii) They are used in chemical sensors to detect gases. Scanning electron microscope (sem): Principle: The surface of a sample is scanned using a high energy beam of electrons. This gives rise to secondary electrons, back scattered electrons and X-rays. The secondary electrons give topographical information of the surface. Back scattered electron give topographical information and chemical composition. X-rays give the elemental composition. h 1.226 =  nm V 2meV This is the basic principle made use of in the working of all kinds of electron microscopes.

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Construction: An electron gun is used to produce high energy electrons. Two magnetic lenses are used as condenser lenses to convert the diverging electron beam into a fine beam of spot diameter of the order of a few nanometers. A scanning coil is used to deflect the electron beam to scan the sample. The objective lens is used to focus the scanning beam on a desired spot on the sample. The intensities of secondary electron, back scattered electrons and the X-rays are recorded using detectors. The images are then displayed on TV monitor. Working: When the high energy electron beam strikes the sample, some of the electrons are scattered due to elastic scattering (back scattering electrons), some electrons are knocked off from the surface (secondary electrons) and some electrons penetrate deep into the inner shells of the sample atoms to knock off inner shell electrons due to which characteristic X-rays are produced. These are detected using detectors and the signals are amplified and displayed on a TV monitor. Samples are required to be conducting. Non conducting samples are coated with a thin conducting material. Applications: (i) It gives the information about the surface features of the sample with resolution of the order of a few nanometers. This information can be used to study properties like reflectivity and roughness. (ii) SEM images give the information about the elements and compounds in the sample and their relative abundance. This is used to study properties like hardness and melting point. (iii)SEM is used to study biological specimens like pollen grains. (iv) It is used to study the corroded layers on metal surface.

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