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Properties of Nano-materials

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Properties of Nano-materials. Mainly focusing on mechanical, magnetic, optical and electrical properties.

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Properties of Nano-materials

  1. 1. Properties of A Report on
  2. 2. CONTENTS TOPIC PAGE NO. INTRODUCTION 1-2 MECHANICALPROPERTIES 3-7 MAGNETICPROPERTIES 8-14 OPTICAL PROPERTIES 15-18 ELECTRICALPROPERTIES 19-25
  3. 3. INTRODUCTION • Nanotechnology is the collaboration of the physics ,chemistry,biology,computer and material sciences integrated with engineering entering the nanoscale.This means science and engineering focused on making the particles,things and devices at the atomic and molecular scale. Definition of Nano Particles: Nanomaterials or the Nanoparticles are the set of particles or the substances where atlas one dimension is less than approximately 100nm. or it can be also classically illustrated as the follows: Nanomaterial is an object that has atleast one dimension in the nanometer scale approximately 1-100nm. 1
  4. 4. • • Due to the reduction in the spatial dimension , or confinement of particles or quasi particles in a particular crystallographic direction within a structure generally leads to changes in physical properties of the system in that direction. • Hence classification of the nanostructured materials and systems essentially depends on the number of dimensions which lie within the nanometer range. • a)systems confined in 3 dimensions[Zero dimension structures] Examples:Nanoparticles;Nanograins;Nanoshells;Na nocapsules;Nanorings;Fullerenes;collidal particles;activatedcarbon; nanoporous silicon;quasi crystals. • b)systems confined in 2 dimensions[One dimension structures] Examples:Nanorods;Nanofilaments;Nanotubes;qua ntum wires;nano wires. • c)systems confined in 1 dimension.[two dimension structures] Examples:discs;platelets;ultrathin films;super lattices;quantum wells. • In this report we have discussed mainly on on the following properties of Nanomaterials: • Mechanical Properties • Magnetic Properties • Optical Properties • Electrical Properties Classification of Nanomaterials 2
  5. 5. MECHANICALPROPERTIES • Tensile test Classic Mechanical Properties Determination of mechanical properties Stress: σ = F/S Strain: ε = Δl / l0 Max stress : tensile strength Max elasticity: Yield strength Stress, σ (Mpa) Necking Strain, ε (%)Elastic deformation Plastic deformation Fract ure Tensile Test curve 3
  6. 6. Modulus = slope Strain Stress,σ • Hooke’s law: σ = E ε • E = Young modulus (Pa) • Stiffness of material • Non linear models exist (visco-elastic behaviour) Elastic Deformation Mechanical properties Yield strength: maximum stress before permanent strain Tensile strength: maximum stress Ductility: measure of deformation (Lf – Lo)/ Lo Toughness: ability to absorbe energy: area under curve Hardness Resistance to plastic deformation Measure of depth or size of indentation 4
  7. 7. Nanoparticles • Conventional materials: Grain size micron to mm • Nanoparticles increase grain boundaries • Influence on mechanical properties: Increased hardness, yield strength, elastic modulus, toughness Nanostructured materials Comparison: Al Mg cryomilled (20 nm) Al Mg ultra fine grain (80 nm) Al Mg coarse (2 mm) Cryomilling: Milling in liquid N2 Ultrafine grain: electrodeposition Comparison tensile curves 5
  8. 8. Mechanical properties of nanomaterials compared to coarse grain materials • Higher Young modulus and tensile strength (to 4 times higher) • Lower plastic deformation • More brittle Strength and Hardness with grain size Strength and Hardness of nanostructured material increases with decreasing size Grain boundaries deformation 6
  9. 9. Elongation nanostructured materials • Elongation decreased • Lower density of mobile dislocations • Short distance of dislocation movement Material Young modulus (GPa) Rubber 0.1 Al 70 Fe 200 SiC 440 Fe nanoparticles (100 nm) 800 C nanotubes 1000 Diamond 1200 Comparison of Young modulus 7
  10. 10. MAGNETICPROPERTIES Magnetic properties of nanoparticles • Each spin is a small magnet • Interaction between neighboring spins is dominated by the spin exchange interaction. • In most materials J < 0 and the material is non-magnetic (paramagnetic or diamagnetic) Most people relate magnetics to storage One bit viewed by magnetic force microscopy Is it nano? Well, overall size is ~1mm, but the bit has smaller details. Clearly, nano characterization methods are being used to see this. 8
  11. 11. Apoferritin, your body’s iron storage protein and precision magnetic system. Quaternary structure of the protein. The pieces make an open cavity that can store thousands of Fe atoms Types of Magnetism (Sibel Turksen Thesis) 9
  12. 12. M -M Magnetization Magnetization in opposite direction General Hysteresis Plot Paramagnet, Ferromagnet & Superparamagnet I think of the superparamagnet as a small ferromagnet. Because of its small size, the magnetic moment wanders. When given an order to align (when a magnetic field is imposed) it aligns with the same enthusiasm that a ferromagnet has, which exceeds that of the paramagnet. 10
  13. 13. Paramagnet Zero field Applied field FerromagnetSuperparamagnet Like the paramagnet, the superparamagnet returns to zero magnetization when the field is removed. It does so for a different reason: small size, not intrinsically weak exchange between the individual moments. The bottom line is: Nano scale has a big impact on the magnetic properties! In a normally ferromagnetic material, nano scale reduces the moment, but it can be restored by applying a magnetic field. The good news: switchable interactions! The bad news: There would seem to be a lower limit to the size of a magnetic particle that can hold an alignment for data storage. 11
  14. 14. Superparamagnetic nanoparticles Stabilization a) By surface coating using appropriate polymeric stabilizers/surfactants (carboxylates, phospates, cathecols) b) By deposition of a layer of inorganic metals (e.g., gold), nonmetals (e.g., graphite), or oxides (e.g. SiO2) c) By generating polymeric shells that avoid cluster growth after nucleation (composite particles, nanocapsule). d) By the formation of lipid-like coatings (e.g., liposomes/ lipid NPs) around the magnetic core. 12
  15. 15. MRI imaging T1 spin-lattice relaxation T2 spin-spin relaxation a) SPIO affects T2 b) Gd3+ affects T1 c) Core-shell nanoparticle enable both imaging modes. 13
  16. 16. • They will chain together! • The chain causes high viscosity. •  Magnetorheological effect. Suppose some particles do have magnetic moments. N S N S N S N S A magnetic fluid. Magnetorheological Effect Fluid becomes solid—and reverses! 14
  17. 17. Opticalproperties • So lets start with Optical properties. But first, let me ask you a question. What is the origin of colour? Well its because of SURFACE PLASMONS. • An SP is a natural oscillation of the electron gas inside a gold nanosphere. • If the sphere is small compared to a wavelength of light, and the light has a frequency close to that of the SP, then the SP will absorb energy. • The frequency of the SP depends on the dielectric function of the gold, and the shape of the nanoparticle. For a spherical particle, the frequency is about 0.58 of the bulk plasma frequency. Thus, although the bulk plasma frequency is in the UV, the SP frequency is in the visible (in fact, close to 520 nm) Metallic sphere EM wave Incident electric field is =E o exp(-i w t) Surface plasmon is excited when a long-wavelength electromagnetic wave is incident on a metallic sphere 15
  18. 18. • Calculation of SP Frequency Effective conductivity of a random metal-insulator composite in the effective-medium approximation Effective conductivity of a composite of Drude metal and insulator: dots, numerical; full curves, effective-medium approximation. 16
  19. 19. • Nonlinear optical properties of nanomaterials Suppose we have a suspension of nanoparticles in a host (or some other composite which is structured on the nanoscale). If an EM wave is applied, the local electric field may be hugely enhanced near an SP resonance. Ifso,one expects various nonlinear susceptibilities, which depend on higher powers of the electric field, to be enhanced even more. The Kerr Susceptibility is defined by where D is the electric displacement, E is the electric field, and epsilon and chi are the linear and nonlinear electric susceptibilities. If the electric field is locally large, as near an SP resonance, then its cube is correspondingly larger. Thus, near an SP resonance, one expects a huge enhancement of the cubic nonlinear (Kerr) susceptibility Kerr susceptibility for a dilute suspension of coated spheres Cubic nonlinear (Kerr) susceptibility for a dilute suspension of coated metal particles in a glass host, calculated in Maxwell- Garnett approximation 17
  20. 20. • Kerr enhancement factor for a random metal-insulator composite, assuming (left) metal and (right) insulator is nonlinear. Calculation is carried out numerically, at the metal-insulator percolation threshold. Kerr enhancement factor for metal-insulator composite Faraday Rotation in Composites: enhanced near SP resonance Real and imaginary parts of the Faraday rotation angle in a composite of Drude metal and insulator in a magnetic field (Xia, Hui, Stroud, J. Appl. Phys. 67, 2736 (1990) 18
  21. 21. Quantum confinement In small nanocrystals, the electronic energy levels are not continuous as in the bulk but are discrete (finite density of states), because of the confinement of the electronic wavefunction to the physical dimensions of the particles. This phenomenon is called quantum confinement and therefore nanocrystals are also referred to as quantum dots (QDs). In any material, substantial variation of fundamental electrical and optical properties with reduced size will be observed when the energy spacing between the electronic levels exceeds the thermal energy (kT). Moreover, nanocrystals possess a high surface are and a large fraction of the atoms in a nanocrystal are on its surface. Since this fraction depends largely on the size of the particle (30% for a 1-nm crystal, 15% for a 10-nm crystal), it can give rise to size effects in chemical and physical properties of the nanocrystal. ELECTRICAL PROPERTIES 19
  22. 22. Metal (conductor) Insulator Semiconductor Conduction band (empty) Valence band (full) band gap band gap Electronic band theory Density of states in metal (A) and semiconductor (B) nanocrystals. In each case, the density of states is discrete at the band edges. The Fermi level is in the center of a band in a metal, and so kT will exceed the level spacing even at low temperatures and small sizes. Nevertheless, metal nanoparticles of very small size can exhibit insulating properties. Energy levels in metallic and semiconductor nanoparticles 20
  23. 23. The properties like conductivity or resistivity are come under category of electrical properties. These properties are observed to change at nanoscale level like optical properties. The change in electrical properties in nanomaterials are: 1. Conductivity of a bulk or large material does not depend upon dimensions like diameter or area of cross section and twist in the conducting wire etc. However it is found that in case of carbon nanotubes conductivity changes with change in area of cross section. 2.) It is also observed that conductivity also changes when some shear force (in simple terms twist) is given to nanotube. 3.) Conductivity of a multiwalled carbon nanotube is different than that of single nanotube of same dimensions. 4.) The carbon nanotubes can act as conductor or semiconductor in behaviour but we all know that large carbon (graphite) is good conductor of electricity. These are the important electrical properties of nanomaterials. The electrical properties of the nanomaterial triggered a response in the mesenchymal (adult) stem cells, which we sourced from human bone marrow. In effect, they became electrified, which made them morph into more cardiac-like cells 21
  24. 24. Here we are to discuss about fundamentals of electrical conductivity in nanotubes and nanorods, carbon nanotubes, photoconductivity of nanorods, electrical conductivity of nanocomposites. One interesting method which can be used to demonstrate the steps in conductance is the mechanical thinning of a nanowire and measurement of the electrical current at a constant applied voltage. The important point here is that, with decreasing diameter of the wire, the number of electron wave modes contributing to the electrical conductivity is becoming increasingly smaller by well-defined quantized steps. In electrically conducting carbon nanotubes, only one electron wave mode is observed which transport the electrical current. As the lengths and orientations of the carbon nanotubes are different, they touch the surface of the mercury at different times, which provides two sets of information: (i) the influence of carbon nanotube length on the resistance; and (ii) the resistances of the different nanotubes. As the nanotubes have different lengths, then with increasing protrusion of the fiber bundle an increasing number of carbon nanotubes will touch the surface of the mercury droplet and contribute to the electrical current transport. 22
  25. 25. Electrical conductivity of bulk metals is based on their electronic band structures, and the mobility of electrons is related to their mean free path between two collisions with the lattice. The collective motion of electrons in a bulk metal obeys Ohm’s law, V = RI, where V is the applied voltage, R is the resistance of the material and I is the current. As the electronic band structure changes into discrete energy levels, Ohm’s law is no longer valid. If one electron is transferred to a small particle, the Coulomb energy of the latter increases by E C = e^2 /2C, where C is the capacitance of the particle. If the temperature is low such that kT < e 2 /2C, single electron tunneling processes are observed.* * Thermal motion of the atoms in the particle can initiate a change in the charge and the Coulomb energy so that further electrons may tunnel uncontrolled Hence, the I-V characteristic of a quantum dot is not linear, but staircase-like. No current flows up to V C = ±e/2C. If this value is reached, an electron can be transferred. Following this, an electron tunnelling process occurs if the Coulomb energy of the particle is compensated by an external voltage of V = ±ne/2C. This behaviour is called Coulomb blockade. The charging energy increases with decreasing the size of the quantum dot. I-U characteristic of ideal single electron transport, where Coulomb blockade is shown as the step function. 23
  26. 26. Experimental approaches to measure the Coulomb blockade. Two metallic leads with spacing of a few nm are fabricated. An organic monolayer is then used to bind nanocrystals to the leads. When a nanocrystal bridges the gap between the leads, it can be electrically investigated. (a) Field emission scanning electron micrograph of a lead structure before the nanocrystals are introduced. The light gray region is formed by the angle evaporation and is 10 nm thick. The darker region is from a normal angle evaporation and is 70 nm thick. (b) Schematic cross section of nanocrystals bound via a bifunctional linker molecule to the leads. Transport between the leads occurs through the mottled nanocrystal bridging the gap. Schematic illustration of a single-electron tunnel junction formed by a nanocrystal held between the STM tip and the substrate. 24
  27. 27. • (a) I–V characteristic of an isolated 3.3 nm Pd nanocrystal (dotted • line) and the theoretical fit (solid line) obtained at 300 K using a • semiclassical model. (b) The size dependence of the charging energy. In voltammetric experiments in solution, metal nanoparticles behave as redox active molecules, showing redox cascades that are well known in inorganic and organometallic electrochemistry 25

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