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VI Semester B.Sc. Dr. K S Suresh Page 1
Nanomaterials
Introduction
Nanoscience is a phenomena of manipulation of materials at the atomic, molecular and macromolecular scales, where properties differ significantly from those at larger scales. Nanotechnology is the design, characterisation, production and application of structures, devices and systems by controlling shape and size at the nanometre scale. Nanomaterials are defined as particles in the form of crystals, rods, or spheres having size between 1 nm and 100 nm at least in one dimension. A nanometre is one billionth of a meter, or 10-^9 m. Materials in this range of size exhibit some remarkable specific properties. For example crystals in the nanometre scale have a low melting point and reduced lattice constants. Nanosystems display electronic, photochemical, electrochemical, optical, magnetic, mechanical or catalytic properties that differ significantly not only from those of molecular units, but also from those of macroscopic systems.
Physical properties that make nanomaterials different from bulk (macroscale) materials
- Due to smallness of nanomaterials, their mass is extremely small and gravitational forces become negligible, instead electromagnetic forces are dominant in determining the behaviour of atoms and molecules.
- For objects of very small mass, such as electrons, (wave – particle duality of matter) wave like nature has more pronounced effect. The position of electrons are represented by wave function. Quantum mechanics is used to describe motion and energy instead of classical mechanics.
- The consequence of this is the tunnelling. It is the penetration of an electron into an energy region that is classically forbidden. For a particle having less energy than the energy required to overcome a potential barrier, there is no probability of finding the particle on the other side of the barrier according to classical theory. But quantum mechanically, there is a finite probability of the particle tunnelling through the barrier. The condition for this to happen is that the thickness (energy potential) of the barrier must be comparable to the wavelength of the particle. This is observed at nanometre scale.
- Quantum confinement – In a nanomaterial, such as a metal, electrons are confined in space rather than free to move in the bulk of the material.
- Quantisation of energy – Electrons in a nanomaterial can exist at discrete energy levels.
- At nanoscale, the random motions are of same scale as the size of the material. This has an influence on how particle behave.
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- Increased surface to volume ratio – One of the distinguishing properties of nanomaterials is that they have increased surface area. This leads to unique properties of materials at nanoscale.
Synthesis of nanomaterials
There are two general approaches to the synthesis of nanomaterials and the fabrication of nanostructures. They are (1) top-down method of miniaturizing materials, (2) bottom-up method of building molecular structures atom by atom or molecule by molecule. The top-down approach has been advanced by Richard Feynman in 1959 lecture stating that “there is plenty of room at the bottom” and it is ideal for obtaining structures with long-range order and for making connections with macroscopic world. The bottom-up approach was pioneered by Jean-Marie Lehn (revealing that “there is plenty of room at the top”) and it is best suited for assembly and establishing short-range order at the nanoscale.
Top-down approach
This approach use larger (macroscopic) initial structures, which can be externally controlled in the processing of nanostructures. Typical examples are photolithography, etching through the mask, ball milling and application of severe plastic deformation.
- Top-Down: lithography - The most used top-down approach is photolithography. It has been used to manufacture computer chips and produce structures smaller than 100 nm. Typically, an oxidized silicon (Si) wafer is coated with a 1μm thick photoresist layer. After exposure to ultraviolet (UV) light, the photoresist undergoes a photochemical reaction, which breaks down the polymer by rupturing the polymer chains. Subsequently, when the wafer is rinsed in a developing solution, the exposed areas are removed leading to nanosize material. The other methods are electron-beam lithography and X – ray lithography.
In a ball milling process a powder mixture placed in the ball mill (a cylinder with steel
balls which are rotating at high speeds) is subjected to high-energy collision from the
balls. At the initial stage of ball milling, the powder particles are flattened by the
compressive forces due to the collision of the balls. Micro-forging leads to changes in the
shapes of individual particles, or cluster of particles being impacted repeatedly by the
milling balls with high kinetic energy.At the intermediate stage of the mechanical alloying
process, the intimate mixture of the powder constituents decreases the diffusion distance to the micrometre range. Fracturing and cold welding are the dominant milling processes at this stage. At the final stage of the mechanical alloying process, considerable refinement and reduction in particle size is achieved.
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Zero dimensional nanomaterials
Materials wherein all the dimensions are measured within the nanoscale (no dimensions, or 0-D, are larger than 100 nm). The most common representation of zero-dimensional nanomaterials are nanoparticles and quantum dots or nano-clusters - self-assembled nanoislands and chemically synthesized nanoparticles Nanoparticles can be amorphous or crystalline, be single crystalline or polycrystalline, be composed of single or multi-chemical elements, exhibit various shapes and forms, exist individually or incorporated in a matrix, be metallic, ceramic, or polymeric.
One-dimensional nanomaterials
One dimension is outside the nanoscale. This leads to needle like-shaped nanomaterials. 1 - D materials include nanotubes, nanorods, and nanowires. 1 - D nanomaterials can be amorphous or crystalline, single crystalline or polycrystalline, chemically pure or impure, standalone materials or embedded in within another medium, be metallic, ceramic, or polymeric. Typical example— carbon nanotube and silicon nanowires
Two-dimensional nanomaterials
Two of the dimensions are not confined to the nanoscale. 2 - D nanomaterials exhibit plate-like shapes. They include nanofilms, nanolayers, and nanocoatings. 2 - D nanomaterials can be amorphous or crystalline, made up of various chemical compositions, used as a single layer or as multilayer structures, deposited on a substrate, integrated in a
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VI Semester B.Sc. Dr. K S Suresh Page 5 surrounding matrix material, be metallic, ceramic, or polymeric. Typical example—semiconductor quantum wells.
Three-dimensional nanomaterials
Bulk nanomaterials are materials that are not confined to the nanoscale in any dimension. These materials are thus characterized by having three arbitrarily dimensions above 100 nm. Materials possess a nanocrystalline structure or involve the presence of features at the nanoscale. In terms of nanocrystalline structure, bulk nanomaterials can be composed of a multiple arrangement of nanosize crystals, most typically in different orientations. With respect to the presence of features at the nanoscale, 3 - D nanomaterials can contain dispersions of nanoparticles, bundles of nanowires, and nanotubes as well as multinanolayers.
Fullerenes
Fullerene is any molecule in the form of a hollow sphere, ellipsoid or tubular structure composed entirely of carbon. They are commonly referred to as “Buckyballs” – named after Buckminster Fuller who designed geodesic physical structures and buildings based on this geometry. Discovered in 1985 by Smalley, Curl and Kroto., it is the roundest and most symmetrical large molecule known to man. Using a laser to vapourise graphite rods in an atmosphere of helium gas, these chemists obtained cage like molecules composed of 60 carbon atoms joined together by single and double bonds to form a hollow sphere with 12 pentagonal and 20 hexagonal faces. The C 60 molecule undergoes a wide range of novel chemical reactions. It readily accepts and donates electrons, a behaviour that suggests applications in batteries and advanced electronic devices.
Graphene
It is one-atom-thick planar sheet of sp^2 - bonded carbon atoms that are densely packed in a honeycomb (hexagonal) crystal lattice. It can be viewed as an atomic-scale chicken wire made of carbon atoms and their bonds. The carbon-carbon bond length in graphene is about 0.142 nm. Graphene is the basic structural element of some carbon allotropes including graphite, carbon nanotubes and fullerenes. It has the ability to conduct electrons and is transparent. Those qualities make graphene a tantalizing alternative for use as a transparent conductor, the sort now found in everything from computer displays and flat panel TVs to ATM touch screens and solar cells.
Carbon nanotubes (CNT) - Graphene is the basic
structural building block of carbon nanotubes. Carbon nanotubes (CNT) also known as ‘buckytubes’ have a cylindrical
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VI Semester B.Sc. Dr. K S Suresh Page 7 of the quantum dot are therefore highly tunable. Because the size of the crystals can be controlled during synthesis, the conductive properties can be carefully controlled. Main applications: optical and optoelectronic devices, quantum computing, and information storage. Semiconductors with QDs as Material for Cascade Lasers. Semiconductors with QDs as Material for IR Photodetectors and Injection Lasers with QDs
Nanoparticles
Nanoparticles (NP) are synthesized or machined. They range in size from 2 nm to 100 nm. Nanoparticle materials vary depending on their application. Because Nanoparticles are invisible to the naked eye, they are usually supplied suspended in a liquid. The color is due to the refraction of light the surface area of the particular nanoparticle reflects. Different sized nanoparticles exhibit different colours based on its surface area.
Nanofibers and Nanowires
Nanofibers are slightly larger in diameter than the typical nanomaterial definition, though still invisible to the naked-eye. Their size ranges between 50 nm - 300 nm in diameter and are generally produced by electro spinning in the case of inorganic nanofibers or catalytic synthesis for carbon nanotubes. Nanofibers can be electrostatically aligned and biochemically aligned. Similar to nanofibers are nanowires, though nanowires are considerably smaller in diameter, of the order of 4 nm and conduct electricity.
Electron confinement or Quantum effects
The phenomenon of altering of a material’s electronic properties as it decreases in size is referred to as the quantum size effect. The overall behavior of bulk crystalline materials changes when the dimensions are reduced to the nanoscale. For 0 - D nanomaterials, where all the dimensions are at the nanoscale, an electron is confined in 3- D space. No electron delocalization (freedom to move) occurs. For 1 - D nanomaterials, electron confinement occurs in 2-D, whereas delocalization takes place along the long axis of the nanowire/rod/tube. In the case of 2 - D nanomaterials, the conduction electrons will be confined across the thickness but delocalized in the plane of the sheet. The effect of confinement on the resulting energy states can be calculated by quantum mechanics, as the “particle in the box” problem. An electron is considered to exist inside of an infinitely deep potential well (region of negative energies), from which it cannot escape and is confined by the dimensions of the nanostructure.
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VI Semester B.Sc. Dr. K S Suresh Page 8 The above expressions give the energies in different dimensional nanoparticles. In the expression h is Planck’s constant, m is the mass of the electron, L is the width (confinement) of the infinitely deep potential well, and nx, ny and nz are the principal quantum numbers in the three dimensions x, y, and z. The smaller the dimensions of the nanostructure (smaller L), the wider is the separation between the energy levels, leading to a spectrum of discreet energies.
Density of states – Energy levels
The density of states is the number of quantum states per unit energy. In other words, the density of
states, denoted by g E ( ), indicates how densely packed quantum states in a particular system. Consider
the expression g E dE ( ). Integrating the density of the quantum states over a range of energy will produce
a number of states. (^ )^ (^ )
E E
N E g E dE
=^ . Thus g E dE ( ) represents the number of states between
E and dE. (In the diagram D(E) dE = g(E)dE) The above diagrams show that as we move to 3 D from 0 D the
energy levels become discrete. The number of quantum states become important in the determination of optical properties of a material such as a semiconductor (i.e. carbon nanotubes or quantum dots).
Properties of nanomaterials
1. Surface properties : When a bulk material is subdivided into materials on the nano scale, the total volume remains the same but the collective surface area is increased. This results in increase of surface to volume ratio at nanoscale as compared to bulk materials. The ratio of surface area to volume of a material is given by 𝑎𝑟𝑒𝑎 𝑣𝑜𝑙𝑢𝑚𝑒
4 𝜋𝑟^2 4 3 𝜋𝑟
3 =^
3 𝑟
. Thus as the radius of a given material is decreased, its surface to
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VI Semester B.Sc. Dr. K S Suresh Page 10 magnetisation curves. Thus nanostructuring of bulk magnetic materials leads to changes in the curves which can produce soft or hard magnets with improved properties.
5. Mechanical properties
Mechanical properties of nanomaterials may reach the theoretical strength, which are one
or two orders of magnitude higher than that of single crystals in the bulk form. The
enhancement in mechanical strength is simply due to the reduced probability of defects.
Carbon nanotubes are 100 times stronger than steel but six times lighter.
Applications of nanotechnology in various fields:
Nanotechnology offers an extremely wide range of potential applications from electronics, optical communications and biological systems to new materials. Nanomaterials having wide range of applications in the field of electronics, fuel cells, batteries, agriculture, food industry, and medicines, etc... It is evident that nanomaterials split their conventional counterparts because of their superior chemical, physical, and mechanical properties and of their exceptional formability.
1. Fuel cells
A fuel cell is an electrochemical energy conversion device that converts the chemical energy from fuel (on the anode side) and oxidant (on the cathode side) directly into electricity. The heart of fuel cell is the electrodes. Microbial fuel cell is a device in which bacteria consume water-soluble waste such as sugar, starch and alcohols and produces electricity plus clean water. This technology will make it possible to generate electricity while treating domestic or industrial wastewater. Microbial fuel cell can turn different carbohydrates and complex substrates present in wastewaters into a source of electricity. The efficient electron transfer between the microorganism and the anode of the microbial fuel cell plays a major role in the performance of the fuel cell. The organic molecules present in the wastewater possess a certain amount of chemical energy, which is released when converting them to simpler introduction to Nanomaterials molecules like CO 2 The microbial fuel cell is thus a device that converts the chemical energy present in water-soluble waste into electrical energy by the catalytic reaction of microorganisms. Carbon nanotubes (CNTs) have chemical stability, good mechanical properties and high surface area, making them ideal for the design of sensors and provide very high surface area due to its structural network. Since carbon nanotubes are also suitable supports for cell growth, electrodes of microbial fuel cells can be built using of CNT. Due to three-dimensional architectures and enlarged electrode surface area for the entry of growth medium, bacteria can grow and proliferate and get immobilized. Multi walled CNT scaffolds could offer self-supported structure with large surface area through which hydrogen producing bacteria can eventually grow and proliferate.
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2. Catalysis Higher surface are available with the nanomaterial counterparts, nano-catalysts tend to have exceptional surface activity. For example, reaction rate at nano-aluminum can go so high, that it is utilized as a solid-fuel in rocket propulsion, whereas the bulk aluminum is widely used in utensils. Nano-aluminum becomes highly reactive and supplies the required thrust to send off pay loads in space. Similarly, catalysts assisting or retarding the reaction rates are dependent on the surface activity, and can very well be utilized in manipulating the rate-controlling step. 3. Phosphors for High-Definition TV The resolution of a television, or a monitor, depends greatly on the size of the pixel. These pixels are essentially made of materials called "phosphors," which glow when struck by a stream of electrons inside the cathode ray tube (CRT). The resolution improves with a reduction in the size of the pixel, or the phosphors. Nanocrystalline zinc selenide, zinc sulfide, cadmium sulfide, and lead telluride synthesized by the sol-gel techniques are candidates for improving the resolution of monitors. 4. Next-Generation Computer Chips The microelectronics industry has been emphasizing miniaturization, whereby the circuits, such as transistors, resistors, and capacitors, are reduced in size. By achieving a significant reduction in their size, the microprocessors, which contain these components, can run much faster, thereby enabling computations at far greater speeds. 5. Nanowires for junctionless transistors Transistors are made so tiny to reduce the size of sub assemblies of electronic systems and make smaller and smaller devices, but it is difficult to create high-quality junctions. In particular, it is very difficult to change the doping concentration of a material over distances shorter than about 10 nm. Researchers have succeeded in making the junctionless transistor having nearly ideal electrical properties. It could potentially operate faster and use less power than any conventional transistor on the market today. The device consists of a silicon nanowire in which current flow is perfectly controlled by a silicon gate that is separated from the nanowire by a thin insulating layer. The entire silicon nanowire is heavily n-doped, making it an excellent conductor. However, the gate is p-doped and its presence has the effect of depleting the number of electrons in the region of the nanowire under the gate. The device also has near-ideal electrical properties and behaves like the most perfect of transistors without suffering from current leakage like conventional devices and operates faster and using less energy. 6. Elimination of Pollutants
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