Nanotechnology Module 4 PDF

Nanotechnology Nano size: Generally 1-100 nm Nanoscale was seen to range from 1 to 1000 nm Particle size classification 1 nm Chemical drug 5 nm Protein 10 nm DNA 2...

Nanotechnology Nano size: Generally 1-100 nm Nanoscale was seen to range from 1 to 1000 nm Particle size classification 1 nm Chemical drug 5 nm Protein 10 nm DNA 20-50 nm Blood vessel pore 50 nm Carbon nanotube 100-500 nm Liposome nanoparticle 1000 nm (1 µm) bacteria 10 µm Cell 50 µm Human hair Titanium dioxide nano powder  a b c 1µm 0.5µm 0.5µm d e d f 0.5m 0.5m 0.2m  a b c  0.25 nm 20nm 5nm 2nm d 0.25nm 5nm 5555nm  Nanotechnology Is Not A New Phenomenon The Lycurgus Cup: 4th Century A.D. Green = Reflected Light Red = Transmitted Light Image of silver/gold nanoparticle in the Lycurgus cup The British Museum. http://www.thebritishmuseum.ac.uk/ (March 2004) 7 There’s Plenty of Room at the Bottom Richard Feynman, 1959 8  Quantum confinement effect-an overview The most popular term in the nano world is quantum confinement effect which is essentially due to changes in the atomic structure as a result of direct influence of ultra-small length scale on the energy band structure. The length scale corresponds to the regime of quantum confinement ranges from 1 to 25 nm for typical semiconductor groups of IV, III-V and II-VI. In which the spatial extent of the electronic wave function is comparable with the particle size. As a result of these “geometrical” constraints, electrons “feel” the presence of the particle boundaries and respond to changes in particle size by adjusting their energy. This phenomenon is known as the quantum-size effect. Quantization effects become most important when the particle dimension of a semiconductor near to and below the bulk semiconductor Bohr exciton radius which makes materials properties size dependent. where ε is the dielectric constant of the material, m* is the mass of the particle, m is the rest mass of the electron, and aо is the Bohr radius of the hydrogen atom.  Quantum confinement A particle behaves as if it were free when the confining dimension is large compared to the wavelength of the particle. During this state, the bandgap remains at its original energy due to a continuous energy state. However, as the confining dimension decreases and reaches a certain limit, typically in nanoscale, the energy spectrum turns to discrete. As a result, the bandgap becomes size dependent. This ultimately results in a blue shift in optical illumination as the size of the particles decreases. What happens when the size of the of the nanoparticle becomes smaller than the radius of the orbit of the electron – hole pair (exciton) Two situations : weak and strong confinement Weak confinement : The particle radius is larger than the radius of the electron hole pair. But the range of the motion of the exciton is limited causing a blue shift in the absorption spectrum rp > re,h Strong confinement: When the radius of the particle is smaller than the orbital radius of the electron hole pair, the motion of the electron hole pair becomes independent and the exciton does not exist. The hole and the electron have their own energy levels. rp < r e,h Quantum confinement in semiconductors In an unconfined (bulk) semiconductor, an electron-hole pair is typically bound within a characteristic length called the Bohr exciton radius. If the electron and hole are constrained further, then the semiconductor's properties change. This effect is a form of quantum confinement, and it is a key feature in many emerging electronic structures. Quantum confined semiconductors include: quantum wells, which confine electrons or holes in one dimension and allow free propagation in two dimensions. quantum wires, which confine electrons or holes in two spatial dimensions and allow free propagation in the third. quantum dots, which confine electrons in all three spatial dimensions  Structure Quantum Number of free confinement dimensions Quantum well Quantum well 1 2 Quantum wells are formed in semiconductors by having a material, like gallium arsenide sandwiched between two layers of a material with a wider bandgap, like aluminium arsenide. Because of their quasi-two dimensional nature, electrons in quantum wells have a sharper density of states than bulk materials. As a result quantum wells are in wide use in diode lasers, specifically blue lasers. They are also used to make HEMTs (High Electron Mobility Transistors), which are used in low-noise electronics. Quantum well infrared photodetectors are also based on quantum wells, and are used for infrared imaging.  Structure Quantum Number of free confinement dimensions Quantum wire Quantum wire 2 1 Quantum wires, which confine electrons or holes in two spatial dimensions and allow free propagation in the third. Carbon nanotubes is an excellent example of quantum wires. The advantages of making wires from carbon nanotubes include their high electrical conductivity (due to a high mobility), light weight, small diameter, low chemical reactivity, and high tensile strength. It has been claimed that it is possible to create macroscopic quantum wires. With a rope of carbon nanotubes, it is not necessary for any single fiber to travel the entire length, since quantum tunneling will allow electrons to jump from strand to strand. This makes quantum wires interesting for commercial uses.  Structure Quantum Number of free confinement dimensions Quantum dot 3 0 Quantum dots A quantum dot is a semiconductor whose excitons are confined in all three spatial dimensions. As a result, they have properties that are between those of bulk semiconductors and those of discrete molecules. Different sized quantum dots emit different color light due to quantum confinement. An immediate optical feature of colloidal quantum dots is their coloration. While the material which makes up a quantum dot defines its intrinsic energy signature, the quantum confined size of the nanocrystal is more significant at energies near the band gap. Thus quantum dots of the same material, but with different sizes, can emit light of different colors. The physical reason is quantum confinement effect. The larger the dot, the redder (lower energy) its fluorescence spectrum. Conversely, smaller dots emit bluer (higher energy) light. The coloration is directly related to the energy levels of the quantum dot. Properties of nanoparticles or specialty of nanotechnology Why so special? Very high surface to volume ratio compared to bulk Surface free energy is changed and chemical potential is modified. Quantum effect of the charge particles Examples of some improvement of material properties Quantum confinement in semiconductor nanoparticles increases the band gap and as a result different fluorescent colors comes from such nanoparticles for different band gap Using one dimensional confinement in semiconductor nanoparticles quantum well laser brings much more efficient laser Pd being nanoparticles can occlude huge volume of hydrogen which is very significant for hydrogen storage devices Gold particles being nano in nature the thermodynamic properties is influenced greatly and the melting temperature reduces The storage capacity in computer hard disk has been increased using ferromagnetic nanoparticles E