quantum dots and forms of nanomaterials.ppt

quantum dots and forms of nanomaterials.ppt

• 1.
  DIFFERENT FORMS OFNANOMATERIALS DEFINITION A material with at least one of its dimension in the order of few nanometers is known as nanomaterial. Based on the number of electron confinement directions , their individual shapes and size reduction in various directions, nano materials can be divided into the following classes  Two dimensional nanomaterials  One dimensional nanomaterials Zero dimensional nanomaterials
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  DIFFERENT FORMS OFNANOMATERIALS Two dimensional (2D) materials one dimension at nanoscale, other two dimension at macroscale.  Hence electrons are confined in one dimensional and are free to move in two dimensions parallel to the structure. Definition: Materials whose thickness is at nano scale and length and breadth are at macro scale are known as two dimensional nanomaterials. Example: Quantum well, thin films, nano coatings, nano layers.
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  DIFFERENT FORMS OFNANOMATERIALS One dimensional (1D) materials two dimensions at nanoscale, other one dimension at macroscale.  Hence electrons are confined in two dimensions and are free to move in one dimension along the structure. Definition: Materials whose diameter is at nanoscale and length typically at macro scale. Example: Quantum wire cylinder, nano wires, nanofibers.
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  DIFFERENT FORMS OFNANOMATERIALS Zero dimensional (0D) materials All dimensions at nanoscale, hence electrons are confined in all directions Definition: Materials in which electron motion is confined in all the three dimension are called zero –dimensional nanomaterials. Example: Quantum dot, particles, hollow spheres microcapsules.
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  Forms of Nanomaterials
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  Quantum dots • Quantumdots (QDs) are tiny semiconductor particles 2-10 nm (nanometers, 10^-9) in diameter. Because of their small size, these particles have unique optical and electrical properties. For example, when exposed to light, quantum dot crystals emit light of particular frequencies. • The size and shape of quantum dots can be precisely controlled by adjusting reaction time and conditions, thus making this nanotechnology scalable and useful for display applications.
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  Quantum dots - Working Theprocess of light emission in QDs is called photoluminescence (abbreviated as PL), as it occurs because of the excitation by photons. Under the influence of light, photons get excited and “jump” up to a higher energy band. This is followed by the process of relaxation, during which photons can relax non-radiatively (“fall back”) into a lower-lying state or recombine and re-radiate.
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  Quantum dots –Unique property In regular semiconductors like silicon (also known as bulk matter), the bands are formed by the merger of adjacent energy levels of a very large number of atoms and molecules. However, as the particle size reaches the nano-scale and the quantity of atoms and molecules decreases substantially, the number of overlapping energy levels decreases, causing the width of the band to increase. As QDs are so tiny, they have a higher energy gap between the valence and conduction bands, compared to the bulk matter.
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  Quantum confinement effect Quantumconfinement effect is the change in the atomic structure of the particle observed when the energy band is affected by the shifts in the electronic wave range. Because the wave range is comparable to the particle's size, electrons are constrained by the wavelength boundaries. Hence, quantum dots' properties are size-dependent, and their excitations are confined in all three spatial dimensions. Confinement energy is the key property of a quantum dot that explains the relationship between QDs size and the frequency of light they emit
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  Quantized (or discrete)electronic states of QD • Because of the small size of QD particles, the quantum confinement effect causes a large band gap with observable discrete energy levels. Such quantized energy levels in quantum dots lead to electronic structures that are in between single molecules, which have a single gap, and bulk semiconductors, which have continuous energy levels within bands
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  Unique properties ofquantum dots - caused by their unusually high surface to volume ratio As the size of the crystal decreases, the difference in energy between the highest valence band and the lowest conduction band increases. More energy is then needed to excite the dot, and at the same time, more energy is released when the quantum dot returns to its original relaxed state. Because of this phenomenon, quantum dots can emit any color of light from the same material if their size is altered.