Quantum dots 1
- 1. PRESENTED BY: ASHRAF ALI DEPARTMENT OF NANOTECHNOLOGY NEHU,SHILLONG
- 2. Quantum dots (QD) are nanoparticles/structures that exhibit 3 dimensional quantum confinement, which leads to many unique optical and transport properties. Quantum dots are usually regarded as semiconductors by definition Similar behavior is observed in some metals. Therefore, in some cases it may be acceptable to speak about metal quantum dots. QDs are band gap tunable by size which means their optical and electrical properties can be engineered to meet specific applications
- 3. Quantum Confinement is the spatial confinement of electron-hole pairs (excisions) in one or more dimensions within a material. ◦ 1D confinement: Quantum Wells ◦ 2D confinement: Quantum Wire ◦ 3D confinement: Quantum Dot Quantum confinement is more prominent in semiconductors because they have an energy gap in their electronic band structure. Metals do not have a band gap, so quantum size effects are less prevalent. Quantum confinement is only observed at dimensions below 2 nm
- 4. Nanocrystals (2-10 nm) of semiconductor compounds Small size leads to confinement of excitons (electron-hole pairs) Quantized energy levels and altered relaxation dynamics Examples: CdSe, PbSe, PbTe, InP Absorption and emission occur at specific wavelengths, which are related to QD size
- 5. Electrons in conduction band (and holes in the valence band) are free to move in all three dimensions of space
- 6. Electrons in conduction band (and holes in the valenceband) are free to move in two dimensions. Confined in one dimension by a potential well. Potential well created due to a larger bandgap of thesemiconductors on either side of the thin film. Thinner films lead to higher energy levels.
- 7. Thin semiconductor wire surrounded by a material with a larger bandgap. Surrounding material confines electrons and holes in two dimensions (carriers can only move in one dimension) due to its larger bandgap. Quantum wire acts as a potential well.
- 8. Electrons and holes are confined in all three dimensions of space by a surrounding material with a larger bandgap. Discrete energy levels (artificial atom). A quantum dot has a larger bandgap. Like bulk semiconductor, electrons tend to make transitions near the edges of the bandgap in quantum dots.
- 9. Very small semiconductor particles with a size comparable to the Bohr radius of the excitons (separation of electron and hole). Typical dimensions: 1 – 10 nm Can be as large as several μm. Different shapes (cubes, spheres, pyramids, etc.)
- 10. The energy levels depend on the size, and also the shape, of the quantum dot. Smaller quantum dot: 1. Higher energy required to confine excitons to a smaller volume. 2. Energy levels increase in energy and spread out more. 3. Higher band gap energy.
- 11. 5 nm dots: red 1.5 nm dots: violet
- 12. There are three main ways to confine excitons in semiconductors: 1. Lithography 2. Colloidal synthesis 3. Epitaxy: a) Patterned Growth b) Self-Organized Growth
- 13. Quantum wells are covered with a polymer mask and exposed to an electron or ion beam. The surface is covered with a thin layer of metal, then cleaned and only the exposed areas keep the metal layer. Pillars are etched into the entire surface. Multiple layers are applied this way to build up the properties and size wanted. Disadvantages: slow, contamination, low density, defect formation.
- 14. Immersion of semiconductor microcrystals in glass dielectric matrices. Taking a silicate glass with 1% semiconducting phase (CdS, CuCl, CdSe, or CuBr). Heating for several hours at high temperature. Formation of microcrystals of nearly equal size. Typically group II-VI materials (e.g. CdS, CdSe) Size variations (“size dispersion”).
- 15. Semiconducting compounds with a smaller bandgap (GaAs) are grown on the surface of a compoundwith a larger bandgap (AlGaAs). Growth is restricted by coating it with a masking compound (SiO2) and etching that mask with the shape of the required crystal cell wall shape. Disadvantage: density Of quantum dots limited bymask pattern.
- 16. Uses a large difference in the lattice constants of the substrate and the crystallizing material. When the crystallized layer is thicker than the critical thickness, there is a strong strain on the layers. The breakdown results in randomly distributed islets of regular shape and size. Disadvantages: size and shape fluctuations, ordering
- 17. In many regions of the world there is now, or soon to be, legislation to restrict and in some cases ban heavy metals in many household appliances such as IT & telecommunication equipment, Lighting equipment , Electrical & electronic tools, Toys, leisure & sports equipment. For QDs to be commercially viable in many applications they MUST NOT CONTAIN cadmium or other restricted elements LIKE mercury, lead, chromium. So research has been able to create non-toxic quantum dots using silicon.
- 18. Very narrow spectral line width, depending on the quantum dot’s size. Multiplexed detection Large absorption coefficients across a wide spectral range. Small size / high surface-to-volume ratio. Very high levels of brightness. Blinking.
- 19. Photovoltaic devices: solar cells Biology : biosensors, imaging Light emitting diodes: LEDs Quantum computation Flat-panel displays Memory elements Photodetectors Lasers
- 20. THANK YOU