Quantum dots ppt

By… MEENU CHAUDHARY C08533 Quantum Dots

Introduction Quantum dots are semiconductors whose excitons are confined in all three dimensions of space. Quantum dots have properties combined between Those of bulk semiconductors Those of atoms Different methods to create quantum dots. Multiple applications.

Outline Quantum Confinement and Quantum Dots Fabrication of Quantum Dots Quantum Dot Applications

Bulk Semiconductors Electrons in conduction band (and holes in the valence band) are free to move in all three dimensions of space. B.E.A. Saleh, M.C. Teich. Fundamentals of Photonics . fig. 16.1-10 and 16.1-29.

Thin Film Semiconductors Electrons in conduction band (and holes in the valence band) are free to move in two dimensions. Confined in one dimension by a potential well. Potential well created due to a larger bandgap of the semiconductors on either side of the thin film. Thinner films lead to higher energy levels.

Quantum Wire 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.

Quantum Dot 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.

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 μ m. Different shapes (cubes, spheres, pyramids, etc.) Quantum Dot

Discrete Energy Levels The energy levels depend on the size, and also the shape, of the quantum dot. Smaller quantum dot: Higher energy required to confine excitons to a smaller volume. Energy levels increase in energy and spread out more. Higher band gap energy. .

CdSe Quantum Dot 5 nm dots: red 1.5 nm dots: violet

How to Make Quantum Dots There are three main ways to confine excitons in semiconductors: Lithography Colloidal synthesis Epitaxy: Patterned Growth Self-Organized Growth

Lithography 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.

Colloidal Synthesis 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”).

Epitaxy: Patterned Growth 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 (SiO 2 ) and etching that mask with the shape of the required crystal cell wall shape. Disadvantage: density of quantum dots limited by mask pattern.

Epitaxy: Self-Organized Growth 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.

Cadmium-free quantum dots “CFQD” 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 .

What is it good for??? 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.

Applications Photovoltaic devices: solar cells Biology : biosensors, imaging Light emitting diodes: LEDs Quantum computation Flat-panel displays Memory elements Photodetectors Lasers

Solar Cells Photovoltaic effect: p-n junction. Sunlight excites electrons and creates electron-hole pairs. Electrons concentrate on one side of the cell and holes on the other side. Connecting the 2 sides creates electricity.

Different Generations of Solar Cells First generation: Single crystal silicon wafer. Advantages: high carrier mobility. Disadvantages: most of photon energy is wasted as heat, expensive. Second generation: Thin-film technology. Advantages: less expensive. Disadvantages: efficiency lower compared with silicon solar cells. Third generation: Nanocrystal solar cells. Enhance electrical performances of the second generation while maintaining low production costs.

Solar Cells Efficiency What limits the efficiency: Photons with lower energy than the band gap are not absorbed. Photons with greater energy than the band gap are absorbed but the excess energy is lost as heat.

The quantum dot band gap is tunable and can be used to create intermediate bandgaps. The maximum theoretical efficiency of the solar cell is as high as 63.2% with this method. How Can Quantum Dots Improve the Efficiency?

BIOLOGY: LOCATING CANCER CELL This picture shows silicon quantum dots fluorescing inside cancer cells.

. These quantum dots can be put into single cells, or lots of cells, in the tissue of living organisms. In future, it is planned to attach specific antibodies to the quantum dots – when injected into a body, the quantum dots will find and bind to cancer cells, and illuminate them when they fluoresce.

TARGETED DRUG DELIVERY In this we attach drug molecules to the quantum dots, which will then be able to deliver the drug just to the cancer cells where it is needed. Current anti-cancer drugs tend to have a range of unpleasant side-effects, because they affect the whole body, not just the cancer.

Anti-counterfeiting From consumer goods like music and software, to critical products like drug shipments, and even currency itself, quantum dots provide a method of creating unique, optical barcodes: the precise combinations of wavelengths of light emitted by complex combinations of different quantum dot. Embedded in inks, plastic, glass, and polymers, quantum dots are invisible to the naked eye and impossible to counterfeit.

SENSORS The properties of quantum dots are such that they can be functionalized to emit light on binding to a “target” molecule”, which enables them to be used, for example, to detect specific airborne pollutants.

Conclusion Quantum dot: Semiconductor particle with a size in the order of the Bohr radius of the excitons. Energy levels depend on the size of the dot. Different methods for fabricating quantum dots. Lithography Colloidal synthesis Epitaxy Multiple applications.

Quantum dots ppt

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