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----------------- - 1733 - Quantum Dots and Valleytronics
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- Kindle Fire HDX has a tablet display with brilliant colors, while consuming far less energy. The secret is in “ quantum dots,” manufactured by “ Nanosys”, based in Milpitas , California.
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- The invention started 20 years ago when a lab synthesized spherical nano crystals made from a cadmium selenide core and a cadmium sulfide shell. The small spheres, “ dots”, emit pure light of almost any color, depending on the size of the dot.
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- This technology has spread far beyond tablet displays into computers, TV’s, medical imaging, and ,soon, solar cells.
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- Nanosys’ 100 employees produces 25 tons of quantum dots per year. These dots are inorganic nano crystals made from semiconductors. The color emission lines are exceedingly narrow, 30 nanometers. Therefore, each emission line is a very pure color. Brilliant greens, rich reds, bold blues, ideal for color displays. The wavelength of red light is 700 nanometers, blue light 400 nanometers, so the entire color spectrum is little over 300 nanometers wide. These colors are 30 nanometers wide around each color frequency.
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- Most displays today are made from LCDs , liquid crystal displays. These liquid crystals are essentially “ filters.” A white light is shined through a series of filters to produce a spectrum of colors. This process wastes a lot of energy, even though LCDs are considered very energy efficient compared to incandescent lighting.
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- Quantum dots have a photon conversion efficiency very close to 100%. The total effect for a typical display is 20% more energy efficiency. A display uses half the power of a tablet so 20% is a big deal for extending battery life.
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-Quantum Dot technology is used in luminescent testing for biological tissues. Another exciting medical application has to do with brain research. Quantum dots may be sensitive enough to measure neuron-transmitters in the brain.
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- Another application that becomes obvious is with solar cells. Quantum dots may be used as light concentrators to greatly improve the efficiency of solar cells.
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- The diameters of Quantum Dots range from 2 to 10 nanometers. That is the width of 10 to 50 atoms.
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- The color of the light is a function of quantum confinement. The confinement is due to electrostatic potentials between the interfaces of semiconductor materials.
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- The smaller the dot the higher the energy and the intensity of the light. Dots have been manufactured that can emit the entire light spectrum including infrared and ultraviolet above and below visible light frequencies. The larger the dot the redder the light. The smaller the dot the bluer the light and the higher the energy.
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- LED manufacturing has become so simple you could almost do the manufacturing in your garage. Someday they may be used for all-optical switching and logic gates in computers that could operate up to 15 terabits per second. (10^12) The internet today usually handles 10 megabits (10^6), a 1,000,000 time slower.
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- Billions of quantum dots can set on the head of a pin.
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- Operating speeds of 1 picosecond (10^-12)or less are expected. That is fast but there is still another technology that is even faster, 50 femtoseconds. (10^-15). That is 1 million times faster than a nanosecond (10^-9).
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- This new technology is called Valleytronics. It only requires a single particle per logic state. An electron orbits at different energy levels around a nucleus. The lowest orbit, lowest energy level , is called the “ ground state”. Adding energy to the electron boosts it to higher orbits, higher energy levels. These levels are discrete jumps, quantum leaps, they are not continuous, but discrete steps.
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- There is a similar technology being developed called “ Spintronics” which can store a logic 1 or 0 in a quantum spin number. Electrons have energy levels, spin, and angular momentum quantum properties. “ Valleytronics” stores information in a quantum valley number. Both these changes in states require very little energy.
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- In contrast conventional electronics stores states of high and low electric currents. Keeping information in a particular state requires much more electric power. More power means more heat. So heat becomes a limit for higher and higher computing speeds.
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- The ground states in different materials are different. Some materials have multiple different “valley states” at the ground state energy levels. A diamond , for instance, has 6 possible valleys of minimum energy ground states. Getting between valley states can be controlled using laser pulses to simulate tiny magnetic fields.
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- Getting electrons to jump between valley states is called the “ Optical Stark Effect”. The technical description is using circularly polarized femtosecond light pulses to selectively control the valley degrees of freedom. A mouth full, but, worth it for ultra fast manipulation of valley Exitons in quantum information applications, Another mouth full, but, 50 femtosecond speeds in quantum computing would really change our life. Computing power 1 million times faster. That is faster than your imagination can take you., stay tuned, there is still more to learn.
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