QUANTUM DOTS

Click here to load reader

  • date post

    17-Feb-2017
  • Category

    Documents

  • view

    118
  • download

    0

Embed Size (px)

Transcript of QUANTUM DOTS

TitleQUANTUM DOTS: - Introduction & ExperimentsName: - Aakash Chandrakant MhankaleCWID: - 803000470======================================================Introduction:-Quantum dots are tiny semiconductor crystals this means these can conduct or resist depending upon the temperature and impurity of the semiconductor. Quantum dots ranges is from 2 to 10 Nano meters. Because of the size of the physics is governed by the quantum mechanics.Important properties of quantum dots depends on the following factors: -Size, Energy Levels

By controlling these factors we can use quantum dots for verity of applications. Emission spectrum:-the color depends on the size of quantum dots and not on the material used. Bigger the quantum dots larger the wave length and smaller the frequency. Color depends on the wave length this means the largest quantum dots emits red light spectrum while the smallest quantum dots emits blue light and other colors are between this is the color changing phenomenon of quantum dots. The band gap is different in different size of quantum dots. The band gap of semiconductor is the energy required to enter the exited state. Small QD has large band gap so lots of energy is required to excite the QD i.e. high energy is required that leads to high energy frequency and so small wave length of light. For small dot it is blue and for large dot it is red spectrum.

The fig. Shows the relation between the frequency and Color.Optical properties

In case of semiconductors, when light gets absorbed it generally tends an electron gets excited from valence to the conduction band, hole is left behind. Exciton recombines the excitons energy is emitted as light, this phenomenon is calledFluorescence. In simple words, the energy produced because of the emitted photon is equal to addition of the band gap energy between the highest level and the lowest energy level.

Confinement energy directly depends on the quantum dots size, absorption& fluorescence emission tuned by changing the size of the quantum dot. If the dot is larger it isreddish(i.e.lower energy) its absorption andfluorescencecolour spectrum. Contrariwise, smaller dots absorb and emitbluer(i.e. higher energy) light. Further, the lifetime of fluorescence is determined by the size of the quantum dot. Larger dots are closely spaced. So, electron-hole pairs in larger dots live longer producing larger dots to show a longer lifetime.For enlightening fluorescence quantum, quantum dots are invented that has larger bandgap semiconductor material around them. Band gap energy

Where abis the, m is the mass, and ris the size-dependent dielectric constant (Relative permittivity), Bohr radius=0.053nm, is the reduced mass,

Why Quantum Dots?

Unique properties of QD are used in the many applications such as Medical imaging, light emitting displays, photovoltaic cells. QD includes electrical and nonlinear optical properties. These properties are partly result of high surface to volume ration. The properties make QD important for the electronics industry.

The average distance between an electron and a hole in an exciton is called the Excited Bohr Radius. When the size of the semiconductor falls below the Bohr Radius, the semiconductor is called a quantum dot.

Quantum dots can be simulated by the Ultra Violate rays or electricity. This property makes QD ideal for using solar cell. The verity in size of quantum dots used to synthesize the solar cell which results in greater energy absorption. The photons from the light travels into the cell and striking the quantum dots particles which will raise the energy of electrons in the quantum dots. Electrons will get injected into titanium dioxide and travel through it to the conductive surface of the electrodes and leave holes in the quantum dots. Quantum dots takes electrons from electrolyte and the electron deleted electrolyte in turn takes electron from the counter electrode and this will create a voltage across the cell.

Theoretically, this could boost solar power efficiency from 20-30% to as high as 65%.

So in short, the appropriate area of the solar spectrum absorbs the sunlight and it will tune the band gap of the semiconductor. Further by tuning the size of QD it allows the engineers to enhance the performance of the device. As the performance of the device is increasing by the process then it is the low cost and high-performance devices will make a high impact in energy conversion industry across the world.

Quantum dots used in vivid colour efficient display. Some companies are already working on it and used for flexible screen and vibrant colours. Quantum Dots are used in medical images for high clarity, but still lot of research to be done to see QD as a medical use.

Medical Imaging: - Quantum Dots are useful for monitoring cancerous cells and provides a way to find out the root cause of the cancer. QDs are much more resistant to degradation than other optical imaging probes such as organic dyes, allowing them to track cell processes for longer periods of time.

Quantum Dots in Television Industry

Conventional TV:

Standard LED has a blue backlight and on it has yellow layer of phosphate added to them which make the blue light to change from blue to white. Further, it is filtered through red, green and blue sub pixels at varying intensity and creating pixels in entire area but the performance of white light for Red colour is not that good this is where the Quantum Dots LCD comes into the picture.

Quantum dots Liquid Crystal Display also uses blue as a back light but instead of using yellow phosphate layer to create white light it uses green n red Nano particles which are from 1 to 4nm and they actually emits light rather than just filtering. The blue backlight mixes with the Green and Red to make more bright and clear white light and it efficiently passed thought sub pixels to filter, which allow engineers to create screens which require less operational energy to achieve more vibrant colours. AS QDLCD operates at low energy cost the production companies are interested in this technology. As the cost of QDLCD is or 1/3rd of the OLED TVs.

What are the benefits of quantum dots in Television industry?The tune ability of QDs gives them the ability to emit nearly any frequency of light - a traditional LED lacks this ability.

Higher peak brightnessBetter colour accuracy: - The light produced by QD is closely tied with size so that they can accurately emit the exact kind of light which is required.

Higher colour saturation: - On OLED screen the colours will pop more due to huge colour gamut OLED screen whereas the quantum dots can increase the colour gamut on LCD screen by 40 to 50%.What are the downsides of quantum dots?As there are many difficulties in integrating quantum dots into screen from users point of view there is only one downside seen so far but it is serious the issue is light bleed issue. The reader on kindle HDX which was 1st quantum dots tablet the content was not visible in many condition. It was bleeding blue light instead of white because the backlight is blue in QD. Solving this issue is very important as the money factor is involved in this. Apple is currently working on solution.Nanocrystal displays are 30% more in visible spectrum and using only 335 to 60% less power than LCDs. Further, blue quantum dots require timing control during the reaction, because blue quantum dots are somewhat above the minimum size. Sunlight contains approx. equal luminosities of red, green and blue, a display needs to harvest approximately equal luminosities of red, blue and green.How quantum dots are made?Colloidal Synthesis: This method can be used to create large numbers of quantum dots all at once. Additionally, it is the cheapest method and is able to occur at non-extreme conditions.Electron-Beam Lithography: A pattern is etched by an electron beam device and the semiconducting material is deposited onto it. Electrons are accelerated out of an electron gun and sent through condenser lens optics directly onto a wafer = (12.3 / V) Advantages: generation of micron and submicron resist geometries greater depth of focus than optical lithography masks are unnecessary Optical diffraction limit is not a real concern

Electrons are accelerated out of an electron gun and sent through condenser lens optics directly onto a wafer. = (12.3 / V).

Disadvantage(s):

The lithography is serial (masks arent used; instead the beam itself sweeps across the wafer) Comparatively it has low throughput ~5 wafers per hour at less than 1 micrometer resolution The proximity effect: Electrons scatter because they are relatively low in mass, reducing the resolution. Molecular Beam Epitaxy: Molecular beam epitaxy (MBE) is the deposition of one or more pure materials onto a single crystal wafer one layer of atoms at a time in order to form a perfect crystal. A thin layer of crystals can be produced by heating the constituent elements separately until they begin to evaporate; then allowing them to collect and react on the surface of a wafer.

MBE system consist of: a growth chamber a vacuum pump a effusion (Knudsen) cells a manipulator and substrate heater an in-situ characterization tool RHEED (reflection high energy electron diffraction)

Generally the quantum dots are prepared by the chemical reaction in solution resulting in solid Nano crystals. Chemicals combined to heat at 255 C the more the reaction time it will affects the size of the crystal. As soon as the mixture is out of the heat the particles will retain their size and color.Many semiconductor materials can be used to create Quantum Dots. For example Cadmium Selenide, Lead Selenide, Indium Arsenic Cadmium Sulfide , Lead Sulfide, Indium Phosphorus But, these are the heavy metals and