Presentation Quantum dots

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QUANTUM DOTS

Presented by

Abhisek Banerjee Bishan Mukherjee Somaditya Indu Suman Roy

2Contents

What are Quantum Dots? Bohr exciton radius and quantum

Confinement Why Quantum Dots? Uniqueness of Q Dots Various Fabrication Processes Properties Applications and Uses Future Technologies Acknowledgements

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What are Quantum Dots??

A quantum dot is a semiconductor whose excitons are confined in all three spatial dimensions. As a result, they have properties that are between those of bulk semiconductors and those of discrete molecules.

A crystal of semiconductor compound (eg. CdSe, PbS) with a diameter on the order of the compound's Exciton Bohr Radius. Quantum dots are between 2 and 10 nanometers wide (10 and 50 atoms).

An electromagnetic radiation emitter with an easily tunable bandgap.

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Continued…

In an unconfined (bulk) semiconductor, an electron-hole pair is typically bound within a characteristic length called the Bohr exciton radius. If the electron and hole are constrained further, then the semiconductor's properties change. This effect is a form of quantum confinement, and it is a key feature in many emerging electronic structures.The Quantum dot is such an electronic structure which is based on the principle of Quantum confinement.

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Artificial Atoms

Quantum Dots are more closely related to individual atoms rather than bulk materials because of their discrete quantized energy levels instead of energy bands. Therefore they are also known as artificial atoms.

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Contents What are Quantum Dots? Bohr exciton radius and quantum

Confinement Why Quantum Dots? Uniqueness of Q Dots Various Fabrication Processes Properties Applications and Uses Acknowledgements

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Quantum Confinement

Excitons have an average physical separation between the electron and hole, referred to as the Exciton Bohr Radius this physical distance is different for each material. In bulk, the dimensions of the semiconductor crystal are much larger than the Exciton Bohr Radius, allowing the exciton to extend to its natural limit. However, if the size of a semiconductor crystal becomes small enough that it approaches the size of the material's Exciton Bohr Radius, then the electron energy levels can no longer be treated as continuous - they must be treated as discrete, meaning that there is a small and finite separation between energy levels. This situation of discrete energy levels is called quantum confinement .

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Why Q Dots? Their optical and

electronic qualities are costly to adjust, because their bandgap cannot be easily changed. Their emission frequencies cannot be easily manipulated by engineering. Q Dots exist in a quantum world, where properties are modulated according to needs.

Technological advancements have made it possible to make semiconductors with tunable bandgaps, allowing for unique optical and electronic properties and a broad range of emission frequencies.

Traditional semiconductors have shortcomings, they lack versatility.

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Quantum Dots - A tunable range of energies

Because quantum dots' electron energy levels are discrete rather than continuous, the addition or subtraction of just a few atoms to the quantum dot has the effect of altering the boundaries of the bandgap.

Changing the geometry of the surface of the quantum dot also changes the bandgap energy, owing again to the small size of the dot, and the effects of quantum confinement.

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Size Dependent Control of Bandgap in

Quantum Dots The bandgap in a quantum dot will always be energetically

larger; therefore, we refer to the radiation from quantum dots to be "blue shifted" reflecting the fact that electrons must fall a greater distance in terms of energy and thus produce radiation of a shorter, and therefore "bluer" wavelength.

The quantum Dot allows us to control its band gap by adjusting its size hence controlling the output wavelength with extreme precision

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Fabrication Of Quantum Dots

Q Dots can be synthesized in different ways, these are -----

Colloidal Synthesis:Three components precursors, organic

surfactants, and solvents In this form of synthesis precursor

molecules are dissolved in solvent.Solution is then heated at large

temperature to start creating monomers. Once the monomers reach

a high enough supersaturation level, the nanocrystal growth starts

with a nucleation process by rearranging and annealing of atoms.

For this process the temperature control is necessary.

And is done via heat or laser.

Due to strong quantum confinement, the nanocrystals

show size-tunable absorption and luminescence.

By control of the surface chemistry, we produced

photochemically stable nanocrystals

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Fabrication Continued…

Viral Assembly: In 2002 it was found that using genetically

engineered M13 bacteriophage virusesQ Dots can be created. It is

known that viruses can recognize specific semiconductor surfaces

Through the method of selection by combinatorial phage display.

Therefore using this property and controlling the solution ionic

strength and by applying outside magnetic field we can create

nanocrystals in a controlled environment.

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Fabrication Continued…..Fabrication Continued…..

Electrochemical Assembly: Highly ordered arrays of quantum dots may also be self-assembled by electrochemical techniques. A template is created by causing an ionic reaction at an electrolyte-metal interface which results in the spontaneous assembly of nanostructures, including quantum dots, onto the metal which is then used as a mask for mesa-etching these nanostructures on a chosen substrate.

Cadmium-free quantum dots “CFQD”: In many regions of the world there is now a restriction or ban on the use of heavy metals in many household goods which means that most cadmium based quantum dots are unusable for consumer-goods applications. A range of restricted, heavy metal-free quantum dots has been developed showing bright emissions in the visible and near infra-red region of the spectrum and have similar optical properties to those of CdSe quantum dots.

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Properties

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PropertiesProperties

Quantum Dots - Tunable Emission Pattern An interesting property of quantum dots is that the

peak emission wavelength is independent of the wavelength of the excitation light, assuming that it is shorter than the wavelength of the absorption onset. The bandwidth of the emission spectra, denoted as the Full Width at Half Maximum (FWHM) stems from the temperature, natural spectral line width of the quantum dots, and the size distribution of the population of quantum dots within a solution or matrix material.

Spectral emission broadening due to size distribution is known as inhomogeneous broadening and is the largest contributor to the FWHM. Narrower size distributions yield smaller FWHM. For CdSe, a 5% size distribution corresponds to ~ 30nm FWHM.

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Properties

Colloidally prepared quantum dots are free floating and can be attached to a variety of molecules via metal coordinating functional groups. These groups include but are not limited to thiol, amine, nitrile,phosphine, phosphine oxide, phosphonic acid, carboxylicacid or others ligands. This ability greatly increases the flexibility of quantum dots with respect to the types of environments in which they can be applied. By bonding appropriate molecules to the surface, the quantum dots can be dispersed or dissolved in nearly any solvent or incorporated into a variety of inorganic and organic films. In addition, the surface chemistry can be used to effectively alter the properties of the quantum dot, including brightness and electronic lifetime.

Quantum Dots - Molecular Coupling

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PropertiesProperties

Quantum Dots- Tunable Absorption Pattern

In addition to emissive advantages, quantum dots display advantages in theirabsorptive properties. In contrast to bulk semiconductors, which display a rather uniform absorption spectrum, the absorption spectrum for quantum dots appears as a series of overlapping peaks that get larger at shorter wavelengths. Owing once more to the discrete nature of electron energy levels in quantum dots, each peak corresponds to an energy transition between discrete electron-hole (exciton) energy levels. The quantum dots will not absorb light that has a wavelength longer than that of the first exciton peak, also referred to as the absorption onset. Like all other optical and electronic properties, the wavelength of the first exciton peak (and all subsequent peaks) is a function of the composition and size of the quantum dot. Smaller quantum dots result in a first exciton peak at shorter wavelengths.

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Properties Optical An immediate optical feature of

colloidal quantum dots is their coloration. While the material which makes up a quantum dot defines its intrinsic energy signature, the nanocrystal's quantum confined size is more significant at energies near the band gap. Thus quantum dots of the same material, but with different sizes, can emit light of different colors. The physical reason is the quantum

confinement effect.

The larger the dot, the redder (lower energy) its fluorescence spectrum. Conversely, smaller dots emit bluer (higher energy) light. The coloration is directly related to the energy levels of the quantum dot. Quantitatively speaking, the bandgap energy that determines the energy (and hence color) of the fluorescent light is inversely proportional to the square of the size of the quantum dot. Larger quantum dots have more energy levels which are also more closely spaced. This allows the quantum dot to absorb photons containing less energy, i.e., those closer to the red end of the spectrum .

Recent Observations have shown that the shape of the Crystal lattice also might change the colour

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Properties

Quantum Dots - Quantum Yield

The percentage of absorbed photons that result in an emitted photon is called

Quantum Yield (QY). QY is controlled by the existence of nonradiative transition

of electrons and holes between energy levels- transitions that produce no

electromagnetic radiation. Nonradiative recombination largely occurs at the dot's

surface and is therefore greatly influenced by the surface chemistry.

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Properties Adding Shells to Quantum Dots:Capping a core quantum dot with a shell (several atomic layers of an inorganic wide band semiconductor) reduces non-radiative recombination and results in brighter emission,provided the shell is of a different semiconductor material with awider band gap than the Core semiconductor material. The higher QY of Core-Shell quantum dots comes about due to changes in the surface chemistry of the core quantum dot. The surface of quantum dots that lack a shell has both free (unbonded) electrons, in addition to crystal defects. Both of these characteristics tend to reduce QY by allowing for nonradiative electron energy transitions at the surface. The addition of a shell reduces the opportunities for these nonradiative transitions by giving conduction band electrons an increased probability of directly relaxing to the valence band.The shell also neutralizes the effects of many types of surface defects.

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Applications and uses

Information processing and ComputingQuantum dots have also been implemented as qubits for quantum information processing.By applying small voltages to the leads,

the flow of electrons through the quantum dot can be controlled and

thereby precise measurements of the spin and other properties therein can be

made. With several entangled quantum dots,or qubits, plus a way of

performingoperations, quantum calculations and the computers that would

perform them might be possible.

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Biology and Medicinal sciences: Qdots replacing organic dyes,

Usage of quantum dots for highly sensitive cellular imaging, Extraordinary photostability of quantum dot probes is the real-

time tracking of molecules and cells over extended periods of time, thus is used

cancer technology

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Photovoltaic devices And Nano crystal solar cell Semiconductor nanoparticles

that exhibit size and compositionally tunable bandgaps. Therefore, different types and sizes of quantum dots, engineered to perfectly match and absorb the light of the solar

spectrum, can be brought together into the same cell

Alternative quantum dot based solar cells approaches including, luminescent concentrator cells, quantum dot dye sensitized solar cells, multiple exciton generation, and intermediate band solar cells.

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Electronic applications they have been proven to operate like a single-electron transistor and show the Coulomb blockade effect

Security inks Due to its Colloidal properties Q Dots can be mixed into inks which incorporate quantum dots, nanoscale semiconductor particles,which can be tuned to emit light at specific wavelengths in the visible and infrared portion of the spectra

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Security Ink with Q Dots emmiting greenlight. Combining multiple quantum dots and other pigments to create unique fluorescentspectral barcodes that identify any object or document upon illuminated

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LED: Several advancements have been made in this field the most

significant one being “QD-WLED” or quantum dot white led Laser: Quantum dots are used as active laser medium in its light emitting

region. Due to the tight confinement of charge carriers in quantum dots, they exhibit an electronic structure similar to atoms. Lasers fabricated from such an

active media exhibit device performance that is closer to gas lasers Quantum wire: These dots can be patterned in

the form of wires which acts as good conducting agents and

are lighter in weight.

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Future Technologies

• Imagine a foldable wide screen TV, or a truly foldable phone, right in your pocket

• Furniture coated with a paint, which can be programmed to show act as a computer or TV

• Walls that can change colours to suit the ambience

• Computer memory storage

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Acknowledgements

Presentation Quantum dots

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