Presentation Quantum Dots

QUANTUM DOTS Presented by Abhisek Banerjee Bishan Mukherjee Somaditya Indu Suman Roy 1

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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??

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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.

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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).

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An electromagnetic radiation emitter with an easily tunable bandgap.

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Continued …  In an unconfined (bulk) semiconductor, an electronhole 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. 5

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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

Contents

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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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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

Contents

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 Traditional semiconductors have shortcomings, they lack versatility.

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.

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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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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

Contents

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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…..  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. 16

Properties

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 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  Quantum Dots - Molecular Coupling

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. 19

Properties 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 levelstransitions 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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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 a wider 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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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

Contents

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Applications and uses 

Information processing and Computing

Quantum 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 performing operations, quantum calculations and the computers that would perform them might be possible.

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Applications and uses 

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

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Applications and uses 

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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Applications and uses 

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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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Applications and uses

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

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Applications and uses  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

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wire: These dots can be patterned in

the form of wires which acts as good conducting agents and are lighter in weight. 30

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

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