Quantum Dots

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Transcript of Quantum Dots

The total electron charge density (shown in green) of a quantum dot of gallium arsenide, containing just 465 atoms

Effectively a quantum dot is something capable of confining a single electron or a few, and in which the electrons occupy discrete energy states just as they would in an atom. Quantum dots have been called artificial atoms, and in fact , and the ability to control energy states of the electrons by applying a voltage has led to the exotic idea of a material which can emulate different elements. QUANTUM DOTS SOUND VERY EXOTIC and indeed they are in terms of the way they work which is dictated by the rules of quantum mechanics.

PROPERTIESTunable Emission PatternAn 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.

Molecular CouplingColloidally 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.

Tunable Absorption PatternIn addition to emissive advantages, quantum dots display advantages in their absorptive 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.

Optical PropertiesAn 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) is 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 band gap 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 color

Quantum Dots - Quantum YieldThe 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.

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.

PRODUCTIONThere are several ways to confine excitons in semiconductors, resulting in different methods to produce quantum dots. In general, quantum wires, wells and dots are grown by advanced epitaxial techniques in nanocrystals produced by chemical methods or by ion implantation, or in nanodevices made by state-of-the-art lithographic techniques.

Colloidal synthesisColloidal semiconductor nanocrystals are synthesized from precursor compounds dissolved in solutions, much like traditional chemical processes. The synthesis of colloidal quantum dots is based on a three-component system composed of: precursors, organic surfactants, and solvents. When heating a reaction medium to a sufficiently high temperature, the precursors chemically transform into monomers. Once the monomers reach a high enough supersaturation level, the nanocrystal growth starts with a nucleation process. The temperature during the growth process is one of the critical factors in determining optimal conditions for the nanocrystal growth. It must be high enough to allow for rearrangement and annealing of atoms during the synthesis process while being low enough to promote crystal growth. Another critical factor that has to be stringently controlled during nanocrystal growth is the monomer concentration. The growth process of nanocrystals can occur in two different regimes, focusing and defocusing. At high monomer concentrations, the critical size (the size where nanocrystals neither grow nor shrink) is relatively small, resulting in growth of nearly all particles. In this regime, smaller particles grow faster than large ones (since larger crystals need more atoms to grow than small crystals) resulting in focusing of the size distribution to yield nearly monodisperse particles. The size focusing is optimal when the monomer concentration is kept such that the average nanocrystal size present is always slightly larger than the critical size. When the monomer concentration is depleted during growth, the critical size becomes larger than the average size present, and the distribution defocuses as a result of Ostwald ripening. There are colloidal methods to produce many different semiconductors. Typical dots are made of binary alloys such as cadmium selenide, cadmium sulfide, indium arsenide, and indium phosphide. Although, dots may also be made from ternary alloys such as cadmium selenide sulfide, these quantum dots can contain as few as 100 to 100,000 atoms within the quantum dot volume, with a diameter of 10 to 50 atoms. This

corresponds to about 2 to 10 nanometers, and at 10 nm in diameter, nearly 3 million quantum dots could be lined up end to end and fit within the width of a human thumb. Large batches of quantum dots may be synthesized via colloidal synthesis. Due to this scalability and the convenience of benchtop conditions, colloidal synthetic methods are promising for commercial applications. It is acknowledged to be the least toxic of all the different forms of synthesis.

FabricationSelf-assembled quantum dots are typically between 5 and 50 nm in size. Quantum dots defined by lithographically patterned gate electrodes, or by etching on two-dimensional electron gases in semiconductor heterostructures can have lateral dimensions exceeding 100 nm. Some quantum dots are small regions of one material buried in another with a larger band gap. These can be so-called core-shell structures, e.g., with CdSe in the core and ZnS in the shell or from special forms of silica called ormosil. Quantum dots sometimes occur spontaneously in quantum well structures due to monolayer fluctuations in the well's thickness. Self-assembled quantum dots nucleate spontaneously under certain conditions during molecular beam epitaxy (MBE) and metallorganic vapor phase epitaxy (MOVPE), when a material is grown on a substrate to which it is not lattice matched. The resulting strain produces coherently strained islands on top of a two-dimensional "wetting-layer." This growth mode is known as StranskiKrastanov growth. The islands can be subsequently buried to form the quantum dot. This fabrication method has potential for applications in quantum cryptography (i.e. single photon sources) and quantum computation. The main limitations of this method are the cost of fabrication and the lack of control over positioni