Manual Optical Properties of Semiconductor Quantum Dots

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  1. Introduction
  2. Optical Properties of Semiconductor Quantum Dots by Ulrike Woggon -
  3. Optoelectronic Properties of Semiconductor Quantum Dot Solids for Photovoltaic Applications.

Smaller particles emit blue light because the exciton energy increases as the size decreases Right.

  • Coming attractions for semiconductor quantum dots?
  • Optical Properties of Semiconductor Quantum Dots | SpringerLink?
  • 1. Introduction;
  • Theory Of Semiconductor Quantum Dots: Band Structure, Optical Properties And Applications;
  • Encyclopedia of Prehistory: Volume 8: South and Southwest Asia.

The size-dependence of the emission color comes primarily from a particle-in-a-box effect. The electron and hole that are created when the quantum dot absorbs light are bound together as an exciton by the confines of the "box". Louis Brus used first-order perturbation theory to determine that the bandgap of a semiconductor quantum dot is given approximately by:.


In this equation, the first term after the bulk bandgap is the kinetic energy due to confinement of the exciton, and the second term represents the electrostatic attractive energy between the confined electron and hole. Over time, the monomer concentration diminishes, the critical size becomes larger than the average size present, and the distribution "defocuses".

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  3. Optoelectronic Properties of Semiconductor Quantum Dot Solids for Photovoltaic Applications..
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  5. There are colloidal methods to produce many different semiconductors. Typical dots are made of binary compounds such as lead sulfide , lead selenide , cadmium selenide , cadmium sulfide , cadmium telluride , indium arsenide , and indium phosphide. Dots may also be made from ternary compounds such as cadmium selenide sulfide. 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 [ citation needed ] to be the least toxic of all the different forms of synthesis. Plasma synthesis has evolved to be one of the most popular gas-phase approaches for the production of quantum dots, especially those with covalent bonds. The size, shape, surface and composition of quantum dots can all be controlled in nonthermal plasma. This can lead to excellent dispersion of quantum dots in either organic solvents [24] or water [25] i. The quantum dot absorption features correspond to transitions between discrete, three-dimensional particle in a box states of the electron and the hole, both confined to the same nanometer -size box.

    These discrete transitions are reminiscent of atomic spectra and have resulted in quantum dots also being called artificial atoms. Genetically engineered M13 bacteriophage viruses allow preparation of quantum dot biocomposite structures. Consequently, the specific recognition properties of the virus can be used to organize inorganic nanocrystals , forming ordered arrays over the length scale defined by liquid crystal formation.

    Material designs for control of spontaneous emission of semiconductor quantum dots

    Using this information, Lee et al. This system allowed them to vary both the length of bacteriophage and the type of inorganic material through genetic modification and selection. 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.

    Quantum dot manufacturing relies on a process called "high temperature dual injection" which has been scaled by multiple companies for commercial applications that require large quantities hundreds of kilograms to tonnes of quantum dots. This reproducible production method can be applied to a wide range of quantum dot sizes and compositions. The bonding in certain cadmium-free quantum dots, such as III-V-based quantum dots, is more covalent than that in II-VI materials, therefore it is more difficult to separate nanoparticle nucleation and growth via a high temperature dual injection synthesis.

    An alternative method of quantum dot synthesis, the "molecular seeding" process, provides a reproducible route to the production of high quality quantum dots in large volumes. The process utilises identical molecules of a molecular cluster compound as the nucleation sites for nanoparticle growth, thus avoiding the need for a high temperature injection step. Particle growth is maintained by the periodic addition of precursors at moderate temperatures until the desired particle size is reached.

    Optical Properties of Semiconductor Quantum Dots by Ulrike Woggon -

    Another approach for the mass production of colloidal quantum dots can be seen in the transfer of the well-known hot-injection methodology for the synthesis to a technical continuous flow system. The batch-to-batch variations arising from the needs during the mentioned methodology can be overcome by utilizing technical components for mixing and growth as well as transport and temperature adjustments.

    For the production of CdSe based semiconductor nanoparticles this method has been investigated and tuned to production amounts of kg per month. Since the use of technical components allows for easy interchange in regards of maximum through-put and size, it can be further enhanced to tens or even hundreds of kilograms. In a consortium of U. On January 23, Dow entered into an exclusive licensing agreement with UK-based Nanoco for the use of their low-temperature molecular seeding method for bulk manufacture of cadmium-free quantum dots for electronic displays, and on September 24, Dow commenced work on the production facility in South Korea capable of producing sufficient quantum dots for "millions of cadmium-free televisions and other devices, such as tablets".

    Mass production is due to commence in mid 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. For commercial viability, 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.

    Peptides are being researched as potential quantum dot material. Some quantum dots pose risks to human health and the environment under certain conditions. Assessing their potential toxicity is complex as these factors include properties such as QD size, charge, concentration, chemical composition, capping ligands, and also on their oxidative, mechanical and photolytic stability.

    Many studies have focused on the mechanism of QD cytotoxicity using model cell cultures. It has been demonstrated that after exposure to ultraviolet radiation or oxidation by air, CdSe QDs release free cadmium ions causing cell death. Another aspect of QD toxicity is the process of their size dependent intracellular pathways that concentrate these particles in cellular organelles that are inaccessible by metal ions, which may result in unique patterns of cytotoxicity compared to their constituent metal ions.

    Although concentration of QDs in certain organelles have been reported in in vivo studies using animal models, no alterations in animal behavior, weight, hematological markers or organ damage has been found through either histological or biochemical analysis. Therefore, factors determining the QD endocytosis that determine the effective intracellular concentration, such as QD size, shape and surface chemistry determine their toxicity.

    While significant research efforts have broadened the understanding of toxicity of QDs, there are large discrepancies in the literature and questions still remains to be answered.

    Optoelectronic Properties of Semiconductor Quantum Dot Solids for Photovoltaic Applications.

    Diversity of this class material as compared to normal chemical substances makes the assessment of their toxicity very challenging. As their toxicity may also be dynamic depending on the environmental factors such as pH level, light exposure and cell type, traditional methods of assessing toxicity of chemicals such as LD 50 are not applicable for QDs.

    Therefore, researchers are focusing on introducing novel approaches and adapting existing methods to include this unique class of materials. A recent novelty in the field is the discovery of carbon quantum dots , a new generation of optically-active nanoparticles potentially capable of replacing semiconductor QDs, but with the advantage of much lower toxicity.

    In semiconductors, light absorption generally leads to an electron being excited from the valence to the conduction band, leaving behind a hole.

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    The electron and the hole can bind to each other to form an exciton. When this exciton recombines i.

    This is called fluorescence. In a simplified model, the energy of the emitted photon can be understood as the sum of the band gap energy between the highest occupied level and the lowest unoccupied energy level, the confinement energies of the hole and the excited electron, and the bound energy of the exciton the electron-hole pair :. As the confinement energy depends on the quantum dot's size, both absorption onset and fluorescence emission can be tuned by changing the size of the quantum dot during its synthesis.

    The larger the dot, the redder lower energy its absorption onset and fluorescence spectrum. Conversely, smaller dots absorb and emit bluer higher energy light. Recent articles in Nanotechnology and in other journals have begun to suggest that the shape of the quantum dot may be a factor in the coloration as well, but as yet not enough information is available. Furthermore, it was shown [49] that the lifetime of fluorescence is determined by the size of the quantum dot. Larger dots have more closely spaced energy levels in which the electron-hole pair can be trapped.

    Therefore, electron-hole pairs in larger dots live longer causing larger dots to show a longer lifetime. To improve fluorescence quantum yield , quantum dots can be made with "shells" of a larger bandgap semiconductor material around them. The improvement is suggested to be due to the reduced access of electron and hole to non-radiative surface recombination pathways in some cases, but also due to reduced Auger recombination in others.

    Quantum dots are particularly promising for optical applications due to their high extinction coefficient. Quantum dots have also been suggested as implementations of qubits for quantum information processing , [51] and as active elements for thermoelectrics. Tuning the size of quantum dots is attractive for many potential applications. For instance, larger quantum dots have a greater spectrum-shift towards red compared to smaller dots, and exhibit less pronounced quantum properties.

    Conversely, the smaller particles allow one to take advantage of more subtle quantum effects. Being zero-dimensional , quantum dots have a sharper density of states than higher-dimensional structures. As a result, they have superior transport and optical properties. They have potential uses in diode lasers , amplifiers, and biological sensors. Quantum dots may be excited within a locally enhanced electromagnetic field produced by gold nanoparticles, which can then be observed from the surface plasmon resonance in the photoluminescent excitation spectrum of CdSe ZnS nanocrystals.

    The new generations of quantum dots have far-reaching potential for the study of intracellular processes at the single-molecule level, high-resolution cellular imaging, long-term in vivo observation of cell trafficking, tumor targeting, and diagnostics. CdSe nanocrystals are efficient triplet photosensitizers. In modern biological analysis, various kinds of organic dyes are used.

    However, as technology advances, greater flexibility in these dyes is sought. However, there have been groups which have developed quantum dots which are essentially nonblinking and demonstrated their utility in single molecule tracking experiments. The use of quantum dots for highly sensitive cellular imaging has seen major advances.

    Researchers were able to observe quantum dots in lymph nodes of mice for more than 4 months. Quantum dots can have antibacterial properties similar to nanoparticles and can kill bacteria in a dose-dependent manner. In addition, quantum dots can directly damage the cell wall. Quantum dots have been shown to be effective against both gram- positive and gram-negative bacteria. Semiconductor quantum dots have also been employed for in vitro imaging of pre-labeled cells. The ability to image single-cell migration in real time is expected to be important to several research areas such as embryogenesis , cancer metastasis , stem cell therapeutics, and lymphocyte immunology.

    The use of quantum dots for tumor targeting under in vivo conditions employ two targeting schemes: active targeting and passive targeting. In the case of active targeting, quantum dots are functionalized with tumor-specific binding sites to selectively bind to tumor cells. Passive targeting uses the enhanced permeation and retention of tumor cells for the delivery of quantum dot probes. Fast-growing tumor cells typically have more permeable membranes than healthy cells, allowing the leakage of small nanoparticles into the cell body.

    Moreover, tumor cells lack an effective lymphatic drainage system, which leads to subsequent nanoparticle-accumulation. Quantum dot probes exhibit in vivo toxicity. For example, CdSe nanocrystals are highly toxic to cultured cells under UV illumination, because the particles dissolve, in a process known as photolysis , to release toxic cadmium ions into the culture medium. In the absence of UV irradiation, however, quantum dots with a stable polymer coating have been found to be essentially nontoxic.