Quantum Dots

Quantum dot From Wikipedia, the free encyclopedia Jump to navigationJump to search Colloidal quantum dots irradiated with a UV light. Different sized quantum dots emit different color light due to quantum confinement. Quantum dots (QDs) are tiny semiconductor particles a few nanometres in size, having optical and electronic properties that differ from larger particles due to quantum mechanics. They are a central topic in nanotechnology. When the quantum dots are illuminated by UV light, an electron in the quantum dot can be excited to a state of higher energy. In the case of a semiconducting quantum dot, this process corresponds to the transition of an electron from the valence band to the conductance band. The excited electron can drop back into the valence band releasing its energy by the emission of light. This light emission (photoluminescence) is illustrated in the figure on the right. The color of that light depends on the energy difference between the conductance band and the valence band. In the language of materials science, nanoscale semiconductor materials tightly confine either electrons or electron holes. Quantum dots are sometimes referred to as artificial atoms, emphasizing their singularity, having bound, discrete electronic states, like naturally occurring atoms or molecules.[1][2] It was shown that the electronic wave functions in quantum dots resembles the ones in real atoms.[3] By coupling two or more such quantum dots an artificial molecule can be made, exhibiting hybridization even in room temperature.[4] Quantum dots have properties intermediate between bulk semiconductors and discrete atoms or molecules. Their optoelectronic properties change as a function of both size and shape.[5][6] Larger QDs of 5–6 nm diameter emit longer wavelengths, with colors such as orange or red. Smaller QDs (2–3 nm) emit shorter wavelengths, yielding colors like blue and green. However, the specific colors vary depending on the exact composition of the QD.[7] Potential applications of quantum dots include single-electron transistors, solar cells, LEDs, lasers,[8] single-photon sources,[9][10][11] second-harmonic generation, quantum computing,[12] and medical imaging.[13] Their small size allows for some QDs to be suspended in solution, which may lead to use in inkjet printing and spin-coating.[14] They have been used in Langmuir-Blodgett thin-films.[15][16][17] These processing techniques result in less expensive and less time-consuming methods of semiconductor fabrication. Quantum dot From Wikipedia, the free encyclopedia Jump to navigationJump to search Colloidal quantum dots irradiated with a UV light. Different sized quantum dots emit different color light due to quantum confinement. Quantum dots (QDs) are tiny semiconductor particles a few nanometres in size, having optical and electronic properties that differ from larger particles due to quantum mechanics. They are a central topic in nanotechnology. When the quantum dots are illuminated by UV light, an electron in the quantum dot can be excited to a state of higher energy. In the case of a semiconducting quantum dot, this process corresponds to the transition of an electron from the valence band to the conductance band. The excited electron can drop back into the valence band releasing its energy by the emission of light. This light emission (photoluminescence) is illustrated in the figure on the right. The color of that light depends on the energy difference between the conductance band and the valence band. In the language of materials science, nanoscale semiconductor materials tightly confine either electrons or electron holes. Quantum dots are sometimes referred to as artificial atoms, emphasizing their singularity, having bound, discrete electronic states, like naturally occurring atoms or molecules.[1][2] It was shown that the electronic wave functions in quantum dots resembles the ones in real atoms.[3] By coupling two or more such quantum dots an artificial molecule can be made, exhibiting hybridization even in room temperature.[4] Quantum dots have properties intermediate between bulk semiconductors and discrete atoms or molecules. Their optoelectronic properties change as a function of both size and shape.[5][6] Larger QDs of 5–6 nm diameter emit longer wavelengths, with colors such as orange or red. Smaller QDs (2–3 nm) emit shorter wavelengths, yielding colors like blue and green. However, the specific colors vary depending on the exact composition of the QD.[7] Potential applications of quantum dots include single-electron transistors, solar cells, LEDs, lasers,[8] single-photon sources,[9][10][11] second-harmonic generation, quantum computing,[12] and medical imaging.[13] Their small size allows for some QDs to be suspended in solution, which may lead to use in inkjet printing and spin-coating.[14] They have been used in Langmuir-Blodgett thin-films.[15][16][17] These processing techniques result in less expensive and less time-consuming methods of semiconductor fabrication. Heavy-metal-free quantum dots 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. Among these materials are InP/ZnS, CuInS/ZnS, Si, Ge and C. Peptides are being researched as potential quantum dot material.[41] Since peptides occur naturally in all organisms, such dots would likely be nontoxic and easily biodegraded.[citation needed] Health and safety Main articles: Health and safety hazards of nanomaterials and Nanotoxicology Some quantum dots pose risks to human health and the environment under certain conditions.[42][43][44] Notably, the studies on quantum dot toxicity have focused on cadmium containing particles and have yet to be demonstrated in animal models after physiologically relevant dosing.[44] In vitro studies, based on cell cultures, on quantum dots (QD) toxicity suggest that their toxicity may derive from multiple factors including their physicochemical characteristics (size, shape, composition, surface functional groups, and surface charges) and their environment. 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.[42] 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.[45] Group II-VI QDs also have been reported to induce the formation of reactive oxygen species after exposure to light, which in turn can damage cellular components such as proteins, lipids and DNA.[46] Some studies have also demonstrated that addition of a ZnS shell inhibits the process of reactive oxygen species in CdSe QDs. Another aspect of QD toxicity is that there are, in vivo, 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.[47] The reports of QD localization in the cell nucleus[48] present additional modes of toxicity because they may induce DNA mutation, which in turn will propagate through future generation of cells causing diseases. 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.[49] These finding have led scientists to believe that intracellular dose is the most important deterring factor for QD toxicity. Therefore, factors determining the QD endocytosis that determine the effective intracellular concentration, such as QD size, shape and surface chemistry determine their toxicity. Excretion of QDs through urine in animal models also have demonstrated via injecting radio-labeled ZnS capped CdSe QDs where the ligand shell was labelled with 99mTc.[50] Though multiple other studies have concluded retention of QDs in cellular levels,[44][51] exocytosis of QDs is still poorly studied in the literature. 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. 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 LD50 are not applicable for QDs. Therefore, researchers are focusing on introducing novel approaches and adapting existing methods to include this unique class of materials.[44] Furthermore, novel strategies to engineer safer QDs are still under exploration by the scientific community. 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. Optical properties Fluorescence spectra of CdTe quantum dots of various sizes. Different sized quantum dots emit different color light due to quantum confinement. In semiconductors, light absorption generally leads to an electron being excited from the valence to the conduction band, leaving behind a hole. The electron and the hole can bind to each other to form an exciton. When this exciton recombines (i.e. the electron resumes its ground state), the exciton's energy can be emitted as light. 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): History The term "quantum dot" was coined in 1986.[111] They were first discovered in a glass matrix and in colloidal solutions[112] by Alexey Ekimov[113][114][115][116] and Louis Brus.[117][118]