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33-516: Three crystalline forms of CdSe are known which follow the structures of: wurtzite (hexagonal), sphalerite (cubic) and rock-salt (cubic). The sphalerite CdSe structure is unstable and converts to the wurtzite form upon moderate heating. The transition starts at about 130 °C, and at 700 °C it completes within a day. The rock-salt structure is only observed under high pressure. The production of cadmium selenide has been carried out in two different ways. The preparation of bulk crystalline CdSe

66-416: A CdSe core and a ligand shell. Ligands play important roles in the stability and solubility of the nanoparticles. During synthesis, ligands stabilize growth to prevent aggregation and precipitation of the nanocrystals. These capping ligands also affect the quantum dot's electronic and optical properties by passivating surface electronic states. An application that depends on the nature of the surface ligands

99-605: A description is rarely used. The hexagonal crystal family consists of two crystal systems : trigonal and hexagonal. A crystal system is a set of point groups in which the point groups themselves and their corresponding space groups are assigned to a lattice system (see table in Crystal system#Crystal classes ). The trigonal crystal system consists of the 5 point groups that have a single three-fold rotation axis, which includes space groups 143 to 167. These 5 point groups have 7 corresponding space groups (denoted by R) assigned to

132-455: A few. Production of cadmium selenide by arrested precipitation in solution is performed by introducing alkylcadmium and trioctylphosphine selenide (TOPSe) precursors into a heated solvent under controlled conditions. CdSe nanoparticles can be modified by production of two phase materials with ZnS coatings. The surfaces can be further modified, e.g. with mercaptoacetic acid, to confer solubility. Synthesis in structured environments refers to

165-446: A hexagonal arrangement of rods. High temperature pyrolysis synthesis is usually carried out using an aerosol containing a mixture of volatile cadmium and selenium precursors. The precursor aerosol is then carried through a furnace with an inert gas, such as hydrogen , nitrogen , or argon . In the furnace the precursors react to form CdSe as well as several by-products. CdSe-derived nanoparticles with sizes below 10 nm exhibit

198-463: A primitive hexagonal nickel sublattice and a hexagonal close-packed arsenic sublattice. Each nickel atom is octahedrally coordinated to six arsenic atoms, while each arsenic atom is trigonal prismatically coordinated to six nickel atoms. The structure can also be described as an HCP lattice of arsenic with nickel occupying each octahedral void . Compounds adopting the NiAs structure are generally

231-510: A property known as quantum confinement . Quantum confinement results when the electrons in a material are confined to a very small volume. Quantum confinement is size dependent, meaning the properties of CdSe nanoparticles are tunable based on their size. One type of CdSe nanoparticle is a CdSe quantum dot . This discretization of energy states results in electronic transitions that vary by quantum dot size. Larger quantum dots have closer electronic states than smaller quantum dots which means that

264-438: A rigorous description of the ligand exchange chemistry on the quantum dot surface. A prevailing belief is that trioctylphosphine oxide (TOPO) or trioctylphosphine (TOP), a neutral ligand derived from a common precursor used in the synthesis of CdSe dots, caps the surface of CdSe quantum dots. However, results from recent studies challenge this model. Using NMR, quantum dots have been shown to be nonstoichiometric meaning that

297-408: A wide range of applications including solar cells, light emitting diodes, and biofluorescent tagging. CdSe-based materials also have potential uses in biomedical imaging. Human tissue is permeable to near infra-red light. By injecting appropriately prepared CdSe nanoparticles into injured tissue, it may be possible to image the tissue in those injured areas. CdSe quantum dots are usually composed of

330-401: Is a toxic heavy metal and appropriate precautions should be taken when handling it and its compounds. Selenides are toxic in large amounts. Cadmium selenide is a known carcinogen to humans and medical attention should be sought if swallowed, dust inhaled, or if contact with skin or eyes occurs. Wurtzite (crystal structure) In crystallography , the hexagonal crystal family is one of

363-506: Is done by the High-Pressure Vertical Bridgman method or High-Pressure Vertical Zone Melting. Cadmium selenide may also be produced in the form of nanoparticles . (see applications for explanation) Several methods for the production of CdSe nanoparticles have been developed: arrested precipitation in solution, synthesis in structured media, high temperature pyrolysis, sonochemical, and radiolytic methods are just

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396-630: Is given by a c •sin(60°) Hexagonal close packed (hcp) is one of the two simple types of atomic packing with the highest density, the other being the face-centered cubic (fcc). However, unlike the fcc, it is not a Bravais lattice, as there are two nonequivalent sets of lattice points. Instead, it can be constructed from the hexagonal Bravais lattice by using a two-atom motif (the additional atom at about ( 2 ⁄ 3 ,  1 ⁄ 3 ,  1 ⁄ 2 )) associated with each lattice point. Compounds that consist of more than one element (e.g. binary compounds ) often have crystal structures based on

429-405: Is the synthesis of CdSe thin films. The density of the ligands on the surface and the length of the ligand chain affect the separation between nanocrystal cores which in turn influence stacking and conductivity . Understanding the surface structure of CdSe quantum dots in order to investigate the structure's unique properties and for further functionalization for greater synthetic variety requires

462-563: Is used as an extraction or stabilizing agent. It is an air-stable white solid at room temperature. It is lipophilic, and like other phosphine oxides serves as a Lewis base owing to a partial negative charge at the O atom. TOPO is usually prepared by oxidation of trioctylphosphine, which in turn is produced by alkylation of phosphorus trichloride . The main use of TOPO is in solvent extraction of metals, especially uranium. The high lipophilicity and high polarity are properties key to this application. Its high polarity , which results from

495-429: The chalcogenides , arsenides , antimonides and bismuthides of transition metals . The following are the members of the nickeline group: There is only one hexagonal Bravais lattice in two dimensions: the hexagonal lattice. Trioctylphosphine oxide Trioctylphosphine oxide ( TOPO ) is an organophosphorus compound with the formula OP(C 8 H 17 ) 3 . Frequently referred to as TOPO, this compound

528-481: The 12 point groups such that at least one of their space groups has the hexagonal lattice as underlying lattice, and is the union of the hexagonal crystal system and the trigonal crystal system. There are 52 space groups associated with it, which are exactly those whose Bravais lattice is either hexagonal or rhombohedral. The hexagonal crystal family consists of two lattice systems : hexagonal and rhombohedral. Each lattice system consists of one Bravais lattice . In

561-420: The bulk crystal, but the structure can be favored in some nanocrystal forms of the material. In materials with more than one crystal structure, the prefix "w-" is sometimes added to the empirical formula to denote the wurtzite crystal structure, as in w-BN . Each of the two individual atom types forms a sublattice which is hexagonal close-packed (HCP-type). When viewed all together, the atomic positions are

594-566: The cadmium to selenide ratio is not one to one. CdSe dots have excess cadmium cations on the surface that can form bonds with anionic species such as carboxylate chains. The CdSe quantum dot would be charge unbalanced if TOPO or TOP were indeed the only type of ligand bound to the dot. The CdSe ligand shell may contain both X type ligands which form covalent bonds with the metal and L type ligands that form dative bonds . It has been shown that these ligands can undergo exchange with other ligands. Examples of X type ligands that have been studied in

627-410: The compounds that can take the wurtzite structure are wurtzite itself ( ZnS with up to 8% iron instead of zinc ), silver iodide (AgI), zinc oxide (ZnO), cadmium sulfide (CdS), cadmium selenide (CdSe), silicon carbide (α-SiC), gallium nitride (GaN), aluminium nitride (AlN), boron nitride (w-BN) and other semiconductors . In most of these compounds, wurtzite is not the favored form of

660-561: The context of CdSe nanocrystal surface chemistry are sulfides and thiocyanates. Examples of L type ligands that have been studied are amines and phosphines (ref). A ligand exchange reaction in which tributylphosphine ligands were displaced by primary alkylamine ligands on chloride terminated CdSe dots has been reported. Stoichiometry changes were monitored using proton and phosphorus NMR. Photoluminescence properties were also observed to change with ligand moiety. The amine bound dots had significantly higher photoluminescent quantum yields than

693-413: The dipolar phosphorus-oxygen bond, allows this compound to bind to metal ions. The octyl groups confer solubility in low polarity solvents such as kerosene . In the research laboratory, both trioctylphosphine and TOPO are frequently useful as a capping ligand for the production of quantum dots such as those consisting of CdSe . In these cases, TOPO serves as solvent for the synthesis and solubilizes

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726-468: The energy required to excite an electron from HOMO to the LUMO is lower than the same electronic transition in a smaller quantum dot. This quantum confinement effect can be observed as a red shift in absorbance spectra for nanocrystals with larger diameters. Quantum confinement effects in quantum dots can also result in fluorescence intermittency , called "blinking." CdSe quantum dots have been implemented in

759-861: The hexagonal crystal family. Some of the more common ones are listed here. These structures can be viewed as two or more interpenetrating sublattices where each sublattice occupies the interstitial sites of the others. The wurtzite crystal structure is referred to by the Strukturbericht designation B4 and the Pearson symbol hP4. The corresponding space group is No. 186 (in International Union of Crystallography classification) or P6 3 mc (in Hermann–Mauguin notation ). The Hermann-Mauguin symbols in P6 3 mc can be read as follows: Among

792-478: The hexagonal family, the crystal is conventionally described by a right rhombic prism unit cell with two equal axes ( a by a ), an included angle of 120° ( γ ) and a height ( c , which can be different from a ) perpendicular to the two base axes. The hexagonal unit cell for the rhombohedral Bravais lattice is the R-centered cell, consisting of two additional lattice points which occupy one body diagonal of

825-446: The hexagonal lattice system. The 5 point groups in this crystal system are listed below, with their international number and notation, their space groups in name and example crystals. The 7 point groups ( crystal classes ) in this crystal system are listed below, followed by their representations in Hermann–Mauguin or international notation and Schoenflies notation , and mineral examples, if they exist. The unit cell volume

858-438: The lattice is non-primitive. The Bravais lattices in the hexagonal crystal family can also be described by rhombohedral axes. The unit cell is a rhombohedron (which gives the name for the rhombohedral lattice). This is a unit cell with parameters a = b = c ; α = β = γ ≠ 90°. In practice, the hexagonal description is more commonly used because it is easier to deal with a coordinate system with two 90° angles. However,

891-430: The phosphine bound dots. CdSe material is transparent to infra-red (IR) light and has seen limited use in photoresistors and in windows for instruments utilizing IR light. The material is also highly luminescent. CdSe is a component of the pigment cadmium orange . CdSe can also serve as the n-type semiconductor layer in photovoltaic cells . CdSe occurs in the nature as the very rare mineral cadmoselite . Cadmium

924-414: The production of cadmium selenide in liquid crystal or surfactant solutions. The addition of surfactants to solutions often results in a phase change in the solution leading to a liquid crystallinity. A liquid crystal is similar to a solid crystal in that the solution has long range translational order. Examples of this ordering are layered alternating sheets of solution and surfactant, micelles , or even

957-536: The rhombohedral axes are often shown (for the rhombohedral lattice) in textbooks because this cell reveals the 3 m symmetry of the crystal lattice. The rhombohedral unit cell for the hexagonal Bravais lattice is the D-centered cell, consisting of two additional lattice points which occupy one body diagonal of the unit cell with coordinates ( 1 ⁄ 3 , 1 ⁄ 3 , 1 ⁄ 3 ) and ( 2 ⁄ 3 , 2 ⁄ 3 , 2 ⁄ 3 ). However, such

990-463: The rhombohedral lattice system and 18 corresponding space groups (denoted by P) assigned to the hexagonal lattice system. Hence, the trigonal crystal system is the only crystal system whose point groups have more than one lattice system associated with their space groups. The hexagonal crystal system consists of the 7 point groups that have a single six-fold rotation axis. These 7 point groups have 27 space groups (168 to 194), all of which are assigned to

1023-552: The same as in lonsdaleite (hexagonal diamond ). Each atom is tetrahedrally coordinated. The structure can also be described as an HCP lattice of zinc with sulfur atoms occupying half of the tetrahedral voids or vice versa. The wurtzite structure is non-centrosymmetric (i.e., lacks inversion symmetry ). Due to this, wurtzite crystals can (and generally do) have properties such as piezoelectricity and pyroelectricity , which centrosymmetric crystals lack. The nickel arsenide structure consists of two interpenetrating sublattices:

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1056-456: The six crystal families , which includes two crystal systems (hexagonal and trigonal ) and two lattice systems (hexagonal and rhombohedral ). While commonly confused, the trigonal crystal system and the rhombohedral lattice system are not equivalent (see section crystal systems below). In particular, there are crystals that have trigonal symmetry but belong to the hexagonal lattice (such as α- quartz ). The hexagonal crystal family consists of

1089-578: The unit cell. There are two ways to do this, which can be thought of as two notations which represent the same structure. In the usual so-called obverse setting, the additional lattice points are at coordinates ( 2 ⁄ 3 , 1 ⁄ 3 , 1 ⁄ 3 ) and ( 1 ⁄ 3 , 2 ⁄ 3 , 2 ⁄ 3 ), whereas in the alternative reverse setting they are at the coordinates ( 1 ⁄ 3 , 2 ⁄ 3 , 1 ⁄ 3 ) and ( 2 ⁄ 3 , 1 ⁄ 3 , 2 ⁄ 3 ). In either case, there are 3 lattice points per unit cell in total and

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