In the physical sciences, a particle is a small localized object to which can be ascribed several physical or chemical properties such as volume, density or mass. They vary in size or quantity, from subatomic particles like the electron, to microscopic particles like atoms and molecules, to macroscopic particles like powders and other granular materials. Particles can be used to create scientific models of larger objects depending on their density, such as humans moving in a crowd or celestial bodies in motion; the term'particle' is rather general in meaning, is refined as needed by various scientific fields. Something, composed of particles may be referred to as being particulate. However, the noun'particulate' is most used to refer to pollutants in the Earth's atmosphere, which are a suspension of unconnected particles, rather than a connected particle aggregation; the concept of particles is useful when modelling nature, as the full treatment of many phenomena can be complex and involve difficult computation.
It can be used to make simplifying assumptions concerning the processes involved. Francis Sears and Mark Zemansky, in University Physics, give the example of calculating the landing location and speed of a baseball thrown in the air, they strip the baseball of most of its properties, by first idealizing it as a rigid smooth sphere by neglecting rotation and friction reducing the problem to the ballistics of a classical point particle. The treatment of large numbers of particles is the realm of statistical physics; the term "particle" is applied differently to three classes of sizes. The term macroscopic particle refers to particles much larger than atoms and molecules; these are abstracted as point-like particles though they have volumes, structures, etc. Examples of macroscopic particles would include powder, sand, pieces of debris during a car accident, or objects as big as the stars of a galaxy. Another type, microscopic particles refers to particles of sizes ranging from atoms to molecules, such as carbon dioxide and colloidal particles.
These particles are studied in chemistry, as well as molecular physics. The smallest of particles are the subatomic particles; these would include particles such as the constituents of atoms – protons and electrons – as well as other types of particles which can only be produced in particle accelerators or cosmic rays. These particles are studied in particle physics; because of their small size, the study of microscopic and subatomic particles fall in the realm of quantum mechanics. They will exhibit phenomena demonstrated in the particle in a box model, including wave–particle duality, whether particles can be considered distinct or identical is an important question in many situations. Particles can be classified according to composition. Composite particles refer to particles that have composition –, particles which are made of other particles. For example, a carbon-14 atom is made of six protons, eight neutrons, six electrons. By contrast, elementary particles refer to particles. According to our current understanding of the world, only a small number of these exist, such as leptons and gluons.
However it is possible that some of these might turn up to be composite particles after all, appear to be elementary for the moment. While composite particles can often be considered point-like, elementary particles are punctual. Both elementary and composite particles, are known to undergo particle decay; those that do not are called stable particles, such as a helium-4 nucleus. The lifetime of stable particles can be either infinite or large enough to hinder attempts to observe such decays. In the latter case, those particles are called "observationally stable". In general, a particle decays from a high-energy state to a lower-energy state by emitting some form of radiation, such as the emission of photons. In computational physics, N-body simulations are simulations of dynamical systems of particles under the influence of certain conditions, such as being subject to gravity; these simulations are common in cosmology and computational fluid dynamics. N refers to the number of particles considered.
As simulations with higher N are more computationally intensive, systems with large numbers of actual particles will be approximated to a smaller number of particles, simulation algorithms need to be optimized through various methods. Colloidal particles are the components of a colloid. A colloid is a substance microscopically; such colloidal system can be liquid, or gaseous. The dispersed-phase particles have a diameter of between 5 and 200 nanometers. Soluble particles smaller. Colloidal systems are the subject of colloid science. Suspended solids may be held in a liquid, while solid or liquid particles suspended in a gas together form an aerosol. Particles may be suspended in the form of atmospheric particulate matter, which may constitute air pollution. Larger particles can form marine debris or space debris. A conglomeration of discrete solid, macroscopic particles may be described as a granular material. Particles portal "What is a particle?". University of Florida, Particle Engineering Resea
Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. At room temperature and pressure, another solid form of carbon known as graphite is the chemically stable form, but diamond never converts to it. Diamond has the highest hardness and thermal conductivity of any natural material, properties that are utilized in major industrial applications such as cutting and polishing tools, they are the reason that diamond anvil cells can subject materials to pressures found deep in the Earth. Because the arrangement of atoms in diamond is rigid, few types of impurity can contaminate it. Small numbers of defects or impurities color diamond blue, brown, purple, orange or red. Diamond has high optical dispersion. Most natural diamonds have ages between 1 billion and 3.5 billion years. Most were formed at depths between 150 and 250 kilometers in the Earth's mantle, although a few have come from as deep as 800 kilometers. Under high pressure and temperature, carbon-containing fluids dissolved minerals and replaced them with diamonds.
Much more they were carried to the surface in volcanic eruptions and deposited in igneous rocks known as kimberlites and lamproites. Synthetic diamonds can be grown from high-purity carbon under high pressures and temperatures or from hydrocarbon gas by chemical vapor deposition. Imitation diamonds can be made out of materials such as cubic zirconia and silicon carbide. Natural and imitation diamonds are most distinguished using optical techniques or thermal conductivity measurements. Diamond is a solid form of pure carbon with its atoms arranged in a crystal. Solid carbon comes in different forms known as allotropes depending on the type of chemical bond; the two most common allotropes of pure carbon are graphite. In graphite the bonds are sp2 orbital hybrids and the atoms form in planes with each bound to three nearest neighbors 120 degrees apart. In diamond they are sp3 and the atoms form tetrahedra with each bound to four nearest neighbors. Tetrahedra are rigid, the bonds are strong, of all known substances diamond has the greatest number of atoms per unit volume, why it is both the hardest and the least compressible.
It has a high density, ranging from 3150 to 3530 kilograms per cubic metre in natural diamonds and 3520 kg/m³ in pure diamond. In graphite, the bonds between nearest neighbors are stronger but the bonds between planes are weak, so the planes can slip past each other. Thus, graphite is much softer than diamond. However, the stronger bonds make graphite less flammable. Diamonds have been adapted for many uses because of the material's exceptional physical characteristics. Most notable are its extreme hardness and thermal conductivity, as well as wide bandgap and high optical dispersion. Diamond's ignition point is 720 -- 800 °C in 850 -- 1000 °C in air; the equilibrium pressure and temperature conditions for a transition between graphite and diamond is well established theoretically and experimentally. The pressure changes linearly between 1.7 GPa at 0 K and 12 GPa at 5000 K. However, the phases have a wide region about this line where they can coexist. At normal temperature and pressure, 20 °C and 1 standard atmosphere, the stable phase of carbon is graphite, but diamond is metastable and its rate of conversion to graphite is negligible.
However, at temperatures above about 4500 K, diamond converts to graphite. Rapid conversion of graphite to diamond requires pressures well above the equilibrium line: at 2000 K, a pressure of 35 GPa is needed. Above the triple point, the melting point of diamond increases with increasing pressure. At high pressures and germanium have a BC8 body-centered cubic crystal structure, a similar structure is predicted for carbon at high pressures. At 0 K, the transition is predicted to occur at 1100 GPa; the most common crystal structure of diamond is called diamond cubic. It is formed of unit cells stacked together. Although there are 18 atoms in the figure, each corner atom is shared by eight unit cells and each atom in the center of a face is shared by two, so there are a total of eight atoms per unit cell; each side of the unit cell is 3.57 angstroms in length. A diamond cubic lattice can be thought of as two interpenetrating face-centered cubic lattices with one displaced by 1/4 of the diagonal along a cubic cell, or as one lattice with two atoms associated with each lattice point.
Looked at from a <1 1 1> crystallographic direction, it is formed of layers stacked in a repeating ABCABC... pattern. Diamonds can form an ABAB... structure, known as hexagonal diamond or lonsdaleite, but this is far less common and is formed under different conditions from cubic carbon. Diamonds occur most as euhedral or rounded octahedra and twinned octahedra known as macles; as diamond's crystal structure has a cubic arrangement of the atoms, they have many facets that belong to a cube, rhombicosidodecahedron, tetrakis hexahedron or disdyakis dodecahedron. The crystals can be elongated. Diamonds are found coated in nyf, an opaque gum-like skin; some diamonds have opaque fibers. They are referred to as opaque if the fibers
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group on the periodic table, a reactive nonmetal, an oxidizing agent that forms oxides with most elements as well as with other compounds. By mass, oxygen is the third-most abundant element in the universe, after helium. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O2. Diatomic oxygen gas constitutes 20.8% of the Earth's atmosphere. As compounds including oxides, the element makes up half of the Earth's crust. Dioxygen is used in cellular respiration and many major classes of organic molecules in living organisms contain oxygen, such as proteins, nucleic acids and fats, as do the major constituent inorganic compounds of animal shells and bone. Most of the mass of living organisms is oxygen as a component of water, the major constituent of lifeforms. Oxygen is continuously replenished in Earth's atmosphere by photosynthesis, which uses the energy of sunlight to produce oxygen from water and carbon dioxide.
Oxygen is too chemically reactive to remain a free element in air without being continuously replenished by the photosynthetic action of living organisms. Another form of oxygen, ozone absorbs ultraviolet UVB radiation and the high-altitude ozone layer helps protect the biosphere from ultraviolet radiation. However, ozone present at the surface is a byproduct of thus a pollutant. Oxygen was isolated by Michael Sendivogius before 1604, but it is believed that the element was discovered independently by Carl Wilhelm Scheele, in Uppsala, in 1773 or earlier, Joseph Priestley in Wiltshire, in 1774. Priority is given for Priestley because his work was published first. Priestley, called oxygen "dephlogisticated air", did not recognize it as a chemical element; the name oxygen was coined in 1777 by Antoine Lavoisier, who first recognized oxygen as a chemical element and characterized the role it plays in combustion. Common uses of oxygen include production of steel and textiles, brazing and cutting of steels and other metals, rocket propellant, oxygen therapy, life support systems in aircraft, submarines and diving.
One of the first known experiments on the relationship between combustion and air was conducted by the 2nd century BCE Greek writer on mechanics, Philo of Byzantium. In his work Pneumatica, Philo observed that inverting a vessel over a burning candle and surrounding the vessel's neck with water resulted in some water rising into the neck. Philo incorrectly surmised that parts of the air in the vessel were converted into the classical element fire and thus were able to escape through pores in the glass. Many centuries Leonardo da Vinci built on Philo's work by observing that a portion of air is consumed during combustion and respiration. In the late 17th century, Robert Boyle proved. English chemist John Mayow refined this work by showing that fire requires only a part of air that he called spiritus nitroaereus. In one experiment, he found that placing either a mouse or a lit candle in a closed container over water caused the water to rise and replace one-fourteenth of the air's volume before extinguishing the subjects.
From this he surmised that nitroaereus is consumed in both combustion. Mayow observed that antimony increased in weight when heated, inferred that the nitroaereus must have combined with it, he thought that the lungs separate nitroaereus from air and pass it into the blood and that animal heat and muscle movement result from the reaction of nitroaereus with certain substances in the body. Accounts of these and other experiments and ideas were published in 1668 in his work Tractatus duo in the tract "De respiratione". Robert Hooke, Ole Borch, Mikhail Lomonosov, Pierre Bayen all produced oxygen in experiments in the 17th and the 18th century but none of them recognized it as a chemical element; this may have been in part due to the prevalence of the philosophy of combustion and corrosion called the phlogiston theory, the favored explanation of those processes. Established in 1667 by the German alchemist J. J. Becher, modified by the chemist Georg Ernst Stahl by 1731, phlogiston theory stated that all combustible materials were made of two parts.
One part, called phlogiston, was given off when the substance containing it was burned, while the dephlogisticated part was thought to be its true form, or calx. Combustible materials that leave little residue, such as wood or coal, were thought to be made of phlogiston. Air did not play a role in phlogiston theory, nor were any initial quantitative experiments conducted to test the idea. Polish alchemist and physician Michael Sendivogius in his work De Lapide Philosophorum Tractatus duodecim e naturae fonte et manuali experientia depromti described a substance contained in air, referring to it as'cibus vitae', this substance is identical with oxygen. Sendivogius, during his experiments performed between 1598 and 1604, properly recognized that the substance is equivalent to the gaseous byproduct released by the thermal decomposition of potassium nitrate. In Bugaj’s view, the isolation of oxygen and the proper association of the substance to that part of air, required for life, lends sufficient weight to the discovery of oxygen by Sendivogius.
In chemistry, a salt is an ionic compound that can be formed by the neutralization reaction of an acid and a base. Salts are composed of related numbers of cations and anions so that the product is electrically neutral; these component ions can be inorganic, such as organic, such as acetate. Salts can be classified in a variety of ways. Salts that produce hydroxide ions when dissolved in water are called alkali salts. Salts that produce acidic solutions are acidic salts. Neutral salts are those salts that are neither basic. Zwitterions contain an anionic and a cationic centres in the same molecule, but are not considered to be salts. Examples of zwitterions include amino acids, many metabolites and proteins. Solid salts tend to be transparent. In many cases, the apparent opacity or transparency are only related to the difference in size of the individual monocrystals. Since light reflects from the grain boundaries, larger crystals tend to be transparent, while the polycrystalline aggregates look like white powders.
Salts exist in many different colors, which arise either from the cations. For example: sodium chromate is yellow by virtue of the chromate ion potassium dichromate is orange by virtue of the dichromate ion cobalt nitrate is red owing to the chromophore of hydrated cobalt. copper sulfate is blue because of the copper chromophore potassium permanganate has the violet color of permanganate anion. Nickel chloride is green of sodium chloride, magnesium sulfate heptahydrate are colorless or white because the constituent cations and anions do not absorb in the visible part of the spectrumFew minerals are salts because they would be solubilized by water. Inorganic pigments tend not to be salts, because insolubility is required for fastness; some organic dyes are salts, but they are insoluble in water. Different salts can elicit all five basic tastes, e.g. salty, sour and umami or savory. Salts of strong acids and strong bases are non-volatile and odorless, whereas salts of either weak acids or weak bases may smell like the conjugate acid or the conjugate base of the component ions.
That slow, partial decomposition is accelerated by the presence of water, since hydrolysis is the other half of the reversible reaction equation of formation of weak salts. Many ionic compounds exhibit significant solubility in water or other polar solvents. Unlike molecular compounds, salts dissociate in solution into cationic components; the lattice energy, the cohesive forces between these ions within a solid, determines the solubility. The solubility is dependent on how well each ion interacts with the solvent, so certain patterns become apparent. For example, salts of sodium and ammonium are soluble in water. Notable exceptions include potassium cobaltinitrite. Most nitrates and many sulfates are water-soluble. Exceptions include barium sulfate, calcium sulfate, lead sulfate, where the 2+/2− pairing leads to high lattice energies. For similar reasons, most alkali metal carbonates are not soluble in water; some soluble carbonate salts are: potassium carbonate and ammonium carbonate. Salts are characteristically insulators.
Molten salts or solutions of salts conduct electricity. For this reason, liquified salts and solutions containing dissolved salts are called electrolytes. Salts characteristically have high melting points. For example, sodium chloride melts at 801 °C; some salts with low lattice energies are liquid near room temperature. These include molten salts, which are mixtures of salts, ionic liquids, which contain organic cations; these liquids exhibit unusual properties as solvents. The name of a salt starts with the name of the cation followed by the name of the anion. Salts are referred to only by the name of the cation or by the name of the anion. Common salt-forming cations include: Ammonium NH+4 Calcium Ca2+ Iron Fe2+ and Fe3+ Magnesium Mg2+ Potassium K+ Pyridinium C5H5NH+ Quaternary ammonium NR+4, R being an alkyl group or an aryl group Sodium Na+ Copper Cu2+Common salt-forming anions include: Acetate CH3COO− Carbonate CO2−3 Chloride Cl− Citrate HOC2 Cyanide C≡N− Fluoride F− Nitrate NO−3 Nitrite NO−2 Oxide O2− Phosphate PO3−4 Sulfate SO2−4 Salts with varying number of hydrogen atoms, with respect to the parent acid, replaced by cations can be referred to as monobasic, dibasic or tribasic salts: Sodium phosphate monobasic Sodium phosphate dibasic Sodium phosphate tribasic Salts are formed by a chemical reaction between: A base and an acid, e.g. NH3 + HCl → NH4Cl A metal and an acid, e.g. Mg + H2SO4 → MgSO4 + H2 A metal and a non-metal, e.g. Ca + Cl2 → CaCl2 A base and an a
Merriam-Webster, Inc. is an American company that publishes reference books and is known for its dictionaries. In 1828, George and Charles Merriam founded the company as G & C Merriam Co. in Springfield, Massachusetts. In 1843, after Noah Webster died, the company bought the rights to An American Dictionary of the English Language from Webster's estate. All Merriam-Webster dictionaries trace their lineage to this source. In 1964, Encyclopædia Britannica, Inc. acquired Inc. as a subsidiary. The company adopted its current name in 1982. In 1806, Webster published A Compendious Dictionary of the English Language. In 1807 Webster started two decades of intensive work to expand his publication into a comprehensive dictionary, An American Dictionary of the English Language. To help him trace the etymology of words, Webster learned 26 languages. Webster hoped to standardize American speech, since Americans in different parts of the country used somewhat different vocabularies and spelled and used words differently.
Webster completed his dictionary during his year abroad in 1825 in Paris, at the University of Cambridge. His 1820s book contained 70,000 words, of which about 12,000 had never appeared in a dictionary before; as a spelling reformer, Webster believed that English spelling rules were unnecessarily complex, so his dictionary introduced American English spellings, replacing colour with color, waggon with wagon, centre with center. He added American words, including skunk and squash, that did not appear in British dictionaries. At the age of 70 in 1828, Webster published his dictionary. However, in 1840, he published the second edition in two volumes with much greater success. In 1843, after Webster's death, George Merriam and Charles Merriam secured publishing and revision rights to the 1840 edition of the dictionary, they published a revision in 1847, which did not change any of the main text but added new sections, a second update with illustrations in 1859. In 1864, Merriam published a expanded edition, the first version to change Webster's text overhauling his work yet retaining many of his definitions and the title "An American Dictionary".
This began a series of revisions. In 1884 it contained 118,000 words, "3000 more than any other English dictionary". With the edition of 1890, the dictionary was retitled Webster's International; the vocabulary was vastly expanded in Webster's New International editions of 1909 and 1934, totaling over half a million words, with the 1934 edition retrospectively called Webster's Second International or "The Second Edition" of the New International. The Collegiate Dictionary was introduced in 1898 and the series is now in its eleventh edition. Following the publication of Webster's International in 1890, two Collegiate editions were issued as abridgments of each of their Unabridged editions. With the ninth edition, the Collegiate adopted changes which distinguish it as a separate entity rather than an abridgment of the Third New International; some proper names were returned including names of Knights of the Round Table. The most notable change was the inclusion of the date of the first known citation of each word, to document its entry into the English language.
The eleventh edition includes more than 225,000 definitions, more than 165,000 entries. A CD-ROM of the text is sometimes included; this dictionary is preferred as a source "for general matters of spelling" by the influential The Chicago Manual of Style, followed by many book publishers and magazines in the United States. The Chicago Manual states. Merriam overhauled the dictionary again with the 1961 Webster's Third New International under the direction of Philip B. Gove, making changes that sparked public controversy. Many of these changes were in formatting, omitting needless punctuation, or avoiding complete sentences when a phrase was sufficient. Others, more controversial, signaled a shift from linguistic prescriptivism and towards describing American English as it was used at that time. Since the 1940s, the company has added many specialized dictionaries, language aides, other references to its repertoire; the G. & C. Merriam Company lost its right to exclusive use of the name "Webster" after a series of lawsuits placed that name in public domain.
Its name was changed to "Merriam-Webster, Incorporated", with the publication of Webster's Ninth New Collegiate Dictionary in 1983. Previous publications had used "A Merriam-Webster Dictionary" as a subtitle for many years and will be found on older editions; the company has been a subsidiary of Encyclopædia Britannica, Inc. since 1964. In 1996, Merriam-Webster launched its first website, which provided free access to an online dictionary and thesaurus. Merriam-Webster has published dictionaries of synonyms, English usage, biography, proper names, medical terms, sports terms, Spanish/English, numerous others. Non-dictionary publications include Collegiate Thesaurus, Secretarial Handbook, Manual for Writers and Editors, Collegiate Encyclopedia, Encyclopedia of Literature, Encyclopedia of World Religions. On February 16, 2007, Merriam-Webster announced the launch of a mobile dictionary and thesaurus service developed with mobile search-and-information provider AskMeNow. Consumers use the service to access definitions and synonyms via text message.
Services include Merr
Pentacene is a polycyclic aromatic hydrocarbon consisting of five linearly-fused benzene rings. This conjugated compound is an organic semiconductor; the compound generates excitons upon absorption of visible light. For this reason, this compound, a purple powder degrades upon exposure to air and light. Structurally, pentacene is one of the linear acenes, the previous one being tetracene and the next one being hexacene. In August 2009, a group of researchers from IBM published experimental results of imaging a single molecule of pentacene using an atomic force microscope. In July 2011, they used a modification of scanning tunneling microscopy to experimentally determine the shapes of the highest occupied and lowest unoccupied molecular orbitals. In 2012, pentacene-doped p-terphenyl was shown to be effective as the amplifier medium for a room-temperature maser. Pentacene was first synthesized in 1912 by British chemists William Hobson Mills and Mildred May Gostling. A classic method for pentacene synthesis is by the Elbs reaction.
Pentacenes can be prepared by extrusion of a small volatile component from a suitable precursor at 150 °C. The precursor itself is prepared in three steps from two molecules of α,α,α',α'-tetrabromo-o-xylene with a 7-tert-butoxybicyclohepta-2,5-diene by first heating with sodium iodide in dimethylformamide to undergo a series of elimination and Diels–Alder reactions to form the ring system hydrolysing the tert-butoxy group to an alcohol and followed by its oxidation to the ketone; the product is reported to have some solubility in chloroform and is therefore amenable to spin coating. Pentacene is soluble in hot chlorinated benzenes, such as 1,2,4-trichlorobenzene, from which it can be recrystallized to form platelets. 6,13-Substituted pentacenes are accessible through pentacenequinone by reaction with an aryl or alkynyl nucleophile followed by reductive aromatization. Another method is based on homologization of diynes by transition metals Functionalization of pentacene has allowed for control of the solid-state packing of this chromophore.
The choice of the substituents influences the solid-state packing and can be used to control whether the compound adopts 1-dimensional or 2-dimensional cofacial pi-stacking in the solid-state, as opposed to the herringbone packing observed for pentacene. Although pentacene's structure resembles that of other aromatic compounds like anthracene, its aromatic properties are poorly defined. A tautomeric chemical equilibrium exists between 6-methylene-6,13-dihydropentacene and 6-methylpentacene; this equilibrium is in favor of the methylene compound. Only by heating a solution of the compound to 200 °C does a small amount of the pentacene develop, as evidenced by the emergence of a red-violet color. According to one study the reaction mechanism for this equilibrium is not based on an intramolecular 1,5-hydride shift, but on a bimolecular free radical hydrogen migration. In contrast, isotoluenes with the same central chemical motif aromatize. Pentacene reacts with elemental sulfur in 1,2,4-trichlorobenzene to the compound hexathiapentacene.
X-ray crystallography shows that all the carbon-to-sulfur bond lengths are equal. In the crystal phase the molecules display aromatic stacking interactions, whereby the distance between some sulfur atoms on neighboring molecules can become less than the sum of two Van der Waals radii Like the related tetrathiafulvalene, this compound is studied in the field of organic semiconductors; the acenes may appear as planar and rigid molecules, but in fact they can be distorted. The pentacene depicted below: has an end-to end twist of 144° and is sterically stabilized by the six phenyl groups; the compound can be resolved into its two enantiomers with an unusually high reported optical rotation of 7400° although racemization takes place with a chemical half-life of 9 hours. Oligomers and polymers based on pentacene have been explored both synthetically as well as in device application settings. Polymer light emitting diodes have been constructed using conjugated copolymers containing fluorene and pentacene.
A few other conjugated pentacene polymers have been realized based on Sonogashira and Suzuki coupling reactions of a dibromopentacene monomer. Non-conjugated pentacene-based polymers have been synthesized via esterification of a pentacene diol monomer with bis-acid chlorides to form polymers 4a–b. Various synthetic strategies have been employed to form conjugated oligomers of pentacene 5a–c including a one-pot-four-bond forming procedure which provided a solution-processable conjugated pentacene dimer which exhibited photoconductive gain >10, placing its performance within the same order of magnitude as thermally evaporated films of non-functionalized pentacene which exhibited photoconductive gain >16 using analogous measurement techniques. A modular synthetic method to conjugated pentacene di-, tri- and tetramers has been reported, based on homo- and cross-coupling reactions of robust dehydropentacene intermediates. Non-conjugated oligomers 9–10 based on pentacene have been synthesized, including dendrimers 9–10 with up to 9 pentacene moieties per molecule with molar absorptivity for the most intense absorption > 2,000,000 M−1•cm−1.
Dendrimers 11–12 were shown to have improved perfor
Atomic force microscopy
Atomic force microscopy or scanning force microscopy is a very-high-resolution type of scanning probe microscopy, with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the optical diffraction limit. AFM is a type of scanning probe microscopy, with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the optical diffraction limit; the information is gathered by "feeling" or "touching" the surface with a mechanical probe. Piezoelectric elements that facilitate tiny but accurate and precise movements on command enable precise scanning; the AFM has three major abilities: force measurement and manipulation. In force measurement, AFMs can be used to measure the forces between the probe and the sample as a function of their mutual separation; this can be applied to perform force spectroscopy, to measure the mechanical properties of the sample, such as the sample's Young's modulus, a measure of stiffness.
For imaging, the reaction of the probe to the forces that the sample imposes on it can be used to form an image of the three-dimensional shape of a sample surface at a high resolution. This is achieved by raster scanning the position of the sample with respect to the tip and recording the height of the probe that corresponds to a constant probe-sample interaction; the surface topography is displayed as a pseudocolor plot. In manipulation, the forces between tip and sample can be used to change the properties of the sample in a controlled way. Examples of this include atomic manipulation, scanning probe lithography and local stimulation of cells. Simultaneous with the acquisition of topographical images, other properties of the sample can be measured locally and displayed as an image with high resolution. Examples of such properties are mechanical properties like stiffness or adhesion strength and electrical properties such as conductivity or surface potential. In fact, the majority of SPM techniques are extensions of AFM.
The major difference between atomic force microscopy and competing technologies such as optical microscopy and electron microscopy is that AFM does not use lenses or beam irradiation. Therefore, it does not suffer from a limitation in spatial resolution due to diffraction and aberration, preparing a space for guiding the beam and staining the sample are not necessary. There are several types of scanning microscopy including scanning probe microscopy. Although SNOM and STED use visible, infrared or terahertz light to illuminate the sample, their resolution is not constrained by the diffraction limit. Fig. 3 shows an AFM, which consists of the following features. Numbers in parentheses correspond to numbered features in Fig. 3. Coordinate directions are defined by the coordinate system; the small spring-like cantilever is carried by the support. Optionally, a piezoelectric element oscillates the cantilever; the sharp tip is fixed to the free end of the cantilever. The detector records the motion of the cantilever.
The sample is mounted on the sample stage. An xyz drive permits to displace the sample and the sample stage in x, y, z directions with respect to the tip apex. Although Fig. 3 shows the drive attached to the sample, the drive can be attached to the tip, or independent drives can be attached to both, since it is the relative displacement of the sample and tip that needs to be controlled. Controllers and plotter are not shown in Fig. 3. According to the configuration described above, the interaction between tip and sample, which can be an atomic scale phenomenon, is transduced into changes of the motion of cantilever, a macro scale phenomenon. Several different aspects of the cantilever motion can be used to quantify the interaction between the tip and sample, most the value of the deflection, the amplitude of an imposed oscillation of the cantilever, or the shift in resonance frequency of the cantilever; the detector of AFM measures the deflection of the cantilever and converts it into an electrical signal.
The intensity of this signal will be proportional to the displacement of the cantilever. Various methods of detection can be used, e.g. interferometry, optical levers, the piezoresistive method, the piezoelectric method, STM-based detectors. Note: The following paragraphs assume that'contact mode' is used. For other imaging modes, the process is similar, except that'deflection' should be replaced by the appropriate feedback variable; when using the AFM to image a sample, the tip is brought into contact with the sample, the sample is raster scanned along an x-y grid. Most an electronic feedback loop is employed to keep the probe-sample force constant during scanning; this feedback loop has the cantilever deflection as input, its output controls the distance along the z axis between the probe support and the sample support. As long as the tip remains in contact with the sample, the sample is scanned in the x-y plane, height variations in the sample will change the deflection of the cantilever; the feedback adjusts the height of the probe support so that the deflection is restored to a user-d