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Crystallinity is the degree of structural order in a solid, often represented by a fraction or percentage as a measure of how likely atoms or molecules are to be arranged in a regular pattern, namely into a crystal. This property can often be measured directly using diffraction techniques, such as x-ray diffraction: greater crystallinity yields a sharper diffracted beam.
Amorphous solids such as glass have crystallinity very near zero, while perfect gemstones and semiconductor wafers have a crystallinity approaching one. Most structural materials, including the vast majority of alloys and a slim majority of plastics, show their best performance at some intermediate level of crystallinity.
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Topics Covered | | Background Materials Made from Nanometer-Sized Crystallites are Microstructurally Heterogeneous Grain Boundaries and Deviation of Nanostructured Materials Properties and Benefits of Nanocrystallites Made from Bulk Inorganic Solids Using Nanometals and Oxides as Catalysts, Starting Materials for Reactions, and in Nanocomposites | Background | | Included here are ceramics, metals, and metal oxide nanoparticles. In the last two decades a class of materials with a nanometer-sized microstructure have been synthesized and studied. These materials are assembled from nanometer-sized building blocks, mostly crystallites. The building blocks may differ in their atomic structure, crystallographic orientation, or chemical composition. In cases where the building blocks are crystallites, incoherent or coherent interfaces may be formed between them, depending on the atomic structure, the crystallographic orientation, and the chemical composition of adjacent crystallites. Materials Made from Nanometer-Sized Crystallites are Microstructurally Heterogeneous In other words, materials assembled of nanometer-sized building blocks are microstructurally heterogeneous, consisting of the building blocks (e.g. crystallites) and the regions between adjacent building blocks (e.g. grain boundaries). It is this inherently heterogeneous structure on a nanometer scale that is crucial for many of their properties and distinguishes them from glasses, gels, etc, that are microstructurally homogeneous. Grain Boundaries and Deviation of Nanostructured Materials Grain boundaries make up a major portion of the material at nanoscales, and strongly affect properties and processing. The properties of NsM (Nanostructured Materials) deviate from those of single crystals (or coarse grained polycrystals) and glasses with the same average chemical composition. This deviation results from the reduced size and dimensionality of the nanometer-sized crystallites, as well as from the numerous interfaces between adjacent crystallites. An attempt is made below to summarize the basic physical concepts and the microstructural features of equilibrium and non-equilibrium NsM. Properties and Benefits of Nanocrystallites Made from Bulk Inorganic Solids Nanocrystallites of bulk inorganic solids have been shown to exhibit size-dependent properties, such as lower melting points, higher energy gaps, and non-thermodynamic structures. In comparison to macro-scale powders, increased ductility has been observed in nanopowders of metal alloys. In addition, quantum effects from boundary values become significant, leading to such phenomena as quantum dot lasers. Using Nanometals and Oxides as Catalysts, Starting Materials for Reactions, and in Nanocomposites One of the primary applications of metals in chemistry is their use as heterogeneous catalysts in a variety of reactions. In general, heterogeneous catalyst activity is surface dependent. Due to their vastly increased surface area over macro-scale materials, nanometals and oxides are ultra-high activity catalysts. They are also used as desirable starting materials for a variety of reactions, especially solid-state routes. Nanometals and oxides are also widely used in the formation of nanocomposites. Aside from their synthetic utility, they have many useful and unique magnetic, electrical, and optical properties. |
Nanocrystal
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A nanocrystal is a crystalline material with dimensions measured in nanometers; a nanoparticle with a structure that is mostly crystalline. These materials are of huge technological interest since many of their electrical and thermodynamic properties show strong size dependence, and can therefore be controlled through careful manufacturing processes. Nanocrystals are also of interest since they often provide single domain crystalline systems that can be studied to provide information that can help explain the behaviour of macroscopic samples of similar materials, without the complicating presence of grain boundaries and other defects. Semiconductor nanocrystals in the sub 10nm size range are often referred to as quantum dots.
Nanocrystals made with zeolite are used as a filter to turn crude oil onto diesel fuel at an ExxonMobil oil refinery in Louisiana, a method cheaper than the conventional way.
A layer of nanocrystals is applied to new type of solar panel named SolarPly made by Nanosolar. It is cheaper than other solar panels, more flexible, and claims 12% efficiency. (Conventional solar panels convert 9% of the sun's energy into electricity.) Crystal tetrapods 40 nanometers wide convert photons into electricity, but only have 3% efficiency. (Source: National Geographic June 2006)
The current status of research and development on the structure and properties of nanocrystalline materials is reviewed. Nanocrystalline materials are polycrystalline materials with grain sizes of up to about 100 nm. Because of the extremely small dimensions, a large volume fraction of the atoms is located at the grain boundaries, and this confers special attributes to these materials. Nanocrystalline materials can be prepared by inert gas condensation, mechanical alloying, plasma deposition, spray conversion processing, and many other methods. These are briefly reviewed. A clear picture of the structure of nanocrystalline materials is only now emerging. Whereas the earlier workers had concluded that the structure of grain boundaries in nanocrystalline materials was quite different from that in coarse grained materials, recent studies have shown unambiguously that the structure of the grain boundaries is the same in both nanocrystalline and coarse grained materials. The properties of nanocrystalline materials are very often superior to those of conventional polycrystalline coarse grained materials. Nanocrystalline materials exhibit increased strength/hardness, enhanced diffusivity, improved ductility/toughness, reduced density, reduced elastic modulus, higher electrical resistivity, increased specific heat, higher thermal expansion coefficient, lower thermal conductivity, and superior soft magnetic properties in comparison with conventional coarse grained materials. New concepts of nanocomposites and nanoglasses are also being investigated with special emphasis on ceramic composites to increase their strength and toughness. There appears to be a great potential for applications in the near future for nanocrystalline materials. The extensive investigations in recent years on structure-property correlations in nanocrystalline materials have begun to unravel the complexities of these materials, and pave the way for successful exploitation of alloy design principles to synthesise better materials than hitherto available.
Quantum Dot Introduction Nice read.
Applications
Being quasi-zero dimensional, quantum dots have a sharper density of states than higher-dimensional structures. As a result, they have superior transport and optical properties, and are being researched for use in diode lasers, amplifiers, and biological sensors.
Quantum dots have quickly found their way into homes in many electronics. The new PlayStation 3 and high-definition DVD players (notably Blu-ray and HD-DVD) to come out all use a blue laser for data reading. The blue laser up until only a few years ago was beginning to be seen as something of an impossibility, until the synthesis of a blue quantum dot laser.
Quantum dots are one of the most hopeful candidates for solid-state quantum computation. By applying small voltages to the leads, one can control the flow of electrons through the quantum dot and thereby make precise measurements of the spin and other properties therein.
With several entangled quantum dots, or qubits, plus a way of performing operations, quantum calculations might be possible.
Another cutting edge application of quantum dots is also being researched as potential artificial fluorophore for intra-operative detection of tumors using fluorescence spectroscopy.
In modern biological analysis, various kinds of organic dyes are used. However, with each passing year, more flexibility is being required of these dyes, and the traditional dyes are simply unable to meet the necessary standards at times. To this end, Quantum Dots have quickly filled in the role, being found to be superior to traditional organic dyes on several counts, one of the most immediately obvious being brightness (owing to the high quantum yield) as well as their stability (much less photodestruction). For single particle tracking, the irregular blinking of quantum dots is a minor drawback. Currently under research as well is tuning of the toxicity.
In a paper published in the May 2004 issue of Physical Review Letters a team from Los Alamos National Laboratory found that quantum dots produce as many as three electrons from one high energy photon of sunlight. When today's photovoltaic solar cells absorb a photon of sunlight, the energy gets converted to at most one electron, and the rest is lost as heat. This could boost the efficiency of panels produced in research labs from today's 20-30% to 42%.[1] This work was reproduced one year later by an NREL team.
Another paper, published in the October 18, 2005 issue of the Journal of the American Chemical Society, reports that Michael Bowers II at Vanderbilt University discovered that certain size crystals of cadmium and selenium emit white light when excited by an ultraviolet laser. This emission appears to be coming from the surface of the crystal, rather than the center. The crystals contain either 33 or 34 pairs of atoms. While they are being pyrolytically synthesized, they preferentially form into just this size; so Bowers can make a batch of such crystals in about an hour. Another student then mixed these quantum dots into ordinary varnish, applied it to a blue LED, and observed that the emission is yellowish-white, like a light bulb. The researchers believe that it will be possible to achieve this emission of white light via electrical stimulation as well as photonic, and hope to demonstrate it soon.
There are several inquiries into using quantum dots to make displays and light sources: "QD-LED" displays, and "QD-WLED" (White LED) [1]. In June, 2006, QD Vision announced technical success in making a proof of concept quantum dot display. [2] Quantum dots are valued for displays, because they are very small, they emit colored light in very specific frequencies, and because they require very little power, since they are entirely self-illuminating. [3]
Rice scientists build world's first single-molecule car
'Nanocar' with buckyball wheels paves way for other molecular machines
Rice University scientists have constructed the world's smallest car -- a single molecule "nanocar" that contains a chassis, axles and four buckyball wheels.
The "nanocar" is described in a research paper that is available online and due to appear in an upcoming issue of the journal Nano Letters.
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Photo by Y. Shirai/Rice University |
"The synthesis and testing of nanocars and other molecular machines is providing critical insight in our investigations of bottom-up molecular manufacturing," said one of the two lead researchers, James M. Tour, the Chao Professor of Chemistry, professor of mechanical engineering and materials science and professor of computer science. "We'd eventually like to move objects and do work in a controlled fashion on the molecular scale, and these vehicles are great test beds for that. They're helping us learn the ground rules."
The nanocar consists of a chassis and axles made of well-defined organic groups with pivoting suspension and freely rotating axles. The wheels are buckyballs, spheres of pure carbon containing 60 atoms apiece. The entire car measures just 3-4 nanometers across, making it slightly wider than a strand of DNA. A human hair, by comparison, is about 80,000 nanometers in diameter.
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Photos by T. Sasaki/Rice University |
Other research groups have created nanoscale objects that are shaped like automobiles, but study co-author Kevin F. Kelly, assistant professor of electrical and computer engineering, said Rice's vehicle is the first that actually functions like a car, rolling on four wheels in a direction perpendicular to its axles.
Kelly and his group, experts in scanning tunneling microscopy (STM), provided the measurements and experimental evidence that verified the rolling movement.
"It's fairly easy to build nanoscale objects that slide around on a surface," Kelly said. "Proving that we were rolling - not slipping and sliding - was one of the most difficult parts of this project."
To do that, Kelly and graduate student Andrew Osgood measured the movement of the nanocars across a gold surface. At room temperature, strong electrical bonds hold the buckyball wheels tightly against the gold, but heating to about 200 degrees Celsius frees them to roll. To prove that the cars were rolling rather than sliding, Kelly and Osgood took STM images every minute and watched the cars progress. Because nanocars' axles are slightly longer than the wheelbase - the distance between axles - they could determine the way the cars were oriented and whether they moved perpendicular to the axles.
Shape control of CdSe nanocrystals
Peng X, Manna L, Yang W, Wickham J, Scher E, Kadavanich A, Alivisatos AP.
Department of Chemistry, University of California at Berkeley, and Lawrence Berkeley National Laboratory, 94720, USA.
Nanometre-size inorganic dots, tubes and wires exhibit a wide range of electrical and optical properties that depend sensitively on both size and shape, and are of both fundamental and technological interest. In contrast to the syntheses of zero-dimensional systems, existing preparations of one-dimensional systems often yield networks of tubes or rods which are difficult to separate. And, in the case of optically active II-VI and III-V semiconductors, the resulting rod diameters are too large to exhibit quantum confinement effects. Thus, except for some metal nanocrystals, there are no methods of preparation that yield soluble and monodisperse particles that are quantum-confined in two of their dimensions. For semiconductors, a benchmark preparation is the growth of nearly spherical II-VI and III-V nanocrystals by injection of precursor molecules into a hot surfactant. Here we demonstrate that control of the growth kinetics of the II-VI semiconductor cadmium selenide can be used to vary the shapes of the resulting particles from a nearly spherical morphology to a rod-like one, with aspect ratios as large as ten to one. This method should be useful, not only for testing theories of quantum confinement, but also for obtaining particles with spectroscopic properties that could prove advantageous in biological labelling experiments and as chromophores in light-emitting diodes.
PMID: 10716439 [PubMed - as supplied by publisher]
Mechanics of nanosprings: Stiffness and Young's modulus of molybdenum-based nanocrystals
- C. Durkan, A. Ilie, M. S. M. Saifullah, and M. E. Welland
- Nanoscale Science Laboratory, Department of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, United Kingdom
(Received 5 December 2001; accepted 18 April 2002)
We describe measurements of the stiffness and Young's modulus, Y of single crystals of molybdenum-based compounds. Atomic force microscopy is used first to image, and then to perform stiffness measurements on crystals which are growing up out of a substrate. Y is extracted by comparing the measured stiffness with that calculated from a continuum elasticity model of the crystals, whose experimentally indeterminate parameters are the Young's modulus and the geometry. We find a value for Y in the range 0.8–1.4 TPa, depending on the assumed geometry of the nanocrystal. As these crystals are essentially perfectly ordered on the micron scale, this opens the possibility of forming composite materials of immense strength. ©2002 American Institute of Physics.
doi:10.1063/1.1483927
PACS:
62.20.Dc, 81.40.Jj, 68.37.Ps, 62.25.+g, 61.46.+w
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