files lectures nanotechnology synthesis structures and properties

Nanotechnology: Synthesis, Structures and Properties ‘a review of current carbon nanomaterials and other nanoparticle technologies’ 2 1. Introduction 1.1 introduction to nanotechnology Research on new materials technology is attracting the attention of researchers all over the world. Developments are being made to improve the properties of the materials and to find alternative precursors that can give desirable properties on the materials. Nanotechnology, which is one of the new technologies, refers to the development of devices, structures, and systems whose size varies from 1 to 100 nanometers (nm). The last decade has seen advancement in every side of nanotechnology such as: nanoparticles and powders; nanolayers and coats; electrical, optic and mechanical nanodevices; and nanostructured biological materials. Presently, nanotechnology is estimated to be influential in the next 20-30 years, in all fields of science and technology. Nanotechnology is receiving a lot of attention of late across the globe. The term nano originates etymologically from the Greek, and it means “dwarf.” The term indicates physical dimensions that are in the range of one-billionth of a meter. This scale is called colloquially nanometer scale, or also nanoscale. One nanometer is approximately the length of two hydrogen atoms. Nanotechnology relates to the design, creation, and utilization of materials whose constituent structures exist at the nanoscale; these constituent structures can, by convention, be up to 100 nm in size. Nanotechnology is a growing field that explores electrical, optical, and magnetic activity as well as structural behavior at the molecular and submolecular level. One 3 of the practical applications of nanotechnology (but certainly not the only one) is the science of constructing computer chips and other devices using nanoscale building elements. Nanoscale structures permit the control of fundamental properties of materials without changing the materials’ chemical status. As it might be inferred, nanotechnology is highly interdisciplinary as a field, and it requires knowledge drawn from a variety of scientific and engineering arenas: Designing at the nanoscale is working in a world where physics, chemistry, electrical engineering, mechanical engineering, and even biology become unified into an integrated field. “Building blocks” for nanomaterials include carbon-based components and organics, semiconductors, metals, and metal oxides; nanomaterials are the infrastructure, or building blocks, for nanotechnology. The term nanotechnology was introduced by Nori Taniguchi in 1974 at the Tokyo International Conference on Production Engineering. He used the word to describe ultrafine machining: the processing of a material to nanoscale precision. This work was focused on studying the mechanisms of machining hard and brittle materials such as quartz crystals, silicon, and alumina ceramics by ultrasonic machining. Years earlier, in a lecture at the annual meeting of the American Physical Society in 1959 (There’s Plenty of Room at the Bottom) American Physicist and Nobel Laureate Richard Feynman argued (although he did not coin or use the word nanotechnology) that the scanning electron microscope could be improved in resolution and stability, so that one would be able to “see” atoms. Feynman proceeded to predict the ability to arrange atoms the way a researcher would want them, within the bounds of chemical stability, in order to build tiny structures that in turn would lead to molecular or atomic synthesis of materials [6]. Based on Feynman’s idea, K. E. Drexler advanced 4 the idea of “molecular nanotechnology” in 1986 in the book Engines of Creation, where he postulated the concept of using nanoscale molecular structures to act in a machinelike manner to guide and activate the synthesis of larger molecules. Drexler proposed the use of a large number (billions) of roboticlike machines called “assemblers” (or nanobots) that would form the basis of a molecular manufacturing technology capable of building literally anything atom by atom and molecule by molecule Nanomaterials give impetus to new applications of the (nano)technology becausethey exhibit novel optical, electric ,and/or magnetic properties. The first generation of nanotechnology (late 1990s–early 2000s) focused on performance enhancements to existing micromaterials; the second generation of nanotechnology (slated for2006– 2007) will start employing nanomaterials in much more significant and radical ways. Nanomaterials with structural features at the nanoscale can be found in the form of clusters, thin films, multilayers, and nanocrystalline materials often expressed by the dimensionality of 0,1,2 and 3; the materials of interest include metals, amorphous and crystalline alloys, semiconductors, oxides, nitride and carbide ceramics in the form of clusters, thin films, multilayers, and bulk nanocrystalline materials. All products are manufactured from atoms, however, interestingly, the properties of those products depend on how those atoms are arranged. For example, by rear- ranging the atoms in coal (carbon),one can make diamonds. It should be noted that current manufacturing techniques are very rudimentary at the atomic/molecular level: casting, grinding, milling, and even lithography move atoms in bulk rather than in a “choreographed”or “highly controlled”fashion. On the other hand, with nanotechnology one is able to assemble the fundamental building blocks of nature(atoms, molecules,etc.),within the constraints of the laws of physics, but in 5 ways that may not occur naturally or in ways to create some existing structure but by synthesizing it out of cheaper forms or constituent elements. Nanomaterials often have properties dramatically different from their bulk-scale counterparts; for example, nanocrystalline copper is five times harder than ordinary copper with its micrometer-sized crystalline structure. A goal of nanotechnology is to close the size gap between the smallest lithographically fabricated structures and chemically synthesized large molecules. 2.1 History of fullerenes Fullerenes are large, closed-cage, carbon clusters and have several special properties that were not found in any other compound before. Therefore, fullerenes in general form an interesting class of compounds that surely will be used in future technologies and applications. Before the first synthesis and detection of the smaller fullerenes C60 and C70, it was generally accepted that these large spherical molecules were unstable. However, some Russian scientists 1,2 already had calculated that C60 in the gas phase was stable and had a relatively large band gap. As is the case with numerous, important scientific discoveries, fullerenes were accidentally discovered. In 1985, Kroto and Smalley 3 found strange results in mass spectra of evaporated carbon samples. Herewith, fullerenes were discovered and their stability in the gas phase was proven. The search for other fullerenes had started. 6 Figure 1.1: structures of fullerenes. As is the case with numerous, important scientific discoveries, fullerenes were accidentally discovered. In 1985, Kroto and Smalley found strange results in mass spectra of evaporated carbon samples. Herewith, fullerenes were discovered and their stability in the gas phase was proven. The search for other fullerenes had started. There are many other fullerenes of different shapes and sizes, such as C70, C82 etc. 2.2 Carbon nanotube structure With the revolutionary discovery of so-called fullerenes and carbon nanotubes, different research fields in the domain of carbon experienced an enormous boom. Fullerenes are spherical molecules, the smallest of which composed of 60 carbon atoms that are arranged like the edges of the hexagons and pentagons on a football. Nanotubes can be described as a rolled-up tubular shell of graphene sheet [see Figure 2.2a], which is made of benzene-type hexagonal rings of carbon atoms. The body of 7 the tubular shell is thus mainly made of hexagonal rings (in a sheet) of carbon atoms, whereas the ends are capped by half-dome shaped half-fullerene molecules. Due to their special one-dimensional form, they have interesting physical properties like they have metallic or semiconducting electrical conductivity depending on the chirality’s of the carbon atoms in the tube. Nanotubes have a large geometric aspect ratio and they are the first nanocavities. This and other properties one would like to use in different applications e.g. as electrode material in super capacitors and hydrogen storage material for the fuel storage or as field emitters in flat panel displays [Deepak et al., 2003]. Figure 1.2:(a) A graphene sheet made of C atoms placed at the corners of hexagons forming the lattice with arrows AA and ZZ denoting the rolling direction of the sheet to make (b) a (5,5) armchair nanotube and (c) a (10,0) zigzag nanotube [Deepak et al., 2003]. 8 Carbon nanotubes may be classified into three different types: armchair, zigzag, and chiral nanotubes, depending on how the two-dimensional graphene sheet is "rolled up". The first of the three structural categories is zigzag, which is named for the pattern of hexagons as one move circumferentially around the body of the tubule (Figure 1.3(a)). The second of these nanotube structures is termed armchair, which describes one of the two conformers of cyclohexane, a hexagon of carbon atoms, and describes the shape of the hexagons as one move around the body of the tubule (Figure 1.3(c)). The third form is known as chiral (Figure 1.3(b)) and is believed to be the most commonly occurring SWNT. The name chiral means handedness and indicates that the tubes may twist in either direction. The geometry of the chiral SWCNT lies between that of the armchair and zigzag SWCNTs (see Figure 1.3 (b)) [Ray et al., 2002]. A single walled carbon nanotube (SWCNT) can be described as a rolled up graphene sheet that is closed at each end with half of a fullerene. A nanotube is usually characterized by its diameter d t and the chiral angle θ (0 ≤ |θ| ≤ 30°) (Figure 2.5). The chiral vector C h is defined with the two integers (n, m) and the basis vectors of the graphene sheet [Harris, 2001; Dresselhaus et al., 1998 & 2001; Thomas, 1997and Saito et al., 1993]: C h = n*a 1 +m*a 2 (1.1) The so-called chiral vector of the nanotube, C h , where a 1 and a 2 are unit vectors in the two-dimensional hexagonal lattice, and n and m are integers. Another important parameter is the chiral angle, which is the angle between C h and a [Kiang et al., 1998]. 9 Figure 1.3: (a) Zig-Zag Single-Walled Nanotube. Note the zig-zag pattern around circumference and m = 0. (b) Chiral Single-Walled Nanotube. Note twisting of hexagons around tubule body. (c) Armchair Single-Walled Nanotube. Note the chair- like pattern around circumference and n = m [Harris, 2001]. 10 Figure 1.4: Schematic illustrations of the structures of (A) armchair, (B) zigzag, and(C) chiral SWNTs. Projections normal to the tube axis and perspective views along the tube axis are on the top and bottom, respectively. (D) Tunneling electron microscope image showing the helical structure of a 1.3-nm-diameter chiral SWNT. (E) Transmission electron microscope (TEM) image of a MWNT containing a concentrically nested array of nine SWNTs. (F) TEM micrograph showing the lateral packing of 1.4-nm-diameter SWNTs in a bundle. (G) Scanning electron microscope (SEM) image of an array of MWNTs grown as a nanotube forest [Ray et al., 2002]. To discriminate between different types of carbon nanotubes, the chiral angle and vector play an important role in determining the important properties of nanotubes. Armchair nanotubes are formed when n = m and the chiral angle is 30°. Zigzag nanotubes are formed when either n or m is zero and the chiral angle is 0°. All other nanotubes, with chiral angles intermediate between 0° and 30°, are known as chiral nanotubes. The properties of nanotubes are also determined by their diameter, which depends on n and m. A nanotube is usually characterized by its diameter d t and the [...]... nanotubes and carbon nanoparticles In the case of pure graphite electrodes, MWNTs would be synthesised, but uniform SWNTs could be synthesised if a mixture of graphite with Co, Ni, Fe or Y was used instead of pure graphite SWNTs synthesised this way exist as ', see Figure 2-10 28,30 Laser vaporisation results ropes' in a higher yield for SWNT synthesis and the nanotubes have better properties and a narrower... metallic The differences in conducting properties are caused by the molecular structure that results in a different band structure and thus a different band gap The differences in conductivity can easily be derived from the graphene sheet 8 properties It was shown that a (n,m) nanotube is metallic as accounts that: n=m or (n-m) = 3i, where i is an integer and n and m are defining the nanotube The resistance... very hot vapour plume forms, then expands and cools rapidly As the vaporised species cool, small carbon molecules and atoms quickly condense to form larger clusters, possibly including fullerenes The catalysts also begin to condense, but more slowly at first, and attach to carbon clusters and prevent their closing into 30 cage structures Catalysts may even open cage structures when they attach to them... and schematic representation of carbon nanofibers with their graphite platelets, (a) "perpendicular" and (b) "parallel" to the fiber axis [www.wtec.org, 2002] 2.5 Special properties of carbon nanotubes Electronic, molecular and structural properties of carbon nanotubes are determined to a large extent by their nearly one dimensional structure The most important properties of carbon nano materials and. .. Figure 2-3), decreases the defects and gives cleaner nanotubes, and thus improves the 20 oxidation resistance 27 Figure 2-3: Schematic drawings of the electrode set-ups for (a) the conventional and (b) the new arc discharge electrodes The Raman spectrum of the newly synthesised nanotubes shows that the nanotubes formed are cleaner and less defective compared with those synthesised by conventional methods... plasma arc 15 and the temperature of the deposit form on the carbon electrode Insight in the growth mechanism is increasing and measurements have shown that different diameter distributions have been found depending on the mixture of helium and argon These mixtures have different diffusions coefficients and thermal conductivities These properties affect the speed with which the carbon and catalyst... SWNT synthesis are that the product contains a lot of metal catalyst, SWNTs have defects and purification is hard to perform On the other hand, an advantage is that the diameter can slightly be controlled by changing thermal transfer and diffusion, and hence condensation of atomic carbon and metals between the plasma and the vicinity of the cathode can control nanotube diameter in the arc process This... dissolving, diffusing, and precipitating through the catalyst droplets in CVD process The dissolving, diffusing and precipitating rates of the carbon atoms are affected by both the carbon atoms concentration and the temperature The carbon precipitation region on the Fe catalyst droplets can be distinguished into two areas, surface area and internal area At low temperature , the dissolving and diffusing rates... apparatus under different conditions b) Magnetic field synthesis 23 Synthesis of MWNTs in a magnetic field gives defect-free and high purity MWNTs that can be applied as nanosized electric wires for device fabrication In this case, the arc discharge synthesis was controlled by a magnetic field around the arc plasma Figure 2-6: Schematic diagram of the synthesis system for MWNTs in a magnetic field Extremely... properties affect the speed with which the carbon and catalyst molecules diffuse and cool affecting nanotube diameter in the arc process This implies that single-layer tubules nucleate and grow on metal particles in different sizes depending on the quenching rate in the plasma and it suggests 24 that temperature and carbon and metal catalyst densities affect the diameter distribution of 15 nanotubes . Nanotechnology: Synthesis, Structures and Properties ‘a review of current carbon nanomaterials and other nanoparticle technologies’ . materials and to find alternative precursors that can give desirable properties on the materials. Nanotechnology, which is one of the new technologies, refers to the development of devices, structures, . structures, and systems whose size varies from 1 to 100 nanometers (nm). The last decade has seen advancement in every side of nanotechnology such as: nanoparticles and powders; nanolayers and coats;