introduction to nano science and technology---class lectures (part iii)

8/8/2019 Introduction to Nano Science and Technology---Class Lectures (Part III)

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Part-III


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Allotropes of Carbon

Allotropes : Allotropy or allotropism is a behavior exhibited by certain chemicalelements : these elements can exist in two or more different forms, known as allotropes of

that element. In each allotrope, the element's atoms are bonded together in a different

manner. Allotropes are different structural modifications of an element. Allotropes should

not be confused with isomers, which are chemical compounds that share the same

molecular formula but have different structural formulae.

For example, the element carbon has two common allotropes : diamond, where the

carbon atoms are bonded together in a tetrahedral lattice arrangement, and graphite,where the carbon atoms are bonded together in sheets of a hexagonal lattice.

Note that allotropy refers only to different forms of an element within the same phase or

state of matter (i.e. different solid, liquid or gas forms) - the changes of state between

solid, liquid and gas in themselves are not considered allotropy. For some elements,

allotropes have different molecular formulae which can persist in different phases - forexample, the two allotropes of oxygen (dioxygen, O2 and ozone, O3), can both exist in the

solid, liquid and gaseous states. Conversely, some elements do not maintain distinct

allotropes in different phases : for example phosphorus has numerous solid allotropes,

which all revert to the same P4form when melted to the liquid state.


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Fig. 38. Eight allotropes of carbon : a) diamond, b) graphite, c) Lonsdaleite, d) C60(Buckminsterfullerene or buckyball), e) C540, f) C70, g) amorphous carbon, andh) single-walled carbon nanotube or buckytube.


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Diamond - an extremely hard, transparent crystal, with the carbon atoms arranged in atetrahedral lattice. A poor electrical conductor. An excellent thermal conductor.

Lonsdaleite - also called hexagonal diamond.

Graphite - a soft, black, flaky solid, a moderate electrical conductor. The C atoms arebonded in flat hexagonal lattices (graphene), which are then layered in sheets.

Amorphous carbon

Fullerenes, including "buckyballs", such as C60.

Carbon nanotubes - allotropes of carbon with a cylindrical nanostructure.

Carbon nanobuds

Glassy carbon vitreous carbon

Carbon nanofoam

Carbyne - or linear acetylenic carbon (LAC). Here carbon is in linear modification withsp orbital hybridization.

Theoretically possible forms :

Metallic carbonChaoite

Cubic carbon

Prismane C8


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Fullerene

A fullerene is any molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid,or tube. Spherical fullerenes are also called buckyballs, and cylindrical ones are called carbonnanotubes or buckytubes. Fullerenes are similar in structure to graphite, which is composed ofstacked graphene sheets of linked hexagonal rings; but they may also contain pentagonal (orsometimes heptagonal) rings.

The first fullerene to be discovered, and the family's namesake, was buckminsterfullerene C60, madein 1985 by Robert Curl, Harold Kroto and Richard Smalley. The name was an homage to RichardBuckminster Fuller, whose geodesic domes it resembles. Fullerenes have since been found tooccur (if rarely) in nature.

The discovery of fullerenes greatly expanded the number of known carbon allotropes, which untilrecently were limited to graphite, diamond, and amorphous carbon such as soot and charcoal.Buckyballs and buckytubes have been the subject of intense research, both for their uniquechemistry and for their technological applications, especially in materials science, electronics, andnanotechnology.

With mass spectrometry, discrete peaks were observed corresponding to molecules with the exactmass of sixty or seventy or more carbon atoms. Kroto, Curl, and Smalley were awarded the 1996Nobel Prize in Chemistry for their roles in the discovery of this class of compounds. C

60and other

fullerenes were later noticed occurring outside the laboratory (e.g., in normal candle soot). By 1991,it was relatively easy to produce gram-sized samples of fullerene powder using the techniques ofDonald Huffman and Wolfgan Krtschmer. Fullerene purification remains a challenge to chemistsand to a large extent determines fullerene prices. So-called endohedral fullerenes have ions orsmall molecules incorporated inside the cage atoms. Fullerene is an unusual reactant in manyorganic reactions such as the Bingel reaction discovered in 1993. The first nanotubes wereobtained in 1991.


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Minute quantities of the fullerenes, in the form of C60, C70, C76, and C84 molecules, are produced innature, hidden in soot and formed by lightning discharges in the atmosphere.

Buckminsterfullerene C60 The Icosahedral fullerene C540


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Variations

Since the discovery of fullerenes in 1985, structural variations on fullerenes have evolved well beyondthe individual clusters themselves. Examples include :

Buckyball clusters : smallest member is C20 (unsaturated version of dodecahedrane) and the mostcommon is C60;

Nanotubes : hollow tubes of very small dimensions, having single or multiple walls; potentialapplications in electronics industry;

Megatubes : larger in diameter than nanotubes and prepared with walls of different thickness;potentially used for the transport of a variety of molecules of different sizes;

Polymers : chain, two-dimensional and three-dimensional polymers are formed under high pressurehigh temperature conditions

Nano"onions : spherical particles based on multiple carbon layers surrounding a buckyball core;

proposed for lubricants;

Linked "ball-and-chain" dimers : two buckyballs linked by a carbon chain;

Fullerene rings

Buckyball : Buckminsterfullerene (IUPAC name (C60-Ih)[5,6]fullerene) is the smallest fullerene moleculein which no two pentagons share an edge (which can be destabilizing, as in pentalene). It is also themost common in terms of natural occurrence, as it can often be found in soot.

The structure of C60 is a truncated (T = 3) icosahedron, which resembles a soccer ball of the type madeoftwenty hexagons and twelve pentagons, with a carbon atom at the vertices of each polygon and abond along each polygon edge.The van der Waals diameter of a C60 molecule is about 1 nm. The nucleus to nucleus diameter of a C60molecule is about 0.7 nm.The C60 molecule has two bond lengths. The 6 : 6 ring bonds (between two hexagons) can beconsidered "double bonds" and are shorter than the 6 : 5 bonds (between a hexagon and a pentagon).Its average bond length is 1.4 angstroms.

Silicon buckyballs have been created around metal ions


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Fig. 39. C60

with isosurface of ground state electron density as calculated with DFT.

Fig. 40. An association football is a model of the Buckminsterfullerene C60.
http://upload.wikimedia.org/wikipedia/commons/a/a7/C60_isosurface.png


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Boron buckyballA new type of buckyball utilizing boron atoms instead of the usual carbon has been predicted and describedby researchers at Rice University. The B-80 structure, with each atom forming 5 or 6 bonds, is predicted to bemore stable than the C-60 buckyball. One reason for this given by the researchers is that the B-80 is actuallymore like the original geodesic dome structure popularized by Buckminster Fuller which utilizes trianglesrather than hexagons. However, this work has been subject to much criticism by quantum chemists as it wasconcluded that the predicted Ih symmetric structure was vibrationally unstable and the resulting cageundergoes a spontaneous symmetry break yielding a puckered cage with rare Th symmetry (symmetry of avolleyball). The number of six atom rings in this molecule is 20 and number of five member rings is 12. Thereis an additional atom in the center of each six member ring, bonded to each atom surrounding it.

Variations of buckballs : Another fairly common buckminsterfullerene is C70, but fullerenes with 72, 76, 84 andeven up to 100 carbon atoms are commonly obtained.

In mathematical terms, the structure of a fullerene is a trivalent convex polyhedron with pentagonal and

hexagonal faces. In graph theory, the term fullerene refers to any 3-regular, planar graph with all faces of size5 or 6 (including the external face). It follows from Euler's polyhedron formula, |V|-|E|+|F| = 2, (where |V|, |E|,|F| indicate the number of vertices, edges, and faces), that there are exactly 12 pentagons in a fullerene and|V|/2-10 hexagons.

The smallest fullerene is the dodecahedron- - the unique C20. There are no fullerenes with 22 vertices. Thenumber of fullerenes C2n grows with increasing n = 12,13,14..., roughly in proportion to n

9. For instance, thereare 1812 non-isomorphic fullerenes C60. Note that only one form of C60, the buckminsterfullerene aliastruncated icosahedron, has no pair of adjacent pentagons (the smallest such fullerene). To further illustrate

the growth, there are 214,127,713 non-isomorphic fullerenes C200, 15,655,672 of which have no adjacentpentagons.

Trimetasphere carbon nanomaterials were discovered by researchers at Virginia Tech and licensedexclusively to Luna Innovations. This class of novel molecules comprises 80 carbon atoms (C80) forming asphere which encloses a complex of three metal atoms and one nitrogen atom. These fullerenes encapsulatemetals which puts them in the subset referred to as metallofullerenes. Trimetaspheres have the potential foruse in diagnostics (as safe imaging agents), therapeutics and in organic solar cells.


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Important properties

For the past decade, the chemical and physical properties of fullerenes have been a hot topic in the field ofresearch and development, and are likely to continue to be for a long time.

Researchers have been able to increase the reactivity of fullerenes by attaching active groups to theirsurfaces. Buckminsterfullerene does not exhibit "superaromaticity": that is, the electrons in the hexagonalrings do not delocalize over the whole molecule.

A spherical fullerene ofn carbon atoms has n pi-bonding electrons. These should try to delocalize over thewhole molecule. The quantum mechanics of such an arrangement should be like one shell only of the well-known quantum mechanical structure of a single atom, with a stable filled shell for n = 2, 8, 18, 32, 50, 72, 98,128, etc.; i.e. twice a perfect square number; but this series does not include 60. As a result, C60 in watertends to pick up two more electrons and become an anion. The nC60 may be the result of C60 trying to form aloose metallic bonding.

Fullerenes are stable, but not totally unreactive. The sp2

-hybridized carbon atoms, which are at their energyminimum in planar graphite, must be bent to form the closed sphere or tube, which produces angle strain.The characteristic reaction of fullerenes is electrophilic addition at 6,6-double bonds, which reduces anglestrain by changing sp2-hybridized carbons into sp3-hybridized ones. The change in hybridized orbitalscauses the bond angles to decrease from about 120 in the sp2 orbitals to about 109.5 in the sp3 orbitals.This decrease in bond angles allows for the bonds to bend less when closing the sphere or tube, and thus,the molecule becomes more stable.

Other atoms can be trapped inside fullerenes to form inclusion compounds known as endohedral fullerenes.

An unusual example is the egg shaped fullerene Tb3N@C84, which violates the isolated pentagon rule.Metallofullerene-based inoculates using the rhonditic steel process are beginning production as one of thefirst commercially-viable uses of buckyballs.

Solubility : Fullerenes are sparingly soluble in many solvents. Common solvents for the fullerenes includearomatics, such as toluene, and others like carbon disulfide. Solutions of pure buckminsterfullerene have adeep purple color. Solutions of C70 are a reddish brown. The higher fullerenes C76 to C84 have a variety ofcolors. C76 has two optical forms, while other higher fullerenes have several structural isomers. Fullerenesare the only known allotrope of carbon that can be dissolved in common solvents at room temperature.


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Solubility of C60 in some solvents shows unusual behaviour due to existence of solvate phases(analogues of crystallohydrates). For example, solubility of C60 in benzene solution shows maximum atabout 313 K. Crystallization from benzene solution at temperatures below maximum results information of triclinic solid solvate with four benzene molecules C604C6H6 which is rather unstable inair. Out of solution, this structure decomposes into usual fcc C60 in few minutes' time. At temperatures

above solubility maximum the solvate is not stable even when immersed in saturated solution andmelts with formation of fcc C60. Crystallization at temperatures above the solubility maximum resultsin formation of pure fcc C60. Large millimetre size crystals of C60 and C70 can be grown from solutionboth for solvates and for pure fullerenes.

Superconductivity : Haddon et al. found that intercalation of alkali-metal atoms in solid C60 leads tometallic behavior. In 1991, it was revealed that potassium-doped C60 becomes superconducting at 18K.This was the highest transition temperature for a molecular superconductor. Since then,superconductivity has been reported in fullerene doped with various other alkali metals. It has been

shown that the superconducting transition temperature in alkaline-metal-doped fullerene increaseswith the unit-cell volume. As caesium (Cs) forms the largest alkali ion, Cs-doped fullerene is animportant material in this family. Recently, superconductivity at 38K has been reported in bulk Cs3C60,but only under applied pressure. The highest superconducting transition temperature of 33 K atambient pressure is reported for Cs2RbC60.

Safety and toxicity : When considering toxicological data, care must be taken to distinguish asnecessary between what are normally referred to as fullerenes: (C60, C70,...); fullerene derivatives: C60

or other fullerenes with covalently bonded chemical groups; fullerene complexes (e.g., water-solubilized with surfactants, such as C60-PVP; host-guest complexes, such as with cyclodextrin;,where the fullerene is physically bound to another molecule; C60 nanoparticles, which are extendedsolid-phase aggregates of C60 crystallites; and nanotubes, which are generally much larger (in termsof molecular weight and size) compounds, and are different in shape to the spheroidal fullerenes C60and C70, as well as having different chemical and physical properties.


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C60 in crystallineform.

C60Br24 structure

C60Ph5H spacefill model (left) and crystals ofC60Ph5H (right)

La@C82 structure

PCBM
http://upload.wikimedia.org/wikipedia/commons/8/85/C60-Fulleren-kristallin.JPG


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Fullerene synthesisThe first method of production of fullerenes used laser vaporization of carbon in an inert atmosphere, but thisproduced microscopic amounts of fullerenes. In 1990, a new type of apparatus using an arc to vaporize graphite wasdeveloped in Germany by Kratschmer and Huffmann.

Method : Pump down the system and introduce He gas into the chamber. Repeat (purge). Finally fill the bell-jar withabout 100 Torr of Helium. Connect up the welding kit power supply. Turn the on / off switch on the supply to the on

position for 10 to 15 seconds. Afterwards there should be plenty of black soot like material produced inside the bell-jar.After a 5-10 min cool down period fill the bell-jar to atmospheric pressure. Take the bell-jar off and scrape the glasssurfaces clean, collect all the material. 10% of the soot should be made up of C60. The fullerenes in the soot are thenextracted by solvation in a small amount of toluene. After extraction, the solvent (toluene) is removed using a rotaryevaporator, leaving behind a solid mixture of mostly C60 with small amounts of larger fullerenes. Pure C60 is obtainedby liquid chromatography. The mixture is dissolved in toluene and pumped through a column of activated charcoalmixed with silica gel. The magenta C60 comes off first, followed by the red C70. The different color solutions arecollected separately and the toluene is removed using the rotary evaporator.


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Carbon Nanotubes

Carbon nanotubes (CNTs) are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed

with length-to-diameter ratio of up to 28,000,000 : 1, which is significantly larger than any other material. These

cylindrical carbon molecules have novel properties that make them potentially useful in many applications in

nanotechnology, electronics, optics and other fields of materials science, as well as potential uses in architectural fields.They exhibit extraordinary strength and unique electrical properties, and are efficient thermal conductors. Their final

usage, however, may be limited by their potential toxicity.

Nanotubes are members of the fullerene structural family, which also includes the spherical buckyballs. The ends of a

nanotube might be capped with a hemisphere of the buckyball structure. Their name is derived from their size, since the

diameter of a nanotube is on the order of a few nanometers (approximately 1/50,000th of the width of a human hair), while

they can be up to several millimeters in length (as of 2008). Nanotubes are categorized as single-walled nanotubes (SWNTs)

and multi-walled nanotubes (MWNTs).

The nature of the bonding of a nanotube is described by applied quantum chemistry, specifically, orbital hybridization.The chemical bonding of nanotubes is composed entirely ofsp2 bonds, similar to those of graphite. This bonding structure,

which is stronger than the sp3 bonds found in diamonds, provides the molecules with their unique strength. Nanotubes

naturally align themselves into "ropes" held together by Van der Waals forces. Under high pressure, nanotubes can merge

together, trading some sp2 bonds for sp3 bonds, giving the possibility of producing strong, unlimited-length wires through

high-pressure nanotube linking.


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Types of carbon nanotubes

Single-walled

Most single-walled nanotubes (SWNT) have a diameter of close to 1 nm, with a tube length that can be

many millions of times longer. The structure of a SWNT can be conceptualized by wrapping a one-atom-thick layer of graphite called graphene into a seamless cylinder. The way the graphene sheet iswrapped is represented by a pair of indices (n,m) called the chiral vector. The integers n and m denotethe number of unit vectors along two directions in the honeycomb crystal lattice of graphene. Ifm = 0,the nanotubes are called "zigzag". Ifn = m, the nanotubes are called "armchair". Otherwise, they arecalled "chiral.

Fig. 41. The (n,m) nanotube naming scheme can be thought of as a vector (Ch) in an infinite graphenesheet that describes how to "roll up" the graphene sheet to make the nanotube. T denotes the tube

axis, and a1 and a2 are the unit vectors of graphene in real space.


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Armchair (n,n) Chiral (n,m) Zigzag (n,0)

Graphene nanoribbon n and m can be countedat the end of the tube


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Single-walled nanotubes are an important variety of carbon nanotube because they exhibit electricproperties that are not shared by the multi-walled carbon nanotube (MWNT) variants. Single-wallednanotubes are the most likely candidate for miniaturizing electronics beyond the microelectromechanical scale currently used in electronics. The most basic building block of these systemsis the electric wire, and SWNTs can be excellent conductors. One useful application of SWNTs is in the

development of the first intramolecular field effect transistors. Production of the first intramolecularlogic gate using SWNT FETs has recently become possible as well. To create a logic gate you musthave both a p-FET and an n-FET. Because SWNTs are p-FETs when exposed to oxygen and n-FETsotherwise, it is possible to protect half of an SWNT from oxygen exposure, while exposing the otherhalf to oxygen. This results in a single SWNT that acts as a NOT logic gate with both p and n-type FETswithin the same molecule.

Multi-walledMulti-walled nanotubes (MWNT) consist of multiple rolled layers (concentric tubes) of graphite. Thereare two models which can be used to describe the structures of multi-walled nanotubes. In theRussian Doll model, sheets of graphite are arranged in concentric cylinders, e.g. a (0,8) single-wallednanotube (SWNT) within a larger (0,10) single-walled nanotube. In the Parchmentmodel, a single sheetof graphite is rolled in around itself, resembling a scroll of parchment or a rolled newspaper. Theinterlayer distance in multi-walled nanotubes is close to the distance between graphene layers in

graphite, approximately 3.3 (330 pm).The morphology and properties of DWNT are similar to SWNT but their resistance to chemicals issignificantly improved. This is especially important when functionalization is required (this meansgrafting of chemical functions at the surface of the nanotubes) to add new properties to the CNT. In thecase of SWNT, covalent functionalization will break some C=C double bonds, leaving "holes" in thestructure on the nanotube and thus modifying both its mechanical and electrical properties. In thecase of DWNT, only the outer wall is modified. DWNT synthesis on the gram-scale was first proposed

in 2003 by the CCVD technique, from the selective reduction of oxide solutions in methane andhydrogen.
http://en.wikipedia.org/wiki/Matryoshka_dollhttp://en.wikipedia.org/wiki/Scroll_%28parchment%29http://en.wikipedia.org/wiki/Scroll_%28parchment%29http://en.wikipedia.org/wiki/Matryoshka_dollhttp://en.wikipedia.org/wiki/Matryoshka_dollhttp://en.wikipedia.org/wiki/Matryoshka_doll


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A nanotorus is theoretically described as carbon nanotube bent into a torus (doughnut shape).Nanotori are predicted to have many unique properties, such as magnetic moments 1000 times largerthan previously expected for certain specific radii. Properties such as magnetic moment, thermalstability etc. vary widely depending on radius of the torus and radius of the tube.

Torus

NanobudCarbon nanobuds are a newly created material combining two previously discovered allotropes ofcarbon: carbon nanotubes and fullerenes. In this new material fullerene-like "buds" are covalentlybonded to the outer sidewalls of the underlying carbon nanotube. This hybrid material has usefulproperties of both fullerenes and carbon nanotubes. In particular, they have been found to beexceptionally good field emitters. In composite materials, the attached fullerene molecules mayfunction as molecular anchors preventing slipping of the nanotubes, thus improving the composites

mechanical properties.


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Important properties

Carbon nanotubes are the strongest and stiffest materials yet discovered in terms of tensile strength andelastic modulus respectively. This strength results from the covalent sp bonds formed between theindividual carbon atoms. In 2000, a multi-walled carbon nanotube was tested to have a tensile strength of 63(GPa). (This, for illustration, translates into the ability to endure tension of 6300 kg on a cable with cross-section of 1 mm2.) Since carbon nanotubes have a low density for a solid of 1.3 to 1.4 gcm 3, its specificstrength of up to 48,000 kNmkg1 is the best of known materials, compared to high-carbon steel's154 kNmkg1. Under excessive tensile strain, the tubes will undergo plastic deformation, which means thedeformation is permanent. This deformation begins at strains of approximately 5% and can increase themaximum strain the tubes undergo before fracture by releasing strain energy. CNTs are not nearly as strongunder compression. Because of their hollow structure and high aspect ratio, they tend to undergo bucklingwhen placed under compressive, torsional or bending stress.

Strength

Kinetic

Multi-walled nanotubes, multiple concentric nanotubes precisely nested within one another, exhibit a strikingtelescoping property whereby an inner nanotube core may slide, almost without friction, within its outernanotube shell thus creating an atomically perfect linear or rotational bearing. This is one of the first trueexamples of molecular nanotechnology, the precise positioning of atoms to create useful machines. Already

this property has been utilized to create the world's smallest rotational motor. Future applications such as agigahertz mechanical oscillator are also envisaged.

Thermal

All nanotubes are expected to be very good thermal conductors along the tube, exhibiting a property knownas "ballistic conduction," but good insulators laterally to the tube axis. It is predicted that carbon nanotubeswill be able to transmit up to 6000 Wm1K1 at room temperature; compare this to copper, a metal well-knownfor its good thermal conductivity, which transmits 385 Wm1K1. The temperature stability of carbon

nanotubes is estimated to be up to 2800 C in vacuum and about 750 C in air.


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Electrical

Because of the symmetry and unique electronic structure of graphene, the structure of a nanotube stronglyaffects its electrical properties. For a given (n,m) nanotube, ifn = m, the nanotube is metallic; ifn m is amultiple of 3, then the nanotube is semiconducting with a very small band gap, otherwise the nanotube is amoderate semiconductor. Thus all armchair (n = m) nanotubes are metallic, and nanotubes (5,0), (6,4), (9,1),

etc. are semiconducting. In theory, metallic nanotubes can carry an electrical current density of 4 109

A/cm2

which is more than 1,000 times greater than metals such as copper.

Defects

As with any material, the existence of a crystallographic defect affects the material properties. Defects canoccur in the form of atomic vacancies. High levels of such defects can lower the tensile strength by up to85%. Another form of carbon nanotube defect is the Stone Wales defect, which creates a pentagon andheptagon pair by rearrangement of the bonds. Because of the very small structure of CNTs, the tensilestrength of the tube is dependent on its weakest segment in a similar manner to a chain, where the strengthof the weakest link becomes the maximum strength of the chain.

Crystallographic defects also affect the tube's electrical properties. A common result is lowered conductivitythrough the defective region of the tube. A defect in armchair-type tubes (which can conduct electricity) cancause the surrounding region to become semiconducting, and single monoatomic vacancies inducemagnetic properties.

Crystallographic defects strongly affect the tube's thermal properties. Such defects lead to phonon

scattering, which in turn increases the relaxation rate of the phonons. This reduces the mean free path andreduces the thermal conductivity of nanotube structures. Phonon transport simulations indicate thatsubstitutional defects such as nitrogen or boron will primarily lead to scattering of high-frequency opticalphonons. However, larger-scale defects such as Stone Wales defects cause phonon scattering over a widerange of frequencies, leading to a greater reduction in thermal conductivity.


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One-dimensional transportDue to their nanoscale dimensions, electron transport in carbon nanotubes will take place through quantum effectsand will only propagate along the axis of the tube. Because of this special transport property, carbon nanotubes arefrequently referred to as one-dimensional in scientific articles.

Synthesis techniquesTechniques have been developed to produce nanotubes in sizeable quantities, including arc discharge, laser ablation,

high pressure carbon monoxide (HiPCO), and CVD. Most of these processes take place in vacuum or with processgases. CVD growth of CNTs can occur in vacuum or at atmospheric pressure. Large quantities of nanotubes can besynthesized by these methods; advances in catalysis and continuous growth processes are making CNTs more

commercially viable.

Arc dischargeNanotubes were observed in 1991 in the carbon soot of graphite electrodes during an arc discharge, by using a currentof 100 A, that was intended to produce fullerenes. However the first macroscopic production of carbon nanotubes was

made in 1992 by two researchers at NEC's Fundamental Research Laboratory. The method used was the same as in1991. During this process, the carbon contained in the negative electrode sublimates because of the high dischargetemperatures. Because nanotubes were initially discovered using this technique, it has been the most widely-usedmethod of nanotube synthesis.

The yield for this method is up to 30% by weight and it produces both single- and multi-walled nanotubes with lengthsof up to 50 micrometers with few structural defects.

Laser ablations

In the laser ablation process, a pulsed laser vaporizes a graphite target in a high-temperature reactor while an inert gasis bled into the chamber. Nanotubes develop on the cooler surfaces of the reactor as the vaporized carbon condenses.A water-cooled surface may be included in the system to collect the nanotubes.This process was developed by Dr. Richard Smalley and co-workers at Rice University, who at the time of the discoveryof carbon nanotubes, were blasting metals with a laser to produce various metal molecules. When they heard of theexistence of nanotubes they replaced the metals with graphite to create multi-walled carbon nanotubes. Later that yearthe team used a composite of graphite and metal catalyst particles (the best yield was from a cobalt and nickel mixture)to synthesize single-walled carbon nanotubes.The laser ablation method yields around 70% and produces primarily single-walled carbon nanotubes with a

controllable diameter determined by the reaction temperature. However, it is more expensive than either arc dischargeor chemical vapor deposition.


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CVD synthesis

The catalytic vapor phase deposition of carbon was first reported in 1959, but it was not until 1993 thatcarbon nanotubes were formed by this process. In 2007, researchers at the University of Cincinnati (UC)developed a process to grow aligned carbon nanotube arrays of 18 mm length on a FirstNano ET3000 carbonnanotube growth system.

During CVD, a substrate is prepared with a layer of metal catalyst particles, most commonly nickel, cobalt,iron, or a combination. The metal nanoparticles can also be produced by other ways, including reduction ofoxides or oxides solid solutions. The diameters of the nanotubes that are to be grown are related to the sizeof the metal particles. This can be controlled by patterned (or masked) deposition of the metal, annealing, orby plasma etching of a metal layer. The substrate is heated to approximately 700C. To initiate the growth ofnanotubes, two gases are bled into the reactor: a process gas (such as ammonia, nitrogen or hydrogen) anda carbon-containing gas (such as acetylene, ethylene, ethanol or methane). Nanotubes grow at the sites ofthe metal catalyst; the carbon-containing gas is broken apart at the surface of the catalyst particle, and the

carbon is transported to the edges of the particle, where it forms the nanotubes. This mechanism is stillbeing studied. The catalyst particles can stay at the tips of the growing nanotube during the growth process,or remain at the nanotube base, depending on the adhesion between the catalyst particle and the substrate.

CVD is a common method for the commercial production of carbon nanotubes. For this purpose, the metalnanoparticles are mixed with a catalyst support such as MgO or Al2O3 to increase the surface area for higheryield of the catalytic reaction of the carbon feedstock with the metal particles. One issue in this synthesisroute is the removal of the catalyst support via an acid treatment, which sometimes could destroy the originalstructure of the carbon nanotubes. However, alternative catalyst supports that are soluble in water haveproven effective for nanotube growth.

If a plasma is generated by the application of a strong electric field during the growth process (PECVD), thenthe nanotube growth will follow the direction of the electric field. By adjusting the geometry of the reactor it ispossible to synthesize vertically aligned carbon nanotubes (i.e., perpendicular to the substrate), amorphology that has been of interest to researchers interested in the electron emission from nanotubes.Without the plasma, the resulting nanotubes are often randomly oriented. Under certain reaction conditions,even in the absence of a plasma, closely spaced nanotubes will maintain a vertical growth direction resulting

in a dense array of tubes resembling a carpet or forest.


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Potential and current applications

The strength and flexibility of carbon nanotubes makes them of potential use in controlling othernanoscale structures, which suggests that they will have an important role in nanoengineering. Thehighest tensile strength in an individual multi-walled carbon nanotube has been tested to be is 63 GPa.

Carbon nanotubes were found in Damascus steel, possibly helping to account for the legendarystrength of the (almost ancient) swords made of it.

Structural

Because of the carbon nanotube's superior mechanical properties, many structures have been proposedranging from everyday items like clothes and sports gear to combat jackets and space elevators. However,the space elevator will require further efforts in refining carbon nanotube technology, as the practical tensile

strength of carbon nanotubes can still be greatly improved.ElectronicsCarbon nanotubes have many propertiesfrom their unique dimensions to an unusual current conductionmechanismthat make them ideal components of electrical circuits. For example, they have shown to exhibitstrong electron-phonon resonances, which indicate that under certain direct current (DC) bias and dopingconditions their current and the average electron velocity, as well as the electron concentration on the tubeoscillate at terahertz frequencies. These resonances could potentially be used to make terahertz sources orsensors.

Nanotube based transistors have been made that operate at room temperature and that are capable of digitalswitching using a single electron.

In 2001 IBM researchers demonstrated how nanotube transistors can be grown in bulk, somewhat like silicontransistors. Their process is called "constructive destruction" which includes the automatic destruction ofdefective nanotubes on the wafer.

The IBM process has been developed further and single-chip wafers with over ten billion correctly alignednanotube junctions have been created. In addition it has been demonstrated that incorrectly aligned

nanotubes can be removed automatically using standard photolithography equipment.


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The first nanotube integrated memory circuit was made in 2004. One of the main challenges has beenregulating the conductivity of nanotubes. Depending on subtle surface features a nanotube may act as aplain conductor or as a semiconductor. A fully automated method has however been developed to remove

non-semiconductor tubes.

Paper batteriesA paper battery is a battery engineered to use a paper-thin sheet of cellulose (which is the major constituentof regular paper, among other things) infused with aligned carbon nanotubes. The nanotubes act aselectrodes; allowing the storage devices to conduct electricity. The battery, which functions as both a lithium-ion battery and a supercapacitor, can provide a long, steady power output comparable to a conventionalbattery, as well as a supercapacitors quick burst of high energyand while a conventional battery contains anumber of separate components, the paper battery integrates all of the battery components in a singlestructure, making it more energy efficient.

Drug delivery

The nanotubes versatile structure allows it to be used for a variety of tasks in and around the body. Althoughoften seen especially in cancer-related incidents, the carbon nanotube is often used as a vessel fortransporting drugs into the body. The nanotube application potentially allows for the drug dosage to belowered by localizing its distribution. The nanotube commonly carries the drug one of two ways: the drug canbe attached to the side or trailed behind, or the drug can actually be placed inside the nanotube. Both ofthese methods are effective for the delivery and distribution of drugs inside the body.

Solar cells

Solar cells developed at the New Jersey Institute of Technology use a carbon nanotube complex, formed by amixture of light sensitive polymers, carbon nanotubes and carbon buckyballs to form snake-like structures.Buckyballs trap electrons, although they can't make electrons flow. Add sunlight to excite the light sensitivepolymers, and the buckyballs will grab the electrons. Nanotubes, behaving like copper wires, will then be ableto make the electrons or current flow.


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Ultracapacitors

MIT Laboratory for Electromagnetic and Electronic Systems uses nanotubes to improve ultracapacitors. Theactivated charcoal used in conventional ultracapacitors has many small hollow spaces of various size, whichcreate together a large surface to store electric charge. But as charge is quantized into elementary charges,i.e. electrons, and each such elementary charge needs a minimum space, a significant fraction of the

electrode surface is not available for storage because the hollow spaces are not compatible with the charge'srequirements. With a nanotube electrode the spaces may be tailored to sizefew too large or too smallandconsequently the capacity should be increased considerably.

Other applications

Carbon nanotubes have been implemented in nanoelectromechanical systems (NMES), including mechanicalmemory elements (NRAM being developed by Nantero Inc.) and nanoscale electric motors.

Carbon nanotubes have been proposed as a possible gene delivery vehicle and for use in combination withradiofrequency fields to destroy cancer cells.

In May 2005, Nanomix Inc placed on the market a hydrogen sensor which integrated carbon nanotubes on asilicon platform. Since then Nanomix has been patenting many such sensor applications such as in the fieldof carbon dioxide, nitrous oxide, glucose, DNA detection, etc.


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Fig. 42. The joining of two carbon nanotubes with different electrical properties to form a diode has

been proposed.
http://upload.wikimedia.org/wikipedia/commons/4/46/Louie_nanotube.jpg


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Quantum dotsQuantum dot is a semiconductor whose excitons are confined in all three spatial dimensions. As a result,they have properties that are between those of bulk semiconductors and those of discrete molecules. Theywere discovered by Louis E. Brus, who was then at Bell Labs. The term "Quantum Dot" was coined by MarkReed.

Researchers have studied quantum dots in transistors, solar cells, LEDs, and diode lasers. They have alsoinvestigated quantum dots as agents for medical imaging and hope to use them as qubits.

In layman's terms, quantum dots are semiconductors whose conducting characteristics are closely related tothe size and shape of the individual crystal. Generally, the smaller the size of the crystal, the larger thebandgap, the greater the difference in energy between the highest valence band and the lowest conductionband becomes, therefore more energy is needed to excite the dot, and concurrently, more energy is releasedwhen the crystal returns to its resting state. For example, in fluorescent dye applications, this equates to

higher frequencies of light emitted after excitation of the dot as the crystal size grows smaller, resulting in acolor shift from red to blue in the light emitted. The main advantages in using quantum dots is that becauseof the high level of control possible over the size of the crystals produced, it is possible to have very precisecontrol over the conductive properties of the material.

Quantum confinement in semiconductors :

In an unconfined (bulk) semiconductor, an electron-hole pair is typically bound within a characteristic length,which is called the exciton Bohr radius and is estimated by replacing the positively charged atomic core with

the hole in the Bohr formula. If the electron and hole are constrained further, then properties of thesemiconductor change. This effect is a form of quantum confinement, and it is a key feature in manyemerging electronic structures.

Qther quantum confined semiconductors include :

1. quantum wires, which confine electrons or holes in two spatial dimensions and allow free propagation inthe third,2. quantum wells, which confine electrons or holes in one dimension and allow free propagation in twodimensions


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Synthesis techniques

There are several ways to confine excitons in semiconductors, resulting in different methods to producequantum dots. In general, quantum wires, wells and dots are grown by advanced epitaxial techniques innanocrystals produced by chemical methods or by ion implantation, or in nanodevices made by state-of-the-art lithographic techniques,

Colloidal synthesis :Colloidal semiconductor nanocrystals are synthesized from precursor compounds dissolved in solutions,much like traditional chemical processes. The synthesis of colloidal quantum dots is based on a three-component system composed of : precursors, organic surfactants, and solvents. When heating a reactionmedium to a sufficiently high temperature, the precursors chemically transform into monomers. Once themonomers reach a high enough supersaturation level, the nanocrystal growth starts with a nucleationprocess.

The temperature during the growth process is one of the critical factors in determining optimal conditions forthe nanocrystal growth. It must be high enough to allow for rearrangement and annealing of atoms during thesynthesis process while being low enough to promote crystal growth.

Another critical factor that has to be stringently controlled during nanocrystal growth is the monomerconcentration. The growth process of nanocrystals can occur in two different regimes, focusing anddefocusing. At high monomer concentrations, the critical size (the size where nanocrystals neither grownor shrink) is relatively small, resulting in growth of nearly all particles. In this regime, smaller particles growfaster than large ones (since larger crystals need more atoms to grow than small crystals) resulting in

focusing of the size distribution to yield nearly monodisperse particles. The size focusing is optimal whenthe monomer concentration is kept such that the average nanocrystal size present is always slightly largerthan the critical size. When the monomer concentration is depleted during growth, the critical size becomeslarger than the average size present, and the distribution defocuses as a result of Ostwald ripening.

There are colloidal methods to produce many different semiconductors, including CdSe, CdS, InAs, and InP.These quantum dots can contain as few as 100 to 100,000 atoms within the quantum dot volume, with adiameter of 10 to 50 atoms. This corresponds to about 2 to 10 nm, and at 10 nm in diameter, nearly 3 millionquantum dots could be lined up end to end and fit within the width of a human thumb.


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Viral assembly :

Lee et al. (2002) reported using genetically engineered M13 bacteriophage viruses to create quantum dotbiocomposite structures. As a background to this work, it has previously been shown that geneticallyengineered viruses can recognize specific semiconductor surfaces through the method of selection bycombinatorial phage display. Additionally, it is known that liquid crystalline structures of wild-type viruses(Fd, M13, and TMV) are adjustable by controlling the solution concentrations, solution ionic strength, and the

external magnetic field applied to the solutions. Consequently, the specific recognition properties of the viruscan be used to organize inorganic nanocrystals, forming ordered arrays over the length scale defined byliquid crystal formation. Using this information, Lee et al. (2000) were able to create self-assembled, highlyoriented, self-supporting films from a phage and ZnS precursor solution. This system allowed them to varyboth the length of bacteriophage and the type of inorganic material through genetic modification andselection.

Electrochemical assembly :

Highly ordered arrays of quantum dots may also be self-assembled by electrochemical techniques. Atemplate is created by causing an ionic reaction at an electrolyte-metal interface which results in thespontaneous assembly of nanostructures, including quantum dots, onto the metal which is then used as amask for mesa-etching these nanostructures on a chosen substrate.

Bulk manufacturing of quantum dots :

Conventional, small-scale quantum dot manufacturing relies on a process called high temperature dualinjection which is impractical for most commercial applications that require large quantities of quantumdots. A reproducible method for creating larger quantities of consistent, high-quality quantum dots involves

producing nanoparticles from chemical precursors in the presence of a molecular cluster compound underconditions whereby the integrity of the molecular cluster is maintained and acts as a prefabricated seedtemplate. Individual molecules of a cluster compound act as a seed or nucleation point upon whichnanoparticle growth can be initiated. In this way, a high temperature nucleation step is not necessary toinitiate nanoparticle growth because suitable nucleation sites are already provided in the system by themolecular clusters. A significant advantage of this method is that it is highly scalable.


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Novel optical properties

An immediate optical feature of colloidal quantum dots is their coloration. While the material which makes upa quantum dot defines its intrinsic energy signature, the nanocrystal's quantum confined size is moresignificant at energies near the band gap. Thus quantum dots of the same material, but with different sizes,

can emit light of different colors. The physical reason is the quantum confinement effect.The larger the dot, the redder (lower energy) its fluorescence spectrum. Conversely, smaller dots emit bluer(higher energy) light. The coloration is directly related to the energy levels of the quantum dot. Quantitativelyspeaking, the bandgap energy that determines the energy (and hence color) of the fluorescent light isinversely proportional to the square of the size of the quantum dot. Larger quantum dots have more energylevels which are also more closely spaced. This allows the quantum dot to absorb photons containing lessenergy, i.e., those closer to the red end of the spectrum. Furthermore, it was shown that the lifetime offluorescence is determined by the size of the quantum dot. Larger dots have more closely spaced energy

levels in which the electron-hole pair can be trapped. Therefore, electron-hole pairs in larger dots live longercausing larger dots to show a longer lifetime.

As with any crystalline semiconductor, a quantum dot's electronic wave functions extend over the crystallattice. Similar to a molecule, a quantum dot has both a quantized energy spectrum and a quantized densityof electronic states near the edge of the band gap.

Qdots can be synthesized with larger (thicker) shells (CdSe qdots with CdS shells). The shell thickness hasshown direct correlation to the lifetime and emission intensity.

Fig. 43. Colloidal quantum dotsirradiated with a UV light.Different sized quantum dotsemit different color light due toquantum confinement.
http://upload.wikimedia.org/wikipedia/en/f/f4/QD_mini_rainbow.jpg


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Potential and current applications

Quantum dots are particularly significant for optical applications due to their theoretically highquantum yield. In electronic applications they have been proven to operate like a single-electrontransistor and show the Coulomb blockade effect. Quantum dots have also been suggested as

implementations of qubits for quantum information processing.The ability to tune the size of quantum dots is advantageous for many applications. For instance,larger quantum dots have a greater spectrum-shift towards red compared to smaller dots, and exhibitless pronounced quantum properties. Conversely, the smaller particles allow one to take advantage ofmore subtle quantum effects.

Being zero dimensional, quantum dots have a sharper density of states than higher-dimensionalstructures. 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 may be excited within thelocally enhanced electromagnetic field produced by the gold nanoparticles, which can then beobserved from the surface Plasmon resonance in the photoluminescent excitation spectrum of(CdSe)ZnS nanocrystals. High-quality quantum dots are well suited for optical encoding andmultiplexing applications due to their broad excitation profiles and narrow/symmetric emissionspectra. The new generations of quantum dots have far-reaching potential for the study of intracellularprocesses at the single-molecule level, high-resolution cellular imaging, long-term in vivo observationof cell trafficking, tumor targeting, and diagnostics.


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Fig. 44. Researchers at Los Alamos National Laboratory have developed a wireless device thatefficiently produces visible light, through energy transfer from thin layers of quantum wells to crystalsabove the layers.

Computing :Quantum dot technology is one of the most promising candidates for use in solid-state quantumcomputation. By applying small voltages to the leads, the flow of electrons through the quantum dotcan be controlled and thereby precise measurements of the spin and other properties therein can bemade. With several entangled quantum dots, or qubits, plus a way of performing operations, quantumcalculations and the computers that would perform them might be possible.

Bi l
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Biology :

In modern biological analysis, various kinds of organic dyes are used. However, with each passingyear, more flexibility is being required of these dyes, and the traditional dyes are often unable to meetthe expectations. To this end, quantum dots have quickly filled in the role, being found to be superiorto 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 (allowing much less photobleaching). It has

been estimated that quantum dots are 20 times brighter and 100 times more stable than traditionalfluorescent reporters. For single-particle tracking, the irregular blinking of quantum dots is a minor

drawback.

Semiconductor quantum dots have also been employed for in vitro imaging of pre-labeled cells. Theability to image single-cell migration in real time is expected to be important to several research areassuch as embryogenesis, cancer metastasis, stem-cell therapeutics, and lymphocyte immunology.

Scientists have proven that quantum dots are dramatically better than existing methods for delivering

a gene-silencing tool, known as siRNA, into cells.Another potential cutting-edge application of quantum dots is being researched, with quantum dotsacting as the inorganic fluorophore for intra-operative detection of tumors using fluorescencespectroscopy.

Photovoltaic devices :

Quantum dots may be able to increase the efficiency and reduce the cost of today's typical silicon

photovoltaic cells. According to an experimental proof from 2006, quantum dots of PbSe can produceas many as seven excitons from one high energy photon of sunlight (7.8 times the bandgap energy).This compares favourably to today's photovoltaic cells which can only manage one exciton per high-energy photon, with high kinetic energy carriers losing their energy as heat. This would not result in a7-fold increase in final output however, could boost the maximum theoretical efficiency from 31% to42%. Quantum dot photovoltaics would theoretically be cheaper to manufacture, as they can be made"using simple chemical reactions." The generation of more than one exciton by a single photon iscalled multiple exciton generation (MEG) or carrier multiplication.

Li ht itti d i


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Light emitting devices :

There are several inquiries into using quantum dots as light-emitting diodes to make displays andother light sources, such as "QD-LED" displays, and "QD-WLED" (White LED). In June, 2006, QDVision announced technical success in making a proof-of-concept quantum dot display and show abright emission in the visible and near infra-red region of the spectrum. Quantum dots are valued fordisplays, because they emit light in very specific gaussian distributions. This can result in a display

that more accurately renders the colors that the human eye can perceive. Quantum dots also requirevery little power since they are not color filtered. Additionally, since the discovery of "white-lightemitting" QD, general solid-state lighting applications appear closer than ever. A color liquid crystaldisplay (LCD), for example, is powered by a single fluorescent lamp that is color filtered to producered, green, and blue pixels. Displays that intrinsically produce monochromatic light can be moreefficient, since more of the light produced reaches the eye.


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Self-assembled monolayers (SAM)A self assembled monolayer (SAM) is an organized layer of amphiphilic (both hydrophilic andlipophilic) molecules in which one end of the molecule, the head group shows a special affinity for asubstrate. SAMs also consist of a tail with a functional group at the terminal end.

SAMs are created by the chemisorption of hydrophilic head groups onto a substrate from either thevapor or liquid phase followed by a slow two-dimensional organization of hydrophobic tail groups.Initially, adsorbate molecules form either a disordered mass of molecules or form a lying downphase, and over a period of hours, begin to form crystalline or semi-crystalline structures on the

substrate surface. The hydrophilic head groups assemble together on the substrate, while thehydrophobic tail groups assemble far from the substrate. Areas of close-packed molecules nucleateand grow until the surface of the substrate is covered in a single monolayer.

Adsorbate molecules adsorb readily because they lower the surface energy of the substrate and arestable due to the strong chemisorption of the head groups. These bonds create monolayers that aremore stable than the physisorbed bonds of Langmuir-Blodgett films. Thiol-metal bonds, for example,are on the order of 100 kJ/mol, making the bond stable in a wide variety of temperature, solvents, and

potentials. The monolayer packs tightly due to van der Waals interactions, thereby reducing its ownfree energy.
http://upload.wikimedia.org/wikipedia/commons/2/28/SAM_schematic.jpeg


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Selecting the type of head group depends on the application of the SAM. Typically, head groups areconnected to an alkyl chain in which the terminal end can be functionalized (i.e. adding OH, NH3, or

COOH groups) to vary the wetting and interfacial properties. An appropriate substrate is chosen toreact with the head group. Substrates can be planar surfaces, such as silicon and metals, or curvedsurfaces, such as nanoparticles. Thiols and disulfides are the most commonly used molecules forSAMs on noble metal substrates because of the strong affinity of sulfur for these metals. In addition,

gold is an inert and biocompatible material that is easy to acquire. It is also easy to pattern vialithography, a useful feature for applications in nanoelectromechanical systems (NEMS). Additionally,it can withstand harsh chemical cleaning treatments. Silanes are generally used on nonmetallic oxidesurfaces.

Areas of application for SAMs include biology, electrochemistry and electronics,nanoelectromechanical systems (NEMS) and microelectromechanical systems (MEMS), and everydayhousehold goods. SAMs can serve as models for studying membrane properties of cells and

organelles and cell attachment on surfaces. SAMs can also be used to modify the surface propertiesof electrodes for electrochemistry, general electronics, and various NEMS and MEMS. For example,the properties of SAMs can be used to control electron transfer in electrochemistry. They can serve toprotect metals from harsh chemicals and etchants. SAMs can also reduce sticking of NEMS andMEMS components in humid environments. In the same way, SAMs can alter the properties of glass. Acommon household product, Rain-X, utilizes SAMs to create a hydrophobic monolayer on carwindshields to keep them clear of rain.


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Nanocomposites

A nanocomposite is a multiphase solid material where one of the phases has one, two or threedimensions of less than 100 nm, or structures having nano-scale repeat distances between thedifferent phases that make up the material. In the broadest sense this definition can include porous

media, colloids, gels and copolymers, but is more usually taken to mean the solid combination of abulk matrix and nano-dimensional phase (s) differing in properties due to dissimilarities in structureand chemistry. The mechanical, electrical, thermal, optical, electrochemical, catalytic properties of thenanocomposite will differ markedly from that of the component materials. Size limits for these effectshave been proposed,


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In general, the nano reinforcement is dispersed into the matrix during processing. The percentage byweight (called mass fraction) of the nanoparticulates introduced can remain very low (on the order of0.5% to 5%) due to the low filler percolation threshold, especially for the most commonly used non-spherical, high aspect ratio fillers (e.g. nanometer-thin platelets, such as clays, or nanometer-diametercylinders, such as carbon nanotubes).

Ceramic-matrix nanocomposites :

In this group of composites the main part of the volume is occupied by a ceramic, i.e. a chemicalcompound from the group of oxides, nitrides, borides, silicides etc.. In most cases, ceramic-matrixnanocomposites encompass a metal as the second component. Ideally both components, the metallicone and the ceramic one, are finely dispersed in each other in order to elicit the particular nanoscopicproperties. Nanocomposite from these combinations were demonstrated in improving their optical,electrical and magnetic properties as well as tribological, corrosion-resistance and other protectiveproperties.

Metal-matrix nanocomposites :Polymer-matrix nanocomposites :

In the simplest case, appropriately adding nanoparticulates to a polymer matrix can enhance itsperformance, often in very dramatic degree, by simply capitalizing on the nature and properties of thenanoscale filler (these materials are better described by the term nanofilled polymer composites). Thisstrategy is particularly effective in yielding high performance composites, when good dispersion ofthe filler is achieved and the properties of the nanoscale filler are substantially different or better than

those of the matrix, for example, reinforcing a polymer matrix by much stiffer nanoparticles ofceramics, clays, or carbon nanotubes. Alternatively, the enhanced properties of high performancenanocomposites may be mainly due to the high aspect ratio and/or the high surface area of the fillers,since nanoparticulates have extremely high surface area to volume ratios when good dispersion isachieved.

Nanowires


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NanowiresA nanowire is a nanostructure, with the diameter of the order of a nm. Alternatively, nanowires can bedefined as structures that have a thickness or diameter constrained to tens of nms or less and anunconstrained length. At these scales, quantum mechanical effects are important which coined theterm "quantum wires". Many different types of nanowires exist, including metallic (e.g., Ni, Pt, Au),semiconducting (e.g., Si, InP, GaN, etc.), and insulating (e.g., SiO

2, TiO

2). Molecular nanowires are

composed of repeating molecular units either organic (e.g. DNA) or inorganic (e.g. Mo6S9-xIx).

The nanowires could be used, in the near future, to link tiny components into extremely small circuits.Using nanotechnology, such components could be created out of chemical compounds.

Typical nanowires exhibit aspect ratios (length-to-width ratio) of 1000 or more. As such they are oftenreferred to as one-dimensional (1-D) materials. Nanowires have many interesting properties that arenot seen in bulk or 3-D materials. This is because electrons in nanowires are quantum confinedlaterally and thus occupy energy levels that are different from the traditional continuum of energylevels or bands found in bulk materials.

Peculiar features of this quantum confinement exhibited by certain nanowires manifest themselves indiscrete values of the electrical conductance. Such discrete values arise from a quantum mechanicalrestraint on the number of electrons that can travel through the wire at the nanometer scale. Thesediscrete values are often referred to as the quantum of conductance and are integer values of 2e2/h ~12.9 k-1.

They are inverse of the well-known resistance unit h/e2, which is roughly equal to 25812.8 ohms, andreferred to as the von Klitzing constant RK (after Klaus von Klitzing, the discoverer of exactquantization). Since 1990, a fixed conventional value RK-90 is accepted.

Examples of nanowires include inorganic molecular nanowires (Mo6S9-xIx, Li2Mo6Se6), which can havea diameter of 0.9 nm and hundreds of micrometers long. Other important examples are based onsemiconductors such as InP, Si, GaN, etc., dielectrics (e.g. SiO2,TiO2), or metals (e.g. Ni, Pt).

There are many applications where nanowires may become important in electronic, opto-electronicand nanoelectromechanical devices, as additives in advanced composites, for metallic interconnectsin nanoscale quantum devices, as field-emitters and as leads for biomolecular nanosensors.


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Nanowires still belong to the experimental world of laboratories. However, they may complement or
http://upload.wikimedia.org/wikipedia/commons/8/83/Epitaxial_Nanowire_Heterostructures_SEM_image.jpg


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g p , y y preplace carbon nanotubes in some applications. Some early experiments have shown how they can beused to build the next generation of computing devices.

To create active electronic elements, the first key step was to chemically dope a semiconductornanowire. This has already been done to individual nanowires to create p-type and n-typesemiconductors.

The next step was to find a way to create a p-n junction, one of the simplest electronic devices. Thiswas achieved in two ways. The first way was to physically cross a p-type wire over an n-type wire. Thesecond method involved chemically doping a single wire with different dopants along the length. Thismethod created a p-n junction with only one wire.

After p-n junctions were built with nanowires, the next logical step was to build logic gates. Byconnecting several p-n junctions together, researchers have been able to create the basis of all logiccircuits: the AND, OR, and NOT gates have all been built from semiconductor nanowire crossings.

It is possible that semiconductor nanowire crossings will be important to the future of digitalcomputing. Though there are other uses for nanowires beyond these, the only ones that actually takeadvantage of physics in the nanometer regime are electronic.

Nanowires are being studied for use as photon ballistic waveguides as interconnects in quantumdot/quantum effect well photon logic arrays. Photons travel inside the tube, electrons travel on theoutside shell.

When two nanowires acting as photon waveguides cross each other the juncture acts as a quantumdot.

Conducting nanowires offer the possibility of connecting molecular-scale entities in a molecularcomputer. Dispersions of conducting nanowires in different polymers are being investigated for useas transparent electrodes for flexible flat-screen displays.

Because of their high Young's moduli, their use in mechanically enhancing composites is beinginvestigated. Because nanowires appear in bundles, they may be used as tribological additives toimprove friction characteristics and reliability of electronic transducers and actuators.

Because of their high aspect ratio, nanowires are also uniquely suited to dielectrophoreticmanipulation.


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Gas phase clustersGas-phase clusters, which are weakly bound aggregates comprised of either atoms or molecules,often display properties that lie between those of the gaseous and condensed states. Interestingquestions arise concerning how large a cluster must be before it will display bulk properties. Currentlythere is extensive research activity directed toward studies of their formation and varying propertiesand reactivity as a function of the degree of aggregation. Results serve to elucidate at the molecularlevel the course of change of a system to be followed from the gas to the condensed state, therebyenabling a spanning of the states of matter.

Clusters belong to a new category of materials ; in size they fall between bulk materials and theiratomic or molecular constituents. Sometimes they are considered to constitute a new form of mater,as their properties are fundamentally different from those of discrete molecules and bulk solids.

Schematic representation of cluster placed in between atom molecule and bulk material.


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Gas phase clusters are generated in cluster sources. There are many kinds of cluster sources, e.g.,Laser-vaporization flow condensation source, pulsed arc cluster ion source, laser ablation clustersource, supersonic nozzle source, Knudsen cell, ion sputtering source, etc.

Various cluster sources.

Detection and analysis of gas phase clusters :


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Detection and analysis of gas phase clusters :

Wien filter, Quadrupole mass filter, Time of flight mass filter

Schematic of (a) Wien filter and (b) quadrupole mass analyzer.

Various cluster types and their properties : metal clusters semiconductor clusters ionic clusters


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Various cluster types and their properties : metal clusters, semiconductor clusters, ionic clusters,metcars,


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Metcars

Metcars are closed-cage clusters made of metals and carbon. Various such clusters, such as Mo-C, Ti-C, Hf-C, V-C, Cr-C, etc. are known.

M l t t d t l ti l


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Monolayer protected metal nanoparticlesBrust, et. al., prepared dodecanethiol-protected gold nanoparticles with a core size in the range of 1-3nm. AuCl4

- was transferred to toluene using tetra octyl ammonium bromide as the phase transferagent.

Schematic showing the Brust method of preparing monolayer-protected clusters.

Functionalized metal nanoparticles


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Functionalized metal nanoparticles


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S l tti S l tti i i di th ti lti l h i it ll i ti f


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Superlattices : Superlattice is a periodic, synthetic multi-layer, wherein a unit cell, consisting ofsuccessive layers that are chemically different from their adjacent neighbors, is repeated. Thesematerials are characterized by their double periodicity in the structure, periodicity of atoms in theangstrom level, and periodicity of nanocrystals in the nanometer level.

Pictorial representation of a super lattice.


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Core-shell nanostructures/nanoshells

Core-shell nanoparticles, i.e., particles with a well defined core and a shell both in the nm range, havedemanding applications in pharmaceuticals, chemical engineering, biology, optics, drug delivery andmany other related areas in addition to chemistry. Investigations on these types of materials have

been catalyzed by their applicability in modern science and their technological edge overconventional materials. These are also ideal systems used for probing the interfaces of thenanoparticle core and the shell.

M t l t l id h ll ti l


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Metal- metal oxide core-shell nanoparticles


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Bimetallic core shell nanoparticles


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Bimetallic core-shell nanoparticles

Semiconductor core-shell nanoparticles :CdSe-CdS, CdSe-ZnS, CdTe-ZnTe, etc.

Polymer coated core shell nanoparticles


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Polymer coated core-shell nanoparticles


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Applications : Catalytic, biological, drug delivery, magnetism, sensing,chemical reactivity, etc.

N h ll


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NanoshellsA nanoshell is a type of spherical nanoparticle consisting of a dielectric core which is covered by athin metallic shell. These nanoshells involve a quasiparticle called plasmon which is a collectiveexcitation or quantum plasma oscillation where the electrons simultaneously oscillate with respect toall the ions.The simultaneous oscillation can be called plasmon hybridization where the tunability of theoscillation is associated with mixture of the inner and outer shell where they hybridize to give a lowerenergy or higher energy. This lower energy couples strongly to incident light whereas, the higherenergy is an anti-bonding and weakly combines to incident light. The hybridization interaction isstronger for thinner shell layers, hence, the thickness of the shell and overall particle radiusdetermines which wavelength of light it couples with. Nanoshells can be varied across a broad rangeof the light spectrum that spans the visible and near infrared regions. The interaction of light and

nanoparticles affects the placements of charges which affects the coupling strength. Incident lightpolarized parallel to the substrate gives a s-polarization, hence the charges are further from thesubstrate surface which gives a stronger interaction between the shell and core. Otherwise, a p-polarization is formed which gives a more strongly shifted plasmon energy causing a weakerinteraction and coupling.

Since nanoshells possess highly favorable optical and chemical properties it is often used for
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Since nanoshells possess highly favorable optical and chemical properties it is often used forbiomedical imaging, therapeutic applications, fluorescence enhancement of weak molecular emitters,surface enhanced Raman spectroscopy and surface enhanced infrared absorption spectroscopy.


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introduction to nano science and technology---class lectures (part iii)

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