Nanoscience & Nanotechnology-II What is happening at a very, very small length scale?

Transcription

1 Nanoscience & Nanotechnology-II What is happening at a very, very small length scale?

2 Plan of the talk Fullerenes Graphene Carbon Nanotubes

3 Properties Fullerenes The most symmetrical large molecule Discovered in Nobel prize Chemistry 1996, Curl, Kroto, and Smalley C 60, also 70, 76 and facets (12 pentagons and 20 hexagons) - prototype Epcot center, Paris ~1 nm Architect: R. Buckminster Fuller

4 Buckminsterfullerene Molecule consisting of 60 C atoms sp 2 hybridized bonds Has 20 hexagons, 12 pentagons Other related structures have 70 or 84 C atoms from sciencedaily.com

5 from unusualife.com C 60 is named for Buckminster Fuller who designed geodesic domes.

6 Bucky Balls Bucky Ball (C 60 ) C 240 colliding with C 60 at 300 ev (Kinetic energy)

7 Original report of C60

8 1996 Nobel Prize in Chemistry Robert Curl, Sir Harold Kroto, Richard Smalley for their discovery of fullerenes. from Nobelprize.org

9 Buckyball Discovery 1985: British chemist Harry Kroto studied molecules with exactly sixty carbon atoms found near red giant stars Kroto collaborated with Richard Smalley and Robert Curl to recreate the conditions in the laboratory and form C 60 molecules by laser vaporization of graphite The scientists hypothesized that the molecules were made of hexagonal carbon rings blasted apart from the graphite structure, and that the molecule must be spheroid to satisfy valence requirements

10 Determining the C 60 Structure After considerable work, Kroto, Smalley, and Curl determined that the structure of the C 60 buckyball was a combination of 12 pentagonal and 20 hexagonal rings, forming a spheroid shape with 60 vertices for the 60 carbons. The pentagonal rings sit at the vertices of an icosahedron such that no 2 pentagonal rings are next to each other Curl, Kroto, and Smalley received the Nobel Prize in 1996 for their work. The architect R. Buckminster Fuller designed a geodesic dome for the 1967 Montreal World Exhibition with the same structure; the scientists thus named the new molecule Buckminsterfullerene, which was shortened to fullerene when referring to the family of molecules. Haymet, A. D. J. "Footballene: a Theoretical Prediction for the Stable, Truncated Icosahedral Molecule C60. Journal of the American Chemical Society 108 (1986):

11 Properties Fullerene Symmetric shape lubricant Large surface area catalyst High temperature (~500 o C) High pressure Hollow caging particles Ferromagnet? - polymerized C 60 - up to 220 o C

12 Properties Fullerene Chemically stable as graphite - most reactive at pentagons Crystal by weak van der Waals force Superconductivity - K 3 C 60 : 19.2 K - RbCs 2 C 60 : 33 K Kittel, Introduction to Solid State Physics, 7the ed

13 Bucky Ball properties Arranged in pentagons and hexagons A one atom thick seperation of two spaces; inside the ball and outside Highest tensile strength of any known 2D structure or element, including cross-section of diamonds which have the highest tensile strength of all known 3D structures (which is also a formation of carbon atoms) Also has the highest packing density of all known structures (including diamonds) Impenetrable to all elements under normal circumstances, even a helium atom with an energy of 5eV (electron Volt)

14 Synthesis The fullerenes were prepared by evaporation of carbon electrodes in an electric arc discharge process in helium atmosphere. The main part of the deposition system consists in a stainless steel, double walled, cylindrical chamber. Between the two chamber walls is flowing the cooling agent, the temperature being automatically controlled. The two electrodes were horizontally mounted near the bottom of a reaction chamber. For the anode electrode pure graphite electrodes of a diameter of 6 mm and the length of 140 mm are used. The second electrode consists in a pure graphite disk, mounted at the bottom of the reaction chamber. The reaction chamber acts also like a soot collector. The anode is mounted in a guiding system, controlled by a mechanical system in order to assure a constant distance between the two electrodes during arc discharge.

15 Synthesis of fullerenes

16 DC power supply unit was used and operated at the voltage of V and the current of A. To obtain the carbon soot, the chamber is evacuated of the air. After that, the chamber was filled in with helium gas at the pressure between 50 and 200 Torr. As current flows between electrodes, graphite electrode gets vaporized. The arc intensity is controlled by the distance between the electrodes. Fullerene C60 was extracted from the carbon soot collected in the chamber. The main technological parameters that control the process efficiency are the discharge current, the distance between the electrodes, the pressure and nature of the working gas, the electrodes composition, shape and dimensions.

17 Species of fullerenes Alkali-doped fullerenes Alkali atoms doped on fullerenes Exohedral Fullerenes Atoms, molecules, and complexes are attached to the exterior of the cage Endohedral Fullerenes Molecules are enclosed within the cage

18 Species of fullerenes Alkali-doped fullerenes As fullerene molecule is highly electronegative, it readily forms compounds with electron donating atoms, the most common examples being alkali metals forming alkali-doped fullerides, wherein alkali metal atoms fill in the space between Buckyballs and donate valence electron to the neighbouring C60 molecule. If alkali atoms are potassium or rubidium, the compounds are superconductors, and they conduct electric current without any resistance at temperatures below K, e.g., K 3 C 60, Rb 3 C 60.

19 Endohedral fullerenes & Exohedral Fullerenes When the atom trapped inside is a metal, they are known as metallo fullerenes. Most of endohedral materials are made out of C 82, C 84 or even higher fullerenes. The atoms that form stable endohedral compounds include lanthanum, yttrium, scandium, and some of the noble gases. As it is very difficult to open up carbon cage molecules to enclose a foreign atom inside, endohedral material must be synthesized while formation of the cage itself. (The accepted notation for endohedral material is to use symbol to show that the first material is inside the second, e.g., La@C 82 and Sc 84.) Exohedral fullerenes or fullerene derivatives, which are molecules formed by a chemical reaction between fullerenes and other chemical groups. Fullerene derivatives are also known as functionalized fullerenes. As fullerenes possess the conjugated p- system of electron, two main types of primary chemical transformations are possible on fullerene surface: addition reactions and redox reactions, which lead to covalent exohedral adducts and salts respectively. As fullerenes are insoluble in water, numerous derivatives of fullerenes have been synthesized with improved solubility profile.

20 Fullerenes Applications Hydrogen or oxygen storage: hydrogenation of fullerene produces hydrides. The reaction is reversible and can be catalyzed with metals (low pressure). Catalyst: fullerene promotes the conversion of methane into higher hydrocarbons and inhibits coking reactions. Sensor: fullerene based capacitors can be used to detect ppm of H2S in N2, ppm of water in isopropanol. Diamond precursor: fullerene can be transformed to diamond at high pressure (RT) or can be used as a diamond nucleation center during CVD. Alloy strengthening/hardening (Ti), improvement of electrical conductivity of Cu alloys. Biomedical field: inhibition of human HIV replication and HIV-1 protease. Biological antioxidant (radical sponge).

21 What is graphene? In late 2004, graphene was discovered by Andre Geim and Kostya Novoselov (Univ. of Manchester) Nobel Prize in Physics Q1. How thick is it? a million times thinner than paper (The interlayer spacing : 0.33~0.36 nm) Q2. How strong is it? stronger than diamond (Maximum Young's modulus : ~1.3 TPa) Q3. How conductive is it? better than copper (The resistivity : 10 6 Ω cm) (Mobility: 200,000 cm 2 V -1 s -1 ) But, weak bonding between layers Seperated by mechanical exfoliation of 3D graphite crystals. 21

22 Graphene is a 2D building block for carbon-based materials

23 Electronic structure of graphene Effective mass (related with 2 nd derivative of E(k) ) Massless Graphene charged particle is massless Dirac fermion. Zero gap semiconductor or Semi-metal Pz anti bonding Conduction band Fermi energy Ef Pz bonding Valence band K K 2DEG K K

24 Electrical properties of graphene High electron mobility at room temperature: Electronic device. Si Transistor, HEMT devices are using 2D electron or hole. μ (mobility) = v avg / E (velocity/electric field) J drift ~ ρ x v avg Graphene is great conductor; electrons are able to flow through graphene more easily than through even copper. The electrons travel through the graphene sheet as if they carry no mass, as fast as just one hundredth that of the speed of light. Graphene is a semi-metal and is a zero-gap semiconductor.

25 Thermal properties of graphene Graphene is a perfect thermal conductor. Its thermal conductivity is much higher than all the other carbon structures as carbon nanotubes, graphite and diamond (> 5000 W/m/K). The ballistic thermal conductance of graphene is isotropic, i.e. same in all directions. The study of thermal conductivity in graphene may have important implications in graphene-based electronic devices. As devices continue to shrink and circuit density increases, high thermal conductivity, which is essential for dissipating heat efficiently to keep electronics cool, plays an increasingly larger role in device reliability.

26 Optical properties of graphene Optical transmittance control: transparent electrode Reduction of single layer: 2.3% Graphene is almost completely transparent, yet so dense that even the smallest atom helium cannot pass through it. F. Bonaccorso et al. Nat. Photon. 4, 611 (2010)

27 Mechanical properties of graphene It was found that graphene is harder than diamond and about 300 times harder than steel. The tensile strength of graphene exceeds 1 TPa. Even though graphene is so robust, it is also very stretchable to make use in flexible and stretchable devices Young s modulus =tensile stress/tensile strain Diamond ~ 1200 GPa Force-displacement measurement C. Lee et al. Science 321, 385 (2008)

28 Chemical properties of graphene Similar to the surface of graphite, graphene can adsorb and desorb various atoms and molecules (for example, NO2, NH3, K, and OH). Weakly attached adsorbates often act as donors or acceptors and lead to changes in the carrier concentration, so graphene remains highly conductive. This can be exploited for applications as sensors for chemicals. Other than weakly attached adsorbates, graphene can be functionalized by several chemical groups (for instances OH-, F-) forming graphene oxide and fluorinated graphene. It has also been revealed that single-layer graphene is much more reactive than 2, 3 or higher numbers or layers. Also, the edge of graphene has been shown to be more reactive than the surface. Unless exposed to reasonably harsh reaction conditions, graphene is a fairly inert material, and does not react readily despite every atom being exposed and vulnerable to it's surroundings.

29 Preparation and characterization graphene Preparation methods Top-down approach (From graphite) - Micromechanical exfoliation of graphite (Scotch tape or peel-off method) - Creation of colloidal suspensions from graphite oxide or graphite intercalation compounds (GICs) Bottom up approach (from carbon precursors) - By chemical vapour deposition (CVD) of hydrocarbon - By epitaxial growth on electrically insulating surfaces such as SiC - Total Organic Synthesis Ref: Carbon, 4 8, ( )

30 Graphene synthesis Mechanical Exfoliation (Scotch-tape method) Repeated peeling of graphite to get an atom-thick layer of graphite, which is called grapheme. This simple, low-budget technique has been widely credited for the explosive growth of interest in graphene. Unfortunately, however, they are usually available at a size of several-microns (or tens of microns at best), have irregular shapes, and their azimuthal orientation is not deterministically controlled.

31 Direct exfoliation of graphite Graphene sheets ionic-liquid-modified by electrochemistry using graphite electrodes. Liu, N. et al. One-step ionic-liquid-assisted electrochemical synthesis of ionicliquidfunctionalized graphene sheets directly from graphite. Adv. Funct. Mater. 18, (2008).

32 Graphene synthesis Chemical Vapor Deposition Graphene and few-layer graphene (FLG) have been grown by chemical vapor deposition (CVD) from C-containing gases on catalytic metal surfaces and/or by surface segregation of C dissolved in the bulk of such metals. Depending on the solubility of C in the metal, the former or the latter can be the dominant growth process, or they can coexist. The electrical properties of CVD graphene cannot be tested in situ on the conductive metal substrates. Thus, processes to transfer graphene on an appropriate insulating substrate have been developed. The ability to select the host substrate independently of the sacrificial growth substrate is a major advantage for graphene grown on metals. At the same time, the transfer process often affects negatively graphene's integrity, properties, and performance. Wrinkle formation, impurities, graphene tearing, and other structural defects, can occur during transfer

33 Potential application of graphene - Single molecule gas detection - Graphene transistors - Integrated circuits - Transparent conducting electrodes for the replacement of ITO - Ultracapacitors - Graphene biodevices - Reinforcement for polymer nanocomposites: Electrical, thermally conductive nanocomposites, antistatic coating, transparent conductive composites..ect

34 Graphene Nanoribbon (GNR) Graphene is a zero-gap semiconductor. To extend the real applications, an energy gap is needed, which enables the basic electric logic states: on and off. When graphene is etched or patterned along one specific direction, a novel quasi one-dimensional (1D) structure is obtained, which is a strip of graphene, referred as graphene nanoribbon (GNR).Depends on the termination style, normally, GNR can be divided into two kinds: Armchair and Zigzag The width of armchair GNRs is classified by the number of dimer lines (N a ) across the ribbons. The width of zigzag GNRs is classified by the number of zigzag chains (Nz) across the ribbons. Perpendicular to the direction of defined width, GNRs repeat their geometric structures, and form one-dimensional periodic structures.

35 Carbon Nanotubes Types Fabrication Structure Properties Applications

36 What are Carbon Nanotubes? Carbon nanotubes are fullerene-related structures which consist of graphene cylinders closed at either end with caps containing pentagonal rings Rotating Carbon Nanotube

37 Discovery They were discovered in 1991 by the Japanese electron microscopist Sumio Iijima who was studying the material deposited on the cathode during the arc-evaporation synthesis of fullerenes. He found that the central core of the cathodic deposit contained a variety of closed graphitic structures including nanoparticles and nanotubes, of a type which had never previously been observed

38 What is it? Sheet of graphite rolled into a tube Single-Walled (SWNT) and Multi-Walled (MWNT) Large application potential, metallic, semiconducting armchair SWNT zigzag MWNT chiral

39 Why Carbon Nanotubes? Small Dimensions Chemically Stable Mechanically Robust High Thermal Conductivity High Specific Surface Area (Good Adsorbents) Low Resistivity (Ballistic Electron Conduction) Ideal materials for applications in conductive and high-strength composites; energy storage and energy conversion devices; sensors; field emission displays and radiation sources; hydrogen storage media; and nanometer-sized semiconductor devices, probes, and interconnects.

40 Types of Carbon nanotubes Two main types of carbon nanotubes: Single-walled nanotubes (SWNTs) consist of a single graphite sheet seamlessly wrapped into a cylindrical tube. Multiwalled nanotubes (MWNTs) comprise an array of such nanotubes (more than one wall) that are concentrically nested with in.

41 Carbon nanotube Properties depending on how it is rolled up. a 1, a 2 are the graphene vectors. OB/AB overlaps after rolling up. OA is the rolling up vector. OA na 1 ma 2 Institute of Optics, University of Rochester 41

42 Structure of Single Walled Carbon Nanotubes Structure depends on rolling direction (chirality) Metallic Semi-conducting

43 Figure 6.1. Diagram explaining the relationship of a SWNT to a graphene sheet. The wrapping vector for an (8,4) nanotube, which is perpendicular to the tube axis, is shown as an example. Those tubes which are metallic have indices shown in red. All other tubes are semiconducting.

44 Three Forms of CNTs Chiral Zigzag Armchair Vectors describe the rolling process that occurs when a graphite sheet is transformed into a tube

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46 Fabrication/Nanotube Synthesis SWNTs and MWNTs are usually made by carbon-arc discharge methods C electrodes, 20-25V potential, 1mm, 500 torr, C ejected from + electrode forms NT on electrode (Co, Ni or Fe for SWNTs, 1-5nm, 1µm length, no catalyst = MWNTs,) laser ablation of carbon 1200 C, pulsed laser, catalysts (Co, Ni), condensation ( 10-20nm, 100µm length) chemical vapor deposition (typically on catalytic particles) 1100 C, decomposition of hydrocarbon gas (e.g. CH 4 ), open NTs, catalyst on substrate, industrial scale up, and length can vary Nanotube diameters range from 0.4 to > 3 nm for SWNTs and from ~1.4 to at least 100 nm for MWNTs

47 Carbon-arc discharge Gas Inlet Graphite rod CNT Deposit Water cooled chamber The schematic diagram of and arc chamber for CNT production is shown. After evacuating the chamber, an appropriate ambient gas is introduced at the desired pressure, and then a dc arc voltage is applied between the two graphite rods. When pure graphite rods are used, the anode evaporates and the is deposited on the cathode, which contains C N Ts. T h e s e C N Ts, a r e M W N Ts. When a graphite rod containing metal catalyst (Fe, Co, etc.) is used as the anode with a pure graphite cathode, SWNTs are generated in the form of soot. Figure shows typical setup used for laser ablation of carbon, which consists of a furnace, a quartz tube with a window, a target carbon composite doped with catalytic metals, a water-cooled trap, and flow systems for the buffer gas to maintain constant pressures and flow rates. A laser beam (typically a YAG or CO2 laser) is introduced through the window and focused onto the target located in the center of the furnace. The target is vaporized in high-temperature Ar buffer gas and forms SWNTs. The SWNTs produced are conveyed by the b u f f e r gas to t h e t r a p, w h e r e t h e y a r e c o l l e c t e d. The method has several advantages, such as high-quality SWNT production, diameter control, investigation of growth dynamics, and the production of new materials. High-quality SWNTs with minimal defects and contaminants, such as amorphous carbon and catalytic metals, have been produced using the laser-furnace method. Laser Gas Inlet Target Rod Furnace SWNT Water Inlet Pump Laser ablation of carbon for CNT growth

48 Typical CVD Furnace Schematics Parameters Used: Ferrocene/Xylene: 1gm in 100 ml Gas flow rate: 100 sccm Growth temperature: C Figure 1: Schematics of the nanotube growth apparatus The CVD method can be used for growing controlled architectures (aligned as well as patterned) of carbon nanotubes on various substrates. 40 micron Aligned CNTs

49 Properties Chemical Reactivity Electrical Optical Mechanical Strength

50 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 CNTs and their molecular background are stated below. Chemical reactivity. The chemical reactivity of a CNT is, compared with a graphene sheet, enhanced as a direct result of the curvature of the CNT surface. Carbon nanotube reactivity is directly related to the pi-orbital mismatch caused by an increased curvature. Therefore, a distinction must be made between the sidewall and the end caps of a nanotube. For the same reason, a smaller nanotube diameter results in increased reactivity. Covalent chemical modification of either sidewalls or end caps has shown to be possible. For example, the solubility of CNTs in different solvents can be controlled this way. Though, direct investigation of chemical modifications on nanotube behaviour is difficult as the crude nanotube samples are still not pure enough.

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53 Electrical Properties If the nanotube structure is armchair then the electrical properties are metallic If the nanotube structure is chiral then the electrical properties can be either semiconducting with a very small band gap, otherwise the nanotube is a moderate semiconductor In theory, metallic nanotubes can carry an electrical current density of A/cm 2 which is more than 1,000 times greater than metals such as copper

54 Electrical Properties Electrical Properties Metallic armchair structure conductive Semi-conductors zigzag and chiral Depends on diameter (quantum effects) Ropes of SWNTs (R=10-4 cm -1 at 27 C) Combinations transistors Bent molecules Response to stretching Chirality and diameter of nanotubes are important parameters!!!

55 Carbon nanostructures may be used in new electronic devices from nanotechweb.org

56 Devices made with carbon nanotubes Figure Atomic force microscopy image of an isolated SWNT deposited onto seven Pt electrodes by spin-coating from dichloromethane solution. The substrate is SiO 2. An auxiliary electrode is used for electrostatic gating. (Reproduced with kind permission of C. Dekker.)

57 Devices made with carbon nanotubes

58 Optical activity Theoretical studies have revealed that the optical activity of chiral nanotubes disappears if the nanotubes become larger. Therefore, it is expected that other physical properties are influenced by these parameters too. Use of the optical activity might result in optical devices in which CNTs play an important role.

59 Mechanical Strength Carbon nanotubes have the strongest tensile strength of any material known. It also has the highest modulus of elasticity. Material Young's Modulus (TPa) Tensile Strength (GPa) Elongation at Break (%) SWNT ~1 (from 1 to 5) E 16 Armchair SWNT 0.94 T T 23.1 Zigzag SWNT 0.94 T 94.5 T Chiral SWNT 0.92 MWNT E 150 Stainless Steel ~0.2 ~

60 Mechanical Strength Mechanical Properties Young s modulus E = TPa (steel 0.21TPa) Strength R m = 45,000 MPa (high strength steel 2,000 MPa) Buckling no fracture change in hybridization (from sp 2 ) Molecular dynamics simulations of a (10,10) nanotube under axial tension (J. Bernholc, M. Buongiorno Nardelli and B. Yakobson). Plastic flow behavior is shown after 2.5 ns at T = 3,000 K and 3% strain. The blue area indicates the migration path (in the direction of the arrow) of the edge dislocation (green). This sort of behavior might help make composite materials that are really tough (as measured by their ability to absorb energy).

61 Application of Nanotubes VFD (Vacuum Fluorescent Display) LCD (Liquid Crystal Display) CRT (Cathode Ray Tube) FED (Field Emission Display) SET (single Electron Transistor) STM (Scanning tunneling Miceoscope) AFM (Atomic force Microscope)

62 Energy storage a) Hydrogen storage The advantage of hydrogen as energy source is that its combustion product is water. In addition,hydrogen can be easily regenerated. The two commonly used means to store hydrogen are : 1. Gas phase due to cylindrical and hollow geometry, and nano scale diameters carbon nanotubes can store a liquid or a gas in the inner cores through a capillary effect. 2. Electrochemical adsorption-h atom is adsorbed. This is called chemisorption.

63 Hydrogen storage

64 Carbon nanotubes Hydrogen carriers based on nanostructured carbon (such as carbon buckyballs and nanotubes) have been proposed. Despite initial claims of greater than 50 wt% hydrogen storage, it was later accepted that a realistic number is less than 1 wt%. "Spillover" Mechanism in Carbon Nanotube Hydrogen Storage June 7, 2011 Schematic of the "spillover" mechanism by which platinum nanoparticles (tan) helps make it possible to store hydrogen (purple) in single-walled carbon nanotubes (gray).

65 b) Lithium intercalation The basic principle of rechargeable lithium batteries is electrochemical intercalation and deintercalation of lithium in both electrodes. An ideal battery has a high-energy capacity, fast charging time and a long cycle time determined by determined by the lithium saturation concentration of the electrode materials. For Li, this is the highest in nanotubes if all the interstitial sites (inter-shell van der Waals spaces, inter-tube channels and inner cores) are accessible for Li intercalation. SWNTs have shown to possess both highly reversible and irreversible capacities. Because of the large observed voltage hysteresis. Li-intercalation in nanotubes is still unsuitable for battery application. This feature can potentially be reduced or eliminated by processing, i.e. cutting, the nanotubes to short segments

66 Lithium intercalation Use of MWNTs in Li-ion batteries Laptop notebooks Mobile phones MWNT powder blended with active materials Increases electrical connectivity Increases mechanical strength Enhances cycle life and rate capability

67 c) Electrochemical supercapacitors Supercapacitors have a high capacitance and potentially applicable in electronic devices. Typically, they are comprised two electrodes separated by an insulating material that is ionically conducting in electrochemical devices. The capacity of an electrochemical supercapacitor inversely depends on the separation between the charge on the electrode and the counter charge in the electrolyte. Because this separation is about a nanometre for nanotubes in electrodes, very large capacities result from the high nanotube surface area accessible to the electrolyte. In this way, a large amount of charge injection occurs if only a small voltage is applied. This charge injection is used for energy storage in nanotube supercapacitors.

68 Electrochemical supercapacitors Study on packaged cells utilizing forest-grown SWNTs revealed remarkable performance 16 Wh kg-1 energy density 10 kw kg-1 power density 16 year lifetime forecast The only drawback is the high cost of SWNTs

69 Molecular electronics with CNTs a) Field emitting devices If a solid is subjected to a sufficiently high electric field, electrons near the Fermi level can be extracted from the solid by tunneling through the surface potential barrier. This emission current depends on the strength of the local electric field at the emission surface and its work function (which denotes the energy necessary to extract an electron from its highest bounded state into the vacuum level). The applied electric field must be very high in order to extract an electron. This condition is fulfilled for carbon nanotubes, because their elongated shape ensures a very large field amplification.

70 Molecular electronics FEDs CNTFETs SETs

71 Application Flat screen displays Field emission Saito et al., Jpn. J. Appl. Phys. 37 (1998) L346.

72 FED Principles Field emission displays, electrons coming from millions of tiny microtips pass through gates and light up pixels on a screen. This principle is similar to that of cathode-ray tubes in television sets. The difference: Instead of just one "gun" spraying electrons against the inside of the screens face, there are as many as 500 million of them (microtips).

73 Field Emitting Devices Single Emitter Film Emitter

74 Field Emitting Devices Single Emitter Film Emitter

75 Field Emitting Devices Single Emitter Film Emitter

76 Patterned Film Field Emitters Etching and lithography Conventional CVD Soft lithography

77 b) Transistors The field-effect transistor a three-terminal switching device can be constructed of only one semiconducting SWNT. By applying a voltage to a gate electrode, the nanotube can be switched from a conducting to an insulating state. Such carbon nanotube transistors can be coupled together, working as a logical switch, which is the basic component of computers.

78 Transistors Nanotubes hold the promise of creating novel devices, such as carbon-based single-electron transistors, that significantly smaller than conventional transistors.

79 c) Nanoprobes and sensors Because of their flexibility, nanotubes can also be used in scanning probe instruments. Since MWNT tips are conducting, they can be used in STM and AFM instruments. Advantages are the improved resolution in comparison with conventional Si or metal tips and the tips do not suffer from crashes with the surfaces because of their high elasticity. However, nanotube vibration, due to their large length, will remain an important issue until shorter nanotubes can be grown controllably. Nanotube tips can be modified chemically by attachment of functional groups. Because of this, nanotubes can be used as molecular probes, with potential applications in chemistry and biology.other applications are the following: A pair of nanotubes can be used as tweezers to move nanoscale structures on surfaces. Sheets of SWNTs can be used as electromechanical actuators, mimicking the actuator mechanism present in natural muscles. SWNTs may be used as miniaturised chemical sensors. On exposure to environments, which contain NO 2, NH 3 or O 2, the electrical resistance changes.

80 Biotechnology CNTs have been investigated as components of Biosensors Medical devices Appeal due to compatibility with biomolecules (DNA/proteins) from two aspects: Dimensional chemical

81 Biotechnology CNTs enable biological functions like Fluoroscence Photoacoustic imaging Localized heating via near-infrared radiation

82 Biotechnology (SWNT biosensors) Adsorption of target molecules on CNT surface allow for large changes in Electrical impedance Optical properties Application include Gas and toxin detection in industry and military Test strips for hormones and biological markers (NO2,troponin, estrogen, progesterone)

83 d) Composite materials Because of the stiffness of carbon nanotubes, they are ideal candidates for structural applications. For example, they may be used as reinforcements in high strength, low weight, and high performance composites. Theoretically, SWNTs could have a Young s Modulus of 1 TPa. MWNTs are weaker because the individual cylinders slide with respect to each other. Ropes of SWNTs are also less strong. The individual tubes can pull out by shearing and at last the whole rope will break. This happens at stresses far below the tensile strength of individual nanotubes &month=1&ct=a&pc=25&sp=25

84 Nanotubes also sustain large strains in tension without showing signs of fracture. In other directions, nanotubes are highly flexible. One of the most important applications of nanotubes based on their properties will be as reinforcements in composite materials. However, there have not been many successful experiments that show that nanotubes are better fillers than the traditionally used carbon fibres. A main advantage of using nanotubes for structural polymer composites is that nanotube reinforcements will increase the toughness of the composites by absorbing energy during their highly flexible elastic behaviour. Other advantages are the low density of the nanotubes, an increased electrical conduction and better performance during compressive load. Another possibility, which is an example of a non-structural application, is filling of photoactive polymers with nanotubes. PPV (Poly-p-phenylenevinylene) filled with MWNTs and SWNTs is a composite, which has been used for several experiments. These composites show a large increase in conductivity with only a little loss in photoluminescence and electro-luminescence yields. Another benefit is that the composite is more robust than the pure polymer. Of course, nanotube-polymer composites could be used also in other areas.

85 For instance, they could be used in the biochemical field as membranes for molecular separations or for osteointegration (growth of bone cells). However, these areas are less explored. The most important thing we have to know about nanotubes for efficient use of them as reinforcing fibres is knowledge on how to manipulate the surfaces chemically to enhance interfacial behaviour between the individual nanotubes and the matrix material e) Templates Because of the small channels, strong capillary forces exist in nanotubes. These forces are strong enough to hold gases and fluids in nanotubes. In this way, it may be possible to fill the cavities of the nanotubes to create nanowires.

86 The Space Elevator Pictures from

87 The Space Elevator The Solution: Carbon Nanotubes 10x the tensile strengh (30GPa) 1 atm = kPA 10-30% fracture strain Further Obstacles Production of Nanofibers Record length 4cm Investment Capital: $10 billion

88 Nanotubes excellent strength to weight ratio creates the potential to build an elevator to space.

89 Health Hazards According to scientists at the National Institute of Standards and Technology, carbon nanotubes shorter than about 200 nanometers readily enter into human lung cells similar to the way asbestos does, and may pose an increased risk to health. Carbon nanotubes along with the majority of nanotechnology, are an unexplored matter, and many of the possible health hazards are still unknown.

90 Conclusions Films Biotechnology Energy storage Coating Microelectronics Environment Water filters Transistors Biosensors screens

91 Thank You!