Outline - General concepts of nanoscale particles - BN tubes & - PDF document

One- and zero-dimensional nanoscale materials derived from two-dimensional materials Markus Ahlskog University of Jyväskylä, Finland Nanoscience Center Outline - General concepts of nanoscale particles - BN tubes & MX 2 tubes - Carbon allotropes - Carbon nanotube: basic properties - Carbon nanotube: applications 1

Formation of nanoscale particles Alternative: Downsize any clump of Take 2D sheet and wrap material. From 3D to 0D: into sphere or cylinder Cluster with solid core - Hollow core ! - Covalent bonding ! From layers to nanotubes Layer of atoms rolled over (seamlessly !) to nanotube Examples include: - Graphene → Carbon nanotube - BN layer → BN nanotube - MX 2 layer → MX 2 nanotube (M: metal, X: chalcogenide WS 2 , MoS 2 ,… ) 2

Boron nitride nanotubes - Theoretically predicted after discovery of carbon nanotubes. (A. Rubio et al., Phys. Rev. B 49 , 508 (1994)). - Even stiffer than carbon nanotube - Only semiconducting among various symmetries. Metal chalcogenide nanotubes MoS 2 tube MX 2 M: metal (Mo, W,…) X: chalgonide (S, Se,…) - Presently not found as single walled, only multi- walled. 3

Carbon C: 1s 2 , 2s 2 , 2p 2 Valence electrons: 2s 2 2p 2 Hybridization: sp, sp 2 , sp 3 ; σ σ σ σ and π π π electrons π CH 4 CH 2 =CH 2 sp 3 : 4 × σ sp 2 : 3 × σ + 1 × π (methane) (ethylene) π -electrons determine the electronic properties of carbon materials Carbon allotropes Diamond 3D Graphite 2D (Graphene) Diamond Graphite - sp 3 - sp 2 Carbon - insulating, E g = 5.47 eV - semimetallic nanotube - layered material Polymers 1D (single chain) Fullerene (C 60 ) Fullerene - sp 2 0D (single) 4

Linear conjugated carbon chains: conducting polymers An instructive case: the benzene molecule (sp 2 ) the linear case: polyacetylene E g = 1.5 eV the sp 3 version: polyethylene Carbon history - Diamond & Graphite known since ancient times, though only later (~ 1800) that they are basic forms of pure carbon. - Fullerene (C 60 ): - discovery in 1985 - available in pure form in macroscopic amounts since 1991 - Nobel Prize in Chemistry awarded in 1996 (Curl, Kroto, Smalley) - Other fullerenes found (C 70 ,…) - Carbon nanotubes: - Multiwalled 1991 (S. Iijima) - Single walled 1993 - Graphene (single layer graphite) 2004 5

Fullerene discovery 1985 Rice University, Texas, 1985 Geometry of the fullerene The fullerene consists of hexagons Graphene sheet consists and pentagons of hexagons: Pentagon Hexagon Euler’s rule : f + v – e = 2 Applied to the fullerene: f: number of faces p: number of pentagons v: vertices h: hexagons e: edges f = p+h; 2e = 5p+6h; 3v = 5p+6h p = 12 6

Carbon nanotubes: basic structure Single-walled (SWNT) and multiwalled (MWNT) nanotubes MWNT SWNT Symmetry specified by vector along circumference of the tube: 1-5 nm 2-30 nm C h = na 1 +ma 2 . R. Saito, M. Dresselhaus, G. Dresselhaus, Physical Properties of Carbon Nanotubes 7

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Basic electronic structure C h = na 1 +ma 2 . Tight binding calculations give gap-structure , neglecting curvature a) Armchair (n,n): metallic b) Zig-Zag (n,0): metallic & semiconducting c) Chiral (n,m): metallic & semiconducting Of all configurations: 1/3 metallic, 2/3 semiconducting ta E = c − c g d d : diameter a) Semiconducting t : overlap energy b) Metallic a c-c : bond length Advantages of carbon nanotubes Mechanically rigid Electronically versatile Young’s modulus of - Both metallic and semiconducting graphite in-plane - Ballistic conduction possible ~ 1 TPa ⇒ Among the strongest materials Chemically stable Can be functionalized with Various chemical sidegroups 9

Synthesis of Carbon Nanotubes I The main methods: - in arc-discharge of graphite electrodes (AD-method) - laser vaporization of graphite target - catalytic decomposition of hydrocarbon gases (CVD-method) Major differences: -is catalyst used ? -growth temperature (600-2000 C) Different size and degree of perfection can be expected Chirality is not under control Synthesis of Carbon Nanotubes II Arc-discharge & laser vaporization CVD synthesis Carbon atoms vaporize in intense A carbon containing gas is lead heat and coalesce to form fullerenes at a high temperature over nano- and nanotubes. scale catalyst particles. However, SWNTs form only when The gas cracks up and loses its catalyst particles are present. carbon atoms which diffuse into the nanoparticles. Nanotubes in turn grow from the particles as they get saturated with carbon. 10

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Some real carbon nanotubes Individual SWNTs grown SWNT ”rope” on surface. (Rice Univ.) (Stanford Univ.) MWNTs Impurities CVD-grown AD-grown 12

Carbon nanotube applications Nanotubes in bulk form Individual nanotubes - Ultra-strong fibers - Field emission source - Space elevator !!?? - (Field-effect) transistor - Conducting composites - Sensors - Conducting & transparent coating - Many more… - Molecular filter High performance fibers from CNTs Koziol et al., Science 318 , 1892 (2007) Space elevator concept still highly hypothetical 13

Other bulk applications CNT Membrane CNT transparent film Holt et al., Science 312 , 1034 (2006). M. Zhang et al., Science 309 , 1215 (2005). Single CNT applications Field emitter CNT field-effect transistor A CNT for each pixel ! 14

The ”field-effect” creates the conducting channel. Doping with electric field instead of dopants ! The transistor The first transistor from 1947 (Bardeen,Brattain, Shockley; Bell Labs.) The size of the individual transistors in an IC has decreased according A modern integrated to ”Moore’s Law” circuit (IC) contains millions of transistors 15