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

1 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 & MX2 tubes
• Carbon allotropes
• Carbon nanotube: basic properties
• Carbon nanotube: applications

2

Formation of nanoscale particles

Downsize any clump of

• material. From 3D to 0D:

Cluster with solid core Alternative: Take 2D sheet and wrap into sphere or cylinder

• 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
• MX2 layer → MX2 nanotube

(M: metal, X: chalcogenide WS2, MoS2,… )

3

Boron nitride nanotubes

• Theoretically predicted after discovery
• f 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

MoS2 tube MX2 M: metal (Mo, W,…) X: chalgonide (S, Se,…)

• Presently not found as

single walled, only multi- walled.

4

Carbon

C: 1s2, 2s2, 2p2 Valence electrons: 2s22p2 Hybridization: sp, sp2, sp3; σ σ σ σ and π π π π electrons CH4 sp3: 4×σ (methane) CH2=CH2 sp2: 3×σ + 1×π (ethylene) π-electrons determine the electronic properties of carbon materials

Carbon allotropes

Diamond

• sp3
• insulating, Eg = 5.47 eV

Graphite

• sp2
• semimetallic
• layered material

Fullerene (C60)

• sp2

3D 2D 1D 0D Diamond Graphite (Graphene) Fullerene (single) Polymers (single chain) Carbon nanotube

5

Linear conjugated carbon chains: conducting polymers

An instructive case: the benzene molecule (sp2) the linear case: polyacetylene the sp3 version: polyethylene Eg = 1.5 eV

Carbon history

• Diamond & Graphite known since ancient times, though only later

(~ 1800) that they are basic forms of pure carbon.

• Fullerene (C60): - 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 (C70,…)
• Carbon nanotubes: - Multiwalled 1991 (S. Iijima)
• Single walled 1993
• Graphene (single layer graphite) 2004

6

Fullerene discovery 1985

Rice University, Texas, 1985

Geometry of the fullerene

Graphene sheet consists

• f hexagons:

Euler’s rule: f + v – e = 2 f: number of faces v: vertices e: edges Applied to the fullerene: p: number of pentagons h: hexagons f = p+h; 2e = 5p+6h; 3v = 5p+6h

p = 12

Pentagon Hexagon

The fullerene consists of hexagons and pentagons

7

Carbon nanotubes: basic structure

• R. Saito, M. Dresselhaus, G. Dresselhaus, Physical Properties of Carbon Nanotubes

Symmetry specified by vector along circumference

• f the tube:

Ch = na1+ma2.

Single-walled (SWNT) and multiwalled (MWNT) nanotubes

1-5 nm 2-30 nm

SWNT MWNT

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• Energy
• !

" !

#$%&'

• ($%&'

! " )

• "!

"!

• *+
• ,"

"

• !

9

Basic electronic structure

Ch = na1+ma2. 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 a) Semiconducting b) Metallic

d ta E

c c g −

=

d: diameter t: overlap energy ac-c: bond length

Advantages of carbon nanotubes

Mechanically rigid Young’s modulus of graphite in-plane ~ 1 TPa ⇒ Among the strongest materials Electronically versatile

• Both metallic and semiconducting
• Ballistic conduction possible

Chemically stable Can be functionalized with Various chemical sidegroups

10

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 Carbon atoms vaporize in intense heat and coalesce to form fullerenes and nanotubes. However, SWNTs form only when catalyst particles are present. CVD synthesis A carbon containing gas is lead at a high temperature over nano- scale catalyst particles. The gas cracks up and loses its carbon atoms which diffuse into the nanoparticles. Nanotubes in turn grow from the particles as they get saturated with carbon.

11

12

Some real carbon nanotubes

SWNT ”rope” (Rice Univ.)

Impurities

Individual SWNTs grown

• n surface.

(Stanford Univ.)

MWNTs CVD-grown AD-grown

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Carbon nanotube applications

Nanotubes in bulk form

• Ultra-strong fibers
• Space elevator !!??
• Conducting composites
• Conducting & transparent

coating

• Molecular filter

Individual nanotubes

• Field emission source
• (Field-effect) transistor
• Sensors
• Many more…

High performance fibers from CNTs

Koziol et al., Science 318, 1892 (2007)

Space elevator concept still highly hypothetical

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Other bulk applications

Holt et al., Science 312, 1034 (2006).

• M. Zhang et al., Science 309,

1215 (2005).

CNT Membrane CNT transparent film

Single CNT applications

Field emitter CNT field-effect transistor A CNT for each pixel !

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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.) A modern integrated circuit (IC) contains millions of transistors The size of the individual transistors in an IC has decreased according to ”Moore’s Law”