The wondrous world

of carbon nanotubes

March 26, 2014

Nanocarbon:Properties and Applications

Overview

• Introduction

• Synthesis & Purification

• Overview of applications

• Single nanotube measurements

• Energy storage

• Molecular electronics

• Conclusion and future outlook

Carbon

• Melting point: ~ 3500oC

• Atomic radius: 0.077 nm

• Basis in all organic componds

• 10 mill. carbon componds

Introduction

Nanocarbon

• Fullerene• Tubes • Cones• Carbon black• Horns• Rods• Foams• Nanodiamonds

Introduction

BondingProperties

Graphite – sp2 Diamond – sp3

Nanocarbon

Shenderova et al. Nanotechnology 12 (2001) 191.

Properties

Nanocarbon

Properties

12 pentagons

6 + 6 pentagons

1 – 5 pentagons

Fullerene

”The most symmetrical large molecule”• Discovered in 1985 - Nobel prize Chemistry 1996, Curl, Kroto, and Smalley

Properties

Epcot center, Paris

~1 nm

Architect: R. Buckminster Fuller

• C60, also 70, 76 and 84.

- 32 facets (12 pentagons and 20 hexagons)

- prototype

Fullerene• Symmetric shape → lubricant

• Large surface area → catalyst

Properties

Fullerene• Symmetric shape → lubricant

• Large surface area → catalyst

• High temperature (~500oC)• High pressure

Properties

Fullerene• Symmetric shape → lubricant

• Large surface area → catalyst

• High temperature (~500oC)• High pressure• Hollow → caging particles

Properties

Fullerene

• Chemically stable as graphite - most reactive at pentagons

• Crystal by weak van der Waals force • Superconductivity - K3C60: 19.2 K

- RbCs2C60: 33 K

Kittel, Introduction to Solid State Physics, 7the ed. 1996.

Properties

Introduction: common facts

• Discovered in 1991 by Iijima

• Unique material properties

• Nearly one-dimensional structures

• Single- and multi-walled

NanotubeProperties

Roll-up vector:

21 amanCh

• Discovered 1991, Iijima

NanotubeProperties

Roll-up vector:

21 amanCh

• Discovered 1991, Iijima

Nanotube

Electrical conductanse depending on helicity

21 amanCh

Properties

If , then metallicelse semiconductor

imn

3

2

Nanotube

Electrical conductanse depending on helicity

21 amanCh

Properties

• Current capacity

Carbon nanotube 1 GAmps / cm2

Copper wire 1 MAmps / cm2

• Heat transmission

Comparable to pure diamond (3320 W / m.K)

• Temperature stability

Carbon nanotube 750 oC (in air)

Metal wires in microchips 600 – 1000 oC

• Caging

May change electrical properties

→ sensor

imn

3

2If , then metallicelse semiconductor

Introduction: nanotube structure• Roll a graphene sheet in a certain direction:

• Armchair structure

• Zigzag structure

• Chiral structure

• Defects result in bends and transitions

Geometry

• Rollup Vector– (n,m)– n-m=3d

• Chiral Angle– tan(θ) =

√3m/(2√(n2+m2+nm))• Arm Chair (n,n), θ=30

○

• Zig-zag (n,0), θ=0 ○

• Chiral, 0○< θ<30 ○

Nanotube

Properties

Diameter:

as low as 1 nm

Length:

typical few μm

High aspect ratio:

1000diameter

length

→ quasi 1D solid

Nanotube

Zheng et al. Nature Materials 3 (2004) 673.

Properties

SWCNT – 1.9 nm

Diameter:

as low as 1 nm

Length:

typical few μm

High aspect ratio:

1000diameter

length

→ quasi 1D solid

NanotubesCarbon nanotubes are the strongest ever known material.• Young Modulus (stiffness): Carbon nanotubes 1250 GPa Carbon fibers 425 GPa (max.) High strength steel 200 GPa

• Tensile strength (breaking strength)

Carbon nanotubes 11- 63 GPa

Carbon fibers 3.5 - 6 GPa

High strength steel ~ 2 GPa

Properties

Carbon nanotubes are very flexible

Nanoscience Research Group University of North Carolina (USA)

http://www.physics.unc.edu/~rsuper/research/

http://www.ipt.arc.nasa.gov/gallery.html

Properties

Mechanical

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 4×109 A/cm2 which is more than 1,000 times greater than metals such as copper

Synthesis: overview• Commonly applied techniques:

– Chemical Vapor Deposition (CVD)– Arc-Discharge– Laser ablation

• Techniques differ in:– Type of nanotubes (SWNT / MWNT / Aligned)– Catalyst used– Yield– Purity

Synthesis: arc discharge

• MWNTs and SWNTs• Batch process

• Relatively cheap

• Many side-products

28

Research ApproachChemical vapor deposition set up

Parameters to be varied are:

Temperature , Mass Flow Rate, Hydrocarbon source, Carrier gas, Deposition Metal

Synthesis: CVD

•Gas phase deposition

•Large scale possible

•Relatively cheap

•SWNTs / MWNTs

•Aligned nanotubes

•Patterned substrates

Synthesis: laser ablation• Catalyst / no catalyst

• MWNTs / SWNTs

• Yield <70%

• Use of very strong laser

• Expensive (energy costs)

• Commonly applied

Synthesis: growth mechanism• Metal catalyst

• Tip growth / extrusion growth

Purification: techniques• Removal of catalyst:

– Acidic treatment (+ sonication)– Thermal oxidation– Magnetic separation (Fe)

• Removal of small fullerenes– Micro filtration– Extraction with CS2

• Removal of other carbonaceous impurities– Thermal oxidation– Selective functionalisation of nanotubes– Annealing

Field Effect Transistors• FETs work because of applied

voltage on gate changes the amount of majority carriers decreasing Source-Drain Current

• SWCNT and MWCNT used– Differences will be discussed

• Gold Electrodes• Holes main carriers

– Positive applied voltage should reduce current

SWCNT Transport Properties

• Current shape consistent with FET

• Bias VSD = 10 mA• G(S) conductance varies by ~5

orders of magnitude• Mobility and Hole concentration

determined to be large– Q=CVG,T (VG,T voltage to

deplete CNT of holes)– C calculated from physical

parameters of CNT– p=Q/eL

Overview of potential applications

< Energy storage:

•Li-intercalation

•Hydrogen storage

•Supercaps

> FED devices:

•Displays

< AFM Tip

> Molecular electronics

•Transistor

< Others

• Composites

• Biomedical

• Catalyst support

• Conductive materials

• ???

Overview of potential applications

< Energy storage:

•Li-intercalation

•Hydrogen storage

•Supercaps

> FED devices:

•Displays

< AFM Tip

> Molecular electronics

•Transistor

< Others

• Composites

• Biomedical

• Catalyst support

• Conductive materials

• ???

Overview of potential applications

< Energy storage:

•Li-intercalation

•Hydrogen storage

•Supercaps

> FED devices:

•Displays

< AFM Tip

> Molecular electronics

•Transistor

< Others

• Composites

• Biomedical

• Catalyst support

• Conductive materials

• ???

Energy Storage

Experiments & Modelling

• Electrochemical Storage of Lithium

• Electrochemical Storage of Hydrogen

• Gas Phase Intercalation of Hydrogen

• Supercapacitors

Energy Storage3-electrode cell

- + -2

reduction

oxidationCNT H O e CNT H OHx x x x

+ -

2

reduction

oxidationNi OH NiOOH H e

Work Electrode

Counter Electrode

Molecular electronics

• FEDs

•CNTFETs

•SETs

Field Emitting Devices

Single Emitter

Film Emitter

Field Emitting Devices

Single Emitter

Film Emitter

Field Emitting Devices

Single Emitter

Film Emitter

Patterned Film Field Emitters

•Etching and lithography•Conventional CVD•Soft lithography

Single Electron transistor

Conclusions

• Mass production is nowadays too expensive• Many different techniques can be applied for

investigation• Large scale purification is possible• FEDs and CNTFETs have proven• Positioning of molecular electronics is difficult• Energy storage is still doubtful, fundamental

investigations are needed