The wondrous world of carbon nanotubes · 2010. 2. 12. · Synthesis of single wall carbon...

Zentrum für Mikro- und Nanotechnologien

Technische Universität Ilmenau, FG Nanotechnologie

The wondrous world of carbon nanotubes

Overview• Introduction• Synthesis & Purification• Overview of applications• Single nanotube measurements• Energy storage• Molecular electronics• Conclusion and future outlook

• Discovered in 1991 by Iijima• Unique material properties• Nearly one-dimensional structures• Single- and multi-walled

What is a nanotube?

• Responsible bond:• Unit cell: honeycomb pattern• Wrapping these patterns back on top

of themselves and joining the edgesCarbon nanotube

Single and Multi-wall nanotube • Single wall nanotube:

– SWNT– single atomic layer wall,

diameter of 1-5 nm – excellent mechanical

property– hot topic now

• Multi wall nanotube:– MWNT– Inner diameter: 1.5 – 15 nm– Outer diameter: 2.5 – 30 nm– ~ 50 layers – containing more structure

defects

(http://www.lbl.gov)

Basis contains two atoms

nmdda 14.0,3 ==

2D graphite sheet

Orientation defined by Hamada vector (n,m) r = n a + m b

Common orientations:

• armchair (5,5)

• zigzag (9,0)

• chiral (10,5)

The Electrical Properties of nanotube• Can be either Metal or semiconductor

– Controlled by Rolling DirectionCh (rolling vector)= na + mb (unit vector)

Rule: = integer metallic

non-integer semiconductor

• Electrical Conductivity– Four Point Probe Method to determine sheet resistance and conductivity

History Buckyball ( )

• The discovery of Buckyball is by accident, from Radio-astronomy

• Around 1970s

• The discovery of nanotubes comes from Buckyball

(http://www.slb.com )

The first computer circuit composed of only one single carbon nanotube

IBM2001

Flat Panel display prototypeSamsung1999

Nobel Prize in Chemistry for the discovery of Buckyball

Robert F. Curl, Harry Kroto ,

Richard E. Smalley

Synthesis of single wall carbon nanotubesS, Iijima and T, Ichihashi1993

Discovery of multi wall carbon nanotubes

Japanese Scientist, Sumio Iijima1991

Buckyball was synthesized and confirmed as C60

Scientists around the worldLate 1980s

Try to synthesize long carbon chainsHarry Kroto & Dave Walton1970s

EventsWhoWhen

Mechanical:Young Modulus ~ 1 TPa (SWNT) and 1.25 TPa (MWNT)

(Steel: 230 GPa)

High Aspect Ratio: 1000 – 10.000

Density: 1.3 – 1.4 g/cm3

Maximum Tensile Strength: 30 GPa

Thermal:Conductivity: 2000 W/mK (Copper has 400 W/mK)

Physical Properties

Special Properties

• Difference in chemical reactivity for end caps and side wall

• High axial mechanical strength• Special electrical properties:

– Metallic– Semi conducting

Methods of Fabrication

• Carbon Arc or Arc Discharge• Laser Ablation or Pulsed Laser Vaporization

(PLV)• Chemical Vapor Deposition (CVD)• High-pressure CO conversion (HiPCO)

Carbon Arc

• Carbon Atoms are evaporated by a plasma of Helium gas that is ignited by high currents passed through opposing carbon anode and cathode.

Arc-Discharge I

Arc-Discharge

• Excellent Method for the production of both MWNT and SWNT

• Carbon Nanotubes are obtained by controlling growth conditions– Pressure of gas in discharge chamber– Arcing Current

Carbon Arc

Pulsed Laser Vaporization

• Used for the production of SWNTs• Uses laser pulses to ablate (or evaporate) a carbon

target– Target contains 0.5 atomic percent nickel and/or

cobalt• The target is placed in a tube-furnace

– Flow tube is heated to ~1200°C

• A flow of inert gas is passed through the chamber to carry the nanotubes “downstream”– SWNTs condense from

the laser vaporization plume and are deposited on a collector outside the furnace zone

Pulsed Laser Vaporization

• MWNTs / SWNTs

• Yield <70%

• Catalyst / no catalyst

• Use of very strong laser

• Expensive (energy costs)

• Commonly applied

Chemical Vapor Deposition• Involves heating a catalyst material to high

temperatures in a tube furnace and flowing hydrocarbon gas through the tube reactor

• The materials are grown over the catalyst and are collected when the system is cooled to room temperature

• Key parameters are:– Hydrocarbons– Catalysts– Growth Temperature

Chemical Vapor Deposition- CVD process involves the dissociation of

hydrocarbon molecules catalyzed by the transition metal, and the dissolution and saturation of carbon atoms in the metal nanoparticle

- Both MWNT and SWNT can be grown byCVD methods

- MWNTs use acetylene gas for the carbon source and a growth temperature between 600 – 800°C

- SWNTs use carbon monoxide or methane for a carbon source and a much higher growth temperature (900 – 1200°C)

Steps of CVD

• Deposit photoresist• Expose resist• Deposit catalyst• Etch resist• CVD growth of Carbon

Nanotube on catalyst

Synthesis: growth mechanism

• Gas phase deposition

• Large scale possible

• Relatively cheap

• SWNTs / MWNTs

• Aligned nanotubes

• Patterned substrates

High-pressure CO conversion (HiPCO)

• New method of growing SWNT• Primary carbon source is carbon monoxide• Catalytic particles are generated in-situ

– Thermal decomposition of iron pentacarbonyl in a reactor heated to 800 - 1200°C

• Process is done at a high pressure to speed up the growth (~10 atm)

• Promising method for bulk production of SWNTs

Fabrication HiPCO

2CO• Single wall nanotube in gas phase (1200oC, 10 atm)• CO+CO C+ catalyst: (25 mTorr)• Flow high pressure carbon monoxide past catalyst particles at high temperatures• Can now produce largely single-walled nanotubes in kilogram quantities• Purification steps are unnecessary due to use CO instead of hydrocarbons

Synthesized SWNTs

• A metal catalyst needs to be present to produce a SWNT– Catalysts include

cobalt, iron or nickel• SWNTs are usually

assembled in ropes but some single tubes can also be found

Purification• Contaminants:

– Catalyst particles– Carbon clusters– Smaller fullerenes: C60 / C70

• Impossibilities:– Completely retain nanotube structure– Single-step purification

• Only possible on very small scale:– Isolation of either semi-conducting SWNTs

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

Cees Dekker, Delft University

Nanotube transistor

Wind et. al, Appl. Phys. Lett., May 20, 2002http://www.research.ibm.com/resources/news/20020520_nanotubes.shtml

• Logic gates, oscillators, … using many nanotubeson a single wafer has been demonstrated.

Nanotube transistor

• Field Effect transistor– Similar to MOSFET– Formation of p-type– Annealing or doping with

K to form n-type– Use both n and p to

make CMOS typecircuits

Directed Growth of SWNTs

• SWNT can be grown as suspended networks

• Grown from silicon pillars that have been patterned with a catalyst material

< Energy storage:

• Li-intercalation

• Hydrogen storage

> FED devices:

• Displays

< AFM Tip

> Molecular electronics

• Transistor

< Others

• Composites

• Biomedical

• Catalyst support

• Conductive materials

• ???

Super CapacitorElectrochemical double layer

Electrode (+) Electrode (-)

Separator

Field Emitting Devices

Single Emitter

Film Emitter

Patterned Film Field Emitters

• Etching and lithography• Conventional CVD• Soft lithography

Field Emission Display Device• Take advantage of the properties

of high current containing and high aspect ratio

• Useful as the electron source for Flat Panel Display with lower power consuming and high voltage circuit is unneeded

• Samsung has shown the prototype of 9” full color display with 576 X 242 pixels

• The first nanotube flat screen TV is expected to be manufactured by the end of 2003

The Samsung 4.5” full-color nanotube display

Schematic structure of nanotube flat panel display.(Choi et al.)

(Cathode)

(Anode)

Upon deformation

sp2 to sp3

Stretching of bonds

Opens bandgap in most nanotubes[Phys. Rev. B, vol. 60 (1999)]

Electromechanical Switch?

Resonator, Nanobalance• Deflection of MWNT ∝ Vs

• If Vs is alternating: resonance occurs (same as elastic theory of beams)

• Nanobalance: can measure picogram – femtogram masses.– Picture shows v = 968 KHz,

m = 22±6 fg

Poncharal et al., Science (1999)

Nanotweezers

• Separation between tweezer arms depends on applied potential

• Measure electrical properties of nanoparticles (SiC), nanowires (GaAs)

Kim and Lieber, Science (1999)

Atomic Force Microscopy

Chemical Sensor• Advantages

– High adsorption surface area

– Changing electrical properties at room temp.

– Detect very small concentrations (ppm) of O2, NO2, NH3

• Semiconducting nanotube • Depletion or accumulation of

carriers depending on species

Change of electrical resistance of SWNT with exposure to O2

V-I curves for NH3 and NO2exposure

Collins et al., Science (2000); Kong et al., Science (2000)

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 to work• Positioning of molecular electronics is difficult• Energy storage is still doubtful, fundamental

investigations are needed

ReferencesDresselhaus, et al. “Carbon Nanotubes: Synthesis, Structure, Properties and Applications, 2001

K. Tanaka, et al. “The Science and Technology of Carbon Nanotubes,” 1999

H. Dai, et al. “Single-Wall Nanotubes Produced by Metal-Catalyzed Disproportionation of Carbon Monoxide,” Chemical Physical Letters 260 1996

N.R. Franklin, et al. “An enhanced CVD approach to extensive nanotube networks with directionality,” Advanced Materials, vol.12, June 2000

C. Journet, et al. “Large Scale Production of Single-wall Carbon Nanotubes by the Electric Arc Technique,” Nature 388 (1997).

P. Nikolaev, et al. "Gas-phase Catalytic Growth of Single-Walled Carbon Nanotubes from Carbon Monoxide,“Chemical Physics Letters 313 (1999).

C.D. Scott, et al. “Growth Mechanisms for Single-Wall Carbon Nanotubes in a Laser-Ablation Process” Applied Physics A 72 2001

The Nanotube Site: http://www.pa.msu.edu/cmp/csc/nanotube.htm

Institute of Theoretical Physics: http://www.itp.ucsb.edu

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