SCIENCE August 19, 2005 (Vol. 309)

¹ NanoTech Institute

University of Texas, USA

&

² CSIRO (textile and fibre technology),

Australia

Why this paper ?Why this paper ?

Large, transparent sheets of carbon nanotubes can be produced at a fast rate.

Solid state processSimple & Elegant techniqueMass productionWide application (transparent

electrical conductors)

Carbon NanotubesCarbon Nanotubes

Tiny cylinders of carbon atomsLightStrongGood conductivitySingle walled and multi-walled nanotubes

Carbon nanotube sheets- How Carbon nanotube sheets- How are they made?are they made?

Ancient art of paper making filtration of nanotubes dispersed in water for a week and peeling the dried nanotubes layer (usually isotropic) from the filter

Partial alignment can be achieved by:

- application of high magnetic fields during filtration process (OR)

- mechanical rubbing of nanotubes that are trapped in the filter pores.

Other methods to make Other methods to make Carbon nanotube sheetsCarbon nanotube sheets

Nanotube aerogel Langmuir Blodgett deposition Casting from oleum(  Oleum, SO3 dissolved in H2SO4

Chemical formula H2SO4 x SO3

Molecular weights H2SO4:  98.08

g/mol SO3:      80.06

g/mol ) Spin coating

The paper presents..The paper presents.. A Method to produce carbon nanotube sheets that are:

– Highly oriented– Free standing (self support)

With high rate of production that appears to be scalable Method is based on the previous advances in:

* Spinning of carbon nanotube yarns from forests &* Introduction of twist to increase by a factor of 1000

Ref: Multifunctional carbon nanotube yarns by down sizing an ancient technology- M. Zhang et. Al.

Science Vol. 306 November 19, 2004 page 135

Nanotube forestsNanotube forests

Multiwalled nanotube (MWNT) forest is synthesized by catalytic chemical vapor deposition using acetylene gas as the carbon source

Diameter of the MWNT are ~10 nm Height of the forests range from 70 to 300 µm

Draw-Twist Process for Draw-Twist Process for Nanotube yarnNanotube yarn

SEM- micrographsSEM- micrographs

Nanosheet TeamNanosheet Team

From left: Nanotechnologists Mei Zhang, Ray Baughman and ShaoliFang rest while Ali Aliev, Sergey Lee and Anvar Zakhidov remainstanding. The team sees potential commercial uses for their discovery.

Making Carbon nanosheetsMaking Carbon nanosheets

A) Self supportingsheet 3.4cm x 1m. @ 1m/min.

B) SEM Image @35 degrees

C) 90 degree rotationto form a sheet

D) Four sheets with 45 deg. Rotation

E) Two sheets that are @90 degrees

supporting drops of: water, orange juice andGrape juice. 50,000

times the mass of the nanotube sheet.

MWNT aerogel sheetsMWNT aerogel sheets

Contacting the forest sidewall with adhesive strip (like 3M Post-it) and drawing it at away at a steady rate.

1m/min. for hand drawn sheets5m/min. with linear translation stage10/min. by winding the sheet on a cylinderThe width of the sheet decreased above

7m/min.

MWNT aerogel sheets …MWNT aerogel sheets …

Nanotubes are oriented in the direction in which the sheet is drawn (anisotropy)

Thickness of the MWNT sheets increased with increase in forest height

Typical thickness of ~18mWeight 2.7µg/cm² (density of 0.0015g/cm³)

Increasing the densityIncreasing the density

By dipping it in a liquid (like ethanol)Thickness reduces to 50 nm and density increases

to 0.5g/cc ( factor of 360)Surface tension effects during evaporation of

ethanol reduces the aero gel to thin sheetSheet resistance of 700 Decrease in sheet resistance is less than 10%

although the anisotropy ratio decreases from the range (50 –70) to about (10-20)

Increasing the density….Increasing the density….

Transmission 85% for polarization and 65% for ll polarization (400nm to 2µm)

85% for non-polarized radiation in the range of 2 to 10 µm

Adhesion between substrate and MWNT sheet is better after shrinking because collapse aerogel increases the contact area

Strength - ComparisonStrength - Comparison 18 sheets of MWNT oriented 465 MPa/(g/cm³)

in the same direction Same with neighboring sheets in 175 Mpa/(g/cm³)

orthogonal direction (biaxial) Mylar and Kapton films ~ 160 Mpa/(g/cm³)

Ultra-high strength steel ~ 125 Mpa/(g/cm³)

Aluminum alloy ~ 250 Mpa/(g/cm³)

SEM of MWNT sheet after SEM of MWNT sheet after shrinkingshrinking

Resistance of MWNT sheetResistance of MWNT sheet

Resistance does not change Resistance does not change when the MWNT sheet is when the MWNT sheet is

flexedflexed

* For flexible displays

A) Sheet resistance of MWNT sheet has a low temperature dependence

B) Transmission vs. Wavelength

C) Noise power density lowest for the MWNT sheet (40 ohms)

ApplicationsApplications

Electrodes in flexible display (replacement for Indium Tin Oxide) -- OLEDs

Solar cellsSolid state lighting Embedded Antenna or heater in the car

windowsPolarized light source

MWNT sheet in OLEDMWNT sheet in OLED

ConclusionConclusion

Amazingly simple process coupled with the high rate of

production will lead to the use of MWNT sheets in many

applications

Thank you!Thank you!

Transparent conducting sheetTransparent conducting sheet