SCIENCE August 19, 2005 (Vol. 309) ¹ NanoTech Institute University of Texas, USA & ² CSIRO (textile and fibre technology), Australia

Why this paper ? Large, transparent sheets of carbon nanotubes can be produced at a fast rate. Solid state process Simple & Elegant technique Mass production Wide application (transparent electrical conductors)

Carbon Nanotubes Tiny cylinders of carbon atoms Light Strong Good conductivity Single walled and multi-walled nanotubes

Carbon nanotube sheets- How 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 Carbon nanotube sheets Nanotube aerogel Langmuir Blodgett deposition Casting from oleum( Oleum, SO 3 dissolved in H 2 SO 4 Chemical formula H 2 SO 4 x SO 3 Molecular weights H 2 SO 4 : g/mol SO 3 : g/mol ) Spin coating

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 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 Nanotube yarn

SEM- micrographs

Nanosheet Team From left: Nanotechnologists Mei Zhang, Ray Baughman and Shaoli Fang rest while Ali Aliev, Sergey Lee and Anvar Zakhidov remain standing. The team sees potential commercial uses for their discovery.

Making Carbon nanosheets

A)Self supporting sheet 3.4cm x 1m/min. B) SEM 35 degrees C)90 degree rotation to form a sheet D)Four sheets with 45 deg. Rotation E)Two sheets that degrees supporting drops of: water, orange juice and Grape juice. 50,000 times the mass of the nanotube sheet.

MWNT 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 sheets 5m/min. with linear translation stage 10/min. by winding the sheet on a cylinder The width of the sheet decreased above 7m/min.

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 ~18  m Weight 2.7µg/cm² (density of g/cm³)

Increasing 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 sheet Sheet 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…. 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 - Comparison 18 sheets of MWNT oriented465 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 shrinking

Resistance of MWNT sheet

Resistance does not change when the MWNT sheet is flexed * 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)

Applications Electrodes in flexible display (replacement for Indium Tin Oxide) -- OLEDs Solar cells Solid state lighting Embedded Antenna or heater in the car windows Polarized light source

MWNT sheet in OLED

Conclusion Amazingly simple process coupled with the high rate of production will lead to the use of MWNT sheets in many applications

Thank you!

Transparent conducting sheet