• When subjected to high E field, electrons near the Fermi level can overcome the energy barrier to escape to the vacuum level

• Common tips: Mo, Si, diamond• Applications:

- Cathode ray lighting elements- Flat panel displays- Gas discharge tubes in telecom networks- Electron guns in electron microscopy- Microwave amplifiers

• Fowler - Nordheim equation: φ is work function, β is field enhancement factor

• Plot of ln (I/V2) vs. (1/V) should be linear• At low emission levels, linearity seen; in the high

field region, current saturates• Critical: low threshold E field, high current density, high emission site density (for

high resolution displays)

I = aV 2 exp(−bφ1.5 /βV )

• Cathode and anode enclosed in an evacuated cell at a vacuum of10-9 - 10-8 Torr

• Cathode: glass or polytetrafluoroethylene substrate with metal-patterned lines

- nanotube film tranferred to substrateor grown directly on it

• Anode located 20-500 µm from cathode

• Turn-on field: electric-field required togenerate 1 nA

- should be small

• Threshold field: electric field required to yield 10 mA/cm2

• Needs- For displays, 1-10 mA/cm2

- For microwave amplifiers, > 500 mA/cm2

• To obtain low threshold field- Low work function (φ)- Large field enhancement factor (β) ⇒ depends on geometry

of the emitter; β ~ 1/5r

• Threshold field values (in V/µm) for 10 mA/cm2

- Mo - 50-100- Si - 50-100- P-type diamond - 130- Graphite Powder - 17- Carbon nanotubes - 1-3 (stable at 1 A/cm2)

_

• Working full color flat panel displays and CRT-lighting elements have beendemonstrated in Japan and Korea

• Display- Working anode, a glass substrate with phosphor coated ITO stripes- Anode and cathode perpedicular to each other to form pixels at the

intersection- Phosphors such as Y2O2S: Eu (red), Zns: Cu, Al (green),

ZnS: Ag, Cl (blue)- 4.5” display showing a uniform and stable image

• Lighting Element- Phophor screen printed on the inner surface of the glass and backed by

a thin Al film (~100 nm) to give electrical conductivity- Lifetime testing of the lighting element shows a lifespan over 1000 hrs.

• Needed for composites, hydrogen storage, other applications which need bulk material

• Floating catalysts (instead of supported catalysts)• Carbon source (CO, hydrocarbons)• Floating catalyst source (Iron pentacarbonl, ferrocene…)• Typically, a carrier gas picks up the catalyst source and goes

through first stage furnace (~200° C)• Precursor injected directly into the 2nd stage furnace• Decomposition of catalyst source, source gas pysolysis, catalyzed

reactions all occur in the 2nd stage• Products: Nanotubes, catalyst particles, impurities

• Purpose: To remove all unwanted material and obtain the highest yield with no damage to the nanotubes

• Several processes have been reported in the literature; a typical process involves the following steps:

- A sample (50 mg) is transferred to a 50 ml flask, with 25 ml concentrated HCl and 10 ml of concentrated HNO3

- Solution heated for 3 hrs, constantly stirred with a magnetic stirrer in a reflux apparatus equipped with a water-cooled condenser ⇒ removes metal and graphite particles

- Suspension transferred into centrifuge tubes and spun at 3220 g for 30 min.- After pouring of the supernatant, solids are resuspended,

spun three times in deionized water- The solid is treated with NaOH (0.01 µ) and centrifuged for

30 minutes ⇒ nanotube bundles with tube ends capped with half fullerenes- Sample dried overnight in a vacuum oven at 60° C

• More & more components are packaged in smaller spaces where electromagneticinterference can become a problem

- Ex: Digital electronics coupled with high power transmitters as in manymicrowave systems or even cellular phone systems

• Growing need for thin coatings which can help isolate critical components fromother components of the system and external world

• Carbon nanofibers have been tested for EMI shielding; nanotubes have potentialas well

- Act as absorber/scatterer of radar and microwave radiation- High aspect ratio is advantageous- Efficiency is boosted by small diameter. Large d will have material too

deep inside to affect the process. At sub-100 nm, all of the materialparticipate in the absorption

- Carbon fibers and nanotubes (< 2 g/cc) have better specific conductivitythan metal fillers, sometimes used as radar absorbing materials.

• Fully automated control of vehicles to enhance safety and mobility

• Lateral control ⇒ steering to control position relative to thecenter of the traffic lane

• Longitudinal control ⇒ speed & headway

• Original contender:Magnetic sensors together with magnetic highway markings for(lateral) + radar technology (for longitudinal)

• Cement paste with 0.5 vol% carbon filaments exhibits reflectivity at 1 GHz that is 29 dB higher than transmittivity [ D.D.L. Chung,in Carbon Filaments and Nanotubes, NATO Science Series, Kluwer Academic 2001].

Electromagnetic technology is better than magnetic technology: Why?

1. Low material cost- Reflecting concrete is only 30% more than regular concrete- Still much less than concrete with magnetic strips or embedded magnets

2. Low labor cost3. Low peripheral electronics cost (off-the-shelf oscillators and detectors)4. Reflecting concrete exhibits better mechanical properties and lower

drying shrinkage than conventional concrete; embedded magnets weaken concrete

5. Good reliability, less affected by weather as frequency, impedance and power selectivity provide tuning capability

6. High durability; demagnetization and marking detachments are not issues7. Magnetic field from a magnetic marking can be shielded by electrical

conductors (such as steel) between the marking and the vehicle, whereas electromagnetic field cannot be shielded easily.

• Impediments to commercialization of fuel cells: safe storage and delivery of hydrogen fuel

• Potential solution: adsorption of H2 in a solid support ⇒ storage at relatively low pressures and high T

• DOE Target: 6.5 wt%, 62 kg H2/m3

• Carbon nanotubes may be attractive for H2 storage- porous structure- low density

• Storage mechanisms: physisorption?• To date, several groups have confirmed 1% uptake easily• Higher % claims (5-8%) are not verifiable or reproducible

• Rechargeable lithium batteries: work by intercalation and de-intercalation of lithium between two electrodes

- Transition metal oxide cathode and graphite anode• Production improvement: high energy capacity, fast charging time, long

cycle time• How do you get high energy capacity?

- Determined by the saturation Li concentration of the electrode material

• For graphite, this concentration is LiC6 ⇒ yields a capacity of 372 mA h/g• For nanotubes ⇒ inner cores, inter-tube channels, interstitial sites

(inter-shell van der Waals spaces) all are available for Li intercalation• To date, a reversible capacity of 1000 mA h/g has been demonstrated• Exact locations of Li ions still unknown

• Attaching chemical groups to the sidewall of CNTs to modify theproperties as needed for applications

- Chemical modification of the sidewall may improve theadhesion characteristics of CNTs in a host matrix to makecomposites

- Chemical or biosensors

• Saturation of 2% the C atoms in SWNTs with C-Cl sufficient to change electronic band structures dramatically; done with reactingSWNTs with dichlorocarbene (Chen et al, Science, 282, 95 (1998).

• Fluorination of SWNTs with F2 gas flow at 250-600°C for 5 hrs.(Michelson et al, CPL, 296, 188 (1998) has been shown to attach Fcovalently to the sidewall

• Cold plasma approach to functionalization(Khare et al, NanoLett. 2, 73, (2002)

• Carbon nanotubes viewed as the “ultimate” nanofibers ever made• Carbon fibers have been already used as reinforcement in high strength, light

weight, high performace composites:- Expensive tennis rackets, air-craft body parts…

• Nanotubes are expected to be even better reinforcement- C-C covalent bonds are one of the strongest in nature- Young’s modulus ~ 1 TPa ⇒ the in-plane value for defect-free graphite

• Problems- Creating good interface between CNTs and polymer matrix necessary

for effective load transfer

CNTs are atomically smooth; h/d ~ same as for polymer chainsCNTs are largely in aggregates ⇒ behave differently from individuals

• Solutions- Breakup aggregates, disperse or cross-link to avoid slippage- Chemical modification of the surface to obtain strong interface with

surrounding polymer chains

WHY?

From P. Ajayan and O. Zhou

• Electronic properties are independent of helicity and the number of layers

• Applications: Nanoelectronic devices, composites

• Techniques: Arc discharge, laser ablation

• Also: B2O3 + C (CNT) + N2 → 2 BN (nanotubes) + 3 CO