Carbon Nanotube

An Old Thing With New Name ?

Carbon nanotube ?

C60-FULLERENE SINGLE WALL NANOTUBEC60-FULLERENE SINGLE WALL NANOTUBE

Carbon Nanotube TechnologyCarbon Nanotube TechnologyCarbon Nanotube TechnologyCarbon Nanotube Technology

1.2 nm

Single WalledSingle Walled

Carbon NanotubeCarbon Nanotube

SWNTSWNT

Multi WalledMulti Walled

Carbon NanotubeCarbon Nanotube

MWNTMWNT

10-200 nm

Fullerene Cap

•Resistivity 10-4 cm•Maximum Current Density 1013 A/m2

•Thermal Conductivity ~ 2000 W/m/K•Young's Modulus (SWNT) ~ 1 TPa•Young's Modulus (MWNT) 1.28 TPa•Maximum Tensile Strength ~ 100 GPa

Properties of InterestProperties of Interest

1889 1889 US Patent 405,480: Fe catalyst grows Carbon Fibers

1948 military interests in fibers and whiskers1948 military interests in fibers and whiskers

1955 1955 CVD synthesis of Carbon Nanofibers with Fe, Ni, Co

1976 1976 Hollow tube structure of Carbon Nanofibers

1983 1983 CCVD synthesis of hollow Carbon Nanofilaments

1991 1991 Discovery of Carbon Nanotubes in Arc Discharge

1992- NANOTUBE GOLD RUSH: 500$/gramm1992- NANOTUBE GOLD RUSH: 500$/gramm

2001 Nanotube based FED2001 Nanotube based FED

2002 2002 NANOTUBE COMPOSITES, BATTERIES, TRANSISTOR

2018 2018 NASA nanotube space elevator ?

A BRIEF HISTORY OF A BRIEF HISTORY OF CARBON NANOTUBESCARBON NANOTUBES

Properties Arc Discharge Laser Ablation Chemical Vapor Deposition

Nanotube generator Requires high-voltage arc discharge, not very expensive

Requires high-power lasers, which are very expensive

Carbon containing precursor vapors and catalysts, low cost

Moving parts inside reaction chamber

None Worm gear to reposition target at 1000 C

None

Precursor Solid carbon (graphite) Carbon target Carbon monoxide, acetylene, ethylene, xylene, etc.

Process Batch – requires harvesting nanotubes

Batch – requires changing targets and harvesting nanotubes

Batch – direct deposition on product or harvesting for other applications

Carrier gas Argon, helium Argon, helium Argon, nitrogen

Diameters of nanotubes 1 – 10 nm 1 – 100 nm 1 – 1000 nm

Length of nanotubes 0.01 – 0.1 µm 0.1 – 1 µm 1 – 1000 µm

Yields ~ 50% ~ 70% ~ 60 – 90%

Contaminants Soot, fullerenes, graphenes, amorphous carbon. High amount

Fullerenes, amorphous carbon. Medium amount

Almost none. Theoretically clean process

Production quantities 10 g / day < 1 g / day 1 kg / day

SUMMARY OF NANOTUBE PROCESS ROUTESSUMMARY OF NANOTUBE PROCESS ROUTES

H

M

MxC M+CFilaments

HCCatalyst

Hydro-Carbon

Carbon-Layer

fishbone multiwalledcarbides soot

SUMMARY OF CATALYTIC CVD PROCESSSUMMARY OF CATALYTIC CVD PROCESS

Nanofiber ApplicationsNanofiber Applications

Thermionic cathode

Electron sources of high intensity and high efficiency are desirable for a variety of applications.

The most common electron source is the thermionic cathode such as the cathode-ray tube (CRT).

The disadvantages of heating the cathode are the wasted energy and the relatively large minimum separation that can be reliably achieved between the cathode and current modulating electrodes.

Cold cathodesas the name implies, do not rely on the heating of a material to emit electrons over the vacuum barrier.

There are several types of cold cathodes which can be classified based on their geometry.

Field emitters rely on field enhancement created by sharp points or edges to facilitate tunneling of electrons.

Planar cold cathodes fall into a large variety of types but in general do not rely on field enhancement for their operation.

Field Emitter as Cold Cathode The most common type of microelectronic cold cathode is the field emitter.

Field emitters operate by tunnelling of electrons from a solid material into vacuum as the result of an impressed electric field.

The high electric field serves to thin the vacuum barrier and allow electrons to tunnel.

As the field emission process is dependent on a high electric field at the surface, a sharp, tip-like geometry is needed.

Mo, W, Si and diamond are used as the Emitter.

Field emission wastes no energy in the emission process and thus can approach 100% efficiency.

Three important Parameters of a Field Emitter

The field enhancement factor: A high field enhancement will lower the applied voltage necessary for emission

The emission area: It is determined by the geometry, and in general, it decreases as field enhancement is increased. One method to increase emission area without taking a large field enhancement penalty is to produce an array of field emitters

The work function or electron affinity: the height of the barrier that the electrons must tunnel through, contributes to determining the operating voltage of the field emitter.

Micro-tips

CATHODE

2.5 mm

FIELD EMISSIONFIELD EMISSION

FED: field emission display

The most commonly researched vacuum microelectronic display is the field emission display.

In an FED, each pixel of the display is supplied electrons from an array of field emitters. The array of pixels on the screen is directly in front of the arrays of field emitters, and thus, the voltage necessary to excite the phosphors governs the required thickness of the display, with higher voltage phosphors requiring larger front-to-back spacing.

The main components of the display are the cold cathode, the anode screen, and the spacers that separate the anode from the cathode.

The cold cathode is composed of the field emitter arrays and the gate electrodes which turn on and off the emission for the individual pixels.  

Cathode Ray Tube

L

large L= big housing

Field Emission Display

L

small L= flat panel display

Display technologies

Objective: patterned surface with discrete emission sites

Field emission displays

Edge emitters

• Negative photolithographic patterning of „forbidden“ sites• Graphitising of these sites• CVD process; decomposition of organometallic precursors provide both catalyst and carbon from the gas phase•CNF grow on „positve“ sites

CSIROprocess:

Bottom Up process: patterned growth

Objective:patterned surface with discrete emission sites

Flat Panel Display

Conventional field emitter for FEDs

The first field emission electron sources were realized by the fabrication of microtips.

The pioneer in the fabrication of vacuum microelectronic field emitter cathodes is Charles Spindt. The most common structure for microelectronic field emission cathodes now carries his name

Pixtech, Motorola ,Futuba ,Samsung and Candescent Technologies had developed or already sold field emission displays (FEDs) based on microtips, which are usually made from silicon or molybdenum.

Disadvantages of such cathodes:the expensive fabrication the threat of tip degradation due to the residual gas.

The latter is especially a problem for applications where high emission current densities are required.

Pixtech: one of the famous company in this field

In PixTech's field emission displays, electrons coming from millions of tiny microtips pass through gates and light up pixels on a screen. This principle is similar to that of cathode-ray tubes in television sets. The difference: Instead of just one "gun" spraying electrons against the inside of the screens face, there are as many as 500 million of them (microtips).

PixTech designed, developed and manufactured field emission displays. The Company operated a display pilot manufacturing facility in France, R&D facilities and offices in the USA. It was developing high-volume manufacturing capabilities for its FEDs in Taiwan, and PixTech had also established a marketing partnership with Sumitomo Corporation.

Unfortunately, it announced its bankruptcy a few months ago.

Challenges and Progresses on Carbon Nanotubes as Electron Field Emitter

The ideal material for a field emitter would have high electron concentration, high thermal conductivity, and would be hard and non-reactive in the vacuum environment.

High electron concentration is necessary for high emission current . A secondary concern for achieving high current density is the thermal conductivity of the emitter material. lifetime of field emitters is determined by the reactivity and mechanical strength of the emitter material.

The advantages of the carbon nanotube as an electron field emitter tip include its small radius of curvature, high aspect ratio (L/R>100), low resistivity (10E-4Ohm-cm) , high thermal conductivity (~2000w/mK), high chemical resistance and strong mechanical strength. It has been shown that CNT is an excellent field emitter.

Fowler-Nordheim EquationFowler-Nordheim formula for emission current

J =k1E2 /ø exp(-k2 ø

3/2/ E)E = U/ r

J: Current density of field emission

r: Size of tip

ø: Work function

By excellent electrical properties (lower work function, ø)

as well as unique topology and structure (lower tip radius, r), carbon nanotube is an ideal field emitter.

Samsung announced the first 9-inch carbon-nanotube based FED

The first 9-inch carbon nanotube based colour field emission displays (FEDs) are integrated using a paste squeeze technique.

The panel is composed of 576 x 242 lines with implementation of low voltage phosphors.

The uniform and moving images are achieved only at 2 V/µm. This demonstrates a turning point of nanotube for large area and full colour applications.

The CNT-FEDs will be one of the very promising flat panel displays towards the new millenium.

Current EU Projects related to FED

Carbon Nanotubes for Large Area Displays: CANADISProject starting date: 31/08/2000

Deposition Equipment for Flat panel Display Fabrication: DEPODISProject starting date: 01/09/2001

Printable Technologies for Consumer Priced Large Area Thin Flat Panel Color TV Display: PRINDISProject starting date: 01/07/2001

Technologies and Advanced Materials for Kick-OFF in FED Manufacture: TAKEOFFProject starting date: 01/10/2001

Carbon nanotubes for large area displays

Milestones of the CANADIS project:* In situ growth of localized CNT using CVD methods and catalyst.

* Selection of the best method for reliable growth on large area substrate at low temperature. * Fabrication of a CVD reactor for growth of CNT on large area substrates. * Selection of the best triode structure for high performance FED. * Comparison between experimental data and theoretical models. * Modeling of field emission properties. * Fabrication of a 6' colored CNT based FED. * Fabrication of a 15' colored CNT based FED

Technologies and Advanced materials for Kick-OFF in FED Manufacture

Carbon nanotubes on glass are used as the cathode. The direct deposition process is studied by two original routes: polymer-dispersed CNT deposited through a mask and chemical vapour deposition assisted by an extremely local heating.

Cathodes using indirect CNT will be fabricated and tested in order to select the best option between the different gate technologies for triode-type FED: under-gate, remote-gate and normal-gate structures.

The final packages cover the assembly of 18 inches CNT demonstrators and their final testing and evaluation. Samsung is one of the participaters.