7/31/2019 Field Emission From Nanotubes

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Fieldemission(1)Field

emission

basics

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emission

f rom

nanotube

f i lms

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Electr on sour ces are becoming increasingly import ant in r esearch and everyday lif e. Although the

conventional, t hermoelect ronic emit t er seems to be declining, cold elect r on emit ters using f ield

emission show great potent ial f or numerous applications.

How would t he ideal f ield emit t er look li ke? I t should be very long and very t hin, made of conduct ive

mater ial wit h high mechanical st rength, be robust, and cheap and easy to process... yes, something

very close to a carbon nanotube!

The power of car bon nanot ubes as elect ron f ield emit t ers was alr eady apparent f rom the f ir st

ar t icles repor t ing ext remely low turn-on f ields and high cur rent densit ies in 1995 . Only a f ew

studies f ollowed dur ing the next two year s. From 1998 on, the perspective to use nanotubes as f ieldemission devices spur r ed ef f ort s worldwide: a f ir st cr ude display as well as a light ing element were

presented. Lat er studies addressed issues of large scale product ion compat ible wit h

micr of abricat ion technology and str ove towards bett er under standing of t he emission mechanism.

Carbon nanot ubes can be used as electr on sources in two dif f erent t ypes of set- ups, namely single

and mult iple elect ron beam devices. One possible applicat ion of a single elect ron beam inst rument is

an elect r on micr oscope that uses a single nanot ube as a f ield emission elect ron gun t o produce a

highly coherent electr on beam. Conversely, f lat panel displays are t he most popular example of

mult iple beam inst rument s where a cont inuous or pat t erned f ilm of nanot ubes provides a large

number of independent electr on beams.

We discuss these t wo t ypes of emit t ers separat ely, and begin wit h a short description of f ieldemission.

Field emission basics

Field emission involves the ext r acti on of elect rons f r om a solid by t unneling thr ough the sur f ace

potential bar r ier. The emit t ed cur rent depends dir ect ly on t he local elect r ic f ield at t he emit t ing

sur f ace, E, and on it s work f unct ion, f, as shown below. I n f act , a simple model (t he Fowler- Nordheim

model) shows that t he dependence of t he emit ted cur rent on the local electr ic f ield and t he

wor kf uncti on is exponential-li ke. As a consequence, a small var iat ion of the shape or surr ounding of

t he emit ter (geometr ic f ield enhancement ) and/ or t he chemical st at e of t he sur f ace has a st rong

impact on the emitted current.

d emission from nanotubes http://ipn2.epfl.ch/CHBU/NTfieldemission1.ht

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St andard f ield emission model f r om a metallic emit t er , showing the potential bar r ier and t he corr esponding energy

dist r ibution (energy on the vert ical axi s, cur r ent on t he hor izontal logar it hmic axis).

The f ield E has to be very high, on the order of 2- 3 V/ nm. To reach this value, one takes advantage

of the f ield amplif icat ion ef f ect : the elct r ic f ield lines are concentr at ed around a sharp obj ect . As

t he f ield amplif icat ion increases wit h decreasing radius of curvat ure, the sharper the bet ter.

Nanot ubes are thus ideally suit ed as f ield emit t ers, as their elongat ed shape ensur es a ver y high

f ield amplif ication. Elect r on emission can be obser ved wit h about 100V alr eady on a single nanotube!

Typical set -up f or f ield emission: a potenti al di f f erence is applied between a nanotube (or an assembly of nanotubes) anda count erelectr ode.

To know more about f ield emission f rom nanotube f ilms, see the

d emission from nanotubes http://ipn2.epfl.ch/CHBU/NTfieldemission1.ht

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