Contacting single bundles of carbon Contacting single bundles of carbon nanotubes with alternating electric fieldsnanotubes with alternating electric fields

Marcella De CarloMarcella De Carlo

Danilo ZampettiDanilo Zampetti

IntroductionIntroduction

They can be:They can be:1.1. Multiwall (firstly observed in 1991)Multiwall (firstly observed in 1991)

2.2. Single-wall (observed in 1993)Single-wall (observed in 1993)

They play a crucial role in miniaturization at theThey play a crucial role in miniaturization at the

nanometre scale thanks to their great properties.nanometre scale thanks to their great properties.

Carbon nanotubes are quasi-one dimensional solid.Carbon nanotubes are quasi-one dimensional solid.

Properties of CNTsProperties of CNTs

They canThey can::

Conduct electricity better than copper Conduct electricity better than copper

Transmit heat better than diamondTransmit heat better than diamond

Tolerate stress better than steelTolerate stress better than steel

Now, we focus our attention on CNTs’ electricalNow, we focus our attention on CNTs’ electrical

properties. properties.

Electrical propertiesElectrical properties

We canWe can have:have:1.1. Metallic nanotubes Metallic nanotubes

(ideal nanoscale (ideal nanoscale wires)wires)

2.2. Semiconducting Semiconducting nanotubes (enable us nanotubes (enable us to build nanoscale to build nanoscale field-effect transistor)field-effect transistor)

This distinction depends on the way the grafiteThis distinction depends on the way the grafite

sheet, that constitutes the nanotube, is rolled up.sheet, that constitutes the nanotube, is rolled up.

Molecular WiresMolecular WiresThanks to their high electrical conductivity and Thanks to their high electrical conductivity and

chemical stability, we can use nanotubes to realize chemical stability, we can use nanotubes to realize molecular wires.molecular wires.

HOW CAN WE CONTACT AND POSITIONHOW CAN WE CONTACT AND POSITION

NANOTUBES IN A CONTROLLED WAY?NANOTUBES IN A CONTROLLED WAY?

There are some methods:There are some methods:Spraying of nanotubesSpraying of nanotubes

Catalytic growth of nanotubesCatalytic growth of nanotubes

Self-assembling on chemically modified surfacesSelf-assembling on chemically modified surfaces

Why don’t we tell nanotubes where Why don’t we tell nanotubes where to go?to go?

Recently, a new method has been proposed…Recently, a new method has been proposed…

Applying an alternating electric field, we canApplying an alternating electric field, we can

place a bundle of carbon nanotubes betweenplace a bundle of carbon nanotubes between

two metal electrodes!two metal electrodes!

Nagahara’ s AdvantagesNagahara’ s Advantages

As shown by As shown by Nagahara et al., Nagahara et al., this can be athis can be a

viable technique for wiring up nanotubesviable technique for wiring up nanotubes

into circuits. In fact:into circuits. In fact:After the initial patterning of the metal electrodes, After the initial patterning of the metal electrodes, no further lithography steps are necessary.no further lithography steps are necessary.

We can selectively place CNTs separating them We can selectively place CNTs separating them from contaminant species.from contaminant species.

Nagahara’s issueNagahara’s issue

……he can’t control the number of trapped nanotubes.he can’t control the number of trapped nanotubes.

SOLUTION:SOLUTION:Choose a different electrode material!Choose a different electrode material!

Now, we are going to describe a process toNow, we are going to describe a process to

contact a single bundle of carbon nanotubes… contact a single bundle of carbon nanotubes…

Getting the nanotubes…Getting the nanotubes…

In order to obtain single-walled nanotubesIn order to obtain single-walled nanotubes

we can use a we can use a laser ablationlaser ablation system. system.

The next step is to The next step is to purify the materialpurify the material from from

contaminant species.contaminant species.

Purification ProcessPurification Process

The purification is performed following a procedureThe purification is performed following a procedurethat uses:that uses:

AcidsAcidsCentrifugal machineCentrifugal machineSonicationSonicationFilteringFiltering

At the end of this process we obtain CNTs having -COOH functional groups associated with the acid treatment.

Substrate’s PreparationSubstrate’s Preparation

The silicon surface is thermally oxidized The silicon surface is thermally oxidized

Electrodes are prepared with an e-beam Electrodes are prepared with an e-beam lithography and lift-off technique.lithography and lift-off technique.

……Both Ag and Au electrodes are prepared usingBoth Ag and Au electrodes are prepared using

this method.this method.

Experimental SetupExperimental Setup

The structure has been bondedThe structure has been bonded

onto a chip carrier and wired uponto a chip carrier and wired up

as sketched in the figure.as sketched in the figure.

The circuit is powered by aThe circuit is powered by a

frequency generator with a dc-offsetfrequency generator with a dc-offset

voltage. The electric current isvoltage. The electric current is

monitored by the ac or dc voltage.monitored by the ac or dc voltage.

Let’s trap them…Let’s trap them…

Switch on the frequency generatorSwitch on the frequency generator

Apply a drop of nanotube suspensionApply a drop of nanotube suspension

Wait a minute …Wait a minute …

A single bundle of carbon nanotubes trapped on four Au electrodes

Frequency and AlignmentFrequency and Alignment

In order to have an excellent alignment, it isIn order to have an excellent alignment, it is

necessary to use frequencies above 1kHz. necessary to use frequencies above 1kHz.

This fact is due to effects of the suspension. In fact,This fact is due to effects of the suspension. In fact,

at lower frequencies ions act as an electric shield,at lower frequencies ions act as an electric shield,

not allowing CNTs to align precisely.not allowing CNTs to align precisely.

What’s the Number of Trapped What’s the Number of Trapped Bundles ?Bundles ?

The control over the number of trappedThe control over the number of trapped

bundles depends on the electrode material.bundles depends on the electrode material.

Au electrodesAu electrodes: N : N increases with time.increases with time.

Ag electrodesAg electrodes: only a : only a very few bundles are very few bundles are trapped indipendent of trapped indipendent of time and concentration.time and concentration.

What’s the difference?What’s the difference?

Looking at the previous figure, we can seeLooking at the previous figure, we can seethat the two-terminal resistence is lowerthat the two-terminal resistence is lowerwhen using Ag instead of Au electrodes.when using Ag instead of Au electrodes.

Therefore:Therefore: Lower resistenceLower resistence

Lower voltageLower voltage

Electric field collapsesElectric field collapses

No more CNTs are trappedNo more CNTs are trapped

Resistence MeasuramentsResistence Measuraments

The previous measuraments have beenThe previous measuraments have been

performed after removal of the solvent.performed after removal of the solvent.

BUT:BUT:

WHAT HAPPENS IN THE PRESENCE OF WHAT HAPPENS IN THE PRESENCE OF THE SOLVENT?THE SOLVENT?

To answer this question, we need other measurements.To answer this question, we need other measurements.

Wet Resistence MeasurementsWet Resistence Measurements

After adding a small DC offset, the VAfter adding a small DC offset, the VDCDC isismeasured giving the following results:measured giving the following results:

We can see that CNTs form an electric contact with Ag in the presence of the suspension, while they don’t with Au.

A Chemical ExplanationA Chemical Explanation

The origin of this fact may be found in theThe origin of this fact may be found in the

strong affinity of the –COOH groups ofstrong affinity of the –COOH groups of

acid-treated nanotubes to Ag surface.acid-treated nanotubes to Ag surface.

In fact, it’s known that n-alkanoic acids form a In fact, it’s known that n-alkanoic acids form a SAM on Ag surfaces.SAM on Ag surfaces.

On the other hand, they have no affinity to Au On the other hand, they have no affinity to Au surfaces.surfaces.

ConclusionsConclusions

It is possible to control the number ofIt is possible to control the number of

trapped bundles because it depends ontrapped bundles because it depends on

chemical bonds. chemical bonds.

Such chemical properties can be controlledSuch chemical properties can be controlled

using different electrode materials. using different electrode materials.

To Be Done...To Be Done...

Try to repeat the experiment using isolatedTry to repeat the experiment using isolated

carbon nanotubes ( not bundles )!!!carbon nanotubes ( not bundles )!!!

This method uses acid-treated nanotubes…This method uses acid-treated nanotubes…

……what about different treatments?what about different treatments?

Other methods…Other methods…

Several methods have been proposed toSeveral methods have been proposed to

position carbon nanotubes, but none of theseposition carbon nanotubes, but none of these

has been efficient enough to supersede thehas been efficient enough to supersede the

others.others.

For example, we have found that at least 38 people will speak about CCVD at the “International Conference on the Science and Application of Nanotubes” that will begin in July.