Contacting single bundles of carbon nanotubes with alternating electric fields
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Transcript of Contacting single bundles of carbon nanotubes with alternating electric fields
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.