Alberto Caviglia & Enri Alberto Caviglia & Enri co Costa co Costa Crossed nanotube Crossed nanotube Junctions Junctions M. S. Fuhrer, J. Nygård, L. Shih, M. Forero, M. S. Fuhrer, J. Nygård, L. Shih, M. Forero, Young-Gui Yoon, Young-Gui Yoon, M. S. C. Mazzoni, Hyoung Joon Choi, Jisoon Ihm, M. S. C. Mazzoni, Hyoung Joon Choi, Jisoon Ihm, Steven G. Louie, A. Zettl, Paul L. McEuen Steven G. Louie, A. Zettl, Paul L. McEuen
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Alberto Caviglia & Enrico Costa Crossed nanotube Junctions M. S. Fuhrer, J. Nygård, L. Shih, M. Forero, Young-Gui Yoon, M. S. C. Mazzoni, Hyoung Joon Choi,
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Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa
M. S. Fuhrer, J. Nygård, L. Shih, M. Forero, Young-Gui M. S. Fuhrer, J. Nygård, L. Shih, M. Forero, Young-Gui Yoon,Yoon,
M. S. C. Mazzoni, Hyoung Joon Choi, Jisoon Ihm,M. S. C. Mazzoni, Hyoung Joon Choi, Jisoon Ihm,Steven G. Louie, A. Zettl, Paul L. McEuenSteven G. Louie, A. Zettl, Paul L. McEuen
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa
Presentation byPresentation by Alberto Caviglia & Enrico Alberto Caviglia & Enrico
CostaCosta
for the course offor the course of
Nanotecnologie 1Nanotecnologie 1
held by held by
Prof. Ermanno Di ZittiProf. Ermanno Di Zitti
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 33
The principal methods for obtaining high-quality single-walled nanotube are arc discharge and laser vaporization.
Both methods involve evaporating carbon atoms from solid carbon sources at ≥3000 °C, which limits the scale-up of SWNTs.
Nanotubes synthesized by the evaporation methods are in tangled forms that are difficult to purify, manipulate, and assemble for building addressable nanotubes architectures.
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 55
A direct current of 50 to 100 A driven by approximately 20 V DC A direct current of 50 to 100 A driven by approximately 20 V DC creates a high temperature discharge between the two electrodes.creates a high temperature discharge between the two electrodes.
The discharge vaporises one of the carbon rods and forms a small rod The discharge vaporises one of the carbon rods and forms a small rod shaped deposit on the other rod. shaped deposit on the other rod.
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 66
The laser vaporisation apparatus used by Smalley's The laser vaporisation apparatus used by Smalley's group is used to vaporise a graphite target in an oven group is used to vaporise a graphite target in an oven at 1200 °C. at 1200 °C.
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 77
Growth of SWNT (2)Growth of SWNT (2)(78 IEEE Transaction on nanotechnology, VOL. 1, N° 1, March 2002, pg.80)(78 IEEE Transaction on nanotechnology, VOL. 1, N° 1, March 2002, pg.80)
((a) Schematic of a SWNT a) Schematic of a SWNT growing from a catalyst seed growing from a catalyst seed particle. particle.
(b) Atomic force microscope (b) Atomic force microscope images of a single nanotube images of a single nanotube device fabricated using electron device fabricated using electron beam lithography. beam lithography.
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 88
Chemical vapor deposition methods have been very successful in synthesizing carbon fibers, filaments.
CVD synthesis of high-quality SWNTs is only recent by using methane as carbon feedstock and iron oxide nanoparticles supported on high surface area alumina as the catalyst.
It is found that methane is stable at the elevated growth temperatures without appreciable self-pyrolysis.
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 99
In a typical procedure, a raw sample of nanotubes (8.5 g) was first refluxed in 1.2 liters of 2.6 M nitric acid for 45 hours.
Upon cooling, the solution was transferred to polytetrafluoroethylene centrifuge tubes and spun at 2400g for 2 hours.
The supernatant acid was decanted off, replaced by deionized water, and vigorously shaken to resuspend the solids, followed by a second centrifuge-decant cycle.
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 1212
Experiments and theory have shown that these Experiments and theory have shown that these tubes can be either metals or semiconductors, tubes can be either metals or semiconductors, depending on their chirality and their electrical depending on their chirality and their electrical properties can rival, or even exceed, the best properties can rival, or even exceed, the best metals or semiconductors known.metals or semiconductors known.
Individual SWNTs may act as device such as Individual SWNTs may act as device such as field-effect transistors , single-electron-tunneling field-effect transistors , single-electron-tunneling transistors or rectifiers.transistors or rectifiers.
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 1414
How can individual SWNTs be joined together How can individual SWNTs be joined together to form multimedial devices and, ultimately, to form multimedial devices and, ultimately, complex circuits ?complex circuits ?
How can, in particular, SWNT-SWNT How can, in particular, SWNT-SWNT junctions, formed by nanotubes that lie across junctions, formed by nanotubes that lie across one another on a substrate, be used to solve the one another on a substrate, be used to solve the previous question ?previous question ?
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 1515
Consist of two crossed individual SWNT’s or Consist of two crossed individual SWNT’s or small bundles (diameter<3nm) of SWNTs with small bundles (diameter<3nm) of SWNTs with four electrical contacts, one on each end of each four electrical contacts, one on each end of each SWNT or bundle.SWNT or bundle.
This type of junction is easily constructed and, This type of junction is easily constructed and, with the development of techniques to place with the development of techniques to place nanotubes with precision on substrates, could be nanotubes with precision on substrates, could be mass produced.mass produced.
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 1616
On the left we can see the On the left we can see the atomic force microscope atomic force microscope (AFM) image of a (AFM) image of a completed crossed completed crossed nanotube device.nanotube device.
Two crossed SWNTs Two crossed SWNTs (green) interconnect the (green) interconnect the Cr/Au contacts (yellow). Cr/Au contacts (yellow).
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 1717
Dependence by gate Dependence by gate voltagevoltage
At room temperature the two type of SWNTs have At room temperature the two type of SWNTs have
different behavior .different behavior .
Metallic SWNTs have a finite conductance that is nearly independent of Vg.
Semiconducting Semiconducting SWNTs are found to be SWNTs are found to be p-type:p-type:Conducting for VConducting for Vgg<0<0Insulating for VInsulating for Vgg>0>0
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 1919
Mesure of the conductancesMesure of the conductances In order to measure the In order to measure the
four-terminal four-terminal conductances and so conductances and so accurately determinate accurately determinate the junction conductance, the junction conductance, current is passed, for current is passed, for example, into the 1->3 example, into the 1->3 circuit and the contactcircuit and the contact 2 & 4 act as probe 2 & 4 act as probe. .
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 2222
Landauer-Buttiker Formula (1)Landauer-Buttiker Formula (1)(78 IEEE Transaction on nanotechnology, VOL. 1, N° 1, March 2002, pg.78)(78 IEEE Transaction on nanotechnology, VOL. 1, N° 1, March 2002, pg.78)
The two terminal Landauer-Buttiker formula states The two terminal Landauer-Buttiker formula states that, for a system with N 1-D channels in parallel: that, for a system with N 1-D channels in parallel: G= (NeG= (Ne22/h)T, where T is the transmission coefficient /h)T, where T is the transmission coefficient for electrons through the sample.for electrons through the sample.
For a SWNT at low doping levels such that only one For a SWNT at low doping levels such that only one transverse sub-band is occupied, N=4. Each channel transverse sub-band is occupied, N=4. Each channel is fourfold degenerate, due to spin degeneracy and is fourfold degenerate, due to spin degeneracy and the sublattice degeneracy of electrons in graphene.the sublattice degeneracy of electrons in graphene.
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 2323
Landauer-Buttiker Formula (2)Landauer-Buttiker Formula (2)(78 IEEE Transaction on nanotechnology, VOL. 1, N° 1, March 2002, pg.78)(78 IEEE Transaction on nanotechnology, VOL. 1, N° 1, March 2002, pg.78)
The conductance of ballistic SWNT with perfect The conductance of ballistic SWNT with perfect contacts (T=1) is then 4econtacts (T=1) is then 4e22/h = 155 /h = 155 µS, or about 6.5 kµS, or about 6.5 kΩΩ..
Two-terminal conductances of metallic SWNTs at Two-terminal conductances of metallic SWNTs at room temperature can vary significantly, ranging from room temperature can vary significantly, ranging from as small as ~ 6 kas small as ~ 6 kΩΩ to many Mto many MΩΩ. .
Most of this variation is due to variations in contact Most of this variation is due to variations in contact resistance between the electrodes and the tube. resistance between the electrodes and the tube.
The best contacts have been obtained by evaporating The best contacts have been obtained by evaporating Au or Pt over the tube, often followed by a subsequent Au or Pt over the tube, often followed by a subsequent anneal. anneal.
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 2424
Transmission probababilityTransmission probabability The transmission The transmission probabilityprobability for the junction is:for the junction is:
T = G/(4eT = G/(4e22/h)/h) The previous values of G gave that:The previous values of G gave that:
0.02 <0.02 < TTj j < 0.06< 0.06 MM junctions make surprisingly good tunnel MM junctions make surprisingly good tunnel
contacts, despite the extremely small junction area contacts, despite the extremely small junction area (on the order of 1 nm(on the order of 1 nm22).).
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 2626
For two SWMTs with For two SWMTs with wrapping indices (5,5) wrapping indices (5,5) separated by the Van der separated by the Van der Waals distance of 0,34 Waals distance of 0,34 nm experiments gave:nm experiments gave:
TTj j ≈ 2 x 10≈ 2 x 10-4-4
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 2727
Crossed junction composition (1) If the nanotubes interact If the nanotubes interact
with the SiO substrate with the SiO substrate included, they deforms included, they deforms significantly at the junction.significantly at the junction.
In a following article (Structural Deformation and Intertube Conductance of Crossed Carbon Nanotube Junctions) is performed a study of the conductance as a function of the contact force.
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 2828
Crossed junction composition (2) In a crossed junction composed of SWNTs:
for a diameter of 1.4 nm, this contact force must be about 5 nN.
for a diameter of 5.5 nm, this contact force has been estimated to be about 15 nN
The junction geometry is determined by performing a constrained total energy minimization in a supercell, in which they fully relax the position of the atoms near the junction while fixing the center-to-center intertube distance at the boundaries to produce the desired contact force.
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 2929
The structure needs to be accurate, since the interatomic in the experimental devices, they expect that the strong adhesion of the SWNTs to the SiO2 substrate generates a significant contact force between the two crossed SWNTs.
In this case:
Tj ≈ 0,04
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 3030
Charge density in MM Charge density in MM SWNTsSWNTs
The charge density in the contact region became The charge density in the contact region became sizeable as the nanotubes became closer and more sizeable as the nanotubes became closer and more deformed, resulting from a significant overlap of deformed, resulting from a significant overlap of intertube wave function.intertube wave function.
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 3131
The measurement of SS junctions are complicated The measurement of SS junctions are complicated by the often very resistive contacts to by the often very resistive contacts to semiconducting SWNTs.semiconducting SWNTs.
The SS junction, like MM junctions, make The SS junction, like MM junctions, make
relatively good tunnel contactsrelatively good tunnel contacts..
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 3232
SS junctions (2)SS junctions (2) The two terminal The two terminal
conductances of SS conductances of SS junctions measure at junctions measure at 200 K give:200 K give: G = 0.011 eG = 0.011 e22/h/h G = 0.06 eG = 0.06 e22/h/h
In the figure on the left In the figure on the left is represented the is represented the higher measure with higher measure with open circles.open circles.
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 3333
MM & SS comparison MM & SS comparison In the two figures are represented In the two figures are represented
the expected band structures near the expected band structures near the junctions of the MM & SS. the junctions of the MM & SS.
Both junctions are expected to Both junctions are expected to have a finite density of states have a finite density of states available for tunneling on either available for tunneling on either side of the junction. side of the junction.
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 3434
MS junctions (1)MS junctions (1) In this case charge transfer at the junction between a In this case charge transfer at the junction between a
dropped semiconducting SWNT and a metallic dropped semiconducting SWNT and a metallic SWNT is expected to form a Schottky barrier.SWNT is expected to form a Schottky barrier.
Semiconducting & metallic SWNTs both share the Semiconducting & metallic SWNTs both share the same graphene band structure.same graphene band structure.
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 3535
The Fermi level EThe Fermi level EF F of the metallic SWNT should align of the metallic SWNT should align
within the band gap of the semiconducting SWNT at within the band gap of the semiconducting SWNT at the junction, depleting the doping of the the junction, depleting the doping of the semiconducting SWNT at the junction.semiconducting SWNT at the junction.
EEFF far for the junction is in the valence band of the far for the junction is in the valence band of the
semiconducting SWNT.semiconducting SWNT.
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 3636
Total barrier transmission Total barrier transmission probability probability
The depletion region in the semiconducting SWNT The depletion region in the semiconducting SWNT associated with the Schottky barrier represent an associated with the Schottky barrier represent an additional tunneling barrier.additional tunneling barrier.
The total barrier transmission probability is given by:The total barrier transmission probability is given by:
TTMSMS = T = Tj j TTdd
where:where: Td is the transmission probability for tunneling through the
depletion region to the location of the metal SWNT Tj is the probability of tunneling between the SWNTs
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 3737
MS conductance (1)MS conductance (1) The I-V curves for two The I-V curves for two
MS junctions, measured MS junctions, measured at a temperature of 50 K at a temperature of 50 K and Vand Vgg = -25 V is shown = -25 V is shown in the figure.in the figure.
The low-bias The low-bias conductances are for both conductances are for both devices:devices:
TTMSMS ≈ 2 x 10 ≈ 2 x 10-4-4
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 3838
TTj j = 0.04 (as the MS case)= 0.04 (as the MS case)
then must be:then must be:
TTdd = 5 x 10 = 5 x 10-3-3
This is in agreement with a recent calculation by This is in agreement with a recent calculation by Odintsov and a corresponding depletion width of 7 Odintsov and a corresponding depletion width of 7 nm, for a similar doping level.nm, for a similar doping level.
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 3939
MS I-V characteristicsMS I-V characteristics The I-V characteristics of The I-V characteristics of
the two MS devices are the two MS devices are here shown over an here shown over an expanded bias range.expanded bias range.
The dotted lines are linear The dotted lines are linear fits to the forward bias fits to the forward bias data (bias applied to the data (bias applied to the semiconducting SWNT).semiconducting SWNT).
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 4040
The conductance grows with The conductance grows with increasing bias and is greater for increasing bias and is greater for positive biases applied to the positive biases applied to the semiconducting SWNT than for semiconducting SWNT than for negative biases.negative biases.
The forward bias I-V curve The forward bias I-V curve saturates to a linear behavior.saturates to a linear behavior.
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 4141
The V intercept of the linear region gives a measure of the The V intercept of the linear region gives a measure of the barrier height for the two devices: barrier height for the two devices:
EEbarrier1barrier1 = 190 meV = 190 meV
EEbarrier2barrier2 = 290 meV = 290 meV
This value agrees with the expected value for 1 to 1.5 nm This value agrees with the expected value for 1 to 1.5 nm semiconducting SWNTs :semiconducting SWNTs :
EEbarrierbarrier = E = Egg/2/2
500 meV<E500 meV<Egg<700 meV<700 meV
So: 250meV<ESo: 250meV<Ebarrierbarrier<350 meV<350 meV
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 4242
Reverse biases increase the Reverse biases increase the Schottky barrier, but because the Schottky barrier, but because the depletion region is small, depletion region is small, tunneling still occurs through the tunneling still occurs through the barrier and leads to a measurable barrier and leads to a measurable current flow that increase with current flow that increase with increasing reverse bias.increasing reverse bias.
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 4343
G through a semiconducting G through a semiconducting SWNT (1)SWNT (1)
As shown in the figure (bias = 0), carriers charge As shown in the figure (bias = 0), carriers charge in the depletion region in the semiconducting in the depletion region in the semiconducting SWNT must pass a barrier twice as wide as in SWNT must pass a barrier twice as wide as in tunneling to the metallic SWNT. tunneling to the metallic SWNT.
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 4444
G through a semiconducting G through a semiconducting SWNT (2)SWNT (2)
The semiconducting SWNT contains two barriers, The semiconducting SWNT contains two barriers, each with transmission probability Td, corresponding each with transmission probability Td, corresponding to the depletion regions in the semiconducting SWNT to the depletion regions in the semiconducting SWNT on either side of the metallic SWNT. on either side of the metallic SWNT.
The total direct transmission through the The total direct transmission through the semiconducting SWNT is thensemiconducting SWNT is then::
TTss ≈ T ≈ T22dd
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 4545
MeasureMeasure of of TTs s (1)(1) To determinate TTo determinate Ts, s,
excluding the process excluding the process that involve the metal, it that involve the metal, it used the device used the device geometry shown in the geometry shown in the figure.figure.
A voltage V was applied A voltage V was applied to one end of the to one end of the semiconducting SWNT semiconducting SWNT in a MS device.in a MS device.
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 4646
Three-terminal behavior (1)Three-terminal behavior (1) The test circuit is the same of the determination of TThe test circuit is the same of the determination of Ts.s.
Two cases are possible:Two cases are possible: VVbiasbias <0 : the barrier to holes will remain intact, because <0 : the barrier to holes will remain intact, because
the metallic SWNT remains at roughly the same the metallic SWNT remains at roughly the same potential as the grounded end of the semiconducting potential as the grounded end of the semiconducting SWNT.SWNT.
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 4949
VVbiasbias >0 : the potential difference between the metallic >0 : the potential difference between the metallic
and semiconducting SWNT is greater than the barrier and semiconducting SWNT is greater than the barrier height, holes may pass the barrier and current will height, holes may pass the barrier and current will flow through the semiconducting SWNTflow through the semiconducting SWNT
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 5050
ConclusionsConclusions It has been noted that the ineffective screening
inherent in one-dimensional systems poses problems for nanotube Schottky devices: Nanometer-scale depletion regions are likely to be leaky barriers to tunneling.
The existence of a third terminal in their MS junction device offer a solution to this problem, allowing to construct a good rectifier from narrow Schottky barriers.
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 5252
The active length of the device is on the order of 15 nm, demonstrating that useful devices consisting of only a few thousands of atoms can be constructed from SWNTs
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 5353
Challenges and future Challenges and future prospects (1)prospects (1)
(78 IEEE Transaction on nanotechnology, VOL. 1, N° 1, March 2002, pg.83)(78 IEEE Transaction on nanotechnology, VOL. 1, N° 1, March 2002, pg.83)
Metallic tubes have conductivities and current Metallic tubes have conductivities and current densities that meet or exceed the best metals.densities that meet or exceed the best metals.
Semiconducting tubes have mobilities and Semiconducting tubes have mobilities and transconductances that meet or exceed the best transconductances that meet or exceed the best semiconductor.semiconductor.
Nanotube electronic devices function perfectly well Nanotube electronic devices function perfectly well under biological condictions and have dimensions under biological condictions and have dimensions comparable to typical biomolecules.comparable to typical biomolecules.
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 5454
Challenges and future Challenges and future prospects (2)prospects (2)
(78 IEEE Transaction on nanotechnology, VOL. 1, N° 1, March 2002, pg.83)(78 IEEE Transaction on nanotechnology, VOL. 1, N° 1, March 2002, pg.83)
Issue of device manufacturability. Issue of device manufacturability. In CVD In CVD
The general location for tube growth can be controlled The general location for tube growth can be controlled by patterning the catalyst material. by patterning the catalyst material.
The number of tubes and their orientation relative to the The number of tubes and their orientation relative to the substrate are still not well defined.substrate are still not well defined.
The high growth temperature (900°) for CVD tubes is The high growth temperature (900°) for CVD tubes is incompatible with many other standard Si process.incompatible with many other standard Si process.
Alberto Caviglia & Enrico CostaAlberto Caviglia & Enrico Costa 5555
Challenges and future Challenges and future prospects (3)prospects (3)
(78 IEEE Transaction on nanotechnology, VOL. 1, N° 1, March 2002, pg.83)(78 IEEE Transaction on nanotechnology, VOL. 1, N° 1, March 2002, pg.83) Depositing tubes on a substrate after growth Depositing tubes on a substrate after growth
Avoids the high temperature issue of CVDAvoids the high temperature issue of CVD Suffers from the chirality and positioning limitatiion.Suffers from the chirality and positioning limitatiion.
To date, there are no reliable, rapid , and reproducible To date, there are no reliable, rapid , and reproducible approaches to creating complex arrays of nanotube approaches to creating complex arrays of nanotube devices. devices.
However, there appear to be no fundamental barriers However, there appear to be no fundamental barriers to the development of a technology. The science of to the development of a technology. The science of nanotubes has come a long way in five years and nanotubes has come a long way in five years and perhaps it will see similar progress in the next five.perhaps it will see similar progress in the next five.