Carbon Nanotube Intramolecular Junctions. Nanotubes A graphene sheet with a hexagonal lattice…
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Transcript of Carbon Nanotube Intramolecular Junctions. Nanotubes A graphene sheet with a hexagonal lattice…
Carbon Nanotube Intramolecular
Junctions
Nanotubes
• A graphene sheet with a hexagonal lattice…
Nanotubes
• …wrapped up into a cylinder
Structure
The structure of a nanotube is characterized by:
• Diameter (1-2 nm)• Chirality
Structure
• Diameter and chirality are characterized by the vector ch=na1+ma2= (n,m)
• a1 and a2 are the graphene lattice vectors
• n and m are integers
Structure
• Armchair (n,n)• Zig-zag (n,0)• Chiral (n,m)
Structure
• Armchair (n,n)• Zig-zag (n,0)• Chiral (n,m)
Structure
• Armchair (n,n)• Zig-zag (n,0)• Chiral (n,m)
Structure
• Singlewall (SWNT)
• Multiwall (MWNT)
Structure
• Singlewall (SWNT)
• Multiwall (MWNT)
Electronic properties
Nanotubes can be:• Metallic (n-m a multiple of 3)• Semiconducting
depending on their diameter and chirality
Nanotube heterojunctions
Nanotubes can be used to realize functional devices on individual molecules, for example to create intra-molecular junctions
• Metal-Metal• Metal-Semiconductor• Semiconductor-Semiconductor
What is an heterostructure?
It is a structure that contains an heterojunction in order to build quantum structures like tunnel barrier and quantum wells.
In an heterostructure :
The interface IS the device
Nanotube heterojunctions
It consists in:a change in the chirality within a
single nanotubeIt can be obtained by:• Local mechanical deformation• A pentagon-heptagon (5-7)
topological defect pair
Nanotube heterojunctions
The insertion of a (5-7) topological
defect pair creates a kink. In order to generate a kink of a large angle this pair must be placed
on opposite sides of the kink.
Nanotube heterojunctions
They obtained nanotubes that contain:
• A single kink of 36° (M-S heterojunction)
• A single kink of 41° (M-M heterojunction)
(M-S) Nanotube heterojunction
• The nanotube is lying on 3 electrodes.
• The upper straight segment has a resistance of 110 kΏ with no gate-voltage dependence (it is metallic)
(M-S) Nanotube heterojunction
• The lower straight segment is a semiconductor.
(M-S) Nanotube heterojunction
This is the I-V characteristic
across the kink: nonlinear and asymmetric
resembling that of a diode
(M-S) Nanotube heterojunction
The strong gate modulation
demonstrates that the lower
nanotube segment is
semiconducting.
(M-M) Nanotube heterojunction
• The nanotube is lying on 4 electrodes.
• At room temperature
Rupper= 56kΏ Rlower=101kΏ Rjunction=608kΏ
(M-M) Nanotube heterojunction
• Conductances depend on temperature
• There is no gate-voltage dependence, demonstrating that both are metallic
• The conductance across the junction is much more temperature dependent then that of the 2 straight segments
(M-M) Nanotube heterojunction
• The data are plotted on a double-logarithmic scale.
• The data can be fitted with a power-law function
(if eV<<kBT)
TG
(M-M) Nanotube heterojunction
• Power-law behaviour of G versus T was interpreted as a signature for electron-electron correlation.
• The nanotube behaves as a Luttinger liquid
Luttinger liquid
• An LL is a one-dimensional correlated electron state characterized by a parameter g that measures the strength of the interaction between electrons.
g<<1 for strong repulsive interactions g=1 for non-interacting electron gas• In SWNTs gtheory ≈ 0.28.• The tunnelling amplitude vanishes as a
power-law function of energy:
EE )(
Luttinger liquid
• Tunnelling into the end of an LL is more strongly suppressed than into the bulk
• αend > αbulk
αend = (g-1-1)/4
αbulk = (g-1+g-2)/8• When a tunnel junction is placed between
2 LLs the tunnelling conductance is :
αend-end = 2 αend
(M-M) Nanotube heterojunction
At large bias (eV>>kBT)
VdVdI
(M-M) Nanotube heterojunction
If we scale dI/dV by Tα and
V by T the curves obtained
at different temperatures collapse onto one universal
curve
Conclusions
• SWNTs are promising candidates for obtaining individual molecules as functional devices using their particular electronic properties .
Future development
• A better process of fabrication of SWNTs and their junctions is necessary.
• These junctions could be the building blocks of nanoscale electronic devices made entirerly of carbon.