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.