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 c h =na 1 +ma 2 = (n,m) a 1 and a 2 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 nanotube It 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 R upper = 56kΏ R lower =101kΏ R junction =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<<k B T)

(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 g theory ≈ The tunnelling amplitude vanishes as a power-law function of energy:

Luttinger liquid Tunnelling into the end of an LL is more strongly suppressed than into the bulk α end > α bulk α end = (g -1 -1)/4 α bulk = (g -1 +g-2)/8 When a tunnel junction is placed between 2 LLs the tunnelling conductance is : α end-end = 2 α end

(M-M) Nanotube heterojunction At large bias (eV>>k B T)

(M-M) Nanotube heterojunction If we scale dI/dV by T α 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.