Introduction to Molecular Electronics2 - Aalborg...

Molecular electronics

Lecture 2

Molecular electronics approach

Electrodes and contacts

• Basic requirement for molecular electronics: connection of the molecule of interest to the outside world, i.e. electrode

type of links

Covalent bond

van der Waals

Au-thiol bond is the most used oneAu-Se and Ag-S under investigation

deposition of Langmuir-Blodget films+ only planar surface with proper wetting properties required- poor stability

Bond length effect

• If the distant is short enough that delocalized state of the molecule overlaps with the metallic electronic states, than the common delocalized state is formed and electron can be transmitted through the system.

• If the overlap is not achieved the wave function can be treated independently and the whole situation can be modeled as tunneling of an electron from electrode to a molecule.

Molecular wires

• polyene – alternating system of single and double bonds;

• polythiophene

• polyphenylenevinylene

• polyphenyleneethynylene

• thyophenylsubstituted benzene

• Molecular wires are, generally, rod-like structures with delocalized p-system, the longer the structure the lesser the difference between the frontier orbitalsand the Fermi level of the electrode

Molecular insulator (spacer)• insulating molecular structures could be used as spacers i.e. have to be

significantly insulating to preserve energy difference, but still allow tunneling.

• alkanes – good insulating properties, lack rigidity;

• adamantyl cage – good rigidity and insulating properties, synthetically demanding

• tetramethylsubstituted bephenyl - single bonding connecting two rings with perpendicular p-systems

• metal-organic insulator (?)

• meta connected aromatic are insulators opposite to ortho- and para- connections

relative position is important

torsion angle is important

Molecular rectifier (Diode)• The idea (Aviram and Ratner,

1974): donor-acceptor system separated by a spacer so their p-systems don’t overlap.

• the system will have preferential charge transfer direction

V

Switches and Memory

• Bistable molecular systems: molecules that can exist in 2 (meta)stable states with different properties

• switches can be triggered by light, pH etc.

Switches and Memory• light triggered switch

Switches and Memory• Voltage triggered

switch: catenane molecule can be switched between two state (rotation of a ring) by applying positive (+2V) or negative pulses (-2V), reading voltage is 0.1V

Collier et al, Science 289, 1172 (2000)

Theory considerations: Resonance transport

• Landauer-Buttiker theory: electron is transmitted through a state with a certain probability (transparency) Tr(t,t’)

electron states in the molecule

electrodes

EF

Theory considerations: Contacts and MO overlap• contribution of different MO to the current may very strongly

depending on their spatial arrangement• issue of contact is important: a “supermolecule” involving last

few metal atoms should be in the calculations

HOMO: depleted at the contacts

LUMO: depleted in the middle

lower lying MO (~1eV) is important for current

Role of contacts

Theory considerations: Coulomb blockade

• Charging effects on the nanoscale are important

Geometrical effect:depends on the particle size and geometry of the contacts

1nm cluster: Ec ~ 0.5 eV

Design Rules for Molecular Circuits

• Applicability of superposition principle is restricted as molecular parts can not be treated independently. Effect of molecular structure on density of states and geometry of MO should be considered

• Coulomb blockade effects: conductance will depend on charge on subunits and the capacitance to the gate

• Interference effects

≠

Molecular devices

• The challenges:–how to attach molecules to the

electrodes–how to arrange them in the same

direction

Langmuir-Blodgett Approach• Molecular rectifier:

Metzger, R. M.; Xu, T.; Peterson, I. R.,. Journal of Physical Chemistry B 2001, 105, (30), 7280-7290.

arachidic acid

“rectifier” acid

Molecular rectifier• Very fragile device!

Need for “cold” evaporation technique

Scanning probe methods

• advanced possibility: – molecules can be selectively desorbed

by applying a voltage pulse or– by mechanical forces (AFM)

• molecular rods protruding from dodecylmercaptan layer

SAM domains

Scanning probe approach• advanced possibilities:

– molecules can be selectively desorbed by applying a voltage pulse or

– by mechanical forces (AFM)

Molecular rectifier: pore approach

• Molecular layer is sandwiched between gold layers in a SiN nanopore

Mechanically controlled break junction

Mechanically controlled break junction

Fixed three terminal technique

Electromigration break junctions• electromigration technique: small current applied to a notched

e-beam fabricated wire, electromigration causes thinning of the notched part.

various gate voltages