Do molecular rectifiers exist?

Fatemeh Gholamrezaie

June 2006

RuG

Contents

• History ( Molecule as Electronic Device)

• Principles • Aviram and Ratner Model • Metal- Molecule Contacts• Conformational Molecular Rectifier

• Conclusion

• 1940’s - 1950’s: Inorganic Semiconductors

- Make p-doped and n-doped materials

History Perspective

• 1960’s: Organic Molecules- Inorganic semiconductor have their own organic molecular counterparts. Molecules can be designed as electron-rich donors (D) or electron-poor acceptors (A)

• 1970’s: Single Molecule Devices?- Organic synthetic techniques start to grow up prompting the idea that device function can be combined into a single molecule.

- Aviram and Ratner suggest a molecular rectifier.

- But, no idea how this molecule can connect to the outside world.

History Perspective

• 1980’s: Single Molecule Detection- Scanning Probe Microscopy: STM , AFM

• 1990’s: Single Molecule Devices- New synthetic and characterization techniques , also advanced devices

• 2000’s: - More reliable device geometries are introduced- Molecules are incorporated in small circuits

moleculeMetal1 Metal 2

V

IWhy molecules?

Molecules are small. Molecules are inexpensive. Molecules can be self-assembled. Molecules can be engineered.

Principles

Electron Delocalization

Which molecules?

Conduction

Electron Delocalization

Benzene, Overlap of p orbitals to form a pi bonds

66HC

Pi bondSigma bonds orbital

p orbital p orbital

Proceedings of the IEEE,VOL.88, NO.3, March 2000

Which Molecules?

• Polyphenylene molecules - Conjugated molecule• Extended overlap of p orbitals and electron delocalization.

Proceedings of the IEEE,VOL.88, NO.3, March 2000

Schematic diagrams

Conduction• Different mechanism:

• Tunneling

• Hopping

• Thermionic emission

Applied bias change the electronic structure of the system.

Aviram and Ratner Model

Molecular Rectifier

Forward and Reverse Bias

Aviram and Ratner Model

• Molecular Diode (1974): proposed for first time the use of a single molecule containing two electrodes to rectify the current through the molecule.

• Similar to p-n junction.

• Rectifier I-V curve

Idea : By Modifying pi electron density of the

organic molecules similar system made.

Examples of Molecular Rectifier

• Electron donors elements: (n-type)

- Increase the pi density- Lower ionization potential ( Raise the HOMO)

• Electron acceptor elements: (p-type)

- Decrease the pi density- Raise electron affinity (lower the LUMO)

• Separation of two pi-system Quino Group Methoxy Group

Methylene

Another Rectifier Molecule

TCNQ : Acceptor

TTF : Donor

Sigma bonded segment

Pi conjugated segments

Pi conjugated segments

Pi conjugated region have different energies due to electron donors and acceptors.

Aviram and Ratner Model

• Polyphenylene-based molecular rectifying diode

Proceedings of the IEEE,VOL.88, NO.3, March 2000

Forward and Reverse bias

• Forward: The voltage must be sufficient to increase the Fermi energy of the electrodes on the right as high as LUMO of the acceptor.

• Reverse: The voltage should be relatively high compare to the forward bias, because the total energy of the donor is raised.

• rectification behavior

Proceedings of the IEEE,VOL.88, NO.3, March 2000

Metal- Molecule Contact

Role of the Metal-Molecule Contacts

Single organic Molecules (Break Junction)

OPEs Molecules • To investigate the effect of the metal-molecule contact on the rectification Kushmerick and co-workers (2004)

• Oligo phenylene ethynylene

Role of Metal-Molecule Contacts

Au/1/Au , Asymmetric Au/2/Au , Symmetric

The negative bias is mirror imaged onto the positive bias axis.

Positive bias

Role of Metal-Molecule ContactsRectification at a metal-molecule interface happens due to the poor contact.

Charge density , DFT( density functional theory)

In molecule 2 , Charge density is the same from two terminals so the charge injection is much more symmetric.

Role of Metal-Molecule Contacts

Rectification ratio is the forward current divided by the reverse current.

Rectification increases as coupling decreases at right interface

Single organic Molecules

Reichert and colleagues (2002)

Symmetry Molecule – Symmetry I-V

Asymmetry Molecule – Asymmetry I-V

Mechanically controlled break junctions

Transport data of the asymmetric molecule

Current- Voltage and the dI/dU curves.

system in unstable situation system in stable situation

This experiment shows the effect of the molecule and electrode junction on the I-Vs

Difference in these two graphs is because of the metal-molecule contact

Transport data of the symmetric molecule

Sequence of I-V

The results show that the sample molecules was really measured.

Symmetry

Asymmetry

Asymmetry

Conformational molecular rectifier

CMR (Conformational molecular rectifier )

Conformational motions driven by the electric field might lead a molecular junction to exhibit switching behavior.

CMR has two parts, one connected to the electrode and the other part is mobile and has strong dipole , Cyanomethyl CNCH 2

Ratner and Troisi (2004)

CMR (Conformational molecular rectifier )

• Different conformations have large difference in conductance.

• Metal-molecule interaction can make different in the conductance.

Relative conductance as a function of the dihedral angle α

Simulated I/V curve at different temperatures Rectification at room temperature is much reduced because all the

conformations become populated.

Conclusion

Two views:

1) Rectification due to the molecule

2) Rectification due to the metal-molecule contacts

More accurate measurements and devices need to solve this mystery!

Do Molecular rectifiers exist ??