Transcript F. (Fatemeh) Gholamrezaie

Do molecular rectifiers exist?
Fatemeh Gholamrezaie
June 2006
RuG
Contents
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History ( Molecule as Electronic Device)
Principles
Aviram and Ratner Model
Metal- Molecule Contacts
Conformational Molecular Rectifier
Conclusion
History Perspective
• 1940’s - 1950’s: Inorganic Semiconductors
- Make p-doped and n-doped materials
• 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
I
Metal1 molecule
V
Metal 2
Why molecules?
Molecules are small.
Molecules are inexpensive.
Molecules can be self-assembled.
Molecules can be engineered.
Principles
Electron Delocalization
Which molecules?
Conduction
Electron Delocalization
Sigma bond
Pi bond
s orbital
p orbital
p orbital
Benzene, C6 H 6
Overlap of p orbitals to form a pi bonds
Proceedings of the IEEE,VOL.88, NO.3, March 2000
Which Molecules?
• Polyphenylene molecules - Conjugated molecule
• Extended overlap of p orbitals and electron delocalization.
Schematic diagrams
Proceedings of the IEEE,VOL.88, NO.3, March 2000
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
Methylene
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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
Another Rectifier Molecule
TCNQ : Acceptor
Pi conjugated
segments
TTF : Donor
Sigma bonded
segment
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/2/Au , Symmetric
Au/1/Au , Asymmetric
Positive bias
The negative bias is mirror imaged onto the positive bias axis.
Role of Metal-Molecule Contacts
Rectification 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 increases
as coupling decreases
at right interface
Rectification ratio is the forward current divided by the reverse current.
Single organic Molecules
Reichert and colleagues (2002)
Mechanically controlled break junctions
Symmetry Molecule – Symmetry I-V
Asymmetry Molecule – Asymmetry I-V
Transport data of the asymmetric molecule
Current- Voltage and the dI/dU curves.
system in stable situation
system in unstable situation
Difference in these two graphs is because of the metal-molecule contact
This experiment shows the effect of the molecule and electrode junction on the I-Vs
Transport data of the symmetric molecule
Sequence of I-V
Asymmetry
Symmetry
The results show that the sample molecules
was really measured.
Asymmetry
Conformational molecular rectifier
CMR (Conformational molecular rectifier )
Ratner and Troisi (2004)
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  CH 2CN
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
Do Molecular rectifiers exist ??
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!