Do molecular rectifiers exist

1
Do molecular rectifiers exist?

• Fatemeh Gholamrezaie
• June 2006

RuG
2
Contents

• History ( Molecule as Electronic Device)
• Principles
• Aviram and Ratner Model
• Metal- Molecule Contacts
• Conformational Molecular Rectifier
• Conclusion

3
History Perspective

• 1940s - 1950s Inorganic Semiconductors
• - Make p-doped and n-doped materials

• 1960s Organic Molecules
• - Inorganic semiconductor have their own organic
  molecular counterparts. Molecules can be designed
  as electron-rich donors (D) or electron-poor
  acceptors (A)
• 1970s 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.

4
History Perspective

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

• 1990s Single Molecule Devices
• - New synthetic and characterization techniques
  , also advanced devices
• 2000s
• - More reliable device geometries are introduced
• - Molecules are incorporated in small circuits

• Why molecules?
• Molecules are small.
• Molecules are inexpensive.
• Molecules can be self-assembled.
• Molecules can be engineered.

5

• Principles
• Electron Delocalization
• Which molecules?
• Conduction

6
Electron Delocalization
Pi bond
Sigma bond
s orbital
p orbital
p orbital
Benzene, Overlap of p orbitals to form a pi bonds
Proceedings of the IEEE,VOL.88, NO.3, March 2000
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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
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Conduction

• Different mechanism
• Tunneling
• Hopping
• Thermionic emission

Applied bias change the electronic structure of
the system.
9

• Aviram and Ratner Model
• Molecular Rectifier
• Forward and Reverse Bias

10
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.

11
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

Methylene
Quino Group
Methoxy Group
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Another Rectifier Molecule

• TCNQ Acceptor
• TTF
  Donor

Pi conjugated segments
Sigma bonded segment
Pi conjugated segments
Pi conjugated region have different energies due
to electron donors and acceptors.
13
Aviram and Ratner Model

• Polyphenylene-based molecular rectifying
  diode

Proceedings of the IEEE,VOL.88, NO.3, March 2000
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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
15

• Metal- Molecule Contact
• Role of the Metal-Molecule Contacts
• Single organic Molecules (Break Junction)

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OPEs Molecules

• To investigate the effect of the metal-molecule
  contact on the rectification
• Kushmerick and co-workers (2004)
• Oligo phenylene ethynylene

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Role of Metal-Molecule Contacts
Au/1/Au , Asymmetric
Au/2/Au , Symmetric
Positive bias
The negative bias is mirror imaged onto the
positive bias axis.
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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.
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Role of Metal-Molecule Contacts

Rectification increases as coupling decreases at
right interface
Rectification ratio is the forward current
divided by the reverse current.
20
Single organic Molecules
Reichert and colleagues (2002)
Mechanically controlled break junctions
Symmetry Molecule Symmetry I-V
Asymmetry Molecule Asymmetry I-V
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Transport data of the asymmetric molecule

• Current- Voltage and the dI/dU curves.

system in unstable situation
system in stable 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
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Transport data of the symmetric molecule

• Sequence of I-V

Asymmetry
Symmetry
The results show that the sample molecules was
really measured.
Asymmetry
23

• Conformational molecular rectifier

24
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
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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 a
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Simulated I/V curve at different temperatures

• Rectification at room temperature is much reduced
  because all the conformations become populated.

27
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!