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
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.
I molecule Metal1 Metal 2 V 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 • 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 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
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 • 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
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.
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 Positive bias The negative bias is mirror imaged onto the positive bias axis.
Role of Metal-Molecule Contacts Rectificationat a metal-molecule interface happens due to thepoor 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
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
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!