Do molecular rectifiers exist? Fatemeh Gholamrezaie June 2006 RuGRuG

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

1940’s ’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 molecule Metal1 Metal 2 V I 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 Benzene, Overlap of p orbitals to form a pi bonds Pi bond Sigma bond s 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 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 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 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 situationsystem 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

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 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 ??