Precision control of single molecule electrical junctions Iain Grace & Colin Lambert

Collaborators University of Liverpool W. Haiss R. Nicholls R. Schiffron Durham University Durham University C. Wang M. Bryce A. Batsanov Smeagol team A. R. Rocha S. Sanvito V. Garcia Suarez J. Ferrer S. Bailey C. Lambert

Modelling of Molecular Electronics at Lancaster Controlling transport through single molecules Molecule Synthesis ~ Durham University M. Bryce & C. Wang Molecule Characterization STM measurements Cranfield ~ G. Ashwell, W. Tyrell Liverpool ~ W. Haiss, R. Nicholls Device fabrication ~ QinetiQ  Scalable technology Gold contacts Theory ~ Lancaster University I. Grace, T. Papadopolous, C. Finch S. Sirichantaropass, V. Garcia Suarez C. Lambert

Experimental Details ~ I(t) Method X-Ray Crystallogaphy structure Length ~ 2nm I W ~ Measured current through the molecule W. Haiss et al, Phys. Chem. Chem. Phys., 2004, 6, 4330.

Tilting Molecules No temperature dependence ~ molecule is rigid ~ for non-rigid molecules there is a strong temperature dependence Theory on molecule tilting Kornilovitch et al, PRB 64, (2001) Geng et al, App. Phys. Lett. 85, 5992 (2004)

Theoretical Modelling  Relax geometry of the isolated molecular wire ( SIESTA DFT code)  Extend the molecule to include surface layers of gold  Using SIESTA extract a tight binding Hamiltonian describing the extended molecule  Compute zero-bias transport with a greens function scattering approach. Employ a simpler form of the SMEAGOL code ~ developed to study very long molecules (10nm) efficiently

Theoretical tilt dependence Hollow site Top site Artificially shifted LUMO resonances

Surfaces of constant LDOS Increasing the tilt angle alters the strength of the contact coupling

Molecule Geometry Geometry A ~ α = 0 Geometry B ~ α = 60 Rings are free to rotate about the molecule axis

Rotational Dependence Rotate the whole molecule about its axis by an angle Φ  

Conclusions Developed a theoretical method, based on the SMEAGOL code, to compute efficiently the zero bias conductance of molecular wires. Good agreement with the measured magnitude of conductance. The behaviour of the tilt dependence of the conductance is determined by geometry of the molecule between the contacts.