1. Self-assembled molecular corrals on a semiconductor surface: charge corralling and conductance switching S. Dobrin, K.R. Harikumar, I.R. McNab, J.C. Polanyi, P.A. Sloan, Z. Waqar, J. Yang Department of Chemistry and Institute of Optical Sciences, University of Toronto, Toronto,ON, M5S 3H6, Canada S. Ayissi, W.A. Hofer. Surface Science Research Centre, The University of Liverpool, Liverpool L69 3BX, UK Simulation of corrals Simulation finds corralled adatom is lowered (darkened) by 0.4 Å. Dimers self-assemble at RT  Precursor state. No residue after desorption  Physisorbed Darkening of adatom at (x)  Charge-transfer (Dipole-induced) Dimer formation around a single adatom (x) 30 Å (x) Nano-corrals for capturing surface electrons are of interest in molecular electronics. The favoured substrate for molecular electronics is a semiconductor surface. Haloalkane long-chain molecules, e.g., 1- chlorododecane, physisorbed on Si(111)-(7x7) self-assemble to form dimers stable to 100ºC which corral silicon adatoms. Corral size is governed by the haloalkane chain-length. Theoretical evidence shows that haloalkane dimers induce electron transfer to the corralled adatom. Two mechanisms for the formation of corrals are suggested: v + h → h 2 and v + v → h 2. At > 373 K the corrals desorb or react locally to imprint a halogen atom, X-Si and an adjacent alkyl residue, R-Si. For certain types of corrals the corralled adatom switches spontaneously between high and low conductance during STM imaging. Conductance switching is caused by surprisingly small changes in the dimer molecular configuration 230 Å Self-Assembly Chlorododecane (C 12 ) on Si(111)-(7x7) at RT + 1.5 V Charge transfer What happened to the dangling bond (x)? Lowered DOS above Fermi level, to +1.2 V (empty states) S. Ayissi and Werner A. Hofer University of Liverpool DFT-VASP (x) Corralled adatom Corral formation II I I I h h I ‘Corralled’ adatom switches between high and low conductance states Bright = ON Dark = OFF Self-assembled haloalkanes on a semiconductor surface CONCLUSIONS (1) Corralling is common for long- chain haloalkanes on Si(111)-(7x7) due to stability of the dimer (2) Corralling alters the entrapped adatom due to charge transfer (0.6 e - ). (3) Corrals reacts locally by C-X bond scission imprinting X-Si and an adjacent alkyl-residue, R-Si. (4) Corralling can be switched on and off thermally by very small changes in the molecule configuration Conductance Switching Simulated switch Red molecule: on-state Blue molecule: off-state Small molecular change  Big electronic change K. R. Harikumar, J. C. Polanyi, P. A. Sloan, S. Ayissi and W. A. Hofer, submitted JACS. Corral around a corner-hole CIPI CIAR NSERC PRO Thank you + 2.5 V I Type I corral Type II corral (x) Theory predicts corral induced charge-transfer of about 0.6 electrons into the corralled adatom’s DB. This creates the extra dipole associated with the dimer. S. Dobrin, K. R. Harikumar, R. V. Jones, N. Li, I. R. McNab, J. C. Polanyi, P. A. Sloan, Z. Waqar, J. Yang, S. Ayissi and W. A. Hofer, Surf. Sci. Lett. 600, L43 (2006). Chlorododecane (C 12 ) on Si(111)-(7x7) 130 ×150 Å Deposited and imaged at 220 K Vertical mobile molecules v, monomers Horizontal immobile molecules h, monomers v → h h v + h → h 2 (mainly type II) v + v → h 2 (both type I and II) 130 ×150 Å at 50 K No dimer formation at 50 K Diffusing lines of molecules Pre-cursor to dimer formation Dimer formation at RT No dimer formation at 220 K Mostly h, monomers v + v → h 2 (corral) 1.5 V 20 min Thermal reaction Imprints one Br atom at the open end of the corral Alkyl residue Daughter Br After reaction Br-Si. Daughter Br Alkyl residue B 3.0 V A Bromododecane, C 12 Br Br-Si + C 12 Bromododecane, 373 K; Imaging, 373 K Corral Reaction Parent BrC 12 corral 3.0 V 80 Å S. Dobrin, K. R. Harikumar, and J. C. Polanyi, J. Phys. Chem. B 110, 8010 (2006). S. Dobrin, K. R. Harikumar, I. R. McNab, J. C. Polanyi, Z. Waqar, and J. Yang, J. Chem. Phys (submitted). (x) Since corral becomes unstable, second molecule desorbs. Effect of Sodium Presence of Na on the surface PREVENTS corral formation and DESTROYS already existing corrals! Experiment: After 0.25 ML of Na has been deposited on Si(111)-7x7 with preadsorbed ClC 12 corrals, the corrals become unstable and form monomers, h. Work in progress 130 × 150 Å h 2.0 V LOCALISED ATOMIC REACTION 30 Å v 1.5 V