Self-assembled molecular corrals on a semiconductor surface: charge corralling and conductance switching

1. Thermal reaction 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 Dimer formation around a single adatom (x) Self-Assembly Simulation of corrals • 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 → h2and v + v → h2. • 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 Charge transfer Chlorododecane (C12) on Si(111)-(7x7) at RT II + 1.5 V (x) Corralled adatom II (x) S. Ayissi and Werner A. Hofer University of Liverpool h I I II Lowered What happened to the dangling bond (x)? I h 30 Å Dimers self-assemble at RT  Precursor state. No residue after desorption  Physisorbed Darkening of adatom at (x) Charge-transfer (Dipole-induced) DOS above Fermi level, to +1.2 V (empty states) I II Simulation finds corralled adatom is lowered (darkened) by 0.4 Å. DFT-VASP 230 Å 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). Corral formation Corral Reaction Bromododecane, C12Br Br-Si + C12 Chlorododecane (C12) on Si(111)-(7x7) Bromododecane, 373 K; Imaging, 373 K 3.0 V 3.0 V A B Deposited and imaged Parent BrC12 corral Daughter Br at 50 K at 220 K 1.5 V 1.5 V Alkyl residue h 20 min v 80 Å 80 Å After reaction Br-Si. Daughter Br Parent BrC12 corral Imprints one Br atom at the open end of the corral 130 ×150 Å 130 ×150 Å Horizontal immobile molecules h, monomers Vertical mobile molecules v, monomers LOCALISED ATOMIC REACTION Alkyl residue Since corral becomes unstable, second molecule desorbs. No dimer formation at 50 K Diffusing lines of molecules Pre-cursor to dimer formation No dimer formation at 220 K Mostly h, monomers S. Dobrin, K. R. Harikumar, and J. C. Polanyi, J. Phys. Chem. B110, 8010 (2006). v→ h Effect of Sodium Type I corral Type II corral 2.0 V Presence of Na on the surface PREVENTS corral formation and DESTROYS already existing corrals! h Dimer formation at RT (x) (x) Experiment: After 0.25 ML of Na has been deposited on Si(111)-7x7 with preadsorbed ClC12 corrals, the corrals become unstable and form monomers, h. v + v→ h2(both type I and II) v + h → h2(mainly type II) v + v → h2 (corral) Work in progress 130 × 150 Å S. Dobrin, K. R. Harikumar, I. R. McNab, J. C. Polanyi, Z. Waqar, and J. Yang, J. Chem. Phys (submitted). Simulated switch 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 Red molecule: on-state Blue molecule: off-state ‘Corralled’ adatom switches between high and low conductance states I + 2.5 V Bright = ON Thank you 30 Å Dark = OFF Corral around a corner-hole CIAR CIPI Small molecular change Big electronic change NSERC PRO K. R. Harikumar, J. C. Polanyi, P. A. Sloan, S. Ayissi and W. A. Hofer, submitted JACS.