Appl Phys A (2008) 93: 313–318 DOI 10.1007/s00339-008-4836-0 Bistability of single 1,5 cyclooctadiene molecules on Si(001) Christophe Nacci · Jérôme Lagoute · Xi Liu · Stefan Fölsch Received: 23 January 2008 / Accepted: 9 July 2008 / Published online: 9 August 2008 © Springer-Verlag 2008 Abstract The adsorption and current-induced bistability of single 1,5 cyclooctadiene molecules on Si(001) were studied in ultrahigh vacuum by low-temperature scanning tunneling microscopy (STM). After a dosage of ≈0.05 L at room tem- perature followed by cooling to the measuring temperature of 7 K, we find that the cyclic alkene molecule preferably ad- sorbs in the bridge structure with both C=C double bonds re- acting with two adjacent Si dimers via [2 + 2] cycloaddition reaction. The time-dependent current measured upon tunnel- ing through the adsorbed molecule at fixed STM tip height displays a switching between two current levels with the same mean residence time in each level. Higher bias and/or reduced tip height—and therefore higher current—increase the switching rate, suggesting that the reversible switching is due to inelastic electron tunneling. The observed bistabil- ity is interpreted as a dynamic interconversion between two degenerate conformations of the adsorbed molecule. PACS 68.37.Ef · 68.43.Fg · 68.47.Fg · 82.37.Gk 1 Introduction Apart from its relevance to fundamental surface chemistry, the interaction of organic molecules with Si surfaces [1–4] is in focus with a view to new device concepts combin- ing conventional semiconductors with functionalities based on molecular structures. For the technologically relevant Si(001) surface, Hamers and coworkers [5–8] showed that C. Nacci · J. Lagoute · X. Liu · S. Fölsch ( ) Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany e-mail: foelsch@pdi-berlin.de cyclic alkenes (having one or more C=C double bonds) ad- sorb via a [2 + 2] cycloaddition reaction [7] between the π bond of the unsaturated hydrocarbon and the dangling bonds of a Si dimer. The cycloaddition reaction is highly selective and forms stable Si–C covalent bonds [9] between the mole- cule and the Si template, avoiding the molecular fragmen- tation that often takes place in the chemisorption of organic molecules on the reactive Si(001) surface. In this paper, we concentrate on single 1,5 cyclooctadi- ene (COD) molecules adsorbed on Si(001) and study the conformational bistability of this cyclic alkene upon ex- ternal excitation. The excitation is carried out by vertical charge transport through the molecule in the tunnel junc- tion of a low-temperature scanning tunneling microscope. On Si(001), the prototype functionality of single-molecule switching was recently observed by scanning tunneling mi- croscopy and spectroscopy (STM/STS) for single biphenyl molecules, which undergo a reversible rotational movement across the surface upon excitation by inelastic electron tun- neling (IET) [10, 11]. STM work by Wolkow and cowork- ers [12] on styrene and cyclopentene on Si(001) revealed IET-induced dynamic processes like lateral molecular dis- placement, adsorbate structural changes, and desorption. The present system highlights an alternative process trigger- ing single-molecule switching, namely a reversible confor- mational interconversion of the molecule without any bond breaking and rebonding effects being involved. 2 Experimental The investigations were performed under ultrahigh vacuum (UHV) conditions in a low-temperature STM system op- erated at a base temperature of 7 K. We used As-doped n-type Si(001) samples [<0.01  cm], which were degassed