Molecule-based functional devices on surfaces may take advantage of bistable molecular switches. The conformational dynamics and efficiency of switches are radically different on surfaces compared to the liquid phase. We present a design of molecular layers which enables bistable switching on a surface and, for the first time, demonstrate control of a single switch in a dense and ordered array at the spatial limit. Up and down motion of a central Sn ion through the frame of a phthalocyanine molecule is achieved via resonant electron or hole injection into molecular orbitals.

The tip of a low-temperature scanning tunneling microscope is brought into contact with individual cobalt atoms adsorbed on Cu(100). A smooth transition from the tunneling regime to contact occurs at a conductance of G approximately G0. Spectroscopy in the contact regime, i.e., at currents in a muA range, was achieved and indicated a significant change of the Kondo temperature TK. Calculations indicate that the proximity of the tip shifts the cobalt d band and thus affects TK.

The point contact of a tunnel tip approaching towards Ag(111) and Cu(111) surfaces is investigated with a low temperature scanning tunneling microscope. A sharp jump to contact, random in nature, is observed in the conductance. After point contact, the tip-apex atom is transferred to the surface, indicating that a one-atom contact is formed during the approach. In sharp contrast, the conductance over single silver and copper adatoms exhibits a smooth and reproducible transition from tunneling to contact regime. Numerical simulations show that this is a consequence of the additional dipolar bonding between the adatom and the surface atoms.

Low-temperature scanning tunneling spectroscopy of magnetic and non-magnetic metal atoms on Ag(111) and on Cu(111) surfaces reveals the existence of a common electronic resonance at an energy below the binding energies of the surface states. Using an extended Newns-Anderson model, we assign this resonance to an adsorbate-induced bound state, split off from the bottom of the surface-state band, and broadened by the interaction with bulk states. A lineshape analysis of the bound state indicates that native adatoms decrease the surface-state lifetime, while a cobalt adatom causes no significant change. PACS numbers: 73.20.Fz, 68.37.Ef, 72.15.Qm The unique ability of scanning tunneling microscopy and spectroscopy (STS) to access locally the density of states of single adsorbed atoms and clusters has been recently used to investigate magnetic adatoms which -owing to the Kondo effect -exhibit a sharp spectroscopic structure close to the Fermi energy E F [1,2,3,4,5,6]. Surprisingly, only few studies of metal surfaces have been reported for non-magnetic adatoms and for a wider energy range around the Fermi level [4,7,8,9,10]. There is a good reason to explore the physics beyond a narrow range around E F . For instance, it is known that a localized attractive perturbation of a two-dimensional electron gas should result in the appearance of a bound state, split off from the bottom of the continuum, and with a wave function localized around the perturbation [11]. Surface and image-potential states represent an opportunity to investigate this scenario with STS. In fact, it has been predicted that bound states should appear around single alkali atoms on metal surfaces as a consequence of their attractive perturbation on these twodimensional electron gases [12].In this Letter, we present a comparative lowtemperature STS study of a set of adsorbates on the Ag(111) and the Cu(111) surfaces. We show that the density of states (DOS) of single silver and cobalt atoms adsorbed on Ag(111), as well as single copper and cobalt atoms adsorbed on Cu(111), exhibit a resonance below the binding energy E 0 of the surface states of these substrates. Within the framework of a Newns-Anderson model extended to a two-band interaction, we assign this resonance to a bound state split-off from the bottom edge of the surface-state band. A lineshape analysis suggests that the scattering of the surface state at the adsorbate affects its lifetime, depending on the adatom nature. This appears to be a general property of atoms interacting with a two-dimensional electron gas. The measurements were performed in a homebuilt ultrahigh vacuum scanning tunneling microscope at a working temperature of T = 4.6 K. The Ag(111) and the Cu(111) surfaces were cleaned by Ar + sputter/anneal cycles. The single copper and silver adatoms were created by controlled tip-sample contact, whereas the single cobalt atoms were evaporated onto the cold substrates by heating a degassed cobalt wire wound around a pure W wire (> 99.95 %). The evaporation, through an opening of ...

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Heads or tails? The evolution of structural and electronic properties of tin-phthalocyanine films has been analyzed for sub-monolayer to multilayer coverage using low-temperature scanning tunneling microscopy. Two molecular conformations are observed: randomly dispersed for the first layer, and islands with a single conformation in subsequent layers.

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Experiments using the tip of a scanning tunnelling microscope to contact atoms and molecules adsorbed on surfaces are reviewed. Conductance quantization upon forming or breaking a contact between the tip and surfaces as well as between the tip and specifically chosen atoms and molecular orbitals is addressed. Imaging the contact area prior to and after contact measurements allows one to monitor the status of the contacted object as well as that of the contacting electrodes. Spectroscopy with the tip in contact with individual atoms or molecules reveals the reproducibility of and control over such experiments today. (Some figures in this article are in colour only in the electronic version) 4.3. Orientation change in a single-molecule contact 11 5. Conclusion and outlook 13 Acknowledgments 13 References 13