The nature and control of individual metal atoms on insulators are of great importance in emerging atomic-scale technologies. Individual gold atoms on an ultrathin insulating sodium chloride film supported by a copper surface exhibit two different charge states, which are stabilized by the large ionic polarizability of the film. The charge state and associated physical and chemical properties such as diffusion can be controlled by adding or removing a single electron to or from the adatom with a scanning tunneling microscope tip. The simple physical mechanism behind the charge bistability in this case suggests that this is a common phenomenon for adsorbates on polar insulating films.

A covalent bond between an individual pentacene molecule and a gold atom was formed by means of single-molecule chemistry inside a scanning tunneling microscope junction. The bond formation is reversible, and different structural isomers can be produced. The single-molecule synthesis was done on ultrathin insulating films that electronically isolated the reactants and products from their environment. Direct imaging of the orbital hybridization upon bond formation provides insight into the energetic shifts and occupation of the molecular resonances.

A combined study using density functional calculations and scanning tunneling microscopy experiments shows that individual silver adatoms on ultrathin sodium chloride films on copper surfaces are stable in three different charge states-neutral, negatively, and positively charged adatoms. The charge states of the individual adatoms were manipulated by voltage pulses. The key parameters determining the stability of various charge states are identified and discussed within a simple model.

Broad Gaussian line shapes are observed in scanning tunneling spectroscopy of single, localized electronic states induced by Cl vacancies in ultrathin NaCl films on Cu surfaces. Using a simple inelastic resonance tunneling model, we show that the observed broad line shapes are caused by a strong coupling between the localized state and the optical phonons in the film. The parameters for the model are obtained from density functional calculations, in which the occupation of the vacancy state temporarily taking place in the experiment has also been accounted for. DOI: 10.1103/PhysRevLett.95.225503 PACS numbers: 63.20.Mt, 68.37.Ef, 71.15.Mb, 73.20.Hb The recent spectacular progress in carrying out scanning tunneling microscopy (STM) and spectroscopy (STS) of adsorbates on ultrathin, insulating films supported by metal surfaces has opened up a new fascinating field in atomicscale science [1][2][3][4]. This field is of key importance, for example, in the realization of any future atomic-scale electronics that will necessarily be based on nanostructures hosting confined electronic states, which are insulated from the environment by nanostructured, wide-band-gap materials. A fundamental issue is the degree of decoupling of localized electronic states by an insulating film. The well-known lifetime broadening -the dominant broadening mechanism for localized electronic states on metal surfaces -should be dramatically reduced by an insulating film. Therefore, it could be expected that the adsorbate and defect-induced states on insulators exhibit very small linewidths. However, recent STS experiments revealed the contrary [5,6]. This apparent contradiction triggered this study on the different broadening mechanisms and resulting line shapes and widths in STS.An ideal model system for the study of tunneling through a localized electronic state decoupled by a polar insulator from the metal substrate and the associated line shape is provided by a single Cl vacancy in NaCl films on Cu surfaces. This atomic-scale defect in an otherwise defect-free environment is fairly simple, well defined, and stable. The intrinsic defects in bulk and at surfaces of NaCl have been studied both experimentally and theoretically in detail [7][8][9]. These studies have shown that a Cl vacancy, which is commonly referred to as a color or F center, introduces a singly occupied defect state in the bulk band gap.In this Letter, we show from STM and STS measurements that a single Cl vacancy in a NaCl film on a Cu surface introduces an unoccupied vacancy state (VS) with a broad Gaussian line shape in the dI=dV spectra. Using a simple inelastic resonance tunneling model with parameters from density functional calculations, we show that the observed line shape is caused by a strong electron-phonon (e-ph) coupling of the VS to the ionic lattice of the insulating film. This broadening mechanism should be a general phenomenon for defect and adsorbate states on polar, insulating films. For example, it provides an explanation of the recently observed, Gauss...

The Cu adatom-induced localization of the two-dimensional Shockley surface state at the Cu(111) surface was identified from experimental and simulated scanning tunneling microscopy spectra. The localization gives rise to a resonance located just below the surface state band edge. The adatom-induced surface state localization is discussed in terms of the existence theorem for bound states in any attractive two-dimensional potential. We also identify adatom-induced resonance states deriving from atomic orbitals in both experimental and simulated spectra.

Using density functional theory, the nature of the overlayer formed by dissociation of water on an oxygen covered Pt(111) surface has been studied. This overlayer has been argued to be equivalent to the reaction intermediate in the low-temperature water production reaction on Pt(111). The overlayer is assumed to be formed through either of the two reactions, 2H2Oad+Oad→3OHad+Had, or 2H2Oad+Oad→2OHad+H2Oad. Based on experimental data the products of these two reactions are reduced to two adsorbate compositions labeled OH+H2O and 2OH+H. The two compositions are analyzed in terms of their energetics, vibrational spectra, work functions, and simulated scanning tunneling microscopy (STM) images. The OH+H2O composition is found to be 1.4 eV more stable per OH+H2O compared to the 2OH+H composition. Only the vibrational spectrum corresponding to the OH+H2O composition agrees well with experiments, which provides strong additional support for this composition of the overlayer. A large redshift of the water scissoring mode is revealed for this composition. We also find a significant difference between the work functions of the two compositions, which could be used to discriminate between the two compositions. In contrast, we find our simulated STM images of both compositions to be consistent with the observed STM images.

The Scandinavian wolf population descends from only five individuals, is isolated, highly inbred and exhibits inbreeding depression. To meet international conservation goals, suggestions include managing subdivided wolf populations over Fennoscandia as a metapopulation; a genetically effective population size of Ne⩾500, in line with the widely accepted long-term genetic viability target, might be attainable with gene flow among subpopulations of Scandinavia, Finland and Russian parts of Fennoscandia. Analytical means for modeling Ne of subdivided populations under such non-idealized situations have been missing, but we recently developed new mathematical methods for exploring inbreeding dynamics and effective population size of complex metapopulations. We apply this theory to the Fennoscandian wolves using empirical estimates of demographic parameters. We suggest that the long-term conservation genetic target for metapopulations should imply that inbreeding rates in the total system and in the separate subpopulations should not exceed Δf=0.001. This implies a meta-Ne of NeMeta⩾500 and a realized effective size of each subpopulation of NeRx⩾500. With current local effective population sizes and one migrant per generation, as recommended by management guidelines, the meta-Ne that can be reached is ~250. Unidirectional gene flow from Finland to Scandinavia reduces meta-Ne to ~130. Our results indicate that both local subpopulation effective sizes and migration among subpopulations must increase substantially from current levels to meet the conservation target. Alternatively, immigration from a large (Ne⩾500) population in northwestern Russia could support the Fennoscandian metapopulation, but immigration must be substantial (5–10 effective immigrants per generation) and migration among Fennoscandian subpopulations must nevertheless increase.

The physical properties of ultrathin NaCl overlayers on the stepped Cu͑311͒ surface have been characterized using scanning tunneling microscopy ͑STM͒ and spectroscopy, and density-functional calculations. Simulations of STM images and differential conductance spectra were based on the Tersoff-Hamann approximation for tunneling with corrections for the modified tunneling barrier at larger voltages and calculated Kohn-Sham states. Characteristic features observed in the STM images can be directly related to calculated electronic and geometric properties of the overlayers. The measured apparent barrier heights for the mono-, bi-, and trilayers of NaCl and the corresponding adsorption-induced changes in the work function, as obtained from the distance dependence of the tunneling current, are well reproduced and explained by the calculated results. The measurements revealed a large reduction of the tunneling conductance in a large voltage range, resembling a band gap. However, the simulated spectrum showed that only the onset at positive sample voltages may be viewed as a valence-band edge, whereas the onset at negative voltages is caused by the drastic effect of the electric field from the tip on the tunneling barrier.