Werner A. Hofer

Buckled Silicene Formation on Ir(111)

Silicene, a two-dimensional (2D) honeycomb structure similar to graphene, has been successfully fabricated on an Ir(111) substrate. It is characterized as a (√7×√7) superstructure with respect to the substrate lattice, as revealed by low energy electron diffraction and scanning tunneling microscopy. Such a superstructure coincides with the (√3×√3) superlattice of silicene. First-principles calculations confirm that this is a (√3×√3)silicene/(√7×√7)Ir(111) configuration and that it has a buckled conformation. Importantly, the calculated electron localization function shows that the silicon adlayer on the Ir(111) substrate has 2D continuity. This work provides a method to fabricate high-quality silicene and an explanation for the formation of the buckled silicene sheet.

Field regulation of single-molecule conductivity by a charged surface atom

Electrical transport through molecules has been much studied since it was proposed that individual molecules might behave like basic electronic devices, and intriguing single-molecule electronic effects have been demonstrated. But because transport properties are sensitive to structural variations on the atomic scale, further progress calls for detailed knowledge of how the functional properties of molecules depend on structural features. The characterization of two-terminal structures has become increasingly robust and reproducible, and for some systems detailed structural characterization of molecules on electrodes or insulators is available. Here we present scanning tunnelling microscopy observations and classical electrostatic and quantum mechanical modelling results that show that the electrostatic field emanating from a fixed point charge regulates the conductivity of nearby substrate-bound molecules. We find that the onset of molecular conduction is shifted by changing the charge state of a silicon surface atom, or by varying the spatial relationship between the molecule and that charged centre. Because the shifting results in conductivity changes of substantial magnitude, these effects are easily observed at room temperature.

Electron traps and their effect on the surface chemistry of TiO2(110)

Oxygen vacancies on metal oxide surfaces have long been thought to play a key role in the surface chemistry. Such processes have been directly visualized in the case of the model photocatalyst surface TiO2(110) in reactions with water and molecular oxygen. These vacancies have been assumed to be neutral in calculations of the surface properties. However, by comparing experimental and simulated scanning tunneling microscopy images and spectra, we show that oxygen vacancies act as trapping centers and are negatively charged. We demonstrate that charging the defect significantly affects the reactivity by following the reaction of molecular oxygen with surface hydroxyl formed by water dissociation at the vacancies. Calculations with electronically charged hydroxyl favor a condensation reaction forming water and surface oxygen adatoms, in line with experimental observations. This contrasts with simulations using neutral hydroxyl where hydrogen peroxide is found to be the most stable product.

Pushing and pulling a Sn ion through an adsorbed phthalocyanine molecule.

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.

Atom transfer and single-adatom contacts

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.

Challenges and errors: interpreting high resolution images in scanning tunneling microscopy

With the availability of first principles methods to simulate the operation of a scanning tunneling microscope (STM) theory has moved from the qualitative and topographic to the quantitative and dynamic. Simulations in effect predict the influence of a model-tip or chemical interactions between tip and sample in the actual imaging process. By comparing experiments and simulations, the information about the analyzed system can be substantially extended. We give an overview of recent work, where the combination of first principles simulations with high resolution measurements was decisive to arrive at consistent results. This concerns the resolution of single wavefunctions by STM, force effects in high resolution scans, contrast reversal due to the field of the tip, the imaging of magnetic properties by spin-polarized STM, and the analysis of dynamic processes on surfaces.

Structural and Electronic Properties of Ultrathin Tin–Phthalocyanine Films on Ag(111) at the Single-Molecule Level†

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.

Surface relaxations, current enhancements, and absolute distances in high resolution scanning tunneling microscopy

We have performed the most realistic simulation to date of the operation of a scanning tunneling microscope. Probe-sample distances from beyond tunneling to actual surface contact are covered. We simultaneously calculate forces, atomic displacements, and tunneling currents, allowing quantitative comparison with experimental values. A distance regime below which the probe becomes unstable is identified. It is shown that the real distance differs substantially from previous estimates because of large atomic displacements on the surface and at the probe tip.

Reversible Single Spin Control of Individual Magnetic Molecule by Hydrogen Atom Adsorption

The reversible control of a single spin of an atom or a molecule is of great interest in Kondo physics and a potential application in spin based electronics. Here we demonstrate that the Kondo resonance of manganese phthalocyanine molecules on a Au(111) substrate have been reversibly switched off and on via a robust route through attachment and detachment of single hydrogen atom to the magnetic core of the molecule. As further revealed by density functional theory calculations, even though the total number of electrons of the Mn ion remains almost the same in the process, gaining one single hydrogen atom leads to redistribution of charges within 3d orbitals with a reduction of the molecular spin state from S = 3/2 to S = 1 that directly contributes to the Kondo resonance disappearance. This process is reversed by a local voltage pulse or thermal annealing to desorb the hydrogen atom.

Scanning tunneling microscopy of binary alloys: first principles calculation of the current for PtX (100) surfaces.

To probe the influence of realistic tip models on the tunnel current and the corrugation of binary alloy surfaces we have calculated the electronic structure of PtX (100) sample surfaces and realistic STM tips with different tip atoms. We then used the Bardeen integral to calculate the tunnel current from the electronic structure of sample and tip numerically. Apart from the usual approximations of the perturbation approach the method developed is therefore fully ab initio. It can be shown that the currents obtained in the limit of low bias voltage are within the range of measurements, and equally, that including a realistic tip improves the agreement between measurements and calculations.