We outline calibrated measurements of the microwave reflection coefficient from the tunnel junction of an ultra-high vacuum low temperature scanning tunneling microscope. The microwave circuit design is described in detail, including an interferometer for an enhanced signal-to-noise ratio and a demodulation scheme for lock-in detection. A quantitative, in situ procedure for impedance calibration based on the numerical three-error-term model is presented. Our procedure exploits the response of the microwave reflection signal due to the change of the tunneling conductance caused by sub-nm variation of the tunneling distance. Experimental calibration is achieved by a least-squares numerical fit of simultaneously measured conductance and microwave reflection retraction curves at finite conductance. Our method paves the way for nanoscale microscopy and spectroscopy of dielectric surface properties at GHz frequencies and cryogenic temperatures. This opens a promising pathway even for dielectric fingerprinting at the single molecule limit.

Diverse spectroscopic methods operating at radio frequency depend on a reliable calibration to compensate for the frequency dependent damping of the transmission lines. Calibration may be impeded by the existence of a sensitive interdependence of two or more experimental parameters. Here, we show by combined scanning tunnelling microscopy measurements and numerical simulations how a frequency-dependent conductance response is affected by different DC conductance behaviours of the tunnel junction. Distinct and well-defined DC-conductance behaviour is provided by our experimental model systems, which include C60 molecules on Au(111), exhibiting electronic configurations distinct from the well-known dim and bright C60's reported so far. We investigate specific combinations of experimental parameters. Variations of the modulation amplitude as small as only a few percent may result in systematic conductance deviations as large as one order of magnitude. We provide practical guidelines for calibrating respective measurements, which are relevant to RF spectroscopic measurements.

Dielectric relaxation lies at the heart of well-established techniques of dielectric spectroscopy essential to diverse fields of research and technology. We report an experimental route for increasing the sensitivity of dielectric spectroscopy ultimately towards the scale of a single molecule. We use the method of radio frequency scanning tunneling microscopy to excite a single molecule junction based on a polar substituted helicene molecule by an electric field oscillating at 2-5 GHz. We detect the dielectric relaxation of the single molecule junction indirectly via its effect of power dissipation, which causes lateral displacement. From our data we determine a corresponding relaxation time of about 300 ps-consistent with literature values of similar helicene derivatives obtained by conventional methods of dielectric spectroscopy.

Radio-frequency (rf) scanning tunneling microscopy has recently been advanced to methods such as single-atom spin resonance. Such methods benefit from a frequency-independent rf voltage amplitude across the tunnel junction, which is challenging to achieve due to the strong frequency dependence of the rf attenuation in a transmission line. Two calibration methods for the rf amplitude have been reported to date. In this Note, we present an alternative method to achieve a frequency-independent rf voltage amplitude across the tunnel junction and show the results of this calibration. The presented procedure is applicable to devices that can deliver rf voltage to a tunnel junction.

Active optical control over matter is desirable in many scientific disciplines, with prominent examples in all-optical magnetic switching(1,2), light-induced metastable or exotic phases of solids(3-8) and the coherent control of chemical reactions(9,10). Typically, these approaches dynamically steer a system towards states or reaction products far from equilibrium. In solids, metal-to-insulator transitions are an important target for optical manipulation, offering ultrafast changes of the electronic(4) and lattice(11-16) properties. The impact of coherences on the efficiencies and thresholds of such transitions, however, remains a largely open subject. Here, we demonstrate coherent control over a metal-insulator structural phase transition in a quasi-one-dimensional solid-state surface system. A femtosecond double-pulse excitation scheme(17-20) is used to switch the system from the insulating to a metastable metallic state, and the corresponding structural changes are monitored by ultrafast low-energy electron diffraction(21,22). To govern the transition, we harness vibrational coherence in key structural modes connecting both phases, and observe delay-dependent oscillations in the double-pulse switching efficiency. Mode-selective coherent control of solids and surfaces could open new routesto switching chemical and physical functionalities, enabled by metastable and non-equilibrium states.

We use ultrafast low-energy electron diffraction (ULEED) and multi-pulse optical excitation to demonstrate coherent control over the metal-insulator structural phase transition in atomic indium wires on the (111) surface of silicon.

The structure of a physisorbed sub‐monolayer of 1,2‐bis(4‐pyridyl)ethylene (bpe) on epitaxial graphene is investigated by low‐energy electron diffraction and scanning tunneling microscopy. Additionally, nonequilibrium heat‐transfer between bpe and the surface is studied by ultrafast low‐energy electron diffraction. Bpe arranges in an oblique unit cell which is not commensurate with the substrate. Six different rotational and/or mirror domains, in which the molecular unit cell is rotated by 28 ± 0.1° with respect to the graphene surface, are identified. The molecules are weakly physisorbed, as evidenced by the fact that they readily desorb at room temperature. At liquid nitrogen temperature, however, the layers are stable and time‐resolved experiments can be performed. The temperature changes of the molecules and the surface can be measured independently through the Debye–Waller factor of their individual diffraction features. Thus, the heat flow between bpe and the surface can be monitored on a picosecond timescale. The time‐resolved measurements, in combination with model simulations, show the existence of three relevant thermal barriers between the different layers. The thermal boundary resistance between the molecular layer and graphene is found to be 2 ± 1 × 10 −8 K m 2 W −1 .

We demonstrate the capability of ultrafast low-energy electron diffraction to resolve phase-ordering kinetics and structural phase transitions on their intrinsic time scales with ultimate surface sensitivity.

The desire to exert active optical control over matter is a unifying theme across multiple scientific disciplines, as exemplified by all-optical magnetic switching, light-induced metastable or exotic phases of solids and the coherent control of chemical reactions. Typically, these approaches dynamically steer a system towards states or reaction products far from equilibrium. In solids, metal-insulator transitions are an important target for optical manipulation, offering dramatic and ultrafast changes of the electronic and lattice properties. In this context, essential questions concern the role of coherence in the efficiencies and thresholds of such transitions. Here, we demonstrate coherent vibrational control over a metal-insulator structural phase transition in a quasi-one-dimensional solid-state surface system. An optical double-pulse excitation scheme is used to drive the system from the insulating to a metastable metallic state, and the corresponding structural changes are monitored by ultrafast low-energy electron diffraction. We observe strong oscillations in the switching efficiency as a function of the double-pulse delay, revealing the importance of vibrational coherence in two key structural modes governing the transition on a femtosecond timescale. This mode-selective coherent control of solids and surfaces could open new routes to switching chemical and physical functionalities, facilitated by metastable and non-equilibrium states.

The covalent coupling of porphine molecules on Au(111) is studied by scanning probe microscopy experiments and density functional theory. At sufficient temperatures, dehydrogenative C–C coupling of the unfunctionalized molecules occurs directly on the surface. Characteristic dimer structures between individual porphine molecules are observed and assigned to various binding motifs that are distinguished by specific intermolecular connections. Different preparations show that the relative abundance of these motifs depends on the temperature of the gold sample during deposition and is explained by calculated free energies and kinetic aspects that are relevant during the linking process. Observations on the gold terraces are completed by studying polymerization at step edges, giving insight into their role during the covalent linking process.

This work describes three systems in the emerging field of on-surface chemistry. The aim of on-surface chemistry is to employ covalent bond forming reactions to create complex well-defined molecular architectures directly on surfaces from simple precursors. However, the number of viable on-surface reactions is still very limited and the resulting oligomers, polymers and networks are often highly defective due to the irreversible nature of the reactions. The present work aims to increase the knowledge of different aspects of on-surface chemistry and its potential applications by examining three distinct systems. The first system employs the established on-surface reaction of porphyrins on Cu(110) to form a barrier to the diffusion of a second mobile component, 1,3-bis(imidazol-1-yl methyl)benzene. The diffusion of this molecule is highly anisotropic; it diffuses readily in the direction but hardly in the direction. Moreover, it was found through calculations that the diffusion of this molecule is of a walking type due to the specifics of the molecule-surface interaction. The ‘walker’ binds with two identical ‘feet’ to the surface and it can only detach one ‘foot’ at a time. The barriers are linear one dimensional organometallic chains grown exclusively in the direction, perpendicular to the diffusion direction of 1,3-bis(imidazol-1-yl methyl)benzene, and as such these porphyrin chains can act as ‘fences’ to the diffusion of the walker. Additionally, there is some attractive interaction between the porphyrin fences and the walker. Most notably, the walker can be trapped temporarily by the fences, creating something resembling stations for the walker. The second system investigated is that of two porphyrins, either porphine or zinc diphenyl porphyrin, on Au(111). In this case, the porphyrin derivatives are connected on the relatively inert Au(111) surface even though these porphyrins lack marked functional groups. All available peripheral C-H bonds were sufficiently activated such that the hydrogen could be cleaved off and C-C bonds could be formed upon annealing. Unfortunately, the selectivity that was found on Cu(110) among the different C-H bonds of diphenyl porphyrins was not observed on Au(111). The reaction initiates at step edges, but propagates on the terraces at slightly higher temperatures. This results in large scale irregular networks. In the third system the effect of chirality in on-surface reactions between helicenes on Cu(110) is investigated. Two different hexahelicenes, one with a single methyl group and one with two ethynyl groups, were used. It was found that both helicenes can undergo an indiscriminate reaction of the peripheral C-H bonds, forming random oligomers. However, the helicenes bearing the ethynyl functionalities can also undergo on-surface Glaser coupling at room temperature. For this reaction, the two types of dimers could be distinguished in scanning tunnelling microscopy images which revealed a significant difference between the observed and expected ratio of the products: the formation of heterochiral dimers is favoured over the formation of homochiral dimers.

An on-surface bimolecular system is described, comprising a simple divalent bis(imidazolyl) molecule that is shown to "walk" at room temperature via an inchworm mechanism along a specific pathway terminated at each end by oligomeric "fences" constructed on a monocrystalline copper surface. Scanning tunneling microscopy shows that the motion of the walker occurs along the [110] direction of the Cu surface with remarkably high selectivity and is effectively confined by the orthogonal construction of covalent porphyrin oligomers along the [001] surface direction, which serve as barriers. Density functional theory shows that the mobile molecule walks by attaching and detaching the nitrogen atoms in its imidazolyl "legs" to and from the protruding close-packed rows of the metal surface and that it can transit between two energetically equivalent extended and contracted conformations by overcoming a small energy barrier.

We have investigated hot electron transmission across epitaxial metal-disilicide/n-Si(111) interfaces using ballistic electron emission microscopy (BEEM). Different crystal orientations of epitaxial NiSi2 were grown on a Si(111) substrate using molecular beam epitaxy. The presence of different interfaces of NiSi2 on Si(111) were confirmed by high resolution transmission electron microscopy. Electrical transport measurements reveal a clear rectifying Schottky interface with a barrier height of 0.69 eV. However, using BEEM, three different regions with different transmissions and Schottky barrier heights of 0.65 eV, 0.78 eV, and 0.71 eV are found. The addition of a thin Ni film on the NiSi2 layer strongly reduces the transmission in all the three regions and interestingly, almost equalizes the transmission across them.