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In the last decade, hydrogen (H-)tautomerization, that is, a reaction that involves simple intramolecular proton transfer, has been studied in single phthalocyanine, porphyrin, and porphycene derivatives as a prototypical single molecular conductance switch. Here, by means of low-temperature scanning tunneling microscopy and density functional theory calculations, we report a binary H-tautomerism and stereoisomeric conformational switch in (amino-functionalized) porphyrins assembled in molecular chains on a gold surface. We show that the formation of the chain is crucial for the binary tautomeric switch mechanism as the single molecule switches differently. Our findings suggest that the (amino-)functionalization of molecules can be exploited not only to drive the formation of molecular self-assemblies but also to steer their switching properties.

In a continued quest to monitor subsecond surface dynamics on the atomic scale and to improve imaging resolution, a FAST module to accelerate existing scanning probe microscopy setups was previously presented. Hereby, the speedup is enabled by external electronics without modification of the actual instrument. The resulting one-dimensional (1D) data stream, recorded while the tip oscillates in a sinusoidal motion, has to be reconstructed into a layered rectangular matrix in order to visualize the movie. The Pythonbased pyfastspm package performs this conversion, while also correcting for sample tilt, noise frequencies, piezo creep, and thermal drift. Quick automatic conversion even of considerable batches of data is achieved by efficient algorithms that bundle time-expensive steps, such as interpolation based on Delaunay triangulation.

Crown ethers, assembled into a regular 2D array via a chemical guesthost recognition process, have been successfully employed to trap sodium atoms on a surface, under ultra- high vacuum conditions.

Here, we report on the conceptual design, the hardware realization, and the first experimental results of a novel and compact x-ray polarimeter capable of a single-pulse linear polarization angle detection in the extreme ultraviolet photon energy range. The polarimeter is tested by performing time resolved pump-probe experiments on a Ni80Fe20 Permalloy film at the M-2,M-3 Ni edge at an externally seeded free-electron laser source. Comparison with similar experiments reported in the literature shows the advantages of our approach also in view of future experiments.

The photoionization dynamics of OsO4 and RuO4, chosen as model systems of small-size mononuclear heavy-metal complexes, has been theoretically studied by the time-dependent density functional theory (TDDFT). Accurate experimental measurements of photoionization dynamics as a benchmarking test for the theory are reported for the photoelectron asymmetry parameters of outer valence ionizations of OsO4, measured in the 17–90 eV photon energy range. The theoretical results are in good agreement with the available experimental data. The observed dynamical behavior of partial cross sections and asymmetry parameters has been related to both the coupling to the continuum of discrete excited states, giving strong modulations in the photon energy dependency, and the atomic composition of the initial ionized states, which determines the rate of decay of ionization probability for increasing excitation energies. Overall, an extensive analysis of the photoionization dynamics for valence and core orbitals is presented, showing good agreement with all the available experimental data. This provides confidence for the validity of the TDDFT approach in describing photoionization of heavy transition element compounds, with the perspective of being used for larger systems. Further experimental work is suggested for RuO4 to gather evidence of the sensitivity of the theoretical method to the nature of the metal atom., In this work, the time-dependent density functional theory is used to calculate the photoionization dynamics of the valence and core ionizations of OsO4 and RuO4 complexes, which are chosen as model systems of small-size mononuclear heavy-metal complexes. Accurate experimental measurements of the branching ratios and photoelectron asymmetry parameters of outer valence ionizations of OsO4 provide a sound benchmark of the computational methodology.

The photoionization dynamics of OsO4 and RuO4, chosen as model systems of small-size mononuclear heavy-metal complexes, has been theoretically studied by the time-dependent density functional theory (TDDFT). Accurate experimental measurements of photoionization dynamics as a benchmarking test for the theory are reported for the photoelectron asymmetry parameters of outer valence ionizations of OsO4, measured in the 17-90 eV photon energy range. The theoretical results are in good agreement with the available experimental data. The observed dynamical behavior of partial cross sections and asymmetry parameters has been related to both the coupling to the continuum of discrete excited states, giving strong modulations in the photon energy dependency, and the atomic composition of the initial ionized states, which determines the rate of decay of ionization probability for increasing excitation energies. Overall, an extensive analysis of the photoionization dynamics for valence and core orbitals is presented, showing good agreement with all the available experimental data. This provides confidence for the validity of the TDDFT approach in describing photoionization of heavy transition element compounds, with the perspective of being used for larger systems. Further experimental work is suggested for RuO4 to gather evidence of the sensitivity of the theoretical method to the nature of the metal atom.

The thermal treatment of a monolayer of 1-naphthylmethylamine (NMA) on Ni(1 1 1) surface reveals a complex chemistry promoted by the high surface reactivity. Both the amino-terminated functional group and the naphthalene body of NMA undergo a progressive dehydrogenation, which eventually leads to the coalescence of the molecules. As a result, 2D structures of increasing size are synthesized on the surface, characterized by a graphene-like lattice in which nitrogen atoms are mainly incorporated as pyridinic substitutional defects. In this paper, we show the evidence of the formation of extended nitrogen-doped graphene (N-GR) domains at temperatures as low as 300 °C, with band dispersion measurements that reveal p-type doping.

The valence and core-level photoelectron spectra of gaseous indole, 2,3-dihydro-7-azaindole, and 3-formylindole have been investigated using VUV and soft X-ray radiation supported by both an ab initio electron propagator and density functional theory calculations. Three methods were used to calculate the outer valence band photoemission spectra: outer valence Green function, partial third order, and renormalized partial third order. While all gave an acceptable description of the valence spectra, the last method yielded very accurate agreement, especially for indole and 3-formylindole. The carbon, nitrogen, and oxygen is core-level spectra of these heterocycles were measured and assigned. The double ionization appearance potential for indole has been determined to be 21.8 +/- 0.2 eV by C is and N is Auger photoelectron spectroscopy. Theoretical analysis identifies the doubly ionized states as a band consisting of two overlapping singlet states and one triplet state with dominant configurations corresponding to holes in the two uppermost molecular orbitals. One of the singlet states and the triplet state can be described as consisting largely of a single configuration, but other doubly ionized states are heavily mixed by configuration interactions. This work provides full assignment of the relative binding energies of the core level features and an analysis of the electronic structure of substituted indoles in comparison with the parent indole.

In many experiments involving detection of events, information is represented both by position and time of occurrence of the interaction. For the latter, the resolution is a primary demand that must remain compliant with global versatility and fast real-time computing of the system.At the basis of this kind of measurements are detectors providing at the same time X-Y position and the arrival time t of each detected event. Among these detectors, Cross Delay-Lines (CDL) detectors are one of the reference solutions and use the information of detection time also for determining the position of the event.The requests of precise time measurement and fast computing time-resolved experiments are nowadays fulfilled at best by the combination of Time-to-Digital Converter (TDC) architectures with spatial computing environment of Field Programmable Gate Array (FPGA) devices. Although very promising results have been achieved, time resolution necessary to imaging applications is still a serious limiting factor.Starting from a previous system introduced in [5], in which a multi-channel TDC is combined with efficient readout logic to maximize the overall efficiency, we propose a re-design of this system both in the hardware and firmware parts. In particular, the TDC reduces the Integral Non-Linearity error (INL) from 80 ps over 50 ns of full scale-range to 4 ps over 0.5 ms, maintaining resolution up to 1 ps and single-shot precision of the channel lower than 12 ps r.m.s.This has greatly improved the overall performance of the detected image of the CDL detector, allowing a spatial X/Y precision up to 35/50 mm r.m.s. respectively without local aberrations due to the INL.

Time resolution is one of the most severe limitations of scanning probe microscopies (SPMs), since the typical image acquisition times are in the order of several seconds or even few minutes. As a consequence, the characterization of dynamical processes occurring at surfaces (e.g. surface diffusion, film growth, self-assembly and chemical reactions) cannot be thoroughly addressed by conventional SPMs. To overcome this limitation, several years ago we developed a first prototype of the FAST module, an add-on instrument capable of driving a commercial scanning tunneling microscope (STM) at and beyond video rate frequencies. Here we report on a fully redesigned version of the FAST module, featuring improved performance and user experience, which can be used both with STMs and atomic force microscopes (AFMs), and offers additional capabilities such as an atom tracking mode. All the new features of the FAST module, including portability between different commercial instruments, are described in detail and practically demonstrated., The FAST project has received funding from the EU-H2020 research and innovation programme under grant agreement no. 654360 NFFA-Europe. The ICN2 is funded by the CERCA program/ Generalitat de Catalunya. The ICN2 is supported by the Severo Ochoa program of MINECO (Grant SEV-2017-0706). We acknowledge S. Gottlieb, M. Fernández-Regúlez and F. Pérez-Murano for providing us with the silicon and block copolymer samples. Work at TUM was supported by the Deutsche Forschungsgemeinschaft (Research grants ES 349/1-2 and HE 3454/18-2). BAJL acknowledges a Research Fellowship from the Alexander von Humboldt Foundation and a Marie Skłodowska-Curie Individual Fellowship under grant ClusterDynamics (no. 703972) from the EU Horizon 2020 Programme.

This work reports on the recent activities carried out by the Detector and Instrumentation Laboratory of Elettra Sincrotrone Trieste. Since both the Elettra synchrotron and the Fermi free electron laser are generating photons in the low to medium x-ray energy range from some eV to tenths of keV the activities of the detector and instrumentation laboratory focuses on spectroscopic and imaging photon detectors, which feature high quantum efficiency from below the carbon edge and are operated also in UHV environments. Special focus will be drawn on custom made monolithic and multi element silicon drift detectors for the Twinmic and the XRF beam line at Elettra. Regarding low energy imaging detectors the PERCIVAL CMOS (‘Pixelated Energy Resolving CMOS Imager, Versatile and Large’) will be discussed which is currently being developed by a collaboration of DESY, RAL, Elettra, PAL and DLS to address the need for this type of detectors for free electron lasers in the soft X-ray regime. The majorities of Elettra’s soft x-ray beam lines are employing fast and spatially resolving electron detectors and their associated readout electronics, which have been developed in-house and have been tailored to the specific needs of the respective beam line. In addition devices for in situ beam diagnostics and dose monitoring for synchrotron radiation and FEL beams have been developed and are operated on a daily basis. Moreover, some recent results in basic research on room temperature semiconductors will be discussed. It is noteworthy that Elettra Sincrotrone Trieste through its Industrial Liaison Office commercializes many of these devices and the associated read-out electronics. In this presentation an overview of these devices and their application to specific scientific applications will be given and in view of upgrade programs future directions will be discussed.

Stable hydrocarbon surface species in the carbon dioxide hydrogenation reaction on Ir(111) were identified by means of infrared-visible sum-frequency generation vibrational spectroscopy and X-ray photoelectron spectroscopy under near-ambient pressure conditions (0.1 mbar). By introducing gas phase binary and ternary mixtures of CO2, CO, and H-2 into the reaction chamber, stable ethylidyne and ethynyl species were found at the metal surface above 425 K, in remarkable analogy with that observed during the ethylene decomposition process yielding graphene. In addition, upon increasing temperature (up to 600 K depending on the reaction conditions), vibrational and electronic spectroscopic fingerprints appeared that could be attributed to the nucleation of aromatic hydrocarbons at the edge of metastable graphenic clusters interacting with the metal surface.

A regular 2D array of crown molecules, which would spontaneously self-assemble into disordered molecular clusters, is obtained by exploiting a guest–host process, based on the chemical affinity between amino and carboxylic groups on a gold surface. First a carboxylic organic template is formed, which then serves as a host for amino-functionalized crown molecules. The amino-carboxylic interaction thereby drives the formation of a monolayer of guest molecules, regularly distributed at the nanometer scale, preventing their aggregation in unordered clusters observed on a bare gold surface. This method, which can be applied to other guest molecules, represents a novel route to overcome the shape-matching requirements of the standard guest–host architectures. Furthermore, it is intrinsically selective, due to the chemical nature of the anchoring process.

Time-resolved experiments often need detection devices able to provide information about position and arrival time of each detected event. Cross Delay-Lines detectors (CDL), which use arrival time information also for position detection, are the most suited instruments for this task and time resolution is a fundamental requirement, along with the high versatility and fast real-time computing. Typical acquisition architectures, based on Time-to-Digital Converters (TDC) followed by a Field Programmable Gate Array (FPGA), combine very fast parallel computing with a high time precision, allowing to perform state-of-the-art time-resolved experiments. In these devices, time precision as much as time accuracy are fundamental in order to guarantee a homogenous time-to-space conversion. Nevertheless, time resolution is still a limiting factor when time resolved experiments are associated to imaging.In 2017 at Nuclear Science Symposium, inside ″Development of fully FPGA-based 3D (X, Y, t) detection systems using multichannel Tapped Delay-Line Time-to-Digital Converter with Cross Delay-Line detectors″ we presented a new approach, combining FPGA-based multi-channel TDC and an efficient multipurpose readout logic addressed to CDL detector. In this contribution, thanks to the modularity and reconfigurability offered by the FPGA, we redesigned the firmware that describes the TDC increasing the resolution from 10 ps to 1 ps, the channel precision from 15 ps r.m.s. to 9.7 ps r.m.s. and, reducing the accuracy from 200 ps to 40 ps.As result, we have greatly improved the overall performance and versatility of CDL detector systems allowing image acquisition with spatial precision better than 30 μm.These make possible the application in a multidisciplinary research center like the Elettra and FERMI lightsources where electron analyzers and Time-of-Flight devices endowed with CDL detectors allow very high spatial and time resolved spectroscopy experiments.

We report the formation of dimethyl sulfoxide (DMSO) molecular complexes on Au(111) enabled by native gold adatoms unusually linking the molecules via a bonding of ionic nature, yielding a mutual stabilization between molecules and adatom(s). DMSO is a widely used polar, aprotic solvent whose interaction with metal surfaces is not fully understood. By combining X-ray photoelectron spectroscopy, low temperature scanning tunneling microscopy, and density functional theory (DFT) calculations, we show that DMSO molecules form complexes made by up to four molecules arranged with adjacent oxygen terminations. DFT calculations reveal that most of the observed structures are accurately reproduced if, and only if, the negatively charged oxygen terminations are linked by one or two positively charged Au adatoms. A similar behavior was previously observed only in nonstoichiometric organic salt layers, fabricated using linkage alkali atoms and strongly electronegative molecules. These findings suggest a motif for anchoring organic adlayers of polar molecules on metal substrates and also provide nanoscale insight into the interaction of DMSO with gold.

Atomic-scale description of the structure of graphene edges on Ni(111), both during and post growth, is obtained by scanning tunneling microscopy (STM) in combination with density functional theory (DFT). During growth, at 470 °C, fast STM images (250 ms/image) evidence graphene flakes anchored to the substrate, with the edges exhibiting zigzag or Klein structure depending on the orientation. If growth is frozen, the flake edges hydrogenate and detach from the substrate, with hydrogen reconstructing the Klein edges.

The bottom-up assembly of chiral structures usually relies on a cascade of molecular recognition interactions. A thorough description of these complex stereochemical mechanisms requires the capability of imaging multilevel coordination in real-time. Here we report the first direct observation of hierarchical expression of supramolecular chirality at work, for 10,10′-dibromo-9,9′-bianthryl (DBBA) on Cu(111). Molecular recognition first steers the growth of chiral organometallic chains and then leads to the formation of enantiopure islands. The structure of the networks was determined by noncontact atomic force microscopy (nc-AFM), while high-speed scanning tunnelling microscopy (STM) revealed details of the assembly mechanisms at the ms time-scale. The direct observation of the chirality transfer pathways allowed us to evaluate the enantioselectivity of the interchain coupling.

The adsorption of CO on Pt nanoclusters grown in a regular array on a template provided by the graphene/Ir(111) Moire was investigated by means of infrared-visible sum frequency generation vibronic spectroscopy, scanning tunneling microscopy, X-ray photoelectron spectroscopy from ultrahigh vacuum to near-ambient pressure, and ab initio simulations. Both terminally and bridge bonded CO species populate nonequivalent sites of the clusters, spanning from first to second-layer terraces to borders and edges, depending on the particle size and morphology and on the adsorption conditions. By combining experimental information and the results of the simulations, we observe a significant restructuring of the clusters. Additionally, above room temperature and at 0.1 mbar, Pt clusters catalyze the spillover of CO to the underlying graphene/Ir(111) interface.

A combined theoretical and experimental study of the assembly of triphenylboroxines on Au(111) reveals the charge delocalization properties of the interface., We performed a combined experimental and theoretical study of the assembly of phenylboronic acid on the Au(111) surface, which is found to lead to the formation of triphenylboroxines by spontaneous condensation of trimers of molecules. The interface between the boroxine group and the gold surface has been characterized in terms of its electronic properties, revealing the existence of an ultra-fast charge delocalization channel in the proximity of the oxygen atoms of the heterocyclic group. More specifically, the DFT calculations show the presence of an unoccupied electronic state localized on both the oxygen atoms of the adsorbed triphenylboroxine and the gold atoms of the topmost layer. By means of resonant Auger electron spectroscopy, we demonstrate that this interface state represents an ultra-fast charge delocalization channel. Boroxine groups are among the most widely adopted building blocks in the synthesis of covalent organic frameworks on surfaces. Our findings indicate that such systems, typically employed as templates for the growth of organic films, can also act as active interlayers that provide an efficient electronic transport channel bridging the inorganic substrate and organic overlayer.

Time-resolved experiments often need detection devices able to provide information about position and arrival time of each detected event. Time resolution is a fundamental requirement for these devices, along with the high versatility and fast real-time computing of the acquisition system. Typical architectures, based on Time-to-Digital Converters (TDC) followed by a FPGA, combine very fast parallel computing with a time precision better than hundreds of picoseconds, allowing to perform state-of-the-art time-resolved experiments. Nevertheless, time resolution is still a limiting factor, in particular for imaging applications where the detector spatial resolution is determined through time measurement. Furthermore, the separation between the TDC devices and a readout FPGA has some drawbacks in terms of versatility of the system. In this article, we present a new approach, combining FPGA-based multi-channel Tapped-Delay-Line TDC and an efficient multipurpose readout logic, to greatly improve the overall performance and versatility of Cross Delay-Line (CDL) detector systems.

The adsorption energy of carbon monoxide on Ni ad-islands and ultra-thin films grown on the Cu(110) surface can be finely tuned via a complex interplay among diffusion, site change mechanisms, and coverage effects. The observed features of CO desorption can be explained in terms of migration of CO molecules from Cu to Ni islands, competition between bridge and on-top adsorption sites, and repulsive lateral adsorbate-adsorbate interactions. While the CO adsorption energy on clean Cu(110) is of the order of 0.5 eV, Ni-alloying allows for its controlled, continuous tunability in the 0.98-1.15 eV range with Ni coverage. Since CO is a fundamental reactant and intermediate in many heterogeneous catalytic (electro)-conversion reactions, insight into these aspects with atomic level detail provides useful information to potentially drive applicative developments. The tunability range of the CO adsorption energy that we measure is compatible with the already observed tuning of conversion rates by Ni doping of Cu single crystal catalysts for methanol synthesis from a CO2, CO, and H2 stream under ambient pressure conditions.

The interaction of CO, CO2, CO + H2, CO2 + H2, and CO + CO2 + H2 with the nickel (110) single crystal termination has been investigated at 10(-1) mbar in situ as a function of the surface temperature in the 300-525 K range by means of infrared-visible sum frequency generation (IR-vis SFG) vibrational spectroscopy and by near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). Several stable surface species have been observed and identified. Besides atomic carbon and precursors for graphenic C phases, five nonequivalent CO species have been distinguished, evidencing the role of coadsorption effects with H and C atoms, of H-induced activation of CO, and of surface reconstruction. At low temperature, carbonate species produced by the interaction of CO2 with atomic oxygen, which stems from the dissociation of CO2 into CO + O, are found on the surface. A metastable activated CO2(-) species is also detected, being at the same time a precursor state toward dissociation into CO and O in the reverse water-gas shift mechanism and a reactive species that undergoes direct conversion in the Sabatier methanation process. Finally, the stability of ethylidyne is deduced on the basis of our spectroscopic observations.

We report the observation of spontaneous chiral symmetry breaking within ordered, racemic monolayers of theophylline, manifesting itself as extended, nanoscale unichiral stripes at the interface between molecular domains. Theophylline is a xanthine derivative playing an important role in several biochemical processes. Molecular chirality is induced by adsorption on the Au(111) surface, resulting in extended domains with two different racemic, ordered structures, coexisting with a disordered phase. By combining low-temperature scanning tunneling microscopy (LT-STM) and ab initio density functional theory calculations, we first provide a detailed picture of the interactions within the ordered assemblies, and we uncover the origin of the distinct contrast features in STM images. Secondly, experiments reveal the existence of nanoscale stripes of unichiral molecules separating racemic domains of one of the two ordered phases, giving rise to a local enantiomeric imbalance. Systematic theoretical investigation of their structure and chiral composition confirm their unichirality, with the specific handedness related to the registry between the two ordered domains facing the stripes. These findings can open the way to new insights into the elusive mechanisms leading to local chiral imbalances in racemic systems, possibly at the origin of biomolecular homochirality, as well as suggest novel approaches for stereoselective heterogeneous catalysis.

The self-assembly of the naphthylmethylamine molecules (NMA) on the Au(111) surface is investigated by a combined experimental and theoretical approach. Three well-defined phases are observed upon different thermal treatments at the monolayer stage. The role played by the methylamine termination is evidenced in both the molecule–molecule and molecule–substrate interactions. The autorecognition process of the amino groups is identified as the driving factor for the formation of a complex hydrogen bonding scheme in small molecular clusters, possibly acting also as a precursor of a denitrogenation condensation process induced by thermal annealing.

The original version of this Article contained an error in the spelling of the author Claus Michael Schneider, which was incorrectly given as Claus Michael Schneidery. This has now been corrected in both the PDF and HTML versions of the Article.

An amino-carboxylic motif is identified as a novel synthon in the formation of 2D hetero-organic architectures at surfaces. The well-defined interacting scheme we describe herein represents an ideal prototypical system for further investigation of the interaction at surfaces of the two functional groups. This journal is

The interaction of carbon monoxide with an ordered array of copper nanoclusters was investigated under ultrahigh vacuum conditions by means of in situ X-ray photoelectron spectroscopy in combination with density functional theory calculations. The Cu clusters were supported on an alumina template grown on the Ni3Al(111) termination. Adsorption and dissociation of carbon monoxide occur at the copper clusters, yielding accumulation of carbidic carbon at the metal particles through the Boudouard process. The involved mechanisms are investigated at the atomic level, unveiling the effects of cluster finite size, reconstruction, support, and of local CO coverage. It is found that the high coverage of CO at the cluster surface, which considerably exceeds that achievable on single crystal surfaces, facilitates the metal restructuring and the reaction, yielding carbon incorporation into the bulk of the particles.

The catalytic conversion of carbon dioxide to synthetic fuels and other valuable chemicals is an issue of global environmental and economic impact. In this report we show by means of X-ray photoelectron spectroscopy in the millibar range that, on a Ni surface, the reduction of carbon dioxide is indirectly governed by the CO chemistry. While the growth of graphene and the carbide-graphene conversion can be controlled by selecting the reaction temperature, oxygen is mainly removed by CO, since oxygen reduction by hydrogen is a slow process on Ni. Even though there is still a consistent pressure gap with respect to industrial reaction conditions, the observed phenomena provide a plausible interpretation of the behavior of Ni/Cu based catalysts for CO2 conversion and account for a possible role of CO in the methanol synthesis process.

Coadsorption of ammonia and nitric oxide on the (111) surface of platinum causes the mutual stabilization of the two adsorbed species, arranged in an ordered 2 × 2 mixed layer. Furthermore, their interaction leads also to stable, isolated triangular units, which we observe on the surface after annealing to 345 K. Having provided in the preceding article (10.1021/jp406068y) a detailed structural description of the NH3-NO mixed layer, we focus here on the stabilizing intermolecular interactions. By combining scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations, we identify the isolated triangular units as formed by one NH3 and three NO molecules, and we characterize them in terms of structure, energetics, and charge rearrangement. Eventually, we investigate the nature of the chemical bond between the coadsorbed NH3 and NO both in the mixed layer and in the isolated triangular units, pointing out the essential role of the surface mediation in inducing attractive dipole-dipole interactions and the presence of hydrogen bonds.

In the selective catalytic reduction (SCR) process, nitrogen oxides are selectively transformed to N2 by reductants such as ammonia. The specificity of this reaction on platinum-based catalysts was tentatively attributed to the formation of NH3–NO coadsorption complexes, as indicated by several surface science techniques. Here we combine scanning tunneling microscopy (STM) and density functional theory (DFT) calculations to characterize the NH3–NO complex at the atomic scale on the (111) surface of platinum, investigating the intermolecular interactions that tune the selectivity. In this first article, we analyze the structures that arise upon coadsorption of NH3 and NO in terms of adsorption sites, geometry, energetics, and charge rearrangement. An ordered 2 × 2 adlayer forms, where the two molecules are arranged in a configuration that maximizes mutual interactions. In this structure, NH3 adsorbs on-top and NO on fcc-hollow sites, leading to a cohesional stabilization of the extended layer, calculated to be 0.29 eV/unit cell. The calculated vibrational energies of the coadsorption structure agree with the experimental values found in the literature.

A key challenge in the field of nanotechnology, in particular in the design of molecular machines, novel materials or molecular electronics, is the bottom-up construction of covalently bound molecular architectures in a well-defined arrangement. To date, only rather simple structures have been obtained because of the limitation of one-step connection processes. Indeed, for the formation of sophisticated structures, step-by-step connection of molecules is required. Here, we present a strategy for the covalent connection of molecules in a hierarchical manner by the selective and sequential activation of specific sites, thereby generating species with a programmed reactivity. This approach leads to improved network quality and enables the fabrication of heterogeneous architectures with high selectivity. Furthermore, substrate-directed growth and a preferred orientation of the molecular nanostructures are achieved on an anisotropic surface. The demonstrated control over reactivity and diffusion during covalent bond formation constitutes a promising route towards the creation of sophisticated multi-component molecular nanostructures.

The implementation of fast measurement modes in conventional scanning tunneling microscopes (STM) generally implies that at least the fast scanning frequency reaches or exceeds the first resonance frequency of the scanning stage. We present a straightforward protocol for the determination of accessible frequency windows, where high spatial resolution can be routinely achieved and maintained during the fast scanning movement. This protocol relies on a simple, in situ method to locate these frequency windows by measuring the response in the characteristic probe signal while varying the tip-sample distance. The method is compared to other approaches used to characterize the resonant behavior of STMs. In principle, the protocol can also be applied to other types of scanning probe microscopes with sufficiently fast probe signal detection, as a general approach to upgrade these instruments to faster imaging rates.

By means of a joint experimental and theoretical approach we provide the first single molecule imaging and characterization of CO2 on Ni(110), chemisorbed with high charge transfer from the substrate in an activated state that plays a crucial role in the hydrogenation process. Low temper- ature scanning tunneling microscopy images of single CO2 molecules are combined with ab-initio density functional theory calculations, where dispersive forces and zero point energy estimates have been explicitly accounted for. We obtain a detailed characterization of the adsorption geometries and an estimate of the energies corresponding to the different adsorbed states. A consistent pic- ture of CO2 chemisorption on Ni(110) is provided on the basis of the newly available information, yielding a deeper insight into the previously existing spectroscopic and theoretical data.

By means of scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we characterize at the single-atom level the mechanism of the water formation reaction on the (10 × 2)-O/Rh(110) surface, a prototype of a one-dimensional (1D) oxide where the lattice expansion and the segmentation of the surface play a fundamental role. When the reaction is imaged in the 238-263 K temperature range (35 s/image acquisition time), a peculiar comblike propagation mechanism for the reaction front is found. Fast STM measurements (33 ms/image) prove that this mechanism holds also at room temperature, being therefore an intrinsic characteristic of the reaction on the 1D oxide. DFT calculations explain the observed behavior as due to the interplay between the lattice expansion in the initial surface and its relaxation during the reaction that leads to varying configurations for the reactants. At low temperatures, the reaction produces, in its final stages, a low-coverage, ordered patterning of the surface with residual oxygen. The pattern formation is related to the segmentation of the oxide phase.

The ultimate miniaturization of future devices will require the use of functional molecules at the nanoscale and their integration into larger architectures(1,2). Switches represent a prototype of such functional molecules because they exhibit characteristic states of different physical/chemical properties, which can be addressed reversibly(3). Recently, various switching entities have been studied and switching of single molecules on surfaces has been demonstrated(4-13). However, for functional molecules to be used in a future device, it will be necessary to selectively address individual molecules, preferentially in an ordered pattern. Here, we show that azobenzene derivatives in the trans form, adsorbed in a homogeneous two-dimensional layer, can be collectively switched with spatial selectivity, thus forming a periodic pattern of cis isomers. We find that the probability of a molecule switching is not equally distributed, but is strongly dependent on both the surrounding molecules and the supporting surface, which precisely determine the switching capability of each individual molecule. Consequently, exactly the same lattices of cis isomers are created in repeated erasing and reswitching cycles. Our results demonstrate a conceptually new approach to spatially addressing single functional molecules.

The initial oxidation of the Rh(110) surface was studied by scanning tunneling microscopy, core level spectroscopy, and density functional theory. The experiments were carried out exposing the Rh(110) surface to molecular or atomic oxygen at temperatures in the 500-700 K range. In molecular oxygen ambient, the oxidation terminates at oxygen coverage close to a monolayer with the formation of alternating islands of the (10x2) one-dimensional surface oxide and (2x1)p2mg adsorption phases. The use of atomic oxygen facilitates further oxidation until a structure with a c(2x4) periodicity develops. The experimental and theoretical results reveal that the c(2x4) structure is a "surface oxide" very similar to the hexagonal O-Rh-O trilayer structures formed on the Rh(111) and Rh(100) substrates. Some of the experimentally found adsorption phases appear unstable in the phase diagram predicted by thermodynamics, which might reflect kinetic hindrance. The structural details, core level spectra, and stability of the surface oxides formed on the three basal planes are compared with those of the bulk RhO2 and Rh2O3.

The formation conditions, morphology, and reactivity of thin oxide films, grown on a Rh(110) surface in the ambient of atomic or molecular oxygen, have been studied by means of laterally resolved core level spectroscopy, scanning tunneling microscopy and low energy electron diffraction. Exposures of Rh(110) to atomic oxygen lead to subsurface incorporation of oxygen even at room temperature and facile formation of an ordered, laterally uniform surface oxide at approximately 520 K, with a quasi-hexagonal structure and stoichiometry close to that of RhO(2). In the intermediate oxidation stages, the surface oxide coexists with areas of high coverage adsorption phases. After a long induction period, the reduction of the Rh oxide film with H(2) is very rapid and independent of the coexisting adsorption phases. The growth of the oxide film by exposure of a Rh(110) surface to molecular oxygen requires higher pressures and temperatures. The important role of the O(2) dissociation step in the oxidation process is reflected by the complex morphology of the oxide films grown in O(2) ambient, consisting of microscopic patches of different Rh and oxygen atomic density.

Phase-sensitive detection with a lock-in amplifier is a powerful, general measurement scheme finding widespread applications in science and technology. In several cases, however, its performance is limited by the modulation broadening, i.e. the resolution loss due to the finite amplitude of the modulation that, in this technique, is applied to the system under study. We demonstrate that this broadening can be reduced by employing a modulation function different from the commonly used sinusoid, which is non-optimal in this respect. On the basis of a mathematical analysis of the lock-in output, we provide a modulation function optimized for the detection at twice its frequency, testing the resulting performance in an STM-IETS experiment. Our findings can be directly applied whenever a lock-in is used to measure a second derivative, and are easily extendible to other derivatives as well.

We present the design and the performance of the FAST (Fast Acquisition of SPM Timeseries) module, an add-on instrument that can drive commercial scanning probe microscopes (SPM) at and beyond video rate image frequencies. In the design of this module, we adopted and integrated several technical solutions previously proposed by different groups in order to overcome the problems encountered when driving SPMs at high scanning frequencies. The fast probe motion control and signal acquisition are implemented in a way that is totally transparent to the existing control electronics, allowing the user to switch immediately and seamlessly to the fast scanning mode when imaging in the conventional slow mode. The unit provides a completely non-invasive, fast scanning upgrade to common SPM instruments that are not specifically designed for high speed scanning. To test its performance, we used this module to drive a commercial scanning tunneling microscope (STM) system in a quasi-constant height mode to frame rates of 100 Hz and above, demonstrating extremely stable and high resolution imaging capabilities. The module is extremely versatile and its application is not limited to STM setups but can, in principle, be generalized to any scanning probe instrument.

The chemical and physical properties of nanoclusters largely depend on their sizes and shapes. This is partly due to finite size effects influencing the local electronic structure of the nanocluster atoms which are located on the nanofacets and on their edges. Here we present a thorough study on graphene-supported Rh nanocluster assemblies and their geometry-dependent electronic structure obtained by combining high-energy resolution core level photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory. We demonstrate the possibility to finely control the morphology and the degree of structural order of Rh clusters grown in register with the template surface of graphene/Ir(111). By comparing measured and calculated core electron binding energies, we identify edge, facet, and bulk atoms of the nanoclusters. We describe how small interatomic distance changes occur while varying the nanocluster size, substantially modifying the properties of surface atoms. The properties of under-coordinated Rh atoms are discussed in view of their importance in heterogeneous catalysis and magnetism.

Achieving control of the nanoscale structure of binary alloys is of paramount importance for the design of novel materials with specific properties, leading to, for example, improved reaction rates and selectivity in catalysis, tailored magnetic behavior in electronics, and controlled growth of nanostructured materials such as graphene. By means of a combined experimental and theoretical approach, we show that the complex self-diffusion mechanisms determining these key properties can be mostly defined by kinetic rather than energetic effects. We explain how in the Ni−Cu system nanoscale control of self-diffusion and segregation processes close to the surface can be achieved by finely tuning the relative concentration of the alloy constituents. This allows tailoring the material functionality and provides a clear explanation of previously observed effects involved, for example, in the growth of graphene films and in the catalytic reduction of carbon dioxide.