Carbon dioxide reduction on Ir(111): Stable hydrocarbon surface species at near-ambient pressure

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 H2 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

Temperature-Driven Changes of the Graphene Edge Structure on Ni(111): Substrate vs Hydrogen Passivation

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 \ub0C, 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

CO on supported Cu nanoclusters: Coverage and finite size contributions to the formation of carbide via the boudouard process

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

Imaging on-surface hierarchical assembly of chiral supramolecular networks

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

Agreement on classification of clinical photographs of pigmentary lesions: exercise after a training course with young dermatologists.

Smartphone apps may help promoting the early diagnosis of melanoma. The reliability of specialist judgment on lesions should be assessed. Hereby, we evaluated the agreement of 6 young dermatologists, after a specific training. Clinical judgment was evaluated during 2 online sessions, 1 month apart, on a series of 45 pigmentary lesions. Lesions were classified as highly suspicious, suspicious, non-suspicious or not assessable. Cohen's and Fleiss' kappa were used to calculate intra- and inter-rater agreement. The overall intra-rater agreement was 0.42 (95% confidence interval - CI: 0.33-0.50), varying between 0.12-0.59 on single raters. The inter-rater agreement during the first phase was 0.29 (95% CI: 0.24-0.34). When considering the agreement for each category of judgment, kappa varied from 0.19 for not assessable to 0.48 for highly suspicious lesions. Similar results were obtained in the second exercise. The study showed a less than satisfactory agreement among young dermatologists. Our data point to the need for improving the reliability of the clinical diagnoses of melanoma especially when assessing small lesions and when dealing with thin melanomas at a population level

Electronic properties of the boroxine–gold interface: evidence of ultra-fast charge delocalization

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

How to select fast scanning frequencies for high-resolution fast STM measurements with a conventional microscope

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

Caratterizzazione e manipolazione di singole molecole e complessi di molecole tramite microscopia a scansione a effetto tunnel a bassa temperatura

2006/2007The first part of this thesis work concerns the development of an ultra–high–vacuum experimental system, hosting a commercial Low Temperature Scanning Tunneling Microscope (LT-STM), that has been built for characterizing and manipulating single molecules and molecular complexes adsorbed on metal surfaces. The design from scratch, the commissioning and the performance tests of the various components of the system are presented. The preparation chamber that has been developed provides the fundamental surface preparation and analysis instruments, combining the STM analysis with other experimental techniques and allowing for easy setup of additional preparation/analysis instruments. The manipulator sample stage allows for a wide temperature range, needed for the preparation of a variety of surfaces to be investigated in the LT-STM. In the second part of this work, we present two examples of how molecules and molecular complexes can be studied and manipulated with the LT-STM technique. In the first example, within a collaboration established with the group of Dr. Leonhard Grill at the Freie Universität in Berlin, we have characterized an azobenzene derivative adsorbed on the Au(111) surface, a known molecular switch based on a trans–cis isomerization, which can be reversibly induced by the tip of the STM with controlled voltage pulses. We could show how the molecule–molecule interactions play a critical role in determining the switching abilities of the molecules between different self–assembled molecular islands, and how the molecule–substrate interaction can efficiently determine two different spatial periodicities of the switching molecules. In the second example, we have presented preliminary results regarding the characterization of the NH3-NO complex which forms on the Pt(111) surface, showing that by exploiting the manipulation and spectroscopic tools of the LT-STM, it will be possible to investigate at the atomic scale the properties of the hydrogen bond most likely involved the complex.La prima parte di questa tesi descrive lo sviluppo di un sistema sperimentale da ultra–alto–vuoto, dotato di un microscopio a scansione a effetto tunnel a bassa temperatura (LT-STM) disponibile in commercio. Il sistema è stato concepito e costruito per caratterizzare e manipolare singole molecole e complessi di molecole adsorbiti su superfici metalliche. Viene descritta la realizzazione del sistema sperimentale, a partire dalla fase di progettazione fino alla messa in opera e ai vari test di funzionamento che sono stati effettuati. La camera di preparazione che è stata costruita è provvista della strumentazione di base per la preparazione e l’analisi dei campioni di misura, e permette di integrare la tecnica STM con altre tecniche sperimentali e di installare in modo semplice e veloce nuovi strumenti. Il manipolatore è dotato di un sistema di riscaldamento campioni che permette di raggiungere un largo intervallo di temperature, necessario per la preparazione di svariati tipi di campioni da poter studiare tramite l’LT-STM. Nella seconda parte della tesi, vengono presentati due esempi di come, grazie alle potenzialità dell’LT-STM, è possibile caratterizzare e manipolare molecole e complessi di molecole. Nel primo dei due esempi, nell’ambito di una collaborazione con il gruppo del Dr. Leonhard Grill presso la Freie Universität a Berlino, è stato studiato sulla superficie (111) dell’oro il comportamento di un derivato dell’azobenzene, un “interruttore molecolare” basato sul processo di isomerizzazione trans–cis che può essere indotto in modo reversibile tramite impulsi di voltaggio dalla punta del microscopio STM. Si è dimostrato come le interazioni inter–molecolari giocano un ruolo fondamentale nel determinare se, all’interno di isole molecolari con diversa struttura, le molecole possono isomerizzare o meno; inoltre è stato scoperto che l’interazione delle molecole con il substrato può determinare l’ordinamento delle molecole in grado di isomerizzare, in strutture di periodicità definita all’interno delle isole molecolari. Nel secondo esempio che viene descritto, vengono presentati i risultati preliminari della caratterizzazione del complesso NH3-NO che si forma sulla superficie (111) del platino, e viene indicato come sarà possibile caratterizzare la natura dei legami idrogeno, che probabilmente stabilizzano il complesso, tramite le tecniche spettroscopiche e di manipolazione messe a disposizione dall’LT-STM.XX Ciclo198

Improving the resolution of lock-in measurements by tailoring the modulation

3Phase-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.nonenoneA. Peronio;C. Dri;G. ComelliPeronio, Angelo; Dri, Carlo; Comelli, Giovann