Thierry Visart de Bocarmé

Early Stages in the Nucleation Process of Carbon Nanotubes

The early stages of carbon nanotube nucleation are investigated using field ion/electron microscopy along with in situ local chemical probing of a single nanosized nickel crystal. To go beyond experiments, tight-binding Monte Carlo simulations are performed on oriented Ni slabs. Real-time field electron imaging demonstrates a carbon-induced increase of the number density of steps in the truncated vertices of a polyhedral Ni nanoparticle. The necessary diffusion and step-site trapping of adsorbed carbon atoms are observed in the simulations and precede the nucleation of graphene-based sheets in these steps. Chemical probing of selected nanofacets of the Ni crystal reveals the occurrence of Cn (n=1-4) surface species. Kinetic studies prove C2+ species are formed from C1 with a delay time of several milliseconds at 623 K. Carbon dimers, C2, must not necessarily be formed on the Ni surface. Tight-binding Monte Carlo simulations reveal the high stability of such dimers in the first layer beneath the surface.

Nanometric chemical clocks

Field ion microscopy combined with video techniques and chemical probing reveals the existence of catalytic oscillatory patterns at the nanoscale. This is the case when a rhodium nanosized crystal—conditioned as a field emitter tip—is exposed to hydrogen and oxygen. Here, we show that these nonequilibrium oscillatory patterns find their origin in the different catalytic properties of all of the nanofacets that are simultaneously exposed at the tips surface. These results suggest that the underlying surface anisotropy, rather than a standard reaction–diffusion mechanism, plays a major role in determining the self-organizational behavior of multifaceted nanostructured surfaces. Surprisingly, this nanoreactor, composed of the tip crystal and a constant molecular flow of reactants, is large enough for the emergence of regular oscillations from the molecular fluctuations.

Fluctuating Dynamics of Nanoscale Chemical Oscillations: Theory and Experiments

Chemical oscillations are observed in a variety of reactive systems, including biological cells, for the functionality of which they play a central role. However, at such scales, molecular fluctuations are expected to endanger the regularity of these behaviors. The question of the mechanism by which robust oscillations can nevertheless emerge is still open. In this work, we report on the experimental investigation of nanoscale chemical oscillations observed during the NO2 + H2 reaction on platinum, using field electron microscopy. We show that the correlation time and the variance of the period of oscillations are connected by a universal constraint, as predicted theoretically for systems subjected to a phenomenon called phase diffusion. These results open the way to a better understanding, modeling, and control of nanoscale oscillators.

Influence of Step Geometry on the Reconstruction of Stepped Platinum Surfaces under Coadsorption of Ethylene and CO

We demonstrate the critical role of the specific atomic arrangement at step sites in the restructuring processes of low-coordinated surface atoms at high adsorbate coverage. By using high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), we have investigated the reconstruction of Pt(332) (with (111)-oriented triangular steps) and Pt(557) surfaces (with (100)-oriented square steps) in the mixture of CO and C2H4 in the Torr pressure range at room temperature. CO creates Pt clusters at the step edges on both surfaces, although the clusters have different shapes and densities. A subsequent exposure to a similar partial pressure of C2H4 partially reverts the clusters on Pt(332). In contrast, the cluster structure is barely changed on Pt(557). These different reconstruction phenomena are attributed to the fact that the 3-fold (111)-step sites on Pt(332) allows for adsorption of ethylidyne-a strong adsorbate formed from ethylene-that does not form on the 4-fold (100)-step sites on Pt(557).

Formation of neutral and charged gold carbonyls on highly facetted gold nanostructures

We show that gold mono- and di-carbonyls are formed on gold field emitter tips during interaction with carbon monoxide gas at room temperature and in the presence of high electrostatic fields. The experiments are done in a time-of-flight atom probe to obtain mass spectra. The yield of monocarbonyl cations is about twice that of di-carbonyl ions. Density functional theory calculations are reported that explain the field stabilization of adsorbed carbonyls and the desorption yield of their cations.

Imaging and chemically probing catalytic processes using field emission techniques: a study of NO hydrogenation on Pd and Pd–Au catalysts

Nitric oxide hydrogenation is investigated on palladium and gold–palladium alloy crystallites, i.e. the extremity of sharp tip samples aimed at modelling a single catalytic grain. Field ion microscopy and field emission microscopy are used to monitor adsorption and reaction in real time. One-dimensional atom probe and atom probe tomography are used on the same samples to unravel the surface composition of the adsorbed layers and the composition of the very first atomic layers of the Pd–Au surface. At constant NO pressure and at 450 K, the surface composition of the adsorbed layer on Pd samples shows a strong hysteresis behavior when H2 gas is varied. Under oxidizing conditions, N2O is formed via the occurrence of surface (NO)2 dimers. In the presence of Pd–Au alloys, the NO–H2 interaction comprises a simple NO dissociation causing the formation of surface NO2 species. On Pd–Au tip samples, atom probe tomography proves the occurrence of significant surface enrichment of palladium atoms in the presence of NO gas, but it is not sufficient to drift the behavior of the surface to that of pure palladium. Accordingly, external control parameters could be changed to tune the surface composition of Pd–Au catalysts and thus their activity and/or selectivity.

Adsorption‐Induced Restructuring and Early Stages of Carbon‐Nanotube Growth on Ni Nanoparticles

Carbon adsorption on various Ni surfaces is investigated as a function of coverage via a combination of first-principles simulations and field emission microscope experiments. It is found that carbon can be efficiently stored as subsurface carbides, but with different energetics on differently oriented surfaces depending on their compactness and density of adsorption sites. In the resulting morphological reshaping, {113} facets are predicted to grow at the expense of {111} and {100} facets, in excellent agreement with experimental observations. Moreover, at high coverage on the {113} surface the carbon adsorption energy passes through a maximum after which a structural crossover is realized such that carbon atoms tend to ascend to the surface to form one-dimensional chains (which are the precursors of graphitic nanostructures). This rationalizes the experimental observation of an incubation time between carbon storage and the beginning of catalytic growth, and provides insight into the early stages (nucleation mechanism) of carbon nanotubes on Ni nanoparticles.

Nanoscale Chiral Recognition Using Field Ion and Field Emission Microscopy

Chirality at surfaces has become an active research area targeting possible applications of enantioselective separation or detection. In this context, significant success has been achieved these past decades by developing new methods for a better understanding of enantiospecific interactions of chiral adsorbate with surfaces. Here, we propose a promising route for obtaining fundamental understanding of enantiospecific interaction of chiral molecules on metal surfaces using field emission based techniques. This technique has been chosen for its particular advantage to expose a wide range of structurally different facets in one atomically resolved picture. This diversity allows us to screen with one sample the interactions between a chemical species and a number of facets during the adsorption process.In the present study, we envisage the adsorption of alanine on platinum surfaces modelled as sharp tip using field emission and field microscopy along with theoretical studies using density functional theory.In order to observe the adsorption pattern of the adsorption, the Pt surface is kept at temperatures between 150 and 300K, and then exposed to vapors of D or L-alanine. The in-situ FEM is filmed with a high-speed camera. The whole process is also performed in absence of alanine molecules to perform reference experiments. The subtraction of the results before and after the adsorption gives us a net image of the adsorption sites. Our results show a clear preference of the alanine to adsorb on chiral facets. Although the 20 A resolution of the FEM does not allow to unravel the edges of the facets of interest, the net images after exposures to one enantiomer of alanine show the occurrence of an enantioselective adsorption over sector of the same chiral symmetry. The results show that L-alanine has a strong tendency to adsorb onto R facets. Conversely, D-alanine adsorbs onto the S facets.

Field Emission Microscopy to Study the Catalytic Reactivity of Binary Alloys at the Nanoscale

Catalysis plays a crucial role in modern industrial applications. The aim in every process involving catalysis is to obtain a high and sustainable conversion along with a high selectivity towards the desired product(s). In the case of heterogeneous catalysis, one of the ways to reach this goal is to design tailored nanoparticles that present a specific composition, shape and morphology. Such engineering of catalysts only works if one understands how the reaction proceeds on different morphologies and how the reaction may induce structural changes. Another way to improve the efficiency relies in the control of the catalytic reaction. For this, the study of the dynamics occurring at the surface of the catalyst is used to determine the reaction mechanism with better accuracy, which in turn opens the way to a rationale for assessing the reproducibility, the predictability and the controllability of the reaction. To improve a catalytic process, a fundamental understanding of the catalytic behavior of the active materials is thus required. Surface science studies had, and still have, a great impact on the understanding of catalytic systems. These studies are mainly performed on catalytic reactions occurring at the surface of pure metals. There is, however, an increasing interest in using alloy catalysts in industrial applications, which calls for in situ studies providing a fundamental understanding of the properties of alloy catalysts.

Imaging Graphene by Field Ion Microscopy

This work shows that field ion microscopy can be used to image 2D materials by using a rather simple fishing method. The appearance of streaked spots is assigned to the presence of graphene, and it is possible to study their evolution as a function of the applied electrical field.