Claude R. Henry

Recent Trends in Surface Characterization and Chemistry with High‐Resolution Scanning Force Methods

The current status and future prospects of non-contact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFM) for studying insulating surfaces and thin insulating films in high resolution are discussed. The rapid development of these techniques and their use in combination with other scanning probe microscopy methods over the last few years has made them increasingly relevant for studying, controlling, and functionalizing the surfaces of many key materials. After introducing the instruments and the basic terminology associated with them, state-of-the-art experimental and theoretical studies of insulating surfaces and thin films are discussed, with specific focus on defects, atomic and molecular adsorbates, doping, and metallic nanoclusters. The latest achievements in atomic site-specific force spectroscopy and the identification of defects by crystal doping, work function, and surface charge imaging are reviewed and recent progress being made in high-resolution imaging in air and liquids is detailed. Finally, some of the key challenges for the future development of the considered fields are identified.

Surface Segregation of Pd from TiO2‐Supported AuPd Nanoalloys under CO Oxidation Conditions Observed In situ by ETEM and DRIFTS

A TiO2‐supported AuPd bimetallic catalyst with an Au/Pd atomic ratio of 8 was prepared by deposition‐precipitation with urea, and its activity in CO oxidation at room temperature was compared to the one of a monometallic Au/TiO2 catalyst. X‐ray photoelectron spectroscopy (XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analyses suggest that Au‐Pd/TiO2 contains bimetallic nanoparticles after reduction under H2 at 500u2009°C although the presence of monometallic Au particles cannot be totally excluded. The evolution of the AuPd nanoparticles surface composition during exposure to O2 and CO/O2 was studied inu2005situ by environmental high resolution electron microscopy (ETEM) and DRIFTS. Pd segregation at the surface of the bimetallic nanoparticles was evidenced by DRIFTS and directly observed by ETEM under O2 and CO/O2 with the formation of Aucore‐Pdshell structure. The changes in the surface composition of the Au‐Pd nanoparticles under CO/O2 was paralleled with the higher rate of deactivation in the first reaction stages observed for Au‐Pd/TiO2 compared to Au/TiO2, which could be related to the possible replacement of Au in low coordination sites, at the origin of the high activity in CO oxidation, by Pd atoms. These results noticeably underline the modifications induced by the reactant that can undergo a bimetallic AuPd catalyst.

2D-Arrays of Nanoparticles as Model Catalysts

From studies on supported model catalysts, we know that the reactivity of catalysts depends on the size and shape of the metal nanoparticles but also for some reactions it can depend on the distance between them. We discuss recent methods to prepare regular arrays of supported model catalysts. One of these methods is based on the growth of regular arrays of metal clusters on a template (graphene or h-BN monolayers or ultrathin oxide film on a metal single crystal) and allows the study of size effects down to clusters of a few atoms. A second method uses self-assembled micelles from diblock co-polymers, and produces hexagonal arrays of clusters on large area flat supports which are well suited to study effects of the size (1–10xa0nm) and of the density of clusters. A third method using colloidal synthesis is able to produce in large quantity metal nanoparticles with a very narrow size distribution and a single shape which spontaneously self-organize in regular arrays on a large area. This method is very promising for studying the effect of the shape of metal particles on catalytic activity and selectivity. We discuss also some other methods like electron beam or nanosphere lithography which are less promising for the fabrication of model catalysts. Finally we make some remarks about the likely evolution of studies on supported model catalysts.Graphical Abstract

Two-dimensional nanostructured growth of nanoclusters and molecules on insulating surfaces.

Noncontact atomic force microscopy (nc-AFM), Kelvin probe force microscopy (KPFM) and first principle calculations show that the nanostructured (001) Suzuki surface of Cd(2+) doped NaCl can be used to confine the growth of palladium clusters and functionalized brominated pentahelicene molecules into only the Suzuki regions, which contain the impurities. The Suzuki surface is an ideal model surface for nanostructuring metal clusters and molecules.

TEM-assisted dynamic scanning force microscope imaging of (001) antigorite: Surfaces and steps on a modulated silicate

Abstract Ultra-high vacuum dynamic scanning force microscopy (dynamic SFM) has been performed on in situ cleaved and as-grown (001) surfaces of low-T, m = 18 and m = 20, antigorite from the Kovdor Mine, Russia. The internal microstructure of the same crystal before cleavage has been checked by conventional TEM on FIB-cut cross-sections. The structural wave is imaged by dynamic SFM with a ~0.25 nm topographic amplitude (outcropping tetrahedral sheet) on cleaved and as-grown surfaces, and with a ~0.5 nm topographic amplitude (outcropping tetrahedral + octahedral sheets) mostly found on cleaved surfaces. Atomic resolution imaging was successfully applied on the cleavage surface through imaging individual atomic features on the outer hexagonal net of the emerging (Mg, O, OH) octahedra of the half-wave. The antigorite cleavage crack undulates through a single octahedral sheet, thereby avoiding rupture of strong Si-O bonds. The two tetrahedral reversals, which form the edges of the modulation repeat, are found to be strongly non-equivalent in structure: across <010>, one reversal is sharp as expected from the standard models of the antigorite structure, whereas the other reversal is unexpectedly “extended.” The latter suggests some scheme of anti-polar positioning of silicate tetrahedra along <010> at the 6-membered ring reversal. High-resolution transmission electron microscopy (HRTEM) structure imaging of antigorite viewed down to <010> confirms spread out electron densities at this reversal. Numerous step height measurements on (001) surfaces show incremental results as integral multiples of 0.25 nm, the spacing between O,OH surfaces along c*. Many of them differ in height from integral multiples of the unit cell repeat along c* and could be explained from carving the bulk wave structure. For all surfaces and steps, local stoichiometry and global electro-neutrality of the surface are satisfied.

Nonisotropic Self‐Assembly of Nanoparticles: From Compact Packing to Functional Aggregates

Quantum strongly correlated systems that exhibit interesting features in condensed matter physics often need an unachievable temperature or pressure range in classical materials. One solution is to introduce a scaling factor, namely, the lattice parameter. Synthetic heterostructures named superlattices or supracrystals are synthesized by the assembling of colloidal atoms. These include semiconductors, metals, and insulators for the exploitation of their unique properties. Most of them are currently limited to dense packing. However, some of desired properties need to adjust the colloidal atoms neighboring number. Here, the current state of research in nondense packing is summarized, discussing the benefits, outlining possible scenarios and methodologies, describing examples reported in the literature, briefly discussing the challenges, and offering preliminary conclusions. Penetrating such new and intriguing research fields demands a multidisciplinary approach accounting for the coupling of statistic physics, solid state and quantum physics, chemistry, computational science, and mathematics. Standard interactions between colloidal atoms and emerging fields, such as the use of Casimir forces, are reported. In particular, the focus is on the novelty of patchy colloidal atoms to meet this challenge.

Water adsorption by a sensitive calibrated gold plasmonic nanosensor.

We demonstrate in this work that using nanoplasmonic sensing it is possible to follow the adsorption/desorption of water molecules on gold nanodisks nanofabricated by electron beam lithography. This quantitative method is highly sensitive allowing the detection of a few hundredths of adsorbed monolayer. Disk parameters (height, diameter, and interdisk distance) have been optimized after finite-difference time-domain (FDTD) simulations in order to obtain the best localized surface plasmon resonance (LSPR) signal-to-noise ratio. Finally, we have precisely measured the adsorption kinetics of water on gold as a function of the relative humidity of the surrounding medium.

Molecular Beam Study of the CO Adsorption on a Regular Array of PdAu Clusters on Alumina

The adsorption kinetics of CO on PdAu bimetallic clusters, containing 140 ± 12 atoms and a composition varying between 55% and 100% of Pd atoms, is investigated by a pulsed molecular beam method (MBRS). The clusters are grown on a nanostructured ultrathin film of alumina on Ni3Al(111), playing the role of a template which gives a hexagonal array of bimetallic clusters having a sharp size distribution and a uniform composition. The surface concentration calculated, assuming segregation of gold to the surface, varies between 0 and 90% of Au atoms. From the adsorption–desorption kinetics of CO, the lifetime of CO is measured at various temperatures. At low coverage, plotting the CO lifetime in an Arrhenius diagram, one obtains the adsorption energy of CO. When the surface concentration of Au increases, the adsorption energy of CO on the PdAu clusters decreases. This evolution of the adsorption energy is discussed, from previous studies, in terms of ligand and ensemble effects. We find that the ensemble effec...

Charging C60 islands with the AFM tip

We show that electrons can be transferred on demand from an AFM tip into single bulk-like C60 islands, which are supported on the insulating NaCl(001) surface. We exemplify this by controlled charge-manipulation experiments conducted in ultrahigh vacuum by noncontact AFM (nc-AFM), electrostatic force microscopy (EFM) and Kelvin probe force microscopy (KPFM). KPFM shows a homogeneous contrast at the islands, which is a signature for an equal distribution of the electrons in the T1u band. The charge dissipates during half a day due to an interaction of the charged C60 islands with defects in the near surface region of NaCl. Our results open the perspective in photo-voltaics to study charge attachment, stability and charge exchange with the environment of any C60 bulk-like system.