A molecular beam study of CO oxidation on Pd clusters supported on alumina: the effect of cluster size.

The activity in CO oxidation of regular arrays of Pd clusters, in the size range of 5-180 atoms, supported on alumina has been studied by using a molecular beam method. The Pd clusters are grown on a nanostructured ultrathin alumina film on Ni3Al(111) providing a hexagonal array of clusters with a narrow size distribution. The steady state turnover frequency (TOF) is measured as a function of the mean cluster size. On average the TOF increases with size. Below about 20 atoms, the TOF increases rapidly and for larger clusters it grows slowly and reaches a value close to those obtained on bulk single crystals at 180 atoms. The size effect is explained by the smaller activity of Pd atoms in contact with the alumina support.

CO and O2 Adsorption and CO Oxidation on Pt Nanoparticles by Indirect Nanoplasmonic Sensing.

We used indirect nanoplasmonic sensing (INPS) coupled with mass spectrometry to study CO and oxygen adsorption as well as CO oxidation, on Pt nanoparticles, in the Torr pressure range. Due to an optimization of the physical parameters of our plasmonic sample, we obtain a highly sensitive probe that can detect gas adsorption of a few hundredths of a monolayer, even with a very low number density of Pt particles. Moreover and for the first time, a similarity is observed between the sign and the evolution of the localized surface plasmon resonance (LSPR) peak shift and the work function measurements for CO and oxygen chemisorption. Controlling the size, shape, and surface density of Pt particles, the turnover frequency (TOF) has also been accurately determined. For similar experimental conditions, the TOF is close to those measured on Pt/oxide powder catalysts and Pt(100) single crystals.

Particle size effect on the Langmuir-Hinshelwood barrier for CO oxidation on regular arrays of Pd clusters supported on ultrathin alumina films.

The Langmuir-Hinshelwood barrier (ELH) and the pre-exponential factor (νLH) for CO oxidation have been measured at high temperatures on hexagonal arrays of Pd clusters supported on an ultrathin alumina film on Ni3Al (111). The Pd clusters have a sharp size distribution, and the mean sizes are 174 ± 13, 360 ± 19, and 768 ± 28 atoms. ELH and νLH are determined from the initial reaction rate of a CO molecular beam with a saturation layer of adsorbed oxygen on the Pd clusters measured at different temperatures [493 ≤ T(K) ≤ 613]. The largest particles (3.5 nm) give values of ELH and νLH similar to those measured on Pd (111) [T. Engel and G. Ertl, J. Chem. Phys. 69, 1267 (1978)]. However, smaller particles (2.7 and 2.1 nm) show very different behaviors. The origin of this size effect is discussed in terms of variation of the electronic structure and of the atomic structure of the Pd clusters.

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.

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.

Indirect Nanoplasmonic Sensing to Probe with a High Sensitivity the Interaction of Water Vapor with Soot Aerosols.

We demonstrate in this work that the indirect nanoplasmonic sensing lets us follow the adsorption/desorption of water molecules on soot particles that are a major contributor of the global warming. Increasing the relative humidity of the surrounding medium we measure a shift in wavelength of the localized surface plasmon resonance response of gold nanodisks on which soot particles are deposited. We show a singular and reversible blue shift with hydrophilic aircraft soot particles interpreted from a basic model as a reversible morphological change of the soot aggregates. This new method is highly sensitive and interesting to follow the change of optical properties of aerosols during their aging in the atmosphere, where they can adsorb and react with different gas molecules.

Growth of Pt-Pd Nanoparticles Studied In Situ by HRTEM in a Liquid Cell.

The growth of Pt-Pd nanoparticles from organometallic precursors is studied in situ in real time by HRTEM in a graphene oxide liquid cell. The reduction of the metal precursors is induced by the electron beam. During the growth, the particles rearrange their internal structure to form faceted single crystals. The growth is compatible with the Lifshitz-Slyozov-Wagner (LSW) mechanism in the limiting case of a reaction-limited process. The same particles are also synthesized ex situ by using a chemical reducing agent and observed in HRTEM.

Transition from molecule to solid state: reactivity of supported metal clusters.

The evolution of the adsorption energy of carbon monoxide (CO) molecules on palladium (Pd) clusters as function of Pd particle size from the molecular regime (less than ~100 atoms per particle) to the bulk regime has been revealed. This adsorption energy is retrieved from the residence time of CO molecules on the Pd clusters, measured by a pulsed molecular beam technique, versus temperature. Unprecedented accuracy on the determination of the particle size has been achieved here by using a regular array of metal clusters exhibiting a size dispersion down to the ultimate limit of a Poisson distribution. This allows getting rid of the convolution effects that generally occur when considering particles grown through other techniques.

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.

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.

TEM and HRTEM evidence for the role of ligands in the formation of shape-controlled platinum nanoparticles.

The morphology of platinum nanoparticles synthesized using an organometallic approach from PtMe(2) (C(8) H(12) ) is influenced by the nature of the ligands used as stabilizing agents. The use of long alkyl chain amines leads to the formation of multipodal nanoparticles that transform into compact nano-objects, adopting cubic, truncated cubic, or cuboctahedral shapes. In contrast, the use of diamine ligands allows the growth of compact (111) arrowlike faces, forming polycrystalline nanoparticles of an overall desert-rose aspect. Different reaction parameters are studied ([ligand]/[metal] ratio, temperature, solvent identity) in order to optimize the various shapes.

Expeditive syntheses of functionalized pentahelicenes and NC-AFM on Ag(001).

One of the shortest and most efficient routes toward a series of functionalized pentahelicenes is reported. Benzylic (dibromo)methine coupling is an important entry into functional helicene chemistry. It allowed a mono- or a double functionalization by some metal-catalyzed Ar-C, Ar-S, Ar-CN, and Ar-I bond formations. Those functions offer new avenues for further applications. For instance, helicene (4) can be supported on a Ag(001) surface, which was characterized by high-resolution NC-AFM imaging.

Chemical identification of ions in doped NaCl by scanning force microscopy.

A quantitative comparison between experiment and theory is presented, which shows that all ions of the Suzuki structure on (001) surfaces of Mg2+ or Cd2+ doped NaCl crystals can be identified despite the tip-surface distance, differences in impurity chemistry, and surface termination. The identification can be used to calibrate the potential of the tip's last atom, and it is proposed to use these surfaces for better characterization of deposited nano-objects.

Imaging Suzuki precipitates on NaCl : Mg2+ (001) by scanning force microscopy.

We present a dynamic scanning force and Kelvin microscopy study of Suzuki precipitates on clean (001) surfaces of UHV cleaved NaCl : Mg2+. Nanometer sized precipitates with a rectangular shape on flat terraces and also at steps were found. Images with atomic resolution confirm that the precipitates are embedded in the NaCl matrix and permit the identification of each type of ion in the unit cell of the Suzuki structure. Kelvin microscopy images show always a strong bright contrast at the precipitates, which is probably due to the negative cation vacancies in the Suzuki structure.

Cluster chemistry: size-dependent reactivity induced by reverse spill-over.

In the present work, the CO oxidation rate on size-selected Pd clusters supported on thin MgO films is investigated in pulsed molecular beam experiments. By varying the cluster coverage independent of the cluster size, we were able to change the ratio of direct and diffusion flux (reverse spill-over) of CO onto the cluster catalyst and thus probe the influence of reverse spill-over on the reaction rate for different cluster sizes (Pd(8) and Pd(30)). The experimental results show that the change in reaction rate as a function of cluster coverage is different for Pd(8) and Pd(30). In order to explain these findings, the CO flux onto the clusters is modeled utilizing the collection zone model for the given experimental conditions. The results indicate that, for small clusters (Pd(8)), the reaction probability of an impinging CO molecule is independent of whether it is supplied by diffusion or direct flux. By contrast, for larger clusters (Pd(30)) a reduced reaction probability is found for CO supplied by reverse spill-over compared to CO supplied by direct flux.

Surface double layer on (001) surfaces of alkali halide crystals: a scanning force microscopy study.

In this Letter we consider the surface double layer on (001) surfaces of UHV cleaved and annealed alkali halide crystals (KCl, NaCl), which we studied with dynamic scanning force microscopy and Kelvin probe force microscopy. Kelvin images show bright and round patches at corner sites and steps. Images with atomic resolution always show kinks at the latter step sites. Our findings are in perfect agreement with the long-standing picture that, due to the presence of divalent impurity ions, the surface carries a net negative surface charge originating from negative cation vacancies at kinks.

Surface structure of an ultrathin alumina film on Ni3Al(111): a dynamic scanning force microscopy study.

The surface structure of an ultrathin alumina film on a Ni3Al(111) substrate has been studied by dynamic scanning force microscopy. The alumina film exhibits a hexagonal superstructure with a lattice parameter of 4.14 nm and a (1/sqrt[3] x 1/sqrt[3])R30 degrees substructure. Two domains rotated by 24 degrees are present. The film is terminated by a hexagonal lattice of oxygen ions with a lattice parameter of 0.293 nm, which is rotated by 30 degrees with respect to the substrate lattice. The nodes of the 4.14 nm superstructure and the 2.39 nm substructure are pinned on points of the substrate lattice, where the surface atomic lattice is almost commensurable. The oxygen lattice is perfectly hexagonal close to these nodes and disordered in the surrounding regions.

Imaging nanoclusters in the constant height mode of the dynamic SFM.

For the first time, high quality images of metal nanoclusters which were recorded in the constant height mode of a dynamic scanning force microscope (dynamic SFM) are shown. Surfaces of highly ordered pyrolytic graphite (HOPG) were used as a test substrate since metal nanoclusters with well defined and symmetric shapes can be created by epitaxial growth. We performed imaging of gold clusters with sizes between 5 and 15 nm in both scanning modes, constant Δf mode and constant height mode, and compared the image contrast. We notice that clusters in constant height images appear much sharper, and exhibit more reasonable lateral shapes and sizes in comparison to images recorded in the constant Δf mode. With the help of numerical simulations we show that only a microscopically small part of the tip apex (nanotip) is probably the main contributor for the image contrast formation. In principle, the constant height mode can be used for imaging surfaces of any material, e.g. ionic crystals, as shown for the system Au/NaCl(001).