We have investigated self-assembled island formation, molecular detail and interesting contrast reversal effects for Er3N@C80 and Sc3N@C80 on Au(111) and Ag/Si(111) surfaces using variable temperature scanning tunnelling microscopy (STM) and spectroscopy (STS). The trinitride containing fullerenes have been evaporated onto Ag-passivated Si(111) at room temperature and self-assembled into close-packed 2-D islands. Gentle annealing at 200 – 300°C is required for the formation of close-packed islands with 20 – 50 nm diameter on the Au(111) surface. Variable-voltage STM reveals bias-dependent contrast anomalies within the islands, and at low temperatures (< 90K), intra-molecular resolution of the fullerenes has been achieved. STS measurements indicate that the bright/dark anomalies may be caused by different electron densities of states for the ‘normal’ and ‘anomalous’ fullerenes.

Recent years have seen a rising interest in the study of surface properties of metal and metaloxide nanoparticles according to their use in a wide range of applications as heterogeneous catalysis, magnetic nanostructures and gas sensing devices. In particular atomic clusters supported on different substrates represent an interesting system for the study of cluster structural and electronic properties by means of scanning probe microscopies or advanced spectroscopic techniques. A fact which is very attractive in this field is certainly the transition of these properties when going from bulk to small clusters. Since these differences are expected to affect the catalytic behaviour of a given material, the investigation of the basic mechanisms of catalysis and their dependence on the structure and size of the clusters are of great interest to those who investigate catalytic processes.

A simple, reliable method for preparation of bulk Cr tips for Scanning Tunneling Microscopy (STM) is proposed and its potentialities in performing high-quality and high-resolution STM and Spin Polarized-STM (SP-STM) are investigated. Cr tips show atomic resolution on ordered surfaces. Contrary to what happens with conventional W tips, rest atoms of the Si(111)-7x7 reconstruction can be routinely observed, probably due to a different electronic structure of the tip apex. SP-STM measurements of the Cr(001) surface showing magnetic contrast are reported. Our results reveal that the peculiar properties of these tips can be suited in a number of STM experimental situations.

We report on how different cluster deposition regimes can be obtained and observed by in situ scanning tunneling microscopy by exploiting deposition parameters in a pulsed laser deposition process. Tungsten clusters were produced by nanosecond pulsed laser ablation in Ar atmosphere at different pressures and deposited on Au(1 1 1) and HOPG surfaces. Deposition regimes including cluster deposition–diffusion–aggregation, cluster melting and coalescence and cluster implantation were observed, depending on background gas pressure and target-to-substrate distance which influence the kinetic energy of the ablated species. These parameters can thus be easily employed for surface modification by cluster bombardment, deposition of supported clusters and growth of films with different morphologies. The variation in cluster mobility on different substrates and its influence on aggregation and growth mechanisms has also been investigated.

A simple, reliable method for the preparation of bulk Cr tips for scanning tunneling microscopy (STM) is proposed and its potentialities in performing high-quality and high-resolution STM and spin-polarized STM (SP-STM) are investigated. Cr tips show atomic resolution on ordered surfaces. Contrary to what happens with conventional W tips, rest atoms of the Si(111)-7×7 reconstruction can be routinely observed, probably due to a different electronic structure of the tip apex. SP-STM measurements of the Cr(001) surface showing magnetic contrast are reported. Our results reveal that the peculiar properties of these tips can be suited in a number of STM experimental situations.

In the synthesis of nanostructured thin films the characterization of the growth processes plays a fundamental role for the control of the film and surface properties. Moreover when the deposition technique is based on the production and the assembling of nanoparticles/clusters the characterization of the precursor size distribution is of fundamental importance.We have designed a pulsed laser deposition (PLD) apparatus for the production of nanostructured thin films and surfaces, connected to a UHV variable temperature scanning tunneling microscope (STM). The whole system is devoted to the synthesis and in situ study of nanostructured and nanoporous functional metal and metal oxide films and surfaces. We have deposited W nanoparticles produced by a few hundreds laser pulses in order to investigate the initial mechanisms of the film growth. Different deposition conditions have been explored by controlling the laser generated plasma expansion through a background gas in the PLD chamber. STM measurements have been performed on W thin films deposited on different substrates to study both the size distribution and the aggregation of the precursors on the surface. Although substrate effects must be taken into account, the control of the background gas pressure and of the target-to-substrate distance allows to produce surfaces with different morphologies. This opens the possibility to tailor the material properties through the control of the size and deposition energy of the building nano-units. (c) 2007 Elsevier B.V. All rights reserved.

We report the characterization by Raman spectroscopy of rutile nanocrystals embedded in cluster-assembled TiO2 films produced by supersonic cluster beam deposition. Thermal annealing of the as-deposited films promotes the growth of rutile and anatase nanoparticles with a dimension distribution peaked around 10 nm. We show that a phonon confinement model is not adequate to explain the features of the lattice dynamics of rutile nanocrystals. The coexistence of the two crystalline phases allows us to identify the role of chemical and structural inhomogeneities in the film potentially affecting the Raman spectra. Our results suggest that the lattice dynamics of rutile nanocrystals is influenced by the dielectric constant of the cluster-assembled films.

Tungsten oxide thin films are interesting for their electrochromic and gas sensing properties. For technological applications, the control of morphology (effective surface, porosity) and structure (crystallinity, stoichiometry) at the nanoscale is of paramount interest to deliver distinctive properties.Nanostructured tungsten and tungsten oxide films have been synthesized by nanosecond pulsed laser deposition (PLD) starting from a W metallic target. Both inert and reactive buffer gases (He, Ar, dry air) have been exploited to induce cluster formation and/or oxidation and to vary the deposition energies of ablated species. A wide range of morphologies extending from compact and smooth, up to extremely porous and spongy-like structures have been obtained.The structure, the oxide phase and the degree of crystallinity of the films have been investigated by Raman spectroscopy. Films deposited in the presence of inert buffer gases reveal a spontaneous ex-situ oxidation, when exposed to ambient atmosphere, which is strongly related to the nanostructure. Instead, deposition in a varying pressure of dry air permits to grow tungsten oxide films and to tune their structure from amorphous to nano- and microcrystalline with different coexisting oxide phases. (C) 2006 Elsevier B.V. All rights reserved.

Nanostructured tungsten and tungsten oxide films have been synthesized by pulsed laser deposition (PLD) in different atmospheres (He, Ar, dry air). The control of the gas pressure in the deposition chamber allows to vary the morphology of the deposited films (studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM)) ranging from a compact ultrasmooth structure to a porous nano and mesostructure characterized by a high fraction of voids and by a large specific area. The presence, the structure and the degree of crystallinity of tungsten oxide have been investigated by Raman spectroscopy. By varying the pressure of the background inert gas (He, Ar) in the 1–1000 Pa range we obtain metallic films with different degrees of spontaneous oxidation when exposed to the atmosphere. Deposition in dry air permits to grow nanostructured tungsten oxide films characterized by different degrees of crystallinity, ranging from amorphous to nanocrystalline.

Trimetallic nitride template fullerenes have been deposited onto a variety of substrates in order to elucidate the substrate–fullerene interactions. We have investigated self-assembled island formation and molecular detail of Er3N@C80 and Sc3N@C80 on Ag/Si(111), Au(111)/mica, Si(111), and Si(001) using variable temperature scanning tunnelling microscopy (STM). At room temperature, the fullerenes self-assemble into monolayer-high hexagonal close-packed islands on Ag-passivated Si(111) whereas annealing at elevated temperatures (250–300°C) is necessary for the self-assembly of close-packed islands on Au(111). Intra-molecular resolution of the fullerenes has been achieved at liquid nitrogen temperature on Ag/Si(111) and already at room temperature on Si(001), when the rotation of the fullerenes is frozen. Whereas the bonding between the fullerenes and Si surfaces is mainly covalent, it appears to be mainly van-der-Waals on the other surfaces.

We report the fabrication of libraries of nanostructured TiO2 films with a gradient in nanoparticle crystalline phase and dimensions. By supersonic cluster beam deposition it is possible to produce rutile nanoparticles with a diameter smaller than 10nm and to spread them into a co-deposited amorphous titania matrix. Upon thermal annealing rutile nanocrystals act as growth seeds of a film with a spatially controlled rutile/anatase ratio. Films with controlled crystalline phase variation have been deposited on micropatterned substrate to produce arrays of chemoresistive sensors of volatile organic compounds. Devices with different nanocrystalline structures and performances are obtained by a simple one-step thermal treatment after deposition.

A systematic study of the shift and linewidth of the Eg Raman peak at 144cm−1 of anatase TiO2 nanopowders, produced by a flame aerosol technique, is here presented. The analysis was performed as a function of the crystal domain size and of the degree of oxidation. In the nanopowders, a clear contribution of the stoichiometry defects to the peak shift was evidenced, while the peak width seems to be less affected by the oxygen content. The Raman peak behavior due to size reduction has been interpreted in the framework of a phonon quantum confinement model. A critical review of the different approaches to this model, adopted in the literature to explain the behavior of the anatase Raman spectra as a function of the domain size, is presented. In particular, the hypothesis of a three-dimensional isotropic model for the dispersion relations is discussed. This analysis evidences general limits in the application of the phonon confinement model to the study and characterization of nanoparticles and nanostructured materials, showing how an uncritical use of the confinement theory can yield distorted results.

Thin films synthesized by assembling clusters present interesting chemical and physical properties and a large specific surface, and are appealing for functional applications (e.g. sensing and catalysis). Also, clusters supported on surfaces are interesting both for nanocatalysis applications and for fundamental research. By means of pulsed laser deposition (PLD) in a background atmosphere we can induce cluster aggregation in the ablation plume and control the deposition kinetic energy of the clusters. These phenomena depend on the plume expansion dynamics and their influence on the properties of the deposited films has been investigated as a function of the background gas mass and pressure. The control of these parameters permits variation of the film surface morphology, from a compact structure with a very smooth surface, to a film with a controlled roughness at the nanoscale, to an open, low density meso- and nanostructure characterized by a high fraction of voids and by a large specific area. Thin films of WO x , TiO x , Pd/PdO, and Ag were deposited and characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM) and Raman spectroscopy. Post-deposition annealing permits control of the crystalline degree of the films, which in the case of tungsten and titanium oxide is found to depend on the original nanostructure, while a different degree of oxidation can be induced by controlling the amount of oxygen in the deposition chamber. In-situ scanning tunneling microscopy (STM) was employed to study the first stages of growth of W films on different substrates. This opens the possibility to tailor the material properties through the control of the building nano-units.