Nils Blanc

Functional nanostructures from clusters

Functional cluster-assembled nanostructures with original structures and properties are prepared using the Low Energy Cluster Beam Deposition method (LECBD). This technique consists in depositing supersonic clusters produced in the gas phase using a combined laser vapourisation-inert gas condensation source. Low energy clusters with typical sizes ranging from ∼1 to a few nm are not fragmented upon impact on the substrate (soft landing regime) leading to the formation of cluster-assembled nanostructures which retain the original structures and properties of the incident free clusters. Model nanostructured systems of any kind of materials (metallic, covalent, oxides) well suited for fundamental studies in various fields (nanoelectronics, nanomagnetism, nanophotonics, catalysis or nanobiology) and for applications to very high integration-density devices (∼Tbits/in/²) are prepared using this method. After a brief review of techniques to produce, analyse, mass select, and deposit clusters in the LECBD-regime, the specific aspects of the nucleation and growth process which govern the formation of cluster-assembled nanostructures on the substrate are presented, especially the preparation of 2D-organised arrays of cluster-assembled dots by depositing low energy clusters on FIB-functionalised substrates. Characteristic examples of cluster systems prepared by LECBD are also described: i) metallic (Au, Ag, Au-Ag, Ag-Ni, Ag-Pt)) and oxide (Gd2O3, ZnO) cluster-assembled nanostructures for applications to linear and non linear nano-optics; ii) magnetic nanostructures from Co-based nanoclusters (i.e., Co-Pt) exhibiting a high magnetic anisotropy which is well suited for applications to high density data storage devices; iii) gold or Pd-Pt or Au-Ti clusters for chemical reactivity and catalysis applications. In some specific cases, we were able to perform studies from an isolated individual nanocluster up to 2D or 3D-collections of non-interacting or interacting particles leading to a rather good understanding of the intrinsic as well as the collective properties at nanoscale.

Strains Induced by Point Defects in Graphene on a Metal

Strains strongly affect the properties of low-dimensional materials, such as graphene. By combining in situ, in operando, reflection high-energy electron diffraction experiments with first-principles calculations, we show that large strains, above 2%, are present in graphene during its growth by chemical vapor deposition on Ir(111) and when it is subjected to oxygen etching and ion bombardment. Our results unravel the microscopic relationship between point defects and strains in epitaxial graphene and suggest new avenues for graphene nanostructuring and engineering its properties through introduction of defects and intercalation of atoms and molecules between graphene and its metal substrate.

Coalescence-free L10 ordering of embedded CoPt nanoparticles

We have synthesized diluted samples of CoPt clusters and investigated by various techniques the effect of a 2 h anneal at 750 K. Transmission electron microscopy observations have put into evidence L10 chemical ordering without any detectable coalescence upon annealing. Magnetic measurements on CoPt clusters embedded in amorphous carbon have been used to provide additional and stronger evidence of the absence of coalescence. We show how the analysis of normalized ZFC curves can be used as a convenient and powerful technique to detect subtle variations in the cluster size distribution.

Local deformations and incommensurability of high quality epitaxial graphene on a weakly interacting transition metal

We investigate the fine structure of graphene on iridium, which is a model for graphene weakly interacting with a transition metal substrate. Even the highest quality epitaxial graphene displays tiny imperfections, \textit{i.e.} small biaxial strains, ca. 0.3%, rotations, ca. 0.5°, and shears over distances of ca. 100 nm, and is found incommensurate, as revealed by X-ray diffraction and scanning tunneling microscopy. These structural variations are mostly induced by the increase of the lattice parameter mismatch when cooling down the sample from the graphene preparation temperature to the measurement temperature. Although graphene weakly interacts with iridium, its thermal expansion is found positive, contrary to free-standing graphene. The structure of graphene and its variations are very sensitive to the preparation conditions. All these effects are consistent with initial growth and subsequent pining of graphene at steps.

Topography of the graphene/Ir(111) moiré studied by surface x-ray diffraction

The structure of a graphene monolayer on Ir(111) has been investigated {\it in situ} in the growth chamber by surface x-ray diffraction including the specular rod, which allows disentangling the effect of the sample roughness from that of the nanorippling of graphene and iridium along the moire-like pattern between graphene and Ir(111). Accordingly we are able to provide precise estimates of the undulation associated with this nanorippling, which is small in this weakly interacting graphene/metal system and thus proved difficult to assess in the past. The nanoripplings of graphene and iridium are found in phase, i.e. the in-plane position of their height maxima coincide, but the amplitude of the height modulation is much larger for graphene (\(0.379 \pm 0.044\) \AA) than, {\it e.g.}, for the topmost Ir layer (\(0.017 \pm 0.002\) \AA). The average graphene-Ir distance is found to be \(3.38 \pm 0.04\) \AA.

Signature of multimers on magnetic susceptibility curves for mass-selected Co particles

Even if efforts are currently made to produce nanoparticle samples by deposition of preformed clusters with a size dispersion as low as possible, the incident particle size distribution is necessarily degraded because of the statistical formation of multimers. Here we study diluted Co cluster samples synthesized by mass-selected low energy cluster beam deposition. Transmission electron microscopy is used to determine the cluster size distribution and, in particular, the proportion of dimers. We then show how multimers can have a strong impact on magnetic measurements even if they constitute only a small proportion of the total particles. However, a thorough analysis can be used to determine the respective proportion of dimers and trimers just from the magnetization curves. These proportions are found to be in excellent agreement with the model of random cluster deposition.

Local Order and Magnetic Properties in Mass-Selected

Because of high magnetocrystalline anisotropy in bulk phase, binary nanoalloys from Co 3d and Pt 5d metal are expected to offer the opportunity to obtain room temperature nanomagnets. In this paper, we report a quantitative study combining X-ray absorption and magnetic measurements on well-defined size-selected CoPt clusters assemblies in the 2-4 nm range, diluted in an inert carbon matrix. The particles undergo A1 to L10 phase transition without coalescence upon annealing under vacuum. For the as-prepared samples, using a fcc chemically disordered A1 model, we have found a contracted lattice parameter in the whole sizes range, while the cobalt-platinum stacking in L10 chemically ordered phase corresponds to a larger tetragonalization at the nanoscale compared to the bulk phase. Such quantitative characterizations will be helpful for future theoretical calculations that may elucidate size and alloy effects on the magnetic behavior at nanoscale.

{\rm L}1_{0}

Over the past few years, magnetic bimetallic nanoparticles (NPs) have attracted considerable attention as potential candidates for various applications from catalysis, magnetism, optics, to nanomedicine [1].