Alessandro Coati

Combinatorial growth and anisotropy control of self-assembled epitaxial ultrathin alloy nanowires.

Self-assembled vertical epitaxial nanostructures form a new class of heterostructured materials that has emerged in recent years. Interestingly, such kind of architectures can be grown using combinatorial processes, implying sequential deposition of distinct materials. Although opening many perspectives, this combinatorial nature has not been fully exploited yet. This work demonstrates that the combinatorial character of the growth can be further exploited in order to obtain alloy nanowires coherently embedded in a matrix. This issue is illustrated in the case of a fully epitaxial system: CoxNi1-x nanowires in CeO2/SrTiO3(001). The advantage brought by the ability to grow alloys is illustrated by the control of the magnetic anisotropy of the nanowires when passing from pure Ni wires to CoxNi1-x alloys. Further exploitation of this combinatorial approach may pave the way toward full three-dimensional heteroepitaxial architectures through axial structuring of the wires.

Independent tuning of size and coverage of supported Pt nanoparticles using atomic layer deposition

Synthetic methods that allow for the controlled design of well-defined Pt nanoparticles are highly desirable for fundamental catalysis research. In this work, we propose a strategy that allows precise and independent control of the Pt particle size and coverage. Our approach exploits the versatility of the atomic layer deposition (ALD) technique by combining two ALD processes for Pt using different reactants. The particle areal density is controlled by tailoring the number of ALD cycles using trimethyl(methylcyclopentadienyl)platinum and oxygen, while subsequent growth using the same Pt precursor in combination with nitrogen plasma allows for tuning of the particle size at the atomic level. The excellent control over the particle morphology is clearly demonstrated by means of in situ and ex situ X-ray fluorescence and grazing incidence small angle X-ray scattering experiments, providing information about the Pt loading, average particle dimensions, and mean center-to-center particle distance.The performance of supported nanoparticle catalysts is closely related to their size, shape and interparticle distance. Here, the authors introduce an atomic layer deposition-based strategy to independently tune the size and coverage of platinum nanoparticles with atomic-level precision.

Mobile setup for synchrotron based in situ characterization during thermal and plasma-enhanced atomic layer deposition

We report the design of a mobile setup for synchrotron based in situ studies during atomic layer processing. The system was designed to facilitate in situ grazing incidence small angle x-ray scattering (GISAXS), x-ray fluorescence (XRF), and x-ray absorption spectroscopy measurements at synchrotron facilities. The setup consists of a compact high vacuum pump-type reactor for atomic layer deposition (ALD). The presence of a remote radio frequency plasma source enables in situ experiments during both thermal as well as plasma-enhanced ALD. The system has been successfully installed at different beam line end stations at the European Synchrotron Radiation Facility and SOLEIL synchrotrons. Examples are discussed of in situ GISAXS and XRF measurements during thermal and plasma-enhanced ALD growth of ruthenium from RuO4 (ToRuS™, Air Liquide) and H2 or H2 plasma, providing insights in the nucleation behavior of these processes.

Direct observation of elastic displacement modes by grazing-incidence X-ray diffraction

It is demonstrated that grazing-incidence X-ray diffraction is a direct tool for measuring the elastic displacement modes near the surface of a crystal. Due to the fact that X-ray diffraction is a Fourier transform of the electronic density, and thus, of the atomic positions, elastic displacement modes appear as additional spots in the reciprocal space. Their characteristics can be directly derived from the elastic constants of the material. Measuring the amplitude of the diffracted wave for these peaks allows direct determination of the force distribution at the surface, which is at the origin of the elastic displacements. Various examples of such determinations are given for self-organized surfaces and for vicinal surfaces.

Huge metastable axial strain in ultrathin heteroepitaxial vertically aligned nanowires

Strain engineering is a powerful tool to tailor the physical properties of materials coherently stacked in an epitaxial heterostructure. Such an approach, applied to the mature field of planar heteroepitaxy, has yielded a variety of new phenomena and devices. Recently, heteroepitaxial vertically aligned nanocomposites have emerged as alternatives to planar structures. Owing to the peculiar geometry of such nanoarchitectures, efficient strain control can be achieved, opening the way to novel functionalities. In this paper, we report a very large tensile axial strain in epitaxial transition metal nanowires embedded in an oxide matrix. We show that axial strains in excess of 1.5% can be sustained over a large thickness (a few hundred nanometers) in epitaxial nanowires having ultrasmall diameters (∼3–6 nm). The axial strain depends on the diameter of the nanowires, reflecting its epitaxial nature and the balance of interface and elastic energies. Furthermore, it is experimentally shown that such strain is metastable, in agreement with the calculations performed in the framework of the Frenkel-Kontorova model. The diameter dependence and metastability provide effective ways to control the strain, an appealing feature for the design of functional nanoarchitectures.

Structure and evolution of semiconducting buffer graphene grown on SiC(0001)

Using highly controlled coverages of graphene on SiC(0001), we have studied the structure of the first graphene layer that grows on the SiC interface. This layer, known as the buffer layer, is semiconducting. Using x-ray reflectivity and x-ray standing wave analysis, we have performed a comparative study of the buffer layer structure with and without an additional monolayer graphene layer above it. We show that no more than 26% of the buffer carbon is covalently bonded to Si in the SiC interface. We also show that the top SiC bilayer is Si depleted and is likely the cause of the incommensuration previously observed in this system. When a monolayer graphene layer forms above the buffer, the buffer layer becomes less corrugated with signs of a change in the bonding geometry with the SiC interface. At the same time, the entire SiC interface becomes more disordered, presumably due to entropy associated with the higher growth temperature.

Enhanced magnetization at the Cr/MgO(001) interface

We report on the magnetization at the Cr/MgO interface, which we studied through two complementary techniques: angle-resolved photoemission spectroscopy and polarized neutron reflectivity. We experimentally observe an enhanced interface magnetization at the interface, yet with values much smaller than the ones reported so far by theoretical and experimental studies on Cr(001) surfaces. Our findings cast some doubts on the interpretations on previous works and could be useful in antiferromagnetic spin torque studies.

In Situ Investigation of the Early-Stage Growth of Nanoporous Alumina

We report the successful use of in situ grazing incidence small-angle X-ray scattering to follow the anodization of aluminum. A dedicated electrochemical cell was designed and developed for this purpose with low X-ray absorption, with the possibility to access all azimuthal angles (360°) and to remotely control the temperature of the electrolyte. Three well-known fabrication techniques of nanoporous alumina, i.e., single, double, and pretextured, were investigated. The differences in the evolution of the scattering images are described and explained. From these measurements, we could determine at which moment the pores start growing even for very short anodization times. Furthermore, we could follow the thickness of the alumina layer as a function of the anodization time by monitoring the period of the Kiessig fringes. This work is aimed at helping to understand the different steps taking place during the anodization of aluminum at the very early stages of nanoporous alumina formation.

Fermi surface symmetry and evolution of the electronic structure across the paramagnetic-helimagnetic transition in MnSi/Si(111)

MnSi has been extensively studied for five decades, nonetheless detailed information on the Fermi surface (FS) symmetry is still lacking. This missed information prevented from a comprehensive understanding the nature of the magnetic interaction in this material. Here, by performing angle-resolved photoemission spectroscopy on high-quality MnSi films epitaxially grown on Si(111), we unveil the FS symmetry and the evolution of the electronic structure across the paramagnetic-helimagnetic transition at T

Synchrotron based in situ characterization during atomic layer deposition

_C