C. de Julián Fernández

Structure and optical properties of Au-polyimide nanocomposite films prepared by ion implantation

Au-polyimide nanocomposites have been synthesized by implanting different doses of Au+ ions in 100nm thick films of pyromellitic dianhydride-4,4′ oxydianiline polyimide, prepared by glow discharge vapor deposition polymerization. Unambiguous evidence of Au nanoclusters growth is found only at the highest implantation doses (5×1016Au+∕cm2). Structural, compositional, and optical characterizations show that the implantation induces the compactation of the initial film due to H and C loss. The resulting structure is a composite containing 2–3nm gold nanoparticles arranged in a layer of about 40nm and, just beneath the sample surface, a 15nm thick carbon-rich layer. Optical simulations suggest the presence of a gold-carbon core-shell structure in the nanoparticles.

Coupling between magnetic and optical properties of stable Au–Fe solid solution nanoparticles

Au-Fe nanoparticles constitute one of the simplest prototypes of a multifunctional nanomaterial that can exhibit both magnetic and optical (plasmonic) properties. This solid solution, not feasible in the bulk phase diagram in thermal equilibrium, can be formed as a nanostructure by out-of-equilibrium processes. Here, the novel magnetic, optical and magneto-optical properties of ion-implanted Au-Fe solid solution nanoparticles dispersed in a SiO(2) matrix are investigated and correlated. The surface plasmon resonance of the Au-Fe nanoparticles with almost equicomposition is strongly damped when compared to pure Au and to Au-rich Au-Fe nanoparticles. In all cases, the Au atoms are magnetically polarized, as measured by x-ray magnetic circular dichroism, and ferromagnetically coupled with Fe atoms. Although the chemical stability of Au-Fe nanoparticles is larger than that of Fe nanoparticles, both the magnetic moment per Fe atom and the order temperature are smaller. These results suggest that electronic and magnetic properties are more influenced by the hybridization of the electronic bands in the Au-Fe solid solution than by size effects. On the other hand, the magneto-optical transitions allowed in the vis-nIR spectral regions are very similar. In addition, we also observe, after studying the properties of thermally treated samples, that the Au-Fe alloy is stabilized, not by surface effects, but by the combination of the out-of-equilibrium nature of the ion implantation technique and by changes in the properties due to size effects.

Influence of annealing atmosphere on metal and metal alloy nanoclusters produced by ion implantation in silica

Abstract The evolution of binary alloy clusters in sequentially ion-implanted Au/Cu or Ag/Au silica glasses has been studied as a function of the annealing atmosphere. Alloy formation has been unambiguously evidenced in the as-implanted samples. In the case of Au/Cu system, the selective effect of reducing (hydrogen containing) or oxidizing (air) atmosphere during subsequent annealing can be used to either modify the structure of the alloy or to separate the two species, respectively. On the contrary, no remarkable difference between reducing or oxidizing post-implantation annealing can be evidenced in the Au/Ag system.

MAGNETIC PROPERTIES OF Co AND Ni BASED ALLOY NANOPARTICLES DISPERSED IN A SILICA MATRIX

Abstract A comparative study of the magnetic properties of Co and Ni based alloy nanoparticles (Ni–Co, Ni–Cu and Co–Cu) formed in a silica matrix by ion implantation is presented. Different ion doses and implantation sequences were realized in order to obtain different nanostructures. The structural and magnetic properties observed for the Cu50Ni50 nanoparticles are similar to those of the Cu60Ni40 bulk alloy. The crystal structure of CoxNi1−x (0⩽x⩽1) nanoparticles is similar to that of the corresponding bulk alloy. The magnetic properties depend on the ion-implanted dose and on the alloy composition. The samples prepared by implanting a 15×10 16 ions / cm 2 total dose contain nanoparticles, which are superparamagnetic at room temperature and their magnetic behavior is influenced by dipolar interparticle interactions. The magnetization of the CoNi samples at high magnetic field is larger than that of the corresponding bulk alloy and follows the same composition dependence of that quantity measured in the alloy.

Synthesis of wide band gap nanocrystals by ion implantation

Abstract Nanocrystals of wide band gap materials (GaN and In2O3) were synthesized by sequential ion implantation in dielectric substrates, followed by thermal annealing of the samples. Transmission electron microscopy, extended X-ray absorption fine structure spectroscopy and grazing incidence X-ray diffraction analyses confirmed the formation of GaN and In2O3 crystalline nanoparticles. Blue shift of the near-edge photoluminescence (PL) band (quantum confinement effect) was observed for GaN nanocrystals. A strong PL band peaked at 3.35 eV was detected upon excitation of In2O3 nanocrystals at 5.20 eV.

Influence of post-implantation thermal and laser annealing on the stability of metal-alloy nanoclusters in silica

Abstract The effect of thermal or laser annealing on the evolution of binary alloy clusters in sequentially ion-implanted Au/Cu or Ag/Au silica glasses has been studied. In the case of Au/Cu system the selective effect of oxidizing atmosphere on alloy decomposition has been investigated as a function of the temperature and time, evidencing that alloy decomposition takes place with Cu oxidation. In the AuAg system the decomposition process is slower than in the AuCu case, due to lower silver–oxygen reactivity. On the contrary, laser annealing is able to promote alloy formation and/or composition modification even in systems with partially or completely separated components.

Structure and magnetic properties of alloy-based nanoparticles silica composites prepared by ion-implantation and sol-gel techniques

Abstract Ni–Co nanoparticles embedded in an SiO2 matrix with different Co/Ni atomic ratio have been prepared by ion-implantation and sol–gel techniques. The structural and magnetic characterizations were realized to determine the alloy formation at the nanoparticle level. The magnetic properties of these materials are presented and analyzed considering the metal composition and the nanostructural features.

Sequential ion implantation of copper and cobalt in silica glass: A study by synchrotron radiation techniques

Abstract Copper–cobalt nanostructures are expected to exhibit peculiar magnetic and optical properties that make them interesting for several application fields. In particular, nanoclusters of these metals can be formed upon ion implantation in dielectric matrices. In this work, Co + and Cu + ions were sequentially implanted into fused silica. Samples were analyzed by different synchrotron radiation-based techniques, namely, extended X-ray absorption fine structure spectroscopy and grazing incidence X-ray diffraction. The obtained cluster structure, and so its magnetic features, are observed to depend on the sequential implantation order.

MAGNETIC PROPERTIES OF ORGANIC COATED GOLD SURFACES

We review here our recent results of experimental observation of room temperature magnetism in gold nanoparticles (NPs) and thin films. Capping gold surfaces with certain organic molecules leads to the appearance of magnetism at room temperature. The surface bonds between the organic molecules and Au atoms give rise to magnetic moments. These magnetic moments are blocked along the bond direction showing huge anisotropy. In the case of atomically flat surfaces, the magnetic moments are giants. An explanation of this orbital ferromagnetism is given. These results point out the possibility to observe magnetism at nanoscale in materials without typical magnetic atoms (transition metals and rare earths), and are of fundamental value to understand the magnetic properties of surfaces.

Gold/titania nanocomposites thin films for optical gas sensing devices

Nanostructured TiO2 films and Au-TiO2 nanocomposite thin films prepared by sol-gel method have been deposited onto gold covered glass substrates and glass substrates in order to study their optical properties using Surface Plasmon Resonance and Optical Absorption measurements. Both techniques have been used to study the sensing features of both kind of films to different vapour organic compounds. A comparative study of the two techniques has allowed us to know the possible benefits than can be found when the sensing material is a nanocomposite thin film.