Quantum confinement effect on Gd2O3 clusters

Doping nanoparticles using pulsed laser ablation in a liquid containing the doping agent

While doping of semiconductors or oxides is crucial for numerous technological applications, its control remains difficult especially when the material is reduced down to the nanometric scale. In this paper, we show that pulsed laser ablation of an undoped solid target in an aqueous solution containing activator ions offers a new way to synthesise doped-nanoparticles. The doping efficiency is evaluated for laser ablation of an undoped Gd₂O₃ target in aqueous solutions of EuCl₃ with molar concentration from 10^-5 mol L^-1 to 10^-3 mol L^-1. Thanks to luminescence experiments, we show that the europium ions penetrate the core of the synthesised monoclinic Gd₂O₃ nanoparticles. We also show that the concentration of the activators in the nanoparticles is proportional to the initial concentration in europium ions in the aqueous solution, and a doping of about 1% ([Eu]/[Gd] atomic ratio) is reached. On the one hand, this work could open new ways for the synthesis of doped nanomaterials. On the other hand, it also raises the question of undesired penetration of impurities in laser-generated nanoparticles in liquids.

Ab initio prediction of the electronic, magnetic and topological properties of Ln2O3 clusters

The structural, electronic and magnetic properties of the lanthanide oxide Ln₂O₃ clusters, where Ln signifies lanthanides from La to Lu, have been calculated using spin-polarized density functional theory with the B3LYP hybrid functional. The intensities of ferromagnetic RKKY interaction are found comparable with that of antiferromagnetic superexchange interaction in Ce₂O₃ / Pr₂O₃ / Nd₂O₃ / Gd₂O₃ / Tb₂O₃ / Tm₂O₃ clusters, while the other Ln₂O₃ clusters prefer ferromagnetic states to antiferromagnetic states in energy, except nonmagnetic La₂O₃ and Lu₂O₃ clusters. The theoretical spin magnetic moments, calculated three-dimensional spin density maps and dipole moments of Ln₂O₃ clusters suggest that the induced polarizations of oxygen atoms in Sm₂O₃, Eu₂O₃ and Yb₂O₃ clusters remarkably lead to the elongated Ln-O bond lengths in these clusters. The partial density of states of Ln₂O₃ clusters reveals that Sm³⁺ /Eu³⁺ /Yb³⁺ ions are distinctive from other Ln³⁺ ions in that their Ln-4f electrons are strongly hybrid with O-2 p electrons. The topological analysis of the electron density was also performed with quantum theory of atoms in molecules, which indicates the ionic Ln-O bonds have partial covalent characteristics.

Multifunctional role of rare earth doping in optical materials: nonaqueous sol-gel synthesis of stabilized cubic HfO2 luminescent nanoparticles

In this work a strategy for the control of structure and optical properties of inorganic luminescent oxide-based nanoparticles is presented. The nonaqueous sol-gel route is found to be suitable for the synthesis of hafnia nanoparticles and their doping with rare earths (RE) ions, which gives rise to their luminescence either under UV and X-ray irradiation. Moreover, we have revealed the capability of the technique to achieve the low-temperature stabilization of the cubic phase through the effective incorporation of trivalent RE ions into the crystal lattice. Particular attention has been paid to doping with europium, causing a red luminescence, and with lutetium. Structure and morphology characterization by XRD, TEM/SEM, elemental analysis, and Raman/IR vibrational spectroscopies have confirmed the occurrence of the HfO2 cubic polymorph for dopant concentrations exceeding a threshold value of nominal 5 mol %, for either Lu(3+) or Eu(3+). The optical properties of the nanopowders were investigated by room temperature radio- and photoluminescence experiments. Specific features of Eu(3+) luminescence sensitive to the local crystal field were employed for probing the lattice modifications at the atomic scale. Moreover, we detected an intrinsic blue emission, allowing for a luminescence color switch depending on excitation wavelength in the UV region. We also demonstrate the possibility of changing the emission spectrum by multiple RE doping in minor concentration, while deputing the cubic phase stabilization to a larger concentration of optically inactive Lu(3+) ions. The peculiar properties arising from the solvothermal nonaqueous synthesis here used are described through the comparison with thermally treated powders.

Shells of crystal field symmetries evidenced in oxide nano-crystals

By the use of a point charge model based on the Judd-Ofelt transition theory, the luminescence from Eu(3+) ions embedded in Gd(2)O(3) clusters is calculated and compared to the experimental data. The main result of the numerical study is that without invoking any other mechanisms such as crystal disorder, the pure geometrical argument of the symmetry breaking induced by the particle surface has an influence on the energy level splitting. The modifications are also predicted to be observable in realistic conditions where unavoidable size dispersion has to be taken into account. The emission spectrum results from the contribution of three distinct regions; a cluster core, a cluster shell and the very surface, the latter being almost completely quenched in realistic conditions. Eventually, by detailing the spectra of the ions embedded at different positions in the cluster we get an estimate of about 0.5 nm for the extent of the crystal field induced Stark effect. Due to the similarity between Y (2)O(3) and Gd(2)O(3), these results also apply to Eu(3+) doped Y(2)O(3) nanoparticles.

Facile and rapid synthesis of highly luminescent nanoparticles via pulsed laser ablation in liquid

This paper demonstrates the usefulness of pulsed laser ablation in liquids as a fast screening synthesis method able to prepare even complex compositions at the nanoscale. Nanoparticles of Y2O3:Eu3+, Lu2O2S:Eu3+, Gd2SiO5:Ce3+, Lu3TaO7:Gd3+ and Tb3+ are successfully synthesized by pulsed laser ablation in liquids. The phase and stoichiometries of the original materials are preserved while the sizes are reduced down to 5-10 nm. The optical properties of the materials are also preserved but show some small variations and some additional structures which are attributed to the specificities of the nanoscale (internal pressure, inhomogeneous broadening, surface states, etc).

Is a highly ionic material still ionic as a nanoparticle?

The evolution of ionicity with size in highly ionic nanoparticles is investigated in small sesquioxide clusters. Representative clusters (Y2O3)N (N < 50) are theoretically analyzed by first-principle calculations within the density functional theory within the local-density approximation (DFT-LDA) framework and compared to experimental results obtained in an ultrahigh vacuum environment. By studying the structural relaxation and the electronic density of states as a function of size, the respective roles of ionicity and covalency are elucidated. For compounds as ionic as rare earth sesquioxides, the highly ionic bond essentially governs and preserves the crystalline structure. Particular attention is paid to the mechanism responsible for the surface relaxation. The role of the ions at the corners and edges appears prominent, especially in reducing the dipole carried by the particles. Eventually, contrary to the observations and computations concerning ionic surfaces, the mean ionicity remains constant as the size is reduced. It emphasizes that the description of highly ionic nanoparticles cannot be directly inferred from knowledge regarding the ionic surface reconstruction.

Functionalization of atomic force microscope tips by dielectrophoretic assembly of Gd(2)O(3):Eu(3+) nanorods

An atomic force microscopy (AFM) tip has been coated with photoluminescent Eu(3+)-doped Gd(2)O(3) nanorods using a dielectrophoresis technique, which preserves the red emission of the nanorods (quantum yield 0.47). The performance of the modified tips has been tested by using them for regular topography imaging in tapping and contact modes. Both a regular AFM standard grid and a patterned surface (of an organic-inorganic methacrylate Zr-based oxo-cluster and poly(oxyethylene)/siloxane hybrid) have been used. Similar depth values have been measured using a conventional silicon tip and the nanorod-modified tip. The tips before and after use exhibit similar SEM images and photoluminescence spectra and, thus, seem to be stable under working conditions. These tips should find applications in scanning near-field optical microscopy and other scanning techniques.

Optimized Geometry of the Cluster Gd2O3 and Proposed Antiferromagnetic Alignment of f-Electron Magnetic Moment

There is currently experimental interest in assemblies of Gd2O3 clusters. This has motivated the present study in which a single such cluster in free space is examined quantitatively by spin-density functional theory, with appropriate relativistic corrections incorporated for Gd. First, the nuclear geometry of the cluster is optimized, and it is found to be such that the two Gd atoms lie in a symmetry axis perpendicular to the isosceles triangle formed by the O atoms. Then, a careful study is made of the magnetic arrangement of the localized f-electron moments on the two Gd atoms. The prediction of the present treatment is that the localized spins are aligned antiferromagnetically. An alternative picture using superexchange ideas leads to the same conclusion.