Massimo F. Bertino

Ferromagnetism in Li doped ZnO nanoparticles: The role of interstitial Li

ZnO nanoparticles doped with Li (Zn1−yLiyO, y ≤ 0.1) have been investigated with emphasis on the correlation between their magnetic, electronic, and structural properties. In particular, defects such as interstitial Li and Zn atoms, substitutional Li atoms, and oxygen vacancies have been identified by X-ray photoelectron spectroscopy (XPS) and their respective roles in stabilization of the magnetic moment are discussed. X-ray diffraction (XRD) and XPS give clear evidence of Li presence at both substitutional and interstitial sites. XPS studies further show that the amount of substitutional Li defects (Lizn) and interstitial Li defects (Lii) vary non-monotonically with the Li concentration, with the Lii defects being noticeably high for the y = 0.02, 0.08, and 0.10 concentrations, in agreement with the XRD results. Magnetization studies show room temperature ferromagnetism in these nanoparticles with the moment being largest for the particles with high concentration of interstitial lithium and vice versa. Both interstitial Zn (Zni) defects and Zn-O bonds were determined from the Zn LMM Auger peaks; however, the variation of these with Li concentrations was not large. Oxygen vacancies (Vo) concentrations are estimated to be relatively constant over the entire Li concentration range. We relate the Lii and Zni defects to the formation and stabilization of Zn vacancies and thus stabilizing the p-type ferromagnetism predicted for cation (zinc) vacancy in the ZnO type oxides.ZnO nanoparticles doped with Li (Zn1−yLiyO, y ≤ 0.1) have been investigated with emphasis on the correlation between their magnetic, electronic, and structural properties. In particular, defects such as interstitial Li and Zn atoms, substitutional Li atoms, and oxygen vacancies have been identified by X-ray photoelectron spectroscopy (XPS) and their respective roles in stabilization of the magnetic moment are discussed. X-ray diffraction (XRD) and XPS give clear evidence of Li presence at both substitutional and interstitial sites. XPS studies further show that the amount of substitutional Li defects (Lizn) and interstitial Li defects (Lii) vary non-monotonically with the Li concentration, with the Lii defects being noticeably high for the y = 0.02, 0.08, and 0.10 concentrations, in agreement with the XRD results. Magnetization studies show room temperature ferromagnetism in these nanoparticles with the moment being largest for the particles with high concentration of interstitial lithium and vice versa. ...

Magnetic response of core-shell cobalt ferrite nanoparticles at low temperature

Cobalt ferrite nanoparticles (size: 26±4nm) have been synthesized by coprecipitation route. The coercivity of nanoparticles follows a simple model of thermal activation of particle moments over the anisotropy barrier in the temperature range of 30–300K in accordance with Kneller’s law; however, at low temperatures (<30K), the coercivity shows some deviation from this law. The saturation magnetization follows the modified Bloch’s law in the temperature range of 10–300K. Exchange bias (Hex) studies of the samples show that Hex increases with decreasing temperature of the sample. A strong increase in the Hex values is found below 30K for the low applied field (±20kOe), while a smaller increase is found for the high applied field (±90kOe). The slow increase in the exchange bias at high applied field has been attributed to the high field effects on the surface (shell) spins. These shell spins align along the field direction that weakens the core-shell interface interactions leading to the reduction in the exch...

Quantum dots by ultraviolet and x-ray lithography

Highly luminescent semiconductor quantum dots have been synthesized in porous materials with ultraviolet and x-ray lithography. For this, the pore-filling solvent of silica hydrogels is exchanged with an aqueous solution of a group II metal ion together with a chalcogenide precursor such as 2-mercaptoethanol, thioacetamide or selenourea. The chalcogenide precursor is photodissociated in the exposed regions, yielding metal chalcogenide nanoparticles. Patterns are obtained by using masks appropriate to the type of radiation employed. The mean size of the quantum dots is controlled by adding capping agents such as citrate or thioglycerol to the precursor solution, and the quantum yield of the composites can be increased to up to about 30% by photoactivation. Our technique is water-based, uses readily available reagents, and highly luminescent patterned composites are obtained in a few simple processing steps. Polydispersity, however, is high (around 50%), preventing large-scale usage of the technique for the time being. Future developments that aim at a reduction of the polydispersity are presented.

Photocatalytic performances of mesoporous TiO2 films doped with gold clusters

We report on the single-pot fabrication of ordered mesoporous crystallized titania films doped with gold. Au is incorporated in TiO2 films by adding to the coating solution precursors such as AuCl3 or monodisperse Au113+ nanoclusters, and Au nanoparticles are formed by calcination. A systematic study is performed to correlate structure, Au doping and photocatalytic activity of such films. Two-dimensional small angle X-ray scattering (2D-SAXS), transmission electronic microscopy (TEM), scanning electronic microscopy (SEM) and porosimetry–ellipsometry show that the films retain their mesoporous order even for doping levels as high as 1% Au : Ti atomic ratio. Wide angle X-ray scattering (WAXS), and cyclovoltammetry (CV) show that Au113+ nanoclusters promote the formation of the TiO2 (B) phase in competition with the anatase phase. AuCl3 stabilizes instead only the anatase phase. The highest photocatalytic activity is exhibited by films where Au113+ is employed as a precursor, which we attribute to the combination of the mixed anatase/TiO2 (B) phase, of Au nanoparticle doping and of a well-ordered mesoporous TiO2 matrix.

Photolithographic synthesis of polyaniline nanofibres

Thin films of polyaniline nanofibres were synthesized using ultraviolet irradiation of aqueous solutions of aniline, nitric acid, and ammonium peroxydisulfate. The parent solution was spin coated on a planar substrate, and allowed to polymerize in the dark for 4–5 min. The substrate was then exposed to ultraviolet light for 6–10 min. Irradiation was carried out either with a frequency-tripled Nd:YAG laser, or with a mercury vapour lamp. The polyaniline fibres and films were characterized with scanning and transmission electron microscopy, and with Fourier transform infrared spectroscopy. Fibres had typical diameters between 20 and 150 nm, and lengths of the order of microns. Bulk polyaniline formed in the unirradiated portion of the samples. Using a masking technique, alternating stripes of bulk polyaniline and polyaniline nanofibres were produced.

The effect of dissociative chemisorption on the diffraction of D2 from Ni(110)

Absolute scattering probabilities of nearly monoenergetic D2 and He beams are compared for the highly reactive clean Ni(110) surface at a surface temperature Ts=700 K along the more corrugated [001] direction. At incident energies between 20 and 110 meV the total reflectivity of D2 is about a factor 200 smaller than for He, whereas the first order diffraction intensities relative to the specular peak are a factor 7 larger. The D2 angular distributions also show clear evidence of rotationally inelastic diffraction peaks. The diffraction intensities of both He and D2 can be accounted for by a conventional hard wall model with reasonable values of the corrugation amplitudes of 0.060 A for He and 0.091 A for D2 without including a lateral variation in the probability for chemisorption. The reflectivity results when extrapolated to Ts=0 indicate that for He only 33% of the incident atoms are coherently reflected. For D2 only 9% are coherently scattered and approximately 24% are chemisorbed. The coherently scat...

DIFFRACTION AND ROTATIONAL TRANSITIONS IN THE SCATTERING OF D2 FROM CU(001) AT ENERGIES UP TO 250 MEV

Absolute diffraction probabilities for the scattering of D2 from a clean Cu(001) surface along the [100] azimuth have been measured at incident kinetic energies between 20 and 250 meV. The measured attenuation of the diffraction intensities with surface temperature corresponds to a surface Debye temperature of ΘD=341 K. The high-resolution angular distributions show clear evidence of rotationally inelastic diffraction (RID) peaks. The RID probability increases with incident energy and represents as much as 30% of the elastic diffraction probability at energies above Ei=200 meV. An Eikonal approximation analysis gives a value h=0.075 A for the surface corrugation which is independent of incident energy. The rotational transition probabilities correspond to an effective value of δ=0.3 for the molecular eccentricity. The experimental results indicate that diffraction of D2 from Cu(001) can be accounted for by a hard-wall collision mechanism over the whole range of investigated energies.

Experimental determination of a longitudinal phonon dispersion curve in a quasi-two-dimensional system

The structural and dynamical properties of a monolayer of Xe atoms adsorbed on a Cu(001) surface have been studied with high-resolution He-atom scattering. Angular distributions reveal a phase-transition from the high-temperature in commensurate phase to a new, more densely packed, phase upon cooling below 65 K. For both phases the strongly dispersive longitudinal phonon mode could be detected. A lattice-dynamical analysis yields a substrate mediated softening of the Xe–Xe radial force constant down to 25% of that obtained from gas-phase potentials and a fit of Xe bulk phonon data. This decrease is much larger than assumed in previous work for noble gas atoms adsorbed on metals.

Quasielastic helium atom scattering measurements of microscopic diffusion of CO on the Ni(110) surface

The microscopic diffusion of CO on the Ni(110) surface has been studied by quasielastic helium atom scattering. From the temperature dependence of the energetic broadening of the quasielastic peak measured at a parallel wave vector near the Brillouin zone boundary, the activation energies for diffusion have been determined to be Ediff=57±4 and 35±4 meV for diffusion parallel to the rows (〈110〉) and perpendicular to the rows (〈001〉), respectively. The activation energies are a factor 2–6 smaller and the preexponential diffusion coefficients orders of magnitude larger than obtained in recent laser measurements of chemical diffusion coefficients, indicating that diffusion over distances from about a few Angstroms up to 30 A is much faster than over the much larger distances probed in macroscopic diffusion measurements. The difference is attributed to the impeding effect of step edges or impurities on the latter measurements.

Shortened aerogel fabrication times using an ethanol–water azeotrope as a gelation and drying solvent

Native and cross-linked aerogel monoliths were fabricated in a few hours using a technique that does not require solvent exchange prior to supercritical drying. Native oxide alcogels were synthesized by alkoxide hydrolysis–condensation using an ethanol–water azeotrope mixture as the gelation solvent. Cross-linked alcogels were synthesized by replacing part of the gelation solvent with a monomer, followed by visible light photopolymerization for which Eosin Y was used as an initiator and a tertiary amine used as a coinitiator. After aging for 2 hours, the alcogels were removed from the molds, placed in a pressure vessel, and dried using a supercritical ethanol–water azeotrope mixture. Starting from the sol, dried aerogels could be fabricated in about 6 hours. Most importantly, since solvent exchange was not required, native oxide and polymer cross-linked aerogels could be fabricated at the same rate. A systematic study was carried out to confirm that monoliths produced with our technique had density, surface area and Young’s modulus comparable to those of aerogels produced following more conventional pathways, such as supercritical CO2 drying. We synthesized samples using base- and acid-catalyzed chemistries, varied alkoxide concentration and, for cross-linked aerogels, monomer concentration. Depending on alkoxide concentration, native oxide aerogels had densities between about 0.06 and 0.17 g cm−3 and surface areas between about 300 and 500 m2 g−1. Depending on monomer type and concentration, cross-linked monoliths had a modulus between about 10 and 400 MPa, a density between 0.25 and 0.5 g cm−3 and a surface area between 150 m2 g−1 and 350 m2 g−1. Shrinkage was about 5% for base-catalyzed synthesis, about 20% for acid-catalyzed synthesis and about 10% for cross-linked monoliths. Infrared and Raman spectroscopies, solid state NMR and thermogravimetric analysis confirmed that drying in a supercritical ethanol–water azeotrope did not significantly affect the cross-linking polymer used to produce mechanically strong aerogels.