L. G. Jacobsohn

Y2O3:Bi nanophosphor: Solution combustion synthesis, structure, and luminescence

Photoluminescence (PL), radioluminescence (RL), and thermoluminescence (TL) investigation of Y2O3:Bi nanophosphors prepared by solution combustion synthesis using urea, glycine, and hexamethylenetetramine (HMT) as fuels was carried out. The as-prepared nanopowders have increasing crystallinity and average crystallite sizes for urea, glycine, and HMT, respectively. Luminescence is composed of two emission bands centered at 408 and 505 nm due to two nonequivalent Bi3+ sites with symmetry S6 and C2, respectively. The occupancy of these sites depends on the synthesis conditions, in agreement with theoretical predictions. Annealing at 1000 °C for 1 h improves PL and RL efficiency due to enhanced crystallinity of the nanopowders and activation of recombination centers (Bi3+ ions). No shift in the PL peak position was observed as a function of average crystallite size. The concentration quenching was experimentally determined to have a maximum emission of around 3 mol % of the dopant. TL spectra present several ...

Structural and mechanical characterization of fluorinated amorphous-carbon films deposited by plasma decomposition of CF4–CH4 gas mixtures

Fluorinated amorphous-carbon films (a-C:F:H) were deposited by low-power rf capacitively coupled plasma-enhanced chemical-vapor deposition using CH4–CF4 gas mixtures. Different series of films were deposited, changing one parameter at a time: the CF4 partial pressure from 0% to 100%, the self-bias voltage from −50 to −700 V, and the total deposition pressure from 5 to 15 Pa. The composition was determined by ion-beam analysis (IBA): Rutherford backscattering spectrometry, elastic recoil detection analysis, and nuclear reaction analysis. The atomic density of the films was evaluated by combining the IBA results with the thickness value measured by stylus profilometry. Film structure was investigated by infrared transmission and Raman scattering spectroscopies. The internal stress and Vickers hardness were also measured. For a fixed self-bias, the increase of the CF4 partial pressure leads to a higher fluorine incorporation and the decrease of both hardness and internal stress. The film microstructure chang...

Multifunction Gd2O3:Eu nanocrystals produced by solution combustion synthesis: Structural, luminescent, and magnetic characterization

The synthesis, structural, luminescent, and magnetic properties of multifunction 8 at. % Eu-doped Gd2O3 nanocrystals were investigated. The material was synthesized by the solution combustion technique and characterized by x-ray diffraction (XRD), transmission electron microscopy, and photoluminescence and magnetization measurements. The as-prepared material presents predominant base-centered monoclinic structure with average crystallite size of 35 nm. Isothermal annealing at 1000 °C for up to 152 h induced gradual structural transition toward the body-centered cubic structure. Debye–Scherrer analysis of XRD results showed that annealing did not induce grain growth of monoclinic nanocrystals, while cubic nanocrystals reached 47 nm after annealing for 152 h. The luminescent behavior was monitored as a function of annealing time and related to the structural transformation. In particular, an inversion of the predominant photoluminescence emission line from 621.4 to 609.5 nm was observed and related to chang...

Synthesis of cobalt nanoparticles by ion implantation and effects of postimplantation annealing

A study of the synthesis of Co nanoparticles by ion implantation and the effects of postimplantation annealing was carried out. Silica was implanted with 35keV Co+ ion beams to doses ranging from 8×1015to1×1017atoms∕cm2. Nanoparticle size, distribution, and structure were ascertained via transmission electron microscopy measurements, which reveal the presence of spherical nanoparticles in both as-implanted and annealed samples. X-ray photoelectron spectroscopy show the chemical state of the nanoparticles in both as-implanted and annealed samples to be metallic. Temperature-dependent field-cooled and zero-field-cooled, susceptibility measurements indicated superparamagnetic behavior, which is analyzed by accounting for the size dispersion of the nanoparticles. Results showed that the magnetic properties of the specimen can be tailored by implantation conditions. Annealing in vacuum at 900°C for up to 10h leads to an increase of the average nanoparticle size and a broader size distribution. Mie scattering a...

Science and Application of Oxyorthosilicate Nanophosphors

Nanophosphor Y2SiO5:Ce (n-YSO), Lu2SiO5:Ce (n-LSO), and Gd2SiO5:Ce (n-GSO) were prepared by solution-combustion synthesis yielding nanophosphor crystallite sizes between 20 nm - 80 nm. Ce dopant concentrations were varied between 0.1%-10% for each the nanophosphors and concentration quenching curves were measured by radioluminescence (RL) and photoluminescence (PL). n-YSO exhibits concentration quenching at 1 at% and 4 at% under UV and X-ray excitation, respectively. Red shifted emission with a larger Stokes shift is observed for nanophosphors as compared to bulk crystals. The measured PL lifetime depended on the refractive index of the media, indicating that the PL originates from the surface. Measurements of the RL/PL intensity indicate that the light output of these materials is comparable to the bulk crystal.

Quantum confinement contribution to porous silicon photoluminescence spectra

Photoluminescence (PL) spectra of freshly etched and ambient-aged porous silicon specimens have been measured and analyzed to extract spectral contributions due to quantum confinement (QC) and non-QC effects. It is shown that all spectra can be deconvolved into five Gaussian bands with typical peak energies 1.59, 1.76, 1.84, 1.94, and 2.07 eV. Ambient aging induces ∼0.1 eV blueshift in each of the three highest energy peaks, which is attributed to QC effects. In contrast, the two lowest energy peaks remain unshifted as expected for non-QC effects. Because size of the porous silicon nanoparticles is deduced from the magnitude of blueshift, it is imperative to correctly identify the spectral components associated solely with QC. The three closely spaced Gaussian bands are summed and the resulting single Gaussian band is analyzed with the model of Islam and Kumar [J. Appl. Phys. 93, 1753 (2003)]. Results show that peak energy of the freshly etched sample is 1.86 eV with average nanoparticle size L0=3.90 nm a...

Hard amorphous carbon–fluorine films deposited by PECVD using C2H2–CF4 gas mixtures as precursor atmospheres

Abstract An investigation of the structural arrangement and mechanical properties of a-C:F:H films obtained by plasma enhanced chemical vapor deposition (PECVD) using C 2 H 2 –CF 4 gas mixtures as precursor atmospheres was carried out. The results indicate that fluorine incorporation increases the hydrophobicity of the films and relaxes the internal stress, while reducing the hardness also. It was shown that it is possible to obtain fluorinated carbon films with hydrophobicity comparable to that of polytetrafluoroethylene (TFE) but with much higher hardness, that is in the range of a few GPa.

Investigation on the chemical, structural and mechanical properties of carbon-germanium films deposited by dc-magnetron sputtering

Abstract Amorphous carbon-germanium films were deposited by de-magnetron sputtering onto Si substrates leading to a set of films with Ge/C ratios ranging from 0 to 2. Nuclear techniques, Rutherford backscattering spectrometry and elastic recoil detection analysis, provided both the composition and the atomic density of the films. Raman results suggest that carbon and germanium atoms are segregated into distinct amorphous domains. X-ray diffraction analysis was also performed and the results confirmed the amorphous character of the films, already determined by Raman spectroscopy. The internal stress of the films was obtained by measuring the bending of the substrates and the hardness was measured by a nanoindentation technique. These mechanical properties were correlated to the Ge content of the films; an important reduction in both hardness and internal stress with the Ge incorporation was observed.

The role of trapped Ar atoms in the mechanical properties of boron carbide films deposited by dc-magnetron sputtering

Boron carbide films were deposited by dc-magnetron sputtering and their composition, chemical bonding and mechanical properties were investigated as a function of the substrate bias voltage. The films are stoichiometric within the entire range of substrate bias voltages employed as determined by Rutherford backscattering spectrometry. As the substrate bias voltage increases, higher amounts of trapped Ar atoms are found in the films. A decrease of the number of B–C bonds is observed while the hardness decreases and the compressive stress increases. It is suggested that the trapped Ar atoms are in substitutional sites and act to inhibit chemical bonds within the amorphous network which leads to softer films. The trapped Ar atoms also induce local expansion of the amorphous network and a correlation between the amount of trapped Ar atoms and the magnitude of the compressive internal stress is suggested.

Plasma deposition of amorphous carbon films from CH4 atmospheres highly diluted in Ar

Abstract The deposition, structure and mechanical properties of hydrogenated amorphous carbon films grown in highly Ar-diluted CH4 atmospheres were investigated for a total pressure of 13 Pa. Films were investigated as a function of the self-bias voltage between −50 and −500 V for two extreme CH4 partial pressures, 2 and 100%. For the self-bias voltage that optimizes the diamond-like properties of the films, −350 V, we carried out an investigation as a function of the Ar partial pressure, which ranged from 0 to 99%. The deposition rate and the hydrogen content decreased with progressive Ar dilution. The density and the compressive internal stress are nearly constant. The hardness decreased for Ar-rich precursor atmospheres. The surface roughness was independent of the CH4 partial pressure.