1,721,077 research outputs found
Photluminescence properties of orgainic-inorganic sol-gel films doped with semiconductor quantum dots
No full textHybrid organic-inorganic thin films doped with lead sulfide nanocrystallites were synthesized by a combination of colloidal chemistry and sol-gel processing. In order to study the influence on the spectroscopic properties of the crystallite surface, and hence the related defect states, PbS-doped films with different sulfur to lead ratios and different surface capping agent concentrations were fabricated. X-ray diffraction measurements showed the presence of nanoparticles with a mean diameter raging from 3 to 5 nm. The absorption spectra showed a large blue shift of the absorption edge to shorter wavelength, indicating strong quantum confinement. Strong photoluminescence emission in the near infrared was found by pumping at 514 or 532 nm. The emission intensity and its position were found to be dependent on the elaboration parameters. The film fabrication process influences only slightly the good particle size distribution of the colloidal PbS solutions. Best results were obtained for films with low sulfur to lead ratio or with high capping agent concentration
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Sol-gel films doped with semiconductor nanocrystals for optical applications
No full textSol-gel films doped with MeS (Me=Cd or Pb) nanocrystals or with MxOy (M=Ni or Co) nanocrystals have been fabricated. Two different strategies were used: for the sulfides a colloidal doping sol was prepared at room temperature and then mixed with the matrix sol. For the oxides a bifunctional ligand was used, bearing either an amine group capable of coordinating the M ions and hydrolysable siloxane groups for anchoring the metal complex moiety to the silicate matrix. In this case the oxide nanocrystals precipitate at 500°C. Films containing MeS nanocrystals showed both non-linear and active optical properties. Nanocomposite films doped with MxOy showed optical gas sensing properties
Nonlinear charcterization of PbS nanocrystals embedded in sol-gel planar waveguides
No full textThin silica-titania planar waveguides doped with different concentrations of lead sulfide (10 - 25 mol%) have been prepared by a sol-gel process. It consist of three steps: (a) preparation of a colloidal sol of semiconductor particles; (b) preparation of an alkoxide solution, precursor of the glass- like matrix; (c) mixing of the colloidal sol and the alkoxide solution. Films were deposited on fused silica by dipping and densified for 1 h at 300 degrees Celsius in nitrogen. The mean particle size is 3 nm and the optical absorption edge is situated around 1100 nm. The nonlinear properties have been investigated using degenerated four wave mixing (DFWM) an a nonlinear m-line technique. Different Nd:YAG lasers with pico- and nanosecond pulses at 1064 nm have been used. Depending on the PbS concentration we measured a high negative nonlinear refractive index of n2 equals -3 to -9 10-8 cm2kW for nanosecond pulses (m-lines) and -1 to -10 10-10 cm2/kW for picosecond pulses (m-lines and DFWM). The response time of the nonlinearity is below 30 ps. All observed nonlinear effects are fully reversible and we did not observe any photodarkening. Straight, monomode channel waveguides have been fabricated on theses films
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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