1,721,068 research outputs found
Generation of white LED light by frequency downconversion using a perylene-based dye
No full textA high efficiency white light emitting diode (LED) was fabricated by generation of frequency down-conversion from a GaN/InGaN blue LED. In place of conventional inorganic phosphors, a perylene-based dye was used for colour conversion. The resulting hybrid structure is analysed by focusing on the visual performance of the realised LEDs
employing the most relevant photometric parameters of a light source.
Preparation of the organic polymer is described as well. The thermal
stability of the dye was investigated and a simple structure which avoids
colour degradation is proposed
Well-aligned hydrothermally synthesized zinc oxide nanorods on p-GaN without a seed layer
No full textZinc oxide nanorods have great potential for the realization of high efficiency heterostructure LEDs based on pdoped gallium nitride. In order to obtain a good confinement of the light, a well-aligned nanorod waveguiding structure is desirable. This paper reports on the fabrication of vertical zinc oxide nanorods using a solution-based growth process that does not require a seed layer. The nanorods obtained follow the crystalline growth direction of the GaN layer along the c-axis. Various results with different reagent concentrations are reported
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
Hybrid LEDs pave way to new lighting applications
Full text linkMany analysts agree that the global lighting market is close to a real revolution: the LED revolution. One of the most widespread solutions for the production of white LEDs is the conversion of a part of the light, coming from a known source, by exciting one or more materials that emit at a longer wavelength. The result is an emission spectrum given by the superposition of the single source and the photoexcited material; the big advantage is that the phenomenon of photoluminescence replaces the further integration of other solid-state devices of different colors. The photoluminescence of a perylene-based polymer dye turned out to be a good substitute for conventional inorganic color conversion. The dye used was Lumogen, a product commercialized by BASF and often employed in graphic arts; it is available in a large palette of colors from the UV to the IR at the same low cost
Warm white LEDs based on Lumogen® Red and Yellow
No full textOne of the most widespread solutions for the production of white LEDs is the frequency downconversion of a part of the light, coming from a blue source, by exciting one or more materials
(typically Ce:YAG) that emit at longer wavelength [1]. In this work we report ona simple and less
expensive method to fabricate warmwhite-light LEDsusingthe photoluminescence of Lumogen®,
a perylene-based polymer dyecommercialized by BASF,that has already beenprovedto be a good
substitute for conventi onal inorganic colour conversion [2],[3].
Standard InGaN-based blue LEDs (~ 450 nm) were fabricated on a sapphire substrate by metal organic chemical vapour deposition. Both Lumogen® Yellow, and Red dyes weredissolved in a
poly-methyl-methacrylate (PMMA) solution, which was in turn dissolved in ethyl acetate (16%
w/w concentration). The bare side of the sapphire substrate was coated with the dye solution by
dippingat different extraction speed,so that different coating thicknesses couldbe obtained.
Dipping solutions were prepared by mixing the content of yellow and red dyes, with Lumogen®
Yellow concentration fixed at0.5 mg/mL and Lumogen® Red in a concentration variable from 0.05
to 0.5 mg/mL. In the experiments here reported, five different solutions were used,each with the
following couples of concentrations of yellow and red dyes in PMMA (mg/mL of Lumogen®
Yellow; mg/mL of Lumogen® Red):solution YELL (0.5 ; 0), solution MIX1 (0.5 ; 0.05), solution
MIX2 (0.5 ; 0.1), solution MIX3 (0.5 ; 0.2), solution MIX4 (0.5 ; 0.5).
The wafer was three-times dipped, the first two times in the solution of Lumogen® Yellow (0.5
mg/mL), and finally in a sol ution of mixed dyes (Lumogen® Yellow and Lumogen® Red in
different proportions). The second layer was rapidly coated above the first one, after thishad been
completely dried, in order to minimize the partial dissolution of the first layer during the second
dipping.The same procedure was carried out for the third layer.Four samples were prepared, the
first and the second coating being prepared by dipping the wafer in thesolution YELL, the thirdone
by dipping on different solutionsas following: Sample W1, dipped in MIX1; sample W 2, dipped in
MIX2; sample W3, dipped in MIX3; sample W 4, dipped in MIX4.
As shown both by the chromaticity diagram (Fig. 1), and the emission spectra (Fig. 2), with the
increase of red dye concentration, the emitted white light (at 20 mA of driving current) became
warmer and the chromatic coordinates moved awayfrom the blue region.
The data reporting the correlated colour temperature and colour rendering index (CRI)are shown in
Table 1. Withthe increaseof the red content, the correlated color temperaturedecreased while the
CRI increased. In particular, sample W4 emitteda warm white light with a high CRI (87.2) and a
low correlated color temperature (3912 K). The chromatic coordinates (x; y) were(0.381 ; 0.369),
very close to the Planckian locus, as provedby the high value of CRI.
This work showed that Lumogen® Yellow and Lumogen® Red can be advantageously used to set
up a simple and cheap technique for fabricating warm white LEDs. In particular, warm white LED
light with high values of CRI and low correlated color temperature was obtained and the results
were reported
Stability improvement of PMMA and Lumogen® coatings for hybrid white LEDs
Full text linkHybrid white LEDs employing perylene-based dyes for the frequency down-conversion of blue light, generated by a standard inorganic source, suffer from colour rendering variations due to the degradation of the organic molecule under prolonged irradiation. To avoid such inconvenient, proper encapsulation of the dyes in resins or other polymer matrices can prevent their accelerated ageing; nevertheless, embedding polymers can also exhibit significant bleaching caused by chemico-physical agents. Among all, polymethyl methacrilate (PMMA) is one of the most used materials for the fabrication of hybrid LEDs' colour conversion coatings, therefore its stability needs to be investigated
Warm white LED light by frequency down- conversion of mixed perylene-based dyes
No full textThe growing demand of the solid-state lighting market for the development of sources for illumination has led to the fabrication of the first white LED in 1997, which employed a blue LED coated by a Ce:YAG phosphor to mix the down-converted yellow light with the blue one. The white light appears cold due to the weakness of red components in the emission spectrum. In order to obtain a warmer white, one possible solution is to add a red phosphor to the yellow one to move the chromatic coordinates properly, though the luminous efficiency drastically decreases due to the increased light absorption of the coating layer. It is generally believed that the low efficacy of warm white LEDs is the main issue today.
Using photoluminescence of Lumogen® F Yellow 083 (BASF), a perylene-based polymer dye, we obtained a high efficiency cold white LED by generation of yellow down-conversion from a standard 450 nm GaN/InGaN blue LED with record values of 9.37 lm of
luminous flux and 118.23 lm/W of luminous efficiency. The intense cold white light turned warmer, by adding a small quantity of another perylene-based dye, Lumogen® F Red 305 (BASF). Different weight proportions of dyes were dissolved in solutions with equal amounts of poly-methyl-methacrylate (PMMA) in ethyl acetate. Finally, the LEDs were dip-coated in each solution and optically characterised.
Adding 2 mg of red dye to 10 mL of PMMA solution with 5 mg of yellow dye, the light turned into a purplish white with a dramatic decrease of the luminous efficiency (46.72 lm/W). Decreasing the
amount of red dye to 0.5 mg, the white light appeared warm with a negligible decrease of the efficiency (116.11 lm/W) and a luminous flux of 8.03 l
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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