HYBRID WHITE LIGHT-EMITTING DIODES: STUDY AND FABRICATION OF THIN-FILM AND NANOWIRE-BASED DEVICES

I diodi ad emissione luminosa sono dei dispositivi a semiconduttore che posseggono straordinarie proprietà fisiche di generazione della luce. Il primo LED bianco si è mosso silenziosamente dalla metà degli anni 90 fino a un punto in cui, oggi, il mercato dell’illuminazione è stato totalmente rivoluzionato. A fronte dell’industria dell’illuminazione a stato solido che richiede LED sempre più luminosi ed economici, la ricerca scientifica risponde con strutture innovative, nuovi materiali più efficienti, anche combinati con l’uso delle nanotecnologie applicate. I diodi bianchi ibridi ad emissione luminosa, ad esempio, fanno anch’essi uso di un LED blu, come in un tradizionale diodo bianco, ma in essi il materiale inorganico convenzionale per la conversione del colore è sostituito da un colorante organico cha una maggiore resa quantica. La combinazione di una sorgente blu nanostrutturata con tali coloranti costituisce un’interessante struttura non ancora esaustivamente studiata in letteratura. In questo lavoro, due strutture per diodi bianchi ibridi innovativi sono state progettate, fabbricate e caratterizzate. La prima consiste in un dispositivo a eterogiunzione ZnO/GaN, basato su nanofili di ossido di zinco, ottenuto con una tecnica di crescita in soluzione acquosa a basso costo; la conversione del colore, ottenuta tramite un colorante organico giallo a base di perilene, ha portato alla realizzazione di un LED ibrido bianco a luce fredda. Il secondo dispositivo ha sfruttato al massimo le proprietà dello strato organico di conversione del colore, facendo uso di una sorgente blu di pompa basata su una struttura GaN/InGaN ad alta luminosità. Inoltre, combinando opportunamente coloranti organici gialli e rossi, è possibile ottenere il controllo fine sul colore di emissione della struttura ibrida. Infine, viene presentato uno studio dettagliato sulla stabilità dello strato di conversione, insieme ad alcune soluzioni che possono essere adottate per ottenere le migliori prestazioni dal diodo ibrido bianco.Light emitting diodes are very compact and durable semiconductor devices that possess extraordinary physical properties for the generation of light. The white LED quietly moved from the middle of the 1990s to a point where, today, the lighting market is completely revolutionized. With the industry of solid state lighting demanding for always brighter and economic LEDs, the scientific research responds with innovative layouts and more efficient materials also mixed with applied nanotechnologies. Hybrid white light emitting diodes, for example, also use blue LEDs but replace the conventional inorganic material for color conversion with an organic dye that has a higher quantum conversion yield. The combination of a nanostructured blue light source with organic dyes can lead to another interesting structure that it is worth investigating on. In this work, two layouts for novel hybrid white light emitting diodes were designed, fabricated and characterized. The first one consisted in a heterojunction ZnO/GaN nanowire-based hybrid device that showed an easy processing with a low-cost acqueous solution growth technique; the color conversion achieved with an organic perylene-based yellow dye coating led to the realization of a cold white hybrid LED. The second device exploited the properties of the organic conversion layer at the maximum of its capabilities with the use of an high-brightness GaN/InGaN blue pump source. Moreover, mixing yellow and red organic dyes resulted in the color tuning of the hybrid structure’s emission. Finally, a detailed study of the conversion layer stability was also reported together with the solutions that can be adopted to obtain the best performances out of the hybrid white light emitting diode

Biofilm-induced bioclogging produces sharp interfaces in hyporheic flow, redox conditions, and microbial community structure

Riverbed sediments host important biogeochemical processes that play a key role in nutrient dynamics. Sedimentary nutrient transformations are mediated by bacteria in the form of attached biofilms. The influence of microbial metabolic activity on the hydrochemical conditions within the hyporheic zone is poorly understood. We present a hydrobiogeochemical model to assess how the growth of heterotrophic and autotrophic biomass affects the transport and transformation of dissolved nitrogen compounds in bedform-induced hyporheic zones. Coupling between hyporheic exchange, nitrogen metabolism, and biomass growth leads to an equilibrium between permeability reduction and microbial metabolism that yields shallow hyporheic flows in a region with low permeability and high rates of microbial metabolism near the stream-sediment interface. The results show that the bioclogging caused by microbial growth can constrain rates and patterns of hyporheic fluxes and microbial transformation rate in many streams

Stability improvement of PMMA and Lumogen® coatings for hybrid white LEDs

Hybrid 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

Progress in Violet Light-Emitting Diodes Based on ZnO/GaN Heterojunction

Progress in light-emitting diodes (LEDs) based on ZnO/GaN heterojunctions has run into several obstacles during the last twenty years. While both the energy bandgap and lattice parameter of the two semiconductors are favorable to the development of such devices, other features related to the electrical and structural properties of the GaN layer prevent an efficient radiative recombination. This work illustrates some advances made on ZnO/GaN-based LEDs, by using high-thickness GaN layers for the p-region of the device and an ad hoc device topology. Heterojunction LEDs consist of a quasicoalesced non-intentionally doped ZnO nanorod layer deposited by chemical bath deposition onto a metal–organic vapor-phase epitaxy -grown epitaxial layer of p-doped GaN. Circular 200 μm-sized violet-emitting LEDs with a p-n contact distance as low as 3 μm exhibit a turn-on voltage of 3 V, and an emitting optical power at 395 nm of a few microwatts. Electroluminescence spectrum investigation shows that the emissive process can be ascribed to four different recombination transitions, dominated by the electron-hole recombinations on the ZnO side

Assessment of the Treatment Performance of an Open-Air Green Wall Fed with Graywater under Winter Conditions

Graywater (GW), i.e., the portion of household wastewater that excludes toilet flushes, is an interesting wastewater type because it requires only mild treatment. Green walls have been proposed as example of a nature-based solution for GW treatment due to low energy requirement and high ecological/societal benefits; however, indications about their treatment performances remain limited. This work presents experimental results of a laboratory modular green wall for GW treatment. Experiments have been performed outdoors during the winter season for three months. Each panel included four vertical columns of planted pots, and it was fed with 100 L of synthetic GW per day. Removal efficiencies were as follows (average values): 40% chemical oxygen demand, 97% biochemical oxygen demand, 61% total Kjeldhal nitrogen, 56% NO3–-N, 57% total phosphorus, 99% Escherichia coli, and 63% anionic surfactants. This work proved the potential of an open-air green wall for treating GW, even under challenging conditions for biological treatment processes and with high hydraulic loading rates

Evaluation of the influence of filter medium composition on treatment performances in an open-air green wall fed with greywater.

Abstract According to the European Research and Innovation Policy Agenda, nature-based solutions (NBSs) are key technologies to improve the sustainability of urban areas. Among NBSs, green walls have been recently studied for several applications, among the others the treatment of lowly polluted wastewater flows as greywater (GW, e.g. domestic wastewater excluding toilet flushes). This work is aimed at the evaluation of the influence of four additives (compost, biochar, granular activated carbon, polyacrylate) mixed with a base filter medium made of coconut fibre and perlite, on the performances of a green wall fed in batch mode with synthetic GW. The green wall was operated with a high hydraulic loading rate of GW (740.8 L/m2/day) in open-air winter conditions (3.5–15 °C measured for GW) between January and April. The performances of the green wall have been assessed though the monitoring every 1–2 weeks of physicochemical and biological parameters (pH, electric conductivity, total suspended solids, dissolved oxygen, BOD5 and COD, nitrogen and phosporus compounds, chlorides and sulphates, anionic surfactants and E. coli). Removal performances were excellent for BOD5 (>95%) and E.coli (>98%) for all additives; compared to the base medium, biochar was the best performing additive over the highest number of parameters, achieving removals equal to 51% for COD, 47% for TKN and nitric nitrogen and 71% for anionic surfactants. Compost also achieved high removal performances, but the frequent clogging events occurred during the monitoring period do not make its use recommendable. Granular activated carbon and the combination of biochar and polyacrylate performed better than the base medium, but only about the removal of nitric nitrogen. These results demonstrated that, in the considered experimental boundaries, biochar could improve the overall treatment performances of a green wall fed by GW and operated in challenging conditions

A review of nature-based solutions for greywater treatment: Applications, hydraulic design, and environmental benefits

Abstract Recognizing greywater as a relevant secondary source of water and nutrients represents an important chance for the sustainable management of water resource. In the last two decades, many studies analysed the environmental, economic, and energetic benefits of the reuse of greywater treated by nature-based solutions (NBS). This work reviews existing case studies of traditional constructed wetlands and new integrated technologies (e.g., green roofs and green walls) for greywater treatment and reuse, with a specific focus on their treatment performance as a function of hydraulic operating parameters. The aim of this work is to understand if the application of NBS can represent a valid alternative to conventional treatment technologies, providing quantitative indications for their design. Specifically, indications concerning threshold values of hydraulic design parameters to guarantee high removal performance are suggested. Finally, the existing literature on life cycle analysis of NBS for greywater treatment has been examined, confirming the provided environmental benefits

Effects of biophysical stimulation in patients undergoing arthroscopic reconstruction of anterior cruciate ligament: prospective, randomized and double blind study

Pre-clinical studies have shown that treatment by pulsed electromagnetic fields (PEMFs) can limit the catabolic effects of pro-inflammatory cytokines on articular cartilage and favour the anabolic activity of the chondrocytes. Anterior cruciate ligament (ACL) reconstruction is usually performed by arthroscopic procedure that, even if minimally invasive, may elicit an inflammatory joint reaction detrimental to articular cartilage. In this study the effect of I-ONE PEMFs treatment in patients undergoing ACL reconstruction was investigated. The study end-points were (1) evaluation of patients’ functional recovery by International Knee Documentation Committee (IKDC) Form; (2) use of non-steroidal anti-inflammatory drugs (NSAIDs), necessary to control joint pain and inflammation. The study design was prospective, randomized and double blind. Sixty-nine patients were included in the study at baseline. Follow-up visits were scheduled at 30, 60 and 180 days, followed by 2-year follow-up interview. Patients were evaluated by IKDC Form and were asked to report on the use of NSAIDs. Patients were randomized to active or placebo treatments; active device generated a magnetic field of 1.5 mT at 75 Hz. Patients were instructed to use the stimulator (I-ONE) for 4 h per day for 60 days. All patients underwent ACL reconstruction with use of quadruple hamstrings semitendinosus and gracilis technique. At baseline there were no differences in the IKDC scores between the two groups. At follow-up visits the SF-36 Health Survey score showed a statistically significant faster recovery in the group of patients treated with I-ONE stimulator (P < 0.05). NSAIDs use was less frequent among active patients than controls (P < 0.05). Joint swelling resolution and return to normal range of motion occurred faster in the active treated group (P < 0.05) too. The 2-year follow-up did not shown statistically significant difference between the two groups. Furthermore for longitudinal analysis the generalized linear mixed effects model was applied to calculate the group × time interaction coefficient; this interaction showed a significant difference (P < 0.0001) between the active and placebo groups for all investigated variables: SF-36 Health Survey, IKDC Subjective Knee Evaluation and VAS. Twenty-nine patients (15 in the active group; 14 in the placebo group) underwent both ACL reconstruction and meniscectomy; when they were analysed separately the differences in SF-36 Health Survey scores between the two groups were larger then what observed in the whole study group (P < 0.05). The results of this study show that patient’s functional recovery occurs earlier in the active group. No side effects were observed and the treatment was well tolerated. The use of I-ONE should always be considered after ACL reconstruction, particularly in professional athletes, to shorten the recovery time, to limit joint inflammatory reaction and its catabolic effects on articular cartilage and ultimately for joint preservation