169,728 research outputs found
Carbon nanotubes for solar energy conversion
No full textE’ stato dimostrato che la nanotecnologia ed i nanomateriali possono offrire valide soluzioni per migliorare l’efficienza di conversione sfruttando gli effetti indotti a scala mesoscopica. In questo ambito di ricerche, l’attività sui nano tubi di carbonio CNT come elementi costituenti di dispositivi di conversione dell’energia solare è in grande sviluppo. Il lavoro presentato in questa tesi è stato dedicato all’investigazione dei meccanismi fisici alla base della foto-risposta dei nanotubi di carbonio a multi parete (MWCNT) quando vengono utilizzati in celle elettrochimiche e in dispositivi a stato solido. Larga parte dell’attività è consistita nello sviluppo di strategie per la crescita controllata dei MWCNT.
MWCNTs sono stati sintetizzati mediante CVD termico variando i parametri di crescita sperimentali. Utilizzando tecniche di caratterizzazione morfologica e strutturale, è stato possibile fissare i parametri di sintesi appropriati per ottenere strutture di MWCNT controllate su specifici substrati.
Esperimenti di generazione di corrente fotochimica da parte dei MWCNT cresciuti su substrati SiO2/Si sono stati effettuati così come misure di fotoconduttività a stato solido. Tali esperimenti hanno confermato la capacità dei MWCNT di generare una corrente elettronica sotto illuminazione, nonostante la loro stretta similarità con la grafite metallica. Spettri EELS mostrano una spalla ad energie comprese tra 2 - 4 eV, sotto il tipico plasmone π -π* della grafite altamente orientata. Questa può essere associata, in analogia con i nano tubi a singola parete SWNT, alla presenza di singolarità di Van Hove nella densità degli stati elettronici. Tali transizioni permettono la generazione sotto illuminazione di un eccitone. L’elettrolita all’interno della cella chimica e/o la presenza di giunzioni Schottky tra MWCNT differenti separano le cariche.
I risultati di fotoconducibilità a stato solido sono stati confrontati con quelli ottenuti negli esperimenti fotochimici, e quelli riportati in letteratura. Sulla base di calcoli teorici della DOS di nanotubi multi pareti (4WCNT), è possibile affermare che i MWCNT possono creare delle giunzioni locali p-n e/o Schottky con il substrato di silicio, dando luogo ad una estesa regione di svuotamento. Questo implica una modifica delle caratteristiche degli spettri di foto corrente attesi per i MWCNT.
MWCNT sono stati decorati con nanoparticelle di rame e la loro foto risposta è stata studiata e confrontata con quella dei soli nano tubi. L’efficienza quantica dei sistemi ibridi aumenta su tutto lo spettro e due modelli sono proposti per spiegare questo risultato. Infine una prima misura sulle prestazioni di un dispositivo MWCNTs/SiO2/Si come cella fotovoltaica è stata effettuata mediante un simulatore solare. La caratteristica I-V sotto illuminazione non è la ben nota curva aspettata per le celle PV, ma mostra come dispositivi a stato solido basati sui MWCNT possano operare come generatore di potenza attivo sotto illuminazione solare.It has been demonstrated that nanotechnology and nanomaterials could offer valid solutions to improve the conversion efficiency by exploiting effects induced at mesoscopic scales. In this research field, the activity on carbon nanotubes CNTs as building blocks for solar energy conversion devices is developing. The work presented in this thesis has been devoted to investigate the physical mechanisms underlying the photoresponse generation of multiwall carbon nanotubes (MWCNTs) when exploited in electrochemical cells and solid-state devices. A large part of the activity has consisted of the development of suitable strategies for the controlled growth of MWCNTs.
MWCNTs have been synthesised by thermal CVD varying the experimental growth parameters. By using morphological and structural characterization techniques, it has been possible to fix the proper synthesis parameters to obtain controlled MWCNTs structures on specific substrates.
Experiments of photochemical current generation by MWCNTs grown on SiO2/Si substrates have been performed as well as solid-state photoconductivity measurements. They have confirmed the capability of MWCNTs to generate a current of electrons under light irradiation, despite their close similarity to metallic graphite. EELS spectra show a shoulder at energies 2-4 eV, below the typical plasmon π -π* peak for HOPG. That can be associated, similarly to SWCNTs, to the presence of van Hove singularities in electronic density of states. These transitions allow the generation upon illumination of an exciton. The electrolyte inside the chemical cell and/or the presence of localized Schottky junctions between different MWCNTs separates the charges.
The solid-state photoconductivity results have been compared with those obtained in photochemical experiments, and with other described in literature. On the basis of theoretical calculations of DOS for a specific four-wall carbon nanotube (4WCNT), it is possible to affirm that MWCNTs are able to locally create p-n and/or Schottky junctions with the silicon substrate underneath, giving rise to an extended depletion layer. This implies a modification of the feature in the expected photocurrent spectrum of MWCNTs.
MWCNTs have been decorated by Cu nanoparticles and their photoresponse has been studied and compared with that of bare nanotubes. The quantum efficiency of the hybrid system increases over the entire spectrum and two models have been proposed to explain these results. Finally, a first measurement of the performance of a MWCNTs/SiO2/Si device as PV cell has been performed by using a solar simulator. The I-V characteristic under illumination is not the expected well-shaped curve of a PV cell, but it demonstrates that the MWCNT solid-state device can operate as an active power supply element under solar illumination
Il ruolo del welfare civile nel welfare mix. Bisogni non evasi dal welfare pubblico. Rapporto di ricerca relativo ad un progetto svolto in collaborazione fra l’Università Ca’ Foscari Venezia – Dipartimento di Economia e Direzione Aziendale e la CCIAA di Treviso
No full textRole of growth temperature on nanostructure and field emission properties of PLD thin carbon films
No full textThin carbon films have been deposited in vacuum
(∼10−4 Pa) on Si substrates by pulsed laser ablation of a
graphite target using a Nd:YAG laser operating in the near
infrared region (λ = 1064 nm). The samples have been deposited
at different substrate temperatures (T sub) ranging
from room temperature (RT) to 800°C. X-ray diffraction
analysis established the progressive formation of nanosized
graphene structures as T sub increased. In fact, film structure
evolves from almost amorphous to nanostructured phase
characterized by graphene layers oriented perpendicularly to
the film plane. The film density, evaluated by X-ray reflectivity
measurements, is strongly affected by T sub. At RT the
film density is similar to the graphite one, while it decreases
at higher T sub. The electrical properties of the samples have
been characterized by field emission measurements. The parameters
describing the emitter properties (threshold field
Eth and field enhancement factor β) have been evaluated
using variable anode-to-cathode distance method. Samples
deposited at low T sub have shown the best emission properties,
presenting lower Eth and larger β values than those
deposited at higher T sub. This is mainly attributed to thesensible density variation, which is in competition with the
slighter augment of mean nanoparticle size
Comparison of the local order in highly oriented pyrolitic graphite and bundles of single-wall carbon nanotubes by nanoscale extended energy loss spectra
No full textWe compare the local order in single-wall carbon nanotubes (SWCNTs) and highly oriented pyrolitic graphite (HOPG) by means of nanoscale transmission extended energy loss fine structure (EXELFS) measurements above the carbon K edge. The HOPG EXELFS spectra and their Fourier transform were compared to their synchrotron extended X-ray absorption fine structure (EXAFS) counterpart and discussed within a multiple scattering framework. By comparing the experimental EXELFS data to spectra calculated using a theoretical model based on a single scattering approach, we showed the fundamental importance of considering multiple paths (involving up to eight body scattering) to reproduce the fine details of EXELFS features. Simulating EXELFS spectra of SWCNTs by our theoretical model is shown to represent a measure of their chiralities. Finally, the shrinkage of the nearest-neighbor distance in the Fourier transform observed for SWCNTs (absent in graphite) is interpreted within a simple model invoking anharmonic effects
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe 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
Photon harvesting with multi wall carbon nanotubes
No full textIn this work we show that a sizeable enhancement in the photocurrent over the visible and near ultraviolet energy range can be obtained from multi-wall carbon nanotubes when decorated with Cu-nanoparticles. The result has been obtained both from electrochemical and solid state cells measurements. The photoactive metal nanoparticles do not change the intrinsic ability of multi-wall carbon nanotubes to behave as an efficient low dimensional media for generating e-h carriers. This result further confirms that multi wall carbon nanotubes can be used in photovoltaic nano-devices and solar energy conversion applications. (C) 2009 Elsevier Ltd. All rights reserved
Multiwall carbon nanotubes decorated with copper nanoparticles: Effect on the photocurrent response
No full textPhotocurrent generation measured with an electrochemical cell on carbon nanotubes has been obtained mainly from single-walled nanostructures. In fact, multiwall carbon nanotubes (MWCNTs) were not expected to show analogous low-dimensional effects, due to their close similarity to metallic graphite. Recently we reported on the ability of MWCNTs to generate photocurrent in the visible and ultraviolet spectral range. In this paper we show a significant enhancement in the photocurrent generation when the walls of the MWCNTs were decorated with dispersed Cu nanoparticles. This result is of particular relevance for photovoltaic nanodevices and solar energy conversion applications
Mitomycin C in highly myopic eyes - Author reply
No full textOphthalmology. 2005 Feb;112(2):208-18; discussion 219.
Mitomycin C modulation of corneal wound healing after photorefractive keratectomy in highly myopic eyes.
Gambato C, Ghirlando A, Moretto E, Busato F, Midena E.
SourceRefractive Surgery Service and Antimetabolite Therapy Research Unit, Department of Ophthalmology, University of Padova, Padova, Italy.
Abstract
PURPOSE: To evaluate the role of topical mitomycin C in corneal wound healing (CWH) after photorefractive keratectomy (PRK) in highly myopic eyes.
DESIGN: Prospective, double-masked, randomized clinical trial.
PARTICIPANTS: Seventy-two eyes of 36 patients affected by high (>7 diopters) myopia.
METHODS: In each patient, one eye was randomly assigned to PRK with intraoperative topical 0.02% mitomycin C application, and the fellow eye was treated with a placebo. Postoperatively, mitomycin C-treated eyes received artificial tears (3 times daily, tapered in 3 months), whereas the fellow eye was treated with fluorometholone sodium 2% and artificial tears (3 times daily, tapered in 3 months).
MAIN OUTCOME MEASURES: Uncorrected visual acuity (UCVA) and best-corrected visual acuity (BCVA), contrast sensitivity, manifest refraction, and biomicroscopy. Contrast sensitivity was determined using the Pelli-Robson chart. Corneal confocal microscopy documented CWH.
RESULTS: Mean follow-up was 18 months (range, 12-36). No side effects or toxic effects were documented. At 12-month follow-up examination, UCVAs (logarithm of the minimum angle of resolution) were 0.4+/-0.48 and 0.5+/-0.53 (P = .03) in mitomycin C-treated eyes and corticosteroid-treated eyes, respectively. At 1 year, corneal haze developed in 20% of corticosteroid-treated eyes, versus 0% of mitomycin C-treated eyes. At 12, 24, and 36 months, corneal confocal microscopy showed activated keratocytes and extracellular matrix significantly more evident in untreated eyes (Ps = 0.004, 0.024, and 0.046, respectively).
CONCLUSION: Topical intraoperative application of 0.02% mitomycin C can reduce haze formation in highly myopic eyes undergoing PRK.
Comment in
Ophthalmology. 2006 Feb;113(2):357; author reply 357-8
Dispelling the Myths Behind First-author Citation Counts
No full textWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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