1,721,075 research outputs found
La spettroscopia in riflettanza anisotropa (RAS) appliacta allo studio di film organici sottili e ultra-sottili
Full text linkLa tesi di dottorato che presento è il risultato di un lungo lavoro sviluppatosi nell’arco di tre anni. L’obiettivo è dimostrare le potenzialità della tecnica in Riflettanza Anisotropa (RAS, Reflectance Anisotropy Spectroscopy), quando applicata allo studio delle proprietà elettroniche e morfologiche di film sottili o ultra-sottili di materiali organici, cresciuti con differenti tecniche di deposizione. I vantaggi di una caratterizzazione ottica dei film organici erano noti da tempo: in questa tesi mostrerò come la RAS sia stata estremamente utile nello studio di film depositati con tecniche Langmuir-Blodgett (LB), LangmuirScheffer e in camere a crescita epitassiale (OMBE), aprendo la reale possibilità di uno studio in-situ ed in tempo reale del campione durante la crescita.
In questa tesi la RAS è stata applicata in modo nuovo ed originale allo studio di porfirine e metallo-porfirine, depositate con tecnica LB ed LS su diversi tipi di substrati: l’idea era quella di caratterizzare da un lato l’evoluzione del segnale ottico in funzione del ricoprimento, e dall’altro di monitorare eventuali cambiamenti durante l’esposizione dei film LB ad alcuni vapori di solventi organici. La RAS ha misurato per la prima volta piccole variazioni morfologiche che intervengono mano a mano che lo spessore del film aumenta e possibili nuove transizioni ottiche che si accendono a seguito di questi cambiamenti strutturali. La variazione del segnale di anisotropia in funzione dell’esposizione a vapori di analiti è stata studiata ed analizzata, per cercare così di capire in dettaglio i differenti meccanismi che intervengono durante l’interazione fra le molecole organiche del film e quelle del gas organico. Infine un modello interpretativo dell’andamento del segnale spettroscopico, in funzione di variazioni strutturali del film depositato, viene proposto all’attenzione del lettore. Lo studio è ancora lungi dall’essere concluso e nuove misure sono già in programma; nonostante
ciò i risultati ottenuti sono organizzati e commentati nel capitolo III della presente tesi
Stability of Organic Cations in Solution-Processed CH3NH3PbI3 Perovskites: Formation of Modified Surface Layers
Full text linkCalloni A, Abate A, Bussetti G, et al. Stability of Organic Cations in Solution-Processed CH NH PbI Perovskites: Formation of Modified Surface Layers. The Journal of Physical Chemistry C. 2015;119(37):21329-21335.We report on a combined X-ray and UV photoemission spectroscopy study (XPS and UPS) of organic–inorganic perovskites prepared from a solution of lead chloride (PbCl2) and methylammonium iodide (CH3NH3I). The XPS intensities are consistent with a pure iodide perovskite (CH3NH3PbI3), with no detectable chloride left. However, we found that the elimination of chloride results in residual methylamine molecules (CH3NH2) trapped within the perovskite crystal lattice. Furthermore, we show that vacuum annealing or sputtering induces the formation of a thin PbI2 layer at the crystal surface which acts as a surface barrier blocking electron transfer from the underlying perovskite film
Investigative photometry experiments on planar extended-light sources
Full text linkThe inverse-square decay law of the illuminance of a point light source with distance is a common notion of basic optics theory, which is readily demonstrated to be a direct consequence of the propagation of spherical wave fronts with the centre at the light source. It is far less common to address the experimental verification of this law and, even less, to study the illuminance decay with the distance of extended light sources, which somehow represent an unknown topic. We propose a scientific experiment where the light sensor of a smartphone is used to collect illuminance data as a function of the source-to-sensor distance and orientation. Through this procedure, students can realize the limit of validity of the inverse-square law and determine the luminance flux of the chosen point-like light source (e.g. the white LED flashlight of a smartphone). More interestingly, when dealing with extended sources (e.g. the LCD of a laptop displaying a white image) subtle characteristics of the decay trend emerge, particularly for distances lower that the source size. A detailed analysis of these characteristics is presented though a process allowing student engagement in a real scientific investigation, envisaging steps of data acquisition through experimental measurements, model construction on the basis of the observed patterns, and finally model testing. We provide a guided formulation for the general modelling of planar emitters, starting from the theoretical treatment of Lambertian sources. In this way, students are able to quantify the luminous emission also for extended sources and their deviation from a Lambertian behaviour
Macroscopic and microscopic electron transfer kinetics of HOPG and graphite intercalated compound investigated by cyclic voltammetry and SECM
Full text linkHighly oriented pyrolytic graphite (HOPG) is one of the most used host materials for obtaining and investigating graphite intercalated compounds, because of the high degree structural order of this polycrystal. Experiments on electrochemically intercalated HOPG in sulphuric acid have a model character, as the results obtained can be usefully generalised, not only with respect to other graphite compounds but also for the intercalation of other layered host lattices. In addition, the HOPG/H2SO4 system has an attractive potential for the possibility of electrochemically producing graphite oxide, ideally, by reversible oxidation/reduction cycles, which is of interest for energy storage and graphene production on an industrial scale. However, the oxidation/reduction cycles in such electrochemical intercalation process are not reversible and topotactic, so that the HOPG structure is considerably altered. This alteration may affect, for instance, the quality of the electrochemically produced graphene. In particular, the impact the electrochemical intercalation has on the conductivity of basal planes of HOPG, and so on graphene sheets, is still debated. In this work, we investigated both the macroscopic and microscopic electron transfer (ET) kinetics of the HOPG surface, before and after the intercalation of 1 M H2SO4 to obtain graphite intercalated compound, by using cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM), respectively. The heterogeneous kinetic constant (k0) of the HOPG was evaluated quantitatively by using the redox systems [Fe(CN)6]3–/4– and [Ru(NH3)6]3+/2+. The morphology of the samples was also investigated by atomic force microscopy (AFM), which revealed a widespread formation of blisters and precipitates during the HOPG intercalation process. The CV and SECM results indicate that, upon intercalation, the electrochemical behaviour of the HOPG changes sensibly and the ET decreases sensibly. However, this effect depends on the redox mediators employed and it results more dramatic for the [Fe(CN)6]3–/4– system, for which a decrease of k0 by orders of magnitude was obtained. The decrease of ET can be correlated to the blisters and precipitates, which occur during the HOPG intercalation, as observed by AFM
Uniaxial Alignment of a Monolayer of Flat-on Free-Base Porphyrins on an Exfoliable Insulating Substrate
No full textPorphyrins are an extremely valuable class of molecules engaged in a variety of roles spanning from biology to optoelectronics. Manipulation of the chemical and physical properties of the inner cavity of porphyrins has been recognized as crucial for the exploitation of these systems in organic devices, particularly when porphyrins self-organize at the interface with a flat-on orientation of the macrocycle. Such an orientation has been mostly observed on metallic surfaces. Unfortunately, the physical-chemical properties of the molecules result in being largely perturbed due to the molecule-metal interaction. In addition, conducting substrates are unsuited to exploit electrically driven devices based on organic layers. To overcome these issues, we performed a topology-based analysis of insulating organic single crystal structures to identify a surface which (i) ensures easy exfoliation through mechanical methods, (ii) ensures epitaxial match with an overlayer of close-packed flat-on porphyrin molecules, and (iii) displays chirality. The outcome of this work is represented by a unique crystal of mixed 2,5-diketopiperazine and fumaric acid in a 1:1 ratio. We demonstrate that the (110) surface of this crystal fulfills the aforementioned requirements and, thanks to its peculiar subnanometric corrugations, allows one to grow uniaxially aligned monolayers of flat-on porphyrin molecules assembled through van der Waals interactions
How does cobalt phosphate modify the structure of TiO2 nanotube array photoanodes for solar water splitting?
No full textTiO2 nanotube arrays (TNA) have been modified by cobalt phosphate (CoPi) through potentiostatic electrodeposition method. Different samples have been prepared by changing the loaded CoPi through the deposition time from 10 to 960 min. Formed catalytic materials have been characterized by different methods. Although charge transfer resistance of the CoPi/TNA photoanodes have been decreased from 5.5 to 4.0 kΩ by increasing the deposition time from 5 to 60 min, the maximum photoresponse was obtained for 10 min CoPi deposition leading to 24% more photocurrent compare to bare TNA which proposed optimum value for cobalt phosphate decoration. Based on field emission scanning electron microscopy (FESEM) and X-ray photoelectron spectroscopy (XPS) results, by depositing the electrocatalyst, nanotube walls have been spread and holes have been covered by CoPi. Correlation between observed change in surface morphology and Raman spectra of the samples along with electrocatalytic tests revealed how decoration time can influence structural properties and photoelectrochemical performance of the final CoPi/TNA photoanodes in solar water splitting system
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
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