DSpace Collection:

Functionalization of zinc oxide nanostructures

Sat, 31 Dec 2011 23:00:00 GMT

Title: Functionalization of zinc oxide nanostructures Authors: Villani, Marco Abstract: ZnO is nowadays a key technological material and a promising building block for functional devices. The lack of a center of symmetry in the wurtzite structure lend to an extremely interesting material, which exhibits both semiconducting and piezoelectric behavior together with a large exciton binding energy. Moreover ZnO probably has the richest family of nanostructures among all materials, both in structures and in properties. Although ZnO properties are much interesting themselves, the functionalization of ZnO nanostructures – by means of surface functionalization as well as the creation of heterostructures combining different materials (organic / inorganic, metals, semiconductors, insulators…) – paves the way to the creation of novel functional materials that exhibit unique new properties, or the enhancement of the pre-existent ones. This thesis deals with the functionalization of ZnO nanostructures, tetrapods (TP) and nanorods (NR) in particular, which have been grown by our group at IMEM Institute through a non-catalyzed CVD technique. My research, in particular, focused on different kinds of functionalization in order to tailor the properties of the resulting material towards different applicative fields, namely gas sensing devices, photocatalytic systems and photovoltaic cells. Different materials, belonging to three specific classes, have been chosen to tailor the properties of ZnO nanostructures: (1) cadmium sulphide (II-VI semiconductor), (2) magnetite (magnetic materials), (3) phthalocyanine and porphyrin (organic semiconductors). More in detail, the cadmium sulphide (CdS) functionalization of ZnO TP is aimed to create an heterojunction for photoenergy conversion applications. Although the use of CdS to extend the fraction of visible light to be collected by the heterostructure is not new, the present work focuses on a new synthetic approach. This is required since, despite of the huge variety of literature-proposed methods for CdS synthesis, none of them can be considered optimal for ZnO functionalization. In fact, on the one hand, CdS aqueous-based synthesis requires alkaline or acidic environments that invariably etch the ZnO surface; on the other, high temperature syntheses in organic solvents require the presence of ligands or surfactant to effectively control the CdS dimension. Unfortunately such polymers / molecules would likely be trapped at the interface limiting the heterojunction performance. Hence, an in–situ deposition of CdS on ZnO TP, without the need of any surface passivating agent, is presented. The CdS functionalization allows to extend the ZnO TP light absorption into the visible range, and the formation of a type–II heterojunction promotes exciton dissociation at the interface and electron transfer from CdS to the ZnO conduction band. Thus the composite material can be employed as photoanode in photovoltaic devices or photo electro–chemical cells, as well as in photocatalytic systems (degradation of organic pollutants, photo-induced water splitting), and gas sensing applications. The following section briefly extends the CdS deposition to ZnO NR, which are promising nanostructures for photovoltaic applications, since they show excellent transport properties and allow a continuous path for the electrons, as opposite to ZnO TP which shows hopping/percolative transport. The second part of the Thesis, deals with the functionalization ZnO TP with magnetite nanoparticles. Small superparamagnetic magnetite nanoparticles, with mean dimensions up to 10 nm, have been synthesized in both aqueous solution and organic solvents. All the prepared nanoparticles have good magnetic properties: i.e. superparamagnetic behavior with saturation magnetizations above 50 A m2 Kg–1. These nanoparticles have been used as building blocks for the functionalization of ZnO tetrapods: this results in the creation of a coupled material which still possess all ZnO own attractive features, e.g. surface reactivity, strong UV–emission, piezoelectricity etc., together with added magnetism. The magnetic response of the coupled compound is ferromagnetic but, considering the very small remanence (MR), the Fe3O4–ZnO coupled nanostructures, dispersed in a liquid, don’t aggregate because of magnetic dipolar interactions: this paves the way for future use in photocatalytic systems, as well as biomedical applications. Finally, the last section deals with the functionalization of ZnO nanorods with organic molecules: the aim of this work is to create and characterize a hybrid photoanode to be used in photovoltaic applications. One of the challenging issues in excitonic photovoltaic cells deals with the optimization of the absorbing layer in order to collect a vast fraction of the incident light. Owing to the limited absorption width of organic molecules and polymers, only a small fraction of the solar flux can be harvested by a single-layer bulk heterojunction photovoltaic cell. In addition to the aforementioned issues, as a matter of fact, ZnO solar cells have shown relatively low overall conversion efficiencies when compared with TiO2-based systems. The limited performance in ZnO-based DSSC may be explained by the instability of ZnO in acidic dye and the slow electron-injection kinetics from dye to ZnO. Considering these open issues, on the one side, one promising strategy to overcome both the limited absorption and the low exciton diffusion in organic materials, is to couple organics with inorganic semiconductor nanostructures (e.g. ZnO). On the other, ZnO functionalization has been carried out by means of molecular beam deposition (SuMBD), which is an attractive growth technique, alternative to wet impregnation, that doesn’t affect ZnO surface, on the contrary, by varying the kinetic energy of the beam, it is possible to promote chemical bonds at the interface. Driven by the high directionality of SuMBD deposition, the goal is to deposit two distinct molecules, with convenient absorption range, on the surface of a single ZnO NR, in order to collect a wider spectrum of incident light. Photovoltaic, however, is not the only applicative field where sensitized ZnO NR can be employed in. According to literature, either porphyrin, ZnO and the resulting hybrid material have been well established as materials for singlet oxygen production, hence they can be used in photocatalytic systems as well as in nano-medicine.

Organic thin films as active materials in field effect transistors and electrochemical sensing

Thu, 29 Mar 2012 22:00:00 GMT

Title: Organic thin films as active materials in field effect transistors and electrochemical sensing Authors: Tarabella, Giuseppe Abstract: This PhD thesis is focused on Organic Electronics, an emerging field where different disciplines converge to gain insights into the properties of organic materials and their applications. Under the present work different organic materials have been realized and analysed for application both in Organic Field Effect Transistors and electrochemical sensing with Organic Electrochemical Transistors. An overview about Organic Electronic is reported with the most recent advancement of the last years: a state of the art of research about Organic Field Effect Transistors (OFETs) and Organic Electrochemical Transistors (OECTs) is given, with an overview on the emerging Organic Bioelectronics. The main motifs of the research performed are reported along the discussion. In the application of the supersonic molecular beam epitaxy method, thin films of Copper Phthalocyanine have been grown, reaching an unprecedented order in the crystalline structure, as the characterization by Raman spectroscopy and AFM have shown. A modified-pentacene molecule (2,3-CN2-TIPS-Pn films) has been used as active layer for the building of an OFET device, which showed an ambipolar behaviour with balanced electrons and holes mobility on the order of 2⋅10-3cm2/Vs. The charge transport properties of 2,3-CN2-TIPS-Pn films show the effectiveness of TIPS-Pn functionalization with cyano e− withdrawing groups to promote e- transport while maintaining equivalent h− transport. A second OFET device has been realized with tetracene organic thin films deposited on different dielectrics substrates: the devices have been characterized and the mobility measured. For the tetracene film deposited on the polystyrene substrate, we have found a mobility of 2⋅10-1 cm2/Vs, the highest retrieved up to now in literature for tetracene. The molecular structures of all the organic molecules used, have been deeply investigated by means AFM analysis and XRD-advanced algorithm tools. For the films made with the TIPS molecule, the GIXRD analysis revealed a favourable arrangement of the molecules in the TFT channel. The XRD analysis performed on the tetracene films revealed interesting correlation between the mobility of the film and the AFM and structural parameters: in particular the polystyrene film shows the best surface coverage and the highest alpha phase percentage of the molecular structure. New insights into the device physics of OECT have been discovered: in the sensing experiments with OECTs, the role of the gate electrode has been investigated. This clarified the two working principles an OECT can operate (faradaic or non-faradaic mode). We found that an OECT can switch between these two modes of operation simply changing the metal wire acting as gate electrode. In particular the faradaic operational mode lead to the possibility to exploit the transistor as a halide sensor, able to detect Na+ ions in solution with a sensibility up to 10μM. Then the role of electrolyte has been studied with micellar structures, which open unexplored horizons for the application of OECT with a new class of electrolytes. The ability of micelles to dope/dedope efficiently the PEDOT:PSS permitted to investigate the doping process of the polymer, that is one of the main issue today in organic electronics. The modulation signals have been correlated with the surface charge of the micelles, measured by the zeta-potential techniques and the injection of micelles into the polymer structure has been probed by an optical spectroscopy measurement, performed in-situ during the OECT current acquisition. As a consequence of the micelle experiment, bilayer structure, like liposomes, have been tested and detected for the first time with an OECT. Although this experiment is currently in progress, it seems particularly promising, mainly because the opportunity to exploit the ability of liposomes to trap and release drugs in a controlled way. A new nanoparticles-based sensor has been developed, able to detect the presence in solution of iron-oxide magnetic nanoparticles functionalised with different polymeric coatings: we provide the ability of OECTs to detect and monitor selectively, with an appropriate choice of the electrolyte, different nanosystems. We demonstrate an on-line sensing based on OECTs, with an easy sampling/sample preparation, for the detection of functionalized magnetic nanoparticles. OECTs have promising applications in bioelectronics as well as in nanomedicine or neuroscience. They are becoming an ideal platform for both in-vitro and in-vivo biomedical applications, as well as for the development of protocells inside miniaturized electro-chemical laboratory.

Study of Molecular Sensitization Processes of Nanostructured Metal Oxides

Wed, 29 Feb 2012 23:00:00 GMT

Title: Study of Molecular Sensitization Processes of Nanostructured Metal Oxides Authors: Detto, Francesca Abstract: Functional hybrids are nano-composite materials lying at the interface of organic and inorganic realms, combining properties and advantages of both materials, and possibly minimizing their disadvantages. Nanostructured titanium dioxide is one of the most investigated material for hybrid systems, as its application areas range from photovoltaics and photocatalysis to photo-/electrochromics, optoelectronics and sensors. Among organic molecules, metal-phthalocyanines (MPcs) are of particular interest due their great versatility and high thermal and chemical stability. A critical and very challenging issue in hybrid system engineering is the contemporary achievement of the metal oxide nano-crystals and the molecular sensitization process. As the high crystalline quality of TiO2 is commonly obtained through thermal treatments, its organic decoration is usually carried out in successive separate steps. Supersonic beams techniques (SuMBD) offer to overcome this challenge, thanks to the possibility of working with both organic molecules and inorganic aggregates, and to the ability to achieve physical and chemical processes at surfaces. The final aim of this thesis work is the study of the molecular sensitization processes, induced by supersonic beams approaches, occurring at the titanium dioxide/copper phthalocyanine interface. A preliminary extensive analysis of the nanostructured TiO2 thin films grown by PMCS is performed, in order to test the material peculiarities resulting from the non conventionality of the deposition technique. At the Parma IMEM-CNR Laboratories, the as-grown TiO2 thin films were investigated by means of several electron microscopy techniques. The morphological and luminescence properties have been evaluated by Scanning Electron Microscopy and Cathodoluminescence, while Transmission Electron Microscopy analyses provided the structural information. These studies have been integrated by surface photoelectron spectroscopies (XPS and UPS), that the candidate performed at the Trento IMEM-CNR Laboratories in order to study the material electronic properties. Moreover, the effects of annealing treatments on the observed properties have been investigated and discussed. Photoelectron emission experiments on both copper and free-base phthalocyanine, grown by SuMBD. These experiments have been carried out at synchrotron ELETTRA (Trieste). The aim of this study was to understand the real influence of the metal core on the molecule electronic properties. Taking advantages of the high resolution of the synchrotron facility, it was possible to determine well suitable fit models for both molecules. In particular, the deconvolution model related to the CuPc was decisive in the last part of this thesis work, to analyze the reactivity processes at the inorganic/organic interface. After achieving a comprehensive knowledge of the electronic properties of both the organic and the inorganic counterparts, we focused our attention to the chemical reactivity phenomena occurring at their interface, which is the final aim of this thesis work. In the Trento IMEM-CNR Laboratories, we got two hybrid systems with the same configuration (CuPc/TiO2) in a combined PMCS/SuMBD approach. In both cases the nanocrystalline metal oxide has been synthesized by PMCS, at room temperature without the need of any thermal treatment. In order to understand the role of the kinetic energy during the sensitization process, the CuPc deposition has been performed by molecular seeded beams at high (experiment A) and low (experiment B) kinetic energies. For both hybrid systems, a complete characterization of core levels and valence band states have been performed at increasing organic coverages, in order to better emphasize the dynamic of the chemical bonds formation at the interface, induced by different kinetic energies.; Combinando materiali organici ed inorganici è possibile ottenere materiali funzionali ibridi con proprietà fisico-chimiche modulate al tipo di applicazione desiderata. Un sistema ibrido particolarmente promettente per l’energetica e la sensoristica è quello costituito da biossido di titanio nanostrutturato e metalloftalocianine. Una delle sfide più attuali nell’ingegnerizzazione di sistemi ibridi è la realizzazione contemporanea dell’ossido metallico nanostrutturato e della sensitizzazione molecolare. Infatti i comuni metodi di crescita del TiO2 si servono di processi termici al fine di attivarne la cristallizzazione; affinchè ciò non vada a danneggiare le molecole organiche funzionalizzanti, i processi di sensitizzazione molecolare vengono solitamente effettuati in una fase successiva. Le tecniche di deposizione basate su fasci supersonici possono superare questo ostacolo, grazie alla possibilità di lavorare con materiali sia organici (SuMBD) che inorganici (PMCS), e alla capacità di attivare processi chimici e fisici alle interfacce. Lo scopo di questo lavoro di tesi è appunto lo studio dei processi di sensitizzazione molecolare indotti da fasci supersonici, realizzato sull’interfaccia biossido di titanio/rameftalocianina (n-TiO2/CuPc). Studi preliminari sono stati effettuati separatamente sia sul materiale inorganico che sulle molecole organiche, al fine di analizzarne le peculiarità risultanti dalla non convenzionalità della tecnica di crescita. I film sottili di TiO2 as-grown sono stati analizzati presso I laboratori del IMEM-CNR di Parma, mediante diverse tecniche di microscopia elettronica. Le proprietà morfologiche e di luminescenza sono stati osservate mediante Microscopio Elettronico a Scansione e Catodoluminescenza, mentre le informazioni strutturali sono state ottenute da misure di Microscopia Elettronica in Trasmissione. Tali studi sono stati integrati da Spettroscopie di superficie (XPS and UPS) effettuate presso I laboratori IMEM-CNR di Trento, al fine di analizzare le proprietà elettroniche del materiale. Inoltre, sono stati analizzati gli effetti dei trattamenti termici sulle proprietà appena descritte. Ulteriori esperimenti di Spettroscopie da fotoelettroni sono state effettuati su film di ftalocianina-rame (CuPc) e free-base (H2Pc), al fine di individuare l’influenza del centro metallico sulle proprietà elettroniche della molecola. Tali analisi sono state effettuate presso il sincrotrone ELETTRA (Trieste). Dopo aver studiato le proprietà elettroniche di entrambi I materiali, sono stati analizzati i fenomeni di reattività alle loro interfacce. Presso i laboratori IMEM-CNR di Trento abbiamo ottenuto due sistemi ibridi con la stessa configurazione CuPc/TiO2. In entrambi i casi l’ossido metallico nanostrutturato è stato sintetizzato a temperatura ambiente con la sorgente PMCS, senza ulteriori post-processing termici. Al fine di studiare il ruolo dell’energia cinetica durante il processo di sensitizzazione, la deposizione della CuPc è stata effettuata mediante fasci molecolari supersonici sia ad alta (esperimento A) che a bassa (esperimento B) energia cinetica. Entrambi I sistemi ibridi sono stati analizzati acquisendo i core levels e gli stati della banda di valenza durante le varie fasi della deposizione. In questo modo è stato possible osservare la dinamica della formazione di legami chimici indotti dalla diversa energia cinetica

Heterostructures based on L10-FePt for spintronics and magnetic recording.

Wed, 29 Feb 2012 23:00:00 GMT

Title: Heterostructures based on L10-FePt for spintronics and magnetic recording. Authors: Lupo, Pierpaolo Abstract: This Thesis is focused on the design, growth and characterization of thin films and heterostructures, based on FePt, with potential applications in magnetic recording and spintronics, where the nanometric scale phenomena play a important role. With this aim, heterostructures with peculiar properties like high perpendicular anisotropy, exchange-spring effect, magnetoresistance and spin polarization have been developed. The studied systems have been produced mainly by means of physical deposition techniques, such as radio frequency sputtering and pulsed laser deposition. The structural, morphological and magnetic studies have been performed through a variety of techniques, allowing a constant feedback with the design and preparation of materials. This work has involved a considerable effort to reach a better understanding of various and innovative scientific aspects. The study of new nanocomposites recording media, based on FePt, has highlighted the dependence of the exchange-spring behavior on the soft magnetic layer properties. For example, varying its thickness and chemical com- position it is possible to change the effect of exchange interaction between hard and soft layers. Furthermore, the lattice mismatching with the substrate, which influences the morphology and chemical order of the nanostructure, can be exploited to tailor the magnetic behavior. Moreover, ion irradiation on L10-FePt thin film has been studied as possible technique to develop exchange-spring media with suitable characteristics closely defined by the irradiation parameters. The objective is to study innovative per- pendicular recording media that exploit the exchange-spring interaction between a hard and soft magnetic phase, in bilayers or graded systems, to increase the storage density in future hard disks. In spintronics, the original choice of ferromagnetic electrodes (i.e., L10-FePt and Fe3O4) and their magnetization configuration in the heterostructure for magnetic tunnel junctions (MTJs) has led to deepen the extrinsic properties effects on the electrical transport through the tunnel barrier and on tunnel magnetic resistance. The inverse tunneling effect, as expected for Fe3O4/MgO interface, has been studied as a function of film thickness and junction area. The perpendicular direction between the easy magnetization axes of the electrodes gives rise to a suitable configuration in spin-torque devices. The first part of my work (chapter 3) starts describing the optimization of thin films growth conditions, with emphasis on L10-FePt films grown on MgO and SrTiO3 substrates, studying the correlation among morphology, magnetism, crystalline order and substrate lattice mismatch. Chapter number 4 describes the next implementation of FePt layers with optimized properties, tailored interfaces and morphology in nanos- tructures for future recording media. Moreover, a numerical micromagnetic model is proposed to describe the reversal magnetization mechanism in these systems. The final chapter (number 5) is focused to the growth and characterization of heterostructures, based on L10-FePt, and their lithography process in order to obtain MTJs with new characteristics for application in spintronics. The first chapter presents some basic concepts of magnetism, in order to understand the underlying physics and a brief summary of the employed experimental techniques is given in the second chapter.; Questa tesi è incentrata sulla crescita, progettazione e caratterizzazione di film sottili ed eterostrutture, basati su L10-FePt, con potenziali applicazioni nella registrazione magnetica e spintronica, dove i fenomeni su scala nanometrica giocano un ruolo importante. Con questo obiettivo, sono state sviluppate eterostrutture con caratteristiche peculiari come elevata anisotropia perpendicolare,comportamento tipo exchange-spring, magnetoresistenza e polarizzazione di spin. I sistemi studiati sono stati prodotti principalmente per mezzo di tecniche di deposizione fisica, come lo sputtering a radio frequenza e deposizione laser pulsata. Studi morfologici e magnetici sono stati eseguiti attraverso una varietà di tecniche, permettendo un feedback costante con la progettazione e la preparazione dei materiali. Questo lavoro ha comportato un notevole sforzo per raggiungere una migliore comprensione dei vari aspetti scientifici e tecnologici emersi. Lo studio dei nuovi supporti di registrazione nanocompositi ha messo in evidenza la dipendenza dello scambio dalle proprietà dello strato magneticamente soft.Per esempio, variando il suo spessore e la percentuale in Fe, è possibile cambiare l'effetto di interazione di scambio tra gli strati hard e soft. Anche il misfit con il substrato, che influenza la morfologia e l'ordine chimico della nanostruttura, può essere sfruttato per cambiare il comportamento magnetico. Inoltre, l'irraggiamento con ioni Ar di un film sottile di L10-FePt è stato studiato come possibile tecnica per lo sviluppare di un sistema exchange-spring graded, con caratteristiche strettamente definite dai parametri d'irradiazione. L'obiettivo è quello di studiare un innovativo materiale per la registrazione perpendicolare che sfruttano lo scambio tra le due fasi magnetiche, in bilayers o sistemi graded, possa aumentare la densità di memorizzazione dei futuri hard disk. Nella spintronica, la scelta originale di elettrodi ferromagnetici (ad esempio, L10-FePt e Fe3O4) e la loro direzione di magnetizzazione sfruttati in eterostrutture per tunnel magnetic junction (MTJ), ha permesso di approfondire le proprietà di trasporto attraverso la barriera tunnel in sistemi a magnetizzazione perpendicolare. L'effetto inverso tunneling, come previsto per interfacce tipo Fe3O4/MgO, è stato studiato in funzione dello spessore degli strati e l'area di giunzione. La direzione perpendicolare tra gli assi di facile magnetizzazione degli elettrodi dà luogo a una configurazione adatta a dispositivi tipo spin-torque. La prima parte del mio lavoro (capitolo 3) descrive l'ottimizzazione delle condizioni di crescita di film sottili, con particolare attenzione ai films di L10-FePt cresciuti su substrati di MgO e SrTiO3, studiando la correlazione tra la morfologia, il magnetismo, l'ordine strutturale e il misfit. Nel capitolo 4 è descritta l'utilizzo successiva di strati FePt con proprietà, interfacce e morfologia ottimizzate, in nanostrutture ideali per il magnetic recording. Inoltre, un modello numerico micromagnetico è stato proposto per descrivere il meccanismo di inversione di magnetizzazione in questi sistemi. Il capitolo finale (numero 5) è focalizzata sulla crescita e caratterizzazione di eterostrutture, basate su L10-FePt, e il loro processo di litografia per ottenere MTJ con caratteristiche nuove per applicazioni in spintronica. Il primo capitolo presenta alcuni concetti di base del magnetismo e un breve riassunto delle tecniche impiegate sperimentale è dato fornito nel secondo capitolo.

Self-assembled network via molecular recognition

Wed, 29 Feb 2012 23:00:00 GMT

Title: Self-assembled network via molecular recognition Authors: Dionisio, Marco Abstract: Supramolecular chemistry is the ultimate strategy for the fabrication of innovative materials devote to accomplish advanced tasks. Mastering the weak interactions that held together the molecular components, a sophisticated structures appear. The present thesis deals with the formation of supramolecular networks via molecular recognition, covering three main topics: 1) the realization of hybrid organic-inorganic material, by polymer controlled association of gold nanoparticles; 2) the fabrication of sensor device, by combining molecular recognition functions with electrical properties of carbon nanotubes, and 3) the formation of polymer blending, as macroscopic expression of molecular recognition. All the topics have been exploited moving in the frame of cavitand chemistry. Several and complementary techniques, both in solution and in the solid state, allow for the comprehensive characterization of the formed networks, at different scale, ranging from molecular to macroscopic level.; La chimica supramolecolare è la miglior strategia per la fabbricazione di materiali innovativi, importarti per svolgere funzioni avanzate. Grazie alla possibilità di scegliere e prevedere le interazioni che legano i componenti molecolari, è possibile costruire architetture molto sofisticate. Questa tesi si colloca all’interno di questo contesto affrontando tre linee di ricerca: 1) la realizzazione di materiali ibridi organici-inorganici, attraverso l’associazione controllata di nano particelle d’oro; 2) la fabbricazione di un sensore, inserendo funzioni di riconoscimento molecolare, in materiali grafici quali nano tubi di carbonio e 3) la formazione di miscele polimerica, per mezzo dell’espressione macroscopica del riconoscimento molecolare. Le nuove strutture ottenute sono state studiate attraverso diverse e complementari tecniche di caratterizzazione, muovendosi lungo la scala che collega il mondo microscopico a quello macroscopico.

Muons and Hydrogen in Graphene

Thu, 17 Mar 2011 23:00:00 GMT

Title: Muons and Hydrogen in Graphene Authors: Mazzani, Marcello Abstract: Macroscopic quantities of graphenes have been prepared by diff erent chemical methods and characterized by Muon Spin Rotation spectroscopy, which proved a useful tool to study the interactions of the hydrogen atom with the defective graphene plane. A clear muon spin precession is observed in all the samples, contrary to the standard behaviour of graphite. Its origin lies in the magnetic dipolar interactions of hydrogen nuclei present at defects and reveal the formation of an extremely stable CHMu (CH2) state. The signal amplitude suggests that vacancies saturated by hydrogen have an extraordinary hydrogen capture cross-section. In addition the Muon Spin Rotation results, together with our SQUID investigations, pose important limits on the debated possibility of magnetism in graphene: Muon Spin Rotation, indeed, is very sensitive to the local internal field and does not show the presence of any magnetization.

Cavitand receptors for chiral recognition and fluorescent sensing

Thu, 17 Mar 2011 23:00:00 GMT

Title: Cavitand receptors for chiral recognition and fluorescent sensing Authors: Maffei, Francesca Abstract: The need to develop sensors specific for different classes of analytes is well recognized and confirmed by the considerable research efforts spent for the preparation of more and more efficient devices. The crucial parameter to define the success of a given sensor is therefore selectivity, and for this reason the strategy to prepare the sensing material following the principle of supramolecular chemistry has quickly gained increasing importance. Within this context, the present thesis focused on the realization and characterization of supramolecular receptors as material for sensing chemicals vapors such as alcohols and nitroaromatics. In detail, we have conceived and prepared mixed bridged phosphonate cavitands and quinoxaline cavitands, probing their molecular recognition properties toward suitable guest at the gas solid interface and in solution. A new highly selective fluorescent sensor for C1-C4 alcohols has been prepared, in which only the specific interactions between the analyte and the sensing layer are transduced into a readable signals. Inherently chiral cavitands were also prepared, capable of enantiospecific recognition in the solid state toward chiral alcohols and in solution toward L-amino acids.; La necessità di sviluppare sensori specifici per diverse classi di analiti è confermata dai considerevoli sforzi spesi per la realizzazione di sistemi sempre più efficienti. Il parametro cruciale che definisce il successo di un dato sensore è quindi la selettività, e per questa ragione la realizzazione di materiali basati sui principi della chimica supramolecolare ha un’importanza sempre più rilevante. In questo contesto, il presente lavoro di tesi è focalizzato sulla realizzazione e caratterizzazione di recettori come materiali per lo sviluppo di sensori all’interfaccia gas-solido. Nel dettaglio, abbiamo preparato cavitandi fosfonati e chinossalinici a ponti misti dimostrando le loro proprietà di riconoscimento molecolare verso opportuni analiti all’interfaccia gas solido e in soluzione. E’ stato realizzato un nuovo sensore fluorescente altamente selettivo per alcoli a catena corta (C1-C4), in cui solo le interazioni specifiche tra analita e sensing layer sono tradotte in un segnale leggibile. Sono stati inoltre preparati cavitandi inerentemente chirali che hanno dimostrato una buona enantioselettività allo stato solido verso alcoli chirali e in soluzione verso L-ammino acidi.

MOVPE Growth and Study of III-V Multi-Junction Structures for Advanced Photovoltaic Applications

Thu, 17 Mar 2011 23:00:00 GMT

Title: MOVPE Growth and Study of III-V Multi-Junction Structures for Advanced Photovoltaic Applications Authors: Baldini, Michele Abstract: The energy question is one of the main problem of modern society and is particularly urgent because of the drawbacks of fossil fuel exploitation, which provide 80% of the total energy we currently consume. The only realistic and far-seeing solution to current energy crisis is represented by the employment of renewable energy sources, assisted by global energy conservation policies. Photovoltaic represents one of the most interesting renewable technologies, since it’s the only one that can convert solar energy directly into electricity, without the use of any moving parts. Solar radiation, moreover, is abundant, inexhaustible and diffuse all over the world. Since the 1950s, when the first silicon solar cell was produced, three generations of devices were conceived with the purpose of improving the production cost/conversion efficiency ($/W) ratio to become market-competitive. In particular, the third generation is based on innovative devices that mean to exceed the theoretical efficiency limit for single junction p-n solar cells, by reducing their main energy loss mechanisms. This PhD thesis deals with the study of two types of third generation structures based on multiple band gaps. The first structure is based on a InGaP p-i-n junction with an intrinsic region consisting of 30 periods of 8 nm thick GaAs quantum wells (QW) and 12 nm thick InGaP barriers, conceived to be part of a quantum well solar cell (QWSC). This heterostructure was grown by a low pressure MOVPE reactor, with the employment of liquid alternative metalorganic precursors for the group V elements, terbutylarsine (TBAs) and terbutylphosphine (TBP). In particular, it was investigated the light response of the structure by an accurate photoelectric spectroscopy (PES) study: both the photocurrent (PC) and photovoltage (PV) signals were detected by a standard lock-in technique, as a function of the wavelength, at different sample temperatures and for different frequencies of the exciting light, modulated by a chopper. A second type of photovoltaic structure was designed and realized, consisting in a relatively simple monolithic GaAs-based tandem structure grown on GaSb substrates. By taking advantage of the high temperature of the growth process (T=600-650° C), the deposition of a highly Zn doped GaAs layer enabled the Zn diffusion into the Te-doped (n-type) GaSb substrate, forming a buried GaSb p-n homojunction. By depositing additional GaAs layers with appropriate doping levels, a tunnel and a top junction were stacked to obtain the final tandem structure. The originality of the proposal is related both to the method employed to activate the Zn diffusion in GaSb, and also to the assessment of the GaAs-on-GaSb epitaxial growth. The possibility to realize a tandem cell by properly modulating the doping of the same compound (GaAs), thus making the fabrication process very simple, is the main advantage of this structure.

Realization and Characterization of CZT X-Ray and gamma ray detectors

Fri, 31 Dec 2010 23:00:00 GMT

Title: Realization and Characterization of CZT X-Ray and gamma ray detectors Authors: Marchini, Laura Abstract: Radiation detection is a very important tool for every application field that includes monitoring and detection of radiation sources. In the case of gamma and X-ray detectors, a wide number of application fields are involved in the detector developing, this list comprehends medical imaging, environmental monitoring, homeland security and astrophysics. In recent years CdZnTe(CZT) has emerged as an interesting material for the realization of room temperature X- and gamma ray detectors. In this work IMEM grown CZT ingots and detectors were studied with several techniques, in order to understand the mechanisms occurring in the device functioning. The results were pushed together to create a direct feedback with the crystal growers of the group.

Synthesis and characterization of new multiferroic materials

Wed, 17 Mar 2010 23:00:00 GMT

Title: Synthesis and characterization of new multiferroic materials Authors: Mezzadri, Francesco Abstract: I materiali multiferroici, cioè sistemi in cui coesistono almeno due fra gli ordinamenti ferromagnetico, ferroelettrico e ferroelastico, ottengono oggi larga attenzione a causa sia delle promettenti possibilità offerte nel campo applicativo, sia delle sfide proposte per quanto riguarda lo studio della fisica dello stato solido, con riferimento in particolare alle condizioni di coesistenza e dei possibili meccanismi di accoppiamento degli ordini ferroici. Il presente lavoro tratta la sintesi e caratterizzazione di nuovi materiali multiferroici, ponendo particolare attenzione al rapporto struttura-proprietà, grazie all’uso sinergico di diverse tecniche di diffrazione e di numerose tecniche per la determinazione delle caratteristiche fisiche. La ricerca di nuovi materiali multiferroici ha seguito due linee pricipali: la prima riguarda l’inserimento di ioni magnetici in tipiche strutture ferroelettriche mentre la seconda implica l’utilizzo di ioni stereochimicamente per indurre deformazioni in strutture magnetiche. La prima linea di ricerca ha portato a fluoruri ed ossidi appartenenti alla classe strutturale dei bronzi di tungsteno tetragonali. Fra questi, i composti KxFe5F15 sono stati studiati in tutto il range di stabilità della fase (2