Photonic crystal slabs for low-cost biosensors

Biosensors are devices that utilize biological recognition elements to selectively detect and analyze specific biological and chemical analyte substances. In this work a technology platform for label-free optical biosensors based on surface-functionalized photonic crystal slabs is proposed. Using this technology platform, low-cost solutions for three biotechnical questions are presented

Flexible oxide thin film transistors: fabrication and photoresponse

Gli ossidi amorfi semiconduttori (AOS) sono nuovi candidati per l’elettronica flessibile e su grandi aree: grazie ai loro legami prevalentemente ionici hanno una mobilità relativamente alta (µ > 10cm^2/Vs) anche nella fase amorfa. Transistor a film sottile (TFT) basati sugli AOS saranno quindi più performanti di tecnologie a base di a-Si e più economici di quelle a base di silicio policristallino. Essendo amorfi, possono essere depositati a basse temperature e su substrati polimerici, caratteristica chiave per l’elettronica flessibile e su grandi aree. Per questa tesi, diversi TFT sono stati fabbricati e caratterizzati nei laboratori del CENIMAT all’Università Nova di Lisbona sotto la supervisione del Prof. P. Barquinha. Questi dispositivi sono composti di contatti in molibdeno, un canale semiconduttivo di ossido di zinco, gallio e indio (IGZO) e un dielettrico composto da 7 strati alternati di SiO2 e SiO2+Ta2O5. Tutti i dispositivi sono stati depositati mediante sputtering su sostrati flessibili (fogli di PEN). Le misure tensione-corrente mostrano che i dispositivi mantengono alte mobilità (decine di 10cm^2/Vs) anche quando fabbricati a temperature inferiori a 200°C. Si è analizzato il funzionamento dei dispositivi come fototransistor rilevando la risposta alla luce ultravioletta e in particolare la loro responsività e spostamento della tensione di soglia in funzione della lunghezza d’onda incidente. Questi risultati consentono di formulare ipotesi sul comportamento dei dispositivi alla scala microscopica. In particolare, indicano che i) la mobilità del canale non è influenzata dall’illuminazione, ii) sia l'IGZO sia il Ta2O5 contribuiscono al processo di fotoconduttività e iii) il processo di fotogenerazione non è adiabatico. La tesi contiene inoltre una descrizione del processo di ricombinazione e presenta un’applicazione pratica di tali dispositivi in un circuito per RFID. Infine, esplora la possibilità di migliorarne la flessibilità e le prestazioni

Dopamine modification of interfaces between polymers and metals

Increasingly in science inspiration is drawn from Nature that provides many outstanding examples of adhesive strategies. One of them is the adhesion of marine organisms, which present strong binding to virtually all inorganic and organic surfaces in aqueous environments in which most synthetic adhesives function poorly. The attachment occurs by secreting adhesive proteins that harden and cross-link in situ. Previous studies of these functionally unique proteins have revealed the presence of an unusual amino acid, 3,4-dihydroxy-L-phenylalanine (DOPA), which is responsible for the cohesive and adhesive strength of this natural glue and gives to marine organisms the ability to bond with numerous substrates including glass, Teflon, wood, concrete, plastics, metals, biological cell lines, bone, teeth, and others. Although the precise mechanism for assembly of the proteins is not understood, the obtained knowledge broadens biomimetic strategies for synthesizing new practical adhesives. In this PhD thesis our efforts to understand the adherence behavior of dopamine-based promoters on metals are described. Firstly a simple DOPA protein mimics – dopamine was used to coat metal substrates and the properties of dopamine and polydopamine coatings on metals were studied in detail. Dip coating from aqueous dopamine hydrochloride solutions at pH 4 and pH 8 was performed on gold (having nor or hardly any native oxide) and aluminium (having a native aluminium oxide layer). The formed coatings were analyzed with surface sensitive techniques dealing with coating composition and the oxidation state, as well as with mechanical measurements characterising the adhesion of coatings. The outcomes of these studies emphasized a need to design a better defined system. As a result two model molecules, N-stearoyldopamine and 4-stearylcatechol, were synthesized. These molecules contain a catechol residue that is capable of binding to metals and metal oxides and a hydrophobic alkyl-chain that gives an ability to coordinate molecules on surfaces using the Langmuir-Blodgett preparation technique. By using these catechol-containing molecules the role of amide functionality on packing, orientation and interaction with the metal surface could be compared. Attention was devoted in studying the adsorption behavior onto gold and aluminum oxide in terms of monolayer characteristics. The organization of Langmuir-Blodgett monolayers on a molecular scale was examined by several surface sensitive techniques. The experimental work was supported with molecular dynamic simulations on gold in order to get a more complete understanding of the monolayer configuration near the interface. Our study shows that within the monolayer the catechols functions as a surface anchor on gold and the alkyl-chains appear to be tilted within the monolayer. Moreover, the irregularity of the 4-stearylcatechol film on gold leads to micelle type structures that are caused by the absence of the amide functionality. On the contrary, from the molecular simulations it appeared that for both types of molecules parallel orientations of the catechols with the gold are also present. However, hydrogen bonds formed between the amide functionality and the catechol hydroxyl groups have a profound influence on the structure and regularity on the adsorbed layer. Attempts were made to quantify the adhesion strength of the anchoring catechols on metal oxide. A covalently bound top-coating was used to determine the adhesion of an Nlinoleoyldopamine primer on an aluminum alloy. Several application conditions were tested for the top-coating and the primer. The resulting tensile strength values of the topcoating having an N-linoleoyldopamine monolayer primer formed by the Langmuir-Blodgett technique showed an obvious improvement in adhesion whereas a negative impact on adhesion occurred when the primer was not applied in a uniform and controlled manner. According to the results of this work and the literature data, synthetic adhesives inspired by mussel adhesive proteins can be used successfully to improve the adhesive properties between polymers and metals. Consequently, the outcomes of the research will give an input in the development of useful synthetic polymer adhesives that exhibit similar wet adhesive capabilities as mussel foot proteins

DESIGN AND ANALYSIS OF NANO-GAP ENHANCED SURFACE PLASMON RESONANCE SENSORS

Surface plasmon resonance (SPR) sensors are advantageous to other techniques of sensing chemical binding, offering quantitative, real-time, label-free results. Previous work has demonstrated the effectiveness of using dual-mode SPR sensors to differentiate between surface and background effects, making the sensors more robust to dynamic environments. This work demonstrates a technique that improves upon a previously optimized planar film dual-mode SPR sensor’s LOD by introducing a periodic array of subwavelength nano-gaps throughout the plasmon supporting material. First, general figures of merit for a sensor having an arbitrary number of modes are studied. Next, the mode effective index dispersion and magnetic field profiles of the two strongly bound modes found using a gap width of 20nm are analyzed. Qualitative analysis of the results demonstrates how such a design can enable better LODs in terms of each figure of merit. By optimizing a nano-gap enhanced sensor containing 20nm gaps, it is quantitatively demonstrated that the resulting modes improve upon almost every figure of merit, especially with respect to the orthogonality and magnitude of the sensitivity vectors, resulting in LODs approximately a factor of five less than the optimal planar design

DIRECT LASER FABRICATION OF DIFFRACTION GRATINGS ON POLYMER THIN FILMS AND WAVEGUIDES

With the increasing interest in polymer integrated optical devices, techniques for fabrication of efficient polymer diffraction gratings are of critical importance to aid in coupling of light into and out of polymer waveguides. We have been developing direct laser fabrication techniques for diffraction gratings and standard photo-lithographic techniques for polymer waveguides. The investigation of the laser fabrication is carried out using a 193 nm ArF excimer laser. Symmetric and blazed gratings were fabricated on polymer thin films and characterized. Diffraction efficiencies into a single order as high as 79% have been observed. In addition, gratings were fabricated on multi-mode polymer dispensed waveguides for use as input couplers and on single mode photo-lithographically fabricated polymer waveguides for use as output couplers. Output coupling efficiencies of 32% have been observed

X-ray telescope mirrors from surface profile to point spread function.

X-ray astronomy was born on 1962 when Giacconi decided to put a Geiger counter onto a rocket, in hope of measuring the X-ray emission from the Sun. Even if the Sun emission was disappointing, he made the discovery that changed our view on the Universe. An unknown background of X-ray emission that later turned out to contain millions of X-ray sources, both galactic and extra-galactic. Owing to the development of increasingly sophisticated instruments, the sensitivity and the resolution to detect X-ray sources has improved significantly over the last 50 years. One of the major technological improvement was the development of focusing telescope, which allowed to enhance the angular resolution and sensitivity of several orders of magnitude. The angular resolution of an X-ray imaging telescope is mainly determined by the quality of its focusing optics. These generally consist of a number of nested shells of grazing incidence mirrors. The typical configuration used, which minimizes the effect of coma aberration and reduces the focal length, is the so-called Wolter-I (paraboloid-hyperboloid mirror configuration). In order to keep the mass to levels comparable with the launcher (because X-ray absorption in the atmosphere prevents observation from ground) the optics have to be lightweight, hence the mirrors have to be thin. The final performance of a mirror module is always subject to degradation, provided in the realization phase. During the different stages of production (under construction and integration) there may be distortions. In addition, the mirror surface is not ideally smooth but is characterized by a certain roughness topography. Both these types of imperfections combine to determine the degradation of the Point Spread Function (PSF), i.e. the annular integral of focused intensity around the focal spot, which generally characterizes the quality of the optics. Regarding the characterization of an X-ray mirror, one of the basic objectives is to establish the relationship between the imperfections of the mirrors and their PSFas a function of the incident wave energy. The aimis to predict the angular resolution of a mirror, given measurements of profiles and microroughness, or to establish the level of tolerable imperfections of a mirror given a certain angular resolution required by the project specifications. The study of the topography of the mirror surface is done through several methods. It is generally divided into two different kinds of analysis: the study of the profile, i.e. large spatial wavelengths (comparable with the mirror length) and the study of microroughness, i.e. short spatial wavelengths. The first ones, also called the figure errors, are often due to deformation of the mirror that occurs during construction and integration and are responsible for the degradation of the PSF and can be treated by geometrical optics. The second ones are due, for example, to the limits of the polishing mandrel methods, from which the shells are replicated, and the deposition technology of the reflective coating. These imperfections are responsible for a diffusion (called scattering), which degrades the PSF at increasing energy and can not be treated by geometrical optics, but using physical optics under some assumptions. This problem is much more important in Xray than in optical astronomy, because X-ray have a 1000 times smaller wavelengths and are sensitive to surface defects 1000 times smaller. The surface polishing is thereby a fundamental point in X-ray mirrors. The characterization of the microroughness is made in terms of power spectrum as a function of the spatial frequency on the surface (PSD - Power Spectral Density). The PSD is a fundamental quantity in the characterization of X-ray telescope optics because is proportional to the scattering. The measure of roughness is done with different instruments in order to have a range of spatial frequencies as more wide as possible, from a few millimetres to a few tens of nanometres. There is also a range of intermediate frequencies, at the limit of microroughness, which generates a degradation of the PSF that can neither be predicted by geometrical optics and nor by the scattering theory. For this reason, it is difficult in general to predict accurately its effect on the PSF. My PhD activity is included in the mission project NHXM financed by ASI and in the development of X-ray mirrors for ATHENA mission project financed by ESA. The first part of my PhD project has been therefore aimed at the characterization of microroughness and reflectivity of the mirrors, at INAF/OAB, in order to determine the topography of the surface and to support the industry (Media Lario Technology, leader company of manufacturing optical components) in setting the process. The second part of the project was instead dedicated to the development of a selfconsistent general method, based on physical optics, to compute the PSF of X-ray 2 mirrors from their profile metrology. The third part, which is the merging of these two parts of the project, consist in the applications to real cases through verifications with calibration tests. My research work can be divided into three phases: \u2022 First phase - I performed measurements of mirror profiles and roughness of several samples of mirrors for the missions NHXMand ATHENA, using different instruments available at INAF/OAB. The roughness measurements, at spatial scales smaller than 1 mm, can be achieved with different instruments that have different spatial wavelength ranges (i.e. optical interferometer WYKO and Atomic Force Microscope). However, the effects of roughness can also be directly observed by performing X-ray reflectivity measurements using an X-ray diffractometer. I used the X-ray diffractometer available at INAF/OAB for scattering measurements, with particular attention to the effects in large angle scattering and modulation interference introduced by the multilayer. By merging these different data I derived the complete roughness surface PSD. Bymeans of the X-ray diffractometer Imade reflectivity measurements of samples of mirrors with multilayer coating, obtaining the reflectivity curve as a function of the angle of incidence and as a function of the energy. Using a program to fit the reflectivity curves (PPM), I estimated the thickness of the layers and their uniformity, then assessing the compliance with design specifications. In summary, from one hand surface roughness (from direct topography measurements and scattering measurements) to obtain the PSD, on the other hand, measurements of reflectivity (as a function of both the incidence angle and the energy) for the characterization of the structure of the multilayer. The feedback provided to the industry in a commons way and the isolation of the critical points has lead to the deposition of coatings with excellent reflectivity. I performed reflectivity measurements also within the study of the crystallization of gold during the evaporation process, which contributes to worsen the surface roughness. The gold layer is deposited on the mandrel, which is then electroformed a Nickel-Cobalt shell (the mirror). The gold layer serves to detach the shell from mandrel and it should minimize the microroughness increasing. In this regard, I performed diffraction measurements of different gold deposits with different thickness. Studying the Bragg peaks I obtained an estimation of the size of the gold crystallites as a function of the thickness. Larger are the crystallites, higher is the value of the microroughness. The conclusion is that more the gold layer is thick, larger are the crystallites and larger crystallites means microroughness increasing. \u2022 Second phase - I developed a new method to calculate the PSF of an X-ray mirror (e.g. Wolter-I configuration, in double reflection) at any energy by applying the principle of Huygens-Fresnel from real profile and roughness data. In other words, the X-ray reflection is treated by the undulatory theory, building the wavefront deformed by the mirror imperfections. In this interpretation, even the deformed geometry are treated by the physical optics. This allows to obtain the PSF determined from both contributions (figure error and scattering) at any energy in a self-consistent way, without considering different separated energy regimes treated with different methods. This method, never used before, ultimately solves the problem of PSF computation, starting from the complete surface topography of an X-ray mirror. \u2022 Third phase - I performed several calibrations over mirror shells in different configurations as demonstrator for the NHXM hard X-ray imaging telescope (0.3 - 80 keV). Prototypes of NHXM mirror modules with a few mirror shells were manufactured, aiming at demonstrating the feasibility of mirrors. Imade the direct performance verification by measuring the X-ray PSF (Point Spread Function) up to 50 keV in full-illumination setup at PANTER (MPE, Germany) and in pencil-beam set up at monochromatic X-ray energies from 15 to 63 keV at the BL20B2 beamline of the SPring-8 synchrotron radiation facility. Moreover, I simulated PSF from the metrology profile of mirror shell using Fresnel diffraction method. The calibration measured data and the simulated data (obtained with my Fresnel method) match perfectly. This provide the experimental proof of the correctness of the method, that therefore will represent, from now on, a powerful prediction tool in X-ray optics. The Fresnel diffraction method is easily extendible to other optical systems, also out of astrophysical applications, even with a number of more than two reflections, e.g Syncrotron and FEL facilities. For the future, I plan to implement the Fresnel diffraction method improving the simulations of mirrors coated with multilayer. In this case in order to increase the prediction accuracy, we have to taking into account the scattering from multilayer interfaces