Multicentre study for a robust protocol in single-voxel spectroscopy: quantification of MRS signals by time-domain fitting algorithms

Magnetic Resonance Spectroscopy (MRS) is an advanced clinical and research application which guarantees a specific biochemical and metabolic characterization of tissues by the detection and quantification of key metabolites for diagnosis and disease staging. The "Associazione Italiana di Fisica Medica (AIFM)" has promoted the activity of the "Interconfronto di spettroscopia in RM" working group. The purpose of the study is to compare and analyze results obtained by perfoming MRS on scanners of different manufacturing in order to compile a robust protocol for spectroscopic examinations in clinical routines. This thesis takes part into this project by using the GE Signa HDxt 1.5 T at the Pavillion no. 11 of the S.Orsola-Malpighi hospital in Bologna. The spectral analyses have been performed with the jMRUI package, which includes a wide range of preprocessing and quantification algorithms for signal analysis in the time domain. After the quality assurance on the scanner with standard and innovative methods, both spectra with and without suppression of the water peak have been acquired on the GE test phantom. The comparison of the ratios of the metabolite amplitudes over Creatine computed by the workstation software, which works on the frequencies, and jMRUI shows good agreement, suggesting that quantifications in both domains may lead to consistent results. The characterization of an in-house phantom provided by the working group has achieved its goal of assessing the solution content and the metabolite concentrations with good accuracy. The goodness of the experimental procedure and data analysis has been demonstrated by the correct estimation of the T2 of water, the observed biexponential relaxation curve of Creatine and the correct TE value at which the modulation by J coupling causes the Lactate doublet to be inverted in the spectrum. The work of this thesis has demonstrated that it is possible to perform measurements and establish protocols for data analysis, based on the physical principles of NMR, which are able to provide robust values for the spectral parameters of clinical use.

13C NMR of a single molecule magnet: analysis of pseudocontact shifts and residual dipolar couplings

Paramagnetic triple decker complexes of lanthanides are promising Single Molecule Magnets (SMMs), with many potential uses. Some of them show preferable relaxation behavior, which enables the recording of well resolved NMR spectra. These axially symmetric complexes are also strongly magnetically anisotropic, and this property can be described with the axial component of the magnetic susceptibility tensor, χa. For triple decker complexes with phthalocyanine based ligands, the Fermi˗contact contribution is small. Hence, together with the axial symmetry, the experimental chemical shifts in 1H and 13C NMR spectra can be modeled easily by considering pseudocontact and orbital shifts alone. This results in the determination of the χa value, which is also responsible for molecular alignment and consequently for the observation of residual dipolar couplings (RDCs). A detailed analysis of the experimental 1H-13C and 1H-1H couplings revealed that contributions from RDCs (positive and negative) and from dynamic frequency shifts (negative for all observed couplings) have to be considered. Whilst the pseudocontact shifts depend on the average positions of 1H and 13C nuclei relative to the lanthanide ions, the RDCs are related to the mobility of nuclei they correspond to. This phenomenon allows for the measurement of the internal mobility of the various groups in the SMMs.

Electrical spectroscopy of rubrene bulk and thin film crystals

One of the most diffused electronic device is the field effect transistor (FET), contained in number of billions in each electronic device. Organic optoelectronics is an emerging field that exploits the unique properties of conjugated organic materials to develop new applications that require a combination of performance, low cost and processability. Organic single crystals are the material with best performances and purity among the variety of different form of organic semiconductors. This thesis is focused on electrical and optical characterization of Rubrene single crystal bulk and thin films. Rubrene bulk is well known but for the first time we studied thin films. The first Current-voltage characterization has been performed for the first time on three Rubrene thin films with three different thickness to extract the charge carriers mobility and to assess its crystalline structure. As results we see that mobility increase with thickness. Field effect transistor based on Rubrene thin films on $SiO_2$ have been characterize by current-voltage (I-V) analyses (at several temperatures) and reveals a hopping conduction. Hopping behavior probably is due to the lattice mismatch with the substrate or intrinsic defectivity of the thin films. To understand effects of contact resistance we tested thin films with the Transmission Line Method (TLM) method. The TLM method revealeds that contact resistance is negligible but evidenced a Schottky behavior in a limited but well determined range of T. To avoid this effect we carried out annealing treatment after the electrode evaporation iswe performed a compete I-V characterization as a function of in temperature to extract the electronic density of states (DOS) distribution through the Space Charge Limited Current (SCLC) method. The results show a DOS with an exponential trenddistribution, as expected. The measured mobility of thin films is about 0.1cm^2/Vs and it increases with the film thickness. Further studies are necessary to investigate the reason and improve performances. From photocurrent spectrum we calculated an Eg of about 2.2eV and both thin films and bulk have a good crystal order. Further measurement are necessary to solve some open problems

Defect states in MAPbBr3 by photo-induced current transient spectroscopy

In the past decade, perovskites have been under the spotlight as promising semicon- ductors with unique properties. Hybrid halide perovskites show excellent characteristic properties suitable for optoelectronic applications as tunable band gap, high absorption coefficient, large mobility and long carrier recombination lifetime. However, a complete understanding of environmental instability and the nature of defects in these materials is still lacking, hindering the development of perovskite-based technologies. In this work we studied MAPbBr3 single crystals, fabricated with Inverse Temperature Crystallization (ITC) technique, with Photo-Induced Current Transient Spectroscopy (PICTS). PICTS is a transient photocurrent measurement rarely employed for studying perovskites mate- rials, that allows for the defects characterization in high resistivity materials. We studied the samples under different conditions, such as negative and positive voltage biases, bias stress, different contact geometries and different illumination wavelengths, in order to study their effect on the material physical properties and to evaluate the trap activation energies and their behavior under different working conditions.

Characterization of in-gap electronic states in two-dimensional single crystal PEA2PbBr4 perovskite for X-ray detection

Hybrid Organic-Inorganic Halide Perovskites (HOIPs) include a large class of materials described with the general formula ABX3, where A is an organic cation, B an inorganic cation and X an halide anion. HOIPs show excellent optoelectronic characteristics such as tunable band gap, high adsorption coefficient and great mobility life-time. A subclass of these materials, the so-called two- dimensional (2D) layered HOIPs, have emerged as potential alternatives to traditional 3D analogs to enhance the stability and increase performance of perovskite devices, with particular regard in the area of ionizing radiation detectors, where these materials have reached truly remarkable milestones. One of the key challenges for future development of efficient and stable 2D perovskite X-ray detector is a complete understanding of the nature of defects that lead to the formation of deep states. Deep states act as non-radiative recombination centers for charge carriers and are one of the factors that most hinder the development of efficient 2D HOIPs-based X-ray detectors. In this work, deep states in PEA2PbBr4 were studied through Photo-Induced Current Transient Spectroscopy (PICTS), a highly sensitive spectroscopic technique capable of detecting the presence of deep states in highly resistive ohmic materials, and characterizing their activation energy, capture cross section and, under stringent conditions, the concentration of these states. The evolution of deep states in PEA 2 PbBr 4 was evaluated after exposure of the material to high doses of ionizing radiation and during aging (one year). The data obtained allowed us to evaluate the contribution of ion migration in PEA2PbBr4. This work represents an important starting point for a better understanding of transport and recombination phenomena in 2D perovskites. To date, the PICTS technique applied to 2D perovskites has not yet been reported in the scientific literature.