Solution processed quantum dot photodetectors

To find sustainable solutions for the production of energy, it is necessary to create photovoltaic technologies that make every photon count. To pursue this necessity, in the present work photodetectors of zinc oxide embedded with nano-structured materials, that significantly raise the conversion of solar energy to electric energy, were developed. The novelty of this work is on the development of processing methodologies in which all steps are in solution: quantum dots synthesis, passivation of their surface and sol-gel deposition. The quantum dot solutions with different capping agents were characterized by UVvisible absorption spectroscopy, spectrofluorimetry, dynamic light scattering and transmission electron microscopy. The obtained quantum dots have dimensions between 2 and 3nm. These particles were suspended in zinc acetate solutions and used to produce doped zinc oxide films with embedded quantum dots, whose electric response was tested. The produced nano-structured zinc oxide materials have a superior performance than the bulk, in terms of the produced photo-current. This indicates that an intermediate band material should have been produced that acts as a photovoltaic medium for solar cells. The results are currently being compiled in a scientific article, that is being prepared for possible submission to Energy and Environmental Science or Nanoscale journals.

Rely-guarantee protocols for safe interference over shared memory

Mutable state can be useful in certain algorithms, to structure programs, or for efficiency purposes. However, when shared mutable state is used in non-local or nonobvious ways, the interactions that can occur via aliases to that shared memory can be a source of program errors. Undisciplined uses of shared state may unsafely interfere with local reasoning as other aliases may interleave their changes to the shared state in unexpected ways. We propose a novel technique, rely-guarantee protocols, that structures the interactions between aliases and ensures that only safe interference is possible. We present a linear type system outfitted with our novel sharing mechanism that enables controlled interference over shared mutable resources. Each alias is assigned separate, local roles encoded in a protocol abstraction that constrains how an alias can legally use that shared state. By following the spirit of rely-guarantee reasoning, our rely-guarantee protocols ensure that only safe interference can occur but still allow many interesting uses of shared state, such as going beyond invariant and monotonic usages. This thesis describes the three core mechanisms that enable our type-based technique to work: 1) we show how a protocol models an alias’s perspective on how the shared state evolves and constrains that alias’s interactions with the shared state; 2) we show how protocols can be used while enforcing the agreed interference contract; and finally, 3) we show how to check that all local protocols to some shared state can be safely composed to ensure globally safe interference over that shared memory. The interference caused by shared state is rooted at how the uses of di↵erent aliases to that state may be interleaved (perhaps even in non-deterministic ways) at run-time. Therefore, our technique is mostly agnostic as to whether this interference was the result of alias interleaving caused by sequential or concurrent semantics. We show implementations of our technique in both settings, and highlight their di↵erences. Because sharing is “first-class” (and not tied to a module), we show a polymorphic procedure that enables abstract compositions of protocols. Thus, protocols can be specialized or extended without requiring specific knowledge of the interference produce by other protocols to that state. We show that protocol composition can ensure safety even when considering abstracted protocols. We show that this core composition mechanism is sound, decidable (without the need for manual intervention), and provide an algorithm implementation.

Comparison of catheter-related infection risk in two different long-term venous devices in adult hematology-oncology patients

PURPOSE: Infection is the leading complication of long-term central venous catheters, and its incidence may vary according to catheter type. The objective of this study was to compare the frequency and probability of infection between two types of long-term intravenous devices. METHODS: Retrospective study in 96 onco-hematology patients with partially implanted catheters (n = 55) or completely implanted ones (n = 42). Demographic data and catheter care were similar in both groups. Infection incidence and infection-free survival were used for the comparison of the two devices. RESULTS: In a median follow-up time of 210 days, the catheter-related infection incidence was 0.2102/100 catheter-days for the partially implanted devices and 0.0045/100 catheter-days for the completely implanted devices; the infection incidence rate was 46.7 (CI 95% = 6.2 to 348.8). The 1-year first infection-free survival ratio was 45% versus 97%, and the 1-year removal due to infection-free survival ratio was 42% versus 97% for partially and totally implanted catheters, respectively (P <.001 for both comparisons). CONCLUSION: In the present study, the infection risk was lower in completely implanted devices than in partially implanted ones.

Perylene Diimide acceptors: Fabrication and characterization of electron-only, hole-only devices and solar cells

The thrust towards energy conservation and reduced environmental footprint has fueled intensive research for alternative low cost sources of renewable energy. Organic photovoltaic cells (OPVs), with their low fabrication costs, easy processing and flexibility, represent a possible viable alternative. Perylene diimides (PDIs) are promising electron-acceptor candidates for bulk heterojunction (BHJ) OPVs, as they combine higher absorption and stability with tunable material properties, such as solubility and position of the lowest unoccupied molecular orbital (LUMO) level. A prerequisite for trap free electron transport is for the LUMO to be located at a level deeper than 3.7 eV since electron trapping in organic semiconductors is universal and dominated by a trap level located at 3.6 eV. Although the mostly used fullerene acceptors in polymer:fullerene solar cells feature trap-free electron transport, low optical absorption of fullerene derivatives limits maximum attainable efficiency. In this thesis, we try to get a better understanding of the electronic properties of PDIs, with a focus on charge carrier transport characteristics and the effect of different processing conditions such as annealing temperature and top contact (cathode) material. We report on a commercially available PDI and three PDI derivatives as acceptor materials, and its blends with MEH-PPV (Poly[2-methoxy 5-(2-ethylhexyloxy)-1,4-phenylenevinylene]) and P3HT (Poly(3-hexylthiophene-2,5-diyl)) donor materials in single carrier devices (electron-only and hole-only) and in solar cells. Space-charge limited current measurements and modelling of temperature dependent J-V characteristics confirmed that the electron transport is essentially trap-free in such materials. Different blend ratios of P3HT:PDI-1 (1:1) and (1:3) show increase in the device performance with increasing PDI-1 ratio. Furthermore, thermal annealing of the devices have a significant effect in the solar cells that decreases open-circuit voltage (Voc) and fill factor FF, but increases short-circuit current (Jsc) and overall device performance. Morphological studies show that over-aggregation in traditional donor:PDI blend systems is still a big problem, which hinders charge carrier transport and performance in solar cells.

Saloplastic membranes as green devices for soft tissue regeneration

Implantable devices must exhibit mechanical properties similar to native tissues to promote appropriate cellular behavior and regeneration. Herein, we report a new membrane manufacture method based on the synthesis of polyelectrolyte complexes (PECs) that exhibit saloplasticity, i.e. variable physical-chemistry using salt as a plasticizer. This is a Green Chemistry approach, as PECs generate structures that are stabilized solely by reversible electrostatic interactions, avoiding the use of harmful crosslinkers completely. Furthermore, natural polyelectrolytes - chitosan and alginate - were used. Upon mixing them, membranes were obtained by drying the PECs at 37ºC, yielding compact PECs without resorting to organicsolvents. The plasticizing effect of salt after synthesis was shown by measuring tensile mechanical properties, which were lower when samples were immersed in high ionic strength solutions.Salt was also used during membrane synthesis in different quan- tities (0 M, 0.15 M and 0.5 M in NaCl) yielding structures with no significant differences in morphology and degradation (around 15% after 3 months in lysozyme). However, swelling was higher (about 10x) when synthesized in the presence of salt. In vitro cell studies using L929 fibroblasts showed that cells adhered and proliferated preferentially in membranes fabricated in the presence of salt (i.e. the membranes with lower tensile strength). Structures with physical-chemical properties controlled with precision open a path to tissue engineering strategies depending on fine tuning mechanical properties and cellular adhesion simply by changing ionic strength during membrane manufacture

Identifying finger postures when interacting with the touch screen of mobile devices

This study presents the results of preliminary test on the interaction between fingertip and touch screen. The objective of this study is to identify the fingertip posture when interacting with touch screen devices. Ten participants, 7 males and 3 females, participated in this study. The participants were asked to touch targets on the mobile devices screen by tapping them sequentially and connecting them. The participants performed the tasks in a sitting posture. A tablet with 10 inches screen and a mobile phone with 4 inches screen were used in the study. The results showed that all participants dominantly used their thumb to interact with the mobile phone in single and two hands postures. The common thumb posture adopted by the participants is the combination of the 60° pitch and 0° roll angles. While for interaction with tablet in various postures observed in the study, the participants commonly used their index fingers in the combination of 60° pitch and 0° roll angles. This study also observed the participant with long finger nails touched targets on the mobile devices screen by using her index or middle fingers very low pitch.

Effect of polymeric matrix on morphology and photoelectric properties of polymer-quantum dots nanocomposites

Recently, CdTe semiconductor quantum dots (QDs) have attracted great interest due to their unique properties [1]. Their dispersion into polymeric matrices would be very for several optoelectronics applications. Despite its importance, there has been relatively little work done on charge transport in the QD polymeric films [2], which is mainly affected by their structural and morphological properties. In the present work, polymer-quantum dot nanocomposites films based on optically transparent polymers in the visible spectral range and CdTe QDs with controlled particle size and emission wavelength, were prepared via solvent casting. Photoluminescent (PL) measurements indicate different emission intensity of the nanocomposites. A blue shift of the emission peak compared to that of QDs in solution occurred, which is attributed to the QDs environment changes. The morphological and structural properties of the CdTe nanocomposites were evaluated. Since better QDs dispersion was achieved, PMMA seemed to be the most promising matrix. Electrical properties measurements indicate an ohmic behavior.

Electrically tunable resonant scattering in fluorinated bilayer graphene

Adatom-decorated graphene offers a promising new path towards spintronics in the ultrathin limit. We combine experiment and theory to investigate the electronic properties of dilutely fluorinated bilayer graphene, where the fluorine adatoms covalently bond to the top graphene layer. We show that fluorine adatoms give rise to resonant impurity states near the charge neutrality point of the bilayer, leading to strong scattering of charge carriers and hopping conduction inside a field-induced band gap. Remarkably, the application of an electric field across the layers is shown to tune the resonant scattering amplitude from fluorine adatoms by nearly twofold. The experimental observations are well explained by a theoretical analysis combining Boltzmann transport equations and fully quantum-mechanical methods. This paradigm can be generalized to many bilayer graphene-adatom materials, and we envision that the realization of electrically tunable resonance may be a key advantage in graphene-based spintronic devices.

Strain induced edge magnetism at zigzag edge of a graphene quantum dot

We study the temperature dependent magnetic susceptibility of a strained graphene quantum dot by using the determinant quantum Monte Carlo method. Within the Hubbard model on a honeycomb lattice, our unbiased numerical results show that a relative small interaction $U$ may lead to a edge ferromagnetic like behavior in the strained graphene quantum dot, and a possible room temperature transition is suggested. Around half filling, the ferromagnetic fluctuations at the zigzag edge is strengthened both markedly by the on-site Coulomb interaction and the strain, especially in low temperature region. The resultant strongly enhanced ferromagnetic like behavior may be important for the development of many applications.

Performance assessment of electrically small antennas for implantable microsystems with wireless power and communications

Dissertação de mestrado integrado em Engenharia Biomédica (área de especialização em Eletrónica Médica)

Assistive locomotion strategies for active lower limb devices

Dissertação de mestrado integrado em Engenharia Biomédica (área de especialização em Eletrónica Médica)

Electrochromic devices incorporating biohybrid electrolytes doped with a lithium salt, an ionic liquid or a mixture of both

The sol-gel method was employed in the synthesis of di-urethane cross-linked poly( caprolactone) (PCL(530)/siloxane biohybrid ormolytes incorporating either a mixture of lithium triflate (LiCF3SO3) and the ionic liquid (IL) 1-ethyl-3-methyl imidazolium tetrafluoroborate ([Emim]BF4), or solely with [Emim]BF4 or LiCF3SO3. The ormolyte doped with [Emim]BF4 is thermally more stable and exhibits higher ionic conductivity (4 x 10-4 and 2 x 10-3 S cm-1 at 36 and 98 ºC, respectively) than those containing the LiCF3SO3/[Emim]BF4 mixture or just LiCF3SO3. The three ormolytes were employed in the production of glass/ITO/ormolyte/WO3/ITO/glass electrochromic devices (ECDs) designated as ECD@Y with Y = Li-[Emim]BF4, [Emim]BF4 and Li. The three ECDs displayed fast switching speed (ca. 30 s). ECD@Li-[Emim]BF4 exhibited an electrochromic contrast of 18.4 % and an optical density change of 0.11 in the visible region, the coloration efficiency attained at 555 nm was 159 and 80.2 cm-2 C-1 in the “on” and “off” states, respectively, and the open circuit memory was 48 hours. In the “on” state the CIE 1931 color space coordinates were x = 0.29 and y = 0.30, corresponding to blue color.

Eco-friendly luminescent hybrid materials based on EuIII and LiI co-doped chitosan

Biopolymer-based materials have been of particular interest as alternatives do synthetic polymers due to their low toxicity, biodegradability and biocompatibility. Among them, chitosan is one of the most studied ones and has recently been investigated for the application as solid state polymer electrolytes. Furthermore, it can serve as a host for luminescent species such as rare earth ions, giving rise to materials with increased functionality, of particular interest for electrochemical devices. In this study, we investigate chitosan based luminescent materials doped wit Eu3+ and Li+ triflate salts from the structural, photophysical and conductivity points of view. Because the host presents a broad emission band in the blue to green, while Eu3+ emits in the red, fine tuning of emission colour and/or generation of white light is possible by optimizing composition and excitation scheme. Europium lifetimes (5D0) are in the range 270 – 350 µs and quantum yields are as high as 2%. Although Li+ does not interfere with the luminescent properties, it grants ion-conducting properties to the material suggesting that a combination of both properties could be further explored in multifunctional device.

Polymer electrolytes for electrochromic devices

Polymer electrolytes are currently the focus of much attention as potential electrolytes in electrochemical devices such as batteries, display devices and sensors. Generically, solid polymer electrolytes (SPEs) are mixtures of salts with soft polar polymers. SPEs have many advantages including high energy density, no risk of leakage, no issues related to the presence of solvent, wide electrochemical stability windows, simplified processability and light weight. With the goal of developing a new family of environmentally friendly multifunctional biohybrid materials displaying high ionic conductivity we have produced in the present work, flexible films based on different polymers or hybrids incorporating different salts. The polymer electrolytes studied here have been characterized by means of Differential Scanning Calorimetry, Thermogravimetric Analysis, X-ray diffraction, Polarized Optical Microscopy, complex impedance spectroscopy and cyclic voltammetry. An evaluation of the performance of the sample with the highest conductivity as electrolyte in all solid-state ECDs was performed.

Lithium-doped silk fibroin films for application in electrochromic devices

Silk fibroin (SF) is a commonly available natural biopolymer produced in specialized glands of arthropods, with a long history of use in textile production and also in health cares. The exceptional intrinsic properties of these fibers, such as self-assembly, machinability, biocompatibility, biodegradation or non-toxicity, offer a wide range of exciting opportunities [1]. It has long been recognized that silk can be a rich source of inspiration for designing new materials with tailored properties, enhanced performance and high added value for targeted applications, opening exciting new prospects in the domain of materials science and related technological fields, including bio-friendly integration, miniaturization and multifunctionalization. In recent years it has been demonstrated that fibroin is an excellent material for active components in optics and photonics devices. Progress in new technological fields such as optics, photonics and electronics are emerging [2,3]. The incorporation of polymer electrolytes as components of various devices (advanced batteries, smart windows, displays and supercapacitors) offers significant advantages with respect to traditional electrolytes, including enhanced reliability and improved safety. SF films are particularly attractive in this context. They have near-perfect transparency across the VIS range, surface flatness (together with outstanding mechanical robustness), ability to replicate patterned substrates and their thickness may be easily tailored from a few nanometers to hundreds of micrometers through spin-casting of a silk solution into subtract. Moreover, fibroin can be added to other biocomponents or salts in order to modify the biomaterial properties leading to optimized and total different functions. Preliminary tests performed with a prototype electrochromic device (ECD) incorporating SF films doped with lithium triflate and lithium tetrafluoroborate (LiTFSI and LiBF4, respectively) as electrolyte and WO3 as cathodic electrochromic layer, are extremely encouraging. Aiming to evaluate the performance of the ion conducting SF membranes doped with LiTFSI and LiBF4 (SF-Li), small ECDs with glass/ITO/WO3/SF-Li/CeO2-TiO2/ITO/glass configuration were assembled and characterized. The device exhibited, after 4500 cycles, the insertion of charge at -3.0 V reached –1.1 mC.cm-2 in 15 s. After 4500 cycles the window glass-staining, glass/ITO/WO3/Fibrin-Li salts electrolyte/CeO2-TiO2/ITO/glass configuration was reversible and featured a T  8 % at λ = 686 nm