Minimal energy control of a nanoelectromechanical memory element

The Pontryagin minimal energy control approach has been applied to minimise the switching energy in a nanoelectromechanical memory system and to characterise global stability of the oscillatory states of the bistable memory element. A comparison of two previously experimentally determined pulse-type control signals with Pontryagin control function has been performed, and the superiority of the Pontryagin approach with regard to power consumption has been demonstrated. An analysis of global stability shows how values of minimal energy can be utilized in order to specify equally stable states

Towards bioinspired superhydrophobic poly(L-lactic acid) surfaces using phase inversion-based methods

The water repellency and self-cleaning ability of many biological surfaces has inspired many fundamental and practical studies related to the development of synthetic superhydrophobic surfaces. However, the investigation of such substrates made of biodegradable polymers has been scarce. Simple approaches based on a single step, performed at room temperature (and pressure), were implemented to obtain superhydrophobic poly(L-lactic acid) (PLLA) surfaces via phase inversion-based methods, without addition of low-surface-energy compounds. Water contact angles above 150◦ were obtained using some processing conditions. In such cases scanning electronic microscopy micrographs of such surfaces revealed a clear rough texture composed by leafy clusters with micro-nano binary structures. Such materials could be used in specific environmental and biomedical applications, namely in implantable materials or in antibacterial or antithrombogenic surfaces

pH responsive biomineralization onto chitosan grafted biodegradable substrates

Bioactive composites that enable the formation of an apatite layer onto the surface are important in the development of osteoconductive biomaterials in orthopaedic applications. In this work, the surface of biodegradable and bioactive substrates, composed of poly(L-lactic acid) (PLLA) reinforced with Bioglass , was modified by coupling chitosan to the surface, using plasma activation. The wettability of the modified films was analysed by contact angle (CA) measurements as a function of pH. It was found that this surface property changed significantly with pH. Moreover, the apatite formation on the surface upon immersion of the modified films in a simulated body fluid (SBF) solution was analysed at pH 5.4 and pH 7.4 by scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS). It was found that such modification, together with the effect of pH, could block the formation of apatite onto the biodegradable substrate upon immersion in a simulated body fluid solution when the pH changed to 5.4. On the other hand, a dense apatite layer was formed at pH 7.4. For the unmodified substrates an apatite layer was formed at both pHs. These results suggest that the formation of apatite or possibly other kinds of minerals could be controlled by such a smart surface, in this case pH-responsive

Size dependent line broadening in the emission spectra of single GaAs quantum dots: Impact of surface charges on spectral diffusion

Making use of droplet epitaxy, we systematically controlled the height of self-assembled GaAs quantum dots by more than one order of magnitude. The photoluminescence spectra of single quantum dots revealed the strong dependence of the spectral linewidth on the dot height. Tall dots with a height of ~30 nm showed broad spectral peaks with an average width as large as ~5 meV, but shallow dots with a height of ~2 nm showed resolution-limited spectral lines (<120 micro eV). The measured height dependence of the linewidths is in good agreement with Stark coefficients calculated for the experimental shape variation. We attribute the microscopic source of fluctuating electric fields to the random motion of surface charges at the vacuum-semiconductor interface. Our results offer guidelines for creating frequency-locked photon sources, which will serve as key devices for long-distance quantum key distribution.Comment: 6 pages, 6 figures; updated figs and their description

Biomimetic polysaccharide/bioactive glass nanoparticles multilayer membranes for guided tissue regeneration

Nowadays, guided tissue regeneration (GTR) research is centred in the development of composite bioabsorbable membranes with enhanced bioactivity and with processing controlled at the nanoscale. Inspired by this new focus of GTR research and also by nacre structure, layered freestanding membranes were produced using the Layer-by-Layer (LbL) deposition technique, combining chitosan (CHI), hyaluronic acid (HA) and bioactive glass nanoparticles (BGNPs). It is expected that the combination of these materials processed by this particular technique will result in nanostructured membranes with enhanced mechanical performance as well as improved bioactivity. Moreover, the effect of the modification of HA with catechol groups (HAD) on the adhesive properties of the membranes was also analysed. The results showed that it was possible to produce biomimetic membranes with different surface properties, improved adhesive strength and the ability to induce the formation of apatite, necessary for the formation of new bone. It was also possible to control the BGNPs content of the membranes by use of HAD instead of unmodified HA and changing the number of BGNPs' deposition steps. Moreover, it was shown that membranes with different concentrations of BGNPs possess different mechanical performance, swelling properties and degradation behaviour, which indicates the possibility to tune the membranes' properties by controlling the deposition of BGNPs onto the membranes

Thermal Transport Imaging in the Quantum Hall Edge Channel

Research focused on heat transport in the quantum Hall (QH) edge channel has successfully addressed fundamental theoretical questions surrounding the QH physics. However, the picture of the edge channel is complicated by the phenomenon of energy dissipation out of the edge, and theories treating this dissipation are lacking. More experimental data is also needed to determine the coupling mechanism by which energy leaves the edge channel. We developed a method to map the heat transport in the QH edge to study the dissipation of heat. We locally heated the QH edge and locally detected the temperature increase while continuously varying the distance between heater and thermometer. We thereby obtained the thermal decay length of the edge state, which we found to depend on magnetic field strength

Enthalpy relaxation studies in polymethyl methacrylate networks with different crosslinking degrees

Structural relaxation of PMMA networks with distinct crosslink density has been studied by differential scanning calorimetry (DSC). The crosslinking agent used was ethylene glycol dimethacrylate (EGDMA). The experiments were carried out on heating after the samples have been subjected to distinct thermal histories, namely isothermal stages at different temperatures below the glass transition temperature for distinct times and cooling at different rates. These studies revealed a broadening of the glass transition with increasing crosslinking degree due to the constraints imposed by the crosslinks and suggested the presence of crosslink heterogeneity in the networks. A phenomenological model based on the configurational entropy concept was used to simulate the structural relaxation phenomenon and to evaluate the temperature dependence and distribution of the relaxation times of the conformational rearrangements for these networks. The agreement between the experimental results and the simulated thermograms was quite satisfactory. In addition, the kinetic fragility of the networks was evaluated from the results corresponding to the thermal treatments at distinct cooling rates. It was found an increase of the fragility index m with increasing crosslinking degree

Analysis of the thermal environment inside the furnace of a dynamic mechanical analyser

In this work, the thermal environment inside the furnace of a dynamic mechanical analyser was investigated by putting a standard sample in distinct positions inside the furnace. Penetration experiments were carried out in order to compare the measured melting temperature of the sample with the theoretical melting temperature. The thermal gradients were investigated for three distinct modes: compression, three-point bending and extension. In the compression mode a small variation of the measured melting point was found as a function of the radial distance. However, for both extension and three-point bending modes quite important variations were measured along the relevant directions. In the former case the bottom clamp was found to be warmer than the upper one and in the latter mode higher temperatures are found in the extremities of the samples (closer to the furnace wall). Other factors, such as the influence of the heating rate and the purge gas flow rate, were also investigated. In the particular case of the extension mode, it was found that the measured melting temperature decreased with increasing scanning rate. A simple model was used in order to investigate the influence of the temperature gradients on DMA measurements. For temperature gradients along the sample length below 10°C the differences in the viscoelastic parameters (tan d, D∗ and E∗) from the homogeneous case are small when compared with typical experimental errors