A.M. Beltrán

Micro/Macroporous System: MFI‐Type Zeolite Crystals with Embedded Macropores

Zeolite crystals with an embedded and interconnected macropore system are prepared by using mesoporous silica particles as a silica source and as a sacrificial macroporogen. These novel hierarchical zeolite crystals are expected to reduce diffusion limitations in all zeolite-catalyzed reactions, especially in the transformation of larger molecules like in the catalytic cracking of polymers and the conversion of biomass.

Incorporation of Sb in InAs∕GaAs quantum dots

The formation of a quaternary InGaAsSb alloy is shown to occur in the core of epitaxial GaSb capped InAs∕GaAs quantum dots emitting at 1.3μm. The existence of the four constituent elements is demonstrated by using spatially resolved low-loss electron energy loss spectroscopy and aberration-corrected high angle annular dark field scanning transmission electron microscopy. The intermixing process giving rise to the formation of this quaternary alloy takes place despite the large miscibility gap between InAs and GaSb binary compounds, and is probably driven by the existence of strain in the quantum dots.

Uniform Surface Modification of 3D Bioglass(®)-Based Scaffolds with Mesoporous Silica Particles (MCM-41) for Enhancing Drug Delivery Capability.

The design and characterization of a new family of multifunctional scaffolds based on bioactive glass (BG) of 45S5 composition for bone tissue engineering and drug delivery applications are presented. These BG-based scaffolds are developed via a replication method of polyurethane packaging foam. In order to increase the therapeutic functionality, the scaffolds were coated with mesoporous silica particles (MCM-41), which act as an in situ drug delivery system. These sub-micron spheres are characterized by large surface area and pore volume with a narrow pore diameter distribution. The solution used for the synthesis of the silica mesoporous particles was designed to obtain a high-ordered mesoporous structure and spherical shape – both are key factors for achieving the desired controlled drug release. The MCM-41 particles were synthesized directly inside the BG-based scaffolds, and the drug-release capability of this combined system was evaluated. Moreover, the effect of MCM-41 particle coating on the bioactivity of the BG-based scaffolds was assessed. The results indicate that it is possible to obtain a multifunctional scaffold system characterized by high and interconnected porosity, high bioactivity, and sustained drug delivery capability.

Three dimensional atom probe imaging of GaAsSb quantum rings

Unambiguous evidence of ring-shaped self-assembled GaSb nanostructures grown by molecular beam epitaxy is presented on the basis of atom-probe tomography reconstructions and dark field transmission electron microscopy imaging. The GaAs capping process causes a strong segregation of Sb out of the center of GaSb quantum dots, leading to the self-assembled GaAs(x)Sb(1-x) quantum rings of 20-30 nm in diameter with x ∼ 0.33.

Comparative Study of Electrical and Microstructural Properties of 4H-SiC MOSFETs

N-channel MOSFETs were manufactured on p-type and on p-implanted, n-type 4H-SiC substrates. The electron mobility in the inversion channel was measured to be correlated with the structural and chemical properties determined by transmission electron microscopy. With regard to what was previously discussed in the literature, interfacial layer formation and carbon distribution across the SiC/SiO2 interface were considered in relation with the measured Hall electron mobility.

Blocking of indium incorporation by antimony in III-V-Sb nanostructures

The addition of antimony to III-V nanostructures is expected to give greater freedom in bandgap engineering for device applications. One of the main challenges to overcome is the effect of indium and antimony surface segregation. Using several very high resolution analysis techniques we clearly demonstrate blocking of indium incorporation by antimony. Furthermore, indium incorporation resumes when the antimony concentration drops below a critical level. This leads to major differences between nominal and actual structures.

High resolution electron microscopy of GaAs capped GaSb nanostructures

We show in this work that GaAs capping of 2 ML of GaSb grown by molecular beam epitaxy results in the formation of very small (with heights of about 1 nm) GaAsxSb1−x nanostructures surrounded by a GaAs rich layer. This conclusion is obtained by analyzing the morphology of the GaAsxSb1−x nanostructures by high resolution scanning transmission electron microscopy in Z-contrast mode. This result shows that a significant fraction of the Sb atoms must segregate along the growth direction during the GaAs capping process.

Development of Novel Mesoporous Silica-Based Bioactive Glass Scaffolds with Drug Delivery Capabilities

Novel silica-based bioactive glasses were successfully prepared by the sol-gel method. The optimized glass composition for fabrication of the scaffolds was (in mol.%) 60% SiO2 – 30% CaO - 5% Na2O - 5% P2O5 (60S30C5N5P). This composition was confirmed to develop a thick hydroxycarbonate apatite (HCA) layer in Simulated Body Fluid (SBF) after 7 days, as revealed by Fourier Transform Infrared Spectroscopy (FTIR), indicating the bioactive character of the scaffolds. The mesoporous nature of the glass structure allows the load of tetracycline and a sustained release of the drug in PBS during 7 days was measured.

Incorporation of Calcium Containing Mesoporous (MCM-41-Type) Particles in Electrospun PCL Fibers by Using Benign Solvents

The electrospinning technique is a versatile method for the production of fibrous scaffolds able to resemble the morphology of the native extra cellular matrix. In the present paper, electrospinning is used to fabricate novel SiO2 particles (type MCM-41) containing poly(epsilon-caprolactone) (PCL) fibers. The main aims of the present work are both the optimization of the particle synthesis and the fabrication of composite fibers, obtained using benign solvents, suitable as drug delivery systems and scaffolds for soft tissue engineering applications. The optimized synthesis and characterization of calcium-containing MCM-41 particles are reported. Homogeneous bead-free composite electrospun mats were obtained by using acetic acid and formic acid as solvents; neat PCL electrospun mats were used as control. Initially, an optimization of the electrospinning environmental parameters, like relative humidity, was performed. The obtained composite nanofibers were characterized from the morphological, chemical and mechanical points of view, the acellular bioactivity of the composite nanofibers was also investigated. Positive results were obtained in terms of mesoporous particle incorporation in the fibers and no significant differences in terms of average fiber diameter were detected between the neat and composite electrospun fibers. Even if the Ca-containing MCM-41 particles are bioactive, this property is not preserved in the composite fibers. In fact, during the bioactivity assessment, the particles were released confirming the potential application of the composite fibers as a drug delivery system. Preliminary in vitro tests with bone marrow stromal cells were performed to investigate cell adhesion on the fabricated composite mats, the positive obtained results confirmed the suitability of the composite fibers as scaffolds for soft tissue engineering.

Nano-Analytical and Electrical Characterization of 4H-SiC MOSFETs

4H-SiC presents great advantages for its use in power electronic devices working at particular conditions. However the development of MOSFETs based on this material is limited by mobility degradation. N-channel SiC MOSFETs were manufactured on p-type epitaxial and p-implanted substrates and the electron mobility in the inversion channels was measured to be correlated with their structural and chemical properties determined by transmission electron microscopy methods. With regard to what was previously discussed in the literature, transition layer formation and carbon distribution across the SiC-SiO2 interface are considered in relation with the measured low electron mobility of the MOSFETS.