J. S. de Sousa

Thickness-corrected model for nanoindentation of thin films with conical indenters

Nanoindentation of soft materials is a growing research field, demanding sophisticated models to extract accurate information from these materials. In this work we investigate the nanoindentation of thin soft films by sharp conical indenters using Finite Elements Modeling. Based on the work of Dimitriadis et al. [Biophys. J. 82, 2798 (2002)], we propose that load-displacement (F × δ) curves for conical indenters can be described by F = FHertz(δ)g[χ(δ,h)], where FHertz(δ) is the regular Hertz model, and g[χ(δ,h)] is a correction function that includes finite thickness effects. To test the applicability of the model, we analyze the elastic modulus of fibroblast cells as measured by Atomic Force Microscopy. The elastic modulus obtained with Hertz model is overestimated by 50% (when compared to our thickness-corrected model) in the thickest parts of the cell (3.67 μm), and by approximately 128% in the lamellipodia region (0.45 μm), illustrating the importance of the sample thickness for the evaluation of the mechanical properties.

Hole-versus electron-based operations in SiGe nanocrystal nonvolatile memories

A comparative study between electron- and hole-based data storage operations in Si1−xGex nanocrystal nonvolatile memories is presented. The authors show that the valence band-edge alignment is ideally suited for holes storage, which allows for extremely long retention times for alloy composition x>0 without affecting the programming performances.

Radiative transitions in P- and B-doped silicon nanocrystals

The radiative transitions in P- and B-doped Si nanocrystals are investigated by means of first-principle calculations. Using a three-level model, we show that the radiative lifetimes and oscillator strengths of the transitions between the conduction and the impurity bands, as well as the transitions between the impurity and the valence bands are strongly affected by the impurity position. On the other hand, the direct conduction-to-valence band decay is practically unchanged due to the presence of the impurity. In addition, the emission intensity of P(B)-doped nanocrystals with impurities positioned in the surface (anywhere) is higher (lower) than for pure nanocrystals.

Intraband absorption in silicon nanocrystals: The combined effect of shape and crystal orientation

We investigate intraband transitions in Si∕SiO2 quantum dots (QD’s) by using a tridimensional quantum mechanical model that takes into account the six-valley structure of silicon. The interplay between QD orientation and shape strongly affects the infrared absorption spectra of Si QD’s. In particular, we show the orientation of the Si valleys dramatically changes the optical properties of Si QD’s.

CaO first-principles electronic properties and MOS device simulation

The carrier effective masses of CaO in the cubic phase are estimated by ab initio calculations, which are used for the simulation of Si/CaO metal–oxide–semiconductor (MOS) devices by solving Schrodinger and Poisson equations self-consistently. The possibility of using CaO as a gate dielectric material for MOS device applications is then discussed. The theoretical simulations point to the possibility of using CaO as a gate dielectric, but thin films of CaO on silicon still present roughness that precludes its actual use as a gate dielectric material.

Micromorphology and microrheology of modified bitumen by atomic force microscopy

The rheological study of bitumen is essential to improve the performance of pavements. Bitumen properties are traditionally determined by means of dynamic shear rheometer measurements. In this work, we aim to demonstrate that atomic force microscopy (AFM) can be a powerful complementary tool to the characterisation of bitumen. In particular, we compare the mechanical properties of pure and modified bitumen. Two different anti-oxidant additives were used: PPA (poly-phosphoric acid) and LCC (cashew nut oil) at concentrations of 1% and 2%, respectively. We will show that the AFM technique is able to distinguish the effects of both additives in the morphological and micromechanical properties of bitumen films.

Excitons in Si1−xGex nanocrystals: Ab initio calculations

The electronic structure of hydrogen-passivated Si1−xGex nanocrystals is investigated by means of ab initio calculations. Our calculated optical gaps and electron-hole binding energies decrease linearly with x, while the exciton exchange energy increases with x due to the increase of the spatial extent of the electron and hole wave functions. This also increases the electron-hole wave functions overlap, leading to recombination lifetimes that are very sensitive to the Ge content. Our results exhibited very good agreement with available theoretical data.

Light-induced programming of Si nanocrystal flash memories

We show theoretically that it is possible to achieve much faster programming performances in nanocrystal flash memory devices by means of a light-induced mechanism that inverts the direction of charge transfer between nanocrystals and device substrate in comparison to conventional voltage-induced programming. Our method is based on the self-consistent solutions of Hartree and Poisson equation for both electrons and holes with open boundary conditions.

Substrate effects on the exciton fine structure of black phosphorus quantum dots

We study the size-dependent exciton fine structure in monolayer black phosphorus quantum dots (BPQDs) deposited on different substrates (isolated, Si and SiO

On the interplay between quantum confinement and dielectric mismatch in high-k based quantum wells

_2