In this article, we demonstrate that the thermal conductivity of nanostructured porous silicon is reduced by amorphization and also that this amorphous phase in porous silicon can be created by swift (high-energy) heavy ion irradiation. Porous silicon samples with 41%-75% porosity are irradiated with 110 MeV uranium ions at six different fluences. Structural characterisation by micro-Raman spectroscopy and SEM imaging show that swift heavy ion irradiation causes the creation of an amorphous phase in porous Si but without suppressing its porous structure. We demonstrate that the amorphization of porous silicon is caused by electronic-regime interactions, which is the first time such an effect is obtained in crystalline silicon with single-ion species. Furthermore, the impact on the thermal conductivity of porous silicon is studied by micro-Raman spectroscopy and scanning thermal microscopy. The creation of an amorphous phase in porous silicon leads to a reduction of its thermal conductivity, up to a factor of 3 compared to the non-irradiated sample. Therefore, this technique could be used to enhance the thermal insulation properties of porous Si. Finally, we show that this treatment can be combined with pre-oxidation at 300 °C, which is known to lower the thermal conductivity of porous Si, in order to obtain an even greater reduction.

We report the thermal transport properties of wurtzite GaN in the presence of dislocations using molecular dynamics simulations. A variety of isolated dislocations in a nanowire configuration are analyzed and found to considerably reduce the thermal conductivity while impacting its temperature dependence in a different manner. Isolated screw dislocations reduce the thermal conductivity by a factor of two, while the influence of edge dislocations is less pronounced. The relative reduction of thermal conductivity is correlated with the strain energy of each of the five studied types of dislocations and the nature of the bonds around the dislocation core. The temperature dependence of the thermal conductivity follows a physical law described by a T⁻¹ variation in combination with an exponent factor that depends on the material's nature, type and the structural characteristics of the dislocation core. Furthermore, the impact of the dislocation density on the thermal conductivity of bulk GaN is examined. The variation and absolute values of the total thermal conductivity as a function of the dislocation density are similar for defected systems with both screw and edge dislocations. Nevertheless, we reveal that the thermal conductivity tensors along the parallel and perpendicular directions to the dislocation lines are different. The discrepancy of the anisotropy of the thermal conductivity grows with increasing density of dislocations and it is more pronounced for the systems with edge dislocations. Besides the fundamental insights of the presented results, these could also be used for the identification of the type of dislocations when one experimentally obtains the evolution of thermal conductivity with temperature since each type of dislocation has a different signature, or one could extract the density of dislocations with a simple measurement of thermal anisotropy.

Spherical and cylindrical water droplets on silicon surface are studied to tune the silicon–oxygen interaction. We use molecular dynamics simulations to estimate the contact angle of two different shaped droplets. We found that the cylindrical droplets are independent of the line tension as their three phases curvature is equal zero. Additionally, we compare an analytical model, taking into account or not the Tolman length and we show that for spherical small size droplets, this length is important to be included, in contrast to cylindrical droplets in which the influence of the Tolman length is negligible. We demonstrate that the usual convenient way to exclude linear tension in the general case can give wrong results. Here, we consider cylindrical droplets, since their contact angle does not depend on the droplet size in the range of few to 10ths of nanometres. The droplets are stabilised due to the periodic boundary conditions. This allows us to propose a new parameterisation for nanoscale droplets, which is independent the size of the droplets or its shape, minimising at the same time the calculation procedure. With the proposed methodology, we can extract the epsilon parameter of the interaction potential between a liquid and a solid from the nanoscaled molecular simulation with only as input the macrosized experimental wetting angle for a given temperature.

We report on the size dependence of the surface tension of a free surface of an isotropic fluid. The size dependence of the surface tension is evaluated based on the Gibbs-Tolman-Koenig-Buff equation for positive and negative values of curvatures and the Tolman lengths. For all combinations of positive and negative signs of curvature and the Tolman length, we succeed to have a continuous function, avoiding the existing discontinuity at zero curvature (flat interfaces). As an example, a water droplet in the thermodynamical equilibrium with the vapor is analyzed in detail. The size dependence of the surface tension and the Tolman length are evaluated with the use of experimental data of the International Association for the Properties of Water and Steam. The evaluated Tolman length of our approach is in good agreement with molecular dynamics and experimental data.

Thermal conductivity value of sponge-like meso-porous germanium (meso-PGe) layers measured by means of photoacoustic technique is reported. The room temperature thermal conductivity value is found to be equal to 0.6 W/(m K). The experimental results are in excellent agreement with molecular dynamic and Monte Carlo simulations. Both experiments and simulations show an important thermal conductivity reduction of the meso-PGe layers compared to the bulk Ge. The obtained results reveal meso-PGe as an interesting candidate for both thermoelectric and photovoltaic applications in which thermal transport is a really crucial issue.