Claudio Melis

The effect of selective interactions at the interface of polymer–oxide hybrid solar cells

The working mechanisms of excitonic solar cells are strongly dominated by interface processes, which influence the final device efficiency. However, it is still very challenging to clearly track the effects of inter-molecular processes at a mesoscopic level. We report on the realization of polymer-based hybrid solar cells made of prototypical materials, namely, poly(3-hexylthiophene) (P3HT) finely infiltrated in a TiO2 scaffold, with power conversion efficiency exceeding 1%. A step-change improvement in the device performance is enabled by engineering the hybrid interface by the insertion of an appropriate molecular interlayer. An unprecedented set of characterization techniques, including time-resolved optical spectroscopy, X-ray photoemission spectroscopy, positron annihilation spectroscopy and atomistic simulations, allows us to rationalize our findings. We show that a suitable chemical structure of the interlayer molecule induces selective intermolecular interactions, and thus a preferential surface energetic landscape and morphological order at the interface which consequently drives a strong improvement in charge generation and a decrease in recombination losses.

Patterning of gold-polydimethylsiloxane (Au-PDMS) nanocomposites by supersonic cluster beam implantation

Patterned gold–polydimethylsiloxane (Au–PDMS) nanocomposites were fabricated by supersonic cluster beam implantation (SCBI) of neutral gold nanoparticles in PDMS through stencil masks. The influence of nanoparticle dose on the surface roughness and morphology of the micropatterned regions of the nanocomposite was characterized. Nanoparticle implantation causes the swelling of PDMS without affecting substantially the lateral resolution of the patterns. In order to have an insight on the mechanism and the influence of nanoparticle implantation on the polymeric matrix, large-scale molecular dynamics simulations of the implantation process have been performed. The simulations show that even a single cluster impact on PDMS substrate strongly affects the polymer local temperature and density. Our results show that SCBI is a promising methodology for the efficient fabrication of nanocomposite microstructures on polymers with interesting morphological, structural and functional properties.

Thermal transport in porous Si nanowires from approach-to-equilibrium molecular dynamics calculations

We study thermal transport in porous Si nanowires (SiNWs) by means of approach-to-equilibrium molecular dynamics simulations. We show that the presence of pores greatly reduces the thermal conductivity, κ, of the SiNWs as long mean free path phonons are suppressed. We address explicitly the dependence of κ on different features of the pore topology—such as the porosity and the pore diameter—and on the nanowire (NW) geometry—diameter and length. We use the results of the molecular dynamics calculations to tune an effective model, which is capable of capturing the dependence of κ on porosity and NW diameter. The model illustrates the failure of Matthiessens rule to describe the coupling between boundary and pore scattering, which we account for by the inclusion of an additional empirical term.

Anomalous energetics and defect-assisted diffusion of Ga in silicon

We study via first-principles calculations the energetics and diffusion of Ga in c-Si. In contrast to B and In, the favored Ga/self-interstitial complex is the tetrahedral interstitial GaT. Thus in the presence of self-interstitials Ga becomes interstitial, and is electrically deactivated as an acceptor. Studying the native-defect assisted diffusion, we find a self-interstitial-assisted mechanism to be favored; vacancy-assisted diffusion has a sizably larger activation energy, in agreement with the observed transient enhanced diffusion behavior.

Stretchable nanocomposite electrodes with tunable mechanical properties by supersonic cluster beam implantation in elastomers

We demonstrate the fabrication of gold-polydimethylsiloxane nanocomposite electrodes, by supersonic cluster beam implantation, with tunable Youngs modulus depending solely on the amount of metal clusters implanted in the elastomeric matrix. We show both experimentally and by atomistic simulations that the mechanical properties of the nanocomposite can be maintained close to that of the bare elastomer for significant metal volume concentrations. Moreover, the elastic properties of the nanocomposite, as experimentally characterized by nanoindentation and modeled with molecular dynamics simulations, are also well described by the Guth-Gold classical model for nanoparticle-filled rubbers, which depends on the presence, concentration, and aspect ratio of metal nanoparticles, and not on the physical and chemical modification of the polymeric matrix due to the embedding process. The elastic properties of the nanocomposite can therefore be determined and engineered a priori, by controlling only the nanoparticle concentration.

Self-Assembling of Zinc Phthalocyanines on ZnO (1010) Surface through Multiple Time Scales

We adopt a hierarchic combination of theoretical methods to study the assembling of zinc phthalocyanines (ZnPcs) on a ZnO (1010) surface through multiple time scales. Atomistic simulations, such as model potential molecular dynamics and metadynamics, are used to study the energetics and short time evolution (up to ∼100 ns) of small ZnPc aggregates. The stability and the lifetime of large clusters is then studied by means of an atomistically informed coarse-grained model using classical molecular dynamics. Finally, the macroscopic time scale clustering phenomenon is studied by Metropolis Monte Carlo algorithms as a function of temperature and surface coverage. We provide evidence that at room temperature the aggregation is likely to occur at sufficiently high coverage, and we characterize the nature, morphology, and lifetime of ZnPcs clusters. We identify the molecular stripes oriented along [010] crystallographic directions as the most energetically stable aggregates.

Neutral-cluster implantation in polymers by computer experiments

In this work, we perform atomistic model potential molecular dynamics simulations by means of state-of-the art force-fields to study the implantation of a single Au nanocluster on a polydimethylsiloxane substrate. All the simulations have been performed on realistic substrate models containing up to ∼4.6 × 106 of atoms having depths up to ∼90 nm and lateral dimensions up to ∼25 nm. We consider both entangled-melt and cross-linked polydimethylsiloxane amorphous structures. We show that even a single cluster impact on the polydimethylsiloxane substrate remarkably changes the polymer local temperature and pressure. Moreover, we observe the presence of craters created on the polymer surface having lateral dimensions comparable to the cluster radius and depths strongly dependent on the implantation energy. Present simulations suggest that the substrate morphology is largely affected by the cluster impact and that most-likely such modifications favour the penetration of the next impinging clusters.

Thermal rectification in silicon by a graded distribution of defects

We discuss about computer experiments based on nonequilibrium molecular dynamics simulations providing evidence that thermal rectification can be obtained in bulk Si by a non-uniform distribution of defects. We consider a graded population of both Ge substitutional defects and nanovoids, distributed along the direction of an applied thermal bias, and predict a rectification factor comparable to what is observed in other low–dimensional Si–based nanostructures. By considering several defect distribution profiles, thermal bias conditions, and sample sizes, the present results suggest that a possible way for tuning the thermal rectification is by defect engineering.

Lattice strain at c-Si surfaces: a density functional theory calculation

The measurement of the Avogadro constant by counting Si atoms is based on the assumption that Si balls of about 94 mm diameter have a perfect crystal structure up to the outermost atom layers. This not the case because of the surface relaxation and reconstruction, the possible presence of an amorphous layer, and the oxidation process due to the interaction with the ambient. This paper gives the results of density functional calculations of the strain components orthogonal to crystal surface in a number of configurations likely found in real samples.

Thermal boundary resistance in semiconductors by non-equilibrium thermodynamics

We critically address the problem of predicting the thermal boundary resistance at the interface between two semiconductors by atomistic simulations. After reviewing the available models, lattice dynamics calculations and molecular dynamics simulation protocols, we reformulate this problem in the language of non-equilibrium thermodynamics, providing an elegant, robust and valuable theoretical framework for the direct calculation of the thermal boundary resistance through molecular dynamics simulations. The foundation of the method, as well as its subtleties and the details of its actual implementation are presented. Finally, the Si/Ge interface showcase is discussed as the prototypical example of semiconductor heterojunction whose thermal properties are paramount in many front-edge nanotechnologies. Graphical Abstract