Traian Dumitrica

Thermal Transport across Surfactant Layers on Gold Nanorods in Aqueous Solution

Ultrafast transient absorption experiments and molecular dynamics simulations are utilized to investigate the thermal transport between aqueous solutions and cetyltrimethylammonium bromide (CTAB)- or polyethylene glycol (PEG)-functionalized gold nanorods (GNRs). The transient absorption measurement data are interpreted with a multiscale heat diffusion model, which incorporates the interfacial thermal conductances predicted by molecular dynamics. According to our observations, the effective thermal conductance of the GNR/PEG/water system is higher than that of the GNR/CTAB/water system with a surfactant layer of the same length. We attribute the enhancement of thermal transport to the larger thermal conductance at the GNR/PEG interface as compared with that at the GNR/CTAB interface, in addition to the water penetration into the hydrophilic PEG layer. Our results highlight the role of the GNR/polymer thermal interfaces in designing biological and composite-based heat transfer applications of GNRs, and the importance of multiscale analysis in interpreting transient absorption data in systems consisting of low interfacial thermal conductances.

Molecular dynamics simulations of nanoparticle-surface collisions in crystalline silicon

We present a microscopic description for the impacting process of silicon nanospheres onto a silicon substrate. In spite of the relatively low energy regime considered (up to 1 eV/atom), the impacting process exhibits a rich behavior: A rigid Hertzian model is valid for speeds below 500 m/s, while a quasi-ellipsoidal deformation regime emerges at larger speeds. Furthermore, for speeds up to 1000 m/s the particle undergoes a soft landing and creates a long-lived coherent surface phonon. Higher speeds lead to a rapid attenuation of the coherent phonon due to a partial diamond cubic to-tin phase transformation occurring in the particle.

Excluded Volume Approach for Ultrathin Carbon Nanotube Network Stabilization: A Mesoscopic Distinct Element Method Study

Ultrathin carbon nanotube films have gathered attention for flexible electronics applications. Unfortunately, their network structure changes significantly even under small applied strains. We perform mesoscopic distinct element method simulations and develop an atomic-scale picture of the network stress relaxation. On this basis, we put forward the concept of mesoscale design by the addition of excluded-volume interactions. We integrate silicon nanoparticles into our model and show that the nanoparticle-filled networks present superior stability and mechanical response relative to those of pure films. The approach opens new possibilities for tuning the network microstructure in a manner that is compatible with flexible electronics applications.

Nonthermal transition of GaAs in ultra-intense laser radiation field

Using the technique of tight-binding electron-ion dynamics, we have calculated the response of crystalline GaAs when a femtosecond laser pulse excites 1-20% of the valence electrons. Above a threshold fluence, which corresponds to promotion of about 12% of the valence electrons to the conduction band, the lattice is destabilized and the band gap collapses to zero. This result supports the conclusion that structural changes on a subpicosecond time scale observed in pump-probe experiments are of a nonthermal nature.

Screw-Dislocated Nanostructures

Nanostructures grown by screw dislocations have been successfully synthesized in a range of materials, including thermoelectric materials, but the impact of these extended crystallographic defects on thermal properties of nanostructures had not been known. In this chapter, thermal transport in nanowires storing screw dislocations is investigated via molecular dynamics simulations. The inherent one-dimensionality and the combined presence of a reconstructed surface and dislocation yield ultralow thermal conductivity values. Molecular dynamics (MD) simulations suggest that the large dislocation strain field in nanowires may play a key role in suppressing the thermal conductivity of thermoelectric nanomaterials further to enhance their thermoelectricity.

Amorphous Silicon-Boron-Nitride Networks

In recent years, atomistic simulations are assuming a guiding role in the effort of optimizing the properties of advanced coating materials (Lawson et al., J Appl Phys 110:083507, 2011; Kindlund et al., APL Mater 1:042104, 2013; Tang et al., J Phys Chem C 119:24649–24656, 2015; Zhang et al., Surf Coat Technol 277:136–143, 2015; Ni et al., Appl Phys Lett 107:031603, 2015). In amorphous Silicon-Boron-Nitride networks (a-Si-B-N), understanding the role played by composition is of great importance for the future design of this new material. So far, a-Si-B-N structures have been explored to understand the impact of the BN:Si3N4 ratio onto mechanical properties (Tang et al., Chem Eur J 16:6458–6462, 2010; Schon et al., Process Appl Ceram 5:49–61, 2011; Griebel and Hamaekers, Comput Mater Sci 39:502–517, 2007; Ge et al., Adv Appl Ceram 113:367–371, 2014). Using classical molecular dynamics (MD) simulations, Griebel and Hamaekers (Comput Mater Sci 39:502–517, 2007) derived strain-stress curves of selected a-Si3BN5, a-Si3B2N6, and a-Si3B3N7 models and found that increasing the B content increases Young’s modulus. In this chapter, we extend the scope of the previous studies by revealing how composition and structure might influence a combination of properties desirable for coating applications. Using a combination of atomistic numerical methods, we screen a library of low enthalpy a-Si-B-N networks (a-Si3BN5, a-Si3B3N7, and a-Si3B9N13) to predict from extensive atomistic simulations the thermal conductivity (κ) and mechanical stiffness with different BN contents.

Deformed carbon nanotubes

Carbon nanotubes’ resilience to mechanical deformation is a potentially important feature for imparting tunable properties at the nanoscale. The influence of mechanical deformation on the thermal transport of carbon nanotubes is studied by non-equilibrium molecular dynamics. Nanotubes of different bending angles, lengths, diameters, chiralities, and degrees of twist are simulated in the regime in which the thermal transport extends from ballistic to diffusive. The study in purely bent carbon nanotubes settles the controversy around the differences between the current experimental and molecular dynamics measurements of the thermal transport in bent nanotubes. Collapsed carbon nanotubes, in contrast with graphene nanoribbons, which are known to exhibit substantial rough-edge and cross-plain phonon scatterings, preserve the quasiballistic phononic transport encountered in cylindrical nanotubes. Stacked-collapsed nanotube architectures, closely related with the strain-induced aligned tubes occurring in stretched nanotube sheets, are shown to inherit the ultrahigh thermal conductivities of individual tubes and are therefore proposed to form highways for efficient heat transport in lightweight composite materials.

Computational Nanomechanics of Quasi-one-dimensional Structures in a Symmetry-Adapted Tight Binding Framework

The success of many nanotechnologies depends on our ability to understand and control the mechanics of nano objects, such as nanotubes and nanobelts. Because of the numerous experimental difficulties encountered at this scale, simulation can emerge as a powerful predictive tool. For this, new multiscale simulation methods are needed in which a continuum model emerges from a precise, quantum mechanical description of the atomic scale. Because computing nanomechanical responses requires large systems, computationally affordable but less accurate classical atomistic treatments of the atomic scale are widely adopted and only multiscale classical atomistic-to-continuum bridging is achieved. As a first step towards achieving accurate multiscale models for nano objects, based on a quantum-mechanical description of chemical bonding, here we present an ingenious symmetry-adapted atomistic scheme that performs calculations under helical boundary conditions. The utility of the microscopic method is illustrated with examples discussing the nanomechanical response of carbon nanotubes and thermodynamical stability of silicon nanowires.

NANOMECHANICS OF SILICON NANOWIRES VIA SYMMETRY-ADAPTED TIGHT-BINDING AND CLASSICAL OBJECTIVE MOLECULAR DYNAMICS

Stability and elastic response of the most promising ground state candidate Si nanowires with less than 10 nm in diameter are comparatively studied with objective molecular dynamics coupled with non-orthogonal tight-binding and classical potential models. The computationally-expensive tight-binding treatment becomes tractable due to the substantial simplifications introduced by the presented symmetry-adapted scheme. Quantitative differences regarding stability with the classical model description are noted. Using a Wulff energy decomposition approach it is revealed that these differences are caused by the inability of the classical potential to accurately describe the interaction of Si atoms on surfaces. Differences between the results of the two atomistic treatments are also noted in the elastic response in elongation.

Femtosecond-scale response of semiconductors to laser pulses

We report simulations of the response of InSb, GaAs, and Si to 70-femtosecond laser pulses of various intensities. In agreement with the experiments of Mazur and coworkers, and other groups, there is a nonthermal phase transition for each of these semiconductors above a threshold intensity. Our simulations employ tight-binding electron-ion dynamics (TED), a technique which is briefly described in the text. In the experimental pump-probe observations, the dielectric function (epsilon) ((omega) ) and the second-order susceptibility (chi) (2) can be measured. These same quantities can be calculated during a TED simulation, and there is good agreement in the behavior with respect to both time and frequency. The simulations provide much additional microscopic information which is experimentally inaccessible: for example, the time-dependence of the atomic pair-correlation function, electronic energy bands, occupancies of excited states, kinetic energy of the atoms, and excursions of atoms from their initial positions.