Pan Xiao

Influence of microstructures on mechanical behaviours of SiC nanowires: a molecular dynamics study

The tensile behaviours of [111]-oriented SiC nanowires with various microstructures are investigated by using molecular dynamics simulations. The results revealed the influence of microstructures on the brittleness and plasticity of SiC nanowires. Plastic deformation is mainly induced by the anti-parallel sliding of 3C grains along an intergranular amorphous film parallel to the plane and inclined at an angle of 19.47° with respect to the nanowire axis. Our study suggests that the wide dispersion of mechanical properties of SiC nanowires observed in experiments might be attributed to their diverse microstructures.

Wurtzite-to-tetragonal structure phase transformation and size effect in ZnO nanorods

The deformation of [0001]-oriented ZnO nanorods with hexagonal cross sections under uniaxial tensile loading is analyzed through molecular statistical thermodynamics (MST) simulations. The focus is on the size dependence of mechanical behavior in ZnO nanorods with diameters ranging from 1.95 to 17.5 nm. An irreversible phase transformation from the wurtzite (P6(3)mc space group) structure to a tetragonal structure (P4(2)/mnm space group) occurs during the tensile loading process. Youngs modulus before the transformation demonstrates a size dependence consistent with what is observed in experiments. A stronger size dependence of response is seen after the transformation and is attributed to the polycrystalline nature of the transformed structure. A comparison of the MST and molecular dynamics (MD) methods shows that MST is 60 times faster than MD and yields results consistent with the results of MD.

Understanding large plastic deformation of SiC nanowires at room temperature

Tensile behaviors of SiC [111] nanowires with various possible microstructures have been investigated by molecular-dynamics simulations. The results show that the large plastic deformation in these nanowires is induced by the anti-parallel sliding of 3C grains along an ultrathin intergranular amorphous film parallel to the (11 (1) over bar) plane and inclined at an angle of 19.47 degrees. with respect to the nanowire axis. The resulting large plastic deformation of SiC nanowires at room temperature is attributed to the stretching, breaking and re-forming of Si-C bonds in the intergranular amorphous film, which is also evident from the sawtooth jumps in the stress-strain response. Copyright (C) EPLA, 2011

Surface transformation and inversion domain boundaries in gallium nitride nanorods

Phase transformation and subdomain structure in [0001]-oriented gallium nitride (GaN) nanorods of different sizes are studied using molecular dynamics simulations. The analysis concerns the structure of GaN nanorods at 300 K without external loading. Calculations show that a transformation from wurtzite to a tetragonal structure occurs along {0110} lateral surfaces, leading to the formation of a six-sided columnar inversion domain boundary (IDB) in the [0001] direction of the nanorods. This structural configuration is similar to the IDB structure observed experimentally in GaN epitaxial layers. The transformation is significantly dependent on the size of the nanorods.

Molecular-dynamics investigation on polarization retention of barium titanate nanofilm arising from ordered oxygen vacancy

Molecular-dynamics simulations have been carried out to investigate the electric hysteresis of barium titanate nanofilm containing oxygen vacancy ordering array parallel to the {101} crystal plane. The results obtained show a significant weakening of polarization retention from non-zero value to zero as the size of the array was reduced to a critical level, which was attributed to the formation and motion of head-to-head domain wall structure under external field loading process. By comparing with materials containing isolated oxygen vacancies, it was found that the zero retention was due to the oxygen vacancy ordering array rather than to the concentration of oxygen vacancy.

Exploration of CdTe quantum dots as mesoscale pressure sensors via time-resolved shock-compression photoluminescent emission spectroscopy

The nanometer size of CdTe quantum dots (QDs) and their unique optical properties, including size-tunable narrow photoluminescent emission, broad absorption, fast photoluminescence decay, and negligible light scattering, are ideal features for spectrally tagging the shock response of localized regions in highly heterogeneous materials such as particulate media. In this work, the time-resolved laser-excited photoluminescence response of QDs to shock-compression was investigated to explore their utilization as mesoscale sensors for pressure measurements and in situ diagnostics during shock loading experiments. Laser-driven shock-compression experiments with steady-state shock pressures ranging from 2.0 to 13 GPa were performed on nanocomposite films of CdTe QDs dispersed in a soft polyvinyl alcohol polymer matrix and in a hard inorganic sodium silicate glass matrix. Time-resolved photoluminescent emission spectroscopy was used to correlate photoluminescence changes with the history of shock pressure and the...

Laser-excited optical emission response of CdTe quantum dot/polymer nanocomposite under shock compression

Laser-driven shock compression experiments and corresponding finite element method simulations are carried out to investigate the blueshift in the optical emission spectra under continuous laser excitation of a dilute composite consisting of 0.15% CdTe quantum dots by weight embedded in polyvinyl alcohol polymer. This material is a potential candidate for use as internal stress sensors. The analyses focus on the time histories of the wavelength blue-shift for shock loading with pressures up to 7.3 GPa. The combined measurements and calculations allow a relation between the wavelength blueshift and pressure for the loading conditions to be extracted. It is found that the blueshift first increases with pressure to a maximum and subsequently decreases with pressure. This trend is different from the monotonic increase of blueshift with pressure observed under conditions of quasistatic hydrostatic compression. Additionally, the blueshift in the shock experiments is much smaller than that in hydrostatic experim...

Self-healing of fractured one-dimensional brittle nanostructures

Recent experiments have shown that fractured GaAs nanowires can heal spontaneously inside a transmission electron microscope. Here we perform molecular-dynamics simulations to investigate the atomic mechanism of this self-healing process. As the distance between two fracture surfaces becomes less than 1.0 nm, a strong surface attraction is generated by the electrostatic interaction, which results in Ga-As re-bonding at the fracture site and restoration of the nanowire. The results suggest that self-healing might be prevalent in ultrathin one-dimensional nanostructures under near vacuum conditions.

Two opposite size effects of hardness at real nano-scale and their distinct origins

Although it has been well known that hardness of metals obtained with conical indenter remains a constant of about 3 times yield strength in conventional tests, and hardness will show a size effect of increasing hardness with decreasing indentation depth in micro-scale beyond 100 nm, the nano-indentation hardness experiments within 100 nm indentation depth usually show a large deviation and unclear trends. We report the cross-validated experimental and numerical results of two opposite depth-dependences of hardness at real nano-scale. That is to say, the indentation size effect (ISE) of hardness of single-crystal copper shows a rapid increase and then a slow decrease with increasing indentation depth within 100 nm depth. All of the results were coss-checked by means of both elaborated nano-indentation experiments with calibrated indenter tips and large scale molecular dynamics (MD) simulations. Further analysis of the MD results and experimental data reveal that the two opposite ISE of nano-hardness should be attributed to the finite roundness of the indenter tip and the intrinsic transition governing property of the material.