Z. D. Sha

Perfect spin-filter and spin-valve in carbon atomic chains

We report ab initio calculations of spin-dependent transport in single atomic carbon chains bridging two zigzag graphene nanoribbon electrodes. Our calculations show that carbon atomic chains coupled to graphene electrodes are perfect spin-filters with almost 100% spin polarization. Moreover, the carbon atomic chains show bias-dependent magnetoresistance as large as 106% which make them perfect spin-valves. These two spin-related properties are independent of the length of carbon chains. The simultaneous occurrence of spin-filter and spin-valve in a single device opens the possibilities of all-carbon composite spintronics.

Structure and photoluminescence properties of Fe-doped ZnO thin films

Zn1−xFexO films were prepared by the radio-frequency (rf) magnetron sputtering technique on n-Si substrates with a composite target of a ceramic polycrystalline ZnO containing several Fe pieces on the surface. X-ray photoelectron spectroscopy study of the Zn1−xFexO films shows that Fe in the as-deposited film exists mainly in the form of Fe2+. And x-ray diffraction spectra show that all the films exhibited c-axis orientation. The room temperature photoluminescence (PL) properties of the Zn1−xFexO films were also discussed. Two obvious PL peaks appear at 378 nm and 414 nm, respectively. It is interesting that there is a tendency of redshift for the peak at 378 nm and that the PL intensity increases slightly for the peak at 414 nm as the Fe concentration increases. Discussions have been given to explain the different phenomena.

Effects of edge passivation by hydrogen on electronic structure of armchair graphene nanoribbon and band gap engineering

We investigated effects of hydrogen passivation of edges of armchair graphene nanoribbons (AGNRs) on their electronic properties using first-principles method. The calculated band gaps of the AGNRs vary continually over a range of 1.6 eV as a function of a percentage of sp3-like bonds at the edges. This provides a simple way for band gap engineering of graphene as the relative stability of sp2 and sp3-like bonds at the edges of the AGNRs depends on the chemical potential of hydrogen gas, and the composition of the sp2 and sp3-like bonds at the edges of the AGNRs can be easily controlled experimentally via temperature and pressure of H2 gas.

Tuning the thermal conductivity of silicene with tensile strain and isotopic doping: A molecular dynamics study

Silicene is a monolayer of silicon atoms arranged in honeycomb lattice similar to graphene. We study the thermal transport in silicene by using non-equilibrium molecular dynamics simulations. We focus on the effects of tensile strain and isotopic doping on the thermal conductivity, in order to tune the thermal conductivity of silicene. We find that the thermal conductivity of silicene, which is shown to be only about 20% of that of bulk silicon, increases at small tensile strains but decreases at large strains. We also find that isotopic doping of silicene results in a U-shaped change of the thermal conductivity for the isotope concentration varying from 0% to 100%. We further show that ordered doping (isotope superlattice) leads to a much larger reduction in thermal conductivity than random doping. Our findings are important for the thermal management in silicene-based electronic devices and for thermoelectric applications of silicene.

Mechanical properties and fracture behavior of single-layer phosphorene at finite temperatures

Phosphorene, a new two-dimensional (2D) material beyond graphene, has attracted great attention in recent years due to its superior physical and electrical properties. However, compared to graphene and other 2D materials, phosphorene has a relatively low Youngs modulus and fracture strength, which may limit its applications due to possible structure failures. For the mechanical reliability of future phosphorene-based nanodevices, it is necessary to have a deep understanding of the mechanical properties and fracture behaviors of phosphorene. Previous studies on the mechanical properties of phosphorene were based on first principles calculations at 0 K. In this work, we employ molecular dynamics simulations to explore the mechanical properties and fracture behaviors of phosphorene at finite temperatures. It is found that temperature has a significant effect on the mechanical properties of phosphorene. The fracture strength and strain reduce by more than 65% when the temperature increases from 0 K to 450 K. Moreover, the fracture strength and strain in the zigzag direction is more sensitive to the temperature rise than that in the armchair direction. More interestingly, the failure crack propagates preferably along the groove in the puckered structure when uniaxial tension is applied in the armchair direction. In contrast, when the uniaxial tension is applied in the zigzag direction, multiple cracks are observed with rough fracture surfaces. Our present work provides useful information about the mechanical properties and failure behaviors of phosphorene at finite temperatures.

Carbon isotope doping induced interfacial thermal resistance and thermal rectification in graphene

We investigate the thermal transport properties of carbon isotope doped graphene using nonequilibrium molecular dynamics simulations. We find that the interfacial thermal resistance between graphene and the isotope atoms causes severe reduction in thermal conductivity of the doped graphene. Furthermore, we find that thermal rectification occurs in the interface. Tensile strain leads to an increase in the interfacial thermal resistance and thermal rectification, while increasing temperature decreases these parameters. We calculate the phonon spectra and find that the thermal rectification is associated with the overlap areas in the phonon spectra.

Statistical composition-structure-property correlation and glass-forming ability based on the full icosahedra in Cu–Zr metallic glasses

Using the large-scale atomic/molecular massively parallel simulator, fraction of the Cu-centered ⟨0,0,12,0⟩ full icosahedra (fico) is obtained from a statistical analysis over a broad compositional range with high resolution in the Cu–Zr binary system. Weak but significant peaks are observed at certain compositions which coincide with good glass formers. This correlation implies that the change in fico is a fundamental structural factor in determining the ease of glass formation. In this regard, fico can be an indicator of glass-forming ability. Our work provides further understanding on the atomic structure of the Cu–Zr system and its effect on glass formation.

A transition from localized shear banding to homogeneous superplastic flow in nanoglass

A promising remedy to the failure of metallic glasses (MGs) by shear banding is the use of a dense network of glass-glass interfaces, i.e., a nanoglass (NG). Here we investigate the effect of grain size (d) on the failure of NG by performing molecular dynamics simulations of tensile-loading on Cu50Zr50 NG with d = 5 to 15 nm. Our results reveal a drastic change in deformation mode from a single shear band (d ∼ 15 to 10 nm), to cooperative shear failure (d ∼ 10 to 5 nm), to homogeneous superplastic flow (d ≤ 5 nm). Our results suggest that grain size can be an effective design parameter to tune the mechanical properties of MGs.

Effects of temperature and strain rate on the mechanical properties of silicene

Silicene, a graphene-like two-dimensional silicon, has attracted great attention due to its fascinating electronic properties similar to graphene and its compatibility with existing semiconducting technology. So far, the effects of temperature and strain rate on its mechanical properties remain unexplored. We investigate the mechanical properties of silicene under uniaxial tensile deformation by using molecular dynamics simulations. We find that the fracture strength and fracture strain of silicene are much higher than those of bulk silicon, though the Youngs modulus of silicene is lower than that of bulk silicon. An increase in temperature decreases the fracture strength and fracture strain of silicene significantly, while an increase in strain rate enhances them slightly. The fracture process of silicene is also studied and brittle fracture behavior is observed in the simulations.

On the notch sensitivity of CuZr metallic glasses

Atomistic simulations are performed to study the effects of size and shape of a superficial or internal notch on the strength and failure mechanism of CuZr metallic glass (MG) under tensile loading. Our results show that plastic deformation originating at the notch root reduces the stress concentration there and leads to a notch-insensitive normalized tensile strength. The notch, however, dictates the failure location as the plastic zone at the notch root serves as a nucleation site for shear band (SB) formation. It is shown that when the plastic zone size reaches a critical value, a SB starts to propagate from the notch root across the entire sample, causing the material failure. These results provide useful guidelines for the design, testing, and engineering of MG for structural applications.