Using a top-down approach, we report a theoretical investigation of the melting temperature at the nanoscale, T m , for different shapes of "free-standing" nanostructures. To easily calculate the nanoscale melting temperature for a wide range of metals and semiconductors, a convenient shape parameter called R shape is defined. Considering this parameter, we argue why smaller size effects are observed in high bulk melting temperature materials. Using T m , a phase transition stress model is proposed to evaluate the intrinsic strain and stress during the first steps of solidification. Then, the size effect on the Thornton & Hoffman's criterion at the nanoscale is discussed and the intrinsic residual stress determination in nanostructures is found to be essential for sizes below 100 nm. Furthermore, the inverse Hall-Petch effect, for sizes below ∼15 nm, can be understood by this model. Finally, the residual strain in hexagonal zinc oxide nanowires is calculated as a function of the wire dimensions.
We present a rigorous analysis of the thermal conductivity of bulk silicon (Si) and Si nanowires (Si NWs) which takes into account the exact physical nature of the various acoustic and optical phonon mechanisms. Following the Callaway solution for the Boltzmann equation, where resistive and nonresistive phonon mechanisms are discriminated, we derived formalism for the lattice thermal conductivity that takes into account the phonon incidence angles. The phonon scattering processes are represented by frequency-dependent relaxation time. In addition to the commonly considered acoustic three-phonon processes, a detailed analysis of the role of the optical phonon decay into acoustic phonons is performed. This optical phonon decay mechanism is considered to act as acoustic phonon generation rate partially counteracting the acoustic phonon scattering rates. We have derived the analytical expression describing this physical mechanism which should be included in the general formalism as a correction to the resistive phonon-point-defects and phonon-boundary scattering expressions. The phonon-boundary scattering mechanism is taken as a function of the phonon frequency, incidence angles, and surface roughness. The importance of all the mechanisms we have involved in the model is demonstrated clearly with reference to reported data regarding the isotopic composition effect in bulk Si and Si NW samples. Namely, our model accounts for previously unexplained experimental results regarding (i) the isotope composition effect on the thermal conductivity of bulk silicon reported by Ruf et al. [Solid State Commun. 115, 243 (2000)], (ii) the size effect on κ(T) of individual Si NWs reported by Li et al. [Appl. Phys. Lett. 83, 2934 (2003)], and (iii) the dramatic decrease in the thermal conductivity for rough Si NWs reported by Hochbaum et al. [Nature (London) 451, 163 (2008)].
We report a theoretical investigation concerning the melting temperature, T m , of ZnO and Zn nanoparticles (NPs), nanowires (NWs) and nanotubes (NTs). The shapes considered here for the zinc oxide low dimensional structures include spherical NPs, NWs with circular, rectangular (nanobelts) and hexagonal sections and NTs with circular and hexagonal sections. A comparison between ZnO and Zn nanostructures demonstrates a higher stability of ZnO for most size and shape ranges considered. Moreover, the size effect on the melting temperature for ZnO is found to be quite strong: for a spherical ZnO NP with a radius of 5 nm, the size effect on T m corresponds to a decrease of ∼36% relative to the bulk melting temperature, whereas the reduction for the case of a metallic Zn NP with the same dimension is ∼13%. Based on T m estimations as a function of size and shape, we predict that certain ZnO nanostructures, such as small (<10 nm) NTs, may not be viable for nanoelectronics or nanophotonic devices, since T m is too close to, or in some cases even below, room temperature. The influence of the surface tension uncertainties on the calculated melting temperatures is also discussed. Finally, based on the determination of T m at the nanoscale, the maximal intrinsic residual stress in a hexagonal ZnO NW and in a cylindrical Zn NW is estimated to be ∼45 MPa and ∼1.9 GPa, respectively.
The structural properties of InGaN have attracted interest on account of the recent widespread use of the material in visible light-emitting devices. A key topic has been the indirect determination of the composition by x-ray diffraction (XRD). We examine critically the several levels of approximation involved in this procedure. It is shown by extended x-ray absorption fine structure (EXAFS) measurements that the local structure of InGaN is independent of the composition, in the range of InN fraction, from about 15 to 40%, that corresponds to blue to infrared light emission from this material. EXAFS-determined ratios of the numbers of indium and gallium atoms in the first metal co-ordination shell, M1, show very good agreement with the composition measured by established techniques, both structural and chemical, on similar samples. On the other hand, the atomic separations deviate markedly from values calculated using Vegard's law. In particular, the average radial separations, In-N1=2.11(2) Å and In-M1=3.28(3) Å, do not vary significantly with In/Ga ratio in the examined composition range. We conclude with some brief comments on the uncertain but challenging topic of InGaN nanostructure.
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