C. Z. Wang

The geometry of small fullerene cages: C20 to C70

The ground‐state structures of small fullerenes below C70 were determined by tight‐binding molecular‐dynamics total energy optimization. An efficient simulated annealing scheme was used to generate closed, hollow, spheroidal cage structures for all even‐numbered carbon clusters from C20 to C70. As a general trend, fullerenes prefer geometries which separate the pentagonal rings as far apart as possible. Except for C60, C70, and C50, most fullerenes have relatively low symmetries.

Ab initio molecular dynamics simulation of liquid Al88Si12 alloys

First-principles molecular dynamics simulations are carried out to study the structures, dynamics, and electronic properties of liquid Al88Si12 in the temperature ranging from 898 to 1298 K. The temperature dependence of static structure factors, pair correlation functions, and electronic density-of-states are investigated. The structural properties obtained from the simulations are in good agreement with the x-ray diffraction experimental results.

High efficiency of photon-to-heat conversion with a 6-layered metal/dielectric film structure in the 250-1200 nm wavelength region

The optical properties and thermal stability of a 6-layered metal/dielectric film structure are investigated in this work. A high optical absorption average of > 98% is achieved in the broad spectral range of 250-1200 nm with experiment results, in good agreement with our simulated results. The samples have a typical layered structure of: SiO(2)(57.3 nm)/Ti(5.7 nm)/SiO(2) (67.1 nm)/Ti(11.6 nm)/SiO(2)(51.4 nm)/Cu(>100 nm), deposited on optically polished Si or K9-glass substrates by magnetron sputtering. The sample of the 6-layered metal/dielectric film structure has an AM1.5G solar absorptance of 95.5% with the features of low thermal emittance of 0.136 at 700K and good thermal stability, and will be potentially suitable for practical application in high-efficiency solar absorber devices in many fields.

Addimer diffusion along the trough between dimer rows on Si(001)

Abstract The diffusion pathways along the trough on the Si(001) surface are investigated by tight-binding molecular dynamics calculations using the environment-dependent tight-binding silicon potential and by ab-initio calculations using the Car–Parrinello method. A new diffusion pathway is discovered, consisting of rotation of the addimer. The energy barrier is calculated to be 1.22xa0eV per dimer, which is in excellent agreement with the experiment.

High photon-to-heat conversion efficiency in the wavelength region of 250–1200 nm based on a thermoelectric Bi 2 Te 3 film structure

In this work, 4-layered SiO2/Bi2Te3/SiO2/Cu film structures were designed and fabricated and the optical properties investigated in the wavelength region of 250–1200u2009nm for their promising applications for direct solar-thermal-electric conversion. A typical 4-layered film sample with the structure SiO2 (66.6u2009nm)/Bi2Te3 (7.0u2009nm)/SiO2 (67.0u2009nm)/Cu (>100.0u2009nm) was deposited on a Si or K9-glass substrate by magnetron sputtering. The experimental results agree well with the simulated ones showing an average optical absorption of 96.5%, except in the shorter wavelength region, 250–500u2009nm, which demonstrates the superior absorption property of the 4-layered film due to the randomly rough surface of the Cu layer resulting from the higher deposition power. The high reflectance of the film structure in the long wavelength region of 2–20u2009μm will result in a low thermal emittance, 0.064 at 600u2009K. The simpler 4-layered structure with the thermoelectric Bi2Te3 used as the absorption layer may provide a straightforward way to obtain solar-thermal-electric conversion more efficiently through future study.

Predicting the Atomic Configuration of 1- and 2-Dimensional Nanostructures via Global Optimization Methods

In the cluster structure community, global optimization methods are common tools for arriving at the atomic structure of molecular and atomic clusters. The large number of local minima of the potential energy surface of these clusters, and the fact that these local minima proliferate exponentially with the number of atoms in the cluster simply demands the use of fast stochastic methods to find the optimum atomic configuration. Therefore, much of the development work has come from (and mostly stayed within) the cluster structure community. Partly due to wide availability and landmark successes of high resolution microscopy techniques, finding the structure of periodically reconstructed semiconductor surfaces was not posed as a problem of stochastic optimization until recently, when we have shown that high-index semiconductor surfaces can posses a rather large number of local minima with such low surface energies that the identification of the global minimum becomes problematic. We have therefore set out to develop global optimization methods for systems other than clusters, focusing on periodic systems in one- and two- dimensions as such systems currently occupy a central place in the field of nanoscience. In this article, we review some of our recent work on global optimization methods (the parallel-tempering Monte Carlo method and the genetic algorithm) and show examples/results from two main problem categories: (1) the two-dimensional problem of determining the atomic configuration of clean semiconductor surfaces, and (2) finding the structure of freestanding nanowires. While focused mainly on optimization the atomic structure for a system with set periodic boundary conditions, our account also reviews a recent example of using genetic algorithms for growth of nanostructures into their global energy minima compatible with given confinement conditions