Songyou Wang

Bio: Songyou Wang is an academic researcher from Fudan University. The author has contributed to research in topics: Thin film & Band gap. The author has an hindex of 25, co-authored 184 publications receiving 2475 citations. Previous affiliations of Songyou Wang include Chinese Ministry of Education & United States Department of Energy.

Broadband optical absorption by tunable Mie resonances in silicon nanocone arrays

Abstract: Nanostructure arrays such as nanowire, nanopillar, and nanocone arrays have been proposed to be promising antireflection structures for photovoltaic applications due to their great light trapping ability. In this paper, the optical properties of Si nanopillar and nanocone arrays in visible and infrared region were studied by both theoretical calculations and experiments. The results show that the Mie resonance can be continuously tuned across a wide range of wavelength by varying the diameter of the nanopillars. However, Si nanopillar array with uniform diameter exhibits only discrete resonance mode, thus can't achieve a high broadband absorption. On the other hand, the Mie resonance wavelength in a Si nanocone array can vary continuously as the diameters of the cross sections increase from the apex to the base. Therefore Si nanocone arrays can strongly interact with the incident light in the broadband spectrum and the absorbance by Si nanocone arrays is higher than 95% over the wavelength from 300 to 2000 nm. In addition to the Mie resonance, the broadband optical absorption of Si nanocone arrays is also affected by Wood-Rayleigh anomaly effect and metal impurities introduced in the fabrication process.

Optical properties of cubic Ti3N4, Zr3N4, and Hf3N4

Abstract: A systematic theoretical study is presented for the electronic, mechanical, and optical properties of cubic Ti3N4, Zr3N4, and Hf3N4 with the Th3P4 structure in the framework of density functional theory. The calculated band structures of Ti3N4, Zr3N4, and Hf3N4 show the indirect band gaps of 0.268, 0.909, and 1.00eV, respectively. Furthermore, the optical properties for all three materials were calculated and analyzed in detail. The calculated results are well consistent with available experimental data. Also, it is shown that all these materials have relatively large static dielectric constants at zero frequency, rendering them potential applications in microelectronic devices.

Third-generation plasma immersion ion implanter for biomedical materials and research

Abstract: A third generation plasma immersion ion implanter dedicated to biomedical materials and research has been designed and constructed. The distinct improvement over first and second generation multipurpose plasma immersion ion implantation equipment is that hybrid and combination techniques utilizing metal and gas plasmas, sputter deposition, and ion beam enhanced deposition can be effectively conducted in the same machine. The machine consists of four sets of high-efficiency metal arc plasma sources with magnetic filters, a custom designed high voltage modulator for operation up to 60 kV, a separate high-frequency, low-voltage power supply for hybrid treatment processes, special rotating sample stage for samples with an irregular shape, and other advanced features. The machine has been installed at Southwest Jiaotong University and operated reliably for 6 months. This article describes the design principles and performances of the machine as well as pertinent biomedical applications.

Experimental and ab initio molecular dynamics simulation studies of liquid Al 60 Cu 40 alloy

Abstract: Solidification of crystalline phases from the liquid is a highly complex, yet critically important process involving phase selection and growth that is of academic interest and also has many practical considerations. 1 In order to fully model this process at the atomic level, we must begin with an accurate description of the local atomic structure of the liquid and further describe how it changes as a function of temperature. Accurately describing the liquid state is complicated by its lack of long-range order and the inherently statistical nature of its short-range order SRO. However, the local atomic arrangement of the nearest neighbors is expected to play an important role in the properties of liquid and amorphous materials. 2‐4 Recently, ab initio molecular dynamics MD simulations of the amorphous solids and liquid have provided profound insights into the structures and properties of these disordered materials, especially when the calculated data closely correspond to the experimental data. The consistency between the computer simulation and experimental data lends strong support to the further analysis of the structural and electronic properties of the amorphous or liquid systems. In this paper liquid structures of an Al60Cu40 alloy throughout a 450 K span of superheat temperature are studied both by experiments and ab initio MD simulations. The theoretical atomic configurations of the liquid structures are verified by comparing the calculated pair-correlation functions with the experimental results. The atomic configurations derived are further investigated using standard geometric methods. The local atomic structure of the liquid is then compared to various crystalline phases reported for Al-Cu alloys to check for topological similarities.

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Ferromagnetic materials

Abstract: In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Crystal structures and elastic properties of superhard IrN2 and IrN3 from first principles

Abstract: First principles calculations were performed to investigate the structural, elastic, and electronic properties of IrN2 for various space groups: cubic Fm-3m and Pa-3, hexagonal P3(2)21, tetragonal P4(2)/mnm, orthorhombic Pmmn, Pnnm, and Pnn2, and monoclinic P2(1)/c. Our calculation indicates that the P2(1)/c phase with arsenopyrite-type structure is energetically more stable than the other phases. It is semiconducting (the remaining phases are metallic) and contains diatomic N-N with the bond distance of 1.414 A. These characters are consistent with the experimental facts that IrN2 is in lower symmetry and nonmetallic. Our conclusion is also in agreement with the recent theoretical studies that the most stable phase of IrN2 is monoclinic P2(1)/c. The calculated bulk modulus of 373 GPa is also the highest among the considered space groups. It matches the recent theoretical values of 357 GPa within 4.3% and of 402 GPa within 7.8%, but smaller than the experimental value of 428 GPa by 14.7%. Chemical bonding and potential displacive phase transitions are discussed for IrN2. For IrN3, cubic skutterudite structure (Im-3) was assumed.

Laser-induced breakdown spectroscopy (LIBS), part II: review of instrumental and methodological approaches to material analysis and applications to different fields.

Abstract: The first part of this two-part review focused on the fundamental and diagnostics aspects of laser-induced plasmas, only touching briefly upon concepts such as sensitivity and detection limits and largely omitting any discussion of the vast panorama of the practical applications of the technique. Clearly a true LIBS community has emerged, which promises to quicken the pace of LIBS developments, applications, and implementations. With this second part, a more applied flavor is taken, and its intended goal is summarizing the current state-of-the-art of analytical LIBS, providing a contemporary snapshot of LIBS applications, and highlighting new directions in laser-induced breakdown spectroscopy, such as novel approaches, instrumental developments, and advanced use of chemometric tools. More specifically, we discuss instrumental and analytical approaches (e.g., double- and multi-pulse LIBS to improve the sensitivity), calibration-free approaches, hyphenated approaches in which techniques such as Raman and fluorescence are coupled with LIBS to increase sensitivity and information power, resonantly enhanced LIBS approaches, signal processing and optimization (e.g., signal-to-noise analysis), and finally applications. An attempt is made to provide an updated view of the role played by LIBS in the various fields, with emphasis on applications considered to be unique. We finally try to assess where LIBS is going as an analytical field, where in our opinion it should go, and what should still be done for consolidating the technique as a mature method of chemical analysis.