R. J. Zhang

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 250–1200 nm for their promising applications for direct solar-thermal-electric conversion. A typical 4-layered film sample with the structure SiO2 (66.6 nm)/Bi2Te3 (7.0 nm)/SiO2 (67.0 nm)/Cu (>100.0 nm) 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, 250–500 nm, 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 2–20 μm will result in a low thermal emittance, 0.064 at 600 K. 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.

Magneto-optical Kerr effect in Co-Ag granular films

Abstract Polar Kerr rotation spectra of Co-Ag granular films were measured at room temperature in the wavelength region between 400 and 850 nm. A maximum Kerr rotation angle (θk) of 0.28° was obtained when a magnetic field of 10 kOe was applied perpendicular to the film plane.

OSCILLATION OF POLAR KERR ROTATION IN PTCU/CO MULTILAYERS

The effect of Cu doped into Pt spacer layers on magneto-optical properties for sputtered Pt/Co multilayers has been investigated. The polar Kerr rotation θk oscillates with the content of Cu in Pt spacer layers, and an obvious enhancement of θk at the first and second peaks on its oscillation curve can be seen. The numerical calculation indicates that the components of conductivity tensor in PtCu layers make a simultaneous oscillation with the polar Kerr rotation. These behaviors may result from the spin polarization in PtCu layers.

Evolution of short- and medium-range order in the melt-quenching amorphization of Ge2Sb2Te5

Phase-change memory takes advantage of the fast phase transition between amorphous and crystalline phases of phase-change materials (e.g., Ge2Sb2Te5 or GST). To date, while the “SET” process (crystallization of GST glass) has been intensively studied, studies on the “RESET” process (melt-quenching amorphization of GST) are still limited. In this work, we explored the structural changes of GST upon rapid cooling by ab initio molecular dynamics simulations and atomistic cluster alignment (ACA) analysis. Different from other methods which only focus on the nearest bonding atoms, the ACA method can study both the short- and medium-range order clusters containing atoms beyond the first-neighboring shell and enables us to explore the changes of cluster structures in a larger scale. The results reveal that low-coordinated octahedral clusters tend to become high-coordinated ones, and Ge-centered octahedral structures change to tetrahedrons whereas Sb-centered tetrahedrons transform to octahedral structures during the amorphization process. Interestingly, tetrahedrons show aggregation in liquid and supercooled liquid in contrast to 6-fold octahedrons which present notable aggregation in amorphous GST. Moreover, our study showed that wrong bonds (Ge–Ge, Sb–Sb, Ge–Sb and Te–Te bonds) can promote the formation of large rings, and irreducible rings tend to separate into smaller and larger rings as the temperature is decreased. Our findings provide useful insights into the formation process and the structure of amorphous GST, which is valuable for facilitating the application of phase change materials.

Magneto-optical characteristics of SiN/GdFe films

The magneto-optical and optical properties of GdFe single layer films, which are covered with a thin Si3N4 layer, were studied in the visible wavelength region. A Kerr rotation peak and a reflectivity drop were observed near 4.1 eV in GdFe alloyed films and attributed to the Gd element. Compared with the single thick GdFe film, the Kerr effect of SiN/GdFe bilayers was enhanced, due to the optical interference between Si3N4 and GdFe. The Kerr rotation of GdFe films showed a nonlinear function of the compositions in the whole measured wavelength range. Magneto-optical measurements directly evidence the spin–flip in the GdFe films as the Gd content increased from 20.7 to 24.2 at. %, which showed advantages over conventional magnetometry.

Si-centered capped trigonal prism ordering in liquid Pd82Si18 alloy study by first-principles calculations

First-principles molecular dynamic (MD) simulation and X-ray diffraction were employed to study the local structures of Pd–Si liquid at the eutectic composition (Pd82Si18). A strong repulsion is found between Si atoms, and Si atoms prefer to be evenly distributed in the liquid. The dominate local structures around Si atoms are found to be with of a trigonal prism capped by three half-octahedra and an archimedean anti-prism. The populations of these clusters increase significantly upon cooling, and may play an important role in the formation of Pd82Si18 alloy glass.

Experimental study on light transmission through Au wedge-shaped films

Summary form only given. Metal film materials have been widely used in optical devices, green energy field, biosensor technologies, and so on. In these applications, the properties of metal surface play the significant role. However, propagation characteristics of light in metal-based materials are still confusing. Wavelength-dependent refraction going from negative to positive at air/metal interface was reported in previous researches. The experiment results show that classical Snells law fails to describe light transmission at metal surface. This is because the optical constants of metals are presented as a complex form, whose imaginary part can not be neglected. Its necessary to get more experimental evidences for exploring more deeply mechanisms. In this work, we observed that refraction continuously changes from positive to negative at Au/air interface in the spectrum from 470 nm to 700 nm. In the experiment, a series of Au wedge-shaped films with different wedge angles were radio frequency sputtered on K9 glass. Continuity and uniformity of the wedge-shaped samples were verified by AFM and step profiler. A collimated laser beam with continuously variable wavelength generated by SuperK EXTREME supercontinuum laser was normal incident on the glass substrate then emerged at air/Au interface, where refraction took place. The measurement interval was 10 nm. The shifts of spot Δx on screen at far-field (14.6 m) were detected by CCD. Then effective index of refraction ne was calculated. The results indicate that ne of samples with different wedge-shaped angles correspond well with each other, which means ne is an intrinsic characteristic of the material and irrelevant to the shape of samples. Different from the real part of complex refractive n, which is always positive in natural materials, ne reaches to negative in the spectrum starting from about 520 nm. The positive and negative in the experiment can be explained qualitatively by the real valued boundary conditions. The results might also be conducive to the further quantitative study on negative refraction at metal/dielectric interface.

Dynamics and Diffusion Mechanism of Low-Density Liquid Silicon

A first-order phase transition from a high-density liquid to a low-density liquid has been proposed to explain the various thermodynamic anomies of water. It also has been proposed that such liquid-liquid phase transition would exist in supercooled silicon. Computer simulation studies show that, across the transition, the diffusivity drops roughly 2 orders of magnitude, and the structures exhibit considerable tetrahedral ordering. The resulting phase is a highly viscous, low-density liquid silicon. Investigations on the atomic diffusion of such a novel form of liquid silicon are of high interest. Here we report such diffusion results from molecular dynamics simulations using the classical Stillinger-Weber (SW) potential of silicon. We show that the atomic diffusion of the low-density liquid is highly correlated with local tetrahedral geometries. We also show that atoms diffuse through hopping processes within short ranges, which gradually accumulate to an overall random motion for long ranges as in normal liquids. There is a close relationship between dynamical heterogeneity and hopping process. We point out that the above diffusion mechanism is closely related to the strong directional bonding nature of the distorted tetrahedral network. Our work offers new insights into the complex behavior of the highly viscous low density liquid silicon, suggesting similar diffusion behaviors in other tetrahedral coordinated liquids that exhibit liquid-liquid phase transition such as carbon and germanium.

Optical properties of Fe(1-x)CoxSi2 thin films

A series of Fe(1-x)CoxSi2 thin films with variation of x was prepared by reactive deposition epitaxy (RDE) method. The optical properties of the samples are reported in this paper. The dielectric function of the samples was measured by spectroscopic ellipsometer in the photon energy range of 0.26 - 4.8 eV at room temperature. Its interesting to find that the dielectric function of Fe(1- x)CoxSi2 films is strongly dependent on the phase of the films: (1) The dielectric function spectra show interference peaks in the low photon energy range for the beta phase Fe(1-x)CoxSi2 samples. (2) The dielectric function spectra show a feature between the semiconductor and metal feature for the samples containing both beta and sigma phase Fe(1-x)CoxSi2. (3) The dielectric function spectra show metal feature for the sigma phase Fe(1-x)CoxSi2 samples. According to the x-ray diffraction results, the variation of the dielectric spectra is arisen form the change of the Fe-Si phase in the samples, rather than that from the variation of x.

Temperature dependence of magneto-optical properties of Zn1-xMnxSe

This paper reports the temperature dependence of the magneto-optical properties of Zn1-xMnxSe samples prepared by molecular beam epitaxy method. In the magneto- optical spectra, there exist several Faraday rotation peaks. The peaks located at approximately 2.18, approximately 2.36 and 2.45 - 2.57 eV are attributed to Mn2+ d yields d* transitions. The peak located at approximately 2.7 eV is attributed to the interband transitions and higher order Mn2+ d yields d* transitions, which are blue-shifted with decreasing temperature. The positions of the rotation peaks induced by Mn2+ d yields d* transitions show weak temperature dependence.