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31.
We present a photoluminescence study of single‐layer MoS2 flakes on SiO2 surfaces. We demonstrate that the luminescence peak position of flakes prepared from natural MoS2, which varies by up to 25 meV between individual flakes, can be homogenized by annealing in vacuum. We use HfO2 and Al2O3 layers prepared by atomic layer deposition to cover some of our flakes. In these flakes, we observe a suppression of the low‐energy luminescence peak which appears in asprepared flakes at low temperatures. We infer that this peak originates from excitons bound to surface adsorbates. We also observe different temperature‐induced shifts of the luminescence peaks for the oxide‐covered flakes. This effect stems from the different thermal expansion coefficients of the oxide layers and the MoS2 flakes. It indicates that the single‐layer MoS2 flakes strongly adhere to the oxide layers and are therefore strained. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   
32.
This paper presents the latest development of a lead-free piezoelectric ceramic and its application to transducers suitable for high-frequency ultrasonic imaging. A lead-free piezoelectric ceramic with formula of (K0.5Na0.5)0.97Li0.03(Nb0.9 Ta0.1)O3 (abbreviated as KNLNT-0.03/0.10) was fabricated and characterized. The material was found to have a clamped dielectric constant ε33S/ε0 = 890, piezoelectric coefficient d33 = 245 pC/N, electromechanical coupling factor kt = 0.42 and Curie temperature Tc > 300 °C. High-frequency (40 MHz) ultrasound transducers were successfully fabricated with the lead-free material. A representative lead-free transducer had a bandwidth of 45%, two-way insertion loss of -18 dB. This performance is comparable to reported performances of popular lead-based transducers. The comparison results suggest that the lead-free piezoelectric material may serve as an alternative to lead-based piezoelectric materials for high-frequency ultrasonic transducer applications.  相似文献   
33.
Pattern formation and self-organization are phenomena that occur across the board, in animate and inanimate systems. In this paper, we rely on the constructal law to explain the generation of patterns (shapes, structures) in aggregates of organisms-pedestrian crowds and stony corals. In pedestrian crowds a variety of patterns are often observed, from ‘chaotic’ appearances to spontaneous organization in lanes of uniform walking direction. Stony corals and other organisms also present intraspecific variability in shape. We show that flow systems develop in time patterns which provide easier access to the nutrients and space, within a set of constraints imposed by each situation. Flow systems have the freedom to morph their shape in search for architectures that allows them to have greater access to the space that they inhabit. We identify the mechanisms allowing pedestrians to evolve in space and time. We also show that stony corals may develop branched or spherical shapes, depending on which shape performs best in response to the environmental conditions. The constructal law allows systems with complex internal flows to be described and understood for a unified view.  相似文献   
34.
Plasma-boundaries floating in an ionized gas are usually negatively charged. They accumulate electrons more efficiently than ions leading to the formation of a quasi-stationary electron film at the boundaries. We propose to interpret the build-up of surface charges at inert plasma boundaries, where other surface modifications, for instance, implantation of particles and reconstruction or destruction of the surface due to impact of high energy particles can be neglected, as a physisorption process in front of the wall. The electron sticking coefficient se and the electron desorption time τe, which play an important role in determining the quasi-stationary surface charge, and about which little is empirically and theoretically known, can then be calculated from microscopic models for the electron-wall interaction. Irrespective of the sophistication of the models, the static part of the electron-wall interaction determines the binding energy of the electron, whereas inelastic processes at the wall determine se and τe. As an illustration, we calculate se and τe for a metal, using the simplest model in which the static part of the electron-metal interaction is approximated by the classical image potential. Assuming electrons from the plasma to loose (gain) energy at the surface by creating (annihilating) electron-hole pairs in the metal, which is treated as a jellium half-space with an infinitely high workfunction, we obtain se≈10-4 and τe≈10-2 s. The product seτe≈10-6 s has the order of magnitude expected from our earlier results for the charge of dust particles in a plasma but individually se is unexpectedly small and τe is somewhat large. The former is a consequence of the small matrix elements occurring in the simple model while the latter is due to the large binding energy of the electron. More sophisticated theoretical investigations, but also experimental support, are clearly needed because if se is indeed as small as our exploratory calculation suggests, it would have severe consequences for the understanding of the formation of surface charges at plasma boundaries. To identify what we believe are key issues of the electronic microphysics at inert plasma boundaries and to inspire other groups to join us on our journey is the purpose of this colloquial presentation.  相似文献   
35.
The theory is developed for piezoelectric scattering rate of carriers in a degenerate surface layer under the condition of low temperature when the approximations of the well-known traditional theory are not valid. The scattering rates thus obtained are then used to estimate the zero-field mobility characteristics for the surface layers under similar condition of low temperature. The results for the surface layers in GaAs and ZnO show that when one takes into account either the degeneracy of the carrier ensemble or the finite energy of the phonons or both, the energy dependence of the scattering rates changes significantly from what follows for a non-degenerate ensemble or from the traditional theory, where one makes use of the high-temperature approximation and thus assumes equipartition law for the phonon distribution, and neglects the phonon energy in the energy balance equation of the electron–phonon system. It is observed that the zero-field mobility characteristics that follow from these scattering rates are interesting in that they are quite different from what turns out either for a non-degenerate ensemble or in the high-temperature approximations.  相似文献   
36.
We theoretically and numerically investigate the modulation efficiencies of integrated electro-optic (EO) modulators based on ferroelectric crystals with the point symmetry of orthorhombic mm2. An analytical expression describing the index-ellipsoid under the EO effect is derived, where all 5 non-zero linear EO coefficients of mm2-crystals are considered. As practical candidates for integrated EO platform, three mm2-crystals (KNbO3, KTiOPO4, and Ba2NaNb5O15) that are well-established and commercially available are chosen for numerical simulations. The refractive-index (RI) changes caused by an external electric field applied to each crystal with an arbitrary direction are analyzed in detail for the given optical polarization states of input light wave. For each crystal, the modulation efficiency is evaluated in terms of the half-wave voltage required for a π-phase shift of input wave, which is compared with the established counterpart based on LiNbO3. The results indicate that higher modulation efficiencies are achievable in the two schemes using KNbO3 and Ba2NaNb5O15 than those for other cases, meaning that either lower power-consuming or larger beam-window integrated EO devices are feasible.  相似文献   
37.
磁化等离子体光子晶体缺陷态的研究   总被引:6,自引:0,他引:6       下载免费PDF全文
章海锋  马力  刘少斌 《物理学报》2009,58(2):1071-1076
采用磁化等离子体的分段线性电流密度卷积时域有限差分(PLCDRC-FDTD)算法研究具有单一缺陷层的一维磁化等离子体光子晶体的缺陷模特性. 从频域角度分析得到微分高斯脉冲的透射率,并讨论该光子晶体的缺陷层厚度、位置、周期常数和等离子体参数对其缺陷模的影响. 结果表明,改变位置和周期常数不会影响缺陷模的频率,改变缺陷层的厚度可以增加缺陷模数,改变等离子体参数能同时影响缺陷模的频率和峰值. 关键词: 磁化等离子体光子晶体 光子晶体 缺陷模 时域有限差分法  相似文献   
38.
Optical absorption spectra of tungsten diselenide (WSe2) single crystals subjected to different values of pressure (0, 2, 4 and 6 GPa) were obtained in the spectral range 700–1450 nm with the help of a UV–VIS–NIR spectrophotometer. The spectra were thoroughly analyzed in the absorption edge region for obtaining direct as well as indirect band gaps in this material. The high temperature resistivity and thermoelectric power on WSe2 single crystals at various pressures were also studied. The results and their implications are discussed in the article.  相似文献   
39.
40.
Single crystals of the organic nonlinear optical material γ-glycine have been grown in the presence of Zinc sulphate by slow evaporation technique at ambient temperature for the first time. Bulk growth of γ-glycine single crystals was grown by Top-seeded solution growth method. The γ-phase of glycine was confirmed by powder X-ray diffraction and the FTIR analysis. Elemental analysis CHN was performed to confirm the non-inclusion of zinc sulphate species into the solution. Inductively coupled plasma optical emission spectrometry study (ICP-OES) was employed to quantify the concentration of Zinc element in the grown γ-glycine single crystals. The optical transmission was ascertained from UV–Vis–NIR spectrum. The optical band gap was estimated for γ-glycine single crystal using UV–Vis–NIR study. Differential scanning calorimetry analysis was employed to explore information about thermal stability, phase transition and melting point of the grown crystal. The second harmonic generation relative efficiency was measured by Kurtz and Perry powder technique.  相似文献   
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