Electric field effect tuning of electron-phonon coupling in graphene

Gate-modulated low-temperature Raman spectra reveal that the electric field effect (EFE), pervasive in contemporary electronics, has marked impacts on long-wavelength optical phonons of graphene. The EFE in this two-dimensional honeycomb lattice of carbon atoms creates large density modulations of carriers with linear dispersion (known as Dirac fermions). Our EFE Raman spectra display the interactions of lattice vibrations with these unusual carriers. The changes of phonon frequency and linewidth demonstrate optically the particle-hole symmetry about the charge-neutral Dirac point. The linear dependence of the phonon frequency on the EFE-modulated Fermi energy is explained as the electron-phonon coupling of massless Dirac fermions.     

Nonlinear dynamics of a bistable piezoelectric-composite energy harvester for broadband application

The continuing need for reduced power requirements for small electronic components, such as wireless sensor networks, has prompted renewed interest in recent years for energy harvesting technologies capable of capturing energy from ambient vibrations. A particular focus has been placed on piezoelectric materials and devices due to the simplicity of the mechanical to electrical energy conversion and their high strain energy densities compared to electrostatic and electromagnetic equivalents. In this paper an arrangement of piezoelectric layers attached to a bistable asymmetric laminate is investigated experimentally to understand the dynamic response of the structure and power generation characteristics. The inherent bistability of the underlying structure is exploited for energy harvesting since a transition from one stable configuration to another, or “snap-through”, is used to repeatedly strain the surface bonded piezoelectric and generate electrical energy. This approach has been shown to exhibit high levels of power extraction over a wide range of vibrational frequencies. Using high speed digital image correlation, a variety of dynamic modes of oscillation are identified in the harvester. The sensitivity of such modes to changes in vibration frequency and amplitude are investigated. Power outputs are measured for repeatable snap-through events of the device and are correlated with the measured modes of oscillation. The typical power generated is approximately 3.2?mW, comparing well with the needs of typical wireless senor node applications.     

Formation and photoluminescence of Y2O3–H3BO3:Eu3+ powders by mechanical alloying

Y₂O₃–H₃BO₃:Eu³⁺ powders were synthesized by the mechanical alloying (MA) method, and their structural and photoluminescent characteristics were investigated by X-ray diffractometry (XRD), scanning electron microscopy (SEM), thermogravimetric/differential thermal analysis (TG/DTA), and luminescence spectrophotometer. The crystallite size of the powder mixture milling for 30 minutes (min) by the Willaimson–Hall method was approximately 58.8 nm with strain of 0.00141; overall, the internal strain increased with the milling time (t_m). The morphology of the powder mixture with t_m, as observed by SEM, divided into three different stages: agglomeration (0 < t_m ≤ 30 min), disintegration (30min < t_m ≤ 120 min), and homogenization (120min < t_m ≤ 300 min). The transition temperature and the weight reduction rate of the sample powders were 645.58 °C and 2.851%, respectively. Furthermore, the photoluminescence of the powder mixture excited to 240 nm by a zenon discharge lamp (20 kW) was detected near 592 nm(⁵D_o → ⁷F₁), 613 nm, 628  (⁵D_o → ⁷F₂), and 650 nm (⁵D_o → ⁷F₃).     

Heterojunction light emitting diodes fabricated with different n-layer oxide structures on p-GaN layers by magnetron sputtering

We grew heterojunction light emitting diode (LED) structures with various n-type semiconducting layers by magnetron sputtering on p-type GaN at high temperature. Because the undoped ZnO used as an active layer was grown under oxygen rich atmosphere, all LED devices showed the EL characteristics corresponding to orange-red wavelength due to high density of oxygen interstitial, which was coincident with the deep level photoluminescence emission of undoped ZnO. The use of the Ga doped layers as a top layer provided the sufficient electron carriers to active region and resulted in the intense EL emission. The LED sample with small quantity of Mg incorporated in MgZnO as an n-type top layer showed more intense emission than the LED with ZnO, in spite of the deteriorated electrical and structural properties of the MgZnO film. This might be due to the improvement of output extraction efficiency induced by rough surface.     

Anisotropic electrical properties of Bi0.5(Na0.75K0.25)0.5TiO3 ceramics fabricated by reactive templated grain growth (RTGG)

We have fabricated lead-free Bi_0.5(Na_0.75K_0.25)_0.5TiO₃ (BNKT) ceramics by a conventional process (CP) and reactive templated grain growth (RTGG) methods. The effect of grain orientation on structure, dielectric, complex impedance and electrical properties was investigated. The phase formation and grain morphology of BNKT ceramics were examined by X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. High dielectric constant and low dielectric loss was observed for grain oriented (textured) BNKT ceramics. Complex impedance, temperature dependent ac and dc conductivity were performed to explore the conduction behavior of the prepared BNKT ceramics.     

Comparison of Differences in Brain Structure Between Teenagers and Twenties Brain Using 3 T Magnetic Resonance Imaging

Applied Magnetic Resonance - Total cerebral volume increases very rapidly in childhood, peaking in early teenage years then declining in adolescence. However, most studies quantified only one or...     

Evaluation of aerodynamic performance enhancement of Risø_B1 airfoil with an optimized cavity by PIV measurement

Journal of Visualization - Airfoils are mostly inefficient in their off-design conditions. In order to improve the aerodynamic performance of airfoils in these conditions, using an optimized cavity...     

Fabrication of stabilized piezoelectric thick film for silicon-based MEMS device

Electrical properties of piezoelectric thick films with controlled microstructure were investigated. In order to enhance the electromechanical properties (e.g. d₃₁, d₃₃) of a thick film by control of its microstructure, a mixed powder, referred to as BNP, consisting of both nano-sized and micro-sized piezoelectric particles, was employed as a starting precursor in the film fabrication process. According to a scanning electron microscopy study, it is shown that a BNP thick film exhibits the densest homogeneous microstructures. According to surface area measurements, the BNP thick film was sufficiently densified without an additional infiltration process of Pb(Zr_1-xTi_x)O₃ sol for densification. The screen-printed BNP thick film possesses a dielectric constant and a remanent polarization much higher than those of a thick film composed of only micro-sized piezoelectric particles by a factor of more than two. This suggests the potential application of the BNP thick film, in conjunction with a silicon substrate, to a micromachined monolithic PZT thick film device on the silicon substrate. PACS 85.40.Xx; 85.85.+j; 85.50.-n; 77.65.-j; 68.37.-d