Free carrier absorption and lattice vibrational modes in bulk ZnO

The electronic properties of semiconductors are highly dependent on carrier scattering mechanisms determined by crystalline structure, band structure, and defects in the material. Experimental characteristics of lattice vibrational modes and free carrier absorption in single-crystal ZnO samples obtained from different sources are presented in this work to provide a further understanding of carrier scattering processes pertaining to electronic properties. Infrared absorption measurements indicate strong absorption peaks due to a combination of optical and nonpolar phonon modes in the 9–13 μm spectral region. The Raman spectra obtained for these samples similarly reveal the presence of these phonon modes. Infrared absorption measurements also demonstrate free carrier absorption in the 3–9 μm spectral region for higher conductivity samples, where a λ^m dependence is observed with m=2.7–3, indicating both longitudinal optical phonon scattering and ionized impurity scattering. From these results, we show that infrared absorption can be used as a routine nondestructive technique to determine the material characteristics and quality of bulk ZnO.     

Modelling of electrostatic gate operations in the Kane solid state quantum computer

The nuclear spin quantum computer proposed by Kane [Nature 393 (1998) 133] exploits as a qubit array ³¹P dopants embedded within a silicon matrix. Single-qubit operations are controlled by the application of electrostatic potentials via a set of metallic ‘A’ gates, situated above the donors, on the silicon surface, that tune the resonance frequency of individual nuclear spins, and a globally applied RF magnetic field that flips spins at resonance. Coupling between qubits is controlled by the application of potentials via a set of ‘J’ gates, between the donors, that induce an electron-mediated coupling between nuclear spins. We report the results of the study of the electric field and potential profiles arising within the Kane device from typical gate operations. The extent to which a single nuclear spin can be tuned independently of its neighbours, by operation of an associated A-gate, is examined and key design parameters in the Kane architecture are addressed. Implications for current fabrication strategies involving the implantation of ³¹P atoms are discussed. Solution of the Poisson equation has been carried out by simulation using a TCAD modelling package (Integrated Systems Engineering AG).     

Atomic Ni Anchored Covalent Triazine Framework as High Efficient Electrocatalyst for Carbon Dioxide Conversion

Electrochemically driven carbon dioxide (CO₂) conversion is an emerging research field due to the global warming and energy crisis. Carbon monoxide (CO) is one key product during electroreduction of CO₂; however, this reduction process suffers from tardy kinetics due to low local concentration of CO₂ on a catalyst's surface and low density of active sites. Herein, presented is a combination of experimental and theoretical validation of a Ni porphyrin‐based covalent triazine framework (NiPor‐CTF) with atomically dispersed NiN₄ centers as an efficient electrocatalyst for CO₂ reduction reaction (CO₂RR). The high density and atomically distributed NiN₄ centers are confirmed by aberration‐corrected high‐angle annular dark field scanning transmission electron microscopy and extended X‐ray absorption fine structure. As a result, NiPor‐CTF exhibits high selectivity toward CO₂RR with a Faradaic efficiency of >90% over the range from ?0.6 to ?0.9 V for CO conversion and achieves a maximum Faradaic efficiency of 97% at ?0.9 V with a high current density of 52.9 mA cm^?2, as well as good long‐term stability. Further calculation by the density functional theory method reveals that the kinetic energy barriers decreasing for *CO₂ transition to *COOH on NiN₄ active sites boosts the performance.     

Near-Field Drives Long-Lived Shallow Trapping of Polymeric C3N4 for Efficient Photocatalytic Hydrogen Evolution

Undesired photoelectronic dormancy through active species decay is adverse to photoactivity enhancement. An insufficient extrinsic driving force leads to ultrafast deep charge trapping and photoactive species depopulation in carbon nitride (g-C₃N₄). Excitation of shallow trapping in g-C₃N₄ with long-lived excited states opens up the possibility of pursuing high-efficiency photocatalysis. Herein, a near-field-assisted model is constructed consisting of an In₂O₃-cube/g-C₃N₄ heterojunction associated with ultrafast photodynamic coupling. This In₂O₃-cube-induced near-field assistance system provides catalytic “hot areas”, efficiently enhances the lifetimes of excited states and shallow trapping in g-C₃N₄ and this favors an increased active species density. Optical simulations combined with time-resolved transient absorption spectroscopy shows there is a built-in charge transfer and the active species lifetimes are longer in the In₂O₃-cube/g-C₃N₄ hybrid. Besides these properties, the estimated overpotential and interfacial kinetics of the In₂O₃-cube/g-C₃N₄ hybrid co-promotes the liquid phase reaction and also helps in boosting the photocatalytic performance. The photocatalytic results exhibit a tremendous improvement (34-fold) for visible-light-driven hydrogen production. Near-field-assisted long-lived active species and the influences of trap states is a novel finding for enhancing (g-C₃N₄)-based photocatalytic performance.     

Stable Light-Emitting Electrochemical Cells Using Hyperbranched Polymer Electrolyte

The choice of an adequate electrolyte is a fundamental aspect in polymer light-emitting electrochemical cells (PLECs) as it provides the in situ electrochemical doping and influences the performance of these devices. In this study, a hyperbranched polymer (Hybrane DEO750 8500) blended with a Li salt is used as a novel electrolyte in state-of-the-art Super Yellow (a polyphenylenevinylene) based LECs. Due to the desirable properties of the hyperbranched polymer and the homogeneous and smooth films that it forms with the emitting polymer, PLEC with excellent electroluminescent properties are obtained using a pulsed current bias scheme. The devices are very stable, with lifetimes in excess of 2000 h with initial luminance values above 450 cd m⁻², a peak efficiency of 12.6 lm W⁻¹, and sub-minute turn-on times. The stability of the devices is also studied by measuring the photoluminescence (PL) of the semiconductor during electroluminescent operation. The findings suggest that it is possible to observe the quenching of the PL in vertically stacked devices due to the advancement of the doped fronts in the film and an immediate PL recovery when the bias is removed.     

Imaging performance of A-PET: a small animal PET camera

The evolution of positron emission tomography (PET) imaging for small animals has led to the development of dedicated PET scanner designs with high resolution and sensitivity. The animal PET scanner achieves these goals for imaging small animals such as mice and rats. The scanner uses a pixelated Anger-logic detector for discriminating 2 x 2 x 10 mm3 crystals with 19-mm-diameter photomultiplier tubes. With a 19.7-cm ring diameter, the scanner has an axial length of 11.9 cm and operates exclusively in three-dimensional imaging mode, leading to very high sensitivity. Measurements show that the scanner design achieves a spatial resolution of 1.9 mm at the center of the field-of-view. Initially designed with gadolinium orthosilicate but changed to lutetium- yttrium orthosilicate, the scanner now achieves a sensitivity of 3.6% for a point source at the center of the field-of-view with an energy window of 250-665 keV. Iterative image reconstruction, together with accurate data corrections for scatter, random, and attenuation, are incorporated to achieve high-quality images and quantitative data. These results are demonstrated through our contrast recovery measurements as well as sample animal studies.     

Chloride and Indium‐Chloride‐Complex Inorganic Ligands for Efficient Stabilization of Nanocrystals in Solution and Doping of Nanocrystal Solids

Here, the surface functionalization of CdSe and CdSe/CdS core/shell nanocrystals (NCs) with compact chloride and indium‐chloride‐complex ligands is reported. The ligands provide not only short interparticle distances but additionally control doping and passivation of surface trap states, leading to enhanced electronic coupling in NC‐based arrays. The solids based on these NCs show an excellent electronic transport behavior after heat treatment at the relatively low temperature of 190 °C. Indeed, the indium‐chlorido‐capped 4.5 nm CdSe NC based thin‐film field‐effect transistor reaches a saturation mobility of μ = 4.1 cm² (V s)^?1 accompanied by a low hysteresis, while retaining the typical features of strongly quantum confined semiconductor NCs. The capping with chloride ions preserves the high photoluminescence quantum yield ( ≈ 66%) of CdSe/CdS core/shell NCs even when the CdS shell is relatively thin (six monolayers). The simplicity of the chemical incorporation of chlorine and indium species via solution ligand exchange, the efficient electronic passivation of the NC surface, as well as their high stability as dispersions make these materials especially attractive for wide‐area solution‐processable fabrication of NC‐based devices.     

Printed Sub‐2 V Gel‐Electrolyte‐Gated Polymer Transistors and Circuits

The fabrication and characterization of printed ion‐gel‐gated poly(3‐hexylthiophene) (P3HT) transistors and integrated circuits is reported, with emphasis on demonstrating both function and performance at supply voltages below 2 V. The key to achieving fast sub‐2 V operation is an unusual gel electrolyte based on an ionic liquid and a gelating block copolymer. This gel electrolyte serves as the gate dielectric and has both a short polarization response time (<1 ms) and a large specific capacitance (>10 µF cm^?2), which leads simultaneously to high output conductance (>2 mS mm^?1), low threshold voltage (<1 V) and high inverter switching frequencies (1–10 kHz). Aerosol‐jet‐printed inverters, ring oscillators, NAND gates, and flip‐flop circuits are demonstrated. The five‐stage ring oscillator operates at frequencies up to 150 Hz, corresponding to a propagation delay of 0.7 ms per stage. These printed gel electrolyte gated circuits compare favorably with other reported printed circuits that often require much larger operating voltages. Materials factors influencing the performance of the devices are discussed.     

用单个按钮实现多种功能

如果你拥有一部智能电话（例如,BlackBerry、Palm…等）、个人数字助理（PDA）、笔记本电脑或PC,那么你也许已经熟悉了用单一按钮就可以利用的多种电源功能。这些设备仅是这类例子中的几个,在这类设备中,用来给设备加电和断电的主按钮常常具有附加功能,通常是“备用”或“冬眠”模式,这种模式将设备置于一种省电模式。在笔记本电脑中,当你暂时离开时,备用模式切断对硬盘驱动器和显示屏的供电,将数据存储到存储器中,保持笔记本电脑基本部分的运行。在冬眠模式,笔记本电脑完全停机,将数据保存到硬盘驱动器而不是存储器中,     

Application of coupled electro-thermal and physics-of-failure-based analysis to the design of accelerated life tests for power modules

In the reliability theme a central activity is to investigate, characterize and understand the contributory wear-out and overstress mechanisms to meet through-life reliability targets. For power modules, it is critical to understand the response of typical wear-out mechanisms, for example wire-bond lifting and solder degradation, to in-service environmental and load-induced thermal cycling. This paper presents the use of a reduced-order thermal model coupled with physics-of-failure-based life models to quantify the wear-out rates and life consumption for the dominant failure mechanisms under prospective in-service and qualification test conditions. When applied in the design of accelerated life and qualification tests it can be used to design tests that separate the failure mechanisms (e.g. wire-bond and substrate-solder) and provide predictions of conditions that yield a minimum elapsed test time. The combined approach provides a useful tool for reliability assessment and estimation of remaining useful life which can be used at the design stage or in-service. An example case study shows that it is possible to determine the actual power cycling frequency for which failure occurs in the shortest elapsed time. The results demonstrate that bond-wire degradation is the dominant failure mechanism for all power cycling conditions whereas substrate-solder failure dominates for externally applied (ambient or passive) thermal cycling.