Effect of cobalt substitution on microstructure and magnetic properties in ZnO nanoparticles

Ferromagentic semiconductors have been actively pursued because of their potential as spin polarized carrier sources and easy integration into semiconductor technology. One such material, ZnO has been shown to be a potential Diluted Magnetic Semiconductor (DMS). The appearance of ferromagnetism, however, is found to be sensitive to the processing conditions. We report synthesis of ZnO nanoparticles of size ∼20 nm by a simple co-precipitation technique using metal nitrates and NaOH as precipitant. The particles are self-organised and reveal single crystalline behaviour in electron diffraction pattern. Incorporation of Co in ZnO matrix leads not only to the reduction in crystallite size but also to the modification of the structure. At 5% Co, the particles are highly textured. The particles also aggregate and the aggregated mass have nearly rectangular shape as seen through TEM. Increasing Co to 10%, results into further reduction of particle size and the particles self organize in a line, which looks like nanofibers. This alignment of particles increases by increasing the Co content further. This type of growth of nanofibers above Co ≥ 10% is well correlated with the anisotropic peak broadening observed in the XRD spectra. In addition, Co substitute Zn site up to 20% without showing any extra phase in XRD spectra as compared to 7 to 10% in case of bulk. Transport and magnetic studies indicate that conductivity increases with increasing Co content, but carrier mediated ferromagnetism is absent down to 10 K.      

High temperature ferromagnetism of the vacuum-annealed (In1−xFex)2O3 powders

(In_1−xFe_x)₂O₃ (x = 0.02, 0.05, 0.2) powders were prepared by a solid state reaction method and a vacuum annealing process. A systematic study was done on the structural and magnetic properties of (In_1−xFe_x)₂O₃ powders as a function of Fe concentration and annealing temperature. The X-ray diffraction and high-resolution transmission electron microscopy results confirmed that there were not any Fe or Fe oxide secondary phases in vacuum-annealed (In_1−xFe_x)₂O₃ samples and the Fe element was incorporated into the indium oxide lattice by substituting the position of indium atoms. The X-ray photoelectron spectroscopy revealed that both Fe²⁺ and Fe³⁺ ions existed in the samples. Magnetic measurements indicated that all samples were ferromagnetic with the magnetic moment of 0.49-1.73 μ_B/Fe and the Curie temperature around 783 K. The appearance of ferromagnetism was attributed to the ferromagnetic coupling of Fe²⁺ and Fe³⁺ ions via an electron trapped in a bridging oxygen vacancy.     

Channel length scaling and the impact of metal gate work function on the performance of double gate-metal oxide semiconductor field-effect transistors

In this paper, we study the effects of short channel on double gate MOSFETs. We evaluate the variation of the threshold voltage, the subthreshold slope, the leakage current and the drain-induced barrier lowering when channel length L _CH decreases. Furthermore, quantum effects on the performance of DG-MOSFETs are addressed and discussed. We also study the influence of metal gate work function on the performance of nanoscale MOSFETs. We use a self-consistent Poisson-Schrödinger solver in two dimensions over the entire device. A good agreement with numerical simulation results is obtained.     

Combinatorial fabrication and magnetic properties of homoepitaxial Co and Li co-doped NiO thin-film nanostructures

Magnetic properties of nanostructured epitaxial thin layers of a series of Co and Li co-doped NiO on MgO(1 0 0) substrate with NiO buffer layer have been investigated. Thin films were synthesized by combinatorial laser molecular beam epitaxy (CLMBE) in the continuous binary composition spread approach. Large and linear variation of x was achieved in the growth of Co_xLi_0.2Ni_0.8−xO_, onto 9 mm of single substrate. Homoepitaxial growth with smooth surface morphology was confirmed by grazing incidence X-ray diffraction (GIXRD) and atomic force microscopy (AFM). Linear decrease in the band gap and optical transparency was observed with increasing cobalt concentration. The magneto-optical Kerr effect revealed a strong photon energy dependency with negative Kerr rotation for all the Co-concentrations in the film, suggesting intra-valence charge transfer (IVCT) between low spin state Co²⁺ with host Ni²⁺. Ferromagnetic (FM)-like ordering was observed at low temperatures, while antiferromagnetism predominates at room temperature in the Co and Li co-doped nickel oxide epitaxial films.     

Impact of interface geometric structure on organic–metal interface energetics and subsequent films electronic structure

We present the electronic structure of various pentacene thin films grown on Au(1 1 1), Cu(1 1 1), Cu(1 0 0), and Cu(1 1 0) surfaces studied by angle-resolved ultraviolet photoemission spectroscopy using synchrotron radiation. A systematic variation of the metal surface such as the substrate metal and its surface symmetry allows a comprehensive discussion on the correlation between the electronic structure and the interface geometric structure. In the monolayer regime, we observed the evidence of the formation of the organic–metal interface state depending on the metal surface, i.e., the interface geometric structure. This evidence is explained by the different organic–metal and intermolecular interactions, which originate from the hybridization of the molecular orbitals with the metal wavefunction. These interface geometric and electronic phenomena can be a seed for the subsequent film growth and resultant films electronic structure.     

Growth, properties and dye-sensitized solar cells–applications of ZnO nanorods grown by low-temperature solution process

Hexagonal-shaped small ZnO nanorods were grown in a large-quantity via simple aqueous solution process by using zinc nitrate as a source of zinc ions at low temperature under stirring. The as-grown hexagonal-shaped ZnO nanorods were characterized in detail in terms of their structural, optical and photovoltaic properties. The detailed structural investigations by HRTEM, SAED and FFT revealed that the as-synthesized ZnO nanorods are well-crystalline, possessing a perfect hexagonal ideal growth habits of wurtzite zinc oxide and grown along the [0001] direction in preference. The optical properties, composition and quality of the as-synthesized nanorods were examined by using UV-visible and FTIR spectroscopy. Moreover, films of as-grown nanorods were used as photoanode materials to fabricate the dye sensitized solar cells (DSSCs). An overall light to electricity conversion efficiency of 0.70% with a fill factor of 47.2%, short-circuit current of 1.8 mA/cm² and open-circuit voltage of 0.76 V were achieved for the solar cell based on hexagonal-shaped small ZnO nanorods.     

减反膜外腔半导体激光器特性的研究

研究了镀减反膜GaAs半导体激光器在Littrow 式外腔结构中的输出特性.采用电流补偿技术,得到了约30 GHz的连续无跳模调谐范围.利用外差拍探测技术获得激光器输出线宽约为120 kHz.还研究了激光器的强度噪声分别随着电流与波长变化的特点并对此进行了分析.该类激光器可以广泛应用在冷原子物理、激光光谱以及量子光学等许多领域. 关键词： 减反膜 半导体激光器 线宽 强度噪声     

矩阵理论在半导体激光器耦合中的应用

傍轴近似下的光学矩阵理论,可以简化光束传输计算过程,使光学系统设计更为方便。将ABCD变换矩阵方法引入到耦合光学系统的设计中,运用高斯光束的ABCD法则,详尽地给出了某一耦合方式下的半导体激光器耦合入单模光纤系统的设计;另一方面,对系统的耦合损耗与耦合距离的关系进行了理论计算,并把计算结果与最近的实验报道做了比较,它们基本相吻合,说明此方法是可行的、合理的。从整个设计及理论计算来看,ABCD矩阵方法减少了复杂的计算,从而简化了设计过程,与通常的衍射计算相比,它不失为一种方便、有效的方法,同时它对生产半导体激光耦合器也有实际指导意义。     

基于半导体激光器的全光波长转换的理论研究

全光波长转换是全光网络的关键技术之一。利用多模半导体激光器的速率方程,建立了基于半导体激光器的全光波长转换器的理论模型,该模型不仅考虑了注入信号光对激光腔内载流子的消耗,而且考虑了激光腔对注入信号光的放大作用。利用该模型分析了半导体激光器工作电流、注入信号光功率以及码率各参数对波长转换特性的影响。结果对于确定基于半导体激光器的全光波长转换器件的最佳工作条件有一定的参考价值。     

一种获得半导体薄膜光学参数的方法

实现薄膜光学参数的简便测量对于薄膜的制备和应用具有重要意义。引入适用于半导体材料的Forouhi_Bloomer模型,用其表征薄膜折射率与色散的关系。考虑到粗糙度的影响,假设薄膜厚度服从正态分布,给出了模拟退火法与迭代法相结合、由可见光光谱测定薄膜光学参数的方法。作为尝试,以硅系薄膜为例进行了计算。结果表明,获得的厚度与用椭偏仪测量的结果较为吻合。该方法适用于研究和测量半导体薄膜的光学性能和膜厚,具有很高的实用价值。