Optical and electrical properties of two-dimensional anisotropic materials

Two-dimensional(2D) anisotropic materials, such as B-P, B-As, GeSe, GeAs, ReSe2, KP15 and their hybrid systems, exhibit unique crystal structures and extraordinary anisotropy. This review presents a comprehensive comparison of various 2D anisotropic crystals as well as relevant FETs and photodetectors, especially on their particular anisotropy in optical and electrical properties. First, the structure of typical 2D anisotropic crystal as well as the analysis of structural anisotropy is provided. Then, recent researches on anisotropic Raman spectra are reviewed. Particularly, a brief measurement principle of Raman spectra under three typical polarized measurement configurations is introduced. Finally, recent progress on the electrical and photoelectrical properties of FETs and polarization-sensitive photodetectors based on 2D anisotropic materials is summarized for the comparison between different 2D anisotropic materials. Beyond the high response speed, sensitivity and on/off ratio, these 2D anisotropic crystals exhibit highly conduction ratio and dichroic ratio which can be applied in terms of polarization sensors, polarization spectroscopy imaging, optical radar and remote sensing.     

Preface to the Special Issue on 2D-Materials-Related Physical Properties and Optoelectronic Devices

Recent advances in two-dimensional (2D) materials following the successful fabrication of graphene in 2004 by Novoselov and Geim is expected to grow into the new silicon, offering a lifeline for Moore’s law. With the rapid development of the synthesis methods, more and more 2D materials, such as transition metal dichalcogenides (TMDs, MX2), black phosphorus (BP) and InSe with a finite gap are reported to be more promising for achieving this dream since they often offer alternative solutions to compensate for the gapless graphene’s weaknesses.     

Symmetry Breaking in Monometallic Nanocrystals toward Broadband and Direct Electron Transfer Enhanced Plasmonic Photocatalysis

Metallic nanocrystals manifest themselves as fascinating light absorbers for applications in plasmon-enhanced photocatalysis and solar energy harvesting. The essential challenges lie in harvesting the full-spectrum solar light and harnessing the plasmon-induced hot carriers at the metal–acceptor interface. To this end, a cooperative overpotential and underpotential deposition strategy is proposed to mitigate both the challenges. Specifically, by utilizing both ionic additive and thiol passivator to introduce symmetry-breaking growth over gold icosahedral nanocrystals, the microscopic origin can be attributed to the site-specific nucleation of stacking faults and dislocations. By adopting asymmetric crystal shape and unique surface facets, such nanocrystals attain high activity toward photocatalytic ammonia borane hydrolysis, arising from combined broadband plasmonic properties and enhanced direct transfer of hot electrons across the metal–adsorbate interface.     

三势垒共振隧穿结构中极大增强的光生空穴共振隧穿

研究了电子隧穿出射端嵌入1.2 μm厚n型弱掺杂GaAs层的三势垒双阱隧穿结构,观察到了隧穿峰谷比高达36的光生空穴共振隧穿峰.研究证实1.2 μm厚n型弱掺杂GaAs层在光照下产生的大量光生空穴以及空穴隧穿出射端的23 nm宽的量子阱中量子化的空穴能级对空穴隧穿谷电流的限制作用,是导致高峰谷比的光生空穴隧穿现象的主要原因.     

GaNAs／GaAs中的激子局域化和发光特性

通过多种光谱手段研究了GaNAs量子阱和体材料中的局域态和非局域态的不同光学特性．在超短激光脉冲激发下，第一次在GaNAs／GaAs量子阱发光光谱中，观察到非局域激子发光．选择激发光谱表明，局域中心主要聚集在GaNAs、GaAs异质结界面．在低N含量的GaNAs体材料发光光谱中，除了与N相关的局域态发光外，也发现发光特性完全不同的GaNAs合金态发光．这些结果为理解Ⅲ-Ⅴ-N族半导体的异常能带特性具有十分重要的意义．     

Economic potential of bioremediation using immobilized microalgae-based microbial fuel cells

Clean Technologies and Environmental Policy - The cathodic microalgae-based MFC converts the nutrients within wastewater and produces oxygen as oxygen supply for cathodic reactions, leading to the...     

Potential of bio-hydrogen production by C. pasteurianum co-immobilized with selected nano-metal particles

Currently, green energy has gained considerable interest as an economical source of energy. Compared with other alternative energy sources, hydrogen has a considerable environment-friendly potential. Microorganisms that possess hydrogen production ability have been investigated. Direct mixing of nano-metals with microorganisms under suspended growth has been proven to increase hydrogen production. In this study, a novel approach of co-immobilization of nano-metals and hydrogen-producing microorganism was proposed. Accumulative gas production and composition showed that co-immobilization with nano-metal nickel (NP–Ni) and iron (NP–Fe) improved system performance. Adding 400 ml/L of NP-Ni and 200 ml/L of NP-Fe enhanced composition percentage of hydrogen gas to 46.90% and 42.03%, respectively. Co-immobilized granules could endure up to 14 days of operation, and some cracked into two halves at 22 days. However, hydrogen production remained consistent throughout the experiment. Thus, results demonstrated that Clostridium pasteurianum co-immobilized with selected nanometal particles could be a feasible method to increase hydrogen production.     

Breakdown of Raman selection rules by Fröhlich interaction in few-layer WS2

The polarization selection rule of Raman scattering is crucial in symmetry analysis of elementary excitations in semiconductors and correlated electron systems. Here we reported the observation of breakdown of Raman selection rules in few-layer WS₂ by using resonant Raman spectroscopy. When the excitation energy is close to the dark A exciton state, we observed some infrared active modes and backscattering forbidden modes. Importantly, we found that all observed phonon modes follow the same paralleled-polarization behavior. According to the electron-phonon coupling near the band edge in WS₂, we proposed a theoretical model based on the intraband Fröhlich interaction. In this case, the polarization response of the scattering signal is no longer determined by the original Raman tensor of scattered phonons. Instead, it is determined by a new isotropic Raman tensor that generated from this intraband Fröhlich interaction between dark A exciton and phonons. We found that this theoretical model is in excellent agreement with the observed results. The breakdown of Raman selection rules can violate the conventional limitations of the optical response and provide an effective method to control the polarization of Raman scattering signals in two-dimensional materials.

     

Joint Density for Eigenvalues of Two Correlated Complex Wishart Matrices: Characterization of MIMO Systems

In this letter, the joint probability density function (PDF) for the eigenvalues of a complex Wishart matrix and a perturbed version of it are derived. The latter version can be used to model channel estimation errors and variations over time or frequency. As an example, the joint PDF is used to calculate the transition probabilities between modulation states in an adaptive MIMO system. This leads to a Markov model for the system. We then use the model to investigate the modulation state entering rates (MSER), the average stay duration (ASD), and the effects of feedback delay on the accuracy of modulation state selection in mobile radio systems. Other applications of this PDF are also discussed.