Direct visualization of structural defects in 2D semiconductors

Direct visualization of the structural defects in two-dimensional (2D) semiconductors at a large scale plays a significant role in understanding their electrical/optical/magnetic properties, but is challenging. Although traditional atomic resolution imaging techniques, such as transmission electron microscopy and scanning tunneling microscopy, can directly image the structural defects, they provide only local-scale information and require complex setups. Here, we develop a simple, non-invasive wet etching method to directly visualize the structural defects in 2D semiconductors at a large scale, including both point defects and grain boundaries. Utilizing this method, we extract successfully the defects density in several different types of monolayer molybdenum disulfide samples, providing key insights into the device functions. Furthermore, the etching method we developed is anisotropic and tunable, opening up opportunities to obtain exotic edge states on demand.     

Effect of oxygen on regulation of properties of moderately boron-doped diamond films

Regulation of oxygen on properties of moderately boron-doped diamond films is fully investigated. Results show that, with adding a small amount of oxygen (oxygen-to-carbon ratio < 5.0%), the crystal quality of diamond is improved, and a suppression effect of residual nitrogen is observed. With increasing ratio of O/C from 2.5% to 20.0%, the hole concentration is firstly increased then reduced. This change of hole concentration is also explained. Moreover, the results of Hall effect measurement with temperatures from 300 K to 825 K show that, with adding a small amount of oxygen, boron and oxygen complex structures (especially B₃O and B₄O) are formed and exhibit as shallow donor in diamond, which results in increase of donor concentration. With further increase of ratio of O/C, the inhibitory behaviors of oxygen on boron leads to decrease of acceptor concentration (the optical emission spectroscopy has shown that it is decreased with ratio of O/C more than 10.0%). This work demonstrates that oxygen-doping induced increasement of the crystalline and surface quality could be restored by the co-doping with oxygen. The technique could achieve boron-doped diamond films with both high quality and acceptable hole concentration, which is applicable to electronic level of usage.     

Theoretical study on the improvement of the doping efficiency of Al in 4H-SiC by co-doping group-IVB elements

The p-type doping efficiency of 4H silicon carbide (4H-SiC) is rather low due to the large ionization energies of p-type dopants. Such an issue impedes the exploration of the full advantage of 4H-SiC for semiconductor devices. In this study, we show that co-doping group-IVB elements effectively decreases the ionization energy of the most widely used p-type dopant, i.e., aluminum (Al), through the defect-level repulsion between the energy levels of group-IVB elements and that of Al in 4H-SiC. Among group-IVB elements Ti has the most prominent effectiveness. Ti decreases the ionization energy of Al by nearly 50%, leading to a value as low as ~0.13 eV. As a result, the ionization rate of Al with Ti co-doping is up to ~5 times larger than that without co-doping at room temperature when the doping concentration is up to 10¹⁸ cm^-3. This work may encourage the experimental co-doping of group-IVB elements such as Ti and Al to significantly improve the p-type doping efficiency of 4H-SiC.     

稀薄气体动力学现状

自从1946年钱学森的开创性文章发表以来,对稀薄气体动力学的兴趣主要在于高速稀薄气体对物体绕流的空气动力学问题。二十余年来这门学科发展很快,直到最近其发展势头仍未减弱,这点从历届稀薄气体动力学国际会议的情况也可以看出来(其中第七次会议文集未到,仅见会议较详介绍,第八次1972年夏刚刚开过)。会议文集     

The Gerber-Shiu discounted penalty function for a compound binomial risk model with by-claims

A recursive formula of the Gerber-Shiu discounted penalty function for a compound binomial risk model with by-claims is obtained. In the discount-free case, an explicit formula is given. Utilizing such an explicit expression, we derive some useful insurance quantities, including the ruin probability, the density of the deficit at ruin, the joint density of the surplus immediately before ruin and the deficit at ruin, and the density of the claim causing ruin.     

MORPHOLOGY AND ELEMENTAL COMPOSITION OF DUSTFALL PARTICLES INSIDE EMPEROR QIN＇S TERRA-COTTA WARRIORS AND HORSES MUSEUM

Indoor dusffall samples were collected in Pit 1 and Pit 2 of Emperor Qin＇s Terra-cotta Warriors and Horses Museum in Xi＇an, Shaanxi Province, China. The morphological and elemental analysis for long-term （〉10 years） and short-term （～0.5 year） dusffall particles and lacquer pieces on the faces of partly assembled warriors were performed with scanning electron microscopy with energy dispersive X-ray spectrometer （SEM/EDX）. The results showed that the majority of dusffall samples appeared to be composed of mineral dust particles, fly ash, and biological particles with different elemental compositions. Most of the mineral particles had irregular shapes with smooth edges. Small floccules of calcium sulfate were more frequently observed in long-term dusffall samples than in short-term samples, implying a progressive chemical reaction between continuous dusffall particles and atmospheric sulfur dioxide. Crystals of calcium sulfate were also found near pits on both inner and outer surfaces of lacquer pieces, revealing an acid chemical reaction between sulfur dioxide and lacquer material as the cause of the formation of erosion pits and cracks on surface of lacquer pieces.     

关于风沙层中颗粒垂向浓度分布规律等的思考

本文采用统计平均方法和二相流连续介质模型方法研究了风沙流场中颗粒浓度和输运通量沿高度的分布规律．结果表明：即使对均匀沙这一简单情形，现有的理论模型也很难证明这两个量一定服从负指数分布．针对负指数分布是经验规律这一事实，我们对现阶段常用的测算颗粒浓度的方法进行讨论，指出引进更高精度实验手段的必要性．     

伯努利方程应用中的若干问题

在定常流动中,若流体重力势能的变化不重要,则伯努利方程为1/2 v~2+∫dp/ρ=c_1(常数) (1)在不可压缩流动中,ρ是常数,则有∫dp/ρ=p/ρ.伯努利方程形式简单,却很重要,有着广泛的应用.但是,若不注意其使用条件,就会造成误用.     

Necessary and Suffcient Conditions for Distinguishability of Linear Control Systems

Distinguishability plays a crucial rule in studying observability of hybrid system such as switched system. Recently, for two linear systems, Lou and Si gave a condition not only necessary but also sufficient to the distinguishability of linear systems. However, the condition is not easy enough to verify. This paper will give a new equivalent condition which is relatively easy to verify.     

Thermally induced structural transformation of polytriazoleimide to polyindoleimide

A new kind of polytriazoleimide containing bisphenyl-l,2,3-triazole（BPT） was synthesized by copper-catalyzed 1,3-dipolar cycloaddition of azides and alkynes（CuAAC） and polycondensation.The thermal stability and degradation mechanism of the polytriazoleimide were investigated.The results show that the structure of BPT in polytriazoleimide transforms to phenylindole after thermal treatment,accompanying the release of N₂.