Phonon and Raman scattering of two-dimensional transition metal dichalcogenides from monolayer, multilayer to bulk material

Two-dimensional (2D) transition metal dichalcogenide (TMD) nanosheets exhibit remarkable electronic and optical properties. The 2D features, sizable bandgaps, and recent advances in the synthesis, characterization, and device fabrication of the representative MoS$_2$, WS$_2$, WSe$_2$, and MoSe$_2$ TMDs make TMDs very attractive in nanoelectronics and optoelectronics. Similar to graphite and graphene, the atoms within each layer in 2D TMDs are joined together by covalent bonds, while van der Waals interactions keep the layers together. This makes the physical and chemical properties of 2D TMDs layer dependent. In this review, we discuss the basic lattice vibrations of monolayer, multilayer, and bulk TMDs, including high-frequency optical phonons, interlayer shear and layer breathing phonons, the Raman selection rule, layer-number evolution of phonons, multiple phonon replica, and phonons at the edge of the Brillouin zone. The extensive capabilities of Raman spectroscopy in investigating the properties of TMDs are discussed, such as interlayer coupling, spin--orbit splitting, and external perturbations. The interlayer vibrational modes are used in rapid and substrate-free characterization of the layer number of multilayer TMDs and in probing interface coupling in TMD heterostructures. The success of Raman spectroscopy in investigating TMD nanosheets paves the way for experiments on other 2D crystals and related van der Waals heterostructures.Comment: 30 pages, 23 figure

General stationary charged black holes as charged particle accelerators

We study the possibility of getting infinite energy in the center of mass frame of colliding charged particles in a general stationary charged black hole. For black holes with two-fold degenerate horizon, it is found that arbitrary high center-of-mass energy can be attained, provided that one of the particle has critical angular momentum or critical charge, and the remained parameters of particles and black holes satisfy certain restriction. For black holes with multiple-fold degenerate event horizons, the restriction is released. For non-degenerate black holes, the ultra-high center-of-mass is possible to be reached by invoking the multiple scattering mechanism. We obtain a condition for the existence of innermost stable circular orbit with critical angular momentum or charge on any-fold degenerate horizons, which is essential to get ultra-high center-of-mass energy without fine-tuning problem. We also discuss the proper time spending by the particle to reach the horizon and the duality between frame dragging effect and electromagnetic interaction. Some of these general results are applied to braneworld small black holes.Comment: 23 pages, no figures, revised version accepted for publication in Phys. Rev.

What Motivates People to Donate in Medical Charitable Crowdfunding Projects? A Trust Theory Perspective

In recent years, the micro-public welfare model based on social network has been widely used in medical crowdfunding, providing a novel solution for patients facing financial difficulties. Using social networks to eliminate time and space constraints on offline donations, more potential donors can be found. Based on the trust theory, this paper collected the medical crowdfunding projects data of public welfare on Weibo from 2012 to 2018, combined with the number of followers, regions and identity information of the project promoters on Weibo social media, and explored the influencing factors of the amount of public crowdfunding projects by using multi-source data. In the fundraising process of medical crowdfunding projects, the subject of trust construction is the donors, and the object of trust includes not only the relevant information of the patient, but also the promoter and the information characteristics of the project. This paper aims to build a theoretical model of trust from promoters, patients, project information characteristics, so as to explore the factors influencing the fundraising ability of medical projects under the micro-public welfare model. The results show that promoters, patients and project information characteristics can affect the trust of donors, and thus affect the amount of money raised by medical crowdfunding projects. On the basis of the above empirical research, this paper also puts forward some suggestions for the healthy development of micro-public welfare based on the problems found

Effects of genetically modified herbicide-tolerant (GMHT) rice on biodiversity of weed in paddy fields

To detect potential changes in properties of weed communities in fields of GMHT rice Bar68-1, trials were carried out from 2007-2008 at Changsha, China with conventional indica rice D68 served as control. The average richness of weed community measured by species accumulation curve tended to be identical in the fields of Bar68-1 and D68 as the number of sampling points increased. There were no significant differences (p > 0.05) in diversity indices which included numbers of species(S), Shannon-Wiener (H’), Pielou evenness (J’), Simpson diversity (D) and evenness (E) indices. Species composition for these two weed communities was roughly comparable. The top four weed species, sorted by individual abundance, were Monochoria vaginalis (Burm. f.) Presl ex Kunth, Lindernia procumbens(Krock.) Philcox, Cyperus diformis L. and Juncellus serotinus (Rottb.) .B. Clarke in the fields of Bar68-1 and D68. ABC curves showed that the weed communities were “unpolluted”. The data above confirm the hypothesis that the difference between the effect of GMHT rice Bar68-1 on biodiversity of weed in paddy fields and that of non-GM rice D68 was not significant