Exploration on performance of two-dimensional GaN photocathodes with uniform-doping and variable-doping structure

In this paper, the properties of two-dimensional (2D) gallium nitride (GaN) photocathodes with a uniform doping and variable doping structure are studied by using Mg as a doping element based on first principles. The stability, bandstructure, work function, density of state and optical properties of the GaN bilayer and GaN trilayer in two-doped ways are investigated. The results show that formation energy of variable doping structure is less than that of the uniform doping structure, which means that the variable doping structure is more stable. At the same time, the formation energy increases with increase of layers. The pristine GaN bilayer has an indirect bandgap, while the doped GaN bilayer transforms into a direct bandgap. The impurity levels appear in a forbidden band of doped GaN trilayers, which is favorable for electron transition. The results of work function reveal that variable doping structure has lower vacuum barriers and more electron escape numbers, which proves that it can improve the quantum efficiency of photocathodes. Finally, the analysis of optical properties shows that the uniform doping structure has better optical properties than that of the variable doping structure.

Quantitative Evaluation on Street Vitality: A Case Study of Zhoujiadu Community in Shanghai

Streets functioning as important components of urban public space are not only the primary carriers of traffic but also essential spaces for individuals’ daily activities, including recreation and communication. The paper highlights the social characteristics of streets by integrating them into one single index of vitality. The application of open recourse data combined with empirical research forms the foundation of a quantitative exploration on the street vitality of Zhoujiadu Community in Shanghai. Supported by the ideology of street urbanism, this paper defines the concept of “street vitality”, and then constructs a quantitative evaluation index system. Afterwards, a multiple linear regression model is developed to explore the main influential factors of street vitality. This work evidences the relationship between the environment and citizens’ activities and is beneficial to the potential improvement of street space quality and the enhancement of streets with higher vitality. Results from this work proved that the constituent factors of social function density, mixing degree of social functions, distance from the nearest subway station and green view have strong impacts on street vitality, among which the social function density and mixing degree of social functions are paramount.

Monolayer InSe photodetector with strong anisotropy and surface-bound exciton

The in-plane anisotropy of monolayer InSe plays a critical role in the application of photodetectors. In this work, through nonequilibrium Green’s function density functional theory (NEGF-DFT) and time-dependent density functional...

The Cause and Evolution of Urban Street Vitality under the Time Dimension: Nine Cases of Streets in Nanjing City, China

Street vitality is associated with a comfortable human-based public environment and urban sustainability. In most current studies, street vitality is assessed considering single or multi factors; however, the impact of time dimension is ignored. This study selects nine different year-built streets in old, main, and new urban areas, in Nanjing, China, proposes a framework to assess street vitality considering the different time dimensions and selects the following factors: street form, including building density, continuity, and height-width; street business type, including store density, function density, and permeation rate; and street accessibility, including location, the number of entrances/exits, transportation, and walkability. After calculating the values of the subfactors, a ranking method was applied to assign the ranking of impact of all factors for a comprehensive analysis. The results showed that Pipa Street, Wufu Street in a main urban area, and Hongmiao Street had the highest street vitality and the highest rankings of almost all the factors. Street vitality in different periods demonstrated that street vitality in new urban areas is lower compared with old and main urban areas.

Macromolecularab initiophasing enforcing secondary and tertiary structure

Ab initiophasing of macromolecular structures, from the native intensities alone with no experimental phase information or previous particular structural knowledge, has been the object of a long quest, limited by two main barriers: structure size and resolution of the data. Current approaches to extend the scope ofab initiophasing include use of the Patterson function, density modification and data extrapolation. The authors' approach relies on the combination of locating model fragments such as polyalanine α-helices with the programPHASERand density modification with the programSHELXE. Given the difficulties in discriminating correct small substructures, many putative groups of fragments have to be tested in parallel; thus calculations are performed in a grid or supercomputer. The method has been named after the Italian painter Arcimboldo, who used to compose portraits out of fruit and vegetables. WithARCIMBOLDO, most collections of fragments remain a `still-life', but some are correct enough for density modification and main-chain tracing to reveal the protein's true portrait. Beyond α-helices, other fragments can be exploited in an analogous way: libraries of helices with modelled side chains, β-strands, predictable fragments such as DNA-binding folds or fragments selected from distant homologues up to libraries of small local folds that are used to enforce nonspecific tertiary structure; thus restoring theab initionature of the method. Using these methods, a number of unknown macromolecules with a few thousand atoms and resolutions around 2 Å have been solved. In the 2014 release, use of the program has been simplified. The software mediates the use of massive computing to automate the grid access required in difficult cases but may also run on a single multicore workstation (http://chango.ibmb.csic.es/ARCIMBOLDO_LITE) to solve straightforward cases.

Macromolecular Ab Initio Phasing Enforcing Secondary and Tertiary Structure

Ab Initio phasing of macromolecular structures, from the native intensities alone with no experimental phase information or previous particular structural knowledge has been the object of a long quest, limited by two main barriers: structure size and resolution of the data. Beyond the first successful atomic resolution scenario, current approaches have been developed, exploiting alternative constraints to atomicity, through use of the Patterson function, density modification and data extrapolation [1]. Our own approach relies on the combination of locating model fragments such as polyalanine alpha-helices with the program PHASER [2] and density modification with the program SHELXE [3]. Given the difficulties in discriminating correctly positioned fragments, many putative groups of fragments have to be tested in parallel, thus calculations are performed in a grid or supercomputer. The method has been called after the Italian painter Arcimboldo, who used to compose portraits out of fruits and vegetables. In the case of our program, most collections of fragments remain a "still-life", but some are correct enough for density modification to reveal the protein's true portrait (http://chango.ibmb.csic.es/ARCIMBOLDO). Beyond alpha-helices, any unknown structure should contain fragments already seen in the PDB, but how to retrieve and exploit this information? Our program BORGES identifies, retrieves and exploits this geometric information The PDB database contains a vast amount of information and for any unknown structure, given small enough fragments (e.g. two helices or three strands in a particular disposition), similar models (<0.5Å rmsd) are bound to occur in some of the deposited entries. In analogy to Borges' "Library of Babel" that enclosed books with all random combinations of letters and therefore held any possible book, the information required to phase a structure through fragment search and density modification should already be present somewhere in the PDB. The more so as unlike "Borges library", the PDB is non-random, containing in all sort of structural contexts only meaningful structural units. In addition, to bootstrap phasing our method requires small sentences instead of complete volumes, that is, a small fraction of perfect mainchain rather than a complete description of the structure (http://chango.ibmb.csic.es/BORGES). Using these methods, a number of unknown macromolecules with a few thousand atoms and resolutions around 2 Å have been solved.