Pulickel M. Ajayan

Bio: Pulickel M. Ajayan is an academic researcher from Rice University. The author has contributed to research in topics: Carbon nanotube & Graphene. The author has an hindex of 176, co-authored 1223 publications receiving 136241 citations. Previous affiliations of Pulickel M. Ajayan include University of Hawaii at Manoa & University of Florida.

Effects of Ball Milling on the Electrochemical Capacity and Interfacial Stability of Li2MnO3 Cathode Materials

Abstract: The cycling mechanism of Li2MnO3 cathode materials synthesized by conventional solid-state methods at high temperatures (800–900 °C) has been intensively investigated. Previous studies showed that CO2 and O2 gas evolution accounts for most of the charge capacity, followed by some Mn reduction during discharge. In this work, we analyze the effects of ball milling on the structure, surface contaminant, and electrochemical capacity of Li2MnO3 cathode material, with or without a graphitic fluoride (C–F) additive. At the same time, C–F is added to form a protective coating layer that reduces unwanted reactions with the electrolyte during later electrochemical cycling. We find that the C–F ball-milled material shows Li2MnO3/LiMnO2 composite phases, while the purely ball-milled material shows a single Li2MnO3 phase. Furthermore, we characterize surface species and gas evolution during the first cycle, which reveals the decomposition of Li2CO3 and the carbonate electrolyte during the first charge, especially during the high potential region (>4.4 V), and the electrochemical reduction of only a small fraction of the evolved gas on the first discharge (<2.75 V). The appearance further demonstrates the repetitive nature of this process during charge and disappearance during discharge of Mn 2p3/2 X-ray photoelectron spectroscopy (XPS) spectra signals during the first two cycles. These processes result in first discharge specific capacities of only 155 and 170 mAh/g after first charge specific capacities of 210 and 320 mAh/g for the pure ball-milled and ball-milled with C–F materials, respectively. These studies demonstrate the interfacial instability introduced by ball milling. However, the electrochemical capacity is significantly increased, necessitating further investigation to determine whether ball milling can activate Mn-containing cathode materials.

Solvent-Induced Incremental Pore Collapse in Two-Dimensional Covalent Organic Frameworks

Abstract: Nanoporous materials such as covalent organic frameworks (COFs) are attractive due to their extremely high surface areas and porosities, but they are susceptible to pore collapse and loss of crystallinity. Prior studies reported complete pore collapse above a threshold surface tension for the activation solvents. In this study, we show that the degree of pore collapse can be precisely controlled by using a mixture of high and low surface tension solvents for activation. We further demonstrate that pore collapse is an irreversible process, in which partially collapsed COFs can collapse further using high surface tension solvents but cannot recover their structural integrity by activation with low surface tension solvents. Finally, we show that the solvent surface tension is the most important characteristic that governs pore collapse of COFs during solvent activation, and the activation temperature, pressure, washing time, and number of activation cycles has negligible impact on the final COF porosity and crystallinity. This work provides novel insight into pore collapse of COFs, a method to systematically tune surface area, and guidance on the development of effective activation methods to produce highly crystalline and porous COFs. These findings will be useful for the preparation of COFs and other nanoporous materials.

U-carbon: metallic and magnetic

Abstract: We report the discovery of a pristine crystalline 3D carbon that is magnetic, electrically conductive and stable under ambient conditions. This carbon material, which has remained elusive for decades, is synthesized by using the chemical vapor deposition (CVD) technique with a particular organic molecular precursor 3,3-dimethyl-1-butene (C6H12). An exhaustive computational search of the potential energy surface reveals its unique sp2-sp3 hybrid bonding topology. Synergistic studies involving a large number of experimental techniques and multi-scale first-principles calculations reveal the origin of its novel properties due to the special arrangement of sp2 carbon atoms in lattice. The discovery of this U-carbon, named such because of its unusual structure and properties, can open a new chapter in carbon science.

Synthesis of Large-Area and Monolayer of Graphene: Role of Transition Metal Support and Growth Time by CVD Method

Abstract: Continuous monolayer graphene sheet with large area has been synthesized via chemical vapor deposition (CVD) method using liquid hydrocarbon as precursor. Synthesis parameters including growth substrate and growth time have been investigated to assess their influence on monolayer graphene synthesis. Raman spectroscopy and high resolution transmission electron microscopy (HRTEM) reveal that the number of layers and quality of graphene sheet depend greatly on the varied synthesis parameter. The study could be used to improve understanding the growth of graphene by CVD method in order to meet the needs of graphene in various electronic applications.

Quaternary two-dimensional (2D) transition metal dichalcogenides (TMDs) with tunable bandgap

Abstract: Alloying/doping in two-dimensional material has been important due to wide range band gap tunability. Increasing the number of components would increase the degree of freedom which can provide more flexibility in tuning the band gap and also reduced the growth temperature. Here, we report synthesis of quaternary alloys MoxW1-xS2ySe2(1-y) using chemical vapour deposition. The composition of alloys has been tuned by changing the growth temperatures. As a result, we can tune the bandgap which varies from 1.73 eV to 1.84 eV. The detailed theoretical calculation supports the experimental observation and shows a possibility of wide tunability of bandgap.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

The electronic properties of graphene

Abstract: This article reviews the basic theoretical aspects of graphene, a one-atom-thick allotrope of carbon, with unusual two-dimensional Dirac-like electronic excitations. The Dirac electrons can be controlled by application of external electric and magnetic fields, or by altering sample geometry and/or topology. The Dirac electrons behave in unusual ways in tunneling, confinement, and the integer quantum Hall effect. The electronic properties of graphene stacks are discussed and vary with stacking order and number of layers. Edge (surface) states in graphene depend on the edge termination (zigzag or armchair) and affect the physical properties of nanoribbons. Different types of disorder modify the Dirac equation leading to unusual spectroscopic and transport properties. The effects of electron-electron and electron-phonon interactions in single layer and multilayer graphene are also presented.

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Abstract: 抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。