604 research outputs found
Raman spectroscopic characterization of stacking configuration and interlayer coupling of twisted multilayer graphene grown by chemical vapor deposition
No full textMultilayer graphene (MLG) grown by chemical vapor deposition (CVD) is a promising material for electronic and optoelectronic devices. Understanding the stacking configuration and interlayer coupling of MLGs is technologically relevant and of importance for the device applications. Here, we reported a kind of twisted MLGs (tMLGs), in which only one twist angle was revealed from the twist-related modes, R and R'. With increasing the total layer number of N (N > 2), the observed interlayer shear modes in the tMLG flake always follow those of AB-stacked (N-1)LG, while the observed interlayer breathing modes always follow those of AB-stacked NLG, independent of its twist angle. The tMLGs are identified as t(1+n) LGs, which are formed by stacking one graphene monolayer on the top of AB-stacked n layer graphenes by rotating a certain angle between them. The layer breathing coupling of the t(l+n)LGs is almost identical to that of mechanically-exfoliated tMLGs, which demonstrates the high quality of MLGs grown by CVD. This study provides an applicable approach to probe the stacking configuration and interlayer coupling of MLGs grown by CVD or related methods. This work also demonstrates the possibility to grow MLG flakes with a fixed stacking configuration, e.g., t(1+n)LG, by the CVD method. (C) 2016 Elsevier Ltd. All rights reserved.National Basic Research Program of China [2016YFA0301200, 2014CB932500]; National Natural Science Foundation of China [11225421, 11434010, 11474277, 11504077, 21525310]SCI(E)[email protected]
Spinel LiMn2O4 Nanorods as Lithium Ion Battery Cathodes
No full textSpinel LiMn2O4 is a low-cost, environmentally friendly, and highly abundant material for Li-ion battery cathodes. Here, we report the hydrothermal synthesis of single-crystalline beta-MnO2 nanorods and their chemical conversion into free-standing single-crystalline LiMn2O4 nanorods using a simple solid-state reaction. The LiMn2O4 nanorods have an average diameter of 130 nm and length of 1.2 mu m. Galvanostatic battery testing showed that LiMn2O4 nanorods have a high charge storage capacity at high power rates compared with commercially available powders. More than 85% of the initial charge storage capacity was maintained for over 100 cycles. The structural transformation studies showed that the Li ions intercalated into the cubic phase of the LiMn2O4 with a small change of lattice parameter, followed by the coexistence of two nearly identical cubic phases in the potential range of 3.5 to 4.3V.The work is supported by the Global
Climate and Energy Project at Stanford and King Abdullah
University of Science and Technology. C.K.C. acknowledges
support from a National Science Foundation graduate fellowship
and Stanford Graduate Fellowship. D.K.K. would
like to thank the SBS Foundation and Korea Research
Foundation (KRF-2005-005-JO9701) for supporting his
sabbatical leave
Large Single-Crystal Cu Foils with High-Index Facets by Strain-Engineered Anomalous Grain Growth
No full textThe rich and complex arrangements of metal atoms in high-index metal facets afford appealing physical and chemical properties, which attracts extensive research interest in material science for the applications in catalysis and surface chemistry. However, it is still a challenge to prepare large-area high-index single crystals in a controllable and cost-efficient manner. Herein, entire commercially available decimeter-sized polycrystalline Cu foils are successfully transformed into single crystals with a series of high-index facets, relying on a strain-engineered anomalous grain growth technique. The introduction of a moderate thermal-contact stress upon the Cu foil during the annealing leads to the formation of high-index grains dominated by the thermal strain of the Cu foils, rather than the (111) surface driven by the surface energy. Besides, the designed static gradient of the temperature enables the as-formed high-index grain seed to expand throughout the entire Cu foil. The as-received high-index Cu foils can serve as the templates for producing high-index single-crystal Cu-based alloys. This work provides an appealing material basis for the epitaxial growth of 2D materials, and the applications that require the unique surface structures of high-index metal foils and their alloys
Chemistry Makes Graphene beyond Graphene
No full textAlthough graphene is extremely inert in chemistry because of the giant delocalized pi electron system, various methods have been developed to achieve its efficient chemical modification. Covalent chemistry is effective to modulate the physical properties of graphene. By converting the sp(2) hybridized carbon atoms to sp(3) ones, new two-dimensional (2D) materials and 2D superlattices with fascinating features beyond mother graphene could be built from the graphene scaffold, greatly expanding the graphene family and its attraction. In this Perspective, the power of covalent chemistry is demonstrated from the viewpoint of tailoring graphene's energy band structure as well as creating new 2D materials and 2D superlattices. A specific focus is laid on the general consideration and understanding of covalent graphene chemistry toward electronic devices and material science.http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcApp=PARTNER_APP&SrcAuth=LinksAMR&KeyUT=WOS:000341226000001&DestLinkType=FullRecord&DestApp=ALL_WOS&UsrCustomerID=8e1609b174ce4e31116a60747a720701Chemistry, MultidisciplinarySCI(E)[email protected]; [email protected]
Controlled Synthesis of High-Mobility Atomically Thin Bismuth Oxyselenide Crystals
No full textNon-neutral layered crystals, another group of two-dimensional (2D) materials that lack a well-defined van der Waals (vdWs) gap, are those that form strong chemical bonds in-plane but display weak out-of-plane electrostatic interactions, exhibiting intriguing properties for the bulk counterpart. However, investigation of the properties of their atomically thin counterpart are very rare presumably due to the absence of efficient ways to achieve large-area high-quality 2D crystals. Here, high-mobility atomically thin Bi2O2Se, a typical non-neutral layered crystal without a standard vdWs gap, was synthesized via a facial chemical vapor deposition (CVD) method, showing excellent controllability for thickness, domain size, nucleation site, and crystal-phase evolution. Atomically thin, large single crystals of Bi2O2Se with lateral size up to similar to 200 mu m and thickness down to a bilayer were obtained. Moreover, optical and electrical properties of the CVD-grown 2D Bi2O2Se crystals were investigated, displaying a size-tunable band gap upon thinning and an ultrahigh Hall mobility of >20000 cm(2) V-1 s(-1) at 2 K. Our results on the high-mobility 2D Bi2O2Se semiconductor may activate the synthesis and related fundamental research of other non-neutral 2D materials.National Natural Science Foundation of China [21525310]; National Basic Research Program of China [2014CB932500]; National Program for Support of Top-Notch Young ProfessionalsSCI(E)ARTICLE53021-30261
Reaction kinetics in low -dimensional diffusion -limited systems: Experiments and simulations.
No full textThe non-classical kinetics of elementary reactions in some basic diffusion-limited systems are studied in this thesis. Two reaction types are explored in detail: (1) Trapping reaction A + T → T and (2) elementary bimolecular reaction A + B → C. Different methods are used in both systems: experiments, Monte Carlo simulations, exact enumeration, and analytical theory. Experimentally the trapping reaction A + T → T was realized by photobleaching fluorescein molecules in solution by a focused laser beam. By changing the shape of the reactor and the laser beam, different dimensionalities are achieved: line trap in gaps between glass slides (1D behavior), point traps in gaps in-between glass slides (2D behavior), point trap in a flat rectangular capillary (2D to 1D behavior) and short line trap in a flat rectangular capillary (1D to 2D to 1D behavior). Reaction front kinetics of the bimolecular reaction A + B → C was discussed in quasi-1D confined systems. An experimental manifestation was the reaction between calcium ions and Calcium-Green-1 (Dextran attached) dye molecules, initially separated by a semi-permeable membrane. The experimental and theoretical approaches agree well with each other. The results show that non-classical kinetics in diffusion-limited systems differ from classical kinetics in two main aspects: an overall slowing down of the reaction rate with time and a dependency of the rate law on the dimensionality of the systems.PhDPhysical chemistryPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/124913/2/3163910.pd
Product-specific active site motifs of Cu for electrochemical CO₂ reduction
No full textElectrochemical CO₂ reduction (CO₂R) to fuels is a promising route to close the anthropogenic carbon cycle and store renewable energy. Cu is the only metal catalyst that produces C₂₊ fuels, yet challenges remain in the improvement of electrosynthesis pathways for highly selective fuel production. To achieve this, mechanistically understanding CO₂R on Cu, particularly identifying the product-specific active sites, is crucial. We rationally designed and fabricated nine large-area single-crystal Cu foils with various surface orientations as electrocatalysts and monitored their surface reconstructions using operando grazing incidence X-ray diffraction (GIXRD) and electron back-scattered diffraction (EBSD). We quantitatively established correlations between the Cu atomic configurations and the selectivities toward multiple products and provide a paradigm to understand the structure-function correlation in catalysis.This research was supported by the National Natural Science Foundation of China (grants 21872039, 51991340, and 51991342), Science and Technology Commission of Shanghai Municipality (grant 18JC1411700), National Key Research and Development Program of China (2016YFA0300903 and 2016YFA0300804), Beijing Natural Science Foundation (JQ19004), Beijing Excellent Talents Training Support (2017000026833ZK11), Beijing Municipal Science & Technology Commission (Z191100007219005), Beijing Graphene Innovation Program (Z181100004818003), and the Key Research and Development Program of Guangdong Province (2020B010189001, 2019B010931001, and 2018B030327001). We sincerely thank Dr. Bing Deng, Prof. Hailin Peng, and Prof. Zhongfan Liu for providing some low-index single-crystal Cu electrodes when initiating the work
- …
