Large negative thermal expansion (NTE) has been discovered during the last decade in materials of various kinds, particularly materials associated with a magnetic, ferroelectric or charge-transfer phase transition. Such NTE materials have attracted considerable attention for use as thermal-expansion compensators. Here, we report the discovery of giant NTE for reduced layered ruthenate. The total volume change related to NTE reaches 6.7% in dilatometry, a value twice as large as the largest volume change reported to date. We observed a giant negative coefficient of linear thermal expansion α=−115 × 10−6 K−1 over 200 K interval below 345 K. This dilatometric NTE is too large to be attributable to the crystallographic unit-cell volume variation with temperature. The highly anisotropic thermal expansion of the crystal grains might underlie giant bulk NTE via microstructural effects consuming open spaces in the sintered body on heating.
TitleAtomically precise graphene nanoribbon heterojunctions from a single molecular precursor
AbstractThe rational bottom-up synthesis of atomically defined graphene nanoribbon (GNR)heterojunctions represents a key enabling technology for the design of nanoscale electronic devices. Synthetic strategies have thus far relied on the random copolymerization of two electronically distinctive molecular precursors to yield a segmented band structure within a GNR. Here we report the fabrication and electronic characterization of atomically precise GNR heterojunctions prepared through a late-stage functionalization of chevron GNRs obtained from a single precursor that features fluorenone substituents along the convex edges. Excitation of the GNR induces cleavage of sacrificial carbonyl groups at the GNR edge, thus giving rise to atomically well-defined heterojunctions comprised of segments of fluorenone GNR and unfunctionalized chevron GNR. The structure of fluorenone/unfunctionalized GNR heterojunctions was characterized using bond-resolved STM (BRSTM) which enables chemical bonds to be imaged via STM at T = 4.5 K. Scanning tunneling spectroscopy (STS) reveals that the band alignment across the interface yields a staggered gap Type II heterojunction and is consistent with first-principles calculations. Detailed spectroscopic and theoretical studies reveal that the band realignment at the interface between fluorenone and unfunctionalized chevron GNRs proceeds over a distance less than 1nm, leading to extremely large effective fields.
International audienceHighly dispersed crystalline/amorphous LiFePO4 (LFP) nanoparticles encapsulated within hollow-structured graphitic carbon were synthesized using an in situ ultracentrifugation process. Ultracentrifugation triggered an in situ sol–gel reaction that led to the formation of core–shell LFP simultaneously hybridized with fractured graphitic carbon. The structure has double cores that contain a crystalline LFP (core 1) covered by an amorphous LFP containing Fe3+ defects (core 2), which are encapsulated by graphitic carbon (shell). These core–shell LFP nanocomposites show improved Li+ diffusivity thanks to the presence of an amorphous LFP phase. This material enables ultrafast discharge rates (60 mA h g-1 at 100C and 36 mA h g-1 at 300C) as well as ultrafast charge rates (60 mA h g-1 at 100C and 36 mA h g-1 at 300C). The synthesized core–shell nanocomposites overcome the inherent one-dimensional diffusion limitation in LFP and yet deliver/store high electrochemical capacity in both ways symmetrically up to 480C. Such a high rate symmetric capacity for both charge and discharge has never been reported so far for LFP cathode materials. This offers new opportunities for designing high-energy and high-power hybrid supercapacitors
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