53 research outputs found
Catalyst-Free Growth of Millimeter-Long Topological Insulator Bi2Se3 Nanoribbons and the Observation of pi Berry Phase
We report the growth of single-crystalline Bi2Se3 nanoribbons with lengths up
to several millimeters via a catalyst-free physical vapor deposition method.
Scanning transmission electron microscopy analysis reveals that the nanoribbons
grow along the (1120) direction. We obtain a detailed characterization of the
electronic structure of the Bi2Se3 nanoribbons from measurements of
Shubnikov-de Haas (SdH) quantum oscillations. Angular dependent
magneto-transport measurements reveal a dominant two-dimensional contribution
originating from surface states and weak contribution from the bulk states. The
catalyst-free synthesis yields high-purity nanocrystals enabling the
observation of a large number of SdH oscillation periods and allowing for an
accurate determination of the pi-Berry phase, one of the key features of Dirac
fermions in topological insulators. The long-length nanoribbons can empower the
potential for fabricating multiple nanoelectronic devices on a single
nanoribbon.Comment: 22 pages, 5 figure
Guided Growth of Horizontal GaN Nanowires on Spinel with Orientation-Controlled Morphologies
Critical Factors Driving the High Volumetric Uptake of Methane in Cu-3(btc)(2)
A thorough experimental and computational study has been carried out to elucidate the mechanistic reasons for the high volumetric uptake of methane in the metal-organic framework Cu-3(btc)(2) (btc(3-) = 1,3,5-benzenetricarboxylate; HKUST-1). Methane adsorption data measured at several temperatures for Cu-3(btc)(2), and its isostructural analogue Cr-3(btc)(2), show that there is little difference in volumetric adsorption capacity when the metal center is changed. In situ neutron powder diffraction data obtained for both materials were used to locate four CD4 adsorption sites that fill sequentially. This data unequivocally shows that primary adsorption sites around, and within, the small octahedral cage in the structure are favored over the exposed Cu2+ or Cr2+ cations. These results are supported by an exhaustive parallel computational study, and contradict results recently reported using a time-resolved diffraction structure envelope (TRDSE) method. Moreover, the computational study reveals that strong methane binding at the open metal sites is largely due to methane-methane interactions with adjacent molecules adsorbed at the primary sites instead of an electronic interaction with the metal center. Simulated methane adsorption isotherms for Cu-3(btc)(2) are shown to exhibit excellent agreement with experimental isotherms, allowing for additional simulations that show that modifications to the metal center, ligand, or even tuning the overall binding enthalpy would not improve the working capacity for methane storage over that measured for Cu-3(btc)(2) itself
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Critical Factors Driving the High Volumetric Uptake of Methane in Cuâ(btc)â.
A thorough experimental and computational study has been carried out to elucidate the mechanistic reasons for the high volumetric uptake of methane in the metal-organic framework Cu3(btc)2 (btc(3-) = 1,3,5-benzenetricarboxylate; HKUST-1). Methane adsorption data measured at several temperatures for Cu3(btc)2, and its isostructural analogue Cr3(btc)2, show that there is little difference in volumetric adsorption capacity when the metal center is changed. In situ neutron powder diffraction data obtained for both materials were used to locate four CD4 adsorption sites that fill sequentially. This data unequivocally shows that primary adsorption sites around, and within, the small octahedral cage in the structure are favored over the exposed Cu(2+) or Cr(2+) cations. These results are supported by an exhaustive parallel computational study, and contradict results recently reported using a time-resolved diffraction structure envelope (TRDSE) method. Moreover, the computational study reveals that strong methane binding at the open metal sites is largely due to methane-methane interactions with adjacent molecules adsorbed at the primary sites instead of an electronic interaction with the metal center. Simulated methane adsorption isotherms for Cu3(btc)2 are shown to exhibit excellent agreement with experimental isotherms, allowing for additional simulations that show that modifications to the metal center, ligand, or even tuning the overall binding enthalpy would not improve the working capacity for methane storage over that measured for Cu3(btc)2 itself
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