Microsymposia C149 MS given in this presentation with results obtained with an ultrastable double aberration-corrected and monochromated electron microscope. First of all, we will demonstrate the detection of low-loss features in plasmonic nanostructures down to the infrared part of the electron energy loss spectrum by directly imaging resonances down to 0.5eV, the lowest features currently detected with EELS [1]. Using momentum resolved near-edge structures we will discuss the detection of the strong anisotropy in bonding in carbon nanotubes. After an overview of the imaging conditions used to detect ordering changes in alloy nanoparticles using a combination of X-ray diffraction techniques and high-angle annular dark-field STEM imaging and simulations, we will discuss the study the application of atomic-resolved EELS mapping in the study of interfaces [2], [3]. We will demonstrate how this powerful technique can be used in the study of the structure and substitutional effects on the atomic structure of interfaces and electronic states changes within one or two unit cells from the interface. We will demonstrate how such spectroscopic technique can be used to detect changes in valence and electronic structure as well as the termination of substrate surfaces in contact with epitaxial films. Examples will show how the stability of microscopes, coupled with atomic resolution, can be used to not only obtain spectroscopic information but aso to determine, directly from high angle annular dark-field images, the local strain at interfaces and at dislocations [4]. Additional examples will highlight the application of microscopy technique to the analysis of clusters, multiferroic materials based on the perovskite structures, and interfaces in complex oxides. These examples demonstrate that compositional and chemical state (valence and coordination) information can be obtained down to the Ångstrom level. Silica nanowires (SiO x-NWs) embedded with Au peapods have been studied by energy-filtered scanning transmission electron microscopy (EFTEM), Au L 3-and O K-edge x-ray absorption near-edge structure (XANES) and x-ray emission spectroscopy (XES). XANES and XES data show that band gaps of Au-peapod embedded and pure SiO x-NWs were 6.8 eV. XANES results indicate illumination induced electron transfer from Au peapod to SiO x-NWs. Photo-response and EFTEM measurements show that green light has more significant enhancement of photo conductivity than red and blue light due to surface plasmon resonance.
Previous investigations [H. L. Zhuang and R. G. Hennig, J. Phys. Chem. C, 2013, 117, 20440-20445; J. Kang, S. Tongay, J. Zhou, J. Li and J. Wu, Appl. Phys. Lett., 2013, 102, 012111] demonstrated that molybdenum disulfide (MoS2) is a potential photocatalyst for water splitting. However, the photogenerated electron-hole pairs in MoS2 remain in the same spatial regions, resulting in a high rate of recombination. Using first-principles calculations, we designed a MoS2-based heterostructure by stacking MoS2 on two-dimensional zinc oxide (ZnO) and investigated its structural, electronic, and optical properties. The interaction at the MoS2/ZnO interface was found to be dominated by van der Waals (vdW) forces. The energy levels of both water oxidation and reduction lie within the bandgap of the MoS2/ZnO vdW heterostructure, which guarantee their occurrence for water splitting. Moreover, a type-II band alignment and a large built-in electric field are formed at the MoS2/ZnO interface, which ensure the enhanced separation of the photogenerated electron-hole pairs. In addition, strong optical absorption in the visible region was also found in the MoS2/ZnO vdW heterostructure, indicating that it has potential for application in photovoltaic and photocatalytic devices.
The structural, electronic, and optical properties of heterostructures formed by transition metal dichalcogenides MX2 (M = Mo, W; X = S, Se) and graphene-like zinc oxide (ZnO) were investigated using first-principles calculations. The interlayer interaction in all heterostructures was characterized by van der Waals forces. Type-II band alignment occurs at the MoS2/ZnO and WS2/ZnO interfaces, together with the large built-in electric field across the interface, suggesting effective photogenerated-charge separation. Meanwhile, type-I band alignment occurs at the MoSe2/ZnO and WSe2/ZnO interfaces. Moreover, all heterostructures exhibit excellent optical absorption in the visible and infrared regions, which is vital for optical applications.
Herein we present a systematic study of the structures and magnetic properties of six coordination compounds with mixed azide and zwitterionic carboxylate ligands, [M(N(3) )(2) (2-mpc)] (2-mpc=N-methylpyridinium-2-carboxylate; M=Co for 1 and Mn for 2), [M(N(3) )(2) (4-mpc)] (4-mpc=N-methylpyridinium-4-carboxylate; M=Co for 3 and Mn for 4), [Co(3) (N(3) )(6) (3-mpc)(2) (CH(3) OH)(2) ] (5), and [Mn(3) (N(3) )(6) (3-mpc)(2) ] (6; 3-mpc=N-methylpyridinium-3-carboxylate). Compounds 1-3 consist of one-dimensional uniform chains with (μ-EO-N(3) )(2) (μ-COO) triple bridges (EO=end-on); 5 is also a chain compound but with alternating [(μ-EO-N(3) )(2) (μ-COO)] triple and [(EO-N(3) )(2) ] double bridges; Compound 4 contains two-dimensional layers with alternating [(μ-EO-N(3) )(2) (μ-COO)] triple, [(μ-EO-N(3) )(μ-COO)] double, and (EE-N(3) ) single bridges (EE=end-to-end); 6 is a layer compound in which chains similar to those in 5 are cross-linked by a μ(3) -1,1,3-N(3) azido group. Magnetically, the three Co(II) compounds (1, 3, and 5) all exhibit intrachain ferromagnetic interactions but show distinct bulk properties: 1 displays relaxation dynamics at very low temperature, 3 is an antiferromagnet with field-induced metamagnetism due to weak antiferromagnetic interchain interactions, and 5 behaves as a noninnocent single-chain magnet influenced by weak antiferromagnetic interchain interactions. The magnetic differences can be related to the interchain interactions through π-π stacking influenced by different substitution positions in the ligands and/or different magnitudes of intrachain coupling. All of the Mn(II) compounds show overall intrachain/intralayer antiferromagnetic interactions. Compound 2 shows the usual one-dimensional antiferromagnetism, whereas 4 and 6 exhibit different weak ferromagnetism due to spin canting below 13.8 and 4.6 K, respectively.
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