Qiao L.,University of Manchester |
Qiao L.,Center for Nanophase Materials science |
Jang J.H.,Oak Ridge National Laboratory |
Singh D.J.,Oak Ridge National Laboratory |
And 13 more authors.
Epitaxial strain provides a powerful approach to manipulate physical properties of materials through rigid compression or extension of their chemical bonds via lattice-mismatch. Although symmetry-mismatch can lead to new physics by stabilizing novel interfacial structures, challenges in obtaining atomic-level structural information as well as lack of a suitable approach to separate it from the parasitical lattice-mismatch have limited the development of this field. Here, we present unambiguous experimental evidence that the symmetry-mismatch can be strongly controlled by dimensionality and significantly impact the collective electronic and magnetic functionalities in ultrathin perovskite LaCoO3/SrTiO3 heterojunctions. State-of-art diffraction and microscopy reveal that symmetry breaking dramatically modifies the interfacial structure of CoO6 octahedral building-blocks, resulting in expanded octahedron volume, reduced covalent screening, and stronger electron correlations. Such phenomena fundamentally alter the electronic and magnetic behaviors of LaCoO3 thin-films. We conclude that for epitaxial systems, correlation strength can be tuned by changing orbital hybridization, thus affecting the Coulomb repulsion, U, instead of by changing the band structure as the common paradigm in bulks. These results clarify the origin of magnetic ordering for epitaxial LaCoO3 and provide a route to manipulate electron correlation and magnetic functionality by orbital engineering at oxide heterojunctions. © 2015 American Chemical Society. Source
Ji Z.,Ohio State University |
Natu G.,Ohio State University |
Huang Z.,Ohio State University |
Kokhan O.,Chemical science and Engineering Division |
And 2 more authors.
Journal of Physical Chemistry C
We report the first application of cyclometalated ruthenium complexes of the type Ru[(N ̂N) 2(C ̂N)] + as sensitizers for p-type NiO dye-sensitized solar cells (NiO p-DSCs). These dyes exhibit broad absorption in the visible region. The carboxylic anchoring group is attached to the phenylpyridine ligand, which results in efficient hole injection. Moreover, the distance between the Ru[(N ̂N) 2(C ̂N)] + core and the carboxylic anchoring group is systematically varied by inserting rigid phenylene linkers. Femtosecond transient absorption (TA) studies reveal that the interfacial charge recombination rate between reduced sensitizers and holes in the valence band of NiO decreases as the number of phenylene linkers increases across the series. As a result, the solar cell made of the dye with the longest spacer (O12) exhibits the highest efficiency with both increased short-circuit current (J sc) and open-circuit voltage (V oc). The incident photon-to-current conversion efficiency (IPCE) spectra match well with the absorption spectra of sensitizers, suggesting the observed cathodic current is generated from the dye sensitization. In addition, the absorbed photon-to-current conversion efficiencies (APCEs) display an increment across the series. We further studied the interfacial charge recombination of our solar cells by electrochemical impedance spectroscopy (EIS). The results reveal an enhanced hole lifetime as the number of phenylene linkers increases. This study opens up opportunities of using cyclometalated Ru complexes for p-DSCs. © 2012 American Chemical Society. Source
Lin C.-K.,Chemical science and Engineering Division |
Piao Y.,Chemical science and Engineering Division |
Piao Y.,Jilin University |
Kan Y.,Chemical science and Engineering Division |
And 6 more authors.
ACS Applied Materials and Interfaces
Safety of lithium-ion batteries has been a major barrier to large-scale applications. For better understanding the failure mechanism of battery materials under thermal abuse, the decomposition of a delithiated high energy cathode material, Li1.2-xNi0.15Mn0.55Co 0.1O2, in the stainless-steel high pressure capsules was investigated by in situ high energy X-ray diffraction. The data revealed that the thermally induced decomposition of the delithiated transition metal (TM) oxide was strongly influenced by the presence of electrolyte components. When there was no electrolyte, the layered structure for the delithiated TM oxide was changed to a disordered Li1-xM2O4-type spinel, which started at ca. 266 °C. The disordered Li1-xM 2O4-type spinel was decomposed to a disordered M 3O4-type spinel phase, which started at ca. 327 °C. In the presence of organic solvent, the layered structure was decomposed to a disordered M3O4-type spinel phase, and the onset temperature of the decomposition was ca. 216 °C. When the LiPF6 salt was also present, the onset temperature of the decomposition was changed to ca. 249 °C with the formation of MnF2 phase. The results suggest that a proper optimization of the electrolyte component, that is, the organic solvent and the lithium salt, can alter the decomposition pathway of delithiated cathodes, leading to improved safety of lithium-ion batteries. © 2014 American Chemical Society. Source
Lipson A.L.,Chemical science and Engineering Division |
Lipson A.L.,Argonne National Laboratory |
Pan B.,Chemical science and Engineering Division |
Pan B.,Argonne National Laboratory |
And 7 more authors.
Chemistry of Materials
As new uses for larger scale energy storage systems are realized, new chemistries that are less expensive or have higher energy density are needed. While lithium-ion systems have been well studied, the availability of new energy storage chemistries opens up the possibilities for more diverse strategies and uses. One potential path to achieving this goal is to explore chemistries where a multivalent ion such as Ca2+ or Mg2+ is the active species. Herein, we demonstrate this concept for a Ca-ion system utilizing manganese hexacyanoferrate (MFCN) as the cathode to intercalate Ca reversibly in a dry nonaqueous electrolyte. Through characterization via X-ray absorption near-edge spectroscopy, it is determined that only the manganese changes oxidation state during cycling with Ca. X-ray diffraction indicates the MFCN maintains its crystallinity during cycling, with only minor structural changes associated with expansion and contraction. Furthermore, we have demonstrated the first rechargeable Ca-ion battery utilizing MFCN as the cathode and elemental tin as the anode. © 2015 American Chemical Society. Source
Mara M.W.,Northwestern University |
Bowman D.N.,North Carolina State University |
Buyukcakir O.,Korea Advanced Institute of Science and Technology |
Shelby M.L.,Northwestern University |
And 9 more authors.
Journal of the American Chemical Society
Copper(I) diimine complexes have emerged as low cost replacements for ruthenium complexes as light sensitizers and electron donors, but their shorter metal-to-ligand-charge-transfer (MLCT) states lifetimes and lability of transient Cu(II) species impede their intended functions. Two carboxylated Cu(I) bis-2,9-diphenylphenanthroline (dpp) complexes [Cu(I)(dpp-O(CH2CH2O)5)(dpp-(COOH)2)]+ and [Cu(I)(dpp-O(CH2CH2O)5)(dpp-(φ-COOH)2)]+ (φ = tolyl) with different linker lengths were synthesized in which the MLCT-state solvent quenching pathways are effectively blocked, the lifetime of the singlet MLCT state is prolonged, and the transient Cu(II) ligands are stabilized. Aiming at understanding the mechanisms of structural influence to the interfacial charge transfer in the dye-sensitized solar cell mimics, electronic and geometric structures as well as dynamics for the MLCT state of these complexes and their hybrid with TiO2 nanoparticles were investigated using optical transient spectroscopy, X-ray transient absorption spectroscopy, time-dependent density functional theory, and quantum dynamics simulations. The combined results show that these complexes exhibit strong absorption throughout the visible spectrum due to the severely flattened ground state, and a long-lived charge-separated Cu(II) has been achieved via ultrafast electron injection (<300 fs) from the 1MLCT state into TiO2 nanoparticles. The results also indicate that the TiO2-phen distance in these systems does not have significant effect on the efficiency of the interfacial electron-transfer process. The mechanisms for electron transfer in these systems are discussed and used to develop new strategies in optimizing copper(I) diimine complexes in solar energy conversion devices. © 2015 American Chemical Society. Source