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- Szymanski, N. J., et al.
(författare)
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Dynamical stabilization in delafossite nitrides for solar energy conversion
- 2018
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Ingår i: Journal of Materials Chemistry A. - : ROYAL SOC CHEMISTRY. - 2050-7488 .- 2050-7496. ; 6:42, s. 20852-20860
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Tidskriftsartikel (refereegranskat)abstract
- Delafossite structured ternary nitrides, ABN(2), have been of recent experimental investigation for applications such as tandem solar and photoelectrochemical cells. We present a thorough first principles computational investigation of their stability, electronic structure, and optical properties. Nine compounds, where A = Cu, Ag, Au and B = V, Nb, Ta, were studied. For three of these compounds, CuTaN2, CuNbN2, and AgTaN2, our computations agree well with experimental results. Optimized lattice parameters, formation energies, and mechanical properties have been computed using the generalized gradient approximation (GGA). Phonon density of states computed at zero-temperature shows that all compounds are dynamically unstable at low temperatures. Including finite-temperature anharmonic effects stabilizes all compounds at 300 K, with the exception of AgVN2. Analysis of Crystal Orbital Hamiltonian Populations (COHP) provides insight into the bonding and antibonding characters of A-N and B-N pairs. Instability at low temperatures can be attributed to strong A-N antibonding character near the Fermi energy. B-N bonding is found to be crucial in maintaining stability of the structure. AgVN2 is the only compound to display significant B-N antibonding below the Fermi energy, as well as the strongest degree of A-N antibonding, both of which provide explanation for the sustained instability of this compound up to 900 K. Hybrid functional calculations of electronic and optical properties show that real static dielectric constants in the semiconductors are related to corresponding band gaps through the Moss relation. CuTaN2, CuNbN2, AgTaN2, AgNbN2, AgVN2, AuTaN2, and AuNbN2 exhibit indirect electronic band gaps while CuVN2 and AuVN2 are metallic. Imaginary parts of the dielectric function are characterized by d-d interband transitions in the semiconductors and d-d intraband transitions in the metals. Four compounds, CuTaN2, CuNbN2, AgTaN2, and AgNbN2, are predicted to exhibit large light absorption in the range of 1.0 to 1.7 eV, therefore making these materials good candidates for solar-energy conversion applications. Two compounds, AuTaN2 and AuNbN2, have band gaps and absorption onsets near the ideal range for obtaining high solar-cell conversion efficiency, suggesting that these compounds could become potential candidates as absorber materials in tandem solar cells or for band-gap engineering by alloying.
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| 2. |
- Wang, L.-L., et al.
(författare)
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Origin of bulklike structure and bond length disorder of Pt37 and Pt6Ru31 clusters on carbon : Comparison of theory and experiment
- 2006
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 128:1, s. 131-142
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Tidskriftsartikel (refereegranskat)abstract
- We describe a theoretical analysis of the structures of self-organizing nanoparticles formed by Pt and Ru-Pt on carbon support. The calculations provide insights into the nature of these metal particle systems-ones of current interest for use as the electrocatalytic materials of direct oxidation fuel cells- and clarify complex behaviors noted in earlier experimental studies. With clusters deposited via metalloorganic Pt or PtRu5 complexes, previous experiments [Nashner et al. J. Am. Chem. Soc. 1997, 119, 7760, Nashner et al. J. Am. Chem. Soc. 1998, 120, 8093, Frenkel et al. J. Phys. Chem. B 2001, 105, 12689] showed that the Pt and Pt-Ru based clusters are formed with fcc(111)-stacked cuboctahedral geometry and essentially bulklike metal-metal bond lengths, even for the smallest (few atom) nanoparticles for which the average coordination number is much smaller than that in the bulk, and that Pt in bimetallic [PtRu5] clusters segregates to the ambient surface of the supported nanoparticles. We explain these observations and characterize the cluster structures and bond length distributions using density functional theory calculations with graphite as a model for the support. The present study reveals the origin of the observed metal-metal bond length disorder, distinctively different for each system, and demonstrates the profound consequences that result from the cluster/carbon-support interactions and their key role in the structure and electronic properties of supported metallic nanoparticles. © 2006 American Chemical Society.
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