Physical and Chemical Nature of the Scaling Relations between Adsorption Energies of Atoms on Metal Surfaces

Catalysis and Surface Chemistr

Optical and vibrational properties of (MnF6)4- complexes in cubic fluoroperovskites: insight through embedding calculations using Kohn-Sham equations with constrained electron density

The local structure and optical and vibrational properties associated with Mn2+-doped cubic AMF3 (A = K, Rb; M = Mg, Zn, Cd) fluoroperovskites are studied by means of embedding calculations using Kohn–Sham equations with constrained electron density. It is shown that while an electronic parameter like 10Dq essentially depends on the Mn2+–F− distance, the local vibration frequencies ωi (i = a1g, eg modes) are dominated by the interaction between F− ligands and nearest M2+ ions lying along bonding directions. The high ωa values observed for KMgF3:Mn2+ and KZnF3:Mn2+, the huge variations of ωe and ωa frequencies when the host lattice is changed, as well as the increase of Huang–Rhys factors and the Stokes shift following the host lattice parameter, are shown to be related to this elastic coupling of the MnF64− complex to the rest of the host lattice. The present results support the conclusion that the Stokes shift is determined by the interaction of the excited 4T1g state with a1g and eg local modes while the coupling with the t2g shear mode is not relevant. The variations of local vibrational frequencies and the Stokes shift induced by a hydrostatic pressure on a given system are shown to be rather different to those produced by the chemical pressure associated with distinct host lattices

Design of solar cell materials via soft X-ray spectroscopy

This overview illustrates how spectroscopy with soft X-rays can assist the development of new materials and new designs for solar cells. The starting point is the general layout of a solar cell, which consists of a light absorber sandwiched between an electron donor and an electron acceptor. There are four relevant energy levels that can be measured with a combination of X-ray absorption spectroscopy and photoelectron spectroscopy, as illustrated for an organic dye as absorber attached to a p-doped diamond film as donor. Systematic measurements of organometallic dyes (phthalocyanines and porphyrins) as a function of the metal atom are presented for the metal 2p and N 1s absorption edges. In combination with density functional theory one can discern trends that are useful for tailoring absorber molecules. A customized porphyrin molecule is investigated that combines an absorber with a donor and a linker to an oxide acceptor. The bridge to device fabrication is crossed by correlating spectroscopic features with the photocurrent in hematite photoanodes for water splitting. For speeding up the development of new materials and designs of solar cells a feedback loop between spectroscopy, theory, synthesis and device fabrication is envisioned. © 2012 Elsevier B.V.This work was supported by the NSF with the awards CHE-1026245, DMR-1121288 (MRSEC), and DMR-0537588 (SRC), by the DOE under the contracts DE-SC0006931, DE-AC02-05CH11231 (ALS), and DE-FG02-01ER45917 (end station), by the Spanish Ministerio de Economia y Competitividad (MAT2010-21156-C03-01, C03-03, and PIB2010US-00652), and by the Basque Government (IT-257-07). RER and RJH acknowledge support from the NSF with grants CHE-0613010 and CHE-0911543.Peer Reviewe