Linköping University Postprint

Near-edge x-ray absorption studies of Na-doped tetracyanoethylene films: A model system for the V(TCNE) x room-temperature molecular magnet, 2008, Physical Review B, (77), 054420. V͑TCNE͒ x , with TCNE= tetracyanoethylene and x ϳ 2, is an organic-based molecular magnet with potential to be used in spintronic devices. With the aim of shedding light on the unoccupied frontier electronic structure of V͑TCNE͒ x we have studied pristine TCNE and sodium-intercalated TCNE by near edge x-ray absorption fine structure ͑NEXAFS͒ spectroscopy as well as with theoretical calculations. Sodium-intercalated TCNE was used as a model system of the more complex V͑TCNE͒ x and both experimental and theoretical results of the model compound have been used to interpret the NEXAFS spectra of V͑TCNE͒ x. By comparing the experimental and theoretical C K-edge of pristine TCNE, the contributions from the various carbon species ͑cyano and vinyl͒ could be disentangled. Upon fully sodium intercalation, TCNE is n doped with one electron per molecule and the features in the C and N K-edge spectra of pristine TCNE undergo strong modification caused by partially filling the TCNE lowest unoccupied molecular orbital ͑LUMO͒. When comparing the C and N K-edge NEXAFS spectra of fully sodium-doped TCNE with V͑TCNE͒ x , the spectra are similar except for broadening of the features which originates from structural disorder of the V͑TCNE͒ x films. The combined results from the model system and V͑TCNE͒ x suggest that the lowest unoccupied molecular orbital with density on the nitrogen atoms in V͑TCNE͒ x has no significant hybridization with vanadium and is similar to the so-called singly occupied molecular orbital of the TCNE anion. This suggests that the LUMO of V͑TCNE͒ x is TCNE − or vanadiumlike, in contrast to the frontier occupied electronic structure where the highest occupied molecular orbital is a hybridization between V͑3d͒ and cyano carbons. The completely different nature of the unoccupied and occupied frontier electronic structure of the material will most likely affect both charge injection and transport properties of a spintronic device featuring V͑TCNE͒ x .