Near-edge x-ray absorption studies of Na-doped tetracyanoethylene films : A model system for the V(TCNE)x room-temperature molecular magnet

• V(TCNE)(x), with TCNE=tetracyanoethylene and x similar to 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 V(TCNE)(x) we have studied pristine TCNE and sodium-intercalated TCNE by near edge x-ray absorption fine structure (NEXAFS) spectroscopy as well as with theoretical calculations. Sodium-intercalated TCNE was used as a model system of the more complex V(TCNE)(x) and both experimental and theoretical results of the model compound have been used to interpret the NEXAFS spectra of V(TCNE)(x). By comparing the experimental and theoretical C K-edge of pristine TCNE, the contributions from the various carbon species (cyano and vinyl) 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 (LUMO). When comparing the C and N K-edge NEXAFS spectra of fully sodium-doped TCNE with V(TCNE)(x), the spectra are similar except for broadening of the features which originates from structural disorder of the V(TCNE)(x) films. The combined results from the model system and V(TCNE)(x) suggest that the lowest unoccupied molecular orbital with density on the nitrogen atoms in V(TCNE)(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 V(TCNE)(x) is TCNE- or vanadiumlike, in contrast to the frontier occupied electronic structure where the highest occupied molecular orbital is a hybridization between V(3d) 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 V(TCNE)(x).

Nyckelord

ELECTRONIC-STRUCTURE; SPINTRONICS; MILLENNIUM

NATURAL SCIENCES

NATURVETENSKAP

TECHNOLOGY

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