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

Abstract

$\mathrm{V}{(\mathrm{TCNE})}_{x}$, with $\mathrm{TCNE}=\text{tetracyanoethylene}$ and $x\ensuremath{\sim}2$, 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 $\mathrm{V}{(\mathrm{TCNE})}_{x}$ 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 $\mathrm{V}{(\mathrm{TCNE})}_{x}$ and both experimental and theoretical results of the model compound have been used to interpret the NEXAFS spectra of $\mathrm{V}{(\mathrm{TCNE})}_{x}$. 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 $\mathrm{V}{(\mathrm{TCNE})}_{x}$, the spectra are similar except for broadening of the features which originates from structural disorder of the $\mathrm{V}{(\mathrm{TCNE})}_{x}$ films. The combined results from the model system and $\mathrm{V}{(\mathrm{TCNE})}_{x}$ suggest that the lowest unoccupied molecular orbital with density on the nitrogen atoms in $\mathrm{V}{(\mathrm{TCNE})}_{x}$ 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 $\mathrm{V}{(\mathrm{TCNE})}_{x}$ is ${\mathrm{TCNE}}^{\ensuremath{-}}$ or vanadiumlike, in contrast to the frontier occupied electronic structure where the highest occupied molecular orbital is a hybridization between $\mathrm{V}(3d)$ 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 $\mathrm{V}{(\mathrm{TCNE})}_{x}$.