Electronic structure of thin film cobalt tetracyanoethylene, Co(TCNE)x

Abstract

V(TCNE)x, TCNE=tetracyanoethylene, x∼2, is a semiconducting organic-based magnet and one of very few organic-based magnets with critical temperature above room temperature (RT). With the aim to understand the key design criteria for achieving RT organic-based magnets we have started to study the electronic and chemical structure of members of the M(TCNE)x family with significantly lower critical temperatures than V(TCNE)x. In this paper, Co(TCNE)x, x∼2, (TC∼44K, derived from its powder form) were prepared by a method based on physical vapor deposition, resulting in oxygen-free thin films. By using a variety of photoemission and X-ray absorption techniques the highest occupied molecular orbital (HOMO) of Co(TCNE)x was determined to mainly be TCNE−-derived while the states originating from Co(3d) are localized at higher binding energies. This is in stark contrast to V(TCNE)x where V(3d) is mainly responsible for the HOMO, but in line with the results of Fe(TCNE)x (TC∼121K, derived from its powder form) for which the HOMO also is TCNE−-derived. Moreover, the results propose Co(TCNE)x to contain large amounts of local bonding disorder in contrast to V(TCNE)x which can be grown virtually defect free. We speculate that cobalt binds to vinyl – instead of cyano groups, hence creating a disordered bonding arrangement which deviates from octahedral. The very weak (<1eV) crystal field splitting and a large spread in the local ligand field strengths due to disorder in Co(TCNE)x are in agreement with the absence of fine structure in the Co L-edge spectra.