Thin films of molecule-based charge transfer complex cobalt tetracyanoethylene: In situ X-ray photoemission study

Thin films of molecule-based charge transfer magnet, cobalt tetracyanoethylene Co(TCNE)_x, x ~ 2] consisting of the transition metal Co, and an organic molecule viz. tetracyanoethylene (TCNE) have been deposited by using physical vapor deposition method under ultra-high vacuum conditions at room temperature. X-ray photoelectron spectroscopy (XPS) technique has been used extensively to investigate the electronic properties of the Co(TCNE)_x thin films. The XPS measurements show that the prepared Co(TCNE)_x films are clean, and oxygen free. The stoichiometries of the films, based on atomic sensitive factors, are obtained, and yields a ~ 1:2 ratio between metal Co and TCNE for all films. Interestingly, the positive shift of binding energy position for Co(2p), and negative shifts for C(1s) and N(1s) peaks suggest a charge-transfer from Co to TCNE, and cobalt is assigned to its Co(II) valence state. In the valence band investigation, the highest occupied molecular orbital (HOMO) of Co(TCNE)_x is found to be at ~ 2.4 eV with respect to the Fermi level, and it is derived either from the TCNE^? singly occupied molecular orbital (SOMO) or Co(3d) states. The peaks located at ~ 6.8 eV and ~ 8.8 eV are due to TCNE derived electronic states. The obtained core level and valence band results of Co(TCNE)_x, films are compared with those of V(TCNE)_x thin film magnet: a well known system of M(TCNE)_x type of organic magnet, and important points regarding their electronic properties have been brought out.