Organometallic MTCNQ films: a comparative study of CuTCNQ versus AgTCNQ

We performed a systematic study of electron-acceptor molecules in two closely related organometallic solids, namely, CuTCNQ and AgTCNQ. These studies were performed using both an experimental approach, via the use of electron spectroscopies (XPS and UPS), and a theoretical approach, via the use of ab initio DFT calculations. From these results, a complete description of the electronic structure of these molecular solid-films could be given, identifying the characteristic electronic and structural features of each part of the molecules and their contribution to the final electronic structure. Empty states were found close to the Fermi level in both solids. The presence of an electronic band close to the Fermi level is related to the magnetic behavior predicted for both MTCNQ solids for their isolated monolayers. However, the lower work function of the MTCNQ with respect to the metal substrate one implies that the MTCNQ film accepts electron from the metal substrate, thus fulfilling its Fermi level band. This occupied band explains the absence of shake-up features in the core level spectra in opposition to the TCNQ. The UPS experiments indicated that the MTCNQ film was doped by a small excess of metal from the substrate, shifting the electron Fermi level close to the MTCNQ conduction band. Thus, the MTCNQ film becomes an n-type semiconductor, opening up a very interesting field in the technological applications of this system.