Tuning the hole injection barrier in the intermolecular charge-transfer compound DTBDT-F(4)TCNQ at metal interfaces

Molecular monolayers of the charge-transfer salt dithienobenzodithiophene-tetrafluorotetracyanoquinodimethane (DTBDT-F${}_{4}$TCNQ) have been deposited on C(R$15\ifmmode\times\else\texttimes\fi{}3$)/W(110), Co/W(110), and hcp Co(0001) using molecular beam epitaxy in an ultrahigh vacuum. The integrity of the deposited molecules has been confirmed by scanning tunneling microscopy. Scanning tunneling spectroscopy has been used to determine the energetic positions of the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbital of acceptor and donor in the pure and in the mixed phase. The mixed charge transfer phase exhibits a new HOMO close to the Fermi edge depicting a charge transfer of less than one elementary charge. The total amount of the transferred charge depends on the underlying metal substrate. The binding energy of the HOMO decreases from $0.4$ eV for C(R$15\ifmmode\times\else\texttimes\fi{}3$)/W(110) to $0.16$ eV for Co(0001), thus revealing the tunability of the hole injection barrier. The energy gap between HOMO and LUMO decreases from $0.96$ eV ($0.81$ eV) for C(R$15\ifmmode\times\else\texttimes\fi{}3$)/W(110) [ps-ML Co/W(110)] to $0.66$ eV for Co(0001), demonstrating a strong reduction compared to the value of the pure moieties.