Electron escape depth, surface composition, and charge transfer in tetrathiafulvalene tetracyanoquinodimethane (TTF-TCNQ) and related compounds: Photoemission studies

Ultraviolet photoemission spectroscopy (UPS) has been used to study in-situ-prepared tetracyanoquinodimethane (TCNQ), ${\mathrm{Cs}}_2$ ${\mathrm{}\left(\mathrm{TCNQ}\right)\mathrm{}}_3$, and tetrathiafulvalene-TCNQ (TTF-TCNQ) thin films. The measurments on ${\mathrm{Cs}}_2$ ${\mathrm{}\left(\mathrm{TCNQ}\right)\mathrm{}}_3$ were made during the charge-transfer reaction between Cs vapor and TCNQ solid, which allows an unambiguous determination of the energy levels near $E_F$ of the TCN${\mathrm{Q}}^{-}$ anion. Our results demonstrate that the excess charge of TCN${\mathrm{Q}}^{-}$ is localized to a molecular scale and that the TCNQ molecule in solids will normally exist in either the neutral or -1 charge state, and not in a state of shared fractional charge. Quantitative comparison of the UPS spectra between ${\mathrm{Cs}}_2$ ${\mathrm{}\left(\mathrm{TCNQ}\right)\mathrm{}}_3$ and TTF-TCNQ permits a rather complete assignment of the structure in the TTF-TCNQ data. The structures near -1 and -2 eV are assigned to TCN${\mathrm{Q}}^{-}$, those near -3 eV to TT${\mathrm{F}}^{+}$ and those near -4 eV to both TCN${\mathrm{Q}}^0$ and TCN${\mathrm{Q}}^{-}$. In addition, the data suggest the presence of TT${\mathrm{F}}^{+}$ states immediately below $E_F$. From studies of ultrathin films, the electron escape depth is estimated to be less than 10Å in these solids, which indicates a strong representation of the surface electronic structure in our data. TTF-TCNQ deposited at 77°K is found to undergo irreversible changes in both its UPS spectra and its bulk thermal properties upon annealing to room temperature. A model is presented which accounts for these changes in terms of molecular rearrangements upon annealing which are accompanied by altered surface composition of the films and increased ionization of the TTF-TCNQ complex. In room-temperature TTF-TCNQ there is evidence for significant charge transfer, for the presence of some TCN${\mathrm{Q}}^0$ at the surface, and evidence against any significant surface concentration of TT${\mathrm{F}}^0$. In view of this work we suggest (i) that prior uv-photoemission work may have sampled principally the surface and (ii) that the surface of room-temperature TTF-TCNQ films may have a different composition from the bulk.