Formation of Dispersive Hybrid Bands at an Organic-Metal Interface

An electronic band with quasi-one-dimensional dispersion is found at the interface between a monolayer of a charge-transfer complex (TTF-TCNQ) and a Au(111) surface. Combined local spectroscopy and numerical calculations show that the band results from a complex mixing of metal and molecular states. The molecular layer folds the underlying metal states and mixes with them selectively, through the TTF component, giving rise to anisotropic hybrid bands. Our results suggest that, by tuning the components of such molecular layers, the dimensionality and dispersion of organic-metal interface states can be engineered.

Figure 1

(a) STM image of TTF-TCNQ domains on Au(111). Excess TCNQ molecules appear segregated in pure islands ($V_s=1.7\mathrm{V}$; $I_t=0.1\mathrm{nA}$). (b) Intramolecular structure of TTF and TCNQ resembling the shape of the HOMO and LUMO, respectively ($V_s=0.3\mathrm{V}$; $I_t=0.4\mathrm{nA}$). Data analyzed using WSxM [22]. (c) Simulated constant current STM image ($V_s=1.0\mathrm{V}$) using the Tersoff-Hamann approach [23] on the DFT minimized structure shown in (d). TTF and TCNQ stand for ${\mathrm{C}}_6{\mathrm{H}}_4{\mathrm{S}}_4$ and ${\mathrm{C}}_{12}{\mathrm{H}}_4{\mathrm{N}}_4$, respectively. The vectors a1 and a2 define the surface unit cell (inset shows the corresponding surface Brillouin zone [20]).

Figure 2

Comparison of differential conductance spectra of (a) TCNQ pure islands [spectrum taken on a nearby clean Au(111) region shown for comparison] and (b) TTF and TCNQ in mixed TTF-TCNQ islands. (d),(e) The differential conductance spectra is mapped along the dashed lines shown in the STM image (c) ($V_s=1.9\mathrm{V}$; $I_t=0.07\mathrm{nA}$). The maps clearly show the spatial localization of IS1 and IS2 features on TCNQ and TTF rows, respectively.

Figure 3

(a)–(c) $dI/dV$ maps of a TTF-TCNQ island, shown in (d), measured at the indicated sample bias values, lying above the onset of the IS1 resonance ($I_t=1.0\mathrm{nA}$; $16\times 16{\mathrm{nm}}^2$). (e) Dispersion relation $E(k)$ obtained from data sets similar to (a)–(c). The dashed line is a parabolic fit to the data points, including the IS1 peak position.

Figure 4

(a) DOS of the $\mathrm{TTF}\mathrm{\text{−}}\mathrm{TCNQ}/\mathrm{Au}(111)$ system projected on TTF and TCNQ orbitals around $E_F$, compared with the PDOS on free molecular chains of each component (leftmost panel, TTF; rightmost panel, TCNQ). A Gaussian broadening of 250 mV width is employed. (b) Wave vector resolved PDOS on TTF orbitals along $\overline{\Gamma XM}$. The PDOS map adds the contribution of six molecular states around $E_F$, since some higher lying resonances also show some weight in this energy region.

Figure 5

Calculated electronic structure of the $\mathrm{TTF}\mathrm{\text{−}}\mathrm{TCNQ}/\mathrm{Au}(111)$ system. (a) Interface band structure along $\overline{\Gamma XM}$ directions [20]. Green (●) and red (▪) dots mark states with charge at the interface. Dashed lines along $\overline{XM}$ show their parabolic fit ($m^{*}=0.44m_e$ and $0.54m_e$, respectively). Mixing with folded bulk states prevents to continue the green band from $\overline{X}$ to $\overline{\Gamma }$. The blue bands (▴) correspond to the bare Au(111) SS, for comparison. The state is folded due to the use of a unit cell with the periodicity of the molecular layer [see Fig. 1d]. (b) Constant charge density isosurfaces for two characteristic states located at the $\overline{X}$ point of the SBZ.