Gating the Charge State of Single Molecules by Local Electric Fields

The electron-acceptor molecule TCNQ is found in either of two distinct integer charge states when embedded into a monolayer of a charge transfer complex on a gold surface. Scanning tunneling spectroscopy measurements identify these states through the presence or absence of a zero-bias Kondo resonance. Increasing the (tip-induced) electric field allows us to reversibly induce the oxidation or reduction of TCNQ species from their anionic or neutral ground state, respectively. We show that the different ground states arise from slight variations in the underlying surface potential, pictured here as the gate of a three-terminal device.

Figure 1

Figure 1: (a) STM image [36] of self-assembled TMTTF-TCNQ domains on Au(111) (the molecular structure of both species are shown in the image) ($V_s=1\mathrm{V}$; $I_t=0.4\mathrm{nA}$). The topographic height difference is $\Delta z\sim 20\mathrm{pm}$. (b) Intramolecular structure of TMTTF and TCNQ molecules resembles the shape of the free molecule HOMO and LUMO, respectively, ($V_s=90\mathrm{mV}$; $I_t=0.17\mathrm{nA}$). Inset: TCNQ molecules switching under the influence of the scanning tip between Type I and Type II ($V_s=90\mathrm{mV}$; $I_t=0.17\mathrm{nA}$). (c,d) Spectra acquired over the center of “darker” (Type I) and “brighter” (Type II) TCNQ molecules, respectively, ($V_s=150\mathrm{mV}$; $I_t=0.5\mathrm{nA}$, The lock-in modulation is 2 mV rms at 723 Hz). The peak features in the former correspond to a Kondo resonance, at $V_s=0\mathrm{mV}$, with two sidebands at $V_s=\sim \pm 40\mathrm{mV}$ due to the excitation of a molecular vibration, as in Ref. [16]. The flat spectrum in (d) suggests a neutral ground state.

Figure 2

(a) Current and $dI/dV$ spectrum acquired over the center of Type I molecules. The spectrum shows the onset of the hybrid metal-organic interface state (IS), and a sharp peak ($P$) due to the change of the charge state of the TCNQ molecule, as indicated. (b) Potential energy model depicting schematically the field-induced discharging of a ${\mathrm{TCNQ}}^{-}$ anion at positive sample bias. (c) Spectral maps of $dI/dV$ vs $V_s$ and lateral distance, showing the shift of peak $P$ as a the tip approaches the center of a molecule. (d) Constant current $dI/dV$ maps at different bias voltages evidence an elliptical equipotential contour tracing the onset for discharging ($I_t=0.7\mathrm{nA}$).

Figure 3

(a) Current and $dI/dV$ spectrum acquired over the center of Type II molecules. The interface state (IS) lies below the Fermi level, the LUMO is found above $E_F$. A sharp dip ($D$) results from the blocking of the tunneling current upon charging. (b) Spectral $dI/dV$ ($V_s$, distance) map showing the shift of dip D with varying tip-molecule distance. (c) Model of the voltage drop in the DBTJ explaining the charging of the TCNQ molecules.

Figure 4

(a) $dI/dV$ vs distance map along eight TCNQ molecules in an extended molecular row. Dashed lines guide the gradual shift of the charging dip, discharging peak, interface state and LUMO with the molecular site. (top) STM image with numbers labeling the different sites within the molecular chain ($V_s=1.5\mathrm{V}$, $I_t=0.23\mathrm{nA}$). (b) $dI/dV$ spectra acquired over the center of the eight molecules. (c) Scheme of the variation of features in (b) and (c) with the lateral molecular site within the row, resembling charging or discharging diamonds as a function of sample bias and a gate voltage (here the molecular site). The asymmetry of the molecular junction in the STM configuration is reflected in the asymmetry of the Coulomb diamond.