Probing Molecular Excited States by Atomic Force Microscopy

By employing single charge injections with an atomic force microscope, we investigated redox reactions of a molecule on a multilayer insulating film. First, we charged the molecule positively by attaching a single hole. Then we neutralized it by attaching an electron and observed three channels for the neutralization. We rationalize that the three channels correspond to transitions to the neutral ground state, to the lowest energy triplet excited states and to the lowest energy singlet excited states. By single-electron tunneling spectroscopy we measured the energy differences between the transitions obtaining triplet and singlet excited state energies. The experimental values are compared with density functional theory calculations of the excited state energies. Our results show that molecules in excited states can be prepared and that energies of optical gaps can be quantified by controlled single-charge injections. Our work demonstrates the access to, and provides insight into, ubiquitous electron-attachment processes related to excited-state transitions important in electron transfer and molecular optoelectronics phenomena on surfaces.

Figure 1

(a) Sketch of the experimental arrangement. The molecule (NPc) is adsorbed on top of a multilayer insulating film. Electron transfer is only possible between the tip and NPc, not between NPc and the metal substrate, Cu(111). (b) Top view of the atomic model of NPc on top of NaCl. Blue, black, and gray solid circles represent N, C, and H elements, respectively. Purple and green solid circles represent the ions in the film, Na and Cl, respectively. (c) Constant-$\mathrm{\Delta }f$ AFM image ($\mathrm{\Delta }f=-1.7\mathrm{Hz}$ and $V=1\mathrm{V}$) of NPc on multilayer NaCl. The scale bar is 10 Å. Black to white color scale corresponds to 3 Å tip-height range. The black circle represents the position where the tunneling spectroscopy experiments were performed.

Figure 2

(a) Fraction $r$ (time in the initial positive charge state divided by the probe time) as a function of $V_{\text{probe}}$ for different $z_{\text{probe}}$ for NPc on a 14 ML NaCl. Each data point is extracted from $80\mathrm{\Delta }f(t)$ probe traces. The error bars correspond to the standard error of the mean $r$ for each individual set of measurements of $r$. The data for $z_{\text{probe}}=z_0$ is reproduced from [27]. (b) Same as (a), with a different tip at larger $V_{\text{probe}}$. Each data point is extracted from $40\mathrm{\Delta }f(t)$ probe traces.

Figure 3

(a) Analysis of the ${\mathrm{NPc}}^{+}\to {\mathrm{NPc}}^0$ transition as a function of $V_{\text{probe}}$ at $z_{\text{probe}}=z_0+0.7\AA$. Black circles represent the extracted tunneling rate $\mathrm{\Gamma }$ (the corresponding value of tunneling current $I$ is displayed on the right axis) along with the fitted sum of two error functions (black line) and its derivative (red line). The peak positions of ${\mathrm{red}}_1^{+}$ and ${\mathrm{red}}_2^{+}$ (dashed vertical lines) are indicated in the graph. The areas corresponding to each fitted error function are colored orange for the ${\mathrm{red}}_1^{+}$ and purple for the ${\mathrm{red}}_2^{+}$ transition, respectively. (b) Same analysis as in (a) but for larger $V_{\text{probe}}$ and a different tip. The peak position of ${\mathrm{red}}_3^{+}$ (dashed vertical line) is indicated in the graph. The area corresponding to the fitted error function is colored green.

Figure 4

Diagrams of the single-electron transfer from the tip to the cation, resulting in ${\mathrm{NPc}}^0$ in the (a) ground state $S_0$, (b) lowest energy triplet excited states $T_{1,2}$, and (c) lowest energy singlet excited states $S_{1,2}$, respectively. Labels $\mathrm{LUMO}+1$, LUMO and HOMO correspond to the second lowest unoccupied, lowest unoccupied and highest occupied molecular orbital, respectively, of the neutral molecule. Because of their small energy separation, LUMO and $\mathrm{LUMO}+1$ are represented as one energy level in panels (b)–(d). The relaxation energies related to the respective charge transitions are labeled ${\mathrm{\Delta }}_1$, ${\mathrm{\Delta }}_2$, and ${\mathrm{\Delta }}_3$. (d) Many-body electron picture associated with the measured charge-state transitions (orange, purple, and green). The free energy, with respect to the Fermi level of the tip, is shown for $V=0\mathrm{V}$. An increase (decrease) of $V$ shifts the ${\mathrm{NPc}}^{+}$ level upwards (downwards) with respect to the ${\mathrm{NPc}}^0$ levels. The controlled, initial cation $D_0^{\text{cation}}$ formation is highlighted with a curved black arrow. The electronic structure of each state is shown in the gray insets. The relaxation energy related to the electron detachment from NPc is labeled ${\mathrm{\Delta }}_0$.