Fluorescence and Phosphorescence from Individual ${\mathrm{C}}_{60}$ Molecules Excited by Local Electron Tunneling

Using the highly localized current of electrons tunneling through a double barrier scanning tunneling microscope junction, we excite luminescence from a selected ${\mathrm{C}}_{60}$ molecule in the surface layer of fullerene nanocrystals grown on an ultrathin NaCl film on Au(111). In the observed fluorescence and phosphorescence spectra, pure electronic as well as vibronically induced transitions of an individual ${\mathrm{C}}_{60}$ molecule are identified, leading to unambiguous chemical recognition on the single-molecular scale.

Figure 1

(a) STM image of ${\mathrm{C}}_{60}$ nanocrystals formed on a NaCl ultrathin film grown on Au(111) ($V=-3\mathrm{V}$, $I=0.02\mathrm{nA}$). Island $\mathbf{A}$ is a ${\mathrm{C}}_{60}$ monolayer on Au(111); the small blue triangle below $\mathbf{A}$ is part of the bare Au surface. Hexagonal island $\mathbf{B}$ and truncated triangular islands $\mathbf{C}$ and $\mathbf{D}$ consist of up to two, three, and four ${\mathrm{C}}_{60}$ molecular layers, respectively, on NaCl. (b) Submolecular resolution on island $\mathbf{B}$ ($V=-3\mathrm{V}$, $I=0.1\mathrm{nA}$).

Figure 2

STM-induced photon emission spectra acquired with (a) negative ($V=-3\mathrm{V}$, $I=1\mathrm{nA}$) and (b) positive ($V=+3\mathrm{V}$, $I=1\mathrm{nA}$) bias polarity. Spectra acquired on bare Au(111) and on NaCl thin film reveal characteristic emission from a localized surface plasmon. Emission from ${\mathrm{C}}_{60}$ can be excited only at negative voltages. Spectra are vertically shifted for clarity.

Figure 3

Schematic energy diagram of the double barrier tunneling junction at (a) zero-bias voltage and (b) applied negative voltage, corresponding to the conditions for luminescence.

Figure 4

(a) STM-induced light emission spectrum assigned to ${\mathrm{C}}_{60}$ fluorescence ($V=-3\mathrm{V}$, $I=1\mathrm{nA}$) and calculated spectrum. (b) Schematic diagram of the lowest singlet ($S_i$) and triplet ($T_i$) states. Horizontal solid lines: pure electronic levels; horizontal dashed lines: vibrational levels. Solid arrows represent electronic transitions; dashed arrows represent radiationless mechanisms of relaxation (internal conversion, intersystem crossing, vibrational relaxation). (c) STM-induced light emission spectrum assigned to ${\mathrm{C}}_{60}$ phosphorescence ($V=-3\mathrm{V}$, $I=1\mathrm{nA}$) and calculated spectrum. For both simulations, experimentally determined frequencies for the vibronically induced $S_0\leftarrow S_1$ ($T_{1g}$, $T_{2g}$, $G_g$) and $S_0\leftarrow T_1$ (${}^{3}T_{2g}$), $T_2$ (${}^{3}T_{1g}$) transitions are used; see Table  [14, 27]. Each component has a Lorentzian line shape, broadened by $150{\mathrm{cm}}^{-1}$ (a) and $200{\mathrm{cm}}^{-1}$ (c) to obtain the calculated spectra (red curve).