Electronic structure of organic films in the first excited states determined using scanning tunneling spectroscopy: An experimental and theoretical study

We report a pilot study of whether laser-assisted scanning tunneling spectroscopy (STS) can be used to investigate the changes in electronic structure between the ground state and first singlet $\left(S_1\right)$ excited states of thin films of organic molecules. STS measurements of $4^{″},3^{″}$-dimethylsexithiophene and pentacene, excited into their $S_1$ states using suitable continuous wave laser excitation, show a narrowing of the electronic gap. These results are consistent with theoretical calculations of the electronic structure of these molecules in their $S_1$ states.

Figure 1

Molecular structures of pentacene and T6Me2.

Figure 2

Schematic representation of the electronic configuration for the states with energies $E_N$, $E_{N-1}$, $E_{N+1}$, $E_N^{*}$, $E_{N-1}^{*}$, and $E_{N+1}^{*}$ (see text for details).

Figure 3

UV-vis absorption spectra of pentacene and T6Me2 films on quartz glass. The arrows indicate the wavelengths of the two lasers used for the light-excited STS measurements.

Figure 4

STM images of films of (a) pentacene evaporated in situ ($4.5\mathrm{V}$, $0.05\mathrm{nA}$), (b) T6Me2 ($3\mathrm{V}$, $0.1\mathrm{nA}$), and (c) pentacene evaporated ex situ ($4.5\mathrm{V}$, $0.05\mathrm{nA}$).

Figure 5

(a) Area-averaged current-voltage characteristics of the T6Me2 film in dark conditions and under laser illumination. $\lambda =450\mathrm{nm}$. Set point $-3\mathrm{V}$, $0.5\mathrm{nA}$; (b) area-averaged normalized conductance spectra for the T6Me2 film in dark conditions, and under laser illumination, derived from the data of Fig. 5a.

Figure 6

(a) Area-averaged current-voltage characteristics of the pentacene film produced ex situ in dark conditions and with under laser illumination. $\lambda =663\mathrm{nm}$. Set point $-1.5\mathrm{V}$, $0.02\mathrm{nA}$; (b) area-averaged normalized conductance spectra for the pentacene film produced ex situ in dark conditions and under laser illumination, derived from the data of (a).

Figure 7

(a) Area-averaged current-voltage characteristics of the pentacene film evaporated in situ, in dark conditions, and under laser illumination at different powers. $\lambda =663\mathrm{nm}$. Set point $-1.5\mathrm{V}$, $0.02\mathrm{nA}$; (b) area-averaged normalized conductance spectra for the pentacene film evaporated in situ, in dark conditions, and under laser illumination, derived from the data of (a).

Figure 8

Normalized conductance curves of T6Me2 obtained by subtracting the data under laser irradiation from the data under dark condition. The solid line corresponds to a wavelength $\left(663\mathrm{nm}\right)$ where no absorption phenomena occur, while the dotted one corresponds to a wavelength $\left(454\mathrm{nm}\right)$ where the T6Me2 sample absorbs.