Many-Body Description of STM-Induced Fluorescence of Charged Molecules

A scanning tunneling microscope is used to study the fluorescence of a model charged molecule (quinacridone) adsorbed on a sodium chloride (NaCl)-covered metallic sample. Fluorescence from the neutral and positively charged species is reported and imaged using hyperresolved fluorescence microscopy. A many-body model is established based on a detailed analysis of voltage, current, and spatial dependences of the fluorescence and electron transport features. This model reveals that quinacridone adopts a palette of charge states, transient or not, depending on the voltage used and the nature of the underlying substrate. This model has a universal character and clarifies the transport and fluorescence mechanisms of molecules adsorbed on thin insulators.

Figure 1

(a) Sketch of the STML experiment on QA where neutral and charged fluorescence are represented. (b) STM image ($V=2.8\mathrm{V}$, $I=5\mathrm{pA}$) of QA molecules adsorbed on a 4 ML-NaCl island on Ag(111). (c) $dI/dV$ spectrum recorded at the center of a QA molecule on 4 ML-NaCl. Inset: STM image of QA on 4 ML-NaCl/Ag(111) (left part: $V=0.4\mathrm{V}$ and $I=500\mathrm{pA}$, and right part: $V=2.6\mathrm{V}$ and $I=5\mathrm{pA}$). (d) DFT calculations (details in Supplemental Material [20]) of the frontier orbitals of the neutral molecule and corresponding maps of their partial density of states at constant height shown alongside with experimental $dI/dV$ maps acquired at voltages corresponding to $dI/dV$ resonances in (c).

Figure 2

(a) STML spectra acquired ($V=2.7\mathrm{V}$, $I=100\mathrm{pA}$, acquisition time $t=300\mathrm{s}$) at the positions identified in the inset by crosses and reference spectrum acquired on Ag(111) ($V=2.7\mathrm{V}$, $I=200\mathrm{pA}$, $t=30\mathrm{s}$). (b) HRFMs of QA on 4 ML-NaCl/Ag(111) for the neutral ($X^0$) and charged emission ($X^{+}$) ($3.5\mathrm{nm}\times 2.4\mathrm{nm}$, $V=2.6\mathrm{V}$, $t=30\mathrm{s}/\mathrm{pixel}$). (c) Transition electron density of $S_0\to S_1$ and $D_0^{+}\to D_1^{+}$ calculated using TDDFT. (d) Simulations of the HRFMs (see text for details).

Figure 3

(a) Voltage dependences of the neutral ($X^0$, blue squares) and charged ($X^{+}$, red circles) emission intensities measured at a constant current ($I=50\mathrm{pA}$) and $dI/dV$ spectrum of a QA/NaCl(4 ML)/Ag(111) (bottom panel). Current dependences of the neutral ($X^0$, blue squares) and charged ($X^{+}$, red dots) emission intensities measured at (b) $V=2.6\mathrm{V}$ and (c) $V=3.2\mathrm{V}$. (d) Sketches of the luminescence mechanisms for the neutral and charged QA emission.

Figure 4

(a) STM image ($V=2.9\mathrm{V}$, $I=5\mathrm{pA}$) of QA adsorbed on a 3 ML-NaCl/Au(111). (b) Voltage dependence of the $X^{+}$ emission (top) and $dI/dV$ spectrum (bottom) recorded for QA/NaCl(3 ML)/Au(111). (c) DFT simulations of the partial density of states derived from the LUMO, HOMO, and HOMO-1 of the neutral QA evaluated at a constant height above the molecule (left), and constant height $dI/dV$ maps (right) acquired at voltages corresponding to the resonances in the $dI/dV$ in (b). (d) STML spectra acquired ($V=2.9\mathrm{V}$, $I=50\mathrm{pA}$, $t=180\mathrm{s}$) at the short and long axis of the molecule. Inset: HRFM of $X^{+}$ for QA on 3 ML-NaCl/Au(111) ($3.5\mathrm{nm}\times 2.4\mathrm{nm}$, $V=2.9\mathrm{V}$, $t=30\mathrm{s}/\mathrm{pixel}$.) (e) Sketch of the luminescence mechanisms for QA on 3 ML-NaCl/Au(111).