Energy-level alignment of a single molecule on ultrathin insulating film

Elucidation of the energy-level alignment mechanism at a molecule/insulator/metal interface is a key to understanding the molecular and interfacial phenomena. Herein, we provide a detailed investigation into the electronic structures of a free-base phthalocyanine on NaCl films of various thicknesses using scanning tunneling microscopy/spectroscopy (STM/STS). The energy of the ionization and the affinity levels of the molecule were deduced from the STS spectra, and we determined their dependence on the NaCl-film thickness, which can be explained based on three effects: a voltage drop within the NaCl films, the degree of electric-field screening around the molecule, and a variation in the work function of the substrates. We further found that the energy levels relative to the vacuum level are independent of the work function of the substrate, and that the size of the energy gap increases with the thickness. Our results suggest that it is possible to predict the energy levels at the interfaces based on the energy levels of the molecules in a gas phase, the work function of the substrate, and the thickness of the insulating films.

Figure 1

(a), (b) STM images of NaCl films on Au(111) surface (a) before and (b) after annealing (sample voltage, $V_{\mathrm{s}}=-2.0\mathrm{V}$; tunneling current, $I_{\mathrm{t}}=4\mathrm{pA})$, and (c) an STM image of ${\mathrm{H}}_2\mathrm{Pc}$/NaCl(2 ML)/Au(111) $\left(V_{\mathrm{s}}=+1.3\mathrm{V},I_{\mathrm{t}}=4\mathrm{pA}\right)$.

Figure 2

(a) dI/dV spectra measured on bare Au(111), ${\mathrm{H}}_2\mathrm{Pc}$/Au(111), and ${\mathrm{H}}_2\mathrm{Pc}$/NaCl(1,2,4 ML)/Au(111). (b), (c) dI/dV maps of (b) ${\mathrm{H}}_2\mathrm{Pc}$/NaCl(1 ML)/Au(111) and (c) ${\mathrm{H}}_2\mathrm{Pc}$/Au(111) imaged at $V_{\mathrm{s}}$ pointed at in (a). The measurement was conducted in constant height mode. The image sizes are $3.5\times 3.5\mathrm{n}{\mathrm{m}}^2$. (d) Molecular structure of ${\mathrm{H}}_2\mathrm{Pc}$ and DFT calculation results of the spatial distributions of the HOMO, LUMO, and LUMO+1 of ${\mathrm{H}}_2\mathrm{Pc}$ in the gas phase.

Figure 3

(a) Plot of the peak top position of the HOMO $\left(V_{\mathrm{i}}\right)$ in the dI/dV spectrum as a function of 1/$d_{\mathrm{vac}}$, where $d_{\mathrm{vac}}$ is the distance between the tip and the NaCl films as defined in (b). (b), (c) Schematic illustration of the energy diagrams at $V_{\mathrm{s}}=0$, and at $V_{\mathrm{s}}=V_{\mathrm{i}}$, respectively. The error bars indicate a two-sided 95% confidence interval.

Figure 4

(a) Energies of the ionization and affinity levels with respect to $E_{\mathrm{F}}$. (b) dZ/dV spectra of 1 ML (blue), 2 ML (green), and 4 ML (red) NaCl films and bare Au(111) (black) as a reference. The orders of the field emission resonances are assigned from the lowest voltage. (c) Energies of the ionization and affinity levels with respect to $E_{\mathrm{v}}$. (d) A schematic illustration of the ELA of a molecule on an insulating film supported by a metal substrate as a function of the thickness of the insulating film. The two extremes, $\mathrm{thickness}=0$ and infinity, correspond to the cases with a molecule adsorbed directly on metal substrate or on the bulk insulator, respectively. The dashed lines represent the ionization potential (blue), and the electron affinity (red) of the molecule in the gas phase. The Fermi levels of the bare metal substrate are shown by a black line.