Electrostatic screening mediated by interfacial charge transfer in molecular assemblies on semiconductor substrates

Using scanning tunneling microscopy and spectroscopy (STM, STS), we report the electronic structures of self-assembled zinc phthalocyanine (ZnPc) and hexadecafluorinated zinc phthalocyanine $\left({\mathrm{F}}_{16}\mathrm{ZnPc}\right)$ monolayers on the Si(111)-B surface. We show that interfacial charge transfer occurs in the ${\mathrm{F}}_{16}\mathrm{ZnPc}$ monolayer, which gives rise to a pronounced spatial variation of the occupied molecular state across the molecular assembly, a feature not observed in the molecular states of the ZnPc overlayer without the presence of interfacial charge transfer. We attribute this observation to the inhomogeneous electrostatic screening of the intraorbital Coulomb interaction in molecular adsorbates arising from the substrate boron distribution. This study highlights the impact of the substrate electrostatic environment on molecular electronic structures, an essential aspect in the applications of organic and molecular electronic devices.

Figure 1

STM topography images of (a) Si(111)-B $\left(V_s=2\mathrm{V},I_t=5\mathrm{pA}\right)$, (c) ZnPc $\left(V_s=1.8\mathrm{V},I_t=60\mathrm{pA}\right)$, and (d) ${\mathrm{F}}_{16}\mathrm{ZnPc}$ $\left(V_s=2\mathrm{V},I_t=5\mathrm{pA}\right)$ taken at 77 K. Scale bars represent 2 nm. Lattice parameters of the three unit cells are given by (a) $a_1=a_2=0.665\pm 0.005\mathrm{nm},\alpha ={60}^{\mathrm{o}};$ (c) $b_1=1.23\pm 0.01\mathrm{nm},b_2=0.67\pm 0.01\mathrm{nm},\beta =92\pm 1^{\mathrm{o}};$ and (d)$c_1=1.56\pm 0.02\mathrm{nm},c_2=0.58\pm 0.01\mathrm{nm},\gamma =89\pm 1^{\mathrm{o}}$. (b) Schematics of Si(111)-B in the top view (top) and side view (bottom), adapted from Ref. [30]. The free radicals in the Si adatoms are deactivated by boron atoms located on the 3rd atomic layer directly beneath the adatoms.

Figure 2

(a) STS data taken on Si(111)-B (set point: $V_s=-2\mathrm{V},I_t=100\mathrm{pA}$, red), ZnPc molecular overlayer $(V_s=-2\mathrm{V},I_t=100\mathrm{pA}$, magenta) and ${\mathrm{F}}_{16}\mathrm{ZnPc}$ molecular overlayer $(V_s=2\mathrm{V},I_t=50\mathrm{pA}$, blue). Energy-band diagrams illustrated for (b) Si(111)-B/ZnPc and (c) Si(111)-B/${\mathrm{F}}_{16}\mathrm{ZnPc}$. Energy levels are defined by STS peaks unless specified. For simplicity, it is assumed that there is no interface dipole so that the vacuum level is continuous at the ${\mathrm{F}}_{16}\mathrm{ZnPc}$/Si(111)-B heterointerface. The energy range $(\sim 0.3\mathrm{eV})$ of the occupied molecular orbital of the ${\mathrm{F}}_{16}\mathrm{ZnPc}$ overlayer, as provided in (c), is derived based on the analysis of Fig. 3.

Figure 3

(a) Multiple STS spectra (set point: $-2\mathrm{V}$, 100 pA) taken at various locations on the ZnPc overlayer. No peak variation is observed among the curves. The positive DOS feature is a convolution of the LUMO, $\mathrm{LUMO}+1$, and Si surface states. The two well-resolved occupied molecular states are located at $\sim -1.4\mathrm{V}$ and $-1.1\mathrm{V}$. Inset:$dI/dV$ map of ZnPc obtained at $\left(I_t=100\mathrm{pA}\right)$: $-1.5\mathrm{V}$ (left), $-1\mathrm{V}$ (middle), 1.4 V (right). Magenta lines denote individual ZnPc molecules. (b) Four characteristic STS spectra (set point: 2 V, 50 pA) taken at multiple locations on the ${\mathrm{F}}_{16}\mathrm{ZnPc}$ overlayer. The positive DOS feature that remains consistent (centered at $\sim 1\mathrm{V})$ is a convolution of the Si surface states and molecular LUMO. The occupied molecular orbital observed on the negative sample bias, however, is shifted in energy position. Inset:$dI/dV$ map of ${\mathrm{F}}_{16}\mathrm{ZnPc}$ at 1 V, 50 pA. Blue lines denote individual ${\mathrm{F}}_{16}\mathrm{ZnPc}$ molecules. Scale bars represent 1 nm.

Figure 4

STM topography images of Si(111)-B obtained at 77 K at (a) $V_s=-0.1\mathrm{V},I_t=100\mathrm{pA}$ and (b) $V_s=1.3\mathrm{V},I_t=300\mathrm{pA}$. Subsurface boron dopant (beyond the 3rd atomic layer) and surface dangling bond defect are indicated by the red and green arrows, respectively. The blue arrow points to a region with a low concentration of subsurface boron. (c) Averaged STS spectra taken at $V_s=-0.5\mathrm{V},I_t=300\mathrm{pA}$ on the bright and dark regions indicated by the red and blue arrows in (a), respectively. Insets (i) and (ii) illustrate the tip-induced band bending modulated by the Coulomb potential of thermally ionized boron dopants under the filled-state and empty-state tunneling conditions, respectively, where the dotted line in the schematics refers to the band structure on the bright areas (hillocks) with accumulated subsurface boron and the solid line refers to the dark areas where the boron accumulation is minimal. (d) STS taken at $V_s=-2\mathrm{V},I_t=100\mathrm{pA}$ on bright (red) and dark (blue) areas as indicated by colored arrows in (a). The curves are vertically offset for clarity, and there is no noticeable modulation on the apparent band gap or density-of-states features.

Figure 5

STM topography images of ${\mathrm{F}}_{16}\mathrm{ZnPc}$ obtained at 77 K at (a) $V_s=-2.5\mathrm{V},I_t=1\mathrm{pA}$ and (b) $V_s=2\mathrm{V},I_t=5\mathrm{pA}$. Significant contrast variation is observed in the filled-state image across the molecular structure, which is likely correlated to the subsurface boron inhomogeneity observed in Fig. 4. Uniform features are observed in the empty-state image, although small-scale contrast variation, resulting from the epitaxial registration between the molecular assembly and the underlying Si lattice, can be identified from molecule to molecule. Blue bars represent individual ${\mathrm{F}}_{16}\mathrm{ZnPc}$ molecules.