Emergence and Visualization of an Interface State during Contact Formation with a Single Molecule

Contact formation dynamics and electronic perturbations arising from the interaction of a metallic probe and a single molecule (1,3 cyclohexadiene) bound on the Si (100) surface are examined using a series of plane wave, density functional theory calculations. The approach of the probe induces a relaxation of the molecule that ultimately leads to the formation of an interface state due to a specific interaction between the probe apex atom and the $\mathrm{C}\mathrm{C}$ bond of the molecule. The calculated interface state is located 0.2 eV above the Fermi energy, in agreement with low temperature scanning tunneling spectroscopy local density of states data (0.35 eV), and is responsible for the contrast observed in low bias empty-state STM images.

Figure 1

(a)–(e) are empty-state STM images of the five different 1,3-CHD attachment geometries $V_{\mathrm{sample}}=1.3\mathrm{V}$. (f) is a close-up of product (c) in which STS spectra (g) are recorded at positions 1–3 at two different heights. Peaks and inflection features (shaded area) in $\frac{d^{2}I}{d{V}^2}$ correspond to vibrational modes and LDOS peaks, respectively. Vibrational mode assignment in (g) is based on published IR and HREELS data [31, 32, 33]. The spectra in (g) demonstrate the localization of the new interface state to the probe position that corresponds to the center of the remaining $\mathrm{C}\mathrm{C}$ bond [position 2 in (f)].

Figure 2

(a) Interaction potential (left $y$ axis) and Mulliken analysis (right $y$ axis) as a function of the probe to $\mathrm{C}\mathrm{C}$ distance, (b) $\mathrm{C}\mathrm{C}$ bond length as a function of the probe to $\mathrm{C}\mathrm{C}$ distance, and (right $y$ axis) $\mathrm{C}\mathrm{C}$ bond height relative to probe position.

Figure 3

(a) Schematic of probe-molecule-surface system used in DFT geometry optimization calculations. (b) Tracking the LDOS for a slice positioned 1 Å above the $\mathrm{C}\mathrm{C}$ bond as a function of probe position. Charge density from the 3–4 eV energy window is pushed to lower energy as the probe approaches the molecule.

Figure 4

(a) PDOS for a C $2p$ state centered on one of the atoms of the $\mathrm{C}\mathrm{C}$ bond as a function of probe-molecule separation. Localization of the $\pi$ and ${\pi }^{*}$ states is seen to decrease and is accompanied by the growth of a state with C $2p$ character 0.2 eV above $E_f$. (b) A conceptual molecular orbital diagram appropriate for ${\eta }^2$-alkene complexes. Competition between the $\Psi$ states can account for the Mulliken analysis of the Pt-C bond of Fig. 2a.