Molecules on Insulating Films: Scanning-Tunneling Microscopy Imaging of Individual Molecular Orbitals

Ultrathin insulating NaCl films have been employed to decouple individual pentacene molecules electronically from the metallic substrate. This allows the inherent electronic structure of the free molecule to be preserved and studied by means of low-temperature scanning-tunneling microscopy. Thereby direct images of the unperturbed molecular orbitals of the individual pentacene molecules are obtained. Elastic scattering quantum chemistry calculations substantiate the experimental findings.

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Figure 1

Pentacene molecules on one and two layers of NaCl on Cu(111). Numbers indicate the layer thickness. The long axis of a pentacene molecule is aligned parallel to one of the polar $⟨011⟩$ directions of the NaCl(100) films, as deduced from the orientation of the nonpolar NaCl step edge. The center of a pentacene molecule is located on top of a Cl ion of the NaCl film, which was determined indirectly with the help of coadsorbed Au adatoms (inset).

Figure 2

$dI/dV$ spectroscopy at the center of a pentacene molecule on NaCl. The $dI/dV$ signal shows two distinct peaks that can be attributed to the NIR and PIR states, and a broad gap of low conductance in between the peaks [11, 12].

Figure 3

From top to bottom, the images are STM images acquired with a metal and a pentacene tip, and contours of constant orbital probability distribution ($|\Psi {|}^2=5\times {10}^{-4}{\AA }^{-3}$) of the free molecule. Whereas the STM images for bias voltages in the HOMO-LUMO band gap are relatively featureless (center), the images at bias voltages exceeding the HOMO (left) or LUMO (right) exhibit very pronounced features, resembling the electron density of the HOMO (left) and LUMO (right) of the free molecule [32]. The geometry of the free pentacene molecule is displayed in the lower center image.

Figure 4

Calculated images of the pentacene molecule adsorbed on $\mathrm{N}\mathrm{a}\mathrm{C}\mathrm{l}/\mathrm{C}\mathrm{u}(111)$ using ESQC. The left (right) images are calculated at the energy corresponding to the HOMO (LUMO) of the adsorbed molecule. The images for a current of 1 (upper panel) and 100 pA (lower panel) correspond nicely to the experimental images acquired with the metal and pentacene tip, respectively. In the center the geometry of the adsorbed molecule is shown.