Origin of High-Resolution IETS-STM Images of Organic Molecules with Functionalized Tips

Recently, the family of high-resolution scanning probe imaging techniques using decorated tips has been complemented by a method based on inelastic electron tunneling spectroscopy (IETS). The new technique resolves the inner structure of organic molecules by mapping the vibrational energy of a single carbon monoxide (CO) molecule positioned at the apex of a scanning tunneling microscope (STM) tip. Here, we explain high-resolution IETS imaging by extending a model developed earlier for STM and atomic force microscopy (AFM) imaging with decorated tips. In particular, we show that the tip decorated with CO acts as a nanoscale sensor that changes the energy of its frustrated translation mode in response to changes of the local curvature of the surface potential. In addition, we show that high resolution AFM, STM, and IETS-STM images can deliver information about the charge distribution within molecules deposited on a surface. To demonstrate this, we extend our mechanical model by taking into account electrostatic forces acting on the decorated tip in the surface Hartree potential.

Figure 1

1D IETS-STM model. (a) Individual contributions to the potential energy of the probe particle, plotted as a function of its lateral position ${\mathrm{x}}_{\text{PROBE}}$: The full line shows the harmonic potential of the tip (${\mathrm{V}}_{\text{SPRING}}$) responsible for the lateral confinement of the probe particle beneath the apex. The dashed lines show the L-J potential of the interaction of the probe particle with the surface atom (${\mathrm{V}}_{\text{ATOM}}$) or three different distances between probe particle and surface ${\mathrm{z}}_{\text{PROBE}}$= 5.5 Å (pink dashed line), 3.5 Å (blue dash-dot line) and 2.9 Å (green dotted line). (b) Corresponding evolution of the vibration energy eigenvalue $\epsilon$ of the probe particle (red line) and probe particle potential energy ${\mathrm{V}}_{\text{ATOM}}$ (black line) as a function of ${\mathrm{z}}_{\text{PROBE}}$. Relaxations of the probe particle are not taken into the account in the model. (c) Schematic explanation how the variation of the vibration energy eigenvalue $\epsilon$ (red line) affects the intensity of the IETS-STM signal (blue peak) when the tip scans over a surface protrusion (e.g. a bond ridge or an atom). At close tip-surface distances (repulsive regime), the proximity of the surface atom/bond ridge produces a softening of the vibration energy $\epsilon$ with respect to the unperturbed vibration energy ${\epsilon }_0$ (dashed blue line). Thus the characteristic IETS peak [21] (represented by the red gaussian) centered at the vibration energy $\epsilon$ crosses the bias set point (green line) at a certain ${\mathrm{x}}_{\text{TIP}}$ accordingly changing the IETS-STM signal (blue peak). We used L-J parameters of oxygen atom for both probe particle and surface atom (see [28]) for (a),(b).

Figure 2

Hartree potential of a CoPc molecule adsorbed on Ag(110), obtained from DFT calculations in the $xy$ plane 2.00 Å above the Co atom of the molecule. The Hartree potential reveals substantial partial charging between the pyrrole rings and imine nitrogens. Color scale bar in eV. The following color code for atoms is used: N: blue, C: gray, H: turquoise, Co: pink.

Figure 3

Simulated constant-height IETS-STM images calculated at tip-sample distance $z=7.3\AA$ at $V=1.5\mathrm{mV}$ for different point charge values on the probe particle, $Q=-0.4e$, 0, and $+0.4e$. (a) IETS-STM images. (b) The same images as in panel (a), overlaid with the molecular skeleton (red line) of the relaxed CoPc molecule on Ag(110) obtained by the DFT calculations. Bright color indicates a large intensity of the IETS signal; see [29].

Figure 4

Simulated constant-height AFM, STM, and IETS-STM images obtained with the extended simulation model assuming probe particle charge of $-0.4e$. (a) Red dots indicate the relaxed $xy$ position of the probe particle; AFM (b), IETS-STM (c), and STM (d) channels for different tip-sample distances $z=8.0$, 7.3, and 7.0 Å are shown. Color scales in all images except (a) are renormalized to obtain the best molecular contrast.