Submolecular Resolution by Variation of the Inelastic Electron Tunneling Spectroscopy Amplitude and its Relation to the AFM/STM Signal

Here we show scanning tunneling microscopy (STM), noncontact atomic force microscopy (AFM), and inelastic electron tunneling spectroscopy (IETS) measurements on an organic molecule with a CO-terminated tip at 5 K. The high-resolution contrast observed simultaneously in all channels unambiguously demonstrates the common imaging mechanism in STM/AFM/IETS, related to the lateral bending of the CO-functionalized tip. The IETS spectroscopy reveals that the submolecular contrast at 5 K consists of both renormalization of vibrational frequency and variation of the amplitude of the IETS signal. This finding is also corroborated by first principles simulations. We extend accordingly the probe-particle AFM/STM/IETS model to include these two main ingredients necessary to reproduce the high-resolution IETS contrast. We also employ the first principles simulations to get more insight into a different response of frustrated translation and rotational modes of the CO tip during imaging.

Figure 1

Constant current STM images of FePc on Au(111) and IETS signal of a CO-functionalized tip. (a) $16\times 16{\mathrm{nm}}^2$ constant-current STM image of the Au(111) surface and the coadsorbed CO and FePc molecules, acquired at the $-200\mathrm{mV}$ and 10 pA set point. (b) STM-IETS of the CO adsorbed on the tip apex taken at the bare Au surface, showing the vibrational modes. The FR and FT modes of the CO are resolved. Bias voltage modulation was 3 mV at 963 Hz frequency. Stabilization set point $V=50.0\mathrm{mV}$ and $I=3.0\mathrm{nA}$.

Figure 2

Simultaneous AFM/STM/IETS images of FePc/Au(111) with a CO-functionalized tip and spatial variation of the FT mode. (a) Set of simultaneous ($1.6\times 1.6{\mathrm{nm}}^2$) current, $\mathrm{\Delta }f$, IETS, and normalized-IETS constant-height images acquired at three different tip-sample distances with a CO decorated tip. The images were acquired with bias voltage of 3.0 mV. Both $\mathrm{\Delta }f$ and IETS images show the sharp edges related with the geometric structure of the FePc molecule. (b) Spatial dependence of the frustrated translational mode for the CO-functionalized tip above the FePc molecule.

Figure 3

Variation of IETS signal with tip position. Experimental (a) and calculated (b) IETS of the FT and FR modes above a carbon bond (black) and a benzene ring (red). Both experimental and calculated plots show the variation of the amplitude in the FT mode responsible for the IETS contrast. (c) Origin of the inelastic signal (as a fraction of the total inelastic amplitude) above the benzene ring. The signal for FR modes originates mainly in the CO and tip. However, for FT modes, the CO-benzene coupling also plays a significant role.

Figure 4

Theoretical simulations of STM/AFM/IETS of FePc on the Au(111) surface at three different tip-sample distances. The tunneling current (top), $\mathrm{\Delta }f$ (middle), and IETS (bottom) images were calculated using the PP-SPM model.