Hovering and Twirling of Tethered Molecules by Confinement between Surfaces

Abstract

Through STM images, we show that azobenzene-terminated alkanethiols hover and twirl when confined between the Ag tip and Au(111) substrate of an STM junction. In contrast with mechanisms of activation used to drive molecular rotors, twirling is induced by the effective elimination of lateral corrugation in the energy landscape when molecules hover by their van der Waals attraction to the approaching tip. While in the stationary state the benzenes of the head group lie flat with an inter-ring separation of 7.5 Å, they stand on-edge as the molecule twirls and their separation contracts to 5.2 Å, close to the value of the free molecule. The captured images of motion allow the characterization of physisorption potentials.

Ion-Selective, Atom-Resolved Imaging of a 2D Cu2N Insulator: Field and Current Driven Tip-Enhanced Raman Spectromicroscopy Using a Molecule-Terminated Tip

“…The classical Stark effect, induced by a DC field along a dipolar mode with interaction potential V ′ = −μ· E , cannot be trivially invoked since the DC field from the applied bias is normal to the plane of the molecule and the observed lines are in-plane vibrations of even ( g ) symmetry that do not carry net dipole. Doming of the molecule, where the aromatic macrocycle snaps onto the substrate while the Co remains bonded to Ag, and subsequent replanarization when it is squeezed in the hovering potential between tip and substrate, gives a consistent explanation of the discontinuity and turnover in frequency shifts, respectively. Nevertheless, it is useful to note that in conductive junctions, intramolecular fields induced by charging of the molecule allow Stark shifts along modes orthogonal to the external field.…”

Tip-Enhanced Raman Spectromicroscopy of Co(II)-Tetraphenylporphyrin on Au(111): Toward the Chemists’ Microscope

“…The gap dependence of the signal intensity can be approximated as exponential, with a decay constant of 3.5 Å. The scatter in the retraction data is due to motion of the molecule, both thermal and lift-off by the van der Waals attraction to the tip . The spectrum (Figure d) consists of three distinct components: the exponentially decaying background of electronic Raman scattering on the metal at shifts larger than 1700 cm –1 (better shown in Figure c), a flat background with a cutoff near 1600 cm –1 , and a set of thermally broadened molecular vibrational lines within this window.…”

“…The scatter in the retraction data is due to motion of the molecule, both thermal and lift-off by the van der Waals attraction to the tip. 32 The spectrum (Figure 1d) consists of three distinct components: the exponentially decaying background of electronic Raman scattering on the metal at shifts larger than 1700 cm −1 (better shown in Figure 6c), a flat background with a cutoff near 1600 cm −1 , and a set of thermally broadened molecular vibrational lines within this window. The presented spatial maps are the spectrally integrated TERS intensity (Figure 1b) and the mode-specific map of the vibration at 1576 cm −1 (Figure 1c).…”

The Raman Spectrum of a Single Molecule on an Electrochemically Etched Silver Tip

“…In effect, these images capture orientational dynamics assisted by the tip-induced hovering potential. 46 Both images underscore that the ABT molecules preferentially bind to reactive sites: step edges in Fig. 1a and domain boundaries in Fig.…”