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Exploring Intermolecular Interactions with the Scanning Tunneling Microscope

  • Author(s): Han, Zhumin
  • Advisor(s): Ho, Wilson
  • et al.
Abstract

Compared to intramolecular interactions, intermolecular interactions are relatively weak but they lay the foundation for research involving molecular recognition, self-assembly and surface adsorption in chemical and physical systems. This dissertation describes three different experimental approaches based on the detection of molecular vibrations to provide direct insights into intermolecular interactions at the sub-Ångström spatial resolution with a home built sub-Kelvin scanning tunneling microscope (STM).

First, the intermolecular interaction can be evaluated by measuring the coupled vibrational mode of two interacting molecules with STM inelastic electron tunneling spectroscopy (IETS). The measurement of intermolecular coupled vibrations with tunable vertical and lateral displacements offers a direct assessment of the short range intermolecular repulsion in three dimensions.

Second, the self-assembled molecular bonding structures can be imaged by the inelastic tunneling probe (itProbe). In itProbe, a carbon monoxide (CO) molecule is transferred onto the STM tip. The hindered translation energy of the CO-tip varies when it is positioned over different locations of the self-assembled structures. By recording the intensity variations of the inelastic electron tunneling signal caused by the energy shift, the geometric structure of each molecule and intermolecular interactions can be imaged in real space.

Third, a sample surface can be considered as a giant molecule with infinite mass, and the binding strength of an adsorbed molecule on the surface is reflected in the external vibrations. The combination of the itProbe and IETS enable the determination of the adsorption configuration and low energy external vibrational modes of individual physisorbed benzene on an inert metal for the first time.

All the approaches mentioned above rely on resolving fine spectral features induced by intermolecular interactions. The unprecedented spectral sensitivity, energy resolution and thermal stability achieved in 600 mK create the opportunity to study these weak intermolecular interaction effects at the single molecule level, which are otherwise obscured at higher temperatures.

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