UC Berkeley

This dissertation focuses on the optomechanical behavior of molecular adsorbates on semiconducting and metal surfaces. Specifically, light-induced conformational changes of an azobenzene derivative on a semiconducting surface and infrared active vibrational modes of diamondoid molecules on a metal surface were studied in a scanning tunneling microscopy setup.

Using conventional scanning tunneling microscopy, the self-assembly and light-induced mechanical switching of 3,3',5,5'-tetra-tert-butylazobenzene deposited on GaAs(110) were explored at the single-molecule level. After exposure to ultraviolet light, tetra-tert-butylazobenzene molecules exhibited conformational changes attributed to trans to cis photoisomerization. Photoisomerization of the molecules was observed to occur preferentially in one-dimensional stripes, a behavior that is significantly different from optically induced switching behavior observed when these molecules were placed on a gold surface.

To characterize infrared absorption of submonolayers of molecules on electrically conducting crystals, a new scanning-tunneling-microscopy-based spectroscopy technique was developed. The technique employs a scanning tunneling microscope as a precise detector to measure the expansion of a molecule-decorated crystal that is irradiated by infrared light from a tunable laser source. Using this technique, the infrared absorption spectra of adamantane, [121]tetramantane, and [123]tetramantane on a Au(111) surface were obtained. Significant differences between the infrared spectra for the two tetramantane isomers show the power of the new technique to differentiate chemical structures even when single-molecule-resolved scanning tunneling microscopy images look quite similar. For adamantane on Au(111), the simplest studied system, ab initio calculations allowed for the interpretation of the microscopic vibrational dynamics revealed by the measurements. The infrared spectrum of an adamantane monolayer on Au(111) was found to be substantially modified with respect to the gas-phase infrared spectrum due to both the adamantane-adamantane interaction and adamantane-gold interactions.

The results presented in this dissertation improve our understanding of photoisomerizational and optically induced vibrational properties of molecules placed in a condensed matter environment, and highlight the important role played by the molecule-surface and molecule-molecule interactions. These results are highly relevant to the field of molecular electronics and optically actuated molecular nanomachines.