College of Sciences





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Mechanical properties of molecules adsorbed on materials surfaces are increasingly vital for the applications of molecular thin films. Here, we conduct a fundamental research to induce conformational change mechanically on a single molecule and quantify the driving force needed for such molecular shape switch via a low temperature (~ 5K) Scanning Tunneling Microscope (STM) and Qplus Atomic Force Microscope (Q+AFM). Our measurement maps a three-dimensional landscape for mechanical potential and force at single molecule level with high spatial resolution in all three dimensions of a few angstrom (10-10 m).

Molecule TBrPP-Co (a cobalt porphyrin) deposited on an atomically clean gold substrate typically has two of its pentagon rings tilted upward and the other two downward. An atomically sharp tip of the STM/Q+AFM, which vibrates with a high frequency (~ 30kHz), is employed to scan the molecule at different heights with 0.1Å increment and meanwhile record tip-molecule interaction strength in the form of tip frequency change. When tip approaches to the threshold distance to the molecule, mechanical force become large enough and cause pentagon rings flip their direction. Due to the sensitive nature of tip-molecule interaction, the rings flipping can be directly visualized by STM, as rings tilting upward exhibit two bright protrusions in contrast to rings downward in image. By processing frequency change, we obtain a three-dimensional mechanical potential and force map for a single molecule with the resolution of angstrom level in all three dimensions. Our preliminary results indicate that an energy barrier of ~400meV needs to be overcome for rings flipping of TBrPP-Co.


3D energy map, Single molecular switch, Scanning Tunneling Microscope, Qplus Atomic Force Microscope, Material Surface Science


Atomic, Molecular and Optical Physics



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Direct Visualization of 3-Dimensional Force and Energy Map of a Single Molecular Switch