Within the past few years, mechanical probe microscopy has made remarkable progress in the direct imaging of the covalent bond in organic molecules. In particular, images from non-contact force microscopy (nc-AFM) have yielded high resolution snapshots of covalent bonds and the formation thereof, including claims of directly imaging hydrogen bond formation. The mechanical nature of the AFM probe inhibits the theoretical understanding and simulation of probe images. Most theoretical approaches to simulate nc-AFM images depend critically on the atomic structure of the tip, which is typically unknown, and the calculation of accurate forces between the tip and specimen, which is computational intense. The corresponding lack of reliable models for nc-AFM has significantly detracted from its routine application and interpretation. We propose a new approach to efficiently and accurately simulate AFM imagines by dramatically reducing the computational load in calculating atomic forces between the tip probe and the molecular species of interest. A novel aspect of our approach is that it does <i>not</i> require extensive computations of the electronic structure of the specimen. Instead, we compute the electronic structure of the molecule and ``freeze’’ the computed charge density. Density embedding theory is used to compute the force on the AFM tip as it is moved across the molecule. Changes in the forces can be used to render an accurate simulated image. Our approach builds on our previously funded work in that we can now address the role of chemical functionalization for the AFM tip, which is required to generate subatomic images. We will examine chemical bond images in important and widely studied molecules such as complex hydrocarbons. Our approach will allow us to resolve issues in chemical-structure determinations, the investigation of chemical reactions, the role of hydrogen bonding, the prediction of reaction sites, the characterization of atomic defects and the role of enhance resolution by AFM tip functionalization and orientation.

Simulating direct images of the covalent bond from atomic force microscopy

2019 Fall National Meeting

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