"Seeing'' the covalent bond: Simulating Atomic Force Microscopy Images
| Date/Time: | Thursday, 05 Oct 2017 from 10:00 am to 10:50 am |
|---|---|
| Location: | 205 TASF |
| Phone: | 515-294-7377 |
| Channel: | College of Liberal Arts and Sciences |
| Actions: | Download iCal/vCal | Email Reminder |
Advances in atomic force microscopy (AFM) have made it possible to achieve
unprecedented images of covalent bonds, in some cases even to resolve the bond order in
polycyclic aromatics. However, fundamental questions remain about interpreting the
images and modeling the AFM tip. For example, the bright spots in non-contact AFM
images can have a close correspondence to the atomic structure of a given specimen, but
there can be contrast changes with tip height that cannot be interpreted directly by atomic
positions. While the nature of the tip can be crucial in understanding the details of the
image, the atomic structure of the tip is often unknown. This situation is compounded by
the difficulty in simulating AFM images. In order to perform computational studies of
AFM, one must determine the interatomic forces as a function of the tip height on a fine
grid above the specimen.
I will present an efficient first-principles method [1] for simulating noncontact atomic force
microscopy (nc-AFM) images using a "frozen density" embedding theory. Frozen density
embedding theory enables one to efficiently compute the tip-sample interaction by
considering a sample as a frozen external field. This method reduces the extensive
computational load of first-principles AFM simulations by avoiding consideration of the
entire tip-sample system and focusing on the tip alone. I will demonstrate that our
simulation with frozen density embedding theory accurately reproduces full density
functional theory simulations of freestanding hydrocarbon molecules while the
computational time is significantly reduced. Our method also captures the electronic effect
of a Cu(111) substrate on the AFM image of pentacene and reproduces the experimental
AFM image of Cu 2 N on a Cu(100) surface. This approach is applicable for theoretical
imaging applications on large molecules, two-dimensional materials, and materials
surfaces.