Donovan SungEE235 Student Presentation

4/16/08

Chemical identification of individual surface atoms by atomic

force microscopy

Atomic Fingerprinting

Goal: Identify different

elements on a surface quickly and reproducibly using only a mechanical probe

Challenges:1. The AFM tip tends to

change shape over time

2. Difficult to precisely position the AFM tip

Sn on Si(111) Pb on Si(111)

SiliconLead Tin

Atomic Force Microscopy

Principle:An AFM operated in ultrahigh vacuum (UHV) in noncontact mode can be used to detect the chemical forces between the outermost atom of the AFM tip and the atoms on the surface.

Dynamic Force Microscopy

As the tip approaches the surface, different forces come into play depending on the distance between tip and surface.

Atom Tracking

Atom tracking allows for a precise positioning of the tip on different locations over a single molecule, using a feedback system

The radius δR must be smaller than the dimensions of the surface atom

Compensation of Lateral Thermal Drift

The precision of the positioning control is better than 0.2 Å peak to peak

The interaction force can be obtained from the detuning of the resonance frequency of the cantilever with the tip-sample vertical distance

Probing short-range interaction forces

Lead, tin, and silicon have similar electronic structures and tend to sit in similar positions on the surface

Regardless of the structure and chemistry of the tip, the ratio of the force for lead and tin relative to silicon remains the same

(Structure) (Chemistry)

Single-atom chemical identification

Lead and tin atoms are nearly indistinguishable in topography in (a), while silicon and lead are indistinguishable in (b). However, the ratio of forces remains the same, allowing easy identification of the atoms in each case.

(a) Si, Sn, Pb mixed equally (b) Pb surrounded by Si

Conclusion

This work helps to establish the AFM as a metrology tool on the atomic scale

Quick and reproducible way to identify atoms quickly, which should be widely applicable

Developed a “standard” for identifying atoms, similar to element-specific forms of spectroscopy

References

Abe, M. et al. “Room-temperature reproducible spatial force spectroscopy using atom-tracking technique.” Applied Physics Letters 87, 173503 (2005).

Sugimoto, Y. et al. “Chemical identification of individual surface atoms by atomic force microscopy.” Nature 446, 64-67 (2007).