Advantages of Force Microscopy with a Vertically-Oriented Cantilever

Session

Atomic and Molecular Resolution Phenomena via AFM, STM and Scanning Probes

Authors

Prof Mervyn Miles (1), Dr. Massimo Antognozzi (1), Dr. Robert Harniman (1), Dr. Loren Picco (1), Dr. Ashvin Cherodian (deceased) (1)

Affiliations

1. University of Bristol

Keywords

Vertical probe microscopy, shear force microscopy, transverse dynamic force microscopy, lateral molecular force microscopy, AFM, biomolecules, force spectroscopy

Abstract text

Vertically-oriented-probe force microscopy (VOP FM) consists of a cantilever probe oriented perpendicularly to the sample surface rather than the usual ‘horizontal’ cantilever of conventional AFM. This presentation highlights the advantages the vertical configuration has over the ‘horizontal’ and their geometric origins:

With the probe perpendicular to the sample surface, it has essentially infinite rigidity in this direction and so is inherently extremely stable vertically.  With a cylindrical vertical cantilever, the probe can have circular symmetry.  This permits force measurements in two dimensions.

With a vertically rigid probe, contact mode is not possible, and so imaging is always in a non-contact mode.  The cantilever is made to oscillate in the plane parallel to the sample surface at a distance of 1 to 2 nm, resulting in an alternating force measurement, thus providing a complex information.

 Some examples of the various advantages of VOP FM will be presented, beginning with one-dimensional measurements such as the following:

• single-molecule complex force-extension measurements and transitions,
• the mechanics of water layers adsorbed to a surface, 
• and phase transitions in confined-geometry liquid-crystal systems.

An example of two-dimensional imaging is that of local magnetic fields at the nanoscale.  Here two perpendicular components of the field can be measured using either eddy current damping or with a magnetic moment of the vertical probe.

With a tip-sample separation of  < 2 nm, under ambient conditions, this tip-sample gap results in capillary condensation of water in the gap, assisting in determining the tip-sample separation.  The VOP FM can, of course, also be operated in water environment in a liquid cell. The probe is typically driven at its fundamental flexural resonant frequency at ~ 1 nm amplitude at the free end.  As the tip approaches the surface, the amplitude of oscillation decreases and is accompanied by a frequency shift.  Usually, the amplitude is used as the controlling parameter in feedback to maintain the tip-sample separation during imaging.    

The VOP FM imposes only extremely small normal forces on the sample, resulting in much lower deformations of molecular structures.  An initially surprising observation was that the resolution in VOP images can be significantly better than the 1 nm oscillation amplitude.  Some images containing  atomic and submolecular details will be presented.