ISPM conference Toronto 2012

June 15-18 2012

Dr. Nic Mullin will give a talk about some of his work on torsional resonance AFM, entitles "True molecular resolution studies of soft and biological samples in ambient conditions using torsional tapping atomic force microscopy"


True molecular resolution studies of soft and biological samples in ambient conditions using torsional tapping atomic force microscopy

Nic Mullin and Jamie K. Hobbs

Since their invention in the early 1980s, scanning probe methods have offered the promise of visualizing surfaces with atomic and molecular resolution in real space. Initially, this was realized[1] under vacuum with scanning tunneling microscopy on conducting samples. Later atomic force microscopy (AFM) allowed similar resolution to be obtained[2] on insulating surfaces under vacuum. More recently, instrumentation has progressed to allow true atomic resolution to be obtained in liquids[3]. However, severe constraints still preclude the application of these methods to many systems of interest. Two of the most prevalent of these are the difficulty of high resolution imaging in ambient air, and the difficulty in obtaining sub-nanometre resolution on surfaces with a topographic roughness of more than a few Ångstroms.
A relatively new variation on dynamic AFM, torsional tapping[4] (TT), is used to address these constraints and obtain true molecular resolution on topographically rough, soft surfaces in ambient conditions.
In TT, a T-shaped cantilever, with a tip offset from its long axis, is driven into torsional oscillation to yield a tapping motion at the tip. The favourable dynamics of torsional resonance, coupled with only a modest increase in spring constant, result in increased force sensitivity and signal-to-noise ratio (SNR). Furthermore, the optical lever detection system is more effective for torsional cantilever bending, further increasing the SNR. Finally, as the torsional mode of the cantilever is mounted on the (softer) flexural mode, the tip-sample force under feedback error signal is reduced, allowing sharp tips to be used on rough samples without blunting.

Studies of synthetic polymers (including polyethylene and polypropylene) in which single polymer chains[5], defects, monomer repeat units, chiral twist of chains are resolved will be presented. Data on biological systems, including measurement of the helical pitch of DNA and sub-nanometre resolution on 2D protein crystals[6] will also be shown.

[1] G. Binnig, H. Rohrer, Ch. Gerber and E. Weibel, Phys. Rev. Lett. 50 (1983) 120.
[2] F. J. Giessibl, Science 267 (1995) 68.
[3] T. Fukuma, K. Kobayashi, K. Matsushige and H. Yamada, Appl. Phys. Lett. 87 (2005) 034101.
[4] N. Mullin et al. Appl. Phys. Lett. 94 (2009) 173109.