International Conference on Nanoscience + Technology (ICN+T2012)

July 23-27, 2012, Paris

Dr. Nic Mullin will present a talk on his work with torsional tapping atomic force microscopy


Torsional tapping [1] mode Atomic Force Microscopy (AFM) is a relatively new variant of intermittent contact AFM that allows true molecular resolution to be routinely obtained on rough, soft surfaces in ambient air. Recent results obtained with this technique include resolving different crystal planes and sub-molecular defects in polyethylene crystals with 3.7Å resolution [2] and the observation of polypropylene molecules in bulk crystals as individual helices whose handedness and 6.5Å pitch is clearly resolved. 

In torsional tapping, a T-shaped cantilever with the tip offset from the long axis is driven into torsional oscillation close to its first torsional resonance, yielding a tapping motion at the tip. The torsional amplitude is detected and used for feedback and height, error-signal and phase images are recorded as normal.

The use of torsional oscillations has a variety of benefits that allow considerably increased resolution, force sensitivity and scan speed. As the mass of the cantilever lies close to the oscillation axis, its moment of inertia is considerably reduced (as compared to the same cantilever undergoing flexural oscillation). This yields an increase in resonant frequency out of proportion to the increase in spring constant. Furthermore, as only a small volume is swept out by the cantilever during torsional oscillation, the viscous damping of the cantilever by the surrounding medium is significantly reduced, giving a corresponding increase in Quality factor (Q). These characteristics lead to the force sensitivity being increased by approximately a factor of 3. The increase of the resonant frequency by a larger factor than the Q also increases the bandwidth of the cantilever (and hence the available scan rate) by a factor of 3.

In addition to improved cantilever dynamics, the geometry of the AFM under torsional tapping is improved. As the tip offset is smaller than the length of the cantilever, the optical lever detection system is proportionally more sensitive to torsional deflection. This, combined with the increases in Q and resonant frequency, reduces the detection noise floor by a factor of 12 when comparing the flexural and torsional modes of the same cantilever.

Finally, the properties of the torsional mode of the cantilever are effectively decoupled from that of the flexural mode. This means that the spring used for imaging (the torsional mode) may be optimised to give optimal characteristics for oscillation, while the flexural mode may be kept soft to allow passive deflection (limiting the tip sample force) under feedback error signal. This allows the use of fragile (but extremely sharp) carbon “whisker” tips on rough surfaces.

Using the technique described above, it has been possible to repeatably obtain sub-nanometre resolution on soft matter and biological samples including semicrystalline polymers, 2D protein crystals, organic semiconductors and molecular networks. For several of these systems this is the first demonstration of real-space imaging at true molecular resolution.  Measurements of polymer chain helicity, crystalline stem length, and single molecular statistics will be presented. The function, optimisation and further development of the instrument, including cantilever design, will also be discussed.

[1] N. Mullin et al “”Torsional tapping” atomic force microscopy using T-shaped cantilevers” Appl. Phys. Lett. 94(17), 173109 (2009)
[2] N. Mullin and J. K. Hobbs “Imaging thick polyethylene films at 3.7 Å resolution with torsional tapping atomic force microscopy” Phys. Rev. Lett. 107(19), 197801 (2011)