The 15th European Microscopy Congress

16th - 21st September 2012, Manchester

Dr. Nic Mullin will present a talk on "Molecular resolution studies of soft matter by torsional tapping atomic force microscopy"

 

Molecular resolution studies of soft matter by torsional tapping atomic force microscopy

Nic Mullin and Jamie K. Hobbs


Since the introduction of Atomic Force Microscopy (AFM) in 1986[1] there has been great progress towards the visualization of insulating surfaces, in real space, at sub-nanometre resolution. While the instrumentation has progressed to allow true atomic resolution to be obtained outside of vacuum[2], there are still severe constraints upon the types of samples that may be imaged and the conditions that they may be observed under – namely that the surface must be close to atomically flat and under liquid. This largely limits the systems that may be studied to hard, crystalline materials. In the work presented here, a relatively new refinement to dynamic AFM – “torsional tapping”[3] - which allows true molecular resolution to be obtained on rough, soft surfaces, in air, will be described.
Torsional tapping utilizes T-shaped cantilevers with the tip offset from the long axis, driven into torsional oscillation, to provide a tapping motion at the tip. The torsional amplitude is used for feedback and the torsional phase and flexural deflection may also be recorded. The favourable dynamics (high Quality factor (Q) and resonant frequency combined with a low spring constant) of torsional AFM cantilever resonances maximise the sensitivity of the cantilever to tip-sample forces. The high frequency and Q, combined with improved optical lever sensitivity in torsion also minimise the influence of noise arising both from thermal fluctuations and from noise sources inherent in the detection system. The cantilever is free to deflect flexurally, and this passively limits tip-sample forces under error signal, allowing extremely sharp tips to be used without blunting on rough surfaces. Analytical measurements of the performance of the instrument will be presented, including discussion of current and future developments for increased spatial and temporal resolution and broader application of the technique.


High resolution images of a variety of soft matter and biological systems will also be presented. Torsional tapping data are used to complement reciprocal space techniques such as electron and X-ray diffraction to provide information concerning the orientation of proteins in 2D crystals[4] and the nature of defects and grain boundaries in liquid crystal samples[5]. DNA is studied and the 3.4 nm helical pitch of the molecule is clearly resolved. True molecular resolution data on semicrystalline polymers will also be presented. These include rough, thick samples of polyethylene – a model system for the crystallization of long chain molecules, and the most widely used synthetic polymer - in which chain folds, single molecular crystal defects and disordered “loose” molecular loops are observed for the first time[6]. Crystalline regions with intermolecular spacings as small as 3.7 Å also directly resolved. Images of semicrystalline isotactic polypropylene samples, in which submolecular details such as the helical pitch and handedness of the molecules are resolved, will also be presented. The data obtained on these systems will be discussed both within the framework of scanning probe instrumentation and in terms of the implications of the results obtained in the fields of biology, materials science and polymer physics.
 

References:
[1] G. Binnig, Ch. Gerber and C.F. Quate, Phys. Rev Lett. 56 (1986), p.930.
[2] T. Fukuma, K. Kobayashi, K. Matsushige and H. Yamada,  Appl. Phys. Lett. 87 (2005), p.034101.
[3] N. Mullin et al., Appl. Phys. Lett. 94 (2009), p.173109.
[4] L. Kailas et al., PNAS 108 (2011), p.16014.
[5] C. Weber et al., Soft Matter 6 (2010), p.5390.
[6] N. Mullin & J.K. Hobbs, Phys. Rev. Lett. 107 (2011), p.197801.

[1] G. Binnig, Ch. Gerber and C.F. Quate, Phys. Rev Lett. 56 (1986), p.930.

 

[2] T. Fukuma, K. Kobayashi, K. Matsushige and H. Yamada, Appl. Phys. Lett. 87 (2005), p.034101.

 

[3] N. Mullin et al., Appl. Phys. Lett. 94 (2009), p.173109.

 

[4] L. Kailas et al., PNAS 108 (2011), p.16014.

 

[5] C. Weber et al., Soft Matter 6 (2010), p.5390.

 

[6] N. Mullin & J.K. Hobbs, Phys. Rev. Lett. 107 (2011), p.197801.