26 - 31 August, 2012
Dr Nic Mullin will present a talk on "TRUE MOLECULAR RESOLUTION STUDIES OF SOFT MATTER AND BIOLOGICAL SYSTEMS BY TORSIONAL TAPPING ATOMIC FORCE MICROSCOPY"
Since the introduction of Atomic Force Microscopy (AFM) in 1986 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, 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” - 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 cantilever dynamics of torsional AFM maximise the sensitivity of the cantilever to tip-sample forces. A combination of the cantilever dynamics and the geometry of the torsional configuration 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.
Torsional tapping images 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. True molecular resolution data on semicrystalline polymers will also be presented. Including 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. 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.