Oct 27, 2011
AFM model evaluates true height of water layers
Under ambient conditions, nanometric water layers form on hydrophilic surfaces, covering them and modifying sample properties and characteristics. Water layers can have a great impact on the surface chemistry, reactivity and the overall performance of a substrate. As a result, instruments with the capability of measuring the thickness of water layers at the nanoscale in a routine fashion are of paramount importance in understanding these effects.
In a recent study, researchers based in the UAE, Spain and the UK have teamed up to deal with the apparent height of water layers as measured by the Atomic Force Microscope (AFM) in one of the apparatus' most widely used modes; amplitude modulation. The group has modelled hydrophilic and hydrophobic surfaces and the interaction between these and a nanoscale hydrated oscillator, in other words the tip of the AFM probe.
In this way, the scientists show that system dynamics can lead to drastically different measurements for the apparent height of water layers. The researchers differentiate between four cases where, in one extreme, the water layers are never perturbed (termed pure non-contact mode). In the other extreme there is mechanical contact between tip and samples, and the apparent height is zero or close to zero. In transition regions the measured apparent height can be up to four times larger than the true height.
The team theoretically predicts the water height measurements in each mode and shows how to monitor each interaction experimentally. Predictions match experimental outcomes thus validating the method, and the development of a pure non-contact mode of imaging implies that consistent measurements can be now found.
The work will have future applications in modes where water perturbation negates the validity of measurements such as Kelvin Probe AFM.
Further details including supplementary data can be found in the journal Nanotechnology.
About the author
The team is based at the Laboratory for Energy and Nanosciences, Masdar Institute of Science & Technology, UAE; Centre d' Investigacio en Nanociencia i Nanotecnologia (CIN2) (CSIC-ICN), Esfera UAB, Spain; and the School of Physics and Astronomy and Department of Oral Biology, University of Leeds, UK.