The advancements in material technology have necessitated the development of sophisticated instruments to study materials at the scale of atoms and molecules. One such popular instrument is an Atomic Force Microscope (AFM). It measures and controls the forces from the surface atoms of a substance- also called the sample. A spatial map of these regulated forces generates a 2-dimensional topography image of its surface. The AFM measures these forces using micro-meter sized mechanical structures called ‘probes’, which have sharp tips to explore the surface and a recording device to store these measurements.
Besides the topography of the surface, the AFM can also measure the material properties of the sample, by studying the behaviour of these atomic forces. Hence, one can characterize the sample by measuring physical, structural and mechanical quantities like the spacing between atoms, length of a polymer chain, binding forces between molecules, stiffness, hardness and adhesion. Given these abilities, AFMs are widely used in studying biological samples, drug research, nanomechanics, nanolithography, food research, polymer engineering and thermal analysis.
However at such small scales, measuring the material properties accurately is a challenge. To address this challenge, researchers from the Indian Institute of Science are building an AFM system that can allow them to precisely measure these properties, while having a peek at the atoms.
In their system, the probe is vibrated at a particular frequency and the parameters of its oscillation get modified due to the interaction with the sample. “The design of the AFM probe depends on the frequency of its vibration. Better readings are obtained at its ‘Eigen frequencies’ – the frequencies at which the probe is more sensitive. The probe is usually set to vibrate at its lowest Eigen frequency”, says Prof. Jayanth, Department of Instrumentation and Applied Physics, Indian Institute of Science, Bengaluru. Typically, to obtain information about the material properties, the response of the probe has to be measured at integral multiples of the vibrated frequency, called the Harmonic Frequencies. However, the response cannot be measured precisely at these frequencies since the sensitivity of the conventional probe is poor. “We designed the probe in a way that the higher Eigen frequencies coincide with the higher harmonic frequencies. This significantly enhances the sensitivity of the probe”, explains Sriramshankar, a Ph.D student about the design of the probe.
The team at IISc has more reasons to be proud. The physical design of the probe differed from conventional designs; it was designed to undergo twisting motion upon oscillation, as opposed to the bending motion used by conventional designs. Further, specially developed models and algorithms were used to iteratively correct the geometry of the probe to reach the perfect design. To quickly fabricate prototypes of the designed probe, they used a technology called “focused ion-beam milling”. The probe was fabricated and tested in the Micro and Nano Characterization Facility (MNCF) at the Centre of Nano Science and Engineering (CeNSE), IISc.
What’s in the future? Prof. Jayanth says: “We hope to develop a novel AFM that can simultaneously map the surface topography and mechanical properties of a material in real-time with atomic-scale resolution. This can also study hybrid materials, where two dissimilar components are distributed among each other. Changes in physical and atomic structure of the materials in response to radiation, electric field and chemicals can also be explored,” he signs off.
About the authors
G. R. Jayanth is an Assistant Professor and R. Sriramshankar is working towards his PhD degree. The paper appeared in IEEE/ASME Transactions on Mechatronics, vol. 20 on 4th August 2015.
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