Imaging the fragile SEI layer in real time over multiple charging cycles is challenging in liquid electrolytes. So the team employed PeakForce Tapping because it exerts less shear force on the fragile SEI layer. 18 They also simultaneously measured the mechanical response of the SEI layer with PeakForce QNM. We wanted to investigate the elasticity of the material that forms to understand the stresses that develop in these layers due to expansion and contraction of underlying silicon, says Kumar. A highly compliant SEI layer can better withstand expansion and contraction. PeakForce QNM allows quantitative high-resolution nanomechanical data to be collected at the same time as standard AFM images, without compromising imaging rates. 1.2 m 50.0 nm 2 m

AFM images showing opening and closing of SEI cracks that formed during (ac) the first cycle, (ce) the second cycle and (eg) the third cycle. Reprinted from Kumar R, Tokranov A, Sheldon BW, et al. In situ and operando investigations of failure mechanisms of the solid electrolyte interphase on silicon electrodes. ACS Energy Letters 2016;1:68997. Copyright 2016 American Chemical Society Kumar R, Tokranov A, Sheldon BW, et al. In situ and operando investigations of failure mechanisms of the solid electrolyte interphase on silicon electrodes. ACS Energy Lett 2016;1:68997. ( acsenergylett.6b00284) 19

CASE STUDY 2. Rare earth magnets Yin Yao at the University of New South Wales in Australia uses AFM to study the magnetic properties of rare earth (RE) magnets, which are an essential component of advanced electric motors such as the direct-drive generators in wind turbines. However, the correlation between the microstructure, magnetic properties and performance of RE magnets remains poorly understood. Better understanding of the microstructure of these magnetic materials is the key to improving their properties, says Yao. The grain size, grain boundary and elemental composition are very important as they can have a major influence on the materials performance.

In a recent study, Yao and his collaborators Hansheng Chen and Rongkun Zheng used MFM, a secondary AFM mode, to study the magnetic domains of neodymium, iron and boron magnets. MFM is a variant of tapping mode that employs a magnetized tip, which first taps on the sample surface to obtain topographic information before being lifted off to obtain information about the magnetic field above the surface. This way, the localized microstructure, topography and magnetic information can all be obtained at the same time. More over, the typical resolution of the magnetic domain images is less than 100nm, allowing very small features to be captured.