#High-sensitivity atomic force microscopy opens up for photosensitive materials

To demonstrate the applicability of their approach they measured cantilever resonance curves and the atomic scale topography of a mica surface in phosphate buffered saline solution with various customized cantilevers including those with a megahertz-order resonance frequency.
Atomic force microscopy
The first image using AFM was reported by Gerd Binnig, Calvin Quate and Christoph Gerber in 1986, five years after the scanning tunneling microscope. The technique is capable of atomic scale resolution and generates images by measuring the sum strength of a number of forces at play between tip and sample, including van der Waals and electrostatic.
AFM uses a cantilever with a tiny tip attached at the end. For static AFM the tip is dragged over the surface and the cantilever deflection is measured or, the cantilever height is adjusted to maintain a constant deflection. In dynamic AFM, where the cantilever oscillates at its resonance frequency and taps the surface with the tip, contact between the tip and surface is causing less damage to the sample. It is capable of high sensitivity imaging without making contact with the surface at all in non-contact mode, by monitoring the impact of interactions with the surface on the amplitude and frequency of the cantilever oscillations.
Besides piezo actuated and photothermal cantilever excitation electrostatic and electrostrictive interactions can be used by applying a bias voltage between tip and surface or both sides of a cantilever. However, in many of the liquids used to house samples, this can cause uncontrolled chemical reactions.
Closed loop versus open loop with differentiation circuits
When using magnetic fields to excite oscillations in the cantilever, the circuit supplying current to the solenoid coil needs to maintain a constant current amplitude. However, the impedance of the circuit increases with frequency, so that a higher voltage signal is needed to maintain a constant current amplitude. This is usually achieved with a feedback loop, which converts the coil current to a voltage and compares it with the input voltage. However, this feedback loop becomes unstable at megahertz frequencies.
In the open-loop circuit used instead, the input voltage is fed into a differentiation circuit that returns a complex coil voltage that is proportional to the input voltage and the frequency (V_coil = iωV_in, where V_coil is the coil voltage, V_in is the input voltage and ω is the frequency.) This way the coil voltage automatically scales with the frequency, compensating for the frequency-dependent impedance changes.