Self-Sensing & Self-Actuating AFM Probes

One of the challenges in AFM instrumentation is the implementation of non-optical sensing systems, which will open many new opportunities for using Atomic Force Microscopy (AFM) in different conditions and environments.

In the history of Atomic Force Microscopy, AFM cantilevers featuring piezo resistors are most likely the first ones which have opened the epoch of integrated non-optical sensing in an AFM. Many AFM probes with different sensing principles, like capacitive, piezoelectric, MOS transistor etc., have been published so far.

A special subgroup of self-actuating and self-sensing AFM probes is based on quartz tuning forks (TF) which are mainly produced for watches. Their size and availability as well as stable mechanical oscillation with high quality factor make them very attractive also as oscillatory force sensors in dynamic mode AFM. The self-sensing and self-actuating capabilities, which make the use of optics or an external piezo actuator not necessary, as well as tuning forks’ inherent low power consumption, make this kind of AFM probes very interesting for use in ultrahigh vacuum, liquids or for low temperature applications.

Several designs and concepts for TF scanning probe microscopes (SPM) have been introduced. The first reported application was as “distance” sensors in acoustic near field microscopy. In scanning near field optical microscope (SNOM), TFs are often used for controlling the distance between the optical near field probe and the surface.

The most popular type for AFM imaging is using one of the two prongs as a “cantilever”. In this concept, the other prong is entirely fixed on a substrate in order to reduce the quality factor, which is too high in some cases, if both prongs are free to vibrate. A probing AFM tip is usually glued on the end of the prong.

The prongs are generally very stiff, in the rage of a few kilonewton per meter. On one hand, this stiffer spring enables stable AFM tip vibration with sub-nanometer amplitude and is therefore an advantage. For example, when atomic resolution in vacuum is desired, a stable oscillation with 0.3–1 nm amplitude is preferred. On the other hand, if one would like to use this AFM probe for topographic AFM imaging in atmospheric environments, the amplitude is too small and the AFM tip would be easily crashed on a sample surface due to the high stiffness of the AFM cantilever. A contrivance is required for the standard AFM application.

The probing AFM tip is one of the critical points in adapting tuning forks for AFM. The lateral resolution of AFM images is determined by the quality of the AFM tip. Initially, an edge of one of the prongs was used for probing. It was soon replaced by electrochemically sharpened W, Ni, or Pt/Ir wires glued on one of the two prongs. Standard microfabricated AFM cantilevers with sharp AFM tips have also been applied in the same manner. In the SNOM configuration, the tapered end of an optical fiber attached to the tuning fork is used as a tip. Also, the use of a diamond and a multiwall carbon nanotube have been reported.

NANOSENSORS™ Akiyama-Probe (also sometimes called A-Probe) is a self-sensing and self-actuating (-exciting) AFM probe based on a quartz tuning fork combined with a micromachined AFM cantilever for dynamic mode AFM. The great advantage of this AFM probe is that one can benefit from both the tuning fork's extremely stable oscillation and the silicon AFM cantilever's reasonable spring constant with one AFM probe. Akiyama-Probe is equipped with a high-end sharp silicon AFM tip and has an excellent imaging capability on various samples with different properties, which is as high as a conventional optical AFM lever system. The Akiyama-Probe requires neither optical detection, nor external shaker and occupies only a small space above the sample.

The recommended operation mode for Akiyama-Probe is dynamic mode with the frequency modulation (FM) detection, where the TF is self-excited (oscillating) at its first resonance frequency. The TF is used as an oscillatory force sensor similar to a quartz microbalance. Its frequency and amplitude are influenced by those of the AFM tip motion. Consequently, the AFM tip- sample interaction can be electrically detected with the TF. During sample imaging, the resonance frequency is tracked by a phase locked loop (PLL) and kept at a set value by adjusting the AFM tip-sample distance with a feedback loop of AFM.

To benefit from the advantages of Akiyama-Probe, a proper operation setup is required: (i) an electrical circuit which makes a self-oscillation of the AFM probe, and (ii) a Phase-locked Loop (PLL) to measure the oscillating frequency. In order to save some time and effort, a commercially available instrument should be the first choice. One can obtain the best performance in a shorter time when using finished instruments and components compared to having to start from scratch. An alternative to the commercially available/offered solutions is building one’s own setup. Many examples and suggestions are available at NanoAndMore also offers a basic Tuning Fork Sensor controller (TFSC) which is an electronic device for controlling the self-oscillation of tuning fork sensors and measuring their frequency. This device can be useful if you plan on building your own setup.

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