Atomic Force Microscopes: How They Work Without a Light Source

Atomic Force Microscopes (AFMs) are powerful tools used to visualize surfaces with atomic-level resolution. Unlike optical microscopes that rely on light, AFMs operate based on the interaction between a probe and the sample surface. This article explores how AFMs function without a light source and highlights the key components and techniques involved in their operation.

Key Components of an AFM

An AFM consists of a sharp tip that scans over a sample surface, measuring the interaction forces between the tip and the sample. The forces acting on the sharp tip cause it to deflect, and these deflections are detected to create an image of the surface. Unlike optical microscopes, AFMs do not require illumination and can operate in varied environments, including air and liquid.

Optical Beam Deflection (OBD)

The most common detection system for AFMs today is the Optical Beam Deflection (OBD) system. This system utilizes a laser that bounces off the top of the cantilever and is detected by a photodetector. As the cantilever deflects, the laser's position on the photodetector changes, allowing the AFM to measure the X-Y-Z position of the sample. This data is then used to create a digital image of the sample's surface.

History and Evolutions

Early AFMs used non-optical methods like the Scanning Tunneling Microscope (STM) to detect the deflection of a metal cantilever. The tip of the STM was positioned behind the metal cantilever, and the bending of the cantilever was measured by the STM's servo. However, this method was quickly replaced by OBD and optical interferometer methods. These later systems allowed for more precise and versatile measurements.

Non-Optical Detection Systems

Even today, non-optical detection systems such as piezoresistive cantilevers and piezoelectric sensors are still utilized. These methods are particularly useful in situations where optical alignments are challenging, such as in ultrahigh vacuum (UHV) environments or low-temperature cryostats. Here's a closer look at each:

Piezoresistive Cantilevers

These cantilevers are made from silicon with doped resistive channels on the back. The resistance of these channels changes slightly as the cantilever bends, making them effective for measuring both DC and AC components of cantilever deflection. The change in resistance is detected using a Wheatstone bridge circuit.

Piezoelectric Sensors

Among the non-optical AFM detection systems, piezoelectric sensors are the most commonly used. One early design involved an etched tungsten tip glued to a quartz shear mode crystal resonator. Although this design did not gain widespread use, quartz tuning fork probes are now widely employed in UHV and low-temperature applications. The resonant frequency of the quartz tuning fork changes as the tip approaches the sample surface, and these frequency changes are measured using a phase-locked loop to maintain a constant force between the tip and the sample.

Advantages of AFMs

AFMs offer several advantages over electron microscopes (EMs). Most notably, EMs require high vacuum environments and materials that can deflect electrons, such as metals. In contrast, AFMs can image living matter and materials that cannot deflect electrons, providing unprecedented insights into biological and chemical processes. The ability of AFMs to capture live matter in action has been a driving force behind many recent advances in bioscience.

Conclusion

In summary, AFMs operate on the principles of detecting surface interactions without the need for a light source. By utilizing various detection systems like OBD, piezoresistive cantilevers, and piezoelectric sensors, these microscopes offer unparalleled resolution and versatility. Whether in a research lab or industrial setting, AFMs have proven to be indispensable tools for visualizing and understanding the intricate details of surfaces at the atomic scale.